Toshiyuki Itoh

Abstract Cellulose consists of linear glucose polymer chains that form a very tight hydrogen-bonded supramolecular structure making it highly resistant to enzymatic degradation. The ionic liquid, 1-butyl-3-methylimidazolium chloride ([C4mim]Cl), has been found to dissolve cellulose and the regenerated cellulose from the IL solution is less crystalline. To design ionic liquids that dissolve cellulose, Kamlet — Abboud-Taft p-values can be used as a solvent indicator. Amino acid anions have strong interactions between hydroxyl groups in the cellulose molecule: N, N-diethyl, N-methyl, N-(2-methoxy)ethylammonium alanate ([N221(ME)][Ala]) thus they are studied in this chapter for cellulose dissolution. Addition of an anti-solvent like water or ethanol to the cellulose/IL solution caused precipitation of cellulose dissolved and the structure of the regenerated cellulose to change to a disordered form. Crystal form of the regenerated cellulose depends on the dissolution solvent; the disordered chain region seems to increase in the order of [N221(ME)][Ala] < [C2mim][OAc] < [C2mim][(EtO)2PO2] < [C2mim]Cl. On the other hand, the order of degree of polymerization of the cellulose is [N221(ME)][Ala] > [C2mim] [OAc] > [C2mim][(EtO)2PO2] > [C2mim]Cl. Treatment with [N221(ME)][Ala] is therefore much more suitable to use in preparing regenerated cellulose fibers than other commonly used ionic liquids.

Keywords Cellulose dissolution • Amino acid ionic liquids • Regenerated cellu­lose • A mixed solvent

T. Itoh (*)

Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Koyama Minami, Tottori, Japan e-mail: titoh@chem. tottori-u. ac. jp

Z. Fang et al. (eds.), Production of Biofuels and Chemicals with Ionic Liquids, Biofuels and Biorefineries 1, DOI 10.1007/978-94-007-7711-8_4,

© Springer Science+Business Media Dordrecht 2014

4.1 Introduction

Cellulose is an important renewable resource for production of biocomposites and biofuel alcohols. However, since it consists of linear glucose polymer chains that form a very tight hydrogen-bonded supramolecular structure, cellulose resists enzymatic degradation. There has been growing interest in the development of a means of modifying cellulose structure to an easily digestible form by biodegrada­tion [1]. Multiple hydrogen bonding among cellulose molecules results in the formation of highly ordered crystalline regions [2]. Therefore, cellulose does not dissolve in water and common organic solvents at ambient conditions. The chal­lenge for dissolving cellulose has a long history [3]. The first attempt was reported early in the 1920s and some mixed solvent systems for cellulose dissolution were developed [1,3]: sodium hydroxide/carbon disulfide (CS2) [4] and sodium hydrox — ide/urea [5] are well known as commercial cellulose derivatizing solvents. Rosenau et al. [6] reported using N-methylmorphorine-N-oxide monohydrate (NMMO) as a solvent for direct dissolution of cellulose in an industrial fiber-making process [6]. Combination of a polar molecular solvent with a salt was also reported to dissolve cellulose: N, N-dimethyl acetoamide (DMA) in combination with LiCl [7], a mixture of DMSO and tetrabutylammonium fluoride (TBAF) [8] were thus developed as cellulose dissolution solvents. Recently, Ohno and co-workers reported an interesting cellulose dissolution system of a mixed solvent of tetrabutyl- phosphonium hydroxide (TBPH) containing 40 wt% water [9]. In all cases, an appropriate combination of organic salts and polar solvents was essential to realiz­ing high dissolution of cellulose. Fischer et al. [10] reported that molten salt hydrates (LiXmH2O; X = Г, NO3“ CH3CO2 , ClO4~) dissolved cellulose [10]. It is now well recognized that very high polarity of the solvent system might be the key to breaking down the cellulose network and dissolving cellulose even if these solvents have no ionic character [3] (Fig. 4.1). However, there is a serious environmental drawback to such traditional solvent systems: they generally require large quantities of hazardous chemicals and high temperatures. From the standpoint of green chemistry, development of a safe and efficient cellulose disso­lution process can be anticipated [3].

Ionic liquids (ILs) usually melt below 100 °C and are becoming attractive alternatives to volatile and unstable organic solvents due to their high thermal stability and nearly non-volatility. The most fascinating nature of ILs is their structural diversity. We are able to design their physicochemical properties, includ­ing viscosity, polarity, and hydrophobicity. Numerous papers and several reviews on the ILs have been published [11], and it is now widely recognized that ILs are applicable to the media for many types of chemical reactions [11] and even for enzymatic reactions [12]. ILs have consequently show a unique solubility in many inorganic and organic materials, and it is anticipated that ILs might dissolve insoluble compounds, including cellulose, which is impossible with conventional molecular liquids [11].

Swatloski et al. [13] reported a breakthrough on this issue using ionic liquid technology: they found that cellulose dissolved in an ionic liquid, 1-butyl-3- methylimidazolium chloride ([C4mim]Cl), and that the regenerated cellulose from the IL solution was less crystalline [13]. An increased reaction rate of cellulase — mediated hydrolysis was realized when regenerated cellulose from the ionic liquid solution was subjected to the enzymatic reaction resulting from reduced crystallin­ity [13]. Since then, extensive investigations have been carried out to develop an IL that possesses the capability to dissolve cellulose and reduce its crystallinity [1,3,

14] . Most reported ILs are imidazolium based or alkyloxyalkyl-substituted ammo­nium salts with chloride, formate, acetate, propionate, or phosphate as counter anion (Fig. 4.2) [1, 3, 14].