DIFFUSION MEASUREMENTS THROUGH EMBEDDED ZEOLITE CRYSTALS

For the first time, a macroscopic method was used to measure directly diffusivity of fast diffusing species such as methane in silicalite. A form of Wicke-Kallenbach technique was applied to measure intracrysta lline diffusivity. The technique uses a single embedded zeolite crysta l as a membrane and a mass-selective detector to determine the transie nt mass response and hence the diffusion flux passing through the memb rane. A concentration-difference driving force is used rather than a p ressure-difference driving force. The diffusivities calculated from bo th adsorption and desorption transients were in excellent agreement, t hus providing a check for the self-consistency of the data. The diffus ivities of carbon dioxide, methane, ethane, propane and butane in sili calite between 30 and 70 degrees C have been compared with those from the literature. The diffusivities reported here are between those meas ured with microscopic methods and those measured with other macroscopi c methods.