Carbon materials as microwave absorbers for microwave-assisted conversion of sugars to 5-hydroxymethylfurfural in dimethyl carbonate-water solvents

The microwave-assisted process intensification of the conversion of sugars represents a promising approach for the energy-efficient production of value-added chemicals and fuels. In this study, we employed organic-water solvents utilizing dimethyl carbonate (DMC) as the organic phase, with hydrochloric acid (HCl) or citric acid (CA) as catalysts, alongside multi-walled carbon nanotube (MWCNT) or multi-layered graphene (MLG) as microwave absorbers to facilitate the microwave-assisted conversion of sugars (sucrose, fructose, glucose, and cellulose) into 5-hydroxymethylfurfural (HMF). Additionally, the influence of reaction temperature, reaction time, catalyst dosage, and absorber dosage on HMF synthesis was investigated. A volume ratio of DMC to water at 1:1—using CA as catalyst and MWCNT as absorber under microwave heating at 200 °C for 20 min—yielded fructose-to-HMF conversion with yield reaching 73.5 % and selectivity at 74.5 %. The use of a DMC-water biphasic solvent effectively reduces side reactions, thereby significantly enhancing the selectivity towards HMF. The incorporation of MWCNT, with its superior microwave absorption properties and large specific surface area, boosts the yield of HMF by up to 18.1 %, and notably MWCNT can be reused at least three times with sustained absorption performance.