The Effect of Acidic Ternary Deep Eutectic Solvent Treatment on Native Lignin

Abstract

Ternary deep eutectic solvents (DESs) are gaining increased attention to serve as a cheap green alternative medium for the processing of lignocellulosic biomass. For example, mixtures of choline chloride (ChCl), ethylene glycol (EG), and oxalic acid (OA) were recently explored for the fractionation of lignocellulosic biomass into its main components. Interestingly, during this processing, the recovered lignin was structurally modified by incorporation of EG, which altered its solubility properties and led to the need for different lignin recovery strategies. This offers an excellent starting point for a deeper investigation of the effect of acidic DES systems on the structure of lignin. In particular, nativelike residual enzyme lignins (RELs) that are hard to completely dissolve in organic solvents are specifically suitable for this task. Here, a ternary DES is used consisting of ChCl/EG with OA or trifluoromethanesulfonic acid (HOTf) as a third component. The results showed that both solvent systems led to high EG incorporation into REL. The HOTf system showed a lesser extent of lignin depolymerization at similar modification levels as it already induced modification at lower temperature (25−30 °C). Low recovery yields from typical acidic precipitation were observed for treatment with both acidic DES systems. Analysis of THF and DCM extracts showed that the products in the water phase included small EG modified lignin fragments and aromatic monomers released from lignin aryl ether linkage cleavage. This analysis details the types of other products that can be expected and where these will end up during fractionation. These results show that the treatment of lignin with acidic DES in the presence of alcohols leads to low-and high-molecular-weight products that are not effectively recovered by typical precipitation procedures.

Physicochemical Properties, Thermal Stability, and Pyrolysis Behavior of Antioxidative Lignin from Water Chestnut Shell Obtained with Ternary Deep Eutectic Solvents

“…This result may be because the main connecting bonds (ether bonds) in the lignin were not destroyed, thus preserving its original structure. Previous studies reported that the addition of ethylene glycol to DES can effectively preserve the β- O -4 bond in lignin and inhibit lignin condensation [ 26 , 27 ]. Moreover, the polydispersity index (PDI) can directly reflect homogeneity in the molecular weight distribution of lignin; when the PDI is lower than 3, lignin is considered to be a homogeneous polymer [ 28 ].…”

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

“…4-Propyl catechol was selectively extracted through the depolymerization of lignin. 153 Wang et al 154 explored a brilliant idea to process 4 different lignocellulosic biomasses (birch, pine, reed, and walnut) with ternary DESs composed of ChCl : EG with OA and HOTf at different reaction conditions. The discussed DESs are in situ capable of incorporating the EG (80-85%), and large linkage content (60-80% aryl ether linkage) into the residual enzymatic lignin.…”

Oxidized cellulose as bio‐adhesive for hydrophobic lignocellulosic bioplastic

“…The rich hydrogen bonding and acidity of acid DES swell the structure of corn stover by disrupting the hydrogen bonding between cellulose fibers and dissolving hemicellulose and acid-insoluble lignin. [24][25][26] 2D-HSQC NMR can determine the specific carbon-hydrogen functionalities to identify the structural features of low molecular weight lignin fragments, which can predict the cleavage of bonds. The aromatic region (δ C /δ H 135-95/6.3-8.0) and side-chain region (δ C /δ H 90-50/3.0-5.5) of the HSQC NMR spectra of DES extracted lignin and low-alkali extracted lignin are shown in Figure 2A,B.…”