Note: Descriptions are shown in the official language in which they were submitted.
CA 02193259 2004-08-10
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~ ZEOLITE CONTAINING COMPOSITION WITH A SELECTIVITY
ENHANCING COATING
FIELD OF THE INVENTION
The present invention is directed toward a supported zeolite
composition containing a coating which enhances the permselective properties
of
the composition.
BACKGRO TNf7 F THE INVENTION
Fabrication of practical zeolite membranes has long been a goal of
separation science. For a zeolite membrane to be practical. it must have a
high
flux and selectivity for the desired permeate molecule(s). Obtaining such a
membrane has been difficult because of defects and interparticle voids
inherent
to the zeolite film. This is especially true for membranes grown from
classical
zeolite synthesis routes described in the literature. These membranes have a
heterogeneous crystal structure within the membrane layer and require an
enormous (> 50 micrometers) layer thickness to seal pinholes and void
structures. What is needed in the art is a thin, continuous zeolite layer with
few
defects, and a method, or methods, to 'heal' any remaining defects and void
structures and stabilize these to subsequent severe conditions of operation.
Membranes described in the literature are formed with several
zones (larger crystals grown on top of smaller crystals) across the membrane
thickness. In several zones, the crystals are not grown into a dense mat that
is
free of intercrystalline voids To obtain a permselective zeolite membrane, the
above zeolite layers (comprised of zones) must be grown to an excessive
thickness (> 50 micrometers) to seal off voids and defects within the membrane
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system. This introduces a great mass transfer resistance resulting in very low
fluxes through the membrane layer.
Alternately, a method (or methods) to 'heal' void structures and
defects in the membrane layer, without having a negative effect on permeate
flux
or selectivity, would eliminate the need to grow excessively thick zeolite
layers.
Most successful membrane technologies employ some form of reparation coating
which repairs existing defects or holes in membranes. Reparation techniques
either selectively seal defects and holes or attenuate their effects with a
permeable coating that covers the membrane surface. Reparation techniques
which selectively seal defects are often based on the selective application of
a
film forming material which acts as a diffusion barrier. Reparation
technolognes
which attenuate rather than selectively seal defects or holes apply a
permeable
layer over the entire surface of the membrane. The mass transfer resistance of
this layer must be sufficient to attenuate the effect of defects in the
membrane
and improve the permselective properties of the membrane. In all events
reparation technologies improve the permselective properties of membr~araes
with
existing defects or holes. Defects and holes form nonselective permeation
pathways through the membrane and reparation decreases the flux through these
nonselective pathways relative to permselective pathways through the reset of
the membrane.
A large number of reparation coating technologies have been
developed for organic membranes. Because of differences in materials,
conditions of use and physics of the transport mechanism through different
membranes, reparation coatings developed for organic membranes cannot be
used with any predictability regarding reporting inorganic and zeolite
membranes. Coating materials which have been used to reparate polymer
membranes have been generally polymer and epoxy materials. When they are
used to reparate polymer membranes by selectively sealing defects, film
forming
polymers or epoxies are applied in a manner su ch that they are selectively
absorbed into defects or holes. For example, hollow fiber membrane modules
containing a small percentage of broken fibers have been reparated by
selectively
filling broken fibers with epoxy. When they are used to attenuate defects,
permeable polymers or epoxies are applied as a thin film coating over the
organic membrane surface. The polymer and epoxy materials used to seal and
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attenuate defects in polymer membranes degrade in the harsh chemical
environments
and high temperature operations in which inorganic and zeolite membranes are
used':
Methods used to separate organic membranes are not likely to be
applicable to microporous or mesoporous inorganic membranes. This is
especially true
for zeolite membranes for two reasons. First, since materials used to separate
organic
membranes are organic in nature, wettabiIity is an issue with oxide materials
such as
zeolites, or any system that is heavily hydroxylated at the surface. It is not
obvious that
an organic polymer would adhere to the zeolite or support surface much less
remain
intact to seal voids/defects. Secondly, zeolite membranes take advantage of
the well
defined pore structure of zeolitic materials, reparation using organic
polymers may seal
these pores in addition to the desired voids/defects; it is not likely that
the material will
be able to discriminate beriveen void structures or defects and zeolite pores.
If the
coating material enters the pore structure of the zeolite, it can occlude or
block the pore
structure. When it occludes the pore structure, it hinders diffusion through
the zeolite
and no transport occurs through blocked pores. Even when the coating material
doesi
not enter the pore structure, it can occlude pore mouths at the surface of the
zeolite and
hinder diffusional transport through the membrane. There have been proposals
that '.
zeolite membranes grown by hydrothermal synthesis could be separated by
deposition
of a material that is different from that used to form the zeolite layer in
the membrane
(see for example EP 481,660A1, S.A.I. Barri and G.J. Bratton, and T. de V.
Naylor,
British Petroleum, 1991), however no specific recipe for reparation has been
presented.
Other references relating generally to the use of zeolites include
International's
PCT Application No. WO 93/19840 relating to a deposition process, Application
No.
WO 92/13631 relating to an inorganic composite membrane comprising molecular
sieve crystals, European Patent Application EP 0 254 758 A1 relating to a
pervaporation process and membrane, and copending PCT Applications US95/08511,
US95/08512, and US95/08514.
It has been discovered that thin coatings can preserve or enhance the
permselective properties of continuous, intergrown layers of zeolite crystals
during
separation applications. Certain coatings are capable of preventing the
formation of
defects and voids in the layer during high temperature use or testing which
can lead to a
AMENDED ~~~ET
VJO 96101686 219 3 2 5 9 P~~S95/08513
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zeolite layer, alleviating or dispersing mechanical stresses and deformations
arising from harsh process environments. These coatings can also serve a
second
purpose in that they can act to seal defects in the zeolite layer, although,
as
described above, they can be advantageous when used in the absence of any
defects or voids. It is critical to 'heal' defects and voids since they
provide
nonselective permeation pathways which results in poor separations
performance. These two functions of stabilization and reparation of defects or
voids in the zeo~ite layer enhance the selectivity of the composition, hence,
we
refer to the coating as a selectivity enhancing coating (sec).
Accordingly, this invention provides a new composition useful,
among other things, for separations of molecules and catalytic conversions,
which comprises a substrate, a zeolite or zeoIite-like material in contact
with the
substrate, and a selectivity enhancing coating in contact with the zeolite. In
a
preferred embodiment of the invention a permeable intermediate layer may be
incorporated between the substrate and the zeolite layer. Such compositions
are
often referred to in the art as zeolite membranes and the instant compositions
can
be used as such.
BRIEF DESCRIPT10N OF THE DRAWMGS
Figure 1 is a diagram of the Cold Wall Reactor used for applying a
selectivity enhancing coating using interfacial gaslliquid CVD. In the figure,
(A)
is the air tank, (B) is the ozone generator, (C) is the mass flow controller,
and
(D) is the membrane holder. (E) is the Cold Wall Reaction Vessel, (F) is the
zeolite layer, (G) is the substrate, and (H) is the heating surface.
Figure 2 shows a scanning electron micrograph of the morphology
of an MFI zeolite composition formed by the LAI-ISC Process. The porous
substrate that is formed from a-alumina with 800 A pores is seen in the bottom
third of the figure. The dense intergrown layer of MFI zeolite crystals is
visiblle
in the upper portion of the figure.
Figure 3 shows a scanning electron micrograph of the exterior
surface of a GEL-LAI-ISC MFI zeolite composition. The surface shown is an
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intergrown dense mat of zeolite crystals which are substantially free of
defects
extending through the thickness of the layer.
Figure 4 shows a scanning electron micrograph of the morphology
of an MFI zeolite composition containing a Growth Enhancing Layer (GEL,
optional permeable layer) that was fractured to reveal a cross section. The
layer
labeled a) is the porous substrate that is formed from a-alumina with 800 A
pores. The layer labeled b) is the GEL layer which is mesoporous and is
clearly
discernible in the micrograph. The layer labeled c) contains MFI zeolite
crystals
which are intergrown together into a dense mat which is free of voids and
defects. The columnar nature of zeolite crystals in the layer is readily
apparent
from the morphology of the fracture surface through the zeolite layer(c).
Figure 5 shows a scanning electron micrograph of the exterior
surface of a GEL-LAI-ISC MFI zeolite composition. The surface shown in an
intergrown dense mat of zeolite crystals which are free of defects extending
through the thickness of the layer.
Figure 6 shows a scanning electron micrograph of the cross-section
of a GEL-LAI-ISC MFI with a selectivity enhancing polyimide coating. The
coating covers the entire surface of the zeolite layer and is readily visible
on top
of the zeolite Layer.
DETAILED DESCRIPTION OF THE INVENTION
The present invention describes a new type of supported zeolite
composition with a selectivity enhancing coating (SEC). The selectivity
enhancing coating prevents the composition's zeolite layer from developing
defects during process applications. These defects formed during process
application on the compositions not containing the selectivity enhancing
coating
are found to degrade the permselective properties of the composition. The
selectivity enhancing coating can also reparate defects, voids, and
interparticle
spaces that existed in the zeolite layer of the composition before if was used
for
process applications. Process applications in which the compositions can be
used include separations of molecular species, catalytic reactors and reactors
which combine reaction and separation. The composition contains at least a
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porous substrate, a zeolite layer, and a selectivity enchaining coating. The
substrate may also be referred to herein as a support. The composition may
also
contain a permeable intermediate layer between the support and the zeolite
Layer.
The selectivity enhancing coating is in contact with the zeolite layer. The
zeolite
layer is in contact with either the substrate or the intermediate layer. In
apreferred composition the zeolite crystals of the zeolite layer are grown so
that
they form a continuous film.
The present invention has a continuous zeolite layer formed on a
support (substrate), or on a permeable layer intermediate between the zeolite
and
support. Advantageously, the crystals are contiguous, i.e., substantially
every
crystal is in contact with one or more of its neighbors. Such contact may be
such
that neighboring crystals are intergrown, provided they retain their identity
as
individual crystals. In a preferred embodiment, the crystals in the layer are
closely packed.
It is preferred that the zeolite crystals are intergrown in the zeolite
layer so that nonselective permeation paths through the layer are blocked by
the
narrowest point of approach between crystals. Non-selective permeation
pathways are taken to be permeation pathways which exist at room temperature
that do not pass through the zeolite crystals. This blockage of nonselective
permeation pathways can exist at room temperature after a template which
occludes the pore structure is removed from the zeolite crystals. Templates
which are used to aid in the crystallization of zeolites are typically removed
by a
calcination step; intergrown zeolite crystals (within a layer) should exhibit
a
blockage of nonselective permeation pathways after the template is removed. A
preferred dense zeolite layer is formed such that there exists at least one
point on
a crystal that is less than 20 A from a point on an adjacent crystal. Between
these points can be inorganic oxide material that restricts nonselective
permeation of molecules through the layer. The spacing between zeolite
crystals
in this dense intergrown zeolite layer can be established by TEM or with dye
permeation tests.
The absence of nonselective permeation paths can be detected by
the ability to prevent the permeation at room temperature (-20°C) of
dye
molecules through the layer after any template is removed from the pore
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structure. Dye molecules which can be chosen to detect non-selective
permeation pathways through the layer should have minimum dimensions which
are larger than the controlling aperture through the zeolite and the size of
the dye
molecule should be less than 20 A in at least one dimension. Nonselective
pathways transport dye molecules which are larger than the pore size of the
zeolite. The dye molecules should be carried in a solution made with a solvent
which can be transported through the zeolite pore structure and the zeolite
layer
should not be allowed to pick up foreign contanvnants (such as water) before
being tested. The most preferred zeolite layers in the present invention block
the
permeation of dye molecules at room temperature through the zeolite layer. All
of the dye molecules chosen have sizes less than 20 A. The lack of permeation
at room temperature of dye molecules with sizes less than ~20A demonstrates
that non-selective permeation pathways with sizes less than -20A are blocked.
It
should be noted that this test does not have to be performed with a dye
molecule,
and any molecular species that can be detected having a size less than 20 A
and
greater than the zeolite pore dimension can be used. The advantage of using a
dye molecule is that it can be readily detected by optical means. An example
of
dyes that can be detected by optical means (and have molecular sizes less than
20 A) include Rhodamine B and other laser dyes.
The zeolite crystals fornvng the zeolite layer of the composition
may be true zeolites or zeolite-like materials including molecular sieve
materials.
As molecular sieves, there may be mentioned a silicate, an aluminosilicate, an
aluminophosphate, a silicoaluminophosphate, a metalloaluminophosphate, a
metalloaluminophosphosilicate, or a stannosilicate. The preferred molecular
sieve will depend on the chosen application for example, separation, catalytic
applications, and combined reaction and separation. These 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, 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 to include such materials.
..
! _ 8 _ 2193259 ..: ;.;
The thickness of the zeolite sieve layer is advantageously within
the range 0.1 to I~0 pm, preferably from 0.5 to 20 pm.
Advantageously, the zeolite layer is substantially free of voids.
Voids used herein means spaces beriveen adjacent zeolite crystals larger than
5000 ~.. It is preferred that the voids between zeolite crystals are less than
40
A. Layers which are substantially free of voids contain less than 20 volume %
of
voids in the zeolite layer, and preferably less than 0.1% voids in the zeolite
layer.
Such zeolite layers which are substantially free of voids are taken to be
dense.
Advantageously, zeolite layers used with the present invention
should be substantially free of defects. Defects are connected voids and are
spaces between zeolite crystals extending through the thickness of the zeolite
layer. It is preferred that any defects in the zeolite layer be formed from
small
voids. It is preferred that the number of defects with sizes greater than 40
.~. be
<10000 per square inch (6.4516 cmz), and preferably <1000 per square inch
(6.4516 cmz). The number of defects with sizes larger than ?000 A should be
less than 1000 per square inch (6.4516 cmZ) and preferably less than 100 per
square inch (6.4516 cm2).
The zeolite layer can have either a shape preferred orientation, a
crystallographically preferred orientation, or both. Shape or
crystallographically
preferred orientations occur because of the control of the relative rates of
nucleation and growth offered by the synthesis procedure. Specifically, during
synthesis, the rate of growth can be made to dominate the rate of surface
nucleation of new crystals or incorporation of new crystals. Incorporation of
new crystals is defined as attachment onto the surface of the growing zeolite
layer of a crystal formed in the synthesis mixture. Since the growth rate can
dominate renucleation or incorporation, crystals can competitively grow for
long
periods of time without significant addition of new crystals into the growing
layer. Since the growing layer is composed of individual crystals and the
synthesis method seeks to prevent renucleation or incorporation of crystals,
the
resulting composition can have shape, crystallographically preferred
orientation,
or both. Shape orientation occurs because the crystals grow with preferred
regular habits (or morphology) at the surface of the zeolite layer. A regular
habit
(or morphology) is taken to be a regularly shaped outline of a particular
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21~32~9
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crystallographic grain in the layer. Regularly shaped outlines are defined as
those which can be fitted or packed together so that there are no
interconnected
spaces or voids between crystals. Interconnected voids will form a pore
structure. A few examples of regular habits with regular shapes are columnar,
cubic, rectangular, and prismatic. Spherical, irregular and elliptical shapes
are
not considered to be regular habits. In a shape preferred orientation, defined
Layers will have the same regular habit. This can be measured by cleaving or
fracturing the substrate on which the layer is grown and examining the cross-
sectional morphology of the zeolite layer with a scanning electron microscope.
Examining the surface of the as grown zeolite layer can also give additional
information concerning the shape preferred orientation in the layer. A layer
with
shape preferred orientation is taken to be one which has more than 90% of the
crystals within one layer inside the zeolite layer exhibiting self similar
regular
habits. The self similar requirement means that the same regular habit it
exhibited within a layer that can be drawn in the electron micrograph of the
cross-section of the zeolite layer, however even though the shapes are the
same,
they do not all have to be the same size. Because of the growth mechanism of
the zeolite layer, it is possible to have one shape preferred orientation at
the
bottom (base) of the layer and another shape preferred orientation in a layer
drawn near the surface. An example of this is an MFI zeolite layer which has a
columnar habit at the base of the layer and a rectangular habit at the surface
of
the layer. Many MFI zeolite layers grown in accordance with the present
invention exhibit only one habit throughout the thickness of the zeolite
layer.
Usually MFI zeolite layers with a preferred C-axis orientation exhibit a
columnar
habit (or morphology) throughout the entire thickenss of the zeolite layer.
Often
shape preferred orientational layers have a preferred crystallographic
orientation.
The dense zeolite layer is formed on either a support or a
permeable intermediate layer supported on a support. The support (substrate)
can be porous or non-porous. If the support is porous, it will be porous
material
throughout its entire thickness. Preferably, an inorganic oxide or stainless
steel
will be utilized. The porous substrate, hence can be ceramic, metal, carbide,
polymer, or mixtures thereof. For example, refracting oxides, such as alumina,
titania, silica, as well as materials such as cordierite, mullite, stainless
steel,
pyrex, zeolite, silica, silicon carbide, carbon, graphite, and silicon nitride
or
mixture thereof can be utilized. The substrate hence can have a uniform pore
aMErlol:o ~>a~~~r
:,
size throughout or may be asymmetrical, having a larger pore structure
throughout the bulk of the substrate with a smaller pore structure at the
surface.
The substrate pore size is dictated by mass transfer considerations. It is
preferred
that the pore structure and thickness of the substrate be chosen such that
mass
transfer resistance does not limit the flux of material permeating throughout
the
membrane during use. The substrate will hence display a porosity of about ~ to
70%, preferably about 20 to ~0% and an average pore size of about 0.001 to
about I00 micrometers, preferably about .OS to ~ micrometers.
Additionally, non-porous supports will include quartz, silicon,
glass, borosilicate glasses, dense ceramics, i.e., clays, metals, polymers,
graphite
and mixtures thereof. When non-porous supports are utilized, the compositions
are useful as sensors. Most preferably, the support will be a porous ceramic
or a
porous metal.
It is preferred that the surface of the substrate, porous or non-
porous, on which the zeolite layer is grown be smooth. Roughness in the
substrate leads todefects in the zeolite layer. The substrate should have an
average roughness with an amplitude of less than IO prn with an aspect ratio
of
the roughness less than 1:1. It is preferable that the average roughness of
the
substrate be less than .5 pm with an aspect ratio of the roughness less than
1:1.
Though non-porous substrates may be utilized, porous substrates are preferred.
If a permeable intermediate layer is present, it may be mesoporous
or microporous. Microporous and mesoporous intermediate layers can contain
zeolites. Mesoporous intermediate layers can contain inorganic oxides, metals
or
carbides. Zeolites in the permeable intermediate layer can be used to nucleate
the gowth of the zeolite layer. When the intermediate layer nucleates the
growth of the zeolite layer in a hydrothermal synthesis process, the
morphology
of crystals in the zeolite layer can be columnar platelike or mixtures of the
two
and a crystallographically preferred orientation may be observed. When the
intermediate layer is mesoporous and contains nanocrystalline zeolites, it is
referred to as a growth enhancing layer (GEL). When the intermeditate layer is
microporous and contains nanocrystalline zeolites it is referred to as a
Seeding
layer. A growth enhancing layer (GEL) contains identifiable particles with
interstices between said particles of colloidal zeolites; or colloidal
zeolites and
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metal oxides; colloidal zeolites and colloidal metal; and mixtures thereof.
The
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. Said interstices in the GEL are
mesoporous. In a preferred embodiment, said mesoporous interstices in the GEL
have sizes of about 20 to about 2000 ~.. Interstices in this size range
provide a
permeation path for molecules through the GEL layer. Molecules can permeate
through these interstices because they are devoid of any material which would
hinder mass transport during the compositions use. GEL and Seeding layers
nucleate the formation of the dense mat of zeolite crystals grown on the
surfaces
of these layers. This dense mat of crystals is closely packed together such
that
99% of the grain boundary zones stretching between top and bottom of the
zeolite layer have at least one point along their length where the width does
not
exceed 20 A. As the zeolite layer grows from the interface at the GEL layer,
crystal width may increase, however the individual crystals remain separated
at
their boundaries by a distance between adjacent crystals of <_ 20 A. This
densely
packed mat can have a columnar morphology. Zeolite membranes grown
without the use of seeding or growth enhancing layers do not have the degree
of
perfection of such columnar compositions.
Both seeding and GEL layers contain nanocrystalline zeolites.
Growth enhancing layers can be formed from a solution containing a
nanocrystalline zeolite; a mixture of metal oxide and nanocrystaIline zeolite;
a mixture of colloidal zeolite and colloidal metal; or mixtures thereof.
Preferably, colloidal zeolite or a mixture of colloidal zeolite and metal
oxide
will be used to form the GEL layer. Seeding layers are formed from
solutions containing nanocrystalline zeolites. Nanocrystalline zeolites have
sizes between SO and 5,000 A and form a stable dispersion or solution of
discrete particles. 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 silica source is advantageously particulate silica or
polymeric silica which is prepared by methods known in the art; see for
example Brinker and Scherer, Sol-Gel Science, Academic Press, 1990. A
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preferred method for preparing polymeric silica is by the acid hydrolysis of
tetraethylorthosilicate and a preferred method for preparing colloidal silica
is
by the base hydrolysis of tetraethylorthosilicate. 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. The structure
directing
agent may be, for example, the hydroxide or salt of tetramethylammonium
(TMA), tetraethylammonium (TEA), triethylmethylammonium (TEMA),
tetrapropylammonium (TPA), tetrabutylammonitun (TBA),
tetrabutylphosphonium (TBP), trimethylbenzylammonium (TMBA),
trimethylcetylammonium (TMCA), trimethylneopentylammonium (TNNA),
triphenylbenzylphosphonium (TPBP), bispyrrolidinium (BP),
ethylpyridinium (EP), diethylpiperidinium (DEPP) or a substituted
azoniabicyclooctane, e.g. methyl or ethyl substituted quinuclidine or 1,4-
diazoniabicyclo-(2,2,2)octane. Preferred structure- directing agents are the
hydroxides and halides of TMA, TEA, TPA and TBA, and mixtures of
hydroxides and halides thereof.
Either GEL or seeding layers can be formed from solutions
containing the colloidal zeolites. Composition of the solutions and
processing conditions determine if a GEL or seeding layer is obtained. For
the GEL layer, it is preferable to calcine it at temperatures < 1000°C,
preferably from about 300 - 600°C. When a GEL layer is formed, it is
maintained in the final composition as a distinct layer having a thiclrness of
about .1 to 20 pm, preferably about 1 to 5 um. This layer contains interstices
as described above.
By adjusting the ratio of colloidal zeolite and metal oxide, the
density of nucleation sites on the GEL can be controlled. This density
controls the morphology of the zeolite layer grown over the growth
enhancing layer in a subsequent hydrothermal synthesis step. The higher the
nucleation density, the narrower the zeolite- crystal width the layer will
exhibit at the GEL zeolite interface. Nucleation density can be controlled by
the relative proportions of nanocrystalline zeolites and metal oxides (with
the
density decreasing as the amount of the metal oxide utilized increases) as
well as the size of the nanocrystalline zeolites in the GEL. NanocrystalIine
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sized zeolites in the range of from 50 - 10,000 .4 are thus used in the GEL.
The larger the nanocrystalline zeolite crystals utilized in the GEL, the wider
the zeolite columns in the membrane will be. Applicants believe that the
addition of metal oxide, colloidal metal or mixtures thereof to the colloidal
zeolite in the GEL layer provides spaces between nucleation sites allowing
for control of the crystal width in the zeolite layer.
GEL's containing pure metal oxides or colloidal metal fail to
produce nucleation sites. A preferred formulation of GEL contains 100-x wt%
of colloidal metal or metal oxide and x wt% of colloidal zeolite, where x is
at
least 0.01 when the GEL is not formed from pure colloidal zeolite. Hence,
the nucleation density is set by the above formula as well as the size of the
particles of colloidal zeolite, colloidal metal and metal oxide. The smaller
the
particle size of the colloidal zeolite particles, the denser the nucleation
sites
which produces narrower zeolite crystal widths.
The hydrothermal treatment to form the crystalline zeolite layer is
advantageously carried out by contacting the GEL or seed layer with a zeolite
synthesis mixture, and heating for a time and at the temperature sufficient to
effect crystallization. Contacting as used herein includes total and partial
immersion. Heating times may be, for example, in the range of from 1 hour
to 10 days, preferably from 24 hours to 4 days. Temperatures may be, for
example, from 50 to 300°C, preferably from 1~0 to 200°C.
Contacting of the
substrate with GEL or seed layer must be carried out such that there is no
settling of crystals formed in the synthesis mixture during hydrothermal
treatment onto the GEL or seed layer. When a true zeolite material is
formed, the synthesis mixture contains a source of silica; optimally a
structure directing agent, and a source of any other component desired in the
resulting zeolite. It may also contain nanocrystalline zeolites or seed
crystals.
Synthesis mixtures from which zeolite crystals are grown are well known in
the art (see e.g., Handbook of Molecular Sieves, Rosemarie Szostak, Van
Nostrand Reinhold, NY 1992). A preferred route for MFI zeolites, e.g., is
from a Low Alkaline synthesis mixture having a pH of about 6 to about 13
preferably about 8 to about 13, and from which MFI zeotite crystals can be
grown. Such mixtures are readily prepared by those skilled in the art. For
example, suitable mixtures include Na20, TPABr,
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tetrapropylatnntoniumbromide, Si02 and water. The compositions are
grown by contacting the GEL coated substrate of choice in the low alkaline
synthesis mixture. The synthesis mixture is then heated to about 50 to about
300°C, preferably about 180°C for a period of about 30 minutes
to about 300
hours, preferably for about 30 minutes. After crystallization, the supported
layer may be washed, dried, and calcined by methods known in the art.
A preferred synthesis technique used with this invention is the
growth of zeolite crystals on the face of a support or intermediate layer
which is oriented from 90 to 270 degrees in a synthesis mixture. In the 180
degree orientation, the preferred orientation, the surface on which the
zeolite
layer is to be grown is horizontal and facing downward, thus being referred
to as inverted. Applicants believe this prevents zeolites which are
homogeneously nucleated in the synthesis mixture from settling by
gravitation and incorporating into the growing zeolite layer. Thus, the
zeolite
layer is not perturbed during the growth process. This synthesis technique is
referred to as an Inverted In-Situ-Crystallization (I-ISC) process. Growth on
a GEL from a Low-Alkaline-synthesis solution using the I-ISC process is
referred to as a GEL-LAI-ISC process or composition. Growth using a
Seeding Layer from a Low-Alkaline-synthesis solution using the I-ISC
process is referred to as a S-LAI-ISC process or composition. For I-ISC,
GEL-LAI-ISC and S-LAI-ISC processes, the crystal width in the zeolite layer
is between 0. I and 20 llm. It has been found that when MFI layers are
grown by the LAI-ISC process, a degree of crystallographic orientation
occurs. Much more significant crystallographic orientation occurs when
MFI layers are grown by the GEL-LAI-ISC process or the S-LAI-ISC
process. Preferred orientation will be different depending on the zeolite
grown, however, a preferred orientation can be exhibited. When MFI zeolite
layers are grown using a I-ISC process from a Low Alkaline synthesis
solution, the crystallographic orientation of the MFI zeolites is such that at
least 75% of the crystals in the zeolite layer are aligned in the orientation
(with the c-axis parallel to the growth direction within 15° preferably
5° of
the normal to the surface of the zeolite layer), preferably at least 90% of
the
crystals will display the preferred orientation. A measure of the proportion
of the crystals that have the longest axis normal to the plane of the layer
may
be obtained by comparison of the X-ray diffraction pattern of the layer with
W096/01686 PG1'/US951085I3
-IS-
that of a randomly oriented zeolite powder. In the case of an MFI-type
zeolite, for example, the longest edge corresponding to the c axis, the ratio
of
the intensity of the 002-peak to the combined 200 and 020 peak is divided by
the same ratio for randomly oriented powder; the quotient is termed the
crystallographic preferred orientation (CPO). Measured in this way, layers in
accordance with the invention have a CPO of at least 2, and may have a CPO
higherthan 105.
Zeolite compositions fabricated using the above described LAI-
ISC, GEL-LAI-ISC, or S-LAI-ISC techniques can have dense zeolite layers in
which the zeolite crystals are intergrown such that non-selective permeation
pathways in these as-synthesized zeolite layers are non-existent. Absence of
these non-selective pathways can be established using the previously
described Dye Test.
In this state, it would be expected that these continuous layers
of intergrown zeolite crystals would effect selective separations of
molecules.
An example of an expected selective separation is through molecular sieving
mechanisms. Such separations are realized when specific molecules are
sterically unrestricted by the zeolite pores (becoming permeate molecules)
while larger molecules (retentate molecules) are rejected because of steric
constraints. Thus the effective zeolite pore diameter becomes the controlling
aperture of the selective permeation pathway. An example of such
separations effected through molecular sieving would be the separation of
pare-xylene from a mixture of xylenes isomers utilizing an MFI-based zeolite
composition. The controlling pore aperture for the selective permeation
pathway in the MFI system is larger than the kinetic diameter of pare-xylene
and smaller than the kinetic diameters of ortho-xylene and mete-xylene.
Contrary to expectations, the previously described zeolite
compositions produced using LAI-ISC, GEL-LAI-ISC, or S-LAI-ISC
techniques, and having dense, intergrown, continuous zeolite layers, exhibit a
separation with selectivities less than expected. This is very evident when
the compositions with dense, intergrown, continuous zeoiite layers are tested
in molecular sieving. Generally, expected separation performance can be
estimated from nndependent measurements of flee diffusivity of molecular
2193259
R'O 96101686 PCT/US95108513
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species in the zeolites and the partition coefficient for molecules entering
the
zeoIite form a molecular mixture being processed. Techniques to measure
these quantities have been described J. Karger and D. Ruthven in "Diffusion
In Zeolites And Other Microporous Solids" (John Wiley And Sons Inc. NY,
1992).
Applicants ascribe the less than ideal performance in
separations applications for these as-synthesized zeolite compositions to
physical changes which occur in the zeolite layer during separations testing
or use. These physical changes in the zeolite layer which arises during use
may be due to mechanical stresses and deformations which occur because of
exposure of the zeolite layer to elevated temperatures and pressures in the
presence of hydrocarbon molecules. Applicants believe that stress or
mechanical deformations in a layer of closely packed zeolite crystals may
introduce defects or voids in the layer which can degrade the permselective
properties of the layer. Specifically the deformation or movement of one
crystal will affect the neighboring crystals as well as the grain boundary
separating the crystals.
Although, most of these dense intergrown zeolite layers
completely reject dye (using the Dye Test procedures described above) prior
to testing (indicating continuous zeolite layers), they then exhibit less than
expected performance characteristics during use for high temperature
molecular separations, and finally fail the dye test in post-testing
evaluations
by passing dye through the zeolite layer. Dense intergrown zeoIite layers
which do not reject dye in testing prior to testing in separations typically
have
a small number of isolated defects.
In order for coatings to be effective, they must be applied
directly to the as-synthesized zeolite layer. The coating can be applied to
either surface of the zeolite layer. In a preferred embodiment the coating is
applied to the surface of the zeolite layer which is not in contact with the
support or intermediate layer. Additionally, these coatings must interact with
the zeolite layer without blocking or impeding molecular transport through
pore openings of the zeolite layer. Interaction of the coating with the
zeolite
layer should be such that the coating does not block transport through more
2195259
w0 96101686 PCT/US95108513
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than nine - tenths of the surface-exposed pore openings of the zeolite layer.
In a preferred embodiment, interaction of the coating with the zeolite layer
should be such that the coating does not block transport through more than
one-half of the surface-exposed pore openings of the zeolite layer. An
occlusion does not completely block the diffusion of molecules through the
pores, but significantly hinders diffusion of the molecular species of
interest
through the pore. As such occlusions increase the mass transfer resistance of
the composition.
In order to successfully perform as a coating layer with the
previously mentioned characteristics, a coating must possess several other
properties. Fast, the coating must be stable at the application temperature of
the compositions. Stability is taken to be both mechanical and chemical
stability. The coating can change its physical characteristics within the
first
200 hours of use, after which the coating material should be stable.
Secondly, the coating must adhere to the zeolite layer and should not impede
molecular transport through a significant number of surface-exposed pores of
the zeolite layer. This means that the coating layer precursor material must
adhere to the zeolite without diffusing significantly into the pore structure
of
the zeolite crystallites, inhibiting transport of the desired molecular
species
through the zeolite. For example many polymeric coating materials cannot
be used even though they are stable under process conditions because their
precursor molecules have molecular sizes which permit their introduction
into the zeolite pore system. As such many polymeric coating materials
inhibit transport of the desired molecules through the zeolite. It is
possible,
however, to choose polymeric coating precursor materials which are
sterically incapable of entering the pores of the zeolite crystals. An example
of such a coating layer precursor is a polyimide resin which can be thermally
cured to produce a polyimide coaring. Polyimide films have been extensively
used in the semiconductor industry and are known to be thermally stable in
non-oxidizing atmospheres to temperatures as high as 500 °C.
Additionally,
these polymers have excellent mechanical properties and adhere well to oxide
materials. This is especially true when an adhesion promoter is incorporated
in the precursor resin. These characteristics indicate that polyimide
represents an excellent candidate material for a selectivity enhancing
coating.
W096101686 ~ PCT/U595108513
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For the composition to have an adequate flux, the selectivity
enhancing coating should increase the mass transfer resistance the
composition offers to molecules permeating through the zeolite layer by no
more than a factor of five. In a preferred embodiment, the selectivity
enhancing coating should increase the mass transfer resistance the
composition offers to molecules permeating through the zeolite layer by no
more than a factor of two. The coating can increase the mass transfer
resistance of the composition offers to molecules permeating through the
zeolite crystals by either occluding the zeolite pore structure, or covering
the
zeolite pore structure. Invariably some portion of the zeolite pore structure
exposed on one of the sides of the zeolite layer is covered by the coating.
The increase of mass transfer resistance due to the coverage of the zeolite
pore structure is determined by the material used to form the selectivity
enhancing coating.
The selectivity enhancing coating material can be a permeable
or impermeable material. or a material which in some regions is permeable
and in other regions impermeable. The permeability of the coating is taken to
mean its ability to pass the molecules transported through the zeolite layer
at
the temperature of composition use. Some coating materials are almost
impermeable at low temperatures and become permeable at high
temperatures. An example of a polymeric coating material which has a
temperature dependent permeability is polyimide. At room temperature,
polyimide films with thicknesses greater than .5 ltm have a very low
permeability too hydrocarbon molecules containing between 5 and 15 carbon
atoms. At elevated temperatures (between 100°C and 500°C) these
hydrocarbon molecules can permeate through the free volume left in dense
polyimide films. As such, polyimide films meet the requirements for a
perneable selectivity enhancing coating.
If the selectivity enhancing coating is made from a permeable
material, the material can be coated over the entire surface of the zeolite
layer. Permeable materials can either have a pore structure which transports
molecules that can pass through the zeolite or it can be a dense material
which has a free volume sufficient to permeate molecules which can pass
through the zeolite. Examples of permeable inorganic materials containing a
w0 96/01686 219 3 2 5 9 1,C.1.~S95108513
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pore structure are materials produced by sol-gel processing. Examples of a
dense material which has a free volume sufficient to permeate molecules
which can pass through the zeolite are polymers. A preferred polymeric
material used for the selectivity enhancing coating is polyimide. Preferably
the selectivity enhancing material is formed into a film which has well
mechanical properties suffi; Tent to stabilize the zeolite layer during use.
If the material used for the selectivity enhancing coating is
made from an impermeable material, it cannot completely cover and block
the pore structure of the zeolite membrane. Impermeable materials must be
applied so that they cover only a fraction of the zeolite layer. Ideally an
impermeable coating must l;ridge the boundaries of enough zeolite crystals to
prevent defects and voids from arising without blocking a significant fraction
of the zeolite composition. Preferably, this type of selectivity enhancing
coating bridges boundaries bet<veen at least 10% of the surface-exposed
crystals in the zeolite layer and, more preferably, the coating bridges he
boundaries between at least 50% of the surface-exposed crystals in the
zeolite layer.
A third type of selectivity enhancing coating may combine the
characteristics of the two previously described coating systems. It may be
both permeable (to process molecules) in some regions of the layer and
impermeable in other region of the composition. An example of such a
coating is a thin silica film which when deposited on the surface of a zeolite
nucleates and grows a porous film in some areas and a dense film in others.
Applicants believe that such silica films can be deposited by interfacial
ozone
assisted CVD processes. In the bulk phase, silica is a dense impermeable
material for the permeation of hydrocarbon molecules, however when
deposited as a thin film, some regions of the film can be formed into pores or
isolated molecular or nanosaale aggregates of the coating material on the
surface of the zeolite layer. In the region of the zeolite layer covered by
either pores in the silica film or isolated molecular or nanoscale aggregates
of
the silica coating material, t;°re coating is said to be porous. The
porous
region may occur at the edgy of a dense region in the film.
w0 96101686 ~ r 9 3 2 ~ ~ PCTIUS95108513
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The selectivity enhancing coating can serve two functions.
First, it can attenuate the flow through defects in a composition, thereby
providing a reparation of existing defects. Secondly, it can prevent the
formation of defects during use. The second of these functions is especially
important and is referred to as stabilization of the composition. Defects
which are observed to form during use are crack structures in the zeolite
layer
which degrade the performance of the composition. In many cases, these
cracks extend into the growth enhancing layer. Cracks in the zeoliter layer
provide a very permeable nonselective pathway that reduces the overall
selectivity of the composition. Isolated crack structures may form or grow by
temperature cycling, or by cycling the traps-composition pressure, or by
cycling the total system pressure, or by introducing a liquid phase feed onto
the composition. Crack structures grow by either branching or by extending
their length. As crack structures grow they form an intergrown crack network
in the zeolite layer. The selectivir<~ enhancing coating prevents these cracks
and crack networks from fonnin~~. As such, the selectivity enhancing coating
stabilizes the composition, preventing defect structures (i.e. cracks) which
were not in the as synthesized composition forming during use or changing of
the physical conditions. By preventing defect structures from forming, the
selecitivity enhancing coating improves the separation properties of the
zeolite composirion.
All selectivity enhancing coating materials must be able to
improve the separation properties of the zeolite layer of the composition.
Separation properties are measured as the ability of a composition to pass one
molecular species through the zeolite layer while impeding the transport of at
least one other molecular species. Because flux through zeolite layers tends
to increase with temperature, it is preferred to conduct these separations at
elevated temperatures above room temperature, preferable above 100°C.
For
hydrocarbon molecules other than methane and ethane, it is preferred to
conduct these separations at temperatures less than 600°C to avoid
thermal
degradation of the hydrocarbons.
The compositions with selectivity enhancing coatings are useful
for a variety of separations. The compositions are useful for separation
processes whereby feedstock derived from petroleum, natural gas,
2~ ~3z~~
W 0 96101686 PC1'IUS95I08513
-21-
hydrocarbons, or air comprising at least two molecular species is contacted
with the composition comprising a substrate, a zeolite or zeolite-like layer,
a
selectivity enhancing coating in contact with the zeolite layer, and
optionally
a permeable intermediate layer in contact with the substrate, the zeolite
layer
or zeolite-like layer being in contact with the substrate or the optional
intermediate layer, wherein at least one molecular species of said feedstock
is
separated from said feedstock by said composition and wherein said
hydrocarbon feedstocks, are coal, bitumen and kerogen derived feedstocks.
Separations which may be carried out using a composition comprising a
zeolite layer in accordance with the invention include, for example,
separation of normal alkanes from co-boiling hydrocarbons, especially n-C 10
to C 16 alkanes from kerosene; also normal alkanes and alkenes from the
corresponding branched alkane and alkene isomers; also alcohols from other
hydrocarbons, particularly alkanes and alkenes that may be present in
mixtures formed during the manufacture of the alcohols; separation of
aromatic compounds from one another, especially separation of Cg aromatic
isomers from each other, more especially para-xylene from a mixture of
xylenes and, optionally, ethylbenzene, and separation of aromatics of
different carbon numbers, for example, mixtures of benzene, toluene, and
mixed Cg aromatics; separation of aromatic compotmds from aliphatic
compounds, especially aromatic molecules with from 6 to 8 carbon atoms
from Cg to C1~ (naphtha range) aliphatics; separation of olefinic compotmds
from saturated compounds, especially light alkenes from alkane / alkene
mixtures, more especially ethene from ethane and propene from propane;
removing hydrogen from hydrogen-containing streams, especially from light
refinery and petrochemical gas streams, more especially from C2 and lighter
components; and alcohols from aqueous streams.
Specifically, the following table shows some possible feedstocks
derived from petroleum, natural gas or air and Qlle molecular species
separated
therefrom by use of the instant compositions. The table is not meant to be
limiting.
w0 96/01686 ~ ~ ~ ~ ~ J ~ PCTIUS951118513
-22-
F ck a crated Molecular i
Mixed ~ lenes ortho, arc, meta and ethvlbenzeneParaxvlene
Mixture ofhvdro en, HAS, and ammonia H dro en
Mixture of normal and isobutanes Normal butane
Mixture of normal and isobutenes Normal butene
Kerosene containin C to C normal araffinsC to C normal araffins
Mixture of rtitro en and o en Nitro en or o en
Mixture of h dro en and methane H dro en
Mixture of h dro en, ethane, and ethyleneHvdro en and/or ethylene
Coker naphtha containing CS to C 10 CS to C 10 normal olefins
normal olefins and and
araffins araffins
Methane and ethane mixtures containing Helium, neon, and/or
argon, helium, argon
neon, or vitro en
Intermediate reactor catalytic reformerHydrogen, and/or light
products gases
coniainin hvdroeen and/or lieht eases C -C
Fluid Catalytic Cracking products containingHydrogen, and/or light
H~ and/or gases
lieht ases
Na htha containin CS to C normal araffinsC to C normal araffins
Light coker gas oil containing Cg to Cg to Clg normal olefins
Clg normal olefins and
and araffins araffins
Mixture of normal and iso entanes Normal entane
Mixture of normal and iso entenes Normal entene
Mixture of ammonia, h dro en, and vitroH dro en and vitro en
en
Mixture of A10 (10 carbon) aromatics e.g. Paragiethylbenzene
DEB
Mixed butenes n-butenes
Sulfur and/or nitrogen com ounds HAS and/or NH
Mixtures containin Benzene (Toluene) Benzene
H~, ro ane and ro vlene H dro en and/or ro lene
Applicants believe that molecular diffusion is responsible for the
above separations. Additionally, the compositions can be used to ettect a
chemical reaction to yield at least one reaction product by contacting the
feedstocks as described above or below with the compositions having a catalyst
incorporated within the zeolite layer, support, or intermediate layer or by
placing
the catalyst in close enough proximity with the composition to form a module.
A
module would react the feedstock just prior to its entrance into the
composition
or just after its exit from the composition. In this manner one can separate
at
least one reaction product or reactant from the feedstocks. The catalysts of
choice for particular process fluids are well known to those skilled in the
art and
Z~~1JGJ~
w0 96/01686 PCT'/US95/08513
-23-
are readily incorporated into the instant compositions or formed into modules
by
one skilled in the art. The following table represents some of the possible
feedstocks/processes, in addition to those above which can be reacted and some
possible products yielded. The table is not meant to be limiting.
Feedstock/ rocess Product Yielded
Mixed xylenes (para, ortho, Pwaxylene and/or ethylbenzene
meta) and
ethvlbenzene
Ethane deh dro enation to Hvdro en and/or eth lene
eth lene
Ethylbenzene dehydrogenation Hydrogen
to
styrene
Butanes dehydrogenation butenesHydrogen
(iso's and normals)
Propane dehydrogenation to Hydrogen and/or propylene
ro lene
C 1 p-C 1 g normal paraffin Hydrogen
dehvdro enation to olefins
H dro en Sulfide decom ositionH dro en
Reforming Hydrogen, light hydrocarbons
dehvdro enation/aromatization(C 1-
C
Light Petroleum Gas Hydrogen
deh dro enation/aromatization
Mixed Butenes n-butenes
The supported zeolite layer 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°C,
and it
is an advantage of the supported layer 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 zeolite layer according to the invention under conditions such
that at least one component of the mixture has a different steady state
permeability through the layer from that of another component and
2193?59
w0 96101686 PGTlU895108513
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recovering a component of mixture of components from the other face of the
Layer.
The invention further provides a process for catalyzing a
chemical reaction which comprises contacting a feedstock with a zeolite layer
according to the invention which is in active catalytic form under catalytic
conversion conditions and recovering a composition comprising at least one
conversion product.
The invention further provides a process for catalyzing a
chemical reaction which comprises contacting a feedstock with one face of a
zeolite layer according to the invention, in active catalytic form, under
catalytic conversion conditions, and recovering from an opposite face of the
layer at least one conversion product, advantageously in a concentration
differing from its equilibrium concentration in the reaction mixture. For
example, a para-xylene rich mixture from the reactor or reactor product in a
xylenes isomerization process; aromatic compounds from aliphatics and
hydrogen in a reforming reactor; hydrogen removal from refinery and
chemicals processes such as alkane dehydrogenation in the formation of
alkenes, light alkane / alkene dehydrocyclisation in the fotanation of
aromatics (e.g., Cyclar), ethylbenzene dehydrogenation to styrene.
The invention further provides a process for catalyzing a
chemical reaction which comprises contacting one reactant of a bimolecular
reaction with one face of a zeolite layer according to the invention, in
active
catalytic form, under catalytic conversion conditions, and controlling the
addition of a second reactant by diffusion from the opposite face of the layer
in order to more precisely control reaction conditions. Facamples include:
controlling ethylene, propylene or hydrogen addition to benzene in the
formation of ethylbenzene, cumene or cyclohexane respectively.
The invention further contemplates separation of a feedstock as
described herein wherein the separated species reacts as it leaves the zeolite
layer or as it passes through the membrane and thus for another product.
This is believed to increase the driving force for diffusion through the
zeolite
layer.
219259
t WO 96/01686 PCT/US95/08513
_ ~5 _
Catalytic functions can be incorporated.into the compositions.
_ When a catalytic function is incorporated into the compositions, it can be
used as an active element in a reactor. Several different reactor
architecture's
can be constructed depending on the location of the catalytic site in the
composition. In one case the catalytic function can be located within the
zeolite layer, while in another case the catalytic function can be located
within the support, and in another case the catalytic function can be
distributed throughout the support, GEL layer or seed layer and the zeolite
layer. Impregnating with catalytically active metals such as Pt can impart
the catalytic function to the composition. In addition, catalytic function can
be incorporated into a membrane reactor by locating conventional catalyst
particles near one or more surfaces of the composition such that specific
reaction products or reactants are continuously and selectively removed or
added to the reaction zone throughout the reactor.
CA 02193259 2004-08-10
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Example 1:
Xvlene Separation Using MFI Compositions Synthesized With The LAI-ISC
Process
Several different MFI compositions were prepared and tested
for the separation of xylene isomers. The tests were conducted on as
synthesized membranes which did not have a selectivity stabilizing coating.
The materials used for synthesis of LAI-ISC compositions will first be
described, followed by the hydrothermal synthesis conditions used, followed
by a description of the products made and then by a summary of results from
dye testing before and after ~cviene separation experiments which exhibited
low selectivities. The results of the xvlene separation experiments showed
selectivities which were less than expected. For the range of conditions
under which the xvlenes testing experiments were conducted, the selectivities
for the separation of para from either the metal or ortho isomer were expected
to be greater than 2.5 over the entire range temperatures used. This lower
linut for the expected selectivity was determined from measurements of
diffusion coefficients of the xyiene isomers in the type of high silica MFI
crystals present in the membrane.
1. Materials
The hydrothermal experiments were perfotTned using mixtures
of the following reagents: NaOH(Baker), Al(N03)3.9H20(Baker), Ludox~AS-
40 (Dupont), tetrapropylamonium bromide (98% Aldrich), and distilled
water.
Porous aiumina and stainless steel substrates were used for the
support of the zeolite layers. The alumina substrates alpha phase and has an
average pore size and porosity of about 800A and 32%., respectively. Porous
sintered stainless steel substrates from Mott (0.25 um) and Pall (M020, 2 Itm)
were obtained. All of the substrates were cleaned with acetone in an ultra-
sonic bath, dried at 120°C and then cooled down to room temperature
before
use.
* trade-mark
CA 02193259 2004-08-10
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2. Hydrothermal Reaction
MFI compositions were prepared from two different reaction
batch mixtures, one contained silica only to make silicalite and the other was
doped with alumina to make ZSM-5. They have the general formulation x
M20 : 10 Si02 : (0-0.045) A12O3 : 0.5 TPA Br : yH20; where M can be Li,
Na, K, Rb, and Cs, x varied from 0 to 0.5, and y varied from 50 to 3000.
TPABr may be replaced with tetrapropylammonium hydroxide if desired.
All the results shown in the next section have the composition of 0.22
Na20:10 Si02:280 H20:0.5 TP.A Br and doping with 0.05 A 1203 for the
ZSM-5 sample. The 1.74 g of TPA Br and 0.45g of NaOH (50 wt%) were
dissolved in 52 ml of distilled water with stirring. To this solution, 18.8 g
of
Ludox AS-~10 was then added with agitation for at least 15 minutes until a
uniform solution was formed.
The substrates were placed inverted in a Teflon lined reaction
vessel supported on a stainless steel wire frame. The distance between the
substrate and the bottom of reactor was at least 5 mm. The synthesis solution
was then poured into the reactor to cover the whole substrate. The autoclave
was sealed and placed in an oven which was preheated at the desired
temperature. The reaction bombs were removed from the oven after reaction
and cooled to room temperature. The coated substrates were washed with hot
water for at least 6 hours. then calcined at 500°C for 6 hours in air.
The
heating rate was controlled at 10°C/hour.
3. Products
The following table shows some typical examples synthesized
under different experimental conditions, such as reaction time, and substrate.
4- Physical Characterization Of Compositions
The resulting compositions were characterized by x-ray
diffraction and electron microscopy. X-ray diffraction revealed that the
zeolite grown was MFI with no other phases detected. The x-ray diffraction
analysis also revealed a degree of preferred orientation in the as-synthesized
* trade-mark
WO 96/01686 2 ~ ~ ~ ~ ~ ~ pC'gyUS95108513
-as-
membrane. To establish that the zeolite layer was substantially free of voids,
membranes were cleaved and the cross-sectional profile of the zeolite layer
was viewed in the scanning electron microscope. Figure 2 shows a cross
sectional view of a LAI-ISC composition grown on an alpha-alumina
substrate with 800 t~ pores. It is seen that the zeolite layer is continuous ,
and substantially free of voids. Figure 3 shows a plan view of the surface of
a LAI-ISC composition showing that the crystals are intergrown.
Dye Testing Procedure Used To Characterize Compositions
The absence of voids in the zeolite layer is reflected in the
inability of large dye molecules such as rhodamine B to permeate through the
layer into the substrate. Permeation of dye through the zeolite layer can be
detected by observing the way it soaks into the porous substrate. Dye
permeating through the columnar zeolite layer will be wicked into the porous
substrate in a manner similar to the way ink soaks into a piece of blotter
paper. The presence of dye in the porous substrate is especially easy to
detect when the substtate is made from a porous ceramic that does not absorb
light. Light scattering in such porous substrates causes them to appear white
when viewed with the naked eye. Any dye which wicks into the substrate
through a small pinhole in the zeolite layer will be readily visible as a
coloration change in the ceramic substrate. Dye molecules with sizes large
than the zeolite pore structure will not transport into the underlying
substrate
except through defect sites. The rate at which the dye transports through a
pinhole is different from the rate at which dye spread over the underlying
porous ceramic can be removed through the same pinhole in a subsequent
washing step. Thus a simple way of detecting dye permeation into the porous
substrate consists of applying a dye solution to the zeolite layer and
subsequently washing excess dye away to reveal pinholes in the zeolite layer.
The test can be conducted with a soluble dye which has a physical size larger
than the controlling aperture size of the zeolite. A preferred dye is
Rhodamine B ,which is a commonly used laser dye. A convenient solvent to
make a solution of this dye is methanol, however other solvents such as
toluene can also be used as long as they tend to wet (or wick into) the
ceramic substrate. Water is not a recommended solvent for this test. This
dye should be applied from a concentrated solution which has a coloration
W096101686 ~ ~ ~ J ~ ~ ~ PCT1U995/08513
-29-
similar to that of commonly available red wines. This range of colorations
can be obtained from solutions with Rhodamine B concentrations in the
range of .OS-5 wt%. With different concentrations, the amount of time
required for the dye soaking thorough to the substrate to become visible
changes. A preferred procedure for dye testing zeolite membranes formed
on white porous ceramic substrates is;
1) Make 20 ml of a stock solution containing 0.5 wt% of Rhodamine B in
methanol. This solution should be stored in a sealed bottle to prevent
evaporation of the methanol.
2) From the stock dye solution remove 5 drops with an eye dropper and
apply the dye dropwise to the center of the dry ( 120°C for 30 minutes)
membrane so that the surface is covered. '1 ypically 2 ~ drops are
needed to cover the membrane surface. Each drop contains -0.25
milligrams of dye. Do not let the dye wick over the edge of the
membrane into the porous ceramic exposed at the rim of the membrane.
3) Let the dye covered membrane stand for -30 seconds and then blot off
the excess dye solution with a paper towel.
4) Apply methanol to a clean section of the paper towel and blot the
surface again with the towel to remove more dye.
5) Rinse the disk surface with methanol for 10-30 seconds. A convenient
way to do this is with a methanol wash bottle.
6) Visually examine the substrate for any red colorations
6 Dve Testine Of ComQositions Before Separations Testing
As-synthesizes LAI-ISC MFI zeolite compositions were dye tested
before being used in separations experiments. It was determined that the
compositions were essentially free of defects and that dye molecules did not
permeate through boundaries between crystals.
7 Xvlene Separations Testing
LAI-ISC MFI zeolite composition fabricated on 3mm thick 1 inch
diameter alpha-alumina supports (800 A pore size) were mounted into a
J
2~9~2~~
-- 30 -
permeation cell and sealed with a graphite o-ring. The membranes chosen
for separations testing passed the dye test. The permeation cell was heated to
temperatures between 170 °C and 300 °C while a mixture of xylene
isomers
flowed across the face of the composition which had the zeolite layer. The
flow rate of the xylene feed was 1 ml/ minute. Across the opposite face of
the membrane an argon sweep was flowed at rates between 100 and 400
cc/minute. Pressure of both the argon sweep and the xylene feed was fixed at
1 atmosphere (absolute) (1.013 bar). Observed selectivities for separation of
the xylene isomers by the LAI-ISC compositions was less than 1.75
(para/ortho or para/meta) at all temperatures tested. Typical selectivities
were in the range between 1.0 and 1.4. A total of four LAI-ISC
compositions were tested in this manner. The LAI-ISC compositions tested
had zeolite layer thicltnesses. in a range between 5 and 25 Vim.
8. Dve Testing Of Compositions After Separations Testing
LAI-ISC MFI zeolite compositions were dye tested after being used
in xylene separations experiments. It was determined that compositions pass
dye into the substrate. A reddish color not present in the dye test before
using
in xylene separation experiments was clearly visible in the substrate. This
indicated that defects formed during separations testing.
Xvlne Separation Bv An MFI Composition Sy nthesized Llsing A GEL-LAI-
ISC Process-
Several different MFI compositions were prepared and tested
for the separation of xylene isomers. The tests were conducted on as
synthesized compositions which did not have a selectivity stabilizing coating.
The materials used for the GEL layer will first be described, followed by the
technique used to form the GEL layer, followed by the hydrothermal
synthesis conditions used, and a description of the final membrane structure.
Finally results are presented from dye testing before and after xylene
separation experiments which exhibited low selectivities. The results of the
xylene separation experiments with these GEL-LAI-ISC compositions
r 17~~~
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W096101686 ~ ~ ~ pC1'/US9S/08513
-31-
showed selectivities which were less than expected. For the range of
conditions under which the xylenes testing experiments were conducted, the
selectivities for the separation of para from either the meta or ortho isomer
should be greater than 2.5 over the entire range of temperatures used. This
lower limit for the expected selectivity was determines from measurements
of diffusion coe~cients of the xylene isomers in the type of high silica MFI
crystals present in the compositions.
1. GEL Coating Materials
The following reagents were used in preparing GEL coatings:
colloidal alutnina solution, colloidal titanic prepared from a sol-gel
process,
colloidal silicalite solutions, and distilled water. Several batches of
colloidal
silicalite solutions prepared in accordance with the previously described
nanocrystalline synthesis method were used for the preparation of GEL
coatings. More information on these solutions are shown below:
Batch Silicalite SilicaliteFinal SilicaliteSolidsParticle
#
Synthesis Washed? Solution (%) Size.
Temp. pH nm
I 68 ves 10.3 8.7 -30
2 509 no >13 -9 -~0
3 81 ves 9.9 9.1 -90
4 50 no >13 -9 -60
4 50 ves -10 -9 -60
Remarks:
1. All suspensions were prepared from the same type of synthesis solutions
with the same raw materials.
2. Batch 4 was a duplication of 2. The solids content of batches 2 and 4
were calculated assuming 55% conversion of amorphous silica to
zeolite. The actual solids content of these 2 unwashed samples is of
course higher, e.g. for batch 4 the solids content (evaporation to
219329
- 32 - : :'
dryness) was 23.3 wt%, but this includes zeolite, amorphous silica and
residual TPAOH-NaOH.
Porous alumina and stainless steel substrates were used for the
support of GEL and zeolite coatings. The average pore size and porosity of
the alumina is about 800A and 32%, respectively. Porous sintered stainless
steel substrates from Mott (0.25 micrometer) and Pall (M020, 2 micrometer)
were obtained. All the substrates were cleaned with acetone in an ultra-sonic
bath, dried at 120°C and then cooled down to room temperature before
use.
2. GEL Coatine
In general, a dilute solution is preferred to produce a high
quality growth enhancing layer. Dilution with distilled water to obtain a
solids concentration less than 1 wt% is generally preferred. Colloidal
silicalites and metal oxides are first diluted separately with distilled water
to
the concentration of 0.5 wt%. The diluted colloidal silicalite solution was
slowly added into the desired amount of metal oxide solution with continuous
stirring. The resulting solutions with the desired wt% of colloidal silicalite
and metal oxide were then degassed for 15 minutes to remove the trapped air
in solutions.
The substrates were then spin coated with these solutions at
4000 rpm and calcined at 500°C for 6 hours in au. The heating rate was
controlled at 20°C/hr.
3. Svnthesi~ Of Zeolite Compositions Llnder Hvdrothermal Conditions
The hydrothermal experiments were performed using mixtures
of the following reagents: NaOH (Baker), Al(N03)3~9H20 (Baker), Ludox
AS-40 (Dupont), tetrapropylammonium bromide (98%, Aldrich), and
distilled water.
MFI compositions were prepared from two different reaction
batch mixtures, one containing silica only to make silicalite and the other
doped with alumina to make ZSM-5. They have the general formulation
~n ~,4.1~C'f
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R'O 96101686 ~ ~ ~ PCT/US95/08513
- 33 -
xM20 : 10 Si02 : (0-0.045) A1203 : 0.5 TPA Br : y H20; M can be Na, Li,
K, Rb, and Cs, x was varied from 0 to 0.5, and y varied from 50 to 3000. All
the results shown in the next section have tile composition 0.22 Na20:10
Si02:280 H20:0.5 TPABr and doping with 0.05 AI203 for the ZSM-5
sample. The 1.74g of TPABr and 0.45g of NaOH (50 wt%) were dissolved in
52 ml of distilled water with stirring. To this solution, 18.8g of Ludox AS-40
was then added with agitation for at least 15 minutes until a uniform solution
was formed. TPA Br can be replaced with aqueous tetrapropylammonium
hydroxide if desired. Substrates with GEL coating were placed inverted
(180° orientation) in a Teflon lined autoclave by supporting them on
the
stainless steel wire frame. The distance between the substrate and the bottom
of autoclave reactor was at least 5 mm. The s5mthesis solution was then
poured into the reactor to cover the whole substrate. The autoclave was
sealed and placed in an oven, which was preheated at the desired
temperature. The autoclaves were removed from the oven after reaction and
cooled to room temperature. The coated substrates were washed with hot
water for at least 6 hours, then calcined at 500°C for 6 hours in air.
The
heating rate was controlled at 10°C/hour.
4. Products
The following table shows some typical examples synthesized
under different experimental conditions, such as GEL composition, reaction
time and substrate.
5. General Observations
The x-ray diffraction pattern of typical inverted zeolite
composition (LAI-ISC), noninverted zeolite composition (LA-ISC),
noninverted zeolite composition grown on a GEL coated substrate (LA-
GEL-ISC), and inverted zeolite composition grown on a GEL coated
substrate (LAI-GEL-ISC) were observed. Reflections of MFI type zeolite
were identified in all diagrams. No zeolite second phase was observed. The
only lines in the patterns not associated with the zeolite identified with the
porous support. The pattern associated with the GEL-LAI-ISC membrane was
dramatically different from other samples. It is seen that the MFI crystal
layer
2193259 ...
- 34 -
prepared from GEL-LAI-ISC exhibits pronounced 001 peaks with no other
significant zeolite peaks occurring in the pattern. This is strong evidence
that
a preferred orientation of (001) directions parallel to the growth direction
exists in the membrane. Another way of saying this is that the MFI crystal
layer in GEL-LAI-ISC membranes shows very strong orientation with c-axis
norriial to the GEL layer.
Figure 5 shows a plan view of a typical GEL-LAI-ISC
membrane (sample #2). The cauliflower-like top surface and columnar cross-
sectional morphologies were observed in the membrane. Figure 4a shows the
zeolite layer (C), the growth enhancing layer (B) and porous support (A). The
major part of Figure 4a shows the continuous growth of zeolite that
completely covers the surface of the GEL layer. The formation of a columnar
structure in the zeolite layer is apparent. The width of the columns right on
the growth enhancing layer is very narrow and becomes larger and larger as
the layer grows. As such, the average grain size of zeolite crystals increases
with increasing layer thickness. The columnar nature of the microstructure is
consistent with the x-ray powder diffraction pattern. The c-axis is the
fastest
growth for the MFI zeolite in the membrane. In Figure 5b, it is clear that
zeoIite surface consists of a continuous array of densely packed zeolite
crystals, which are <10 ltm in width.
6. Dye Permeation Test Before Xvlene Separation Testing
The dye permeation test described in Example 1 was applied to
test the integrity of GEL-LAI-ISC compositions grown on porous alpha-
alumina supports. It was found that compositions grown with MFI zeolite
layers thicker than 5 um did not permeate dye into the substrate even after
they had been calcined. For 1 inch (2.54 cm) diameter compositions grown
on 3 mm thick alpha-alumina supports, even isolated defects were not
detected in over-95% ofthe membranes tested.
7. Xvlene Separations Testine
GEL- LAI-ISC MFI zeolite compositions fabricated on 3mm
thick 1 inch diameter alpha-alumina supports (800 ~ pore size) were
1. ~rr,~' C". .y ~~r
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WO 96101686 219 3 ~ ~ 9 p~'/US95/OSSI3
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mounted into a permeation cell and sealed with a graphite o-ring. Xylene
separations tests were conducted using the method described in example 1.
Observed selectivities for separation of the xylene isomers by the GEL-LAI-
ISC membranes was less than 1.75 (para/ortho or para/meta) at all
temperatures tested. Typical selectivities were in the range between I.0 and
1.4. A total of five GEL- LAI-ISC compositions were tested in this manner.
The GEL - LAI-ISC compositions tested had zeolite layer thicirnesses in a
range between 5 and 25 pm.
8. Dye Testing Of Compositions After Separations Testing
GEL-LAI-ISC MFI zeolite compositions' were dye tested
after being used in xylene separations experiments. It was determined that
membranes pass dye into the substrate. A reddish color not present in the
dye test before using in xylene separation experiments was clearly visible in
the substrate. This indicated that defects formed during separations testing.
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EXAMPLE 3
cPiP~rivity Enhanicne Coatines Made From Polvimide
A selectivity enhancing coating made from polyimide was
applied to zeolite membrane layers (produced using one of the LAI-ISC,
GEL/LAI-ISC, or S/LAI-ISC methods described above). The zeolite layer
thicknesses ranged from 2 to 25 pm. All of the compositions chosen passed
the dye test described in example 1. As such they appeared to be free of
defects.
Zeolite compositions were coated utilizing a spin coating
apparatus (Headway Research, Inc.). The coating material was a polyimide
precursor resin, PLY-8203, from Hitachi Chemical Company, which
contained an adhesion promoter. The composition material was initially
dried at 120°C and any dust or foreign material removed with high
pressure
filtered nitrogen just prior to applying the permeation stabilization coating.
The compositions were placed on the spinner and the apparatus set to the
desired spin speed and duration, two of the principle factors which
determine coating thickness. As the spin speed increases, the coating
thickness decreases.
The entire surface of the zeolite layer is next covered with
polyimide precursor resin (neat, as received) and allowed to sit, without
spinning, in air, for 10 to 20 seconds. Next, the spinner is turned on which
spins the material, removing any excess polymer resin from the surface. As
an example, the material is spun at a rate of 6000 rpm for 30 seconds which
results in a coating layer thickness of 1 mm.
The coated composition in then placed in a 120°C oven, in
argon, for 30 minutes to remove any solvents (inherent to the as received
polymer precursor: N-methyl pyrrolidone and/or dimethylacetamide). The
oven temperature is then raised (over a one hour period) to 400°C and
held
at this temperature for 2 hours. A cross sectional view taken in the
scanning electron microscope of a GEL-LAI-ISC composition with a
2;9~2~
'~ w0 96/01686 PCT1US95/OSSI3
-39-
thermally cured polyimide selectivity enhancing coating is shown in Figure
5.
The method of example I and 2 was used to measure the
selectivity for the separation of xylene isomers. The maximum selectivities
for the separation the xylene isomers was determined. This maximum
selectivity is taken to be either the separation factor for the pare with
respect
to the ortho isomer or the separation factor for the pare with respect to the
mete- isomer, whichever is higher. Three polyimide coated LAI-ISC
compositions were tested which showed maximum selectivities in the range
from 2.5 to 5. The average maximum selectivity was approximately 3.5.
Six poiyimide coated GEL-LAI-ISC compositions were measured which
showed a maximum selectivity between 4 and 10. The average of these
maximum selectivities was approximately 6.5. One S-LAI-ISC composition
was measured which showed a maximum selectivity near 5. These
maximum selectivities are significantly higher than the selectivities observed
for LAI-ISC and GEL-LAI-ISC compositions (described in Examples 1 and
2) that were tested without the selectivity enhancing coating. The
selectivites measured are in line with the expected maximum selectivity.
Alternately, different coating thicknesses can be effected by
varying the spin rate of the spinner and/or the spinning time duration. A
thicker coatings was produced on a GEL-LAI-ISC composition by spinning
at 5000. The maximum selectivity for separation of the pare isomer from
either the mete- or ortho- isomer was 6.
EXAMPLE 4
Repartion Using Silicone Oil
Selectivity enhancing coatings were applied to zeolite
membrane layers (produced using one of the LAI-ISC, GEL/LAI-ISC, or
S/L,AI-ISC methods described above). The zeolite layer thicknesses ranged
from 2 to 25 ltln. All of the membranes chosen passed the dye test
described in Example 1. As such they appeared to be free of defects.
CA 02193259 2004-08-10
-40-
The apparatus depicted in Figure 1 (Cold Wall Reactor which
minimizes ozone degradation outside of the reaction zone) was used to apply
selectivity enhancing coatings to the compositions. The Reactor eliminates
the need for O-rings in the system and is amenable to large scale fabrication
operations.
A model GL-1 ozone generator manufactured by PCI Ozone and
Control Systems was used for the experiments. The instrument utilized a
plasma discharge system to produce ozone (four weight percent in flowing air,
0.2 SCFM, 5000 cm2/min). This stream was split by a mass flow controller so
that 100 - 300 cm2/min reached the reaction zone. The residence time of the
ozone in the reaction zone is governed by regulating the reactor temperature,
since ozone has a specific half -ife at given temperatures.
With a reactor volume of 50 cm'', a reactor pressure of 1 atm
( 1.013 bar), and temperature of 200°C, the residence time of ozone
flowing at
300 cm2/min was 10 seconds. The liquid phase oxidized by ozone to produce
the silicon oxide SEC was Invoil 940 silicone diffusion pump fluid (Inland
Vacuum Industries).
The use of this technique to produce selectivity enhancing
coatings is illustrated for to ozone assisted CVD coating methods. The first
method involves application of a liquid selectivity enhancing coating
precursor to the front side of a zeolite layer followed by ozone treatment in
the Cold Wall Reactor. The front side of the zeolite layer is taken to be the
side of the zeolite layer opposite to the side nearest the porous substrate.
The
second example illustrates a application of a liquid selectivity enhancing
coating precursor to the back side of a zeolite layer followed by ozone
treatment in the Cold Wall Reactor.
A) Formation of an Selectivity Enhancing Coating from Front Side
Treatment of a GEL/LAI-ISC Zeolite Composition
A GEL/LAI-ISC zeolite layer (supported by a porous A I 6
alumina support) was coated with a thin layer of Invoil 940 silicone diffusion
pump fluid (Inland Vacuum Industries) applied by pouring the material
directly on top of the zeolite layer. This oil was allowed to stand, in air,
on
* trade-mark
W O 96!01686 ~ ~ ~ ~ ~ ~ PCTYUS95108513
-31 -
the zeolite crystals, partly permeating into the intercrystalline defects and
voids in addition to coating the surfaces of the zeolite crystals. After 5
minutes, any remaining silicone oil was removed from the ceramic by blotting
with a towel.
The treated disk was inserted into the Cold Wall Reactor, the
entire system placed on a heating plate (at room temperature), and the
ozone/sir mixture flowed over the surface. The temperature was then raised to
200°C and held at these conditions for 0.1 - 4 hours. After ozone
treatment,
the membrane was cooled and washed with technical grade toluene (drawn
through the membrane).
The method of example 1 and 2 was used to measure the
selectivity for the separation of xylene isomers, and the maximum
selectivities for the separation the xyiene isomers was determined. This
maximum selectivity is taken to be either the separation factor for the para
with respect to the ortho isomer or the separation factor for the para with
respect to the meta isomer, whichever is higher. Three GEL-LAI-ISC
membranes tested showed maximum selectivities in the range from 3.5 to 5.
The average maximum selectivity was approximately --4. These maximum
selectivities are significantly higher than the selectivities observed for LAI-
ISC and GEL-LAI-ISC membranes (described in Examples 1 and 2) that
were tested without the selectivity enhancing coating. The selectivites
measured are in line with the expected maximum selectivity.
B) Formation of a Selectivity Enhancing Coating from Back Side Treatment
of a GEL/LAI-ISC Zeolite Composition
A GEL/LAI-ISC zeolite layer (supported by a porous AI6
alumina support) was placed in Invoil 940 silicone diffusion pump fluid
(Inland Vacuum Industries) such that the oil covered the back side of the
disk,
but did not contact the zeolite layer. The disk was allowed to stand, in sir,
in
the oil, which permeated into the substrate by capillary action. The oil
eventually permeated into the intercrystalline defects and voids in the
zeolite
layer. After 30 minutes, the disk was removed from the oil and any remaining
silicone oil was removed from the ceramic by blotting with a towel.
2193259 -
VVO 96/01686 PC'lYITS95108513
i2 -
The treated disk was inserted into the Cold Wall Reactor, the
entire system placed on a heating surface (at room temperature), and the
ozone/air mixture flowed over the surface. The temperature was then raised to
200°C and held at these conditions for 0.1 - 4 hours. After ozone
treatment,
the membrane was cooled and washed with technical grade toluene (drawn
through the membrane).
The method of example l and 2 was used to measure the
selectivity for the separation of xylene isomers. The maximum selectivities
for
the separation the xylene isomers were determined. One GEL-LAI-ISC
membrane tested showed a maximum selectivity near 4.5. Comparing these
separation factors with example 2, it is seen that applying the interfacial
gas
phase reparation technology to GEL-LAI-ISC zeolite membranes, the
selectivity for the separation of xylene isomers has been improved.
