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

Abstract

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.

Dual Metal–Acid Pd-Br Catalyst for Selective Hydrodeoxygenation of 5-Hydroxymethylfurfural (HMF) to 2,5-Dimethylfuran at Ambient Temperature

“…The by-products formation can also lead to catalyst deactivation. Indeed, even heating of HMF to 100 o C in the presence of water leads to generation of humins [34] .…”

“…The homogenous acids provide intimacy of acid sites around the metal surface, which could protonate the substrate hydroxyl group and polarize the C-O bond and favor the cleavage of the C-O bond over Pd [58] . However, the addition of homogeneous organic or inorganic acid at the same time could result in the side self-condensation of HMF, which lowers the DMF productivity [34] . Besides, the presence of liquid acids in the reaction mixture leads to the corrosion of equipment.…”

LiCl-promoted-dehydration of fructose-based carbohydrates into 5-hydroxymethylfurfural in isopropanol

“…Meanwhile, the HMF yield presented an increasing trend as the reaction time prolonged, and reached the peak value of 79.1% at 2.0 h. Further prolonging the reaction time, the HMF yield declined, reecting the weak stability of HMF at higher reaction temperature (>130 C) due to some sidereactions occurred as reaction time extended. 30,43 Hence, to obtain the higher HMF yield, the optimized reaction temperature and time would be set to 130 C for 2 h.…”

Hydrolysis of Sucrose over Sulfonic Acid Resins

“…Key compounds of that complex system are 5‐hydroxymethylfurural, 2‐hydroxyacetylfuran, and 3,5‐Dihydroxy‐2‐methyl‐5,6‐dihydropyran‐4‐one [68] . Formation of caramel colors is related to the formation of humins currently of interest as by‐product in the isolation of bulk HMF [82] . The color of an acid HMF mixture changes from dark brown to black during storage, until the emergence of humic solids [82] .…”

“…Formation of caramel colors is related to the formation of humins currently of interest as by‐product in the isolation of bulk HMF [82] . The color of an acid HMF mixture changes from dark brown to black during storage, until the emergence of humic solids [82] . Caramelization can therefore be regarded as a very early stage of the evolution of high polymeric humins [83] …”