Systematic Study of Ionic Liquids Based Coal Extraction: Selectivity in Extract Molecular Weights and Targeted Functional Groups

Ionic liquids (ILs) are environmentally friendly solvents that have been used to extract valuable compounds from coal and biomass. In this work, eight methylimidazolium-based ionic liquids were selected based on their different capabilities of interactions with solutes. ζ-Potential characterization was used as a descriptor to reveal the type of interactions (e.g., hydrogen bonding, π–π interaction, and dispersion force) between the chosen ILs and coal particles. Consequently, the different interactions with certain strengths between the selected cations/anions and coal particles provide unprecedented selectivity of ILs in coal extraction. For aromatic-rich bituminous coal, the ILs associated with Cl^– preferably extracted polycyclic aromatic hydrocarbons (PAHs). In contrast, for the same cations, BF₄^– and SbF₆^– anions did not show significant PAH extraction for bituminous coal. On the other hand, for oxygen-rich subbituminous coal, the fatty acids (FAs) with more than 20 carbon atoms were found to be rich in the extracts from the ILs associated with Cl^–, whereas the same C₄mim⁺ cation with BF₄^–, PF₆^–, and SbF₆^– anions extracted unique tricyclic diterpanes. As such, this study provides systematic experimental results demonstrating that ILs may be custom designed to extract targeted compounds from coal or other complex feedstock such as lignocellulosic biomass.