Investigation into Lewis and Brønsted acid interactions between metal chloride and aqueous choline chloride-oxalic acid for enhanced furfural production from lignocellulosic biomass

Synergistic release of lignin-carbohydrate structure by Lewis acid cations and anions to enhance delignification in deep eutectic solvents

Lewis acids are commonly used in deep eutectic solvents (DESs) to enhance delignification in lignocellulose pretreatment, though the mechanism is not well understood. This study compares the effects of alkali metal chlorides, acidic metal chlorides, and acidic metal non-chlorides on DESs delignification, focusing on metal ions and DESs properties. Results indicate that delignification enhancement follows the order: acidic metal chlorides > acidic metal non-chlorides > alkali metal chlorides. Key factors include pH (Pearson correlation coefficient: -0.88) and hydrogen bond donor activity (correlation: 0.91), and other DESs properties, such as viscosity, surface tension, hydrogen bond acceptor activity, and polarity, provide limited explanatory for delignification performance. The anion (Cl-) forms hydrogen bonds with lignin and lignin-carbohydrate complexes (LCCs) hydroxyl groups and further participates in ether bond cleavage. The destruction of β-O-4 linkages in recovered lignin confirms this finding, alongside the disruption of LCCs. Cations primarily facilitate the hydrolysis of hemicellulose glycosidic bonds by directly cracking and lowering the bond energy (-3.02 kJ/mol). This leads to the release and further degradation of LCCs, especially in γ-ester, p-coumaric acid, and ferulic acid components. These findings provide insight into Lewis acid-enhanced DESs delignification mechanisms.

In-situ conversion of hemicellulose to furfural by Lewis acid-enhanced deep eutectic solvents to maintain stable pretreatment performance and trigger profitable biorefining processes

Deep eutectic solvents (DESs) are gaining attention for lignocellulose pretreatment, yet screening methods and stable cyclic processes remain underexplored. This study compared solubility and machine learning to predict delignification, screening the optimal DESs combination from 168 recombinant ternary DESs. The selected DESs were utilized to develop a stable, recyclable pretreatment process (delignification and hemicellulose removal) via Lewis acid-catalyzed conversion of hemicellulose to furfural. Results suggested the multilayer perceptron model within the machine learning framework achieved the highest accuracy (R2 = 0.96, RMSE = 4.13) and generalization ability for delignification prediction. Lewis acid was employed to enhance the screened DESs (chloride: lactic acid: glycol = 1:5:1) for catalyzing the in situ conversion of hemicellulose to furfural (89.92 %), enhanced delignification (93.15 %) and maintained stable pretreatment performance even after 10 cycles. The cellulose-rich material exhibited higher enzymatic hydrolysis efficiency (78.17 %) and can be used to prepare nanocellulose with a narrower diameter (5.59 nm). Additionally, the lignin isolated by Lewis acid-enhanced DESs exhibited stronger antioxidant activity (IC50 = 0.03 mg/mL) and ultraviolet shielding capability. This work conducts a comprehensive investigation, from DESs screening to establishing a stable and recyclable pretreatment process, advancing the scalable application of DESs pretreatment for biomass processing.

Efficient separation of lignocellulose component and furfural production from hemicellulose using a γ-valerolactone/H2O system

This study investigates a one-step process for the simultaneous separation of cellulose, hemicellulose, and lignin from lignocellulosic biomass using γ-valerolactone (GVL)/H2O pretreatment, which also converts hemicellulose to furfural (FF). A response surface methodology (RSM) was applied to optimize the process parameters. Under optimal conditions (163 °C for 1.5 h), the maximum FF yield reached 85.36 %, and cellulose purity was 80.80 %. The extracted GVL-lignin was characterized as low-molecular-weight guaiacyl-syringyl (G-S) type nanospheres with significant antioxidant activity. The conversion of hemicellulose to FF was enhanced by the synergistic interaction of Lewis acid (MgCl2) and Brønsted acid (H2SO4), which promoted the isomerization of xylose and dehydration to FF. The high-purity cellulose yielded 18.87 g/L ethanol through separate hydrolysis and fermentation (SHF), 27 times higher than untreated solids. Additionally, GVL exhibited consistent performance in lignocellulose separation after six cycles. This method offers an efficient and sustainable approach to valorize lignocellulosic biomass, advancing the production of high-value products.

A novel efficient liquefaction process for corn starch through ternary deep eutectic solvent: Products characterization and liquefaction mechanism

A novel approach for the solvothermal liquefaction of corn starch (CS) was investigated, using ternary deep eutectic solvent (TDES) as both an acidic catalyst and a source of liquefaction reagent. Synergistic effects from multi-component TDES were observed, leading to milder reaction conditions (110 °C, 35 min) and improved product selectivity (relative content of polyhydroxy compounds up to 97.83 %). As a result, a viscosity of 351.76 mPa·s, hydroxyl number of 131.98 mg KOH/g, and acid number of 5.56 mg KOH/g of LP-60 with comparable polyol properties were obtained. Liquefaction residue analysis revealed a significant enhancement in the reactivity of CS by disrupting its recalcitrant structure through electrostatic interactions and hydrogen bonding in TDES, as evidenced by the significant reduction of short-range ordering (R1047/1022 and R995/1022) and C1 (C-C/C-H), C3 (C=O/O-C-O) components of the starch granule surface. According to the gas chromatography-mass spectroscopy result, the reaction pathway and mass balance for CS liquefaction in the TDES system were proposed. This study provides an efficient and mild route for starch liquefaction using TDES.

An integrated biorefinery strategy for Eucalyptus fractionation and co-producing glucose, furfural, and lignin based on deep eutectic solvent/cyclopentyl methyl ether system

A novel biphasic system containing water-soluble deep eutectic solvent (DES) and cyclopentyl methyl ether (CPME) was developed to treat Eucalyptus for furfural production, extracting lignin and enhancing cellulose enzymatic hydrolysis. Herein effect of DES type, water content in DES, temperature and time on furfural yield in water-soluble DES/CPME pretreatment process was firstly evaluated. A maximum furfural yield of 80.6 % was attained in 10 min at 150 °C with choline chloride (ChCl)/citric acid monohydrate (CAM)/CPME system containing 30 wt% water and 2.5 wt% SnCl4·5H2O, which was higher than that obtained from ChCl/CAM/CPME system without water (55.5 %) and H2O/CPME system (49.7 %). These results demonstrated that the water-soluble DES/CPME system was a powerful method enhancing the furfural production. Under the optimal pretreatment conditions, the delignification and glucose yield were reached to 72.7 % and 94.3 %, respectively. The extracted lignin showed low molecular weight and β-aryl-ether was obviously cleaved. Additionally, water-soluble DES/CPME pretreatment led to a significant removal of hemicelluloses (100.0 %) and lignin (72.7 %) and introduced morphological changes on cell walls, especially from the cell corner (CC) and secondary wall (SW) layers. Overall, this work proposed a practical one-step fractionation strategy for co-producing furfural, lignin and fermentable sugar, providing a way to biorefinery.

One-pot, microwave (MW)-assisted production of furfural from almond-, oil-, and wine-derived co-products through biorefinery-based approaches

This work outlines the first microwave (MW)-assisted protocol for the production of biofuel precursor furfural (FF) from the raw agricultural waste almond hull (AH), olive stone (OS), and the winemaking-derived grape stalk (GS), grape marc (GM) and exhausted grape marc (EGM) through a one-pot synthesis process. To enhance the overall yield, a catalytic process was firstly developed from xylose, major constituent of hemicellulose present in lignocellulosic biomass. This method afforded FF with 100 % selectivity, yielding over 85 % in isolated product when using H2SO4, as opposed to a 37 % yield with AlCl3·6H2O, at 150 °C in only 10 min. For both catalysts, the developed methodology was further validated, proving adaptable and efficient in producing the targeted FF from the aforementioned lignocellulosic raw materials. More specifically, the employment of AlCl3·6H2O resulted in the highest selectivity (up to 89 % from GM) and FF yield (42 % and 39 % molar from OS and AH, respectively), maintaining notable selectivity for the latter (61 and 48 % from AH and OS). At this regard, and considering the environmental factor of sustainability, it is important to point out the role of AlCl3·6H2O in contrast to H2SO4, thus mitigating detrimental substances. This study provides an important management of agricultural waste through sustainable practises for the development of potential bio-based chemicals, aligning with Green Chemistry and process intensification principles.

Fractionation of oil palm fronds using ethanol-assisted deep eutectic solvent: Influence of ethanol concentration on enhancing enzymatic saccharification and lignin β-O-4 content

Numerous fractionation methods have been developed in recent years for separating components such as cellulose, hemicellulose, and lignin from lignocellulosic biomass wastes. Deep eutectic solvents (DES) have recently been widely investigated as captivating green solvents for biomass fractionation. However, most acidic-based deep eutectic solvent fractionation produces condensed lignin with low β-O-4 content. Besides, most DESs exhibit high viscosity, which results in poor mass transfer properties. This study aimed to address the challenges above by incorporating ethanol into the deep eutectic solvent at various concentrations (10-50 wt%) to fractionate oil palm fronds at a mild condition, i.e., 80 °C, 1 atm. Cellulose residues fractionated with ethanol-assisted deep eutectic solvent showed a maximum glucose yield of 85.8% when 20 wt% of ethanol was incorporated in the deep eutectic solvent, significantly higher than that achieved by pure DES (44.8%). Lignin extracted with ethanol-assisted deep eutectic solvent is lighter in color and higher in β-O-4 contents (up to 44 β-O-4 per 100 aromatic units) than pure DES-extracted lignin. Overall, this study has demonstrated that incorporating ethanol into deep eutectic solvents could enhance the applicability of deep eutectic solvents in the complete valorization of lignocellulosic biomass. Highly enzymatic digestible cellulose-rich solid and β-O-4-rich lignin attained from the fractionation could serve as sustainable precursors for the production of biofuels.

Recent advances in biorefineries based on lignin extraction using deep eutectic solvents: A review

Considering the urgent need for alternative biorefinery schemes based on sustainable development, this review aims to summarize the state-of-the-art in the use of deep eutectic solvent pretreatment to fractionate lignocellulose, with a focus on lignin recovery. For that, the key parameters influencing the process are discussed, as well as various strategies to enhance this pretreatment efficiency are explored. Moreover, this review describes the challenges and opportunities associated with the valorization of extraction-derived streams and highlights recent advancements in solvent recovery techniques. Furthermore, the utilization of computational models for process design and optimization is introduced, as the initial attempts at the economic and environmental assessment of this lignocellulosic bioprocess based on deep eutectic solvents. Overall, this review offers a comprehensive perspective on the recent advances in this emerging field and serves as a foundation for further research on the potential integration of deep eutectic pretreatment in sustainable multi-product biorefinery schemes.

FeIII -Based Eutectic Mixtures as Multi-task and Reusable Reaction Media for Efficient and Selective Conversion of Alkynes into Carbonyl Compounds

An efficient, simple and general protocol for the selective hydration of terminal alkynes into the corresponding methyl ketones has been developed by using a cheap, easy-to-synthesise and sustainable FeIII -based eutectic mixture [FeCl3  ⋅ 6H2 O/Gly (3 : 1)] as both promoter and solvent for the hydration reaction, working: i) under mild (45 °C) and bench-type reaction conditions (air); and ii) in the absence of ligands, co-catalysts, co-solvents or toxic, non-abundant and expensive noble transition metals (Au, Ru, Pd). When the final methyl ketones are solid/insoluble in the eutectic mixture, the hydration reaction takes place in 30 min, and the obtained methyl ketones can be isolated by simply decanting the liquid FeIII -DES, allowing the direct isolation of the desired ketones without VOC solvents. By using this straightforward and simple isolation protocol, we have been able to recycle the FeIII -based eutectic mixture system up to eight consecutive times. Furthermore, the FeIII -eutectic mixture is able to promote the selective and efficient formal oxidation of internal alkynes into 1,2-diketones, with the possibility of recycling this system up to three consecutive times. Preliminary investigations into a possible mechanism for the oxidation of the internal alkynes seem to indicate that it proceeds through the formation of the corresponding methyl ketones and α-chloroketones.

Recent advances in the application of alcohols in extracting lignin with preserved β-O-4 content from lignocellulosic biomass

Utilizing lignocellulosic biomass wastes to produce bioproducts is essential to address the reliance on depleting fossil fuels. However, lignin is often treated as a low-value-added component in lignocellulosic wastes. Valorization of lignin into value-added products is crucial to improve the economic competitiveness of lignocellulosic biorefinery. Monomers obtained from lignin depolymerization could be upgraded into fuel-related products. However, lignins obtained from conventional methods are low in β-O-4 content and, therefore, unsuitable for monomer production. Recent literature has demonstrated that lignins extracted with alcohol-based solvents exhibit preserved structures with high β-O-4 content. This review discusses the recent advances in utilizing alcohols to extract β-O-4-rich lignin, where discussion based on different alcohol groups is considered. Emerging strategies in employing alcohols for β-O-4-rich lignin extraction, including alcohol-based deep eutectic solvent, flow-through fractionation, and microwave-assisted fractionation, are reviewed. Finally, strategies for recycling or utilizing the spent alcohol solvents are also discussed.

Microwave-assisted choline chloride/1,2-propanediol/methyl isobutyl ketone biphasic system for one-pot fractionation and valorization of Eucalyptus biomass

The developing of pretreatment method to break the biomass barrier of lignocellulosic is a challenging task for achieve high value utilization. A fast microwave-assisted choline chloride/1,2-propanediol/methyl isobutyl ketone biphasic system was constructed for pretreating Eucalyptus to the production of furfural and cellulose-rich residues and the extraction of lignin. Results showed that the combination of AlCl₃·6H₂O and HCl had the best catalytic ability for furfural production among the examined catalysts. Under the optimal conditions (140 °C, 15 min, 0.075 M AlCl₃·6H₂O, 0.05 M HCl), the furfural yield of 55.4 %, the glucose yield of 90.3 % and the delignification rate of 92.4 % could be achieved. Moreover, the extracted lignin samples with a low polydispersity (1.55-1.73) and molecular weight (1380-2040 g/mol) are promising to act as precursor for the value-add products processing. These findings demonstrated an ultrafast pretreatment process with excellent results in biomass fractionation and comprehensive utilization of biomass components.

Chemoenzymatic catalytic synthesis of furfurylamine from hemicellulose in biomasses

Recently, efficient synthesis of furan-based chemicals from biomacromolecule via chemoenzymatic approaches have been widely recognized. In this work, an efficient conversion of biomacromolecule (e.g., xylan in biomass) to furfurylamine (FLA) was developed in a tandem reaction by bridging with chemocatalysis and biocatalysis. Various biomasses (e.g., corncob, bagasse, bamboo shoot shell, corn stalk, rice straw stalk, reed, water bamboo and sunflower stalk) could produce different titer of furfural due to the diverse xylan content in biomass. After being catalyzed by shrimp shell-supported solid acid catalyst (Sn-DAT-SS) in deep eutectic solvent choline chloride:ethylene glycol (ChCl:EG) - water (10:90, v/v) at 170 °C after 30 min, corncob gave the highest furfural yield of 52.4 %. The potential catalytic mechanism for Sn-DAT-SS-catalyzing the conversion of biomass into furfural in ChCl:EG - water was proposed. It was found that by-products (formic acid, levulinic acid, 5-hydroxymethylfurfural) and soluble sugars (glucose, xylose, arabinose, cellobiose) produced during the conversion of biomass to furfural had certain inhibition effects on the biotransamination of furfural to FLA. Biomass-derived furfural (36.7-92.3 mM) could be fully aminated to FLA by E. coli CCZU-XLS160 cells harboring ω-transaminase after 24-72 h. The established chemoenzymatic strategy for converting biomacromolecules into valuable furan-based products was successfully developed in an eco-friendly system.