Selective hydrogenolysis of sorbitol to ethylene glycol and propylene glycol on ZrO 2 -supported bimetallic Pd-Cu catalysts

Sorbitol is one of the key building blocks in catalytic conversion of biomass, and its selective hydrogenolysis to ethylene glycol and propylene glycol provides a viable and sustainable route towards the synthesis of the two glycols. Herein, the hydrogenolysis of biomass-derived sorbitol was studied on Pd-modified Cu/monoclinic zirconia (Pd-Cu/ZrO2) catalysts with a wide range of Cu/Pd atomic ratios in the presence of La(OH)3. The bimetallic Pd-Cu/ZrO2 catalysts showed superior activities and selectivities to the two target glycols, compared with the monometallic Cu/ZrO2 and Pd/ZrO2 catalysts under identical conditions. At nearly 100% sorbitol conversion, a combined selectivity of 61.7% to ethylene glycol, propylene glycol, and glycerol was obtained on Pd-Cu/ZrO2 (Cu/Pd=5) at 493 K under 5.0 MPa H2. Pd-Cu/ZrO2 was also stable and recyclable, in contrast to Cu/ZrO2, which suffered severe deactivation because of agglomeration of Cu particles during sorbitol hydrogenolysis. Clearly, the presence of Pd improved not only the activity and selectivity of the Cu catalyst, but also the hydrothermal stability. Characterization of these catalysts by X-ray diffraction, diffuse-reflectance infrared Fourier transform spectroscopy of CO adsorption, and H2 temperature-programmed reduction suggests that the Cu particles deposited on the Pd surfaces with close contact and strong interaction between the two metals, most likely involving electron transfer from Pd to Cu. Such structural and electronic effects are proposed as the critical contributors to the significant promoting effect of Pd on the activity and stability of Pd-Cu catalysts in sorbitol hydrogenolysis. These findings provide useful information for design of new Cu-based catalysts with higher efficiency and stability for selective hydrogenolysis of polyols and other biomass-derived reactants under hydrothermal conditions.