Comparison of the Degradation Performance of Seven Different Choline Chloride-Based DES Systems on Alkaline Lignin

Abstract

Lignin is a natural polymer second only to cellulose in natural reserves, whose structure is an aromatic macromolecule composed of benzene propane monomers connected by chemical bonds such as carbon–carbon bonds and ether bonds. Degradation is one of the ways to achieve the high-value conversion of lignin, among which the heating degradation of lignin by deep eutectic solvent (DES) can be an excellent green degradation method. In this study, choline chloride (CC) was used as the hydrogen bond acceptor, and urea (UR), ethylene glycol (GC), glycerol (GE), acetic acid (AA), methyl acetate mixed acid (MA), oxalic acid (OX), and p-toluenesulfonic acid (TA) were used as hydrogen bond donors to degrade lignin. NMR hydrogen spectroscopy was used for the simple and rapid determination of phenolic hydroxyl groups in lignin. FT-IR spectroscopy was used to characterize the changes of functional groups of lignin during DES treatment. GPC observed the molecular weight of lignin after degradation and found a significant increase in the homogeneity (1.6–2.0) and a significant decrease in the molecular weight Mw (2478–4330) of the regenerated lignin. It was found that acidic DES was more effective in depolymerizing alkaline lignin, especially for the toluene–choline chloride. Seven DES solutions were recovered, and it was found that the recovery of DES still reached more than 80% at the first recovery.

Recent advances in catalytic conversion of lignin to value-added chemicals using ionic liquids and deep eutectic solvents: a critical review

“…As a result, ChCl : PTSA produced larger, whereas ChCl : Gly produced a smaller content of phenolic hydroxyl groups. 157 By employing three distinct DESs, namely ChCl : OA (1 : 1), ChCl : LA (1 : 1), and ChCl : FA (1 : 2), under microwave irradiation at varied temperatures and times, Muley et al 158 were able to deconstruct biomass and depolymerize kraft lignin. According to the results, the maximum lignin yields were obtained with ChCl : FA at 150 °C for 15 min and ChCl : OA DES at 130 °C.…”

Composition of Lignocellulose Hydrolysate in Different Biorefinery Strategies: Nutrients and Inhibitors

“…They are composed of different molar ratios of a hydrogen bond acceptor (such as choline chloride) and a hydrogen bond donor (like lactic acid, urea, ethylene glycol, etc.) [119,120]. The DES pretreatment can increase both the digestibility and solubility of lignocellulose and reduce its resistance to enzymatic digestion; it can also selectively break ether bonds to separate lignins and celluloses, thus deconstructing plant cell walls effectively [121].…”

Structural Characterization of Acid DES-Modified Alkaline Lignin and Evaluation of Antioxidant Properties

“…The intense absorption band at 3405 cm −1 is caused by the O-H stretching vibration of the OH group in the product, which is where we need to focus our attention, and the unique peak signal of the symmetric and asymmetric vibration of the different peaks C-H at 2930 cm −1 . The spectrum exhibits a weaker peak at 1705 cm −1 , suggesting that the DES treatment process may have resulted in more β-O-4′ bond cleavage [23,24]. Consequently, the relatively…”

“…The product obtained from laccase treatment had the highest phenolic hydroxyl content and lower molecular weight, and showed the exceptional antioxidant activity (IC 50 = 28.8 µg/mL), even compared to the commercially available antioxidants BHT (3,5-di-tert-butyl-4-hydroxytoluene, IC 50 = 38.2 µg/mL) and BHA (3-tert-butyl-4-hydroxyanisole, IC 50 = 56.3 µg/mL). Our group also studied the effect of deep eutectic solvents (DES) on the degradation of AL in the early stage and found that the acidic choline chloride-based DES system was effective in obtaining lignin monomers, with more phenolic hydroxyl groups, lower molecular weight and more homogeneity, which have good prospects for improving their antioxidant properties [23,24].…”