High-Performance Bifunctional Cu–Zn/γ-Al2O3 Chromium-Free Catalysts for Selective Hydrogenolysis of Glycerol to 1,2-Propanediol

Abstract

Glycerol, a major biodiesel byproduct, represents a renewable feedstock for producing high-value chemicals such as 1,2-propanediol (1,2-PDO) via selective hydrogenolysis. However, current catalysts often suffer from insufficient selectivity, limited stability, or reliance on toxic promoters, limiting industrial adoption. This study presents the development of a high-performance, chromium-free bifunctional Cu–Zn/γ-Al2O3 catalyst through systematic optimization, combining one-factor-at-a-time screening and multivariate statistical analysis. Catalysts were prepared by varying the support type (pseudoboehmite-derived γ-Al2O3), copper precursor, promoters (e.g., Zn), peptizing agents, organic additives (e.g., PEG-4), and metal loadings, followed by characterization via XRD, N2 physisorption, H2-TPR, NH3-TPD, SEM-EDS, ICP-OES, and N2O chemisorption. Performance was evaluated in continuous-flow fixed-bed reactors under industrially relevant conditions (220–240 °C, 8 MPa, 50 cm3 catalyst bed, LHSV = 0.33 h–1). The optimized formulation (15%Cu–1%Zn/γ-Al2O3 prepared from Cu(NO3)2 precursor with 3% PEG-4 as additive) achieved a very high TOF of 2136 h–1, 86.5% glycerol conversion, and 95.9% selectivity to 1,2-propanediol (1,2-PDO) maintained after 1000 h on stream at 220 °C and 8 MPa, with only minor copper leaching (8%) and no detectable coke formation or sintering. Successful scale-up to a 200 cm3 catalyst bed was also demonstrated. These results—superior long-term stability under mild conditions—highlight the catalyst's potential for sustainable, bio-based 1,2-propanediol production and provide mechanistic insights into the roles of metal dispersion, acidity, and promoter effects in glycerol hydrogenolysis.