JPS6311045B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a gas separation membrane made of polyimide. In recent years, gas separation using organic polymer membranes, especially oxygen enrichment of air, has been attracting attention from the viewpoint of resource and energy conservation. Since the permeability coefficient ratio of oxygen to nitrogen is small, it is not suitable for oxygen enrichment on an industrial scale. For example, polydimethylsiloxane has an oxygen permeability coefficient of 10 ^-8 cm ³
(STP)・cm/cm ²・sec・cmHg, which is the largest of the conventionally known polymer films, but this film has low mechanical strength and cannot be put into practical use. The film thickness must be 100μm or more,
As a result, even if the gas permeation coefficient through the membrane itself is large, the permeation rate, which determines the efficiency of gas separation by the membrane, cannot be increased. In addition, the permeability coefficient ratio of oxygen to nitrogen is at most about 2,
The selective separation of oxygen is poor, and if a high concentration of oxygen is to be obtained, multistage membrane treatment is required, making it impractical in terms of both equipment and cost. For this reason, Japanese Patent Publication No. 47-51715 proposes an oxygen-enriching membrane made of polyvinyltrimethylsilane, which improves the permeability coefficient ratio of oxygen to nitrogen to about twice that of polydimethylsiloxane. It has poor chemical resistance and is susceptible to deterioration due to airborne contaminants, oil from pumps, etc. In recent years, in addition to oxygen enrichment, with the development of so-called _C1 chemistry, gas separation membranes for synthesis gas have been required, and in particular, gas separation membranes for such purposes are Since it is used at high temperatures of 100 to 200°C or higher, extremely high heat resistance is required. As a result of intensive research to solve the above-mentioned problems, the present inventors found that a membrane made of polyimide has excellent gas separation properties, heat resistance, mechanical strength,
Discovered that it has excellent chemical resistance and processability,
This led to the present invention. The gas separation membrane according to the invention has substantially the general formula (However, R represents a divalent organic group.) Made of polyimide consisting of a repeating unit represented by
Moreover, it is characterized by being composed of a film having a homogeneous layer having a thickness of 10 μm or less. A polyimide consisting essentially of repeating units represented by the above general formula is disclosed in, for example, JP-A-55-152507.
Preferably, 1,2,3,4-butanetetracarboxylic acid and approximately equimolar amount of general formula H ₂ NR—NH ₂ (however, , R is the same as above.) It can be obtained by condensation polymerization of a diamine represented by the following in a solvent under heating. Examples of such diamines include m-phenylenediamine, p-phenylenediamine, 4,
4'-diaminodiphenylmethane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'diaminophenyl sulfide, 4,
4'diaminophenyl sulfone, 3,3'-diaminodiphenylsulfone, p-bis(4-aminophenoxy)benzene, m-bis(4-aminophenoxy)benzene, m-xylylenediamine, p-
xylylenediamine, di(4-aminocyclohexyl)methane, hexamethylenediamine, heptamethylenediamine, octamethylenediamine,
1,4-diaminocyclohexane, bis(p-aminophenyl)phosphine oxide, bis-(p
-aminophenyl)dimethylsilane and the like. In particular, in the present invention, a polyimide obtained by using a diamine in which R in the general formula is an organic group containing an aromatic ring is preferred. The polyimide having an organic group R having an aromatic ring obtained in this way has particularly excellent heat resistance, so it can be used for gas separation at high temperatures, and gases generally have a high permeability coefficient at high temperatures. This is because efficient gas separation can be performed. Preferred specific examples of the organic group having an aromatic group include, for example,

【formula】

[Formula] etc., where X is a divalent organic group, and preferred specific examples thereof include -CH ₂ - -C(CH ₃ ) ₂ - -O- -SO ₂ -
-S- -CO- -Si(CH ₃ ) ₂ - etc. can be mentioned. In the present invention, the number of imide rings/the number of imide rings + the number of amide bonds x 100
(%) It is possible to use a polyimide consisting essentially of the above-mentioned repeating units with an imidization rate defined as 70% or more, but preferably the imidization rate is 90% or more, particularly preferably 98% or more. ~100%. Further, the intrinsic viscosity of the polyimide (measured as an N-methyl-2-pyrrolidone solution at 30°C, the same applies hereinafter) is 0.6 to 4, preferably 0.8 to 2.
If the intrinsic viscosity is too low, the self-supporting properties will be poor when formed into a film, and if it is too high, as will be described later, a homogeneous solution of polyimide,
That is, it becomes difficult to prepare a membrane-forming solution and, therefore, it becomes difficult to prepare a uniform gas separation membrane. The gas separation membrane according to the present invention can be produced by various methods, but usually, the above polyimide is dissolved in an organic solvent to prepare a uniform membrane-forming solution, and this is spread on a suitable support substrate. After casting coating,
It can be obtained by heat treatment under normal pressure or reduced pressure to evaporate the solvent. Here, the organic solvent for preparing the film forming solution is usually N.
-Methyl-2-pyrrolidone, N-methyl-2-piperidone, dimethylacetamide, dimethylformamide and the like are preferably used. The support base material for casting the membrane forming solution is not particularly limited, but a plate member with a smooth surface, exemplified by glass, stainless steel, aluminum, polyethylene, polypropylene, etc., is usually used. The heating temperature after coating the film-forming solution on the support substrate depends on the solvent of the film-forming solution, but in the case of a polar organic solvent with a relatively high boiling point as mentioned above, it is 80 to 80°C.
The temperature is 140°C, preferably 100-120°C. especially,
In the present invention, after most of the solvent is evaporated in this temperature range, the solvent is further evaporated by heating to a temperature of 150 to 200°C. The membrane obtained by completely removing the solvent by evaporation in this manner can be immediately used as a gas separation membrane. Furthermore, even when the solvent is almost completely removed by evaporation, the resulting membrane can be used for gas separation as is, but if necessary, the membrane formed on the substrate can be immersed together with the substrate in water.
The solvent remaining in the membrane can be completely extracted and removed with water, and the membrane can be easily peeled off from the base material, and it can also be dried to form a gas separation membrane. The gas separation membrane thus obtained has a substantially homogeneous and dense layer with substantially no pores having a diameter of 25 Å or more. This can be confirmed by observing the film with an electron microscope at a magnification of 20,000 times or more. In order to increase the gas permeation rate, the thinner the thickness of the above-mentioned homogeneous layer is, the better; however, from the point of view of mechanical strength, the thicker the better, and from these points of view, the thickness of the above-mentioned homogeneous layer is about 0.05 to 10 μm. preferable. In the gas separation membrane of the present invention, the entire membrane may be a homogeneous membrane consisting only of the above-mentioned homogeneous layer, or a part of the membrane, usually only the surface layer, may be a homogeneous membrane consisting of only the above-mentioned homogeneous layer. The homogeneous layer may be an anisotropic membrane supported by a porous layer.
To prepare a homogeneous membrane, for example, a dilute membrane forming solution is used. For example, by using a membrane-forming solution with a polyimide concentration of 5% by weight or less, it is possible to obtain a gas separation membrane with a thickness of several μm, the entire membrane consisting of only a homogeneous layer as described above. Further, in the anisotropic film, the homogeneous layer is composed of a dense layer having substantially no pores with a diameter of 25 Å or more, and this dense layer is substantially continuous and integrated with the porous layer made of the same resin. The film may be an anisotropic film with a so-called asymmetric structure supported by It may also be a composite membrane that is attached. In any case, even in the case of an anisotropic film, the thickness of the homogeneous layer is preferably 10 μm or less, but especially
The thickness is preferably about 0.001 to 1 μm. As described above, the gas separation membrane according to the present invention has the following features:
In addition to having excellent chemical resistance and heat resistance, it also has large gas permeability coefficients for many gases and a large ratio of permeability coefficients between them.It also has excellent mechanical strength, so it can be used not only for oxygen enrichment. , it can also be suitably used for gas separation at high temperatures in _C1 chemistry. In particular, even though many conventionally known gas separation membranes have large gas permeability coefficients at room temperature, they generally become small at high temperatures.
The gas separation membrane according to the present invention maintains a high permeability coefficient ratio even at high temperatures, and is therefore suitable for synthesis gas separation and composition adjustment that require gas separation at high temperatures, and as described above. In addition, since gases generally have a large permeability coefficient at high temperatures, the gas separation membrane of the present invention allows efficient gas separation. For example, according to conventionally known gas separation membranes, when separating mixed gases, due to the limit of heat resistance of the membrane, the pressure is ^about 100 Kg/cm2, preferably 10 to 80 Kg/cm2. ^However , with the gas separation membrane of the present invention, gas separation is possible at temperatures up to about 400°C, and in particular It has a great advantage that it can be practically used for gas separation, such as separation of mixed gases, in the temperature range of 100 to 300°C. The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way. In the following examples, the permeability coefficient P of a gas was determined by a high vacuum method, and the permeability coefficient ratio α was determined by dividing the permeability coefficient of the gas by the permeability coefficient of a reference gas. Reference example (Preparation of polyimide) N-methyl-2 was placed in two reactors equipped with a stirrer, a nitrogen gas introduction device, a reflux condenser with a reaction product water extraction device, and a jacket bath capable of heating up to a temperature of 250°C.
- After charging 1500 g of pyrrolidone, 281 g of 1,2,3,4-butanetetracarboxylic acid and 240 g of 4,4'-diaminodiphenyl ether, it was heated to about 70°C to form a homogeneous solution. After this, 170g of xylene was added as an azeotropic dehydration solvent, and the mixture was heated under a nitrogen stream.
The reaction was continued for 17 hours while heating to 190° C., refluxing xylene, and continuously removing reaction product water from the reaction vessel by azeotropy. Next, the azeotropic solvent xylene was distilled out of the reaction system to obtain a viscous solution of polyimide in N-methyl-2-pyrrolidone. This polyimide solution was poured into vigorously stirred water to coagulate and precipitate the polyimide, then filtered and isolated, poured into acetone, and thoroughly washed.
It was dried at 50° C. for 10 hours under a reduced pressure of 10 mmHg. In the polyimide thus obtained, R is It was confirmed from the nuclear magnetic resonance spectrum and infrared absorption spectrum that the imidization rate was 99% or more. Moreover, its limiting viscosity was 1.51. Example 1 3 g of the polyimide obtained above was mixed with N-methyl-2
- Average pore size 4μm after dissolving in 97g of pyrrolidone
The mixture was filtered through filter paper to remove foreign substances and the like to obtain a uniform membrane forming solution. This film-forming solution was cast onto a glass plate, and then dried at 25° C. for 5 hours and then at 90° C. for 10 hours under reduced pressure of 10 mmHg to remove the solvent. The film thus formed on the glass plate was put into water together with the glass plate, immersed for 5 hours, peeled off from the glass plate, and vacuum dried at 80°C for 10 hours to form a homogeneous film with a thickness of 3μ. I got it. By observing the surface of this film with an electron microscope at a magnification of 20,000 times, it was confirmed that it did not have pores with a pore diameter of 25 Å or more. Table 1 shows the permeability coefficient P (cc(STP)·cm/cm ² ·sec·cmHg) of this membrane for various gases at 25°C and the permeability coefficient ratio α for nitrogen.

[Table] Also, for the gas separation membrane obtained above, the permeability coefficient P of hydrogen and carbon monoxide, which are syngas components, at 25℃
and 100°C, and from these values, the permeability coefficient ratio α of hydrogen to carbon monoxide was determined.
The results are shown in Table 2. It is clear that the gas separation membrane according to the present invention has a high separation coefficient not only at room temperature but also at high temperature.

[Table] Example 2 In the same manner as in Reference Example, except that 238 g of 4,4'-diaminodiphenyl methane was used instead of 4,4'-diaminodiphenyl ether, R was A polyimide with an imidization rate of 99% or more and an intrinsic viscosity of 0.85 was obtained. Using this polyimide, a gas separation membrane was prepared in the same manner as in Example 1, consisting of only a homogeneous layer with a thickness of 3 μm and having no pores with a pore size of 25 Å or more as observed by an electron microscope. This membrane has an oxygen permeability coefficient of 1.3× at 25℃
10 ^-10 , and the permeability coefficient ratio for nitrogen was 5.5. Example 3 In the same manner as in Reference Example, except that 290 g of bis(p-aminophenyl)dimethylsilane was used instead of 4,4'-diaminodiphenyl ether, R was A polyimide with an imidization rate of 99% or more and an intrinsic viscosity of 0.66 was obtained. Using this polyimide, a gas separation membrane was prepared in the same manner as in Example 1, consisting of only a homogeneous layer with a thickness of 3 μm and having no pores with a pore size of 25 Å or more as observed by an electron microscope. This membrane has an oxygen permeability coefficient of 9.6× at 25℃
10 ^-10 , and the permeability coefficient ratio for nitrogen was 4.9.

Claims (1)

[Claims] 1. Substantially general formula (However, R represents a divalent organic group.) Made of polyimide consisting of a repeating unit represented by
A gas separation membrane comprising a homogeneous layer having a thickness of 10 μm or less.