Impact of Mesopores on Reaction-Diffusion-Deactivation of Bulky Molecule Conversions in Hierarchical Zeotypes With Differing Frameworks and Crystal Sizes
Michele L. Sarazen
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 "hierarchical" 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.