Microstructures of the Sulfonic Acid-Functionalized Ionic Liquid/Sulfuric Acid and Their Interactions: A Perspective from the Isobutane Alkylation

The all-atom force field for concentrated sulfuric acid (98.30 wt %) was developed in this work based on ab initio calculations. The structural and dynamical properties of sulfuric acid and the mixing behaviors of sulfuric acid with ionic liquids (ILs), i.e., SFIL (1-methyl-3-(propyl-3-sulfonate) imidazolium bisulfate ([PSMim]­[HSO<sub>4</sub>])) and non-SFIL (1-methyl-3-propyl imidazolium bisulfate ([PMim]­[HSO<sub>4</sub>])), were investigated using a molecular dynamics simulation. For sulfuric acid, most H<sub>3</sub>O<sup>+</sup> ions were found beside HSO<sub>4</sub><sup>–</sup> ions, forming a contact ion pair with the HSO<sub>4</sub><sup>–</sup> ions, and three-dimensional hydrogen-bonding networks existed in the sulfuric acid. Analyses indicate that both ILs could be miscible with sulfuric acid with a strong exothermic character. The new strong interaction site between the sulfonic acid group of SFIL and an H<sub>2</sub>SO<sub>4</sub> molecule through a strong hydrogen-bonding interaction was observed, which was beneficial to the catalytic activity and stability of the sulfuric acid. This observation is in good agreement with the experimental results that indicate SFILs could enhance the reusability of sulfuric acid for the isobutane alkylation about 4-fold compared to that of non-SFILs. Hopefully this work will provide insights into the screening and designing of new isobutane alkylation catalysts based on sulfuric acid and SFILs.