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.

The evolution process of humins derived from glucose was studied dynamically from the feedstock, the intermediates in solution, till humins solid formed. The key intermediates resulting in the formation of humins were identified, unveiling two elementary reactions including etherification of dehydrated glucose and electrophilic substitution of furfuryl alcohol (FAL). The characteristic IR peaks at 1583 and 1616 cm−1 could be attributed to the C=C conjugated with C=C and the C=C conjugated with C=O, respectively, whereas the shift from 1583 to 1616 cm−1 could be promoted by O2 or adding kojic acid. Unexpectedly, the participations of HMF and small aliphatic acids were excluded. Herein, through characterizations such as HPLC‐MS/MS, IR and SEM, the structural models of glucose‐derived humins at different temperatures and a successive growth diagram were proposed.

The Cover Feature shows the evolution process of HMF‐derived humins. By the low‐temperature approach, the key intermediates towards HMF‐derived humins were captured, revealing multiple elementary reactions such as etherification, esterification, aldol condensation, and acetalization. Furthermore, the humins structures with varying content of aldol condensation could be controllably synthesized at different temperatures and times. More information can be found in the Full Paper by H. Shen et al. on page 513 in Issue 3, 2020 (DOI: 10.1002/cssc.201902799).