Unveiling the Migration and Transformation Mechanism of Lignin in Eucalyptus During Deep Eutectic Solvent Pretreatment

Abstract

Deep eutectic solvents (DESs) have unique advantages in biomass conversion. However, the migration and transformation mechanism of lignin in the cell wall during the DES pretreatment is still elusive. In this work, Eucalyptus blocks were pretreated in choline chloride/lactic acid DES to reveal the lignin migration. Meanwhile, the remaining lignin in the pretreated residue, the regenerated DES lignin, and the solubilized degraded lignin in the recovered DES were investigated to decipher the lignin transformation. Results showed that the DES pretreatment resulted in the penetration of DES from the cell lumen to the cell wall, and lignin in the secondary wall was more easily dissolved than that in the cell corner middle lamella. The syringyl unit of lignin was better stabilized in the DES than the guaiacyl unit of lignin. The condensed lignin fraction mainly remained in the pretreated residue, while the solubilized degraded lignin fraction was monomeric aromatic ketone compounds. This study elucidates the fate of lignin during the DES pretreatment, which could also promote the development of a modern lignocellulosic pretreatment technique.

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

“…During the subsequent degradation at 350–400 °C, the degradation rate of lignin mainly depends on the side-chain oxidation of lignin, such as carbonylation, carboxylation, and dehydrogenation reactions. At temperatures greater than 400 °C, the rate of pyrolysis gradually slows down, indicating a relatively stable aromatic ring structure and slow weight loss, with a series of reactions occurring mainly on the aromatic ring of lignin, such as saturation of the aromatic ring, carbon–carbon bond breakage, and degradation of lignin to CO 2 , CO, and H 2 O [ 41 ]. At a given endpoint of testing at 700 °C, the weight loss was 71.45% for TA/CC, 49.09% for alkaline lignin, and ranged from 41.71% to 46.51% for GE/CC, OX/CC, GC/CC, MA/CC, UR/CC, and AA/CC.…”

Ultrasound‐assisted formic acid–choline chloride deep eutectic solvent pretreatment of cotton straw to extracted lignin

“…22 In all the above studies, acidic DES was used to pretreat lignocellulose, which showed that acidic DES could effectively realize the fractionation of biomass. 23 Although DES has many advantages in the process of separating lignocellulose, low lignin yield and long processing time are still the main problems that limit its industrial development. The traditional treatment (such as heat treatment) requires a longer time to achieve better results.…”

“…Tan et al used DESs with lactic maleate citrate as HBD and choline chloride as HBA to treat empty fruit clusters of oil palm, and found that all acidic DESs pretreatments removed a large amount of xylan 22 . In all the above studies, acidic DES was used to pretreat lignocellulose, which showed that acidic DES could effectively realize the fractionation of biomass 23 …”

New Parameters to Model Microwave-Assisted Deep Eutectic Solvent Extraction of Lignin Using Analytical Pyrolysis–GC/MS

“…26,27 In most of the available literature, optimal extraction conditions are established by determining the mass yield of lignin as a function of experimental parameters. Structural features of the lignins obtained under optimal conditions are evaluated with infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, 12,28,29 thermogravimetric (TG) analysis, 19 size-exclusion or gel permeation chromatography, 26 and analytical pyrolysis−gas chromatography−mass spectrometry (Py−GC/MS). 15,20,30 However, very limited information is available on how these features are affected by the experimental parameters.…”

“…In most of the available literature, optimal extraction conditions are established by determining the mass yield of lignin as a function of experimental parameters. Structural features of the lignins obtained under optimal conditions are evaluated with infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, ,, thermogravimetric (TG) analysis, size-exclusion or gel permeation chromatography, and analytical pyrolysis–gas chromatography–mass spectrometry (Py–GC/MS). ,, However, very limited information is available on how these features are affected by the experimental parameters . A more detailed knowledge of the structure and composition of DES lignin as a function of the process parameters could disclose new strategies to obtain lignin with desirable properties that could prove more suitable for specific upgrading or valorization strategies.…”