Reaction-Diffusion-Deactivation in Hierarchical Zeolite Catalysts for Liquid-Phase Hydrocarbons Upgrading

Abstract

The industrial ubiquity of crystalline Brønsted acid zeolite catalysts incentivizes their adaptation for emerging conversions, including for upgrading biomass-derived platform molecules to fuels and chemicals conventionally produced from nonrenewable petroleum. The aluminosilicate backbones of zeolites form periodic architectures with micropores of molecular diameters (dpore < 1 nm). Micropores can sieve molecules and reduce activation energies through van der Waals stabilization of confined moieties, but they can also inhibit diffusion of desired bulky species. Expanded zeolite applications necessitate structure-function studies of modified hierarchical zeolites containing auxiliary mesopores (dpore = 2-50 nm) capable of increasing catalytic efficiency by enhancing diffusion of bulky molecules. This dissertation investigated impacts of mesopores on reaction, diffusion, and deactivation in hierarchical zeolites by probing competing parallel reactions with kinetically relevant differences in effective diffusivities (De): alkylation of 1,3,5-trimethylbenzene (TMB) by benzyl alcohol (BA) to 1,3,5-trimethyl-2-benzylbenzene (TM2B), or self-etherification of BA to dibenzyl ether (DBE). These probes exclusively interrogated how steric differences (predicted from calculated confinement energies for adsorbed molecules using periodic density-functional theory) impacted observed activity, by precluding complicating interactions between zeolites’ hydrophilic silanol defects and heavily oxygenated biomass. Proton-normalized reaction rates were measured for (hierarchical) zeolites synthesized from MFI, MOR, and BEA parent architectures of different crystal sizes, using post-synthetic demetallation treatments under ambient or autogenous pressures. Extracted rate constants demonstrated prevailing kinetic control for hierarchical zeolites that increased De,TMB relative to microporous zeolites with crystal radii (R) below thresholds that otherwise overinflated the relevant diffusion timescale (De,TMB/R2). Subsequent thermogravimetric analyses of spent catalysts revealed that graphitic coke formed from undesired polyalkylation of products preferentially deposited in micropores and dampened activity by irreversibly blocking protons. Subsequently extracted deactivation rate constants (kD) scaled logarithmically with coke accumulation. In contrast, BA, TMB, TM2B, DBE unbiasedly saturated all pores without inducing kinetically relevant deactivation. Hierarchical zeolites accommodated more coke than microporous parents at equivalent BA conversions while delaying onsets of measurable kD. These findings extend to many zeolite- and zeotype-catalyzed reactions, including ketone reduction on tin-type BEA explored in this dissertation. Ultimately, careful kinetics analysis complemented thorough catalyst characterization to inform future selection and design of heterogeneous catalysts.