INTERPRETATION OF THE NATURE OF OXIDATION SITES OF SILICA ACTIVE IN AMMOXIMATION REACTION OF CYCLOHEXANONE BY SURFACE-POTENTIAL MEASUREMENTS

The catalytic behaviour in the ammoximation reaction of cyclohexanone to cyclohexanone oxime and the presence of activated forms of molecula r oxygen on different commercial silicas (Akzo F-7, Degussa A200, Cabo sil) was investigated. The presence of O-2(-) species in the same rang e of temperature at which the silicas are active in ammoximation react ion was detected. The investigated silicas presented different selecti vities in cyclohexanone oxime but all of them gave a high amount of ta rs as by products in presence of molecular oxygen. It has been propose d that the O-2(-) adsorbed on the surface of silica are only involved in the beginning a radical reaction responsible of the formation of ta rs and that the high activity in oxime formation presented by Akzo F-7 is mainly related to its mesoporosity and the presence of tars.