Method for manufacturing luminescent materials derived from lignin

Abstract

To provide a method for treating lignin that enables production of lignin-derived luminescent materials.SOLUTION: A method for treating lignin involves treating the lignin with inorganic particles.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a method for producing a luminescent material derived from lignin. [Background technology]

[0002] More than 90% of wood is composed of cell wall components, which are mainly made up of cellulose, hemicellulose, and lignin. Of these main components, lignin is usually present in wood at a concentration of 20-30%, and it adheres cell membranes together to form the intermediate layer. Some of the lignin in wood is also present in the cell membranes. Lignin is a polymer compound formed by condensation using hydroxyphenylpropane as the basic unit. Lignin has a chain of π-conjugated molecules, possessing an aromatic main chain structure and phenolic hydroxyl groups that can act as organic radicals. Due to this structure, lignin has 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 polymer compounds like lignin are also expected to function as environmentally sustainable materials.

[0003] However, typical lignin is colored brown or black. Therefore, its applications are limited due to factors such as causing color changes in the medium to which it is added, and the resulting low light transmittance of the medium. Therefore, with the aim of expanding the applications of lignin as a material, the inventors have developed and published a method for decolorizing colored lignin (see Patent Documents 1 and 2). Furthermore, lignin-derived materials that absorb ultraviolet light and exhibit luminescence are being researched (see Non-Patent Document 1), and are expected to have applications in ultraviolet sensors and induced light-emitting materials. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2021-017582 [Patent Document 2] Japanese Patent Publication No. 2022-103080 [Non-patent literature]

[0005] [Non-Patent Document 1] ACS Sustainable Chem. Eng., 2018, vol. 6, No. 3, p. 3169-3175 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] As mentioned above, lignin is a promising functional material, and by decolorizing lignin, which is brown or black in color, it becomes possible to apply lignin to various uses. If such decolorized lignin can be made to emit light, the types and range of devices and articles to which lignin can be applied will expand, and its usefulness as an ultraviolet sensor or induced light-emitting material can be enhanced. Therefore, the present invention aims to provide a method for processing lignin and a method for producing a luminescent material derived from lignin. [Means for solving the problem]

[0007] In view of the above problems, the inventors conducted an investigation. As a result, they found that by treating lignin with particles of inorganic substances such as silicates, lignin can be made to emit light, and a lignin-derived light-emitting material can be produced. This invention was completed based on these findings.

[0008] The above-mentioned problems of the present invention were solved by the following means. (1) A method for treating lignin using inorganic particles. (2) The method for treating lignin according to item (1), wherein the lignin is subjected to a decolorization treatment. (3) The method for treating lignin according to item (1) or (2) above, wherein the lignin is subjected to saccharification treatment. (4) The method for treating lignin according to any one of (1) to (3) above, wherein the inorganic particles are particles made of a swellable layered silicate. (5) A method for treating lignin according to any one of the above (1) to (4), wherein the particle size of the inorganic particles is 10 nm or more and 100 nm or less. (6) A method for treating lignin according to any one of (1) to (5) above, comprising: mixing the lignin and the inorganic particles in a solvent to prepare a dispersion; and treating the dispersion with inorganic particles by homogenization treatment using ultrasonic diffusion. (7) A method for treating lignin according to any one of (1) to (5) above, comprising: mixing the lignin and the inorganic particles in a solvent to prepare a dispersion; homogenizing the dispersion by ultrasonic diffusion and drying it; and subjecting the resulting dried powder to a heat press treatment, thereby treating the lignin with inorganic particles. (8) The method for treating lignin according to item (6) or (7), wherein the lignin and inorganic particles are mixed by weight in a ratio of (lignin):(inorganic particles) = 0.5:9.5 to 6.5:3.5.

[0009] (9) A method for producing a light-emitting material, comprising treating lignin with inorganic particles to obtain a light-emitting material containing lignin. (10) The method for producing the light-emitting material according to item (9), wherein the lignin is subjected to a decolorization treatment. (11) The method for producing a luminescent material according to item (9) or (10), wherein the lignin is subjected to a saccharification treatment. (12) A method for producing a light-emitting material according to any one of (9) to (11), wherein the inorganic particles are particles made of a swellable layered silicate. (13) The method for producing a luminescent material according to any one of the above (9) to (12), wherein the particle size of the inorganic particles is 10 nm or more and 100 nm or less. (14) The method for producing a luminescent material according to any one of the above (9) to (13), wherein the lignin and the inorganic particles are mixed in a solvent to prepare a dispersion, and the dispersion is homogenized by ultrasonic diffusion to treat the lignin with the inorganic particles. (15) The method for producing a luminescent material according to any one of the above (9) to (13), wherein the lignin and the inorganic particles are mixed in a solvent to prepare a dispersion, the dispersion is homogenized by ultrasonic diffusion and dried, and the obtained dried powder is subjected to a hot pressing treatment to treat the lignin with the inorganic particles. (16) The method for producing a luminescent material according to the above (14) or (15), wherein the lignin and the inorganic particles are mixed at a weight ratio of (lignin):(inorganic particles) = 0.5:9.5 to 6.5:3.5. (17) The method for producing a luminescent material according to any one of the above (9) to (16), wherein the luminescent material is a material that absorbs ultraviolet light and emits light when irradiated with ultraviolet light.

[0010] (18) A luminescent material containing lignin treated with inorganic particles. (19) The luminescent material according to the above (18), wherein the lignin is subjected to a decolorization treatment. (20) The luminescent material according to the above (18) or (19), wherein the lignin is subjected to a saccharification treatment. (21) The luminescent material according to any one of the above (18) to (20), wherein the inorganic particles are particles composed of a swellable layered silicate. (22) The luminescent material according to any one of the above (18) to (21), wherein the particle size of the inorganic particles is 10 nm or more and 100 nm or less. (23) The light-emitting material according to any one of (18) to (22), wherein the lignin is treated with inorganic particles by mixing the lignin and the inorganic particles in a solvent to prepare a dispersion, and then homogenizing the dispersion by ultrasonic diffusion. (twenty four) The light-emitting material according to any one of (18) to (22), wherein the lignin and the inorganic particles are mixed in a solvent to prepare a dispersion, the dispersion is homogenized by ultrasonic diffusion and dried, and the resulting dried powder is subjected to a heat press treatment, thereby treating the lignin with inorganic particles. (twenty five) The light-emitting material according to item (23) or (24), wherein the lignin and inorganic particles are mixed in the dispersion in a weight ratio of (lignin):(inorganic particles) = 0.5:9.5 to 6.5:3.5. (26) A light-emitting material according to any one of items (18) to (25) above, which is a material that emits light by absorbing ultraviolet light when irradiated with ambient light. [Effects of the Invention]

[0011] According to the present invention, a luminescent material derived from lignin can be produced. The lignin-containing luminescent material obtained by the present invention can be applied as a functional substance to various media such as resin compositions, polymer materials, coating materials, cosmetic compositions, automotive components, building materials, adhesives, and heat-resistant fillers. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1(A) is a substitute photograph of the white lignin ethanol dispersion prepared in Example 1, Figure 1(B) is a substitute photograph showing the luminescence behavior of the white lignin ethanol dispersion before UV light irradiation, and Figure 1(C) is a substitute photograph showing the luminescence behavior of the white lignin ethanol dispersion after UV light irradiation. [Figure 2]Figure 2(A) is a photograph used as a substitute for a diagram showing the luminescence behavior of white lignin powder and molten material before UV light irradiation, and Figure 2(B) is a photograph used as a substitute for a diagram showing the luminescence behavior of white lignin powder and molten material after UV light irradiation. [Figure 3] Figure 3(A) shows the fluorescence spectra of the white lignin-ethanol dispersion under different concentration conditions, and Figure 3(B) shows the fluorescence spectra of the white lignin-ethanol dispersion under different excitation wavelength conditions. [Figure 4] This is the ultraviolet-visible absorption spectrum of a white lignin-ethanol solution. [Figure 5] Figure 5(A) is a substitute photograph of the white lignin-imidazolium-modified synthetic saponite composite powder prepared in Example 2, Figure 5(B) is a substitute photograph showing the luminescence behavior of the white lignin-imidazolium-modified synthetic saponite composite powder before UV light irradiation, and Figure 5(C) is a substitute photograph showing the luminescence behavior of the white lignin-imidazolium-modified synthetic saponite composite powder after UV light irradiation. [Figure 6] Figure 6(A) is a substitute photograph showing the luminescence behavior of white lignin-imidazolium-modified clay composite powders with different heat treatment histories before UV light irradiation, and Figure 6(B) is a substitute photograph showing the luminescence behavior of white lignin-imidazolium-modified clay composite powders with different heat treatment histories after UV light irradiation. [Figure 7] Figure 7(A) is a photographic substitute for a drawing showing the luminescence behavior of the white lignin-imidazolium-modified synthetic saponite composite powder prepared in Example 4 before UV light irradiation, and Figure 7(B) is a photographic substitute for a drawing showing the luminescence behavior of the white lignin-imidazolium-modified synthetic saponite composite powder prepared in Example 4 after UV light irradiation. [Figure 8] Figure 8(A) is a photographic substitute for a drawing showing the luminescence behavior of the white lignin-imidazolium-modified synthetic saponite composite powder prepared in Examples 4 and 5 before UV light irradiation, and Figure 8(B) is a photographic substitute for a drawing showing the luminescence behavior of the white lignin-imidazolium-modified synthetic saponite composite powder prepared in Examples 4 and 5 after UV light irradiation. [Figure 9]This graph shows the X-ray diffraction behavior of the white lignin-imidazolium-modified synthetic saponite composite powders prepared in Examples 4 and 5. [Modes for carrying out the invention]

[0013] In this invention, lignin is treated with a predetermined amount of inorganic particles to obtain a luminescent material containing lignin. The present invention will be described below based on preferred embodiments. However, the present invention is not limited thereto.

[0014] The lignin that is the target of treatment in this invention is a high-molecular-weight compound found in the cell walls and cell membranes of plants. Lignin is composed of hydroxyphenylpropane as its basic unit. The type and composition of substituted aromatic substances that make up lignin vary depending on the plant species, such as conifers, broad-leaved trees, and grasses. The lignin used as the target of treatment in this invention may be obtained from any plant, as long as it contains lignin. Furthermore, the object to be treated in this invention is not particularly limited as long as it contains lignin, and may also include components that make up cell walls and cell membranes, such as cellulose and hemicellulose.

[0015] The reason (mechanism) why lignin emits light when treated with inorganic particles is not clear. However, it is thought that when lignin is dispersed in inorganic particles, the crystallization of hexyl groups within the lignin is suppressed, resulting in a uniform dispersion of lignin. This suppresses intramolecular rotation and vibration in the modified hexyl groups, reducing molecular mobility and leading to a dominance of radiative deactivation. As a result, the lignin aromatic main chain, consisting of guaiacyl groups, syringyl groups, etc., absorbs UV light and emits light, increasing fluorescence intensity.

[0016] In the method of the present invention, commercially available lignin may be used as the starting material to be treated with inorganic particles, or a plant or plant-treated material containing lignin may be used.

[0017] For plants containing lignin, the amount of lignin can be quantified according to conventional methods, and plant raw materials containing lignin can be used. Alternatively, one can refer to Shiro Saka et al. (2013) "Latest Trends in Lignin Utilization," supervised by Shiro Saka, Chapter 1 "Classification and Chemical Composition of Biomass," etc., and appropriately select plant raw materials containing lignin to use in the present invention. Specific examples of lignin-containing plants that can be used in the present invention include Japanese cedar, beech, pine, balsa, Myriophyllum aquaticum, Moso bamboo, rice (preferably rice straw, rice hulls), bread wheat, maize, Erianthus, Miscanthus, sugarcane (preferably bagasse). Bagasse Examples include sugarcane pulp residue, reeds, giant reeds, oil palms, nipa palms, sugar palms, water hyacinths, clematis, Canadian waterweed, hydrangea, dwarf hydrangea, red oak, Sargassum, sea lettuce, sea lettuce, sea lettuce, sea lettuce, sea lettuce, sea lettuce, sea lettuce, and plants of the genus Echinocactus. Of these, cedar, beech, pine, balsa, moso bamboo, rice, bread wheat, maize, elianthus, miscanthus, sugarcane, reeds, giant reeds, oil palms, nipa palms, and sugar palms have a high lignin content and can be preferably used in the present invention. Any part of the plant can be used as a raw material in this invention, including the whole plant, roots, tubers, rhizomes, trunks, branches, stems, leaves (leaf blades, petioles, etc.), bark, sap, resin, flowers (petals, ovaries, etc.), fruits, seeds, etc. Multiple parts of these can also be used in combination. Of these parts, it is preferable to use the rhizomes, trunks, branches, stems, leaves (leaf blades, petioles, etc.), and bark of the plant.

[0018] In the present invention, the aforementioned plants may be used as they are, or a plant-treated product in which the plants have been subjected to a predetermined treatment may be used. By subjecting the plants to a predetermined treatment, the amount of lignin contained in the plant raw material can be increased. The treatments applied to the plants can be appropriately selected within the limits that do not impair the effects of the present invention, with reference to Chapter 1, "Classification and Chemical Composition of Biomass," of "Latest Trends in Lignin Utilization," supervised by Shiro Saka et al. (2013). Examples include chipping, dry grinding, wet grinding, crushing, saccharification, fermentation, pulverization, explosion, subcritical water treatment, decomposition with ionic liquid, acid treatment, base treatment, and microwave treatment. It is preferable that the lignin used in the present invention has been subjected to saccharification treatment (monosaccharification or low-saccharification treatment) using cellulase or the like.

[0019] Lignin is said to possess an ultraviolet chromophore because the vinyl group at the para position of the phenolic hydroxyl group within the aromatic compound residue skeleton has lost electron conjugation (see Green Chem., 2016, vol. 18, p. 1175-1200; see Keiji Takabe (2013) "Latest Trends in Lignin Utilization," supervised by Shiro Saka, Chapter 2 "Lignin Distribution and Structural Diversity in Biomass Cells," etc.). It is presumed that lignin is colored brown or black because of the presence of such an ultraviolet chromophore. The lignin-containing luminescent material obtained by the present invention is preferably applied as a functional substance to various media such as resin compositions, polymer materials, coating materials, cosmetic compositions, automotive components, building materials, adhesives, and heat-resistant fillers. Considering the application of the luminescent material obtained by the present invention to various media, it is preferable that the lignin, which is colored brown or black, is decolorized. There are no particular restrictions on the method of decolorizing lignin, and the methods described in Japanese Patent Publication No. 2021-017582 and Japanese Patent Publication No. 2022-103080 can be referenced.

[0020] The inorganic particles used in this invention are not particularly limited as long as they do not impair the effects of the present invention, but it is preferable that they consist of particles made of at least one inorganic substance selected from silicates, colloidal silica, alumina hydrate, carbonates, magnesium oxide, magnesium hydroxide, magnesium carbonate, magnesium silicate, aluminum silicate, aluminum hydroxide, magnesium oxide-aluminum oxide solid solution, hydrotalcite, and hydroxyapatite. The inorganic particles used in this invention may be of only one type, or two or more types may be mixed and used. Of these, particles made of at least one inorganic substance selected from the group consisting of silicates, colloidal silica, and alumina hydrate are preferred, and particles made of silicates are more preferred. Among the particles made of silicates, particles made of swollen layered silicates are particularly preferred.

[0021] The term "silicate" as used in this invention refers to a compound containing an anion that has a structure in which one or more silicon atoms are at the center, surrounded by electronegative ligands. In most silicates, the silicon atoms take on a tetrahedral structure surrounded by four oxygen atoms. Depending on the type of silicate, the degree to which these tetrahedra are linked differs, and the way in which the tetrahedra are linked results in a wide variety of structures, including pairs, clusters, rings, chains, double-chains, layered structures, and three-dimensional networks. Of the silicates mentioned above, the "swellable layered silicate" that can be particularly used in the present invention refers to a compound that exhibits water swelling and has a layered structure in which many two-dimensional unit layers are stacked, and the layered structure is composed of at least silicon atoms and electrically negative ligands. The layered silicate may have single layers of tetrahedral sheets and / or octahedral sheets, or mixed sheets thereof. The tetrahedral sheets contain silicon ions (Si) as standard elements. 4+ ) into four oxygen ions (O 2- The octahedron sheet is enclosed by a structure that shares three vertices with the neighboring tetrahedron, forming a hexagonal network that is linked together in a sheet-like manner. 2+ ), or aluminum ions (Al 3+ ) into six oxygen ions (O2- ) or hydroxide ions (OH - ), in which octahedrons surrounded by them share edges and extend two-dimensionally. When the tetrahedral sheet and the octahedral sheet are combined, the oxygen ions at the vertices of the tetrahedral sheet are shared. Furthermore, in the tetrahedral sheet, part of them are aluminum ions, and in the octahedral sheet, part of them are aluminum ions, magnesium ions, iron ions (Fe 2+ , Fe 3+ ), or lithium ions (Li + ), etc. are isomorphously substituted, resulting in the generation of negative charges. Alternatively, negative charges are also generated due to the presence of voids in part of the tetrahedral sheet and the octahedral sheet. Cations exist in the interlayer in a form that neutralizes these negative charges. The swelling layered silicate has various properties such as swelling property, thickening property, thixotropy, cation exchange property, etc. Furthermore, since the swelling layered silicate is an inorganic substance, it is hardly decomposed or altered by microorganisms and is a material friendly to the human body. Furthermore, since the layered silicate that can be used in the present invention has water swelling property, it is difficult to sediment naturally even in a suspension. Therefore, it has excellent uniform dispersibility and can stably maintain a uniform dispersion state of lignin for a long time.

[0022] The swelling layered silicate that can be used in the present invention is preferably derived from a mineral belonging to the smectite group. Minerals belonging to the smectite group have a 2:1 type layer structure. The primary particles of smectite are plate-like crystals with a thickness of 1 nm and a spread of 20 nm to 2 μm. As described above, in an aqueous dispersion, cations such as sodium ions are incorporated into the crystal layer in a form that compensates for the permanent negative charges possessed by the crystal layer itself of smectite (see "Clay Handbook", Third Edition, edited by the Japan Clay Society, May 2009, p. 65). Specific examples of the swelling layered silicate that can be used in the present invention include montmorillonite, hectorite, stevensite, saponite, beidellite, nontronite, sauconite, and swelling mica.

[0023] The swellable layered silicate that can be used in the present invention may have any cation exchange capacity as long as it does not impair the effects of the present invention. The cation exchange capacity of the swellable layered silicate is preferably 10 meq / 100g or more, and more preferably 30 meq / 100g or more. A cation exchange capacity within this preferred range imparts a negative charge to the swellable layered silicate that is suitable for the adsorption of proteins (serum components). From the viewpoint of achieving excellent swelling and dispersion stability, the swellable layered silicate that can be used in the present invention is preferably a monovalent cationic silicate such as sodium, and more preferably a monovalent cationic swellable layered silicate such as sodium. The cation exchange capacity of the swellable layered silicate can be measured by a method in accordance with the Schollenberger method (Clay Handbook, 3rd Edition, edited by the Clay Society of Japan, May 2009, pp. 453-454). More specifically, it can be measured by the method described in the Japan Bentonite Industry Association Standard Test Method JBAS-106-77. For example, the amount of leached cations from montmorillonite can be calculated by leaching interlayer cations from montmorillonite using 100 mL of 1 M ammonium acetate aqueous solution per 0.5 g of montmorillonite for 4 hours or more, and then measuring the concentrations of various cations in the resulting solution by ICP emission spectrometry or atomic absorption spectrometry. Furthermore, there is no particular upper limit to the cation exchange capacity of the swellable layered silicate that can be used in this invention, but 120 meq / 100 g or less is practical.

[0024] The particle size of the inorganic particles used in this invention varies depending on the measurement conditions, but is generally determined by particle size distribution measurement using water as the dispersion medium, and can be set arbitrarily within a range that does not impair the effects of this invention. From the viewpoint of efficiently and uniformly dispersing lignin, the median diameter of the inorganic particles is preferably 10 nm or more, more preferably 20 nm or more, preferably 100 nm or less, and more preferably 80 nm or less. The particle size of the inorganic particles used in this invention can be adjusted to a desired range by conventional methods such as grinding. For example, commercially available inorganic particles can be made even smaller by using a jet mill for dry grinding, a bead mill for wet grinding, a pressurized wet grinding apparatus, etc. as appropriate. In measuring the median diameter of inorganic particles, when the dispersed particle size is nanoscale, measurement by photon correlation is preferred, and the median diameter can be expressed using the particle size distribution obtained as the diffusion coefficient equivalent diameter. For measurement, inorganic particles are dispersed in water and then diluted to about 0.1 mass% for measurement. Any commercially available photon correlation apparatus can be used as the measuring device. For example, the SZ-100 series from Horiba, Ltd. Other examples include the Zetasizer Nano series from Malvern Corporation and the DLS-6500 series from Otsuka Electronics Co., Ltd. When inorganic particles are on a microscale, the median diameter can be measured using laser diffraction and scattering methods. Examples of measuring devices include the LA-960 series from Horiba, Ltd., the Mastersizer series from Malvern, and the ELSZ series from Otsuka Electronics Co., Ltd.

[0025] The inorganic particles used in this invention may be natural products or can be synthesized according to conventional methods. When natural products are used as inorganic particles, they may contain impurities and contaminants, but it is preferable that these impurities and contaminants are removed. Methods for synthesizing swollen layered silicates include, for example, hydrothermal synthesis, melt synthesis, high-pressure synthesis, solid-state reaction, flame melting, and alteration. For a method of synthesizing swollen layered silicates, refer to the method described in Japanese Patent Publication No. 2008-13401. Representative methods for synthesizing colloidal silica include gas-phase synthesis methods such as aerosil synthesis by thermal decomposition of silicon tetrachloride, liquid-phase synthesis methods such as those using water glass as a raw material, and hydrolysis of alkoxides. Alumina hydrate is generally produced by the Bayer process, in which bauxite is dissolved in sodium hydroxide at high temperatures.

[0026] Furthermore, the present invention can also use commercially available inorganic particles. For example, Kunipia-F (median diameter: 347.4 nm, BSA adsorption capacity: 222 μg / mg, cation exchange capacity: 108 meq / 100 g, viscosity of 1% by mass aqueous dispersion at 25°C: 4 mPa·s), Smecton-SA (median diameter: 85.7 nm, BSA adsorption capacity: 437 μg / mg, cation exchange capacity: 70 meq / 100 g, viscosity of 1% by mass aqueous dispersion at 25°C: 5 mPa·s), Smecton-SWN (median diameter: 64.4 nm, BSA adsorption capacity: 485 μg / mg, cation exchange capacity: 49 meq / 100 g, viscosity of 1% by mass aqueous dispersion at 25°C: 6 mPa·s), S Examples include Mecton-SWF (median diameter: 69.8 nm, BSA adsorption capacity: 388 μg / mg, cation exchange capacity: 73 meq / 100 g, viscosity of 1% by mass aqueous dispersion at 25°C: 16 mPa·s), Smecton-ST (median diameter: 42.4 nm, BSA adsorption capacity: 474 μg / mg, cation exchange capacity: 30 meq / 100 g, viscosity of 1% by mass aqueous dispersion at 25°C: 2 mPa·s) (product name, manufactured by Kunimine Industries Co., Ltd.), Somasif ME, Somasif MEB-3 (both product names, manufactured by Katakura Coop Agri Co., Ltd.), PDM-5B, PDM-800 (both product names, manufactured by Topy Industries Co., Ltd.).

[0027] As long as a luminescent material can be obtained, there are no particular restrictions on the conditions for treating lignin with inorganic particles, and conditions that are commonly used can be appropriately selected. The lignin treatment conditions in the present invention will be described below based on preferred embodiments. However, the present invention is not limited to these.

[0028] For example, a solvent can be 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. The selected solvent is then mixed with lignin and inorganic particles to prepare a dispersion containing lignin and inorganic particles. In this invention, ethanol is preferred as the solvent used from the viewpoint of the efficiency of manufacturing the luminescent material.

[0029] The concentrations of lignin and inorganic particles in the dispersion can be set as appropriate. For example, it is practical to set the lignin concentration in the dispersion to 0.1% by mass or more, preferably 0.2% by mass or more, and more preferably 0.6% by mass or more. It is practical to set the upper limit of the lignin concentration to 1.2% by mass or less, preferably 1.0% by mass or less, more preferably 0.8% by mass or less, more preferably 0.7% by mass or less, and even more preferably 0.6% by mass or less. It is practical to set the solid content concentration of inorganic particles in the dispersion to 0.8% by mass or more, preferably 1.0% by mass or more, and more preferably 1.4% by mass or more. It is practical to set the upper limit of the solid content concentration to 1.9% by mass or less, preferably 1.8% by mass or less, more preferably 1.6% by mass or less, more preferably 1.5% by mass or less, and even more preferably 1.4% by mass or less. Furthermore, the mixing ratio of lignin and inorganic particles can be appropriately selected, and it is preferable to adjust the mixing ratio of inorganic particles to the weight of lignin in order to obtain a luminescent material. For example, the mixing ratio of lignin and inorganic particles is preferably (lignin):(inorganic particles) = 0.5:9.5 to 6.5:3.5 by weight, and more preferably 1:9 to 6:4.

[0030] In the present invention, a light-emitting material can be manufactured by appropriately selecting a process that is normally performed in the manufacture of a light-emitting material. Examples of processes that can be employed in the present invention include homogenization, drying of precipitates, and redispersion. A preferred manufacturing process in the present invention will be described in detail. For example, a dispersion of lignin and inorganic particles is prepared and homogenized by ultrasonic diffusion at 150W for 15 minutes. After that, a drying process is performed, and the lignin is melted by hot pressing at 150°C and 10MPa on the dried powder. By going through such a process, the luminescent material of the present invention can be produced by treating lignin with inorganic particles. However, the present invention is not limited thereto.

[0031] By following the process described above, a light-emitting material containing lignin treated with inorganic particles can be obtained. Specifically, a light-emitting material that absorbs ultraviolet light and emits light when irradiated with ultraviolet light can be manufactured. Furthermore, in order to apply the light-emitting material obtained by the present invention to various media such as ultraviolet sensors and induced light-emitting materials, it is preferable that the lignin used as the starting material has high whiteness, and white lignin with an L* value of 80 or higher in the L*a*b* color space is more preferable. The L*a*b* color space is a type of complementary color space, having a dimension L* that indicates lightness and complementary color dimensions a* and b*, and is based on a non-linearly compressed coordinate system of the CIE XYZ color space. In this specification, "white" is defined as having an L* value of 80 or higher in the L*a*b* color space. The L* value, a* value, and b* value can be measured in accordance with JIS Z 8781-4:2013. The whiteness (L* value) of lignin can be adjusted by appropriately selecting the raw material preparation method. [Examples]

[0032] The present invention will be described in more detail below based on examples, but the present invention is not limited thereto.

[0033] Example 1 <Preparation of lignin> Japanese cedar was crushed to a size of approximately 0.02-5 mm using a cutter mill or jet mill to obtain plant powder. 500 g of the obtained plant powder was soaked overnight in 4.5 L of 100 mM phosphate buffer (pH=4-6) and then placed in a wet milling apparatus (LMZ4, manufactured by Ashizawa Finetech Co., Ltd.) along with the buffer. A cellulase / hemicellulase mixture (50 mL each of Optimash XL and Optimash BG, manufactured by DuPont Genencore) was added, and wet milling was performed using 0.5 mm diameter zirconia beads while maintaining a temperature of 50°C. During the wet grinding process described above, the average particle size of the plant powder was measured as needed, and when the average particle size reached 10 μm, the beads were replaced with 0.1 mm diameter zirconia beads. The wet grinding process described above was carried out for a total of 4 hours. As the wet grinding progressed, the viscosity of the plant powder suspension decreased. The average primary particle size of the particles in the suspension was 30-40 nm.

[0034] After grinding, the supernatant and residue were separated by centrifugation. The residue was washed with water, and then a cellulase / hemicellulase mixture and 1 L of phosphate buffer were added to the residue. The mixture was stirred at 50°C for 12 hours to carry out the saccharification reaction. After the reaction was complete, the supernatant and residue were separated by centrifugation, and brown lignin was obtained as the residue.

[0035] <Preparation of lignin dispersion> After measuring the concentration of the recovered lignin residue using a moisture meter (MS-70, A&D), ultrapure water and ethanol were added dropwise to the lignin residue so that the mixing ratio of water to ethanol was 1:1 (by weight) to prepare a 1% by mass lignin dispersion.

[0036] <Synthesis of white lignin> To 1 mL of the obtained lignin dispersion, hexyl isocyanate (manufactured by Tokyo Chemical Industry Co., Ltd.) in an amount approximately 9 times the weight of lignin (100 μL) was added dropwise, and the mixture was stirred at 50°C for 5 hours to whiten the lignin through a urethane bond formation reaction. After stirring, approximately 10 mL of ethanol was added to wash the mixture, and unreacted material was filtered out using filter paper (Kiriyama Seisakusho Co., Ltd. No. 5B (21φ mm)). The residue on the filter paper was vacuum-dried, and the white lignin powder was recovered.

[0037] <Preparation of a white lignin-ethanol dispersion> The concentration of white lignin is 4.9 × 10 -3 A predetermined amount of white lignin was mixed and dissolved in 10 mL of ethanol to prepare a white lignin ethanol dispersion, resulting in a concentration of mg / mL.

[0038] Example 2 <Preparation of imidazolium-modified clay> 2.0 g of synthetic saponite (Smecton SA; average particle size: 20 nm, manufactured by Kunimine Industries Co., Ltd.) was mixed with 98 g of ultrapure water. The mixture was then treated with a homogenizer (IKA UltraTax T50) at 6000 rpm for 10 minutes and with an ultrasonic homogenizer (BRANSON Sonifier, Model 450A) at 75 W for 10 minutes. 1 g of 1-ethyl-3-methylimidazolium methanesulfonate (manufactured by Tokyo Chemical Industry Co., Ltd.) and 74 g of ultrapure water were added. The sample was then treated with a homogenizer at 2000 rpm for 30 minutes, and centrifuged at 10000 rpm for 10 minutes to collect the precipitate. The precipitate was mixed with 150 mL of water and treated with an ultrasonic homogenizer at 75 W for 15 minutes. The resulting sample was then washed by centrifugation at 16000 rpm for 30 minutes. This procedure was repeated once to obtain a gel-like imidazolium-modified synthetic saponite (30.2 wt%).

[0039] <Preparation of imidazolium-modified clay ethanol dispersion> An imidazolium-modified synthetic saponite gel (0.5 g of solid content) was mixed with 18 mL of ethanol. Ultrasonic diffusion (Chinky NanoPremixer PR-1) was performed at 150 W for 15 minutes, and the sample was centrifuged at 10,000 rpm for 15 minutes to collect the precipitate. The same procedure was repeated once to obtain an imidazolium-modified synthetic saponite ethanol dispersion (10.8 wt%).

[0040] <Preparation of White Lignin-Imidazolium Modified Clay Composite Powder> 1.3 g of imidazolium-modified synthetic saponite ethanol dispersion (10.8 wt%) was mixed with 8.0 g of ethanol, and ultrasonic diffusion (Chinky NanoPremixer PR-1) was performed at 150 W for 15 minutes. Subsequently, a dispersion consisting of 0.06 g of white lignin and 5.94 g of ethanol prepared in Example 1 was mixed with the above sample, ultrasonic diffusion was performed at 150 W for 15 minutes, and the solvent was evaporated by drying. A portion of the dried sample was hot-pressed at 150°C and 10 MPa to obtain a white lignin-imidazolium-modified clay composite powder. The weight ratio of white lignin to imidazolium-modified synthetic saponite was 30% white lignin and 70% imidazolium-modified synthetic saponite.

[0041] Example 3 An untreated lignin-imidazolium-modified clay composite powder was prepared in the same manner as in Example 2, except that untreated brown lignin was used instead of white lignin.

[0042] Example 4 A white lignin-imidazolium-modified clay composite powder was prepared in the same manner as in Example 2, except that synthetic smectite (stevensite ST; average particle size: approximately 50 nm, manufactured by Kunimine Industries Co., Ltd.) was used instead of synthetic saponite.

[0043] Example 5 White lignin-imidazolium-modified clay composite powder was prepared in the same manner as in Example 2, except that synthetic smectite (stevensite ST; average particle size: approximately 50 nm, manufactured by Kunimine Industries Co., Ltd.) was used instead of synthetic saponite, and the ratio of white lignin to clay was changed to 60%:40%, 50%:50%, 40%:60%, 20%:80%, or 10%:90%.

[0044] <Test Example 1> Luminescence behavior of white lignin ethanol dispersion (1) UV irradiation experiment The luminescence behavior of the white lignin-ethanol dispersion prepared in Example 1 was evaluated using a handy UV lamp (AS ONE; LUV-4). The results are shown in Figure 1. The white lignin-ethanol dispersion prepared in Example 1 was transparent (Figure 1(A)), but showed blue emission upon irradiation with 4W UV light (330-380nm) (Figures 1(B) and (C)). Furthermore, ethanol, the control substance, did not exhibit blue emission when irradiated with UV light. Also, as shown in Figure 2, white lignin alone did not exhibit emission upon UV irradiation, confirming that dispersion in a medium is necessary for white lignin to emit light.

[0045] (2) Fluorescence Spectrum Measurement Experiment The white lignin-ethanol dispersion prepared in Example 1 was diluted with ethanol to a predetermined concentration, and the fluorescence spectrum was measured using a fluorescence spectrophotometer (JASCO; F-4500). The results are shown in Figure 3. As shown in Figure 3(A), emission peaks were observed in the 390-560 nm wavelength range upon excitation with 340 nm wavelength light. The intensity of these peaks was proportional to the concentration of white lignin, suggesting that they originated from white lignin. Furthermore, as shown in Figure 3(B), a clear emission peak was observed at excitation wavelengths of 300-380 nm.

[0046] (3) Experiment to measure ultraviolet-visible absorption spectroscopy The white lignin-ethanol dispersion prepared in Example 1 was diluted with ethanol to a predetermined concentration, and the ultraviolet-visible absorption spectrum was measured using a spectrophotometer (Hitachi High-Technologies Corporation; U-2910). The results are shown in Figure 4. Absorption originating from lignin monomers (guaiacyl group / syringyl group) with intensity corresponding to the concentration of white lignin was observed around wavelengths of 250-290 nm, and it was considered that the luminescence phenomenon originated from light absorption by these monomers. Furthermore, no absorption was observed around wavelength 320 nm in this spectrum, which indicates aggregation of lignin molecular chains (e.g., aromatic ring π-π stack). Based on these results, it is hypothesized that the hexyl group in white lignin constrains the lignin aromatic main chain, suppressing intramolecular rotation and vibration, reducing molecular mobility, and leading to a dominance of radiative inactivation, thus enhancing fluorescence intensity and causing white lignin to emit light (Reference: Int. J. Biol. Macromol., 2020, 154, 981).

[0047] <Test Example 2> Luminescence behavior of white lignin-imidazolium-modified clay composite powder (1) UV irradiation experiment The luminescence behavior of the white lignin-imidazolium-modified synthetic saponite (SA) composite powder prepared in Example 2 was evaluated using a handy UV lamp (AS ONE; LUV-4). The results are shown in Figure 5. Of the samples shown in Figure 5(A), only the white lignin-SA composite powder showed blue emission when irradiated with 4W UV light (330-380nm) (see Figure 5(C)). The control substance, SA alone, or the untreated lignin-SA composite powder prepared in Example 3, did not show blue emission (see Figure 5(C)).

[0048] In the white lignin-imidazolium-modified clay composite powder prepared in Example 2, only the sample heat-treated by hot pressing as shown in Figure 6 exhibited blue emission upon irradiation with UV light (330-380 nm). Since white lignin exhibits thermal melting, this suggests that the uniformity of the mixture of white lignin and clay, depending on whether or not white lignin melts, affects the presence or absence of emission.

[0049] The luminescence behavior of the white lignin-imidazolium-modified clay composite powder prepared in Example 4 was similarly evaluated, and the mixture of synthetic smectite and white lignin showed blue luminescence (see Figure 7). Furthermore, the white lignin-imidazolium-modified clay composite powders prepared in Examples 4 and 5 exhibited blue luminescence in the range of white lignin:synthetic clay = 60:40 to 10:90 (%), as shown in Figure 8 (see Figure 8). Visual observation revealed a tendency for the luminescence intensity to be strongest at a white lignin to synthetic clay ratio of 30:70 (%).

[0050] (2) X-ray diffraction experiment X-ray diffraction measurements were performed on the white lignin-imidazolium-modified clay composite powders prepared in Examples 2 to 5 using an X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation). The results are shown in Figure 9. For the white lignin-imidazolium-modified clay composite powder composed of synthetic saponite, only diffraction peaks originating from the clay were observed, and no peaks associated with hexyl group crystallization within the white lignin were present. Since the synthetic clay has a small and uniform particle size (reference: US Pat. 5763345, 1998), it was possible to finely and uniformly disperse the white lignin, suppressing hexyl group crystallization within the white lignin and reducing intermolecular interactions within the white lignin, thus enabling luminescence in the solid state.

[0051] As described above, lignin-derived light-emitting materials can be manufactured by treating lignin with inorganic particles.

Claims

1. A method for producing a light-emitting material, comprising hot-pressing a composite dry powder containing decolorized lignin, which is lignin that has been decolorized, and swellable layered silicate, which is an inorganic particle, to obtain a light-emitting material containing lignin.

2. The method for producing a luminescent material according to claim 1, wherein the lignin is subjected to a saccharification treatment.

3. A method for producing a light-emitting material according to claim 1 or 2, wherein the particle size of the swollen layered silicate is 10 nm or more and 100 nm or less.

4. A method for producing a light-emitting material according to claim 1 or 2, comprising drying a dispersion containing the decolorized lignin and the swellable layered silicate to obtain the composite dried powder.

5. The method for producing a light-emitting material according to Claim 4, wherein the weight ratio of the decolorized lignin to the swellable layered silicate in the composite dried powder is (decolorized lignin):(swellable layered silicate) = 0.5:9.5 to 6.5:3.

5.

6. The method for manufacturing a light-emitting material according to claim 1 or 2, wherein the light-emitting material is a material that emits light by absorbing ultraviolet light when irradiated with ultraviolet light.

7. A luminescent material containing lignin, wherein a composite dried powder containing decolorized lignin, which is lignin that has been decolorized, and a swellable layered silicate, which is an inorganic particle, is subjected to a heat press treatment.

8. The luminescent material according to claim 7, wherein the lignin has been subjected to a saccharification treatment.

9. The light-emitting material according to claim 7 or 8, wherein the particle size of the swollen layered silicate is 10 nm or more and 100 nm or less.

10. The luminescent material according to claim 7 or 8, wherein the composite dried powder is obtained by drying a dispersion containing the decolorized lignin and the swellable layered silicate.

11. The luminescent material according to claim 10, wherein in the composite dried powder, the weight ratio of the decolorized lignin to the swellable layered silicate is (decolorized lignin):(swellable layered silicate) = 0.5:9.5 to 6.5:3.

5.

12. The light-emitting material according to claim 7 or 8, which is a material that emits light by absorbing ultraviolet light when irradiated with ultraviolet light.

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