Molecular dynamics simulation of phosphonium-based ionic liquids for natural gas sweetening

Sour natural gas (SNG) contains significant amounts of H2S, CO2, and other sulfur-containing components which need to be reduced to the required level suitable for safe transportation and usage. In this work, we conducted molecular dynamics simulations of five Phosphonium-based Ionic Liquids (PILs) with a formulated gas mixture containing 80% CH4, 10% CO2, 5% H2S, and 5% N2, reflecting a typical composition of SNG. The simulation was performed at 300 K and 50 bar for 100 ns and five PILs studied were tetrabutylphosphonium acetate ([P4444][ACE]), tetrabutylphosphonium formate ([P4444][FOR]), tetrabutylphosphonium methyl carbonate ([P4444][MTC]), tetrabutylphosphonium dimethylphosphate ([P4444][DMP]), and tetrabutylphosphonium diethylphosphate ([P4444][DEP]). Analysis of radial distribution functions (RDFs) indicated that PILs have a higher affinity for and absorption capacity of acid gases over CH4. Among all the simulated PIL+SNG systems, [P4444][FOR]+SNG has the highest absolute value of the electrostatic energy of -47,564.2 kJ mol-1 and the lowest absolute value of van der Waals energy of -21,447.8 kJ mol-1 both at 300 K and 50 bar. As a result, [P4444][FOR] has the lowest CH4 absorption capacity of 0.128 mol/mol and the highest acid gas selectivity of 7.750 mol/mol and 9.523 mol/mol for CO2 and H2S, respectively. [P4444][ACE] has the highest acid gas absorption capacity of 1.181 mol/mol for CO2 and 1.373 mol/mol for H2S. This result showed that a carboxylate functional group enhances acid gas absorption capacity while higher electrostatic energy favors their selectivity. The self-diffusivity of CH4 in the PILs is higher than that of CO2 or H2S. This manifested an unfavorable interaction between the PILs and CH4, leading to its low absorption capacity. This study demonstrated that PILs with a carboxylate functional group on the anion are preferable over those with a phosphate group in formulating sour natural gas sweetening absorbents.