Evaluation of Interaction between Aromatic Penetrants and Acidic OH Groups of Solvent-Swollen Coals by Inverse Liquid Chromatography

The objective of the present study is to clarify the role of OH-π interaction in selective recognition of aromatic rings by Morwell brown coal. Low-volatile Pocahontas No. 3 bituminous coal was used as a reference. The microstructure of Morwell coal was modified by swelling in various polar and nonpolar solvents. The interaction between coal and aromatic penetrants was evaluated with an interaction index, Sar, that was determined by an inverse liquid chromatography technique. Swelling ratio and Sar for Morwell coal were strongly affected by solvents, while for Pocahontas coal they were influenced little. The lowest value of Sar for Morwell coal in n-hexane, which did not swell the coal (swelling ratio Q = 1.02), shows that aromatic molecules hardly penetrated into the micropores and therefore could not access self-associated OH groups. Sar showed a maximum in acetonitrile (Q = 1.25) and methanol (Q = 1.40), both of which formed moderate hydrogen bonds with acidic OH groups in the coal. Then the swelling induced by the disruption of hydrogen bonds was essential for the selective recognition of aromatic rings by Morwell coal. THF gave a higher swelling ratio Q (=2.0) than acetonitrile and methanol, but it lowered Sar. However, excessively strong hydrogen bond between OH groups and the solvent interfered with the interaction of aromatic molecules with OH groups. Pyridine added to acetonitrile reduced Sar significantly and obstructed the OH-π interaction by forming stable hydrogen bonds with acidic OH groups in the coal. Chemical removal of OH groups by O-butylation also greatly decreased Sar. These results indicate that selective recognition of aromatic rings by Morwell coal is ascribed to the formation of OH-π interaction between coal- OH and aromatic plane.