Synthesis of cresols by alkylation of phenol with methanol on solid acids

Abstract

The gas-phase alkylation of phenol with methanol to obtain cresols was studied at 473 K on SiO 2–Al 2O 3, Al-MCM-41, tungstophosphoric acid (HPA) supported on silica, and zeolites HBEA, HZSM5 and HY. The nature, density and strength of surface acid sites were probed by temperature-programmed desorption (TPD) of NH 3 coupled with infrared spectra of adsorbed pyridine. Anisole, o-cresol and p-cresol were primary reaction products while m-cresol, xylenols and methylanisoles were secondary products. O- and p-cresols were formed via direct C-alkylation of phenol, and also by conversion of anisole intermediate obtained by O-alkylation of phenol. C-alkylation of phenol was predominant as compared to O-alkylation over all the catalysts, excepting HPA/SiO 2. Zeolites HBEA and HY were the most active catalysts for obtaining cresols because both reaction pathways leading from phenol to cresols were particularly promoted by the simultaneous presence of strong Lewis and Brönsted acid sites. However, zeolite HY was rapid and severely deactivated on stream. Phenol methylation was controlled by intracrystalline diffusion on HZSM5 and the narrow channels of this zeolite hindered the formation of bulky intermediates involved in the overalkylation of cresols to xylenols. In contrast, the cresol isomer distribution was practically the same on zeolites HBEA and HZSM5 because HZSM5 did not improve the relative formation of p-cresol by shape selectivity.