The Role of Active Oxygen in the AMM-VxSi-Catalysed Selective Oxidation of Toluene

Abstract

The selective catalytic oxidation of toluene with air to benzaldehyde in the gas phase has been studied over amorphous microporous mixed oxide catalysts containing isolated V-centres in microporous silica (AMM-VxSi). The homogeneous distribution of the vanadium on atomic scale in these catalysts has been proven by XRD, HRTEM/EDX, light microscopy, and FTIR. Benzaldehyde selectivities up to 94% at Xtoluene=1.7% (SBA=60% at Xtoluene=6%) could be obtained. Experiments under kinetically controlled conditions, i.e., in the absence of pore diffusional or external mass transport limitations, showed that the AMM-VxSi catalysts are more active than the reference bulk material V2O5. The role of active oxygen in this reaction has been elucidated using vacuum transient experiments with labelled 18O2 in the TAP reactor. The results revealed that even in the presence of isolated vanadium centres in the silica matrix the active oxygen is lattice oxygen, and the selective oxidation mechanism of toluene is therefore identical to that of bulk V-oxide phases (Mars–van Krevelen type). For the reoxidation of the active site, a new mechanism is proposed on the basis of the mechanistic studies of the present work.