AMMOXIMATION REACTION WITH TITANIUM-DOPED AMORPHOUS SILICA

The catalytic behavior and the role of the acidic sites and of the act ivated forms of molecular oxygen in the ammoximation reaction of cyclo hexanone to cyclohexanone oxime were investigated on pure and titanium doped amorphous silicas. The samples were prepared by the sol-gel met hod under acidic and basic conditions. The introduction of low amounts of titanium on silicas, prepared under basic conditions, strongly inc reased the oxime yield and conversion. The sample containing 0.25% tit anium resulted in a very active and selective catalyst. A further incr ease of the titanium content decreased the catalytic activity and also the catalyst deactivation, because of a decrease of the tars formatio n rate. The comparison of the catalytic data and the results of FT-IR and EPR analysis confirmed the bifunctional nature of the amorphous si lica catalyst. On the first step of the reaction pathway, the imine: f ormation, the paramount parameter is an intrinsic property of pure sil ica, related to its textural and structural features. The adsorption e xperiments showed that the silica prepared under basic conditions, the only active in ammoximation, was able to form surface imine on acidic sites. The nature and the role of the oxidizing sites seem more compl ex. EPR results showed the presence of radical species on titanium mod ified silica and also, in small amount, on silica itself; however, the oxime formation did not seem to be a property related only to the pre sence of activated oxygen species anchored to titanium sites. It has b een proposed that the formation of oxime is due to a combined effect o f the presence of tars and of the activated oxygen species due to the titanium, which are able to oxidize the imine to oxime.