📄 chemistry

Influence Mechanism of Carboxylic Acid Hydrogen Bond Donors on 2-Methylimidazole-Based Deep Eutectic Solvent Pretreatment of Glycyrrhiza Residue

This study demonstrates that a deep eutectic solvent composed of 2-methylimidazole and lactic acid, characterized by a low net hydrogen bond acidity, effectively pretreats glycyrrhiza residue by selectively removing lignin and modifying its structure to significantly enhance enzymatic hydrolysis efficiency.

Original authors: Yu Zhang, Mengqiao Cao, Yinghu Guo, Wei Liu, Shiyou Xing, Xuesong Tan, Xiaoyan Chen

Published 2026-06-26

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Yu Zhang, Mengqiao Cao, Yinghu Guo, Wei Liu, Shiyou Xing, Xuesong Tan, Xiaoyan Chen

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Unlocking a Tough Nut

Imagine Glycyrrhiza residue (the leftover bits from making licorice medicine) as a tightly packed bundle of sticks. Inside this bundle, you have three main ingredients:

Cellulose (Glucan): The valuable "sugar sticks" we want to turn into fuel or chemicals.
Lignin: The tough, sticky "glue" and "armor" that holds the bundle together and protects the sugar.
Hemicellulose: The filler material between the sticks.

The problem is that the "glue" (lignin) is so strong it hides the "sugar sticks" (cellulose), making it impossible for enzymes to get to them. To fix this, scientists need to break the bundle apart without breaking the sugar sticks themselves.

The Solution: A "Smart Solvent" Team

The researchers created a special liquid mixture called a Deep Eutectic Solvent (DES). Think of this not as a single chemical, but as a team of two partners working together:

The Host (2-Methylimidazole): A stable base that holds the team together.
The Guest (Carboxylic Acid): A variable partner that does the heavy lifting. The researchers tried many different "Guests" (like acetic acid, lactic acid, benzoic acid, etc.) to see which one was the best at breaking the glue without hurting the sugar.

The Experiment: Finding the Perfect "Guest"

The team tested 10 different types of carboxylic acids, changing their shapes and features (like adding a methyl group, a hydroxyl group, or changing the length of their carbon chain). They wanted to see which "Guest" could:

Dissolve the most lignin (remove the glue).
Keep the most glucan (save the sugar).

The Winner:
The best performer was a team made of Lactic Acid (a common acid found in sour milk) and the Host.

The Result: This team removed 61.6% of the tough lignin glue while saving 90.9% of the valuable sugar.
Why it won: Other acids were either too weak (didn't remove enough glue) or too aggressive (dissolved the glue but also ate away some of the sugar). Lactic acid found the "Goldilocks" zone.

The Secret Sauce: The "Personality" of the Liquid

The researchers didn't just guess; they measured the "personality" of these liquids using something called Kamlet-Taft parameters. Imagine these as personality traits:

Acidity (α): How "aggressive" the liquid is at attacking bonds.
Basicity (β): How "protective" the liquid is of the sugar.
The Net Score (α - β): This is the most important number.

The Analogy:
Think of the pretreatment like a dance.

If the liquid is too aggressive (high acidity), it grabs the sugar sticks and breaks them.
If it's too passive, it can't break the glue.
The Lactic Acid team had a perfect balance. It was aggressive enough to break the lignin glue but gentle enough to hold onto the sugar sticks. The researchers found that a low "Net Score" (α - β) meant the liquid was smart enough to target only the glue, leaving the sugar alone.

What Happened to the Bundle?

After the Lactic Acid team did its work, the researchers looked at the results with powerful microscopes and scanners:

The Glue is Gone: The lignin was successfully stripped away.
The Sugar is Stronger: The remaining sugar structure became more organized and crystalline (like turning a messy pile of yarn into a neat spool), which actually helps enzymes work better later.
The Surface is Rough: The smooth, compact surface of the original residue became rough and porous (like a sponge). This gave enzymes a much larger surface area to grab onto.

The Payoff:
Because the bundle was opened up and the glue removed, the enzymatic hydrolysis (the process of turning sugar into fuel) became twice as efficient. The raw material only yielded 42% sugar, but the treated material yielded 80%.

What Happened to the Removed Glue?

The researchers also looked at the lignin they removed. They found that the Lactic Acid team was picky:

It preferentially dissolved a specific type of lignin unit (called S-type) while leaving others behind.
It also changed the chemical bonds inside the lignin, making the removed glue more stable and less likely to fall apart on its own. This is important because it tells us exactly what kind of "recycled glue" we have for future use.

Summary

This paper is about finding the perfect chemical "key" to unlock a tough plant bundle. By mixing a specific acid (Lactic Acid) with a base, the scientists created a solvent that acts like a smart surgeon: it cuts away the tough protective armor (lignin) with high precision, leaving the valuable core (cellulose) perfectly intact and ready for conversion into useful products. The key to this success was balancing the liquid's "aggression" and "protection" so it only attacked what it was supposed to.

Technical Summary: Influence Mechanism of Carboxylic Acid Hydrogen Bond Donors on 2-Methylimidazole-Based Deep Eutectic Solvent Pretreatment of Glycyrrhiza Residue

Problem Statement
Lignocellulosic biomass, such as Glycyrrhiza residue (GR), possesses a highly recalcitrant structure due to the intricate cross-linking of cellulose, hemicellulose, and lignin via ester, ether, and hydrogen bonds. This structure severely limits enzyme accessibility, hindering efficient biomass conversion. While Deep Eutectic Solvents (DESs) have emerged as promising green solvents for biomass pretreatment due to their tunability and biocompatibility, the systematic understanding of how the molecular structure of Hydrogen Bond Donors (HBDs) influences DES performance and selectivity remains elusive. Specifically, the mechanisms by which functional groups (e.g., methyl, hydroxyl), carbon chain length, and the number of carboxyl groups affect lignin removal and carbohydrate preservation in GR pretreatment require deeper investigation.

Methodology
The study employed a systematic approach to evaluate the influence of HBD structure on DES pretreatment efficiency:

DES Synthesis: Ten different carboxylic acids (formic, acetic, propionic, glycolic, lactic, malonic, oxalic, malic, benzoic, and p-toluic acids) were used as HBDs to construct DESs with 2-methylimidazole (MI) as the Hydrogen Bond Acceptor (HBA). Synthesis was conducted at molar ratios of 0.5:1, 1:1, and 2:1 (HBD:HBA) at 80°C.
Pretreatment: GR was pretreated with the synthesized DESs at 80°C for 1 hour. Solid recovery, glucan recovery, and lignin removal were quantified using NREL standard procedures.
Microscopic Characterization: Kamlet-Taft (K-T) solvatochromic parameters ( $\alpha$ , $\beta$ , $\pi^*$ ) were determined to quantify the microscopic solvent properties (hydrogen bond acidity, basicity, and dipolarity/polarizability).
Structural Analysis: The structural changes in both the pretreated solid residue and the recovered lignin were analyzed using Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and 2D-Nuclear Magnetic Resonance (2D-NMR HSQC).
Bioconversion Assessment: Enzymatic hydrolysis assays were conducted to evaluate the accessibility of cellulose to cellulases following pretreatment.

Key Contributions and Results
The study identified that the structural characteristics of the HBD critically determine the selectivity and efficiency of the DES pretreatment:

Optimal Performance: The DES composed of Lactic Acid (LA) and MI at a 2:1 molar ratio (LA-DES) demonstrated the highest performance, achieving 61.59% lignin removal while maintaining a 90.89% glucan recovery.
Structural Influence of HBDs:
- Hydroxyl Groups: The introduction of a hydroxyl group (as in LA and Glycolic Acid) significantly enhanced delignification efficiency. The hydroxyl group acts as an additional hydrogen-bonding site and enhances acidity via the inductive effect, facilitating the disruption of lignin-carbohydrate complexes.
- Methyl Groups: For aliphatic acids, methyl groups (e.g., Acetic vs. Formic) reduced acidity, thereby protecting cellulose from acid hydrolysis. For aromatic acids, methyl groups (e.g., p-Toluic vs. Benzoic) introduced steric hindrance that reduced penetration efficiency and delignification.
- Carbon Chain Length: Medium chain lengths (C2, e.g., Acetic Acid) offered the best balance between hydrophobicity (for lignin interaction) and polarity (for biomass penetration). Longer chains reduced penetration efficiency.
- Carboxyl Groups: Dicarboxylic acids generally increased acidity and lignin removal but often at the expense of carbohydrate retention due to non-selective hydrolysis.
Kamlet-Taft Parameter Correlation: The study established that the net hydrogen bond acidity ( $\alpha - \beta$ ) is a key predictive indicator for pretreatment selectivity. DESs with low $\alpha - \beta$ values (e.g., LA-DES at 0.038, Acetic Acid-DES at 0.041) achieved highly selective separation. These systems possess a balanced hydrogen-bonding capability where the HBA component ( $\beta$ ) protects cellulose by forming a stabilizing layer, while the HBD component ( $\alpha$ ) selectively disrupts lignin structures.
Structural Transformations:
- Solid Residue: LA-DES pretreatment increased cellulose crystallinity (from 48.41% to 55.62%) and induced a partial transformation from Cellulose I to Cellulose II. SEM analysis revealed a loosened, porous surface morphology, which correlated with a significant increase in enzymatic hydrolysis yield (from 42.38% to 80.80%).
- Recovered Lignin: The pretreatment induced specific structural changes in the recovered lignin. It preferentially removed Syringyl (S) units, leaving the lignin enriched in Guaiacyl (G) units. Additionally, the labile $\beta-O-4$ ether linkages were partially converted into more stable $\beta-\beta$ (resinol) linkages, indicating a recondensation process.

Significance
This research provides a theoretical foundation for the rational design of highly selective DESs for biomass pretreatment. By elucidating the structure-activity relationship between HBD molecular features and macroscopic pretreatment outcomes, the study demonstrates that optimizing the net hydrogen bond acidity ( $\alpha - \beta$ ) is crucial for achieving efficient delignification without compromising carbohydrate integrity. Furthermore, the detailed characterization of the recovered lignin highlights the specific molecular transformations (S-unit removal and $\beta-O-4$ to $\beta-\beta$ conversion) that occur during the process, offering essential insights for the subsequent valorization of lignin. The findings offer practical guidance for developing efficient, green pretreatment technologies for Glycyrrhiza residue and similar lignocellulosic feedstocks.