Cationic functionalisation of cellulose using a choline based ionic liquid analogue{

The cationic functionalisation of cellulose is of considerable industrial importance and has found applications in the paper industry, cosmetics, textiles, in flotation and flocculation and in drilling fluids. In particular, it is used in the removal of acidic dyes from aqueous effluent produced by the textile industry, which significantly reduces the environmental impact of such processes. These dyes are of an anionic nature comprising of groups such as sulfonates, carboxylates or sulfates, and so suit the use of a cationic substrate. The form of cellulose often used, cotton, is natural, inexpensive and renewable. The preferred route to the formation of these materials is by the etherification of cellulose using glycidyl ammonium salts or alkylene epoxides in the presence of a suitable alkaline catalyst, usually sodium hydroxide. Cationic substitution values between 0.034 and 0.5 moles of quaternary nitrogen per mole of glucose unit have been reported. However, such approaches entail the use of large amounts of organic solvents, and result in low yields and the loss of product as a result of solubility in the reaction mixture. Two other approaches, coupling oligo-ionomers to cellulose fibres and grafting on cationic polymer chains, have also been reported, but again yields tend to be poor. We have previously reported the development of ionic liquids based on a combination of the inexpensive and readily available components choline chloride (ChCl; HOCH2CH2N(Me)3Cl) and zinc chloride as alternatives to the more commonly employed alkyl imidazolium–aluminium chloride mixtures. These new Lewis acidic solvents have been successfully employed for a variety of reactions. We have also reported a eutectic mixture of choline chloride and urea which forms an ionic liquid analogue with the advantage of being non-toxic and readily biodegradable but has no Lewis acidity. Two uses of this mixture for inorganic applications have recently been reported. In this current study we report the first successful use for organic synthetic processes of a deep eutectic solvent, consisting of a choline derivative and urea, which can be employed as both a reagent and solvent for the cationic functionalisation of cellulose.