New types of ILs specifically designed to wood fractionation applications have been introduced to the field, and some of the new generations of ILs have also being designed to be more suitable for processing steps, such as recycling of the IL after the treatment. Promising alternatives have been found to the most commonly used dialkylimidazolium acetates and chlorides.

One successful effort that has been made towards more sustainable systems is the development of ‘distillable’ ILs capable of dissolving cellulose, by King et al. [15] This group of ILs is based on the acid-base conjugates of 1,1,3,3- tetramethylguanidine (TMG) and common carboxylic acids, such as acetic or propionic acid. The distillation ability arises from the fact that the acid-base equilibrium can be shifted to a sufficient extent, at high temperature, to produce volatile neutral species. Yet, the applicability to wood fractionation has not yet been examined. Anugwom et al. have achieved the selective extraction of hemicelluloses using ‘switchable’ ILs (SIL), that are not capable of dissolving cellulose or lignin from the wood matrix [16, 17]. SILs can be formed by reacting CO2, and 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) with alcohols. They can be converted back to neutral solvents by removal of CO2, under reduced pressure or bubbling with nitrogen. Reversing the IL equilibrium back to volatile components is an interesting potential method for solvent recycling.

In other areas of the IL field, work has been done with the aim of tuning the hydrogen-bonding properties of traditional ILs, to be more selective towards specific wood components. An example of this, by Froschauer et al. [18] demon­strates modified properties of dialkyl phosphate ILs by using sulfur or selenium to replace one of the oxygen atoms in the anion structure. As a result, the hydrogen — bond accepting ability, as described by the Kamlet-Taft parameter p, was reduced. The new type of anions showed a selective dissolution of hemicelluloses out of hemicellulose-rich pulp. In this regard, Kamlet-Taft parameterization of a range of ILs and co-solvents is starting to allow for a better understanding of solvent and wood biopolymer interactions. In work by Hauru et al., the fundamentals of cellulose dissolution and recovery were presented. They have proposed a different approach to interpret the solvent properties of ILs. Rather than simple evaluation of hydrogen bonding accepting properties ф), the donor ability (a) should be consid­ered as well, resulting in an effective or net basicity value ф—a) that better describes the cellulose dissolution capability of ILs [19]. Undoubtedly an in-depth understanding of the hydrogen-bonding properties has a key role in sophisticated design of ILs that can selectively extract various main wood compo­nents. For lignin isolation, Pinkert et al. have applied dialkylimidazolium sulfamates [20], which do not possess any ability to dissolve cellulose and thus can be used to extract lignin out of biomass. Of note, for this class of IL’s, is an earlier application of acesulfamate as a food additive, which makes them promising from an ecotoxicity point-of-view.