Hydrogenation of furfural to 1,5-pentanediol over CuCo bimetallic catalysts

A series of Cu_xCo_3–xAl hydrotalcite-like catalysts with different Cu/Co molar ratios were synthesized by the urea homogeneous precipitation method and used for the direct hydrogenation–hydrogenolysis of furfural to 1,5-pentanediol. The results showed that the Cu/Co molar ratio of the catalyst had a significant effect on its textural properties and catalytic performance. The catalyst exhibited excellent catalytic performance when the molar ratio of Cu/Co was 1∶29 (Cu_0.1Co_2.9Al), and the conversion of furfural was 100% together with 51.1% yield of pentanediol among which the yield of 1,5-pentanediol was 41.1%, under the reaction condition of 140^oC, 4 MPa H₂ for 6 h. Extensive characterization techniques, including temperature-programmed reduction (H₂-TPR), temperature-programmed desorption (H₂-TPD), X-Ray photoelectron spectroscopy (XPS) and Raman confirmed that the excellent catalytic activity of Cu_0.1Co_2.9Al catalyst was attributed to the highest content of Cu⁰ and CoO_x on its surface, and a synergistic catalytic effect was present between them. Typically, Cu⁰ was used to adsorb and activate H₂, and CoO_x with much oxygen vacancies promoted the adsorption and activation of C=O groups in furfural molecules, leading to the quick conversion of furfural to furfuryl alcohol. In addition, the oxygen vacancies anchored the –OH in the intermediate furfuryl alcohol to produce a C2-terminal oblique adsorption on the catalyst surface. Then it promoted the hydrogenation of C2=C3 with the weakening and cleavage of C2–O1 bond, and enhanced the selectivity of 1,5-pentanediol.