Method for producing a cellulose porous body

Abstract

To provide a method for producing a cellulose porous body that has a high transmittancy while having the advantages of being producible by general-purpose equipment and being easy to scale up.SOLUTION: A method for producing a cellulose porous body according to the present invention includes the steps of: preparing a liquid mixture comprising a cellulose nanofiber with a substituent having an ammonium ion as a counterion and a dispersion medium; and using a gelator to turn the liquid mixture into a gel; and lyophilizing the gel.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a method for producing a porous body containing cellulose nanofibers, and particularly to a method for producing a cellulose porous body having high light transmittance.

Background Art

[0002] When the cellulose porous body is a porous body obtained by supercritical drying of a wet gel, it is also called an aerogel. It has attracted attention because it has a low density, a high specific surface area, and is also derived from natural products that are present in large quantities. Porous bodies made of organic substances generally have higher strength than those made of inorganic materials at the same density. Those having a high specific surface area are excellent in adsorbability and can also become carbon-based porous bodies by carbonization by heating. The present inventors have studied the use of cellulose nanofibers having a number average fiber diameter of 1 to 1000 nm in such technological trends.

[0003] In this specification, cellulose nanofibers refer to (1) fine cellulose nanofibers (cellulose fibers) or (2) chemically treated (modified) fine cellulose nanofibers having a number average fiber diameter of 1 to 1000 nm, and are sometimes called nanocellulose in recent years. As the cellulose nanofibers of (1), for example, microfibrillated cellulose obtained by shearing and defibrating cellulose fibers under high pressure (hereinafter abbreviated as MFC) or fine bacterial cellulose produced by microorganisms (hereinafter abbreviated as BC). As the modified cellulose nanofibers of (2), for example, cellulose nanocrystals (hereinafter abbreviated as CNC) obtained by treating natural cellulose with concentrated sulfuric acid of 40% or more, or microfibrils which are the minimum units of the fibers constituting wood pulp are isolated as an aqueous dispersion by mild chemical treatment and slight mechanical treatment at normal temperature and pressure. They are ultra-fine and fine cellulose fibers having a uniform fiber diameter (see, for example, Patent Document 1).

[0004] Since cellulose nanofibers are materials derived from animals and plants, they have the feature of imposing less environmental burden during production and disposal than nanofibers made of petroleum-derived thermoplastic polymers. Therefore, it is expected to form a porous body using cellulose nanofibers and apply it to various fields and applications such as functional filters, electronic device materials, regenerative medical materials, and even carbon materials.

[0005] However, due to the cohesive force that acts during drying, the dried body obtained by drying the aqueous dispersion of cellulose nanofibers becomes a high-density material with low fluid permeability. In particular, the cellulose nanofibers with a number average fiber diameter of several nm described in Patent Document 1 have a part or all of the C6-position hydroxyl groups of the cellulose molecules on the fiber surface replaced with carboxyl groups that are more hydrophilic than hydroxyl groups. Also, generally, the smaller the fiber diameter, the greater the surface free energy per unit mass, so the cohesive force between fibers that stabilizes the surface when dried increases. Therefore, if the aqueous dispersion of the cellulose nanofibers described in Patent Document 1 is dried as it is, it will aggregate due to the hydrophilicity derived from the cellulose hydroxyl groups and carboxyl groups and the strong surface tension of water, and become a non-porous gas barrier film (see, for example, Patent Document 2).

[0006] As a method for not causing aggregation during drying of cellulose microfibers (microfibrils), a method of producing a porous body of cellulose microfibers by spraying an aqueous dispersion containing cellulose microfibers onto a cooled metal plate and rapidly freezing it, and then sublimating, and a method of producing a porous body of cellulose microfibers by substituting the dispersion medium with ethanol and then t-butyl alcohol and then freeze-drying are known (see, for example, Patent Document 3).

[0007] As a method for obtaining a cellulose porous body, a method is known in which water, which is the dispersion medium in the physical gel of cellulose nanofibers, is replaced with hydrous ethanol, ethanol, or t-butyl alcohol, and then freeze-dried (see, for example, Patent Document 4).

[0008] As a method for obtaining a cellulose porous body, a method is known in which cellulose nanofibers are dispersed in a mixed dispersion medium of water and an organic solvent soluble in water, and then freeze-dried (see, for example, Patent Document 5 or Patent Document 6).

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0011] According to the manufacturing methods disclosed in Patent Documents 3 to 6, a porous body of cellulose microfibers can be obtained. Here, when obtaining a porous body of cellulose microfibers from a wet gel, there are three methods depending on the drying method.

[0012] First, when attempting to obtain a porous body by supercritically drying a wet gel, a cellulose porous body with a high porosity and a high specific surface area is likely to be obtained. Therefore, the cellulose porous body is likely to have a high light transmittance. However, in order to perform supercritical drying, special pressure-resistant heating equipment is required, and there is a problem that scale-up cannot be achieved.

[0013] Also, when attempting to obtain a porous body by evaporatively drying a wet gel, it can be manufactured with general-purpose equipment and is also easy to scale up. However, the resulting cellulose porous body is a porous body with a low porosity and a low specific surface area, and there is a problem that the light transmittance is low.

[0014] Furthermore, when attempting to obtain a porous body by freeze-drying a wet gel, it can be manufactured with general-purpose equipment and is also easy to scale up. However, the resulting cellulose porous body is a porous body with a high porosity, and there is a problem that the light transmittance is insufficient.

[0015] Note that the cellulose porous body is characterized in that it is likely to have high light transmittance when it has a high porosity and a high specific surface area.

[0016] This disclosure has been made in view of such problems, and the object of this disclosure is to provide a method for manufacturing a cellulose porous body having a high light transmittance while having the advantages of being manufacturable with general-purpose equipment and being easy to scale up.

Means for Solving the Problems

[0017] As a result of repeated studies to solve the above problems, the present inventors have found that in a method for producing a porous body by freeze-drying a wet gel, the above problems can be solved by gelling a mixed solution containing a specific cellulose nanofiber with a gelling agent and then performing freeze-drying, and have thus completed the present invention. That is, the method for producing a cellulose porous body according to the present invention includes a step of preparing a mixed solution containing a cellulose nanofiber having an ammonium ion as a counter ion of a substituent and a dispersion medium, a step of gelling the mixed solution with a gelling agent to obtain a gel, and a step of freeze-drying the gel. as NH 4 + It is characterized by comprising. and the gelling agent is an organic acid having a boiling point of 210 °C or lower It is characterized in that.

[0018] placement A mixed solution containing a cellulose nanofiber having an ammonium ion as a counter ion of a substituent and a dispersion medium can be uniformly gelled.

[0019] In the method for producing a cellulose porous body according to the present invention, it is preferable that the organic acid contains formic acid, acetic acid, or both formic acid and acetic acid. A mixed solution containing a cellulose nanofiber having an ammonium ion as a counter ion of a substituent and a dispersion medium can be gelled more uniformly.

[0020] In the method for producing a cellulose porous body according to the present invention, it is preferable that the substituent of the cellulose nanofiber is at least one selected from the group consisting of a carboxyl group, a sulfate group, and a phosphate group. A uniform gel is easily obtained.

[0021] In the method for producing a cellulose porous body according to the present invention, it is preferable that the solid content concentration of the cellulose nanofiber in the mixed solution is 0.1 to 3.0% by mass. A cellulose porous body having a desired light transmittance is easily obtained.

[0022] In the method for producing a cellulose porous body according to the present invention, it is preferable that the dispersion medium is a mixed dispersion medium of water and an organic solvent miscible with water. When water freezes, the crystals (ice crystals) formed can be made smaller, and a porous body with a larger specific surface area can be obtained.

[0023] In the method for producing a cellulose porous body according to the present invention, in the step of freeze-drying, it is preferable that the temperature around the gel is 20°C or more lower than the melting point of the dispersion medium in the gel. A cellulose porous body having a desired light transmittance can be easily obtained.

[0024] In the method for producing a cellulose porous body according to the present invention, in the step of obtaining the gel, it is preferable to contact the surface of the mixed liquid with the vapor or mist of the gelling agent or the mist of an aqueous solution containing the gelling agent to gel the mixed liquid. A mixed liquid containing cellulose nanofibers having ammonium ions as counterions of substituents and a dispersion medium can be gelled more uniformly.

Advantages of the Invention

[0025] According to the present disclosure, it is possible to provide a method for producing a cellulose porous body having a high light transmittance while having the advantages of being manufacturable with general-purpose equipment and being easy to scale up.

Embodiments for Carrying Out the Invention

[0026] Next, embodiments of the present invention will be shown and described in detail, but the present invention is not construed as being limited to these descriptions. As long as the effects of the present invention are achieved, the embodiments may be variously modified.

[0027] The method for producing a cellulose porous body according to the present embodiment includes a step of preparing a mixed liquid containing cellulose nanofibers having ammonium ions as counterions of substituents and a dispersion medium, a step of gelling the mixed liquid with a gelling agent to obtain a gel, and a step of freeze-drying the gel.

[0028] <Cellulose Nanofibers> In this embodiment, the cellulose nanofibers include chemically treated (modified) cellulose nanofibers. In the cellulose nanofibers, two or more cellulose molecular chains form bundles. The fact that two or more cellulose molecular chains form bundles means a state in which two or more cellulose molecular chains aggregate to form an aggregate called a microfibril. In this embodiment, the cellulose molecular chain is one in which part or all of the C6 hydroxyl group in the molecule is oxidized to an aldehyde group, a carboxyl group, etc., one in which part or all of the hydroxyl groups including hydroxyl groups other than the C6 position are oxidized, esterified such as sulfate ester, nitrate ester, acetate ester, phosphate ester, etc., and substituted with other functional groups such as methyl ether, hydroxypropyl ether, carboxymethyl ether, etc. It includes the form which has been.

[0029] The number average fiber diameter of the cellulose nanofibers is not particularly limited, but is preferably 1 to 100 nm. More preferably, it is 2 to 50 nm, and particularly preferably, it is 2 to 10 nm. By using cellulose nanofibers with a number average fiber diameter of 1 to 100 nm, it is easy to obtain a porous body with low light scattering and high light transmittance. If the number average fiber diameter is less than 1 nm, the single fiber strength of the nanofibers is weak, and it may be difficult to maintain the structure of the porous body. If it exceeds 100 nm, the light transmittance of the porous body may be insufficient. Here, the number average fiber diameter is calculated as follows. Observation of the cellulose nanofibers is performed by electron microscope images using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). For the obtained observation images, two random axes, two vertical and two horizontal, are drawn per image, and the fiber diameters of the fibers intersecting the axes are visually read. At this time, it is performed at a magnification of either 5000 times, 10000 times, or 50000 times according to the size of the constituent fibers. Note that the sample or magnification should be such that 20 or more fibers intersect the axes. In this way, at least three non-overlapping surface part images are taken with an electron microscope, and the fiber diameter values of the fibers intersecting the two axes are read respectively. Therefore, at least 20 × 2 × 3 = 120 fiber information is obtained. The number average fiber diameter is calculated from the fiber diameter data thus obtained. For branched fibers, if the length of the branched part is 50 nm or more, it is incorporated into the calculation of the fiber diameter as one fiber.

[0030] Also, the number average fiber length of the cellulose nanofibers is not particularly limited, but is preferably 0.01 to 20 μm. More preferably, it is 0.05 to 10 μm. If the number average fiber length is less than 0.01 μm, the nanofibers may become close to particles, and the entanglement of the porous body may become weak. If it exceeds 20 μm, the entanglement between the nanofibers may increase, and the fluidity of the liquid when dispersed in the solvent may decrease. The number average fiber length is calculated from observations by an electron microscope image using SEM of a thin cast of the cellulose nanofiber dispersion on a substrate and freeze-dried. For the obtained observation image, 10 independent fibers are randomly selected per image, and the fiber length is visually read. At this time, it is carried out at a magnification of either 5000 times or 10000 times according to the length of the constituent fibers. The sample or magnification is targeted at those in which the start and end points of the fiber are within the same image. In this way, at least 12 non-overlapping surface part images are taken by SEM, and the fiber length is read. Therefore, at least 10 × 12 = 120 fiber information can be obtained. The number average fiber length can be calculated from the fiber diameter data thus obtained. For branched fibers, the length of the longest part of the fiber is taken as the fiber length.

[0031] The types of cellulose nanofibers include, for example, the aforementioned BC, CNC, and cellulose nanofibers that have undergone chemical treatment before nanofiberization. BC is characterized by having a relatively uniform fiber diameter. CNC has a relatively uniform fiber diameter but is characterized by a short fiber length of 0.1 - 0.2 μm. In order to reduce the mechanical treatment energy when nanofiberizing cellulose fibers, chemical pretreatment such as enzymatic treatment, esterification treatment, etherification treatment, and oxidation treatment may be performed on the cellulose fibers. As described in Patent Document 1, cellulose nanofibers that have undergone chemical treatment before nanofiberization are produced as an aqueous dispersion by subjecting a cellulose raw material to oxidation treatment using an oxidizing agent in the presence of an N - oxyl compound, bromide, iodide, or a mixture thereof, and further subjecting the oxidized cellulose to wet micronization treatment to defibrate and nanofiberize, and are characterized by having a uniform fiber diameter. Among these, the fine cellulose described in Patent Document 1 is particularly preferable in terms of requiring less energy for production than other cellulose fibers and having high productivity.

[0032] The cellulose nanofibers described in Patent Document 1 are cellulose single microfibrils. Natural cellulose constructs a higher - order structure with multiple bundles of microfibrils. Here, between the microfibrils, they are strongly aggregated by hydrogen bonds derived from hydroxyl groups in the cellulose molecule. Cellulose single microfibrils refer to microfibrils isolated by performing chemical treatment and mild mechanical treatment on natural cellulose. The cellulose nanofibers described in Patent Document 1 have a part of the hydroxyl groups of the cellulose molecule oxidized to at least one functional group selected from the group consisting of carboxyl groups and aldehyde groups, and have a cellulose I - type crystal structure. The maximum fiber diameter is 1000 nm or less. This cellulose nanofiber becomes a transparent liquid when dispersed in water.

[0033] In this embodiment, the cellulose nanofiber is the cellulose nanofiber described in Patent Document 1, and preferably has a number average fiber diameter of 1 to 100 nm. More preferably, it is 2 to 10 nm. If the number average fiber diameter is less than 1 nm, the single fiber strength of the nanofiber is weak, and it may be difficult to maintain the structure of the porous body. If it exceeds 100 nm, the light transmittance of the porous body may decrease. Further, the number average fiber length of the cellulose nanofiber described in Patent Document 1 is not particularly limited, but is preferably 0.01 to 20 μm. More preferably, it is 0.05 to 10 μm. If the number average fiber length is less than 0.01 μm, the nanofiber becomes close to particles, and the entanglement of the porous body may become weak. If it exceeds 20 μm, the entanglement between the nanofibers may be too strong, and the fluidity of the liquid when dispersed in the solvent may decrease.

[0034] The cellulose raw material serving as the raw material of the cellulose nanofiber is not particularly limited. For example, kraft pulp derived from various woods such as hardwood bleached kraft pulp (LBKP) and softwood bleached kraft pulp (NBKP), sulfite pulp, wastepaper pulp such as deinked pulp (DIP), ground pulp (GP), pressure groundwood pulp (PGW), refiner groundwood pulp (RMP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), chemimechanical pulp (CMP), chemiground pulp (CGP), etc. mechanical pulp, powdered cellulose obtained by pulverizing them with a high-pressure homogenizer, mill, etc., and microcrystalline cellulose powder obtained by purifying them by chemical treatment such as acid hydrolysis. In addition, plant celluloses such as kenaf, hemp, rice, bagasse, bamboo, cotton, etc., and further, cellulose produced by bacteria, cellulose of the mantle of jellyfish, etc. may be used. This embodiment is not limited to the raw material and manufacturing method of the cellulose nanofiber.

[0035] The method for producing cellulose nanofibers is, for example, the production method described in Patent Document 1. According to Patent Document 1, the method for producing cellulose nanofibers uses natural cellulose as a raw material, oxidizes the natural cellulose by allowing an N-oxyl compound to act as an oxidation catalyst and a co-oxidant in water to obtain reaction product fibers, a purification step of removing impurities to obtain reaction product fibers containing water, and a fibrillation step of dispersing the reaction product fibers containing water as cellulose nanofibers in a dispersion medium.

[0036] In the oxidation reaction step, a dispersion liquid in which natural cellulose is dispersed in water is prepared. The dispersion medium of natural cellulose in the reaction is water. And the concentration of natural cellulose in the reaction dispersion liquid is arbitrary as long as the sufficient diffusion of the reagent is possible, but usually it is 5% by mass or less based on the mass of the reaction dispersion liquid.

[0037] Numerous N-oxyl compounds that can be used as oxidation catalysts for cellulose have been reported. As a reported example, there is an article titled "Catalytic Oxidation of Cellulose Using TEMPO Derivatives: HPSEC and NMR Analyses of Oxidation Products" (see, for example, Non-Patent Document 1). Among N-oxyl compounds, in particular, TEMPO, 4-acetamido-TEMPO, 4-carboxy-TEMPO, or 4-phosphonooxy-TEMPO is preferable in that the reaction rate at room temperature in water is high. The addition amount of the N-oxyl compound is sufficient in a catalytic amount. That is, the addition amount of the N-oxyl compound to the reaction aqueous solution is preferably 0.1 to 4 mmol / l, and more preferably 0.2 to 2 mmol / l. If it is less than 0.1 mmol / l, the catalytic effect may be inferior. If it exceeds 4 mmol / l, it may not dissolve in water.

[0038] The co-oxidizing agent is, for example, hypohalous acid or its salt, halous acid or its salt, perhalic acid or its salt, hydrogen peroxide, or perorganic acid. Preferably it is an alkali metal hypohalite. The alkali metal hypohalite is, for example, sodium hypochlorite, sodium hypobromite. When using sodium hypochlorite, it is preferable in terms of reaction rate to proceed the reaction in the presence of an alkali metal bromide, for example, sodium bromide. The addition amount of this alkali metal bromide is preferably 1 to 40 times the molar amount relative to the N-oxyl compound. More preferably, it is 10 to 20 times the molar amount. If it is less than 1 times the molar amount, the reaction rate may be inferior. If it exceeds 40 times the molar amount, the reaction rate may be inferior. The pH of the reaction aqueous solution is preferably maintained in the range of 8 to 11 using an alkaline aqueous solution. The temperature of the aqueous solution is arbitrary at 4 to 40 °C, but the reaction can be carried out at room temperature and particularly does not require temperature control. The addition amount of the co-oxidizing agent is preferably in the range of 0.5 to 16 mmol per 1 g of natural cellulose. The reaction is preferably carried out for 5 to 120 minutes and is completed within 240 minutes at the longest.

[0039] The purification step is a step of removing impurities such as unreacted hypochlorous acid and various by-products from the oxidized cellulose slurry obtained in the oxidation reaction step and purifying it. At the stage after passing through the oxidation reaction step, since it is not usually dispersed into individual nanofibers, a normal purification method, that is, repeating a water washing step and a filtration step, is used to obtain a purified oxidized cellulose slurry with high purity (99% by mass or more). The purified oxidized cellulose slurry thus obtained preferably has a solid content (cellulose) concentration in the range of 10 to 50% by mass in a squeezed state. More preferably, it is 15 to 30% by mass. Considering the subsequent fibrillation step, if the solid content concentration is higher than 50% by mass, it is not preferable because extremely high energy is required for fibrillation.

[0040] The fibrillation process is a process of further dispersing the oxidized cellulose slurry obtained in the purification process in water to obtain a cellulose nanofiber dispersion. In addition to water, an organic solvent and other components can also be included in this dispersion. As the fibrillation machine, a general-purpose fibrillation machine as an industrial production machine can be used. General-purpose fibrillation machines include, for example, screw-type mixers, paddle mixers, disperser-type mixers, and turbine-type mixers. Furthermore, by using more powerful and beating-capable devices such as homomixers under high-speed rotation, high-pressure homogenizers, ultra-high-pressure homogenizers, ultrasonic dispersers, beaters, disk-type refiners, conical-type refiners, double-disk-type refiners, grinders, etc., more efficient and advanced downsizing becomes possible.

[0041] The solid content concentration of the oxidized cellulose slurry before fibrillation is preferably 0.01 to 4.00% by mass. More preferably, it is 0.10 to 1.00% by mass. When the solid content concentration exceeds 4.00% by mass, the viscosity of the cellulose nanofiber dispersion increases, resulting in a decrease in fluidity and possibly a decrease in fibrillation efficiency. When the solid content concentration is less than 0.01% by mass, the proportion of water in the dispersion becomes very large, and the fibrillation efficiency may decrease. When a cellulose nanofiber dispersion with a high solid content concentration is required, it can also be obtained by concentrating the dispersion with a low solid content concentration.

[0042] In this embodiment, the cellulose nanofiber has an ammonium ion as the counter ion of the substituent. Here, it is preferable that the substituent of the cellulose nanofiber is at least one selected from the group consisting of a carboxyl group, a sulfate group, and a phosphate group. For example, it is more preferable to use a cellulose nanofiber in which the counter ion of the carboxyl group (usually -COO - Na + ) of TEMPO-oxidized cellulose nanofiber is replaced with an ammonium ion (-COO - NH4 + ).

[0043] In the method for producing a cellulose porous body according to the present embodiment, it is necessary to remove the dispersion medium in the mixed solution. As the dispersion medium, water or a mixed dispersion medium obtained by adding a small amount of an organic solvent to water can be used. A more preferable dispersion medium is a mixed dispersion medium obtained by adding a small amount of an organic solvent to water. By using a mixed dispersion medium obtained by adding a small amount of an organic solvent to water as the dispersion medium, the growth of ice crystals generated during freezing of the dispersion medium can be suppressed, and the dispersion medium can be solidified in a state close to an amorphous state. Then, by sublimating the dispersion medium solidified in a state close to an amorphous state, a cellulose porous body with high light transmittance can be obtained.

[0044] <organic solvent> The organic solvent referred to in the present invention means an organic compound that is liquid at normal temperature and pressure. Also, being soluble in water means that in a mixed dispersion medium obtained by mixing water and an organic solvent, the mixed mass ratio of water and the organic solvent is within the range of 98:2 to 50:50, and the two are mixed at the molecular level and do not phase-separate. Note that the mixed dispersion medium refers to the liquid part in the mixed solution that does not contain cellulose nanofibers having ammonium ions as counterions of substituents and other components. In the cellulose porous body according to the present embodiment, water, an organic solvent, cellulose nanofibers having ammonium ions as counterions of substituents, and other components as necessary can be mixed to form a mixed solution. The concentration of the organic solvent in the mixed dispersion medium is preferably more than 0 and 50% by mass or less. More preferably, it is 10 to 40% by mass. If the concentration of the organic solvent exceeds 50% by mass, the dispersion medium becomes highly hydrophobic, and there is a possibility that the hydrophilic cellulose nanofibers will not be uniformly dispersed in the mixed solution. In freeze-drying with complete substitution of the dispersion medium with an organic solvent, although a porous body can be obtained, a great deal of effort is required for the substitution. On the other hand, when a mixed dispersion medium in which the dispersion medium is a mixture of water and a water-miscible organic solvent and the concentration of the organic solvent in the mixed dispersion medium is 15 to 50% by mass is used, aggregation of cellulose nanofibers does not occur, and a mixed solution uniformly dispersed in the dispersion medium can be obtained. Furthermore, it is possible to obtain a cellulose porous body by a simple method that does not require complete substitution of the dispersion medium.

[0045] In this embodiment, it is preferable that the organic solvent contains at least one of alcohols, carboxylic acids or carbonyl compounds. By containing such an organic solvent, crystals (ice crystals) formed when water freezes can be made smaller, and the specific surface area of the porous body can be enlarged. Further, from the viewpoint of compatibility with water, it is more preferable that the organic solvent contains at least one of (1) methanol, (2) ethanol, (3) 1-propanol or (4) t-butyl alcohol as alcohols, (5) acetic acid as carboxylic acids, and (6) acetone as carbonyl compounds. Among these, it is particularly preferable that the organic solvent is only t-butyl alcohol. The complete freezing point of the mixed dispersion medium of water and t-butyl alcohol is about -10°C at the lowest, which is higher than that of the mixed dispersion medium of other organic solvents and water, and it is easy to freeze. Further, it is easy to adjust the ambient temperature of the sample. In an aqueous solution of t-butyl alcohol, it is known that when the t-butyl alcohol concentration is around 20% by mass, water and t-butyl alcohol form a eutectic, and the crystal size at the time of freezing becomes the smallest. When the organic solvent is only t-butyl alcohol, the concentration of t-butyl alcohol in the mixed dispersion medium is preferably 10 to 40% by mass, and more preferably 15 to 30% by mass.

[0046] <Other components> The other components referred to in the present invention are substances such as surface modifiers, strength modifiers, and freeze-drying stabilizers. As the surface modifier, for example, a cationic surfactant, an anionic surfactant, a nonionic surfactant or an amphoteric surfactant can be used. As the strength modifier, for example, various latex emulsions such as acrylic latex, NBR latex, vinyl acetate latex or olefin latex, or water-soluble polymers such as polyacrylamide, polyamide epichlorohydrin, polyvinyl alcohol or starch can be used. As the freeze-drying stabilizer, for example, sucrose, trehalose, L-arginine or L-histidine can be used.

[0047] In this embodiment, other components can be added by any method, such as a method of adding them to the cellulose slurry in advance, or a method of adding them in a dispersion / mixing step such as a fibrillation step of cellulose nanofibers or a preparation step of a mixed solution. In this embodiment, it is preferable that other components are contained in an amount of 0.1 to 1000% by mass based on the mass of cellulose nanofibers. More preferably, it is 0.5 to 500% by mass, and still more preferably 1 to 100% by mass. When the other components exceed 1000% by mass, the ratio of cellulose nanofibers in the cellulose porous body decreases, so that the strength and specific surface area of the cellulose porous body may decrease. On the other hand, when the other components are less than 0.1% by mass, the effects of surface modification, strength modification, or freeze-drying stabilization may not be obtained.

[0048] Next, each step of the method for producing a cellulose porous body according to this embodiment will be described.

[0049] <Step of converting the counter ion of the substituent to an ammonium ion> The step of converting the counter ion of the substituent to an ammonium ion is performed before the step of nanofibrillating the cellulose raw material. The counter ion of at least one substituent selected from the group consisting of a carboxyl group, a sulfate group, and a phosphate group is converted to an ammonium ion. Specifically, there are two methods: (i) a method of using an aqueous ammonia solution when maintaining the pH of the aqueous solution during the oxidation reaction in the range of 8 to 11, and (ii) a method of washing the oxidized cellulose slurry with an acid once and then neutralizing it with an aqueous ammonia solution. The method (i) is more preferable.

[0050] <Step of nanofibrillating the cellulose raw material> As described above, for example, a cellulose raw material is nanofibrillated by the method for producing cellulose nanofibers described in Patent Document 1. This production method uses natural cellulose as a raw material, oxidizes the natural cellulose by allowing an N-oxyl compound to act as an oxidation catalyst and a co-oxidant in water to obtain reaction product fibers, a purification step of removing impurities to obtain reaction product fibers containing water, and a fibrillation step of dispersing the reaction product fibers containing water as cellulose nanofibers in a dispersion medium. Through these steps, a cellulose nanofiber dispersion liquid in which cellulose nanofibers having ammonium ions as counter ions of substituents are dispersed in a dispersion medium is obtained.

[0051] <Preparation step of mixed solution> In the method for producing a cellulose porous body according to this embodiment, water, cellulose nanofibers having an ammonium ion as a counter ion of a substituent (hereinafter, also simply referred to as cellulose nanofibers), and, if necessary, an organic solvent and other components are mixed to form a mixed solution. The form of the cellulose nanofibers in the mixed solution is, for example, a form in which the cellulose nanofibers are dispersed in a scattered manner. The preparation of the mixed solution can be carried out by (1) a method in which after preparing a cellulose nanofiber dispersion liquid in which cellulose nanofibers are dispersed in water in a defibrillation step, an organic solvent is added to the cellulose nanofiber dispersion liquid, or (2) a method in which cellulose nanofibers are dispersed in a mixed liquid of water and an organic solvent in a defibrillation step to prepare a cellulose nanofiber dispersion liquid. Among these, in order to disperse the cellulose nanofibers more uniformly, the method (1) is preferable. The other components can be added by any method, such as a method of adding them in advance to the cellulose slurry or a method of adding them in the mixing step after defibrillation of the cellulose nanofibers. The method for homogenizing the mixed solution is not particularly limited. For example, a method using a magnetic stirrer, a propeller type mixer, a screw type mixer, a paddle mixer, a disperser type mixer, or a turbine type mixer can be used. Further, more powerful devices such as a homomixer under high-speed rotation, a high-pressure homogenizer, an ultra-high pressure homogenizer, an ultrasonic disperser, a beater, a disk type refiner, a conical type refiner, a double disk type refiner, and a grinder can also be used. In the process of preparing the mixed solution, when a cellulose nanofiber aqueous dispersion is added to an organic solvent, aggregates may occur.

[0052] In the method for producing a cellulose porous body according to the present embodiment, the solid content concentration of cellulose nanofibers in a mixed liquid obtained by mixing water, cellulose nanofibers having an ammonium ion as a counter ion of a substituent, and, if necessary, an organic solvent and other components is 0.1 to 3.0% by mass. More preferably, it is 0.1 to 1.0% by mass, and particularly preferably, it is 0.1 to 0.5% by mass. When the solid content concentration of cellulose nanofibers in the mixed liquid exceeds 3.0% by mass, the voids between cellulose nanofibers decrease, and a porous body having a large specific surface area cannot be obtained. Further, the haze of the dried porous body obtained may increase. On the other hand, when the solid content concentration of cellulose nanofibers in the mixed liquid is less than 0.1% by mass, the entanglement between cellulose nanofibers decreases, and the structure cannot be maintained as a porous body.

[0053] <Gelation step> Next, a gelling agent is added to a mixed liquid containing cellulose nanofibers having an ammonium ion as a counter ion of a substituent and a dispersion medium to cause gelation.

[0054] The gelling agent is preferably at least one selected from the group consisting of organic acids, sulfuric acid, and phosphoric acid having a boiling point of 210°C or lower. Examples of the organic acid having a boiling point of 210°C or lower include carboxylic acids having 6 or less carbon atoms, and include saturated carboxylic acids, unsaturated carboxylic acids, hydroxycarboxylic acids, and oxocarboxylic acids. For example, formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, citric acid, lactic acid, and maleic acid are preferable. Among these, it is more preferable that the organic acid is formic acid or acetic acid. The organic acid may be used as a mixture of two or more kinds. For example, it is preferable that the organic acid contains both formic acid and acetic acid.

[0055] As a method of adding a gelling agent to a mixed solution containing cellulose nanofibers having an ammonium ion as a counter ion of a substituent and a dispersion medium, there are: (1) a first method of dropping a liquid gelling agent little by little into the mixed solution; (2) a second method of bringing the vapor or mist of the gelling agent or the mist of an aqueous solution containing the gelling agent into contact with the liquid surface of the mixed solution. Among these, it is preferable to perform the second method as a preliminary gelling step and then perform the first method as a gelling step. As a method of bringing the vapor of the gelling agent into contact with the liquid surface of the mixed solution, for example, the case where acetic acid is used as the gelling agent will be exemplified. The boiling point of acetic acid is 118°C, but it liquefies if the melting point is 16.7°C or higher and vaporizes even at a temperature below the boiling point such as room temperature. By allowing the mixed solution placed in a container and acetic acid placed in a container to stand together in a sealed space, the vapor of acetic acid can be brought into contact with the liquid surface of the mixed solution. Instead of acetic acid, for example, an aqueous acetic acid solution of 5 to 20% by mass may be used to bring the vapor of acetic acid into contact with the liquid surface of the mixed solution. Also, acetic acid or an aqueous acetic acid solution may be sprayed to form a mist and the mist may be brought into contact with the liquid surface of the mixed solution. By the second method, when the gelling agent comes into contact with the liquid surface of the mixed solution, gelation proceeds gently, so a highly smooth cellulose porous body is easily obtained. Also, in the first method, an aqueous solution of a gelling agent containing an organic solvent and water, which is the same kind of organic solvent as the organic solvent contained in the mixed solution containing the gelling agent, an organic solvent, preferably cellulose nanofibers having an ammonium ion as a counter ion of a substituent, and a dispersion medium, for example, an aqueous acetic acid solution containing acetic acid, t-butyl alcohol, and water, may be dropped into the mixed solution containing cellulose nanofibers having an ammonium ion as a counter ion of a substituent and a dispersion medium. Also, when performing the first method after performing the second method, the aqueous solution of the gelling agent may be dropped into the preliminarily gelled gel obtained by the second method. The concentration of the organic solvent contained in the aqueous solution of the gelling agent is preferably a concentration close to the range of (X - 20) to (X + 20)% by mass, more preferably a concentration close to the range of (X - 10) to (X + 10)% by mass, and even more preferably the same concentration, based on the concentration (X% by mass) of the organic solvent contained in the above mixed solution.For example, if the concentration of the organic solvent contained in the mixed solution is 30% by mass, the concentration of the organic solvent to be contained in the aqueous solution of the gelling agent is preferably 10 to 50% by mass, more preferably 20 to 40% by mass, and even more preferably 30% by mass.

[0056] <Freeze-drying process> In the method for producing the cellulose porous body according to the present embodiment, the gel is freeze-dried. Freeze-drying is a method of drying by freezing the gel and reducing the pressure in the frozen state to sublime the dispersion medium. The method for freezing the gel in freeze-drying is not particularly limited. For example, there are a method of freezing the gel by placing it in a refrigerant, a method of freezing the gel by placing it in a low-temperature atmosphere, and a method of freezing the gel by placing it under reduced pressure. Preferably, it is a method of freezing the gel by placing it in a refrigerant. The freezing temperature of the gel must be below the freezing point of the dispersion medium in the gel, preferably -50°C or lower, and more preferably -100°C or lower. If the freezing temperature is high, that is, if the freezing rate is slow, even when a mixed dispersion medium in which water and an organic solvent are mixed is used, the crystals of the dispersion medium may become large, and cellulose nanofibers may be concentrated around the crystals to form aggregates. In addition, the haze of the dried porous body obtained may increase. On the other hand, by lowering the freezing temperature, that is, by increasing the freezing rate, the dispersion medium can be frozen in a state close to an amorphous state. It is easy to obtain a dried porous body while maintaining a cellulose porous body with a high porosity and a high specific surface area.

[0057] In the method for producing the cellulose porous body according to the present embodiment, the gel is freeze-dried, but the form of freeze-drying is not limited. The method for freeze-drying the gel is, for example, a method of freeze-drying the gel alone in a container. In the method of freeze-drying the mixed solution alone in a container, a thin sheet-like or thick plate-like cellulose porous body can be obtained.

[0058] In the method for producing a porous body according to this embodiment, in the freeze-drying step, the ambient temperature of the sample must be set to a temperature equal to or lower than the melting point of the dispersion medium in the gel, and the dispersion medium in the frozen gel must be sublimated under reduced pressure. If the ambient temperature of the sample exceeds the melting point of the dispersion medium in the gel, a part of the dispersion medium in the frozen gel may melt, and the specific surface area of the porous body may decrease. Generally, in the freeze-drying step, even if the ambient temperature of the sample exceeds the melting point of the dispersion medium in the gel, the sample is cooled by the heat of vaporization when the dispersion medium in the gel vaporizes, so the dispersion medium in the gel can maintain a frozen state. However, when the ambient temperature is significantly higher than the melting point of the dispersion medium in the gel, or when the volume of the sample is large, melting of the dispersion medium in the gel or coarsening of ice crystals may occur in a portion far from the interface where sublimation is occurring. When the dispersion medium in the gel melts, the sample deforms, and when coarsening of ice crystals occurs, the haze of the porous body increases. To prevent this, it is conceivable to control the ambient temperature of the sample (also called the shelf temperature) in the freeze-drying step. The ambient temperature of the sample in the freeze-drying step usually becomes room temperature unless specifically controlled. The inventors of the present invention have found that by controlling the ambient temperature of the sample in the freeze-drying step to be equal to or lower than the melting point of the dispersion medium in the gel, not only can deformation of the sample be prevented, but also the haze of the porous body can be kept low. The ambient temperature of the sample in the freeze-drying step is preferably 20°C or more lower than the melting point of the dispersion medium in the gel, more preferably 40°C or more lower, and even more preferably 50°C or more lower. For example, when using a mixed dispersion medium of water and t-butyl alcohol, -20°C or lower is preferable, and -50°C or lower is more preferable. The lower limit of the ambient temperature of the sample in the freeze-drying step varies depending on the type of dispersion medium. For example, when using a mixed dispersion medium of water and t-butyl alcohol, it is preferably -90°C or higher. Here, the melting point can be determined from the first endothermic point that occurs when the mixture changes from a solid to a liquid. The melting point of the mixture may be measured from the peak of the first endothermic peak of the DSC curve obtained by, for example, a differential scanning calorimeter (DSC). Further, in the method for producing a porous body according to this embodiment, the pressure during decompression in the freeze-drying step is preferably 200 Pa or less, and more preferably 50 Pa or less.When the pressure exceeds 200 Pa, the dispersion medium in the frozen mixture may melt.

[0059] In the method for producing a porous body according to the present embodiment, in the freeze-drying step, after sublimating the dispersion medium in the frozen gel with the ambient temperature of the sample being set to a temperature equal to or lower than the melting point of the dispersion medium in the gel under reduced pressure, the ambient temperature of the sample is set to a temperature equal to or higher than room temperature, and the residual moisture in the gel is continuously volatilized under reduced pressure. In the method for producing a porous body according to the present embodiment, the former is called primary drying, and the latter is called secondary drying. Secondary drying not only volatilizes the residual moisture but also has the effect of preventing dew condensation due to the temperature difference. The ambient temperature of the sample during secondary drying is preferably equal to or higher than room temperature, more preferably 30°C or higher. The upper limit of the ambient temperature of the sample during secondary drying is not particularly limited, but for example, it is 50°C.

[0060] In the method for producing a cellulose porous body according to the present embodiment, the freeze-dried cellulose porous body can also be heat-treated. Heat treatment may make it easier to exhibit moisture absorption resistance and strength of the cellulose porous body. The heat treatment temperature is preferably, for example, 50 to 200°C, more preferably 70 to 170°C. When the heat treatment temperature exceeds 200°C, the cellulose nanofibers may be thermally decomposed.

[0061] The cellulose porous body obtained in this way is a dried body, with almost no cracks or shrinkage and high transparency. The total light transmittance (converted to a thickness of 1 mm) is 60% or more, and the haze value is 85% or less.

Examples

[0062] Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. In addition, "parts" and "% " in the examples indicate "parts by mass" and "mass% ", respectively, unless otherwise specified. The number of added parts is the value in terms of solid content.

[0063] (Example 1) <Preparation method of cellulose nanofibers> 2.00 g equivalent of NBKP (mainly composed of fibers with a fiber diameter exceeding 1000 nm), 0.025 g of TEMPO (2,2,6,6 - tetramethylpiperidine - 1 - oxyl radical), and 0.25 g of sodium bromide were dispersed in 150 ml of water. Then, an aqueous solution of 13% sodium hypochlorite was added to 1.00 g of pulp (NBKP) such that the amount of sodium hypochlorite was 5.00 mmol to initiate the reaction. During the reaction, a 0.50 mol / l aqueous sodium hydroxide solution was added dropwise to maintain the pH at 10. After reacting for 2 hours, the reaction product was filtered and thoroughly washed with water to obtain sodium salt - type TEMPO - oxidized pulp. Thereafter, 1 mol / l hydrochloric acid was added dropwise to the pulp to adjust the pH to 2. After dehydrating this pulp slurry, pure water was added, stirred, and dehydrated again. This washing operation was repeated 5 times to obtain acid - type TEMPO - oxidized pulp. This acid - type TEMPO - oxidized pulp was diluted to 2.0% with pure water, and an aqueous ammonia solution was added dropwise thereto to adjust the pH to 7.0, thereby obtaining ammonium salt - type TEMPO - oxidized pulp. The ammonium salt - type TEMPO - oxidized pulp slurry adjusted to 0.5% was treated 3 times at a pressure of 150 MPa using a high - pressure homogenizer (Starburst HJP - 25005X, manufactured by Sugino Machine Ltd.). Thereafter, it was passed through a nylon mesh with an opening of 7 μm twice to remove coarse fibers, and a transparent ammonium salt - type TEMPO - oxidized cellulose nanofiber aqueous dispersion was obtained. In the case of samples with a high set value of the cellulose nanofiber concentration in the mixed solution, the cellulose nanofiber aqueous dispersion was concentrated using a rotary evaporator before preparing the mixed solution. <Mixed solution preparation step> The ammonium salt - type TEMPO - oxidized cellulose nanofiber aqueous dispersion obtained above was adjusted using distilled water and t - butyl alcohol so that the cellulose nanofiber concentration was 0.2% and the t - butyl alcohol concentration of the dispersion medium was 30% to obtain a cellulose nanofiber mixed solution. <Pre - gelation step> A 10-cm diameter stainless steel petri dish containing 31.4 g of the cellulose nanofiber mixture obtained in the mixture preparation step and a petri dish containing 20 g of acetic acid stock solution (17.5 mol / l) were placed in the same sealed container and left for 30 minutes to pre-gel the cellulose nanofiber mixture. <Gelation step> To the pre-gelled cellulose nanofiber mixture, 3 g of a 6 mol / l acetic acid aqueous solution containing 30% t-butyl alcohol was dropped onto the surface to cause gelation. <Freezing step> After the gelation step, the cellulose nanofiber gel was frozen for 1 hour in a freeze dryer (CS-100-9 Super XS, manufactured by Sakuma Seisakusho) with the shelf temperature adjusted to -90°C. <Drying step> After the freezing step, the freeze dryer was depressurized, and primary drying was carried out at a shelf temperature of -50°C and secondary drying at a shelf temperature of 40°C. Drying was terminated when the vacuum pressure reached 0.4 Pa or less, and a porous cellulose body was obtained.

[0064] (Example 2) A porous body was produced in the same manner as in Example 1, except that the cellulose nanofiber concentration in the mixture preparation step was changed from 0.2% to 0.4%.

[0065] (Example 3) A porous body was produced in the same manner as in Example 1, except that the cellulose nanofiber concentration in the mixture preparation step was changed from 0.2% to 0.6%.

[0066] (Example 4) A porous body was produced in the same manner as in Example 1, except that the cellulose nanofiber concentration in the mixture preparation step was changed from 0.2% to 0.3%.

[0067] (Example 5) A porous body was produced in the same manner as in Example 4, except that the t-butyl alcohol concentration of the dispersion medium in the mixture preparation step was changed from 30% to 15%.

[0068] (Example 6) A porous body was produced in the same manner as in Example 1, except that the standing time in the preliminary gelation step was changed from 30 minutes to 2 hours and the gelation step was omitted.

[0069] (Example 7) A porous body was produced in the same manner as in Example 1, except that the preliminary gelation step was omitted and the dropping amount in the gelation step was changed from 3 g to 5 g.

[0070] (Example 8) A porous body was produced in the same manner as in Example 6, except that the concentration of cellulose nanofibers in the mixed solution preparation step was changed from 0.2% to 0.3% and the gelling agent in the preliminary gelation step was changed from glacial acetic acid solution (17.5 mol / l) to formic acid (16.5 mol / l).

[0071] (Comparative Example 1) A porous body was produced in the same manner as in Example 2, except that preliminary gelation and gelation were not performed.

[0072] (Comparative Example 2) (Preparation method of cellulose nanofibers) 2.00 g equivalent of NBKP (mainly composed of fibers with a fiber diameter exceeding 1000 nm), 0.025 g of TEMPO (2,2,6,6 - tetramethylpiperidine - 1 - oxyl radical), and 0.25 g of sodium bromide were dispersed in 150 ml of water. Then, an aqueous solution of 13% sodium hypochlorite was added so that the amount of sodium hypochlorite was 5.00 mmol per 1.00 g of pulp (NBKP) to initiate the reaction. During the reaction, a 0.50 mol / l aqueous sodium hydroxide solution was added dropwise to maintain the pH at 10. After reacting for 2 hours, the reaction product was filtered and thoroughly washed with water to obtain sodium salt - type TEMPO - oxidized pulp. The sodium salt - type TEMPO - oxidized pulp slurry adjusted to 0.5% was treated 3 times at a pressure of 150 MPa using a high - pressure homogenizer (Starburst HJP - 25005X, manufactured by Sugino Machine Limited). Then, it was passed through a nylon mesh with an opening size of 7 μm twice to remove coarse fibers, and a transparent aqueous dispersion of sodium salt - type TEMPO - oxidized cellulose nanofibers was obtained. In the case of samples with a high set value of the cellulose nanofiber concentration in the mixed solution, the aqueous dispersion of cellulose nanofibers was concentrated using a rotary evaporator before preparing the mixed solution. <Mixed solution preparation process>~<Drying process> The cellulose nanofibers used in the mixed solution preparation process were sodium salt - type TEMPO - oxidized cellulose nanofibers obtained by the above <Method for preparing cellulose nanofibers>. A porous body was produced in the same manner as in Example 2, except that pre - gelation and gelation were not performed.

[0073] (Comparative Example 3) The cellulose nanofibers used in the mixed solution preparation process were the same sodium salt - type TEMPO - oxidized cellulose nanofibers as in Comparative Example 2. A porous body was produced in the same manner as in Example 2, except that pre - gelation was not performed and the acetic acid used in the gelation process was changed to 0.1 mol / l hydrochloric acid.

[0074] (Comparative Example 4) The cellulose nanofibers used in the mixed solution preparation step were the same sodium salt type TEMPO oxidized cellulose nanofibers as in Comparative Example 2. Without performing pre-gelation, the acetic acid used in the gelation step was changed to 0.1 mol / l hydrochloric acid, and the cellulose nanofiber mixed solution was gelled in the same manner as in Example 2. Thereafter, the gel of the cellulose nanofiber mixed solution was immersed in an excessive t-butyl alcohol solution for 24 hours. Further, this operation was repeated twice using fresh t-butyl alcohol to replace the water contained in the cellulose nanofiber mixed solution with t-butyl alcohol and to remove as much as possible the NaCl, which was the neutralization salt remaining in the gel. Thereafter, a cellulose porous body was produced in the same manner as in Example 2.

[0075] (Comparative Example 5) A porous body was produced in the same manner as in Example 4, except that pre-gelation was not performed and the acetic acid used in the gelation step was changed to hydrochloric acid.

[0076] (Comparative Example 6) A porous body was produced in the same manner as in Example 4, except that the cellulose nanofibers used in the mixed solution preparation step were the same sodium salt type TEMPO oxidized cellulose nanofibers as in Comparative Example 2.

[0077] (Evaluation Method) <State of the Dried Body> The state of the cellulose porous body was visually evaluated, and cracking and shrinkage were evaluated as follows. 〇 → No cracking or shrinkage × → Cracking and shrinkage <Thickness> The thickness of the cellulose porous body was measured with a scale. <Total Light Transmittance> For the measurement of the total light transmittance, a spectrophotometer with an integrating sphere (device name: V-670, manufactured by JASCO Corporation) was used. Light with wavelengths from 220 to 850 nm was transmitted through the sample and collected by the integrating sphere to measure the transmittance at each wavelength. The transmittance at a wavelength of 600 nm was taken as the total light transmittance. The measured values were converted to the transmittance of a 1-mm thickness according to Equation (1) based on Lambert-Beer's law, where t is the thickness (mm). (Number 1) Total light transmittance (%) = 100 × (measured value ^ (1 / t)) <Haze For the measurement of haze, an integrating sphere spectrophotometer (equipment name: V-670, manufactured by JASCO Corporation) was used, and the ratio of the diffuse transmittance to the total light transmittance was defined as the haze value.

[0078] For Examples 1 to 8 and Comparative Examples 1 to 6, the evaluation results are shown in Table 1.

[0079]

Table 1

[0080] The cellulose porous bodies of Examples 1 to 8 had no cracks or shrinkage, and all had a total light transmittance of 60% or more, preferably 75% or more, more preferably 85% or more, and a haze of 85% or less, preferably 66% or less, more preferably 55% or less, indicating high light transmittance. In Comparative Example 1, since pre-gelation and gelation were not performed, cracks and shrinkage occurred. In Comparative Example 2, since cellulose nanofibers having an ammonium ion as a counter ion of the substituent were not used and pre-gelation and gelation were not performed, cracks and shrinkage occurred. In Comparative Example 3, since cellulose nanofibers having an ammonium ion as a counter ion of the substituent were not used and hydrochloric acid was used as a gelling agent, cracks and shrinkage occurred. In Comparative Example 4, since cellulose nanofibers having an ammonium ion as a counter ion of the substituent were not used, hydrochloric acid was used as a gelling agent, and the remaining NaCl in the gel was removed as much as possible, cracks and shrinkage did not occur, but the total light transmittance was low and the haze was high. In Comparative Example 5, although cellulose nanofibers having an ammonium ion were used, pre-gelation was not performed and hydrochloric acid used in the gelation process was used, resulting in cracks and shrinkage. In Comparative Example 6, although acetic acid was used as a gelling agent, since cellulose nanofibers having an ammonium ion were not used, cracks and shrinkage did not occur, but the haze was high.

Claims

1. A step of preparing a mixed solution containing cellulose nanofibers having NH₄⁺ as an ammonium ion for a counter ion of a substituent and a dispersion medium; A step of gelling the mixed solution with a gelling agent to obtain a gel; A step of lyophilizing the gel, and having, The method for producing a porous cellulose body, characterized in that the gelling agent is an organic acid having a boiling point of 210 ° C or lower.

2. The method for producing a porous cellulose body according to claim 1, characterized in that the organic acid contains formic acid, acetic acid, or both formic acid and acetic acid.

3. The method for producing a porous cellulose body according to claim 1 or 2, characterized in that the substituent of the cellulose nanofibers is at least one selected from the group consisting of a carboxyl group, a sulfate group, and a phosphate group.

4. The method for producing a porous cellulose body according to any one of claims 1 to 3, characterized in that the solid content concentration of the cellulose nanofibers in the mixed solution is 0.1 to 3.0% by mass.

5. The method for producing a porous cellulose body according to any one of claims 1 to 4, characterized in that the dispersion medium is a mixed dispersion medium of water and an organic solvent miscible with water.

6. The method for producing a porous cellulose body according to any one of claims 1 to 5, characterized in that in the step of lyophilizing, the ambient temperature of the gel is 20 ° C or more lower than the melting point of the dispersion medium in the gel.

7. The method for producing a porous cellulose body according to any one of claims 1 to 6, characterized in that in the step of obtaining the gel, the vapor or mist of the gelling agent or the mist of an aqueous solution containing the gelling agent is brought into contact with the liquid surface of the mixed solution to gel the mixed solution.

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