Sequential extraction and separation of soluble humins from fructose conversion for structural and evolutional understanding

Abstract

Understanding the evolution, structure, and separation of humins (i.e., carbonaceous polymeric side-products) during the acid-catalyzed conversion of fructose, especially at high substrate loading, is of crucial importance for the future inhibition or application of humins. Herein, we report the sequential extraction and separation of soluble humins formed in the acid-catalyzed conversion of high-concentration fructose (20 wt%) to 5-ethoxymethylfurfural (EMF) and 5-hydroxymethylfurfural (HMF) in a mixed solvent of ethanol and 1,4-dioxane under microwave heating to understand the structural characteristics and evolution pathways of humins. We demonstrate the formation of four types of soluble humins during fructose-to-EMF/HMF conversion, wherein three of them were primarily composed of furan rings imported by self-etherification/hemiacetalation of HMF and subsequent aldol-condensation with ethyl levulinate (named humins I); cross-etherification between HMF, fructose, and ethanol (named humins III); and self-etherification/hemiacetalation of HMF and subsequent hemiacetalation with ethanol (named humins IV). In comparison, another type of humins (named humins II) was composed of fructose units introduced via self-etherification and subsequent dehydrations, wherein the as-formed difructose anhydride intermediates can be reversibly transformed back to fructose and finally into EMF and HMF by extending the reaction time, increasing the reaction temperature, or increasing the amount of the acid catalyst used. The above-mentioned structural diversity of humins accounted for their difference in polarity (II > III > IV > I) and allowed their sequential extraction and separation from the product mixture. This separation offers an opportunity to understand humin chemistry for the future sustainable production of value-added platform chemicals in biorefineries.