Low-Temperature Gas-Phase Oxidation of Benzyl Alcohol on Mesoporous K-Cu-TiO ₂ through Oxidative Dehydrogenation

A highly selective, gas-phase benzyl alcohol-to-benzaldehyde transformation is achieved over mesoporous multi-component metal oxide K-Cu-TiO ₂ (prepared by AcHE process) at a surprisingly low temperature: the b.p. of benzyl alcohol, 203°C. We propose that highly efficient selective oxidation over K-Cu-TiO ₂ occurs through an oxidative dehydrogenation mechanism based on the catalytic performance with controlled oxygen addition and the observation of surface hydride species on the catalyst surface by electron paramagnetic resonance spectroscopy and spin-trapping techniques. It was observed that the benzyl alcohol conversion increased sharply with the increase in pO2 and reached a maximum of 80.4% at pO2/p _BA=0.4-0.6. A further increase in the pO2 slightly decreased the benzyl alcohol conversion. The fact that the reaction rate was independent of the oxygen pressure after the stoichiometric pO2/p _BA ratio (0.5) could indicate a fast reoxidation of Cu ^IH by molecular oxygen, which is, therefore, not the rate-controlling step. No hydride species have been detected on the catalyst that was collected from the alcohol oxidation reaction performed with added O ₂, suggesting that O ₂ acts as efficient hydrogen acceptor and accelerate the reaction by liberating free active Cu ^I sites. Based on understanding of the reaction mechanism, in addition to the low-temperature reaction condition, stoichiometrical addition of O ₂ proves effective in the stabilisation of the Cu ^I oxidation state for highly selective benzyl alcohol oxidative dehydrogenation over a wide reaction temperature range (up to 310°C).