MOLECULAR SIMULATION OF PURE AND MIXED GASES OF CO_2, H_2, AND Ar PHYSICALLY ABSORBED IN THE IONIC LIQUID l-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyI)imide (hmim Tf_2N)

摘要

Understanding the absorption mechanism of H_2 and CO_2 in ionic liquids is highly important for rational design of supported ionic liquid membranes used for CO_2 pre-combustion capture applications. Recently, Noble et al. [1] and Maurer et al. [2] have determined experimentally the H_2 solubility in [hmim][Tf_2N],but the two sets of results present significant differences among each other. In this study, we used osmotic continuous fractional component (CFC) Monte Carlo simulations and Gibbs CFC technique [3]; to calculate the absorption isotherms of CO_2, Ar and H_2 gases and the selectivity of the mixed H_2/Ar gases. These simulation techniques have been shown to predict solubility in quantitative agreement with experiments for many solute gases such as CO_2, N_2,O_2, SO_2, NH_3, andH_2O [4]. Our simulations show that the H_2 solubility in [hmim][Tf_2N] is in good agreement with the experiment data obtained by Maurer et al. [2], but differ significantly from the results obtained by Noble et al. [1]. Energy analysis indicates that the interaction between H_2 and the cation/anion pair is about six times smaller than that between CO_2 and the same cation/anion system. Additionally, the heat of mixing for H_2 absorption in [hmim][Tf_2N] is very small, and in some instances it is even positive, implying that H_2 absorption in this ionic liquid is endothermic. The absorption enthalpy at infinite dilution was found to be 3.0 ± 0.6 kJ/mol from simulations, close to the experimental value of 4.09 土 0.05 kJ/mol obtained by Maurer et al. [2], but significantly different from the experimental one of 16.6 土 1.5 kJ/mol obtained by Noble et al. [1]. Overall, our simulations suggest that the molar free volume of the ionic liquid is the determining factor in establishing the H_2 solubility. We also studied the effect of H_2 polarizability, different H_2 molecular models, and different bond force constants on the H_2 solubility. It was found that the polarizability and the bond force constants have a small effect, generally less than 10 % on the H_2 solubility. The Ar gas has been widely used in the feeding and as a sweeping gas in the supported ionic liquid membranes [5]. Our simulations indicate that Ar has a higher solubility, about three times larger than H_2 in [hmim][Tf_2N]. The energy analysis shows that Ar interaction with the cation/anion pair is about three times larger than that between H_2 and the same cation/anion pair. When the solubility of H_2 and Ar are appreciable, we observed that H_2 solubility in the mixed gas was decreased by about 20% relative to the solubility of pure H_2 at the same temperature and fugacity conditions, due to the presence of Ar. This effect is due to the competing filling for the available free molar volume of the ionic liquid between H_2 and Ar.