Single-walled carbon nanotubes: efficient nanomaterials for separation and on-board vehicle storage of hydrogen and methane mixture at room temperature?

Abstract

We investigate possible usage of single-walled carbon nanotubes (SWNTs) as an efficient storage and separation device of hydrogen-methane mixtures at room temperature. The study has been done using Grand Canonical Monte Carlo simulations for modeling storage and separation of hydrogen-methane mixtures in idealized SWNTs bundles. These mixtures have been studied at several pressures, up to 12 MPa. We have found that the values of the stored volumetric energy and equilibrium selectivity greatly depend on the chiral vector (i.e., pore diameter) of the nanotubes. The bundle formed by [5,4] SWNTs (nanotube diameter of 6.2 A) can be regarded as a threshold value. Below that value the densification of hydrogen or methane is negligible. Bundles with wider nanotube diameter (i.e., 12.2, 13.6, 24.4 A) seem to be promising nanomaterials for hydrogen-methane storage and separation at 293 K. SWNTs with pore diameters greater than 24.4 A (i.e., [18,18]) are less efficient for both on-board vehicle energy storage and separation of hydrogen-methane mixture at 293 K with pressures up to 12 MPa. SWNTs comprised of cylindrical pores of 8.2 and 6.8 A in diameter (equivalent chiral vector [6,6] and [5,5], respectively) are the most promising for separation of the hydrogen-methane mixture at room temperature, with the former selectively adsorbing methane and the latter selectively adsorbing hydrogen. We observed that inside the pores of [6,6] nanotubes absorbed methane forms a quasi-one-dimensional crystal when the system has thermalized. The average intermolecular distance of such a crystal is smaller than the one of liquid methane in bulk at 111.5 K, therefore exhibiting the quasi-one-dimensional system clear compression characteristics. On the other hand, for a smaller nanotube diameter of 6.8 A the hydrogen can enter into the tubes and methane remaining in bulk. We found that in the interior of [5,5] nanotubes adsorbed/compressed hydrogen forms a quasi-one-dimensional crystal.