The Alkylation of Phenol with Methanol: The Influence of Acid–Base Properties of X Zeolite on the Selectivity of para- and meta-Cresol

The vapor-phase alkylation of phenol with methanol was investigated on X zeolites and modified X zeolites. First, the difference of product distribution was tested between acid zeolite (HZSM-5, HX, HMCM-22, and Hβ) and basic zeolite X (KX and CsX). Then, X zeolites were modified with Li, K, Cs, Ca, Mg, La, and Ce ion exchange to adjust the acid–base properties of the zeolites. Finally, the type of acid sites and strength of acid–base sites on zeolite catalysts were determined by characterization techniques such as Py-IR and TPD, and the alkylation of phenol with methanol was tested. The results showed that O-alkylation products convert to the C-alkylation products on the B acid sites, which enhances the isomerization reaction, thereby increasing the proportion of meta-cresol in cresol. The results of TPD and IR indicated that weak basic sites on the zeolite promote the vertical adsorption of the aromatic ring and strong basic sites promote side-chain activation, while acid sites determine whether ring substitution or side-chain substitution occurs. During the reaction of phenol with methanol, the phenolic hydroxyl group strongly interacts with the surface of zeolites, leading to differences in the adsorption mode of the aromatic ring (vertical or parallel), which, in turn, alters the position of alkyl substitution. It is found that the proper acid–base property of X zeolites can selectively determine the desired alkylated products.