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Review

Comprehensive Review of Recent Trends in the Use of Deep Eutectic Solvents for the Valorization of Secondary Lignocellulosic Biomass

by
Akerke Toleugazykyzy
1,2,
Kairat Bekbayev
2,*,
Bakytzhan Bolkenov
2,*,
Duried Alwazeer
3,
Berdikul Rskeldiyev
2,4,
Kairat Kuterbekov
5,
Kenzhebatyr Bekmyrza
5,
Asset Kabyshev
5,
Marzhan Kubenova
5 and
Serikzhan Opakhai
5
1
Department of Food Technology and Processing Products, Technical Faculty, S. Seifullin Kazakh Agortechnical University, 010011 Astana, Kazakhstan
2
Department of Technological Equipment, Shakarim University, 071410 Semey, Kazakhstan
3
Department of Nutrition and Dietetics, Faculty of Health Sciences, Igdir University, 760000 Igdir, Turkey
4
Department of Food Technology, Almaty Technological University, 050012 Almaty, Kazakhstan
5
Faculty of Transport and Energy, L.N. Gumilev Eurasian National University, 010008 Astana, Kazakhstan
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(21), 9492; https://doi.org/10.3390/su17219492 (registering DOI)
Submission received: 31 August 2025 / Revised: 16 October 2025 / Accepted: 20 October 2025 / Published: 24 October 2025
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Review Reports Versions Notes

Abstract

Conventional solvents remain the most used media for lignocellulosic biomass valorization. However, these solvents exhibit many limitations and have a negative environmental impact. In the last decade, Deep Eutectic Solvents (DESs) have emerged as a multifaceted tool in biomass valorization, with a promising perspective in the application of lignocellulosic biomass valorization. DESs have gained attention in the last decade as an alternative solvent in biomass valorization and biorefinery processes due to their high efficiency; eco-friendliness; low cost; and numerous other advantages, such as recyclability, non-volatility, and stability. This paper discusses the latest research on the potential applications of DESs in the valorization of secondary lignocellulosic biomass.

1. Introduction

In recent years, with the increased demand for energy worldwide and growing global warming concerns, attention has been drawn to lignocellulosic biomass as a potential source for biofuels [1]. It is expected that global demand for biofuel will increase by 28% in the upcoming years [2]. Lignocellulosic biomass has been considered as a sustainable alternative to the depleting fossil fuels to meet the increased demand for energy. In addition, the valorization of lignocellulosic biomass can give a wide range of other value-added products. Secondary lignocellulosic biomass refers to residues and by-products generated from agriculture, agro-processing, and food industries, which remain rich in valuable biopolymers and bioactive compounds. The potential economic benefit of valorizing secondary lignocellulosic biomass lies in obtaining value-added products such as biofuels, bioplastics, fertilizers, phenolic compounds, flavonoids, peptides, polyphenolic antioxidants, and plant proteins [1,3].
From a circular economy perspective, turning waste biomass from the agriculture and food industry into chemicals and fuel is a winning issue as it reduces environmental burdens while creating economic opportunities [4]. Every year, around the world, millions of tons of secondary lignocellulosic residues are discarded or openly burned, causing severe environmental problems such as air pollution, greenhouse gas emissions, and soil degradation [4,5,6]. Also, in our globe, 10–50% of agricultural products end up as waste each year [6]. Despite their valuable constituents, secondary lignocellulosic biomass in many countries around the world is not valorized due to the complex, highly recalcitrant composition of lignocellulosic biomass [7]. The primary barriers of lignocellulosic biomass valorization on a large scale are the high cost of pretreatment technologies and the use of conventional solvents, which are often expensive and environmentally harmful [8,9]. The challenges of lignocellulosic residue handling are more prevalent in developing countries, due to the limited infrastructure and the lack of investments [10]. Therefore, there is an urgent need for innovative, cost-effective, and environmentally friendly solutions to utilize the full potential of lignocellulosic biomass.
Solvents play an important role in biomass valorization and biorefinery processes by helping to convert the biomass into value-added products such as biofuels, different chemicals, and materials. Solvents are used at different stages of biomass valorization and biorefinery. These stages or processes include pretreatment, fractionation, extraction, enzymatic saccharification, and catalytic conversion. Conventional solvents are inefficient, costly, and environmentally harmful. Therefore, green solvents with low cost and higher efficiency, such as Deep Eutectic Solvents (DESs), have gained the attention of researchers in the last decade. DESs share similar characteristics and physical properties with ionic liquids, but they offer extra advantages such as low cost, ease of preparation, biodegradability, and non-toxicity [11]. A large body of research on DES application for the valorization of lignocellulosic biomass has been accumulated during this period.
In most cases, DESs have been used as a solvent for pretreatment and extraction purposes [12,13,14]. In addition to solvents, DESs can serve as catalysts and reaction media. Additionally, DESs can be utilized in CO2 separation and enzymatic saccharification [15,16]. Moreover, ongoing research might further increase the areas of application of DES. For example, the DESs can be applied to produce carbohydrate-derived chemicals such as furfural, 5-hydroxymethylfurfural (HMF), and levulinic acid in carbohydrate conversion processes [16,17].
This review will provide an overview of DES applications in the valorization of secondary lignocellulosic biomass, which is not managed properly worldwide, especially in developing countries. This review covers literature published from January 2000 to October 2025. Manuscript preparation and analysis were conducted from March 2024 to October 2025.

2. Current DES Types, Composition, and Characteristics

Current DESs consist of at least two components, one of which acts as a hydrogen bond donor (HBD) and the other as a hydrogen bond acceptor (HBA) species. The interaction of HBD and HBA lowers the melting point of the DES solution compared to individual components. This unique quality of DES makes it attractive as a solvent for various applications, including pretreatment and extraction processes of different biomasses [18]. In Table 1, the most common hydrogen bond acceptors and donors for DES are presented. Despite the wide range of combinations, only a few DESs have shown significant effectiveness. Some studies have reported that choline chloride-based DESs are more efficient than other types of DESs [19].
Based on the chemical composition, DESs can be categorized into five main types. DES types I–III mostly include quaternary ammonium salts, such as choline chloride, combined with hydrogen bond donors, including urea, glycerol, ethylene glycol, or organic acids (formic, lactic, citric, or malonic). Type IV DESs are formed by metal salts and hydrogen bond donors, exhibiting higher ionic strength, and often used for catalytic or electrochemical purposes [20,21]. Type V DESs consist of non-ionic constituents, primarily molecular hydrogen-bond donors and acceptors [22]. In addition to these five types of DESs, recently, natural deep eutectic solvents (NADESs) have been developed, consisting entirely of naturally derived and biodegradable components such as sugars, amino acids, organic acids, and choline derivatives [23]. Their biocompatibility and non-toxicity make them particularly attractive for food, pharmaceutical, and biorefinery applications [23,24].
The efficiency of solvents in biomass valorization is dictated by their fundamental physicochemical properties, such as their ability to form hydrogen bonds, acid-base interactions, polarity, viscosity, and specific molecular interactions between the solvent and solute [25,26]. Hydrogen bond formation and disruption are considered fundamental solvent action mechanisms. For example, hydrogen bonding between the solvent and hydroxyl groups of polysaccharides can weaken or completely disrupt the extensive hydrogen-bonding network, thereby enhancing solubility and reducing recalcitrance in DES.
The physicochemical properties of DES depend strongly on the type and molar ratio of their constituents. Compared with ionic liquids, DESs generally exhibit lower melting points, moderate viscosities, and tunable polarity, allowing selective solubilization of lignin and hemicellulose while maintaining cellulose integrity. Their hydrogen-bonding networks contribute to low volatility and non-flammability, while their high thermal and chemical stability make them attractive for biomass pretreatment and extraction. For example, choline chloride–lactic acid (1:2) and choline chloride–formic acid (1:2) are widely used for delignification due to their high acidity and efficient hydrogen-bonding capability, whereas choline chloride–glycerol (1:2) provides mild conditions suitable for extracting phenolic compounds. The ability to tailor DES composition to achieve specific polarity and viscosity ranges underpins their versatility as green solvents for biomass valorization.
The mechanisms explaining the DES’s role in the extraction process typically involve breaking down the complex, resistant structure through molecular interactions such as hydrogen bonds and acid-base reactions [27]. Lignocellulosic biomass is primarily composed of various polysaccharides, including cellulose, hemicellulose, and lignin, forming a recalcitrant network stabilized by hydrogen bonds. The use of DESs destabilizes this structure through their functional groups, which act as both Hydrogen Bond Donors (HBD) and Hydrogen Bond Acceptors (HBA), thereby affecting these interactions and leading to the separation of the components in a process called fractionation [27]. The DES molecules can enter the biomass and form new hydrogen bonds with the lignin-polysaccharide network, allowing for the disruption of internal interactions and destabilization of the biomass structure [27]. Additionally, the ionic structure of DESs leads to breaking the hydrogen bonds within the internal structure of biomass by cleaving the ether bonds and ether-ester bonds found in the lignin-polysaccharide structure [28]. The chloride ions of choline chloride form strong hydrogen bonds with the hydroxyl groups of lignin and polysaccharides, providing the basis for the delignification mechanism [28]. Since DESs are good solvents of lignin, they can solubilize lignin and extract it from the cellulosic structure [29]. The combination of DESs with acids, especially organic acids such as lactic acid, oxalic acid, and formic acid, can enhance lignin extraction by protonating benzylic carbon atoms, resulting in the cleavage of ester and ether bonds [30]. Additionally, the acidification of DESs, especially HBDs, can promote the degradation of hemicellulose [29]. On the other hand, basic (alkaline) DESs have been reported to have the ability to break the ether bonds in lignin and ester bonds between lignin and hemicellulose [29]. Moreover, DESs weakly solubilize cellulose compared with lignin and hemicellulose. Regarding the extraction of proteins from plant biomass, the neutral DESs like choline-glycerol are commonly used [31]. These DESs can stabilize the protein structure and help unfolded or denatured proteins to refold in a soluble configuration. However, for animal proteins like collagen and keratin, the acidic DESs are more effective due to their ability to break the disulfide bonds, allowing for the disruption of the fibrous structure and swelling it, which enables DES to penetrate and hydrolyze peptide bonds in soluble fragments [31].

3. Conventional Solvents vs. DES in the Biomass Valorization and Biorefinery

Pretreatment of lignocellulolytic biomass is where solvents are used in large amounts compared to other valorization areas. In some cases, pretreatment costs reach up to 40% of the total costs of the valorization and biorefinery processes [32]. Therefore, the role of solvents is vital in pretreatment, as they significantly affect the efficiency and cost of breaking down lignocellulosic biomass. Also, extraction requires a significant amount of solvent, as conventional solvents, such as hexane, ethanol, and methanol, have been widely used to extract biocomponents from biomass [33]. However, worldwide practice has shown that conventional solvents used in extraction can cause environmental and safety problems, such as toxicity, flammability, and the generation of hazardous waste [34].
Conventional biomass pretreatment methods can be categorized as physical, chemical, physicochemical, and biological [35,36]. Additionally, a wide range of hybrid approaches of multiple pretreatment techniques were documented in previous studies [37,38,39,40,41,42,43]. Physical pretreatment methods require high energy and equipment, making the pretreatment process expensive [44]. Chemical pretreatment methods, such as acid or alkali, are considered the most established and widely used pretreatment methods in biomass valorization due to their effectiveness and lower cost [45,46,47]. Conventional solvents used in the biomass valorization process have considerable flaws despite their wide application, such as high energy consumption, toxicity, environmental impact, low selectivity, and poor biodegradability by reducing product yields [33,48]. In addition to these limitations, the formation of fermentation inhibitors can adversely impact the microbial community essential for subsequent downstream processes and reduce product yields. [49,50]. Acid-based solvents often generate fermentation inhibitors such as acetic acid, furfural, hydroxymethylfurfural (HMF), phenolic acids, and aldehydes [49,51]. For example, acid-based pretreatment can reduce the yield of bioethanol fermentation by up to 30% through the inhibition of Saccharomyces cerevisiae by inhibiting compounds [52]. Moreover, the corrosive nature of many conventional solvents presents another challenge, as it can lead to a reduction in reactor lifespan or require the use of expensive corrosion-resistant materials, which in turn increases the capital costs in biorefinery processes [53,54].
Additionally, there are other less widely used pretreatment methods, such as organosolv, ionic liquids, and ozonolysis [55,56,57]. To recover organosolvents such as ethanol and acetone energy-intensive distillation is required in lignocellulosic biomass processing [58]. Moreover, many conventional organic solvents are both flammable and environmentally toxic, thereby posing significant safety concerns and environmental risks [59,60]. In recent years, ionic liquids (ILs) have demonstrated remarkable efficiency in lignocellulosic biomass treatment owing to their low vapor pressure, high thermal stability and high tunable properties [57,61]. However, despite the hype surrounding ionic liquids (ILs), their high cost, toxicity, poor biodegradability, high-water solubility, and recovery difficulties reduce their economic viability [61,62]. In addition, another drawback of ionic liquids is that their high boiling temperatures, strong solute interactions, and high viscosity at room temperatures make separation processes challenging [63,64].
The low efficiency of conventional solvents has led researchers to explore other alternatives. One of the twelve principles of green chemistry proposed by Anastas and Eghbali (2010) emphasizes the development of environmentally friendly solvents, and in this context, the emergence of DES has been particularly promising [65,66]. Moreover, compared to conventional solvents, DESs have shown high efficiency by effectively breaking down lignocellulosic biomass. The increase in the efficiency of the dissociation of lignin in the biomass could be obtained by adjusting the proportions of the components of the solvents. It has been reported that DES requires less energy than the widely used processes in pretreatment. The lower amount of energy used in DES pretreatment is due to its ability to solubilize the lignin and make cellulose available at lower temperatures and pressures [14,67]. For example, a study has shown that the DES technique requires about 28% less energy and can help remove 40% of lignin from lignocellulosic biomass compared to traditional methods of pretreatment [17]. Other advantages of DES, such as low vapor pressure, high tunability, low volatility, and biodegradability, have attracted growing interest in this green solvent [60]. Additionally, DES can impact inhibitors of lignin extraction and works in milder conditions (lower temperature and pH) compared to other pretreatment methods [68]. A hypothesized figure (Figure 1) illustrates how conventional lignocellulosic biomass reactions differ from those occurring when DESs are used.
As with most solvents currently in use, DES also exhibits certain limitations that should be recognized to ensure an objective evaluation. A major limitation of DES is high viscosity, which can lower the solubility of sugars [69,70]. Despite the simplicity of recovery and recycling of DES compared to ionic liquids, which is considered another promising solvent, there are challenges, such as repeated cycles may alter DES composition or lead to degradation of components, raising concerns about long-term stability and performance [71].
To sum up, while DES represents a promising green alternative to conventional, harmful, and toxic solvents, it still possesses certain limitations that must be addressed through further experimental research and process optimization.

4. Application of DES in Secondary Lignocellulosic Biomass Pretreatment

Pretreatments are a vital step as they help to overcome the recalcitrance of lignocellulosic biomass and unlock cellulose and hemicellulose from the grasp of lignin, which in turn heightens the efficiency of biofuel production by enhancing sugar yields [72]. Additionally, there are other purposes for using pretreatment, such as preventing the degradation of sugars, minimizing the formation of inhibitors that might affect the downstream fermentation process, helping the versatile utilization of biomass components, and preparing biomass for subsequent unit operations [73].
In recent years, pretreating lignocellulosic biomass with green solvents has become an attractive pretreatment technique. One of the potential pretreatment techniques, DESs, received a great amount of attention in biomass processing [74]. The first application of DES for the treatment of lignocellulosic biomass was conducted by Francisco, Van Den Bruinhorst, and Kroon (2012) [75], and the authors reported the efficient removal of lignin. After this study, a great body of research has been conducted to evaluate the efficiency of DES as a pretreatment tool for lignocellulosic biomass.
Choosing the right pretreatment method for each lignocellulosic biomass is a challenging task [73]. However, the DES method for biomass pretreatment has been shown to be effective in many studies that targeted different types of lignocellulosic biomass (Table 2). For example, without DES pretreatment, the sugar recovery was estimated at around 20% during enzymatic hydrolysis due to the crystalline structure of cellulose and the presence of lignin, according to Singhvi, Chaudhari, and Gokhale (2014) [76]. However, the application of DES as a pretreatment solvent increased the sugar recovery yield by up to 96% compared to conventional pretreatment method. The studies presented in Table 2 show that the DES efficiency of the pretreatment process depends on various factors, such as the DES type [77,78,79,80,81], pretreatment time and temperature, and solid to solvent ratio [12]. Acidic DES systems, such as ChCl–formic acid, ChCl–lactic acid, and ChCl–oxalic acid, consistently showed the highest lignin removal efficiencies, reaching up to 98.5% in the case of ChCl–oxalic acid (1:1) for corn cob and 83.6% for ChCl–oxalic acid (3:1) in rapeseed straw. In contrast, DESs containing neutral or polyol-based donors, including glycerol and ethylene glycol, favored cellulose preservation and enhanced enzymatic hydrolysis, yielding up to 96.4% glucose with ChCl–glycerol (1:2) while maintaining the structural integrity of the polysaccharides. When comparing solvent families, DESs with choline chloride as the hydrogen bond acceptor were generally more effective than those containing betaine or proline. Acidic DESs provided strong lignin dissolution through hydrogen bonding and cleavage of ether linkages. Higher temperatures (120–155 °C) were generally required for optimal delignification; some studies reported substantial biomass fractionation at lower temperatures (70–90 °C), offering avenues for low-energy pretreatment conditions. Optimum delignification was generally observed at temperatures between 90 and 155 °C for 2–12 h. Lower temperatures (60–70 °C) required extended durations, as observed for wheat straw treated with ChCl–monoethanolamine, where 71–81% lignin removal was achieved after 9–12 h. Conversely, short pretreatment times of 1–3 h at elevated temperatures (120–155 °C) proved sufficient for effective delignification of rice hulls, sugarcane bagasse, and grape pomace. Many studies reported that the efficient pretreatment temperature using the DES is around 110 °C. Moreover, a large body of research reported that the DES efficiency depends on the type of biomass or raw materials [82].
Besides the removal of solid-phase lignin, DES pretreatment can also dissolve the lignin into the liquid phase via hydrogen bond interactions and the cleavage of lignin-carbohydrate complexes [85,86]. However, the liquid phase lignin and its derived phenolic compounds such as furfural, and hydroxymethylfurfural (HMF) can act as inhibitory compounds during subsequent enzymatic hydrolysis and fermentation stages [87,88]. These inhibitory compounds can adsorb onto enzyme active sites, disrupt catalytic activity, or interfere with microbial cell membranes and metabolic enzymes [89]. Studies indicate that the extent of lignin solubilization depends on DES composition and process severity: highly acidic DESs tend to enhance delignification but also increase the formation of soluble lignin-derived aromatics [90,91]. For example, results of a study showed that lignin solubilization was enhanced by higher HBD content, shorter HBD chains, and fewer functional groups, while carboxylic acid–based DESs proved particularly effective in cleaving β-O-4 linkages. And the authors claim that elevated temperature and controlled water addition further facilitated lignin dissolution [92]. Thus, optimizing DES composition and process temperature to balance efficient delignification with minimal formation of soluble lignin fragments is a key research priority for improving enzymatic hydrolysis and fermentation performance. Studies on the structure–activity relationship of soluble lignin in DES-pretreated hydrolysates are still limited, and further research is needed to clarify how DES–lignin interactions influence subsequent enzymatic and microbial processes.
The DES type or composition was the most significant factor that affected the positive outcome. Literature review showed that incorporating choline chloride as the hydrogen bond acceptor (HBA) in the formation of the DES resulted in better lignin solubilization and cellulose preservation compared to other HBAs. For example, from Table 2, we can summarize that when combining choline chloride with hydrogen bond donors (HBDs) such as formic acid, lactic and acetic acids, lignin, removal efficiency improved up to 93.1% and sugar yield exceeded 95% as a result of pretreatment for corncob, rice straw, and brewers’ spent grain. It seems this efficiency is due to the synergetic effects of choline chloride’s ionic character and the acidity of HBDs when combined disrupt the ester and ether bonds in the lignin–carbohydrate complex. Also, it is noticed that organic acids (formic and lactic acid) contributed to deep delignification while polyol-based HBDs (e.g., glycerol, ethylene glycol) facilitated higher sugar recovery. These observations demonstrate that when targeting specific fractions such as lignin and cellulose, appropriate formulations of DES should be employed. For example, Zhang et al. (2016) reported that a choline chloride and lactic acid DES (1:15) enabled 93.1% delignification of corncob, while choline chloride and glycerol DES helped to 96.4% sugar recovery from the same biomass [80]. DES conditions with high HBD content or water addition (e.g., choline chloride: lactic acid with a ratio 1:15 or choline chloride: formic acid with 10% water) resulted in higher lignin solubilization at lower temperatures. Possible cause might be improved solvent penetration and viscosity reduction.
Some studies have revealed the effectiveness of deep eutectic solvents (DESs) in biomass pretreatment for improving biofuel production. For example, Jing et al. (2022) used the DES method to pretreat corncob to enhance biohydrogen production, and they found that the efficiency of delignification was increased by 83.12% [1]. The authors also noticed an increase in cellulose levels in the pretreated corncob. These results highlight the potential of DES as a more environmentally friendly and effective alternative to traditional biomass pretreatment methods.

5. Applications of DES in the Extraction Process

The most prevalent solvents used in the extraction process are volatile organic compounds from non-renewable sources that are environmentally harmful [33]. Due to the toxicity of conventional solvents, new green non-toxic solvents have been searched by scientists [13]. A large body of studies in the last two decades showed that the extraction of bioactive compounds using DESs, such as phenolic compounds, sugars, lipids and fatty acids, alkaloids, vitamins, carbohydrates, proteins and peptides, terpenoids and essential oils, pigments, bioactive compounds, chitin, etc., is a promising method [48]. The extraction of bioactive compounds from biomass depends on many parameters, including the physicochemical properties of choline chloride-based deep eutectic solvents, extraction temperature, solid-to-liquid ratio (water content), and extraction time and temperature [33].
Table 3 summarizes the application of DES for the extraction of bioactive compounds from secondary lignocellulosic biomass. Several patterns were revealed. In all studies presented, choline chloride (ChCl) serves as the primary hydrogen bond acceptor (HBA), confirming its versatility as a hydrogen bond acceptor, while the choice of hydrogen bond donor (HBD)—particularly organic acids (lactic, citric, malic, oxalic) or polyols (glycerol, ethylene glycol)—governs solvent polarity and hydrogen-bonding ability. Acidic DESs generally achieve higher recovery of phenolic compounds (e.g., grape skin, apple pomace, orange peel), likely due to their enhanced capacity to disrupt ester linkages in lignin and hemicellulose. In contrast, hydrophobic or alcohol-based DESs such as menthol: lactic acid and ethyl acetate: ethyl lactate proved more effective for carotenoid extraction from lipid-rich matrices like tomato pomace and pumpkin peels. Extraction methods also play a crucial role: ultrasound- and microwave-assisted DES extractions consistently outperform conventional shaking or heating due to improved mass transfer and cell disruption, while moderate water addition (10–30%) reduces viscosity, aligning with optimization for tunability [93]. Combinatorial DES systems (ternary NADESs) significantly increase yield and antioxidant activity.

6. Application of DES for Catalytic Conversion

In recent years, DESs have emerged as green and promising catalysts in biomass valorization and upgrading reactions [107]. A study reported that the DES composed of choline chloride and oxalic acid acted as both a solvent and a catalyst for the conversion of xylan from corncob into furfural [108]. Moreover, deep eutectic solvents were recycled up to three times, facilitating the high-value production of furfuryl alcohol and ensuring the comprehensive utilization of corncob. Arslanoğlu and Sert (2019) reported that DES can effectively convert cellulose extracted from sunflower stalks into valuable products, achieving 38.4% efficiency at 170 °C in 5 min, making DES a promising green catalyst for biomass valorization [109]. Another study reported excellent catalytic performance using choline chloride/oxalic acid-based DES with a 89% conversion rate of cellulose to reducing sugars. used a choline chloride/lactic acid DES for the conversion of arabinoxylan to arabinose and xylose, achieving high sugar yields [110,111]. Gawade and Yadav (2018) [112] synthesized 5-EMF from fructose under microwave heating using various DESs. ChCl:oxalic acid showed the best performance, with 92% fructose conversion and 74% 5-EMF yield in 3 h [112]. All these studies highlight the effectiveness of DES as a better catalyst compared to conventional catalysts.

7. Integration of DES with Other Techniques in Valorization of Secondary Lignocellulosic Biomass

In recent years, research on the application of DES in combination with other methods has gained wide attention in lignocellulosic biomass valorization. Many studies reported that synergetic methods can enhance the efficiency of pretreatment and increase the biomass conversion [113].
The synergetic integration of DES with physical methods has been studied to improve the efficiency of DES. Microwave-assisted DES has shown good results in recent years. For example, a study revealed that microwave-assisted DES was very efficient in solubilizing xylan in a very short time compared to the traditional DES pretreatment method, which in turn improved the digestibility of the biomass [38]. In another study, microwave-assisted DES was utilized on bamboo biomass fractionation, resulting in cellulose-rich residue, lignin, and recovered DES. Consequently, value-added bio-based nanomaterials (lignin-containing cellulose nanofibrils (LCNFs), lignin nanoparticles (LNPs), and carbon quantum dots (CDs)) were obtained as end products [79]. Integrating the DES and ultrasonication is another synergetic method used by scientists to improve the efficiency of biomass valorization [113]. The authors reported that pretreatment of oil palm empty fruit bunch using the combination of ultrasonication and DES increased cellulose digestibility and decreased the lignin content and biomass crystallinity. The synergistic combination of the DES with biological methods promotes environmental friendliness and improves the efficiency of the valorization of biomass. Enzymes and microorganisms have been deployed in combination with DES in many studies [114].
Some studies integrated both promising green solvents (Ionic liquids and DES) in the valorization of lignocellulosic biomass and achieved good results. For example, a study utilized both DES and Ionic liquids (IL) to fully valorize chestnut shell waste [115]. The authors reported that ChCl:oxalic acid dihydrate (ChCl:Oax2H2O) performed best in the extraction of polyphenol and ionic liquid was used to further separate lignin and cellulose. Combining green chemistry using DES with biological conversion processes is a promising step towards making efficient biomass valorization. The integration of fermentative hydrolysis with DES in biomass valorization has been explored by scientists, as both DES and fermentative hydrolysis are environmentally friendly. For example, Manivannan and Anguraj (2023) used DES for pretreatment of banana peel waste; then, reducing sugars were fermented into bioethanol using Saccharomyces cerevisiae [116]. Another study also integrated DES with fermentative hydrolysis and concluded that the integration was a facile and effective approach for whole valorization of industrial xylose residue [117].
Integration of DES with supercritical fluids for the purpose of lignocellulosic biomass valorization has been explored by some researchers [118,119]. This type of integration can improve the extraction process by enhancing extraction efficiency, improved selectivity, process intensification and reducing solvent consumption. Carbon dioxide is the most widely used supercritical fluid due to its mild critical conditions, non-toxic and environmentally friendly nature, chemical inertness, ease of removal, selective extraction capabilities, and recyclability.

8. Challenges and Future Directions

Despite the growing application of DES in biomass valorization processes, it comes with some challenges. While DES is more efficient than traditional solvents for lignocellulosic biomass treatment, the sequential valorization of lignin is still challenging [16]. Despite the growing number of studies, the industrial application of DES for biomass valorization remains in its early stages [9]. Purification and recycling pose significant challenges for the industrial-scale use of DES in biomass pretreatment [120]. However, there is ongoing research on the recovery of DES after valorization processes to solve the cost-effectiveness problem and the sustainability issue [121]. Recent studies have introduced methods for recovering and recycling DES, with recycling rates reaching up to six times [111,121,122,123,124]. However, a recent study has reported the need to purify recycled DES prior to biomass pretreatment to enhance its efficiency [125].
Despite extensive research on the application of DES in lignocellulosic biomass valorization, most studies remain at the laboratory scale. The economic feasibility of DES remains challenging, particularly at the industrial level. Although the components used for DES synthesis are relatively inexpensive, the overall cost increases due to energy-intensive processing and purification steps, as well as solvent recovery required [126]. Therefore, further research and process optimization are needed to achieve economic feasibility.
Most studies claim that DESs are green and sustainable because of their biodegradability and low toxicity; however, these studies do not provide sufficient justification or make assumptions [127,128]. Moreover, a review of the literature showed that only a few studies have conducted life cycle assessments (LCAs) and techno-economic analysis related to DESs [129,130,131]. A study reported that only by recovering DES can the high consumption of energy and greenhouse gas emissions be decreased [131]. Another study’s conclusion is optimistic, indicating a competitive pathway based on techno-economic analysis of biomass pretreatment using deep eutectic solvents (DESs) for the co-production of 2,3-butanediol, furfural, and technical lignin. However, the model offered by the authors does not guarantee that it will work at an industrial scale, due to barriers such as solvent stability, mass transfer scaling, and separation energy, which might add additional costs [130].
Another challenge is the methodological variations in different studies affecting the outcomes of these studies. Moreover, the calculation of recovery and yield of main compounds (cellulose, hemicellulose, and lignin) is not sufficient for determining the success of biomass pretreatment as some pretreatment, methods might not be appropriate for post-processing steps such as fermentation, enzymatic hydrolysis, and purification. For example, the level of inhibitors, toxic compounds, or phenolic compounds might affect the post-processing steps [73].
Despite the challenges mentioned earlier, DES demonstrates considerable promise as an alternative to traditional solvents, fostering environmentally friendly and effective practices in the pretreatment of lignocellulosic biomass. Since the use of DESs in biomass valorization is still in its early stages, additional research is essential to enhance the extraction and pretreatment methods for both raw materials and bioactive compounds.

9. Conclusions

This paper presents an updated review on the application of Deep Eutectic Solvents (DESs) as alternative solvents for the valorization of diverse lignocellulosic biomass. The review highlights the potential of DES as an effective green solvent in the valorization processes. Additionally, numerous studies indicate that DES can be recycled up to six times, making it a cost-effective alternative compared to other solvents. Despite the numerous advantages of DES over traditional solvents, there are still challenges that must be addressed to optimize the usage conditions of DES for different raw materials and intended outcomes products (Figure 2).

Funding

This research and the article processing charge (APC) were funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan under Grant No. BR21882359 and No. BR24992914.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. A hypothesized figure illustrating the effect of selective DES pretreatment on lignocellulosic biomass fractionation and inhibition minimization compared to conventional harsh treatments.
Figure 1. A hypothesized figure illustrating the effect of selective DES pretreatment on lignocellulosic biomass fractionation and inhibition minimization compared to conventional harsh treatments.
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Figure 2. Valorization of lignocellulosic biomass in the context of circular economy and sustainability. The numbers 1, 2, 3, 4 are the sequence of valorization processes.
Figure 2. Valorization of lignocellulosic biomass in the context of circular economy and sustainability. The numbers 1, 2, 3, 4 are the sequence of valorization processes.
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Table 1. The most common hydrogen bond acceptors and donors for Deep Eutectic Solvents (DES).
Table 1. The most common hydrogen bond acceptors and donors for Deep Eutectic Solvents (DES).
Hydrogen Bond AcceptorHydrogen Bond Donor
Choline ChlorideUrea
Tetrabutylammonium ChlorideGlycerol
Methyltriphenylphosphonium BromideMalonic Acid
Imidazolium ChlorideCitric Acid
Ethylammonium NitrateLactic Acid
DimethylsulfoxideTartaric Acid
Ethylene GlycolAcetic Acid
Triethylammonium ChlorideLevulinic Acid
Tetraethylammonium BromidePropionic Acid
DimethylacetamideGlucose
DimethylformamideSorbitol
Diethyl EtherMannitol
Dimethyl CarbonatePolyethylene Glycol (PEG)
AcetonitrileEthylene Glycol
N, N-DimethylformamideXylitol
Table 2. Summary of Deep Eutectic Solvent (DES) pretreatment studies on various biomass types.
Table 2. Summary of Deep Eutectic Solvent (DES) pretreatment studies on various biomass types.
Biomass TypeDES CompositionPretreatment Time
(h)		Pretreatment Temperature (°C)Biomass to Solvent RationLignin Removal (%)Cellulose/Sugar Yield (%)Key Observations/TrendsReferences
1Rice strawChCl–acetic acid2.512601:03.683.1-ChCl–acetic acid
pretreatment produced cellulose-rich rice straw, yielding high reducing sugar and proving its efficacy		[77]
2Wheat StrawCholine chloride–Monoethanolamine9701:2071.493.7Choline chloride: monoethanolamine at pretreatment time 9 h and 70 °C was the best solvent among these DES’s.[12]
Choline chloride–Monoethanolamine12901:208190.8
Choline chloride–Glycerol12901:2024.797.8
Choline chloride–Urea12901:2027.795.9
Choline chloride–Diethanolamine12901:0273.598
3Brewers’ spent grainCholine chloride–glycerol (1:2)3601:08-81In both DES treatments low temperature and small solid to solvent ratio—increased the amount of recovered biomass.[14]
3601:16-80
3601:32-80
31151:08-75
31151:16-74
31151:32-73
Choline chloride–ethylene glycol (1:2)3601:08-80
3601:16-80
3601:32-79
31151:08-76
31151:16-76
31151:32-74
4Rice hullsCholine chloride-Formic acid (1:2)21551:104860Overall, the biomass that underwent pretreatment was more cellulosic, and less concentrated in lignin compared to raw biomass. DES composed of Choline Chloride-Formic acid was more efficient as pretreatment solution for rice hulls.[78]
Choline chloride-Lactic acid (1:10)21551:102455
Choline chloride-Acetic acid (1:2)21551:102858
Betaine–Lactic acid (1:2)21551:10044
Proline -Lactic acid (1:3.3)21551:10043
5Sugarcane Bagasse Choline chloride-Formic acid (1:2)21551:107960DESs with choline chloride as hydrogen bond acceptor were effective compared to proline as hydrogen bond acceptor.[78]
Choline chloride-Lactic acid (1:10)21551:107960
Choline chloride-Acetic acid (1:2)21551:107662
Betaine–Lactic acid (1:2)21551:10--
Proline -Lactic acid (1:3.3)21551:104247
6Corn stoverCholine chloride:formic acid21301:2023.848Choline chloride–formic acid was more efficient as DES.[79]
1-Butyl-3-methylimidazolium chloride21301:208.536
7CorncobCholine chloride–Lactic acid (1:2)-901:2064.781.6Choline chloride–Lactic acid (1:15) was the most efficient solvent in removing lignin, while Choline chloride–Glycerol (1:2) was more successful for yield of glucose.[80]
Choline chloride–Lactic acid (1:5)-901:2077.983.5
Choline chloride–Lactic acid (1:10)-901:2086.183.2
Choline chloride–Lactic acid (1:15)-901:2093.179.1
Choline chloride–Oxalic acid (1:1)-901:2098.545.2
Choline chloride–Ethylene glycol (1:2)-901:2087.685.3
Choline chloride–Glycerol (1:2)-901:2071.396.4
8Grape pomace and stalksCholine chloride–Formic acid (1:2)21201:1047.9-Choline chloride–Formic acid (1:2) and Choline chloride–Lactic acid (1:10) were more effective in lignin removal.[81]
Choline chloride–Acetic acid (1:2)21201:1022.5-
Choline chloride–Oxalic acid (1:1)21201:107.8-
Choline chloride–Lactic acid (1:10)21201:1047.1-
Choline chloride–Lactic acid (1:2)21201:1019.6-
Choline chloride–Lactic acid (1:5)21201:1022.7-
Choline chloride–Lactic acid (1:15)21201:1043-
9Potato peelsCholine chloride–glycerol3601:083385Lower solid-to-solvent ratio (1:8) and lower temperature (60 °C) yielded higher compound recovery.[14]
1:3283
1151:0880
1:3275
1:0853
1501:3252
10Pineapple peelCholine chloride–Oxalic acid (1:1)199.651:0172.488.35Acidic DES (CC–OA) showed superior cellulose recovery and hemicellulose/lignin removal compared with other DESs.[83]
Choline chloride–Lactic acid (1:2)
Choline chloride–Ethylene glycol (1:2)
Choline chloride–Ethylene glycol (1:2)
Choline chloride–Glycerol (1:2)
11Rape strawCholine chloride–Oxalic acid (1:1)11301:1083.6 Choline chloride–Oxalic acid (3:1) gave highest delignification (83.6%) and enzymatic efficiency (96%) with improved cellulose accessibility.[84]
Choline chloride–Oxalic acid (3:1)
Table 3. Applications of DES in the extraction of bioactive compounds.
Table 3. Applications of DES in the extraction of bioactive compounds.
#Biomass TypeDES CompositionTarget CompoundsExtraction ConditionsExtraction EfficiencyAdditional NotesReferences
1Grape skinChCl/glucosePhenolics80 °C for 2–6 hAmong the DES tested, ChCl/oxalic acid with 25% water was the most effective solvent for extracting grape skin phenolic compounds, outperforming conventional organic solvents. [94]
ChCl/sorbose
ChCl/glycerol
ChCl/proline
ChCl/malic acid
ChCl/oxalic acid
2Orange peel wasteChCl/EG (1:4)Phenolic compounds60 °C, 100 minDES outperformed the conventional solvents by providing higher total phenolic content (TPC) and antioxidant potential. [95]
3Apple pomaceCholine chloride-lactic acid (1:1)Phenolic compounds60 °C The highest total flavonoid content, 17.30 mg EPE/g apple pomace, was obtained with choline chloride:urea, while the NADES composed of choline chloride:lactic acid exhibited significant antioxidant activity.In addition to deep eutectic solvent, ultrasound-assisted extraction was employed[96]
Choline chloride-lactic acid (1:6)60 °C
Choline chloride-lactic acid (1:9)60 °C
Choline chloride-malic acid (1:1)70 °C
Choline chloride-citric acid (1:1)80 °C
Choline chloride-citric acid (3:1)80 °C
Choline chloride–sucrose-water (1:1:11)50 °C
Glucose-fructose-water (1:1:11)50 °C
Fructose-sucrose-water (1:1:11)50 °C
Glucose-sucrose-water (1:1:11)50 °C
Choline chloride-ethylene glycol (1:2)80 °C
Choline chloride-glycerol (1:2)80 °C
Choline chloride-urea (1:2)80 °C
4Buckwheat huskCholine chloride/Citric acid (1:1) Glucose/Citric acid (1:1)Antioxidants80 °C for 2–6 hGlucose/Urea (1:1) solvent with various amounts of water achieved the best extraction efficiency for antioxidants from Buckwheat husk [97]
Glucose/Urea (1:1)
Glucose/Urea (1:1)
Betaine/Citric acid (1:1)
Betaine/Urea (1:1)
5Wheat branCholine chloride-glycerol (1:1)Ferulic acid80 °C for 2 hThe DES with choline chloride-glycerol at a 1:2 molar ratio provided the highest extraction yield of ferulic acid from wheat bran under optimal conditions of 50% DES, 15 mL/g liquid-to-material ratio, and 35 min of extraction [98]
Choline chloride-glycerol (1:2)
Choline chloride-glycerol (1:3)
Choline chloride-glycerol (1:4)
Choline chloride-glycerol (1:5)
6Spent coffee groundsCholine chloride-Urea (1:2)Phenolic compoundsThe mixtures was vigorously agitated at 80 °C until a homogeneous liquid formedThe testing and adjusting of solvents showed that a DES made of 1,6-hexanediol and ChCl in a 7:1 ratio (called HC-6) was the most effective.Additionally, UAE (Ultrasound-Assisted Extraction) was used for extraction.[99]
Choline chloride-Acetamide(1:2)
Choline chloride-Glycerol (1:2)
Choline chloride-Sorbitol (1:2)
Choline chloride-Ethylene glycol (1:2)
Choline chloride-1,4-Butanediol (1:2)
Choline chloride-1,6-Hexanediol (1:2)
Choline chloride-Malonic acid (1:2)
Choline chloride-Citric acid (1:2)
Choline chloride-Fructose-Water (5:2:5)
Choline chloride-Xylose-Water (2:1:2)
Choline chloride-Sucrose-Water (4:1:4)
Choline chloride-Glucose-Water (5:2:5)
7Brewer’s Spent GrainNaAcO:Urea (1:2)Protein80 °C for 20 hThe extraction using 90 wt% sodium acetate and urea (1:2) was more effective than choline chloride and urea (1:2), achieving up to 79% protein yield from brewer’s spent grain. [100]
NaAcO:Urea (1:3)
KAcO:Urea (1:2)
KAcO:Urea (1:3)
NaForm:Urea (1:2)
NaForm:Urea (1:3)
8Brewer’s Spent Grain and malt dustmalic acid-choline chloride (1:1)Phenolic compounds120 °CUsing high-temperature hydrothermal extraction with acidic NADESs significantly boosts polyphenolic compound yields from brewer’s spent grain and malt dust compared to other methods [101]
glycerol-choline chloride (1:1)
9Apricot Kernel BiomassGlycerol–choline chloride (2:1)Polyphenol rich extractThe mixture was heated at 80–90 °C for 90 min under stirring until a transparent liquid was formed. Using DES increased polyphenol content by approximately 70%, and combining DES with PEF resulted in a 173% increasePulsed electric field (PEF) extraction methods used as complementary.[102]
10Tomato pomaceEthyl acetate:ethyl lactatecarotenoidsUltrasound-assisted extraction; solvent preheated; 60 °C, 20 minEthyl acetate:ethyl lactate with non-thermal air-drying yielded the highest lycopene (75.86 μg/g) and β-carotene (3950.08 μg/g) contents.Non-thermal air-drying[103]
11Sour cherry pomaceChCl:malic acid (1:1); ChCl:urea (1:2); ChCl:fructose (1:1)PolyphenolsMicrowave-assisted; 80–100 °C, 5–10 minYields up to 35 mg GAE/g dry weight with ChCl:malic acid, 1.5–2x higher than conventional solventsMicrowave-assisted NADES preparation and extraction; extracts exhibited >80% DPPH inhibition and antimicrobial effects against E. coli.[104]
12Pumpkin peelsDL-menthol:lactic acid (1:2); menthol:acetic acid (1:1)β-caroteneConventional shaking; 50 °C, 30 min, 10 mL/g0.823 mg/mL β-carotene yield (93.95% of acetone reference), with menthol:lactic acid (1:2) most effective.Natural Hydrophobic Deep Eutectic Solvents as sustainable alternative; high efficiency without toxic solvents; potential for direct food/cosmetic use.[105]
13Blueberry leavesLactic acid:sodium acetate:water (3:1:2)Phenolic compoundsSamples were sonicated 45 min at 65 °C using 15:1 and 75:1 (v/w) solvent ratios.1.6–2.2x higher phenolics and 1.6–2.8x antioxidants than conventional solvents. [105]
ChCl:oxalic acid (1:1)
14Strawberry and raspberry waste Choline chloride-Glycerol (1:2)Phenolic compoundsDES preparation: Heated to 60 °C with agitation until viscous liquid formedCholine chloride:Glycolic acid:Oxalic acid (1:1.7:0.3, 0% H2O) yielded highest phenolics from raspberry and strawberry [106]
Choline chloride:Sucrose (1:2, 25% H2O)
Choline chloride:1,4-Butanediol (1:5, 0% H2O)
Choline chloride:1,2-Propanediol (1:1, 7.5% H2O)
Betaine:Sucrose (2:1, 13% H2O)
Betaine:Levulinic acid (1:2, 0% H2O)
Choline chloride:Glycolic acid:Oxalic acid (1:1.7:0.3, 0% H2O)
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Toleugazykyzy, A.; Bekbayev, K.; Bolkenov, B.; Alwazeer, D.; Rskeldiyev, B.; Kuterbekov, K.; Bekmyrza, K.; Kabyshev, A.; Kubenova, M.; Opakhai, S. Comprehensive Review of Recent Trends in the Use of Deep Eutectic Solvents for the Valorization of Secondary Lignocellulosic Biomass. Sustainability 2025, 17, 9492. https://doi.org/10.3390/su17219492

AMA Style

Toleugazykyzy A, Bekbayev K, Bolkenov B, Alwazeer D, Rskeldiyev B, Kuterbekov K, Bekmyrza K, Kabyshev A, Kubenova M, Opakhai S. Comprehensive Review of Recent Trends in the Use of Deep Eutectic Solvents for the Valorization of Secondary Lignocellulosic Biomass. Sustainability. 2025; 17(21):9492. https://doi.org/10.3390/su17219492

Chicago/Turabian Style

Toleugazykyzy, Akerke, Kairat Bekbayev, Bakytzhan Bolkenov, Duried Alwazeer, Berdikul Rskeldiyev, Kairat Kuterbekov, Kenzhebatyr Bekmyrza, Asset Kabyshev, Marzhan Kubenova, and Serikzhan Opakhai. 2025. "Comprehensive Review of Recent Trends in the Use of Deep Eutectic Solvents for the Valorization of Secondary Lignocellulosic Biomass" Sustainability 17, no. 21: 9492. https://doi.org/10.3390/su17219492

APA Style

Toleugazykyzy, A., Bekbayev, K., Bolkenov, B., Alwazeer, D., Rskeldiyev, B., Kuterbekov, K., Bekmyrza, K., Kabyshev, A., Kubenova, M., & Opakhai, S. (2025). Comprehensive Review of Recent Trends in the Use of Deep Eutectic Solvents for the Valorization of Secondary Lignocellulosic Biomass. Sustainability, 17(21), 9492. https://doi.org/10.3390/su17219492

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