Influence of Surface Groups on the Diffusion of Gases in MCM-41: A Molecular Dynamics Study

The diffusion of binary mixtures of CO2 and N2 in a series of functionalized MCM-41 materials was studied using molecular dynamics (MD) simulations. Transport diffusion coefficients were obtained in order to assess the effect of surface functionalization with organic groups. Unmodified MCM-41 was found to be transport selective for N2. The spatial distribution of the two coadsorbed species showed that N2 molecules permeated faster and occupied the central pore region to a greater extent than CO2 molecules. CO2 was found to have a much lower permeability than N2 due to stronger interactions with the surface of MCM-41 and a tendency to become trapped in nooks in the amorphous pore wall. This effect was not reproduced when using a more simplistic model pore with a smooth surface demonstrating the importance of using a realistic pore model. Larger aminophenyl surface groups were found to significantly reduce the N2 selectivity due to a decrease in the flux of N2 and an increase in the flux of CO2. We explain ...