Molecular sieves and processes for their manufacture

Details Molecular sieves and processes for their manufacture Molecular sieves and processes for their manufacture

1999-01-13

2000-06-13

Spitzer, Robert H.

Gas separation: processes

Selective diffusion of gases

Selective diffusion of gases through substantially solid...

96 11, 55524, B01D 5322, B01D 7102

Patent

active

06074457&

DESCRIPTION:

BRIEF SUMMARY
This invention relates to molecular sieves, especially to crystalline molecular sieves, and to layers containing them. More especially, the invention relates to a layer, especially a supported layer, containing a crystalline molecular sieve and a structure comprising such a layer.
Molecular sieves find many uses in physical, physicochemical, and chemical processes, most notably as selective sorbents, effecting separation of components in mixtures, and as catalysts. In these applications, the crystallographically-defined pore structure within the molecular sieve material is normally required to be open, and it is then a prerequisite that any structure-directing agent, or template, that has been employed in the manufacture of the molecular sieve be removed, usually by calcination.
Numerous materials are known to act as molecular sieves, among which zeolites form a well-known class. Examples of zeolites and other materials suitable for use in the present invention will be given below.
In our earlier International Application WO 94/25151, the disclosure of which is incorporated by reference, we have described an inorganic layer comprising optionally contiguous particles of a crystalline molecular sieve, the mean particle size being within the range of from 20 nm to 1 .mu.m on a substrate, which is advantageously porous. When the pores of the substrate are covered to the extent that they are effectively closed, and the substrate is continuous, a molecular sieve membrane results; such membranes have the advantage that they may perform catalysts and separation simultaneously if desired.
In our earlier International Application No. PCT/EP95/02704, the disclosure of which is incorporated herein by reference, we have described such a layer in which the crystals of the sieve extend predominantly in a single direction, especially the direction perpendicular to the plane of the layer, the structure including an additional relatively thin layer between the molecular sieve layer and the substrate. Since the crystals of the layer are ordered, diffusion of materials through the membrane is hampered by grain boundaries to a much reduced extent.
To achieve effective separation of the components of a mixture containing a plurality of molecular species, the structure must be substantially free of voids in the sieve extending through the thickness of the layer that are of sufficient cross-section to allow undiscriminated passage of all species through them. In this specification, the term "void" refers to any break in the continuity of the molecular sieve structure, whether in the form of a crack, an inter-particle space, or otherwise, including, for example, a zone of amorphous material which may appear solid but which actually has a molecular structure that allows passage of all species from one surface of the layer to the other.
The presence or absence of such voids may be established by the so-called "dye-permeation test", in which a dyestuff, having a molecular size greater than that of the zeolite pores, but smaller than that of the voids e.g., smaller than about 2 nm, is applied to one face of the structure in a solution in a solvent the molecules of which are small enough to pass through the structure. After being allowed to stand for some seconds, excess dye is washed off with more solvent. The face is then inspected; no coloration indicates absence of voids while their presence is shown by coloration, the depth of color giving a qualitative indication of void concentration. Another test for the insignificance of voids is a pX/mX selectivity, as defined below, of at least 2.0 at a substantially zero pressure differential between opposite faces.
It has been found that while some membrane structures, including those of the International applications mentioned above, pass the test after manufacture, or after calcination to remove any organic template employed in zeolite-forming process, and before use at elevated temperature, re-testing after such use shows that voids have developed. Tests of separation selectivity also i

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