Permeation through a single crystal of Zeolite NaX

Steady state permeation through a 120 μm single crystal of zeolite NaX has for the first time been observed. Each crystal was imbedded in an epoxy film, polished to expose opposite faces to feed and permeate gases, and mounted in an apparatus with pmol/s permeation rate sensitivity. Permeation was constrained to occur by an intracrystalline diffusion mechanism, confirmed by temperature dependence and selectivity measurements. Rates decreased at higher temperatures due to opposing sorption and diffusion effects. Intracrystalline butane diffusivity at 25°C was 3 x 10^?4 cm²/sec, in agreement with other methods. Anomalously long time lags for approach to steady state are explained by a transition between two states of the zeolite, differing in permeability by 10²-10⁴. The state with lower permeability was selectively permeable to isobutane from a methaneísobutane mixture.