Insights into the NaCl-Induced Formation of Soluble Humins during Fructose Dehydration to 5-Hydroxymethylfurfural

NaCl is often added to facilitate phase separation for constructing an effective biphasic solvent and promote the dehydration of biomass-derived fructose to 5-hydroxymethylfurfural (HMF). In this work, we demonstrate that the presence of NaCl in water leads to the occurrence of hydrogen bonding between Cl– and fructose, which inhibits the C–C cleavage of fructose, thereby alleviating the formation of unwanted oligomerized side products (i.e., soluble humins) through the degradative condensation pathway. In addition to the above positive effects, the presence of NaCl in the aqueous solution also slows down the mutarotation of fructose, increases the proportion of β-fructopyranose tautomer, and facilitates the aggregation of fructose in solution, thereby favoring the formation of soluble humins via an etherification–dehydration–condensation path. This insightful understanding of the NaCl-induced formation of humins is of great importance for constructing sustainable solvent systems in future for inhibiting the formation of humins in HMF production.