Activities of V-Ti-O Catalysts in the Ammoxidation of 3-Picoline. II. Acid-Base Properties and Infrared Spectra
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The acidity and basicity of prereduced V-Ti-O catalysts were measured by adsorption of NH3 and CO2. It was found that a high activity in the ammoxidation of 3-picoline corresponds to a relatively small amount of acidic sites. A catalyst selective in the formation of nicotinonitrile requires high concentrations of both acidic and basic sites. These results are discussed, and can be correlated if the acidity is considered to be a measure of the hydroxyl group concentration, and the basicity a measure of oxygen vacancies in the vicinity of VO groups. Also, the infrared spectra of the prereduced catalysts were recorded. A new band was found at 995 cm−1. The same band appeared in a homogeneous mixture of V2O5 and V6O13. It is proposed that this band can be assigned to VO bonds in a reduced V2O5phase with a number of disordered vacancies in the lattice, or a nonstoichiometric V6O13 phase. The shift in frequency relative to that of (VO)3+ shows that these bonds are weaker, and probably also more active. The effect of TiO2 is to increase the amount of V4+ in the vanadium oxide lattice. This is caused by dissolution of Ti4+ in the V2O5 melt during the catalyst preparation.