Liquid-phase efficient oxidation of cyclohexane over cobalt promoted VPO catalyst using tert-butylhydroperoxide

Abstract

A series of cobalt-doped vanadium phosphorus oxides (VPO-Co) catalysts, as well as unpromoted sample (VPO) was prepared using classical organic method via VOHPO4·0.5H2O precursor followed by calcinations in 1.5% butane/air environment at 400 °C for 24 h. 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 VPO catalyst were contained crystalline form of V4+ (vanadyl pyrophosphate, (VO)2P2O7), CoPO4 phase and a small amount of V5+ (β-VOPO4 phase). However, Co promoted VPO contained poorer crystallinity compared to the unpromoted catalysts. Co was found to increase the average oxidation number of vanadium from 4.28 to 4.43 due to increase of V5+ oxidation state from 28 to 43%.Oxidation of cyclohexane, 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 cyclohexane. At 90 °C, the obtained activities were 0.076 and 0.491 gpro/gVPO/h over the VPO and VPO-Co (molar ratio Co/V = 0.1) catalysts, respectively. The effects of Co loading, TBHP/cyclohexane molar ratio, amount of the catalyst, solvents and catalyst recycling were investigated. The kinetic of cyclohexane oxidation was investigated at different temperatures using VPO-Co(0.1) and excess TBHP. The order of reaction with respect to cyclohexane was determined to be pseudo-first order. The value of the apparent activation energy was also determined.