Green chemistry design of one-step extraction of multi-functionalized lignin-precursors directly from biomass

Abstract

The reliance on non-renewable fossil fuel-based polymers presents significant environmental and humanitarian challenges, including climate change and pollution. Urgent replacement with sustainable alternatives is imperative to mitigate these issues and ensure a carbon-neutral footprint. Lignocellulose, derived from biomass, emerges as a promising substitute due to its abundance and non-edible nature. Comprising lignin, cellulose, and hemicellulose, lignocellulose offers a structural basis for plant biomass, with lignin particularly standing out for its unique properties such as mechanical strength and rigidity due to its aromatic backbone. Despite its potential, effective valorization of lignin remains limited, with a significant portion still used primarily as a fuel source due to its heterogeneity and extensive condensation of lignin structure during existing pulping processes. This study aims to address this challenge by developing a one-step process for synthesizing multi-functionalized lignin precursors directly from biomass, aligning with green chemistry principles. By utilizing lignin as a biobased alternative and implementing a one-step process, this study aims to enhance the sustainability of polymer production while minimizing waste and eliminating the use of toxic chemicals. The novelty of the current study involves sourcing reactants from renewable resources and employing organic carbonates and organic acids to alter lignin's chemistry during extraction, paving the way for a safer and more sustainable synthesis of multi-functionalized precursors in one-step from biomass. This study also includes an environmental impact analysis and industrial scalability assessment of the one-step lignin extraction process. The analysis compares the environmental footprint of the one-step process with conventional methods like Kraft and Organosolv pulping, as well as sustainable methods such as Gamma-Valerolactone (GVL) and Deep Eutectic Solvent (DES) extraction. Key metrics such as the Waste E factor and Greenhouse Gas (GHG) emissions are calculated to evaluate waste and carbon emissions. Additionally, the scalability assessment explores the feasibility of implementing the one-step process at an industrial scale, focusing on efficiency, cost, and sustainability.

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Green foundation

1. This work introduces a one-step extraction and in situ functionalization of lignin directly from pine biomass, eliminating multiple processing steps. Using maleic acid–diallyl carbonate solvent system under mild conditions enables simultaneous lignin isolation and chemical activation, advancing waste minimization, energy efficiency, and renewable feedstock utilization.

2. The process achieves >80% lignin yield with high functionality (up to 5.6 mmol g⁻¹ allyl, 3.1 mmol g⁻¹ aldehyde, and 1.1 mmol g⁻¹ carboxyl). A low E-factor (7.23), reduced GHS weight, and >90% solvent/catalyst recovery demonstrate strong environmental performance.

3. Greenness could be enhanced by improving diallyl carbonate recovery, replacing TBAB with biobased catalysts, implementing continuous processing, and valorizing the cellulose-rich residue as a co-product.

Introduction

The prevalent use of non-renewable fossil fuel-based polymers poses significant environmental and humanitarian challenges like climate change leading to societal imbalance, and pollution. Urgent replacements with sustainable alternatives are imperative to avoid competition with edible resources and to ensure a carbon-neutral footprint.^1–5 Lignocellulose, derived from biomass, emerges as a promising and renewable substitute, particularly due to its abundance and non-edible nature. Lignocellulose comprises three key biopolymers—lignin, cellulose, and hemicellulose—forming the structural basis of plant biomass. Notably, lignin, the most naturally available aromatic biopolymer, has unique chemistry and impressive properties such as mechanical strength, thermal stability, and chemical resistance.^1–5 In contrast to carbohydrates like cellulose and hemicellulose, which have limitations in processability and thermal resistance, lignin presents an opportunity to create polymers with enhanced mechanical, thermal, and chemical resistance.⁵ Lignin, composed of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units derived from respective alcoholic precursors, exhibits varying compositions depending on the biomass source.⁵ The lignin structure is comprised of different linkages comprising of ether bonds, the β-O-4 bond representing 50% of ether linkages which is the most targeted for depolymerization and other C–C linkages that are difficult to break.⁵ Other functionalities like aliphatic and aromatic hydroxyl groups, carboxylic acid and aldehyde groups along the backbone enable the tailoring of lignin-based polymers for specific applications.

Despite research focusing on lignin extraction and utilization, particularly in paper and pulp industries where it is abundant, effective valorization of this natural, aromatic polymer has been limited.^2–5 Although lignin has potential as a valuable polymer source, a significant portion is still primarily used as a fuel source because of its condensed chemical structure due to harsh processing conditions. Enhanced strategies are essential to harness its inherent properties for developing value-added polymers. For decades research has been done to understand how to valorize lignin, but there are relatively few commercial examples of actual polymer or plastic products. There still exists a great potential to valorize lignin for commercial products, however, simplified and industrially viable techniques for lignin valorization are needed. The extraction and functionalization of lignin is often one of the most difficult processes to commercialize due to lignin's intractable and complicated structure. Hence, it is important to understand the greenness of different methodologies that are employed to valorize lignin on the way to polymer synthesis and product development to determine the sustainability of the process.

Current procedures for transforming lignin, including extraction, depolymerization, and upgrading into valuable chemicals, typically follow a three-step process.¹ Industrial extraction methods, such as Kraft pulping, sulfite pulping, soda pulping, and organosolv pulping, often involve harsh chemicals and reaction conditions. These processes lead to the depolymerization and condensation of the native lignin structure, reducing ether bonds and resulting in a less reactive form of lignin.^1,6–10 Alternative approaches, like pyrolysis and enzyme-based methods, also have drawbacks, such as low yield and lengthy processing times.¹ As a result, secondary processes like depolymerization and functionalization are required to produce more reactive lignin, which can be further upgraded into useful polymers.¹ However, many lignin depolymerization techniques face significant challenges, including low yield, poor selectivity, toxicity, the high cost of metal catalysts, and difficulties in catalyst recovery.^1,11–14

The synthesis of lignin-based epoxies traditionally involves two primary routes, both addressing the challenges posed by lignin's unique chemistry. The first approach incorporates the oxirane ring by reacting lignin with compounds such as epichlorohydrin or diglycidyl ethers. This condensation mechanism eliminates phenolic hydrogen as hydrochloric acid (HCl), forming the basis for lignin modification. The second route introduces unsaturated allyl bonds onto the lignin structure, often using allyl bromide, followed by conversion to oxirane rings through the Prilezhaev reaction. However, the use of allyl bromide raises health concerns due to its carcinogenic and toxic nature. These strategies are adopted because the lignin lacks inherent unsaturated bonds, making it challenging to synthesize non-toxic epoxies without resorting to potentially hazardous compounds like epichlorohydrin and allyl bromide.^15–19 Apart from epoxies, the oxirane rings could be converted into cyclic carbonates by reaction with carbon-dioxide that could be utilized for sustainable synthesis of polyurethanes without employing isocyanates.

Similarly, there are a variety of biobased polyamides made from vegetable oils and from aromatic sources like furfuran, vanillin and not much lignin-based polyamides. Altering lignin's chemistry in a suitable way could enable making more sustainable biobased polyamides from the functionalized lignin precursors.^20–24 Also introducing unsaturated bonds onto lignin structure could be utilized in polymerization strategies that could be used in later stages for polymer development to make highly functional polymers with better mechanical and thermal properties. Therefore, there is a need to develop a process that can extract and multi-functionalize lignin simultaneously, streamlining the process and yielding a highly valorized lignin product which could be upgraded into a variety of polymers.

A one-step process would serve as an efficient strategy to produce multifunctional precursors from lignin minimizing solvent usage, chemical waste, energy, time and money. Bertella et al. developed a protocol to simultaneously extract and chemically functionalize lignin with acetals introducing aldehyde structures in lignin's backbone.²⁵ Lignin was functionalized in one-step using multifunctional terephthalic aldehyde that prevents condensation and other side reactions during lignin extraction.²⁴ The main disadvantage here is that terephthalic aldehyde is considered hazardous and acutely toxic (category 3 specific organ toxicity).^26,27 Another study by Angelini et al. extracts and modifies lignin in one-pot using γ-valerolactone (GVL) which is a green solvent from pretreated lignocellulosic biomass.²⁸ Biomass is subjected to steam explosion that disintegrates lignin and carbohydrates and delignifies it to greater than 70%. This is further extracted and modified through acetylation by adding acetic anhydride and Mannich condensation in GVL solution by adding formaldehyde and diethyl amine solution.²⁸ Although this method employs sustainable solvents for lignin extraction it still employs harsh solvents like formaldehyde and diethyl ether solution for lignin modification in GVL solution and involves multiple steps like pretreatment of biomass, lignin extraction in GVL solution and lignin modification.

This study aims to develop a one-step process for lignin extraction and functionalization, creating a variety of functional groups that can be harnessed for tailored applications. The approach of the current study addresses sustainability by sourcing reactants from renewable resources, minimizing waste, and eliminating the use of toxic chemicals in regard to green chemistry principles as shown in Table 1. This involved designing a solvent system to extract and functionalize lignin in one-step using COSMO-RS (conductor like screening model for real solvents) software. Organic carbonates and acids were employed to design the system to ensure sustainability in the process.

Table 1 Summary of methods adopted addressing the green chemistry principles in this one-step process

Method adopted	Green chemistry principle addressed
Raw material: biomass feedstock	7. Use of renewable raw material
Reagents: less harsh and non-toxic	3. Safer chemical synthesis
	4. Safer chemicals design
Solvents: less harsh and non-toxic	5. Use of safer solvents and auxiliaries
Mild reaction conditions and reduction in steps to make functionalized precursor	6. Energy efficiency
	8. Reduction of derivatives

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In this context, maleic acid (MA) has emerged as a promising green solvent for lignin extraction due to its hydrotropic behavior and esterifying reactivity, which facilitate effective fractionation of wood under mild conditions (≤100 °C, atmospheric pressure). Cai et al. demonstrated that MA enables simultaneous lignin solubilization and functionalization, making it advantageous for low-temperature, sustainable biorefinery operations without requiring elevated pressures or harsh reagents. While MA's relatively low operational temperature and moderate acidity (pK_a ≈ 1.9) are expected to reduce equipment, stress compared to stronger mineral acids, direct corrosion data are not reported in the study. As most experimental setups in such systems use glassware, the potential for corrosion in metal reactors or during downstream evaporation (where acid may concentrate) cannot be definitively ruled out. So, Generally Recognized As Safe (GRAS) under Title 21 of the U.S. Code of Federal Regulations, only holds true for low concentrations of MA. Therefore, corrosion considerations, particularly in metal components like distillation columns or heat exchangers—should be addressed in future studies. Nonetheless, compared to hydrotropes like p-toluenesulfonic acid, MA has a higher minimal hydrotropic concentration (MHC), potentially lowering water demand and simplifying acid recovery.²⁹

The second component in the solvent system, diallyl carbonate (DAC), provides a sustainable and non-toxic route for lignin functionalization. It can be synthesized in situ from dimethyl carbonate and allyl alcohol, offering an alternative to conventional valorization strategies.³⁰ The combination of MA and DAC creates a solvent system that efficiently extracts lignin and introduces multiple functional groups, resulting in multi-functionalized lignin precursors in one step from biomass.

Experimental

Materials

Maleic acid (MA) and formic acid (FA) were procured from spectrum, while tetra butyl ammonium bromide (TBAB), methanol, and diallyl carbonate (DAC) 99% were obtained from Sigma Aldrich. Additionally, other chemicals such as dimethyl carbonate (>99%), allyl alcohol (≥99%), chloroform anhydrous (≥99%), dimethylsulfoxide-d6 (DMSO-d6, 99.96 atom% D), 1,5,7-triazabicyclo[4.40]dec-5-ene (TBD, 98%), 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (phosphorylating agent, 95%), cholesterol (>99%), chromium(III) acetylacetonate (97%), and absolute ethanol were also sourced from Sigma Aldrich Inc. The natural fine pinewood sawdust was procured from Etsy.

The procedure involved the use of varying reactors depending on the sample size. Smaller batches were processed using pressure tubes and compact reactors, while the parr reactor was dedicated to larger batches. Notably, the parr reactor and compact reactor are designed to withstand maximum pressures of 345 bar and 206 bar, respectively.

Solvent design using COSMO-RS

COSMO-RS model is a quantum chemical method that computes thermodynamic properties of various solvent systems for various applications using density functional theory (DFT) methods. It predicts the molecular surface charges in an ideal conductor using quantum chemical calculation and performs thermodynamic calculations. Different thermodynamic properties like activity coefficients, solubilities, excess enthalpies are estimated based on intermolecular interaction. Using the software, different solvent systems were modelled, and the structure was geometrically optimized. COSMO-RS compound task was run to generate solvent files to calculate thermodynamic properties and other solvent interaction parameters.^31,32 Various combinations of solvent systems, including MA, FA, glycerol, and DAC, were analyzed based on their hydrogen bond acceptor and donor counts to predict the most effective solvent combinations for coordination and system formation. These combinations were then modeled using software, minimizing waste of solvents, time, and energy in experimental trials to identify the optimal solvent system for the one-step process.

Extraction and functionalization protocol

Using COSMO-RS software DAC + MA and DAC + FA solvent system was selected. Larger sample size was done in 500 ml Parr reactor and smaller size was done in 100 ml compact pressure reactor. Majority of trials were done in compact reactors to minimize solvent usage during optimization of the process. 1 g of pinewood saw dust was fed into the 100 ml compact pressure reactor. To this sawdust, 4.5 g of TBAB catalyst, 3 to 10 g of DAC in the range of 1 : 3 to 1 : 10 weight ratio of biomass: DAC and 10 g of MA/FA solution (5 g maleic acid/formic acid in 5 ml deionized water) were added. The reaction was done over a temperature range of 75 to 130 °C for 5 hours. The reaction was pressurized due to internally developed pressure from the solvent mixture which aided in dissolution of biomass in the solvent mixture.

DAC recovery

After the reaction, the hot mixture was filtered to separate the solid and liquid fractions. The solid was washed with 10–15 mL of acetone to remove any adhered lignin. The liquid filtrate was transferred to a round-bottom flask and subjected to rotary evaporation at 70 °C. The recovered DAC was collected and reused for subsequent trials to improve process sustainability.

Lignin recovery

The residue remaining after DAC recovery via rotary evaporation was redissolved in 20–30 mL of acetone to aid transfer and solubilization. Subsequently, 170 mL of deionized water was added to induce lignin precipitation. The solution was stirred and left to settle overnight. Precipitated lignin was then recovered by filtration, washed 2–3 times with 20 mL of deionized water, and vacuum-dried at 45 °C to yield multi-functionalized lignin precursor.

MA and TBAB recovery

Following lignin precipitation and separation, the aqueous filtrate was distilled under reduced pressure to concentrate the maleic acid (MA) content to approximately 50 wt% (Fig. 1). Quantitative and qualitative ¹H NMR analyses were then carried out on the concentrated filtrate to determine the amounts of MA and tetrabutylammonium bromide (TBAB) present, thereby assessing their recovery and reusability.

Fig. 1 Overall process of lignin extraction and functionalization and products recovered.

Compositional analysis of pinewood and cellulose-rich residue

Prior to compositional analysis, pinewood sawdust (40–60 mesh) was subjected to solvent extraction adapted from TAPPI T204 cm-07 and TAPPI T264 cm-07 to remove extractives. Approximately 2 g of oven-dry sawdust was placed in 50 mL centrifuge tubes and sequentially extracted with acetone (3 cycles), absolute ethanol (1 cycle), and deionized water (2 cycles), using sonication and centrifugation between cycles. The extracted solids were dried overnight in a vacuum oven at 60 °C (∼50 mbar) and equilibrated in a vacuum desiccator prior to analysis.^33,34

The extractive-free pinewood and the cellulose-rich residue obtained after MA + DAC treatment were then subjected to two-step acid hydrolysis, and the hydrolysates were analyzed by high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC/PAD) to quantify monosaccharides. Anhydro correction factors of 0.88 (pentoses) and 0.90 (hexoses) were applied to convert monosaccharide contents to polysaccharide equivalents. Acid-insoluble lignin was determined gravimetrically from the hydrolysis residue, while acid-soluble lignin was quantified spectrophotometrically at 240 nm using an absorption coefficient of 12 L g⁻¹ cm⁻¹. Each sample was analyzed in duplicates.³⁵

Characterization

The ¹H and ³¹P NMR spectroscopy procedures were conducted on a Bruker Avance-300 MHz spectrometer, equipped with a quadro nucleus probe. For the H-NMR analysis, a lignin sample weighing 40 mg was dissolved in 1000 microliters of deuterated dimethyl sulfoxide (d-DMSO). A total of 32 scans were performed, providing detailed insights into the molecular composition of the lignin sample. The quantification of allyl groups, a crucial aspect of the analysis, was accomplished in the H-NMR spectrum, with syringaldehyde serving as the internal standard.

In the ³¹P NMR analysis, the lignin sample was dissolved in 0.5 ml of a solvent mixture consisting of pyridine and chloroform-d6 in a ratio of 1.6 : 1. Cholesterol was introduced as an internal standard, and Cr(III) was incorporated as a relaxation agent. The phosphorylating agent used in this process was 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane. This approach allowed for the identification and quantification of aliphatic hydroxyl (OH), aromatic OH, and carboxylic acid (COOH) groups present in the lignin structure according to established standards.³⁶

Additionally, the ¹H and ¹³C-HSQC experiment was carried out on a Bruker Avance 500 MHz spectrometer, utilizing a BBO probe and the standard Bruker pulse sequence ‘hsqcetgpsp.3’ (phase-sensitive gradient-edited-2D HSQC using adiabatic pulses for inversion and refocusing).³⁷ A total of 2000 data points were acquired over a 12 ppm spectral width in the F² dimension, with an acquisition time of 170 milliseconds. In the F¹ dimension, 165 ppm of spectral width and an acquisition time of 6.36 ms were employed.

GPC was done on TOSOH Elite GPC instrument (HLC-8420 EcoSEC) with a refractive index (RI) detector and four columns in series (1× TSKgel SuperAW-L guard column, 2× TSKgel SuperAWM-H columns, and 1× TSKgel SuperAW2500 column). The mobile phase was DMAc with 0.5 wt% LiBr. The flow rate of the mobile phase was 0.4 mL min⁻¹ with an injection volume of 80 µL. Narrow-dispersity poly (methyl methacrylate) [PMMA] standards from Agilent Technologies were used to make a calibration curve. The column temperature was 50 °C.

FTIR was carried out on Thermo Nicolet Nexus 670 FT-IR with a 2 mm diameter diamond ATR crystal and the spectra were collected in absorbance mode from 4000–400 cm⁻¹ using 32 scans with a spectral resolution of 2 cm⁻¹. The high-performance anion exchange chromatographic method with pulsed amperometric detection (HPAEC/PAD) was used for the analysis of monosaccharides in the hydrolysate obtained after the standard two-step acid hydrolysis.

Results and discussion

Solubility modeling

Examining the chemical properties of MA and DAC reveals complementary features, enabling them to coordinate as a solvent system. The addition of water to MA enhances compatibility with the lignin. The solubility parameters for this solvent system and lignin, estimated using COSMOS, guide the selection of an effective dissolution agent for lignin from biomass. Other solvent systems, including DAC + Glycerol, DAC + FA, and DAC + MA, were also modeled in COSMO's, and their solubility parameters were estimated (Fig. 2). The values of different solvent systems are reported in Table 2. Further experiments using the DAC + MA and DAC + FA systems were conducted, given their solubility parameters were closer to a lignin structure assumed to consist of 100% coniferyl alcohol. These solvents, tested under various reaction conditions, aim to evaluate their effectiveness in extracting and functionalizing lignin from biomass.

Fig. 2 COSMO-RS models of MA + DAC system.

Table 2 Solubility parameters for different solvent systems

Solvent system	Solubility parameter
DAC – Glycerol	13.767
DAC–FA	12.168
DAC–MA	13.172
Lignin (100% coniferyl alcohol)	12.827

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Consequently, the initial experimentation involved the application of both solvent systems, although formic acid is toxic and categorized as level 1 specific organ toxin.³⁸ The reason was to determine the efficacy of MA in extraction and functionalization in comparison to FA which has a similar solubility parameter coordinated with DAC as that of MA. Their respective yield and functionalization efficiency were compared in the initial trials.

Comparison of DAC + FA and DAC + MA System

To evaluate the extraction and functionalization efficiency of diallyl carbonate (DAC)-based systems, two solvent systems—DAC + FA and DAC + MA were initially screened using a Taguchi design of experiments. Preliminary trials at a biomass-to-solvent ratio of 1 : 10 showed negligible lignin recovery at 90 °C, highlighting the need for higher solvent loadings; subsequent trials were carried out at 1 : 20 (w/w). Based on the initial trials on a small reaction scale, a reduced Taguchi orthogonal array (L4 design) was implemented (Table 3), to minimize the number of experiments while still capturing the influence of key parameters such as solvent ratio and temperature. This reduced model allowed efficient screening of solvent systems with limited resources while identifying temperature as a dominant factor.

Table 3 Reduced Taguchi orthogonal array design for DAC : acid systems

Trial	Solvent system	DAC : acid ratio (w/w)	Temperature (°C)	Time (h)	Biomass : solvent (w/w)
1	DAC + FA	1 : 3	90	5	1 : 20
2	DAC + MA	1 : 3	90	5	1 : 20
3	DAC + MA	1 : 3	120	5	1 : 20
4	DAC + MA	1 : 3	130	5	1 : 20

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To quantify the chemical modifications on lignin, ¹H-NMR analysis was performed. As reported by Lena C. Over et al., allylic proton signals Ha, Hb, and Hc appear between 4.7–4.2 ppm, 6.3–5.8 ppm, and 5.7–4.8 ppm, respectively, while aromatic protons are observed between 8–6 ppm (Fig. 3).³⁹ These spectral regions were used to quantify the degree of allylation in both systems. ¹H NMR confirmed the presence of allyl groups introduced by DAC. According to Over et al., complete allylation results in an integral ratio of 1.45 : 1 (aromatic protons to Hb), representing two allyl groups per aromatic unit. In this study, integration of the aromatic and allyl proton peaks showed that the DAC + FA system achieved a ratio of one allyl group per six aromatic units (0.16), while DAC + MA showed one per seven (0.14). These values were notably lower than the benchmark set by Over et al., likely due to the absence of a phase transfer catalyst like TBAB and lower weight ratio of DAC : MA (1 : 3) (Table 4).

Fig. 3 H-NMR of allylated organosolv lignin based on Lena C Over et al.³⁹ (Reproduced with permission from the Royal Society of Chemistry Publishing, license id: 1558297-1.)

Table 4 Lignin yield and allylation efficiency for DAC + FA and DAC + MA systems at 90 °C, 5 h (biomass : solvent = 1 : 20, DAC : acid = 1 : 3)

Solvent system	DAC : acid (w/w)	T (°C)	Time (h)	Lignin (g)	Allyl/aromatic ratio
DAC + FA	1 : 3	90	5	0.172	0.16
DAC + MA	1 : 3	90	5	0.100	0.14

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³¹P NMR further revealed that DAC + MA produced higher levels of carboxylic acid groups compared to DAC + FA (Table 5). MA's esterification activity converts hydroxyl and carbonyl groups into additional –COOH groups, enhancing functionalization. Both solvent systems retained comparable levels of aliphatic and aromatic hydroxyl groups (∼2.2 mmol g⁻¹ lignin), but DAC + MA showed distinctly higher –COOH incorporation.

Table 5 Functional group distribution in lignin extracted with DAC + FA vs. DAC + MA, quantified by ³¹P NMR at 90 °C, 5 h (biomass : solvent = 1 : 20, DAC : acid = 1 : 3)

Functional group	DAC + FA (mmol g^−1 lignin)	DAC + MA (mmol g^−1 lignin)
Aliphatic –OH	1.08	0.579
Aromatic –OH	1.25	1.74
–COOH	0.30	1.11

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At identical conditions (90 °C, 5 h, DAC : acid = 1 : 3), DAC + FA outperformed DAC + MA in lignin yield and degree of allylation, which is attributed to formic acid's stronger acidity and enhanced lignin solubilization. However, due to the known toxicity and limited sustainability of FA, it was excluded from further studies, and subsequent optimization focused solely on DAC + MA.

Temperature strongly influenced lignin recovery in DAC + MA, with yields rising from <7% at 75 °C to ∼60% at 130 °C (Table 6). This confirmed temperature as a dominant factor in extraction efficiency.

Table 6 Effect of temperature on lignin yield in the DAC + MA system (biomass : solvent = 1 : 20, DAC : MA = 1 : 3, 5 h)

Trial	Biomass (g)	Time (h)	T (°C)	Lignin (g)
1	1	5	75	0.022
2	1	5	90	0.134
3	1	5	120	0.200
4	1	5	130	0.213

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Overall, while DAC + FA initially provided higher yields, the DAC + MA system was pursued as the more sustainable route. Subsequent optimization with TBAB at elevated temperatures and improved DAC : MA ratios (1 : 1) led to enhanced allylation and functional group incorporation, establishing DAC + MA as the preferred system for further development.

Reaction pathway and optimization of DAC + MA system

Another key factor contributing to the conversion efficiency was the solubility of the extracted lignin in DAC. The one-step process of extraction and functionalization made the limited solubility of lignin in the solvent system impactful on the degree of allylation by DAC on the extracted lignin structure. Therefore, it became essential to introduce a medium that aids in the dissolution of lignin in DAC and encourages allylation. TBAB was chosen as the phase transfer catalyst that enables better dissolution of biomass in the solvent system by forming a homogeneous mixture and hence thereby improving allylation on lignin structure. Based on a study by Over et al. (2015), TBAB works best for allylation compared to other bases like K₂CO₃, Cs₂CO₃, NaOH and DBU due to high phase transfer activity of tetrabutylammonium salt.³⁹

To systematically explore the effects of process variables, a reduced Taguchi design was implemented to minimize the number of trials while still probing the influence of key variables: reaction temperature (120–130 °C), time (5–6 h), DAC : MA ratio (1 : 1), and the use of TBAB as a phase transfer catalyst. This approach allowed us to efficiently identify the dominant factors affecting lignin yield and functionalization without executing a full factorial matrix.

The biomass loading (4–5.5 wt%) is lower compared to other processes, which typically use biomass loadings in the range of 10–20 wt%. This lower loading was intentionally chosen to ensure effective lignin extraction and functionalization under optimized conditions. Higher biomass loadings can result in mass transfer limitations and incomplete reactions, which may reduce the overall efficiency of the process. Strategies for solvent recycling will further enhance the sustainability and efficiency of the process.

Table 7 summarizes the results of four representative trials. Increasing temperature from 120 to 130 °C enhanced both lignin yield and degree of allylation. Longer reaction time (6 h vs. 5 h) further increased allyl incorporation (from 1.39 to 2.05 mmol g⁻¹ lignin at 120 °C). Notably, trial 4 with higher biomass input (3 g) at 130 °C achieved a lignin yield of 1.44 g, increased COOH incorporation (3.66 mmol g⁻¹ lignin), and reasonable solids yield (85%), suggesting that elevated pressure at higher loadings may promote depolymerization and generate additional reactive sites.

Table 7 Taguchi-style optimization of DAC + MA system: effect of temperature, time, and biomass loading on lignin yield and functionalization

Trial no.	Amount of biomass (g)	Temp. (°C)	Time (h)	Amount of lignin obtained (g)	Allyl group (mmol g^−1 lignin)	–CHO (mmol g^−1 lignin)	COOH (mmol g^−1 lignin)	OHaliphtatic (mmol g^−1 lignin)	OHaromatic (mmol g^−1 lignin)	Cellulose rich residue (g)	Solids yield (%)
1	1	120	5	0.42	1.39	0.9	0.86	0.88	2.35	0.38	80
2	1	120	6	0.44	2.05	5.5	0.95	0.9	2.2	0.32	76
3	1	130	6	0.52	3.1	2.8	1.1	0.23	0.6	0.51	87
4	3	130	6	1.435	5.6	2.7	3.66	0.28	0.72	1.12	85

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Fig. 4 illustrates the reaction pathway of biomass treated with the DAC + MA system. MA disrupts lignin-carbohydrate complexes, separating lignin from carbohydrates. MA acts both as a hydrotrope to solubilize lignin and as a reactive dicarboxylic acid, esterifying lignin hydroxyl groups while also introducing additional carboxylic acid functionalities. Cai et al. demonstrated that these ester linkages can undergo partial hydrolysis under reaction conditions, liberating free –COOH groups and thereby increasing the carboxyl content of the lignin.²⁸ At the same time, diallyl carbonate (DAC) introduces allyl groups onto lignin hydroxyls, particularly in the presence of TBAB, significantly broadening the potential for downstream polymer applications.³⁷ In addition, consistent with the general principles described by Schutyser et al., partial depolymerization of lignin under acidic conditions generates reactive benzylic intermediates.¹ Unlike the stabilizing aldehyde-addition strategy employed in Luterbacher's work, which is performed under aldehyde-assisted acidic fractionation, our DAC/MA system lacks an external stabilizing agent.²⁵ As a result, these intermediates can undergo oxidation during the reaction, leading to the in situ formation of aldehyde groups directly in the lignin structure. While Schutyser's RCF studies were conducted under reductive catalytic conditions, our interpretation here is restricted to acidic fractionation pathways. The yield of the process is defined by total solids yield relative to the biomass by equation below and the values for different processes are summarized in Table 7.

Total solids yield = Lignin yield + Cellulose rich residue yield

Fig. 4 Reaction pathway for one step extraction and functionalization of biomass using DAC + MA system.

The amount of lignin obtained is greater than the amount of lignin present in pinewood saw dust (35% as estimated by (HPAEC/PAD)). This could be attributed to simultaneous functionalization of lignin with diallyl carbonate (mol. wt is 142.15 g mol⁻¹) and maleic acid (mol. wt is 116 g mol⁻¹). As the extent of functionalization increases, the more is the incorporation of allyl, ester, acid and aldehyde groups in the lignin structure, contributing to increase in the obtained lignin weight with subsequent trials. Such apparent yield increases are consistent with prior studies, for example Over et al., who observed that allylation of lignin with DAC introduced carbonate/allyl groups.³⁹ In our case, aldehyde groups were formed in situ during the one-step extraction process, under acidic and elevated-temperature conditions. The aldehyde functionality observed in our lignin is generated as a by-product of the extraction process rather than through intentional stabilization. Nevertheless, these aldehyde groups are valuable for downstream upgrading, as they can serve as reactive handles for polymer development. This parallels Luterbacher's TALD-lignins, where externally introduced aldehyde groups produced lignins that were more reactive than Kraft or mild organosolv lignins, thereby facilitating their use in the development of more sustainable resins and materials.^24,39 These results confirm that the higher-than-expected lignin recovery reflects not only extraction but also chemical modification during the one-step process, which increases the functionalized lignin's weight while simultaneously producing a carbohydrate-enriched residue.

FTIR was done to quickly analyze the lignin samples from trials to estimation allylation. The strong broad peak between 3550–3200 cm⁻¹ corresponds to O–H stretching in the alcohol group, indicating the presence of aliphatic and aromatic hydroxyl groups in the lignin samples. Trials 1 and 2 reveal more distinct peaks in this region compared to trials 3 and 4, aligning with higher total OH content in the former trials. This suggests less intensive reaction conditions in trials 1 and 2, leading to more hydroxyl groups and poor functionalization, supported by smaller allyl peaks. Allyl peaks appear in the 1000–900 cm⁻¹ region, with trials 3 and 4 showing strong peaks, confirming effective lignin allylation. The double peak between 3000 and 2700 cm⁻¹ originates from C–H stretching in the aldehyde group. However, trials 3 and 4 exhibit a peak centered at 3000 cm⁻¹ due to O–H stretching from carboxylic acid. In trial 2, the peak at 1720 cm⁻¹ may result from C=O stretching in α,β-unsaturated ester introduced by maleic acid. Under severe conditions, these ester bonds could undergo acid hydrolysis, forming COOH groups, evident in trials 3 and 4 at 1716 and 1713 cm⁻¹, respectively.

A detailed analysis on the amount of allylation with consecutive trials was done using H-NMR. The amount of allyl groups was 0.79, 1.2, 2.67 and 5.6 mmol g⁻¹ lignin for trials 1, 2, 3 and 4 respectively. The number of allyl groups increased with an increase in reaction temperature and time. Another important finding was that although trials 3 and 4 had similar reaction conditions, the amount of biomass was 3 times more for trial 4 compared to trial 3. This means the amount of solvent system was thrice for trial 4, leading to an increased pressured condition to around 30 bars, causing more depolymerization and formation of more reactive sites leading to increased allylation.

The peak at 9.8 ppm was from the proton in the aldehyde group of the standard and the peak at 9.6 ppm was from the proton of the aldehyde group of one-step lignin samples. This shows that the one-step lignin also had considerable amount of aldehyde groups apart from allyl groups A reduction in the aldehyde peak at 9.6 ppm from lignin samples with an increase in severity of reaction conditions was observed which could be due to the side reactions of aldehyde groups at elevated pressures and temperatures.

Other functional groups like carboxylic acid, aliphatic, and aromatic hydroxyl groups were studied and quantified using P-NMR analysis. P-NMR analysis revealed that trials 1, 2, and 3 displayed a reduction in aliphatic and aromatic hydroxyl groups, attributed to increased allylation efficiency with higher temperature and time. Trial 4 exhibited an unusual increase in aromatic hydroxyl groups, possibly due to elevated pressure causing extensive depolymerization. Trials 3 and 4 showed higher carboxylic content than total hydroxyl, suggesting incorporation of carboxylic acid groups from maleic acid under severe conditions.

As shown in Table 7, H and P-NMR revealed the presence of key functional groups, such as allyl, aldehyde, aliphatic and aromatic hydroxyl, and carboxylic acid, which suggest a broad potential for polymer modifications and advancements. The allyl group offers the possibility for free radical polymerization, synthesis of polymers like epoxies and cyclic carbonates which could be transformed into polyurethanes through non-toxic methods, avoiding the use of epichlorohydrin and isocyanates respectively. The abundance of carboxylic acid groups can also be exploited to produce polyamides, expanding the versatility of these precursors. Additionally, aldehydes present an opportunity for the synthesis of lignin-based acetals and other polymer types. Altogether, these functional groups provide a platform for developing a wide range of sustainable polymers.

2D HSQC NMR and mechanistic analysis

Two-dimensional ¹H–¹³C HSQC NMR spectroscopy was used to understand how lignin's chemical structure changed during the one-step treatment with MA and DAC. The spectra provide direct evidence that lignin's side-chain hydroxyl groups reacted with both solvents, leading to the formation of new ester and allyl groups.

Based on Sternberg et al. there are a variety of inter-unit linkages observed in pine organosolv lignin like resinol, phenylcoumaran and β-O-4 units.⁴⁰ None of these inter-unit linkages appear in the lignin samples, implying the condensation of ether bonds and alteration in lignin chemistry during allylation. At the same time, new peaks appeared at δH/δC ≈ 4.1–4.4/63–66 ppm and 6.5–7.0/130–140 ppm, which correspond to maleated esters and the vinyl (C=C–H) bonds from maleic acid. These new signals show that MA formed ester bonds with the lignin side chains.

Additional new peaks appeared at δH/δC ≈ 4.2–4.7/63–70 ppm, 4.8–5.7/115–130 ppm, and 5.8–6.3/128–135 ppm, which are characteristic of allyl carbonate groups introduced by DAC. These peaks became stronger in the higher-temperature and higher-loading trials, especially trial 4, showing that the reaction conditions promoted more extensive allylation. The methoxy (–OCH₃) peak at δH/δC ≈ 3.6–3.8/55–57 ppm remained unchanged in all trials.

The reaction proceeds through selective activation and transformation of lignin's γ-hydroxyl groups under the mildly acidic conditions generated by MA. As reported by Cai et al., MA functions both as a hydrotrope and a reactive dicarboxylic acid. It protonates β-O-4 ether oxygens, increasing lignin solubility and simultaneously activating the γ-CH₂OH groups for esterification.²⁹ These primary hydroxyls, being more nucleophilic and less sterically hindered than the α-OH groups adjacent to the aromatic ring, preferentially react with the carboxylate group of MA to form γ-maleated esters while releasing water. This step introduces new ester and conjugated carboxylic functionalities into the lignin side chain, consistent with the new cross-peaks observed in the HSQC spectra at δ_H/δ_C ≈ 4.1–4.4/63–66 ppm and 6.5–7.0/130–140 ppm.

At the same time, the bulk of the cellulose remains in the water-insoluble solid (WIS) fraction of the MA hydrotropic fractionation system, as directly demonstrated by Cai et al., where the retained solids are enriched in cellulose fibers that remain suitable for both enzymatic hydrolysis and mechanical nanofibrillation.²⁹ In addition, direct studies applying MA-based hydrotropic fractionation to herbaceous biomass show that maleic acid preferentially esterifies lignin through γ-hydroxyl functionalization, introducing carboxyl functionality into the lignin structure while the residual water-insoluble solid remains highly enriched in cellulose.⁴¹ This behavior originates from the fundamentally different supramolecular organizations of lignin and cellulose. Cellulose exists as highly crystalline microfibrils stabilized by dense intra- and intermolecular hydrogen-bonding networks, which severely restricts acid penetration and suppresses the nucleophilicity of its hydroxyl groups. In contrast, lignin is an amorphous, three-dimensional aromatic polymer with accessible phenolic and aliphatic hydroxyl groups, flexible side-chain mobility, and significantly lower hydrogen-bonding density. As a result, maleic acid can readily penetrate, solubilize, and chemically modify lignin domains, while the tightly hydrogen-bonded cellulose framework remains chemically and structurally preserved.

Moreover, recent comprehensive reviews on hydrotropic solvent systems demonstrate that hydrotropes—including organic acids such as maleic acid—operate primarily through reversible disruption of π–π interactions, hydrogen bonding, and hydrophobic associations within lignin domains, enabling efficient lignin extraction without extensive hydrolysis or depolymerization of cellulose.^42–44 These studies consistently report high recoveries of both isolated lignin and the cellulose-rich solid fraction, confirming that cellulose undergoes minimal chemical modification under mild hydrotropic conditions.

Simultaneously, DAC undergoes transesterification with the same γ-hydroxyl and phenolic –OH groups, forming γ-allyl carbonate and allyl aryl ether linkages and liberating allyl alcohol as a by-product as reported by Over et al.³⁹ The resulting allylic correlations observed at δ_H/δ_C ≈ 4.2–4.7/63–70 ppm, 4.8–5.7/115–130 ppm, and 5.8–6.3/128–135 ppm confirm the introduction of allyl functionalities. These transformations, along with the loss of native γ-CH₂OH signals, clearly indicate that lignin is not merely solubilized but chemically modified during the DAC + MA treatment, yielding a γ-maleated and γ-allylated lignin structure with enhanced reactivity for downstream polymer synthesis (Fig. 5 and 6).

Fig. 5 2D ¹H–¹³C HSQC spectra of lignin extracted and functionalized in the one-step DAC + MA system under different reaction conditions (trials 1–4).

Fig. 6 Proposed reaction mechanism for the one-step lignin extraction and functionalization using DAC and MA adapted from Cai et al. and Over et al.^29,39 Step 1: Lignin solubilization by protonation of ether oxygen by MA. Step 2: γ-OH esterification by maleic acid. Step 3: aromatic and γ-OH allylation and carbonylation by DAC.

GPC analysis

The molecular weight of lignin obtained from different trials were determined from GPC. The overall molecular size distribution is estimated in the lignin samples. It also helps in assisting the polydispersity index (PDI) of the lignin precursor, vital for further upgradation to polymers from these precursors with enhanced performance.

From Table 8, it can be inferred that the M_n is almost similar in all the lignin samples from all four trials, roughly around 2.6–2.8 kg mol⁻¹. This is slightly higher compared to other industrially available lignin like Kraft lignin, organosolv lignin, soda lignin whose values ranges from 1.5–2.5 kg mol⁻¹.⁵ This mainly is because of functionalization of lignin with allyl groups and formation of new functionals groups like aldehydes and carboxylic acid groups as stated before in Table 7, leading to a slightly higher M_n value than industrial grade lignins (Fig. 7).

Fig. 7 Plot of refraction index intensity vs. retention time for lignin samples from all four trials from GPC with RI detectors [GPC solvent: DMAc + 0.5 wt% LiBr].

Table 8 Molecular weights of lignin fractions from all four trials using GPC

Sample	M n (kg mol^−1)	M w (kg mol^−1)	Dispersity
Trial 1	2.6	4.42	1.7
Trial 2	2.6	3.9	1.5
Trial 3	2.8	5.04	1.8
Trial 4	2.6	4.16	1.6

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Additionally, these extracted and functionalized lignin samples demonstrate a notably lower polydispersity index (PDI) ranging from 1.5 to 1.8, in contrast to the higher PDI observed in industrial grade lignin. Kraft and soda lignin has PDI in the range of 2–5 to 3.5 while lignosulfonates have a higher PDI of 6 to 8.³⁸ Low PDI is an important characteristic for their potential to be used as a high-quality polymer precursor material, facilitating further enhancement into epoxies, polyurethanes, free-radical polymerized resins, polyamides etc. (Table 8).

Analysis of recovered DAC, MA and TBAB

Quantitative ¹H NMR analysis using 1,4-dinitrobenzene as an internal standard confirmed the successful recovery and high purity of DAC from the lignin functionalization process. From an initial charge of 10 mL DAC, 5 g MA, and 4.5 g TBAB, the recovered fractions were evaluated. DAC recovery was 68% (≈6.8 mL), with ¹H NMR quantification indicating 6.76 g of pure DAC. Using the density of DAC (0.991 g mL⁻¹), this corresponds to 6.8 mL of pure DAC. Thus, the recovered DAC was confirmed to be of high purity by quantitative ¹H NMR (see SI), though with moderate recovery (69%) due to small-scale operation (1 g biomass), transfer losses, and volatilization during hot discharge of the pulp without water cooling. These recovery values are consistent with literature, where Over et al. reported DAC recovery yields of 60% during organosolv lignin allylation, noting that larger-scale operation improves solvent recovery.³⁹

TBAB was completely recovered quantitatively (4.5 g from 4.5 g) by quantitative ¹H NMR with no signs of decomposition (see SI), confirming its excellent recyclability. MA recovery reached ∼90% (∼4.36 g from 5 g). To maximize reusability, the filtrate can be distilled to concentrate MA to ∼50 wt% and then directly reused for the next cycle of fractionation. Following the strategy reported by Cai et al., a small spike of 5–10% fresh MA relative to the original charge can be added to compensate for inevitable process losses, including MA retained in the water-insoluble solids (≈1–2%) after washing.²⁹ This reuse strategy minimizes acid consumption and supports process sustainability. TBAB was co-recovered from the same stream, with a typical recovery efficiency of almost 100%, enhancing the overall economic and environmental viability of the process.

Overall, although DAC recovery was somewhat lower than MA and TBAB, the very high solvent purity and demonstrated reuse strategies indicate that all three solvents can be effectively recycled with only minor supplementation. This strongly supports the reproducibility and sustainability of the process.

Compositional profiles of pinewood and cellulose-rich residue

Compositional analysis confirmed that significant structural changes occurred in the biomass following the one-step DAC + MA lignin extraction and functionalization. On a 1 g pinewood basis, the native biomass contained approximately 35 wt% total lignin (∼0.35 g), 42% glucan, 5.8% xylan, and 10.8% mannan. After treatment, about 0.52 g of cellulose-rich residue was recovered, corresponding to a total solids recovery of ∼85%. Based on the carbohydrate composition (59 wt% glucan in the residue versus 42 wt% in native pinewood), this corresponds to a cellulose recovery of approximately 73%, calculated as:

The residue retained 23–24 wt% lignin (∼0.12–0.13 g), giving a delignification efficiency of ∼64%, calculated as:

This substantial lignin removal was accompanied by enrichment of structural carbohydrates. The glucan content increased from ∼42% in native pinewood to ∼59% in the cellulose-rich residue, while xylan and mannan decreased from ∼5.8% and ∼10.8% to ∼3% and ∼7.6%, respectively. Minor sugars such as arabinan and galactan, initially present at 1.2% and 2.5%, were largely removed (≤0.3%), confirming selective solubilization of hemicelluloses and lignin over cellulose. The relatively high solid yield and increased glucan proportion indicate that cellulose remained largely preserved and structurally intact under the DAC + MA conditions, consistent with a mild yet selective extraction mechanism (Table 9).

Table 9 Composition of pinewood saw dust extractives and cellulose rich residue duplicates

Sample	% OD pinewood/cellulose-residue
	Lignin				Polysaccharides
	Ins	Sol	Total	Ash	Ara	Gal	Rha	Glc	Xyl	Man
Pinewood_1	30.60	3.59	34.19	0.18	1.18	2.47	0.23	42.12	5.86	10.82
Pinewood_2	31.00	3.79	34.79	0.19	1.17	2.44	0.22	41.84	5.82	10.73
Cellulose_Res_1	15.95	7.82	23.77	0.19	0.10	0.33	0.0	59.53	3.18	7.63
Cellulose_Res_2	16.06	7.75	23.81	0.36	0.12	0.34	0.0	59.03	3.15	7.54

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Environmental impact assessment

Environmental impact assessment of various lignin extraction strategies was carried out using the Waste E-factor and Globally Harmonized System (GHS) weight analysis for chemical toxicity and hazards. The evaluation included the present one-step extraction and functionalization process, Luterbacher's work with γ-valerolactone (GVL)-based extraction, formaldehyde–dioxane stabilization systems, and more recent alternatives such as deep eutectic solvents (DES).^24,45–52E-Factor quantifies the total waste generated per unit mass of product, while the GHS weight factor captures the overall hazard potential by summing the hazard scores of each chemical, weighted by its mass contribution. Lower values for both parameters indicate more environmentally favorable and inherently safer processes.

E-Factor and GHS calculations in biomass valorization systems are inherently approximate because they depend on assumptions about solvent recovery efficiency, reaction scale, and data availability for certain reagents. In this study, these indicators are therefore used as comparative sustainability metrics rather than absolute measures of process impact. The calculation basis—including chemical inputs, recovery rates, waste generation, and hazard scoring—is provided in the SI. Summarized in Table 10 and 11, these results allow side-by-side comparison of the different approaches. Together, the metrics highlight the environmental performance of each system, with particular emphasis on the one-step method's ability to reduce waste and chemical hazards relative to both conventional and emerging lignin extraction techniques.

Table 10 Waste factor calculation (calculations included in SI)^24,46–53

Solvent system	Biomass	Extraction and functionalization	Lignin yield	Temperature and time	Waste (E-factor)
MA + DAC	Pine wood saw dust	Yes	>80%	120–1300 °C, 5 h	7.23
GVL, H2SO4	Norway spruce chips	No	50%	140–1800 °C, 2 h	1.89
GVL, H2SO4	NM6 (populus nigra × populus maximowiczii)	No	56.5%	80–1200 °C, 3 h	2.4
Formaldehyde, 1,4-dioxane, HCL	Birchwood	Yes	80–90%	950 °C, 3.5 h	56.6
Terephthalic aldehyde, HCL, 1,4-dioxane	Birchwood	Yes	80–90%	850 °C, 3 h	342.57
ChCl, FA, LA	Pinewood	No	72%	1300 °C, 6 h	13.7

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Table 11 Hazard score and GHS weight calculations for different processes^54–60

Solvent system	Chemicals	Mass (g)	E + Z + B score	Contributing mass (g) (unrecovered mass)	Contributing GHS weight (mass*(E + B + Z) scores)	Total GHS weight (g)
MA + DAC	Maleic acid	5	25	0.64	16	69.55
	Diallyl carbonate	9.91	17	3.15	53.55
	TBAB (90% recovered)	4.5	18	—	—
GVL, H2SO4	γ-Valerolactone (GVL)	8.8	12	0.1	1.2	1.242
	Sulfuric acid	0.002	21	0.002	0.042
GVL, H2SO4	γ-Valerolactone (GVL)	11.11	12	0.11	1.32	2.895
	Sulfuric acid	0.075	21	0.075	1.575
282.946	Formaldehyde	0.465	42	0.465	19.53
	Hydrochloric acid	0.207	20	0.207	4.14
	Methanol	3.484	19	3.484	66.196
	1,4-Dioxane	10.31	18	10.31	185.58
	Sodium bicarbonate	0.75	10	0.75	7.5
Terephthalic aldehyde, HCL, 1,4-dioxane	Diethyl ether	57	12	57	684	935.6
	Terephthalic aldehyde	1.89	12	1.89	22.68
	Hydrochloric acid	0.207	20	0.207	4.14
	1,4-Dioxane	12.36	18	12.36	222.48
	Sodium bicarbonate	0.23	10	0.23	2.3
ChCl : LA : FA	Choline chloride	5	16	0	0	36
	Lactic acid	3.3	10	0	0
	Formic acid	1.7	21	0	0
	Ethanol	18	10	3.6	36

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Industrial feasibility

When evaluating the industrial feasibility of lignin fractionation technologies, solvent recovery, sustainability metrics, and the quality of co-product streams must be considered together. In this one-pot process using maleic acid (MA), diallyl carbonate (DAC), and tetrabutylammonium bromide (TBAB), solvent recovery yields were 90% for MA, ∼100% for TBAB, and 68% for DAC. Importantly, the recovered DAC exhibited exceptionally high purity (almost 100% from NMR analysis), supporting direct reuse with minimal supplementation. Waste factor (E-factor) analysis further highlights the trade-offs: the MA + DAC process yielded an E-factor of 7.23, which is higher than γ-valerolactone (GVL)/H₂SO₄ systems (1.9–2.4) but dramatically lower than formaldehyde–dioxane systems (56–342). Similarly, GHS-weighted solvent burden calculations show that while the MA + DAC system carries a higher hazard score (≈70 g) than GVL/H₂SO₄ (<3 g), it remains an order of magnitude greener than aldehyde–dioxane systems (>280–900 g). Thus, the one-pot approach balances yield, functionality, and sustainability more effectively than many state-of-the-art solvent systems.

The distinct advantage of this process lies in the simultaneous extraction and in situ functionalization of lignin, rather than yield alone. Benchmark systems such as GVL organosolv or aldehyde-stabilization routes developed by Luterbacher and co-workers generally extract lignin first and require downstream functionalization, which adds process complexity and additional solvent/energy demands.^24,46,50,61 By integrating these steps, the MA + DAC system streamlines lignin valorization, potentially lowering both capital and operating costs. This aligns with Over et al., who reported DAC recoveries of 60% during lignin allylation and noted that solvent closure improves substantially at scale.³⁹ Likewise Cai et al., demonstrated that MA can be distilled to ∼50 wt% and reused directly with a ∼5% spike to compensate for process losses, validating solvent recyclability in similar systems.²⁹

In terms of solid fraction valorization, approximately 0.51 g of cellulose-rich residue was recovered from 1 g of pinewood with roughly around 70% delignification. Compositional analysis confirmed substantial carbohydrate enrichment, with glucan content rising to ∼59% in the residue compared to ∼42% in pinewood, while total lignin content decreased from ∼34–35% to ∼23–24%. Hemicellulose components (xylan and mannan) were partially preserved, indicating selective lignin removal while retaining structural polysaccharides.

This demonstrates that the one-pot process yields not only functionalized lignin but also a carbohydrate-enriched solid fraction suitable for downstream valorization (e.g., enzymatic hydrolysis, nanocellulose production).

Finally, the energy requirements of solvent recovery remain a critical consideration. While this process avoids high-pressure reactors used in reductive catalytic fractionation (RCF), all solvents are still recovered by evaporation, which can be energy intensive given the high solvent-to-biomass ratios (5 wt% biomass loading). Technoeconomic assessments of RCF have shown that solvent recovery and water removal account for ∼73% of the biomass energy content and ∼35% of total operating costs.⁶¹ Comparable analyses using Aspen Plus modeling are needed here to quantify energy demands and confirm whether the evaporative recovery used in this work is less intensive than distillation-based systems.

In summary, the MA + DAC system is not superior in terms of yield alone but offers a unique integration of lignin extraction and functionalization, strong solvent recoverability, and a cellulose-rich residue enriched in glucan. With improvements in biomass loading, energy analysis, and validation of carbohydrate quality, this approach could represent a scalable and sustainable alternative for lignin valorization in integrated biorefineries.

Conclusion

The innovative one-step strategy developed for the simultaneous extraction and functionalization of lignin directly from biomass represents a meaningful advancement in sustainable materials synthesis. Using a specifically designed solvent system under controlled conditions in the Parr reactor, the process produced lignin with distinct features—high allyl and carboxylic content, low molecular weight, and narrow polydispersity—as confirmed by GPC, ¹H, ³¹P, 2D HSQC, and FTIR analyses. In contrast to conventional commercial lignins, the lignin obtained here contained abundant allyl, carboxylic, and aldehyde groups, enabling diverse downstream chemistries. Allyl functionalities can be employed in epoxy synthesis or free-radical polymerization, while aldehyde and carboxylic groups enable access to acetals, imines, and amides, broadening the polymer design space. Thus, this one-step approach not only eliminates the need for additional functionalization but also provides a sustainable, non-toxic route to multi-functional lignin precursors with versatile upgrading potential.

From an industrial feasibility perspective, the advantage of the MA + DAC system lies not in lignin yield alone—which is comparable to other state-of-the-art methods—but in its integration of extraction and functionalization. Solvent recovery efficiencies were high (90% for MA, ∼100% for TBAB, 68% for DAC with >99% purity), and the process achieved ∼70% delignification while producing a cellulose-rich residue enriched in glucan (∼59%). Sustainability metrics place the process between GVL organosolv and aldehyde–dioxane systems: the E-factor (7.23) and GHS-weight (∼70 g) are higher than GVL/H₂SO₄ (<3 g) but far lower than formaldehyde–dioxane systems (>280–900 g). Importantly, simultaneous functionalization reduces downstream processing steps and chemical inputs. Remaining challenges include the energy intensity of solvent recovery, optimization of biomass loading, and validation of cellulose/hemicellulose quality for high-value applications. With improved recovery strategies, scale-up, and technoeconomic assessment, the MA + DAC route offers a practical and sustainable pathway for early-stage deployment in integrated biorefineries, combining multifunctional lignin precursors with valorization of carbohydrate-rich residues.

Author contributions

K. G., J. S., and S. P. contributed equally to this paper. J. S., and S. P. conceptualized the scope of study. S. P. guided the research progress, acquired funding for the research, and administered the project. J. S. and S. P. contributed to manuscript review and editing. K. G. conducted the experiments, performed material characterization, and wrote the manuscript.

Conflicts of interest

The authors declare no competing interests.

Data availability

The authors confirm that all data supporting the results are included in the article and its supplementary information (SI). The supplementary information includes COSMO-RS modeling data, FTIR and NMR characterization data, and supporting E-factor and GHS hazard calculations. Supplementary information is available. See DOI: https://doi.org/10.1039/d5gc02571k.

Raw data underlying the study's findings can be obtained from the corresponding author upon reasonable request.

Acknowledgements

Research supported as part of the AIM for Composites, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences at the University of Delaware under award #DE-SC0023389 (Literature review, Material synthesis, Characterization and Data analysis) and by the Department of Energy, Basic Energy Sciences under award #DE-SC0021367 (Solvent design, and Characterization). The authors gratefully acknowledge Carlos Baez, USDA Forest Service, Forest Products Laboratory, Madison, WI, for conducting the HPAEC/PAD compositional analyses, and Dr Govind Sharma, Postdoctoral Researcher, University of Delaware, for his assistance with biomass fractionation trials.

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