Catalytic transfer hydrogenation of ethyl levulinate to γ-valerolactone over zirconium (IV) Schiff base complexes on mesoporous silica with isopropanol as hydrogen source

Zr(IV)-salphen-MCM-41 was prepared by immobilizing a zirconium(IV) Schiff base complex onto the surface of MCM-41. The structural properties of the heterogeneous catalysts were characterized by FTIR, UV, TG, XRD, SEM, TEM, BET, EDX, CO2-TPD, NH3-TPD and ICP-OES. Studies on the catalytic properties of the immobilized Zr(IV)-Schiff base complexes were performed using the reduction and cyclization of ethyl levulinate (EL) to gamma-valerolactone (GVL) with isopropanol as the hydrogen source. Various reaction parameters, such as temperature, reaction time, solvent and catalyst loading were carefully investigated. The optimal conditions (GVL yield up to 90%) involved the utilization of Zr(Salphen)-MCM-41 as the catalyst in isopropanol. The key to heightened reaction performance was the incorporation of the phenolate and aniline groups in the ligands of bound to Zr. This increased the basicity of the catalyst, which subsequently enhances the efficiency of the transfer hydrogenation reported herein. Meanwhile, the immobilized catalyst can be easily separated and efficiently recycled at least five times without significant loss of activity. Finally, we proposed a plausible reaction mechanism based on experimental and characterization data. (C) 2017 Elsevier B.V. All rights reserved.