(691b) Structurally Diverse Hierarchical Zeolites for Hydrocarbon Upgrading

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 (or zoning). For poly-substituted aromatics alkylation on Brønsted acid zeolites (H-Al-MFI, H-Al-MOR, H-Al-BEA), 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. While reactivity and selectivity are highly sensitive to subtle nuances in pore structure and connectivity, deactivation and subsequent coke formation follow collapsible trends as functions of surface area, mesopore volume, and extent of reaction. These insights into reaction engineering in diffusion-limited extremes according to mesopore accessibility highlights the flexibility of zeolites in novel reaction systems.