A non-alkoxide sol–gel route to highly active and selective Cu–Cr catalysts for glycerol conversion

Abstract

A non-alkoxide sol–gel route to highly active and selective Cu–Cr catalysts for glycerol conversion is presented. The addition of propylene oxide to ethanol solutions of Cr(NO₃)₃·9H₂O and Cu(NO₃)₂·3H₂O resulted in the formation of transparent Cu–Cr gels. The resulting gels were converted to the Cu–Cr catalysts by atmospheric drying and calcination. The Cu–Cr catalysts are characterized by X-ray diffraction (XRD), N₂ physisorption, temperature-programmed reduction (TPR), and transmission electron microscopy (TEM). The results show that the surface area of the Cu–Cr catalyst is adjusted by the hydrolysis conditions, Cu/Cr molar ratio, and treatment conditions (such as gas atmosphere and final temperature). For the sample with Cu/Cr = 0.5, the surface area of Cu–Cr xerogel can reach 94 m²/g and decreased to only 31 m²/g after calcination at 500 °C. The catalysts show significant catalytic activity and selectivity in glycerol conversion, i.e. above 52% conversion of glycerol and above 88% selectivity to 1,2-propanediol at 210 °C and 4.15 MPa H₂ pressure. CuCr₂O₄ supported Cu catalysts are much more active than Cr₂O₃ supported Cu catalysts. This indicates a strong interaction between Cu and CuCr₂O₄ that is significantly improving the effectiveness of the catalyst for glycerol conversion.