Cellulose-dissolving ionic liquids

Hence, we postulated that it should be possible to use binary mixtures consisting of a little (but sufficient) cellulose-dissolving ionic liquid (e.g., [C2mim] [OAc]) and an excess of a non-dissolving ionic liquid (e.g., [C2mim][Br]). In fact, an extensive experimental study demonstrated that only equimolar amounts of acetate anion (relative to cellulose hydroxyl moieties) need to be present to achieve dissolution, and the second ionic liquid, which has no cellulose-dissolving ability on its own, provides the fluid medium and does not lead to precipitation of the cellulose even if present in tenfold excess (at 100°C) [87, 88]. 

Hedicke-Hbchstotter K, T.im GT, Altstadt V (2009) Novel polyamide nanocomposites based on silicate nanotubes of the mineral halloysite. Compos Sci Technol 69(3-4) 330-334 Hermanutz F, Gahr F, Utadingen E, Meister F, Kosan B (2008) New developments in dissolving and processing of cellulose in ionic liquids. Macromol Symp 262(l) 23-27 Hyden WL (1929) Manufacture and poperties of regenerated cellulose films. Ind Eng Chem 21 (5) 405-410... 

Rogers, R.D. (2005) A Platform Strategy Using Ionic Liquids to Dissolve and Process Cellulose for Advanced New Materials. The University of Alabama. 

More recently, ionic liquids capable of dissolving cellulose have been prepared. The best known is l-A-butyl-3-methylimidazolium chloride (BMIMC1). No significant industrial development of yam fabrication with this process is known so far. 

Ionic liquids are able to dissolve carbohydrates to high concentrations as mentioned in the previous section. The use of ionic liquids in cellulose dissolution and functionalisation is particularly significant considering the problems associated with conventional processes such as the cupramonium and xanthate processes [117, 118, 120, 134-136], Lignin is soluble in ionic liquids, as discussed in Sect. 4.2. 

Abstract Cellulose is the most important biopolymer in Nature and is used in preparation of new compounds. Molecular structure of cellulose is a repeating unit of p-D-glucopyranose molecules forming a linear chain that can have a crystallographic or an amorphous form. Cellulose is insoluble in water, but can dissolve in ionic liquids. Hemicelluloses are the second most abundant polysaccharides in Nature, in which xylan is one of the major constituents of this polymer. There are several sources of cellulose and hemicelluloses, but the most important source is wood. Typical chemical modifications are esterifications and etherifications of hydroxyl groups. TEMPO-mediated oxidation is a good method to promote oxidation of primary hydroxyl groups to aldehyde and carboxylic acids, selectively. Modified cellulose can be used in the pharmaceutical industry as a metal adsorbent. It is used in the preparation of cellulosic fibers and biocomposites such as nanofibrils and as biofuels. 

It has recently been found that salts which melt at or near room temperature, so-called ionic liquids, can form physical solutions of cellulose and starch. l-A -Butyl-3-methylimidazolium chloride dissolved plant and bacterial cellulose with no apparent loss of DP, and cellulose in the resulting solutions was much more readily derivatised to various esters than in the solid (Figure 4.34d). The same applied to l-A -allyl-3-methylimidazolium chloride in both solvents, NMR indicated that the cellulose chains were disordered in solution.Studies... 

Recently, cellulose was dissolved in a variety of imidazolium chloride-based ionic liquids (IL s) bearing a series of substituents, which imparted varying degrees... 

An important process is the manufacture of regenerated cellulose applied to make fibers (e.g., rayon) and films (e.g., Cellophane). Solvents used classically in cellulose regeneration are a mixture of carbon disulfide and sodium hydroxide or ammoniacal copper solutions. More recent solvents include N-methylmorpholine-N-oxide and phosphoric acid [4]. The cellulose solution is extruded through nozzles into an acidic precipitation bath and is spun into fibers. Recently, the partly toxic and strongsmelling solvents have been replaced by ionic liquids which are even able to improve the solubility of the slightly soluble cellulose [10]. For example, 1 1 of l-butyl-3-methylimidazolium chloride dissolves lOOg of cellulose at 100°C [11],... 

Schobitz, M.M. RHeinze, T. Unconventional reactivity of cellulose dissolved in ionic liquids. Macromol. 

Blends were prepared with cellulose or silk as soon as a common solvent was available [63, 69-71]. Recently, ionic liquids were used. The solvent l-ethyl-3-methyl-imidazolium acetate completely dissolves raw crustacean shells allowing to recover high purity chitin powder or films and fibres by direct spinning [72]. Films of poly(e-caprolactone) (PCL) blends with a-chitin and chitosan were produced. They are completely biodegradable and the crystallinity of PCL is suppressed in the blends due to hydrogen bond interaction between PCL and polysaccharides [73]. Blends were also realized with poly (3-hydroxybutyric acid) (PHB) and chitin or chitosan. They show faster biodegradation than the pure-state component polymers [74,75]. 

HFIP is a commonly used fluorinated organic solvent for silks. HHP easily dissolves dehydrated RSF films (silk l), draglme silk fibers, and recombinant silk proteins. HFIP causes silk to prefer an a-helical stmcture, and the solutions are easily spinnable. However, HFIP is an extremely hazardous and expensive organic solvent and preferably avoided. Other solvents that have been successftilly used to dissolve silk are formic acid, trifluoroacetic acid, and hydrofluoric add. More environmentally friendly organic salts, commonly used for dissolving cellulose, have recently been employed for dissolving silks. N-Methylmorpholine N-oxide (NMMO) and ionic liquids are nontoxic and recydable and have been shown to dissolve silk I films and silk fibers and make them spinnable into fibers. 

Since, it was found that an ionic liquid, 1-butyl-3-methylimidazolium chloride, dissolved cellulose (Swatloski et al., 2002), and various ionic liquids have been found to be good solvents for cellulose (Seoud et al, 2007, Liebert and Heinze, 2008, Feng and Chen, 2008, Pinkert et al., 2009, Zakrzewska et al., 2010). Therefore, ionic liquids are considered as powerful solvents and are used for derivatization and material processing of cellulose. 

The reaction is done at a pFl of 9-11. There the hydrolysis of the product chitin is much suppressed. The degree of polymerization of synthetic chitin is some 10-20 depending on the reaction conditions (50). Peracetyloxazolines can be obtained from peracetyl saccharides (51). A facile preparation method of chitin cellulose composite films has been described (52). Hereby ionic liquids are used, l-aUyl-3-methylimidazolium bromide and l-butyl-3-methyl-imidazoUum chloride. The former liquid dissolves chitin and the latter Uquid dissolves cellulose. 

More recently, some ionic liquids (IL) were shown to be good cellulose solvents over a wide range of DP values without covalent interaction, especially those based on substituted imidazolium ions (Table 16.1) [17]. Even bacterial cellulose with a DP of 6 500 was found to dissolve in one of these solvents [18]. 

Ionic liquids are promising new solvents in the field of cellulose shaping and functionalization. The acylation of cellulose dissolved in an ionic liquid can be carried out with acetic anhydride. The reaction succeeds without an additional catalyst. Starting from DS 1.86, the cellulose acetates obtained are acetone soluble [21], The control of the DS by the prolongation of the reaction time is possible. When acetyl chloride is used, complete acetylation is achieved in 20 min (Table 16.1) [18, 22, 23], This method may lead to a widely applicable acylation procedure for polysaccharides, if the regeneration of the solvent becomes possible. 

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