Diffusion of Binary CO2/CH4 Mixtures in the MIL-47(V) and MIL-53(Cr) Metal-Organic Framework Type Solids: A Combination of Neutron Scattering Measurements and Molecular Dynamics Simulations

The dynamics of CO2 and CH4 in a mixture of different compositions has been explored in two metal-organic frameworks, namely, MIL-47(V) and MIL-53(Cr), by combining molecular dynamics (MD) simulations and quasi-elastic neutron scattering (QENS) measurements. The experimental and simulated self-diffusion coefficient (D-s) values for CH4 are in very good agreement in the whole range of the CO2 explored loadings. It is clearly stated that CH4 which shows a fast diffusivity at low loading becomes significantly slower in both metal-organic frameworks (MOFs) when CO2 molecules are introduced within the porosities of these materials. Further, compared to its behavior in a single component, CH4 tends to diffuse slightly faster in the presence of CO2. The MD simulations revealed that this speeding up is concomitant with a mutual speeding up or a slowing down of the slower CO2 molecules in MIL-47(V) and MIL-53(Cr), respectively. Analysis of the MD trajectories emphasizes that both gases in the mixture follow individually a 1D-type diffusion mechanism in both MOFs, where the CO2 molecules diffuse close to the pore wall while the motions of CH4 are restricted in the central region of the tunnel.