The dissolution of cellulose was usually attributed to the ability of the IL to disrupt the hydrogen-bond network in cellulose by forming hydrogen bonds with cellulose. For example, in [AMIM][Cl], the chloride anion is a hydrogen-bond acceptor, while the proton at the 2-position of the imidazolium ring is a hydrogen — bond donor [32]. NMR studies of [BMIM][Cl] showed that the chloride anion has an active role in the solubility of cellulose through hydrogen bonding with the hydroxyl groups of cellulose [84]. Density-functional theory calculations showed that the anions in imidazolium-based ILs tended to form hydrogen bonds with the O2 and O3 hydroxyl groups of cellulose. The strength of the hydrogen bonds increased for the following anions in the order: hexafluorophosphate < tetrafluo — roborate < alkyl phosphate < acetate. The trend matched the one observed in the dissolution of cellulose in the corresponding imidazolium-based ILs, where cellulose solubility was highest with the acetate anion [25, 92]. The strong hydrogen bonding ability of ILs means that they can disrupt the hydrogen bonding network in lignocellulosic biomass by displacing the lignocellulose components to form stronger hydrogen bonds [34].

Fig. 4.3 A time series of X-ray diffraction images recorded from a radial section of Poplar sp. As [EMIM][OAc] is applied and then expelled with water; a Untreated sample, b-e application of [EMIM][OAc], f-i application of water.

The fiber diffraction direction is approximately vertical. Note the presence of two superimposed equators in a, h, and i with a relative orientation of approximately 25°. Reprinted from [71], copyright (2011), with permission from Elsevier

Molecular dynamics simulations were conducted to study the interaction between [EMIM][OAc] with glucose oligomers (5-20 units). The total interaction energy between [EMIM][OAc] with cellulose (around -75 kcal/mol) was larger than the one between water and cellulose (around -50 kcal/mol) and the one between methanol and cellulose (around -45 kcal/mol). The difference between [EMIM][OAc] and water/methanol became larger with the cellulose chain length [49]. The acetate anion is also a hydrogen-bond acceptor, with the potential to form hydrogen bonds with the three hydroxyl groups of each unit of cellulose. The strength of these hydrogen bonds (14 kcal/mol) was estimated to be three times higher than the hydrogen bonds in water (5 kcal/mol) and methanol (4 kcal/mol). The simulations showed that the imidazolium cation interacts strongly with the glucose ring structure via van der Waals forces. Also, the interactions between [EMIM][OAc] and cellulose led to conformation changes in the cellulose chains, which can explain the loss in crystallinity and structural changes in regenerated cellulose [49].

The hydrogen bonding ability of ILs was probed by IR spectroscopy. ILs were prepared with the same anion [Tf2N]- and different cations with increasing hydrogen bonding ability: 1,2,3-trimethylimidazolium, 1,3-dimethylimidazolium, 1,2-dimethylimidazolium, and 1-methylimidazolium. The increasing strength of hydrogen bonds was indicated by a shift of the IR absorption band below 150 cm-1 toward higher wave numbers. This band shifted from 62 cm-1 for the 1,2,3-trimethylimidazolium cation to 101 cm-1 for the 1-methylimidazolium cation. There was a linear relationship between the measured peak position and the average interaction energies in IL clusters from ab initio calculations. Ab initio calculations also showed that the interaction energy is minimal for the 1,2,3,4,5- pentamethylimidazolium cation where all protons were substituted by methyl groups and the hydrogen bonding ability was reduced [77].

Formation of hydrogen bonding between [EMIM][OAc] and cellobiose was also studied by :H NMR. A broadening of the OH resonances was observed as the molar ratio between [EMIM] [OAc] and cellobiose was increased, which was explained by the interaction between the O atoms in the hydroxyl groups and the protons of the imidazolium ring. The accompanying downshift of the OH resonances was attributed to the hydrogen bonding between the acetate anion and the hydrogen atoms in the cellulose hydroxyl groups. NMR spectra of [EMIM][OAc] with increasing cellobiose concentration indicated that the strongest hydrogen bonding between the imidazolium cation and cellobiose involves the proton at the 2-position of the imidazolium ring. The next stron­gest hydrogen bonds involve the protons at the 4- and 5-position, which are much weaker hydrogen-bond donors [84]. When all the hydroxyl groups in cellobiose were acetylated, the NMR spectra of the [EMIM][OAc]/cellobiose octaacetate remained unchanged as the IL concentration increased. This showed that hydrogen bonding between cellobiose and the IL cation/anion is the main reason cellobiose dissolves in [EMIM][OAc]. In order to dissolve cellulose effectively, it was proposed that the IL must have an anion that is a good hydrogen acceptor, and a cation that is a moderate hydrogen-bond donor and not too large [84].