Composite membrane
Composite membranes comprising at least one porous substrate layer and at least one microporous separation layer, comprising at least one zeolite of the MFI type, are described, the separation layer being produced by a hydrothermal synthesis in which the molar ratio of silicon to aluminum is greater than 120 and the separation layer contains less than 10% by weight of aluminum in elemental or chemically bound form in a zone of at least 100 nm adjacent to the separation layer.
1. A process for the production of a composite membrane comprising at least one porous substrate and at least one porous separation layer wherein the separation layer contains at least one zeolithe of the MFI type and the molar ratio of silicon to aluminum in the separation layer is greater than 120 and wherein the substrate contains less than 10% by weight of aluminum in elemental or chemically bound form in a zone of at least 100 nm adjacent to the separation layer and the substrate material has an asymmetrical structure, comprising the following process steps:
(a) hydrothermal synthesis of a synthesis solution, in which the molar ratio of silicon to aluminum is greater than 120, on a substrate by bringing the substrate into contact with the synthesis solution over a period of from 1 to 100 hours at from 100 to 250° C.,
(b) washing of the membrane resulting from process step (a) with water or an acidic solution for a period of from 5 to 120 minutes,
(c) drying of the membrane at from 5 to 40° C. over a period of from 1 to 100 hours in the presence of a flowing or stationary gas,
(d) calcination of the membrane at a heating rate of from 0.1 to 1 K/min to a temperature of 200 to 600° C., residence being effected at the final temperature for from 30 to 500 minutes and then cooling being effected at a rate of 0.1 K/min to 10 K/min.
2. A process according to claim 1 , wherein the substrate material used in process step (a) is subjected, before the hydrothermal synthesis, to a seeding step in which a layer of seed particles which at least partly covers that side of the substrate which is to be coated is applied to said side.
3. A process according to claim 1 , wherein, in the hydrothermal synthesis of the synthesis solution on the substrate, access of the synthesis solution to a surface not to be coated or to surfaces of the substrate which are not to be coated coated is substantially prevented by a medium which is present in the pores of the substrate and on the surface not to be coated or the surfaces not to be coated.