Method for manufacturing gels and gel composites

Abstract

The present invention addresses the problem of providing a novel and simple method for removing the water content from a gel. The present invention provides a method for producing a gel, the method being characterized by having a gel dehydration step in which a hydrous gel is immersed in a nonaqueous solvent in the presence of a dehydration agent.

Description

Technical Field

[0001] The present invention relates to a method for producing a gel and a gel composite.

Background Art

[0002] A gel generally refers to a solidified state in which solutes have lost their independent mobility due to interactions and aggregated. A typical gel is one in which polymers are crosslinked to form a network structure. Here, the gel has a dispersion medium in its structure, and when water is contained in the dispersion medium, there may be a requirement to remove the water. The same applies to gel composites. For example, the solvent replacement step in the production process of aerogel is such a case. Aerogel is a porous material obtained, for example, through supercritical drying after producing a wet gel by the sol-gel method. Here, in the production process of aerogel, in order to suppress gel shrinkage during drying, a solvent replacement step for removing the water contained in the water-containing gel is carried out multiple times.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0006] Furthermore, the present invention relates to a method for producing a composite in which a gel is filled into a porous substrate, characterized by comprising a dehydration step of immersing a water-containing composite in which a gel is filled into a porous substrate in a non-aqueous solvent in the presence of a dehydrating agent. [Effects of the Invention]

[0007] According to the present invention, a novel and simple method for removing water contained in a gel or gel composite can be provided.

[0008] Furthermore, in the case of an aerogel, conventional methods require multiple water removal steps (solvent replacement steps) to treat the water in the gel (for example, water generated during the sol-gel reaction and water originally present in the solvent). As a result, a large amount of waste solvent is generated at each step, leading to poor work efficiency and a large environmental burden. On the other hand, the present invention makes it possible to provide a method for producing aerogels and aerogel composites that can suppress the generation of large amounts of waste solvent. [Modes for carrying out the invention]

[0009] The method for producing a gel according to this embodiment includes a gel dehydration step of immersing a water-containing gel in a non-aqueous solvent in the presence of a dehydrating agent. Furthermore, the method for producing a composite (a composite in which a gel is packed into a porous substrate) according to this embodiment includes a dehydration step of immersing a water-containing composite in which a gel is packed into a porous substrate into a non-aqueous solvent in the presence of a dehydrating agent. Here, "non-aqueous solvent" refers to a solvent other than water. Note that the non-aqueous solvent may contain trace amounts of water. Furthermore, "gel" is not particularly limited as long as it has a structure in which solutes have aggregated due to interactions and have solidified, and is not limited to inorganic gels, organic polymer gels, physical gels, chemical gels, dry gels {e.g., xerogels, aerogels (e.g., silica aerogels, carbon aerogels, alumina aerogels, polymer aerogels)}, or wet gels. Moreover, "water-containing gel" is not particularly limited as long as it is a gel containing water. Similarly, "water-containing composite" is not particularly limited as long as it is a composite containing water, for example, a gel and / or a porous substrate containing water. The present invention will be described in detail below, using the methods for producing silica aerogel and silica aerogel composite as examples of methods for producing gels and composites. However, the present invention is not limited to the following examples.

[0010] ≪Method for manufacturing silica aerogel≫ A method for producing silica aerogel, which is an example of this embodiment, comprises: a sol preparation step for preparing a sol solution; a gel preparation step for gelling the sol solution to prepare a wet gel; a solvent replacement step (corresponding to the "gel dehydration step" described above) for replacing the water in the wet gel with a non-aqueous solvent in the presence of a dehydrating agent after the gel preparation step; and a drying step for drying the wet gel to prepare an aerogel after the solvent replacement step. Each step will be described in order below.

[0011] <Solu preparation process> The sol preparation process involves adding various raw materials, including silica raw material (main component), to a predetermined solvent and stirring to mix them, thereby preparing a sol solution. The raw materials are described in detail below.

[0012] (Main ingredient) The main agent is not particularly limited as long as it is a component that serves as a raw material for silica aerogel. For example, alkali metal silicate salts and alkoxysilanes can be mentioned. These components will be described in detail below.

[0013] Examples of alkali metal silicate salts include potassium silicate and sodium silicate. For example, sodium silicate is represented by the molecular formula Na2O·nSiO2·mH2O. The coefficient n is the molar ratio of SiO2·Na2O, and the coefficient m is the molar ratio of H2O to Na2O. The relationship between the weight ratio and the molar ratio of the SiO2 and Na2O components is shown by the following formula 1. (Formula 1) Molar ratio = (a / b) × 1.032

[0014] Here, a is the mass of SiO2, and b is the mass of Na2O. Also, the constant 1.032 is the ratio of the molecular weight of SiO2 to the molecular weight of Na2O. Generally, the molar ratio (n value) of commercially produced sodium silicate is 0.5 to 5.0. Sodium silicate only needs to have a structure of Na2O·nSiO2, and the n value is not particularly limited. Therefore, the n value of sodium silicate may be outside the range of 0.5 to 5.0 that is not generally produced, but 0.5 to 5.0 is preferred because it is easily obtainable. Sodium silicate can be dissolved in water, for example, before being mixed with other raw materials and used as an aqueous sodium silicate solution. In that case, when the n value is less than 1, it is crystalline and not easily soluble in water, so 1.0 to 5.0, which is easily soluble in water, is more preferred.

[0015] Furthermore, the alkoxysilane is not particularly limited, and difunctional, trifunctional, or tetrafunctional alkoxysilanes can be used individually or in combination. Examples of difunctional alkoxysilanes include dimethyldimethoxysilane, dimethyldiethoxysilane, diphenyldiethoxysilane, diphenyldimethoxysilane, methylphenyldiethoxysilane, methylphenyldimethoxysilane, diethyldiethoxysilane, and diethyldimethoxysilane. Examples of trifunctional alkoxysilanes include methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane, and phenyltriethoxysilane. Examples of tetrafunctional alkoxysilanes include tetramethoxysilane and tetraethoxysilane. In addition, bistrimethylsilylmethane, bistrimethylsilylethane, bistrimethylsilylhexane, vinyltrimethoxysilane, etc. can also be used as alkoxysilanes. Furthermore, partial hydrolysates of alkoxysilanes may be used as raw materials.

[0016] (solvent) Examples of solvents include water, alcohols (methanol, ethanol, isopropanol, tert-butanol, etc.), aprotic polar organic solvents (N,N-dimethylformamide, dimethyl sulfoxide, N,N-dimethylacetamide, etc.), hydrocarbons (n-hexane, heptane, etc.), fluorine-containing solvents (2H,3H-decafluoropentane, 1,1,2,2,3,3,4-heptafluorocyclopentane, etc.), and mixtures thereof. When alkoxysilane is used as the main agent, the hydrolysis and polymerization of the alkoxysilane are preferably carried out in the presence of water, and more preferably using a mixture of water and an organic solvent that is compatible with water and dissolves the alkoxysilane (this allows the hydrolysis and polymerization steps to be carried out consecutively). Here, the solvent that is compatible with water and dissolves the alkoxysilane is not particularly limited, and examples include alcohols such as methanol, ethanol, n-propanol, 2-propanol, n-butanol, 2-butanol, and t-butanol, as well as acetone and N,N-dimethylformamide. One of these may be used alone, or two or more may be used in combination.

[0017] (Surfactants) Surfactants may be added as various raw materials other than the main component. Surfactants contribute to the formation of the bulk portion and pore portion that constitute the aerogel in the gel preparation process described later. Examples of surfactants include nonionic surfactants and ionic surfactants (e.g., cationic surfactants, anionic surfactants, and zwionic surfactants).

[0018] <Gel preparation process> The gel preparation step involves adding a catalyst to the sol solution obtained in the sol preparation step described above to obtain a wet gel. Specific examples of basic catalysts include ammonia; tetraalkylammonium hydroxides such as tetramethylammonium hydroxide (TMAH); amines such as trimethylamine; alkali hydroxides such as sodium hydroxide; alkali metal carbonates such as sodium carbonate and sodium bicarbonate; and alkali metal silicates. Examples of acidic catalysts include hydrochloric acid, citric acid, nitric acid, sulfuric acid, and ammonium fluoride. Of these, ammonia, tetraalkylammonium hydroxides, or amines are preferred, with ammonia being particularly preferred, because they do not contain metal elements and do not require washing. However, the use of a catalyst is not essential; hydrolysis or condensation polymerization may be carried out by methods such as heating to high temperatures. Alternatively, the catalyst may be added in the sol preparation step (in this case, the process transitions from the sol preparation step to the gel preparation step, so the two steps together may be referred to as the "sol preparation step / gel preparation step").

[0019] Furthermore, the gel preparation step may include a step of forming a W / O emulsion in order to prepare a spherical aerogel. Specifically, this step is to disperse an aqueous sol solution in a hydrophobic solvent to form a W / O emulsion. That is, the aqueous sol solution is used as the dispersion phase and the hydrophobic solvent as the dispersion medium to form an emulsion. By forming such a W / O emulsion, the sol solution, which is the dispersed phase, becomes spherical due to surface tension, etc. In this state, a spherical gel can be obtained by gelling the sol solution dispersed in the hydrophobic solvent in this spherical shape. Here, it is preferable to add a surfactant when forming the W / O emulsion. Any of anionic surfactants, cationic surfactants, or nonionic surfactants can be used.

[0020] <Solvent replacement process> The solvent replacement step is a step of replacing water (or water and an organic solvent) on the surface and inside of a gel with a non-aqueous solvent in order to suppress shrinkage of the gel (wet gel) in the drying step performed after the solvent replacement step (or after the hydrophobization step). Here, the non-aqueous solvent is a solvent other than water, for example, preferably a polar solvent, and more preferably a solvent that is compatible with water. Specific examples include methanol, ethanol, n-propanol, isopropanol, 1-butanol, acetone, methyl ethyl ketone, methyl isobutyl ketone, xylene, 1,2-dimethoxyethane, acetonitrile, hexane, toluene, diethyl ether, chloroform, ethyl acetate, tetrahydrofuran, methylene chloride, N,N-dimethylformamide, dimethyl sulfoxide, fluorinated solvents such as perfluorohexane, perfluorooctane, methyl nonafluorobutyl ether, and the like. The non-aqueous solvent may be used alone or in combination of two or more kinds.

[0021] Here, the solvent replacement step may be performed once or multiple times. From the viewpoint of environmental impact, workability, and cost reduction, one solvent replacement is preferable. If multiple solvent replacements are performed, different solvents may be used in each replacement. For example, an organic solvent with a surface tension of 45 mN / m or less at 20°C may be used as the solvent immediately before the drying step. Examples of such organic solvents include dimethyl sulfoxide (43.5 mN / m), cyclohexane (25.2 mN / m), isopropanol (21 mN / m), heptane (20.2 mN / m), pentane (15.5 mN / m), ethanol (22.4 mN / m), methanol (22.6 mN / m), perfluorohexane (12 mN / m), perfluorooctane (15 mN / m), methyl nonafluorobutyl ether (13.6 mN / m), and other fluorine-based solvents. Furthermore, the amount of solvent used in each solvent replacement step is, for example, between 2 and 100 times the volume of the gel (wet gel). In addition, the solvent replacement method may be total replacement, partial replacement, or circulating replacement. Moreover, the immersion time of the gel in the solvent replacement step is not particularly limited and may be, for example, 1 to 72 hours, 6 to 60 hours, 12 to 48 hours, 18 to 45 hours, or 30 to 40 hours.

[0022] Here, in this embodiment, a dehydrating agent is used in any one or all of the one or more times. Here, the dehydrating agent is not particularly limited as long as it can dehydrate water from the solvent. For example, the dehydrating agent is a component capable of absorbing 0.01 g or more, 0.05 g or more, 0.1 g or more, 0.15 g or more, 0.2 g or more of water per 1 g of the dry mass of the dehydrating agent (the upper limit is not limited at all, for example, 1000 g or less, 100 g or less, 50 g or less, 10 g or less, 5 g or less, 2 g or less, 1 g or less). As the dehydrating agent, substances capable of chemically or physically dehydrating the moisture contained in the wet gel are used. More specific examples include at least one selected from zeolite, activated alumina, silica gel, calcium oxide, magnesium oxide, aluminum oxide, sodium sulfate, calcium sulfate, magnesium sulfate, potassium carbonate, sodium hydroxide, potassium hydroxide, and calcium chloride. Among these, zeolite is particularly suitable because it is easy to separate from the system (easy to remove) after dehydration completion, easy to reuse (can be regenerated by heat treatment), and does not affect the physical properties of the gel.

[0023] Here, zeolite capable of adsorbing molecules with an effective diameter of less than 1 nm is preferred, zeolite capable of adsorbing molecules with an effective diameter of less than 0.5 nm is more preferred, zeolite capable of adsorbing molecules with an effective diameter of less than 0.4 nm is further preferred, and zeolite capable of adsorbing molecules with an effective diameter of less than 0.3 nm is most preferred.

[0024] In addition, the shape of the dehydrating agent is not particularly limited, and examples include powder form, pellet form, spherical form, and cartridge form.

[0025] Furthermore, the amount of dehydrating agent used (dry mass) is preferably 5 times or more, 7.5 times or more, 10 times or more, 12.5 times or more, 15 times or more, 17.5 times or more, 20 times or more, 22.5 times or more, 25 times or more, 27.5 times or more, 30 times or more, 32.5 times or more, 35 times or more, 37.5 times or more, 40 times or more, 42.5 times or more, 45 times or more, 47.5 times or more, and 50 times or more, based on the amount of water (mass) contained in the gel. The upper limit is not particularly limited, for example, 1000 times or less, 500 times or less, and 100 times or less.

[0026] Furthermore, some of the dehydrating agents used in the solvent replacement process can be regenerated by drying them through heating or other means. Such dehydrating agents are preferably reused after regeneration. Similarly, non-aqueous solvents used in the solvent replacement process are also preferably reused.

[0027] <Hydrophobicization process> The method for producing silica aerogel according to this embodiment may include a hydrophobicization step. Here, the hydrophobicization step is a step in which the surface of the aerogel is hydrophobicized with a silylating agent or a functional silane in order to prevent dehydration condensation and shrinkage of hydroxyl groups present on the inner wall of the gel during drying. Here, examples of functional silanes include a group of silicon compounds composed of chlorosilane, alkoxysilane, and silazane.

[0028] <Drying process> The drying step involves drying the gel after the solvent replacement step (or after the hydrophobicization step, if the process includes the hydrophobicization step described above) to obtain a silica aerogel. The drying method is not particularly limited and can include supercritical drying, atmospheric pressure drying, freeze-drying, etc. Of these, the manufacturing method according to this embodiment is particularly suitable when using supercritical drying.

[0029] <Processing process> The method for producing silica aerogel according to this embodiment may further include a step of processing the silica aerogel obtained in the drying step into a different shape (for example, processing from a plate, rectangular parallelepiped, or film into various shapes such as particles, rectangular or circular plates, cubes, spheres, cylinders, pyramidal cones, or granular shapes).

[0030] ≪Method for manufacturing silica aerogel composite≫ The method for producing the silica aerogel composite of this embodiment comprises: a sol preparation step of preparing a sol solution; an impregnation step of impregnating a porous substrate with the sol solution; a gel composite preparation step of gelling the sol solution after the impregnation step to obtain a gel composite in which a wet gel is filled into the porous substrate; a solvent replacement step of replacing the water in the gel composite with a non-aqueous solvent in the presence of a dehydrating agent after the gel composite preparation step (corresponding to the "dehydration step" described above); and a drying step of drying the gel composite after the solvent replacement step to prepare a silica aerogel composite. Each step will be described in detail below.

[0031] <Solu preparation process> The sol preparation step is a process of preparing a sol solution by adding various raw materials, including a silica raw material (main component), to a predetermined solvent and stirring to mix them. The main component, solvent, and surfactants, which are examples of various raw materials, are as described in the method for producing silica aerosol.

[0032] <Impregnation process> The impregnation step is a step of impregnating a porous substrate with a sol solution. Vibration may be applied during this step. Here, "porous substrate" is a general term for materials that contain many pores (micropores) (e.g., microporous materials, mesoporous materials, macroporous materials). Examples of porous substrates include foamed substrates, substrates made of fibrous materials, and substrates that form a complex three-dimensional skeleton. More specific examples of porous substrates include nonwoven fabrics and porous sheets having a porous structure. Furthermore, the pores in the porous structure are preferably interconnected pores in order to sufficiently fill with aerogel. Here, the porous sheet is not particularly limited and can be selected from olefin resin foam, acrylic resin foam, urethane resin foam, vinyl acetate resin foam, vinyl chloride resin foam, epoxy resin foam, rubber foam, silicone resin foam, melamine resin foam, imide resin foam, etc. Furthermore, the nonwoven fabric is not particularly limited and can be selected from metal fibers such as stainless steel fibers and aluminum fibers, polyolefin resins such as polyethylene terephthalate (PET) resin, polyethylene, and polypropylene, polyvinyl chloride resin, acrylic resin, polyimide resin, organic fibers such as cellulose and nylon, and inorganic fibers such as glass fibers, carbon fibers, silica fibers, rock wool, and ceramic fibers.

[0033] <Gel Composite Preparation Process> The gel composite preparation step involves adding a catalyst to the sol solution obtained in the sol preparation step described above to obtain a wet gel composite in which the wet gel is filled into a porous substrate. Here, the gel composite preparation step is the same as described in the method for producing silica aerogel, except that a substrate is used. Also, as described in the method for producing silica aerogel, the catalyst may be added in advance in the sol preparation step.

[0034] <Solvent replacement process> The solvent replacement step is the same as described in the method for producing silica aerogel, except that a substrate is used. The amount of solvent used in each solvent replacement step is, for example, 2 to 1000 times the volume of the gel complex. Furthermore, the amount of dehydrating agent used (dry mass) is preferably 2.5 times or more, 3 times or more, 4 times or more, 5 times or more, 7.5 times or more, 10 times or more, 12.5 times or more, 15 times or more, 17.5 times or more, 20 times or more, 22.5 times or more, 25 times or more, 27.5 times or more, 30 times or more, 32.5 times or more, 35 times or more, 37.5 times or more, 40 times or more, 42.5 times or more, 45 times or more, 47.5 times or more, or 50 times or more, based on the amount of water (mass) contained in the gel complex. Furthermore, there is no particular upper limit; for example, it can be 1000 times or less, 500 times or less, 100 times or less, 50 times or less, 40 times or less, 30 times or less, 25 times or less, 20 times or less, 15 times or less, or 10 times or less. In addition, there is no particular limit to the immersion time of the gel composite in the solvent replacement step; for example, it can be 1 to 72 hours, 6 to 60 hours, 12 to 48 hours, 18 to 45 hours, or 30 to 40 hours.

[0035] <Hydrophobicization process> The method for producing the silica aerogel composite according to this embodiment may include a hydrophobicization step. The hydrophobicization step is the same as described in the method for producing silica aerogel, except that a substrate is used.

[0036] <Drying process> The drying step is a step in which the gel composite after the solvent replacement step (or after the hydrophobicization step, if the method includes the hydrophobicization step described above) is dried to obtain a silica aerogel composite. Here, the drying step is the same as described in the method for producing silica aerogel, except that a substrate is used.

[0037] <Processing process> The method for producing the silica aerogel composite according to this embodiment may further include a step of processing the shape of the silica aerogel composite obtained in the drying step into another shape. Here, the processing step is the same as the matter described in the method for producing silica aerogel, except that a substrate is used. [Examples]

[0038] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples. Below, a silica aerogel composite will be described in detail as an example.

[0039] <<Preparation example of silica aerogel composite according to Example 1>> <Solu preparation process> Tetramethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd.) is used as the main ingredient, and 33 moles of methanol (manufactured by Wako Pure Chemical Industries, Ltd.) and 14 moles of ion-exchanged water (electrical resistivity 1 × 10⁻¹⁶) are added per mole of the main ingredient. 10 A sol solution was prepared by mixing a Ω·cm or larger oz oz oz oz with 0.01 moles of catalyst {25% aqueous ammonia (manufactured by Wako Pure Chemical Industries, Ltd.)}.

[0040] <Impregnation process / Gel composite preparation process> The foam substrate described below was cut to a width of 210 mm and a length of 3 m, placed in a separable flask, and then the sol solution was added until the foam substrate was completely immersed (impregnation step). The mixture was left to stand under atmospheric pressure for 3 hours to obtain a foam substrate filled with a wet gel (gel composite preparation step).

[0041] (Method for preparing foamed substrate) Foamed base material: 58 parts by mass of random-type polypropylene, 15 parts by mass of low-density polyethylene, 20 parts by mass of EPDM (ethylene content 29.5%, diene content 5%), 1.5 parts by mass of polyoxyethylene stearylamine, 5 parts by mass of wet silica, and 0.2 parts by mass of a phenolic antioxidant were melt-kneaded, impregnated with carbon dioxide under supercritical conditions, and then the pressure was released to foam and extrude, thereby obtaining a foamed base material that is a polyolefin foam. The manufacturing conditions were an impregnation temperature of 180°C, an impregnation pressure of 13 MPa, and an impregnation time of 20 minutes.

[0042] <Solvent replacement process> As per Table 1, a foamed substrate filled with wet gel and zeolite {Molecular Sieve 3A, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.} were immersed in methanol, and solvent replacement was performed for 24 hours while stirring (8.75 times the amount of dehydrating agent relative to the water content in the wet gel by mass ratio). At this point, the water concentration in methanol at the end of the process, measured using a Hiranuma trace moisture analyzer (HIRANUMA Corporation: AQ-2200A) in accordance with JIS K 0068:2001 "Method for measuring the moisture content of chemical products", was 0.09%.

[0043] <Hydrophobicization process> To hydrophobize the gel surface within the foamed substrate after solvent replacement, the substrate was immersed in a methanol solution of hexamethyldisilazane (20% by mass) and subjected to a hydrophobic treatment while stirring for 24 hours.

[0044] <Drying process> The foamed substrate after the hydrophobicization process was impregnated in carbon dioxide at 80°C and 20 MPa, and supercritical fluid drying was performed for 12 hours to obtain the silica aerogel composite according to Example 1.

[0045] <<Preparation example of silica aerogel composite according to Example 2>> As shown in Table 1, the silica aerogel composite according to Example 2 was obtained using the same method as in Example 1, except that the amount of dehydrating agent was changed (by mass ratio, 10 times the amount of dehydrating agent relative to the water content in the gel was used). The water concentration in methanol at the end of the solvent replacement step was 0.04%.

[0046] <<Preparation example of silica aerogel composite according to Example 3>> As shown in Table 1, the silica aerogel composite according to Example 3 was obtained using the same method as in Example 1, except that the solvent replacement was performed for 36 hours. The water concentration in methanol at the end of the solvent replacement step was 0.03%.

[0047] <<Preparation example of silica aerogel composite according to Comparative Example 1>> A silica aerogel composite according to Comparative Example 1 was obtained using the same method as in Example 1, except that the substitution step was repeated three times without the use of a dehydrating agent. The water concentrations in methanol at the end of each substitution step were 4.0%, 1.5%, and 0.4%, respectively.

[0048] <<Rating>> The silica aerogel composites of Examples 1-3 and Comparative Example 1 were evaluated according to the method described below.

[0049] <Density Measurement> The density of the silica aerogel composite was measured in accordance with JIS K7222:2005 "Foamed plastics and rubber - Method for determining apparent density".

[0050] <Measurement of thermal conductivity> The thermal conductivity was measured in accordance with JIS A1412-2:1999 "Method for measuring the thermal resistance and thermal conductivity of thermal insulating materials - Part 2: Heat flow meter method (HFM method)" using a thermal conductivity measuring device (HC-72, manufactured by Eiko Seiki Co., Ltd.).

[0051] The results are shown in Tables 1 and 2. Note that in the tables, the water content in the gel (88.6g) was calculated by multiplying the amount of impregnated gel (571.6g) by the water concentration in the sol solution (232.4g / 1500g = 15.5%).

[0052] [Table 1]

[0053] [Table 2]

Claims

1. A method for producing an aerogel, characterized by comprising a gel dehydration step of immersing a water-containing gel in a non-aqueous solvent in the presence of a zeolite.

2. A method for producing a composite in which aerogel is filled into a porous substrate, A method for producing an aerogel composite, characterized by comprising a dehydration step of immersing a water-containing composite in which the gel is filled into a porous substrate in a non-aqueous solvent in the presence of a dehydrating agent.

3. The manufacturing method according to claim 2, wherein the dehydrating agent is at least one selected from zeolite, activated alumina, silica gel, calcium oxide, magnesium oxide, aluminum oxide, sodium sulfate, calcium sulfate, magnesium sulfate, potassium carbonate, sodium hydroxide, potassium hydroxide, and calcium chloride.

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