(735d) Mesopore Effects on Kinetics of Zeolite-Catalyzed Liquid-Phase Aromatics Alkylation

Diffusion constraints imposed by narrow zeolite micropores (d_pore < 2 nm) on relatively bulky molecules exacerbate transport limitations already presented by denser, liquid-phase reactions relative to the gas phase. Mitigation or absence of otherwise advantageous van der Waals stabilization of bulky reaction moieties within micropores, due to massively decreased diffusivities, manifests in reduced selectivity to bulky products, premature catalyst deactivation, and/or low conversions. Brønsted acid-catalyzed reactions in liquid media deserve further study in the context of hybrid, micro/mesopore “hierarchical” zeolites investigating how intrinsic diffusion barriers present in parent zeolites – arising from crystal size and parent architecture – impact mesopore efficacy in mitigating deactivation and controlling selectivity. The presented work demonstrates how mesopores (d_pore = 2-50 nm) introduced via post-synthetic acid and/or base leaching of parent zeolites can yield kinetically-controlled catalysts that facilitate ingress/egress of bulky molecules and redistribute coke deposition during liquid-phase, batch Friedel-Crafts alkylation of 1,3,5-trimethylbenzene (TMB; d_vdW = 0.72 nm) with benzyl alcohol (BA; d_vdW = 0.60 nm) to produce 1,3,5-trimethyl-2-benzylbenzene (TM2B; d_vdW = 0.75 nm) at 373 K. This work demonstrates how a complex reaction network involving secondary reactions, including the self-etherification of BA to form dibenzyl ether (DBE; d_vdW = 0.63 nm), may be systematically deduced under liquid batch conditions without facile, gaseous plug-flow kinetics by leveraging neat conditions ([TMB]₀:[BA]₀ = 35:1). Lab-synthesized hierarchical analogs MFI (3D, d_pore = 0.45 nm), BEA (3D, d_pore = 0.60 nm), and MOR (1D, d_pore = 0.65 nm) zeolites all significantly increased BA conversions and TM2B production, even in scenarios of inhibited internal diffusion within large microcrystals (d_crystal = 1-20 μm). Selectivities to TM2B compared to DBE exhibit nuanced dependence on mesopore accessibility, proton siting, and parent pore network architecture. Results from these reactions yield fundamental insight that may guide materials exploration for similarly diffusion-constrained systems in liquid media.