Preparation of MoO3/SiO2–Al2O3 metathesis catalysts via wet impregnation with different Mo precursors

Molybdenum oxide supported on mesoporous silica–alumina is an efficient catalyst for the light olefins metathesis. Preparation via the classical wet impregnation with ammonium heptamolybdate (AHM) suffers from several downsides related to the control of the species formed on the surface and the build up of inactive species (MoO3 crystals and Al2(MoO4)3). This study explores the synthesis, properties and propene metathesis activity of MoO3/SiO2–Al2O3 catalysts prepared with two Mo precursors chosen as alternatives to AHM. Characterization is done on the precursor solutions, on the dried catalyst precursors and on the calcined catalysts (ICP-AES, N2-physisorption, XRD, Raman spectroscopy, XPS and 27Al MAS NMR). Firstly, oxalic acid is used as an additive with the objective to perturb the Mo speciation in the AHM impregnation solution. It affects the catalysts texture and impedes the formation of Al2(MoO4)3. Samples are totally amorphous and outcompete the reference samples prepared with AHM. Secondly, a molybdenum oxide hydrates solution is used as impregnation solution. The MoO3 phase formed is also totally amorphous, the texture is preserved, the spreading of the Mo phase is enhanced and the activity is high. However, at high Mo loading the build-up of Al2(MoO4)3 provokes a decrease in activity.

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► Classical wet impregnation of silica–alumina with AHM has several downsides.
► Preparation of MoO3/SiO2–Al2O3 can be improved by changing the Mo precursor.
► Oxalic acid added to AHM impedes the formation of inactive species.
► Mo hydrates favours the uniform dispersion of Mo.
► Higher metathesis activities are reached.