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) (H2tbip = 5-tert-butyl isophthalic acid), a representative of metal-organic frameworks, show an inflection at a loading, qi = 6 molecules per unit cell. This inflection causes the inverse thermodynamic factor, 1/Γi, to display a minimum at qi = 6, along with a maximum at qi ≈ 10. Molecular Dynamics (MD) simulations of the self-diffusivity Di,self, taking account of the framework flexibility of Zn(tbip) show that the Di,self − qi dependence follows that of 1/Γi − qi, with a minimum at qi = 6. Remarkably, MD simulations assuming a rigid framework yield significantly lower values of the self-diffusivities, and show a monotonic decrease of Di,self with qi.