JP2014001272A - Method of producing polyuronic acid salt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing polyuronic acid salt, the method for improving the efficiency of production by electrolytic oxidation and easily separating a product material from an electrolyte.SOLUTION: In a method of producing polyuronic acid salt, cellulose with 30% or less of crystallinity is electrolytically oxidized in hydrous electrolyte that contains the following (1) to (3), wherein (1) is hydrous solvent, (2) is one or more mediator selected from N-oxyl compound, N-hydroxy compound, and nitroso compound, and (3) alkali metal halide of 0.3 mol or more relative to the hydrous solvent 1 kg.

Description

  The present invention relates to a method for producing a polyuronic acid salt.

  Water-soluble polymer materials have functions such as particle dispersion, stabilization, aggregation, viscosity adjustment, and adhesion, and are applied in various fields. In particular, many kinds of polycarboxylic acids are manufactured and used.

  In addition, as environmental awareness increases, various biodegradable water-soluble polymers that can be produced from natural raw materials that are renewable and have a low environmental impact and that are excellent in biodegradability have been proposed and developed. One of them is a water-soluble polysaccharide and its derivatives. As specific examples, xanthan gum, carboxymethyl cellulose, hydroxyethyl cellulose, alginic acid, pectinic acid, hyaluronic acid, polyuronic acid such as celouronic acid, and the like are known. Among them, polyuronic acid is expected to be applied to builders, dispersants, stabilizers, flocculants, viscosity modifiers, adhesives, film forming agents, and the like.

  As a method for producing polyuronic acid and polyuronic acid salt, various methods are known. For example, Patent Document 1 discloses amylose or starch in the presence of an N-oxyl compound such as 2,2,6,6-tetramethyl-1-piperidine-1-oxyl (TEMPO) and an alkali metal bromide and A method for producing water-soluble polyuronic acid that is oxidized using an oxidizing agent is described.

  In Patent Document 2, a polysaccharide is dispersed or dissolved in an aqueous solvent, oxidized with a co-oxidant in the presence of an N-oxyl compound, then added with polyvalent cations to precipitate, washed with water and desalted. A process for producing water-soluble polyuronic acid is described.

  Patent Document 3 describes a method for producing a polyuronic acid salt that oxidizes low crystalline powdery cellulose having a crystallinity of 30% or less.

  In Non-Patent Documents 1 and 2, the oxidization method using TEMPO as a mediator (electron mediator) selectively oxidizes the C6 hydroxyl group of potato starch (starch) or alkali cellulose, and the amount of carboxylic acid is high ( It has been reported that a (high oxidation degree) product was obtained.

  Patent Document 4 and Non-Patent Document 2 describe a method for producing a polyuronic acid salt by electrolytic oxidation of cellulose using low-crystallized cellulose or alkali-cellulose cellulose as a raw material.

  From the viewpoint of waste suppression and catalyst recycling, Patent Document 5 describes an electrolytic oxidation method in which TEMPO, which is a mediator, is adsorbed on a resin to be reacted, recovered, and reused.

JP 2004-189924 A JP 2006-124598 A JP 2009-263641 A JP 2011-256352 A Japanese Patent Laid-Open No. 11-170443

Karsten Schnatbaum, et al., Synthesis, 5, p.864〜872, 1999 P. Parport, et al., Cellulose, 17, p.815, 2010

  However, the methods described in Patent Documents 1 to 3 have a problem that a large amount of waste derived from the oxidizing agent or co-oxidizing agent used in the reaction is generated.

  The methods described in Non-Patent Documents 1 and 2 are advantageous in terms of reducing waste because they do not require a stoichiometric amount of oxidant. However, when a raw material that does not dissolve in water such as cellulose is used, the reaction does not proceed sufficiently, and it is difficult to produce polyuronic acid with high water solubility and high oxidation degree. Further, since the oxidation reaction proceeds at the optimum potential of TEMPO, it is necessary to insert a reference electrode for measuring the potential, which is a burden in industrial production.

  In the methods described in Patent Literature 4 and Non-Patent Literature 2, the electrolytic oxidation reaction is compared with the oxidation method (chemical oxidation) using an oxidizing agent or a co-oxidizing agent described in Patent Literatures 1 and 2 and the like. There is a problem that time is long.

  In the method described in Patent Document 5, since TEMPO is not in a dissolved state, a special solvent containing a specific salt capable of dissolving cellulose is required to allow the reaction to proceed efficiently. In addition, a large load is required to separate the product polyuronic acid from the solvent.

  An object of the present invention is to improve the efficiency of the production of polyuronic acid salt by electrolytic oxidation and to easily separate the product from the electrolytic solution.

In the method for producing a polyuronic acid salt of the present invention, cellulose having a crystallinity of 30% or less is electrolytically oxidized in an electrolytic solution containing the following 1) to 3).
1) Hydrous solvent 2) One or more mediators selected from N-oxyl compounds, N-hydroxy compounds, and nitroso compounds 3) 0.3 mol or more of alkali metal halide per 1 kg of hydrous solvent

  According to the method for producing polyuronic acid of the present invention, a polyuronic acid salt can be produced by efficiently electrolytically oxidizing cellulose, and the produced polyuronic acid salt can be easily separated from the reaction mixture.

  The inventor of the present application uses a special solvent and an oxidizing agent of a stoichiometric amount or more by performing electrolytic oxidation under the condition where a specific amount of alkali metal halide and mediator is present using cellulose having low crystallinity as a raw material. It was found that water-soluble polyuronic acid salts can be efficiently produced.

The polyuronic acid salt production method of the present embodiment (hereinafter also simply referred to as the production method of the present embodiment) is a cellulose having a crystallinity of 30% or less in an electrolytic solution containing the following 1) to 3). Hereinafter, this is also referred to as “low crystalline cellulose”).
1) Hydrous solvent 2) One or more mediators selected from N-oxyl compounds, N-hydroxy compounds, and nitroso compounds 3) 0.3 mol or more of alkali metal halides per 1 kg of hydrous solvent The polyuronic acid salt in which part or all of the hydroxyl group at the 6-position of anhydroglucose is oxidized can be obtained efficiently. Hereafter, each component used for the manufacturing method of this embodiment, manufacturing conditions, etc. are demonstrated.

(Alkali metal halide)
The alkali metal halide contained in the electrolytic solution of this embodiment is converted into a halogen oxoacid such as hypochlorite or a salt thereof by electrolytic oxidation. The converted halogen oxo acid or salt thereof is used for efficiently oxidizing the mediator described later. Further, the halogen oxo acid reacts with the mediator to return to the original alkali metal halide, and can be reused for the oxidation reaction.

  The alkali metal halide may be any compound as long as it is a compound of an alkali metal element and a halogen element, but an alkali metal chloride, bromide, or iodide is preferred. From the viewpoint of the selectivity of the oxidation reaction, chloride or bromide is more preferable. The alkali metal is preferably lithium, potassium, or sodium. Specific examples of the alkali metal halide include lithium chloride, sodium chloride, potassium chloride, lithium bromide, sodium bromide, and potassium bromide. These may be used alone or in combination of two or more.

  The amount of the alkali metal halide in the electrolytic solution is preferably high from the viewpoint of efficiently performing the electrolytic oxidation of the low crystalline cellulose. If the amount of the alkali metal halide is 0.3 mol or more with respect to 1 kg of the water-containing solvent in the electrolytic solution, the electrolytic oxidation proceeds efficiently, but is preferably 0.4 mol or more, and more preferably 0.5 mol or more. It is more preferable. By such an amount, even if the low crystalline cellulose is not dissolved in the water-containing electrolyte, the electrolytic oxidation proceeds efficiently. Moreover, since the polyuronic acid salt is obtained in a state where it is not dissolved in the aqueous electrolyte solution, the produced polyuronic acid salt can be easily separated from the aqueous electrolyte solution.

  On the other hand, from the viewpoint of suppressing side reactions, the amount of alkali metal halide in the electrolytic solution is preferably 3 mol or less, more preferably 2 mol or less, relative to 1 kg of the hydrous solvent in the electrolytic solution. More preferably, it is 5 mol or less.

(Mediator)
The mediator of this embodiment is one or more selected from N-oxyl compounds, oxime compounds, N-hydroxy compounds, and nitroso compounds from the viewpoint of selectively oxidizing the hydroxyl group at the C6 position of the anhydroglucose unit of cellulose. Compounds are preferred. Among these, N-oxyl compounds, oxime compounds, and N-hydroxy compounds are more preferable, and N-oxyl compounds are more preferable.

  The N-oxyl compound is a hindered amine N-oxide, and a heterocyclic N-oxyl compound having a bulky group at the α-position of the amino group is particularly preferable.

  The heterocyclic N-oxyl compound is preferably a piperidine oxyl compound, a pyrrolidine oxyl compound, or an imidazoline oxyl compound having an alkyl group having 1 or 2 carbon atoms. From the viewpoint of reaction selectivity, the alkyl having 1 or 2 carbon atoms is preferred. A piperidine oxyl compound or a pyrrolidine oxyl compound having a group is more preferable.

Specific examples of the piperidineoxyl compound having an alkyl group having 1 or 2 carbon atoms include 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO), 4-hydroxy-2,2,6,6 -Tetramethyl-piperidine-1-oxyl, 4-amino-2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-oxo-2,2,6,6-tetramethyl-piperidine-1- Oxyl, 4-methoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-acetoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-acetamido-2, 2,6,6-tetramethyl-piperidine-1-oxyl, 4- (isothiocyanato) -2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-maleimide 2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-benzoyloxy-2,2,6,6-tetramethyl-piperidine-1-oxyl, 4- (4-nitrobenzoyloxy) -2 , 2,6,6-tetramethyl-piperidine-1-oxyl, 4- (phosphonooxy) -2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-cyano-2,2,6,6 - tetramethyl - piperidine-1-oxyl, PIPO (polymer-bound piperidinyloxy Le), SiO ₂ supported TEMPO, polystyrene supported TEMPO, and polyacrylic acid-supported TEMPO, and the like.

  Specific examples of the pyrrolidine oxyl compound having an alkyl group having 1 or 2 carbon atoms include 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolidine-1-oxyl.

  Specific examples of the imidazoline oxyl compound having an alkyl group having 1 or 2 carbon atoms include 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl, 4-carbamoyl- Examples include 2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl and 4-phenacridene-2,2,5,5-tetramethyl-imidazolidine-1-oxyl.

  Among these, a piperidineoxyl compound having a methyl group is preferable from the viewpoint of solubility in a solvent and reactivity. Among these, 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-amino-2,2,6,6-tetramethyl-piperidine-1-oxyl, 4-methoxy-2,2, 6,6-tetramethyl-piperidine-1-oxyl, 4-acetoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl, and 4-benzoyloxy-2,2,6,6-tetramethyl -Piperidine-1-oxyl is preferred, 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetra Tyl-piperidine-1-oxyl and 4-methoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl are more preferred, and 2,2,6,6-tetramethyl-piperidine-1-oxyl ( More preferred is TEMPO).

  In the present embodiment, these mediators may be used alone or in combination of two or more.

  In the production method of the present embodiment, the amount of mediator may be a catalytic amount. Specifically, 0.001 mass% or more is preferable with respect to the low crystalline cellulose used as a raw material, 0.1 mass% or more is more preferable, and 1 mass% or more is further more preferable. Moreover, it is preferable that it is 30 mass% or less, It is more preferable that it is 20 mass% or less, It is further more preferable that it is 15 mass% or less.

(Hydrous solvent)
As a hydrous solvent contained in the electrolytic solution of the present embodiment, water or a mixed solvent of water and an organic solvent can be used. Among these, water is preferable from the viewpoint of dissolving the alkali metal halide and reducing the environmental load. The ratio of water and organic solvent in the mixed solvent is not particularly limited, but the ratio of water to the mixed solvent is preferably 30% by mass or more, more preferably 40% by mass or more, and 50% by mass or more. More preferably.

  The organic solvent is an alcohol having 1 to 3 carbon atoms, a ketone having 1 to 6 carbon atoms, a linear or branched saturated hydrocarbon having 1 to 6 carbon atoms, a linear or branched unsaturated group having 1 to 6 carbon atoms. Hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, polar solvents and the like can be used. Examples of the alcohol having 1 to 3 carbon atoms include methanol, ethanol, and propanol. Examples of the ketone having 1 to 6 carbon atoms include acetone and methyl isobutyl ketone. Examples of the linear or branched saturated hydrocarbon having 1 to 6 carbon atoms include n-pentane and n-hexane. Examples of the linear or branched unsaturated hydrocarbon having 1 to 6 carbon atoms include 2-pentene and 1-hexene. Examples of the aromatic hydrocarbon include benzene and toluene. Examples of the halogenated hydrocarbon include methylene chloride and chloroform. Examples of the polar solvent include esters, lower alkyl ethers, cyclic ethers, N, N-dimethylformamide, and dimethyl sulfoxide. Among these organic solvents, alcohols having 1 to 6 carbon atoms, ketones having 1 to 6 carbon atoms, and polar solvents are preferable from the viewpoint of reactivity. These solvents may be used alone or in combination of two or more.

  The usage-amount of a hydrous solvent is not specifically limited, What is necessary is just to select suitably according to reaction conditions etc. Usually, it is 2 mass times or more, preferably 5 mass times or more, 2000 mass times or less, preferably 300 mass times or less with respect to the low crystalline cellulose which is a raw material.

  The hydrogen ion concentration (pH) of the electrolytic solution is preferably pH 7.5 or higher, more preferably pH 8.0 or higher from the viewpoint of suppressing the degree of polymerization of the low crystalline cellulose or the polyuronic acid salt to be generated, or from the viewpoint of the oxidation reaction. Moreover, pH 10.0 or less is preferable. In the present embodiment, the pH of the electrolytic solution is a value measured at room temperature using a pH meter equipped with a glass electrode or the like.

(PH buffer)
In the method for producing the polyuronic acid salt according to this embodiment, a pH buffering agent may be added to the water-containing solvent in order to bring the pH of the electrolytic solution into a suitable range. By adding a pH buffer to the water-containing solvent, the effect of further suppressing the decrease in the degree of polymerization of the low crystalline cellulose or the produced polyuronic acid salt while maintaining the pH of the electrolytic solution can be obtained.

  The pH buffer added to the water-containing solvent is not particularly limited. For example, alkali metal carbonate, alkaline earth metal carbonate, alkali metal phosphate, alkaline earth metal phosphate, various amines, amino acids, and the like are used. be able to.

  As the alkali metal carbonate, for example, lithium carbonate, sodium carbonate, potassium carbonate and the like can be used. These pH buffering agents can be used alone or in combination of two or more. Among these, a combination of an alkali metal carbonate and an alkali metal bicarbonate is preferable from the viewpoint of solubility in a solvent and a desired pH buffering ability. Examples of the alkaline earth metal carbonate that can be used include magnesium carbonate and calcium carbonate. As the alkali metal phosphate, for example, sodium dihydrogen phosphate, disodium phosphate, potassium dihydrogen phosphate, dipotassium phosphate, and the like can be used. Examples of alkaline earth metal phosphates that can be used include magnesium phosphate and calcium phosphate.

  Although there is no restriction | limiting in particular about the usage-amount of a pH buffer, It is preferable that it is 0.05 mol or more with respect to 1 kg of hydrous solvent from a viewpoint of a pH buffer effect, and it is more preferable that it is 0.1 mol or more. Further, the amount of the pH buffer used is preferably 2 mol or less, more preferably 1 mol or less with respect to 1 kg of the hydrous solvent.

(Low crystalline cellulose)
The cellulose used as a raw material in the production method of the present embodiment is cellulose having a crystallinity of 30% or less from the viewpoint of reactivity.

In general, cellulose has a plurality of crystal structures, and it is known that an amorphous part and a crystal part coexist. Generally known cellulose also has an extremely small amount of amorphous part. The degree of crystallization can be quantified as a degree of crystallinity by putting a numerical value obtained from an X-ray crystal diffraction spectrum into the following calculation formula (1).
Crystallinity (%) = [(I _22.6 −I _18.5 ) / I _22.6 ] × 100 Formula (1)
However, I _22.6 indicates the diffraction intensity of the lattice plane (002 plane) (diffraction angle 2θ = 22.6 °) in X-ray diffraction, and I _18.5 indicates the amorphous portion (diffraction angle 2θ = 18.5 °). The diffraction intensity is shown.

  This index uses the change in the X-ray diffraction intensity on the 002 plane of the cellulose I-type crystal accompanying the change from crystal to amorphous as its index. Therefore, if the crystal form contained in cellulose is only type I, the crystallinity is theoretically a value of 0 to 100%. In fact, since there are a plurality of crystal forms in cellulose, a negative value can also be taken when crystals other than type I are sufficiently destroyed and made amorphous. In this embodiment, when a negative value is obtained in the formula (1), the crystallinity is regarded as 0%.

  In the production method of the present embodiment, the crystallinity of cellulose used as a raw material is 30% or less. Even if the crystallinity of cellulose is 30% or less, it does not dissolve in a reaction solvent such as water. However, if the crystallinity is 30% or less, the electrolytic oxidation reaction of cellulose proceeds extremely well, and a polyuronic acid salt having a high degree of oxidation can be obtained efficiently. From this viewpoint, the crystallinity of cellulose is preferably as low as possible, preferably 25% or less, more preferably 20% or less, and even more preferably 10% or less. In particular, in the present embodiment, it is most preferable to use cellulose whose crystallinity obtained by the formula (1) is 0%.

  The median diameter of the low crystalline cellulose used as a raw material is not particularly limited, but is preferably 300 μm or less, more preferably 150 μm, and even more preferably 50 μm or less from the viewpoint of allowing the electrolytic oxidation reaction to proceed in a slurry state. However, from the viewpoint of operability when carried out industrially, it is preferably 20 μm or more, more preferably 25 μm or more. In addition, the median diameter of the low crystalline cellulose in the present embodiment can be obtained by laser diffraction / scattering particle size distribution measurement in a state where cellulose is dispersed in ion-exchanged water.

(Method for producing low crystalline cellulose)
The manufacturing method of the low crystalline cellulose used as a raw material is not specifically limited. Examples thereof include the methods described in JP-A-62-236801, JP-A-2003-64184, JP-A-2004-331918, and the like. Among these, a method of pulverizing a cellulose-containing raw material with a pulverizer is preferable.

  Cellulose-containing raw materials include wood chips, pruned branches of various trees, thinned wood, branch wood, building waste, factory waste, wood pulp produced from wood, and fibers around cotton seeds Pulp such as cotton linter pulp; Paper such as newspaper, corrugated cardboard, magazine, and fine paper; Plant stems and leaves such as rice straw and corn stalk; Plant shells such as rice husk, palm husk and coconut husk be able to. Among these, pulps and woods are preferable. A cellulose containing raw material may be used independently, or 2 or more types may be mixed and used for it.

  Although there is no restriction | limiting in particular in the magnitude | size and shape of a cellulose containing raw material processed with a grinder, From a viewpoint on operation, 1 micrometer square or more is preferable, 5 micrometers square or more are more preferable, and 7 micrometers square or more are further more preferable. Moreover, 50 mm square or less is preferable, 20 mm square or less is more preferable, and 10 mm square or less is more preferable.

  The cellulose-containing raw material may be subjected to a drying process in advance depending on the water content before the pulverization process is performed with a pulverizer from the viewpoint of improving the rate of decrease in crystallinity due to the pulverization process. The drying process can be performed by heating or decompression.

  Depending on the size and shape of the cellulose-containing raw material, the raw material may be preliminarily cut using a cutting machine and then pulverized. For example, when the cellulose-containing raw material is a sheet-like pulp or the like, it is possible to reduce a load required for pulverization in the subsequent pulverization process by performing a cutting process in advance. Further, if the drying process is performed after the cutting process, the drying process can be efficiently and easily performed.

  The pulverizer used for producing the low crystalline cellulose is preferably a medium pulverizer from the viewpoint of the rate of decrease in crystallinity. The medium pulverizer may be a container driven pulverizer or a medium agitating pulverizer. Examples of the container-driven pulverizer include a rolling mill, a vibration mill, a planetary mill, and a centrifugal fluid mill. Among these, a vibration mill with high crushing efficiency is preferable from the viewpoint of productivity. Examples of the medium stirring pulverizer include tower pulverizers such as tower mills; stirring tank pulverizers such as attritors, aquamizers and sand grinders; distribution tank pulverizers such as visco mills and pearl mills; distribution pipe pulverizers; Examples thereof include an annular pulverizer such as a coball mill; a continuous dynamic pulverizer and the like. Among these, from the viewpoint of productivity, an agitation tank type pulverizer having high pulverization efficiency is preferable.

  Of the above-described pulverizers, a vibration mill or a planetary ball mill is more preferable from the viewpoint of the effect of reducing the crystallinity of cellulose by pulverization. The medium used for the vibration mill is preferably a rod or a ball. A vibration rod mill is more preferable from the viewpoint of productivity. The pulverization process may be either a batch type or a continuous type.

  The pulverization time may be appropriately determined according to the type of pulverizer, the type of medium, the size, the filling rate, etc., but from the viewpoint of reducing the crystallinity of cellulose to a desired value, 0.01 hour The above is preferable, 0.05 hours or more is more preferable, and 0.1 hours or more is more preferable. Moreover, from a viewpoint of productivity, 50 hours or less are preferable, 20 hours or less are more preferable, and 10 hours or less are more preferable.

  The pulverization temperature is not particularly limited, but is preferably 5 ° C or higher, and more preferably 10 ° C or higher. Moreover, from a viewpoint of preventing deterioration by heat, 250 degrees C or less is preferable and 200 degrees C or less is more preferable.

  In the pulverization treatment as described above, the degree of polymerization of the obtained low crystalline cellulose can be controlled. As a result, it is possible to easily produce cellulose having a high degree of polymerization and low crystallinity that is generally difficult to obtain. The average degree of polymerization of the low crystalline cellulose is not particularly limited, but is preferably 10 or more, more preferably 20 or more, and still more preferably 30 or more. Moreover, it is preferable that it is 1000 or less, It is more preferable that it is 500 or less, It is further more preferable that it is 200 or less. The average degree of polymerization of the low crystalline cellulose in the present embodiment is a viscosity average degree of polymerization that can be determined by a copper-ammonia method.

(Polyuronic acid salt production method)
In the manufacturing method of this embodiment, an electrode to be an anode and a cathode is inserted into an electrolytic solution, electricity is passed between the anode and the cathode, and electrons are emitted from the cellulose via an alkali metal halide and a mediator on the anode side. To oxidize cellulose.

  In the manufacturing method of this embodiment, the low crystalline cellulose which is a raw material should just be disperse | distributed in electrolyte solution and made into the slurry form. In a slurry state, the oxidation reaction proceeds efficiently and a polyuronic acid salt is obtained. Moreover, since the polyuronic acid salt obtained can be made into the state which is not melt | dissolved in electrolyte solution, it can collect | recover easily.

(Electrolysis device)
As the electrolyzer, a single tank electrolyzer having two ordinary electrodes (anode and cathode) can be used. Further, a filter press type electrolysis device or the like can also be used. In order to improve the efficiency of the electrolytic oxidation reaction, the same electrolytic cell may have two or more anodes and cathodes.

  There are no particular restrictions on the material and shape of the electrodes that serve as the anode and the cathode, and electrodes that are used in ordinary electrolytic reactions can be widely used. For example, examples of the anode material include platinum, gold, titanium, platinized titanium, stainless steel, nickel, ruthenium, lead oxide, iron oxide, and carbon. Examples of the cathode material include platinum, gold, titanium, platinized titanium, tin, aluminum, stainless steel, zinc, lead, copper, and carbon. From the viewpoint of corrosion resistance and reactivity, the anode and cathode materials are preferably selected and combined from platinum, platinum-plated titanium, graphite, and glassy carbon.

  The electrode interval between the anode and the cathode is not particularly limited as long as a necessary current can be applied, but is usually set at an interval of 5 mm to 10 cm, and can be used at such an electrode interval also in this embodiment.

(Reaction conditions)
The temperature of the electrolytic reaction is preferably 50 ° C. or lower, more preferably 40 ° C. or lower, and even more preferably 30 ° C. or lower, from the viewpoint of reaction selectivity and side reaction suppression. The lower limit of the reaction temperature is preferably −5 ° C. or more from the viewpoint of reactivity.

The current density in the electrolytic oxidation reaction is a value obtained by dividing the current flowing through the reaction system by the electrode area of the anode. The lower limit of the current density from the viewpoint of reaction selectivity is preferably at 50 mA / cm ² or more, more preferably 60 mA / cm ² or more, and still more preferably 70 mA / cm ² or more. The upper limit of the current density is preferably at 200 mA / cm ² or less, more preferably 150 mA / cm ² or less, and more preferably 120 mA / cm ² or less.

  The amount of electricity used in the reaction (= current value × time) varies depending on the desired amount of oxidation, but is preferably 3F or more per mole of anhydroglucose unit (hereinafter also referred to as “AGU”) of the low crystalline cellulose, 5F or more is more preferable. The upper limit of the amount of electricity is preferably 60F or less, and more preferably 50F or less.

  There is no particular limitation on the reaction time, and the reaction time may be adjusted so that the amount of electricity falls within a preferable range.

(Purification)
In the production method of the present embodiment, electrolytic oxidation of low crystalline cellulose proceeds in a slurry state, and polyuronic acid salt is obtained in a solid state. For this reason, it is possible to isolate the polyuronic acid salt by a simple operation such as filtration or centrifugation. The isolated crude purified polyuronate can be used as it is in various applications, but may be purified. In the case of performing purification, purification used in the production of polyuronic acid salt by ordinary electrolytic oxidation can be performed. Specifically, reprecipitation using water as a good solvent, methanol, ethanol, acetone, or the like as a poor solvent, extraction of a mediator into a solvent phase-separated with water, ion exchange of a salt, purification by dialysis, or the like is used. be able to. Moreover, you may wash | clean the crudely purified polyuronate obtained by filtration or centrifugation etc. with a water-containing organic solvent etc. as needed.

  The electrolytic solution recovered by removing the product polyuronic acid salt and unreacted cellulose by filtration or centrifugation can be used again for electrolytic oxidation of low crystalline cellulose. The recovered electrolytic solution can be used as it is, but a water-containing solvent, a mediator, and an alkali metal halide may be added as necessary. Further, an unused electrolytic solution and a recovered electrolytic solution can be mixed and used.

(Polyuronic acid salt)
The polyuronic acid salt obtained by the above method is a polymer salt in which D-glucuronic acid and glucose are linked by a β-glycoside bond, and is typically represented by the following structural formula (1).

  In the structural formula (1), X represents a cation, and examples thereof include hydrogen ions, alkali metal ions, alkaline earth metal ions, various amines, and protonated products of amino acids. When the valence (a) of the cation is 2 or more, the number of X per carboxy group is 1 / a. X is preferably an alkali metal ion, more preferably a sodium ion, from the viewpoint of the water solubility of the produced polyuronic acid salt.

  In Structural Formula (1), m represents the molar fraction of anhydroglucuronate units in the polyuronic acid salt, and n represents the molar fraction of anhydroglucose units in the polyuronic acid salt.

  The weight average molecular weight of the obtained polyuronic acid salt is not particularly limited, but is preferably 500 or more, more preferably 1000 or more, and further preferably 5000 or more. Moreover, from a viewpoint of providing water solubility and biodegradability, 500000 or less are preferable, 200000 or less are more preferable, and 100000 or less are more preferable. In addition, in this embodiment, a weight average molecular weight is a molecular weight of pullulan conversion which can be calculated | required by gel permeation chromatography (GPC).

The degree of oxidation of the resulting polyuronic acid salt is preferably 0.6 or higher, more preferably 0.7 or higher, from the viewpoint of water solubility of the polyuronic acid salt to be produced. Moreover, 1.0 or less is preferable. In this embodiment, the degree of oxidation of polyuronic acid salt is a number obtained by dividing the number of carboxy groups per molecule of polyuronic acid salt by the number of sugar units constituting the main chain of polyuronic acid salt. It is calculated using the number of equivalents of the basic compound used in the sum. Specifically, it is a value obtained by the following calculation formula (2) from the amount of carboxylic acid per unit mass of polyuronic acid measured by the neutralization titration method described in the examples.
Oxidation degree = [(162.1 × A) / (1-14.0 × A)] (2)
Here, A is the amount of carboxylic acid (mol / g) determined by neutralization titration. The basic compound used for neutralization can be alkali metal or alkaline earth metal hydroxides such as sodium hydroxide, potassium hydroxide, and magnesium hydroxide, and ammonia or amine compounds.

  The polyuronic acid salt obtained by the production method of the present embodiment is used as a biodegradable water-soluble polymer material in a wide range of applications such as a dispersing / stabilizing agent, a flocculant, a viscosity modifier, an adhesive, and a film forming agent. It can be used suitably.

(Optional component)
In the production method of the present embodiment, the electrolyte is an alkali metal salt such as a perchloric acid metal salt or a borofluoride metal salt and an inorganic compound other than a buffer, an anion activator such as an alkyl sulfonate or a long-chain alkyl fatty acid salt. , Cationic surfactants such as benzalkonium chloride and distearyldimethylammonium chloride, nonionic surfactants such as polyoxyethylene alkyl ether (POE alkyl ether) and polyoxyethylene alkyl phenyl ether (POE alkyl phenyl ether), An amphoteric surfactant surfactant, a polymer type dispersant such as polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylate, carboxymethyl cellulose, and the like may be included.

≪Evaluation method≫
(1) Calculation of crystallinity of cellulose The crystallinity of cellulose is calculated from the peak intensity of the diffraction spectrum measured under the following conditions using “Rigaku RINT 2500VC X-RAY diffractometer” manufactured by Rigaku Corporation. It was calculated by the formula (1).
X-ray light source: Cu / Kα-radiation
Tube voltage: 40 kV, tube current: 120 mA, measurement range: 2θ = 5-45 °
Measurement sample: compressed by pellets with an area of 320 mm ² × 1 mm thickness X-ray scanning speed: 10 ° / min

(2) Measurement of weight average molecular weight of polyuronic acid salt The weight average molecular weight of polyuronic acid salt was measured under the following conditions using gel permeation chromatography (GPC).
Column: Tosoh Corporation G4000PWXL + G2500PWXL
Eluent: 0.2M phosphate buffer / acetonitrile = 9/1 (volume ratio)
Measurement temperature: 40 ° C
Flow rate: 1.0 mL / min
Detector: UV or RI
Standard polymer: Pullulan

(3) Measurement of degree of oxidation of polyuronic acid salt Polyuronic acid sodium salt was prepared in a 2% aqueous solution, and the pH was adjusted to 1 or less with 6N hydrochloric acid. This acidic aqueous solution was poured into ethanol, and the resulting precipitate was collected, washed several times with ethanol, and dried. 0.05 g of the obtained polyuronic acid is precisely weighed and dissolved in 30 mL of ion exchange water, and titrated with 0.1N sodium hydroxide aqueous solution using phenolphthalein as an indicator to determine the amount of carboxylic acid per unit mass of polyuronic acid salt. It was. From the obtained amount of carboxylic acid, the degree of oxidation of polyuronic acid was determined by the above formula (2).

≪Production of low crystalline cellulose≫
About 35 g of powdered cellulose (Nippon Paper Industries Co., Ltd .: KC Flock W400G) was charged into a planetary ball mill (FRITSCH: Pulveristte 5) filled with 150 zirconia balls (498 g) having a diameter of 10 mm, and the rotational speed was 400 r / min. Then, pulverization was performed for about 240 minutes. The pulverization process was repeated for 10 minutes and 10 minutes. The yield was almost 100%, and the crystallinity of the obtained low crystalline cellulose powder was 0%.

≪Manufacture of polyuronic acid salt≫
Example 1
As a water-containing solvent containing a pH adjuster, 35.6 g of a 0.7 M carbonate buffer solution (pH 8.2) was placed in a 40 mL cylindrical glass cell containing a magnetic stir bar. In a carbonate buffer, 0.026 g of 2,2,6,6-tetramethyl-1-piperidine-1-oxyl (TEMPO) as a mediator and 1.46 g (25 mmol) of sodium chloride (NaCl) as an alkali metal halide were added. Dissolved. The amount of alkali metal halide with respect to 1 kg of the hydrous solvent is 0.7 mol. Thereto was added 0.26 g of the low crystalline cellulose powder obtained in “Production of Low Crystalline Cellulose”, and the mixture was stirred and dispersed to form a slurry. The amount of mediator is 10% by mass relative to the polysaccharide. A platinum electrode (3.4 cm ² , manufactured by Techno Sigma) as an anode and a cathode was immersed in the reaction solution with a spacing of 1 cm.

The terminal of the electrode and a DC stabilized power source (Matsusada Precision Co., Ltd .: PLE-36-1.2) were connected by a conducting wire, and a current of 300 mA was passed to conduct the electrolytic oxidation reaction at 24 ° C. The current density at this time, since the area of the anode is 3.4cm ^2, a ^{300mA / 3.4cm 2 = 88mA / cm} 2. Even after 5 hours of electrolytic oxidation, the reaction solution remained in a slurry state. The amount of electricity from the start of the reaction to the end of the reaction for 5 hours is 36 F per mole of AGU. The reaction solution was put into a dialysis tube (manufactured by AS ONE: Visking tube) and dialyzed for 3 days using water as a dialysis solution. The reaction solution became transparent. The reaction solution purified by dialysis was freeze-dried to obtain a powdery polyuronic acid salt (sodium polyuronic acid). The yield was 61%, the degree of oxidation was 0.78, and the weight average molecular weight was 25500.

(Example 2)
Sodium polyuronate was obtained in the same manner as in Example 1 except that the pH of the electrolytic solution was 8.8. The reaction temperature at this time was 24 ° C. The yield was 78%, the degree of oxidation was 0.72, and the weight average molecular weight was 23400.

(Example 3)
Sodium polyuronate was obtained in the same manner as in Example 1 except that the voltage was controlled to set the current density to 118 mA / cm ² . The reaction temperature at this time was 24 ° C. The amount of electricity in this case is 45.5 F per mole of AGU. The yield was 88%, the oxidation degree was 0.77, and the weight average molecular weight was 26000.

Example 4
Sodium polyuronate was obtained in the same manner as in Example 1 except that the alkali metal halide was 2.57 g of sodium bromide (NaBr). The reaction temperature at this time was 24 ° C. In this case, the amount of alkali metal halide with respect to 1 kg of the hydrous solvent is 0.7 mol. The yield was 100%, the degree of oxidation was 0.63, and the weight average molecular weight was 11500.

(Example 5)
Sodium polyuronate was obtained in the same manner as in Example 4 except that the pH of the electrolyte was 9.7. The reaction temperature at this time was 24 ° C. The yield was 77%, the oxidation degree was 0.85, and the weight average molecular weight was 12,500.

(Example 6)
The reaction solution produced under the same conditions as in Example 1 was centrifuged (3000 r / min, 30 minutes), then washed several times with 80% ethanol, filtered and dried to obtain sodium polyuronic acid. The yield was 45%.

(Comparative Example 1)
The electrolytic oxidation was performed under the same conditions as in Example 1 except that the low crystalline cellulose of Example 1 was changed to crystalline cellulose (KC floc W400). The reaction temperature at this time was 24 ° C. Formation of sodium polyuronic acid could not be confirmed.

(Comparative Example 2)
A 200 mL beaker containing a magnetic stirring bar was charged with 105.94 g of 0.7 M carbonate buffer (pH 8.5) as a reaction solvent, and 2,2,6,6-tetramethyl-1-piperidine-1-oxyl ( TEMPO) 0.16 g was dissolved. Thereto was added 0.81 g of low crystalline cellulose powder, and the mixture was stirred and dispersed. An electrode having a glassy carbon (made by BAS Co., Ltd., area: 25 mm × 25 mm) as the anode and platinum-plated titanium (Denbo Kogyo Co., Ltd., 40 mm × 45 mm) attached as a cathode with a wormworm clip is 1 cm apart. It was immersed in the reaction solution. Moreover, the calomel electrode (made by BAS Co., Ltd.) was inserted in the reaction solution as a reference electrode, and each electrode was attached to the terminal of potentiostat (Hokuto Denko Co., Ltd .: HA-151).

The electrolytic oxidation reaction was performed so that the set potential was set to 0.53 V with respect to the reference electrode so as to be a constant potential. The reaction temperature at this time was 24 ° C. After energization for 112.5 hours (electricity of 3.1 F per mol of AGU was energized), the electrolytic oxidation reaction was stopped. The average current density during electrolytic oxidation was 0.6 mA / cm ² . The reaction solution was filtered using a filter paper (manufactured by Advantech: No. 2), and the filtrate was adjusted to pH 5 or less with 6M hydrochloric acid, and then reprecipitated with ethanol. The precipitated product was washed three times with ethanol, then washed with acetone, and dried under reduced pressure. The resulting product was sodium polyuronate (0.12 g). Moreover, the solid which remained on the filter paper at the time of filtration of a reaction liquid was unreacted cellulose. The yield of the obtained sodium polyuronic acid was 15%, the degree of oxidation was 0.63, and the weight average molecular weight was 1640.

  Table 1 summarizes the results of each example and comparative example. In each embodiment, the polyuronic acid salt could be efficiently produced with a shorter electrolytic oxidation time than in the comparative example. Further, the produced polyuronic acid salt was easily purified, and the yield was high.

  The production method of the present invention can perform efficient electrolytic oxidation of cellulose and can easily separate the produced polyuronic acid salt from the reaction mixture.

Claims (6)

A method for producing a polyuronic acid salt, in which cellulose having a crystallinity of 30% or less is electrolytically oxidized in an electrolytic solution containing the following 1) to 3).
1) Hydrous solvent 2) One or more mediators selected from N-oxyl compounds, N-hydroxy compounds, and nitroso compounds 3) 0.3 mol or more of alkali metal halide per 1 kg of the hydrous solvent   The method for producing a polyuronic acid salt according to claim 1, wherein the electrolytic solution has a pH of 7.5 or more and a pH of 10 or less. The method for producing a polyuronic acid salt according to claim 1 or 2, wherein the electrolytic oxidation is performed at a current density of 50 mA / cm ² or more and 200 mA / cm ² or less.   The method for producing a polyuronic acid salt according to any one of claims 1 to 3, wherein the mediator is an N-oxyl compound.   The manufacturing method of the polyuronic acid salt of any one of Claims 1-4 in which the said electrolyte solution contains a pH buffer.   The method for producing a polyuronic acid salt according to any one of claims 1 to 5, wherein the cellulose having a crystallinity of 30% or less is produced by pulverizing a cellulose-containing raw material using a pulverizer.