Methylation of phenol over high-silica beta zeolite: effect of zeolite acidity and crystal size on catalyst behaviour

A systematic investigation was carried out to elucidate several aspects of the gas/solid methylation of phenol over high Si/Al ratio beta-structured zeolite in protonated form, characterised by various techniques, including XRD, SEM, BET, ICP, FTIR, TGA, microcalorimetry, and modeling by ab initio calculations. Data on the characteristics and the kinetic and mechanistic features of the catalytic reaction, as well as on catalyst deactivation, show that these zeolites, besides being very active for the present reaction, lead to cresols and anisole as primary products. As catalyst deactivation proceeds, the selectivity to cresols and anisole increases substantially, accompanied by a rapid decrease in selectivity to polyalkylated species. Medium- to low-strength silanols are the main contributors to catalyst surface acidity. High-strength Lewis acid sites either are virtually absent (especially when metal cations partially substitute for protons) or play a role essentially in catalyst deactivation. Stacking faults in the zeolite framework, generated by the intergrowth of at least two beta polymorphs, lead to an increased concentration of silanol-based Brønsted acid sites. Deactivation is due to the interaction of phenol and oxygenated products with the silanol-based acid sites and of methanol only with the strong acid sites of both Lewis and Brønsted nature. Self-oligomerisation–cyclisation of methanol to olefins and aromatics, followed by further alkylation to aromatic C atoms, contributes to catalyst deactivation. At any conversion level and at any temperature, the anisole/cresol ratio is systematically lower for the larger-crystal size zeolite, because the secondary transformations of anisole to cresols by both intramolecular rearrangement and intermolecular alkylation of phenol is favoured by the longer residence time of anisole within the zeolite pores.