Switchable Ionic Liquid Fractionation The main objective of the work was to devel-

op a process to fractionate the main biomass components to enable their utilization in the production of materials, chemicals and fuels for the future bioeconomy.

Several Switchable Ionic Liquids (SIL) were synthesized and characterized. The prepared SILs were used in a novel process involving the fractionation of Nordic woody biomass by se- lective dissolution. The process was discov- ered and extensively developed during the FuBio programme, with the duration of frac- tionation successfully reduced from five days to just two hours. In addition, relatively high purity of the resulting fractions was achieved; for example, cellulose-rich material contain- ing 79 wt-% cellulose, 11 wt-% hemicelluloses and 5 wt-% lignin was obtained from spruce chips. All of the produced fractions were char- acterized in detail using several methods. The SIL treatment enables the fractionation of biomass into relatively pure fractions un- der milder conditions than in current industri- al processes, thus consuming less energy. The process could be utilized where the production of hemicelluloses and lignin together with cel- lulose fibres is targeted. Hemicelluloses and lignin fractions could be processed further as bio-based chemical and materials, thus adding value to these raw materials compared to the

current kraft process, in which they are burnt for energy recovery.

The SILs were synthesized by bubbling CO₂ or SO₂ through the liquid mixture with the ratio of amidine/guanidine to hydroxyl-containing compound calculated based on the number of hydroxyl groups in the alcohol or alkanol. Here a sample case is presented as an example procedure. DBU-MEA-SO₂-SIL was prepared by passing SO₂ through the mixture containing 1:1 molar amounts of 1,8-diazabicyclo-[5.4.0]- undec-7-ene (DBU) and monoethanolamine (MEA). The weight increase with the SIL corresponds to a molar ratio of 1:1:1 of all components. These results indicate that all OH groups of the MEA react upon formation of the ionic liquid according to Scheme 2. This observation is supported by NMR and FTIR studies.

In previous studies it has been demonstrated that SILs are potential novel solvents for the fractionation of lignocellulosic material into suitable fractions. However, the fractionation efficiency has not been comparable to those obtained with conventional ILs mainly due to the low treatment temperature. Therefore, the fractionation of lignocellulosic material was investigated based on new types of SILs. These were based on glycerol or alkanol amine, CO₂ or SO₂ and an amidine (DBU). The new SILs have one major advantage

Scheme 2. Proposed reaction scheme for the formation of DBU-MEA-SO₂-SIL (adapted from Anugwom et.al. 2014. ChemSusChem 7, 1170).

compared to previously presented SILs: their decomposition temperatures are significantly higher, thus allowing higher treatment temperatures, which leads to more efficient fractionation. Wood was fractionated without any mechanical agitation under normal pressure and at 100-120°C. However, the treatment time was found to be too long and too cost-intensive to implement at an industrial scale. Furthermore, the use of large amounts of SILs and the need for drying of the wood raw material would also add to the process cost. However, addition of water to the wood/SIL mixture or/and use of fresh non-dried wood was found to be effective in reducing SIL consumption. The optimization of conditions for the selective fractionation of woody biomass was investigated via a novel and economically feasible fractionation method using an alkanol amine (MEA) and an organic superbase (DBU) derived SIL with water. The Short Time High Temperature (STHT) approach was used, where the wood was immersed in the SILs and water added to achieve a 1:3:5 weight ratio. The mixture was kept at 160°C under normal atmospheric pressure for 2 hours without stirring. While the mixture was still hot, the undissolved wood fraction was separated using vacuum

filtration. The undissolved wood material was washed several times with isopropanol at about 40°C until all the SIL was visually removed. After the dissolution/extraction of wood, the undissolved residues as well as precipitated materials from the SIL were analysed by various methods. Weight losses were recorded and the materials were analysed in order to determine their composition. The latest results showed that the STHT method can be used to remove almost all lignin and the majority of hemicelluloses from wood in just two hours. For example, DBU/MEA/ CO₂-water treated spruce contained 79 wt-% cellulose, and only 11 wt-% hemicelluloses and 5 wt-% lignin. In addition, the resulting fibres are very light in colour (Figure 1).

Significant further improvements to the process can be made. Firstly, the recovery of dissolved hemicelluloses and lignin from the spent SIL should be improved. In addition, the process is still far from optimized, thus, substantially improved results could be achieved through optimization. Furthermore, the number of new SIL designs is considerable and remains so far unexplored. Most importantly, SILs ‘triggered’ with acid gases other than those studied here could give rise to a rich family of yet unknown potential.

Figure 1. Undissolved fluffy material recovered from A) birch and B) spruce after SIL treatment applying

4.2 Recyclable Ionic Liquid Design Throughout FuBio 1 & JR2, emphasis was placed on the development of recyclable systems for biomass processing at the University of Helsinki. The motivation for this was that no workable strategies were available for recycling ionic liquids after a fractionation step. Typically, oligomeric materials and inorganics are present that are very difficult to remove due to the non-volatility of traditional ionic liquids. Therefore, we sought to introduce recyclability to the ionic liquid in the form of the classical purification methods of distillation and phase separation (Figure 2). In this case the distillation of [TMGH][CO₂Et] required 130°C and 5 mbar in a Kugelrohr. In the case of recycling by phase- separation, dissolved cellulose and [P₈₈₈₁][OAc] could be recovered by addition of pure water or kosmotropic salts. [P₈₈₈₁][OAc] could then be reused in the dissolution of cellulose.

After continued optimization of different distillable acid-base conjugate ionic liquids for biomass processing, it was found that [DBNH] [OAc] was a highly effective ionic liquid for cellulose dissolution. This was a result of the very low viscosity achieved by the combination of 1,5-diazabicyclo(4.3.0)non-5-ene (DBN, superbase) and acetic acid (organoacid), which was similar to that of the benchmark ionic liquid [emim][OAc] (Figure 3). When combining different bases with acetic and propionic acid, a clear trend developed concerning cellulose solubility. Combinations of these acids with the superbase range proved effective at dissolving cellulose, whereas their combination with normal organic bases was ineffective. This was rationalized by the fact that the basicity of the unconjugated base is also a measure of the ionic liquids’ cation acidity. It was concluded that if the cations are too acidic they stabilize the anion to such an extent that solvation of cellulose, with significant enthalpy of dissolution gain through cellulose hydrogen bond breakage, is inhibited.

[P₈₈₈₁][OAc], which was demonstrated to be phase-separable upon addition of water or kosmotropic electrolyte solutions (e.g. sodium acetate solution), was also found to be an excellent solvent for cellulose as the DMSO electrolyte. Its ability to form isotropic solutions was so strong that it was possible to obtain 1_H-13_{C heteronuclear} single-quantum correlation (HSQC) NMR spectra where the cellulose backbone was free from any ionic liquid resonances (Figure 4). To the best of our knowledge, this was the first time that this has been achieved with high DP cellulose (MCC). Moreover, we were able to assign the terminal glycosidic C1 and anomeric C1 signals from the chain ends in the 1_{H NMR spectra (Figure 5).} Cellulose regenerated from this solution mainly as the more thermodynamically stable cellulose II crystalline polymorph, together with some residual cellulose I and amorphous cellulose (Figure 4). Therefore, these are highly suitable media for further biomass pre-treatments, fractionations or chemical modification. This will be thoroughly investigated in the future.

Figure 2. Distillation of the cellulose-dissolving [TMGH][CO₂Et] (a) and the recycling strategy for phase- separable ionic liquids, such as [P8881][OAc] (b), which dissolve cellulose as their DMSO electrolytes (Adapted

from King et al. 2011, Angew. Chem. Int. Ed. 50, 6301 and Holding et al. 2014. ChemSusChem 9, 1565). a)

Figure 4. (a) Solution-state HSQC NMR of MCC dissolved in [P8881][OAc]:d6-DMSO. (b) XRD of untreated

and regenerated MCC (adapted from Holding et al. 2014. ChemSusChem 9, 1565).

Figure 5. Assigned 1_{H NMR of MCC dissolved in [P}

8881][OAc]:d6-DMSO showing polymeric and terminal

C1 peaks.

Figure 3. Viscosities vs. temp. of a range of acid-base conjugate ionic liquids (a), with [DBNH][OAc]

having the same viscosities as the benchmark [emim][OAc]. X-ray crystal structure of [DBNH][OAc] (b) indicating strong hydrogen bonding between anion and cation. Proposed model for how cation acidity influences the enthalpy gain for Gibbs free energy of dissolution (c). (Adapted from Parviainen et al. 2013 ChemSusChem 6, 2161).

a) b) c)

4.3 Homogeneous Ionic Liquid-Aided