Synergistic Sequential Phosphoric Acid-Deep Eutectic Solvent Pretreatment-Based Biorefinery: Selective Fractionation of Sugarcane Bagasse and Isolation of Structurally Preserved Lignin

Technical Summary: Synergistic Sequential Phosphoric Acid-Deep Eutectic Solvent Pretreatment-Based Biorefinery

Problem Statement
Lignocellulosic biorefineries face significant challenges due to the inherent recalcitrance of plant cell walls and the heterogeneity of biomass composition. Conventional pretreatment methods, such as steam explosion, dilute acid/alkali hydrolysis, and ionic liquid systems, often suffer from high energy demands, corrosive reagents, expensive solvents, and the generation of inhibitory compounds (e.g., furfural, HMF) that hinder downstream enzymatic and microbial processes. Furthermore, existing strategies frequently fail to achieve selective fractionation of cellulose, hemicellulose, and lignin. A critical limitation is the underutilization of lignin, which is often degraded into condensed, low-value byproducts or burned for heat, rather than being isolated as a structurally preserved precursor for advanced materials. There is a need for a sustainable, selective, and integrated approach that maximizes component recovery while preserving lignin functionality.

Methodology
The study proposes a synergistic sequential pretreatment strategy applied to sugarcane bagasse (SCB) to achieve selective fractionation. The process involves three main stages:

1. Extractive Removal: Raw SCB undergoes exhaustive extraction with water and ethanol to remove soluble impurities.
2. Phosphoric Acid (PA) Pretreatment: Extractive-free SCB is treated with 2% dilute phosphoric acid at 160 ± 10 °C for 2 hours. This step targets the selective hydrolysis and solubilization of hemicellulose while swelling the cellulose matrix.
3. Deep Eutectic Solvent (DES) Treatment: The PA-pretreated residue is subsequently treated with a Choline Chloride-Lactic Acid (ChCl-LA) DES system (molar ratio 1:2.5) at 90 ± 5 °C for 6 hours. This step facilitates the solubilization and extraction of lignin.
4. Fraction Recovery: The process yields three distinct fractions: a hemicellulose-rich filtrate from the PA step, a cellulose-rich pulp (DES-cellulose) from the DES step, and a precipitated lignin fraction (DES-lignin) recovered by adding water to the DES liquor. The DES system is recovered via acetone precipitation and vacuum distillation.

Key Results

• Fractionation Efficiency: The sequential PA-DES strategy achieved high selectivity. The PA step removed approximately 93% of hemicellulose. The subsequent DES treatment recovered over 93% of the lignin (specifically 93.55% of Klason lignin) and yielded a cellulose-rich pulp containing ~84% α-cellulose.
• Lignin Characterization: The isolated DES-lignin exhibited nanoscale features with a z-average particle size of 63.3 ± 18.3 nm and a near-spherical morphology, as confirmed by Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM).
• Structural Integrity: HSQC-NMR and 1H/13C-NMR analyses revealed that the DES-lignin preserved key native linkages, particularly the β-O-4 aryl ether bonds. While some lactylation (esterification) of aliphatic/phenolic hydroxyl groups by lactic acid was observed, the process minimized condensation reactions. The lignin retained a mixed guaiacyl-syringyl-p-hydroxyphenyl (G-S-H) structure.
• Cellulose Properties: X-ray Diffraction (XRD) analysis of the recovered pulp indicated a transformation from native Cellulose I to Cellulose II, characterized by new peaks at 12.2° and 20.3°. This shift, along with reduced crystallinity and disrupted hydrogen bonding, suggests improved accessibility for downstream processing.
• Thermal Behavior: Thermogravimetric analysis (TGA) showed a high char yield (~42% at 800 °C), indicating substantial aromatic retention and potential for use in carbonaceous materials.

Significance and Claims
The authors claim that this integrated biorefinery system offers a controlled and selective deconstruction pathway for lignocellulosic biomass using environmentally benign solvents. The primary significance lies in the synergistic effect of the sequential treatment: the initial acid step opens the biomass matrix and removes hemicellulose, thereby enhancing the accessibility of lignin to the DES without the severe degradation associated with single-step harsh treatments.

The study highlights that this approach successfully isolates structurally preserved lignin with nanoscale dimensions, distinguishing it from conventional methods that often degrade lignin into condensed, less valuable forms. The preservation of native linkages (e.g., β-O-4) and the specific modification (lactylation) without extensive condensation position the recovered lignin as a promising feedstock for high-value applications such as bio-based composites, resins, and carbon precursors. Additionally, the process demonstrates the feasibility of recovering high-purity cellulose pulp with altered crystallinity suitable for enzymatic hydrolysis or nanocellulose production.

The paper concludes that this strategy provides a scalable, mild, and recyclable platform for the comprehensive valorization of all major biomass components, aligning with the principles of a circular bioeconomy. Future work is noted to focus on the reusability of the DES system and comprehensive techno-economic and life-cycle assessments to validate industrial viability.