Sorbitol Hydrogenolysis over Hybrid Cu/CaO-Al2O3 Catalysts: Tunable Activity and Selectivity with Solid Base Incorporation

We report for the first time the performance of hybridized Cu/CaO-Al2O3 catalysts for aqueous-phase hydrogenolysis of sorbitol to ethylene glycol (EG), 1,2-propanediol (1,2-PDO), and 1,2-butanediol (1,2-BDO) with linear alcohols as coproducts in a base-free liquid phase. These supported Cu catalysts with solid bases as promoters show significant activity for C-C cleavage and high selectivity (similar to 84%) to glycols and linear alcohols. The effects of Cu loading, catalyst pretreatment conditions, H-2 pressure, and temperature on activity and selectivity of Cu/CaO-Al2O3 catalysts were investigated. The strong interaction between Cu and Ca2+ cations in the solid support is found to facilitate C-C and C-O cleavage of sorbitol, as evidenced from TEM, SEM, and TPR studies of the catalysts. Surface characterization and activity tests further suggest that CaxCuyAlzOp (Phase I) promotes dehydrogenation and isomerization reactions, whereas spinal CuAl2O4 (Phase II) species facilitates hydrogenation reactions. In addition, the overall activity and selectivity of the Cu catalysts may be easily tuned by the Cu/Ca2+ molar ratio and catalyst preparation conditions. Cu/CaO-Al2O3 catalysts also give higher overall yields of value-added glycols (63-82%) for facile conversion of various other sugar polyols such as glycerol (C-3), erythritol (C-4), xylitol (C-5), and mannitol (C-6) under similar reaction conditions. A surface reaction mechanism involving the formation of beta-ketoses on multifunctional Cu-Ca2+ sites is proposed.