Note: Descriptions are shown in the official language in which they were submitted.
!' - !,, ',,~, , "'
204~3~0~
OXYGEN-S~PARATING POROUS MEMBRANES
BACKGROUND OF THE INVENTION
~ his invention relates to oxygen-separating porous mem-
branes to be used in oxygen-enriching processes, typically for
combustion gas production, for medical treatment. More partic-
ularly, the invention concerns porous membranes which contain,
as dispersed in the pores, a metal complex capable of adsorbing
and desorbing oxygen rapidly and reversibly.
Oxygen is one of the chemicals most widely used on indus-
trial scales, specifically in the manufacture of iron, steel,
and other metals and glass, in chemical oxidation and combus-
tion, and in wastewater disposal. In the field of medical care
too, it has very wide applications including the therapy for
lung disease patients by means of oxygen inhalation. ~or these
reasons the development of processes for concentrating oxygen
out of air is an important problem with far-reaching effects on
various sectors of industry. While dominant industrial pro-
cesses for atmospheric oxygen concentration today are low-
temperature and adsorption techniques, membrane separation is
considered promising from the energy-saving viewpoint.
Success of membrane separation depends primarily on the
discovery of a membrane material that would permit selective
and efficient oxygen permeation relative to nitrogen from air.
Currently available membranes capable of permeating and
2049~00
concentrating atmospheric oxygen (known as oxygen-enriching
membranes) are those of silicone, silicone polycarbonate, and
the like. Some of them are in practical service. They do not
have high oxygen-permeation selectivity (oxygen-permeability
coefficient/nitrogen-permeability coefficient, or ratio ~), the
value being approximately 2, and y~t exhibit high permeability
coefficient (10-8 [cm3 (STP) cm/cm2 sec cmHg]). With this
feature the membranes are incorporated in modules, multistage
processes, and other systems to obtain oxygen-enri~hed air,
with oxygen concentrations of approximately 30~.
Gas separation by the use of microporous membranes ranging
in pore size fr~m several ten to several hundred angstroms is
also extensively performed. Gas permeation through a porous
mass is dictated by the ratio of the distance over which the
particular gas particles impinge upon one another, or the mean
free path, A to the pore diameter r, (r/A). When the pore
diameter is small, being r/A<l, the mutual impingement of the
gas particles is ignored. The permeation conforms to the
Knudsen flow in which it is inversely proportional to the
square root of the molecular weight of the gas. Gas permeation
based on this permeation mechanism attains a strikingly high
permeability coefficient. Nevertheless, the process is unsuit-
able for the oxygen separation by permeation from air, because,
when separating gases of dissimilar molecular diameter, such as
oxygen and nitrogen, the selectivity becomes less than 1.
2049~0V
It has been reported that generally gas molecules once
adsorbed by the pore surface of a porous membrane will diffuse
over the adsorption layer for permeation, resulting in a
substantial increase in permeability. However, the phenomenon
is limited to operations handling lower hydrocarbons, carbonic
gas, and other gases of relatively high boiling points. The
~henomenon also is observed only when membranes with pore
diameters from about 30 to about 300 ~ are used. Oxygen
permeation from air has not in the least been known in the art.
In order to obtain highly oxygen-rich air useful for
industrial and medical applications by a single permeable-
membrane pass, it is essential that the separating membrane
have a high oxygen-permeability coefficient, of the order of
10-8, and an ~ of at least 5.
Polymeric membranes of silicone and the like exhibit
oxygen-permeability coefficients as high as about lo~8/ but
their oxygen selectivities are low. The porous membranes that
rely upon the Knudsen flow for gas permeation are incapable of
separating oxygen and nitrogen, although they show greater
permeability than polymeric membranes.
We have hitherto continued the synthesis of metal complex-
es capable of rapid, reversible adsorption and desorption of
oxygen molecules. As a result, we clarified essential require-
ments of the metal complexes that can adsorb and desorb oxygen
molecules selectively, rapidly, and reversibly, even in a
Z049~20~
solid-phase polymer. We successfully synthesized the novel
complexes and taught their use for oxygen-separating membranes
(Patent Application Public Disclosure No. 171730/1987).
However, polymeric membranes incorporating such complexes, when
used in air permeation, did not always achieve the object
satisfactorily. Although the ~ value exceeded the target value
of 5, the permeability coefficient was only 10-9. For the
treatment of a sufficiently large volume of air for oxygen
enrichment, an additional step, for example, of providing an
extra thin film, was required.
SU2~RY OF THE INVENTION
In view of the above, we have made further intensive
research and have now successfully produced membranes having
oxygen-permeation selectivity while maintaining high gas
permeability, by allowing a porous support to hold a porphyrin
metal complex uniformly in the pores under certain conditions.
Thus, the invention relates to oxygen-separating porous
membranes as follows:
1. An oxygen-separating porous membrane characterized by a
complex comprising (a) a transition metal (II) ion, and (b) a
ligand taken from the group consisting of (1) porphyrins, (2)
Schiff bases, (3) cyclidenes, and (4) amine-like macrocycles,
and (c) an aromatic amine, said complex retained in the pores
of a porous substrate, the mean free pore diameter of said
porous membrane being in the range of 3.5 to 100 A.
204~
2. The membrane of 1 above in which said ligand is a
porphyrin.
3. The membrane of 2 above in which said porphyrin is
meso-tetrakis(~ -o-pivalamidophenyl)porphyrinato.
4. The membrane of 1 above in which said transition metal
(II) comprises cobalt (II).
5. The membrane of 1 above in which said aromatic amine
comprises (1) copolymers of a vinyl aromatic amine and either
(a) an alkyl acrylate or (b) an alkyl methacrylate, or (2) a
low-molecular-weight aromatic amine.
6. The membrane of 5 above in which said aromatic amine is
a copolymer of a vinyl aromatic amine and either (i) an alkyl
acrylate or (ii) an alkyl methacrylate, containing 1 to 15
carbon atoms in the alkyl group thereof.
7. The membrane of 1 above in which said transition metal
(II) comprises from about 0.02 to 1.7 millimoles per gram of
said complex.
8. The membrane of 7 above in which said transition metal
(II) comprises from about 0.20 to 1.7 millimoles per gram of
said complex.
9. The membrane of 1 above in which said porous substrate
comprises an inorganic porous membrane.
10. The membrane of 1 above in which said porous substrate
comprises an organic porous membrane.
11. The membrane of 10 above in which said porous
2049~00
substrate comprises polysulfone.
12. The membrane of 10 above in which said porous
substrate comprises polyimides.
13. The membrane of 1 above in which said porous membrane
comprises a flat film or a hollow fiber membrane.
14. The membrane of 1 above in which said transition metal
(II) comprises cobalt (II), said porphyrin is meso-tetrakis-
~ -o-pivalamidophenyl)porphyrinato, and said porous
membrane comprises hollow fibers.
15. The membrane of 1 above in which said mean free pore
diameter is in the range of 3.8 to 60 A.
16. The membrane of 14 above in which said mean free pore
diameter is in the range of 3.8 to 60 ~.
17. The membrane of 14 above in which said aromatic amine
comprises (1) copolymers of a vinyl aromatic amine and either
(a) an alkyl acrylate or (b) an alkyl methacrylate, or (2) a
low-molecular-weight aromatic amine.
DETAILED DESCRIPTION OF THE INVENTION
Metal complexes capable of reversible oxygen adsorption
and desorption usually are complexes consisting of a metal ion
of a low oxidation number and a ligand of conjugated system
combined with an aromatic amine. The present invention prefer-
ably utilizes a complex consisting of a meso-tetrakis(~
o-pivalamidophenyl)porphyrinato metal (II) as the first compo-
nent and either a copolymer of a vinyl aromatic amine and an
Z~4~
alkyl acrylate or alkyl methacrylate or a low-molecular-weight
aromatic amine as the second component. The metal in the metal
complex is a bivalent metal element, preferably cobalt.
As the ligand that constitutes the metal complex, any of
those mentioned above may be used.
Among other examples of porphyrins is "PRIXDME", protopor-
phyrin IX dimethy] ester.
Examples of Schiff bases include "salen", bis(salicyl-
ideneiminato)ethylenediamine, and "3-methoxysaltmen", N,N'-bis-
(3-methoxysalicylideneiminato)tetramethylethylenediamine.
Cyclidenes are, for example, "lacunar methyl, methyl-C6-
cyclidene", 2,3,10, 11, 13, 19-hexamethyl-3, 10, 14, 18, 21,
25-hexaazabicyclo[10.7.7]hexacosa-1,11,13,18,25-hexene~4N, and
"lacunar phenyl,benzyl-metaxylyl-cyclidene", 3,11-dibenzyl-
2,12-diphenyl-3,11,15,19,22,26-hexaazatricyclo[11.7.7.15 9]-
octacosa-1,5,7,9(28),12,14,19,21,26-nonaene~4N.
Examples of amine-like macrycycles are "lacunar Me2~p-
xylylene)Me2malMeDPT", 7,19-Diacetyl-6,20-diketo-8,13,18-tri-
methyl-26,33-dioxa-9,13,17-triazatricyclo[23.8.228~3l.1l 5.-
121~25]heptatriaconta-1,3,5(36),7,18,21,23,25(37),28,30,34-
undecaenato-~3N-~20, and "salMeDPT", bis-(salicylideneiminato)-
N-methyl-dipropylenetriamine.
The transition metal (II) ion, especially cobalt (II),
fcrms a complex which has reversible interactions with 2~
The aromatic amine functions as the axial base in the
204~3~00
complex, "activating" the complex for reversible interactions
with 2 The amine residues, such as derivatives of pyridine
or imidazole, may be present in either high-molecular-weight
polymers as pendant groups, or in low-molecular-weight
individual molecules.
Such a complex is dissolved in a dichloromethane solution,
and a porous support is immersed in the solution. After full
retention of the complex in the pores has been confirmed, the
impregnated support is dried in vacuum to obtain a porous
membrane. The porous support for this purpose may be any of
materials in which each pore is open on one side and extends
backward to open also on the opposite side. An inorganic
support of porous glass, porous alumina, porous carbon or the
like is a good choice. The porous membrane has a mean pore
diameter of 100 A or less, desirably 50 A or less, provided the
complex can be retained in the pores without clogging the
latter. The mean pore diameter is limited to 100 A or less
because if it exceeds 100 A the Knudsen flow becomes dominant,
reducing the ~ value of oxygen-permeation selectivity. The
specific composition and conditions for preparation as will be
described later permit the pores of the porous membrane to be
kept unclogged. Consequently, the membrane maintains high gas
permeability (the Knudsen flow) and attains high oxygen
selectivity since the complex dispersedly held on the pore
surface causes selective, rapid adsorption and desorption of
20~3;20~)
oxygen, which in turn produces a surface diffused flow that
adds to the selectivity.
The introduction of a complex capable of rapid, reversible
oxygen adsorption and desorption has now rendered it possible
for the first time to observe a surface diffused flow on a
porous membrane. It is apparently for this reason that an
oxygen-separating membrane is obtained which exhibits excep-
tionally efficient performance (an oxygen-permeability coeffi-
cient of approximately 10-6 and selectivity of 5 or more).
For use in the present invention it is desirable that the
complex comprises, as a metal complex of a porphyrin compound,
a meso-tetra(~ o-pivalamidophenyl)porphyrinato metal (II)
and, as an aromatic amine ligand, a copolymer of a vinyl
aromatic amine and an alkyl acrylate or alkyl methacrylate,
typified by poly(N-vinylimidazole-co-octyl methacrylate) or the
like, or N-methylimidazole or pyridine.
The metal ion and the ligand residue that constitute a
complex are in a molar ratio appropriately in the range from
1:1 to 1:50.
A porphyrin and a ligand are separately dissolved
uniformly in an organic solvent such as dichloromethane,
thoroughly deoxidized, and mixed up. Into this mixed solution
is immersed a porous support in an oxygen-free atmosphere.
After the complex has been adequately supported in the pores,
the porous membrane is finished by vacuum drying. In this case
o~
the porphyrin content is desirably chosen from the range of
about 1 to about 30% by weight. There is no specific
limitation to the form of the porous membrane, but a flat or
tubular shape is desirable. For the manufacture of the
membrane thorough oxygen removal from the complex solution in
advance is advisable.
The use of the membrane according to the invention permits
oxygen enrichment with a high selectivity, at the ~ value of 5
or upwards. Also, because of the extremely high permeability
coefficient, single-step concentration by a membrane with an
effective area of one square meter can yield as much as air
with an oxygen concentration of at least 60% per hour. In a
system for removing residual oxygen from nitrogen in which the
oxygen concentration has been reduced down to 1%, the membrane
makes it possible to afford 99.99%-pure nitrogen. For the
purposes of the invention the measurements of gas permeability
with oxygen-enriching membranes are evaluated by gas
chromatography.
E X A M P L E S
The invention will be more fully described below in
connection with examples thereof which, of course, are in no
way limitative.
Example ~
For the manufacture of a tubular porous membrane, a tube
of porous glass 7 mm in outside diameter and 1.1 mm in wall
-- 10 --
2~D4~0~
thickness was employed. The glass (marketed by Corning Glass
Works under the trade designation "Vicor #7930") had a porosity
of 28% and an average pore diameter of 40 A, the size ranging
from 40 to 70 A. The porous glass was conditioned and prepared
in the following way. Test pieces of the glass, cut to lengths
of 11 cm each, were immersed in 5N hydrochloric acid for 2 to 3
days and then washed with pure water for one full day. They
were heated in a nitrogen atmosphere at 80C until the porous
glass became clear, further heated up to 180C while the
pressure was reduced to 10-3 mmHg, and dried.
Twelve milliliters of a dichloromethane solution contain-
ing 100 mg meso-tetra(~ -o-pivalamidophenyl)porphyrinato
metal ~II) (hereinafter called "CoP" for brevity) and 20 ml of
a dichloromethane solution containing 600 mg poly(N-vinyl-
imidazole-co-octyl methacrylate) were mixed. After one hour of
nitrogen gas injection into the mixed solution, the activated
tubular porous supports were immersed in the solution for 2 to
3 days. Following the confirmation that the complex had been
retained in the pores, the porous memhranes were taken out into
a dry box under a nitrogen atmosphere and then dried in vacuum.
Red, clear porous membranes were obtained which contained 3% by
weight of the complex and had pores 40 ~ or less in diameter,
with adequate mechanical strength.
Thorough introduction of the CoP complex into the porous
glass was confirmed by electron spectroscopy for chemical
~ ~ ~e~
analysis (ESCA). Nitrogen adsorption indicated that the
surface area decreased with the introduction of the complex.
Reversible oxygen adsorption and desorption of the porphy-
rin complex in the membranes could be confirmed from changes in
the visible spectrum (oxygen-combined type: 545 nm; deoxygen-
ation type: 528 nm).
The porous membranes thus prepared were tested for their
mixed oxygen-nitrogen gas permeability by gas chromatography.
When a mixed gas with an oxygen concentration of 2.6% was
supplied, the permeability coefficient was 4.1x10-6 cm3 (STP) -
cm/cm2 sec cmHg and ~ = 7, achieving efficient permeation of
oxygen. The comparative values of a complex-free porous mem-
brane determined under identical conditions were: permeability
coefficient 7.8x10-6 cm3 (STP) cm/cm2 sec cmHg and ~ = 0.98.
Obviously, the membranes according to the invention were
superior in performance. The oxygen permeability of the
membranes remained stable with little change one month later.
Example 2
In Example 1, a combination of CoP and poly~N-vinyl-
imidazole-co-lauryl methacrylate) was used instead, otherwise
the same procedure was followed. Porous membranes, red and
clear, which contained 3% by weight of the resulting complex
and had a pore size of 40 ~ or less and adequate mechanical
strength were obtained. Permeability tests of the membranes,
conducted in the same way as in Example 1, indicated their
26~14~
ability of efficient oxygen production, with a permeability
coefficient of 4.2x10-6 cm3 (STP~ cm/cm2 sec cmHg and ~ = 6.
Example 3
The procedure of Example 1 was repeated with the exception
that the ligand was replaced by poly(N-vinylimidazole-co-butyl
methacrylate). Porous membranes containing 3% by weight of the
complex, red and clear, and which possessed a pore size of 40 A
or less and adequate mechanical strength resulted. Permeabili-
ty measurements made in the same way as in Example l gave a
permeability coefficient of 4.5x10-6 cm3 (STP) cm/cm2 sec cmHg
and ~ = 7, indicating efficient oxygen production.
Example 4
Excepting the use of N-methylimidazole as the ligand, the
procedure of Example l was followed to obtain red, clear porous
membranes containing 3% by weight of the complex and having a
pore size of 40 ~ or less and satisfactory mechanical strength.
Permeability measurements performed similarly to Example 1
indicated efficient oxygen production, with a permeability
coefficient of 8.5x10-6 cm3 (STP) cmlcm2 sec cmHg and ~ = 5.
The oxygen-separating porous membranes according to the
present invention comprise a porous membrane and a certain
porphyrin complex dispersed on the pore surface of the mem-
brane. Their oxygen-permeability coefficients are as high as
approximately 103 times those of conventional polymeric mem-
branes containing or not containing metal complexes. They can,
~4~
therefore, treat by far the larger volume of gases, with the
selectivity value ~ as oxygen-separating membranes in excess of
5. The membranes are capable of collecting oxygen-rich gases
from lean feed gases or even recovering high-purity nitrogen
gas by single-stage permeation. Further outstanding advantages
are that they do not deteriorate with time but maintain good
durability and heat resistance.
