Rapid Transport of Gases in Carbon Nanotubes
Physical Review Letters
We report atomistic simulations for both self-and transport diffusivities of light gases in carbon nanotubes and in two zeolites with comparable pore sizes. We find that transport rates in nanotubes are orders of magnitude faster than in the zeolites we have studied or in any microporous material for which experimental data are available. The exceptionally high transport rates in nanotubes are shown to be a result of the inherent smoothness of the nanotubes. We predict that carbon nanotube
... arbon nanotube membranes will have fluxes that are orders of magnitude greater than crystalline zeolite membranes. Single walled carbon nanotubes (SWNTs) have many potential applications as molecular sieves, membranes, sensors, and "nanopipes" for the precise delivery of gases or liquids        . As with all microporous materials, molecular transport rates inside SWNTs may have a large impact on the usefulness of these materials  . If molecular transport through SWNTs is slow, their usefulness may be severely constrained. Here we show using atomistic simulations that diffusion rates of light gases inside defect-free SWNTs are orders of magnitude higher than in crystalline microporous zeolites with similar pore sizes. This observation applies to both the self-diffusion of individual molecules and to the transport diffusivity, the quantity that describes macroscopic diffusion. We compare gas fluxes through a model SWNT membrane with experimental and theoretical results for analogous zeolite membranes and find that SWNTs membranes may exhibit exceptionally high fluxes. Since nanotubes are predicted to have high selectivity for separating gases , our predictions indicate that a nanotube membrane would have both extremely high selectivity and flux, a long-standing goal of membrane technology  . We used atomistic simulations to predict the diffusive transport rates of CH 4 and H 2 in a range of SWNTs. Computer simulations of adsorption phenomena in SWNTs have proved to be a useful complement to experimental studies, providing insight into novel structures and dynamics of adsorbed H 2 O, for example [1, 10] . In this study we compare our SWNT results to analogous calculations for two siliceous zeolites: silicalite and ZSM-12. Transport of gases in silicalite has been widely studied both experimentally and with atomistic simulations, so this system aids us in estimating how closely our simulations mimic reality. ZSM-12 has roughly the same pore size as silicalite but has the same unidimensional pore topology as SWNTs, so it provides an excellent probe for the effect of pore dimensionality on transport. Zeolites such as these two are already widely used in many practical applications  , so comparing the proper-ties of SWNTs to them provides a practical benchmark. Each adsorbent was simulated as a rigid structure. Silicalite and ZSM-12 were fixed in their known crystallographic structures  , the former in its orthorhombic form  . Both zeolites were modeled with chemical composition SiO 2 . The pore diameters of these zeolites are approximately 0.8 nm (atom to atom)  . Two different nanotubes were considered, namely, the (10,10) and (6,6). The (10,10) and (6,6) nanotubes have diameters of 1.36 and 0.81 nm, respectively (atom to atom). Thus, the (6,6) nanotube has a very similar diameter to the zeolites considered here, while the (10,10) nanotube is slightly larger. We consider only adsorption inside the SWNTs, not adsorption in the interstices or the external surfaces of nanotube bundles. CH 4 and H 2 were each treated as spherical particles with pairwise Lennard-Jones interactions between adsorbates and the O (C) atoms in the zeolites (SWNTs). Potential parameters for the adsorbed species were taken from the literature [13, 14] . Predicted adsorption isotherms and self-diffusivities for CH 4 using this model for silicalite are in quantitative agreement with experimental measurements [12, 15] . The same potential parameters were used in our ZSM-12 simulations. The fluid-nanotube potentials were based on the potential parameters for fluid-graphite interactions, which reproduce the experimental CH 4 -and H 2 -graphite isotherms and isosteric heats [16, 17] . We first computed the equilibrium adsorption isotherms for single-component H 2 and CH 4 in (10,10) SWNTs, silicalite, and ZSM-12 using standard grand canonical Monte Carlo techniques . These three materials have similar volumetric adsorption capacities, as can be seen from the computed isotherms in Fig. 1 . The adsorption of CH 4 in silicalite has been measured in multiple experiments and our computed isotherm is in quantitative agreement with experimental results [13, 15] . Several different quantities are commonly used to describe single-component diffusion  .