Electron paramagnetic resonance characterisation of silica-dispersed copper molybdate obtained by sol-gel and impregnation methods

Silica-dispersed copper molybdates having different Cu:Mo and Cu:Si molar r atios were prepared both by a sol-gel route and a conventional impregnation technique. These materials, used as catalysts for the propene oxidation, w ere studied in order to assess whether the different preparation methods an d/or different Cu:Mo and Cu:Si molar ratios affected the copper(II) coordin ation. Electron paramagnetic resonance spectroscopy showed that oxo/hydroxo -bridged copper(II) dimers were present in xerogel catalyst precursors and on the surface of fresh impregnated precursor materials, their amount incre asing with Cu:Si molar ratio and in impregnated samples. The value of the z ero field interaction is very low and decreases with increase in temperatur e. This means that Cu(II) centres in dimers are spaced fairly far apart and lie in a strongly distorted symmetry held which varies because of the flex ible host matrix. Thermal treatment of both xerogels and impregnated sample s gave silica-dispersed Cu3Mo2O9 catalysts from samples with Cu:Mo molar ra tio 1 : 1 and silica-dispersed CuMoO4 from those with Cu : Mo 1 : 2. The pr esence of polynuclear metal compounds in the crystalline phase seems to be connected to the presence of dimeric species in the precursor. The sol-gel method is more efficient than impregnation to produce catalysts with small and homogeneous particle dimensions of the active phase, for a wide range o f Cu: Si molar ratios.