The catalytic scope of solid acid zeotypes has grown to span reactions of increasingly bulky aromatics and oxygenates, owing to advancing (post)synthetic strategies for modifying conventionally microporous (pore diameters < 2 nm) zeotypes with auxiliary mesopores (2-50 nm) that alleviate diffusional barriers otherwise hindering access to confined active sites, reducing selectivity to desired bulky products, and/or accelerating deactivation. Despite generally enhanced bulk performance of mesopore-modified âhierarchicalâ zeotypes in upgrading bulky molecules, we show that diffusivity enhancements must be contextualized with crystal sizes, microporous framework architectures of parent zeotypes, and active site distributions. For poly-substituted aromatics alkylation on Brønsted acid zeolites (H-Al-MFI, H-Al-MOR, H-Al-BEA) in the liquid phase, as probed via batch alkylation of 1,3,5-trimethylbenzene (TMB) with benzyl alcohol, kinetic control (Thiele modulus ⤠1) dominates in hierarchical zeolites for all studied mesopore synthesis strategies (recrystallization, desilication, and/or dealumination) unless a high crystal radius (R ~ 10 µm) oversaturates the Thiele modulus beyond feasibly compensatory increases in effective diffusivity. We further conclude that mesopores provide identical diffusional environments to crystal surfaces, consistent with converged TMB alkylation rates on H-Al-MFI and a mesoporous aluminosilicate control (Al-MCM-41) when alkylation rates on H-Al-MFI are normalized by surface proton density. These outcomes highlight mass transport as primarily responsible for relative activity between microporous and hierarchical zeolites for hydrocarbon upgrading; however, investigation of hierarchical Lewis acid zeotypes for upgrading bulky oxygenates in protic solvents introduces additional considerations of mesopore surface hydrophilicity, which enhances hydrogen-bound solvent networks that block access of limiting reactants to active sites and/or impede their diffusion through mesopores. We demonstrate these effects via batch reduction of cyclohexanone with 2-butanol on hierarchical Sn-BEA, where mesopore surface hydrophilicity increases with silanol density. These investigations of hierarchical zeotype structure-function relations, with emphasis on transport and adsorption within mesopores, extend broadly to hydrocarbon and oxygenate upgrading reactions.
