Selective Hydrogenolysis of Glycerol to Propylene Glycol on MgO-Al2O3 Dispersed Cu Catalysts

Cu/MgO-Al2O3 (Mg/Al atomic ratio=1/1, 3/1, 4/1), Cu/MgO and Cu/Al2O3 catalysts with high Cu dispersions were prepared by an impregnation method using ethanol as solvent instead of water. Compared to water, ethanol as solvent was favorable not only for obtaining the high dispersions of the Cu clusters on the basic oxide supports, but also for retaining the basicity and structures of the supports during the catalyst preparation. The density of the basic sites for the dispersed Cu catalysts increased with increasing the MgO contents in the basic supports. These catalysts catalyzed glycerol hydrogenolysis to propylene glycol with a high selectivity (>90%), and showed slight deactivation in dioxane at 200 degrees C and 6.0 MPa H-2. Their reaction rates normalized by per exposed Cu atom increased with increasing the ratios of the basic sites to Cu atoms on the catalyst surfaces. The intrinsic hydrogenolysis activity of the Cu atoms were probed by N2O chemisorption-H-2 temperature programmed reduction, which did not change essentially with the Cu loadings (2-6 wt%) and the Mg/Al atomic ratios of the MgO-Al2O3 supports. Meanwhile, the basic sites alone on the supports were inactive for the glycerol conversion. Taken together, we propose a synergetic effect that the basic sites at the interfaces between the Cu sites and the supports assist the cleavage of alpha-C-H bonds of glycerol on the Cu surfaces leading to the faster dehydrogenation of glycerol to glyceraldehyde, a kinetically-relevant step in glycerol hydrogenolysis to propylene glycol. These active basic sites appear to be the Bronsted OH- sites, i.e. the hydroxyl groups bonded to the surface Mg2+ cations on MgO-Al2O3. The understanding on such synergy between the basic sites and Cu sites in glycerol hydrogenolysis provides a basis for the rational design of superior hydrogenolysis catalysts and selective removal of oxygen from biomass-derived feedstocks.