Predicting Cellulose Solvating Capabilities of Acid-Base Conjugate Ionic Liquids

Different acid–base conjugates were made by combining a range of bases and superbases with acetic and propionic acid. Only the combinations that contained superbases were capable of dissolving cellulose. Proton affinities were calculated for the bases. A range, within which cellulose dissolution occurred, when combined with acetic or propionic acid, was defined for further use. This was above a proton affinity value of about 240 kcal mol−1 at the MP2/6‐311+G(d,p)//MP2/ 6‐311+G(d,p) ab initio level. Understanding dissolution allowed us to determine that cation acidity contributed considerably to the ability of ionic liquids to dissolve cellulose and not just the basicity of the anion. By XRD analyses of suitable crystals, hydrogen bonding interactions between anion and cation were found to be the dominant interactions in the crystalline state. From determination of viscosities of these conjugates over a temperature range, certain structures were found to have as low a viscosity as 1‐ethyl‐3‐methylimidazolium acetate, which was reflected in their high rate of cellulose dissolution but not necessarily the quantitative solubility of cellulose in those ionic liquids. 1,5‐Diazabicyclo[4.3.0]non‐5‐enium propionate, which is one of the best structures for cellulose dissolution, was then distilled using laboratory equipment to demonstrate its recyclability. Distillation and solubility: Rational design of protic ionic liquids that are both distillable (as a means of recycling) and are excellent solvents for cellulose has been achieved. Ab initio and experimental studies highlight the role of a protic cation in the thermodynamics of cellulose dissolution. Structures described herein are comparable with commercial materials.