Solid-state 31P magic-angle spinning NMR relaxation study of silica-supported polyoxoanions of the Keggin and Dawson structures

Abstract

Solid-state 31P magic-angle spinning NMR investigations performed on silica-supported phosphopolyoxoanions [PMo 11Vo 40] 4−, [PW 12O 40] 3− and [P 2W 18O 62] 6− reveal a similar trend as already observed with [PMo 12O 40] 3− — the 31P spin-lattice relaxation rate increases with decreasing concentration of the polyanions at the silica surface. However the relaxation time T 1 decreases with increasing the age of the samples, especially for the tungstic compounds. More generally T 1 depends on the history of the sample after having been supported on silica; a vacuum treatment increases T 1 whereas exposure to water vapour has an opposite effect, showing the influence of water molecules assisting the relaxation. In addition the chemical shift anisotropy for the less symmetrical anions ([PMo 11Vo 40] 4− and [P 2W 18O 62] 6−) is partially averaged after water exposure, which could be explained by a “pseudoliquid phase”, already invoked for the interpretation of the “spillover” effect at the silica surface.