A quantitative study of the structure–activity relationship in hierarchical zeolites using liquid‐phase reactions

Xu, Dandan; Abdelrahman, Omar A.; Ahn, Sang Hyun; Guefrachi, Yasmine; Kuznetsov, Anatoliy; Ren, Limin; Hwang, Son‐Jong; Khaleel, Maryam; Hassan, Saeed Al; Liu, Dongxia; Hong, Suk Bong; Dauenhauer, Paul J.; Tsapatsis, Michael

doi:10.1002/aic.16503

Cited by 37 publications

(47 citation statements)

References 42 publications

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“…Turn over frequency (TOF) of ethanol dehydration to diethyl ether in the absence of DTBP remains similar across all materials tested indicating that the nature of active sites as probed by this reaction remains unaltered (Table S3). The fraction of external sites of SPP (47 %) is much higher than that of tk‐SPP (21 %) and both conform with the expected (developed earlier based on geometric arguments) dependence on high‐aspect‐ratio nanosheet half‐thickness along b ( x p ) shown in Figure a. The external Brønsted acid site fractions of SPP and tk‐SPP increase upon steam treatment from 47 % to 51 % and from 21 % to 27 %, respectively.…”

Section: Resultssupporting

confidence: 85%

“…Understanding and controlling the water vapor‐induced structural rearrangements at the nanometer (single‐unit‐cell) level is of particular significance for two‐dimensional (2D) zeolites and thicker nanosheets that constitute an emerging class of catalysts, adsorbents and membranes . Herein, we demonstrate that an all‐silica single‐unit‐cell meso /microporous MFI‐type zeolite (SPP: self‐pillared pentasil) retains its crystallinity and micro‐ and mesoporosity under steaming at 350 °C, while small but detectable changes take place in the content of silanol groups and the enthalpy of transition (Δ H Transition ) relative to α‐quartz (the most stable polymorph of silica under ambient conditions). Electron microscopy reveals major changes in the nanosheet dimensions: increase in thickness along the b ‐direction (straight pore channels) and reduction of basal ((010) plane) dimensions along the c ‐direction.…”

Section: Introductionmentioning

confidence: 96%

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Steam‐Induced Coarsening of Single‐Unit‐Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

Guefrachi

Sharma

et al. 2020

Angewandte Chemie

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Commonly used methods to assess crystallinity, micro‐/mesoporosity, Brønsted acid site density and distribution (in micro‐ vs. mesopores), and catalytic activity suggest nearly invariant structure and function for aluminosilicate zeolite MFI two‐dimensional nanosheets before and after superheated steam treatment. Yet, pronounced reaction rate decrease for benzyl alcohol alkylation with mesitylene, a reaction that cannot take place in the zeolite micropores, is observed. Transmission electron microscopy images reveal pronounced changes in nanosheet thickness, aspect ratio and roughness indicating that nanosheet coarsening and the associated changes in the external (mesoporous) surface structure are responsible for the changes in the external surface catalytic activity. Superheated steam treatment of hierarchical zeolites can be used to alter nanosheet morphology and regulate external surface catalytic activity while preserving micro‐ and mesoporosity, and micropore reaction rates.

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Section: Resultssupporting

confidence: 85%

Section: Introductionmentioning

confidence: 96%

Steam‐Induced Coarsening of Single‐Unit‐Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

Guefrachi

Sharma

et al. 2020

Angewandte Chemie

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“…It is observed that the uptake of benzyl alcohol per acid site is lower for the 2D MFI (25) catalyst than the 2D MFI (144) catalyst. The reaction studied in this work proceeds through the adsorption of benzyl alcohol on the catalyst surface . Assuming the strength of the acid sites does not change with varying the Si/Al ratio, the higher availability of the reactant molecule (benzyl alcohol) per active site in the micropores for the 2D MFI (144) catalyst is consistent with its higher effectiveness factor in the case of the reaction with the DTBP poisoning and its higher TOF, as compared to the 2D MFI (25) catalyst.…”

Section: Resultsmentioning

confidence: 72%

“…The reaction studied in this work proceeds through the adsorption of benzyl alcohol on the catalyst surface. [33] Assuming the strength of the acid sites does not change with varying the Si/Al ratio, [34] the higher availability of the reactant molecule (benzyl alcohol) per active site in the micropores for the 2D MFI (144) catalyst is consistent with its higher effectiveness factor in the case of the reaction with the DTBP poisoning and its higher TOF, as compared to the 2D MFI (25) catalyst. This is possibly due to steric crowding or repulsive interactions between the adsorbed reactant molecules, similar to what has been suggested for other reactions.…”

Section: Benzyl Alcohol Adsorption Isothermsmentioning

confidence: 87%

“…As shown in previous works, the alkylation of mesitylene occurs exclusively on the external surface of the 2D MFI catalysts since the bulky mesitylene cannot diffuse into the micropores, while the etherification of benzyl alcohol occurs on the external surface as well as the micropores. [31][32][33] Both of these reactions are catalyzed by the Brønsted acid sites in the catalysts. In each case, sufficient catalyst was added to the reaction to ensure conversions that could be reliably measured.…”

Section: Catalytic Activity Of 2d Mfi Nanosheetsmentioning

confidence: 99%

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Effect of Si/Al Ratio on the Catalytic Activity of Two‐Dimensional MFI Nanosheets in Aromatic Alkylation and Alcohol Etherification

et al. 2019

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Zeolite catalysts with pronounced two‐dimensional (2D) morphologies, specifically 2D MFI nanosheets composed of multilamellar stacks of MFI nanosheets, are hypothesized to offer accessibility advantages in catalytic reactions involving large, bulky molecules. The Si/Al ratio of zeolite catalysts is an important parameter determining their performance. However, the effect of Si/Al ratio on catalytic reactivity is not well understood in 2D zeolite catalysts. To provide further insight into this issue, the catalytic performance of 2D MFI nanosheet materials with different Si/Al ratios is compared using the liquid phase Friedel‐Crafts alkylation of mesitylene with benzyl alcohol & the self‐etherification of benzyl alcohol that occurs in parallel, both of which are catalyzed by Brønsted acid sites. The turnover frequency (TOF) of the catalysts is found to decrease with decreasing Si/Al ratio, with the etherification reaction being the main contributor to this trend. When the same reaction is carried out in the presence of a bulky poison, 2,6 di‐tert‐butylpyridine (DTBP), to selectively deactivate the external acid sites, only the etherification reaction of benzyl alcohol takes place in the micropores of the 2D MFI catalyst and the effectiveness factor is found to decrease with decreasing Si/Al ratio. The single component adsorption isotherms of benzyl alcohol show that the uptake normalized per acid site decreases with decreasing Si/Al ratio. Thus, increasing the density of acid sites in the micropores by decreasing the Si/Al ratio makes it more difficult for the reactant molecules to access them, as demonstrated by the decrease in TOF and the effectiveness factor. While the micropore reactions (benzene alkylation, alcohol etherification) showed a clear trend with Si/Al ratio, the reaction catalyzed on the external surface (mesitylene alkylation) did not, showing distinct differences in reactivity.

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Spontaneous Pillaring of Pentasil Zeolites

et al. 2021

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Conventional methods to prepare hierarchical zeolites depend upon the use of organic structure‐directing agents and often require multiple synthesis steps with limited product yield and Brønsted acid concentration. Here, it is shown that the use of MEL‐ or MFI‐type zeolites as crystalline seeds induces the spontaneous formation of self‐pillared pentasil zeolites, thus avoiding the use of any organic or branching template for the crystallization of these hierarchical structures. The mechanism of formation is evaluated by time‐resolved electron microscopy to provide evidence for the heterogeneous nucleation and growth of sequentially branched nanosheets from amorphous precursors. The resulting hierarchical zeolites have large external surface area and high percentages of external acid sites, which markedly improves their catalytic performance in the Friedel–Crafts alkylation and methanol to hydrocarbons reactions. These findings highlight a facile, commercially viable synthesis method to reduce mass‐transport limitations and improve the performance of zeolite catalysts.

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A quantitative study of the structure–activity relationship in hierarchical zeolites using liquid‐phase reactions

Cited by 37 publications

References 42 publications

Steam‐Induced Coarsening of Single‐Unit‐Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

Steam‐Induced Coarsening of Single‐Unit‐Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

Effect of Si/Al Ratio on the Catalytic Activity of Two‐Dimensional MFI Nanosheets in Aromatic Alkylation and Alcohol Etherification

Spontaneous Pillaring of Pentasil Zeolites

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