Porous gel and method for producing porous gel

Abstract

Provided is a porous gel including cellulose nanocrystals dispersed therein. Also provided is a method for producing such porous gel, the method comprising a dispersion step in which cellulose nanocrystals are dispersed in a solvent to obtain a dispersion, a mixing step in which an inorganic feed material is added to and mixed with the cellulose nanocrystal dispersion to obtain a wet gel, and a drying step in which the solvent contained in the wet gel is removed.

Description

Porous gel and method for producing the same

[0001] The present invention relates to a porous gel and a method for producing the porous gel, and more particularly to a porous gel with improved strength and a method for producing the same.

[0002] In a broad sense, a dry gel obtained by applying supercritical drying to a wet gel is called an aerogel, a dry gel obtained by drying under atmospheric pressure is called a xerogel, and a dry gel obtained by freeze-drying is called a cryogel. However, in this embodiment, regardless of these drying methods for wet gels, a low-density dry gel composed of a microporous solid whose dispersed phase is gas is called an "aerogel." In addition, a porous gel is generally a general term for a gel body having a plurality of pores (voids) inside the material. In this embodiment, however, in a nitrogen adsorption isotherm obtained by nitrogen adsorption measurement performed by the following method, a low-density dry gel composed of a microporous solid whose dispersed phase is gas is called an "aerogel." ０ The relative pressure P / P of the equilibrium pressure P to ０ The maximum amount of nitrogen adsorption when the value is 0 to 0.99 is 100 cm ３ (STP) / g or more.

[0003] Porous gels such as aerogels are known as materials with many excellent functions, such as a large specific surface area, low density, high thermal insulation, unique optical properties, and unique electrical properties, due to the large number of micropores they contain, and are expected to be used in catalysts, gas sensors, thermal insulation materials, electronic circuit board materials, etc. Furthermore, because they have high transparency to visible light depending on the material composition, they are being considered for use in optical materials that take advantage of their low refractive index, and transparent insulation materials such as insulating window materials for homes and insulating materials for solar heat collector panels.

[0004] However, aerogel has the drawback of being brittle and has low material strength. As a result, bulk and thin-film aerogels are prone to cracking and breaking due to impact or vibration, while granular and powdered aerogels can have their pore structure damaged by stresses applied during mixing or kneading with resin.

[0005] Another issue is the need for a special high-pressure process for the production of aerogel. Typically, porous gels such as aerogel are produced using the sol-gel method, a liquid-phase reaction (see, for example, Patent Document 1). A sol consisting of starting materials is solidified into a wet gel (sol-gel transition), and then dispersed components such as solvents in the resulting wet gel are replaced with gases by drying, forming a porous structure. Note that "sol" refers to the state before gelation occurs, in which the sol-like starting material or a product consisting of the starting material is dissolved or dispersed in a liquid medium and has fluidity. Furthermore, "wet gel" refers to a gel solid in a wet state that contains a liquid medium but does not have fluidity.

[0006] It is known that aerogels with large pore volumes can be obtained by using a method called supercritical drying, in which the solvent contained in the wet gel is discharged as a supercritical fluid under a temperature and pressure environment above the critical point of the solvent. However, because this method is a high-pressure process, it poses challenges, such as the need for expensive equipment and sophisticated safety measures. Therefore, if a method for producing porous gels with large pore volumes were established without using supercritical drying, it would be more advantageous from a production standpoint, since it would allow the use of general-purpose dryers using hot air or halogen heaters.

[0007] When a porous gel such as an aerogel is produced by drying at a temperature and pressure below the critical point of the solvent in the wet gel (hereinafter sometimes referred to as "atmospheric pressure drying") instead of supercritical drying, a capillary force represented by the following formula is generated within the gel during drying of the wet gel: P Ｃ =-2γ cos(θ) / a (where P Ｃ is the capillary force, γ is the surface tension of the solvent, θ is the contact angle between the solvent and the capillary wall, and a is the pore radius.

[0008] Therefore, in order to obtain a porous gel with a large pore volume by atmospheric pressure drying, it is necessary to minimize the decrease in pore volume that occurs due to the shrinkage of the wet gel caused by the capillary force described above. Furthermore, in order to produce a porous gel in bulk, it is necessary to prevent cracking and destruction during drying.

[0009] For example, in order to reduce the capillary force, it is possible to consider methods such as increasing the pore radius a of the gel, decreasing the surface tension γ of the solvent, or increasing the contact angle θ (making it less wettable) based on the above-mentioned formula for capillary force. However, it is difficult to obtain a porous gel with a large pore volume such as an aerogel by these ingenious measures alone, and in particular, when obtaining a large bulk porous gel, cracks and breakage are likely to occur during drying.

[0010] For the reasons stated above, it is industrially desirable to produce porous gels such as aerogels that have practical strength and large pore volumes by atmospheric pressure drying, and it is necessary to improve the skeletal strength of wet gels and porous gels. For inorganic aerogels, for example, an organic-inorganic hybrid method, in which inorganic and organic components are combined, is known to be effective as a method for improving the skeletal strength of porous gels. In particular, to improve the strength of aerogels while maintaining their transparency, it is desirable to combine inorganic and organic components at the molecular level. Specific examples include a method using a metal alkoxide with an organic modifying group such as an alkyl group, and a method of combining polymer components.

[0011] As a method of using a metal alkoxide having an organic modifying group such as an alkyl group, a method for producing a porous gel has been proposed in Patent Document 2 and elsewhere, in which a nonionic surfactant is dissolved in an acidic solution, an alkyl silicon alkoxide is added thereto, the product is solidified, and then dried at atmospheric pressure. However, since alkyl silicon alkoxides are relatively expensive among alkoxysilanes, it would be cheaper and more preferable if a method for producing a porous gel that does not primarily use alkyl silicon alkoxides could be established.

[0012] One known method for compounding the above-mentioned polymer components is to mix a polymer component with a liquid inorganic component, but this method is prone to opacity due to macroscopic phase separation between the inorganic component and the polymer component, making it difficult to produce a transparent, highly strong porous gel. Therefore, to improve the strength of a porous gel by compounding the polymer component, it is important that the polymer component is rigid and nanosized, is resistant to macroscopic phase separation from the precursor sol, and is dispersed in the porous gel so as not to impair transparency.

[0013] JP-A-10-182261 Patent No. 5250900

[0014] An object of the present invention is to provide a porous gel having improved strength and a large pore volume, and a method for producing the same.

[0015] The present inventors have discovered that by forming a porous gel having a structure in which cellulose nanocrystals are dispersed, a porous gel with a large pore volume and improved strength can be obtained, and that the gel can be produced by drying at atmospheric pressure, and have thus completed the present invention. That is, the present invention provides the following inventions [1] to

[15] .

[0016] [1] A porous gel in which cellulose nanocrystals are dispersed. [2] The porous gel according to [1], wherein the material skeleton of the porous gel contains silicon oxide as a main component. [3] The porous gel according to [2], wherein the silicon oxide and the cellulose nanocrystals are bonded via Si—O—C bonds. [4] The porous gel according to any one of [1] to [3], which contains the cellulose nanocrystals in a proportion of 0.1% by weight or more and less than 50% by weight. [5] The porous gel according to any one of [1] to [4], wherein the light transmittance of linear transmitted light at a wavelength of 600 nm at a thickness of 3 mm of the porous gel is 10% or more. [6] The porous gel according to any one of [1] to [4], wherein the maximum nitrogen adsorption amount is 500 cm ３ [7] The porous gel according to any one of [1] to [5], wherein the solid content is measured using a DD / MAS method and is (STP) / g or more. ２９[8] The porous gel according to any one of [1] to [6], wherein in a Si-NMR spectrum, the ratio of the peak area attributable to Q units to the total area of ​​peaks attributable to silicon-containing bond units, M units, D units, T units and Q units, is 10 to 100%. ２９ In Si-NMR, Q １ Peak, Q ２ Peak, Q ３ Peak, and Q ４ Q for the total area of ​​the peak ４ The porous gel according to any one of [1] to [7], wherein the peak area ratio is 50% or more. [9] The porous gel according to any one of [1] to [8], wherein the porous gel has an irregular structure connected in a granular form.

[10] The method for producing the porous gel according to any one of [1] to [9], comprising: a dispersing step of dispersing cellulose nanocrystals in a solvent to obtain a dispersion; a mixing step of adding an inorganic material to the cellulose nanocrystal dispersion and mixing them to obtain a wet gel; and a drying step of drying the solvent contained in the wet gel.

[11] The method for producing the porous gel according to

[10] , wherein the mixing step comprises a first mixing step of adding an inorganic material and an acid catalyst to the cellulose nanocrystal dispersion, and a second mixing step of adding a basic catalyst to the mixture obtained by the first mixing step.

[12] The method for producing the porous gel according to

[10] or

[11] , wherein the mixing step is performed in the presence of water, and the ratio of the total amount of water to 100 g of inorganic material is 40 to 400 g.

[13] The method for producing a porous gel according to any one of

[10] to

[12] , wherein the mixing step is performed in the presence of water and formamide.

[14] The method for producing a porous gel according to any one of

[10] to

[13] , wherein the period from the dispersing step to obtaining a wet gel in the mixing step is within 72 hours.

[15] The method for producing a porous gel according to any one of

[10] to

[14] , wherein the removal of the solvent in the drying step is performed at a temperature and / or under pressurized conditions below the critical point of the solvent.

[0017] According to the present invention, it is possible to provide a porous gel having improved strength and a large pore volume, and a method for producing the same.

[0018] 1 is a photograph showing the appearance of the bulk porous gel obtained in Example 1. FIG. 2 is a SEM image of the porous gel obtained in Example 6.

[0019] <Porous Gel> The porous gel of the present invention is a porous gel in which cellulose nanocrystals are dispersed.

[0020] The porous gel of the present invention may be a solid having a plurality of pores (voids) therein and containing dispersed cellulose nanocrystals. Dispersion refers to the fact that the cellulose nanocrystals or cellulose nanocrystal-derived fibrous structural units in the porous gel are randomly distributed in rod-like shapes on the order of nanometers, without forming a localized arrangement on the order of micrometers (e.g., a striped structure). It is more preferable that the cellulose nanocrystals or cellulose nanocrystal-derived fibrous structural units in the porous gel of the present invention are randomly distributed without forming a cholesteric liquid crystal phase. More specifically, it is preferable that the cellulose nanocrystals or cellulose nanocrystal-derived fibrous structural units are distributed throughout the porous gel, and it is preferable that the cellulose nanocrystals or cellulose nanocrystal-derived fibrous structural units are distributed at approximately the same content throughout the porous gel. Dispersing the cellulose nanocrystals as described above can suppress the decrease in transparency and heat insulating properties caused by the cellulose nanocrystals. Dispersing the cellulose nanocrystals as described above can suppress the decrease in transparency and heat insulating properties caused by the cellulose nanocrystals. Whether cellulose nanocrystals or fibrous structural units derived from cellulose nanocrystals are dispersed throughout the porous gel can be confirmed, for example, by SEM observation, AFM observation, or XRD analysis. The porous gel of the present invention may be one in which cellulose nanocrystals or fibrous structural units derived from cellulose nanocrystals are substantially dispersed, and a small portion of the cellulose nanocrystals or fibrous structural units derived from cellulose nanocrystals may form a liquid crystal phase, provided that the effects of the present invention are not impaired.

[0021] The porous gel of the present invention, which contains dispersed cellulose nanocrystals, can be obtained with excellent transparency to visible light, while also exhibiting heat insulation and high strength. The action of the cellulose nanocrystals can improve the strength of the porous gel, thereby suppressing cracking during drying of the wet gel and enabling the production of a bulk aerogel with high strength. Furthermore, when the porous gel of the present invention is used in granular or powder form, it is possible to obtain a porous gel whose pore structure is resistant to damage due to stresses applied during mixing or kneading with a resin. The porous gel of the present invention can be obtained, for example, by a sol-gel method, and does not require a high-pressure process such as supercritical drying. Because bulk or powdered porous gels can be produced by atmospheric pressure drying, it is possible to produce the porous gel relatively easily and inexpensively.

[0022] Cellulose nanocrystals are rod-shaped cellulose crystalline fibers obtained by acid hydrolysis of cellulose fibers such as pulp with sulfuric acid or hydrochloric acid. In the present invention, cellulose nanocrystals containing sulfate groups, which are anionic functional groups, obtained by treating cellulose fibers with sulfuric acid are preferred. The sulfate groups may be sulfate ester groups. Furthermore, cellulose nanocrystals having a fiber diameter of 50 nm or less, for example, in the range of 1 to 50 nm, particularly in the range of 2 to 20 nm, a fiber length of 50 to 1000 nm, particularly in the range of 100 to 500 nm, an aspect ratio of 2 to 100, particularly in the range of 5 to 50, and a crystallinity of 50% or more, preferably 60% or more, particularly 70% or more, can be suitably used. The fiber diameter, fiber length, and aspect ratio of nanocellulose can be determined by SEM observation of an aqueous dispersion of nanocellulose or dispersed fibers, and the average values ​​measured for 10, more preferably 100, selected fibers.

[0023] Furthermore, cellulose nanocrystals containing sulfate groups can be further subjected to hydrophilization treatment to introduce anionic functional groups such as carboxyl groups or phosphate groups into hydroxyl groups at positions such as the 6th position of cellulose, thereby producing anionic functional group-containing cellulose nanocrystals in which the total amount of anionic functional groups such as carboxyl groups or phosphate groups is adjusted to more than 0.17 mmol / g and not more than 4.0 mmol / g, particularly in the range of 0.17 to 2.0 mmol / g. Hydrophilization treatment of cellulose nanocrystals improves their dispersibility in water or highly polar organic solvents, allowing the production of porous gels such as wet gels and aerogels in which cellulose nanocrystals are dispersed.

[0024] The hydrophilization treatment of cellulose nanocrystals is not particularly limited, but may be a never-dry treatment, or a combination of a never-dry treatment and a treatment using a water-soluble carbodiimide, sulfuric acid, a sulfur trioxide-pyridine complex, phosphoric acid-urea, a TEMPO catalyst, or an oxidizing agent. The hydrophilization treatment using a carbodiimide, sulfuric acid, or a sulfur trioxide-pyridine complex introduces hydrophilic functional groups into the cellulose nanocrystals and further shortens the cellulose nanocrystals. Furthermore, the treatment using phosphoric acid-urea, a TEMPO catalyst, or an oxidizing agent introduces anionic functional groups such as phosphoric acid groups or carboxyl groups, adjusting the total amount of anionic functional groups in the cellulose nanocrystals to fall within the above range.

[0025] Furthermore, if necessary, hydrophobized cellulose nanocrystals may be used. The hydrophobization method is not particularly limited, but includes acylation of some of the hydroxyl groups, such as those at the 6-position, of the cellulose nanocrystals with an acid or the like, treatment with a hydrophobizing agent such as trimethylchlorosilane, hexamethyldisilazane, or hexamethyldisiloxane, or mixing the carboxyl-introduced cellulose nanocrystals with a cationic surfactant to substitute the hydrophobic functional groups, followed by washing to remove impurities and unreacted materials, thereby obtaining modified cellulose nanocrystals. Hydrophobization of cellulose nanocrystals improves their dispersibility in nonpolar organic solvents, allowing the production of porous gels, such as wet gels and aerogels, in which the cellulose nanocrystals are dispersed.

[0026] The porous gel of the present invention may be any gel in which cellulose nanocrystals are dispersed, and the content of cellulose nanocrystals (content of cellulose nanocrystals or fiber structural units derived from cellulose nanocrystals) relative to the total solid weight of the porous gel is not particularly limited, but is preferably 0.1% by weight or more and less than 50% by weight, more preferably 1 to 30% by weight, even more preferably 2 to 20% by weight, and particularly preferably 3 to 10% by weight, relative to the total solid weight of the porous gel. By ensuring that the content of cellulose nanocrystals falls within the above range, the strength, transparency, and heat insulation of the porous gel can be further improved.

[0027] The porous gel of the present invention may be any gel in which cellulose nanocrystals are dispersed, but may also contain components that form the material skeleton in addition to cellulose nanocrystals or fibrous structural units derived from cellulose nanocrystals.

[0028] The components constituting the material skeleton refer to all components constituting the material skeleton of the porous gel other than cellulose nanocrystals and fiber structural units derived from cellulose nanocrystals, and the component with the largest amount among them is referred to as the "main component." The content of the main component among the components constituting the material skeleton relative to the total solid weight of the porous gel is preferably 30% by weight or more, more preferably 50% by weight or more, even more preferably 60% by weight or more, particularly preferably 80% by weight or more, and most preferably 90% by weight or more.

[0029] The components forming the material skeleton for obtaining the porous gel of the present invention are not particularly limited, but are preferably composed of inorganic raw materials. The inorganic raw materials are not particularly limited, but are preferably those that form a wet gel by a liquid-phase reaction, and suitable examples include metal compounds such as metal alkoxides and silicates. That is, the porous gel of the present invention preferably contains, as the inorganic raw material, silicon oxide, titanium oxide, or zirconium oxide derived from metal alkoxides or silicates such as sodium silicate as the main component. Of these, metal alkoxides are particularly preferred from the viewpoint of having chemical reaction sites with cellulose nanocrystals.

[0030] The metal atoms constituting the metal alkoxide are not particularly limited, and may be any metal atoms capable of forming a metal alkoxide. However, Si, Ti, Zr, Mg, Zn, Pb, Al, Ba, Y, W, V, Na, Li, In, etc. are preferred. From the viewpoint of having high transparency to visible light and thus being suitable for applications requiring optical properties, and also being able to obtain a porous gel with a large pore volume, Si, Ti, and Zr are more preferred, and Si is particularly preferred. That is, alkoxysilane is preferred as the metal alkoxide. That is, the porous gel of the present invention preferably contains Si as the main component. The metal alkoxide may be a single type, or may contain two or more types.

[0031] The metal alkoxide is not particularly limited, but includes compounds represented by the following general formula (1), oligomers thereof, and hydrolysates thereof, and these can be suitably used. Among the compounds represented by the general formula (1), alkoxysilane, alkoxytitanium, and zirconium alkoxides can be suitably used. Among these, alkoxysilane is preferred. M(R １ ) ｋ (OR ２ ) ４－ｋ (1) (In the above general formula (1), M is Si, Ti or Zr, R １ is any organic group having one or more carbon atoms, and R ２ is an alkyl group having 1 or more carbon atoms, and k is an integer of 0 to 3.

[0032] In the above general formula (1), R １ is an organic group having one or more carbon atoms, and is not particularly limited, and may have a functional group such as an alkyl group, an epoxy group, a glycidyl group, an isocyanate group, or a thiol group, or a halogen atom such as F, Cl, Br, or I. It is preferably an alkyl group, more preferably an alkyl group having 1 to 10 carbon atoms, even more preferably an alkyl group having 1 to 3 carbon atoms, and particularly preferably a methyl group having 1 carbon atom.

[0033] In addition, in the general formula (1), the alkoxy group OR ２ is preferably an alkoxy group having 1 to 10 carbon atoms (i.e., R ２ is preferably an alkyl group having 1 to 10 carbon atoms, more preferably an alkyl group having 1 to 4 carbon atoms (methoxy group, ethoxy group, propoxy group, butoxy group), still more preferably an alkyl group having 1 to 2 carbon atoms (methoxy group, ethoxy group), and particularly preferably an alkyl group having 1 carbon atom (methoxy group). Furthermore, k is an integer of 0 to 3, preferably an integer of 0 to 2, more preferably 0 or 1, and particularly preferably 0. That is, M(OR ２ ) ４ is particularly preferred, and in the case of alkoxysilane, Si(OR ２ ) ４ It is particularly preferred that:

[0034] In the above general formula (1), the alkoxysilane where k = 1 to 3 is not particularly limited, but examples include methyltrimethoxysilane, ethyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-glycidyloxypropyltrimethoxysilane, (3-mercaptopropyl)trimethoxysilane, vinyltrimethoxysilane, etc. Compounds in which some or all of the methoxy groups have been substituted with other alkoxy groups such as ethoxy groups can also be used.

[0035] In the above general formula (1), specific examples of alkoxysilanes where k = 0 include, but are not limited to, tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, etc. In the above general formula (1), specific examples of zirconium alkoxides where k = 0 include tetramethoxyzirconium, tetraethoxyzirconium, tetrapropoxyzirconium, tetrabutoxyzirconium, etc. In the above general formula (1), specific examples of alkoxytitaniums where k = 0 include tetramethoxytitanium, tetraethoxytitanium, tetrapropoxytitanium, tetrabutoxytitanium, etc. Among these, tetramethoxysilane and tetraethoxysilane are preferred, and tetramethoxysilane is more preferred.

[0036] It is also possible to use alkoxysilanes in which silicon atoms are directly crosslinked with an organic component, and specific examples include bistrimethoxysilylmethane, bistrimethoxysilylethane, and bistrimethoxysilylhexane.

[0037] The alkoxysilane may be used alone or in combination of two or more depending on the purpose. Furthermore, oligomers of these may also be used, but to obtain a porous gel with transparency and a large pore volume, it is preferable to use tetramethoxysilane or tetraethoxysilane alone. Furthermore, from the viewpoint of raw material costs, it is preferable to use tetramethoxysilane or tetraethoxysilane.

[0038] The silicate salt is not particularly limited, but includes sodium silicate, potassium silicate, calcium silicate, magnesium silicate, and silicic anhydride, among others. In particular, Na ２ O.nSiO ２ ・mH ２ Preferably, water glass is represented by the composition formula: O. Commercially available water glass can be used, and examples thereof include water glass No. 1, water glass No. 2, and water glass No. 3 specified in JIS K1408.

[0039] When a metal alkoxide is used as the inorganic raw material for the porous gel of the present invention, some of the structural units derived from the metal alkoxide may be chemically bonded to C atoms derived from the cellulose nanocrystals and may exist as a reaction product in the porous gel of the present invention. Metal alkoxides containing functional groups reactive with OH groups derived from the cellulose nanocrystals can be suitably used, but among such reaction products, those having an M-O-C bond (M is a metal atom) derived from the reaction between the M-OH derived from the metal alkoxide and an OH group at the 6th position or the like derived from the cellulose nanocrystals are preferred. That is, at least some of the reaction products may have the M atom derived from the metal alkoxide and the cellulose nanocrystal bonded via an M-O-C bond, and by bonding these via an M-O-C bond, the porous gel of the present invention can be made to have improved strength. Whether an M-O-C bond is formed can be determined, for example, by examining the solid １３ This can be confirmed by C-NMR measurement, FT-IR measurement, or XPS measurement. When M is Si, the porous gel of the present invention preferably has a structural unit derived from alkoxysilane and an Si—O—C bond derived from the reaction of cellulose nanocrystals.

[0040] For example, the M-O-C bond between the metal alkoxide and the cellulose nanocrystals is preferably formed by a condensation reaction between the M-OH group derived from the metal alkoxide and the OH group derived from the cellulose nanocrystals under heating conditions or in the presence of a catalyst: M-OH + HO-Rc → M-O-Rc + H ２O (2) (In the above formula (2), M is a metal atom, R is any organic group, and Rc is any organic structure derived from cellulose nanocrystals.)

[0041] The porous gel of the present application is preferably a porous silica gel containing a reaction product of a metal alkoxide hydrolysate and cellulose nanocrystals (a porous gel containing silica-derived components in a proportion of preferably 50% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more).

[0042] The porous gel of the present invention may also contain a structure represented by the following general formula (3). That is, it may have a structure in which two metal atoms M are bonded via one oxygen atom: -M-O-M- (3) (In the above general formula (3), M is a metal atom.)

[0043] The structure represented by the general formula (3) above is not particularly limited, but when a metal alkoxide is used as an inorganic raw material, it can be formed through a hydrolysis reaction and a polycondensation reaction of the metal alkoxide shown in the following formulas (4) to (6): Z-M-OR + H ２ O → Z-M-OH + ROH (4) Z-M-OH + Z'-M-OH → Z-M-O-M-Z' + H ２ O (5) Z-M-OH + Z'-M-OR → Z-M-OM-Z' + ROH (6) (In the above formulas (4) to (6), M is a metal atom, R is any organic group, and Z and Z' are any groups.)

[0044] In addition to the metal alkoxide, the inorganic raw material may contain a compound having a total of two or more alcoholic hydroxyl groups and / or ester bonds as a crosslinkable compound. When the crosslinkable compound contains a compound having a total of two or more alcoholic hydroxyl groups and / or ester bonds, the porous gel of the present invention may contain a structure represented by the following general formula (7): -M-(OX) ｎ -OM- (7) (In the above general formula (7), M is a metal atom, X is any organic group having a valence of 2 or more, and n is an integer of 1 or more.)

[0045] Furthermore, X in formula (7) may be any divalent or higher organic group and is not particularly limited, but is preferably a linear, branched, or cyclic divalent or higher alcohol residue or ester residue, more preferably a linear, branched, or cyclic divalent or higher alkylene group, even more preferably a linear or branched divalent or higher alkylene group, and preferably has 2 or more carbon atoms, more preferably 2 to 10, even more preferably 2 to 6, even more preferably 2 to 4, and particularly preferably 2. ｍ H ２ｍ (m is preferably 2 or more, more preferably 2 to 10, even more preferably 2 to 6, still more preferably 2 to 4, and particularly preferably 2.) Specific examples of X include a 1,1-ethylene group, a 1,2-ethylene group, a 1,2-propylene group, a 1,3-propylene group, a 1,2-butylene group, a 1,3-butylene group, a 1,4-butylene group, a 2,3-butylene group, a 1,2-pentylene group, a 1,3-pentylene group, a 1,4-pentylene group, a 1,5-pentylene group, a 2,3-pentylene group, a 2,4-pentylene group, a 1,2-hexylene group, a 1,3-hexylene group, a 1 , 4-hexylene group, 1,5-hexylene group, 1,6-hexylene group, 2,3-hexylene group, 2,4-hexylene group, 2,5-hexylene group, 3,4-hexylene group, 1,2-cyclohexylene group, 1,3-cyclohexylene group, 1,4-cyclohexylene group, 1,2-cyclopropylene group, 1,3-cyclopropylene group, 1,2-cyclobutylene group, 1,2-cyclopropylene group, and the like. Among these, 1,2-ethylene group, 1,2-propylene group, 1,2-butylene group, 1,2-pentylene group, and 1,2-hexylene group are preferred, 1,2-ethylene group, 1,2-propylene group, 1,2-butylene group, and 1,2-hexylene group are more preferred, 1,2-ethylene group and 1,2-propylene group are even more preferred, and 1,2-ethylene group is particularly preferred. X may be one type alone or may be a combination of two or more types.

[0046] n is an integer of 1 or more, preferably 1 to 100, more preferably 1 to 10, even more preferably 1 to 4, and particularly preferably 1.

[0047] The structure represented by the general formula (7) is formed by reacting a metal alkoxide with the crosslinkable compound by a transesterification reaction according to the following formula (8) and / or formula (9). Specifically, the transesterification reaction according to the following formula (8) and / or formula (9) proceeds in multiple stages to form the structure represented by the general formula (7). Z-Si-OR + R １ OH → Z-Si-OR １ + ROH (8) Z-Si-OR + R １ COOR ２ → Z-Si-OR ２ + R １ COOR (9) (In the above formulas (8) and (9), R, R １ , R ２ is any organic group, and Z is any group.

[0048] The crosslinkable compound may be any compound having a total of two or more alcoholic hydroxyl groups or ester bonds, and is preferably a compound having two alcoholic hydroxyl groups. Specific examples of compounds having two or more alcoholic hydroxyl groups include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, and 2,3-pentanediol. ethanol, 2,4-pentanediol, 1,2-hexanediol, 1,3-hexanediol, 1,4-hexanediol, 1,5-hexanediol, 1,6-hexanediol, 2,3-hexanediol, 2,4-hexanediol, 2,5-hexanediol, 3,4-hexanediol, 1,2-heptanediol, 1,3-heptanediol, 1,4-heptanediol, 1,5-heptanediol, 1,6-heptanediol Examples of suitable polysaccharides include 1,7-heptanediol, 2,3-heptanediol, 2,4-heptanediol, 2,5-heptanediol, 3,4-heptanediol, 3,5-heptanediol, 1,2-cyclohexanediol, 1,3-cyclohexanediol, 1,4-cyclohexanediol, 3-allyloxy-1,2-propanediol, catechol, resorcinol, hydroquinone, glycerin, diethylene glycol, dipropylene glycol, dibutyl glycol, triethylene glycol, polyvinyl alcohol, polyethylene glycol, sugar alcohols such as erythritol and sorbitol, chicken, chitosan, hydroxypropyl cellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, and carboxymethyl cellulose, and metal salts of the above polysaccharides.Among these, compounds having two adjacent hydroxy groups and having 6 or less carbon atoms, i.e., ethylene glycol, 1,2-propanediol, 1,2-butanediol, 2,3-butanediol, 1,2-pentanediol, 2,3-pentanediol, 1,2-hexanediol, 2,3-hexanediol, and 3,4-hexanediol, are preferred, and compounds having two adjacent hydroxy groups and having 4 or less carbon atoms, i.e., ethylene glycol, 1,2-propanediol, 1,2-butanediol, and 2,3-butanediol, are more preferred, with ethylene glycol being even more preferred. Use of the above compounds having 6 or less carbon atoms is preferred because it allows the production of a bulk porous gel that is free from cracks and fractures, or a porous gel that has a large pore volume.

[0049] Furthermore, as the crosslinkable compound, a compound having two or more ester bonds can also be used, and specific examples thereof include diacylglycerol, triacylglycerol, cellulose acetate, polyester resin, polyvinyl acetate resin, ethylene-polyvinyl acetate copolymer, etc. Among these, polyvinyl acetate resin and ethylene-polyvinyl acetate copolymer can be used to introduce a long alkyl chain, making them suitable for use in terms of imparting flexibility. Furthermore, specific examples of compounds having both an ester bond and a hydroxyl group include methyl glycolate and ethyl glycolate.

[0050] Furthermore, the porous gel of the present invention may also be subjected to a transesterification reaction between a metal alkoxide and a compound having one hydroxy group or one ester bond. The reaction with a compound having one hydroxy group (monohydric alcohol) is shown in the following formula (10), and the reaction with a compound having one ester bond is shown in the following formula (11): Z-M-OR + R １ OH → Z-M-OR １ + ROH (10) Z-M-OR + R ２ COOR ３ → Z-M-OR ３ + R ２ COOR (11) (In the above formulas (10) and (11), M is a metal atom, and R, R １ , R ２ , R３ is any organic group, and Z is any group.

[0051] When an organic chain is introduced by a compound having one hydroxy group or an ester bond, a crosslinked structure is not formed, inhibiting the development of the three-dimensional network structure of the resulting porous gel, which is expected to control physical properties such as mechanical strength. It is also possible to introduce reactive functional groups such as vinyl groups, allyl groups, epoxy groups, and carboxylic acid groups, or halogen elements such as F, Cr, Br, and I.

[0052] The compound having one hydroxy group or ester bond is more preferably a monohydric alcohol, and is not particularly limited to, but includes 2-propanol, tertiary butanol, allyl alcohol, 2-chloroethanol, 2,2,2-trifluoroethanol, 5-fluoropropanol (2,2,3,3,3-pentafluoro-1-propanol), 2-(perfluoro-n-octyl)ethanol, pentafluorophenol, 1-(pentafluorophenyl)ethanol, 1,1,1,3,3,3-hexafluoro-2-propanol, and the like. Among these, alcohols having fluorine atoms, such as trifluoroethanol, are effective in imparting properties such as water repellency, low refractive index, and low dielectric constant derived from fluorine atoms. These monohydric alcohols may be used alone or in combination of two or more.

[0053] The ratio of the metal alkoxide to the crosslinkable compound used is not particularly limited and may be adjusted depending on the amount of the structure represented by general formula (7) introduced into the resulting porous gel, but the metal alkoxide:crosslinkable compound charge ratio (weight ratio) is preferably in the range of 100:1 to 1:1000, more preferably 100:100 to 100:300, and even more preferably 100:120 to 100:200. Furthermore, the ratio of the metal alkoxide to the crosslinkable compound used, in terms of the molar ratio of "total amount of alkoxide groups in the metal alkoxide:total amount of alcoholic hydroxyl groups and ester bonds in the crosslinkable compound", is preferably in the range of 1:0.25 to 1:5.0, more preferably 1:1.5 to 1:3, and even more preferably 1:1.75 to 1:2.25.

[0054] Furthermore, the porous gel of the present invention preferably has a light transmittance of 10% or more, more preferably 30% or more, and even more preferably 50% or more, for in-line transmitted light at a wavelength of 600 nm when converted into a film having a thickness of 3 mm. The light transmittance of in-line transmitted light through the porous gel can be measured by the method described in the Examples below.

[0055] The porous gel of the present invention also exhibits a relative pressure P / P ０ Nitrogen adsorption amount [g / g or cm ３ (STP) / g] is preferably 500 cm ３ (STP) / g or more, more preferably 1000 cm ３ (STP) / g or more, and more preferably 1200 cm ３ (STP) / g or more, and ３ It is most preferable that the relative pressure P / P is not less than 77 Kelvin. ０ Nitrogen adsorption amount [g / g or cm ３ When the change in [(STP) / g] is measured, it is preferable that the nitrogen adsorption isotherm is classified as Type IV as defined by IUPAC. Type IV nitrogen adsorption isotherm indicates the presence of mesopores of 2 to 50 nm, and also shows a phenomenon (hysteresis) in which the adsorption and desorption processes do not coincide, suggesting that a relatively strong interaction occurs between the porous gel and the adsorbate.

[0056] The porous gel of the present invention preferably has a BET specific surface area of ​​200 m ２ / g or more, more preferably 400m ２ / g or more, and more preferably 600m ２ / g or more, particularly preferably 700m ２ The BET specific surface area can be measured, for example, by the method described in the examples below.

[0057] The porous gel of the present invention is a solid ２９In Si-NMR, the proportion of the peak area attributable to Q units to the total area of ​​peaks attributable to M units, D units, T units, and Q units, which are the basic structural units of the silica compound, is 10 to 100%, preferably 40 to 100%, more preferably 60 to 100%, and particularly preferably 70 to 100%. The M units, D units, T units, and Q units are classified as follows: M: a silicon-containing bond unit having one oxygen atom bonded to one silicon atom and three hydrogen atoms or monovalent organic groups; D: a silicon-containing bond unit having two oxygen atoms bonded to one silicon atom and two hydrogen atoms or monovalent organic groups; T: a silicon-containing bond unit having three oxygen atoms bonded to one silicon atom and one hydrogen atom or monovalent organic group; Q: a silicon-containing bond unit having four oxygen atoms bonded to one silicon atom. The organic group is a monovalent organic group in which the atom bonded to the silicon atom is a carbon atom. The ratio of the peak area derived from Q units to the total area of ​​the peaks derived from M units, D units, T units and Q units can be measured, for example, by the method described in the Examples below.

[0058] The porous gel of the present invention is a solid ２９ In Si-NMR, in the signal derived from four silicon-containing bond units Q in which an oxygen atom bonded to one silicon atom, Q １ Peak, Q ２ Peak, Q ３ Peak and Q ４ Q for the total area of ​​the peak ４ The peak area ratio is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more. １ ~Q ４ is SiO ４ Depending on the degree of peak shift caused by the electronegativity of the atoms connected to the tetrahedron, they can be classified as follows: １ : A solid that has one structure in which a Si atom is surrounded by an O atom and another Si atom or C atom. ２９ A structural unit having a peak at around -80 ppm in the Si-DD / MAS-NMR spectrum. Q ２: A solid that has two structures with Si atoms and C atoms around a Si atom via an O atom. ２９ The Si-DD / MAS-NMR spectrum has a peak at around -91 ppm. ３ : Three structures in which Si atoms and C atoms are located around a Si atom via an O atom, and the solid ２９ The Si-DD / MAS-NMR spectrum has a peak at around -101 ppm. ４ : It has four structures in which Si atoms and C atoms are arranged around a Si atom via an O atom, and is a solid. ２９ The Si-DD / MAS-NMR spectrum has a peak at around -110 ppm. １ Peak, Q ２ Peak, Q ３ Peak and Q ４ Q for the total area of ​​the peak ４ The peak area ratio can be measured, for example, by the method described in the Examples below.

[0059] The porous gel of the present invention preferably has a pore volume of 0.5 cm3 or less as measured by the BJH method for nitrogen adsorption. ３ / g or more, more preferably 1.0 cm ３ / g or more, more preferably 2.0 cm ３ / g or more, particularly preferably 2.5 cm ３ When the pore volume is in the above range, the thermal conductivity can be further reduced.

[0060] The "pore volume by the BJH method" is the pore volume derived from pores with a pore radius of 1 nm or more and 100 nm or less, which is obtained by drying a sample to be measured at a temperature of 150°C for 2 hours or more under a vacuum of 1 kPa or less, obtaining a nitrogen adsorption isotherm at liquid nitrogen temperature, and analyzing the resultant isotherm by the BJH method (Barrett, E.P.; Joyner, L.G.; Halenda, P.P., J. Am. Chem. Soc. 73, 373 (1951)), and can be measured by the method described in the examples below.

[0061] Furthermore, the porous gel of the present invention has a large pore volume, and from the viewpoint of minimizing heat conduction through the solid phase and improving thermal insulation, it is desirable for the porous gel to have an irregular structure in which the particles of the porous gel are randomly connected. Methods for confirming the irregular structure include, for example, SEM observation and AFM observation. In order to form the irregular structure, it is preferable to obtain a porous gel in which cellulose nanocrystals are dispersed while suppressing the formation of a liquid crystal phase such as a cholesteric liquid crystal of the cellulose nanocrystals, and further it is preferable to develop a three-dimensional network structure by undergoing the mixing process and aging process described below.

[0062] <Method for producing porous gel> There are no particular limitations on the method for producing the porous gel of the present invention, but a method for producing a porous gel that includes a dispersing step of dispersing cellulose nanocrystals in a solvent to obtain a dispersion, a mixing step of adding an inorganic material to the cellulose nanocrystal dispersion and mixing it to obtain a wet gel, and a drying step of drying the solvent contained in the wet gel is preferred.

[0063] (Dispersion Step) The dispersion step is a step of preparing a dispersion liquid of cellulose nanocrystals by dispersing cellulose nanocrystals in a solvent.

[0064] The solvent used in the dispersion step is not particularly limited as long as it is a solvent capable of dispersing cellulose nanocrystals, and examples thereof include water, polar solvents, non-polar solvents, etc. Polar solvents include, but are not particularly limited to, alcoholic solvents such as methanol, ethanol, propanol, butanol, formic acid, nitromethane, formamide, etc. Two or more of the above solvents may be mixed and used depending on the purpose.

[0065] Furthermore, the method for dispersing cellulose nanocrystals in a solvent is not particularly limited, but examples include a method using a dispersing machine such as an ultrasonic disperser, homogenizer, or mixer, or a method of stirring with a stirring rod, stirring bar, or the like.

[0066] The concentration of cellulose nanocrystals in the cellulose nanocrystal dispersion prepared in the dispersion step is preferably 10% by weight or less, more preferably 8% by weight or less, and even more preferably 6% by weight or less. If the concentration exceeds this range, the dispersibility of the cellulose nanocrystals may decrease, and the viscosity of the resulting dispersion may be so high that it may be difficult to handle industrially.

[0067] Furthermore, when a cellulose nanocrystal dispersion is used at a concentration of 2.5 wt % or more to suppress the formation of a liquid crystal phase in the cellulose nanocrystals, it is desirable to disperse the cellulose nanocrystals in water and then quickly mix them with inorganic raw materials to form a wet gel. More specifically, after preparing an aqueous dispersion of cellulose nanocrystals in the dispersion step, the time required for gelation to obtain a wet gel in the mixing step described below (i.e., the fluid solution containing cellulose nanocrystals loses its fluidity and solidifies) is preferably within 72 hours, more preferably within 60 hours, even more preferably within 48 hours, and particularly preferably within 36 hours.

[0068] The dispersion temperature in the dispersion step varies depending on the solvent, but is preferably 5 to 100°C, more preferably 20 to 80°C, and the dispersion time may be the shortest time required for cellulose nanocrystal aggregates to become undetectable by visual inspection, but is preferably 30 seconds to 5 minutes, more preferably 1 to 3 minutes. Furthermore, when dispersing cellulose nanocrystals using a homogenizer, it is possible to repeat short-term treatment multiple times to prevent the liquid temperature from rising excessively during dispersion.

[0069] (Mixing step) The mixing step is a step of adding an inorganic raw material to the dispersion liquid of cellulose nanocrystals prepared in the above-mentioned dispersion step and mixing them to obtain a wet gel. In the mixing step, mixing is preferably performed in the presence of water. In this case, if water is used as a solvent in the dispersion step, the water used in the dispersion step can be used as is, or if water is not used as a solvent in the dispersion step, water can be added in the mixing step.

[0070] In the mixing step, the cellulose nanocrystal dispersion and the inorganic raw materials are mixed under conditions such that the inorganic raw materials, such as metal alkoxides, undergo hydrolysis and polycondensation reactions to form a wet gel. At this time, as shown in the reaction of formula (2), the M-OH derived from the metal alkoxide and the hydroxyl groups derived from the cellulose nanocrystals undergo a condensation reaction, forming an M-O-C bond between the metal alkoxide and the cellulose nanocrystals. When an alkoxysilane is used as the metal alkoxide, an Si-O-C bond is formed between the silicon oxide of the material skeleton and the cellulose nanocrystals. Furthermore, for example, the above-described mixing step may include a first mixing step in which the inorganic raw materials and an acid catalyst are added to the cellulose nanocrystal dispersion, and a second mixing step in which a basic catalyst is added to the mixture obtained in the first mixing step. In particular, when the inorganic raw material is a metal alkoxide, after the first mixing step in which water and the metal alkoxide are mixed in an acidic environment, the pH of the resulting sol-like product is adjusted to the neutral to basic side in the second mixing step, whereby the hydrolysis reaction and polycondensation reaction as shown in the following formulas (12) and (13) or (14) proceed efficiently, and by further passing through the aging step described below, a three-dimensional network structure based on the M-O-M structure is obtained. By developing the three-dimensional network structure based on the M-O-M structure, an irregular structure in which particles are randomly linked is formed, making it possible to obtain a porous gel with a large pore volume. In other words, it is preferable to react the metal alkoxide under acidic conditions in the first mixing step, and then raise the pH to neutral to basic in the second mixing step and subsequent steps. M-OR + H ２ O → MOH + ROH (12) M-OR + M-OH → M-OM + ROH (13) M-OH + M-OH → M-OM + H ２ O (14) (In the above formulas (12) to (14), M is a metal atom, and R is any organic group.)

[0071] Furthermore, when a compound having a total of two or more alcoholic hydroxyl groups and / or ester bonds is further contained as a crosslinking compound in the porous gel, the crosslinking compound may be mixed with the cellulose nanocrystal dispersion together with the metal alkoxide in the mixing step. The amount of the crosslinking compound used may be an amount that satisfies the above-mentioned range relative to the metal alkoxide.

[0072] The acid catalyst used in the first mixing step is not particularly limited, and examples thereof include mineral acids, heteropolyacids, organic sulfonic acids, organic carboxylic acids, inorganic solid acids, and acidic ion exchange resins. More specifically, examples thereof include sulfuric acid, hydrochloric acid, nitric acid, boric acid, phosphoric acid, dodecylbenzenesulfonic acid, cumenesulfonic acid, xylenesulfonic acid, dinonylnaphthalenesulfonic acid, paratoluenesulfonic acid, oxalic acid, malonic acid, formic acid, propionic acid, and acetic acid. Among these, preferred are organic sulfonic acids such as sulfuric acid and dodecylbenzenesulfonic acid, with sulfuric acid being more preferred. Furthermore, by using an acidic metal salt, it is also possible to introduce metal atoms into the skeleton.

[0073] The acid catalyst can be used in a state diluted with water, an organic solvent, etc., as needed. For example, when sulfuric acid is added, the concentration is preferably 0.1 to 80% by weight, more preferably 0.5 to 20% by weight, and even more preferably 1 to 5% by weight, after dilution with water.

[0074] The amount of acid catalyst used varies depending on the type of inorganic raw material, the type of catalyst, the reaction temperature, etc., but when sulfuric acid is used, the amount of sulfuric acid component is preferably in the range of 0.1 mg to 10 g, more preferably 1.0 mg to 1000 mg, still more preferably 5.0 mg to 500 mg, and particularly preferably 10.0 mg to 100 mg per 100 g of inorganic raw material. These catalysts can be used either alone or in combination of two or more.

[0075] Furthermore, in the first mixing step described above, by mixing an acidic or basic catalyst together with the metal alkoxide and cellulose nanocrystals, the reaction of the above formula (2) can proceed efficiently, and a homogeneous solution of the metal alkoxide and cellulose nanocrystals can be easily obtained.

[0076] In the first mixing step described above, the ratio of inorganic raw materials to water is preferably 40 to 400 g, more preferably 120 to 400 g, even more preferably 140 to 350 g, even more preferably 160 to 300 g, and particularly preferably 180 to 280 g, per 100 g of inorganic raw materials. Note that this ratio is based on the total amount of water present in the first mixing step. When a water-containing dispersion is used as the cellulose nanocrystal dispersion, this amount includes the amount of water contained in the cellulose nanocrystal dispersion. For example, if the amount of water is less than the above range, the hydrolysis reaction and polycondensation reaction of the inorganic raw materials may not proceed efficiently, and the dispersibility of the cellulose nanocrystals may be poor, leading to the risk of opacification. If the amount of water is greater than the above range, the resulting wet gel may become brittle, prone to cracking, fracture, pore shrinkage, etc. during drying, and the resulting porous gel may become brittle or opaque due to the roughened skeleton of the material.

[0077] Furthermore, although not particularly limited, additives such as surfactants can be added to the sol-state reaction solution depending on the purpose, such as improving the dispersibility of the cellulose nanocrystals. Both nonionic and ionic surfactants can be used as surfactants, and cationic surfactants, anionic surfactants, and zwitterionic surfactants can be used as ionic surfactants.

[0078] Examples of nonionic surfactants include polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene alkyl ether, polyoxypropylene alkyl ether, and block copolymers of polyoxyethylene and polyoxypropylene.

[0079] Examples of ionic surfactants include cationic surfactants such as cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, and benzalkonium chloride, and examples of anionic surfactants include sodium dodecylsulfonate. Examples of amphoteric surfactants include acyl glutamic acid, lauryl dimethylaminoacetic acid betaine, lauryl dimethylaminoacetic acid betaine, stearyl dimethylaminoacetic acid betaine, and lauryl dimethylamine oxide.

[0080] It is also possible to use a crosslinking agent for chemically crosslinking with the hydroxyl groups of the cellulose nanocrystals. Specific examples include, but are not limited to, polycarboxylic acid crosslinkers selected from the group consisting of tartaric acid, malic acid, poly(acrylic acid), poly(methacrylic acid), poly(maleic acid), poly(methyl vinyl ether-co-maleate) copolymer, poly(methyl vinyl ether-co-itaconate) copolymer, and mixtures thereof; epoxy resins having a glycidyl ether structure; metal salts consisting of divalent or higher metal cations (magnesium ions, calcium ions, barium ions, zinc ions, copper ions, cobalt ions, nickel ions, aluminum ions, iron ions, etc.); and metal alkoxides.

[0081] The reaction temperature in the first mixing step is not particularly limited, but is preferably 5 to 120°C, more preferably 20 to 80°C, and even more preferably 20 to 40°C. If the temperature is in a range equal to or higher than the boiling point of the solvent, it is preferable to prevent evaporation of the solvent by reflux or the like. If necessary, the reaction can also be carried out under vacuum or while sealing in an inert gas such as nitrogen or argon. The reaction time is also not particularly limited, but is preferably 1 second to 6 hours, more preferably 30 minutes to 3 hours, and even more preferably 45 minutes to 90 minutes.

[0082] The base catalyst used in the second mixing step is not particularly limited, but examples thereof include carbonates such as calcium carbonate, potassium carbonate, sodium carbonate, barium carbonate, magnesium carbonate, lithium carbonate, ammonium carbonate, copper (II) carbonate, iron (II) carbonate, silver (I) carbonate, and aqueous solutions of ammonium zirconium carbonate; hydrogen carbonates such as calcium hydrogen carbonate, potassium hydrogen carbonate, sodium hydrogen carbonate, and ammonium hydrogen carbonate; alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide; hydroxides of tetraalkylammonium such as tetramethylammonium hydroxide and tetraethylammonium hydroxide; metal hydroxides such as magnesium hydroxide and calcium hydroxide; ammonium compounds such as ammonium hydroxide, ammonium fluoride, ammonium chloride, and ammonium bromide; sodium metaphosphate, sodium pyrophosphate and basic sodium phosphates such as sodium polyphosphate; aliphatic amines such as allylamine, diallylamine, triallylamine, isopropylamine, diisopropylamine, ethylamine, diethylamine, trimethylamine, triethylamine, n-octylamine, 2-ethylhexylamine, 3-ethoxypropylamine, diisobutylamine, 3-(diethylamino)propylamine, di-2-ethylhexylamine, 3-(dibutylamino)propylamine, tetramethylethylenediamine, t-butylamine, sec-butylamine, propylamine, 3-(methylamino)propylamine, 3-(dimethylamino)propylamine, 3-methoxyamine, dimethylethanolamine, methyldiethanolamine, diethanolamine, and triethanolamine; aqueous ammonia; and ammonium acetate. Among these, organic strong base catalysts are preferred; that is, when an organic base having an acid dissociation constant (PKa) of 9.5 or more at room temperature is used, a porous gel with high strength tends to be easily obtained, and tetramethylammonium hydroxide, triethylamine, etc. are preferably used.

[0083] In the second mixing step, the pH of the solution when the base catalyst is added is not particularly limited, but the pH is preferably 3.0 to 10.0, more preferably 5.0 to 9.0, even more preferably 6.0 to 8.0, and particularly preferably 6.6 to 7.4. By setting the pH within the above range, the reactions of the above formulas (12) to (14) can be effectively carried out, and a bulk porous gel can be suitably obtained.

[0084] The amount of base catalyst used is not particularly limited as long as it can adjust the pH to the above range. When aqueous ammonia is used, the amount of ammonia component is preferably in the range of 0.01 to 1.0 g, more preferably 0.02 to 0.1 g, and even more preferably 0.04 to 0.08 g per 100 g of inorganic raw materials. When tetramethylammonium hydroxide is used, the amount of tetramethylammonium hydroxide component is preferably in the range of 0.001 g to 1.0 g, more preferably 0.01 g to 0.50 g, and particularly preferably 0.05 g to 0.10 g per 100 g of inorganic raw materials. When triethylamine is used, the amount of triethylamine component is preferably in the range of 0.001 g to 1.0 g, more preferably 0.01 g to 0.10 g, and particularly preferably 0.08 g to 0.16 g per 100 g of inorganic raw materials. Addition of the base catalyst in the above amounts allows the reaction to proceed efficiently and uniformly, resulting in a transparent, highly strong porous gel.

[0085] Furthermore, as the base catalyst, a hydrolyzable compound that generates a basic product by hydrolysis or the like can also be used as a catalyst. Specifically, acid amides such as urea, formamide, N-methylformamide, N,N-dimethylformamide, acetamide, N-methylacetamide, and N,N-dimethylacetamide, as well as the cyclic nitrogen compound hexamethylenetetramine can be used, but there are no particular limitations as long as the compound makes the solvent basic after hydrolysis. Furthermore, the hydrolyzable compound can be used in combination with another of the above-mentioned base catalysts.

[0086] When the hydrolyzed compound is used as a catalyst, the amount used may be any amount that can adjust the pH to the above range, and in the case of urea, for example, the amount used is preferably 0.1 to 30 g, more preferably 0.2 to 10 g, even more preferably 0.4 to 2.0 g, and particularly preferably 1.0 to 1.2 g per 100 g of inorganic raw material. If the amount is less than the above range, the reactions of the above formulas (12) to (14) may not proceed smoothly, and if the amount is more than the above range, the pH may become excessively high, causing the porous gel to whiten.

[0087] Furthermore, by adding the above-mentioned hydrolyzable compound in advance to the first mixing step, a basic product is generated during the aging step described below, eliminating the need for the second mixing step in which a basic catalyst is added, making it possible to appropriately control the pH. In other words, using a hydrolyzable compound as a basic catalyst makes it possible to omit the second mixing step, thereby reducing the number of steps. Furthermore, by gradually generating a basic product from the hydrolyzable compound, the pH can be increased stepwise without a sudden increase, which also serves to suppress aggregation of cellulose nanocrystals due to pH changes and improve the appearance of the resulting porous gel. Furthermore, in order to efficiently hydrolyze the hydrolyzable compound, it is preferable to heat the mixture to a temperature equal to or higher than the hydrolysis temperature of the hydrolyzable compound in the aging step described below.

[0088] In the mixing step, the ratio of the total amount of inorganic raw materials to the total amount of water is preferably 40 to 400 g per 100 g of inorganic raw materials, more preferably 140 to 350 g, more preferably 160 to 300 g, and particularly preferably 180 to 280 g. In this case, the total amount of water refers to the total amount of water contained in the system when synthesizing the wet gel, and is the sum of the water contained in the cellulose nanocrystal dispersion and the water added in the mixing step, and refers to the total amount of water in the reaction system when synthesizing the wet gel in the mixing step. If the amount is greater than the above range, the resulting wet gel will be brittle and prone to cracks, breakage, pore shrinkage, etc. during drying. Furthermore, the material skeleton will become coarse, which may cause the resulting porous gel to become brittle or opaque. If the amount is less than the above range, the above formulas (12) to (14) will not proceed easily, and a porous gel may not be obtained.

[0089] In the mixing step, a polar solvent may be further mixed to improve the dispersibility of cellulose, promote the reaction of inorganic raw materials, etc. The polar solvent is preferably a solvent capable of uniformly dispersing cellulose nanocrystals and inorganic raw materials, specifically methanol, ethanol, formamide, dimethylformamide, etc. Formamide, in particular, can be used to obtain a porous gel with high strength and excellent transparency. The amount of formamide used is preferably 1 g to 400 g, more preferably 10 g to 100 g, more preferably 30 to 50 g, and particularly preferably 35 to 45 g per 100 g of inorganic raw materials. When formamide is used, the total amount of water is preferably 40 to 400 g, more preferably 60 to 280 g, and more preferably 80 to 140 g per 100 g of inorganic raw materials. Addition within the above ranges suppresses shrinkage of the wet gel during drying, resulting in a highly transparent porous gel. While the details of how polar solvents affect the physical properties of porous gels are unclear, mixing polar solvents can cause the polar solvent to strongly solvate with water, etc., temporarily releasing OH groups. － It is presumed that the reaction of the above formulas (12) to (14) is promoted by generating

[0090] (Aging Process) The wet gel obtained in the mixing process of the present application may be further subjected to an aging process, if necessary. The aging process involves applying a predetermined amount of energy over a predetermined period of time to further promote the reaction of the above formula (2) and further promote the reactions of the above formulas (12) to (14). One example of the energy is heat (temperature). Specifically, the obtained wet gel can be aged by placing it in a sealed container at an appropriate temperature for an appropriate period of time. The aging process allows the three-dimensional network structure in the wet gel to be further developed. The aging conditions vary depending on the type of inorganic raw material, the boiling point of the solvent, pH, volume, etc., but the aging temperature is preferably 5 to 120°C, more preferably 40 to 100°C, and even more preferably 50 to 80°C. The aging time is preferably 1 to 120 hours, more preferably 24 to 108 hours, and even more preferably 48 to 96 hours. Aging within the above ranges allows for the production of a wet gel with more suitable strength.

[0091] Since it is often difficult to determine the end point of gelation of the sol, the mixing step and the subsequent aging step may be carried out continuously as a series of operations. That is, by carrying out the aging step before the precursor sol is gelled in the mixing step, it is possible to simultaneously proceed with gelation and aging of the sol during the aging step.

[0092] Alternatively, the mixing step may be a step of obtaining a wet gel without using an acid catalyst and / or a basic catalyst. In this case, a wet gel can be obtained by mixing an inorganic raw material such as a metal alkoxysilane with a dispersion of cellulose nanocrystals for a predetermined period of time and allowing the mixture to stand. In this case, an aging step may also be carried out after the mixing step, and the aging step conditions may be the same as those described above.

[0093] (Drying Step) The drying step is a step of obtaining a porous gel by drying the solvent contained in the wet gel of the precursor obtained through the mixing step.

[0094] The method for drying the wet gel obtained by the above method is not particularly limited, and includes atmospheric pressure drying, i.e., drying under a temperature and pressure environment below the critical point of the solvent contained in the wet gel; CO ２ Examples of drying methods include a method of drying through a supercritical state at a temperature and pressure equal to or higher than the critical point of the drying atmosphere, such as the above, and a method of sublimating the solvent by freeze-drying. However, from the viewpoint of productivity, atmospheric pressure drying is most preferred.

[0095] When atmospheric pressure drying is used, the capillary force generated during drying of a wet gel is proportional to the surface tension of the solvent contained in the wet gel, according to the capillary force formula described above. Therefore, drying a wet gel containing a solvent with high surface tension, such as water, is undesirable because it generates a large capillary force, which can easily cause the porous gel to crack or break or reduce its pore volume. Therefore, in order to prevent the porous gel from cracking or breaking or reducing its pore volume, it is extremely important to incorporate a solvent with a surface tension of 30 mN / m or less at 20°C (hereinafter referred to as a "low surface tension solvent") into the wet gel before performing the drying process. Examples of methods for incorporating a low surface tension solvent into a wet gel include the following: replacing the solvent in the wet gel with a low surface tension solvent.

[0096] In the present invention, before the drying step, a solvent substitution treatment may be performed to substitute another solvent for the solvent contained in the precursor wet gel. By performing the solvent substitution treatment, water remaining in the wet gel, catalysts, unreacted inorganic raw materials, surfactants, hydrolyzable compounds, and other additive residues can be removed, and further, by incorporating a low surface tension solvent into the wet gel, the capillary force acting on the wet gel during drying can be weakened.

[0097] The solvent substitution process may involve substituting a different solvent for the solvent contained in the wet gel, followed by further substituting a different solvent. That is, the solvent substitution process may be a multi-step process using different solvents. Furthermore, the solvent substitution process may involve multiple cycles of substitution with the same solvent to ensure sufficient solvent substitution. That is, the solvent contained in the wet gel due to the sol-gel transition may be substituted with a different solvent multiple times (e.g., three times), followed by further substituting with a different solvent multiple times (e.g., three times). For example, the wet gel obtained by the above method may be immersed in a sufficient amount of methanol and heated for approximately 24 hours. After cooling to room temperature, the immersed methanol is removed, and the resulting wet gel is again immersed in fresh methanol and heated. This process is repeated three times, and then the same process as the solvent substitution process using methanol is repeated three times, using n-hexane instead of methanol, to complete the solvent substitution with n-hexane.

[0098] The solvent used in the solvent substitution process is not particularly limited, but can be first substituted with an organic polar solvent or an alcohol such as methanol, ethanol, or propanol, and then preferably used as a low surface tension solvent, such as a chain saturated hydrocarbon such as n-pentane or n-hexane, or a fluorine-based solvent containing at least one fluorine atom in the molecule, such as 2,3-dihydrodecafluoropentane, perfluorohexane, perfluoroheptane, perfluorooctane, methyl nonafluorobutyl ether, or ethyl nonafluorobutyl ether. That is, by first substituting an organic solvent that exhibits affinity for both the solvent added to the sol made of the starting material and the low surface tension solvent, and then substituting it with the low surface tension solvent, substitution with the low surface tension solvent can be efficiently achieved.

[0099] Furthermore, if necessary, various additives can be added to the solvent used in the solvent substitution treatment to modify the resulting porous gel. In particular, a method known as hydrophobization, in which functional groups such as -OH and -COOH in the wet gel are reacted with a hydrophobizing agent such as a silyl ether compound to introduce an organosilicon structure, is useful for improving the moisture stability and heat insulation of the porous gel. The hydrophobizing treatment can be performed by immersing the wet gel in a solvent containing a hydrophobizing agent, and the hydrophobizing agent is not particularly limited, but examples thereof include chlorotrimethylsilane, hexamethyldisilazane, hexamethyldisiloxane, trimethylsilyl trifluoromethanesulfonate, chlorotriethylsilane, tertiarybutyldimethylchlorosilane, chlorotriisopropylsilane, 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane, chloromethyltrimethylsilane, 3-methacryloxypropyltrichlorosilane, trichloromethylsilane, 3-methacryloxypropylmethyldichlorosilane, tris(N,N-dimethylamino)methylsilane, bis(N,N-dimethylamino)dimethylsilane, (N,N-dimethylamino)trimethylsilane, methyltrimethoxysilane, dimethyldimethoxysilane, trimethylmethoxysilane, etc. These hydrophobizing agents may be used alone or in combination of two or more.

[0100] In order to efficiently carry out the hydrophobization treatment, it is desirable to carry out the treatment in an environment where the water in the wet gel has been removed by the above-mentioned solvent substitution treatment. It is preferable to first substitute the solvent contained in the wet gel with a solvent such as an alcohol, e.g., methanol, ethanol, or propanol, or a chain saturated hydrocarbon, e.g., n-pentane or n-hexane, and then immerse the wet gel in a solvent containing a hydrophobizing agent.

[0101] The drying conditions in the drying step are not particularly limited and may be selected appropriately depending on the type of solvent contained in the precursor wet gel. Typically, the pressure is 0.01 MPa to 0.30 MPa and the temperature is -30°C to 150°C, more preferably 0.05 MPa to 0.2 MPa and the temperature is 0°C to 120°C, even more preferably 0.08 MPa to 0.12 MPa and the temperature is 20°C to 80°C, and particularly preferably 0.08 MPa to 0.12 MPa and the temperature is 20°C to 40°C. The drying time may be the shortest time required for the solvent in the wet gel to be completely removed, but is usually in the range of 1 hour to 120 hours, preferably 24 hours to 96 hours, more preferably 48 hours to 72 hours. In order to control the drying rate, the wet gel can be dried while immersed in a solvent with low surface tension such as n-heptane or n-hexane. Furthermore, a heat treatment at a temperature higher than that in the drying step can be performed after the drying step in order to further develop the three-dimensional network structure of the porous gel.

[0102] In this manner, the porous gel of the present invention can be produced.

[0103] The porous gel of the present invention is porous and therefore has a large pore volume, which not only gives it excellent heat insulation, soundproofing properties, and a low dielectric constant, but also allows for improved strength, making it suitable for use as a heat insulating material, soundproofing material, low dielectric constant material, etc. Furthermore, by utilizing its high specific surface area, the porous gel of the present invention can also be used as a catalyst, gas sensor, oil adsorbent, etc. Furthermore, due to its excellent transparency, it can also be used in various optical material applications such as low refractive index materials, and can also be used as a transparent heat insulating material for insulating window materials for homes and solar heat collector panels.

[0104] The shape of the porous gel of the present invention is not particularly limited and may be appropriately selected depending on the application to be used. The porous gel may be a molded body having a specific shape, or may be in the form of a panel, a thin film, granules, or powder. Furthermore, the porous gel may be dissolved in a solvent and used as a coating material, or the coating material may be composited with a substrate such as cloth, paper, a foam, a film, a glass plate, or a metal plate. In these forms, the porous gel can also be suitably used as a heat insulating material, a soundproofing material, a low dielectric constant material, etc.

[0105] Furthermore, the method for producing a porous gel of the present invention does not require a high-pressure process such as supercritical drying, and drying at normal pressure is applicable. Furthermore, it is possible to suppress a decrease in pore volume during drying and cracking or destruction of the porous gel, making it possible to produce a porous gel with a large pore volume relatively easily and inexpensively.

[0106] Next, the present invention will be specifically explained by way of examples, but the present invention is not limited to these examples.

[0107] Example 1 (Preparation of an aqueous dispersion of anionic functional group-containing cellulose nanocrystals) Pulp was decomposed and purified using a 64 wt% aqueous sulfuric acid solution, followed by drying to obtain anionic functional group-containing cellulose nanocrystals (CNC). Ion-exchanged water was then added to the obtained anionic functional group-containing cellulose nanocrystals, and the mixture was dispersed using an ultrasonic homogenizer (Hielscher, UP400st) to obtain a 6.0 wt% aqueous dispersion of anionic functional group-containing cellulose nanocrystals. The amount of anionic functional groups in the obtained anionic functional group-containing cellulose nanocrystals was 0.17 mmol / g.

[0108] (Preparation of Wet Gel) A mixed solution was obtained by mixing 3.2 g of the aqueous dispersion of 6.0 wt% anionic functional group-containing cellulose nanocrystals obtained above, 5.0 g of ion-exchanged water, 0.5 g of 40 wt% urea aqueous solution, and 3.2 g of tetramethoxysilane (TMOS). Stirring of the resulting mixed solution was then initiated at room temperature (23°C). 19.2 μL of a 2 wt% aqueous sulfuric acid solution was added dropwise to the stirred mixed solution, and stirring was continued for 1 hour at room temperature. The resulting solution was allowed to stand at 60°C for 72 hours in a sealed polypropylene resin container to age the solution, thereby producing a wet gel. After dispersing the anionic functional group-containing cellulose nanocrystals obtained by the above method in water, it took less than 24 hours to obtain a wet gel.

[0109] (Solvent substitution treatment) Next, as the first solvent substitution treatment, the obtained wet gel was immersed in methanol at room temperature and allowed to stand in a sealed container at 60° C. for 24 hours. Next, as the second solvent substitution treatment, the methanol used for the immersion was drained, and the obtained wet gel was again immersed in fresh methanol and allowed to stand in a sealed container at 60° C. for 24 hours. Next, as the third solvent substitution treatment, the same operation as in the second solvent substitution treatment was performed.

[0110] Next, the wet gel that had been subjected to the solvent substitution treatment with methanol three times in total as described above was removed from the sealed container, and as the first solvent substitution treatment with a low surface tension solvent, the removed wet gel was immersed in n-hexane, a low surface tension solvent, at room temperature and allowed to stand in a sealed container at 50°C for 24 hours. Next, as the second solvent substitution treatment with a low surface tension solvent, the n-hexane used for immersion was drained, and the obtained wet gel was again immersed in fresh n-hexane and allowed to stand in a sealed container at 50°C for 24 hours. Next, as the third solvent substitution treatment with a low surface tension solvent, the same procedures as those for the second solvent substitution treatment were performed.

[0111] (Drying Step) Next, the wet gel that had been subjected to the solvent substitution treatment with n-hexane three times in total as described above was placed in a dryer and dried. Specifically, the wet gel was completely immersed in fresh n-hexane and left to stand in an atmosphere of 23°C and 50% RH for 72 hours to dry, and the solvent was removed to obtain a porous gel without defects such as cracks or breaks, as shown in Figure 1. Table 1 shows the amounts of each material used in the production and the evaluation results. Each evaluation was performed as follows.

[0112] In Example 1, SEM observation confirmed the presence of nanorod-shaped structures in which anionic functional group-containing cellulose nanocrystals or fibrous structural units derived from anionic functional group-containing cellulose nanocrystals were randomly distributed throughout the porous gel (the same applies to Examples 2 to 17 described below), confirming that the porous gel contained dispersed cellulose nanocrystals. Dispersing cellulose nanocrystals prevented a decrease in transparency, and high transmittance was confirmed in the light transmittance evaluation described below. Furthermore, it was confirmed that the resulting porous gel had an irregular structure in which particles were randomly connected to each other.

[0113] ·solid ２９ Si-DD / MAS-NMR Measurement The obtained porous gel was measured under the following conditions using a Fourier transform nuclear magnetic resonance apparatus (product name: JNM-ECA400, manufactured by JEOL Ltd.): ２９ Si-DD / MAS-NMR spectra were obtained and Q １ Peak, Q ２ Peak, Q ３ Peak, and Q ４ Q for the total area of ​​the peak ４ The peak area ratio was calculated. Data analysis was carried out by calculating the peak area for each peak after waveform processing of the spectrum after Fourier transformation. Temperature: Room temperature Observation nucleus: ２９ Si Rotation speed: 5 kHz Number of integrations: 8000 Measurement mode: DD / MAS method Waiting time: 10 s

[0114] Appearance evaluation (aggregates) After drying, each of the 4 mm thick samples obtained in the Examples and Comparative Examples was visually observed and rated as follows: ◯: No aggregates were visually observed in the obtained solid matter Δ: Aggregates were visually observed in the obtained solid matter ×: No solid matter was obtained or the obtained solid matter was whitened overall, making it impossible to evaluate the aggregates

[0115] Appearance evaluation (whitening, cracks) After drying, each of the 4 mm thick samples obtained in the Examples and Comparative Examples was visually observed and rated as follows: ◎: The obtained solid was transparent, and no defects such as cracks were visually observed. ◯: The obtained solid was transparent, but defects such as cracks were visually observed. △: No defects such as cracks were visually observed in the obtained solid, but some or all of the solid was whitened and opaque. ×: Defects such as cracks were visually observed in the obtained solid, and some or all of the solid was whitened and opaque.

[0116] Evaluation of mechanical strength A cylindrical porous gel with a diameter of 8 mm and a height of 10 mm was prepared and used as a measurement sample. Then, using an autograph (product name "AG-IS", manufactured by Shimadzu Corporation) as a compression tester, a load was applied in the compression direction to the upper surface of the cylindrical porous gel at a rate of 1 mm / min, and the maximum strength until the measurement sample broke was evaluated as follows: ◎+: Maximum strength is 700 N or more ◎: Maximum strength is 500 N or more and less than 700 N ○: Maximum strength is 200 N or more and less than 500 N △: Maximum strength is 50 N or more and less than 200 N ×: Maximum strength is less than 50 N

[0117] Evaluation of nitrogen adsorption amount For the obtained porous gel, a specific surface area / pore size distribution measuring device (product name "BELSORP MAX II", manufactured by Microtrac-Bell Co., Ltd.) was used to heat treat the measurement sample under vacuum at 150°C for 2 hours, and then nitrogen adsorption / desorption measurement was performed at an adsorption temperature of 77 Kelvin to obtain a nitrogen adsorption / desorption isotherm. In the nitrogen adsorption / desorption isotherm obtained by the above method, the saturated vapor pressure P ０ The relative pressure P / P of the equilibrium pressure P to ０ The maximum amount of nitrogen adsorption was calculated when the value was between 0 and 0.99.

[0118] Evaluation of Pore Volume The pore volume was calculated by the BJH method from the adsorption / desorption isotherm obtained by the above method.

[0119] - Evaluation of BET specific surface area The saturated vapor pressure P ０ The relative pressure P / P of the equilibrium pressure P to ０ The BET specific surface area was calculated using the BET method in the region of excellent linearity of 0.05 to 0.30.

[0120] Evaluation of Light Transmittance The light transmittance of the obtained porous gel was measured for linear transmitted light using an ultraviolet / visible spectrophotometer ("UV-3600 i plus", manufactured by Shimadzu Corporation) under the conditions of a slit width of 2.0 nm and a measurement wavelength range of 800 nm to 200 nm. The light transmittance of the porous gel was measured using a value for a wavelength of 600 nm (visible light) and corrected to a value when the thickness of the porous gel was 3 mm. The light transmittance T after thickness correction Ｃ was calculated by the following formula: Tc=(T' / 100) ３／ｔ × (1−r / 100) ２ Here, Tc is the transmittance (%) after thickness correction, t is the measured thickness (mm) of the porous gel, T' is the internal transmittance (%) relative to the measured light transmittance T, and r is the surface reflectance (%), and T' and r are calculated from the following formula: T'=T / (1-r / 100). ２ r (%) = 100×((n-1) / (n+1)) ２ Here, T is the measured light transmittance at a wavelength of 600 nm, n is the refractive index, and r, T', and Tc were calculated assuming n = 1.03. The light transmittance was evaluated as follows from the light transmittance after thickness correction calculated by the above method. ⊚+: Light transmittance Tc after thickness correction is 70% or more ⊚: Light transmittance Tc after thickness correction is 50% or more and less than 70% ◯: Light transmittance Tc after thickness correction is 30% or more and less than 50% △: Light transmittance Tc after thickness correction is 10% or more and less than 30% ×: Light transmittance Tc after thickness correction is less than 10%

[0121] Example 2 A porous gel was obtained and evaluated in the same manner as in Example 1, except that ion-exchanged water and ethylene glycol were added to the mixed solution at the blending ratio shown in Table 1. The results are shown in Table 1. After the anionic functional group-containing cellulose nanocrystals obtained by the above method were dispersed in water, it took less than 24 hours for a wet gel to be obtained.

[0122] Examples 3 and 4 A porous gel was obtained and evaluated in the same manner as in Example 1, except that no urea aqueous solution was used, and ion-exchanged water was added to the mixed solution to the blending ratio shown in Table 1, followed by stirring for 1 hour, and then a 0.64 wt % ammonia aqueous solution was added dropwise to the mixed solution to the blending ratio shown in Table 1. The results are shown in Table 1. After dispersing the anionic functional group-containing cellulose nanocrystals obtained by the above method in water, it took less than 2 hours for a wet gel to be obtained.

[0123] Example 5 A porous gel was obtained and evaluated in the same manner as in Example 1, except that no aqueous sulfuric acid solution or aqueous urea solution was used, and ion-exchanged water was added to the mixed solution at the blending ratio shown in Table 1. The results are shown in Table 1. After the anionic functional group-containing cellulose nanocrystals obtained by the above method were dispersed in water, it took 48 hours or less for a wet gel to be obtained.

[0124] Examples 6 and 7 A porous gel was obtained in the same manner as in Example 1, except that 6.0 wt% of an aqueous dispersion of anionic functional group-containing cellulose nanocrystals and ion-exchanged water were added to the mixed solution in the blending ratio shown in Table 1, and the gel was evaluated in the same manner. Furthermore, in Example 6, SEM observation of the resulting porous gel confirmed the presence of nanorod-shaped structures in which cellulose nanocrystals, or fibrous structural units derived from anionic functional group-containing cellulose nanocrystals, as shown in FIG. 2 were randomly distributed. Furthermore, it was confirmed that the resulting porous gel had an irregular structure in which particles were randomly connected to each other. The results are shown in Table 1. After dispersing the anionic functional group-containing cellulose nanocrystals obtained by the above method in water, it took less than 24 hours for a wet gel to be obtained.

[0125] Example 8 A porous gel was obtained and evaluated in the same manner as in Example 1, except that a 2 wt % aqueous solution of dodecylbenzenesulfonic acid (DBS) was used instead of the aqueous sulfuric acid solution, no aqueous urea solution was used, and ion-exchanged water was added to the mixed solution so as to achieve the blending ratio shown in Table 1. The results are shown in Table 1. After the anionic functional group-containing cellulose nanocrystals obtained by the above method were dispersed in water, it took within 48 hours for a wet gel to be obtained.

[0126] Example 9 A porous gel was obtained and evaluated in the same manner as in Example 1, except that 6.0 wt % of an aqueous dispersion of anionic functional group-containing cellulose nanocrystals, ion-exchanged water, and 1,2-propanediol were added to a mixed solution in the blending ratio shown in Table 1. The results are shown in Table 1. After dispersing the anionic functional group-containing cellulose nanocrystals obtained by the above method in water, it took less than 24 hours for a wet gel to be obtained.

[0127] Example 10 A porous gel was obtained and evaluated in the same manner as in Example 1, except that 6.0 wt% of an aqueous dispersion of anionic functional group-containing cellulose nanocrystals, ion-exchanged water, and ethylene glycol were added to a mixed solution in the blending ratio shown in Table 1. The results are shown in Table 1. After dispersing the anionic functional group-containing cellulose nanocrystals obtained by the above method in water, it took less than 24 hours for a wet gel to be obtained.

[0128] Examples 11 and 12 A porous gel was obtained and evaluated in the same manner as in Example 1, except that 6.0 wt% of an aqueous dispersion of anionic functional group-containing cellulose nanocrystals and ion-exchanged water were added to the mixed solution in the blending ratio shown in Table 1. The results are shown in Table 1. After dispersing the anionic functional group-containing cellulose nanocrystals obtained by the above method in water, it took less than 24 hours for a wet gel to be obtained.

[0129] Example 13 A porous gel was obtained and evaluated in the same manner as in Example 1, except that an aqueous urea solution was added to the mixed solution at the blending ratio shown in Table 1. The results are shown in Table 1. After the anionic functional group-containing cellulose nanocrystals obtained by the above method were dispersed in water, it took less than 24 hours for a wet gel to be obtained.

[0130] Example 14 A porous gel was obtained and evaluated in the same manner as in Example 1, except that a 1 wt % aqueous solution of tetramethylammonium hydroxide (TMAOH) was added to the mixed solution instead of the urea aqueous solution to the blending ratio shown in Table 1. The results are shown in Table 1. After the anionic functional group-containing cellulose nanocrystals obtained by the above method were dispersed in water, it took less than 2 hours for a wet gel to be obtained.

[0131] Example 15 A porous gel was obtained and evaluated in the same manner as in Example 1, except that a 1 wt % aqueous triethylamine solution was added to the mixed solution instead of the urea aqueous solution to achieve the blending ratio shown in Table 1. The results are shown in Table 1. After the anionic functional group-containing cellulose nanocrystals obtained by the above method were dispersed in water, it took less than 2 hours for a wet gel to be obtained.

[0132] Example 16 A porous gel was obtained and evaluated in the same manner as in Example 1, except that no urea aqueous solution was used and formamide and ion-exchanged water were added to the mixed solution in the blending ratio shown in Table 1. The results are shown in Table 1. After the anionic functional group-containing cellulose nanocrystals obtained by the above method were dispersed in water, it took less than 24 hours for a wet gel to be obtained.

[0133] Example 17 A porous gel was obtained and evaluated in the same manner as in Example 1, except that no urea aqueous solution was used and a 1 wt % aqueous solution of tetramethylammonium hydroxide (TMAOH), formamide, and ion-exchanged water were added to the mixed solution in the blending ratios shown in Table 1. The results are shown in Table 1. After dispersing the anionic functional group-containing cellulose nanocrystals obtained by the above method in water, it took less than 2 hours to obtain a wet gel.

[0134] Comparative Example 1: A 4.5 wt% aqueous dispersion of anionic functional group-containing cellulose nanocrystals was prepared and allowed to stand for 4 days in an environment of 23°C and 50% RH. Phase separation occurred due to the formation of a liquid crystal phase of the anionic functional group-containing cellulose nanocrystals. The lower phase was used, and a porous gel was obtained and evaluated in the same manner as in Example 1, except that the aqueous dispersion of anionic functional group-containing cellulose nanocrystals and ion-exchanged water were added to the mixed solution in the blending ratio shown in Table 1. The results are shown in Table 2. The time required to obtain a wet gel after dispersing the anionic functional group-containing cellulose nanocrystals obtained by the above method in water was within 96 to 120 hours. The resulting porous gel had significantly reduced transparency due to poor dispersion of the cellulose nanocrystals, and its light transmittance was also significantly inferior to that of the porous gel of Example 6.

[0135] Comparative Example 2 A porous gel was obtained and evaluated in the same manner as in Example 1, except that no anionic functional group-containing cellulose nanocrystals were used, ion-exchanged water was added to the mixed solution to the blending ratio shown in Table 2, the mixture was stirred for 1 hour, and then a 0.64 wt % aqueous ammonia solution was added dropwise to the mixed solution to the blending ratio shown in Table 2. The results are shown in Table 2.

[0136] Comparative Example 3 A porous gel was produced in the same manner as in Example 1, except that the anionic functional group-containing cellulose nanocrystals were replaced with a 1.0 wt % aqueous dispersion of "LEOCRYSTA I-2SX" (product name, cellulose nanofiber (CNF) manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) and ion-exchanged water in the blending ratio shown in Table 2. However, the gel was destroyed during drying, and a bulk porous gel could not be obtained. The results are shown in Table 2.

[0137] Comparative Example 4 A porous gel was produced in the same manner as in Example 1, except that a 3.0 wt% aqueous dispersion of Chitosan 10 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and ion-exchanged water were used in the blending ratio shown in Table 2 instead of the anionic functional group-containing cellulose nanocrystals. However, the gel was destroyed during drying, and a bulk porous gel could not be obtained. The results are shown in Table 2.

[0138]

[0139] In Table 1, each compound is as follows: TMOS: tetramethoxysilane CNC: cellulose nanocrystal containing anionic functional groups CNF: cellulose nanofiber DBS: dodecylbenzenesulfonic acid TMAOH: tetramethylammonium hydroxide In Tables 1 and 2, the amount of each component is expressed in parts by weight, where the total weight of the metal alkoxide is taken as 100 parts by weight. The total amount of water refers to the total parts by weight including the amount of ion-exchanged water added in advance as well as the amount of water contained in the polysaccharide, acid catalyst, and base catalyst.

[0140] As shown in Table 1, it was confirmed from Examples 1 to 17 that the porous gels in which cellulose nanocrystals were dispersed were porous gels that had both high strength and large pore volume.

Claims

1. Porous gel with dispersed cellulose nanocrystals.

2. The porous gel according to claim 1, wherein the material skeleton of the porous gel contains silicon oxide as a main component.

3. The porous gel according to claim 2, wherein the silicon oxide and the cellulose nanocrystals are bonded via Si—O—C bonds.

4. A porous gel according to any one of claims 1 to 3, containing the cellulose nanocrystals in a proportion of 0.1% by weight or more but less than 50% by weight.

5. The porous gel according to any one of claims 1 to 3, wherein the light transmittance of the porous gel at a thickness of 3 mm for in-line transmitted light of 600 nm wavelength is 10% or more.

6. The maximum nitrogen adsorption amount is 500 cm ３ The porous gel according to any one of claims 1 to 3, wherein the viscosity is (STP) / g or more.

7. Solid ２９ 4. The porous gel according to claim 1, wherein in a Si-NMR spectrum, the ratio of the peak area attributable to Q units to the total area of ​​peaks attributable to silicon-containing bond units, M units, D units, T units and Q units, is 10 to 100%.

8. Solid ２９ In Si-NMR, Q １ Peak, Q ２ Peak, Q ３ Peak, and Q ４ Q for the total area of ​​the peak ４ The porous gel according to any one of claims 1 to 3, wherein the peak area ratio is 50% or more.

9. The porous gel according to any one of claims 1 to 3, which has an irregular structure connected in a granular manner.

10. A method for producing a porous gel as described in claim 1, comprising: a dispersing step of dispersing cellulose nanocrystals in a solvent to obtain a dispersion; a mixing step of adding an inorganic material to the cellulose nanocrystal dispersion and mixing it to obtain a wet gel; and a drying step of drying the solvent contained in the wet gel.

11. A method for producing a porous gel as described in claim 10, wherein the mixing step includes a first mixing step of adding inorganic raw materials and an acid catalyst to the dispersion of cellulose nanocrystals, and a second mixing step of adding a basic catalyst to the mixture obtained by the first mixing step.

12. The method for producing a porous gel according to claim 10, wherein the mixing step is carried out in the presence of water, and the ratio of the total amount of inorganic raw materials to water is 40 to 400 g per 100 g of inorganic raw materials.

13. The method for producing a porous gel according to claim 10, wherein the mixing step is carried out in the presence of water and formamide.

14. The method for producing a porous gel according to claim 10, wherein the time required from the dispersing step to the gelling step in the mixing step to obtain a wet gel is within 72 hours.

15. The method for producing a porous gel according to claim 10, wherein the solvent is removed in the drying step under conditions of a temperature and / or pressure below the critical point of the solvent.

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