Preparation Of Vanadium Doped Mesoporous TiO₂for Direct Hydroxylation Of Benzene

Phenol is an important industrial intermediate and is widely used for the production ofphenolic resins, bisphenol A, epoxy resins, caprolactam, and aniline, for example. The directoxidation of benzene to phenol using H2O₂as an oxidant is widely accepted as a greenprocess, with water as the only byproduct, and this may be one of the most useful processes inthe future. Recently, much effort has been devoted to finding higher activity catalysts, such asV-based catalysts.In this work, a series of TiO₂-supported VOxcatalysts with various amounts of vanadiumoxide were synthesized using a hydrothermal synthesis method and used for hydroxylation ofbenzene to phenol, with Cu and Ag as the second metals loaded with VOxwere investigated.The samples were characterized using thermogravimetry-differential thermal analysis,N2-adsorption, scanning electron microscopy, H2temperature-programmed reduction, X-raydiffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. It wasfound that vanadium exists as monomeric and polymeric VOxand V2O5crystal phases,depending on the amount of vanadium, accompanied by transformation of the TiO₂carrierfrom anatase to rutile, because the active VOxcomponents would aggregate and easily form aVxTi1-xO₂rutile-phase solid-solution with TiO₂. Meanwhile, the influence of temperature onthe anatase to rutile transformation was strong. Monodisperse vanadia/TiO₂（mainly anatase）catalysts are highly active in benzene hydroxylation to phenol. When the calcinationtemperature was550°C, the yield and selectivity of phenol of benzene, initially increasedwith vanadium species loading, and the maximum phenol yield,23.7%, was obtained whenthe vanadium loadings was4.30wt%. The yield of phenol decreased slightly with furtherincreases in the vanadium species loading, bacause the active VOxcomponents wouldaggregate and easily form a VxTi1-xO₂rutile-phase solid-solution with TiO₂. The formation ofa solid solution would have an unfavorable effect on the total catalytic activity and on theoxidation product.After the addition of Cu, the Cu/VOx-TiO₂catalyst had a more ordered mesoporousstructure as compared with the VOx-TiO₂catalyst, and the vanadium was monodispersed onthe TiO₂carrier. These Cu2+ions probably contributed to the dispersion of vanadium on thesurface of the TiO₂carrier, and the more facile reduction of VOx. When the copper species loading was0.75wt%, the isolated VOxand copper species were highly dispersed on thecarrier surface. The Cu（0.75）/V（4.3）TiO₂-450catalyst has highly catalytic activity for thehydroxylation of benzene to phenol, and gave a phenol yield of25.6%and phenol selectivityof91.4%.When the Ag as a second metal was loaded with VOx, the Ag/VOx-TiO₂catalyst had aordered mesoporous structure as compared with the VOx-TiO₂catalyst, The results fromH2-TPR revealed that the addition of Ag promoted the reduction of vanadium species in thecomplex catalysts. The Ag-V-O complex oxides were found to exhibit good catalyticreactivity for the selective oxidation of benzene to phenol.A possible reaction mechanism was discussed in this paper, using V-based catalysts.