(568e) Correlation of Zeolite Membrane Defect Fluxes and Zeolite Unit Cell Parameters

Changes in zeolite unit cells upon molecular adsorption have been reported since the mid-1900's. However, relating this phenomenon to zeolite membranes has not been reported until recently. Studies using permporosimetry measurements, a method used to determine the amount of flow through membrane defects, found that n-hexane, n-pentane, n-butane, propane, and SF₆ decreased the flux through defects in MFI membranes by orders of magnitude, indicating that the zeolite crystals expanded upon adsorption of all of these molecules. In contrast, the helium flux through defects increased when low concentrations of i-butane was in the gas phase, and this increased flux was attributed to contraction of the MFI crystals, and thus enlargement of the membrane defects.

In this study we correlate in-situ powder XRD measurements with permporosimetry data from zeolite membranes. X-ray diffraction measurements showed that MFI zeolite unit cells change when i-butane, SF₆, and n-hexane molecules adsorb at loadings less than saturation at room temperature. For example, low i-butane loadings contracted the MFI crystals and higher loadings expanded the structure. The percent volume change measured by XRD correlated with the increase and decrease of helium flux measured by permporosimetry for zeolite membranes. Measurements were also made on SAPO-34, a CHA zeolite structure, with carbon dioxide and methanol.