Synthesis of a Thermally and Hydrothermally Stable Copper-Based Catalyst via Alloying of Cu with Ni and Zn for Catalyzing Conversion of Furfural into Cyclopentanone

Copper-based catalysts are active for hydrogenation, while they are prone to sintering in hydrothermal conditions. In this study, alloying Cu with Ni and Zn was adopted to suppress sintering of copper species in the catalyzed conversion of furfural to cyclopentanone (CPO) and cyclopentanol (CPL) in the aqueous phase. The results indicated that Zn acted as a "glue" for aiding the formation of CuZn or CuNiZn alloy. The formation of the ternary CuNiZn alloy in CuNiZn/Al2O3 catalyst created the developed porous structure, induced the formation of abundant Lewis acidic sites and Brønsted acidic sites, and enhanced the dispersion and the total surface area of Cu species. These together rendered CuNiZn/Al2O3 with superior activity to monometallic or bimetallic Cu-based catalysts for the production of CPO and CPL (sum yield of 86% over Cu1.0Ni1.0Zn1.0/Al2O3), which was attributed to the synergy between CuNiZn alloy and Brønsted acidic sites. Alloying Ni with Cu and Zn weakened the adsorption of the C═C bond in the furan ring of furfural, while enhancing the activation of C═O and C–O–C bonds in furfural, avoiding the deep hydrogenation of the furan ring, facilitating the rearrangement of furfuryl alcohol to CPO, and improving the turnover frequency for the production of CPO from furfural remarkably (1011.3 h–1 over Cu1.0Ni1.0Zn1.0/Al2O3 vs 23.9 h–1 over Cu1.0/Al2O3). The formation of ternary CuNiZn alloy could not only suppress the sintering of metallic Cu species during the reduction in hydrogen but also effectively prevent the aggregation of Cu species in the aqueous phase, enhancing the catalytic stability under hydrothermal conditions.