Computational Study of Hexamethylguanidinium Lactate Ionic Liquid: A Candidate for Natural Gas Sweetening

Abstract

Considering the technological and economical importance of natural gas sweetening, new methods have to be developed to replace the aqueous amine processes currently used in most of the gas processing industry, with the objective of improving the treatment of sour gases, avoiding the problems arising from the amine technologies to fulfill the present technological, economical, and environmental requirements of the natural gas industry. Ionic liquids have been proposed in the literature as a suitable alternative for sour gas capture through physical/chemical absorption mechanisms. Guanidinium/lactate ionic liquids have showed adequate absorption ability, leading to low Henry’s constants, for both CO₂ and H₂S. Moreover, this ionic liquid shows very favorable economical, technological, and environmental properties that would make it suitable for sour gas absorption on a large scale. Nevertheless, the mechanism of the interaction between the considered acid compounds and the ionic liquid is still not fully understood. Therefore, we report in this work a computational study, using classical molecular dynamics methods, to characterize the properties and molecular level structure of hexamethylguanidinium lactate ionic liquid, [HMG]LAC, pure and after absorption of CO₂, H₂S, and CH₄, to infer the mechanism of interaction between these gases and the ionic liquid, leading to the absorption of these compounds, and to analyze the effect of absorbed gases on the ionic liquid structure from a molecular viewpoint. Results show a strong interaction between CO₂, H₂S, and the LAC anion, arising from the presence of hydroxyl and carboxylate groups, which govern the absorption of these gases on the studied ionic liquid. The reported results should stimulate further studies, including accurate experimental measurements, on the absorption ability of the considered ionic liquid and the related family of compounds