Y. Zhang et al., FTIR INVESTIGATION OF SURFACE INTERMEDIATES FORMED DURING THE AMMOXIDATION OF TOLUENE OVER VANADYL PYROPHOSPHATE, Journal of molecular catalysis. A, Chemical, 118(2), 1997, pp. 205-214
The mechanism of formation of surface intermediates, appearing during
the interaction of vanadyl pyrophosphate (VO)(2)P2O7 with feed compone
nts of the toluene ammoxidation was studied by FTIR spectroscopy. The
investigation of ammonia adsorption at elevated temperature showed pro
tonated 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 inte
rmediate. Afterwards, benzylimine surface species were probably formed
by a N-insertion, using NH4+ surface species followed by the formatio
n of benzonitrile by subsequent oxidative dehydrogenation. However, no
benzamide species were observed. The surface species generated upon a
dsorption of benzaldehyde were similar to those formed from toluene, i
ndicating the role of the former as intermediate in the nitrile format
ion path. Otherwise, the adsorption of benzylamine in the presence of
oxygen did not lead to the formation of benzonitrile. Therefore, an am
moxidation mechanism of toluene via a benzaldehyde intermediate is pre
ferred and reaction pathways via benzamide or benzylamine as intermedi
ates seem to be improbable.