Evolution Process of Humins Derived from Hemicellulose

As one of the three components in lignocellulose, the conversion of hemicellulose was seriously hindered by the formation of humins. In this study, xylan from corn cob was used as the model to study the formation mechanism of humins from hemicellulose (53.4% in yield), whereas a key soluble intermediate of 9 was separated by dialysis with a yield of 0.15%. Furthermore, by means of HPLC-MS/MS analysis, IR, 13C solid-state NMR, gel permeation chromatography (GPC), MALDI-TOF, and SEM, it was proposed that six types of elementary reactions were involved in the formation of xylan-derived humins, including depolymerization, etherification, electrophilic substitution, aldol condensation, dehydration, and thermal oxidation. In the early stage, xylan depolymerization results in xylose, which undertakes continuous dehydration and etherification to form the early humins via the small-molecule mechanism, accompanied by electrophilic substitution of furfural/furfuryl alcohol (FAL) and aldol condensation of the furfural fragment with levulinic acid (LA). In the later stage, xylo-oligosaccharides from xylan depolymerization undergo etherification to form larger polysaccharides, which are incorporated into the humins via the oligomer mechanism. Meanwhile, the degree of dehydration and thermal oxidation deepens, resulting in the enhancement of C═C and C═O conjugation. For the first time, the oligomer mechanism was further visualized by atomic force microscopy (AFM), demonstrating a new approach of humin evolution at the single-molecule level.