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Gas separation membranes: A novel application for conducting polymers
Synthetic Metals, 41--43 (1991) 1151-1154
1151
GAS SEPARATION MEMBRANES: A NOVEL APPLICATION FOR CONDUCTING POLYMERS
M. R. ANDERSON, B.R. MATTES, H. REISS and R.B. KANER Department of Chemistry and Biochemistry University of California at Los Angeles, Los Angeles, California 90024-1569, U.S.A.
ABSTRACT Gas permeability values are reported for films of the conducting polymer polyaniline in the emeraldine oxidation state. Substantial
improvements in gas
selectivity, when compared to literature values, were observed for all gas pairs studied (He/N2, H2/N2, O2/N2, CO2/CH4). The doping procedure which enables conductivity in the film was found to cause significant concurrent changes in the film morphology which resulted
in improved gas separations.
These morphological changes primarily
influence the permeant diffusivity as shown by comparison to the kinetic diameter of the permeant. The potential for development of conducting polymer membranes with precisely controlled morphology is clearly indicated, allowing optimization of films for any gas separation. INTRODUCTION Membrane based gas separation systems may offer enormous savings in energy over standard
cryogenic
separation
processes 1,2. Although great progress has been
made in membrane technology, an inverse relationship is commonly found between selectivity
and
permeability 3. This has severely limited potential applications. Here we
report a new concept in tailoring the selectivity of membranes which may overcome this paradox. Conjugated polymers are employed which can be doped and undoped to open up size selective pores. Dopable conjugated polymers have been studied extensively for their electrical p r o p e r t i e s 4. Gas separations represent a new application for these materials which does not depend on their conductivity, but rather utilizes the reversible doping possible with conducting polymers. Polyaniline was chosen for study because of its environmental stability, its simple reversible acid/base doping chemistry and its ability to be formed into high quality films. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1152
EXPERIMENTAL Prenaration
o f p01yaniline
films
A 50g batch of polyaniline hydrochloride, in the emeraldine oxidation state, was synthesized following known
procedures 5. The polymer powder was purified by
successive equilibration cycles of acid(lM HCI) and base(0.1M NH4OH) in solution with subsequent filtration. The dried emeraldine base powder was manually ground to a fine powder, dried under vacuum, and stored in a desiccator for further use. 5g of emeraldine powder was added incrementally to a mortar and pestle containing 50 ml of n-methyl pyrrolidone. The slurry was ground for 45 minutes which resulted in a homogeneous, viscous solution. The solution was cast onto glass plates with a spreader bar to give films. The plates were heated to 135°C for 3 hours in a drying oven to cure the polymer films. The resulting films were released by immersion in water and then dried under dynamic vacuum for at least 24 hours prior to use. Measurement of ~as nermeabilitv A sample of the film was mounted in the test cell and thoroughly degassed under high vacuum before being subjected to the permeability test. The test system designed and constructed for the measurement utilizes the manometric method 6, i.e. constant permeation volume, to measure the gas flux on the low pressure side of the membrane. Pure gas permeabilities for N2, O2, CO2, and He were measured on the film as cast at 40 psi. The same sample was then removed from the test cell and doped by immersion in 4M HC1 solution for 15 hours. It was then dried under vacuum, remounted in the test cell and degassed for 24 hours before running the second series of tests( including H2
and CH4).
The sample was removed again and undoped by immersion in 1M NH40H for 24 hours. After degassing the film, the test series was repeated again. The film was removed a third time and doped from 0.01M HCI solution as before and tested a fourth time. The data for all series are shown in Table 1. RESULTS AND DISCUSSION From the data in the first column of Table I (as cast), it is clear that polyaniline film has some permeability to all gases which is size dependent, thus leading to modest separation ratios. Heavy doping of the film substantially reduces the permeability of each gas (2nd column). Undoping the film noticeably increases the permeability of the small gases relative to their as cast values (3rd column). Light redoping of the film blocks the permeation of the larger gases more effectively than the smaller ones(4th column), leading to very high separation ratios.
1153
TABLE 1 Permeabilitva--(P) of a nolvaniline film (~a8
g~ cast
4M doped
undoDed
0.01M redoDed
He
4.90
2.06
11.5
7.31
H2
....
1.40
10.6
6.54
(302
1.44
<0.005 b
1.59
0.732
02
0.280
<0.005 b
0.172
0.110
N2
0.0382
<0.005 b
0.00323
0.00253
CH~
....
0,608
<0.005 b
a Values in Barters(10 -10 c m 3 ( S T P ) / c m . s e c . c m H g ) b Below detection limit The separating ability of polyaniline films may be adjusted for a variety of gas mixtures as a function of the dopant concentration and subsequent changes in film morphology. Table 2 shows the optimum separation factors calculated from Table 1, compared to the best literature values for each separation. TABLE 2 Separation
factors-
calculated
versus
literature
Gas oair
oolvaniline
literatureT, 8
He/N2
3560
(undoped)
2200(poly(triflurochloroethylene))
H2/N2
3281
(undoped)
313(poly(triflurochloroethylene))
O2/N2
53
COg/CHa
146 (redooed)
16(cellulose
(undoped)
60(fluorinated
nitrate) Dolvimide)
TABLE 3 Kinetic diameter versus vermeabilitv of a doDed/undoned nolvaniline film Gas CI-Ia
He
H9
CO?
Q7
~?
e(A)
2.6
2.89
3.3
3.46
3.64
3.8
['(barrers)
11.5
10.6
1.59
0,172
0.00323
0.608
The general trend in the order of the permeability of the gases correlates strongly with the kinetic diameter9as shown in Table 3. This correlation indicates that the permeability of polyaniline films are primarily influenced by diffusion and not s o l u b i l i t y 10,11, although CH4 appears to have a non-negligable solubility component. It is clear that protonic doping with acid and undoping with base changes the morphology of the films which alters their average effective pore size. This enables polyaniline films to function as gas separation membranes. Potential technological applications are now being studied 12
1154
ACKNOWLEDGEMENTS This work was funded by the Air Force Office of Scientific Research under Contract No. F4962-086-C-0060 and the National Science Foundation Grant No. CHE-86-57882. REFERENCES 1
J.M.S. Henis and M.K. Tripodi, Science, 220 (1983), p.4592
2
J. Haggin, Chem. Eng. News, 66 (1988), p.7
3
R.T. Chern,W.J. Koros, H.B. Hopfenburg and V.T. Stannett, in D.R. Lloyd (ed.),
Materials Science of Synthetic Membranes, ACS Symposium Series No. 269, American Chemical Society, Washington, D.C., 1985
4
Handbook of Conducting Polymers, Vols 1 & 2, T.J. Skotheim (ed.), Marcel Dekker,
5
W.S. Huang, B.D. Humphrey and A.G. MacDiarmid, J. Chem Soc., Faraday
New York, 1986
Trans.,82(1986),p.2385 6
ASTM Designation D1434-82 Determining Gas Permeability Characteristics of Plastic
Film and Sheeting, Annual Book of ASTM Standards, 1982 7
H. Yasuda and V.T. Stannett, in J. Brandrup and E.H. Immergut (eds.), Polymer
Handbook, 2nd Ed., Wiley and Sons, New York, 1975 8
T.H. Kim, W.J. Koros, G.R. Husk, K.C. O'Bden, J. Memb. Sci., 37 (1988) 45
9
D.W. Breck, in Zeolite Molecular Sieves, Wiley and Sons, New York,1974, p.636
10
W.J. Koros and D.R. Paul, in M.B. Chenoweth (ed.), Synthetic Membranes, Harwood Academic Publishers, Chur, Switzerland,
11
E.S. Sanders, J. Memb. Sci. ,37 (1988) 63
12
Patent
Pending
1986
1151
GAS SEPARATION MEMBRANES: A NOVEL APPLICATION FOR CONDUCTING POLYMERS
M. R. ANDERSON, B.R. MATTES, H. REISS and R.B. KANER Department of Chemistry and Biochemistry University of California at Los Angeles, Los Angeles, California 90024-1569, U.S.A.
ABSTRACT Gas permeability values are reported for films of the conducting polymer polyaniline in the emeraldine oxidation state. Substantial
improvements in gas
selectivity, when compared to literature values, were observed for all gas pairs studied (He/N2, H2/N2, O2/N2, CO2/CH4). The doping procedure which enables conductivity in the film was found to cause significant concurrent changes in the film morphology which resulted
in improved gas separations.
These morphological changes primarily
influence the permeant diffusivity as shown by comparison to the kinetic diameter of the permeant. The potential for development of conducting polymer membranes with precisely controlled morphology is clearly indicated, allowing optimization of films for any gas separation. INTRODUCTION Membrane based gas separation systems may offer enormous savings in energy over standard
cryogenic
separation
processes 1,2. Although great progress has been
made in membrane technology, an inverse relationship is commonly found between selectivity
and
permeability 3. This has severely limited potential applications. Here we
report a new concept in tailoring the selectivity of membranes which may overcome this paradox. Conjugated polymers are employed which can be doped and undoped to open up size selective pores. Dopable conjugated polymers have been studied extensively for their electrical p r o p e r t i e s 4. Gas separations represent a new application for these materials which does not depend on their conductivity, but rather utilizes the reversible doping possible with conducting polymers. Polyaniline was chosen for study because of its environmental stability, its simple reversible acid/base doping chemistry and its ability to be formed into high quality films. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1152
EXPERIMENTAL Prenaration
o f p01yaniline
films
A 50g batch of polyaniline hydrochloride, in the emeraldine oxidation state, was synthesized following known
procedures 5. The polymer powder was purified by
successive equilibration cycles of acid(lM HCI) and base(0.1M NH4OH) in solution with subsequent filtration. The dried emeraldine base powder was manually ground to a fine powder, dried under vacuum, and stored in a desiccator for further use. 5g of emeraldine powder was added incrementally to a mortar and pestle containing 50 ml of n-methyl pyrrolidone. The slurry was ground for 45 minutes which resulted in a homogeneous, viscous solution. The solution was cast onto glass plates with a spreader bar to give films. The plates were heated to 135°C for 3 hours in a drying oven to cure the polymer films. The resulting films were released by immersion in water and then dried under dynamic vacuum for at least 24 hours prior to use. Measurement of ~as nermeabilitv A sample of the film was mounted in the test cell and thoroughly degassed under high vacuum before being subjected to the permeability test. The test system designed and constructed for the measurement utilizes the manometric method 6, i.e. constant permeation volume, to measure the gas flux on the low pressure side of the membrane. Pure gas permeabilities for N2, O2, CO2, and He were measured on the film as cast at 40 psi. The same sample was then removed from the test cell and doped by immersion in 4M HC1 solution for 15 hours. It was then dried under vacuum, remounted in the test cell and degassed for 24 hours before running the second series of tests( including H2
and CH4).
The sample was removed again and undoped by immersion in 1M NH40H for 24 hours. After degassing the film, the test series was repeated again. The film was removed a third time and doped from 0.01M HCI solution as before and tested a fourth time. The data for all series are shown in Table 1. RESULTS AND DISCUSSION From the data in the first column of Table I (as cast), it is clear that polyaniline film has some permeability to all gases which is size dependent, thus leading to modest separation ratios. Heavy doping of the film substantially reduces the permeability of each gas (2nd column). Undoping the film noticeably increases the permeability of the small gases relative to their as cast values (3rd column). Light redoping of the film blocks the permeation of the larger gases more effectively than the smaller ones(4th column), leading to very high separation ratios.
1153
TABLE 1 Permeabilitva--(P) of a nolvaniline film (~a8
g~ cast
4M doped
undoDed
0.01M redoDed
He
4.90
2.06
11.5
7.31
H2
....
1.40
10.6
6.54
(302
1.44
<0.005 b
1.59
0.732
02
0.280
<0.005 b
0.172
0.110
N2
0.0382
<0.005 b
0.00323
0.00253
CH~
....
0,608
<0.005 b
a Values in Barters(10 -10 c m 3 ( S T P ) / c m . s e c . c m H g ) b Below detection limit The separating ability of polyaniline films may be adjusted for a variety of gas mixtures as a function of the dopant concentration and subsequent changes in film morphology. Table 2 shows the optimum separation factors calculated from Table 1, compared to the best literature values for each separation. TABLE 2 Separation
factors-
calculated
versus
literature
Gas oair
oolvaniline
literatureT, 8
He/N2
3560
(undoped)
2200(poly(triflurochloroethylene))
H2/N2
3281
(undoped)
313(poly(triflurochloroethylene))
O2/N2
53
COg/CHa
146 (redooed)
16(cellulose
(undoped)
60(fluorinated
nitrate) Dolvimide)
TABLE 3 Kinetic diameter versus vermeabilitv of a doDed/undoned nolvaniline film Gas CI-Ia
He
H9
CO?
Q7
~?
e(A)
2.6
2.89
3.3
3.46
3.64
3.8
['(barrers)
11.5
10.6
1.59
0,172
0.00323
0.608
The general trend in the order of the permeability of the gases correlates strongly with the kinetic diameter9as shown in Table 3. This correlation indicates that the permeability of polyaniline films are primarily influenced by diffusion and not s o l u b i l i t y 10,11, although CH4 appears to have a non-negligable solubility component. It is clear that protonic doping with acid and undoping with base changes the morphology of the films which alters their average effective pore size. This enables polyaniline films to function as gas separation membranes. Potential technological applications are now being studied 12
1154
ACKNOWLEDGEMENTS This work was funded by the Air Force Office of Scientific Research under Contract No. F4962-086-C-0060 and the National Science Foundation Grant No. CHE-86-57882. REFERENCES 1
J.M.S. Henis and M.K. Tripodi, Science, 220 (1983), p.4592
2
J. Haggin, Chem. Eng. News, 66 (1988), p.7
3
R.T. Chern,W.J. Koros, H.B. Hopfenburg and V.T. Stannett, in D.R. Lloyd (ed.),
Materials Science of Synthetic Membranes, ACS Symposium Series No. 269, American Chemical Society, Washington, D.C., 1985
4
Handbook of Conducting Polymers, Vols 1 & 2, T.J. Skotheim (ed.), Marcel Dekker,
5
W.S. Huang, B.D. Humphrey and A.G. MacDiarmid, J. Chem Soc., Faraday
New York, 1986
Trans.,82(1986),p.2385 6
ASTM Designation D1434-82 Determining Gas Permeability Characteristics of Plastic
Film and Sheeting, Annual Book of ASTM Standards, 1982 7
H. Yasuda and V.T. Stannett, in J. Brandrup and E.H. Immergut (eds.), Polymer
Handbook, 2nd Ed., Wiley and Sons, New York, 1975 8
T.H. Kim, W.J. Koros, G.R. Husk, K.C. O'Bden, J. Memb. Sci., 37 (1988) 45
9
D.W. Breck, in Zeolite Molecular Sieves, Wiley and Sons, New York,1974, p.636
10
W.J. Koros and D.R. Paul, in M.B. Chenoweth (ed.), Synthetic Membranes, Harwood Academic Publishers, Chur, Switzerland,
11
E.S. Sanders, J. Memb. Sci. ,37 (1988) 63
12
Patent
Pending
1986