Full, Reactive Solubilization of Humin Byproducts by Alkaline Treatment and Characterization of the Alkali-Treated Humins Formed

Abstract

The valorization of the humin by-products 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 13 C solid-state NMR, and pyrolysis-GC-MS 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 is thought to be the reason behind the improved solubility.

Spray Combustion Analysis of Humins

“…Although the humins production process is currently still under development, a higher carbon content and thus higher heating value is preferred when applying this resource for marine fuel applications. Further dehydrogenation and alkaline treatment of humins are being considered by others [44,45], but their economics have yet to be assessed.…”

“…The structure and properties of humins have been studied extensively by Van Zandvoort et al [29,[43][44][45], Constant et al [42] and others [46][47][48][49][50]. In the former two sets of studies, the molecular structure was estimated with techniques including elemental analysis, IR and various 1D and 2D NMR methods.…”

A New Technology for Utilization of Biomass Hydrolysis Residual Humins for Acetic Acid Production

“…Besides, the cleavage of the ether bond is conducive to the reduction of the molecular weight of humins. It was considered that the reduction of molecular weight and formation of polar functional groups were the reasons for the dissolution of humins (van Zandvoort et al 2015). However, the humins conversion ratio resulting from direct alkaline catalytic hydrothermal treatment was only 33% for a holding time of 12 h (Fig.…”

“…Therefore, the presence of NaOH during the wet oxidation was good for humins degradation, because it could decrease the molecular weight of humins and lead to the formation of a homogeneous solution, which is beneficial to the mass transfer of oxidation reactions. However, the dissolution and degradation of humins in reaction conditions need a certain of reaction time, and longer reaction time results in much more low molecular compounds (van Zandvoort et al 2015). On the other hand, H2O2 is unstable at elevated temperatures (Casson et al 2012).…”

“…Hydrothermal alkaline catalytic treatment has been applied for the dissolution and decomposition of water-insoluble macromolecular polymers (e.g., lignin) (Kang et al 2013). This technology has also been reported to be useful in the dissolution and degradation of humins (van Zandvoort et al 2015), which may be beneficial for wet oxidation. Therefore, an original two-step technology involving alkaline catalytic hydrothermal treatment followed by wet oxidation was conducted in this work.…”