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

Abstract

A series of Pd-supported metal oxides (Al₂O₃, Fe₂O₃ and CeO₂) have been prepared by a single step solution combustion synthesis (SCS) method. Their catalytic performance was evaluated for the 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. X-ray diffraction and high resolution transmission electron microscopy analyses of the as prepared SCS sample show fine dispersion of PdO over the γ-Al₂O₃ support. On ageing, a major portion of PdO is reduced to metallic Pd (Pd²⁺:Pd⁰ = 36:64 for the SCS catalyst and Pd²⁺:Pd⁰ = 26:74 for the IWI catalyst from X-ray photoelectron spectroscopy 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.