Molecular sieve abstract
Layers comprising a molecular sieve layer on a porous or non-porous
support, having uniform properties and allowing high flux are prepared
from colloidal solutions of zeolite or other molecular sieve precursors
(particle size less than 100 nm), by deposition, e.g., by spin or
dip-coating, or by in situ crystallization.
Molecular sieve claims
What is claimed is:
1. A supported inorganic layer comprising contiguous particles
of a crystalline molecular sieve, the particles having a means particle
size within the range of from 20 nm to 1 .mu.m, wherein the support
is selected from the group consisting of glass, fused quartz, silica,
silicon, clay, metal, porous glass, sintered porous metal, titania,
and cordierite, and wherein the particle size distribution is such
that at least 95% of the particles have a size within .+-.33% of
the mean.
2. A supported inorganic layer comprising contiguous particles
of a crystalline molecular sieve, the particles having a mean particle
size within the range of from 20 nm to 1 .mu.m, wherein the support
is selected from the group consisting of glass, fused quartz, silica,
silicon, clay, metal, porous glass, sintered porous metal, titania,
and cordierite, and wherein the layer primarily contains nanopores
having size of between 1 and 10 nm.
3. A supported inorganic layer comprising contiguous particles
of a crystalline molecular sieve, the particles having a mean particle
size within the range of from 20 nm to 1 .mu.m, wherein the support
is selected from the group consisting of glass, fused quartz, silica,
silicon, clay, metal, porous glass, sintered porous metal, titania,
and cordierite, and wherein the layer primarily contains micropores
having a size of between 0.2 and 1 nm.
4. A supported inorganic layer comprising contiguous particles
of a crystalline molecular sieve, the particles having a mean particle
size within the range of from 20 nm to 1 .mu.m, wherein the support
is selected from the group consisting of glass, fused quartz, silica,
silicon, clay, metal, porous glass, sintered porous metal, titania,
and cordierite, and wherein the layer comprises molecular sieve
crystals in a particulate matrix, the pore structure being defined
by the interstices between the particles, between the crystals,
and between the particles and the crystals, the pore structure advantageously
being between 0.2 and 1 nm in size.
5. A supported inorganic layer comprising contiguous particles
of a crystalline molecular sieve, the particles hang a mean particle
size within the range of from 20 nm to 1 .mu.m, wherein the layer
primarily contains nanopores 1 and 10 nm.
6. A supported inorganic layer comprising contiguous particles
of a crystalline molecular sieve, the particles having a mean particle
size within the range of from 20 nm to 1 .mu.m, wherein the layer
primarily contains micropores having a size of between 0.2 and 1
nm.
7. A layer as claimed in claim 6 wherein the layer comprises molecular
sieve crystals in a particular matrix, the pore structure being
defined by the interstices between the particles, between the crystals,
and between the particles and the crystals.
Molecular sieve description
This invention relates to molecular sieves, more especially to
crystalline molecular sieves, and to layers containing them. More
especially, the invention relates to a layer, especially a supported
layer, containing particles of a crystalline molecular sieve.
Molecular sieves find many uses in physical, physico-chemical,
and chemical processes, most notably as selective sorbents, effecting
separation of components in mixtures, and as catalysts. In these
applications, the crystallographically-defined pore structure within
the molecular sieve material is normally required to be open, and
it is then a prerequisite that any structure-directing agent, or
template, that has been employed in the manufacture of the molecular
sieve be removed, usually by calcination.
Numerous materials are known to act as molecular sieves, among
which zeolites form a well-known class. Examples of zeolites and
other materials suitable for use in the invention will be given
below.
When molecular sieves are used as sorbents or catalysts they are
often in granular form. Such granules may be composed entirely of
the molecular sieve or be a composite of a binder or support and
the molecular sieve, with the latter distributed throughout the
entire volume of the granule. In any event, the granule usually
contains a non-molecular sieve pore structure which improves mass
transfer through the granule.
The support may be continuous, e.g., in the form of a plate, or
it may be discontinuous, e.g., in the form of granules. The molecular
sieve crystals may be of such a size that, although the pores of
the support are occupied by the crystals, the pores remain open.
Alternatively, the molecular sieve may occupy the pores to an extent
that the pores are effectively closed; in this case, when the support
is continuous a molecular sieve membrane may result.
Thus, depending on the arrangement chosen and the nature and size
of the material to be contacted by the molecular sieve, material
may pass through the bulk of the molecular sieve material entirely
through the pores of the molecular sieve material, or entirely through
interstices between individual particles of the molecular sieve
material, or partly through the pores and partly through the interstices.
Molecular sieve layers having the permeation path entirely through
the the molecular sieve crystals have been proposed for a variety
of size and shape selective separations. Membranes containing molecular
sieve crystals have also been proposed as catalysts having the advantage
that they may perform catalysis and separation simultaneously if
desired.
In EP-A-135069 there is disclosed a composite membrane comprising
a porous support, which may be a metal, e.g., sintered stainless
steel, an inorganic material, or a polymer, one surface of which
is combined with an ultra thin (less than 25 nm) film of a zeolite.
In the corresponding U.S. Pat. No. 4699892 it is specifically
stated that the zeolite is non-granular. In EP-A-180200 a composite
membrane is disclosed, employing a zeolite that has been subjected
to microfiltration to remove all particles of 7.5 nm and above.
The membrane is made by impregnation of a porous support by the
ultrafiltered zeolite solution, resulting in a distribution of the
zeolite crystals within the pore structure.
In EP-A-481660 which contains an extensive discussion of earlier
references to membranes, there is disclosed a zeolite membrane on
a porous support, in which the zeolite crystals are stated to form
an essentially continuous layer over and be directly bonded to the
support. The membrane is formed by immersing the support in a synthesis
gel, multiple immersions being employed to ensure that any pinholes
are occluded by the zeolite crystals being formed within the pores.
Zeolites with a small particle size and narrow size distribution
are disclosed for use in composite poly-dimethylsiloxane membranes
in J. Mem. Sci. 73 (1992) p 119 to 128 by Meng-Dong Jia et al;
however, the crystal size, though uniform, is within the range of
200 to 500 nm. Bein et al, in Zeolites, Facts, Figures, Future,
Elsevier, 1989 pp 887 to 896 disclose the manufacture of zeolite
Y crystals of a size of about 250 nm and embedding them in a glassy
silica matrix. Even smaller sizes such as 2 to 10 nm are envisaged
in WO 92/19574.
In Zeolites, 1992 Vol. 12 p 126 Tsikoyiannis and Haag describe
the formation of membranes from zeolite synthesis gels on both porous
and non-porous supports; when the support is non-porous, e.g., poly-tetrafluorethylene
or silver, the membrane is separable from the support. When the
support is porous, e.g., a Vycor (a trademark) porous glass disk,
the membrane is strongly bonded to the surface, zeolite crystallization
within the pores being prevented by presoaking the disk in water.
Numerous other techniques for forming membranes have been proposed.
In EP-A-397216 methods of making crack- and pinhole-free alumina
films of a thickness within the range of from 0.01 to 2 .mu.m on
a porous support layer are described, the methods including brush,
spray, dip, spin coating, electrophoretic and thermophoretic techniques.
The membranes may be pretreated.
Despite the proposals in these literature and patent references,
there still remains a need for a supported inorganic molecular sieve
layer having a controllable thickness that may, if desired, be of
a thickness of the order of only a few microns. There accordingly
also remains a need for a process of manufacturing such a layer
whereby the uniformity of the layer thickness may be controlled,
even when the layer is thin.
Such a layer and a process for its manufacture make possible the
production of a number of useful products, including membranes,
which because of their uniformity and thinness will have predictable
properties, and will permit a high flux.
It has now been found that such a supported layer is obtainable
using as starting material a crystalline molecular sieve of very
small particle size, preferably of a size that a true colloidal
dispersion of the particles may be obtained, and preferably also
of a narrow particle size distribution.
BRIEF DESCRIPTION OF THE FIGURE
FIG. 1 is a Scanning Electron Microscopy ("SEM") image
of the cross-section of a silica/zeolite layer manufactured on a
porous alpha-alumina support by spin-coating in conjunction with
the use of a temporary barrier layer.
FIG. 2 is a SEM image of the cross-section of a silica/zeolite
layer manufactured on a porous alpha-alumina support by spin-coating
in conjunction without the use of a temporary barrier layer.
FIG. 3 is a SEM image of the cross-section of a silica/zeolite
layer manufactured on a alpha-alumina support by spin-coating in
conjunction with the use of a permanent barrier layer.
FIG. 4 is a SEM image of the top-view of a silica/zeolite layer
manufactured on a alpha-alumina support by spin-coating in conjunction
with the use of a permanent barrier layer.
FIG. 5 is a data pilot illustrating the separation properties of
a silica/zeolite layer manufactured on a alpha-alumina support by
spin-coating in conjunction with the use of a permanent barrier
layer as shown in FIGS. 3 and 4. The data plot shows of a relative
molar concentrations of the permeate obtained from subjecting the
structure to an equimolar mixture of toluene, m-xylene, n-octane,
and i-octane.
FIG. 6 is an SEM image of the cross-section of a silica/zeolite
layer manufactured on a porous alpha-alumina support by spin-coating
in conjunction with the use of a temporary barrier layer and hydrothermal
crystallization techniques.
FIG. 7 is a SEM image of the top-view of a silica/zeolite layer
manufactured on alpha-alumina support by dipping the support into
a silica/zeolite mixture in conjunction with the use of an aging
solution and heart treatment.
FIG. 8 is a SEM image of the cross-section of a silica/zeolite
layer manufactured on alpha-alumina support by dipping the support
into a silica/zeolite mixture in conjunction with the use of an
aging solution and heat treatment.
FIG. 9 is an SEM image of an alpha-alumina support surface prior
to in-situ formation of zeolite crystals on the support.
FIG. 10 is an SEM image of an alpha-alumina support surface following
in-situ formation of zeolite crystals on the support at 150.degree.
C. followed by calcining.
FIG. 11 is an SEM image of an alpha-alumina support surface following
in-situ formation of zeolite crystals on the support at 98.degree.
C. followed by calcining.
FIG. 12 is an SEM image (at 156.times.magnification) of a alpha-alumina
support surface following in-situ formation of zeolite crystals
on the support at 120.degree. C. followed by calcining.
FIG. 13 is an SEM image (at 10000.times.magnification) of the
same alpha-alumina support surface as FIGS. 12 and 14.
FIG. 14 is an SEM image (at 80000.times.magnification) of the
same alpha-alumina support surface as FIGS. 12 and 13.
FIG. 15 is an SEM image of a cross-section of a alpha-alumina support
surface following in-situ formation of zeolite crystals on the support
at 120.degree. C. followed by calcining as shown in FIGS. 12 14.
In a first aspect of the invention, there is provided a layer comprising
a supported inorganic layer comprising contiguous particles of a
crystalline molecular sieve, the particles having a mean particle
size within the range of from 20 nm to 1 .mu.m.
Advantageously, in the first aspect of the invention, the mean
particle size is within the range of from 20 to 500 nm, preferably
it is within the range of from 20 to 300 nm and most preferably
within the range of from 20 to 200 nm. Alternatively, the mean particle
size is advantageously such that at least 5% of the unit cells of
the crystal are at the crystal surface.
In a second aspect of the invention, there is provided a supported
inorganic layer comprising particles of a crystalline molecular
sieve, the particles having a mean particle size within the range
of from 20 to 200 nm.
In both the first and second aspects of the invention, the layer
comprises molecular sieve particles optionally coated with skin
of a different material; these are identifiable as individual particles
(although they may be intergrown as indicated below) by electron
microscopy. The layer, at least after activation, is mechanically
cohesive and rigid. Within the interstices between the particles
in this rigid layer, there may exist a plethora of non-molecular
sieve pores, which may be open, or partially open, to permit passage
of material through or within the layer, or may be completely sealed,
permitting passage through the layer only through the pores in the
particles.
Advantageously, the particle size distribution is such that 95%
of the particles have a size within .+-.33% of the mean, preferably
95% are within .+-.15% of the mean, preferably +10% of the mean
and most preferably 95% are within .+-.7.5% of the mean.
It will be understood that the particle size of the molecular sieve
material forming the layer may vary continuously or stepwise with
distance from the support. In such a case, the requirement for uniformity
is met if the particle size distribution is within the defined limit
at one given distance from the support, although advantageously
the particle size distribution will be within the defined limit
at each given distance from the support.
The use of molecular sieve crystals of small particle size and
preferably of homogeneous size distribution facilitates the manufacture
of a three-dimensional structure which may if desired be thin but
which is still of controlled thickness.
In the first aspect of the invention, the particles are contiguous,
i.e., substantially every particle is in contact with one or more
of its neighbours as evidenced by electron microscopy preferably
high resolution microscopy, although not necessarily in contact
with all its closest neighbours. Such contact may be such in some
embodiments that neighbouring crystal particles are intergrown,
provided they retain their identity as individual crystalline particles.
Advantageously, the resulting three dimensional structure is grain-supported,
rather than matrix-supported, in the embodiments where the layer
does not consist essentially of the crystalline molecular sieve
particles. In a preferred embodiment, the particles in the layer
are closely packed.
In the second aspect of the invention, the particles may be contiguous,
but need not be.
A layer in accordance with either the first or the second aspect
of the invention may be constructed to contain passageways between
the particles that provide a non-molecular sieve pore structure
through or into the layer. Such a layer may consist essentially
of the particles or may contain another component, which may be
loosely termed a matrix which, while surrounding the particles,
does not so completely or closely do so that all pathways round
the particles are closed. Alternatively, the layer may be constructed
so that a matrix present completely closes such pathways, with the
result that the only path through or into the layer is through the
particles themselves.
It will be understood that references herein to the support of
a layer include both continuous and discontinuous supports.
References to particle size are throughout this specification to
the longest dimension of the particle and particle sizes are as
measured by direct imaging with electron microscopy. Particle size
distribution may be determined by inspection of scanning or transmission
electron micrograph images preferably on lattice images, and analysing
an appropriately sized population of particles for particle size.
As molecular sieve, there may be mentioned a silicate, metallosilicates
an aluminosilicate, an aluminophosphate, a silicoaluminophosphate,
a metalloaluminophosphate, or a metalloaluminophosphosilicate or
a gallosilicate.
The preferred molecular sieve will depend on the chosen application,
for example, separation, catalytic applications, and combined reaction
separation. There are many known ways to tailor the properties of
the molecular sieves, for example, structure type, chemical composition,
ion-exchange, and activation procedures.
Representative examples are molecular sieves/zeolites of the structure
types AFI, AEL, BEA, CHA, EUO, FAU, FER, KFI, LTA, LTL, MAZ, MOR,
MFI, MEL, MTW, OFF and TON.
Some of the above materials while not being true zeolites are frequently
referred to in the literature as such, and this term will be used
broadly in the specification below.
A supported layer according to the invention may be manufactured
in a number of different ways. In one embodiment the invention provides
a process of making a layer by deposition on a support from a colloidal
zeolite suspension obtainable by preparing an aqueous synthesis
mixture comprising a source of silica and an organic structure directing
agent in a proportion sufficient to effect substantially complete
dissolution of the silica source in the mixture at the boiling temperature
of the mixture, and crystallization from the synthesis mixture.
The synthesis mixture will contain, in addition, a source of the
other component or components, if any, in the zeolite.
The particle size of the crystals formed may be controlled by the
crystallization temperature, or any other process capable of giving
crystals of highly uniform particle size, in a size such that a
stable colloidal suspension may be obtained. A stable colloidal
suspension is one in which no visible separation occurs on standing
for a prolonged period, e.g., one month. Details of the procedure
for preparing the colloidal suspension mentioned above are given
in our co-pending Application No. PCT/EP92/02386 the entire disclosure
of which is incorporated by reference herein.
The invention also provides a supported layer made by the above
process.
In accordance with preferred processes according to the invention,
the silica is advantageously introduced into the synthesis mixture
as silicic acid powder.
The organic structure directing agent is advantageously introduced
into the synthesis mixture in the form of a base, specifically in
the form of a hydroxide, but a salt, e.g, a halide, especially a
bromide, may be employed.
The structure directing agent may be, for example, the hydroxide
or salt of tetramethylammonium (TMA), tetraethylammonium (TEA),
triethylmethylammonium (TEMA), tetrapropylammonium (TPA), tetrabutylammonium
(TBA), tetrabutylphosphonium (TBP), trimethylbenzylammonium (TMBA),
trimethylcetylammonium (TMCA), trimethylneo-pentylammonium (TMNA),
triphenylbenzylphosphonium (TPBP), bispyrrolidinium (BP), ethylpyridinium
(EP), diethylpiperidinium (DEPP) or a substituted azoniabicyclooctane,
e.g. methyl or ethyl substituted quinuclidine or 14-diazoniabicyclo-(222)octane.
Preferred structure directing agents are the hydroxides of TMA,
TEA, TPA and TBA.
Further processes for the manufacture of layers according to the
invention, including specific methods of depositing the molecular
sieve on the support and post-treatment of the resulting layer,
will be given below.
The thickness of the molecular sieve layer is advantageously within
the range of 0.1 to 20 .mu.m preferably 0.1 to 15 .mu.m, more preferably
from 0.1 to 2 .mu.m. Advantageously, the thickness of the layer
and the particle size of the molecular sieve are such that the layer
thickness is at least twice the particle size, resulting in a layer
several particles thick rather than a monolayer of particles.
Advantageously, the layer is substantially free of pinholes, i.e.,
substantially free from apertures of greatest dimension greater
than 0.1 .mu.m. Advantageously, at most 0.1% and preferably at most
0.0001% of the surface area is occupied by such apertures.
Depending on the intended end use of the layer, a greater or smaller
proportion of the area of the layer may be occupied by macropores,
apertures having a greatest dimension less than 0.1 .mu.m but greater
than 1 nm. These macropores may be formed by the interstices between
the crystals of the molecular sieve, if the layer consists essentially
of the molecular sieve, and elsewhere, if the layer comprises the
molecular sieve and other components. Such layers may be used, inter
alia, for ultrafiltration, catalytic conversion, and separations
based on differences in molecular mass (Knudsen diffusion), and
indeed for any processes in which a high surface area is important.
The layer advantageously has a large proportion of its area occupied
by crystalline-bounded micropores, i.e., pores of a size between
0.2 and 1 nm, depending on the particular molecular sieve being
employed. Pores of size within the micropore range result, for example,
when the layer contains a component in addition to one derived from
colloidal molecular sieve particles. In another embodiment especially
suitable for ultrafiltration, the layer contains nanopores, i.e.,
pores of a size between 1 and 10 nm.
The layer support may be either non-porous or, preferably, porous,
and may be continuous or particulate. As examples of non-porous
supports there may be mentioned glass, fused quartz, and silica,
silicon, dense ceramic, for example, clay, and metals. As examples
of porous supports, there may be mentioned porous glass, sintered
porous metals, e.g., steel or nickel (which have pore sizes typically
within the range of 0.2 to 15 .mu.m), and, especially, an inorganic
oxide, e.g., alpha-alumina, titania, an alumina/zirconia mixture,
or Cordierite.
At the surface in contact with the layer, the support may have
pores of dimensions up to 50 times the layer thickness, but preferably
the pore dimensions are comparable to the layer thickness.
Advantageously, the support is porous alpha-alumina with a surface
pore size within the range of from 0.08 to 10 .mu.m, preferably
from 0.08 to 1 .mu.m, most preferably from 0.08 to 0.16 .mu.m, and
advantageously with a narrow pore size distribution. The support
may be multilayered; for example, to improve the mass transfer characteristics
of the layer, only the surface region of the support in contact
with the layer may have small diameter pores, while the bulk of
the support, toward the surface remote from the layer, may have
large diameter pores. An example of such a multilayer support is
an alpha-alumina disk having pores of about 1 .mu.m diameter coated
with a layer of alpha-alumina with pore size about 0.08 .mu.m.
The invention also provides a structure in which the support, especially
a continuous porous support, has a molecular sieve layer on each
side of the support, the layers on the two sides being the same
or different.
The layer may, and for many uses advantageously does, consist essentially
of the molecular sieve material, or it may be a composite of the
molecular sieve material and intercalating material which is also
inorganic. The intercalating material may be the material of the
support. If the layer is a composite it may, as indicated above,
contain macropores and/or micropores, bounded by molecular sieve
portions, by portions of intercalating material, or by both molecular
sieve and intercalating material. The material may be applied to
the support simultaneously with or after deposition of the molecular
sieve, and may be applied, for example, by a sol-gel process followed
by thermal curing. Suitable materials include, for example, inorganic
oxides, e.g., silica, alumina, and titania.
The intercalating material is advantageously present in sufficiently
low a proportion of the total material of the layer that the molecular
sieve crystals remain contiguous.
The invention further provides additional preferred processes for
manufacturing a layer.
The present invention accordingly also provides a process for the
manufacture of a layer comprising a crystalline molecular sieve
on a porous support, which comprises pre-treating the porous support
to form at a surface thereof a barrier layer, and applying to the
support a reaction mixture comprising a colloidal suspension of
molecular sieve crystals, having a mean particle size of at most
100 nm and advantageously a particle size distribution such that
at least 95% of the particles have a size within .+-.15%, preferably
.+-.10%, more preferably within .+-.7.5%, of the mean, colloidal
silica and optionally an organic structure directing agent, to form
a supported molecular sieve layer, and if desired or required activating
the resulting layer.
Activation removes the template and can be achieved by calcination,
ozone treatment, plasma treatment or chemical extraction such as
acid extraction.
The invention also provides a supported layer formed by the process.
The barrier layer functions to prevent the water in the aqueous
reaction mixture from preferentially entering the pores of the support
to an extent such that the silica and zeolite particles form a thick
gel layer on the support.
The barrier layer may be temporary or permanent. As a temporary
layer, there may be mentioned an impregnating fluid that is capable
of being retained in the pores during application of the reaction
mixture, and readily removed after such application and any subsequent
treatment.
As indicated below, spin coating is an advantageous technique for
applying the reaction mixture to the support according to this and
other aspects of the invention. The impregnating fluid should accordingly
be one that will be retained in the pores during spinning if that
technique is used; accordingly the rate of rotation, pore size,
and physical properties of the fluid need to be taken into account
in choosing the fluid.
The fluid should also be compatible with the reaction mixture,
for example if the reaction mixture is polar, the barrier fluid
should also be polar. As the reaction mixture is advantageously
an aqueous reaction mixture, water is advantageously used as the
barrier layer.
To improve penetration, the fluid barrier may be applied at reduced
pressure or elevated temperature. If spin-coating is used, the support
treated with the barrier fluid is advantageously spun for a time
and at a rate that will remove excess surface fluid, but not remove
fluid from the pores. Premature evaporation of fluid from the outermost
pores during treatment may be prevented by providing an atmosphere
saturated with the liquid vapour.
As a temporary barrier layer suitable, for example, for an alpha-alumina
support there may be especially mentioned water or glycol. As a
permanent barrier suitable for an alpha-alumina support there may
be mentioned titania, gamma-alumina or an alpha-alumina coating
of smaller pore size.
The colloidal suspension of molecular sieve crystals is advantageously
prepared by the process indicated above, i.e., that described in
PCT Application EP/92/02386. The colloidal silica may be prepared
by methods known in the art; see for example Brinker and Scherer,
Sol-Gel Science, Academic Press, 1990. A preferred method is by
the acid hydrolysis of tetraethyl orthosilicate. The organic structure
directing agent, if used, is advantageously one of those mentioned
above.
As indicated above, the reaction mixture is advantageously applied
to the support by spin-coating, the viscosity of the mixture and
the spin rate controlling coating thickness. The mixture is advantageously
first contacted with the stationary support, then after a short
contact time the support is spun at the desired rate. After spinning,
the silica is advantageously aged by retaining the supported layer
in a high humidity environment, and subsequently dried, advantageously
first at room temperature and then in an oven.
In a further embodiment of the invention, there is provided a process
for the manufacture of a layer comprising a crystalline molecular
sieve on a porous support which comprises applying to the support
by dip-coating a colloidal suspension of molecular sieve crystals,
having a mean particle size of at most 100 nm and advantageously
a particle size distribution such that at least 95% of the particles
have a size within .+-.15%, preferably .+-.10%, more preferably
.+-.7.5%, of the mean, drying the resulting gel on the support and
if desired or required activating the resulting layer.
The invention also provides a layer made by the process.
In this embodiment of the invention, the pH of the suspension is
an important factor. For example, at a pH above 12 colloidal silicalite
crystals tend to dissolve in the medium. Adhesion of the layer to
the support improves as pH is reduced, with acceptable adhesion
being obtained between pH 7 and 11 good adhesion between pH 4.0
and 7 and very good adhesion below pH 4.0 although agglomeration
of particles may occur at too low a pH.
Adhesion of the layer to its support may be enhanced by the inclusion
in the suspension of an organic binder or surfactant, the presence
of an appropriate proportion of which may also reduce the incidence
of cracks in the final layer. Among binders there may be mentioned
polyvinyl alcohol (PVA), advantageously with a molecular weight
of from 1000 to 100000 preferably from 2000 to 10000 and most
preferably in the region of 3000 and hydroxyalkyl cellulose, especially
hydroxypropyl cellulose (HPC), advantageously with a molecular weight
of from 50000 to 150000 and preferably in the region of 100000.
An appropriate proportion of crystals in the suspension may readily
be determined by routine experiment; if the proportion is too low
a continuous layer will not be reliably formed while if it is too
high the layer will tend to contain cracks after activation. For
silicalite, advantageous lower and upper limits are 0.5% (preferably
0.75%) and 1.5% respectively.
The time spent by the support immersed in the suspension also affects
the thickness of the layer and its quality. Advantageously the dip-time
is at most 15 seconds with a solution containing 1.1% by weight
silicalite crystals; an immersion of from 1 to 10 seconds gives
a crack-free layer of thickness 0.7 to 3 .mu.m.
In our co-pending Application No. PCT/EP92/02330 the entire disclosure
of which is incorporated by reference herein, there is disclosed
the formation of an aqueous synthesis mixture comprising a source
of particulate silica in which the particles advantageously have
a mean diameter of at most 1 .mu.m, seeds of an MFI zeolite having
a mean particle size of at most 100 nm in the form of a colloidal
suspension, an organic structure directing agent, and a source of
fluorine or of an alkali metal, the synthesis mixture having an
alkalinity, expressed as a molar ratio of OH.sup.-:SiO.sub.2 of
at most 0.1. Crystallization of this synthesis mixture produces
very uniform, small, zeolite crystals. The proportion of seed, based
on the weight of the mixture, is given as from 0.05 to 1700 wppm.
The synthesis mixture will additionally contain a source of any
other zeolite component.
In a further embodiment of the present invention, a seeding technique
may be used. In this embodiment, the invention provides a process
for the manufacture of a layer comprising a crystalline molecular
sieve on a porous support, which comprises applying to or forming
on the support a layer comprising amorphous silica containing seeds
of a zeolite having a mean particle size of at most 100 nm, and
advantageously having a particle size distribution such that at
least 95% of the particle have a size within .+-.15%, preferably
.+-.10%, more preferably within .+-.7.5%, of the mean, subjecting
the layer to hydrothermal crystallization, and if desired or required
activating the crystallized layer.
Again, other components useful in forming the zeolite layer may
be present. Such components may include, for example, an organic
structure directing agent, which may be in salt form.
The invention also provides a supported layer made by the process.
The layer is advantageously applied to or formed on the support
by dipcoating or spincoating, advantageously substantially as described
above.
If dipcoating is used, the support is advantageously dipped into
a solution containing the amorphous silica in colloidal form, advantageously
with a particle size at most 0.1 .mu.m; the solution may if desired
contain other components useful in forming the final zeolite layer.
If spincoating is used, the silica may be of larger particle size
but is advantageously colloidal.
The layer thickness at this stage, after dipcoating or spincoating,
is advantageously within the range of from 0.1 to 20 .mu.m.
Hydrothermal crystallization to form the zeolite layer is advantageously
carried out by immersing the layer in a solution described below,
and heating for a time and at the temperature necessary to effect
crystallization.
The solution advantageously contains either all the components
necessary to form a zeolite or only those components necessary but
which are not already present in the layer on the support. In the
latter case, crystals do not form in the solution, which remains
clear and may be re-used.
After crystallization, the supported layer may be washed, dried,
and calcined in the normal way.
By this embodiment of the invention, a dense, homogeneous, and
crack-free supported layer may be obtained. A 1 .mu.m thick zeolite
layer may readily be obtained, with a grain size of 100 to 300 nm.
In a further embodiment of the invention, molecular sieve crystals
are synthesized in situ on the support. According to this embodiment,
the invention provides a process for the manufacture of a layer
comprising a crystalline molecular sieve on a porous support, which
comprises preparing a synthesis mixture comprising a source of silica
and an organic structure directing agent preferably in the form
of a hydroxide in a proportion sufficient to effect substantially
complete dissolution of the silica source in the mixture at the
boiling temperature of the mixture, immersing the support in the
synthesis mixture, crystallizing zeolite from the synthesis mixture
onto the support, and if desired or required activating the crystallized
layer.
The invention also provides a supported layer made by the process.
The synthesis mixture will also contain a source of other components,
if any, in the zeolite.
Advantageously, to obtain colloidal material, crystallization is
effected at a temperature less than 120.degree. C. As indicated
in PCT/EP92/02386 the lower the crystallization temperature the
smaller the resulting particle size of the crystals. For zeolites
made in the presence of an alumina source, the particle size may
also be varied by varying the alumina content. The effect of varying
the alumina content is, however, not the same for all zeolites;
for example, for zeolite beta, the particle size varies inversely
with alumina content while for an MFI-structured zeolite the relationship
is direct.
The substrate used in accordance with this aspect of the invention
may be any one of those described above in connexion with other
processes; an alpha-alumina support is advantageously used; the
pore size may vary with the intended use of the layer; a pore size
within the range 100 nm to 1.5 .mu.m may conveniently be used. Care
should be taken to avoid undue weakening of the support by for example,
controlling prolonged exposure to high temperature and alkalinity.
Although the various processes of the invention described above
yield a supported layer of good quality, the resulting layer may
still contain apertures of greater size than desired for the intended
use of the product. For example, apertures greater than those through
the molecular sieve itself are undesirable if the supported layer
is to be used for certain types of separation process since they
result in a flux greater than desired and impaired separation. If
this is the case, the supported layer may be subjected to a reparation
procedure. In this procedure, the supported layer may be subjected
to one of the various reparation techniques known to those skilled
in the art.
It is therefore in accordance with the invention to manufacture
a supported layer by first carrying out one of the layer-forming
processes according to the invention and described above and following
it by reparation of the layer by a method known per se.
Preferably, however, the reparation is carried out by again subjecting
the supported layer to a manufacturing process of the invention.
The invention accordingly also provides a process for the manufacture
of a supported layer in which one of the layer-forming processes
above is carried out two or more times, or in which one of the processes
above carried out one or more times is followed by another of the
processes above, carried out one or more times, or in which one
of the processes above is carried out two or more times with another
or others of the processes above, carried out one or more times,
intervening. The invention also provides a supported layer, especially
a membrane, made by such a process.
The layers according to the invention and produced in accordance
with the processes of the invention may be treated in manners known
per se to adjust their properties, e.g., by steaming or ion exchange
to introduce different cations or anions, by chemical modification,
e.g., deposition of organic compounds on the crystals or into the
pores of the molecular sieve, or by introduction of a metal.
The layers may be used in the form of a membrane, used herein to
describe a barrier having separation properties, for separation
of fluid (gaseous, liquid, or mixed) mixtures, for example, separation
of a feed for a reaction from a feedstock mixture, or in catalytic
applications, which may if desired combine catalysed conversion
of a reactant or reactants and separation of reaction products.
Separations which may be carried out using a membrane comprising
a layer in accordance with the invention include, for example, separation
of normal alkanes from co-boiling hydrocarbons, for example normal
alkanes from isoalkanes such as C.sub.4 to C.sub.6 mixtures and
n-C.sub.10 to C.sub.16 alkanes from kerosene; separation of aromatic
compounds from one another, especially separation of C.sub.8 aromatic
isomers from each other, more especially para-xylene from a mixture
of xylenes and, optionally, ethylbenzene, and separation of aromatics
of different carbon numbers, for example, mixtures of benzene, toluene,
and mixed C.sub.8 aromatics; separation of aromatic compounds from
aliphatic compounds, especially aromatic molecules with from 6 to
8 carbon atoms from C.sub.5 to C.sub.10 (naphtha range) aliphatics;
separation of olefinic compounds from saturated compounds, especially
light alkenes from alkane/alkene mixtures, more especially ethene
from ethane and propene from propane; removing hydrogen from hydrogen-containing
streams, especially from light refinery and petrochemical gas streams,
more especially from C.sub.2 and lighter components; and alcohols
from aqueous streams.
Separation of heteroatomic compounds from hydrocarbons such as
alcohols and sulphur containing materials such as H.sub.2S and mercaptans.
The supported 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.degree.
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 layer according to the invention in the form
of a membrane under conditions such that at least one component
of the mixture has a different steady state permeability through
the layer from that of another component and recovering a component
or mixture of components from the other face of the layer.
Some specific reaction systems where these membranes would be advantageous
for selective separation either in the reactor or on reactor effluent
include: selective removal of a Para-Xylene rich mixture from the
reactor, reactor product, reactor feed or other locations in a Xylenes
isomerization process; selective separation of aromatics fractions
or specific aromatics molecule rich streams from catalytic reforming
or other aromatics generation processes such as light alkane and
alkene dehydrocyclization processes (e.g. C.sub.3 C.sub.7 paraffins
to aromatics from processes such as Cyclar), methanol to gasoline
and catalytic cracking processes; selective separation of benzene
rich fractions from refinery and chemical plant streams and processes;
selective separation of olefins or specific olefin fractions from
refinery and chemicals processing units including catalytic and
thermal cracking, olefins isomerization processes, methanol to olefins
processes, naphtha to olefins conversion processes, alkane dehydrogenation
processes such as propane dehydrogenation to propylene; selective
removal of hydrogen from refinery and chemicals streams and processes
such as catlytic reforming, alkane dehydrogenation, catalytic cracking,
thermal cracking, light alkane/alkene dehydrocyclization, ethylbenzene
dehydrogenation, paraffin dehydrogenation; selective separation
of molecular isomers in processes such as butane isomerization,
butylene isomerization, paraffin isomerization, olefin isomerization;
selective separation of alcohols from aqueous streams and/or other
hydrocarbons; selective separation of products of bimolecular reactions
where equilibrium limits conversion to the desired products, e.g.
MTBE production from methanol and isobutylene, ethylbenzene from
ethylene and benzene, and cumene from propylene and benzene; selective
removal of 26 dimethyl naphthalene from mixtures of alkane substituted
naphthalenes during alkylation and/or isomerization.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting a feedstock with a layer according
to the invention which is in active catalytic form under catalytic
conversion conditions and recovering a composition comprising at
least one conversion product.
The invention further provides a process for catalysing a chemical
reaction which comprises contacting a feedstock with one face of
a layer according to the invention, that is in the form of a membrane
and in active catalytic form, under catalytic conversion conditions,
and 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. |