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

The structures of mesoporous solids of abbreviated formula Al100PxVy where x, y=0, 5, 10 have been characterised by multinuclear magnetic resonance techniques. P-31 MAS spectra indicate that two phosphorus environments (threefold or fourfold O-Al coordination) are incorporated within the aluminium network. V-51 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. Al-27 MAS spectra show that amorphous AlPO4 species are formed upon addition of phosphorus, already occurring for the lower phosphorus input. However, Al-27 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. P-31-{Al-27} and P-31-{V-51} 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.