Molecular sieve abstract
The invention is comprised of a procedure for applying molecular
sieve films to the surface of different substrates. This procedure
is characterized in that microcrystals of the molecular sieves in
question bind to the substrate surface in a first step as a monolayer,
and are then, in a second step, allowed to grow into a thin, continuous
and dense film. Molecular sieve films are of interest in application
areas such as preparing membranes, catalysts, sensor devices and
polymer reinforcing fillers.
Molecular sieve claims
What is claimed is:
1. A method of preparing a monolayer structure comprising molecular
sieve microcrystals which method comprises;
a) preparing a dispersion comprising discrete microcrystals of
molecular sieve which have a surface charge,
b) selecting or preparing a substrate with a surface charge which
is opposite to that of the discrete microcrystals in the dispersion,
c) contacting the substrate with the dispersion comprising discrete
microcrystals of molecular sieve such that discrete microcrystals
of molecular sieve are attracted to and adhere to the substrate
as a monolayer.
2. The method recited in claim 1 wherein the molecular sieve is
zeolite or a microporous metal silicate or a metal phosphate.
3. The method recited in claim 2 wherein the molecular sieve is
silicalite, hydroxysodalite, Ti-silicalite, mordenite, or one of
the zeolites A, Beta, L, X, Y, ZSM-5 ZSM-2 ZSM-11 ZSM-22 or SAPO-34.
4. The method recited in claim 3 wherein the substrate is a non-porous
substrate silicon, silica, aluminum oxide, aluminum silicate, titanium
dioxide, zirconium dioxide, or a metal.
5. The method recited in claim 3 wherein the substrate is a porous
substrate silica, zirconia, titania, aluminum oxide, aluminum silicate,
metal or organic polymer substrate.
6. The method of claim 5 wherein the crystal size of said microcrystals
is larger than the pore sizes of said porous support.
7. The method recited in claim 2 wherein the substrate is an inorganic
or organic fiber.
8. The method recited in claim 1 wherein the microcrystals are
bonded by electrostatic adsorption to a substrate surface.
9. The method recited in claim 1 wherein the substrate is pre-treated.
10. The method recited in claim 1 wherein the substrate is charge
reversed.
11. The method recited in claim 10 wherein the charge reversal
is achieved by contact of the substrate with a solution comprising
a cationic polymer.
12. The method recited in claim 1 wherein the microcrystals are
charge reversed.
13. The method recited in claim 1 wherein the microcrystals are
colloidal molecular sieve microcrystals and have a particle size
of 10 to 500 nm.
14. The method recited in claim 1 wherein the monolayer consists
essentially of molecular sieve microcrystals which are in contact
with one or more of their neighbors.
15. The method recited in claim 1 wherein the substrate comprising
a monolayer of molecular sieve is calcined in steam.
16. The method of claim 1 wherein said microcrystals have a crystal
size of at most 500 nm.
17. The method of claim 16 wherein said microcrystals have a crystal
size of 10 to 300 nm.
18. The method of claim 17 wherein said microcrystals have a crystal
size of 20 to 200 nm.
19. The method of claim 18 wherein said microcrystals have a crystal
size of 20 to 120 nm.
20. The method of claim 1 further including the step:
(d) heating said substrate having adhered microcrystals to a temperature
in the range of about 200 to 1000.degree. C. for a period of time
sufficient to fix said monolayer in place on said substrate.
21. A method of preparing a structure comprising a molecular sieve
film which method comprises:
(a) depositing on a substrate a monolayer comprising molecular
sieve microcrystals which are capable of nucleating the growth of
a molecular sieve film, said microcrystals having a crystal size
of at most 500 nm;
(b) forming a molecular sieve synthesis solution; and
(c) contacting (a) and (b) and hydrothermally growing molecular
sieve to form a molecular sieve film on the substrate.
22. The method recited in claim 21 wherein the molecular sieve
is zeolite or a microporous metal silicate or a metal phosphate.
23. The method recited in claim 22 wherein the molecular sieve
is sillicalite, hydroxysodalite, Ti-silicalite, mordenite, or one
of the zeolites A, Beta, L, X, Y, ZSM-5 ZSM-2 ZSM-1 1 ZSM-22
or SAPO-34.
24. The method recited in claim 23 wherein the substrate is a
non-porous substrate silicon, silica, aluminum oxide, aluminum silicate,
titanium dioxide, zirconium dioxide, or a metal.
25. The method recited in claim 23 wherein the substrate is a
porous substrate silica, zirconia, titania, aluminum oxide, aluminum
silicate, metal or organic polymer substrate.
26. The method of claim 25 wherein the crystal size of said microcrystals
is larger than the pore sizes of said porous support.
27. The method recited in claim 21 wherein the substrate is an
inorganic or organic fiber.
28. The method recited in claim 21 wherein the microcrystals are
bonded by electrostatic adsorption to a substrate surface.
29. The method recited in claim 21 wherein the substrate is pre-treated.
30. The method recited in claim 21 wherein the substrate is charge
reversed.
31. The method as claimed in claim 30 wherein the charge reversal
is achieved by contact of the substrate with a solution comprising
a cationic polymer.
32. The method recited in claim 21 wherein the microcrystals are
charge reversed.
33. The method recited in claim 21 wherein the substrate is pre-treated
by means of a coupling agent.
34. The method recited in claim 21 wherein the monolayer consists
essentially of molecular sieve microcrystals which are in contact
with one or more of their neighbors.
35. The method recited in claim 21 wherein the synthesis solution
is clear.
36. The method recited in claim 21 wherein the substrate comprising
a monolayer of molecular sieve is calcined in steam.
37. The method recited in claim 21 wherein the molecular sieve
film has a thickness of less than 2 .mu.m.
38. The method of claim 37 wherein said molecular sieve film has
a thickness of less than 0.25 microns.
39. The method of claim 21 wherein said microcrystals have a crystal
size of 10 to 300 nm.
40. The method of claim 39 wherein said microcrystals have a crystal
size of 20 to 200 nm.
41. The method of claim 21 wherein said microcrystals have a crystal
size of 20 to 120 nm.
42. The method of claim 21 wherein said contacting comprises immersion
or partial immersion of said substrate in said molecular sieve synthesis
solution.
43. The method of claim 21 wherein said molecular sieve synthesis
solution has a molar composition within the ranges:
TBL M.sub.2 O:SiO.sub.2 0 to 0.7:1 SiO.sub.2 :Al.sub.2 O.sub.3
12 to infinity:1 (TPA).sub.2 O:SiO.sub.2 0 to 0.2:1 H.sub.2 O:SiO.sub.2
7 to 1000:1
44. The method of claim 21 wherein said substrate comprising a
monolayer of molecular sieve is heated to a temperature in the range
of about 200 to 1000.degree. C. for a period of time sufficient
to fix said monolayer in place on said substrate prior to said contacting.
Molecular sieve description
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. 4699892 (1987)), a substrate is first impregnated
with a synthesis gel that is subsequently transformed into zeolite
under hydrothermal synthesis conditions. U.S. Pat. No. 4800187
(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 .mu.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 .mu.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 crack-free both immediately after production and
after exposure to high temperatures.
Another problem associated with the production of membranes via
the formation of molecular sieve films on porous substrates according
to the known techniques, is blocking of the substrate pore system
due to deposits of molecular sieve material in this pore system,
causing an effectively thicker film to be made.
A further problem associated with the production of molecular sieve
films on substrates according to the known techniques is that in
practice the number of possible substrate/molecular sieve combinations
is limited by the fact that the conditions required for synthesizing
numerous molecular sieves are so severe that the substrate is dissolved
or etched.
A problem with the use of fibers in composite materials or as fillers
in polymers and plastics is the low degree of compatibility between
polymer and filler, ultimately resulting In a material with unsatisfactory
mechanical properties. However, this compatibility problem can be
wholly or partly overcome by using known techniques, but only at
the expense of significantly increased production costs.
The present invention provides a means for avoiding the problems
associated with the known procedures for preparing molecular sieve
films on substrates and describes a new procedure for preparing
molecular sieve films on substrates especially very thin films.
A purpose of the present invention is to avoid the disadvantages
associated with the known methods used for preparing molecular sieve
films and to present a new procedure allowing to prepare in particular
very thin continuous films of this type. Another purpose of the
present invention is to develop a procedure for depositing very
thin molecular sieve films on the surface of a porous substrate
without simultaneously blocking the substrate pores by depositing
molecular sieve material in those pores. A further purpose of the
present invention is to develop a procedure whereby it is possible
to prepare very thin molecular sieve films on substrates that are
likely to dissolve or be etched under the conditions normally employed
for synthesizing the molecular sieve in question. It is also the
purpose of the present invention to develop a procedure for preparing
very thin molecular sieve films that are crack-free both immediately
after preparation and after thermal treatment.
It is further the purpose of the present invention to be able to
apply the prepared very thin molecular sieve films to both flat
and fibrous substrates.
The present invention deals with a procedure for preparing molecular
sieve films, especially very thin films, in which procedure discrete
molecular sieve microcrystals are bound to the surface of a support
to form a monolayer comprising molecular sieve microcrystals, which
is subsequently made to grow into a continuous film of molecular
sieve.
The key aspect in the process of the present invention is the formation
of an intermediate product which comprises as a key component a
monolayer comprising molecular sieve microcrystals on a suitable
support.
The present invention therefore in a first aspect provides a molecular
sieve monolayer structure which comprises;
a) a substrate and deposited thereon
b) a monolayer comprising molecular sieve microcrystals.
The term monolayer in the context of the present invention is taken
to mean a layer comprising microcrystals which are substantially
in the same plane deposited on a substrate. The microcrystals and
other materials if present may be close packed to provide a classical
monolayer. Alternatively the microcrystals and other materials if
present are not close packed and therefore are present as a sub-monolayer.
The exact packing density required depends to a certain degree on
the nature of the molecular sieve microcrystals and the desired
molecular sieve film to be grown from these microcrystals. The packing
density of microcrystals in the monolayer should in any event be
such as to enable a thin layer of molecular sieve film to be grown
from the layer. If the microcrystals are of an Inadequate density
in the monolayer the growth of molecular sieve film crystals from
such a monolayer to ensure a dense molecular sieve film would be
such as to provide a thick molecular sieve film.
The present invention in a second aspect provides for a method
of preparing a monolayer structure comprising molecular sieve microcrystals
which method comprises;
a) preparing a dispersion comprising discrete microcrystals of
molecular sieve which have a surface charge,
b) selecting or preparing a substrate with a surface charge which
is opposite to that of the discrete microcrystals in the dispersion,
c) contacting the substrate with the dispersion comprising discrete
microcrystals of molecular sieve such that discrete microcrystals
of molecular sieve are attracted to and adhere to the substrate
as a monolayer.
The present invention in a third aspect provides a method of preparing
a structure comprising a molecular sieve film which method comprises;
a) depositing on a substrate a monolayer comprising molecular sieve
microcrystals which are capable of nucleating the growth of a molecular
sieve film,
b) forming a molecular sieve synthesis solution,
c) contacting a) and b) and hydrothermally growing molecular sieve
to form a molecular sieve film on the substrate.
The present invention in a fourth aspect provides a structure comprising
a support and a film comprising a crystalline molecular sieve in
wherein the molecular sieve film has incorporated within its structure
a monolayer comprising molecular sieve microcrystals.
The present invention is particularly suitable for the production
of very thin molecular sieve films. By the term very thin films
is meant films with a thickness of less than 2 .mu.m, ideally less
than 1 .mu.m, preferably less than 0.25 .mu.m and most preferably
less than 0.1 .mu.m.
The term microcrystal as used in the relation to the present invention
refers to molecular sieve crystals with a size of less than 500
nm preferably less than 200 nm, the crystal structure of which can
be identified by X-ray diffraction.
International Application PCT/SE93/00715 published as WO 94/05597
the teaching of which is hearby incorporated by reference, describes
a method whereby it is possible to synthesize colloidal suspensions
of discrete molecular sieve microcrystals suitable for use in the
preparation of monolayer structures according to the present invention.
Molecular sieves such as zeolites or crystalline microporous metal
silicates are generally synthesized by hydrothermal treatment of
a silicate solution with a well-defined composition. This composition,
as well as the synthesis parameters such as temperature, time and
pressure, determine the type of product and the crystal shape obtained.
Suitable molecular sieve microcrystals for use in the present invention
include nanocrystalline zeolites which are crystallites having sizes
from about 10 .ANG. to 1 .mu.m. Nanocrystalline zeolites can, e.g.,
be prepared in accordance with the methods set forth in PCT-EP92-02386
published as WO 93/08125 the teaching of which is hereby incorporated
by reference, or other methods known to those skilled in the art.
Colloidal sized particles are between 50 and 10000 .ANG. and form
a stable dispersion or solution of discrete particles. Preferably,
the colloidal particles will be 250 to 5000 .ANG., most preferably
less than 1000 .ANG.. Colloidal zeolites with sizes <5000 .ANG.
are readily obtainable. Following calcination the zeolite will be
nanocrystalline or colloidal sized zeolite. Representative of molecular
sieves (zeolites) which can be used include but are not limited
to those of structure types AFI, AEL, BEA, CHA, EUO, FAU, FER, KFI,
LTA, LTL, MAZ, MOR, MEL, MTN, MTT, MTW, OFF, TON includes zeolite
X and zeolite Y) zeolite beta and especially MFI zeolites. Preferably,
a MFI zeolite with a silicon to aluminium ratio greater than 30
will be used including compositions with no aluminium. MFI zeolites
with Si/Al ratios greater than 300 are herein referred to as silicate.
Some of the above materials, while not being true zeolites are frequently
referred to in the literature as such, and the term zeolite will
herein be used broadly to include such materials.
A synthesis mixture to form the molecular sieve microcrystals that
are advantageously applied to the support is advantageously prepared
by the process described in International Application WO93/08125.
In that process, a synthesis mixture is prepared by boiling an aqueous
solution of a silica source and an organic structure directing agent
in a proportion sufficient to cause substantially complete dissolution
of the silica source. The organic structure directing agent, if
used, is advantageously introduced into the synthesis mixture in
the form of a base, specifically in the form of a hydroxide, or
in the form of a salt, e.g. a halide, especially a bromide. Mixtures
of a base and a salt thereof may be used, if desired or required,
to adjust the pH of the mixture.
Other suitable molecular sieve microcrystals may be prepared by
the methods described in PCT/EP96103096 PCT/EP96/03097 and PCT/EP96/0309698
the disclosures of which are hereby incorporated by reference.
The structure directing agent may be, for example, the hydroxide
or salt of tetramethylammonium (TMA), tetraethylammonium (TEA),
triethylmethylammonium (TEMA), tetrapropylammonium (TPA), tetrabutylammonium
(TBA), tetrabutylphosphonium (TBP), trimethylbenzylammonium ITMEA),
trimethylcetylammonium (TMCA), trimethyineo- pentylammonium (TMNA),
triphenylbenzylphosphonium (TPBP), bispyrrolidinium (BP), ethylpyridinium
(EP), diethylpiperidinium (DEPP) or a substituted azoniabicyclooctane,
e.g. methyl or ethyl substituted quinuclidine or 14-diazoniabicyclo-(222)octane.
16- diaminohexane, 18-diaminooctane, or a crown ether may also
be used.
Preferred structure directing agents are the hydroxides and halides
of TMA, TEA, TPA and TBA.
The monolayer of the present invention may further comprise additional
materials such as silica and/or metal oxide; metal particles; metal
particles with metal oxides and/or silica. The monolayer may be
formed from a solution containing a nanocrystalline or colloidal
zeolite or a mixture of metal oxide and nanocrystalline or colloidal
zeolite or a mixture of nanocrystalline or colloidal zeolite and
colloidal metal. Preferably, nanocrystalline or colloidal zeolite
or a mixture of nanocrystalline or colloidal zeolite and metal oxide
will be used to form the monolayer layer. The metal oxides from
which the monolayer is prepared are colloidal metal oxides or polymeric
metal oxides prepared from sol-gel processing. In this aspect of
the present invention the metal oxides which can be used herein
are selected from the group consisting of colloidal alumina, colloidal
silica, colloidal zirconia, colloidal titania and polymeric metal
oxides prepared from sol-gel processing and mixtures thereof. Preferably
colloidal alumina will be used. The colloidal metals which can be
used include copper, platinum and silver. If used these additional
materials must also have the required surface charge in relation
to the substrate.
By adjusting the ratio of colloidal zeolite and metal oxide, the
density of nucleation sites on the monolayer can be controlled.
This density controls the morphology of the zeolite film grown over
this layer in a subsequent hydrothermal synthesis step. The higher
the nucleation density, the narrower the molecular sieve crystal
width the crystals will exhibit at the molecular sieve film/substrate
interface. Nucleation density can be controlled by the relative
proportions of microcrystals and metal oxides (with the density
decreasing as the amount of the metal oxide utilised increases)
as well as the size of the microcrystals in the monolayer. Microcrystals
in the range of 50-10000 .ANG. are thus used in this layer. The
larger the microcrystals utilised in this layer, the wider the zeolite
crystals in the upper layer will be. It is If preferred that the
monolayer consists substantially of molecular sieve microcrystals
and most preferably that these molecular sieve microcrystals are
zeolite crystals and are colloidal in nature preferably less than
100 nm.
The first step in preparing molecular sieve films according to
the present invention entails bonding to the substrate surface of
one or more nearly continuous monolayers of discrete microcrystals
that will ultimately form and become an integral part of the molecular
sieve film. To obtain the best results, at the time of deposition,
the microcrystals should form discrete entities and not aggregates
consisting of several particles.
Advantageously, the crystal size of the molecular sieve microcrystals
in the monolayer is at most 500 nm preferably at most 300 nm, ideally
within the range 10 to 300 nm, 20 to 200 nm and most preferably
20 to 120 nm.
Advantageously the size of the additional particles if present
are the same or similar to that of the molecular sieve microcrystals.
A key aspect of this stage of the process is the selection or preparation
of the substrate onto which the monolayer is to be deposited.
Selection of the appropriate substrate assumes that the substrate
surface has or may be imparted a surface charge that is sufficiently
strong and of the opposite sign to that of the species to be adsorbed.
The majority of the molecular sieves of interest in the present
invention are metal silicates, that may be characterized as having
a negative charge in neutral or alkaline aqueous suspensions. The
magnitude of the surface charge is generally at its highest in the
pH range 8-12 and hence this pH range is suitable for adsorbing
microcrystals onto substrate surfaces.
Certain types of molecular sieves are prepared in the presence
of tetraalkyl ammonium ions in stoechiometric excess. In such cases,
the adsorption on several types of surfaces is promoted if the excess
tetraalkyl ammonium ions are replaced by, for instance, ammonium
ions. This may be achieved by allowing the microcrystal suspension
to pass through a column packed with an organic ion exchange resin
in the ammonium form, or by adding ion exchange resin in such form
to a microcrystals containing suspension and, after complete ion
exchange, separating the ion exchange resin from the suspension
through for example filtration or centrifugation.
The support for use in the present invention may be may be either
non-porous or porous. As examples of non-porous support there may
be mentioned glass, fused quartz, and silica, silicon, dense ceramic,
for example, clay, and metals. As examples of porous supports, there
may be mentioned porous glass, sintered porous metals, e.g., steel
or nickel or precious metals, and, especially, an inorganic oxide,
e.g., alpha-alumina, titania, an alumina/zirconia mixture, Cordierite,
or zeolite as herein defined.
Examples of substrates that are of major interest for coating with
thin molecular sieve films, in order to be used in the field of
sensor technology include solid silicon wafers, quartz, aluminium
oxide, aluminium silicate and precious metals. Porous aluminium
oxide, silica, aluminium silicate, sintered metal and polymer substrates
are examples of materials that may be used for preparing membranes
for separation processes and, to a certain extent, catalytic membranes.
Within the electrochemistry and electronics application fields,
metal and alloy substrates are of prime interest. Examples of fibrous
materials that may be coated according to the procedures described
in the present invention include glass fibers, ceramic fibers, carbon
fibers, graphite fibers, cellulose fibers and various polymer fibers.
All the above mentioned substrates may be produced according to
known methods and certain substrates are commercially available.
The support may be any material compatible with the monolayer deposition
and film synthesis techniques, as described, for example below,
e.g., porous alpha-alumina with a surface pore size within the range
of from 0.004 to 100 .mu.m, more advantageously 0.05 to 10 .mu.m,
preferably from 0.08 to 1 .mu.m, most preferably from 0.08 to 0.16
.mu.m, and advantageously with a narrow pore size distribution.
The support may be multilayered; for example, to improve the mass
transfer characteristics of the support, only the surface region
of the support in contact with the monolayer may have small diameter
pores, while the bulk of the support, toward the surface remote
from the layer, may have large diameter pores. An example of such
a multilayer support is an alpha-alumina disk having pores of about
1 .mu.m diameter coated with a layer of alpha-alumina with pore
size about 0.08 .mu.m.
For building up films on porous substrates, it is often advantageous
to choose the microcrystal size or the porosity of the support so
that microcrystal size is slightly larger than the substrate pores,
to avoid forming a film in the substrate pore structure and blocking
the substrate pore system.
Many substrates of interest for the preparation of thin molecular
sieve films according to the present invention are oxides or have
a surface covered with an oxide layer, which means that the substrate
also has a negative charge in an aqueous suspension in the pH range
of interest. In such cases, to make the substrate surface suitable
for adsorbing negatively charged microcrystals, it may be charge
reversed to obtain a positive charge. Such a charge reversal can
be achieved by treating the substrate with a solution containing
0.1-4 weight % cationic polymer. The pH value for the charge reversal
is selected after considering both the substrate and the polymer
chemistry. However, cationic polymers may be used within a wide
pH range. The repeat unit in such polymers can be quaternary amines
with hydroxyl groups in the main chain. An example of such a polymer
is Berocell 6100 a water soluble polymer with a repeat unit [CH.sub.2
CH(OH)CH.sub.2 N(CH.sub.3).sub.2 ].sub.n.sup.+ and a molecular weight
of 50000 g/mol, marketed by Akzo Nobel AB, Sweden. Other suitable
polymers a well known in the art.
An alternative, but less preferred method, is to charge reverse
the molecular sieve microcrystals instead of the substrate surface.
This may be achieved according to known methods, similar to those
used for charge reversing the substrate.
For certain substrates it may be advantageous, in order to impart
to them satisfactory surface properties, to submit them to one or
more pretreatment steps, aimed at cleaning their surface or modifying
their surface chemistry. In such cases, it is advantageous to treat
the substrate in one or more alkaline, acid or oxidizing cleaning
steps, or combinations of such steps. Adsorption of molecular sieves
on quartz and aluminium oxide is often promoted by a treatment of
the substrates involving deposing a thin silica layer, that provides
a surface with a high hydroxyl group density and higher surface
charge density under the conditions where the adsorption takes place.
Another way of enhancing microcrystal deposit is to carry out the
adsorption in two or more steps, as the case may be, with an intermediate
charge reversal. For certain substrate types, that may be of interest
for coating with very thin zeolite films according to the present
invention, the procedure described above for depositing a more or
less complete microcrystal monolayer is not satisfactory, since
it is not possible to achieve a sufficiently high surface charge
on the substrate surface. Examples of such surfaces are precious
metals and the majority of the organic polymers. In such cases coupling
agents, for instance of the silane type, may be used according to
known techniques. Such coupling agents are characterized by the
fact that they consist of two functional groups, one of them having
affinity for the substrate surface and the second one binding to
the microcrystal surface. For bonding zeolite or metal silicate
microcrystals to precious metal surfaces, a silane containing a
thiol group is often suitable. Coupling agents are made by for example
Dow Corning and Union Carbide and they are generally used for incorporating
inorganic fillers and reinforcing agents into organic polymers.
The coupling agent may be deposited on the substrate and then hydrolised
to provide the required surface charge or it may have inherent functionality
which provides the required charge. Suitable coupling agents are
chemicals which are well known in the art such as those supplied
by OSi specialties is "Silquest Silanes" and as indicated
in their 1994 brochure for these products. The coupling agent may
be utilised in conjunction with the cationic polymers as indicated
above to provide the required surface charge. Thus the charge reversal
or control may be achieved by; utilisation of the appropriate pH
of the solution into which the substrate is immersed and which contains
the microcrystals to induce opposite charges on crystals and substrate
surface; deposition of a cationic polymer which imparts appropriate
charge reversal in relation to the microcrystals; or utilisation
of a coupling agent with or without hydrolysis and/or with a suitable
cationic polymer.
The monolayer deposition process may be repeated a number of times
in order to ensure the complete formation of a true monolayer or
to achieve the desired density of coverage of the substrate surface
with a sub-monolayer.
In one aspect of the present invention the support with monolayer
deposited thereon is placed in the synthesis mixture without any
further treatment of the monolayer. Even when submerged in the synthesis
mixture, the microcrystals of the monolayer remain adhered to the
support and facilitate growth of the molecular sieve film. However,
under some circumstances, e.g. during stirring or agitation of the
synthesis mixture during hydrothermal synthesis, the adhesion between
the particles and the support may be insufficient and steps must
be taken to stabilise the monolayer and fix its position.
Preferably therefore the monolayer is stabilised or fixed in place
before being placed into the synthesis mixture. This stabilisation
can be achieved in one aspect by heat-treating the monolayer, e.g.
at temperatures between 30 and 1000.degree. C., preferably greater
than 50.degree. C. and more preferably between 200.degree. C. and
1000.degree. C. and most preferably greater than 300.degree. C.
A preferred range would be 400 to 600.degree. C. This heat treating
is for preferably at least two hours with or without steam.
In an alternative method of stabilisation the monolayer may be
treated with a solution that modifies the surface characteristics
of the microcrystals in the monolayer. For example, the layer may
be washed with a solution that would cause the microcrystal particles
in the monolayer to flocculate; without wishing to be bound by theory
it is believed that a processes similar to flocculation in colloidal
solutions may also bind the microcrystals in the monolayers more
strongly together. Suitable solutions include those which comprise
materials which will ion-exchange with the monolayer. These include
solutions of divalent metal ions such as for example solutions comprising
alkaline earth metal salts. As an example, a wash with a diluted
Ca salt e.g. CaCl.sub.2 solution may be mentioned. In this aspect
there may be included the additional step of heating of the treated
layer at a temperature of up to 300.degree. C. and preferably up
to 200.degree. C. Those skilled in the art will appreciate that
many other solutions or treatments may be used to stabilise the
monolayer.
The monolayer structure of the present invention is utilised in
the manufacture of a molecular sieve film according to the present
invention. In this process the monolayer structure is contacted
with a synthesis solution for the required molecular sieve.
In the manufacture of molecular sieve films according to the present
invention, the deposited microcrystals are allowed to grow on the
substrate surface. The growth of the initially discrete crystals
leads to their intermeshing and the more or less complete discrete
microcrystal monolayer is transformed into a continuous and dense
molecular sieve film on the substrate surface. The thickness of
the thinnest film necessary to obtain a continuous and dense film
is dictated by both the size of the deposited crystals and the degree
of close-packing of such crystals on the substrate surface and their
orientation on the substrate surface. With maximum close-packing,
it is In most cases sufficient to grow the crystals to a film thickness
corresponding to one and a half times the thickness of the monolayer
which when the crystals are approximately spherical corresponds
to one and a half times the diameter of the initially deposited
crystals, in order to obtain a continuous and crack-free film. When
the crystals have a geometric shape other than spherical then they
may be deposited on the substrate to form a monolayer which consists
of crystals which are oriented and may also be close packed. In
this case the faces of the crystals which are in a plane other than
the plane of the monolayer surface may in fact be the faces of the
crystals from which the new molecular sieve growth is greatest for
film formation. The result of this arrangement is that the crystals
in the monolayer grow from these faces towards each other and form
a dense thin film with little or no growth from the surface plane
of the monolayer. Thus in this case a sufficiently dense film may
be produced which has a thickness which corresponds substantially
to the thickness of the original monolayer.
The molecular sieve film is formed by placing the substrate with
the adsorbed-molecular sieve microcrystals in a solution with a
composition suitable for synthesizing the desired molecular sieve,
and subsequently processing it under conditions suitable for synthesizing
the desired molecular sieve. Since most molecular sieve synthesis
takes place in gels and not in solutions, it should be stressed
that the procedure according to the present invention is applicable
in cases where crystallization is preferably achieved in a clear
solution. In such case, the solution used leads to crystallisation
of the molecular sieve in question, even in the absence of substrate.
The composition of the synthesis mixture varies according to the
process; the mixture often contains a source of silicon, and usually
contains a structure directing agent, for example one of those mentioned
above, and a source of any other component desired in the resulting
zeolite. In some processes according to the invention, a source
of potassium is required. A preferred silicon source is colloidal
silica, especially an ammonia-stabilised colloidal silica, e.g.,
that available from du Pont under the trade mark Ludox AS-40.
The source of silicon may also be the source of potassium, in the
form of potassium silicate. Such a silicate is conveniently in the
form of an aqueous solution such, for example, as sold by Aremco
Products, Inc. under the trade mark CERAMA-BIND, which is available
as a solution of pH 11.3 specific gravity 1.26 and viscosity 40
mPas. Other sources of silicon include, for example, silicic acid.
As other sources of potassium, when present, there may be mentioned
the hydroxide. Whether or not the synthesis mixture contains a potassium
source, it may also contain sodium hydroxide to give the desired
alkalinity.
The structure directing agent, when present, may be any of those
listed above for the synthesis mixture for forming the intermediate
layer crystals.
Suitable molecular sieve synthesis solutions are well known in
the art and are described in for example In International Application
WO 94/25151 International Application PCT/EP93/01209 International
Application PCT/US95/08514 published as WO 96/01687 and International
Application PCT/US95/08511 published as WO 96/01685 the teachings
of which are all incorporated herein by reference.
As molecular sieves for the molecular sieve film, there may be
mentioned a silicate, an aluminosilicate, an aluminophosphate, a
silicoaluminophosphate, a metalloaluminophosphate, or a metalloaluminophosphosilicate.
The preferred molecular sieve will depend on the chosen application,
for example, separation, catalytic applications, and combined reaction
and separation, and on the size of the molecules being treated.
There are many known ways to tailor the properties of the molecular
sieves, for example, structure type, chemical composition, ion-exchange,
and activation procedures.
Representative examples are molecular sieves/zeolites of the structure
types AFI, AEL, BEA, CHA, EUO, FAU, FER, KFI, LTA, LTL, MAZ, MOR,
MEL, MTT, MTW, OFF, TON and, especially, MFI. Some of these materials
while not being true zeolites are frequently referred to in the
literature as such, and this term will be used broadly in the specification
below. Examples of molecular sieves that are of major interest for
the present invention include silicalite, hydroxysodalite, TS-1
as well as the zeolites A, Beta, L, X, Y, ZSM-2 ZSM-1 1 ZSM-22
ZSM-5 and SAPO-34.
For the manufacture of an MFI type zeolite, especially ZSM-5 or
silicalite e.g silicalite-1 the synthesis mixture is advantageously
of a molar composition, calculated in terms of oxides, within the
ranges:
M.sub.2 O:SiO.sub.2 0 to 0.7 to:1 preferably 0.016 to 0.350:1 SiO.sub.2
:Al.sub.2 O.sub.3 12 to infinity:1 (TPA).sub.2 O:SiO.sub.2 0 to
0.2:1 preferably 0 to 0.075:1 H.sub.2 O:SiO.sub.2 7 to 1000:1 preferably
9 to 300:1
wherein TPA represents tetrapropylammonium and M an alkali metal,
preferably sodium or potassium, also Li, Cs and ammonia. Other template
agents may be used in these ratios. For the manufacture of an MFI
layer, a tetrapropylammonium hydroxide or halide is preferably used.
In this specification ratios with infinity as the value indicate
that one of the ratio materials is not present in the mixture.
The process described above when using specific amounts of sodium
generally results in an MFI zeolite layer in which the CPO (as defined
below) is such that the crystallographic c-axis is perpendicular
to the plane of the layer. In the MFI structure, the channel system
comprising straight channels, which lie parallel to the b-axis,
and sinusoidal channels, which lie parallel to the a-axis, lies
parallel to the plane of the layer.
Contacting of the coated support is advantageously carried out
by immersion or partial immersion and with the support in an orientation
and location in the synthesis mixture such that the influence of
settling of crystals formed in the reaction mixture itself, rather
than on the coated surface, is minimised. For example, the surface
to be coated is advantageously at least 5 mm, and preferably at
least 8 mm, from a wall or, especially, the base, of the vessel
to avoid interference from crystals settling and local depletion
of the mixture by a high concentration of growing crystals. Further,
the coated surface is advantageously oriented at an angle within
the range of from 90.degree. to 270.degree., preferably 180.degree.,
180.degree. representing the coating surface horizontal and facing
downward. Especially if the coated surface of the structure is three
dimensional, e.g., a honeycomb, other means may be used to inhibit
settling, for example, agitation, stirring or pumping.
In relation to the processes described herein contacting is to
be understood to include immersion or partial immersion of the substrate
in the relevant zeolite synthesis mixture.
The hydrothermal treatment to form the molecular sieve film is
advantageously carried out by contacting the support carrying the
monolayer in a synthesis mixture, and heating for a time and at
the temperature necessary to effect crystallisation, advantageously
in an autoclave under autogenous pressure. Heating times may be,
for example, in the range of from 1 hour to 14 days, advantageously
from 1 hour to 6 days. If microwave heating is used the time may
be reduced to a matter of minutes. Temperatures are below 200.degree.
C. advantageously below 150.degree. C. and within the range of 80
to 150.degree. C., preferably within the range 80 to 125.degree.
C. and most preferably less than 100.degree. C.
If desired, the formation of zeolite crystals within the synthesis
mixture itself may be inhibited by maintaining the pH of the synthesis
mixture in the range of from 6 to 13. In such low-alkaline synthesis
mixtures the effectiveness of the molecular sieve crystals in the
monolayer in acting as seed crystals is enhanced, thereby facilitating
the growth of the molecular sieve film. On the other hand, if so
desired, the formation of molecular sieve crystals within the synthesis
mixture itself may be controlled by adding very small quantities
of colloidal size seed crystals to the synthesis mixture, thereby
reducing the growth of the molecular sieve film. It is believed
that the addition of controlled amounts of colloidal molecular sieves
to the synthesis mixture enables the thickness of the molecular
sieve film to be controlled without changing the pH of the synthesis
mixture, the crystallisation time or the crystallisation temperature.
Apart from the formation of a molecular sieve film on the substrate
surface, crystals of the same molecular sieve type are formed in
the solution phase. The conditions used in this step of the procedure
can in normal cases be detrimental to the substrate surface, for
example etching and dissolving, in the case of certain substrate/molecular
sieve combinations when highly alkaline solutions are used. The
adsorbed layers of cationic polymer and molecular sieve microcrystals
provide some protection against such attack.
For certain types of molecular sieves, a final calcination step
is necessary to burn off the organic molecules in the pore structure,
thus providing an internal pore structure available for adsorption,
catalysis or ion exchange. Calcination of films prepared according
to the present inventions, and most often comprised of a treatment
in air at a temperature exceeding 400.degree. C., does not lead
to cracks that can be observed with a scanning electron microscope.
If the support is porous then, advantageously, before the molecular
sieve microcrystals are applied from an aqueous reaction mixture
or the monolayer structure is contacted with the synthesis solution,
the support is treated with a barrier layer.
The barrier layer functions to prevent the coating mixture or components
thereof from preferentially entering the pores of the support e.g.
to such an extent that the zeolite crystals form a thick gel layer
on the support.
The barrier layer may be temporary or permanent. As a temporary
layer, there may be mentioned an impregnating fluid that is capable
of being retained in the pores during application of the reaction
mixture, and readily removed after such application and any subsequent
treatment.
To improve penetration, the fluid barrier may be applied at reduced
pressure or elevated temperature. Premature evaporation of fluid
from the outermost pores during treatment may be reduced by providing
an atmosphere saturated with the liquid vapour.
As a temporary barrier layer suitable, for example, for an alpha-alumina
support there may be especially mentioned water or glycol. As a
permanent barrier suitable for an alpha-alumina support there may
be mentioned titania, gamma-alumina or an alpha-alumina coating
of smaller pore size.
Larger supports, for example, honeycomb reactor sections, may be
treated by sealing the support in its reactor housing, either before
or after applying the monolayer, and the synthesis mixture then
poured into the housing, or pumped through it, crystallisation,
washing and calcining taking place with the support already in its
housing.
If desired or required, areas of the support whether porous or
non-porous upon which it is not wanted or needed to form the monolayer
and/or the molecular sieve film may be masked before application,
using, e.g., wax, or unwanted zeolite on such areas may be removed
after application.
The molecular sieve film structure of the present invention despite
being relatively thin (less than 2 .mu.m) is found to be continuous
and dense and to consist of intergrown molecular sieve crystals.
In addition in contrast with some prior art methods although a nucleation
seeding layer is utilised in its preparation this is no longer visible
in the final product as a distinct layer because the monolayer microcrystals
are incorporated into the molecular sieve film. The resultant product
has the attributes of prior art molecular sieve films prepared using
multi-layer nucleation layers without the problems associated with
such layers which are for example reduced flux of the final membrane
and being prone to crack formation.
It will be appreciated that the structure may be of any shape,
and may be, for example, planar, cylindrical, especially cylindrical
with a circular cross-section, or may be a honeycomb structure.
For clarity, however, the following description will refer to the
structure as if it were planar, and references will be made to the
plane of a layer.
This molecular sieve film in this structure is also found to exhibit
a certain degree of CPO (as defined below) and SPO (as defined below).
The film despite being relatively thin is found to be columnar in
nature.
What is meant by columnar in this context is that the molecular
sieve film comprises a crystalline molecular sieve in which at least
75%, as viewed by scanning electron microscopy (SEM), and advantageously
at least 85%, of the crystallites at the uppermost face extend to
the interface between the film and the substrate.
Advantageously, at least 75%, as viewed by scanning electron microscopy
(SEM), of the grain boundaries in the upper layer are, at least
in the region of the uppermost face, within 30.degree. of the perpendicular
to the layer plane, more advantageously at least 90% being within
that angle, and preferably at least 90% are within 25.degree. and
most preferably 15.degree. of perpendicular.
The directions of grain boundaries of the crystals in the upper
layer indicate the extent to which the crystals have a shape preferred
orientation (SPO).
Materials comprising non-spherical grains may exhibit a dimensional
preferred orientation or shape preferred orientation (SPO). An SPO
may be defined, for example, as a non-random orientation distribution
of the longest dimensions of the grains or crystals. Such an SPO
may be detected, for instance, on cross-sectional electron micrographs;
only the outline of the grains or crystals is considered, the orientation
of the longest dimension of each is determined and this is used
to determine the orientation distribution.
Because the shape of a grain or crystal is not necessarily related
to its crystallographic orientation, SPO is in principle independent
from CPO, although in many cases SPO and CPO are related.
The products of the invention may be characterised by X-Ray Diffraction
(XRD) among other techniques. For this purpose a conventional powder.
diffraction technique may be used, where the supported layered structure
in the shape of a disk is mounted in a modified powder sample holder
and a conventional .theta./2.theta. scan is performed. The intensities
of the zeolite reflections thus measured are compared to the intensities
of reflections of a randomly oriented powder of a zeolite of the
same structure and composition. If one or more sets of reflections,
related to one or more specific orientations of the crystal, are
significantly stronger than the remaining reflections as compared
to the diffractogram of a randomly oriented powder, this Indicates
that the orientation distribution In the sample deviates from random.
This is referred to as a crystallographic preferred orientation
or CPO. An example of a simple CPO is the case where the 001 reflections
(e.g., 002 004 006 etc. for MFI) are strong while all other reflections
are weak or absent. In this case the majority of the crystals has
the crystallographic c-axis close to the normal to the plane of
the layer; it is often referred to as a c-axis CPO. Another example
is a diffractogram where the hOO reflections (200 400 600 800
etc. for MFI) are dominant; this is referred to as an a-axis CPO.
More complex situations may also occur, for example a diffractogram
where both the OkO and 00l reflections dominate, which is referred
to as a mixed b- and c-axis CPO.
In the case of a CPO, a unique identification of the crystal structure
type based on the XRD diffractogram of the layer alone may not be
possible, because only a limited number of reflections may be detected.
In principle, the material of the layer should be separated from
the substrate, ground to a powder and a randomly oriented powder
diffractogram should be obtained to verify the structure type. In
practice this is often difficult. Therefore, if the synthesis has
yielded any powder product or deposits on the walls or bottom of
the autoclave, this material is used for the identification of the
structure type. If all the reflections in the diffractogram of the
layer can be attributed to specific sets of reflections in the indexed
powder diffractogram (e.g., the 00l reflections), it is good indication
that the layer has the same structure type as the powder.
The quantification of the degree of CPO may be based on the comparison
between the observed XRD diffractogram with that of a randomly oriented
powder. For each type of crystal structure and CPO a specific set
of reflections may be selected to define a number that can be used
as a parameter to describe the degree of CPO. For example, in the
case of a structure in which the uppermost layer has the MFI zeolite
structure type, and the crystals have a c-axis CPO, a CPO-parameter
C.sub.ool may be defined using the intensities, I, of the 002-reflexion
and the combined 200 and 020 reflections, as follows: ##EQU1##
where I.sub.200020 and I.sub.002 are the background-corrected
heights of the combined MFI-200020 reflections and of the MFI-002
reflection, respectively, for a randomly oriented powder R and for
the sample S under investigation, before calcination.
A value for the parameter C.sub.ool of 0 represents random orientation,
while 100 represents the virtual absence of 100 and 010 planes parallel
to the layer plane. The absence of all MFI reflexions except the
00l reflections indicates a nearly perfect alignment of 001 planes
parallel to the layer.
Similarly, in the case of an a-axis CPO, a parameter C.sub.h00
may be defined using the intensity of the 10 0 0 reflection relative
to the intensity of, for instance, the sum of the 002 and 0 10 0
reflections, or the 133 reflection (before calcination) as in the
following definition: ##EQU2##
For other types of CPO other parameters may be defined. Other ways
to measure CPO may also be used, for example, texture goniometry.
Advantageously, for a c-axis CPO, a structure according to the
invention has a parameter C.sub.ool of at least 50 and preferably
at least 95. Advantageously, however, the molecular sieve film exhibits
strong CPO and SPO.
Advantageously, In the molecular sieve film, the crystals are contiguous,
i.e., substantially every crystal is in contact with its neighbours,
although not necessarily in contact with its neighbours throughout
its length. (A crystal is in contact with its neighbour if the space
between them is less than 2 nm wide.) Preferably, the molecular
sieve film is substantially free from defects greater than 4 nm
in cross-section, extending through its thickness.
The process of the present invention allows the preparation of
thin films (leos than 2 .mu.m) of molecular sieve on a substrate
with acceptable performance and properties without the visible presence
of an intermediate seeding layer. However the process is equally
applicable to the preparation of thicker films of molecular sieve
on a substrate e.g films of up to 150 .mu.m may be prepared using
this technique. It is expected that such films will also have superior
properties and performance. Advantageously, the thickness of the
molecular sieve film layer and the crystallite size of the molecular
sieve are such that the layer thicknesses approximately the size
of the longest edges of the crystals, giving essentially a monolayer
with a columnar structure.
The invention also provides a structure in which the support, especially
a porous support, has molecular sieve films according to the invention
on each side, the layers on the two sides being the same or different;
it is also within the scope of the invention to provide a film not
in accordance with the invention on one side of the support, or
to incorporate other materials in the support if it is porous.
The molecular sieve film may, and for many uses advantageously
do, consist essentially of the molecular sieve material, or may
be a composite of the molecular sieve material and intercalating
material which may be organic or inorganic. The intercalating material
may be the same material as the support. The material may be applied
simultaneously with or after deposition of the molecular sieve,
and may be applied, for example, by a sol-gel process followed by
thermal curing. Suitable materials include, for example, inorganic
oxides , e.g., silica, alumina, and titania. The intercalating material
is advantageously present in sufficiently low a proportion of the
total material of the film that the molecular sieve crystals remain
contiguous.
Although it is expected that in many cases the molecular sieve
film will be crack and defect free it is possible that during preparation
of the film or during post-treatment or handling or use of the film
cracks or defects may be formed. The remainder of the film under
these circumstances may be intact and of high quality and performance
although the film as a whole will be deficient due to these local
defects. In these cases it will be necessary to separate the film.
Such reparation techniques for inorganic membranes are known in
the art.
A catalytic function may be imparted to the molecular sieve film
structure of the present invention either by bonding of a catalyst
to the support or the free surface of the molecular sieve film,
or its location within a tube or honeycomb formed of the structure,
by its incorporation in the support, e.g., by forming the support
from a mixture of support-forming and catalytic site-forming materials
or in the monolayer or molecular sieve film itself. If the support
is porous a catalyst may be incorporated into the pores, the catalyst
optionally being a zeolite. For certain applications, it suffices
for the structure of the invention to be in close proximity to,
or in contact with, a catalyst, e.g. in particulate form on a face
of the structure.
Catalytically active sites may be incorporated in the molecular
sieve film of the structure, e.g., by selecting as zeolite one with
a finite SiO.sub.2 :Al.sub.2 O.sub.3 ratio, preferably lower than
300. The strength of these sites may also be tailored by ion-exchange.
Metal or metal oxide precursors may be included in the synthesis
mixture for the monolayer microcrystals or molecular sieve film,
or both, or metal, metal oxides, salts or organic complexes may
be incorporated by impregnation of or ion-exchange with the pre-formed
molecular sieve film. The structure may also be steamed, or treated
in other manners known per se, to adjust properties.
The layers may be configured as a membrane, a term used herein
to describe a barrier having separation properties, for separation
of fluid Igaseous, liquid, or mixed) mixtures, for example, separation
of a feed for a reaction from a feedstock mixture, or in catalytic
applications, which may if desired combine catalysed conversion
of a reactant or reactants and separation of reaction products.
Separations which may be carried out using a membrane comprising
a structure in accordance with the invention include, for example,
separation of normal alkanes from co-boiling hydrocarbons, for example,
normal alkanes from isoalkanes in C.sub.4 to C.sub.6 mixtures and
n-C.sub.10 to C.sub.16 alkanes from kerosene; separation of normal
alkanes and alkenes from the corresponding branched alkane and alkene
isomers; 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
(e.g. separation of p-xylene from a p-xylene-rich mixture produced
in a xylene isomerization process), 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 aromatic compounds from aliphatic compounds and hydrogen
in a reforming reactor; separation of olefinic compounds from saturated
compounds, especially light alkenes from alkanelalkene 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; removing hydrogen from the products of refinery
and chemical processes such as the dehydrogenation of alkanes to
give alkenes, the dehydrocyclization of light alkanes or alkenes
to give aromatic compounds and the dehydrogenation of ethylbenzene
to give styrene; removing alcohols from aqueous streams; and removing
alcohols from hydrocarbons, especially alkanes and alkenes, that
may be present in mixtures formed during the manufacture of the
alcohols.
Conversions which may be effected include isomerizations, e.g.,
of alkanes and alkenes, conversion of methanol or naphtha to alkenes,
hydrogenation, dehydrogenation, e.g., of alkanes, for example propane
to propylene, oxidation, catalytic reforming or cracking and thermal
cracking.
Feedstocks derived from hydrocarbons, e.g., in the form of petroleum
or natural gas or feedstocks derived from coal, bitumen or kerogen,
or from air, the feedstocks containing at least two different molecular
species, may be subjected to separation, e.g., by molecular diffusion,
by contact with a structure according to the invention, advantageously
one configured as a membrane, at least one species of the feedstock
being separated from at least one other species.
The following table gives examples of such separations.
Separated Molecular Feedstock Species Mixed xylenes (ortho, para,
meta) and Paraxylene ethylbenzene Mixture of hydrogen, H.sub.2 S,
and ammonia Hydrogen Mixture of normal and isobutanes Normal butane
Mixture of normal and isobutenes Normal butene Kerosene containing
C.sub.9 to C.sub.18 normal C.sub.9 to C.sub.18 normal paraffins
paraffins Mixture of nitrogen and oxygen Nitrogen (or oxygen) Mixture
of hydrogen and methane Hydrogen Mixture of hydrogen, propane, and
propylene Hydrogen and/or propylene Mixture of hydrogen, ethane,
and ethylene Hydrogen and/or ethylene Coker naphtha containing C.sub.5
to C.sub.10 normal C.sub.5 to C.sub.10 normal olefins olefins and
paraffins and paraffins Methane and ethane mixtures containing argon,
Helium, neon, and/or helium, neon, or nitrogen argon Intermediate
reactor catalytic reformer products Hydrogen, and/or light containing
hydrogen and/or light gases gases (C.sub.1 -C.sub.7) Fluid Catalytic
Cracking products containing Hydrogen, and/or light H.sub.2 and/or
light gases gases Naphtha containing C.sub.5 to C.sub.10 normal
paraffins C.sub.5 to C.sub.10 normal paraffins Light coker gas oil
containing C.sub.9 to C.sub.18 normal C.sub.9 to C.sub.18 normal
olefins olefins and paraffins and paraffins Mixture of normal and
isopentanes Normal pentane Mixture of normal and isopentenes Normal
pentene Mixture of ammonia, hydrogen, and nitrogen Hydrogen and
nitrogen Mixture of A10 (10 carbon) aromatics e.g. Paradiethylbenzene
(PDEB) Mixed butenes n-butene Sulfur and/or nitrogen compounds H.sub.2
S and or NH.sub.3 Mixtures containing benzene (Toluene Benzene mixtures)
Examples of chemical reactions which may be effected by the structure
of the invention, advantageously one configured as a membrane, in
association with a catalyst, (e.g. the catalyst is a module with
the structure) or treated to impart catalytic activity to the structure,
are given in the following table:
Feedstock/process Product Yielded Mixed xylenes (para, ortho, meta)
Paraxylene and/or ethylbenzene and ethylbenzene Ethane dehydrogenation
to ethylene Hydrogen and/or ethylene Ethylbenzene dehydrogenation
to Hydrogen styrene Butanes dehydrogenation butenes Hydrogen (iso's
and normals) Propane dehydrogenation to Hydrogen and/or propylene
propylene C.sub.10 -C.sub.18 normal paraffin Hydrogen dehydrogenation
to olefins Hydrogen Sulfide decomposition Hydrogen Reforming Hydrogen,
light hydrocarbons dehydrogenation/aromatization (C.sub.1 -C.sub.7)
Light Petroleum Gas Hydrogen dehydrogenation/aromatization Mixed
Butenes n-Butene
The structure of the invention may be employed as a membrane 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 structure of the present invention
that it may be used at such elevated temperatures.
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 structure according to the invention in the form
of a membrane under conditions such that at least one component
of the mixture has a different steady state permeability through
the structure from that of another component and recovering a component
of mixture of components from the other face of the structure.
The present invention accordingly also provides a process for the
separation of a fluid mixture which comprises contacting the mixture
with a structure according to the invention in one embodiment in
the form of a membrane under conditions such that at least one component
of the mixture is removed from the mixture by adsorption. Optionally
the adsorbed component is recovered and used in a chemical reaction
or may be reacted as an adsorbed species on the structure according
to the invention.
The invention further provides such processes for catalysing a
chemical reaction in which the structure is in close proximity or
in contact with a catalyst.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting a feedstock with a structure
according to the invention which is in active catalytic form under
catalytic conversion conditions and recovering a composition comprising
at least one conversion product.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting a feedstock with one face of
a structure according to the invention, that is in the form of a
membrane and in active catalytic form, under catalytic conversion
conditions, and recovering from an opposite face of the structure
at least one conversion product, advantageously in a concentration
differing from its equilibrium concentration in the reaction mixture.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting a feedstock with one face of
a structure according to the invention that is in the form of a
membrane under conditions such that, at least one component of said
feedstock is removed from the feedstock through the structure to
contact a catalyst on the opposite side of the structure under catalytic
conversion conditions.
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 structure 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
structure 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.
DESCRIPTION OF THE ILLUSTRATIONS
FIG. 1 shows a micrograph of a monolayer of silicalite-1 bound
to the surface of a monocrystal Si substrate by means of electrostatic
adsorption, obtained with a scanning electron microscope.
FIG. 2 shows a micrograph of a film of TPA-silicalite-1 on a monocrystal
Si substrate, obtained with a scanning electron microscope.
FIG. 3 shows a micrograph, obtained with a scanning electron microscope,
of of hollow fibers of silicalite-1 (left hand side image) and zeolite
Y right hand side image), prepared by building up a molecular sieve
films on the surface of the carbon fibers, followed by removal of
the carbon fiber through calcination.
CHARACTERIZATION
The procedure according to the present invention was evaluated
by examining the materials by means of scanning electron microscopy
(SEM), powder X-ray diffraction (XRD), spectroscopy, as well as
specific surface area measurements using N.sub.2 and Kr adsorption.
The scanning electron microscopy studies were performed using samples
coated with carbon or gold (by means of respectively vapour deposition
and sputtering techniques). A scanning electron microscope of the
Philips XL 30 type fitted with a LaB.sub.6 emission source was used
in these studies.
X-ray diffraction studies were performed on untreated film samples,
using a Philips PW 1710-00 powder diffractometer.
Kr-adsorption measurements to determine the specific surface area
were performed by means of an ASAP 2010 apparatus from Micromeritics
Instruments Inc. Prior to these measurements, the films were degassed
at 250.degree.C. for 3 hours.
Spectroscopic studies of certain samples prepared according to
the present invention were performed with a Perkin Elmer 2000 FT
i.r. spectrometer (infrared spectroscopy), as well as a Perking
Elmer Lambda 2S UV-VIS spectrometer (spectroscopy within the visible
and ultraviolet wavelength regions).
Particle size and particle size distribution analyses of the colloidal
suspensions of molecular sieves used in preparing the thin molecular
sieve films described in the present invention were made using dynamic
light scattering (ZetaPlus, Brookhaven Instruments).
The invention will be described below by means of several examples.
However, the latter should not be considered as limiting the invention.
EXAMPLE 1
The example illustrates the preparation of a silicalite-1 film
with a thickness of about 100 nm on a crystalline silicon substrate.
A colloidal suspension of discrete silicalite-1 particles was prepared
by hydrothermal synthesis at 55.degree. C. of a synthesis solution
with the following composition : 9 TPAOH: 25 SiO.sub.2 : 480 H.sub.2
O: 100 EtOH, TPAOH representing tetrapropyl ammonium hydroxide and
EtOH representing ethanol. The silicic acid was added as tetraethoxy
silane, hydrolyzed in an aqueous TPAOH solution. After synthesis,
the resulting sol was purified by separation from the mother liquor
by centrifugation, after which the sol particles were re-dispersed
in distilled water. The pH was adjusted to 10.5 by adding 0.10 M
ammonia. The size of the resulting colloidal particles was found
to be 51 nm by dynamic light scattering.
A crystalline silicon substrate in the form of a wafer (40.times.9.times.0.4
mm) was mounted vertically in a teflon holder and cleaned with acetone
in an ultrasonic bath for five minutes. The silicon substrate was
then boiled in a solution with the following composition (on a volume
basis) : (5 H.sub.2 O: 1 H.sub.2 O.sub.2 (30 wt-%): 1 NH.sub.3 (25
wt-%) for five minutes and subsequently in a solution with the following
composition (on a volume basis) : (6 H.sub.2 O: 1 H.sub.2 O.sub.2
(30 wt-%): 1 HCl (37 wt-%)) for five minutes. Between each cleaning
step, the substrate was rinsed with distilled water. After the cleaning
procedure, the substrate was treated for one hour with a solution
adjusted to pH 8.0 and containing 0.4 wt.-% cationic polymer (Berocell
6100 Akzo Nobel AB, Sweden), in order to reverse the surface charge
of the substrate from an initially negative to a positive value.
Excess cationic polymer was rinsed off with 0.1 M ammonia. The surface
modified substrate was transferred to the above described sol (with
a solids content of 2.5 wt-%), containing colloidal silicalite-1
crystals with a size of 51 nm and these crystals were allowed to
adsorb onto the substrate surface for one hour. The colloidal crystals
in excess, if any. were rinsed off with an 0.1 M ammonia solution.
Through this treatment, a monolayer of silicalite crystals was built
up on the substrate surface. FIG. 1 shows an electron micrograph
of the adsorbed silicalite crystals on the substrate surface.
The substrate with the adsorbed silicalite-1 crystal monolayer
was then first processed at 550.degree. C. in a 100% steam atmosphere
for one hour. After cooling, the substrate was further treated with
a synthesis solution of the following composition : 3 TPAOH : 25
SiO.sub.2 : 1500 H.sub.2 O: 100 EtOH, at 100.degree. C. for 13 hours.
This treatment resulted in a continued growth and intermeshing of
the adsorbed crystals and the formation of a dense and continuous
silicalite film on the substrate surface. FIG. 2 shows an SEM micrograph
(lateral view) of this film. From this SEM micrograph, the film
thickness was estimated to be 100 nm.
A sample of the product was analyzed by X-ray diffractometry and
by FTIR spectroscopy. By both these analysis techniques, the film
was identified as consisting of silicalite1. A sample of the product
was calcined at 600.degree. C. in air in a preheated muffle furnace,
to remove organic material (TPA.sup.+) and to make the silicalite
pore structure available for gas adsorption. The specific surface
area of the sample was then determined by means of Kr-adsorption
and found to be 72 m.sup.2 /(m.sup.2 substrate surface), a value
corresponding well with what is to be expected for a 100 nm silicalite
film on this type of silicon substrate.
EXAMPLE 2
This example illustrates the preparation of films with a thickness
of about 100 nm on alumina and quartz substrates.
Monocrystalline structures in the form of sapphire (.alpha.-aluminium
oxide) and of quartz disks (10.times.10.times.1 mm) were vertically
mounted in teflon holders and cleaned with acetone in an ultrasonic
bath for five minutes. The substrates were then boiled in a solution
with the following composition (on a volume basis): (5 H.sub.2 O:
1 H.sub.2 O.sub.2 (30 wt-%): 1 NH.sub.3 (25 wt-%) for five minutes
and subsequently in a solution with the following composition (on
a volume basis): (6 H.sub.2 O: 1 H.sub.2 O.sub.2 (30 wt-%): 1 HCl
(37 wt-%)) for five minutes. Between each cleaning step, the substrates
were rinsed with distilled water. After the cleaning procedure,
the substrates were treated for one hour with a solution adjusted
to pH 8.0 and containing 0.4 wt.-% cationic polymer (Berocell 6100
Akzo Nobel AB, Sweden), in order to reverse the surface charge of
the substrate from an initially negative to a positive value. Excess
cationic polymer was rinsed off with 0.1 M ammonia. The surface
modified substrates were transferred to the above described sol
(with a solids content of 2.5 wt-%), containing colloidal silicalite-1
crystals (see example 1) with a size of 51 nm and these crystals
were allowed to adsorb onto the substrate surface for one hour.
The colloidal crystals in excess, if any, were rinsed off with an
0.1 M ammonia solution. Through this treatment, a monolayer of silicalite
crystals was built up on the substrate surfaces.
The substrates with the adsorbed silicalite-1 crystal monolayer
were then first processed at 550.degree. C. in a 100% steam atmosphere
for one hour. After cooling, the substrate was further treated with
a synthesis solution of the following composition : 3 TPAOH : 25
SiO.sub.2 : 1500 H.sub.2 O: 100 EtOH, at 100.degree. C. for 13 hours.
This treatment resulted in a continued growth and intermeshing of
the adsorbed crystals and the formation of a dense and continuous
silicalite film on the substrate surfaces, as confirmed by analysis
through of scanning electron microscopy, x-ray diffractometry and
specific surface area measurements by Kr-adsorption (after calcining
at 600.degree. C. in air). All these analyses yielded results analogous
to those obtained when using crystalline silicon as the substrate
(see implementation example 1) and show that the method is insensitive
to the chemical properties of the substrate.
EXAMPLE 3
This example illustrates the preparation of a silicalite-1 film
on a carbon fiber substrate.
A colloidal suspension of discrete silicalite-1 particles was prepared
by hydrothermal treatment (100.degree. C.) of a synthesis solution
with the following molar composition: 9 TPAOH : 25 SiO.sub.2 : 480
H.sub.2 O: 100 EtOH, TPAOH representing tetrapropyl ammonium hydroxide
and EtOH ethanol. The silicic acid was added as tetraethoxy silane,
hydrolyzed in water. After synthesis, the resulting sol was purified
by separation from the mother liquor by centrifugation, followed
by re-dispersing the sol particles in distilled water. The pH of
the purified sol was adjusted to 9.5 by adding 0.10 M ammonia. The
size of the silicalite-1 crystals in the resulting sol was found
to be 98 nm by dynamic light scattering.
Continuous carbon fibers, with a diameter of 7-15 micrometers were
first cleaned with acetone in an ultrasonic bath and then with a
solution having the following composition (on a volume basis): (5
H.sub.2 O: 1 H.sub.2 O.sub.2 (30 wt-%): 1 HCl (37 wt-%)). After
both these cleaning steps, the fibers were separated by vacuum filtration
and rinsed with distilled water on the filter paper. After completing
the cleaning procedure, the fibers were treated for one hour with
a solution containing 1.0 wt-% cationic polymer (Berocell 6100
Akzo Nobel AB, Sweden), adjusted to pH 8.0 for inverting the substrate
surface charge from an initially negative to a positive value. The
charge reversed fibers were transferred to a colloidal sol (solids
content: 4.6 wt-%), containing 98 nm silicalite-1 crystals, that
were adsorbed as a monolayer onto the fiber surface. After one hour's
contact between fibers and sol, the fibers were separated by filtration
and washed with an 0.1 M ammonia solution. The fibers were then
transferred to a synthesis solution with the following molar composition
: 9 TPAOH : 25 SiO.sub.2 : 480 H.sub.2 O: 100 EtOH and hydrothermally
treated with this solution for 24 hours at 100.degree. C. Scanning
electron microscopy examinations of the fibers showed this treatment
to provide a dense and continuous silicalite film on the carbon
fiber surface. In order to further characterize the film, a sample
of the product was calcined at 600.degree. C. in air for three hours.
This treatment removed both the organic material in the silicalite
pore structure and part of the carbon fiber around which the film
was formed. As a consequence, there remained after this treatment
a fibrous product consisting of the thin silicalite film built up
on the fiber surface. FIG. 3 shows a scanning electron micrograph
of the resulting material. From the micrograph it can be clearly
seen that the film is indeed continuous and dense. This film was
further characterized by means of x-ray diffractometry, FTIR spectroscopy
and specific surface area measurement using nitrogen adsorption.
These analyses showed the film to consist solely of silicalite-1.
EXAMPLE 4
This example illustrates the preparation of a zeolite Y film on
a carbon fiber substrate.
A colloidal suspension of zeolite Y was prepared by hydrothermal
treatment of a synthesis mixture with the following molar composition
: 2.46 (TMA).sub.2 O: 0.04 Na.sub.2 O: 1.0 Al.sub.2 O.sub.3 : 3.4
SiO.sub.2 : 370 H.sub.2 O, TMA representing the tetramethyl ammonium
ion, which was added as tetramethyl ammonium hydroxide pentahydrate.
The silicic acid was added as sodium stabilized silica sol, and
the aluminate as an alkali stabilized aluminate solution. The alkali
stabilized aluminate solution was prepared as follows: Al.sub.2
(SO.sub.4).sub.3.18 H.sub.2 O was dissolved in distilled water under
mild heating. After complete dissolution, an ammonia solution containing
25 wt-% ammonia in water was added under vigorous stirring, in order
to precipitate Al(OH).sub.3. The resulting gel was separated by
vacuum filtration and the filter cake was dispersed in distilled
water to dissolve the remaining sulfate. The gel was then again
separated by filtration. This procedure was repeated until no sulfate
ions could be detected in the filtrate by adding BaCl.sub.2. The
sulfate-free aluminum hydroxide was dissolved in an a aqueous TMAOH
solution. The thus obtained alkali stabilized aluminate solution
was added to the silica sol under vigorous stirring and treated
at 100.degree. C. until zeolite Y crystals with a size of 100 nm
(as determined by dynamic light scattering) were formed. After completion
of the synthesis the sol was purified by separation from the mother
liquor through centrifugation, followed by re-dispersion in distilled
water. The pH was adjusted to 9.5. by adding 0.1 M ammonia.
Charge reversed carbon fibers, prepared in the manner described
in example 3 were transferred to the colloidal sol (solids content:
2.4 wt-%) containing 100 nm zeolite Y crystals, that were adsorbed
as a monolayer onto the fiber surface. The fibers were in contact
with the sol for one hour. After the sol treatment, the fibers were
separated by filtration and the excess sol was rinsed off with a
0.1 M ammonia solution. The fibers were transferred to a synthesis
solution with the following composition: 2.46 (TMA).sub.2 O: 0.04
Na.sub.2 O: 1.0 Al.sub.2 O.sub.3 : 3.4 SiO.sub.2 : 370 H.sub.2 O,
prepared as described above. The fibers were hydrothermally treated
with this solution for 96 hours at 100.degree. C. Analysis of the
product by means of scanning electron microscopy revealed that this
provided a dense and continuous zeolite Y film on the surface of
the carbon fibers, see FIG. 3. The crystal structure of the film
material was confirmed by further characterization using x-ray diffractometry
and Kr-adsorption.
EXAMPLE 5
This example illustrates the application of a zeolite A film to
a carbon fiber substrate.
A colloidal suspension of zeolite A was prepared by hydrothermally
treating a synthesis mixture with the following molar composition:
1.2 (TMA).sub.2 O: 0.4 Na.sub.2 O: 1.0 Al.sub.2 O.sub.3 : 3.4 SiO.sub.2
: 246 H.sub.2 O, TMA representing the tetramethyl ammonium cation.
The synthesis solution was prepared by the procedure described example
4. After completing the synthesis, the resulting sol was purified
according to the procedure described in example 1 and the pH of
the purified sol was adjusted to 9.5. by adding 0.1 M ammonia. Charge
reversed carbon fibers, prepared in the manner described in example
3 were transferred to the colloidal sot containing 120 nm zeolite
A crystals (solids content: 2.8 wt-%). This resulted in the adsorption
of a monolayer of zeolite A crystals onto the carbon fiber surface.
After this treatment, the fibers were separated by filtration and
rinsed with 0. 1 M ammonia to remove excess sol. The fibers were
transferred to a synthesis solution with the following composition:
1.2 (TMA).sub.2 O: 0.4 Na.sub.2 O: 1.0 Al.sub.2 O.sub.3 : 3.6 SiO.sub.2
: 246 H.sub.2 O, prepared according to the procedure described in
example 4. The fibers were treated with this solution for 60 hrs
at 100.degree. C. This treatment resulted in the formation of a
dense and continuous zeolite A film on the carbon fiber surface,
as shown by scanning electron microscopy and x-ray diffractometry
analysis.
EXAMPLE 6
This example illustrates the application of a Ti-silicalite-1 film
to a carbon fiber substrate.
A colloidal suspension of Ti-silicalite-1 was prepared by hydrothermal
treatment of a synthesis solution with the following composition:
9 TPAOH: 1.46 TiO.sub.2 : 25 SiO.sub.2 : 404 H.sub.2 O: 100 EtOH,
TPAOH being the tetrapropyl ammonium cation added as a solution
of tetrapropyl ammonium hydroxide in water. TiO.sub.2 was added
as tetraethyl orthotitanate (34.1 wt-% TiO.sub.2).
The synthesis solution was prepared as follows. Tetraethyl orthotitanate
was added to tetraethoxy silane under stirring. A solution of TPAOH
was added dropwise to this solution under vigorous stirring. The
resulting synthesis solution was hydrothermally treated at 100.degree.
C. for 20 hours. This resulted in the crystallization of a colloidal
suspension of discrete Ti-silicalite-1 crystals. After completing
the synthesis, the colloidal particles were separated from the mother
liquor by centrifugation, after which the particles were re-dispersed
in distilled water and the pH adjusted to 9.5. by adding 0.1 M ammonia.
The average particle size of the colloidal Ti-silicate crystals
was found to be 90 nm by dynamic light scattering.
Charge reversed carbon fibers, prepared in the manner described
in example 3 were immersed in the colloidal sol (solids content:
1.9 wt-%) of Ti-silicalite-1 microcrystals, that were allowed to
adsorb onto the fiber surface. The duration of the contact between
sol and fibers was one hour. After completing the treatment, the
fibers were washed according to the procedure of example 3.
The fibers were then transferred to a synthesis solution with the
following composition: 9 TPAOH: 1.46 TiO.sub.2 : 25 SiO.sub.2 :
404 H.sub.2 O: 100 EtOH. The synthesis solution was prepared according
to the procedure described above. The fibers were hydrothermally
treated with this solution at 100.degree. C. for 24 hours. Scanning
electron microscopy analysis of this product showed that this treatment
provided a dense and continuous Ti-silicalite-1 film on the carbon
fiber surface. Further analysis of the composite material prepared
was performed by means of x-ray diffractometry, DRIFT (diffuse reflectance
infrared spectroscopy) and UV-VIS spectroscopy. These analyses confirmed
that the film obtained consisted of Ti-silicalite-1.
EXAMPLE 7
This example demonstrates the use of repeated hydrothermal treatments
in adequate synthesis solutions to increase the thickness of the
final film to a desired value.
A substrate of crystalline silicon in the form of a plate (40.times.9.times.0.4
mml was washed in the manner described in example 1. After the washing
procedure, the substrate was treated for five minutes with a solution,
adjusted to pH 8.0 and containing 0.4 wt % cationic polymer (Berocell
6100 Akzo Nobel, Sweden) and subsequently washed with a 0.10 M
ammonia solution. The surface modified substrate was transferred
to a sol, (with a solids content of 2.5 wt %) containing silicalite-1
microcrystals with a size of ca. 30 nm, and contacted with this
sol for five minutes in order to absorb a monolayer of microcrystals
on the substrate surface. After rinsing with 0.10 M ammonia and
drying in air, the substrate with absorbed microcrystals was calcined
at 250.degree. C. in air for 10 minutes and then allowed to cool
to room temperature.
The sample was immersed into a synthesis solution with the composition:
3TPAOH: 25 SiO.sub.2 : 1500 H.sub.2 O: 100 EtOH and treated with
this solution at 100.degree. C. for 62 hours. This treatment resulted
in a continued growth and intergrowth of the adsorbed crystals and
in the formation of a dense and continuous film of silicilate on
the surface of the substrate. The film thickness after this treatment
was determined to 530 nm by SEM. This sample was then hydrothermally
treated in the same manner (for 80 h) with a freshly prepared synthesis
solution having the same composition as that described above. After
this second hydrothermal treatment the film thickness was determined
to ca. 1200 nm by SEM.
EXAMPLE 8
This example demonstrates the preparation of a dense and continuous
film of silicalite-1 on the surface of a porous .gamma./.alpha.-aluminium
oxide membrane.
A conventional .gamma./.alpha.-aluminiumoxide membrane with a nominal
pore size of 5 nm was first treated with O.sub.2 -plasma and then
washed according to the procedure described in example 1. After
the washing procedure, the sample was rinsed with a 0.10 M ammonia
solution filtered through a 0.1 micron PVDF-membrane. The aluminium
oxide membrane was then treated for 10 minutes with a solution,
adjusted to pH 8.0 and containing 0.4 wt % cationic polymer (Berocell
6100 Akzo Nobel, Sweden) and subsequently rinsed with a filtered
0.10 M ammonia solution. The surface modified membrane was transferred
to a sol, (with a solids content of 2.5 wt %) containing silicalite-1
microcrystals with a size of ca. 30 nm, and contacted with this
sol for 10 minutes in order to adsorb a monolayer of microcrystals
on the membrane surface. After rinsing with filtered 0.10 M ammonia
and drying in air, the membrane with adsorbed microcrystals was
calcined by placing in it a cool furnace, heating the furnace to
425.degree. C. over a period of 20 minutes and keeping this temperature
for 10 minutes. The furnace was then switched off and allowed to
cool to room temperature before the membrane was taken out. The
membrane was then mounted in a teflon holder designed to protect
the reverse side of the membrane from the synthesis solution. The
mounted membrane was hydrothermally treated with a synthesis solution
having the composition: 3TPAOH: 25 SiO.sub.2 : 1500 H.sub.2 O: 100
EtOH at 100.degree. C. for 77 hours. Prior to use, the synthesis
solution was filtered through a PVDF-membrane with a pore size of
0.1 micron. After the hydrothermal treatment, the membrane was taken
out and rinsed with 0.10 M ammonia. The synthesis procedure was
then repeated using the same conditions and the same (freshly prepared)
synthesis solution as above. The membrane was finally rinsed with
0.10 M ammonia and dried in air. The non-calcined membrane was tested
in a permeation experiment at 70.degree. C. and with .DELTA.P=1.0
bar. The measured flux was <1.7 10.sup.-5 l/(m.sup.2 h bar) showing
that the film was essentially gas tight.
EXAMPLE 9
This example demonstrates the preparation of a thin film of hydroxysodalite
on a quartz substrate.
A colloidal suspension of discrete crystals of hydroxysodalite
was prepared by hydrothermal treatment of a solution with the molar
composition: 14(TMA).sub.2 O: 0.85Na.sub.2 O: 1.0Al.sub.2 O.sub.3
: 40SiO.sub.2 : 805H.sub.2 O. The sol was purified using the procedure
described in example 1 and the final pH of the sol was adjusted
to 10.5 by addition of 0.1 M NaOH. The size of the colloidal microcrystals
of hydroxysodalite was determined to 26 nm by dynamic light scattering.
A monocrystalline substrate in the form of a plate (10.times.10.times.1
mm) of quartz was vertically mounted in a teflon holder and washed
according to the procedure described in example 1. After the washing
procedure, the substrate was treated for 10 minutes with a solution,
adjusted to pH 8.0 and containing 0.4 wt % cationic polymer (Berocell
6100 Akzo Nobel, Sweden) and subsequently washed with a 0.10 M
ammonia solution. The surface modified substrate was transferred
to a sol, (with a solids content of 2.5 wt %) containing the hydroxysodalite
microcrystals, and contacted with this sol for 10 minutes. In order
to improve the adsorption of microcrystals on the substrate surface,
the treatment with the cationic polymer and the subsequent adsorption
of microcrystals was repeated once. After rinsing with 0.10 M ammonia
and drying in air, the substrate with adsorbed microcrystals was
calcined at 425.degree. C. in air for 30 minutes and then allowed
to cool to room temperature. The sample was immersed into a synthesis
solution with the composition: 14(TMA).sub.2 O: 1.9Na.sub.2 O:1.0Al.sub.2
O.sub.3 : 40SiO.sub.2 : 815H.sub.2 O and treated with this solution
at 100.degree. C. for 16 hours. This treatment resulted in a continued
growth and intergrowth of the adsorbed crystals and in the formation
of a film of hydroxysodalite on the surface of the substrate.
EXAMPLE 10
This example illustrates the preparation of a thin continuous film
of zeolite Beta on a tantalum substrate.
The substrates used for growing zeolite Beta films were Ta plates
(Plansee, Austria, 99.9%) with dimensions 10.times.30 mm. Prior
to use, the Ta surface was cleaned at room temperature for 15 min
with acetone (p.a.) under ultrasonic action and then for 10 min
with a solution having the molar composition 9H.sub.2 O.sub.2 :
10 HCl: 350H.sub.2 O. After these cleaning procedures, the Ta plates
were rinsed several times with distilled water. The surface charge
of the Ta plates was reversed by treatment with a cationic polymer
(Berocell 6100 Akzo Nobel, Sweden) with the repeating unit (CH.sub.2
CHOHCH.sub.2 H(CH.sub.3).sub.2).sub.n.sup.+ 0 and an average molecular
weight of 50000 g/mol. The Ta plates were immersed for 1 h in 30
ml of a solution containing 0.5 wt % of the cationic polymer and
then the excess polymer was rinsed with a 0.1 M ammonia solution.
A colloidal suspension of zeolite Beta (average crystal size, 90
nm) was prepared by hydrothermally treating a precursor solution
with the molar composition 0.35Na.sub.2 O: 9TEAOH: 0.25Al.sub.2
O.sub.3 : 25SiO.sub.2 : 295H.sub.2 O for 6 days at 100.degree. C.
The crystals thus obtained were separated form the mother liquor
by centrifugation and redispersed in distilled water whereafter
the suspensions were centrifuged again. This procedure was repeated
until the pH of the zeolite suspension was in the range 9.5-10.0.
The surface modified Ta substrate was contacted with the purified
zeolite Beta suspension (3.5 wt %, pH adjusted to 10 with a 0.1
M ammonia solution) for 1 h whereafter excess zeolite was rinsed
off with a 0.1 M NH.sub.3 solution. SEM analysis of the composite
shows a monolayer of zeolite Beta crystals on the Ta substrate.
The Ta-substrate with adsorbed zeolite Beta crystals was calcined
in air at 300.degree. C. for 1 h whereafter the composite was immersed
in a zeolite Beta precursor solution with the molar composition
given above and hydrothermally treated under reflux for 6 days at
100.degree. C. Scanning electron microscopy (SEM) micrographs of
the zeolite film after hydrothermal treatment shows a polycrystalline
morphology and side-view images show a continuous film with an average
thickness of 200 nm. The Beta film was characterised by Reflection-Absorption
Infrared Spectroscopy (RAIR) using a Perkin Elmer PE 2000 FT-IR
spectrometer. The angle of incidence was 83.degree. and 500 interterograms
at 4 cm.sup.-1 resolution was performed for each spectrum. The absorbance
bands characteristic of BEA-type molecular structure at 450 520
570 1150 1175 and 1230 cm.sup.-1 were seen in the spectrum of
zeolite Beta film sample. XRD analysis of the substrate and film
showed that the deposited material is zeolite Beta.
EXAMPLE 11
Colloidal suspensions of TPA-slilicilite-1 were synthesized using
tetraethoxysilane (TEOS, >98% GC grade, <3 ppm A1 Aldrich-Chemie),
tetrapropylammonium hydroxide (TPAOH, 1.0 M in water, 143 ppm Na,
4200 ppm K, <10 ppm A1 Sigma) and distilled water.
Preparation of thin silicalite-1 films
The surface of gold substrates (prepared by depositing gold on
pretreated TiN-covered silicon (100) wafers using a BAL-TEC MED
020 Coating System operating at a base pressure of ca. 2 10.sup.-2
mbar) was modified by contacting the substrate with a solution of
10 mM MPS (gamma-mercaptopropyltrimethoxysilane (MPS, Osi Specialities))
in methanol for 3 hours at room temperature. The surface-attached
silane was hydrolyzed at room temperature in an acidic solution
(0.10 M HCl) for 15 hours. The excess silane was removed by rinsing
with ethanol.
A suspension of descrete colloidal crystals with an average size
of 90 nm was prepared in a similar manner to earlier examples. The
molar composition of the seed precursor sol was 9 TPAOH: 25 SiO.sub.2
: 480 H.sub.2 O: 100 ethanol. The pH of the purified sol was reduced
to 3.4 by the addition of a strong cationic exchange resin (DOWEX
HCR-S(H.sup.+), Dow Chemical). A monolayer of silicaliate-1 was
adsorbed by contacting the silane-covered substrate |