The role of active oxygen in the AMM-VxSi-catalysed selective oxidation oftoluene

The selective catalytic oxidation of toluene with air to benzaldehyde in th e gas phase has been studied over amorphous microporous mixed oxide catalys ts containing isolated V-centres in microporous silica (AMM-VxSi). The homo geneous distribution of the vanadium on atomic scale in these catalysts has been proven by XRD, HRTEM/EDX, light microscopy, and FTIR. Benzaldehyde se lectivities up to 94% at X-toluene = 1.7% (S-BA = 60% at X-toluene = 6%) co uld 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 elucida ted using vacuum transient experiments with labelled O-18(2) in the TAP rea ctor. The results revealed that even in the presence of isolated vanadium c entres in the silica matrix the active oxygen is lattice oxygen, and the se lective oxidation mechanism of toluene is therefore identical to that of bu lk V-oxide phases (Mars-van Krevelen type). For the reoxidation of the acti ve site, a new mechanism is proposed on the basis of the mechanistic studie s of the present work. (C) 1999 Academic Press.