**3.1 Brief summary of ionic liquids**

Ionic liquids are commonly defined as molten salts; these salts are composed of inorganic or organic anions and organic cations. In ILs, the oppositely charged ions are held together by columbic forces [28–30]. Most widely studied ILs are room temperature ionic liquids (RTILs) having melting point of below 100°C [31–34]. The ILs are liquids at room temperature because they are composed of polyatomic, bulk, asymmetric organic cations and charge-diffuse ions [17, 35], and their ions are not packed well; hence, they remain liquid [36] at room temperature. The

*Solvents, Ionic Liquids and Solvent Effects*

research and produced first cellulosic thermoplastics [4, 6, 7]. The research of cellulose modification has been developing, from one process to another, trying to find as the most eco-friendly and eco-efficient method as possible from the time when it

Ever since cellulose has been discovered, many researches are being carried out on the most efficient ways to dissolve this natural polymer in various common solvents [8]. The first attempt toward cellulose dissolution was made by Hyatt Manufacturing Co. in which cellulose was converted into nitrocellulose with the aid of nitric acid. The nitric acid treatment of the cellulose gives cellulosic-based thermoplastic polymer that has several industrial and biological applications [4]. As time progresses a new and a better method, namely, "viscose," was developed which became effective from the 1890s and is the most widely used method in processing cellulose to date. Approximately 3.2 million tons of cellulose are processed per year using this method [9–11]. This method makes the use of alkali (NaOH) and carbon disulfide (CS2) and produces cellulose xanthate. The detail methodology about the

was discovered as cellulose is the most abundant natural material on earth.

*Pictorial presentation of inter- and intramolecular H bonding in cellulose.*

**2. Cellulose dissolution history**

**174**

**Figure 2.**

**Figure 1.**

*Chemical structure of cellulose.*

#### **Table 1.**

*Structures and abbreviations of some common cations and anions of ionic liquids employed as solvents.*

RTILs are considered to be designer solvents since one can alter the physicochemical properties of an IL by simply varying anions' type or alky chain length of the cations [37–40]. The RTILs are composed of nitrogen or phosphorus containing dissymmetrical organic cations such as imidazolium, pyridinium, or ammonium or phosphonium cations and wide variety of simple anions such as chloride, nitrate, bromide, tetrafluoroborate, acetate, triflate, etc. [29, 41, 42] as shown in **Table 1**. So far, the ILs have been found to have numerous applications in various areas including solvent science for manufacturing of different materials including dissolution of biomass, electrochemistry for electrolytes in batteries, polymer chemistry for plasticizers, and separation technology for extractions and separations [26, 36, 37, 43]. Since ILs have been discovered, they are widely becoming advanced; in addition, the efficient methods of synthesis ILs are being developed. The application of ILs is vastly increasing yearly; recently, ILs have been started to be researched toward biomass processing, particularly cellulose, since cellulose is the most abundant biomaterial on earth [1, 17]; therefore, the use of ILs in cellulose chemistry can bring about economic sufficient developments archiving of the United Nation (UN) mission of moving away from depending on fossils fuels as well as using of harmful convectional solvents.

#### **3.2 Cellulose dissolution using ionic liquids**

The very first cellulose dissolution research was carried out by Richard P. Swatloski in the year 2002 [44]. Swatloski along with his coworkers reported that 1-butyl-3-methylimidazolium chloride ([C4MIM]+ ) with Cl<sup>−</sup>, Br<sup>−</sup>, and SCN<sup>−</sup> could dissolve cellulose whereas with [BF4] <sup>−</sup> and [PF6] <sup>−</sup> could not. They were further

**177**

**Table 2.**

Degree of polymerization

*Ionic Liquids as Environmental Benign Solvents for Cellulose Chemistry: A Review*

**Factor Explanation Reference**

electronegativity such as halides, e.g., Cl<sup>−</sup>, have better dissolution properties unlike noncoordinating anions such as [BF4] and [PF6]

carbons were found to have high solubilizing power than the one with odd number and pyridiniumbased ionic liquids showed that as alkyl chain length of the cation increases, the solubility decreases

Solubility rate of cellulose decreases as degree of

Swatloski et al. [44], Muhammad et al. [62]

Erdmenger et al. [52], Sashina et al. [43], Olivier-Bourdigou et al. [65]

Kilpeläinen et al. [51]

[46]

Kosan et al. [48], Zhang et al.

Tywabi [24], Fort et al. [66], Kilpelainen et al. [51]

Rinaldi et al. [67], Zhao et al. [68], Xu et al. [69, 70], Xu and Zhang [71], Bengtsson [72], Andanson et al. [73], Holding et al. [74], Swatloski et al. [44]

Holm et al. [63], Dadi et al. [64], Swatloski et al. [44]

Presence of water Water content should be very low in both IL and cellulose, typically less than 1%

Alkyl chain length Methylimidazolium cations with even number of

polymerization increases

Dissolution time Dissolution time should be short typically around 12 h at low heating temperature

IL viscosity Low viscosity promotes higher dissolution since it promotes greater ions mobility

> quicker dissolution at low temperature Polar protic is non-solvent which causes precipitation of the cellulose from the IL solution

Cosolvent Polar aprotic cosolvents promote higher and

*Detailed summary of factors affecting cellulose solubility.*

Anions type ILs containing anions that have strong

observed that as increase in the alkyl chain length of the ILs decreases the solubility of the investigated ionic liquids as [C4MIM][Cl], [C6MIM][Cl], and [C8MIM] [Cl] since [C4MIM][Cl] dissolved 10 wt%, [C6MIM][Cl] dissolved 5 wt% and [C8MIM][Cl] were slightly soluble. In addition the dissolution could be significantly enhanced by heating the solution using oil bath, microwave, or ultrasonic technique. This research indicated that "ionic liquids" can be used as solvents for cellulose and opened a new horizon in green chemistry [45]. Ever since Swatloski et al. [44] breakthrough in cellulose chemistry, many researchers proceed with his outcomes to further understand the dynamics and factors associated with the dissolution of cellulose using ionic liquids. To determine dynamics and factors that play a role in cellulose dissolution using ILs, properties such as different structures of the ionic liquids, degree of polymerization of cellulose, dissolution time, temperature, water content, and cosolvent were investigated starting from Zhang et al. [46] to Meenatchi et al. [47] including Kosan et al. [48], Heinze et al. [49], Lee et al. [50], Kilpeläinen et al. [51], Erdmenger et al. [52], Zavrel et al. [53], Sun et al. [54], Vits et al. [55], Fukaya et al. [21, 56], Sashina et al. [43], Xu et al. [57], Sescousse et al. [58], Zhen et al. [59], and Freire et al. [60], and the detailed summary of the factors affecting cellulose solubility is given in **Table 2**. It can be concluded that most of the studied ionic liquids are imidazolium-based ionic liquids; this is due to a fact that imidazolium ILs have the finest properties among other ILs; they have the lowest melting points; many of them are liquid at room temperature; they have high conductivity and a wide electrical window stability that makes them suitable for a variety of applications including solvents for the dissolution of cellulose [61]. Among the most widely studied imidazolium-based ILs [BMIM][Cl], [BMIM][Ac], [BMIM][Fmt], and [BMIM][OAc] together with [AMIM][Cl] and [AMIM][Fmt]

*DOI: http://dx.doi.org/10.5772/intechopen.76433*
