Various types of cellulose fibers are used as essential materials for our modern life. The first step in manufacturing cellulose fiber is the dissolution of cellulose in an appropriate solvent, and numerous solvent systems for such dissolution have been developed [1]. As mentioned before, ILs are now well acknowledged as cellulose dissolution agents. Although a great deal of such research has been carried out to develop a pretreatment method of cellulose for bioethanol production, ILs are now seen as attractive solvents for cellulose fiber production [1, 2]. We developed a novel amino acid ionic liquid, N-(2-methoxyethyl),N, N-diethyl, N-methylammonium alanine ([N221(ME)][Ala]), and demonstrated that it dissolved cellulose very well [24].

Rinaldi [34] reported that addition of [C4mim]Cl or [C4mim][OAc] to DMI, DMF, sulfolane, or DMSO caused effective dissolution of cellulose [34]. Some traditional solvents were composed of a highly polar molecular solvent and an appropriate salt material, such as DMAc/LiCl, DMI/LiCl, or DMSO/Bu4NF as noted earlier [7, 8]. Inspired by these results, we investigated an appropriate combination of polar solvent, such as DMSO or DMF, with an ionic liquid as a cellulose dissolution solvent. The dissolving property of a mixed solvent of DMSO and [C4mim]Cl (1:1 (w/w)) against cellulose was first investigated using micro­crystalline cellulose as a model compound. However, no dissolution of cellulose took place in the solvent. We next prepared various types of mixed solvent of DMSO with hydrophobic ILs (1:1 (w/w)): [C4mim][NTf2], [C4mim][PF6], [C4mim][C5F8] [35], [N221(ME)][NTf2], [N221^™, [N221(me)][C5F8][35], [P444(ME)][NTf2], [P444(ME)][PF6], [P444(ME)][C5F8][35], [C4Py][NTf2], [C4Py] [PF6], and [C4Py][C5F8][35]. These mixed solvents did not dissolve cellulose at all even at 100 °C. On the other hand, it was found that by switching the IL to a hydrophilic liquid like [C4mim][OAc] or [N221(ME)][OAc], the corresponding mixed solvent (DMSO : IL = 1:1 (w/w)) did slightly dissolve cellulose (5 and 7 wt% vs. solvent, respectively) at 100 °C. Since [N221ME][Ala] showed the best dissolution among amino acid ILs [24], we prepared a 1:1 mixed solvent of DMSO and [N221ME][Ala] and found that the resulting solution dissolved cellulose very well (11 wt%) after just 10 min of stirring at room temperature (25 °C); a total of 22 wt% of cellulose was dissolved in this solvent at 100 °C. Furthermore, 23 wt% of cellulose dissolved even at room temperature with 6 h stirring [36].

The solubility depended significantly on the ratio of the IL to DMSO solvent ratio as shown in Fig. 4.7: cellulose did not dissolve in pure DMSO at all, and the highest solubility was recorded for ca. a 1:1 mixture of DMSO and [N221(ME)][Ala] (IL molar ratio (xIL) was 0.25) which coincidentally was the same ratio as our initial testing solvent. The resulting solution coagulated in water or methanol to obtain a transparent regenerated cellulose in quantitative yield and XRD analysis confirmed that the cellulose regenerated from this solution was only Type II form [36].

Rinaldi also reported that p-value of the mixed solvent of DMI/[C2mim] [OAc] increased when the ratio of the IL was increased and reached the highest value at the XIL = ca, 0.1 for the DMI/[C2mim][OAc] solvent system, where xIL indicates the molar ratio of the IL in the solvent system [34]. From around xIL = 0.10, the values were identical to those of the neat IL [34]. Since xIL of 1:1 (w/w) mixture of DMSO/ [N221(ME)][Ala] was calculated as 0.25, the p-value of the solvent might be the same as [N221(ME)][Ala]. [N221(ME)][Ala] has ahigh p-value (1.041) [36], which is almost the same as that reported for [C2mim][Ala] (1.036) [27]. High hydrogen bond basicity of the mixed solvent of DMSO/[N221(ME)][Ala] might contribute to break­ing the inter or intramolecular hydrogen bonds of cellulose and causing its disso­lution in the solvent as proposed by Ohno et al. [14, 18].

It was also reported that instantaneous dissolution of the cellulose (10 wt%) took place when [C2mim][OAc] was added to DMI at a ratio of over xIL 0.4 at 100 °C [34]. We confirmed that a 1:1 mixture of DMSO and [C4mim][OAc] (xIL 0.31) caused 5.0 wt% dissolution of cellulose at 100 °C, while no dissolution of cellulose took place at room temperature in this solvent. On the other hand, the mixture of DMSO/[N221(ME)][Ala] (xIL 0.25) dissolved cellulose even at room temperature. These results clearly indicated that there was a clear contrast in the dissolution

Fig. 4.7 Change in cellulose solubility for a mixed solvent composed of [N221(ME)][Ala] and DMSO at different temperature conditions with 2 h stirring

property between [N221(ME)][OAc] and [N221(ME)][Ala]. Amino acid-based IL is obviously so effective as a co-solvent or salt that it dissolves more cellulose in DMSO than conventional acetate-based ILs [36]. We anticipate that the hydrogen bond acceptor property of [Ala] and [OAc] might be different in the mixed solvent and reflect a different solubility. As mentioned previously, that free amino group of alanine was essential to realize high cellulose dissolution. Therefore, it was sup­posed that the amino group of [Ala] may interact with a certain part of cellulose and contribute to breaking its hydrogen bond network. However, since the cellulose solubility is also modified by the cationic part of the IL, cation might play an important co-operative role in the mechanism for cellulose dissolution. Further investigation of the scope and limitation of our ionic liquid technology will make it even more beneficial in cellulose science.

4.2 Conclusion

Development of an efficient means to dissolve cellulose in a simple solvent has been a long-standing goal in cellulose chemistry. ILs are now acknowledged as cellulose dissolution agents and are also seen as the most attractive solvents for cellulose fiber production. Strong hydrogen-bonding basicity ф-value in KAT values) is now recognized as the most important property of ILs with high cellulose
dissolution. Viscosity of ILs is the second key factor causing cellulose dissolution at low temperature conditions. However, cellulose solubility was not determined only by the physical characteristics of the solvent shown as KAT values, but that affinity of a certain component of ionic liquids with cellulose was an important factor of cellulose dissolution in the ILs. We hypothesized that we might be able to obtain a hint on how to design such anion or cation from nature. Focusing on the structure of hydrolyzing enzyme of cellulose (cellulase), we found that amino acid ILs were strongly capable of dissolving cellulose: N, N-diethyl, N-methyl, N-(2-methoxy) ethylammonium alanate ([N221(ME)][Ala]) worked as an excellent solvent for cel­lulose dissolution among ILs whose anion part was natural amino acid. It should be emphasized that amino acid IL, [N221(ME)][Ala], is a halogen free and safe solvent, consisting of non-toxic ammonium cation and natural amino acid. Furthermore, the present results seem to provide an important hint to cellulose chemists to consider the mechanism of how an enzyme interacts with the cellulose surface.