**2. Cellulose dissolution history**

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

**175**

**3. Ionic liquids**

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

viscose process has been described elsewhere [12–14]. During the late 1960s, lyocell (amine oxides) technique was invented mainly to overcome the challenges of viscose method which is found to be more effective in processing of cellulose. Lyocell technique employs the use of N-methylmorpholine N-oxide (NMMO). NMMO dissolves cellulose directly due to its strong N-O dipole [15]. However lyocell technique was not as effective as "viscose" technique as it had significant engineering complications; hence viscose remained to be the most widely used technique [16]. Both of those cellulose dissolution techniques retain major environmental complications

Those challenges lead to further research in cellulose dissolution process in search of solvents which can be as environmental friendly as possible. Therefore the new solvents should be associated with low volatile, easily recyclable, high thermal stability, nontoxic, non-derivative, etc. [21]. The solvents which were found to be most effective were NaOH/thiourea and urea aqueous solvents as well as molten salts. The NaOH/thiourea was found to dissolve cellulose at low temperatures ranging from −8 to −5°C as described elsewhere in detail [22, 23]. However this method was found to produce cellulose fibers with high degree of crystallinity but lower degree of crystal orientation when compared to fibers obtained from viscose process [23, 24]. In order to advance this method, thiourea was replaced by urea forming a new solvent system NaOH/urea [12], and this solvent was found to have enhanced cellulose solubility by precooling the solution to −12°C. The advantage of the method is based on the fact that the by-products so formed were nontoxic. The methodologies related to the preparation of NaOH/Urea system can be elsewhere [11, 12, 24]. Another eco-friendly solvent that is still in the academia is ionic liquids (designated by ILs). Recently, the ILs have been identified as the most universal solvents for the future due to their several fascinating properties such as high polarity, negligible vapor pressure, nonflammability, low melting point, good thermal stability, tunable viscosity, broad liquid range, high thermal conductivity, good dissolution properties, etc. [25–27]. Because of the association of ILs with these fascinating properties, they (ILs) can be considered as most important, cost-effective,

• Poor thermal stability of NMMO [17, 18] which leads to major investment in safety measures of the industry and difficulties in recycling the solvent [16] • NMMO requires high temperatures which leads to

degradation of cellulose [19, 20]

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

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

(some of which are listed below).

and also not sustainable [10]

• For every kilogram of cellulose produced, there is two kilograms of waste [11, 17] • Makes use of carbon disulfide and dihydrogen sulfide which are both environmental rivals

**Viscose Lyocell**

and environmental benign solvents for cellulose.

**3.1 Brief summary of ionic liquids**
