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Distribution and Mobility of Amines Confined in Porous Silica Supports Assessed via Neutron Scattering, NMR, and MD Simulations: Impacts on CO2 Sorption Kinetics and Capacities
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Accounts of Chemical Research

Cite this: Acc. Chem. Res. 2023, 56, 19, 2620–2630
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https://doi.org/10.1021/acs.accounts.3c00363
Published September 18, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Conspectus

Solid-supported amines are a promising class of CO2 sorbents capable of selectively capturing CO2 from diverse sources. The chemical interactions between the amine groups and CO2 give rise to the formation of strong CO2 adducts, such as alkylammonium carbamates, carbamic acids, and bicarbonates, which enable CO2 capture even at low driving force, such as with ultradilute CO2 streams. Among various solid-supported amine sorbents, oligomeric amines infused into oxide solid supports (noncovalently supported) are widely studied due to their ease of synthesis and low cost. This method allows for the construction of amine-rich sorbents while minimizing problems, such as leaching or evaporation, that occur with supported molecular amines.

Researchers have pursued improved sorbents by tuning the physical and chemical properties of solid supports and amine phases. In terms of CO2 uptake, the amine efficiency, or the moles of sorbed CO2 per mole of amine sites, and uptake rate (CO2 capture per unit time) are the most critical factors determining the effectiveness of the material. While structure–property relationships have been developed for different porous oxide supports, the interaction(s) of the amine phase with the solid support, the structure and distribution of the organic phase within the pores, and the mobility of the amine phase within the pores are not well understood. These factors are important, because the kinetics of CO2 sorption, particularly when using the prototypical amine oligomer branched poly(ethylenimine) (PEI), follow an unconventional trend, with rapid initial uptake followed by a very slow, asymptotic approach to equilibrium. This suggests that the uptake of CO2 within such solid-supported amines is mass transfer-limited. Therefore, improving sorption performance can be facilitated by better understanding the amine structure and distribution within the pores.

In this context, model solid-supported amine sorbents were constructed from a highly ordered, mesoporous silica SBA-15 support, and an array of techniques was used to probe the soft matter domains within these hybrid materials. The choice of SBA-15 as the model support was based on its ordered arrangement of mesopores with tunable physical and chemical properties, including pore size, particle lengths, and surface chemistries. Branched PEI─the most common amine phase used in solid CO2 sorbents─and its linear, low molecular weight analogue, tetraethylenepentamine (TEPA), were deployed as the amine phases. Neutron scattering (NS), including small angle neutron scattering (SANS) and quasielastic neutron scattering (QENS), alongside solid-state NMR (ssNMR) and molecular dynamics (MD) simulations, was used to elucidate the structure and mobility of the amine phases within the pores of the support. Together, these tools, which have previously not been applied to such materials, provided new information regarding how the amine phases filled the support pores as the loading increased and the mobility of those amine phases. Varying pore surface-amine interactions led to unique trends for amine distributions and mobility; for instance, hydrophilic walls (i.e., attractive to amines) resulted in hampered motions with more intimate coordination to the walls, while amines around hydrophobic walls or walls with grafted chains that interrupt amine-wall coordination showed recovered mobility, with amines being more liberated from the walls. By correlating the structural and dynamic properties with CO2 sorption properties, novel relationships were identified, shedding light on the performance of the amine sorbents, and providing valuable guidance for the design of more effective supported amine sorbents.

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Published as part of the Accounts of Chemical Research special issue “Opportunities and Challenges of Nanomaterials in Sustainability: Pursuing Carbon Neutrality”.

Key References

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  • Holewinski, A.; Sakwa-Novak, M. A.; Jones, C. W. Linking CO2 sorption performance to polymer morphology in aminopolymer/silica composites through neutron scattering. J. Am. Chem. Soc. 2015, 137, 11749–11759. (1) This article illustrates how small-angle neutron scattering (SANS) was used as an effective tool for characterizing PEI morphologies in SBA-15 and the impacts of the morphologies on CO2 uptake performance.

  • Holewinski, A.; Sakwa-Novak, M. A.; Carrillo, J.-M. Y.; Potter, M. E.; Ellebracht, N.; Rother, G.; Sumpter, B. G.; Jones, C. W. Aminopolymer mobility and support interactions in silica–PEI composites for CO2 capture applications: A quasielastic neutron scattering study. J. Phys. Chem. B 2017, 121, 6721–6731. (2) This article demonstrates that QENS is a robust method for quantitatively analyzing diffusive dynamic properties of mesopore confined PEI and the effect of the attraction of the PEI toward the pore walls.

  • Moon, H. J.; Carrillo, J.-M. Y.; Leisen, J.; Sumpter, B. G.; Osti, N. C.; Tyagi, M.; Jones, C. W. Understanding the impacts of support-polymer interactions on the dynamics of poly(ethylenimine) confined in mesoporous SBA-15. J. Am. Chem. Soc. 2022, 144, 11664–11675. (3) This article presents combining neutron scattering and solid-state NMR studies to yield comprehensive understanding of polymer mobility in SBA-15 under varied wall–polymer interaction conditions.

  • Moon, H. J.; Sekiya, R.; Jones, C. W. Probing the morphology and mobility of amines in porous silica CO2 sorbents by 1H T1-T2 relaxation correlation NMR. J. Phys. Chem. C 2023, 127, 11652–11665. (4) This article describes that 1H relaxation time measurements can capture morphology and mobility of amines with different chain topologies as well as varied pore fill fraction.

1. Introduction

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Solid-supported amines are among the most widely studied CO2 adsorbents and are the backbones of many emerging CO2 separation applications. Many types of solid supported amines exist─MOFs with pendant amines tethered to organic ligands or metal centers, (5−9) porous polymers or COFs containing backbone or pendant amines, (10−13) as well as porous solid supports on which (or in which) amines are incorporated. (14−17) One widely studied class of supported amine materials is mesoporous oxide supports (i.e., silica, alumina, mixed oxides, etc.) functionalized with organic moieties containing alkyl amine groups. (15,17−22) With these supports, amines can be either covalently bound to the pore surfaces (15,18−22) or physically contained in the pores. (17) While covalent bonding yields excellent stability to moderate temperature swings and exposure to humidity, physical confinement of amines enables simpler syntheses and often, higher CO2 uptakes due to higher amine loadings compared with sorbents containing only chemically grafted amines (for grafting, amine loading scales with surface area, whereas for amine impregnation, amine loading scales with mesopore volume), (23) with potential limitations around leaching or evaporation of amines under suboptimal conditions. These sorbents are often used in temperature swing adsorption (TSA) cycles or temperature vacuum swing adsorption (TVSA) cycles, making the volatility of the amine component an important parameter for sorbents based on noncovalent bonding of amines to supports. Indeed, many classical amine solvents (e.g., monoethanolamine, diethanolamine, etc.) and low molecular weight amine oligomers (e.g., triethylenetetramine (TETA), tetraethylenepentamine (TEPA), etc.) are likely too volatile to be effectively deployed in solid sorbents at ambient and higher temperatures, despite numerous academic studies exploring such materials.
A representative example of this type of supported amine sorbent is branched poly(ethylenimine) (PEI) (MW ∼ 800 g/mol) physically impregnated into the pores of ordered mesoporous SBA-15 silica (PEI/SBA-15) (Figure 1a). While more scalable, practical sorbents use disordered mesoporous supports, ordered supports like SBA-15 and MCM-41 facilitate fundamental scientific investigations, as described in this article. Song and co-workers (17) discovered that inclusion of aminopolymers in porous silica supports exhibited promising CO2 uptake performance. However, they observed an unusual, antithermodynamic behavior of the materials, where CO2 uptake initially increased with temperature before reducing upon further heating. The authors attributed this to diffusion limitations imposed by the polymer at low temperatures that were alleviated by adding heat to the system. When polymer chains were given sufficient mobility to allow for suitable CO2 mass transfer, a further temperature increase resulted in the expected thermodynamic behavior. Thus, from the earliest studies on these materials, the mobility of the supported amines within the mesopores of the support was recognized as an important factor. Further research on those materials aimed to improve their CO2 uptake properties by varying pore sizes, (24−26) leaving behind surfactants (e.g., CTAB) (24−26) around pore walls, using additives (e.g., PEG) (27−29) or smaller, molecular amines (e.g., diethanolamine), (30,31) and using combinations of grafted amines together with physically impregnated amines. (32,33) Researchers observed varied gas capture performance under these different circumstances. In many cases, the authors suggested that a more favorable distribution and motions of amines were the main driving forces behind the improved performance.

Figure 1

Figure 1. Schematic structure and the CO2 capture mechanism in PEI/SBA-15. (a) Schematic showing PEI/SBA-15. (b) CO2 diffusion pathways reaching amine groups and interaction with amines resulting in CO2 sorption. (16,42,43) CO2–amine interaction reprinted with permission from ref (16). Copyright 2011 Royal Society of Chemistry.

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During CO2 capture on such supported amines, CO2 molecules initially enter the pore openings of the solid materials, diffuse through the available free volume, reach amine sites, and then interact with the amines through adsorption or chemical reaction (Figure 1b). (34) As CO2 adsorption consumes amine sites, CO2 must diffuse through the amine compounds within the pores to access additional free amine sites. However, this diffusion process is often slower due to mass transfer barriers imposed by the amine polymer. Studies of the dynamics of CO2 adsorption on these materials show two distinct sorption regimes; an initial rapid phase followed by a slower approach to pseudoequilibrium. (35) When CO2 reacts with amine groups under dilute (≤10% CO2) or ultradilute conditions (≤1% CO2), it typically reacts to form carbamic acid species (secondarily) and alkyl ammonium carbamates (primarily). (17,18) The formation of carbamates involve the deprotonation of a carbamic acid species by an adjacent amine. (36,37) Carbamates are thermodynamically favored, and most amine sorbents that display high CO2 uptake capacities adsorb CO2 primarily as carbamates. Carbamate formation has the potential to cross-link amine chains due to the amine:CO2 stoichiometry (2:1), which can rigidify the organic phase. This phenomenon has been hypothesized to be a key driver of the slow approach to equilibrium observed in experimental studies. (38−40) Under humid conditions, CO2 uptakes often improve, and this has been associated with an additional CO2 adsorption manifold, bicarbonate formation, (41) which has 1:1 amine:CO2 stoichiometry, and also to enhanced polyamine mobility due to water disrupting hydrogen bonding networks. One gauge of the efficiency of these materials is amine efficiency (AE), defined as the amount of CO2 captured per total amine loading, typically represented by mmol of CO2/ mmol of N. The interactions between CO2 and amines mentioned above lead to the theoretical AE maxima ranging from 0.5 (when forming ammonium carbamate) to 1.0 (bicarbonate or carbamic acid).
Supported amines typically exhibit limited AE, reaching up to ∼0.2 under dry 400 ppm of CO2, simulating DAC. (23,44) This value is far from the theoretical maxima (0.5). Rationalizing these behaviors requires knowledge of the local structure of the polyamines within the support pores as well as insight into amine mobility, including center of mass diffusion, local chain mobility, and bond dynamics. Unfortunately, conventional laboratory characterization techniques for these hard/soft composite materials predominantly provide information about the hard oxide domains. The PEI used in these materials is low molecular weight and exhibits a honeylike consistency under ambient conditions. While researchers have systematically altered key structural aspects of the support such as average support pore diameter, support pore volume and support channel length and particle size, (45−48) limited insight is available regarding the structure and dynamics of the supported, low MW branched PEI phase. Some empirical knowledge has been gathered correlating CO2 uptake behavior to the structure of aminopolymers with varied structures (e.g., branched vs linear, MW, grafted PEI), (49−52) but little is known about how these polymers fill the pores or move within the pores. To address this gap, we employed techniques from the soft matter community to investigate supported amine adsorbents with the goal of directly probing the polymer domains in these composite materials.

2. Model Supported Amine CO2 Sorbents and Techniques to Understand Physical Properties of Sorbent Materials

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There are numerous practical and scalable supported amine CO2 sorbents that utilize mesoporous oxide supports with a limited structural order. However, to understand the structural and dynamic properties of supported amines, well-defined support materials are necessary. SBA-15 silica consists of regular arrays of mesopores, and literature synthesis protocols were used to ensure well-controlled mesopore sizes and particle morphologies.
Direct characterization of solid-supported PEI can be effectively achieved using experimental tools, such as neutron scattering (NS) and solid-state NMR (ssNMR). To aid in the interpretation of the experimental findings, we utilized coarse-grained molecular dynamics (MD) simulations. These approaches enable characterization of various important PEI properties, as listed in Table 1. Detailed discussions about the techniques and methods can be found in the following examples.
Table 1. Approaches Used for Understanding PEI Properties within Mesoporous Silica
ApproachCharacteristicsAccessible PEI propertiesScales (length, time)
Small-angle neutron scatteringPenetrable, contrast (PEI vs silica)Morphology0.1–1000 nm (53,54)
Quasielastic neutron scatteringPenetrable, selective (PEI mainly)Mobility (center-of-mass or segmental diffusion)1 ps–1 ns (55,56)
Solid-state NMR (1H relaxation)Penetrable, selective (isotopes, spins)Morphologya and mobilitya0.1–100 nm (57)
ns–ms (42)
MD simulationPredict PEI behavior, crosscheck NS and ssNMR resultsMorphology and mobility0.1–100 nm
1 ps–1000 ns (58)
a

ssNMR yields qualitative information and trends, whereas NS and MD simulation give quantitative results.

3. Distribution and Mobility of Amines Supported in SBA-15 Explored via NS, ssNMR, and MD Simulation

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3.1. Structure and Distribution of PEI in Mesoporous SBA-15 Silica

The limited AE of PEI/SBA-15 systems may originate from the physical properties of confined amines, and one contributing factor may be the unfavorable distribution of amines within the mesopores. Branched PEI contains different types of amine groups, with primary amines dispersed in the peripheral region of PEI domains and tertiary amines hidden in the core. These domains, containing hydrophilic and basic amine groups, may strongly coordinate with the hydrophilic parts of the pore walls of the solid support. SBA-15 surfaces contain slightly acidic silanol groups, suggesting possible amine-silanol interactions that attract PEI toward the pore walls. This attraction may result in loss of the ability of wall-coordinating amine groups to capture CO2. Our observations support this interpretation, as PEI/SBA-15 with low amine loadings showed limited AE (∼0.05 mol CO2/mol N), whereas sorbents with intermediate or high amine loadings yielded much higher AE (0.1–0.15), with a smaller fraction of total PEI anchored to the pore walls. (1,2,4) Therefore, we assert that amines closely located to the support surface are not effective sorption sites for CO2 capture, and the amount of available active amines is governed by the morphology of PEI within the support. Hence, understanding the amine structure and morphology at different amine loadings is crucial.
Small-angle neutron scattering (SANS) is a technique well-suited for probing the structural properties of PEI/SBA-15 systems for two key reasons. First, neutrons can effectively pass-through condensed silica matter and reach the soft PEI. (53) Second, we can leverage differences in the effectiveness of interactions with incident neutrons, known as “contrast,” to enhance scattering from specific domains. By using deuterated PEI (dPEI), a significant difference in neutron scattering length density (SLD) can be achieved (dPEI ∼ 8.2, SiO2 ∼ 3.5, units: 10–6 Å–2). (59) Maximizing coherent neutron scattering (53,60,61) through deuteration also helps acquire SANS spectra with suitable signal-to-noise ratio.
We sought to understand how PEI fills the mesopores of SBA-15 and used neutron scattering to generate this understanding. SANS spectra of dPEI/SBA-15 with varying amounts of dPEI were obtained. Upon introduction of dPEI to the silica mesopores, noticeable changes were observed in the ratios of the silica diffraction peaks [10], [11], and [20]. These changes were attributed to changes of the SLD distribution around the wall-polymer interface and within the previously evacuated pore spaces (Figure 2a). The spectra were subsequently compared to theoretical models, considering various contributions, such as particle surface scatter (Porod’s law), the form factor of a single mesopore, a structure factor that is related to the arrangement of repeated structures of the mesopores, and diffuse scattering from structural inhomogeneities (Figure 2b). This analysis allowed the estimation of key structural aspects such as (i) morphology of PEI within mesopores of SBA-15, (ii) the characteristic lengths of different domains, and (iii) the probable structures of PEI around the wall-polymer interface.

Figure 2

Figure 2. Structural characteristics of PEI/SBA-15 captured via SANS. (a) SANS spectra (intensity vs scattering vector Q) of bare SBA-15 support and PEI/SBA-15 composites with varied PEI loadings, with diffraction peaks highlighted. (b) Key structural features captured via SANS, such as particle surface scatter, form factor of a mesopore, structure factor showing arrangement of mesopores, and structural inhomogeneities. (53,62) (c) PEI morphologies considered, where i represents consistent PEI deposition on the pore walls, ii represents formation of PEI plugs in the pores, and iii indicates mixed cases of the formers. (d) Possible PEI morphologies around the wall–PEI interfaces with different extents of surface roughness. Particle length of SBA-15 is approximately 1 μm. (1) Reproduced with permission from ref (1). Copyright 2015 American Chemical Society.

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We first focused on understanding how PEI fills the mesopore space. Three different types of models were considered (Figure 2c)─wall coating dominant phases, plug dominant phases, and mixed cases. Among these morphologies, the best fits were associated with PEI initially forming conformal coating layers (referred to as pore-coating PEI) around the pore walls, occupying up to ∼20 vol % of the pore. As more PEI was added, it subsequently formed aggregates within the pores. By analysis of the characteristic lengths of the pore-coating and aggregate PEI domains, SANS curve fitting yielded plausible thicknesses of these domains, with the pore-coating thickness roughly 10–15 Å. Furthermore, by calculating the SLD values of the wall–PEI interfaces (i.e., called the “corona,” associated with silica surface roughness, Figure 2d), it was possible to estimate the likely structure of PEI around the wall–PEI interfaces. In supports with relatively rough surfaces, a clear increase in the SLD was observed due to the formation of a PEI-rich layer (Figure 2d, upper case). On the other hand, SBA-15 with relatively smooth pore surfaces (Figure 2d, lower case) had relatively lower SLD values, indicating a lower volume fraction of PEI at the interfaces compared to rough surfaces. (38)

3.2. Mobility of Confined PEI in SBA-15 and Effects of Wall–PEI Interactions (2,3)

Attractive interactions between the pore wall and polymer may also affect amine mobility, which can further affect the CO2 diffusivity. To probe PEI mobility, we conducted quasielastic neutron scattering (QENS) experiments on PEI/SBA-15 systems with two distinctly different types of PEI–wall interactions. In one system, the pore walls possessed native silanol groups, which are slightly acidic and strongly interact with the basic amine groups of PEI. In the second system, the pore surfaces were capped with hexamethyldisilazane (HMDS), resulting in the presence of trimethylsilyl groups that repel the hydrophilic PEI end groups. QENS enabled us to experimentally explore the dynamics of PEI, with incoherent neutron scattering providing data that could be analyzed to determine self-correlation functions related to the dynamics of the PEI. Hydrogen has a significantly larger extent of incoherent scattering compared to other nuclei present in PEI/SBA-15 systems (i.e., C, N, Si, and O), and therefore, using PEI with the usual natural abundance of 1H (∼99.99%) was beneficial to the study of PEI dynamics.
Different amounts of PEI were loaded in SBA-15 targeting sorbent materials with different fractions of pore-coating and aggregate PEI. Both native silanol-containing walls and trimethylsilyl-capped supports were used, producing different wall–PEI interactions. QENS experiments were conducted in two modes: first, yielding the mean-square displacement (MSD) as a function of temperature (Figure 3a) and then producing QENS spectra (Figure 3b). The notation used here includes P100 for nonconfined free PEI, P20 representing a composite with 20 wt % PEI mostly representing wall-coating PEI, P40 denoting a composite with 40 wt % PEI consisting of both aggregates and wall-coating PEI, and P40H indicating a composite with 40 wt % PEI and hydrophobized pore walls.

Figure 3

Figure 3. Characterization of PEI mobility using QENS and impacts on CO2 uptake. (a) Mean-square displacement as a function of temperature taken by QENS, showing trend of PEI mobility. (b) QENS spectra taken at 375 K with the momentum transfer (Q) of 0.87 Å–1 (normalized intensities vs energy transfer). (c) QENS spectral widths (half-width at half-maximum; HWHM) fitted against jump-mediated diffusion model. (d) Amine efficiencies as a function of volumetric fill fraction of PEI/SBA-15, showing relationship between PEI mobility and CO2 uptake. (2) Reproduced with permission from ref (2). Copyright 2017 American Chemical Society.

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The MSD as a function of temperature shown in Figure 3a shows a greater extent of diffusive motions (i.e., larger MSD at given temperatures) for PEI as the sorbents are loaded with larger amounts of PEI aggregates (P20 vs P40) and under conditions with reduced attraction to the pore walls (P40 vs P40H). Consistent with the MSD versus temperature plot, the QENS spectra (Figure 3b) showed varying degrees of broadening for different materials (where broader spectra indicate faster motions), which are related to the different diffusive mobilities of PEI in each case. Figure 3c demonstrates extracted spectral widths that were fitted to a jump-mediated diffusion model, eventually yielding dynamic parameters, such as the time scale, jump lengths, and diffusivity (Table 2). PEI mobility was shown to correlate with the effectiveness of the CO2 sorbents. It was observed that improved PEI mobility yielded a better AE (Figure 3d). Once the pore fill fraction reached a certain extent, further improved PEI mobility did not yield better CO2 sorption performance, probably due to limited CO2 diffusivity through PEI aggregates located far from the pore walls.
Table 2. Characteristics of Center-of-Mass Diffusive Motions of PEI (2)
 ⟨L2⟩1/2 (Å)τ (ns)D (×10–11 m2/s)
T (K)325350375325350375325350375
P100 (bulk PEI)3.72.02.10.290.090.067.07.07.0
P20 (20 wt % PEI)  3.2  0.22  9.4
P40 (40 wt % PEI)6.96.67.90.260.230.22323232
P40H (40 wt % PEI)9.24.36.50.270.170.193.80.531.3
Reproduced with permission from ref (2). Copyright 2017 American Chemical Society.
We further diversified the range of PEI–wall interactions and investigated the impact of different silica surface treatments on the PEI mobility. Silanes with varying end groups were grafted on the pore walls to modulate the PEI–wall interactions (Figure 4a). The mobility of PEI in 40 wt % PEI loaded composites (P40Cl, P40SiOH, P40NH2, P40CH3) was probed by QENS, where we hypothesized that less attractive PEI–wall interactions would give rise to faster PEI motions (i.e., P40Cl showing the slowest motions, as this functional group can form covalent bonds with the PEI, followed by P40SiOH, P40NH2, with P40CH3 giving the fastest motions). Upon analysis of QENS spectra, this hypothesis was refuted (Figure 4b). P40NH2 displayed faster motions than P40CH3, for example. The data led to a new hypothesis that the conformation of the wall-grafted chains may differ depending on the end groups (e.g., hydrophilic NH2 vs hydrophobic CH3) (Figure 4c). This hypothesis was tested by ssNMR 1H T1–T2 relaxation correlation measurements from which we determined plausible chain configurations for the wall-grafted chains (Figure 4d). The proton relaxation correlation for the PEI domains close to the silica support walls also supported the hypothesized structures. T1–T2 plots for 20 wt % PEI, which represent PEI around the pore walls, suggested that alkylamine groups on the walls effectively cap hydrophilic area of the walls, blocking PEI–wall interactions and creating a continuous distribution of T2 (i.e., effective 1H spins’ dipolar coupling) (Figure 4e). On the other hand, hydrophobic chains grafted on the walls did not coordinate closely to hydrophilic residues of the walls, and PEI–wall interactions were less affected compared to P20NH2. That in turn created an energy penalty when PEI located close to walls (and surrounded by hydrophobic alkyl chains) moved diffusively. The discontinuous T2 signal contributions suggested the existence of diffusion barriers in the PEI domains adjacent to the silica walls (Figure 4e). Additionally, the density distribution calculated by MD simulations reflecting the interplay between wall-grafted chains and the silica surface (including hydrophilic portions such as silanols) captured a similar trend, as shown in the T1–T2 NMR (Figure 4f, dotted lines). The solid lines representing the population of periphery primary amines of branched PEI showed subtle differences in the P40CH3 and P40NH2 materials at r ∼ 11σ, suggesting that the PEI distribution was also affected (Figure 4f, solid lines).

Figure 4

Figure 4. Impacts of wall–PEI interactions in PEI mobility and distribution. (a) Different types of wall–PEI interactions. (b) Trend of PEI mobility understood by analyzing the spectral widths of QENS spectra (curve fitting done by jump-mediated diffusion). (c) Hypothesized configurations of wall-grafted alkylamine chains and alkyl chains and PEI distribution around pore walls. (f) 1H T1–T2 correlation plots for wall-grafted chains. (e) 1H T1–T2 plots for PEI around pore walls. (f) Density distribution function calculated by MD simulation (probability vs distance, where r ∼ 11 refers to the walls), where dotted lines represent wall-grafted chains and solid lines represent primary amine groups of PEI. (g) Comparison of CO2 uptake rates based on fractional uptake versus time and underlying causes. Reproduced with permission from ref (3). Copyright 2022 American Chemical Society.

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The effects of PEI mobility were then interpreted in terms of the CO2 uptake rates (Figure 4g). The data show that the macroscopic structures described above affected the initial uptake rates. The presence of PEI coating the external surface of the SBA-15 particles rapidly captured CO2 (Figure 4g, P40Cl), and faster uptake rates were observed for the sorbents that had more free volume within the mesopores (Figure 4g, P40SiOH vs P40CH3 and P40NH2). For materials with similar macroscopic structures and extents of free volume in the mesopores, the CO2 uptake was faster in the sorbents having faster PEI mobilities (Figure 4g, P40CH3 vs P40NH2).

3.3. Behavior of Supported Amines as a Function of Chain Topologies (4)

The literature shows that oligomeric or polymeric amines with different chain topologies show distinct CO2 capture behavior. Among widely used amines, TEPA and PEI share similar chemical identities (i.e., amine groups separated by ethylene spacers) but exhibit discernible behavior in CO2 sorption when supported in SBA-15. TEPA generally shows faster uptake but less stability of repeated cycling. These differences are ascribed to their unique chain topologies. TEPA has short (MW ∼ 200 g/mol) and relatively linear structures, (63,64) whereas PEI has larger MW (∼800 g/mol) with highly branched structures.
To probe the structure and mobility of TEPA and PEI supported in SBA-15 and link their physical properties to their gas sorption behavior, we conducted 1H T1–T2 correlation NMR studies on TEPA/SBA-15 and PEI/SBA-15 with varied weight loadings of amine. Figure 5a and 5b shows the T1–T2 correlation plots along with expected amine distributions and amine mobility trends. Considering the 20TEPA and 20PEI materials (both ∼20 wt % amines and ∼40% pore filling), TEPA showed a more diffuse signal for the pore-coating domain compared to PEI. These differences can be related to their unique chain topologies. Branched PEI, with multiple flexible arms, sticks to the silica surfaces via multidentate interactions, while the short and linear TEPA likely adopts only monodentate or bidentate binding. When comparing the cases with intermediate pore fill fractions (∼60%; 45TEPA, 35PEI), the signal linked to the pore-coating TEPA showed notably increased T2 values with decreased T1, suggesting faster mobility. On the other hand, the pore-coating signal appearing in 35PEI did not show significant T1 and T2 shifts from the lower amine loading case. We suggest that a significant jump in wall-bound TEPA mobility may arise from the interactions between mobile TEPA molecules that are less engaged in wall–TEPA interactions (TEPA aggregates) and TEPA on the walls. The data suggest that mobile TEPA can at least partly detach wall-bound TEPA chains and increase the mobility of the wall domains. However, PEI molecules having multiple arms bound to the walls may have a larger barrier to such enhanced mobility. Lastly, when considering the composites almost completely loaded with amines (70TEPA and 60PEI, pore fill fraction ∼100%), a significant jump in molecular mobility in both the TEPA and PEI was observed. This unexpected finding suggests that pores that are nearly filled with amines give enhanced spin coupling and spin diffusion, indicative of enhanced molecular mobility.

Figure 5

Figure 5. Morphology and mobility of amines with different chain topologies probed by 1H T1–T2 NMR and relationship to CO2 sorption behavior. (a) 1H T1–T2 plots for TEPA/SBA-15 with varied loadings. (b) T1–T2 plots for PEI/SBA-15 with matching pore fill fraction to TEPA cases. (c) CO2 sorption results shown by fractional uptake, pseudoequilibrium capacities, and amine efficiencies. (d) CO2 uptake curves at early stages and the initial uptake rates calculated in the semilinear region approximately 0–10 min. (4) Reproduced with permission from ref (4). Copyright 2023 American Chemical Society.

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The qualitative information about amine distribution and dynamics taken from NMR was then correlated to the CO2 uptake of the sorbents. Figure 5c shows the fractional CO2 uptake curves and AEs. The cases with lower amine content (20TEPA and 20PEI) showed considerably lower AE compared to other cases. This can be explained as noted above, with pore-coating layers resulting in buried amines toward the pore walls. (1) Compared with low amine loadings, materials with intermediate and high amine loadings (45–70TEPA and 35–60PEI) showed much higher AE, presumably caused by more PEI in aggregates. In the case of PEI, further increases in the aggregate content (60PEI, pore-occluded case) did not return better AE. In this case, CO2 mainly diffuses through fully amine-packed pores, where the formation of ammonium carbamate further reduced the molecular mobility of PEI by cross-linking amine groups. In contrast, 70TEPA showed a remarkably larger AE, likely due to the greater extent of molecular mobility and less pronounced effect of CO2-induced cross-linking. Lastly, Figure 5d shows CO2 uptake curves over the early sorption period and initial uptake rates. Comparing lower and intermediate loadings, we found that faster amine mobility correlates to faster CO2 uptake. However, sorbents fully loaded with amines showed slower uptake due to the congestion of the pores.

3.4. Prediction of Molecular Behavior of Amines via MD Simulation (65)

To aid in the interpretation of the NS and ssNMR results mentioned earlier, we used coarse-grained molecular dynamics simulations at the bead–spring level. In this approach, a single Lennard-Jones bead represents a PEI monomer, and the interactions among beads are chosen to mimic hydrophobic and hydrophilic interactions by adjusting the cutoff and the energy well-depth, ε, of the shifted and truncated Lennard-Jones potential, which describes the interaction between a pair of beads. A larger value of ε represents a stronger attraction, while a cutoff at the minimum of the potential represents purely repulsive interactions. The probabilities of selective adsorption of beads toward a bead belonging to the mesoporous support can also be estimated by comparing ε using the Boltzmann factor. The simulation was designed to incorporate a strong attraction between silanols and the primary amines, while the model retains the branched architecture of the PEI.
Despite the primitive nature of the model and the lack of specific chemical details, the simulation approach could describe the structure and dynamics of the adsorbed chain by calculating the density distributions and dynamic structure factors at different length and time scales. This description shows reasonable agreement with the results of SANS and QENS experiments. In particular, the simulations observe the conformal coating and aggregation of the polymers at mesopores, which can be directly linked to the fast and slow segmental dynamics of the chain. (66) This approach allows for the investigation of the morphology of the support, ranging from cylinders to double gyroids. (65) By analysis of the dynamics of the chains in the simulation, a correlation between the dynamics of the chain and the mesopore morphology was elucidated.

4. Conclusions and Outlook

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Solid-supported amines are a promising platform for CO2 capture with the potential to enable negative emission technologies by maximizing mass transfer and CO2 affinity toward sorbent materials. Among diverse materials available, amine oligomers physically loaded in mesoporous supports are scalable and robust systems. Researchers have explored the role of support and amine composition and structure on the CO2 uptake performance, introduced additives, humidity, and altered operating conditions to gather new knowledge and promote understanding of these materials. However, less is known about the physical structure and mobility of the soft supported amine phase.
We chose PEI/SBA-15 systems as model sorbents due to their regular structure and the extensive literature data on these materials. Through a series of NS, ssNMR, and MD simulation studies, we learned that PEI/SBA-15 and similar amine/SBA-15 composites yield different amine structures and domains depending on the loading with some compositions dominated by an unfavorable distribution of amines. Amines impregnated into the solid supports form pore-coating domains driven by (typically) attractive wall-amine interactions. Consequently, a portion of the amines becomes nearly inactive toward CO2 capture, as they are tied around the pore walls. We further learned that attractive interactions between the wall and amines hamper amine mobility, thereby affecting CO2 diffusion through amine-loaded phases. Third, the presence of grafted chemical moieties on the pore walls brought about complex interplay between the native silica walls, chemically tethered groups, and PEI chains, which affected the amine structure and mobility and hence the CO2 sorption. Fourth, we observed that amines with different chain topologies exhibit unique distributions and mobility patterns that influence the diffusive properties of the amine molecules as well as CO2. Here are some key design principles based on the findings mentioned in this paper. First, retaining enough pore volume is crucial for the diffusion of CO2. Second, lessening the extent of pore-coating domains or making them easily turn from the walls can result in better CO2 uptake. This can be done by tuning the supports’ structures (e.g., structured supports with less extent of surface area per pore volume), (47,48) pore surface chemistries, and the structures of aminopolymers. However, a careful approach should be made─too loosely bound amines or fast-moving amines may lessen materials’ stability.
We anticipate that the systematic experimental and simulation approach presented in this account will contribute to the formulation of more nuanced structure–property relationships, enabling a deeper understanding of these materials. We further envision that additional studies aimed at understanding the structures and dynamics of these materials will eventually lead to the development of a theoretical model or predictive framework for sorption and diffusion that can accelerate materials development.

Author Information

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  • Corresponding Author
  • Authors
    • Hyun June Moon - School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
    • Jan Michael Y. Carrillo - Center of Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States
  • Funding

    This work was supported by UNCAGE-ME, a U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) Energy Frontier Research Center, under award no. DE-SC0012577. H.J.M. was additionally supported by Kwanjeong Educational Foundation.

  • Notes
    The authors declare the following competing financial interest(s): C.W.J. has a financial interest in Global Thermostat, which seeks to commercialize carbon dioxide capture from air. This work is not affiliated with Global Thermostat. C.W.J. has a conflict-of-interest management plan in place at Georgia Tech.

Biographies

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Hyun June Moon is a PhD candidate under Prof. Christopher W. Jones at Georgia Institute of Technology. He focuses on understanding physical properties and behavior of solid-supported amines using neutron scattering and solid-state NMR. He gained a BS and MS degree (under Prof. Ki Wan Bong) from the Korea University in 2015 and 2018, respectively.

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Jan Michael Y. Carrillo earned a BS degree in chemical engineering and an MS degree in environmental engineering from the University of the Philippines Diliman in 1998 and 2003, respectively. He then obtained his PhD degree in polymer science from the University of Connecticut under the supervision of Prof. Andrey Dobrynin in 2009. His research focused on the physics of polymers and computational aspects of modeling polymers. After a brief postdoctoral period at UConn, he pursued a postdoctoral fellowship at the Oak Ridge National Laboratory (ORNL), where he worked with Dr. Bobby Sumpter and Dr. W. Michael Brown on large-scale coarse-grained molecular dynamics simulations of organic photovoltaics. Currently, he is a staff scientist at the Center for Nanophase Materials Sciences at ORNL, where he focuses on multiscale simulations of soft matter systems.

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Christopher W. Jones earned a BSE degree in chemical engineering at the University of Michigan in 1995. He subsequently studied under Prof. Mark E. Davis at Caltech, where he earned M.S. and Ph.D. degrees in chemical engineering in 1997 and 1999, respectively, where he focused on zeolite synthesis and catalysis. He completed a brief postdoctoral period in chemistry at Caltech working with Davis and Prof. John E. Bercaw, studying supported olefin polymerization catalysts. Since 2000, Jones has been on the faculty at Georgia Tech, where today he is the John F. Brock III School Chair and Professor of Chemical & Biomolecular Engineering. His research program focuses on materials for catalysis and separations, with a special emphasis on direct air capture of CO2.

References

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This article references 66 other publications.

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    Amine-grafted porous polymer networks were assessed for the direct capture of CO2 from air (400 ppm CO2, 78.96% N2, 21% O2). Under these ultra-dil. conditions, PPN-6-CH2DETA had an extraordinarily high CO2 selectivity (3.6 × 1010) and loading capacity (1.04 mol/kg), calcd. using ideal adsorption soln. theory. This material also out-performed other materials based on simulated break-through calcns., showing great potential for use in direct air capture applications.
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    Lu, W.; Sculley, J. P.; Yuan, D.; Krishna, R.; Wei, Z.; Zhou, H. Polyamine-tethered Porous Polymer Networks for Carbon Dioxide Capture from Flue Gas. Angew. Chem., Int. Ed. 2012, 51, 7480– 7484,  DOI: 10.1002/anie.201202176
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    Polyamine-Tethered Porous Polymer Networks for Carbon Dioxide Capture from Flue Gas
    Lu, Weigang; Sculley, Julian P.; Yuan, Daqiang; Krishna, Rajamani; Wei, Zhangwen; Zhou, Hong-Cai
    Angewandte Chemie, International Edition (2012), 51 (30), 7480-7484, S7480/1-S7480/12CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Porous polymer networks prepd. by polymn. of tetrakis(4-bromophenyl)methane with subsequent functionalization by chloromethylation and reaction with diethylenetriamine had high photochem. and thermal stability, high CO2 adsorption capacity and low regeneration costs.
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    Kumar, P.; Guliants, V. V. Periodic Mesoporous Organic-Inorganic Hybrid Materials: Applications in Membrane Separations and Adsorption. Microporous Mesoporous Mater. 2010, 132, 1– 14,  DOI: 10.1016/j.micromeso.2010.02.007
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    Periodic mesoporous organic-inorganic hybrid materials: Applications in membrane separations and adsorption
    Kumar, Parveen; Guliants, Vadim V.
    Microporous and Mesoporous Materials (2010), 132 (1-2), 1-14CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier Inc.)
    A review on the state of the art on the synthesis, functionalization and emerging applications of mesoporous SiO2 materials. Mesoporous SiO2 materials can be synthesized as membranes or powders with controlled pore size and geometry depending on the synthesis conditions. Mesoporous membranes are generally grown on porous supports by solvent evapn. or hydrothermal synthesis techniques. Synthesis of powd. mesoporous SiO2 materials with controlled pore sizes in the range 2-30 nm and various different pore geometries was an active area of research over last 15 years. Functionalization of the pore channels of ordered mesoporous SiO2 with org. groups provides new opportunities for fine-tuning the chem., phys., mech., and dielec. properties of these intriguing materials. This led to an interest in application of these materials as sepn. membranes for the removal of environmental pollutants, e.g. greenhouse CO2 emissions, and the sepn. of bioethanol from H2O; heterogeneous catalysts; adsorbents for removal of environmental pollutants, such as Hg; as well as other advanced nanotechnol. applications.
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  15. 15
    Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G.; Jones, C. W. Designing Adsorbents for CO2 Capture from Flue Gas-Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly. J. Am. Chem. Soc. 2008, 130, 2902– 2903,  DOI: 10.1021/ja077795v
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    Designing Adsorbents for CO2 Capture from Flue Gas - Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly
    Hicks, Jason C.; Drese, Jeffrey H.; Fauth, Daniel J.; Gray, McMahan L.; Qi, Genggeng; Jones, Christopher W.
    Journal of the American Chemical Society (2008), 130 (10), 2902-2903CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Carbon dioxide adsorption from a simulated flue gas stream was successfully performed with a hyperbranched aminosilica (HAS) material. The HAS was synthesized by a one-step reaction, spontaneous aziridine ring-opening polymn. off of surface silanols, to form a 32 wt % org./inorg. hybrid material. The adsorption measurements were performed in a fixed-bed flow reactor using humidified CO2. The advantage of this adsorbent over previously reported adsorbents is the stability of the org. groups covalently bound to the silica support compared to those made by physisorbed methods. Furthermore, a large CO2 capacity (∼3 mmol CO2/g adsorbent) assocd. with the high loading of amines was obsd.
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    Bollini, P.; Didas, S. A.; Jones, C. W. Amine-Oxide Hybrid Materials for Acid Gas Separations. J. Mater. Chem. 2011, 21, 15100– 15120,  DOI: 10.1039/c1jm12522b
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    Amine-oxide hybrid materials for acid gas separations
    Bollini, Praveen; Didas, Stephanie A.; Jones, Christopher W.
    Journal of Materials Chemistry (2011), 21 (39), 15100-15120CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    A review of the development and use of org.-inorg. hybrid materials based on porous silica functionalized with amines and amino group-contg. organc compds., esp. for adsorptive sepn. of acid gases from dil. gas streams. Topics discussed include adsorption of CO2 from simulated flue gases (e.g., for post-combustion carbon capture), CO2 adsorption capacities, adsorption and desorption kinetics, material stability, amino-functionalized silanes, alkanolamines, polyethylenepolyamines, melamine, etc., CO2 adsorption mechanism, effect of oxide support, effect of accompanying org. groups, temp.-swing adsorption with inert gas purge or CO2 purge, steam stripping, pressure-swing adsorption, and degrdn. mechanisms of the hybrid materials, the nature of adsorbed CO2, adsorption of CO2 in the presence of H2S, and structure-adsorption kinetics.
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    Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Novel Polyethylenimine-Modified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO2 Capture. Energy Fuels 2002, 16, 1463– 1469,  DOI: 10.1021/ef020058u
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    Novel Polyethylenimine-Modified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO2 Capture
    Xu, Xiaochun; Song, Chunshan; Andresen, John M.; Miller, Bruce G.; Scaroni, Alan W.
    Energy & Fuels (2002), 16 (6), 1463-1469CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    A nanoporous solid adsorbent suitable for "mol. basket-type" adsorption of CO2 in the condensed form was prepd. by impregnation of polyethylenimine (PEI) onto MCM-41 mesoporous mol. sieve. The phys. properties of the adsorbents were characterized by x-ray powder diffraction, N2 adsorption/desorption, and thermogravimetric anal. The structure of the MCM-41 was preserved after loading the PEI, and the PEI was uniformly dispersed into the channels of the mol. sieve. The CO2 adsorption/desorption performance was tested in a flow system using a microbalance to track the wt. change. The mesoporous mol. sieve had a synergetic effect on the adsorption of CO2 by PEI. A CO2 adsorption capacity as high as 215 mg-CO2/g-PEI was obtained with MCM-41-PEI-50 at 75°, which is 24 times higher than that of the MCM-41 and is even 2 times that of the pure PEI. With an increase in the CO2 concn. in the CO2/N2 gas mixt., the CO2 adsorption capacity increased. The cyclic adsorption/desorption operation indicated that the performance of the adsorbent was stable. The adsorbent has interest in the sequestration and storage of CO2 from combustion processes.
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    Tsuda, T.; Fujiwara, T. Polyethyleneimine and Macrocyclic Polyamine Silica Gels Acting as Carbon Dioxide Absorbents. J. Chem. Soc., Chem. Commun. 1992, 22, 1659– 1661,  DOI: 10.1039/c39920001659
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    Huang, H. Y.; Yang, R. T.; Chinn, D.; Munson, C. L. Amine-Grafted MCM-48 and Silica Xerogel as Superior Sorbents for Acidic Gas Removal from Natural Gas. Ind. Eng. Chem. Res. 2003, 42, 2427– 2433,  DOI: 10.1021/ie020440u
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    Amine-Grafted MCM-48 and Silica Xerogel as Superior Sorbents for Acidic Gas Removal from Natural Gas
    Huang, Helen Y.; Yang, Ralph T.; Chinn, Daniel; Munson, Curtis L.
    Industrial & Engineering Chemistry Research (2003), 42 (12), 2427-2433CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    In an effort to develop selective solid sorbents for acidic gas (CO2 and H2S) removal from natural gas mixts., we synthesized amine-surface-modified silica xerogel and MCM-48 materials. With large amts. of basic amine groups on the surface, the sorbents are able to selectively bind the acidic gases CO2 and H2S. High adsorption capacities and adsorption rates were obtained for both gases. The adsorption-desorption isotherms of the gases and thermogravimetric anal. of the sorbents showed that these sorbents can be regenerated completely under mild conditions such as those used in pressure swing or temp. swing adsorption processes. We have also investigated the effect of moisture on the adsorption of CO2 and H2S by TPD-MS and IR spectroscopy. The results indicated that the presence of water vapor doubled the amt. of CO2 adsorbed and barely affected the H2S adsorption.
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    Harlick, P. J. E.; Sayari, A. Applications of Pore-Expanded Mesoporous Silicas. 3. Triamine Silane Grafting for Enhanced CO2 Adsorption. Ind. Eng. Chem. Res. 2006, 45, 3248– 3255,  DOI: 10.1021/ie051286p
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    Applications of Pore-Expanded Mesoporous Silicas. 3. Triamine Silane Grafting for Enhanced CO2 Adsorption
    Harlick, Peter J. E.; Sayari, Abdelhamid
    Industrial & Engineering Chemistry Research (2006), 45 (9), 3248-3255CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    Conventional MCM-41 and pore-expanded MCM-41 (PE-MCM-41) silicas were used as supports for grafting 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (I) and tested for CO2 adsorption capacity and activity. The effects of the quantity of I added to the grafting mixt. on the CO2 adsorption capacity and apparent adsorption rate adsorption rate were examd. When both supports were grafted under the same conditions, PE-MCM-41 was grafted with slightly larger quantities of amine than MCM-41, for all controlled silane addns. Based on the adsorption performance of the materials using a dry 5% CO2/N2 feed mixt., the optimal quantity of I added to the grafting mixt. was detd. to be ∼3.0 cm3/g(SiO2), for both MCM-41 and PE-MCM-41. The CO2 adsorption capacity of I-PE-MCM-41 was significantly higher than that of I-MCM-41. Furthermore, the dynamic adsorption performance of I-PE-MCM-41 was far superior to I-MCM-41. In comparison to 13X zeolite, I-PE-MCM-41 exhibited higher adsorption capacities in the initial time frame of exposure, even though 13X zeolite exhibited a higher equil. adsorption capacity. The behavior was largely due to the rapid CO2-amine interaction and the open pore structure of I-PE-MCM-41 over that of 13X zeolite. When these adsorbents were exposed to a humid stream of 5% CO2/N2 (28% relative humidity), both grafted materials exhibited a slight increase in the adsorption capacity, whereas 13X zeolite did not retain any significant CO2 adsorption capacity. Thus, I-PE-MCM-41 material may be most suitable for use in a rapid cyclic adsorption process under humid feed conditions.
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  21. 21
    Harlick, P. J. E.; Sayari, A. Applications of Pore-Expanded Mesoporous Silica. 5. Triamine Grafted Material with Exceptional CO2 Dynamic and Equilibrium Adsorption Performance. Ind. Eng. Chem. Res. 2007, 46, 446– 458,  DOI: 10.1021/ie060774+
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    Applications of Pore-Expanded Mesoporous Silica. 5. Triamine Grafted Material with Exceptional CO2 Dynamic and Equilibrium Adsorption Performance
    Harlick, Peter J. E.; Sayari, Abdelhamid
    Industrial & Engineering Chemistry Research (2007), 46 (2), 446-458CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    Application of pore-expanded MCM-41 (PE-MCM-41) mesoporous silica coated with 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (TRI) was extensively examd. for the adsorption of CO2 from N2. A systematic study of the amine loading as a function of the relative amts. of TRI and water used during the grafting procedure and the temp. of the grafting reaction was carried out. Extremely high levels of active amine content were achieved using prehydrated silica surfaces at grafting temps. below reflux to facilitate thermally controlled water-aided surface polymn. of the aminosilanes. The CO2 adsorption capacities and rates were detd. for all materials as a function of the amt. of TRI and water per g of support added to the grafting mixt. The optimal TRI grafted PE-MCM-41 adsorbent exhibited a 2.65 mmol/g adsorption capacity at 25° and 1.0 atm for a dry 5% CO2 in N2 feed mixt., which exceeded all literature reported values, for both meso- and microporous materials under the conditions used. Further, the apparent adsorption and desorption rates with the amine functionalized materials were exceedingly high. When considering the grafted amine quantity, the adsorption capacity and rate are mutually dependent on each other, exhibiting an apparent optimal combination. In comparison to zeolite 13X, the optimally loaded TRI-PE-MCM-41 was far superior in terms of dynamic adsorption and desorption performance. These results were further enhanced when the adsorbents were challenged with a humid stream of 5% CO2/N2. The TRI-PE-MCM-41 exhibited a 10% increase in CO2 adsorption capacity, whereas the 13X zeolite did not retain any significant CO2 adsorption capacity.
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    Serna-Guerrero, R.; Da’na, E.; Sayari, A. New Insights into the Interactions of CO2 with Amine-Functionalized Silica. Ind. Eng. Chem. Res. 2008, 47, 9406– 9412,  DOI: 10.1021/ie801186g
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    New Insights into the Interactions of CO2 with Amine-Functionalized Silica
    Serna-Guerrero, Rodrigo; Da'na, Enshirah; Sayari, Abdelhamid
    Industrial & Engineering Chemistry Research (2008), 47 (23), 9406-9412CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    The CO2-amine chem. in gas-solid processes was investigated under both humid and dry conditions using aminopropyl-grafted pore-expanded MCM-41 silica (MONO-PE-MCM-41). To draw accurate conclusions, a set of conditions had to be met including the use of an adsorbent with open pore structure and readily accessible adsorption sites; e.g., MONO-PE-MCM-41 with a mean pore size of 7.2 nm, the CO2 concn. in the feed should be high enough to achieve satn. via chemisorption; but low enough to avoid any addnl. physisorption, e.g., 5% CO2 in N2; the use of a reliable method for the accurate measurement of CO2/N ratio. Under such conditions, the obtained CO2/N ratios were reminiscent of those obtained in the CO2 scrubbing process using ethanolamine solns. Under dry conditions, the CO2/N ratio was close to 0.5, consistent with the formation of carbamate. Streams with relative humidity (RH) of 27, 61, and 74% were studied as well. As RH in the feed increased, CO2/N ratio increased from 0.57-0.88, in line with the gradual formation of bicarbonate. As for the detn. of CO2/N ratio under dry conditions, both thermogravimetry (TG) and mass spectrometry (MS) were suitable, whereas in the presence of moisture, TG was found to drastically underestimate the CO2 uptake. The seemingly disparate CO2/N ratios reported in the literature for various propylamine-bearing adsorbents were rationalized on the basis of the adsorbent pore structure and/or the exptl. conditions used.
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    Didas, S. A.; Choi, S.; Chaikittisilp, W.; Jones, C. W. Amine-Oxide Hybrid Materials for CO2 Capture from Ambient Air. Acc. Chem. Res. 2015, 48, 2680– 2687,  DOI: 10.1021/acs.accounts.5b00284
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    Amine-Oxide Hybrid Materials for CO2 Capture from Ambient Air
    Didas, Stephanie A.; Choi, Sunho; Chaikittisilp, Watcharop; Jones, Christopher W.
    Accounts of Chemical Research (2015), 48 (10), 2680-2687CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review describing the evolution of activities to design amine-functionalized SiO2 materials for catalysis and for design, characterization, and utilization of these materials for CO2 sepn. is given. Topics discussed include: introduction; supported amine materials for catalysis and CO2 capture; supported amine materials for CO2 capture from air; summary and conclusions; and supporting information (debate over air capture).
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    Heydari-Gorji, A.; Sayari, A. CO2 Capture on Polyethylenimine-Impregnated Hydrophobic Mesoporous Silica: Experimental and Kinetic Modeling. Chem. Eng. J. 2011, 173, 72– 79,  DOI: 10.1016/j.cej.2011.07.038
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    CO2 capture on polyethylenimine-impregnated hydrophobic mesoporous silica: Experimental and kinetic modeling
    Heydari-Gorji, Aliakbar; Sayari, Abdelhamid
    Chemical Engineering Journal (Amsterdam, Netherlands) (2011), 173 (1), 72-79CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    CO2 adsorption measurements for polyethylenimine (PEI)-impregnated, pore-expanded MCM-41 were conducted by gravimetry to assess the effect of amine load, CO2 partial pressure, and adsorption/desorption temps. Amine impregnation was conducted on ethanol-extd., pore-expanded MCM-41 (PME), a meso-porous SiO2 whose internal surface is laden by a layer of cetyltrimethylammonium cations. Well-dispersed PEI inside the PME hydrophobic channels exhibited a CO2 adsorption capacity at 75° as high as 206 mg/g for a 55 wt.% PEI load. Current PEI-impregnated PME materials had higher CO2 adsorption efficiency (g CO2/g PEI) vs. any other PEI-contg. adsorbent reported in the literature. In contrast to most PEI-impregnated materials reported in the literature, which due to diffusion resistance exhibited little or no CO2 adsorption at room temp., PEI-impregnated PME material displayed high potential for CO2 removal at ambient temp. An adsorption kinetic model was proposed to describe CO2 adsorption by amine-impregnated materials. Model results agreed well with exptl. data under a wide range of conditions, including different PEI loads, CO2 pressures, and adsorption temps.
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    Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Y.; Wang, Z. J.; Zhu, J. H. Efficient CO2 Capturer Derived from As-synthesized MCM-41 Modified with Amine. Chem. Eur. J. 2008, 14, 3442– 3451,  DOI: 10.1002/chem.200701467
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    Efficient CO2 capturer derived from as-synthesized MCM-41 modified with amine
    Yue, Ming Bo; Sun, Lin Bing; Cao, Yi; Wang, Ying; Wang, Zhu Ji; Zhu, Jian Hua
    Chemistry - A European Journal (2008), 14 (11), 3442-3451CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A new strategy to synthesize a highly efficient CO2 capturer by incorporating tetraethylenepentamine (TEPA) into as-synthesized MCM-41 (AM) is reported. The amine guest can be distributed in the micelle of the support, forming a web within the mesopore to trap CO2 mols. and resulting in a high adsorption capacity for CO2 up to 237 mg g-1. Four samples of the as-synthesized MCM-41 with a different amt. or type of surfactant are employed as supports to investigate the influence of micelles on the CO2 adsorption, and the spokelike structure of the micelle in the channel of the support is proven to be essential to the distribution of guest amine. Among these supports, the AM sample is the most competitive due to the advantages of energy and time saving in prepn. of the support along with the resulting higher CO2 adsorption capacity. At the optimal loading of 50 wt% TEPA, the AM-50 sample exhibits a high adsorption capacity of 183 mg g-1 in the sixth adsorption cycle at 5% CO2 concn.
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    Heydari-Gorji, A.; Belmabkhout, Y.; Sayari, A. Polyethylenimine-Impregnated Mesoporous Silica: Effect of Amine Loading and Surface Alkyl Chains on CO2 Adsorption. Langmuir 2011, 27, 12411– 12416,  DOI: 10.1021/la202972t
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    Polyethylenimine-Impregnated Mesoporous Silica: Effect of Amine Loading and Surface Alkyl Chains on CO2 Adsorption
    Heydari-Gorji, Aliakbar; Belmabkhout, Youssef; Sayari, Abdelhamid
    Langmuir (2011), 27 (20), 12411-12416CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Poly(ethyleneimine) (PEI) supported on pore-expanded MCM-41 whose surface is covered with a layer of long-alkyl chains is a more efficient CO2 adsorbent than PEI supported on the corresponding calcined silica and all PEI-impregnated materials reported in the literature. The layer of surface alkyl chains plays an important role in enhancing the dispersion of PEI, thus decreasing the diffusion resistance. Also at low temp., adsorbents with relatively low PEI contents are more efficient than their highly loaded counterparts because of the increased adsorption rate. Extensive CO2 adsorption-desorption cycling showed that the use of humidified feed and purge gases affords materials with enhanced stability, despite limited loss due to amine evapn.
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    Wang, L.; Al-Aufi, M.; Pacheco, C. N.; Xie, L.; Rioux, R. M. Polyethylene Glycol (PEG) Addition to Polyethylenimine (PEI)-Impregnated Silica Increases Amine Accessibility during CO2 Sorption. ACS. Sustain. Chem. Eng. 2019, 7, 14785– 14795,  DOI: 10.1021/acssuschemeng.9b02798
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    Tanthana, J.; Chuang, S. S. C. In Situ Infrared Study of the Role of PEG in Stabilizing Silica-Supported Amines for CO2 Capture. ChemSusChem 2010, 3, 957– 964,  DOI: 10.1002/cssc.201000090
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    In Situ Infrared Study of the Role of PEG in Stabilizing Silica-Supported Amines for CO2 Capture
    Tanthana, Jak; Chuang, Steven S. C.
    ChemSusChem (2010), 3 (8), 957-964CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The CO2 capture capacity, adsorption mechanism, and degrdn. characteristics of two sorbents, silica-supported tetraethylenepentamine (TEPA/SiO2) and polyethylene-glycol-modified TEPA/SiO2 (PEG/TEPA/SiO2), are studied by diffuse reflectance IR Fourier transform spectroscopy and mass spectrometry. The CO2 capture capacities of TEPA/SiO2 and PEG/TEPA/SiO2 are detd. to be 2087 and 1110 μmol CO2 g-1 sorbent, resp. Both sorbents adsorb CO2 as hydrogen-bonding species, NH2[bond]O, and carbamate/carboxylate species. The CO2 adsorption half-time increases with the no. of CO2 capture cycles. IR results suggest that the increased adsorption half-time is a result of diffusion limitation, caused by accumulation of TEPA and PEG species on the surface of the sorbent particles. The degrdn. of TEPA/SiO2 is found to correlate with the accumulation of carboxylate/carbamic species. The addn. of PEG decreases the degrdn. rate of the sorbent and slows down the formation of carboxylate species. These carboxylate species can block CO2 capture on amine (NH2/NH) sites. The stabilizing role of PEG on TEPA/SiO2 can be attributed to hydrogen-bonding between TEPA (NH2/NH)and PEG (OH).
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    Sakwa-Novak, M. A.; Tan, S.; Jones, C. W. Role of Additives in Composite PEI/Oxide CO2 Adsorbents: Enhancement in the Amine Efficiency of Supported PEI by PEG in CO2 Capture from Simulated Ambient Air. ACS. Appl. Mater. Interfaces 2015, 7, 24748– 24759,  DOI: 10.1021/acsami.5b07545
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    Role of Additives in Composite PEI/Oxide CO2 Adsorbents: Enhancement in the Amine Efficiency of Supported PEI by PEG in CO2 Capture from Simulated Ambient Air
    Sakwa-Novak, Miles A.; Tan, Shuai; Jones, Christopher W.
    ACS Applied Materials & Interfaces (2015), 7 (44), 24748-24759CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Supported amines are promising candidate adsorbents for the removal of CO2 from flue gases and directly from ambient air. The incorporation of additives into polymeric amines such as poly(ethylenimine) (PEI) supported on mesoporous oxides is an effective strategy to improve the performance of the materials. Here, several practical aspects of this strategy are addressed with regards to direct air capture. The influence of three additives (CTAB, PEG200, PEG1000) was systematically explored under dry simulated air capture conditions (400 ppm of CO2, 30 °C). With SBA-15 as a model support for poly(ethylenimine) (PEI), the nature of the additive induced heterogeneities in the deposition of org. on the interior and exterior of the particles, an important consideration for future scale up to practical systems. The PEG200 additive increased the obsd. thermodn. performance (∼60% increase in amine efficiency) of the adsorbents regardless of the PEI content, while the other mols. had less pos. effects. A threshold PEG200/PEI value was identified at which the diffusional limitations of CO2 within the materials were nearly eliminated. The threshold PEG/PEI ratio may have phys. origin in the interactions between PEI and PEG, as the optimal ratio corresponded to nearly equimolar OH/reactive (1°, 2°) amine ratios. The strategy is shown to be robust to the characteristics of the host support, as PEG200 improved the amine efficiency of PEI when supported on two varieties of mesoporous γ-alumina with PEI.
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  30. 30
    Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Z. J.; Wang, Y.; Yu, Q.; Zhu, J. H. Promoting the CO2 Adsorption in the Amine-Containing SBA-15 by Hydroxyl Group. Microporous Mesoporous Mater. 2008, 114, 74– 81,  DOI: 10.1016/j.micromeso.2007.12.016
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    30
    Promoting the CO2 adsorption in the amine-containing SBA-15 by hydroxyl group
    Yue, Ming Bo; Sun, Lin Bing; Cao, Yi; Wang, Zhu Ji; Wang, Ying; Yu, Qing; Zhu, Jian Hua
    Microporous and Mesoporous Materials (2008), 114 (1-3), 74-81CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier)
    Novel CO2 capturer with a high efficiency is fabricated through dispersing the amine mixt. of tetraethylenepentamine (TEPA) and diethanolamine (DEA) or glycerol within the as-synthesized mesoporous SiO2 SBA-15, and the resulting sample is characterized by low angle x-ray diffraction and N2 adsorption to evaluate the distribution of the guest. The influence of hydroxyl group on the CO2 adsorption capacity of the composite is studied by using CO2-TPD and TG-MS techniques. The hydroxyl group of the P123 ((EO)20(PO)70(EO)20, template preserved in as-synthesized SBA-15) and the guest could promote the capture of CO2 by the amine through changing the interaction mechanism. The presence of hydroxyl group promotes the formation of the intermediate between CO2 and the amine with a lower thermal stability hence the CO2 trapped by the composite is easier to be desorbed and thus the regeneration of adsorbent is facilitated. Therefore, using this mixed amine (TEPA and DEA) modified as-synthesized SBA-15 as CO2 capturer not only saves the energy for removal of template, but also cut down the cost in the prepn. and regeneration of CO2 capturer, which is crit. in CO2 sepn. and capture.
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  31. 31
    Miao, Y.; Wang, Y.; Ge, B.; He, Z.; Zhu, X.; Li, J.; Liu, S.; Yu, L. Mixed Diethanolamine and Polyethyleneimine with Enhanced CO2 Capture Capacity from Air. Adv. Sci. 2023, 10, 2207253,  DOI: 10.1002/advs.202207253
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    31
    Mixed Diethanolamine and Polyethyleneimine with Enhanced CO2 Capture Capacity from Air
    Miao, Yihe; Wang, Yaozu; Ge, Bingyao; He, Zhijun; Zhu, Xuancan; Li, Jia; Liu, Shanke; Yu, Lijun
    Advanced Science (Weinheim, Germany) (2023), 10 (16), 2207253CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Supported polyethyleneimine (PEI) adsorbent is one of the most promising com. direct air capture (DAC) adsorbents with a long research history since 2002. Although great efforts have been input, there are still limited improvements for this material in its CO2 capacity and adsorption kinetics under ultradilute conditions. Supported PEI also suffers significantly reduced adsorption capacities when working at sub-ambient temps. This study reports that mixing diethanolamine (DEA) into supported PEI can increase 46% and 176% of pseudoequil. CO2 capacities at DAC conditions compared to the supported PEI and DEA, resp. The mixed DEA/PEI functionalized adsorbents maintain the adsorption capacity at sub-ambient temps. of -5 to 25°C. In comparison, a 55% redn. of CO2 capacity is obsd. for supported PEI when the operating temp. decreases from 25 to -5°C. In addn., the supported mixed DEA/PEI with a ratio of 1:1 also shows fast desorption kinetics at temps. as low as 70°C, resulting in maintaining high thermal and chem. stability over 50 DAC cycles with a high av. CO2 working capacity of 1.29 mmol g-1. These findings suggest that the concept of "mixed amine", widely studied in the solvent system, is also practical to supported amine for DAC applications.
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  32. 32
    Choi, S.; Gray, M. L.; Jones, C. W. Amine-tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture from Ambient Air. ChemSusChem 2011, 4, 628– 635,  DOI: 10.1002/cssc.201000355
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    32
    Amine-Tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture From Ambient Air
    Choi, Sunho; Gray, McMahan L.; Jones, Christopher W.
    ChemSusChem (2011), 4 (5), 628-635CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Impregnation-synthesized, SiO2-supported poly(ethylenimine) (PEI) materials are demonstrated to be promising adsorbents for CO2 capture from ultra-dil. gas streams, e.g., ambient air. A prototypical class 1 adsorbent contg. 45 wt.% PEI (PEI/SiO2) and 2 modified PEI-based aminosilica adsorbents derived from PEI modified by 3-aminopropyltrimethoxysilane (A-PEI/SiO2) or tetra-Et orthotitanate (T-PEI/SiO2), were synthesized and characterized by thermogravimetric anal. and Fourier transform IR spectroscopy. The modifiers were shown to enhance the thermal stability of the polymer-oxide composites, leading to higher PEI decompn. temps. Modified adsorbents had extremely high CO2 adsorption capacity under conditions simulating ambient air (400 ppm CO2 in inert gas), with >2 molCO2/kgsorbent-1 and enhanced adsorption kinetics vs. conventional class 1 sorbents. The new sorbents have excellent stability in cyclic adsorption/desorption operations, even under dry conditions in which aminosilica adsorbents are known to lose capacity due to urea formation. Thus, this type of adsorbent is considered a promising material for the direct capture of CO2 from ultra-dil. gas streams.
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  33. 33
    Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Pérez, E. S. Development of High Efficiency Adsorbents for CO2 Capture Based on a Double-Functionalization Method of Grafting and Impregnation. J. Mater. Chem. A 2013, 1, 1956– 1962,  DOI: 10.1039/c2ta01343f
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    Development of high efficiency adsorbents for CO2 capture based on a double-functionalization method of grafting and impregnation
    Sanz, Raul; Calleja, Guillermo; Arencibia, Amaya; Sanz-Perez, Eloy S.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (6), 1956-1962CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A functionalization method based on the impregnation of previously grafted, pore-expanded SBA-15 is presented. The combination of tethered and mobile amino groups led to a synergic effect, yielding samples with CO2 uptake of up to 235 mg CO2/g (5.34 mmol CO2/g) at 45°, 0.15 bar CO2, and high adsorption efficiency.
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  34. 34

    We consider that adsorption is the main driving force to capture CO2 via amine–CO2 interactions. For supported-amines at low CO2 partial pressures, most amine–CO2 association occurs via chemical reaction, creating covalent or ionic bonds between CO2 and the amines.

    There is no corresponding record for this reference.
  35. 35

    Pseudoequilibrium is often used in studies of CO2 adsorption over solid-supported amines because the approach to true thermodynamic equilibrium is too slow to allow for reasonable data collection. Equilibrium may require days to weeks of equilibration.

    There is no corresponding record for this reference.
  36. 36
    Caplow, M. Kinetics of Carbamate Formation and Breakdown. J. Am. Chem. Soc. 1968, 90, 6795– 6803,  DOI: 10.1021/ja01026a041
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    Kinetics of carbamate formation and breakdown
    Caplow, Michael
    Journal of the American Chemical Society (1968), 90 (24), 6795-803CODEN: JACSAT; ISSN:0002-7863.
    The rate law for reaction of amines with CO2 is rate = kamine(R2NH)(CO2) + kamine'(R2NH)(OH)(CO2), where the first and second terms are for uncatalyzed and hydroxide-catalyzed pathways. The latter reaction, which involves proton abstraction in the rate-detg. step, is not observed with all amines. Values for kamine at 10° follow the Bronsted relationship log kamine (M-1 sec.-1) = mpK + Y, with values of m and Y equal to 0.43 and -1.50 for reactions of primary and secondary amines, and 0.48 and -0.20 for the reactions of hydrazine and hydroxylamine derivs. Second-order rate consts. for H ion catalyzed decarboxylation of carbamates formed from amines of pK - 1.05 to approx. 5 may be fitted to a Bronsted relationship log kH+ (M-1 sec.-1) = 0.77 pK + 3.6 at 10°. Rates for carbamates formed from more basic amines are virtually independent of basicity and are approx. 108 M-1 sec.-1. The rate-limiting step in carbamate formation and breakdown with weakly basic amines involves C-N bond formation and cleavage. It is suggested that proton transfer may be rate limiting in the synthesis and breakdown of carbamates formed from basic amines.
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  37. 37
    Danckwerts, P. V. The Reaction of CO2 with Ethanolamines. Chem. Eng. Sci. 1979, 34, 443– 446,  DOI: 10.1016/0009-2509(79)85087-3
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    The reaction of carbon dioxide with ethanolamines
    Danckwerts, P. V.
    Chemical Engineering Science (1979), 34 (4), 443-6CODEN: CESCAC; ISSN:0009-2509.
    A crit. review of measurements of the rate of absorption of CO2 by solns. contg. [HO(CH2)2]nNH3-n (n=1,2,3). 11 Refs.
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  38. 38
    Kuwahara, Y.; Kang, D.; Copeland, J. R.; Bollini, P.; Sievers, C.; Kamegawa, T.; Yamashita, H.; Jones, C. W. Enhanced CO2 Adsorption over Polymeric Amines Supported on Heteroatom-incorporated SBA-15 Silica: Impact of Heteroatom Type and Loading on Sorbent Structure and Adsorption Performance. Chem. Eur. J. 2012, 18, 16649– 16664,  DOI: 10.1002/chem.201203144
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    Enhanced CO2 Adsorption over Polymeric Amines Supported on Heteroatom-Incorporated SBA-15 Silica: Impact of Heteroatom Type and Loading on Sorbent Structure and Adsorption Performance
    Kuwahara, Yasutaka; Kang, Dun-Yen; Copeland, John R.; Bollini, Praveen; Sievers, Carsten; Kamegawa, Takashi; Yamashita, Hiromi; Jones, Christopher W.
    Chemistry - A European Journal (2012), 18 (52), 16649-16664CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Silica supported amine materials are promising compns. that can be used to effectively remove CO2 from large stationary sources, such as flue gas generated from coal-fired power plants (ca. 10% CO2) and potentially from ambient air (ca. 400 ppm CO2). The CO2 adsorption characteristics of prototypical poly(ethyleneimine)-silica composite adsorbents can be significantly enhanced by altering the acid/base properties of the silica support by heteroatom incorporation into the silica matrix. In this study, an array of poly(ethyleneimine)-impregnated mesoporous silica SBA-15 materials contg. heteroatoms (Al, Ti, Zr, and Ce) in their silica matrixes are prepd. and examd. in adsorption expts. under conditions simulating flue gas (10 % CO2 in Ar) and ambient air (400 ppm CO2 in Ar) to assess the effects of heteroatom incorporation on the CO2 adsorption properties. The structure of the composite adsorbents, including local information concerning the state of the incorporated heteroatoms and the overall surface properties of the silicate supports, are investigated in detail to draw a relationship between the adsorbent structure and CO2 adsorption/desorption performance. The CO2 adsorption/desorption kinetics are assessed by thermogravimetric anal. and in situ FT-IR measurements. These combined results, coupled with data on adsorbent regenerability, demonstrate a stabilizing effect of the heteroatoms on the poly(ethyleneimine), enhancing adsorbent capacity, adsorption kinetics, regenerability, and stability of the supported aminopolymers over continued cycling. It is suggested that the CO2 adsorption performance of silica-aminopolymer composites may be further enhanced in the future by more precisely tuning the acid/base properties of the support.
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    Wilfong, W. C.; Srikanth, C. S.; Chuang, S. S. C. In Situ ATR and DRIFTS Studies of the Nature of Adsorbed CO2 on Tetraethylenepentamine Films. ACS. Appl. Mater. Interfaces 2014, 6, 13617– 13626,  DOI: 10.1021/am5031006
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    39
    In Situ ATR and DRIFTS studies of nature of adsorbed CO2 on tetraethylenepentamine films
    Wilfong, Walter Christopher; Srikanth, Chakravartula S.; Chuang, Steven S. C.
    ACS Applied Materials & Interfaces (2014), 6 (16), 13617-13626CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    CO2 adsorption/desorption onto/from tetraethylenepentamine (TEPA) films of 4, 10, and 20 μm thicknesses were studied by in situ attenuated total reflectance (ATR) and diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) techniques under transient conditions. Molar adsorption coeffs. for adsorbed CO2 were used to det. the CO2 capture capacities and amine efficiencies (CO2/N) of the films in the DRIFTS system. Adsorption of CO2 onto surface and bulk NH2 groups of the 4 μm film produced weakly adsorbed CO2, which can be desorbed at 50 °C by reducing the CO2 partial pressure. These weakly adsorbed CO2 exhibit low ammonium ion intensities and could be in the form of ammonium-carbamate ion pairs and zwitterions. Increasing the film thickness enhanced the surface amine-amine interactions, resulting in strongly adsorbed ion pairs and zwitterions assocd. with NH and NH2 groups of neighboring amines. These adsorbed species may form an interconnected surface network, which slowed CO2 gas diffusion into and diminished access of the bulk amine groups (or amine efficiency) of the 20 μm film by a min. of 65%. Desorption of strongly adsorbed CO2 comprising the surface network could occur via dissocn. of NH3+/NH2+···NH2/NH ionic hydrogen bonds beginning from 60 to 80 °C, followed by decompn. of NHCOO-/NCOO- at 100 °C. These results suggest that faster CO2 diffusion and adsorption/desorption kinetics could be achieved by thinner layers of liq. or immobilized amines.
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  40. 40
    Taniguchi, I.; Urai, H.; Kai, T.; Duan, S.; Kazama, S. A CO2-Selective Molecular Gate of Poly (Amidoamine) Dendrimer Immobilized in a Poly (Ethylene Glycol) Network. J. Membr. Sci. 2013, 444, 96– 100,  DOI: 10.1016/j.memsci.2013.05.017
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    40
    A CO2-selective molecular gate of poly(amidoamine) dendrimer immobilized in a poly(ethylene glycol) network
    Taniguchi, Ikuo; Urai, Hiromi; Kai, Teruhiko; Duan, Shuhong; Kazama, Shingo
    Journal of Membrane Science (2013), 444 (), 96-100CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    A polymeric membrane composed of poly(amidoamine) (PAMAM) dendrimer immobilized in a poly(ethylene glycol) (PEG) network expresses excellent CO2 sepn. properties over smaller H2. The preferential CO2 permeation can be explained by specific interaction between CO2 and primary amine of the dendrimer, which enhances CO2 soly. into the polymeric membrane. CO2 forms carbamate with the amines or bicarbonate in the presence of H2O detd. by inverse-gate decoupled 13C NMR. The resulting carbamate ion pair works to form a quasi-crosslinking, which would suppress H2 permeation by a CO2-selective Mol. Gate, while bicarbonate ion can be a major moving species to pass through the polymeric membrane. Attenuated total reflection (ATR) indicates the formation of carbamate. Small-angle x-ray scattering (SAXS) reveals increase in scattering intensity under CO2 atmosphere due to the formation of scattering particles, which can be a cluster of the dendrimer-CO2 crosslinks. Tensile testing of the membrane exhibits increase in both Young's modulus and elongation-to-break by CO2 treatment, suggesting that the crosslinking is reversible and rearrangeable. DSC also shows an exothermic peak at 120°, which is assocd. with dissocn. of the crosslinks.
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  41. 41
    Donaldson, T. L.; Nguyen, Y. N. Carbon Dioxide Reaction Kinetics and Transport in Aqueous Amine Membranes. Ind. Eng. Chem. Fundam. 1980, 19, 260– 266,  DOI: 10.1021/i160075a005
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    Carbon dioxide reaction kinetics and transport in aqueous amine membranes
    Donaldson, Terrence L.; Nguyen, Yen N.
    Industrial & Engineering Chemistry Fundamentals (1980), 19 (3), 260-6CODEN: IECFA7; ISSN:0196-4313.
    Reaction kinetics of CO2 with mono-, di- and triethanolamine (I) and Et3N were studied using a tracer 14CO2 membrane-transport technique. At low concns. of mono- and diethanolamine, the results are consistent with carbamate formation. At higher concns. the chem. appears to be more complex. Transport results are discussed in terms of various potential phenomena. Although triamines do not form carbamates with CO2, both I and Et3N increased the membrane-transport flux of CO2. The only reasonable reaction mechanism for I consistent with the data is basic catalysis of CO2 hydration. Et3N, on the other hand, appears to act only as a weak base to produce free OH-, which reacts with CO2.
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  42. 42
    Russell-Parks, G. A.; Leick, N.; Marple, M. A. T.; Strange, N. A.; Trewyn, B. G.; Pang, S. H.; Braunecker, W. A. Fundamental Insight into Humid CO2 Uptake in Direct Air Capture Nanocomposites Using Fluorescence and Portable NMR Relaxometry. J. Phys. Chem. C 2023, 127, 15363– 15374,  DOI: 10.1021/acs.jpcc.3c03653
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    Fundamental Insight into Humid CO2 Uptake in Direct Air Capture Nanocomposites Using Fluorescence and Portable NMR Relaxometry
    Russell-Parks, Glory A.; Leick, Noemi; Marple, Maxwell A. T.; Strange, Nicholas A.; Trewyn, Brian G.; Pang, Simon H.; Braunecker, Wade A.
    Journal of Physical Chemistry C (2023), 127 (31), 15363-15374CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Direct air capture (DAC) technol. is being explored as a pathway for reducing greenhouse gas emissions through the efficient removal of CO2 from the atm. However, there remains a knowledge gap regarding structure-property-performance factors that impact the behavior of these systems in diverse, real-world environments. In aminopolymer-based DAC systems, gas diffusion is tightly coupled with polymer mobility, which is in turn affected by a large matrix of variables, including interactions with the pore wall of the support, nanoconfinement, the presence of co-adsorbates (moisture), and electrostatic cross-links that develop as a function of CO2 chemisorption. Higher-throughput, benchtop techniques for studying and understanding mobility in these systems would lead to more rapid advances in the field. Here, we demonstrate the value of a fluorescence technique for monitoring polymer mobility within nanocomposite capture materials as a function of CO2 and water adsorption in a series of humidified polyethylenimine-Al2O3 composite materials. The approach allows us to correlate changes in mobility with CO2 adsorption kinetics as a function of relative humidity. We further couple this information with NMR relaxometry data attained using a portable single-sided magnetic resonance device, and we employ diffuse reflectance IR Fourier transform spectroscopy to correlate the formation of different relative amts. of carbamates and carbonates with the environmental conditions. These results provide a blueprint for using benchtop techniques to promote fundamental understanding in DAC systems that can in turn enable more efficient operation in real-world conditions.
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    Yu, J.; Zhai, Y.; Chuang, S. S. C. Water Enhancement in CO2 Capture by Amines: An Insight into CO2–H2O Interactions on Amine Films and Sorbents. Ind. Eng. Chem. Res. 2018, 57, 4052– 4062,  DOI: 10.1021/acs.iecr.7b05114
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    Water Enhancement in CO2 Capture by Amines: An Insight into CO2-H2O Interactions on Amine Films and Sorbents
    Yu, Jie; Zhai, Yuxin; Chuang, Steven S. C.
    Industrial & Engineering Chemistry Research (2018), 57 (11), 4052-4062CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    Water, a component in flue gas, plays a significant role in CO2 capture through a complex interaction between water mols. and adsorbed CO2 on amine sorbents. To det. how the H2O-CO2-amine interactions affect amine efficiency and the binding energy of adsorbed CO2, we used in situ IR spectroscopy (IR) to det. the structure of adsorbed CO2 and H2O as well as their relations to adsorption/desorption kinetics and CO2 capture capacity on tetraethylenepentamine (TEPA) films and Class I amine (i.e., impregnated) sorbents. H2O enhanced amine efficiency of TEPA films and sorbents by increasing the accessibility of secondary amine sites to CO2 and promoting the formation of hydronium carbamate and carbamic acid. CO2 adsorbed on the surface of the TEPA film as a weakly adsorbed CO2 in the form of hydronium and ammonium-carbamate with a low IR intensity of hydrogen bonding (-OH···-OOC or -NH···-OOC) between hydronium/ammonium ions and carbamate ions. CO2 adsorbed on the middle layers (i.e., 0.2-0.4 μm below the surface) of TEPA films produced a strongly adsorbed species that exhibits an intensive hydrogen bonding band of ammonium-water-carbamate desorbing at temps. above 120 °C. Comparison of IR spectra shows that the kinetic behaviors of adsorbed CO2 on amine films are correlated well with those of adsorbed CO2 on Class I amine sorbents. Thick amine films and high-amine-loading sorbents contain high-d. amine sites that produce mainly strongly adsorbed CO2. Adsorbed H2O further increased amine efficiency and the binding energy of strongly adsorbed CO2 through the formation of hydronium carbamate.
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  44. 44
    Rim, G.; Priyadarshini, P.; Song, M.; Wang, Y.; Bai, A.; Realff, M. J.; Lively, R. P.; Jones, C. W. Support Pore Structure and Composition Strongly Influence the Direct Air Capture of CO2 on Supported Amines. J. Am. Chem. Soc. 2023, 145, 7190– 7204,  DOI: 10.1021/jacs.2c12707
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    Support Pore Structure and Composition Strongly Influence the Direct Air Capture of CO2 on Supported Amines
    Rim, Guanhe; Priyadarshini, Pranjali; Song, MinGyu; Wang, Yuxiang; Bai, Andrew; Realff, Matthew J.; Lively, Ryan P.; Jones, Christopher W.
    Journal of the American Chemical Society (2023), 145 (13), 7190-7204CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A variety of amine-impregnated porous solid sorbents for direct air capture (DAC) of CO2 were developed, yet the effect of amine-solid support interactions on the CO2 adsorption behavior is still poorly understood. When tetraethylenepentamine (TEPA) is impregnated on two different supports, com. γ-Al2O3 and MIL-101(Cr), they show different trends in CO2 sorption when the temp. (-20 to 25°) and humidity (0-70% RH) of the simulated air stream are varied. In situ IR spectroscopy is used to probe the mechanism of CO2 sorption on the two supported amine materials, with weak chemisorption (formation of carbamic acid) being the dominant pathway over MIL-101(Cr)-supported TEPA and strong chemisorption (formation of carbamate) occurring over γ-Al2O3-supported TEPA. Formation of both carbamic acid and carbamate species is enhanced over the supported TEPA materials under humid conditions, with the most significant enhancement obsd. at -20°. However, while equil. H2O sorption is high at cold temps. (e.g., -20°), the effect of humidity on a practical cyclic DAC process is expected to be minimal due to slow H2O uptake kinetics. This work suggests that the CO2 capture mechanisms of impregnated amines can be controlled by adjusting the degree of amine-solid support interaction and that H2O adsorption behavior is strongly affected by the properties of the support materials. Thus, proper selection of solid support materials for amine impregnation will be important for achieving optimized DAC performance under varied deployment conditions, such as cold (e.g., -20°) or ambient temp. (e.g., 25°) operations.
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    Franchi, R. S.; Harlick, P. J. E.; Sayari, A. Applications of Pore-Expanded Mesoporous Silica. 2. Development of a High-Capacity, Water-Tolerant Adsorbent for CO2. Ind. Eng. Chem. Res. 2005, 44, 8007– 8013,  DOI: 10.1021/ie0504194
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    Applications of Pore-Expanded Mesoporous Silica. 2. Development of a High-Capacity, Water-Tolerant Adsorbent for CO2
    Franchi, Robert S.; Harlick, Peter J. E.; Sayari, Abdelhamid
    Industrial & Engineering Chemistry Research (2005), 44 (21), 8007-8013CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    A novel high-capacity, water-tolerant adsorbent for CO2 was developed. It consisted of diethanolamine (DEA) loaded pore-expanded MCM-41 silica (PE-MCM-41). Due to its very large pore vol., PE-MCM-41 silica was capable of accommodating a greater quantity of amine resulting in higher CO2 adsorption capacity compared to the other supports including activated carbon, silica gel, and std. MCM-41 silica. Adsorption measurements were conducted by gravimetry using dry CO2 to obtain uptake curves and apparent rate data. The capacity and uptake rate reached maxima with respect to amine content and then declined due to the deposition of excess amine on the particle's external surface and within the interparticle voids. At CO2 partial pressures below 0.15 atm, the current DEA loaded PE-MCM-41 adsorbent was superior to the more conventional zeolite 13X. Adsorption studies with humid CO2 revealed that the adsorption capacity of the PE-MCM-41-based material was insensitive to the presence of moisture, which represents a major advantage over zeolite 13X. Repeated adsorption-desorption cycles revealed that our novel adsorbent exhibited good cyclic stability.
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    Heydari-Gorji, A.; Yang, Y.; Sayari, A. Effect of the Pore Length on CO2 Adsorption over Amine-Modified Mesoporous Silicas. Energy Fuels 2011, 25, 4206– 4210,  DOI: 10.1021/ef200765f
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    46
    Effect of the Pore Length on CO2 Adsorption over Amine-Modified Mesoporous Silicas
    Heydari-Gorji, Aliakbar; Yang, Yong; Sayari, Abdelhamid
    Energy & Fuels (2011), 25 (9), 4206-4210CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    Carbon dioxide adsorption was investigated in the presence of polyethylenimine (PEI)-impregnated mesoporous silicas with different pore lengths, namely, pore-expanded MCM-41, conventional SBA-15 with different pore diams. (7.2 and 10.5 nm), and SBA-15 with platelet morphol. The pore lengths of the silica supports were ca. 25, 1.5, and 0.2 μm, resp. Under comparable conditions, the adsorption performance was found to be strongly dependent upon the pore length. The materials with the shortest channels showed the highest capacity and fastest adsorption. These findings were assocd. with diminished diffusion resistance and enhanced amine accessibility inside the pores.
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  47. 47
    Kwon, H. T.; Sakwa-Novak, M. A.; Pang, S. H.; Sujan, A. R.; Ping, E. W.; Jones, C. W. Aminopolymer-Impregnated Hierarchical Silica Structures: Unexpected Equivalent CO2 Uptake under Simulated Air Capture and Flue Gas Capture Conditions. Chem. Mater. 2019, 31, 5229– 5237,  DOI: 10.1021/acs.chemmater.9b01474
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    47
    Aminopolymer-Impregnated Hierarchical Silica Structures: Unexpected Equivalent CO2 Uptake under Simulated Air Capture and Flue Gas Capture Conditions
    Kwon, Hyuk Taek; Sakwa-Novak, Miles A.; Pang, Simon H.; Sujan, Achintya R.; Ping, Eric W.; Jones, Christopher W.
    Chemistry of Materials (2019), 31 (14), 5229-5237CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Poly(ethyleneimine)-impregnated sorbents are prepd. using a hierarchical SiO2 support with bimodal meso-/macroporosity. The sorbents behave unexpectedly during CO2 adsorption from simulated air and flue gases (400 ppm and 10% CO2) at a fixed temp., as compared to systems built on commonly studied mesoporous materials. The results demonstrate that (1) impregnation methods influence the efficacy of sorption performance and (2) the sorbents show almost similar uptake capacities under 400 ppm and 10% dry CO2 at 30°, exhibiting step-like CO2 adsorption isotherms. These unusual observations are rationalized via control expts. and a hypothesized sorption mechanism. While the sorption performance near room temp. is unexpectedly identical under 400 ppm and 10% CO2 conditions, there is an optimal temp. at each gas concn. where the uptake is maximized. The max. sorption capacities are 2.6 and 4.1 mmol CO2/g sorbent at the optimized sorption temps. using 400 ppm and 10% dry CO2, resp. The presence of H2O vapor under 400 ppm CO2 conditions further improves the sorption capacity to 3.4 mmol/g sorbent, which is the highest capacity under direct air capture conditions among known amine sorbents impregnated with a similar polymer, to the best of the knowledge.
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  48. 48
    Min, K.; Choi, W.; Choi, M. Macroporous Silica with Thick Framework for Steam-Stable and High-Performance Poly (Ethyleneimine)/Silica CO2 Adsorbent. ChemSusChem 2017, 10, 2518– 2526,  DOI: 10.1002/cssc.201700398
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    48
    Macroporous Silica with Thick Framework for Steam-Stable and High-Performance Poly(ethyleneimine)/Silica CO2 Adsorbent
    Min, Kyungmin; Choi, Woosung; Choi, Minkee
    ChemSusChem (2017), 10 (11), 2518-2526CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Polyethyleneimine (PEI)/SiO2 has been widely studied as a solid adsorbent for post-combustion CO2 capture. This work synthesized a highly macroporous SiO2 (MacS) by secondary sintering of fumed SiO2 and compared it with various mesoporous SiO2 with different pore structures as a support for PEI. SiO2 with large pore diam. and vol. enabled high CO2 adsorption kinetics and capacity, since pore occlusion by the supported PEI was minimized. Steam stability of SiO2 structures increased with SiO2 wall thickness due to suppressed framework ripening. SiO2 with low steam stability displayed rapid PEI leaching, indicating PEI squeezed out of collapsed SiO2 pores leached more readily. Consequently, MacS with an extra-large pore vol. (1.80 cm3/g), pore diam. (56.0 nm), and thick wall (>10 nm), exhibited the most promising CO2 adsorption kinetics and capacity and steam stability.
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  49. 49
    Hack, J.; Frazzetto, S.; Evers, L.; Maeda, N.; Meier, D. M. Branched versus Linear Structure: Lowering the CO2 Desorption Temperature of Polyethylenimine-Functionalized Silica Adsorbents. Energies (Basel) 2022, 15, 1075,  DOI: 10.3390/en15031075
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    49
    Branched versus Linear Structure: Lowering the CO2 Desorption Temperature of Polyethylenimine-Functionalized Silica Adsorbents
    Hack, Jannis; Frazzetto, Seraina; Evers, Leon; Maeda, Nobutaka; Meier, Daniel M.
    Energies (Basel, Switzerland) (2022), 15 (3), 1075CODEN: ENERGA; ISSN:1996-1073. (MDPI AG)
    Lowering the regeneration temp. for solid CO2-capture materials is one of the crit. tasks for economizing CO2-capturing processes. Based on reported pKa values and nucleophilicity, we compared two different polyethylenimines (PEIs): branched PEI (BPEI) and linear PEI (LPEI). LPEI outperformed BPEI in terms of adsorption and desorption properties. Because LPEI is a solid below 73-75°C, even a high loading amt. of LPEI can effectively adsorb CO2 without diffusive barriers. Temp.-programmed desorption (TPD) demonstrated that the desorption peak top dropped to 50.8°C for LPEI, compared to 78.0°C for BPEI. We also revisited the classical adsorption model of CO2 on secondary amines by using in situ modulation excitation IR spectroscopy, and proposed a new adsorption configuration, R1(R2)-NCOOH. Even though LPEI is more expensive than BPEI, considering the long-term operation of a CO2-capturing system, the low regeneration temp. makes LPEI attractive for industrial applications.
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    Sayari, A.; Heydari-Gorji, A.; Yang, Y. CO2-Induced Degradation of Amine-Containing Adsorbents: Reaction Products and Pathways. J. Am. Chem. Soc. 2012, 134, 13834– 13842,  DOI: 10.1021/ja304888a
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    CO2-Induced Degradation of Amine-Containing Adsorbents: Reaction Products and Pathways
    Sayari, Abdelhamid; Heydari-Gorji, Aliakbar; Yang, Yong
    Journal of the American Chemical Society (2012), 134 (33), 13834-13842CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A comprehensive study was conducted to study the stability of a wide variety of mesoporous SiO2-supported amine-contg. adsorbents in the presence of CO2 under dry conditions. CO2-induced degrdn. of grafted primary and secondary monoamines (pMono, sMono), diamines with one primary and one secondary amines (Diamine) and triamine with one primary and 2 secondary amines (TRI) as well as different impregnated polyamines such as branched and linear polyethylenimine (BPEI and LPEI) and polyallylamine (PALL) was studied using extensive CO2 adsorption-desorption cycling as well as diffuse reflectance IR Fourier transform (DRIFT) and 13C CP MASNMR measurements. Except for sMono, all other supported amines underwent significant deactivation in the presence of dry CO2 under mild conditions. In all cases, the decrease in CO2 uptake was assocd. with the formation of urea linkages at the expense of amine groups. The urea-contg. species were identified, and the deactivation pathways were delineated.
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    Fan, Y.; Jia, X. Progress in Amine-Functionalized Silica for CO2 Capture: Important Roles of Support and Amine Structure. Energy Fuels 2022, 36, 1252– 1270,  DOI: 10.1021/acs.energyfuels.1c03788
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    Progress in amine-functionalized silica for CO2 capture: important roles of support and amine structure
    Fan, Yanfang; Jia, Xiaohao
    Energy & Fuels (2022), 36 (3), 1252-1270CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    A review. In the past decades, a huge development in rational design, synthesis and application of solid amine CO2 adsorbents has been reported. Since the reports on polyethylenimine impregnated silica adsorbents, there has been tremendous growth in this area along with the increase of supported amine adsorbents related publications. Owing to high adsorption capacity, facile synthesis procedure and low cost, amine-functionalized silica adsorbents demonstrate the great promise for CO2 capture technologies. To push their industrial application forward, the stability must be improved further to mitigate acidic impurities, CO2, O2 and thermal-induced deactivation. Furthermore, fundamental understanding of relationship between amine/support structure and adsorption performance is needed to guide the design of stable amine silica adsorbents. This summarizes recent researches that mainly focus on important roles of support and amine structure in CO2 adsorption performance and improved stability. The recent progress in promising reactor design for solid amine adsorbents is discussed to provide insights on developing the continuous CO2 capture process for sepg. CO2 from flue gas and ambient air.
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    Choi, W.; Min, K.; Kim, C.; Ko, Y. S.; Jeon, J. W.; Seo, H.; Park, Y.-K.; Choi, M. Epoxide-Functionalization of Polyethyleneimine for Synthesis of Stable Carbon Dioxide Adsorbent in Temperature Swing Adsorption. Nat. Commun. 2016, 7, 12640,  DOI: 10.1038/ncomms12640
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    Epoxide-functionalization of polyethyleneimine for synthesis of stable carbon dioxide adsorbent in temperature swing adsorption
    Choi, Woosung; Min, Kyungmin; Kim, Chaehoon; Ko, Young Soo; Jeon, Jae Wan; Seo, Hwimin; Park, Yong-Ki; Choi, Minkee
    Nature Communications (2016), 7 (), 12640CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Amine-contg. adsorbents have been extensively investigated for post-combustion carbon dioxide capture due to their ability to chemisorb low-concn. carbon dioxide from a wet flue gas. However, earlier studies have focused primarily on the carbon dioxide uptake of adsorbents, and have not demonstrated effective adsorbent regeneration and long-term stability under such conditions. Here, we report the versatile and scalable synthesis of a functionalized-polyethyleneimine (PEI)/silica adsorbent which simultaneously exhibits a large working capacity (2.2 mmol g-1) and long-term stability in a practical temp. swing adsorption process (regeneration under 100% carbon dioxide at 120°C), enabling the sepn. of concd. carbon dioxide. We demonstrate that the functionalization of PEI with 1,2-epoxybutane reduces the heat of adsorption and facilitates carbon dioxide desorption (>99%) during regeneration compared with unmodified PEI (76%). Moreover, the functionalization significantly improves long-term adsorbent stability over repeated temp. swing adsorption cycles due to the suppression of urea formation and oxidative amine degrdn.
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    Hammouda, B. Probing Nanoscale Structures-the sans Toolbox; National Institute of Standards and Technology, 2008; pp 1– 717.
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    Zhao, J. K.; Gao, C. Y.; Liu, D. The Extended Q-Range Small-Angle Neutron Scattering Diffractometer at the SNS. J. Appl. Crystallogr. 2010, 43, 1068– 1077,  DOI: 10.1107/S002188981002217X
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    The extended Q-range small-angle neutron scattering diffractometer at the SNS
    Zhao, J. K.; Gao, C. Y.; Liu, D.
    Journal of Applied Crystallography (2010), 43 (5, Pt. 1), 1068-1077CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
    The extended Q-range small-angle neutron scattering diffractometer (EQ-SANS) at the Spallation Neutron Source (SNS), Oak Ridge, is designed for wide neutron momentum transfer (Q) coverage, high neutron beam intensity and good wavelength resoln. In addn., the design and construction of the instrument aim to achieve a max. signal-to-noise ratio by minimizing the background. The instrument is located on the high-power target station at the SNS. One of the key components in the primary flight path is the neutron optics, consisting of a curved multichannel beam bender and sections of straight neutron guides. They are optimized to minimize neutron transport loss, thereby maximizing the available flux on the sample. They also enable the avoidance of a direct line of sight to the neutron moderator at downstream locations. The instrument has three bandwidth-limiting choppers. They allow a novel frame-skipping operation, which enables the EQ-SANS diffractometer to achieve a dynamic Q range equiv. to that of a similar machine on a 20 Hz source. The two-dimensional low-angle detector, based on 3He tube technologies, offers very high counting rates and counting efficiency. Initial operations have shown that the instrument has achieved its design goals.
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    Mamontov, E.; Herwig, K. W. A Time-of-Flight Backscattering Spectrometer at the Spallation Neutron Source, BASIS. Rev. Sci. Instrum. 2011, 82, 085109,  DOI: 10.1063/1.3626214
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    A time-of-flight backscattering spectrometer at the Spallation Neutron Source, BASIS
    Mamontov, E.; Herwig, K. W.
    Review of Scientific Instruments (2011), 82 (8), 085109/1-085109/10CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
    We describe the design and current performance of the backscattering silicon spectrometer (BASIS), a time-of-flight backscattering spectrometer built at the spallation neutron source (SNS) of the Oak Ridge National Lab. (ORNL). BASIS is the first silicon-based backscattering spectrometer installed at a spallation neutron source. In addn. to high intensity, it offers a high-energy resoln. of about 3.5 μeV and a large and variable energy transfer range. These ensure an excellent overlap with the dynamic ranges accessible at other inelastic spectrometers at the SNS. (c) 2011 American Institute of Physics.
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    Meyer, A.; Dimeo, R. M.; Gehring, P. M.; Neumann, D. A. The High-Flux Backscattering Spectrometer at the NIST Center for Neutron Research. Rev. Sci. Instrum. 2003, 74, 2759– 2777,  DOI: 10.1063/1.1568557
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    The high-flux backscattering spectrometer at the NIST Center for Neutron Research
    Meyer, A.; Dimeo, R. M.; Gehring, P. M.; Neumann, D. A.
    Review of Scientific Instruments (2003), 74 (5), 2759-2777CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
    A review. We describe the design and current performance of the high-flux backscattering spectrometer located at the NIST Center for Neutron Research. The design incorporates several state-of-the-art neutron optical devices to achieve the highest flux on sample possible while maintaining an energy resoln. of less than 1 μeV. Foremost among these is a novel phase-space transformation chopper that significantly reduces the mismatch between the beam divergences of the primary and secondary parts of the instrument. This resolves a long-standing problem of backscattering spectrometers, and produces a relative gain in neutron flux of 4.2. A high-speed Doppler-driven monochromator system has been built that is capable of achieving energy transfers of up to ±50 μeV, thereby extending the dynamic range of this type of spectrometer by more than a factor of 2 over that of other reactor-based backscattering instruments.
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    Yan, Z.; Zhang, R. Rapid Structural Analysis of Minute Quantities of Organic Solids by Exhausting 1H Polarization in Solid-State NMR Spectroscopy Under Fast Magic Angle Spinning. J. Phys. Chem. Lett. 2021, 12, 12067– 12074,  DOI: 10.1021/acs.jpclett.1c03672
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    Rapid Structural Analysis of Minute Quantities of Organic Solids by Exhausting 1H Polarization in Solid-State NMR Spectroscopy Under Fast Magic Angle Spinning
    Yan, Zhiwei; Zhang, Rongchun
    Journal of Physical Chemistry Letters (2021), 12 (50), 12067-12074CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Solid-state NMR (NMR) often suffers from significant limitations due to the inherent low signal sensitivity when low-γ nuclei are involved. Herein, we report an elegant solid-state NMR approach for rapid structural anal. of minute amts. of org. solids. By encoding staggered chem. shift evolution in the indirect dimension and staggered acquisition in the 1H dimension, a proton-detected homonuclear 1H/1H and heteronuclear 13C/1H chem. shift correlation (HETCOR) spectrum can be obtained simultaneously in a single expt. at a fast magic-angle-spinning (MAS) condition with barely increasing the exptl. time. We further show that during the conventional 1H-detected HETCOR exptl. time, multiple homonuclear 1H/1H correlation spectra can be recorded in addn. to the HETCOR spectrum, enabling the detn. of 1H-1H distances. We establish that abundant 1H polarization can be efficiently manipulated and fully utilized in proton-detected solid-state NMR spectroscopy for extn. of more crit. structural information and thus redn. of the total exptl. time.
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    Carrillo, J.-M. Y.; Sumpter, B. G. Structure and Dynamics of Confined Flexible and Unentangled Polymer Melts in Highly Adsorbing Cylindrical Pores. J. Chem. Phys. 2014, 141, 074904,  DOI: 10.1063/1.4893055
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    Structure and dynamics of confined flexible and unentangled polymer melts in highly adsorbing cylindrical pores
    Carrillo, Jan-Michael Y.; Sumpter, Bobby G.
    Journal of Chemical Physics (2014), 141 (7), 074904/1-074904/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Coarse-grained mol. dynamics simulations are used to probe the dynamic phenomena of polymer melts confined in nanopores. The simulation results show excellent agreement in the values obtained for the normalized coherent single chain dynamic structure factor, ((S(Q,Δt))/(S(Q,0))). In the bulk configuration, both simulations and expts. confirm that the polymer chains follow Rouse dynamics. However, under confinement, the Rouse modes are suppressed. The mean-square radius of gyration 〈R2g〉 and the av. relative shape anisotropy 〈κ2〉 of the conformation of the polymer chains indicate a pancake-like conformation near the surface and a bulk-like conformation near the center of the confining cylinder. This was confirmed by direct visualization of the polymer chains. Despite the presence of these different conformations, the av. form factor of the confined chains still follows the Debye function which describes linear ideal chains, which is in agreement with small angle neutron scattering expts. (SANS). The exptl. inaccessible mean-square displacement (MSD) of the confined monomers, calcd. as a function of radial distance from the pore surface, was obtained in the simulations. The simulations show a gradual increase of the MSD from the adsorbed, but mobile layer, to that similar to the bulk far away from the surface. (c) 2014 American Institute of Physics.
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    The neutron scattering length density (SLD) of molecules can be calculated based on the properties of the constituent atoms (scattering lengths) and the molecular properties (molecular volume). For further information, please refer to the NIST webpage (https://www.ncnr.nist.gov/resources/sldcalc.html).
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    Coherent neutron scattering gives rise to constructive diffraction patterns, which can aid analysis of structure and morphology. One can refer to scattering lengths of various atoms (isomer-specific) where larger positive values refer to stronger coherent scattering upon neutrons scattering with nuclei. A full listing of the scattering lengths can be found in ref (46).

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    Sears, V. F. Neutron Scattering Lengths and Cross Sections. Neutron news 1992, 3, 26– 37,  DOI: 10.1080/10448639208218770
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    Chiang, W.-S.; Fratini, E.; Baglioni, P.; Georgi, D.; Chen, J.-H.; Liu, Y. Methane Adsorption in Model Mesoporous Material, SBA-15, Studied by Small-Angle Neutron Scattering. J. Phys. Chem. C 2016, 120, 4354– 4363,  DOI: 10.1021/acs.jpcc.5b10688
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    Methane Adsorption in Model Mesoporous Material, SBA-15, Studied by Small-Angle Neutron Scattering
    Chiang, Wei-Shan; Fratini, Emiliano; Baglioni, Piero; Georgi, Daniel; Chen, Jin-Hong; Liu, Yun
    Journal of Physical Chemistry C (2016), 120 (8), 4354-4363CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The understanding of methane adsorption is important for many industrial applications, esp. for the shale gas prodn., where it is crit. to understand the adsorption/desorption of methane in pores even as small as a few nanometers. Using small-angle neutron scattering (SANS), the authors have studied the adsorption of deuterated methane (CD4) into one model mesoporous material, SBA-15, with pore diam. approx. 6.8 nm at the temp. range from 20 to 295 K at low pressure (≈100 kPa). A new scattering model is developed to analyze the SANS patterns of gas adsorption in SBA-15. The surface roughness of the SBA-15 matrix is estd. The gas adsorption behaviors on the surface regions are extd. from the fitting. The rough surface of the pores is found to retain a large amt. of CD4 at the temp. above the capillary condensation temp. (Tc). At temps. below Tc, the confined liq. and solid methane are estd. to be less dense than the corresponding bulk liq. and solid methane. Detailed theor. anal. and exptl. verification also show that SANS patterns at temps. higher than Tc are much more sensitive to the change of the excess adsorption, εads, rather than the av. d. of adsorbed layers commonly used in many studies. The model we establish can be used to analyze future SANS/SAXS data for gas confined in similar model porous materials.
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    Commercial tetraethylenepentamine (TEPA) contains a mixture of branched and cyclic isomers. Please see ref (64) for details.

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  64. 64
    Numaguchi, R.; Fujiki, J.; Yamada, H.; Chowdhury, A.; Kida, K.; Goto, K.; Okumura, T.; Yoshizawa, K.; Yogo, K. Development of Post-Combustion CO2 Capture System Using Amine-Impregnated Solid Sorbent. Energy Procedia 2017, 114, 2304– 2312,  DOI: 10.1016/j.egypro.2017.03.1371
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    Development of Post-combustion CO2 Capture System Using Amine-impregnated Solid Sorbent
    Numaguchi, Ryohei; Fujiki, Junpei; Yamada, Hidetaka; Firoz; Chowdhury, A.; Kida, Koji; Goto, Kazuya; Okumura, Takeshi; Yoshizawa, Katsuhiro; Yogo, Katsunori
    Energy Procedia (2017), 114 (), 2304-2312CODEN: EPNRCV; ISSN:1876-6102. (Elsevier Ltd.)
    A new synthetic polyamine was found as a suitable compd. for amine-impregnated solid sorbent through screening of amine compds. with the aid of d. functional theory calcns. CO2 capture performance of the sorbent was evaluated on a lab-scale CO2 capture system. The solid sorbent was excellently regenerable even at low temp. condition. Then, Bench-scale demonstration will be started on current project, using the sorbent developed by Research Institute of Innovative Technol. for the Earth (RITE) and moving bed system of Kawasaki Heavy Industries, Ltd (KHI).
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    Carrillo, J.-M. Y.; Potter, M. E.; Sakwa-Novak, M. A.; Pang, S. H.; Jones, C. W.; Sumpter, B. G. Linking Silica Support Morphology to the Dynamics of Aminopolymers in Composites. Langmuir 2017, 33, 5412– 5422,  DOI: 10.1021/acs.langmuir.7b00283
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    Linking Silica Support Morphology to the Dynamics of Aminopolymers in Composites
    Carrillo, Jan-Michael Y.; Potter, Matthew E.; Sakwa-Novak, Miles A.; Pang, Simon H.; Jones, Christopher W.; Sumpter, Bobby G.
    Langmuir (2017), 33 (22), 5412-5422CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    A combined computational and exptl. approach is used to elucidate the effect of silica support morphol. on polymer dynamics and CO2 adsorption capacities in aminopolymer/silica composites. Simulations are based on coarse-grained mol. dynamics simulations of aminopolymer composites where a branched aminopolymer, representing poly(ethylenimine) (PEI), is impregnated into different silica mesoporous supports. The morphol. of the mesoporous supports varies from hexagonally packed cylindrical pores representing SBA-15, double gyroids representing KIT-6 and MCM-48, and cagelike structures representing SBA-16. In parallel, composites of PEI and the silica supports SBA-15, KIT-6, MCM-48, and SBA-16 are synthesized and characterized, including measuring their CO2 uptake. Simulations predict that a 3D pore morphol., such as those of KIT-6, MCM-48, and SBA-16, will have faster segmental mobility and have lower probability of primary amine and surface silanol assocns., which should translate to higher CO2 uptake in comparison to a 2D pore morphol. such as that of SBA-15. Indeed, it is found that KIT-6 has higher CO2 uptake than SBA-15 at equiv. PEI loading, even though both supports have similar surface area and pore vol. However, this is not the case for the MCM-48 support, which has smaller pores, and SBA-16, whose pore structure rapidly degrades after PEI impregnation.
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    Carrillo, J.-M. Y.; Sakwa-Novak, M. A.; Holewinski, A.; Potter, M. E.; Rother, G.; Jones, C. W.; Sumpter, B. G. Unraveling the Dynamics of Aminopolymer/Silica Composites. Langmuir 2016, 32, 2617– 2625,  DOI: 10.1021/acs.langmuir.5b04299
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    Unraveling the Dynamics of Aminopolymer/Silica Composites
    Carrillo, Jan-Michael Y.; Sakwa-Novak, Miles A.; Holewinski, Adam; Potter, Matthew E.; Rother, Gernot; Jones, Christopher W.; Sumpter, Bobby G.
    Langmuir (2016), 32 (11), 2617-2625CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The structure and dynamics of a model branched polymer was investigated through mol. dynamics simulations and neutron scattering expts. The polymer confinement, monomer concn., and solvent quality were varied in the simulations and detailed comparisons between the calcd. structural and dynamical properties of the unconfined polymer and those confined within an adsorbing and nonadsorbing cylindrical pore, representing the silica based structural support of the composite, were made. The simulations show a direct relationship in the structure of the polymer and the nonmonotonic dynamics as a function of monomer concn. within an adsorbing cylindrical pore. However, the nonmonotonic behavior disappears for the case of the branched polymer within a nonadsorbing cylindrical pore. Overall, the simulation results are in good agreement with quasi-elastic neutron scattering (QENS) studies of branched poly(ethylenimine) in mesoporous silica (SBA-15) of comparable size, suggesting an approach that can be a useful guide for understanding how to tune porous polymer composites for enhancing desired dynamical and structural behavior targeting carbon dioxide adsorption.
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  • Abstract

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    Figure 1

    Figure 1. Schematic structure and the CO2 capture mechanism in PEI/SBA-15. (a) Schematic showing PEI/SBA-15. (b) CO2 diffusion pathways reaching amine groups and interaction with amines resulting in CO2 sorption. (16,42,43) CO2–amine interaction reprinted with permission from ref (16). Copyright 2011 Royal Society of Chemistry.

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    Figure 2

    Figure 2. Structural characteristics of PEI/SBA-15 captured via SANS. (a) SANS spectra (intensity vs scattering vector Q) of bare SBA-15 support and PEI/SBA-15 composites with varied PEI loadings, with diffraction peaks highlighted. (b) Key structural features captured via SANS, such as particle surface scatter, form factor of a mesopore, structure factor showing arrangement of mesopores, and structural inhomogeneities. (53,62) (c) PEI morphologies considered, where i represents consistent PEI deposition on the pore walls, ii represents formation of PEI plugs in the pores, and iii indicates mixed cases of the formers. (d) Possible PEI morphologies around the wall–PEI interfaces with different extents of surface roughness. Particle length of SBA-15 is approximately 1 μm. (1) Reproduced with permission from ref (1). Copyright 2015 American Chemical Society.

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    Figure 3

    Figure 3. Characterization of PEI mobility using QENS and impacts on CO2 uptake. (a) Mean-square displacement as a function of temperature taken by QENS, showing trend of PEI mobility. (b) QENS spectra taken at 375 K with the momentum transfer (Q) of 0.87 Å–1 (normalized intensities vs energy transfer). (c) QENS spectral widths (half-width at half-maximum; HWHM) fitted against jump-mediated diffusion model. (d) Amine efficiencies as a function of volumetric fill fraction of PEI/SBA-15, showing relationship between PEI mobility and CO2 uptake. (2) Reproduced with permission from ref (2). Copyright 2017 American Chemical Society.

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    Figure 4

    Figure 4. Impacts of wall–PEI interactions in PEI mobility and distribution. (a) Different types of wall–PEI interactions. (b) Trend of PEI mobility understood by analyzing the spectral widths of QENS spectra (curve fitting done by jump-mediated diffusion). (c) Hypothesized configurations of wall-grafted alkylamine chains and alkyl chains and PEI distribution around pore walls. (f) 1H T1–T2 correlation plots for wall-grafted chains. (e) 1H T1–T2 plots for PEI around pore walls. (f) Density distribution function calculated by MD simulation (probability vs distance, where r ∼ 11 refers to the walls), where dotted lines represent wall-grafted chains and solid lines represent primary amine groups of PEI. (g) Comparison of CO2 uptake rates based on fractional uptake versus time and underlying causes. Reproduced with permission from ref (3). Copyright 2022 American Chemical Society.

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    Figure 5

    Figure 5. Morphology and mobility of amines with different chain topologies probed by 1H T1–T2 NMR and relationship to CO2 sorption behavior. (a) 1H T1–T2 plots for TEPA/SBA-15 with varied loadings. (b) T1–T2 plots for PEI/SBA-15 with matching pore fill fraction to TEPA cases. (c) CO2 sorption results shown by fractional uptake, pseudoequilibrium capacities, and amine efficiencies. (d) CO2 uptake curves at early stages and the initial uptake rates calculated in the semilinear region approximately 0–10 min. (4) Reproduced with permission from ref (4). Copyright 2023 American Chemical Society.

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  • References

    This article references 66 other publications.

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      Linking CO2 Sorption Performance to Polymer Morphology in Aminopolymer/Silica Composites through Neutron Scattering
      Holewinski, Adam; Sakwa-Novak, Miles A.; Jones, Christopher W.
      Journal of the American Chemical Society (2015), 137 (36), 11749-11759CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Composites of poly(ethylenimine) (PEI) and mesoporous silica are effective, reversible adsorbents for CO2, both from flue gas and in direct air-capture applications. The morphol. of the PEI within the silica can strongly impact the overall carbon capture efficiency and rate of satn. Here, we directly probe the spatial distribution of the supported polymer through small-angle neutron scattering (SANS). Combined with textural characterization from physisorption anal., the data indicate that PEI first forms a thin conformal coating on the pore walls, but all addnl. polymer aggregates into plug(s) that grow along the pore axis. This model is consistent with obsd. trends in amine-efficiency (CO2/N binding ratio) and pore size distributions, and points to a trade-off between achieving high chem. accessibility of the amine binding sites, which are inaccessible when they strongly interact with the silica, and high accessibility for mass transport, which can be hampered by diffusion through PEI plugs. We illustrate this design principle by demonstrating higher CO2 capacity and uptake rate for PEI supported in a hydrophobically modified silica, which exhibits repulsive interactions with the PEI, freeing up binding sites.
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      Holewinski, A.; Sakwa-Novak, M. A.; Carrillo, J.-M. Y.; Potter, M. E.; Ellebracht, N.; Rother, G.; Sumpter, B. G.; Jones, C. W. Aminopolymer Mobility and Support Interactions in Silica–PEI Composites for CO2 Capture Applications: A Quasielastic Neutron Scattering Study. J. Phys. Chem. B 2017, 121, 6721– 6731,  DOI: 10.1021/acs.jpcb.7b04106
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      Aminopolymer Mobility and Support Interactions in Silica-PEI Composites for CO2 Capture Applications: A Quasielastic Neutron Scattering Study
      Holewinski, Adam; Sakwa-Novak, Miles A.; Carrillo, Jan-Michael Y.; Potter, Matthew E.; Ellebracht, Nathan; Rother, Gernot; Sumpter, Bobby G.; Jones, Christopher W.
      Journal of Physical Chemistry B (2017), 121 (27), 6721-6731CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      Composite gas sorbents, with an active polymer phase and a porous support, are promising materials to sep. acid gases from a variety of gas streams. Significant changes in sorption performance (capacity, rate, stability) can be achieved by tuning the polymer properties and the nature of interactions between polymer and support. This work used quasi-elastic neutron scattering (QENS) and coarse-grained mol. dynamics (MD) simulations to characterize the dynamic behavior of the most commonly reported polymer in such materials, poly(ethylenimine) (PEI), in bulk form and when supported in a mesoporous SiO2 framework. Polymer chain dynamics (rotational and translational diffusion) were characterized using two neutron back-scattering spectrometers which had overlapping time scales, ranging from pico-seconds to several nanoseconds. Two motion modes were detected for the PEI mol. in QENS. At low energy transfer, a slow process (∼200 ps) was obsd. and attributed to jump-mediated, center-of-mass diffusion. A second, fast process at ∼20 ps was obsd. and is attributed to locally confined, jump-diffusion. Characteristic data (time scale, spectral wt.) of these processes were compared to those characterized by MD; reasonable agreement was achieved. For nanopore-confined PEI, a significant redn. in polymer motion time scale was obsd. vs. bulk. The effect of SiO2 surface functionalization and polymer fill fraction in SiO2 pores (controlling the portion of polymer mols. in contact with pore walls), were examd. in detail. Hydrophobic functionalization of SiO2 led to an increased PEI mobility, above that of native silanol-terminated SiO2; however, the dynamics were still slower than those in bulk PEI. Sorbents with faster PEI dynamics were also more efficient for CO2 capture, possibly because sorption sites were more accessible than those in systems with slower PEI dynamics. Results supported the existence of a link between the support affinity for PEI and the accessibility of active sorbent functional groups.
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      Moon, H. J.; Carrillo, J.-M.; Leisen, J.; Sumpter, B. G.; Osti, N. C.; Tyagi, M.; Jones, C. W. Understanding the Impacts of Support-Polymer Interactions on the Dynamics of Poly (Ethyleneimine) Confined in Mesoporous SBA-15. J. Am. Chem. Soc. 2022, 144, 11664– 11675,  DOI: 10.1021/jacs.2c03028
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      Understanding the Impacts of Support-Polymer Interactions on the Dynamics of Poly(ethyleneimine) Confined in Mesoporous SBA-15
      Moon, Hyun June; Carrillo, Jan-Michael; Leisen, Johannes; Sumpter, Bobby G.; Osti, Naresh C.; Tyagi, Madhusudan; Jones, Christopher W.
      Journal of the American Chemical Society (2022), 144 (26), 11664-11675CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Supported amines are a promising class of CO2 sorbents offering large uptake capacities and fast uptake rates. Among supported amines, poly(ethyleneimine) (PEI) phys. impregnated in the mesopores of SBA-15 silica is widely used. Within these composite materials, the chain dynamics and morphologies of PEI strongly influence the CO2 capture performance, yet little is known about chain and macromol. mobility in confined pores. Here, we probe the impact of the support-PEI interactions on the dynamics and structures of PEI at the support interface and the corresponding impact on CO2 uptake performance, which yields crit. structure-property relationships. The pore walls of the support are grafted with organosilanes with different chem. end groups to differentiate interaction modes (spanning from strong attraction to repulsion) between the pore surface and PEI. Combinations of techniques, such as quasi-elastic neutron scattering (QENS), 1H T1-T2 relaxation correlation solid-state NMR, and mol. dynamics (MD) simulations, are used to comprehensively assess the phys. properties of confined PEI. We hypothesized that PEI would have faster dynamics when subjected to less attractive or repulsive interactions. However, we discover that complex interfacial interactions resulted in complex structure-property relationships. Indeed, both the chain conformation of the surface-grafted chains and of the PEI around the surface influenced the chain mobility and CO2 uptake performance. By coupling knowledge of the dynamics and distributions of PEI with CO2 sorption performance and other characteristics, we det. that the macroscopic structures of the hybrid materials dictate the first rapid CO2 uptake, and the rate of CO2 sorption during the subsequent gradual uptake stage is detd. by PEI chain motions that promote diffusive jumps of CO2 through PEI-packed domains.
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      Moon, H. J.; Sekiya, R.-S.; Jones, C. W. Probing the Morphology and Mobility of Amines in Porous Silica CO2 Sorbents by 1H T 1-T 2 Relaxation Correlation NMR. J. Phys. Chem. C 2023, 127, 11652– 11665,  DOI: 10.1021/acs.jpcc.3c02441
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      Probing the Morphology and Mobility of Amines in Porous Silica CO2 Sorbents by 1H T1-T2 Relaxation Correlation NMR
      Moon, Hyun June; Sekiya, Ryoh-Suke; Jones, Christopher W.
      Journal of Physical Chemistry C (2023), 127 (24), 11652-11665CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Mol. and oligomeric amines supported in porous oxide supports are a promising class of CO2 sorbent materials studied for CO2 removal from diverse streams such as flue gas and ambient air. Among the various amines investigated, low mol. wt., hyperbranched poly(ethyleneimine) (PEI), and tetraethylenepentamine (TEPA) are among the most extensively studied. While macroscopic structure-performance relationships relating the support structure, amine loading, and other factors affecting CO2 sorption capacities and kinetics have been developed, structural and dynamic information about the org. amine phase in the porous support is less plentiful. The structure and mobility of amines impregnated in the pores of porous supports directly impact gas sorption, as the accessibility of amine sites in the pores directly relates to amine distribution in the pores and overall pore filling as well as the dynamics of the amine chains. Here, we prep. a family of mesoporous silica SBA-15 materials contg. varying loadings of oligomeric (PEI) and mol. (TEPA) amines. 1H T1-T2 relaxation correlation solid-state NMR expts. are used to characterize the structural and dynamic properties of the confined amines. Both TEPA and PEI are shown to form multiple different domains in the pores, each with distinguishable dynamic properties. TEPA and PEI form more rigid layers around the silica support walls at lower org. loading fractions, characterized by lower mobilities, followed by the formation of more mobile domains less engaged in pore wall interactions at higher loadings. TEPA shows faster mobilities than PEI because of its lower mol. wt. TEPA also appears to more easily transfer between domains within the pores, leading to generally faster CO2 uptake rates with higher sorption capacities, while PEI located closer to the pore walls remained much less mobile and is thus less engaged in CO2 capture.
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      Carbon Dioxide Capture: prospects for New Materials
      D'Alessandro, Deanna M.; Smit, Berend; Long, Jeffrey R.
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      A review. The escalating level of atm. carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO2 emissions; however, currently the capture alone will increase the energy requirements of a plant by 25-40%. This review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO2 emissions to the atm., namely postcombustion (predominantly CO2/N2 sepn.), precombustion (CO2/H2) capture, and natural gas sweetening (CO2/CH4). The key factor which underlies significant advancements lies in improved materials that perform the sepns. In this regard, the most recent developments and emerging concepts in CO2 sepns. by solvent absorption, chem. and phys. adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal-org. frameworks.
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      Functionalizing the well-known MIL-53(Al) metal-org. framework with amino groups increases its selectivity in CO2/CH4 sepns. by orders of magnitude while maintaining a very high capacity for CO2 capture.
      >> More from SciFinder ®
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      Different alkylamine mols. were post-synthetically tethered to unsatd. Cr3+ centers in the metal-org. framework, MIL-101. The resultant metal-org. frameworks displayed almost no N2 adsorption but significantly enhanced CO2 capture under ambient conditions due to the interaction between amine groups and CO2 mols. Given its extraordinary stability, high CO2 uptake, ultra-high CO2/N2 selectivity, and mild regeneration energy, MIL-101/diethylenetriamine holds exceptional promise for post-combustion CO2 capture and CO2/N2 sepn.
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      Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal-Organic Framework mmen-Mg2(dobpdc)
      McDonald, Thomas M.; Lee, Woo Ram; Mason, Jarad A.; Wiers, Brian M.; Hong, Chang Seop; Long, Jeffrey R.
      Journal of the American Chemical Society (2012), 134 (16), 7056-7065CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Two new metal-org. frameworks, M2(dobpdc) (M = Zn (1), Mg (2); dobpdc4- = 4,4'-dioxido-3,3'-biphenyldicarboxylate), adopting an expanded MOF-74 structure type, were synthesized via solvothermal and microwave methods. Coordinatively unsatd. Mg2+ cations lining the 18.4-Å-diam. channels of 2 were functionalized with N,N'-dimethylethylenediamine (mmen) to afford Mg2(dobpdc)(mmen)1.6(H2O)0.4 (mmen-Mg2(dobpdc)). This compd. displays an exceptional capacity for CO2 adsorption at low pressures, taking up 2.0 mmol/g (8.1 wt %) at 0.39 mbar and 25 °C, conditions relevant to removal of CO2 from air, and 3.14 mmol/g (12.1 wt %) at 0.15 bar and 40 °C, conditions relevant to CO2 capture from flue gas. Dynamic gas adsorption/desorption cycling expts. demonstrate that mmen-Mg2(dobpdc) can be regenerated upon repeated exposures to simulated air and flue gas mixts., with cycling capacities of 1.05 mmol/g (4.4 wt %) after 1 h of exposure to flowing 390 ppm CO2 in simulated air at 25 °C and 2.52 mmol/g (9.9 wt %) after 15 min of exposure to flowing 15% CO2 in N2 at 40 °C. The purity of the CO2 removed from dry air and flue gas in these processes was estd. to be 96% and 98%, resp. As a flue gas adsorbent, the regeneration energy was estd. through differential scanning calorimetry expts. to be 2.34 MJ/kg CO2 adsorbed. Overall, the performance characteristics of mmen-Mg2(dobpdc) indicate it to be an exceptional new adsorbent for CO2 capture, comparing favorably with both amine-grafted silicas and aq. amine solns.
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    10. 10
      Lu, W.; Yuan, D.; Zhao, D.; Schilling, C. I.; Plietzsch, O.; Muller, T.; Brase, S.; Guenther, J.; Blumel, J.; Krishna, R. Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/Separation. Chem. Mater. 2010, 22, 5964– 5972,  DOI: 10.1021/cm1021068
      10
      Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/Separation
      Lu, Weigang; Yuan, Daqiang; Zhao, Dan; Schilling, Christine Inge; Plietzsch, Oliver; Muller, Thierry; Brase, Stefan; Guenther, Johannes; Blumel, Janet; Krishna, Rajamani; Li, Zhen; Zhou, Hong-Cai
      Chemistry of Materials (2010), 22 (21), 5964-5972CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Three porous polymer networks (PPNs) have been synthesized by the homocoupling of tetrahedral monomers. Like other hyper-crosslinked polymer networks, these materials are insol. in conventional solvents and exhibit high thermal and chem. stability. Their porosity was confirmed by N2 sorption isotherms at 77 K. One of these materials, PPN-3, has a Langmuir surface area of 5323 m2 g-1. Their clean energy applications, esp. in H2, CH4, and CO2 storage, as well as CO2/CH4 sepn., have been carefully investigated. Although PPN-1 has the highest gas affinity because of its smaller pore size, the maximal gas uptake capacity is directly proportional to their surface area. PPN-3 has the highest H2 uptake capacity among these three (4.28 wt %, 77 K). Although possessing the lowest surface area, PPN-1 shows the best CO2/CH4 selectivity among them.
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    11. 11
      Ben, T.; Ren, H.; Ma, S.; Cao, D.; Lan, J.; Jing, X.; Wang, W.; Xu, J.; Deng, F.; Simmons, J. M. Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area. Angew. Chem., Int. Ed. 2009, 48, 9457– 9460,  DOI: 10.1002/anie.200904637
      11
      Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area
      Ben, Teng; Ren, Hao; Ma, Shengqian; Cao, Dapeng; Lan, Jianhui; Jing, Xiaofei; Wang, Wenchuan; Xu, Jun; Deng, Feng; Simmons, Jason M.; Qiu, Shilun; Zhu, Guangshan
      Angewandte Chemie, International Edition (2009), 48 (50), 9457-9460, S9457/1-S9457/16CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      With the assistance of computational design, a porous arom. framework with an unprecedented high surface area of 7100 m2g-1 has been successfully synthesized. PAF-1 possesses local diamond-like tetrahedral bonding of tetraphenylenemethane building units to produce exceptional thermal and hydrothermal stabilities. In addn., PAF-I demonstrates high uptake capacities of hydrogen and carbon dioxide, as well as benzene and toluene vapors. The strategy presented in this work opens a new avenue for the design and construction of highly porous materials with exceptional stabilities for clean energy and environmental applications.
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    12. 12
      Lu, W.; Sculley, J. P.; Yuan, D.; Krishna, R.; Zhou, H.-C. Carbon Dioxide Capture from Air Using Amine-Grafted Porous Polymer Networks. J. Phys. Chem. C 2013, 117, 4057– 4061,  DOI: 10.1021/jp311512q
      12
      Carbon Dioxide Capture from Air Using Amine-Grafted Porous Polymer Networks
      Lu, Weigang; Sculley, Julian P.; Yuan, Daqiang; Krishna, Rajamani; Zhou, Hong-Cai
      Journal of Physical Chemistry C (2013), 117 (8), 4057-4061CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Amine-grafted porous polymer networks were assessed for the direct capture of CO2 from air (400 ppm CO2, 78.96% N2, 21% O2). Under these ultra-dil. conditions, PPN-6-CH2DETA had an extraordinarily high CO2 selectivity (3.6 × 1010) and loading capacity (1.04 mol/kg), calcd. using ideal adsorption soln. theory. This material also out-performed other materials based on simulated break-through calcns., showing great potential for use in direct air capture applications.
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    13. 13
      Lu, W.; Sculley, J. P.; Yuan, D.; Krishna, R.; Wei, Z.; Zhou, H. Polyamine-tethered Porous Polymer Networks for Carbon Dioxide Capture from Flue Gas. Angew. Chem., Int. Ed. 2012, 51, 7480– 7484,  DOI: 10.1002/anie.201202176
      13
      Polyamine-Tethered Porous Polymer Networks for Carbon Dioxide Capture from Flue Gas
      Lu, Weigang; Sculley, Julian P.; Yuan, Daqiang; Krishna, Rajamani; Wei, Zhangwen; Zhou, Hong-Cai
      Angewandte Chemie, International Edition (2012), 51 (30), 7480-7484, S7480/1-S7480/12CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Porous polymer networks prepd. by polymn. of tetrakis(4-bromophenyl)methane with subsequent functionalization by chloromethylation and reaction with diethylenetriamine had high photochem. and thermal stability, high CO2 adsorption capacity and low regeneration costs.
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    14. 14
      Kumar, P.; Guliants, V. V. Periodic Mesoporous Organic-Inorganic Hybrid Materials: Applications in Membrane Separations and Adsorption. Microporous Mesoporous Mater. 2010, 132, 1– 14,  DOI: 10.1016/j.micromeso.2010.02.007
      14
      Periodic mesoporous organic-inorganic hybrid materials: Applications in membrane separations and adsorption
      Kumar, Parveen; Guliants, Vadim V.
      Microporous and Mesoporous Materials (2010), 132 (1-2), 1-14CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier Inc.)
      A review on the state of the art on the synthesis, functionalization and emerging applications of mesoporous SiO2 materials. Mesoporous SiO2 materials can be synthesized as membranes or powders with controlled pore size and geometry depending on the synthesis conditions. Mesoporous membranes are generally grown on porous supports by solvent evapn. or hydrothermal synthesis techniques. Synthesis of powd. mesoporous SiO2 materials with controlled pore sizes in the range 2-30 nm and various different pore geometries was an active area of research over last 15 years. Functionalization of the pore channels of ordered mesoporous SiO2 with org. groups provides new opportunities for fine-tuning the chem., phys., mech., and dielec. properties of these intriguing materials. This led to an interest in application of these materials as sepn. membranes for the removal of environmental pollutants, e.g. greenhouse CO2 emissions, and the sepn. of bioethanol from H2O; heterogeneous catalysts; adsorbents for removal of environmental pollutants, such as Hg; as well as other advanced nanotechnol. applications.
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    15. 15
      Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G.; Jones, C. W. Designing Adsorbents for CO2 Capture from Flue Gas-Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly. J. Am. Chem. Soc. 2008, 130, 2902– 2903,  DOI: 10.1021/ja077795v
      15
      Designing Adsorbents for CO2 Capture from Flue Gas - Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly
      Hicks, Jason C.; Drese, Jeffrey H.; Fauth, Daniel J.; Gray, McMahan L.; Qi, Genggeng; Jones, Christopher W.
      Journal of the American Chemical Society (2008), 130 (10), 2902-2903CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Carbon dioxide adsorption from a simulated flue gas stream was successfully performed with a hyperbranched aminosilica (HAS) material. The HAS was synthesized by a one-step reaction, spontaneous aziridine ring-opening polymn. off of surface silanols, to form a 32 wt % org./inorg. hybrid material. The adsorption measurements were performed in a fixed-bed flow reactor using humidified CO2. The advantage of this adsorbent over previously reported adsorbents is the stability of the org. groups covalently bound to the silica support compared to those made by physisorbed methods. Furthermore, a large CO2 capacity (∼3 mmol CO2/g adsorbent) assocd. with the high loading of amines was obsd.
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    16. 16
      Bollini, P.; Didas, S. A.; Jones, C. W. Amine-Oxide Hybrid Materials for Acid Gas Separations. J. Mater. Chem. 2011, 21, 15100– 15120,  DOI: 10.1039/c1jm12522b
      16
      Amine-oxide hybrid materials for acid gas separations
      Bollini, Praveen; Didas, Stephanie A.; Jones, Christopher W.
      Journal of Materials Chemistry (2011), 21 (39), 15100-15120CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      A review of the development and use of org.-inorg. hybrid materials based on porous silica functionalized with amines and amino group-contg. organc compds., esp. for adsorptive sepn. of acid gases from dil. gas streams. Topics discussed include adsorption of CO2 from simulated flue gases (e.g., for post-combustion carbon capture), CO2 adsorption capacities, adsorption and desorption kinetics, material stability, amino-functionalized silanes, alkanolamines, polyethylenepolyamines, melamine, etc., CO2 adsorption mechanism, effect of oxide support, effect of accompanying org. groups, temp.-swing adsorption with inert gas purge or CO2 purge, steam stripping, pressure-swing adsorption, and degrdn. mechanisms of the hybrid materials, the nature of adsorbed CO2, adsorption of CO2 in the presence of H2S, and structure-adsorption kinetics.
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    17. 17
      Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Novel Polyethylenimine-Modified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO2 Capture. Energy Fuels 2002, 16, 1463– 1469,  DOI: 10.1021/ef020058u
      17
      Novel Polyethylenimine-Modified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO2 Capture
      Xu, Xiaochun; Song, Chunshan; Andresen, John M.; Miller, Bruce G.; Scaroni, Alan W.
      Energy & Fuels (2002), 16 (6), 1463-1469CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      A nanoporous solid adsorbent suitable for "mol. basket-type" adsorption of CO2 in the condensed form was prepd. by impregnation of polyethylenimine (PEI) onto MCM-41 mesoporous mol. sieve. The phys. properties of the adsorbents were characterized by x-ray powder diffraction, N2 adsorption/desorption, and thermogravimetric anal. The structure of the MCM-41 was preserved after loading the PEI, and the PEI was uniformly dispersed into the channels of the mol. sieve. The CO2 adsorption/desorption performance was tested in a flow system using a microbalance to track the wt. change. The mesoporous mol. sieve had a synergetic effect on the adsorption of CO2 by PEI. A CO2 adsorption capacity as high as 215 mg-CO2/g-PEI was obtained with MCM-41-PEI-50 at 75°, which is 24 times higher than that of the MCM-41 and is even 2 times that of the pure PEI. With an increase in the CO2 concn. in the CO2/N2 gas mixt., the CO2 adsorption capacity increased. The cyclic adsorption/desorption operation indicated that the performance of the adsorbent was stable. The adsorbent has interest in the sequestration and storage of CO2 from combustion processes.
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    18. 18
      Tsuda, T.; Fujiwara, T. Polyethyleneimine and Macrocyclic Polyamine Silica Gels Acting as Carbon Dioxide Absorbents. J. Chem. Soc., Chem. Commun. 1992, 22, 1659– 1661,  DOI: 10.1039/c39920001659
      There is no corresponding record for this reference.
    19. 19
      Huang, H. Y.; Yang, R. T.; Chinn, D.; Munson, C. L. Amine-Grafted MCM-48 and Silica Xerogel as Superior Sorbents for Acidic Gas Removal from Natural Gas. Ind. Eng. Chem. Res. 2003, 42, 2427– 2433,  DOI: 10.1021/ie020440u
      19
      Amine-Grafted MCM-48 and Silica Xerogel as Superior Sorbents for Acidic Gas Removal from Natural Gas
      Huang, Helen Y.; Yang, Ralph T.; Chinn, Daniel; Munson, Curtis L.
      Industrial & Engineering Chemistry Research (2003), 42 (12), 2427-2433CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      In an effort to develop selective solid sorbents for acidic gas (CO2 and H2S) removal from natural gas mixts., we synthesized amine-surface-modified silica xerogel and MCM-48 materials. With large amts. of basic amine groups on the surface, the sorbents are able to selectively bind the acidic gases CO2 and H2S. High adsorption capacities and adsorption rates were obtained for both gases. The adsorption-desorption isotherms of the gases and thermogravimetric anal. of the sorbents showed that these sorbents can be regenerated completely under mild conditions such as those used in pressure swing or temp. swing adsorption processes. We have also investigated the effect of moisture on the adsorption of CO2 and H2S by TPD-MS and IR spectroscopy. The results indicated that the presence of water vapor doubled the amt. of CO2 adsorbed and barely affected the H2S adsorption.
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    20. 20
      Harlick, P. J. E.; Sayari, A. Applications of Pore-Expanded Mesoporous Silicas. 3. Triamine Silane Grafting for Enhanced CO2 Adsorption. Ind. Eng. Chem. Res. 2006, 45, 3248– 3255,  DOI: 10.1021/ie051286p
      20
      Applications of Pore-Expanded Mesoporous Silicas. 3. Triamine Silane Grafting for Enhanced CO2 Adsorption
      Harlick, Peter J. E.; Sayari, Abdelhamid
      Industrial & Engineering Chemistry Research (2006), 45 (9), 3248-3255CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Conventional MCM-41 and pore-expanded MCM-41 (PE-MCM-41) silicas were used as supports for grafting 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (I) and tested for CO2 adsorption capacity and activity. The effects of the quantity of I added to the grafting mixt. on the CO2 adsorption capacity and apparent adsorption rate adsorption rate were examd. When both supports were grafted under the same conditions, PE-MCM-41 was grafted with slightly larger quantities of amine than MCM-41, for all controlled silane addns. Based on the adsorption performance of the materials using a dry 5% CO2/N2 feed mixt., the optimal quantity of I added to the grafting mixt. was detd. to be ∼3.0 cm3/g(SiO2), for both MCM-41 and PE-MCM-41. The CO2 adsorption capacity of I-PE-MCM-41 was significantly higher than that of I-MCM-41. Furthermore, the dynamic adsorption performance of I-PE-MCM-41 was far superior to I-MCM-41. In comparison to 13X zeolite, I-PE-MCM-41 exhibited higher adsorption capacities in the initial time frame of exposure, even though 13X zeolite exhibited a higher equil. adsorption capacity. The behavior was largely due to the rapid CO2-amine interaction and the open pore structure of I-PE-MCM-41 over that of 13X zeolite. When these adsorbents were exposed to a humid stream of 5% CO2/N2 (28% relative humidity), both grafted materials exhibited a slight increase in the adsorption capacity, whereas 13X zeolite did not retain any significant CO2 adsorption capacity. Thus, I-PE-MCM-41 material may be most suitable for use in a rapid cyclic adsorption process under humid feed conditions.
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    21. 21
      Harlick, P. J. E.; Sayari, A. Applications of Pore-Expanded Mesoporous Silica. 5. Triamine Grafted Material with Exceptional CO2 Dynamic and Equilibrium Adsorption Performance. Ind. Eng. Chem. Res. 2007, 46, 446– 458,  DOI: 10.1021/ie060774+
      21
      Applications of Pore-Expanded Mesoporous Silica. 5. Triamine Grafted Material with Exceptional CO2 Dynamic and Equilibrium Adsorption Performance
      Harlick, Peter J. E.; Sayari, Abdelhamid
      Industrial & Engineering Chemistry Research (2007), 46 (2), 446-458CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Application of pore-expanded MCM-41 (PE-MCM-41) mesoporous silica coated with 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (TRI) was extensively examd. for the adsorption of CO2 from N2. A systematic study of the amine loading as a function of the relative amts. of TRI and water used during the grafting procedure and the temp. of the grafting reaction was carried out. Extremely high levels of active amine content were achieved using prehydrated silica surfaces at grafting temps. below reflux to facilitate thermally controlled water-aided surface polymn. of the aminosilanes. The CO2 adsorption capacities and rates were detd. for all materials as a function of the amt. of TRI and water per g of support added to the grafting mixt. The optimal TRI grafted PE-MCM-41 adsorbent exhibited a 2.65 mmol/g adsorption capacity at 25° and 1.0 atm for a dry 5% CO2 in N2 feed mixt., which exceeded all literature reported values, for both meso- and microporous materials under the conditions used. Further, the apparent adsorption and desorption rates with the amine functionalized materials were exceedingly high. When considering the grafted amine quantity, the adsorption capacity and rate are mutually dependent on each other, exhibiting an apparent optimal combination. In comparison to zeolite 13X, the optimally loaded TRI-PE-MCM-41 was far superior in terms of dynamic adsorption and desorption performance. These results were further enhanced when the adsorbents were challenged with a humid stream of 5% CO2/N2. The TRI-PE-MCM-41 exhibited a 10% increase in CO2 adsorption capacity, whereas the 13X zeolite did not retain any significant CO2 adsorption capacity.
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    22. 22
      Serna-Guerrero, R.; Da’na, E.; Sayari, A. New Insights into the Interactions of CO2 with Amine-Functionalized Silica. Ind. Eng. Chem. Res. 2008, 47, 9406– 9412,  DOI: 10.1021/ie801186g
      22
      New Insights into the Interactions of CO2 with Amine-Functionalized Silica
      Serna-Guerrero, Rodrigo; Da'na, Enshirah; Sayari, Abdelhamid
      Industrial & Engineering Chemistry Research (2008), 47 (23), 9406-9412CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      The CO2-amine chem. in gas-solid processes was investigated under both humid and dry conditions using aminopropyl-grafted pore-expanded MCM-41 silica (MONO-PE-MCM-41). To draw accurate conclusions, a set of conditions had to be met including the use of an adsorbent with open pore structure and readily accessible adsorption sites; e.g., MONO-PE-MCM-41 with a mean pore size of 7.2 nm, the CO2 concn. in the feed should be high enough to achieve satn. via chemisorption; but low enough to avoid any addnl. physisorption, e.g., 5% CO2 in N2; the use of a reliable method for the accurate measurement of CO2/N ratio. Under such conditions, the obtained CO2/N ratios were reminiscent of those obtained in the CO2 scrubbing process using ethanolamine solns. Under dry conditions, the CO2/N ratio was close to 0.5, consistent with the formation of carbamate. Streams with relative humidity (RH) of 27, 61, and 74% were studied as well. As RH in the feed increased, CO2/N ratio increased from 0.57-0.88, in line with the gradual formation of bicarbonate. As for the detn. of CO2/N ratio under dry conditions, both thermogravimetry (TG) and mass spectrometry (MS) were suitable, whereas in the presence of moisture, TG was found to drastically underestimate the CO2 uptake. The seemingly disparate CO2/N ratios reported in the literature for various propylamine-bearing adsorbents were rationalized on the basis of the adsorbent pore structure and/or the exptl. conditions used.
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    23. 23
      Didas, S. A.; Choi, S.; Chaikittisilp, W.; Jones, C. W. Amine-Oxide Hybrid Materials for CO2 Capture from Ambient Air. Acc. Chem. Res. 2015, 48, 2680– 2687,  DOI: 10.1021/acs.accounts.5b00284
      23
      Amine-Oxide Hybrid Materials for CO2 Capture from Ambient Air
      Didas, Stephanie A.; Choi, Sunho; Chaikittisilp, Watcharop; Jones, Christopher W.
      Accounts of Chemical Research (2015), 48 (10), 2680-2687CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review describing the evolution of activities to design amine-functionalized SiO2 materials for catalysis and for design, characterization, and utilization of these materials for CO2 sepn. is given. Topics discussed include: introduction; supported amine materials for catalysis and CO2 capture; supported amine materials for CO2 capture from air; summary and conclusions; and supporting information (debate over air capture).
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    24. 24
      Heydari-Gorji, A.; Sayari, A. CO2 Capture on Polyethylenimine-Impregnated Hydrophobic Mesoporous Silica: Experimental and Kinetic Modeling. Chem. Eng. J. 2011, 173, 72– 79,  DOI: 10.1016/j.cej.2011.07.038
      24
      CO2 capture on polyethylenimine-impregnated hydrophobic mesoporous silica: Experimental and kinetic modeling
      Heydari-Gorji, Aliakbar; Sayari, Abdelhamid
      Chemical Engineering Journal (Amsterdam, Netherlands) (2011), 173 (1), 72-79CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      CO2 adsorption measurements for polyethylenimine (PEI)-impregnated, pore-expanded MCM-41 were conducted by gravimetry to assess the effect of amine load, CO2 partial pressure, and adsorption/desorption temps. Amine impregnation was conducted on ethanol-extd., pore-expanded MCM-41 (PME), a meso-porous SiO2 whose internal surface is laden by a layer of cetyltrimethylammonium cations. Well-dispersed PEI inside the PME hydrophobic channels exhibited a CO2 adsorption capacity at 75° as high as 206 mg/g for a 55 wt.% PEI load. Current PEI-impregnated PME materials had higher CO2 adsorption efficiency (g CO2/g PEI) vs. any other PEI-contg. adsorbent reported in the literature. In contrast to most PEI-impregnated materials reported in the literature, which due to diffusion resistance exhibited little or no CO2 adsorption at room temp., PEI-impregnated PME material displayed high potential for CO2 removal at ambient temp. An adsorption kinetic model was proposed to describe CO2 adsorption by amine-impregnated materials. Model results agreed well with exptl. data under a wide range of conditions, including different PEI loads, CO2 pressures, and adsorption temps.
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    25. 25
      Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Y.; Wang, Z. J.; Zhu, J. H. Efficient CO2 Capturer Derived from As-synthesized MCM-41 Modified with Amine. Chem. Eur. J. 2008, 14, 3442– 3451,  DOI: 10.1002/chem.200701467
      25
      Efficient CO2 capturer derived from as-synthesized MCM-41 modified with amine
      Yue, Ming Bo; Sun, Lin Bing; Cao, Yi; Wang, Ying; Wang, Zhu Ji; Zhu, Jian Hua
      Chemistry - A European Journal (2008), 14 (11), 3442-3451CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A new strategy to synthesize a highly efficient CO2 capturer by incorporating tetraethylenepentamine (TEPA) into as-synthesized MCM-41 (AM) is reported. The amine guest can be distributed in the micelle of the support, forming a web within the mesopore to trap CO2 mols. and resulting in a high adsorption capacity for CO2 up to 237 mg g-1. Four samples of the as-synthesized MCM-41 with a different amt. or type of surfactant are employed as supports to investigate the influence of micelles on the CO2 adsorption, and the spokelike structure of the micelle in the channel of the support is proven to be essential to the distribution of guest amine. Among these supports, the AM sample is the most competitive due to the advantages of energy and time saving in prepn. of the support along with the resulting higher CO2 adsorption capacity. At the optimal loading of 50 wt% TEPA, the AM-50 sample exhibits a high adsorption capacity of 183 mg g-1 in the sixth adsorption cycle at 5% CO2 concn.
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    26. 26
      Heydari-Gorji, A.; Belmabkhout, Y.; Sayari, A. Polyethylenimine-Impregnated Mesoporous Silica: Effect of Amine Loading and Surface Alkyl Chains on CO2 Adsorption. Langmuir 2011, 27, 12411– 12416,  DOI: 10.1021/la202972t
      26
      Polyethylenimine-Impregnated Mesoporous Silica: Effect of Amine Loading and Surface Alkyl Chains on CO2 Adsorption
      Heydari-Gorji, Aliakbar; Belmabkhout, Youssef; Sayari, Abdelhamid
      Langmuir (2011), 27 (20), 12411-12416CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Poly(ethyleneimine) (PEI) supported on pore-expanded MCM-41 whose surface is covered with a layer of long-alkyl chains is a more efficient CO2 adsorbent than PEI supported on the corresponding calcined silica and all PEI-impregnated materials reported in the literature. The layer of surface alkyl chains plays an important role in enhancing the dispersion of PEI, thus decreasing the diffusion resistance. Also at low temp., adsorbents with relatively low PEI contents are more efficient than their highly loaded counterparts because of the increased adsorption rate. Extensive CO2 adsorption-desorption cycling showed that the use of humidified feed and purge gases affords materials with enhanced stability, despite limited loss due to amine evapn.
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    27. 27
      Wang, L.; Al-Aufi, M.; Pacheco, C. N.; Xie, L.; Rioux, R. M. Polyethylene Glycol (PEG) Addition to Polyethylenimine (PEI)-Impregnated Silica Increases Amine Accessibility during CO2 Sorption. ACS. Sustain. Chem. Eng. 2019, 7, 14785– 14795,  DOI: 10.1021/acssuschemeng.9b02798
      There is no corresponding record for this reference.
    28. 28
      Tanthana, J.; Chuang, S. S. C. In Situ Infrared Study of the Role of PEG in Stabilizing Silica-Supported Amines for CO2 Capture. ChemSusChem 2010, 3, 957– 964,  DOI: 10.1002/cssc.201000090
      28
      In Situ Infrared Study of the Role of PEG in Stabilizing Silica-Supported Amines for CO2 Capture
      Tanthana, Jak; Chuang, Steven S. C.
      ChemSusChem (2010), 3 (8), 957-964CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The CO2 capture capacity, adsorption mechanism, and degrdn. characteristics of two sorbents, silica-supported tetraethylenepentamine (TEPA/SiO2) and polyethylene-glycol-modified TEPA/SiO2 (PEG/TEPA/SiO2), are studied by diffuse reflectance IR Fourier transform spectroscopy and mass spectrometry. The CO2 capture capacities of TEPA/SiO2 and PEG/TEPA/SiO2 are detd. to be 2087 and 1110 μmol CO2 g-1 sorbent, resp. Both sorbents adsorb CO2 as hydrogen-bonding species, NH2[bond]O, and carbamate/carboxylate species. The CO2 adsorption half-time increases with the no. of CO2 capture cycles. IR results suggest that the increased adsorption half-time is a result of diffusion limitation, caused by accumulation of TEPA and PEG species on the surface of the sorbent particles. The degrdn. of TEPA/SiO2 is found to correlate with the accumulation of carboxylate/carbamic species. The addn. of PEG decreases the degrdn. rate of the sorbent and slows down the formation of carboxylate species. These carboxylate species can block CO2 capture on amine (NH2/NH) sites. The stabilizing role of PEG on TEPA/SiO2 can be attributed to hydrogen-bonding between TEPA (NH2/NH)and PEG (OH).
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    29. 29
      Sakwa-Novak, M. A.; Tan, S.; Jones, C. W. Role of Additives in Composite PEI/Oxide CO2 Adsorbents: Enhancement in the Amine Efficiency of Supported PEI by PEG in CO2 Capture from Simulated Ambient Air. ACS. Appl. Mater. Interfaces 2015, 7, 24748– 24759,  DOI: 10.1021/acsami.5b07545
      29
      Role of Additives in Composite PEI/Oxide CO2 Adsorbents: Enhancement in the Amine Efficiency of Supported PEI by PEG in CO2 Capture from Simulated Ambient Air
      Sakwa-Novak, Miles A.; Tan, Shuai; Jones, Christopher W.
      ACS Applied Materials & Interfaces (2015), 7 (44), 24748-24759CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Supported amines are promising candidate adsorbents for the removal of CO2 from flue gases and directly from ambient air. The incorporation of additives into polymeric amines such as poly(ethylenimine) (PEI) supported on mesoporous oxides is an effective strategy to improve the performance of the materials. Here, several practical aspects of this strategy are addressed with regards to direct air capture. The influence of three additives (CTAB, PEG200, PEG1000) was systematically explored under dry simulated air capture conditions (400 ppm of CO2, 30 °C). With SBA-15 as a model support for poly(ethylenimine) (PEI), the nature of the additive induced heterogeneities in the deposition of org. on the interior and exterior of the particles, an important consideration for future scale up to practical systems. The PEG200 additive increased the obsd. thermodn. performance (∼60% increase in amine efficiency) of the adsorbents regardless of the PEI content, while the other mols. had less pos. effects. A threshold PEG200/PEI value was identified at which the diffusional limitations of CO2 within the materials were nearly eliminated. The threshold PEG/PEI ratio may have phys. origin in the interactions between PEI and PEG, as the optimal ratio corresponded to nearly equimolar OH/reactive (1°, 2°) amine ratios. The strategy is shown to be robust to the characteristics of the host support, as PEG200 improved the amine efficiency of PEI when supported on two varieties of mesoporous γ-alumina with PEI.
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    30. 30
      Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Z. J.; Wang, Y.; Yu, Q.; Zhu, J. H. Promoting the CO2 Adsorption in the Amine-Containing SBA-15 by Hydroxyl Group. Microporous Mesoporous Mater. 2008, 114, 74– 81,  DOI: 10.1016/j.micromeso.2007.12.016
      30
      Promoting the CO2 adsorption in the amine-containing SBA-15 by hydroxyl group
      Yue, Ming Bo; Sun, Lin Bing; Cao, Yi; Wang, Zhu Ji; Wang, Ying; Yu, Qing; Zhu, Jian Hua
      Microporous and Mesoporous Materials (2008), 114 (1-3), 74-81CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier)
      Novel CO2 capturer with a high efficiency is fabricated through dispersing the amine mixt. of tetraethylenepentamine (TEPA) and diethanolamine (DEA) or glycerol within the as-synthesized mesoporous SiO2 SBA-15, and the resulting sample is characterized by low angle x-ray diffraction and N2 adsorption to evaluate the distribution of the guest. The influence of hydroxyl group on the CO2 adsorption capacity of the composite is studied by using CO2-TPD and TG-MS techniques. The hydroxyl group of the P123 ((EO)20(PO)70(EO)20, template preserved in as-synthesized SBA-15) and the guest could promote the capture of CO2 by the amine through changing the interaction mechanism. The presence of hydroxyl group promotes the formation of the intermediate between CO2 and the amine with a lower thermal stability hence the CO2 trapped by the composite is easier to be desorbed and thus the regeneration of adsorbent is facilitated. Therefore, using this mixed amine (TEPA and DEA) modified as-synthesized SBA-15 as CO2 capturer not only saves the energy for removal of template, but also cut down the cost in the prepn. and regeneration of CO2 capturer, which is crit. in CO2 sepn. and capture.
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    31. 31
      Miao, Y.; Wang, Y.; Ge, B.; He, Z.; Zhu, X.; Li, J.; Liu, S.; Yu, L. Mixed Diethanolamine and Polyethyleneimine with Enhanced CO2 Capture Capacity from Air. Adv. Sci. 2023, 10, 2207253,  DOI: 10.1002/advs.202207253
      31
      Mixed Diethanolamine and Polyethyleneimine with Enhanced CO2 Capture Capacity from Air
      Miao, Yihe; Wang, Yaozu; Ge, Bingyao; He, Zhijun; Zhu, Xuancan; Li, Jia; Liu, Shanke; Yu, Lijun
      Advanced Science (Weinheim, Germany) (2023), 10 (16), 2207253CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Supported polyethyleneimine (PEI) adsorbent is one of the most promising com. direct air capture (DAC) adsorbents with a long research history since 2002. Although great efforts have been input, there are still limited improvements for this material in its CO2 capacity and adsorption kinetics under ultradilute conditions. Supported PEI also suffers significantly reduced adsorption capacities when working at sub-ambient temps. This study reports that mixing diethanolamine (DEA) into supported PEI can increase 46% and 176% of pseudoequil. CO2 capacities at DAC conditions compared to the supported PEI and DEA, resp. The mixed DEA/PEI functionalized adsorbents maintain the adsorption capacity at sub-ambient temps. of -5 to 25°C. In comparison, a 55% redn. of CO2 capacity is obsd. for supported PEI when the operating temp. decreases from 25 to -5°C. In addn., the supported mixed DEA/PEI with a ratio of 1:1 also shows fast desorption kinetics at temps. as low as 70°C, resulting in maintaining high thermal and chem. stability over 50 DAC cycles with a high av. CO2 working capacity of 1.29 mmol g-1. These findings suggest that the concept of "mixed amine", widely studied in the solvent system, is also practical to supported amine for DAC applications.
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    32. 32
      Choi, S.; Gray, M. L.; Jones, C. W. Amine-tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture from Ambient Air. ChemSusChem 2011, 4, 628– 635,  DOI: 10.1002/cssc.201000355
      32
      Amine-Tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture From Ambient Air
      Choi, Sunho; Gray, McMahan L.; Jones, Christopher W.
      ChemSusChem (2011), 4 (5), 628-635CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Impregnation-synthesized, SiO2-supported poly(ethylenimine) (PEI) materials are demonstrated to be promising adsorbents for CO2 capture from ultra-dil. gas streams, e.g., ambient air. A prototypical class 1 adsorbent contg. 45 wt.% PEI (PEI/SiO2) and 2 modified PEI-based aminosilica adsorbents derived from PEI modified by 3-aminopropyltrimethoxysilane (A-PEI/SiO2) or tetra-Et orthotitanate (T-PEI/SiO2), were synthesized and characterized by thermogravimetric anal. and Fourier transform IR spectroscopy. The modifiers were shown to enhance the thermal stability of the polymer-oxide composites, leading to higher PEI decompn. temps. Modified adsorbents had extremely high CO2 adsorption capacity under conditions simulating ambient air (400 ppm CO2 in inert gas), with >2 molCO2/kgsorbent-1 and enhanced adsorption kinetics vs. conventional class 1 sorbents. The new sorbents have excellent stability in cyclic adsorption/desorption operations, even under dry conditions in which aminosilica adsorbents are known to lose capacity due to urea formation. Thus, this type of adsorbent is considered a promising material for the direct capture of CO2 from ultra-dil. gas streams.
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    33. 33
      Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Pérez, E. S. Development of High Efficiency Adsorbents for CO2 Capture Based on a Double-Functionalization Method of Grafting and Impregnation. J. Mater. Chem. A 2013, 1, 1956– 1962,  DOI: 10.1039/c2ta01343f
      33
      Development of high efficiency adsorbents for CO2 capture based on a double-functionalization method of grafting and impregnation
      Sanz, Raul; Calleja, Guillermo; Arencibia, Amaya; Sanz-Perez, Eloy S.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (6), 1956-1962CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A functionalization method based on the impregnation of previously grafted, pore-expanded SBA-15 is presented. The combination of tethered and mobile amino groups led to a synergic effect, yielding samples with CO2 uptake of up to 235 mg CO2/g (5.34 mmol CO2/g) at 45°, 0.15 bar CO2, and high adsorption efficiency.
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    34. 34

      We consider that adsorption is the main driving force to capture CO2 via amine–CO2 interactions. For supported-amines at low CO2 partial pressures, most amine–CO2 association occurs via chemical reaction, creating covalent or ionic bonds between CO2 and the amines.

      There is no corresponding record for this reference.
    35. 35

      Pseudoequilibrium is often used in studies of CO2 adsorption over solid-supported amines because the approach to true thermodynamic equilibrium is too slow to allow for reasonable data collection. Equilibrium may require days to weeks of equilibration.

      There is no corresponding record for this reference.
    36. 36
      Caplow, M. Kinetics of Carbamate Formation and Breakdown. J. Am. Chem. Soc. 1968, 90, 6795– 6803,  DOI: 10.1021/ja01026a041
      36
      Kinetics of carbamate formation and breakdown
      Caplow, Michael
      Journal of the American Chemical Society (1968), 90 (24), 6795-803CODEN: JACSAT; ISSN:0002-7863.
      The rate law for reaction of amines with CO2 is rate = kamine(R2NH)(CO2) + kamine'(R2NH)(OH)(CO2), where the first and second terms are for uncatalyzed and hydroxide-catalyzed pathways. The latter reaction, which involves proton abstraction in the rate-detg. step, is not observed with all amines. Values for kamine at 10° follow the Bronsted relationship log kamine (M-1 sec.-1) = mpK + Y, with values of m and Y equal to 0.43 and -1.50 for reactions of primary and secondary amines, and 0.48 and -0.20 for the reactions of hydrazine and hydroxylamine derivs. Second-order rate consts. for H ion catalyzed decarboxylation of carbamates formed from amines of pK - 1.05 to approx. 5 may be fitted to a Bronsted relationship log kH+ (M-1 sec.-1) = 0.77 pK + 3.6 at 10°. Rates for carbamates formed from more basic amines are virtually independent of basicity and are approx. 108 M-1 sec.-1. The rate-limiting step in carbamate formation and breakdown with weakly basic amines involves C-N bond formation and cleavage. It is suggested that proton transfer may be rate limiting in the synthesis and breakdown of carbamates formed from basic amines.
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    37. 37
      Danckwerts, P. V. The Reaction of CO2 with Ethanolamines. Chem. Eng. Sci. 1979, 34, 443– 446,  DOI: 10.1016/0009-2509(79)85087-3
      37
      The reaction of carbon dioxide with ethanolamines
      Danckwerts, P. V.
      Chemical Engineering Science (1979), 34 (4), 443-6CODEN: CESCAC; ISSN:0009-2509.
      A crit. review of measurements of the rate of absorption of CO2 by solns. contg. [HO(CH2)2]nNH3-n (n=1,2,3). 11 Refs.
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    38. 38
      Kuwahara, Y.; Kang, D.; Copeland, J. R.; Bollini, P.; Sievers, C.; Kamegawa, T.; Yamashita, H.; Jones, C. W. Enhanced CO2 Adsorption over Polymeric Amines Supported on Heteroatom-incorporated SBA-15 Silica: Impact of Heteroatom Type and Loading on Sorbent Structure and Adsorption Performance. Chem. Eur. J. 2012, 18, 16649– 16664,  DOI: 10.1002/chem.201203144
      38
      Enhanced CO2 Adsorption over Polymeric Amines Supported on Heteroatom-Incorporated SBA-15 Silica: Impact of Heteroatom Type and Loading on Sorbent Structure and Adsorption Performance
      Kuwahara, Yasutaka; Kang, Dun-Yen; Copeland, John R.; Bollini, Praveen; Sievers, Carsten; Kamegawa, Takashi; Yamashita, Hiromi; Jones, Christopher W.
      Chemistry - A European Journal (2012), 18 (52), 16649-16664CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Silica supported amine materials are promising compns. that can be used to effectively remove CO2 from large stationary sources, such as flue gas generated from coal-fired power plants (ca. 10% CO2) and potentially from ambient air (ca. 400 ppm CO2). The CO2 adsorption characteristics of prototypical poly(ethyleneimine)-silica composite adsorbents can be significantly enhanced by altering the acid/base properties of the silica support by heteroatom incorporation into the silica matrix. In this study, an array of poly(ethyleneimine)-impregnated mesoporous silica SBA-15 materials contg. heteroatoms (Al, Ti, Zr, and Ce) in their silica matrixes are prepd. and examd. in adsorption expts. under conditions simulating flue gas (10 % CO2 in Ar) and ambient air (400 ppm CO2 in Ar) to assess the effects of heteroatom incorporation on the CO2 adsorption properties. The structure of the composite adsorbents, including local information concerning the state of the incorporated heteroatoms and the overall surface properties of the silicate supports, are investigated in detail to draw a relationship between the adsorbent structure and CO2 adsorption/desorption performance. The CO2 adsorption/desorption kinetics are assessed by thermogravimetric anal. and in situ FT-IR measurements. These combined results, coupled with data on adsorbent regenerability, demonstrate a stabilizing effect of the heteroatoms on the poly(ethyleneimine), enhancing adsorbent capacity, adsorption kinetics, regenerability, and stability of the supported aminopolymers over continued cycling. It is suggested that the CO2 adsorption performance of silica-aminopolymer composites may be further enhanced in the future by more precisely tuning the acid/base properties of the support.
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    39. 39
      Wilfong, W. C.; Srikanth, C. S.; Chuang, S. S. C. In Situ ATR and DRIFTS Studies of the Nature of Adsorbed CO2 on Tetraethylenepentamine Films. ACS. Appl. Mater. Interfaces 2014, 6, 13617– 13626,  DOI: 10.1021/am5031006
      39
      In Situ ATR and DRIFTS studies of nature of adsorbed CO2 on tetraethylenepentamine films
      Wilfong, Walter Christopher; Srikanth, Chakravartula S.; Chuang, Steven S. C.
      ACS Applied Materials & Interfaces (2014), 6 (16), 13617-13626CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      CO2 adsorption/desorption onto/from tetraethylenepentamine (TEPA) films of 4, 10, and 20 μm thicknesses were studied by in situ attenuated total reflectance (ATR) and diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) techniques under transient conditions. Molar adsorption coeffs. for adsorbed CO2 were used to det. the CO2 capture capacities and amine efficiencies (CO2/N) of the films in the DRIFTS system. Adsorption of CO2 onto surface and bulk NH2 groups of the 4 μm film produced weakly adsorbed CO2, which can be desorbed at 50 °C by reducing the CO2 partial pressure. These weakly adsorbed CO2 exhibit low ammonium ion intensities and could be in the form of ammonium-carbamate ion pairs and zwitterions. Increasing the film thickness enhanced the surface amine-amine interactions, resulting in strongly adsorbed ion pairs and zwitterions assocd. with NH and NH2 groups of neighboring amines. These adsorbed species may form an interconnected surface network, which slowed CO2 gas diffusion into and diminished access of the bulk amine groups (or amine efficiency) of the 20 μm film by a min. of 65%. Desorption of strongly adsorbed CO2 comprising the surface network could occur via dissocn. of NH3+/NH2+···NH2/NH ionic hydrogen bonds beginning from 60 to 80 °C, followed by decompn. of NHCOO-/NCOO- at 100 °C. These results suggest that faster CO2 diffusion and adsorption/desorption kinetics could be achieved by thinner layers of liq. or immobilized amines.
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    40. 40
      Taniguchi, I.; Urai, H.; Kai, T.; Duan, S.; Kazama, S. A CO2-Selective Molecular Gate of Poly (Amidoamine) Dendrimer Immobilized in a Poly (Ethylene Glycol) Network. J. Membr. Sci. 2013, 444, 96– 100,  DOI: 10.1016/j.memsci.2013.05.017
      40
      A CO2-selective molecular gate of poly(amidoamine) dendrimer immobilized in a poly(ethylene glycol) network
      Taniguchi, Ikuo; Urai, Hiromi; Kai, Teruhiko; Duan, Shuhong; Kazama, Shingo
      Journal of Membrane Science (2013), 444 (), 96-100CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      A polymeric membrane composed of poly(amidoamine) (PAMAM) dendrimer immobilized in a poly(ethylene glycol) (PEG) network expresses excellent CO2 sepn. properties over smaller H2. The preferential CO2 permeation can be explained by specific interaction between CO2 and primary amine of the dendrimer, which enhances CO2 soly. into the polymeric membrane. CO2 forms carbamate with the amines or bicarbonate in the presence of H2O detd. by inverse-gate decoupled 13C NMR. The resulting carbamate ion pair works to form a quasi-crosslinking, which would suppress H2 permeation by a CO2-selective Mol. Gate, while bicarbonate ion can be a major moving species to pass through the polymeric membrane. Attenuated total reflection (ATR) indicates the formation of carbamate. Small-angle x-ray scattering (SAXS) reveals increase in scattering intensity under CO2 atmosphere due to the formation of scattering particles, which can be a cluster of the dendrimer-CO2 crosslinks. Tensile testing of the membrane exhibits increase in both Young's modulus and elongation-to-break by CO2 treatment, suggesting that the crosslinking is reversible and rearrangeable. DSC also shows an exothermic peak at 120°, which is assocd. with dissocn. of the crosslinks.
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    41. 41
      Donaldson, T. L.; Nguyen, Y. N. Carbon Dioxide Reaction Kinetics and Transport in Aqueous Amine Membranes. Ind. Eng. Chem. Fundam. 1980, 19, 260– 266,  DOI: 10.1021/i160075a005
      41
      Carbon dioxide reaction kinetics and transport in aqueous amine membranes
      Donaldson, Terrence L.; Nguyen, Yen N.
      Industrial & Engineering Chemistry Fundamentals (1980), 19 (3), 260-6CODEN: IECFA7; ISSN:0196-4313.
      Reaction kinetics of CO2 with mono-, di- and triethanolamine (I) and Et3N were studied using a tracer 14CO2 membrane-transport technique. At low concns. of mono- and diethanolamine, the results are consistent with carbamate formation. At higher concns. the chem. appears to be more complex. Transport results are discussed in terms of various potential phenomena. Although triamines do not form carbamates with CO2, both I and Et3N increased the membrane-transport flux of CO2. The only reasonable reaction mechanism for I consistent with the data is basic catalysis of CO2 hydration. Et3N, on the other hand, appears to act only as a weak base to produce free OH-, which reacts with CO2.
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    42. 42
      Russell-Parks, G. A.; Leick, N.; Marple, M. A. T.; Strange, N. A.; Trewyn, B. G.; Pang, S. H.; Braunecker, W. A. Fundamental Insight into Humid CO2 Uptake in Direct Air Capture Nanocomposites Using Fluorescence and Portable NMR Relaxometry. J. Phys. Chem. C 2023, 127, 15363– 15374,  DOI: 10.1021/acs.jpcc.3c03653
      42
      Fundamental Insight into Humid CO2 Uptake in Direct Air Capture Nanocomposites Using Fluorescence and Portable NMR Relaxometry
      Russell-Parks, Glory A.; Leick, Noemi; Marple, Maxwell A. T.; Strange, Nicholas A.; Trewyn, Brian G.; Pang, Simon H.; Braunecker, Wade A.
      Journal of Physical Chemistry C (2023), 127 (31), 15363-15374CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Direct air capture (DAC) technol. is being explored as a pathway for reducing greenhouse gas emissions through the efficient removal of CO2 from the atm. However, there remains a knowledge gap regarding structure-property-performance factors that impact the behavior of these systems in diverse, real-world environments. In aminopolymer-based DAC systems, gas diffusion is tightly coupled with polymer mobility, which is in turn affected by a large matrix of variables, including interactions with the pore wall of the support, nanoconfinement, the presence of co-adsorbates (moisture), and electrostatic cross-links that develop as a function of CO2 chemisorption. Higher-throughput, benchtop techniques for studying and understanding mobility in these systems would lead to more rapid advances in the field. Here, we demonstrate the value of a fluorescence technique for monitoring polymer mobility within nanocomposite capture materials as a function of CO2 and water adsorption in a series of humidified polyethylenimine-Al2O3 composite materials. The approach allows us to correlate changes in mobility with CO2 adsorption kinetics as a function of relative humidity. We further couple this information with NMR relaxometry data attained using a portable single-sided magnetic resonance device, and we employ diffuse reflectance IR Fourier transform spectroscopy to correlate the formation of different relative amts. of carbamates and carbonates with the environmental conditions. These results provide a blueprint for using benchtop techniques to promote fundamental understanding in DAC systems that can in turn enable more efficient operation in real-world conditions.
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    43. 43
      Yu, J.; Zhai, Y.; Chuang, S. S. C. Water Enhancement in CO2 Capture by Amines: An Insight into CO2–H2O Interactions on Amine Films and Sorbents. Ind. Eng. Chem. Res. 2018, 57, 4052– 4062,  DOI: 10.1021/acs.iecr.7b05114
      43
      Water Enhancement in CO2 Capture by Amines: An Insight into CO2-H2O Interactions on Amine Films and Sorbents
      Yu, Jie; Zhai, Yuxin; Chuang, Steven S. C.
      Industrial & Engineering Chemistry Research (2018), 57 (11), 4052-4062CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Water, a component in flue gas, plays a significant role in CO2 capture through a complex interaction between water mols. and adsorbed CO2 on amine sorbents. To det. how the H2O-CO2-amine interactions affect amine efficiency and the binding energy of adsorbed CO2, we used in situ IR spectroscopy (IR) to det. the structure of adsorbed CO2 and H2O as well as their relations to adsorption/desorption kinetics and CO2 capture capacity on tetraethylenepentamine (TEPA) films and Class I amine (i.e., impregnated) sorbents. H2O enhanced amine efficiency of TEPA films and sorbents by increasing the accessibility of secondary amine sites to CO2 and promoting the formation of hydronium carbamate and carbamic acid. CO2 adsorbed on the surface of the TEPA film as a weakly adsorbed CO2 in the form of hydronium and ammonium-carbamate with a low IR intensity of hydrogen bonding (-OH···-OOC or -NH···-OOC) between hydronium/ammonium ions and carbamate ions. CO2 adsorbed on the middle layers (i.e., 0.2-0.4 μm below the surface) of TEPA films produced a strongly adsorbed species that exhibits an intensive hydrogen bonding band of ammonium-water-carbamate desorbing at temps. above 120 °C. Comparison of IR spectra shows that the kinetic behaviors of adsorbed CO2 on amine films are correlated well with those of adsorbed CO2 on Class I amine sorbents. Thick amine films and high-amine-loading sorbents contain high-d. amine sites that produce mainly strongly adsorbed CO2. Adsorbed H2O further increased amine efficiency and the binding energy of strongly adsorbed CO2 through the formation of hydronium carbamate.
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    44. 44
      Rim, G.; Priyadarshini, P.; Song, M.; Wang, Y.; Bai, A.; Realff, M. J.; Lively, R. P.; Jones, C. W. Support Pore Structure and Composition Strongly Influence the Direct Air Capture of CO2 on Supported Amines. J. Am. Chem. Soc. 2023, 145, 7190– 7204,  DOI: 10.1021/jacs.2c12707
      44
      Support Pore Structure and Composition Strongly Influence the Direct Air Capture of CO2 on Supported Amines
      Rim, Guanhe; Priyadarshini, Pranjali; Song, MinGyu; Wang, Yuxiang; Bai, Andrew; Realff, Matthew J.; Lively, Ryan P.; Jones, Christopher W.
      Journal of the American Chemical Society (2023), 145 (13), 7190-7204CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A variety of amine-impregnated porous solid sorbents for direct air capture (DAC) of CO2 were developed, yet the effect of amine-solid support interactions on the CO2 adsorption behavior is still poorly understood. When tetraethylenepentamine (TEPA) is impregnated on two different supports, com. γ-Al2O3 and MIL-101(Cr), they show different trends in CO2 sorption when the temp. (-20 to 25°) and humidity (0-70% RH) of the simulated air stream are varied. In situ IR spectroscopy is used to probe the mechanism of CO2 sorption on the two supported amine materials, with weak chemisorption (formation of carbamic acid) being the dominant pathway over MIL-101(Cr)-supported TEPA and strong chemisorption (formation of carbamate) occurring over γ-Al2O3-supported TEPA. Formation of both carbamic acid and carbamate species is enhanced over the supported TEPA materials under humid conditions, with the most significant enhancement obsd. at -20°. However, while equil. H2O sorption is high at cold temps. (e.g., -20°), the effect of humidity on a practical cyclic DAC process is expected to be minimal due to slow H2O uptake kinetics. This work suggests that the CO2 capture mechanisms of impregnated amines can be controlled by adjusting the degree of amine-solid support interaction and that H2O adsorption behavior is strongly affected by the properties of the support materials. Thus, proper selection of solid support materials for amine impregnation will be important for achieving optimized DAC performance under varied deployment conditions, such as cold (e.g., -20°) or ambient temp. (e.g., 25°) operations.
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    45. 45
      Franchi, R. S.; Harlick, P. J. E.; Sayari, A. Applications of Pore-Expanded Mesoporous Silica. 2. Development of a High-Capacity, Water-Tolerant Adsorbent for CO2. Ind. Eng. Chem. Res. 2005, 44, 8007– 8013,  DOI: 10.1021/ie0504194
      45
      Applications of Pore-Expanded Mesoporous Silica. 2. Development of a High-Capacity, Water-Tolerant Adsorbent for CO2
      Franchi, Robert S.; Harlick, Peter J. E.; Sayari, Abdelhamid
      Industrial & Engineering Chemistry Research (2005), 44 (21), 8007-8013CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      A novel high-capacity, water-tolerant adsorbent for CO2 was developed. It consisted of diethanolamine (DEA) loaded pore-expanded MCM-41 silica (PE-MCM-41). Due to its very large pore vol., PE-MCM-41 silica was capable of accommodating a greater quantity of amine resulting in higher CO2 adsorption capacity compared to the other supports including activated carbon, silica gel, and std. MCM-41 silica. Adsorption measurements were conducted by gravimetry using dry CO2 to obtain uptake curves and apparent rate data. The capacity and uptake rate reached maxima with respect to amine content and then declined due to the deposition of excess amine on the particle's external surface and within the interparticle voids. At CO2 partial pressures below 0.15 atm, the current DEA loaded PE-MCM-41 adsorbent was superior to the more conventional zeolite 13X. Adsorption studies with humid CO2 revealed that the adsorption capacity of the PE-MCM-41-based material was insensitive to the presence of moisture, which represents a major advantage over zeolite 13X. Repeated adsorption-desorption cycles revealed that our novel adsorbent exhibited good cyclic stability.
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    46. 46
      Heydari-Gorji, A.; Yang, Y.; Sayari, A. Effect of the Pore Length on CO2 Adsorption over Amine-Modified Mesoporous Silicas. Energy Fuels 2011, 25, 4206– 4210,  DOI: 10.1021/ef200765f
      46
      Effect of the Pore Length on CO2 Adsorption over Amine-Modified Mesoporous Silicas
      Heydari-Gorji, Aliakbar; Yang, Yong; Sayari, Abdelhamid
      Energy & Fuels (2011), 25 (9), 4206-4210CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      Carbon dioxide adsorption was investigated in the presence of polyethylenimine (PEI)-impregnated mesoporous silicas with different pore lengths, namely, pore-expanded MCM-41, conventional SBA-15 with different pore diams. (7.2 and 10.5 nm), and SBA-15 with platelet morphol. The pore lengths of the silica supports were ca. 25, 1.5, and 0.2 μm, resp. Under comparable conditions, the adsorption performance was found to be strongly dependent upon the pore length. The materials with the shortest channels showed the highest capacity and fastest adsorption. These findings were assocd. with diminished diffusion resistance and enhanced amine accessibility inside the pores.
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    47. 47
      Kwon, H. T.; Sakwa-Novak, M. A.; Pang, S. H.; Sujan, A. R.; Ping, E. W.; Jones, C. W. Aminopolymer-Impregnated Hierarchical Silica Structures: Unexpected Equivalent CO2 Uptake under Simulated Air Capture and Flue Gas Capture Conditions. Chem. Mater. 2019, 31, 5229– 5237,  DOI: 10.1021/acs.chemmater.9b01474
      47
      Aminopolymer-Impregnated Hierarchical Silica Structures: Unexpected Equivalent CO2 Uptake under Simulated Air Capture and Flue Gas Capture Conditions
      Kwon, Hyuk Taek; Sakwa-Novak, Miles A.; Pang, Simon H.; Sujan, Achintya R.; Ping, Eric W.; Jones, Christopher W.
      Chemistry of Materials (2019), 31 (14), 5229-5237CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Poly(ethyleneimine)-impregnated sorbents are prepd. using a hierarchical SiO2 support with bimodal meso-/macroporosity. The sorbents behave unexpectedly during CO2 adsorption from simulated air and flue gases (400 ppm and 10% CO2) at a fixed temp., as compared to systems built on commonly studied mesoporous materials. The results demonstrate that (1) impregnation methods influence the efficacy of sorption performance and (2) the sorbents show almost similar uptake capacities under 400 ppm and 10% dry CO2 at 30°, exhibiting step-like CO2 adsorption isotherms. These unusual observations are rationalized via control expts. and a hypothesized sorption mechanism. While the sorption performance near room temp. is unexpectedly identical under 400 ppm and 10% CO2 conditions, there is an optimal temp. at each gas concn. where the uptake is maximized. The max. sorption capacities are 2.6 and 4.1 mmol CO2/g sorbent at the optimized sorption temps. using 400 ppm and 10% dry CO2, resp. The presence of H2O vapor under 400 ppm CO2 conditions further improves the sorption capacity to 3.4 mmol/g sorbent, which is the highest capacity under direct air capture conditions among known amine sorbents impregnated with a similar polymer, to the best of the knowledge.
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    48. 48
      Min, K.; Choi, W.; Choi, M. Macroporous Silica with Thick Framework for Steam-Stable and High-Performance Poly (Ethyleneimine)/Silica CO2 Adsorbent. ChemSusChem 2017, 10, 2518– 2526,  DOI: 10.1002/cssc.201700398
      48
      Macroporous Silica with Thick Framework for Steam-Stable and High-Performance Poly(ethyleneimine)/Silica CO2 Adsorbent
      Min, Kyungmin; Choi, Woosung; Choi, Minkee
      ChemSusChem (2017), 10 (11), 2518-2526CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Polyethyleneimine (PEI)/SiO2 has been widely studied as a solid adsorbent for post-combustion CO2 capture. This work synthesized a highly macroporous SiO2 (MacS) by secondary sintering of fumed SiO2 and compared it with various mesoporous SiO2 with different pore structures as a support for PEI. SiO2 with large pore diam. and vol. enabled high CO2 adsorption kinetics and capacity, since pore occlusion by the supported PEI was minimized. Steam stability of SiO2 structures increased with SiO2 wall thickness due to suppressed framework ripening. SiO2 with low steam stability displayed rapid PEI leaching, indicating PEI squeezed out of collapsed SiO2 pores leached more readily. Consequently, MacS with an extra-large pore vol. (1.80 cm3/g), pore diam. (56.0 nm), and thick wall (>10 nm), exhibited the most promising CO2 adsorption kinetics and capacity and steam stability.
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    49. 49
      Hack, J.; Frazzetto, S.; Evers, L.; Maeda, N.; Meier, D. M. Branched versus Linear Structure: Lowering the CO2 Desorption Temperature of Polyethylenimine-Functionalized Silica Adsorbents. Energies (Basel) 2022, 15, 1075,  DOI: 10.3390/en15031075
      49
      Branched versus Linear Structure: Lowering the CO2 Desorption Temperature of Polyethylenimine-Functionalized Silica Adsorbents
      Hack, Jannis; Frazzetto, Seraina; Evers, Leon; Maeda, Nobutaka; Meier, Daniel M.
      Energies (Basel, Switzerland) (2022), 15 (3), 1075CODEN: ENERGA; ISSN:1996-1073. (MDPI AG)
      Lowering the regeneration temp. for solid CO2-capture materials is one of the crit. tasks for economizing CO2-capturing processes. Based on reported pKa values and nucleophilicity, we compared two different polyethylenimines (PEIs): branched PEI (BPEI) and linear PEI (LPEI). LPEI outperformed BPEI in terms of adsorption and desorption properties. Because LPEI is a solid below 73-75°C, even a high loading amt. of LPEI can effectively adsorb CO2 without diffusive barriers. Temp.-programmed desorption (TPD) demonstrated that the desorption peak top dropped to 50.8°C for LPEI, compared to 78.0°C for BPEI. We also revisited the classical adsorption model of CO2 on secondary amines by using in situ modulation excitation IR spectroscopy, and proposed a new adsorption configuration, R1(R2)-NCOOH. Even though LPEI is more expensive than BPEI, considering the long-term operation of a CO2-capturing system, the low regeneration temp. makes LPEI attractive for industrial applications.
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    50. 50
      Sayari, A.; Heydari-Gorji, A.; Yang, Y. CO2-Induced Degradation of Amine-Containing Adsorbents: Reaction Products and Pathways. J. Am. Chem. Soc. 2012, 134, 13834– 13842,  DOI: 10.1021/ja304888a
      50
      CO2-Induced Degradation of Amine-Containing Adsorbents: Reaction Products and Pathways
      Sayari, Abdelhamid; Heydari-Gorji, Aliakbar; Yang, Yong
      Journal of the American Chemical Society (2012), 134 (33), 13834-13842CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A comprehensive study was conducted to study the stability of a wide variety of mesoporous SiO2-supported amine-contg. adsorbents in the presence of CO2 under dry conditions. CO2-induced degrdn. of grafted primary and secondary monoamines (pMono, sMono), diamines with one primary and one secondary amines (Diamine) and triamine with one primary and 2 secondary amines (TRI) as well as different impregnated polyamines such as branched and linear polyethylenimine (BPEI and LPEI) and polyallylamine (PALL) was studied using extensive CO2 adsorption-desorption cycling as well as diffuse reflectance IR Fourier transform (DRIFT) and 13C CP MASNMR measurements. Except for sMono, all other supported amines underwent significant deactivation in the presence of dry CO2 under mild conditions. In all cases, the decrease in CO2 uptake was assocd. with the formation of urea linkages at the expense of amine groups. The urea-contg. species were identified, and the deactivation pathways were delineated.
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    51. 51
      Fan, Y.; Jia, X. Progress in Amine-Functionalized Silica for CO2 Capture: Important Roles of Support and Amine Structure. Energy Fuels 2022, 36, 1252– 1270,  DOI: 10.1021/acs.energyfuels.1c03788
      51
      Progress in amine-functionalized silica for CO2 capture: important roles of support and amine structure
      Fan, Yanfang; Jia, Xiaohao
      Energy & Fuels (2022), 36 (3), 1252-1270CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      A review. In the past decades, a huge development in rational design, synthesis and application of solid amine CO2 adsorbents has been reported. Since the reports on polyethylenimine impregnated silica adsorbents, there has been tremendous growth in this area along with the increase of supported amine adsorbents related publications. Owing to high adsorption capacity, facile synthesis procedure and low cost, amine-functionalized silica adsorbents demonstrate the great promise for CO2 capture technologies. To push their industrial application forward, the stability must be improved further to mitigate acidic impurities, CO2, O2 and thermal-induced deactivation. Furthermore, fundamental understanding of relationship between amine/support structure and adsorption performance is needed to guide the design of stable amine silica adsorbents. This summarizes recent researches that mainly focus on important roles of support and amine structure in CO2 adsorption performance and improved stability. The recent progress in promising reactor design for solid amine adsorbents is discussed to provide insights on developing the continuous CO2 capture process for sepg. CO2 from flue gas and ambient air.
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    52. 52
      Choi, W.; Min, K.; Kim, C.; Ko, Y. S.; Jeon, J. W.; Seo, H.; Park, Y.-K.; Choi, M. Epoxide-Functionalization of Polyethyleneimine for Synthesis of Stable Carbon Dioxide Adsorbent in Temperature Swing Adsorption. Nat. Commun. 2016, 7, 12640,  DOI: 10.1038/ncomms12640
      52
      Epoxide-functionalization of polyethyleneimine for synthesis of stable carbon dioxide adsorbent in temperature swing adsorption
      Choi, Woosung; Min, Kyungmin; Kim, Chaehoon; Ko, Young Soo; Jeon, Jae Wan; Seo, Hwimin; Park, Yong-Ki; Choi, Minkee
      Nature Communications (2016), 7 (), 12640CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Amine-contg. adsorbents have been extensively investigated for post-combustion carbon dioxide capture due to their ability to chemisorb low-concn. carbon dioxide from a wet flue gas. However, earlier studies have focused primarily on the carbon dioxide uptake of adsorbents, and have not demonstrated effective adsorbent regeneration and long-term stability under such conditions. Here, we report the versatile and scalable synthesis of a functionalized-polyethyleneimine (PEI)/silica adsorbent which simultaneously exhibits a large working capacity (2.2 mmol g-1) and long-term stability in a practical temp. swing adsorption process (regeneration under 100% carbon dioxide at 120°C), enabling the sepn. of concd. carbon dioxide. We demonstrate that the functionalization of PEI with 1,2-epoxybutane reduces the heat of adsorption and facilitates carbon dioxide desorption (>99%) during regeneration compared with unmodified PEI (76%). Moreover, the functionalization significantly improves long-term adsorbent stability over repeated temp. swing adsorption cycles due to the suppression of urea formation and oxidative amine degrdn.
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    53. 53
      Hammouda, B. Probing Nanoscale Structures-the sans Toolbox; National Institute of Standards and Technology, 2008; pp 1– 717.
      There is no corresponding record for this reference.
    54. 54
      Zhao, J. K.; Gao, C. Y.; Liu, D. The Extended Q-Range Small-Angle Neutron Scattering Diffractometer at the SNS. J. Appl. Crystallogr. 2010, 43, 1068– 1077,  DOI: 10.1107/S002188981002217X
      54
      The extended Q-range small-angle neutron scattering diffractometer at the SNS
      Zhao, J. K.; Gao, C. Y.; Liu, D.
      Journal of Applied Crystallography (2010), 43 (5, Pt. 1), 1068-1077CODEN: JACGAR; ISSN:0021-8898. (International Union of Crystallography)
      The extended Q-range small-angle neutron scattering diffractometer (EQ-SANS) at the Spallation Neutron Source (SNS), Oak Ridge, is designed for wide neutron momentum transfer (Q) coverage, high neutron beam intensity and good wavelength resoln. In addn., the design and construction of the instrument aim to achieve a max. signal-to-noise ratio by minimizing the background. The instrument is located on the high-power target station at the SNS. One of the key components in the primary flight path is the neutron optics, consisting of a curved multichannel beam bender and sections of straight neutron guides. They are optimized to minimize neutron transport loss, thereby maximizing the available flux on the sample. They also enable the avoidance of a direct line of sight to the neutron moderator at downstream locations. The instrument has three bandwidth-limiting choppers. They allow a novel frame-skipping operation, which enables the EQ-SANS diffractometer to achieve a dynamic Q range equiv. to that of a similar machine on a 20 Hz source. The two-dimensional low-angle detector, based on 3He tube technologies, offers very high counting rates and counting efficiency. Initial operations have shown that the instrument has achieved its design goals.
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    55. 55
      Mamontov, E.; Herwig, K. W. A Time-of-Flight Backscattering Spectrometer at the Spallation Neutron Source, BASIS. Rev. Sci. Instrum. 2011, 82, 085109,  DOI: 10.1063/1.3626214
      55
      A time-of-flight backscattering spectrometer at the Spallation Neutron Source, BASIS
      Mamontov, E.; Herwig, K. W.
      Review of Scientific Instruments (2011), 82 (8), 085109/1-085109/10CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      We describe the design and current performance of the backscattering silicon spectrometer (BASIS), a time-of-flight backscattering spectrometer built at the spallation neutron source (SNS) of the Oak Ridge National Lab. (ORNL). BASIS is the first silicon-based backscattering spectrometer installed at a spallation neutron source. In addn. to high intensity, it offers a high-energy resoln. of about 3.5 μeV and a large and variable energy transfer range. These ensure an excellent overlap with the dynamic ranges accessible at other inelastic spectrometers at the SNS. (c) 2011 American Institute of Physics.
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    56. 56
      Meyer, A.; Dimeo, R. M.; Gehring, P. M.; Neumann, D. A. The High-Flux Backscattering Spectrometer at the NIST Center for Neutron Research. Rev. Sci. Instrum. 2003, 74, 2759– 2777,  DOI: 10.1063/1.1568557
      56
      The high-flux backscattering spectrometer at the NIST Center for Neutron Research
      Meyer, A.; Dimeo, R. M.; Gehring, P. M.; Neumann, D. A.
      Review of Scientific Instruments (2003), 74 (5), 2759-2777CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      A review. We describe the design and current performance of the high-flux backscattering spectrometer located at the NIST Center for Neutron Research. The design incorporates several state-of-the-art neutron optical devices to achieve the highest flux on sample possible while maintaining an energy resoln. of less than 1 μeV. Foremost among these is a novel phase-space transformation chopper that significantly reduces the mismatch between the beam divergences of the primary and secondary parts of the instrument. This resolves a long-standing problem of backscattering spectrometers, and produces a relative gain in neutron flux of 4.2. A high-speed Doppler-driven monochromator system has been built that is capable of achieving energy transfers of up to ±50 μeV, thereby extending the dynamic range of this type of spectrometer by more than a factor of 2 over that of other reactor-based backscattering instruments.
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    57. 57
      Yan, Z.; Zhang, R. Rapid Structural Analysis of Minute Quantities of Organic Solids by Exhausting 1H Polarization in Solid-State NMR Spectroscopy Under Fast Magic Angle Spinning. J. Phys. Chem. Lett. 2021, 12, 12067– 12074,  DOI: 10.1021/acs.jpclett.1c03672
      57
      Rapid Structural Analysis of Minute Quantities of Organic Solids by Exhausting 1H Polarization in Solid-State NMR Spectroscopy Under Fast Magic Angle Spinning
      Yan, Zhiwei; Zhang, Rongchun
      Journal of Physical Chemistry Letters (2021), 12 (50), 12067-12074CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Solid-state NMR (NMR) often suffers from significant limitations due to the inherent low signal sensitivity when low-γ nuclei are involved. Herein, we report an elegant solid-state NMR approach for rapid structural anal. of minute amts. of org. solids. By encoding staggered chem. shift evolution in the indirect dimension and staggered acquisition in the 1H dimension, a proton-detected homonuclear 1H/1H and heteronuclear 13C/1H chem. shift correlation (HETCOR) spectrum can be obtained simultaneously in a single expt. at a fast magic-angle-spinning (MAS) condition with barely increasing the exptl. time. We further show that during the conventional 1H-detected HETCOR exptl. time, multiple homonuclear 1H/1H correlation spectra can be recorded in addn. to the HETCOR spectrum, enabling the detn. of 1H-1H distances. We establish that abundant 1H polarization can be efficiently manipulated and fully utilized in proton-detected solid-state NMR spectroscopy for extn. of more crit. structural information and thus redn. of the total exptl. time.
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    58. 58
      Carrillo, J.-M. Y.; Sumpter, B. G. Structure and Dynamics of Confined Flexible and Unentangled Polymer Melts in Highly Adsorbing Cylindrical Pores. J. Chem. Phys. 2014, 141, 074904,  DOI: 10.1063/1.4893055
      58
      Structure and dynamics of confined flexible and unentangled polymer melts in highly adsorbing cylindrical pores
      Carrillo, Jan-Michael Y.; Sumpter, Bobby G.
      Journal of Chemical Physics (2014), 141 (7), 074904/1-074904/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      Coarse-grained mol. dynamics simulations are used to probe the dynamic phenomena of polymer melts confined in nanopores. The simulation results show excellent agreement in the values obtained for the normalized coherent single chain dynamic structure factor, ((S(Q,Δt))/(S(Q,0))). In the bulk configuration, both simulations and expts. confirm that the polymer chains follow Rouse dynamics. However, under confinement, the Rouse modes are suppressed. The mean-square radius of gyration 〈R2g〉 and the av. relative shape anisotropy 〈κ2〉 of the conformation of the polymer chains indicate a pancake-like conformation near the surface and a bulk-like conformation near the center of the confining cylinder. This was confirmed by direct visualization of the polymer chains. Despite the presence of these different conformations, the av. form factor of the confined chains still follows the Debye function which describes linear ideal chains, which is in agreement with small angle neutron scattering expts. (SANS). The exptl. inaccessible mean-square displacement (MSD) of the confined monomers, calcd. as a function of radial distance from the pore surface, was obtained in the simulations. The simulations show a gradual increase of the MSD from the adsorbed, but mobile layer, to that similar to the bulk far away from the surface. (c) 2014 American Institute of Physics.
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    59. 59
      The neutron scattering length density (SLD) of molecules can be calculated based on the properties of the constituent atoms (scattering lengths) and the molecular properties (molecular volume). For further information, please refer to the NIST webpage (https://www.ncnr.nist.gov/resources/sldcalc.html).
      There is no corresponding record for this reference.
    60. 60

      Coherent neutron scattering gives rise to constructive diffraction patterns, which can aid analysis of structure and morphology. One can refer to scattering lengths of various atoms (isomer-specific) where larger positive values refer to stronger coherent scattering upon neutrons scattering with nuclei. A full listing of the scattering lengths can be found in ref (46).

      There is no corresponding record for this reference.
    61. 61
      Sears, V. F. Neutron Scattering Lengths and Cross Sections. Neutron news 1992, 3, 26– 37,  DOI: 10.1080/10448639208218770
      There is no corresponding record for this reference.
    62. 62
      Chiang, W.-S.; Fratini, E.; Baglioni, P.; Georgi, D.; Chen, J.-H.; Liu, Y. Methane Adsorption in Model Mesoporous Material, SBA-15, Studied by Small-Angle Neutron Scattering. J. Phys. Chem. C 2016, 120, 4354– 4363,  DOI: 10.1021/acs.jpcc.5b10688
      62
      Methane Adsorption in Model Mesoporous Material, SBA-15, Studied by Small-Angle Neutron Scattering
      Chiang, Wei-Shan; Fratini, Emiliano; Baglioni, Piero; Georgi, Daniel; Chen, Jin-Hong; Liu, Yun
      Journal of Physical Chemistry C (2016), 120 (8), 4354-4363CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The understanding of methane adsorption is important for many industrial applications, esp. for the shale gas prodn., where it is crit. to understand the adsorption/desorption of methane in pores even as small as a few nanometers. Using small-angle neutron scattering (SANS), the authors have studied the adsorption of deuterated methane (CD4) into one model mesoporous material, SBA-15, with pore diam. approx. 6.8 nm at the temp. range from 20 to 295 K at low pressure (≈100 kPa). A new scattering model is developed to analyze the SANS patterns of gas adsorption in SBA-15. The surface roughness of the SBA-15 matrix is estd. The gas adsorption behaviors on the surface regions are extd. from the fitting. The rough surface of the pores is found to retain a large amt. of CD4 at the temp. above the capillary condensation temp. (Tc). At temps. below Tc, the confined liq. and solid methane are estd. to be less dense than the corresponding bulk liq. and solid methane. Detailed theor. anal. and exptl. verification also show that SANS patterns at temps. higher than Tc are much more sensitive to the change of the excess adsorption, εads, rather than the av. d. of adsorbed layers commonly used in many studies. The model we establish can be used to analyze future SANS/SAXS data for gas confined in similar model porous materials.
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    63. 63

      Commercial tetraethylenepentamine (TEPA) contains a mixture of branched and cyclic isomers. Please see ref (64) for details.

      There is no corresponding record for this reference.
    64. 64
      Numaguchi, R.; Fujiki, J.; Yamada, H.; Chowdhury, A.; Kida, K.; Goto, K.; Okumura, T.; Yoshizawa, K.; Yogo, K. Development of Post-Combustion CO2 Capture System Using Amine-Impregnated Solid Sorbent. Energy Procedia 2017, 114, 2304– 2312,  DOI: 10.1016/j.egypro.2017.03.1371
      64
      Development of Post-combustion CO2 Capture System Using Amine-impregnated Solid Sorbent
      Numaguchi, Ryohei; Fujiki, Junpei; Yamada, Hidetaka; Firoz; Chowdhury, A.; Kida, Koji; Goto, Kazuya; Okumura, Takeshi; Yoshizawa, Katsuhiro; Yogo, Katsunori
      Energy Procedia (2017), 114 (), 2304-2312CODEN: EPNRCV; ISSN:1876-6102. (Elsevier Ltd.)
      A new synthetic polyamine was found as a suitable compd. for amine-impregnated solid sorbent through screening of amine compds. with the aid of d. functional theory calcns. CO2 capture performance of the sorbent was evaluated on a lab-scale CO2 capture system. The solid sorbent was excellently regenerable even at low temp. condition. Then, Bench-scale demonstration will be started on current project, using the sorbent developed by Research Institute of Innovative Technol. for the Earth (RITE) and moving bed system of Kawasaki Heavy Industries, Ltd (KHI).
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    65. 65
      Carrillo, J.-M. Y.; Potter, M. E.; Sakwa-Novak, M. A.; Pang, S. H.; Jones, C. W.; Sumpter, B. G. Linking Silica Support Morphology to the Dynamics of Aminopolymers in Composites. Langmuir 2017, 33, 5412– 5422,  DOI: 10.1021/acs.langmuir.7b00283
      65
      Linking Silica Support Morphology to the Dynamics of Aminopolymers in Composites
      Carrillo, Jan-Michael Y.; Potter, Matthew E.; Sakwa-Novak, Miles A.; Pang, Simon H.; Jones, Christopher W.; Sumpter, Bobby G.
      Langmuir (2017), 33 (22), 5412-5422CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      A combined computational and exptl. approach is used to elucidate the effect of silica support morphol. on polymer dynamics and CO2 adsorption capacities in aminopolymer/silica composites. Simulations are based on coarse-grained mol. dynamics simulations of aminopolymer composites where a branched aminopolymer, representing poly(ethylenimine) (PEI), is impregnated into different silica mesoporous supports. The morphol. of the mesoporous supports varies from hexagonally packed cylindrical pores representing SBA-15, double gyroids representing KIT-6 and MCM-48, and cagelike structures representing SBA-16. In parallel, composites of PEI and the silica supports SBA-15, KIT-6, MCM-48, and SBA-16 are synthesized and characterized, including measuring their CO2 uptake. Simulations predict that a 3D pore morphol., such as those of KIT-6, MCM-48, and SBA-16, will have faster segmental mobility and have lower probability of primary amine and surface silanol assocns., which should translate to higher CO2 uptake in comparison to a 2D pore morphol. such as that of SBA-15. Indeed, it is found that KIT-6 has higher CO2 uptake than SBA-15 at equiv. PEI loading, even though both supports have similar surface area and pore vol. However, this is not the case for the MCM-48 support, which has smaller pores, and SBA-16, whose pore structure rapidly degrades after PEI impregnation.
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    66. 66
      Carrillo, J.-M. Y.; Sakwa-Novak, M. A.; Holewinski, A.; Potter, M. E.; Rother, G.; Jones, C. W.; Sumpter, B. G. Unraveling the Dynamics of Aminopolymer/Silica Composites. Langmuir 2016, 32, 2617– 2625,  DOI: 10.1021/acs.langmuir.5b04299
      66
      Unraveling the Dynamics of Aminopolymer/Silica Composites
      Carrillo, Jan-Michael Y.; Sakwa-Novak, Miles A.; Holewinski, Adam; Potter, Matthew E.; Rother, Gernot; Jones, Christopher W.; Sumpter, Bobby G.
      Langmuir (2016), 32 (11), 2617-2625CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The structure and dynamics of a model branched polymer was investigated through mol. dynamics simulations and neutron scattering expts. The polymer confinement, monomer concn., and solvent quality were varied in the simulations and detailed comparisons between the calcd. structural and dynamical properties of the unconfined polymer and those confined within an adsorbing and nonadsorbing cylindrical pore, representing the silica based structural support of the composite, were made. The simulations show a direct relationship in the structure of the polymer and the nonmonotonic dynamics as a function of monomer concn. within an adsorbing cylindrical pore. However, the nonmonotonic behavior disappears for the case of the branched polymer within a nonadsorbing cylindrical pore. Overall, the simulation results are in good agreement with quasi-elastic neutron scattering (QENS) studies of branched poly(ethylenimine) in mesoporous silica (SBA-15) of comparable size, suggesting an approach that can be a useful guide for understanding how to tune porous polymer composites for enhancing desired dynamical and structural behavior targeting carbon dioxide adsorption.
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