Chemoselective hydrogenation of cinnamaldehyde at atmospheric pressure over combustion synthesized Pd catalysts

A series of Pd-supported metal oxides (Al2O3, Fe2O3 and CeO2) have been prepared by a single step solution combustion synthesis (SCS). Their catalytic performance was evaluated for selective hydrogenation of cinnamaldehyde (CAL) to hydrocinnamaldehyde (HCAL) under atmospheric pressure of hydrogen at 100 °C. Among these materials, combustion synthesized Pd (2 at.%)/Al2O3 catalyst exhibits the highest CAL conversion (69 %) with complete HCAL selectivity. The analogous catalyst prepared by the incipient wetness impregnation (IWI) method shows an initially similar activity. XRD and HRTEM analyses of the as prepared SCS sample show fine dispersion of PdO over the -Al2O3 support. On ageing, a major portion of PdO is reduced to metallic Pd (Pd:Pd=36:64 for the SCS catalyst and Pd:Pd=26:74 for the IWI catalyst from XPS studies) suggesting comparatively more ionic character of palladium in the SCS catalyst. In the hydrogen atmosphere, without distinguishing the reductive pretreatment of catalyst and the beginning of hydrogenation subsequent to CAL addition, the Pd-species undergoes rearrangement to form a core-shell like structure of Pd (core)-PdO (periphery) covered with alumina layer, bringing in additional stability to the Pd-species in the SCS catalyst and making it highly recyclable. The analogous IWI catalyst, on the contrary, contains a mixed Pd-PdO ensemble that does not increase the stability causing continuous loss of activity in the consecutive cycles of hydrogenation.