# Patent

## Title
Ceramic porous membrane and ceramic filter

## Abstract

There are disclosed a ceramic porous membrane formed with less membrane formation times and having less defects, a small and uniform thickness and a high flux, and a ceramic filter. A silica membrane is formed on a titania UF membrane as an ultrafiltration membrane (a UF membrane) formed on a porous base member which is a microfiltration membrane (also referred to as an MF membrane) and having an average pore diameter smaller than that of the porous base member, and the silica membrane has an average pore diameter smaller than that of the titania UF membrane, and does not substantially permeates the titania UF membrane.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. application Ser. No. 11/850,693, filed Sep. 6, 2007, the entirety of which is incorporated herein by reference.


FIELD OF THE INVENTION
The present invention relates to a ceramic porous membrane and a ceramic filter. More particularly, it relates to a ceramic porous membrane having less defects and having a small and uniform thickness, and a ceramic filter.
BACKGROUND OF THE INVENTION
Heretofore, various methods of forming a ceramic porous membrane on a porous base member have been known. For example, a hot coating process is known (see Non-Patent Document 1). This is a method of rubbing a tube base member with cloth containing a silica sol to apply the silica sol and thereby form a porous membrane on an outer surface of the heated tube base member.
A method of forming a porous membrane on an inner surface of a porous base member having a tubular shape or a cylindrical lotus-root-like monolith shape by filtering membrane formation is also known (see Patent Document 1). The outer surface of the porous base member is held at a pressure lower than that of an inner surface thereof which comes in contact with a sol liquid to form the membrane on the inner surface of the porous base member.

    Patent Document 1: Japanese Patent Application Laid-Open No. 3-267129
    Patent Document 2: Japanese Patent Application Laid-Open No. 61-238315
    Non-Patent Document 1: Tsuru et al., “Silica-Zirconia Membranes for Nanofiltration,” Journal of Membrane Science, 149 (1988), pp. 127 to 135


However, the hot coating process has a problem that the membrane cannot uniformly be formed on the whole base surface, and the membrane can be formed on the only outer surface of the tube base member. The process cannot be applied to any monolith-type base. On the other hand, in the filtering membrane formation process, during drying of the formed membrane, a solvent present in base pores sometimes flows out on a membrane side to cause membrane peeling. As a result, there is a problem that a defect is generated in the porous membrane formed on the fired base surface. A dip coating process can be applied to the monolith type base, but the number of membrane formation times is large.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a ceramic porous membrane formed with less membrane formation times and having less defects, a small and uniform thickness and a high flux, and a ceramic filter.
The present inventors have found that the above-mentioned object can be achieved using a constitution where the ceramic porous membrane which does not substantially permeate pores of the ultrafiltration membrane is formed on the ultrafiltration membrane. That is, according to the present invention, the following ceramic porous membrane and ceramic filter are provided.
[1] A ceramic porous membrane which is formed on an ultrafiltration membrane having an average pore diameter of 2 to 20 nm and which does not substantially permeate pores of the ultrafiltration membrane.
[2] The ceramic porous membrane according to the above [1], which is formed on an intermediate layer having an average pore diameter of 1 to 5 nm on at least a surface that comes in contact with the ultrafiltration membrane having the average pore diameter of 2 to 20 nm, and which does not substantially permeate the pores of the ultrafiltration membrane.
[3] The ceramic porous membrane according to the above [1] or [2], wherein the ultrafiltration membrane is a titania membrane.
[4] The ceramic porous membrane according to any one of the above [1] to [3], which is a silica membrane.
[5] A ceramic filter comprising: a porous base member; an ultrafiltration membrane formed on the porous base member and having an average pore diameter of 2 to 20 nm; and a ceramic porous membrane which is formed on the ultrafiltration membrane and which does not substantially permeate pores of the ultrafiltration membrane.
[6] The ceramic filter according to the above [5], which has an intermediate layer having an average pore diameter of 1 to 5 nm on at least a surface that comes in contact with the ultrafiltration membrane between the ultrafiltration membrane and the ceramic porous membrane.
[7] The ceramic filter according to the above [5] or [6], wherein the ultrafiltration membrane is a titania membrane.
[8] The ceramic filter according to any one of the above [5] to [7], wherein the ceramic porous membrane is a silica membrane.
In a case where a constitution in which the ceramic porous membrane is formed on the ultrafiltration membrane is employed, the membrane is formed on the ultrafiltration membrane having smoothness, and hence the thin ceramic porous membrane having less defects can be formed. In a case where a structure in which the ceramic porous membrane does not infiltrate the ultrafiltration membrane is used, a ceramic sol forming the ceramic porous membrane is not consumed for the infiltration, and can securely be formed into a membrane, so that the number of membrane formation times can be reduced. Since pressure losses in an ultrafiltration membrane portion are not increased, the ceramic porous membrane having a high flux can be prepared. When the titania membrane is employed as the ultrafiltration membrane, titania has a high durability against acid, alkali and water vapor as compared with another ceramic membrane. That is, the silica membrane having high separability, high flux and high durability can be prepared with reduced costs. Furthermore, since the titania membrane is formed on the porous base member and the silica membrane is formed on the titania membrane, the ceramic filter having high separability, high flux and high durability can be manufactured with reduced costs. When the ceramic porous membrane is the silica membrane, the membrane is suitable for an application of dehydration of alcohol such as ethanol or isopropyl alcohol or an organic acid such as acetic acid.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of a ceramic filter according to one embodiment of the present invention;
FIG. 2 is a perspective view showing a ceramic filter according to one embodiment of the present invention;
FIGS. 3(a)(b) are schematic diagrams schematically showing one example of a method of manufacturing a silica membrane of the ceramic filter according to the present invention;
FIGS. 4(a)(b)(c) are explanatory views of a silica membrane in a case where a titania UF membrane is formed;
FIGS. 5(a) to 5(e) are explanatory views of a silica membrane in a case where any titania UF membrane is not formed; and
FIG. 6 is a diagram showing a flux with respect to a separation coefficient.



DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will hereinafter be described with reference to the drawings. The present invention is not limited to the following embodiment, can be changed, modified or improved without departing from the scope of the present invention.
FIG. 1 shows a silica membrane 1 which is a ceramic porous membrane of the present invention. The silica membrane 1 is formed on a titania UF membrane 14 which is an ultrafiltration membrane (also referred to as the UF membrane) formed on a porous base member 11 as a microfiltration membrane (also referred to as the MF membrane) and having an average pore diameter smaller than that of the porous base member 11. The silica membrane does not substantially permeate the titania UF membrane 14. Here, in a case where the silica membrane does not substantially permeate the titania UF membrane 14, it is indicated according to EDX element analysis that a portion having a silica/titania oxide weight ratio of 0.2 or less has a thickness of 1⁄2 or more of that of the UF membrane from the lowermost surface (an interface between UF and MF) of the UF membrane. It is assumed that the silica/titania oxide weight ratio is an average value of ten measurements of spot analysis based on the EDX element analysis.
It is preferable that the porous base member 11 is the microfiltration membrane (the MF membrane) having pore diameters of about 0.1 to 0.6 μm at an outermost layer.
Moreover, the titania UF membrane 14 which is an ultrafiltration membrane having pore diameters of about 2 to 20 nm (preferably about 8 nm) is formed on the microfiltration membrane (the MF membrane) 11, and the silica membrane 1 is formed on the titania UF membrane 14. It is assumed that the silica membrane 1 has a multilayered structure in which a silica sol is laminated a plurality of times and the silica membrane 1 does not substantially permeate the titania UF membrane 14.
In a case where the silica membrane 1 is formed on the titania UF membrane 14 having pore diameters of about 2 to 20 nm as described above, when a membrane surface of the titania UF membrane 14 is smooth and has less defects, the silica membrane 1 can be formed to be thin without any defect. That is, the silica membrane 1 having a high separability and a high flux (a transmitted and filtered flux) can be prepared with reduced costs.
On the other hand, when the silica membrane 1 is formed on titania having pore diameters of 20 nm or more, owing to the unevenness of the surface, a silica layer constitutes a thick membrane in order to cover the whole surface with the silica membrane 1, thereby resulting in a low flux. Owing to the unevenness of the surface, the silica membrane 1 becomes non-uniform, and defects such as cracks are easily generated. That is, a low separation performance is obtained. Furthermore, to prevent the generation of the cracks, an only thin membrane is formed once. The number of steps increases, and hence the costs increase.
In a case where the titania UF membrane 14 is used as a base member for the formation of the silica membrane 1 and the silica membrane 1 is formed on the titania UF membrane 14 to constitute a structure in which a predetermined amount of silica or more silica does not infiltrate the titania UF membrane 14, influences of the unevenness of the MF membrane are reduced, and the silica membrane 1 having less defects, that is, the silica membrane 1 having a high separability can be formed.
To form the structure in which the predetermined amount of silica or more silica does not infiltrate the titania UF membrane 14, one layer of the silica membrane may first be formed as an intermediate layer on the titania UF membrane 14 having pore diameters of about 2 to 20 μm by use of a silica sol having an average particle diameter in a range of about 1 to 50 nm. It is to be noted that the intermediate layer may be formed so that the average pore diameter has a range of 1 to 5 nm on at least a surface which comes in contact with the titania UF membrane 14 as the ultrafiltration membrane. Moreover, when the silica membrane 1 is further formed on the intermediate layer, the membrane does not permeate the titania UF membrane 14. After forming the titania UF membrane 14, the silica membrane 1 may be formed without being fired. Alternatively, after firing the titania UF membrane 14 at a firing temperature of the silica membrane 1 or less, the silica sol may be formed into a membrane, and the membrane may be fired. Alternatively, to form a first membrane of the silica sol, an organic binder such as PVA may be mixed and formed into the membrane, and then usual membrane formation by use of the silica sol (which does not contain any organic binder) may be performed.
Next, one embodiment of a ceramic filter 10 in which the silica membrane 1 is formed according to the present invention will be described with reference to FIG. 2. The ceramic filter 10 of the present invention has a monolith shape including a plurality of cells 23 defined by partition walls 22 to form channel passages in an axial direction. In the present embodiment, the cells 23 have a circular section, and the silica membrane 1 shown in FIG. 1 is formed on an inner wall surface of each of the cells. The cells 23 may be formed so as to have a hexagonal or quadrangular section. According to such a structure, for example, when a mixture (e.g., water and acetic acid) is introduced into the cells 23 from an inlet-side end surface 25, one of constituting elements of the mixture is separated at the silica membrane 1 formed on an inner wall of each cell 23, transmitted through the porous partition walls 22 and discharged from an outermost wall of the ceramic filter 10, so that the mixture can be separated. That is, the silica membrane 1 formed in the ceramic filter 10 can be used as a separation membrane, and has a high separation characteristic with respect to, for example, water and acetic acid.
The porous base member 11 which is a base member main body is formed as a columnar monolith-type filter element formed of a porous material by extrusion or the like. As the porous material, for example, alumina can be used, because this material has a resistance to corrosion, pore diameters of a filtering portion scarcely change even with a temperature change and a sufficient strength can be obtained. However, instead of alumina, a ceramic material such as cordierite, mullite or silicon carbide may be used.
Since the silica membrane 1 of the present invention is formed on an inner peripheral surface (the inner wall surface) of the porous base member 11, a comparatively long cylindrical base having a length of 50 cm or more, or a porous base member having a lotus-root-like shape can preferably be used.
Moreover, the titania UF membrane 14 is formed on the porous base member 11, and the silica membrane 1 is formed on the titania UF membrane 14. That is, an ultrafiltration membrane (the UF membrane) is formed on at least a silica membrane 1 forming surface of the base member formed of the porous material. It is preferable to form, as the ultrafiltration membrane, a titania membrane which inhibits generation of particles or polymers in a range of 0.1 μm to 2 nm. It is assumed that an average pore diameter of the titania membrane is smaller than that of the porous material.
Next, a method of manufacturing the silica membrane 1 will be described with reference to FIGS. 3(a) and 3(b). First, a coating liquid (a silica sol liquid) 40 for forming the silica membrane 1 is prepared. To prepare the coating liquid 40, tetraethoxy silane is hydrolyzed in the presence of nitric acid to form a sol liquid, and the sol liquid is diluted with ethanol. The liquid may be diluted with water instead of ethanol.
Next, as shown in FIG. 3(a), an outer peripheral surface of the porous base member 11 provided with the titania UF membrane 14 is sealed with a masking tape 41. The porous base member 11 is fixed to, for example, a lower end of a wide-mouthed rotor (not shown), and the coating liquid (the silica sol liquid) 40 is passed through the cells 23 from an upper portion of the base member.
Subsequently, as shown in, for example, FIG. 3(b), cold air is sent into the cells with a drier or the like to dry the cells. Since the cells are dried with the cold air in this manner, the structure where the silica membrane 1 does not substantially permeate the titania UF membrane 14 can be obtained.
Subsequently, a temperature is raised at a ratio of 100° C./hr, retained at 500° C. for one hour, and then lowered at a ratio of 100° C./hr. Operations such as the passing of the coating liquid (the silica sol liquid) 40, drying, temperature raising and temperature lowering are repeated three to five times.
According to the above steps, the silica membrane 1 is formed on the titania UF membrane 14. That is, as shown in FIG. 4(b), the titania UF membrane 14 is formed on the porous base member 11 shown in FIG. 4(a). In consequence, influences of the unevenness of the surface of the porous base member are reduced by the titania UF membrane 14. Therefore, as shown in FIG. 4(c), the silica membrane can be formed as a thin membrane having less defects. That is, the silica membrane 1 having a high flux and a high separability can be formed with reduced costs.
On the other hand, in a case where the silica membrane 1 is directly formed on the surface of the porous base member 11 shown in FIG. 5(a), even when a silica membrane 1a is formed as shown in FIG. 5(b), the whole surface cannot be covered, and cracks are easily generated in the silica membrane 1 owing to unevenness.
As shown in FIGS. 5(c) to 5(e), when silica membranes 1b, 1c and 1d are superimposed to form a thick membrane, the silica membrane 1 can be flattened, but in this case, a low flux results. Since the number of the steps increases, the costs increase.
The ceramic filter 10 obtained as described above and including the nano-level thin-membrane-like silica membrane 1 formed on the inner wall surface thereof can preferably be used as a filter which separates a mixed liquid or the like. It is to be noted that when the cells 23 are submerged into acetic acid or acetic acid is passed through the cells, a separation coefficient can be improved. In the above embodiment, the case where the silica membrane is formed as the ceramic porous membrane has been described, but the present invention is not limited to this embodiment, and a titania membrane, a zirconia membrane, a zeolite membrane or the like may be formed.
EXAMPLES
A manufacturing method of the present invention will hereinafter be described in accordance with examples in more detail, but the present invention is not limited to these examples. First, a porous base member, a ceramic sol liquid, a membrane forming method and the like used in the present example will be described.
Example 1
(1) Porous Base Member
A material provided with an alumina membrane having an average pore diameter of 0.2 μm and having a monolith shape (an outer diameter of 30 mm, a cell inner diameter 3 mm×37 cells and a length of 500 mm) was used as a base member. It is to be noted that opposite end portions of the base member were sealed with glass. The average pore diameter of the base member was measured based on an air flow process described in ASTM F306.
(2) Titania Sol Liquid
Titanium isopropoxide was hydrolyzed in the presence of nitric acid to obtain a titania sol liquid. A sol particle diameter measured by a dynamic optical scattering process was 100 nm.
(3) Titania UF Membrane Formation
The titania sol liquid was diluted with water to obtain a sol liquid for membrane formation. The liquid was circulated through base cells to come in contact with the cells, whereby the membrane was formed in the cells.
(4) Drying, Firing
After a sample was dried, the sample was thermally treated at 500° C. This sample was used as a titania UF base provided with the titania UF membrane. When pore diameters of the titania UF base were measured, an average pore diameter was 8 nm. A measurement principle of the pore diameters is the same as that of the method described in Non-Patent Document 1, but in the Non-Patent Document 1, water vapor and nitrogen were used, whereas in the measurement method used in the present invention, n-hexane and nitrogen were used.
(5) First Silica Membrane
A silica sol liquid having an average particle diameter of 20 nm was circulated through titania UF base cells to come in contact with the cells, whereby the silica membrane was formed in the cells. An average pore diameter of the first silica membrane was 3 nm.
(6) Silica Sol Liquid
Tetraethoxy silane was hydrolyzed in the presence of nitric acid to obtain a silica sol liquid. The silica sol liquid was diluted with ethanol, and regulated into 0.7 wt % in terms of silica to prepare a sol liquid for membrane formation.
(7) Membrane Formation
An outer peripheral surface of the sample (the porous base member) was sealed with a masking tape. The porous base member was fixed to a lower end of a wide-mouthed rotor, and a silica sol liquid was passed through the cells from an upper portion of the base member. It was confirmed that the membrane was formed on the whole inner wall by this membrane formation step.
(8) Drying
The cells of the porous base member through which the silica sol was circulated were dried using a drier so that cold air passed through the cells.
(9) Firing
A temperature was raised at a ratio of 100° C./hr, retained at 500° C. for one hour and lowered at the ratio of 100° C./hr. It is to be noted that the operations of (6) to (8) were repeated three to five times to obtain Example 1.
Example 2
During titania UF preparation of (1) to (4) of Example 1, titania was not fired unlike (4), and used as a base member, and then silica membrane formation of (6) to (9) was performed.
Example 3
During titania UF preparation of (1) to (4) of Example 1, titania was fired at 300° C. in (4), and then silica membrane formation of (6) to (9) was performed.
Example 4
Titania UF of (1) to (4) of Example 1 was used as a base member, and then silica membrane formation of (6) to (9) was performed, but at the first silica membrane formation, the silica sol liquid (6) was mixed with a PVA solid content which was equal to a silica solid content. At second and third silica membrane formations, the membrane was formed using a usual silica sol liquid.
Comparative Example 1
A titania UF membrane was formed as a base member in the same manner as in (1) to (4) of Example 1, and subsequently silica membrane formation of (6) to (9) was performed.
Comparative Example 2
During titania UF preparation of (1) to (4) of Example 1, titania was fired at 700° C. in (4), and this membrane was formed as a titania UF base provided with a titania UF membrane. When pore diameters of the titania UF base were measured, an average pore diameter was 40 nm. This base was subjected to silica sol membrane formation of (6) to (9).
Comparative Example 3
In the membrane formation of Example 1, any titania UF membrane was not formed, and an alumina porous base member was directly subjected to silica sol membrane formation of (6) to (9).
In Example and Comparative Example, an infiltration depth into titania UF was measured. In Examples 1 to 4, a portion in which a silica/titania oxide weight ratio according to EDX element analysis was 0.2 or less reached 3⁄4 of a UF thickness from the lowermost surface (an interface between UF and MF) of the UF membrane. In Comparative Example 1, a portion in which a silica/titania oxide weight ratio according to EDX element analysis was 0.2 or less only reached 1/10 of a UF thickness from the lowermost surface (the interface between UF and MF) of the UF membrane. Therefore, Comparative Example 1 had a structure where silica infiltrated the titania UF.
In Comparative Examples 2, 3, when a membrane was formed using a membrane formation sol liquid having a silica concentration of 0.7 wt %, cracks were generated in the membrane surface, and any membrane could not be formed. Therefore, in Comparative Example 2, a membrane was formed with 0.3 wt %, but cracks were generated. Furthermore, a membrane was formed with 0.1 wt %, the cracks were reduced, and membranes were formed 20 times. In Comparative Example 3, the cracks were reduced at 0.3 wt %, and membranes were formed seven times. However, in either case, it was recognized that micro cracks were left in the membrane surface, and any sound membrane could not be formed.
In Examples 1 to 4 and Comparative Examples 1 to 3, a permeation evaporating separation test was conducted for two hours. In the test, 90% ethanol was circulated through monolith cells at 70° C. and a liquid flow rate of 10 L/min, and the outside of monolith was evacuated in a range of 2 to 10 Pa. The sampling was performed four times every 30 minutes. As results of the separation tests of Examples 1 to 4 and Comparative Examples 1 to 3, a relation between a separation coefficient and a flux is shown in FIG. 6.
Examples 1 to 4 indicated a high flux as compared with Comparative Example 1, and indicated a high a (separation coefficient) and a high flux as compared with Comparative Examples 2, 3. In Comparative Examples 2, 3, to form the membrane surface without any crack, a slurry having a small silica concentration as compared with the example needs to be used. As a result, the number of membrane formation times increases. When the number of the membrane formation times increases, steps lengthen, and costs increase.
As described above, when the titania UF membrane is formed on the MF membrane and the silica membrane is formed on the titania UF membrane, a silica dehydration membrane having a high performance can be obtained with reduced costs.
According to the present invention, a thin and uniform membrane having less coarse and large pores and less defects can be obtained with less membrane formation times. Therefore, a ceramic filter provided with such a silica membrane can preferably be used as a filter. A ceramic filter including a nano-level thin-membrane-like silica membrane formed on the inner wall surface thereof can be used in a portion where an organic filter cannot be used, for example, separation removal or the like in an acidic or alkaline solution or an organic solvent.
Description of Reference Numerals


    1: silica membrane
    10: ceramic filter
    11: porous base member
    14: titania UF membrane
    22: partition wall
    23: cell
    25: inlet-side end surface
    40: coating liquid (silica sol liquid)
    41: masking tape.

## Claims

1. A method for removing water from an aqueous mixture solution using a ceramic porous membrane, the method comprising:
providing a ceramic porous membrane that includes
forming an ultra-filtration membrane of titania having an average pore diameter of 2 to 20 nm;
forming an intermediate layer of silica on the ultra-filtration membrane, the intermediate silica layer having an average pore diameter of 1 to 5 nm on at least a surface in contact with the ultra-filtration membrane; and
forming the ceramic porous membrane of silica on the intermediate layer, wherein the ceramic porous membrane does not substantially permeate the ultra-filtration membrane pores; and

contacting the aqueous mixture solution with the ceramic porous membrane to remove water from the aqueous mixture solution.

2. The method of claim 1, wherein the aqueous mixture solution comprises water and an alcohol.

3. The method of claim 2, wherein the alcohol is an isopropyl alcohol.

4. The method of claim 1, wherein the aqueous mixture solution comprises water and an ethanol.

5. The method of claim 1, wherein the aqueous mixture solution comprises water and an organic acid.

6. The method of claim 1, wherein the aqueous mixture solution comprises water and an acetic acid.

7. The method of claim 1, wherein during formation of the ceramic porous membrane of silica, the method further comprises blowing cold air across the silica forming the ceramic porous membrane to dry the silica forming the ceramic porous membrane onto the intermediate layer without the silica forming the ceramic porous membrane substantially permeating the ultra-filtration membrane pores and then sintering the ceramic porous membrane by raising a temperature at a rate of 100° C/hr, remaining at a temperature of 500° C. for one hour and then lowering the temperature at a rate of 100° C/hr.