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

Abstract

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.

MWW and MFI Frameworks as Model Layered Zeolites: Structures, Transformations, Properties, and Activity

“… a Based on refs ,− ,,,,,,− . Abbreviations: n/p–not provided, pm–pore mouth. b Brønsted + Lewis acid sites. …”

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

“…Although ethanol dehydration indicates unaltered catalytic activity upon steam treatment, a drastically different outcome is observed when we use benzyl alcohol etherification to dibenzyl ether and benzyl alcohol alkylation with mesitylene as probe reactions under the reaction conditions specified in Section SI in the Supporting Information. The first reaction takes place both in micro‐ and mesopores of SPP, while alkylation can only take place in the mesopores (external surfaces of nanosheets) . In earlier work, we developed a reaction‐diffusion mathematical model (Section SIII in the Supporting Information) for these reactions and obtained the reaction rate and equilibrium constants that determine the observed etherification and alkylation rates .…”

Cooperative Interplay of Micropores/Mesopores of Hierarchical Zeolite in Chemical Production

“…A wide variety of reactions, including methanol-to-hydrocarbon conversion, ,,,− (hydro)­cracking, ,,− hydroisomerization, ,− aromatization, − alkylation, ,,− C–C coupling reactions, ,,, esterification, ,, acetalization, , etc., have been investigated to assess the catalytic properties of hierarchical zeolites. Many of these studies have reported enhanced catalytic activity for bulky reactants and retarded catalyst deactivation due to suppressed coke formation.…”