Process for the production of an inorganic porous composite membrane

Abstract: A multilayer inorganic porous membrane consisting of a monolayer or multilayer porous support having at least one surface integrally formed thereon with a porous thin layer having an average pore size less than that of the porous support, wherein a maximum pore size in the support surface is defined to be from 1 to 250 times an average pore size of the thin layer. The multilayer porous membrane is superior to prevent the occurrence of pin holes and cracks therein thereby to enhance precision of the filtration and separation.

Description: The present invention relates to a process for the production of an inorganic porous membrane on a porous support, adapted for use in filtration, gas separation or the like. As one of the inorganic porous membranes described above, there has been proposed a multilayer inorganic porous membrane consisting of a monolayer or multilayer porous support integrally provided on at least one surface with a porous thin layer having an average pore size smaller than that of the porous support. In general, these inorganic porous membranes are required to have high corrosion resistance and heat resistance. In the case that high precision is required for filtration and separation, it is important to make the inorganic porous membrane without causing any pin holes and cracks in its porous thin layer. If the inorganic porous membrane is exposed to high temperatures, acid, alkali or the like in use for filtration and separation or in its regenerative process, it is required to be superior particularly in heat resistance. In a particular process for the production of superpure water in the semiconductor field or the production of pyrogen-free water in the pharmaceutical field, however, a problem will be caused due to dissolution of components from the inorganic porous membrane. In a regenerative process of the inorganic porous membrane for eliminating contaminants adhered thereto in a field of ultrafiltration or microfiltration, high resistance to corrosion is required for washing the porous membrane with acid and alkali. In a field where the inorganic porous membrane is used for ultrafiltration or microfiltration of fine particles such as organic colloid, the inorganic porous membrane, for instance, is washed with acid and alkali and with steam for sterilization. If pin holes, cracks or the like exsist in the thin layer of the porous membrane, they will increase during washing, resulting in deterioration of precision of the filtration and the resistance to corrosion. Inorganic porous membranes as described above are known from the following publications. US-A-4,711,719 discloses a process for the production of an inorganic semi-permeable membrane by coating a porous support of sintered inorganic oxides with a suspension of coating material forming an inorganic thin layer and then heating the intermediate product. In this production process, the suitability of the porous support for forming the thin layer is determined mainly by the average pore size of the support. If the average pore size is too large, sol particles in the suspension permeate into the porous support and do not form any thin layer on the support. For this reason, it is pointed out in the publication that a preferable pore size of the support is about from 0.1 to 0.5 µm. For the purpose of preventing the forming of cracks during the production process, the intermediate product is dried by a complicated supercritical drying for a long time and is gradually heated during the firing process thereof so that a porous thin layer of γ-alumina is formed on the porous support to obtain an ultrafiltration membrane. In JP-A-52-94572 (see also CH-A-604 826) there is disclosed a multilayer inorganic porous membrane consisting of a porous support coated with a porous thin layer, the porous support having an average pore size of from 10 to 200,000 times, preferably from 200 to 20,000 times that of the thin layer. A heat resistant inorganic porous membrane for gas separation is disclosed in JP-B-61-27091, and a filter in the form of a heat resistant inorganic porous membrane for liquid and gas filtration is disclosed in JP-A-61-500221. The former inorganic porous membrane consists of a porous support of heat resistant oxide having a pore size of from 150 to 500 nm integrally formed thereon with a sintered layer having an average pore size of from 20 to 120 nm, the sintered layer containing aluminum oxide particles amounts of 97% by weight and having particle sizes of less than 0.5 µm. However, there is not disclosed any definition concerning the composition of the porous support and sintered layer. With regard to the composition of the support, it is noted that an example of the support contains mullite of 8% matrix and aluminum oxide with an impurity of 0.5% by weight. In the support, however, a big problem will be caused in corrosion resistance. With regard to the composition of the sintered layer, it is noted that an impurity in the layer is 3% by weight maximum. However, the sintered layer as well as the support has a big problem in corrosion resistance and in dissolution of the impurity. With regard to the aluminum oxide forming the sintered layer, it is noted that α-alumina of less than 0.5 µm is preferably used for the layer. In general, however, α-alumina having a particle size of more than 0.2 µm has a specific surface area less than 10m 2 /g, and γ-alumina having a particle size of less than 0.2 µm has a specific surface area more than 10m 2 /g. Accordingly, γ-alumina having a particle size of less than 0.2 µm must be used to form a sintered layer having an average pore size of less than 0.1 µm. This will cause a problem in corrosion resistance of the sintered layer. The inorganic porous membrane disclosed in JP-A-61-500221 (see also WO-A-85/01937) is in the form of a tubular filter made of 99.9 % alumina by weight and having an average pore size of from 2 to 20 µm, the filter having formed thereon a filtration layer of titanum oxide having an average pore size of 0.2 µm. The publication does not disclose any porous membrane having an average pore size of less than 0.1 µm and superior in corrosion resistance. With the foregoing background, the problems to be solved by the present invention are summarized as follows. In general, pin holes and cracks in the thin layer of the multilayer inorganic porous membrane occur during the forming process of the thin layer. The porous thin layer is formed by coating a porous support by immersion in a sol liquid of fine particles, removal of the support from the sol liquid, drying in air, and firing. In such a process for forming the thin layer, the sol liquid permeates into pores of the support and is concentrated in the surface of the support. If the particles in the sol liquid is partly sucked into the support, pin holes will occur in the thin layer. If the thin layer is partly thickened, cracks will occur by shrinking through drying and firing. The inventors have found the fact that the pin holes and cracks will occur if the particles in the sol liquid permeate easily into a portion of maximum pores in the porous support. It is, therefore, important to define the maximum pore size of the porous support in relation to the thin layer for preventing the forming of pin holes and cracks. In the prior art, however, the maximum pore size of the support has not been considered to prevent the forming of pin holes and cracks. The pin holes and cracks in the thin layer not only reduce precision of filtration but also cause the peeling off of the thin layer during washing with acid and alkali and steam sterilization. To improve the corrosion resistance of the multilayer inorganic porous membrane, it has been considered to enhance the purity of materials. However, an impurity is inevitably contained in the material during the production process, while it is very difficult to control the sintering condition of a high purity material. In the sintering process of the high purity material, it is required to contain a sintering agent and a sintering restrainer into the material for obtaining a desired mechanical strength of the porous membrane. For these reasons, it has been found that a specific composition should be determined to obtain an inorganic porous membrane superior in corrosion resistance and capable of minimizing dissolution of components therefrom. In a process for the production of an inorganic porous membrane having an average pore size of less than 0.1 µm, fine particles with a particle size of less than 0.2 µm or relatively large particles with a particle size of more than 0.2 µm are used as a material for the porous membrane. In a production process using the particles less than 0.2 µm, it is advantageous that spaces among the particles can be utilized to form pores in the membrane. It is, however, required to define a composition superior in corrosion resistance because of activity of the material itself. In a production process using the particles more than 0.2 µm, the material itself is relatively stable. In this process, however, spaces among the particles are shrunk by sintering to form pores in the membrane. It is, therefore, difficult to form a desired number of pores in the membrane. Moreover, such thermal shrinkage of the material will cause cracks in the membrane. EP-A-136937 discusses a process of manufacture of microfiltration elements comprising composite inorganic membranes, including the coating of a porous support with a sol of oxide of aluminium, titanium, zirconium or silicon obtained by hydrolysis of a precursor. Alumina is employed in the examples. EP-A-242208 describes manufacture of alumina and titania coatings on porous supports. In the case of titania, a titanium polymer solution is made by partial hydrolysis of alkoxide and peptization. It is stated that this solution may be concentrated, or may be used at its concentration as formed (10 gl -1 ). The support is coated with the sol, followed by drying and firing to obtain an inorganic composite membrane. EP-A-188950 describes examples of manufacture of composite inorganic membranes of zirconia and titania. In the case of titania, one example describes concentration of a colloidal suspension obtained by hydrolysis of a precursor. The coated sol is dried and fired. In the light of these above prior art, the present invention is particularly concerned to provide a process of making a porous inorganic membrane in which titanium oxide forms the thin layer, enabling formation of cracks or pinholes to be avoided or reduced. The invention provides a process for the production of an inorganic porous membrane on a porous support, as set out in claim 1. The thin layer may be made of titanium oxide of 99.5% purity. The porous membrane may be characterized in that the thin layer is formed to have an average pore size of less than 80 nm and a thickness of less than 2 µm and that the average pore size in the surface layer of the porous support formed on which the thin layer is formed is less than 0.1 µm. In practical embodiments of the present invention, the porous support may be in the form of a pipe, a flat plate or a honeycomb structure consisting of a sintered body made of ceramic such as alumina, zirconia or titania, glass such as boro-silicate glass, metal such as nickel or carbon. The porous support may be formed in a monolayer or multi layer structure. In a multilayer porous support, a porous layer is formed as part of the support in such a manner that an average pore size of the support becomes smaller successively. In this case, the porous layer is made of the same material as that of the support, for example, hydrophilic alumina, titania or similar compound. Some useful results obtainable with the present invention are summarized as follows: In a multilayer inorganic porous membrane wherein a maximum pore size of a porous support for the membrane is defined to be from 1 to 250 times an average pore size of a porous thin layer, it has been found that any pin holes and cracks does not occur in the thin layer. The average pore size of the thin layer was determined in accordance with use of the membrane and could be adjusted by the particle size of the raw material and in a process of the production of the porous membrane. The porous support may have one of two types of structure, that is, a monolayer porous support having an average pore size of Dav and a multilayer porous support consisting of a basic support having an average pore size of Dav and an additional support attached onto the one side surface of the basic support and having an average pore size of Dav' which is smaller than Dav. In the multilayer support, it is desirable that each layer of the support has the same composition and the same or nearly same coefficient of thermal expansion. The average pore sizes of the porous support Dav and Dav' each are determined to be larger than that of the thin layer dav, the preferable values of Dav and Dav' being varied depending on the value dav of the thin layer attached thereto. In a porous membrane for ultrafiltration and gas separation, it is desirable that the average pore size (dav) of the thin layer is less than 0.1 µm. In the case that a support for the membrane is in the form of a monolayer structure, it is desirable that the average pore size (Dav) of the support is from 0.05 to 3 µm. In the case that a support for the membrane is in the form of a multilayer structure, it is desirable that the average pore size (Dav) of a basic support corresponding with the monolayer structure is from 0.1 to 30 µm and that the average pore size (Dav') of an additional support is determined to be less than 1 µm. Each minimum value of the average pore sizes (Dav, Dav') is determined in consideration of diffusion resistance of fluid, and each maximum value of them is determined in consideration of mechanical strength of the support and a formation property of the thin layer. In addition, the thin layer of the multilayer support can be thinner than that of the monolayer support. This is useful to reduce diffusion resistance of fluid. It is desirable that the porosity of the porous support is from 25 to 45% by volume. If the porosity is less than 25% by volume, there will occur a problem in adherence of the thin layer. If the porosity is more than 45% by volume, there will occur a problem in the mechanical strength of the support. It is also desirable in strength that the thickness of the thin layer is from 0.5 to 2 mm, and it is desirable in diffusion resistance that the thickness of the intermediate layer in the multilayer structure is from 10 to 150 µm. As described above, the respective maximum pore sizes (Dmax, Dmax') of the monolayer support and the multilayer support should be from 1 to 250 times the average pore size of the thin layer. This is very important to prevent the forming of pin holes and cracks in the porous membrane. In the multilayer support, the maximum pore size (Dmax) of the basic support layer should be determined larger than or equal to that (Dmax') of the additional support layer. As will be described later, the maximum pore size (Dmax, Dmax') can be measured by the well-known bubble point method. A preferable ratio between the maximum pore size (Dmax, Dmax') of the support and the average pore size (dav) of the thin layer is determined in accordance with the average pore size (dav). In the case that the average pore size (dav) of the thin layer is less than 0.1 µm, the average pore size ratio (Dmax/dav, D'max/dav) should be from 1 to 100. Such average pore size ratios are effective in eliminating the occurrence of pin holes and cracks in the thin layer and minimizing the thickness of the porous membrane thereby to remarkably improve the precision and efficiency in filtration and separation. In the inorganic porous membrane of the present invention, it is desirable to define each relationship between the maximum pore sizes (Dmax, Dmax') of the support and the thickness of the thin layer (t) and between the average pore sizes (Dav, Dav') of the support and the average pore size (dav) of the thin layer. A minimum ratio (t/Dmax, t/Dmax') between the maximum pore size of the support and the thickness of the thin layer is effective to prevent the occurrence of pin holes, and a maximum ratio between them is effective to prevent the forming of cracks. It is, therefore, desirable that the ratio (t/Dmax, t/Dmax') is from 1 to 5. In a process of successively forming a thin layer onto the support, it is preferable to take into consideration a maximum pore size of the thin layer formed as an intermediate layer. A ratio (Dav/dav, Dav'/dav) between the average pore sizes of the support and the thin layer affects the adherence of the thin layer to the support. It is desirable that the average pore size ratio (Dav/dav, Dav'/dav) is from 1 to 200. In the case that the average pore size (dav) of the thin layer is less than 0.1 µm, the average pore size ratio should be from 1 to 50. The thin layer is formed onto the porous support and then stabilized by heat treatment such as firing. To enhance the corrosion resistance of the porous thin layer and eliminate dissolution of the components from the thin layer, a hydrosol containing titanium hydroxide or titanium oxide is prepared. The hydrosol is prepared by hydrolysis of a precursor forming titanium hydroxide or titanium oxide under the presence of acid. The precursor can be selected from an organic titanate compound of alkoxide, acylate, chelate and the like, titanium salt of titanium tetrachloride, titanyl sulfate and the like, and meta and ortho titanoxide or titania. Particularly, the organic titanate compound is suitable for the precursor because of purity and handling thereof. The acid is selected from nitric acid, hydrochloric acid, acetic acid, lower fatty acid and the like, and the addition amount of the acid is from 0.01 to 0.5 mol/l. Preferably, when nitric acid or hydrochloric acid is used as the acid, the addition amount of the acid is from 0.05 to 0.15 mol/l, and pH of the hydrosol is from 0.5 to 2. If the addition amount of the acid is out of the suitable range, particles of the sol will coagulate and precipitate. Or the particles of the sol becomes too fine. In this instance, a first important point is that a molar ratio of the precursor to water is adjusted to be more than 1/200, preferably from 1/200 to 1/40. With such an adjustment, the precursor is properly hydrolyzed in the presence of the acid. If the molar ratio is more than 1/40, the sol will tend to become gel. A second important point is to control conditions of the hydrolysis. For this purpose, the precursor is first entered into hot water of temperaturs of above 50°C for ageing for at least ten minutes. Thereafter, the acid is added to the precursor at temperatures above 70°C for ageing for at least thirty minutes. During such a process, white suspension of the precursor is prepared when entered into the hot water and, it becomes finally a hydrosol of translucent cream color. The particles of the hydrosol have a particle size of from 3 to 100 nm which can be adjusted by the temperature and time of hydrolysis. Finally, the hydrosol is diluted with water to obtain a hydrosol suitable for coating. The concentration of the coating hydrosol is adjusted to a molar ratio of less than 1/500. If the molar ratio of the coating hydrosol is adjusted to more than 1/500, pin holes and cracks will occur in the process of forming the porous thin layer. A preferable range of the molar ratio is from 1/800 to 1/2000, whereby a uniform thin layer of thickness less than 2 µm is obtained by one or two coating processes. When the molar ratio is lower than the above-described range, the thickness of a layer formed by one coating process is so thin that the coating process should be repeated several times to obtain a uniform thin layer of a desired thickness. In the case that titanium salt is used as the precursor, the titanium salt is first neutralized to remove salt of impurity and is subsequently hydrolyzed under the presence of acid to obtain a hydrosol which is diluted with water to prepare a hydrosol for coating. In the case that particles of titania are used as the precursor, an anatase having a specific surface area of more than 50m 2 /g is used to facilitate hydrolysis for obtaining a hydrosol. The hydrosol is coated onto at least one surface of the porous support and fired after drying to form a porous thin layer on the support. The drying is preferably carried out in air at about 100°C for at least two hours, and the firing is carried out at temperatures of from 300 to 700°C. Thus, a porous thin layer having an average pore size of less than 80 nm and superior in corrosion resistance is obtained without any pin holes and cracks therein. If the firing temperature was below 300°C, desired corrosion resistance would not be obtained due to insufficient adherence of the thin layer onto the support surface. If the firing temperature exceeded 700°C, an average pore size of the thin layer would become more than 80 nm. Hereinafter, practical embodiments of the present invention will be described in detail with reference to Tables 1 to 3 attached thereto. Examples 1) Preparation of Porous Supports Two kinds of porous supports I and II were provided in the form of a multilayer structure having a tubular basic support and a thin additional support. To prepare the basic support, a kneading body was extruded to form a pipe body having an outer diameter of 10mm, an inner diameter of 7mm and a length of 150mm. After drying, the pipe body was fired at 1500°C for three hours to obtain a monolayer porous support. The monolayer porous support was coated at its external surface with a slurry. After drying, the coated porous support was fired at 1300°C to obtain a mutilayer porous support. Thus, an additional support layer was newly formed on the basic support to have a thickness of 30 micron. The basic support was made of α-alumina of 99.92% purity, while the additional support was made of α-alumina of more than 99.99% purity. Porous support I: average pore size of the basic support was 3 µm, an average pore size of the additional support being 80 nm, and maximum pore size Dmax' 0.4 µm. Porous support II: average pore size of the basic support was 3 µm, average pore size of the additional support was 0.2 µm, and a maximum pore size Dmax' was 1.5 µm. 2) Preparation of hydrosol before dilution Titanium isopropoxide or titanium tetrachloride as the precursor was hydrolyzed at 80°C for two hours under the presence of hydrochloric acid (added 0.1 mol/l) to obtain a hydrosol before dilution as listed on Table 1 attached hereto. In the case that titanium tetrachloride was used as the precursor, the titanium tetrachloride was first hydrolyzed with ammonia water to obtain white precipitate. The white precipitate was washed with water until chlorine ion was not detected, and then hydrolyzed as described above. 3) Preparation of coating sol The hydrosol prepared as described above was diluted to obtain a hydrosol for coating as listed on Table 2 attached hereto. 4) Preparation of porous membranes The multilayer porous support was coated at its external surface with the coating sol. After drying at room temperature for two hours and then at 100°C for two hours, the coated porous support was fired at various temperatures for three hours to form a porous thin layer. The average pore size of the thin layer was adjusted by the firing temperature. 5) Measurement of membrane characteristics of the porous membranes With respect to the multilayer porous membrane prepared as described above, the following tests were made to obtain test results shown in Table 3. In Table 3, porous membranes nos. 3, 7, 9 and 12 are comparative examples. Crossflow filtration: A solution containing a marker at 100ppm was circulated through a bore of the porous membrane at a speed of 2.5m/sec and at an inlet pressure of 3kg/cm 2 to analyze permeate flux of the membrane thereby to calculate a rejection efficiency of the marker. As the marker, bovine serum albumin (average molecular weight of 65000) was used for the thin layer with an average pore size of 3.5 or 5 nm, while uniform latex (particle diameter of 0.8 µm) was used for the thin layer with an average pore size of 68 or 85 nm. Corrosion resistance: The porous membranes were immersed in a solution of HCl (pH = 0) at 90°C and a solution of NaoH (pH = 14) for 168 hours to observe the presence of pin holes and cracks in the thin layers by means of an electron microscope of the scanning type. Rejection efficiency was measured before and after the corrosion test. Peel-off of thin layers: Porous membranes after corrosion resistance test was set in an ultrasonic washer to observe peel-off of the porous membranes by ultrasonic waves. Pin holes and cracks: Pin holes and cracks were observed before and after the corrosion resistance test. 6) Consideration As is clearly seen from Table 3, a porous membrane No. 9 with a thin layer having an average pore size of more than 80 nm, Nos. 3 and 12 with a thin layer having a thickness of more than 2 µm, and No. 14 with a thin layer of 0.5 µm thickness and average pore diameter 5 nm coated onto a porous support having an average pore size of more than 0.1 µm, all these membranes have lower values of membrane characteristics than desired. Other porous membranes have high values of membrane characteristics as desired except No. 7 which was fired at a very low temperature. In other words, to minimize the thickness of porous membranes with a thin layer having an average pore size dav of less than 0.1 µm, it is necessary that Dmax'/dav is determined within a range of from 1 to 100 and t/Dmax' is determined within a range of from 1 to 5. This is imporant especially in the case of oxides such as TiO 2 which is less flexible and more apt to sinter than γ-Al 2 O 3 . It was also confirmed that ion permeated from the thin layer was not detected when porous membranes had high values of other membrane characteristics as desired. In preparation of porous membranes, porous membranes with desired membrane characteristics could not be obtained when the molar ratio of precursor to water in hydrosol is less than 1/200 (No. 12) or the molar ratio after dilution is more than 1/500 (No. 3). In contrast, porous membranes with all the desired membrane characteristics could be obtained when the molar ratio in hydrosol is more than 1/200 and the molar ratio after dilution is less than 1/500 except No. 7. A more preferable range of the molar ratio is from 1/200 to 1/40 in the hydrosol and from 1/800 to 1/2000 after dilution. When the molar ratio in hydrosol is 1/30 (No. 1), the hydrosol should be handled with care due to rapid gelation thereof. When the molar ratio after dilution is 1/3000 (No. 11), coating should be repeated several times. Appropriate temperature for firing thin layers is within a range of from 300 to 700°C. At a lower firing temperature (No. 7), the porous membrane is low in corrosion resistance, while the membrane is low in rejection efficiency at a higher firing temperature (No. 9).

Claims: A process for the production of an inorganic porous membrane on a porous support, comprising the steps of: (i) forming a hydrosol containing titanium hydroxide or titanium oxide, by hydrolysing a precursor forming the hydrosol in the presence of acid at a molar ratio of the precursor to water of more than 1/200; (ii) coating at least one surface of said porous support by immersion thereof in said hydrosol, so as to form a porous thin layer on said support; (iii) drying the hydrosol coated porous support; and (iv) firing the hydrosol coated porous support in the temperature range 300°-700°C, characterised in that the molar ratio of titanium compound contained in the hydrosol to water is reduced to less than 1/500 by dilution prior to step (ii). The process of claim 1, wherein said hydrosol coated porous support is fired at a temperature in the range 400°-700°C. The process of claim 1 or 2, wherein the molar ratio of the precursor to water is 1/200-1/40 and the molar ratio of titanium compound contained in the hydrosol to water is diluted to 1/800-1/2000.

Inventors: ABE FUMIO, MORI HIROSHI, TAKAGI SHIGEKAZU, ABE, FUMIO, MORI, HIROSHI, TAKAGI, SHIGEKAZU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1989
Main classification: B01D71
IPC codes: B01D71, C04B41