Molecular sieve abstract
A method is provided for the preparation of a synthetic, non-composited
microporous membrane comprising a continuous array of crystalline
molecular sieve material. A non-porous forming surface is contacted,
under crystallization conditions, with a chemical mixture capable
of forming the molecular sieve material. After a layer of the molecular
sieve material is crystallized on the forming surface, the layer
and the forming surface are recovered from the chemical mixture
and the layer is separated from the forming surface.
Molecular sieve claims
What is claimed is:
1. A method for preparing a noncomposited, microporous membrane
which comprises a continuous array of crystalline molecular sieve
material the method comprising preparing a chemical mixture capable
of forming the crystalline molecular sieve material, contacting
a non-porous forming surface with the chemical mixture under crystallization
conditions until crystallization of a continuous layer of crystalline
molecular sieve material occurs on the non-porous forming surface,
recovering the layer and the non-porous forming surface from the
chemical mixture, rinsing the layer, and separating the layer from
the forming surface to obtain said non-composited, microporous membrane.
2. The method of claim 1 which further comprises calcining the
crystalline molecular sieve material before or after the layer of
crystalline molecular sieve material is separated from the forming
surface.
3. The method of claim 1 which further comprises steaming the crystalline
molecular sieve material at a temperature of from about 200.degree.
C. to 800.degree. C. for from about 1 to 50 hours before or after
the layer of crystalline molecular sieve material is separated from
the forming surface.
4. The method of claim 1 which further comprises ion exchanging
the crystalline molecular sieve material by contacting the material
with a salt of replacing ions before or after the layer of crystalline
molecular sieve material is separated from the forming surface.
5. The method of claim 4 wherein the ion is an alkali or alkaline
earth metal.
6. The method of claim 5 wherein the ion is selected from the group
consisting of Mg, Ca, Sr, Ba, Na, K, Li, Rb, and Cs.
7. The method of claim 1 which further comprises incorporating
a metal function into the membrane.
8. The method of claim 7 wherein the metal function is selected
from the group consisting of Pd, Pt, Ru, Mo, W, Ni, Fe and Ag.
9. The method of claim 1 which further comprises depositing into
the membrane a compound selected from the group consisting of metal
oxides, phosphorous compounds, silicon compounds, organic compounds,
and coke.
10. The method of claim 1 wherein said chemical mixture capable
of forming the crystalline material has a composition in terms of
moles per mole YO.sub.2 including:
wherein X is a trivalent element and Y is a tetravalent element.
11. The method of claim 10 wherein X comprises at least one member
selected from the group consisting of aluminum, boron, iron and
gallium; and Y comprises at least one member selected from the group
consisting of silicon, germanium and titanium.
12. The method of claim 10 wherein X comprises aluminum and Y comprises
silicon.
13. The method of claim 10 wherein said chemical mixture has a
composition including:
14. The mixture of claim 10 wherein said chemical mixture has a
composition including:
15. The method of claim 1 wherein said molecular sieve material
comprises a zeolite.
16. The method of claim 1 wherein said molecular sieve material
comprises an aluminophosphate, silicoaluminophosphate, metaloaluminophosphate
or metaloaluminophosphosilicate.
17. The method of claim 1 wherein said molecular sieve material
consists essentially of silica.
18. The method of claim 1 wherein said molecular sieve material
has composition in terms of mole ratios of oxides as follows:
wherein X is a trivalent element selected from the group consisting
of aluminum, boron, iron and gallium and combinations thereof; Y
is a tetravalent element selected from the group consisting of silicon,
germanium, titanium and combinations thereof; and n is at least
about 2.
19. The method of claim 18 wherein X comprises aluminum and Y comprises
silicon.
20. The method of claim 18 wherein n is from about 20 to about
10000.
Molecular sieve description
BACKGROUND OF THE INVENTION
The invention relates to membranes having molecular sieve properties
and/or catalytic activity and a process for producing the membranes.
Membrane separation technology is a rapidly expanding field. Organic
and inorganic materials have been used as membranes in a variety
of separation processes, such as microfiltration, ultrafiltration,
dialysis, electrodialysis, reverse osmosis and gas permeation. Most
membranes have been made from organic polymers with pore sizes ranging
from 10 to 1000 angstroms. Membranes have also been made from inorganic
materials such as ceramics, metals, clay and glasses.
Synthetic zeolites have been used as adsorptive separating agents
for gases or liquids or as catalysts and have usually been sued
in the form of granules or pellets often incorporated with a binder
such as clay or alumina. The potential of zeolites as components
in microporous membranes has not been fully explored.
Zeolites have also been used as components in composite membranes.
In such membranes, in addition to the presence of a zeolite phase,
the membrane material always contains a second phase with distinctly
different chemical composition, physical properties, chemical properties
and morphology. As a result of the presence of different phases,
the separation properties of composite membranes are determined
by the individual properties of the different phases and of the
phase boundaries. (Demertzes et al., J. Chem. Soc., Faraday Trans.
1 82 3647 (1986)). Examples of such non-zeolitic phases are polymeric
materials and inorganic materials such as glasses, silica or alumina.
Composite membranes or filters of materials such as paper and polymers
which may contain dispersed particles of zeolites have been described
,for example, in U.S. Pat. Nos. 3266973; 3791969; 4012206;
4735193; 4740219 and European Patent Application 254758.
U.S. Pat. No. 4699892 describes a composite membrane having an
ultrathin film of a cage-shaped zeolite of from 10 to several hundred
angstroms in thickness on a porous support of metal, inorganic material
or polymeric material.
Non-composited inorganic membranes are described, for example,
in U.S. Pat. Nos. 3392103; 3499537; 3628669 and 3791969.
U.S. Pat. No. 3392103 describes membranes made from hydrous metal
oxide ceramics such as aluminum oxide. U.S. Pat. No. 3499537 discloses
membranes of pressed and sintered aluminum vanadate powder. U.S.
Pat. No. 3628669 discloses silica membranes made by leaching thin
inorganic glass films. U.S. Pat. No. 3791969 describes membranes
of flocculated sodium exfoliated vermiculite.
Other non-composited membranes described in U.S. Pat. Nos. 3413219
and 4238590 require some manner of supporting material U S. Pat.
No 3413219 discloses the preparation of membranes from colloidal
hydrous oxide which is formed on a permeable substrate. U.S. Pat.
No. 4238590 discloses silicic acid heteropolycondensates suitable
for use as membranes but which are not self-supporting and are stretched
over porous or net-like supporting material
It is therefore an object of the invention to provide a pure and
spatially continuous molecular sieve membrane It is also an object
to provide a material of macroscopic dimensions, composed only of
a zeolitic phase, and having adequate mechanical strength to maintain
its macroscopic structural integrity and capable of carrying out
molecular sieve action
SUMMARY OF THE INVENTION
The invention is manufacture of a synthetic, non-composited, microporous
membrane comprising a continuous array of crystalline molecular
sieve material. The molecular sieve may have a composition in terms
of mole ratios of oxides as follows:
wherein X is a trivalent element of at least one member selected
from the group consisting of aluminum, boron, iron and gallium;
Y is a tetravalent element of at least one member selected from
the group consisting of silicon, germanium and titanium; and, n
is at least about 2.
The crystalline material may also be an aluminophosphate, silicoaluminophosphate,
metaloaluminophosphate or metaloaluminophosphosilicate.
In the method for preparing the microporous membrane, a chemical
mixture capable of forming the crystalline molecular sieve material
is prepared and the mixture is formed into a thin, uncomposited,
cohesive, continuous membrane, dried and calcined.
A method is also provided for using the membrane for the separation
of the components of a gaseous or liquid mixture having at least
two components. The mixture is contacted with an upstream face of
the membrane under separation conditions such that at least one
component of the mixture has a greater steady state permeability
through the membrane than at least one of the remaining component(s)
of the mixture. After contact of the mixture with the membrane and
passage through the membrane, the component with the greater permeability
is collected on the downstream side of the membrane.
A method is also provided for using the membrane as a catalyst.
The membrane is rendered catalytically active and a feedstock is
passed through the upstream face of the membrane under catalytic
conditions. For cases where all or at least one of the reaction
products have higher permeability than the reactant(s), they will
emerge from the downstream side of the membrane. In equilibrium
limited reactions, this will lead to higher single-pass conversion
of the reactant(s) than normally observed and allowed by thermodynamic
equilibrium constraints. At least one or all of the reaction products
are collected on the downstream side of the membrane. Other advantages
can be realized, for example, when one or all of the products inhibit
or poison the desired reaction, or when they would undergo undesired
secondary reactions.
The microporous zeolitic membranes prepared by the method of the
invention advantageously have unique molecular sieve and/or catalytic
properties due to the well defined pore structure of zeolites. The
membranes have the advantages of having different properties from
traditionally used granular form zeolites, and from composited membranes
which include zeolites. These different properties result from the
sheet-like structure of the membranes and the composition of pure
zeolite in the membrane.
For a better understanding of the present invention, together with
other and further objects, reference is made to the following description,
taken together with the accompanying drawings, and its scope will
be pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of a Si-NMR spectrum of the crystalline membrane.
FIG. 2a shows the membrane surface which was exposed to the non-porous
substrate during crystallization.
FIG. 2b shows the membrane surface which was exposed to the synthesis
mixture.
FIG. 2c shows crystal intergrowth on the surface of a large single
crystal.
FIG. 2d is a higher magnification of the view of FIG. 2c.
FIG. 3 illustrates the membrane affixed in a Wicke-Kallenbach cell.
DETAILED DESCRIPTION OF THE INVENTION
Zeolite materials, both natural and synthetic, have been demonstrated
in the past to have catalytic properties for various types of hydrocarbon
conversion Certain zeolitic materials are ordered, porous, crystalline
aluminosilicates having a definite crystalline structure as determined
by X-ray diffraction, within which there are a large number of smaller
cavities and channels or pores. These cavities and pores are uniform
in size within a specific zeolitic material. Since the dimensions
of these pores are such as to accept for adsorption molecules of
certain dimensions while rejecting those of larger dimensions, these
materials have come to be known as "molecular sieves"
and are utilized in a variety of ways to take advantage of these
properties.
Zeolites typically have uniform pore diameters of about 3 angstroms
to about 10 angstroms. The chemical composition of zeolites can
vary widely and they typically consist of SiO.sub.2 in which some
of the silicon atoms may be replaced by tetravalent ions such as
Ti or Ge, by trivalent ions such as Al, B, Ga, Fe, or by bivalent
ions such as Be, or by a combination of any of the aforementioned
ions. When there is substitution by bivalent or trivalent ions,
cations such as Na, K, Ca, NH.sub.4 or H are also present.
Representative examples of siliceous zeolites are small pore zeolites
such as NaA, CaA, Erionite; medium pore zeolites such as ZSM-5
ZSM-11 ZSM-22 ZSM-23 ZSM-48 ZSM-12 zeolite beta; and large
pore zeolites such as zeolite L, ZSM-4 (omega), NaX, NaY, CaY, REY,
US-Y, ZSM-20 and mordenite.
Zeolites include a wide variety of positive ion-containing crystalline
aluminosilicates. These aluminosilicates can be described as a rigid
three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which
the tetrahedra are cross-linked by the sharing of oxygen atoms whereby
the ratio of the total aluminum and silicon atoms to oxygen atoms
is 1:2. The electrovalence of the tetrahedra containing aluminum
is balanced by the inclusion in the crystal of the cation, for example
an alkali metal or an alkaline earth metal cation. This can be expressed
wherein the ratio of aluminum to the number of various cations,
such as Ca/2 Sr/2 Na, K or Li, is equal to unity. One type of
cation may be exchanged either entirely or partially with another
type of cation utilizing ion exchange techniques in a conventional
manner. By means of such cation exchange, it has been possible to
vary the properties of a given aluminosilicate by suitable selection
of the cation. The spaces between the tetrahedra are occupied by
molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great
variety of synthetic zeolites. The zeolites have come to be designated
by letter or other convenient symbols, as illustrated by zeolite
A (U.S. Pat. No. 2882243); zeolite X (U.S. Pat. No. 2882244);
zeolite Y (U.S. Pat. No. 3130007); zeolite beta (U.S. Pat. No.
3308069); zeolite ZK-5 (U.S. Pat. No. 3247195); zeolite ZK-4
(U.S. Pat. No. 3314752); zeolite ZSM-5 (U.S. Pat. No. 3702886);
ZSM-5/ZSM-11 intermediate (U.S. Pat. No. 4229424); zeolite ZSM-23
(U.S. Pat. No. 4076842); zeolite ZSM-11 (U.S. Pat. No. 3709979);
zeolite ZSM-12 (U.S. Pat. No. 3832449); zeolite ZSM-20 (U.S. Pat.
No. 3972983); ZSM-35 (U.S. Pat. No. 4016245), ZSM-38 (U.S. Pat.
No. 4046859); and zeolite ZSM-48 (U.S. Pat. No. 4375573), merely
to name a few. All of the above patents are incorporated herein
by reference.
The silicon/aluminum atomic ratio of a given zeolite is often variable.
For example, zeolite X can be synthesized with silicon/aluminum
atomic ratios of from 1 to 1.5; zeolite Y, from 1.5 to about 3.
In some zeolites, the upper limit of the silicon/aluminum atomic
ratio is unbounded. ZSM-5 is one such example wherein the silicon/aluminum
atomic ratio is at least 12. U.S. Pat. No. 3941871 (Re. 29948)
discloses a porous crystalline silicate made from a reaction mixture
containing no deliberately added aluminum in the recipe and exhibiting
the X-ray diffraction pattern characteristic of ZSM-5 type zeolites.
U.S. Pat. Nos. 4061724; 4073865 and 4104294 describe crystalline
silicas of varying aluminum and metal content. These zeolites can
consist essentially of silica, containing only trace amounts of
aluminum.
Another class of molecular sieves consists of AlO.sub.2 and PO.sub.2
units (AlPO.sub.4) whose Al or P constituents optionally may be
substituted by other elements such as Si (called silicoaluminophosphates
or SAPO's), or metals (called metaloaluminophosphates or MeAPO's)
or combinations thereof (called metaloaluminophosphosilicates or
MeAPSO's). As with aluminosilicates, the ALPO.sub.4 's, SAPO's,
MeAPO's and MeAPSO's are crystalline and have ordered pore structures
which accept certain molecules while rejecting others and they are
often considered to be zeolitic materials.
Aluminum phosphates are taught in U.S. Pat. Nos. 4310440 and
4385994 for example. These aluminum phosphate materials have essentially
electroneutral lattices. U.S. Pat. No. 3801704 teaches an aluminum
phosphate treated in a certain way to impart acidity.
The crystalline silicoaluminophosphates useful for the membranes
of the invention have molecular sieve framework which may exhibit
ion-exchange properties and may be converted to material having
intrinsic catalytic activity.
Silicoaluminophosphates of various structures are taught in U.S.
Pat. No. 4440871. Aluminosilicates containing phosphorous, i.e.
silicoaluminophosphates of particular structures are taught in U.S.
Pat. Nos. 3355246 (i.e. ZK-21) and 3791964 (i.e. ZK-22). Other
teachings of silicoaluminophosphates and their synthesis include
U.S. Pat. Nos. 4673559 (two-phase synthesis method); 4623527
(MCM-10); 4639358 (MCM-1); 4647442 (MCM-2); 4664897 (MCM-4);
4638357 (MCM-5) and 4632811 (MCM-3). All of the above patents
are incorporated herein by reference.
A method for synthesizing crystalline metaloaluminophosphates (MeAPO's)
is shown in U.S. Pat. No. 4713227 and an antimonophosphoaluminate
and the method for its synthesis are taught in U.S. Pat. No. 4619818.
U.S. Pat. No. 4567029 teaches metalloaluminophosphates, and titaniumaluminophosphate
and the method for its synthesis are taught in U.S. Pat. No. 4500651.
Compositions comprising crystals having a framework topology after
heating at 110.degree. C. or higher giving an X-ray diffraction
pattern indicating pore windows formed by 18 tetrahedral members
of about 12-13 Angstroms in diameter are taught in U.S. Pat. No.
4880611 incorporated herein by reference.
The membranes of the invention consist essentially of only molecular
sieve material, as contrasted with prior art composite membranes
which can contain various amounts of molecular sieve material composited
with other materials. The zeolitic membrane may contain a single
zeolite or mixtures of zeolites. The membrane can be monocrystalline
or polycrystalline. "Monocrystalline" is intended to mean
consisting of a single crystal. "Polycrystalline" is intended
to mean consisting of a continuous intergrowth of more than a single
crystal, e.g. many crystals.
The membrane can be produced, for example, by synthesis under hydrothermal
conditions on a non-porous substrate forming surface, such as a
polymer, a metal or glass. Suitable polymer surfaces are, for example,
fluorocarbon polymers such as tetrafluoroethylene (TFE) and fluorinated
ethylene-propylene polymers (FEP). Suitable metal surfaces are,
for example, silver, nickel, aluminum and stainless steel. A thin
layer of metal on glass or an organic polymer or other material
may be used as the forming surface. A thin layer of a polymer film
on glass or other material may also be used as the forming surface.
The forming surface may have various configurations. For example,
the surface may be flat, curved, a hollow cylinder or honeycomb-shaped.
Although amorphous materials can be used as substrates for crystal
growth, monocrystalline surfaces can also be used. The synthesis
can also be achieved by mechanical compression of a powder form
zeolite, followed by chemical treatment.
In forming the membranes of the invention, a non-porous surface
is contacted with a chemical mixture capable of forming the desired
crystalline material under crystallization conditions. After a period
of time under suitable conditions, a cohesive membrane of crystallized
material forms on the non-porous substrate surface. The thickness
dimension of the membrane may vary from about 0.1 micron to about
400 microns depending upon the length of time the surface is contacted
with the chemical mixture and the amount of mixture provided. Other
means such as varying the temperature or the ratio of crystallization
mixture to forming surface area are also effective in adjusting
the membrane thickness to a desired dimension.
The chemical composition of the forming mixture, in terms of moles
per mole YO.sub.2 includes:
______________________________________ Most Broad Preferred Preferred
______________________________________ H.sub.2 O/YO.sub.2 5 to 500
>20 to 500 >40 to 500 X.sub.2 O.sub.3/YO.sub.2 0 to 0.2 0
to 0.014 0 to 0.010 OH.sup.- /YO.sub.2 0 to 2 0.02 to 0.15 0.02
to 0.15 ______________________________________
In general, preferred forming mixture compositions are those combining
a high H.sub.2 O/YO.sub.2 ratio (towards the upper end of the indicated
range), a low X.sub.2 O.sub.3 ratio and a low OH.sup.- /YO.sub.2
ratio. Also microporous crystals with a tendency to form twinned
crystals are particularly prone to form membranes.
To form membrane film of uniform thickness and avoid homogeneous
crystallization, a minimum H.sub.2 O/YO.sub.2 ratio is preferred,
which depends on the X.sub.2 O.sub.3 /YO.sub.2 ratio. This minimum
ratio is about 20 for X.sub.2 O.sub.3 /YO.sub.2.ltoreq. 0.0025
70 for X.sub.2 O.sub.3 /YO.sub.2= 0.01 and 130 for X.sub.2 O.sub.3
/YO.sub.2= 0.014.
For crystallizing a membrane having the structure of ZSM-5 for
example, it is preferred to use an organic matrix such as tetrapropylammonium
(TPA) in a ratio TPA/YO.sub.2 of 0-2 preferably 0.05-1. Other well-known
organic matrices can be used for the syntheses of other zeolite
membranes.
The method of synthesis can be either in a batch process, semicontinuous
or continuous process. In a batch process, it is preferred to use
static conditions, i.e. absence of stirring, to promote formation
of a membrane. In a continuous or semicontinuous process, a forming
solution or slurry is passed through the reaction zone with or without
recycle. In the recycle mode, the composition of the solution can
be adjusted and maintained at optimal concentrations. This mode
of operation is particularly advantageous when very high H.sub.2
O/YO.sub.2 ratios are employed.
The thickness of the membrane can be controlled by varying the
crystallization time or by adjusting the total nutrient (e.g. SiO.sub.2)
provided per surface area of the membrane-forming surface.
The time of contacting of the surface with the reaction mixture
may be from about 0.5 hrs. to about 1000 hrs., preferably from about
1 hr. to about 100 hrs.; at a temperature of from about 50.degree.
C. to about 250.degree. C., preferably from about 110.degree. C.
to about 200.degree. C.; and at a pressure from about 1 atm to about
100 atm, preferably from about 1 atm to about 15 atm.
After the desired period of time, the substrate, now coated with
crystalline material, is removed from contact with the chemical
mixture, washed with distilled water and allowed to dry.
The layer of crystalline material may be removed from the non-porous
surface by various means depending upon the material chosen for
the forming surface. The layer may be separated from polymeric surfaces,
for example, by mechanical means such as careful scraping or peeling.
Removal from metal surfaces may be accomplished with the use of
solvents such as acetone, or by dissolving the metal with acid such
as aqueous hydrochloric or nitric acid. With a support consisting
of metal or metallized material such as aluminum on glass or teflon,
treatment with an aqueous mineral acid can be employed.
The membrane material may also be calcined before or after removal
from the substrate for example in an inert atmosphere or in air
at from about 200.degree. to about 700.degree. C. for about 1 hr.
to about 50 hrs.
The membrane may also be treated to adjust its catalytic properties
before or after removal from the surface, for example by steaming
and/or ion exchange. Low or zero catalytic activity can be obtained
by incorporating alkali or alkaline earth cations into the membrane.
Catalytic activity can be increased by methods known in the art
such as by increasing the aluminum content or by introducing a hydrogenation-dehydrogenation
function into the membrane
The original ions, i.e. cations or anions, of the synthesized membrane
can be replaced in accordance with techniques well known in the
art, at least in part, by ion exchange with other cations or anions
Preferred replacing cations include metal ions, hydrogen ions, hydrogen
precursor, e.g. ammonium ions and mixtures thereof Particularly
preferred cations include hydrogen, rare earth metals and metals
of Groups IIA, IIIA, IVA, IB, IIB, IIIB, IVB, VIB and VIII of the
Periodic Table of the Elements
Typical ion exchange technique would be to contact the synthesized
membrane with a salt of the desired replacing ion or ions. Examples
of such salts of cations include the halides, e.g., chlorides, nitrates
and sulfates.
Cations may be incorporated into the membrane to neutralize acid
sites or to adjust the diffusion properties; preferred cations to
be incorporated for these purposes include metals of Groups IA and
IIA of the Periodic Table of the Elements, for example, sodium,
potassium, magnesium, barium, lithium, strontium, rubidium and cesium.
Siliceous membranes containing a relatively high concentration
of aluminum (SiO.sub.2 /Al.sub.2 O.sub.3< 100) can be prepared
directly by synthesis. Alternatively, a high SiO.sub.2 /Al.sub.2
O.sub.3 membrane can be prepared first and aluminum incorporated
by post-synthesis treatment, using known methods, e.g. treatment
with reactive aluminum compounds such as AlCl.sub.3 at elevated
temperature; by adding aluminum oxide or hydroxide and treatment
under hydrothermal conditions; or by treating with small amounts
of sodium aluminate
Other metals can be incorporated during synthesis (e.g. titanium,
tin, iron, gallium, transition metals) or post-synthetically via
known processes such as impregnation, ion exchange, vapor deposition
and the like.
The diffusive properties of the membrane such as permeation rate
and selectivity, depend on the geometric properties, particularly
the thickness, and the particular zeolite that constitutes the membrane
A given membrane can be further modified by subsequent treatment
that changes the diffusion properties. Examples of such treatments
are: deposition of coke or organic compounds, such as pyridine or
other carbonaceous material, at the exterior or interior of the
zeolite pores, deposition of silica or silicon compounds via treatment
with SiCl.sub.4 or Si(OR).sub.4 followed by calcination, treatment
with phosphorus compounds, incorporation of metal salts or oxides,
such as of Mg, Mo, W, Sb, or other oxides such as silicon dioxide,
or ion exchange, e.g., with K, Rb, Cs, or Ag.
It is also contemplated that a metal function can be incorporated
into the membrane, such as Pd, Pt, Ru, Mo, W, Ni, Fe, Ag, etc. These
metal-containing membranes may have essentially no acid activity,
or they may have substantial acid activity to provide for dual-functional
catalysis. The catalytic activity of the membrane can be adjusted
from essentially zero to high activity, depending on the particular
use thereof.
The membranes can be used for separation of gaseous or liquid mixtures
or catalytic applications which combine chemical conversion of the
reactant with in situ separation of the products.
A variety of gaseous or liquid mixtures may be separated using
the membrane Examples of mixtures advantageously separated are oxygen
and nitrogen, hydrogen and carbon monoxide, linear and branched
paraffins, hydrogen and methane, p-xylene and m- and/or o-xylene.
Further examples of desirable separations to be carried out with
the membranes described herein include: (1) removal of waxy components
from distillate and lube oil fractions and of linear and slightly
branched paraffins from mixtures such as reformate; (2) removal
of organics from aqueous streams in which high silica (SiO.sub.2
/Al.sub.2 O.sub.3> 100) microporous materials are particularly
useful; examples include removal of ethanol from fermentation mixtures
used to produce beer or wine and removal of harmful organic contaminants
from ground water or waste streams; and (3) removal of paraffins
from aromatics using a high silica zeolite membrane, and aromatics
from paraffins, using low SiO.sub.2 /Al.sub.2 O.sub.3 zeolite membrane
in the alkali-exchanged form.
When separation of the components of a gaseous or liquid mixture
is to be accomplished, a low or zero activity zeolitic membrane
is preferably used. Siliceous zeolites of low or zero activity contain
no or only trace amounts of two-or three-valent metal ions; or when
they contain substantial amount of such ions, their catalytic activity
can be reduced to the desired low level by cation exchange with
alkali or alkaline earth cations, by thermal or steam treatment,
by treatment with phosphorus compounds and steaming, or by replacement
of the three-valent ions, e.g., Al, by four-valent ions, e.g., Si,
by treatment with hexafluorosilicate, SiCl4 etc. Aluminophosphate
molecular sieves also have low if any catalytic activity.
Catalytic applications can combine chemical conversion of one or
more reactants with in situ separation Such separation may involve,
for example, the separation of one or all of the products from the
reactant(s).
For use in a catalytic process, (i) a catalytically inactive membrane
may be combined with an active catalyst, or (ii) the membrane itself
may be catalytically active. As an example of the first case (i),
a Pt on Al.sub.2 O.sub.3 catalyst is contained in a tubular reactor
whose walls consist at least partly of a catalytically inactive
zeolitic membrane chosen to selectively permeate hydrogen.
Dehydrogenation of alkanes is an example of a catalytic process
which may be accomplished by passing an alkane feed through the
tubular reactor; the effluent contains alkene in greater than equilibrium
concentration. In this way, for example, propane may be converted
to propylene. In the second case (ii), the zeolitic membrane possesses
catalytic activity, either acid activity, or metal activity, or
both. The acid activity of siliceous zeolites can be adjusted by
the amount of three-valent substituents, especially aluminum, by
the degree of cation exchange from salt form to hydrogen form or
by thermal or steam treatment. The acid activity of AlPO4 type zeolites
can be increased by incorporation of activating agents such as silica
An example utilizing the acid activity of the membrane is the dealkylation
of ethylbenzene to benzene and ethylene Utilizing the higher permeation
rate of ethylene alone or of ethylene and benzene, a higher degree
of dealkylation in greater selectivity is obtained.
Activity may be correlated with acid character. Silicious zeolites
may be considered to contain SiO.sub.4 tetrahedra. Substitution
by a trivalent element such as aluminum introduces a negative charge
which must be balanced If this is done by a proton, the material
is acidic. The charge may also be balanced by cation exchange with
alkali or alkaline earth metal cations.
One measure of catalytic activity may be termed the Alpha Value.
When Alpha Value is examined, it is noted that the Alpha Value is
an approximate indication of the catalytic cracking activity of
the catalyst compared to a standard catalyst and it gives the relative
rate constant (rate of normal hexane conversion per volume of catalyst
per unit time). It is based on the activity of silica-alumina cracking
catalyst taken as an Alpha of 1 (Rate Constant=0.016 sec .sup.-1).
The Alpha Test is described in U.S. Pat. No. 3354078; in the Journal
of Catalysis, Vol. 4 p. 527 (1965); Vol. 6 p. 278 (1966); and
Vol. 61 p. 395 (1980), each incorporated herein by reference as
to that description. The experimental conditions of the test used
herein include a constant temperature of 538.degree. C. and a variable
flow rate as described in detail in the Journal of Catalysis, vol.
61 p. 395.
The crystalline membranes of the present invention are readily
convertible to catalytically active material for a variety of organic,
e.g. hydrocarbon, compound conversion processes. Such conversion
processes include, as non-limiting examples, cracking hydrocarbons
with reaction conditions including a temperature of from about 300.degree.
C. to about 700.degree. C., a pressure of from about 0.1 atmosphere
(bar) to about 30 atmospheres and a weight hourly space velocity
of from about 0.1 hr.sup.-1 to about 20 hr.sup.-1 dehydrogenating
hydrocarbon compounds with reaction conditions including a temperature
of from about 300.degree. C. to about 700.degree. C., a pressure
of from about 0.1 atmosphere to about 10 atmospheres and a weight
hourly space velocity of from about 0.1 to about 20; converting
paraffins to aromatics with reaction conditions including a temperature
of from about 100.degree. C. to about 700.degree. C., a pressure
of from about 0.1 atmosphere to about 60 atmospheres, a weight hourly
space velocity of from about 0.5 to about 400 and a hydrogen/hydrocarbon
mole ratio of from about 0 to about 20; converting olefins to aromatics,
e.g. benzene, toluene and xylenes, with reaction conditions including
a temperature of from about 100.degree. C. to about 700.degree.0
C., a pressure of from about 0.1 atmosphere to about 60 atmospheres,
a weight hourly space velocity of from about 0.5 to about 400 and
a hydrogen/hydrocarbon mole ratio of from about 0 to about 20; converting
alcohols, e.g. methanol, or ethers, e.g. dimethylether, or mixtures
thereof to hydrocarbons including olefins and/or aromatics with
reaction conditions including a temperature of from about 275.degree.
C. to about 600.degree. C., a pressure of from about 0.5 atmosphere
to about 50 atmospheres and a liquid hourly space velocity of from
about 0.5 to about 100; isomerizing xylene feedstock components
with reaction conditions including a temperature of from about 230.degree.
C. to about 510.degree. C., a pressure of from about 3 atmospheres
to about 35 atmospheres, a weight hourly space velocity of from
about 0.1 to about 200 and a hydrogen/hydrocarbon mole ratio of
from about 0 to about 100; disproportionating toluene with reaction
conditions including a temperature of from about 200.degree. C.
to about 760.degree. C., a pressure of from about atmospheric to
about 60 atmospheres and a weight hourly space velocity of from
about 0.08 to about 20; alkylating aromatic hydrocarbons, e.g. benzene
and alkylbenzenes in the presence of an alkylating agent, e.g. olefins,
formaldehyde, alkyl halides and alcohols, with reaction conditions
including a temperature of from about 250.degree. C. to about 500.degree.
C., a pressure of from about atmospheric to about 200 atmospheres,
a weight hourly space velocity of from about 2 to about 2000 and
an aromatic hydrocarbon/alkylating agent mole ratio of from about
1/1 to about 20/1; and transalkylating aromatic hydrocarbons in
the presence of polyalkylaromatic hydrocarbons with reaction conditions
including a temperature of from about 340.degree. C. to about 500.degree.
C., a pressure of from about atmospheric to about 200 atmospheres,
a weight hourly space velocity of from about 10 to about 1000 and
an aromatic hydrocarbon/polyalkylaromatic hydrocarbon mole ratio
of from about 1/1 to about 16/1.
In general, therefore, catalytic conversion conditions over a catalyst
comprising the membrane in active form include a temperature of
from about 100.degree. C. to about 760.degree. C., a pressure of
from about 0.1 atmosphere (bar) to about 200 atmospheres (bar),
a weight hourly space velocity of from about 0.08 hr to about 2000
hr and a hydrogen/organic, e.g. hydrocarbon compound mole ratio
of from 0 (no added hydrogen) to about 100.
In order to more fully illustrate the nature of the invention and
the manner of practicing same, the following examples are presented.
EXAMPLE A
To prepare the initial batch composition, the procedure outlined
by Hayhurst and Lee (Proceedings of the 7th International Zeolite
Conference, Murakami, Y., Iijima, A, Ward, I.W., Ed., p. 113 Elsevier,
Tokyo, 1986) was followed. The following is a list of the reagents
used: Ludox AS-40 (DuPont) aqueous colloidal silica solution as
the silica source, 50% by weight sodium hydroxide solution (Baker,
reagent grade), tetrapropylammonium bromide (Aldrich), distilled
water.
The tetrapropylammonium bromide, 3.475 gm, was dissolved in 50.0
gm distilled water by stirring in a polyethylene container. A 0.90
gm quantity of sodium hydroxide solution was added. The resulting
solution was subsequently diluted with 54.6 gm distilled water.
A 37.55 gm quantity of the silica source was slowly added with continuous
stirring. About 40 ml of the suspension was poured into a 45 ml
polytetrafluoroethylene (TN) (Teflon) lined autoclave (Parr Model
4744). The molar composition was:
A polytetrafluoroethylene (TN) slab (80 mm.times.25 mm.times.2.5
mm) was immersed in the solution and placed vertically along the
axis of the cylindrical vessel. The autoclave was sealed and placed
in a convection oven, which was preheated at 180.degree. C.
In a second autoclave, a vycor frit, approximately 1.5 cm in diameter,
already mounted inside a pyrex tube (Corning Glass) was immersed
in the synthesis solution.
The autoclaves were removed from the oven after 9 days and quenched
with water. The TN slab was recovered, washed with distilled water
and dried at room temperature. The slab and the vycor frit with
surrounding pyrex tube were observed to be covered with a uniform
layer of crystallized material.
The layer of crystalline material was removed from the TN surfaces
by carefully scraping with a spatula. No solid particles were found
suspended in the remaining solution or settled on the bottom of
the container.
The resulting membranous material was calcined in nitrogen at 560.degree.
C. and in air at 600.degree. C. to decompose and burn the organic
template. No cracks generated by the calcination process were observed.
Segments of the zeolite membrane greater than 1 cm.sup.2 were selected
for characterization by electron microscopy, X-ray diffraction,
Silicon-NMR and hexane sorption. The membrane was crushed to powder
form for X-ray and Silicon-NMR tests. It was introduced uncrushed
into the sorption apparatus for hexane sorption measurements.
X-ray Diffraction, Si-NMR and Hexane Sorotion
The X-ray diffraction pattern was shown to be that of pure ZSM-5.
The Si-NMR spectrum, FIG. 1 showed silanols as the only non-ZSM-5
framework silicons. Spectra with 30 and 300s relaxation delays were
obtained to be certain no dense phases were present. Silanols, whose
identity was established by crosspolarization, are seen as a broad
peak at about -103 ppm. Their concentration is about 1.5 SiOH/unit
cell or about 1/3 what is normally present in a high silica/alumina
ZSM-5 made from TPABr. The hexane sorption capacity, measured at
90.degree. C. and 110.8 torr hexane partial pressure, was found
to be 112.0 mg/gm of zeolite, i.e., slightly higher than the sorption
capacity of standard small ZSM-5 crystals.
Electron Microscopy
FIG. 2 shows the distinct morphologies of the two membrane surfaces.
FIG. 2a corresponds to the membrane surface exposed to the teflon
support. FIG. 2b corresponds to the surface exposed to the synthesis
mixture. Although the teflon side surface consists of a layer of
apparently loosely held crystals of less than 0.1 .mu.m size, the
solution surface consists of a continuous array of densely packed
and intergrown (twinned) crystals, 10 to 100 .mu.m in size. The
intergrowth is better shown on the surface of a large single crystal
(FIG. 2c). The surface of the same crystal at even higher magnification
is shown in FIG. 2d. This is partly coated with small particles,
which may or may not have the ZSM-5 microstructure. |