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

MoO3 was added to Rh/SiO2 catalysts in order to improve both activity and s electivity towards cinnamyl alcohol during the hydrogenation of cinnamaldeh yde 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 c onventional impregnation techniques and characterised by hydrogen chemisorp tion, X-ray diffraction, transmission electron microscopy, temperature-prog rammed reduction and X-ray photoelectron spectroscopy. The catalytic behavi our in cinnamaldehyde hydrogenation, after in situ reduction treatments in flowing hydrogen at 773 K shows that Mo oxide clearly promotes the hydrogen ation 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 b ond, (ii) the presence of flat surfaces of molybdenum oxides strongly inter acting with Rh particles, that provide the appropriate morphology to enhanc e the selectivity to the unsaturated alcohol, and (iii) the poisoning effec t of MoOx on rhodium species by partial coverage of the metallic particles that produce a decrease in the hydrogenation ability and therefore reduce t he formation of the saturated alcohol.