Vanadium phosphorus oxide catalyst promoted by cobalt doping for mild oxidation of benzyl alcohol to benzaldehyde in the liquid phase

Abstract

A series of cobalt-doped vanadium phosphorus oxides (VPO-Co) catalysts, as well as unpromoted sample (VPO) were prepared using classical organic method via VOHPO4·0.5H2O precursor followed by calcinations in n-butane/air environment at 400°C for 24h. Techniques such as XRD, BET surface area, chemical titration, SEM, TPR and FT-IR were used for characterization of the catalysts. The results showed that promoted and unpromoted VPO catalyst were contained crystalline form of V4+ (vanadyl pyrophosphate, (VO)2P2O7) and also a small amount of V5+ (β-VOPO4 phase). However, Co promoted VPO contained poorer crystallinity compared to the unpromoted catalysts. CoPO4 phase also observed in the VPO-Co series and the intensities of the X-ray lines of CoPO4 increased with the increase of the Co/V molar ratio. In addition, the promotion of Co resulted in larger amount of V5+ species. Co was found to increase the average oxidation number of vanadium from 4.28 to 4.35 due to an addition of V5+ oxidation state from 28% to 35%.Oxidation of benzyl alcohol, for the first time, was studied in the liquid phase over VPO and VPO-Co catalysts, using tert-butylhydroperoxide (TBHP) as an oxidant. The catalytic tests showed that cobalt doping significantly increased the overall activity for the oxidation of benzyl alcohol. At 90°C, the obtained activities were 0.662 and 1.014gprog−1VPOh−1 over the VPO and VPO-Co (molar ratio Co/V=0.01) catalysts, respectively. The effects of reaction temperature, amount of the catalyst, solvents, and catalyst recycling of VPO-Co in liquid phase were also investigated.