Direct Hydroxylation of Benzene to Phenol Over Mixed-Crystal Particles of Mesoporous VO_x/TiO₂ Catalyst Mixed-Crystal VO_x/TiO₂ for Benzene Hydroxylation

摘要

Abstract The influence of vanadium loading and calcination temperature on the catalytic performance of vanadia/TiO₂ (mixed-crystal) catalysts for the selective hydroxylation of benzene was investigated. A series of VO _x /TiO₂ catalysts were prepared using a range of vanadium loadings (2.27–9.06 wt %) and calcination temperatures of 450–650 °C. The samples were characterized using thermogravimetry–differential thermal analysis, N₂-adsorption, scanning electron microscopy, H₂ temperature-programmed reduction, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. It was found that vanadium exists as monomeric and polymeric VO _x and V₂O₅ crystal phases, depending on the amount of vanadium, accompanied by transformation of the TiO₂ carrier from anatase to rutile. The influence of temperature on the anatase to rutile transformation was strong, and only a little anatase was transformed to rutile at temperatures below 550 °C. When the temperature was raised to 650 °C, rutile became the main crystal phase. Monodisperse vanadia/TiO₂ (mainly anatase) catalysts are highly active in benzene hydroxylation to phenol, but aggregation of VO _x and crystalline V₂O₅ supported on a rutile carrier lowers the catalytic activity. In addition, the catalytic performances of the various catalysts in hydroxylation of benzene to phenol were investigated and possible reaction mechanisms are discussed.