Dissolution of Cellose with Ionic Liquids

Cellulose is the most abundant biorenewable material, with a long and well-established technological base.1 Derivitized products have many important applications in the fiber, paper, membrane, polymer, and paints industries. Cellulose consists of polydisperse linear glucose polymer chains (Figure 1) which form hydrogen-bonded supramolecular structures;2 cellulose is insoluble in water and most common organic liquids. The growing willingness to develop new cellulosic materials results from the fact that cellulose is a renewable resource, although many of the technologies currently used in cellulose processing are decidedly nongreen.3 For example, viscose rayon is prepared from cellulose xanthate (production over 3,000,000 tons per year) utilizing carbon disulfide as both reagent and solvent. Most recently, processes using more environmentally acceptable nonderivitizing solvents (N-methylmorpholine-N-oxide (NMNO) and phosphoric acid) have been commercialized. Solvents are needed for dissolution that enable homogeneous phase reactions without prior derivitization.4 Graenacher5 first suggested in 1934 that molten N-ethylpyridinium chloride, in the presence of nitrogen-containing bases, could be used to dissolve cellulose; however, this seems to have been treated as a novelty of little practical value since the molten salt system was, at the time, somewhat esoteric and has a relatively high melting point (118 °C). We were interested in examining whether other solvents that would now be described as ionic liquids (ILs)6 would dissolve cellulose and, especially, whether the availability of a wide and varied range of ILs, coupled with the current understanding of their solvent properties,7 would allow flexibility and control in the processing methodology, with increased solution efficiency and reduction or elimination of undesirable solvents. Ionic liquids, containing 1-butyl-3-methylimidazolium cations ([C4mim]) were screened with a range of anions, from small, hydrogen-bond acceptors (Cl-) to large, noncoordinating anions ([PF6]) also including Br-, SCN-, and [BF4]. In addition, variations in cation alkyl-substituent from butyl through octyl were investigated for the chloride salts. Dissolution experiments were carried out using cellulose-dissolving pulps (from cellulose acetate, lyocell, and rayon production lines), fibrous cellulose (Aldrich), and Whatman cellulose filter papers. The cellulose samples were added to the ionic liquids without pretreatment, in glass vials, and heated without agitation on a heating plate or in a domestic microwave oven. Table 1 summarizes the results obtained using high MW dissolving pulp (DP ≈ 1000). Stirring cellulose in the ILs under ambient conditions did not lead to dissolution, although the cellulose fibers were wetted by the ILs. However, on heating to 100-110 °C, cellulose slowly dissolved in the Cl--, Br--, and SCN-containing ILs to yield increasingly viscous solutions. Dissolution rates could be significantly improved by heating in a microwave oven. In a typical procedure to prepare a 10 wt % solution, 0.5-1.0 g of fibrous cellulose was placed in a glass vial and [C4mim]Cl ionic liquid (10 g) was added as a liquid at 70 °C (i.e., above the melting point). The vial was then loosely capped, placed in a microwave oven, and heated with 3-5 s pulses at full power. Between pulses, the vial was removed, shaken or vortexed, and replaced in the oven. A clear, colorless, viscous solution was obtained. ILs are heated with exceptional efficiency by microwaves,8 and care must be taken to avoid excessive heating that induces cellulose pyrolysis. The decomposition appears to be more rapid in contact with the ILs than for isolated cellulose under equivalent conditions. Solutions containing up to 25 wt % cellulose can be formed as viscous pastes in the chloride-containing ILs, although compositions between 5 and 10 wt % cellulose are more readily prepared. The greatest solubility was obtained using [C4mim]Cl as the solvent. When high concentrations of cellulose (>10 wt %) were dissolved in [C4mim]Cl, the viscous solutions obtained were optically anisotropic between crossed polarizing filters and displayed birefringence. The formation of liquid crystalline solutions of cellulose may have useful applications for the generation of new, advanced materials.9 High-strength materials that conserve anisotropy in the solid phase are especially desirable, yielding enhanced mechanical properties. Nonderivitizing solvents for cellulose effect dissolution by disrupting and breaking the intramolecular hydrogen-bonding network. For dimethylacetamide (DMAC)/LiCl solvents, complexation of lithium ions by DMAC mobilizes chloride ions which * To whom correspondence should be addressed. E-mail: rdrogers@ bama.ua.edu. Figure 1. A cellulose polymer chain, n is typically 400-1000.