Coating processes – Foraminous product produced – Filter – sponge – or foam
Reexamination Certificate
1998-09-25
2002-05-07
Beck, Shrive P. (Department: 1762)
Coating processes
Foraminous product produced
Filter, sponge, or foam
C427S245000, C427S299000, C427S387000, C427S397700, C427S419200, C427S419700, C427S419800
Reexamination Certificate
active
06383563
ABSTRACT:
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 A1 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 vapour 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 gelmainly round the wires of the support with little improvement in filling the voids after a single growth. EP 0481 659 A1 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 A1 discloses treating the porous support a plurality of times with the synthesis gel and crystallising a zeo-type material from tinc 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 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 tree 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 imp roved 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 silicon or other metal compound capable of reacting with the membrane either before, after or instead of the treatment of the membrane with a silicic acid or polysilicic acid as set out in patent application PCT/GB95/00956.
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 materialsare disclosed as crystalline aluminophosphate, silicoalumina phosphates and other metallo-alumino phosphates.
Typical zeolites which can be used in the present invention are include but are not limited to, 3A, 4A, 5A, 13X, X, Y, ZSM5, MPOs, SAPOs, Silicalite, &bgr; or theta or theta-1, etc.
The porous supports on which zeo-type membranes are formed and which can be used in the present invention include those formed of metals, ceramics, glass, mineral, carbon or polymer fibres or cellulosic or organic or inorganic polymers. Suitable metals include titanium, chromium and alloys such as those sold under the Trade Marks “Fecralloy” and “Hastalloy” and stainless steels. The porous supports may be formed of a mesh or from sintered metal particles or a mixture of both. These are commonly sold in the form of filters.
Porous ceramics, glass mineral or carbon materials can be used including porous silicon and other carbides, clays and other silicates and porous silica. If desired, the support can be a zeolite formed by compression or using a binder, or by the conversion of meta kaolin to a zeolite. The shape of the support is not critical, for example, flat sheet, tubular, wound spiral, etc. can be used. If polymeric materials are used, these can optionally be film coated with metal or metal oxide or a silicic acid as herein defined.
The porous support can be also be a granular solid e.g. formed of particles of a closely packed material such as a pellitised catalyst.
The present invention can be used with porous supports of any suitable size although, for large flux rates through a membrane, large pore sizes are preferred. Preferably pore sizes of 0.01 to 2,000 microns, more preferably of 0.01 to 200 and ideally of 0.01 to 5 microns are used. Pore sizes up to 300 microns can be determined by bubble point pressure as specified in ISO 4003. Larger pore sizes can be measured by microscopic methods. The larger the relative amount of the surface which is composed of voids in general the more suitable the porous support.
The membranes which can be treated by the method of the present invention can be formed by any method, for example by crystallisation from a gel or solution, by plasma deposition or by any other method such as electrodeposition of crystals on conducting substrates e.g. as described in DE 4109037 or the conversion of meta kaolin to a zeolite.
When the membrane comprising a film of zeo-type material is prepared by crystallisation from a synthesis gel, any of the methods described in the prior art can be used.
The synthesis gel used in the process can be any gel which is capable of producing the desired crystalline zeo-type material. Gels for the synthesis of zeo-type materials are well known and are described in the prior art given above or, for example, in EP-A-57049, EP-A-104800, EP-A-2899 and EP-A-2900. Standard text books by D W Breck (“Zeolites Molecular Sieves, Structure Chemistry and Use”) published by John Wiley (1974) and P. A Jacobs and J. A Martens (Studies in Surface Science and Catalysis No. 33, Synthesis of High Silica Alumino silicate Zeolites” published by Elsevier (1987), describe many such synthesis gels. The process which can be used includes conventional syntheses of zeo-type materials, except that the synthesis is carried out in the presence of the porous support. Most commonly, gels are crystallised by the application of heat.
The membrane which is treated by the process of the invention can be prepared by a process which comprises deposition or crystallisation from a Larger pore medium. In one embodiment of the invention the growth medium can be used in two different methods. In the gel method (method 1) for forming the membrane the gel used to form the membrane preferably has a molar composition in the range of
(1.5-3.0)Na
2
O:(1)Al
2
O
3
:(2.0)SiO
2
:(50-200)H
2
O
and the method used can be used in any of the methods disclosed in the references listed above. In the liquid solution method (method 2) the liquid solution used to form the membrane preferably has a molar composition in
Bratton Graham John
Buck Karon Doreen
De Villiers Naylor Timothy
Beck Shrive P.
Crockford Kirsten A.
Smart (Isle of Man) Limited
Wenderoth , Lind & Ponack, L.L.P.
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