Surface basicity of a new family of catalysts: Aluminophosphate oxynitride (AlPON)

A new family of basic catalysts, amorphous aluminophosphate oxynitrides (AlPONs) with variable nitrogen content, have been synthesized by nitridation of amorphous AlPO4 (AlPO). The samples were analysed by X-ray photoelectron spectroscopy (XPS) and diffuse reflectance IR spectroscopy (DRIFTS). XPS indicates that nitrogen is preferentially bonded to phosphorus in the AlPON framework. Residual surface NH4+ ions coming from the nitridation reaction were used to evaluate the Bronsted acidity of these new catalysts. The decrease in the Bronsted acidity along the series is parallel to a shift of the P-O stretching to lower wavenumbers. Malonitrile-benzaldehyde condensation was employed as a test reaction to estimate the basicity of AlPON samples. The conversion trend observed also correlates with the decrease in the frequency of the P-O stretching. Consequently, the position of the P-O stretching reflects both the reduction of the Bronsted acidity and the enhancement of the basicity of the AlPON specimen.