Ionic liquid: current developments, potential and ... - Lenzing

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magnesium on a silver surface was realised in the IL [BuImMe][BF4] [116]. The hydrophobic ILs [BuImMe][PF6] and [Tf2N] were used in developing a highly efficient anode system for lithium/seawater batteries [117]. Materials science The unique adaptability and flexibility of ILs provides an attractive alternative for synthetic protocols which are otherwise inaccessible. Thus, exciting new materials such as TiO2 microspheres, Bi2S3 nanorods, or CuCl nanoplatelets with unique and tunable properties have been obtained [118]. Carbohydrates and other biomacromolecules, wood, pulp, and paper ILs offer promise as solvents for the dissolution of highly polar compounds such as carbohydrates. Over the past few years, carbohydrate chemists have witnessed the use of ILs in a variety of important reactions [5]. There is an increasing willingness to develop new cellulose-based materials due to the fact that cellulose is the earth's most abundant biorenewable macromolecular resource. The efficient dissolution of cellulose is a longstanding goal in cellulose research and development and is still of great importance. There are various solvents known [119], however, among quaternised nitrogen bases only N-methylmorpholine-N-oxide monohydrate is applied commercially. The cellulose is processed via direct dissolution to produce fibers (Lyocell process) [120]. Side reactions and byproduct formation in the system NMMO/cellulose have been reviewed [120]. Many solvents have been studied with regard to their application as medium for homogeneous phase chemistry under lab scale conditions. The application of N,N-dimethylacetamide/LiCl and dimethylsulfoxide in combination with tetrabutylammonium fluoride trihydrate has broadened the diversity of synthetic paths. In addition, the degree of substitution and the distribution of functional groups may be controlled. In his pioneering work, Graenacher [121] 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 Lenzinger Berichte, 84 (2005) 71-85 76 novelty of little practical value since the molten salt system was, at the time, somewhat exotic and had a relatively high melting point (118°C). The unconventional dissolution of cellulose in molten lithium perchlorate trihydrate was demonstrated to be efficient for acetylation and carboxymethylation reactions [122]. Cellulose can be dissolved in a number of ILs (so-called green solvents) and easily regenerated by contacting with water or alcohols. This allows a simple, benign system for the processing of cellulose into fibers, monoliths, and films by forming into an aqueous phase. This has potential environmental and cost advantages over current processing methodologies which make use of volatile or hazardous solvents [123]. The dissolution of cellulose in [BuImMe]Cl [124] and the production of cellulose films reconstituted from [BuImMe]Cl [125] looked promising. The ILs used have the ability to dissolve cellulose with a degree of polymerisation (DP) in the range from 290 to 1200 in high concentrations. Using [BuImMe]Cl, no degradation of the polymer was observed. By 13 C NMR measurement, it was concluded that this IL is a so-called non-derivatising solvent [126]. However, our results are to the contrary. It is evident, that 13 C NMR in not a suitable method to determine molar mass distributions. "New" and powerful non-derivatising solvents (1-allyl-3-methylimidazolium and 1,3-diallylimidazolium chlorides) for cellulose were described [119, 127], and conductivity and viscosity data were given [128]. Homogeneous acetylation of cellulose in the "new" IL [AllImMe]Cl was reported [129]. However, this IL is not new at all [23, 24]. ILs that dissolve carbohydrates in high concentrations include alkyloxyalkyl-substituted imidazolium dicyanamides [130]. Other imidazolium-based RTILs containing ether and alcohol groups in the side chain [131, 132, 133, 134], even poly(ethyleneglycol)imidazolium salts (grafted system) [135], were described. The oligo-ether units are expected to act as Lewisbasic/hydrogen-accepting functionalities [136]. The suitability of ILs for dissolving and regenerating silk was studied. Thus, [BuImMe] Cl, [Bu(2-Me)ImMe]Cl, and [EtImMe]Cl were found to be able to disrupt the hydrogen bonding
network in silk fibroin [137, 138]. Starch was selectively depolymerised by mixing with 1butyl-3-methylimidazolium chloride. Stirring native barley starch at 100°C gave a product mixture containing monomeric products of depolymerised amylose, but amylopectin remained intact. However, all the starch was converted into monomeric products at 150°C [139]. This is an interesting example of selective degradation of biopolymers. [BuImMe]Cl was also used for the dissolution and regeneration of wool keratin fibers, which can be used to prepare wool/cellulose blended materials directly [140]. Solubilisation and derivatisation of the amino polysaccharide chitin [141] in RTILs clearly has potential for exciting new products [142, 143]. Certain ILs were proposed for wood preservation [144, 145, 146]. Impregnation of paper using ILs as curing agents reportedly decreased strength, improved wettability, and provided resistance to moulds and fungi [147]. Let us take a look at the patent situation: there are of course numerous patents for ILs, e.g. [148, 149], but only a very limited number concerning ILs and cellulose [150, 151, 152, 153, 154, 155]. It is surprising which patents are granted today, taking into account that they involve long-known facts. Drawbacks Lack of exhaustive physical parameters such as conductivity, viscosity etc. are a serious drawback for the industrial use of ILs at the moment. The establishment of a comprehensive database was proposed [156], and in 2003, an international effort was launched through the IUPAC to meet this important need. Among the different generations of ILs, hydrolytically stable yet hydrophobic ILs would allow easiest recovery from biphasic processes (such as spinning cellulose fibers into an aqueous precipitation bath). However, these systems can obviously be never designed to dissolve carbohydrate-based macromolecules, since their solubilisation relies on the competitive replacement of intermolecular hydrogen bridges which is only feasible with hydrophilic solvent systems. Another potential problem with ILs is the possible pathway into the environment through waste water. However, this situation is common with every solvent. Lenzinger Berichte, 84 (2005) 71-85 77 Experimental Synthesis of 1-allyl-3-butylimidazolium chloride To 99.5 g of freshly destilled 1-allylimidazole (0.92 mol, 1 equiv.) in a round-bottomed flask equipped with reflux condenser was added an excess of 1-chlorobutane (93.7 g, 1.01 mol, 1.1 equiv.). The reaction mixture was refluxed for 24 h. Excess 1-chlorobutane was removed by means of an oil pump (80°C) yielding 182.7 g of a slightly yellow, viscous liquid (98% of theory). 1 H NMR (CDCl3): δ 1.12 (3H, t, J 7.3 Hz), 1.54 (2H, m), 2.10 (2H, m), 4.50 (2H, t, J 7.3 Hz), 5.13 (2H, d, J 6.3 Hz), 5.65 (2H, m), 6.30 ppm (1H, m), 7.91 (1H, m), 8.00 (1H, m), 9.38 (1H, s). 13 C NMR (CDCl3): δ 13.3, 19.3, 32.9, 49.5, 51.7, 122.0 (2C), 122.6, 130.1, 137.0 ppm. IR (neat, ATR): 3047, 2957, 2934, 2871, 1644, 1561, 1458, 1424, 1163, 994, 942 cm –1 . Mp.: not crystalline down to –50°C, turns into a glass. Synthesis of 1-allyl-3-butylimidazolium dicyanamide To a solution of 18.5 g 1-allyl-3-butylimidazolium chloride (0.092 mol, 1 equiv.) in 80 ml acetone were added 8.17 g of sodium dicyanamide (0.092 mol, 1 equiv.). The suspension was stirred for 24 h at room temperature, then sodium sulfate was added. The mixture was stirred for additional 20 minutes and then filtered with suction through a G4 frit. The solvent was removed by means of a rotary evaporator and the product finally dried on a vacuum line (oil pump, 70°C, 8 h) yielding 20.7 g of a slightly yellow liquid (97% of theory). 1 H NMR (neat, C6D6 external locking capillary): δ 1.06 (3H, t, J 7.3 Hz), 1.48 (2H, m), 2.04 (2H, m), 4.44 (2H, t, J 7.3 Hz), 5.07 (2H, d, J 6.3 Hz), 5.59 (2H, m), 6.25 ppm (1H, m), 7.85 (1H, m), 7.94 (1H, m), 9.32 (1H, s). 13 C NMR (neat, C6D6 external locking capillary): δ 13.7, 19.7, 32.2, 49.9, 52.0, 119.9, 121.7, 122.9, 123.2, 131.5, 136.6 ppm. IR (neat, ATR): 3136, 3093, 2961, 2874, 2225, 2191, 2124, 1561, 1461, 1304, 1160, 993, 944 cm –1 . Synthesis of 1-allyl-3-methylimidazolium dicyanamide To a solution of 27.9 g 1-allyl-3-methylimidazolium chloride (0.175 mol, 1 equiv.) in 80 ml acetone were added 15.6 g of sodium dicyanamide (0.175 mol, 1 equiv.). The mixture
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Ionic liquid: current developments, potential and ... - Lenzing

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