Theoretical investigations of NMR chemical shieldings on the AlPON catalyst system

Using molecular cluster models we present a theoretical ab initio study of the 27Al and 31P nuclear magnetic resonance (NMR) chemical shieldings aimed at obtaining short-range structural information on the aluminophosphate oxynitride (AlPON) catalyst system. We have employed orthophosphate-like molecular models with P/Al ratio equal to one and varying N/O ratios to simulate the experimentally obtained compositions. The computed NMR chemical shieldings reproduce quantitatively the observed features in the 31P magic angle spinning NMR (MAS-NMR) spectra and permit the explanation of these modifications in terms of a progressive and preferential nitridation on the first coordination shell of the phosphorus atom leading to a mixture of [POxN4−x] units in the solid. The comparison of 27Al isotropic chemical shieldings and the experimental spectra allow us to conclude that N/O substitution does not happen in the first coordination shell of aluminum atoms. The experimentally observed broadening of the tetrahedral aluminum band can be explained by a nitridation of aluminum environment, but not by a nitridation of its first coordination shell.