High Conversion of Concentrated Sugars to 5-Hydroxymethylfurfural over a Metal-free Carbon Catalyst: Role of Glucose-Fructose Dimers.

To reduce the production cost of chemicals from renewable resources, the feedstock loading must be high and the catalyst must be of low cost and efficient. In this study, at a very short reaction time of 10 min at 125 °C, concentrated sugar solutions (20 wt %, 101 wt % on solvent) were converted to 5-hydroxymethylfurfural (HMF) over a cotton gin trash (CGT)-derived sulfonated carbon catalyst in a 1-butyl-3-methyl-imidazolium chloride ([BMIM]Cl) and 2-methyltetrahydrofuran (MeTHF) biphasic system. We report, for the first time, that the presence of glucose either as a covalently bonded monomer in sucrose or in a mixture with fructose achieved yields of HMF up to 62 mol % compared to a value of only 39 mol % obtained with fructose on its own. In the concentrated reaction medium, glucose, fructose, and sucrose molecules produce difructose anhydrides, dimers/reversion products, and sucrose isomers. The glucose-fructose dimers formed in sucrose and glucose/fructose reaction systems play a critical role in the transformation of the sugars to a higher-than-expected HMF yield. Thus, a strategy of using cellulosic glucose, where it is partially converted to fructose content and the high sugar concentration sugar mixture is then converted to HMF, should be exploited for future biorefineries.