The biological method mainly refers to the pretreatment of lignocellulose using microbial technology to destroy its structure, followed by removing lignin, which significantly improves the utilization rate of cellulose (Chen et al., 2017). Microorganisms, such as white rot fungi, brown rot fungi, soft rot fungi, or their combinations, are applied into the pretreatment process (Dias et al., 2010; Tian et al., 2018). Singh et al. pretreated wheat straw with Phanerochaete chrysosporium, which achieved 30% of total lignin removal rate within 3 weeks, while the total sugar content was reduced by about 27% compared with untreated wheatgrass (Singh et al., 2011b). Saha et al. (2016) pretreated corn stover with white rot fungus strain Pycnoporus sanguineus NRRL-FP-103506-Sp at 28°C and 74% humidity for 30 days and found that the lignin loss rate reached 51 ± 1.2%, and the hemicellulose and cellulose loss rates were 50.7 ± 2.1% and 25.4 ± 0.3%, respectively. Despite the mild and green reaction conditions of the biological method, it is still difficult to achieve large-scale industrial applications because of extreme long treatment time, low efficiency, few types of microorganisms that degrade lignin, and strict degradation conditions (Chen et al., 2017).

Physical method mainly includes mechanical comminution (Sun and Cheng, 2002), radiation pretreatment (Yang and Wang, 2018), microwave pretreatment (Liu et al., 2021), freeze pretreatment (Chang et al., 2011), etc., which significantly improves the enzymatic hydrolysis efficiency and tunes the reaction time. However, the physical method still has some disadvantages, such as high energy consumption and low lignin removal rate (Aguilar-Reynosa et al., 2017). It has been reported that the sugar yield of glucose and xylose/mannose was 59.67 and 23.82%, respectively, after 30 min of planetary ball milling of Douglas-fir forest residuals (Gu et al., 2018). Monschein adopted the steam explosion method for pretreatment of wheat straw (Monschein and Nidetzky, 2016). Inevitably, only 36% of the removal rate of lignin was achieved, accompanying with 24.1 and 38.7% of hemicelluloses and cellulose loss rates, respectively. In order to receive effective pretreatment, most of the existing physical pretreatment methods are conducted in combination with chemical or biological methods (Bals et al., 2010). Compared with wet ball milling, the enzymatic hydrolysis efficiency of the pretreatment of corn stover by the alkaline milling method was increased by 110% (Kumar and Sharma, 2017). The combination method has the characteristics of greatly destroying the structure of lignocellulose, effectively removing lignin, improving the efficiency of enzymolysis, and reducing the loss of equipment, but there are still some shortcomings that limit its industrial application, such as insufficient technology, high energy consumption, difficulty to operate, and pollution (Teymouri et al., 2005).

The chemical method is usually defined as the process of destroying lignocellulosic dense cell wall structures with chemical reagents, such as acid, alkali, and organic solvent to remove hemicellulose or lignin.

Sulfolane is a cheap polar aprotic solvent with high boiling and flashing points, extremely low vapor pressure (1.93 kPa at 150°C), high thermochemical stability, and low level of volatility and toxicity, and it is originally used in the extraction of aromatic hydrocarbons in the petroleum industry (Singh et al., 2011a; Tilstam, 2012; Huang et al., 2014; Wang et al., 2016). Therefore, sulfolane could be defined as a green solvent according to the 12 principles. The basic structure diagram of sulfolane is shown in Figure 3. Because of the existence of sulfoxide double bond, it can be miscible with water and most organic solvents, etc. Notably, its special structure gives sulfolane the selective ability to extract aromatic hydrocarbons (Clark, 2000), and its outstanding compatibility with lignin instead of carbohydrates. Thus, sulfolane has a better industrial application prospect in delignification of lignocellulosic biomass (Tilstam, 2012).

Sulfolane is usually applied in the pretreatment of lignocellulosic biomass with the presence of water and catalysts. Previous studies by this research group have initially put forward that sulfolane is an effective and promising solvent for pretreatment of lignocellulosic biomass (Wang et al., 2016). When shrub willow was pretreated using a sulfolane solvent system with a ratio of substrate to solvent of 1:5 at 170°C for 1.5 h, the production rate of glucose and xylose could reach 78.2 and 56.6%, respectively. Notably, 84.7% of lignin removal rate could be obtained, accompanied with 13.9% of hemicellulose loss rate. Meanwhile, we investigated the effect of water and acid on the system and found that the anhydrous and acid-free conditions are conducive to the retention of cellulose. Finally, 80% of lignin removal rate could still be obtained after reusing sulfolane five times. Zhong et al. pretreated willow by alkaline catalysis with a sulfolane/water solvent system and prepared antioxidant lignin with high purity. The results showed that the lignin removal rate was 54% at 170°C with the sulfolane/water solvent system (mass ratio of 50/50; Zhong et al., 2020). Interestingly, when 4 wt% of NaOH was added as alkali catalyst, the lignin removal rate could gradually increase up to 94%, and the losses in glucose and xylose were controlled around 1.62 and 4.08%, respectively. The low molecular weight lignins collected by this pretreatment system had a phenolic hydroxyl group content (0.86–2.02 mmol/g), which was higher than that reported in the literature (0.2–0.45 mmol/g), which may be due to the breaking of aryl ether bonds or the phenolic hydroxyl group being more stable under relatively mild conditions. Furthermore, lignin components with high phenolic hydroxyl content had good free radical scavenging ability, indicating that they all had high antioxidant performance, which further increased the functionality of lignin. The research results showed that lignin extracted by sulfolane pretreatment has great potential to replace the expensive and relatively low efficiency antioxidants on the current market. Barahona et al. adopted sulfolane as an organic solvent for the pretreatment of bagasse to efficiently convert lignocellulosic biomass into hydrolyzed cellulose and high-purity lignin, and allow the recovery of valuable byproducts and improve the economic feasibility of the whole cellulosic ethanol process (Portero-Barahona et al., 2019). This research has shown that a total reductive sugar yield of 62.9% could be obtained with a 1:1 (v/v) water: sulfolane and 5% NaOH solution for 5 min at 140°C. The saccharification condition was to use 10 FPU cellulase/g for enzymatic hydrolysis of dry substrate at 50°C for 72 h, and the final glucose yield was 80.5%. Compared with the pretreatment of bagasse with water as medium, which also showed lower energy consumption, shorter reaction time, and higher glucose yield, the total reducing sugar and saccharinic acid production rate of sulfolane-TiO₂ increased by 5 and 33%, respectively.

These studies indicated that sulfolane could selectively remove large amounts of lignin during the pretreatment process of lignocellulosic biomass. Furthermore, cellulose and hemicellulose can be preserved in large quantities by selecting the appropriate catalyst and dosage of water, which would also benefit the subsequent enzymatic hydrolysis saccharification process. Based on the physical and chemical properties and reported experimental studies, it is believed that sulfolane can be defined as a green solvent with low toxicity, high thermo-chemical stability, easy separation and recycling characteristics, high efficiency, low cost, and low steam pressure, etc. Therefore, sulfolane has broad application prospects in biomass pretreatment.

GVL is a well-known renewable organic compound (Yang et al., 2020), and it can be directly obtained from cellulose and lignocellulosic biomass (Alonso et al., 2013a,b; Cai et al., 2017). As shown in Figure 4, Alonso et al. introduced the general path for the preparation of GVL from lignocellulosic biomass by hydrogenation of levulinic acid. GVL is a thermo-chemical stable solvent with a low melting point of −31°C, high boiling point of 207°C, high flash point of 96°C and very low vapor pressure even at higher temperatures (0.65 kPa at 25°C; Alonso et al., 2013b). On the other hand, due to the following important characteristics, namely, easy and safe to store and transport in large quantities on a global scale, miscible with water, and contributor to biodegradation, GVL does not suffer from environmental problems and most risk factors, making it a safe material for large-scale use (Horváth et al., 2008). In addition, in the GVL/water system, it is easy to separate and recycle the GVL in pretreated waste liquid. By the combination of two-step lignin precipitation and vacuum distillation, more than 90% of lignin and GVL could be recycled from waste liquid (Lê et al., 2018).

It has been reported that a 80/20 (w/w) GVL/water composite system was used to pretreat hardwood with the presence of 0.75 wt% H₂SO₄ at 120°C for 1 h (Shuai et al., 2016). Up to 80% of the original lignin and 77% of the xylan could be selectively removed. Notably, more than 96–99% of the original cellulose can be retained. An enzyme loading of 15 FPU g⁻¹ glucan was used to quantify the sugar conversion of the pretreated substrate, and the total glucose and xylose production rate was 99 and 100%, respectively. Moreover, by performing liquid carbon dioxide extraction on the pretreated slurry, more than 99% of sugars can be retained, accompanied with up to 99.5% recycle rate of GVL. This study found that most of the dissolved xylan was recycled in the form of oligomers in the pretreatment liquor, indicating that the hydrolysis of xylose and glycosidic bonds was very slow under these reaction conditions, which is benefit for reducing the degradation of sugar (Shuai et al., 2016). Wu et al. proposed a GVL/water system to combine hybrid pennisetum lignocellulosic biomass pretreatment with anaerobic digestion for biogas production and synthesis of lignin nanoparticles (LNPs; Wu et al., 2020). Notably, around 33% of lignin could be selectively removed without acid catalysis in the GVL/H₂O = 50/50 (w/w) system at 150°C for 90 min, retaining about 92% of dextran and 85% of xylan sugar. Subsequently, the pretreated solid was used for anaerobic fermentation to produce methane. When the proportion of GVL in the solvent system reached 50%, the sample was hydrothermally pretreated to obtain the highest methane production (204 ml/g VS) because of the increase in solubility of lignin. Finally, a simple water sedimentation method was used to collect nearly 95% of LNPs from the pretreatment solution. After characterization, it was proved that LNPs can be prepared from the pretreatment waste of lignocellulosic biomass. Furthermore, the solvent in this system could be further recycled efficiently and proved that the GVL/H₂O system is a sustainable green pretreatment method. Interestingly, GVL and the p-TsOH system were also adopted by Yang et al. for the pretreatment of hybrid poplar wood powders, and the optimal conditions were confirmed by characterizing the enzyme saccharification and lignin residues. The removal rates of lignin and hemicellulose reached 86.14 and 91.67%, respectively (Yang et al., 2020). Meanwhile, the enzymatic saccharification conversion rate of the cellulose residue could reach up to 84.84%. Finally, the related characterization of the separated lignin confirmed that the basic structure of lignin was generally maintained though a certain extent lignin was destroyed during the process. As a result, the lignin separated by this method has low average molecular weight, narrow molecular weight distribution, and high hydroxyl content, and can be easily used to produce fuels and chemicals, especially for lignin-phenolic resin synthesis (Pang et al., 2017).

In conclusion, GVL basically meets all the requirements of a green solvent, which could separate the three components efficiently and selectively. Though the cost of GVL is relatively high (53–80 dollars/kg), which partly limit its industrial application, the outstanding thermo-chemical stability endows GVL with excellent retrievability (Alonso et al., 2013b), especially in the production of value-added chemicals by direct liquefaction of lignocelluloses with the presence of a suitable heterogeneous catalyst (Liu et al., 2020). Thus, the GVL/H₂O system pretreatment method has broad prospects in industrial applications.

Cyrene is a promising novel bio-based solvent derived from cellulose through two simple paths, which are shown in Figure 5 (Zhang et al., 2016b). Just like GVL, Cyrene has characteristics, such as regeneration and economic feasibility and high boiling point (about 203°C). In addition, Table 3 shows the similarities in the physical properties of N-methyl pyrrolidone (NMP), DMF, sulfolane, and Cyrene (Sherwood et al., 2014; Zhang et al., 2016b). Moreover, studies also shown that Cyrene has no mutagenic properties and is relatively non-toxic, which is one of the green organic solvents that can replace toxic polar solvents (Sherwood et al., 2014; Zhang et al., 2016b; Camp, 2018). In the pharmaceutical industry (Camp et al., 2020), synthetic chemistry (Zhang et al., 2016b; Bousfield et al., 2019), materials chemistry and other fields (Marino et al., 2019; Poon and Zhitomirsky, 2020), Cyrene has already been used as a sustainable solvent. Previous studies have shown that Cyrene has good lignin solubility due to its relatively high HBA capacity (0.61) based on the Kamlet–Abboud–Taft parameter. Furthermore, it is also highly miscible with water without forming an azeotrope, so it is possible to recycle it from its mixture with water by simple distillation (Camp, 2018; Meng et al., 2020b).

The research group of Meng treated poplar in a Cyrene/water solution with a solvent ratio of 4:1 at 120°C for 60 min, and found that the strong hydrogen bond that formed between Cyrene and water is beneficial for the cleavage of lignin–carbohydrate linkages (Meng et al., 2020b). Therefore, the Cyrene/water ratio played a key role in the solubility of lignin. In addition, they also found that the pretreatment process could be conducted under mild conditions to reduce lignin condensation and β-O-4 bond breakage without affecting the removal and dissolution of lignin during the pretreatment of Populus trichocarpa x deltoids with the Cyrene/water system. Interestingly, some unwanted inhibitors formed during longer pretreatment time because of the high concentration of Cyrene in the cosolvent mixture. Subsequent studies have found that dilute alkali incubation was a good method to overcome the negative effects of high Cyrene concentration or long pretreatment time. In order to better understand how Cyrene works during the pretreatment of biomass, the researchers also tested the degree of polymerization of cellulose and lignin. Lignin integrity is well preserved, and the recovery rate of original lignin is over 60%. Moreover, all the Cyrene lignin has high contents of syringyl units and β-O-4 linkages. At present, however, a large amount of water must be distilled to recover pure Cyrene. Therefore, how to effectively recycle Cyrene from the cosolvent system and reuse it efficiently still needs further study.

Combined with the current research, although Cyrene is currently relatively expensive, and the cost of water used during the retrieval of Cyrene is comparatively high (Clarke et al., 2018; Camp et al., 2020), its outstanding performance on selective degradation of lignin cannot be ignored. As a biodegradable and bio-based green solvent, Cyrene has a great prospect in the pretreatment of lignocellulosic biomass (such as grass, softwood, and agricultural energy crops), and the byproduct of Cyrene lignin can be used as a high value-added product in aromatic compounds, carbon fiber, polyurethane precursors, etc (Meng et al., 2020b).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling editor declared a past co-authorship with several of the authors (JJ and KW).