Reactive and Mechanistic Insights into the Acid-Catalyzed Conversion of Concentrated C5/C6 Sugars in Ethylene Glycol

It remains a challenging task for the efficient conversion of carbohydrates to value-added chemicals at concentrated feedstock solution. In this contribution, the acid-catalyzed processing of high-concentration C5/C6 sugars (180 g/L) in the ethylene glycol (EG) solvent system was investigated, and prominent discrepancy was noticed between C5 and C6 sugars. A large amount of liquefied products was achieved in the concentrated C6 sugars EG solution, whereas the main products in the case of C5 sugars were found to be solid humins. The reaction mechanism studies revealed that EG can shelter fructose and reaction intermediates (5-hydroxymethylfurfural and levulinic acid) from the cross-polymerization and self-etherification reactions, thereby reducing the generation of solid humins during the fructose liquidation process. However, in the case of xylose, the copolymerization between EG and furfural (FF) generates a solid resin, thus rendering a high solid product yield. Moreover, the produced FF–EG resin was found to be a polymer with high molecular weight formed by connecting the furan ring and the alkyl structure through an ether bond. This finding provides insights into the acid-catalyzed conversion of concentrated C5/C6 sugars in EG, facilitating rationally designed processes for the valorization of concentrated carbohydrates.