ACID BLOCK ANION MEMBRANE
FIELD OF INVENTION
[0001] The invention pertains to an electrodialysis anion selective membrane
and to methods for producing such membranes.
BACKGROUND OF THE INVENTION
[0002] Bipolar electrodialysis (BPED) is a membrane separation process in
which high purity acid and base solutions may be generated from a salt solution by the
electrodialysis water splitting process. Typically, apparatus designed to perform such
ED function consists of a stack containing a plurality of cation-selective membranes,
bipolar membranes, and anion selective membranes positioned between a pair of
electrodes. The stack may itself comprise an assembly of unit cells in which each unit
cell comprises the above membranes arranged in such fashion to provide a plurality of
flow paths or channels between adjacent membranes.
[0003] When a direct electrical current is applied to the bipolar membrane,
water is split into OH ions and H+ ions which migrate to the anode and cathode
respectively. The cation selective membrane readily allows passage of the cations
(positively charged ions, such as Na+, H+) therethrough while blocking passage of
anions. Conversely, the anion selective membranes readily permit passage of the
anions (negatively charged ions, such as Cl , OH ) while retarding cation migration.
If a salt solution such as NaCl is directed through the channel between the cationic
selective and anionic selective membranes, the concentration of that salt solution is
depleted with HC1 and NaOH being formed in adjacent acid and base containing
channels.
[0004] In connection with some prior art ED devices, H+ migration across the
anion select membrane has proven problematic. This proton leak through the anion
selective membrane is well known and is referred to as the Grotthuss mechanism by
which protons diffuse through the hydrogen bonding network of water molecules.
Anion selective membranes demonstrating substantial passage or migration of H+ aredetrimental to electrodialysis processes, resulting in poor anion transfer current
efficiency, low concentration of acid and base and high equipment and energy costs.
SUMMARY OF THE INVENTION
[0005] In one exemplary embodiment, a method is provided for preparing an
acid block anion selective polymeric membrane of the type having a woven or non-
woven cloth reinforcing structure. The polymer of the membrane is prepared by the
process comprising copolymerization of components of (I) an ethylenically unsaturated
aliphatic or aromatic tertiary or quaternary amine monomer, (II) a cross-linking
monomer, and (III) vinylbenzyl chloride in the presence of a free radical
polymerization initiator. In one exemplary embodiment, the molar ratio of
components (I): (II): (III) is from about 20-60:30-70: 1-19. In another exemplary
embodiment, the molar ratio of components I:II:III is from about 35-45:45-55: 10-15.
The foregoing percentages equal 100 molar %.
[0006] In another aspect of the invention, the copolymerization is conducted in
the absence of nonpolymerizable solvent.
[0007] In another exemplary embodiment, the membrane is characterized as
having a water content percent of from about 12-20 wt and a current efficiency of
greater than about 93%.
[0008] In another aspect of the invention, an anionic exchange membrane is
provided of the type that is used in electrodialysis apparatus. The membrane
comprises a woven or non-woven cloth with the cloth being impregnated with a
copolymer comprising the reaction products of components I , II, and III wherein I is
an ethylenically unsaturated aliphatic or aromatic tertiary or quaternary amine
monomer, II is a cross-linking monomer, and III is a vinylbenzyl chloride.
DETAILED DESCRIPTION
[0009] In one aspect of the invention, the acid block anion selective membrane
is prepared by copolymerizing (I) an ethylenically unsaturated aliphatic or aromatic
tertiary or quaternary amine monomer and a (II) cross-linking monomer in the
presence of (III) vinylbenzyl chloride (VBC). A free radical polymerization initiator(IV) is also present. Preferably no solvent is used. As used herein, "acid block"
means an anion selective membrane that has enhanced capacity to retard the migration
of H+ therethrough. Typically, this is accomplished by reducing the water content of
the membrane and/or increasing the cross linking of the polymer.
[0010] The components I , II, III, and initiator IV are mixed to form a
homogenous solution. The resulting solution is used to impregnate a piece of cloth
such as a polypropylene, polyester, acrylic, or modacrylic type cloth. The thus
impregnated cloth is placed between glass plates, and this glass sandwich construction
is then heated to initiate the polymerization. After completion of the polymerization
reaction, the glass sheets are removed, leaving the polymer impregnated cloth.
[0011] The polymer impregnated cloth is then allowed to swell by soaking in
an aqueous acidic bath such as a 2N HCl solution. The polymer impregnated cloth or
sheet may then be used as an acid block anion selective membrane in electrodialysis
and other applications.
[0012] As to the component I , exemplary aliphatic tertiary amines may be
encompassed by the Formula A.
Formula A - aliphatic tert-amine
Rl
CH2= C
C= 0
X
R2
wherein Rl is H or CHs, X is O or NH, R2 is lower (Ci-Ce) alkylene and R3 and R4
are independently chosen from lower (Ci-Ce) alkyl.
[0013] Exemplary component I aliphatic quaternary amine monomers may be
encompassed by the Formula B.Formula B - aliphatic quaternary amine
Rl
CH2= C
C= 0
O
R2
R3 - N R4
R5
wherein Rl, R2, R3, and R4 are as defined in Formula A; R5 is chosen from lower
(Ci-Ce) alkyl; and A is an anion chosen from halo, nitrite, sulfate and other inorganic
or organic anions.
[0014] Exemplary component I aromatic tertiary amine monomers may be
encompassed by the Formula C.
ula C - aromatic tertiary amine
N
wherein R6 is vinyl.
[0015] Exemplary component I aromatic quaternary ammonium monomers
may be encompassed by the Formula D.
Formula D
Iwherein R6 is the same as in Formula C, R2, when present, is the same as in Formula
A; R3, R4, and R5 are the same as in Formula B and A is the same as defined in
Formula B.
[0016] Specific members of (I) the ethylenically unsaturated aliphatic or
aromatic tertiary or quaternary amine monomers that may be mentioned include
i) trimethylaminoethylmethacrylate chloride (TMAEMC)
Formula B - Rl = CH3, R2 = Et, R3, R4, and R5 are CH3 and A= C 1 ;
ii) vinylbenzyltrimethylammonium chloride (VBTMAC)
Formula D - R2 = —CH2—, R3, R4, and R5 = CH3, R6 = vinyl,
A =cr ;
iii) vinyl pyridine
Formula C - R6 = vinyl;
iv) dimethylaminoethylmethacrylate (DMAEMA)
Formula A - R 1= CH3, X= 0 , R2 = Et, R3 and R4 = CH3;
v) dimethylaminopropylmethacrylamide (DMAPMA)
Formula A - R 1= CH3, X= NH, R2= propyl, R3 and R4 = CH3.
[0017] The cross linking monomers II may generally be described as
containing ethylenically unsaturated functionality and may be chosen from a wide
class of known cross linking agents such as divinylbenzene (DVB), ethylene glycol
dimethacrylate (EGDM), ethylene glycol diacrylate, 1,10-decane diol diacrylate or
dimethacrylate, methylene bis acrylamide or bis methacrylamide, dodecamethylene bis
acrylamide or bis methacrylamide, diethylene glycol diacrylate or dimethacrylate, 1,4
butane diol divinyl ether, triethylene glycol divinyl ether, divinyl succinate, subernate
or sebacate, etc.
[0018] As to the polymerization initiators that may be used, these include the
azo initiators such as 2,2-azo bis (2-methylpropionitrile) 2,2'-Azobis(2-methyl-
propionamidine) dihydrochloride; 1, l'-Azobis(cyclohexane carbonitrile); 4,4'-
Azobis(4-cyanovaleric acid) purum etc., peroxide initiators such as benzoyl peroxide,
and t-butylperoxy-2-ethylhexanoate, l,l-Bis(tert-amylperoxy)cyclohexane; 1,1-Bis
(tert-butylperoxy) -3,3,5-trimethylcyclohexane; l,l-Bis(tert-butylperoxy) cyclohexane;
2,4-Pentanedione peroxide; 2,5-Bis(tert butylperoxy) -2,5-dimethylhexane; 2-Butanone peroxide; di-tert-amyl peroxide; Di cumyl peroxide; Lauroyl peroxide, tert-
Butylperoxy-2-ethylhexyl carbonate, tert-Butyl per acetate, tert-Butyl peroxide, and
tert-Butyl peroxybenzoate, etc.
[0019] In one embodiment, ranges of addition for the components (I), (II), and
(III) are as follows: Component I:II:III - (37-40%): (48-50%): 12-13%... molar ratio.
EXAMPLES
Example 1 (A-l)
ABA-091509- vinylpyridine/DVB/VBC/PP 470
[0020] To a mixture of 12 grams (0. 108 moles) of 4-vinyl pyridine, 6 grams
(0.039 moles) of vinyl benzyl chloride (VBC) and 24 grams (0.147 moles) of 80%
divinyl benzene (DVB), 0.8 grams (1.9% by weight) of t-butyl peroxy-2-
ethyhexanoate is added. The resulting solution is a clear solution without the addition
of any non-polymerizable solvents. The solution is poured into a Mylar tray sized
6.5" x 6.5" with a piece of glass under the Mylar tray, a piece of polyester cloth is
laid into the solution then a piece of Mylar sheet on the cloth. Alternate cloth and
Mylar sheet to form a package of 3 layers and a piece of glass on the top of the
assembly. The package is then moved to an oven with temperature at 90 °C for 2
hours. The package is taken apart to get 3 pieces of reinforced membrane sheets.
The membrane sheets are then placed into a 2 N hydrochloric acid solution and
allowed to swell until equilibrium. The final membrane is an acid efficiency anion
membrane that can be used in bipolar electrodialysis for recovering mineral acid from
salt.
Example 2 (A-2)
ABA-092409 - DMAEMA/DVB/VBC/PP 447
[0021] To a mixture of 10 grams (.0604 moles) of dimethylamino ethyl
methacrylate (DMAEMA), 3 grams (.0189 moles) of vinyl benzyl chloride (VBC) and
12 grams (0.0735 mole) of divinylbenzene (DVB), 0.5 grams (2.0% by weight) of t-
butylperoxy-2-ethylhexanoate is added. The resulting solution is a clear solution
without the addition of any non-polymerizable solvents. The solution is poured into aMylar tray sized 6.5" x 6.5" with a piece of glass under the Mylar tray, a piece of
polypropylene (#477) cloth is laid into the solution. Then, a piece of Mylar sheet is
placed over the cloth. Another cloth and Mylar sheet are placed in the solution. The
cloth and Mylar sheet are alternately spaced to form a package of 3 layers, and a piece
of glass is placed on top of the assembly. The package is then moved to an oven and
maintained at a temperature of 90 °C for 2 hours. The package is taken apart to get 3
pieces of reinforced membrane sheets. The membrane sheets are then placed into a
2N HC1 solution and allowed to swell until equilibrium. The final membrane is an
acid efficiency anion membrane that can be used in bipolar electrodialysis for
recovering mineral acid from salt.
Example 3 (A-3)
ABA-100609 - DMAEMA/DVB/VBC/PE
[0022] To a mixture of 60 grams (0.363 moles) of dimethylamino ethyl
methacrylate (DMAEMA), 18 grams (0. 114 moles) of vinyl benzyl chloride (VBC)
and 72 grams (0.442 moles) of 80% divinyl benzene (DVB), 3 grams (2% by weight)
of t-butyl peroxy-2-ethyhexanoate is added. The resulting solution is a clear solution
without the addition of any non-polymerizable solvents. The solution is poured into a
Mylar tray sized 10" x 11" with a piece of glass under the Mylar tray; a piece of
polyester cloth is laid into the solution then a piece of Mylar sheet on the cloth.
Alternate cloth and Mylar sheet to form a package of 3 layers and a piece of glass on
the top of the assembly. The package is then moved to an oven with temperature at
90 °C for 2 hours. The package is taken apart to get 3 pieces of reinforced membrane
sheets. The membrane sheets are then placed into a 2 N hydrochloric acid solution
and allowed to swell until equilibrium. The final membrane is an acid efficiency
anion membrane that can be used in bipolar electrodialysis for recovering mineral acid
from salt.
Example 4
[0023] Membrane cell testing with the above membranes and several other
commercially available membranes was conducted. Membrane current efficiency wasmeasured by the method set forth in Example 2 of U.S. Patent 4,822,471. The
percent of current efficiency means that the given percent amount of current is used to
transport anion through the anion membrane. The remaining percent of current
represents the hydrogen leak through the anion membrane. The notation "strong"
base means quaternary amine functionality with "weak" base denoting tertiary amine
functionality. The quat amine in a membrane is measured by first converting the
membrane into CI (chloride) form by soaking the membrane in 2N NaCl solution and
washing out the free CI ion. Then another salt such as sodium nitrate solution is used
to exchange the CI ion out from the membrane and titrate the CI . It is thus possible
to calculate the capacity of quat amine in the membrane in meq/per dry gram resin.
To determine the tertiary amine in the membrane, the total amines including quat and
tert amines is determined. Then the amount of quat amine is subtracted from the total
amine amount present. In order to determine the total amines including quat and tert
amines present in the membrane, first tert amines are converted to salt by adding HCl.
(This also converts the quat amines to CI form). Then, a 100% ethanol solution is
used to wash out excess HCl. Then sodium nitrate solution is used to exchange out
the CI ions and titrate CI ions. The total amount of amines can then be calculated.
The following results were obtained and are shown in Table I .
Table I
- poly(vinyl pyridine) radio graft in fluoro-copolymers; available Solvay-
Solvay ARA
*Data from Journal o f Membrane Science, 326 (2009) 650-658.C-4 - poly(vinyl pyridine) radio graft in fluoro-copolymers; available Solvay-
Solvay AW
**Data from Journal o f Membrane Science, 110 (1996) 181-190.
C-5 - vinylpyridine/DVD/poly vinyl chloride membrane; available Asahi
Glass.
Example 5
[0024] (092509-470) To a mixture of 46 grams (0.257 moles) of
dimethylamino propyl methacrylamide (DMAPMA), 12.8 grams (0.084 moles) of
vinyl benzyl chloride (VBC) and 48 grams (0.295 moles) of 80% divinyl benzene
(DVB), 2.1 grams (2% by weight) of t-butyl peroxy-2-ethylhexanoate is added. The
resulting solution is a clear solution without the addition of any non-polymerizable
solvents. The solution is poured into a Mylar tray sized 10" x 11" with a piece of
glass under the Mylar tray, a piece of polypropylene cloth is laid into the solution then
a piece of Mylar sheet on the cloth. Alternate cloth and Mylar sheet to form a
package of 3 layers and a piece of glass on the top of the assembly. The package is
then moved to an oven with temperature at 90 °C for 2 hours. The package is taken
apart to get 3 pieces of reinforced membrane sheets. The membrane sheets are then
placed into a 2 N hydrochloric acid solution and allowed to swell until equilibrium.
The final membrane is an acid efficiency anion membrane that can be used in bipolar
electrodialysis for recovering mineral acid from salt.
Comparison Example: (C-6) (ABA-#082809 VP/DVB/PP 470)
[0025] To a mixture of 15 grams (0. 136 moles) of 4-vinyl pyridine and 30
grams (0.184 moles) of 80% divinyl benzene (DVB), 0.8 grams (1.9% by weight) of
t-butyl peroxy-2-ethylhexanoate is added. No vinyl benzyl chloride (VBC) was added
to the solution. The resulting solution is a clear solution without the addition of any
non-polymerizable solvents. The solution is poured into a Mylar tray sized 6.5" x
6.5", a piece of polypropylene cloth is laid into the solution then a piece of Mylar
sheet on the cloth. Alternate cloth and Mylar sheet to form a package of 3 layers and
a piece of glass on the top of the assembly. The package is then moved to an ovenwith temperature at 90 °C for 2 hours. The package is taken apart to get 3 pieces of
reinforced membrane sheets. The membrane sheets are then placed into a 2 N
hydrochloric acid solution and allowed to swell until equilibrium is reach. The final
membrane is an acid efficiency anion membrane. The properties of the membranes
are listed in the Table II below:
Table II
^Resistance is measured in 0.0 IN NaCl solution at 1000 Hz.
[0026] The membrane A-l contains quaternary amine (strong base) tertiary
amine (weak base) and higher crosslink degree due to the reaction of VBC with
tertiary amine (vinyl pyridine), resulting in the membrane with lower water content,
higher current efficiency but low resistant. The comparison membrane (C-6) contains
only tertiary amine (weak base), resulting in low water content and high current
efficiency, but much higher resistant of the membrane.
Example 6 (Procedure similar to U.S. Patent 4,822,471 - Example 2)
[0027] A three compartment electrodialysis test cell was used to determine the
current efficiency (C.E.) of the membrane of present invention as a comparison with
those prior art when used in an acid solution. The cell comprised a cathode and anode
electrode of platinum coated titanium located at the terminal ends of the cell with two
membranes located there between and position from each other and from electrodes
with gasketed spacers to form liquid containing compartments. Thus the arrangement
was as follows: the anion exchange membrane to be tested, the middle compartment,
a commercially available type cation exchange membrane (GE CR61CMP), the anode
compartment and finally the anode electrode.
[0028] The membranes mounted in the test cell had an active membrane of 25
cm2 and each compartment had a liquid volume of about 40 ml each and a cross-
sectional active area of 25 cm2. The solution in the cell comprised 1 N HC1 in thecathode compartment, 0.5 N HCl in the center compartment and 0.5N E SC in the
anode compartment. The solutions were stirred by use of a magnetic stirrer and
maintained at a temperature of 25 °C. Each test run operated at 20 mA/cm2 for a 20-
minute period at which time the acid concentrations in the cell were determined by
titration. The results are shown in Table III as follows:
Table III
[0029] It is apparent that by adding VBC to the mixes of vinyl pyridine/DVB
or DMAEMA/DVB or DMAPMA/DVB, the mixes were homogenous. Thus, no
solvent was needed for the polymerization. Upon polymerization, the VBC also
reacts with a portion of the tertiary amine, resulting in a membrane with both
quaternary and tertiary amines with an attendant high cross linking degree. The A-3
membranes tested as shown in Table III exhibited a very high 93% and higher acid
current efficacy.
[0030] By adding the VBC to react with a portion of the tertiary amine in the
monomers such as vinyl pyridine, DMAEMA, DMAPMA, etc., we can control the
ratio of quaternary and tertiary amines in the resulting membrane and can also
increase membrane cross linking. A high current efficiency can be obtained from
aliphatic monomers such as DMAEMA, DMAPMA, etc., due to the resulting high
crosslink degree and low water content of the membranes. Resistance is also
decreased due to the introduction of quaternary amines in the membrane.[0031] In one preferred aspect of the invention, the anion block anion selective
membrane is a polymerization reaction product of I) DMAEMC, II (DVB), and III
(VBC). The molar ratio of 1:11:111 may be on the order of 20-60:30-70: 1-19: and
more preferably from 37-45:45-55: 10-15. The preferred initiator is t-butyl peroxy-2-
ethylhexanoate. More specifically, this reaction is shown by the following reaction
scheme shown in Formula E with the resulting polymer shown in Formula F .
Formula E - reaction scheme
CH2
O CH = CH2 CH2
CH2 CI
DVB
CH2 VBC
DMAEMAFormula F - resulting polymer
n
[0032] The acid block anion membranes of the invention are further
characterized as having a water content of about 12-20% , preferably about 13-18%.
Further, they exhibit a current efficiency of about 93% and greater and have a
quaternary amine functionality of about 20-70% based on total amine present with an
even more specific range of about 30-40% demonstrated by the specific examples
herein shown.
[0033] It will be understood that the embodiments described herein are merely
exemplary and that a person skilled in the art may make many variations and
modifications without departing from the spirit and scope of the invention as defined
in the appended claims.CLAIMS
1. Method for preparing an acid block anion selective polymeric
membrane having a woven or non-woven cloth reinforcing structure, the polymer of
said membrane prepared by the process comprising copolymerizing components (I) an
ethylenically unsaturated aliphatic or aromatic tertiary or quaternary amine monomer,
(II) a cross linking monomer, and (III) vinylbenzyl chloride, in the presence of a free
radical polymerization initiator.
2. Method as recited in claim 1 wherein the molar ratio of components
I:I:III is from about 20-60:30-70:1-19 with said I , II and III equating 100 molar
percent.
3. Method as recited in claim 2 wherein the molar ratio of components
I:II:III is from about 35-45:45-55: 10-15.
4. Method as recited in claim 1 said copolymerization is conducted in the
absence of non-polymerizable solvent.
5. Method as recited in claim 1 wherein said component (I) comprises an
ethylenically unsaturated aliphatic tertiary amine.
6. Method as recited in claim 1 wherein said component (I) comprises an
ethylenically unsaturated aromatic tertiary amine.
7. Method as recited in claim 5 wherein said component I comprises a
member selected from the group consisting of DMAEMA and DMAPMA.
8. Method as recited in claim 6 wherein said component I is vinyl
pyridine.9. Method as recited in claim 7 wherein said initiator (IV) comprises a
peroxide initiator.
10. Method as recited in claim 7 wherein said cross linking monomer
comprises a member selected from the group consisting of DVB and EGDM.
11. Method as recited in claim 10 wherein said cloth comprises a member
selected from the group consisting of polypropylene woven cloth, polyester woven
cloth and polyacrylic woven cloth, said membrane having a water content % of from
about 12-20 wt and a resistance R ohm- cm2 of about 50-100 measured in 0.0 IN
NaCl solution at 1,000 Hz.
12. Method as recited in claim 10 wherein said cloth comprises a member
selected from the group consisting of polypropylene woven cloth, polyester woven
cloth, polyester woven cloth, and polyacrylic woven cloth, and modacrylic woven
cloth, said membrane exhibiting a current efficiency of greater than about 93%.
13. An anionic exchange membrane of the type used in an electrodialysis
apparatus, comprising a woven or non-woven cloth, said cloth impregnated with a
copolymer comprising the reaction product of components (I), (II), and (III) wherein
(I) is an ethylenically unsaturated aliphatic or aromatic tertiary or quaternary amine
monomer, (II) a cross linking monomer, and (III) vinyl benzyl chloride.
14. Anionic exchange membrane as recited in claim 13 wherein said
component (I) is chosen from the Formula A-D whereinFormula A - aliphatic tert-amine
Rl
C
C = 0
X
R2
wherein Rl is H or CHs, X is O or NH2, R2 is lower alkyl (Ci-Ce) alkylene and R3
and R4 are independently chosen from lower (Ci-Ce) alkyl.
Formula B - aliphatic quaternary amine
Rl
CH2= C
C = 0
O
R2
I
R3 - N + - R4
R5
wherein Rl, R2, R3, and R4 are as defined in Formula A; R5 is chosen from lower
(Ci-Ce) alkyl, and A is an anion chosen from halo and MeSCn .ula C - aromatic tertiary amine
N
wherein R6 is vinyl.
Formula D
R2 (0-1)
I
N+ - R4
R5
wherein R6 is the same as in Formula C, R2, when present, is the same as in Formula
A; R3, R4, and R5 are the same as in Formula B and A is the same as defined in
Formula B.
15. Anionic exchange membrane as recited in claim 14 wherein the molar
ratio of components I :II: III is from about 20-60:30-70: 1-19% with the foregoing
percentages adding up to 100% .
16. Anionic exchange membrane as recited in claim 15 wherein the molar
ratio of components I:II:III is from about 35-45:45-55% :10-15% .
17. Anionic exchange membrane as recited in claim 15 wherein the molar
ratio of components I:II:III is about 37-40%, 48-50%, 12-13%.
18. Anionic exchange membrane as recited in claim 16 wherein said
component (I) comprises a member selected from the group consisting of DMAEMA
and DMAPMA, said membrane characterized by having a current efficiency of
greater than 93%.19. Anionic exchange membrane as recited in claim 18 wherein cross
linking monomer component (II) is a member selected from the group consisting of
DVB and EGDM.
20. Anionic exchange membrane comprising a polymer reinforced fabric,
said polymer having the structure: