A diffusion study of small hydrocarbons in DDR zeolites by micro-imaging

Intracrystalline diffusion of the light hydrocarbons methane, ethane, ethylene and propylene in three different species of zeolite DDR has been investigated by recording the transient concentration profiles during uptake and release. The measured profiles approximate the form of the appropriate solution of the diffusion equation for radial diffusion in an infinite cylinder. This is as expected since, for an ideal DDR crystal with its 2D pore structure, there should be no flux in the axial direction. Over the range of guest molecules considered, the diffusivities are found to decrease, with increasing critical molecular diameter, over more than three orders of magnitude, i.e. over about one order of magnitude on comparing ethane, ethylene and ethane, and over, additionally, more than two orders of magnitude for propylene. The observed diffusivities are found to be in reasonable agreement (with differences of up to a factor of three) between the three different DDR specimens as well as with values from the literature, obtained mainly from transient macroscopic measurement or microscopic PFG NMR self-diffusion measurements. In the present study, all measurements were performed by the same experimental technique (micro-imaging by interference microscopy), and the derived diffusivities provide a coherent picture of the diffusion process.