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

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.