# Deep Eutectic Solvent Pretreatment and Green Separation of Lignocellulose
## 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 lignocellulosic 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, harbored 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 lignocellulosic. For example, the composition of cellulose, hemicellulose, and lignin of durian shell were 25.7 \(\pm\) 1.9% (\(w/w\)), 18.5 \(\pm\) 1.04% (\(w/w\)), and 15.9 \(\pm\) 0.19% (\(w/w\)), respectively [10]. The separation and purification of lignocellulosic 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 \({}^{\circ}\)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 lignocellulosic, 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 \(\Delta T_{f}\). As illustrated in Figure 1, \(\Delta T_{f}\) is directly correlated with the strength of interaction between components A and B.