Alkylation of Phenol and m-Cresol Over Zeolites

Abstract

The alkylation of phenols and phenol derivatives such as m-cresol are important reactions in a sequence of syntheses for the production of many important fine chemicals. Specifically it is of great importance to develop catalysts and processes, which are able to selectively produce one or other particular isomer or derivative. This paper presents results of an investigation into the alkylation of phenol and m-cresol, respectively, using methanol in the former case and propene in the latter. The catalysts of choice were H-ZSM-5 and H-MCM-22. In the case of phenol methylation it was found that controlling diffusivity by increasing crystal size was most conducive to the formation of p-cresol. H-MCM-22 showed a high selectivity to p-cresol. In the case of m-cresol propylation the selectivity over H-ZSM-5 to thymol was greater than 90% at conversions of around 50%, and 80% at conversions of around 85%. This high selectivity to thymol was considerably greater than reported in previous papers. In both systems mild reaction temperature and pressure conditions were desirable so as to minimize the formation of oligomeric products of methanol (via dimethyl ether) and propene, which could cause deactivation of the zeolite.

An investigation into factors which influence the formation of p-cresol in the methanol alkylation of phenol over MCM-22 and ZSM-5

“…The samples had different crystal sizes and different Si/Al ratios and therefore different activities (indicated by conversion). Details are discussed in [23]. From this figure, it becomes clear that arguably the most dominant variable influencing the p:o-cresol ratio in phenol methylation over zeolite H-MCM-22 is the diffusional path length and activity.…”

Synthesis of (−)‐menthol: Industrial synthesis routes and recent development

“…Alternative versions of some of the reaction steps have been described in the literature. Continuous alkylation of m ‐cresol with propene in the gas phase has been accomplished over the acidic zeolite ZSM‐5 by O'Connor et al with selectivity ( S ) for thymol up to 90% 23 . Besides the directing effects of the hydroxyl and methyl group, the high selectivity is mostly attributed to the shape of the narrow pores of the catalyst, which allow the preferred diffusion of thymol.…”

“…Continuous alkylation of m-cresol with propene in the gas phase has been accomplished over the acidic zeolite ZSM-5 by O'Connor et al with selectivity (S) for thymol up to 90%. 23 Besides the directing effects of the hydroxyl and methyl group, the high selectivity is mostly attributed to the shape of the narrow pores of the catalyst, which allow the preferred diffusion of thymol. In contrast, the equilibrium mixture of the possibly formed isomers contained only 20% thymol and consisted mainly of 3-isopropyl-5-methylphenol.…”

Reaction pathways in the liquid phase alkylation of biomass‐derived phenolic compounds

“…Some kinetics studies have favored an Eley–Rideal type mechanism for these alkylation reactions, in which the alkylating agent is adsorbed on the catalyst surface and reacts with a phenolic molecule from the bulk phase …”