Production of propylene glycol (1,2-propanediol) by the hydrogenolysis of glycerol in a fixed-bed downflow tubular reactor over a highly effective Cu–Zn bifunctional catalyst: effect of an acidic/basic support

Abstract

In this study, different acidic (V₂O₅, ZrO₂, and TiO₂) and basic (CaO and MgO) oxide-supported copper–zinc bimetallic catalysts were prepared by a deposition–precipitation method, and they were evaluated for the vapor-phase hydrogenolysis of glycerol to propylene glycol at 0.1 MPa and at 220 °C. The catalysts were thoroughly characterized by different techniques such as BET, XRD, H₂-TPR, NH₃ and CO₂ TPD, N₂O adsorptive decomposition, TEM, XPS, FE-SEM and TGA. Among all the supported catalysts, the Cu–Zn/MgO catalyst was found to be the most selective to propylene glycol. High copper-metal dispersion (∼5%), surface area (∼23 m² g⁻¹), the highest basicity (0.25 mmol CO₂ g cat⁻¹) and the availability of partially reduced copper species (Cu₂O, CuO and Cu⁰) were the primary reasons for the higher propylene glycol selectivity. At optimum reaction conditions, i.e., at 220 °C, 0.72 MPa and a weight hourly space velocity (WHSV) of 0.073 h⁻¹, ∼98.5% conversion of glycerol with ∼89% selectivity to propylene glycol was obtained over the Cu–Zn/MgO catalyst. This catalyst was also found to be stable for a long period of time (84 h) without much deactivation and without decrease in the selectivity to propylene glycol.