At present, the situation of global climate change is urgent, and the increasingly severe climate crisis requires countries around the world to make joint efforts to deal with it. It is of great significance to build a green, renewable, safe and effective energy system, reduce carbon emissions and achieve carbon neutrality. The development of renewable non-fossil energy is an important response. The development of renewable energy relies significantly on the utilization of naturally abundant and renewable lignocellulosic biomass, which plays an essential role. Lignocellulosic biomass is predominantly constructed of cellulose, hemicellulose and lignin, where hemicellulose and cellulose can be hydrolyzed into fermentable sugars to produce bioethanol through fermentation processes [1], while lignin can be used as a raw material to make biobased materials or chemicals as an aromatic resource, such as particleboard, soil conditioners, stabilizing agents and dimethyl sulfoxide [2]. The complete utilization of these three components is crucial for attaining the maximum value from lignocellulosic biomass. However, cellulose and hemicellulose are tightly bound to lignin via covalent and hydrogen bonds, giving lignocellulose a natural biomass recalcitrance and strong resistance to biological and physical attacks. Thus, the key to achieving effective conversion of lignocellulosic biomass is the advancement of effective pretreatment techniques to break down the biomass recalcitrance for component separation [3]. Some physical, chemical and biological techniques such as ball milling, acid pretreatment, alkali pretreatment, organic dissolution and microbial degradation have been applied on the pretreatment of lignocellulose [4]. These traditional pretreatment technologies face challenges like excessive energy usage, steep expenses, low efficiency and unfriendly environment. As a result, there has been a growing emphasis on developing mild, green and sustainable pretreatment methods.

In recent years, deep eutectic solvent (DES) has shown great potential in the separation and degradation of lignocellulose [5,6]. In general, DES is an eutectic mixture of at least one hydrogen-bonded donor (HBD) and one hydrogen-bonded acceptor (HBA) at a certain molar ratio [7]. DESs exhibit numerous benefits in contrast to conventional solvents, including their non-toxicity, biocompatibility, biodegradability, and recyclability [8]. Francisco et al. [9] first confirmed the solubility of DES to lignocellulosic biomass, and systematically investigated the solubility of lignin, cellulose and starch in 26 kinds of DESs, and found that the DESs composed of malic acid and proline had good solubility to lignin (100 °C, 14.9 mg/g). Subsequent studies demonstrated that the pretreatment of lignocellulosic biomass using DESs led to significant improvements, the hydrogen bond and ether bond of lignin-carbohydrate complex (LCC) were broken under the action of DESs, which was advantageous for the extraction of lignin and hemicellulose [10]. However, DESs showed weak solubility to cellulose due to the strong hydrogen bond network structure of cellulose. DES pretreatment facilitates the extraction of lignin for further high-value utilization [11], and also greatly improves the enzymatic saccharification efficiency of cellulose, resulting in a higher conversion rate of fermentable sugar [12,13]. Besides, the process of lignin regeneration from DES allows for the recovery and reuse of lignin, a valuable resource, thus enhancing the overall sustainability and economic viability of lignocellulosic biomass processing [14]. Furthermore, the regenerated lignin has been found to possess unique properties, such as higher purity and uniformity, which make it suitable for various high-value applications, including the production of bio-based materials and chemicals [15].

According to recent reports [[16], [17], [18], [19], [20], [21], [22]], it has been found that the treatment of lignocellulosic materials with DES often results in a waste of hemicellulose. For example, Gou et al. [23] used benzyl triethyl ammonium chloride (TEBAC) and lactic acid (LA) DES to pretreat corncobs at 140 °C, and found that 63.4 % of the lignin and 80.8 % of the hemicellulose were simultaneously dissolved into the DES. Shen et al. [17] treated eucalyptus wood with choline chloride (ChCl) and LA DES at 130 °C for 6 h, the separation effect of lignin was very satisfactory with the lignin removal rate was 93.2 %, and the 92 % of hemicellulose was also dissolved into the DES. However, the hemicellulose degraded into the DES is often difficult to separate or transform, resulting in underutilization of hemicellulose and affecting the overall economic benefits of DES biorefineries. Fortunately, the treatment of lignocellulose by acidic electrolyzed water (AEW) coupled with FeCl3 could be used for monodirectional separation of hemicellulose [24]. After the hemicellulose is separated, the remaining substrate containing lignin and cellulose is typically used for enzymatic hydrolysis to produce fermentable sugars. However, Tu et al.'s [25] research showed that in the enzymatic hydrolysis of lignocellulose, 49 % of the cellulase was ineffectively adsorbed by lignin, which limited the maximum efficiency of enzymatic hydrolysis and increased the economic cost. Hence, the excellent delignification ability of DES is of great significance to enhance the enzymatic hydrolysis efficiency of cellulose and to fully utilize the whole component.

The purpose of this study is to efficiently separate the three primary components of lignocellulosic biomass, fully exploiting its entire composition. Initially, poplar wood underwent treatment with AEW coupled with FeCl3 to remove hemicellulose, resulting in the poplar hydrolyzed residue (PHR) enriched with lignin and cellulose [26]. The key innovation of this study is the application of a new DES system (benzyl triethyl ammonium chloride-ethylene glycol-FeCl3, T-EG-F) to pretreat PHR, effectively separating lignin and obtaining high-purity cellulose. The investigation extends to understanding the structural changes in poplar and the characteristics of the regenerated lignin after DES pretreatment. Furthermore, the study evaluates the impact of DES pretreatment on the enzymatic hydrolysis of poplar, providing comprehensive insights into sustainable biomass processing.