Nature of active layer in vanadium oxide supported on titanium oxide and control of its reactivity in the selective oxidation and ammoxidation of alkylaromatics

Vanadium oxide supported over titanium oxide is an important example of a transition-metaloxide 'monolayer' catalyst with practical relevance for various industrial applications. After a brief introduction on the concept of monolayer oxide catalysts, this review examines three main topics: (i) the nature of vanadium oxide species over the titanium oxide surface, (ii) their reactivity and structure/activity-selectivity relationships and (iii) modification of the surface properties during the catalytic reactions of o-xylene oxidation and alkylaromatic ammoxidation. An analysis of selected literature data leads to the following main conclusions: (i) the support promotes the formation of an amorphous, hydrated active layer of titanium oxide which may extend up to 3-5 layers; (ii) the reactivity of titanium oxide in this form is enhanced with respect to crystalline vanadium oxide, but the selectivity properties which are a function of the oxidation state of vanadium ion and of the modifications occurring during the catalytic reaction are not significantly different; (iii) in the presence of ammonia, the characteristics of the catalyst surface must be tuned to limit the side reaction of NH3 combustion. The optimal catalyst composition for ammoxidation reactions is thus different from that for oxidation catalysts. Simultaneous O and N insertion requires instead an intermediate catalyst composition between those optimal for oxidation and ammoxidation of alkylaromatics.