Method for producing lignin-derived luminescent material

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] The present 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, and the main components of the cell wall are cellulose, hemicellulose, and lignin. Of the main components, lignin is usually present in wood at about 20 to 30%, and forms an intermediate layer by bonding cell membranes together. A part of the lignin in wood is also present in the cell membranes. Lignin is a polymer compound produced by condensation of hydroxyphenylpropane as a basic unit. Lignin has a series of π-conjugated chains, an aromatic main chain structure, and phenolic hydroxyl groups that can become organic radicals. Lignin with such a structure functions as a heat-resistant filler, ultraviolet absorber, and antioxidant, and is expected to be used as a high-performance resin material such as engineering plastics. In addition, plant-derived polymer compounds such as lignin are expected to function as environmentally friendly materials.

[0003] However, since general lignin is colored brown or black, the uses of lignin are limited due to the reasons such as the color change of the medium to which lignin is added and the low light transmittance of the medium to which lignin is added. Therefore, from the viewpoint of expanding the use of lignin in materials, the present 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 emit light are being researched (see Non-Patent Document 1), and are expected to be used in ultraviolet sensors, induced luminescence materials, and the like. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-017582 [Patent Document 2] 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 Summary of the Invention [Problem to be solved by the invention]

[0006] As mentioned above, lignin is expected to be a functional material, and by decolorizing brown or black lignin, it can be used for various purposes. If such decolorized lignin can be made to emit light, the types and range of devices and items to which lignin can be applied will increase, and its usefulness as an ultraviolet sensor or induced luminescence material will increase. Therefore, an object of the present invention is to provide a method for treating lignin that can produce a luminescent material derived from lignin, and a method for producing a luminescent material. [Means for solving the problem]

[0007] In view of the above problems, the present inventors have conducted research and have found that lignin can be made to emit light by treating it with particles of an inorganic substance such as silicate, and that a luminescent material derived from lignin can be produced. The present invention has been completed based on these findings.

[0008] The above-mentioned object of the present invention has been achieved by the following means. (1) A method for treating lignin, comprising treating the lignin with inorganic particles. (2) The method for treating lignin according to (1) above, wherein the lignin has been subjected to a decolorization treatment. (3) The method for treating lignin according to (1) or (2), wherein the lignin has been subjected to a saccharification treatment. (4) The method for treating lignin according to any one of (1) to (3), wherein the inorganic particles are particles made of a swellable layered silicate. (5) The method for treating lignin according to any one of (1) to (4), wherein the inorganic particles have a particle size of 10 nm or more and 100 nm or less. (6) The method for treating lignin according to any one of (1) to (5), further comprising mixing the lignin and the inorganic particles in a solvent to prepare a dispersion, and subjecting the dispersion to a homogenization treatment by ultrasonic diffusion, thereby treating the lignin with the inorganic particles. (7) The method for treating lignin according to any one of (1) to (5), further comprising mixing the lignin and the inorganic particles in a solvent to prepare a dispersion, homogenizing the dispersion by ultrasonic diffusion and drying the dispersion, and then subjecting the resulting dry powder to a heat press treatment, thereby treating the lignin with the inorganic particles. (8) The method for treating lignin according to (6) or (7) above, 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.

[0009] (9) A method for producing a luminescent material, comprising treating lignin with inorganic particles to obtain a luminescent material containing lignin. (10) The method for producing a luminescent material according to (9) above, wherein the lignin has been subjected to a decolorization treatment. (11) 11. The method for producing a luminescent material according to (9) or (10), wherein the lignin has been subjected to a saccharification treatment. (12) The method for producing a luminescent material according to any one of (9) to (11) above, 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 (9) to (12) above, wherein the inorganic particles have a particle size of 10 nm or more and 100 nm or less. (14) The method for producing a luminescent material according to any one of (9) to (13), further comprising mixing the lignin and the inorganic particles in a solvent to prepare a dispersion, and subjecting the dispersion to a homogenization treatment by ultrasonic diffusion, thereby treating the lignin with the inorganic particles. (15) The method for producing a luminescent material according to any one of (9) to (13), further comprising the steps of: mixing the lignin and the inorganic particles in a solvent to prepare a dispersion; homogenizing the dispersion by ultrasonic diffusion and drying the dispersion; and heat pressing the resulting dried powder to treat the lignin with the inorganic particles. (16) The method for producing a luminescent material according to (14) or (15) above, 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 (9) to (16) above, wherein the luminescent material is a material that absorbs ultraviolet light and emits light when irradiated with ultraviolet light.

[0010] (18) A luminescent material comprising lignin that has been treated with inorganic particles. (19) Item 19. The luminescent material according to item 18, wherein the lignin has been subjected to a decolorizing treatment. (20) The luminescent material according to (18) or (19), wherein the lignin has been subjected to a saccharification treatment. (twenty one) The light emitting material according to any one of (18) to (20) above, wherein the inorganic particles are particles made of a swellable layered silicate. (twenty two) The light emitting material according to any one of (18) to (21) above, wherein the inorganic particles have a particle size of 10 nm or more and 100 nm or less. (twenty three) The luminescent material according to any one of (18) to (22), wherein the lignin is treated with the 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 luminescent material according to any one of (18) to (22), wherein the lignin is treated with the inorganic particles by mixing the lignin and the inorganic particles in a solvent to prepare a dispersion, homogenizing the dispersion by ultrasonic diffusion and drying the dispersion, and then heat pressing the obtained dry powder. (twenty five) 25. The luminescent material according to claim 23 or 24, wherein the lignin and the inorganic particles are mixed in the dispersion at a weight ratio of (lignin):(inorganic particles)=0.5:9.5 to 6.5:3.5. (26) The light-emitting material according to any one of (18) to (25) above, which is a material that emits light by absorbing ultraviolet light when irradiated with external light. Effect of the Invention

[0011] According to the present invention, a luminescent material derived from lignin can be produced. The luminescent material containing lignin 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 parts, building materials, adhesives, and heat-resistant fillers. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1(A) is a photograph of the white lignin ethanol dispersion prepared in Example 1, FIG. 1(B) is a photograph showing the luminescence behavior of the white lignin ethanol dispersion before UV light irradiation, and FIG. 1(C) is a photograph showing the luminescence behavior of the white lignin ethanol dispersion after UV light irradiation. [Diagram 2]Figure 2(A) is a photograph showing the luminescence behavior of white lignin powder and thermally molten material before UV light irradiation, and Figure 2(B) is a photograph showing the luminescence behavior of white lignin powder and thermally molten material after UV light irradiation. [Diagram 3] FIG. 3(A) shows the fluorescence spectrum of a white lignin ethanol dispersion under different concentration conditions, and FIG. 3(B) shows the fluorescence spectrum of a white lignin ethanol dispersion under different excitation wavelength conditions. [Figure 4] 1 is an ultraviolet-visible absorption spectrum of a white lignin ethanol solution. [Diagram 5] Figure 5(A) is a photograph of the white lignin-imidazolium modified synthetic saponite composite powder prepared in Example 2, Figure 5(B) is a photograph showing the luminescence behavior of the white lignin-imidazolium modified synthetic saponite composite powder before irradiation with UV light, and Figure 5(C) is a photograph showing the luminescence behavior of the white lignin-imidazolium modified synthetic saponite composite powder after irradiation with UV light. [Figure 6] Figure 6(A) is a 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 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 photograph 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 photograph 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 photograph showing the luminescence behavior of the white lignin-imidazolium-modified synthetic saponite composite powders prepared in Examples 4 and 5 before UV light irradiation, and Figure 8(B) is a photograph showing the luminescence behavior of the white lignin-imidazolium-modified synthetic saponite composite powders prepared in Examples 4 and 5 after UV light irradiation. [Figure 9]1 is a graph showing the X-ray diffraction behavior of the white lignin-imidazolium modified synthetic saponite composite powders prepared in Examples 4 and 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0014] Lignin, which is the subject of 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 constituent units of lignin, differ depending on the plant species, such as coniferous trees, broad-leaved trees, and grasses. The lignin used as the subject of the present invention may be obtained from any plant, so long as it contains lignin. Furthermore, the subject of treatment in the present invention is not particularly limited as long as it contains lignin, and may also contain components that constitute cell walls and cell membranes, such as cellulose and hemicellulose.

[0015] The reason (mechanism) why lignin emits light when treated with inorganic particles is unclear. However, it is thought that when lignin is dispersed in inorganic particles, the crystallization of the hexyl groups in the lignin is suppressed, the lignin is uniformly dispersed, and the intramolecular rotation and vibration of the modified hexyl groups are suppressed, molecular mobility is reduced, radiation deactivation becomes dominant, and the lignin aromatic main chain, which consists of guaiacyl groups, syringyl groups, etc., with enhanced fluorescence intensity, absorbs UV light and emits light.

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

[0017] For plants that contain lignin, the amount of lignin can be quantified according to standard methods, and plant raw materials containing lignin can be used. Alternatively, by referring to Chapter 1 "Classification and Chemical Composition of Biomass" in "Latest Trends in Lignin Utilization" by Saka Shiro et al. (2013), edited by Saka Shiro, etc., plant raw materials containing lignin can be appropriately selected and used in the present invention. Specific examples of plants containing lignin that can be used in the present invention include cedar, beech, plants of the genus Pinus, balsa, giant oak, moso bamboo, rice (preferably rice straw and rice husks), bread wheat, corn, erianthus, miscanthus, sugarcane (preferably bagasse ( Bagasse , residue after squeezing sugarcane), reed, giant reed, oil palm, nipa palm, sugar palm, water hyacinth, Cinderella crenata, Elodea canadensis, Hydrangea arborescens, Japanese laurel, Japanese laurel, Sargassum serrata, Ulva umbellata, Ilex monadelpha, sea grape, and Eucheuma genus plants. Among these, Japanese cedar, beech, plants of the pine genus, 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. As the plant raw material used in the present invention, any part of the plant can be used, 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. Also, a combination of a plurality of these parts may 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.

[0018] In the present invention, the above-mentioned plant may be used as it is, or a plant treated product obtained by subjecting the plant to a predetermined treatment may be used. By subjecting the plant to a predetermined treatment, 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 "Classification and Chemical Composition of Biomass" edited by Shiro Saka, "Latest Trends in Lignin Utilization" by Shiro Saka et al. (2013). Examples include chipper treatment, dry grinding treatment, wet grinding treatment, grinding treatment, saccharification treatment, fermentation treatment, digestion treatment, explosion treatment, subcritical water treatment, decomposition treatment using ionic liquid, acid treatment, base treatment, and microwave treatment. The lignin used in the present invention is preferably subjected to saccharification treatment (monosaccharide treatment or low-saccharification treatment) using cellulase or the like.

[0019] Lignin is said to have a UV chromophore because the vinyl group at the para-position of the phenolic hydroxyl group in the aromatic compound residue skeleton has lost electronic conjugation (Green Chem., 2016, vol. 18, p. 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). It is presumed that the presence of such a UV chromophore in lignin causes it to be colored brown or black. The luminescent material containing lignin obtainable 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 parts, building materials, adhesives, heat-resistant fillers, etc. In consideration of applying the luminescent material obtained by the present invention to various media, it is preferable that the lignin colored brown to black has been subjected to a decolorization treatment. There are no particular limitations on the method for decolorizing lignin, and the methods described in JP-A-2021-017582 and JP-A-2022-103080 can be referenced.

[0020] The inorganic particles used in the present invention are not particularly limited as long as they do not impair the effects of the present invention, but are preferably particles made of at least one inorganic substance selected from silicate, colloidal silica, alumina hydrate, carbonate, 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 the present invention may be of one type only, or may be a mixture of two or more types. Among these, particles made of at least one inorganic substance selected from the group consisting of silicate, colloidal silica, and alumina hydrate are preferred, and particles made of silicate are more preferred. Among particles made of silicate, particles made of swellable layered silicate are particularly preferred.

[0021] The term "silicate" that can be used in the present invention refers to a compound containing an anion having a structure in which one or several silicon atoms are at the center and electronegative ligands surround the silicon atom. In most silicates, the silicon atom has a tetrahedral structure surrounded by four oxygen atoms. The degree to which the tetrahedrons are connected varies depending on the type of silicate, and the structure can vary widely depending on the way the tetrahedrons are connected, such as pairs, clusters, rings, chains, double chains, layers, and three-dimensional networks. Among the silicates, the "swellable layered silicate" that can be particularly preferably used in the present invention refers to a compound that exhibits water swelling and has a layer structure in which many two-dimensional unit layers are stacked, and the layer structure is composed of at least silicon atoms and electronegative ligands. The layered silicate may have a single layer of tetrahedral sheets and / or octahedral sheets, or a mixture of these sheets. The tetrahedral sheets have silicon ions (Si 4+ ) into four oxygen ions (O 2- ) are surrounded by adjacent tetrahedrons, which share three vertices, forming a hexagonal network that is connected in a sheet shape. The octahedral sheets are connected by magnesium ions (Mg 2+ ), or aluminum ions (Al 3+ ) to six oxygen ions (O2- ), or hydroxide ion (OH - ) are octahedra surrounded by tetrahedral sheets that share their edges and spread out two-dimensionally. When a tetrahedral sheet and an octahedral sheet are combined, the oxygen ions at the vertices of the tetrahedral sheets are shared. Furthermore, some of the tetrahedral sheets are aluminum ions, and some of the octahedral sheets are aluminum ions, magnesium ions, and iron ions (Fe 2+ , Fe 3+ ), or lithium-ion (Li + Negative charges are generated by isomorphous substitution of cations such as tetrahedral and octahedral. Negative charges are also generated by the presence of voids in some of the tetrahedral and octahedral sheets. Cations exist between the layers to neutralize these negative charges. The swellable layered silicate has various properties such as swelling, viscosity, thixotropy, and cation exchange. Furthermore, since the swellable layered silicate is an inorganic substance, it is hardly decomposed or altered by microorganisms, and is a material that is gentle on the human body. Furthermore, since the layered silicate that can be used in the present invention has water swelling properties, it is difficult to naturally settle even in a suspension. Therefore, it has excellent uniform dispersibility and can stably maintain the uniform dispersion state of lignin for a long time.

[0022] The swellable 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 particle of smectite is a plate-like crystal with a thickness of 1 nm and an extent of 20 nm to 2 μm. As mentioned above, in an aqueous dispersion, cations such as sodium ions are incorporated into the crystal layer of smectite in a form that compensates for the permanent negative charge that the crystal layer itself has (see "Clay Handbook", Third Edition, Edited by the Clay Science Society of Japan, May 2009, p. 65). Specific examples of swellable layered silicates that can be used in the present invention include montmorillonite, hectorite, stevensite, saponite, beidellite, nontrite, sauconite and swellable mica.

[0023] The swellable layered silicate that can be used in the present invention may have any cation exchange capacity within a range that 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, more preferably 30 meq / 100g or more. The cation exchange capacity in a suitable range imparts a negative charge suitable for adsorption of proteins (serum components) to the swellable layered silicate. The swellable layered silicate that can be used in the present invention is preferably a monovalent cation type silicate such as sodium from the viewpoint of achieving excellent swelling properties and dispersion stability, and more preferably a monovalent cation type swellable layered silicate such as sodium from the viewpoint of achieving excellent swelling properties and dispersion stability. The cation exchange capacity of the swellable layered silicate can be measured by a method conforming to the Schollenberger method (Clay Handbook 3rd Edition, edited by the Clay Science Society of Japan, May 2009, p.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 in montmorillonite can be calculated by leaching interlayer cations in montmorillonite for 4 hours or more using 100 mL of 1M ammonium acetate aqueous solution per 0.5 g of montmorillonite, measuring the concentration of various cations in the resulting solution by ICP emission spectrometry, atomic absorption spectrometry, or the like. In addition, there is no particular limit to the upper limit of the cation exchange capacity of the swellable layered silicate that can be used in the present invention, but 120 meq / 100 g or less is practical.

[0024] The particle size of the inorganic particles used in the present invention varies depending on the measurement conditions, but is generally determined by particle size distribution measurement using water as a dispersion medium, and can be set arbitrarily within the range that does not impair the effects of the present invention. From the viewpoint of efficiently dispersing lignin uniformly, 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 the present invention can be adjusted to a desired range by a conventional method such as pulverization. For example, the particle size of commercially available inorganic particles can be further reduced by appropriately using a jet mill such as a dry pulverizer, a bead mill such as a wet pulverizer, a pressurized wet pulverizer, or the like. In measuring the median diameter of inorganic particles, when the dispersed particle size is nanoscale, the measurement by the photon correlation method is preferable, and the median diameter in the particle size distribution obtained as the diffusion coefficient equivalent diameter can be used. In the measurement, the inorganic particles are dispersed in water, and then diluted to about 0.1 mass % and then measured. The measuring device may be any commercially available device using the photon correlation method. For example, the SZ-100 series manufactured by Horiba, Ltd. may be mentioned. Other examples include the Zetasizer Nano series manufactured by Malvern Instruments, and the DLS-6500 series manufactured by Otsuka Electronics Co., Ltd. When inorganic particles are microscale, the median diameter can be measured by a laser diffraction / scattering method using a measuring device such as the LA-960 series manufactured by Horiba, the Mastersizer series manufactured by Malvern, or the ELSZ series manufactured by Otsuka Electronics.

[0025] The inorganic particles used in the present invention may be natural products or may be synthesized according to a conventional method. When natural products are used as the inorganic particles, the natural products may contain foreign matter and impurities, but it is preferable that the foreign matter and impurities have been removed. Examples of the synthesis method of the swellable layered silicate include hydrothermal synthesis, melt synthesis, high pressure synthesis, solid reaction, flame fusion and alteration. The synthesis method of the swellable layered silicate can be referred to the method described in JP 2008-13401 A. The main synthesis methods of colloidal silica are gas phase synthesis such as aerosil synthesis by thermal decomposition of silicon tetrachloride, a method using water glass as a raw material, and liquid phase synthesis such as hydrolysis of alkoxide. Alumina hydrate is generally produced by the Bayer process, which dissolves bauxite in sodium hydroxide at high temperature.

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

[0027] As long as a luminescent material can be obtained, there are no particular limitations on the conditions for treating lignin with inorganic particles, and conditions commonly used in conventional methods can be appropriately selected. The conditions for treating lignin 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 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 selected solvent is mixed with lignin and inorganic particles to prepare a dispersion liquid containing lignin and inorganic particles. The solvent used in the present invention is preferably ethanol from the viewpoint of production efficiency of the luminescent material.

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

[0030] In the present invention, the luminescent material can be produced by appropriately selecting a treatment that is usually performed in the production of the luminescent material. Treatments that can be adopted in the present invention include homogenization treatment, drying of precipitates, redispersion, etc. A preferred manufacturing process in the present invention will be specifically described. For example, a dispersion of lignin and inorganic particles is prepared, and homogenization is performed by ultrasonic diffusion at 150W for 15 minutes. A drying process is then performed, and the lignin is melted by a heat press process at 150°C and 10MPa for the dried powder. Through such a process, the lignin is treated with inorganic particles, and the luminescent material of the present invention can be manufactured. However, the present invention is not limited to this.

[0031] By going through the above-mentioned steps, a luminescent material containing lignin treated with inorganic particles can be obtained. Specifically, a luminescent material that absorbs ultraviolet light and emits light when irradiated with ultraviolet light can be produced. In order to apply the luminescent material obtained by the present invention to various media such as ultraviolet sensors and induced luminescence materials, it is preferable that the lignin used as the starting material has a high whiteness, and white lignin having an L* value of 80 or more in the L*a*b* color space is more preferable. The L*a*b* color space is a type of complementary color space, and has a dimension L* 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 more in the L*a*b* color space. The L* value, a* value, and b* value can be measured according to JIS Z 8781-4:2013. The whiteness (L* value) of the lignin can be adjusted by appropriately selecting the preparation method of the raw material. EXAMPLES

[0032] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

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

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

[0035] <Preparation of lignin dispersion> The concentration of the recovered lignin residue was measured using a moisture meter (MS-70 manufactured by A&D Co., Ltd.), and then ultrapure water and ethanol were added dropwise to the lignin residue so that the water to ethanol ratio 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.) was added dropwise in an amount (100 μL) of approximately 9 times the weight of the lignin, and the mixture was stirred at 50°C for 5 hours to whiten the lignin through a urethane bond formation reaction. After stirring, about 10 mL of ethanol was added to wash the mixture, and unreacted matter was filtered using filter paper (Kiriyama Seisakusho No. 5B (21φmm)). The residue on the filter paper was vacuum dried to recover white lignin powder.

[0037] <Preparation of 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 so as to give a concentration of 1 mg / mL, to prepare a white lignin ethanol dispersion.

[0038] Example 2 <Preparation of imidazolium-modified clay> 2.0 g of synthetic saponite (Sumecton SA; average particle size: 20 nm, Kunimine Kogyo Co., Ltd.) was mixed with 98 g of ultrapure water, and then treated with a homogenizer (IKA Ultratax T50) at 6000 rpm for 10 minutes and an ultrasonic homogenizer (BRANSON Sonifier, Model 450A) at 75 W for 10 minutes, respectively, and 1 g of 1-ethyl-3-methylimidazolium methanesulfonate (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, treated with an ultrasonic homogenizer at 75 W for 15 minutes, and the obtained sample was centrifuged at 16000 rpm for 30 minutes to wash it. This operation was repeated once to obtain a gel-like imidazolium-modified synthetic saponite (30.2 wt%).

[0039] <Preparation of imidazolium-modified clay ethanol dispersion> The imidazolium-modified synthetic saponite gel (0.5 g solid content) was mixed with 18 mL of ethanol. Ultrasonic diffusion (Chinkey 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> 1.3g of the imidazolium-modified synthetic saponite ethanol dispersion (10.8wt%) was mixed with 8.0g of ethanol, and ultrasonic diffusion (Chinkey Nanopremixer PR-1) was performed at 150W for 15 minutes. Then, the dispersion consisting of 0.06g of white lignin and 5.94g of ethanol prepared in Example 1 was mixed with the above sample, and ultrasonic diffusion was performed at 150W for 15 minutes, and the solvent was evaporated by drying. A part of the dried sample was heat pressed at 150°C and 10MPa to obtain a white lignin-imidazolium-modified clay composite powder. The weight ratio of the white lignin and the 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: about 50 nm, manufactured by Kunimine Kogyo Co., Ltd.) was used instead of synthetic saponite.

[0043] Example 5 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 Kogyo Co., Ltd.) was used instead of synthetic saponite, and the white lignin:clay ratio 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 (manufactured by AS ONE; LUV-4). The results are shown in FIG. The white lignin ethanol dispersion prepared in Example 1 was transparent (FIG. 1(A)), but emitted blue light when irradiated with 4 W of UV light (330-380 nm) (FIGS. 1(B) and (C)). The control substance, ethanol, did not emit blue light even when irradiated with UV light. As shown in Figure 2, white lignin alone did not emit light when irradiated with UV light, 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 FIG. As shown in Figure 3(A), an emission peak was confirmed in the wavelength region of 390-560 nm when irradiated with excitation light of 340 nm. The intensity of this peak was proportional to the concentration of white lignin, suggesting that it was an emission peak derived from white lignin. In addition, as shown in Figure 3(B), a clear emission peak was confirmed at an excitation wavelength of 300-380 nm.

[0046] (3) UV-Visible Absorption Spectrum Measurement Experiment The white lignin ethanol dispersion prepared in Example 1 was diluted with ethanol to a predetermined concentration, and ultraviolet-visible absorption spectrum was measured using a spectrophotometer (Hitachi High-Technologies Corporation; U-2910). The results are shown in FIG. Absorption from lignin monomers (guaiacyl and syringyl groups) with an intensity corresponding to the concentration of white lignin was observed in the wavelength range of 250 to 290 nm, and it was considered that the emission phenomenon was due to the light absorption by the monomers. In addition, no absorption was observed in the wavelength range of 320 nm, which indicates the aggregation of lignin molecular chains (e.g., aromatic ring π-π stacks). From these results, it is considered that the hexyl groups in white lignin constrain the aromatic main chain of lignin, suppressing intramolecular rotation and vibration, reducing molecular mobility, and causing radiative deactivation to prevail, thereby increasing the 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. Among the samples shown in Fig. 5(A), only the white lignin-SA composite powder emitted blue light when irradiated with 4 W of UV light (330-380 nm) (see Fig. 5(C)). The control substances, SA alone and the untreated lignin-SA composite powder prepared in Example 3, did not emit blue light (see Fig. 5(C)).

[0048] In the white lignin-imidazolium modified clay composite powder prepared in Example 2, only the sample that was heat-treated by heat pressing emitted blue light when irradiated with UV light (330-380 nm), as shown in Figure 6. Since white lignin exhibits thermal melting, it is suggested that the uniformity of the mixture of white lignin and clay, which depends on whether or not the white lignin melts, affects whether or not light is emitted.

[0049] The luminescence behavior of the white lignin-imidazolium modified clay composite powder prepared in Example 4 was also evaluated in a similar manner, and the mixture of synthetic smectite and white lignin emitted blue light (see FIG. 7). Furthermore, the white lignin-imidazolium modified clay composite powders prepared in Examples 4 and 5 emitted blue light in the white lignin:synthetic clay blend ratio range of 60:40 to 10:90 (%) as shown in Fig. 8. It was visually confirmed that the luminescence intensity tended to be strongest when the white lignin:synthetic clay blend ratio was 30:70 (%).

[0050] (2) X-ray diffraction experiment X-ray diffraction measurements were performed using an X-ray diffractometer (Rigaku Corporation; SmartLab) on the white lignin-imidazolium-modified clay composite powders prepared in Examples 2 to 5. The results are shown in FIG. For the white lignin-imidazolium modified clay composite powder made of synthetic saponite, only the diffraction peaks derived from the clay were confirmed, and no peaks associated with the crystallization of hexyl groups in the white lignin were present. Since the synthetic clay has a small and uniform particle size (Reference: US Pat. 5763345, 1998), the white lignin can be finely and uniformly dispersed, suppressing the crystallization of hexyl groups in the white lignin and reducing the interactions between molecules in the white lignin, which is thought to have enabled luminescence in the solid state.

[0051] As described above, by treating lignin with particles of an inorganic substance, a luminescent material derived from lignin can be produced.

Claims

1. A method for treating lignin by treating lignin with inorganic particles.

2. The method for treating lignin according to claim 1, wherein the lignin has been subjected to a decolorization treatment.

3. The method for treating lignin according to claim 1 or 2, wherein the lignin has been subjected to a saccharification treatment.

4. The method for treating lignin according to claim 1 or 2, wherein the inorganic particles are particles composed of a swellable layered silicate.

5. The method for treating lignin according to claim 1 or 2, wherein the particle size of the inorganic particles is 10 nm or more and 100 nm or less.

6. The method for treating lignin according to claim 1, 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.

7. The method for treating lignin according to claim 1, 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.

8. The method for treating lignin according to claim 6 or 7, 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.

9. A method for producing a luminescent material, which treats lignin with inorganic particles to obtain a luminescent material containing lignin.

10. The method for producing a luminescent material according to claim 9, wherein the lignin has been subjected to a decolorization treatment.

11. The method for producing a luminescent material according to claim 9 or 10, wherein the lignin has been subjected to a saccharification treatment.

12. The method for producing a luminescent material according to claim 9 or 10, wherein the inorganic particles are particles composed of a swellable layered silicate.

13. The method for producing a luminescent material according to claim 9 or 10, 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 claim 9, 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 claim 9, 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 claim 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 claim 9 or 10, wherein the luminescent material is a material that absorbs ultraviolet light and emits light when irradiated with ultraviolet light.

18. A luminescent material containing lignin treated with inorganic particles.

19. The luminescent material according to claim 18, wherein the lignin has been subjected to a decolorization treatment.

20. The luminescent material according to claim 18 or 19, wherein the lignin has been subjected to a saccharification treatment.

21. The luminescent material according to claim 18 or 19, wherein the inorganic particles are particles composed of a swellable layered silicate.

22. The luminescent material according to claim 18 or 19, wherein the particle size of the inorganic particles is 10 nm or more and 100 nm or less.

23. The luminescent material according to claim 18, wherein the lignin and the inorganic particles are mixed in a solvent to prepare a dispersion, and the dispersion is homogenized by ultrasonic diffusion, whereby the lignin is treated with the inorganic particles.

24. The luminescent material according to claim 18, wherein the lignin and the inorganic particles are mixed in a solvent to prepare a dispersion, the dispersion is homogenized by ultrasonic diffusion and then dried, and the obtained dried powder is subjected to a hot press treatment, whereby the lignin is treated with the inorganic particles.

25. The luminescent material according to claim 23 or 24, wherein in the dispersion, 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.

26. The luminescent material according to claim 18 or 19, which is a material that absorbs ultraviolet light and emits light when irradiated with external light.

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