Simulations of binary mixture adsorption of carbon dioxide and methane in carbon nanotubes: Temperature, pressure, and pore size effects

Grand canonical Monte Carlo (GCMC) simulations were performed to investigate the adsorption behavior of an equimolar CO₂/CH ₄ mixture in carbon nanotubes (CNTs). Five CNTs [(6, 6), (7, 7), (8, 8), (9, 9), and (10, 10)] with diameters varying from 0.678 to 1.356 nm, seven temperatures (283, 293, 303, 313, 323, 333, 343 K), and seven pressures (1, 5, 10, 15, 20, 25, 30 MPa) were chosen to investigate the effect of temperature, pressure, and pore size on the adsorption behavior. The results show that the CNTs have a preferential adsorption of CO₂ in the binary CO ₂/CH₄ mixture. For pore size effect on the adsorption behavior, we found that the adsorption of CO2 is much larger than that of CH₄ in the same CNT and that the CO₂ adsorption in the CNTs increases dramatically with an increase of CNT's diameter, whereas the absolute amount of adsorbed CH₄ changes little with the CNT's pore size. In the investigated temperature and pressure ranges, we observed that in the CNTs with diameters less than 1.1 nm, the temperature and pressure have little effect on the adsorption behavior of the binary mixture, whereas in larger CNTs the adsorption behavior changes with temperature and pressure significantly. In addition, the CNTs demonstrated a higher selectivity of CO₂ than other materials (activated carbons, zeolites 13X, and metal-organic frameworks) reported in the literature. For example, in (6, 6) CNT at 343 K and 1 MPa, the selectivity reaches 11.2, larger than that reported in activated carbon. The selectivity in narrow CNTs (<1 nm in diameter) is fluctuating with temperature and pressure, but it remains almost the same in larger CNTs at the investigated temperature and pressure.