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Reversible and Site-Dependent Proton-Transfer in Zeolites Uncovered at the Single-Molecule Level

  • Zoran Ristanović
    Zoran Ristanović
    Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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  • , 
  • Abhishek Dutta Chowdhury
    Abhishek Dutta Chowdhury
    Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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    Orcidhttp://orcid.org/0000-0002-4121-7375
  • , 
  • Rasmus Y. Brogaard
    Rasmus Y. Brogaard
    Department of Chemistry, University of Oslo, Postboks 1126 Blindern, 0318 Oslo, Norway
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  • Klaartje Houben
    Klaartje Houben
    NMR Research Group, Bijvoet Centre for Biomolecular Research, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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  • , 
  • Marc Baldus
    Marc Baldus
    NMR Research Group, Bijvoet Centre for Biomolecular Research, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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  • Johan Hofkens
    Johan Hofkens
    Department of Chemistry, KU Leuven, Celestijnenlaan 200 F, B-3001 Leuven, Belgium
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    Orcidhttp://orcid.org/0000-0002-9101-0567
  • , 
  • Maarten B. J. Roeffaers
    Maarten B. J. Roeffaers
    Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
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  • , and 
  • Bert M. Weckhuysen*
    Bert M. Weckhuysen
    Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
    *[email protected]
    More by Bert M. Weckhuysen
    Orcidhttp://orcid.org/0000-0001-5245-1426
Cite this: J. Am. Chem. Soc. 2018, 140, 43, 14195–14205
Publication Date (Web):October 3, 2018
https://doi.org/10.1021/jacs.8b08041

Copyright © 2022 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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Abstract

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Zeolite activity and selectivity is often determined by the underlying proton and hydrogen-transfer reaction pathways. For the first time, we use single-molecule fluorescence microscopy to directly follow the real-time behavior of individual styrene-derived carbocationic species formed within zeolite ZSM-5. We find that intermittent fluorescence and remarkable photostability of carbocationic intermediates strongly depend on the local chemical environment imposed by zeolite framework and guest solvent molecules. The carbocationic stability can be additionally altered by changing para-substituent on the styrene moiety, as suggested by DFT calculations. Thermodynamically unstable carbocations are more likely to switch between fluorescent (carbocationic) and dark (neutral) states. However, the rate constants of this reversible change can significantly differ among individual carbocations, depending on their exact location in the zeolite framework. The lifetimes of fluorescent states and reversibility of the process can be additionally altered by changing the interaction between dimeric carbocations and solvated Brønsted acid sites in the MFI framework. Advanced multidimensional magic angle spinning solid-state NMR spectroscopy has been employed for the accurate structural elucidation of the reaction products during the zeolite-catalyzed dimerization of styrene in order to corroborate the single-molecule fluorescence microscopy data. This complementary approach of single-molecule fluorescence microscopy, NMR, and DFT collectively indicates that the relative stability of the carbocationic and the neutral states largely depends on the substituent and the local position of the Brønsted acid site within the zeolite framework. As a consequence, new insights into the host–guest chemistry between the zeolite and aromatics, in terms of their surface mobility and reactivity, have been obtained.

Introduction

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Zeolites are microporous aluminosilicates with tunable acidity and microporous structure that can accommodate and transform numerous organic molecules to valuable chemicals and fuels. (1−3) The Si–O tetrahedra define a highly crystalline and porous matrix that has acidic properties if silicon is substituted with aluminum. A proton transfer and carbocation formation related to the bridging hydroxyl groups in the vicinity of Al, commonly known as Brønsted acid sites (BAS), is of fundamental importance for numerous, large scale acid-catalyzed processes. (4,5) The location of BAS is related to the exact spatial distribution of Al over the available crystallographic sites (known as T-sites). (6,7) In addition to the structural parameters of a zeolite catalyst, the chemical nature of adsorbed reactants, physisorbed intermediates, and solvent environment may significantly affect the proton transfer rates and carbocationic chemistry. (8)
Conventional methods to characterize the chemical nature of acid sites in zeolites include solid-state nuclear magnetic resonance (NMR), (9,10) infrared spectroscopy (IR) based on probe molecules, (11,12) and X-ray absorption spectroscopy (XAS). (13,14) These methods have been used to characterize the heterogeneous population of T-sites in the zeolite framework. Recent advances in atom probe tomography (APT) have opened up new horizons in the sensitive structural characterization of single zeolite particles. This method can be used to map the 3-D distribution of atomic constituents (Si, Al, O, and C) before and after a catalytic reaction. (15,16) Nevertheless, it is more difficult to obtain the dynamic picture of organic transformations taking place in zeolite micropores with the ultimate single-molecule sensitivity. For example, industrially important zeolite ZSM-5 contains 12 distinct T-sites that may have nonrandom distribution of Al atoms. (10) There is very little experimental knowledge about the real-time behavior of organic molecules and local proton-transfer processes taking place at individual T-sites. In this respect, single-molecule fluorescence microscopy can provide high spatiotemporal resolution and single-molecule sensitivity. (17−20) In recent years, the technique has provided ensemble-free information about mechanistic single-particle/single-molecule studies of a number of catalytic processes, (21−25) including studies of the related transport phenomena. (26−29) Zeolite catalysis has also benefited from the recent single-molecule fluorescence microscopy studies. By localizing fluorescent products within pores of zeolites, we have elucidated important inter- and intraparticle heterogeneities in acid-catalyzed zeolite chemistry. (30−37) These studies mainly focused on quantitative imaging of single particle reactivity, whereas the potential of single-molecule fluorescence microscopy to disclose intrinsic time-dependent heterogeneities in behavior of individual molecules remained largely unexplored. To the best of our knowledge, this is the first experimental report that studies single-molecule blinking dynamics in heterogeneous catalysis, following the ideas from enzymatic single-molecule catalysis (38) and single nanoparticle catalysis. (22,29) It will be shown that it is possible to use fluorescent states of trapped organic species in order to study their real-time interaction with BAS and zeolite framework in an ensemble-free manner.
We have used single-molecule fluorescence microscopy to follow the real-time behavior of carbocationic species confined in the pores of zeolite ZSM-5. To achieve this goal, we employed the Brønsted-acid catalyzed oligomerization of styrene derivatives as a fluorogenic probe reaction. (39−43) Two styrene derivatives, namely 4-methoxystyrene and 4-fluorostyrene, were chosen as probes with different proton affinities (214 kcal/mol vs 200 kcal/mol, (44) respectively) and markedly different proton-transfer rates. (35) Importantly, the probe molecules can fit inside ∼0.5 nm large pores of zeolite ZSM-5, and form highly fluorescent carbocationic species that are additionally stabilized by a tight sterical protection from nucleophilic attacks. Recently, we have shown that by changing the substituent on the styrene moiety, as well as the solvent medium, it is possible to directly study the turnover rates of proton transfer processes with single turnover sensitivity. (35) Here, we take a crucial next step to study real-time dynamics of individual carbocationic species confined in the pores of ZSM-5. As positively charged species, these carbocations are adsorbed in the vicinity of Al atoms and can be used as real-time reporters of interactions between Brønsted acid sites and guest molecules. However, accurate structural elucidation of such guest molecules is also essential to provide in-depth understanding of the host–guest chemistry between the inorganic zeolite and organic guest molecules. In this context, we corroborated the findings from the single-molecule fluorescence microscopy with a detailed investigation of the reaction mechanism by using advanced magic angle spinning (MAS) solid-state NMR spectroscopy and density functional theory (DFT) calculations. These approaches collectively point toward a large diversity of reaction pathways and proton/hydrogen-transfer dynamics taking place within zeolite ZSM-5 crystals. More specifically, a detailed reaction mechanism, including the identification of several mobile and immobile reaction intermediates, as well as the various reaction products during the zeolite-catalyzed oligomerization of styrene is presented.

Results

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Single-Molecule Fluorescence Microscopy

The mechanism of the styrene oligomerization involves the formation of fluorescent dimeric and trimeric carbocations that can be efficiently excited with visible light. The spectral properties of fluorescent species formed have been reported previously, (35,43) and recently addressed in a TD-DFT study. (44) Taking into account the reaction mechanism resolved by solid-state NMR (vide infra), we have hypothesized that the fluorescent zeolite-entrapped linear dimeric carbocations (Figure 1a, right) can transfer a proton to the zeolite framework and yield neutral and nonfluorescent dimeric 1,3-diphenyl-1-butene species (Figure 1a, left). The products are typically formed at the intersections of zeolite ZSM-5 (Figure 1b), leading to a well-defined orientation of fluorescence transition dipole moments along the straight pores of the zeolite. (33,43)

Figure 1

Figure 1. (a) Schematic of the approach to detect protonated, fluorescent dimeric carbocation (bright state on the right); the dark, nonfluorescent state (left) is a neutral dimer that is formed after a proton transfer to the zeolite framework. (b) Representation of the MFI framework (view along b axis) and a dimeric carbocation (ball and stick model) trapped along the straight pores; additional monomeric styrene residing in a sinusoidal pore is shown (wire model). (c,d) Blinking (c) and nonblinking (d) fluorescence intensity trajectories of individual carbocationic species of 4-methoxystyrene in heptane. (e) A zoom-in of the trajectory shown in (c), indicating the definition of τon and τoff times.

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We based our analysis of single-molecule dynamics in zeolites on a statistical description of single-molecule fluorescence intensity trajectories. A wide-field epi-fluorescence microscope was used to simultaneously detect the fluorescent products within ∼25 × 25 μm2 field of view. In this way, we were able to follow in real time fluorescence intensity of individual emitters, with 30 ms temporal resolution and ∼20 nm localization accuracy. (33)Figure 1 shows an example of two distinctly different fluorescence intensity trajectories. Emitter #1 (Figure 1c) exhibited pronounced blinking, i.e., intermittently present fluorescent and dark states. In contrast, emitter #2 showed constant fluorescence followed by permanent photobleaching (Figure 1d). Careful examination of the scatter plots of individual molecules confirms that the observed blinking behavior can be attributed to isolated single molecules (Supporting Information (SI), Figure S1, Supporting Movie S1). We exclude the possibility that the blinking may be explained by the formation or diffusion of new fluorescent molecules, as blinking events were highly localized—i.e., no large single-molecule jumps were recorded within the localization precision of the method (20 nm). Subsequent turnover events happening at one specific acid site are highly unlikely under the conditions of low reactivity, considering also the bulkiness of fluorescent products. As is illustrated in Figure 1a, we initially hypothesize that a single proton-exchange between an individual carbocation and the zeolite framework is possible to be observed by our approach after the dimerization of styrene over zeolitic BAS sites.
We further define the time intervals within the fluorescence intensity trajectories that exhibit alternating fluorescent (protonated, “on”) and dark (“off”) states, denoted further as τon and τoff lifetimes, as illustrated in Figure 1e. We only considered “blinking” trajectories (Figure 1c) with at least 5 intermitted fluorescent/dark states; hence, molecules that did not show visible blinking (Figure 1d) were not taken into account with our statistical approach; from now on, we will consider all nonblinking molecules as belonging to one distinct subpopulation of emitters with similar chemical properties.
We have analyzed a total of 110 (4-fluorostyrene) and 60 (4-methoxystyrerene) fluorescence intensity trajectories that exhibited significant blinking behavior. This resulted in more than 1500 pairs of on/off times for 4-fluorostyrene and 3000 pairs for 4-methoxystyrene. The complex nature of individual trajectories is highlighted by typical examples of trajectories recorded for 4-fluorostyrene (Figure 2a) and 4-methoxystyrene carbocations (Figure 2b). The averaged fluorescent ⟨τon⟩ and dark ⟨τoff⟩ times for all analyzed molecules are presented in Figure 2c. Strikingly, the average lifetimes span 2 orders of magnitude and differ significantly from molecule to molecule. The intensity trajectories of 4-fluorostyrene exhibited strong blinking behavior, with typically <1 s short fluorescent and dark times, ⟨τon⟩ = 0.5 ± 0.7 s and ⟨τoff⟩ = 0.7 ± 1.2 s), as shown in Figure 2a. Interestingly, the analyzed emitters prefer residing in one of the two states and with slight preference for their dark states, ⟨τon⟩ < ⟨τoff⟩. In stark contrast, 4-methoxystyrene carbocationic products largely preferred fluorescent states with two times longer average ⟨τon⟩ times, 2.3 ± 5.7 s, as compared to ⟨τoff⟩ times, 1.1 ± 3.3 s. Additionally, the mean duration of 4-methoxystyrene blinking events was more than 10 times longer (195 s) as compared to 4-fluorostyrene (17 s). Our observations can be explained by the electronic effects of the substituents on the protonated aromatic ring known from electrophilic aromatic substitution. The net effect of the fluoro- group is destabilization, agreeing well with the slight preference toward the dark neutral state. The methoxy- group on the other hand is stabilizing, agreeing with the preference toward the bright cationic state, including some exceptionally photostable emitters that did not show visible blinking.

Figure 2

Figure 2. Statistical description of blinking properties for fluorescent products originating from 4-methoxystyrene and 4-fluorostyrene. (a,b) Examples of intermittent fluorescence intensity trajectories for 4-fluorostyrene (a) and 4-methoxystyrene (b), for different ratios of average ⟨τon⟩ and ⟨τoff⟩ times. (c) Plot of ⟨τon⟩ and ⟨τoff⟩ lifetimes of fluorescent and dark states for 4-fluorostyrene (blue) and 4-methoxystyrene (red) trajectories. See the SI Figure S2 for cumulative frequency histograms of τon and τoff lifetimes.

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The averaged τon and τoff lifetimes do not accurately describe the blinking process as a whole, as fluorescence intensity trajectories consist of multiple alternating states of different and highly dispersed lengths in time. The normalized 2-D histograms of subsequent pairs of “on–off” times, i.e., (τon(1), τoff(1)), (τon(2), τoff(2)), etc., provide more quantitative information about statistical distribution of long and short fluorescent events, and their correlation in time. As expected from averaged properties presented in Figure 2c, 4-fluorostyrene predominantly exhibits 100–500 ms long on–off pairs, slightly skewed toward longer “off states” (Figure 3a). This suggests, in average, higher stability of the dark dimeric states of 4-fluorostyrene. The additional autocorrelation analysis of 47 fluorescence intensity trajectories indicated large dispersion of monoexponential correlation times ranging from 0.05 to 1.2 s (SI Figure S3). The large time span indicates diversity of molecular environments that set fluorescent molecules into preferentially bright or dark states (examples shown in Figure 2a).

Figure 3

Figure 3. (a,b) Normalized 2-D histograms of all pairs of subsequent τon and τoff lifetimes collected for 4-fluorostyrene (110 trajectories, 1500 pairs) and 4-methoxystyrene (60 trajectories, 3000 pairs). The histograms are normalized in respect to the (τon, τoff) pair with the highest occurrence (i.e., 100 ms, 100 ms). The color bars denote the normalized number of detected events for specific (τon, τoff) values. (c,d) Normalized scatter plots of subsequent (τon, τoff) pairs of lifetimes for 4-methoxystyrene trajectories categorized based on the average ⟨τon⟩ and ⟨τoff⟩ times; (c) ⟨τon⟩ > 5 × ⟨τoff⟩, and (d) ⟨τon⟩ < ⟨τoff⟩. Plots in (c) and (d) contain all (τon, τoff) pairs recorded for 10 trajectories with the specified criteria.

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A normalized 2-D histogram for 4-methoxystyrene shows much broader distribution of “on states” (Figure 3b). The detected fluorescent states span 4 orders of magnitude in measured τon lifetimes and ⟨τon⟩/⟨τoff⟩ ratios from 0.3 to 100 (SI Figures S2 and S7). Interestingly, we notice two distinct classes of blinking trajectories for 4-methoxystyrene-based carbocations. The first class includes emitters with very long fluorescent states and short dark states. The subsequent (τon, τoff) pairs of representative 10 molecules of this class with ⟨τon⟩ > 5 × ⟨τoff⟩ are collectively plotted in Figure 3c. The (τon, τoff) pairs of lifetimes are strongly scattered along τon axis, with ⟨τon⟩ = 4.6 ± 7.9 s and ⟨τoff⟩ = 0.5 ± 0.8 s. The second class consists of 10 highly blinking emitters presented in Figure 3d. In this case, the dispersion is larger for the dark states (⟨τon⟩ = 1.3 ± 1.8 s and ⟨τoff⟩ = 3.2 ± 6.7 s). These two classes encompass at least 40% of analyzed carbocations. Other analyzed trajectories fall in between these two categories, typically with larger fluorescence intervals, ⟨τon⟩ > ⟨τoff⟩. It should be noted that observed classes of trajectories do not have the expected random distribution of (τon, τoff) times, but rather highly clustered values of the two parameters are observed. This fact points toward differences among individual emitters that we relate to the carbocation stability at specific T-adsorption sites.

Effect of Solvent Polarity on Blinking Dynamics

The described approach can be generalized and extended to study framework-carbocation interactions in the presence of different guest molecules, such as solvents. Previously, we have shown that the rate constants of 4-methoxystyrene oligomerization can differ dramatically in n-heptane and 1-butanol. (35) Fluorescence intensity trajectories of 4-methoxystyrene recorded on parent zeolite crystals in n-heptane showed predominantly stable (nonblinking) trajectories with typical lifetimes of 0.1–10 s. However, under the conditions of higher acid site accessibility, such as in steamed zeolites crystals, (35) highly stable and blinking fluorescence intensity trajectories were present. Qualitatively, this behavior was very similar to observed trajectories recorded in solvent-free 4-methoxystyrene (Figure 2). Figure 4 presents fluorescence intensity trajectory and related statistical distribution of τon and τoff lifetimes for one blinking emitter of 4-methoxystyrene oligomer in n-heptane (see also Supporting Movie S2). The average lifetime of fluorescent τon states was 0.46 ± 0.57 s, approximately 2 times longer than that of dark τoff states 0.23 ± 0.55 s (Figure 4b). The autocorrelation function of the trajectory reveals two processes with different durations (Figure 4b, inset). The fast decay component (0.16 s) corresponds to the large number of short off-states, and the slow decay component (3.6 s) designates a large number of long on-states. Consequently, the distribution of subsequent (τon, τoff) pairs in their 2-D lifetime histogram is stretched toward longer τon times (Figure 4c). Interestingly, in the case of 4-methoxystyrene in n-heptane, we consistently observe trajectories with ⟨τon⟩ > ⟨τoff⟩, indicating that carbocationic form of these species is more stable that the neutral (nonfluorescent) state.

Figure 4

Figure 4. Single-molecule blinking dynamics of the zeolite H-ZSM-5 trapped 4-methoxystyrene-derived emitters in n-heptane. (a) Fluorescence intensity trajectory of a highly blinking molecule. (b) Histogram of lifetimes of the fluorescent (red dots) and dark states (blue dots) measured for the single-molecule trajectory shown in (a). Inset: autocorrelation function for the trajectory in (a). Red line is a biexponential fit, with exponential decay constants of t1 = 0.16 ± 0.02 s (fast-decay component) and t2 = 3.6 ± 0.8 s (slow-decay component). (c) Real 2-D frequency histogram of the (τon, τoff) pairs as measured for the trajectory in (a). The color bar denotes the number of detected events for specific (τon, τoff) values. The simulated 2-D histogram based on the experimentally measured (τon, τoff) distributions in (b) is shown in SI Figure S4.

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An additional experiment in 1-butanol confirms that we are measuring true carbocation-framework interactions. In this case, fluorescence intensity trajectories typically consisted of short <100 ms bursts (Figure 5a). Blinking events were rare, with the distributions of fluorescent and dark states typically the opposite of the ones recorded in n-heptane. The histograms of τon and τoff times are characterized with short fluorescent lifetimes ⟨τon⟩ = (0.18 ± 0.11 s) and in average 10 times longer dark states ⟨τoff⟩ = (1.6 ± 2.2 s), Figure 5b. Similarly, autocorrelation function of the trajectory has fast decay time of 0.11 s, most likely limited by the acquisition time in our experiments. In this case, the activation energy for the formation of fluorescent carbocations is increased due to solvation of Brønsted acid sites, (35) clearly suggesting that the “vicinity of Al-site” is playing an important role in the stabilization of carbocations.

Figure 5

Figure 5. Single-molecule blinking dynamics of the zeolite H-ZSM-5 trapped 4-methoxystyrene-derived emitters in 1-butanol. (a) Fluorescence intensity trajectory of a highly blinking molecule. (b) Histogram of lifetimes of the fluorescent (red dots) and dark states (blue dots) measured for the single-molecule trajectory shown in (a). Inset: autocorrelation function for the trajectory in (a). Red line is a monoexponential fit, with exponential decay constant of t1 = 0.11 ± 0.02 s. (c) Real 2-D frequency histogram of the (τon, τoff) pairs as measured for the trajectory in (a). The color bar denotes the number of detected events for specific (τon, τoff) values. The simulated 2-D histogram based on the experimentally measured (τon, τoff) distributions in (b) is shown in SI Figure S4.

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Solid-State Nuclear Magnetic Resonance

With the aim of understanding the mechanism as well as to identify the molecular structures of reaction products of zeolite-catalyzed styrene oligomerization reaction, we employed advanced solid-state NMR spectroscopy on zeolite H-ZSM-5 after being exposed to 13C8-styrene. Using of fully isotope-enriched styrene not only significantly increased the NMR signal intensities, but also allowed us to perform multidimensional solid-state NMR correlation experiments to elucidate accurate molecular structures of zeolite-trapped species. The 1H–13C cross-polarization (CP), 1H–13C insensitive nuclei enhanced by polarization transfer (INEPT) and 13C direct excitation (DE) solid-state NMR spectra of the postreacted catalyst are presented in Figure 6. The following three features were primarily observed: (i) 13–52 ppm aliphatic moieties and methyl groups, (ii) 65–75 ppm alkoxy species, and (iii) 112–152 ppm (methylated) aromatic/olefinic groups (Tables S1 and S2). The different solid-state NMR magnetization transfer techniques were previously used for spectral separation of zeolite-trapped species and biomolecules on the basis of mobility. (45−48) As a result, both mobile (i.e., molecule or group with fast tumbling or rotation) and rigid (i.e., molecule physisorbed in/on zeolite) versions of zeolite-trapped organics have been distinguished in the present case. This spectral separation of mobile and rigid species was envisioned by using through-bond (scalar interactions such as in INEPT) (49) and through-space (dipolar transfer such as in CP) (50) magnetization transfer schemes, respectively. Alternatively, all chemical species, including those that exhibit intermediate dynamics, can be characterized using DE experiments. Comparison of the 1D spectra (Figure 6) shows that the DE spectrum (red) is dominated by mobile zeolite-trapped molecules, as also observed in the INEPT spectrum (green). In addition, line-widths of the rigid molecules (observed in CP, blue) are significantly broader, suggesting heterogeneity in the molecular environment of the physisorbed reactant and product molecules. This observation is consistent with our single-molecule fluorescence results presented in Figure 3.

Figure 6

Figure 6. 1H–13C CP (blue, NS = 4096), 13C DE (red, NS = 2048), and 1H–13C INEPT (green, NS = 2048) solid-state NMR spectra (at 15 kHz MAS) of trapped products obtained on H-ZSM-5 after being exposed to 13C8-styrene at 393 K (MAS = magic angle spinning, NS = number of scans).

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In the solid-state NMR spectra probing rigid molecules, signals corresponding to the untreated reactant (green lines in Figure 7) are broad or show more than one peak for the same resonance. This can be explained by the same molecule existing in different molecular environments inside the zeolite framework. Interestingly, the signals corresponding to the reactant are absent from the spectra probing the mobile molecules (Figure 6). At least two (solid and dashed green lines in Figure 7) full spin-systems could be identified for the reactant, which we putatively attribute to the (i) “physisorbed state” in zeolite pores by dispersion forces (A, Scheme 1) and (ii) the “π-complex” between Brønsted acid sites of zeolites and styrene (A′, Scheme 1). (8) In both cases, covalent bonds were neither broken nor formed. This finding actually provides experimental support to a long-standing speculation of multiple adsorption modes of olefin on zeolite, as was recently also emphasized by Lercher et al. (8) The prominent role of a π-complex between zeolitic Brønsted acid-sites and aromatics during zeolite catalyzed hydrocarbon conversion has recently been recognized by us. (48) In addition to the “physisorbed” and “π-complex”, we could also identify the zeolite-alkoxy species (B, Scheme 1), formed upon adsorption of styrene onto a Brønsted acid site of zeolite (red lines in Figure 7). Also this form of the reactant exists in multiple conformations or molecular environments as doubling or even tripling of the resonance lines is observed. The formation of surface-alkoxy species (B, Scheme 1) provides evidence in support of the formation of chemisorbed carbenium species (A″, Scheme 1) in the process. The identification of A–A′′ species further supports the influential role of the “vicinity of Al-site” in the stabilization of carbocations during dimerization of styrene reaction, as was also visible from the diverse dynamics of single-molecule fluorescence blinking (Figure 2). Moreover, detection of ethylbenzene (BHT, Scheme 1) can presumably be attributed to the ongoing hydrogen-transfer (HT) side reaction during dimerization of styrene. (51)

Figure 7

Figure 7. Zooms of 2D MAS solid-state NMR spectra of rigid zeolite trapped molecules. Spectra were recorded on H-ZSM-5 after being exposed to 13C8-styrene at 393 K, using 15 kHz MAS. Polarization of the 13C atoms was achieved through cross-polarization (CP) and a 120 ms PARIS (52) mixing period was used. Lines indicate identified spin-systems, in red (surface alkoxide) and in green (styrene), where for the latter at least two forms could be identified as indicated by the solid and dashed lines.

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Scheme 1

Scheme 1. Proposed Catalytic Cycle of the ZSM-5 Catalyzed Oligomerization of Styrene Based on Reaction Intermediates and Products Detected in This Studya
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aFour distinct adsorption and reaction routes are denoted in different colors. The dimeric carbocation formation route is the most relevant for this study. The dimeric fluorescent species C′ are highlighted in the yellow frame.

Alternatively, products after the reaction, 1,3 diphenyl-1-butene (linear dimeric species C, Scheme 1, red spheres Figure 8) and 1-methyl-3-phenylindane (cyclic dimeric species D, Scheme 1, in syn- and anti-stereoisomeric fashion, cyan cross and pink rectangle in Figure 8), were observed in the 2D solid-state NMR spectra using direct excitation, which, as discussed above, for this sample displayed dominantly mobile molecules. Identification of the molecules was achieved, starting from the methyl resonances as observed in panel IV of Figure 8 and concomitant connection to other resonances via the observed cross peaks. For example, an intense cross peak of the methyl resonance at 21.6 ppm (Cd of molecule D, Scheme 1) with a resonance at 40.5 ppm (Cc of molecule D, Scheme 1) is observed (cyan cross, panel IV Figure 8) and this aliphatic resonance further correlates through a strong peak with 49.0 ppm (Cb of molecule D, Scheme 1) and a low intense peak at 52.9 ppm (Ca of molecule D, Scheme 1; panel III Figure 8). Further correlations of these aliphatic carbons with aromatic carbons and corresponding intra-aromatic correlations could be identified in panel II and V, respectively. In most cases, correlations with both direct and next neighbors could be identified, as indicated by the solid and dashed lines in Figure 8 panel VI, respectively. As expected, the chemical shifts of the syn- and antistereoisomers of D (Scheme 1) significantly differ, for both magnetically nonequivalent aliphatic carbons as well as their adjacent aromatic carbons (i.e., C1, C1′ and C2′ of molecule D). On the basis of the intensities of signals, occurrence of the two stereoisomers is close to equal (cf. ∼47 ± 1% and 53 ± 1% for pink spin-system of syn-D and cyan spin-system of anti-D, respectively, based on methyl resonances). The linear dimer (molecule C, red circles in Figure 8) could be identified through the strong cross-peaks observed at 44.7 ppm (Cc of molecule C): a methyl at 23.5 ppm (Cd of molecule C, Scheme 1) in panel IV, an aromatic carbon at 147.7 ppm (C1′ of molecule C, Scheme 1) and an olefinic carbon at 137.3 ppm (Cb of molecule C, Scheme 1) in panel II. The latter carbon could be further connected (panel V) to its neighboring carbon with a chemical shift of 131.1 ppm (Ca of molecule C, Scheme 1), whose chemical shift correlates with the ring carbon (C1 of molecule C, Scheme 1) at 139.7 ppm. The formation of 1,3-diphenylbutane (CHT, Scheme 1) from linear dimer (C, Scheme 1) is another example of HT-side reaction during styrene dimerization (SI Figure S5). (51)

Figure 8

Figure 8. Zooms of 2D MAS solid-state 13C–13C correlation NMR spectrum of zeolite trapped products. Spectra were obtained on H-ZSM-5 after being exposed to 13C8-styrene at 393 K. 13C atoms were polarized by direct excitation (DE) and a 120 ms PARIS (52) mixing period was used. The MAS frequency was 10 kHz. The gray box in panel IV marks spinning side-bands.

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Interestingly, unlike for the rigid molecules observed in the CP spectra, we do not observe line broadening of the 13C lines of the mobile molecules, as expected due to their mobile nature. However, we do see line-broadening of the proton lines in the J-based (INEPT) 13C–1H correlation spectrum (SI Figure S6), which can best be described as a superposition of 2 or more overlapping resonances. This observation explains why our single-molecule fluorescence trajectories behave differently depending on the exact local environment of individual molecules. Additional spin-systems identified in the 2D spectra that could not be assigned to a molecular structure are listed in the Supporting Information (SI Table S2). Herein, the presence of more than four bridging carbon atoms (molecule R1 and M4 in SI Table S2, also Figure 8) are indicative of the formation of trimeric species as was also envisioned by our fluorescence study. Even though these correlations are consistent with the existence of a trimeric product as a result of styrene oligomerization reaction, the absence of additional correlations prohibited us to construct its complete molecular structure. Such partial elucidation (weak/absent NMR signals) could be attributed to its lower quantity and/or overlapping of certain signals.

DFT Calculations: Reversibility of the Proton-Transfer Processes

On the basis of our experimental results, we hypothesized that the stability of the fluorescent carbocations determines the blinking dynamics. We have conducted DFT calculations in order to support this reasoning. The calculations treated the crystallographic MFI unit cell with periodic boundary conditions, using the BEEF-vdW functional (53) to take into account both ion-pair interactions with acid sites and van der Waals interactions with the framework. We calculated electronic energy differences, assuming that they reflect the same trends as free energies for the rigid, confined species considered in this work. First, we determined if the calculations predict a clear preference for adsorption of linear dimeric species in certain locations. We considered the linear dimer of 4-fluorostyrene adsorbed in the following configurations (Table 1): sinusoidal channel (Sinus), straight channel (Straight), intersection region oriented along straight channel (IntStraight), and intersection region oriented along sinusoidal channel (IntSinus). As it can be seen from the Table 1, 4-fluorostyrene is generally more stable in the intersection region directed along the straight pores of MFI, than in other locations, which is the orientation determined experimentally. (40,43) The results indicate that steric interactions outweigh the influence of ion-pair interactions in chemisorption. On the basis of the results from the 4-fluoro species, we calculated chemisorption energies of the 4-methoxy analogue at sites being favorably located for adsorption in the IntStraight configuration (SI Table S3).
Table 1. Relative Chemisorption Energies ΔΔEchem (kJ/mol) Calculated for the 4-Fluorostyrene Linear Dimeric Carbocation (C′ in Scheme 1) in H-ZSM-5a
a

T-site numbering of 12 possible crystallographic tetrahedral sites of ZSM-5 follows Kim et al. (54) Numbers are indicated relative to the IntStraight adsorption configuration at the T9 site (ΔEchem = −153 kJ/mol); The IntStraight configuration is highlighted as the most stable. Values can be considered identical if they deviate less than 10 kJ/mol. The adsorption configurations are depicted in the first column. The straight and sinusoidal channels are indicated with dark and light gray, respectively. The black double-arrows indicate the adsorbate position and orientation of its longest axis. For molecules as large as the oligomers considered in this work, there is significant overlap between the locations.

We further compared the dimeric species of 4-fluorostyrene and 4-methoxystyrene situated in the intersection region and oriented along the straight channels of the MFI zeolite framework. The calculated protonation energies at individual T sites are indeed consistently smaller for the dimeric species formed from 4-fluorostyrene than those originating from 4-methoxystyrene by about 50 kJ mol–1 (Table 2). This agrees with the experimental observation that the dimeric species of 4-methoxystyrene has a higher preference toward the bright carbocationic state than the 4-fluoro analogue does. Note that the calculations aimed at locating the most stable isomers of the carbocations. The neutral dimeric diphenyl alkenes were created as the products from deprotonation of the corresponding fluorescent cations. Hence, the neutral species are not necessarily the most stable isomers, which can explain remarkable photochemistry and stability of the styrene-derived carbocationic species. Whereas the energy of a protonated dimer varies 20 kJ/mol across the investigated acid sites, the energy of the corresponding neutral state varies by 60 kJ/mol. This suggests that the protonation energies are mainly determined by acid-site relaxation and stability of the neutral dimer. As such, we expect proton-transfer-induced blinking behavior to be more pronounced at Brønsted sites that are easily accessible. For example, in the calculations conducted for 4-fluorostyrene, T7 and T8 sites significantly stabilize the neutral dimer of 4-fluorostyrene, relative to the protonated species (Table 2). Hence, these sites are plausible locations of the proton transfer process leading to the blinking fluorescence signal observed experimentally for approximately 50% of the trajectories. In fact, at these acid sites the 4-fluorostyrene-derived dimeric species are equally stable in the neutral and protonated states (within the estimated accuracy of the results). This agrees well with the averaged ratio of ⟨τon⟩/⟨τoff⟩ ∼ 1.2 (Figure S7), corresponding to free energy differences of 0–5 kJ/mol between the carbocationic and neutral states.
Table 2. Calculated Energies (kJ/mol) of Protonation of the Para-Substituted Styrene-Derived Dimers at Different Acid Sites within Zeolite H-ZSM-5a
R-StyT1T2T3T5T7T8T9T12
4-F–35–38–68–28–29–35–45
4-Meo–86–81–120–87–38–32–81–104
a

All species are situated in the intersection region, oriented along the straight channel.

Finally, we have addressed the possibility of formation of the trimeric species. The DFT calculations suggest that it is possible to create trimeric species at the intersections of straight and sinusoidal pores, although the reaction energies generally do not favor trimerization of the 4-fluorostyrene species (SI Table S4). We expect the same to be the case for the 4-methoxystyrene trimers, although in this case the formation is more likely at the crystalline defects. (35)

Discussion

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Reaction Mechanism

On the basis of the results described above, we are now in the position to give more general description of the styrene oligomerization reaction mechanism and related blinking dynamics. A reaction pathway for the zeolite (denoted as ZeOH) catalyzed oligomerization of styrene is proposed in Scheme 1. As indicated previously, styrene is first adsorbed on the zeolite and equilibrated between physisorbed (species A on non-Brønsted acid site, Scheme 1) and π-complex state (species A′ on Brønsted acid site, Scheme 1). The reaction is initiated with the subsequent formation of chemisorbed carbenium ion (an ion-pair species A″, Scheme 1) and followed by covalently bonded surface-alkoxide (species B, Scheme 1). This step is proposed to be the rate-determining step during the zeolite catalyzed dimerization of alkene reaction. (8) The surface-alkoxy species then react with another styrene molecule to form both linear (C) and cyclic (D) dimers, via linear (C′) and cyclic (D′) chemisorbed carbenium species, respectively. Two other side products, ethylbenzene (BHT) and 1,3-diphenylbutane (CHT), were formed from chemisorbed A″ and C′ carbenium species respectively, as a result of hydrogen-transfer reactions over zeolite. (51) Our fluorescence microscopy blinking observations (Figure 2) and DFT calculations (Table 2) indicate that the single proton transfer/exchange between protonated linear dimeric fluorescent carbocation (C′) and nonfluorescent 1,3 diphenyl-1-butene (C) can be reversible in nature. Thus, it is safe to assume that dimeric carbocation C′ simultaneously undergoes through two different chemical transformation: (i) reversible single-proton transfer to C and (ii) irreversible hydrogen transfer to CHT. This phenomenon not only indicates the “site-dependent” claim from the fluorescence intermittency results in the single-molecule fluorescence experiments, but also suggests the hydrogen-transfer pathway as a possible quenching route. Moreover, the simultaneous existence of both linear (C) and cyclic (D) dimerized products further supports heterogeneities in acid site strengths within a zeolite crystal, as the cyclic product is supposed to form on stronger acid sites and most likely close to crystalline defects. (42,55) Similarly, the observed line-broadening of the proton lines of syn-D and anti-D isomers in the INEPT 13C–1H correlation spectrum (as illustrated in Figure S6) could be attributed to their presence in the nonidentical zeolite environment. It should be noted that our NMR studies failed to detect 1,3-diphenyl-1,3-butadiene (i.e., the absence of any cross peak between 110 and 115 ppm and 145–150 ppm in the DE based spectra, Figure 7), which inevitably excludes the allylic species as a possible candidate for fluorescent carbocationic species in the present study. This observation is consistent with the recent TD-DFT calculations, where it has been suggested that the linear dimeric carbocations C′ can absorb light in the visible region. (44)

Site-Dependent Stability and Blinking of the Fluorescent Carbocationic Species

We relate the observed fluorescence blinking behavior of 4-methoxystyrene and 4-fluorostyrene to differences in the proton transfer between linear carbocationic species C′ (or their trimeric analogues) and the BAS. As shown in our earlier work, zeolite ZSM-5 crystals exposed to 4-fluorostyrene do not show visible reactivity (coloration) in bulk UV–vis experiments at room temperature, whereas 4-methoxystyrene reacts immediately producing blue coloration of the crystals. (40,41) Nevertheless, single-molecule fluorescence microscopy can detect extremely low reactivity of 4-fluorostyrene, with 4 orders of magnitude lower turnover rates than of 4-methoxystyrene. (35) This instability of 4-fluorostyrene carbocations explains why fluorescence of these emitters was accompanied with significant blinking, also in line with the DFT calculations. Our fluorescence blinking analysis (Figures 2 and 3) and solid-state NMR spectroscopy (Figure 6) suggests that individual fluorescent molecules may experience different local environments, as reflected in their blinking behavior (or the absence of blinking). This observation also provides indirect justification that side reactions (i.e., cyclization and/or hydrogen-transfer) of fluorescent linear dimeric products occur over zeolite acid sites, as illustrated in Scheme 1.
Several arguments point toward an observation of proton-transfer and/or related hydrogen-transfer processes. First, we experimentally detect distinct subpopulations of carbocations with different fluorescence behavior. As we observe both blinking and nonblinking subpopulations, the results suggest that blinking properties are closely related to chemical stability of fluorescent carbocations at specific crystallographic locations in the framework. Second, fluorescence intermittence patterns alone can be further classified into subgroups of quantitatively similar behavior, according to the duration and frequency of fluorescent and dark states. Another possibility that we considered is hopping of fluorescent species between acid sites and in and out of polarization plane (i.e., between sinusoidal and straight pores). This scenario is difficult to rationalize as we did not observe appreciable fluorescence along sinusoidal pores. In addition, site-hopping would be followed by measurable diffusion (over 10 min interval) and blinking patterns would not have distinct signatures. In contrast, analyzed emitters were static in all cases to the positional accuracy of 20 nm.
It is interesting to compare the equilibrium constant, K, of the reversible protonation reaction, K = ⟨τon⟩/⟨τoff⟩, for both reactants (Figure S7). For 4-fluorostyrene emitters, the median value is at K = 0.7, with relatively small dispersion. For 4-methoxystyrene emitters, the values range from K = 0.3 (larger dark lifetimes) to K = 100 (significantly larger fluorescent times). Nevertheless, the measured equilibrium constants can only account for the free energy differences in the range of ±10 kJ/mol. Whereas these values fit well with the DFT calculations for 4-fluorostyryrene, they are smaller than the calculated values for 4-methoxystyrene (Table 2). In this case, the dimeric carbocationic species were found to be more stable than the neutral dimers at all T-site locations, when the species were situated along straight pores at the intersection region (Table 2); these sites could be the plausible candidates for nonblinking trajectories. Keeping in mind that we only considered a small fraction of the large number of proton locations, adsorption configurations and adsorbate isomers, other less-stable sites may be able to induce the blinking behavior of 4-methoxystyrene emitters observed in Figures 2–4.
The behavior of 4-methoxystyrene is more complex to rationalize in terms of fluorescence behavior and dispersion of lifetimes, as this compound is highly reactive and leads to the formation of at least two types of distinguishable fluorescence species. (35) This is in line with many emitters of 4-methoxystyrene observed without blinking, especially on parent zeolite crystals and in highly diluted n-heptane solutions. Previously, we reported on reaction conditions that lead to the formation of highly photostable fluorescent species of 4-methoxystyrene. (35) The fluorescent products of this reactant only showed pronounced blinking at the conditions of high acid-site accessibility and presumably formation of trimeric (or other more photostable) fluorescent species. Therefore, it is likely that the trimeric species are also contributing to the highly photostable and blinking trajectories. At present, we can only speculate if the existence of other types of fluorophores may contribute to the observed fluorescence behavior, although the number of candidates is limited due to spatial constrains of ZSM-5. As the length and diversity of trajectories grow with introducing crystalline defects in zeolite framework, we speculate that several distinct nanoenvironments exist around fluorescent carbocationic species. The solid-state NMR studies also indicate simultaneous existence of hydrogen-transfer product of neutral and nonfluorescent 1,3-diphenyl-1-butene and its corresponding carbocation, supporting further the site-heterogeneity of zeolite ZSM-5.
Finally, we show that zeolites can be excellent hosts to stabilize protonated organic intermediates (56,57) and that this chemistry can lead to highly fluorescent carbocationic species protected from the nucleophilic attacks. This suggests that stability of carbocations in zeolites can be significantly different from those in solution, leading to their remarkable fluorescence properties. This especially holds true for thermodynamically more stable 4-methoxystyrene, where we observed fluorescent carbocationic emitters stable for longer than 20 min of constant laser radiation.

Conclusion

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We have demonstrated a powerful fluorescence microscopy approach to study single-molecule blinking dynamics and stability of individual carbocationic species in the highly confined spaces of a zeolite host. Supported by advanced solid-state NMR spectroscopy and DFT calculations, our results indicate that the relative stability and location of the formed fluorescent carbocationic species determines their blinking behavior and statistical distribution fluorescent and dark states. Depending on the nearest-neighbor molecular environment, fluorescence switching can span large time scales of carbocationic stability and proton-transfer rates. In this respect, the single-site/single-molecule approach clearly shows that the equilibrium between carbocation and neutral states can be significantly shifted by changing substituents and solvents.
Simultaneously, solid-state NMR spectroscopy was utilized for the accurate structural elucidation of the reaction products after the dimerization of styrene reaction over H-ZSM-5. Multiple adsorption modes of olefin (i.e., styrene in the present case) along with both linear and cyclic dimeric products were primarily observed by solid-state NMR spectroscopy. Additionally, the results also identify the mobility-dependent features within the zeolitic framework and an additional hydrogen transfer-based side-reaction route.
Our combined spectroscopic and theory approach collectively advocates the concept of a hybrid inorganic–organic nature of the working catalyst material, constituted by the inorganic zeolite and organic trapped species during a catalytic reaction. The vicinity of the framework Al and the distinctive host–guest chemistry between the zeolite and the trapped hydrocarbons play a governing role in the product formation. In a broader perspective, our findings will not only have important implications for understanding proton/hydrogen transfer processes in zeolites and the formation/behavior of highly photostable emitters confined in microporous hosts, but will also contribute to the basic understanding of zeolite-catalyzed hydrocarbon conversion chemistry.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b08041.

  • Supporting figures and tables, single molecule fluorescence trajectories, NMR data, and DFT calculations (PDF)

  • Movie S1 (AVI)

  • Movie S2 (AVI)

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

Author Information

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  • Corresponding Author
  • Authors
    • Zoran Ristanović - Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
    • Abhishek Dutta Chowdhury - Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands;  Orcidhttp://orcid.org/0000-0002-4121-7375
    • Rasmus Y. Brogaard - Department of Chemistry, University of Oslo, Postboks 1126 Blindern, 0318 Oslo, Norway
    • Klaartje Houben - NMR Research Group, Bijvoet Centre for Biomolecular Research, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
    • Marc Baldus - NMR Research Group, Bijvoet Centre for Biomolecular Research, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
    • Johan Hofkens - Department of Chemistry, KU Leuven, Celestijnenlaan 200 F, B-3001 Leuven, Belgium;  Orcidhttp://orcid.org/0000-0002-9101-0567
    • Maarten B. J. Roeffaers - Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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B.M.W. acknowledges The Netherlands Organization for Scientific Research (NWO) for a Gravitation program (Netherlands Center for Multiscale Catalytic Energy Conversion, MCEC) and a European Research Council (ERC) Advanced Grant (321140). This project has also received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement (no. 704544 to A.D.C.), a Middelgroot program (no. 700.58.102 to M.B.), and uNMR-NL, an NWO-funded National Roadmap Large-Scale Facility for The Netherlands (no. 184.032.207). R.Y.B acknowledges The Norwegian High Performance Computing program for resources at the Abel cluster under project no. nn4683k, and the USIT center at University of Oslo for support.

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    Determining the location and nearest neighbours of aluminium in zeolites with atom probe tomography
    Perea Daniel E; Liu Jia; Kovarik Libor; Arey Bruce W; Arslan Ilke; Lercher Johannes A; Ristanovic Zoran; Weckhuysen Bert M; Lercher Johannes A; Bare Simon R
    Nature communications (2015), 6 (), 7589 ISSN:.
    Zeolite catalysis is determined by a combination of pore architecture and Bronsted acidity. As Bronsted acid sites are formed by the substitution of AlO4 for SiO4 tetrahedra, it is of utmost importance to have information on the number as well as the location and neighbouring sites of framework aluminium. Unfortunately, such detailed information has not yet been obtained, mainly due to the lack of suitable characterization methods. Here we report, using the powerful atomic-scale analysis technique known as atom probe tomography, the quantitative spatial distribution of individual aluminium atoms, including their three-dimensional extent of segregation. Using a nearest-neighbour statistical analysis, we precisely determine the short-range distribution of aluminium over the different T-sites and determine the most probable Al-Al neighbouring distance within parent and steamed ZSM-5 crystals, as well as assess the long-range redistribution of aluminium upon zeolite steaming.
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  16. 16
    Schmidt, J. E.; Oord, R.; Guo, W.; Poplawsky, J. D.; Weckhuysen, B. M. Nanoscale Tomography Reveals the Deactivation of Automotive Copper-Exchanged Zeolite Catalysts. Nat. Commun. 2017, 8, 1666,  DOI: 10.1038/s41467-017-01765-0
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    16
    Nanoscale tomography reveals the deactivation of automotive copper-exchanged zeolite catalysts
    Schmidt Joel E; Oord Ramon; Weckhuysen Bert M; Guo Wei; Poplawsky Jonathan D
    Nature communications (2017), 8 (1), 1666 ISSN:.
    Copper-exchanged zeolite chabazite (Cu-SSZ-13) was recently commercialized for the selective catalytic reduction of NO X with ammonia in vehicle emissions as it exhibits superior reaction performance and stability compared to all other catalysts, notably Cu-ZSM-5. Herein, the 3D distributions of Cu as well as framework elements (Al, O, Si) in both fresh and aged Cu-SSZ-13 and Cu-ZSM-5 are determined with nanometer resolution using atom probe tomography (APT), and correlated with catalytic activity and other characterizations. Both fresh catalysts contain a heterogeneous Cu distribution, which is only identified due to the single atom sensitivity of APT. After the industry standard 135,000 mile simulation, Cu-SSZ-13 shows Cu and Al clustering, whereas Cu-ZSM-5 is characterized by severe Cu and Al aggregation into a copper aluminate phase (CuAl2O4 spinel). The application of APT as a sensitive and local characterization method provides identification of nanometer scale heterogeneities that lead to catalytic activity and material deactivation.
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  17. 17
    Cremer, G. D.; Sels, B. F.; De Vos, D. E.; Hofkens, J.; Roeffaers, M. B. J. Fluorescence Micro(Spectro)Scopy as a Tool to Study Catalytic Materials in Action. Chem. Soc. Rev. 2010, 39 (12), 4703– 4717,  DOI: 10.1039/c0cs00047g
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    Chen, P.; Zhou, X.; Shen, H.; Andoy, N. M.; Choudhary, E.; Han, K.-S.; Liu, G.; Meng, W. Single-Molecule Fluorescence Imaging of Nanocatalytic Processes. Chem. Soc. Rev. 2010, 39 (12), 4560– 4570,  DOI: 10.1039/b909052p
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    18
    Single-molecule fluorescence imaging of nanocatalytic processes
    Chen, Peng; Zhou, Xiaochun; Shen, Hao; Andoy, Nesha May; Choudhary, Eric; Han, Kyu-Sung; Liu, Guokun; Meng, Weilin
    Chemical Society Reviews (2010), 39 (12), 4560-4570CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    This tutorial review covers recent developments in using single-mol. fluorescence microscopy to study nanoscale catalysis. The single-mol. approach enables following catalytic and electrocatalytic reactions on nanocatalysts, including metal nanoparticles and carbon nanotubes, at single-reaction temporal resoln. and nanometer spatial precision. Real-time, in situ, multiplexed measurements are readily achievable under ambient soln. conditions. These studies provide unprecedented insights into catalytic mechanism, reactivity, selectivity, and dynamics in spite of the inhomogeneity and temporal variations of catalyst structures. Prospects, generality, and limitations of the single-mol. fluorescence approach for studying nanocatalysis are also discussed.
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  19. 19
    Cordes, T.; Blum, S. A. Opportunities and Challenges in Single-Molecule and Single-Particle Fluorescence Microscopy for Mechanistic Studies of Chemical Reactions. Nat. Chem. 2013, 5 (12), 993– 999,  DOI: 10.1038/nchem.1800
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    19
    Opportunities and challenges in single-molecule and single-particle fluorescence microscopy for mechanistic studies of chemical reactions
    Cordes, Thorben; Blum, Suzanne A.
    Nature Chemistry (2013), 5 (12), 993-999CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    In recent years, single-mol. and single-particle fluorescence microscopy has emerged as a tool to investigate chem. systems. After an initial lag of over a decade with respect to biophys. studies, this powerful imaging technique is now revealing mechanisms of 'classical' org. reactions, spatial distribution of chem. reactivity on surfaces and the phase of active catalysts. The recent advance into com. imaging systems obviates the need for home-built laser systems and thus opens this technique to traditionally trained synthetic chemists. We discuss the requisite photophys. and chem. properties of fluorescent reporters and highlight the main challenges in applying single-mol. techniques to chem. questions. The goal of this Perspective is to provide a snapshot of an emerging multidisciplinary field and to encourage broader use of this young exptl. approach that aids the observation of chem. reactions as depicted in many textbooks: mol. by mol.
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  20. 20
    Chen, T.; Dong, B.; Chen, K.; Zhao, F.; Cheng, X.; Ma, C.; Lee, S.; Zhang, P.; Kang, S. H.; Ha, J. W.; Xu, W.; Fang, N. Optical Super-Resolution Imaging of Surface Reactions. Chem. Rev. 2017, 117 (11), 7510– 7537,  DOI: 10.1021/acs.chemrev.6b00673
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    20
    Optical Super-Resolution Imaging of Surface Reactions
    Chen, Tao; Dong, Bin; Chen, Kuangcai; Zhao, Fei; Cheng, Xiaodong; Ma, Changbei; Lee, Seungah; Zhang, Peng; Kang, Seong Ho; Ha, Ji Won; Xu, Weilin; Fang, Ning
    Chemical Reviews (Washington, DC, United States) (2017), 117 (11), 7510-7537CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Optical super-resoln. imaging has gained momentum in studies of heterogeneous and homogeneous chem. reactions at the single-mol. level. Thanks to its exceptional spatial resoln. and ability to monitor dynamic systems, much detailed information on single-mol. reaction/adsorption processes and single-particle catalytic processes was revealed, including chem. kinetics and reaction dynamics; active-site distributions on single-particle surfaces; and size-, shape-, and facet-dependent catalytic activities of individual nanocatalysts. In this review, the authors provide an overview of recent advances in super-resoln. chem. imaging of surface reactions.
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  21. 21
    Roeffaers, M. B. J.; Sels, B. F.; Uji-i, H.; De Schryver, F. C.; Jacobs, P. A.; De Vos, D. E.; Hofkens, J. Spatially Resolved Observation of Crystal-Face-Dependent Catalysis by Single Turnover Counting. Nature 2006, 439 (7076), 572– 575,  DOI: 10.1038/nature04502
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    21
    Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting
    Roeffaers, Maarten B. J.; Sels, Bert F.; Uji-i, Hiroshi; De Schryver, Frans C.; Jacobs, Pierre A.; De Vos, Dirk E.; Hofkens, Johan
    Nature (London, United Kingdom) (2006), 439 (7076), 572-575CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quant. correlations between surface features and catalytic activity-ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with at. resoln., but are limited by the need for low-pressure conditions and conductive surfaces, resp. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single mol. level. Here we adapt real-time monitoring of the chem. transformation of individual org. mols. by fluorescence microscopy to monitor reactions catalyzed by crystals of a layered double hydroxide immersed in reagent soln. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {10‾10} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temp. and pressure and in a condensed phase, it can be applied to the growing no. of liq.-phase industrial org. transformations to localize catalytic activity on and in inorg. solids. An exciting opportunity is the use of probe mols. with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.
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  22. 22
    Xu, W.; Kong, J. S.; Yeh, Y.-T. E.; Chen, P. Single-Molecule Nanocatalysis Reveals Heterogeneous Reaction Pathways and Catalytic Dynamics. Nat. Mater. 2008, 7 (12), 992– 996,  DOI: 10.1038/nmat2319
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    22
    Single-molecule nanocatalysis reveals heterogeneous reaction pathways and catalytic dynamics
    Xu, Weilin; Kong, Jason S.; Yeh, Yun-Ting E.; Chen, Peng
    Nature Materials (2008), 7 (12), 992-996CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Nanoparticles are important catalysts for many chem. transformations. However, owing to their structural dispersions, heterogeneous distribution of surface sites and surface restructuring dynamics, nanoparticles are intrinsically heterogeneous and challenging to characterize in ensemble measurements. Using a single-nanoparticle single-turnover approach, we study the redox catalysis of individual colloidal Au nanoparticles in soln., using single-mol. detection of fluorogenic reactions. We find that for product generation, all Au nanoparticles follow a Langmuir-Hinshelwood mechanism but with heterogeneous reactivity; and for product dissocn., three nanoparticle subpopulations are present that show heterogeneous reactivity between multiple dissocn. pathways with distinct kinetics. Correlation analyses of single-turnover waiting times further reveal activity fluctuations of individual Au nanoparticles, attributable to both catalysis-induced and spontaneous dynamic surface restructuring that occurs at different timescales at the surface catalytic and product docking sites. The results exemplify the power of the single-mol. approach in revealing the interplay of catalysis, heterogeneous reactivity and surface structural dynamics in nanocatalysis.
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  23. 23
    Tachikawa, T.; Majima, T. Exploring the Spatial Distribution and Transport Behavior of Charge Carriers in a Single Titania Nanowire. J. Am. Chem. Soc. 2009, 131 (24), 8485– 8495,  DOI: 10.1021/ja900194m
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    23
    Exploring the Spatial Distribution and Transport Behavior of Charge Carriers in a Single Titania Nanowire
    Tachikawa, Takashi; Majima, Tetsuro
    Journal of the American Chemical Society (2009), 131 (24), 8485-8495CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    One-dimensional nanostructures of metal oxide semiconductors have both potential and demonstrated applications for use in light waveguides, photodetectors, solar energy conversion, photocatalysis, etc. The transport and reaction dynamics were studied of the photogenerated charge carriers in individual TiO2 nanowires using single-particle luminescence (PL) spectroscopy. Examn. of the spectral and kinetic characteristics revealed that the luminescence bands originating from defects in the bulk and/or on the surface appeared in the visible region with numerous photon bursts by photoirradn. using a 405-nm laser under an Ar atm. From the single-mol. kinetic anal. of the bursts, the quenching reaction of trapped electrons by mol. O follows a Langmuir-Hinshelwood mechanism. A novel spectroscopic method, i.e., single-mol. spectroelectrochem., was used to explore the nature of the defect states inherent in the wires. The spatially resolved PL imaging techniques thus enable one to ascertain the location of the luminescent active sites that are related to the heterogeneously distributed defects and to present exptl. evidence of the long-distance transport of charge carriers in the wire. Consequently, this study provides a great opportunity to understand the role of defects in the behavior of charge carriers in TiO2 nanomaterials with various morphologies.
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  24. 24
    Hensle, E. M.; Blum, S. A. Phase Separation Polymerization of Dicyclopentadiene Characterized by In Operando Fluorescence Microscopy. J. Am. Chem. Soc. 2013, 135 (33), 12324– 12328,  DOI: 10.1021/ja405283k
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    24
    Phase Separation Polymerization of Dicyclopentadiene Characterized by In Operando Fluorescence Microscopy
    Hensle, Eva M.; Blum, Suzanne A.
    Journal of the American Chemical Society (2013), 135 (33), 12324-12328CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Phase sepn. polymn. of dicyclopentadiene has been characterized from initiation to bulk material formation for the first time via in operando fluorescence microscopy imaging. The morphol. of the pptd. polymers at early reaction stages persists in the bulk polymer after completion of the reaction. Two-fluorophore expts. revealed the mechanistic origin of the dumbbell morphol. as phys. strand aggregation/pptn. rather than chem. attachment and revealed that strand aggregation was slow and irreversible relative to pptn. These data highlight the complementary information available through the single-particle sensitivity and in operando microscopy nature of this technique.
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  25. 25
    Sambur, J. B.; Chen, T.-Y.; Choudhary, E.; Chen, G.; Nissen, E. J.; Thomas, E. M.; Zou, N.; Chen, P. Sub-Particle Reaction and Photocurrent Mapping to Optimize Catalyst-Modified Photoanodes. Nature 2016, 530 (7588), 77– 80,  DOI: 10.1038/nature16534
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    25
    Sub-particle reaction and photocurrent mapping to optimize catalyst-modified photoanodes
    Sambur, Justin B.; Chen, Tai-Yen; Choudhary, Eric; Chen, Guanqun; Nissen, Erin J.; Thomas, Elayne M.; Zou, Ningmu; Chen, Peng
    Nature (London, United Kingdom) (2016), 530 (7588), 77-80CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    The splitting of water photoelectrochem. into hydrogen and oxygen represents a promising technol. for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled-yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalyzing water oxidn. also contribute to competing charge-carrier recombination with photogenerated electrons. Surface heterogeneity exacerbates these uncertainties, esp. for nanostructured photoanodes benefiting from small charge-carrier transport distances. Here we use super-resoln. imaging, operated in a charge-carrier-selective manner and with a spatiotemporal resoln. of approx. 30 nm and 15 ms, to map both the electron- and hole-driven photoelectrocatalytic activities on single titanium oxide nanorods. We then map, with sub-particle resoln. (about 390 nm), the photocurrent assocd. with water oxidn., and find that the most active sites for water oxidn. are also the most important sites for charge-carrier recombination. Site-selective deposition of an OEC, guided by the activity maps, improves the overall performance of a given nanorod-even though more improvement in photocurrent efficiency correlates with less redn. in onset potential (and vice versa) at the sub-particle level. Moreover, the optimal catalyst deposition sites for photocurrent enhancement are the lower-activity sites, and for onset potential redn. the optimal sites are the sites with more pos. onset potential, contrary to what is obtainable under typical deposition conditions. These findings allow us to suggest an activity-based strategy for rationally engineering catalyst-improved photoelectrodes, which should be widely applicable because our measurements can be performed for many different semiconductor and catalyst materials.
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    Zürner, A.; Kirstein, J.; Döblinger, M.; Bräuchle, C.; Bein, T. Visualizing Single-Molecule Diffusion in Mesoporous Materials. Nature 2007, 450 (7170), 705– 708,  DOI: 10.1038/nature06398
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    26
    Visualizing single-molecule diffusion in mesoporous materials
    Zuerner, Andreas; Kirstein, Johanna; Doeblinger, Markus; Braeuchle, Christoph; Bein, Thomas
    Nature (London, United Kingdom) (2007), 450 (7170), 705-708CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Periodic mesoporous materials formed through the cooperative self-assembly of surfactants and framework building blocks can assume a variety of structures, and their widely tunable properties make them attractive hosts for numerous applications. Because the mol. movement in the pore system is the most important and defining characteristic of porous materials, it is of interest to learn about this behavior as a function of local structure. Generally, individual fluorescent dye mols. can be used as mol. beacons with which to explore the structure of-and the dynamics within-these porous hosts, and single-mol. fluorescence techniques provide detailed insights into the dynamics of various processes, ranging from biol. to heterogeneous catalysis. However, optical microscopy methods cannot directly image the mesoporous structure of the host system accommodating the diffusing mols., whereas transmission electron microscopy provides detailed images of the porous structure, but no dynamic information. It has therefore not been possible to see how mols. diffuse in a real nanoscale pore structure. Here we present a combination of electron microscopic mapping and optical single-mol. tracking expts. to reveal how a single luminescent dye mol. travels through linear or strongly curved sections of a mesoporous channel system. In our approach we directly correlate porous structures detected by transmission electron microscopy with the diffusion dynamics of single mols. detected by optical microscopy. This opens up new ways of understanding the interactions of host and guest.
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    De Cremer, G.; Roeffaers, M. B. J.; Bartholomeeusen, E.; Lin, K.; Dedecker, P.; Pescarmona, P. P.; Jacobs, P. A.; De Vos, D. E.; Hofkens, J.; Sels, B. F. High-Resolution Single-Turnover Mapping Reveals Intraparticle Diffusion Limitation in Ti-MCM-41-Catalyzed Epoxidation. Angew. Chem., Int. Ed. 2010, 49 (5), 908– 911,  DOI: 10.1002/anie.200905039
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    Hendriks, F. C.; Meirer, F.; Kubarev, A. V.; Ristanović, Z.; Roeffaers, M. B. J.; Vogt, E. T. C.; Bruijnincx, P. C. A.; Weckhuysen, B. M. Single-Molecule Fluorescence Microscopy Reveals Local Diffusion Coefficients in the Pore Network of an Individual Catalyst Particle. J. Am. Chem. Soc. 2017, 139 (39), 13632– 13635,  DOI: 10.1021/jacs.7b07139
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    28
    Single-Molecule Fluorescence Microscopy Reveals Local Diffusion Coefficients in the Pore Network of an Individual Catalyst Particle
    Hendriks, Frank C.; Meirer, Florian; Kubarev, Alexey V.; Ristanovic, Zoran; Roeffaers, Maarten B. J.; Vogt, Eelco T. C.; Bruijnincx, Pieter C. A.; Weckhuysen, Bert M.
    Journal of the American Chemical Society (2017), 139 (39), 13632-13635CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We used single-mol. fluorescence microscopy to study self-diffusion of a feedstock-like probe mol. with nanometer accuracy in the macropores of a micrometer-sized, real-life fluid catalytic cracking (FCC) particle. Movies of single fluorescent mols. allowed their movement through the pore network to be reconstructed. The obsd. tracks were classified into three different states by machine learning and all found to be distributed homogeneously over the particle. Most probe mols. (88%) were immobile, with the mol. most likely being physisorbed or trapped; the remainder was either mobile (8%), with the mol. moving inside the macropores, or showed hybrid behavior (4%). Mobile tracks had an av. diffusion coeff. of D = 8 × 10-14 ± 1 × 10-13 m2 s-1, with the std. deviation thought to be related to the large range of pore sizes found in FCC particles. The developed methodol. can be used to evaluate, quantify and map heterogeneities in diffusional properties within complex hierarchically porous materials.
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    Zou, N.; Zhou, X.; Chen, G.; Andoy, N. M.; Jung, W.; Liu, G.; Chen, P. Cooperative Communication within and between Single Nanocatalysts. Nat. Chem. 2018, 1, 607,  DOI: 10.1038/s41557-018-0022-y
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    Roeffaers, M. B. J.; De Cremer, G.; Libeert, J.; Ameloot, R.; Dedecker, P.; Bons, A.-J.; Bückins, M.; Martens, J. A.; Sels, B. F.; De Vos, D. E.; Hofkens, J. Super-Resolution Reactivity Mapping of Nanostructured Catalyst Particles. Angew. Chem., Int. Ed. 2009, 48 (49), 9285– 9289,  DOI: 10.1002/anie.200904944
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    30
    Super-Resolution Reactivity Mapping of Nanostructured Catalyst Particles
    Roeffaers, Maarten B. J.; De Cremer, Gert; Libeert, Julien; Ameloot, Rob; Dedecker, Peter; Bons, Anton-Jan; Bueckins, Matthias; Martens, Johan A.; Sels, Bert F.; De Vos, Dirk E.; Hofkens, Johan
    Angewandte Chemie, International Edition (2009), 48 (49), 9285-9289, S9285/1-S9285/9CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Chromophore formation by nanostructured catalyst condensation reaction.
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  31. 31
    Ristanović, Z.; Kerssens, M. M.; Kubarev, A. V.; Hendriks, F. C.; Dedecker, P.; Hofkens, J.; Roeffaers, M. B. J.; Weckhuysen, B. M. High-Resolution Single-Molecule Fluorescence Imaging of Zeolite Aggregates within Real-Life Fluid Catalytic Cracking Particles. Angew. Chem., Int. Ed. 2015, 54 (6), 1836– 1840,  DOI: 10.1002/anie.201410236
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    31
    High-Resolution Single-Molecule Fluorescence Imaging of Zeolite Aggregates within Real-Life Fluid Catalytic Cracking Particles
    Ristanovic, Zoran; Kerssens, Marleen M.; Kubarev, Alexey V.; Hendriks, Frank C.; Dedecker, Peter; Hofkens, Johan; Roeffaers, Maarten B. J.; Weckhuysen, Bert M.
    Angewandte Chemie, International Edition (2015), 54 (6), 1836-1840CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Fluid catalytic cracking (FCC) is a major process in oil refineries to produce gasoline and base chems. from crude oil fractions. The spatial distribution and acidity of zeolite aggregates embedded within the 50-150 μm-sized FCC spheres heavily influence their catalytic performance. Single-mol. fluorescence-based imaging methods, namely nanometer accuracy by stochastic chem. reactions (NASCA) and super-resoln. optical fluctuation imaging (SOFI) were used to study the catalytic activity of sub-micrometer zeolite ZSM-5 domains within real-life FCC catalyst particles. The formation of fluorescent product mols. taking place at Bronsted acid sites was monitored with single turnover sensitivity and high spatiotemporal resoln., providing detailed insight in dispersion and catalytic activity of zeolite ZSM-5 aggregates. The results point towards substantial differences in turnover frequencies between the zeolite aggregates, revealing significant intraparticle heterogeneities in Bronsted reactivity.
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    Liu, K.-L.; Kubarev, A. V.; Van Loon, J.; Uji-i, H.; De Vos, D. E.; Hofkens, J.; Roeffaers, M. B. J. Rationalizing Inter- and Intracrystal Heterogeneities in Dealuminated Acid Mordenite Zeolites by Stimulated Raman Scattering Microscopy Correlated with Super-Resolution Fluorescence Microscopy. ACS Nano 2014, 8 (12), 12650– 12659,  DOI: 10.1021/nn505576p
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    32
    Rationalizing Inter- and Intracrystal Heterogeneities in Dealuminated Acid Mordenite Zeolites by Stimulated Raman Scattering Microscopy Correlated with Super-resolution Fluorescence Microscopy
    Liu, Kuan-Lin; Kubarev, Alexey V.; Van Loon, Jordi; Uji-i, Hiroshi; De Vos, Dirk E.; Hofkens, Johan; Roeffaers, Maarten B. J.
    ACS Nano (2014), 8 (12), 12650-12659CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Dealuminated zeolites are widely used acid catalysts in research and the chem. industry. Bulk-level studies have revealed that the improved catalytic performance results from an enhanced mol. transport as well as from changes in the active sites. However, fully exploiting this information in rational catalyst design still requires insight in the intricate interplay between both. The authors used fluorescence and stimulated Raman scattering microscopy to quantify sub-crystal reactivity as well as acid site distribution and to probe site accessibility in the set of individual mordenite zeolites. Dealumination effectively introduces significant heterogeneities between different particles and even within individual crystals. Besides enabling direct rationalization of the nanoscale catalytic performance, these observations reveal valuable information on the industrial dealumination process itself.
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  33. 33
    Ristanović, Z.; Hofmann, J. P.; De Cremer, G.; Kubarev, A. V.; Rohnke, M.; Meirer, F.; Hofkens, J.; Roeffaers, M. B. J.; Weckhuysen, B. M. Quantitative 3D Fluorescence Imaging of Single Catalytic Turnovers Reveals Spatiotemporal Gradients in Reactivity of Zeolite H-ZSM-5 Crystals upon Steaming. J. Am. Chem. Soc. 2015, 137 (20), 6559– 6568,  DOI: 10.1021/jacs.5b01698
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    Quantitative 3D Fluorescence Imaging of Single Catalytic Turnovers Reveals Spatiotemporal Gradients in Reactivity of Zeolite H-ZSM-5 Crystals upon Steaming
    Ristanovic, Zoran; Hofmann, Jan P.; De Cremer, Gert; Kubarev, Alexey V.; Rohnke, Marcus; Meirer, Florian; Hofkens, Johan; Roeffaers, Maarten B. J.; Weckhuysen, Bert M.
    Journal of the American Chemical Society (2015), 137 (20), 6559-6568CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Optimizing the no., distribution, and accessibility of Bronsted acid sites in zeolite-based catalysts is of a paramount importance to further improve their catalytic performance. However, it remains challenging to measure real-time changes in reactivity of single zeolite catalyst particles by ensemble-averaging characterization methods. In this work, a detailed 3D single mol., single turnover sensitive fluorescence microscopy study is presented to quantify the reactivity of Bronsted acid sites in zeolite H-ZSM-5 crystals upon steaming. This approach, in combination with the oligomerization of furfuryl alc. as a probe reaction, allowed the stochastic behavior of single catalytic turnovers and temporally resolved turnover frequencies of zeolite domains smaller than the diffraction limited resoln. to be investigated with great precision. It was found that the single turnover kinetics of the parent zeolite crystal proceeds with significant spatial differences in turnover frequencies on the nanoscale and noncorrelated temporal fluctuations. Mild steaming of zeolite H-ZSM-5 crystals at 500 °C led to an enhanced surface reactivity, with up to 4 times higher local turnover rates than those of the parent H-ZSM-5 crystals, and revealed remarkable heterogeneities in surface reactivity. In strong contrast, severe steaming at 700 °C significantly dealuminated the zeolite H-ZSM-5 material, leading to a 460 times lower turnover rate. The differences in measured turnover activities are explained by changes in the 3D aluminum distribution due to migration of extraframework Al-species and their subsequent effect on pore accessibility, as corroborated by time-of-flight secondary ion mass spectrometry (TOF-SIMS) sputter depth profiling data.
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  34. 34
    Kubarev, A. V.; Janssen, K. P. F.; Roeffaers, M. B. J. Noninvasive Nanoscopy Uncovers the Impact of the Hierarchical Porous Structure on the Catalytic Activity of Single Dealuminated Mordenite Crystals. ChemCatChem 2015, 7 (22), 3646– 3650,  DOI: 10.1002/cctc.201500708
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    Noninvasive Nanoscopy Uncovers the Impact of the Hierarchical Porous Structure on the Catalytic Activity of Single Dealuminated Mordenite Crystals
    Kubarev, Alexey V.; Janssen, Kris P. F.; Roeffaers, Maarten B. J.
    ChemCatChem (2015), 7 (22), 3646-3650CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Spatial restrictions around catalytic sites, provided by mol.-sized micropores, are beneficial to reaction selectivity but also inherently limit diffusion. The mol. transport can be enhanced by introducing meso- and macropores. However, the impact of this extraframework porosity on the local nanoscale reactivity is relatively unexplored. Herein we show that the area of enhanced reactivity in hierarchical zeolite, examd. with super-resoln. fluorescence microscopy, is spatially restricted to narrow zones around meso- and macropores, as obsd. with focused ion-beam-assisted SEM. This comparison indicates that reagent mols. efficiently reach catalytic active sites only in the micropores surrounding extraframework porosity and that extensive macroporosity does not warrant optimal reactivity distribution throughout a hierarchical porous zeolite.
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    Ristanović, Z.; Kubarev, A. V.; Hofkens, J.; Roeffaers, M. B. J.; Weckhuysen, B. M. Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal. J. Am. Chem. Soc. 2016, 138 (41), 13586– 13596,  DOI: 10.1021/jacs.6b06083
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    Kubarev, A. V.; Breynaert, E.; Van Loon, J.; Layek, A.; Fleury, G.; Radhakrishnan, S.; Martens, J.; Roeffaers, M. B. J. Solvent Polarity-Induced Pore Selectivity in H-ZSM-5 Catalysis. ACS Catal. 2017, 7 (7), 4248– 4252,  DOI: 10.1021/acscatal.7b00782
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    Solvent Polarity-Induced Pore Selectivity in H-ZSM-5 Catalysis
    Kubarev, Alexey V.; Breynaert, Eric; Van Loon, Jordi; Layek, Arunasish; Fleury, Guillaume; Radhakrishnan, Sambhu; Martens, Johan; Roeffaers, Maarten B. J.
    ACS Catalysis (2017), 7 (7), 4248-4252CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Mol.-sized micropores of ZSM-5 zeolite catalysts provide spatial restrictions around catalytic sites that allow for shape-selective catalysis. However, the fact that ZSM-5 has two main pore systems with different geometries is relatively unexploited as a potential source of addnl. shape selectivity. Here, we use confocal laser-scanning microscopy to show that by changing the polarity of the solvent, the acid-catalyzed furfuryl alc. oligomerization can be directed to selectively occur within either of two locations in the microporous network. This finding is confirmed for H-ZSM-5 particles with different Si/Al ratios and indicates a general trend for shape-selective catalytic reactions.
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    Van Loon, J.; Janssen, K. P. F.; Franklin, T.; Kubarev, A. V.; Steele, J. A.; Debroye, E.; Breynaert, E.; Martens, J. A.; Roeffaers, M. B. J. Rationalizing Acid Zeolite Performance on the Nanoscale by Correlative Fluorescence and Electron Microscopy. ACS Catal. 2017, 7 (8), 5234– 5242,  DOI: 10.1021/acscatal.7b01148
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    Rationalizing Acid Zeolite Performance on the Nanoscale by Correlative Fluorescence and Electron Microscopy
    Van Loon, Jordi; Janssen, Kris P. F.; Franklin, Thomas; Kubarev, Alexey V.; Steele, Julian A.; Debroye, Elke; Breynaert, Eric; Martens, Johan A.; Roeffaers, Maarten B. J.
    ACS Catalysis (2017), 7 (8), 5234-5242CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The performance of zeolites as solid acid catalysts is strongly influenced by the accessibility of active sites. However, synthetic zeolites typically grow as complex aggregates of small nanocrystallites rather than perfect single crystals. The structural complexity must therefore play a decisive role in zeolite catalyst applicability. Traditional tools for the characterization of heterogeneous catalysts are unable to directly relate nanometer-scale structural properties to the corresponding catalytic performance. In this work, an innovative correlative super-resoln. fluorescence and scanning electron microscope is applied, and the appropriate anal. procedures are developed to investigate the effect of small-port H-mordenite (H-MOR) morphol. on the catalytic performance, along with the effects of extensive acid leaching. These correlative measurements revealed catalytic activity at the interface between intergrown H-MOR crystallites that was assumed inaccessible, without compromising the shape selective properties. Furthermore, it was found that extensive acid leaching led to an etching of the originally accessible microporous structure, rather than the formation of an extended mesoporous structure. The assocd. transition of small-port to large-port H-MOR therefore did not render the full catalyst particle functional for catalysis. The applied characterization technique allows a straightforward investigation of the zeolite structure-activity relationship beyond the single-particle level. We conclude that such information will ultimately lead to an accurate understanding of the relationship between the bulk scale catalyst behavior and the nanoscale structural features, enabling a rationalization of catalyst design.
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    Science (Washington, D. C.) (1998), 282 (5395), 1877-1882CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Enzymic turnovers of single cholesterol oxidase mols. were obsd. in real time by monitoring the emission from the enzyme's fluorescent active site, FAD. Statistical analyses of single-mol. trajectories revealed a significant and slow fluctuation in the rate of cholesterol oxidn. by FAD. The static disorder and dynamic disorder of reaction rates, which are essentially indistinguishable in ensemble-averaged expts., were detd. sep. by the real-time single-mol. approach. A mol. memory phenomenon, in which an enzymic turnover was not independent of its previous turnovers because of a slow fluctuation of protein conformation, was evidenced by spontaneous spectral fluctuation of FAD.
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    Kox, M. H. F.; Stavitski, E.; Weckhuysen, B. M. Nonuniform Catalytic Behavior of Zeolite Crystals as Revealed by In Situ Optical Microspectroscopy. Angew. Chem., Int. Ed. 2007, 46 (20), 3652– 3655,  DOI: 10.1002/anie.200700246
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    Stavitski, E.; Kox, M. H. F.; Weckhuysen, B. M. Revealing Shape Selectivity and Catalytic Activity Trends Within the Pores of H-ZSM-5 Crystals by Time- and Space-Resolved Optical and Fluorescence Microspectroscopy. Chem. - Eur. J. 2007, 13 (25), 7057– 7065,  DOI: 10.1002/chem.200700568
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    Revealing shape selectivity and catalytic activity trends within the pores of H-ZSM-5 crystals by time- and space-resolved optical and fluorescence microspectroscopy
    Stavitski, Eli; Kox, Marianne H. F.; Weckhuysen, Bert M.
    Chemistry - A European Journal (2007), 13 (25), 7057-7065CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A combination of in-situ optical and fluorescence microspectroscopy has been employed to investigate the oligomerization of styrene derivs. occurring in the micropores of coffin-shaped H-ZSM-5 zeolite crystals in a space- and time-resolved manner. The carbocationic intermediates in this reaction act as reporter mols. for catalytic activity, since they exhibit strong optical absorption and fluorescence. In this way, reactant selectivity and restricted transition-state selectivity for 14 substituted styrene mols. can be visualized and quantified. Based on a thorough anal. of the time- and space-resolved UV/Vis spectra, it has been revealed that two main parameters affect the reaction rates, namely, the carbocation stabilization effect and the diffusion hindrance. The stabilization effect was tested by comparison of the reaction rates for 4-methoxystyrene vs. 4-methylstyrene and in the series 4-bromo-, 4-chloro and 4-fluorostyrene; in both cases less electroneg. substituents were found to accelerate the reaction. As to the steric effect, bulkier chem. groups bring down the reaction rate, as evident from the observation that 4-methoxystyrene is more reactive than 4-ethoxystyrene due to differences in their diffusivity, while heavily substituted styrenes, such as 3,4-dichlorostyrene and 2,3,4,5,6-pentafluorostyrene, cannot enter the zeolite pore system and therefore do not display any reactivity. Furthermore, β-methoxystyrene and trans-β-methylstyrene show limited reactivity as well as restricted reaction-product formation due to steric constraints imposed by the H-ZSM-5 channel system. Finally, polarized-light optical microspectroscopy and fluorescence microscopy demonstrate that dimeric styrene compds. are predominantly formed and aligned within the straight channels at the edges of the crystals, whereas a large fraction of trimeric carbocations along with dimeric compds. are present in the straight channels of the main body of the H-ZSM-5 crystals. Our results reinforce the observation of a non-uniform catalytic behavior within zeolite crystals, with specific parts of the zeolite grains being less accessible and reactive towards reactant mols. The prospects and potential of this combined in-situ approach for studying large zeolite crystals in the act will be discussed.
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    Buurmans, I. L. C.; Pidko, E. A.; Groot, J. M. de; Stavitski, E.; Santen, R. A. v.; Weckhuysen, B. M. Styrene Oligomerization as a Molecular Probe Reaction for Zeolite Acidity: A UV-Vis Spectroscopy and DFT Study. Phys. Chem. Chem. Phys. 2010, 12 (26), 7032– 7040,  DOI: 10.1039/c002442b
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    Styrene oligomerization as a molecular probe reaction for zeolite acidity: a UV-Vis spectroscopy and DFT study
    Buurmans, Inge L. C.; Pidko, Evgeny A.; de Groot, Jennifer M.; Stavitski, Eli; van Santen, Rutger A.; Weckhuysen, Bert M.
    Physical Chemistry Chemical Physics (2010), 12 (26), 7032-7040CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A series of H-ZSM-5 crystallites with different framework Si/Al ratios was studied by analyzing the kinetics and reaction mechanism of the oligomerization of 4-fluorostyrene as mol. probe reaction for Bronsted acidity. The formation of carbocationic species was followed by UV-Vis spectroscopy. Three carbocationic products were obsd., namely a cyclic dimer, a conjugated linear dimer and a larger, more conjugated carbocation. Rate consts. for the formation of all three products show a max. at a Si/Al ratio of 25. Oligomerization of 4-fluorostyrene within the larger supercages of zeolite H-Y leads solely to cyclic dimers. The exptl. observations were rationalized by DFT calcns., which show that the selectivity of the styrene oligomerization is controlled by the steric properties of the intrazeolite micropore voids. Two reaction pathways were considered for the formation of the conjugated linear carbocation. The conventional mechanism involves a hydride transfer between two dimeric hydrocarbons (HCs) in the zeolite pores. We propose an alternative monomol. path, in which the hydride transfer takes place between a hydrogen atom of a dimeric HC and a zeolitic proton, yielding a conjugated carbocation and mol. H2. Computed free energies indicate that the preference for a particular reaction mechanism is detd. by the local shape of the zeolite micropores.
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    Sprung, C.; Weckhuysen, B. M. Differences in the Location of Guest Molecules within Zeolite Pores As Revealed by Multilaser Excitation Confocal Fluorescence Microscopy: Which Molecule Is Where?. J. Am. Chem. Soc. 2015, 137 (5), 1916– 1928,  DOI: 10.1021/ja511381f
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    Valencia, D. Elucidating the Structure of Light Absorbing Styrene Carbocation Species Formed within Zeolites. Phys. Chem. Chem. Phys. 2017, 19 (23), 15050– 15058,  DOI: 10.1039/C7CP02344H
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    Andronesi, O. C.; Becker, S.; Seidel, K.; Heise, H.; Young, H. S.; Baldus, M. Determination of Membrane Protein Structure and Dynamics by Magic-Angle-Spinning Solid-State NMR Spectroscopy. J. Am. Chem. Soc. 2005, 127, 12965– 12974,  DOI: 10.1021/ja0530164
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    Determination of Membrane Protein Structure and Dynamics by Magic-Angle-Spinning Solid-State NMR Spectroscopy
    Andronesi, Ovidiu C.; Becker, Stefan; Seidel, Karsten; Heise, Henrike; Young, Howard S.; Baldus, Marc
    Journal of the American Chemical Society (2005), 127 (37), 12965-12974CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    It is shown that mol. structure and dynamics of a uniformly labeled membrane protein can be studied under magic-angle-spinning conditions. For this purpose, dipolar recoupling expts. are combined with novel through-bond correlation schemes that probe mobile protein segments. These NMR schemes are demonstrated on a uniformly [13C,15N] variant of the 52-residue polypeptide phospholamban. When reconstituted in lipid bilayers, the NMR data are consistent with an α-helical transmembrane segment and a cytoplasmic domain that exhibits a high degree of structural disorder.
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    Labokha, A. A.; Gradmann, S.; Frey, S.; Hülsmann, B. B.; Urlaub, H.; Baldus, M.; Görlich, D. Systematic Analysis of Barrier-Forming FG Hydrogels from Xenopus Nuclear Pore Complexes. EMBO J. 2013, 32 (4), 204– 218,  DOI: 10.1038/emboj.2012.302
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    Chowdhury, A. D.; Houben, K.; Whiting, G. T.; Mokhtar, M.; Asiri, A. M.; Al-Thabaiti, S. A.; Baldus, M.; Weckhuysen, B. M. Initial Carbon–carbon Bond Formation during the Early Stages of the Methanol-to-Olefin Process Proven by Zeolite-Trapped Acetate and Methyl Acetate. Angew. Chem., Int. Ed. 2016, 55, 15840– 15845,  DOI: 10.1002/anie.201608643
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    Initial Carbon-Carbon Bond Formation during the Early Stages of the Methanol-to-Olefin Process Proven by Zeolite-Trapped Acetate and Methyl Acetate
    Chowdhury, Abhishek Dutta; Houben, Klaartje; Whiting, Gareth T.; Mokhtar, Mohamed; Asiri, Abdullah M.; Al-Thabaiti, Shaeel A.; Basahel, Suliman N.; Baldus, Marc; Weckhuysen, Bert M.
    Angewandte Chemie, International Edition (2016), 55 (51), 15840-15845CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Methanol-to-olefin (MTO) catalysis is a very active field of research because there is a wide variety of sometimes conflicting mechanistic proposals. An example is the ongoing discussion on the initial C-C bond formation from methanol during the induction period of the MTO process. By employing a combination of solid-state NMR spectroscopy with UV/visible diffuse reflectance spectroscopy and mass spectrometry on an active H-SAPO-34 catalyst, the authors provide spectroscopic evidence for the formation of surface acetate and Me acetate, as well as dimethoxymethane during the MTO process. As a consequence, new insights in the formation of the first C-C bond are provided, suggesting a direct mechanism may be operative, at least in the early stages of the MTO reaction.
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    Chowdhury, A. D.; Houben, K.; Whiting, G. T.; Chung, S.-H.; Baldus, M.; Weckhuysen, B. M. Electrophilic Aromatic Substitution over Zeolites Generates Wheland-Type Reaction Intermediates. Nat. Catal. 2018, 1, 23– 31,  DOI: 10.1038/s41929-017-0002-4
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    Electrophilic aromatic substitution over zeolites generates Wheland-type reaction intermediates
    Chowdhury, Abhishek Dutta; Houben, Klaartje; Whiting, Gareth T.; Chung, Sang-Ho; Baldus, Marc; Weckhuysen, Bert M.
    Nature Catalysis (2018), 1 (1), 23-31CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
    The synthesis of many industrial bulk and fine chems. frequently involves electrophilic arom. substitution (SEAr) reactions. The most widely practiced example of the SEAr mechanism is the zeolite-catalyzed ethylation of benzene, using ethylene as an alkylating agent. However, the current prodn. route towards ethylbenzene is completely dependent on fossil resources, making the recent com. successes in the zeolite-catalyzed benzene ethylation process using bioethanol (instead of ethylene) very encouraging and noteworthy. Unfortunately, there is no information available on the reaction mechanism of this alternative synthesis route. Here, by employing a combination of advanced solid-state NMR spectroscopy and operando UV-Vis diffuse reflectance spectroscopy with online mass spectrometry, we have obtained detailed mechanistic insights into the bioethanol-mediated benzene ethylation process through the identification of active surface ethoxy species, surface-adsorbed zeolite-arom. π-complexes, as well as the more controversial Wheland-type σ-complex. Moreover, we distinguish between rigid and mobile zeolite-trapped org. species, providing further evidence for distinctive host-guest chem. during catalysis.
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    Morris, G. A.; Freeman, R. Enhancement of Nuclear Magnetic Resonance Signals by Polarization Transfer. J. Am. Chem. Soc. 1979, 101 (3), 760– 762,  DOI: 10.1021/ja00497a058
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    Journal of the American Chemical Society (1979), 101 (3), 760-2CODEN: JACSAT; ISSN:0002-7863.
    A technique is described for enhancing the NMR sensitivity of nuclei with low magnetogyric ratios by transferring polarization from a 2nd nuclear species, usually protons. It constitutes an alternative to the nuclear Overhauser effect, commonly used when recording 13C spectra, but has the advantage that it does not rely on any particular relaxation mechanism. It is expected to be esp. useful for nuclei such as 15N or 29Si which have neg. magnetogyric ratios, so than an incomplete nuclear Overhauser effect can lead to signal cancellation. The technique is illustrated for the proton-coupled 13C spectrum of pyridine.
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    Pines, A.; Gibby, M. G.; Waugh, J. S. Proton-Enhanced NMR of Dilute Spins in Solids. J. Chem. Phys. 1973, 59 (2), 569– 590,  DOI: 10.1063/1.1680061
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    Pines, A.; Gibby, M. G.; Waugh, J. S.
    Journal of Chemical Physics (1973), 59 (2), 569-90CODEN: JCPSA6; ISSN:0021-9606.
    The NMR signals of isotopically or chem. dil. nuclear spins S in solids can be enhanced by repeatedly transferring polarization from a more abundant species I of high abundance (usually protons) to which they are coupled. The gain in power sensitivity as compared with conventional observation of the rare spins approaches NII(I + 1)γI2/NSS(S + 1)γS2, or ∼103 for S = 13C, I = H in org. solids. The transfer of polarization is accomplished by any of a no. of double resonance methods. High-frequency resoln. of the S-spin signal was obtained by decoupling of the abundant spins. The exptl. requirements of the technique are discussed and a brief comparison of its sensitivity with other procedures is made. Representative applications and exptl. results are mentioned.
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    Müller, S.; Liu, Y.; Kirchberger, F. M.; Tonigold, M.; Sanchez-Sanchez, M.; Lercher, J. A. Hydrogen Transfer Pathways during Zeolite Catalyzed Methanol Conversion to Hydrocarbons. J. Am. Chem. Soc. 2016, 138 (49), 15994– 16003,  DOI: 10.1021/jacs.6b09605
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    Hydrogen Transfer Pathways during Zeolite Catalyzed Methanol Conversion to Hydrocarbons
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    Journal of the American Chemical Society (2016), 138 (49), 15994-16003CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
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  • Abstract

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    Figure 1

    Figure 1. (a) Schematic of the approach to detect protonated, fluorescent dimeric carbocation (bright state on the right); the dark, nonfluorescent state (left) is a neutral dimer that is formed after a proton transfer to the zeolite framework. (b) Representation of the MFI framework (view along b axis) and a dimeric carbocation (ball and stick model) trapped along the straight pores; additional monomeric styrene residing in a sinusoidal pore is shown (wire model). (c,d) Blinking (c) and nonblinking (d) fluorescence intensity trajectories of individual carbocationic species of 4-methoxystyrene in heptane. (e) A zoom-in of the trajectory shown in (c), indicating the definition of τon and τoff times.

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    Figure 2

    Figure 2. Statistical description of blinking properties for fluorescent products originating from 4-methoxystyrene and 4-fluorostyrene. (a,b) Examples of intermittent fluorescence intensity trajectories for 4-fluorostyrene (a) and 4-methoxystyrene (b), for different ratios of average ⟨τon⟩ and ⟨τoff⟩ times. (c) Plot of ⟨τon⟩ and ⟨τoff⟩ lifetimes of fluorescent and dark states for 4-fluorostyrene (blue) and 4-methoxystyrene (red) trajectories. See the SI Figure S2 for cumulative frequency histograms of τon and τoff lifetimes.

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    Figure 3

    Figure 3. (a,b) Normalized 2-D histograms of all pairs of subsequent τon and τoff lifetimes collected for 4-fluorostyrene (110 trajectories, 1500 pairs) and 4-methoxystyrene (60 trajectories, 3000 pairs). The histograms are normalized in respect to the (τon, τoff) pair with the highest occurrence (i.e., 100 ms, 100 ms). The color bars denote the normalized number of detected events for specific (τon, τoff) values. (c,d) Normalized scatter plots of subsequent (τon, τoff) pairs of lifetimes for 4-methoxystyrene trajectories categorized based on the average ⟨τon⟩ and ⟨τoff⟩ times; (c) ⟨τon⟩ > 5 × ⟨τoff⟩, and (d) ⟨τon⟩ < ⟨τoff⟩. Plots in (c) and (d) contain all (τon, τoff) pairs recorded for 10 trajectories with the specified criteria.

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    Figure 4

    Figure 4. Single-molecule blinking dynamics of the zeolite H-ZSM-5 trapped 4-methoxystyrene-derived emitters in n-heptane. (a) Fluorescence intensity trajectory of a highly blinking molecule. (b) Histogram of lifetimes of the fluorescent (red dots) and dark states (blue dots) measured for the single-molecule trajectory shown in (a). Inset: autocorrelation function for the trajectory in (a). Red line is a biexponential fit, with exponential decay constants of t1 = 0.16 ± 0.02 s (fast-decay component) and t2 = 3.6 ± 0.8 s (slow-decay component). (c) Real 2-D frequency histogram of the (τon, τoff) pairs as measured for the trajectory in (a). The color bar denotes the number of detected events for specific (τon, τoff) values. The simulated 2-D histogram based on the experimentally measured (τon, τoff) distributions in (b) is shown in SI Figure S4.

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    Figure 5

    Figure 5. Single-molecule blinking dynamics of the zeolite H-ZSM-5 trapped 4-methoxystyrene-derived emitters in 1-butanol. (a) Fluorescence intensity trajectory of a highly blinking molecule. (b) Histogram of lifetimes of the fluorescent (red dots) and dark states (blue dots) measured for the single-molecule trajectory shown in (a). Inset: autocorrelation function for the trajectory in (a). Red line is a monoexponential fit, with exponential decay constant of t1 = 0.11 ± 0.02 s. (c) Real 2-D frequency histogram of the (τon, τoff) pairs as measured for the trajectory in (a). The color bar denotes the number of detected events for specific (τon, τoff) values. The simulated 2-D histogram based on the experimentally measured (τon, τoff) distributions in (b) is shown in SI Figure S4.

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    Figure 6

    Figure 6. 1H–13C CP (blue, NS = 4096), 13C DE (red, NS = 2048), and 1H–13C INEPT (green, NS = 2048) solid-state NMR spectra (at 15 kHz MAS) of trapped products obtained on H-ZSM-5 after being exposed to 13C8-styrene at 393 K (MAS = magic angle spinning, NS = number of scans).

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    Figure 7

    Figure 7. Zooms of 2D MAS solid-state NMR spectra of rigid zeolite trapped molecules. Spectra were recorded on H-ZSM-5 after being exposed to 13C8-styrene at 393 K, using 15 kHz MAS. Polarization of the 13C atoms was achieved through cross-polarization (CP) and a 120 ms PARIS (52) mixing period was used. Lines indicate identified spin-systems, in red (surface alkoxide) and in green (styrene), where for the latter at least two forms could be identified as indicated by the solid and dashed lines.

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    Scheme 1

    Scheme 1. Proposed Catalytic Cycle of the ZSM-5 Catalyzed Oligomerization of Styrene Based on Reaction Intermediates and Products Detected in This Studya
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    aFour distinct adsorption and reaction routes are denoted in different colors. The dimeric carbocation formation route is the most relevant for this study. The dimeric fluorescent species C′ are highlighted in the yellow frame.

    Figure 8

    Figure 8. Zooms of 2D MAS solid-state 13C–13C correlation NMR spectrum of zeolite trapped products. Spectra were obtained on H-ZSM-5 after being exposed to 13C8-styrene at 393 K. 13C atoms were polarized by direct excitation (DE) and a 120 ms PARIS (52) mixing period was used. The MAS frequency was 10 kHz. The gray box in panel IV marks spinning side-bands.

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      Han, Oc Hee; Kim, Chang-Sam; Hong, Suk Bong
      Angewandte Chemie, International Edition (2002), 41 (3), 469-472CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
      27Al MAS and MQ MAS NMR measurements were carried out on a series of as-made ZSM-5 zeolites with 0.4-6.4 Al atoms per unit cell (Si/Al = 250-14). All 27Al MAS NMR spectra of the as-made zeolites consisted of two peaks at δ = 51.0-51.4 and 52.9-53.9. The same result was obtained for the 27Al MAS NMR spectra of the corresponding calcined ZSM-5 samples, although these two peaks were shifted slightly downfield relative to the as-made samples. This indicated that the two peaks of each spectrum resulted from the presence of at least two crystallog. distinct Al sites in the ZSM-5 framework, rather than arising from possible interactions between the framework Al atoms and encapsulated tetrapropylammonium ions. The preference of Al atoms for particular T-sites in the zeolites depended strongly on the framework Si/Al ratio, i.e., the extent of Al substitution. NMR results suggested that the substitution of Al atoms into the ZSM-5 framework during the crystn. process is kinetically, rather than energetically, controlled, and thus occurred in a nonrandom manner.
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    11. 11
      Stavitski, E.; Weckhuysen, B. M. Vibrational Spectroscopy and Related In Situ Studies of Catalytic Reactions within Molecular Sieves. In Zeolites and Catalysis: Synthesis, Reactions and Applications; Čejka, J., Corma, A., Zones, S., Eds.; Wiley-VCH: Weinheim, 2010; pp 209– 236.
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      Bordiga, S.; Lamberti, C.; Bonino, F.; Travert, A.; Thibault-Starzyk, F. Probing Zeolites by Vibrational Spectroscopies. Chem. Soc. Rev. 2015, 44 (20), 7262– 7341,  DOI: 10.1039/C5CS00396B
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      12
      Probing zeolites by vibrational spectroscopies
      Bordiga, Silvia; Lamberti, Carlo; Bonino, Francesca; Travert, Arnaud; Thibault-Starzyk, Frederic
      Chemical Society Reviews (2015), 44 (20), 7262-7341CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Bronsted and Lewis acidity and surface basicity are treated in detail. The role of probe mols. and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely det. the IR absorption coeffs. is given, making IR a quant. technique. The thermodn. parameters of the adsorption process that can be extd. from a variable-temp. IR study are described. Finally, cutting-edge space- and time-resolved expts. are reviewed. All aspects are discussed by reporting relevant examples. When available, the theor. literature related to the reviewed exptl. results is reported to support the interpretation of the vibrational spectra on an at. level.
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    13. 13
      Vjunov, A.; Fulton, J. L.; Huthwelker, T.; Pin, S.; Mei, D.; Schenter, G. K.; Govind, N.; Camaioni, D. M.; Hu, J. Z.; Lercher, J. A. Quantitatively Probing the Al Distribution in Zeolites. J. Am. Chem. Soc. 2014, 136 (23), 8296– 8306,  DOI: 10.1021/ja501361v
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      13
      Quantitatively Probing the Al Distribution in Zeolites
      Vjunov, Aleksei; Fulton, John L.; Huthwelker, Thomas; Pin, Sonia; Mei, Donghai; Schenter, Gregory K.; Govind, Niranjan; Camaioni, Donald M.; Hu, Jian Zhi; Lercher, Johannes A.
      Journal of the American Chemical Society (2014), 136 (23), 8296-8306CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The degree of substitution of Si4+ by Al3+ in the oxygen-terminated tetrahedra (Al T-sites) of zeolites dets. the concn. of ion-exchange and Bronsted acid sites. Because the location of the tetrahedra and the assocd. subtle variations in bond angles influence the acid strength, quant. information about Al T-sites in the framework is crit. to rationalize catalytic properties and to design new catalysts. A quant. anal. is reported that uses a combination of extended X-ray absorption fine structure (EXAFS) anal. and 27Al MAS NMR spectroscopy supported by DFT-based mol. dynamics simulations. To discriminate individual Al atoms, sets of ab initio EXAFS spectra for various T-sites are generated from DFT-based mol. dynamics simulations, allowing quant. treatment of the EXAFS single- and multiple-photoelectron scattering processes out to 3-4 atom shells surrounding the Al absorption center. It is obsd. that identical zeolite types show dramatically different Al distributions. A preference of Al for T-sites that are part of one or more 4-member rings in the framework over those T-sites that are part of only 5- and 6-member rings in an HBEA150 zeolite has been detd. using this anal.
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    14. 14
      van Bokhoven, J. A.; van der Eerden, A. M. J.; Koningsberger, D. C. Three-Coordinate Aluminum in Zeolites Observed with In Situ X-Ray Absorption Near-Edge Spectroscopy at the Al K-Edge:  Flexibility of Aluminum Coordinations in Zeolites. J. Am. Chem. Soc. 2003, 125 (24), 7435– 7442,  DOI: 10.1021/ja0292905
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    15. 15
      Perea, D. E.; Arslan, I.; Liu, J.; Ristanović, Z.; Kovarik, L.; Arey, B. W.; Lercher, J. A.; Bare, S. R.; Weckhuysen, B. M. Determining the Location and Nearest Neighbours of Aluminium in Zeolites with Atom Probe Tomography. Nat. Commun. 2015, 6, 7589,  DOI: 10.1038/ncomms8589
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      15
      Determining the location and nearest neighbours of aluminium in zeolites with atom probe tomography
      Perea Daniel E; Liu Jia; Kovarik Libor; Arey Bruce W; Arslan Ilke; Lercher Johannes A; Ristanovic Zoran; Weckhuysen Bert M; Lercher Johannes A; Bare Simon R
      Nature communications (2015), 6 (), 7589 ISSN:.
      Zeolite catalysis is determined by a combination of pore architecture and Bronsted acidity. As Bronsted acid sites are formed by the substitution of AlO4 for SiO4 tetrahedra, it is of utmost importance to have information on the number as well as the location and neighbouring sites of framework aluminium. Unfortunately, such detailed information has not yet been obtained, mainly due to the lack of suitable characterization methods. Here we report, using the powerful atomic-scale analysis technique known as atom probe tomography, the quantitative spatial distribution of individual aluminium atoms, including their three-dimensional extent of segregation. Using a nearest-neighbour statistical analysis, we precisely determine the short-range distribution of aluminium over the different T-sites and determine the most probable Al-Al neighbouring distance within parent and steamed ZSM-5 crystals, as well as assess the long-range redistribution of aluminium upon zeolite steaming.
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    16. 16
      Schmidt, J. E.; Oord, R.; Guo, W.; Poplawsky, J. D.; Weckhuysen, B. M. Nanoscale Tomography Reveals the Deactivation of Automotive Copper-Exchanged Zeolite Catalysts. Nat. Commun. 2017, 8, 1666,  DOI: 10.1038/s41467-017-01765-0
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      16
      Nanoscale tomography reveals the deactivation of automotive copper-exchanged zeolite catalysts
      Schmidt Joel E; Oord Ramon; Weckhuysen Bert M; Guo Wei; Poplawsky Jonathan D
      Nature communications (2017), 8 (1), 1666 ISSN:.
      Copper-exchanged zeolite chabazite (Cu-SSZ-13) was recently commercialized for the selective catalytic reduction of NO X with ammonia in vehicle emissions as it exhibits superior reaction performance and stability compared to all other catalysts, notably Cu-ZSM-5. Herein, the 3D distributions of Cu as well as framework elements (Al, O, Si) in both fresh and aged Cu-SSZ-13 and Cu-ZSM-5 are determined with nanometer resolution using atom probe tomography (APT), and correlated with catalytic activity and other characterizations. Both fresh catalysts contain a heterogeneous Cu distribution, which is only identified due to the single atom sensitivity of APT. After the industry standard 135,000 mile simulation, Cu-SSZ-13 shows Cu and Al clustering, whereas Cu-ZSM-5 is characterized by severe Cu and Al aggregation into a copper aluminate phase (CuAl2O4 spinel). The application of APT as a sensitive and local characterization method provides identification of nanometer scale heterogeneities that lead to catalytic activity and material deactivation.
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    17. 17
      Cremer, G. D.; Sels, B. F.; De Vos, D. E.; Hofkens, J.; Roeffaers, M. B. J. Fluorescence Micro(Spectro)Scopy as a Tool to Study Catalytic Materials in Action. Chem. Soc. Rev. 2010, 39 (12), 4703– 4717,  DOI: 10.1039/c0cs00047g
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    18. 18
      Chen, P.; Zhou, X.; Shen, H.; Andoy, N. M.; Choudhary, E.; Han, K.-S.; Liu, G.; Meng, W. Single-Molecule Fluorescence Imaging of Nanocatalytic Processes. Chem. Soc. Rev. 2010, 39 (12), 4560– 4570,  DOI: 10.1039/b909052p
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      18
      Single-molecule fluorescence imaging of nanocatalytic processes
      Chen, Peng; Zhou, Xiaochun; Shen, Hao; Andoy, Nesha May; Choudhary, Eric; Han, Kyu-Sung; Liu, Guokun; Meng, Weilin
      Chemical Society Reviews (2010), 39 (12), 4560-4570CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This tutorial review covers recent developments in using single-mol. fluorescence microscopy to study nanoscale catalysis. The single-mol. approach enables following catalytic and electrocatalytic reactions on nanocatalysts, including metal nanoparticles and carbon nanotubes, at single-reaction temporal resoln. and nanometer spatial precision. Real-time, in situ, multiplexed measurements are readily achievable under ambient soln. conditions. These studies provide unprecedented insights into catalytic mechanism, reactivity, selectivity, and dynamics in spite of the inhomogeneity and temporal variations of catalyst structures. Prospects, generality, and limitations of the single-mol. fluorescence approach for studying nanocatalysis are also discussed.
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    19. 19
      Cordes, T.; Blum, S. A. Opportunities and Challenges in Single-Molecule and Single-Particle Fluorescence Microscopy for Mechanistic Studies of Chemical Reactions. Nat. Chem. 2013, 5 (12), 993– 999,  DOI: 10.1038/nchem.1800
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      19
      Opportunities and challenges in single-molecule and single-particle fluorescence microscopy for mechanistic studies of chemical reactions
      Cordes, Thorben; Blum, Suzanne A.
      Nature Chemistry (2013), 5 (12), 993-999CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      In recent years, single-mol. and single-particle fluorescence microscopy has emerged as a tool to investigate chem. systems. After an initial lag of over a decade with respect to biophys. studies, this powerful imaging technique is now revealing mechanisms of 'classical' org. reactions, spatial distribution of chem. reactivity on surfaces and the phase of active catalysts. The recent advance into com. imaging systems obviates the need for home-built laser systems and thus opens this technique to traditionally trained synthetic chemists. We discuss the requisite photophys. and chem. properties of fluorescent reporters and highlight the main challenges in applying single-mol. techniques to chem. questions. The goal of this Perspective is to provide a snapshot of an emerging multidisciplinary field and to encourage broader use of this young exptl. approach that aids the observation of chem. reactions as depicted in many textbooks: mol. by mol.
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    20. 20
      Chen, T.; Dong, B.; Chen, K.; Zhao, F.; Cheng, X.; Ma, C.; Lee, S.; Zhang, P.; Kang, S. H.; Ha, J. W.; Xu, W.; Fang, N. Optical Super-Resolution Imaging of Surface Reactions. Chem. Rev. 2017, 117 (11), 7510– 7537,  DOI: 10.1021/acs.chemrev.6b00673
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      20
      Optical Super-Resolution Imaging of Surface Reactions
      Chen, Tao; Dong, Bin; Chen, Kuangcai; Zhao, Fei; Cheng, Xiaodong; Ma, Changbei; Lee, Seungah; Zhang, Peng; Kang, Seong Ho; Ha, Ji Won; Xu, Weilin; Fang, Ning
      Chemical Reviews (Washington, DC, United States) (2017), 117 (11), 7510-7537CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Optical super-resoln. imaging has gained momentum in studies of heterogeneous and homogeneous chem. reactions at the single-mol. level. Thanks to its exceptional spatial resoln. and ability to monitor dynamic systems, much detailed information on single-mol. reaction/adsorption processes and single-particle catalytic processes was revealed, including chem. kinetics and reaction dynamics; active-site distributions on single-particle surfaces; and size-, shape-, and facet-dependent catalytic activities of individual nanocatalysts. In this review, the authors provide an overview of recent advances in super-resoln. chem. imaging of surface reactions.
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    21. 21
      Roeffaers, M. B. J.; Sels, B. F.; Uji-i, H.; De Schryver, F. C.; Jacobs, P. A.; De Vos, D. E.; Hofkens, J. Spatially Resolved Observation of Crystal-Face-Dependent Catalysis by Single Turnover Counting. Nature 2006, 439 (7076), 572– 575,  DOI: 10.1038/nature04502
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      21
      Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting
      Roeffaers, Maarten B. J.; Sels, Bert F.; Uji-i, Hiroshi; De Schryver, Frans C.; Jacobs, Pierre A.; De Vos, Dirk E.; Hofkens, Johan
      Nature (London, United Kingdom) (2006), 439 (7076), 572-575CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quant. correlations between surface features and catalytic activity-ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with at. resoln., but are limited by the need for low-pressure conditions and conductive surfaces, resp. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single mol. level. Here we adapt real-time monitoring of the chem. transformation of individual org. mols. by fluorescence microscopy to monitor reactions catalyzed by crystals of a layered double hydroxide immersed in reagent soln. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {10‾10} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temp. and pressure and in a condensed phase, it can be applied to the growing no. of liq.-phase industrial org. transformations to localize catalytic activity on and in inorg. solids. An exciting opportunity is the use of probe mols. with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.
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    22. 22
      Xu, W.; Kong, J. S.; Yeh, Y.-T. E.; Chen, P. Single-Molecule Nanocatalysis Reveals Heterogeneous Reaction Pathways and Catalytic Dynamics. Nat. Mater. 2008, 7 (12), 992– 996,  DOI: 10.1038/nmat2319
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      22
      Single-molecule nanocatalysis reveals heterogeneous reaction pathways and catalytic dynamics
      Xu, Weilin; Kong, Jason S.; Yeh, Yun-Ting E.; Chen, Peng
      Nature Materials (2008), 7 (12), 992-996CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Nanoparticles are important catalysts for many chem. transformations. However, owing to their structural dispersions, heterogeneous distribution of surface sites and surface restructuring dynamics, nanoparticles are intrinsically heterogeneous and challenging to characterize in ensemble measurements. Using a single-nanoparticle single-turnover approach, we study the redox catalysis of individual colloidal Au nanoparticles in soln., using single-mol. detection of fluorogenic reactions. We find that for product generation, all Au nanoparticles follow a Langmuir-Hinshelwood mechanism but with heterogeneous reactivity; and for product dissocn., three nanoparticle subpopulations are present that show heterogeneous reactivity between multiple dissocn. pathways with distinct kinetics. Correlation analyses of single-turnover waiting times further reveal activity fluctuations of individual Au nanoparticles, attributable to both catalysis-induced and spontaneous dynamic surface restructuring that occurs at different timescales at the surface catalytic and product docking sites. The results exemplify the power of the single-mol. approach in revealing the interplay of catalysis, heterogeneous reactivity and surface structural dynamics in nanocatalysis.
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    23. 23
      Tachikawa, T.; Majima, T. Exploring the Spatial Distribution and Transport Behavior of Charge Carriers in a Single Titania Nanowire. J. Am. Chem. Soc. 2009, 131 (24), 8485– 8495,  DOI: 10.1021/ja900194m
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      23
      Exploring the Spatial Distribution and Transport Behavior of Charge Carriers in a Single Titania Nanowire
      Tachikawa, Takashi; Majima, Tetsuro
      Journal of the American Chemical Society (2009), 131 (24), 8485-8495CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      One-dimensional nanostructures of metal oxide semiconductors have both potential and demonstrated applications for use in light waveguides, photodetectors, solar energy conversion, photocatalysis, etc. The transport and reaction dynamics were studied of the photogenerated charge carriers in individual TiO2 nanowires using single-particle luminescence (PL) spectroscopy. Examn. of the spectral and kinetic characteristics revealed that the luminescence bands originating from defects in the bulk and/or on the surface appeared in the visible region with numerous photon bursts by photoirradn. using a 405-nm laser under an Ar atm. From the single-mol. kinetic anal. of the bursts, the quenching reaction of trapped electrons by mol. O follows a Langmuir-Hinshelwood mechanism. A novel spectroscopic method, i.e., single-mol. spectroelectrochem., was used to explore the nature of the defect states inherent in the wires. The spatially resolved PL imaging techniques thus enable one to ascertain the location of the luminescent active sites that are related to the heterogeneously distributed defects and to present exptl. evidence of the long-distance transport of charge carriers in the wire. Consequently, this study provides a great opportunity to understand the role of defects in the behavior of charge carriers in TiO2 nanomaterials with various morphologies.
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    24. 24
      Hensle, E. M.; Blum, S. A. Phase Separation Polymerization of Dicyclopentadiene Characterized by In Operando Fluorescence Microscopy. J. Am. Chem. Soc. 2013, 135 (33), 12324– 12328,  DOI: 10.1021/ja405283k
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      24
      Phase Separation Polymerization of Dicyclopentadiene Characterized by In Operando Fluorescence Microscopy
      Hensle, Eva M.; Blum, Suzanne A.
      Journal of the American Chemical Society (2013), 135 (33), 12324-12328CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Phase sepn. polymn. of dicyclopentadiene has been characterized from initiation to bulk material formation for the first time via in operando fluorescence microscopy imaging. The morphol. of the pptd. polymers at early reaction stages persists in the bulk polymer after completion of the reaction. Two-fluorophore expts. revealed the mechanistic origin of the dumbbell morphol. as phys. strand aggregation/pptn. rather than chem. attachment and revealed that strand aggregation was slow and irreversible relative to pptn. These data highlight the complementary information available through the single-particle sensitivity and in operando microscopy nature of this technique.
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    25. 25
      Sambur, J. B.; Chen, T.-Y.; Choudhary, E.; Chen, G.; Nissen, E. J.; Thomas, E. M.; Zou, N.; Chen, P. Sub-Particle Reaction and Photocurrent Mapping to Optimize Catalyst-Modified Photoanodes. Nature 2016, 530 (7588), 77– 80,  DOI: 10.1038/nature16534
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      25
      Sub-particle reaction and photocurrent mapping to optimize catalyst-modified photoanodes
      Sambur, Justin B.; Chen, Tai-Yen; Choudhary, Eric; Chen, Guanqun; Nissen, Erin J.; Thomas, Elayne M.; Zou, Ningmu; Chen, Peng
      Nature (London, United Kingdom) (2016), 530 (7588), 77-80CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      The splitting of water photoelectrochem. into hydrogen and oxygen represents a promising technol. for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled-yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalyzing water oxidn. also contribute to competing charge-carrier recombination with photogenerated electrons. Surface heterogeneity exacerbates these uncertainties, esp. for nanostructured photoanodes benefiting from small charge-carrier transport distances. Here we use super-resoln. imaging, operated in a charge-carrier-selective manner and with a spatiotemporal resoln. of approx. 30 nm and 15 ms, to map both the electron- and hole-driven photoelectrocatalytic activities on single titanium oxide nanorods. We then map, with sub-particle resoln. (about 390 nm), the photocurrent assocd. with water oxidn., and find that the most active sites for water oxidn. are also the most important sites for charge-carrier recombination. Site-selective deposition of an OEC, guided by the activity maps, improves the overall performance of a given nanorod-even though more improvement in photocurrent efficiency correlates with less redn. in onset potential (and vice versa) at the sub-particle level. Moreover, the optimal catalyst deposition sites for photocurrent enhancement are the lower-activity sites, and for onset potential redn. the optimal sites are the sites with more pos. onset potential, contrary to what is obtainable under typical deposition conditions. These findings allow us to suggest an activity-based strategy for rationally engineering catalyst-improved photoelectrodes, which should be widely applicable because our measurements can be performed for many different semiconductor and catalyst materials.
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    26. 26
      Zürner, A.; Kirstein, J.; Döblinger, M.; Bräuchle, C.; Bein, T. Visualizing Single-Molecule Diffusion in Mesoporous Materials. Nature 2007, 450 (7170), 705– 708,  DOI: 10.1038/nature06398
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      26
      Visualizing single-molecule diffusion in mesoporous materials
      Zuerner, Andreas; Kirstein, Johanna; Doeblinger, Markus; Braeuchle, Christoph; Bein, Thomas
      Nature (London, United Kingdom) (2007), 450 (7170), 705-708CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Periodic mesoporous materials formed through the cooperative self-assembly of surfactants and framework building blocks can assume a variety of structures, and their widely tunable properties make them attractive hosts for numerous applications. Because the mol. movement in the pore system is the most important and defining characteristic of porous materials, it is of interest to learn about this behavior as a function of local structure. Generally, individual fluorescent dye mols. can be used as mol. beacons with which to explore the structure of-and the dynamics within-these porous hosts, and single-mol. fluorescence techniques provide detailed insights into the dynamics of various processes, ranging from biol. to heterogeneous catalysis. However, optical microscopy methods cannot directly image the mesoporous structure of the host system accommodating the diffusing mols., whereas transmission electron microscopy provides detailed images of the porous structure, but no dynamic information. It has therefore not been possible to see how mols. diffuse in a real nanoscale pore structure. Here we present a combination of electron microscopic mapping and optical single-mol. tracking expts. to reveal how a single luminescent dye mol. travels through linear or strongly curved sections of a mesoporous channel system. In our approach we directly correlate porous structures detected by transmission electron microscopy with the diffusion dynamics of single mols. detected by optical microscopy. This opens up new ways of understanding the interactions of host and guest.
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    27. 27
      De Cremer, G.; Roeffaers, M. B. J.; Bartholomeeusen, E.; Lin, K.; Dedecker, P.; Pescarmona, P. P.; Jacobs, P. A.; De Vos, D. E.; Hofkens, J.; Sels, B. F. High-Resolution Single-Turnover Mapping Reveals Intraparticle Diffusion Limitation in Ti-MCM-41-Catalyzed Epoxidation. Angew. Chem., Int. Ed. 2010, 49 (5), 908– 911,  DOI: 10.1002/anie.200905039
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    28. 28
      Hendriks, F. C.; Meirer, F.; Kubarev, A. V.; Ristanović, Z.; Roeffaers, M. B. J.; Vogt, E. T. C.; Bruijnincx, P. C. A.; Weckhuysen, B. M. Single-Molecule Fluorescence Microscopy Reveals Local Diffusion Coefficients in the Pore Network of an Individual Catalyst Particle. J. Am. Chem. Soc. 2017, 139 (39), 13632– 13635,  DOI: 10.1021/jacs.7b07139
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      28
      Single-Molecule Fluorescence Microscopy Reveals Local Diffusion Coefficients in the Pore Network of an Individual Catalyst Particle
      Hendriks, Frank C.; Meirer, Florian; Kubarev, Alexey V.; Ristanovic, Zoran; Roeffaers, Maarten B. J.; Vogt, Eelco T. C.; Bruijnincx, Pieter C. A.; Weckhuysen, Bert M.
      Journal of the American Chemical Society (2017), 139 (39), 13632-13635CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We used single-mol. fluorescence microscopy to study self-diffusion of a feedstock-like probe mol. with nanometer accuracy in the macropores of a micrometer-sized, real-life fluid catalytic cracking (FCC) particle. Movies of single fluorescent mols. allowed their movement through the pore network to be reconstructed. The obsd. tracks were classified into three different states by machine learning and all found to be distributed homogeneously over the particle. Most probe mols. (88%) were immobile, with the mol. most likely being physisorbed or trapped; the remainder was either mobile (8%), with the mol. moving inside the macropores, or showed hybrid behavior (4%). Mobile tracks had an av. diffusion coeff. of D = 8 × 10-14 ± 1 × 10-13 m2 s-1, with the std. deviation thought to be related to the large range of pore sizes found in FCC particles. The developed methodol. can be used to evaluate, quantify and map heterogeneities in diffusional properties within complex hierarchically porous materials.
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      Zou, N.; Zhou, X.; Chen, G.; Andoy, N. M.; Jung, W.; Liu, G.; Chen, P. Cooperative Communication within and between Single Nanocatalysts. Nat. Chem. 2018, 1, 607,  DOI: 10.1038/s41557-018-0022-y
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      Roeffaers, M. B. J.; De Cremer, G.; Libeert, J.; Ameloot, R.; Dedecker, P.; Bons, A.-J.; Bückins, M.; Martens, J. A.; Sels, B. F.; De Vos, D. E.; Hofkens, J. Super-Resolution Reactivity Mapping of Nanostructured Catalyst Particles. Angew. Chem., Int. Ed. 2009, 48 (49), 9285– 9289,  DOI: 10.1002/anie.200904944
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      Super-Resolution Reactivity Mapping of Nanostructured Catalyst Particles
      Roeffaers, Maarten B. J.; De Cremer, Gert; Libeert, Julien; Ameloot, Rob; Dedecker, Peter; Bons, Anton-Jan; Bueckins, Matthias; Martens, Johan A.; Sels, Bert F.; De Vos, Dirk E.; Hofkens, Johan
      Angewandte Chemie, International Edition (2009), 48 (49), 9285-9289, S9285/1-S9285/9CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Chromophore formation by nanostructured catalyst condensation reaction.
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    31. 31
      Ristanović, Z.; Kerssens, M. M.; Kubarev, A. V.; Hendriks, F. C.; Dedecker, P.; Hofkens, J.; Roeffaers, M. B. J.; Weckhuysen, B. M. High-Resolution Single-Molecule Fluorescence Imaging of Zeolite Aggregates within Real-Life Fluid Catalytic Cracking Particles. Angew. Chem., Int. Ed. 2015, 54 (6), 1836– 1840,  DOI: 10.1002/anie.201410236
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      High-Resolution Single-Molecule Fluorescence Imaging of Zeolite Aggregates within Real-Life Fluid Catalytic Cracking Particles
      Ristanovic, Zoran; Kerssens, Marleen M.; Kubarev, Alexey V.; Hendriks, Frank C.; Dedecker, Peter; Hofkens, Johan; Roeffaers, Maarten B. J.; Weckhuysen, Bert M.
      Angewandte Chemie, International Edition (2015), 54 (6), 1836-1840CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Fluid catalytic cracking (FCC) is a major process in oil refineries to produce gasoline and base chems. from crude oil fractions. The spatial distribution and acidity of zeolite aggregates embedded within the 50-150 μm-sized FCC spheres heavily influence their catalytic performance. Single-mol. fluorescence-based imaging methods, namely nanometer accuracy by stochastic chem. reactions (NASCA) and super-resoln. optical fluctuation imaging (SOFI) were used to study the catalytic activity of sub-micrometer zeolite ZSM-5 domains within real-life FCC catalyst particles. The formation of fluorescent product mols. taking place at Bronsted acid sites was monitored with single turnover sensitivity and high spatiotemporal resoln., providing detailed insight in dispersion and catalytic activity of zeolite ZSM-5 aggregates. The results point towards substantial differences in turnover frequencies between the zeolite aggregates, revealing significant intraparticle heterogeneities in Bronsted reactivity.
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      Liu, K.-L.; Kubarev, A. V.; Van Loon, J.; Uji-i, H.; De Vos, D. E.; Hofkens, J.; Roeffaers, M. B. J. Rationalizing Inter- and Intracrystal Heterogeneities in Dealuminated Acid Mordenite Zeolites by Stimulated Raman Scattering Microscopy Correlated with Super-Resolution Fluorescence Microscopy. ACS Nano 2014, 8 (12), 12650– 12659,  DOI: 10.1021/nn505576p
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      Rationalizing Inter- and Intracrystal Heterogeneities in Dealuminated Acid Mordenite Zeolites by Stimulated Raman Scattering Microscopy Correlated with Super-resolution Fluorescence Microscopy
      Liu, Kuan-Lin; Kubarev, Alexey V.; Van Loon, Jordi; Uji-i, Hiroshi; De Vos, Dirk E.; Hofkens, Johan; Roeffaers, Maarten B. J.
      ACS Nano (2014), 8 (12), 12650-12659CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Dealuminated zeolites are widely used acid catalysts in research and the chem. industry. Bulk-level studies have revealed that the improved catalytic performance results from an enhanced mol. transport as well as from changes in the active sites. However, fully exploiting this information in rational catalyst design still requires insight in the intricate interplay between both. The authors used fluorescence and stimulated Raman scattering microscopy to quantify sub-crystal reactivity as well as acid site distribution and to probe site accessibility in the set of individual mordenite zeolites. Dealumination effectively introduces significant heterogeneities between different particles and even within individual crystals. Besides enabling direct rationalization of the nanoscale catalytic performance, these observations reveal valuable information on the industrial dealumination process itself.
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    33. 33
      Ristanović, Z.; Hofmann, J. P.; De Cremer, G.; Kubarev, A. V.; Rohnke, M.; Meirer, F.; Hofkens, J.; Roeffaers, M. B. J.; Weckhuysen, B. M. Quantitative 3D Fluorescence Imaging of Single Catalytic Turnovers Reveals Spatiotemporal Gradients in Reactivity of Zeolite H-ZSM-5 Crystals upon Steaming. J. Am. Chem. Soc. 2015, 137 (20), 6559– 6568,  DOI: 10.1021/jacs.5b01698
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      Quantitative 3D Fluorescence Imaging of Single Catalytic Turnovers Reveals Spatiotemporal Gradients in Reactivity of Zeolite H-ZSM-5 Crystals upon Steaming
      Ristanovic, Zoran; Hofmann, Jan P.; De Cremer, Gert; Kubarev, Alexey V.; Rohnke, Marcus; Meirer, Florian; Hofkens, Johan; Roeffaers, Maarten B. J.; Weckhuysen, Bert M.
      Journal of the American Chemical Society (2015), 137 (20), 6559-6568CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Optimizing the no., distribution, and accessibility of Bronsted acid sites in zeolite-based catalysts is of a paramount importance to further improve their catalytic performance. However, it remains challenging to measure real-time changes in reactivity of single zeolite catalyst particles by ensemble-averaging characterization methods. In this work, a detailed 3D single mol., single turnover sensitive fluorescence microscopy study is presented to quantify the reactivity of Bronsted acid sites in zeolite H-ZSM-5 crystals upon steaming. This approach, in combination with the oligomerization of furfuryl alc. as a probe reaction, allowed the stochastic behavior of single catalytic turnovers and temporally resolved turnover frequencies of zeolite domains smaller than the diffraction limited resoln. to be investigated with great precision. It was found that the single turnover kinetics of the parent zeolite crystal proceeds with significant spatial differences in turnover frequencies on the nanoscale and noncorrelated temporal fluctuations. Mild steaming of zeolite H-ZSM-5 crystals at 500 °C led to an enhanced surface reactivity, with up to 4 times higher local turnover rates than those of the parent H-ZSM-5 crystals, and revealed remarkable heterogeneities in surface reactivity. In strong contrast, severe steaming at 700 °C significantly dealuminated the zeolite H-ZSM-5 material, leading to a 460 times lower turnover rate. The differences in measured turnover activities are explained by changes in the 3D aluminum distribution due to migration of extraframework Al-species and their subsequent effect on pore accessibility, as corroborated by time-of-flight secondary ion mass spectrometry (TOF-SIMS) sputter depth profiling data.
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      Kubarev, A. V.; Janssen, K. P. F.; Roeffaers, M. B. J. Noninvasive Nanoscopy Uncovers the Impact of the Hierarchical Porous Structure on the Catalytic Activity of Single Dealuminated Mordenite Crystals. ChemCatChem 2015, 7 (22), 3646– 3650,  DOI: 10.1002/cctc.201500708
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      Noninvasive Nanoscopy Uncovers the Impact of the Hierarchical Porous Structure on the Catalytic Activity of Single Dealuminated Mordenite Crystals
      Kubarev, Alexey V.; Janssen, Kris P. F.; Roeffaers, Maarten B. J.
      ChemCatChem (2015), 7 (22), 3646-3650CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Spatial restrictions around catalytic sites, provided by mol.-sized micropores, are beneficial to reaction selectivity but also inherently limit diffusion. The mol. transport can be enhanced by introducing meso- and macropores. However, the impact of this extraframework porosity on the local nanoscale reactivity is relatively unexplored. Herein we show that the area of enhanced reactivity in hierarchical zeolite, examd. with super-resoln. fluorescence microscopy, is spatially restricted to narrow zones around meso- and macropores, as obsd. with focused ion-beam-assisted SEM. This comparison indicates that reagent mols. efficiently reach catalytic active sites only in the micropores surrounding extraframework porosity and that extensive macroporosity does not warrant optimal reactivity distribution throughout a hierarchical porous zeolite.
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      Ristanović, Z.; Kubarev, A. V.; Hofkens, J.; Roeffaers, M. B. J.; Weckhuysen, B. M. Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal. J. Am. Chem. Soc. 2016, 138 (41), 13586– 13596,  DOI: 10.1021/jacs.6b06083
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      Kubarev, A. V.; Breynaert, E.; Van Loon, J.; Layek, A.; Fleury, G.; Radhakrishnan, S.; Martens, J.; Roeffaers, M. B. J. Solvent Polarity-Induced Pore Selectivity in H-ZSM-5 Catalysis. ACS Catal. 2017, 7 (7), 4248– 4252,  DOI: 10.1021/acscatal.7b00782
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      Solvent Polarity-Induced Pore Selectivity in H-ZSM-5 Catalysis
      Kubarev, Alexey V.; Breynaert, Eric; Van Loon, Jordi; Layek, Arunasish; Fleury, Guillaume; Radhakrishnan, Sambhu; Martens, Johan; Roeffaers, Maarten B. J.
      ACS Catalysis (2017), 7 (7), 4248-4252CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Mol.-sized micropores of ZSM-5 zeolite catalysts provide spatial restrictions around catalytic sites that allow for shape-selective catalysis. However, the fact that ZSM-5 has two main pore systems with different geometries is relatively unexploited as a potential source of addnl. shape selectivity. Here, we use confocal laser-scanning microscopy to show that by changing the polarity of the solvent, the acid-catalyzed furfuryl alc. oligomerization can be directed to selectively occur within either of two locations in the microporous network. This finding is confirmed for H-ZSM-5 particles with different Si/Al ratios and indicates a general trend for shape-selective catalytic reactions.
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      Van Loon, J.; Janssen, K. P. F.; Franklin, T.; Kubarev, A. V.; Steele, J. A.; Debroye, E.; Breynaert, E.; Martens, J. A.; Roeffaers, M. B. J. Rationalizing Acid Zeolite Performance on the Nanoscale by Correlative Fluorescence and Electron Microscopy. ACS Catal. 2017, 7 (8), 5234– 5242,  DOI: 10.1021/acscatal.7b01148
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      Rationalizing Acid Zeolite Performance on the Nanoscale by Correlative Fluorescence and Electron Microscopy
      Van Loon, Jordi; Janssen, Kris P. F.; Franklin, Thomas; Kubarev, Alexey V.; Steele, Julian A.; Debroye, Elke; Breynaert, Eric; Martens, Johan A.; Roeffaers, Maarten B. J.
      ACS Catalysis (2017), 7 (8), 5234-5242CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The performance of zeolites as solid acid catalysts is strongly influenced by the accessibility of active sites. However, synthetic zeolites typically grow as complex aggregates of small nanocrystallites rather than perfect single crystals. The structural complexity must therefore play a decisive role in zeolite catalyst applicability. Traditional tools for the characterization of heterogeneous catalysts are unable to directly relate nanometer-scale structural properties to the corresponding catalytic performance. In this work, an innovative correlative super-resoln. fluorescence and scanning electron microscope is applied, and the appropriate anal. procedures are developed to investigate the effect of small-port H-mordenite (H-MOR) morphol. on the catalytic performance, along with the effects of extensive acid leaching. These correlative measurements revealed catalytic activity at the interface between intergrown H-MOR crystallites that was assumed inaccessible, without compromising the shape selective properties. Furthermore, it was found that extensive acid leaching led to an etching of the originally accessible microporous structure, rather than the formation of an extended mesoporous structure. The assocd. transition of small-port to large-port H-MOR therefore did not render the full catalyst particle functional for catalysis. The applied characterization technique allows a straightforward investigation of the zeolite structure-activity relationship beyond the single-particle level. We conclude that such information will ultimately lead to an accurate understanding of the relationship between the bulk scale catalyst behavior and the nanoscale structural features, enabling a rationalization of catalyst design.
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      Enzymic turnovers of single cholesterol oxidase mols. were obsd. in real time by monitoring the emission from the enzyme's fluorescent active site, FAD. Statistical analyses of single-mol. trajectories revealed a significant and slow fluctuation in the rate of cholesterol oxidn. by FAD. The static disorder and dynamic disorder of reaction rates, which are essentially indistinguishable in ensemble-averaged expts., were detd. sep. by the real-time single-mol. approach. A mol. memory phenomenon, in which an enzymic turnover was not independent of its previous turnovers because of a slow fluctuation of protein conformation, was evidenced by spontaneous spectral fluctuation of FAD.
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      Kox, M. H. F.; Stavitski, E.; Weckhuysen, B. M. Nonuniform Catalytic Behavior of Zeolite Crystals as Revealed by In Situ Optical Microspectroscopy. Angew. Chem., Int. Ed. 2007, 46 (20), 3652– 3655,  DOI: 10.1002/anie.200700246
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      Stavitski, E.; Kox, M. H. F.; Weckhuysen, B. M. Revealing Shape Selectivity and Catalytic Activity Trends Within the Pores of H-ZSM-5 Crystals by Time- and Space-Resolved Optical and Fluorescence Microspectroscopy. Chem. - Eur. J. 2007, 13 (25), 7057– 7065,  DOI: 10.1002/chem.200700568
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      Revealing shape selectivity and catalytic activity trends within the pores of H-ZSM-5 crystals by time- and space-resolved optical and fluorescence microspectroscopy
      Stavitski, Eli; Kox, Marianne H. F.; Weckhuysen, Bert M.
      Chemistry - A European Journal (2007), 13 (25), 7057-7065CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A combination of in-situ optical and fluorescence microspectroscopy has been employed to investigate the oligomerization of styrene derivs. occurring in the micropores of coffin-shaped H-ZSM-5 zeolite crystals in a space- and time-resolved manner. The carbocationic intermediates in this reaction act as reporter mols. for catalytic activity, since they exhibit strong optical absorption and fluorescence. In this way, reactant selectivity and restricted transition-state selectivity for 14 substituted styrene mols. can be visualized and quantified. Based on a thorough anal. of the time- and space-resolved UV/Vis spectra, it has been revealed that two main parameters affect the reaction rates, namely, the carbocation stabilization effect and the diffusion hindrance. The stabilization effect was tested by comparison of the reaction rates for 4-methoxystyrene vs. 4-methylstyrene and in the series 4-bromo-, 4-chloro and 4-fluorostyrene; in both cases less electroneg. substituents were found to accelerate the reaction. As to the steric effect, bulkier chem. groups bring down the reaction rate, as evident from the observation that 4-methoxystyrene is more reactive than 4-ethoxystyrene due to differences in their diffusivity, while heavily substituted styrenes, such as 3,4-dichlorostyrene and 2,3,4,5,6-pentafluorostyrene, cannot enter the zeolite pore system and therefore do not display any reactivity. Furthermore, β-methoxystyrene and trans-β-methylstyrene show limited reactivity as well as restricted reaction-product formation due to steric constraints imposed by the H-ZSM-5 channel system. Finally, polarized-light optical microspectroscopy and fluorescence microscopy demonstrate that dimeric styrene compds. are predominantly formed and aligned within the straight channels at the edges of the crystals, whereas a large fraction of trimeric carbocations along with dimeric compds. are present in the straight channels of the main body of the H-ZSM-5 crystals. Our results reinforce the observation of a non-uniform catalytic behavior within zeolite crystals, with specific parts of the zeolite grains being less accessible and reactive towards reactant mols. The prospects and potential of this combined in-situ approach for studying large zeolite crystals in the act will be discussed.
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      Buurmans, I. L. C.; Pidko, E. A.; Groot, J. M. de; Stavitski, E.; Santen, R. A. v.; Weckhuysen, B. M. Styrene Oligomerization as a Molecular Probe Reaction for Zeolite Acidity: A UV-Vis Spectroscopy and DFT Study. Phys. Chem. Chem. Phys. 2010, 12 (26), 7032– 7040,  DOI: 10.1039/c002442b
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      Styrene oligomerization as a molecular probe reaction for zeolite acidity: a UV-Vis spectroscopy and DFT study
      Buurmans, Inge L. C.; Pidko, Evgeny A.; de Groot, Jennifer M.; Stavitski, Eli; van Santen, Rutger A.; Weckhuysen, Bert M.
      Physical Chemistry Chemical Physics (2010), 12 (26), 7032-7040CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A series of H-ZSM-5 crystallites with different framework Si/Al ratios was studied by analyzing the kinetics and reaction mechanism of the oligomerization of 4-fluorostyrene as mol. probe reaction for Bronsted acidity. The formation of carbocationic species was followed by UV-Vis spectroscopy. Three carbocationic products were obsd., namely a cyclic dimer, a conjugated linear dimer and a larger, more conjugated carbocation. Rate consts. for the formation of all three products show a max. at a Si/Al ratio of 25. Oligomerization of 4-fluorostyrene within the larger supercages of zeolite H-Y leads solely to cyclic dimers. The exptl. observations were rationalized by DFT calcns., which show that the selectivity of the styrene oligomerization is controlled by the steric properties of the intrazeolite micropore voids. Two reaction pathways were considered for the formation of the conjugated linear carbocation. The conventional mechanism involves a hydride transfer between two dimeric hydrocarbons (HCs) in the zeolite pores. We propose an alternative monomol. path, in which the hydride transfer takes place between a hydrogen atom of a dimeric HC and a zeolitic proton, yielding a conjugated carbocation and mol. H2. Computed free energies indicate that the preference for a particular reaction mechanism is detd. by the local shape of the zeolite micropores.
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      Sprung, C.; Weckhuysen, B. M. Differences in the Location of Guest Molecules within Zeolite Pores As Revealed by Multilaser Excitation Confocal Fluorescence Microscopy: Which Molecule Is Where?. J. Am. Chem. Soc. 2015, 137 (5), 1916– 1928,  DOI: 10.1021/ja511381f
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      Valencia, D. Elucidating the Structure of Light Absorbing Styrene Carbocation Species Formed within Zeolites. Phys. Chem. Chem. Phys. 2017, 19 (23), 15050– 15058,  DOI: 10.1039/C7CP02344H
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      Andronesi, O. C.; Becker, S.; Seidel, K.; Heise, H.; Young, H. S.; Baldus, M. Determination of Membrane Protein Structure and Dynamics by Magic-Angle-Spinning Solid-State NMR Spectroscopy. J. Am. Chem. Soc. 2005, 127, 12965– 12974,  DOI: 10.1021/ja0530164
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      Determination of Membrane Protein Structure and Dynamics by Magic-Angle-Spinning Solid-State NMR Spectroscopy
      Andronesi, Ovidiu C.; Becker, Stefan; Seidel, Karsten; Heise, Henrike; Young, Howard S.; Baldus, Marc
      Journal of the American Chemical Society (2005), 127 (37), 12965-12974CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      It is shown that mol. structure and dynamics of a uniformly labeled membrane protein can be studied under magic-angle-spinning conditions. For this purpose, dipolar recoupling expts. are combined with novel through-bond correlation schemes that probe mobile protein segments. These NMR schemes are demonstrated on a uniformly [13C,15N] variant of the 52-residue polypeptide phospholamban. When reconstituted in lipid bilayers, the NMR data are consistent with an α-helical transmembrane segment and a cytoplasmic domain that exhibits a high degree of structural disorder.
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      Labokha, A. A.; Gradmann, S.; Frey, S.; Hülsmann, B. B.; Urlaub, H.; Baldus, M.; Görlich, D. Systematic Analysis of Barrier-Forming FG Hydrogels from Xenopus Nuclear Pore Complexes. EMBO J. 2013, 32 (4), 204– 218,  DOI: 10.1038/emboj.2012.302
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      Chowdhury, A. D.; Houben, K.; Whiting, G. T.; Mokhtar, M.; Asiri, A. M.; Al-Thabaiti, S. A.; Baldus, M.; Weckhuysen, B. M. Initial Carbon–carbon Bond Formation during the Early Stages of the Methanol-to-Olefin Process Proven by Zeolite-Trapped Acetate and Methyl Acetate. Angew. Chem., Int. Ed. 2016, 55, 15840– 15845,  DOI: 10.1002/anie.201608643
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      Initial Carbon-Carbon Bond Formation during the Early Stages of the Methanol-to-Olefin Process Proven by Zeolite-Trapped Acetate and Methyl Acetate
      Chowdhury, Abhishek Dutta; Houben, Klaartje; Whiting, Gareth T.; Mokhtar, Mohamed; Asiri, Abdullah M.; Al-Thabaiti, Shaeel A.; Basahel, Suliman N.; Baldus, Marc; Weckhuysen, Bert M.
      Angewandte Chemie, International Edition (2016), 55 (51), 15840-15845CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Methanol-to-olefin (MTO) catalysis is a very active field of research because there is a wide variety of sometimes conflicting mechanistic proposals. An example is the ongoing discussion on the initial C-C bond formation from methanol during the induction period of the MTO process. By employing a combination of solid-state NMR spectroscopy with UV/visible diffuse reflectance spectroscopy and mass spectrometry on an active H-SAPO-34 catalyst, the authors provide spectroscopic evidence for the formation of surface acetate and Me acetate, as well as dimethoxymethane during the MTO process. As a consequence, new insights in the formation of the first C-C bond are provided, suggesting a direct mechanism may be operative, at least in the early stages of the MTO reaction.
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      Chowdhury, A. D.; Houben, K.; Whiting, G. T.; Chung, S.-H.; Baldus, M.; Weckhuysen, B. M. Electrophilic Aromatic Substitution over Zeolites Generates Wheland-Type Reaction Intermediates. Nat. Catal. 2018, 1, 23– 31,  DOI: 10.1038/s41929-017-0002-4
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      Electrophilic aromatic substitution over zeolites generates Wheland-type reaction intermediates
      Chowdhury, Abhishek Dutta; Houben, Klaartje; Whiting, Gareth T.; Chung, Sang-Ho; Baldus, Marc; Weckhuysen, Bert M.
      Nature Catalysis (2018), 1 (1), 23-31CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
      The synthesis of many industrial bulk and fine chems. frequently involves electrophilic arom. substitution (SEAr) reactions. The most widely practiced example of the SEAr mechanism is the zeolite-catalyzed ethylation of benzene, using ethylene as an alkylating agent. However, the current prodn. route towards ethylbenzene is completely dependent on fossil resources, making the recent com. successes in the zeolite-catalyzed benzene ethylation process using bioethanol (instead of ethylene) very encouraging and noteworthy. Unfortunately, there is no information available on the reaction mechanism of this alternative synthesis route. Here, by employing a combination of advanced solid-state NMR spectroscopy and operando UV-Vis diffuse reflectance spectroscopy with online mass spectrometry, we have obtained detailed mechanistic insights into the bioethanol-mediated benzene ethylation process through the identification of active surface ethoxy species, surface-adsorbed zeolite-arom. π-complexes, as well as the more controversial Wheland-type σ-complex. Moreover, we distinguish between rigid and mobile zeolite-trapped org. species, providing further evidence for distinctive host-guest chem. during catalysis.
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      A technique is described for enhancing the NMR sensitivity of nuclei with low magnetogyric ratios by transferring polarization from a 2nd nuclear species, usually protons. It constitutes an alternative to the nuclear Overhauser effect, commonly used when recording 13C spectra, but has the advantage that it does not rely on any particular relaxation mechanism. It is expected to be esp. useful for nuclei such as 15N or 29Si which have neg. magnetogyric ratios, so than an incomplete nuclear Overhauser effect can lead to signal cancellation. The technique is illustrated for the proton-coupled 13C spectrum of pyridine.
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      Pines, A.; Gibby, M. G.; Waugh, J. S. Proton-Enhanced NMR of Dilute Spins in Solids. J. Chem. Phys. 1973, 59 (2), 569– 590,  DOI: 10.1063/1.1680061
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      Journal of Chemical Physics (1973), 59 (2), 569-90CODEN: JCPSA6; ISSN:0021-9606.
      The NMR signals of isotopically or chem. dil. nuclear spins S in solids can be enhanced by repeatedly transferring polarization from a more abundant species I of high abundance (usually protons) to which they are coupled. The gain in power sensitivity as compared with conventional observation of the rare spins approaches NII(I + 1)γI2/NSS(S + 1)γS2, or ∼103 for S = 13C, I = H in org. solids. The transfer of polarization is accomplished by any of a no. of double resonance methods. High-frequency resoln. of the S-spin signal was obtained by decoupling of the abundant spins. The exptl. requirements of the technique are discussed and a brief comparison of its sensitivity with other procedures is made. Representative applications and exptl. results are mentioned.
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      Müller, S.; Liu, Y.; Kirchberger, F. M.; Tonigold, M.; Sanchez-Sanchez, M.; Lercher, J. A. Hydrogen Transfer Pathways during Zeolite Catalyzed Methanol Conversion to Hydrocarbons. J. Am. Chem. Soc. 2016, 138 (49), 15994– 16003,  DOI: 10.1021/jacs.6b09605
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      Hydrogen Transfer Pathways during Zeolite Catalyzed Methanol Conversion to Hydrocarbons
      Mueller, Sebastian; Liu, Yue; Kirchberger, Felix M.; Tonigold, Markus; Sanchez-Sanchez, Maricruz; Lercher, Johannes A.
      Journal of the American Chemical Society (2016), 138 (49), 15994-16003CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Hydrogen transfer is the major route in catalytic conversion of methanol to olefins (MTO) for the formation of nonolefinic byproducts, including alkanes and aroms. Two sep., noninterlinked hydrogen transfer pathways have been identified. In the absence of methanol, hydrogen transfer occurs between olefins and naphthenes via protonation of the olefin and the transfer of the hydride to the carbenium ion. A hitherto unidentified hydride transfer pathway involving Lewis and Bronsted acid sites dominates as long as methanol is present in the reacting mixt., leading to aroms. and alkanes. Expts. with purely Lewis acidic ZSM-5 showed that methanol and propene react on Lewis acid sites to HCHO and propane. In turn, HCHO reacts with olefins stepwise to arom. mols. on Bronsted acid sites. The arom. mols. formed at Bronsted acid sites have a high tendency to convert to irreversibly adsorbed carbonaceous deposits and are responsible for the crit. deactivation in the methanol to olefin process.
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      Weingarth, M.; Demco, D. E.; Bodenhausen, G.; Tekely, P. Improved Magnetization Transfer in Solid-State NMR with Fast Magic Angle Spinning. Chem. Phys. Lett. 2009, 469, 342– 348,  DOI: 10.1016/j.cplett.2008.12.084
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      Chemical Physics Letters (2009), 469 (4-6), 342-348CODEN: CHPLBC; ISSN:0009-2614. (Elsevier B.V.)
      The efficiency of magnetization transfer between different spins S such as chem. inequivalent carbon-13 nuclei in solid samples that are spinning at high frequencies about the magic angle can be enhanced by a phase-alternated recoupling irradn. scheme (PARIS). Dipolar recoupling is assisted by a radiofrequency (rf) field applied to the abundant I (proton) spins. In contrast to rotary resonance-based recoupling schemes, the new method does not depend critically on the radiofrequency amplitude, which need not be matched with the spinning frequency. Modest radiofrequency amplitudes suffice to bring about efficient magnetization transfer even at high spinning speeds, thus avoiding excessive sample heating. The new method compensates efficiently for radiofrequency field inhomogeneity, so that the full sample vol. was used more effectively.
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      Wellendorff, J.; Lundgaard, K. T.; Møgelhøj, A.; Petzold, V.; Landis, D. D.; Nørskov, J. K.; Bligaard, T.; Jacobsen, K. W. Density Functionals for Surface Science: Exchange-Correlation Model Development with Bayesian Error Estimation. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85 (23), 235149,  DOI: 10.1103/PhysRevB.85.235149
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      Physical Review B: Condensed Matter and Materials Physics (2012), 85 (23), 235149/1-235149/23CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      A methodol. for semiempirical d. functional optimization, using regularization and cross-validation methods from machine learning, is developed. We demonstrate that such methods enable well-behaved exchange-correlation approxns. in very flexible model spaces, thus avoiding the overfitting found when std. least-squares methods are applied to high-order polynomial expansions. A general-purpose d. functional for surface science and catalysis studies should accurately describe bond breaking and formation in chem., solid state physics, and surface chem., and should preferably also include van der Waals dispersion interactions. Such a functional necessarily compromises between describing fundamentally different types of interactions, making transferability of the d. functional approxn. a key issue. We investigate this trade-off between describing the energetics of intramol. and intermol., bulk solid, and surface chem. bonding, and the developed optimization method explicitly handles making the compromise based on the directions in model space favored by different materials properties. The approach is applied to designing the Bayesian error estn. functional with van der Waals correlation (BEEF-vdW), a semilocal approxn. with an addnl. nonlocal correlation term. Furthermore, an ensemble of functionals around BEEF-vdW comes out naturally, offering an est. of the computational error. An extensive assessment on a range of data sets validates the applicability of BEEF-vdW to studies in chem. and condensed matter physics. Applications of the approxn. and its Bayesian ensemble error est. to two intricate surface science problems support this.
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      Kim, C. W.; Heo, N. H.; Seff, K. Framework Sites Preferred by Aluminum in Zeolite ZSM-5. Structure of a Fully Dehydrated, Fully Cs+-Exchanged ZSM-5 Crystal (MFI, Si/Al = 24). J. Phys. Chem. C 2011, 115 (50), 24823– 24838,  DOI: 10.1021/jp208246q
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      Benito, A.; Corma, A.; García, H.; Primo, J. Dimerization of Styrene Catalyzed by Acid 12-Membered Ring Zeolites. Appl. Catal., A 1994, 116 (1–2), 127– 135,  DOI: 10.1016/0926-860X(94)80284-X
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      Dimerization of styrene catalyzed by acid 12-membered ring zeolites
      Benito, Angel; Corma, Avelino; Garcia, Hermenegildo; Primo, Jaime
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      Dimerization of styrene has been carried out in the presence of acid large-pore zeolites with Y and β structure. Besides trimers and oligomers, variable amts. of the three known dimers are also formed depending on the different level of Na+-to-H+ exchange, framework Si/Al ratio and crystal size. The av. activity per acid site of zeolites is lower than that measured for H2SO4. This fact is not likely due to the acid strength demands of the dimerization, but more probably to diffusion limitations of the org. compds. through the zeolite micropores. Finally, depending on the acidity of the zeolite, a consecutive cyclization of the open dimer to the indane dimers, which are formed with a significant degree of diastereoselectivity, was obsd.
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      Bjørgen, M.; Bonino, F.; Kolboe, S.; Lillerud, K.-P.; Zecchina, A.; Bordiga, S. Spectroscopic Evidence for a Persistent Benzenium Cation in Zeolite H-Beta. J. Am. Chem. Soc. 2003, 125 (51), 15863– 15868,  DOI: 10.1021/ja037073d
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      Spectroscopic Evidence for a Persistent Benzenium Cation in Zeolite H-Beta
      Bjorgen, Morten; Bonino, Francesca; Kolboe, Stein; Lillerud, Karl-Petter; Zecchina, Adriano; Bordiga, Silvia
      Journal of the American Chemical Society (2003), 125 (51), 15863-15868CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Exptl. evidence of protonation of an arom. ring by a zeolite is hereby presented for the first time. The changes in vibrational properties and electronic transitions of the highest polymethylbenzene homolog, i.e., hexamethylbenzene, were investigated after introducing the compd. directly into a H-beta zeolite. Protonation of the arom. ring, and thus the loss of symmetry, activated a vibrational ring mode at 1600 cm-1. Furthermore, an electronic transition around 26 000 cm-1, which was totally absent for the neutral species, was an obvious consequence of protonation. A parallel study of hexamethylbenzene adsorbed on a beta zeolite virtually free from protons did not show those distinctive spectral features. On the basis of the gas-phase proton affinity of hexamethylbenzene, a complete proton transfer from the zeolite framework to the mol. is, according to conventional considerations, not expected. The hexamethylbenzenium ion is stable in the zeolite cavities at least up to 200 °C. The remarkable persistence of this carbenium ion may be attributed to spatial constraints imposed by the tight fit of the cation inside the zeolite channels. Hexamethylbenzene is a relevant reaction intermediate in the methanol-to-hydrocarbons reaction and also plays a central role as a coke precursor in zeolite-catalyzed reactions that involve polymethylbenzenes.
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      Bjørgen, M.; Bonino, F.; Arstad, B.; Kolboe, S.; Lillerud, K.-P.; Zecchina, A.; Bordiga, S. Persistent Methylbenzenium Ions in Protonated Zeolites: The Required Proton Affinity of the Guest Hydrocarbon. ChemPhysChem 2005, 6 (2), 232– 235,  DOI: 10.1002/cphc.200400422
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