Permeation of single gases and gas mixtures through faujasite-type molecular sieve membranes

Composite membranes that consist of thin faujasite layers on the surfaces of porous Y-Al2O3 support disks were prepared by using a two-step synthesis. First, a seed layer of externally synthesized Na-Y nanocrystals was deposited on the surface of the support, Second, the polycrystalline faujasite layer was grown hydrothermally. Characterization by X-ray diffraction, field emission-scanning electron microscopy and energy dispersive X-ray spectroscopy showed dense intergrown FAU layers with Si/Al ratios of 1.3-1.8 and layer thicknesses in the range of 0,8-6 mum depending on the synthesis conditions. Two different kinds of heating were used in the second synthesis step, either a conventional oven or a microwave oven. The synthesis conditions like crystallization time and temperature as well as the kind of heating influenced the permeation properties of the supported faujasite membranes. The thicker the FAU layers, the lower were the permeances of the single gases which were found to be of the order H-2 > CH4 > N-2 > O-2 > CO2 > nC(4)H(10) > SF6 at 23 degreesC, The separation factors a of equimolar binary gas mixtures were measured at 23 degreesC. For the gas mixture N-2/CO2 an alpha of 8.4 was found. for the mixture CH4/CO2 alpha was 3.5. From these permeation characteristics one can draw the conclusion that the separation of these small gas molecules by the large and heteropolar FAU (Na-X) pore system with 0.74 nm pore opening diameters is mainly based on differences of adsorption or diffusion rates, rather than on size exclusion like in LTA- or MFI-type zeolite membranes. (C) 2002 Elsevier Science Inc. All rights reserved.