TITLE: Fluorinated polymeric membranes for gas separation processes
European Patent EP0226141
 B1

ABSTRACT:
Abstract not available for EP0226141
Abstract of corresponding document: US4657564
The present invention is a treated, semi-permeable, polymeric membrane
having improved selectivity for separating components of a gas mixture.
The membrane is provided by fluorinating a polymer cast into membrane
form, having the general structural formula: wherein R1 is a linear or
branched C1-C4 alkyl group; R2 and R3 are independently linear or
branched C1-C6 alkyl groups; R4 is a linear or branched C1-C12 alkyl or
aryl group; X is a C1-C3 alkyl group or m is at least 100; and n is 0
or 1.

INVENTORS:
Langsam, Michael (1114 North 26th Street, Allentown, Pennsylvania,
18104, US)

APPLICATION NUMBER: EP19860116924
PUBLICATION DATE: 03/27/1991
FILING DATE: 12/05/1986

ASSIGNEE: AIR PRODUCTS AND CHEMICALS, INC. (P.O. Box 538, Allentown,
Pennsylvania, 18105, US)
INTERNATIONAL CLASSES: C08J5/18; B01D53/22; B01D67/00; B01D71/44; B01D71/70; C08F8/20;
C08F8/22; C08J5/18; B01D53/22; B01D67/00; B01D71/00; C08F8/00;
(IPC1-7): B01D53/22; B01D71/70; C08F8/22; C08F138/00; C08J7/12
EUROPEAN CLASSES: B01D67/00R18; B01D71/44; C08F8/22

FOREIGN REFERENCES:
WO/1983/003419A CONTROLLED SURFACE-FLUORINATION PROCESS
JP60232205A  GAS SEPARATING MOLDED BODY

OTHER REFERENCES:
PATENT ABSTRACTS OF JAPAN, vol. 9, no. 45 (C-268)[1768], 26th February
1985; & JP-A-59 189 913 (NIPPON OIL SEAL KOGYO K.K.) 27-10-1984
Attorney, Agent or Firm:
Kador & Partner (Corneliusstrasse 15, München, 80469, DE)

CLAIMS:
1. A method of producing a semi-permeable, polymeric membrane having
improved selectivity for use in separating components of a feed gas
mixture, comprising casting into membrane form a polymer formed from a
1-alkyl-2-silyl-substituted acetylene monomer and treating the membrane
with an inorganic gas, characterized in that the polymer has the
general structural formula: wherein R[1] is a linear or branched
C[1]-C[4] alkyl group; R[2] and R[3] are independently linear or
branched C[1]-C[6] alkyl groups; R[4] is a linear or branched
C[1]-C[1][2] alkyl or aryl group; X is a C[1]-C[3] alkylene group or m
is at least 100; and n is 0 or 1; and the membrane is treated with
elemental fluorine for a time sufficient to modify the membrane such
that the O[2]/N[2] selectivity ratio of the membrane is increased by at
least 50% over that of the membrane prior to treatment.
2. A method according to claim 1 wherein the fluorine is provided by a
gas stream containing 0.01-25% fluorine gas.
3. A method according to claim 2 wherein the said gas stream contains
0.1-2% fluorine gas.
4. A method according to any of claims 1 to 3 wherein the membrane is
treated with the fluorine for from 10 seconds to 24 hours.
5. A method according to claim 4 wherein the membrane is treated with
the fluorine for from 0.5 to 120 minutes.
6. A method according to any preceding claim wherein, in the general
structural formula, R[1], R[2] and R[3] are CH[3] groups, R[4] is a
linear or branched C[1]-C[1][2] alkyl group and n = 0.
7. A method according to any preceding claim wherein the polymer has
the following structural formula: wherein m is at least 100.
8. A method according to any preceding claim wherein the membrane is
treated with the fluorine under ambient conditions of temperature and
pressure.
9. A method according to any preceding claim wherein the polymer is
cast into the form of an asymmetric membrane having a thin dense layer
over a porous layer.
10. A method according to any preceding claim wherein the polymer,
prior to treatment with elemental fluorine, is first coated onto the
surface of a porous substrate.
11. A method according to claim 10 wherein the porous substrate is in
the form of a flat sheet or hollow fiber prior to being coated.
12. A method according to claim 10 or to claim 11 wherein the porous
substrate is a polyolefin.
13. A method according to claim 12 wherein the polyolefin is
polyethylene or polypropylene.
14. A treated, semi-permeable, polymeric membrane having improved
selectivity for use in separating components of a feed gas mixture,
said membrane comprising a polymer, cast into membrane form, having the
general structural formula: wherein R[1] is a linear or branched
C[1]-C[4] alkyl group; R[2] and R[3] are independently linear or
branched C[1]-C[6] alkyl groups; R[4] is a linear or branched
C[1]-C[1][2] alkyl or aryl group; X is a C[1]-C[3] alkylene group or m
is at least 100; and n is 0 or 1: said membrane having been produced by
a method in accordance with any of claims 1 to 13 and possessing in
consequence of the treatment with elemental fluorine an O[2]/N[2]
selectivity ratio increased by at least 50% over that of the membrane
prior to said treatment.
15. Use of a polymeric membrane in accordance with claim 14 in
separating a feed gas mixture containing at least two components.

DESCRIPTION:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to polymeric membranes which are used to
separate components of a gas mixture. It also relates to gas separation
processes using the polymeric membranes.
BACKGROUND OF THE INVENTION
A review of emerging technology using membranes to separate acid gases
such as CO[2], H[2]S and SO[2] from gas streams is disclosed by S.
Kulkarni, et al. in an article entitled, "Membrane Separation Processes
for Acid Gases, "AIChE Symposium Series (1983). Both currently
available and potential polymer membranes for the separation of CO[2]
from natural gas are discussed. The permeation characteristics of
various types of membranes, such as asymmetric cellulose esters,
multi-component polysulfone/silicon rubber, ultrathin polyetherimide,
and ultrathin silicone rubber/polycarbonate were calculated for
CO[2]/CH[4] gas mixtures.
U.S. patent 4,486,202 discloses gas separation membranes exhibiting
improved gas separation selectivity. A preformed, asymmetrical gas
separation membrane having selective permeation of at least one gas in
a gaseous mixture over that of one or more remaining gases in the
gaseous mixture, is contacted on one or both sides with a Lewis acid.
Contacting the asymmetrical membrane with a Lewis acid results in
improved separation factors for the permeating gases. The patent also
discloses a method for producing improved, asymmetrical membranes in
flat film or hollow fiber form having improved gas separation
properties by treatment with a volatile Lewis acid.
U.S. Patent 4,472,175 discloses gas separation membranes exhibiting
improved gas separation selectivity. In this patent, a preformed,
asymmetrical gas separation membrane, having selective permeation for
at least one gas in a gaseous mixture over that of one or more
remaining gases in a gaseous mixture, is contacted on one or both sides
with a Bronsted-Lowry acid. Contacting the asymmetrical membrane with a
Bronsted-Lowry acid results in improved separation for the permeating
gases. Additionally, this patent discloses a method for producing
improved, asymmetric membranes in flat film or hollow fiber form having
improved gas separation properties by treatment with a Bronsted-Lowry
acid.
U.K. Patent Application 2135319A discloses a membrane having improved
permeability for a variety of gases. The membrane is formed from a
polymer having repeating units of the formula: wherein R is an alkyl
radical having 1-12 carbon atoms. The polymer is dissolved in one or
more solvents, such as aliphatic hydrocarbons, to form a polymer
solution which is cast to form a film. The membranes may be produced in
any form, such as plain film, tubular and hollow fibrous forms and, if
necessary, may be supported on one or more backing layers to form
composites.
U.S. Patent 4,020,223 discloses a method of modifying the surface of
synthetic resins selected from the group consisting of polyolefins and
polyacrylonitriles by treatment with a fluorine-containing gas. The
fluorinated resin fibers exhibit good soil release and good water
adsorption or moisture transport properties.
JP-A-60 232 205 (prior art according to Art.54(3)EPC) describes the
treatment of semi-permeable membranes, such as those derived from
1-methyl-2-trimethylsilyl acetylene, by a fluorine-containing low
temperature plasma.

BRIEF SUMMARY OF THE INVENTION
The present invention is a treated, semi-permeable, polymeric membrane
having improved selectivity for use in separating components of a feed
gas mixture. The membrane is a polymer cast into membrane form, having
the general structural formula: Wherein R[1] is a linear or branched
C[1]-C[4] alkyl group; R[2] and R[3] are independently linear or
branched C[1]-C[6] alkyl groups; R[4] is a linear or branched
C[1]-C[1][2] alkyl or aryl group; X is a C[1]-C[3] alkylene group or m
is at least 100; and n is 0 or 1, which has been treated by contacting
it with elemental fluorine for a time sufficient to modify the membrane
such that the O[2]/N[2] selectivity ratio of the membrane is increased
by at least 50% over that of the membrane prior to contact with the
reactive fluorine source.
The present treated; i.e., fluorinated, polymeric membrane exhibits
good gas permeability properties with a significant increase in gas
selectivity over the unfluorinated polymer. Increased selectivity of
the membrane is achieved for a wide variety of gas streams which
contain at least two components having different permeability rates
through the membrane.
The present invention is also a process for separating feed gas
mixtures containing at least two components having different
permeabilities through the membrane, by bringing said feed gas mixture
into contact with a treated, semi-permeable, polymeric membrane as
described above.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a treated, semi-permeable, polymeric membrane
which exhibits both good permeability and high selectivity for a wide
variety of gas mixtures. The membrane comprises a polymeric membrane
which has been treated with elemental fluorine to form an ultra thin,
selective surface. The polymeric membrane has the general structural
formula: wherein R[1] is a linear or branched C[1]-C[4] alkyl group;
R[2] and R[3] are independently linear or branched C-C[6] alkyl groups;
R[4] is a linear or branched C[1]-C[1][2] alkyl or aryl group; X is a
C[1]-C[3] alkylene group or m is at least 100, and n is 0 or 1.
The basic polymer can be produced by any conventional polymerization
method which is capable of synthesizing this type of polymer, for
example, by polymerizing monomer units in an organic solvent using a
suitable catalyst such as TaCl[5], MoCl[5], NbCl[5] etc. While the
polymer can have a wide range of molecular weights wherein m is at
least 100, for handling and synthesis purposes it is preferred that m
is less than 50,000. After it is synthesized, the polymer is cast into
membrane form. The membrane form may be any conventional type of
membrane, such as a flat sheet, hollow fibers or spiral wound flat
sheets. In addition to self-supporting layers, the polymer may be cast
onto a suitable support to form a composite structure.
The untreated polymeric membrane generally has high permeability values
for a wide range of gases, but typically exhibits relatively poor gas
selectivity and therefore is not suitable for many gas separation
operations. To increase selectivity, after the polymer is cast into
membrane form it is fluorinated by contacting it with elemental
fluorine source. One such fluorination method involves contacting the
membrane with a gas stream containing between 0.01%-25% fluorine gas
for a period of time between 10 seconds and 24 hours. Preferred
fluorination techniques include a contact time between 0.5 and 120
minutes with a gas stream having a fluorine concentration between
0.1%-2% fluorine gas. In any case, fluorination should be sufficient to
increase the O[2]/N[2] selectivity ratio of the membrane at ambient
temperature by at least 50%. A wide variety of fluorine-containing gas
streams can be used to fluorinate the film, such as F[2]/O[2],
F[2]/N[2], F[2]/Cl[2], F[2]/O[2]/N[2], F[2]/Cl[2]/N[2],
F[2]/SO[2]/N[2], F[2]/SO[3]/N[2], F[2]/SO[2]Cl[2] and F[2]/SO[2]Cl/N[2]
etc. If a high concentration, i.e. 10%-25%, of fluorine is to be used
in the fluorination step, the fluorine concentration should be
incrementally staged-up slowly to avoid burning the membrane. In
addition to the above-described gas-phase fluorination, other
fluorination techniques can be used. For example, a liquid containing
fluorinating agents may be either volatized into a reactive gas
atomsphere or the membrane may be coated or dipped into a dilute
solution of a fluorine containing agent followed by a gas phase
volatilization. While both sides of the polymeric membrane can be
subjected to the fluorine treatment, it is preferred that only one
surface of the membrane be treated, thereby forming an ultra-thin
selective surface only on that side of the membrane, with the remainder
of the membrane consisting of the highly permeable polymeric structure.
The interaction between the silicon containing polymer and the reactive
atmosphere can be carried out under ambient conditions of temperature
and pressure.
The fluorinated membrane exhibits greatly enhanced permselectivity for
various gas mixtures, making it useful in many different gas separation
operations. A gas stream containing two or more components is brought
into contact with the membrane, and the permeate stream from the
membrane is analyzed and measured to determine the permeability
coefficient (P) of the various gaseous components. Permeability
coefficient can be measured by the following relationship: Where:
J
 is Flux
A
 is Area
L
 is Thickness
P
 is Pressure
This relationship can be conveniently expressed in units of measurement
termed Barrers. The relationship for Barrers is:
Additionally, the permeance (P/L), as defined by Henis and Tripodi in
their paper on resistence models, J. Memb. Sci. 8, 223 (1981), of the
composite structure is also measured taking into account the area of
the ultra thin surface layer. By comparing the permability end/or
permeance values for different gaseous components, a selectivity (α)
ratio for various gas mixtures can be calculated. It was found that the
treated membrane structure of the present invention significantly
increased the selectivity ratios of a wide number of gas mixtures.
Examples of such gas mixtures include: He/CH[4], He/N[2], H[2]/CH[4],
H[2]/CO, H[2]/N[2], O[2]/N[2] and CO[2]/CH[4]. While the selectivity
ratios of the above gas mixtures demonstrated a significant increase,
it is expected that many other gas mixtures, both binary and
multi-component mixtures, would also exhibit increased selectivity
ratios.
Experimental:
Synthesis of Poly Trimethyl Silyl Propyne (PTMSP)
100 grams of Toluene was mixed with TaCl[5] catalyst and stirred for
about 5 minutes until it dissolved to form a bright yellow solution,
About 25 grams of trimethyl silyl propyne (TMSP) monomer was added and
the solution immediately turned dark brown. Within two hours there was
a noticeable increase in solution viscosity. After 24 hours the
reaction mixture was quenched in methanol, washed with about 1000 ml of
methanol and then dried, leaving a PTMSP Polymer.
The polymer produced, polytrimethyl silyl propyne, (PTMSP), has the
structure: Wherein m is at least 100.
By varying the monomer (TMPS) to catalyst (TaCl[5]) ratio, it was
possible to control the molecular weight of the polymer. The yields and
Brookfield viscosities of 1.7% toluene solutions of several of the
polymers synthesized according to the above procedure are listed in
Table 1 below:
Both flat sheet PTMSP membranes and PTMSP membranes coated on a porous
hollow fiber substrate were fabricated by dissolving the polymer in
toluene at a weight ratio of 1/40 to form a 2.5% solution by weight. A
portion of the toluene-polymer solution was cast on a clean, smooth
glass surface using a 40 mil. (1.016 mm) doctor knife, and air dried
using a stream of dry nitrogen. The polymer film ranged from about
25-30 micro-meters in total thickness. The flat sheet membranes were
removed from the solid glass support by soaking in water. The films
easily floated off of the glass surface. The flat sheet membranes were
mounted in a CSC-135 Permeation Cell (manufactured by Custom Scientific
Corporation, Whippany, NJ) using the procedure described in an article
by S. A. Stern, et al. in Modern Plastics, October 1964.
The same toluene-polymer solution was used for coating Celgard®
polypropylene porous hollow fiber using grade #X-20 of Celgard®
material manufactured by Celanese Chemical Corporation. The Celgard®
hollow fibers were dipped into the toluene-polymer solution twice to
insure complete coverage of the outer surface of the fiber.
Several of the PTMSP membranes while still attached to the glass
supports, were fluorinated in a gas phase batch reactor with various
fluorine/nitrogen mixtures. The membranes were placed in the reactor
and the gas space was purged for 4 hours with nitrogen to remove
ambient air. Pre-set ratios of F[2]/N[2] were then flowed through the
reactor space for pre-determined periods of time.
Three PTMSP membranes were fluorinated for a period of five minutes
using different fluorine gas concentrations. A study of the surface
composition of the PTMSP membranes before and after fluorination
indicates a drastic alteration in the surface of the membrane. The
surface compositions of the three fluorinated membranes and one
unfluorinated PTMSP membrane were analyzed, and the results are
reported in Table 2 below.
The surface analysis reported in Table 2 shows a significant decrease
in both the carbon and silicon contents on the surface of the
fluorinated membranes. It is postulated that the reduction of silicon
on the surface of the fluorinated polymeric membranes is due to the
formation of volatile silicon compounds, such as SiF̅[4], which are
removed from the polymeric chain resulting in improved membrane
selectivity.
Several other PTMSP membranes were synthesized and fluorinated in a gas
phase batch reactor in accordance with the above procedures. The
fluorine content of the treating gas stream ranged from 0.1-0.5% F[2]
with total F[2] exposure ranging from 0.5-60 ml. Contact times of the
membranes with the treating gas ranged from 1-60 m.
The fluorinated PTMSP membranes were recovered from the reactor and
subsequently removed from the glass supports by a water wedge
technique. The membranes were measured for total thickness and
subsequently mounted in the CSC-135 Permeation cells for gas
permeability and selectivity studies.
Gas permeability and selectivity studies using the PTMSP membranes
treated with various fluorine concentrations and contact times were
carried out and are reported in the examples below. These examples are
meant only to illustrate the present invention and are not meant to be
limiting.
Example 1
Two unfluornated and nine fluorinated flat sheet PTMSP polymer membrane
samples were mounted in Separate CSC permeation cells such that
pressurized gas mixtures could be passed over the membrane surface and
the permeate stream could be measured on the permeate side of the
membrane by a volumetric flow device.
The fluorinated samples differed in the fluorine concentration used as
well as the contact time for fluorination. The permeability (P),
permeance (P/L), and selectivity (α) of various gases through the
membranes are reported in Tables 3 and 4 below.
The results reported in Tables 3 and 4 above for the gas permeability
and selectivity tests, show a significant increase in membrane
selectivity of the fluorinated membranes for all seven gas mixtures
tested. For example, the O[2]/N[2] selectivity ratio of the PTMSP
membrane showed over a two-fold increase for the membrane when
fluorinated with 0.1% F[2] gas for 5 minutes, and over a three-fold
increase for the membrane when fluorinated with 0.1% F[2] for 15
minutes. The largest selectivity increase was exhibited by the PTMSP
membranes which were fluorinated with a 0.5% fluorine gas for 60
minutes, although a significant selectivity increase was observed even
with the membranes which were fluorinated with only 0.1% fluorine gas.
Table 3 also shows that PTMSP membranes, when fluorinated under the
proper conditions, maintain good permeability characteristics while
increasing selectivity.
Example 2
The fluorination techniques used to treat the polytrimethyl silyl
propyne polymers were also used to treat silicone rubber and poly
2-nonyne polymers.
Silicone rubber which is a crosslinked polymer having the general
structural formula: when formed into a membrane has been shown to be
very permeable for many gases yet exhibits relatively low
selectivities. A 5 mil (0.127 mm) thick membrane of commercial silicone
rubber (MEM-100, lot #B-163, manufactured by General Electric Comany)
was fluorinated with a gas stream containing 0.5% F[2] gas for 45
minutes. The permeabilities and selectivities for various gases were
tested for both the fluorinated membrane and an unfluorinated membrane.
Gas permeability values and surface analysis data for the fluorinated
and unfluorinated membranes are reported in Table 5 below.
The above permeability coefficient and surface analysis data indicate
that the silicone rubber membrane is fluorinated, but that surface
fluorination did not have a significant effect on permeability or
selectivity of the membrane for the gases tested. Additionally, the
fluorinated membrane eroded over time, making this polymer unsuitable
for surface fluorination.
A sample of poly 2-nonyne was polymerized using a mixed
MoCl[5]/P(Ph)[4]catalyst system. The resulting polymer, having the
general structural formula: was formed into a dense membrane and
treated with a F[2]/N[2] gas stream comprising 0.5% F[2] gas for 15
minutes. Fluorinated and unfluorinated membrane samples were tested for
permeability and selectivity for various gases, and a surface analysis
was performed on both samples. The results of the tests and analyses
are reported in Table 6 below.
The poly-2-nonyne membrane, when treated with an F[2]/N[2] reactive
mixture, exhibited a highly fluorinated surface, but demonstrated no
significant change in either permeability coefficient or selectivity
for the gases tested.
The results of the above examples demonstrates the importance of both
the basic polymer structure and the fluorination step in synthesizing a
membrane having both high permeability and high selectivity for a wide
range of gas mixtures.
Having this described the present invention, what is now deemed
appropriate for Letters Patent is set out in the following appended
claims.