Deactivation kinetics of V/Ti-oxide in toluene partial oxidation

Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxida tion of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523-573 K. T he catalyst consisted of 0.37 monolayer of VOx species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectr oscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed carboxylate and benzoate species was observed accompanied by disap pearance of the band of the monomeric species (2038 cm(-1)) (polymeric spec ies were not controlled). Slow accumulation of maleic anhydride, coupling p roducts and/or BAc on the surface caused deactivation of the catalyst durin g the reaction. Temperature-programmed oxidation (TPO) after the reaction s howed formation of high amounts of CO, CO2 and water. Rate constants for th e steps of the toluene oxidation were derived via mathematical modelling of reaction kinetics at low conversion and constant oxygen/toluene ratio of 2 0:1. The model allows predicting deactivation dynamics, steady-state rates and selectivity. The highest rate constant was found for the transformation of BA into BAc explaining a low BA yield in the reaction. (C) 2001 Elsevie r Science B.V. All rights reserved.