Catalysis by polyoxometalates. Part 2.—29Si nuclear magnetic resonance evidence for 12-molybdosilicate in silica-supported molybdenum catalysts

Abstract

²⁹ Si MAS NMR has been applied to unsupported and silica-supported (impregnation method) dodecamolybdosilicic acid H₄SiMo₁₂O₄₀. The ²⁹Si line of the polyanion is isotropic. The relaxation times (T₁) of the supported polyanions are strongly reduced with respect to that of the unsupported polyanion. The short T₁ values (down to 0.3 s) for low-loaded supported samples are attributed to [SiMo₁₂O₄₀]^4– anions in interaction with the silica support. The relative amount of this species, with respect to unperturbed H₄SiMo₁₂O₄₀ acid, simply deposited on the silica surface, is correlated with the behaviour of the sample in the catalytic methanol oxidation. The [SiMo₁₂O₄₀]^4– anion is detected down to 2 wt.% Mo, but quantitative determination by NMR using the SiO₂ resonance as an internal standard requires a long spectrometer time for the low-loaded catalysts. ²⁹Si NMR reveals that mechanical mixtures obtained by grinding the dodecamolybdosilicic acid and silica gives a more homogeneous dispersion than the impregnation method. In addition the H₄SiMo₁₂O₄₀ acid, supported on silica, is destroyed after thermal treatment at 500 °C. At room temperature, [SiMo₁₂O₄₀]^4– anions are reconstructed at the silica surface from silica and molybdate or MoO₃ with the assistance of water vapour.