ILs used in cellulose and biomass pretreatment has attracted much interest due to their excellent properties, such as the low melting point, high thermal stability and solvation capacity, especially the hydrogen-bonds among the cations and anions [43]. Moreover, the promising diversity of ILs suggests that appropriate ILs will be non-volatile polar solvents for carbohydrate dissolution and biomass pretreatment.

1.5.1 Melting Point

The melting point of a compound represents the lower limit of the liquid range and together with thermal stability defines the temperature window in which it can be used as a solvent [44].

The melting point is defined as the temperature of equilibrium between solid and liquid state in thermodynamic. Since the change of the Gibbs free energy equals zero at the equilibrium, the melting point (Tm) is defined as follows [45]:

Recently, many researchers study the effect of structural features on melting point because the melting point is an important factor for employment of ILs as a reaction media. Factors influencing in ILs melting point are charge, size and the distribution of charge on the constituent ions. For the same class of ILs, small changes in the shape of the uncharged, covalent regions of the ions need to be considered [46]. For example, the melting point of imidazolium based ILs is influenced by four factors: electron delocalization, H-bonding ability, the symmetry of the ions, and van der Waals interactions. Both alkyl substitutions on the cation and ion asymmetry have been shown to interfere with the packing efficiency of ions in the crystalline lattice [47]. Researchers are always try to find that how the chemical structure affects the melting point by many different measuring methods. The following trends can be concluded from open literature:

1. The sizes and shapes of cations of ILs are important factors influencing the melting points of ILs. In general, as the size of the cation increases, the melting point of the salts decreases.

2. With increasing the size of the anion, the melting point of the salts decreases, which reflects the weaker columbic interactions in the crystal lattice. For instance, from Cl — to [BF4]- to [PF6]- to [AlCl4]-, the melting points of the sodium salts decrease from 801 to 185 °C with increasing thermochemical radius of the anion [47].

3. With the same anion, symmetry of the alkyl substitution also affects the melting point of ILs. Generally, highly asymmetric alkyl substitution has been identified as important for obtaining high melting point. For example, the melting point of [Mmim]Cl is 124.5 °C while the melting point of [Bmim]Cl is 65 °C.

4. The alkyl chain length of cations also affect melting point of ILs. For example, as for 1-alkyl-3-methylimidazolium tetrafluoroborate, with an increase in the alkyl chain length (up to n = 8), the melting point decrease, where n is the number of carbon atoms. But the melting point of ILs increases gradually with increasing chain length when n > 8. The same condition occurs in 1-alkyl-3- methylimidazoliumbis (trifyl) imide [46].

5. For ILs in which the only difference is the degree of branching within the alkyl chain at the imidazolium ring, higher degree of branching within the alkyl chain, higher melting point of ILs.

Owing to their unique properties, ILs are widely used as a kind of versatile solvents in biomass separation/conversion. Most ILs used for biomass pretreatment have low melting points, so that they are liquid at room temperature, the melting points of the ILs listed in Fig. 1.2 are all lower than 350 K [26]. That’s because

lower melting points of ILs make them easy to handle and are known as a technical advantage on the recyclability of the solvents. In recent years, some new types of ILs that have lower melting points and a sufficient polarity to further process carbohydrates have been claimed to replace chloride-based ILs [48]. These ILs are formate, acetate or phosphate salts with imidazolium based cations. They have been proven to be potential solvents to dissolve cellulose under mild condition.