Alkylation of poly-substituted aromatics to probe effects of mesopores in hierarchical zeolites with differing frameworks and crystal sizes

Abstract

This study examines how the inherent diffusion constraints of MFI (3D, pore-limiting diameter (PLD) = 0.45 nm), BEA (3D, PLD = 0.60 nm), and MOR (1D, PLD = 0.65 nm) zeolite architectures, at both nanocrystal (nMFI, nBEA, nMOR; d_crystal < 0.5 μm) and microcrystal (μBEA, μMOR; d_crystal > 0.5 μm) scales, impact functions of mesopores in their hierarchical analogs. Reactivities, deactivation rates, and product selectivities were compared among zeolites, as well as to a mesoporous aluminosilicate control (Al-MCM-41; PLD = 6.2 nm), during Friedel–Crafts alkylation of 1,3,5-trimethylbenzene (TMB; d_vdW = 0.72 nm) with benzyl alcohol (BA; d_vdW = 0.58 nm) to form 1,3,5-trimethyl-2-benzylbenzene (TM2B; d_vdW = 0.75 nm). Operation in the neat liquid phase ([TMB]₀ : [BA]₀ = 35 : 1, 393 K) ensured that the parallel BA self-etherification to yield dibenzyl ether (DBE; d_vdW = 0.58 nm) occurred only at the expense of TM2B production when the alkylation reaction was impeded due to hindered access of TMB to confined protons. Investigation of secondary TM2B formation from reaction of DBE with TMB at low [BA]/[DBE] indicates an additional route of selectivity control for hierarchical zeolites that can achieve high BA conversion (X_BA > 0.9) with no DBE cofeed. These findings highlight a compounding advantage of increased diffusivity in mesopores that alter rates, extend lifetimes, and subsequently permit secondary reactions that enable significant shifts in product distribution. Fundamental insights into hierarchical zeolite reaction–diffusion–deactivation for alkylation of poly-substituted aromatics, as detailed here, can be applied broadly to reactions of other bulky species, including biomass-derived oxygenates, for more atom-efficient chemical and fuel production.