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Review

Deep Eutectic Solvent Pretreatment and Green Separation of Lignocellulose

by
Zhengyuan Yao
1,2,
Gunhean Chong
2,* and
Haixin Guo
1,*
1
Agro-Environmental Protection Institute, Ministry of Agriculture and Rural Affairs, No. 31 Fukang Road, Nankai District, Tianjin 300191, China
2
Faculty of Food Science and Technology, Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2024, 14(17), 7662; https://doi.org/10.3390/app14177662 (registering DOI)
Submission received: 15 July 2024 / Revised: 24 August 2024 / Accepted: 27 August 2024 / Published: 30 August 2024
(This article belongs to the Special Issue Recent Advances in Green Chemistry and Sustainable Catalysis)
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Abstract

:
Plant-based waste biomass with lignocellulose as an important component is produced in large quantities worldwide every year. The components of lignocellulose that typically exhibit high utilization value include cellulose and hemicellulose, as well as pentoses and hexoses derived from their hydrolysis. As a pretreatment for the hydrolysis process, delignification is a pivotal step to enhance cellulose/hemicellulose accessibility and achieve high yields of fermentable sugars. Additionally, deep eutectic solvents (DESs) are the most widely used solvents for delignification during biomass fractionation due to their clean and environmentally friendly attributes. DESs dissolve lignin by inducing a large amount of β-O-4 bond cleavage and partial carbon–carbon bond cleavage, retaining cellulose in the solid residue, while most of the hemicellulose is hydrolyzed in DES pretreatment. This article provides a comprehensive review of the influence of DESs in the lignocellulose separation process. Key factors such as lignin removal rate, sugar conversion rate, and product chemical structure are critically reviewed to assess the feasibility of employing DESs for lignocellulose separation.

1. Introduction

Lignocellulosic biomass stands as a plentiful and renewable resource, poised to play a pivotal role in facilitating the sustainable production of fuels and chemicals in the foreseeable future [1]. According to 2022 statistics, the global annual production of lignocellulosic biomass is about 181.5 billion tons, of which only 8.2 billion tons of lignocellulosic biomass are utilized in productive applications rather than being treated as waste [2]. In 2024, the global lignocellulosic biomass market size is estimated at $4.3 billion and is expected to reach $9.1 billion by 2034. As one of the lignocellulosic components, the market for lignin is expected to grow at a compound annual growth rate (CAGR) of 4.5 from 2024 to 2030. [3]. Lignocellulosic biomass comprises cellulose, hemicellulose, and lignin, with the latter exhibiting a notably more intricate structure compared to cellulose and hemicellulose [4,5]. The typical structure of lignocellulose in plant-based biomass is a shell of cellulose and hemicellulose surrounded by interconnected lignin molecules [6]. The proportion of cellulose, hemicellulose, and lignin is slightly different in different parts of plants due to functional differentiation [7]. For example, hardwood biomass typically possesses a cellulose content ranging from 43% to 47%, hemicellulose content between 25% and 35%, and lignin content spanning from 16% to 24% [8]. Conversely, herbaceous biomass exhibits a composition characterized by cellulose levels of 33% to 38%, hemicellulose content ranging from 26% to 32%, and lignin content comprising 17% to 19% of its composition [9]. Agricultural and vegetable wastes such as fruit shells and pits often contain high amounts of lignocellulose. For example, the composition of cellulose, hemicellulose, and lignin of durian shell were 25.7 ± 1.9% (w/w), 18.5 ± 1.04% (w/w), and 15.9 ± 0.19% (w/w), respectively [10].
The separation and purification of lignocellulose components provide a possibility for efficient utilization and valorization of plant-based biomass waste [11]. A range of chemical treatments has been suggested for the separation of lignin components during biomass fractionation, including methods employing organic solvents [12] and ionic liquids [13,14]. Ionic liquids [13] are generally defined as compounds consisting entirely of ions with a melting point below 100 °C. They are preferred over conventional volatile solvents and catalysts from both “green” and “design” perspectives [15]). While ILs have demonstrated high separation rates for lignocellulose, their high cost and environmental unfriendliness preclude their widespread adoption as solvents for large-scale green production processes [16,17]. Recently, deep eutectic solvents (DESs) have emerged as viable alternatives, demonstrating their efficacy as environmentally friendly and sustainable biomass solvents [18,19]. The interaction between DESs and biomass can be understood as an acid–base catalytic mechanism [20]. The unstable ether–alkyl bonds between the phenylpropane units of lignin, as well as the hydrogen bonds and covalent bonds between lignin and hemicellulose are cleaved under the catalysis of hydrogen protons [21,22]. DESs typically consist of transparent fluids composed of at least two components, including one hydrogen bond donor (HBD) and one hydrogen bond acceptor (HBA) [23,24]. DES comprises large and asymmetric ions, and the reduction in lattice energy due to hydrogen bonds and charge delocalization leads to a lower melting point of the DES mixture compared to its individual components [25,26]. The degree of decline in melting point can be quantified as the difference between the melting point of binary mixture of component A and component B and that of the theoretically ideal mixture, denoted as ΔTf. As illustrated in Figure 1, ΔTf is directly correlated with the strength of interaction between components A and B. Greater interaction results in a lower melting point of the mixture, leading to a larger ΔTf. Mixtures at specific molar ratios will exhibit the lowest melting point, known as the eutectic point [27]. The low melting point and high fluidity of DESs, along with its capacity to induce numerous β-O-4 bond cleavages in lignin structure and partial carbon-carbon bond cleavages (such as β-β′, β-5′), render it an efficient solvent for delignification, which is important in lignocellulosic separation processes [28]. There have been more than 1000 studies on the application of DESs in lignocellulosic separation in the past decade and it has received increasing attention from researchers since 2018. The cellulose separated and purified from lignocellulose is depolymerized into glucose and further converted into bio-based chemicals such as ethanol, furan dicarboxylic acid, caprolactam, sorbitol, levulinic acid, γ-valerolactone [29,30,31,32], 5-hydroxymethylfurfural (5-HMF) [33,34], 2-furfural (2-F) [33], and 2,5-Bishydroxymethylfuran (BHMF) [35], which become an important direction of value-added conversion of waste biomass. DES has been widely discussed and studied in recent years and has been applied in many practical applications as a new solvent. For example, Lee et al. investigated the ability of hydrophobic DES to extract non-polar compounds [36]. Vasil et al. investigated the superiority of DES as an early sample preparation step for atomic absorption spectrometry [37].
This paper provides a comprehensive review of the research and application of deep eutectic solvent (DES) in lignocellulose separation over the past decade (2014–2024). The review focuses on assessing the impact of DES on delignification efficiency and subsequent sugar conversion. Investigations delve into understanding the effects of DES composition, treatment conditions, and pretreatment measures on lignocellulose separation. Furthermore, the recycling capabilities and environmental sustainability of various DES formulations are evaluated.

2. Deep Eutectic Solvent in Separation of Lignocellulose

Various types of deep eutectic solvents (DESs) have proven effective in facilitating the separation of lignocellulosic materials [38]. Nonetheless, mechanical and chemical pretreatments have been employed in conjunction to enhance the separation efficiency and refine the process conditions, including the duration and temperature of the separation [39]. Table 1 provides an overview of the typical DES-based lignocellulosic separation systems that have been utilized in recent years, highlighting their key features and applications. It is noteworthy that type II DES (ChCl:AlCl3·6H2O/FeCl3·6H2O) and type III DES (ChCl:lactic acid/p-TsOH) are commonly employed in the lignin removal process. This is due to the limited low-melting-point range of non-hydrated metal halides forming type I DES, which results in higher costs compared to metal salts meeting the requirements of type II DES. The preparation of type III DES is simple, relatively unreactive with water, and cost effective. Additionally, many are biodegradable in line with Green Chem principles. The discovery of type IV DES has demonstrated that transition metal halides can also form eutectic crystals. However, their high melting point makes them challenging for use in lignocellulose separation [40,41]. Among these, type III DES has shown outstanding in effectively removing lignin [42].
By exploring the separation of lignocellulose with different DESs, researchers have observed that various classes and ratios of DES can effectively separate lignocellulose from different plant biomass. The specific separation effect is influenced by both the raw materials and types of DES used. Increasing the separation temperature and extending the separation time can improve the removal rate of lignin. Different chemical and mechanical pretreatments assist in enhancing the efficiency of lignocellulose separation.

3. Definition of Deep Eutectic Solvent Effects

In the current research on lignocellulose separation by DES, it is widely recognized that DES acts by disrupting chemical bonds, hydrogen bonds, and covalent bonds within lignin in biomass materials as well as between lignin and hemicellulose, thereby facilitating the separation of lignocellulose components. By comparing the lignin removal efficiency of different DES, it is evident that the binary systems of ChCl (choline chloride) + Alcoholamine/benzene sulfonic acid and some quaternary ammonium salt + Brønsted acid/Lewis acid have higher lignin removal capabilities [21,78]. The principle can be attributed to several factors: alkaline and acidic DES have excellent lignin solubility, and alkaline DES has the additional benefit of protecting polysaccharides in the material [23,79]. Benzene sulfonic acid demonstrates a significant lignin removal rate at relatively low temperatures due to its strong proton-supplying capacity, which increases its ability to solubilize lignin [6,80]. Furthermore, ternary DES is often used to increase lignin and hemicellulose removal rates [81], and when Brønsted and Lewis acids coexist in DESs, these two different acids can work synergically to destroy the lignocellulose structure and catalyze the cleavage of hemicellulose β-1, 4-glucoside bonds [77]. This combination of properties and synergistic effects makes these DES formulations particularly effective for lignocellulosic separations.
Due to the easy degradation of hemicellulose into toxic compounds in a strong acidic environment, such as furfural or acetic acid, and the low yield of aromatic monomers caused by lignin recondensation [82], recent research has focused on reducing the intensity of component degradation and condensation while maintaining a high degree of lignocellulose separation [83]. This approach aims to enhance lignocellulose reuse and the downstream conversion of compounds in crop residues. One promising strategy is to introduce polyols into acidic DES to form a mixed-solvent system that preserves the chemical structure of the isolated components. For instance, Ji et al. developed a polyol-based DES composed of ChCl, glycerol, and AlCl3·6H2O, which successfully removed over 90% of lignin from garlic skin [48]. Shen et al. utilized a DES composed of ChCl, oxalic acid, and ethylene glycol to remove lignin from corn stover, which obtained better cellulose preservation and shortened the reaction time [74]. Similarly, Wang et al. used the neutral DES of ChCl/Xylitol and moso bamboo to obtain a lignin removal rate of about 85% [49]. The enhanced performance of these DES systems can be attributed to the presence of hydroxyl groups in the structure of polyols, which significantly boost the hydrogen bond supply capacity of the DES. This increased capacity for hydrogen bonding facilitates higher lignin removal rates [84]. As a binary neutral DES composed of HBD, polyols exhibit high cellulose preservation and monosaccharide conversion due to their mild chemical properties [85]. However, to overcome the limitation on lignin removal, dilute acids are usually used to pretreat raw materials [86]. On the other hand, polyols can be combined with quaternary ammonium salts and Brønsted/Lewis acids to form ternary acid DES. In these systems, polyols can be used as pH regulators to balance the lignin removal ability and component structure protection ability. Furthermore, DESs typically exhibit high viscosity, especially at close to room temperature, which can hinder mass transfer and prolong lignin removal times. The addition of polyols effectively reduce the viscosity of most DESs to improve its mass transfer efficiency and shorten the lignin removal time [87].
Many kinds of assisted treatments are used in lignocellulosic separation processes [88]. Different treatments often have varied effects on the separation process and the material [89,90]. For example, microwave and ultrasonic treatments provide distinct advantages over conventional conduction and convective heating methods [91,92]. Microwave radiation can transfer heat more quickly and evenly through dipole rotation and ion conduction with lower energy loss, which greatly reduces the reaction/extraction time without compromising the ease of control [93]. In addition to directly interacting with polar molecules in DES, microwave energy can selectively heat polar portions of biomass. At the same time, it improves the interaction between DES and lignin, and microwave-induced molecular scale stirring and mixing increases the mass transfer rate, making lignin easier to extract and dissolve [94]. The special cavitation effect of ultrasound can change the structure and properties of biomass during treatment [95]. For lignocellulose, ultrasound treatment can destroy the outer structure of the lignin, separate out the hemicellulose and increase the accessibility of cellulose molecules [96]. Some studies have found that the yield of reducing sugar from biomass is increased by enzymatic hydrolysis or acid hydrolysis after ultrasonic pretreatment [97].
Pretreatment or assisted separation of lignocellulosic raw materials using chemical agents such as acids, bases, or alcohols has been shown to be an effective way to improve the separation efficiency. Huang et al. [50] added 1 wt% hydroxyethyl sulfonic acid to DES of ChCl/PEG 600 to construct an acid lignocellulosic separation system and isolated more than 83% lignin and almost all hemicellulose from sugarcane bagasse. This proved that acidic environment can significantly improve the separation effect of lignocellulose [98]. Zhou et al. [6] achieved more than 97% and 87% removal rates of lignin and hemicellulose from poplar by KOH pretreatment. It was found that the BET (Brunauer–Emmett-Teller) surface area of the pretreated material increased and the contact angle between poplar and DES decreased, which increased the subsequent lignin removal rate [6].
Alkaline hydrogen peroxide pretreatment (AHP) has good performance in biomass deconstruction. It is beneficial to moderate the processing conditions of DES. The main mechanism of action is the dissociation of H2O2 under alkaline conditions to produce highly reactive oxygen species (ROS), such as hyperhydroxylated anions (HOO-) and free radicals (HO·). ROS depolymerizes lignin by attacking the lignin side chain and reacts with the enone chromophore (such as the quinone structure) in lignin to increase the dissolution of lignin and reduce the polymerization of lignin [99]. Ma et al. [16] used hydrogen peroxide to pretreat industrial xylose residue at 60 °C, removing more than 97% of lignin. This has greatly reduced the energy consumption in the separation process compared to similar raw materials without pretreatment.
Mechanochemical pretreatment processes have been increasingly recognized as green methods for deconstructing lignocellulose through mechanical forces such as shock, compression, shear, and friction [100]. Ball milling has been widely used in research and industry as a mechanical treatment to break and reduce sample particle size. In biomass pretreatment, ball milling can effectively deconstruct materials and increase the contact area between lignocellulosic components and DES, thus accelerating the separation rate [101]. This characteristic makes ball milling a method to simplify the separation conditions of lignocellulose [102]. Impressively, Sun et al. used DES to remove lignin from poplar wood during ball milling, achieving a removal rate of over 99% at room temperature [54].
Hydrothermal pretreatment is a straightforward and effective method that relies solely on the addition of water, making it widely used in the processing of lignocellulosic biomass [103]. Its simplicity offers an opportunity to reduce production costs [104]. Hydrothermal pretreatment is a chemical-free, economical and environmentally friendly process with little effect on the structure of lignin and cellulose. In addition, hydrothermal pretreatment uses water at high temperatures and pressures to hydrolyze the lignocellulosic structure, especially hemicellulose, which is more easily hydrolyzed due to its highly amorphous structural properties [105]. Compared to biomass material, the pretreatment significantly shortened the treatment time while maintaining the same lignin removal rate [57,64].
Overall, different types of DESs have varied separation effects on biomass due to their specific compositions and ability to disrupt and catalyze the breakdown of lignocellulosic structures. By modifying the components of DESs and introducing assisted solvent, the lignin solubility of DES can be adjusted, thereby protecting the structure and function of raw materials after separation [106]. Numerous of assisted treatment methods have been applied to lignocellulosic separation process [107]. The assisted treatment can improve the separation capacity of DES or simplify the separation conditions through chemical or physical means [108]. By comparing the different kinds of auxiliary treatment on the separation effect of lignocellulose, it can be found that microwave-assisted pretreatment and ball-milling pretreatment often have a higher lignin removal rate and cellulose retention rate. This is due to the potential of these two methods to be applied in industry.
Figure 2 provides a comprehensive and intuitive overview of the steps involved in the separation of lignocellulosic biomaterials using DES. This includes the composition and type of DES, pretreatment of biomass, separation of lignocellulose by DES, and the resulting cellulose-enriched residue and lignin-enriched filtrate obtained through filtration. Due to the high viscosity of DES, an organic solvent-assisted filtration process is necessary to effectively separate the cellulose-enriched component from the DES + lignin/hemicellulose component. Commonly used organic solvents in such studies include aqueous solutions of ethanol or acetone [49,51], which serve to reduce the viscosity of the filtrate without causing dissolution of solid cellulose residues. The changes in biomass microstructure during DES treatment were analyzed by SEM after drying the solid residue. At present, DES is widely used as a catalyst, co-solvent, and extraction solvent for various conversion and downstream processes [109].
The recycling of DES is one of the major challenges in achieving cost-effective industrial applications [110,111]. The relatively low solvent consumption and recyclability of DES make it an important green solvent in the field of green separation of lignocellulose [112]. There are many methods to recover and purify DES, including the addition of anti-solvent, crystallization, membrane filtration, solid–liquid extraction, liquid–liquid extraction, short path distillation, supercritical fluid extraction, and density differences-based separation [24,113,114,115,116,117]. Specific recovery methods can be selected based on the types and characteristics of DES, the properties of target compounds, energy consumption, and equipment costs [118,119,120,121,122]. Although DES is widely known as a green solvent, it does not remain environmentally friendly under all conditions due to decomposition and volatilization. Therefore, many strategies have been studied and applied to improve the greenness of DES as potential solutions to achieve Green Chem [123]. For example, strategies include choosing more natural ingredients [121,124], specific functionalization [125], increasing molecular weight and surface area by polymerization for separation, sensing and catalysis [126], tuning types of intermolecular interaction and component numbers [127], constructing a switchable system for efficient separation [128], avoiding air, water, high temperature, reduced pressure, strong oxidizers/deoxidizers, long-term exposure, and organic solvents [129].

4. Effect of DES on Sugar Conversion of Substrate

It is found that the reducing sugar yields from the pretreated cellulose-enriched solid residue differ from those of the untreated biomass [130]. Untreated biomass contains both cellulose and hemicellulose. However, after pretreatment with DES, the biomass generally no longer contains hemicellulose. Figure 3 completely shows the enzymatic hydrolysis process of lignocellulose isolated after DES pretreatment to produce glucose. It is worth noting that DES treatment removes lignin and hemicellulose in biomass, which significantly improves the enzymatic hydrolysis rate of cellulose enriched residue after treatment.
Table 2 shows the changes in enzymolysis glucose yield of lignocellulosic biomass substrates before and after pretreatment with different types of DES. It is evident that each DES pretreatment significantly improves the substrate enzymatic hydrolysis efficiency between (174–883)% with different types of cellulase [16,23,56,131,132,133,134]. It is worth noting that adding appropriate metal chloride salts or benzene sulfonic acid to DES as an auxiliary solvent can further significantly improve the glucose yield compared to conventional binary DES. Wu et al. added 1.6 wt% p-TsOH to soybean straw as pretreatment in BTEAC/glycerol system, which increased the yield of substrate glucose hydrolysis from 9.17% to 90.18% [131]. These findings are interpreted as the disruption of the complex structure of biomass, degradation of hemicellulose, and removal of lignin after pretreatment with DES. This pretreatment strategy resulted in the destruction of the hemicellulose–cellulose binding protein matrix, exposing more enzyme attack sites, and improving the accessibility of substrate enzymes [72,133].

5. Effect of DES Treatment on the Chemical Structure of Lignocellulosic Separation Products

FT-IR spectroscopy can be used to study the effect of DES pretreatment on the internal chemical structure of lignocellulose by comparing the characteristic absorption peaks of chemical bonds such as CH, C-C, and CO [135]. This technique allows for the identification and quantification of functional groups present in the lignocellulosic material, providing insights into the chemical changes induced by the pretreatment process [63]. Xie et al. [136] pretreated Moso bamboo with ChCl/zinc acetate DES under microwave-assisted conditions and performed FT-IR characterization of the samples before and after treatment. The FT-IR results showed changes in lignocellulosic characteristic peaks associated with OH, CH, CO, and CO bond stretching. After DES pretreatment, the characteristic peak at 3331 cm−1 almost disappeared. This suggested that bands of intramolecular and intermolecular hydrogen bonds between 3500 and 3100 cm−1 were destroyed by DES. In addition, the absorbance of DES pretreated biomass decreased at 1734cm−1. These peaks indicate removal of hemicellulose because they suggest stretching vibrations of acetyl groups in hemicellulose [137].
Li et al. [138] pretreated rape straw with ChCl/oxalate acid DES and performed FT-IR characterization of the samples before and after treatment. It was found that peaks near 3420 cm−1 were associated with tensile vibrations of OH in biomass lignin, and -CH2 vibration was detected near 2930 cm−1. The ester bond stretching vibration between hemicellulose and lignin was close to 1732 cm−1. After pretreatment, the characteristic peaks at 3420 cm−1 and 2930 cm−1 were significantly reduced, indicating that the hydrogen bond within lignocellulose was broken. Additionally, the tensile vibration of aromatic ring-associated CO near 1625 cm−1 was significantly reduced, indicating the degradation of lignin in the raw material [139]. The peak near 1420 cm−1 was associated with the bending of -CH2 in cellulose. Peaks near 1050 cm−1 corresponded to CO stretching in the hemicellulose–cellulose–lignin polymers. The peak near 896 cm−1 was associated with the β-glucoside bond. These FT-IR data indicated that DES pretreatment can effectively remove hemicellulose and lignin from biomass [140]. Table 3 clearly summarizes the chemical bond changes in the biomass after DES treatment of lignocellulose.
DES pretreatment can significantly affect the types and contents of functional groups in lignocellulosic biomass, playing a crucial role in disrupting the interactions between lignocellulosic components and lignin structure [142]. The structural differences of regenerated lignin are usually analyzed by 2D-HSQC NMR [143]. In general, the major substructures of lignin include methoxy [120], aryl ether bonds (A, β-O-4), and β-β resinol (B). As Figure 4 shows, during the pretreatment process, the cleavage of β-O-4 linkage in lignin linkage significantly increased with the increase in pretreatment temperature. It was found that the β-O-4 bond disappeared above 110 °C, indicating that the depolymerization of lignin was achieved by destroying the β-O-4 substructure during DES pretreatment. This is also consistent with the results of acidic DES pretreatment and the breakdown of most lignocellulose. The lignin–carbohydrate complex (LCC) bond, as described above, shows different signals in the aromatic region corresponding to guaiacol (G) and syringyl (S) units, which are clearly visible. This phenomenon indicates the process of lignin polycondensation during pretreatment [144].
Morphological changes of lignocellulosic biomass during DES pretreatment can be analyzed by SEM. This technique provides detailed images of the surface structure and morphology of the biomass, allowing researchers to observe alterations at the microstructural level [145]. Moran-Aguilar et al. [146] pretreated sugarcane bagasse (SCB) with ChCl/carboxylic acid. The morphological changes of SCB surface after pretreatment were shown in Figure 5. Pictures of the natural samples showed smooth, complete, and ordered fiber surfaces (Figure 5a), while SEM analysis of the pretreated samples revealed structural differences, with rough and exposed structural morphology. Micrographs of ChCl/LA (lactic acid) showed smooth and consistent surfaces (Figure 5b), mainly indicating the presence of crystalline cellulose. The pretreated samples had higher crystallinity and more ordered cellulose structure than the natural samples [147]. In addition, images of ChCl/CA (citric acid) pretreated biomass indicated a planar porous structure and non-uniform surface formed by various fiber fragments (Figure 5c). Finally, images of the SCB pretreated with ChCl/AA (acetic acid) showed that the deformed structure had wide cracks and holes (Figure 5d). The loss of fibers and the increase in porous surfaces can be observed [146].
The effects of DES with different characteristics and the addition of auxiliary solvents on the morphology of biomass were also demonstrated by SEM analysis [148]. This technique showed that mild acidic DES pretreatment can improve cellulose reactivity by decomposing and swelling the cellulose, removing lignin and hemicellulose (mainly xylan), thereby better exposing the innermost cellulose component of the biomass and making it accessible to enzymes [149]. In contrast, alkaline DES can lead to expansion and partial dissolution of lignocellulosic materials. These DES systems typically effectively remove lignin while preserving the cellulose structure [150]. Shen et al. added FeCl3 to Moso bamboo pretreated with ChCl/lactic acid DES. SEM illustrated varying degree of cracks and fragments on the surface of the fibers after pretreatment. FeCl3-mediated pretreatment caused more damage to cellulose, indicating that adding FeCl3 can improve the intensity of biomass pretreatment [151]. Ma et al. added hydrogen peroxide to industrial xylose residue after ChCl/Isopropanolamine pretreatment. SEM images showed that as the hydrogen peroxide content increased, the pores in the pretreated substrate also increased. This indicated that hydrogen peroxide had destroyed the morphology of the residue while removing lignin, which helped improve the yield of fermentable glucose [16]. Grzegorz et al. explained the effect of microwave-assisted treatment on the microstructure of lignocellulose through SEM analysis. The results showed that microwave-treated biomass particles had larger porous structures on their surfaces, which exposed more cellulose structures. The pore structure and cellulose exposure contribute to the hydrolysis efficiency of cellulase [152]. Chemical treatment usually achieves the effect of separating components through targeted chemical bonding. However, ultrasonic and microwave treatments tend to destroy the biomass structure with physical impact. Both of them can increase the porosity of lignocellulose and improve the efficiency of enzymatic hydrolysis.
Overall, FT-IR spectral analysis can be utilized to compare the presence or absence of characteristic peaks associated with specific functional groups before and after DES pretreatment. This alteration is employed to ascertain the chemical bonds that have been broken and new bonds that have been formed during the process. The side chain and aromatic regions of the 2D-HSQC spectra were analyzed. Trends in the increase and decrease in chemical bonds and units in polymers during DES treatment were determined by evaluating the sizes of the corresponding functional group graphs. SEM analysis can effectively demonstrate the effect of DES pretreatment on the morphology of lignocellulosic biomass. It can be observed that most DESs can cause the surface of the biomass to form porous and non-uniform structure by destroying the bonding bonds in the biomass. Moreover, the addition of auxiliary solvents in the DES pretreatment process can also enhance the damage ability of the biomass surface, which can also be confirmed in the improvement effect of lignocellulose separation ability corresponding to this method in the previous section.

6. Conclusions

Lignocellulose, which accounts for the majority of the waste biomass in crops and other plants, has become an important raw material for platform chemicals. To effectively use this waste biomass, it is necessary to separate lignocellulose into purified cellulose, hemicellulose, and lignin. In recent years, DES, as a green and easy-to-synthesize solvent, has received extensive attention from researchers. Different kinds of DESs have shown different lignin separation efficiencies due to their ability to supply proton and cleave hydrogen bonds and covalent bonds. Mechanical- or chemical-assisted treatments have been shown to optimize the separation conditions of lignocellulose, including reducing the reaction temperature and shortening the reaction time. The separation mechanism of DES on lignocellulose can be better understood by analyzing the biomass before and after treatment through various characterization methods, paving the way for more effective and sustainable biomass conversion technologies. Although extensive research has been conducted on the separation and purification of lignocellulose from waste biomass and the reuse of products, there are still several challenges in optimizing the pre-/post-processing steps of separation from a green and economic perspective:
  • Prior to DES pretreatment, fat-soluble components in biomass can be eliminated through organic solvent or supercritical fluid extraction to enhance the efficiency of DES treatment.
  • Variations in the separation effect between different types of DESs and biomass have been observed, which are generally attributed to differences in their physical and chemical properties, including solvent polarity, proton-providing capacity, hydrogen bonding capacity, and electrical conductivity. The specific impacts of these properties require systematic exploration.
  • While the DES processing temperature has gradually decreased in previous studies, further investigation is needed to explore the potential for lower temperatures or even room temperature separation.
  • Attention should be given to assessing the impact of DES treatment on the chemical structure and crystallinity of lignin products as this influence can guide lignin towards meeting specific deep processing requirements.

Author Contributions

Conceptualization, G.C. and H.G.; methodology, H.G.; software, Z.Y.; validation, Z.Y., G.C. and H.G.; formal analysis, H.G.; investigation, Z.Y.; resources, H.G.; data curation, Z.Y.; writing—original draft preparation, Z.Y.; writing—review and editing, H.G. and G.C.; visualization, Z.Y.; supervision, H.G.; project administration, H.G.; funding acquisition, H.G. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful for Elite Youth program of the Chinese Academy of Agricultural Sciences (to Haixin Guo).

Data Availability Statement

No data were used for the research described in the article.

Conflicts of Interest

The authors have no competing interests to declare.

Abbreviations

DESDeep eutectic solvent
CAGRCompound annual growth rate
ILsIonic liquids
HBDHydrogen bond donor
HBAHydrogen bond acceptor
HMF5-hydroxymethylfurfural
F2-furfural
BHMF2,5-Bishydroxymethylfuran
ChClCholine chloride
LALactic acid
p-TsOHp-Toluenesulfonic Acid
MEAMonoethanolamine
GLGlycerol
AAAcetic acid
BTEACBenzyltriethylammonium chloride
FAFormic acid
GndHClGuanidine hydrochloride
4-CSA4-chlorobenzene sulfonic acid
OAOxalic acid
EGEthylene glycol
CACitric acid
HESHydroxyethyl sulfonic acid
IPAIsopropanolamine
PTAPhosphotungstic acid
CTABCetyltrimethylammonium bromide
BETBrunauer-Emmett-Teller
AHPAlkaline hydrogen peroxide
ROSReactive oxygen species
FT-IRFourier Transform Infrared Spectroscopy
LCCLignin-carbohydrate complex
G unitGuaiacol
S unitSyringyl
2D-HSQC2D-Heteronuclear Single Quantum Coherence
SCBSugarcane bagasse
SEMScanning electron microscope

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Figure 1. Schematic representation of a eutectic point on a two-components phase diagram.
Figure 1. Schematic representation of a eutectic point on a two-components phase diagram.
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Figure 2. DES separation process of lignocellulose. Cat+ for ammonium and other cation, X− for Lewis base, generally a halide anion, Y stands for Lewis or Brønsted acid (z refers to the number of Y molecules that interact with the anion).
Figure 2. DES separation process of lignocellulose. Cat+ for ammonium and other cation, X− for Lewis base, generally a halide anion, Y stands for Lewis or Brønsted acid (z refers to the number of Y molecules that interact with the anion).
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Figure 3. Schematic diagram of enzymatic hydrolysis of cellulose enrichment residue after DES pretreatment.
Figure 3. Schematic diagram of enzymatic hydrolysis of cellulose enrichment residue after DES pretreatment.
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Figure 4. (a) Side-chain regions in 2D-HSQC spectra of the lignin, (b) aromatic regions in 2D-HSQC spectra of the lignin, (c) the identified main structures in the lignin (Reprinted/adapted with permission from Ref. [144]. 2023, Xuefeng Yao) [144].
Figure 4. (a) Side-chain regions in 2D-HSQC spectra of the lignin, (b) aromatic regions in 2D-HSQC spectra of the lignin, (c) the identified main structures in the lignin (Reprinted/adapted with permission from Ref. [144]. 2023, Xuefeng Yao) [144].
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Figure 5. Images of (a) the native sugarcane bagasse (SCB) and SCB pretreated with different HBD of DES: (b) ChCl/lactic acid, (c) ChCl/citric acid and (d) ChCl/acetic acid. Micrographs were obtained with variable magnification: (I) ×50; (II) ×200; (III) ×1500 (Reprinted/adapted with permission from Ref. [146]. 2022, María Guadalupe Morán-Aguilar) [146].
Figure 5. Images of (a) the native sugarcane bagasse (SCB) and SCB pretreated with different HBD of DES: (b) ChCl/lactic acid, (c) ChCl/citric acid and (d) ChCl/acetic acid. Micrographs were obtained with variable magnification: (I) ×50; (II) ×200; (III) ×1500 (Reprinted/adapted with permission from Ref. [146]. 2022, María Guadalupe Morán-Aguilar) [146].
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Table 1. Delignification rate of different DESs for biomass and component ratio of solid residue.
Table 1. Delignification rate of different DESs for biomass and component ratio of solid residue.
SubstrateSolvent (mol)Assisted
Treatment		ConditionDelignification
Rate
(%)		Lignin in
Solid Residue (%)		Cellulose in
Solid Residue
(%)		Hemicellulose
in Solid Residue
(%)		Cellulose Preservation
(%)		Ref.
Alkaline DESs
Industrial xylose residueChCl: MEA (1:6)Microwave-assisted80 °C, 10 min91.953.5995.600.4298.73[23]
Wheat strawLA: Pyrazole
(1:1)		-145 °C, 9 h93.702.15--51.17[43]
PopulusChCl: Imidazole (3:7)-150 °C, 0.67 h75.506.5029.902.9069.25[44]
Rice strawChCl: Urea (1:2)-130 °C, 6 h43.198.4325.0611.5570.61[45]
Wheat strawGL: Urea (1:2)Hydrothermal pretreatment90 °C, 3 h75.6019.8076.13nd99.20[46]
Neutral DESs
Garlic skinChCl: GL: AlCl3·6H2O
(1:2:0.2)		Ultrasound +
microwave assisted		Room
temperature,
30 min;
80 °C, 20 min		90.145.1548.5229.4479.75[47]
Sugarcane bagasseChCl: GL: FeCl3·6H2O (1:1:0.3)Ultrasound ethanol pretreatment240 W, 1 h;
DES: 120 °C, 3 h		86.399.0466.17--[48]
BambooChCl: Xylitol (1:2)1 wt% H2SO4120 °C, 4 h84.9110.7273.301.8272.68[49]
Sugarcane bagasseChCl: PEG 600
(1:4)		1 wt% HES 170 °C, 2 h83.2311.4175.66nd73.63[50]
Corn strawChCl: 1,4-butanediol (1:2)NaOH assisted100 °C, 3 h81.30---71.50[51]
Industrial xylose residueChCl: IPA
(1:6)		Hydrogen peroxide assisted60 °C, 2 h97.111.3191.882.52-[16]
Acid DESs
Poplar sawdustChCl: p-TsOH (1:2)NaOH treatment100 °C, 40 min92.204.6987.501.6192.98[52]
Poplar residues ChCl: p-TsOH
(1:2)		Ethanol90 °C, 1 h90.99---90.47[53]
Poplar
wood chips 		ChCl: p-TsOH
(1:2)		Ball milling
Room temperature, 3 h99.78---82.19[54]
Sorghum strawChCl: LA (1:1)NaOH
0.75 wt% pretreatment		NaOH:121 °C,
1 h
DES:140 °C, 0.67 h		79.3011.2053.4029.3079.67[55]
Corn stoverChCl: LA (1:2)Microwave-assisted800 W, 45 s79.609.2265.785.0375.11[56]
Sugarcane bagasseChCl: LA (1:2)Hydrothermal pretreatment130 °C, 1.5 h84.303.5078.5010.6056.76[57]
CorncobChCl: LA (1:2)-110 °C, 24 h95.50-77.80--[58]
Eucalyptus grandisChCl: LA (1:2)-150 °C, 1.5 h93.008.9078.4nd49.47[59]
Oil palm empty fruit bunchChCl: LA (1:5)-120 °C, 8 h88.004.7071.40nd-[60]
Sugarcane bagasseChCl: LA
(1:10)		Ultrasound40 kHz, 1.5 h
DES:120 °C, 3 h		86.826.8965.624.4767.57[61]
CorncobChCl: LA (1:10)-130 °C, 4 h95.901.5 074.907.0089.03[62]
Poplar ChCl: LA
(1:10)		PTA120 °C, 4 h82.20----[63]
Poplar sawdustChCl: LA (1:10)Hydrothermal pretreatment130 °C, 1.5 h84.407.8079.2010.2086.10[64]
Poplar ChCl: acetic acid (AA) (1:2)-130 °C, 3 h76.509.5085.406.9099.00[65]
BambooChCl: acetic acid (1:2)-140 °C, 6 h80.107.30--92.70[66]
Reed strawBTEAC: FA (1:6)1,4-dioxane/water
(96:4 v/v)		130 °C, 3 h94.109.0089.321.6888.57[67]
Radiata pineBTEAC: FA (1:2)-150 °C, 2 h87.007.4182.696.0288.23[68]
Corn stoverBTEAC: LA (1:7)-140 °C, 1.5 h83.519.4767.074.6687.71[69]
Pine wood powderChCl:LA: FA
(1:1:1)		-130 °C, 6 h72.0015.2063.4010.3083.84[70]
R. glutinisChCl: GL: Acetic acid (1:2:1.5)-140 °C, 4 h63.767.2955.623.9178.00[71]
BambooGL: GndHCl: FeCl3·6H2O (2:1:0.2,)Microwave-assisted120 °C, 10 min81.1711.1374.376.0390.96[72]
PoplarChCl: EG: 4-CSA (1:2:0.7)KOH pretreatment90 °C, 0.5 h97.011.0593.334.3895.02[6]
Pine needlesLA: Oxalic acid: ChCl (4:1:1)Toluene-ethanol pretreatment120 °C, 3 h95.80----[73]
Corn stoverChCl: Oxalic acid: EG (1:0.2:2)Densification pretreatment
130 °C, 1 h75.277.8559.4311.0988.75[74]
Wheat strawChCl: CA: EG
(1:1:2, mol)		-100 °C, 12 h92.375.7189.123.0489.18[75]
Paddy husksEG: CA (1:2)-90 °C, 16 h52.3515.2544.0017.37-[13]
Poplar sawdustZnCl2: LA (1:10)-140 °C, 3 h97.502.2092.502.7093.84[76]
Canola strawCTAB: LA: FeCl3 (1:4:0.012)-180 °C, 2 h87.408.7059.402.1053.89[77]
Choline chloride: ChCl; lactic acid: LA; citric acid: CA; ethylene glycol: EG; glycerol: GL; formic acid: FA; monoethanolamine: MEA; 4-chlorobenzene sulfonic acid: 4-CSA; Isopropanolamine: IPA; Guanidine hydrochloride: GndHCl; Hydroxyethyl sulfonic acid: HES; Phosphotungstic acid: PTA; benzyltriethylammonium chloride: BTEAC.
Table 2. Effects of pretreatment of lignocellulosic biomass with DES pretreatment system on reducing glucose yield.
Table 2. Effects of pretreatment of lignocellulosic biomass with DES pretreatment system on reducing glucose yield.
SubstrateDESConditionUntreated Glucose Yield (%)DES-Treated Glucose Yield (%)Increase Rate (100×%)Ref.
SwitchgrassChCl: lactic acid 1:2Solid loading 1.5 wt%, 20 mg protein/g solid, CTec2:HTec2 = 1:9 (vol%)13.6175.04.51[56]
Industrial xylose residueChCl: monoethanolamine
1:6		Solid loading, 5 wt%, 10 FPU/g CTec2 15.9790.124.64[23]
Industrial xylose residueChCl: monoethanolamine
1:6		10 FPU/g CTec234.9595.891.74[16]
SugarcaneChCl: lactic acid
1:2		Solid loading, 10 wt%, 15 mg protein/g solid, cellulase enzymes NS2225719.7493.993.76[132]
Corn stoverFeCl3/ChCl: N-(2-hydroxyethyl) Ethylenediamine
1:4		Solid loading, 5 wt%, 30 FPU/g CTec216.5498.64.96[134]
Soybean strawp-TsOH/BTEAC: glycerol
1:12		Solid loading, 5 wt%, 10 FPU/g, CTec29.1790.188.83[131]
Eucalyptus wood chipsAlCl3/Betaine: lactic acid
1:2		30 FPU/g, CTec213.2096.006.27[133]
Table 3. The effect of DES pretreatment on the internal chemical structure of lignocellulose was studied by comparing the characteristic absorption peaks of chemical bonds with FT-IR spectroscopy.
Table 3. The effect of DES pretreatment on the internal chemical structure of lignocellulose was studied by comparing the characteristic absorption peaks of chemical bonds with FT-IR spectroscopy.
SubstrateDESChemical BondsAbsorption Peaks (cm−1)TrendAffected ComponentRef.
Moso bamboo ChCl: zinc acetate = 1:6OH/C-H3331/2891Almost disappearedHydrogen bonds[136]
C=O1734DecreasedHemicellulose
Rape strawChCl: oxalic acid = 3:1CO1625Significantly reducedLignin[138]
CO stretching1050ReducedHemicellulose-cellulose-lignin polymers
CH or CH2 bending896Reducedβ-glucoside bond
WillowChCl: lactic acid = 1:10CH bending vibration of aliphatic compounds1373Not presentCarbohydrates[141]
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Yao, Z.; Chong, G.; Guo, H. Deep Eutectic Solvent Pretreatment and Green Separation of Lignocellulose. Appl. Sci. 2024, 14, 7662. https://doi.org/10.3390/app14177662

AMA Style

Yao Z, Chong G, Guo H. Deep Eutectic Solvent Pretreatment and Green Separation of Lignocellulose. Applied Sciences. 2024; 14(17):7662. https://doi.org/10.3390/app14177662

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Yao, Zhengyuan, Gunhean Chong, and Haixin Guo. 2024. "Deep Eutectic Solvent Pretreatment and Green Separation of Lignocellulose" Applied Sciences 14, no. 17: 7662. https://doi.org/10.3390/app14177662

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