Porous gel and manufacturing method of porous gel

Abstract

To provide a porous gel having improved strength.SOLUTION: A porous gel contains reactant of a metal alkoxide and a cellulose nanocrystal.SELECTED DRAWING: None

Description

[Technical Field]

[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. [Background technology]

[0002] In a broad sense, a dry gel obtained by supercritical drying of 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 of a wet gel, a low-density dry gel composed of a microporous solid whose dispersed phase is gas is called an "aerogel." Furthermore, a porous gel is generally a general term for a gel body having multiple pores (voids) inside the material. In this embodiment, however, in the nitrogen adsorption isotherm obtained by nitrogen adsorption measurement performed by the following method, the maximum nitrogen adsorption amount is 100 cm when the relative pressure P / P0 of the equilibrium pressure P to the saturated vapor pressure P0 is 0 to 0.99. 3 It is defined as a structure having a mass of at least (STP) / g.

[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 aerogels. Typically, porous gels such as aerogels 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 converted to gas 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. A "wet gel" refers to a wet gel solid 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 solvent's critical point. 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 expressed by the following formula is generated within the gel during drying of the wet gel. P C =-2γcos(θ) / a (In the above formula, 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, 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, these ingenious methods alone make it difficult to obtain a porous gel with a large pore volume such as aerogel, 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 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 metal alkoxides having organic modifying groups such as alkyl groups, 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 to the solution, 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 tends to cause opacity due to macroscopic phase separation between the inorganic and polymer components, making it difficult to produce a transparent, 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. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 10-182261 [Patent Document 2] Patent No. 5250900 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[0015] The inventors discovered that by making the porous gel contain a reaction product of a metal alkoxide and cellulose nanocrystals, a porous gel with a large pore volume and improved strength can be obtained, and that it can be produced by drying at atmospheric pressure, thereby completing the present invention. That is, the present invention provides the following inventions [1] to

[15] .

[0016] [1] A porous gel containing the reaction product of metal alkoxide and cellulose nanocrystals. [2] The porous gel according to [1], wherein the metal alkoxide contains at least an alkoxysilane. [3] The porous gel according to either [1] or [2], containing the cellulose nanocrystals in an amount of 0.1% by weight or more and less than 50% by weight. [4] The porous gel according to any one of [1] to [3], wherein the light transmittance of the porous gel at a thickness of 3 mm for in-line transmitted light of a wavelength of 600 nm is 10% or more. [5] The pore volume measured by the BJH method for nitrogen adsorption is 0.5 cm 3 The porous gel according to any one of [1] to [4], wherein the porous gel has a viscosity of 1 / g or more. [6] Solid 29 The porous gel according to any one of [1] to [5], 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%. [7] Solid 29 In Si-NMR, Q 1 Peak, Q 2 Peak, Q 3 Peak, and Q 4 Q relative to the total area of ​​the peak 4 The porous gel according to any one of [1] to [6], wherein the peak area ratio is 50% or more. [8] The porous gel according to any one of [1] to [7], which has an irregular structure connected in a granular manner. [9] A method for producing a porous gel containing a reaction product of a metal alkoxide and a cellulose nanocrystal, comprising: A dispersing step of dispersing the cellulose nanocrystals in a solvent to obtain a dispersion; a mixing step of adding the metal alkoxide to the dispersion of the cellulose nanocrystals and mixing them to obtain a wet gel; a drying step of drying the solvent contained in the wet gel.

[10] The method for producing a porous gel according to [9], wherein the solvent used in the dispersion step is an aqueous solvent.

[11] The method for producing a porous gel according to [9] or

[10] , wherein a hydrolytic catalyst is further added and mixed in the mixing step to obtain a wet gel.

[12] The method for producing a porous gel according to any one of [9] to

[11] , wherein the mixing step comprises a first mixing step of adding the metal alkoxide 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.

[13] The method for producing a porous gel according to any one of [9] to

[12] , wherein the wet gel is obtained without using an acid catalyst or a basic catalyst in the mixing step.

[14] The method for producing a porous gel according to any one of [9] to

[13] , wherein the mixing step is performed in the presence of water, and the ratio of the total amount of the metal alkoxide to the total amount of water is 40 to 400 g per 100 g of the metal alkoxide.

[15] The method for producing a porous gel according to any one of [9] to

[14] , wherein the mixing step is carried out in the presence of water and formamide.

[16] The method for producing a porous gel according to any one of [9] to

[15] , wherein the removal of the solvent in the drying step is carried out at a temperature and / or under pressure below the critical point of the solvent. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a photograph showing the appearance of the bulk porous gel obtained in Example 1. [Figure 2] FIG. 1 is an SEM image of the porous gel obtained in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0019] <Porous gel> The porous gel of the present invention is a porous gel containing a reaction product of a metal alkoxide and a cellulose nanocrystal.

[0020] The porous gel of the present invention may be a solid having pores (voids) therein and containing a reaction product of a metal alkoxide and cellulose nanocrystals. Because the porous gel of the present invention contains a reaction product of a metal alkoxide and cellulose nanocrystals, it is possible to obtain a porous gel that has excellent transparency to visible light, as well as 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 less likely to be damaged by 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 can be produced relatively easily and inexpensively because bulk or powdery porous gels can be produced by atmospheric pressure drying without requiring high-pressure processes such as supercritical drying.

[0021] Cellulose nanocrystals are rod-shaped crystalline cellulose 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 sulfuric acid treatment of cellulose fibers 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, and particularly in the range of 2 to 20 nm, a fiber length of 50 to 1000 nm, and particularly in the range of 100 to 500 nm, an aspect ratio of 2 to 100, and particularly in the range of 5 to 50, and a crystallinity of 50% or more, preferably 60% or more, and particularly 70% or more are preferred. The fiber diameter, fiber length, and aspect ratio of nanocellulose can be determined by observing the nanocellulose aqueous dispersion or dispersed fibers with an SEM, selecting 10, or more preferably 100, fibers, and measuring the average values.

[0022] 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, resulting in 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.

[0023] 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, sulfur trioxide-pyridine complex, phosphate-urea, TEMPO catalyst, or oxidizing agent. The hydrophilization treatment using carbodiimide, sulfuric acid, or sulfur trioxide-pyridine complex introduces hydrophilic functional groups into the cellulose nanocrystals and further shortens the cellulose nanocrystals. Furthermore, the treatment using phosphate-urea, a TEMPO catalyst, or an oxidizing agent introduces anionic functional groups such as phosphate groups or carboxyl groups, adjusting the total amount of anionic functional groups in the cellulose nanocrystals to fall within the above range.

[0024] If necessary, cellulose nanocrystals may be hydrophobized. The hydrophobization method is not particularly limited, but includes acylation of some of the hydroxyl groups (e.g., at the 6-position) of the cellulose nanocrystals with an acid or other suitable agent, treatment with a hydrophobizing agent such as trimethylchlorosilane, hexamethyldisilazane, or hexamethyldisiloxane. Modified cellulose nanocrystals can also be obtained by 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. Hydrophobization of cellulose nanocrystals improves their dispersibility in nonpolar organic solvents, enabling the production of porous gels, such as wet gels and aerogels, in which the cellulose nanocrystals are dispersed.

[0025] The porous gel of the present invention may contain any material containing a reaction product of a metal alkoxide and cellulose nanocrystals. The content of cellulose nanocrystals (the 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 wt% or more but less than 50 wt%, more preferably 1 to 30 wt%, even more preferably 2 to 20 wt%, and particularly preferably 3 to 10 wt% relative to the total solid weight of the porous gel. By setting the content of cellulose nanocrystals within the above range, the strength, transparency, and heat insulation of the porous gel can be further improved. If the content is less than the above range, it is difficult to obtain the strength-enhancing effect derived from the reaction product of the metal alkoxide and cellulose nanocrystals. If the content is greater than the above range, the cellulose nanocrystals in the porous gel may be difficult to disperse, resulting in the gel becoming opaque.

[0026] Furthermore, the porous gel of the present invention contains a reaction product of a metal alkoxide and cellulose nanocrystals. The metal alkoxide is not particularly limited, but it is desirable that it forms a wet gel by a liquid phase reaction, and includes compounds represented by the following general formula (1), their oligomers, or their hydrolysates, which can be suitably used. M(R 1 ) j (OR 2 ) k (1) (In the above general formula (1), M is a metal atom, 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, j is an integer of 0 or more, and k is an integer of 1 or more.

[0027] The metal atom M constituting the metal alkoxide is not particularly limited, and may be any metal atom capable of forming a metal alkoxide. Si, Ti, Zr, Mg, Zn, Pb, Al, Ba, Y, W, V, Na, Li, In, etc. are preferred. Si, Ti, and Zr are more preferred, with Si being particularly preferred, from the viewpoint of having high transparency to visible light, making it suitable for applications requiring optical properties, and obtaining a porous gel with a large pore volume. That is, the metal alkoxide is preferably an alkoxysilane, alkoxytitanium, or zirconium alkoxide, with alkoxysilane being particularly preferred. That is, the porous gel of the present invention preferably contains Si as a main component. The metal alkoxide may be a single type, or two or more types may be contained.

[0028] 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.

[0029] 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 2is 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). In addition, j in the above general formula (1) is an integer of 0 or more, preferably an integer of 0 to 2, more preferably 0 or 1, and particularly preferably 0. That is, M(OR 2 ) k In the case of alkoxysilane, Si(OR 2 In the general formula (1), k is an integer of 1 or more, preferably an integer of 1 to 4, more preferably an integer of 2 to 4, even more preferably an integer of 3 to 4, and particularly preferably 4.

[0030] In the above general formula (1), the alkoxysilane where j = 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.

[0031] In the above general formula (1), specific examples of alkoxysilanes where j=0 include, but are not limited to, tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane. In the above general formula (1), specific examples of zirconium alkoxides where j=0 include tetramethoxyzirconium, tetraethoxyzirconium, tetrapropoxyzirconium, and tetrabutoxyzirconium. In the above general formula (1), specific examples of alkoxytitanium where j=0 include tetramethoxytitanium, tetraethoxytitanium, tetrapropoxytitanium, and tetrabutoxytitanium. Among these, tetramethoxysilane and tetraethoxysilane are preferred, and tetramethoxysilane is more preferred.

[0032] 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.

[0033] 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.

[0034] The porous gel of the present invention may contain a reaction product of a metal alkoxide and cellulose nanocrystals. The reaction product of a metal alkoxide and cellulose nanocrystals refers to a product in which some structural units derived from the metal alkoxide are chemically bonded to C atoms derived from the cellulose nanocrystals. 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 MOC 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, it is preferred that at least some of the reaction products have the M atom derived from the metal alkoxide bonded to the cellulose nanocrystals via an MOC bond. By bonding these via an MOC bond, the porous gel of the present invention can be made to have improved strength. Whether an MOC 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. Preferably, the porous gel of the present invention has Si-O-C bonds derived from the reaction between alkoxysilane-derived structural units and cellulose nanocrystals.

[0035] For example, the MOC bond between the above-mentioned metal alkoxide and cellulose nanocrystals is preferably formed by a condensation reaction between the M-OH derived from the metal alkoxide and the OH group derived from the cellulose nanocrystals under heating conditions or with the addition of a catalyst. M-OH + HO-Rc → MO-Rc + H2O (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.) In particular, 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).

[0036] 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. -MOM- (3) (In the above general formula (3), M is a metal atom.)

[0037] The structure represented by the above general formula (3) is not particularly limited, but can be formed through a hydrolysis reaction and a polycondensation reaction of a metal alkoxide shown in the following formulas (4) to (6). ZM-OR + H2O → ZM-OH + ROH (4) ZM-OH + Z'-M-OH → ZMOM-Z' + H2O (5) ZM-OH + Z'-M-OR → ZMOM-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.)

[0038] In the porous gel of the present invention, the content of structural units derived from metal alkoxides is not particularly limited, but is preferably 50% by weight or more, more preferably 70 to 99% by weight, even more preferably 80 to 98% by weight, and particularly preferably 90 to 97% by weight, based on the total solid weight of the porous gel.

[0039] In the porous gel of the present invention, it is sufficient that at least a portion of the metal alkoxide contained in the porous gel of the present invention reacts with at least a portion of the cellulose nanocrystals to form a reaction product, and the content ratio of the reaction product is not particularly limited.

[0040] The porous gel of the present invention may contain a compound having a total of two or more alcoholic hydroxyl groups and / or ester bonds as a crosslinkable compound. When the porous gel of the present invention contains a compound having a total of two or more alcoholic hydroxyl groups and / or ester bonds as a crosslinkable compound, 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 two or more, and n is an integer of 1 or more.)

[0041] 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, and even more preferably a linear or branched divalent or higher alkylene group. X 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. X is C 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 Examples of cyclohexylene include 1,4-hexylene, 1,5-hexylene, 1,6-hexylene, 2,3-hexylene, 2,4-hexylene, 2,5-hexylene, 3,4-hexylene, 1,2-cyclohexylene, 1,3-cyclohexylene, 1,4-cyclohexylene, 1,2-cyclopropylene, 1,3-cyclopropylene, 1,2-cyclobutylene, and 1,2-cyclopropylene groups. 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 contain two or more types.

[0042] 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.

[0043] 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 structure represented by the general formula (7) is formed by the transesterification reaction according to the following formula (8) and / or formula (9) proceeding in multiple steps. Z-Si-OR + R 1OH → 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 and R 1 ,R 2 is any organic group, and Z is any group.

[0044] The crosslinkable compound may be any compound having a total of two or more alcoholic hydroxyl groups or ester bonds, and is particularly 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. ol, 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, which are polysaccharides formed by the polymerization of a large number of monosaccharide molecules through glycosidic bonds, and metal salts of the above polysaccharides.Among these, compounds having two adjacent hydroxy groups and 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 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 bulk porous gels free from cracks and fractures, or porous gels with large pore volumes.

[0045] Furthermore, the crosslinkable compound may be a compound having two or more ester bonds, specific examples of which 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 imparting flexibility. Specific examples of compounds having both an ester bond and a hydroxyl group include methyl glycolate and ethyl glycolate.

[0046] 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). ZM-OR + R 1 OH → ZM-OR 1 + ROH (10) ZM-OR + R 2 COOR 3 → ZM-OR 3 + R 2 COOR (11) (In the above formulas (10) and (11), M is a metal atom, and R, R 1 ,R 2 ,R 3 is any organic group, and Z is any group.

[0047] When organic chains are introduced using compounds with a single hydroxyl group or ester bond, cross-linking structures are not formed, inhibiting the development of the three-dimensional network structure of the resulting porous gel, which is expected to allow for control of 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, as well as halogen elements such as F, Cr, Br, and I.

[0048] The compound having one hydroxy group or ester bond is preferably a monohydric alcohol, and examples thereof include, but are not limited to, 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, and 1,1,1,3,3,3-hexafluoro-2-propanol. Among these, alcohols containing fluorine atoms, such as trifluoroethanol, are effective in imparting properties such as water repellency, low refractive index, and low dielectric constant, which are derived from fluorine atoms. These monohydric alcohols may be used alone or in combination of two or more.

[0049] The ratio of the metal alkoxide to the crosslinkable compound 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 100:1000, 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, as a 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.

[0050] Furthermore, the porous gel of the present invention may contain any reaction product of a metal alkoxide and cellulose nanocrystals, but it is preferable that the cellulose nanocrystals be dispersed in the porous gel. It is preferable that the cellulose nanocrystals or fibrous structural units derived from the cellulose nanocrystals are dispersed or distributed randomly in rod-like shapes on the order of nm, without forming a local arrangement on the order of μm (e.g., a striped structure), and it is even more preferable that the cellulose nanocrystals or fibrous structural units derived from the cellulose nanocrystals are distributed randomly without forming a cholesteric liquid crystal phase. More specifically, it is preferable that cellulose nanocrystals or fibrous structural units derived from cellulose nanocrystals are distributed throughout the porous gel, and it is preferable that the cellulose nanocrystals or fibrous structural units derived from cellulose nanocrystals are distributed at approximately the same content throughout the porous gel. By dispersing cellulose nanocrystals as described above, it is possible to suppress the decrease in transparency and heat insulating properties caused by 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. It is sufficient that the porous gel of the present invention is substantially dispersed with cellulose nanocrystals or fibrous structural units derived from cellulose nanocrystals. As long as the effects of the present invention are not impaired, a small portion of the cellulose nanocrystals or fibrous structural units derived from cellulose nanocrystals may form a liquid crystal phase.

[0051] 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 3 mm thickness. The light transmittance of in-line transmitted light through the porous gel can be measured by the method described in the Examples below.

[0052] The porous gel of the present invention has a nitrogen adsorption amount [g / g or cm] at a temperature of 77 Kelvin and a relative pressure P / P0 of 0 to 0.99. 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 1500cm 3 It is most preferable that the porous gel of the present invention has a nitrogen adsorption capacity [g / g or cm] at a temperature of 77 Kelvin relative to a relative pressure P / P0.3 When measuring the change in [(STP) / g], it is preferable that the nitrogen adsorption isotherm is classified as Type IV as defined by IUPAC. Type IV nitrogen adsorption isotherms indicate the presence of mesopores of 2 to 50 nm, and also show 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.

[0053] 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, 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.

[0054] The porous gel of the present invention is a solid 29 In Si-NMR, the ratio of the peak area derived from Q units to the total area of ​​peaks derived from M units, D units, T units and Q units, which are 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 classification of M units, D units, T units and Q units is 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 with 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 proportion 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.

[0055] The porous gel of the present invention is a solid 29 In Si-NMR, the signal derived from four silicon-containing bond units Q, where an oxygen atom bonded to one silicon atom, 1 Peak, Q 2 Peak, Q 3 Peak and Q 4 Q relative to 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 can be classified as follows, depending on the degree of peak shift caused by the electronegativity of the atoms connected to the SiO4 tetrahedron: Q 1 : A structure in which Si atoms and C atoms are arranged around a Si atom via an O atom, and the solid 29 A structural unit with a peak around -80 ppm in the Si-DD / MAS-NMR spectrum. Q 2 : It has two structures with Si atoms and C atoms around Si atoms via O atoms, and is a solid 29 The Si-DD / MAS-NMR spectrum has a peak at around -91 ppm. Q 3 : There are three structures in which Si atoms and C atoms are located around Si atoms via O atoms, and the solid 29 The Si-DD / MAS-NMR spectrum has a peak around -101 ppm. Q 4 : It has four structures in which Si atoms and C atoms are surrounded by O atoms around a Si atom, and is a solid. 29 The Si-DD / MAS-NMR spectrum has a peak at around -110 ppm. Q 1 Peak, Q 2 Peak, Q 3Peak and Q 4 Q relative to the total area of ​​the peak 4 The peak area ratio can be measured, for example, by the method described in the Examples below.

[0056] 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, and more preferably 1.0 cm 3 / g or more, more preferably 2.0 cm 3 / g or more, particularly preferably 2.5 cm 3 / g or more. When the pore volume is in the above range, the thermal conductivity can be further reduced.

[0057] 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 the 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, EP; Joyner, LG; Halenda, PP, J. Am. Chem. Soc. 73, 373 (1951)), and can be measured by the method described in the examples below.

[0058] 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. 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. Furthermore, it is preferable to develop a three-dimensional network structure by undergoing the mixing and aging processes described below.

[0059] <Method of manufacturing porous gel> The method for producing the porous gel of the present invention is not particularly limited, but may be a dispersing step of dispersing cellulose nanocrystals in a solvent to obtain a dispersion; a mixing step of adding a metal alkoxide to the dispersion of cellulose nanocrystals and mixing them to obtain a wet gel; and a drying step of drying the solvent contained in the wet gel.

[0060] (Dispersion process) The dispersion step is a step of preparing a dispersion of cellulose nanocrystals by dispersing cellulose nanocrystals in a solvent.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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 in a metal alkoxide 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 not particularly limited, but is preferably within 72 hours, more preferably within 60 hours, even more preferably within 48 hours, and particularly preferably within 36 hours.

[0065] 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 with the naked eye, but is preferably 30 seconds to 5 minutes, more preferably 1 to 3 minutes. 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.

[0066] (Mixing process) The mixing step is a step of adding a metal alkoxide to the dispersion of cellulose nanocrystals prepared in the 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. Alternatively, if water is not used as a solvent in the dispersion step, water can be added in the mixing step.

[0067] In the mixing process, the cellulose nanocrystal dispersion and the metal alkoxide are mixed under conditions that allow the metal alkoxide to 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 MOC bond between the metal alkoxide and the cellulose nanocrystals. When an alkoxysilane is used as the metal alkoxide, an Si-OC bond is formed between the silicon oxide of the material skeleton and the cellulose nanocrystals. Furthermore, for example, the above-mentioned mixing step may include a first mixing step in which a metal alkoxide, an acid catalyst, and water are added to a dispersion of cellulose nanocrystals, and a second mixing step in which a basic catalyst is added to the mixture obtained by the first mixing step. In particular, after the first mixing step in which water and a metal alkoxide are mixed under acidic conditions, the pH of the resulting sol-like product is adjusted to neutral or basic in the second mixing step. This allows the hydrolysis and polycondensation reactions shown in the following formulas (12) and (13) or (14) to proceed efficiently, and the product undergoes the aging step described below to form a three-dimensional network structure based on the MOM structure. The development of the three-dimensional network structure based on the MOM structure leads to the formation of an irregular structure in which particles are randomly linked, 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 or basic in the second mixing step or later. M-OR + HO → MOH + ROH (12) M-OR + M-OH → MOM + ROH (13) M-OH + M-OH → MOM + H2O (14) (In the above formulas (12) to (14), M is a metal atom, and R is any organic group.)

[0068] 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 metal alkoxide in the cellulose nanocrystal dispersion in the mixing step. The amount of the crosslinking compound used may be set to be within the above-mentioned range relative to the metal alkoxide.

[0069] 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.

[0070] 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.

[0071] The amount of acid catalyst used varies depending on the type of metal alkoxide, 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 metal alkoxide. These catalysts can be used either alone or in combination of two or more.

[0072] 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 solution containing the reaction product of the metal alkoxide and cellulose nanocrystals can be easily obtained.

[0073] In the first mixing step, the ratio of metal alkoxide 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 metal alkoxide. 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 metal alkoxide 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.

[0074] 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.

[0075] Examples of nonionic surfactants include polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene alkyl ether, polyoxypropylene alkyl ether, and block copolymers of polyoxyethylene and polyoxypropylene.

[0076] 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.

[0077] It is also possible to use a crosslinking agent to chemically crosslink 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 of divalent or higher metal cations (magnesium ion, calcium ion, barium ion, zinc ion, copper ion, cobalt ion, nickel ion, aluminum ion, iron ion, etc.); and metal alkoxides.

[0078] 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 above 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.

[0079] 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; tetraalkylammonium hydroxides 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, organic bases with an acid dissociation constant (PKa) of 9.5 or more at room temperature tend to produce porous gels with high strength, and tetramethylammonium hydroxide, triethylamine, and the like are preferably used.

[0080] 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.

[0081] The amount of base catalyst used is not particularly limited as long as it can be used to adjust the pH to the above range. When aqueous ammonia is used, the amount of the ammonia component is preferably 0.1 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 metal alkoxide. When tetramethylammonium hydroxide is used, the amount of the tetramethylammonium hydroxide component is preferably 0.001 g to 1.0 g, more preferably 0.01 g to 0.50 g, and especially preferably 0.05 g to 0.10 g per 100 g of inorganic raw materials. When triethylamine is used, the amount of the triethylamine component is preferably 0.001 g to 1.0 g, more preferably 0.01 g to 0.1 g, and especially preferably 0.08 g to 0.16 g per 100 g of inorganic raw materials. Adding the above amounts allows the reaction to proceed efficiently and uniformly, and a transparent, high-strength porous gel can be obtained.

[0082] 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 other base catalysts.

[0083] When the hydrolyzed compound is used as a catalyst, the amount used may be adjusted to the above pH, but in the case of urea, for example, the amount 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 metal alkoxide. 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.

[0084] By adding the hydrolyzable compound in advance to the first mixing step, a basic product can be generated during the aging step described below, eliminating the need for the second mixing step in which a basic catalyst is added, thereby enabling appropriate control of the pH. In other words, using the hydrolyzable compound as a basic catalyst allows the second mixing step to be omitted, 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.

[0085] In the mixing step, the ratio of the total amount of water to the metal alkoxide is preferably 40 to 400 g, more preferably 140 to 350 g, even more preferably 160 to 300 g, and particularly preferably 180 to 280 g, per 100 g of metal alkoxide. The total amount of water refers to the total amount of water contained in the system when synthesizing the wet gel, including the total amount of water contained in the cellulose nanocrystal dispersion and the water added during the mixing step. If the amount is greater than this range, the resulting wet gel becomes brittle and prone to cracking, fracture, pore shrinkage, etc. during drying. Furthermore, the resulting porous gel may become brittle or opaque due to the coarse skeleton of the material. If the amount is less than this range, the processes represented by formulas (12) to (14) described above may not proceed smoothly, and a porous gel may not be obtained.

[0086] 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 produce 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 g to 50 g, and particularly preferably 35 g to 45 g per 100 g of inorganic raw materials. When formamide is used, the total amount of water is preferably 40 g to 400 g, more preferably 60 g to 280 g, and more preferably 80 g 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

[0087] (ripening process) The wet gel obtained in the mixing step of the present invention may be further subjected to an aging step, if necessary. The aging process involves applying a predetermined amount of energy for a predetermined period of time to further promote the reaction of formula (2) above, and further promoting the reactions of formulas (12) to (14) above. One example of the energy is heat (temperature). Specifically, the obtained wet gel can be aged 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 metal alkoxide, 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 the production of a wet gel with suitable strength.

[0088] 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.

[0089] 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 metal alkoxide is mixed with a dispersion of cellulose nanocrystals for a predetermined time, and the mixture is allowed to stand to obtain a wet gel. In this case, an aging step may be carried out after the mixing step, and the aging step conditions may be the same as those described above.

[0090] (drying process) 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.

[0091] The method for drying the wet gel obtained by the above method is not particularly limited, and examples include atmospheric pressure drying, i.e., a method of drying under an environment of temperature and pressure below the critical point of the solvent contained in the wet gel, a method of drying through a supercritical state under a temperature and pressure above the critical point of a dry atmosphere such as CO2, and a method of sublimating the solvent by freeze-drying. From the viewpoint of productivity, however, atmospheric pressure drying is most preferable.

[0092] 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 equation. 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, 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 the drying process. Methods for incorporating a low surface tension solvent into a wet gel include replacing the solvent in the wet gel with a low surface tension solvent, as described below.

[0093] 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, catalyst, unreacted metal alkoxide, surfactant, hydrolyzable compound, 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.

[0094] 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 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.

[0095] The solvent used in the solvent substitution process is not particularly limited, but it is preferable to first substitute an organic polar solvent or an alcohol such as methanol, ethanol, or propanol, and then use 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 has 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 carried out efficiently.

[0096] 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 thermal insulation of the porous gel. The hydrophobization treatment can be performed by immersing the wet gel in a solvent containing a hydrophobizing agent. 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, and trimethylmethoxysilane. These hydrophobizing agents may be used alone or in combination of two or more.

[0097] To efficiently perform the hydrophobization treatment, it is desirable to perform the treatment in an environment where the water in the wet gel has been removed by the solvent substitution treatment described above. It is preferable to first substitute the solvent contained in the wet gel with a solvent such as an alcohol, such as methanol, ethanol, or propanol, or a chain saturated hydrocarbon, such as n-pentane or n-hexane, and then immerse the wet gel in a solvent containing a hydrophobizing agent.

[0098] The drying conditions for 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 drying conditions are 0.01 MPa to 0.3 MPa and -30°C to 150°C, more preferably 0.05 MPa to 0.2 MPa and 0°C to 120°C, even more preferably 0.08 MPa to 0.12 MPa and 20°C to 80°C, and particularly preferably 0.08 MPa to 0.12 MPa and 20°C to 40°C. The drying time is the shortest time required to completely remove the solvent from the wet gel, but is typically 1 hour to 120 hours, preferably 24 hours to 96 hours, and more preferably 48 hours to 72 hours. To control the drying rate, the wet gel can be dried while immersed in a low-surface-tension solvent such as n-heptane or n-hexane. Furthermore, a heat treatment at a higher temperature than that of the drying step can be performed after the drying step in order to further develop the three-dimensional network structure of the porous gel.

[0099] In this manner, the porous gel of the present invention can be produced.

[0100] 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.

[0101] The shape of the porous gel of the present invention is not particularly limited and may be appropriately selected depending on the application. The porous gel may be in the form of 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.

[0102] 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. [Example]

[0103] Next, the present invention will be specifically explained by way of examples, but the present invention is not limited to these examples.

[0104] Example 1 (Preparation of aqueous dispersions of cellulose nanocrystals containing anionic functional groups) 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). The resulting anionic functional group-containing cellulose nanocrystals were then mixed with ion-exchanged water and dispersed using an ultrasonic homogenizer (Hielscher UP400st) to obtain a 6.0 wt% aqueous dispersion of anionic functional group-containing cellulose nanocrystals. The anionic functional group content of the resulting anionic functional group-containing cellulose nanocrystals was 0.17 mmol / g.

[0105] (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% sulfuric acid aqueous 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, 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 for the wet gel to be obtained.

[0106] (Solvent replacement treatment) Next, as the first solvent substitution treatment, the obtained wet gel was immersed in methanol at room temperature and left to stand in a sealed container at 60°C for 24 hours. Then, as the second solvent substitution treatment, the methanol used for immersion was drained, and the obtained wet gel was again immersed in fresh methanol and left to stand in a sealed container at 60°C for 24 hours. Next, as the third solvent substitution treatment, the same operation as the second solvent substitution treatment was performed.

[0107] Next, the wet gel that had undergone the solvent substitution treatment with methanol three times as described above was removed from the sealed container. For 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 left to stand in a sealed container at 50°C for 24 hours. For the second solvent substitution treatment with a low surface tension solvent, the n-hexane used for immersion was drained, and the resulting wet gel was again immersed in fresh n-hexane and left to stand in a sealed container at 50°C for 24 hours. For the third solvent substitution treatment with a low surface tension solvent, the same procedures as for the second solvent substitution treatment were performed.

[0108] (drying process) Next, the wet gel that had undergone the solvent substitution treatment with n-hexane three times 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.

[0109] 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, as shown in Figure 2 (the same applies to Examples 2 to 13 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.

[0110] ·solid 29 Si-DD / MAS-NMR measurement The obtained porous gel was measured under the following conditions using a Fourier transform nuclear magnetic resonance spectrometer (product name: JNM-ECA400, manufactured by JEOL Ltd.): 29 Obtain Si-DD / MAS-NMR spectra and Q 1 Peak, Q 2 Peak, Q 3 Peak, and Q 4 Q relative to the total area of ​​the peak 4 The peak area ratio was calculated. Data analysis was carried out by calculating the peak area of ​​each peak after waveform processing of the spectrum after Fourier transformation. Temperature: room temperature Observation kernel: 29 Si Rotation speed: 5kHz Accumulation count: 8000 times Measurement mode: DD / MAS method Waiting time: 10 seconds

[0111] Appearance evaluation (aggregates) After drying, each of the 4 mm thick samples obtained in the Examples and Comparative Examples was visually observed and evaluated as follows. 〇: No agglomerates of the obtained solid material are visually observed △: Aggregates of the obtained solid matter are visually observed ×: No solid matter was obtained or the obtained solid matter was entirely whitened, making it impossible to evaluate the presence of aggregates.

[0112] Appearance evaluation (whitening, cracks) After drying, each of the 4 mm thick samples obtained in the Examples and Comparative Examples was visually observed and evaluated as follows. ◎: The obtained solid is transparent and no defects such as cracks are visually observed. ◯: The obtained solid is transparent, but defects such as cracks are visually observed. △: No cracks or other defects were visually observed in the obtained solid, but it was partially or entirely whitened and opaque. ×: Defects such as cracks are visually observed in the obtained solid, and the solid is partially or entirely whitened and opaque.

[0113] Mechanical strength evaluation A cylindrical porous gel with a diameter of 8 mm and a height of 10 mm was prepared as a measurement sample. A load was applied to the top surface of the cylindrical porous gel in the compressive direction at a rate of 1 mm / min using an Autograph (product name "AG-IS" manufactured by Shimadzu Corporation) as a compression tester, and the maximum strength until the measurement sample broke was evaluated as follows. ◎+: Maximum strength is 700N or more ◎: Maximum strength is 500N or more but less than 700N 〇: Maximum strength is 200N or more but less than 500N △: Maximum strength is 50N or more but less than 200N ×: Maximum strength is less than 50N

[0114] Evaluation of nitrogen adsorption amount The obtained porous gel was subjected to a specific surface area and pore size distribution measurement device (product name "BELSORP MAX II", manufactured by Microtrac-Bell Corporation) by heating the measurement sample at 150°C for 2 hours under vacuum, and then nitrogen adsorption and desorption measurements were performed at an adsorption temperature of 77 Kelvin to obtain a nitrogen adsorption and desorption isotherm. In the nitrogen adsorption / desorption isotherm obtained by the above method, the maximum amount of nitrogen adsorption was calculated when the relative pressure P / P0 of the equilibrium pressure P to the saturated vapor pressure P0 was 0 to 0.99.

[0115] Pore ​​volume evaluation The pore volume was calculated by the BJH method from the adsorption / desorption isotherm obtained by the above method.

[0116] ·BET specific surface area evaluation From the adsorption / desorption isotherms obtained by the above method, the BET specific surface area was calculated using the BET method in the region where the relative pressure P / P0 of the equilibrium pressure P to the saturated vapor pressure P0 is 0.05 to 0.30, which is a highly linear region.

[0117] Light transmittance evaluation The light transmittance of the obtained porous gel for in-line transmitted light was measured using an ultraviolet / visible spectrophotometer (UV-3600 i plus, manufactured by Shimadzu Corporation) with 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 at a wavelength of 600 nm (visible light) and corrected to a value when the thickness of the porous gel was 3 mm. C was calculated using the following formula: Tc=(T' / 100) 3 / t ×(1-r / 100) 2 Here, Tc is the light transmittance (%) after thickness correction, t is the measured thickness of the porous gel (mm), T' is the internal transmittance (%) relative to the measured light transmittance T, and r is the surface reflectance (%). T' and r are calculated using the following formulas. 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 based on 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 but less than 30% ×: Light transmittance Tc after thickness correction is less than 10%

[0118] <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 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.

[0119] <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 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 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.

[0120] <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 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 48 hours for a wet gel to be obtained.

[0121] <Examples 6 and 7> 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.

[0122] Example 8 A 4.5 wt% aqueous dispersion of anionic functional group-containing cellulose nanocrystals was prepared and left to stand for 4 days in an environment of 23°C and 50% RH, and the lower phase that separated due to the formation of a liquid crystal phase of the anionic functional group-containing cellulose nanocrystals was used to obtain a porous gel, which was then 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 1.

[0123] Example 9 A porous gel was obtained and evaluated in the same manner as in Example 1, except that the urea aqueous solution was added to the mixed solution so as to have the compounding 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.

[0124] Example 10 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.

[0125] Example 11 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.

[0126] Example 12 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.

[0127] Example 13 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.

[0128] <Comparative Example 1> The same procedure as in Example 1 was followed except that 3.2 g of titanium oxide (IV) (Fujifilm Wako Pure Chemical Industries, Ltd.) was used instead of tetramethoxysilane (TMOS), but a wet gel could not be obtained, and a porous gel could not be produced. The results are shown in Table 1.

[0129] <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 1, 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 1. The results are shown in Table 1.

[0130] <Comparative Example 3> A porous gel was produced in the same manner as in Example 1, except that a 1.0 wt% aqueous dispersion of cellulose nanofiber (CNF) ("Rheocrysta I-2SX, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.") was used in place of the anionic functional group-containing cellulose nanocrystals, and ion-exchanged water was used 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 1.

[0131] <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 place of the anionic functional group-containing cellulose nanocrystals, in a blending ratio shown in Table 1. However, the gel was destroyed during drying, and a bulk porous gel could not be obtained. The results are shown in Table 1.

[0132] [Table 1]

[0133] In Table 1, the compounds are as follows: TMOS: Tetramethoxysilane CNC: Cellulose nanocrystals containing anionic functional groups CNF: Cellulose nanofiber TMAOH: tetramethylammonium hydroxide In Table 1, the amount of each component is expressed in parts by weight, where the total weight of tetramethoxysilane (TMOS) or titanium(IV) oxide is taken as 100 parts by weight. The 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.

[0134] As shown in Table 1, it was confirmed from Examples 1 to 13 that the porous gels in which cellulose nanocrystals were dispersed were porous gels that had both high strength and large pore volume.

Claims

1. A porous gel containing a reaction product of a metal alkoxide and cellulose nanocrystals.

2. The porous gel according to claim 1 , wherein the metal alkoxide contains at least an alkoxysilane.

3. 3. The porous gel according to claim 1, wherein the cellulose nanocrystals are contained in an amount of 0.1% by weight or more and less than 50% by weight.

4. 3. The porous gel according to claim 1, 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.

5. The pore volume measured by the BJH method for nitrogen adsorption is 0.5 cm ３ 3. The porous gel according to claim 1, wherein the molecular weight is 1 / g or more.

6. solid ２９ 3. 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, namely M units, D units, T units and Q units, is 10 to 100%.

7. 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 3. The porous gel according to claim 1, wherein the peak area ratio is 50% or more.

8. 3. The porous gel according to claim 1, which has an irregular structure connected in a granular manner.

9. 1. A method for producing a porous gel comprising a reaction product of a metal alkoxide and cellulose nanocrystals, comprising: A dispersing step of dispersing the cellulose nanocrystals in a solvent to obtain a dispersion; a mixing step of adding the metal alkoxide to the dispersion of the cellulose nanocrystals and mixing them to obtain a wet gel; a drying step of drying the solvent contained in the wet gel.

10. The method for producing a porous gel according to claim 9, wherein the solvent used in the dispersion step is an aqueous solvent.

11. The method for producing a porous gel according to claim 9 or 10, wherein a hydrolytic catalyst is further added and mixed in the mixing step to obtain a wet gel.

12. The method for producing a porous gel according to claim 9 or 10, wherein the mixing step comprises a first mixing step of adding the metal alkoxide 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.

13. The method for producing a porous gel according to claim 9 or 10, wherein the wet gel is obtained without using an acid catalyst or a basic catalyst in the mixing step.

14. The method for producing a porous gel according to claim 9 or 10, wherein in the mixing step, mixing is performed in the presence of water, and the ratio of the total amount of the metal alkoxide to the total amount of water is 40 to 400 g per 100 g of the metal alkoxide.

15. 11. The method for producing a porous gel according to claim 9, wherein the mixing step is carried out in the presence of water and formamide.

16. The method for producing a porous gel according to claim 9 or 10, wherein the removal of the solvent in the drying step is carried out at a temperature and / or under pressure that is lower than the critical point of the solvent.

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