Catalyst for lignin decomposition and method for acquiring vanillin

Abstract

A catalyst for lignin decomposition, which contains a metal oxide.

Description

Catalyst for lignin decomposition and method for obtaining vanillin

[0001] The present invention relates to a catalyst for lignin decomposition and a method for obtaining vanillin.

[0002] Since lignin is a refractory substance whose molecules are three-dimensionally crosslinked, it does not dissolve in common solvents and has no melting point. Various techniques have been proposed so far for the purpose of effective utilization of such lignin. For example, Patent Document 1 proposes a method of treating lignin with supercritical water. Patent Document 2 proposes reacting lignin with hydroxy radicals. Patent Documents 3 and 4 propose methods for producing compounds derived from low-molecular-weight aromatic lignin. Non-Patent Documents 1 and 2 propose methods for decomposing lignin by light irradiation.

[0003] Japanese Patent Application Laid-Open No. 11-292799Japanese Patent Application Laid-Open No. 2012-6857International Publication No. WO 2017 / 174098International Publication No. WO 2017 / 174206

[0004] ACS Sustainable Chem. Eng. 2018, 6, 11, 13968‐13976ACS Catal. 2016, 6, 11, 7716‐7721

[0005] However, even with the prior arts such as Patent Documents 1 to 4 and Non-Patent Documents 1 and 2, obtaining vanillin without requiring a large amount of energy for lignin decomposition has not been sufficiently studied.

[0006] The present invention has been made to solve at least part of the above-described problems, and an object of the present invention is to provide a technology that can selectively obtain high-concentration vanillin without requiring a large amount of energy for lignin decomposition.

[0007] The present invention has been made to solve at least part of the above-described problems, and can be implemented as the following embodiments.

[0008] (1) According to one aspect of the present invention, a catalyst for lignin decomposition is provided. This catalyst for lignin decomposition is a catalyst for lignin decomposition containing a metal oxide.

[0009] This configuration allows for the selective acquisition of high concentrations of vanillin without requiring a large amount of energy for lignin degradation.

[0010] (2) In the above-described form of lignin decomposition catalyst, the metal oxide may be titanium oxide. With this configuration, even when titanium oxide is included as the metal oxide, a high concentration of vanillin can be obtained selectively without requiring a great deal of energy for lignin decomposition.

[0011] (3) In the above-described form of lignin decomposition catalyst, the metal oxide may have a metal other than the metal contained in the metal oxide supported on it. With this configuration, a higher concentration of vanillin can be obtained compared to a lignin decomposition catalyst containing a metal oxide that does not have another metal supported on it.

[0012] (4) In the above-described form of the lignin decomposition catalyst, the metal oxide may be doped with nitrogen. With this configuration, a higher concentration of vanillin can be obtained compared to a lignin decomposition catalyst containing a metal oxide that is not doped with nitrogen.

[0013] (5) In the above-described form of lignin decomposition catalyst, the metal oxide may be doped with nitrogen and may also have another metal supported on it that is different from the metal contained in the metal oxide. With this configuration, a higher concentration of vanillin can be obtained compared to a lignin decomposition catalyst containing a nitrogen-doped metal oxide and a lignin decomposition catalyst containing a metal oxide with another metal supported on it.

[0014] (6) Another embodiment of the present invention provides a method for obtaining vanillin. This method comprises the step of obtaining vanillin by adding a lignin decomposition catalyst containing a metal oxide to water in which lignin is dispersed. With this configuration, a high concentration of vanillin can be obtained selectively without requiring a large amount of energy for the decomposition of lignin.

[0015] (7) Another embodiment of the present invention provides a method for obtaining vanillin. This method comprises the steps of preparing a mixed liquid by mixing water and an organic solvent, and obtaining vanillin by adding a lignin-degrading catalyst containing a metal oxide to the mixed liquid in which lignin is dispersed. With this configuration, it is possible to obtain vanillin at an even higher concentration selectively without requiring a great deal of energy for the degradation of lignin.

[0016] Furthermore, the present invention can be realized in various forms, for example, in the form of a lignin decomposition apparatus or lignin decomposition system containing a lignin decomposition catalyst, a lignin decomposition method, a computer program for executing these apparatuses and methods, a server device for distributing this computer program, or a non-temporary storage medium storing the computer program.

[0017] This is a table showing the details of each lignin decomposition catalyst that underwent evaluation testing. This is a figure showing the results of the evaluation test for the lignin decomposition catalyst of the first embodiment. This is a figure showing the results of the evaluation test for the lignin decomposition catalyst of the second embodiment. This is a figure showing the results of the evaluation test for the lignin decomposition catalyst of the third embodiment. This is a figure showing the results of the evaluation test for the lignin decomposition catalyst of the fourth embodiment. This is a table showing the details of each lignin decomposition catalyst that underwent evaluation testing. This is a figure showing the results of the evaluation test for the lignin decomposition catalyst of the third embodiment. This is a figure showing the results of the evaluation test for the lignin decomposition catalyst of the fourth embodiment. This is a figure showing the results of the evaluation test for the lignin decomposition catalyst of the fifth embodiment. This is a table showing the details of each lignin decomposition catalyst that underwent another evaluation test. This is a figure showing the results of the evaluation test for the mixed liquid. This is a figure showing the results of the evaluation test for the mixed liquid. This is a figure showing the results of the evaluation test for the mixed liquid.

[0018] <Embodiments> Figure 1 is a table showing the details of the lignin decomposition catalysts of the first to fourth embodiments that underwent evaluation testing. The evaluation test was carried out in the following steps. First, the evaluator prepared a reaction solution by adding each lignin decomposition catalyst to water in which lignin was dispersed (hereinafter referred to as lignin dispersed water). Next, the evaluator evaluated the various components contained in the reaction solution, which had been left for one week at room temperature and atmospheric pressure under visible light irradiation, using gas chromatography-mass spectrometry (GC-MS). Here, room temperature is 5 to 45°C, preferably 10 to 40°C, more preferably 15 to 35°C, but is not limited to this temperature and may be other temperatures. Atmospheric pressure is atmospheric pressure. Visible light is light with a wavelength of 360 to 800 nm, but may also be light with a wavelength of 380 to 790 nm or light with a wavelength of 400 to 780 nm. Furthermore, when using a white LED as the visible light source, a white LED emitting light with a wavelength of 400 to 800 nm may be used, and it is more preferable to use a white LED emitting light with a wavelength of 415 to 700 nm.

[0019] In the table in Figure 1, "Catalyst" refers to the type of substance used as a catalyst for lignin decomposition in the first to fourth embodiments. In the table in Figure 1, "Iron-supported" indicates whether or not iron is supported on the substance used as a catalyst for lignin decomposition. A "-" under "Iron-supported" indicates that iron is not supported on the substance used as a catalyst for lignin decomposition, while a "○" under "Iron-supported" indicates that iron is supported on the substance used as a catalyst for lignin decomposition.

[0020] In the table in Figure 1, "Nitrogen Doping" indicates whether or not the substance used as the lignin decomposition catalyst is doped with nitrogen. A "-" in "Nitrogen Doping" indicates that the substance used as the lignin decomposition catalyst is not doped with nitrogen, while a "○" indicates that the substance used as the lignin decomposition catalyst is doped with nitrogen. In the table in Figure 1, "Catalyst Weight" indicates the weight of the lignin decomposition catalyst added to the reaction solution.

[0021] In the table in Figure 1, "Lignin Addition Amount" indicates the amount of lignin added to water when preparing the lignin-dispersed water. In the table in Figure 1, "Water Weight" indicates the weight of water to which lignin is added when preparing the lignin-dispersed water.

[0022] As shown in the table in Figure 1 under "Catalyst," the type of substance used as the lignin decomposition catalyst in the first to fourth embodiments is titanium dioxide. However, the "iron support" and "nitrogen doping" differ in each of the lignin decomposition catalysts in the first to fourth embodiments. Specifically, the titanium dioxide used as the lignin decomposition catalyst in the first embodiment is not iron-supported and is not nitrogen-doped. The titanium dioxide used as the lignin decomposition catalyst in the second embodiment is iron-supported but not nitrogen-doped. The titanium dioxide used as the lignin decomposition catalyst in the third embodiment is not iron-supported but is nitrogen-doped. The titanium dioxide used as the lignin decomposition catalyst in the fourth embodiment is iron-supported and nitrogen-doped.

[0023] As shown in the "Catalyst Weight" column in the table in Figure 1, 10 mg of each of the lignin decomposition catalysts of the first to fourth embodiments was added to a lignin dispersion to prepare a reaction solution. After standing for one week, the reaction solution was subjected to gas chromatography-mass spectrometry. The lignin dispersion to which each of the lignin decomposition catalysts of the first to fourth embodiments was added was prepared by dispersing 2.0 g of lignin in 35.0 g of water, as shown in the "Amount of Lignin Added" and "Weight of Water" columns in the table in Figure 1. In the comparative examples to be compared with the lignin decomposition catalysts of the first to fourth embodiments, gas chromatography-mass spectrometry was performed on lignin dispersions that were left standing for one week without the addition of a lignin decomposition catalyst. The lignin dispersed in the water during the preparation of the lignin dispersion was lignin dissolved in the strong acid or strong alkali by subjecting wood to strong acid or strong alkali treatment.

[0024] Figure 2 shows the results of an evaluation test of the lignin decomposition catalyst of the first embodiment. More specifically, Figure 2 shows a portion of chromatogram C1 obtained by performing gas chromatography-mass spectrometry on a reaction solution (after standing for one week) to which the lignin decomposition catalyst of the first embodiment had been added. In addition to a portion of chromatogram C1, Figure 2 also shows a dashed chromatogram Cp. Chromatogram Cp is a chromatogram (results of an evaluation test of a comparative example) obtained by performing gas chromatography-mass spectrometry on lignin dispersion water (after standing for one week) to which the lignin decomposition catalyst had not been added.

[0025] The horizontal axis in Figure 2 represents retention time, and the vertical axis represents relative intensity. The same applies to the horizontal and vertical axes in Figures 3 and beyond. The time point Tv on the horizontal axis in Figure 2 indicates the timing at which the vanillin peak is detected. The same applies to the time point Tv in Figures 3 and beyond. As shown in Figure 2, it was confirmed that when the lignin decomposition catalyst of the first embodiment was added to the lignin dispersion (chromatogram C1) compared to when the lignin decomposition catalyst was not added to the lignin dispersion (chromatogram Cp), the decomposition of lignin was promoted and a high concentration of vanillin was obtained. It is presumed that the reason a peak was detected near time Tv in chromatogram Cp, although lower than the peak detected near time Tv in chromatogram C1, is because the lignin eluted by strong acid or strong alkali treatment of wood contains low molecular weight compounds such as vanillin.

[0026] Figure 3 shows the results of an evaluation test of the lignin decomposition catalyst of the second embodiment. More specifically, Figure 3 shows a portion of chromatogram C2 obtained by performing gas chromatography-mass spectrometry on the reaction solution to which the lignin decomposition catalyst of the second embodiment was added (after standing for one week). In addition to a portion of chromatogram C2, Figure 3 also shows a portion of chromatogram C1 (see Figure 2). As shown in Figure 3, it was confirmed that the decomposition of lignin was more accelerated and a higher concentration of vanillin was obtained when the lignin decomposition catalyst of the second embodiment was added to the lignin dispersion (chromatogram C2) compared to when the lignin decomposition catalyst of the first embodiment was added to the lignin dispersion (chromatogram C1). In other words, it was confirmed that titanium oxide with iron support has higher catalytic activity for the lignin decomposition reaction than titanium oxide without iron support.

[0027] Figure 4 shows the results of an evaluation test of the lignin decomposition catalyst of the third embodiment. More specifically, Figure 4 shows a portion of chromatogram C3 obtained by performing gas chromatography-mass spectrometry on a reaction solution (after standing for one week) to which the lignin decomposition catalyst of the third embodiment had been added. In addition to a portion of chromatogram C3, Figure 4 also shows a portion of chromatogram C1 (see Figure 2). As shown in Figure 4, it was confirmed that the decomposition of lignin was more accelerated and a higher concentration of vanillin was obtained when the lignin decomposition catalyst of the third embodiment was added to the lignin dispersion (chromatogram C3) compared to when the lignin decomposition catalyst of the first embodiment was added to the lignin dispersion (chromatogram C1). In other words, it was confirmed that nitrogen-doped titanium dioxide has higher catalytic activity for the lignin decomposition reaction than nitrogen-doped titanium dioxide.

[0028] Figure 5 shows the results of an evaluation test of the lignin decomposition catalyst of the fourth embodiment. More specifically, Figure 5 shows a portion of chromatogram C4 obtained by performing gas chromatography-mass spectrometry on a reaction solution to which the lignin decomposition catalyst of the fourth embodiment was added (after standing for one week). In addition to a portion of chromatogram C4, Figure 5 also shows a portion of chromatogram C1 (see Figure 2). As shown in Figure 5, it was confirmed that the decomposition of lignin was more accelerated and a higher concentration of vanillin was obtained when the lignin decomposition catalyst of the fourth embodiment was added to the lignin dispersion (chromatogram C4) compared to when the lignin decomposition catalyst of the first embodiment was added to the lignin dispersion (chromatogram C1). In other words, it was confirmed that titanium oxide with iron supported and nitrogen doped has higher catalytic activity for the lignin decomposition reaction than titanium oxide without iron support and without nitrogen doping. Furthermore, a comparison of chromatograms C2 and C3 (see Figures 3 and 4) with chromatogram C4 (see Figure 5) confirmed that titanium oxide treated with both iron support and nitrogen doping exhibited higher catalytic activity for lignin decomposition than titanium oxide treated with either iron support or nitrogen doping alone.

[0029] According to the results of the evaluation tests described above, the lignin decomposition catalyst (titanium dioxide) of the first embodiment promotes the decomposition of lignin under normal temperature and pressure conditions, thereby obtaining a high concentration of vanillin. This means that a high concentration of vanillin can be obtained selectively without requiring a large amount of energy for lignin decomposition.

[0030] Furthermore, according to the results of the evaluation tests described above, the lignin decomposition catalyst of the second embodiment (titanium oxide supported with iron) and the lignin decomposition catalyst of the third embodiment (titanium oxide doped with nitrogen) can produce a higher concentration of vanillin while reducing the energy required for lignin decomposition, compared to the lignin decomposition catalyst of the first embodiment (titanium oxide that is neither supported with iron nor doped with nitrogen).

[0031] Furthermore, according to the results of the evaluation tests described above, the lignin decomposition catalyst of the fourth embodiment (titanium oxide supported with iron and doped with nitrogen) can produce an even higher concentration of vanillin while keeping the energy required for lignin decomposition lower compared to the lignin decomposition catalysts of the second and third embodiments (titanium oxide treated with either iron support or nitrogen doping).

[0032] Although the above evaluation tests were conducted under visible light irradiation, it has been confirmed that vanillin can also be obtained from the decomposition of lignin by the lignin decomposition catalysts of the first to fourth embodiments when similar evaluation tests are conducted under ultraviolet light irradiation.

[0033] Figure 6 is a table showing the details of the lignin-degrading catalysts of the third to fifth embodiments, which underwent evaluation tests under ultraviolet light irradiation. The evaluation tests described in Figures 6 to 9 are the same as those described in Figures 1 to 5, except that they are conducted under ultraviolet light irradiation instead of visible light irradiation.

[0034] The lignin decomposition catalysts and comparative examples of the third and fourth embodiments in the table in Figure 6 are the same as the lignin decomposition catalysts and comparative examples of the third and fourth embodiments in the table in Figure 1. Also, the terms "catalyst substance," "nitrogen doping," "catalyst weight," "lignin addition amount," and "water weight" in the table in Figure 6 are the same as in the table in Figure 1. In the table in Figure 6, "supported" indicates the substance supported on the substance used as the lignin decomposition catalyst. "Iron" in "supported" indicates that iron is supported on the substance used as the lignin decomposition catalyst, "copper" in "supported" indicates that copper is supported on the substance used as the lignin decomposition catalyst, and "-" in "supported" indicates that neither iron nor copper is supported on the substance used as the lignin decomposition catalyst. The lignin decomposition catalyst of the fifth embodiment is titanium oxide that is supported with copper and doped with nitrogen.

[0035] As shown in Figure 6, in the case of the lignin decomposition catalyst of the third embodiment, the reaction solution was prepared by adding 2.3 mg to the lignin dispersion. In the case of the lignin decomposition catalyst of the fourth embodiment, the reaction solution was prepared by adding 2.0 mg to the lignin dispersion. In the case of the lignin decomposition catalyst of the fifth embodiment, the reaction solution was prepared by adding 2.1 mg to the lignin dispersion. The lignin dispersion to which each of the lignin decomposition catalysts of the third to fifth embodiments is added was prepared by dispersing 0.20 g of lignin in 2.0 g of water, as shown in the "Amount of Lignin Added" and "Weight of Water" in the table in Figure 6.

[0036] Figures 7 to 9 show the results of evaluation tests of the lignin-decomposing catalysts of the third to fifth embodiments. More specifically, each of Figures 7 to 9 shows portions of chromatograms C3 to C5 obtained by performing gas chromatography-mass spectrometry on the reaction solution (after standing for one week) to which the lignin-decomposing catalysts of the third to fifth embodiments were added. In addition to portions of chromatograms C3 to C5, each of Figures 7 to 9 also shows a portion of the dashed chromatogram Cp. Chromatogram Cp, similar to Figure 2, is a chromatogram showing the results of the evaluation test of the comparative example.

[0037] As shown in Figures 7-9, even under ultraviolet light irradiation, it was confirmed that when the lignin decomposition catalyst of the third to fifth embodiments was added to the lignin dispersion (chromatograms C3-C5), the decomposition of lignin was accelerated and a high concentration of vanillin could be obtained, compared to when the lignin decomposition catalyst was not added to the lignin dispersion (chromatogram Cp).

[0038] Figure 10 is a table showing the details of another evaluation test using the lignin-decomposing catalyst of the fourth embodiment. The evaluation tests described in Figures 10 to 13 use the lignin-decomposing catalyst of the fourth embodiment as shown in the evaluation test in Figure 1, but they differ from the evaluation test described in Figure 1. Specifically, in the evaluation test described in Figure 1, the solvent for the lignin-decomposing catalyst of the fourth embodiment was water, but in the evaluation tests described in Figures 10 to 13, the solvent for the lignin-decomposing catalyst of the fourth embodiment is a mixed liquid of water and an organic solvent.

[0039] The evaluation test was conducted in the following steps. First, the evaluator prepared a mixed liquid by mixing water and an organic solvent. Next, the evaluator dispersed lignin in the mixed liquid (hereinafter referred to as the lignin dispersion). Next, the evaluator prepared a reaction solution by adding titanium dioxide (the catalyst for lignin decomposition in the fourth embodiment), which is supported with iron and doped with nitrogen, to the lignin dispersion. Next, the evaluator evaluated the various components contained in the reaction solution, which had been left for one week at room temperature and pressure under visible light irradiation, using gas chromatography-mass spectrometry (GC-MS). Note that the room temperature and pressure referred to here are the same as those in the evaluation test described in Figure 1. A white LED may also be used as the visible light irradiation source.

[0040] The terms "Catalyst," "Iron Support," "Nitrogen Doping," "Catalyst Weight," and "Lignin Addition Amount" in the table of Figure 10 are the same as in the table of Figure 1. On the other hand, the "Weight of Water" in the "Mixed Liquid" section of the table of Figure 10 indicates the weight of water contained in the mixed liquid. The "Name and Weight of Organic Solvent" in the "Mixed Liquid" section of the table of Figure 10 indicates the name and weight of the organic solvent contained in the mixed liquid. As shown in the "Mixed Liquid" section of the table of Figure 10, three types of mixed liquids were prepared for the evaluation test in Figure 10: a mixed liquid of 17.5 g of water and 17.5 g of toluene, a mixed liquid of 17.5 g of water and 17.5 g of xylene, and a mixed liquid of 17.5 g of water and 17.5 g of chloroform.

[0041] As shown in the "Catalyst Weight" column in the table in Figure 10, 10 mg of the lignin-decomposing catalyst of the fourth embodiment was added to each mixed liquid to prepare the reaction solution, and gas chromatography-mass spectrometry was performed on the reaction solution after it had stood for one week. Also, similar to the evaluation test in Figure 1 described above, gas chromatography-mass spectrometry was performed on the lignin dispersion liquid that was to be compared with the lignin-decomposing catalyst of the fourth embodiment, after it had stood for one week without the addition of the lignin-decomposing catalyst. Note that the lignin dispersed in water when preparing the lignin dispersion liquid was the same as in the evaluation test in Figure 1 described above.

[0042] Figure 11 shows the results of an evaluation test for a lignin dispersion liquid obtained by dispersing lignin in a mixed liquid of water and toluene. More specifically, Figure 11 shows a portion of the chromatogram (CT) obtained by performing gas chromatography-mass spectrometry on a reaction solution (after standing for one week) in which the lignin decomposition catalyst of the fourth embodiment was added to the lignin dispersion liquid (containing water and toluene). Note that the vertical axis of Figure 11 is approximately 20 times longer than the vertical axis of Figure 5.

[0043] As shown in Figures 5 and 11, it was confirmed that a higher concentration of vanillin could be obtained when the lignin decomposition catalyst of the fourth embodiment was added to a lignin dispersion liquid (containing water and toluene) (chromatogram CT) compared to when the lignin decomposition catalyst of the fourth embodiment was added to a lignin dispersion water (chromatogram C4). The comparison between chromatogram CT and chromatogram C4 is based on the difference between using only water as the solvent and using a mixed liquid of water and an organic solvent. In other words, it was confirmed that a mixed liquid of water and an organic solvent such as toluene is preferable as the solvent to which the lignin decomposition catalyst is added, rather than using only water.

[0044] Figure 12 shows the results of an evaluation test on a lignin dispersion liquid obtained by dispersing lignin in a mixed liquid of water and xylene. Specifically, Figure 12 shows a portion of the chromatogram CX obtained by performing gas chromatography-mass spectrometry on a reaction solution (after standing for one week) to which the lignin-degrading catalyst of the fourth embodiment was added to the lignin dispersion liquid (containing water and xylene). The vertical axis of Figure 12 is also approximately 20 times longer than the vertical axis of Figure 5. As shown in Figures 5 and 12, it was confirmed that a higher concentration of vanillin could be obtained when the lignin-degrading catalyst of the fourth embodiment was added to the lignin dispersion liquid (containing water and xylene) (chromatogram CX) compared to when the lignin-degrading catalyst of the fourth embodiment was added to lignin-dispersed water (chromatogram C4). In other words, it was confirmed that a mixed liquid of water and xylene, an organic solvent, is also preferable as the solvent to which the lignin-degrading catalyst is added.

[0045] Figure 13 is a diagram showing results of an evaluation test on a lignin dispersion liquid obtained by dispersing lignin in a mixed liquid of water and chloroform. Specifically, Figure 13 shows a part of a chromatogram CC obtained by performing gas chromatography-mass spectrometry on a reaction liquid (after being left standing for one week), which is obtained by adding the lignin decomposition catalyst of the fourth embodiment to the lignin dispersion liquid (containing water and chloroform). It should be noted that the vertical axis of Figure 13 is also about 20 times longer than the vertical axis of Figure 5. As shown in Figures 5 and 13, compared with the case where the lignin decomposition catalyst of the fourth embodiment is added to lignin-dispersed water (chromatogram C4), it was confirmed that a higher concentration of vanillin can be obtained in the case where the lignin decomposition catalyst of the fourth embodiment is added to the lignin dispersion liquid (containing water and chloroform) (chromatogram CT). That is, it was confirmed that, as a solvent to which the lignin decomposition catalyst is added, a mixed liquid obtained by adding chloroform, which is an organic solvent, to water is also preferable.

[0046] As shown in Figures 11 to 13, it was confirmed that as a solvent to which the lignin decomposition catalyst is added, a mixed liquid obtained by mixing water and an organic solvent can selectively obtain vanillin at a higher concentration than that in the case of using only water. Such an effect is estimated to be based on the following reasons. That is, when a lignin decomposition catalyst is contained in a solvent, hydroxyl radicals are easily generated in the solvent. While hydroxyl radicals promote the decomposition of lignin, they also promote the decomposition of vanillin produced by the decomposition of lignin. However, when a mixed liquid of water and an organic solvent is used as the solvent to which the lignin decomposition catalyst is added, vanillin is more soluble in the organic solvent phase than in the aqueous phase. Therefore, vanillin produced from lignin by the action of hydroxyl radicals in the aqueous phase does not remain in the aqueous phase but moves to the organic solvent phase, and thus can avoid decomposition by hydroxyl radicals. Of course, hydroxyl radicals are localized in the aqueous phase and hardly exist in the organic solvent phase.

[0047] The evaluation test shown in FIG. 10 described above was carried out under visible light irradiation. Similarly, when carried out under ultraviolet light irradiation, it was confirmed that a higher concentration of vanillin can be obtained when the solvent to which the lignin decomposition catalyst is added is a mixed liquid of water and an organic solvent than when the solvent is only water. Also, similarly with the lignin decomposition catalysts of the first to third and fifth embodiments, it was confirmed that a higher concentration of vanillin can be obtained both under visible light irradiation and under ultraviolet light irradiation when the solvent to which the lignin decomposition catalyst is added is a mixed liquid of water and an organic solvent than when the solvent is only water.

[0048] <Modification of the Present Embodiment> The present invention is not limited to the above embodiments, and can be implemented in various aspects without departing from the scope of the gist thereof. For example, the following modifications are also possible.

[0049] In the above embodiment, titanium oxide is used as the lignin decomposition catalyst, but the present invention is not limited thereto. The lignin decomposition catalyst only needs to contain titanium oxide having lignin decomposition activity. In other words, the lignin decomposition catalyst is not limited to 100% by weight of titanium oxide, and it is only required that the lignin decomposition catalyst contains titanium oxide in a proportion higher than 0% by weight. Of course, the lignin decomposition catalyst may have titanium oxide as a main component. The main component as used herein refers to the component with the highest weight percentage.

[0050] In the above embodiment, titanium oxide is used as the lignin decomposition catalyst, but the present invention is not limited thereto. As long as the lignin decomposition catalyst has lignin decomposition activity, it may be a metal oxide different from titanium oxide. In addition, the metal oxide different from titanium oxide only needs to be contained in the lignin decomposition catalyst in any weight percentage as long as the proportion thereof is higher than 0% by weight. Of course, the lignin decomposition catalyst may have a metal oxide different from titanium oxide as a main component. Further, the lignin decomposition catalyst may contain a plurality of types of metal oxides having lignin decomposition activity, and may contain titanium oxide together with a metal oxide different from titanium oxide.

[0051] In the above embodiment, titanium oxide treated with at least one of the following treatments—supporting iron (or copper) and doping with nitrogen—was exemplified as a catalyst for lignin decomposition, but the invention is not limited thereto. A metal oxide other than titanium oxide, which has been treated with at least one of the following treatments—supporting iron (or copper) and doping with nitrogen—may also be used as a catalyst for lignin decomposition.

[0052] In the above embodiment, titanium oxide supported with iron (or copper) was given as an example of a catalyst for lignin decomposition, but it is not limited to this. Metal oxides having lignin decomposition activity (titanium oxide or other metal oxides) may also have other metals supported on them that are different from the metal contained in the metal oxide and iron (or copper), as metals that improve the decomposition of lignin, similar to iron (or copper). Of course, iron (or copper) and other metals may both be supported on a metal oxide having lignin decomposition activity, or iron and copper and other metals may both be supported on a metal oxide having lignin decomposition activity.

[0053] The embodiments of this specification have been described above based on the embodiments and modifications described above. The embodiments described above are for the purpose of facilitating understanding of this specification and do not limit it. This specification may be modified and improved without departing from its spirit and the scope of the claims, and equivalents thereof are included in this specification. Furthermore, any technical features that are not described as essential in this specification may be deleted as appropriate.

[0054] The present invention can also be realized in the following forms: [Example 1] A lignin decomposition catalyst containing a metal oxide. [Example 2] A lignin decomposition catalyst according to Example 1, wherein the metal oxide is titanium oxide. [Example 3] A lignin decomposition catalyst according to Example 1 or Example 2, wherein the metal oxide has a different metal supported on it than the metal contained in the metal oxide. [Example 4] A lignin decomposition catalyst according to any one of Examples 1 to 3, wherein the metal oxide is doped with nitrogen. [Example 5] A lignin decomposition catalyst according to any one of Examples 1 to 4, wherein the metal oxide is doped with nitrogen and has a different metal supported on it than the metal contained in the metal oxide. [Application Example 6] A method for obtaining vanillin, comprising the step of obtaining vanillin by adding a lignin-decomposing catalyst containing a metal oxide to water in which lignin is dispersed. [Application Example 7] A method for obtaining vanillin, comprising the step of preparing a mixed liquid by mixing water and an organic solvent, and the step of obtaining vanillin by adding a lignin-decomposing catalyst containing a metal oxide to the mixed liquid in which lignin is dispersed.

[0055] C1-C4...Chromatograms Cp...Chromatogram Tv...Time point

Claims

1. A catalyst for lignin decomposition containing a metal oxide.

2. A lignin decomposition catalyst according to claim 1, wherein the metal oxide is titanium oxide.

3. A lignin decomposition catalyst according to claim 1, wherein the metal oxide has a metal other than the metal contained in the metal oxide supported on it.

4. A lignin decomposition catalyst according to claim 1, wherein the metal oxide is doped with nitrogen.

5. A lignin decomposition catalyst according to claim 1 or claim 2, wherein the metal oxide is doped with nitrogen and has a metal other than the metal contained in the metal oxide supported on it.

6. A method for obtaining vanillin, comprising the step of obtaining vanillin by adding a lignin decomposition catalyst containing a metal oxide to water in which lignin is dispersed.

7. A method for obtaining vanillin, comprising the steps of: preparing a mixed liquid by mixing water and an organic solvent; and obtaining vanillin by adding a lignin-decomposing catalyst containing a metal oxide to the mixed liquid in which lignin is dispersed.

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