In-Situ Crystallized Zeolite Containing Composition (LAI-ISC)

The present invention is related to a new zeolite containing composition, its preparation, and use.

U.S. Pat. No. 5,110,478 describes the direct synthesis of zeolite membranes. The membranes produced in accordance with the teachings of U.S. Pat. No. 5,110,478 were discussed in "Synthesis and Characterization of a Pure Zeolite Membrane," J. G. Tsikoyiannis and W. Haag, Zeolites (VOL. 12, p. 126., 1992). Such membranes are free standing and are not affixed or attached as layers to any supports. Furthermore, the membranes have a gradient of crystal sizes across the thickness of the membrane. Such a gradient precludes growth of a thin membrane with a minimum number of nonselective permeation paths.

Zeolite membranes have also been grown on supports. See e.g. "High temperature stainless steel supported zeolite (MFI) membranes: Preparation, Module, Construction and Permeation Experiments," E. R. Geus, H. vanBekkum, J. A. Moulyin, Microporous Materials, Vol. 1, p. 137, 1993; Netherlands Patent Application 91011048; European Patent Application 91309239.1 and U.S. Pat. No. 4,099,692.

All of the above prepared membranes have nonuniform sized zeolite crystals and are noncontinuous, exhibiting many voids. Obtaining functional zeolite membranes from low alkaline synthesis routes is difficult because the heterogeneous crystals in the membrane require an enormous membrane thickness to seal pinholes and void structures which lower the membrane selectivity.

FIG. 1 is an electron micrograph showing a cross-sectional view of a zeolite layer grown on the inverted face of a porous .alpha. alumina substrate.

FIG. 2 is an electron micrograph of a top view of a zeolite layer grown on the inverted face of a porous .alpha. alumina substrate.

FIG. 3 is an X-ray diffraction pattern showing the preferred orientation of a composition of the invention. The x-axis is 2 theta and the y-axis is intensity in CPS.

FIG. 4 is a schematic view of a zeolite layer on a porous substrate. (A) is the porous substrate, (B) the zeolite layer, (C) a grain boundary, (W) the width at one point along a zeolite crystal, and (T) the thickness of one crystal.

Applicant has discovered a new composition of matter containing zeolite and a process for preparing the same. The composition is unique in that the zeolite crystals making up part of the composition pack in a manner such that the zeolite forms a layer which is essentially continuous with no large scale voids even when the zeolite layer is <10 .mu.m thick.

Hence, one aspect of the present invention is directed toward a composition of matter comprised of a substrate (also referred to herein as a support) and a polycrystalline layer of zeolite crystals wherein at least 99% of said zeolite crystals have at least one point between adjacent zeolite crystals that is .ltoreq.20 .ANG. and wherein at least 90% of said zeolite crystals have widths of from about 0.2 to about 100 microns and wherein at least 75% of said zeolite crystals have a thickness of within 20% of the average crystal thickness. Preferably, the zeolite layer will have at most 1 Volume % voids and the zeolite crystal will range from about 2 to about 50 microns.

Another aspect of the present invention is directed toward a process of making the instant composition comprising the steps of:

(a) contacting a substrate with a zeolite synthesis mixture;

(b) hydrothermally treating said substrate and said zeolite synthesis mixture for a time and at a temperature sufficient to form a zeolite layer on said substrate and wherein settling on to said zeolite layer of particles produced from said zeolite synthesis mixture is prevented;

(c) removing any unreacted zeolite synthesis mixture.

Contacting as used herein includes total and partial immersion.

The process further comprises calcining said composition of step (c) at a temperature of about 400.degree. to about 600.degree. C. for at least about 10 minutes when said zeolite synthesis mixture contains an organic template.

The compositions thus produced are useful, for example, for size exclusion separations such as separation of dye molecules from alcohol and oligomer separation from hexane.

Additionally, the compositions described herein are often referred to as zeolite membranes in the art and can be used as such.

Applicant has discovered a new composition comprising a polycrystalline zeolite layer on a support. The support can be porous or non-porous, preferably porous supports will be used. The zeolite layer can be grown on any porous supports including but not limited to alumina, titania, cordierite, zeolite, mullite, stainless steel, pyrex, silica, silicon carbide, silicon nitride, carbon, graphite and mixtures thereof. Additionally, non-porous supports will include quartz, silicon, glass, borosilicate glasses, dense ceramics, i.e., clays, metals, polymers, graphite and mixtures thereof. When non-porous supports are utilized, the compositions are useful as sensors or catalysts. Most preferably, the support will be a porous ceramic or porous metal.

Growth of the zeolite layer may be carried out by contacting the substrate in a zeolite synthesis mixture for a time and at a temperature sufficient to effect crystallization. Hydrothermally treating may, for example, in an autoclave under autogenous pressure. Contacting of the substrate must be carried out such that there is no settling of crystals formed in the synthesis mixture during autoclaving onto the substrate. The synthesis mixture may thus be handled in a manner to prevent such settling.

In a preferred embodiment, the zeolite layer is grown on a support which is inverted in the zeolite synthesis mixture. Inverted as used herein means that the zeolite layer is grown on the side of the substrate oriented from 90 to 270 degrees in the synthesis mixture. In the 180 degree orientation, the side of the substrate onto which the zeolite layer is grown, is horizontal and facing downward. This is referred to as inverted. Preferably, the membrane will be grown on the 180 degree oriented side. In the inverted orientation, the surface of the substrate to be coated should be at least 5 mm, from the bottom and sides of the vessel containing the zeolite synthesis mixture, preferably at least 8 mm at its lowest point during the process of preparation. Applicant believes that inversion of the substrate prevents zeolite crystals, which are homogeneously nucleated in the zeolite synthesis mixture, from settling onto the substrate where the zeolite layer is grown. Thus, the crystals do not incorporate into the growing zeolite layer or perturb the growth process. Hence, at least 99%, preferably at least 99.9% of the crystals in the zeolite layer have at least one point between adjacent zeolite crystals that is .ltoreq.20 .ANG.. In the instant invention, the spacing between adjacent crystals is set by a grain boundary zone and the maximum grain boundary zone, absent voids or defects, will be .ltoreq.40 .ANG.. Additionally, at least 90%, preferably at least 95% of the zeolite crystals in the zeolite layer have widths of from about 0.2 to 100 microns, preferably 2 to about 50 microns. As used herein, grain boundary zone is the width of the disordered zone between two adjacent ordered crystals. The zeolite crystals in the zeolite layer are intergrown in the membrane so that nonselective permeation paths through the membrane are blocked by the narrowest point of approach between crystals. Nonselective permeation pathways are taken to be permeation pathways which exist at room temperature that do not pass through the zeolite crystals. This blockage of non-permeation pathways exists at room temperature after a template which occludes the pore structure is removed from the zeolite crystals. Templates which are used to grow the zeolite are often removed by a calcination step. From transmission electron microscopy (TEM) the narrowest point of approach between crystals of less than 20 .ANG. after the template is removed is established. The space between crystals at this point can contain inorganic oxide material that restricts non-selective permeation of molecules through the membrane. The absence of non-selective permeation paths can be detected by the ability to prevent the permeation at room temperature (.about.20.degree. C.) of dye molecules through the membrane after any template is removed from the pore structure. Dye molecules which can be chosen to detect non-selective permeation pathways through the membrane should have minimum dimensions which are larger than the controlling aperture through the zeolite and the size of the dye molecule should also be less than 20 .ANG.. Non-selective pathways transport dye molecules which are larger than the pore size of the zeolite. The dye molecules should be carried in a solution made with a solvent which can be transported through the zeolite pore structure and the zeolite layer should not be allowed to pick up foreign contaminants (such as water) before being tested. It is found that the compositions made in accordance with the present invention block the permeation of dye molecules at room temperature through the zeolite layer. All of the dye molecules chosen have sizes less than 20 .ANG.. The lack of permeation at room temperature of dye molecules with sizes less than .about.20 .ANG. demonstrates that non-selective permeation pathways with sizes less than 20 .ANG. are blocked. It should be noted that this test does not have to be performed with a dye molecule and any molecular species that can be detected having a size less than 20 .ANG. and greater than the zeolite pore dimension can be used. The advantage of using a dye molecule is that it can be readily detected by optical means.

The habitus of MFI zeolite compositions grown in accordance with the instant invention preferably display a degree of C-orientation (within 30.degree. of the normal to the surface of the substrate in the zeolite layer). More especially, the longest edges (thickness) of at least 75% of the crystals are within 30.degree. of perpendicular to the layer plane, advantageously at least 90% of the crystals being within that angle.

A measure of the crystallographic preferred direction of the unit cell and of the proportion of the crystals that have the longest axis (thickness) normal to the plane of the layer may be obtained by comparison of the X-ray diffraction pattern of the layer with that of a randomly oriented zeolite powder. In the case of an MFI-type zeolite with predominant C-axis orientation, for example, the longest edge corresponding to the c axis, the ratio of the intensity of the 002-peak to the combined 200 and 020 peak is divided by the same ratio for randomly oriented powder; the quotient is termed the crystallographic preferred orientation (CPO). Measured in this way, zeolite layers in accordance with the invention have a CPO of at least 1, and may have a CPO as high as 500. The crystals in the zeolite layer range in thickness from about 2 to about 100.mu., preferably about 5 to 100.mu., more preferably about 30 to about 60.mu., most preferably about 30.mu.. Although the crystals may range in thickness from 2 to 100.mu., for any given membrane, 75 Volume %, preferably 90 Volume % of the crystals will all have the crystal thicknesses of within 20% of the average crystal thickness. Thickness is herein defined as the length of the crystals from the substrate surface to the uppermost edge of the zeolite crystal perpendicular to the substrate. Compositions made from zeolites other than MFI will also display a degree of orientation in habitus and/or of the unit cell, however, such orientation may not be in the c direction. For example, A, B, and C orientations are possible, as well as mixtures thereof. Other orientations are also possible depending on the zeolite selected.

The instant compositions are virtually free of voids. They preferably exhibit at most about 1 Volume % voids, preferably less than 0.5 Volume % voids.

Void as used herein means spaces between the zeolite crystals in the zeolite layer along grain boundaries larger than 40 .ANG.. Defects are spaces between adjacent zeolite crystals extending through the thickness of the zeolite layer. In the instant membrane, the total number of defects in the zeolite layer with sizes >40 .ANG. is <10,000 per square inch, preferably <100 per square inch. The number of defects having spacing between adjacent zeolite crystals larger than about 2000 .ANG. is <100/inch.sup.2, preferably <0.1/inch.sup.2. Voids and defects can be detected from cross-sectional images of the zeolite layer made in the scanning or transmission electron microscope. In the most preferable case, the zeolite layer will be substantially free of voids and defects.

Defects of the type described can be detected in dye permeation experiments. Isolated points at which dye permeates into the substrate reveal such defects. Defects can also be determined by examining cross-sections of the zeolites membrane in the scanning electron microscope. Gas permeation can also be used to reveal defects in the membrane. If the permeability of the zeolite layer to nitrogen at room temperature is less than 5.times.10.sup.-6 moles/m(m.sup.2 -sec-Pascal) for each micron of thickness of the zeolite layer, the membrane can be considered to have an acceptable defect density. More preferably, the permeability of the zeolite layer to nitrogen at room temperature is less than 5.times.10.sup.-7 moles/m.sup.2 -sec-Pascal) for each micron of thickness of the zeolite layer. The zeolite membranes of the instant invention are prepared from zeolite synthesis mixtures. Zeolite synthesis mixtures are any mixtures from which zeolite crystals are grown and are well known in the art. (See, e.g., Handbook of Molecular Sieves, Rosemary Szostak, Van Nostrand Reinhold, NY, 1992 and Zeolite Molecular Sieves, D. W. Breck, R. E. Kreiger Publishing Co., Malabar, Fla., 1984, ISBN 0-89874-648-5). For example, for MFI zeolites, the synthesis mixture can be a mixture having a pH of about 8 to about 12 and is readily prepared by those skilled in the art. For example, suitable mixtures include NA.sub.2 O, TPABr (tetrapropyl ammonium bromide), SiO.sub.2 and water. The membranes are grown by contacting the support material of choice in the zeolite synthesis mixture. The synthesis mixture is then heated to about 50.degree. to about 300.degree. C., preferably about 100.degree. to about 250.degree. C., preferably about 180.degree. C. for a period of about 30 minutes to about 300 hours. Any undesired growth on the substrate can be easily removed by known techniques. For example, grinding can be used. Undesired growth does not refer to the zeolite layer of the invention, but refers to growth on other surfaces of the substrate.

The zeolite layer compositions which can be prepared in accordance with the instant invention include silicates, aluminosilicates, aluminophosphates, silicoalumino-phosphates, metalloaluminophosphates, stanosilicates and mixtures thereof. Representative examples of such zeolites are MFI, FAU (including zeolite x, zeolite y), zeolite beta, MAZ, LTA, LTL, CHA, AFI, AEL, BEA, EUO, FER, KFI, MOR, MEL, MTW, OFF, TON, AFS, AFY, APC, APD, MTN, MTT, AEL and mixtures thereof, preferably MFI zeolite with a silicon to aluminum ratio greater than 30 will be used including compositions with no aluminum. MFI zeolites with Si/Al ratios greater than 30 are herein referred to as silicalite.

Some of the above materials while not being true zeolites are frequently referred to in the literature as such, and this term will be used herein to include such materials.

The zeolite layer can have either a shape preferred orientation, a crystallographically preferred orientation, or both. Shape or crystallographically preferred orientations occur because of the control of the relative rates of nucleation and growth offered by the synthesis procedure. Specifically, during synthesis, the rate of growth can be made to dominate the rate of surface nucleation of new crystals or incorporation of new crystals. Incorporation of new crystals is defined as attachment onto the surface of the growing zeolite layer of a crystal formed in the synthesis mixture. Since the growth rate can dominate renucleation or incorporation, crystals can competitively grow for long periods of time without significant addition of new crystals into the growing layer. Since the growing layer is composed of individual crystals and the synthesis method seeks to prevent renucleation or incorporation of crystals, the resulting composition can have shape, crystallographically preferred orientation, or both. Shape orientation occurs because the crystals grow with preferred regular habits (or morphology) at the surface of the zeolite layer. A regular habit (or morphology) is taken to be a regularly shaped outline of a particular crystallographic grain in the layer. Regularly shaped outlines are defined as those which can be fitted or packed together so that there are no interconnected spaces or voids between crystals. Interconnected voids will form a pore structure. A few examples of regular habits with regular shapes are columnar, cubic, rectangular, and prismatic. Spherical, irregular and elliptical shapes are not considered to be regular habits. In a shape preferred orientational, defined layers will have the same regular habit. This can be measured by cleaving or fracturing the substrate on which the layer is grown and examining the cross-sectional morphology of the zeolite layer with a scanning electron microscope. Examining the surface of the as grown zeolite layer can also give additional information concerning the shape preferred orientation in the layer. A layer with shape preferred orientation is taken to be one which has more than 90% of the crystals within one layer inside the zeolite layer exhibiting self similar regular habits. The self similar requirement means that the same regular habit is exhibited within a layer that can be drawn in the electron micrograph of the cross-section of the zeolite layer, however, even though the shapes are the same, they do not all have to be the same size. Because of the growth mechanism of the zeolite layer, it is possible to have one shape preferred orientation at the bottom (base) of the layer and another shape preferred orientation in a layer drawn near the surface. An example of this is an MFI zeolite layer which has a columnar habit at the base of the layer and a rectangular habit at the surface of the layer. Many MFI zeolite layers grown in accordance with the present invention exhibit only one habit throughout the thickness of the zeolite layer. Usually MFI zeolite layers grown in accordance with the present invention exhibit only one habit throughout the thickness of the zeolite layer. Usually MFI zeolite layers with a preferred C-axis orientation exhibit a columnar habit (or morphology) throughout the entire thickness of the zeolite layer. Often shape preferred orientational layers have a preferred crystallographic orientation.

The substrate on which the zeolite layer is grown may be porous or non-porous. If the substrate is porous, it will be a porous material throughout its entire thickness. Preferably an inorganic oxide or stainless steel will be utilized. The porous substrate, hence can be a ceramic, metal, carbide, polymer or mixture thereof The porous substrate, hence may have a uniform pore size throughout or may be asymmetrical, having a larger pore structure throughout the bulk of the substrate with a smaller pore structure at the surface on which the zeolite layer is to be grown. The substrate pore size is dictated by mass transfer considerations. It is preferred that the pore structure and thickness of the substrate be chosen such that the mass transfer resistance does not limit the flux of material permeating through the zeolite membrane during use. The porous substrate will hence display a porosity of about 5 to about 70%, preferably about 20 to about 50% and an average pore size of about 0.004 to about 100.mu., preferably about t 0.05 to about 2 microns.

It is preferred that the surface of the substrate, porous or non-porous, on which the zeolite layer is grown be smooth. Roughness in the substrate leads to defects in the zeolite layer. The substrate should have an average roughness with an amplitude of less than 10 .mu.m with an aspect ratio of the roughness less than 1:1. It is preferable that the average roughness of the substrate be less than 0.5 .mu.m with an aspect ratio of the roughness less than 1:1. Though non-porous substrates may be utilized, porous substrates are preferred.

Once the zeolite layer has been grown, the substrate, with attached zeolite layer may preferably be washed with water for a time and at a temperature sufficient to remove any zeolite synthesis mixture which did not react during hydrothermal treatment. Hence, washing may be conducted at a temperature of about 15.degree. to 100.degree. C., preferably about 80.degree. to about 100.degree. C., for at least 10 minutes, preferably at least six hours. Excess washing for longer periods will not affect the compositions separation capabilities. Additionally, any liquids or solutions capable of removing the excess zeolite synthesis mixture may be used.

Once washed, if the zeolite synthesis mixture contained an organic template, the composition is calcined at about 400.degree. to 600.degree. C. for at least one hour, preferably at least about six hours. Longer calcination times will not affect the performance of the membrane.

The compositions are useful for separation processes whereby feedstock derived from petroleum, natural gas, hydrocarbons, or air comprising at least two molecular species is contacted with the composition of the invention and at least one molecular species of said feedstock is separated from said feedstock by said composition and wherein said hydrocarbon feedstocks are coal, bitumen and kerogen derived feedstocks.

Specifically, the following table shows some possible feedstocks derived from petroleum, natural gas, air, or hydrocarbons and the molecular species separated therefrom by use of the instant compositions. The table is not meant to be limiting.

Applicants believe that molecular diffusion is responsible for the above separations. Additionally, the compositions can be used to effect a chemical reaction to yield at least one reaction product by contacting the feedstocks as described above or below with the compositions having a catalyst incorporated within the zeolite layer, or support, or by placing the catalyst in close enough proximity with the composition to form a module. A module would react the feedstock just prior to its entrance into the composition or just after its exit from the composition. In this manner one can separate at least one reaction product or reactant from the feedstocks. The catalysts of choice for particular process fluids are well known to those skilled in the art and are readily incorporated into the instant compositions or formed into modules with the compositions by one skilled in the art. The following table represents some of the possible feedstocks/processes, in addition to those above which can be reacted and some possible products yielded. The table is not meant to be limiting.

The supported zeolite layer of the invention may be employed in such separations without the problem of being damaged by contact with the materials to be separated. Furthermore, many of these separations are carried out at elevated temperatures, as high as 500.degree. C., and it is an advantage of the supported layer of the present invention that it may be used at such elevated temperatures.

Further separations which may be carried out using the composition in accordance with the invention include, for example, separation of normal alkanes from co-boiling hydrocarbons, for example normal alkanes from iso alkanes such as C.sub.4 to C.sub.6 mixtures and n-C.sub.10 to C.sub.16 alkanes from kerosene; separation of aromatic compounds from one another, especially separation of C.sub.8 aromatic isomers from each other, more especially para-xylene from a mixture of xylenes and, optionally, ethylbenzene, and separation of aromatics of different carbon numbers, for example, mixtures of benzene, toluene, and mixed C.sub.8 aromatics; separation of aromatic compounds from aliphatic compounds, especially aromatic molecules with from 6 to 8 carbon atoms from C.sub.5 to C.sub.10 (naphtha range) aliphatics; separation of olefinic compounds from saturated compounds, especially light alkenes from alkane/alkene mixtures, more especially ethene from ethane and propene from propane; removing hydrogen from hydrogen-containing streams, especially from light refinery and petrochemical gas streams, more especially from C.sub.2 and lighter components; and alcohols from aqueous streams; alcohols from other hydrocarbons, particularly alkanes and alkenes that may be present in mixtures formed during the manufacture of alcohols and separation of heteroatomic compounds from hydrocarbons such as alcohols and sulphur containing materials such as H.sub.2 S and mercaptans.

The present invention accordingly also provides a process for the separation of a fluid mixture which comprises contacting the mixture with one face of a layer according to the invention under conditions such that at least one component of the mixture has a different steady state permeability through the layer from that of another component and recovering a component or mixture of components from the other face of the layer.

The invention further provides a process for catalysing a chemical reaction which comprises contacting a feedstock with a layer according to the invention which is in active catalytic form under catalytic conversion conditions and recovering a composition comprising at least one conversion product, advantageously in a concentration differing from its equilibrium concentration in the reaction mixture. For example, a p-xylene rich mixture from the reactor or reactor product in a xylenes isomerization process; aromatic compounds from aliphatics and hydrogen in a reforming reactor; hydrogen removal from refinery and chemicals processes such as alkane dehydrogenation in the formation of alkenes, light alkane/alkene dehydrocyclization in the formation of aromatics (e.g., Cyclar), ethylbenzene dehydrogenation to styrene. The invention further provides a process for catalysing a chemical reaction which comprises contacting one reactant of a bimolecular reaction with one face of a layer according to the invention, that is in the form of a membrane and in active catalytic form, under catalytic conversion conditions, and controlling the addition of a second reactant by diffusion from the opposite face of the layer in order to more precisely control reaction conditions. Examples include controlling ethylene, propylene or hydrogen addition to benzene in the formation of ethylbenzene, cumene or cyclohexane, respectively.

The invention further contemplates separation of a feedstock as described herein wherein the separated species reacts as it leaves the composition or as it passes through the composition and thus forms another product. This is believed to increase the driving force for diffusion through the zeolite layer.

Catalytic functions can be incorporated into the instant compositions. When a catalytic function is incorporated into the composition, it can be used as an active element in a reactor. Several different reactor architectures can be constructed depending on the location of the catalytic site in the composition. In one case the catalytic function can be located within the zeolite layer, while in another case the catalytic function can be located within the support, and in another case the catalytic function can be distributed throughout the support and the zeolite layer. In addition, catalytic function can be incorporated into a reactor by locating conventional catalyst particles near one or more surfaces of the composition such that specific products or reactants are continuously and selectively removed or added to the reaction zone throughout the reactor. Impregnating with catalytically active metals such as Group VIII noble metals, e.g. Pt, can impart the catalytic function to the composition. The catalytically active metals can be incorporated by techniques known in the art such as incipient wetness. The amount of Group VIII noble metal to be incorporated will range from about 0.01 to about 10 wt %.

Some specific reaction systems where these compositions would be advantageous for selective separation either in the reactor or on reactor effluent include: selective removal of a para-xylene rich mixture from the reactor, reactor product, reactor feed or other locations in a xylenes isomerization process; selective separation of aromatics fractions or specific aromatics molecule rich streams from catalytic reforming or other aromatics generation processes such as light alkane and alkene dehydrocyclization processes (e.g., C.sub.3 -C.sub.7 paraffins to aromatics from processes such as Cyclar), methanol to gasoline and catalytic cracking processes; selective separation of benzene rich fractions from refinery and chemical plant streams and processes; selective separation of olefins or specific olefin fractions from refinery and chemicals processing units including catalytic and thermal cracking, olefins isomerization processes, methanol to olefins processes, naphtha to olefins conversion processes, alkane dehydrogenation processes such as propane dehydrogenation to propylene; selective removal of hydrogen from refinery and chemicals streams and processes such as catalytic reforming, alkane dehydrogenation, catalytic cracking, thermal cracking, light alkane/alkene dehydrocyclization, ethylbenzene dehydrogenation, paraffin dehydrogenation; selective separation of molecular isomers in processes such as butane isomerization, paraffin isomerization, olefin isomerization, selective separation of alcohols from aqueous streams and/or other hydrocarbons.

The following examples are for illustration and are not meant to be limiting.

1. Materials

The hydrothermal experiments were performed using mixtures of the following reagents: NaOH (Baker), Al(NO.sub.3).sub.3, 9H.sub.2 O(Baker), Ludox AS-40 (Dupont), tetrapropylammonium bromide (98%, Aldrich), and distilled water.

Porous alumina and stainless steel substrates were used for the support of the zeolite layers. The average pore size and porosity of the alumina substrate was about 800 .ANG. and 32%, respectively. Porous sintered stainless steel substrates from Mott's (0.25 .mu.m) and Pall (M020, 2 .mu.m) were obtained. All of the substrates were cleaned with acetone in an ultra-sonic bath, dried at 120.degree. C. and then cooled down to room temperature before use.

2. Hydrothermal Reaction

MFI membranes were prepared from two different reaction batch mixtures, one contained silica only to make high silica MFI and the other with added alumina to make ZSM-5. they have the general formulation.times.M.sub.2 O:10 SiO.sub.2 : z Al.sub.2 O.sub.3 :pTPABr:y H.sub.2 O; M can be NA, K, Rb, Cs, x varied from 0.1 to 0.15 and p varied from 0.2 to 1. all the results shown in the next section have the composition of 0.22 Na.sub.2 O:10 SiO.sub.2 :OAl.sub.2 O.sub.3 :280 H.sub.2 O:0.5 TPABr (tetrapropylammoniumbromide) with the exception of the ZSM-5 sample which contained 0.05 Al.sub.2 O.sub.3, the 1.74 g of TPABr and 0.45 g of NaOH (50 wt %) were dissolved in 52 ml of distilled water with stirring. To this solution, 18.8 g of Ludox AS-40 was then added with agitation for at least 15 minutes until a uniform solution was formed.

The substrates were placed inverted (180 degree orientation) in a Teflon lined reaction vessel supported on a stainless steel wire frame. The distance between the substrate and the bottom of reactor was at least 5 mm. The synthesis solution was then poured into the reactor to cover the whole substrate. The autoclave was sealed and placed in an oven, which preheated at the desired temperature. The reaction bombs were removed from the oven after reaction and cooled to room temperature. The coated substrates were washed with hot water for at least 6 hours, then calcined at 500.degree. C. for 6 hours in air. The heating rate was controlled at 10.degree. C./hour.

3. Analysis

The resulting membranes were characterized by x-ray diffraction, electron microscopy and permeability measurement.

Results and Discussion

1. Products

The following table shows some typical examples synthesized under different experimental conditions, such as reaction time, and substrate.