Membrane

Details Membrane Membrane

1997-03-14

1999-08-10

Fortuna, Ana

Liquid purification or separation

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Material

21050026, 2105021, 264 451, 264 48, 264 49, 96 4, B01D 3900, B29D 6500

Patent

active

059354402

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to an improved membrane and a process for producing improved membranes.
It is well known to use zeolites and similar crystalline zeolitic materials in separations and as catalysts. Zeolitic membranes and membranes which incorporate zeolites are also well known and can come in a range of different types. European Patent Application 0481660 discloses and discusses prior art zeo-type membranes and refers in particular to U.S. Pat. Nos. 3,244,643, 3,730,910 and 4,578,372, Applied Catalysts 49(1989) 1-25, DE-A-3827049, CA1235684, JP-A-63287504, JP-A-63291809, EP-A-180200, EP-A-135069.
It is disclosed in EP 0481 658 Al to surface coat a porous support on which a zeo-type material is deposited by crystallisation from a synthesis gel with a surface coating of nickel cobalt or molybdenum in the form of the metal and/or oxide.
The methods disclosed for forming this surface coating include vapor deposition, vacuum evaporation, Rf sputtering or electroplating or deposition of a salt from a liquid and oxidising the salt. These methods give a surface coating on the porous support of varying thicknesses and provide for improved crystal deposition from the gel.sub.-- mainly round the wires of the support with little improvement in filling the voids after a single growth. EP 0481 659 Al discloses a similar process for pre-treating a porous support, except that the porous support is pre-treated with an acid.
The process of EP 0481660 Al discloses treating the porous support a plurality of times with the synthesis gel and crystalising a zeo-type material from the gel in order to obtain an improved coating. However, this process can leave gel and other debris blocking the pores of the porous support and thus prevent complete coverage, even wiping between zeolite growths will leave debris behind.
However, these processes do not produce a defect free membrane.sub.-- without a plurality of growths and they do not bridge the voids after a single growth, and even though these patent applications disclose repeated retreatment with the gel as being required to block "pin holes", these existing methods have not proved successful.
Even small defects or pinholes can have a marked deleterious effect on the performance of membranes and can render them of substantially little value in many operations. This is because in many separation operations the effect of defects is essentially to provide a channel where the unseparated products can pass.
Some existing methods claim that a defect free membrane is obtained on a laboratory scale, but attempts to provide a substantially defect free membrane on a larger scale have proved unsuccessful.
In order to provide an improved membrane with better performance characteristics, we have devised a treatment for such membranes.
According to the invention, there is provided a process for treating a membrane comprising a film of a crystalline zeo-type material which process comprises treating the membrane with a silicic acid and/or polysilicic acid or a mixture of silicic and/or poysilicic acids.
Zeo-type materials are also known as molecular sieves which are widely known and used. They comprise an extended network of channels formed from silicon/oxygen tetrahedrons joined through the oxygen atoms. Zeolites and alumino-silicates are the most commonly known form of zeo-type materials and the present invention is applicable to any membrane formed from zeo-type materials and particularly applicable to zeolites and alumino-silicates. In the "Atlas of Zeolite Structure Types", Meier and Ofsen, 1987, Polycrystal Book Service, Pittsburg USA, various types of structure are described and, for example, those described as having LTA, MEL, MFI or TON structure can be used.
In "New Developments in Zeolite Science and Technology Proceedings of the 7th International Conference, Tokyo, 1986, page 103, another class of zeo-type materials are disclosed as crystalline aluminophosphate, silicoalumina phosphates and other metalloalumino phosphates.
Typical zeolites which can be used in the presen

REFERENCES:
patent: 5019263 (1991-05-01), Haag Et al.
patent: 5362522 (1994-11-01), Barri et al.
patent: 5716527 (1998-02-01), Deckman et al.
High Permselectivities of Microporous Silica-Modified Y-Alumina Membranes, Journal of Material Science Letters--1989.

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