Solvent Screening for Separation of Lignin-Derived Molecules Using the NIST-UNIFAC Model

Liquid–liquid phase equilibria were calculated to investigate the potential separations of value-added components from products obtained by lignin depolymerization. In this study, the ability of the group-contribution model was evaluated in the prediction of mutual solubility and liquid–liquid phase equilibria of phenolic compounds. The phase equilibria behaviors of quaternary systems were evaluated by the NIST-UNIFAC model so that the predicted results were in good agreement with the available experimental data. In sequence, the partition coefficients of 29 lignin-derived molecules with complex and polar functional groups were predicted by the model with good accuracy (RMSE = 0.7424). The abilities of binary, ternary, and quaternary solvent systems were evaluated in the counter-current chromatography (CCC) separations of the products reported in the literature that are obtained through lignin depolymerization processes. Based on the empirical solvent selection criteria for CCC measurements, promising solvent systems were found for some of the lignin products. The difficult separations of some products in other cases can be attributed to the very similar chemical structures of the monomers. Finally, it was found that the NIST-UNIFAC model could qualitatively predict the solvent systems from the Arizona series, suitable for the separation of 4-hydroxybenzoic acid, vanillin, acetovanillone, syringaldehyde, acetosyringone, vanillic acid, and syringic acid.