Surface basicity of a new family of catalysts: aluminophosphate oxynitride (ALPON)

Abstract

A new family of basic catalysts, amorphous aluminophosphate oxynitrides (AIPONs) with variable nitrogen content, have been synthesized by nitridation of amorphous AlPO₄(AIPO). 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 Brønsted acidity of these new catalysts. The decrease in the Brønsted 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 AIPON 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 Brønsted acidity and the enhancement of the basicity of the AlPON specimen.