Method for decolorizing lignin and method for preparing decolorized lignin

Abstract

To provide a method for decolorizing lignin that decolorizes lignin to the extent that it can be applied to various uses.SOLUTION: A method for decolorizing lignin includes reacting a plant or a processed plant containing lignin of 10-50 mass% with a compound represented by one of the general formulae (11)-(13). General formula (11) R-N=C=O, general formula (12) R-COOH, and general formula (13) R-OH (where, R is an alkyl group, an aralkyl group, an aryl group, a halogenated alkyl group, a halogenated aryl group, a sulfonyl group, or a silyl group).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for decolorizing lignin and a method for preparing decolorized lignin. [Background technology]

[0002] More than 90% of wood is composed of cell wall components, and the cell walls are mainly composed of cellulose, hemicellulose, and lignin. Of these main components, lignin usually accounts for about 20 to 30% in wood, and forms an intermediate layer by bonding cell membranes together. In addition, some of the lignin in wood is also present in the cell membranes. Lignin is a polymeric compound formed by the condensation of hydroxyphenylpropane units. It has a series of π-conjugated chains, an aromatic main chain structure, and phenolic hydroxyl groups that can become organic radicals. Lignin with this structure functions as a heat-resistant filler, UV absorber, and antioxidant, and is expected to be used as a high-performance resin material, such as engineering plastics. Furthermore, plant-derived polymeric compounds such as lignin are also expected to function as environmentally friendly materials.

[0003] However, typical lignin is colored brown or black, which causes color changes in the medium to which it is added and reduces the light transmittance of the medium to which it is added, limiting the uses of lignin. Therefore, from the viewpoint of expanding the use of lignin in materials, a method for decolorizing lignin is a very important technology.

[0004] As a method for decolorizing lignin, Azotobacter ( Azotobacter Patent Document 1 proposes a biological decolorization method using microorganisms and enzymes belonging to the genus Bacillus subtilis. However, the microorganisms used in the method described in Patent Document 1 are microorganisms that contribute to the decomposition of lignin in plants. Therefore, what actually occurs in the method described in Patent Document 1 is the decolorization of plants due to the decomposition of lignin. Therefore, it is difficult to utilize the usefulness of lignin and expand its use to various materials using the method described in Patent Document 1.

[0005] Furthermore, a technology has been reported that improves the heat resistance, hardness, compatibility with other components, etc. of resins by chemically modifying the hydroxyl groups (hydroxyl groups of the guaiacol structure and alcoholic hydroxyl groups) of lignin obtained from pulp waste liquor, etc. (see Non-Patent Document 1). However, lignin extracted from pulp waste liquor is unstable due to denaturation during extraction, making it difficult to use as a material. Furthermore, the use of harmful chemicals such as strong acids and alkalis and heating during extraction creates a significant environmental burden, making industrial deployment difficult. Furthermore, no technology has been reported for decolorizing and whitening lignin by modifying its hydroxyl groups. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 9-67785 [Non-patent literature]

[0007] [Non-Patent Document 1] Green Chem., 2016, vol. 18, p. 1175-1200 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, lignin is expected to be a functional material, but since lignin itself is colored, the applications to which lignin can be applied are limited. Therefore, an object of the present invention is to provide a method for decolorizing lignin that decolorizes lignin to an extent that makes it applicable to various uses. Another object of the present invention is to provide a method for preparing decolorized lignin that can be used for various purposes. [Means for solving the problem]

[0009] In view of the above-mentioned problems, the present inventors have conducted extensive research into methods for decolorizing colored lignin. It has been speculated that lignin is colored because the vinyl group at the para-position of the phenolic hydroxyl group loses electronic conjugation and a UV chromophore is present. As a result of extensive research, the inventors have discovered that by reacting a plant or treated plant material containing a predetermined amount of lignin with a reactive compound such as an isocyanate compound to modify the hydroxyl groups of the lignin, the degree of coloration of the lignin is reduced, and decolorized lignin can be prepared (extracted). The present invention has been completed based on these findings.

[0010] The above-mentioned problems of the present invention have been solved by the following means. (1) A method for decolorizing lignin, comprising reacting a plant or treated plant material containing 10 to 50 mass % of lignin with a compound represented by any one of the following general formulas (11) to (13): RN=C=O general formula (11) R-COOH general formula (12) R-OH general formula (13) (In the general formulae (11) to (13), R represents an alkyl group, an aralkyl group, an aryl group, a halogenated alkyl group, a halogenated aryl group, a sulfonyl group, or a silyl group.)

[0011] (2) A method for preparing decolorized lignin, comprising reacting a plant or a treated plant product containing 10 to 50% by mass of lignin with a compound represented by any one of the general formulae (11) to (13) to obtain decolorized lignin.

[0012] (3) The method according to (1) or (2) above, wherein the compound represented by any one of the general formulas (11) to (13) above is an isocyanate compound represented by the following general formula (21): R'-N=C=O General formula (21) (In general formula (21), R' represents an alkyl group, an aralkyl group, or an aryl group.) (4) The method according to any one of (1) to (3) above, wherein the obtained decolorized lignin is in powder form. (5) The plant is a cedar ( Cryptomeria japonica ), beech ( Fagus crenata Blume), Pinus spp. ( Pinus ) Plants, Balsa ( Ochroma lagopus ), giant pheasant ( Myriophyllum aquaticum ), Moso bamboo ( Phyllostachys heterocycla ), rice ( Oryza sativa ), bread wheat ( Triticum aestivum ), corn ( Zea mays subsp. mays (L.) Iltis), Erianthus ( Erianthus arundinaceus ), Miscanthus ( Miscanthus sinensis ),sugar cane( Saccharum officinarum ), Yoshi ( Phragmites australis ), Giant Lead ( Arundo donax ), oil palm, Elaeis ), Nipa palm ( Nypa fruticans Wurmb), sugar palm ( Arenga pinnata or Arenga saccharifera ), Water hyacinth ( Eichhornia crassipes ), Senninmo ( Potamogeton maackianus ), Elodea canadensis ( Egeria densa ), Kuromo (Hydrilla verticillata ), Konakadamo ( Elodea nuttallii ), Akamoku ( Sargassum horneri ), Sargassum ( Sargassum fulvellum ), Ulva ( Ulva ), Ichiijita ( Caulerpa taxifolia ), sea grapes ( Caulerpa lentillifera ), and Eucheuma genus ( Eucheuma The method according to any one of (1) to (4), wherein the plant is at least one kind of plant selected from the group consisting of: (6) The method according to any one of (1) to (5), wherein the plant material has been subjected to at least one treatment selected from the group consisting of chipper treatment, dry grinding treatment, wet grinding treatment, grinding treatment, saccharification treatment, fermentation treatment, digestion treatment, explosion treatment, subcritical water treatment, decomposition treatment with an ionic liquid, acid treatment, base treatment, and microwave treatment. [Effects of the Invention]

[0013] According to the present invention, colored lignin can be decolorized and decolorized lignin can be produced. The decolorized lignin obtained by the present invention can be used as a functional substance in various media such as resin compositions, polymer materials, coating materials, cosmetic compositions, automotive parts, building materials, adhesives, and heat-resistant fillers. [Brief explanation of the drawings]

[0014]

Figure 1

Figure 2

Figure 3

[0015] In the present invention, a plant or treated plant product (hereinafter also referred to as "plant raw material") containing 10 to 50 mass % of lignin is reacted with a specific compound having a functional group reactive with the hydroxyl groups of lignin to modify these hydroxyl groups, thereby decolorizing the colored lignin. The present invention will be described below based on preferred embodiments, but the present invention is not limited to these.

[0016] Lignin, which is the target of treatment in the present invention, is a polymeric compound present in the cell walls and cell membranes of plants. Lignin is composed of hydroxyphenylpropane as a basic unit. The type and composition of substituted aromatic substances, which are the structural units of lignin, vary depending on the plant species, such as coniferous trees, broad-leaved trees, and grasses. The lignin used as the target of treatment in the present invention may be obtained from any plant, as long as it contains a predetermined amount of lignin. Furthermore, the object to be treated in the present invention is not particularly limited as long as it contains a predetermined amount of lignin, and may also contain components that constitute cell walls and cell membranes, such as cellulose and hemicellulose.

[0017] In the method of the present invention, a plant or treated plant product containing 10 to 50 mass % of lignin is used as a starting material to be reacted with the compound described below. The starting materials used in the present invention will be described in detail below.

[0018] For plants containing a predetermined amount of lignin, the amount of lignin can be quantified according to the method described below, and plant materials containing the predetermined amount can be used as plants for use in the present invention. Alternatively, by referring to Chapter 1 "Biomass Classification and Chemical Composition" in "Latest Trends in Lignin Utilization" by Shiro Saka et al. (2013), edited by Shiro Saka, with reference to such, plant materials containing the predetermined amount can be appropriately selected and used in the present invention. Specific examples of plants containing a predetermined amount of lignin that can be used in the present invention include cedar, beech, pine plants, balsa, giant oak, moso bamboo, rice (preferably rice straw and rice husks), bread wheat, corn, Erianthus, Miscanthus, and sugarcane (preferably bagasse ( Bagasse , sugarcane juice residue), reed, giant reed, oil palm, nipa palm, sugar palm, water hyacinth, Celtis crenata, Elodea canadensis, Hydrangea chinensis, Asclepias chinensis, Sargassum serrata, Ulva lettuce, yarrow, sea grape, and Eucheuma plants. Of these, cedar, beech, plants of the pine family, balsa, moso bamboo, rice, wheat, corn, Erianthus, Miscanthus, sugarcane, reed, giant reed, oil palm, nipa palm, and sugar palm have a high lignin content and can be preferably used in the present invention. Any part of the plant can be used as the plant raw material used in the present invention, including the whole plant, roots, tuberous roots, rhizomes, trunks, branches, stems, leaves (leaf blades, petioles, etc.), bark, sap, resin, flowers (petals, ovaries, etc.), fruits, seeds, etc. A combination of these parts may also be used. Of these parts, it is preferable to use the rhizomes, trunks, branches, stems, leaves (leaf blades, petioles, etc.), and bark of the plant.

[0019] In the present invention, the above-mentioned plants may be used as they are, or may be treated plants that have been subjected to a predetermined treatment. By subjecting the plants to the predetermined treatment, the reactivity with the compounds described below can be improved and the amount of lignin contained in the plant material can be increased. The treatment applied to the plant can be appropriately selected within a range that does not impair the effects of the present invention, with reference to Chapter 1, "Biomass Classification and Chemical Composition," in "Latest Trends in Lignin Utilization" by Shiro Saka et al. (2013), edited by Shiro Saka. Examples include chipper treatment, dry grinding treatment, wet grinding treatment, grinding treatment, saccharification treatment, fermentation treatment, cooking treatment, explosion treatment, subcritical water treatment, decomposition treatment using ionic liquid, acid treatment, base treatment, and microwave treatment. Among these, saccharification treatment (monosaccharification treatment or low-saccharification treatment) using cellulase or the like, and fermentation treatment using alcohol-fermenting yeast are preferred.

[0020] When natural plants are used as starting materials in the present invention, the lignin content can be determined by referring to "Latest Trends in Lignin Utilization" by Shiro Saka et al. (2013), edited by Shiro Saka, Chapter 1, "Biomass Classification and Chemical Composition." Alternatively, the amount of lignin contained in a treated product can be measured by the method described in the Examples below.

[0021] Lignin is believed to possess a UV-ray chromophore due to the loss of electronic conjugation of the vinyl group para to the phenolic hydroxyl group within the aromatic compound residue backbone (Green Chem., 2016, vol. 18, pp. 1175-1200; see Chapter 2, "Lignin Distribution and Structural Diversity in Biomass Cells," in "Latest Trends in Lignin Utilization" by Keiji Takabe (2013), edited by Shiro Saka). The presence of such a UV-ray chromophore in lignin is presumed to be responsible for its brown or black color. Therefore, we proposed that decolorization could be achieved by covalently modifying the hydroxyl groups of lignin with organic side chains and encapsulating the UV-ray chromophore. Therefore, in the present invention, lignin is reacted with a compound represented by any one of the following general formulas (11) to (13) (hereinafter simply referred to as a "reactive compound") to chemically modify the hydroxyl groups of the lignin.

[0022] RN=C=O general formula (11) R-COOH general formula (12) R-OH general formula (13) In the general formulae (11) to (13), R represents an alkyl group, an aralkyl group, an aryl group, a halogenated alkyl group, a halogenated aryl group, a sulfonyl group, or a silyl group.

[0023] Although the details of the mechanism by which lignin is decolorized by the present invention are unclear, it is believed that the following occurs. Specifically, modification of the phenolic hydroxyl groups restores electronic conjugation and inhibits the π-π interactions of the aromatic rings of lignin. Furthermore, organic side chains chemically modified via the hydroxyl groups of lignin cover the UV chromophores of lignin. As a result, it is presumed that light absorption by lignin is suppressed, resulting in decolorization of lignin. The reactive compounds used in the present invention may be used singly or in combination of two or more.

[0024] In general formula (11), R represents an alkyl group, an aralkyl group, an aryl group, a halogenated alkyl group, a halogenated aryl group, a sulfonyl group, or a silyl group. Among these, in view of the solubility of the reaction product in an organic solvent, R is preferably an alkyl group, an aralkyl group, or an aryl group, more preferably an alkyl group having 2 to 18 carbon atoms, an aralkyl group having 7 to 24 carbon atoms, or an aryl group having 7 to 24 carbon atoms, and more preferably an alkyl group having 6 to 12 carbon atoms, an aralkyl group having 7 to 18 carbon atoms, or an aryl group having 7 to 18 carbon atoms. Specific examples of the reactive compound (isocyanate compound) represented by general formula (11) include alkyl isocyanates such as ethyl isocyanate, propyl isocyanate, butyl isocyanate, pentyl isocyanate, hexyl isocyanate, hexyl diisocyanate, heptyl isocyanate, octyl isocyanate, decyl isocyanate, dodecyl isocyanate, tetradecyl isocyanate, and octadecyl isocyanate, benzyl isocyanate, phenethyl isocyanate, and 1,3-bis(2-isocyanato-2-propyl)benzoyl isocyanate. Aralkyl isocyanates such as phenyl, naphthyl ethyl isocyanate, methyl benzyl isocyanate, 3-isopropyl-α,α-dimethyl benzyl isocyanate, tosyl isocyanate, and xylene diisocyanate, phenyl isocyanate, phenylene diisocyanate, dimethyl phenyl isocyanate, ethoxyphenyl isocyanate, acetyl phenyl isocyanate, butyl phenyl isocyanate, diisopropyl phenyl isocyanate, naphthyl isocyanate, nitrophenyl isocyanate, and biphenyl isocyanate. Examples of the halogenated alkyl isocyanates include tosyl-2,6-diisocyanate, 4-ethylphenyl isocyanate, methoxyphenyl isocyanate, naphthalene-1,5-diisocyanate, and 2-methoxyphenyl isocyanate; halogenated alkyl isocyanates such as chloropropyl isocyanate and trichloroacetyl isocyanate; halogenated aryl isocyanates such as chlorophenyl isocyanate, bromophenyl isocyanate, dichlorophenyl isocyanate, trichlorophenyl isocyanate, chloromethylphenyl isocyanate, chloronitrophenyl isocyanate, fluorophenyl isocyanate, difluorophenyl isocyanate, trifluoromethylphenyl isocyanate, trifluoromethoxyphenyl isocyanate, bis(trifluoromethyl)phenyl isocyanate, and chloro(trifluoromethyl)phenyl isocyanate; sulfonyl isocyanates such as chlorosulfonyl isocyanate, benzylsulfonyl isocyanate, and toluenesulfonyl isocyanate; and silyl isocyanates such as trimethylsilyl isocyanate.Of these, alkyl isocyanates, aralkyl isocyanates, and aryl isocyanates are preferred, and hexyl isocyanate, hexyl diisocyanate, heptyl isocyanate, octadecyl isocyanate, dodecyl isocyanate, benzyl isocyanate, phenethyl isocyanate, and diisopropylphenyl isocyanate are more preferred.

[0025] R in the general formula (12) has the same meaning as R in the general formula (11), and the preferred range is also the same. Specific examples of the reactive compound (carboxylic acid) used in general formula (12) include propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, 3-(2-amino-2-oxoethyl)-5-methylhexane, 5-azidopentanoic acid, crotonic acid, cyanoacetic acid, aminocinnamic acid, atrolactic acid, (aminomethyl)phenylacetic acid, 2-phenylacrylic acid, 3-aminocinnamic acid, benzoic acid, 2-amino-4,5-dimethylbenzoic acid, and anthracene. carboxylic acid, 3-chloropropionic acid, 5-chloropentanoic acid, 2,3-dichloroisobutyric acid, 3-bromopropionic acid, 3-bromo-2-oxopropionic acid, 2-bromoisobutyric acid, 9-bromononanoic acid, 2,3-dibromopropionic acid, 3-iodopropionic acid, 3-amino-3-(4-chlorophenyl)propionic acid, 2-acetamido-5-bromobenzoic acid, 4-(bromomethyl)phenylacetic acid, 4-bromocinnamic acid, 4-(2-bromoethyl)benzoic acid, 2-(p-toluenesulfonyl)acetic acid, and 3-(trimethylsilyl)propiolic acid. Of these, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, and dodecanoic acid are preferred.

[0026] R in the general formula (13) has the same meaning as R in the general formula (11), and the preferred range is also the same. Specific examples of the reactive compound (alcohol) used in general formula (13) include ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, dodecanol, tetradecanol, hexadecanol, heptadecanol, octadecanol, docosanol, eicosanol, benzyloxypropanol, cinnamyl alcohol, cyclohexylpropanol, phenoxypropanol, (chloroiodo)bromoethanol, (chloro)bromopropanol, (chloro)bromopentanol, (chloro)bromohexanol, bromoundecyl alcohol, bromodecanol, and 2-[(3-aminophenyl)sulfonyl]ethanol. Among these, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, and dodecanol are preferred.

[0027] In the present invention, the reactive compound is preferably an isocyanate compound represented by the following general formula (21). R'-N=C=O General formula (21) In general formula (21), R' represents an alkyl group, an aralkyl group, or an aryl group. The alkyl group, aralkyl group, and aryl group represented by R' have the same meanings as the alkyl group, aralkyl group, and aryl group represented by R in general formulas (11) to (13), respectively, and the preferred ranges are also the same.

[0028] The method for modifying the hydroxyl groups of lignin is not particularly limited, and for example, when modifying phenolic hydroxyl groups, a modification method that restores the electronic conjugation of the vinyl group and inhibits the π-π interaction of the aromatic rings of lignin is preferred. In the present invention, the reaction of lignin with a reactive compound forms at least one bond selected from the group consisting of a urethane bond, an ester bond, and an ether bond. The formation of urethane bonds, ester bonds, and ether bonds in the present invention will be described in detail based on the following general formulas (1) to (3). Lignin is a large polymeric compound that forms a complex structure by highly polymerizing through random radical coupling reactions. Therefore, the structure of lignin has not yet been clearly elucidated. Therefore, when describing the chemical structure of lignin in this specification, only the guaiacol structure and the vinyl group at the para-position of the phenolic hydroxyl group will be described in detail, and other parts will be omitted or simplified. Furthermore, in actual lignin, a substituent is bonded to the vinyl group at the para-position of the phenolic hydroxyl group, forming a complex aromatic main chain structure as a whole. However, in this specification, the bond of such a substituent to the vinyl group will also be omitted. Furthermore, the substituent bonded to the vinyl group may have an alcoholic hydroxyl group. Such an alcoholic hydroxyl group will also be omitted. In this specification, the chemical structure of lignin is depicted focusing on one guaiacol structure among lignins that form a complex aromatic main chain structure. Therefore, when the reactive compound used in the present invention has two or more reactive functional groups in the molecule, one reactive functional group is reacted with a phenolic hydroxyl group, and the remaining reactive functional groups are described as unreacted for convenience. However, since lignin contains many phenolic hydroxyl groups and alcoholic hydroxyl groups, the remaining reactive functional groups are not actually unreacted, but react with other phenolic hydroxyl groups or alcoholic hydroxyl groups, forming a complex structure.

[0029] [ka]

[0030] General formula (1) shows the reaction formula when a reactive compound represented by general formula (11) is used. In general formula (1), a urethane bond is formed by an addition reaction between the hydroxyl group of the guaiacol structure of lignin and an isocyanate compound. For alcoholic hydroxyl groups not shown in general formula (1), a urethane bond is also formed by an addition reaction with the reactive compound represented by general formula (11).

[0031] [ka]

[0032] General formula (2) shows the reaction formula when a reactive compound represented by general formula (12) is used. In general formula (2), an ester bond is formed by a condensation reaction between a hydroxyl group of the guaiacol structure of lignin and a carboxylic acid. An ester bond is also formed by a condensation reaction between an alcoholic hydroxyl group not shown in general formula (2) and a reactive compound represented by general formula (12).

[0033] [ka]

[0034] General formula (3) shows the reaction formula when a reactive compound represented by general formula (13) is used. In general formula (3), an ether bond is formed by a condensation reaction between the hydroxyl group of the guaiacol structure of lignin and an alcohol. An ether bond is also formed by a condensation reaction between an alcoholic hydroxyl group not shown in general formula (3) and a reactive compound represented by general formula (13).

[0035] The structure of lignin after reaction with a reactive compound is specifically shown below, but the present invention is not limited thereto.

[0036] [ka]

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[0051] The reaction conditions for modifying the hydroxyl groups of lignin are not particularly limited, and conditions used in conventional methods can be appropriately selected. For example, a solvent is appropriately selected from water, alcohols such as ethanol, methanol, isopropanol, and n-butanol, carboxylic acids such as formic acid and acetic acid, dimethyl sulfoxide, acetonitrile, dimethylformamide, and N-methylpyrrolidone, and the plant material and reactive compound are mixed in the selected solvent. From the viewpoint of solubility, the solvent for the reaction system used in the present invention is preferably water, ethanol, or a mixture thereof. The mixing ratio of the plant material and the reactive compound can also be appropriately selected, and it is preferable to mix the plant material with a reactive compound in a stoichiometrically equimolar amount or more relative to the hydroxyl groups of the lignin contained in the plant material. For example, it is preferable to mix the plant material with an equal to nine-fold amount of reactive compound by mass relative to the plant material.

[0052] After mixing the plant raw material and the reactive compound, the mixture may be left to stand to react the lignin with the reactive compound, but it is preferable to stir the mixture to react the lignin with the reactive compound. The reaction temperature is preferably 20 to 150°C, more preferably 20 to 60°C. The reaction time is preferably 3 to 24 hours, more preferably 3 to 5 hours.

[0053] The reaction between the plant raw material and the reactive compound can be terminated by adding an excess amount of solvent to the reaction system. The product after the chemical reaction can be separated and purified by removing unreacted materials using standard methods such as filter filtration or column chromatography to separate and purify the decolorized lignin. While there are no particular restrictions on the solvent used for separating and purifying the reaction product, it is preferable to use an alcohol solvent such as ethanol to maintain the degree of decolorization of the reaction product. Furthermore, powdered decolorized lignin can be obtained by removing the solvent from the reaction system using standard methods after the chemical reaction.

[0054] By undergoing the above-described steps, decolorized lignin can be obtained. As used herein, "decolorization" refers to a reduction in the degree of coloration of lignin, and decolorization of lignin can be confirmed using a coloration measurement device or by visually inspecting the appearance.

[0055] Since lignin can be used in a variety of media, including resin compositions, polymeric materials, coating materials, cosmetic compositions, automotive components, building materials, adhesives, and heat-resistant fillers, the lignin obtained by the method of the present invention preferably has high whiteness. White lignin having an L* value of 80 or greater in the L*a*b* color space is more preferable. The L*a*b* color space is a type of complementary color space, with an L* dimension indicating lightness and complementary color dimensions a* and b*, and is based on a nonlinear compression of the coordinates of the CIE XYZ color space. In this specification, "white" is defined as an L* value of 80 or greater in the L*a*b* color space. The L*, a*, and b* values ​​can be measured according to JIS Z 8781-4:2013. The whiteness (L* value) of lignin can be adjusted by appropriately selecting the plant raw material, reactive compounds, reaction solvent, reaction temperature and time, and separation and purification method of the reaction product. For example, if the plant raw material is prepared from wood or other materials in the presence of bases and sulfur, and the raw material contains these components or chromophores produced by chemical reactions induced by these components, the degree of decolorization of the reaction product may be reduced, making it impossible to achieve the desired L* value. In this case, it is preferable to remove components such as bases and sulfur from the plant raw material.

[0056] Furthermore, a solution of the decolorized lignin obtained by the present invention can be applied to a suitable support, such as a PET plate or a glass plate, and then dried to produce a transparent film containing lignin. The decolorized lignin obtained by the present invention also functions as a heat-resistant filler, adhesive, and the like. Therefore, by incorporating the decolorized lignin obtained by the present invention into media such as resin compositions, polymeric materials, coating materials, cosmetic compositions, automotive components, and building materials, the functions of lignin can be imparted to the media without causing any inherent color change. In the present invention, the solvent for preparing the lignin solution is not particularly limited and can be appropriately selected depending on the physical properties of the reactive compound used in the present invention. Specific examples include ethanol, acetone, chloroform, acetamide, acetonitrile, isopropanol, 1,4-dioxane, dimethyl sulfoxide, tetrahydrofuran, toluene, nitrobenzene, hexane, and methanol.

[0057] The reaction in the method of the present invention is carried out under milder reaction conditions than existing technologies, such as cooking treatments using strong acids or strong bases under high temperatures and pressures. Therefore, in addition to decolorizing and whitening the lignin, denaturation of the lignin components is suppressed. Therefore, the lignin obtained by the method of the present invention is more suitable for use as a variety of materials than lignin obtained by conventional methods. [Example]

[0058] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0059] Example 1 <Preparation of cedar sawdust dispersion> A cedar sawdust dispersion was prepared by mixing 0.6 g of cedar sawdust (obtained from Daiso Co., Ltd.) with 9.4 mL of ultrapure water and 10 mL of ethanol to a concentration of 3 wt %.

[0060] <Formation of urethane bond> To 20 mL of the resulting dispersion, 2 mL of hexyl isocyanate (Tokyo Chemical Industry Co., Ltd.) was added dropwise, and the mixture was stirred at 50° C. and 1 atmosphere for 5 hours to carry out a urethane bond forming reaction. The mixture was then washed with an excess amount of ethanol, and the precipitate (unreacted cedar sawdust) was decanted. After that, unreacted materials (unreacted isocyanate, etc.) were removed by filtration, and the mixture was dried to recover a white powder.

[0061] The cedar sawdust was colored before the urethane bond formation reaction. The colored cedar sawdust (lignin) was whitened by performing a urethane bond formation reaction on this cedar sawdust using hexyl isocyanate. Furthermore, when 0.05 g of the resulting powder (lignin) was added to 1 mL each of chloroform and ethanol, the powder (lignin) dissolved in all of these organic solvents.

[0062] Example 2 A powder was prepared in the same manner as in Example 1, except that the urethane bond-forming reaction was carried out using dodecyl isocyanate (manufactured by Tokyo Chemical Industry Co., Ltd.) instead of hexyl isocyanate. As a result, the obtained powder (lignin) was white. Furthermore, when 0.05 g of the obtained powder (lignin) was added to 1 mL each of chloroform and ethanol, the powder (lignin) dissolved in all of these organic solvents.

[0063] Example 3 <Preparation of saccharification residue from cedar wood flour> 2 kg of cedar wood powder with a diameter of approximately 0.7 mm, prepared by grinding using a chopper mill and a hammer mill, was dispersed in water (18 L) to a concentration of 10 wt %. The resulting sample was wet-ground for 6 hours in a wet bead mill (LME4; manufactured by Ashizawa Finetech Co., Ltd.) containing 2 mm zirconia beads. A saccharifying enzyme (cellulase, GODO-TCF; Godo Shusei Co., Ltd.) was added to the resulting slurry sample at a concentration of 0.2 mL / g relative to the plant material, and enzymatic saccharification was carried out at 50°C for 24 hours. An alcoholic fermentation enzyme (sake dry yeast, model number 901, obtained from Kyokai Yeast Co., Ltd.) was then added, and parallel fermentation was carried out at 30°C for 5 days. The resulting slurry sample was centrifuged at 9000 rpm for 10 minutes, and the precipitate was collected as saccharification residue (lignin-polysaccharide complex).

[0064] According to Shiro Saka et al. (2013) "Latest Trends in Lignin Utilization," edited by Shiro Saka, Chapter 1, "Biomass Classification and Chemical Composition," 2 kg of cedar wood flour, a plant raw material, contains 1,338 g of polysaccharides and 662 g of lignin. HPLC (LC-20AD; Shimadzu Corporation) analysis of the supernatant of centrifuged saccharification residue revealed that 745 g of polysaccharides were liberated from the raw cedar wood flour. Therefore, the aforementioned saccharification residue was determined to contain 593 g of polysaccharides and 662 g of lignin (54% by mass).

[0065] <Formation of urethane bond> A powder was prepared in the same manner as in Example 1, except that the obtained saccharification residue was used. As a result, the obtained powder (lignin) was white. Furthermore, when 0.05 g of the obtained powder (lignin) was added to 1 mL each of chloroform and ethanol, the powder (lignin) was dissolved in all of these organic solvents.

[0066] Example 4 A powder was prepared in the same manner as in Example 3, except that the urethane bond-forming reaction was carried out using dodecyl isocyanate (manufactured by Tokyo Chemical Industry Co., Ltd.) instead of hexyl isocyanate. As a result, the obtained powder (lignin) was white. Furthermore, when 0.05 g of the obtained powder (lignin) was added to 1 mL each of chloroform and ethanol, the powder (lignin) dissolved in all of these organic solvents.

[0067] Example 5 <Preparation of beech sawdust dispersion> 0.6 g of beech sawdust (obtained from Toei Scientific Industry Co., Ltd.) was mixed with 9.4 mL of ultrapure water and 10 mL of ethanol to give a 3 wt % beech sawdust dispersion.

[0068] <Formation of urethane bond> To 20 mL of the resulting dispersion, 2 mL of dodecyl isocyanate (Tokyo Chemical Industry Co., Ltd.) was added dropwise, and the mixture was stirred at 50° C. and 1 atmosphere for 5 hours to carry out a urethane bond forming reaction. The mixture was then washed with an excess amount of ethanol, and the precipitate (unreacted beech sawdust) was decanted. After that, unreacted materials (unreacted isocyanate, etc.) were removed by filtration, and the mixture was dried to recover a white powder.

[0069] Beech sawdust was colored before the urethane bond formation reaction. By carrying out a urethane bond formation reaction using dodecyl isocyanate on this beech sawdust, the colored beech sawdust (lignin) turned white. Furthermore, when 0.05 g of the resulting powder (lignin) was added to 1 mL each of chloroform and ethanol, the powder (lignin) dissolved in both of these organic solvents.

[0070] Example 6 <Preparation of pine flour dispersion> Pine wood chips (obtained from Monotaro Co., Ltd.) were ground using a belt and disc sander (obtained from Sankyo Corporation) equipped with a 60-mesh file to prepare 0.03 g of pine powder. 0.03 g of the prepared pine powder was mixed with 5 mL of ultrapure water and 5 mL of ethanol to prepare a pine powder dispersion at a concentration of 0.3 wt %.

[0071] <Formation of urethane bond> To 10 mL of the resulting dispersion, 0.2 mL of dodecyl isocyanate (Tokyo Chemical Industry Co., Ltd.) was added dropwise, and the mixture was stirred at 50° C. and 1 atmosphere for 5 hours to carry out a urethane bond forming reaction. The mixture was then washed with an excess amount of ethanol, and the precipitate (unreacted pine powder) was decanted. After that, unreacted materials (unreacted isocyanate, etc.) were removed by filtration, and the mixture was dried to recover a white powder.

[0072] Pine powder was colored before the urethane bond formation reaction. By carrying out a urethane bond formation reaction using dodecyl isocyanate, the colored pine powder (lignin) was whitened. Furthermore, when 0.05 g of the resulting powder (lignin) was added to 1 mL each of chloroform and ethanol, the powder (lignin) dissolved in both of these organic solvents.

[0073] Example 7 <Preparation of balsa powder dispersion> A piece of balsa (obtained from Monotaro Co., Ltd.) was ground using a dust-collecting sander (obtained from Takagi Co., Ltd.) equipped with an 80-mesh file to prepare 1.00 g of balsa powder. 0.3 g of the prepared balsa powder was mixed with 10 mL of ultrapure water and 10 mL of ethanol to give a 3 wt% dispersion of balsa powder.

[0074] <Formation of urethane bond> To 20 mL of the resulting dispersion, 0.6 mL of dodecyl isocyanate (Tokyo Chemical Industry Co., Ltd.) was added dropwise, and the mixture was stirred at 50° C. and 1 atmosphere for 5 hours to carry out a urethane bond forming reaction. The mixture was then washed with an excess amount of ethanol, and the precipitate (unreacted balsa powder) was decanted. After that, unreacted materials (unreacted isocyanate, etc.) were removed by filtration, and the mixture was dried to recover a white powder.

[0075] The balsa powder was colored before the urethane bond formation reaction. The colored balsa powder (lignin) was whitened by performing a urethane bond formation reaction on this balsa powder using dodecyl isocyanate. Furthermore, when 0.05 g of the resulting powder (lignin) was added to 1 mL each of chloroform and ethanol, the powder (lignin) dissolved in both of these organic solvents.

[0076] <Test Example 1> The color difference and color space of the powders prepared in Examples 1 to 7 and the starting material before urethane bond formation were measured in the L*a*b* color space in reflection mode using a spectrophotometer (CR-5) manufactured by Konica Minolta. The measurement results are shown in Table 1.

[0077] [Table 1]

[0078] As shown in Table 1, various plant materials were highly colored before the urethane bond formation reaction. By reacting these materials with a compound represented by one of general formulas (11) to (13), a powder (lignin) with a higher whiteness (L*) than the raw material was recovered. Furthermore, the color space varied depending on the modifying group of the reactive compound.

[0079] <Test Example 2> Molecular structure of reaction product (1) FT-IR measurement A sample of isolated lignin modified with dodecyl isocyanate (hereinafter referred to as "Dod-I-modified isolated lignin") was prepared based on Green Chem., 2016, 18, 5962 and JP 2019-154381 A. Specifically, 500 g of plant powder obtained by grinding Japanese cedar (Cryptomeria japonica) to a size of approximately 0.02–5 mm using a cutter mill or jet mill was soaked overnight in 4.5 L of 100 mM phosphate buffer (pH = 5.0) and then placed in a wet grinding apparatus (LMZ4, manufactured by Ashizawa Finetech Co., Ltd.) along with the buffer solution. A cellulase / hemicellulase mixture (50 mL each, Optimash XL and Optimash BG, manufactured by DuPont Genencor) was then added, and the mixture was wet-ground using 0.5 mm diameter zirconia beads while maintaining the temperature at 50 °C. When the average particle size reached 10 μm, the beads were replaced with 0.1 mm diameter zirconia beads. The wet milling was carried out for a total of 4 hours. Lignin was obtained as a residue by centrifugation (10,000 × g, 30 minutes). The obtained lignin and dodecyl isocyanate were used in a urethane bond formation reaction in the same manner as in Example 3 to prepare powder (Dod-I modified isolated lignin). The lignin powders prepared in Examples 2 and 4 and the Dod-I-modified isolated lignin were subjected to FT-IR measurement in total reflection mode using a Thermo Fisher Scientific FT-IR instrument (NICOLET6700). The results are shown in Figure 1. As shown in Figure 1, an NH stretching vibration peak and a group of peaks thought to be derived from the lignin aromatic ring were confirmed. The NH stretching vibration peak is derived from the urethane bond formed by the reaction of the hydroxyl group or alcoholic hydroxyl group of the guaiacol structure with the isocyanate group. Furthermore, the peaks of the lignin powders prepared in Examples 1 and 2 matched the peaks of the existing Dod-I-modified isolated lignin. Therefore, these results confirmed that an addition reaction occurred between lignin and isocyanate groups in all reaction systems.

[0080] (2) Measurement of UV-visible absorption spectrum The white powder of lignin prepared in Example 1 was dissolved in ethanol, and the resulting ethanol solution was poured into a quartz cell. The ultraviolet-visible absorption spectrum was measured at room temperature using a spectrophotometer (U-2910, manufactured by Hitachi Koki).

[0081] The results are shown in Figure 2. As shown in Figure 2, peaks due to the absorption of the guaiacyl skeleton (280 nm) and the syringyl skeleton (270 nm) of lignin were confirmed (see IAWA Bull. ns 1992, 13(1), 105).

[0082] From the results of FT-IR measurement and ultraviolet-visible absorption spectrum measurement, the white solids prepared in Examples 1 and 2 were identified as lignin compositions.

[0083] Test Example 3: Properties of reaction products The lignin powder prepared in Example 2 was dissolved and mixed in chloroform at a ratio of 5 wt% with polyepsilon caprolactam (PCL, Sigma-Aldrich), and the mixture was cast onto a glass substrate and dried to prepare a composite. The weight loss due to heating was measured in an air atmosphere at a heating rate of 10°C using a thermogravimetric analyzer (Thermo plus EVO2, Rigaku). The results are shown in Figure 3. As shown in Figure 3, the thermal decomposition temperature of the composite of lignin powder and PCL increased. Specifically, the temperature at which the composite reached half of its total weight loss was approximately 60°C higher than that of PCL alone. This suggests that the lignin obtained by this invention may have potential applications as a heat-resistant filler.

[0084] As described above, according to the present invention, colored lignin can be decolorized and decolorized lignin can be produced. Furthermore, since lignin is expected to be a functional substance, the decolorized lignin obtained by the present invention can be used in various applications.

Claims

1. A method for decolorizing lignin, comprising reacting a plant or treated plant material containing 10 to 50 mass % of lignin with a compound represented by the following general formula (11): RN=C=O General formula (11) (In general formula (11), R represents an alkyl group, an aralkyl group, an aryl group, a halogenated alkyl group, a halogenated aryl group, a sulfonyl group, or a silyl group.)

2. A method for preparing bleached lignin, comprising reacting a plant or treated plant material containing 10 to 50 mass % of lignin with a compound represented by the following general formula (11), to obtain bleached lignin: RN=C=O General formula (11) (In general formula (11), R represents an alkyl group, an aralkyl group, an aryl group, a halogenated alkyl group, a halogenated aryl group, a sulfonyl group, or a silyl group.)

3. The method according to claim 1 or 2, wherein the compound represented by the general formula (11) is an isocyanate compound represented by the following general formula (21): R'-N=C=O General formula (21) (In general formula (21), R′ represents an alkyl group, an aralkyl group, or an aryl group.)

4. 4. The method according to claim 1, wherein the obtained decolorized lignin is in powder form.

5. The plant is selected from the group consisting of cedar (Cryptomeria japonica), beech (Fagus crenata Blume), pine (Pinus), balsa (Ochroma lagopus), giant water hyacinth (Myriophyllum aquaticum), moso bamboo (Phyllostachys heterocycla), rice (Oryza sativa), bread wheat (Triticum aestivum), corn (Zea mays subsp. mays (L.) Iltis), Erianthus arundinaceus, miscanthus sinensis, sugarcane (Saccharum officinarum), common reed (Phragmites australis), giant reed (Arundo donax), oil palm (Elaeis), nipa palm (Nypa fruticans Wurmb), sugar palm (Arenga pinnata or Arenga saccharifera), water hyacinth (Eichhornia crassipes), water hyacinth (Potamogeton maackianus), Canada weed (Egeria densa), water hyacinth (Hydrilla verticillata), Elodea nuttallii, Sargassum horneri, Sargassum fulvellum, Ulva, yew (Caulerpa taxifolia), sea grape (Caulerpa lentillifera), and Eucheuma plants.

6. The method according to any one of claims 1 to 5, wherein the treated plant material has been subjected to at least one treatment selected from the group consisting of chipper treatment, dry grinding treatment, wet grinding treatment, attrition treatment, saccharification treatment, fermentation treatment, cooking treatment, explosion treatment, subcritical water treatment, decomposition treatment with an ionic liquid, acid treatment, base treatment, and microwave treatment.

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