Evolution process of humins derived from cellulose by a humin extraction approach†

Cellulose utilization has been seriously hindered by the formation of humins, while the humin structure remains challenging since unconverted cellulose and humins exist as a solid mixture. In this work, we developed a novel strategy to extract humins from unconverted cellulose and disclosed the structural evolution process of cellulose-derived humins for the first time. The key intermediate levoglucosan was successfully captured and identified, which significantly favors the formation of anhydro-sugars followed by polymerization due to its stability. By means of comprehensive HPLC-MS/MS, FT-IR, MALDI-TOF and SEM characterization studies, it was proposed that multiple elementary reactions were involved in the formation of cellulose-derived humins, including cellulose depolymerization, etherification, esterification, aldol condensation, dehydration and thermal oxidation. In the early stage, cellulose depolymerization results in glucose and levoglucosan (LG), which undergo etherification to form the early humins via a small molecule mechanism, accompanied by esterification and dehydration. In the later stage, glucooligosaccharides especially with the LG end from cellulose depolymerization undergo etherification via an oligomer mechanism. Meanwhile, etherification of HMF and aldol condensation with LA take place prominently, together with dehydration and oxidation, resulting in the enhancement of CC and CO conjugation.