Gas-Phase Alkylation of Pyridine and Phenol with Alcohols Over Halide Clusters of Group 5–7 Transition Metals as Solid Acid Catalysts

Abstract

Pyridine is allowed to react with methanol under a hydrogen stream in the presence of (H3O)2[(W6Cl8)Cl6]·6H2O supported on silica gel. When the temperature is raised above 200 °C, the catalytic activity of the cluster appears. Methylation of pyridine proceeds yielding 2-methylpyridine in 61% selectivity at 400 °C. The corresponding hexanuclear chloride clusters of niobium, molybdenum, and tantalum also catalyze the reaction. Ethanol affords the corresponding 2-ethylpyridine. When phenol is allowed to react with methanol in the presence of (H3O)2[(Mo6Cl8)Cl6]·6H2O supported on silica gel in the same manner, selective O-methylation proceeds yielding anisole in 57% selectivity at 150–200 °C. Above 250 °C, C-methylation predominates and provides o-cresol with 67% selectivity at 300 °C. The corresponding clusters of niobium, tantalum, and tungsten also catalyze the reaction. Ethanol and 1-propanol afford the corresponding 2-alkylphenols. Alkyl cations produced over weak Bronsted acid sites (H 0 ≈ +1.3) developed on the clusters are assumed as intermediates for both reactions.