Ammoximation reaction with titanium doped amorphous silica

Abstract

The catalytic behavior and the role of the acidic sites and of the activated forms of molecular oxygen in the ammoximation reaction of cyclohexanone to cyclohexanone oxime were investigated on pure and titanium doped amorphous silicas. The samples were prepared by the sol-gel method under acidic and basic conditions. The introduction of low amounts of titanium on silicas, prepared under basic conditions, strongly increased the oxime yield and conversion. The sample containing 0.25% titanium resulted in a very active and selective catalyst. A further increase of the titanium content decreased the catalytic activity and also the catalyst deactivation, because of a decrease of the tars formation rate. The comparison of the catalytic data and the results of FT-IR and EPR analysis confirmed the bifunctional nature of the amorphous silica catalyst. On the first step of the reaction pathway, the imine formation, the paramount parameter is an intrinsic property of pure silica, related to its textural and structural features. The adsorption experiments 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 complex. EPR results showed the presence of radical species on titanium modified silica and also, in small amount, on silica itself; however, the oxime formation did not seem to be a property related only to the presence of activated oxygen species anchored to titanium sites. It has been proposed that the formation of oxime is due to a combined effect of the presence of tars and of the activated oxygen species due to the titanium, which are able to oxidize the imine to oxime.