Solvation Thermodynamic Properties of Hydrogen Sulfide in [C₄mim][PF₆], [C₄mim][BF₄], and [C₄mim][Cl] Ionic Liquids, Determined by Molecular Simulations

Abstract

Removal of hydrogen sulfide (H2S) and acid gases from natural gas is accomplished by absorption processes using a solvent. The gas solubility in a liquid can be used to measure the degree of removal of the gas and is quantified by the Henry's constant, the free energy of solvation at infinite dilution, or the excess chemical potential. In this work, Henry's constants and thermodynamic properties of solvation of H2S were calculated in three ionic liquids: [C4mim][PF6], [C4mim][BF4], and [C4mim][Cl] ([C4mim], 1-butyl-3-methyl imidazolium). The first step in this work was the evaluation of the force fields for the gas and condensed phases in order to obtain accurate values for the excess chemical potential for H2S on each ionic liquid using free energy perturbation techniques. In the H2S-[C4mim][PF6] and H2S-[C4mim][BF4] systems, the results obtained by molecular simulation agree with the experimental values reported in the literature. However, the solvation free energy calculated for the H2S-[C4mim][Cl] system can be considered predictive because of the lack of experimental data at the simulated conditions. Based on these results, the best solvent for removing H2S is [C4mim][Cl] because it has the highest affinity for this species (lowest value of the Henry's constant). Also, solvation thermodynamic properties such as enthalpy and entropy were calculated in order to evaluate their contribution to the free energy of solvation.

Hydrogen Sulfide Capture: From Absorption in Polar Liquids to Oxide, Zeolite, and Metal–Organic Framework Adsorbents and Membranes

“…With the aid of molecular simulations, deeper insights can be gained about the entropic and enthalpic contributions of H 2 S solubilities in different ILs. Recently, Sánchez-Badillo and co-workers computed solvation thermodynamics of H 2 S in ILs and demonstrated qualitative agreement with experimental data . These calculations showed that, at room temperature, the absolute solvation enthalpy is 2 times higher than the absolute entropic contribution for [bmim + ]­[PF 6 – ] and [bmim + ]­[BF 4 – ] and 3 times higher for [bmim + ]­[Cl – ].…”

Excess chemical potential of thiophene in [C4MIM] [BF4, Cl, Br, CH3COO] ionic liquids, determined by molecular simulations

“…43 In this work, the excess chemical potential of thiophene in a series of imidazolium-based ILs was calculated using classical MD simulations. The ILs studied were selected based on toxicity effects, 27,[44][45][46] 24,47,48 due to its low viscosity 49 and large commercial availability 50 compared with other ILs. Previous experimental and electronic structure calculations reported in the literature, suggested that the extraction of aromatic sulfur compounds in ILs, is related to p-stacking interactions between rings and sulfur-hydrogen interactions with the cation's alkyl chain.…”

Hydrophobic protic ionic liquids tethered with tertiary amine group for highly efficient and selective absorption of H2S from CO2

“…Furthermore, the low heat capacity of ILs (normally lower than 2 J·g −1 ·K −1 ) can potentially reduce the energy consumption of desorption process . A number of literature have reported the solubilities of H 2 S and CO 2 in various normal ILs as physical absorbents, showing that the Henry's law constants of H 2 S in normal ILs are generally in the range of 2∼4 MPa and the ideal selectivities of H 2 S/CO 2 are generally in the range of 2∼4 …”