Tubular MFI zeolite membranes made by secondary (seeded) growth

Zeolite MFI membranes with thickness 15-20 μm were grown on the surface of macroporous α-alumina and stainless steel support tubes (pore size 200 and 500 nm, respectively) by the secondary (seeded) growth technique. The tubular supports were dipped vertically in an aqueous suspension of colloidal silicalite-1 seed crystals (particle size 100 nm) and withdrawn at a speed of 1-2 cm/h to allow for uniform formation of seed layers on the outer (for stainless steel) or inner (for α-alumina) cylindrical surface of the support tubes. The seeded tubes were treated hydrothermally with clear solutions of different composition and the resulting MFI membranes were characterized by SEM and permeation of butane isomers. For the best preparations, binary n-butane/isobutane ratios at 22°C were as high as 28 and 53 for stainless steel and α-alumina tubes, respectively, while the n-butane permeation flux varied in the range 0.8-3.8 mmol m^-2 s^-1. The effect of temperature and feed partial pressure on permeation flux was also studied for binary mixtures of butane isomers and was found similar to that of planar membranes developed previously on α-alumina support disks by the same synthesis procedure.