U.S. patent application number 10/104551 was filed with the patent office on 2002-10-03 for inorganic dual-layer microporous supported membranes.
Invention is credited to Brinker, C. Jeffrey, Lu, Yunfeng, Tsai, Chung-Yi.
Application Number | 20020142172 10/104551 |
Document ID | / |
Family ID | 26838833 |
Filed Date | 2002-10-03 |
United States Patent
Application |
20020142172 |
Kind Code |
A1 |
Brinker, C. Jeffrey ; et
al. |
October 3, 2002 |
Inorganic dual-layer microporous supported membranes
Abstract
The present invention provides for a dual-layer inorganic
microporous membrane capable of molecular sieving, and methods for
production of the membranes. The inorganic microporous supported
membrane includes a porous substrate which supports a first
inorganic porous membrane having an average pore size of less than
about 25 .ANG. and a second inorganic porous membrane coating the
first inorganic membrane having an average pore size of less than
about 6 .ANG.. The dual-layered membrane is produced by contacting
the porous substrate with a surfactant-template polymeric sol,
resulting in a surfactant sol coated membrane support. The
surfactant sol coated membrane support is dried, producing a
surfactant-templated polymer-coated substrate which is calcined to
produce an intermediate layer surfactant-templated membrane. The
intermediate layer surfactant-templated membrane is then contacted
with a second polymeric sol producing a polymeric sol coated
substrate which is dried producing an inorganic polymeric coated
substrate. The inorganic polymeric coated substrate is then
calcined producing an inorganic dual-layered microporous supported
membrane in accordance with the present invention.
Inventors: |
Brinker, C. Jeffrey;
(Albuquerque, NM) ; Tsai, Chung-Yi; (Vernon,
CT) ; Lu, Yunfeng; (San Jose, CA) |
Correspondence
Address: |
Aldo J. Test
FLEHR HOHBACH TEST ALBRITTON & HERBERT LLP
Suite 3400
Four Embarcadero Center
San Francisco
CA
94111-4187
US
|
Family ID: |
26838833 |
Appl. No.: |
10/104551 |
Filed: |
March 22, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10104551 |
Mar 22, 2002 |
|
|
|
09602579 |
Jun 22, 2000 |
|
|
|
60141148 |
Jun 25, 1999 |
|
|
|
Current U.S.
Class: |
428/446 ;
427/350; 427/372.2; 427/402; 428/304.4; 428/336; 428/448 |
Current CPC
Class: |
C04B 41/009 20130101;
B01D 2323/24 20130101; B01D 67/0083 20130101; B01D 69/12 20130101;
B01D 71/027 20130101; C04B 2111/00862 20130101; C04B 41/009
20130101; C04B 2235/441 20130101; C04B 35/14 20130101; C04B 2235/94
20130101; B01D 53/22 20130101; C04B 2237/341 20130101; C04B 35/624
20130101; C04B 41/5035 20130101; B01D 63/06 20130101; C04B 35/10
20130101; C04B 38/0045 20130101; C04B 38/00 20130101; C04B 41/4582
20130101; C04B 41/009 20130101; C04B 41/4537 20130101; Y10T 428/265
20150115; B01D 71/024 20130101; B32B 2315/02 20130101; C04B 41/526
20130101; C04B 2235/3418 20130101; Y10T 428/249953 20150401; B01D
2323/08 20130101; C04B 41/009 20130101; C04B 41/85 20130101; C04B
38/0045 20130101; C04B 2111/00801 20130101; C04B 41/526 20130101;
C04B 2237/343 20130101; B01D 67/0048 20130101; B01D 2313/42
20130101; B01D 71/02 20130101 |
Class at
Publication: |
428/446 ;
427/402; 427/372.2; 427/350; 428/448; 428/336; 428/304.4 |
International
Class: |
B05D 003/02; B05D
001/36; B32B 009/04 |
Goverment Interests
[0002] This invention was made with Government support at Sandia
National Laboratories under Contract No. DE-AC04-94AL85000 awarded
by the Department of Energy. The Government has certain rights in
this invention.
Claims
What is claimed is:
1. An inorganic microporous supported membrane, comprising: a
macroporous support; a templated porous intermediate layer coating
the support; and a microporous layer coating the templated porous
intermediate layer such that the microporous layer is capable of
molecular sieving.
2. The inorganic microporous supported membrane as claimed in claim
1, wherein: the templated porous intermediate layer is an inorganic
surfactant-templated silica layer; and the microporous layer is an
inorganic silica layer.
3. The inorganic microporous supported membrane as claimed in claim
2, wherein: the surfactant-templated porous intermediate layer
having an average pore size of less than about 20 .ANG.; and the
microporous layer having an average pore size of less than about 5
.ANG..
4. The inorganic microporous supported membrane as claimed in claim
1, wherein: the templated porous intermediate layer including an
amphiphilic block copolymer.
5. An inorganic microporous supported membrane, comprising: a
porous substrate; a first inorganic porous membrane coating the
substrate having an average pore size of less than about 25 .ANG.;
and a second inorganic porous membrane coating the first inorganic
membrane having an average pore size of less than about 6
.ANG..
6. The inorganic microporous supported membrane as claimed in claim
5, wherein: the first inorganic porous membrane has a pore diameter
in a range of about 10 to 20 .ANG..
7. The inorganic microporous supported membrane as claimed in claim
6, wherein: the first inorganic porous membrane includes a
surfactant-templated material.
8. The inorganic microporous supported membrane as claimed in claim
7, wherein: the surfactant-templated material is prepared from
silica polymers and a surfactant powder.
9. The inorganic microporous supported membrane as claimed in claim
5, wherein: the second inorganic porous membrane is an inorganic
silica membrane.
10. The inorganic microporous supported membrane as claimed in
claim 5, wherein: the first inorganic porous membrane has a
thickness of less than about 100 .ANG..
11. The inorganic microporous supported membrane as claimed in
claim 5, wherein: the second inorganic porous membrane has an
average pore size of between 2 and 5 .ANG..
12. The inorganic microporous supported membrane as claimed in
claim 5, wherein: the second inorganic porous membrane has an
average pore size of between 3 and 4 .ANG..
13. The inorganic microporous supported membrane as claimed in
claim 5, wherein: the second inorganic porous membrane has a
thickness less than about 100 nm.
14. The inorganic microporous supported membrane as claimed in
claim 5, wherein the porous substrate is an alumina substrate.
15. The inorganic microporous supported membrane as claimed in
claim 6, wherein: the porous substrate has an average pore diameter
ranging between 30 to 60 .ANG..
16. A method for producing an inorganic dual-layered microporous
supported membrane capable of molecular sieving, the method
comprising: contacting a porous substrate with a
surfactant-template polymeric sol resulting in a surfactant sol
coated membrane support; drying the surfactant sol coated membrane
support producing a surfactant-templated polymeric coated
substrate; calcining the surfactant-templated polymeric coated
substrate to produce an intermediate layer surfactant membrane;
contacting the intermediate layer surfactant templated membrane
with a second polymeric sol producing a polymeric sol coated
substrate; and drying the polymeric sol coated substrate producing
an inorganic polymeric coated substrate; calcining the inorganic
polymeric coated substrate producing the inorganic dual-layered
microporous supported membrane.
17. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein: the step of
calcining the inorganic polymeric coated substrate includes:
calcining the inorganic polymeric coated substrate at a first
temperature producing a dual-layered supported membrane; further
calcining the dual-layered supported membrane at a second
temperature to produce the inorganic dual-layered microporous
supported membrane.
18. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 17, wherein: the step of
calcining the inorganic polymeric coated substrate includes
calcining under a vacuum.
19. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 18, wherein: the step of
calcining the inorganic polymeric coated substrate includes
calcining under a vacuum of less than about 6 pounds per square
inch absolute (psia).
20. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 18, wherein: the step of
calcining the inorganic polymeric coated substrate includes
calcining under a vacuum of less than about 4 psia.
21. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 17, wherein: the step of
calcining the inorganic polymeric coated substrate includes
calcining at a temperature ranging from 200 to 400.degree. C.
22. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 21, wherein: the step of
calcining the inorganic polymeric coated substrate includes
calcining at a temperature ranging from 250 to 350.degree. C.
23. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 17, wherein: the step of
calcining the dual-layered inorganic supported membrane includes
calcining at a temperature ranging from 300 to 600.degree. C.
24. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 23, wherein: the step of
calcining the dual-layered inorganic supported membrane includes
calcining at a temperature ranging from 400 to 500.degree. C.
25. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 24, wherein: the step of
calcining the dual-layered inorganic supported membrane includes
calcining for between about 30 to 90 minutes.
26. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein: the step of
calcining the inorganic polymeric coated substrate includes
calcining under a vacuum.
27. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 26, wherein: the step of
calcining the inorganic polymeric coated substrate includes
calcining under a vacuum of less than about 6 pounds per square
inch absolute (psia).
28. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 27, wherein: the step of
calcining the inorganic polymeric coated substrate includes
calcining under a vacuum of less than about 4 pounds per square
inch absolute (psia).
29. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein: the step of
heating the surfactant-template membrane substrate includes heating
at a temperature between 500 to 600.degree. C.
30. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 29, wherein: the step of
heating the surfactant-template membrane substrate includes heating
for between about 30 to 90 minutes.
31. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein: the step of
calcining the surfactant-templated polymeric coated substrate
includes calcining at a temperature between 100-150.degree. C.
32. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein: the
surfactant-template polymeric sol comprises silica polymers.
33. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein: the second
polymeric sol comprises silica polymers.
34. The method for producing the inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein: the
surfactant-template polymeric sol is prepared and deposited under
conditions of low condensation rate; and the second polymeric sol
is prepared and deposited under a condition of low condensation
rate.
35. The method for producing an inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein the method is
performed under Class 100 clean room conditions.
36. The method for producing an inorganic dual-layered microporous
supported membrane as claimed in claim 16, wherein the step of
drying the surfactant sol coated membrane support is performed
under conditions of low relative pressure of the liquid
constituents.
37. A method for producing a supported membrane capable of
molecular sieving, comprising: preventing a subsequently deposited
top microporous sol from penetrating further into the support
including: modifying a surface of the support; and depositing the
top microporous membrane on a modified support producing the
supported membrane capable of molecular sieving.
38. The method for producing a supported membrane as claimed in
claim 37, wherein: the step of modifying a surface of the support
includes: depositing an intermediate membrane on the surface.
39. The method for producing a supported membrane as claimed in
claim 38, wherein: the step of depositing the intermediate membrane
includes depositing an inorganic surfactant-templated silica
intermediate layer having an average pore size of less than 25
.ANG..
40. The method for producing a supported membrane as claimed in
claim 38, wherein: the step of depositing the intermediate membrane
includes: depositing a surfact-template sol onto the support;
drying the surfactant sol coated support; calcining the dried
surfactant sol coated support resulting in a surfactant supported
membrane.
41. The method for producing a supported membrane as claimed in
claim 38, wherein: the step of depositing an intermediate membrane
including: removing the surfactant-template by heating the
surfactant supported membrane producing the modified support.
42. The method for producing a supported membrane as claimed in
claim 38, wherein: the step of depositing the top microporous
membrane includes: depositing an inorganic polymeric sol on the
modified support; drying the polymeric sol coated support resulting
in an inorganic polymeric sol coated support; calcining the
inorganic polymeric sol coated support resulting in a dual-layered
supported membrane; further calcining the dual-layered supported
membrane resulting in the supported membrane capable of molecular
sieving.
Description
PRIORITY APPLICATION
[0001] This application claims priority to Provisional Application
Ser. No. 60/141,148 filed Jun. 25, 1999.
FIELD OF THE INVENTION
[0003] The present invention relates to membranes for use in
molecular sieving, and more particularly to inorganic membranes
providing high sieving flux and selectivity.
BACKGROUND
[0004] Membrane-based separations are energy efficient and cost
effective. They represent promising alternatives to
energy-intensive distillation, cryogenic separation, or pressure
swing adsorption in applications such as purification of
sub-quality natural gas, air separation, removal of VOCs and
NO.sub.X, and hydrogen recovery from processing gases and feed
stocks. Microporous inorganic membranes have attracted considerable
attention for gas separation due to their excellent thermal and
chemical stability, good erosion resistance and high pressure
stability compared to conventional polymeric membranes (e.g.
cellulusic derivative, polysulfone, polyamide, or polyimide
membrane).
[0005] An inorganic membrane system generally consists of a
macroporous support providing mechanical strength for an overlying
thin, either dense or porous, separation membrane. Dense membranes
prepared from palladium or perovskite only allow certain gases
(such as H.sub.2 or O.sub.2) to transport via mechanisms such as
solution-diffusion or solid-state ionic conduction. Such membranes
require high capital investment due to the use of precious metals
and/or extreme synthesis conditions. In contrast, porous silica
membranes with tunable pore sizes can be processed by a simple
dip-coating or spin-coating procedure and can be used potentially
in a large variety of gas separations. Microporous silica membranes
have been demonstrated to show promising molecular sieving
characteristics.
[0006] Certain techniques have been developed to process porous
silica membranes. They include sol-gel synthesis, leaching, and
chemical vapor deposition. Among these, sol-gel processing attracts
the most attention do to its excellent processibility and its
potential to precisely control pore size and pore structure.
Strategies for the fundamental physical and chemical phenomena
involved in the deposition of colloidal ceramic dispersions (sols)
on porous supports for precise pore size and porosity control have
been proposed and discussed in Brinker et al., "Sol-gel Strategies
for Controlled Porosity Inorganic Materials," J. Membr. Sci., 94
(1995) 85, incorporated herein by reference. Three keys to membrane
production are 1) avoidance of cracks, pinholes or other defects
that would reduce the selectivity; 2) precise pore size control
(0.3- 0.4 nm in diameter) so that separation occurs on the basis of
size by molecular filtration or "sieving"; and 3) maximization of
the volume fraction porosity and minimization of the membrane
thickness to maximize flux.
[0007] Forming a microporous silica membrane on top of a home-made
disk-shaped, double-coated .gamma.-alumina support has been
described in the article De Vos et al., "Improved Performance of
Silica Membranes for as Separation," J Membr. Sci., 143 (1-2)
(1998) 37-51, incorporated herein by reference. The coating and
calcination process was repeated once to minimize potential
defects. Both silica sol preparation and membrane processing were
similar to those developed by De Lange, Hekkink and Keizer,
described in De Lange et al., "Formation and Characterization of
Supported Microporous Ceramic Membranes Prepared by So-Gel
Modification Techniques," J. Membr. Sci., 99 (1995) 57-75,
incorporated herein by reference. The improvement of membrane
performance was attributed to the processing under class-10 clean
room conditions, which increases the cost of manufacturing. Prior
art membranes have employed repeated-coating processes to reduce
intrinsic defects. The multi-step coating process results in
reduction of the number of defect sites, thus increasing
selectivity but at the expense of permeation flux and cost of
manufacturing. Although high selectivity can significantly reduce
either feed loss (single stage) or recompression costs (multiple
stage), high permeation flux is necessary to achieve commercially
satisfactory production rates. Therefore, the deadlock of tradeoff
between selectivity and flux needs to be overcome.
SUMMARY OF THE INVENTION
[0008] The present invention provides for an inexpensive supported
membrane capable of molecular sieving.
[0009] The present invention further provides for a uniform
intermediate layer on a substrate to allow the deposition of a
second, top microporous layer which is relatively defect free and a
method for producing the defect free layer.
[0010] The present invention also provides for a supported membrane
with precise pore size to achieve molecular sieving with maximized
flux and selectivity.
[0011] There is described a dual-layer inorganic microporous
membrane capable of molecular sieving, and methods for production
of such membranes. The inorganic microporous supported membrane
includes a porous substrate which supports a first inorganic porous
membrane having an average pore size of less than about 25 .ANG.
and a second inorganic porous membrane coating the first inorganic
membrane having an average pore size of less than about 6
.ANG..
[0012] The dual-layered membrane is produced by contacting the
porous membrane support with a surfactant-containing inorganic
polymeric sol, resulting in a surfactant/inorganic polymer coated
membrane support. The surfactant/inorganic polymer coated membrane
support is dried, producing a self-assembled surfactant-templated
surfactant/inorganic polymer composite film. This supported
composite film is calcined to remove the surfactant templates to
produce a surfactant-templated micro- or mesoporous membrane
substrate, which serves as an intermediate layer
surfactant-templated membrane. The intermediate layer
surfactant-templated membrane is then contacted with a second
inorganic polymeric sol producing an inorganic polymeric sol coated
substrate which is dried, producing a supported inorganic polymer
coated dual layer structure. This supported dual-layered structure
is then calcined to produce a dual-layered microporous supported
membrane in accordance with the present invention.
[0013] In one embodiment, both of the polymeric sols include
silica, aligomers or polymers. The first or intermediate layer of
the dual-layer supported membrane generally has a thickness of less
than 100 mn and the second or top layer has a thickness of less
than about 100 .ANG.. The average pore diameter of the dual-layer
supported membrane gradually decreases in size from about 40 to 60
.ANG. for the support, to about 10 to 25 .ANG. for the intermediate
layer, to about 2 to 5 .ANG. for the top microporous layer.
[0014] The calcining procedure of the second, top layer includes
calcining under a vacuum of less than about 4 psia at a temperature
of between 200 to 400.degree. C. and further calcining at a
temperature of between 300 to 600.degree. C. The calcining of the
first or intermediate layer involves calcining at a temperature
between 100 to 150.degree. C. and further heating between 500 to
600.degree. C. The drying of the sols involves drying under
conditions of low relative pressure of the liquid constituents.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] These and various other features and advantages of the
present invention will be apparent upon reading of the following
detailed description in conjunction with the accompanying drawings,
where:
[0016] FIG. 1 is a graphical representation of weight loss and
differential thermal analysis showing an endotherm peak
corresponding to weight loss, indicating decomposition of the
surfactant template and the oxidative pyrolysis of surfactant and
residual organics.
[0017] FIG. 2 is a graphical representation of nitrogen adsorption
isotherms of three different calcined sol layers after removal of
templates.
[0018] FIG. 3A is an Scanning Electron Microscope (SEM)
cross-sectional electron micrograph of the dual-layer supported
membrane of the present invention.
[0019] FIG. 3B is a zoomed in Transmission Electron Microscope
(TEM) micrograph of the cross-sectional view of the dual-layer
supported membrane of FIG. 3A.
[0020] FIG. 4 shows a schematic diagram of the custom built
automated flow system utilized in gas permeation measurements.
[0021] FIG. 5A is a graphical representation of the separation
factor versus temperature comparing microporous sieving A2** layer
with and without a surfactant-templated intermediate layer.
[0022] FIG. 5B is a graphical representation of gas permeances
versus temperature comparing the A2** membrane with or without a
mesoporous sublayer.
[0023] FIG. 6 is a graphical representation of the CO.sub.2
permeance and CO.sub.2/CH.sub.4 separation factor .alpha. as a
function of temperature for separation of 50/50 (v/v)
CO.sub.2/CH.sub.4 gas mixture.
[0024] FIG. 7A is a graphical representation of the gas permeances
versus temperature of the BTE/TEOS membranes with or without a
surfactant-templated intermediate layer.
[0025] FIG. 7B is a graphical representation of the selectivities
versus temperature of the BTE/TEOS membranes with or without a
surfactant-templated mesoporous C16STS intermediate layer.
[0026] FIG. 8 is a graphical representation of the gas permeance
versus molecular diameter of a variety of molecules of a dual-layer
supported membrane after calcination at 450.degree. C.
[0027] FIG. 9 is a graphical representation of the separation
factor versus permeance of the dual-layer supported membrane of the
present invention in comparison to prior art membranes.
[0028] FIG. 10A is a graphical representation of the %CO.sub.2 in
permeate and CO.sub.2/CH.sub.4 separation factor versus stage cut
of a dual-layer supported A2** membrane.
[0029] FIG. 10B is a graphical representation of the CO.sub.2
permeance and CO.sub.2/CH.sub.4 separation factor versus feed
flow-rate of a dual-layer supported A2** membrane.
[0030] FIG. 11 is a graphical representation of the CO.sub.2
permeance and CO.sub.2/CH.sub.4 separation factor versus change in
pressure of a dual-layer supported A2** membrane.
[0031] FIG. 12 is a graphical representation of the CO.sub.2
permeance and CO.sub.2/CH.sub.4 separation factor versus time of
operation of a dual-layer supported A2** membrane.
DETAILED DESCRIPTION
[0032] In one embodiment, the present invention provides for novel
inorganic dual-layered microporous supported membranes and methods
for fabricating the supported dual-layer membranes. The dual-layer
membranes are constructed by utilizing an underlying support
structure coated with a surfactant-templated micro or mesoporous
intermediate layer, then coated with a top selective microporous
layer. In the production of supported membranes, the quality of the
underlying support determines, to a high degree, the properties and
quality of the top selective microporous silica membrane. Supports
with rough and large-pore surfaces can cause overlying derived
membranes to crack due to stress development on uneven film
coatings or serve to create pin-holes (areas not coated by the
overlying microporous layer). Therefore, the present invention
provides a novel and efficient way for the modification of support
surfaces to allow a stable, even, and substantially defect-free
subsequently deposited overlying microporous membrane. Mechanical
polishing to remove surface roughness is tedious, requiring
repeated processes, and is difficult to perform within the interior
of tubular supports. In the present invention, a membrane, such as
a tube-side .gamma.-alumina surface of a commercial asymmetric
membrane support, is modified with a layer of surfactant-templated
material, for example, surfactant-templated microporous or
mesoporous silica (STS). The STS, configured with high porosity and
narrow pore diameter distribution (.about.1-2 nm), is designed to
minimize additional flow resistance, surface roughness, and
inherent support defects. It also reduces penetration of a
subsequently deposited top layer into the support, thereby reducing
the effective thickness of the top layer. This new dual-layer
approach creates a thin defect-free microporous membrane with both
higher selectivity and higher flux than achieved by the
corresponding single-layer microporous membrane.
[0033] The present invention provides for supported inorganic
membranes capable of molecular sieving, as well as methods for
their preparation and use. In one embodiment, the subject inorganic
membranes have pores with resulting average pore diameters of less
than about 10 .ANG. and usually less than about 5 .ANG. and have a
narrow pore diameter distribution. The subject membranes are
prepared by initially coating a porous substrate with a
surfactant-templated inorganic polymeric sol. After deposition, the
surfactant sol coated substrate is dried and calcined to provide
the intermediate inorganic membrane. The intermediate inorganic
membrane is then coated by a second polymeric sol. After
deposition, the inorganic polymeric sol coated substrate is dried,
preferably under conditions of low relative pressure of the liquid
constituents of the sol, and calcined to provide the subject
inorganic dual-layered microporous supported membrane.
[0034] The inorganic polymeric sols which find use in the present
invention are those sols which are stable under the conditions in
which they are employed, and have a viscosity which provides for
deposition of a thin sol coating on the support. The viscosity of
the sols will generally range from about 1 to 20 cps, usually from
about 1 to 5 cps, more usually from about 1.2 to 1.5 cps.
[0035] The polymeric sols will comprise a colloidal dispersion of
inorganic polymers in a liquid, where the liquid may be a single
component, e.g. H.sub.2O or an alcohol, or a multi-component,
normally miscible, mixture, e.g. alcohol-water and the like. The
inorganic polymers will often have a low mass fractal dimension
(D.sub.f) and be sufficiently large so that the inorganic polymers
are captured, at least in part, on top of the underlying STS
substrate. The diameter of the inorganic polymers as measured by
small angle scattering will often be at least equal to, and will
usually be greater than, the diameter of the pores in the
underlying STS substrate, and may be up to 2 times the diameter,
more usually up to 1.5 times the diameter of the pores of the
underlying STS substrate. The concentration of the inorganic
polymers in the liquid component of the sols will be sufficient to
provide for a sol with the desired viscosity, as described above.
Generally, the concentration of the inorganic polymers in the
liquid component of the sols will range from 0.45 to 0.69, usually
0.59 to 0.63, more usually 0.60 to 0.62 moles/liter.
[0036] The liquid component of the sols will often comprise water
in combination with at least one, usually not more than three, more
usually not more than two, organic solvents. Water present in the
liquid component will usually be deionized and will be present in
about 2.5% to 13% (v/v), usually 5.7 to 6.0% (v/v) of the total
liquid component of the sols. A variety of organic solvents will
normally be combined with the water to produce the liquid component
of the sols. At least one of the organic solvents will be an
alkanol, usually a lower alkanol of from 2 to 8 carbon atoms,
usually 2 to 6 carbon atoms, more usually 2 to 4 carbon atoms. The
alkanol may be an alcohol or polyol, where the number of hydroxy
groups does not exceed the number of carbon atoms, and where there
is usually not more than one hydroxy group for every 1.5 carbon
atoms. Illustrative alkanols which find use as organic solvents in
the subject sols are ethanol, usually absolute ethanol, methanol,
iso-propanol, and the like. The alkanol will be present in the
liquid component of the sols in an amount ranging from about 77 to
82% (v/v), usually from about 80 to 80.5%(v/v) of the liquid
component in the sols. Generally, the ratio of alkanol to water in
the liquid component of the sols will range from 6 to 32, usually
from about 13.5 to 14.5. Other organic solvents, in addition to the
alkanol, may also be present in the subject sols, where the liquid
component of the sols comprises more than one organic solvent.
These other organic solvents may be straight chained, branched or
cyclic, and will usually comprise from about 2 to 10 carbon atoms,
more usually 4 to 8 carbon atoms. The organic solvents may comprise
one or more heteroatoms, usually not more than three heteroatoms,
where the heteroatom may be selected from oxygen, sulfur, nitrogen
and the like. Illustrative non-alkanol organic solvents which may
be present in the liquid component of the sols include hexane,
toluene, tetrahydrofuran, acetone and the like. When present in the
subject sols, these non-alkanol organic solvents will be present in
amounts ranging from 5 to 99% (v/v) of the total liquid
component.
[0037] The subject inorganic polymeric coating sols are prepared by
combining metal alkoxide monomers with the liquid components of the
sols, as described above, in the presence of a catalyst, either
acid or base, with agitation to provide a substantially uniform
dispersion of the metal alkoxide monomers in the liquid components.
Metal alkoxide monomers that find use in preparation of the subject
sols are those monomers having the formula:
M(OR).sub.n or MR'.sub.x(OR).sub.n-x, usually M(OR).sub.n,
[0038] wherein:
[0039] M is a metal having a coordination number in excess of 3,
i.e.>3, and is selected from the group comprising Si, Al, Ti, Zr
and the like, preferably Si;
[0040] OR is a hydrolyzable alkoxy ligand, where R usually has the
formula:
C.sub.zH.sub.2z+1,
[0041] wherein:
[0042] z is from 1 to 4, usually from 1 to 2;
[0043] n is usually from 3 to 4; and
[0044] R' is a non-hydrolyzable organic ligand or other oligomeric
oxoalkoxide having from 1 to 4 carbon atoms, usually from 1 to 2
carbon atoms.
[0045] For electropositive metals, e.g. Ti and Zr, the metal
alkoxide monomer may be modified to reduce the effective
functionality and/or rate of hydrolysis and condensation and
thereby prevent particle formation under the sol preparation
conditions. To reduce the effective functionality and/or rate of
hydrolysis and condensation, the metal alkoxides may be modified by
any convenient means, such as chelation with slowly hydrolyzing
multidentate ligands, e.g. actylacetonate, alcohol amines, and the
like.
[0046] Catalysts that find use include mineral acids and bases,
including HCl, H.sub.2SO.sub.4, HNO.sub.3, NH.sub.4OH, and the
like. In the present invention, often the catalyst is added in
sufficient concentration to cause the average condensation rate of
the hydrolyzed alkoxide species to be minimized.
[0047] Agitation of the metal alkoxide, water, catalyst and organic
solvent(s), i.e. the sol precursors, is maintained for a sufficient
time to provide for a uniform dispersion of the metal alkoxide in
the liquid constituents. Usually the sol precursors will be mixed
for a period of time ranging from 5 to 20 min, more usually 14 to
15 min. Mixing may be accomplished by any convenient means, such as
stirring, shaking and the like.
[0048] The resultant mixture of sol precursors is then allowed to
set or age for a sufficient time under conditions of low
condensation rate to produce a sol comprising inorganic polymers
having the desired cluster size. The condensation conditions, i.e.
the rate at which the monomers are hydrolyzed and then condensed
into the polymeric clusters, depends on a variety of factors, such
as the reactivity of the alkoxy ligands of the metal alkoxide
monomers, the temperature at which the sol is maintained, the pH of
the sol precursor, and the like. Thus, the temperature and pH of
the combined sol precursors will be selected in view of the
reactivity of the alkoxy ligands of the metal alkoxy monomers, to
achieve conditions of low but finite condensation rate. Although
the selected temperature and pH will vary depending on the
particular metal alkoxy monomers employed, generally the
temperature will range from about 35 to 70.degree. C., usually from
about 40 to 60.degree. C., and more usually from about 48 to
52.degree. C. The temperature of the aging precursor sol may be
controlled using any convenient temperature control means, such as
a heating or cooling means, or the like. The effective pH of the
sol precursor composition will generally range from 0 to 5, usually
from about 1 to 3, more usually from about 1.5 to 2.5. For silica
sols, the pH of the precursor solution may be controlled by
introducing to the precursor an acid or basic acatalyst, such as
HCl, HNO.sub.3, NH.sub.4OH and the like, in sufficient amount to
modulate the pH (specifically the -log[H.sub.3O.sup.+]) of the
precursor to the desired value.
[0049] The precursor sol is sealed and allowed to set or age under
the conditions of low condensation rate for a period of time
sufficient to provide for the formation of extended polymeric
networks capable of interpenetration and optionally having the
desired fractal dimensions. Usually, the sols will be allowed to
set in a closed container for a period of time ranging from about 0
to 140 hrs, more usually from about 10 to 24 hours.
[0050] The preparation of a silica polymeric coating sols includes
a two stage process, in which the pH of the sols is decreased from
the first to the second stage. In the first stage of this two stage
sol production process, a silicon alkoxide, e.g. tetraethoxysilane,
is combined with an alkanol and water with agitation to produce a
precursor sol. The amount of silicon alkoxide which is combined
with the alkanol and water to produce the precursor sol will range
from 1.0 to 2.0 mol/liter, usually from about 1.7 to 2.0 M
mol/liter. The amount of alkanol present in the precursor sol will
range from 30 to 70% (v/v), usually 45 to 50% (v/v), and the amount
of water present in the precursor sol will range from 2 to 10%
(v/v), usually 3.5 to 4.5% (v/v). In this first stage, agitation is
continued for a period of 30 to 120 min., usually 60 to 90 min.
During this first stage, the pH of the precursor sol is maintained
at a value between 3 and 6, usually between 4 and 5 as measured by
using an indicator strip, by including in the precursor sol a
sufficient amount of an acid catalyst. Any convenient acid catalyst
may be employed, including HNO.sub.3, HCl and the like. In this
first stage, the molar ratio of the four components of the sol
precursor, i.e. silicon alkoxide: alkanol: water: acid catalyst,
will be 0.8-1.2 : 3.5-4 : 0.8-1.2:
1.0.times.10.sup.-5-9.0.times.10.sup.-5.
[0051] In the second stage of the two stage sol preparation
process, the pH of the precursor sol will be reduced by 1/3to 1/2.
To reduce the pH of the precursor sol, a sufficient amount of water
and acid catalyst will be introduced to the precursor sol.
Following introduction of the water and acid catalyst, the pH of
the precursor sol will be reduced to between about 1 and 3, usually
between about 1.5 and 2.5. In the second stage, a sufficient amount
of water and acid precursor are added to the precursor sol to
change the molar ratio of the four components to 0.8-1.2: 3.5-4:
4.5-5.5: 0.001-0.009. Following introduction of the acid catalyst
and water, the precursor sol will be agitated for a period of time
ranging from 10 to 30 min, usually from 10 to 20 min. Following
agitation, the precursor sol will be allowed to set or age at an
elevated temperature for a period of time sufficient for the
coating sol to be produced. Generally, the incubation temperature
will range from 40 to 60.degree. C., usually from 45 to 55.degree.
C. The setting period will range from 12 to 36 hours, usually 12 to
24 hours. The resultant silica coating sol (referred to below as
the A2** sol) may comprise silica polymers of low fractal
dimension.
[0052] In one embodiment of the present invention, the
surfactant-template polymeric sol utilized to form the intermediate
layer membrane is further prepared from the precursor sol following
substantially similar steps as described above for creating the
A2** sol. However, prior to aging and the second stage, a
sufficient amount of surfactant powder is added to the unaged
precursor sol. Surfactant powders that are used are usually
cationic surfactants, for example C6-surfactant of
triethylhexylammonium bromide and C16-surfactant of
cetyltrimethylammonium bromide. The amount of surfactant powder
that is added is between 2 g and 8 g per 100 g sol, usually between
3 g and 5 g per 100 g/sol. Following the addition of the surfactant
powder, precursor sol is agitated for a period of time between
about 5-30 min., usually between about 10-20 min. Following the
addition of the surfactant powder, the surfactant-template sol is
completed following substantially similar steps as those of aging
and the second stage, described above for the A2** sol.
[0053] In one embodiment, the polymeric sols utilized for both the
intermediate layer and the top or outer layer are formed from the
sol or a derivative of the sol labeled A2** and fully described in
detail in U.S. Pat. No. 5,772,735, incorporated herein by
reference.
[0054] In alternative embodiments, a sol providing substantially
similar sieving characteristics as the A2** sol is utilized as the
top microporous membrane. The alternative sol consists of five
components, i.e., Bis(Triethoxysilyl)Ethane (BTE):silicon
alkoxide:alkanol:water:acid catalyst, with a molar ratio being
0.4-1.2:0.1-0.5:3.5-4:4.8-5.4:1.0.time-
s.10.sup.3-9.0.times.10.sup.3. Further, alternatives to the STS sol
include sols containing amphiphilic block-copolymers. The
amphiphilic block-copolymers, for example
Brij56,CH.sub.3(CH.sub.2).sub.15(CH.sub.2CH- .sub.2O).sub.10OH, 4
wt %, are utilized as the templating agent in replace of the
surfactant in creating the intermediate sol layer, producing
ordered mesoporous silica sublayer membranes. The block-copolymer
templated silica provides both a high porosity of between 40 to
80%, usually between 50 to 60%, and an ordered cubic structure,
resulting in a membrane having an average pore diameter of less
than about 30 .ANG., usually less than 25 .ANG..
[0055] In one embodiment, foreign particles are removed from the
sols (both the intermediate and top layer sols) through the
filtering of the sols prior to being coated onto a substrate. The
top layer or A2** sols are filtered with a 0.2 to 1.0 .mu.m filter,
more usually with a 0.45 82 m filter. The surfactant-template sols
are filled with a 0.45 to 2.0 .mu.m filter, usually with a 1.0
.mu.m filter. Thus, potentially damaging foreign materials are
removed from the sols to further ensure membranes with accurate
sieving capabilities.
[0056] A variety of substrates may be employed as porous supports
for the inorganic membranes. Substrates that may be employed have
pore sizes which are sufficiently large such that the substrate
itself does not contribute to the sieving properties of the
supported membrane. Generally, the pores of the substrate will have
diameters at least 5 times larger than the pores of the inorganic
films to be deposited on the substrate, normally at least 8 times
larger, and not more than 15 times larger, usually not more than 12
times larger than the pores of the inorganic film to be deposited.
The substrate pores usually have diameters ranging in size from
about 30 to 70 .ANG., usually from about 40 to 60 .ANG..
[0057] The substrate is constructed of any suitable material that
is thermally, chemically and mechanically stable during sol
deposition, thermal processing and membrane use including oxides,
e.g. TiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2, hydroxides, e.g. AlOOH,
as well as porous metals, such as stainless, and other materials
know in the art. The substrates may have any convenient shape, such
as square, rectangular and cylindrical, as well as other more
complicated shapes, where the shape chosen will depend primarily on
the intended use of the final supported inorganic membrane. The
substrates may be prepared using methods known in the art, i.e.
deposition of particulate sols, or obtained from commercial
sources. Commercially available substrates capable of acting as
supports for the inorganic films of the subject invention include
those available from U.S. Filter, e.g. Membralox.RTM. (a
cylindrical substrate comprising a .gamma.-Al.sub.2O.sub.3 inside
layer having pores ranging in diameter from 40 to 50 .ANG.), Golden
Technologies, and the like. As necessary and desirable, prior to
use, the substrate may be calcined to eliminate any organic
compounds present on the support and to desorb any water from the
support. Calcination will generally be carried out at temperatures
ranging from 300 to 500.degree. C., usually 350 to 450.degree. C.,
for a period of time between 15 to 180 min., and usually between 45
to 75 min.
[0058] Prior to depositing the STS sublayer onto the membrane
supports, the membrane supports are cleaned, usually by ultrasound.
The membrane supports are then washed with de-ionized water 1 to 7
times, usually 3 to 5 times. The membrane supports are then
calcined at a temperature between 300 and 500.degree. C., usually
350 to 450.degree. C., for a period of time between 15 to 180 min.,
and usually between 45 to 75 min.
[0059] The STS sol is then deposited onto the surface of the
membrane support under substantially clean conditions, usually
under Class 100 clean conditions, to produce a surfactant sol
coated support that is then dried and calcined to yield an STS
inorganic intermediate membrane. The drying is performed in an
environment having between 10-30% relative humidity, usually having
15-25% relative humidity at a temperature between 10 and 40.degree.
C., usually between 20 and 30.degree. C. for a period of time
between 5 and 20 minutes, usually between 10 and 15 minutes
resulting in a surfactant-template polymeric coated substrate. The
calcination is performed by heating the surfactant-template
polymeric coated support at a temperature between 50-200.degree.
C., usually between 100-150.degree. C. for between 30-120 min.,
usually between 45-75 min. producing a surfactant-template membrane
support. Surfactant removal is then performed by heating the STS
membrane support at a temperature between 300 to 700.degree. C.,
usually between 500-600.degree. C. for between 30-90 min., usually
between 45-75 min. producing the intermediate layer surfactant
membrane. Deposition of the surfactant intermediate or sublayer is
achieved by contacting the membrane support with an STS sol-gel so
that the STS sol coats the membrane support in a layer capable of
collapsing with drying to produce a thin film. The support membrane
may be contacted with the STS sol using any convenient means, such
as dip-coating, infiltration, spin-coating, spraying and the
like.
[0060] In one embodiment, the A2** silica top layer is then
deposited onto the surface of the intermediate layer surfactant
membrane, using a similar procedure as described above, producing
an A2** polymeric sol coated substrate that is then dried and
calcined. The drying is performed in an environment having between
10-30% relative humidity, usually having 15-25% relative humidity
at a temperature between 10 and 40.degree. C., usually between 20
and 30.degree. C. for a period of time between 5 and 20 minutes,
usually between 10 and 15 minutes producing an A2** inorganic
polymer coated substrate. The calcination of the A2** coated
substrate is performed under a vacuum of less than about 6 psia
(pounds per square inch absolute), usually less than about 4 psia
(<4 psia), at a temperature between 200 and 400.degree. C.,
usually between 250 and 350.degree. C. During calcination, the rate
at which the temperature is raised will range from 0.5 to 5.degree.
C./min., usually 0.5 to 2.degree. C./min. The coated substrate will
be held isothermally at the calcination temperature for a period of
time ranging from about 4-8 hours, usually 5-7 hours after which
the temperature will be decreased at a rate of 0.5 to 5.0.degree.
C./min., usually between about 0.5 to 3.0.degree. C./min. to
produce a dual-layered inorganic supported membrane having the A2**
top microporous membrane. In an alternative embodiment, further
calcination or second calcination is performed on the A2**
supported membrane at a temperature between 300-600.degree. C.,
usually between 400-500.degree. C. in air for 30 to 90 min.,
usually 45 to 75 min. producing the final A2** inorganic
dual-layered microporous supported membrane. The ends or edges of
the final permeable membrane are then sealed to prevent any defects
or irregularities.
[0061] The final porous inorganic membranes are capable of
molecular sieving, and exhibit high flux and high selectivity. By
capable of molecular sieving is meant that the membranes are
capable of exhibiting substantially molecular sieving behavior,
where molecular sieving behavior exists when the separation factor
for a pair of gases transported through the membrane is greater
than the separation factor for the same pair of gases in a membrane
characterized by Knudsen diffusion. Thus, in the formula where
.alpha..sub.A/B.varies.(molecular weight B/molecular weight
A).sup.1/2, where .alpha..sub.A/B is the separation factor of a
membrane for a pair of gases, in membranes exhibiting Knudsen
separation behavior, the separation factor for the pair of gases
does not typically exceed 10. As the subject membranes are capable
of exhibiting substantially molecular sieving behavior, the
separation factor for pairs of gases will in general greatly exceed
the Knudsen separation values.
[0062] The supported inorganic membranes are substantially defect
free, in that they are substantially free of cracks, pinholes and
the like. By substantially defect free is meant that the supported
inorganic membranes are at least 95% defect free, usually at least
97% defect free, more usually at least 99% defect free. The
supported inorganic membranes are thin, ranging in thickness from
10 to 200 nm, usually from 15 to 150 nm and more usually from 20 to
100 nm. Preferably, the thickness of the supported inorganic
membranes will be less than about 100 nm. The supported inorganic
membranes of the subject invention will have a narrow pore diameter
distribution, i.e. there will be little variance in pore diameter.
The pore diameter of the subject membranes will be sufficiently
small to provide for size exclusion of molecules, i.e. sufficiently
small to provide for selective passage of smaller molecules while
blocking larger molecules. The STS intermediate layer membrane will
have pores with an average diameter, dependent upon the surfactant
utilized, generally in the range of about 5 to 30 .ANG., more
usually from about 10 to 25 .ANG.. The A2** membrane will have
pores with an average diameters generally in the range of about 1
to 1 .ANG., usually from about 2 to 5 .ANG..
[0063] Some of the critical issues in the processing of the sol-gel
derived silica membranes include eliminating defect formation and
controlling pore size. Described below are some of the strategies
employed in one embodiment of the membrane formation to eliminate
defect formation. First, drying-induced stress as high as 200 MPa
in the silica sol system can result in film cracking unless the
film thickness is below a critical cracking thickness h.sub.c.
Through the adjustment of the sol concentration, withdrawal rate or
sol aging time to maintain the membrane film thickness below the
critical thickness h.sub.c (.about.4000 .ANG. for A2** sol),
cracking is avoided. Second, the membrane is processed under clean
conditions to substantially avoid foreign particles causing
pinholes. Third, STS intermediate layer is used to eliminate
inherent defects on commercially-available supports and to
facilitate the formation of subsequently deposited thin selective
membranes (e.g. A2** membrane). The STS intermediate layer is
designed with both high porosity and low tortuosity to avoid
creating additional flow resistance. Examples of strategies for
pore size control, include solvent (water) templating, and
surfactant-templating. Due to preferential alcohol evaporation
during the film deposition or sol contacting process, water is the
dominating solvent at the late drying stage. Water molecules
confined in the stressed film are used as a template to create
pores of molecular dimension needed for molecular sieving. For the
surfactant-templating strategy, surfactants, agregates or liquid
crystalline mesophases (amphiphilic molecules composed of a
hydrophilic head group and hydrophobic tail) are used as templates.
Surfactant-templated silicas are high surface area amorphous solids
(up to approximately 1400 m.sup.2/g) characterized by monosized,
often cylindrical pores organized into periodic arrays that often
mimic the liquid crystalline mesophases exhibited by
surfactant-water systems. Both water and surfactants can be removed
by heating to create uniform pores with various dimensions
depending on a choice of surfactant.
[0064] The resulting inorganic dual-layer membranes find use in a
variety of applications, including purification of sub-quality
natural gas, removal of NO.sub.x from power-plant flue gas,
reduction of greenhouse gases (e.g. CO.sub.2) and hydrogen recovery
from processing gases or hydrogen purification for fuel-cell
applications. The highly : selective top layer will, for example,
remove N.sub.2 and CO.sub.2 efficiently from natural gas without
suffering from CO.sub.2 plasticization commonly seen in dense
polymeric membranes or purify H.sub.2 from reformate for fuel-cell
applications.
[0065] The following examples are offered by way of illustration
and not by way of limitation.
EXPERIMENTAL
EXAMPLE 1
[0066] Synthesis of Micro-/Mesoporous Silica Materials
[0067] 1.A. Preparation of Silica Polymeric Sols
[0068] 1.A.1.--A silica polymeric sol (labeled A2** ) was prepared
as follows. Preparation of the A2** sol consists of two
acid-catalyzed reaction steps designed to minimize the condensation
rates of silica species in order to produce weakly branched
polymeric clusters that interpenetrate and collapse during film
deposition to produce membranes with molecular-sized pores. In the
first step an A2** stock solution was prepared from
tetraethoxysilane (TEOS) (Kodak), 200 proof ethanol (EtOH),
deionized H.sub.2O, and 0.07 N HCl (diluted from `Baker
Analyzed.RTM. 1 N HCl). The four components were mixed in a molar
ratio of 1.0:3.8:1.1:5.0.times.10.sup.-5, respectively, and
refluxed at 60.degree. C. for 90 min. while stirring at 200 rpm
(PMC 730 series DATAPLATE.RTM. or a Whatman DATAPLATE.RTM. model
440 P programmable digital hot plate/Stirrer, 500 ml Pyrex.RTM.
resin reaction kettle with lid, Pyrex.RTM. 24/40 condenser). The pH
of the prepared stock solution was measured using EM Science
ColorpHast.RTM. pH paper and found to be 4.7. This A2** stock
solution is very stable for approximately 90 to 180 days when
stored at -30.degree. C. In the second step, additional water and
HCl were added into the stock sol. The sol was agitated or
hand-shaken for 15 minutes.
[0069] 1.A.2.--The sol is then aged at 50.degree. C. for 22 hours.
The two-step procedure resulted in an A2** standard sol with a
final molar ratio of TEOS: EtOH: H.sub.2O:
HCl=1.0:3.8:5.0:0.004(pH=2.0).
[0070] 1.A.3.--A dip-coating sol was prepared by diluting the A2**
standard sol with two times its volume of ethanol. The dip-coating
A2** sol was filtered with 0.45 .mu.m filter (Nalgene) prior to
dip-coating.
[0071] 1.A.4.--An alternative silica species to the A2** sol which
will also produce membranes with molecular-sized pores is a
BTE/TEOS sol. The dip-coating BTE sol is prepared as described
above in 1.A.1. with the sol comprising
BTE(Bis(Triethoxysilyl)Ethane):TEOS:ethanol:H.sub.2O:HCl in a molar
ratio of 0.8:0.2:3.8:5.1:5.3.times.10.sup.-3.
[0072] 1.B.--Synthesis of Surfactant-Templated Micro-/Mesoporous
Silica Materials
[0073] 1.B.1.--Synthesis of a C6STS Microporous Silica Material. A
surfactant-template silica (STS) sol containing 0.125M
C6-surfactant (triethylhexylammonium bromide, Aldrich) silica sol
is prepared by adding 0.4 g per 100 g/sol amount of surfactant
powder into unaged A2** standard sol as prepared in 1.A.1. The
C6STS sol is then aged according to the process of 1.A.2. The C6STS
sol is then diluted as described in 1.A.3. with two times its
volume of ethanol. The aggregation of the surfactant during drying
does not cause any macroscopic phase separations. The C6STS sol is
filtered with a 1.0 .mu.m filter (Nalgene) prior to
dip-coating.
[0074] 1.B.2.--Synthesis of a C16STS Silica Material. A
surfactant-template silica sol containing a 4.2 wt % C16-surfactant
(cetyltrimethylammonium bromide) is prepared by adding 0.4 g per
100 g/sol amount of surfactant powder into unaged A2** standard sol
as prepared in 1.A.2. and follows the process described in
preparing the C6STS sol in 1.B.1.
[0075] 1 .B.3.--Characterization of the STS Intermediate Layer.
Referring to FIG. 1, for the C6STS xerogel as prepared in 1.B.1.,
differential thermal analysis (DTA) showed an endothermic peak near
200.degree. C. corresponding to the beginning of a drastic weight
loss indicating the decomposition of the C6-surfactant template
while an exothermic peak at around 350.degree. C. accompanied by a
weight loss of about 45%, signified the oxidative pyrolysis of
surfactant and residual organics. Referring to FIG. 2, N.sub.2
sorption isotherms of the calcined C6STS thin film (calcined at
500.degree. C. for one hour), characterized by a surface-acoustic
wave (SAW) technique, appeared to be Type I, characteristic of
microporous materials; while that of calcined C16STS thin film as
prepared in 1.B.2. appeared to be Type II, characteristic of
mesoporous materials. Still referring to FIG. 2, although N.sub.2
sorption isotherms of bulk calcined A2** xerogel powder appeared to
be Type I with a very sharp increase of N.sub.2 volume adsorbed
within a relative pressure of 0.0 to 0.01, indicating a narrow pore
size and pore size distribution, a thin-film form was virtually
inaccessible to N.sub.2 at 77K. due to differences in drying rates
(films dry much more rapidly). Average pore diameters of the C6STS
and C16STS were about 10-12 .ANG. and 18-20 .ANG., respectively.
Furthermore, the surface area and porosity of the C6STS materials
were 575 m.sup.2/g and 28%, respectively. Porosity, determined by
ellipsometry, agreed with the result calculated from N.sub.2
adsorption isotherms.
[0076] 1.B.4.--Synthesis of a Block Copolymer templated Silica
Material. A Brij56,
CH.sub.3(CH.sub.2).sub.15(CH.sub.2CH.sub.2O).sub.10OH, 4 wt %,
amphiphilic block copolymer silica sol is prepared by adding 4
g/100 g sol amount of amphiphilic block copolymer to the unaged
A2** standard sol as prepared in 1.A.2. and follows the process
described in preparing the C6STS sol in 1.B.1.
[0077] 1.C.--Sol Deposition
[0078] Membrane supports were prepared by sectioning a commercial
50 .ANG. .gamma.-Al.sub.2O.sub.3 tube (US filter) into several 5.5
cm-long sections and cleaned by ultrasound. The supports were then
washed with de-ionized water 5 times before calcining at
400.degree. C. for 60 min.
[0079] 1.C.1--Surfactant Sublayer Coating of the Support. A sol-gel
dip-coating process featuring aspects of slip casting was performed
in a laminar flow (150 ft/min) chamber under clean conditions of
Class 100. To prepare a C6STS sublayer, the support tube as
prepared and washed in 1.C. was lowered into the sol containing the
C6-surfactant as prepared and described above in 1.B.1. at a rate
of 8 inches/min. The support was held undisturbed for ten seconds,
the support was then withdrawn at the rate of 3 inches/min. The
surfactant coated support was then dried at a temperature of
25.degree. C. in a 20% relative humidity environment for 15 minutes
resulting in a surfactant-template polymeric layered membrane
support. The membrane was then calcined to 120.degree. C. in a
programmable furnace. In performing the calcination, the membrane
support was heated at a rate of 1.degree. C./min to the target
temperature 120.degree. C. and was held isothermally for 60 min.
The coated membrane support was then cooled at a rate of 1.degree.
C./min. to 25.degree. C. The cooled membrane support was then
confirmed to be substantially impermeable to helium by performing
the permeance measurements. The coated support was then subjected
to surfactant removal by heating the membrane tube in the
programmable furnace at a rate of 1.degree. C./min. to a
temperature of 500.degree. C. resulting in the intermediate
surfactant membrane support. The membrane was held isothermally for
60 min. then cooled at a rate of 1.degree. C./min. to 25.degree. C.
The same procedure could also be used to prepare membranes with
other surfactant-templated silica sublayers.
[0080] 1.C.2--A2** Layer Coating of the Membrane Support. The
support tube with the C6STS layer as prepared in 1.C.1. was lowered
into the A2** sol as prepared in 1A. at a rate of 10 inches/min.
and left undisturbed for 10 seconds. The membrane support was then
withdrawn at a rate of 1 inch/min. The A2** polymeric sol coated
substrate was then dried at a temperature of 25.degree. C. in a 20%
relative humidity environment for 15 minutes resulting in an A2**
layered membrane support. The A2** layered membrane support was
then calcined in a programmable furnace under a vacuum condition of
less than 4 psia (<4 psia) at a heating rate of 1-2.degree.
C./min from room temperature to 300.degree. C. The A2** coated
membrane was held isothermally for six hours at 300.degree. C. to
evacuate solvents and also promote further pore shrinkage. The A2**
coated membrane is then cooled at a rate of 1.degree. C./min. to
25.degree. C. The vacuum calcination procedure also results in the
decomposition of surface ethoxy groups; therefore, a hydrophobic
inner pore surface is formed as evidenced by an increase in the
water contact angle of membrane surface from 17.degree. to
41.degree. C. The imperfections near both ends of the final A2**
dual-layered tube membrane caused by being clamped in the laminar
flow chamber were then sealed by dense silicone (Duraseal 1529,
Cotronics Corp., N.Y.). The final permeable length of the A2**
coated tube is 4 cm.
[0081] 1 .C.3.--Further calcined A2** membrane. The dual layered
membrane as prepared above in 1.C.2. was further calcined, prior to
the coating of the imperfections near both ends, at 450.degree. C.
by heating the membrane at a rate of 1.degree. C./min. to the
target temperature of 450.degree. C., then held isothermally for 60
min. in air and cooled at a rate of 1.degree. C./min. to 25.degree.
C.
[0082] The dual silica coated permeable tube results in both high
flux and selectivity for the membranes with gradual changes of pore
size from 50 .ANG. (commercial .gamma.-alumina support layer) to
10-12 .ANG. (surfactant-templated silica sublayer), and then to 3-4
.ANG. (ultramicroporous silica top-layer). Surfactant templates
embedded in the silica framework are removed by calcination while
solvent templates are evacuated by a low-temperature thermal
treatment under vacuum. FIGS. 3A and 3B show an electron micrograph
view of the cross-section and the surface of the supported
dual-membrane. The thickness of the defect-free ultramicroporous
A2** layer is about 30 nm.
EXAMPLE 2
[0083] Sublayer Effect On Permeation And Separation
[0084] 2.A.--Test Parameters
[0085] The gas permeation through the dual-layer membrane produced
in 1.C. was measured using a custom built automated flow system as
shown in FIG. 4. An outgassing procedure is performed prior to
measuring the permeation because ambient moisture can easily
condense inside silica micropores. The outgassing procedure was
conducted at 80.degree. C. for three hours with dry helium purging
across the membrane. The procedure for determining gas permeance
included evacuating both sides of the membrane and then introducing
pure gas or mixed gas into the tube side of the membrane. Gases
were chosen to give a range of gas molecule sizes. Single
components of He, H.sub.2, CO.sub.2, CO.sub.2, N.sub.2, and
CH.sub.4, and either its dual or multiple component mixture were
tested. The pressure at the tube side was maintained at a constant
pressure (6.5 bar). Meanwhile, the shell-side pressure was
gradually built up due to permeated gases. Upon exceeding
atmosphere pressure, the shell side was exposed to ambient
pressure. To simulate practical operation, no sweeping gas was used
in all experiments and, therefore, the problems of back diffusional
flux were eliminated.
[0086] The flow rates of all inlet and outlet streams were directly
measured by digital bubble flow meters (Humonics). If the permeate
flow rate was below the detectable limit of the digital bubble flow
meter (.about.1 cc/min), it could be calculated from the rate of
pressure increase at a pre-vacuumed permeate side with a closed
outlet.
[0087] For mixed-gas permeation measurement, a pre-mixed gas with
known composition was used and the compositions of both effluent
streams were analyzed as functions of time using an on-line gas
chromatograph. The flow rate and the pressure of the tube-side
stream as well as the temperature of the membrane separator were
varied. The experiments were continued until steady-state
conditions (no change in flow rates and compositions with time)
were reached. The change of gas-phase driving force (the
partial-pressure gradient of component i, .DELTA.P.sub.i, across
the membrane) along the membrane was taken into account; therefore,
the permeance of the component i, P.sub.m,i, was defined as: 1 P m
, i = J i P ln , i where P ln , i = ( P i ) I - ( P i ) I I ln { (
P i ) I / ( P i ) I I }
[0088] where J.sub.i is the steady-state flux of component i
through the membrane. (.DELTA.P.sub.i).sup.I and
(.DELTA.P.sub.i).sup.II are the partial-pressure differences of
component i between the tube-side and shell-side pressures of the
membrane at the gas entrance (I) and exit (II) ends,
respectively.
[0089] The separation factor defined by the ratio of permeabilities
can be equivalent to the ratio of permeances if the membrane
thickness is identical. Thus, for pure-gas permeation, the ideal
separation factor .alpha..sub.1 could be defined by the ratio of
permeances of individual pure gases. Analogous to the definition of
.alpha..sub.I, the true separation factor .alpha..sub.t of mixed
gas is defined by the ratio of permeances of constituent gases.
[0090] 2.B.--Ultramicroporous Silica Membrane with Microporous
C6STS Sublayer
[0091] A coated membrane support as produced in 1.C. utilizing the
C6STS sol as prepared in 1.B.1. is characterized using a single gas
permeability measurement system. The membrane ends were sealed
using Viton.RTM. or Grafoil.RTM. gasket material, and the
compression of the gasket avoided by-passing of the gases. A custom
built automated flow system, as described above in 2.A. was used to
measure the permeance (flux/pressure) of five different gases
through the membrane. The gases were chosen to give a range of gas
molecule sizes. These gases, along with their characteristic
diameters, are: He (2.65 .ANG.), H.sub.2 (2.89.ANG.), CO.sub.2 (3.3
.ANG.), N.sub.2 (3.64 .ANG.), and CH.sub.4 (3.8 .ANG.). Apart from
the different sizes, the inert gases have different chemical
interactions with the membrane surface. Thus, the flow through the
membrane will be a combination of Knudsen diffusion, surface
diffusion and micropore diffusion. The relative contribution from
each of the above flow mechanisms varies according to the gas, as
well as the pore size of the membrane.
[0092] The flow through the membranes was measured with two bubble
meters installed on the exhaust line. The results obtained from the
single gas permeability measurements were reported as permeance
(cm.sup.3/cm.sup.2-s-cm-Hg) vs measurement temperature and the
ideal selectivity .alpha..sub.1/2(flux of pure gas 1/flux of pure
gas 2) vs. temperature.
[0093] The C6STS sublayer as prepared serves to eliminate intrinsic
defects on commercial porous supports and promotes pore uniformity;
therefore increasing selectivity, and preventing penetration of a
subsequently deposited ultramicroporous membrane (e.g. A2**
membrane with an average pore size of about 3-4 .ANG. as described
in 1.D.), thus enhancing flux. Gas permeances and selectivities
were compared for A2** membranes with and without a C6STS sublayer.
Referring to FIG. 5A and 5B, at 60.degree. C., the membrane with a
sublayer exhibited four-fold higher CO.sub.2 permeances and
four-fold higher CO.sub.2/CH.sub.4 selectivities than that without
a sublayer in a single-component gas permeation measurement. Ideal
separation factors of various gas pairs (e.g.
.alpha..sub.I(CO.sub.2/CH.sub.4)=102 at 25.degree. C.) largely
exceeded Knudsen separation factors (e.g.
.alpha..sub.K(CO.sub.2/CH.sub.4)=0.6). The exceptional negative
activation energy of CO.sub.2 (E.sub.a=-3.37 KJ/mol) for the
membrane with a C6STS sublayer indicated the occurrence of CO.sub.2
capillary condensation. Therefore, CO.sub.2 is transported with
high density through narrow pores at lower temperatures. On the
contrary, CO.sub.2 capillary condensation was insignificant for the
membrane without a C6STS sublayer. Therefore, a gradual increase in
CO.sub.2 permeance with temperature (activated transport) is
observed (E.sub.a=2.48 KJ/mol). Thus, a high CO.sub.2 permeance
(3.2.times.10.sup.-4 cm.sup.3(STP)/s/cm.sup.2/cmHg) and a high
CO.sub.2/CH.sub.4 separation factor of approximately 200 can be
achieved with the dual layer membrane at 26.degree. C. for
separation of 50/50 (v/v) CO.sub.2/CH.sub.4 gas mixture, as shown
in FIG. 6. The combination of high permeance and high selectivity
exceeded that of prior art gas separation membranes (e.g.
asymmetric polyimide with a typical CO.sub.2/CH.sub.4 separation
factor of 55 and a CO.sub.2 permeance of 1.7.times.10.sup.-4
cm.sup.3(STP)/s/cm.sup.2/cmHg). Moreover, activation energies of
CH.sub.4 for both A2** membranes with and without a sublayer were
12.92 and 9.64 KJ/mol, respectively, which were higher than those
of CO.sub.2. CH.sub.4 diffused through both the membranes with low
permeance via an activated transport mechanism. Referring back to
FIG. 5A, the combination of CO.sub.2 condensation effect and the
activated transport mechanism help in explaining the increase in
ideal separation factor of CO.sub.2/CH.sub.4 upon the decrease in
temperature for the membrane with a sublayer.
[0094] 2.C.--BTE/TOES Microporous Silica Membrane with a Mesoporous
C16STS Sublayer
[0095] A membrane support prepared as described in 1.C. utilizing
the mesoporous C16STS material as the surfactant-templated silica
sublayer as prepared in 1.B.2. shows substantially the same
substrate effects as the microporous C6STS materials described
above in 2.B. A 5.5 cm commercial 50 .ANG. .gamma.-Al.sub.2O.sub.3
membrane tube was initially cleaned by ultrasound, then washed and
calcined as described in 1.C. The membrane tube was dip-coated into
the surfactant-templated sol as described in 1.C.1. utilizing the
C16STS sol as prepared in 1.B.2. The C16-surfactant was then
removed by heating the C16STS coated support at a rate of 1.degree.
C./min. to the target temperature of 500.degree. C. The coated
support was held isothermally for three hours and then cooled at a
rate of 1.degree. C./min. to 25.degree. C. A microporous membrane
was then formed on the calcined membrane support with the C16STS
intermediate layer by dip-coating the C16STS coated support into a
BTE (Bis(Triethoxysilyl)Ethane) silica sol as prepared in 1.A.4.
and filtered utilizing 0.45 .mu.m filter (Nalgene) prior to
dip-coating. The Ethane ligands (--CH.sub.2CH.sub.2--) of the BTE
membrane embedded in the silica framework are removed by
calcination at 280.degree. C. by heating the BTE coated support at
1.degree. C./min. to the target temperature. The coated support was
held isothermally for three hours and then cooled at a rate of
1.degree. C./min. to 25.degree. C., leaving behind a microporous
membrane. A comparison of the membranes with and without a
mesoporous sublayer is shown in FIGS. 7A and 7B. The results show
that membranes with a mesoporous sublayer exhibit significantly
higher permeance and significantly better selectivity than those
without a sublayer. This is consistent with the observation in
Example 2.B., demonstrating the crucial role of the sublayer in
improving flux and selectivity of an overlying microporous silica
membrane.
[0096] 2.D.--Alternative Intermediate Layers
[0097] The same concept of providing a microporous or mesoporous
intermediate layer can be applied to the use of an ordered
mesoporous silica membrane as a sublayer to minimize transport
resistance. An amphiphilic block copolymer (Brij56,
CH.sub.3(CH.sub.2).sub.15(CH.sub.2 CH.sub.2O).sub.10OH, 4 wt %) was
used as a templating agent in replace of surfactants as prepared
and described in 1.B.4. The copolyer sol is deposited onto
substrate as described in 1.C.1. To facilitate re-organization of
the block-copolymer template, the coating of the substrate with the
copolymer sol is performed in a humid chamber of 40% relative
humidity at a temperature of 25.degree. C. The block copolymer
template silica results in a high porosity (50-65%) and ordered
cubic structure having uniform pore size of about 23 .ANG..
Therefore, the block-copolymer is another candidate which can be
used as the intermediate layer for a subsequently deposited
effective separation layer, such as A2**.
EXAMPLE 3
[0098] Effects Of The Ultramicroporous Silica Membrane Calcined At
450.degree. C.
[0099] 3A.--Ultramicroporous Silica Membrane for Hydrogen
Purification
[0100] A dual layered membrane as prepared in 1.C.3. was calcined
at the 450.degree. C. for one hour in air and cooled at a rate of
1.degree. C./min. to 25.degree. C. Single-component gas permeation
measurements were taken utilizing the measurement system as
described in 2.A. at various temperatures ranging from 20.degree.
C. to 150.degree. C. Pure-gas permeation results at 80.degree. C.
are shown in FIG. 8. Due to the extended calcination for one hour
at 450.degree. C., the pore size of the membrane was further
reduced, resulting in a sharp molecular-size cut-off at about 3.5
.ANG.. The dual layer membrane support obtained through the
extended calcination at 450.degree. C. results in a superior
hydrogen separation factor (H.sub.2/CH.sub.4=1265) as well as
having a high hydrogen permeance (1.times.10.sup.-3
cm.sup.3(STP)/cm.sup.2/s/cmnHg- ). Such a selective membrane
provides a great opportunity in applications such as hydrogen
recovery from petrochemical plants and hydrogen purification for
fuel cells. The membrane selectively separated hydrogen from a
simulated reformate gas mixture consisting of 33.98% N.sub.2,
15.00% CO.sub.2, 0.997% CO, 50.023% H.sub.2 (composition produced
by the partial oxidation of methanol) for fuel cells as evidenced
by the high concentration of hydrogen recovered in the permeate
side stream as shown in Table 1.
1TABLE 1 Permeances of components of a gas mixture* at 80.degree.
C. Permeance .times. 10.sup.6 Separation Pure gas (cm.sup.3
(STP)/s/cm.sup.2/ Factor permeance Permeate Retentate Gas i c/cmHg)
(H.sub.2/Gas i) ratio (mole %) (mole %) H.sub.2 507 -- -- 92.19
42.25 CO.sub.2 101 5.0 5.2 7.36 16.89 N.sub.2 2.15 235.9 316.0 0.37
40.47 CO 3.83 132.2 198.6 0.0193 1.14 *50.023% H.sub.2, 15%
CO.sub.2, 33.98% N.sub.2, 0.997% CO; Tube-side pressure = 6.5 bar;
Stage-cut = 8.2%.
[0101] A 92 mole % H.sub.2 purity was obtained in the permeate
stream at a stage-cut of 8.2% (stage-cut=the ratio of permeate flow
rate to feed flow rate). The CO concentration, (CO is a known PEM
fuel cell poison) in the permeate reduced to at least fifty times
lower than that in the feed. A high H.sub.2 permeance
(6.times.10.sup.-4(STP)/cm.sup.2/s/cmHg) and a high H.sub.2/N.sub.2
separation factor of over 270 for separation of a 50/50 (v/v)
H.sub.2/N.sub.2 gas mixture were achieved as shown in Table 2.
2TABLE 2 The use of a AC450 silica membrane for separation of a
50/50 (v/v) H.sub.2/N.sub.2 gas mixture* at 80.degree. C. Permeance
.times. 10.sup.6 Separation Pure gas (cm.sup.3 (STP)/s/cm.sup.2/
Factor permeance Permeate Retentate Gas i c/cmHg) (H.sub.2/Gas i)
ratio (mole %) (mole %) H.sub.2 606 -- -- 99.41 37.43 N.sub.2 2.21
274.5 316.0 0.59 62.57 *50.023% H.sub.2, 15% CO.sub.2, 33.98%
N.sub.2, 0.997% CO; Tube-side pressure = 6.5 bar; Stage-cut =
11%.
[0102] Besides H.sub.2 purification, the membrane can also be
applied to NO.sub.x removal from fuel gas. A single-component
permeation measurement was also performed in which NO/NO.sub.2
selectivity was measured to be 9.3 cm.sup.3(STP)/cm.sup.2/s/cmHg
and NO permeance reached 3.times.10.sup.-5
cm.sup.3(STP)/cm.sup.2/s/cmHg.
[0103] 3.B.--Comparison of the Ultramicroporous Silica Membrane
[0104] A dual-layer supported membrane as prepared in 3.A. was
further compared to prior art molecular sieving methods and
devices. Operating at 26.degree. C., a moderate pressure gradient
(.DELTA.P=5.5 bar), and high feed flow rate (.about.500 cm.sup.3
(STP)/min), a high CO.sub.2 permeance of about 3.2.times.10.sup.-4
cm.sup.3(STP)/s /cm.sup.2/cmHg and a high CO.sub.2/CH.sub.4
separation factor of over 200 was achieved for the separation of
50/50 (v/v) CO.sub.2/CH.sub.4 gas mixture, as shown in FIG. 9.
Still referring to FIG. 9, the A2** membrane as prepared in 1.C.2.
without further calcination, results in a combination of high
permeance and high selectivity which is superior to prior art
separation membranes. The A2** membrane as prepared in 3.A. with
further calcination results in a higher CO.sub.2/CH.sub.4
separation factor (.about.600) at the expense of CO.sub.2
permeance. The comparisons are made under the assumption that the
best membrane performance was reported under their optimal
operation conditions for each membrane.
[0105] EXAMPLE 4
[0106] Permeance And CO.sub.2CH.sub.4 Separation Factor As Function
Of Feed Flow Rate, Temperature And Pressure Gradient
[0107] The effect of temperature, flow rate and pressure gradient
across the membrane on both CO.sub.2 permeance and
CO.sub.2/CH.sub.4 separation factor was investigated via mixed-gas
permeation experiments. Mixed-gas permeation experiments
representing the true separation were conducted with 50/50 (v/v)
CO.sub.2/CH.sub.4 feed gas under steady-state conditions.
[0108] 4.A.--Feed Flow-Rate Effects
[0109] A 5.5 cm commercial 50 .ANG. .gamma.-Al.sub.2O.sub.3
membrane tube prepared as described in 1.C. was dip-coated into the
surfactant-template sol as described in 1.C.1. utilizing the C6STS
sol as prepared in 1.B.1. The C6STS coated membrane was then
dip-coated in an A2** sol as described in 1.C.2. The experiment for
feed flow-rate effect was operated at a constant temperature of
26.degree. C. and a constant pressure gradient of .DELTA.P=5.5 bar
across the membrane. The stage cut .theta., defined as the ratio of
permeate flow rate to feed flow rate, represents the fraction of
feed gas permeated through the membrane (gas recovery ratio).
Referring to FIGS. 10A and 10B, it is shown that the stage cut
.theta. decreases with the increase in the feed flow rate. The
decrease in the stage cut .theta. resulted in the increase in the
CO.sub.2/CH.sub.4 separation factor, .alpha.(CO.sub.2/CH.sub.4), as
shown in FIG. 10A. Thus, higher purity of CO.sub.2 was recovered at
the permeate side at the lower stage cut .theta.. In a high feed
flow rate (low stage cut), the concentration of the fast permeating
gas, CO.sub.2, remained high at retentate, as shown in FIG. 10B.
Therefore, a high CO.sub.2 driving force across the membrane was
maintained. In addition, CO.sub.2 is allowed to pass through pores
with little chance of being blocked by the larger CH.sub.4
molecules. Alternatively, the CH.sub.4 driving force across the
membrane increases with the decrease in feed flow rate, resulting
in a lower .alpha.(CO.sub.2/CH.sub.4). High
.alpha.(CO.sub.2/CH.sub.4) can be obtained at high feed flow rates
but at the expense of both low gas recovery ratio and pumping
energy. The maximum .alpha.(CO.sub.2/CH.sub.4) at a fixed
temperature and pressure in the tubular membrane can be achieved
when the retentate compositions are equivalent to the feed
compositions. Moreover, due to selective adsorption of CO.sub.2 at
lower temperatures, the mixed-gas .alpha.(CO.sub.2/CH.sub.4) is
higher than the single-component separation factor,
.alpha..sub.1(CO.sub.2/CH.sub.4), under identical operating
conditions. The selectively adsorbed CO.sub.2 would either cover
the pore entrance or further reduce the size of the pore opening,
depending on the size of pores, thus preventing CH.sub.4 from
permeating through the pores.
[0110] 4.B.--Temperature Effects
[0111] A dual-layer membrane as prepared in 4.A. was subjected to a
constant feed flow-rate represented by a stage-cut of 9% and a
constant pressure gradient of .DELTA.P=5.5bar across the membrane
while the temperature was varied from 26.degree. C. to 120.degree.
C. The value of the CO.sub.2 permeance and the separation factor,
.alpha.(CO.sub.2/CH.sub- .4), are inversely related as the
temperature increases. FIG. 6 shows that the
.alpha.(CO.sub.2/CH.sub.4) decreases with the increase in
temperature, while the CO.sub.2 permeance increases slightly with
the increase in temperature. The abrupt increase in the
.alpha.(CO.sub.2/CH.sub.4) at lower temperatures was due to the
combined effects of activated transport and CO.sub.2 selective
adsorption where the capability of CO.sub.2 adsorption on to silica
surfaces diminishes at higher temperature. Due to the CO.sub.2
selective adsorption effect, the .alpha.(CO.sub.2/CH.sub.4) for
mixed gas at lower temperature is higher than the CO.sub.2
separation factor for a single-component gas,
.alpha..sub.1(CO.sub.2/CH.sub.4). The difference between
.alpha.(CO.sub.2/CH.sub.4) and .alpha..sub.1(CO.sub.2/CH.sub.4)
vanishes at higher temperatures as can be seen in comparing FIG. 5A
to FIG. 6.
[0112] 4.C.--The Effects Of Pressure
[0113] The effects of pressure on CO.sub.2 permeance through a
dual-layer membrane was determined. The membrane, as prepared in
4.A., was subjected to a constant feed flow-rate, represented by a
stage-cut of 9%, and a constant temperature of 26.degree. C. The
retentate pressure was varied while permeate pressure was kept at
ambient atmospheric pressure. Referring to FIG. 11 it can be seen
that the .alpha.(CO.sub.2/CH.sub.4) is essentially independent of
the pressure gradient while the CO.sub.2 permeance slightly
increases with pressure gradient and then levels off at a higher
pressure gradient. The increase in CO.sub.2 permeance at the lower
pressure gradient is indicative of the startup of CO.sub.2
adsorption inside the pores. Once the CO.sub.2 is adsorbed on the
pore walls, the CO.sub.2 permeance obeys Henry adsorption
characteristics such that the CO.sub.2 permeance is independent of
the pressure gradient across the membrane.
[0114] 4.D.--Effects of Continuous Operation
[0115] A membrane, as prepared in 4.A., was continuously operated
for 150 hours at a constant temperature, pressure gradient, and
feed flow rate conditions. Referring to FIG. 12, the CO.sub.2
permeance and separation factor, .alpha.(CO.sub.2/CH.sub.4),
gradually increase with time. This is due to a gradual removal of
adsorbed pore fluid by an extensive gas permeation. Nevertheless,
stable and continuous operation is achieved at a temperature of
26.degree. C.
[0116] The foregoing description of a specific embodiment of the
present invention is presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed; obviously many
modifications and variations are possible in view of the above
teachings. The embodiment was chosen and described in order to best
explain the principles of the invention and its practical
applications, to thereby enable others skilled in the art to best
utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
following claims and their equivalents.
* * * * *