Transport properties of alumina-supported MFI membranes made by secondary (seeded) growth

Zeolite MFI membranes with thicknesses in the range of 1-40 μm have been prepared by hydrothermal growth of colloidal silicalite-1 seed crystals of particle sizes ~ 100 nm deposited on the surface of α-Al₂O₃ porous support disks. These membranes exhibit attractive separation characteristics as judged by permeation measurements with butane and xylene isomers. At 22°C, the permeation flux of n-butane and the n-butane/isobutane ratio from 50 kPa/50 kPa binary mixtures vary in the range of 1.0-4.0 mmol m^-2s^-1 and 30-70, respectively. The n-butane flux exhibited a dependence on temperature and feed partial pressure consistent with activated diffusion of an adsorbed phase obeying Langmuir isotherm but was rather insensitive to the membrane thickness. For the best membranes, the single-component p-xylene flux and the p-xylene/o-xylene flux ratio at 100°C was in the range of 10-30 μmol m^-2s^-1 and 30-130, respectively. The permeation flux of o-xylene increases dramatically in the presence of p-xylene resulting in a substantial loss in paralortho selectivity (maximum of ~5), an effect that may be attributed to the flexibility of the MFI framework in the presence of molecules with a close fit to its framework. At 100°C paralortho selectivities (>20) are observed when the p-xylene feed partial pressure is below 0.05 kPa. (C) 2000 Elsevier Science B.V. All rights reserved.