FTIR investigation of surface intermediates formed during the ammoxidation of toluene over vanadyl pyrophosphate

The mechanism of formation of surface intermediates, appearing during the interaction of vanadyl pyrophosphate (VO)(2)P2O7 with feed components of the toluene ammoxidation was studied by FTIR spectroscopy. The investigation of ammonia adsorption at elevated temperature showed protonated and coordinated ammonia as expected as well as the generation of amido species; all could be so called 'nitrogen insertion species' or a source of these at least. The interaction of toluene and possible reaction intermediates such as benzaldehyde and benzylamine with (VO)(2)P2O7 was studied. The investigations revealed that the ammoxidation of toluene probably proceeds via the formation of a benzaldehyde intermediate. Afterwards, benzylimine surface species were probably formed by a N-insertion, using NH4+ surface species followed by the formation of benzonitrile by subsequent oxidative dehydrogenation. However, no benzamide species were observed. The surface species generated upon adsorption of benzaldehyde were similar to those formed from toluene, indicating the role of the former as intermediate in the nitrile formation path. Otherwise, the adsorption of benzylamine in the presence of oxygen did not lead to the formation of benzonitrile. Therefore, an ammoxidation mechanism of toluene via a benzaldehyde intermediate is preferred and reaction pathways via benzamide or benzylamine as intermediates seem to be improbable.