Solid-state NMR study of mesoporous phosphoro-vanado-aluminas

Abstract

The structures of mesoporous solids of abbreviated formula Al100PxVy where x, y = 0, 5, 10 have been characterised by multinuclear magnetic resonance techniques. 31P MAS spectra indicate that two phosphorus environments (threefold or fourfold O–Al coordination) are incorporated within the aluminium network. 51V magic-angle spinning (MAS) spectra suggest that vanadium species are not (or only weakly) incorporated into the alumina network, forming V2O5-like and tetrahedral species at the surface of the pores. 27Al MAS spectra show that amorphous AlPO4 species are formed upon addition of phosphorus, already occurring for the lower phosphorus input. However, 27Al triple-quantum (3Q)-MAS cannot detect this amorphous AlPO4 phase, therefore indicating distorted environments close to the surface. For the Al100P10V10 sample, crystalline AlPO4 is detected. 31P–{27Al} and 31P–{51V} TRAPDOR experiments confirm that all the phosphorus atoms are surrounded by at least three aluminium atoms in the second coordination sphere, and suggest the heterogeneous location of vanadium species at the surface of the mesopores.