Identification of the Basic Site on the Aluminovanadate Oxynitride Catalysts

Abstract

A series of aluminovanadate oxynitride (VAlON) catalysts with increasing nitrogen content was synthesized by thermal nitridation of an oxide precursor with a fixed Al/V atomic ratio of 1.5. The surface of these oxynitrides is characterized by TPD-MS, DRIFT, XPS, and DRIFT of the adsorbed CDCl3. TPD results show that NH3 desorb from all the VAlON samples when they are heated from RT to 673 K under a flow of pure helium. XPS and DRIFT analysis revealed that on the surface of the fresh catalysts, ammonia is adsorbed on Brønsted acid sites, giving two XPS N1s binding energy peaks at 400 and 401.8 eV and two DRIFT absorption bands of δas(NH4+) at 1425 and 1480 cm−1. The VAlON catalysts present basic properties as evidenced by the activity in the Knoevenagel condensation reaction between benzaldehyde and malononitrile. It is shown that the catalytic activity is directly correlated with the amount of ammonium ions measured by DRIFT. This relationship between the basic character and the amount of NH4+ is further confirmed by the adsorption of CDCl3, a weak acid probe, for which the intensity of the ν(C–D) vibration between 2250 and 2238 cm−1 is proportional to the amount of NH4+ remaining after thermal treatment. Since NH4+ is not a basic species, it is proposed that the oxygen of an hydroxyl group whose negative charge is increased by ammonia interaction is the active basic center. The basic center can be described as V–O−+H4N, with the hydroxyls bonded to vanadium atoms. This basic site is thermally unstable and presents a strength similar to that of alumina with a pKa≅7. To explain the high rate of reaction observed, a concerted mechanism with activation of the benzaldehyde by the acid sites of the catalysts and/or the retention by the surface of the water produced by the reaction is proposed.