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 CuOx 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 degrees 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 pi-pi 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 CuOx 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.