U.S. patent application number 10/901401 was filed with the patent office on 2006-02-02 for cross-linked polybenzimidazole membrane for gas separation.
Invention is credited to Brent F. Espinoza, Gregory S. Long, Jennifer S. Young.
Application Number | 20060021502 10/901401 |
Document ID | / |
Family ID | 35730691 |
Filed Date | 2006-02-02 |
United States Patent
Application |
20060021502 |
Kind Code |
A1 |
Young; Jennifer S. ; et
al. |
February 2, 2006 |
CROSS-LINKED POLYBENZIMIDAZOLE MEMBRANE FOR GAS SEPARATION
Abstract
A cross-linked, supported polybenzimidazole membrane for gas
separation is prepared by reacting polybenzimidazole (PBI) with the
sulfone-containing crosslinking agent
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide. The cross-linked
reaction product exhibits enhanced gas permeability to hydrogen,
carbon dioxide, nitrogen, and methane as compared to the unmodified
analog, without significant loss of selectivity, at temperatures
from about 20 degrees Celsius to about 400 degrees Celsius.
Inventors: |
Young; Jennifer S.; (Los
Alamos, NM) ; Long; Gregory S.; (Los Alamos, NM)
; Espinoza; Brent F.; (Los Alamos, NM) |
Correspondence
Address: |
UNIVERSITY OF CALIFORNIA;LOS ALAMOS NATIONAL LABORATORY
P.O. BOX 1663, MS A187
LOS ALAMOS
NM
87545
US
|
Family ID: |
35730691 |
Appl. No.: |
10/901401 |
Filed: |
July 28, 2004 |
Current U.S.
Class: |
95/45 |
Current CPC
Class: |
B01D 2323/30 20130101;
Y02C 10/10 20130101; B01D 2325/20 20130101; B01D 71/62 20130101;
B01D 69/10 20130101; Y02C 20/40 20200801; B01D 67/0006 20130101;
B01D 53/228 20130101 |
Class at
Publication: |
095/045 |
International
Class: |
B01D 53/22 20060101
B01D053/22 |
Goverment Interests
STATEMENT REGARDING FEDERAL RIGHTS
[0001] This invention was made with government support under
Contract No. W-7405-ENG-36 awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
Claims
1. A polymeric reaction product of a polybenzimidazole and a
crosslinking agent having the formula ##STR10## wherein R.sup.1 and
R.sup.2 are independently selected from alkyl having from 1 to 20
carbons, aryl having from 6 tol 8 carbons, substituted aryl;
wherein R.sup.1 and R.sup.2 are connected to form a ring structure
having from 2 to 5 carbons; and wherein X and Z are independently
selected from chloride, bromide, and iodide.
2. The polymeric reaction product of claim 1, wherein the
crosslinking agent comprises
3,4-dihalo-tetrahydro-thiophene-1,1-dioxide, wherein halo is
selected from the group consisting of chloro, bromo, and iodo.
3. The polymeric reaction product of claim 1, further comprising a
porous support for supporting said cross linked polymeric reaction
product.
4. The polymeric reaction product of claim 3, wherein said porous
support comprises metal, metal alloy, ceramic, or combinations
thereof.
5. The Polymeric reaction product of claim 1, wherein said
polybenzimidazole comprises
poly-2,2'-(m-phenylene-5,5'bibenzimidazole).
6. A cross-linked membrane prepared by placing a solution of
polybenzimidazole and a crosslinking agent on a porous support and
removing solvent from the solution, the crosslinking agent having
the formula ##STR11## wherein R.sup.1 and R.sup.2 are independently
selected from alkyl having from 1 to 20 carbons, aryl having from 6
to 18 carbons, substituted aryl; wherein R.sup.1 and R.sup.2 are
connected to form a ring structure having from 2 to 5 carbons; and
wherein X and Z are independently selected from chloride, bromide,
and iodide.
7. The cross-linked membrane of claim 6, wherein the crosslinking
agent comprises 3,4dihalo-tetrahydro-thiophene-1,1-dioxide where
halo is selected from the group consisting of chloro, bromo, and
iodo.
8. The membrane of claim 6, wherein the solution comprises from
greater than zero equivalents to about one equivalent of
crosslinking agent, said cross-linking agent comprising
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide.
9. A method for gas separation, comprising sending a gas mixture
through a membrane comprising the cross-linked polymeric reaction
product of the reaction between polybenzimidazole and a
crosslinking agent having the formula ##STR12## wherein R.sup.1 and
R.sup.2 are independently selected from alkyl having from 1 to 20
carbons, aryl having from 6 to 18 carbons, substituted aryl;
wherein R.sup.1 and R.sup.2 are connected to form a ring structure
having from 2 to 5 carbons; and wherein X and Z are independently
selected from chloride, bromide, and iodide.
10. The method of claim 9, wherein the crosslinking agent comprises
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide.
11. The method of claim 9, wherein the polybenzimidazole comprises
poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole.
12. The method of claim 9, wherein the membrane further comprises a
porous support comprising metal, metal alloys, ceramic, or
combinations thereof.
13. The method of claim 9, wherein the gas mixture comprises at
least one gas selected from the group consisting of hydrogen
sulfide, sulfur dioxide, carbonyl sulfide, carbon monoxide, carbon
dioxide, nitrogen, hydrogen, and methane.
14. The method of claim 9, wherein the membrane is heated to a
temperature from about 25 degrees Celsius to about 400 degrees
Celsius.
15. A method for separating carbon dioxide from a gas mixture,
comprising sending a gas mixture that includes carbon dioxide
through a membrane comprising the cross-linked polymeric reaction
product of the reaction between polybenzimidazole and a
crosslinking agent having the formula ##STR13## wherein R.sup.1 and
R.sup.2 are independently selected from alkyl having from 1 to 20
carbons, aryl having from 6 to 18 carbons, substituted aryl;
wherein R.sup.1 and R.sup.2 are connected to form a ring structure
having from 2 to 5 carbons; and wherein X and Z are independently
selected from chloride, bromide, and iodide.
16. The method of claim 15, wherein the crosslinking agent
comprises 3,4-dihalo-tetrahydro-thiophene-1,1-dioxide where halo is
selected from the group consisting of chloro, bromo, and iodo.
17. The method of claim 15, wherein the membrane further comprises
a porous support comprising a material selected from the group
consisting of metals, metal alloys, ceramic materials, and
combinations thereof.
18. The method of claim 15, wherein the gas mixture comprises at
least one hydrocarbon.
19. The method of claim 15, wherein the gas mixture comprises
methane.
20. The method of claim 15,further comprising heating the membrane
to a temperature from about 25 degrees Celsius to about 400 degrees
Celsius.
21. The method of claim 15, wherein the cross-linked
polybenzimidazole comprises the reaction product of
poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole and
3,4-chloro-tetrahydro-thiophene-1,1-dioxide.
22. A method for separating hydrogen from a gas mixture, comprising
sending a gas mixture that includes hydrogen through a membrane
comprising the cross-linked polymeric reaction product of the
reaction between polybenzimidazole and a crosslinking agent having
the formula ##STR14## wherein R.sup.1 and R.sup.2 are independently
selected from alkyl having from 1 to 20 carbons, aryl having from 6
to 18 carbons, substituted aryl; wherein R.sup.1 and R.sup.2 are
connected to form a ring structure having from 2 to 5 carbons; and
wherein X and Z are independently selected from chloride, bromide,
and iodide.
23. The method of claim 22, wherein the crosslinking agent
comprises 3,4-dihalo-tetrahydro-thiophene-1,1-dioxide where halo is
selected from the group consisting of chloro, bromo, and iodo.
24. The method of claim 22, wherein the membrane further comprises
a porous support comprising a material selected from the group
consisting of metals, metal alloys, ceramic materials, and
combinations thereof.
25. The method of claim 22, wherein the gas mixture comprises at
least one hydrocarbon.
26. The method of claim 22, wherein the gas mixture comprises
methane.
27. The method of claim 22, further comprising heating the membrane
to a temperature from about 25 degrees Celsius to about 400 degrees
Celsius.
28. The method of claim 22, wherein the cross-linked reaction
product comprises the reaction product of
poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole and
3,4-chloro-tetrahydro-thiophene-1,1-dioxide.
Description
FIELD OF THE INVENTION
[0002] The present invention relates generally to gas separation
and more particularly to a cross-linked polybenzimidazole membrane
used for gas separation.
BACKGROUND OF THE INVENTION
[0003] The last decade has seen a dramatic increase in the use of
polymer membranes as effective, economical and flexible tools for
many gas separations. The processability, gas solubility, and
selectivity of several classes of polymers (such as polyimides,
polysulfones, polyesters, and the like) have led to their use in a
number of successful gas separation applications. A drawback to the
use of polymer membranes for gas separation can be their low
permeability or inadequate selectivity. In most instances, the
success of a given membrane rests on achieving an appropriate
combination of adequate permeability and selectivity.
[0004] Polymer membranes can be used for air separation, for the
recovery of hydrogen from mixtures of nitrogen, carbon monoxide and
methane, and for the removal of carbon dioxide from natural gas.
For these applications, glassy polymer membranes provide high
fluxes and excellent selectivities based on size differences of the
gas molecules being separated.
[0005] Separation of hydrogen (H.sub.2) and carbon dioxide
(CO.sub.2) from mixed gas streams is of major industrial interest.
Current separation technologies require cooling of the process gas
to ambient temperatures. Significant economic benefit could be
realized if these separations are performed at elevated
temperatures (greater than 150.degree. C.). Consequently, much
effort is directed at identifying and developing polymers that are
chemically and mechanically stable at elevated temperatures and
high pressures. Linear polybenzimidazole is an example of such a
polymer. Representative patents and papers that describe membranes
of linear polybenzimidazole include U.S. Pat. No. 2,895,948 to K.
C. Brinker et al. entitled "Polybenzimidazoles," which issued Jul.
21, 1959; RE 26,065 entitled "Polybenzimidazoles and Their
Preparation," which reissued to C. S. Marvel et al. on Jul. 19,
1966; "Polybenzimidazoles, New Thermally Stable Polymers," H. Vogel
et al., J. Poly. Sci., vol. L., pp. 511-539, 1961;
"Polybenzimidazoles II," H. Vogel et al., J. Poly. Sci. Part A,
vol. 1, pp. 1531-1541, 1963; U.S. Pat. No. 3,699,038 to A. A. Boom
entitled "Production of Improved Semipermeable Polybenzimidazole
Membranes, which issued Oct. 17, 1972; U.S. Pat. No. 3,720,607 to
W. C. Brinegar entitled "Reverse Osmosis Process Employing
Polybenzimidazole Membranes," which issued Mar. 13, 1973; U.S. Pat.
No. 3,737,042 entitled "Production of Improved Semipermeable
Polybenzimidazole Membranes," which issued to W. C. Brinegar on
Jun. 5,1973; and U.S. Pat. No. 4,933,083 entitled
"Polybenzimidazole Thin Film Composite Membranes," which issued to
R. Sidney Jones Jr. on Jun. 12, 1990, all of which are incorporated
by reference herein. These patents and papers show that, for years,
polybenzimidazole membranes have been useful for liquid phase
separations such as reverse osmosis separations, ion exchange
separations, and ultrafiltration.
[0006] Polybenzimidazole is also useful for gas separations. In
U.S. Pat. No. 6,681,648 to Robert C. Dye et al. entitled "Meniscus
Membranes for Separations," for example, meniscus-shaped
polybenzimidazole supported on a stainless steel substrate was
useful for separating H.sub.2 from an H.sub.2/CO.sub.2 mixture, and
CO.sub.2 from a CO.sub.2/CH.sub.4 mixture, and that membrane
performance improves as the temperature increases from 25.degree.
C. to 250.degree. C.
[0007] The mechanical properties of polybenzimidazole may be
improved by cross-linking (see, for example, U.S. Pat. No.
4,020,142 to Howard J. Davis et al. entitled "Chemical Modification
of Polybenzimidazole Semipermeable Membranes," which issued Apr.
26, 1977). According to the '142 patent, cross-linked
polybenzimidazole is tougher than non-cross-linked analogs and
shows improved compaction resistance during prolonged usage at
higher pressures. While cross-linked polybenzimidazole has been
shown to be useful for liquid separations (separations in acid
waste streams, reverse osmosis separations, ion exchange
separations, and ultrafiltration separations), there is little
information available related to gas separation using cross-linked
polybenzimidazole.
[0008] Accordingly, an object of the present invention is to
provide a method for separating gases using cross-linked
polybenzimidazole.
[0009] Another object of the invention is to provide a cross-linked
polybenzimidazole membrane for gas separation.
[0010] Additional objects, advantages and novel features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art upon examination of the following or may be learned by practice
of the invention. The objects and advantages of the invention may
be realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
[0011] In accordance with the purposes of the present invention, as
embodied and broadly described herein, the present invention
includes the polymeric reaction product of a polybenzimidazole and
a crosslinking agent having the formula ##STR1## wherein R.sup.1
and R.sup.2 are independently selected from alkyl having from 1 to
20 carbons, aryl having from 6 to 18 carbons, substituted aryl;
wherein R.sup.1 and R.sup.2 are connected to form a ring structure
having from 2 to 5 carbons; and wherein X and Z are independently
selected from chloride, bromide, and iodide.
[0012] The invention also includes a method for gas separation. The
method involves sending a gas mixture through a membrane that
includes the cross-linked polymeric reaction product of the
reaction between polybenzimidazole and a crosslinking agent having
the formula ##STR2## wherein R.sup.1 and R.sup.2 are independently
selected from alkyl having from 1 to 20 carbons, aryl having from 6
to 18 carbons, substituted aryl; wherein R.sup.1 and R.sup.2 are
connected to form a ring structure having from 2 to 5 carbons; and
wherein X and Z are independently selected from chloride, bromide,
and iodide.
[0013] The invention also includes a method for separating carbon
dioxide from a gas mixture. The method involves sending a gas
mixture that includes carbon dioxide through a membrane comprising
the cross-linked polymeric reaction product of the reaction between
polybenzimidazole and a crosslinking agent having the formula
##STR3## wherein R.sup.1 and R.sup.2 are independently selected
from alkyl having from 1 to 20 carbons, aryl having from 6 to 18
carbons, substituted aryl; wherein R.sup.1 and R.sup.2 are
connected to form a ring structure having from 2 to 5 carbons; and
wherein X and Z are independently selected from chloride, bromide,
and iodide.
[0014] The invention also includes a method for separating hydrogen
from a gas mixture. The method involves sending a gas mixture that
includes hydrogen through a membrane comprising the cross-linked
polymeric reaction product of the reaction between
polybenzimidazole and a crosslinking agent having the formula
##STR4## wherein R.sup.1 and R.sup.2 are independently selected
from alkyl having from 1 to 20 carbons, aryl having from 6 to 18
carbons, substituted aryl; wherein R.sup.1 and R.sup.2 are
connected to form a ring structure having from 2 to 5 carbons; and
wherein X and Z are independently selected from chloride, bromide,
and iodide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The accompanying drawings, which are incorporated in and
form a part of the specification, illustrate the embodiment(s) of
the present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
[0016] FIG. 1 provides a graph comparing the gas permeabilities of
H.sub.2, N.sub.2, CO.sub.2, and CH.sub.4 as a function of
temperature through the cross-linked reaction product of linear
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole membrane with
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide;
[0017] FIG. 2 provides a graph comparing the volumetric fluxes of
H.sub.2, N.sub.2, CO.sub.2, and CH.sub.4 as a function of
temperature through the cross-linked reaction product of linear
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole membrane with
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide; and
[0018] FIG. 3 provides a graph comparing the gas permeabilities of
H.sub.2, N.sub.2, CO.sub.2, and CH.sub.4 as a function of
temperature through unmodified linear
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole membrane with those for
the cross-linked reaction product of linear
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole membrane with
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Briefly, the present invention relates to a supported,
cross-linked polybenzimidazole membrane, a method for preparing the
membrane, and a method of using the membrane for gas
separation.
[0020] One aspect of the invention relates to membranes prepared
using the polymeric reaction product of a polybenzimidazole and a
crosslinking agent having the formula ##STR5## wherein R.sup.1 and
R.sup.2 are independently selected from alkyl having from 1 to 20
carbons, aryl having from 6 to 18 carbons, substituted aryl;
wherein R.sup.1 and R.sup.2 are connected to form a ring structure
having from 2 to 5 carbons; and wherein X and Z are independently
selected from chloride, bromide, and iodide. Another aspect of the
invention relates to a gas separation method that employs the
membrane.
[0021] An embodiment membrane of the invention was prepared by
reacting a linear polybenzimidazole with
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide cross-linking agent.
These types of crosslinking agents are commonly referred to in the
art as sulfolanes; they have a ring structure with a sulfone group
incorporated into the ring structure. For this particular
embodiment membrane, a solution of
2,2'-(m-phenylene)-5,5'-bibenzimidazole and
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide was cast onto a
porous metal support. Removal of solvent by evaporation resulted in
a supported film of the invention that was found to be especially
useful for gas separation.
[0022] Linear polybenzimidazoles that contain reactive hydrogen
atoms on the imidazole rings may be used to prepare a membrane of
the invention. These reactive hydrogen atoms combine with atoms of
the cross-linking agent to form molecules that are subsequently
released during evaporation of the solvent and/or during heat
cycling. Examples of linear polybenzimidazoles that contain
reactive hydrogens on the imidazole rings include the following:
[0023] poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole; [0024]
poly-2,2'-(pyridylene-3'',5'')-5,5'-bibenzimidazole; [0025]
poly-2,2'-(furylene-2'',5'')-5,5'-bibenzimidazole; [0026]
poly-2,2-(naphthalene-1'',6'')-5,5'-bibenzimidazole; [0027]
poly-2,2'-(biphenylene-4'',4'')-5,5'-bibenzimidazole; p0
poly-2,2'-amylene-5,5'-bibenzimidazole; [0028]
poly-2,2'-octamethylene-5,5'-bibenzimidazole; [0029]
poly-2,6-(m-phenylene)-diimidazobenzene; [0030]
poly-2,2'-cyclohexenyl-5,5'-bibenzimidazole; [0031]
poly-2,2'-(m-phenylene)-5,5'di(benzimidazole)ether; [0032]
poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)sulfide; [0033]
poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)sulfone; [0034]
poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)methane; [0035]
poly-2'-2'-(m-phenylene)-5',5'-(di(benzimidazole)propane-2,2;
[0036] and
poly-2',2'-(m-phenylene)-5',5'-di(benzimidazole)ethylene-1,2 where
the double bonds of the ethylene are intact in the final
polymer.
[0037] Polybenzimidazoles useful with this invention include one or
more imidazoles per repeat unit. An example of polybenzimidazole
with one imidazole per repeat unit shown below left, and an example
of polybenzimidazole with two imidazoles per repeat unit is shown
below right. ##STR6##
[0038] Each repeat unit would react with up to one equivalent of
crosslinking agent per imidazole.
[0039] Without intending to be bound by any particular explanation,
it is believed that the first half equivalent would likely react
with the protonated imidazole nitrogen, resulting in a neutral
polymeric product. This reaction using a preferred
polybenzimidazole, 2,2'-(m-phenylene)-5,5'-bibenzimidazole, is
shown in the equation below. ##STR7##
[0040] The polymer may be cast onto any surface, preferably the
surface of a porous substrate. Porous substrates useful for
preparing supported membranes of the invention include porous metal
substrates and porous ceramic substrates. An example of a suitable
substrate is a commercially available ceramic substrate made from
silicon carbide. A preferred substrate can be formed from a porous
metal medium such as sintered porous stainless steel. Such a porous
metal medium is available from PALL CORPORATION of East Hills, N.Y.
under the trade names PSS (a sintered stainless steel powder metal
medium), PMM (a porous sintered metal membrane including metal
particles sintered to a foraminate support), PMF (a porous sintered
fiber mesh medium), Rigimesh (a sintered woven wire mesh medium),
Supramesh (stainless steel powder sintered to a Rigimesh support),
PMF II (a porous sintered fiber metal medium), and combinations of
more than one of these materials. A sintered metal medium for use
in the present invention may be formed from any of a variety of
metal materials including alloys of various metals such as
chromium, copper, molybdenum, tungsten, zinc, tin, gold, silver,
platinum, aluminum, cobalt, iron, and magnesium, as well as
combinations of metals and alloys, including boron-containing
alloys. Brass, bronze, and nickel/chromium alloys, such as
stainless steels, the Hastelloys, the Monels and the Inconels, as
well as a 50-weight percent chromium alloy, may also be used.
Substrates may include nickel and alloys of nickel, although it is
believed that nickel may react with and degrade the supported
polymer, which would affect the longevity of the invention
membrane. Examples of other suitable high temperature substrates
include those formed of glass fibers.
[0041] A working embodiment of the present invention was prepared
by casting a solution containing
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole (CELANESE CORPORATION,
{overscore (M)}.sub.n=20.times.10.sup.3) and
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide in dimethylacetamide
onto a porous stainless steel substrate. The solution is typically
prepared at elevated temperature and contains about 10 to 15 weight
percent polybenzimidazole in dimethylacetamide and an amount of the
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide to give the
crosslinking density of interest. The following EXAMPLE provides a
procedure for preparing an invention membrane.
EXAMPLE
[0042] Ten grams of a membrane casting solution containing 20
weight percent (wt 15%) of a cross-linking agent was prepared by
dissolving 0.8 gram of poly-2,2'-(m-phenylene)-5,5'bibenzimidazole
(CELANESE CORPORATION, {overscore (M)}.sub.n=20.times.10.sup.3,
0.78 .mu.m-diameter polymer particles) and 0.2 gram of
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide in 9 grams of
N,N-dimethylacetamide. For this EXAMPLE, assuming complete
reaction, the composition containing 20 weight percent of the
cross-linking agent and 80 percent of the polymer would produce a
40 percent crosslinked neutral polymer product (slightly less than
one equivalent of cross-linking agent per two imidazoles). The
reaction between the polymer and crosslinking agent is shown below.
##STR8##
[0043] A 40 microliter (.mu.l) aliquot of the solution was evenly
spread on a stainless steel substrate (PALL CORPORATION). After
drying at room temperature for 15 min. the resulting supported
polymer film was heated to a temperature of 50 degrees Celsius for
60 minutes to allow more complete solvent evaporation. The membrane
was heat-cycled between 50 degrees Celsius and 300 degrees Celsius
(90-mmn cycle time) a total of five times to enhance stability,
resulting in a fully dense supported cross-linked polybenzimidazole
membrane.
[0044] It should be understood that the polymer membranes prepared
from solutions that contain other solvents, and greater and lesser
amounts of the cross-linking agent also fall within the scope of
the invention. Any solvent capable of dissolving polybenzimidazole,
such as N,N-dimethylacetamide, N,N-dimethylformamide or
N-vinylpyrrolidone, can be used with the invention. Membranes of
neutral polymers can be obtained using from about slightly greater
than zero equivalents to about one-half equivalent of crosslinking
agent per imidazole in the starting polymer. Using slightly greater
than about zero equivalents would yield a lightly crosslinked
polymer, while about one half of an equivalent would yield a
neutral crosslinked polymer where all (or most) of the protonated
imidazole nitrogen atoms of the starting polymer have reacted.
[0045] It should be understood that this invention not only
includes neutral cross linked polybenzimidazole as described above,
but also ionic polymers that likely result from reaction of the
crosslinking agent with unprotonated nitrogen atoms of the
imidazole ring, as illustrated in the equation below. ##STR9## A
fully crosslinked ionic polybenzimidazole membrane of the type
shown in the above equation would be obtained by reacting about one
equivalent of crosslinking agent X--R--X per imidazole of the
starting polymer. This reaction stoichiometry would result in
complete reaction of all the imidazole nitrogen atoms with the
crosslinking agent X--R--X.
[0046] In order to demonstrate advantages of the cross-linked
polymer membrane for gas separation, polymer membranes of
unmodified linear poly-2,2'-(m-phenylene)-5,5'bibenzimidazole were
also prepared: The procedure used for preparing unmodified
polybenzimidazole membranes followed that as described for the
cross-linked membrane with the exception that the cross-linking
agent was omitted. Two specific comparison membranes were prepared
from a solution of 10 weight percent
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole in dimethylacetamide. A
40-.mu.L aliquot of the solution was evenly spread on one substrate
and spin coated on another, the substrates used being of the same
type of stainless steel substrate as was used to prepare the
supported cross-linked polymer membrane of the invention described
previously. Each was dried at room temperature for 15 min, and the
resulting supported polymer films were heated to about 50 degrees
Celsius for about 60 minutes to allow more complete solvent
evaporation. Each was heat cycled between 50 degrees Celsius and
300 degrees Celsius (90-min cycle time) a total of five times to
enhance stability, as described for the cross-linked membrane,
which resulted in fully dense supported polybenzimidazole
membranes.
[0047] The gas permeability and gas selectivity of the supported
cross-linked polybenzimidazole membrane was determined and compared
to that for the analogous, unmodified, linear polybenzimidazole
membrane using permeate pressure-rise measurements over a wide
temperature range. Gas permeability is defined herein according to
equation 1 below: P = ( 10 10 ) .times. ( v ) .times. ( L ) ( A )
.times. ( .DELTA. .times. .times. p ) ( 1 ) ##EQU1## where .nu. is
the gas flux in cubic centimeters per second (cm.sup.3/s), L is the
membrane thickness in cm, A is the membrane area in cm.sup.2, and
.DELTA.p is the pressure difference across the membrane in cm
Hg.
[0048] Gas selectivity, .alpha..sub.A/B, is defined herein as the
ratio of the permeability of gas A divided by the permeability of
gas B.
[0049] The practice of the invention can be further understood with
the accompanying FIGURES. FIG. 1 provides a graph comparing the gas
permeabilities of H.sub.2, N.sub.2, CO.sub.2, and CH.sub.4 as a
function of temperature through the cross-linked reaction product
of linear poly-2,2'-(m-phenylene)-5,5'bibenzimidazole membrane with
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide. TABLE 1 below
summarizes the data used in FIG. 1. TABLE-US-00001 TABLE 1
Cross-linked PBI Unmodified, linear PBI Temperature, Permeability,
Temperature, Permeability, .degree. C. barrier .degree. C. barrier
H.sub.2 23 38.43168 17 5.117 111 131.071 95 19.221 211 564.5009 160
33.845 311 2193.375 223 73.057 409 5316.602 313 165.76299 312
2747.458 315 171.1804 208 1216.592 279 125.53064 101 427.6633 181
50.376722 23 132.8588 121 23.689705 22 128.0179 24 4.7438374 100
435.1346 373 263.25309 207 1411.278 306 3020.782 400 5186.646 400
5720.059 N.sub.2 211 3.313368 21 0.0258826 305 28.21815 95 0.077025
304 36.41971 156 0.2030286 404 107.1163 216 0.7087747 304 36.41971
313 2.2544598 206 10.442 313 2.1886325 100 1.943046 279 1.2166992
23 0.291257 181 0.2586471 24 0.300519 121 0.0670755 100 1.895188 23
0.0169855 210 10.36877 369 4.0848769 310 36.8027 400 101.1812 400
98.61064 CO.sub.2 106 4.303982 313 7.6339218 210 22.37276 313
7.5653723 305 107.7932 279 5.3973399 409 270.7548 181 2.1226676 304
133.9686 121 1.1005387 208 65.99256 23 0.3071448 100 29.41249 369
11.299329 23 8.443571 23 7.553125 101 28.78142 208 65.94301 310
139.1192 400 254.186 400 256.5045 CH.sub.4 210 2.593238 315 1.68119
309 22.88694 313 1.6964713 409 94.65714 279 0.9569662 304 29.50894
181 0.1534 208 7.462297 121 0.0093627 101 1.174854 370 4.5872553 21
0.529546 23 0.438549 101 1.001837 210 6.869058 310 30.40814 400
85.98601 400 74.9465
The gas permeability data were collected over a wide temperature
range of from about 20 degrees to about 400 degrees Celsius. Filled
square symbols shown in FIG. 1 relate to permeability data plotted
for hydrogen; filled triangles relate to data plotted for carbon
dioxide; filled circles relate to data plotted for nitrogen; and
filled diamond shaped symbols relate to data plotted for methane.
FIG. 1 shows that the order of gas permeability for this membrane
is H.sub.2>CO.sub.2>N.sub.2>CH.sub.4. This is the order
generally observed for other gas-permeable glassy membranes. This
response of the membrane permeability with increasing temperature
is typical of polymer membranes due to the increased motion of the
polymer chains.
[0050] FIG. 2 provides a graph comparing the volumetric fluxes of
H.sub.2, N.sub.2, CO.sub.2, and CH.sub.4 as a function of
temperature through the cross-linked reaction product of linear
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole membrane with
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide. The symbols used for
FIG. 2 are the same as those for FIG. 1: filled square symbols
shown in FIG. 1 relate to permeability data plotted for hydrogen;
filled triangles relate to data plotted for carbon dioxide; filled
circles relate to data plotted for nitrogen; and filled diamond
shaped symbols relate to data plotted for methane. Ultimately,
volumetric flux controls the economics of the application. The
controlling membrane thickness of these composite membranes is not
easily determined but is estimated at approximately 20 .mu.m.
[0051] FIG. 3 provides a graph comparing the gas permeabilities of
H.sub.2, N.sub.2, CO.sub.2, and CH.sub.4 as a function of
temperature through unmodified linear
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole membrane with those for
the cross-linked reaction product of linear
poly-2,2'-(m-phenylene)-5,5'bibenzimidazole membrane with
3,4-dichloro-tetrahydro-thiophene-1,1-dioxide. The filled symbols
used for FIG. 3 have the same meaning as they do for FIG. 1 and
FIG. 2: filled square symbols shown in FIG. 1 relate to
permeability data plotted for hydrogen; filled triangles relate to
data plotted for carbon dioxide; filled circles relate to data
plotted for nitrogen; and filled diamond shaped symbols relate to
data plotted for methane. The analogous unfilled symbols relate to
the permeability data for these gases for the unmodified polymer.
As FIG. 3 indicates, the permeability of the cross-linked
polybenzimidazole membrane to each gas is significantly higher than
for unmodified PBI. Typically, with increased permeability,
selectivity is adversely affected. However, for this material, the
selectively between H.sub.2 and CO.sub.2 does not appear to be
significantly affected, resulting in an improved combination of
permeability and selectivity that will be more economical in its
application.
[0052] Cross-linking a membrane generally tends to improve
selectivity but decrease permeability. For the membrane of the
invention, neither selectivity nor permeability appears to be
adversely affected by the cross-linking; in fact, permeability,
unexpectedly, has increased upon cross-linking.
[0053] Gas mixtures that include gases such as hydrogen sulfide,
sulfur dioxide (SO.sub.2), carbonyl sulfide (COS), carbon monoxide,
carbon dioxide, nitrogen, hydrogen, chlorine, ammonia, and methane
can be separated using the invention membrane.
[0054] In summary, the cross-linked reaction product of a
polybenzimidazole and a sulfolane displays similar gas selectivity
as the unmodified analog, but is more permeable over a wide
temperature range (measured up to about 400 degrees Celsius). Gas
permeability and selectivity results indicate that the cross-linked
membrane of the invention is useful for separating hydrogen and
carbon dioxide from mixed gas streams, preferably at elevated
temperatures.
[0055] The foregoing description of the invention has been
presented for purposes of illustration and description and is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and obviously many modifications and variations are
possible in light of the above teaching. For example, while
poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole was used with a
dichlorosulfolane crosslinking agent to demonstrate this invention,
it should be understood that other linear polybenzimidazoles that
contain reactive hydrogen atoms can be used, as well as
cross-linking agents that contain bromine and/or iodine reactive
leaving groups.
[0056] The embodiment(s) were chosen and described in order to best
explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto.
* * * * *