Recycling of ILs is one, if not the major technological issue that needs to be solved in order to utilize this novel class of solvents in commercialized procedures in general and for the processing of cellulose in particular. ILs can be considered as rather expensive compounds but reutilization of the solvent for multiple processing cycles decreases the impact of IL prize on the overall process costs. Moreover, preventing pollution with potentially hazardous organic compounds is a matter of general concern when developing environmentally benign processes. ILs are often considered as recyclable mostly because of their very low vapor pressure.

Nevertheless, it needs to be emphasized that efficient recycling strategies for ILs used in (1) shaping, (2) biorefinery, or (3) chemical derivatization reactions are still missing.

The major component that needs to be removed after processing of cellulose is the non-solvent (usually water or an alcohol) used to regenerate cellulose (shaping) or the polysaccharide derivative (chemical derivatization). Additional impurities derive from the chemical derivatization reaction, e. g., residual reagents, side products, and co-solvents, but also from thermal decomposition of the IL and cellulose (see Sect. 5.3.2). Volatile compounds can easily be removed by evapora­tion under reduced pressure yielding crude ILs that, in some cases, could be utilized directly for another dissolution and chemical derivatization cycle. As an example; acylation of cellulose, dissolved in an imidazolium chloride based IL, yields hydrochloric or carboxylic acid when no additional base is added. Both compounds could be removed from the IL together with the volatile precipitation agent and excess acylation reagent [12, 13]. The NMR spectra of the recycled ILs showed no residual impurities and when reused as homogeneous reaction media, results were comparable to derivatization in the initial IL.

IL recycling becomes more complex with increasing number of potential impu­rities. In particular, the removal of non-volatile compounds proved to be more challenging. If a base is applied during the derivatization, the corresponding protonated acid is usually not removable by evaporation due to its higher boiling point [35, 40]. For comparison: the boiling points of pyridine, which has been applied for acylation, tosylation, and tritylation of cellulose in ILs, and pyridinium hydrochloride are 115 and 223 °C. Recycling can be achieved by neutralization of the IL in an aqueous solution, evaporation of water and the deprotonated base, and subsequent extraction of the crude IL with chloroform, which results in precipita­tion of inorganic salts that can be removed by filtration [33]. Finally, treatment of the recycled IL with an anion exchanger might be required in case anionic species are generated that cannot be removed by other means because they are too similar to the original IL’s anion, e. g., carboxylates differing in their alkyl chain, or because their protonated form is not volatile, e. g., tosylate. NMR spectroscopy can be used to follow the individual recycling steps (Fig. 5.7).

So far, most strategies for the recycling of ILs after cellulose processing focused entirely on evaporation to remove the non-solvent, used for regeneration of the polysaccharide, and as well as side product formed upon derivatization. Up to now, it has not been studied whether this rather energy consuming approach is suitable from an economic and ecologic point of view. Thus, alternative approaches for recovery and purification of ILs are constantly studied not only in the field of polysaccharide research. Upon addition of ‘water structuring salts’, e. g., phos­phates, carbonates, and citrates, or of certain organic compounds such as carbohy­drates, amino acids, and surfactants, to an aqueous IL solution, separation into an IL-rich/water-deficient and an IL-deficient/water-rich phase occurs [105,106]. This ‘salting out’ phenomenon has been exploited for recovery of AMIMCl, used as reaction medium for acylation of cellulose, in 85 % yield [16]. 1H-NMR spectros­copy was used to confirm the purity of the recycled IL but no information on

inorganic impurities, in particular the residual amount of Na2HPO4 was provided, which was used to induce phase separation.

Alternative strategies, which are frequently discussed for recycling of ILs in general, are pervaporation, reverse osmosis, and nanofiltration [107, 108]. Only the latter has been studied already for recovery of ILs that were used as cellulose solvent but not as homogeneous reaction medium. A straight forward approach of increasing importance is the development of novel cellulose dissolving ILs that facilitate efficient recycling, e. g., by extraction or induced phase separation (see Sect. 5.3.4). Frequently, utilization of ‘distillable ILs’ such as guanidinium carbox — ylates has been proposed [109]. At high temperatures and low pressure, these ILs decompose into volatile compounds that reconstituted upon cooling. This process yields ILs of high purity but is also very energy consuming. Moreover, most of the impurities need to be removed in advance because they would evaporate prior to the IL.

When developing procedures for the chemical derivatization of cellulose in ILs, it is important to take recycling aspects into consideration as well. To give an example; homogeneous esterification of cellulose with a carboxylic acid chloride (very reactive reagent) in EMIMAc (low viscous room temperature liquid IL) will yield cellulose ester in good yield and high DS. However, purification of the IL by evaporation of volatile compounds is not feasible. Hydrochloric acid, formed as side product, will induce protonation of the less acidic carboxylate anion that is removed under reduced pressure. Thus, partial anion exchange from EMIMAc to EMIMCl will occur. A very complex mixtures of the recycled ILs can also be expected when mixed cellulose esters are prepared [58].