Promoting effect of Mo on the selective hydrogenation of cinnamaldehyde on Rh/SiO2 catalysts

MoO3 was added to Rh/SiO2 catalysts in order to improve both activity and selectivity towards cinnamyl alcohol during the hydrogenation of cinnamaldehyde in the liquid phase. In the present work, four catalysts were studied. Two Rh-Mo/SiO2 catalysts and Rh/SiO2, Mo/SiO2 references were prepared by conventional impregnation techniques and characterised by hydrogen chemisorption, X-ray diffraction, transmission electron microscopy, temperature-programmed reduction and X-ray photoelectron spectroscopy. The catalytic behaviour in cinnamaldehyde hydrogenation, after in situ reduction treatments in flowing hydrogen at 773 K shows that Mo oxide clearly promotes the hydrogenation of the carbonyl bond. This is attributed to (i) the presence of both MoO3-x and Rhdelta+ species which contribute to a polarization of the C=O bond, (ii) the presence of flat surfaces of molybdenum oxides strongly interacting with Rh particles, that provide the appropriate morphology to enhance the selectivity to the unsaturated alcohol, and (iii) the poisoning effect of MoOx on rhodium species by partial coverage of the metallic particles that produce a decrease in the hydrogenation ability and therefore reduce the formation of the saturated alcohol.