Hydrogenolysis of xylose to C2-C3 diols over Cu-Cr2O3/Al2O3 catalyst

Abstract

This study is dedicated to the continuous-flow hydrogenolysis of xylose 10 wt. % solutions in a methanol-water mixture to produce ethylene glycol and propylene glycol over Cu-Cr₂O₃/Al₂O₃ catalyst. A series of catalyst samples with varying CuO loadings (13–19 wt. %) were synthesized through wet impregnation of ?-Al₂O₃ support followed by heat treatment at 350 °C for 4 hours and later reduction of CuO by H₂ to metallic copper. Complete reduction of CuO to metallic copper was confirmed by X-ray diffraction analysis of samples. To investigate the acid-base characteristics of obtained catalysts, desorption mass spectrometry was used. Catalytic experiments were conducted in a stainless steel flow reactor with a fixed catalyst bed (4 cm³, ~2.3 g) under conditions of 180 °C, 4.0 MPa H₂ pressure, and a catalyst load of 1.6 mmol C₅H₁₀O₅/g_cat/h. According to product analysis using ¹³C NMR spectroscopy and gas chromatography, propylene glycol and ethylene glycol as main products, along with minor amounts of xylitol, glycerol, hydroxyacetone, and other by-products were identified. The 19Cu-Cr₂O₃/Al₂O₃ catalyst provides complete xylose conversion with productivities of 1.0 mmol/g_cat/h for propylene glycol and 0.7 mmol/g_cat/h for ethylene glycol. Lowering Cu content in Cu-Cr?O?/Al?O? catalyst resulted in decreasing its productivity. At 13 % Cu content, undesirable hydroxyacetone was observed. Noted, the catalyst exhibited consistent activity for 15 h in the methanol-water system, significantly exceeding the 8 h stability observed in 10 wt. % aqueous xylose solutions. This enhanced stability is attributed to improved hydrogen solubility and effective dissolution of intermediate products in the methanol-water mixture that prevents catalyst deactivation.