Molecular Dynamics Simulations for Loading-Dependent Diffusion of CO₂, SO₂, CH₄, and Their Binary Mixtures in ZIF-10: The Role of Hydrogen Bond

Abstract

The loading-dependent diffusion behavior of CH₄, CO₂, SO₂, and their binary mixtures in ZIF-10 has been investigated in detail by using classical molecular dynamics simulations. Our simulation results demonstrate that the self-diffusion coefficient D_i of CH₄ molecules decreases sharply and monotonically with the loading while those of both CO₂ and SO₂ molecules initially display a slight increase at low uptakes and follow a slow decrease at high uptakes. Accordingly, the interaction energies between CH₄ molecules and ZIF-10 remain nearly constant regardless of the loading due to the absence of hydrogen bonds (HBs), while the interaction energies between CO₂ (or SO₂) and ZIF-10 decease rapidly with the loading, especially at small amounts of gas molecules. Such different loading-dependent diffusion and interaction mechanisms can be attributed to the relevant HB behavior between gas molecules and ZIF-10. At low loadings, both the number and strength of HBs between CO₂ (or SO₂) molecules and ZIF-10 decrease obviously as the loading increases, which is responsible for the slight increase of their diffusion coefficients. However, at high loadings, their HB strength increases with the loading. Similar loading-dependent phenomena of diffusion, interaction, and HB behavior can be observed for CH_4, CO₂, and SO₂ binary mixtures in ZIF-10, only associated with some HB competition between CO₂ and SO₂ molecules in the case of the CO₂/SO₂ mixture.