An IR spectroscopy assessment of the surface acidity of mesoporous VOx-SiO2 catalysts

An IR spectroscopic study is reported of the surface acidity of V-SBA-15 and V-MCF materials with comparable vanadium content (ca. 2.5% by weight), prepared by either direct synthesis or impregnation. Acidity of the V–OH species formed is measured spectroscopically by CO adsorption. Different acidic strength is found depending on the synthesis method and irrespective of the porous morphology: stronger sites occur in impregnated samples, due to the formation of poly-vanadates and/or micro-crystalline vanadia; weaker sites are formed by direct synthesis, during which V atoms enter the silica matrix. Adsorption of ammonia at room temperature reveals the presence of Brønsted acidic sites on hydrated samples and of corresponding Lewis acidic sites on dehydrated samples. By comparing IR data from CO and NH3 adsorption experiments, a scale of Brønsted acidity is drawn for the different surface hydroxyls species, which correlates with the activity in the Oxidative De-Hydrogenation of propane, as a lower acidic strength of the surface species corresponds to a higher selectivity to propene.