Full, reactive solubilization of humin byproducts by alkaline treatment and characterization of the alkali-treated humins formed

Abstract

The valorization of the humin byproducts that are formed during hydrothermal, acid-catalyzed dehydration of carbohydrates is hampered by the insolubility of these byproducts. Here, we report on an alkaline pretreatment method that allows for the insolubility of this highly recalcitrant and structurally complex feed to be overcome. The reactive solubilization of glucose-derived humins was found to require a treatment at 200 °C in 0.5 M NaOH for 3.5 h. Fructose- and xylose-derived humins were found to be more recalcitrant, and complete dissolution required raising the temperature to 240 °C. Gel permeation chromatographic analyses show the relative average molecular weight of the now soluble humins to decrease with increasing temperature and reaction time. The alkali-treated humins are soluble in water of pH 7. Elemental analysis, IR, 2D PASS 13C solid-state NMR and pyrolysis-GC-MS (gas chromatography-mass spectrometry) data indicate that the alkaline pretreatment leads to considerable changes in the molecular structure of the humins. Cleavage of C-O-C bonds and further aromatization of the originally highly furanic humins result in the formation of (polycyclic) aromatic structures decorated with carboxylic acids. The combination of the reduction in Mw and the formation of polar functional groups are thought to be the reasons behind the improved solubility.