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

Abstract

The structures of mesoporous solids of abbreviated formula Al₁₀₀P_xV_y 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.