Probing the diffusional enhancements of mesopores in hierarchical zeolites during liquid-phase Friedel-Crafts alkylation of 1,3,5-trimethylbenzene with benzyl alcohol
Zeolites are crystalline, solid Brønsted acid catalysts with tunable micropore (< 2 nm diameters) networks that act as molecular sieves by selectively accommodating molecules based on size. Despite the ubiquity of zeolites in industrial processes, their utility in catalyzing the production of bulky compounds is limited due to rapid deactivation caused by carbonaceous deposits formed, in part, due to severe diffusion constraints on large species in narrow micropores. Here, we aim to improve diffusion through the introduction of mesopores (≥ 2 nm diameters) in hierarchical micropore-mesopore zeolite architectures. The efficacy of our synthesized materials was probed with liquid-phase, Friedel-Crafts alkylation of 1,3,5-trimethylbenzene (TMB) with benzyl alcohol (BA) to form 1,3,5-trimethyl-2-benzylbenzene (TM2B). A competing reaction, self-etherification of BA to form dibenzyl ether (DBE), dominates when the three methyl substituents on the bulky TMB prevent alkylation by sterically hindering access to active sites within zeolite micropores. These reactions were studied on microporous and hierarchical analogs of lab-synthesized and commercial zeolites with 3D (BEA, MFI) and 1D (MOR) pore networks. Conversions and selectivities quantified using 1H-NMR were consistent with the hypothesis that lower catalyst efficiencies (i.e., lower BA conversions) and lower selectivity to TM2B would be observed for purely microporous zeolites. Accordingly, hierarchical zeolites had increased BA conversions and TM2B fractional selectivity, in agreement with enhanced TMB diffusion. Results from these probe reactions yield fundamental insight into zeolite structure-function relations that extend broadly to other zeolite-catalyzed processes for more efficient chemical and fuel production.