Selective alkylation of phenol to 2,6-xylenol over vanadia—chromia mixed oxide catalysts

Abstract

Vapour-phase alkylation of phenol with methanol was carried out over a series of simple and mixed oxides of vanadia and chromia of different atomic ratios in a fixed-bed, continuous-flow reactor at 573–723 K and 1–20 bar pressure. Preferential orthoalkylation with 90% selectivity took place with the formation of o-cresol and 2,6-xylenol. The acidity of the catalysts was measured by n-butylamine titration and by cyclohexanol dehydration at 523 K, which was also used as an acidity index. Variation of the activity with the atomic ratios of V and Cr in the mixture was correlated with acidity, total phenol conversion and selectivity for o-cresol and 2,6-xylenol. An increase in acidity increases the selectivity for 2,6-xylenol. Pure vanadia is a better alkylation catalyst than pure chromia. However, addition of a small amount of chromia to vanadia improves the acidity and also the total activity and selectivity for 2,6-xylenol.