Selective Hydrogenolysis of Glycerol to Propylene Glycol on Supported Pd Catalysts: Promoting Effects of ZnO and Mechanistic Assessment of Active PdZn Alloy Surfaces

Abstract

Pd catalysts have received increasing attention for selective hydrogenolysis of glycerol to propylene glycol because of their good hydrothermal stability and high selectivity for cleavage of C–O bonds over C–C bonds. Addition of Zn can facilitate glycerol hydrogenolysis to propylene glycol on Pd surface, but the promoting role of Zn, stability of the resulting active PdZn alloys and reaction mechanism remain largely unexplored. Here, we synthesized monoclinic zirconia-supported PdZn (PdZn/m-ZrO2) catalysts via an incipient wetness impregnation method. Glycerol hydrogenolysis turnover rates (normalized per surface Pd atom measured by H2 chemisorption) and propylene glycol selectivity on these PdZn/m-ZrO2 catalysts depended sensitively on their Zn/Pd molar ratios, and Zn leaching from the PdZn alloy phases led to deactivation of PdZn/m-ZrO2. Such deactivation was efficiently inhibited when physical mixtures of Pd/m-ZrO2 and ZnO were directly used in glycerol hydrogenolysis, leading to in situ formation of P...