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

Abstract

Silica-dispersed copper molybdates having different Cu:Mo and Cu:Si molar ratios were prepared both by a sol-gel route and a conventional impregnation technique. These materials, used as catalysts for the propene oxidation, were studied in order to assess whether the different preparation methods and/or different Cu:Mo and Cu:Si molar ratios affected the copper(II) coordination. 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 increasing with Cu:Si molar ratio and in impregnated samples. The value of the zero field interaction is very low and decreases with increase in temperature. This means that Cu(II) centres in dimers are spaced fairly far apart and lie in a strongly distorted symmetry field which varies because of the flexible host matrix. Thermal treatment of both xerogels and impregnated samples gave silica-dispersed Cu ₃ Mo ₂ O ₉ catalysts from samples with Cu:Mo molar ratio 1:1 and silica-dispersed CuMoO ₄ from those with Cu:Mo 1:2. The presence 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 of Cu:Si molar ratios.