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Probing Self-Diffusion of Guest Molecules in a Covalent Organic Framework: Simulation and Experiment
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  • Lars Grunenberg
    Lars Grunenberg
    Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart 70569, Germany
    Department of Chemistry, Ludwig-Maximilians-Universität (LMU), Butenandtstr. 5-13, Munich 81377, Germany
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    Orcidhttps://orcid.org/0000-0002-6831-4626
  • Christopher Keßler
    Christopher Keßler
    Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany
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  • Tiong Wei Teh
    Tiong Wei Teh
    Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany
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  • Robin Schuldt
    Robin Schuldt
    Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
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  • Fabian Heck
    Fabian Heck
    Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart 70569, Germany
    Department of Chemistry, Ludwig-Maximilians-Universität (LMU), Butenandtstr. 5-13, Munich 81377, Germany
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    Orcidhttps://orcid.org/0000-0003-0198-3398
  • Johannes Kästner
    Johannes Kästner
    Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
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    Orcidhttps://orcid.org/0000-0001-6178-7669
  • Joachim Groß
    Joachim Groß
    Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany
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    Orcidhttps://orcid.org/0000-0001-8632-357X
  • Niels Hansen*
    Niels Hansen
    Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-4366-6120
  • Bettina V. Lotsch*
    Bettina V. Lotsch
    Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart 70569, Germany
    Department of Chemistry, Ludwig-Maximilians-Universität (LMU), Butenandtstr. 5-13, Munich 81377, Germany
    E-conversion, Lichtenbergstrasse 4a, Garching 85748, Germany
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-3094-303X
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ACS Nano

Cite this: ACS Nano 2024, 18, 25, 16091–16100
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https://doi.org/10.1021/acsnano.3c12167
Published June 11, 2024

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Abstract

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Covalent organic frameworks (COFs) are a class of porous materials whose sorption properties have so far been studied primarily by physisorption. Quantifying the self-diffusion of guest molecules inside their nanometer-sized pores allows for a better understanding of confinement effects or transport limitations and is thus essential for various applications ranging from molecular separation to catalysis. Using a combination of pulsed field gradient nuclear magnetic resonance measurements and molecular dynamics simulations, we have studied the self-diffusion of acetonitrile and chloroform in the 1D pore channels of two imine-linked COFs (PI-3-COF) with different levels of crystallinity and porosity. The higher crystallinity and porosity sample exhibited anisotropic diffusion for MeCN parallel to the pore direction, with a diffusion coefficient of Dpar = 6.1(3) × 10–10 m2 s–1 at 300 K, indicating 1D transport and a 7.4-fold reduction in self-diffusion compared to the bulk liquid. This finding aligns with molecular dynamics simulations predicting 5.4-fold reduction, assuming an offset-stacked COF layer arrangement. In the low-porosity sample, more frequent diffusion barriers result in isotropic, yet significantly reduced diffusivities (DB = 1.4(1) × 10–11 m2 s–1). Diffusion coefficients for chloroform at 300 K in the pores of the high- (Dpar = 1.1(2) × 10–10 m2 s–1) and low-porosity (DB = 4.5(1) × 10–12 m2 s–1) samples reproduce these trends. Our multimodal study thus highlights the significant influence of real structure effects such as stacking faults and grain boundaries on the long-range diffusivity of molecular guest species while suggesting efficient intracrystalline transport at short diffusion times.

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Introduction

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Two-dimensional (2D) covalent organic frameworks (COFs) are a class of materials that combine a high level of tunability with intrinsic structural porosity on a crystalline, covalently linked polymeric backbone. Their chemical structure can be tuned with atomic precision, rendering these materials an attractive scaffold for diverse applications, including gas storage and separation, sensing, electrochemical energy storage, and heterogeneous (photo)catalysis. (1−9)
The typically large specific surface areas of these materials, and in particular the spatial arrangement of building blocks as encoded in the shape of the pore channels featuring adjustable diameters in the nanometer range, enable the utilization of confinement effects in heterogeneous catalysis, similar to those well-known from enzymes as biological catalysts. (10) Spatial confinement in these pores allows a precise arrangement and relative orientation of catalytic centers and substrates in the pore channels and modulates the local concentration of reactants in the cavities. (11,12) These effects can be used as a handle to tailor product selectivity in catalytic reactions, e.g., by suppressing oligomerization in l-lactide synthesis from lactic acid. (13) Recently, Emmerling et al. demonstrated that the ordered structural porosity of COFs enhances selectivity for (mono)macrocyclization during a ruthenium-catalyzed olefin metathesis reaction, favoring ring closing over oligomerization. (14) While the variation of pore size forms the basis for these effects, interactions between the reactants as well as other molecules in the reaction mixture with the pore wall become more dominant with a reduction in pore size. (15) Acid/base interactions between catalytic substrates and reaction intermediates can affect the reaction rate, while collision events with the pore walls alter the in- and outflow of reactants. (16,17) This can lead either to localized concentration gradients affecting selectivity or, in the limiting case for very small pore diameters that exclude (competing) molecules entirely, enable further areas of application, such as molecular sieving or nanofiltration. (18−22)
Computer simulations are well established in the field of porous media. Molecular dynamics (MD) simulations are a versatile tool to study self- and collective diffusion not only in crystalline porous media such as zeolites, MOFs, and COFs (23) or carbon nanotubes (24) but also in complex amorphous materials if a reasonable structural model is available. (25) In contrast to zeolites (26−30) and MOFs, (31−34) the investigation of self-diffusion in COFs so far focused on light gases such as hydrogen, nitrogen, carbon dioxide, methane, and ethane. (35−39) Molecular simulations in conjunction with experimental investigations have led to a fundamental understanding of nanoconfinement effects, (40) but such combined studies so far exclusively focused on MOFs (41) and zeolites. (42) Therefore, we herein present a combined experimental and computational study of the self-diffusion of acetonitrile and chloroform in the 2D covalent organic network PI-3-COF using pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy and grand canonical Monte Carlo (GCMC) and MD simulations. Using two samples with identical composition but differences in their real structure effects (i.e., crystallinity and porosity), we demonstrate the influence of pore confinement on the diffusivities of molecular probes.

Results and Discussion

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Synthesis and Characterization of COFs

Imine-linked PI-3-COF has been synthesized from 1,3,5-triformyl benzene and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline under solvothermal conditions according to a previously reported procedure. (43) We synthesized two samples of PI-3-COF, named PI-3-COF-lp (low porosity) and PI-3-COF-hp (high porosity) in the following, depending on the selected drying procedure (see Supporting Information for details). Fourier transform infrared (FT-IR) spectra of the yellow powdered materials indicate the successful condensation to the imine-linked frameworks, represented by the imine bond vibration (vC═N) at 1630 cm–1 (Figure S 2). The spectra for both samples appear essentially indistinguishable due to their identical chemical composition. Structural analysis by X-ray powder diffraction (XRPD) with Co–Kα1 radiation shows four narrow reflections at 2θ = 6.6, 11.5, 13.2, and 17.8°, indexed as 100, 110, 200, and 120 reflections (space group P6̅), and a broad stacking reflection (00l) centered at 2θ ≈ 30° (Figures S 4 and S 5). Bragg peaks in the XRPD pattern (Figure 1b) appear essentially at identical positions for both hp- and lp-materials, but with a reduced full width at half-maximum (fwhm) for the hp sample, hinting at a better structural definition and long-range order, i.e. crystallinity, of PI-3-COF-hp compared to its lp derivative. 100, 110, and 200 reflections appear broader, with reduced intensity for PI-3-COF-lp (Figure S 5) and show a slight but visible shift [Δ(2θ) < 0.1°] to higher angles, reminiscent of, but less pronounced than, a reduction in in-plane coherence and contraction due to drying induced stress. (44) A Pawley refinement (Figure S 4) thus gives slightly reduced unit cell parameters of a = b = 17.9 Å and c = 3.47 Å for PI-3-COF-lp, compared to a = b = 18.0 Å and c = 3.48 Å for PI-3-COF-lp. Scanning electron and transmission electron microscopy (SEM/TEM) images show agglomerated polycrystalline spherical particles and a polydisperse distribution of (secondary) particle sizes, approximately centered at ∼300 nm in diameter (Figures S 7 and S 8) for both samples. Some agglomerates show sizes of multiple μm. The surface of these particles is decorated with stings, consisting of crystallites with average diameters of a few tens of nanometers (Figures S 9 and S 10). Nitrogen gas sorption experiments (Figure 1c) show a limited nitrogen uptake for PI-3-COF-lp and reveal BET surface areas of SBET = 442 and 1620 m2 g–1 and total pore volumes of 0.60 and 1.1 cm3 g–1 for PI-3-COF-lp and hp, respectively (Figures S 11 and S 12). Calculated pore size distributions by quenched solid density functional theory (QSDFT) based on a carbon model for cylindrical pores are centered at 1.7 nm for both PI-3-COF-lp and hp (Figures S 11b and S 12b). With respect to the characterization data shown, we find that the difference between lp and hp lies in the extent of crystallinity, i.e., structural definition of the two samples, caused, for example, by inaccessible pores or disorder in PI-3-COF-lp and is not attributed to a difference in chemical composition. This leads to a reduced porosity in the case of PI-3-COF-lp compared to that in PI-3-COF-hp. Based on these findings, we envisaged using these samples as a basis for PFG-NMR diffusion experiments to investigate the effect of real structure effects such as crystallinity and porosity on the diffusivity.

Figure 1

Figure 1. (a) Chemical structure of a single pore of PI-3-COF. (b) XRPD pattern and (c) N2 adsorption isotherm comparison of PI-3-COF-lp (green) and PI-3-COF-hp (blue).

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Probing Diffusion Experimentally by PFG-NMR

Due to its abundant use as a solvent in general organic and COF synthesis, (45) acetonitrile (MeCN) was selected as a proxy to probe the self-diffusion of molecular reactants or intermediates in the pore system of a COF. In particular, MeCN loaded into the pores of PI-3-COFs showed sufficiently long T2 relaxation times for PFG-NMR experiments (s.b., Table S 1) compared to other probe molecules (Table S 3). Excess amounts of liquid on the outer particle surface or in the interparticle space distort the diffusion experiment and result in a major signal in the 1H NMR spectrum with bulk liquid-like mobility. To allow for more selective filling of the pores in the materials, we exposed the vacuum-dried materials to saturated acetonitrile vapor in air. Consequently, condensation of acetonitrile into the pores of the material occurred. As indicated by a single, broadened, and downfield-shifted signal for acetonitrile due to confinement, (46) which is centered at δ = 3.7 ppm in the 1H NMR spectrum compared to the narrow signal for the isolated liquid at δ = 1.9 ppm (Figure S 18), the liquid mainly condensed into the pores, instead of interparticle voids which would yield signals closer to the free liquid. The mass of the samples increased after this solvent vapor treatment, corresponding to loadings of 25 wt % (PI-3-COF-lp) and 39 wt % (PI-3-COF-hp) of acetonitrile, respectively. Despite these high loadings, the appearance of the loaded materials was identical to the (dry) pristine materials. No liquid was visible on the surface.
PFG-NMR is a useful, nondestructive spectroscopic technique capable of tracking molecular motion and transport on a broad range of distances, varying from nanometers to hundreds of micrometers. By probing NMR signal attenuations at different diffusion times (Δ), spatial decoding of different diffusion regions and thus localized as well as long-range information on the structure of porous materials can be obtained. (47,48) Fitting of the NMR signal obtained by the PFG method using the Stejskal-Tanner (49) equation (eq 1) yields the diffusion coefficient (D) as a function of the gradient field strength (g), the gradient pulse duration (δ), and the gyromagnetic ratio of the probed nuclei (γ).
I=I0exp[−γ2g2δ2(Δ−δ3)D]=I0exp[−BD]
(1)
To select appropriate gradient strengths and observation parameters, the relaxation times of the molecules under study are required. (50) Both longitudinal relaxation times (T1,lp = 1.8 s) and transverse relaxation times (T2,lp = 0.54 ms) of acetonitrile loaded onto PI-3-COFs were between 1 and 3 orders of magnitude shorter compared to the bulk liquid at 300 K (Table S 1), diagnostic of smaller molecular mobility within the pores. (51) On the one hand, this observation provides further evidence that MeCN is primarily located in the pores of the material, while, on the other hand, short spin–spin relaxation times (T2) lead to a fast decay of signal intensity in NMR experiments. This sets an experimental upper limit for the gradient pulse duration δ, as well as the diffusion time Δ. Long pulse durations and observation times lead to a bad signal-to-noise ratio because most of the signal has decayed due to relaxation (50) before the signal can be measured. At the same time, uniform and stable gradients in the spectrometer require a technically limited minimum duration for the gradient pulse, setting the lower limits for δ during the PFG experiment. (47)
For the presented PFG experiments, we chose the minimum technically possible value with our spectrometer of δ = 0.3 ms (at high gradients gmax = 900 Gs1 cm–1) to acquire PFG spin–echoes for diffusion times Δ = 20–100 ms using a stimulated echo (ste) pulse sequence (see Supporting Information for details). As shown in Figure 2, the spin–echo attenuations appear nonlinear for both samples, although linearity in the semilogarithmic representation would be expected for regular isotropic diffusion as observed for bulk acetonitrile (Figures 2a and S 16). The course of the signals can be separated into two regimes: a steeply decreasing initial range for small gradients (rA) and then a slowly decaying range toward large gradients (rB) at a fixed pulse duration δ = 0.3 ms. This behavior is characteristic of a distribution of diffusion coefficients, for example, observed in porous materials (52) including zeolites (53,54) and MOFs, (55) where regions with different translational mobilities are found. This behavior can be observed in these materials for example for molecules diffusing inside versus outside of crystallites. (56) A biexponential model (eq 2) can fit attenuations with this behavior, where pi reflects the population of the region (i) with diffusivity Di. (52,57) The diffusivities of both regions appear as linear ranges in the semilogarithmic plot.
II0=pAexp[−BDA]+pBexp[−BDB]
(2)
The observed signal attenuations in PI-3-COF-lp (Figure 2b) are in good agreement with this simple biexponential model. For varying diffusion times, different slopes are visible in the range rB toward high gradients (Figure 2b), indicating a dependence of the diffusivity DB on diffusion time Δ. With variation of Δ, also the population pi, which can be interpreted as the y-intercept of the linear, slowly decaying intensity extrapolated to B = 0 (Figure 2b), changes. The population pB decreases with longer observation times (Figure 2b). This phenomenon indicates a molecular exchange between both regions in the material, expected for open pore channels in PI-3-COF, and can be used by the NMR tracer exchange method (58) to determine the fraction of molecules and their mean lifetime τi within these regions. (59,60) With a defined macroscopic particle geometry, e.g., in single crystals, or for spherical particles, their diffusivity and average lifetime allow to estimate mean particle/crystallite sizes. (55,61,62) Unfortunately, the wide distribution of particle sizes and shapes in our materials, as observed by electron microscopy (Figures S 7 and S 8), does not allow for this analysis. Applying the simple biexponential model to PI-3-COF-lp yields two diffusion coefficients of DA = 1.7(2) × 10–8 m2 s–1 and DB = 1.4(1) × 10–11 m2 s–1, with Δ = 20 ms at T = 300 K. A comparison of the exchange behavior between these regions at reduced temperature down to T = 280 K shows that the population pA drops at reduced temperatures, and the exchange between both regions becomes less prominent (Figure S 21). Analysis of the population pB at temperatures between T = 300 K and T = 280 K as a function of Δ further corroborates this finding, evident from a slower decrease of pB vs Δ at reduced temperature (Figure S 23). The diffusivity DA = 1.7(1) × 10–8 m2 s–1 exceeds the self-diffusion coefficient of pure acetonitrile (Ds = 4.5(1) × 10–9 m2 s–1, Figure S 17) at T = 300 K by 1 order of magnitude. The high diffusivity and strong temperature dependence of pB (Figure S 23) suggest that DA corresponds to an averaged diffusion of liquid acetonitrile molecules, which exchanged with the gas phase during the time of the NMR experiment. (63,64) With reduced temperature, the vapor pressure of acetonitrile and thus the partial pressure of MeCN in the gas phase, as well as the probability for a phase exchange during the observation time, are reduced. Contrary to pA, we conclude that the molecules of the population pB have not exchanged with the gas phase during the time of the NMR experiment and can be labeled as the fraction of molecules remaining within the particle. Their diffusivity DB thus denotes intraparticle diffusion of acetonitrile within the pore channels of the polycrystalline particles of PI-3-COF. A comparison of DB at constant temperature (Figure S 24) shows a decrease of DB with increasing diffusion times Δ. In contrast to this, diffusion in the nonconfined, isotropic bulk liquid is independent of Δ (Figures 2a and S 16). Similar to observations in lithium-ion conductors (65) and polycrystalline faujasite crystals, (66) long-range diffusion of MeCN in PI-3-COF-lp, corresponding to long diffusion times Δ, is limited by transport barriers (e.g., grain boundaries or surface effects (53,67)), whereas at small displacements, these defects have less effect on the diffusion coefficient. (53) To solely observe intracrystalline diffusion and reduce the influence of intercrystallite or interparticle diffusion resistances, DB should ideally be measured at short diffusion times, where the mean square displacements (⟨z2⟩ ≈ 2DΔ) for most diffusing molecules in this time interval are smaller than the average crystallite diameter. However, due to technical limitations, Δ cannot be chosen arbitrarily small for high gradient values. (47) Because the accessible isotropic diffusion lengths in the presented materials (μm range, Table S 2) exceed the observed crystallite size of a few tens of nanometers by SEM/TEM analysis (s. a.), only effective long-range diffusion coefficients can be obtained from the experiments. To estimate the order of magnitude for short-range diffusion in the pores of PI-3-COF, we extrapolated the experimental values for DB toward short diffusion times in a phenomenological log(D)-log(Δ) plot, which has been used to describe, for example, restricted diffusion in zeolites (Figure S 25). (53) The extrapolation suggests that DB may approach values up to the order of 10–10 m2 s–1 for PI-3-COF-lp. The extrapolated values, however, should be interpreted with care as they might overestimate short-range diffusivity: the experimental diffusion coefficients for DB may still contain a contribution of a fraction of acetonitrile diffusing in textural mesopores or small voids between individual crystallites, which would give rise to more bulk-like diffusivities. The presence of such textural pores can be inferred from acetonitrile vapor sorption experiments with PI-3-COF-lp (Figure S 13): the vapor sorption isotherm at 300 K shows a steep uptake at low relative pressures (P/Psat < 0.13), corresponding to the filling of micropores, i.e., pore channels (structural pores). Toward higher relative pressure, a further but less steep uptake with pronounced hysteresis is visible. This uptake is attributed to the filling of textural mesopores. In turn, we conclude that some signal intensity during the NMR experiments may be caused by acetonitrile molecules in small textural pores, besides those in structural pores (pore channels of PI-3-COF). In addition, the linearity of the signal corresponding to intraparticle mobility DB (Figure 2b) as well as the absence of additional signals during relaxation experiments suggests that the obtained PFG attenuation is not amenable to further quantitative differentiation of structural and textural pores. Relaxation times (Table S 1) as well as diffusion coefficients of molecules in these different pore regimes may appear superimposed and thus indistinguishable, likely influenced by a fast exchange between them relative to the NMR experiment time scale.

Figure 2

Figure 2. PFG-NMR spin–echo attenuation for (a) liquid MeCN, (b) MeCN-loaded PI-3-COF-lp, (c) MeCN-loaded PI-3-COF-hp with varying diffusion times (Δ) at T = 300 K. Lines represent fits with a mono- or biexponential model for MeCN and MeCN-loaded COFs, respectively.

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The PFG-NMR signal attenuation curves for MeCN-loaded PI-3-COF-hp (Figure 2c) similarly show two separated ranges and exchanges between the corresponding regions, evident from an offset of the slowly decaying range (rB). In contrast to PI-3-COF-lp, however, the signal attenuation appears to be nonlinear. This phenomenon indicates a contribution of anisotropic diffusion, which is in line with a diffusion along the 1D pore channels of PI-3-COF. Similar attenuations have been observed for anisotropic diffusion in pore channels of aluminum fumarate MOFs (56,68) and mesoporous silica SBA-15. (69) To address this effect of anisotropy on the observed PFG signal attenuation for MeCN in PI-3-COF-hp, the second term of the simple biexponential model was adjusted to a previously developed anisotropic model for hierarchically porous SBA-15 catalysts (eq 3), considering one-dimensional diffusion (channels) in particles, which are randomly oriented in space. (47,70) Notably, this model uses a simplified approximation to account for the molecular exchange between the different regions (pA and paniso) in the material, based on the more complex solution developed by Splith et al., (56) which requires a negligibly small population pA of the region with isotropic diffusion. Similar to the assumptions for the hierarchically ordered SBA-15 materials, however, we need to consider a fraction of MeCN present in small textural pores at the respective loadings of MeCN in the PI-3-COF samples under study, which is in contrast to the boundary conditions of the model by Splith et al.
II0≈pAexp[−BDA]+panisoexp[−BDperp][π2erf{B(Dpar−Dperp)}B(Dpar−Dperp)]
(3)
Applying eq 3 to the observed signal attenuations for PI-3-COF-hp gives an isotropic diffusion coefficient DA and two different anisotropic diffusion coefficients for movement of molecules parallel (Dpar) and perpendicular (Dperp) to the channel direction. The fit shows excellent agreement with experimental data (Figure 2c) and yields two diffusion coefficients of DA = 2.2(1) × 10–8 m2 s–1 and Dpar = 6.1(4) × 10–10 m2 s–1, with Δ = 20 ms at T = 300 K and Dperp → 0, which is in line with the structural model of PI-3-COF consisting of closely stacked 2D layers that restrict diffusion between the layers, e.g., perpendicular to the channel direction. Analogous to the exchange behavior observed for PI-3-COF-lp, the material shows temperature-dependent molecular exchange between both regions, with DA comprising contributions of gas diffusion through gas–liquid exchange during the observation time (Figure S 22). Extrapolation of the experimental values for Dpar in the log(D)-log(Δ) plot (53) gives values up to the order of 10–9 m2 s–1, similar to diffusion in the bulk liquid, toward short diffusion times. In summary, diffusion in the high porosity sample is less affected by defects or limited pore accessibility, resulting in an observable anisotropic diffusion parallel to the channel direction that is on average 1 to 2 orders of magnitude faster compared to PI-3-COF-lp.
To further complement the findings with a different solvent molecule, we assessed relaxation times of other probe molecules (Table S 3) adsorbed by PI-3-COF. Similar to acetonitrile, all solvents are characterized by short T2 relaxation in the μs range, with the shortest values for cyclohexane (T2 = 0.45 ms) and 1,4-dioxane (T2 = 0.31 ms), rendering them essentially unsuitable for diffusion studies by PFG-NMR. Despite the still fast T2 relaxation time (T2 = 0.55 ms), we obtained diffusion coefficients of Dpar = 1.1(2) × 10–10 m2 s–1 (PI-3-COF-hp) and DB = 4.5(1) × 10–12 m2 s–1 (lp) with Δ = 20 ms at 300 K for chloroform as a complementary diffusion probe (Figure S 28). While in the same order of magnitude, the values are smaller compared to diffusion coefficients of MeCN in these samples, in line with 1.7-fold reduced experimental self-diffusion of bulk chloroform (Ds = 2.6(1) × 10–9 m2 s–1) vs bulk MeCN (Figure S 27). Chloroform diffusion in PI-3-COFs shows generally the same qualitative trends as observed for MeCN, i.e., on average 1 to 2 orders of magnitude faster diffusion in the high-porosity sample.

Computational Modeling

Following previous work, (71) the structural model obtained from XRPD experiments was refined by density functional calculations under periodic boundary conditions, as described in more detail in the Supporting Information. Figure 3 shows simulated and experimental excess nitrogen adsorption isotherms for PI-3-COF. The two experimental curves correspond to the lp and hp samples, while the two simulated curves correspond to model structures in which the layers are either perfectly eclipsed (red triangles), or where two adjacent layers are slightly shifted by approximately 1.7 Å in an alternating way (black squares). In the shifted model, the first and the third layer, the second and the fourth layer, and so on are eclipsed. This model mimics the effect of offset layer stacking, often found in COFs, for example, as layer displacement in randomized directions. (71−75) In both cases, the interlayer distance was fixed to a value of 3.65 Å resulting from the DFT optimization for the shifted structure. The two structures are visualized in Figure 4. The good agreement between the simulated isotherms and the experimental curve of the hp sample indicates a high degree of crystallinity and accessibility of the experimental sample. In contrast to our previous work, (71) no scaling factor was required to account for the finding that the simulation usually overestimated the experimental isotherm. The divergence of the experimental isotherms close to the saturation pressure results from condensation of nitrogen in textural macropores and is therefore not captured in the simulation, which is based on an infinite ideal structure. As found previously, (71) the isotherm corresponding to the shifted structure shows a smoother increase in loading with increasing pressure compared to the eclipsed structure. Despite rather small structural differences in the two model COFs, a qualitative difference between the two simulated isotherms is visible in the medium pressure range between p/p0 = 10–4 and 10–2. However, the good agreement between experiment and simulation for the artificial, idealized structural model over the entire pressure range is distinct. This finding suggests that the real structure of the material is characterized by small shifts between the different layers.

Figure 3

Figure 3. Comparison between experimental N2 adsorption isotherms at 77 K (low porosity in blue, high porosity in green) and simulated ones. The isotherm resulting from the shifted structure is depicted in black squares, the one from the eclipsed stacked structure in red triangles.

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Figure 4

Figure 4. Visualization of the eclipsed (a) and the shifted structure (b) of PI 3-COF. Violet surfaces depict the N2 accessible pore surface based on van der Waals parameters.

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The self-diffusion coefficients of acetonitrile in the two structures at 300 K amount to 1.0(2) × 10–9 m2 s–1 in the perfectly eclipsed structure and to 0.7(1) × 10–9 m2 s–1 in the shifted structure. Compared to the bulk liquid value of 3.8(2) × 10–9 m2 s–1 at 298 K, predicted by the molecular model, these coefficients correspond to a reduction by a factor of 3.8 (eclipsed) and 5.4 (shifted). The bulk value is close to the experimentally determined self-diffusion coefficients reported in the literature (D ≈ 4.2 × 10–9 m2 s–1) (76,77) and the present study (Ds = 4.5(1) × 10–9 m2 s–1). However, since the simulated value of bulk diffusion does not exactly match the experimental one, it is reasonable to compare the ratios of the bulk and pore diffusion coefficients in addition to the absolute values. The ratio between the diffusion coefficients of liquid MeCN in bulk (this study) vs PI-3-COF-hp (PFG) of 7.4 obtained in the present work is in good agreement with the ratio of 5.4 for the simulated bulk value vs the simulation result for the shifted structure at 300 K.
In other simulation works, ratios of 2.0 and 3.2 were reported for the diffusion of MeCN in a carbon nanotube of 1.5 nm diameter (78) and an amorphous silica sample of 2.4 nm diameter, (79) respectively. Experimental studies reporting the self-diffusion of acetonitrile under confinement show a more diverse picture. In pores smaller than 1 nm, self-diffusion coefficients on the order of 10–11 m2 s–1 were measured for zeolite NaX (59) and porous carbon, (80) respectively. In a sol–gel glass with a reported diameter of 2.9 nm, a diffusion coefficient of 1.1 × 10–9 m2 s–1 was obtained, (81) i.e., similar to the diffusion coefficient reported for a mesoporous MCM-41 sample (pore size 3.6 nm, D = 9.9 × 10–10 m2 s–1) (82) and larger than the diffusion coefficient reported for a porous carbon (pore size 4.8 nm, D = 6.0 × 10–10 m2 s–1). (80) Experiments probing a pore size similar to the one in the present work are scarce. For a MCM-41 sample with a pore size of 2 nm, a diffusion coefficient of 2.7 × 10–10 m2 s–1 was reported, (82) which is relatively close to the value obtained for the hp sample in the present work. We note that the self-diffusion coefficient under confinement is affected not only by the pore diameter, but also by interactions of the diffusing compound with the pore wall, in particular in narrow pores. Therefore, the comparison with other materials can only provide a qualitative picture.
For chloroform, simulated self-diffusion coefficients of 0.3(1) × 10–9 and 0.2(1) × 10–9 m2 s–1 were obtained in the perfectly eclipsed and in the shifted structures, respectively. Compared to the simulated bulk liquid value of 2.3(1) × 10–9 m2 s–1 at 298 K, predicted by the molecular model, these coefficients correspond to a reduction by a factor of 7.7 (eclipsed) and 11.5 (shifted). The bulk value is close to the experimentally determined self-diffusion coefficient of 2.6(1) × 10–9 m2 s–1 (300 K). The simulated diffusion coefficient for the shifted structure of 0.2(1) × 10–9 m2 s–1 is close to the experimentally determined for PI-3-COF-hp Dpar = 1.1(2) × 10–10 m2 s–1 (PI-3-COF-hp), which is in line with the observations for MeCN as the diffusing molecule.

Experiment vs Simulation

The present work aims at clarifying the comparability of the self-diffusion coefficient of acetonitrile in a COF obtained from MD simulations and PFG-NMR measurements. For this purpose, two model structures were investigated in conjunction with a fluid model that captures the bulk diffusion coefficient reasonably well. The theoretical model, applied to ideal structural models of isolated pore channels, suggests a comparably fast diffusion within the structural pore channels of PI-3-COF in both fully eclipsed and offset-stacked cases, albeit with slightly reduced diffusivity in the offset-stacked case. The obtained simulated diffusion coefficients are slightly lower, yet roughly of the same order of magnitude as isotropic diffusion in the bulk liquid. We complemented our simulation studies with experimental data obtained by PFG-NMR experiments. We observed short T2 relaxation times for the confined liquid in the pores of PI-3-COF. This limits diffusion times and pulse durations applicable during PFG experiments. Due to the limited crystallite sizes in the polycrystalline COF particles, the experimentally observed diffusion coefficients were limited to mid-to-long-range diffusion processes across multiple crystallites, given the technical limits for short pulse durations at high gradient strengths. From the obtained NMR signal attenuation, we identified multicomponent diffusion with open pore channels allowing the equilibrium exchange of molecules between the surrounding vapor phase and pore liquid. PFG measurements at reduced temperatures helped to assign these contributions by limiting the gas–liquid exchange. The experimental diffusivity in PI-3-COF samples was obtained as effective diffusion coefficients and found to decrease for long diffusion times. This behavior points at real structure effects, e.g., defects and surface barriers at crystal boundaries within and between the particles. (66,83) For a sample of PI-3-COF with lower porosity, these effects are more dominant compared to the sample with higher porosity, which led to the observation of effective diffusivities on the order of 10–11 and 10–10 m2 s–1 for lp and hp samples, respectively. Extrapolation of the obtained diffusivities toward short diffusion times, i.e., small mean square displacements, indicates that short-range diffusion may be 1 (hp) to 2 orders (lp) of magnitude faster than the observable long-range diffusion. The high-porosity sample of PI-3-COF showed anisotropy in diffusion, characterized by diffusivities that agree well with the simulated values for the offset-stacked model, both being in the order of 10–10 m2 s–1 for MeCN. In contrast, the reduced structural definition of the lp sample led to the observation of isotropic diffusion only. This observation hints at the dominant influence of diffusion barriers in the material, restricting the diffusion of acetonitrile to shorter displacements and reducing its mobility compared to the hp sample. Thus, we point out that limited structural order not only reduces the accessible pore volume, i.e., porosity, but also restricts the mobility of molecules via diffusion barriers. As these are essentially invisible to typical analytical techniques, including gas sorption experiments, PFG-NMR spectroscopy should be considered as a complementary method to assess diffusivity-dependent parameters, such as turnover frequency or selectivity of reactions with COFs as heterogeneous catalysts.
Comparing the experimental to the simulated results helps to pinpoint important insights into the real structure of the material. Our comparison between calculated nitrogen gas adsorption isotherms for eclipsed and offset structures in a pressure range p/p0 = 10–4 and 10–2 shows profound sensitivity for localized differences in the stacking and suggests small displacements of the layers in the material since the experimentally observed isotherm of PI-3-COF-hp closely resembles the simulated isotherms of the offset-stacked structure with appreciable agreement. Thus, simulated isotherms may serve as a handle to pinpoint local characteristics in the real structure of the material, although these simulations are generally based on artificial, idealized structural models.

Conclusions

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The combined experiments and simulations shine light on prevalent diffusion mechanisms and issues associated with the experimental determination of diffusion coefficients in COFs. However, the direct observation of pure short-range, i.e., undisturbed intracrystalline diffusion within the pore channels, requires large (ideally single crystalline) particles with pore channel lengths in the μm range. Most powdered COF materials obtained from typical synthetic procedures do not meet this requirement, and obtaining crystallite sizes in this range is a rarely tackled and challenging task for imine and other COFs. (84) Nevertheless, our systematic computational and experimental study sets the stage for future exploration of diffusion processes in COFs and related systems. We propose that optimizing the synthesis conditions to obtain domain sizes in the μm range should be the basis for future studies. With these requirements in mind, we expect that the influence of pore sizes and the chemical structure of the pore walls as well as their surface polarity and the impact of meso-/macroporosities on diffusion processes become experimentally accessible.

Methods

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Synthesis of PI-3-COF

PI-3-COFs with low porosity (-lp) and high porosity (-hp) were synthesized according to a previously described procedure. (43) To a mixture of benzene-1,3,5-tricarbaldehyde (22.1 mg, 0.13 mmol, 1.0 equiv) and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (46.8 mg, 0.13 mmol, 1.0 equiv) in mesitylene (2.7 mL) and 1,4-dioxane (1.3 mL), aqueous 6 M AcOH (0.5 mL) was added. The suspension was heated at 120 °C for 72 h. Suction filtration of the precipitate, washing with DMF (20 mL), THF (20 mL), and DCM (20 mL) and drying under reduced pressure, afforded PI-3-COF-lp (55.5 mg, 91%) as a yellow solid. PI-3-COF-hp (53.9 mg, 88%) was obtained with the same procedure extended by an additional Soxhlet-extraction of the material with MeOH and supercritical CO2 drying, instead of drying under reduced pressure.

Vapor-Assisted Loading of COFs

PI-3-COFs (10–15 mg) loaded into a 2 mL vial were exposed to a vapor-saturated air atmosphere of acetonitrile or chloroform at 300 K for 2 h. The mass loading of solvent was determined on a balance. The sample was then quickly transferred to a 5 mm NMR tube and flame-sealed to limit evaporation of the adsorbed liquid. To allow for equilibration within the sample, the sealed tube was stored for at least 24 h prior to the NMR experiment.

PFG-NMR Experiments

Diffusion measurements were performed in flame-sealed 5 mm NMR tubes on a Bruker AVANCE III 400 MHz spectrometer (diff60 probe) with a stimulated-echo sequence (49) (diffSte program, Bruker TopSpin) without sample spinning. Protons served as the observed nuclei. For a typical diffusion experiment with a COF, a gradient pulse with a duration of δ = 0.3 ms (opt shape), repetition times of 3–5 times T1, and diffusion times Δ = 20–100 ms were used. The gradient was varied linearly in 32 steps between 0 and 900 Gs/cm. Further details are described in the Supporting Information.
Diffusion experiments with bulk liquid (acetonitrile or chloroform) were performed in a tube-in-tube setup to reduce convection effects. (85) A small diameter NMR tube filled with acetonitrile or chloroform was immersed in a 5 mm NMR tube with d-chloroform. In addition, the double stimulated-echo pulse (dSte) sequence was used to reduce convection effects on the diffusion experiment. (86) A gradient pulse with a duration of δ = 1 ms (opt shape), repetition times of 3–5 times T1, and diffusion times Δ = 40; 60; 100 ms were used. The gradient was varied linearly in 16 steps between 0 and 75 Gs/cm.

Data Availability

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The data underlying this study, including additional material such as cif files, RASPA and Gromacs input files, and a Jupyter notebook containing the analysis, are available in the Data Repository of the University of Stuttgart (DaRUS) at 10.18419/darus-3269.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.3c12167.

  • Discussions of methods and equipment used, synthetic procedures, FT-IR spectra, XRPD data and structure refinements, (pulsed field gradient) 1H NMR spectra, nitrogen gas sorption isotherms, pore-size distributions, BET plots, and acetonitrile vapor adsorption isotherms (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
    • Niels Hansen - Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany;  Orcidhttps://orcid.org/0000-0003-4366-6120;  Email: [email protected]
    • Bettina V. Lotsch - Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart 70569, Germany;  Department of Chemistry, Ludwig-Maximilians-Universität (LMU), Butenandtstr. 5-13, Munich 81377, Germany;  E-conversion, Lichtenbergstrasse 4a, Garching 85748, Germany;  Orcidhttps://orcid.org/0000-0002-3094-303X;  Email: [email protected]
  • Authors
    • Lars Grunenberg - Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart 70569, Germany;  Department of Chemistry, Ludwig-Maximilians-Universität (LMU), Butenandtstr. 5-13, Munich 81377, Germany;  Orcidhttps://orcid.org/0000-0002-6831-4626
    • Christopher Keßler - Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany
    • Tiong Wei Teh - Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany
    • Robin Schuldt - Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany
    • Fabian Heck - Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart 70569, Germany;  Department of Chemistry, Ludwig-Maximilians-Universität (LMU), Butenandtstr. 5-13, Munich 81377, Germany;  Orcidhttps://orcid.org/0000-0003-0198-3398
    • Johannes Kästner - Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, Stuttgart 70569, Germany;  Orcidhttps://orcid.org/0000-0001-6178-7669
    • Joachim Groß - Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, Stuttgart 70569, Germany;  Orcidhttps://orcid.org/0000-0001-8632-357X
  • Author Contributions

    L.G. synthesized and analyzed the materials and precursors, performed PFG-NMR diffusion experiments, and interpreted experimental data. C.K. performed GCMC and MD simulations with support by T.W.T. R.S. performed DFT calculations. F.H. performed gas and vapor sorption experiments. L.G. wrote the manuscript with input from C.K. and N.H. J.K., J.G., B.V.L., and N.H. supervised the research. All authors read and commented on the manuscript. L.G. and C.K. contributed equally.

  • Funding

    Open access funded by Max Planck Society.

  • Notes
    The authors declare no competing financial interest.

    An initial version of this manuscript has been previously submitted to a preprint server. Grunenberg, L.; Keßler, C.; Teh, T. W.; Schuldt, R.; Heck, F.; Kästner, J.; Groß, J.; Hansen, N.; Lotsch, B. V. Self-Diffusion of Acetonitrile in a Covalent Organic Framework: Simulation and Experiment. 2024. ChemRxiv. 10.26434/chemrxiv-2024-d9rmq (accessed May 13, 2024).

Acknowledgments

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The authors thank Viola Duppel for supporting this work with SEM and TEM images and Christian Schneider for performing thermogravimetric analysis. We are grateful to Igor Moudrakovski for introducing L.G. to PFG-NMR experimentation. This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Germany─Project-ID 358283783─SFB 1333/2 2022. We also thank the DFG for supporting this work by funding via EXC 2075/1─390740016 and EXC 2089/1─390776260 under Germany’s Excellence Strategy. Financial support by the Max Planck Society is gratefully acknowledged. We appreciate the support by the Stuttgart Center for Simulation Science (SimTech), Germany. Monte Carlo and molecular dynamics simulations were performed on the computational resource BinAC at High Performance and Cloud Computing Group at the Zentrum für Datenverarbeitung of the University of Tübingen, funded by the state of Baden-Württemberg through bwHPC and the DFG through grant no INST 37/935-1 FUGG.

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    Biswal, B. P.; Vignolo-Gonzalez, H. A.; Banerjee, T.; Grunenberg, L.; Savasci, G.; Gottschling, K.; Nuss, J.; Ochsenfeld, C.; Lotsch, B. V. Sustained Solar H2 Evolution from a Thiazolo[5,4-d]Thiazole-Bridged Covalent Organic Framework and Nickel-Thiolate Cluster in Water. J. Am. Chem. Soc. 2019, 141, 11082– 11092,  DOI: 10.1021/jacs.9b03243
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    9
    Sustained Solar H2 Evolution from a Thiazolo[5,4-d]thiazole-Bridged Covalent Organic Framework and Nickel-Thiolate Cluster in Water
    Biswal, Bishnu P.; Vignolo-Gonzalez, Hugo A.; Banerjee, Tanmay; Grunenberg, Lars; Savasci, Goekcen; Gottschling, Kerstin; Nuss, Juergen; Ochsenfeld, Christian; Lotsch, Bettina V.
    Journal of the American Chemical Society (2019), 141 (28), 11082-11092CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Solar hydrogen (H2) evolution from water utilizing covalent org. frameworks (COFs) as heterogeneous photosensitizers has gathered significant momentum by virtue of the COFs' predictive structural design, long-range ordering, tunable porosity, and excellent light-harvesting ability. However, most photocatalytic systems involve rare and expensive platinum as the co-catalyst for water redn., which appears to be the bottleneck in the development of economical and environmentally benign solar H2 prodn. systems. Herein, a simple, efficient, and low-cost all-in-one photocatalytic H2 evolution system is reported composed of a thiazolo[5,4-d]thiazole-linked COF (TpDTz) as the photoabsorber and an earth-abundant, noble-metal-free nickel-thiolate hexameric cluster co-catalyst assembled in situ in water, together with triethanolamine (TEoA) as the sacrificial electron donor. The high crystallinity, porosity, photochem. stability, and light absorption ability of the TpDTz COF enables excellent long-term H2 prodn. over 70 h with a max. rate of 941μmol h-1 g-1, turnover no. TONNi > 103, and total projected TONNi > 443 until complete catalyst depletion. The high H2 evolution rate and TON, coupled with long-term photocatalytic operation of this hybrid system in water, surpass those of many previously known org. dyes, carbon nitride, and COF-sensitized photocatalytic H2O redn. systems. Furthermore, unique insights are gathered into the reaction mechanism, enabled by a specifically designed continuous-flow system for non-invasive, direct H2 prodn. rate monitoring, providing higher accuracy in quantification compared to the existing batch measurement methods. Overall, the results presented here open the door toward the rational design of robust and efficient earth-abundant COF-mol. co-catalyst hybrid systems for sustainable solar H2 prodn. in water.
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  10. 10
    Mitschke, B.; Turberg, M.; List, B. Confinement as a Unifying Element in Selective Catalysis. Chem 2020, 6, 2515– 2532,  DOI: 10.1016/j.chempr.2020.09.007
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    Confinement as a Unifying Element in Selective Catalysis
    Mitschke, Benjamin; Turberg, Mathias; List, Benjamin
    Chem (2020), 6 (10), 2515-2532CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    Catalysis has fascinated scientists for centuries and is one of the main pillars of the modern world economy. Achieving high reactivity and selectivity is a crucial requirement of heterogeneous and homogeneous org., metallic, and biol. catalysts. Here, we highlight an underlying principle that is relevant to the reactivity and selectivity of all types of catalysts-''confinement-'' the shaping of a catalyst's active site. While this aspect has been well recognized within the fields of heterogeneous and enzymic catalysis, and has been invoked in supramol. systems, confinement has been less appreciated in the design of small-mol. catalysts. We identify confinement as a unifying element in the science of selective catalysis, reaching beyond the traditional boundaries of the individual subfields. A particular emphasis is given to the latest developments in the area of organocatalysis.
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  11. 11
    Tan, K. T.; Tao, S.; Huang, N.; Jiang, D. Water Cluster in Hydrophobic Crystalline Porous Covalent Organic Frameworks. Nat. Commun. 2021, 12, 6747,  DOI: 10.1038/s41467-021-27128-4
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    Water cluster in hydrophobic crystalline porous covalent organic frameworks
    Tan, Ke Tian; Tao, Shanshan; Huang, Ning; Jiang, Donglin
    Nature Communications (2021), 12 (1), 6747CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Abstr.: Progress over the past decades in water confinement has generated a variety of polymers and porous materials. However, most studies are based on a preconception that small hydrophobic pores eventually repulse water mols., which precludes the exploration of hydrophobic microporous materials for water confinement. Here, we demonstrate water confinement across hydrophobic microporous channels in cryst. covalent org. frameworks. The frameworks are designed to constitute dense, aligned and one-dimensional polygonal channels that are open and accessible to water mols. The hydrophobic microporous frameworks achieve full occupation of pores by water via synergistic nucleation and capillary condensation and deliver quick water exchange at low pressures. Water confinement expts. with large-pore frameworks pinpoint thresholds of pore size where confinement becomes dominated by high uptake pressure and large exchange hysteresis. Our results reveal a platform based on microporous hydrophobic covalent org. frameworks for water confinement.
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  12. 12
    Zeng, W. J.; Wang, K.; Liang, W. B.; Chai, Y. Q.; Yuan, R.; Zhuo, Y. Covalent Organic Frameworks as Micro-Reactors: Confinement-Enhanced Electrochemiluminescence. Chem. Sci. 2020, 11, 5410– 5414,  DOI: 10.1039/D0SC01817A
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    Covalent organic frameworks as micro-reactors: confinement-enhanced electrochemiluminescence
    Zeng, Wei-Jia; Wang, Kun; Liang, Wen-Bin; Chai, Ya-Qin; Yuan, Ruo; Zhuo, Ying
    Chemical Science (2020), 11 (21), 5410-5414CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Electrochemiluminescence (ECL) micro-reactors with enhanced intensity and extreme stability were first established by the assembly of tris(2,2'-bipyridyl) ruthenium(II) (Ru(bpy)32+) onto covalent org. frameworks (COFs), in which a type of imine-linked COF (denoted as COF-LZU1) was employed as a model for ECL micro-reactors. Compared with the dominant ECL system of Ru(bpy)32+/tri-n-propylamine (TPrA), the intensity of the COF-LZU1 micro-reactor-based electrode was significantly increased nearly 5-fold under the same exptl. conditions. This enhancement can be attributed to the large surface area, delimited space, and stable and hydrophobic porous structure of COF-LZU1, which not only enabled a huge amt. of Ru(bpy)32+ to be loaded in/on COF-LZU1, but also enriched a large amt. of TPrA from the aq. soln. into its inner hydrophobic cavity due to the lipophilicity of TPrA. To prove this principle, these Ru@COF-LZU1 micro-reactors were developed to prep. an ECL aptasensor for aflatoxin M1 (AFM1) detection with a wide detection range and a low detection limit. Overall, this work is the first report in which ECL micro-reactors are constructed with COFs to enhance the intensity and stability of the Ru(bpy)32+-based ECL system, and opens a new route to the design of other ECL micro-reactors for bioanal. applications.
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  13. 13
    Zhao, J.; Guo, G.; Wang, D.; Liu, H.; Zhang, Z.; Sun, L.; Ding, N.; Li, Z.; Zhao, Y. A “One-Step” Approach to the Highly Efficient Synthesis of Lactide through the Confinement Catalysis of Covalent Organic Frameworks. Green Chem. 2023, 25, 3103– 3110,  DOI: 10.1039/D2GC04771C
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    Emmerling, S. T.; Ziegler, F.; Fischer, F. R.; Schoch, R.; Bauer, M.; Plietker, B.; Buchmeiser, M. R.; Lotsch, B. V. Olefin Metathesis in Confinement: Towards Covalent Organic Framework Scaffolds for Increased Macrocyclization Selectivity. Chemistry 2022, 28, e202104108  DOI: 10.1002/chem.202104108
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    Gao, W.-Y.; Cardenal, A. D.; Wang, C.-H.; Powers, D. C. In Operando Analysis of Diffusion in Porous Metal-Organic Framework Catalysts. Chem. Eur. J. 2019, 25, 3465– 3476,  DOI: 10.1002/chem.201804490
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    Fang, Q.; Gu, S.; Zheng, J.; Zhuang, Z.; Qiu, S.; Yan, Y. 3D Microporous Base-Functionalized Covalent Organic Frameworks for Size-Selective Catalysis. Angew. Chem., Int. Ed. 2014, 53, 2878– 2882,  DOI: 10.1002/anie.201310500
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    3D Microporous Base-Functionalized Covalent Organic Frameworks for Size-Selective Catalysis
    Fang, Qianrong; Gu, Shuang; Zheng, Jie; Zhuang, Zhongbin; Qiu, Shilun; Yan, Yushan
    Angewandte Chemie, International Edition (2014), 53 (11), 2878-2882CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The design and synthesis of 3D covalent org. frameworks (COFs) were considered a challenge, and the demonstrated applications of 3D COFs have so far been limited to gas adsorption. Herein we describe the design and synthesis of two new 3D microporous base-functionalized COFs, termed BF-COF-1 and BF-COF-2, by the use of a tetrahedral alkyl amine, 1,3,5,7-tetraaminoadamantane (TAA), combined with 1,3,5-triformylbenzene (TFB) or triformylphloroglucinol (TFP). As catalysts, both BF-COFs showed remarkable conversion (96 % for BF-COF-1 and 98 % for BF-COF-2), high size selectivity, and good recyclability in base-catalyzed Knoevenagel condensation reactions. This study suggests that porous functionalized 3D COFs could be a promising new class of shape-selective catalysts.
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    North, A. M. Diffusion-Controlled Reactions. Q. Rev. Chem. Soc. 1966, 20, 421,  DOI: 10.1039/qr9662000421
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    Shinde, D. B.; Sheng, G.; Li, X.; Ostwal, M.; Emwas, A. H.; Huang, K. W.; Lai, Z. Crystalline 2D Covalent Organic Framework Membranes for High-Flux Organic Solvent Nanofiltration. J. Am. Chem. Soc. 2018, 140, 14342– 14349,  DOI: 10.1021/jacs.8b08788
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    Crystalline 2D Covalent Organic Framework Membranes for High-Flux Organic Solvent Nanofiltration
    Shinde, Digambar B.; Sheng, Guan; Li, Xiang; Ostwal, Mayur; Emwas, Abdul-Hamid; Huang, Kuo-Wei; Lai, Zhiping
    Journal of the American Chemical Society (2018), 140 (43), 14342-14349CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Two-dimensional (2D) covalent org. framework (COF) materials have the most suitable microstructure for membrane applications in order to achieve both high flux and high selectivity. Here, we report the synthesis of a cryst. TFP-DHF 2D COF membrane constructed from two precursors of 1,3,5-triformylphloroglucinol (TFP) and 9,9-dihexylfluorene-2,7-diamine (DHF) through the Langmuir-Blodgett (LB) method, for the first timed. A single COF layer is precisely four unit cells thick and can be transferred to different support surfaces layer by layer. The TFP-DHF 2D COF membrane supported on an anodic aluminum oxide (AAO) porous support displayed remarkable permeabilities for both polar and nonpolar org. solvents, which were approx. 100 times higher than that of the amorphous membranes prepd. by the same procedure and similar to that for the best of the reported polymer membranes. The transport mechanism through the TFP-DHF 2D COF membrane was found to be a viscous flow coupled with a strong slip boundary enhancement, which was also different from those of the amorphous polymer membranes. The membrane exhibited a steep mol. sieving with a mol. wt. retention onset (MWRO) of approx. 600 Da and a mol. wt. cutoff (MWCO) of approx. 900 Da. The substantial performance enhancement was attributed to the structural change from an amorphous structure to a well-defined ordered porous structure, which clearly demonstrated the high potential for the application of 2D COFs as the next generation of membrane materials.
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    Shi, X.; Zhang, Z.; Yin, C.; Zhang, X.; Long, J.; Zhang, Z.; Wang, Y. Design of Three-Dimensional Covalent Organic Framework Membranes for Fast and Robust Organic Solvent Nanofiltration. Angew. Chem. 2022, 134, e202207559  DOI: 10.1002/ange.202207559
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    Karak, S.; Dey, K.; Banerjee, R. Maneuvering Applications of Covalent Organic Frameworks Via Framework-Morphology Modulation. Adv. Mater. 2022, 34, e2202751  DOI: 10.1002/adma.202202751
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    Maneuvering Applications of Covalent Organic Frameworks via Framework-Morphology Modulation
    Karak, Suvendu; Dey, Kaushik; Banerjee, Rahul
    Advanced Materials (Weinheim, Germany) (2022), 34 (49), 2202751CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Translating the performance of covalent org. frameworks (COFs) from lab. to macroscopic reality demands specific morphologies. Thus, the advancement in morphol. modulation has recently gained some momentum. A clear understanding of nano- to macroscopic architecture is crit. to det., optimize, and improve performances of this atomically precise porous material. Along with their chem. compns. and mol. frameworks, the prospect of morphol. in various applications should be discussed and highlighted. A thorough insight into morphol. vs. application will help produce better-engineered COFs for practical implications. 2D and 3D frameworks can be transformed into various solids such as nanospheres, thin films, membranes, monoliths, foams, etc., for numerous applications in adsorption, sepn. photocatalysis, the carbon dioxide redn., supercapacitors, and fuel cells. However, the research on COF chem. mainly focuses on correlating structure to property, structure to morphol., and structure to applications. Here, crit. insights on various morphol. evolution and assocd. applications are provided. In each case, the underlying role of morphol. is unveiled. Toward the end, a correlation between morphol. and application is provided for the future development of COFs.
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    Yang, Y.; Yu, L.; Chu, T.; Niu, H.; Wang, J.; Cai, Y. Constructing Chemical Stable 4-Carboxyl-Quinoline Linked Covalent Organic Frameworks via Doebner Reaction for Nanofiltration. Nat. Commun. 2022, 13, 2615,  DOI: 10.1038/s41467-022-30319-2
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    Constructing chemical stable 4-carboxyl-quinoline linked covalent organic frameworks via Doebner reaction for nanofiltration
    Yang, Yongliang; Yu, Ling; Chu, Tiancheng; Niu, Hongyun; Wang, Jun; Cai, Yaqi
    Nature Communications (2022), 13 (1), 2615CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
    Covalent linkages are the key component of covalent org. frameworks (COFs). The development of stable and functional linkages is essential to expand the COFs family and broaden their application prospects. In this work, we report the synthesis of cryst. and chem. stable 4-carboxyl-quinoline linked COFs (QL-COFs) via Doebner reactions in both one-pot (OP) and post-synthetic modification (PSM) methods. Both methods can be universally applied to most of the reported imine COFs family via bottom-up construction or linkage conversion. Owing to the contractive pore size, more hydrophilic structure and better chem. stability than the conventional imine COFs endowed by 4-carboxyl-quinoline linkage, QL-COFs are supposed to possess a wider application range. We further demonstrate the nanofiltration membrane (NFM) based on QL-COF exhibited a desirable sepn. capacity with high rejection for small dye mols. (> 90%), high water permeance (850 L m-2 h-1 MPa-1) and tolerance of extreme conditions (1 M HCl/NaOH), which were benefitted from the enhanced properties of QL-COFs. Addnl., efficient ion sieving properties were also achieved by QL-COF membrane. We anticipate that this work opens up a way for the construction of robust and functional COFs materials for practical applications.
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    Wang, R.; Guo, J.; Xue, J.; Wang, H. Covalent Organic Framework Membranes for Efficient Chemicals Separation. Small Struct. 2021, 2, 2100061,  DOI: 10.1002/sstr.202100061
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    Covalent Organic Framework Membranes for Efficient Chemicals Separation
    Wang, Rui; Guo, Jiaxing; Xue, Jian; Wang, Haihui
    Small Structures (2021), 2 (10), 2100061CODEN: SSMTB2; ISSN:2688-4062. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Covalent org. frameworks (COFs) are an emerging class of org. porous cryst. materials that are composed entirely by org. linkers and connected by strong covalent bonds. The unique characteristics including well-ordered and tailorable pore channels, high and permanent porosity, excellent thermostability and chem. stability, and ease of functionalization enable COFs to perfectly meet the requirements for the fabrication of advanced sepn. membranes. Significant progress has been made in the prepn. and application of COF membranes over the past few years. Herein, the structure-performance relationship of COF materials on membrane sepn. is first proposed. The effects of the intrinsic properties of COF materials on membrane sepn. performance are systematically summarized. The fabrication methods of COF membranes, including both bottom-up strategy (in situ growth and interface-assisted synthesis) and top-down strategy (blending and layer-by-layer [LBL] stacking), are then discussed in detail. The application of COF membranes in gas sepn. (H2 purifn., CO2 capture, hydrocarbon sepn.) and liq. phase sepn. (water treatment, org. solvent nanofiltration, pervaporation) is highlighted. Finally, the remaining challenges and issues to be resolved in the COF membranes are prospected from the perspective of COF materials.
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    Krishna, R. Describing the Diffusion of Guest Molecules inside Porous Structures. J. Phys. Chem. C 2009, 113, 19756– 19781,  DOI: 10.1021/jp906879d
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    Describing the Diffusion of Guest Molecules Inside Porous Structures
    Krishna, Rajamani
    Journal of Physical Chemistry C (2009), 113 (46), 19756-19781CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    A review. The design and development of many emerging sepn. and catalytic process technologies require a proper quant. description of diffusion of mixts. of guest mols. within meso- and microporous structures. In mesoporous materials with pore sizes 2 nm < dp < 50 nm, there is a central core region where the influence of interactions of the mols. with the pore wall is either small or negligible; mesopore diffusion is governed by a combination of mol.-mol. and mol.-pore wall interactions. Within micropores with dp < 2 nm, the guest mols. are always within the influence of the force field exerted with the wall; the authors have to reckon with the motion of adsorbed mols., and there is no bulk fluid region. This article presents a unified, phenomenol., description of diffusion inside meso- and microporous structures using concepts and ideas that originate from James Clerk Maxwell and Josef Stefan. With the aid of extensive data sets of mol. dynamic simulations of unary and mixt. diffusion in a wide variety of materials such as zeolites, metal-org. frameworks, covalent org. frameworks, carbon nanotubes, and cylindrical silica pores with a diverse range of pore topologies and pore sizes, the authors derive a mol.-level understanding of the various coeffs. that arise in the phenomenol. Maxwell-Stefan diffusion formulation. This understanding helps one to explain and describe a variety of exptl. data and observations. Also a mol. level understanding aids sepn. and reaction process development.
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    Jakobtorweihen, S.; Verbeek, M. G.; Lowe, C. P.; Keil, F. J.; Smit, B. Understanding the Loading Dependence of Self-Diffusion in Carbon Nanotubes. Phys. Rev. Lett. 2005, 95, 044501,  DOI: 10.1103/PhysRevLett.95.044501
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    Understanding the Loading Dependence of Self-Diffusion in Carbon Nanotubes
    Jakobtorweihen, S.; Verbeek, M. G.; Lowe, C. P.; Keil, F. J.; Smit, B.
    Physical Review Letters (2005), 95 (4), 044501/1-044501/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    The influence of flexible walls on the self-diffusion of CH4 in an isolated single walled carbon nanotube, as an example, is studied by mol. dynamics simulations. By simulating the carbon nanotube as a flexible framework we demonstrate that the flexibility has a crucial influence on self-diffusion at low loadings. We show how this influence can be incorporated in a simulation of a rigid nanotube by using a Lowe-Andersen thermostat which works on interface-fluid collisions. The reprodn. of the results of a flexible carbon nanotube by a rigid nanotube simulation is excellent.
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    Obliger, A.; Bousige, C.; Coasne, B.; Leyssale, J.-M. Development of Atomistic Kerogen Models and Their Applications for Gas Adsorption and Diffusion: A Mini-Review. Energy Fuels 2023, 37, 1678– 1698,  DOI: 10.1021/acs.energyfuels.2c03633
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    Development of Atomistic Kerogen Models and Their Applications for Gas Adsorption and Diffusion: A Mini-Review
    Obliger, Amael; Bousige, Colin; Coasne, Benoit; Leyssale, Jean-Marc
    Energy & Fuels (2023), 37 (3), 1678-1698CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    A review. With the emergence of shale gas, numerous at.-scale models of kerogen have been proposed in the literature. These models, which attempt to capture the structure, chem., and porosity of kerogens of various types and maturities, are nowadays commonly-if not routinely-used to gain nanoscale insights into the thermodn. and dynamics of complex and important processes such as hydrocarbon recovery and carbon sequestration. However, modeling such a complex, disordered, and heterogeneous material is a particularly challenging task. It implies that important underlying assumptions and simplifications, which can significantly affect the predicted properties, have to be made when constructing the kerogen models. In this mini review, we discuss the existing atomistic models of kerogen by categorizing them according to the different approaches and assumptions used during their construction. For each type of model, we also describe how the construction strategy can impact the prediction of certain properties. Important work on kerogen interactions with gas and oil, from both the point of view of equil. adsorption (including adsorption-induced deformation) and transport, are described. Possible improvements and upscaling strategies-to better account for kerogen in its geol. environment-are also discussed.
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    Dubbeldam, D.; Snurr, R. Q. Recent Developments in the Molecular Modeling of Diffusion in Nanoporous Materials. Mol. Simul. 2007, 33, 305– 325,  DOI: 10.1080/08927020601156418
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    Recent developments in the molecular modeling of diffusion in nanoporous materials
    Dubbeldam, D.; Snurr, R. Q.
    Molecular Simulation (2007), 33 (4-5), 305-325CODEN: MOSIEA; ISSN:0892-7022. (Taylor & Francis Ltd.)
    A review. Mol. modeling has become a useful and widely used tool to predict diffusion coeffs. of mols. adsorbed in the pores of zeolites and other nanoporous materials. These simulations also provide detailed, mol.-level information about sorbate structure, dynamics, and diffusion mechanisms. We review recent advances in this field, including prediction of various transport coeffs. (Fickian, Onsager, Maxwell-Stefan) for single-component and multicomponent systems from equil. and non-equil. mol. dynamics (MD) simulations, elucidation of anomalous diffusion effects induced by the confining pore structure, and prediction of slow diffusion processes beyond the reach of MD simulations.
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    Jobic, H.; Theodorou, D. N. Quasi-Elastic Neutron Scattering and Molecular Dynamics Simulation as Complementary Techniques for Studying Diffusion in Zeolites. Microporous Mesoporous Mater. 2007, 102, 21– 50,  DOI: 10.1016/j.micromeso.2006.12.034
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    Quasi-elastic neutron scattering and molecular dynamics simulation as complementary techniques for studying diffusion in zeolites
    Jobic, Herve; Theodorou, Doros N.
    Microporous and Mesoporous Materials (2007), 102 (1-3), 21-50CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
    A review. Quasi-elastic neutron scattering (QENS) and mol. dynamics (MD) simulations have enabled the quant. study of mol. motion in pure and mixed fluids sorbed in zeolites over length scales of 0.1-100 nm and time scales of 10 fs-100 ns. After an introduction to the statistical mechanics of diffusion, this review describes the principles and practice of time-of-flight (TOF), backscattering (BS), and neutron spin-echo (NSE) measurements, as well as methods to analyze the results. A brief overview of MD simulations is provided, with emphasis on how to calc. QENS observables and how to detect signatures of local anisotropic translational dynamics within the inhomogeneous periodic force field of zeolite crystals. Illustrations of what can be learned from combined application of QENS and mol. simulation are provided from recent work on specific systems and problems: self-diffusion of n-alkanes up to C16 in MFI zeolites, self-diffusion of methane co-adsorbed with n-butane in silicalite-1, and transport diffusion of N2 and CO2 in silicalite-1.
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    Demontis, P.; Suffritti, G. B. Structure and Dynamics of Zeolites Investigated by Molecular Dynamics. Chem. Rev. 1997, 97, 2845– 2878,  DOI: 10.1021/cr950253o
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    Structure and Dynamics of Zeolites Investigated by Molecular Dynamics
    Demontis, Pierfranco; Suffritti, Giuseppe B.
    Chemical Reviews (Washington, D. C.) (1997), 97 (8), 2845-2878CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review, with 246 refs., is given on mol. dynamics simulation of zeolites and covers mol. dynamics simulation technique, simulation of diffusive processes, simulation of the structure and dynamics of the framework and special topics including intramol. dynamics of sorbates, chem. reactions, and activated diffusion.
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    Sholl, D. S. Understanding Macroscopic Diffusion of Adsorbed Molecules in Crystalline Nanoporous Materials via Atomistic Simulations. Acc. Chem. Res. 2006, 39, 403– 411,  DOI: 10.1021/ar0402199
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    Understanding Macroscopic Diffusion of Adsorbed Molecules in Crystalline Nanoporous Materials via Atomistic Simulations
    Sholl, David S.
    Accounts of Chemical Research (2006), 39 (6), 403-411CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    The diffusion rates of mols. inside nanoporous materials lie at the heart of many large-scale industrial applications of these materials. Quant. describing this diffusion, particularly diffusion of chem. mixts. in situations leading to net mass transport, remains challenging. Mol. dynamics (MD) simulations can play an important complementary role to expts. in this area. This Account describes applications of MD to diffusion in nanoporous materials with a particular focus on macroscopic diffusion, i.e., diffusion involving mass transport. These methods have made useful contributions to developing mixing theories for predicting multicomponent diffusion from single-component data and to screening new classes of materials for practical applications.
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    Smit, B.; Maesen, T. L. Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity. Chem. Rev. 2008, 108, 4125– 4184,  DOI: 10.1021/cr8002642
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    Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity
    Smit, Berend; Maesen, Theo L. M.
    Chemical Reviews (Washington, DC, United States) (2008), 108 (10), 4125-4184CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Details of simulation studies for adsorption, diffusion, and shape selectivity on zeolites have been reviewed.
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    Skoulidas, A. I. Molecular Dynamics Simulations of Gas Diffusion in Metal-Organic Frameworks: Argon in Cubtc. J. Am. Chem. Soc. 2004, 126, 1356– 1357,  DOI: 10.1021/ja039215+
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    Molecular dynamics simulations of gas diffusion in metal-organic frameworks: Argon in CuBTC
    Skoulidas, Anastasios I.
    Journal of the American Chemical Society (2004), 126 (5), 1356-1357CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The class of coordination polymers known as metalorg. frameworks (MOFs) has 3D porous structures that are considered as a promising alternative to zeolites and other nanoporous materials for catalysis, gas adsorption, and gas sepn. applications. Here, the first study of gas diffusion inside an MOF is presented and the obsd. diffusion is compared to known behaviors in zeolites. Using grand canonical Monte Carlo and equil. mol. dynamics, the adsorption isotherm and self-, cor., and transport diffusivities for argon in the CuBTC metalorg. framework were calcd. Diffusion of Ar in CuBTC is very similar to Ar diffusion in silica zeolites in magnitude, concn., and temp. dependence. This conclusion appears to apply to a broad range of MOF structures.
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    Skoulidas, A. I.; Sholl, D. S. Self-Diffusion and Transport Diffusion of Light Gases in Metal-Organic Framework Materials Assessed Using Molecular Dynamics Simulations. J. Phys. Chem. B 2005, 109, 15760– 15768,  DOI: 10.1021/jp051771y
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    Self-Diffusion and Transport Diffusion of Light Gases in Metal-Organic Framework Materials Assessed Using Molecular Dynamics Simulations
    Skoulidas, Anastasios I.; Sholl, David S.
    Journal of Physical Chemistry B (2005), 109 (33), 15760-15768CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Metal-org. framework (MOF) materials pose an interesting alternative to more traditional nanoporous materials for a variety of sepn. processes. Sepn. processes involving nanoporous materials can be controlled by either adsorption equil., diffusive transport rates, or a combination of these factors. Adsorption equil. was studied for a variety of gases in MOFs, but almost nothing is currently known about mol. diffusion rates in MOFs. The authors used equil. mol. dynamics (MD) to probe the self-diffusion and transport diffusion of a no. of small gas species in several MOFs as a function of pore loading at room temp. Specifically, the authors have studied Ar, CH4, CO2, N2, and H2 diffusion in MOF-5. The diffusion of Ar in MOF-2, MOF-3, and Cu-BTC was assessed in a similar manner. Results greatly expand the range of MOFs for which data describing mol. diffusion is available. The authors discuss the prospects for exploiting mol. transport properties in MOFs in practical sepn. processes and the future role of MD simulations in screening families of MOFs for these processes.
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    Yang, Q.; Zhong, C. Molecular Simulation of Adsorption and Diffusion of Hydrogen in Metal-Organic Frameworks. J. Phys. Chem. B 2005, 109, 11862– 11864,  DOI: 10.1021/jp051903n
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    Molecular Simulation of Adsorption and Diffusion of Hydrogen in Metal-Organic Frameworks
    Yang, Qingyuan; Zhong, Chongli
    Journal of Physical Chemistry B (2005), 109 (24), 11862-11864CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Metal-org. frameworks (MOFs) are thought to be a set of promising hydrogen storage materials; however, little is known about the interactions between hydrogen mols. and pore walls as well as the diffusivities of hydrogen in MOFs. In this work, we performed a systematic mol. simulation study on the adsorption and diffusion of hydrogen in MOFs to provide insight into mol.-level details of the underlying mechanisms. This work shows that metal-oxygen clusters are preferential adsorption sites for hydrogen in MOFs, and the effect of the org. linkers becomes evident with increasing pressure. The hydrogen storage capacity of MOFs is similar to carbon nanotubes, which is higher than zeolites. Diffusion of hydrogen in MOFs is an activated process that is similar to diffusion in zeolites. The information derived in this work is useful to guide the future rational design and synthesis of tailored MOF materials with improved hydrogen adsorption capability.
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    Düren, T.; Snurr, R. Q. Assessment of Isoreticular Metal-Organic Frameworks for Adsorption Separations: A Molecular Simulation Study of Methane/n-Butane Mixtures. J. Phys. Chem. B 2004, 108, 15703– 15708,  DOI: 10.1021/jp0477856
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    Assessment of Isoreticular Metal-Organic Frameworks for Adsorption Separations: A Molecular Simulation Study of Methane/n-Butane Mixtures
    Dueren, Tina; Snurr, Randall Q.
    Journal of Physical Chemistry B (2004), 108 (40), 15703-15708CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    A wide variety of new nanoporous metal-org. materials are being synthesized by many research groups using supramol. chem. and directed assembly in a building block approach based on corner units and linker mols. These materials may lead to revolutionary advances in adsorption sepns. because the properties of these materials may be tailored in a synthetically predictable manner. In this paper, we use mol. simulations to assess the suitability of one group of metal org. materials, namely, isoreticular metal-org. frameworks (IRMOFs), as adsorbents for mixt. sepns. By using grand canonical Monte Carlo simulations, the influence of the linker mol. on the adsorption of methane, n-butane, and their mixts. is detd. Detailed anal. of the energetics as well as the siting of mols. in the cavities allows us to resolve the impact of the linker mols. on the selectivity and to propose new, not yet synthesized materials, which show even higher selectivities. The predicted selectivities are as good as or better than exptl. obsd. selectivities in other adsorbents, suggesting that IRMOFs are promising materials for the sepn. of hydrocarbons.
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    Garberoglio, G.; Vallauri, R. Adsorption and Diffusion of Hydrogen and Methane in 2d Covalent Organic Frameworks. Microporous Mesoporous Mater. 2008, 116, 540– 547,  DOI: 10.1016/j.micromeso.2008.05.023
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    Keskin, S. Adsorption, Diffusion, and Separation of CH4/H2 Mixtures in Covalent Organic Frameworks: Molecular Simulations and Theoretical Predictions. J. Phys. Chem. C 2012, 116, 1772– 1779,  DOI: 10.1021/jp209804x
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    Adsorption, Diffusion, and Separation of CH4/H2 Mixtures in Covalent Organic Frameworks: Molecular Simulations and Theoretical Predictions
    Keskin, Seda
    Journal of Physical Chemistry C (2012), 116 (2), 1772-1779CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Grand canonical Monte Carlo and equil. mol. dynamics simulations were used to compute adsorption isotherms and self-diffusivities of CH4/H2 mixts. at various compns. in three representative covalent org. frameworks (COFs). Several properties of COFs, such as adsorption selectivity, working capacity, diffusion selectivity, gas permeability, and membrane selectivity were evaluated and were compared with metal org. frameworks (MOFs), zeolites, zeolite imidazolate frameworks (ZIFs), and carbon nanotubes. Results showed that COF-6 outperforms traditional zeolites CHA, LTA, and ITQ-29 and MOFs IRMOF-1, CuBTT, and MOF-177 in adsorption-based CH4 selectivity. Membrane selectivities of COF-5, COF-6, and COF-10 were found to be higher than those of zeolites and similar to ZIFs and MOFs. Adsorption isotherms and diffusivities of CH4/H2 mixts. in the pores of COF-6 were computed using both atomically detailed simulations and theor. correlations. Results showed that theor. correlations based on single component adsorption and diffusion data can be used to accurately predict mixt. adsorption and diffusion of gases in COFs.
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    Yang, Z.; Cao, D. Effect of Li Doping on Diffusion and Separation of Hydrogen and Methane in Covalent Organic Frameworks. J. Phys. Chem. C 2012, 116, 12591– 12598,  DOI: 10.1021/jp302175d
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    Effect of Li Doping on Diffusion and Separation of Hydrogen and Methane in Covalent Organic Frameworks
    Yang, Zhanlei; Cao, Dapeng
    Journal of Physical Chemistry C (2012), 116 (23), 12591-12598CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The authors systematically study the diffusion and sepn. behavior of hydrogen and methane in covalent org. frameworks (COFs) and Li-doped counterparts. The self-diffusivities of hydrogen and methane in COFs decrease monotonically with the increase of pressure. After Li doping into COFs, the self-diffusivities 1st increase at low pressure, and then reach a plateau, and finally decrease slightly at high pressure. This phenomenon stems from the fact that the Li atom has a strong affinity to the gas mols. Also the COFs show a larger self-diffusivity than most of the metal-org. frameworks (MOFs), and the permselectivities of the COFs for H2/CH4 are 1 order of magnitude higher than the Li-doped COFs. In particular, the adsorption selectivity of Li-doped COFs for CH4/H2 gets a significant improvement, compared to undoped ones. To further understand the effect of Li doping on diffusion and sepn. of gases, the isosurface and the contour plots of the center of mass and radial distribution functions of gases are also explored. In short, the Li doping into COFs can increase the adsorption selectivity of COF materials significantly, while for a kinetic sepn. process, the undoped COF-based membranes might be a very promising material.
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    Zeng, H.; Liu, Y.; Liu, H. Adsorption and Diffusion of CO2 and CH4 in Covalent Organic Frameworks: An MC/MD Simulation Study. Mol. Simul. 2018, 44, 1244– 1251,  DOI: 10.1080/08927022.2018.1481959
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    Adsorption and diffusion of CO2 and CH4 in covalent organic frameworks: an MC/MD simulation study
    Zeng, Hongwei; Liu, Yu; Liu, Honglai
    Molecular Simulation (2018), 44 (15), 1244-1251CODEN: MOSIEA; ISSN:0892-7022. (Taylor & Francis Ltd.)
    Covalent org. frameworks (COFs) are a promising gas sepn. material which have been developed recently. In this work, we have used grand canonical Monte Carlo (GCMC) and mol. dynamics (MD) simulations to investigate the adsorption and diffusion properties of CO2 and CH4 in five recent synthesized COF materials. We have also considered the properties of amino-modified COFs by adding -NH2 group to the five COFs. The adsorption isotherm, adsorption/diffusion selectivity, self/transport diffusion coeffs. have been examd. and discussed. All of the five COFs exhibit promising adsorption selectivity which is higher than common nanoporous materials. An S-shaped adsorption isotherm can be found for CO2 instead of CH4 adsorption. The introduction of -NH2 group is effective at low pressure region (<200 kPa). The diffusion coeffs. are similar for TS-COFs but increase with the pore size for PI-COFs, and the diffusion coeffs. seem less dependent on the -NH2 groups.
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    Altundal, O. F.; Haslak, Z. P.; Keskin, S. Combined GCMC, MD, and DFT Approach for Unlocking the Performances of COFs for Methane Purification. Ind. Eng. Chem. Res. 2021, 60, 12999– 13012,  DOI: 10.1021/acs.iecr.1c01742
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    Combined GCMC, MD, and DFT Approach for Unlocking the Performances of COFs for Methane Purification
    Altundal, Omer Faruk; Haslak, Zeynep Pinar; Keskin, Seda
    Industrial & Engineering Chemistry Research (2021), 60 (35), 12999-13012CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    Covalent org. frameworks (COFs) are promising materials for gas storage and sepn.; however, the potential of COFs for sepn. of CH4 from industrially relevant gases such as H2, N2, and C2H6 is yet to be investigated. In this work, we followed a multiscale computational approach to unlock both the adsorption- and membrane-based CH4/H2, CH4/N2, and C2H6/CH4 sepn. potentials of 572 COFs by combining grand canonical Monte Carlo (GCMC) and mol. dynamics (MD) simulations and d. functional theory (DFT) calcns. Adsorbent performance evaluation metrics of COFs, adsorption selectivity, working capacity, regenerability, and adsorbent performance score were calcd. for sepn. of equimolar CH4/H2, CH4/N2, and C2H6/CH4 mixts. at vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) conditions to identify the best-performing COFs for each mixt. Results showed that COFs could achieve selectivities of 2-85, 1-7, and 2-23 for PSA-based CH4/H2, CH4/N2, and C2H6/CH4 sepns., resp., outperforming conventional adsorbents such as zeolites and activated carbons for each mixt. Structure-performance relations revealed that COFs with pore sizes <10 Å are promising adsorbents for all mixts. We identified the gas adsorption sites in the three top-performing COFs commonly identified for each mixt. by DFT calcns. and computed the binding strength of gases, which were found to be on the order of C2H6 > CH4 > N2 > H2, supporting the GCMC results. Nucleus-independent chem. shift (NICS) indexes of aromaticity for adsorption sites were calcd., and the results revealed that the degree of linker aromaticity could be a measure for the selection or design of highly alkane-selective COF adsorbents over N2 and H2. Finally, COF membranes were shown to achieve high H2 permeabilities, 4.57 x 103-1.25 x 106 Barrer, and decent membrane selectivities, as high as 4.3, outperforming polymeric and MOF-based membranes for sepn. of H2 from CH4.
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    Bukowski, B. C.; Keil, F. J.; Ravikovitch, P. I.; Sastre, G.; Snurr, R. Q.; Coppens, M.-O. Connecting Theory and Simulation with Experiment for the Study of Diffusion in Nanoporous Solids. Adsorption 2021, 27, 683– 760,  DOI: 10.1007/s10450-021-00314-y
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    Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids
    Bukowski, Brandon C.; Keil, Frerich J.; Ravikovitch, Peter I.; Sastre, German; Snurr, Randall Q.; Coppens, Marc-Olivier
    Adsorption (2021), 27 (5), 683-760CODEN: ADSOFO; ISSN:0929-5607. (Springer)
    A review. Nanoporous solids are ubiquitous in chem., energy, and environmental processes, where controlled transport of mols. through the pores plays a crucial role. They are used as sorbents, chromatog. or membrane materials for sepns., and as catalysts and catalyst supports. Defined as materials where confinement effects lead to substantial deviations from bulk diffusion, nanoporous materials include cryst. microporous zeotypes and metal-org. frameworks (MOFs), and a no. of semi-cryst. and amorphous mesoporous solids, as well as hierarchically structured materials, contg. both nanopores and wider meso- or macropores to facilitate transport over macroscopic distances. The ranges of pore sizes, shapes, and topologies spanned by these materials represent a considerable challenge for predicting mol. diffusivities, but fundamental understanding also provides an opportunity to guide the design of new nanoporous materials to increase the performance of transport limited processes. Remarkable progress in synthesis increasingly allows these designs to be put into practice. Mol. simulation techniques have been used in conjunction with exptl. measurements to examine in detail the fundamental diffusion processes within nanoporous solids, to provide insight into the free energy landscape navigated by adsorbates, and to better understand nano-confinement effects. Pore network models, discrete particle models and synthesis-mimicking atomistic models allow to tackle diffusion in mesoporous and hierarchically structured porous materials, where multiscale approaches benefit from ever cheaper parallel computing and higher resoln. imaging. Here, we discuss synergistic combinations of simulation and expt. to showcase theor. progress and computational techniques that have been successful in predicting guest diffusion and providing insights. We also outline where new fundamental developments and exptl. techniques are needed to enable more accurate predictions for complex systems.
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    Ford, D. C.; Dubbeldam, D.; Snurr, R. Q.; Kunzel, V.; Wehring, M.; Stallmach, F.; Karger, J.; Muller, U. Self-Diffusion of Chain Molecules in the Metal-Organic Framework IRMOF-1: Simulation and Experiment. J. Phys. Chem. Lett. 2012, 3, 930– 933,  DOI: 10.1021/jz300141n
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    Self-Diffusion of Chain Molecules in the Metal-Organic Framework IRMOF-1: Simulation and Experiment
    Ford, Denise C.; Dubbeldam, David; Snurr, Randall Q.; Kunzel, Volker; Wehring, Markus; Stallmach, Frank; Karger, Jorg; Muller, Ulrich
    Journal of Physical Chemistry Letters (2012), 3 (7), 930-933CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Metal-org. frameworks (MOFs) possess characteristics, such as tunable pore size and chem. functionality, that make them attractive candidates for sepns., catalysis, gas storage, and sensing applications. The rate of diffusion of guest mols. in the pores is an important property for all of these potential applications. The self-diffusion of hydrocarbons in IRMOF-1 was studied as a function of chain length with a combination of mol. dynamics simulations and pulsed field gradient NMR expts. Excellent agreement is seen between the expts. and simulations, and the self-diffusion coeffs. in IRMOF-1 are on the same order as those in the bulk liq. Addnl., the effect of concn. on diffusivity was found to be very small for low to moderate loadings. Mol. dynamics simulations also provided insights about the preferential diffusion pathways of these guests in IRMOF-1.
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    Dutta, S.; Galarneau, A.; Minoux, D.; Aquino, C.; Dath, J. P.; Guenneau, F.; Coasne, B. Molecular Diffusion in Hierarchical Zeolites with Ordered Mesoporosity: Pulsed Field Gradient Nuclear Magnetic Resonance Combined with Thermodynamic Modeling. J. Phys. Chem. C 2023, 127, 1548– 1559,  DOI: 10.1021/acs.jpcc.2c04868
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    Grunenberg, L.; Savasci, G.; Terban, M. W.; Duppel, V.; Moudrakovski, I.; Etter, M.; Dinnebier, R. E.; Ochsenfeld, C.; Lotsch, B. V. Amine-Linked Covalent Organic Frameworks as a Platform for Postsynthetic Structure Interconversion and Pore-Wall Modification. J. Am. Chem. Soc. 2021, 143, 3430– 3438,  DOI: 10.1021/jacs.0c12249
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    Amine-Linked Covalent Organic Frameworks as a Platform for Postsynthetic Structure Interconversion and Pore-Wall Modification
    Grunenberg, Lars; Savasci, Goekcen; Terban, Maxwell W.; Duppel, Viola; Moudrakovski, Igor; Etter, Martin; Dinnebier, Robert E.; Ochsenfeld, Christian; Lotsch, Bettina V.
    Journal of the American Chemical Society (2021), 143 (9), 3430-3438CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Covalent org. frameworks have emerged as a powerful synthetic platform for installing and interconverting dedicated mol. functions on a cryst. polymeric backbone with at. precision. Here, we present a novel strategy to directly access amine-linked covalent org. frameworks, which serve as a scaffold enabling pore-wall modification and linkage-interconversion by new synthetic methods based on Leuckart-Wallach redn. with formic acid and ammonium formate. Frameworks connected entirely by secondary amine linkages, mixed amine/imine bonds, and partially formylated amine linkages are obtained in a single step from imine-linked frameworks or directly from corresponding linkers in a one-pot crystn.-redn. approach. The new, 2D amine-linked covalent org. frameworks, rPI-3-COF, rTTI-COF, and rPy1P-COF, are obtained with high crystallinity and large surface areas. Secondary amines, installed as reactive sites on the pore wall, enable further postsynthetic functionalization to access tailored covalent org. frameworks, with increased hydrolytic stability, as potential heterogeneous catalysts.
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    Grunenberg, L.; Savasci, G.; Emmerling, S. T.; Heck, F.; Bette, S.; Cima Bergesch, A.; Ochsenfeld, C.; Lotsch, B. V. Postsynthetic Transformation of Imine- into Nitrone-Linked Covalent Organic Frameworks for Atmospheric Water Harvesting at Decreased Humidity. J. Am. Chem. Soc. 2023, 145, 13241– 13248,  DOI: 10.1021/jacs.3c02572
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    Postsynthetic Transformation of Imine- into Nitrone-Linked Covalent Organic Frameworks for Atmospheric Water Harvesting at Decreased Humidity
    Grunenberg, Lars; Savasci, Goekcen; Emmerling, Sebastian T.; Heck, Fabian; Bette, Sebastian; Cima Bergesch, Afonso; Ochsenfeld, Christian; Lotsch, Bettina V.
    Journal of the American Chemical Society (2023), 145 (24), 13241-13248CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Herein, we report a facile postsynthetic linkage conversion method giving synthetic access to nitrone-linked covalent org. frameworks (COFs) from imine- and amine-linked COFs. The new two-dimensional (2D) nitrone-linked covalent org. frameworks, NO-PI-3-COF and NO-TTI-COF, are obtained with high crystallinity and large surface areas. Nitrone-modified pore channels induce condensation of water vapor at 20% lower humidity compared to their amine- or imine-linked precursor COFs. Thus, the topochem. transformation to nitrone linkages constitutes an attractive approach to postsynthetically fine-tune water adsorption properties in framework materials.
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    Smith, B. J.; Parent, L. R.; Overholts, A. C.; Beaucage, P. A.; Bisbey, R. P.; Chavez, A. D.; Hwang, N.; Park, C.; Evans, A. M.; Gianneschi, N. C.; Dichtel, W. R. Colloidal Covalent Organic Frameworks. ACS Cent. Sci. 2017, 3, 58– 65,  DOI: 10.1021/acscentsci.6b00331
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    Colloidal Covalent Organic Frameworks
    Smith, Brian J.; Parent, Lucas R.; Overholts, Anna C.; Beaucage, Peter A.; Bisbey, Ryan P.; Chavez, Anton D.; Hwang, Nicky; Park, Chiwoo; Evans, Austin M.; Gianneschi, Nathan C.; Dichtel, William R.
    ACS Central Science (2017), 3 (1), 58-65CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
    Covalent org. frameworks (COFs) are two or three-dimensional polymer networks with designed topol. and chem. functionality, permanent porosity, and high surface areas. These features are potentially useful for a broad range of applications, including catalysis, optoelectronics, and energy storage devices. But current COF syntheses offer poor control over the material's morphol. and final form, generally providing insol. and unprocessable microcryst. powder aggregates. COF polymns. are often performed under conditions in which the monomers are only partially sol. in the reaction solvent, and this heterogeneity has hindered understanding of their polymn. or crystn. processes. Here we report homogeneous polymn. conditions for boronate ester-linked, two-dimensional COFs that inhibit crystallite pptn., resulting in stable colloidal suspensions of 2D COF nanoparticles. The hexagonal, layered structures of the colloids are confirmed by small angle and wide-angle X-ray scattering (SAXS/WAXS), and kinetic characterization provides insight into the growth process. The colloid size is modulated by solvent conditions, and the technique is demonstrated for four 2D boronate ester-linked COFs. The diam. of individual COF nanoparticles in soln. is monitored and quantified during COF growth and stabilization at elevated temp. using in situ variable-temp. liq. cell TEM (VT-LCTEM) imaging, a new characterization technique that complements conventional bulk scattering techniques. Soln. casting of the colloids yields a free-standing transparent COF film with retained crystallinity and porosity, as well as preferential crystallite orientation. Collectively this structural control provides new opportunities for understanding COF formation and designing morphologies for device applications.
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    Das, A.; Jayanthi, S.; Deepak, H. S.; Ramanathan, K. V.; Kumar, A.; Dasgupta, C.; Sood, A. K. Single-File Diffusion of Confined Water inside SWNTs: An NMR Study. ACS Nano 2010, 4, 1687– 1695,  DOI: 10.1021/nn901554h
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    Single-File Diffusion of Confined Water Inside SWNTs: An NMR Study
    Das, Anindya; Jayanthi, Sundaresan; Deepak, Handiganadu Srinivasa Murthy Vinay; Ramanathan, Krishna Venkatachala; Kumar, Anil; Dasgupta, Chandan; Sood, Ajay K.
    ACS Nano (2010), 4 (3), 1687-1695CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We report a NMR study of confined water inside ∼1.4 nm diam. single-walled carbon nanotubes (SWNTs). We show that the confined water does not freeze even up to 223 K. A pulse field gradient (PFG) NMR method is used to det. the mean squared displacement (MSD) of the water mols. inside the nanotubes at temps. below 273 K, where the bulk water outside the nanotubes freezes and hence does not contribute to the proton NMR signal. We show that the mean squared displacement varies as the square root of time, predicted for single-file diffusion in a one-dimensional channel. We propose a qual. understanding of our results based on available mol. dynamics simulations.
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    Kärger, J.; Avramovska, M.; Freude, D.; Haase, J.; Hwang, S.; Valiullin, R. Pulsed Field Gradient NMR Diffusion Measurement in Nanoporous Materials. Adsorption 2021, 27, 453– 484,  DOI: 10.1007/s10450-020-00290-9
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    The stimulated echo in a three-radio-frequency-pulse expt. is useful in extending the range of measurement of diffusion coeffs. to more viscous substances or the measurement of barrier sepns. to wider spacings in systems where the diffusing substance has T1 > T2. The spin-echo attenuation due to self-diffusion is derived for the general case of a time-dependent field gradient, and the result is found exptl. to be correct for the special case of a field gradient applied in 2 equal, square pulse.
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    NMR studies of benzene mobility in metal-organic framework MOF-5
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    European Physical Journal: Applied Physics (2011), 55 (2), 20702/1-20702/10CODEN: EPAPFV; ISSN:1286-0042. (EDP Sciences)
    The concn. and temp. dependence of the self-diffusion of benzene adsorbed in the metal-org. framework MOF-5 (IRMOF-1) is studied by pulsed field gradient (PPG) NMR spectroscopy. When increasing the loading from 10 to 20 mols. per unit cell of MOF-5, the exptl. diffusion data drop by a factor of about 3 while current mol. dynamic (MD) simulations predict slightly increasing diffusion coeffs. for this range of loadings. The observation is rationalized using the recently predicted clustering of adsorbate mols. in microporous systems for temps. well below the adsorbate crit. temp. Necessary improvements of mol. simulation models for predicting diffusivities under such conditions are discussed.
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    Stallmach, F.; Groger, S.; Kunzel, V.; Karger, J.; Yaghi, O. M.; Hesse, M.; Muller, U. NMR Studies on the Diffusion of Hydrocarbons on the Metal-Organic Framework Material MOF-5. Angew. Chem., Int. Ed. 2006, 45, 2123– 2126,  DOI: 10.1002/anie.200502553
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    NMR studies on the diffusion of hydrocarbons on the metal-organic framework material MOF-5
    Stallmach, Frank; Groeger, Stefan; Kuenzel, Volker; Kaerger, Joerg; Yaghi, O. M.; Hesse, Michael; Mueller, Ulrich
    Angewandte Chemie, International Edition (2006), 45 (13), 2123-2126CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    High intracryst. mobilities and fast exchange between the crystal and the surrounding gas phase were found in NMR studies on the diffusion of methane, ethane, n-hexane, and benzene in large crystals of the metal-org. framework MOF-5. The results support future use of metal-org. framework materials as tailorable sorbents for fast gas processing and gas-storage materials in industry.
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    Splith, T.; Frohlich, D.; Henninger, S. K.; Stallmach, F. Development and Application of an Exchange Model for Anisotropic Water Diffusion in the Microporous MOF Aluminum Fumarate. J. Magn. Reson. 2018, 291, 40– 46,  DOI: 10.1016/j.jmr.2018.04.009
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    Development and application of an exchange model for anisotropic water diffusion in the microporous MOF aluminum fumarate
    Splith, Tobias; Froehlich, Dominik; Henninger, Stefan K.; Stallmach, Frank
    Journal of Magnetic Resonance (2018), 291 (), 40-46CODEN: JMARF3; ISSN:1090-7807. (Elsevier B.V.)
    Diffusion of water in aluminum fumarate was studied by means of pulsed field gradient (PFG) NMR (NMR). Due to water mols. exchanging between the intracryst. anisotropic pore space and the isotropic intercryst. void space the model of intracryst. anisotropic diffusion fails to describe the exptl. PFG NMR data at high observation times. Therefore, the two-site exchange model developed by Karger is extended to the case of exchange between an anisotropic and an isotropic site. This extended exchange model is solved by numerical integration. It describes the exptl. data very well and yields values for the intracryst. diffusion coeff. and the mean residence times of the resp. sites. Further PFG NMR studies were performed with coatings consisting of small aluminum fumarate crystals, which are used in adsorptive heat transformation applications. The diffusion coeffs. of water in the small crystal coating are compared to the values expected from the extended two-site exchange model and from the model of long-range diffusion.
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    Zeigermann, P.; Naumov, S.; Mascotto, S.; Karger, J.; Smarsly, B. M.; Valiullin, R. Diffusion in Hierarchical Mesoporous Materials: Applicability and Generalization of the Fast-Exchange Diffusion Model. Langmuir 2012, 28, 3621– 3632,  DOI: 10.1021/la2047432
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    Diffusion in Hierarchical Mesoporous Materials: Applicability and Generalization of the Fast-Exchange Diffusion Model
    Zeigermann, P.; Naumov, S.; Mascotto, S.; Kaerger, J.; Smarsly, B. M.; Valiullin, R.
    Langmuir (2012), 28 (7), 3621-3632CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Transport properties of cyclohexane confined to a silica material with an ordered, bimodal pore structure were studied by pulsed field gradient NMR. A particular organization of the well-defined pore structure, composed of a collection of spatially ordered, spherical mesopores interconnected via narrow worm-like pores, allowed for a quant. anal. of the diffusion process in a medium with spatially ordered distribution of the fluid d. for a broad range of the gas-liq. equil. The measured diffusion data were interpreted in terms of effective diffusivities, which were detd. within a microscopic model considering long-range mol. trajectories constructed by assembling the alternating pieces of displacement in the two constituting pore spaces. It has further been found that for the system under study, in particular, and for mesoporous materials with multiple porosities, in general, this generalized model simplifies to the conventional fast-exchange model used in the literature. Thus, not only was justification of the applicability of the fast-exchange model to a diversity of mesoporous materials provided, but the diffusion parameters entering the fast-exchange model were also exactly defined. The equation resulting in this way was found to nicely reproduce the exptl. detd. diffusivities, establishing a methodol. for targeted fine-tuning of transport properties of fluids in hierarchical materials with multiple porosities.
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    Loskutov, V. V.; Sevriugin, V. A. A Novel Approach to Interpretation of the Time-Dependent Self-Diffusion Coefficient as a Probe of Porous Media Geometry. J. Magn. Reson. 2013, 230, 1– 9,  DOI: 10.1016/j.jmr.2013.01.004
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    A novel approach to interpretation of the time-dependent self-diffusion coefficient as a probe of porous media geometry
    Loskutov, V. V.; Sevriugin, V. A.
    Journal of Magnetic Resonance (2013), 230 (), 1-9CODEN: JMARF3; ISSN:1090-7807. (Elsevier B.V.)
    This article presents a new approxn. describing fluid diffusion in porous media. Time dependence of the self-diffusion coeff. D(t) in the permeable porous medium is studied based on the assumption that diffusant mols. move randomly. An anal. expression for time dependence of the self-diffusion coeff. was obtained in the following form: D(t)=(D0-D∞)exp(-D0t/λ)+D∞, where D0 is the self-diffusion coeff. of bulk fluid, D∞ is the asymptotic value of the self-diffusion coeff. in the limit of long time values (t → ∞), λ is the characteristic parameter of this porous medium with dimensionality of length. Applicability of the soln. obtained to the anal. of exptl. data is shown. The possibility of passing to short-time and long-time regimes is discussed.
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    Latour, L. L.; Mitra, P. P.; Kleinberg, R. L.; Sotak, C. H. Time-Dependent Diffusion Coefficient of Fluids in Porous Media as a Probe of Surface-to-Volume Ratio. J. Magn. Reson. Ser. A 1993, 101, 342– 346,  DOI: 10.1006/jmra.1993.1056
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    Time-dependent diffusion coefficient of fluids in porous media as a probe of surface-to-volume ratio
    Latour, Lawrence L.; Mitra, Partha P.; Kleinberg, Robert L.; Sotak, Christopher H.
    Journal of Magnetic Resonance, Series A (1993), 101 (3), 342-6CODEN: JMRAE2; ISSN:1064-1858.
    The time-dependent diffusion coeff. is independent of the microscopic details of the restricting geometry and depends only on the surface-to-vol. ratio S/V of the pore space. Pulsed field gradient NMR is an ideal tool for measuring the time-dependent diffusion coeff. The min. observation time is detd. by the min. length of gradient pulses and the subsequent recovery of the app. from eddy-current and magnetoacoustic effects. In the current expts., the min. observation time is about 7 ms. The max. time is detd. by the spin-lattice and spin-spin relaxation times of the fluid. For these expts., the max. observation time was chosen to be T1 of the spins. Pulsed field gradient NMR expts. were conducted on fluid-satd. glass sphere packs and on a sample of onion tissue. The diffusion coeff. for the intracellular fluid in an onion was detd. The diffusion coeffs. for distd. water and viscous oil in the pores of a monosized sphere pack were also detd.
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    Forse, A. C.; Colwell, K. A.; Gonzalez, M. I.; Benders, S.; Torres-Gavosto, R. M.; Blümich, B.; Reimer, J. A.; Long, J. R. Influence of Pore Size on Carbon Dioxide Diffusion in Two Isoreticular Metal-Organic Frameworks. Chem. Mater. 2020, 32, 3570– 3576,  DOI: 10.1021/acs.chemmater.0c00745
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    Influence of Pore Size on Carbon Dioxide Diffusion in Two Isoreticular Metal-Organic Frameworks
    Forse, Alexander C.; Colwell, Kristen A.; Gonzalez, Miguel I.; Benders, Stefan; Torres-Gavosto, Rodolfo M.; Blumich, Bernhard; Reimer, Jeffrey A.; Long, Jeffrey R.
    Chemistry of Materials (2020), 32 (8), 3570-3576CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The rapid diffusion of mols. in porous materials is crit. for numerous applications including sepns., energy storage, sensing, and catalysis. A common strategy for tuning guest diffusion rates is to vary the material pore size, although detailed studies that isolate the effect of changing this particular variable are lacking. Here, we begin to address this challenge by measuring the diffusion of carbon dioxide in two isoreticular metal-org. frameworks featuring channels with different diams., Zn2(dobdc) (dobdc4- = 2,5-dioxidobenzene-1,4-dicarboxylate) and Zn2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), using pulsed field gradient NMR spectroscopy. An increase in the pore diam. from 15 Å in Zn2(dobdc) to 22 Å in Zn2(dobpdc) is accompanied by an increase in the self-diffusion of CO2 by a factor of 4 to 6, depending on the gas pressure. Anal. of the diffusion anisotropy in Zn2(dobdc) reveals that the self-diffusion coeff. for motion of CO2 along the framework channels is at least 10000 times greater than for motion between the framework channels. Our findings should aid the design of improved porous materials for a range of applications where diffusion plays a crit. role in detg. performance.
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    Holzmann, T.; Schoop, L. M.; Ali, M. N.; Moudrakovski, I.; Gregori, G.; Maier, J.; Cava, R. J.; Lotsch, B. V. Li0.6[Li0.2Sn0.8S2]─a Layered Lithium Superionic Conductor. Energy Environ. Sci. 2016, 9, 2578– 2585,  DOI: 10.1039/C6EE00633G
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    Beckert, S.; Stallmach, F.; Toufar, H.; Freude, D.; Kärger, J.; Haase, J. Tracing Water and Cation Diffusion in Hydrated Zeolites of Type Li-LSX by Pulsed Field Gradient NMR. J. Phys. Chem. C 2013, 117, 24866– 24872,  DOI: 10.1021/jp408604y
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    Tracing Water and Cation Diffusion in Hydrated Zeolites of Type Li-LSX by Pulsed Field Gradient NMR
    Beckert, Steffen; Stallmach, Frank; Toufar, Helge; Freude, Dieter; Kaerger, Joerg; Haase, Juergen
    Journal of Physical Chemistry C (2013), 117 (47), 24866-24872CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The pulsed field gradient (PFG) technique of NMR is exploited for recording the time-dependent mean diffusion path lengths of both the water mols. (via 1H NMR) and the cations (via 7Li NMR) in hydrated zeolite Li-LSX. The obsd. propagation patterns reveal, for both the water mols. and the cations, two types of transport resistances, acting in addn. to the diffusion resistance of the genuine pore network. They are attributed to the interfaces at the boundary between the purely cryst. regions (crystallites) within the Li-LSX particles (intergrowths) under study and to the external surface of either the particles themselves or crystallite aggregates within these particles. The cation diffusivity is retarded by ∼1 order of magnitude in comparison with the water diffusivity. This notably exceeds the retardation of cation diffusion in comparison with water in free soln., reflecting the particular influence of the zeolite lattice on the guest mobility.
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    Krutyeva, M.; Vasenkov, S.; Yang, X.; Caro, J.; Kärger, J. Surface Barriers on Nanoporous Particles: A New Method of Their Quantitation by PFG NMR. Microporous Mesoporous Mater. 2007, 104, 89– 96,  DOI: 10.1016/j.micromeso.2007.01.010
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    Surface barriers on nanoporous particles: A new method of their quantitation by PFG NMR
    Krutyeva, M.; Vasenkov, S.; Yang, X.; Caro, J.; Kaerger, J.
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    The surface permeability of zeolite crystals NaCaA for small alkane mols. (methane and ethane) was estd. by using a novel method, analyzing NMR tracer desorption measurements. It is based on the measurement of both the relative amt. of tracer exchange and the corresponding effective coeffs. of intracryst. diffusion and their correlation with the results of model calcns. by dynamic Monte Carlo simulations. For the two LTA specimens studied, already in the as-synthesized zeolite crystals notable surface resistances were obsd. Surface permeabilities were studied in dependence on temp., sorbate and crystal size.
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    Peksa, M.; Lang, J.; Stallmach, F. 13C NMR Study of Diffusion Anisotropy of Carbon Dioxide Adsorbed in Nanoporous DMOF-1. Microporous Mesoporous Mater. 2015, 205, 11– 15,  DOI: 10.1016/j.micromeso.2014.09.039
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    13C NMR study of diffusion anisotropy of carbon dioxide adsorbed in nanoporous DMOF-1
    Peksa, Mikulas; Lang, Jan; Stallmach, Frank
    Microporous and Mesoporous Materials (2015), 205 (), 11-15CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier Inc.)
    DMOF-1 (Zn2(bdc)2dabco) is a metal-org. framework with tetragonal symmetry. It contains parallel one-dimensional channels with a diam. of about 0.75nm interconnected by smaller windows. For CO2 adsorbed in the nanochannels the chem. shift tensor and the diffusion tensor were investigated by 13C NMR. The spectra acquired under static conditions reveal a powder pattern with a residual chem. shift anisotropy of 〈Δδ〉 = -55 ppm. The collinearity of the axisym. residual chem. shift and the diffusion tensors was utilized to assess the diffusion anisotropy via the diffusion attenuation of the 13C PFG NMR powder pattern. CO2 is highly mobile in DMOF-1. The apparent diffusion coeff. (the trace of diffusion tensor) is (6.2±1.0) × 10-9m2s-1 at 298 K and the corresponding anisotropy expressed as D‖/D.perp. is ≈3.
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    Naumov, S.; Valiullin, R.; Kärger, J.; Pitchumani, R.; Coppens, M.-O. Tracing Pore Connectivity and Architecture in Nanostructured Silica SBA-15. Microporous Mesoporous Mater. 2008, 110, 37– 40,  DOI: 10.1016/j.micromeso.2007.08.014
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    Isaacs, M. A.; Robinson, N.; Barbero, B.; Durndell, L. J.; Manayil, J. C.; Parlett, C. A.; D’Agostino, C.; Wilson, K.; Lee, A. F. Unravelling Mass Transport in Hierarchically Porous Catalysts. J. Mater. Chem. A 2019, 7, 11814– 11825,  DOI: 10.1039/c9ta01867k
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    Unravelling mass transport in hierarchically porous catalysts
    Isaacs, Mark A.; Robinson, Neil; Barbero, Brunella; Durndell, Lee J.; Manayil, Jinesh C.; Parlett, Christopher M. A.; D'Agostino, Carmine; Wilson, Karen; Lee, Adam F.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (19), 11814-11825CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Bio-derived platform chems. and fuels are important for the development of sustainable manufg. However, their efficient prodn. from biomass necessitates new catalysts and processes optimized for the selective transformation of large mols. Mesoporous and hierarchically porous functional materials are promising catalyst candidates for biomass valorization, but quant. relationships between pore dimensions/connectivity, mass transport, and corresponding catalytic performance are poorly defined. A family of hierarchical macroporous-mesoporous SBA-15 sulfonic acids were prepd. with tunable macropore diams. for carboxylic acid esterification. Turnover frequencies for long-chain (palmitic and erucic) acids were proportional to macropore diam. (≤370 nm), whereas propanoic acid esterification was independent of macropore size. Pulsed field gradient NMR diffusion expts. reveal that larger macropores enhance esterification of bulky carboxylic acids by conferring superior pore interconnectivity and assocd. mass transport.
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  71. 71
    Kessler, C.; Schuldt, R.; Emmerling, S.; Lotsch, B. V.; Kästner, J.; Gross, J.; Hansen, N. Influence of Layer Slipping on Adsorption of Light Gases in Covalent Organic Frameworks: A Combined Experimental and Computational Study. Microporous Mesoporous Mater. 2022, 336, 111796,  DOI: 10.1016/j.micromeso.2022.111796
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    71
    Influence of layer slipping on adsorption of light gases in covalent organic frameworks: A combined experimental and computational study
    Kessler, Christopher; Schuldt, Robin; Emmerling, Sebastian; Lotsch, Bettina V.; Kaestner, Johannes; Gross, Joachim; Hansen, Niels
    Microporous and Mesoporous Materials (2022), 336 (), 111796CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
    Sorption of gases in micro- and mesoporous materials is typically interpreted on the basis of idealized structural models where real structure effects such as defects and disorder are absent. For covalent org. frameworks (COFs) significant discrepancies between measured and simulated adsorption isotherms are often reported but rarely traced back to their origins. This is because little is known about the real structure of COFs and its effect on the sorption properties of these materials. In the present work mol. simulations are used to obtain adsorption isotherms of argon, nitrogen, and carbon dioxide in the COF-LZU1 at various temps. The (perfect) model COF has a BET surface that is higher than the exptl. BET surface by a factor of approx. 1.33, suggesting defects or inclusions are present in the real structure. We find that the satn. adsorption loading of small gaseous species in COF-LZU1, as detd. from grand canonical Monte Carlo simulations, is also higher by approx. the same factor compared to the exptl. satn. loading. The influence of interlayer slipping on the shape of the adsorption isotherm and the adsorption capacity is studied. Comparison between simulation and expt. at lower loadings suggests the layers to be shifted instead of perfectly eclipsed. The sensitivity of the adsorption isotherms in this regime towards the underlying framework topol. shows that real structure effects have significant influence on the gas uptake. Accounting for layer slipping is important to applications such as catalysis, gas storage and sepn.
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  72. 72
    Pütz, A. M.; Terban, M. W.; Bette, S.; Haase, F.; Dinnebier, R. E.; Lotsch, B. V. Total Scattering Reveals the Hidden Stacking Disorder in a 2D Covalent Organic Framework. Chem. Sci. 2020, 11, 12647– 12654,  DOI: 10.1039/D0SC03048A
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    Total scattering reveals the hidden stacking disorder in a 2D covalent organic framework
    Puetz, Alexander M.; Terban, Maxwell W.; Bette, Sebastian; Haase, Frederik; Dinnebier, Robert E.; Lotsch, Bettina V.
    Chemical Science (2020), 11 (47), 12647-12654CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Interactions between extended π-systems are often invoked as the main driving force for stacking and crystn. of 2D org. polymers. In covalent org. frameworks (COFs), the stacking strongly influences properties such as the accessibility of functional sites, pore geometry, and surface states, but the exact nature of the interlayer interactions is mostly elusive. The stacking mode is often identified as eclipsed based on obsd. high symmetry diffraction patterns. However, as pointed out by various studies, the energetics of eclipsed stacking are not favorable and offset stacking is preferred. This work presents lower and higher apparent symmetry modifications of the imine-linked TTI-COF prepd. through high- and low-temp. reactions. Through local structure investigation by pair distribution function anal. and simulations of stacking disorder, we observe random local layer offsets in the low temp. modification. We show that while stacking disorder can be easily overlooked due to the apparent crystallog. symmetry of these materials, total scattering methods can help clarify this information and suggest that defective local structures could be much more prevalent in COFs than previously thought. A detailed anal. of the local structure helps to improve the search for and design of highly porous tailor-made materials.
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  73. 73
    Haase, F.; Gottschling, K.; Stegbauer, L.; Germann, L. S.; Gutzler, R.; Duppel, V.; Vyas, V. S.; Kern, K.; Dinnebier, R. E.; Lotsch, B. V. Tuning the Stacking Behaviour of a 2D Covalent Organic Framework through Non-Covalent Interactions. Mater. Chem. Front. 2017, 1, 1354– 1361,  DOI: 10.1039/C6QM00378H
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    Tuning the stacking behaviour of a 2D covalent organic framework through non-covalent interactions
    Haase, F.; Gottschling, K.; Stegbauer, L.; Germann, L. S.; Gutzler, R.; Duppel, V.; Vyas, V. S.; Kern, K.; Dinnebier, R. E.; Lotsch, B. V.
    Materials Chemistry Frontiers (2017), 1 (7), 1354-1361CODEN: MCFAC5; ISSN:2052-1537. (Royal Society of Chemistry)
    Two-dimensional covalent org. frameworks (COFs) are cryst. porous materials composed of org. building blocks that are connected via covalent bonds within their layers, but through non-covalent interactions between the layers. The exact stacking sequence of the layers is of paramount importance for the optoelectronic, catalytic and sorption properties of these polymeric materials. The weak interlayer interactions lead to a variety of stacking geometries in COFs, which are both hard to characterize and poorly understood due to the low levels of crystallinity. Therefore, detailed insights into the stacking geometry in COFs is still largely elusive. In this work we show that the geometric and electronic features of the COF building blocks can be used to guide the stacking behavior of two related 2D imine COFs (TBI-COF and TTI-COF), which either adopt an averaged "eclipsed" structure with apparent zero-offset stacking or a unidirectionally slip-stacked structure, resp. These structural features are confirmed by XRPD and TEM measurements. Based on theor. calcns., we were able to pinpoint the cause of the uniform slip-stacking geometry and high crystallinity of TTI-COF to the inherent self-complementarity of the building blocks and the resulting donor-acceptor-type stacking of the imine bonds in adjacent layers, which can serve as a more general design principle for the synthesis of highly cryst. COFs.
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    Emmerling, S. T.; Schuldt, R.; Bette, S.; Yao, L.; Dinnebier, R. E.; Kastner, J.; Lotsch, B. V. Interlayer Interactions as Design Tool for Large-Pore COFs. J. Am. Chem. Soc. 2021, 143, 15711– 15722,  DOI: 10.1021/jacs.1c06518
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    Interlayer Interactions as Design Tool for Large-Pore COFs
    Emmerling, Sebastian T.; Schuldt, Robin; Bette, Sebastian; Yao, Liang; Dinnebier, Robert E.; Kaestner, Johannes; Lotsch, Bettina V.
    Journal of the American Chemical Society (2021), 143 (38), 15711-15722CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Covalent org. frameworks (COFs) with a pore size beyond 5 nm are still rarely seen in this emerging field. Besides obvious complications like the elaborated synthesis of large linkers with sufficient soly., more subtle challenges regarding large-pore COF synthesis, including pore occlusion and collapse, prevail. Here we present two isoreticular series of large-pore imine COFs with pore sizes up to 5.8 nm and correlate the interlayer interactions with the structure and thermal behavior of the COFs. By adjusting interlayer interactions through the incorporation of methoxy groups acting as pore-directing "anchors", different stacking modes can be accessed, resulting in modified stacking polytypes and, hence, effective pore sizes. A strong correlation between stacking energy toward highly ordered, nearly eclipsed structures, higher structural integrity during thermal stress, and a novel, thermally induced phase transition of stacking modes in COFs was found, which sheds light on viable design strategies for increased structural control and stability in large-pore COFs.
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    Stahler, C.; Grunenberg, L.; Terban, M. W.; Browne, W. R.; Doellerer, D.; Kathan, M.; Etter, M.; Lotsch, B. V.; Feringa, B. L.; Krause, S. Light-Driven Molecular Motors Embedded in Covalent Organic Frameworks. Chem. Sci. 2022, 13, 8253– 8264,  DOI: 10.1039/D2SC02282F
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    Viel, S.; Ziarelli, F.; Pages, G.; Carrara, C.; Caldarelli, S. Pulsed Field Gradient Magic Angle Spinning NMR Self-Diffusion Measurements in Liquids. J. Magn. Reson. 2008, 190, 113– 123,  DOI: 10.1016/j.jmr.2007.10.010
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    Pulsed field gradient magic angle spinning NMR self-diffusion measurements in liquids
    Viel, Stephane; Ziarelli, Fabio; Pages, Guilhem; Carrara, Caroline; Caldarelli, Stefano
    Journal of Magnetic Resonance (2008), 190 (1), 113-123CODEN: JMARF3; ISSN:1090-7807. (Elsevier)
    Several investigations have recently reported the combined use of pulsed field gradient (PFG) with magic angle spinning (MAS) for the anal. of mol. mobility in heterogeneous materials. In contrast, little attention has been devoted so far to delimiting the role of the extra force field induced by sample rotation on the significance and reliability of self-diffusivity measurements. The main purpose of this work is to examine this phenomenon by focusing on pure liqs. for which its impact is expected to be largest. Specifically, we show that self-diffusion coeffs. can be accurately detd. by PFG MAS NMR diffusion measurements in liqs., provided that specific exptl. conditions are met. First, the methodol. to est. the gradient uniformity and to properly calibrate its abs. strength is briefly reviewed and applied on a MAS probe equipped with a gradient coil aligned along the rotor spinning axis, the so-called magic angle gradient' coil. Second, the influence of MAS on the outcome of PFG MAS diffusion measurements in liqs. is investigated for two distinct typical rotors of different active vols., 12 and 50 μL. While the latter rotor led to totally unreliable results, esp. for low viscosity compds., the former allowed for the detn. of accurate self-diffusion coeffs. both for fast and slowly diffusing species. Potential implications of this work are the possibility to measure accurate self-diffusion coeffs. of sample-limited mixts. or to avoid radiation damping interferences in NMR diffusion measurements. Overall, the outlined methodol. should be of interest to anyone who strives to improve the reliability of MAS diffusion studies, both in homogeneous and heterogeneous media.
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    Cohen, S. R.; Plazanet, M.; Rols, S.; Voneshen, D. J.; Fourkas, J. T.; Coasne, B. Structure and Dynamics of Acetonitrile: Molecular Simulation and Neutron Scattering. J. Mol. Liq. 2022, 348, 118423,  DOI: 10.1016/j.molliq.2021.118423
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    Structure and dynamics of acetonitrile: Molecular simulation and neutron scattering
    Cohen, Samuel R.; Plazanet, Marie; Rols, Stephane; Voneshen, David J.; Fourkas, John T.; Coasne, Benoit
    Journal of Molecular Liquids (2022), 348 (), 118423CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
    We examine the interplay between organization and dynamics in bulk liq. acetonitrile. Using angularly resolved radial distribution functions, g(r,θ), derived from mol. simulations, we identify a complex microscopic structure in which most liq. mols. are assocd. with one or more neighboring mols. in an antiparallel, "octupole-paired" configuration and/or an offset, head-to-tail configuration. A detailed anal. of these structural motifs reveals that the offset head-to-tail dimers are the most prevalent. A time-dependent pairing anal. corroborates this picture of robust mol. assocns. favoring head-to-tail dimers, which last longer than antiparallel dimers. This organization, which is assocd. with pairing times (∼ps) longer than the typical rotational time const., cannot be explained on the basis of the Coulomb interactions in the dimer, and so must arise from collective effects that are neither strong nor specific. Finally, using both neutron-scattering techniques and mol. simulations, we study dynamics in liq. acetonitrile over time scales ranging from subpicosecond (the vibrational d. of states) to picosecond (rotational/translational motions and the generalized d. of states) to tens of picoseconds (self-diffusivity in the Fickian regime).
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    Kalugin, O. N.; Chaban, V. V.; Loskutov, V. V.; Prezhdo, O. V. Uniform Diffusion of Acetonitrile Inside Carbon Nanotubes Favors Supercapacitor Performance. Nano Lett. 2008, 8, 2126– 2130,  DOI: 10.1021/nl072976g
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    Uniform Diffusion of Acetonitrile inside Carbon Nanotubes Favors Supercapacitor Performance
    Kalugin, Oleg N.; Chaban, Vitaly V.; Loskutov, Valentin V.; Prezhdo, Oleg V.
    Nano Letters (2008), 8 (8), 2126-2130CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    An unusual behavior of liq. MeCN (AN) confined inside C nanotubes (CNTs) is predicted by mol. dynamics simulation. In contrast to H2O, which shows inhomogeneous variation of both translational and rotational diffusion with CNT diam., the diffusion coeff. of AN changes uniformly and can be described by a simple analytic model. At the same time, the reorientation dynamics of AN vary irregularly in smaller CNTs because of specific packing structures. The uniform translational diffusion of the nonaq. solvent is crit. for stable performance of the new generation of supercapacitors.
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    Norton, C. D.; Thompson, W. H. On the Diffusion of Acetonitrile in Nanoscale Amorphous Silica Pores. Understanding Anisotropy and the Effects of Hydrogen Bonding. J. Phys. Chem. C 2013, 117, 19107– 19114,  DOI: 10.1021/jp407830f
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    On the Diffusion of Acetonitrile in Nanoscale Amorphous Silica Pores. Understanding Anisotropy and the Effects of Hydrogen Bonding
    Norton, Cassandra D.; Thompson, Ward H.
    Journal of Physical Chemistry C (2013), 117 (37), 19107-19114CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Mol. dynamics simulations are used to examine the diffusion of acetonitrile within ∼2.4 nm diam. amorphous silica pores with a focus on the mechanism. The role of the pore surface chem. is examd. by comparison of a hydrophilic, -OH terminated, silica pore with one that has hydrogen-bonding turned off and with an effectively hydrophobic pore obtained by setting all pore charges to zero. The anisotropy of diffusion, along and perpendicular to the pore axis, is examd. through the mean-squared displacements. The origins of the anisotropy are investigated through the dependence on the acetonitrile position within the pore. The effect of hydrogen bonding of acetonitrile mols. to the hydrophilic pore surface is also probed. The simulations show that acetonitrile mols. do not diffuse axially next to the pore surface. Rather, axial diffusion is preceded by radial diffusion away from the pore surface. The same mechanism is obsd. for mols. independent of their hydrogen-bonding status to surface silanols though hydrogen-bonded mols. diffuse more slowly.
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    Borchardt, L.; Leistenschneider, D.; Haase, J.; Dvoyashkin, M. Revising the Concept of Pore Hierarchy for Ionic Transport in Carbon Materials for Supercapacitors. Adv. Energy Mater. 2018, 8, 1800892,  DOI: 10.1002/aenm.201800892
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    Koone, N.; Shao, Y.; Zerda, T. W. Diffusion of Simple Liquids in Porous Sol-Gel Glass. J. Phys. Chem. 1995, 99, 16976– 16981,  DOI: 10.1021/j100046a025
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    Diffusion of Simple Liquids in Porous Sol-Gel Glass
    Koone, N.; Shao, Y.; Zerda, T. W.
    Journal of Physical Chemistry (1995), 99 (46), 16976-81CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    Diffusion coeffs. of selected solvents (acetone, acetonitrile, chloroform, cyclohexane, toluene, water) in porous sol-gel glass were measured using the radioactive tracer diffusion method and the diaphragm technique. The concns. of the solvents were detd. using IR and scintillation measurements. Selected samples had their internal surface modified by replacing hydroxyl groups with trimethylsilane groups. The mol. translational motion inside the pores is hindered by pure geometrical restrictions and surface interactions. Polar mols. diffuse faster than inert mols. Exptl. results were explained in terms of the model developed by Korb et al. Results obtained for cyclohexane were compared with mol. dynamics computer simulations, and the tortuosity factor was evaluated. The diffusion coeff. for acetone within the pores was studied over the temp. range 284-315 K, and the activation energy was detd.
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    Kittaka, S.; Iwashita, T.; Serizawa, A.; Kranishi, M.; Takahara, S.; Kuroda, Y.; Mori, T.; Yamaguchi, T. Low Temperature Properties of Acetonitrile Confined in MCM-41. J. Phys. Chem. B 2005, 109, 23162– 23169,  DOI: 10.1021/jp052476g
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    Low Temperature Properties of Acetonitrile Confined in MCM-41
    Kittaka, Shigeharu; Iwashita, Takafumi; Serizawa, Akihiro; Kranishi, Miki; Takahara, Shuichi; Kuroda, Yasushige; Mori, Toshinori; Yamaguchi, Toshio
    Journal of Physical Chemistry B (2005), 109 (49), 23162-23169CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 was studied by use of adsorption, FT-IR, DSC, and quasi-elastic neutron scattering (QENS) measurements. Acetonitrile mols. in a monolayer interact strongly with surface hydroxyls to be registered and perturb the triple bond in the C≡N group. Adsorbed mols. above the monolayer through to the central part of the cylindrical pores are capillary condensed mols. (cc-acetonitrile), but they do not show the hysteresis loop in adsorption-desorption isotherms, i.e., second order capillary condensation. FT-IR measurements indicated that the condensed phase is very similar to the bulk liq. The cc-acetonitrile freezes at temps. that depend on the pore size of the MCM-41 down to 29.1 Å (C14), below which it is not frozen. In addn., phase changes between α-type and β-type acetonitriles were obsd. below the m.ps. Application of the Gibbs-Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface free energy differences between the interfaces, i.e., Δγl/α = 22.4 mJ m-2 for the liq./pore surface (ps) and α-type/ps, and Δγα/β = 3.17 mJ m-2 for α-type/ps and β-type/ps, resp. QENS expts. substantiate the differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile mols. are anchored so as not to translate. The two Lorentzian anal. of QENS spectra for cc-acetonitriles showed translational motion but markedly slowed. However, the activation energy for cc-acetonitrile in MCM-41 (C18) is 7.0 kJ mol-1 compared to the bulk value of 12.7 kJ mol-1. The relaxation times for tumbling rotational diffusion of cc-acetonitrile are similar to bulk values.
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    Nivarthi, S. S.; McCormick, A. V.; Davis, H. T. Diffusion Anisotropy in Molecular Sieves. Chem. Phys. Lett. 1994, 229, 297– 301,  DOI: 10.1016/0009-2614(94)01059-5
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    Natraj, A.; Ji, W.; Xin, J.; Castano, I.; Burke, D. W.; Evans, A. M.; Strauss, M. J.; Ateia, M.; Hamachi, L. S.; Gianneschi, N. C.; ALOthman, Z. A.; Sun, J.; Yusuf, K.; Dichtel, W. R. Single-Crystalline Imine-Linked Two-Dimensional Covalent Organic Frameworks Separate Benzene and Cyclohexane Efficiently. J. Am. Chem. Soc. 2022, 144, 19813– 19824,  DOI: 10.1021/jacs.2c07166
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    Single-Crystalline Imine-Linked Two-Dimensional Covalent Organic Frameworks Separate Benzene and Cyclohexane Efficiently
    Natraj, Anusree; Ji, Woojung; Xin, Junjie; Castano, Ioannina; Burke, David W.; Evans, Austin M.; Strauss, Michael J.; Ateia, Mohamed; Hamachi, Leslie S.; Gianneschi, Nathan C.; ALOthman, Zeid A.; Sun, Junliang; Yusuf, Kareem; Dichtel, William R.
    Journal of the American Chemical Society (2022), 144 (43), 19813-19824CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Two-dimensional (2D) covalent org. frameworks (COFs) are composed of structurally precise, permanently porous, layered macromol. sheets, which are traditionally synthesized as polycryst. solids with cryst. domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temp. and ambient pressure. Single crystals of two imine-linked 2D COFs were prepd., consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4μm and 450 nm to 20μm in width, resp. High-resoln. transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-cryst. domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-cryst. in the a-b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatog. sepn. columns and exhibited unusual selective retention of cyclohexane over benzene, with single-cryst. TAPPy-PDA significantly outperforming single-cryst. TAPB-DMPDA. Polycryst. TAPPy-PDA exhibited no sepn., while polycryst. TAPB-DMPDA exhibited poor sepn. and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, cryst. structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for mol. sepns.
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    Sinnaeve, D. The Stejskal-Tanner Equation Generalized for Any Gradient Shape-an Overview of Most Pulse Sequences Measuring Free Diffusion. Concepts Magn. Reson. Part A 2012, 40A, 39– 65,  DOI: 10.1002/cmr.a.21223
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    The Stejskal-Tanner equation generalized for any gradient shape-an overview of most pulse sequences measuring free diffusion
    Sinnaeve, Davy
    Concepts in Magnetic Resonance, Part A: Bridging Education and Research (2012), 40A (2), 39-65CODEN: CMRPC2; ISSN:1546-6086. (Wiley-Blackwell)
    A review. The widely known Stejskal-Tanner (ST) equation is central to any diffusion NMR or so-called pulsed gradient spin-echo expt., describing the signal attenuation due to loss of coherence caused by diffusion as a function of the exptl. parameters. What is less widely known is that the equation itself is not invariable when applying different pulse sequences or different gradient pulse shapes and should thus be modified accordingly. This concept is not new, but nevertheless experimentalists applying diffusion NMR in their research often overlook or are completely oblivious to this fact. In this article, the derivation of the ST equation through the Bloch-Torrey equations is discussed in detail, followed by a discussion of the most basic NMR expts. that measure free diffusion. This derivation is performed here in a novel way, leading to expressions for the ST equation that do not assume any gradient shape beforehand, leaving only a few parameters that are detd. solely by the gradient pulse shape. This new approach to presenting the ST equation increases awareness of its dependence on gradient shape. Moreover, it relieves designers of future diffusion NMR pulse sequences of struggling with the cumbersome task of deriving and reporting the equation for each gradient shape sep. A quick ref. table of the ST equation for the basic diffusion NMR pulse sequences for any gradient shape is given, as well as an overview for most other diffusion NMR pulse sequences described in the literature. © 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part A 40A: 39-65, 2012.
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    Jerschow, A.; Müller, N. Suppression of Convection Artifacts in Stimulated-Echo Diffusion Experiments. Double-Stimulated-Echo Experiments. J. Magn. Reson. 1997, 125, 372– 375,  DOI: 10.1006/jmre.1997.1123
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    Suppression of convection artifacts in stimulated-echo diffusion experiments. Double-stimulated-echo experiments
    Jerschow, Alexej; Muller, Norbert
    Journal of Magnetic Resonance (1997), 125 (2), 372-375CODEN: JMARF3; ISSN:1090-7807. (Academic)
    A pulse sequence is described which suppresses the effects of convection in stimulated-echo (STE) diffusion expts. to first-order provided that the convection current has a const. laminar flow profile during the diffusion interval of the pulse sequence. Turbulent convection cannot be compensated for using this method.
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  1. Samuel Van Gele, Sebastian Bette, Bettina V. Lotsch. The Devil Is in the Details: Pitfalls and Ambiguities in the Analysis of X-ray Powder Diffraction Data of 2D Covalent Organic Frameworks. JACS Au 2024, Article ASAP.
  2. Atazaz Ahsin, Aamna Qamar, Sadegh Kaviani, Khizar Hussain Shah, Dmitrii A. Tayurskii. Unveiling the host-guest interactions: CC2 nanocage as an advanced sensor for detection of nitrogenous bases in DNA. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2025, 709 , 136112. https://doi.org/10.1016/j.colsurfa.2025.136112
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Published June 11, 2024

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  • Abstract

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    Figure 1

    Figure 1. (a) Chemical structure of a single pore of PI-3-COF. (b) XRPD pattern and (c) N2 adsorption isotherm comparison of PI-3-COF-lp (green) and PI-3-COF-hp (blue).

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    Figure 2

    Figure 2. PFG-NMR spin–echo attenuation for (a) liquid MeCN, (b) MeCN-loaded PI-3-COF-lp, (c) MeCN-loaded PI-3-COF-hp with varying diffusion times (Δ) at T = 300 K. Lines represent fits with a mono- or biexponential model for MeCN and MeCN-loaded COFs, respectively.

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    Figure 3

    Figure 3. Comparison between experimental N2 adsorption isotherms at 77 K (low porosity in blue, high porosity in green) and simulated ones. The isotherm resulting from the shifted structure is depicted in black squares, the one from the eclipsed stacked structure in red triangles.

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    Figure 4

    Figure 4. Visualization of the eclipsed (a) and the shifted structure (b) of PI 3-COF. Violet surfaces depict the N2 accessible pore surface based on van der Waals parameters.

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  • References

    This article references 86 other publications.

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      Covalent org. frameworks (COFs) are cryst. porous materials constructed from mol. building blocks using diverse linkage chemistries. Their modular construction system allows not only for tailor-made design but also for an immense variety of building blocks, opening the door to numerous different functionalities and potential applications. As a consequence, a large no. of building blocks that can act as light-harvesters, semiconductors, ligands, binding sites or redox centers have recently been integrated into the scaffolds of COFs. This unique combination of reticular chem. with the mol. control of intrinsic properties paves the way towards the design of new semiconducting materials for (opto-)electronic applications such as sensors, photocatalysts or -electrodes, supercapacitor and battery materials, solar-harvesting devices or light emitting diodes. With new developments regarding the linkage motif, highly stable but still tunable COFs have been developed for applications even under harsh conditions. Further, the mol. stacking modes and distances in the COFs have been investigated as a powerful means to control optical and elec. characteristics of these self-assembled frameworks. Advanced understanding of optoelectronic processes in COFs has enabled their implementation in optoelectronic devices with promising potential for real-world applications. This review highlights the key developments of design concepts for the synthesis of electro- and photoactive COFs as well as our understanding of optoelectronic processes in these frameworks, hence establishing a new paradigm for the rational construction of well-defined novel optoelectronic materials and devices.
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      We report the first covalent incorporation of reactive aliph. amine species into covalent org. frameworks (COFs). This was achieved through the crystn. of an imine-linked COF, termed COF-609-Im, followed by conversion of its imine linkage to base-stable tetrahydroquinoline linkage through aza-Diels-Alder cycloaddn., and finally, the covalent incorporation of tris(3-aminopropyl)amine into the framework. The obtained COF-609 exhibits a 1360-fold increase in CO2 uptake capacity compared to the pristine framework and a further 29% enhancement in the presence of humidity. We confirmed the chem. of framework conversion and corroborated the enhanced CO2 uptake phenomenon with and without humidity through isotope-labeled Fourier transform IR spectroscopy and solid-state NMR spectroscopy. With this study, we established a new synthetic strategy to access a class of chemisorbents characterized by high affinity to CO2 in dil. sources, such as the air.
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      Xu, F.; Yang, S.; Chen, X.; Liu, Q.; Li, H.; Wang, H.; Wei, B.; Jiang, D. Energy-Storage Covalent Organic Frameworks: Improving Performance via Engineering Polysulfide Chains on Walls. Chem. Sci. 2019, 10, 6001– 6006,  DOI: 10.1039/C8SC04518F
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      Energy-storage covalent organic frameworks: improving performance via engineering polysulfide chains on walls
      Xu, Fei; Yang, Shuhao; Chen, Xiong; Liu, Qianhui; Li, Hejun; Wang, Hongqiang; Wei, Bingqing; Jiang, Donglin
      Chemical Science (2019), 10 (23), 6001-6006CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The aligned one-dimensional channels found in covalent org. frameworks offer a unique space for energy storage. However, phys. isolation of sulfur in the channels is not sufficient to prevent the shuttle of lithium-sulfide intermediates that eventually results in a poor performance of lithium-sulfur energy storage. Herein, we report a strategy based on imine-linked frameworks for addressing this shuttle issue by covalently engineering polysulfide chains on the pore walls. The imine linkages can trigger the polymn. of sulfur to form polysulfide chains and anchor them on the channel walls. The immobilized polysulfide chains suppress the shuttle effect and are highly redox active. This structural evolution induces multifold pos. effects on energy storage and achieves improved capacity, sulfur accessibility, rate capability and cycle stability. Our results suggest a porous platform achieved by pore wall engineering for tackling key issues in energy storage.
      >> More from SciFinder ®
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      Xu, F.; Xu, H.; Chen, X.; Wu, D.; Wu, Y.; Liu, H.; Gu, C.; Fu, R.; Jiang, D. Radical Covalent Organic Frameworks: A General Strategy to Immobilize Open-Accessible Polyradicals for High-Performance Capacitive Energy Storage. Angew. Chem., Int. Ed. 2015, 54, 6814– 6818,  DOI: 10.1002/anie.201501706
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      Radical Covalent Organic Frameworks: A General Strategy to Immobilize Open-Accessible Polyradicals for High-Performance Capacitive Energy Storage
      Xu, Fei; Xu, Hong; Chen, Xiong; Wu, Dingcai; Wu, Yang; Liu, Hao; Gu, Cheng; Fu, Ruowen; Jiang, Donglin
      Angewandte Chemie, International Edition (2015), 54 (23), 6814-6818CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Ordered π-columns and open nanochannels found in covalent org. frameworks (COFs) could render them able to store elec. energy. However, the synthetic difficulty in achieving redox-active skeletons has thus far restricted their potential for energy storage. A general strategy is presented for converting a conventional COF into an outstanding platform for energy storage through post-synthetic functionalization with org. radicals. The radical frameworks with openly accessible polyradicals immobilized on the pore walls undergo rapid and reversible redox reactions, leading to capacitive energy storage with high capacitance, high-rate kinetics, and robust cycle stability. Probably channel-wall functional engineering with redox-active species will should be a facile and versatile strategy to explore COFs for energy storage.
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      Furukawa, H.; Yaghi, O. M. Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications. J. Am. Chem. Soc. 2009, 131, 8875– 8883,  DOI: 10.1021/ja9015765
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      Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications
      Furukawa, Hiroyasu; Yaghi, Omar M.
      Journal of the American Chemical Society (2009), 131 (25), 8875-8883CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Hydrogen, methane, and carbon dioxide isotherm measurements were performed at 1-85 bar and 77-298 K on the evacuated forms of seven porous covalent org. frameworks (COFs). The uptake behavior and capacity of the COFs is best described by classifying them into three groups based on their structural dimensions and corresponding pore sizes. Group 1 consists of two-dimensional structures with one-dimensional small pores (9 Å for each of COF-1 and COF-6), group 2 includes two-dimensional structures with large one-dimensional pores (27, 16, and 32 Å for COF-5, COF-8, and COF-10, resp.), and group 3 is comprised of three-dimensional structures with three-dimensional medium-sized pores (12 Å for each of COF-102 and COF-103). Group 3 COFs outperform group 1 and 2 COFs, and rival the best metal-org. frameworks and other porous materials in their uptake capacities. This is exemplified by the excess gas uptake of COF-102 at 35 bar (72 mg/g at 77 K for hydrogen, 187 mg/g at 298 K for methane, and 1180 mg/g at 298 K for carbon dioxide), which is similar to the performance of COF-103 but higher than those obsd. for COF-1, COF-5, COF-6, COF-8, and COF-10 (hydrogen at 77 K, 15 mg/g for COF-1, 36 mg/g for COF-5, 23 mg/g for COF-6, 35 mg/g for COF-8, and 39 mg/g for COF-10; methane at 298 K, 40 mg/g for COF-1, 89 mg/g for COF-5, 65 mg/g for COF-6, 87 mg/g for COF-8, and 80 mg/g for COF-10; carbon dioxide at 298 K, 210 mg/g for COF-1, 779 mg/g for COF-5, 298 mg/g for COF-6, 598 mg/g for COF-8, and 759 mg/g for COF-10). These findings place COFs among the most porous and the best adsorbents for hydrogen, methane, and carbon dioxide.
      >> More from SciFinder ®
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      Diercks, C. S.; Lin, S.; Kornienko, N.; Kapustin, E. A.; Nichols, E. M.; Zhu, C.; Zhao, Y.; Chang, C. J.; Yaghi, O. M. Reticular Electronic Tuning of Porphyrin Active Sites in Covalent Organic Frameworks for Electrocatalytic Carbon Dioxide Reduction. J. Am. Chem. Soc. 2018, 140, 1116– 1122,  DOI: 10.1021/jacs.7b11940
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      Reticular Electronic Tuning of Porphyrin Active Sites in Covalent Organic Frameworks for Electrocatalytic Carbon Dioxide Reduction
      Diercks, Christian S.; Lin, Song; Kornienko, Nikolay; Kapustin, Eugene A.; Nichols, Eva M.; Zhu, Chenhui; Zhao, Yingbo; Chang, Christopher J.; Yaghi, Omar M.
      Journal of the American Chemical Society (2018), 140 (3), 1116-1122CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The electronic character of porphyrin active sites for electrocatalytic redn. of CO2 to CO in a two-dimensional covalent org. framework (COF) was tuned by modification of the reticular structure. Efficient charge transport along the COF backbone promotes electronic connectivity between remote functional groups and the active sites and enables the modulation of the catalytic properties of the system. A series of oriented thin films of these COFs was found to reduce CO2 to CO at low overpotential (550 mV) with high selectivity (faradaic efficiency of 87%) and at high current densities (65 mA/mg), a performance well beyond related mol. catalysts in regard to selectivity and efficiency. The catalysts are stable for more than 12 h without any loss in reactivity. X-ray absorption measurements on the cobalt L-edge for the modified COFs enable correlations between the inductive effects of the appended functionality and the electronic character of the reticulated mol. active sites.
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      Wang, X.; Chen, L.; Chong, S. Y.; Little, M. A.; Wu, Y.; Zhu, W.-H.; Clowes, R.; Yan, Y.; Zwijnenburg, M. A.; Sprick, R. S.; Cooper, A. I. Sulfone-Containing Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution from Water. Nat. Chem. 2018, 10, 1180– 1189,  DOI: 10.1038/s41557-018-0141-5
      8
      Sulfone-containing covalent organic frameworks for photocatalytic hydrogen evolution from water
      Wang, Xiaoyan; Chen, Linjiang; Chong, Samantha Y.; Little, Marc A.; Wu, Yongzhen; Zhu, Wei-Hong; Clowes, Rob; Yan, Yong; Zwijnenburg, Martijn A.; Sprick, Reiner Sebastian; Cooper, Andrew I.
      Nature Chemistry (2018), 10 (12), 1180-1189CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)
      Nature uses org. mols. for light harvesting and photosynthesis, but most man-made water splitting catalysts are inorg. semiconductors. Org. photocatalysts, while attractive because of their synthetic tunability, tend to have low quantum efficiencies for water splitting. Here we present a cryst. covalent org. framework (COF) based on a benzo-bis(benzothiophene sulfone) moiety that shows a much higher activity for photochem. hydrogen evolution than its amorphous or semicryst. counterparts. The COF is stable under long-term visible irradn. and shows steady photochem. hydrogen evolution with a sacrificial electron donor for at least 50 h. We attribute the high quantum efficiency of fused-sulfone-COF to its crystallinity, its strong visible light absorption, and its wettable, hydrophilic 3.2 nm mesopores. These pores allow the framework to be dye-sensitized, leading to a further 61% enhancement in the hydrogen evolution rate up to 16.3 mmol g-1 h-1. The COF also retained its photocatalytic activity when cast as a thin film onto a support.
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    9. 9
      Biswal, B. P.; Vignolo-Gonzalez, H. A.; Banerjee, T.; Grunenberg, L.; Savasci, G.; Gottschling, K.; Nuss, J.; Ochsenfeld, C.; Lotsch, B. V. Sustained Solar H2 Evolution from a Thiazolo[5,4-d]Thiazole-Bridged Covalent Organic Framework and Nickel-Thiolate Cluster in Water. J. Am. Chem. Soc. 2019, 141, 11082– 11092,  DOI: 10.1021/jacs.9b03243
      9
      Sustained Solar H2 Evolution from a Thiazolo[5,4-d]thiazole-Bridged Covalent Organic Framework and Nickel-Thiolate Cluster in Water
      Biswal, Bishnu P.; Vignolo-Gonzalez, Hugo A.; Banerjee, Tanmay; Grunenberg, Lars; Savasci, Goekcen; Gottschling, Kerstin; Nuss, Juergen; Ochsenfeld, Christian; Lotsch, Bettina V.
      Journal of the American Chemical Society (2019), 141 (28), 11082-11092CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Solar hydrogen (H2) evolution from water utilizing covalent org. frameworks (COFs) as heterogeneous photosensitizers has gathered significant momentum by virtue of the COFs' predictive structural design, long-range ordering, tunable porosity, and excellent light-harvesting ability. However, most photocatalytic systems involve rare and expensive platinum as the co-catalyst for water redn., which appears to be the bottleneck in the development of economical and environmentally benign solar H2 prodn. systems. Herein, a simple, efficient, and low-cost all-in-one photocatalytic H2 evolution system is reported composed of a thiazolo[5,4-d]thiazole-linked COF (TpDTz) as the photoabsorber and an earth-abundant, noble-metal-free nickel-thiolate hexameric cluster co-catalyst assembled in situ in water, together with triethanolamine (TEoA) as the sacrificial electron donor. The high crystallinity, porosity, photochem. stability, and light absorption ability of the TpDTz COF enables excellent long-term H2 prodn. over 70 h with a max. rate of 941μmol h-1 g-1, turnover no. TONNi > 103, and total projected TONNi > 443 until complete catalyst depletion. The high H2 evolution rate and TON, coupled with long-term photocatalytic operation of this hybrid system in water, surpass those of many previously known org. dyes, carbon nitride, and COF-sensitized photocatalytic H2O redn. systems. Furthermore, unique insights are gathered into the reaction mechanism, enabled by a specifically designed continuous-flow system for non-invasive, direct H2 prodn. rate monitoring, providing higher accuracy in quantification compared to the existing batch measurement methods. Overall, the results presented here open the door toward the rational design of robust and efficient earth-abundant COF-mol. co-catalyst hybrid systems for sustainable solar H2 prodn. in water.
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    10. 10
      Mitschke, B.; Turberg, M.; List, B. Confinement as a Unifying Element in Selective Catalysis. Chem 2020, 6, 2515– 2532,  DOI: 10.1016/j.chempr.2020.09.007
      10
      Confinement as a Unifying Element in Selective Catalysis
      Mitschke, Benjamin; Turberg, Mathias; List, Benjamin
      Chem (2020), 6 (10), 2515-2532CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      Catalysis has fascinated scientists for centuries and is one of the main pillars of the modern world economy. Achieving high reactivity and selectivity is a crucial requirement of heterogeneous and homogeneous org., metallic, and biol. catalysts. Here, we highlight an underlying principle that is relevant to the reactivity and selectivity of all types of catalysts-''confinement-'' the shaping of a catalyst's active site. While this aspect has been well recognized within the fields of heterogeneous and enzymic catalysis, and has been invoked in supramol. systems, confinement has been less appreciated in the design of small-mol. catalysts. We identify confinement as a unifying element in the science of selective catalysis, reaching beyond the traditional boundaries of the individual subfields. A particular emphasis is given to the latest developments in the area of organocatalysis.
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    11. 11
      Tan, K. T.; Tao, S.; Huang, N.; Jiang, D. Water Cluster in Hydrophobic Crystalline Porous Covalent Organic Frameworks. Nat. Commun. 2021, 12, 6747,  DOI: 10.1038/s41467-021-27128-4
      11
      Water cluster in hydrophobic crystalline porous covalent organic frameworks
      Tan, Ke Tian; Tao, Shanshan; Huang, Ning; Jiang, Donglin
      Nature Communications (2021), 12 (1), 6747CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Abstr.: Progress over the past decades in water confinement has generated a variety of polymers and porous materials. However, most studies are based on a preconception that small hydrophobic pores eventually repulse water mols., which precludes the exploration of hydrophobic microporous materials for water confinement. Here, we demonstrate water confinement across hydrophobic microporous channels in cryst. covalent org. frameworks. The frameworks are designed to constitute dense, aligned and one-dimensional polygonal channels that are open and accessible to water mols. The hydrophobic microporous frameworks achieve full occupation of pores by water via synergistic nucleation and capillary condensation and deliver quick water exchange at low pressures. Water confinement expts. with large-pore frameworks pinpoint thresholds of pore size where confinement becomes dominated by high uptake pressure and large exchange hysteresis. Our results reveal a platform based on microporous hydrophobic covalent org. frameworks for water confinement.
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      Zeng, W. J.; Wang, K.; Liang, W. B.; Chai, Y. Q.; Yuan, R.; Zhuo, Y. Covalent Organic Frameworks as Micro-Reactors: Confinement-Enhanced Electrochemiluminescence. Chem. Sci. 2020, 11, 5410– 5414,  DOI: 10.1039/D0SC01817A
      12
      Covalent organic frameworks as micro-reactors: confinement-enhanced electrochemiluminescence
      Zeng, Wei-Jia; Wang, Kun; Liang, Wen-Bin; Chai, Ya-Qin; Yuan, Ruo; Zhuo, Ying
      Chemical Science (2020), 11 (21), 5410-5414CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Electrochemiluminescence (ECL) micro-reactors with enhanced intensity and extreme stability were first established by the assembly of tris(2,2'-bipyridyl) ruthenium(II) (Ru(bpy)32+) onto covalent org. frameworks (COFs), in which a type of imine-linked COF (denoted as COF-LZU1) was employed as a model for ECL micro-reactors. Compared with the dominant ECL system of Ru(bpy)32+/tri-n-propylamine (TPrA), the intensity of the COF-LZU1 micro-reactor-based electrode was significantly increased nearly 5-fold under the same exptl. conditions. This enhancement can be attributed to the large surface area, delimited space, and stable and hydrophobic porous structure of COF-LZU1, which not only enabled a huge amt. of Ru(bpy)32+ to be loaded in/on COF-LZU1, but also enriched a large amt. of TPrA from the aq. soln. into its inner hydrophobic cavity due to the lipophilicity of TPrA. To prove this principle, these Ru@COF-LZU1 micro-reactors were developed to prep. an ECL aptasensor for aflatoxin M1 (AFM1) detection with a wide detection range and a low detection limit. Overall, this work is the first report in which ECL micro-reactors are constructed with COFs to enhance the intensity and stability of the Ru(bpy)32+-based ECL system, and opens a new route to the design of other ECL micro-reactors for bioanal. applications.
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    13. 13
      Zhao, J.; Guo, G.; Wang, D.; Liu, H.; Zhang, Z.; Sun, L.; Ding, N.; Li, Z.; Zhao, Y. A “One-Step” Approach to the Highly Efficient Synthesis of Lactide through the Confinement Catalysis of Covalent Organic Frameworks. Green Chem. 2023, 25, 3103– 3110,  DOI: 10.1039/D2GC04771C
      There is no corresponding record for this reference.
    14. 14
      Emmerling, S. T.; Ziegler, F.; Fischer, F. R.; Schoch, R.; Bauer, M.; Plietker, B.; Buchmeiser, M. R.; Lotsch, B. V. Olefin Metathesis in Confinement: Towards Covalent Organic Framework Scaffolds for Increased Macrocyclization Selectivity. Chemistry 2022, 28, e202104108  DOI: 10.1002/chem.202104108
      There is no corresponding record for this reference.
    15. 15
      Gao, W.-Y.; Cardenal, A. D.; Wang, C.-H.; Powers, D. C. In Operando Analysis of Diffusion in Porous Metal-Organic Framework Catalysts. Chem. Eur. J. 2019, 25, 3465– 3476,  DOI: 10.1002/chem.201804490
      There is no corresponding record for this reference.
    16. 16
      Fang, Q.; Gu, S.; Zheng, J.; Zhuang, Z.; Qiu, S.; Yan, Y. 3D Microporous Base-Functionalized Covalent Organic Frameworks for Size-Selective Catalysis. Angew. Chem., Int. Ed. 2014, 53, 2878– 2882,  DOI: 10.1002/anie.201310500
      16
      3D Microporous Base-Functionalized Covalent Organic Frameworks for Size-Selective Catalysis
      Fang, Qianrong; Gu, Shuang; Zheng, Jie; Zhuang, Zhongbin; Qiu, Shilun; Yan, Yushan
      Angewandte Chemie, International Edition (2014), 53 (11), 2878-2882CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The design and synthesis of 3D covalent org. frameworks (COFs) were considered a challenge, and the demonstrated applications of 3D COFs have so far been limited to gas adsorption. Herein we describe the design and synthesis of two new 3D microporous base-functionalized COFs, termed BF-COF-1 and BF-COF-2, by the use of a tetrahedral alkyl amine, 1,3,5,7-tetraaminoadamantane (TAA), combined with 1,3,5-triformylbenzene (TFB) or triformylphloroglucinol (TFP). As catalysts, both BF-COFs showed remarkable conversion (96 % for BF-COF-1 and 98 % for BF-COF-2), high size selectivity, and good recyclability in base-catalyzed Knoevenagel condensation reactions. This study suggests that porous functionalized 3D COFs could be a promising new class of shape-selective catalysts.
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      North, A. M. Diffusion-Controlled Reactions. Q. Rev. Chem. Soc. 1966, 20, 421,  DOI: 10.1039/qr9662000421
      There is no corresponding record for this reference.
    18. 18
      Shinde, D. B.; Sheng, G.; Li, X.; Ostwal, M.; Emwas, A. H.; Huang, K. W.; Lai, Z. Crystalline 2D Covalent Organic Framework Membranes for High-Flux Organic Solvent Nanofiltration. J. Am. Chem. Soc. 2018, 140, 14342– 14349,  DOI: 10.1021/jacs.8b08788
      18
      Crystalline 2D Covalent Organic Framework Membranes for High-Flux Organic Solvent Nanofiltration
      Shinde, Digambar B.; Sheng, Guan; Li, Xiang; Ostwal, Mayur; Emwas, Abdul-Hamid; Huang, Kuo-Wei; Lai, Zhiping
      Journal of the American Chemical Society (2018), 140 (43), 14342-14349CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Two-dimensional (2D) covalent org. framework (COF) materials have the most suitable microstructure for membrane applications in order to achieve both high flux and high selectivity. Here, we report the synthesis of a cryst. TFP-DHF 2D COF membrane constructed from two precursors of 1,3,5-triformylphloroglucinol (TFP) and 9,9-dihexylfluorene-2,7-diamine (DHF) through the Langmuir-Blodgett (LB) method, for the first timed. A single COF layer is precisely four unit cells thick and can be transferred to different support surfaces layer by layer. The TFP-DHF 2D COF membrane supported on an anodic aluminum oxide (AAO) porous support displayed remarkable permeabilities for both polar and nonpolar org. solvents, which were approx. 100 times higher than that of the amorphous membranes prepd. by the same procedure and similar to that for the best of the reported polymer membranes. The transport mechanism through the TFP-DHF 2D COF membrane was found to be a viscous flow coupled with a strong slip boundary enhancement, which was also different from those of the amorphous polymer membranes. The membrane exhibited a steep mol. sieving with a mol. wt. retention onset (MWRO) of approx. 600 Da and a mol. wt. cutoff (MWCO) of approx. 900 Da. The substantial performance enhancement was attributed to the structural change from an amorphous structure to a well-defined ordered porous structure, which clearly demonstrated the high potential for the application of 2D COFs as the next generation of membrane materials.
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    19. 19
      Shi, X.; Zhang, Z.; Yin, C.; Zhang, X.; Long, J.; Zhang, Z.; Wang, Y. Design of Three-Dimensional Covalent Organic Framework Membranes for Fast and Robust Organic Solvent Nanofiltration. Angew. Chem. 2022, 134, e202207559  DOI: 10.1002/ange.202207559
      There is no corresponding record for this reference.
    20. 20
      Karak, S.; Dey, K.; Banerjee, R. Maneuvering Applications of Covalent Organic Frameworks Via Framework-Morphology Modulation. Adv. Mater. 2022, 34, e2202751  DOI: 10.1002/adma.202202751
      20
      Maneuvering Applications of Covalent Organic Frameworks via Framework-Morphology Modulation
      Karak, Suvendu; Dey, Kaushik; Banerjee, Rahul
      Advanced Materials (Weinheim, Germany) (2022), 34 (49), 2202751CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Translating the performance of covalent org. frameworks (COFs) from lab. to macroscopic reality demands specific morphologies. Thus, the advancement in morphol. modulation has recently gained some momentum. A clear understanding of nano- to macroscopic architecture is crit. to det., optimize, and improve performances of this atomically precise porous material. Along with their chem. compns. and mol. frameworks, the prospect of morphol. in various applications should be discussed and highlighted. A thorough insight into morphol. vs. application will help produce better-engineered COFs for practical implications. 2D and 3D frameworks can be transformed into various solids such as nanospheres, thin films, membranes, monoliths, foams, etc., for numerous applications in adsorption, sepn. photocatalysis, the carbon dioxide redn., supercapacitors, and fuel cells. However, the research on COF chem. mainly focuses on correlating structure to property, structure to morphol., and structure to applications. Here, crit. insights on various morphol. evolution and assocd. applications are provided. In each case, the underlying role of morphol. is unveiled. Toward the end, a correlation between morphol. and application is provided for the future development of COFs.
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    21. 21
      Yang, Y.; Yu, L.; Chu, T.; Niu, H.; Wang, J.; Cai, Y. Constructing Chemical Stable 4-Carboxyl-Quinoline Linked Covalent Organic Frameworks via Doebner Reaction for Nanofiltration. Nat. Commun. 2022, 13, 2615,  DOI: 10.1038/s41467-022-30319-2
      21
      Constructing chemical stable 4-carboxyl-quinoline linked covalent organic frameworks via Doebner reaction for nanofiltration
      Yang, Yongliang; Yu, Ling; Chu, Tiancheng; Niu, Hongyun; Wang, Jun; Cai, Yaqi
      Nature Communications (2022), 13 (1), 2615CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      Covalent linkages are the key component of covalent org. frameworks (COFs). The development of stable and functional linkages is essential to expand the COFs family and broaden their application prospects. In this work, we report the synthesis of cryst. and chem. stable 4-carboxyl-quinoline linked COFs (QL-COFs) via Doebner reactions in both one-pot (OP) and post-synthetic modification (PSM) methods. Both methods can be universally applied to most of the reported imine COFs family via bottom-up construction or linkage conversion. Owing to the contractive pore size, more hydrophilic structure and better chem. stability than the conventional imine COFs endowed by 4-carboxyl-quinoline linkage, QL-COFs are supposed to possess a wider application range. We further demonstrate the nanofiltration membrane (NFM) based on QL-COF exhibited a desirable sepn. capacity with high rejection for small dye mols. (> 90%), high water permeance (850 L m-2 h-1 MPa-1) and tolerance of extreme conditions (1 M HCl/NaOH), which were benefitted from the enhanced properties of QL-COFs. Addnl., efficient ion sieving properties were also achieved by QL-COF membrane. We anticipate that this work opens up a way for the construction of robust and functional COFs materials for practical applications.
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    22. 22
      Wang, R.; Guo, J.; Xue, J.; Wang, H. Covalent Organic Framework Membranes for Efficient Chemicals Separation. Small Struct. 2021, 2, 2100061,  DOI: 10.1002/sstr.202100061
      22
      Covalent Organic Framework Membranes for Efficient Chemicals Separation
      Wang, Rui; Guo, Jiaxing; Xue, Jian; Wang, Haihui
      Small Structures (2021), 2 (10), 2100061CODEN: SSMTB2; ISSN:2688-4062. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Covalent org. frameworks (COFs) are an emerging class of org. porous cryst. materials that are composed entirely by org. linkers and connected by strong covalent bonds. The unique characteristics including well-ordered and tailorable pore channels, high and permanent porosity, excellent thermostability and chem. stability, and ease of functionalization enable COFs to perfectly meet the requirements for the fabrication of advanced sepn. membranes. Significant progress has been made in the prepn. and application of COF membranes over the past few years. Herein, the structure-performance relationship of COF materials on membrane sepn. is first proposed. The effects of the intrinsic properties of COF materials on membrane sepn. performance are systematically summarized. The fabrication methods of COF membranes, including both bottom-up strategy (in situ growth and interface-assisted synthesis) and top-down strategy (blending and layer-by-layer [LBL] stacking), are then discussed in detail. The application of COF membranes in gas sepn. (H2 purifn., CO2 capture, hydrocarbon sepn.) and liq. phase sepn. (water treatment, org. solvent nanofiltration, pervaporation) is highlighted. Finally, the remaining challenges and issues to be resolved in the COF membranes are prospected from the perspective of COF materials.
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    23. 23
      Krishna, R. Describing the Diffusion of Guest Molecules inside Porous Structures. J. Phys. Chem. C 2009, 113, 19756– 19781,  DOI: 10.1021/jp906879d
      23
      Describing the Diffusion of Guest Molecules Inside Porous Structures
      Krishna, Rajamani
      Journal of Physical Chemistry C (2009), 113 (46), 19756-19781CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      A review. The design and development of many emerging sepn. and catalytic process technologies require a proper quant. description of diffusion of mixts. of guest mols. within meso- and microporous structures. In mesoporous materials with pore sizes 2 nm < dp < 50 nm, there is a central core region where the influence of interactions of the mols. with the pore wall is either small or negligible; mesopore diffusion is governed by a combination of mol.-mol. and mol.-pore wall interactions. Within micropores with dp < 2 nm, the guest mols. are always within the influence of the force field exerted with the wall; the authors have to reckon with the motion of adsorbed mols., and there is no bulk fluid region. This article presents a unified, phenomenol., description of diffusion inside meso- and microporous structures using concepts and ideas that originate from James Clerk Maxwell and Josef Stefan. With the aid of extensive data sets of mol. dynamic simulations of unary and mixt. diffusion in a wide variety of materials such as zeolites, metal-org. frameworks, covalent org. frameworks, carbon nanotubes, and cylindrical silica pores with a diverse range of pore topologies and pore sizes, the authors derive a mol.-level understanding of the various coeffs. that arise in the phenomenol. Maxwell-Stefan diffusion formulation. This understanding helps one to explain and describe a variety of exptl. data and observations. Also a mol. level understanding aids sepn. and reaction process development.
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    24. 24
      Jakobtorweihen, S.; Verbeek, M. G.; Lowe, C. P.; Keil, F. J.; Smit, B. Understanding the Loading Dependence of Self-Diffusion in Carbon Nanotubes. Phys. Rev. Lett. 2005, 95, 044501,  DOI: 10.1103/PhysRevLett.95.044501
      24
      Understanding the Loading Dependence of Self-Diffusion in Carbon Nanotubes
      Jakobtorweihen, S.; Verbeek, M. G.; Lowe, C. P.; Keil, F. J.; Smit, B.
      Physical Review Letters (2005), 95 (4), 044501/1-044501/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      The influence of flexible walls on the self-diffusion of CH4 in an isolated single walled carbon nanotube, as an example, is studied by mol. dynamics simulations. By simulating the carbon nanotube as a flexible framework we demonstrate that the flexibility has a crucial influence on self-diffusion at low loadings. We show how this influence can be incorporated in a simulation of a rigid nanotube by using a Lowe-Andersen thermostat which works on interface-fluid collisions. The reprodn. of the results of a flexible carbon nanotube by a rigid nanotube simulation is excellent.
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    25. 25
      Obliger, A.; Bousige, C.; Coasne, B.; Leyssale, J.-M. Development of Atomistic Kerogen Models and Their Applications for Gas Adsorption and Diffusion: A Mini-Review. Energy Fuels 2023, 37, 1678– 1698,  DOI: 10.1021/acs.energyfuels.2c03633
      25
      Development of Atomistic Kerogen Models and Their Applications for Gas Adsorption and Diffusion: A Mini-Review
      Obliger, Amael; Bousige, Colin; Coasne, Benoit; Leyssale, Jean-Marc
      Energy & Fuels (2023), 37 (3), 1678-1698CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      A review. With the emergence of shale gas, numerous at.-scale models of kerogen have been proposed in the literature. These models, which attempt to capture the structure, chem., and porosity of kerogens of various types and maturities, are nowadays commonly-if not routinely-used to gain nanoscale insights into the thermodn. and dynamics of complex and important processes such as hydrocarbon recovery and carbon sequestration. However, modeling such a complex, disordered, and heterogeneous material is a particularly challenging task. It implies that important underlying assumptions and simplifications, which can significantly affect the predicted properties, have to be made when constructing the kerogen models. In this mini review, we discuss the existing atomistic models of kerogen by categorizing them according to the different approaches and assumptions used during their construction. For each type of model, we also describe how the construction strategy can impact the prediction of certain properties. Important work on kerogen interactions with gas and oil, from both the point of view of equil. adsorption (including adsorption-induced deformation) and transport, are described. Possible improvements and upscaling strategies-to better account for kerogen in its geol. environment-are also discussed.
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    26. 26
      Dubbeldam, D.; Snurr, R. Q. Recent Developments in the Molecular Modeling of Diffusion in Nanoporous Materials. Mol. Simul. 2007, 33, 305– 325,  DOI: 10.1080/08927020601156418
      26
      Recent developments in the molecular modeling of diffusion in nanoporous materials
      Dubbeldam, D.; Snurr, R. Q.
      Molecular Simulation (2007), 33 (4-5), 305-325CODEN: MOSIEA; ISSN:0892-7022. (Taylor & Francis Ltd.)
      A review. Mol. modeling has become a useful and widely used tool to predict diffusion coeffs. of mols. adsorbed in the pores of zeolites and other nanoporous materials. These simulations also provide detailed, mol.-level information about sorbate structure, dynamics, and diffusion mechanisms. We review recent advances in this field, including prediction of various transport coeffs. (Fickian, Onsager, Maxwell-Stefan) for single-component and multicomponent systems from equil. and non-equil. mol. dynamics (MD) simulations, elucidation of anomalous diffusion effects induced by the confining pore structure, and prediction of slow diffusion processes beyond the reach of MD simulations.
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    27. 27
      Jobic, H.; Theodorou, D. N. Quasi-Elastic Neutron Scattering and Molecular Dynamics Simulation as Complementary Techniques for Studying Diffusion in Zeolites. Microporous Mesoporous Mater. 2007, 102, 21– 50,  DOI: 10.1016/j.micromeso.2006.12.034
      27
      Quasi-elastic neutron scattering and molecular dynamics simulation as complementary techniques for studying diffusion in zeolites
      Jobic, Herve; Theodorou, Doros N.
      Microporous and Mesoporous Materials (2007), 102 (1-3), 21-50CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
      A review. Quasi-elastic neutron scattering (QENS) and mol. dynamics (MD) simulations have enabled the quant. study of mol. motion in pure and mixed fluids sorbed in zeolites over length scales of 0.1-100 nm and time scales of 10 fs-100 ns. After an introduction to the statistical mechanics of diffusion, this review describes the principles and practice of time-of-flight (TOF), backscattering (BS), and neutron spin-echo (NSE) measurements, as well as methods to analyze the results. A brief overview of MD simulations is provided, with emphasis on how to calc. QENS observables and how to detect signatures of local anisotropic translational dynamics within the inhomogeneous periodic force field of zeolite crystals. Illustrations of what can be learned from combined application of QENS and mol. simulation are provided from recent work on specific systems and problems: self-diffusion of n-alkanes up to C16 in MFI zeolites, self-diffusion of methane co-adsorbed with n-butane in silicalite-1, and transport diffusion of N2 and CO2 in silicalite-1.
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    28. 28
      Demontis, P.; Suffritti, G. B. Structure and Dynamics of Zeolites Investigated by Molecular Dynamics. Chem. Rev. 1997, 97, 2845– 2878,  DOI: 10.1021/cr950253o
      28
      Structure and Dynamics of Zeolites Investigated by Molecular Dynamics
      Demontis, Pierfranco; Suffritti, Giuseppe B.
      Chemical Reviews (Washington, D. C.) (1997), 97 (8), 2845-2878CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review, with 246 refs., is given on mol. dynamics simulation of zeolites and covers mol. dynamics simulation technique, simulation of diffusive processes, simulation of the structure and dynamics of the framework and special topics including intramol. dynamics of sorbates, chem. reactions, and activated diffusion.
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    29. 29
      Sholl, D. S. Understanding Macroscopic Diffusion of Adsorbed Molecules in Crystalline Nanoporous Materials via Atomistic Simulations. Acc. Chem. Res. 2006, 39, 403– 411,  DOI: 10.1021/ar0402199
      29
      Understanding Macroscopic Diffusion of Adsorbed Molecules in Crystalline Nanoporous Materials via Atomistic Simulations
      Sholl, David S.
      Accounts of Chemical Research (2006), 39 (6), 403-411CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      The diffusion rates of mols. inside nanoporous materials lie at the heart of many large-scale industrial applications of these materials. Quant. describing this diffusion, particularly diffusion of chem. mixts. in situations leading to net mass transport, remains challenging. Mol. dynamics (MD) simulations can play an important complementary role to expts. in this area. This Account describes applications of MD to diffusion in nanoporous materials with a particular focus on macroscopic diffusion, i.e., diffusion involving mass transport. These methods have made useful contributions to developing mixing theories for predicting multicomponent diffusion from single-component data and to screening new classes of materials for practical applications.
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    30. 30
      Smit, B.; Maesen, T. L. Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity. Chem. Rev. 2008, 108, 4125– 4184,  DOI: 10.1021/cr8002642
      30
      Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity
      Smit, Berend; Maesen, Theo L. M.
      Chemical Reviews (Washington, DC, United States) (2008), 108 (10), 4125-4184CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Details of simulation studies for adsorption, diffusion, and shape selectivity on zeolites have been reviewed.
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    31. 31
      Skoulidas, A. I. Molecular Dynamics Simulations of Gas Diffusion in Metal-Organic Frameworks: Argon in Cubtc. J. Am. Chem. Soc. 2004, 126, 1356– 1357,  DOI: 10.1021/ja039215+
      31
      Molecular dynamics simulations of gas diffusion in metal-organic frameworks: Argon in CuBTC
      Skoulidas, Anastasios I.
      Journal of the American Chemical Society (2004), 126 (5), 1356-1357CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The class of coordination polymers known as metalorg. frameworks (MOFs) has 3D porous structures that are considered as a promising alternative to zeolites and other nanoporous materials for catalysis, gas adsorption, and gas sepn. applications. Here, the first study of gas diffusion inside an MOF is presented and the obsd. diffusion is compared to known behaviors in zeolites. Using grand canonical Monte Carlo and equil. mol. dynamics, the adsorption isotherm and self-, cor., and transport diffusivities for argon in the CuBTC metalorg. framework were calcd. Diffusion of Ar in CuBTC is very similar to Ar diffusion in silica zeolites in magnitude, concn., and temp. dependence. This conclusion appears to apply to a broad range of MOF structures.
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    32. 32
      Skoulidas, A. I.; Sholl, D. S. Self-Diffusion and Transport Diffusion of Light Gases in Metal-Organic Framework Materials Assessed Using Molecular Dynamics Simulations. J. Phys. Chem. B 2005, 109, 15760– 15768,  DOI: 10.1021/jp051771y
      32
      Self-Diffusion and Transport Diffusion of Light Gases in Metal-Organic Framework Materials Assessed Using Molecular Dynamics Simulations
      Skoulidas, Anastasios I.; Sholl, David S.
      Journal of Physical Chemistry B (2005), 109 (33), 15760-15768CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Metal-org. framework (MOF) materials pose an interesting alternative to more traditional nanoporous materials for a variety of sepn. processes. Sepn. processes involving nanoporous materials can be controlled by either adsorption equil., diffusive transport rates, or a combination of these factors. Adsorption equil. was studied for a variety of gases in MOFs, but almost nothing is currently known about mol. diffusion rates in MOFs. The authors used equil. mol. dynamics (MD) to probe the self-diffusion and transport diffusion of a no. of small gas species in several MOFs as a function of pore loading at room temp. Specifically, the authors have studied Ar, CH4, CO2, N2, and H2 diffusion in MOF-5. The diffusion of Ar in MOF-2, MOF-3, and Cu-BTC was assessed in a similar manner. Results greatly expand the range of MOFs for which data describing mol. diffusion is available. The authors discuss the prospects for exploiting mol. transport properties in MOFs in practical sepn. processes and the future role of MD simulations in screening families of MOFs for these processes.
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    33. 33
      Yang, Q.; Zhong, C. Molecular Simulation of Adsorption and Diffusion of Hydrogen in Metal-Organic Frameworks. J. Phys. Chem. B 2005, 109, 11862– 11864,  DOI: 10.1021/jp051903n
      33
      Molecular Simulation of Adsorption and Diffusion of Hydrogen in Metal-Organic Frameworks
      Yang, Qingyuan; Zhong, Chongli
      Journal of Physical Chemistry B (2005), 109 (24), 11862-11864CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Metal-org. frameworks (MOFs) are thought to be a set of promising hydrogen storage materials; however, little is known about the interactions between hydrogen mols. and pore walls as well as the diffusivities of hydrogen in MOFs. In this work, we performed a systematic mol. simulation study on the adsorption and diffusion of hydrogen in MOFs to provide insight into mol.-level details of the underlying mechanisms. This work shows that metal-oxygen clusters are preferential adsorption sites for hydrogen in MOFs, and the effect of the org. linkers becomes evident with increasing pressure. The hydrogen storage capacity of MOFs is similar to carbon nanotubes, which is higher than zeolites. Diffusion of hydrogen in MOFs is an activated process that is similar to diffusion in zeolites. The information derived in this work is useful to guide the future rational design and synthesis of tailored MOF materials with improved hydrogen adsorption capability.
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    34. 34
      Düren, T.; Snurr, R. Q. Assessment of Isoreticular Metal-Organic Frameworks for Adsorption Separations: A Molecular Simulation Study of Methane/n-Butane Mixtures. J. Phys. Chem. B 2004, 108, 15703– 15708,  DOI: 10.1021/jp0477856
      34
      Assessment of Isoreticular Metal-Organic Frameworks for Adsorption Separations: A Molecular Simulation Study of Methane/n-Butane Mixtures
      Dueren, Tina; Snurr, Randall Q.
      Journal of Physical Chemistry B (2004), 108 (40), 15703-15708CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      A wide variety of new nanoporous metal-org. materials are being synthesized by many research groups using supramol. chem. and directed assembly in a building block approach based on corner units and linker mols. These materials may lead to revolutionary advances in adsorption sepns. because the properties of these materials may be tailored in a synthetically predictable manner. In this paper, we use mol. simulations to assess the suitability of one group of metal org. materials, namely, isoreticular metal-org. frameworks (IRMOFs), as adsorbents for mixt. sepns. By using grand canonical Monte Carlo simulations, the influence of the linker mol. on the adsorption of methane, n-butane, and their mixts. is detd. Detailed anal. of the energetics as well as the siting of mols. in the cavities allows us to resolve the impact of the linker mols. on the selectivity and to propose new, not yet synthesized materials, which show even higher selectivities. The predicted selectivities are as good as or better than exptl. obsd. selectivities in other adsorbents, suggesting that IRMOFs are promising materials for the sepn. of hydrocarbons.
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    35. 35
      Garberoglio, G.; Vallauri, R. Adsorption and Diffusion of Hydrogen and Methane in 2d Covalent Organic Frameworks. Microporous Mesoporous Mater. 2008, 116, 540– 547,  DOI: 10.1016/j.micromeso.2008.05.023
      There is no corresponding record for this reference.
    36. 36
      Keskin, S. Adsorption, Diffusion, and Separation of CH4/H2 Mixtures in Covalent Organic Frameworks: Molecular Simulations and Theoretical Predictions. J. Phys. Chem. C 2012, 116, 1772– 1779,  DOI: 10.1021/jp209804x
      36
      Adsorption, Diffusion, and Separation of CH4/H2 Mixtures in Covalent Organic Frameworks: Molecular Simulations and Theoretical Predictions
      Keskin, Seda
      Journal of Physical Chemistry C (2012), 116 (2), 1772-1779CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Grand canonical Monte Carlo and equil. mol. dynamics simulations were used to compute adsorption isotherms and self-diffusivities of CH4/H2 mixts. at various compns. in three representative covalent org. frameworks (COFs). Several properties of COFs, such as adsorption selectivity, working capacity, diffusion selectivity, gas permeability, and membrane selectivity were evaluated and were compared with metal org. frameworks (MOFs), zeolites, zeolite imidazolate frameworks (ZIFs), and carbon nanotubes. Results showed that COF-6 outperforms traditional zeolites CHA, LTA, and ITQ-29 and MOFs IRMOF-1, CuBTT, and MOF-177 in adsorption-based CH4 selectivity. Membrane selectivities of COF-5, COF-6, and COF-10 were found to be higher than those of zeolites and similar to ZIFs and MOFs. Adsorption isotherms and diffusivities of CH4/H2 mixts. in the pores of COF-6 were computed using both atomically detailed simulations and theor. correlations. Results showed that theor. correlations based on single component adsorption and diffusion data can be used to accurately predict mixt. adsorption and diffusion of gases in COFs.
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    37. 37
      Yang, Z.; Cao, D. Effect of Li Doping on Diffusion and Separation of Hydrogen and Methane in Covalent Organic Frameworks. J. Phys. Chem. C 2012, 116, 12591– 12598,  DOI: 10.1021/jp302175d
      37
      Effect of Li Doping on Diffusion and Separation of Hydrogen and Methane in Covalent Organic Frameworks
      Yang, Zhanlei; Cao, Dapeng
      Journal of Physical Chemistry C (2012), 116 (23), 12591-12598CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The authors systematically study the diffusion and sepn. behavior of hydrogen and methane in covalent org. frameworks (COFs) and Li-doped counterparts. The self-diffusivities of hydrogen and methane in COFs decrease monotonically with the increase of pressure. After Li doping into COFs, the self-diffusivities 1st increase at low pressure, and then reach a plateau, and finally decrease slightly at high pressure. This phenomenon stems from the fact that the Li atom has a strong affinity to the gas mols. Also the COFs show a larger self-diffusivity than most of the metal-org. frameworks (MOFs), and the permselectivities of the COFs for H2/CH4 are 1 order of magnitude higher than the Li-doped COFs. In particular, the adsorption selectivity of Li-doped COFs for CH4/H2 gets a significant improvement, compared to undoped ones. To further understand the effect of Li doping on diffusion and sepn. of gases, the isosurface and the contour plots of the center of mass and radial distribution functions of gases are also explored. In short, the Li doping into COFs can increase the adsorption selectivity of COF materials significantly, while for a kinetic sepn. process, the undoped COF-based membranes might be a very promising material.
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    38. 38
      Zeng, H.; Liu, Y.; Liu, H. Adsorption and Diffusion of CO2 and CH4 in Covalent Organic Frameworks: An MC/MD Simulation Study. Mol. Simul. 2018, 44, 1244– 1251,  DOI: 10.1080/08927022.2018.1481959
      38
      Adsorption and diffusion of CO2 and CH4 in covalent organic frameworks: an MC/MD simulation study
      Zeng, Hongwei; Liu, Yu; Liu, Honglai
      Molecular Simulation (2018), 44 (15), 1244-1251CODEN: MOSIEA; ISSN:0892-7022. (Taylor & Francis Ltd.)
      Covalent org. frameworks (COFs) are a promising gas sepn. material which have been developed recently. In this work, we have used grand canonical Monte Carlo (GCMC) and mol. dynamics (MD) simulations to investigate the adsorption and diffusion properties of CO2 and CH4 in five recent synthesized COF materials. We have also considered the properties of amino-modified COFs by adding -NH2 group to the five COFs. The adsorption isotherm, adsorption/diffusion selectivity, self/transport diffusion coeffs. have been examd. and discussed. All of the five COFs exhibit promising adsorption selectivity which is higher than common nanoporous materials. An S-shaped adsorption isotherm can be found for CO2 instead of CH4 adsorption. The introduction of -NH2 group is effective at low pressure region (<200 kPa). The diffusion coeffs. are similar for TS-COFs but increase with the pore size for PI-COFs, and the diffusion coeffs. seem less dependent on the -NH2 groups.
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    39. 39
      Altundal, O. F.; Haslak, Z. P.; Keskin, S. Combined GCMC, MD, and DFT Approach for Unlocking the Performances of COFs for Methane Purification. Ind. Eng. Chem. Res. 2021, 60, 12999– 13012,  DOI: 10.1021/acs.iecr.1c01742
      39
      Combined GCMC, MD, and DFT Approach for Unlocking the Performances of COFs for Methane Purification
      Altundal, Omer Faruk; Haslak, Zeynep Pinar; Keskin, Seda
      Industrial & Engineering Chemistry Research (2021), 60 (35), 12999-13012CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Covalent org. frameworks (COFs) are promising materials for gas storage and sepn.; however, the potential of COFs for sepn. of CH4 from industrially relevant gases such as H2, N2, and C2H6 is yet to be investigated. In this work, we followed a multiscale computational approach to unlock both the adsorption- and membrane-based CH4/H2, CH4/N2, and C2H6/CH4 sepn. potentials of 572 COFs by combining grand canonical Monte Carlo (GCMC) and mol. dynamics (MD) simulations and d. functional theory (DFT) calcns. Adsorbent performance evaluation metrics of COFs, adsorption selectivity, working capacity, regenerability, and adsorbent performance score were calcd. for sepn. of equimolar CH4/H2, CH4/N2, and C2H6/CH4 mixts. at vacuum swing adsorption (VSA) and pressure swing adsorption (PSA) conditions to identify the best-performing COFs for each mixt. Results showed that COFs could achieve selectivities of 2-85, 1-7, and 2-23 for PSA-based CH4/H2, CH4/N2, and C2H6/CH4 sepns., resp., outperforming conventional adsorbents such as zeolites and activated carbons for each mixt. Structure-performance relations revealed that COFs with pore sizes <10 Å are promising adsorbents for all mixts. We identified the gas adsorption sites in the three top-performing COFs commonly identified for each mixt. by DFT calcns. and computed the binding strength of gases, which were found to be on the order of C2H6 > CH4 > N2 > H2, supporting the GCMC results. Nucleus-independent chem. shift (NICS) indexes of aromaticity for adsorption sites were calcd., and the results revealed that the degree of linker aromaticity could be a measure for the selection or design of highly alkane-selective COF adsorbents over N2 and H2. Finally, COF membranes were shown to achieve high H2 permeabilities, 4.57 x 103-1.25 x 106 Barrer, and decent membrane selectivities, as high as 4.3, outperforming polymeric and MOF-based membranes for sepn. of H2 from CH4.
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    40. 40
      Bukowski, B. C.; Keil, F. J.; Ravikovitch, P. I.; Sastre, G.; Snurr, R. Q.; Coppens, M.-O. Connecting Theory and Simulation with Experiment for the Study of Diffusion in Nanoporous Solids. Adsorption 2021, 27, 683– 760,  DOI: 10.1007/s10450-021-00314-y
      40
      Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids
      Bukowski, Brandon C.; Keil, Frerich J.; Ravikovitch, Peter I.; Sastre, German; Snurr, Randall Q.; Coppens, Marc-Olivier
      Adsorption (2021), 27 (5), 683-760CODEN: ADSOFO; ISSN:0929-5607. (Springer)
      A review. Nanoporous solids are ubiquitous in chem., energy, and environmental processes, where controlled transport of mols. through the pores plays a crucial role. They are used as sorbents, chromatog. or membrane materials for sepns., and as catalysts and catalyst supports. Defined as materials where confinement effects lead to substantial deviations from bulk diffusion, nanoporous materials include cryst. microporous zeotypes and metal-org. frameworks (MOFs), and a no. of semi-cryst. and amorphous mesoporous solids, as well as hierarchically structured materials, contg. both nanopores and wider meso- or macropores to facilitate transport over macroscopic distances. The ranges of pore sizes, shapes, and topologies spanned by these materials represent a considerable challenge for predicting mol. diffusivities, but fundamental understanding also provides an opportunity to guide the design of new nanoporous materials to increase the performance of transport limited processes. Remarkable progress in synthesis increasingly allows these designs to be put into practice. Mol. simulation techniques have been used in conjunction with exptl. measurements to examine in detail the fundamental diffusion processes within nanoporous solids, to provide insight into the free energy landscape navigated by adsorbates, and to better understand nano-confinement effects. Pore network models, discrete particle models and synthesis-mimicking atomistic models allow to tackle diffusion in mesoporous and hierarchically structured porous materials, where multiscale approaches benefit from ever cheaper parallel computing and higher resoln. imaging. Here, we discuss synergistic combinations of simulation and expt. to showcase theor. progress and computational techniques that have been successful in predicting guest diffusion and providing insights. We also outline where new fundamental developments and exptl. techniques are needed to enable more accurate predictions for complex systems.
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    41. 41
      Ford, D. C.; Dubbeldam, D.; Snurr, R. Q.; Kunzel, V.; Wehring, M.; Stallmach, F.; Karger, J.; Muller, U. Self-Diffusion of Chain Molecules in the Metal-Organic Framework IRMOF-1: Simulation and Experiment. J. Phys. Chem. Lett. 2012, 3, 930– 933,  DOI: 10.1021/jz300141n
      41
      Self-Diffusion of Chain Molecules in the Metal-Organic Framework IRMOF-1: Simulation and Experiment
      Ford, Denise C.; Dubbeldam, David; Snurr, Randall Q.; Kunzel, Volker; Wehring, Markus; Stallmach, Frank; Karger, Jorg; Muller, Ulrich
      Journal of Physical Chemistry Letters (2012), 3 (7), 930-933CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Metal-org. frameworks (MOFs) possess characteristics, such as tunable pore size and chem. functionality, that make them attractive candidates for sepns., catalysis, gas storage, and sensing applications. The rate of diffusion of guest mols. in the pores is an important property for all of these potential applications. The self-diffusion of hydrocarbons in IRMOF-1 was studied as a function of chain length with a combination of mol. dynamics simulations and pulsed field gradient NMR expts. Excellent agreement is seen between the expts. and simulations, and the self-diffusion coeffs. in IRMOF-1 are on the same order as those in the bulk liq. Addnl., the effect of concn. on diffusivity was found to be very small for low to moderate loadings. Mol. dynamics simulations also provided insights about the preferential diffusion pathways of these guests in IRMOF-1.
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    42. 42
      Dutta, S.; Galarneau, A.; Minoux, D.; Aquino, C.; Dath, J. P.; Guenneau, F.; Coasne, B. Molecular Diffusion in Hierarchical Zeolites with Ordered Mesoporosity: Pulsed Field Gradient Nuclear Magnetic Resonance Combined with Thermodynamic Modeling. J. Phys. Chem. C 2023, 127, 1548– 1559,  DOI: 10.1021/acs.jpcc.2c04868
      There is no corresponding record for this reference.
    43. 43
      Grunenberg, L.; Savasci, G.; Terban, M. W.; Duppel, V.; Moudrakovski, I.; Etter, M.; Dinnebier, R. E.; Ochsenfeld, C.; Lotsch, B. V. Amine-Linked Covalent Organic Frameworks as a Platform for Postsynthetic Structure Interconversion and Pore-Wall Modification. J. Am. Chem. Soc. 2021, 143, 3430– 3438,  DOI: 10.1021/jacs.0c12249
      43
      Amine-Linked Covalent Organic Frameworks as a Platform for Postsynthetic Structure Interconversion and Pore-Wall Modification
      Grunenberg, Lars; Savasci, Goekcen; Terban, Maxwell W.; Duppel, Viola; Moudrakovski, Igor; Etter, Martin; Dinnebier, Robert E.; Ochsenfeld, Christian; Lotsch, Bettina V.
      Journal of the American Chemical Society (2021), 143 (9), 3430-3438CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Covalent org. frameworks have emerged as a powerful synthetic platform for installing and interconverting dedicated mol. functions on a cryst. polymeric backbone with at. precision. Here, we present a novel strategy to directly access amine-linked covalent org. frameworks, which serve as a scaffold enabling pore-wall modification and linkage-interconversion by new synthetic methods based on Leuckart-Wallach redn. with formic acid and ammonium formate. Frameworks connected entirely by secondary amine linkages, mixed amine/imine bonds, and partially formylated amine linkages are obtained in a single step from imine-linked frameworks or directly from corresponding linkers in a one-pot crystn.-redn. approach. The new, 2D amine-linked covalent org. frameworks, rPI-3-COF, rTTI-COF, and rPy1P-COF, are obtained with high crystallinity and large surface areas. Secondary amines, installed as reactive sites on the pore wall, enable further postsynthetic functionalization to access tailored covalent org. frameworks, with increased hydrolytic stability, as potential heterogeneous catalysts.
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    44. 44
      Grunenberg, L.; Savasci, G.; Emmerling, S. T.; Heck, F.; Bette, S.; Cima Bergesch, A.; Ochsenfeld, C.; Lotsch, B. V. Postsynthetic Transformation of Imine- into Nitrone-Linked Covalent Organic Frameworks for Atmospheric Water Harvesting at Decreased Humidity. J. Am. Chem. Soc. 2023, 145, 13241– 13248,  DOI: 10.1021/jacs.3c02572
      44
      Postsynthetic Transformation of Imine- into Nitrone-Linked Covalent Organic Frameworks for Atmospheric Water Harvesting at Decreased Humidity
      Grunenberg, Lars; Savasci, Goekcen; Emmerling, Sebastian T.; Heck, Fabian; Bette, Sebastian; Cima Bergesch, Afonso; Ochsenfeld, Christian; Lotsch, Bettina V.
      Journal of the American Chemical Society (2023), 145 (24), 13241-13248CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Herein, we report a facile postsynthetic linkage conversion method giving synthetic access to nitrone-linked covalent org. frameworks (COFs) from imine- and amine-linked COFs. The new two-dimensional (2D) nitrone-linked covalent org. frameworks, NO-PI-3-COF and NO-TTI-COF, are obtained with high crystallinity and large surface areas. Nitrone-modified pore channels induce condensation of water vapor at 20% lower humidity compared to their amine- or imine-linked precursor COFs. Thus, the topochem. transformation to nitrone linkages constitutes an attractive approach to postsynthetically fine-tune water adsorption properties in framework materials.
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    45. 45
      Smith, B. J.; Parent, L. R.; Overholts, A. C.; Beaucage, P. A.; Bisbey, R. P.; Chavez, A. D.; Hwang, N.; Park, C.; Evans, A. M.; Gianneschi, N. C.; Dichtel, W. R. Colloidal Covalent Organic Frameworks. ACS Cent. Sci. 2017, 3, 58– 65,  DOI: 10.1021/acscentsci.6b00331
      45
      Colloidal Covalent Organic Frameworks
      Smith, Brian J.; Parent, Lucas R.; Overholts, Anna C.; Beaucage, Peter A.; Bisbey, Ryan P.; Chavez, Anton D.; Hwang, Nicky; Park, Chiwoo; Evans, Austin M.; Gianneschi, Nathan C.; Dichtel, William R.
      ACS Central Science (2017), 3 (1), 58-65CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
      Covalent org. frameworks (COFs) are two or three-dimensional polymer networks with designed topol. and chem. functionality, permanent porosity, and high surface areas. These features are potentially useful for a broad range of applications, including catalysis, optoelectronics, and energy storage devices. But current COF syntheses offer poor control over the material's morphol. and final form, generally providing insol. and unprocessable microcryst. powder aggregates. COF polymns. are often performed under conditions in which the monomers are only partially sol. in the reaction solvent, and this heterogeneity has hindered understanding of their polymn. or crystn. processes. Here we report homogeneous polymn. conditions for boronate ester-linked, two-dimensional COFs that inhibit crystallite pptn., resulting in stable colloidal suspensions of 2D COF nanoparticles. The hexagonal, layered structures of the colloids are confirmed by small angle and wide-angle X-ray scattering (SAXS/WAXS), and kinetic characterization provides insight into the growth process. The colloid size is modulated by solvent conditions, and the technique is demonstrated for four 2D boronate ester-linked COFs. The diam. of individual COF nanoparticles in soln. is monitored and quantified during COF growth and stabilization at elevated temp. using in situ variable-temp. liq. cell TEM (VT-LCTEM) imaging, a new characterization technique that complements conventional bulk scattering techniques. Soln. casting of the colloids yields a free-standing transparent COF film with retained crystallinity and porosity, as well as preferential crystallite orientation. Collectively this structural control provides new opportunities for understanding COF formation and designing morphologies for device applications.
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    46. 46
      Das, A.; Jayanthi, S.; Deepak, H. S.; Ramanathan, K. V.; Kumar, A.; Dasgupta, C.; Sood, A. K. Single-File Diffusion of Confined Water inside SWNTs: An NMR Study. ACS Nano 2010, 4, 1687– 1695,  DOI: 10.1021/nn901554h
      46
      Single-File Diffusion of Confined Water Inside SWNTs: An NMR Study
      Das, Anindya; Jayanthi, Sundaresan; Deepak, Handiganadu Srinivasa Murthy Vinay; Ramanathan, Krishna Venkatachala; Kumar, Anil; Dasgupta, Chandan; Sood, Ajay K.
      ACS Nano (2010), 4 (3), 1687-1695CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      We report a NMR study of confined water inside ∼1.4 nm diam. single-walled carbon nanotubes (SWNTs). We show that the confined water does not freeze even up to 223 K. A pulse field gradient (PFG) NMR method is used to det. the mean squared displacement (MSD) of the water mols. inside the nanotubes at temps. below 273 K, where the bulk water outside the nanotubes freezes and hence does not contribute to the proton NMR signal. We show that the mean squared displacement varies as the square root of time, predicted for single-file diffusion in a one-dimensional channel. We propose a qual. understanding of our results based on available mol. dynamics simulations.
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    47. 47
      Kärger, J.; Avramovska, M.; Freude, D.; Haase, J.; Hwang, S.; Valiullin, R. Pulsed Field Gradient NMR Diffusion Measurement in Nanoporous Materials. Adsorption 2021, 27, 453– 484,  DOI: 10.1007/s10450-020-00290-9
      There is no corresponding record for this reference.
    48. 48
      Kärger, J.; Chmelik, C.; Heinke, L.; Valiullin, R. A New View of Diffusion in Nanoporous Materials. Chem. Ing. Tech. 2010, 82, 779– 804,  DOI: 10.1002/cite.201000038
      There is no corresponding record for this reference.
    49. 49
      Tanner, J. E. Use of the Stimulated Echo in NMR Diffusion Studies. J. Chem. Phys. 1970, 52, 2523– 2526,  DOI: 10.1063/1.1673336
      49
      Use of the stimulated echo in NMR diffusion studies
      Tanner, John E.
      Journal of Chemical Physics (1970), 52 (5), 2523-6CODEN: JCPSA6; ISSN:0021-9606.
      The stimulated echo in a three-radio-frequency-pulse expt. is useful in extending the range of measurement of diffusion coeffs. to more viscous substances or the measurement of barrier sepns. to wider spacings in systems where the diffusing substance has T1 > T2. The spin-echo attenuation due to self-diffusion is derived for the general case of a time-dependent field gradient, and the result is found exptl. to be correct for the special case of a field gradient applied in 2 equal, square pulse.
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    50. 50
      Kärger, J.; Pfeifer, H.; Heink, W. Principles and Application of Self-Diffusion Measurements by Nuclear Magnetic Resonance. In Advances in Magnetic and Optical Resonance; Waugh, J. S., Ed.; Academic Press, 1988; Vol. 12, pp 1– 89.
      There is no corresponding record for this reference.
    51. 51
      Hertel, S.; Wehring, M.; Amirjalayer, S.; Gratz, M.; Lincke, J.; Krautscheid, H.; Schmid, R.; Stallmach, F. NMR Studies of Benzene Mobility in Metal-Organic Framework MOF-5. Eur. Phys. J. Appl. Phys. 2011, 55, 20702,  DOI: 10.1051/epjap/2011100370
      51
      NMR studies of benzene mobility in metal-organic framework MOF-5
      Hertel, S.; Wehring, M.; Amirjalayer, S.; Gratz, M.; Lincke, J.; Krautscheid, H.; Schmid, R.; Stallmach, F.
      European Physical Journal: Applied Physics (2011), 55 (2), 20702/1-20702/10CODEN: EPAPFV; ISSN:1286-0042. (EDP Sciences)
      The concn. and temp. dependence of the self-diffusion of benzene adsorbed in the metal-org. framework MOF-5 (IRMOF-1) is studied by pulsed field gradient (PPG) NMR spectroscopy. When increasing the loading from 10 to 20 mols. per unit cell of MOF-5, the exptl. diffusion data drop by a factor of about 3 while current mol. dynamic (MD) simulations predict slightly increasing diffusion coeffs. for this range of loadings. The observation is rationalized using the recently predicted clustering of adsorbate mols. in microporous systems for temps. well below the adsorbate crit. temp. Necessary improvements of mol. simulation models for predicting diffusivities under such conditions are discussed.
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    52. 52
      Valiullin, R. R.; Skirda, V. D.; Stapf, S.; Kimmich, R. Molecular Exchange Processes in Partially Filled Porous Glass as Seen with NMR Diffusometry. Phys. Rev. E 1997, 55, 2664– 2671,  DOI: 10.1103/PhysRevE.55.2664
      There is no corresponding record for this reference.
    53. 53
      Hedin, N.; Rzepka, P.; Jasso-Salcedo, A. B.; Church, T. L.; Bernin, D. Intracrystalline Transport Barriers Affecting the Self-Diffusion of CH4 in Zeolites |Na12|-A and |Na12–xKx|-A. Langmuir 2019, 35, 12971– 12978,  DOI: 10.1021/acs.langmuir.9b02574
      There is no corresponding record for this reference.
    54. 54
      Morgan, M.; Cosgrove, T.; Richardson, R. The Diffusion of Benzene in High Silica Zeolite ZSM5 Studied by PFGNMR and QUENS. Colloids Surf. 1989, 36, 209– 219,  DOI: 10.1016/0166-6622(89)80239-2
      There is no corresponding record for this reference.
    55. 55
      Stallmach, F.; Groger, S.; Kunzel, V.; Karger, J.; Yaghi, O. M.; Hesse, M.; Muller, U. NMR Studies on the Diffusion of Hydrocarbons on the Metal-Organic Framework Material MOF-5. Angew. Chem., Int. Ed. 2006, 45, 2123– 2126,  DOI: 10.1002/anie.200502553
      55
      NMR studies on the diffusion of hydrocarbons on the metal-organic framework material MOF-5
      Stallmach, Frank; Groeger, Stefan; Kuenzel, Volker; Kaerger, Joerg; Yaghi, O. M.; Hesse, Michael; Mueller, Ulrich
      Angewandte Chemie, International Edition (2006), 45 (13), 2123-2126CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      High intracryst. mobilities and fast exchange between the crystal and the surrounding gas phase were found in NMR studies on the diffusion of methane, ethane, n-hexane, and benzene in large crystals of the metal-org. framework MOF-5. The results support future use of metal-org. framework materials as tailorable sorbents for fast gas processing and gas-storage materials in industry.
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    56. 56
      Splith, T.; Frohlich, D.; Henninger, S. K.; Stallmach, F. Development and Application of an Exchange Model for Anisotropic Water Diffusion in the Microporous MOF Aluminum Fumarate. J. Magn. Reson. 2018, 291, 40– 46,  DOI: 10.1016/j.jmr.2018.04.009
      56
      Development and application of an exchange model for anisotropic water diffusion in the microporous MOF aluminum fumarate
      Splith, Tobias; Froehlich, Dominik; Henninger, Stefan K.; Stallmach, Frank
      Journal of Magnetic Resonance (2018), 291 (), 40-46CODEN: JMARF3; ISSN:1090-7807. (Elsevier B.V.)
      Diffusion of water in aluminum fumarate was studied by means of pulsed field gradient (PFG) NMR (NMR). Due to water mols. exchanging between the intracryst. anisotropic pore space and the isotropic intercryst. void space the model of intracryst. anisotropic diffusion fails to describe the exptl. PFG NMR data at high observation times. Therefore, the two-site exchange model developed by Karger is extended to the case of exchange between an anisotropic and an isotropic site. This extended exchange model is solved by numerical integration. It describes the exptl. data very well and yields values for the intracryst. diffusion coeff. and the mean residence times of the resp. sites. Further PFG NMR studies were performed with coatings consisting of small aluminum fumarate crystals, which are used in adsorptive heat transformation applications. The diffusion coeffs. of water in the small crystal coating are compared to the values expected from the extended two-site exchange model and from the model of long-range diffusion.
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    57. 57
      Kärger, J. Zur Bestimmung Der Diffusion in Einem Zweibereichsystem Mit Hilfe Von Gepulsten Feldgradienten. Ann. Phys. 1969, 479, 1– 4,  DOI: 10.1002/andp.19694790102
      There is no corresponding record for this reference.
    58. 58
      Kärger, J. A Study of Fast Tracer Desorption in Molecular Sieve Crystals. AlChE J. 1982, 28, 417– 423,  DOI: 10.1002/aic.690280309
      There is no corresponding record for this reference.
    59. 59
      Kärger, J.; Pfeifer, H. N.M.R. Self-Diffusion Studies in Zeolite Science and Technology. Zeolites 1987, 7, 90– 107,  DOI: 10.1016/0144-2449(87)90067-4
      There is no corresponding record for this reference.
    60. 60
      Zeigermann, P.; Naumov, S.; Mascotto, S.; Karger, J.; Smarsly, B. M.; Valiullin, R. Diffusion in Hierarchical Mesoporous Materials: Applicability and Generalization of the Fast-Exchange Diffusion Model. Langmuir 2012, 28, 3621– 3632,  DOI: 10.1021/la2047432
      60
      Diffusion in Hierarchical Mesoporous Materials: Applicability and Generalization of the Fast-Exchange Diffusion Model
      Zeigermann, P.; Naumov, S.; Mascotto, S.; Kaerger, J.; Smarsly, B. M.; Valiullin, R.
      Langmuir (2012), 28 (7), 3621-3632CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Transport properties of cyclohexane confined to a silica material with an ordered, bimodal pore structure were studied by pulsed field gradient NMR. A particular organization of the well-defined pore structure, composed of a collection of spatially ordered, spherical mesopores interconnected via narrow worm-like pores, allowed for a quant. anal. of the diffusion process in a medium with spatially ordered distribution of the fluid d. for a broad range of the gas-liq. equil. The measured diffusion data were interpreted in terms of effective diffusivities, which were detd. within a microscopic model considering long-range mol. trajectories constructed by assembling the alternating pieces of displacement in the two constituting pore spaces. It has further been found that for the system under study, in particular, and for mesoporous materials with multiple porosities, in general, this generalized model simplifies to the conventional fast-exchange model used in the literature. Thus, not only was justification of the applicability of the fast-exchange model to a diversity of mesoporous materials provided, but the diffusion parameters entering the fast-exchange model were also exactly defined. The equation resulting in this way was found to nicely reproduce the exptl. detd. diffusivities, establishing a methodol. for targeted fine-tuning of transport properties of fluids in hierarchical materials with multiple porosities.
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    61. 61
      Loskutov, V. V.; Sevriugin, V. A. A Novel Approach to Interpretation of the Time-Dependent Self-Diffusion Coefficient as a Probe of Porous Media Geometry. J. Magn. Reson. 2013, 230, 1– 9,  DOI: 10.1016/j.jmr.2013.01.004
      61
      A novel approach to interpretation of the time-dependent self-diffusion coefficient as a probe of porous media geometry
      Loskutov, V. V.; Sevriugin, V. A.
      Journal of Magnetic Resonance (2013), 230 (), 1-9CODEN: JMARF3; ISSN:1090-7807. (Elsevier B.V.)
      This article presents a new approxn. describing fluid diffusion in porous media. Time dependence of the self-diffusion coeff. D(t) in the permeable porous medium is studied based on the assumption that diffusant mols. move randomly. An anal. expression for time dependence of the self-diffusion coeff. was obtained in the following form: D(t)=(D0-D∞)exp(-D0t/λ)+D∞, where D0 is the self-diffusion coeff. of bulk fluid, D∞ is the asymptotic value of the self-diffusion coeff. in the limit of long time values (t → ∞), λ is the characteristic parameter of this porous medium with dimensionality of length. Applicability of the soln. obtained to the anal. of exptl. data is shown. The possibility of passing to short-time and long-time regimes is discussed.
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    62. 62
      Latour, L. L.; Mitra, P. P.; Kleinberg, R. L.; Sotak, C. H. Time-Dependent Diffusion Coefficient of Fluids in Porous Media as a Probe of Surface-to-Volume Ratio. J. Magn. Reson. Ser. A 1993, 101, 342– 346,  DOI: 10.1006/jmra.1993.1056
      62
      Time-dependent diffusion coefficient of fluids in porous media as a probe of surface-to-volume ratio
      Latour, Lawrence L.; Mitra, Partha P.; Kleinberg, Robert L.; Sotak, Christopher H.
      Journal of Magnetic Resonance, Series A (1993), 101 (3), 342-6CODEN: JMRAE2; ISSN:1064-1858.
      The time-dependent diffusion coeff. is independent of the microscopic details of the restricting geometry and depends only on the surface-to-vol. ratio S/V of the pore space. Pulsed field gradient NMR is an ideal tool for measuring the time-dependent diffusion coeff. The min. observation time is detd. by the min. length of gradient pulses and the subsequent recovery of the app. from eddy-current and magnetoacoustic effects. In the current expts., the min. observation time is about 7 ms. The max. time is detd. by the spin-lattice and spin-spin relaxation times of the fluid. For these expts., the max. observation time was chosen to be T1 of the spins. Pulsed field gradient NMR expts. were conducted on fluid-satd. glass sphere packs and on a sample of onion tissue. The diffusion coeff. for the intracellular fluid in an onion was detd. The diffusion coeffs. for distd. water and viscous oil in the pores of a monosized sphere pack were also detd.
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    63. 63
      Forse, A. C.; Colwell, K. A.; Gonzalez, M. I.; Benders, S.; Torres-Gavosto, R. M.; Blümich, B.; Reimer, J. A.; Long, J. R. Influence of Pore Size on Carbon Dioxide Diffusion in Two Isoreticular Metal-Organic Frameworks. Chem. Mater. 2020, 32, 3570– 3576,  DOI: 10.1021/acs.chemmater.0c00745
      63
      Influence of Pore Size on Carbon Dioxide Diffusion in Two Isoreticular Metal-Organic Frameworks
      Forse, Alexander C.; Colwell, Kristen A.; Gonzalez, Miguel I.; Benders, Stefan; Torres-Gavosto, Rodolfo M.; Blumich, Bernhard; Reimer, Jeffrey A.; Long, Jeffrey R.
      Chemistry of Materials (2020), 32 (8), 3570-3576CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The rapid diffusion of mols. in porous materials is crit. for numerous applications including sepns., energy storage, sensing, and catalysis. A common strategy for tuning guest diffusion rates is to vary the material pore size, although detailed studies that isolate the effect of changing this particular variable are lacking. Here, we begin to address this challenge by measuring the diffusion of carbon dioxide in two isoreticular metal-org. frameworks featuring channels with different diams., Zn2(dobdc) (dobdc4- = 2,5-dioxidobenzene-1,4-dicarboxylate) and Zn2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), using pulsed field gradient NMR spectroscopy. An increase in the pore diam. from 15 Å in Zn2(dobdc) to 22 Å in Zn2(dobpdc) is accompanied by an increase in the self-diffusion of CO2 by a factor of 4 to 6, depending on the gas pressure. Anal. of the diffusion anisotropy in Zn2(dobdc) reveals that the self-diffusion coeff. for motion of CO2 along the framework channels is at least 10000 times greater than for motion between the framework channels. Our findings should aid the design of improved porous materials for a range of applications where diffusion plays a crit. role in detg. performance.
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    64. 64
      Vasenkov, S.; Geir, O.; Karger, J. Gas Diffusion in Zeolite Beds: PFG NMR Evidence for Different Tortuosity Factors in the Knudsen and Bulk Regimes. Eur. Phys. J. E 2003, 12, S35– S38,  DOI: 10.1140/epjed/e2003-01-009-1
      There is no corresponding record for this reference.
    65. 65
      Holzmann, T.; Schoop, L. M.; Ali, M. N.; Moudrakovski, I.; Gregori, G.; Maier, J.; Cava, R. J.; Lotsch, B. V. Li0.6[Li0.2Sn0.8S2]─a Layered Lithium Superionic Conductor. Energy Environ. Sci. 2016, 9, 2578– 2585,  DOI: 10.1039/C6EE00633G
      There is no corresponding record for this reference.
    66. 66
      Beckert, S.; Stallmach, F.; Toufar, H.; Freude, D.; Kärger, J.; Haase, J. Tracing Water and Cation Diffusion in Hydrated Zeolites of Type Li-LSX by Pulsed Field Gradient NMR. J. Phys. Chem. C 2013, 117, 24866– 24872,  DOI: 10.1021/jp408604y
      66
      Tracing Water and Cation Diffusion in Hydrated Zeolites of Type Li-LSX by Pulsed Field Gradient NMR
      Beckert, Steffen; Stallmach, Frank; Toufar, Helge; Freude, Dieter; Kaerger, Joerg; Haase, Juergen
      Journal of Physical Chemistry C (2013), 117 (47), 24866-24872CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The pulsed field gradient (PFG) technique of NMR is exploited for recording the time-dependent mean diffusion path lengths of both the water mols. (via 1H NMR) and the cations (via 7Li NMR) in hydrated zeolite Li-LSX. The obsd. propagation patterns reveal, for both the water mols. and the cations, two types of transport resistances, acting in addn. to the diffusion resistance of the genuine pore network. They are attributed to the interfaces at the boundary between the purely cryst. regions (crystallites) within the Li-LSX particles (intergrowths) under study and to the external surface of either the particles themselves or crystallite aggregates within these particles. The cation diffusivity is retarded by ∼1 order of magnitude in comparison with the water diffusivity. This notably exceeds the retardation of cation diffusion in comparison with water in free soln., reflecting the particular influence of the zeolite lattice on the guest mobility.
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    67. 67
      Krutyeva, M.; Vasenkov, S.; Yang, X.; Caro, J.; Kärger, J. Surface Barriers on Nanoporous Particles: A New Method of Their Quantitation by PFG NMR. Microporous Mesoporous Mater. 2007, 104, 89– 96,  DOI: 10.1016/j.micromeso.2007.01.010
      67
      Surface barriers on nanoporous particles: A new method of their quantitation by PFG NMR
      Krutyeva, M.; Vasenkov, S.; Yang, X.; Caro, J.; Kaerger, J.
      Microporous and Mesoporous Materials (2007), 104 (1-3), 89-96CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
      The surface permeability of zeolite crystals NaCaA for small alkane mols. (methane and ethane) was estd. by using a novel method, analyzing NMR tracer desorption measurements. It is based on the measurement of both the relative amt. of tracer exchange and the corresponding effective coeffs. of intracryst. diffusion and their correlation with the results of model calcns. by dynamic Monte Carlo simulations. For the two LTA specimens studied, already in the as-synthesized zeolite crystals notable surface resistances were obsd. Surface permeabilities were studied in dependence on temp., sorbate and crystal size.
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    68. 68
      Peksa, M.; Lang, J.; Stallmach, F. 13C NMR Study of Diffusion Anisotropy of Carbon Dioxide Adsorbed in Nanoporous DMOF-1. Microporous Mesoporous Mater. 2015, 205, 11– 15,  DOI: 10.1016/j.micromeso.2014.09.039
      68
      13C NMR study of diffusion anisotropy of carbon dioxide adsorbed in nanoporous DMOF-1
      Peksa, Mikulas; Lang, Jan; Stallmach, Frank
      Microporous and Mesoporous Materials (2015), 205 (), 11-15CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier Inc.)
      DMOF-1 (Zn2(bdc)2dabco) is a metal-org. framework with tetragonal symmetry. It contains parallel one-dimensional channels with a diam. of about 0.75nm interconnected by smaller windows. For CO2 adsorbed in the nanochannels the chem. shift tensor and the diffusion tensor were investigated by 13C NMR. The spectra acquired under static conditions reveal a powder pattern with a residual chem. shift anisotropy of 〈Δδ〉 = -55 ppm. The collinearity of the axisym. residual chem. shift and the diffusion tensors was utilized to assess the diffusion anisotropy via the diffusion attenuation of the 13C PFG NMR powder pattern. CO2 is highly mobile in DMOF-1. The apparent diffusion coeff. (the trace of diffusion tensor) is (6.2±1.0) × 10-9m2s-1 at 298 K and the corresponding anisotropy expressed as D‖/D.perp. is ≈3.
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    69. 69
      Naumov, S.; Valiullin, R.; Kärger, J.; Pitchumani, R.; Coppens, M.-O. Tracing Pore Connectivity and Architecture in Nanostructured Silica SBA-15. Microporous Mesoporous Mater. 2008, 110, 37– 40,  DOI: 10.1016/j.micromeso.2007.08.014
      There is no corresponding record for this reference.
    70. 70
      Isaacs, M. A.; Robinson, N.; Barbero, B.; Durndell, L. J.; Manayil, J. C.; Parlett, C. A.; D’Agostino, C.; Wilson, K.; Lee, A. F. Unravelling Mass Transport in Hierarchically Porous Catalysts. J. Mater. Chem. A 2019, 7, 11814– 11825,  DOI: 10.1039/c9ta01867k
      70
      Unravelling mass transport in hierarchically porous catalysts
      Isaacs, Mark A.; Robinson, Neil; Barbero, Brunella; Durndell, Lee J.; Manayil, Jinesh C.; Parlett, Christopher M. A.; D'Agostino, Carmine; Wilson, Karen; Lee, Adam F.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (19), 11814-11825CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Bio-derived platform chems. and fuels are important for the development of sustainable manufg. However, their efficient prodn. from biomass necessitates new catalysts and processes optimized for the selective transformation of large mols. Mesoporous and hierarchically porous functional materials are promising catalyst candidates for biomass valorization, but quant. relationships between pore dimensions/connectivity, mass transport, and corresponding catalytic performance are poorly defined. A family of hierarchical macroporous-mesoporous SBA-15 sulfonic acids were prepd. with tunable macropore diams. for carboxylic acid esterification. Turnover frequencies for long-chain (palmitic and erucic) acids were proportional to macropore diam. (≤370 nm), whereas propanoic acid esterification was independent of macropore size. Pulsed field gradient NMR diffusion expts. reveal that larger macropores enhance esterification of bulky carboxylic acids by conferring superior pore interconnectivity and assocd. mass transport.
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    71. 71
      Kessler, C.; Schuldt, R.; Emmerling, S.; Lotsch, B. V.; Kästner, J.; Gross, J.; Hansen, N. Influence of Layer Slipping on Adsorption of Light Gases in Covalent Organic Frameworks: A Combined Experimental and Computational Study. Microporous Mesoporous Mater. 2022, 336, 111796,  DOI: 10.1016/j.micromeso.2022.111796
      71
      Influence of layer slipping on adsorption of light gases in covalent organic frameworks: A combined experimental and computational study
      Kessler, Christopher; Schuldt, Robin; Emmerling, Sebastian; Lotsch, Bettina V.; Kaestner, Johannes; Gross, Joachim; Hansen, Niels
      Microporous and Mesoporous Materials (2022), 336 (), 111796CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
      Sorption of gases in micro- and mesoporous materials is typically interpreted on the basis of idealized structural models where real structure effects such as defects and disorder are absent. For covalent org. frameworks (COFs) significant discrepancies between measured and simulated adsorption isotherms are often reported but rarely traced back to their origins. This is because little is known about the real structure of COFs and its effect on the sorption properties of these materials. In the present work mol. simulations are used to obtain adsorption isotherms of argon, nitrogen, and carbon dioxide in the COF-LZU1 at various temps. The (perfect) model COF has a BET surface that is higher than the exptl. BET surface by a factor of approx. 1.33, suggesting defects or inclusions are present in the real structure. We find that the satn. adsorption loading of small gaseous species in COF-LZU1, as detd. from grand canonical Monte Carlo simulations, is also higher by approx. the same factor compared to the exptl. satn. loading. The influence of interlayer slipping on the shape of the adsorption isotherm and the adsorption capacity is studied. Comparison between simulation and expt. at lower loadings suggests the layers to be shifted instead of perfectly eclipsed. The sensitivity of the adsorption isotherms in this regime towards the underlying framework topol. shows that real structure effects have significant influence on the gas uptake. Accounting for layer slipping is important to applications such as catalysis, gas storage and sepn.
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    72. 72
      Pütz, A. M.; Terban, M. W.; Bette, S.; Haase, F.; Dinnebier, R. E.; Lotsch, B. V. Total Scattering Reveals the Hidden Stacking Disorder in a 2D Covalent Organic Framework. Chem. Sci. 2020, 11, 12647– 12654,  DOI: 10.1039/D0SC03048A
      72
      Total scattering reveals the hidden stacking disorder in a 2D covalent organic framework
      Puetz, Alexander M.; Terban, Maxwell W.; Bette, Sebastian; Haase, Frederik; Dinnebier, Robert E.; Lotsch, Bettina V.
      Chemical Science (2020), 11 (47), 12647-12654CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Interactions between extended π-systems are often invoked as the main driving force for stacking and crystn. of 2D org. polymers. In covalent org. frameworks (COFs), the stacking strongly influences properties such as the accessibility of functional sites, pore geometry, and surface states, but the exact nature of the interlayer interactions is mostly elusive. The stacking mode is often identified as eclipsed based on obsd. high symmetry diffraction patterns. However, as pointed out by various studies, the energetics of eclipsed stacking are not favorable and offset stacking is preferred. This work presents lower and higher apparent symmetry modifications of the imine-linked TTI-COF prepd. through high- and low-temp. reactions. Through local structure investigation by pair distribution function anal. and simulations of stacking disorder, we observe random local layer offsets in the low temp. modification. We show that while stacking disorder can be easily overlooked due to the apparent crystallog. symmetry of these materials, total scattering methods can help clarify this information and suggest that defective local structures could be much more prevalent in COFs than previously thought. A detailed anal. of the local structure helps to improve the search for and design of highly porous tailor-made materials.
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    73. 73
      Haase, F.; Gottschling, K.; Stegbauer, L.; Germann, L. S.; Gutzler, R.; Duppel, V.; Vyas, V. S.; Kern, K.; Dinnebier, R. E.; Lotsch, B. V. Tuning the Stacking Behaviour of a 2D Covalent Organic Framework through Non-Covalent Interactions. Mater. Chem. Front. 2017, 1, 1354– 1361,  DOI: 10.1039/C6QM00378H
      73
      Tuning the stacking behaviour of a 2D covalent organic framework through non-covalent interactions
      Haase, F.; Gottschling, K.; Stegbauer, L.; Germann, L. S.; Gutzler, R.; Duppel, V.; Vyas, V. S.; Kern, K.; Dinnebier, R. E.; Lotsch, B. V.
      Materials Chemistry Frontiers (2017), 1 (7), 1354-1361CODEN: MCFAC5; ISSN:2052-1537. (Royal Society of Chemistry)
      Two-dimensional covalent org. frameworks (COFs) are cryst. porous materials composed of org. building blocks that are connected via covalent bonds within their layers, but through non-covalent interactions between the layers. The exact stacking sequence of the layers is of paramount importance for the optoelectronic, catalytic and sorption properties of these polymeric materials. The weak interlayer interactions lead to a variety of stacking geometries in COFs, which are both hard to characterize and poorly understood due to the low levels of crystallinity. Therefore, detailed insights into the stacking geometry in COFs is still largely elusive. In this work we show that the geometric and electronic features of the COF building blocks can be used to guide the stacking behavior of two related 2D imine COFs (TBI-COF and TTI-COF), which either adopt an averaged "eclipsed" structure with apparent zero-offset stacking or a unidirectionally slip-stacked structure, resp. These structural features are confirmed by XRPD and TEM measurements. Based on theor. calcns., we were able to pinpoint the cause of the uniform slip-stacking geometry and high crystallinity of TTI-COF to the inherent self-complementarity of the building blocks and the resulting donor-acceptor-type stacking of the imine bonds in adjacent layers, which can serve as a more general design principle for the synthesis of highly cryst. COFs.
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    74. 74
      Emmerling, S. T.; Schuldt, R.; Bette, S.; Yao, L.; Dinnebier, R. E.; Kastner, J.; Lotsch, B. V. Interlayer Interactions as Design Tool for Large-Pore COFs. J. Am. Chem. Soc. 2021, 143, 15711– 15722,  DOI: 10.1021/jacs.1c06518
      74
      Interlayer Interactions as Design Tool for Large-Pore COFs
      Emmerling, Sebastian T.; Schuldt, Robin; Bette, Sebastian; Yao, Liang; Dinnebier, Robert E.; Kaestner, Johannes; Lotsch, Bettina V.
      Journal of the American Chemical Society (2021), 143 (38), 15711-15722CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Covalent org. frameworks (COFs) with a pore size beyond 5 nm are still rarely seen in this emerging field. Besides obvious complications like the elaborated synthesis of large linkers with sufficient soly., more subtle challenges regarding large-pore COF synthesis, including pore occlusion and collapse, prevail. Here we present two isoreticular series of large-pore imine COFs with pore sizes up to 5.8 nm and correlate the interlayer interactions with the structure and thermal behavior of the COFs. By adjusting interlayer interactions through the incorporation of methoxy groups acting as pore-directing "anchors", different stacking modes can be accessed, resulting in modified stacking polytypes and, hence, effective pore sizes. A strong correlation between stacking energy toward highly ordered, nearly eclipsed structures, higher structural integrity during thermal stress, and a novel, thermally induced phase transition of stacking modes in COFs was found, which sheds light on viable design strategies for increased structural control and stability in large-pore COFs.
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    75. 75
      Stahler, C.; Grunenberg, L.; Terban, M. W.; Browne, W. R.; Doellerer, D.; Kathan, M.; Etter, M.; Lotsch, B. V.; Feringa, B. L.; Krause, S. Light-Driven Molecular Motors Embedded in Covalent Organic Frameworks. Chem. Sci. 2022, 13, 8253– 8264,  DOI: 10.1039/D2SC02282F
      There is no corresponding record for this reference.
    76. 76
      Viel, S.; Ziarelli, F.; Pages, G.; Carrara, C.; Caldarelli, S. Pulsed Field Gradient Magic Angle Spinning NMR Self-Diffusion Measurements in Liquids. J. Magn. Reson. 2008, 190, 113– 123,  DOI: 10.1016/j.jmr.2007.10.010
      76
      Pulsed field gradient magic angle spinning NMR self-diffusion measurements in liquids
      Viel, Stephane; Ziarelli, Fabio; Pages, Guilhem; Carrara, Caroline; Caldarelli, Stefano
      Journal of Magnetic Resonance (2008), 190 (1), 113-123CODEN: JMARF3; ISSN:1090-7807. (Elsevier)
      Several investigations have recently reported the combined use of pulsed field gradient (PFG) with magic angle spinning (MAS) for the anal. of mol. mobility in heterogeneous materials. In contrast, little attention has been devoted so far to delimiting the role of the extra force field induced by sample rotation on the significance and reliability of self-diffusivity measurements. The main purpose of this work is to examine this phenomenon by focusing on pure liqs. for which its impact is expected to be largest. Specifically, we show that self-diffusion coeffs. can be accurately detd. by PFG MAS NMR diffusion measurements in liqs., provided that specific exptl. conditions are met. First, the methodol. to est. the gradient uniformity and to properly calibrate its abs. strength is briefly reviewed and applied on a MAS probe equipped with a gradient coil aligned along the rotor spinning axis, the so-called magic angle gradient' coil. Second, the influence of MAS on the outcome of PFG MAS diffusion measurements in liqs. is investigated for two distinct typical rotors of different active vols., 12 and 50 μL. While the latter rotor led to totally unreliable results, esp. for low viscosity compds., the former allowed for the detn. of accurate self-diffusion coeffs. both for fast and slowly diffusing species. Potential implications of this work are the possibility to measure accurate self-diffusion coeffs. of sample-limited mixts. or to avoid radiation damping interferences in NMR diffusion measurements. Overall, the outlined methodol. should be of interest to anyone who strives to improve the reliability of MAS diffusion studies, both in homogeneous and heterogeneous media.
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    77. 77
      Cohen, S. R.; Plazanet, M.; Rols, S.; Voneshen, D. J.; Fourkas, J. T.; Coasne, B. Structure and Dynamics of Acetonitrile: Molecular Simulation and Neutron Scattering. J. Mol. Liq. 2022, 348, 118423,  DOI: 10.1016/j.molliq.2021.118423
      77
      Structure and dynamics of acetonitrile: Molecular simulation and neutron scattering
      Cohen, Samuel R.; Plazanet, Marie; Rols, Stephane; Voneshen, David J.; Fourkas, John T.; Coasne, Benoit
      Journal of Molecular Liquids (2022), 348 (), 118423CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
      We examine the interplay between organization and dynamics in bulk liq. acetonitrile. Using angularly resolved radial distribution functions, g(r,θ), derived from mol. simulations, we identify a complex microscopic structure in which most liq. mols. are assocd. with one or more neighboring mols. in an antiparallel, "octupole-paired" configuration and/or an offset, head-to-tail configuration. A detailed anal. of these structural motifs reveals that the offset head-to-tail dimers are the most prevalent. A time-dependent pairing anal. corroborates this picture of robust mol. assocns. favoring head-to-tail dimers, which last longer than antiparallel dimers. This organization, which is assocd. with pairing times (∼ps) longer than the typical rotational time const., cannot be explained on the basis of the Coulomb interactions in the dimer, and so must arise from collective effects that are neither strong nor specific. Finally, using both neutron-scattering techniques and mol. simulations, we study dynamics in liq. acetonitrile over time scales ranging from subpicosecond (the vibrational d. of states) to picosecond (rotational/translational motions and the generalized d. of states) to tens of picoseconds (self-diffusivity in the Fickian regime).
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    78. 78
      Kalugin, O. N.; Chaban, V. V.; Loskutov, V. V.; Prezhdo, O. V. Uniform Diffusion of Acetonitrile Inside Carbon Nanotubes Favors Supercapacitor Performance. Nano Lett. 2008, 8, 2126– 2130,  DOI: 10.1021/nl072976g
      78
      Uniform Diffusion of Acetonitrile inside Carbon Nanotubes Favors Supercapacitor Performance
      Kalugin, Oleg N.; Chaban, Vitaly V.; Loskutov, Valentin V.; Prezhdo, Oleg V.
      Nano Letters (2008), 8 (8), 2126-2130CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      An unusual behavior of liq. MeCN (AN) confined inside C nanotubes (CNTs) is predicted by mol. dynamics simulation. In contrast to H2O, which shows inhomogeneous variation of both translational and rotational diffusion with CNT diam., the diffusion coeff. of AN changes uniformly and can be described by a simple analytic model. At the same time, the reorientation dynamics of AN vary irregularly in smaller CNTs because of specific packing structures. The uniform translational diffusion of the nonaq. solvent is crit. for stable performance of the new generation of supercapacitors.
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    79. 79
      Norton, C. D.; Thompson, W. H. On the Diffusion of Acetonitrile in Nanoscale Amorphous Silica Pores. Understanding Anisotropy and the Effects of Hydrogen Bonding. J. Phys. Chem. C 2013, 117, 19107– 19114,  DOI: 10.1021/jp407830f
      79
      On the Diffusion of Acetonitrile in Nanoscale Amorphous Silica Pores. Understanding Anisotropy and the Effects of Hydrogen Bonding
      Norton, Cassandra D.; Thompson, Ward H.
      Journal of Physical Chemistry C (2013), 117 (37), 19107-19114CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Mol. dynamics simulations are used to examine the diffusion of acetonitrile within ∼2.4 nm diam. amorphous silica pores with a focus on the mechanism. The role of the pore surface chem. is examd. by comparison of a hydrophilic, -OH terminated, silica pore with one that has hydrogen-bonding turned off and with an effectively hydrophobic pore obtained by setting all pore charges to zero. The anisotropy of diffusion, along and perpendicular to the pore axis, is examd. through the mean-squared displacements. The origins of the anisotropy are investigated through the dependence on the acetonitrile position within the pore. The effect of hydrogen bonding of acetonitrile mols. to the hydrophilic pore surface is also probed. The simulations show that acetonitrile mols. do not diffuse axially next to the pore surface. Rather, axial diffusion is preceded by radial diffusion away from the pore surface. The same mechanism is obsd. for mols. independent of their hydrogen-bonding status to surface silanols though hydrogen-bonded mols. diffuse more slowly.
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    80. 80
      Borchardt, L.; Leistenschneider, D.; Haase, J.; Dvoyashkin, M. Revising the Concept of Pore Hierarchy for Ionic Transport in Carbon Materials for Supercapacitors. Adv. Energy Mater. 2018, 8, 1800892,  DOI: 10.1002/aenm.201800892
      There is no corresponding record for this reference.
    81. 81
      Koone, N.; Shao, Y.; Zerda, T. W. Diffusion of Simple Liquids in Porous Sol-Gel Glass. J. Phys. Chem. 1995, 99, 16976– 16981,  DOI: 10.1021/j100046a025
      81
      Diffusion of Simple Liquids in Porous Sol-Gel Glass
      Koone, N.; Shao, Y.; Zerda, T. W.
      Journal of Physical Chemistry (1995), 99 (46), 16976-81CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
      Diffusion coeffs. of selected solvents (acetone, acetonitrile, chloroform, cyclohexane, toluene, water) in porous sol-gel glass were measured using the radioactive tracer diffusion method and the diaphragm technique. The concns. of the solvents were detd. using IR and scintillation measurements. Selected samples had their internal surface modified by replacing hydroxyl groups with trimethylsilane groups. The mol. translational motion inside the pores is hindered by pure geometrical restrictions and surface interactions. Polar mols. diffuse faster than inert mols. Exptl. results were explained in terms of the model developed by Korb et al. Results obtained for cyclohexane were compared with mol. dynamics computer simulations, and the tortuosity factor was evaluated. The diffusion coeff. for acetone within the pores was studied over the temp. range 284-315 K, and the activation energy was detd.
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    82. 82
      Kittaka, S.; Iwashita, T.; Serizawa, A.; Kranishi, M.; Takahara, S.; Kuroda, Y.; Mori, T.; Yamaguchi, T. Low Temperature Properties of Acetonitrile Confined in MCM-41. J. Phys. Chem. B 2005, 109, 23162– 23169,  DOI: 10.1021/jp052476g
      82
      Low Temperature Properties of Acetonitrile Confined in MCM-41
      Kittaka, Shigeharu; Iwashita, Takafumi; Serizawa, Akihiro; Kranishi, Miki; Takahara, Shuichi; Kuroda, Yasushige; Mori, Toshinori; Yamaguchi, Toshio
      Journal of Physical Chemistry B (2005), 109 (49), 23162-23169CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 was studied by use of adsorption, FT-IR, DSC, and quasi-elastic neutron scattering (QENS) measurements. Acetonitrile mols. in a monolayer interact strongly with surface hydroxyls to be registered and perturb the triple bond in the C≡N group. Adsorbed mols. above the monolayer through to the central part of the cylindrical pores are capillary condensed mols. (cc-acetonitrile), but they do not show the hysteresis loop in adsorption-desorption isotherms, i.e., second order capillary condensation. FT-IR measurements indicated that the condensed phase is very similar to the bulk liq. The cc-acetonitrile freezes at temps. that depend on the pore size of the MCM-41 down to 29.1 Å (C14), below which it is not frozen. In addn., phase changes between α-type and β-type acetonitriles were obsd. below the m.ps. Application of the Gibbs-Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface free energy differences between the interfaces, i.e., Δγl/α = 22.4 mJ m-2 for the liq./pore surface (ps) and α-type/ps, and Δγα/β = 3.17 mJ m-2 for α-type/ps and β-type/ps, resp. QENS expts. substantiate the differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile mols. are anchored so as not to translate. The two Lorentzian anal. of QENS spectra for cc-acetonitriles showed translational motion but markedly slowed. However, the activation energy for cc-acetonitrile in MCM-41 (C18) is 7.0 kJ mol-1 compared to the bulk value of 12.7 kJ mol-1. The relaxation times for tumbling rotational diffusion of cc-acetonitrile are similar to bulk values.
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    83. 83
      Nivarthi, S. S.; McCormick, A. V.; Davis, H. T. Diffusion Anisotropy in Molecular Sieves. Chem. Phys. Lett. 1994, 229, 297– 301,  DOI: 10.1016/0009-2614(94)01059-5
      There is no corresponding record for this reference.
    84. 84
      Natraj, A.; Ji, W.; Xin, J.; Castano, I.; Burke, D. W.; Evans, A. M.; Strauss, M. J.; Ateia, M.; Hamachi, L. S.; Gianneschi, N. C.; ALOthman, Z. A.; Sun, J.; Yusuf, K.; Dichtel, W. R. Single-Crystalline Imine-Linked Two-Dimensional Covalent Organic Frameworks Separate Benzene and Cyclohexane Efficiently. J. Am. Chem. Soc. 2022, 144, 19813– 19824,  DOI: 10.1021/jacs.2c07166
      84
      Single-Crystalline Imine-Linked Two-Dimensional Covalent Organic Frameworks Separate Benzene and Cyclohexane Efficiently
      Natraj, Anusree; Ji, Woojung; Xin, Junjie; Castano, Ioannina; Burke, David W.; Evans, Austin M.; Strauss, Michael J.; Ateia, Mohamed; Hamachi, Leslie S.; Gianneschi, Nathan C.; ALOthman, Zeid A.; Sun, Junliang; Yusuf, Kareem; Dichtel, William R.
      Journal of the American Chemical Society (2022), 144 (43), 19813-19824CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Two-dimensional (2D) covalent org. frameworks (COFs) are composed of structurally precise, permanently porous, layered macromol. sheets, which are traditionally synthesized as polycryst. solids with cryst. domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temp. and ambient pressure. Single crystals of two imine-linked 2D COFs were prepd., consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4μm and 450 nm to 20μm in width, resp. High-resoln. transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-cryst. domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-cryst. in the a-b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatog. sepn. columns and exhibited unusual selective retention of cyclohexane over benzene, with single-cryst. TAPPy-PDA significantly outperforming single-cryst. TAPB-DMPDA. Polycryst. TAPPy-PDA exhibited no sepn., while polycryst. TAPB-DMPDA exhibited poor sepn. and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, cryst. structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for mol. sepns.
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    85. 85
      Sinnaeve, D. The Stejskal-Tanner Equation Generalized for Any Gradient Shape-an Overview of Most Pulse Sequences Measuring Free Diffusion. Concepts Magn. Reson. Part A 2012, 40A, 39– 65,  DOI: 10.1002/cmr.a.21223
      85
      The Stejskal-Tanner equation generalized for any gradient shape-an overview of most pulse sequences measuring free diffusion
      Sinnaeve, Davy
      Concepts in Magnetic Resonance, Part A: Bridging Education and Research (2012), 40A (2), 39-65CODEN: CMRPC2; ISSN:1546-6086. (Wiley-Blackwell)
      A review. The widely known Stejskal-Tanner (ST) equation is central to any diffusion NMR or so-called pulsed gradient spin-echo expt., describing the signal attenuation due to loss of coherence caused by diffusion as a function of the exptl. parameters. What is less widely known is that the equation itself is not invariable when applying different pulse sequences or different gradient pulse shapes and should thus be modified accordingly. This concept is not new, but nevertheless experimentalists applying diffusion NMR in their research often overlook or are completely oblivious to this fact. In this article, the derivation of the ST equation through the Bloch-Torrey equations is discussed in detail, followed by a discussion of the most basic NMR expts. that measure free diffusion. This derivation is performed here in a novel way, leading to expressions for the ST equation that do not assume any gradient shape beforehand, leaving only a few parameters that are detd. solely by the gradient pulse shape. This new approach to presenting the ST equation increases awareness of its dependence on gradient shape. Moreover, it relieves designers of future diffusion NMR pulse sequences of struggling with the cumbersome task of deriving and reporting the equation for each gradient shape sep. A quick ref. table of the ST equation for the basic diffusion NMR pulse sequences for any gradient shape is given, as well as an overview for most other diffusion NMR pulse sequences described in the literature. © 2012 Wiley Periodicals, Inc. Concepts Magn Reson Part A 40A: 39-65, 2012.
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    86. 86
      Jerschow, A.; Müller, N. Suppression of Convection Artifacts in Stimulated-Echo Diffusion Experiments. Double-Stimulated-Echo Experiments. J. Magn. Reson. 1997, 125, 372– 375,  DOI: 10.1006/jmre.1997.1123
      86
      Suppression of convection artifacts in stimulated-echo diffusion experiments. Double-stimulated-echo experiments
      Jerschow, Alexej; Muller, Norbert
      Journal of Magnetic Resonance (1997), 125 (2), 372-375CODEN: JMARF3; ISSN:1090-7807. (Academic)
      A pulse sequence is described which suppresses the effects of convection in stimulated-echo (STE) diffusion expts. to first-order provided that the convection current has a const. laminar flow profile during the diffusion interval of the pulse sequence. Turbulent convection cannot be compensated for using this method.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.3c12167.

    • Discussions of methods and equipment used, synthetic procedures, FT-IR spectra, XRPD data and structure refinements, (pulsed field gradient) 1H NMR spectra, nitrogen gas sorption isotherms, pore-size distributions, BET plots, and acetonitrile vapor adsorption isotherms (PDF)


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