Modeling the adsorptive selectivity of carbon nanotubes for effective separation of CO₂/N₂ mixtures.

Canonical Monte Carlo (CMC) simulations were carried out to investigate the behavior of CO(2) and N(2) mixtures upon adsorption on single walled carbon nanotubes (CNTs). In the simulation, all the particle-particle interactions between CO(2), N and C were modeled using Lennard-Jones (LJ) potential. To provide deep insight into the effect of pore width, temperature, pressure and bulk composition on the adsorption behavior of CO(2) /N(2) mixtures, five different CNTs [(6,6), (7,7), (8,8) (9,9) and (10,10) CNT] with diameters ranging from 0.807 to 1.35 nm, three temperatures (300 323 and 343 K), six pressures (0.15, 2, 4, 6, 8 and 10 MPa), and three bulk mole compositions of carbon dioxide (0.3 0.5 and 0.7) were tested. The results from all the simulation conditions investigated in this work show that CNTs preferentially adsorb carbon dioxide relative to nitrogen in a binary mixture. The results are consistent with the hypothesis that stronger interaction of one component with the nanotube surface results in a higher adsorption capacity compared to the other component. An optimized pore size of D = 8.07 nm corresponding to (6, 6) CNT, at T = 300 K and P = 0.15 MPa at a bulk mole composition of y(CO2) =0.3 was identified in which carbon nanotubes demonstrate the greatest selectivity for separation of carbon dioxide relative to nitrogen. In addition, it is worth pointing out that, under similar simulation conditions, CNTs exhibit higher selectivity compared to other carbon-based materials [carbon membrane polyimide (PI) and PI/multi-wall carbon nanotubes (MWCNTs)] for CO(2) adsorption. As a prototype, the selectivity of an equimolar mixture of CO(2) /N(2) for adsorption on (6, 6) CNTs at 300 K and 0.15 MPa reaches 9.68, which is considerably larger than that reported in carbon membrane. Therefore, it can be concluded that carbon nanotubes can act as a capable adsorbent for adsorption/desorption of CO(2) in comparison with other carbon-based materials in the literature.