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

Abstract

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.