Insight into the molecular mechanism that controls the solubility of CH4 in ionic liquids

The solubility of methane (CH4) in ionic liquids (ILs) is required in order to develop processes involving CH4, such as methane conversion and CO2/CH4 separation from natural gas or biogas processes. Nevertheless, the solubility of CH4 in ILs is still very rarely achieved and, consequently, fundamental knowledge about the factors that govern the solubility are still poorly understood. Therefore, this work aims to extend the solubility data of CH4 in various ILs and to gain some insights into the factors at a molecular level that play a role in the solubility process through experimental and computational modelling using Conductor-like Screening Model for Real Solvent (COSMO-RS). The solubility of CH4 in 17 commercial ILs was measured experimentally at four different temperatures (298.15 to 343.15 K) and pressures up to 8 MPa. The large number of ILs studied allows the study of the impact of the cation and anion head group and the alkyl chain length on the solubility of CH4. From the experimental solubility data collected, Henry's law constant (KH) values were calculated. The results show that the solubility of CH4 increases with decreasing temperature and increasing pressure. The solubility of CH4 can also be enhanced by increasing the alkyl chain length of the IL cation or anion. Despite the inability of COSMO-RS to make quantitative predictions, the model is able to predict accurately the impact of the IL cation head group, anion, and alkyl chain length on the solubility of CH4. Good correlation between the electrostatic – misfit energy, HE,MF, of CH4 and the experimentally calculated KH values was obtained (R2 = 0.932). This correlation indicates, for the first time, that the electrostatic – misfit energy arising from the repulsive interaction of CH4 plays a dominant role in determining its solubility in ILs. In addition, it is shown that the IL size and van der Waals forces only have marginal influences on the solubility of CH4. The experimental and computational modelling results in this work could pave the way to designing ILs as a medium for gas absorption and separation involving CH4.