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

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 PdZn alloy layers on Pd surfaces with excellent stability and recyclability. Dependence of turnover rates on glycerol and H2 concentrations, combined with the primary kinetic isotope effects (kH/kD = 2.6 at 493 K), reveals the kinetically relevant step of glycerol hydrogenolysis involving the α-C-H cleavage in 2,3-dihydroxypropanoxide intermediate to glyceraldehyde on PdZn alloys and Pd. Measured rate constants show that the transition state of α-C-H cleavage is more stable because of the stronger oxophilicity of Zn on PdZn alloys than on Pd, which thus facilitates α-C-H cleavage of the Zn-bound intermediate by adjacent Pd on PdZn alloys. Such synergy between Zn and Pd sites accounts for the observed superiority of PdZn alloys to Pd in glycerol hydrogenolysis.