A universal solvent effect on the formation of soluble humins in glucose dehydration to 5-hydroxymethylfurfural

Solvents play an important role in controlling the yield and selectivity in carbohydrate conversion. In this work, we report a universal solvent-controlled formation of unwanted soluble humins via degradative-condensation of glucose at the initial stage of glucose dehydration to 5-hydroxymethylfurfural (HMF). We show that the existence of a small amount of polar aprotic organic solvent in water slowed down the mutarotation of glucose, mainly leading to a C1-C2 cleavage of glucose to formic acid and subsequent condensation of C5-fragments with glucose to soluble humins. A miscible solvent favored this humin formation in both catalyst-free and Lewis acid-catalyzed systems because of its facile hydrogen-bonding with water that hampered glucose mutarotation, whereas the more protophilic solvent tended to suppress humin formation on virtue of its reduction on catalyzing C1-C2 cleavage. This work highlights the importance of deliberately tailoring the solvent composition to inhibit the undesired side-product of humins in HMF production. This fundamental understanding of the solvent-controlled formation of soluble humins in glucose dehydration highlights the importance of deliberately tailoring the solvent composition to inhibit the formation of undesired humins in HMF production.