Evolution Process of Humins Derived from Xylose

The rapid development of xylose valorization was severely retarded by the generation of humins. The key intermediate during the formation of xylose-derived humins was successfully captured and identified to be xylobiose through single-crystal analysis. By means of high-performance liquid chromatography–tandem mass spectroscopy, Fourier transform infrared spectroscopy, 13C solid-state nuclear magnetic resonance, elemental analyses, and scanning electron microscopy characterizations, it was speculated that multiple elementary reactions were involved in the formation of xylose-derived humins, such as etherification of xylose, electrophilic substitution, dehydration, and thermal oxidation. The etherification of xylose and electrophilic substitution of furfural occur in the early stage. In the later stage, the aldehyde group of furfural fragments was converted by the electrophilic substitution of furfural alcohol (FAL). Meanwhile, dehydration and thermal oxidation continued to occur, leading to the humins structure being abundant in conjugated CC and CO groups, which is free of the aldehyde group.