Surface acidity of vanadyl pyrophosphate, active phase in n-butane selective oxidation

The surface acidity of two (VO)2P2O7 catalysts with similar specific activities per square meter of surface area in 1-butene selective oxidation, but different specific activities in n-butane selective oxidation, was studied by ammonia, pyridine, acetonitrile, CO, and CO2 adsorption, by ammonia temperature-programmed desorption, and by 2-propanol oxidation. The results for both catalysts indicate the presence of strong Brønsted sites attributed to surface P-OH groups and of medium strong Lewis sites attributed to V(IV) coordinatively unsaturated ions exposed on the surface. The presence of these centers was related to the (VO)2P2O7 structure itself and is fairly independent of the (VO)2P2O7 preparation method. However, in the (VO)2P2O7 prepared in an organic medium and to a lesser extent in the (VO)2P2O7 prepared in an aqueous medium, the presence of very strong Lewis sites also was observed. The enhancement of the rate of n-butane activation in the (VO)2P2O7 prepared in an organic medium was attributed to the presence of these sites. The role of the preparation method in the formation of such very strong Lewis sites also is discussed.