Evolution Process and Controlled Synthesis of Humins with 5-Hydroxymethylfurfural (HMF) as Model Molecule

Elucidation of the chemical structure and formation mechanism of humins is a requisite to further improve the efficiency of acid-catalyzed biomass conversion. Through a low-temperature approach, the key intermediates resulting in the formation of 5-hydroxymethylfurfural (HMF)-derived humins were captured, revealing multiple elementary reactions such as etherification, esterification, aldol condensation, and acetalization. Through humin characterization, it was found out that the aldol condensation moiety between aldehyde group and levulinic acid is critical to justify the characteristic IR peaks (1620 and 1710 cm^-1 ) and aromatic fragments from pyrolysis GC-MS. Based on the investigations by means of HPLC-MS/MS, IR, pyrolysis GC-MS, and SEM, the structural models of humins at different temperatures were proposed, which are comprised of the elementary reaction types confirmed by the key intermediates. Humin structures with varying content of aldol condensation could be controllably synthesized under different reaction conditions (temperature and time), demonstrating the evolution process of HMF-derived humins.