Probing the effects of fructose concentration on the evolution of humins during fructose dehydration

5-Hydroxymethylfurfural (HMF), considered as a “sleeping giant” of sustainable chemistry, is generally produced by fructose dehydration. Till now, high HMF yields have been achieved, whereas large-scale production of HMF is hampered by the formation of undesired humins, especially at higher fructose concentrations (>10 wt%). In this work, we report the effects of fructose concentration (4.5–360.0 wt%) on the evolution pathways of humins during the H2SO4-catalyzed dehydration of fructose in water. We show that both etherification–dehydration–condensation and degradative condensation of fructose and/or HMF are involved in the formation of humins, wherein the increase of fructose concentration promotes the former path and inhibits the latter one because of the promotional effect on the formation of difructose anhydride (DFA) species. The progressive dehydrations and condensations of DFAs under experimental conditions lead to humins, but the reversible hydrolysis of DFAs to fructose favors the HMF formation. Further, we demonstrate that the addition of a typical polar aprotic solvent such as tetrahydrofuran (THF) or 1,4-dioxane (DIO) to water as a co-solvent could stabilize the DFA species and increase the HMF yield by more than 10% in the conversion of high-concentration fructose (72.0 wt%). This understanding provides an indispensable insight on factors influencing humin formation for future advances on HMF biorefineries.