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 membranes will have fluxes that are orders of magnitude greater than crystalline zeolite membranes.

Molecular simulations using standard force fields have been carried out to model the adsorption of various light gases on a number of different metal organic framework-type materials. The results have been compared with the available experimental data to test the validity of the model potentials. We observe good agreement between simulations and experiments for a number of different cases and very poor agreement in other cases. Possible reasons for the discrepancy in simulated and measured isotherms are discussed. We predict hydrogen adsorption isotherms at 77 and 298 K in a number of different metal organic framework materials. The importance of quantum diffraction effects and framework charges on the adsorption of hydrogen at 77 K is discussed. Our calculations indicate that at room temperature none of the materials that we have tested is able to meet the requirements for on-board hydrogen storage for fuel cell vehicles. We have calculated the volume available in a given sorbent at a specified adsorption energy (density of states). We discuss how this density of states can be used to assess the effectiveness of a sorbent material for hydrogen storage.

Correlation effects in diffusion of CH4 and CF4 in MFI zeolite have been investigated with the help of molecular dynamics (MD) simulations and the Maxwell−Stefan (M−S) formulation. For single-component diffusion, the correlations are captured by the self-exchange coefficient ; in the published literature this coefficient has been assumed to be equal to the single-component M−S diffusivity, Đ i . A detailed analysis of single-component diffusivity data from MD, along with published kinetic Monte Carlo (KMC) simulations, reveals that /Đ i is a decreasing function of the molecular loading, depends on the guest−host combination, and is affected by intermolecular repulsion (attraction) forces. A comparison of published KMC simulations for diffusion of various molecules in MFI, with those of primitive square and cubic lattices, shows that the self-exchange coefficient increases with increasing connectivity. Correlations in CH4/CF4 binary mixtures are described by the binary exchange coefficient ; this exchange coefficient has been examined using Onsager transport coefficients computed from MD simulations. Analysis of the MD data leads to the development of a logarithmic interpolation formula to relate with the self-exchange coefficient of the constituents. The suggested procedure for estimation of is validated by comparison with MD simulations of the Onsager and Fick transport coefficients for a variety of loadings and compositions. Our studies show that a combination of the M−S formulation and the ideal adsorbed solution theory allows good predictions of binary mixture transport on the basis of only pure component diffusion and sorption data.

We have used atomistic simulations to examine the adsorption isotherms, self diffusivity, and transport diffusivity of seven light gases, CH 4 , CF 4 , He, Ne, Ar, Xe, and SF 6 , adsorbed as single-components in silicalite at room temperature. By using equilibrium molecular dynamics, the self and transport diffusivities are computed simultaneously. For each species the self diffusivity decreases as pore loading is increased due to steric hindrance from other adsorbed molecules. In contrast, the transport diffusivity is an increasing function of pore loading for each species. Our results are the most extensive collection of transport diffusivities determined from atomistic modeling of adsorption in a zeolite to date, and they allow us to examine the accuracy of several common approximations to the loading-dependent diffusivities. Carefully converged results for the anisotropy of diffusion of CH 4 , CF 4 , He, Ne, Ar, Xe, and SF 6 in silicalite are presented. We discuss the implications of our results for understanding self and transport diffusivities in mesoporous materials and for multi-component mixtures in microporous materials.

We have used equilibrium molecular dynamics (EMD) to study the influences of pore shape and connectivity on single component diffusion of several gases in silica zeolites using atomically detailed models of these materials. Results are presented for CH4, CF4, SF6, Ar, and Ne in silicalite, CH4, Ar, and Ne in ITQ-3, CH4, CF4, Ar, and SF6 in ITQ-7, and CH4, CF4, Ar, and H2 in ZSM-12 at room temperature. This set of four silica zeolites includes one, two, and three-dimensional pore topologies and pore volumes of several different shapes. EMD can be used to simultaneously determine the self-diffusivities and corrected diffusivities as a function of pore loading, and this has been done for every example. In combination with adsorption isotherms computed using grand canonical Monte Carlo, EMD results can also determine the transport diffusivity as a function of pore loading. The resulting transport diffusivities are reported for every example. The broad data set presented here is useful for considering the variety of diffusion behaviors that can occur for small molecules adsorbed in zeolite pores.

We have used atomically detailed simulations to examine the adsorption and transport diffusion of CO2 and N2 in single-walled carbon nanotubes at room temperature as a function of nanotube diameter. Linear and spherical models for CO2 are compared, showing that representing this species as spherical has only a slight impact in the computed diffusion coefficients. Our results support previous predictions that transport diffusivities of molecules inside carbon nanotubes are extremely rapid when compared with other porous materials. By examining carbon nanotubes as large as the (40,40) nanotube, we are able to compare the transport rates predicted by our calculations with recent experimental measurements. The predicted transport rates are in reasonable agreement with experimental observations.

The class of coordination polymers known as metal-organic frameworks (MOFs) has three-dimensional porous structures that are considered as a promising alternative to zeolites and other nanoporous materials for catalysis, gas adsorption, and gas separation applications. In this paper, we present the first study of gas diffusion inside an MOF and compare the observed diffusion to known behaviors in zeolites. Using grand canonical Monte Carlo and equilibrium molecular dynamics, we calculate the adsorption isotherm and self-, corrected, and transport diffusivities for argon in the CuBTC metal-organic framework. Our results indicate that diffusion of Ar in CuBTC is very similar to Ar diffusion in silica zeolites in magnitude, concentration, and temperature dependence. This conclusion appears to apply to a broad range of MOF structures.

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