Z.A.E.P. Vroon

Transport properties of alkanes through ceramic thin zeolite MFI membranes

Polycrystalline randomly oriented defect free zeolite layers on porous -Al2O3 supports are prepared with a thickness of less than 5 μm by in situ crystallisation of silicalite-1. The flux of alkanes is a function of the sorption and intracrystalline diffusion. In mixtures of strongly and weakly adsorbing gases and a high loadings of the strongly adsorbing molecule in the zeolite poze, the flux of the weakly adsorbing molecule is suppressed by the sorption and the mobility of the strongly adsorbing molecule resulting in pore-blocking effects. The separation of these mixtures is mainly based on the sorption and completely different from the permselectivity. At low loadings of the strongly adsorbing molecules the separation is based on the sorption and the diffusion and is the same as the permselectivity. Separation factors for the isomers of butane (n-butane/isobutane) and hexane (hexane/2,2-dimethylbutane) are respectively high (10) and very high (> 2000) at 200°C. These high separation factors are a strong evidence that the membrane shows selectivity by size-exclusion and that transport in pores larger than the zeolite MFI pores (possible defects, etc) can be neglected.

Two component permeation through thin zeolite MFI membranes

Two component permeation measurements have been performed by the Wicke–Kallenbach method on a thin (3 μm) zeolite MFI (Silicalite-1) membrane with molecules of different kinetic diameters, dk. The membrane was supported by a flat porous -Al2O3 substrate. The results obtained could be classified in six regimes based on differences in: • Occupation degrees, θ, on the external surface. • Occupation degrees, θ, in the zeolite pores. • Mobilities in the zeolite pores. High separation factors are obtained for mixtures of “weakly/strongly adsorbing” gases and for mixtures of “small/large” gases. Separation by size-exclusion is demonstrated for 8 kPa n-C6H14/16 kPa 2,2-dimethylbutane (>600, T=298–473 K) and 0.31 kPa p-xylene/0.26 kPa o-xylene (>200, T=400 K). At low θ (high temperature, low pressures) the permeation of both components in a mixture equals the single-gas permeation of these components. At high θ (low temperature and high pressure) the permeation of both components in mixtures is different from single-gas permeation values. The single-gas permeance of “small” molecules with kinetic diameter dk <0.45 nm ranges from 1×10−8 to 15×10−8 mol m−2 s−1 Pa−1 in the temperature range 298–473 K. The permeation decreases with increasing dk of the molecules.

Preparation and characterization of thin zeolite MFI membranes on porous supports

Thin (2–7 μm) polycrystalline randomly-oriented zeolite MFI membranes were prepared on an -Al2O3 support by one or more subsequent hydrothermal treatments. Different particle sizes (275–700 nm) in the layer were achieved by changing the synthesis temperature (371–459 K). Zeolite membranes, prepared by two subsequent hydrothermal treatments have an optimum quality. Membranes prepared by a single hydrothermal treatment have defects and zeolite membranes obtained by three or more hydrothermal treatments become too thick and crack during the removal of the template. The zeolite MFI membranes were characterised by permeation measurements using single gases and mixtures of n-butane (n-C4H10) and iso-butane (i-C4H10). Permselectivities, F, and separation factors, , for n-C4H10/i-C4H10 decrease strongly with an increasing number of defects and hence provide a good indication of membrane quality. The separation, on the other hand, of a CH4/n-C4H10 mixture at high occupation of n-C4H10 is not much affected by small defects. This is ascribed to blocking of those defects by capillary condensation of the butane. Flux values decrease linearly with increasing zeolite crystallite size in the top layer indicating that the effective membrane thickness is only one particle thick. During hydrothermal treatment zeolite particles also grow at the other side of the support but do not decrease significantly the flux through the supported zeolite membrane.

Permeation of single gases in thin zeolite MFI membranes

The permeation of a series of gases with widely different Lennard-Jones kinetic diameters and sorption properties is investigated as a function of feed pressure (up to 100 kPa) and temperature (298–473 K) with two different methods. The membrane system studied consists of an MFI (silicalite) top-layer (thickness 3 μm) directly grown on an -alumina support. Flux values of molecules with a kinetic diameter, dk, <0.45 nm (H2, CO2 and n-alkanes (CH4 to n-butane)) are in the range of 1.5–15 mmol m−2 s−1. Size exclusion is found for o-xylene (dk=0.68 nm) and 2,2-dimethylbutane (dk=0.60 nm). Flux values are obtained for iso-butane, SF6 and benzene, intermediate between those for small molecules and those for molecules with size exclusion. Maxima in the curve of flux versus temperature are found for gases with non-linear Langmuir-type adsorption curves as a function of pressure. The temperature of maxima at 100 kPa pressure increases in the order of CO2