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
1. A process for the separation of a fluid mixture which comprises
contacting the mixture with one face of 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, and wherein the layer primarily contains nanopores
having a size of between 1 and 10 nm, under such conditions 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.
2. The process of claim 1 wherein the separation is of a feed
for a reaction from a feedstock.
3. The process of claim 1 wherein paraxylene is separated from
a mixture of xylenes.
4. The process of claim 1 wherein aromatics are separated from
a hydrocarbon-containing stream from an aromatics generation process.
5. The process of claim 1 wherein the aromatics are separated
from a hydrocarbon-containing stream from a catalytic reforming
process.
6. The process of claim 1 wherein benzene is separated from a
hydrocarbon-containing stream.
7. The process of claim 1 wherein olefins are separated from a
hydrocarbon-containing stream.
8. The process of claim 1 wherein hydrogen is separated from a
hydrocarbon-containing stream.
9. The process of claim 8 wherein the hydrocarbon-containing stream
is produced by a process selected from catalytic reforming, alkane
dehydrogenation, catalytic cracking and thermal cracking.
10. The process of claim 1 wherein the supported inorganic layer
primarily contains micropores having a size of between 0.2 and 1
nm.
11. The process of claim 10 wherein paraxylene is separated from
a mixture of xylenes.
12. A process for the separation of a fluid mixture which comprises
contacting the mixture with one face of 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 a size of
between 1 and 10 nm, under such conditions 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.
13. The process of claim 12 wherein paraxylene is separated from
a mixture of xylenes.
14. The process of claim 12 wherein aromatics are separated from
a hydrocarbon-containing stream from an aromatics generation process.
15. The process of claim 12 wherein olefins are separated from
a hydrocarbon-containing stream.
16. The process of claim 12 wherein hydrogen is separated from
a hydrocarbon-containing stream.
17. The process of claim 12 wherein the supported inorganic layer
primarily contains micropores having a size of between 0.2 and 1
nm.
18. The process of claim 17 wherein paraxylene is separated from
a mixture of xylenes.
19. The process of claim 12 wherein the particle size distribution
of the supported inorganic layer is such that at least 95% of the
particles have a size within .+-.33% of the mean.
20. The process of claim 19 wherein paraxylene is separated from
a mixture of xylenes.
Molecular sieve description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 08/545707 which is the National Stage of International
Application Number PCT/EP94/01301 filed Apr. 25 1994 which claims
the benefit of European Application Number 93303187.4 filed Apr.
23 1993.
BACKGROUND OF THE INVENTION
[0002] 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.
[0003] Molecular sieves find many uses in physical, physicochemical,
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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] Zeolites with a small particle size and narrow size distribution
are disclosed for use in composite polydimethylsiloxane 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
SUMMARY OF THE INVENTION
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
BRIEF DESCRIPTION OF THE FIGURES
[0024] 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.
[0025] FIG. 2 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 without the use of a temporary barrier layer.
[0026] FIG. 3 is an SEM image of the cross-section of a silica/zeolite
layer manufactured on an alpha-alumina support by spin-coating in
conjunction with the use of a permanent barrier layer.
[0027] FIG. 4 is an SEM image of the top-view of a silica/zeolite
layer manufactured on an alpha-alumina support by spin-coating in
conjunction with the use of a permanent barrier layer.
[0028] FIG. 5 is a data plot 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.
[0029] 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.
[0030] FIG. 7 is an 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 heat treatment.
[0031] FIG. 8 is an 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.
[0032] FIG. 9 is an SEM image of an alpha-alumina support surface
prior to in-situ formation of zeolite crystals on the support.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] FIG. 13 is an SEM image (at 10000.times. magnification)
of the same alpha-alumina support surface as FIGS. 12 and 14.
[0037] FIG. 14 is an SEM image (at 80000.times. magnification)
of the same alpha-alumina support surface as FIGS. 12 and 13.
[0038] 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.
DETAILED DESCRIPTION OF THE INVENTION
[0039] 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.
[0040] In the second aspect of the invention, the particles may
be contiguous, but need not be.
[0041] 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.
[0042] It will be understood that references herein to the support
of a layer include both continuous and discontinuous supports.
[0043] 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.
[0044] As molecular sieve, there may be mentioned a silicate, metallosilicate,
an aluminosilicate, an aluminophosphate, a silicoaluminophosphate,
a metalloaluminophosphate, or a metalloaluminophosphosilicate, or
a gallosilicate.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] The invention also provides a supported layer made by the
above process.
[0051] In accordance with preferred processes according to the
invention, the silica is advantageously introduced into the synthesis
mixture as silicic acid powder.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] The invention further provides additional preferred processes
for manufacturing a layer.
[0066] 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.
[0067] Activation removes the template and can be achieved by calcination,
ozone treatment, plasma treatment or chemical extraction such as
acid extraction.
[0068] The invention also provides a supported layer formed by
the process.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] 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 calcining the resulting layer.
[0078] 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.
[0079] 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.
[0080] 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 calcination.
For silicalite, advantageous lower and upper limits are 0.5% (preferably
0.75%) and 1.5% respectively.
[0081] 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.
[0082] 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.
[0083] 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
calcining the crystallized layer.
[0084] 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.
[0085] 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.
[0086] 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.
[0087] The layer thickness at this stage, after dipcoating or spincoating,
is advantageously within the range of from 0.1 to 20 .mu.m.
[0088] 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.
[0089] 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.
[0090] After crystallization, the supported layer may be washed,
dried, and calcined in the normal way.
[0091] 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.
[0092] 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 calcining the
crystallized layer.
[0093] 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.
[0094] 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.
[0095] 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.
[0096] 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.
[0097] 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.
[0098] Preferably, however, the reparation is carried out by again
subjecting the supported layer to a manufacturing process of the
invention.
[0099] 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.
[0100] 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 into the pores of the molecular
sieve, or by introduction of a metal.
[0101] 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.
[0102] 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.
[0103] Separation of heteroatomic compounds from hydrocarbons such
as alcohols and sulphur containing materials such as H.sub.2S and
mercaptans.
[0104] 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.
[0105] 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.
[0106] 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 chemicals
plant streams and processes, selective separations 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 catalytic 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.
[0107] 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.
[0108] 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.
[0109] The following examples illustrate the invention:
EXAMPLES
Example 1
[0110] This example illustrates manufacture of a layer by spin-coating
with a temporary barrier layer.
[0111] A porous alpha-alumina disk, diameter 25 mm, thickness 3
mm, pore size 80 nm, is soaked in demineralized water for 3 days.
The soaked disk is placed in the specimen chuck of a CONVAC Model
MTS-4 Spinner, and hot water is placed in the process cup to increase
the humidity of the atmosphere. The disk is spun at 4000 rpm for
30 seconds. The disk is then immediately covered with a slurry comprising
25% by weight of Ludox (a trademark) AS-40 colloidal silica and
75% by weight of an aqueous dispersion containing 6.5% by weight
colloidal silicalite (MFI) zeolite, mean particle size 50 nm. 10
seconds after contact between the slurry and the disk, the disk
is spun at 4000 rpm for 30 seconds. The disk and the resulting silica-zeolite
layer are kept in a closed vessel at relative humidity close to
100% for 3 hours to age the silica, air dried at room temperature
for 2 hours and subsequently in an oven at 110.degree. C. for 2
hours.
[0112] Under an optical microscope, the resulting silica-zeolite
layer appeared smooth, crack-free, and homogeneous. Scanning Electron
Microscopy (SEM) of a cross-section through the supported layer
shows a layer about 1 .mu.m thick containing uniformly sized zeolite
particles--see FIG. 1. The homogeneity and continuity of the layer,
coupled with its thinness, confirm that the resulting structure
after calcining will form a layer according to the invention.
[0113] In a comparison experiment, instead of soaking the disk,
it was dried at 150.degree. C. in air for 12 hours, other process
steps remaining the same. As can be seen from the SEM cross section
shown in FIG. 2 the resulting layer is about 40 .mu.m thick. It
is also cracked, and not firmly attached to the substrate, making
it unsuitable for use as a layer.
Example 2
[0114] This example illustrates manufacture of a layer by spin-coating
using a permanent barrier layer.
[0115] The support comprised an alpha-alumina base with a barrier
layer of gamma-alumina, and was prepared as follows:
[0116] A slurry was prepared by ball milling 800 g Al.sub.2O.sub.3
in 500 ml distilled water containing 4.3 ml hydrochloric acid for
16 hours to give alumina particles of mean diameter 0.5 .mu.m. The
slurry was degassed, poured into moulds and allowed to dry at ambient
temperature for 3 days. The cast pieces were heated at 5.degree.
C./min to 1200.degree. C., then fired at 1200.degree. C. for 2 hours.
The fired pieces were then polished front and back to a thickness
of about 3 mm. A gamma-alumina coating was applied by dipping the
alpha-alumina piece once into a colloidal suspension of Boehmite,
prepared by hydrolysis of alumina sec-butoxide in 600 ml water and
0.76 ml nitric acid. The Boehmite layer was converted to gamma-alumina
by heating to 400.degree. C. at a rate of 10.degree. C./hour and
holding for 24 hours. The coated product provides a support.
[0117] A silica sol was prepared from tetraethylorthosilicate,
water, and hydrochloric acid and aged at 50.degree. C. for 90 minutes.
[0118] A suspension of silicalite 1 mean particle size 55 nm,
particle size range 40 to 70 nm, containing 8.7% by weight colloidal
crystals in aqueous TPAOH, pH 10.3 was prepared and a coating slurry
formed by mixing equal weights of the suspension and the sol. The
resulting slurry was spin-coated onto the support at 4000 rpm.
[0119] The resulting structure was then heated to 600.degree. C.
at a heating rate of 20.degree. C./hour. The final layer structure
is shown edge on and from the top surface in FIGS. 3 and 4. The
edge on view demonstrates that the layer thickness is about 0.2
.mu.m and the top view shows the organization of the crystals in
the layer, and that the crystals are incorporated into the layer
with little or no change in crystal size and shape.
Example 3
[0120] This example illustrates the use of a layer according to
the invention in the separation of a hydrocarbon mixture.
[0121] The layer of Example 2 was used to separate an equimolar
mixture of toluene, m-xylene, n-octane and iso-octane. The mixture
was applied to the layer side of the layer structure in a continuous
flow. A gas sweep (Argon 40-500 ml/min) was applied to the support
side of the layer structure, and sampled by a gas chromatograph
operating with a 10'.times.1/8.DELTA. (about 3 m.times.3 mm) stainless
steel, GP5% SP1200/5% Bentone 34 on 100/120 Supelcoport column.
The total pressure drop across the layer was 1000 kPa. Analysis
of the gc data shows that the layer permeate had an enhanced aromatics
content relative to feed content. Representative data at a temperature
of 180.degree. C. are shown in FIG. 5. The plot shows the relative
concentrations of toluene, m-xylene, n-octane and iso-octane as
a function of elapsed time. The largest separation factor is observed
for toluene/iso-octane with a value of 10. The total flux of hydrocarbon
through the layer corresponds to 100 kg/m.sup.2/day at the start
and after 16 hours to an average of 40 kg/m.sup.2/day.
Examples 4 To 26
[0122] These examples illustrate manufacture of a layer by dip-coating.
[0123] In each of the following Examples, a colloidal MFI zeolite
crystal suspension having a mean particle size of 70 nm was employed,
together with a gamma-alumina-surfaced alpha-alumina support as
described in Example 2. After dipping, the supported layer was dried
at 40.degree. C. for 3 hours, at a relative humidity of 60%. Each
layer was heated at 10.degree. C./hour to 550.degree. C., maintained
at that temperature for 3 hours to effect calcination, and cooled
to room temperature at 20.degree. C./hour.
Examples 4 to 9
[0124] These examples were conducted at a dip time of 5 seconds,
a concentration of zeolite of 1.1%, and 1.6 g/l of hydroxypropyl
cellulose, varying the pH by adding small amounts of a one molar
HNO.sub.3 solution, the effect of pH on adhesion being shown. TABLE-US-00001
Example No. pH Adhesion 4 3.6 very good 5 5.2 good 6 7.6 acceptable
7 9.1 acceptable 8 10.6 acceptable 9 11.7 poor
[0125] Observation of adhesion standard was subjective; the zeolite
layer thickness varied between 1.5 and 2 .mu.m, as determined by
S.E.M.
Examples 10 to 14
[0126] These examples were conducted at a dip time of 5 seconds,
a zeolite concentration of 1.1%, a pH of 3.5 and with different
binders/surfactants. TABLE-US-00002 Conc. Observation Example No.
Additive g/l Ad; Conty 10 PVA, M = 72000 20 very bad; cracks 11
PVA, M = 3000 20 acceptable; cont. 12 HPC, M = 100000 1.6 very good;
cont. 13 PVA, M = 3000 20} good; cont. HPC, M = 100000 1.6} 14 None
Ad = adherence Conty = continuity of layer Cont = continuous
Examples 15 to 20
[0127] In these examples, the effects on the properties of the
layer resulting from varying the zeolite concentration were studied;
the dip time was 5 seconds, pH was 3.5 additive HPC, 1.6 g/litre.
TABLE-US-00003 Zeolite Conc. Layer Example No. g/l Thickness .mu.m
Observation 15 0.1 -- not continuous 16 0.5 -- not continuous 17
0.8 1.0 continuous, few cracks 18 1.1 2.5 continuous, few cracks
19 1.6 5.0 continuous, cracks 20 2.1 6.5 continuous, cracks
Examples 21 to 26
[0128] In these Examples, the effect of the dipping time was studied;
pH was 3.0 additive was HPC at 1.6 g/litre, zeolite content 1.1%.
TABLE-US-00004 Dipping Time Layer Example No. seconds Thickness
.mu.m Observation 21 1 0.7 to 1.1 no cracks 22 3 1.5 to 2 no cracks
23 6 2 no cracks 24 10 2 to 3 no cracks 25 20 3.5 cracks 26 60 6.5
to 7 cracks
[0129] The experiments show that dipcoating can give good continuous
layers of low thickness; reparation to remove cracks may be effected
by multiple applications.
Example 27
[0130] This and the following example illustrate manufacture of
a layer using hydrothermal crystallization techniques. In this example,
the ageing solution contained all the zeolite-forming ingredients.
[0131] A synthesis mixture was prepared from the following components,
in parts by weight: TABLE-US-00005 Colloidal ZSM-5 suspension, 50
nm mean 18.79 particle size, 6.5% by weight ZSM-5 Tetrapropylammonium
bromide (TPABr) 1.55 Ludox AS-40 colloidal silica 6.25
[0132] Using the barrier-forming and spin-coating procedure of
Example 1 a water-soaked alpha-alumina disk with 80 nm diameter
pores is spincoated with part of the synthesis mixture. The coated
disk is transferred to an autoclave and covered with the remainder
of the synthesis mixture. The autoclave was transferred to an oven,
heated to 160.degree. C. over the course of 2 hours, maintained
at that temperature for 120 hours, and cooled to room temperature.
The cooled coated disk was washed in flowing tap water for 4 hours,
washed twice in demineralized water and then twice more at 80.degree.
C. The disk was dried by heating in an oven at 10.degree. C./hour
to 110.degree. C., maintained at 110.degree. C. for 5 hours, and
allowed to cool at room temperature. Calcining was effected by heating
at 10.degree. C./hour to 550.degree. C., maintaining at that temperature
for 16 hours, and cooling at 60.degree. C. per hour to room temperature.
[0133] From optical and SEM observations--see FIG. 6--the resultant
layer is about 1 .mu.m thick and crack-free, with a final grain
size of from 100 to 300 nm.
Example 28
[0134] In this example, the ageing solution contained only those
ingredients not already in the layer.
[0135] A synthesis mixture was prepared from the following components,
in parts by weight: TABLE-US-00006 Colloidal silicalite 1 suspension,
20 to 30 nm 20.00 particle size distribution, 7.2% by weight solids,
including template present in the zeolite Ludox AS-40 colloidal
silica 20.00 Demineralized water 22.50
[0136] An alpha-alumina disk was dipped into the solution for 5
seconds, and immediately placed in an autoclave and covered with
an ageing solution, pH 11.5 with a molar composition of 6.36 (NH.sub.4).sub.2O/1
TPABr/130 H.sub.2O/0.96 HNO.sub.3. The autoclave was put in an oven
at 152.degree. C. and maintained there for 7 days. After removal
from the autoclave, the disk was repeatedly washed with demineralized
water at 70.degree. C. until the conductivity of the last wash-water
was 10 microSiemens per centimetre. The disk was then dried at 40.degree.
C., relative humidity 60%, for several hours, followed by drying
for 1 hour at 105.degree. C.
[0137] Visual inspection showed the disk to be very homogeneous
and smooth, with no visual terracing or scaling. By SEM it was seen
that the layer had the crystal habit of silicalite--see FIG. 7--with
a mean diameter of 100 nm; the cross-section--FIG. 8--indicating
a layer thickness of about 10 .mu.m.
Example 29
[0138] This example illustrates in situ formation of zeolite crystals
on a support.
[0139] A synthesis solution was prepared from the following components,
the parts being given by weight: TABLE-US-00007 TPAOH (20% by weight
in water) 41.02 NaOH, pellets 0.58 SiO.sub.2 powder (10% of water)
8.94
[0140] The sodium hydroxide was dissolved in the TPAOH solution
at room temperature, the silica added, and the mixture heated to
boiling with vigorous stirring until a clear solution was obtained.
The solution was cooled, weight loss compensated with demineralized
water, and the solution filtered through a 0.45 .mu.m filter. The
molar composition of the synthesis mixture was: 0.52Na.sub.2O/1.50(TPA).sub.2O/10
SiO.sub.2/142 H.sub.2O
[0141] A quarter of an alpha-alumina disk, pore size 1 .mu.m, diameter
47 mm, was air dried for 2 hours at 150.degree. C., and weighed.
25.05 g of synthesis solution was poured onto the disk in a 150
ml stainless steel autoclave. The autoclave was placed in an oven,
heated up to 150.degree. C. in the course of 1 hour and maintained
at that temperature for 24 hours.
[0142] After cooling the autoclave the support was removed, repeatedly
washed with deionized water and air dried at 150.degree. C. for
2 hours. A disk weight increase of 6.9% was noted.
[0143] The dried disk was then heated at 2.degree. C./min to a
temperature of 475.degree. C. and heated in air at that temperature
for 6 hours. Comparison of SEMs of the original alpha-alumina surface--FIG.
9--and of the calcined layer--FIG. 10--shows that the surface of
the disk is homogeneously coated with intergrown spherical crystals
of about 0.4 .mu.m size, which show the typical crystal habit of
silicalite.
Example 30
[0144] Example 29 was repeated except that crystallization took
place at 98.degree. C. for 19 hours. An SEM--FIG. 11--again shows
a homogeneous coating of the disk surface, but the crystal size
is now smaller, between 0.2 and 0.3 .mu.m.
Example 31
[0145] A synthesis solution was prepared as described in Example
29. The support was an alpha-alumina substrate with a pore size
of 160 nm; this was dried at 185.degree. C., placed on the bottom
of a 300 ml stainless steel autoclave, covered with 220.4 g of synthesis
solution, and the autoclave maintained at 120.degree. C. for 24
hours. After washing, drying and calcining at 475.degree. C. for
12 hours in air, the supported layer was examined by SEM. The photographs--FIGS.
12 to 14 show the surface, FIG. 15 shows a cross-section--indicate
a uniform coating of 0.3 .mu.m intergrown silicalite crystals and
a layer thickness of about 0.5 .mu.m.
Example 32
[0146] This example illustrates the manufacture of a zeolite layer
by two in-situ crystallization steps at 120.degree. C.
[0147] The support comprises a porous alpha-alumina disk, having
an average pore diameter of 160 nm, and polished on at least one
side. After polishing the support is stored submerged in demineralised
water until a day before the preparation of the zeolite layer. Then
the support is placed in an oven, heated up at a rate of 1.degree.
C./minute to 400.degree. C., kept at 400.degree. C. for 4 hours,
and cooled down.
[0148] For the first crystallization step, a synthesis mixture
is prepared by mixing silica (Baker, >99.75 pure SiO.sub.2),
Tetrapropyl-ammoniun-hydroxide (TPAOH, Fluka practical grade, 20%
in water), NaOH (Merck, 99.99 pure) and demineralised water to get
100 ml of mixture with the following molar composition;
[0149] 10 SiO.sub.2/1.5 (TPA) 20/0.53 Na.sub.2O/142 H.sub.2O. The
mixture is boiled on a hotplate for 5 minutes while stirring vigorously.
Then the mixture is taken from the hotplate and left to cool down,
after which H.sub.2O is added to compensate for evaporation losses
during boiling. The dry support disk is taken out of the oven and
placed on the bottom of a stainless steel autoclave with the polished
side facing up. The synthesis mixture is poured in the autoclave
next to the disk, which is eventually submerged in the mixture.
The autoclage is closed and placed in an oven at 120.degree. C.
for 72 hours. After removal from the autoclave the disk is washed
5 to 10 times in demineralised water of 70.degree. C.
[0150] For the second crystallization step, a fresh synthesis mixture,
identical to the mixture described for the first step, is prepared.
The disk is placed in a clean autoclave while still wet, in the
same orientation as in the first step, and the fresh synthesis mixture
is poured in the autoclave so that the disk is completely submerged.
The autoclave is closed and put in an oven at 120.degree. C. for
72 hours. After removal from the autoclave the disk is washed 5
to 10 times in demineralised water of 70.degree. C. After washing
the disk is dried in air at 30.degree. C. for 1.5 days. Then the
disk is heated up in air at a rate of 10.degree. C./hour to 550.degree.
C., kept at that temperature for 16 hours, and cooled down to room
temperature at a rate of 20.degree. C./hour.
[0151] X-ray Diffraction (XRD) analysis shows that MFI-type zeolite
crystals have formed on both the top and the bottom surfaces of
the disk, the intensity of the XRD-peaks suggesting a zeolite layer
thickness of a few microns. Scanning Electron Microscope (SEM) micrographs
show that a dense layer, 3 to 5 micrometer in thickness, has formed
at the top surface of the disk, and also at the bottom surface of
the disk.
Examples 33 34 35 and 36
[0152] These examples illustrate the increase in the amount of
zeolite formed on the support with increasing number of crystallization
steps. The preparation is identical to that of Example 32 the number
of crystallization steps varies from one to four.
[0153] XRD patterns have been obtained from these disks after drying
but before the thermal treatment at 550.degree. C. Comparison of
the XRD-patterns shows that with each step the height of the MFI-zeolite
peaks increases while the height of the alpha-alumina peaks decreases,
as shown in the following table, where the intensity ratio refers
to the ratio between the intensity of the MFI (501) (051) the alpha-alumina
(012) peak: TABLE-US-00008 Example Number of Steps Ratio 33 1 0.37
34 2 0.71 35 3 1.41 36 4 2.78
[0154] This indicates that the amount of zeolite on the disk increases
with each crystallization step.
Example 37
[0155] This example describes the Helium permeation characteristics
of disks prepared using one or two crystallization steps similar
to Example 32 the first crystallization step done at 120.degree.
C. and the second crystallization step done at 90.degree. C.
[0156] Helium permeation through the disk has been measured at
total pressures in the range of 1 to 3 bar. Disks prepared using
a single crystallization step at 120.degree. C. show He-permeations
of several hundreds mmol/sm2bar, increasing with pressure. However,
disks prepared using two crystallization steps (120.degree. C. and
90.degree. C.) show He-permeations of a few tens of mmol/sm2bar
that are constant over the pressure range of 1-3 bar.
Example 38
[0157] A membrane fabricated according to the process of example
32 was mounted into a holder and a `Wicke-Kallenbach` experiment
was carried out. A gas mixture of 49.9% n-butane, 49.9% methane
and 0.2% i-butane was passed over one side of the membrane, the
other side being continuously purged with a dry helium stream. Both
sides of the membrane were kept at atmospheric pressure. The analyses
of both gas streams by an on-line gas chromatograph were evaluated
and transformed to the corresponding fluxes through the membrane.
Selectivities are given by:
[0158] S=(Cl (perm)/CL (ret))/(C2 (perm)/C2 (ret)), where C1 and
C2 are concentrations of components 1 and 2 and permeate and retentate
streams are perm and ret, respectively. The calculated fluxes and
selectivities the following table: TABLE-US-00009 Methane flux n-Butane
flux S (n- T [C.] [mol/m.sup.2s] * 10.sup.4 [mol/m.sup.2s] * 10.sup.3
butane/methane) 25 1.35 2.44 18.07 50 2.15 2.67 12.42 75 2.78 2.81
10.11 100 4.94 3.14 6.36 125 8.75 3.36 3.84 150 13.1 3.40 2.60 175
17.1 3.24 1.89 200 21.3 3.07 1.44 (Reference: E. Wicke and R. Kallenbach,
Surface diffusion of carbon dioxide in activated charcoals, Kolloid
Z., 97 (1941), 135).
Example 39
[0159] A membrane fabricated according to the process of example
32 was used for a test similar to that in example 38. A gas mixture
of 48.3% methane and 51.7% i-butane was used as feed stream. The
calculated fluxes and selectivities are given in the following table:
TABLE-US-00010 Methane flux n-Butane flux S (methane/i- T [C.] [mol/m.sup.2s]
* 10.sup.4 [mol/m.sup.2s] * 10.sup.5 butane) 25 1.29 7.18 1.92 50
2.38 7.29 3.49 75 3.76 7.41 5.43 100 4.90 9.38 5.59 125 6.29 13.2
5.10 150 8.42 17.7 5.09 175 12.2 22.3 5.86 200 17.8 25.7 7.41
Example 40
[0160] A membrane fabricated according to the process described
in example 32 was used for test similar to that in example 38. A
gas mixture of 50.0% n-butane and 50.0% i-butane was used as feed
stream. The calculated fluxes and selectivities are given in the
following table: TABLE-US-00011 T n-Butane flux i-Butane flux S
(n-butane/i- [C.] [mol/m.sup.2s] * 10.sup.3 [mol/m.sup.2s] * 10.sup.4
butane) 25 1.33 0.26 51.95 50 1.66 0.71 23.55 75 1.99 0.82 24.21
100 2.29 1.21 18.93 125 2.24 1.60 14.00 150 2.45 1.85 13.24 175
2.28 1.89 12.06 200 2.26 2.06 10.97
Example 41
[0161] A membrane fabricated according to the description in example
32 was used for a test similar to that in example 38. A gas mixture
containing 0.31% p-xylene, 0.26% o-xylene and methane as balance
was used as feed stream. The calculated fluxes and selectivities
are given in the following table: TABLE-US-00012 T p-Xylene flux
o-Xylene flux S (p-xylene/o- [C.] [mol/m.sup.2s] * 10.sup.6 [mol/m.sup.2s]
* 10.sup.7 xylene) 100 3.54 0.49 60.10 150 3.43 0.66 43.46 175 3.33
0.92 30.49 200 3.02 1.22 20.76
Example 42
[0162] A membrane fabricated according to the description in example
32 was used for a test similar to that in example 38. A gas mixture
containing 5.5% benzene, 5.5% cyclohexane and methane as balance
was used as feed stream. The calculated fluxes and selectivities
are given in the following table: TABLE-US-00013 T Benzene flux
Cyclohexane flux S (benzene/ [C.] [mol/m2s] * 107 [mol/m.sup.2s]
* 10.sup.7 cyclohexane) 25 2.64 0.53 5.01 50 3.03 0.66 4.60 75 4.61
0.92 4.99 100 5.67 1.98 2.86 125 9.23 3.20 2.88 150 9.49 4.48 2.12
175 10.9 3.30 3.30 200 17.8 4.48 3.97
Example 43
[0163] A membrane fabricated according to the description in example
32 was used for a test similar to that in example 38. A gas mixture
containing 7.6% n-hexane, 15.4% 22-dimethylbutane and methane as
balance was used as feed stream. The calculated fluxes and selectivities
are given in the following table:
Example 44
[0164] TABLE-US-00014 T n-Hexane flux 22-Dimethylbutane S (benzene/
[C.] [mol/m.sup.2s] * 10.sup.4 flux [mol/m.sup.2s] * 10.sup.7 cyclohexane)
20 1.2 1.9 600 50 1.5 2.3 340 100 3.1 2.7 1150 150 3.0 1.9 1560
200 2.4 1.2 2090
[0165] This example illustrates the growth of zeolite layers by
multiple crystallizations, without refreshing the synthesis mixture
as in example 32 but by increasing the crystallization temperature
stepwise.
[0166] A porous alpha-alumina disk with a pore diameter of 160
nm and polished on one side was cut into four equal-sized parts.
The parts were weighed and placed, polished side up, on Teflon rings
resting on the bottom of a stainless steel autoclave. In the autoclave
was poured 70.22 g of a synthesis solution with a molar composition
of 10 SiO.sub.2/1.56 (TPA)20/0.275 Na.sub.2O/147 H.sub.2O
[0167] The open autoclave was placed in an exsiccator, which was
then evacuated during 0.5 hours to increase the penetration of synthesis
solution into the disks. Then the autoclave was taken out of the
exsiccator, closed, and placed in an oven at room temperature. The
oven was heated up to 90.degree. C. in a few minutes and kept at
that temperature for 48 hours. The autoclave was then cooled to
room temperature, opened and one of the support pieces was removed.
The autoclave was closed again and placed in an oven at room temperature.
The oven was heated up to 110.degree. C. in a few minutes and kept
at that temperature for 24 hours. The autoclave was cooled down
again and the second piece was removed. The temperature cycle was
repeated two more times, first for 24 hours at 130.degree. C. and
then for 24 hours at 150.degree. C. The four pieces of the disk
were all washed with demineralised water of 70.degree. C. until
the washing water had a conductivity of about 6 micro Siemens/cm,
dried at 105.degree. C. and cooled to room temperature in an exsiccator.
It was observed that with each aging step the weight of the disk
pieces increased, as shown in the following table. TABLE-US-00015
Disk Piece # Temperature History .degree. C. Weight increase % 1
90 0.88 2 90 + 110 2.04 3 90 + 110 + 130 3.50 4 90 + 110 + 130 +
150 5.63
[0168] XRD analysis showed that with each aging step the intensity
of the zeolite peaks increased with respect to the intensity of
the alpha-alumina peaks, as shown in the following table: TABLE-US-00016
Peak Intensity ratio: Peak at d = 0.385 nm (MFI)/ Disk Piece # Peak
at d = 0.348 nm (A1.sub.20.sub.3) 1 0.190 2 0.217 3 0.236 4 0.332
These results indicate that with each aging step at a higher temperature
new zeolite crystals are deposited on the supported. |