WO2014125299A1

WO2014125299A1 - Gas separation membranes

Info

Publication number: WO2014125299A1
Application number: PCT/GB2014/050449
Authority: WO
Inventors: Takeshi Umehara; Yujiro Itami
Original assignee: Fujifilm Manufacturing Europe Bv; Fujifilm Imaging Colorants Limited
Priority date: 2013-02-18
Filing date: 2014-02-17
Publication date: 2014-08-21
Also published as: GB201302767D0

Abstract

A gas separation membrane comprising a porous support and a discriminating layer, wherein the discriminating layer comprises (i) a polyimide layer having acid groups and (ii) an organic polyamine, wherein the organic polyamine crosslinks the acid groups of the polyimide predominantly by means of non-covalent crosslinking.

Description

GAS SEPARATION MEMBRANES

This invention relates to gas separation membranes and to their use in the separation of gases.

The use of membranes comprising a polyimide discriminating layer to separate gases is known in the art. The known membranes rely on differences in the relative permeability of the gases through the discriminating layer. Typically a mixture of gasses is brought into contact with one side of the membrane and at least one of the gases permeates through its discriminating layer faster than the other gas(es). In this way, the initial gas stream is separated into two streams, one of which is enriched in the selectively permeating gas(es) and the other of which is depleted.

One of the problems with gas separation membranes is that the discriminating layer can become plasticized, reducing its ability to discriminate between different gases and reducing its selectivity. Furthermore, defects such as craters are often present in the discriminating layer and these can result in surface leaks.

Covalent crosslinking of the polyimide discriminating layer of gas separation membranes was investigated in WO 2006/009520 and also by Tin et al in the article entitled "Chemical cross-linking Modification of Polyimide Membranes for

Gas Separation", Journal of Membrane Science 189 (2001 ) 231-239.

US 5,286,280 describes the preparation of composite membranes comprising a porous support, an intermediate gutter layer and a discriminating layer made from what is known as a "6FDA" type polyimide.

The present invention provides membranes suitable for use in the separation of gases. The process can be performed quickly and provide membranes having a very thin discriminating layer.

According to a first aspect of the present invention there is provided a gas separation membrane comprising a porous support and a discriminating layer, wherein the discriminating layer comprises (i) a polyimide layer having acid groups and (ii) an organic polyamine, wherein the organic polyamine crosslinks the acid groups of the polyimide predominantly by means of non-covalent crosslinking.

Typically the acid groups of the polyimide layer are crosslinked by bringing the polyimide layer into contact with a composition comprising an organic polyamine. In order to achieve crosslinking which is predominantly non-covalent crosslinking, one will generally choose relatively mild conditions for such contact. The precise conditions depend to some extent on the acidity of the acid group, the basicity of the organic polyamine and the reactivity of the organic polyamine towards the polyimide. Generally one will select milder conditions for organic polyamines which have higher reactivity with the polyimide in order to avoid or reduce the extent of covalent bond formation between the polyimide and the organic polyamine.

The acid groups are preferably selected from sulphonic, sulphinic, phosphoric and phosphonic acid groups and especially carboxyl groups. The acid groups may be all the same (e.g. all are carboxyl groups) or the acid groups optionally comprise two or more types of acid groups (e.g. two or more of the aforementioned acid groups, for example sulphonic acid groups and carboxyl groups). Preferably the acid groups comprise carboxyl groups and/or sulphonic acid groups, more preferably the acid groups are carboxy groups or sulphonic acid groups.

The acid groups may be in any form, for example the free acid or salt form, e.g. in the form of a salt with a metal, ammonia or an amine (e.g. a primary, secondary or tertiary amine, preferably comprising six or less carbon atoms).

Preferred carboxyl groups are of the formula -CO₂H of a salt thereof (e.g. one of the aforementioned salts).

Typically the crosslinking is performed entirely at a temperature not exceeding 49^°C, more preferably not exceeding 45^°C, especially 5 to 40^°C, more especially 10 to 30^°C. Low temperatures such as these are preferred because they reduce the chances of covalent crosslinking between the polyimide and the amine groups of the organic polyamine. Furthermore, crosslinking is preferably performed such that the polyimide discriminating layer having acid groups is in contact with a composition comprising the organic polyamine for 1 to 45 minutes, more preferably 2 to 10 minutes, preferably at a temperature above room temperature (e.g. at least 25^°C).

One may determine whether the crosslinking is predominantly non-covalent crosslinking by preparing the member for a second time, under identical conditions, using a membrane which is identical except that its polyimide layer lacks the acid groups, then determining whether the discriminating layer of the membrane prepared the second time has any crosslinking. If crosslinking is detected when the process is performed for the first time (using a membrane having the crosslinkable acid-functional polyimide discriminating layer), but not when it is performed for the second time (using a membrane in which the discriminating layer is free from crosslinkable acid groups), then the conditions under which the process of the invention was performed were such that the crosslinking was predominantly (which can include entirely) non-covalent crosslinking.

One may detect the presence or absence of crosslinking using energy- dispersive X-ray ("EDX") mapping, e.g. using a Jeol JSM-6335F field emission scanning electron microscope. A further technique for determining whether the crosslinking is predominantly non-covalent crosslinking is to observe the infra red spectrum of the polyimide before and after crosslinking. If the infra red spectrum at about 1718, 1783 and 1351 cm^"1 remains substantially the same after crosslinking as before this indicates that the imide ring has remained intact, even after the crosslinking, and hence the crosslinking is predominantly (or entirely) non-covalent.

Furthermore, while ionically bonded and hydrogen bonded organic polyamine crosslinkers may be removed from the membrane by adjusting its pH, covalent bonding is more permanent and the crosslinker is much more strongly bound to the discriminating layer. Thus the removability of the organic polyamine crosslinker by pH adjustment also indicates whether or not the crosslinking is non- covalent.

One may use the following calculation to determine the ratio of non- covalent to covalent crosslinking (i.e. the NCC%) present in the membrane:

NCC% = (Mremovable/Mtotal) X 1 00% wherein:

Mtotai is the total mass per cm² of organic polyamine which is present on the membrane before it is stirred as described in M_removabie below; and

Mremovabie is the mass per cm² of organic polyamine which is removed when the membrane is stirred at 20^°C with ten times its dry weight of 0. 1 M NaOH for 1 0 minutes.

One may determine M_totai by measuring the increase in mass per cm² of membrane resulting from the crosslinking reaction. For example, one may weigh a dry sample of the membrane before and after crosslinking and the increase in weight per cm² is M_totai-

One may use gas-liquid chromatography to determine the concentration of organic polyamine present in the 0.1 M NaOH and, knowing the volume of 0. 1 M NaOH, and the area of the membrane one may calculate M_removabie-

If the NCC% is >50% then the crosslinking is predominantly non-covalent crosslinking. Preferably the NCC% is >75%, more preferably >85%, especially >95%, more especially about 1 00% .

In ionic crosslinking, two or more of the acid groups are in ionised form, such as for example for two or more carboxyl groups(-CO₂ ^_) and are linked by two or more protonated amine groups (e.g. -NH₃ ⁺ groups) present in a molecule of the organic polyamine.

In hydrogen bonding, typically =O groups (e.g. carbonyl groups (C=O) of two or more carboxyl groups) in the polyimide are linked by hydrogen bonding to the hydrogen atoms present in the amino groups (e.g. -NH₂ groups) of the organic polyamine. Ionic crosslinking and hydrogen bond crosslinking are illustrated below in a purely schematic, non-limiting manner, where the substantially vertical line connecting the carboxyl groups is the polyimide backbone and R is the part of the organic olyamine linking amine groups:

ionic crosslinking hydrogen bond crosslinking

Typically the non-covalent crosslinking is a combination of both ionic crosslinking and hydrogen bonding. In contrast, in covalent crosslinking amino groups present in the organic polyamine condense with the polyimide layer to form amide bonds therewith, e.g. the acidl group (e.g. -CO2H) of the polyimide condenses with an amino group (e.g. -NH₂) of the organic polyamine to form an amide and typically water (H₂0) (e.g. -C0₂H + H₂N-→ -CONH- + H₂0).

Typically covalent crosslinking occurs at high temperatures and involves the organic polyamine ring-opening imide rings present in the polyimide discriminating layer to form covalent bonds therewith.

Preferably the gas separation membrane is free from or substantially free from amide groups formed by condensation of the said acid groups with the organic polyamine.

When the acid groups of the polyimide layer are crosslinked by bringing the polyimide layer having acid groups into contact with a composition comprising a organic polyamine, the composition preferably comprises at least 0.1 wt% organic polyamine, e.g. 0.1 to 20wt%, more preferably 0.25 to 15wt%, especially 0.5 to 10wt% of organic polyamine. The identity of the remainder of the composition is not particularly critical, although typically it comprises water and optionally one or more water-miscible organic solvents (e.g. ethanol, methanol, isopropanol and/or n-methyl pyrrolidinone).

One may prepare a polyimide discriminating layer having acid groups by, for example, the reaction of a dianhydride and a diamine, at least one of which has an acid group. Typically the dianhydride and the diamine each comprise an aromatic (e.g. phenylene) group. Examples of diamines include: 2,3,5,6-tetramethyl-1 ,4-phenylenediamine; 4,4'-[1 ,4-phenylenebis(1 -methyl-ethylidene)]bisaniline; 2,4,6-trimethyl- 1 ,3- phenylenediamine; 2,2-bis[4-(4-aminophenoxy)-phenyl]propane; 2,7-bis(4- aminophenoxy)-naphthalene; 4,4,-methylene-bis(2,6-diisopropylaniline); 1 ,4-bis(4- aminophenoxy)benzene; 4,4'-bis(4-aminophenoxy)-biphenyl; 1 ,3-bis(4- aminophenoxy)benzene; 4,4'-(methylethylidene)bisaniline; 4-isopropyl-1 ,3- diaminobenzene; 1 ,5-diaminodiphenylether; diaminonaphthalene; 4,4'- diaminodiphenylether; metaphenylenediamine; paraphenylenediamine; Ν,Ν'- metaphenylenebis(m-aminobenzanilide); and 3,3 -diaminobenzanilide and the aforementioned compounds having an acid group, e.g. a sulphonic, sulphinic, phosphoric or phosphonic acid group or especially a carboxyl group (e.g. 3,5- diaminobenzoic acid).

Examples of dianhydrides include: 3, 4,3', 4'- diphenyldi(trifluoromethyl)methanetetracarboxylicdianhydride (also called 6FDA); pyromellitic dianhydride; 2,3,4,3',4'-diphenylsulfone tetracarboxylic dianhydride; 3,4,3',4'-benzophenone tetra-carboxylic dianhydride; pyrazinetetracarboxylic dianhydride; 3,4,3',4'-diphenyldimethylmethane tetracarboxylic dianhydride; 3,4,3',4'-diphenyldi(trifluoro-methyl) methanetetracarboxylic dianhydride; 2,3,6,7- naphthalenetetracarboxylic dianhydride; 3,4,3',4'-diphenyl tetracarboxylic dianhydride; 3,4,9, 10-perylenetetracarboxylic dianhydride; 3, 4,3', 4'- diphenylethertetra carboxylic dianhydride; 1 ,2,4,5-naphthalenetetracarboxylic dianhydride; 1 ,4,5,8-naphthalenetetracarboxylic dianhydride; 1 ,8,9, 10- phenanthrene tetracarboxylic dianhydride; 3,4,3',4'-diphenylmethane- tetracarboxylic dianhydride; and 2,3,4,5-thiophenetetra-carboxylic dianhydride.

The dianhydride and diamine may be reacted together by any of the known means for forming organic polymers. Discriminating layers in film form may be prepared by melt pressing, melt extrusion, solution casting, and the like. When the discriminating layer is formed from polymer solution in organic solvent, it may be desirable to incorporate up to 100% by weight of soluble salt, based on the total weight of dianhydride and diamine, e.g. LiCI, LiBr, L1N O3 and/or CaC etc..

Preferably the polyimide layer comprises groups of the Formula (1 ) wherein R is an acid group:

Formula (1 ). Preferably R is a carboxyl group or a sulphonic acid group.

US 5,286,280 describes the preparation of composite membranes comprising a porous support, an intermediate gutter layer comprising poly(dimethylsiloxane) groups and a polyimide discriminating layer made from what is known as a "6FDA" type polyimide. In order to prepare the membranes which are subsequently crosslinked in the present invention (i.e. the pre- crosslinked membranes), one may follow the general method of US 5,286,280 except that during formation of the polyimide layer one also includes a monomer which provides an acid group in the resultant polyimide layer (e.g. one may include 3,5-diaminobenzoic acid to make a polyimide having a carboxyl group).

The average thickness increase in the discriminating layer as a result of crosslinking with the organic polyamine is preferably 1 to 100nm, more preferably 2 to 90nm, especially 3 to 80nm, more especially 4 to 60 nm, particularly 5 to 50 nm.

The average thickness of the discriminating layer (i.e. comprising (i) and (ii)) is preferably 50 to 20 pm, more preferably 50nm to 1 pm , especially 50 to 200nm.

The organic polyamine may be any organic polyamine which has amine groups capable of forming non-covalent bonds with the acid groups of the polyimide.

The organic polyamine preferably has 2, 3 or 4 amine groups, preferably 2 to 4 groups selected from the group consisting of primary amine groups and secondary amine groups.

The amine groups present in the organic polyamine may be, for example, in-chain (e.g. as in -CH₂-NH-CH₃), terminal (e.g. as in -CH₂-NH₂ or -C6H₄-NH₂), or form part of a ring (e.g. as in -CH(CH₂CH₂)₂NH or pyridinyl (-CshUN)), or a combination of two or more of such groups.

Preferably the organic polyamine contains amine groups which are quite close together, for example at least two amine groups of the organic polyamine are 2, 3, 4, 5, 6 or 7 bonds apart, e.g. 3, 4, 5, 6 or 7 bonds apart. For example, in 1 ,2-diaminoethane the amine groups are 3 bonds apart (N-C-C-N) and in hexamethylene diamine the amine groups are 7 bonds apart (N-C-C-C-C-C-C-N). Preferably at least two amine groups of the organic polyamine are 7 bonds apart, more preferably 6 bonds apart, especially 5 bonds apart, more especially 4 bonds apart, particularly 3 bonds, more particularly 2 bonds apart.

In one embodiment the organic polyamine has a weight average molecular weight (Mw) of at least 300. Examples of organic polyamines having an Mw of at least 300 include poly(allylamines) (e.g. polyethyleneamine), polyethyleneimines, polyvinylamines, poly(l -vinylpyrrolidone) (PVP) and co-polymers of the foregoing, for example poly(1 -vinylpyrrolidone-co-2-dimethylaminoethyl methacrylate). However the organic polyamine preferably has an Mw below 300, more preferably below 250, especially below 200. The organic polyamine is free from inorganic groups, e.g. free from aluminosilicates. Preferably the organic polyamine is free from silicon atoms. In a particularly preferred embodiment, the organic polyamine contains only carbon, hydrogen, nitrogen and optionally oxygen atoms.

Examples of suitable organic polyamines having a Mw below 300 include unbranched aliphatic diamines (e.g. 1 ,2-diaminoethane, 1 ,3-diaminopropane, 1 ,4- diaminobutane, 1 ,5-diaminopentane and 1 ,6-diaminohexane); branched aliphatic diamines (e.g. 1 ,2-diaminopropane and diaminocyclohexane); xylylenediamines (e.g. ortho- meta- and para-xylylenediamine); aromatic diamines (e.g. ortho- meta- and para-phenylenediamine, 4-bromobenzene-1 ,3-diamine and 2,5- diaminotoluene); amines with two or more aromatic rings (e.g. 4,4'- diaminobiphenyl and 1 ,8-diaminonaphthalene); N-substituted diamines (e.g. Ν, Ν'- methyl phenylenediamine); triamines (e.g. diethylenetriamine and bis(hexamethylene)triamine; and tetraamines (e.g triethylene tetramine and hexamethylene tetramine).

Mixtures comprising two or more of the foregoing organic polyamines may also be used.

The amine groups are preferably primary amine (e.g. -NH₂ or -NH₃ ⁺) or secondary amine (e.g. -NHR or -NRH₂ ⁺) groups, or a combination of two or more thereof, wherein each R independently is an alkyl group (e.g. Ci_-4-alkyl). Preferably the organic polyamine comprises at least two primary amino groups (e.g. -NH₂ or -NH₃ ⁺) because this can result in membranes having particularly good durability.

The gas separation membrane of the present invention optionally further comprises a gutter layer located between the support and the discriminating layer.

The primary purpose of the porous support is to provide mechanical strength to the discriminating layer without materially reducing the flux. Therefore the porous support is typically open pored (before it is coverted into the membrane), relative to the discriminating layer.

The porous support may be, for example, a microporous organic or inorganic membrane, or a woven or non-woven fabric. The porous support may be constructed from any suitable material. Examples of such materials include polysulfones, polyethersulfones, polyimides, polyetherimides, polyamides, polyamideimides, polyacrylonitrile, polycarbonates, polyesters, polyacrylates, cellulose acetate, polyethylene, polypropylene, polyvinylidenefluoride, polytetrafluoroethylene, poly(4-methyl 1 -pentene) and especially polyacrylonitrile. One may use, for example, a commercially available, porous sheet material as the support. Alternatively one may prepare the porous support using techniques generally known in the art for the preparation of microporous materials. One may also use a porous support which has been subjected to a corona discharge treatment, glow discharge treatment, flame treatment, ultraviolet light irradiation treatment or the like, e.g. for the purpose of improving its wettability and/or adhesiveness.

The porous support preferably possesses pores which are as large as possible, consistent with providing a smooth surface for the subsequent gutter layer (when present) or discriminating layer.

The porous support preferably has an average pore size of at least about 50% greater than the average pore size of the discriminating layer, more preferably at least about 100% greater, especially at least about 200% greater, particularly at least about 1000% greater than the average pore size of the discriminating layer.

The pores passing through the porous support typically have an average diameter of 0.001 to 10pm, preferably 0.01 to 1 m. The pores at the surface of the porous support will typically have a diameter of 0.001 to 0.1 m, preferably 0.005 to Ο.Οδμηι. The pore diameter may be determined by, for example, viewing the surface of the porous support before it is converted to the membrane by scanning electron microscopy ("SEM") or by cutting through the support and measuring the diameter of the pores within the porous support, again by SEM.

The porosity at the surface of the porous support may also be expressed as a % porosity, i.e.

% porosity = 100% x (area of the surface which is missing due to pores)

(total surface area)

The areas required for the above calculation may be determined by inspecting the surface of the porous support by SEM. Thus, in a preferred embodiment, the porous support has a % porosity >1 %, more preferably >3%, especially >10%, more especially >20%.

The porosity of the porous support may also be expressed as a CO2 gas permeance (units are m³(STP)/m².s.kPa). When the membrane is intended for use in gas separation the porous support preferably has a CO2 gas permeance of 5 to 150 x 10^"5 m³(STP)/m².s.kPa, more preferably of 5 to 100, most preferably of 7 to 70 x 10^"5 m³(STP)/m².s.kPa.

Alternatively the porosity may be characterised by measuring the N₂ gas flow rate through the porous support. Gas flow rate can be determined by any suitable technique, for example using a Porolux™ 1000 device, available from Porometer.com. Typically the Porolux 1000 is set at the maximum pressure (about 34 bar) and one measures the flow rate (L/min) of N₂ gas through the porous support under test. The N₂ flow rate through the porous support at a pressure of about 34 bar for an effective sample area of 2.69 cm² (effective diameter of 18.5 mm) is preferably >1 L/min, more preferably >5 L/min, especially >10 L/min, more especially >25 L/min. The higher of these flow rates are preferred because this reduces the likelihood of the gas flux of the resultant membrane being reduced by the porous support.

The above pore sizes and porosities refer to the porous support before it has been converted into the gas separation membrane of the present invention.

The porous support preferably has an average thickness of 20 to 500 pm, preferably 50 to 400 μιτη, especially 100 to 300 pm.

The gutter layer usually has the function of providing a smooth and continuous surface for the discriminating layer.

Preferred gutter layers comprises poly(dimethylsiloxane) groups.

The gutter layer preferably has an average thickness 25 to 400nm, preferably 30 to 350nm, especially 50 to 300nm, e.g. 70 to 120nm, or 130 to 170nm, or 180 to 220nm or 230 to 270nm.

The thickness of the gutter layer may be determined by cutting through the membrane and examining its cross section by SEM. The part of the gutter layer which is present within the pores of the support is not taken into account.

The gutter layer is preferably non-porous, i.e. any pores present therein have an average diameter <1 nm.

The gutter layer is preferably a radiation-cured polymer. Such a polymer may be formed between the support and the discriminating layer by a process comprising radiation curing of a radiation-curable composition.

The radiation curing may be performed using any source which provides the wavelength and intensity of radiation necessary to cause the radiation-curable composition to polymerise. For example, electron beam, UV, visible and/or infra red radiation may be used to cure the radiation-curable composition, the appropriate radiation being selected to match the components.

Preferably radiation curing of the radiation-curable composition used to form the optional gutter layer begins within 7 seconds, more preferably within 5 seconds, most preferably within 3 seconds, of the radiation-curable composition being applied to the porous support.

Suitable sources of ultraviolet light include mercury arc lamps, carbon arc lamps, low pressure mercury lamps, medium pressure mercury lamps, high pressure mercury lamps, swirlflow plasma arc lamps, metal halide lamps, xenon lamps, tungsten lamps, halogen lamps, lasers and ultraviolet light emitting diodes. Particularly preferred are ultraviolet light emitting lamps of the medium or high pressure mercury vapour type. In addition, additives such as metal halides may be present to modify the emission spectrum of the lamp. In most cases lamps with emission maxima between 200 and 450nm are particularly suitable.

The energy output of the irradiation source is preferably from 20 to 1000W/cm, preferably from 40 to 500W/cm but may be higher or lower as long as the desired exposure dose can be realized.

The radiation-curable composition used to prepare the optional gutter layer preferably comprises:

(1 ) 0.5 to 50 wt% of radiation-curable component(s);

(2) 0 to 5 wt% of a photo-initiator; and

(3) 50 to 99.5 wt% of inert solvent.

The radiation-curable component(s) typically have at least one radiation- curable group. Radiation curable groups include ethylenically unsaturated groups (e.g. (meth)acrylic groups (e.g. CH₂=CR¹-C(0)- groups), especially (meth)acrylate groups (e.g. CH₂=CR¹-C(0)0- groups), (meth)acrylamide groups (e.g. CH₂=CR¹-C(0)NR¹- groups), wherein each R¹ independently is H or CH₃ ) and especially epoxide groups (e.g. glycidyl and epoxycyclohexyl groups).

The preferred ethylenically unsaturated groups are acrylate groups because of their fast polymerisation rates, especially when the irradiation uses UV light. Many compounds having acrylate groups are also readily available from commercial sources.

Photo-initiators may be included in the radiation-curable composition and are usually required when the curing uses UV radiation. Suitable photo-initiators are those known in the art such as radical type, cation type or anion type photo- initiators.

Cationic photo-initiators are preferred when the radiation-curable component(s) comprises curable groups such as epoxy, oxetane, other ring- opening heterocyclic groups or vinyl ether groups.

Preferred cationic photo-initiators include organic salts of non-nucleophilic anions, e.g. hexafluoroarsinate anion, antimony (V) hexafluoride anion, phosphorus hexafluoride anion, tetrafluoroborate anion and tetrakis(2,3,4,5,6- pentafluorophenyl)boranuide anion. Commercially available cationic photo- initiators include UV-9380c, UV-9390c (manufactured by Momentive performance materials), UVI-6974, UVI-6970, UVI-6990 (manufactured by Union Carbide Corp.), CD-1010, CD-101 1 , CD-1012 (manufactured by Sartomer Corp.), Adekaoptomer™ SP-150, SP-151 , SP-170, SP-171 (manufactured by Asahi Denka Kogyo Co., Ltd.), Irgacure™ 250, Irgacure™ 261 (Ciba Specialty Chemicals Corp.), CI-2481 , CI-2624, CI-2639, CI-2064 (Nippon Soda Co., Ltd.), DTS-102, DTS-103, NAT-103, NDS-103, TPS-103, MDS-103, MPI-103 and BBI- 103 (Midori Chemical Co., Ltd.). The above mentioned cationic photo-initiators can be used either individually or in combination of two or more.

Radical Type I and/or type II photo-initiators may also be used.

Examples of radical type I photo-initiators are as described in WO 2007/018425, page 14, line 23 to page 15, line 26, which are incorporated herein by reference thereto.

Examples of radical type II photo-initiators are as described in WO 2007/018425, page 15, line 27 to page 16, line 27, which are incorporated herein by reference thereto.

Type I photo-initiators are preferred, especially alpha- hydroxyalkylphenones, such as 2-hydroxy-2-methyl-1 -phenyl propan-1 -one, 2- hydroxy-2-methyl-1 -(4-tert-butyl-) phenylpropan-1 -one, 2-hydroxy-[4^'-(2- hydroxypropoxy)phenyl]-2-methylpropan-1 -one, 2-hydroxy-1 -[4-(2- hydroxyethoxy)phenyl]-2-methyl propan-1 -one, 1 -hydroxycyclohexylphenylketone and oligo[2-hydroxy-2-methyl-1 -{4-(1 -methylvinyl)phenyl}propanone], alpha- aminoalkylphenones, alpha-sulfonylalkylphenones and acylphosphine oxides such as 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, ethyl-2,4,6-trimethylbenzoyl- phenylphosphinate and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, are preferred.

Preferably the weight ratio of photo-initiator to radiation-curable components present in the radiation-curable composition is between 0.001 and 0.2 to 1 , more preferably between 0.01 and 0.1 to 1 . A single type of photo-initiator may be used but also a combination of several different types.

When no photo-initiator is included in the radiation-curable composition, the composition can be advantageously cured by electron-beam exposure. Preferably the electron beam output is between 50 and 300keV. Curing can also be achieved by plasma or corona exposure.

The function of the inert solvent (3) is to provide the radiation-curable composition with a viscosity suitable for the particular method used to apply the curable composition to the porous support. For high speed application processes one will usually choose an inert solvent of low viscosity. The number of parts of component (3) is preferably 70 to 99.5wt%, more preferably 80 to 99wt%, especially 90 to 98wt%.

Inert solvents are not radiation-curable. In a specific embodiment there is no solvent present.

The radiation-curable composition may contain other components, for example surfactants, surface tension modifiers, viscosity enhancing agents, biocides and/or other components capable of co-polymerisation with the other ingredients. The radiation-curable composition may be applied to the porous support by any suitable coating technique, for example by curtain coating, meniscus type dip coating, kiss coating, pre-metered slot die coating, reverse or forward kiss gravure coating, multi roll gravure coating, spin coating and/or slide bead coating.

Conveniently the radiation-curable composition may be coated onto the porous support by a multilayer coating method, for example using a consecutive multilayer coating method, optionally along with the components used to form the discriminating layer

In a preferred consecutive multilayer process a layer of the radiation- curable composition and the discriminating layer (or the chemicals used to prepare the discriminating layer) are applied consecutively to the support, with the radiation-curable composition being applied before the discriminating layer (or the chemicals used to prepare the discriminating layer).

In order to produce a sufficiently flowable composition for use in a high speed coating machine, the radiation-curable composition preferably has a viscosity below 4000m Pa s when measured at 25°C, more preferably from 0.4 to l OOOmPa s when measured at 25°C. Most preferably the viscosity of the radiation-curable composition is from 0.4 to 500mPa.s when measured at 25°C. For coating methods such as slide bead coating the preferred viscosity is from 1 to l OOmPa.s when measured at 25°C. The desired viscosity is preferably achieved by controlling the amount of solvent in the radiation-curable composition and/or by the conditions for preparing the radiation curable polymer.

In the multi-layer coating methods mentioned above, one may optionally apply a lower inert solvent layer to the porous support followed by applying the radiation-curable composition.

With suitable coating techniques, coating speeds of at least 5m/min, e.g. at least 10m/min or even higher, such as 15m/min, 20m/min, or even up to 100m/min, can be reached. In a preferred embodiment the radiation-curable composition (and also the discriminating layer) is applied to the support at one of the aforementioned coating speeds.

The thickness of the cured gutter layer on the support may be influenced by controlling the amount of curable composition per unit area applied to the support. For example, as the amount of curable composition per unit area increases, so does the thickness of the resultant gutter layer. The same principle applies to formation of the discriminating layer.

While it is possible to prepare the membranes of the invention on a batch basis with a stationary porous support, it is much preferred to prepare them on a continuous basis using a moving porous support, e.g. the porous support may be in the form of a roll which is unwound continuously or the porous support may rest on a continuously driven belt. Using such techniques the radiation-curable composition can be applied to the porous support on a continuous basis or it can be applied on a large batch basis. Removal of the inert solvent from the radiation-curable composition membrane can be accomplished at any stage after the radiation-curable composition has been applied to the support, e.g. by evaporation.

Thus in a preferred process for making the gas separation membranes of the invention, the radiation-curable composition is applied continuously to the porous support by means of a manufacturing unit comprising a radiation-curable composition application station, curing is performed using an irradiation source located downstream from the radiation-curable composition application station to form a gutter layer, the polyimide layer is formed on the gutter layer by a polyimide layer application station, the crosslinking is then performed by contacting the polyimide layer with a composition comprising the organic polyamine and the resultant gas separation membrane is collected at a collecting station, wherein the manufacturing unit comprises a means for moving the porous support from the radiation-curable composition application station to the irradiation source and to the polyimide layer application station and into contact with the composition comprising the organic polyamine and to the gas separation membrane collecting station.

Optionally the polyimide layer is formed on the gutter layer by a radiation curing process. Under such circumstances, the manufacturing unit preferably further comprises an irradiation source or a heater located downstream from the polyimide layer application station, thereby radiation- or thermally-curing the components used to form the discriminating layer.

The radiation-curable composition application station may be located at an upstream position relative to the irradiation source and the irradiation source is located at an upstream position relative to the polyimide layer application station.

While it is preferred for the gutter layer to be pore-free, the presence of some pores usually does not reduce the permselectivity of the final gas separation membrane because the discriminating layer is often able to fill minor defects in the gutter layer.

According to a second aspect of the present invention there is provided a gas separation module for separating a feed gas containing a target gas into a gas stream rich in the target gas and a gas stream depleted in the target gas, the module comprises a housing and one or more cartridges comprising a gas separation membrane according to the present invention.

A still further aspect of the present invention provides a gas separation cartridge comprising a gas separation membrane according to the present invention. The gas separation membrane is preferably in tubular or, more preferably, in sheet form. Tubular forms of membrane are sometimes referred to as being of the hollow fibre type. Gas separation membranes in sheet form are suitable for use in, for example, spiral-wound, plate-and-frame and envelope cartridges.

Preferred gas separation modules comprising a gas separation membrane according to the present invention are in the form of a spiral-wound cartridge. Such spiral-wound cartridges preferably comprise spacers and outer impermeable support layers, the spacers being positioned on each side of the gas separation membrane and between the gas separation membrane and the impermeable support layer and thereby defining a feed channel on one side of the gas separation membrane and a permeate channel on the other side of the gas separation membrane, wherein the gas separation membrane, spacers and outer impermeable layers are wound in a spiral manner around a core.

The spacers are typically, but not necessarily, made from plastic mesh or netting, which helps to promote turbulent flow in the gas channels. In manufacturing spiral-wound cartridges, care is taken in the choice of spacers. An overly tight mesh may result in pressure drops along the feed or permeate channel that adversely affect separation performance when the cartridge is in use. Likewise, a tight spacer may facilitate the formation of stagnant boundary layers that give rise to concentration polarisation adjacent to the membrane surface. Similar issues affect the manufacture of plate-and-frame cartridges.

In spiral-wound cartridges incorporating mesh spacers, the spacers are preferably sufficiently strong to support the gas separation membrane and hold open the feed and permeate channels, and sufficiently open to limit pressure drops along the channels and concentration polarisation problems.

More details on the manufacture of spiral-wound cartridges can be found in US 3,417,870, US 4,746,430 and US 5,096,584.

When the cartridge is to be used to carry out gas separation using a sweep gas on the permeate side, then the cartridge preferably also includes an inlet to the permeate side of the gas separation membrane by which the sweep gas can be passed into the cartridge.

Hollow fibre cartridges do not normally require spacers because the gas separation membranes may be held in a spaced-apart relationship by a potting compound.

Referring now to the hollow-fibre type gas separation cartridge, the gas separation cartridge preferably comprises:

(a) a tubular gas separation element comprising a wall of gas separation membrane and one or more gas outlets;

(b) a housing accommodating the gas separation element, the housing comprising an external wall and one or more gas outlets; (c) a void between the element wall and the housing external wall;

(d) one or more inlets for introducing feed gas into either the tubular gas separation element or into the void;

wherein:

(i) the gas separation membrane is as defined in the first aspect of the present invention; and

(ii) the cartridge is constructed such that essentially the only way for the target gas to travel between the inside of the tubular gas separation element and the void is through the wall of the gas separation membrane.

In this preferred hollow-fibre type gas separation cartridge, the feed gas containing the target gas may be introduced into either the tubular gas separation element or into the housing void.

Thus in a first embodiment the feed gas containing the target gas is introduced into a near end of the tubular gas separation element through the one or more inlets. The feed gas then passes longitudinally within the tube, with the target gas permeating through the selective gas separation membrane more easily than other gases within the feed gas. A gas stream depleted in target gas may then exit the cartridge through an outlet at the far end of the tubular gas separation element. A gas stream rich in target gas may then exit the cartridge through an outlet at the far end of the housing.

In a second embodiment the feed gas containing the target gas is introduced into a near end of the housing through the one or more inlets. The feed gas then passes longitudinally within the housing, with the target gas permeating through the selective membrane more easily than other gases within the feed gas. A gas stream depleted in target gas may then exit the cartridge through an outlet at the far end of the housing. A gas stream rich in target gas may then exit the cartridge through an outlet at the far end of the tubular gas separation element.

Preferably at least 95%, more preferably all of the target gas which travels between the inside of the tubular gas separation element and the void (whatever the direction of flow) does so through the wall of the gas separation membrane.

The preferred cartridge geometries therefore include plate-and-frame, spiral-wound, hollow-fiber, tubular and envelope type. More information on cartridge geometries can be found in "Membrane Technology in the Chemical Industry", edited by S.P. Nunes and K.-V. Peinemann, page 76-78 and page 101 - 103 and in "Membrane Technology and Applications" (second edition), edited by R. Baker, page 139-155.

While this specification emphasises the usefulness of the gas separation membranes of the present invention for separating gases, especially polar and non-polar gases, it will be understood that the gas separation membranes can also be used for other purposes, for example providing a reducing gas for the direct reduction of iron ore in the steel production industry, dehydration of organic solvents (e.g. ethanol dehydration), pervaporation and vapour separation.

The gas separation membrane of the invention may be used in conjunction with other membranes or with other gas separation techniques if desired, e.g. with solvent absorption (e.g. Selexol, Rectisol, Sulfinol, Benfield), amine absorption (e.g. DEA, MDEA), physical adsorption, e.g. pressure swing adsorption, cryogenic techniques, etc. The membranes are particularly suitable for separating a feed gas containing a target gas into a gas stream rich in the target gas and a gas stream depleted in the target gas. For example, a feed gas comprising polar and non-polar gases may be separated into a gas stream rich in polar gases and a gas stream depleted in polar gases. In many cases the membranes have a high permeability to polar gases, e.g. CO2, H₂S, NH₃, SO_x, and nitrogen oxides, especially NO_x, relative to non-polar gases, e.g. alkanes, H₂, N₂ and water vapour.

The target gas may be, for example, a gas which has value to the user of the membrane and which the user wishes to collect. Alternatively the target gas may be an undesirable gas, e.g. a pollutant or 'greenhouse gas', which the user wishes to separate from a gas stream in order to protect the environment.

The gas separation membranes are particularly useful for purifying natural gas (a mixture which predominantly comprises methane) by removing polar gases (CO2, H₂S); for purifying synthesis gas; and for removing C0₂ from hydrogen and from flue gases. Flue gases typically arise from fireplaces, ovens, furnaces, boilers, combustion engines and power plants. The composition of flue gases depend on what is being burned, but usually they contain mostly nitrogen (typically more than two-thirds) derived from air, carbon dioxide (CO2) derived from combustion and water vapour as well as oxygen. Flue gases also contain a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulphur oxides. Recently the separation and capture of CO2 has attracted attention in relation to environmental issues (global warming).

The gas separation membranes of the invention are particularly useful for separating the following: a feed gas comprising CO2 and N₂ into a gas stream richer in CO2 than the feed gas and a gas stream poorer in CO2 than the feed gas; a feed gas comprising CO2 and CH₄ into a gas stream richer in CO2 than the feed gas and a gas stream poorer in CO2 than the feed gas; a feed gas comprising CO2 and H₂ into a gas stream richer in CO2 than the feed gas and a gas stream poorer in CO2 than the feed gas, a feed gas comprising H₂S and CH₄ into a gas stream richer in H₂S than the feed gas and a gas stream poorer in H₂S than the feed gas; and a feed gas comprising H₂S and H₂ into a gas stream richer in H₂S than the feed gas and a gas stream poorer in H₂S than the feed gas. Preferably the gas separation membrane has a C0₂ CH₄ selectivity (aC0₂/CH₄) >10. Preferably the selectivity is determined by a process comprising exposing the membrane to a 50:50 mixture by volume of CO₂ and CH₄ at a feed pressure of 2000kPa.

The following materials were used in the Examples (all without further purification):

PAN is a porous support (polyacrylonitrile L10 ultrafiltration membrane from GMT Membrantechnik GmbH, Germany),

UV9300 is SilForce™ UV9300 from Momentive Performance Materials

Holdings. This is curable copolymer comprising reactive epoxy groups and linear poly(dimethylsiloxane) chains and is used to prepare a gutter layer.

UV9390C is SilForce™ UV-9390C - a cationic photo-initiator (a solution of a bis(4-alkylaryl)iodonium hexafluoroantimonate salt and photosensitizer in a glycidyl ether reactive diluent).

Ti(OiPr)₄ is titanium (IV) isopropoxide from Dorf Ketal Chemicals.

n-heptane is n-heptane from Brenntag Nederland BV.

MEK is 2-butanone from Brenntag Nederland BV.

MeOH is methanol.

CH is cyclohexanone from Brenntag Nederland BV.

PI1 : is 6FDA-TeMPD_x/DABA;x/y=80/20; obtained from FUJIFILM

Corporation, having the following structure:

PI2: is 6FDA-TeMPD; obtained from FUJIFILM Corporation, having the following structure:

PI3: is (6 FDA- TeMPD)n-(6FDA-DABS)_y n/y = 80/20 obtained from FUJIFILM Corporation, having the following structure:

EDA is 1 ,2-diaminoethane from Aldrich.

PDA is 1 ,3-diaminopropane from Aldrich.

BDA is 1 ,4-diaminobutane from Aldrich.

HMDA is 1 ,6-diaminohexane from Aldrich.

HA is 1 -aminohexane from Aldrich.

PPD is 1 ,4-diaminobenzene from Aldrich.

TEM is 1 ,2-bis(dimethyl amino)ethane from Aldrich.

TETA is triethylene tetramine from Aldrich.

DME is 1 ,2-bis(methyl amino)ethane from Aldrich.

PDMS is poly(dimethylsiloxane), bis(3-amino-propyl)-terminated of Mw 2.5

KDa from Aldrich.

PVP is a 50wt% solution of poly(l -vinylpyrrolidone) of Mw 360 kDa in water, from Aldrich. Evaluation of Gas Flux, Selectivity, Water-contact Angle, Pasticization and

Crosslinking Type

(A) Gas flux

The flux of CH₄ and CO2 through the membranes was measured at 40°C and gas feed pressure of 6000 kPa using a gas permeation cell with a measurement diameter of 3.0 cm and a feed gas composition of 13 v/v % CO2 and 87 v/v % CH₄.

The flux of each gas was calculated based on the following equation:

Qi -(Qperm^' Xperm,i)/( ^' (PFeed^' ^Feed " Pperm^' Xperm,i))

Where:

Q_/= Flux of each gas (m³(STP)/m² kPa s)

Qperm = Permeate flow (m³(STP)/s)

Xperm = Volume fraction of each gas in the permeate

A = Membrane area (m²)

PFeed = Feed gas pressure (kPa) XFeed = Volume fraction of each gas in the feed

Pperm = Permeate gas pressure (kPa)

STP is standard temperature and pressure, which is defined here as 25.0°C and 1 atmosphere (101.325 kPa).

(B) Selectivity

The selectivity (a_Co2 cH ) for the membranes was calculated from Q_Co2 and QCH4 calculated above, based on following equation:

(C) Water Contact Angle (WCA)

The WCA of the membranes was determined by using a VCA 2500 XE (video contact angle analysis system) instrument from AST by the sessile drop method. On several locations at the top of each sample, 1 μΙ_ deionized water was injected and the images were recorded using a video camera system and the surface contact angles were calculated and averaged based on the recorded images of the water drops.

(D) Plasticization versus un-casted membranes

The degree of plasticization was checked as a function of flux-change and the selectivity change for the membranes of the invention versus un-casted membranes (e.g. Comparative Examples 2 and 15) under gas pressures ranging from 8000 to 60000 kPA. Less plasticization means a lower flux from 8000 to 60000 kPA and a higher selectivity from 8000 to 6000 kPa versus the membranes which had not been crosslinked (Comparative Examples 2 and 15).

(E) Crosslinking Type

In order to determine whether the crosslinking was predominantly non- covalent, the discriminating layer was inspected for the reduction of intensity of peaks at approximately 1718cm^"1 (attributed to the C=O symmetric stretch of the imide group), 1783 cm^"1 (attributed to the C=O asymmetric stretch of imide group) and 1351 cm^"1 (attributed to C-N stretch of imide group) indicating a imide ring cleavage and for the presence or appearance of two peaks at 1647 cm^"1 (C=O stretch of the amide group) and 1521 cm^"1 (a C-N stretch band of the C-N-H group) before and after crosslinking. In all Examples, no changes in intensities and no new IR peaks were observed after crosslinking, indicated that there was no discernible imide ring opening and therefore no discernable covalent crosslinking. Analogous techniques were used for detecting the presence of sulphonamide groups. The infra red measurements were done using a PerkinElmer Frontier FT-IR spectrophotometer using Attenuated Total Reflection (ATR) accessory on a germanium topplate from 4000 to 650 cm ^"1 in % transmission (T).

Examples 1 to 9 and Comparative Examples CEX1 and CEX2

A composition C1 having a viscosity of 64,300 mPas at 25°C (at 0.0396 s^"1) was prepared by mixing the components described in Table 1 at 95°C for 105 hours:

Tablel

Viscosity was measured using a Brookfield LVDV-II + PCP viscosity meter, using either spindle CPE-40 or CPE-52 depending on viscosity range.

Radiation-curable composition RCC1 was prepared by cooling the above composition C1 to 20^°C, adding n-heptane to a polymer concentration of 5wt% filtering the resultant solution through a filter paper of 2.7 pm pore size and adding a photo-initiator (UV9390C, 0.50wt%).

Radiation-curable RCC1 composition was then applied to a porous support (PAN) at a speed of 10 m/min by a meniscus dip coating and irradiated using a Light Hammer LH 10 from Fusion UV Systems fitted with a D-bulb with an intensity of 16.8 kW/m (70%). This resulted in a porous support having a gutter layer of dry thickness of about 150nm.

A polyimide discriminating layer having acid groups was formed on the gutter layer by applying thereto a composition comprising PI1 (2wt%), CH (6wt%) and MEK (92 wt%) at 10 m/min by a meniscus type dip coating for Examples 1 to 8 and by applying a composition comprising PI3 (2wt%), CH (6wt%) and MEK (92 wt%) at 10 m/min by a meniscus type dip coating for Example 9. The resultant discriminating layers had an average dry thickness of about 100nm.

(The thickness of the gutter layer and discriminating layer were measured by cutting through the membrane and measuring the thickness from the surface of the porous support or the surface of the gutter layer outwards by SEM).

The acid groups of the polyimide discriminating layers were then non- covalently crosslinked by applying thereto a solution of the organic polyamine indicated in Table 2 at the specified wt% in the solvent indicated in Table 2, at 10 m/min, again by meniscus type dip coating. The resultant membranes had the gas flux, selectivity, water contact angle and anti-plasticization indicated in Table 2. The resultant discriminating layers had an average dry thickness of about 100 nm.

Table 2

Table 2 continued

The infra-red measurements performed on all Examples of the present invention did not show any covalent crosslinking.

Furthermore, it was observed that in the non-crosslinking membranes

CEX1 and CEX2, selectivity (a_Co2 cH ) fell sharply as the gas pressure applied to the membrane increased. This steepness of the fall in selectivity reduced (i.e. selectivity was maintained better at a given pressure) in the concrete Examples, where Example 1 (EDA - amine groups 3 bonds apart) was better than Example 2 (PDA - amine groups 4 bonds apart), Example 2 (PDA - amine groups 4 bonds apart) was better than Example 3 (BDA - amine groups 5 bonds apart), Example 3 (BDA - amine groups 5 bonds apart) was better than Example 4 (HMDA - amine groups 7 bonds apart), and Example 4 (HMDA - amine groups 7 bonds apart) was better than CEX1 and CEX2 (no crosslinking).

Comparative Examples CEX3 to CEX13 Comparative Examples CEX3 to CEX13 were prepared in an analogous manner to Examples 1 to 9 described above, using the ingredients and amounts shown in Table 3, except that in place of PI1 (a polyimide having carboxyl groups, used to provide the polyimide layer having carboxyl groups) or PI3 (a polyimide having sulphonic acid groups, used to provide the polyimide layer having sulphonic acid groups) there was used an identical amount of PI2 (a polyimide which lacked acidgroups, resulting in a polyimide layer which lacked the acid groups). Table 3

Table 3 continued

Ingredient/Test Example

CEX8 CEX9 CEX10 CEX11 CEX12 CEX13

Organic polyamine PPD TEM TETA PDMS (5) DME (5) None

(wt% in solvent) (5) (5) (5)

Solvent H₂0 H₂0 H₂0 n-heptane H₂0 None

Gas Flux 200 204 204 204 200 202

Selectivity(a_C02/cH4) 10 10 10 10 10 10

Water Contact 60.7 61 .2 60.8 60.3 60.1 60.2

Angle

Plasticization same same same same same same

Claims

1 . A gas separation membrane comprising a porous support and a discriminating layer, wherein the discriminating layer comprises (i) a polyimide layer having acid groups and (ii) an organic polyamine, wherein the organic polyamine crosslinks the acid groups of the polyimide predominantly by means of non-covalent crosslinking.

2. A membrane according to claim 1 which is free from or substantially free from amide groups formed by condensation of the said acid groups with the organic polyamine.

3. A membrane according to any one of the previous claims wherein said acid groups comprise carboxyl groups and/or sulphonic acid groups.

4. A membrane according to claim 1 or 2 wherein said acid groups are carboxyl groups.

5. A membrane according to any one of the preceding claims wherein the organic polyamine has a molecular weight below 300.

6. A membrane according to any one of the preceding claims wherein the organic polyamine has 2 to 4 groups selected from the group consisting of primary amine groups and secondary amine groups.

7. A membrane according to any one of the preceding claims wherein at least two amine groups of the organic polyamine are 2, 3, 4, 5, 6 or 7 bonds apart.

8. A membrane according to any one of the preceding claims wherein the average thickness of the discriminating layer after it has been crosslinked is in the range 50 nm to 20 pm.

9. A membrane according to any one of the preceding claims which further comprises a gutter layer between the support and the discriminating layer.

10. A membrane according to claim 9 wherein the gutter layer has an average thickness 25 to 400nm.

1 1 . A membrane according to claim 9 or 10 wherein the gutter layer comprises poly(dimethylsiloxane) groups.

12. A membrane according to any one of claims 9 to 1 1 wherein the gutter layer is a UV-cured gutter layer.

13. A membrane according to any one of the preceding claims wherein the discriminating layer comprises groups of the Formula (1 ) wherein R is an acid group:

Formula (1 )

14. A membrane according to any one of the preceding claims wherein the porous support has surface pores of average diameter 0.001 to 0.1 pm.

15. A membrane according to claim 1 wherein

(a) the organic polyamine has a molecular weight below 300 and has 2 to 4 groups selected from the group consisting of primary amino groups and secondary amino groups; and

(b) the discriminating layer has an average thickness 50 nm to 20 pm and comprises groups of the Formula (1 ) wherein R is a carboxyl group:

Formula (1 )

the membrane further comprising a UV-cured gutter layer between the porous support and the discriminating layer, said gutter layer having an average thickness 25 to 400nm and comprising poly(dimethylsiloxane) groups,

and the porous support having surface pores of average diameter 0.001 to 0.1 pm.

16. A gas separation module for separating a feed gas containing a target gas into a gas stream rich in the target gas and a gas stream depleted in the target gas, the module comprises a housing and one or more cartridges comprising a membrane according to any one of the preceding claims.