Proof of the ISS-concept for LTA and FAU membranes and their characterization by extended gas permeation studies

Abstract

Molecular sieve membranes are expected to separate molecules by size and shape provided that the intercrystalline defect transport is negligible. However, in the case of a non-negligible mass transport through the grain boundaries in the polycrystalline zeolite layer, the separation properties of the membrane become influenced by both the regular and defective transport. Nevertheless, also in this case proper separation performance is found due to adsorptive and/or diffusive interactions of the mixture components with the membrane. In a previous report [M. Noack, P. Kölsch, A. Dittmar, M. Stöhr, G. Georgi, R. Eckelt, J. Caro, Micropor. Mesopor. Mater. 97 (2006) 88] the concept of crystal intergrowth supporting substances (ISS) was demonstrated. The charge neutralization of the negatively charged crystal surfaces of a growing MFI membrane by the positively charged ISS molecules improves the crystal intergrowth and results in a higher permeation selectivity. LTA and FAU membranes can separate water–organic mixtures in an excellent way but they fail in shape-selective gas separations. Therefore, many attempts were made to improve the separation properties of LTA and FAU membranes for gases. Zeta potential measurements on the Al-rich zeolite crystals LTA and FAU show a strong negative surface charge like it was found in [Noack et al., 2006] for MFI. In this work LTA and FAU multi-layer membranes were prepared by using an ISS. These membranes were characterized by SEM, XRD, permporosimetry, single gas permeation in combination with different probe molecules for the selective blocking of the micro and mesopore system. The results are discussed on the basis of pore condensation or pore narrowing according to the Kelvin and Halsey equations. Repeated strong de-watering of the hydrophilic LTA and FAU membranes have changed the membrane structure.