JP7197490B2 - Method for producing anion-modified cellulose nanofiber - Google Patents

Description

The present invention relates to a method for producing anion-modified cellulose nanofibers.

Cellulosic raw material is treated in the presence of 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter referred to as "TEMPO") and sodium hypochlorite, which is an inexpensive oxidizing agent. Then, carboxyl groups can be efficiently introduced onto the surface of cellulose microfibrils (Patent Document 1). Moreover, a carboxymethyl group can be introduced into cellulose by reacting with monochloroacetic acid or sodium monochloroacetate after mercerizing a cellulosic raw material (Patent Document 2). Cellulose into which a carboxyl group or a carboxymethyl group has been introduced is negatively charged in a solvent (hereinafter referred to as "anion-modified cellulose"). It is known that when such anion-modified cellulose is treated with a mixer or the like in a solvent, a cellulose nanofiber dispersion liquid, which is cellulose microfibrils, can be obtained (Patent Document 3).

Cellulose nanofibers are biodegradable, water-dispersible materials. Since the cellulose nanofibers obtained by the above method are in the form of a dispersion, they can be modified by blending with various water-soluble polymers or by combining with organic or inorganic pigments. Cellulose nanofibers can also be made into sheets or fibers. Due to these characteristics, the development of new high-performance products applying cellulose nanofibers to high-performance packaging materials, transparent organic substrate members, high-performance fibers, separation membranes, regenerative medicine materials, etc., is being studied.

In the aqueous dispersion of cellulose nanofibers obtained by the above method, anionic groups such as carboxyl groups present on the surface of the cellulose nanofibers form salts such as sodium salts, resulting in a state of high hydrophilicity. Cellulose nanofibers with reduced hydrophilicity by converting anionic groups into acid forms are required for some applications, such as compounding with resins. In order to convert an anionic group into an acid form, an acid treatment using a mineral acid such as hydrochloric acid or sulfuric acid is performed (see Patent Document 4).

Japanese Unexamined Patent Application Publication No. 2008-001728 JP-A-10-251301 WO2011/115154 WO2010/116795

Acid treatment with a mineral acid to convert an anionic group into an acid form produces by-products such as sodium chloride, which must be removed by washing. In addition, it is necessary to remove the water used for washing by dehydration, and after dehydration, the remaining filter material is collected by passing it through the filter cloth, but cellulose nanofibers with extremely short fiber lengths pass through the filter cloth. Therefore, in some cases, the yield is greatly reduced.
Therefore, as a method for desalting the anion-modified cellulose nanofiber salt, studies have begun on a method using a cation exchange resin that does not generate by-products.

An object of the present invention is to provide a method for producing anion-modified cellulose nanofibers in which used cation exchange resin does not contaminate the cellulose nanofibers.

As a result of intensive studies aimed at solving the above problems, the present inventors have found that the above objects can be achieved by recovering used cation exchange resins, and have completed the present invention.

That is, the present invention provides the following (1) to (12).
(1) A preparation step of preparing anion-modified cellulose, a defibration step of defibrating the anion-modified cellulose obtained in the preparation step to obtain an anion-modified cellulose nanofiber salt, and a desalting step of obtaining anion-modified cellulose nanofibers by subjecting the obtained anion-modified cellulose nanofiber salt to a cation exchange reaction using a cation exchange resin to obtain an anion-modified cellulose nanofiber; and a recovery step of recovering the cation exchange resin from a mixture containing the anion-modified cellulose nanofibers obtained by the above method and the cation-exchange resin.
(2) The method for producing anion-modified cellulose nanofibers according to (1), wherein in the recovery step, pressure filtration at 0.01 MPa or more and 5 MPa or less or vacuum filtration at 0.01 MPa or more and 0.1 MPa or less is performed.
(3) The method for producing anion-modified cellulose nanofibers according to (1) or (2), in which a mesh-like filter medium with an opening of 5 μm or more and 400 μm or less or a slit-like filter medium with a slit width of 5 μm or more and 400 μm or less is used in the collection step. .
(4) The method for producing anion-modified cellulose nanofibers according to any one of (1) to (3), further comprising a microfiltration step before or after the recovery step.
(5) The anion-modified cellulose nanofiber according to any one of (1) to (4), wherein the cation exchange resin recovered in the recovery step is subjected to a cation exchange resin regeneration step by acid treatment. manufacturing method.
(6) including a step of using a positive displacement screw pump for liquid transfer and pressure filtration of the mixture in the recovery step, or liquid transfer between the desalting step and the recovery step (1) to (5) 3. A method for producing an anion-modified cellulose nanofiber according to any one of .
(7) The method for producing anion-modified cellulose nanofibers according to (6), wherein the positive displacement screw pump is a positive displacement uniaxial eccentric screw pump or a positive displacement twin screw pump.
(8) including a step of using a positive displacement reciprocating pump for liquid transfer and pressure filtration of the mixture in the recovery step, or liquid transfer between the desalination step and the recovery step (1) to (5) ).
(9) The method for producing anion-modified cellulose nanofibers according to (8), wherein the positive displacement reciprocating pump is a positive displacement diaphragm pump.
(10) The preparation step includes a step of oxidizing the cellulosic raw material using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromides, iodides, and mixtures thereof. , The method for producing an anion-modified cellulose nanofiber according to any one of (1) to (9), wherein the anion-modified cellulose obtained by the preparation step is cellulose to which a carboxyl group has been introduced.
(11) The preparation step includes a step of mercerizing the cellulosic raw material with a mercerizing agent and then reacting it with a carboxymethylating agent, and the anion-modified cellulose obtained by the preparation step has a carboxymethyl group on the cellulose. The method for producing an anion-modified cellulose nanofiber according to any one of (1) to (9), wherein the is introduced.
(12) The preparation step includes a step of adding a phosphate compound to the cellulosic raw material, and the anion-modified cellulose obtained by the preparation step is obtained by introducing a phosphate group into the cellulose (1). The method for producing an anion-modified cellulose nanofiber according to any one of (9).

According to the present invention, it is possible to provide a method for producing anion-modified cellulose nanofibers in which the cellulose nanofibers are not contaminated with used cation exchange resins.

The present invention will be described in detail below. In the present invention, "-" includes end values. That is, "X through Y" includes the values X and Y at both ends.

The method for producing anion-modified cellulose nanofibers of the present invention comprises a preparation step of preparing anion-modified cellulose, and defibration of the anion-modified cellulose obtained in the preparation step to obtain an anion-modified cellulose nanofiber salt. and the anion-modified cellulose nanofiber salt obtained in the fibrillation step is desalted by performing a cation exchange reaction using a cation exchange resin to obtain anion-modified cellulose nanofibers. It has a desalting step and a recovery step of recovering the cation exchange resin from the mixture containing the anion-modified cellulose nanofibers obtained in the desalting step and the cation exchange resin.

(Preparation process)
In the preparation process of the present invention, a cellulosic raw material is anion-modified to prepare anion-modified cellulose.

(cellulosic raw material)
Cellulosic raw materials include plants (e.g., wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (softwood unbleached kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp). (LUKP), bleached hardwood kraft pulp (LBKP), unbleached softwood sulfite pulp (NUSP), bleached softwood sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.), animals (e.g. sea squirts) , algae, microorganisms (e.g., acetobacter), microbial products, etc. are known, and any of them can be used in the present invention.Cellulose fibers derived from plants or microorganisms are preferred, and cellulose fibers derived from plants are preferred. of cellulose fibers are more preferred.

(anion modification)
Anionic modification is the introduction of anionic groups into cellulose, specifically the introduction of anionic groups into pyranose rings by oxidation or substitution reaction. In the present invention, the oxidation reaction means a reaction of directly oxidizing a hydroxyl group of a pyranose ring to a carboxyl group. Further, in the present invention, the substitution reaction is a reaction for introducing an anionic group into the pyranose ring by a substitution reaction other than the oxidation.

(carboxylation)
Carboxylated (oxidized) cellulose can be used as the anion-modified cellulose. The carboxy group in the present invention means -COOH (acid form) or -COOM (salt form). Here, M is a metal ion such as sodium or potassium. Carboxylated cellulose (also called “oxidized cellulose”) can be obtained by carboxylating (oxidizing) the above cellulosic raw material by a known method. Although not particularly limited, the amount of carboxyl groups is preferably 0.6 to 3.0 mmol/g, more preferably 1.0 to 2.0 mmol/g, relative to the absolute dry mass of the anion-modified cellulose nanofiber. As an example of a carboxylation (oxidation) method, a cellulosic feedstock is treated in water with an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromides, iodides, and mixtures thereof. A method of oxidation can be mentioned. This oxidation reaction selectively oxidizes the primary hydroxyl group at the C6 position of the glucopyranose ring on the cellulose surface, resulting in a cellulose fiber having an aldehyde group and a carboxyl group (--COOH) or carboxylate group ( ^--COO.sup.- ) on the surface. can be obtained. Although the concentration of cellulose during the reaction is not particularly limited, it is preferably 5% by mass or less.

An N-oxyl compound means a compound capable of generating a nitroxy radical. As the N-oxyl compound, any compound can be used as long as it promotes the desired oxidation reaction. Examples include 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) and its derivatives (eg 4-hydroxy TEMPO). The amount of the N-oxyl compound used is not particularly limited as long as it is a catalyst amount capable of oxidizing the cellulosic raw material. For example, it is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, and even more preferably 0.01 to 0.5 mmol, relative to 1 g of absolute dry cellulosic raw material. Further, it is preferable that the concentration is about 0.1 to 4 mmol/L with respect to the reaction system.

Bromides are compounds containing bromine, examples of which include alkali metal bromides that can be dissociated and ionized in water. Also, iodides are compounds containing iodine, examples of which include alkali metal iodides. The amount of bromide or iodide to be used can be selected within a range capable of promoting the oxidation reaction. The total amount of bromide and iodide is, for example, preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol, relative to 1 g of absolute dry cellulosic raw material. The modification is modification by an oxidation reaction.

Known oxidizing agents can be used, for example, halogens, hypohalous acids, halogenous acids, perhalogenates or salts thereof, halogen oxides, peroxides and the like can be used. Among them, sodium hypochlorite is preferable because it is inexpensive and has less environmental load. An appropriate amount of the oxidizing agent to be used is, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, and even more preferably 2.5 to 25 mmol, relative to 1 g of absolute dry cellulosic raw material. Further, for example, 1 to 40 mol is preferable per 1 mol of the N-oxyl compound.

In the process of oxidizing cellulosic raw materials, the reaction can proceed efficiently even under relatively mild conditions. Therefore, the reaction temperature is preferably 4 to 40°C, and may be room temperature of about 15 to 30°C. As the reaction progresses, carboxyl groups are generated in the cellulose, so that the pH of the reaction solution is lowered. In order to allow the oxidation reaction to proceed efficiently, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to the reaction system as needed to maintain the pH of the reaction solution at about 9-12, preferably about 10-11. Water is preferable as the reaction medium because it is easy to handle and less likely to cause side reactions. The reaction time in the oxidation reaction can be appropriately set according to the degree of progress of the oxidation, and is usually about 0.5 to 6 hours, for example about 0.5 to 4 hours.

Moreover, the oxidation reaction may be carried out in two steps. For example, the oxidized cellulose obtained by filtration after the completion of the reaction in the first step is oxidized again under the same or different reaction conditions, so that the cellulose can be obtained without reaction inhibition due to salts produced as a by-product in the reaction in the first step. A carboxyl group can be efficiently introduced into a system raw material.

The amount of carboxyl groups in the oxidized cellulose can be adjusted by controlling the reaction conditions such as the amount of the oxidizing agent added and the reaction time. The amount of carboxyl groups in the anion-modified cellulose nanofibers and the amount of carboxyl groups in the cellulose nanofibers are preferably the same.

In the present invention, in the oxidized cellulose obtained by the above steps, the carboxyl group introduced into the cellulosic raw material is usually in the form of a salt, such as an alkali metal salt such as a sodium salt. Prior to the defibration step, the alkali metal salt of the oxidized cellulose may be replaced with other cationic salts such as phosphonium salts, imidazolinium salts, ammonium salts, sulfonium salts and the like. Substitutions can be carried out by known methods.

(Carboxymethylation)
Preferred anionic groups include carboxyalkyl groups such as carboxymethyl groups. The carboxyalkyl group in the present invention means -RCOOH (acid form) or -RCOOM (salt form). Here, R is an alkylene group such as a methylene group or an ethylene group, and M is a metal ion. Carboxyalkylated cellulose may be obtained by a known method, or a commercially available product may be used. Preferably, the degree of carboxyalkyl substitution per anhydroglucose unit of the cellulose is less than 0.60. Furthermore, when the anionic group is a carboxymethyl group, the degree of carboxymethyl substitution is preferably less than 0.60. If the degree of substitution is 0.60 or more, the crystallinity is lowered and the proportion of dissolved components is increased, so that the function as nanofibers is lost. Also, the lower limit of the degree of carboxyalkyl substitution is preferably 0.01 or more. Considering workability, the degree of substitution is particularly preferably 0.02 to 0.50, more preferably 0.10 to 0.30. An example of a method for producing such carboxyalkylated cellulose includes the following steps. The modification is modification by a substitution reaction. Carboxymethylated cellulose will be described as an example.
i) Mix the raw material, solvent and mercerizing agent, and mercerize at a reaction temperature of 0 to 70° C., preferably 10 to 60° C., for a reaction time of 15 minutes to 8 hours, preferably 30 minutes to 7 hours. process,
ii) Then, a carboxymethylating agent is added in an amount of 0.05 to 10.0 times mol per glucose residue, the reaction temperature is 30 to 90° C., preferably 40 to 80° C., and the reaction time is 30 minutes to 10 hours, preferably A step of performing an etherification reaction for 1 to 4 hours.

As the bottom raw material, the above-mentioned cellulose-based raw material can be used. As the solvent, 3 to 20 times by mass of water or a lower alcohol, specifically water, methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, tertiary butanol, etc. alone, or two Mixed media of more than one species can be used. When a lower alcohol is mixed, the mixing ratio is 60 to 95% by mass. As the mercerizing agent, an alkali metal hydroxide, specifically sodium hydroxide or potassium hydroxide, can be used in an amount of 0.5 to 20 times the moles of the anhydroglucose residue of the starting material.

As described above, the degree of carboxymethyl substitution per glucose unit of cellulose is less than 0.04, preferably 0.01 or more and less than 0.60. By introducing carboxymethyl substituents into cellulose, the celluloses electrically repel each other. Therefore, cellulose into which carboxymethyl substituents have been introduced can be easily nano-fibrillated. If the number of carboxymethyl substituents per glucose unit is less than 0.02, nano-fibrillation may not be sufficient. The degree of substitution in anion-modified cellulose nanofibers and the degree of substitution in cellulose nanofibers are usually the same.

In the present invention, in the carboxyalkylated cellulose obtained by the above steps, the carboxyalkyl group introduced into the cellulosic raw material is usually in the form of a salt, such as an alkali metal salt such as a sodium salt. Prior to the defibration step, the alkali metal salts of the carboxyalkylated cellulose may be replaced with other cationic salts such as phosphonium salts, imidazolinium salts, ammonium salts, sulfonium salts and the like. Substitutions can be carried out by known methods.

(Esterification)
Esterified cellulose can also be used as anion-modified cellulose. Examples include a method of mixing a cellulose-based raw material with a powder or an aqueous solution of the phosphate-based compound A, and a method of adding an aqueous solution of the phosphate-based compound A to a slurry of the cellulose-based raw material. Phosphoric acid compound A includes phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, and esters thereof. These may be in the form of salts. Among the above, a compound having a phosphate group is preferable because it is low cost, easy to handle, and can introduce a phosphate group into the cellulose of the pulp fiber to improve defibration efficiency. Compounds having a phosphate group include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, phosphorus Tripotassium acid, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, ammonium metaphosphate and the like. These may be used singly or in combination of two or more to introduce a phosphate group. Among these, phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, phosphoric acid, from the viewpoint of high efficiency of introduction of phosphate group, easy fibrillation in the fibrillation process described below, and easy industrial application is preferred. Sodium dihydrogen phosphate and disodium hydrogen phosphate are particularly preferred. Further, it is desirable to use the phosphoric acid compound A in the form of an aqueous solution, since the reaction can proceed uniformly and the efficiency of introduction of the phosphoric acid group is increased. The pH of the aqueous solution of the phosphoric acid-based compound A is preferably 7 or less because the efficiency of introducing phosphate groups is high, but from the viewpoint of suppressing hydrolysis of pulp fibers, the pH is preferably 3 to 7.

Examples of the method for producing the phosphorylated cellulose include the following method. A phosphoric acid compound A is added to a suspension of a cellulosic raw material having a solid concentration of 0.1 to 10% by mass while stirring to introduce a phosphoric acid group into the cellulose. When the cellulosic raw material is 100 parts by mass, the amount of phosphoric acid compound A added is preferably 0.2 to 500 parts by mass, more preferably 1 to 400 parts by mass, in terms of elemental amount of phosphorus. . If the proportion of the phosphoric acid-based compound A is at least the lower limit, the yield of fine fibrous cellulose can be further improved. However, if the above upper limit is exceeded, the effect of improving the yield reaches a ceiling, which is not preferable from the viewpoint of cost.

In addition to the phosphoric acid-based compound A, a powder or aqueous solution of the compound B may be mixed. The compound B is not particularly limited, but a basic nitrogen-containing compound is preferable. "Basic" herein is defined as the pink-red color of the aqueous solution in the presence of the phenolphthalein indicator or the pH of the aqueous solution being greater than 7. The basic nitrogen-containing compound used in the present invention is not particularly limited as long as the effect of the present invention is exhibited, but a compound having an amino group is preferable. Examples include urea, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, hexamethylenediamine and the like. Among them, urea is preferred because it is inexpensive and easy to handle. The amount of compound B added is preferably 2 to 1000 parts by mass, more preferably 100 to 700 parts by mass, based on 100 parts by mass of the solid content of the cellulosic raw material. The reaction temperature is preferably 0 to 95°C, more preferably 30 to 90°C. Although the reaction time is not particularly limited, it is about 1 to 600 minutes, more preferably 30 to 480 minutes. When the esterification reaction conditions are within these ranges, cellulose can be prevented from being excessively esterified and easily dissolved, and the yield of phosphate esterified cellulose is improved. After dehydrating the obtained phosphate-esterified cellulose suspension, it is preferable to heat-treat at 100 to 170° C. from the viewpoint of suppressing hydrolysis of cellulose. Further, it is preferable to heat at 130° C. or less, preferably 110° C. or less while water is contained in the heat treatment, and heat at 100 to 170° C. after removing the water.

The degree of phosphate group substitution per glucose unit of the phosphated cellulose is preferably 0.001 or more and less than 0.40. By introducing a phosphate group substituent into cellulose, the celluloses electrically repel each other. Therefore, cellulose into which a phosphate group has been introduced can be easily nano-fibrillated. If the degree of phosphate group substitution per glucose unit is less than 0.001, sufficient nanofibrillation cannot be achieved. On the other hand, if the degree of phosphate group substitution per glucose unit is greater than 0.40, the nanofibers may not be obtained due to swelling or dissolution. In order to perform defibration efficiently, it is preferable that the obtained phosphoric esterified cellulose-based raw material is washed by washing with cold water after boiling. Modification by these esterifications is modification by a substitution reaction. The degree of substitution in the anion-modified cellulose nanofiber and the degree of substitution in the cellulose nanofiber are preferably the same.

In the present invention, in the phosphate-esterified cellulose obtained by the above steps, the phosphate group introduced into the cellulosic raw material is usually in the form of a salt, such as an alkali metal salt such as a sodium salt. Prior to the fibrillation step, the alkali metal salt of the phosphate esterified cellulose may be replaced with other cation salts such as phosphonium salts, imidazolinium salts, ammonium salts, sulfonium salts and the like. Substitutions can be carried out by known methods.

(Fibrillation process)
In the defibration step of the present invention, anion-modified cellulose is defibrated. As defibration treatment, for example, after sufficiently washing the anion-modified cellulose with water, it can be carried out using a known device such as a high-speed shearing mixer or a high-pressure homogenizer. Examples of the types of defibration equipment include high-speed rotation type, colloid mill type, high pressure type, roll mill type, and ultrasonic type. These devices may be used alone, or two or more of them may be used in combination.

When using a high-speed shear mixer, the shear rate is preferably 1000 sec ^-1 or more. When the shear rate is 1000 sec ^-1 or more, the aggregate structure is small and nanofibers can be formed uniformly. When using a high-pressure homogenizer, the applied pressure is preferably 50 MPa or higher, more preferably 100 MPa or higher, and even more preferably 140 MPa or higher. When treated with a wet high-pressure or ultrahigh-pressure homogenizer at this pressure, nanofiber formation proceeds efficiently, and anion-modified cellulose nanofibers with low viscosity can be efficiently obtained in the case of an aqueous dispersion.

Anion-modified cellulose is subjected to defibration treatment as an aqueous dispersion such as water. If the concentration of the anion-modified cellulose in the aqueous dispersion is high, the viscosity may excessively increase during the defibration treatment, and the fibrillation may not be uniform, or the apparatus may stop. Therefore, the concentration of the anion-modified cellulose should be properly set according to the processing conditions for the anion-modified cellulose. As an example, the concentration of anion-modified cellulose is preferably 0.3 to 50% (w/v), more preferably 0.5 to 10% (w/v), and 1.0 to 5% (w/v). is more preferred. In the present invention, an anion-modified cellulose nanofiber salt is obtained after defibrating the anion-modified cellulose.

(Desalination process)
In the desalting step of the present invention, the anion-modified cellulose nanofiber salt is brought into contact with a cation exchange resin to perform a cation exchange reaction, thereby obtaining an anion-modified cellulose nanofiber. In the anion-modified cellulose nanofiber salt, the cation salt is replaced with protons by contacting with a cation exchange resin. Since a cation exchange resin is used, unnecessary by-products such as sodium chloride are not generated.

The anion-modified cellulose nanofiber salt can be directly subjected to the desalting step in the aqueous dispersion obtained in the defibration step. In addition, water can be added as needed to lower the concentration.

As the cation exchange resin, both strongly acidic ion exchange resins and weakly acidic ion exchange resins can be used as long as the counter ion is H ⁺ . Among them, it is preferable to use a strongly acidic ion exchange resin. Examples of strongly acidic ion exchange resins and weakly acidic ion exchange resins include those obtained by introducing sulfonic acid groups or carboxyl groups into styrene resins or acrylic resins.
The shape of the cation exchange resin is not particularly limited, and various shapes such as fine granules (granules), membranes, and fibers can be used. Among them, the granular form is preferable from the viewpoint of efficiently treating the anion-modified cellulose nanofiber salt and facilitating separation after the treatment. A commercial product can be used as such a cation exchange resin. Commercially available products include, for example, Amberjet 1020, Amberjet 1024, Amberjet 1060, Amberjet 1220 (manufactured by Organo Corporation), Amberlite IR-200C, Amberlite IR-120B (manufactured by Tokyo Organic Chemical Co., Ltd.), Lewatit SP 112. , S100 (manufactured by Bayer), GEL CK08P (manufactured by Mitsubishi Chemical), Dowex 50W-X8 (manufactured by Dow Chemical) and the like.

The contact between the anion-modified cellulose nanofiber salt and the cation exchange resin is performed by, for example, mixing an aqueous dispersion of the granular cation exchange resin and the anion-modified cellulose nanofiber salt, stirring and shaking as necessary, and performing anion-modification. It can be carried out by contacting the cellulose nanofiber salt and the cation exchange resin for a certain period of time.

The concentration of the aqueous dispersion and the ratio to the cation exchange resin are not particularly limited, and those skilled in the art can appropriately set them from the viewpoint of efficient proton substitution. As an example, the concentration of the aqueous dispersion is preferably 0.05 to 10% by mass. If the concentration of the aqueous dispersion is less than 0.05% by mass, the time required for proton substitution may be too long. If the concentration of the aqueous dispersion exceeds 10% by mass, a sufficient effect of proton substitution may not be obtained.
The contact time is also not particularly limited, and can be appropriately set by those skilled in the art from the viewpoint of efficient proton substitution. For example, it can be carried out by contacting for 0.2 to 4 hours.

At this time, desalting can be performed by contacting the anion-modified cellulose nanofiber salt with an appropriate amount of cation exchange resin for a sufficient time.

(Step of sending liquid)
In the present invention, a step of pumping a mixture containing anion-modified cellulose nanofibers and a cation exchange resin is provided between the desalting step and the recovery step described later.

As a pump for sending a mixture containing anion-modified cellulose nanofibers and a cation exchange resin to a filtration device used in the recovery process, if it has the ability to send high-viscosity cellulose nanofibers, there is a type However, it is preferable to use a material that does not damage the ion exchange resin as much as possible when sheared by a pump.

Pumps are roughly classified into turbo type (non-displacement) pumps and positive displacement pumps. Turbo pumps include volute pumps, diffuser pumps, centrifugal pumps such as vortex pumps, mixed flow pumps, axial flow pumps, and the like. Positive displacement pumps include reciprocating pumps such as piston pumps, plunger pumps and diaphragm pumps, and rotary pumps such as gear pumps, vane pumps and screw pumps. Among these, it is more preferable to use a positive displacement screw pump or a positive displacement reciprocating pump. Compared to turbo-type (non-displacement type) pumps, which give kinetic energy to the fluid by rotating the impeller at high speed in the casing, this type of pump gives less shearing force to the fluid, Since there is very little leakage between the pumps, the lift is high, and it is characterized by being able to feed fluids with relatively high viscosity.

Examples of positive displacement screw pumps include positive displacement single-axis eccentric screw pumps (Mono pumps), positive displacement twin screw pumps, and the like, and the use of Mono pumps is preferred. In the Mohno pump, a metal rotor with a single-thread structure rotates inside a stator with a double-thread structure, thereby continuously moving the space between the rotor and stator to transfer fluid. is the pump. A positive displacement twin-screw pump is a pump that conveys a fluid confined in a space formed by two screws and a casing in the axial direction of the screws by rotating the screws.

A positive displacement diaphragm pump (membrane pump) is more preferably used as the positive displacement reciprocating pump. A diaphragm pump is composed of a diaphragm and a valve, and is a pump that reciprocates the diaphragm to change its volume, sucks and discharges fluid, and transfers fluid. Diaphragm pumps are superior to foreign matter in terms of seal system and valve system. Since the sliding part of the positive displacement screw pump is in contact with the liquid, it is easy for the resin to bite into it. The sliding parts of positive displacement reciprocating pumps such as plunger pumps, piston pumps, and diaphragm pumps are difficult for resin to get into. In addition, even if resin is caught in the check valve and causes a temporary clogging failure, it is more advantageous because the clogging matter is removed by the next discharge or the like.

(Recovery process)
In the recovery step of the present invention, the cation exchange resin is recovered from the mixture containing the anion-modified cellulose nanofibers and the cation exchange resin. Since a cation exchange resin is used in the desalting step, unnecessary by-products such as sodium chloride are not produced. Therefore, after the acid treatment using the cation exchange resin, the cation exchange resin is recovered from the mixture containing the anion-modified cellulose nanofibers and the cation-exchange resin, and the water of the anion-modified cellulose nanofibers is used as the filtrate. A dispersion is obtained.

A method for recovering the cation exchange resin is not limited, but a method of filtering using a filtering device is preferable. Before or after the recovery step, a step of performing microfiltration such as auxiliary agent filtration may be provided.

The shape of the filter medium used in the filtering device is not limited, and any shape such as a mesh shape or a slit shape can be used. Also, the filter medium may be one with holes. The material of the filter medium is not limited, and any material such as resin, metal, or ceramic can be used, but it is preferable to use a material having acid resistance.

The mesh opening or slit width of the filter medium may be any size as long as it can capture the cation exchange resin, and is preferably 500 μm or less, more preferably 400 μm or less. If the opening is too large, the cation exchange resin passes through the filter medium and cannot be recovered. In addition, the minimum diameter of the mesh opening or slit width of the filter medium depends on the amount of fibrous foreign matter contained in the cellulose nanofibers. That is, when the cellulose nanofibers are preliminarily subjected to microfiltration by auxiliary agent filtration or the like, the thickness is preferably 5 μm or more, and when the microfiltration is not preliminarily performed, the thickness is preferably 10 μm or more, and more preferably 50 μm or more. If the mesh opening is too small, clogging occurs due to undisentangled fibers, and the filtration throughput decreases.

As the filter aid used for the aid filtration, either an inorganic compound or an organic compound may be used, but commonly used aids include diatomaceous earth, silica, perlite, micronized cellulose, activated carbon, and the like. . The type of auxiliary agent can be adjusted according to the filtration conditions.

As the filtration device, any device capable of performing pressure filtration or vacuum filtration can be used regardless of the type. For example, Nutsche type, candle type, leaf disk type, drum type, filter press type, and belt filter type. etc. It is preferable to have a mechanism capable of recovering the cellulose nanofibers in the filter container after filtration. From the viewpoint of facilitating collection and transportation of the cellulose nanofibers and the cation exchange resin, it is preferable to use a candle type or leaf disk type filtration device.

In addition, it is preferable to have a mechanism capable of extracting the cellulose nanofibers by pressurizing with air as necessary, since the recovery rate is improved. The pressure when pressurizing with air is preferably 0.01 to 1.0 MPa, more preferably 0.05 to 0.5 MPa. If the pressure is lower than the lower limit, it is difficult to convey the highly viscous cellulose nanofiber. If the pressure is higher than the upper limit, it is necessary to increase the pressure resistance of the air tank and filtration device. I don't like it.

The pressure for pressure filtration is preferably 0.01 MPa or more and 5 MPa or less, and the pressure for vacuum filtration is preferably 0.01 MPa or more and 0.1 MPa or less.

The filtration device has a mechanism for recovering the cation exchange resin after filtration. From the viewpoint of facilitating recovery of the cation exchange resin, it is preferable that the opening of the filtration device is large. In order to recover the cation exchange resin, the filtration device may be provided with a mechanism for backwashing, a mechanism for scraping by a scraper, a mechanism for tilting and reversing the filtration container, and the like. Moreover, a mechanism for conveying the recovered cation exchange resin by a conveyor may be provided for the filtration device.

The object to be collected as a filter material by a metal mesh or the like is the cation exchange resin, and the anion-modified cellulose nanofibers are difficult to remove due to the diameter of the metal mesh or the like, and almost all of the anion-modified cellulose nanofibers are contained in the filtrate. Therefore, it is considered that the decrease in yield is extremely small.

In addition, in the liquid transfer and pressure filtration of the mixture containing the anion-modified cellulose nanofiber and the cation exchange resin in the recovery process, as in the liquid transfer process, a method that does not damage the cation exchange resin as much as possible should be selected. is desirable, for example positive displacement screw pumps or positive displacement reciprocating pumps are preferably used.

If necessary, the cellulose nanofibers mixed in the collected cation exchange resin in a small amount may be washed, and the collected cation exchange resin may be subjected to acid treatment and deliquoring for regeneration. All of these steps may be performed by one device, or may be performed by a plurality of devices. When using multiple devices, different types of devices may be combined.

The method of carrying out the regeneration step of the cation exchange resin by acid treatment of the recovered cation exchange resin is not limited. A method such as repeating washing with ion-exchanged water four times can be used. Since the cation exchange resin that has undergone the recovery process and the regeneration process can be used repeatedly, costs can be reduced.

EXAMPLES The present invention will be described in detail below with reference to examples. The following examples are intended to better illustrate the present invention and are not intended to limit the present invention. Unless otherwise specified, the methods for measuring physical properties are the methods described below.

(Method for measuring carboxyl group content)
60 mL of a 0.5% by mass slurry (aqueous dispersion) of carboxylated cellulose was prepared, and 0.1 M aqueous hydrochloric acid solution was added to adjust the pH to 2.5. The electrical conductivity was measured until From the amount (a) of sodium hydroxide consumed in the neutralization step of the weak acid, whose electrical conductivity changes slowly, the amount of carboxyl groups was calculated using the following formula:
Carboxyl group amount [mmol/g carboxylated cellulose] = a [mL] x 0.05/carboxylated cellulose mass [g]

(Method for measuring degree of carboxymethyl group substitution)
About 2.0 g of the sample was precisely weighed and put into a 300 mL stoppered Erlenmeyer flask. Add 100 mL of methanol nitrate (100 mL of special grade concentrated nitric acid to 1 L of anhydrous methanol) and shake for 3 hours to convert sodium salt of carboxymethylated cellulose (hereinafter also referred to as "Na-CMC") to carboxymethylated cellulose ( hereinafter also referred to as “H-CMC”). 1.5 to 2.0 g of absolutely dried H-CMC was precisely weighed and placed in a 300 mL stoppered conical flask. The H-CMC was wetted with 15 mL of 80% methanol, added with 100 mL of 0.1N NaOH and shaken at room temperature for 3 hours. Excess NaOH was back-titrated with 0.1N H ₂ SO ₄ using phenolphthalein as an indicator, and the degree of carboxymethyl group substitution was calculated using the following formula.
[{100×F′−(0.1N H ₂ SO ₄ (mL))×F}/(absolute dry mass of H-CMC (g))]×0.1=A degree of carboxymethyl group substitution=0 .162A/(1-0.058A)
A: Amount of 1N NaOH (mL) required to neutralize 1 g of H-CMC
F': Factor of _{0.1N H2SO4} F: Factor of 0.1N NaOH

(Method for measuring type B viscosity (mPa s))
Using a TV-10 type viscometer (Toki Sangyo Co., Ltd.), measurement was performed at 20° C. and 60 rpm.

(Example 1)
(Preparation of carboxylated cellulose)
5 g (absolute dry) of bleached softwood unbeaten pulp (manufactured by Nippon Paper Industries Co., Ltd.) is added to 500 mL of an aqueous solution in which 78 mg (0.5 mmol) of TEMPO (manufactured by Sigma Aldrich) and 754 mg (7.4 mmol) of sodium bromide are dissolved, Stir until the pulp is evenly dispersed. After adding 18 mL of a 2M sodium hypochlorite aqueous solution to the reaction system, the pH was adjusted to 10.3 with a 0.5N hydrochloric acid aqueous solution to initiate an oxidation reaction. Since the pH in the system decreased during the reaction, a 0.5N sodium hydroxide aqueous solution was successively added to maintain the pH at 10. After reacting for 2 hours, the mixture was filtered through a glass filter and thoroughly washed with water to obtain carboxylated cellulose having a carboxyl group content of 1.7 mmol/g.

(Fibrillation process)
Next, the resulting slurry of carboxylated cellulose was adjusted to 1% (w/v) with water and treated with an ultrahigh pressure homogenizer (20°C, 140 MPa) five times to obtain a transparent gel-like carboxylated cellulose nanofiber salt. A dispersion (1% (w/v)) of was obtained.

(Desalination process)
A cation exchange resin (“Amberjet 1024” manufactured by Organo Co., Ltd.) was added to the resulting carboxylated cellulose nanofiber salt dispersion, and the mixture was brought into contact with the mixture while stirring at 20° C. for 0.3 hours.

(Recovery process)
A metal mesh with an opening of 250 μm was attached as a filter medium to a stainless steel holder with a tank (KST-47, manufactured by Advantec, filtration area 12.5 cm ² ) to prepare a filtration device. The mixture of the carboxylated cellulose nanofiber dispersion and the cation exchange resin obtained in the above desalting step was filled in a pressurized tank of a filtration device, pressurized to 0.35 MPa using nitrogen gas, and filtered. Filtration was terminated 30 minutes after the start of filtration, and visual evaluation was performed to determine whether the filtrate was contaminated with the cation exchange resin. When the cation-exchange resin was not mixed in the filtrate, the ion-exchange resin was recovered, and the column of "ion-exchange resin recovery" was marked with "○". In addition, the amount of the obtained filtrate (filtered amount) was confirmed. Table 1 shows the results. In addition, a cellulose nanofiber may be called "CNF."

(Example 2)
A mixture of a carboxylated cellulose nanofiber dispersion and a cation exchange resin was obtained in the same manner as in Example 1, except that the type of cation exchange resin was changed to "Amberlite IR124" manufactured by Organo Corporation, filtered, and made an evaluation. Table 1 shows the results.

(Example 3)
A mixture of a carboxylated cellulose nanofiber dispersion and a cation exchange resin was obtained, filtered, and evaluated in the same manner as in Example 1, except that a wire mesh with an opening of 400 μm was used as the filter medium. Table 1 shows the results.

(Example 4)
A mixture of a carboxylated cellulose nanofiber dispersion and a cation exchange resin was obtained, filtered, and evaluated in the same manner as in Example 1, except that a wire mesh with an opening of 100 μm was used as the filter medium. Table 1 shows the results.

(Example 5)
A mixture of a carboxylated cellulose nanofiber dispersion and a cation exchange resin was obtained, filtered, and evaluated in the same manner as in Example 1, except that a nylon mesh with an opening of 250 μm was used as the filter medium. Table 1 shows the results.

(Example 6)
A metal slit-like filter (slit width: 50 μm) was attached as a filter medium to a candle-type filter (manufactured by Shibata, filtration area: 0.097 m ² ) to prepare a filtering device. A mixture of a carboxylated cellulose nanofiber dispersion and a cation exchange resin was obtained, filtered, and evaluated in the same manner as in Example 1, except that this filtration device was used. Table 1 shows the results.

(Example 7)
Diatomaceous earth (Radiolite 3000, manufactured by Showa Chemical Industry Co., Ltd., particle size 74.9 μm) as a filter aid was diluted with pure water and sent to a stainless steel holder with a tank (KST-47, manufactured by Advantec, filtration area 12.5 cm ² ). The diatomaceous earth was layered by pre-coating by pouring. After that, using the carboxylated cellulose nanofiber salt dispersion liquid before the desalting step in Example 1, the first-stage filtration treatment was performed. The obtained filtrate was desalted in the same manner as in Example 1 to obtain a mixture of a carboxylated cellulose nanofiber dispersion and a cation exchange resin. This mixture was used for the second stage of filtration. In the second-stage filtration, a filter device was prepared by attaching a metal filter with an opening of 5 μm as a filter medium. The mixture was filtered and evaluated in the same manner as in Example 1 except that this filter medium was used. Table 1 shows the results.

(Comparative example 1)
A mixture of a carboxylated cellulose nanofiber dispersion and a cation exchange resin was obtained in the same manner as in Example 1, except that a metal filter with an opening of 5 μm was used as the filter medium, filtered, and evaluated. Table 1 shows the results.

(Comparative example 2)
A mixture of a carboxylated cellulose nanofiber dispersion and a cation exchange resin was obtained, filtered, and evaluated in the same manner as in Example 1, except that a PET filter cloth with an air permeability of 12 was used as the filter medium. Table 1 shows the results.

As can be seen from Table 1, desalting is performed using an anion-modified cellulose nanofiber salt and a cation exchange resin to obtain a mixture containing the anion-modified cellulose nanofibers and the cation exchange resin, and the resulting mixture is filtered. The cation exchange resin could be recovered by filtering using a device. Moreover, the cation exchange resin was not mixed in the obtained filtrate containing the cellulose nanofibers.

In Comparative Examples 1 and 2, the opening of the filter medium was small, and a good filtration throughput could not be obtained due to clogging, whereas Examples 1 to 6 had a large opening or slit width. Clogging due to contained unfibrillated fibers and the like hardly occurred. Moreover, in Example 7, by removing in advance the unfibrillated fibers and the like contained in the CNF, a good filtration throughput was obtained without causing clogging.

(Example 8)
A nylon mesh with an opening of 250 μm was attached to the tip of the outlet hose of the monopump (liquid transfer pump) attached with the hopper. A mixture of the dispersion obtained in the same manner as in Example 1 and the cation exchange resin was charged into the hopper. After that, the solution was fed by a pump, pressure filtration was performed for 30 minutes, and the cation exchange resin and the cellulose nanofibers were separated by filtration. The pressure at the start of filtration was 0.1 MPa, and the pressure at the end of filtration was 0.3 MPa. Moreover, the B-type viscosity of the resulting 1% dispersion of carboxylated cellulose nanofibers was 2500 mPa·s (60 rpm, 20° C.).
When the resulting cellulose nanofiber dispersion was observed under a microscope, no contamination of resin fragments was observed.

(Example 9)
(Preparation of carboxymethylated cellulose)
250 g of dry mass of pulp (LBKP, manufactured by Nippon Paper Industries Co., Ltd.) was placed in a reactor capable of stirring pulp, and 112 g of a 50% by mass sodium hydroxide aqueous solution and 67 g of water were added while stirring. After stirring at 30° C. for 45 minutes and mercerizing, 364 g of a 35 mass % sodium monochloroacetate aqueous solution was added while stirring. The mixture was stirred at 30°C for 60 minutes, heated to 70°C over 30 minutes, and reacted at 70°C for 1 hour. Thereafter, the reactant was taken out to obtain a carboxymethylated pulp having a degree of carboxymethyl substitution of 0.27 per glucose unit (hereinafter also referred to as "carboxymethylated cellulose").

(Fibrillation process)
Carboxymethylated cellulose was adjusted to 1% (w/v) with water and treated with an ultrahigh pressure homogenizer (20° C., 140 MPa) three times to obtain a carboxymethylated cellulose nanofiber salt dispersion.

(Desalination process)
A cation exchange resin ("Amberjet 1024" manufactured by Organo Co., Ltd.) was added to the resulting carboxymethylated cellulose nanofiber salt dispersion, and the mixture was brought into contact with the mixture while stirring at 20°C for 0.3 hours.

A nylon mesh with an opening of 250 μm was attached to the tip of the outlet hose of the monopump (liquid transfer pump) attached with the hopper. A hopper was charged with the mixture of the boxymethylated cellulose nanofiber dispersion obtained above and the cation exchange resin. After that, the solution was fed by a pump, pressure filtration was performed for 30 minutes, and the cation exchange resin and the cellulose nanofibers were separated by filtration. The pressure at the start of filtration was 0.1 MPa, and the pressure at the end of filtration was 0.3 MPa. Moreover, the B-type viscosity of the resulting 1% dispersion of carboxymethylated cellulose nanofibers was 3500 mPa·s (60 rpm, 20° C.).
When the resulting cellulose nanofiber dispersion was observed under a microscope, no contamination of resin fragments was observed.

(Example 10)
A mixture of the cellulose nanofiber dispersion and the cation exchange resin was charged in the same manner as in Example 8, except that the pump in Example 8 was changed to a diaphragm pump, and then the liquid was pumped and filtered under pressure for 30 minutes. Then, the cation exchange resin and cellulose nanofibers were separated by filtration. The pressure at the start of filtration was 0.1 MPa, and the pressure at the end of filtration was 0.3 MPa. Moreover, the B-type viscosity of the resulting 1% dispersion of carboxylated cellulose nanofibers was 2600 mPa·s (60 rpm, 20° C.).
When the resulting cellulose nanofiber dispersion was observed under a microscope, no contamination of resin fragments was observed.

(Comparative Example 3)
The cation exchange resin and the cellulose nanofibers were separated by filtration in the same manner as in Example 10, except that a vortex pump was used as the pump for liquid transfer. The pressure at the start of filtration was 0.1 MPa, and the pressure at the end of filtration was 0.3 MPa. Moreover, the B-type viscosity of the resulting 1% dispersion of carboxylated cellulose nanofibers was 2500 mPa·s (60 rpm, 20° C.).
When the resulting cellulose nanofiber dispersion was observed under a microscope, resin fragments were detected.

(Comparative Example 4)
The cation exchange resin and the cellulose nanofibers were separated by filtration in the same manner as in Example 11, except that a vortex pump was used as the liquid-sending pump. The pressure at the start of filtration was 0.1 MPa, and the pressure at the end of filtration was 0.3 MPa. Moreover, the B-type viscosity of the resulting 1% dispersion of carboxymethylated cellulose nanofibers was 3500 mPa·s (60 rpm, 20° C.).
When the resulting cellulose nanofiber dispersion was observed under a microscope, resin fragments were detected.

As can be seen from Table 2, when a mixture of a cation exchange resin and a cellulose nanofiber dispersion is fed and filtered using a mono-pump, which is a positive displacement uniaxial eccentric screw pump, and a positive displacement diaphragm pump, which is a positive displacement reciprocating pump. No cation exchange resin fragments were found in the filtrate. On the other hand, when the liquid was fed using a vortex pump and filtered, fragments of the cation exchange resin were detected in the filtrate.

When a mixture containing anion-modified cellulose nanofibers and a cation exchange resin is sent to a recovery step for recovering the cation exchange resin, a positive displacement single-axis eccentric screw pump or a positive displacement diaphragm pump is used to transfer the cations. It was found that anion-modified cellulose nanofibers free from contamination by fragments of the exchange resin could be obtained.

Claims (10)

a preparation step of preparing an anion-modified cellulose comprising carboxylated cellulose, carboxymethylated cellulose, or phosphate esterified cellulose ;
a fibrillation step of fibrillating the anion-modified cellulose obtained in the preparation step to obtain an anion-modified cellulose nanofiber salt;
A desalting step of obtaining anion-modified cellulose nanofibers by desalting the anion-modified cellulose nanofiber salt obtained in the defibration step by performing a cation exchange reaction using a cation exchange resin. When,
a recovery step of recovering the cation exchange resin from the mixture containing the anion-modified cellulose nanofibers obtained in the desalting step and the cation exchange resin ;
a step of using a positive displacement screw pump or a positive displacement reciprocating pump for liquid transfer and pressure filtration of the mixture in the recovery step, or for liquid transfer between the desalting step and the recovery step;
A method for producing anion-modified cellulose nanofibers using a mesh-like filter medium with an opening of 10 μm or more and 400 μm or less or a slit-like filter medium with a slit width of 10 μm or more and 400 μm or less in the collecting step . a preparation step of preparing an anion-modified cellulose comprising carboxylated cellulose, carboxymethylated cellulose, or phosphate esterified cellulose;
a fibrillation step of fibrillating the anion-modified cellulose obtained in the preparation step to obtain an anion-modified cellulose nanofiber salt;
A microfiltration step of performing microfiltration on the anion-modified cellulose nanofiber salt obtained in the fibrillation step;
a desalting step of obtaining anion-modified cellulose nanofibers by desalting the anion-modified cellulose nanofiber salt after the microfiltration step by performing a cation exchange reaction using a cation exchange resin;
a recovery step of recovering the cation exchange resin from the mixture containing the anion-modified cellulose nanofibers obtained in the desalting step and the cation exchange resin;
a step of using a positive displacement screw pump or a positive displacement reciprocating pump for liquid transfer and pressure filtration of the mixture in the recovery step, or for liquid transfer between the desalting step and the recovery step;
A method for producing anion-modified cellulose nanofibers using a mesh-like filter medium with an opening of 5 μm or more and 400 μm or less or a slit-like filter medium with a slit width of 5 μm or more and 400 μm or less in the collecting step. The method for producing an anion-modified cellulose nanofiber according to claim 1 or 2 , wherein in the recovery step, pressure filtration at 0.01 MPa or more and 5 MPa or less or vacuum filtration at 0.01 MPa or more and 0.1 MPa or less is performed. The method for producing anion-modified cellulose nanofibers according to claim 1 , further comprising a step of microfiltration before or after the recovery step. The method for producing anion-modified cellulose nanofibers according to any one of claims 1 to 4, wherein the cation exchange resin recovered in the recovery step is subjected to a cation exchange resin regeneration step by acid treatment. . The method for producing anion-modified cellulose nanofibers according to any one of claims 1 to 5, wherein the positive displacement screw pump is a positive displacement uniaxial eccentric screw pump or a positive displacement twin screw pump. The method for producing anion-modified cellulose nanofibers according to any one of claims 1 to 5, wherein the positive displacement reciprocating pump is a positive displacement diaphragm pump. The anion-modified cellulose is the carboxylated cellulose,
The preparation step includes the step of oxidizing the cellulosic raw material using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromides, iodides, and mixtures thereof; The method for producing anion-modified cellulose nanofibers according to any one of claims 1 to 7 , wherein the carboxylated cellulose obtained by the process is cellulose into which a carboxyl group has been introduced. The anion-modified cellulose is the carboxymethylated cellulose,
The preparation step includes a step of mercerizing the cellulosic raw material with a mercerizing agent and then reacting it with a carboxymethylating agent, and the carboxymethylated cellulose obtained by the preparation step has a carboxymethyl group introduced into the cellulose. The method for producing anion-modified cellulose nanofibers according to any one of claims 1 to 7 , wherein the anion-modified cellulose nanofibers are prepared. The anion-modified cellulose is the phosphate-esterified cellulose,
The preparation step includes a step of adding a phosphoric acid compound to the cellulosic raw material, and the phosphorylated cellulose obtained by the preparation step is obtained by introducing a phosphoric acid group into cellulose , and glucose units The degree of phosphate group substitution per unit is 0.01 or more and less than 0.40,
The phosphoric acid compound includes phosphoric acid, polyphosphoric acid, phosphorous acid, phosphonic acid, polyphosphonic acid, phosphate ester, polyphosphate ester, phosphite ester, phosphonate ester, polyphosphonate ester, phosphate, and polyphosphoric acid. Production of anion-modified cellulose nanofibers according to any one of claims 1 to 7 , which is one or more compounds selected from the group consisting of salts, phosphites, phosphonates, and polyphosphonates. Method.