Molecular sieve abstract
A process for modifying surfaces of zeolites and molecular sieve
membranes to decrease effective pore size for separation of materials
includes atomic layer controlled vapor or liquid deposition. The
atomic layer controlled deposition process steps include (i) exposing
the surface to a metal atom coordinated with ligand groups having
bonds that are hydrolyzable to form molecular bonded structures
on the surface, which structures comprise the metal atoms coordinated
with the ligand group or a modified ligand group and then (ii) hydrolyzing
the bonds and possibly, but not necessarily, cross-linking the bonds
in the ligand or modified ligand group.
Molecular sieve claims
The embodiments of the invention in which an exclusive property
or privilege is claimed are defined as follows:
1. A process for modifying selective permeance of a crystalline
membrane with hydroxyl groups (OH) on its surface, comprising:
exposing the membrane to a reaction gas molecule having the formula
M(Z.sub.m L.sub.n), where M is a metal atom coordinated to ligand
group Z.sub.m L.sub.n, wherein said membrane has the formula Y-OII*,
wherein Y is a substrate atom and OH* is a surface hydroxyl group,
and wherein said reaction gas molecule M(Z.sub.m L.sub.n) has sufficient
volatility and reactivity to react with said membrane surface hydroxyl
group, and allowing a solid/vapor phase chemical reaction to proceed
generally as follows:
where Y--O--M(Z.sub.m-1 L.sub.n)is a bonded coordinated group that
results from reaction (1), M(Z.sub.m-1 L.sub.n) coordinated is a
resulting group comprising the metal atom (M) and ligand group Z.sub.m-1
L.sub.n, wherein Z.sub.m-1 L.sub.n is capable of being hydrolyzed
by water, and ZH.uparw. is a gaseous by-product of the reaction
(1); and
exposing the surface of the membrane and the bonded Y--O--M(Z.sub.m-1
L.sub.n) coordinated group that results from reaction (1) to water
(H.sub.2 O) molecules to hydrolyze M(L) bonds according to a hydrolyzing
reaction generally as follows:
where M(Z.sub.m-1 L.sub.n)* is the group on the membrane surface
comprising the coordinated metal atom (M) and ligand group (Z.sub.m-1
L.sub.n), M--OH* is a resulting group comprising the metal atom
M coordinated with a hydroxyl group (OH) formed on the surface of
the membrane, and Z.sub.m-1 L.sub.n H.uparw. is a gaseous by-product
of the hydrolyzing reaction (3), wherein a monomolecular layer of
said M--OH* is deposited on the surface of said membrane.
2. The process of claim 1 including continuing the step of exposing
the membrane to the metal atoms M coordinated with the ligand Z.sub.m
L.sub.n until substantially all available OH* is reacted according
to reaction (1) and the reaction (1) substantially stops.
3. The process of claim 2 including continuing the step of exposing
the surface of the membrane and the bonded Y--O--M(Z.sub.m-1 L.sub.n)
coordinated group of water (H.sub.2 O) molecules to hydrolyze M(Z.sub.m-1
L.sub.n) bonds until substantially all available M(Z.sub.m-1 L.sub.n)
bonds are hydrolyzed.
4. The process of claim 3 including the step of condensing OH
groups of adjacently deposited molecules from reaction (3) to form
a cross-linked monomolecular layer of coordinated groups comprising
at least two metal atoms M bonded with a common oxygen O atom and
coordinated with at least two other oxygen O atoms that are bonded
with substrate atoms Y.
5. The process of claim 1 wherein said membrane is zeolite, said
Y atoms are silicon Si, said M(Z.sub.m L.sub.n) is trimethylaluminum
Al(CH.sub.3).sub.3 and said reaction (1) proceeds generally as:
and said reaction (3) proceeds generally as:
6.
6. The process of claim 5 including continuing the step of exposing
the membrane to the Al(CH.sub.3).sub.3 until substantially all available
OH* is reacted according to reaction (2) and the reaction (2) substantially
stops.
7. The process of claim 6 including continuing the step of exposing
the surface of the membrane and the bonded Si--O--Al(CH.sub.3) coordinated
group to water (H.sub.2 O) molecules to hydrolyze Al--CH.sub.3 bonds
in the Al(CH.sub.3) groups until substantially all available Al--CH.sub.3
bonds are hydrolyzed.
8. The process of claim 7 including the step of condensing the
OH groups of adjacently deposited molecules from reaction (4) to
form a cross-linked monomolecular layer of coordinated groups comprising
at least two Al atoms bonded with a common oxygen O atom and coordinated
with at least two other oxygen O atoms that are bonded with silicon
Si substrate atoms.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to membrane structures for separation of
materials on a molecular scale, such as selective sorption, molecular
sieving, and the like, and more particularly to modification of
such membranes by atomic layer controlled chemical vapor deposition
to increase effectiveness of the membrane for materials separation,
or other purposes.
2. Description of the Prior Art
Materials, such as gases or liquids, can be separated from other
different materials on the molecular level by a number of techniques.
Some well-known examples include evaporation and condensation, which
is used to distill alcohol, electrolysis, which is used to remove
chromium from solution and plate it onto parts, and crystallization,
which is used to purify salts, drugs, or other substances from contaminated
mixtures or solutions. More relevant to this invention, thin membranes
of some solid materials that have very small pores are permeable
to certain gases or liquids comprising small molecules, and impermeable
to other gases or liquids that may comprise larger molecules, different
shaped molecules, molecules with different polarizability, different
absorption properties, or combinations of any of these factors.
In industrial applications, organic polymer membranes are used,
for example, to separate substances such as ethanol from water.
Also of interest, some solid materials are adsorbents for certain
gases or liquids. Some zeolites, such as 4A, are more adsorbent
of molecular nitrogen (N.sub.2) than of molecular oxygen (O.sub.2)
at increasing pressures and desorb the nitrogen when pressure is
decreased. This adsorption/desorption cycle is used in combination
with reversing flow directions to separate nitrogen gas (N.sub.2)
from oxygen (O.sub.2) in air.
Recognizing how such molecular separation materials and mechanisms
work, it is, of course, desirable to improve them to make them more
effective and more efficient for their material separation functions.
For example, in the U.S. Pat. No. 3837500 issued to Nichols et
al., a process is described for reticulating or cross-linking an
organic porous polymer membrane comprising ultrathin layers of polyvinyl
alcohol and polyvinyl pyrrolidone with diisocynate reticulation
agent to decrease and control the pore size of the membrane for
more effective water/salt separation.
Zeolites are microporous crystalline alumina silicates with a narrow
distribution of pore sizes on a molecular scale, and they have high
thermal, chemical, and mechanical stabilities. Molecular sieves
can be, for example, alumina phosphates (ALPO) or silicaaluminaphosphates
(SAPO), which are also microporous, crystalline materials with a
narrow distribution of pore sizes and also have high thermal, chemical,
and mechanical stabilities. Therefore, zeolites and molecular sieves
can be used not only for gas separations in adsorption/desorption
processes, as mentioned above, but also as diffusion membranes when
prepared in thin film form. The size and adsorption properties of
the zeolite pores, however, limit what can be separated with a particular
type of zeolite membrane, even if the crystalline structure is perfect
and defect free. However, perfect and defect free zeolite crystalline
structures are not readily available or easy to prepare, so most
zeolite materials have defects and separations or spaces between
crystals, which can be larger than the pore sizes in the crystalline
structures. Therefore, transport of molecules by diffusion can take
place both within the zeolite crystals and between adjacent crystal
faces. Since the spaces between adjacent crystal faces can be larger
than the pores in the zeolite crystals themselves, it is very difficult
to produce zeolite membrane types with good separation capabilities,
and only a few have been prepared prior to this invention.
There is a substantial need for better and different zeolite membranes.
For example, the sequential adsorption/desorption pressure swing
cycles mentioned above for separating gases require a high capital
investment for any significant volume production in industrial plants,
and such systems have high maintenance costs. A gas separation system
that takes advantage of both the adsorption and diffusion properties
of zeolites, but operates at steady state rather than pressure cycling
would be much more efficient, but such a system would require good,
effective, and efficient continuous zeolite membranes.
In recent years, thin, dense layers of zeolites have been prepared
both in self-supporting thin film membrane structures and on macroporous
substrates or supports. See, for example: W. J. W. Bakker et al.,
"Single and Multi-Component Transport Through Metal Supported
MFI Zeolite Membranes," Precision Process Technology, Eds.
M. P. C. Weiznen and A. A. H. Drinkenburg, Lluwer Academic Publishers,
1993 page 425; W. J. F. Bakker et al., "Doorbreak in Ontwikkeling
Zeolietmembranen, in Dutch (English translation of title: Break-Through
in Development of Zeolite Membranes," Proces Technologies,
Vol. 3 December 1993 page 7; M. Jia et al., "Ceramic-Zeolite
Composite Membranes and Their Application for Separation of Vapor/Gas
Mixtures," J. Membr. Sci., Vol. 90 1994 page 1; Y. Yan et
al., "Zeolite ZSM-5 membranes grown on porous .alpha.-Al.sub.2
O.sub.3" J. Chem Soc. Chem. Commun., 1995 page 227; T. Sano
et al., "Potentials of Silicalite Membranes for Separation
of Alcohol/Water Mixtures," Studies in Surface Science and
Catalysis, 1994 vol. 84 page 1175; J. Tsikoyiannis et al., "Synthesis
and characterization of a pure zeolite membrane," Zeolites,
February 1992 vol. 12 page 126; E. Wu et al., "Hydrocarbon
adsorption characterization of some high silica zeolites,"
Studies in Surface Science and Catalysis, 1986 vol. 28 page 547;
and S. Xiang et al., "Formation and characterization of zeolite
membranes from sols," 3rd International Conference on Inorganic
Membranes, Worcester, Mass., Jul. 10-14 1994. Hydrothermal synthesis
with aqueous solutions of zeolite precursors is the most widely
used method to form the membrane layer. See, for example: M. Jia
et al., supra,; T. Sano, supra; E. Geus et al., "High temperature
stainless steel supported zeolite (MFI) membranes: preparation,
module construction, and permeation experiments," Microporous
Materials, 1993 vol. 1 page 131; Y. Yan et al., "Preparation
of zeolite ZSM-5 membranes by in-situ crystallization of porous
.alpha.-Al.sub.2 O.sub.3 Ind. Eng. Chem. Res., 1995 vol. 34 pages
1652-1661; Y. Yan et al., "Zeolite ZSM-5 Membranes Grown on
Porous .alpha.-Al.sub.2 O.sub.3" J. Chem. Soc., Chem. Commun.,
1995 pages 227-228. Other techniques for forming zeolite membranes
may include: (i) treating a dry layer of zeolite precursors in steam,
N. Nishiyama et al., "A defect-free modernite membrane synthesized
by vapor phase transport method," J. Chem Soc. Chem. Commun.,
1995 page 1967; (ii) embedding zeolite crystal in a metal matrix,
P. Kolsch et al., "Zeolite-in-metal membranes: Preparation
and testing," J. Chem. Soc. Chem. Commun., 1994 vol. 21 page
2491; and (iii) sintering zeolite crystals to form a dense layer,
C. Engelen et al., "Membrane for separation of small molecules
and its manufacture," PCT Int. Appl. WO 93/19841 1993 to
ECN. Such zeolite membranes prepared by these processes have shown
a good potential for the separation of molecules that have sufficient
differences in their respective adsorption and diffusion behavior.
Other commercially important separations, such as nitrogen from
carbon dioxide (N.sub.2 /CO.sub.2) or nitrogen from methane (N.sub.2
/CH.sub.4) have not been obtained with these zeolite membranes,
most likely because the CO.sub.2 and CH.sub.4 molecules are small
and can easily enter and permeate through the regions between the
zeolite crystals. It is noteworthy that polycrystalline silicalite
membranes have been modified with silane coupling reagents to improve
pervaporation performance with aqueous ethanol solutions. The modification
was performed from the liquid phase with a large alkyltrichlorosilane
molecule (alkyl=C.sub.8 H.sub.17 and C.sub.18 H.sub.37). The coupling
agents were dissolved in an inert solvent. The authors concluded
that the change in permeation properties was caused by an altered
hydrophobicity of the external membrane surface. See T. Sano et
al., "Improvement of the pervaporation performance of silicalite
membranes by modification with a silane coupling reagent,"
Microporous Materials, 1995 vol. 5 pages 179-184. However, the
coupling agents used were longer molecules that were not able to
enter the zeolite pores and may be too large to enter gaps between
crystals. Also, diffusion and transport in the fluid phase is different
than in the gas or vapor phase, so different membrane regimes should
be accessed at different rates.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of this invention to provide
a process by which improved molecular sieves and altered membranes
can be fabricated for separation of materials in a mixture that
have different molecular properties, such as size, shape, or adsorption
characteristics.
It is another object of this invention to provide a method by which
passages through zeolite or molecular sieve membranes can be reduced
in a very controlled and effective manner for improved material
separation effectiveness from mixtures.
Still another object of the present invention is to decrease pore
sizes in zeolite or molecular sieve membranes.
A more specific object of this invention is to provide a method
for blocking or partially blocking regions between crystals in crystalline
membranes or molecular sieves to inhibit transfer of larger molecules
through the membranes or molecular sieves, but without substantially
inhibiting transfer of small molecules through pores in the crystalline
structures.
Another specific object of this invention is to improve zeolite
or molecular sieve membranes to effectively separate materials with
smaller molecules than has been feasible with some zeolites, such
as 4A.
Another specific object of this invention is to provide a process
for changing adsorption characteristics of zeolite or molecular
sieve membranes.
Another more specific object of this invention is to block or partially
block regions between zeolite crystals in zeolite membranes without
blocking or substantially affecting or decreasing effective pore
size and molecular transfer through the zeolite crystals.
Additional objects, advantages, and novel features of the invention
shall be set forth in part in the description that follows, and
in part will become apparent to those skilled in the art upon examination
of the following or may be learned by the practice of the invention.
The objects and the advantages may be realized and attained by means
of the instrumentalities and in combinations particularly pointed
out in the appended claims.
To achieve the foregoing and other objects and in accordance with
the purposes and objects of the present invention, as embodied and
broadly described herein, the method of modifying membranes according
to the present invention includes modifying a zeolite or other crystalline
membrane that has surface hydroxyl groups with an atomic layer controlled
chemical vapor deposition (ALC-CVD) process or atomic layer controlled
chemical liquid deposition (ALC-CLD) process. Both the ALC-CVD process
and the ALC-CLD process include (i) exposing the membrane to molecules
comprising a metal atom coordinated with ligand groups having bonds
that are hydrolyzable and allowing them to react with the surface
hydroxyl groups on the zeolite membrane or substrate to form molecular
bonded structures on the zeolite surface that comprise the metal
atoms coordinated with the ligand group or with a modified ligand
group that results from the reaction and bonded to oxygen atoms
that are bonded to zeolite substrate atoms (e.g., silicon atoms)
and then (ii) hydrolyzing and possibly, but not necessarily, cross-linking
the bonds in the ligand or modified ligand group. The resulting
monomolecular layer deposited on the zeolite surface has coordinated
groups of atoms that include (i) the metal atom bonded to oxygen
atoms that are bonded to the zeolite substrate atoms (e.g., silicon
atoms) and (ii) either hydroxyl groups bonded to the metal atoms
or additional oxygen atoms bonded to the metal atoms. These coordinated
groups of atoms that comprise the deposited monomolecular layer
modify the surface of the zeolite by reducing effective sizes of
defects in substrate crystalline structures as well as of gaps or
separations between adjacent crystalline structures that comprise
the zeolite membrane while preferably, but not necessarily, modifying
substantially the effective pore sizes in and through the zeolite
crystals. However, if desired, one or more additional layers can
be deposited substantially in the same manner as described above,
which will substantially modify and possibly even partially or entirely
block the pores in the zeolite crystals.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a
part of the specifications, illustrate the preferred embodiments
of the present invention, and together with the descriptions serve
to explain the principles of the invention. In the Drawings:
FIG. 1 is an enlarged diagrammatic cross-section of a surface of
a crystalline membrane illustrating separate crystal structures
separated between juxtaposed faces or surfaces;
FIG. 2 is a diagrammatic representation and illustration of the
hydroxyl (OH) groups or sites on the surface of a material in a
specific preferred reaction of the reaction gas Al (CH.sub.3).sub.3
to produce intermediate structures that comprise the reactive atom
aluminum (Al) bonded to oxygen atoms remaining from the previous
hydroxyl (OH) sites and also bonded to one or more ligand (CH.sub.3)
groups;
FIG. 3 is a diagrammatic representation and illustration of hydrolyzing
the bond between the reactive atom (Al) and the ligand (CH.sub.3)
to produce monomolecular surface structures as described for FIG.
2 but with each CH.sub.3 ligand replaced with a hydroxyl (OH) group;
FIG. 4 is a diagrammatic representation and illustration of the
monomolecular layer composition after condensing to replace some
hydroxyl (OH) groups;
FIG. 5 is an enlarged isometric view of portions of adjacent crystals
illustrating diagrammatically in general terms the hydroxyl groups
(OH) or sites on the surfaces of the crystals as volatile reaction
gas molecules coordinated with ligands (M(L.sub.3)) are introduced;
FIG. 6 is an enlarged isometric view of the adjacent crystals of
FIG. 5 illustrating in general terms the intermediate molecular
structures after reaction with the gas molecules;
FIG. 7 is an enlarged isometric view of the adjacent crystals of
FIG. 6 as the second step of exposing to molecular structures of
FIG. 6 to water vapor (H.sub.2 O) begins;
FIG. 8 is an enlarged isometric view of the adjacent crystals of
FIG. 7 illustrating in general terms the new hydroxyl groups (OH)
formed on the molecular structures of FIG. 7; and
FIG. 9 is an enlarged isometric view of the adjacent crystals of
FIG. 8 further modified by condensation to produce continuous, cross-linked
monomolecular layers on the surfaces.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An enlarged cross-sectional view of a portion of a crystalline
membrane or molecular sieve 10 is illustrated diagrammatically in
FIG. 1 with a monomolecular layer 20 deposited or grown on the top
surfaces 14 and side surfaces 16 18 of the crystals 12 to modify
the permeability of the membrane or molecular sieve 10 for gases
according to this invention in order to improve the capability of
the membrane to separate mixtures. For purposes of facilitating
the description and explanation of this invention, the illustrations
in the drawings are diagrammatic representations of crystalline
and molecular structures. They are conceptual representations based
on the inventors' understandings and to some extent guesses about
the chemical and physical structures, since there are no direct
microscopy techniques to provide actual images of the structures
involved. Consequently, these diagrammatic representations only
facilitate explanations, but are not intended to provide accurate
depictions, dimensions, or proportions and, more importantly, are
not intended to limit the scope of the invention or of this patent
in any way. At the same time, it is important to note that membranes
can be and have been modified by the process of this invention,
regardless of how the structure actually looks on the atomic scale.
The membrane materials 10 of which this invention is particularly
applicable are crystalline substances, such as zeolites, and molecular
sieves, such as ALPO'S, SAPO's, and MCM41 that are characterized
by pores of molecular dimensions and fairly uniform size that have
the ability to adsorb and/or pass small molecules but not large
ones. Zeolites are one type of such materials and will be used in
the exemplary description of this invention, although this invention
is not limited to zeolites. Zeolites are naturally occurring and
synthetic crystalline substances, composed primarily of silicon,
aluminum, and oxygen and often found in nature. They are naturally
porous, very uniform crystals 12. Silicalite is a particularly useful
zeolite that comprises very uniform, well defined crystal structures
of silicon dioxide (SiO.sub.2), which can be fabricated into membrane
structures.
As illustrated in FIG. 1 the zeolite membrane structures usually
comprise a plurality of crystals 12 which, as mentioned above,
are porous in the crystal lattice. Small molecules, such as gaseous
nitrogen (N.sub.2), can enter and pass through the porous crystal
lattice, while larger molecules, such sulfur hexafluoride (SF.sub.6),
can hardly enter or pass through the crystal lattice of the crystals
12. However, there are defects in individual crystals as well as
microscopic gaps between individual crystals. For purposes of illustration,
the gap or region 22 between the juxtaposed faces 16 18 of adjacent
crystals 12 in FIG. 1 represents diagrammatically such defects and
gaps. Such regions 22 can and do provide conduits through which
larger molecules can pass, thus inhibiting the effectiveness of
the membrane 10 for separation of larger molecules from smaller
molecules.
On a smaller level, even though the size of pore openings 24 in
zeolite and molecular sieve crystals 12 of, for example, (MFI) zeolites,
such as silicalite or ZSM-5 is comparable in size (ovular 0.51.times.0.57
nm) to small organic molecules and are interconnected by a zig-zag
system of straight channels 26 with nearly circular cross-sections
of about 0.54 nm diameter, membranes 10 made of such zeolite materials
are not effective in such commercially important separations as
nitrogen gas from carbon dioxide gas (N.sub.2 /CO.sub.2) or nitrogen
gas from methane gas (N.sub.2 /CH.sub.4). Apparently, smaller molecules,
CO.sub.2 and CH.sub.4 can enter easily and permeate through the
pore channels 26 along with the nitrogen gas molecules.
To alleviate these problems of larger molecules passing through
the gaps 22 between crystals 12 and some significant smaller molecules
from passing through pore channels 26 in the lattice of the crystals
12 according to this invention one or more layers 20 of additional
structures are formed on one or more of the surfaces 14 16 18
of the crystals 12 (such layers 20 can also be formed on the bottom
or other surfaces, which are not shown), which layers 20 are also
porous enough to allow entry and passage of smaller molecules through
the crystal lattice, but which otherwise substantially block or
clog the gaps 22 between the crystals 12 enough to inhibit entry
or diffusion of larger molecules into and through such gaps 22.
Such layer(s) 20 can also decrease the effective size of the pore
openings 24 or otherwise affect adsorbent properties of the crystal
12 surfaces to inhibit migration of some smaller or strongly adsorbing
molecules through the pore channels 26 in the crystal 12 lattice.
Therefore, while the layer(s) 20 cover the top surface 14 as well
as other exposed surfaces 16 18 sufficiently to block or inhibit
larger molecules and even some smaller molecules from flowing through
the membrane 10 such layer(s) do not substantially inhibit entry
of the smallest molecules, such as nitrogen, helium, or hydrogen,
into the pores in the crystals 12. To obtain this feature according
to this invention, the lattice layer(s) 20 have to be kept very
thin--on the order of only a few (1 to 10) angstroms. Even with
such thin lattice layer(s) 20 the fluxes do decrease for all molecules,
but for some more than others. One plausible explanation for this
observation could be that some pores may become totally blocked,
although this explanation is not essential to the invention or to
the practice of this invention.
Conventional thin film deposition processes, such as chemical vapor
deposition (CVD) and chemical liquid deposition (CLD), are very
difficult, if not impossible, to control sufficiently to achieve
the surface modifications required for such separations as those
discussed above, at least on a predictable and dependable basis.
Therefore, according to this invention, atomic layer controlled
chemical vapor depositions (ALC-CVD) can be used to modify the surfaces
14 16 18 of crystalline membranes 10 such as, for example, but
not for limitation, the zeolite crystals 12 illustrated in FIG.
1. ALC-CVD as used in this invention is based on reaction of volatile
molecules with hydroxyl (OH) groups 30 on the surfaces 14 16 18
which are illustrated in FIGS. 2 and 5 to form O--M bonds, where
O is oxygen and M can be any one or more metal atoms, for example,
aluminum (Al), silicon (Si), germanium, (Ge), indium, (In), cadmium
(Cd), and the like, coordinated with ligands (L), such as a alkyl
group (C.sub.n H.sub.2n+1), chlorine (Cl), alkoxy group (O--C.sub.n
H.sub.2n+1), and the like. Any molecules (i) according to this specification,
(ii) that are volatile and reactive enough to react from a gas phase
with surface OH-groups, and (iii) that result in an attached layer
that has remaining ligands that can be hydrolyzed with water to
form new OH-groups are suitable for purposes of this invention.
At least one ligand is necessary if it is desired to form one monomolecular
layer and at least two ligands are necessary if it is desired to
form two or more monomolecular layers.
After completion of this first reaction, the resulting M--L bonds
(chemical bonds between the reactive atoms M and the ligands L)
are hydrolyzed with water (H.sub.2 O) vapor to form new hydroxyl
(OH) groups. The new hydroxyl (OH) groups of adjacent molecular
formations can condense to form a cross-linked monomolecular layer.
These reaction steps are illustrated and described below in more
detail. FIGS. 2-4 illustrate in a graphical manner some exemplary
chemical reactions used according to this invention to modify zeolite
membranes 10 while FIGS. 5-9 illustrate diagrammatically in more
general or generic terms and in three-dimensional views surface
molecular layer formation phases according to these atomic layer
controlled chemical vapor deposition (ALC-CVD) chemical reactions.
While surface modification of zeolite crystal surfaces based on
the presence of hydroxyl (OH) groups on the crystal surfaces is
illustrated in FIGS. 2-9 other crystalline membranes or molecular
sieve materials having surface hydroxyl groups or other reactive
groups, such as NH.sub.2 and SH, can also be accomplished according
to this invention.
Referring now primarily to FIGS. 2 and 5 the top and side surfaces
14 16 18 as well as the bottom surface (not shown), of the zeolite
crystals 12 have a natural abundance of hydroxyl (OH) group sites
30. Some zeolites, such as those that contain some aluminum (Al)
in the crystal 12 lattice, have a high density of surface hydroxyl
(OH) groups 30 while others, especially silica-rich zeolites, such
as, silicalite, have fewer surface OH-groups 30. Usually, however,
even in silica-rich zeolites, there are enough crystal defects that
have OH-groups in sufficient numbers and densities to accommodate
the above-described chemical reactions required for atomic layer
controlled chemical vapor deposition (ALC-CVD) for purposes of this
invention.
In the first step, the volatile, reactive, gaseous compound denoted
generically as M(Z.sub.m L.sub.n) (more specifically for simplicity
of illustration--M(L)--in FIG. 5) and with the exemplary compound
trimethylaluminum Al(CH.sub.3).sub.3 in FIG. 2 is introduced into
a vacuum chamber (not shown) containing the zeolite membrane 10.
The volatile M(Z.sub.m L.sub.n) molecules 32 react with the hydroxyl
(OH) sites 30 on the exposed surfaces 14 16 18 of the zeolite
crystals 12 to form O--M bonds that are coordinated with ligands
according to the reaction expressed in general terms as:
where Y is the substrate material, OH* is the surface hydroxyl
site, M(Z.sub.m L.sub.n) is the reaction gas molecule, M(Z.sub.m-1
L.sub.n) is the resulting coordinated reactive atom and ligand group,
and ZH.uparw. is the gaseous by-product of the reaction as illustrated
in FIG. 6 at 38. In general, there can be any number of ligands
and any combination of ligands for purposes of this invention. Also,
L could be the same ligands as Z, and Z.sub.m L.sub.n can comprise
one of a kind ligand or a combination of different ligand types
in different numbers. The Z.sub.m ligand can be a combination of
different ligands, and L.sub.n can also be a combination of different
ligands. The resulting intermediate crystalline structure, as illustrated
in FIG. 6 can be, for example, one metal atom (M) bonded to two
oxygens (O) and a ligand (L) as illustrated at 34 or one metal
atom (M) bonded to one oxygen (O) and two ligands (L) as illustrated
at 36. Of course, instead of just one ligand (L), there could also
be any combination of ligands Z.sub.m-1 L.sub.n as described above
and as will be understood by persons skilled in the art. However,
these alternatives are not illustrated in FIG. 6 because they would
add unnecessarily to the complexity of the drawing without contributing
substantially to clarity or the understanding of this invention.
The more specific example of this reaction (i), where Al(CH.sub.3).sub.3
is used as the reaction gas to produce O--Al bonds at the hydroxyl
(OH) sites is:
which produces the coordinated group comprising one aluminum (Al)
atom bonded with two oxygen (O) atoms and one CH.sub.3 ligand as
illustrated at 34' in FIG. 2 or the alternative group comprising
one aluminum (Al) atom bonded with one oxygen (O) atom and two CH.sub.3
ligands as illustrated at 36' in FIG. 2. The gaseous by-product
of this reaction (2) is methane gas (CH.sub.4). When all of the
hydroxyl (OH) sites are filled, the reaction stops automatically,
so it is self-limiting, which makes this process very conducive
to use in industrial applications.
After the initial reaction (1) or (2) described above, the M--L
bonds can be hydrolyzed with water (H.sub.2 O) vapor, as indicated
by FIG. 7 to form hydroxyl (OH) groups in place of the ligands
according to the reaction generally expressed as:
The more specific exemplary hydrolyzing reaction is:
This reaction is also self-limiting. When all of the sites are
hydrolyzed and ligands replaced with hydroxyl (OH) groups, as indicated
generally in FIG. 8 and for the exemplary reaction in FIG. 3 this
reaction also stops. The resulting bonded O--M--OH groups of either
one reactive atom M grouped with two oxygen (O) atoms and one hydroxyl
(OH) as illustrated at 38 in FIG. 8 or one reactive atom M grouped
with one oxygen (O) atom and two hydroxyls (OH) as illustrated at
40 in FIG. 8 form a monomolecular layer 20 on the surfaces 14 16
18 of the zeolite crystals 12 as also illustrated in FIG. 1. In
the exemplary reaction (4), the resulting bonded groups of one aluminum
(Al) atom grouped with two oxygen (O) atoms and one hydroxyl (OH)
as illustrated at 38' in FIG. 3 or one aluminum (Al) atom grouped
with one oxygen (O) atom and two hydroxyls (OH) as illustrated at
40' in FIG. 3 form the monomolecular layer 20 on the zeolite crystals
12 in FIG. 1. The gaseous by-product of the exemplary reaction (4)
described above is also methane (CH.sub.4) as illustrated in FIG.
3.
The hydroxyl (OH) groups of adjacently deposited molecules, such
as those illustrated at 38 and 40 in FIG. 8 can also condense to
form a continuous, cross-linked monomolecular layer as illustrated
at 42 in FIG. 9. It is possible, although not necessary to this
invention, for molecules deposited on very closely spaced juxtaposed
surfaces 16 18 of adjacent crystals 12 or in defects to also cross-link,
as illustrated at 44 in FIG. 9. The cross-linked molecular groups
of bonded O--Al--O--Al atoms resulting from condensed adjacently
deposited molecules in the exemplary reaction is illustrated in
FIG. 4. The by-product of such condensation is water (H.sub.2 O)
vapor.
Regardless of whether or how much such condensation actually occurs,
however, the resulting O--M--OH or O--M--O groups that form the
monomolecular layer 20 on the surfaces 14 16 18 of the zeolite
crystals 12 especially those formed on and over juxtaposed surfaces
16 18 of adjacent zeolite crystals 12 or defects in crystals 12
reduce the effective sizes of the separations or gaps 22 between
crystals 12 or defects in crystals 12 to an extent that improves
very substantially selectivity between species and the ability of
the zeolite crystals to block transport of larger molecules, thus
improving molecular separation or sieving capabilities of the zeolite
crystals 12 and of membranes made of such zeolite crystals 12. One
monomolecular layer deposited as described above may also have some
of the resulting O--M--OH or O--M--O groups formed around and over
pore openings 24 in the crystals 12. In general, as described above,
one such monomolecular layer will not substantially inhibit entry
or transport of the smallest gas molecules, such as nitrogen, helium,
or hydrogen into the pore openings 24 and through the pores 26
although larger gas molecules could be inhibited by such structures.
Since the resulting monomolecular layer 20 after the sequence of
reaction (1) or (2) followed by reaction (3) or (4), as described
above, leave hydroxyl (OH) sites 46 as illustrated in FIGS. 4
8 and 9 the same sequence of reaction (1) or (2) followed by reaction
(3) or (4) can be repeated to deposit an additional atomic level
layer (not shown) of the same resulting material, which will reduce
even further the effective pore openings 24 and separation or gap
22 between juxtaposed surfaces 1618. This sequence of reactions
can be repeated as often as desired to obtain the zeolite adsorption
and sieving characteristics desired. However, it has been found
that one of such sequences to produce one monomolecular layer 20
as described above is sufficient and preferred for general gas separation
purposes and that an additional sequence of these reactions to deposit
another layer can block the effective pore openings 24 too much
for practical usage, even for small gas molecules.
It is emphasized again that while the above description and associated
illustrations are explained mostly in terms of relative sizes of
pores, defects, and molecules, which is useful for visualization
and could even be quite accurate, it is not really known for sure
whether size, shape, polarizibility, adsorption properties, or some
combination of these or other characteristics are really affecting
the selective permeability of the membranes. Therefore, the process
of this invention, which does modify the permeability of the membranes,
might be causing modifications in any one or combination of these
factors.
EXAMPLES
Membrane Preparation and Characterization
Silicalite (zeolite) layers were synthesized hydrothermally from
a gel in much the same manner as described by Flanigen et al., "Silicalite,
a new hydrophobic crystalline silica molecular sieve," Nature,
1978 271 512 on the inside of commercially-available porous .gamma.-Al.sub.2
O.sub.3 tubes (US Filters, 5-nm pores). After two synthesis steps,
the membranes were washed, dried, and calcined at 733 K. The detailed
synthesis procedure for the silicalite is described by Funke et
al. 1995 Ind. Eng. Chem. Res. 1996 Vol. 35 page 1575 and Jia
et al. 1994 supra. The membranes were characterized by pure gas
permeation at room temperature and 150 kPa pressure difference between
feed and permeate side. Gases with different molecular sizes were
used as probes. Sulfur hexafluoride (SF.sub.6) has a kinetic diameter
of 0.55 nm, which is close to the pore size of silicalite and hardly
passes the zeolite pores, whereas small molecules such as He, N.sub.2
H.sub.2 or CH.sub.4 readily permeate through silicalite pores.
The pure gas permeance for N.sub.2 (kinetic diameter of 0.36 nm)
and SF.sub.6 and the pure gas permeance ratios of the unmodified
membranes are listed as the first row in Tables 1 and 2 below.
Atomic Layer Controlled Chemical Vapor Deposition (ALC-CVD)
The ALC-CVD depositions were performed in a standard high vacuum
setup with a dosing device to adjust the partial pressure of precursors
in the system. The modification of the zeolite membranes required
four treatment steps. First, an effort was made to remove surface
water from the membrane layer so that the Al(CH.sub.3).sub.3 would
only react with surface OH-groups. Heating to 523 K at 10.sup.-4
torr for several hours was thought to be sufficient for silicalite
membranes, but it was not possible to know for sure that all of
the surface water was actually removed. Because the deposition reaction
is sensitive to adsorbed organic compounds a cleaning step removed
adsorbed hydrocarbons. Treatment for 2 min. with H.sub.2 O plasma
at 0.20 torr at room temperature prior to each deposition was sufficient
to remove adsorbed molecules and allow the deposition. The plasma
was created by an electrical discharge in the system. The dried
and pretreated membranes were exposed to Al(CH.sub.3).sub.3 for
10 min. at 5 torr and 500 K. The progress of the reaction can be
monitored by analyzing the gas composition. At least one molecule
of CH.sub.4 is formed for each reacted Al(CH.sub.3).sub.3. If the
surface OH-density is high, one Al(CH.sub.3).sub.3 molecule can
react with more than one OH group, and more than one CH.sub.4 molecule
forms, as illustrated in FIG. 2 which effect was also shown by
Dillon et al., "Surface chemistry of Al.sub.2 O.sub.3 deposition
using Al(CH.sub.3).sub.3 and H.sub.2 O in a binary reaction sequence,"
Surface Science, 1995 211 2330. The membranes were exposed to
5 torr of water vapor at 500 K to hydrolyze the unreacted Al--CH.sub.3
bonds of the surface and form hydroxyl groups as shown in FIG. 3.
The hydrolysis step can also be accomplished by exposing the membrane
to ambient air that has some moisture content. The hydroxyl groups
of adjacently deposited compounds in close proximity can cross-link
by condensation to form a continuous, monomolecular Al.sub.2 O.sub.3
layer as illustrated in FIG. 4.
Water Plasma Treatment
The influence of the water plasma treatment on the permeation properties
of a zeolite membrane (HSil54) was determined by measuring N.sub.2
and SF.sub.6 permeances of membrane HSil54 before and after a plasma
treatment. The pure gas permeances of N.sub.2 and SF.sub.6 at room
temperature was 640.times.10.sup.-9 moles/(m.sup.2 s Pa) and 10.5.times.10.sup.-9
moles/(m.sup.2 s Pa) respectively for the untreated membrane with
a permeance ratio of 61. The pure gas permeances did not change
within experimental errors after the treatment (650.times.10-9 moles/(m.sup.2
s pa) for N.sub.2 and 11.times.10.sup.-9 moles/(m.sup.2 s Pa) for
SF.sub.6).
Al.sub.2 O.sub.3 -Layer Deposition
The changes of the single gas N.sub.2 and SF.sub.6 permeances through
silicalite zeolite membranes induced by depositing atomic layers
of Al.sub.2 O.sub.3 are listed in Table 1 and 2. Six silicalite
membranes with different initial permeation properties were characterized
before and after the deposition of 1 and 2 layers of Al.sub.2 O.sub.3.
The N.sub.2 permeances through membranes HSil50 HSil52 HSil54
and HSil56 decreased by 30-70% whereas the SF.sub.6 permeance decreased
by factors up to 17. Therefore, the pure gas permeance ratio for
N.sub.2 over SF.sub.6 increased for those four membranes. Membrane
HSil50 showed the best results with an increase in permeance ratio
by a factor of more than five to over 560. The permeance ratio in
the other three membranes increased by factors of two to three.
A second layer deposited on HSil52 and HSil54 further decreased
the N.sub.2 -permeance by factors of 10 to 40 whereas the SF.sub.6
permeance only dropped a factor of 5 for HSil52 and slightly increased
for HSil54. As a result permeance ratios dropped sharply for both
membranes.
For two membranes (HSil51N and HSil53 in Table 2), a single ALC-CVD
layer apparently was sufficient to block a large fraction of the
zeolite pores so that the permeances for N.sub.2 decreased by a
factor of 6 and 20 respectively, but the SF.sub.6 permeances changed
less dramatically. The single gas permeance ratios therefore decreased
for both membranes by a factor of 6.
Single compound permeances and selectivities of gases with different
kinetic diameters through membrane HSil50 and some selected permeance
ratios are listed in Table 3. The smallest molecules (He and H.sub.2)
permeated .about.1500 and 2400 times faster than SF6 the slowest
molecule. Gases with similar kinetic diameters, such as N.sub.2
and CO.sub.2 permeated at similar rates.
The permeance of two fluorinated hydrocarbons (CHF.sub.2 --CH.sub.2
F, kinetic diameter .about.0.49 nm and CF.sub.3 --CH.sub.3 kinetic
diameter --0.51 nm) through HSil56 were measured to determine if
the Al.sub.2 O.sub.3 layer changed the size of the zeolite pore
openings. These kinetic diameters are slightly smaller than the
pore size silicalite. Table 4 indicates that both compounds permeated
at similar rates and only a factor of 1.7 slower than N.sub.2. Apparently,
a sharp size cutoff between 0.55 and 0.51 nm results in permeance
ratios of more than 100 between CF.sub.3 --CH.sub.3 and SF.sub.6.
DISCUSSION OF EXEMPLARY RESULTS
Zeolite membranes 10 can have defects or other non-zeolitic pores
such as grain boundaries 22 between the crystals that are significantly
larger than the zeolite pore openings 24 as indicated by relatively
large permeances of molecules that should not be able to enter the
zeolite pores 24 based on their molecule size. For example isooctane
(kinetic diameter 0.62 nm, see Funke et al. 1996 supra), or m-and
o-xylene (kinetic diameters .about.0.68 nm, see C. D. Baertsch et
al., Permeation of aromatic hydrocarbon vapor through silicalite-zeolite
membranes," J. Phy. Chem., 1996 vol. 100 page 7676) should
not be able to enter silicalite pores 24. Compounds with kinetic
diameters close to the zeolite pore 24 size such as SF.sub.6 should
be highly hindered to enter the small zeolite pores 24. The experimentally
observed fluxes of SF.sub.6 therefore should be mainly attributed
to grain boundaries 22 between the crystals or other non-zeolitic
pores or defects. If the deposition closes or decreases the non-zeolite
pores 22 that allow non-selective permeation, a strong decreases
of the SF.sub.6 permeance is expected. The extent to which each
pathway contributes to the total SF.sub.6 flux, however, is not
known. The high N.sub.2 /SF.sub.6 selectivity of 560 obtained for
membrane HSil50 after one deposition suggests that for an ideal
membrane 10 only a small SF.sub.6 permeance is expected. It is noted
that zeolites exhibit some flexibility in their pore structures,
so molecules that are larger than the pore sizes determined by crystalography
can be accommodated. See E. L. Wu, G. R. Landolt, and A. W. Chester,
"Hydrocarbon adsorption characterization of high silica zeolites,"
Studies in Surface Science and Catalysis, 1986 vol. 28 page 547.
Therefore, fraction of the permeance of the larger molecules can
be due to diffusion through the zeolite pores.
The permeance for N.sub.2 decreases more than expected if ALC-CVD
only blocks the regions or gaps 22 between crystals 12 and N.sub.2
and SF.sub.6 have the same permeances in these pores. Apparently
some Al.sub.2 O.sub.3 overgrows the zeolite pores 24 which are the
main pathway for N.sub.2.
For two membranes (HSil51N, HSil53), one Al.sub.2 O.sub.3 layer
20 was sufficient to block most of the zeolite pore openings 24
and both N.sub.2 and SF.sub.6 permeated at slow but similar rates.
Apparently the deposited layer 20 did not seal all defects, which
are not selective, but the layer 20 did seal zeolite pore openings
24. The density and location of hydroxyl groups on the zeolite surface
may affect whether zeolite pore openings 24 are sealed. The properties
of the initial membrane are not identical, as indicated in Table
1 and 2. Even membranes with similar separation properties do not
necessarily have the same surface structures and density and location
of surface OH-groups.
Growth of the ALC-CVD layer close to the pore openings 24 should
also alter the effective size of the zeolite pores 26. If some SF.sub.6
molecules permeate through silicalite pores 26 a small decrease
in pore opening 24 size should be sufficient to totally exclude
the SF.sub.6 molecules from entering the zeolite pores 26. The observed
permeance decrease therefore could also be partially attributed
to a decrease in pore opening 24 size caused by Al.sub.2 O.sub.3
close to the pore openings 24. The decrease cannot be too large,
however, because the two CFC compounds, which are only slightly
smaller than SF.sub.6 permeated at rates close to N.sub.2 as listed
in Table 3. If the deposition decreased the zeolite pore openings
24 significantly, the CFC's should also be strongly excluded.
The CVD layer can also change the adsorption properties of the
zeolite surface. Barrer, "Porous crystal membranes," J.
Chem. Soc. Faraday Trans., 1990 vol.86 1123 suggests that the
first step for the transport through zeolite membranes is adsorption
on the external membrane surface. A change in external adsorption
properties can therefore significantly alter the overall transport
rates without decreasing the pore sizes. The surface properties
of Al.sub.2 O.sub.3 are sufficiently different from those of the
silica surface of silicalite to change adsorption behavior. |