Triazolium-Based Ionic Liquids: A Novel Class of Cellulose Solvents

Abstract

We present first results on triazolium-based ionic liquids (ILs) as a novel class of nonderivatizing solvents for cellulose. Despite their chemical similarity to imidazolium cations, the 1,2,3-triazolium cation lacks the isolated ring proton, leading to reduced formation of N-heterocyclic carbenes (NHCs) and therefore to lower reactivity and less unwanted side reactions. We synthesized six ILs based on 1,2,3-triazolium and 1,2,4-triazolium cations. The acetates are room-temperature ionic liquids and dissolve a similar amount of cellulose as the corresponding imidazolium salt. From NMR spectroscopy of the solution, we rule out polymer degradation. The cellulose solubility rises with increasing anion basicity, while being almost independent of the cation. We perform molecular dynamics simulations and compute enthalpies of solvation, which quantitatively fit the experimental solubilities. Trajectory analysis reveals strong hydrogen bonds between acetate anions and cellulose hydroxyl groups, while the cations do not form strong hydrogen bonds with cellulose. Thus, the simulations provide an atomistic explanation of our experimental observations.

Dissolving Cellulose in 1,2,3-Triazolium- and Imidazolium-Based Ionic Liquids with Aromatic Anions

“…It is well known that hydrogen bonds play a significant role in imidazolium-and triazolium-based ILs [29][30][31][32][33][34][35]38]. Figure 3 presents the radial distribution functions (RDFs) between several hydrogen bond donors and acceptors in the investigated systems (for the atom labels, see Figure 1).…”

“…Most of the ILs proposed as cellulose solvents are based on imidazolium cations [10,[25][26][27][28], which is not surprising, as it has been known for long that many imidazolium-based ILs form very strong hydrogen bonds [29][30][31][32][33][34][35][36]. Some of these imidazolium-based ILs are able to dissolve more than 20 wt.-% of cellulose if the anions are strong hydrogen bond acceptors [3,37,38].…”

“…Last year, we proposed triazolium-based ILs as a novel class of cellulose solvents [38]. This class of ILs has been discussed in the literature earlier [1,19,21,42,43], but had never been applied to dissolve cellulose before.…”

“…After having introduced a new class of cations for ILs as cellulose solvents in our last article [38], we set the focus on the anions in this study. We combine the two previously studied cations 1-ethyl- 3 [45][46][47], the latter two did not yet appear in the literature.…”

Carbohydrate Based Ionic Liquids (CHILs): Synthesis and Applications

“…The synthesis of new ionic liquids and their potential applications have been studied thoroughly since then. Major applications include the use as solvents and additives in organic synthesis, [ 13–20 ] the processing of cellulose, [ 21–28 ] the use as electrolytes in batteries and electrochemical synthesis [ 29–38 ] and as extracting agents. [ 39–45 ]…”