Many kinds of materials can be dissolved in ILs, such as the metal salts, gases, carbohydrates, sugar alcohols, cellulose and even the biomass [36, 84, 85]. In this chapter, cellulose dissolution with various ILs would be discussed.

Cations and anions play an important role in the cellulose dissolution process [23]. The soluble ability of cellulose in ILs can be modified by changing the cations or anions. Anions that form hydrogen bonds with hydroxyl groups are effective for cellulose dissolution, small size and alkalinity of anions promote to increase cellulose soluble ability, for example, halide, acetate, formate and dialkyl phos­phate [22, 61, 86]. Different imidazolium, pyridinium and pyrrolidinium-based cations are commonly used in cellulose dissolution together with the anions above mentioned. Cellulose soluble ability decreases in the ILs with length of the alky chain. Meanwhile, alkyl chains or anions with hydroxyl groups tend to be adverse to the cellulose dissolution in ILs due to the increase of hydrogen-bond acidity of ILs [22].

The Kamlet-Taft polarity parameters, for example, p is the hydrogen-bond basicity parameter, a is the measure of the hydrogen-bond acidity, which express the ability to donate and accept hydrogen bonds, respectively, and n* is the parameter of the interactions through dipolarity and polarizability. The parameters a and p are similar to the acid and base characteristics according to the definition, while, the a and p are not completely consistent with the acidity and alkalinity of ILs in all conditions, the a and p emphasize the acceptance ability of hydrogen — bond. As for ILs, p has been the most useful parameter in predicting the solubility of cellulose in different ILs with various anions [87, 88].

With ILs, the higher p and dipolarity caused the better ability to dissolve cellulose [22]. With the cation [Bmim]+, some anions showed different trends for dissolving cellulose due to the increasing hydrogen-bond acceptance ability, for example, the p value of some anions are in an order of OAc~ > HCOO~ > (C6H5) COO~ > H2NCH2COO~ > dca~, and the cellulose solubility was about 16 wt% in [Bmim][OAc] (P = 1.161), which was higher than those in [Bmim][HCOO] (P = 1.01), [Bmim][(C6H5)COO] (P = 0.98) [8, 89—92]. For a given cation, the effect of anions on cellulose dissolution changes greatly, this result may contribute to the formula weight of ILs, which means that the cellulose dissolution is close to the mass percent in the given ILs.

The Kamlet-Taft parameters of selected ILs and examples of used in dissolving cellulose and biomass pretreatment are as followed in Table 1.5, and the properties of ILs examples and applications in biomass are shown in Table 1.6. Common ILs used for cellulose/biomass pretreatment are shown with details in Table 1.7.