*Ionic Liquids as Environmental Benign Solvents for Cellulose Chemistry: A Review DOI: http://dx.doi.org/10.5772/intechopen.76433*

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]


#### **Table 2.**

*Solvents, Ionic Liquids and Solvent Effects*

1 Imidazolium

2 Pyridinium

3 Ammonium

4 Phosphonium

**Table 1.**

**Name Structure**

[BMIM]+

[AMIM]+

[C2MIM]+

[C4MPY]+

[BDTAC]+

n = 2– 10), [AМРу]

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

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

**S. no. Cations Anions**

, [AMIM]+

, [HEMIM]+

, [BMPY]+

+

, [EMMIM]+

, [MMIM]+

, [HMIM]+

, ([СnМРу] where

, [EMIM]+ ,

> , [OMIM]+ ,

Cl<sup>−</sup> , Br<sup>−</sup> , SCN<sup>−</sup>

[BF4] − [CF3SO3]

[PF6]

[CH3CO2] − , [PF6] − ,

<sup>−</sup>, [I]<sup>−</sup>, [CH3SO3]

[Ac]<sup>−</sup>, [HSCH2COO]<sup>−</sup>, [HCOO]<sup>−</sup>, [(C6H5] COO]<sup>−</sup>, [H2NCH2COO]<sup>−</sup>, [HOCH2COO]<sup>−</sup>, [CH3CHOHCOO]<sup>−</sup>, [Fmt]<sup>−</sup>, [OAc]<sup>−</sup>, [SCN]<sup>−</sup>, [Tos]<sup>−</sup>, [N(CN)2]

<sup>−</sup>, [BF4] −,

−,

−,

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

<sup>−</sup> and [PF6]

) with Cl<sup>−</sup>, Br<sup>−</sup>, and SCN<sup>−</sup> could

<sup>−</sup> could not. They were further

**176**

convectional solvents.

**3.2 Cellulose dissolution using ionic liquids**

dissolve cellulose whereas with [BF4]

1-butyl-3-methylimidazolium chloride ([C4MIM]+

*Detailed summary of factors affecting cellulose solubility.*

as well as [EMIM][Cl], [EMIM][OAc], and [EMIM][MP] have been documented to be one of the most effective ILs for cellulose dissolution, with [BMIM][Cl] able to dissolve up to 25% wt of cellulose having a DP = 1000 using a microwave heating method. During dissolution process, it has been shown by aid of various analytical techniques that the strong inter- and intramolecular hydrogen bonding of cellulose is broken up by formation of intense hydrogen bonding between ionic liquid anions and cellulose hydroxyls making cellulose to be soluble; hence, ILs with noncoordinating anions are nonsolvents [44].
