Experimental and simulation study on the capture and separation of CO2/CH4 by alkali metal complex ionic liquid

Solutions of [Bmim][BF4] ionic liquid (IL) mixed with three alkali metal tetrafluoroborate salts, i.e. LiBF4, NaBF4 and KBF4, were used to capture and separate CO2/CH4 at 303.15, 313.15 and 323.15 K under different pressures ranged from 500 to 3000 kPa. CO2 solubility in three alkali metal doped solutions decreases with the increase of temperature and decreasing pressure. High concentration of alkali metal salts does not show a very improvement for CO2 absorption, and even reduce CO2 capture instead in the presence of high concentration of LiBF4 in ionic liquid. Unexpectedly, KBF4 has the least solubility in [Bmim][BF4] and LiBF4 dissolved most in [Bmim][BF4] ionic liquid. On the contrary, the solubility of metal complex ionic liquids to CH4 decreased after adding alkali metal salts of any concentration. 0.01 mol.L-1 KBF4-IL showed the best CO2/CH4 solubility selectivity at 303.15 K, up to 54.5. On a basis of Quantum chemical calculations, binding energies between metal-doped ionic liquids and CO2/CH4 were calculated, following an order of KBF4-IL-CO2 > NaBF4-IL-CO2 > LiBF4-IL-CO2 and LiBF4-IL-CH4 > NaBF4-IL-CH4 > KBF4-IL-CH4, which is consistent with the experimental results. [BF4](-) anions are approaching the metal ions and the larger metal cations, i.e. Na+ and K+, have longer distances with the anions than Li+, which explains the reason of reduced CO2 solubility when Li+ becomes more. Generally, high con-centration of metal ions in ionic liquids are inferior for CO2 absorption because of the association of MBF4 with the ionic liquid. Therefore, the results of this paper can lay a theoretical foundation for CO2/CH4 capture separation.