Aluminovanadate oxynitride catalyst: Proposition for the basic site

An aluminovanadate oxynitride (AlVON) catalyst is synthesized. This catalyst is active in the Knoevenagel condensation reaction between malononitrile and benzaldehyde, showing that it has basic sites. DRIFT characterization has evidenced the presence of ammonium ions due to the neutralization by ammonia of hydroxyl Bronsted acid sites. Since the catalyst loses its basic activity when the ammonium ions are removed by thermal treatment, the hypothesis of a negatively charged oxygen,generated by the neutralization of the Bronsted acid hydroxyls by ammonia, like V-O- NH4+, is proposed as a basic center. This hypothesis is strengthened by three observations. (1) The inactive thermally treated AlVON recovers its catalytic activity when the ammonium ions are regenerated by gaseous ammonia adsorption. (2) When CDCl3 adsorption is carried out at increasing pretreatment temperature of the AlVON, a linear correlation is obtained between the amount of deuterated chloroform adsorbed and the ammonium ions staying after thermal desorption. This suggests that a relation can be expected between the ammonium ions and the basicity. (3) The oxide precursor that retains ammonium ions at the surface is active in the Knoevenagel condensation reaction. This provides evidence that the basic site is likely an oxygen atom rather than a nitrogenous species such as nitrides (N3-), -NH-, or -NH2 groups, since the oxide precursor has not been nitrided.