GB2211193A

GB2211193A - Semipermeable membrane of polybenzoxazole for separating gases

Info

Publication number: GB2211193A
Application number: GB8829152A
Authority: GB
Inventors: Yutaka Maruyama; Harukiko Komoriya
Original assignee: Central Glass Co Ltd
Current assignee: Central Glass Co Ltd
Priority date: 1987-12-16
Filing date: 1988-12-14
Publication date: 1989-06-28
Anticipated expiration: 2008-12-14
Also published as: GB8829152D0; GB2211193B; IT8822960A0; JPH01159024A; JPH0477610B2; DE3842093A1; IT1228033B; FR2624756A1

Description

2 2 9 37 SEMIPERMEABLE MEMBRANE OF POLYBENZOXAZOLE FOR SEPARATING GASES

This invention relates to separation of a gas from a gas mixture by contacting the gas mixture with a semipermeable membrane which is relatively high in permeability to the gas to be separated than to the other gas or gases in the mixture, and more particularly to a novel semipermeable membrane formed of a poly- benzoxazole.

Cellulose acetate membranes are known as semipermeable membranes for separating gases, but in many cases cellulose acetate membranes are not suited for practical purposes mainly because of being low in chemical resistance and heat resistance. Polysulfone membranes are industrially produced as gas separating semipermeable membranes of improved heat resistance, but they are not satisfactory in semipermeabilities. Researches have been made on polyimide films with a view to obtaining semipermeable membranes excellent in heat resistance, but reported semipermeabilities are still insufficient.

Semipermeable membranes of silicone resins are known as gas separating membranes high in selective permeability to oxyge.n gas, but silicone resin membranes are not good for industrial applications because silicone resins are insufficient in mechanical strength and are difficult to form into thin films high in permeabilities.

It is an object of the present invention to provide a semipermeable membrane for separating gases, which membrane is high in selective permeabilities for specific gases and is excellent in mechanical strength, heat resistance, chemical ressitance and weather resistance.

From another aspect it is an object of the invention to provide a method of separating a gas from a gas mixture by contacting the gas mixture with a selectively semipermeable membrane, which is a novel membrane high in selective permeability for the gas to be separated and is excellent in mechanical strength, heat resistance, chemical resistance and weather resistance.

According to the invention there is provided a semipermeable membrane for seprating gases, the membrane being formed of a polybenzoxazole having a structure represented by the general formula (l):

c N \A r/ N \C-R 0 / \ 0 / n wherein Ar represents an aromatic group, R represents a divalent aromatic group, and n is an integer from 2 to 200.

In another aspect, this invention provides a method of separating a gas from a gas mixture of said gas and at least one other gas by contacting the gas mixture with a semipermeable membrane which is relatively high in permeability to said gas than to said at least one other gas, the method being characterized in that the semipermeable membrane is formed of a polybenzoxazole having a structure represented by the general formula (1).

A polybenzoxazole of the general formula (1) is obtained by a dehydrating and cyclizing reaction of an aromatic polyamide having a structure represented by the general formula (2):

H H 0 0 N Ar (2) HO / \ OH 7n wherein Ar is the aromatic group in the general formula (1), R is a divalent aromatic group, and n is an integer from 2 to 200.

An aromatic polyamide of the general formula (2) can be prepared by reacting a suitable aromatic diamine with an aromatic dicarboxylic acid or its dihalide or diester.

Some of aromatic polyamides of the general formula (2) and polybenzoxazoles of the general formula (1) are shown in GB 2,188,936A and GB 2,191,496A.

Polybenzoxazoles employed in this invention are high in mechanical strength and excellent in heat resistance, chemical resistance, moisture resistance and weather resistance and can easily be formed into films.

Polybenzoxazole membranes of the invention are fairly high in permeabilities to several kinds of industrially important gases including hydrogen gas and oxygen gas. For example, the permeability of a polybenzoxazole membrane of the invention to hydrogen gas is about 10 to 100 times as high as that of a polyimide membrane known as a heat resistant semipermeable membrane.

By using the present invention it is practicable to efficiently separate hydrogen gas from a mixed gas which is discharged from a petroleum refining process and contains, for example, methane gas together with hydrogen gas. Also it is possible to obtain a gas rich in oxygen from the air.

An aromatic polyamide represented by the general formula (2) can be prepared by either of the following 20 two methods.

The first method uses an aromatic diamine represented by the general formula (3).

H 2 N\A /NH 2 r (3) HO / \ OH wherein Ar is the aromatic group in the general formulas (1) and (2), and each -OH group is at the ortho-position with respect to one of the -NH 2 groups.

The aromatic diamine is reacted with a dicarboxylic acid represented by the general formula (4), a dicarboxylic acid dihalide represented by the general formula (5) or a dicarboxylic acid diester represented by the general formula (6).

0 0 HO-C-R-13-0H (4) wherein R represents a divalent aromatic group; 0 0 X-C R-L (5) wherein R represents a divalent aromatic group, and X represents a halogen atom; 0 0 R'O-C-R-C-R'0 (6) wherein R represents a divalent aromatic group, and R' represents an alkyl group or a phenyl group.

In the present invention good examples of aromatic diamines of the general formula (3) are bis(3-amino- 4hydroxyphenyl)methane, 1-phenyl-l,l-bis(3-amino-4hydroxyphenyl)ethane, 1trifluoromethyl-l,l-bis(3-amino4-hydroxyphenyl)ethane, 1-phenyl-l,l-bis(3amino-4hydroxyphenyl)trifluoroethane, 2,2-bis(3-amino-4hydroxyphenyl)propane, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 4,4'-diamino-3,3'-dihydroxybiphenyl, bis(3-amino-4-hydroxyphenyl)ketone, bis(3-amino-4- hydroxyphenyl)ether, bis(3-amino-4-hydroxyphenyl) sulfide, bis(3-amino-4-hydroxyphenyl)sulfone, 2,5 diamino-3,6-dihydroxybenzene and 2,6-diamino-3,5 dihydroxybenzene.

Preferred examples of dicarboxylic acids of the general formula (4) are isophthalic acid, terephthalic acid, 4,4'-biphenyldicarboxylic acid, biphenylether 4,4'-dicarboxylic acid, benzophenone-4,4'-dicarboxylic acid, benzosulfone-4,4'-dicarboxylic acid, 2,6- naphthalene-dicarboxylic acid, diphenylmethane-4,4'dicarboxylic acid, 4, 4'-isopropylidenediphenyl-1,1'dicarboxylic acid and 4,4'hexafluoroisopropylidenediphenyl-1,1'-dicarboxylic acid.

Preferred examples of dicarboxylic acid dihalides of the general formula (5) are chlorides of the above named ten kinds of carboxylic acids, and good examples of dicarboxylic acid diesters of the general formula (6) are diphenyl esters of the above named ten kinds of carboxylic acids. 20 It is possible to jointly use two or more kinds of dicarboxylic acids, dicarboxylic dihalides or dicarboxylic acid diesters to thereby obtain a copolymerized aromatic polyamide. In the case of reacting an aromatic diamine of the general formula (3) with a dicarboxylic acid, the reaction is carried out in a suitable organic solvent such as, for example, N,N-dimethylformamide, N,N- dimethylacetamide or N-methyl-2-pyrrolidone at a temperature ranging from room temperature to the boiling point of the employed solvent. When a dicarboxylic acid dihalide is used the reaction is also carried out in a 5 similar organic solvent at a temperature ranging from -10 to 50 0 C. In the case of using a dicarboxylic acid diester the reaction is also carreid out in a similar organic solvent at a temperature ranging from 50 to 300 0 C.

The second method uses an aromatic diamine represented by the general formula (7).

H H 11 ' 1 R N\\ /NR Ar (7) 2 / 2 R 0 OR wherein Ar is the aromatic group in the general formulas (1) and (2), R 1 is a monovalent organosilicon group, R 2 is a hydrogen atom or a monovalent organosilicon group, and each -NHR 1 group is at the ortho- position with respect to one of the -OR 2 groups.

The aromatic diamine (7) is reacted with a dicarboxylic acid dihalide represented by the general formula (5) in an organic solvent.

Preferred examples of aromatic diamines represented by the general formula (7) are 2,2-bis(3-trimethylsilylamino-4-trimethylsiloxyphenyl)hexafluoropropane and 2,2-bis(3-triethylsilylamino-4-triethylsiloxyphenyl)- hexafluoropropane. Good examples of the dicarboxylic acid dihalide are the dichlorides named in the description of the first method.

The organic solvent is not particularly limited, and wide selection can be made from, for example, amides such as N,N- dimethylformamide, N,N-dimethylacetamide, Nmethyl-2-pyrrolidone and pyridine, sulfonic solvents such as dimethyl sulfoxide and tetramethyl sulfone, aromatic solvents such as benzene, toluene, anisol, diphenyl ether, nitrobenzene, benzonitrile, cresol and phenol and halogenated hydrocarbons such as chloroform, trichloroethane and carbon tetrachloride.

The reaction between an aromatic diamine of the general formula (7) and a dicarboxylic acid dihalide of the general formula (5) is carried out in a selected organic solvent, under substantially nonaqueous condition, at a temperature ranging from - 100C to reflux temperature of the employed solvent. The reaction time is from several minutes to several hours. The molecular weight of an aromatic polyamide formed by this reaction depends largely on the proportion of the dicarboxylic acid dihalide to the aromatic diamine, and it is favorable to use equimolar quantities of these reactants for preparing an aromatic polyamide of relatively high molecular weight.

An aromatic polyamide of the general formula (2) can easily be converted into a corresponding poly- -g- benzoxazole of the general formula (1) by subjecting the polyamide to a dehydrating and cyclizing reaction. This reaction is accomplished by heating the polyamide in a nonoxidizing gas atmosphere at a.temperature ranging from about 100 0 C to about 500 0 C for a sufficient period of time, which ranges from several seconds to tens of hours. It is optional to heat the polyamide in the presence of a dehydrating agent such as, for example, polyphosphoric acid with a view to accomplishing the dehydrating and cyclizing reaction at a relatively low temperature. Also it is possible to lower the reaction temperature by carrying out the reaction under reduced pressure.

As is apparent from the foregoing description, in polybenzoxazoles used in this invention the aromatic group Ar in the general formula may be -U, or where A is H CH 3 CH 3 CF 3 CF 3 CF 3 0 0 -C- C- -C- 9 -C- -0-, -S- or -S-, H CH 3 UP' 3 H and the divalent aromatic group R may be phenylene group, biphenylene group, naphtylene group or 0 0 H CH 3 CF 3 A' where A' is -C-, -S-, -C-, -u- or -u 0 H Uki 3 W 3 A gas separating membrane according-to the invention can be produced by first forming a membrane or film of an aromatic polyamide of the general formula (2) by a conventional method and then converting the polyamide film into a polybenzoxazole film by the above described heat treatment. When the desired membrane is in sheet form the polyamide film is formed, for example, by melt pressing, melt extrusion or solution casting. Polyamides of the general formula (2) are soluble in many kinds of organic solvents. When the desired membrane is in tube form the polyamide film is formed, for example, by melt spinning or by wet or dry spinning from a solution.

As to semipermeabilities of polybenzoxazole membranes according to the invention, "gas permeation coefficient", P, and "selectivity",i(, are taken in the following examples as representative characteristics.

The gas permeability coefficient is an index indicating the speed of permeation of a specific gas through the semipermeable membrane and is given by the following equation.

V (cm 3 (STP)) x t (cm) A (cm 2) x T (sec) x P (cmHg) where V is the volume (at standard temperature and pressure) of the gas permeated through the membrane, t is the thickness of the membrane, A is the area of the membrane, T is the permeating time and P is the gas pressure.

A large value of the coefficient P indicates that the gas permeates through the membrane at a high rate.

In using the semipermeable membrane for separating a mixture of two kinds of gases, the selectivity,, of the membrane is defined as the ratio of the permeation rate of a gas which permeates relatively easily through the membrane to the permeation rate of the other gas which does not so easily permeates through the membrane. The selectivity can directly be measured by bringing a known mixture of two gases into contact with the membrane and analyzing the gas mixture permeated through the membrane. Alternatively, the selectivity can be approximated by separately measuring the permeation rate of each of the two gases through the same membrane and comparing the permeation rates of the respective gases with each other. For example, if the selectivity of a membrane for H 2 /CH 4 mixture is 200, it means that hydrogen gas in the mixture permeates through this membrane at a rate 200 times as high as the rate of permeation of methane gas in the mixture through this membrane.

The invention is further illustrated by the following nonlimitative examples.

EXAMPLE 1

In a 50-ml three-necked flask, 1.638 g (2.5 milli mol) of 2,2-bis(3-trimethylsilylamino-4-trimethylsiloxy- phenyl)hexafluoropropane was dissolved in 5 ml of dimethylacetamide by stirring in a nitrogen gas atmosphere. The solution was freezed by using a bath of dry ice and acetone, and then 1.073 g (2.5 millimal) of hex'afluoroisopropylidenebiphenyl-4,4'-dicarboxylic acid dichloride was put into the flask. After that the bath was changed to an ice bath, and gentle stirring was commenced to cause the freezed solution to gradually melt. The stirring was continued for 5 hr while maintaining the nitrogen gas atmosphere in the flask.

After that the reaction liquid was poured into a large quantity of water to precipitate a polymer.

The intrinsic viscosity of the obtained polymer was 0.79 dL/g (0.5 g/dL in dimethylacetamide, at 30 0 C). BY infrared absorption spectrum analysis and elementary analysis the polymer was confirmed to be a polyamide having the structure of the following formula.

H H 0 N--; CF N-C CF 0 L3 3 1 0 \ 1 11 C C C HO CF 3 OH t.r- 3 n The above polyamide was dissolved in N-methyl-2pyrrolidone, and the solution was spread on a glass plate followed by evaporation of the solvent to thereby form a film. Then the polyamide film was heated in a nitrogen gas stream at 280-300 0 C for 10 hr. As the result the polyamide film turned into a transparent and very tough film. By infrared absorption spectrum analysis and elementary analysis the film obtained by the heat treatment was confirmed to be of a polybenzoxazole having the structure of the following 5 formula.

N N CF 3 0 c 0 0 c c 4n CF 3 CF 3 Table 1 shows heat resistant characteristics of this polybenzoxazole and two other polybenzoxazoles prepared in the subsequent examples, and Table 2 shows mechanical characteristics of the same polybenzoxazoles. Table 3 shows gas permeation coefficients of the poly- benzoxazole film obtained in each example for oxygen, carbon dioxide, carbon monoxide, methane, nitrogen and hydrogen, and Table 4 shows selectivities of each polybenzoxazole film as a gas separating membrane for separating methane and hydrogen or carbon monoxide or separating nitrogen and oxygen. EXAMPLE 2 To prepare a polyamide, 1.638 g (2.5 millimol) of 2,2-bis(3trimethylsilylamino-4-trimethylsiloxyphenyl)hexafluoropropane was reacted with 0.633 g (2.5 milli- mol) of 2,6-naphthale.ne-dicarboxylic acid dichloride by the same method as in Example 1.

The intrinsic viscosity of the obtained polymer was 0.60 dL/g (0.5 g/dL in dimethylacetamide, at 30 0 c). By analysis the polymer was confirmed to be a polyamide having the structure of the following formula.

H 0 N C: N-C 1 HO OH 00 c 3 0 n A film of this polyamide was formed by the same method as in Example 1, and the film was subjected to the same heat treatment as in Example 1. After the heat treatment the film was transparent and tough. By -analysis this film was confirmed to be of a polybenz- oxazole having the structure of the following formula.

N c N c c 00 c 1 " n CF 3 EXAMPLE 3

To prepare a polyamide, 1.368 g (2.5 millimol) of 2,2-bis(3-trimethylsilylamino-4-trimethylsiloxyphenyl)- propane was reacted with 0.7678 g (2.5 millimol) of benzophenone-4,4'-dicarboxylic acid dichloride by the same method as in Example 1. The intrinsic viscosity of the obtained polymer was 1.00 dL/g (0.5 g/dL in dimethylacetamide, at 30 0 C). BY analysis this polymer was confirmed to be a polyamide having the structure of the following formula.

H H 0 0 0 1 1 11 11 - 11 N:::: C: N-C C C 1 C HO 1 OH CH 3 n A film of this polyamide was formed by the same method as in Example 1, and the film was subjected to the same heat treatment as in Example 1. The heattreated film was transparent and tough. By analysis this film was confirmed to be of a polybenzoxazole having the structure of the following formula.

I N:::: C:1 C 0 C -1 0 0"-- C 0 fl;ti 3 n TABLE 1

Polybenz- Glass Transition 10% Weight Loss oxazele Temperature Temperature 0 C) (OC) in Air in N 2 gas Ex. 1 294 525 530 Ex. 2 325 538 562 Ex. 3 248 485 510 TABLE 2

Polybenz- Tensile Break Elongation Tensile Modulus oxazole Strength at break of Elasticity (MPa) (%) (GPa) Ex. 1 67.8 3.2 2.6 Ex. 2 92.7 5.5 3.2 Ex. 3 150 8.3 3.6 TABLE 3

Gas Permeation Coefficient (P) of Polybenzoxazole Membrane - 10 3 10. cm (STP).cm cm 2 oseccm.Hg Test H 2 CO 2 CO CH 4 0 2 N 2 Temp.

0 C) 2 atm 5 atm 5 atm' 5 atm 5 atm 2 atm Ex. 1 50 142 65.6 6.2 2.84 19.3 4.44 1 atm 5 atm 5 atm 5 atm 5 atm 2 atm 205 66.2 9.96 5.69 25.8 7.49 2 atm 10 atm 10 atm 10 atm 10 atm 10 atm Ex. 2 50 30.6 5.64 0.481 0.195 1.85 0.313 1 atm 10 atm 10 atm 10 atm 10 atm 10 atm 56.9 9.37 1.21 0.554 3.82 0.864 2 atm 5 atm 5 atm 5 atm 5 atm Ex. 3 50 12 x 2.5 x 0.1 x 0.77x 0.12 x 10- 10 10- 10 10- 10 10- 10 10- 10 TABLE 4 Selectivity 1 0 Test.4 2 /CH 4 CO 2 /CH 4 2 IN 2 Temp.

0 C) 50.0 23.1 4.35 Ex. 1 36.0 11.6 3.44 157 28.9 5.91 Ex. 2 103 16.9 4.42 Ex. 3 50 120 25 6.42 EXAMPLE 4

The aromatic polyamide prepared in Example 1 was dissolved in dimethylformamide, and the solution was applied to one side of a disc of high silica glass (Vycol glass of Corning Glass Co.) which was microscopically porous with a mean pore diameter of 40 R and hence was permeable to gases. The disc was 30 mm in diamter and 1 mm in thick-ness. After evaporation of the solvent the polyamide film on the glass disc was heated in a nitrogen gas stream at 280-300 0 C for 3 hr.

As the result, the polyamdie film was converted into a film of the poly-benzoxazole prepared in Example 1. The thickness of the film was about 1)Am.

The polybenzoxazole film supported on the high silica glass disc was.fitted in a device for measuring gas permeabilities to measure the performance of the film for separating oxygen gas from the air (oxygen concentratio is about 21 vol%) at room temperature. The difference in pressure between the front side of the polybenzoxazole film and the opposite side was controlled to 76 cmHg. The composition of the gas permeated through the film was analyzed by gas chromatography to find that the permeated gas contained about 41 vol% of oxygen. In this test the gas flow rate through the polybenzoxazole film was 38 cm 3 per hour. 10 X

Claims

1. A semipermeable membrane for separating gases, characterized in that the membrane is formed of a poly benzoxazole having a structure represented by the general formula (l):

/ N \ / N \O/Ar \-R.

\O/) n wherein Ar represents an aromatic group, R represents a divalent aromatic group, and n is an integer from

2 to 200. 2. A semipermeable membrane according to Claim 1, wherein Ar in the general formula (1) is -U 1 1,5 or where A is H CH

3 CH 3 CF 3 CF 3 CF 3 0 0 -C- -C- 9 -C- 7 -C-0-, -S- or -S-.

H CH 3 ur 3 H 0 3. A semipermeable membrane according to Claim 1 or 2, wherein R in the general formula (1) is phenylene group, biphenylene group, naphthylene group or A 0 0 H CH 3 W 3 where A' is -C-, -S-, -C-, -C- or -C- 0 H CH 3 UP 3

4. A method of separating a gas from a gas mixture of said gas and at least one other gas by contacting said - gas mixture with a semipermeable membrane as claimed in claim 1, 2 or 3, said membrane being relatively more permeable to said gas than to said at least one other gas.

5. A method according to claim 4 wherein said gas to be separated is hydrogen gas and said at least one other gas comprises methane.

6. A method according to claim 4 wherein said gas to be separated is carbon dioxide gas and said at least one other gas comprises methane.

7. A method according to claim 4, wherein said gas to be separated is oxygen gas and said at least one other gas comprises nitrogen.

8. A semipermeable membrane of a polybenzoxazole, substantially as hereinbefore described in any one of Example 1 to 3.

9. A method of concentrating oxygen gas in the air by using a selectively semipermeable membrane, substantially as hereinbefore described in Example 4.

Published 1989 atThe PatentOfnce, StateHouse, 68f71 Holborn, London WC1R4TP. Further copies maybe obtainedfrom ThePatent 0Mce. Sales Branch, St Maxy Cray, Orpington, Rent BR5 31M. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1187