Towards an understanding of the microstructure and interfacial properties of the ionic liquid/sulfuric acid catalyst in liquid-liquid reactions

Abstract

Sulfonic-acid-functionalized Brønsted acidic ionic liquids (SFILs) have been considered as a promising dual solvent-catalyst for numerous applications. The bulk microstructure and interfacial characteristics of different kinds of SFILs and the corresponding SFIL/H2SO4 systems were investigated in this work using molecular dynamics (MD) simulations in terms of the SFILs possessing three anions (e.g. bisulfate, triflate, and bis(trifluoromethylsulfonyl)imide), different alkyl chain lengths, and different introduced position of the sulfonic acid groups (-SO3H). The introduction of the sulfonic acid groups is found to provide a strong hydrogen-bonding interaction site with both the HSO4− and H2SO4, making almost two order of magnitudes larger for average numbers of hydrogen bonds between different pairs. The introduction of sulfonic acid groups onto the shorter side chains contributes to stronger hydrogen-bonding interactions with both the HSO4− and H2SO4 than when introduced onto the longer side chains. The coordination numbers of the equivalent terminal carbon atoms of octyl chain and equivalent sulfur atoms of sulfonic acid groups are more than 4.5 and 2.5, respectively, which suggests that obvious nanostructured aggregation of the polar sulfonic acid groups, as well as nonpolar longer alkyl chains, exists in the pure SFILs and SFIL/H2SO4 systems. In addition, it is found that the ordering preference and interfacial orientation of the side chains and imidazole rings at gas-liquid interfaces are sharply changed following the addition of the sulfonic acid groups. The insights into the bulk structural and interfacial characteristics of the SFIL/H2SO4 systems obtained in this work may inform the design and optimization of novel SFILs for specific catalytic applications.