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

Abstract

Configurational-Bias Monte Carlo simulations of the adsorption isotherm of ethane in Zn(tbip) (H₂tbip = 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/Γ_i, to display a minimum at q_i = 6, along with a maximum at q_i ≈ 10. Molecular Dynamics (MD) simulations of the self-diffusivity D_i,self, taking account of the framework flexibility of Zn(tbip) show that the D_i,self - q_i dependence follows that of 1/Γ_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,self with q_i.