Procedure for preparing molecular sieve films

Details Procedure for preparing molecular sieve films Procedure for preparing molecular sieve films

1999-02-08

2001-01-23

Dunn, Tom (Department: 1755)

Catalyst, solid sorbent, or support therefor: product or process

In form of a membrane

C502S060000, C502S064000

Reexamination Certificate

active

06177373

ABSTRACT:

The present invention is concerned with a method for preparing molecular sieve films on a variety of substrates. Substrates coated with such thin films find their applications in the fields of membrane separation, sensor technology, catalysis, electrochemistry, electronics as well as reinforcing polymer fillers.
Molecular sieves are characterized by the fact that they are microporous materials with pores of a well-defined size in the range of 2-20 A. Most molecules, whether in the gas or liquid phase, both inorganic and organic, have dimensions that fall within this range at room temperature. Selecting a molecular sieve with a suitable pore size therefore allows separation of a molecule from a mixture through selective adsorption, hence the name “molecular sieve”. Apart from the selective adsorption and selective separation of uncharged species, the well-defined pore system of the molecular sieve enables selective ion exchange of charged species and selective catalysis. In the latter two cases, significant properties other than the micropore structure are, for instance, ion exchange capacity, specific surface area and acidity. Molecular sieves can be classified in various categories, for example according to their chemical composition and their structural properties. A group of molecular sieves of commercial interest is the group comprising the zeolites, that are defined as crystalline aluminium silicates. Another category of interest is that of the metal silicates, structurally analogous to zeolites, but for the fact that they do not contain aluminium or only very small amounts thereof). An excellent review of molecular sieves is given in “Molecular Sieves—Principles of Synthesis and Identification” (R. Szostak, Van Reinhold, New York, 1989).
Membrane processes for selective separation have attracted considerable interest, partly due to the fact that they are potentially more effective and more economically advantageous as compared to the currently used separation processes, and partly due to the fact that they may open up new separation possibilities, that are not feasible with the currently available techniques. There is also considerable interest in the development of catalytic membrane reactors and chemical sensors with improved selectivity. The limitations associated with the use of membranes in various applications are primarily due to the membrane itself. The performance of the currently available membrane materials is generally less than optimal as regards capacity, selectivity, thermal and mechanical properties as well as resistance to biodegradation. It is known that significant improvements can be obtained with zeolite based membranes.
Membranes consisting solely of molecular sieve material are known and reported in various patents and publications. Suzuki (Europ. Pat. Appli. 180200 (1986)) describes a method for preparing a zeolite membrane by applying a gel coat to a substrate, followed by hydrothermal treatment of the gel coat to form a zeolite film. In another method (U.S. Pat. No. 4,699,892 (1987)), a substrate is first impregnated with a synthesis gel that is subsequently transformed into zeolite under hydrothermal synthesis conditions. U.S. Pat. No. 4,800,187 (1989) describes a method in which these zeolite films are prepared by reacting the substrate surface with active silica.
In International Application WO 94125151, a supported inorganic layer comprising optionally contiguous particles of a crystalline molecular sieve is deposited, the mean particle size being within the range of from 20 nm to 1 &mgr;m. The support is advantageously porous. When the pores of the support are covered to the extent that they are effectively closed, and the support is continuous, a molecular sieve membrane results; such membranes have the advantage that they may perform catalysis and separation simultaneously if desired. While the products of this earlier application are effective in many separation processes, the crystals of the layer are not ordered, and as a result diffusion of materials through the membrane may be hampered by grain boundaries and voids between the crystals effect selectivity.
International Applications PCT/EP93101209, and PCT/US95/08514 published as WO 96/01687 describe the use of nucleation layers deposited on substrates for the manufacture of molecular sieve layers. These nucleation layers are relatively thick and unordered and the molecular sieve layers are relatively thick.
In International Application PCT/US95/08511 published as WO 96/01685 molecular sieve layers are synthesised without the use of a nucleation layer. The resultant molecular sieve layer is relatively thick from 2 to 100 &mgr;m. This process produces molecular sieve layers which have a relatively low density of molecular sieve at the interface with the substrate.
Furthermore considerable interest has been shown for the development of new and improved composite materials consisting primarily of fibrous inorganic materials in combination with various types of polymers and plastics. In order to obtain advantageous mechanical properties of the composites, efforts are made to ensure compatibility, and where appropriate create the best chemical bonds, between fibers and polymers. For inorganic fibers, this may be achieved by using suitable coupling agents, that is, compounds characterized by the fact that they contain functional groups with a strong affinity towards both the fiber and the polymer. Several methods have been developed to achieve this aim. It is also known that certain types of inorganic molecular sieves show a strong affinity towards organic compounds, such as the monomers used in the production of polymers, and it is known that fibers coated with molecular sieves can impart reinforcing properties when used as fillers in polymer matrixes.
It is possible to prepare continuous molecular sieve films with the techniques known in the art. However, such films have dimensions with a lower thickness limit range of more than 1-10 micrometers, the exact limit depending on the type of molecular sieve being used. Attempts to prepare thinner films with the current available techniques result in discontinuous films, of very limited use in the current fields of application. For example with prior art molecular sieve structures for use in separations there may be restrictions on the flow of material through the membrane due to the presence of flux limiting attributes of the structure and/or there may be defects in the membrane which contribute to non-selective pathways through the membrane. Efforts however are being made to prepare even thinner films, because of the technical advantages such films would offer in several potential applications. However when moving to very thin membranes such problems are compounded; some prior art techniques as indicated above provide low density membrane structures at the interface with the substrate which may only be overcome by the use of thicker membranes or extensive reparation of the membrane. For the use of molecular sieve films as membranes in separation processes, the membrane flux is primarily influenced by the film thickness. The thinner the membrane, the higher the flux. For application as a chemical sensor, the response time is of prime importance. For a given molecular sieve, the response time is shortened by reducing the film thickness. In catalytic processes, the efforts are directed towards avoiding low reaction rates due to limited diffusion transport of species in the catalyst. In catalytic membrane reactors where the molecular sieve is the active phase, the resistance to pore diffusion is lowered as the film thickness is reduced.
It is often desirable to produce thin films of inorganic materials that are crack-free and in most cases this is a prerequisite. For preparing thin films of inorganic materials, it is often desirable and, in many cases required, for use in the envisaged applications, to have films that are free of cracks and large pores. With the currently available techniques, it is difficult to produce zeolite films that are cr

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