Molecular sieve abstract
The invention is directed to a method for preparing a membrane
comprising a homogeneous porous carrier, on which a layer of a molecular
sieve is deposited, comprising the following steps (i) preparing
an inert solution and a precursor solution of the molecular sieve;
(ii) impregnating the porosity of the carrier with the inert solution
and/or the precursor solution; (iii) bringing the region of the
ceramic carrier intended to receive the layer of a molecular sieve
into contact with the precursor solution, and bringing the region
of the carrier not intended to receive the layer of a molecular
sieve into contact with the inert solution; and (iv) forming a molecular
sieve in situ. The invention applies e.g., to filtration or gas
or liquid fluid separation, reverse osmosis or catalysis.
Molecular sieve claims
What is claimed is:
1. A method for preparing a membrane comprising a homogeneous porous
carrier having a pore diameter comprised between 5 nm and 20 .mu.m,
on which a layer of a molecular sieve is deposited, comprising the
following steps: (i) preparing an inert solution and a precursor
solution of the molecular sieve; (ii) impregnating the porosity
of the carrier with the inert solution and/or the precursor solution;
(iii) bringing the region of the carrier intended to receive the
layer of said molecular sieve into contact with the precursor solution,
and bringing the region of the carrier not intended to receive the
layer of said molecular sieve into contact with the inert solution;
and (iv) forming said molecular sieve in situ.
2. The method according to claim 1 in which the carrier is a carrier
comprising at least one channel.
3. The method according to claim 2 in which the steps (ii) and
(iii) comprise: firstly, filling the volume of said at least one
channel and pores of the carrier with the inert solution and, secondly,
filling a volume surrounding the outside of the carrier with the
precursor solution.
4. The method according to claim 2 in which the steps (ii) and
(iii) comprise: firstly, filling a volume surrounding the outside
of the carrier and the pores of the carrier with the inert solution
and, secondly, filling the volume of said at least one channel with
the precursor solution.
5. The method according to claim 2 in which the steps (ii) and
(iii) comprise: firstly, filling the volume of said at least one
channel with the inert solution and, secondly, filling a volume
surrounding the outside of the carrier with the precursor solution.
6. The method according to claim 2 in which the steps (ii) and
(iii) comprise: firstly, filling a volume surrounding the outside
of the carrier with the inert solution and, secondly, filling the
volume of said at least one channel and of the pores of the carrier
with the precursor solution.
7. The method according to claim 1 in which the precursor solution
is an aqueous solution comprising a precursor agent for the molecular
sieve and a structuring agent.
8. The method according to claim 1 in which the precursor solution
is aged prior to its use for a duration comprised between 1 and
96 hours.
9. The method according to claim 8 in which the precursor solution
is aged prior to its use for a duration comprised between 12 and
72 hours.
10. The method according to claim 1 in which the inert solution
is an aqueous solution.
11. The method according to claim 1 in which step (iv) in the
formation of the molecular sieve comprises a hydro thermal synthesis
followed by calcinating.
12. The method according to claim 11 in which the precursor solution
is an aqueous solution comprising a precursor agent for the zeolite
and a structuring agent, which is aged prior to its use for a duration
comprised between 12 and 72 hours, and in which the inert solution
is an aqueous solution.
13. The method according to claim 11 in which step (iv) in the
formation of the zeolite comprises a hydrothermal synthesis followed
by calcinating.
14. A method for preparing a membrane comprising a homogeneous
porous ceramic fiber having a pore diameter comprised between 5
nm and 10 .mu.m, on which a single zero-defect unitary layer of
a zeolite is deposited, said layer having no break in its macroscopic
three-dimensional structure and said layer having a thickness comprised
between 3 and 50 .mu.m, comprising the following steps: (i) preparing
an inert solution and a precursor solution of said zeolite; (ii)
impregnating the porosity of the fiber with the inert solution and/or
the precursor solution; (iii) bringing the region of the ceramic
fiber intended to receive the layer of said zeolite into contact
with the precursor solution, and bringing the region of the fiber
not intended to receive the layer of said zeolite into contact with
the inert solution; and (iv) forming said zeolite in situ.
15. The method according to claim 14 in which the fiber comprises
at least one channel.
16. The method according to claim 15 in which the steps (ii) and
(iii) comprise: firstly, filling the volume of said at least one
channel and pores of the fiber with the inert solution and, secondly,
filling a volume surrounding the outside of the fiber with the precursor
solution.
17. The method according to claim 15 in which steps (ii) and (iii)
comprise: firstly, filling a volume surrounding the outside of the
fiber and the pores of the fiber with the inert solution and, secondly,
filling the volume of said at least one channel with the precursor
solution.
18. The method according to claim 15 in which the steps (ii) and
(iii) comprise: firstly, filling the volume of said at least one
channel with the inert solution and, secondly, filling a volume
surrounding the outside of the fiber with the precursor solution.
19. The method according to claim 15 in which steps (ii) and (iii)
comprise: firstly, filling a volume surrounding the outside of the
fiber with the inert solution and, secondly, filling the volume
of said at least one channel and the pores of the fiber with the
precursor solution.
Molecular sieve description
BACKGROUND OF THE INVENTION
The present invention relates to a novel process for preparing
a membrane comprising a porous carrier and a layer of a molecular
sieve, as well as to novel membranes. The invention applies to filtration
or gas or liquid fluid separation, pervaporation, reverse osmosis
or catalysis.
Zeolite membranes constituted by a (macro)porous portion and a
zeolite are already known. These materials can be obtained principally
by two methods: a method employing a gel and a method employing
a (colloidal or oligomeric) solution, such methods comprising several
steps. First, a film (gel method) or total impregnation (solution
method) is formed, such film or total impregnation containing species
able to form a zeolite, following which the zeolite is crystallized
under hydrothermal conditions.
These two types of process suffer from major drawbacks.
In both cases, the starting pH during the first step is extremely
high. This highly basic pH is not compatible with certain ceramic
materials. In effect, the gamma alumina currently employed as the
carrier layer for the zeolite is soluble in highly basic media,
leading to alumina solubilization in the zeolite precursor, consequently
leading to chemical contamination of the zeolite, the alumina having
penetrated the desired crystalline structure.
In both cases, the use of large amounts of gel or zeolite precursor
solution and the poorly synthesized yield render this process expensive,
particularly when structuring agents of the quaternary ammonium
type are employed (and all the more so as several synthesis cycles
are frequently necessary).
In the case of the gel method, it is difficult to guarantee homogeneity
of the gel composition as the gel is formed from different constituents
which do not mix homogeneously. As the local composition of the
gel varies, the characteristics of the zeolite structure vary and
membrane performance is modified. This defect is additionally clearly
recognized in European Patent Application 0481660 which indicates
that spot defects are present. Thus, European Patent Application
0778076 discloses production of the gel in situ; the porosity
of the carrier is filled with a first solution after which the carrier
is brought into contact with a second solution which is immiscible
with the first one. Gelification occurs locally at the contact of
the two solutions, the gel being essentially formed at the surface
of the porous carrier. Gelification modifies the compositions of
the solutions and consequently it is not possible to guarantee an
identical gel at every point in the porous carrier.
In the article "Characterization and Permeation Properties
of ZSM-5 Tubular membranes", AIChE Journal, July 1997 Vol.
43 No. 7 Coronas et al. studied the influence of the carrier on
zeolite layer deposition. Two asymmetric membranes were tested,
one with a layer of 5 nm pore diameter .gamma.-alumina and the other
with a layer of .alpha.-alumina of pore diameter 0.2 .mu.m. The
method used by Coronas et al. is a gel method. Coronas et al. conclude
that it is easier to form a continuous zeolite layer on an .gamma.-alumina
type carrier (5 nm) than on an .alpha.-alumina type carrier (0.2
.mu.m), which, in the latter case, necessitates repetition of the
process.
Supplementary deposition-crystallisation cycles are in fact always
necessary in the case of gel processes for improving the quality
of the membrane and for thus obtaining a product which effectively
allows separation. The zeolite layer obtained by the gel methods
is consequently in point of fact a multi-layer.
Furthermore, because of their high viscosity, the gels block channels
of a diameter which can reach several millimeters. This technique
is consequently reserved for flat structures or tubes of considerable
inside diameter. Thus, all the examples in European Patent Application
0778076 employ plane-surface carriers as well as the majority
of the examples in European Patent Application 0481660 example
12 of this application employing tubes with an inside diameter of
about 6.5 mm. Now, the use of a ceramic carrier of tubular geometry
(whether this be single- or multi-channel) where the channels are
of significant diameter, or of flat geometry, does not make it possible
to obtain filtration modules or gas separation modules which are
highly compact, in other words which have a large filtering surface
compared to the space they occupy. Indeed, it is accepted that the
compactness for plane-membrane modules is of the order of 150 m.sup.2
/m.sup.3 while that of multi-channel membrane modules only reaches
300 m.sup.2 /m.sup.3 ; these degrees of compactness are very low
when compared to those required for gas separation applications.
In the case of methods employing a solution as in international
application WO-A-9529751 it is also stated that the nucleation
of the zeolite, previously necessary for its formation, cannot be
done for volumes the characteristic dimension of which is greater
than about 10 microns and/or less than 5 nm. According to that document,
it is consequently impossible to obtain nucleation and growth outside
a specific porous material. This consequently rules out the formation
of layers whether this be inside or outside the tube, as well as
for tubes in macroporous carriers, the mean pore diameter of which
is for example higher than 10 microns.
Additionally, the solution method in international application
WO-A-9529751 involves impregnation throughout the total porous volume
(having a suitable dimension), and consequently the zeolite occupies
the totality of the carrier and is not precisely localized (for
example in the form of a layer). This absence of localization is
prejudicial to the efficiency of the composite material at the time
of its use; it is perfectly known that gas permeability through
a zeolite membrane is linked to the thickness of the zeolite. The
thicker the zeolite, the more permeability diminishes for a separation
efficiency, which is not affected.
The solution provided in EP-A-0674939 is similar to the one disclosed
in WO-A-9529751.
Thus, the formation, using a gel method, of a zeolite layer on
a carrier (for example of around 0.2 .mu.m pore diameter) requires
the gel method to be repeated. A solution method, according to WO-A-9529751
does not produce a zeolite layer on the carrier, but in the latter,
to the exclusion of a layer thereon, and does not make layer formation
possible in or on the carrier, for pore diameters greater than 10
microns.
One consequently looks for materials having a zeolite layer, notably
at the inner channels of a multi-channel carrier, this layer requiring
to be homogeneous both from a chemical point of view as well as
from a physical point of view, in the form of a unitary defect-free
layer, the preparation requiring additionally to be simple and economical.
None of the documents cited above offers a solution, nor teaches
or suggests the present invention.
SUMMARY OF THE INVENTION
The present invention discloses a solution for overcoming these
disadvantages.
According to a first aspect, the invention offers new products
as well as a novel production method.
Consequently, the invention provides a membrane comprising a homogeneous
porous carrier having a pore diameter comprised between 5 nm and
20 .mu.m, on which a zero-defect unitary layer of a molecular sieve
is deposited.
In one preferred embodiment, the unitary layer is a single layer.
In a further preferred embodiment, the thickness of the layer of
a molecular sieve is comprised between 1 and 100 .mu.m, for example
between 50 nm and 2 .mu.m, for example between 3 and 50 .mu.m.
The molecular sieve is preferably a zeolite.
According to a preferred embodiment, the carrier has a pore diameter
comprised between 5 nm and 10 .mu.m and preferably between 50 nm
and 2 .mu.m.
In one embodiment, the carrier is a ceramic carrier.
In a further embodiment, the carrier is a fiber, for example a
multi-channel fiber. The layer of molecular sieve can be arranged
on the outside of the fiber, or the layer of molecular sieve can
be arranged inside the channel or channels of the fiber, or the
layer of molecular sieve can be arranged on the outside of the fiber,
the molecular sieve being additionally present within the thickness
of the fiber, or the layer of molecular sieve can be arranged inside
the channel or channels of the fiber, the molecular sieve being
additionally present in the thickness of the fiber.
The invention also provides a module comprising membranes according
to the invention.
The invention also covers the use of this module for gas separation.
The invention also provides a method for separating gas comprising
the step of permeation on a membrane according to the invention.
Thus, the invention provides a method for preparing these membranes,
as well as other conventional membranes, comprising the following
steps: (i) preparing an inert solution and a precursor solution
of the molecular sieve; (ii) impregnating the porosity of the carrier
with the inert solution and/or the precursor solution; (iii) bringing
the region of the ceramic carrier intended to receive the layer
of a molecular sieve into contact with the precursor solution, and
bringing the region of the carrier not intended to receive the layer
of a molecular sieve into contact with the inert solution; and (iv)
forming a molecular sieve in situ.
According to one embodiment of the method, the carrier is a carrier
comprising at least one channel.
In a preferred embodiment, steps (ii) and (iii) comprise: firstly,
filling the volume of the channel or channels and pores of the carrier
with the inert solution and, secondly, filling a volume surrounding
the outside of the carrier with the precursor solution.
In a further preferred embodiment, steps (ii) and (iii) comprise:
firstly, filling a volume surrounding the outside of the carrier
and the pores of the carrier with the inert solution and, secondly,
filling the volume of said channel or channels with the precursor
solution.
In yet a further preferred embodiment, steps (ii) and (iii) comprise:
firstly, filling the volume of the channel or channels with the
inert solution and, secondly, filling a volume surrounding the outside
of the carrier and the pores of the carrier with the precursor solution.
In one preferred embodiment, steps (ii) and (iii) comprise: firstly,
filling a volume surrounding the outside of the carrier with the
inert solution and, secondly, filling the volume of said channel
or channels and of the pores of the carrier with the precursor solution.
In a further preferred embodiment, the precursor solution is an
aqueous solution comprising a precursor agent for the molecular
sieve and a structuring agent.
The precursor solution can be aged prior to its use for a duration
comprised between 1 and 96 hours, for example for a duration comprised
between 12 and 72 hours.
In a preferred embodiment, the inert solution is an aqueous solution.
In a further preferred embodiment, step (iv) in the formation of
the molecular sieve comprises a hydrothermal synthesis followed
by calcinating.
The method is suited to prepare the membranes of the invention
but could also, if necessary, be applied to membranes of the prior
art, starting out from suitable starting materials.
The invention will now be described in more detail below.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Porous carrier
The porous carrier for preparing membranes is homogeneous; it distinguishes
itself from asymmetric membranes employed up until now in the prior
art. Homogeneity should however be evaluated locally in the sense
that, thanks to the method of the invention, zeolite formation can
be localized. Thus, homogeneity is evaluated at the region in contact
(impregnated) with the precursor solution.
The carrier can be a metal, glass, ceramic (for example .alpha.-alumina,
.gamma.-alumina, titanium oxide, zirconium oxide), etc.
In the following, the description is provided with reference to
a "fiber" as the porous carrier; any other type of carriers,
notably flat, is covered by the invention.
The fiber is advantageously a porous multi-channel ceramic fiber;
it can however be single-channel.
This fiber corresponds to a bar of porous ceramic incorporating
one or several channels, said bar of porous ceramic having a porous
(in the conventional sense of the term) structure and variable porosity,
and the axis of said channels is parallel to the axis of the ceramic
bar.
According to one embodiment, the channels are distributed at the
vertices of a regular polygon the order of which is comprised between
3 and 6 a supplementary channel being able to occupy the center
of the polygon where the order is greater than 3 the order being
preferably 5 or 6.
The fiber and/or the channels can have any suitable shape, for
example a circular cross-section; channel cross-sections in the
shape of orange quarters are possible, and the same can apply to
the fiber cross-section, a circular geometry can be replaced by
a multi-lobe geometry. In the case of an orange-quarter geometry
(or where a channel is not circular), the diameter of such a channel
will be defined as the diameter of a circular channel having the
same cross-section. Where the fiber does not have a circular cross-section,
the diameter of such a fiber is similarly defined as the diameter
of a circular fiber having the same cross-section.
The fiber and/or the channels preferably have a circular cross-section.
Preferably again, all the channels are substantially identical;
this is one way of limiting pressure drop and throughput differences
from one channel to another along the fiber.
According to one embodiment, the fiber (multi-channels or single-channel)
according to the invention has the following characteristics: (i)
a channel diameter comprised between 150 and 2000 .mu.m, preferably
between 300 and 1000 .mu.m, and/or (ii) an envelope ratio Re corresponding
to the ratio of porous ceramic fiber diameter to channel diameter
such that Re is comprised between 2.5 and 15 preferably between
4 and 10 and/or (iii) a fill ratio Ro corresponding to a ratio
of the sum of channel cross-sections to porous ceramic fiber cross-section
such that Ro is comprised between 0.03 and 0.45 preferably between
0.04 and 0.35 and advantageously between 0.15 and 0.35 and/or (iv)
a sustain ratio Rs corresponding to a ratio between mean wall thickness
measured along the radius of a fiber and the diameter of a channel
passed through, said mean thickness corresponding to the mean of
channel wall thickness located on a radius of said fiber passing
through a maximum number of channels, such that Rs is comprised
between 0.3 and 2.5 preferably between 0.5 and 1.5 and/or (v)
a thickness ratio Rp corresponding to the ratio between channel
wall thicknesses along a radius of the fiber passing through a maximum
number of channels, such that Rp is comprised between 1/3 and 3
preferably between 1/2 and 2 thickness ratio Rp being advantageously
about 1.
Fiber diameter can extend up to, for example, 25 mm, preferably
up to 15 mm; typically this diameter is comprised between 2 and
10 mm, preferably between 3 and 7 mm.
The fiber has a mean pore diameter comprised between 5 nm and 20
.mu.m, preferably between 5 nm and 10 .mu.m, preferably between
50 nm and 2 .mu.m. According to one alternative embodiment, the
material of the fiber is a homogeneous bulk porous ceramic material,
the mean pore diameter D50 of which is less than 4 .mu.m and the
closed porosity of which is less than 2%. D50 is the volume mean
diameter such that 50% of the pores have a diameter less than D50.
According to one alternative embodiment, the material has a monodisperse
pore diameter distribution; in this embodiment, standard deviation
is less than 35%, preferably 25% of the volume mean diameter D50.
Typically, in this embodiment, the material will have a standard
deviation comprised between 10 and 25% of volume mean diameter D50.
The fiber has a mean porosity comprised, for example, between 10
and 70%, preferably between 35 and 60%.
The fiber can be such that mean pore diameter is comprised between
0.5 and 2 .mu.m and open porosity is comprised between 45 and 60%.
The fibers have a length which may reach several meters; conventionally,
the length of a fiber is comprised between 0.5 and 2 m.
The camber of the fibers according to the invention or extent to
which they are out of true (deformation due to sintering) is generally
low, for example below 0.3%, preferably less than 0.15%, more advantageously
less than 0.05%. This low value favors assembly of the fibers into
a module.
The fiber is conventionally of ceramic material; advantageously,
the ceramic is a metallic oxide.
The method for preparing the fibers comprises three main steps:
(i) Preparation of an inorganic paste comprising an inorganic portion
or filler, a binder and a solvent, with optionally a pore-generating
agent, a deflocculating agent and/or an extrusion agent; (ii) shaping
said paste by extrusion; (iii) consolidating this shape by sintering.
The inorganic portion of said paste comprises particles of a mineral
compound which, after sintering, will form the porous matrix (homogeneous
in its volume). The mineral, preferably metallic, compound is either
a non-oxide compound, or a metal oxide. In the case where this is
a non-oxide derivative, a silicon or aluminium derivative will be
chosen and preferably, silicon carbide, silicon nitride or aluminium
nitride. Where the metallic compound is an oxide, this will be selected
from oxides of aluminium, silicon or metals of groups IVA (titanium
group) or VA (vanadium group) and will preferably be alumina, zirconium
oxide or titanium oxide. These oxides can be used alone or in a
mixture. The metallic compound has, for example, a mean particle
diameter (measured by sedigraph) between 0.15 and 2 .mu.m, and preferably
between 0.15 and 0.6 .mu.m. The paste will contain between 50 and
90% by weight of this, and preferably between 65 and 85% by weight.
This inorganic filler can advantageously consist of particles the
d90 and d50 diameters of which are such that d90/d50<3 and advantageously
d90/d50<2.
The organic binder gives the paste its necessary rheological properties
needed for extrusion and its mechanical properties needed to obtain
good cohesion of the product after extrusion. Said organic binder
is preferably, but not obligatorily, a water-soluble polymer. The
polymer will for example have, for a 2% by weight solution, a viscosity
measured at 20.degree. C. comprised between 4 and 10 Pa/s. This
polymer can be selected from the celluloses and their derivatives
(HEC, CMC, HPC, HPMC, etc.), one can also use a polyacrylic acid,
polyethylene glycol, polyvinyl alcohol, etc. . . One can also use,
as the binder, a binder that is conventionally used as a compression
(or pressing) binder, rather than an extrusion binder, the terms
"compression (or pressing) binder" and "extrusion
binder" having their conventional sense known to the skilled
person. A preferred binder is crystalline, notably a microcrystalline
cellulose which will correspond in whole or in part to the binder.
The paste will for example contain between 2 and 10% by weight of
organic binder and preferably between 3 and 8% by weight.
The role of the solvent is to disperse the inorganic portion and
the binder. Where a water-soluble polymer is employed, water will
be selected as the solvent; where the polymer is not water-soluble,
an alcohol, for example ethanol, will be chosen as solvent. The
concentration of the solvent will be comprised between, for example,
8 and 40% by weight and, preferably, between 10 and 27% by weight.
The pore-generating agent is characterized by a low decomposition
temperature, for example less than 450.degree. C., preferably less
than 250.degree. C. It is additionally characterized by the mean
size of the particles composing it, said size being appropriately
related to the particle size of the metallic filler. This size is
for example comprised between 5 and 30 .mu.m and preferably between
8 and 16 .mu.m. The pore-generating agent is substantially insoluble
in the chosen solvent. A pore-generating agent of natural origin
can be used and, for example dust of husks, carbon black or powder,
or one of artificial origin such as for example low density polyethylene
spheres or a water/oil emulsion and for example mobilcer.RTM. (oil-in-water
emulsion).
The inorganic filler and pore-generating agent particle size can
vary independently of each other to a very high degree.
A deflocculating agent that is soluble in the solvent will improve
dispersion of the particles of the metal compound. Typically, a
polyacrylic acid, a phospho-organic or alkyl-sulfonic acid is chosen.
The deflocculating agent content is of the order of 0.5 to 1% by
weight.
In certain cases, an agent that aids extrusion such as a polyethylene
glycol will be added. The extrusion agent content is of the order
of 0.5 to 1% by weight.
These components are mixed in the form of a paste having a capacity
of being drawn comprised in general between 9 and 30 bar, and preferably
between 10 and 16 bar.
Their bending strength can be modified conventionally by introducing
mineral binders into the composition of the paste, which will react
during sintering to increase the cohesive forces between the particles.
Shaping is carried out conventionally using extrusion. Using a
screw or piston, the paste is forced through a complex die in order
to adopt the die geometry. The membrane preforms are collected at
the outlet from the die, dried in free air in order to eliminate
water or solvent, and are then sintered at a temperature comprised
between 1300 and 1700.degree. C. for, for example, two hours.
Sintering is done under a normal or neutral atmosphere (for example
argon) where the paste is metallic oxide-based, and under a neutral
atmosphere (for example argon or helium) when the metallic compound
is a non-oxide.
The extrusion apparatus is conventional apparatus, specifically
comprising a die with, arranged at the center thereof, a crown supporting
the slugs which will form the channels. The fiber preforms obtained
at the outlet from the extrusion apparatus can be dried and/or sintered
in rotating barrels, for example using a technique described in
French Patent 2229313 in the name of Ceraver.
Molecular sieve
The molecular sieve according to the invention is conventional
and is notably a crystalline structure of the zeolite type. Zeolite
is for example a crystalline solid having a microscopic three-dimensional
structure resulting from the chaining of TO4 tetrahedra (T being
for example selected from Si, Al, B, Ga, Ge, and P), each oxygen
atom being common to two tetrahedra, leading to a network of channels
of molecular dimension (pore diameter varying for example between
3 and 10.ANG.). Structural types are for example FAU, GME, MOR,
OFF, MFI, MEL, FER, LTA, TON and CHA, according to IUPAC nomenclature.
The molecular sieve can also be an oxide of the metallosilicate
type, a portion of the above T elements being replaced, for example
by titanium (for example titanosilicate, such as TS-1), manganese,
molybdenum, gallium (for example a GAPO (gallophosphate)) boron,
zinc, iron and tungsten. The molecular sieve can also be a diatomaceous
earth, a crystalline alumina phosphate (ALPO) or a crystalline silicoaluminophosphate
(SAPO). One particular example of the molecular sieve is the ZSM
zeolite (in particular ZSM-5) or silicalite. The description which
follows refers to a zeolite (in particular silicalite), but can
extend to all molecular sieves to which the invention applies.
The layer thickness is comprised for example between 1 and 100
.mu.m, preferably between 3 and 50 .mu.m, for the portion situated
on the outside of the carrier. Some is in general infiltrated into
the carrier, to a thickness comprised for example between 0.2 and
10 .mu.m, preferably between 0.5 and 5 .mu.m.
This zeolite layer has particular features, notably regarding homogeneity,
in the sense that the composition of the deposited layer is not
modified by a parasitic phenomenon such as gelification or dissolution
of part of the membrane. These characteristics of the zeolite layer
are the following. The zeolite layer is, in the invention, obtained
preferably in a single "deposition-crystallization" step.
One thus obtains a single layer. One can however also proceed using
several steps; but at each step, a "unitary" layer is
deposited having the characteristics listed below. Each "unitary"
layer (or, if appropriate, single layer) is said to have "zero-defect"
in the sense that there is no break in the macroscopic three-dimensional
structure. (The term "macroscopic three-dimensional structure"
is used in contrast to the term "microscopic three-dimensional
structure" which designates the molecular level). This is brought
to light by the test consisting of permeating (or attempting to
permeate) a gas into the dimension corresponding to the characteristic
dimension of the molecular sieve (this dimension being, if need
be, weighted taking account of the carrier dimension, notably for
carriers having high pore diameter, for example greater then 10
microns). In the present case (in the case of the carrier having
a pore dimension for example less than 10 microns) the test gas,
for silicalite, is SF.sub.6 ; the present unitary layer is gas-tight
for this gas. As against this, the gas nitrogen passes through the
layer. In the case of a carrier of 12 micron size, there is also
formation of a crystalline network forming, "without a break"
on the carrier, meaning the layer is homogeneous and is free of
defects, SF.sub.6 -tightness being obtained in this case using two
passes.
The unitary zeolite layer thickness is in general constant at plus
or minus 20% preferably 10%, on the carrier.
Membrane and module according to the invention
The membrane is characterized by a zeolite layer, present for example
on the inner surface of the fiber channels, a (small) portion of
this layer being infiltrated into the porous carrier. This layer
can also be located on the outside of the fiber.
The invention makes it possible to obtain localization of the zeolite
with respect to fiber geometry, specifically in the form of a unitary
layer and no longer in the thickness of the fiber, this unitary
layer having "zero-defect".
The membrane is also characterized by the characteristics of the
zeolite layer deposited, these characteristics being indicated above.
One of the advantages of the invention is that it makes it possible
to obtain highly compact modules by using membranes comprising porous
ceramic (micro)fibers with a zeolite layer.
A further advantage of the invention resides in the high separation
and permeability performance of the membrane. In effect, the present
membrane offers high permeability thanks to the characteristics
of the carrier and the small thickness of the zeolite layer, and
efficient separation thanks to the "zero defect" layer.
The membrane according to the invention has applications in the
fields of fluid, gas or liquid separation, pervaporation, reverse
osmosis or catalysis.
Some examples of gases that can be separated are: n- and iso-hydrocarbons
having 4 to 8 carbon atoms; xylenes; CH.sub.4 /N.sub.2 and CH.sub.4
/CO.sub.2. In the case of gas separation, one embodiment has proven
to be useful. In this embodiment, the zeolite is on the outer surface
of a fiber (preferably single-channel). The gas to be treated thus
permeates from the outer to the inner of the fiber; the gas is injected
at the side of the cartridge. This allows having a tight seal at
the potting: since the potting will be in contact with the outer
surface of the fiber bearing the zeolite, no gas will be able to
permeate through the thickness of the porous support that would
otherwise be free at the potting level. Also, having the zeolite
at the outer surface allows having higher pressures (up to 100 bar),
since the fiber exhibits high compression resistance. Higher pressures
mean higher flowrates and improved yields. Preparation process
The present invention provides a method for preparing a membrane
comprising a porous ceramic carrier and a layer of a molecular sieve,
comprising the steps of: (i) preparing an inert solution and a precursor
solution of the molecular sieve; (ii) impregnating the porosity
of the carrier with the inert solution; (iii) bringing the region
of the ceramic carrier intended to receive the layer of a molecular
sieve into contact with the precursor solution, and bringing the
region of the carrier not intended to receive the layer of a molecular
sieve into contact with the inert solution; and (iv) forming a molecular
sieve in situ.
First, the precursor solution is prepared containing the zeolite
precursor species and the structuring agent, said species and agent
being determined as a function of the final zeolite it is desired
to obtain. In the case of a silicon-based zeolite, i.e. a silicalite,
this solution contains silicon in the form of micronized silica
or silicon alkoxide and, optionally, supplementary metallic species
in the form of salts or alkoxides. Additionally, this solution contains
an organic base such as a quaternary ammonium hydroxide and in particular
ammonium tetrapropyl hydroxide, tetramethyl hydroxide or tetrabutyl
hydroxide, or a mixture of an inorganic base such as soda and an
ammonium tetraalkyl halide such as ammonium tetrapropyl bromide.
The solution obtained is generally left to rest for a duration comprised
for example between 1 and 96 hours, and preferably between 12 and
72 hours in order to lead to the desired precursor solution. The
precursor solution can notably be the one described in international
application WO-A-9529751. This solution is indicated to be an oligomer
solution, the precursor elements being of small size, for example
of nanometer order. This solution, preferably, does not contain
a strong base.
In parallel, an inert solution is prepared; this solution is generally
(distilled) water.
During steps (ii) and (iii), the precursor solution and the carrier
are brought into contact so as to constitute a solution volume which
will produce the zeolite. For this, the fiber is introduced into
a glove finger (for example of PTFE) placed in an autoclave.
According to a first alternative embodiment, the porosity of the
carrier is impregnated with the inert solution; and then, as a function
of the desired structure of the zeolite membrane (in other words
the position of the molecular sieve with respect to the fiber geometry)
hydrothermal synthesis is performed using one or the other of the
alternatives described below: (i) If the zeolite layer is situated
on the outside of the fiber, this embodiment comprises firstly,
filling the volume of said channel(s) with the inert solution and
secondly filling a volume surrounding the outside of the carrier
with the precursor solution, this latter volume being the free volume
of the glove finger. (ii) If the zeolite layer is situated on the
inside of the fiber channels, this embodiment comprises firstly
filling a volume surrounding the outside of the carrier with the
inert solution, this latter volume being the free volume of the
glove finger and then secondly, filling the volume of the channel(s)
with the precursor solution.
According to a second alternative embodiment, the porosity of the
carrier is impregnated with the precursor solution and then, as
a function of the desired structure for the zeolite membrane (in
other words the position of the molecular sieve with respect to
the fiber geometry) hydrothermal synthesis is performed using one
or the other of the alternative embodiments described below: (i)
If the zeolite layer is situated in and on the outside of the fiber,
this embodiment comprises firstly, filling the volume of said channel(s)
with the inert solution and secondly filling a volume surrounding
the outside of the carrier with the precursor solution, this latter
volume being the free volume of the glove finger. (ii) If the zeolite
layer is situated in and on the inside of the fiber channels, this
embodiment comprises firstly filling a volume surrounding the outside
of the carrier with the inert solution, this latter volume being
the free volume of the glove finger and then secondly, filling the
volume of the channel(s) with the precursor solution.
In the above, the steps of impregnation with various solutions
can be concomitant or sequential (when dealing with the same solution).
For example, in the first case of the first alternative embodiment,
the porosity of the carrier can be impregnated and the channels
filled with the inert solution either simultaneously or sequentially.
The impregnation of the porosity with the precursor solution can,
if desired, be only partial. The molecular sieve will then only
be present in the impregnated region. As a function of the "depth"
of impregnation (total or partial impregnation) or lack of impregnation,
of the porosity of the carrier with the precursor solution, the
location of the molecular sieve can be adjusted with precision.
Step (iv) in the formation of the molecular sieve generally comprises
hydrothermal synthesis followed by calcinating.
Hydrothermal synthesis is generally done at a temperature comprised
between 150 and 250.degree. C., for a duration of 12 to 96 hours.
The preferred conditions are a temperature comprised between 170
and 220.degree. C. and a duration comprised between 48 and 84 hours.
After rinsing, generally using water, the membrane is calcined
to eliminate the residual structuring agent. Calcinating is done
in general at a temperature comprised between 300 and 900.degree.
C. and a duration from 2 to 5 hours; preferably, the calcinating
conditions are a temperature of 400 to 600.degree. C., notably 500.degree.
C. and a duration of about three hours. Calcination may or may not
be oxidizing.
The conditions described in international application WO-A-9529751
are similarly suitable.
The present process makes it possible to provide a ceramic membrane
having good gas separation properties in a single deposition-crystallization
cycle.
The present method further offers the advantage of controlling,
firstly, the inherent properties of the zeolite, such as channel
size, acidity, hydrophobicity and, secondly, the properties of the
composite material such as the location of the zeolite at a particular
zone of the fiber geometry, characteristic of the zeolite deposit.
A further advantage of the present method is that of reduced manufacturing
cost made possible firstly through the use of a low-cost porous
carrier and secondly thanks to a limitation of the solution volumes
employed, and finally thanks to an improved yield of the zeolite
deposit. Indeed, the confinement of the reagents in the various
compartments makes it possible to reduce solution volumes and consequently
membrane manufacturing cost (the precursor solution comprising expensive
reagents, notably, for example, the ammonium tetrapropyl hydroxide
solution). Additionally, confinement of the solutions brings about
an increase in crystal deposit yield, in other words in the ratio
between the mass of molecular sieve deposited on the carrier fiber
(and to a lesser degree in the carrier porous fiber) and the mass
of the molecular sieve able to be produced from the volume of solution.
The method according to the invention is applied to the preparation
of membranes according to the invention, but also to any type of
membrane. Notably, the method according to the invention applies
to non-symmetrical carriers.
The following examples illustrate the invention without limiting
it.
In the examples below, a single-channel fiber is employed obtained
as follows:
A paste is prepared constituted by alumina (mean particle size
3 .mu.m), microcellulose, ethylcellulose, low density polyethylene
particles (mean particle size 15 .mu.m) and water, with the following
composition in % by weight:
Alumina 69.7 Microcellulose 3.5 Ethylcellulose 0.3 Low density
polyethylene 7 Water 19.5
The paste thus obtained had a drawability of 15. It was extruded
through a hollow fiber die so as to form a 1.5 mm outside diameter
and 0.8 mm inside diameter tube. The fiber thus obtained was fired
at 1550.degree. C. in a normal atmosphere.
EXAMPLE 1
An aqueous solution was prepared by mixing 60 g silica of the Aerosil
380 type from Degussa with 1000 ml of tetrapropyl ammonium hydroxide
(TPAOH) molar solution. The solution was left to stand overnight.
The porosity of the fiber was impregnated with the distilled water
and the channel was filled with the silica solution. The fiber was
placed in a PTFE glove finger of 1 cm diameter filled with distilled
water, and the whole thing was placed in an autoclave; hydrothermal
synthesis was carried out at 180.degree. C. over 3 days. The fiber
was then rinsed and then calcined at 500.degree. C. for 5 hours.
The mass of zeolite deposited was 20 mg equivalent to a yield of
80% with respect to the mass of silica. Microscopic observation
showed a layer deposited on the inner surface of the channels which
was continuous and homogeneous the thickness of which was about
4 .mu.m, the layer being infiltrated down to about 2 .mu.m into
the fiber.
The nitrogen permeability of this membrane was 4 Nm.sup.3 /h.m.sup.2.bar.
EXAMPLE 2
The solution of example 1 was employed. The porosity of the fiber
was filled with the distilled water as were the channel. The fiber
was then placed in a glove finger of 0.2 cm diameter containing
the precursor solution. The whole thing was placed in an autoclave
and hydrothermal synthesis was carried out at 200.degree. C. for
55 hours. The fiber was rinsed and then calcined at 500.degree.
C. for 5 hours.
The mass of zeolite deposited was 190 mg equivalent to a yield
of 86% with respect to the mass of silica. Microscopic observation
showed an outer layer which was continuous and homogeneous the thickness
of which was about 28 .mu.m, and a portion that had infiltrated
about 1 .mu.m into the fiber.
The nitrogen permeability of this membrane was 0.7 Nm.sup.3 /h.m.sup.2.bar.
EXAMPLE 3
An aqueous solution was prepared by mixing 120 g silica of the
Aerosil 380 type from Degussa with 100 ml of a molar solution of
TPAOH. The solution was left to stand overnight. A ceramic tube
of .alpha.-alumina of 10 mm outside diameter and 7 mm inside diameter
was used, having a homogeneous structure the pore diameter of which
was 12 .mu.m. Initial nitrogen permeability of the ceramic tube
was 7400 Nm.sup.3 /h.m.sup.2.bar. The tube was placed inside a glove
finger. The tube channel was filled with distilled water and the
outside of the tube by the solution of silica and TPAOH. The complete
thing was placed in an autoclave and hydothermal synthesis was performed
at 190.degree. C. for 72 hours. The tube was then rinsed and calcined
at 500.degree. C. for 2 hours.
The nitrogen permeability of the thus-treated tube was 2500 Nm.sup.3
/h.m.sup.2.bar, sulfur hexafluoride permeability was 900 equivalent
to a selectivity of 2.8. This shows that it is possible to have
crystalline growth even for carriers with a very high pore dimension.
After repeating the process, the tube was sulfur hexafluoride-tight.
The fibers of the invention, assembled into modules, thus provide
a very high degree of compactness.
The invention is not limited to the embodiments described but may
be the subject of numerous variations readily accessible to the
person skilled in the art.
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