Relation between differential solubility of cellulose and lignin in ionic liquids and activity coefficients

In this work, the excess enthalpies and activity coefficients were evaluated as reference parameters of the solubility of cellulose and lignin in ionic liquids (ILs) using COSMO-RS. Guided by experimental tests, a variety of solubility behaviours of these compounds in 12 ILs with different anions was reported. Appropriate molecular models able to simulate the behaviour of real cellulose and lignin compounds were selected with the aid of experimental data on their dissolution in ILs. The aim was to develop a reliable COSMO-RS computational approach to predict the ability of an IL to dissolve cellulose and/or lignin. A 3 x 3 cellulose structure was optimized in order to achieve an adequate model for describing cellulose solubility, and nine different models were obtained. Several molecules were evaluated as lignin models in COSMO-RS calculations, such as pinoresinol, guaiacylglycerol-2-coniferyl ether, and p-coumaryl, coniferyl and sinapyl alcohols. The results showed that the activity coefficient was the most consistent property to predict the ability of ILs to dissolve these compounds, allowing a tentative classification of the cellulose and lignin differential solubility in ILs in terms of this parameter value. In addition, the study of the differential solubility of cellulose and lignin, based on the developed COSMO-RS approach, was extrapolated to a wide sample of 750 ionic liquids, including 25 cations and 30 anions.