Carbohydrates, 5-hydroxymethylfurfural, Reaction network, Soluble humins, Tautomer, Renewable energy sources, TJ807-830, Ecology, QH540-549.5

The formation of humins hampers the large-scale production of 5-hydroxymethylfurfural (HMF) in biorefinery. Here, a detailed reaction network of humin formation at the initial stage of fructose-to-HMF dehydration in water is delineated by combined experimental, spectroscopic, and theoretical studies. Three bimolecular reaction pathways to build up soluble humins are demonstrated. That is, the intermolecular etherification of β-furanose at room temperature initiates the C12 path, whereas the C–C cleavage of α-furanose at 130–150 °C leads to C11 path, and that of open-chain fructose at 180 °C to C11′ path. The successive intramolecular dehydrations and condensations of the as-formed bimolecular intermediates lead to three types of soluble humins. We show that the C12 path could be restrained by using HCl or AlCl3 catalyst, and both the C12 and C11′ paths could be effectively inhibited by adding THF as a co-solvent or accelerating heating rate via microwave heating.