Products and coke shape-selectivity during anisole disproportionation over HZSM-5

Abstract

Anisole conversion on an MFI zeolite (SiAl = 43) at atmospheric pressure and 673 K is a model for the upgrading of bio-oils produced by catalytic fast pyrolysis (CFP) of biomass. Catalyst activity and selectivity are measured experimentally, while deactivation is studied experimentally and by DFT calculations. Anisole dismutation is a Friedel-Crafts-type alkylation between two anisole molecules. Consecutive reactions produce phenol and methyl anisoles, cresols and xylenols, as primary, secondary, and tertiary products respectively. The resonance effect of the hydroxyl and methoxy groups, combined with the "product shape selectivity" of the zeolite, results in a high selectivity for p-methyl-anisole, o-cresol and 2,4 xylenol. Coke deposition consists exclusively of methyl phenols with 0–4 methyl groups. Their retention is due to steric hindrance for the larger ones and strong adsorption on stronger acid sites for the smaller ones, as confirmed by DFT calculations. Catalyst deactivation occurs by progressive fouling of the micropores rather than by poisoning.