Sustainable production of gluconic acid and glucuronic acid via microwave-assisted glucose oxidation over low-cost Cu-biochar catalysts

Biomass valorization to chemicals is an important application of this renewable resource. This study reports the production of two versatile value-added organic acids, i.e., gluconic acid (GOA) and glucuronic acid (GUA), from biomass-derived glucose via microwave-assisted catalytic oxidation. A series of novel low-cost biochar-supported Cu-based catalysts prepared under various synthesis conditions were studied with a view to facilitate glucose conversion and improve the product selectivity by tuning the CuO_x species and functionality of the biochar support. The yield of GUA and GOA could reach up to 39.0% and 30.7%, respectively, within 20 min at 160 °C over the CuBC600N catalysts. The enhanced catalytic performance is mainly attributed to a high percentage of Cu and Cu₂O species and abundant O-containing functional groups as well as the less π-π conjugated area of the biochar support. The reaction pathways were elucidated by density functional theory (DFT) simulation. It is found that Cu₂O and Cu sites are favourable for glucose ring-opening and oxidation steps, showing synergistic effects for oxidation of cyclic glucose to GOA and GUA. In the third run of the reaction, the catalytic performance was still 70% higher than those of bare biochar catalysts, demonstrating the importance of the CuO_x species stabilized on the biochar support in catalytic activity improvement. Based on these findings, this study can provide valuable information for the design of low-cost and microwave-absorbable Cu-biochar catalysts for high-efficiency glucose oxidation.