"Selective synthesis of p-cresol by alkylation of phenol with methanol"

MARÍA EUGENIA SAD; CRISTINA LILIANA PADRÓ; CARLOS RODOLFO APESTEGUÍA

Verbania, Italia

Simposio; VIII International Symposium on Catalysis Applied to Fine Chemicals; 2007

p-Cresol is widely employed for the production of pharmaceuticals, herbicides, agrochemicals and dyes, but its current commercial synthesis via toluene dehydration using sulphuric acid poses serious environmental concerns.  A number of studies have been lately devoted to explore the use of solid acids for obtaining selectively p-cresol via the alkylation of phenol with methanol. However, all these studies reported that the para-:ortho-cresol selectivity ratio on amorphous catalysts or wide pore zeolites such as HBeta, HY, and H-mordenites was always lower than 0.7.  In this work, we performed a detailed study of the gas-phase alkylation of phenol with methanol on samples containing only strong Brönsted acid sites (HPA/SiO2) and catalysts containing both Lewis and Brönsted acid sites of either strong (zeolites HZSM5, HBeta and MCM22) or moderate (SiO2-Al2O3) strength.  Zeolites HBeta, HZSM5 and MCM22 were more active than SiO2-Al2O3 or HPA/SiO2, but all the samples deactivated on stream because of coke formation.  At 20% phenol conversion, the selectivity to cresols was clearly higher on MCM22 (90%) than on HBeta, HZSM5 or SiO2-Al2O3 (about 54%); HPA/SiO2 formed predominantly anisole (Scresols = 32%). Production of o-cresol and p-cresol was predominant as compared to m-cresol. The distribution of cresol isomers on HBeta, HZSM5, and SiO2-Al2O3 was similar, being o-cresol the predominant product (p:o ratios between 0.7 and 0.8).  In contrast, the p:o ratio increased to 4 on MCM22. Thus, results show that the primary and secondary reaction pathways leading from phenol to cresols depend on both the pore microstructure and the nature, density, and strength of surface acid sites, but the reaction para-selectivity is only dramatically increased by using zeolite MCM22, because the narrow sinusoidal channels of this zeolite is particularly suitable for improving by shape selectivity the formation of p-cresol. Specifically, we report for the first time that p-cresol yields of 55% and para-:ortho-cresol selectivity ratios of 4 are obtained on MCM22 at 473 K and contact time of 350 g h/mol.