Jinchen Liu

Testing the accuracy of correlations for multicomponent mass transport of adsorbed gases in metal-organic frameworks: diffusion of H2/CH4 mixtures in CuBTC.

Mass transport of chemical mixtures in nanoporous materials is important in applications such as membrane separations, but measuring diffusion of mixtures experimentally is challenging. Methods that can predict multicomponent diffusion coefficients from single-component data can be extremely useful if these methods are known to be accurate. We present the first test of a method of this kind for molecules adsorbed in a metal-organic framework (MOF). Specifically, we examine the method proposed by Skoulidas, Sholl, and Krishna (SSK) ( Langmuir, 2003, 19, 7977) by comparing predictions made with this method to molecular simulations of mixture transport of H 2/CH 4 mixtures in CuBTC. These calculations provide the first direct information on mixture transport of any species in a MOF. The predictions of the SSK approach are in good agreement with our direct simulations of binary diffusion, suggesting that this approach may be a powerful one for examining multicomponent diffusion in MOFs. We also use our molecular simulation data to test the ideal adsorbed solution theory method for predicting binary adsorption isotherms and a method for predicting mixture self-diffusion coefficients.

The importance of charge–quadrupole interactions for H2 adsorption and diffusion in CuBTC

We have assessed the effects of charge–quadrupole interactions (CQIs) between the framework atoms of copper(II) benzene-1,3,5-tricarboxylate (CuBTC) and adsorbed H2 molecules on equilibrium adsorption properties and self- and transport diffusivities. We have also compared charges computed from periodic density functional theory (DFT) on the fully periodic CuBTC structure with charges derived from DFT calculations on cluster models of CuBTC. Our results indicate that carboxylate group atom charges computed from plane wave periodic DFT with the Bader charge analysis formalism are not consistent with the charges computed from the ChelpG method from Gaussian-based DFT cluster calculations. The charges derived from Bader analysis seem to be too large. Adsorption isotherms computed from Monte Carlo simulations and diffusivities computed from molecular dynamics simulations indicate that CQIs have a substantial impact on equilibrium and transport properties of H2 adsorbed in CuBTC at 77 K. Conversely, both adsorption isotherms and diffusivities were shown to be essentially unaffected by CQIs at 298 K.

One-dimensional adsorption and diffusion in Zn(tbip)

We have used grand canonical Monte Carlo simulations to calculate the adsorption isotherms for H2, CH4, Xe and CF4 in Zn(tbip), a metal organic framework material having narrow 1D pores. Simulations show that Xe and CF4 form ordered solid structures when adsorbed in the pores. We have computed the self and transport diffusivities for H2, CH4, Xe and CF4 in Zn(tbip) using equilibrium molecular dynamics simulations. H2 has a diffusivity about one to two order of magnitude higher than CH4, indicating that transport selectivity for H2 over CH4 may be high. Xe and CF4 have very low diffusivities, in the order of 10− 9–10− 8 cm2/s. We have measured experimental adsorption isotherms for H2 at 77 and 298 K for pressures up to 60 bar in Zn(tbip). The H2 isotherms predicted from simulations are in reasonably good agreement with the experiments.

Adsorption and Diffusion of Fluids in Defective Carbon Nanotubes: Insights from Molecular Simulations

Single-walled carbon nanotubes (SWNTs) have been shown from both simulations and experiments to have remarkably low resistance to gas and liquid transport. This has been attributed to the remarkably smooth interior surface of pristine SWNTs. However, real SWNTs are known to have various defects that depend on the synthesis method and procedure used to activate the SWNTs. In this paper, we study adsorption and transport properties of atomic and molecular fluids in SWNTs having vacancy point defects. We construct models of defective nanotubes that have either unrelaxed defects, where the overall structure of the SWNT is not changed, or reconstructed defects, where the bonding topology and therefore the shape of the SWNT is allowed to change. Furthermore, we include partial atomic charges on the SWNT carbon atoms due to the reconstructed defects. We consider adsorption and diffusion of Ar atoms and CO2 and H2O molecules as examples of a noble gas, a linear quadrupolar fluid, and a polar fluid. Adsorption isotherms were found to be fairly insensitive to the defects, even for the case of water in the charged, reconstructed SWNT. We have computed both the self-diffusivities and corrected diffusivities (which are directly related to the transport diffusivities) for each of these fluids. In general, we found that at zero loading that defects can dramatically reduce the self- and corrected diffusivities. However, at high, liquidlike loadings, the self-diffusion coefficients for pristine and defective nanotubes are very similar, indicating that fluid-fluid collisions dominate the dynamics over the fluid-SWNT collisions. In contrast, the corrected diffusion coefficients can be more than an order of magnitude lower for water in defective SWNTs. This dramatic decrease in the transport diffusion is due to the formation of an ordered structure of water, which forms around a local defect site. It is therefore important to properly characterize the level and types of defects when accurate transport diffusivities are needed.