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

摘要

Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523-573 K. The catalyst consisted of 0.37 monolayer of VO_x species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated co nditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed arboxylate and benzoate species was observed accompanied by disappearance of the band of the monomeric species (2038 cm~(-1)) (polymeric species were not controlled). Slow accumulation of maleic anhydride, couling products and/or BAc on the surface caused deactivation of the catalyst during the reaction. temperature-programmed oxidation (TPO) after the reaction showed formation of high amounts of CO, CO_2 and water. Rate constants for the steps of the toluene oxidation were derived via mathematical modelling of reactin kientics at low conversion and constant oxygen/toluene ratio of 20:1. The moel 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.