Coating processes – Foraminous product produced – Microporous coating
Reexamination Certificate
1999-12-02
2002-10-29
Spitzer, Robert H. (Department: 1724)
Coating processes
Foraminous product produced
Microporous coating
C055S524000, C055SDIG005, C096S010000, C096S011000, C502S064000
Reexamination Certificate
active
06472016
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a novel process for preparing a membrane comprising a porous carrier and a layer of a molecular sieve, as well as to novel membranes. The invention applies to filtration or gas or liquid fluid separation, pervaporation, reverse osmosis or catalysis.
Zeolite membranes constituted by a (macro)porous portion and a zeolite are already known. These materials can be obtained principally by two methods: a method employing a gel and a method employing a (colloidal or oligomeric) solution, such methods comprising several steps. First, a film (gel method) or total impregnation (solution method) is formed, such film or total impregnation containing species able to form a zeolite, following which the zeolite is crystallized under hydrothermal conditions.
These two types of process suffer from major drawbacks.
In both cases, the starting pH during the first step is extremely high. This highly basic pH is not compatible with certain ceramic materials. In effect, the gamma alumina currently employed as the carrier layer for the zeolite is soluble in highly basic media, leading to alumina solubilization in the zeolite precursor, consequently leading to chemical contamination of the zeolite, the alumina having penetrated the desired crystalline structure.
In both cases, the use of large amounts of gel or zeolite precursor solution and the poorly synthesized yield render this process expensive, particularly when structuring agents of the quaternary ammonium type are employed (and all the more so as several synthesis cycles are frequently necessary).
In the case of the gel method, it is difficult to guarantee homogeneity of the gel composition as the gel is formed from different constituents which do not mix homogeneously. As the local composition of the gel varies, the characteristics of the zeolite structure vary and membrane performance is modified. This defect is additionally clearly recognized in European Patent Application 0,481,660, which indicates that spot defects are present. Thus, European Patent Application 0,778,076 discloses production of the gel in situ; the porosity of the carrier is filled with a first solution after which the carrier is brought into contact with a second solution which is immiscible with the first one. Gelification occurs locally at the contact of the two solutions, the gel being essentially formed at the surface of the porous carrier. Gelification modifies the compositions of the solutions and consequently it is not possible to guarantee an identical gel at every point in the porous carrier.
In the article “Characterization and Permeation Properties of ZSM-5 Tubular membranes”, AIChE Journal, July 1997, Vol. 43, No. 7, Coronas et al. studied the influence of the carrier on zeolite layer deposition. Two asymmetric membranes were tested, one with a layer of 5 nm pore diameter &ggr;-alumina and the other with a layer of &agr;-alumina of pore diameter 0.2 &mgr;m. The method used by Coronas et al. is a gel method. Coronas et al. conclude that it is easier to form a continuous zeolite layer on an &ggr;-alumina type carrier (5 nm) than on an &agr;-alumina type carrier (0.2 &mgr;m), which, in the latter case, necessitates repetition of the process.
Supplementary deposition-crystallisation cycles are in fact always necessary in the case of gel processes for improving the quality of the membrane and for thus obtaining a product which effectively allows separation. The zeolite layer obtained by the gel methods is consequently in point of fact a multi-layer.
Furthermore, because of their high viscosity, the gels block channels of a diameter which can reach several millimeters. This technique is consequently reserved for flat structures or tubes of considerable inside diameter. Thus, all the examples in European Patent Application 0,778,076 employ plane-surface carriers as well as the majority of the examples in European Patent Application 0,481,660, example 12 of this application employing tubes with an inside diameter of about 6.5 mm. Now, the use of a ceramic carrier of tubular geometry (whether this be single- or multi-channel) where the channels are of significant diameter, or of flat geometry, does not make it possible to obtain filtration modules or gas separation modules which are highly compact, in other words which have a large filtering surface compared to the space they occupy. Indeed, it is accepted that the compactness for plane-membrane modules is of the order of 150 m
2
/m
3
, while that of multi-channel membrane modules only reaches 300 m
2
/m
3
; these degrees of compactness are very low when compared to those required for gas separation applications.
In the case of methods employing a solution as in international application WO-A-9529751, it is also stated that the nucleation of the zeolite, previously necessary for its formation, cannot be done for volumes the characteristic dimension of which is greater than about 10 microns and/or less than 5 nm. According to that document, it is consequently impossible to obtain nucleation and growth outside a specific porous material. This consequently rules out the formation of layers whether this be inside or outside the tube, as well as for tubes in macroporous carriers, the mean pore diameter of which is for example higher than 10 microns.
Additionally, the solution method in international application WO-A-9529751 involves impregnation throughout the total porous volume (having a suitable dimension), and consequently the zeolite occupies the totality of the carrier and is not precisely localized (for example in the form of a layer). This absence of localization is prejudicial to the efficiency of the composite material at the time of its use; it is perfectly known that gas permeability through a zeolite membrane is linked to the thickness of the zeolite. The thicker the zeolite, the more permeability diminishes for a separation efficiency, which is not affected.
The solution provided in EP-A-0674939 is similar to the one disclosed in WO-A-9529751.
Thus, the formation, using a gel method, of a zeolite layer on a carrier (for example of around 0.2 &mgr;m pore diameter) requires the gel method to be repeated. A solution method, according to WO-A-9529751 does not produce a zeolite layer on the carrier, but in the latter, to the exclusion of a layer thereon, and does not make layer formation possible in or on the carrier, for pore diameters greater than 10 microns.
One consequently looks for materials having a zeolite layer, notably at the inner channels of a multi-channel carrier, this layer requiring to be homogeneous both from a chemical point of view as well as from a physical point of view, in the form of a unitary defect-free layer, the preparation requiring additionally to be simple and economical.
None of the documents cited above offers a solution, nor teaches or suggests the present invention.
SUMMARY OF THE INVENTION
The present invention discloses a solution for overcoming these disadvantages.
According to a first aspect, the invention offers new products as well as a novel production method.
Consequently, the invention provides a membrane comprising a homogeneous porous carrier having a pore diameter comprised between 5 nm and 20 &mgr;m, on which a zero-defect unitary layer of a molecular sieve is deposited.
In one preferred embodiment, the unitary layer is a single layer.
In a further preferred embodiment, the thickness of the layer of a molecular sieve is comprised between 1 and 100 &mgr;m, for example between 50 nm and 2 &mgr;m, for example between 3 and 50 &mgr;m.
The molecular sieve is preferably a zeolite.
According to a preferred embodiment, the carrier has a pore diameter comprised between 5 nm and 10 &mgr;m and preferably between 50 nm and 2 &mgr;m.
In one embodiment, the carrier is a ceramic carrier.
In a further embodiment, the carrier is a fiber, for example a multi-channel fiber. The layer of molecular sieve can be arranged on the outside of the fiber, or the layer of molecular sieve can be arranged inside the channel or channels
Chanaud Philippe
Soria Raymond
Anderson Kill & Olick
Lieberstein Eugene
Meller Michael N.
Societe des Ceramiques Techniques
Spitzer Robert H.
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