# Biomass and Cellulose Dissolution--The Important Issue in Renewable Materials Treatment
## 3 Dissolution of Biomass and Its Components
### Ionic Liquids
#### 3.6.1 Ionic Liquids in Biomass Dissolution  
Ionic liquids (ILs) can be applied in biomass pretreatment before further transformations into valuable products, as well as in transformation processes [79]. Plant biomass is a difficult material to dissolve due to its complex structure and differences in material composition and hardness. Soft biomass, e.g., grass, is easier to process than wood biomass [80]. Moreover, the level of biomass fragmentation is crucial--smaller particles have a larger contact surface with the solvent, which increases the efficiency of dissolution [81]. In addition to biomass properties and type, the viscosity of solvent has a major impact on process effectiveness. Lower viscosity facilitates mass transfer, which enhances the dissolution process. [PERSON] et al. (2021) examined ILs with ether-functionalized methylimidazolium cations (e.g., [MOE-mim]Cl 1-(2-methoxyethyl)-3-methylimidazolium chloride), with significantly lower viscosity, than the widely used [bmim]Cl. In [MOE-mim]Cl, the bamboo dissolution level was 96.45%, and in [bmim]Cl, it was 78.35%. Additionally, due to the extra oxygen atom present in the methoxy group, functionalized IL can create more hydrogen bonds with biomass fractions, which additionally enhances its dissolution [82]. Furthermore, the type of anions in IL matters in biomass processing. Solvents with higher basicity of hydrogen bonds, such as [emim][CH\({}_{3}\)COO], are more effective for biomass dissolving [83]. Examples of biomass solubility in different ionic liquids are presented in Table 2.  
The mechanism of biomass dissolution is based on the breakage of hydrogen bonds between lignocellulosic components. At the same time, the solvent's anion creates new hydrogen bonds with hydroxyl groups, especially cellulose. The possible role of the cation in polymer dissolution is connected to its size and hydrophobic character [88; 89]. The biomass component, which is primarily dissolving ILs, is cellulose. Because of that, the effectiveness of biomass dissolution is dependent on the level of cellulose dissolution [80]. [PERSON] et al. (2018) proposed a three-step mechanism of cellulose dissolution--first, IL's anion \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Biomass Type** & **Ionic Liquid** & **Temperature [°C]** & **Solubility [wt.\%]** & **References** \\ \hline Hybrid poplar & [emim][CH\({}_{3}\)COO] & & & 5.3 & \\  & [emim][HCOO] & & & 5.3 & [84] \\  & [amim][CH\({}_{3}\)COO] & & 7.0 & \\  & [amim][HCOO] & & & 7.4 & \\ \hline Miscanthus & [emim][CH\({}_{3}\)COO] & & & 1.7 & \\  & [emim][CH\({}_{3}\)SO\({}_{3}\)] & 100 & & 4.0 & [85] \\  & [emim][HSO\({}_{4}\)] & & 9.6 & \\ \hline Peanut shells & [bmim][Cl\({}_{3}\)COO] & 120 & & 2.3 & \\ \hline Chestnut shells & [bmim][Cl\({}_{3}\)COO] & 120 & & 4.7 & \\ \hline Miscanthus & [bmim][CH\({}_{3}\)COO] & & & 4.0 & \\  & [bmim][Cl\({}_{3}\)COO] & 130 & & 3.0 & [87] \\  & [emim][Cl\({}_{3}\)COO] & & 4.0 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Biomass solubility in ionic liquids. Figure 9: Ionic liquids cations and anions, which are the most frequently applied in biomass transformations (R—hydrogen atom or alkyl group). penetrates through cellulose chains, which promotes hydrogen bond cleavage. At the same time, the cation increases the distance between these chains due to its size. Finally, cellulose chains are scattered [90]. The general idea of the cellulose dissolution mechanism is presented in Figure 10.  
The ability of ionic liquids to dissolve whole biomass results from the fact that some of the solvents can dissolve lignin, rather than only cellulose, e.g., imidazolium and ammonium ionic liquids with Cl\({}^{-}\), Tf\({}_{2}\)N\({}^{-}\), and CH\({}_{3}\)COO\({}^{-}\) anions [24]. In _Eichronia crassipes_ (water hyacinth) dissolution using ionic liquids, the most effective solvent was the mixture of [bmim][CH\({}_{3}\)COO] and water (water content of 40% \(v/v\), and the level of biomass dissolution was approximately 85% of dry biomass).