Unprecedentedly high selective adsorption of Xe/Kr mixtures in carbon nanotubes: A molecular simulation study

By using grand canonical Monte Carlo (GCMC) simulations, we investigated the adsorption of Xe/Kr mixtures with the mole fractions of Xe ranging from 10% to 90% in single-walled carbon nanotubes (CNTs) having diameters of 0.81 to 2.03 nm, at different temperatures. For the (mole ratio, 20/80) Xe/Kr mixture, the Xe/Kr selectivity decreases with diameter of the CNT at 298 K, while demonstrating a complex dependency on pressure in CNTs. Due to the single-file adsorbate configuration, the smaller (6, 6) CNT achieves a higher adsorption capacity than the (7, 7) CNT. At 1 bar and 298 K, the adsorption of Xe and the Xe/Kr selectivity achieve their maxima, 1.69 mol/kg and 36.3, in the (6, 6) CNT among the CNTs considered, and this optimized selectivity is much superior compared to a large number of metal-organic frameworks (MOFs). We find that increasing temperature generally reduces the adsorptions of Xe and Kr, with an exception that the co-adsorption of Kr is enhanced in the small CNTs for obtaining additional adsorption volume released by the reduced adsorption of Xe. While increasing temperature reduces the Xe/Kr selectivity in all the CNTs, the selectivity decreases with the bulk concentration of Xe in the (6, 6) and (7, 7) CNTs and remains almost independent of Xe concentration in the larger CNTs, at 1 bar. As revealed by the performance coefficient, the (6, 6) CNT holds the best performance on the adsorptive separation of Xe/Kr in all the cases considered in this work. Our findings reveal that the (6, 6) CNT has a great potential on preventing the uncontrolled emission of Kr while facilitating the commercial usage of Xe.