Kraft Lignin Solubility and Its Chemical Modification in Deep Eutectic Solvents

Abstract

Lignin stands as a promising raw material to produce commodities and specialty chemicals, yet its poor solubility remains a big challenge. Recently, deep eutectic solvents (DES) have been proposed as sustainable solvents with high potential to dissolve and valorize lignin. In the present study, the ability of DES based on cholinium chloride ([Ch]­Cl) combined with alcohols and carboxylic acids as hydrogen bond donors (HBDs) to dissolve kraft lignin and to change its chemical structure was examined. The influence of the chemical nature of HBDs, water content, and HBD:hydrogen bond acceptor (HBA) molar ratio on the solubility of kraft lignin in DES was studied (313.15 K). The kraft lignin solubility was enhanced by increasing both the HBD’s carbon chain length and the molar ratio, with [Ch]­Cl:HEXA (1,6-hexanediol) and [Ch]­Cl:MaleA (maleic acid) being the best studied solvents for kraft lignin dissolution, while the addition of water was a negative factor. The thermal treatments (393.15 K) of kraft lignin show that carboxylic acid-based DES promote chemical modifications to kraft lignin, including the disruption of several C–O covalent type bonds (e.g., β-O-4, α-O-4 and α-O-α), while alcohol-based DES were found to be nonderivatizing solvents maintaining the lignin chemical structure. These results show the versatility of DES, which, depending on their chemical nature, may offer distinct strategies for lignin valorization.

Biomass delignification with green solvents towards lignin valorisation: ionic liquids vs deep eutectic solvents

“…Although lignin dissolution trials give some perspectives about the required physicochemical properties of DES to perform biomass delignification, their capacity to disrupt the biomass intricate matrix is crucial. For example, the ability of alcohol-based DES, such as [Ch]Cl:glycerol and [Ch]Cl:ethylene glycol, to dissolve high amount of lignin was manifested elsewhere [58]. However, their capacity to extract lignin from biomass is moderate to low.…”

Deep Eutectic Solvents for the Valorisation of Lignocellulosic Biomasses towards Fine Chemicals

“…When water is added to the DES, all HBDs and HBAs are hydrated. Small anions, such as halides, become fully solvated, losing the supramolecular order depicted in Figure 5 and the relevant properties, especially in highly diluted mixtures [50,51].…”

Lignin Nanoparticles and Alginate Gel Beads: Preparation, Characterization and Removal of Methylene Blue

“…In this regard, a simple method based on green solvents along with environmental friendliness and easy handling is highly desirable, and deep eutectic solvents (DES), formed through the combination of hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD), stand out as viable alternatives. In recent years, DES gained considerable attention as a sustainable solution with high potential to dissolve and valorize lignin [ 34 , 35 , 36 , 37 , 38 , 39 ] but with little-to-no exploration of industrial lignins.…”