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-Al 2 O 3 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 (∼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-Al 2 O 3 catalysts were investigated. The strong interaction between Cu and Ca 2+ 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 Ca x Cu y Al z O p (Phase I) promotes dehydrogenation and isomerization reactions, whereas spinal CuAl 2 O 4 (Phase II) species facilitates hydrogenation reactions. In addition, the overall activity and selectivity of the Cu catalysts may be easily tuned by the Cu/Ca 2+ molar ratio and catalyst preparation conditions. Cu/CaO-Al 2 O 3 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 β-ketoses on multifunctional Cu–Ca 2+ sites is proposed.