Microimaging of Transient Intracrystalline Concentration Profiles during Two-Component Uptake of Light Hydrocarbon–Carbon Dioxide Mixtures by DDR-Type Zeolites

We report here the application of interference microscopy to study intracrystalline diffusion in a binary adsorbed phase. Hitherto the application of interference microscopy to the study of intracrystalline diffusion has been limited to single-component systems since only for a single-component system is the measured phase shift directly related to the concentration. However, when the difference in diffusivity between the components is sufficiently large, the faster component reaches equilibrium on a time scale that is short compared with the diffusion time for the slower component. Under these conditions, which are fulfilled for the system studied (CO2-C2H6 in a large crystal of zeolite DDR), it is possible to determine the transient concentration profiles for both components and hence to determine the influence of the fast component on diffusion of the slower species. It is shown that measurements of this type offer a possible approach for experimental determination of the diffusion matrix. For the system studied, diffusion of ethane was found to be essentially unaffected by the presence of CO2, in conformity with the results of an earlier study carried out by the zero length column technique.