Molecular dynamics investigation of the self-diffusion of binary mixture diffusion in the metal organic framework Zn (tbip) accounting for framework flexibility

The self-diffusion of three equimolar mixtures in the metal-organic framework Zn(tbip) has been investigated by molecular dynamics simulations. The simulations take due account of lattice flexibility. The diffusional characteristics are discussed in relation to molecule properties and lattice geometry. The results show that Zn(tbip) may be a useful material for separating methane/ethane and CO2/ethane mixtures at low concentrations, and CO2/methanol mixtures at high concentrations

Investigating the reasons for the significant influence of lattice flexibility on self-diffusivity of ethane in Zn(tbip)

Published molecular dynamics simulations of the self-diffusivity, DS, of ethane within the one-dimensional 4.5 Å channels of Zn(tbip) (H2tbip = 5-tert-butyl isophthalic acid) have shown not only quantitative, but also qualitative, differences in the DS values for fixed and flexible lattices when the concentration of molecules, c, is increased. The reasons for these differences are investigated with the aid of probability density plots, free energy landscapes and barriers, along with a determination of the structural changes accompanying increasing c. It is found that for flexible lattices, the tighter, more constrained parts of the channels become wider at higher c; this allows more molecules to diffuse in the central region of the channels

A Molecular Dynamics investigation of the influence of framework flexibility on self-diffusivity of ethane in Zn(tbip) frameworks

Configurational-Bias Monte Carlo simulations of the adsorption isotherm of ethane in Zn(tbip) (H(2)tbip = 5-tert-butyl isophthalic acid), a representative of metal-organic frameworks, show an inflection at a loading, q(i) = 6 molecules per unit cell. This inflection causes the inverse thermodynamic factor, 1/Gamma(i), to display a minimum at q(i) = 6, along with a maximum at q(i) approximate to 10. Molecular Dynamics (MD) simulations of the self-diffusivity D-i,D-self, taking account of the framework flexibility of Zn(tbip) show that the D-i,D-self - q(i) dependence follows that of 1/Gamma(i) - q(i), with a minimum at q(i) = 6. Remarkably, MD simulations assuming a rigid framework yield significantly lower values of the self-diffusivities, and show a monotonic decrease of D-i,D-self with q(i)