Abstract
We present 1,2,3-triazolium- and imidazolium-based ionic liquids (ILs) with aromatic anions as a new class of cellulose solvents. The two anions in our study, benzoate and salicylate, possess a lower basicity when compared to acetate and therefore should lead to a lower amount of N-heterocyclic carbenes (NHCs) in the ILs. We characterize their physicochemical properties and find that all of them are liquids at room temperature. By applying force field molecular dynamics (MD) simulations, we investigate the structure and dynamics of the liquids and find strong and long-lived hydrogen bonds, as well as significant – stacking between the aromatic anion and cation. Our ILs dissolve up to 8.5 wt.-% cellulose. Via NMR spectroscopy of the solution, we rule out chain degradation or derivatization, even after several weeks at elevated temperature. Based on our MD simulations, we estimate the enthalpy of solvation and derive a simple model for semi-quantitative prediction of cellulose solubility in ILs. With the help of Sankey diagrams, we illustrate the hydrogen bond network topology of the solutions, which is characterized by competing hydrogen bond donors and acceptors. The hydrogen bonds between cellulose and the anions possess average lifetimes in the nanosecond range, which is longer than found in common pure ILs.
Highlights
Within the last few decades of our century, renewable fuels, energies, and materials have gained increasing attention [1]
We synthesized four ionic liquids (ILs) based on the aromatic anions benzoate and salicylate
ILs were obtained as viscous liquids at room temperature
Summary
Within the last few decades of our century, renewable fuels, energies, and materials have gained increasing attention [1]. With an annual production of around 1011 –1012 tons, cellulose is by far the most abundant natural material on Earth [2]. It is obtained in significant amounts as a by-product in the agricultural and food industry, where it is mostly pyrolyzed [3]. Cellulose is a polysaccharide with a degree of polymerization (DP) of around 200–10,000 units in its natural form, as produced by plants It possesses a semi-crystalline structure and a significant amount of intramolecular hydrogen bonds, which contribute to the stiffness of the cellulose chains.
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