Molecular simulation of size-selective gas adsorption in idealised carbon nanotubes

Abstract

Molecular simulations were used to examine the adsorption of diatomic molecules (nitrogen and oxygen) and similarly sized gases (argon and methane) in pores with van der Waals diameters similar in size to the gas diameters. Idealised carbon nanotubes were used to model generic pores, to better understand the effect of pore diameter on guest adsorption in the absence of defects, specific adsorption sites, or variations in pore diameter that often complicate studies of gas adsorption in other porous materials. Molecular dynamics simulations of open nanotubes show that argon and methane are able to enter tubes whose diameters are slightly smaller than the gas diameters. Diatomic gases are able to enter tubes that are significantly smaller than their kinetic diameters with the molecular axis aligned parallel to the nanotube. The results indicate that size-selective adsorption of these gases is theoretically possible, although differences in pore diameters of only a few tenths of an Angstrom are required. Grand canonical Monte Carlo simulations of a 3.38 Å nanotube indicate significant uptake by argon and oxygen, but not nitrogen or methane. The adsorption of nitrogen and methane gradually increases as the nanotube diameter approaches 4.07 Å , and all gases fully saturate a 4.54 Å nanotube. Of the nanotubes studied, the largest adsorption enthalpy for any gas corresponds to the 4.54 Å nanotube, with significantly lower enthalpies seen in the 5.07 Å nanotube. These results suggest an ideal pore diameter for each gas based on the gas-pore van der Waals interaction energies. Trends in the ideal diameter correlate with the minimum tube diameter accessible to each gas.

Monte Carlo Simulations to Examine the Role of Pore Structure on Ambient Air Separation in Metal–Organic Frameworks

“…48 The enhanced selectivities result from a slightly larger increase in adsorbent−adsorbate interaction for O 2 . A direct comparison of our results to those of Greathouse et al 35 in terms of heat of adsorption shows that the MOFs studied here are still well outside the range in which substantially improved selectivity would be expected. The heat of adsorption of both gases in their largest diameter nanotube (5.07 Å) is 15−16 kJ/mol, slightly larger than the range of 14−15 kJ/mol observed in our simulations with VWAA pore diameters near 5.97 Å.…”

“…The VWAA pore diameter, as defined in this paper, was suggested to represent the pore size distribution in a single parameter to establish a clear correlation with adsorptive properties and to relate the work of Greathouse et al on idealized CNTs 35 to more complex pore size distributions. We showed improved selectivity in MOFs with VWAA pore diameters less than 8 Å going from 1.1 to 1.3 at a diameter of 5.97 Å.…”

“…Although this is only about a 20% increase compared to unenhanced MOFs, the principle can be exploited to further improve the selectivity. The selectivity toward oxygen is expected to improve even more dramatically with pore sizes ranging from 3.38 to 4.50 Å based on the heat of adsorption results from Greathouse et al, 35 so this should be the target for future work in developing O 2 -selective materials based on this mechanism.…”

“…The similarity of the interactions can be rationalized by the fact that adsorbates come in contact with the organic ligands on the surface of SMS MOFs. The work of Greathouse et al is significant in this discussion because it shows the enhanced adsorption of O 2 /N 2 from van der Waals interactions in simple CNT systems with pore sizes near the diameter of these diatomic species. However, the complex pore size distributions of real adsorbent materials and the additional interactions that may come into play in MOFs make it difficult to correlate the impact of pore size on adsorption properties of these materials.…”

Molecular dynamics simulation of methane transport in confined organic nanopores with high relative roughness

“…The formation of two distinct layers of adsorbed methane is observed in both the density profiles and density maps. The total thickness of the two adsorption layers is ∼8 Å, which is approximately twice the size of the simulated methane molecule (Greathouse et al, 2015;Wang et al, 2016a). The distance between the first density maximum and the wall is ∼3.5 Å, which corresponds to the hard sphere radius of our coarse-grain methane, as determined by the Lennard-Jones interaction (Wang et al, 2018).…”