The transformations involving methanol in the acid- and base-catalyzed gas-phase methylation of phenol

The alkylation of phenol with methanol was studied using a Brønsted-type acid catalyst (a H-mordenite) and basic/dehydrogenating catalysts (MgO, Fe2O3 and Mg/Fe/O), with the aim of investigating the reaction mechanism. The main difference between the two classes of catalysts concerned the transformations occurring on methanol. Specifically, in the former case the acid-type activation of methanol led to the development of an electrophylic species that gave rise to the formation of anisole and of C-alkylated compounds. With basic catalysts, methanol dehydrogenated to formaldehyde, which then underwent transformation to methylformate and to decomposition products, i.e., CO, CO2, CH4 and H2. In this case, the prevailing compounds obtained by reaction with phenol were o-cresol and 2,6-xylenol. The dehydrogenation of methanol was found to be the key-step in the generation of the active methylating species with basic catalysts.