Microwave-assisted extraction of lignin from coconut coir using deep eutectic solvents and its valorization to aromatics

The role of NMR spectroscopy in lignocellulosic biomass characterisation: A mini review

Lignocellulosic biomass (LB) is promising feedstock for the production of various bio-based products. However, due to its heterogenous character, complex chemical structure and recalcitrance, it is necessary to know its structural composition in order to optimize pretreatment process and further (bio)conversion into bio-based products. Nuclear Magnetic Resonance (NMR) spectroscopy is a fast and reliable method that can provide advanced data on the molecular architecture and composition of lignocellulosic biomass. In this brief overview, characteristic examples of the use of high-resolution NMR spectroscopy for the investigation of various types of LB and their structural units are given and the main drawbacks and future perspectives are outlined.

Deep eutectic solvents for the extraction of polyphenols from food plants

Deep Eutectic Solvents (DESs) offer a promising, sustainable alternative for extracting polyphenols from food plants, known for their health benefits. Traditional extraction methods are often costly and involve toxic solvents. This review discusses the basic concepts, preparation techniques, and factors influencing the effective and safe use of DESs in polyphenol extraction. DESs' adaptability allows integration with other green extraction technologies, such as microwave- and ultrasound-assisted extractions, enhancing their efficiency. This adaptability demonstrates the potential of DESs in the sustainable extraction of bioactive compounds. Current research indicates that DESs could play a significant role in the sustainable procurement of these compounds, marking an important advancement in food science research and development. The review underscores DESs as a realistic, eco-friendly alternative in the realm of natural extraction technologies, offering a significant contribution to sustainable practices in food science.

Hydrogen-transfer strategy in lignin refinery: Towards sustainable and versatile value-added biochemicals

Lignin, the most prevalent natural source of polyphenols on Earth, offers substantial possibilities for the conversion into aromatic compounds, which is critical for attaining sustainability and carbon neutrality. The hydrogen-transfer method has garnered significant interest owing to its environmental compatibility and economic viability. The efficacy of this approach is contingent upon the careful selection of catalytic and hydrogen-donating systems that decisively affect the yield and selectivity of the monomeric products resulting from lignin degradation. This paper highlights the hydrogen-transfer technique in lignin refinery, with a specific focus on the influence of hydrogen donors on the depolymerization pathways of lignin. It delineates the correlation between the structure and activity of catalytic hydrogen-transfer arrangements and the gamut of lignin-derived biochemicals, utilizing data from lignin model compounds, separated lignin, and lignocellulosic biomass. Additionally, the paper delves into the advantages and future directions of employing the hydrogen-transfer approach for lignin conversion. In essence, this concept investigation illuminates the efficacy of the hydrogen-transfer paradigm in lignin valorization, offering key insights and strategic directives to maximize lignin's value sustainably.

Current progress and future perspective of microalgae biomass pretreatment using deep eutectic solvents

Pretreatment process is considered as the most important step for effective microalgae biomass refining and has gained more interest since last decades. However, the main obstacles to commercialize microalgae products are recalcitrant cell wall and lack of cost-effective, green, and sustainable pretreatment approaches. Till now, various microalgae pretreatment approaches have been applied prior to extraction steps to enhance the accessibility of solvent inside the cells. However, high energy consumption and the hazardousness of solvents are considerable problem for these pretreatment methods. In this regard, deep eutectic solvents are recognized as sustainable and green solvents possessing great potential for microalgae biomass processing due to their low toxicity, low cost, biodegradability, easy recycling, and reuse. This article provides the fundamentals of DES composition, synthesis, properties, and the current advances in the application of microalgae biomass process.

Efficient extraction of lignin from moso bamboo by microwave-assisted ternary deep eutectic solvent pretreatment for enhanced enzymatic hydrolysis

Applications of deep eutectic solvent (DES) systems to separate lignocellulosic components are of interest to develop environmentally friendly processes and achieve efficient utilization of biomass. To enhance the performance of a binary neutral DES (glycerol:guanidine hydrochloride), various Lewis acids (e.g., AlCl3·6H2O, FeCl3·6H2O, etc.) were introduced to synthesize a series of ternary DES systems; these were coupled with microwave heating and applied to moso bamboo. Among the ternary DES systems evaluated, the FeCl3-based DES effectively removed lignin (81.17%) and xylan (85.42%), significantly improving enzymatic digestibility of the residual glucan and xylan (90.15% and 99.51%, respectively). Furthermore, 50.74% of the lignin, with high purity and a well-preserved structure, was recovered. A recyclability experiment showed that the pretreatment performance of the FeCl3-based DES was still basically maintained after five cycles. Overall, the microwave-assisted ternary DES pretreatment approach proposed in this study appears to be a promising option for sustainable biorefinery operations.

Natural arginine-based deep eutectic solvents for rapid microwave-assisted pretreatment on crystalline bamboo cellulose with boosting enzymatic conversion performance

Development of amino acid-based natural deep eutectic solvents (DESs) pretreatment technology on lignocellulosic biomass is a promising approach to biorefinery. In this study, for arginine-based DESs with different molar ratios, the viscosity and Kamlet-Taft solvation parameters were quantified to evaluate the pretreatment performance on bamboo biomass. Further, microwave assisted DES pretreatment was eminent, evidenced by 84.8% lignin removal and increased saccharification yield (from 6.3% to 81.9%) in moso bamboo at 120 °C and ratio of 1:7 (arginine: lactic acid). Results showed degradation of lignin molecules together with release of phenolic hydroxyl units after DESs pretreatment, which is conducive to subsequent utilization. Meanwhile, DES-pretreated cellulose exhibited unique structural characteristics including destroyed crystalline region of cellulose (Crystallinity Index from 67.2% to 53.0%), decreased crystallite size (from 3.41 nm to 3.14 nm) and roughened fiber surface. Thus, arginine-based DES pretreatment has excellent potential in bamboo lignocellulose pretreatment.

Structure investigation of light-colored lignin extracted by Lewis acid-based deep eutectic solvent from softwood

Lignin is the most abundant natural phenolic polymer. However, the severe condensations of industrial lignin resulted in an undesirable apparent morphology and darker color, which hindered its application in the field of daily chemicals. Therefore, a ternary deep eutectic solvent is used to obtain lignin with light-color and low condensations from softwood. The results showed that the brightness value of lignin extracted from aluminum chloride-1,4-butanediol-choline chloride at 100 °C and 1.0 h was 77.9, and the lignin yield was 32.2 ± 0.6%. It is important that 95.8% of β-O-4 linkages (β-O-4 and β-O-4') was retained. Lignin is used to prepare sunscreens and is added to physical sunscreens at 5%, with SPF up to 26.95 ± 4.20. Meanwhile, enzyme hydrolysis experiments and reaction liquid composition tests were also conducted. In conclusion, a systematic understanding of this efficient process could facilitate high-value utilization of lignocellulosic biomass in industrial processes.

Valorization of lignin for the production of vanillin by Bacillus aryabhattai NCIM 5503

Coconut coir waste is a rich lignocellulosic biomass. The coconut coir waste generated from temples is resistant to natural degradation, and its accumulation causes environmental pollution. Ferulic acid, a vanillin precursor, was extracted from the coconut coir waste by hydro-distillation extraction. The extracted ferulic acid was used for vanillin synthesis by Bacillus aryabhattai NCIM 5503 under submerged fermentation. In the present study, the Taguchi DOE (design of experiment) software was used to optimize the fermentation process, which resulted in a 1.3 fold increase in vanillin yield (640.96±0.02 mg/L), as compared to the unoptimized yield of 495.96±0.01 mg/L. The optimized media for enhanced vanillin production comprised; fructose 0.75 % (w/v), beef extract 1 % (w/v), pH 9, temperature 30℃, agitation speed 100 rpm, trace metal solution 1 % (v/v), and ferulic acid 2 % (v/v). The results show that the commercial production of vanillin can be envisioned using coconut coir waste.

Ionic liquid as hydrogen bond acceptor in the extraction of nutritional natural products

In-depth studies of the extraction mechanism using deep eutectic solvents (DES), especially extraction through the formation of a deep eutectic system (DESys), revealed commonalities between the DES- and ionic liquids (IL)-based extraction systems. New applications of ILs and DES for extraction of nutritional natural products were presented. In this study, the extraction behavior of choline chloride (ChCl) and 1-(2-hydroxyethyl)-3-methylimidazolium chloride ([HMIm][Cl]) in DES and IL, respectively, in mechanochemical extraction of target compounds from Moringa oleifera leaves was systematically studied. The results suggested that both extraction methods were based on the formation of a DESys, either a normal DESys or an IL DESys. Considering the DESys-based one-step extraction improves the extraction efficiency and reduces the preparation time, the same idea can be used in IL for performance improvement. By formation of a new IL deep eutectic system based on hydrogen bond interaction in extraction, similar improvement was obtained.

Multistage treatment of bamboo powder waste biomass: Highly efficient and selective isolation of lignin components

Bamboo pulp and papermaking produce a lot of bamboo powder waste, and its resource utilization is of great significance for biomass refining and environmental protection. Here, we propose an integrated approach involving mechanical activation, hydrothermal extraction, and deep eutectic solvents (DESs) multiple delignification for the efficient separation of bamboo powder. Among seven carboxylic acids based DESs, choline chloride (ChCl)-lactic acid (La) DES (1:1) is the most effective, with over 78.0% lignin removal and 88.9% cellulose retained after mechanical-hydrothermal (180 °C, 5 h)-DES (110 °C, 12 h) treatment. Notably, 84.7% of delignification is achieved after three times of ChCl-La DES treatment at 70, 90, and 110 °C respectively. The delignification rate is negatively correlated with the amount of carboxyl group in the DESs. The lower the pKa value, the higher the delignification rate. Additionally, the selectivity for lignin is improved with decreasing solvent polarity. DES treatment effectively degrades the guaiacyl unit lignin fractions and disrupts several β-aryl-ether bonds (e.g., β-O-4, β-β, and β-5). Furthermore, DESs exhibit good recyclability, with less than 10% reduction in delignification after three cycles. Theory calculations confirm that ChCl-carboxylic acid DESs could compete with lignin to break hydrogen bonds in lignocellulosic biomass by providing their chloride, hydroxyl, and carboxyl groups. Overall, this study demonstrates the practical significance of multistage treatment for the effective fractionation of biomass into its three components.

Selective degradation of hemicellulose and lignin for improving enzymolysis efficiency via pretreatment using deep eutectic solvents

Deep eutectic solvents (DESs) with different acidity and alkalinity were applied for biomass pretreatment, and the conditions were optimized by response surface methodology. The results showed that lactic acid/betaine hydrochloride had the optimal pretreatment efficiency, where the removal rates of hemicellulose and lignin came up to 89% and 73%, and the enzymolysis efficiency was as high as 92%. Furthermore, eight types of chloride salts with different valence states were introduced into the DESs as the third component. The chloride salts could improve the pretreatment efficiency and positively correlated with the metal valence state. Specifically, AlCl₃ was significantly superior in improving the pretreatment efficiency, where the enzymolysis efficiency reached 96% due to the destruction of crystalline region and the esterification of partial cellulose. Therefore, it is proposed that adding highly valent metal salts to acidic DESs has higher pretreatment and enzymatic efficiency.

Ultrafast process of microwave-assisted deep eutectic solvent to improve properties of bamboo dissolving pulp

Viscosity control and reactivity enhancement are critical to produce high-quality cellulose products, such as dissolving pulp, yet remain challenging. In this work, an ultrafast process, namely microwave-assisted deep eutectic solvent (MW-DES), is proposed for this purpose. It is based on the hypothesis that the MW-DES process can deliver an enhanced synergy: a simultaneous fiber swelling and cellulose depolymerization via hydrogen-bonding break-up and acid hydrolysis from the actions of polar and acidic DES further boosted under MW irradiation. Results showed that after the MW-DES (Choline chloride- oxalic acid, ChCl-OA) treatment for only 40 s, the pulp viscosity decreased from 715 to 453 mL/g, and the reactivity increased from 43.0 % to 84.6 %, which is ultrafast in comparison with those reported work. Furthermore, DES in the process shows a high reusability and chemical stability, thus offering a simple, sustainable and effective alternative for upgrading of dissolving pulp, particularly, using non-wood materials of bamboo.

Process intensification strategies for green solvent mediated biomass pretreatment

Demonstrated to be highly effective for lignocellulosic biomass pretreatment, deep eutectic solvent (DES) has attracted increasing attention owing to its advantages of simple synthesis, relatively low chemical cost, and better biocompatibility as compared to certain ionic liquids. Here we provide a critical review of the status of the design/selection of DES for the pretreatment of biomass feedstocks with an emphasis on the process intensification strategies: 1) integration of microwave, ultrasound, and high solid extrusion for pretreating biomass, 2) one-pot DES pretreatment, enzymatic hydrolysis, and fermentation, 3) strategies for DES recycling and product recovery; and 4) recent progress on molecular simulations toward understanding the interactions between DES and biomass compounds such as lignin and cellulose. Lastly, we provide perspectives toward cost-effective, continuous, high-solid, environmental-benign, and industrial-relevant applications and point to future research directions to address the challenges associated with DES pretreatment.

A review on potential of green solvents in hydrothermal liquefaction (HTL) of lignin

One of the greatest challenges in biorefinery is to reduce biomass' recalcitrance and enable valorization of lignin into higher value compounds. Likewise, green solvents and hydrothermal liquefaction (HTL) with feasible economic viability, functionality, and environmental sustainability have been widely introduced in extraction and conversion of lignin. This review starts with the underscore of disadvantages and limitations of conventional pretreatment approaches and role of green solvents in lignin extraction. Subsequently, the effect of process parameters along with the reaction mechanisms and kinetics on conversion of lignin through HTL were comprehensively reviewed. The limitations of green solvents in extraction and HTL of lignin from biomass were discussed based on the current advancements of the field and future research scopes were also proposed. More details info on HTL of biomass derived lignin which avoid the energy-intensive drying procedures are crucial for the accelerated development and deployment of the advanced lignin biorefinery.

Deep eutectic solvents as promising pretreatment agents for sustainable lignocellulosic biorefineries: A review

Increasing reliance on non-renewable fuels has shifted research attention to environmentally friendly and sustainable energy sources.The inherently recalcitrant nature of lignocellulosic biomass (LCB) makes downstream processing of the bioprocess challenging. Deep eutectic solvents (DESs) are popular and inexpensive green liquids found effective for LCB valorisation. DESs have negligible vapor-pressure and are non-flammable, recyclable, cost-economic, and thermochemically stable. This review provides a detailed overview on the DESs types, properties and their role in effective delignification and enzymatic digestibility of polysaccharides for cost-effective conversion of LCB into biofuels and bioproducts. The conglomeration of DESs with assistive pretreatment techniques can augment the process of biomass deconstruction. The current challenges in upscaling the DESs-based pretreatment technology up to commercial scale is summarized, with possible solutions and future directions. These insights would fill the knowledge-gaps to towards development of lignocellulosic biorefineries and to address the global energy crisis and environment issues.