Influence of solvent on aggregation of metallic Cu in Cu/MgO during hydrogenation in liquid phase

Cu/MgO catalysts generally could selectively hydrogenate C=O in unsaturated aldehydes, but they often suffer from aggregation of Cu species in liquid-phase reactions. In this study, structural change of Cu/MgO catalyst was investigated during the catalysing conversion of ethyl levulinate (EL) to γ-valerolactone (GVL) and 1,4-pentanediol (1,4-PDO) in varied medium. Water as medium could achieve GVL yield of ca. 99%, while ethanol promoted ring-opening of GVL to 1,4-PDO. However, water and ethanol impacted structure of Cu/MgO in distinct ways. Water led to transformation of MgO into Mg(OH)2, destroying interaction of metallic Cu species with MgO. This led to the increase of Cu (111) crystal planes size by ca. 300% and Cu (220) crystal planes size by ca. 200%. The use of water-ethanol as reaction medium further enhanced aggregation of Cu species. Morphology of the Cu/MgO catalyst changed to rope-like structure and abundant additional meso to large pores were created in water or ethanol-water medium. In comparison, ethanol as medium alone suppressed aggregation of metallic Cu and formation of Mg(OH)2, but re-structure of MgO by enlarging pore size also occurred. The aprotic reaction medium such as acetone, tetrahydrofuran and N,N-dimethyl formamide also affected aggregation of metallic Cu species in different ways.