Molecular sieve abstract
Molecular sieve layers on a support are deposited from a synthesis
solution while increasing the solution temperature.
Molecular sieve claims
I claim:
1. A process for the manufacture of a layer comprising a crystalline
molecular sieve on a support comprising:
a) preparing a synthesis mixture comprising a source of silica
and an organic structure-directing agent;
b) immersing said support in said synthesis mixture;
c) heating said synthesis mixture containing said immersed support
to an initial temperature of at least 70.degree. C. and maintaining
said synthesis mixture at said initial temperature for a period
of crystallization time sufficient to deposit zeolite crystals onto
said support; and
d) further heating said synthesis mixture containing said immersed
support to a second temperature higher than said initial temperature
for a period of crystallization time sufficient to deposit additional
zeolite crystals onto said support and to form said layer.
2. The process of claim 1 wherein said crystals in said layer have
a particle size in the range of from 20 to 500 nm.
3. The process of claim 1 wherein said crystals in said layer have
a particle size of in the range of from 20 to 300 nm.
4. The process of claim 1 wherein said layer contains nanopores
having a dimension of 1 to 10 nm.
5. The process of claim 1 wherein said layer contains micropores
having a dimension of 0.2 to 1 nm.
6. The process of claim 1 wherein said synthesis mixture is maintained
at said initial temperature until deposition of zeolite crystals
at said temperature has ceased.
7. The process of claim 1 which is a stepwise process and wherein
said synthesis mixture is maintained at said second temperature
for a period of crystallization time sufficient to deposit additional
zeolite crystals onto said support.
8. The process of claim 7 wherein said synthesis mixture containing
said immersed support is further stepwise heated to at least one
additional temperature higher than said second temperature and wherein
said synthesis mixture is maintained at said at least one additional
temperature for a period of crystallization time sufficient to deposit
additional zeolite crystals onto said support.
9. The process of claim 8 wherein the maximum additional heating
temperature is up to 170.degree. C.
10. The process of claim 8 wherein the maximum additional heating
temperature ranges from 120.degree. to 170.degree. C.
11. The process of claim 1 wherein said synthesis mixture is heated
in step (d) to a maximum temperature of up to 170.degree. C.
12. The process of claim 1 wherein said synthesis mixture is heated
in step (d) to a minimum temperature of at least 120.degree. C.
13. The process of claim 1 wherein said support is porous.
14. The process of claim 13 wherein said layer has a thickness
in the range of 0.1 to 15 .mu.m.
15. A process as claimed in claim 1 wherein the crystallization
period is from 24 hours to 7 days.
16. A process as claimed in claim 15 wherein the period is from
3 to 6 days.
17. A process as claimed in claim 1 wherein after its deposition
on the support the supported zeolite layer is calcined.
18. A process for the manufacture of a layer comprising a crystalline
molecular sieve on a porous support comprising:
a) preparing a synthesis mixture comprising an aqueous solution
of a source of silica and an organic structure-directing agent;
b) immersing said support in said synthesis mixture;
c) heating said synthesis mixture containing said immersed support
in a first step to a first temperature of at least 70.degree. C.
and maintaining said synthesis mixture at said first temperature
for a period of crystallization time sufficient to deposit zeolite
crystals onto said support;
d) further heating said synthesis mixture containing said immersed
support from step (c) in a second step to a second temperature higher
than said first temperature and maintaining said synthesis mixture
at said second temperature for a period of crystallization time
sufficient to deposit additional zeolite crystals onto said support;
and
e) further heating said synthesis mixture containing said immersed
support from step (d) in at least one additional step to at least
one additional temperature higher than said second temperature and
maintaining said synthesis mixture at each of said additional temperatures
for a period of crystallization time sufficient to deposit additional
zeolite crystals onto said support and to form said layer, the maximum
temperature to which said synthesis mixture is heated ranging from
120.degree. to 170.degree. C.
19. The process of claim 18 wherein the temperature increase in
each heating step is at least 20.degree. C.
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, and
a process for its manufacture.
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 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 our earlier European Patent Application No. 93.303 187.4 and
a corresponding PCT Application No. PCT/E94/01301 filed simultaneously
with this application, the disclosures of which are incorporated
herein by reference, there are described 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, and processes
for its manufacture.
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.
One process according to our earlier application comprises preparing
a synthesis mixture comprising a source of silica and an organic
structure directing agent 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
calcining the crystallized layer.
This process gives a uniform layer which is of sufficient thickness
for many purposes. For other purposes, however, a layer of greater
thickness is desirable.
The present 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, immersing the
support in the synthesis mixture and crystallizing zeolite from
the synthesis mixture onto the support, the temperature of the synthesis
mixture being increased during crystallization.
The synthesis mixture will also contain a source of other components,
if any, in the zeolite.
Subsequently to crystallization, if desired or required the supported
layer may be calcined.
Advantageously, the source of silica is in solution in the synthesis
mixture, and dissolution may be facilitated by employing the structure
directing agent in a proportion sufficient to effect substantially
complete dissolution of the source at the boiling temperature of
the mixture.
The synthesis mixture is advantageously prepared as described in
International Application WO93/08125 the disclosure of which is
incorporated herein by reference.
The increase in temperature may be continuous but is advantageously
stepwise. Advantageously, the temperature is increased within the
range of from 70.degree. C. to 170.degree. C., and preferably from
90.degree. C. to 150.degree. C., over the crystallization period.
Advantageously, the temperature at the end of the crystallization
period is at least 120.degree. C. That period may, for example,
range from 24 hours to 7 days, and is preferably between 3 and 6
days. Advantageously, the temperature is increased at least once
after deposition of crystals on the support has commenced. Preferably
the temperature is increased at least twice after deposition has
commenced.
Conveniently, the temperature is increased in steps after a time
long enough to bring about deposition, and advantageously substantial
deposition, at the lower temperature.
Preferably, the synthesis mixture is maintained at each temperature
stage until deposition, as measured by weight increase, has ceased
at that temperature. This time tends to decrease with increase in
temperature but may be, for example, from 4 to 48 hours, advantageously
for from 8 to 24 hours. The increase in temperature at each step
may conveniently be at least 20.degree. C.
The process has advantages over other methods of obtaining thicker
layers which require washing of the support after initial or any
previous deposition and replacement of the synthesis mixture.
Advantageously, the mean particle size of the crystalline molecular
sieve in the layer 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.
Advantageously, the particle size distribution is such that 95%
of the particles have a size within .+-.33% of the mean, preferably
95% are within .+-.10% of the mean and most preferably 95% are within
.+-.7.5% of the mean.
The invention also provides a supported layer made by the process
of the invention. The layer comprises molecular sieve particles;
these are identifiable as individual particles (although they may
be intergrown as indicated below) by electron microscopy. The layer,
at least after calcining, is mechanically cohesive and rigid. Within
the interstices between the particles in this rigid layer, there
may exist 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.
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 layer is considered
to be uniform 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.
Advantageously, the particles are contiguous, i.e., substantially
every particle is in contact with one or more of its neighbours,
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,
if the layer does not consist essentially of the crystalline molecular
sieve particles. In a preferred embodiment, the particles in the
layer are closely packed.
It will be understood that references herein to the support of
a layer include both continuous and discontinuous supports.
References to particle size are 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 electron micrograph images, and analysing
an appropriately sized population of particles for particle size.
As molecular sieve, there may be mentioned a silicate, an aluminosilicate,
an aluminophosphate, a silicoaluminophosphate, a metalloaluminophosphate,
or a metalloaluminophosphosilicate.
The preferred molecular sieve will depend on the chosen application,
for example, separation, catalytic applications, and combined reaction
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, TON and faujasite. The process of the invention
is especially suited to the manufacture of supported layers of MFI,
MEL, and faujasite structures.
Some of the above materials while not being true zeolites are frequently
referred to in the literature as such, and this term is used broadly
in this specification.
Advantageously, the silica is advantageously introduced into the
synthesis mixture in particulate form e.g., 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.
The thickness of the molecular sieve layer is advantageously within
the range of 0.1 to 15 .mu.m, 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
advantageously 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 layer 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 into the pores of the molecular sieve, or by introduction
of a metal.
The layer 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, especially 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.
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.
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.
|