CATALYSIS BY POLYOXOMETALATES .2. SI-29 NUCLEAR-MAGNETIC-RESONANCE EVIDENCE FOR 12-MOLYBDOSILICATE IN SILICA-SUPPORTED MOLYBDENUM CATALYSTS

Si-29 MAS NMR has been applied to unsupported and silica-supported (impregnation method) dodecamolybdosilicic acid H4SiMo12O40. The Si-29 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. Si-29 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-degrees-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.