JPH0582249B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an inorganic porous membrane used for filtration, gas separation, etc., and a method for producing the same. (Prior art) As a type of inorganic porous membrane, a porous thin film having one or more layers of a porous support made of a corrosion-resistant inorganic material and having an average pore diameter smaller than the average pore diameter of the support on at least one side thereof. There is an inorganic porous membrane comprising: This type of porous membrane is used for various filtration and gas separation membranes, and is required to have heat resistance, corrosion resistance, high filtration accuracy, and separation accuracy depending on each application. In this case, it is necessary to give corrosion resistance to the composition of the porous thin film that functions as a filtration membrane or separation membrane, and care must be taken to ensure that there are no pinholes, cracks, etc. in the thin film. . For example, in the field of ultrafiltration and precision filtration, which separate microparticles such as organic colloids, acid or alkaline cleaning is used as a means of regenerating decreased membrane performance due to clogging, etc., and steam cleaning is used as a sterilization means. However, in this case, high corrosion resistance is required. In addition, if pinholes, cracks, etc. exist in the thin film, filtration accuracy and separation accuracy will of course decrease, but the acid, alkali, and steam cleaning described above will increase pinholes, cracks, etc., resulting in poor corrosion resistance and filtration. The accuracy and separation accuracy will further deteriorate. By the way, many technologies related to the above-mentioned multilayered inorganic porous membranes have already been disclosed, and an example of a publication disclosing such technologies is Japanese Patent Application Laid-Open No. 1989-1999.
Publication No. 156510 can be mentioned. The publication describes a method for manufacturing an inorganic semi-permeable membrane that does not cause cracks, specifically, coating a porous support made of sintered inorganic oxide with a suspension (sol solution) of an inorganic membrane-forming coating material and heating it. A method of manufacturing is disclosed. By this manufacturing method, an ultrafiltration membrane in which a porous thin film made of γ-alumina is coated on a porous support is obtained. However, the same publication states that the suitability of a porous support is determined by the pore size (average pore diameter) of the support, and the preferred average pore diameter is 0.10 μm.
~0.50μm is listed. For porous thin films, the film thickness is 20μm or less, and for coating sol solutions, the concentration of the dispersed phase in the medium is 0.01wt%.
It is stated that it is ~25wt%. (Problems to be Solved by the Invention) By the way, pinholes and cracks in a porous thin film in an inorganic porous film having a multilayer structure occur when the thin film is formed. Generally, a porous thin film is formed by supporting a sol solution of microparticles on one side of a porous support, drying, and baking. In this case, the supported sol penetrates into the pores of the porous support and is concentrated and developed on the surface to form a thin film, but if the particles in the sol are locally sucked into the support, Holes are generated, and when the film thickness locally increases, cracks occur due to heat shrinkage during subsequent drying and firing. The present inventor focused on the porous support, the average pore diameter of the porous thin film, the thickness of the thin film, and the composition as factors in the occurrence of pinholes and cracks, and by specifying these, pinholes, cracks, etc. do not exist. It was found that a porous membrane with extremely excellent corrosion resistance in terms of structure and composition and excellent filtration and separation efficiency can be obtained. Therefore, in order to obtain such a porous membrane, it is necessary to specify each of the above-mentioned factors. Regarding this point, when referring to the description in the above-mentioned publication, it is found that the publication only stipulates the average pore diameter of the porous support and the thickness of the porous thin film within a wide range of values; The occurrence of pinholes and cracks is unavoidable. Therefore, a porous membrane with high filtration and separation accuracy and filtration and separation efficiency and excellent corrosion resistance cannot be expected. Furthermore, corrosion resistance cannot be expected from the composition of the thin film. Accordingly, an object of the present invention is to provide a porous membrane that is free from pinholes and cracks, has high filtration and separation accuracy, high filtration and separation efficiency, and has excellent corrosion resistance, and a method for producing the same. (Means for Solving the Problems) A first aspect of the present invention is to provide a porous support of one or more layers made of a corrosion-resistant inorganic material with an average pore diameter smaller than the average pore diameter of the support on at least one side thereof. An inorganic porous membrane comprising a porous thin film having a purity of 99.5% or more, the porous thin film having an average pore diameter of 800 Å or less,
The film thickness is 2 μm or less, and the average pore diameter of the layer of the porous support to which the porous thin film is attached is
It is characterized by being 0.1 μm or less. Further, a second invention of the present invention is based on the first invention, wherein the porous thin film is formed by supporting a hydrosol solution containing titanium hydroxide or titanium oxide on at least one side of the porous support. This is a method for producing such an inorganic porous membrane, in which a precursor forming the hydrosol is hydrolyzed in the presence of an acid at a molar ratio of precursor/water of 1/200 or more, and titanium in the obtained sol is The hydrosol solution is prepared by diluting the compound/water molar ratio to 1/500 or less, and this hydrosol solution is coated on at least one side of the porous support, dried, and then baked at 300°C to 700°C. It is characterized by this. In the present invention, the porous support is made of ceramics such as alumina, zirconia, and titania, glass such as borosilicate glass, and metals such as nickel, and has an appropriate shape such as a pipe shape, a flat plate shape, and a honeycomb shape. In addition, the porous support has an average pore diameter of
A single-layer structure consisting of only one layer of 0.1 μm or less, or a multi-layer structure consisting of the same layer and a layer with a larger average pore diameter, and in the case of a porous support with a multi-layer structure, the average pore diameter layer with a thickness of 0.1 μm or less (intermediate layer)
A porous thin film is carried on the side. Precursors for forming the hydrosol liquid include organic titanate compounds such as alkoxides, acylates, and chelates, titanium salts such as titanium tetrachloride and titanyl sulfate, meta- and orthotitanic acid, and titania. In addition, one side of the porous support means either the inside or the outside in the case of a pipe-shaped support, either the front side or the back side in the case of a flat support, and one side of the honeycomb-shaped support. In the case of a body, it means the entire inner periphery of a certain through hole, the inner periphery of all the through holes, or the outer periphery of the support. (Operations and Effects of the Invention) In the porous film according to the first aspect of the present invention, there are substantially no pinholes or cracks in the porous thin film, and the composition has extremely high corrosion resistance. It has high filtration accuracy and separation accuracy, high corrosion resistance, and does not generate or increase pinholes or cracks due to acid, alkali, steam cleaning, etc., so each of the above characteristics is maintained for a long period of time. Therefore, regarding the porous support constituting the porous membrane, the average pore diameter of the porous support, and the average pore diameter of the intermediate layer in the case of a porous support with a multilayer structure.
The essential requirement is that the pore size is 0.1 μm or less, and as a result, a porous thin film with an average pore diameter of 800 Å or less without pinholes or cracks can be supported, and such a thin film can be made into an extremely thin and uniform film with a thickness of 2 μm or less. It can be done. By specifying the maximum pore diameter of the support, it is possible to support a uniform and extremely thin film free of pinholes and cracks, and the maximum pore diameter is 1% of the average pore diameter of the porous thin film.
~100 times, preferably 0.1 μm or less. In a support with a multilayer structure, a thin film is supported on the intermediate layer side with a small average pore diameter, so
This is preferable because it can reduce the influence of fluid diffusion resistance. In this case, the main layer of the support excluding the intermediate layer may have an average pore diameter of 0.5 μm to 30 μm.
The thickness of the main layer and the average pore diameter are determined by fluid diffusion resistance, mechanical strength, etc. Regarding porous supports, in order to improve corrosion resistance and prevent minute elution of support components, it is necessary to contain alkali metal compounds, alkaline earths, metal compounds, yttrium, lanthanide element compounds, and group element compounds in the support components. It is preferable that the amount of a compound in which the radius of the cations constituting the compound is less than 0.6 Å or more than 0.9 Å and the specific surface area of the compound is 10 m ² /g or more is less than 0.5 wt% in terms of oxide. . Specifically, high-purity α-alumina and titania are used as main raw materials, and an inorganic binder, an organic binder, and a surfactant are added so that the total weight of each of the above compounds is less than 0.5 wt%. The porous thin film is made of titanium oxide with a purity of 99.5% or higher, has excellent film formability and corrosion resistance, and substantially prevents minute elution of thin film components. In the present invention, when forming a porous thin film on at least one side of a porous support, a hydrosol solution containing titanium hydroxide or titanium oxide is first prepared. The hydrosol liquid is obtained by hydrolyzing titanium hydroxide, a precursor for forming titanium oxide, in the presence of an acid. Examples of precursors include organic titanate compounds such as alkoxides, acylates, and chelates; titanium salts such as titanium tetrachloride;
Examples include meta- and orthotitanic acids, fine titania, etc., but organic titanate compounds are preferred from the viewpoint of purity and ease of handling. Examples of acids to be added include nitric acid, hydrochloric acid, acetic acid, lower fatty acids, etc.
The amount added is 0.01 to 0.5 mol/l. Preferably, the acid is nitric acid or hydrochloric acid, and the amount added is 0.05~
It is 0.15 mol/l, and the pH of the liquid is in the range of 0.5 to 2. If the amount of acid added is out of the appropriate range, the sol particles will coagulate and precipitate, but the particle size of the sol particles will become too small to be suitable as a supporting liquid. The first important point is to keep the precursor/water molar ratio at least 1/200, preferably between 1/200 and 1/40, so that the precursor can be properly hydrolyzed in the presence of acid. arise. When the molar ratio is 1/40 or more, the resulting sol tends to gel, which impedes workability. The second important point is to control the conditions of the above-mentioned hydrolysis, first adding the precursor in hot water above 50 °C and aging for at least 10 minutes, then adding acid and aging at 70 °C Aged for at least 30 minutes. As a result, what was a white suspension at the time the precursor was added eventually becomes a transparent, milky-white hydrosol liquid. The sol particles of the obtained hydrosol liquid have a particle size of 30 Å to 1000 Å, and this particle size is adjusted by the hydrolysis temperature, time, etc. By diluting this sol with water, a hydrosol suitable for supporting (hereinafter referred to as a supported sol) can be obtained. It is essential that the concentration (titanium hydroxide + titanium oxide)/water molar ratio of the supported sol solution is 1/500 or less; if this value is exceeded, pinholes and cracks are likely to occur when forming a porous thin film. . The preferred range of the above molar ratio is 1/800 to 1/2000,
At concentrations in this range, the film thickness can be increased by one or two loading operations.
A uniform thin film of 2 μm or less can be obtained. If the concentration is lower than this, the film formed by one carrying operation will be thin, and it will be necessary to repeat the carrying operation to form a uniform thin film of a predetermined thickness. Such a supported sol liquid has the same properties as the hydrosol liquid before dilution, has excellent adhesion to a porous support, uniform film forming properties, and does not generate pinholes or cracks. In addition, when using a titanium salt as a precursor, it is first neutralized with an alkali to remove impurity salts, and then a hydrosol solution is obtained by the same process as the hydrolysis described above in the presence of an acid. Dilute with water to obtain a supported sol solution. In addition, when fine powder titania is used as a precursor, the specific surface area is 50 m ² /g.
It is preferable to use the above-mentioned anatase type, which can be easily hydrolyzed and a hydrosol solution can be obtained. A porous thin film can be obtained by supporting a supported sol on at least one side of a porous support, drying and then firing. Drying is carried out at room temperature for at least 2 hours and then at about 100°C, followed by firing at 300°C to 700°C. As a result, a thin film with an average pore diameter of 800 Å or less and virtually no pinholes or cracks and excellent corrosion resistance can be obtained. Firing temperature is 300
If the temperature is below ℃, the adhesion of the thin film to the support will be insufficient, resulting in poor corrosion resistance.
If the temperature exceeds ℃, the average pore diameter of the thin film becomes larger than 800 Å, and the desired thin film cannot be obtained. (Example) (1) Porous support A support with a multilayer structure consisting of a pipe-shaped main layer and a thin intermediate layer was used as the porous support. The main layer was obtained by extruding a kneaded clay pipe with an outer diameter of 10 mm, an inner diameter of 7 mm, and a length of 150 mm, which was dried and then fired at 1500°C for 3 hours, and the middle layer was made by peptizing the outer periphery of the main layer. It has a thickness of 30 μm and is made by coating a slurry, drying it, and firing it at 1300°C.The main layer is made of α-alumina with a purity of 99.92% and the intermediate layer is made of α-alumina with a purity of 99.99% or more. Support: Average pore diameter of main layer 3 μm, average pore diameter of intermediate layer 800 Å Support: Average pore diameter of main layer 3 μm, average pore diameter of intermediate layer 0.2 μm (2) Preparation of hydrosol solution before dilution Titanium as precursor Using isopropoxide and titanium tetrachloride, in the presence of hydrochloric acid (added amount
0.1 mol/1) was hydrolyzed at 80°C for 2 hours to prepare the hydrosol solution before dilution shown in Table 1. As for titanium tetrasalt, it was first hydrolyzed with aqueous ammonia to form a white precipitate, which was washed with water until no chlorine ions were detected, and then subjected to the above-described hydrolysis. (3) Preparation of supported sol solution The various hydrosol solutions prepared were diluted to prepare supported sols shown in Table 2. (4) Formation of porous thin film The supported sol solution was supported on the outer periphery of a porous support, dried at room temperature for 2 hours, then at 100°C for 2 hours, and then baked at various temperatures for 3 hours. The average pore diameter of the thin film was adjusted by changing the firing temperature. (5) Porous membrane evaluation method Cross-flow filtration, corrosion resistance,
Various tests for thin film peeling, pinhole and crack measurements were conducted, and the results shown in Table 3 were obtained. Cross-flow filtration test: Aqueous solution containing 100 ppm marker at a speed of 2.5 m/sec, inlet pressure 3 Kg/cm ²
Cross-flow filtration is performed by circulating the inner pores of the porous membrane, and the permeate is analyzed to calculate the marker rejection rate. In addition, as a marker, if the average pore diameter of the thin film is 35 Å, if the average pore diameter is 50 Å, bovine serum albumin (average molecular weight 65,000),
In the case of 680 Å and 850 Å, Uniform Terrax (particle size 0.8 μm) was used. Corrosion resistance test: Porous membrane was exposed to HCl aqueous solution (PH=
0), immersed in NaOH aqueous solution (PH = 14) for 168 hours, and then conducted the above cross-flow filtration experiment to measure marker rejection. Peeling test of thin film: The porous membrane after the corrosion resistance test was placed in an ultrasonic cleaner, and the state of peeling of the thin film by ultrasonic waves was observed. Measurement of pinholes and cracks: The presence or absence of pinholes and cracks in the porous membrane before and after the corrosion resistance test was observed using a scanning electron microscope.

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[Table] (6) Discussion As is clear from Table 3, among porous membranes, the average pore diameter of the thin film exceeds 800 Å (porous membrane No. 9), and the thin film thickness exceeds 2 μm. (Porous membranes No. 3 and No. 12), and those with an average pore diameter of more than 0.1 μm in the layer to which the thin film of the porous support is attached ((Porous membrane No. 14), the specified performance cannot be obtained. No. Regarding other porous membranes, except for Porous Membrane No. 7 whose thin film firing temperature is extremely low, all of the specified performances such as membrane thickness, rejection rate, amount of permeated liquid, corrosion resistance, presence or absence of cracks and pinholes were achieved. Furthermore, we measured the elution amount of membrane components into the permeate for these porous membranes that have the specified performance, and it was confirmed that the elution amount was below the detection limit and was extremely small. Next, considering from the manufacturing point of view, for hydrosol liquids and supported sol liquids, when the molar ratio of precursor/water is less than 1/200 (porous membrane No. 12), the molar ratio of precursor/water after dilution is When the ratio exceeds 1/500 (porous membrane No. 13), a porous membrane with the desired performance cannot be obtained.On the other hand, when the molar ratio of precursor/water is 1/
200 or more and the molar ratio of precursor/water after dilution is 1/500 or less, except for porous membrane No. 7, the film thickness, rejection rate, amount of permeated liquid, corrosion resistance, presence or absence of cracks and pinholes. A porous membrane having all the predetermined properties can be obtained. A more preferable molar ratio of precursor/water is 1/200 to 1/40, and a more preferable molar ratio of precursor/water after dilution is 1/800 to 1/2000. In an example where the molar ratio of precursor/water is 1/30 (porous membrane No. 1), a porous membrane with the desired performance can be obtained, but the hydrosol solution gels quickly and must be handled with care.
On the other hand, a porous membrane with the desired performance can be obtained even in an example where the molar ratio of precursor/water after dilution is /3000 (porous membrane No. 11), but the number of times the supported sol solution is applied is increased. There is an inconvenience in having to do so. In addition, the firing temperature of the thin film is 300℃ to 700℃.
Even if the supported sol liquid is appropriate, if the firing temperature is low (porous membrane No. 7), the porous membrane will have low corrosion resistance; on the other hand, if the firing temperature is high (porous membrane No. Film No. 9) has a low rejection rate.

Claims (1)

[Scope of Claims] 1. An inorganic porous support comprising one or more layers of a porous support made of a corrosion-resistant inorganic material, and on at least one side thereof, a porous thin film having an average pore diameter smaller than the average pore diameter of the support. In the porous thin film, the porous thin film is made of titanium oxide with a purity of 99.5% or more, has an average pore diameter of 800 Å or less, a film thickness of 2 μm or less, and has a layer of the porous support to which the porous thin film is attached. An inorganic porous membrane characterized by an average pore diameter of 0.1 μm or less. 2. The method for producing an inorganic porous membrane according to item 1, which comprises forming the porous thin film by supporting a hydrosol solution containing titanium hydroxide or titanium oxide on at least one side of the porous support. , the precursor forming the hydrosol liquid is hydrolyzed in the presence of an acid at a precursor/water molar ratio of 1/200 or more, and the titanium compound/water molar ratio of the obtained sol liquid is reduced to 1/500. The inorganic porous membrane is characterized in that the hydrosol solution is prepared by diluting the hydrosol solution as follows, the hydrosol solution is coated on at least one side of the porous support, and after drying, it is fired at 300°C to 700°C. Manufacturing method.