Tian et al. [42] designed a simple OES system composed of [AMIM]Cl and DMSO, to treat microcrystalline cellulose for enzymatic saccharification. [AMIM]Cl is one of the most effective ILs for dissolving and pretreating wood chips [86]. Compared to [BMIM]Cl, it has a lower melting point. The dissolution mechanism of cellulose in [AMIM]Cl was proposed as: The free Cl — anions associated with cellulose hydroxyl protons and the free cations combined with the cellulose hydroxyl oxygen to form electrovalent bonds, leading to disruption of hydrogen bonding in the cellulose and its consequent dissolution [98]. When an anti solvent, such as water, containing large

quantities of hydrogen bonds was homogeneously mixed with [AMIM]Cl-cellulose solution to remove the ILs, the electrovalent bonds were replaced oppositely by the reforming of hydrogen bonds among cellulose chains. However, the structures consisting of cellulose chains and regenerated hydrogen bonds were never arranged as regularly as the crystalline cellulose prior to the pretreatment (see Fig. 14.3). DMSO is an important polar aprotic solvent that dissolves both polar and non-polar compounds and is miscible in a wide range of organic solvents [99]. Therefore, despite of the relatively lower price, it is an important and efficient co-solvent for cellulose dissolution and pretreatment.

In the OES-dissolving pretreatment study, 5 % microcrystalline cellulose was de­posited in each of these OES which had different molar fractions of [AMIM]Cl (i. e., X [AMIM]Cl = 0.1-0.9). Followed by being kept at 110 °C for 1 h under a continu­ous stirring, the regenerated cellulose was precipitated by water and enzymatically hydrolyzed for 72 h.

Results revealed that the microcrystalline cellulose was rapidly dissolved in the OES (when x [AMIM]Cl > 0.2) within 10 min. During the hydrolysis, with the increase of OES from 0.1 to 0.9, both hydrolysis yield and initial hydrolysis rate of the regenerated cellulose increased gradually. After 72 h, the glucose yield of cellulose treated by OES (at x [AMIM]Cl = 0.7) was 54.1 %, which was 7.2 times that of the untreated cellulose, and was only slightly lower than the value (59.6 %) obtained by using pure [AMIM]Cl. Characterization of the regenerated cellulose samples was conducted subsequently. With increasing molar fractions of [AMIM]Cl, the crystallinity index (CI) of cellulose I decreased from 0.834 to 0.319, whereas the CI of cellulose II stayed at around 0.284. Meanwhile, the average specific surface area

Fig. 14.4 Variable viscosities of OES against the increasing molar fractions of [AMIM]Cl at 110 °C (Calculated according to the Grunberg-Nissan mixing law. Viscosities of DMSO and [AMIM]Cl at 110 °C were caculated according to Arrhenius model and VFT equation, respectively.) [42]

and degree of polymerization remained without significant change, being 4.174 m2/g and 137.2, respectively.

Although DMSO has no positive effect on promoting cellulose dissolution for being unable to donate cations and anions [42, 90], the OES mixed with DMSO and [AMIM]Cl has positive effect on decrystallinity of cellulose I, which may account for the higher hydrolysis yield and rate [18].

The unique advantages of employing OES as a cellulose solvent in the pretreat­ment can be summarized below [42, 85]:

1. Replacing a portion of ILs by less expensive OES, which can lower the processing cost.

2. Shortening the dissolution time to form cellulose homogenerous solutions due to its rapid diffusion efficiency.

3. Being more practical and easier for large-scale operations (stirring and pipeline transportation) due to the reduced viscosity of the OES system. For example, the viscosity of OES with molar fraction of [AMIM]Cl = 0.7, is only 37.28 % of the pure IL at 110 °C (see Fig. 14.4).

4. Enabling a higher cellulose recovery rate (95.37 ± 1.41 %) opposite to significant decomposition of polysaccharides in other chemical pretreatment methods [100].

5. Proving to be a simple but effective process to prepare cellulose with controlled CI than some other methods [101, 102]. The CI of cellulose I in the regenerated samples has a strong negative linear correlation against the molar fraction of IL in OES (i. e., with a correlation coefficient of 0.98).

Referring to the advantages, employing OES as a cellulose solvent has a bright per­spective for efficient pretreatment of lignocellulosic biomass. However, recycling of OES should be taken into account to remove the barriers to large-scale application. It can be achieved by using commercial distillation technology to separate water
from ILs due to their lower vapor pressures. Alternately, using aqueous ethanol or aqueous acetone instead of pure water as an antisolvent would reduce the tempera­ture and vacuum requirement in the distillation. Otherwise, some new technologies, such as nanoflltration, reverse osmosis, and pervaporation [103], three-phase system precipitation [104], and supercritical CO2 extraction [103, 105] may have potential applications in recycling of OES.

Moreover, investigation of OES on the pretreatment of various lignocellulosic biomass, such as corn stover, switchgrass, poplar, and pine, should be performed to determine the chemical components isolation, cellulose decrystallization, and improvement of enzymatic saccharification against the solvent constituents as well as their molar ratio in OES. Physical features of samples, such as moisture, particle size, and homogeneity, which have been proposed to affect the cellulose-dissolving ability in ILs [106], should also be considered and optimized in the OES system.