A comparison of the reaction mechanism in acid-and in basic-catalysed gas-phase methylation of phenol

This paper compares the reaction of gas-phase methylation of phenol with methanol in basic and in acid catalysis, with the aim of investigating how the transformations occurring on methanol affect the catalytic performance and the reaction mechanism. It was found that with the basic catalyst Mg/Fe/O, the true alkylating agent is formaldehyde, obtained by dehydrogenation of methanol. Formaldehyde reacted with phenol to yield salicylic alcohol, which rapidly dehydrogenated to salicylic aldehyde. The latter was isolated in tests made by variation of the residence time, and in tests made by feeding a formalin/phenol aqueous solution. Salicylic aldehyde then transformed to o-cresol, the main product of the basic-catalyzed methylation of phenol, by means of an intermolecular H-transfer. With an H-mordenite catalyst, instead, the activated methanol reacted with phenol to generate anisole, cresols and polyalkylated phenols.