Method for producing hydrogen storage compounds, and method for producing hydrogen

Abstract

This invention provides a method for producing hydrogen storage compounds under mild conditions and with reduced environmental impact. It also provides a method for producing hydrogen through this hydrogen storage compound production method. [Solution] A method for producing a hydrogen storage compound, comprising electrolytically hydrogenating a ketone compound in water containing an electrolyte in the presence of a metal complex catalyst to obtain a secondary alcohol compound, A method for producing a hydrogen storage compound, wherein the ketone compound is at least one of a bicyclomonoketone compound and a polyvalent ketone compound.

Description

[Technical Field]

[0001] This invention relates to a method for producing hydrogen storage compounds and a method for producing hydrogen. [Background technology]

[0002] Hydrogen gas is expected to be a next-generation energy source that does not emit carbon dioxide when used. Safe transportation of hydrogen is essential for its effective utilization. Because hydrogen gas is bulky and highly flammable at room temperature and pressure, transporting it in its original state is difficult. Therefore, in addition to methods of transporting hydrogen gas by compressing or liquefying it, the storage and transport of hydrogen in hydrogen storage materials is currently being considered. Examples of hydrogen storage materials include metal-organic frameworks (MOFs) and hydrogen storage alloys. However, storing and transporting hydrogen using these materials requires low temperature or high pressure conditions, which increases costs and poses safety risks. Organic hydrides are also known as hydrogen storage compounds. Organic hydrides can safely store and transport hydrogen via chemical bonding under normal temperature and pressure conditions. However, the organic hydrides used in previous studies have been petroleum-derived compounds such as methylcyclohexane (dehydrogenated form: toluene) and 9-fluorenol (dehydrogenated form: 9-fluorenone). Furthermore, previous studies have employed methods to produce hydrogen gas from fossil fuels through steam reforming or water electrolysis, and then produce organic hydrides using high-pressure, high-purity hydrogen gas (hydrogenating the dehydrogenated form of organic hydrides). Such conventional methods required multi-stage processing, including hydrogen gas production, purification, and processing under high pressure. In addition, high-temperature conditions were necessary when extracting the hydrogen gas. Furthermore, since the above-mentioned organic hydrides are generally highly volatile liquids, they are difficult to handle during transport and storage. For this reason, solid hydrogen storage materials are being considered. For example, Patent Document 1 discloses a hydrogen carrier (hydrogen storage material) in which a hydrogen storage portion is included in the main chain and / or side chains of an organic polymer, the hydrogen storage portion generates hydrogen molecules in the presence of a catalyst and becomes an oxidation-reduction active portion, and the oxidation-reduction active portion stores hydrogen through reduction and contact with a proton source to become the hydrogen storage portion. In the technology described in Patent Document 1, hydrogen derived from hydrogen gas is not bonded to the oxidation-reduction active portion, but rather hydrogen is introduced to the oxidation-reduction active portion by electrolytic hydrogenation. That is, the oxidation-reduction active portion is put into a reduced state, and the oxidation-reduction active portion is protonated by acting on it with a proton source such as water. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2015 / 005280 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Patent Document 1 describes a method in which a p-quinone-type phenolic resin is electrolytically hydrogenated by using an acetonitrile solution of tetrabutylammonium perchlorate as the electrolyte and adding water as a hydrogen source. However, this method has the problem of having a large environmental impact because it requires the use of an organic solvent as the electrolyte, since it uses an organic hydride with low hydrophilicity as the hydrogen storage material. The present invention aims to provide a method for producing hydrogen storage compounds under mild conditions and with a reduced environmental impact, and to provide a method for producing hydrogen through this method for producing hydrogen storage compounds. [Means for solving the problem]

[0005] The inventors of this invention investigated techniques for obtaining hydrogen storage compounds by electrolytic hydrogenation and discovered that, when using specific ketone compounds, the electrolytic hydrogenation of ketone compounds proceeds efficiently in an aqueous electrolyte in the presence of a metal complex catalyst. They also found that the extraction of hydrogen from the obtained secondary alcohol compounds (dehydrogenation reaction) can be carried out efficiently under mild conditions. This invention was completed based on these findings and further investigations.

[0006] The above-mentioned problems of the present invention were solved by the following means. [1] A method for producing a hydrogen storage compound, comprising electrolytically hydrogenating a ketone compound in water containing an electrolyte in the presence of a metal complex catalyst to obtain a secondary alcohol compound, A method for producing a hydrogen storage compound, wherein the ketone compound is at least one of a bicyclomonoketone compound and a polyvalent ketone compound. [2] The method for producing a hydrogen storage compound according to [1], wherein the ketone compound is at least one of a bicyclomonoketone compound and a diketone compound. [3] The method for producing a hydrogen storage compound according to [1], wherein the ketone compound is at least one of a bicyclomonoketone compound and a β-diketone compound. [4] The method for producing a hydrogen storage compound according to [1], wherein the ketone compound is at least one of dihydrolevoglucocenone, acetylacetone, 2,5-hexanedione, and 1,4-cyclohexanedione. [5] A method for producing a hydrogen storage compound according to any one of [1] to [4], wherein the metal complex catalyst is an iridium complex catalyst. [6] The method for producing a hydrogen storage compound according to [5], wherein the metal complex catalyst is an iridium complex catalyst containing a bipyridine ligand as a ligand. [7] The method for producing a hydrogen storage compound according to [5] or [6], wherein the metal complex catalyst is an iridium complex catalyst containing a bipyridine ligand and a cyclopentadiene ligand in the ligand. [8] The method for producing a hydrogen storage compound according to any one of [1] to [7], wherein the electrolyte is at least one of sodium sulfate, potassium sulfate, potassium nitrate, and sodium chloride. [9] The method for producing a hydrogen storage compound according to any one of [1] to [8], wherein the electrolytic hydrogenation is carried out at a potential at which the metal complex catalyst is reduced and at a potential at which the ketone compound is not reduced.

[10] A method for producing hydrogen, comprising subjecting a secondary alcohol compound obtained by electrolytic hydrogenation of a ketone compound in water containing an electrolyte in the presence of a metal complex catalyst to a dehydrogenation reaction in the presence of a metal complex catalyst to obtain hydrogen. The method for producing hydrogen, wherein the ketone compound is at least one of a bicyclic monoketone compound and a polyvalent ketone compound.

[11] The method for producing hydrogen according to

[10] , comprising repeating a cycle including a step of electrolytic hydrogenation of the ketone compound obtained by the dehydrogenation reaction in water containing an electrolyte in the presence of a metal complex catalyst, and a step of subjecting the secondary alcohol compound obtained by the electrolytic hydrogenation to a dehydrogenation reaction in the presence of a metal complex catalyst to obtain hydrogen, a plurality of times.

[0007] In the present invention, the numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. [Advantages of the Invention]

[0008] According to the method for producing hydrogen of the present invention, the production of a hydrogen storage compound and the dehydrogenation reaction of the produced hydrogen storage compound can be promoted under mild conditions. [Brief Description of the Drawings]

[0009] [Figure 1]FIG. 1 is a cyclic voltammogram of the electrolytic solution a used in Example 1. [Figure 2] FIG. 2 is a cyclic voltammogram of the electrolytic solution b used in Example 1. [Figure 3] FIG. 3 is a cyclic voltammogram of the electrolytic solution c used in Example 1. [Figure 4] FIG. 4 is an overview of the 1H NMR chart used to identify the compound obtained in Example 1. [Mode for Carrying Out the Invention]

[0010] Preferred embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments other than as defined in the present invention.

[0011] [Method for Producing Hydrogen Storage Compound] The method for producing a hydrogen storage compound of the present invention includes electrohydrogenating a ketone compound in water containing an electrolyte in the presence of a metal complex catalyst to obtain a secondary alcohol compound (hydrogen storage compound). In the method for producing a hydrogen storage compound of the present invention, the ketone compound is at least one of a bicyclic monoketone compound and a polyvalent ketone compound. The method for producing a hydrogen storage compound of the present invention performs an electrohydrogenation reaction in the presence of a metal complex catalyst, and further, by adopting a specific ketone compound as a reaction substrate, electrohydrogenation can be carried out with high efficiency without using an organic solvent in the electrolytic solution. As a result, the environmental load can be reduced from both the viewpoints of reducing the amount of organic solvent used and energy efficiency. In addition, in the method for producing a hydrogen storage compound of the present invention, since a ketone compound can be directly converted into a secondary alcohol compound (hydrogen storage compound) by electrohydrogenation using water as a hydrogen source, a hydrogen gas production facility can be made unnecessary. The secondary alcohol compound obtained by the hydrogen storage compound production method of the present invention can be a low-molecular-weight alcohol compound with a relatively high boiling point. In this case, the secondary alcohol compound can be handled as a liquid at or near room temperature. Therefore, conventional transportation methods and facilities that rely on gasoline, etc., can be applied to the transportation and storage of the hydrogen storage compound. As described later, the method for producing hydrogen storage compounds of the present invention can be combined with a hydrogen generation method (dehydrogenation reaction) using a metal complex catalyst, and therefore it is possible to produce hydrogen storage compounds and extract hydrogen at low cost and with high efficiency.

[0012] The method for producing the hydrogen storage compound of the present invention will be described in more detail.

[0013] In the method for producing hydrogen storage compounds of the present invention, a specific ketone compound is hydrogenated by electrolytic hydrogenation to obtain a secondary alcohol compound which is a hydrogen storage compound. In the method for producing hydrogen storage compounds of the present invention, the electrolytic hydrogenation mechanism of the ketone compound is presumed to be as follows: After catalytic active species I is electrochemically reduced, catalyst species II is generated by hydrogenation using water as a hydrogen source, and a secondary alcohol compound is obtained by hydrogen transfer from catalyst species II to the ketone compound via transition state III (Scheme SS). In Scheme SS, R1 and R2 are substituents bonded at carbon atoms.

[0014] [ka]

[0015] In this invention, the electrolytic hydrogenation of the ketone compound is carried out in water. In other words, the hydrogen source for the hydrogenation reaction is water, and hydrogen can be extracted and stored from the inexhaustible supply of water.

[0016] The specific method of electrolytic hydrogenation is not particularly limited, other than using the ketone compound specified in the present invention, using a metal complex catalyst, and performing the process in water. Chronoamperometry, chronopotentiometry, bulk electrolysis, etc., can be used. In the present invention, chronoamperometry is preferred.

[0017] The electrolyte concentration in the electrolyte solution is not particularly limited. The electrolyte concentration in the electrolyte solution is preferably 1 to 5000 mM, more preferably 100 to 2000 mM, and even more preferably 100 to 1000 mM.

[0018] The pH of the electrolyte is not particularly limited. For example, the pH of the electrolyte can be 1 to 14, 5 to 14, or 10 to 14.

[0019] The applied potential during electrolytic hydrogenation is not particularly limited as long as the metal complex catalyst can be reduced. Preferably, this applied potential is such that the ketone compound is not reduced, but the metal complex catalyst is reduced. The applied potential at which the ketone compound is not reduced and the metal complex catalyst is reduced can be determined by the method described in the examples. The applied potential depends on the type of ketone compound and metal complex catalyst used, but is preferably -2 to -0.5V, more preferably -1.5 to -0.8V, and even more preferably -1.2 to -0.8V. The applied Coulomb number during electrolytic hydrogenation is preferably 100 to 200, more preferably 100 to 150, and even more preferably 100 to 110, relative to the theoretically required Coulomb number of 100 for hydrogenating ketone compounds with a 100% reaction conversion rate. Increasing the applied Coulomb number can improve the reaction conversion rate of ketone compounds. The applied Coulomb number during electrolytic hydrogenation can be controlled by adjusting the current value and reaction time. The current value can also be controlled by adjusting the reaction temperature, catalyst amount, electrode area, etc.

[0020] In electrolytic hydrogenation, the amount of metal complex catalyst can be appropriately set according to the type and amount of ketone compound, reaction time, reaction temperature, etc. For example, the molar ratio of the metal complex catalyst to the ketone compound can be [metal complex catalyst] / [ketone compound] = 1 / 1000 to 1 / 5, 1 / 100 to 1 / 5, 1 / 70 to 1 / 5, or 1 / 20 to 1 / 5.

[0021] There are no particular restrictions on the temperature during electrolytic hydrogenation. Electrolytic hydrogenation can be carried out, for example, at 10-50°C or 20-40°C. The reaction time for the electrolytic hydrogenation described above is not particularly limited. The reaction time for electrolytic hydrogenation can be, for example, 1 to 24 hours, 1 to 12 hours, or 1 to 6 hours.

[0022] After electrolytic hydrogenation, the resulting hydrogen storage compound (secondary alcohol compound) may or may not be separated from water. The secondary alcohol compound can be separated, for example, by volatilizing the water.

[0023] The metal complex catalyst after electrolytic hydrogenation can be separated from the solvent by liquid-liquid separation, extraction, column chromatography, etc. The separated metal complex catalyst can be reused as is. If the metal complex catalyst has been degraded, for example, by adding ligands to regenerate the complex structure, it can be reused in the dehydrogenation reaction.

[0024] The following describes the materials used in the method for producing the hydrogen storage compound of the present invention.

[0025] (Ketone compounds) The ketone compound is not particularly limited as long as it is at least one of a bicyclomonoketone compound and a polyvalent ketone compound. In this invention, the ketone compound is a compound that does not have a hydroxyl group.

[0026] The bicyclomonoketone compounds described above are monoketone compounds having a ring structure in which two atoms constituting the ring are linked by bonds outside the ring to form a bridge (i.e., monoketone compounds having two bridgehead atoms and three bridges connecting them). The bicyclomonoketone compound preferably has 4 to 10 ring-constituting atoms, including the atoms forming the bridge, more preferably 5 to 9, and even more preferably 6 to 8. The bicyclomonoketone compound described above preferably has 3 to 10 carbon atoms, more preferably 4 to 9, and even more preferably 4 to 8. The above bicyclomonoketone compounds may contain heteroatoms (oxygen, nitrogen, and sulfur atoms) as ring constituent elements, and it is preferable that they contain an oxygen atom. Here, heteroatoms contained in the ketone group are not considered ring constituent elements. The above bicyclomonoketone compounds preferably have 6 to 8 ring constituent atoms and contain one or two oxygen atoms as ring constituent elements. The molecular weight of the above bicyclomonoketone compound is preferably 120 to 180, more preferably 120 to 160, and even more preferably 120 to 140. Bicyclomonoketone compounds are preferably free of aromatic groups. Bicyclomonoketone compounds may have substituents on the ring atoms. Examples of substituents include alkyl groups having 1 to 10 carbon atoms (preferably 1 to 8 carbon atoms, more preferably 1 to 6 carbon atoms, even more preferably 1 to 4 carbon atoms, and even more preferably methyl or ethyl carbon atoms), and groups having an active hydrogen group (-OH, -NH2, or -SH) (preferably a hydroxyl group). Bicyclomonoketone compounds are preferred, having 4 to 10 ring constituent atoms and containing an oxygen atom as a ring constituent element. Specific examples of bicyclomonoketone compounds include, for example, dihydrolevoglucocenone (Silene®), camphor, 3-methylene-2-norbornanone, and 8-oxabicyclo[3,2,1]octan-3-one. Among these, dihydrolevoglucocenone is preferable from the viewpoint of reducing environmental impact because it is a cellulose-derived compound and is biodegradable.

[0027] As for the polyvalent ketone compound, compounds having 1 to 5 ketone groups are preferred, compounds having 1 to 4 are more preferred, compounds having 1 to 3 are even more preferred, and compounds having 2 (diketone compounds) are even more preferred. The polyvalent ketone compound may be linear or cyclic, with linear diketone compounds being preferred. It is preferable that the polyvalent ketone compound does not have an aromatic ring within the molecule. The polyvalent ketone compound is preferably a ketone compound having 4 to 10 carbon atoms, more preferably a ketone compound having 4 to 8 carbon atoms, even more preferably a ketone compound having 4 to 7 carbon atoms, and even more preferably a ketone compound having 4 to 5 carbon atoms. The polyvalent ketone compound preferably has a molecular weight of 80 to 170, more preferably 80 to 150, even more preferably 80 to 130, even more preferably 80 to 120, and even more preferably 100 to 130. The polyvalent ketone compound may have substituents. Examples of substituents include alkyl groups having 1 to 10 carbon atoms (preferably 1 to 8 carbon atoms, more preferably 1 to 6 carbon atoms, even more preferably 1 to 4 carbon atoms, and even more preferably methyl or ethyl), and groups having an active hydrogen group (-OH, -NH2, or -SH) (preferably a hydroxyl group). From the viewpoint of increasing the mass hydrogen density through hydrogenation reactions, polyvalent ketone compounds are preferred if they have a molecular weight of 80 to 120 and contain two ketone groups (diketone compounds). Diketone compounds are preferred because they have a high boiling point and low volatility, making them easy to handle. Examples of diketone compounds include α-diketone compounds, β-diketone compounds, γ-diketone compounds, and δ-diketone compounds. The diketone compound is preferably a linear diketone compound. From the viewpoint of reaction conversion rate, linear γ-diketone compounds are preferred. The diketone compound is preferably a linear diketone compound having 4 to 10 carbon atoms, more preferably a linear diketone compound having 4 to 8 carbon atoms, even more preferably a linear diketone compound having 4 to 7 carbon atoms, and even more preferably a linear diketone compound having 4 to 5 carbon atoms. Linear γ-diketone compounds having these carbon number configurations are even more preferred. Examples of polyvalent ketone compounds include acetylacetone, 2,5-hexanedione, and 1,4-cyclohexanedione.

[0028] The ketone compound is preferably at least one of a bicyclomonoketone compound and a diketone compound, more preferably at least one of a bicyclomonoketone compound and a β-diketone compound, and even more preferably at least one of dihydrolevoglucocenone, acetylacetone, 2,5-hexanedione, and 1,4-cyclohexanedione.

[0029] (Secondary alcohol compounds) Secondary alcohol compounds are compounds obtained by hydrogenating the above-mentioned ketone compounds. Therefore, their preferred structure is the same as that of the ketone compound, except that the carbonyl group in the ketone group of the ketone compound is converted to a hydroxyl group. For this reason, secondary alcohol compounds corresponding to bicyclomonoketone compounds are sometimes called bicyclomonoalcohol compounds, and secondary alcohol compounds corresponding to polyhydric ketone compounds are sometimes called polyol compounds. For example, the compound corresponding to a diketone compound is a diol compound. In the present invention, a secondary alcohol compound may be both a secondary alcohol compound and a ketone compound. That is, a secondary alcohol compound may have a hydroxyl group bonded to the central carbon atom of three consecutive carbon atoms (a hydroxyl group in a configuration that can be oxidized to a ketone group), as well as an additional ketone group. In the present invention, such a compound is also referred to as a "secondary alcohol compound." The secondary alcohol compound obtained by the above electrolytic hydrogenation is usually a secondary monoalcohol compound, regardless of the number of ketone groups present in the ketone compound.

[0030] (Metal complex catalyst) The metal complex catalyst is not particularly limited as long as it can catalyze the reaction to obtain a secondary alcohol compound by the electrolytic hydrogenation of the ketone compound. Preferably, the metal complex catalyst also functions as a catalyst for the reaction to obtain a ketone compound and hydrogen by the dehydrogenation of the secondary alcohol compound. The central metal of the metal complex catalyst is preferably one of the following: iron, copper, vanadium, cobalt, osmium, rhodium, manganese, nickel, iridium, ruthenium, platinum, palladium, etc., with iridium being more preferred. The ligands for metal complex catalysts include aqua ligands, hydroxide ligands, amine ligands (e.g., aniline, toluidine, anisidine, etc.), diamine ligands (e.g., o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, etc.), pyridine ligands (hydroxypyridine), bipyridine ligands (e.g., 6,6'-dihydroxy-2,2'-bipyridine, 2,2'-bipyridine-6,6'-dionato, 4,4'-bis(dimethylamino)-2,2'-bipyridine-6,6'-dionato, etc.), and acetylacetone ligands (acetyl It is preferable that the ligand is one or a combination thereof of the following: a ruacetonate ligand, a porphyrin ligand, a Schiff base ligand, a phosphine ligand (e.g., triphenylphosphine, trimethylphosphine, triethylphosphine, tributylphosphine, tri-tert-butylphosphine, tricyclohexylphosphine, triethoxyphosphine, etc.), a sulfoxide ligand (e.g., dimethyl sulfoxide, etc.), a benzene ligand, a cyclopentadiene ligand (e.g., cyclopentadiene, pentamethylcyclopentadiene, etc.). The metal complex catalyst may also be in the form of a salt. For example, lithium, sodium, potassium, triflat anion (CF3SO3 - ) or salt may be used. The metal complex catalyst is preferably an iridium complex catalyst containing a bipyridine ligand. Furthermore, it is also preferable to have both a bipyridine ligand and a cyclopentadiene ligand. For example, it is more preferable to have an iridium complex catalyst having aqua, 6,6'-dihydroxy-2,2'-bipyridine, and pentamethylcyclopentadiene; an iridium complex catalyst having aqua, 2,2'-bipyridine-6,6'-dionato, and pentamethylcyclopentadiene; and an iridium complex catalyst having aqua, 4,4'-bis(dimethylamino)-2,2'-bipyridine-6,6'-dionato, and pentamethylcyclopentadiene.

[0031] (electrolyte) The electrolyte used is not particularly limited, as long as it is an electrolyte that dissolves in water. Acids, bases, salts, etc., can be used as electrolytes. For example, acids such as nitric acid, sulfuric acid, hydrochloric acid, and their salts (e.g., sulfates such as sodium sulfate and potassium sulfate) can be used. For example, sodium hydroxide and potassium hydroxide can be used as bases. In the present invention, it is preferable that the electrolyte is at least one of sodium sulfate, potassium sulfate, potassium nitrate, and sodium chloride.

[0032] [Hydrogen production method] The present invention provides a method for producing hydrogen, which includes obtaining a secondary alcohol compound by electrolytically hydrogenating a specific ketone compound in water containing an electrolyte in the presence of a metal complex catalyst, and then subjecting the resulting secondary alcohol compound to a dehydrogenation reaction in the presence of a metal complex catalyst to obtain hydrogen. In this hydrogen production method, the ketone compound is at least one of a bicyclomonoketone compound and a polyvalent ketone compound. That is, the hydrogen production method of the present invention includes a form in which a hydrogen storage compound produced by the hydrogen storage compound production method of the present invention is subjected to a dehydrogenation reaction in the presence of a metal complex catalyst to obtain hydrogen. The hydrogen production method of the present invention may also be configured to repeat multiple cycles that include the steps of electrolytically hydrogenating a ketone compound obtained by a dehydrogenation reaction in water containing an electrolyte in the presence of a metal complex catalyst, and subjecting the secondary alcohol compound obtained by the electrolytic hydrogenation to a dehydrogenation reaction in the presence of a metal complex catalyst to obtain hydrogen. The hydrogen production method of the present invention allows for the use of milder conditions for the dehydrogenation reaction compared to conventional organic hydrides.

[0033] The electrolytic hydrogenation process is as described in the description of the method for producing the hydrogen storage compound of the present invention. The secondary alcohol compounds obtained by electrolytic hydrogenation can be stored. Therefore, the hydrogen production method of the present invention may include a storage step for the secondary alcohol compounds. Furthermore, secondary alcohol compounds can be transported. Therefore, the hydrogen production method of the present invention may include a step of transporting secondary alcohol compounds. Because the hydrogen storage compound is a secondary alcohol compound, it offers excellent handling advantages during storage and transportation, and allows for the storage and transportation of hydrogen using existing transportation methods and facilities.

[0034] The present invention provides a method for producing hydrogen by subjecting the secondary alcohol compound obtained as described above to a dehydrogenation reaction in the presence of a metal complex catalyst. Through the dehydrogenation reaction, hydrogen is released from the secondary alcohol compound. During this process, the secondary alcohol compound is oxidized to a ketone compound.

[0035] The above dehydrogenation reaction may or may not use a solvent. When a solvent is used in the above dehydrogenation reaction, the solvent may be water or an organic solvent. Examples of organic solvents include pentane, hexane, heptane, benzene, toluene, xylene, tetrahydrofuran, diisopropyl ether, dichloromethane, methylformamide, 1-butanol, and γ-valerolactone. From the viewpoint of reducing environmental impact, it is preferable to carry out the reaction without a solvent, or to use water or γ-valerolactone, which is a solvent derived from cellulose.

[0036] The metal complex catalyst used in the dehydrogenation reaction can be appropriately selected from the metal complex catalysts described as metal complex catalysts used in electrolytic hydrogenation. In the hydrogen production method of the present invention, the metal complex catalyst used in electrolytic hydrogenation and the metal complex catalyst used in the dehydrogenation reaction may be the same metal complex catalyst or different metal complex catalysts.

[0037] The amount of metal complex catalyst in the above dehydrogenation reaction can be appropriately set depending on the type and amount of secondary alcohol compound, reaction time, reaction temperature, etc. For example, the molar ratio of the metal complex catalyst to the secondary alcohol compound can be [metal complex catalyst] / [secondary alcohol compound] = 1 / 1000 to 1 / 5, 1 / 100 to 1 / 5, 1 / 70 to 1 / 5, or 1 / 20 to 1 / 5.

[0038] The temperature of the above dehydrogenation reaction is not particularly limited. The dehydrogenation reaction can be carried out, for example, at 100-250°C, or at 150-230°C. From the viewpoint of increasing the efficiency of the dehydrogenation reaction, a high temperature reaction is preferred. It should be noted that the dehydrogenation reaction of methylcyclohexane, a well-known organic hydride, usually requires a high temperature of around 300-400°C. The reaction time for the above dehydrogenation reaction is not particularly limited. The reaction time for the dehydrogenation reaction can be, for example, 1 to 24 hours, 1 to 12 hours, or 1 to 6 hours.

[0039] The pH of the reaction solution that produces the above dehydrogenation reaction is not particularly limited. The pH of this reaction solution can be, for example, 1 to 14, 5 to 14, or 10 to 14.

[0040] In the hydrogen production method of the present invention, the hydrogen gas obtained by the above dehydrogenation reaction can be recovered by conventional methods. After the dehydrogenation reaction described above, the ketone compound may or may not be separated from the solvent. If the solvent is water, it may be subjected to a hydrogenation reaction without separation to convert it into a secondary alcohol compound. The ketone compound produced by the above dehydrogenation reaction can be subjected to the hydrogenation reaction again, as described above. That is, it can be used as a ketone compound in the hydrogen production method of the present invention. Therefore, in one embodiment of the hydrogen production method of the present invention, the ketone compound produced along with hydrogen by the dehydrogenation reaction can be stored as a raw material for the hydrogenation reaction. Accordingly, the hydrogen production method of the present invention may include a step of storing the ketone compound. Furthermore, the ketone compounds produced along with hydrogen by the above dehydrogenation reaction can be transported. Therefore, the hydrogen production method of the present invention may include a step for transporting ketone compounds. When a solvent is used in the above dehydrogenation reaction, the metal complex catalyst after the reaction can be separated from the solvent by liquid-liquid separation and extraction, column chromatography, etc. The separated metal complex catalyst can be reused as is. Furthermore, if the metal complex catalyst has been degraded, for example, a ligand can be added to regenerate the structure of the complex, and it can be used again in the dehydrogenation reaction.

[0041] In the hydrogen production method of the present invention, when a cycle including electrolytic hydrogenation and dehydrogenation is repeated multiple times, ketone compounds and / or secondary alcohol compounds, as well as metal complex catalysts, can be appropriately replenished during the cycle.

[0042] The present invention will be described in more detail based on examples, but the present invention is not to be limited to these examples. [Examples]

[0043] [material] Dihydrolevoglucocenone (Cyrene®): Manufactured by Sigma-Aldrich. Acetylacetone: Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. 2,5-Hexanedione: Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. 1,4-Cyclohexanedione: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,6-Anhydro-3,4-dideoxy-β-D-threo-hexopyranose: Prepared as follows. 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose was prepared by reducing dihydrolevoglucocenone with LiAlH4 (Tokyo Chemical Industries) using anhydrous diethyl ether (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a solvent, and then purifying it by column chromatography. 2,4-Pentanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 2,5-Hexanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,4-Cyclohexanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. Catalyst 1 in schemes S1 and S2, and Catalyst 2 in scheme S4: Aqua(6,6'-dihydroxy-2,2'-bipyridine)(pentamethylcyclopentadienyl)iridium(III)bis(triflate) (manufactured by Kanto Chemical Co., Ltd.) Catalyst 2 in Scheme S1, and Catalyst 1 in Schemes S3-S5: (Aqua(2,2'-bipyridine-6,6'-dionato)(pentamethylcyclopentadienyl)iridium(III)) (manufactured by Kanto Chemical Co., Ltd.) Toluene: Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. γ-Valerolactone: Manufactured by Tokyo Chemical Industry Co., Ltd.

[0044] [Preparation of hydrogen storage compounds (secondary alcohol compounds) (hydrogenation)] Ketone molecules were electrolyzed in water containing an electrolyte in the presence of a metal complex catalyst (see Scheme S1 below). The specific experimental procedure is as follows. The results are shown in Table 1.

[0045] [ka]

[0046] In the above scheme S1, OTf - is the triflat anion (CF3SO3 - This means (the same applies to schemes S2 and S4 below). In scheme S1 above, R1 and R2 in (R1)(R2)C=O (ketone compound) are substituents bonded to a carbon atom, and specifically correspond to the substituents shown in Table 1 (the same applies to schemes S2 to S5 below).

[0047] (Example 1) (1) Determination of applied potential The applied potential for the electrolytic hydrogenation of dihydrolevoglucocenone was determined as follows. Three types of electrolytes a to c were prepared, and oxygen was removed from each electrolyte by bubbling with argon (Ar). Electrolyte a: An aqueous solution containing 10 mM dihydrolevoglucocenon and 100 mM Na2SO4. Electrolyte b: An aqueous solution containing 1 mM of catalyst 1 and 100 mM of Na2SO4. Electrolyte c: An aqueous solution containing 1 mM of catalyst 1, 20 mM of dihydrolevoglucocenone, and 100 mM of Na2SO4. For each of the electrolytes a to c described above, under Ar flow conditions, a potentiostat (electrochemical measurement system HZ-7000 (MODEL: HAG1232m) (product name), manufactured by Meiden Hokuto Co., Ltd.) was used in a three-electrode system consisting of a carbon electrode (working electrode), a platinum electrode (counter electrode), and a silver / silver chloride electrode (reference electrode) to measure the initial potential at 0V, the reversal potential at -1.5V, and the sweep rate at 10mV·s. -1 The cyclic voltammograms (CV, current-potential curves) were obtained by sweeping. The above measurements were performed at room temperature (25°C). Figure 1 is the cyclic voltammogram of electrolyte a, Figure 2 is the cyclic voltammogram of electrolyte b, and Figure 3 is the cyclic voltammogram of electrolyte c. An enlarged view of the cyclic voltammogram in Figure 3, around the potential range of -1.1 to -0.7 V, is shown in conjunction with Figure 3. In the CV waveform of electrolyte a (Figure 1), no reduction wave originating from dihydrolevoglucocenone was observed (no current flowed). In the CV waveforms of electrolytes b and c (Figures 2 and 3), a reduction wave originating from catalyst 1 was observed. From the enlarged view shown in Figure 3, it can be seen that the reduction of catalyst 1 proceeded at potentials lower than -0.80V, and the reduction of substances other than catalyst 1 also proceeded at potentials lower than -0.90V. Therefore, the potential at which only catalyst 1 is reduced is -0.90 to -0.80V, and in order to aim for high Coulomb efficiency and current value, the applied potential for electrolytic hydrogenation of dihydrolevoglucocenone was set to -0.90V.

[0048] (2) Electrolytic hydrogenation The electrolytic hydrogenation of dihydrolevoglucocenone was carried out according to scheme S2 as follows.

[0049] [ka]

[0050] An electrolyte solution containing 0.8 mmol (20 mM) of dihydrolevoglucosenone, 1 mmol of catalyst 1, and sodium sulfate (Na2SO4) (100 mM) in water was prepared, and oxygen in the electrolyte solution was removed by bubbling with Ar. Then, under an Ar flow, chronoamperometry was used to perform constant potential electrolysis at -0.90 V to electrohydrogenate dihydrolevoglucosenone (Scheme S2). At this time, the applied coulomb number can be calculated by integrating the current value over the reaction time. When 10 coulomb numbers were applied with respect to 100, the coulomb number required to hydrogenate 0.8 mmol of dihydrolevoglucosenone with a reaction conversion rate of 100%, the reaction was stopped. Also, the coulomb number required to hydrogenate 0.8 mmol of dihydrolevoglucosenone with a reaction conversion rate of 100% is 154.4 C. The coulomb number required to hydrogenate n moles of a certain ketone compound with a reaction conversion rate of 100% is n × 2 × [Faraday constant: 96500 (C / mol)]. After electrohydrogenation, the 1 1H NMR of the reaction solution was measured at 400 MHz, and the components obtained in the reaction solution were identified by comparing with the 1 1H NMR of dihydrolevoglucosenone and 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose. In Figure 4, (a) is the TM 1H NMR of dihydrolevoglucosenone (described as "Cyrene" in Figure 4), (b) is the 1 1H NMR of the reaction solution, and (c) is the 1 1H NMR of 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose (Cyrene-OH in Figure 4). 1 1H NMR measurement was performed using deuterated dimethyl sulfoxide (DMSO-d6) as the solvent. From Figure 4, the formation of 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose in the reaction solution was confirmed. In Figure 4, only the shape of the chart of each component is shown, and details such as the vertical axis are omitted. 1 When performing 1H NMR measurement, deuterated dimethyl sulfoxide (DMSO-d6) was used as the solvent. From Figure 4, the formation of 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose in the reaction solution was confirmed. In Figure 4, only the shape of the chart of each component is shown, and details such as the vertical axis are omitted. Also, the reaction solution 1 The reaction conversion rate and Coulomb efficiency were calculated from the 1H NMR measurement results using the following formula. The reaction conversion rate was 8.2%, and the Coulomb efficiency was 82%. Reaction conversion rate = 100 × [moles of secondary alcohol compound] / ([moles of secondary alcohol compound] + [moles of ketone compound]) Coulomb efficiency = [Reaction conversion rate] / ([Applied number of Coulombs] / [Number of Coulombs required to hydrogenate 0.8 mmol of dihydrolevoglucocenone with a reaction conversion rate of 100%]) In the above test, in order to determine the Coulomb efficiency, the reaction was deliberately stopped when 10 Coulombs were applied, compared to the 100 Coulombs required to hydrogenate 0.8 mmol of dihydrolevoglucocenone with a 100% conversion rate. The concentration of the resulting secondary alcohol compound was then... 1 The concentration is set to one that can be measured by 1H NMR. However, if the purpose is simply to produce a hydrogen storage compound, the applied Coulomb number can be appropriately set based on the preferred range mentioned above.

[0051] (Examples 2-6) The applied potential was determined in the same manner as in Example 1 (1) above, except that the ketone compound and catalyst were changed to those shown in Table 1. Furthermore, the electrolytic hydrogenation of the ketone compound was performed using this applied potential in the same manner as in Example 1 (2), and the Coulomb efficiency was determined.

[0052] [Table 1]

[0053] Examples 1-6 reveal the following: In the presence of a metal complex catalyst, dihydrolevoglucocenone, acetylacetone, 2,5-hexanedione, and 1,4-cyclohexanedione could all be electrolytically hydrogenated in water containing sodium sulfate to obtain the corresponding secondary alcohol compounds. Therefore, it can be seen that the hydrogen storage compound production method of the present invention allows for the production of hydrogen storage compounds under mild conditions without the use of organic solvents. Furthermore, since the Coulombic efficiency is high when using dihydrolevoglucocenone and acetylacetone, it can be seen that hydrogen storage compounds can be produced more efficiently when using bicyclomonoketone compounds and β-diketone compounds.

[0054] [Hydrogen production (dehydrogenation reaction): Use of organic solvents] Using the above metal complex catalyst, secondary alcohol compounds were subjected to a dehydrogenation reaction in an organic solvent (see scheme S3 below). The experimental procedure was as follows: 1 The reaction conversion rate was calculated using the following formula after 1H NMR measurement. The results are shown in Table 2. Reaction conversion rate = 100 × [moles of ketone compound] / ([moles of secondary alcohol compound] + [moles of ketone compound])

[0055] (Example 7) Under air, 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose (1 mmol) was used as the secondary alcohol compound, and it was added to a flask along with toluene (5 mL) as the solvent and catalyst 1 (20 molar amounts of catalyst 1 per 100 molar amounts of the secondary alcohol compound). The reaction mixture was then heated and stirred under reflux at 150°C for 24 hours to dehydrogenate the secondary alcohol compound, yielding dihydrolevoglucocenone and hydrogen.

[0056] (Examples 8-11) Except for changing the secondary alcohol compound and solvent to those shown in Table 2, using the amount of catalyst shown in Table 2, and using the reaction temperature and reaction time shown in Table 2, the ketone compound and hydrogen shown in Table 2 were obtained in the same manner as in Example 7 above.

[0057] [ka]

[0058] [Table 2]

[0059] Examples 7-11 show the following: Using 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose, ketone compounds and hydrogen can be obtained by dehydrogenation in the presence of an iridium complex catalyst, regardless of the type of organic solvent (Examples 7 and 8). In particular, by using γ-valerolactone, a solvent derived from cellulose, as the organic solvent and subjecting the reaction to high temperature, a reaction conversion rate of 100% was achieved (Example 8). Examples 9-11 use secondary dialcohol compounds instead of secondary monoalcohol compounds as the secondary alcohol compounds. However, considering that the corresponding diketone compounds are obtained from secondary dialcohol compounds, it can be seen that the dehydrogenation reaction will similarly occur and the corresponding diketone compounds will be obtained even when secondary monoalcohol compounds are used as the starting material. It can be inferred that the reaction conversion rate when secondary monoalcohol compounds are used as the starting material is substantially the same as the reaction conversion rate when secondary dialcohol compounds are used, as shown in Table 2 above. The same applies to Examples 12-17 described later. Furthermore, when toluene was used as the solvent, 2,4-pentanediol, 2,5-hexanediol, and 1,4-cyclohexanediol showed higher reaction conversion rates compared to 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose. However, in Examples 10 and 11, it is believed that a 100% reaction conversion rate can be achieved for 2,5-hexanediol and 1,4-cyclohexanediol as well by replacing the solvent with a high-boiling point solvent such as γ-valerolactone and subjecting the reaction to a high temperature. In other words, by hydrogenating 2,5-hexanedione as a chain-like diketone compound to obtain 2,5-hexanediol, and then subjecting it to a dehydrogenation reaction in the presence of a metal complex catalyst to obtain hydrogen, both the hydrogenation and dehydrogenation reactions can be carried out with virtually 100% reaction conversion rates.

[0060] [Hydrogen production (dehydrogenation): Water is used] A secondary alcohol compound was dehydrogenated in water using a metal complex catalyst (see scheme S4 ​​below). In scheme S4, OTf - is the triflat anion (CF3SO3 - This means ). The experimental procedure is as follows. The results are shown in Table 3.

[0061] (Example 12) Under air, 2,4-pentanediol (1 mmol) was used as secondary alcohol compound A1 and added to a flask with water (5 mL) and catalyst 1 (5 molars of catalyst 1 per 100 molars of secondary alcohol compound A1). The reaction mixture was then heated and stirred under reflux at 150°C for 24 hours to dehydrogenate secondary alcohol compound A1, yielding secondary alcohol compound A2, ketone compound K1, and hydrogen as shown in Table 3.

[0062] (Examples 13-14) Except for changing the secondary alcohol compound A1 and catalyst to those shown in Table 3, the same procedure as in Example 12 above was used to obtain the secondary alcohol compound A2, ketone compound K1, and hydrogen shown in Table 3.

[0063] [ka]

[0064] [Table 3]

[0065] Examples 12-14 show that the dehydrogenation reaction proceeds in the presence of an iridium complex catalyst even when water is used as the solvent. Furthermore, it is found that catalyst 1 is more suitable than catalyst 2 for increasing the efficiency of producing ketone compound K1 in which all hydroxyl groups are oxidized to carbonyl groups. It is also found that the reaction using 2,5-hexanediol (the reaction to obtain the chain-like γ-diketone compound 2,5-hexanedione) has a higher reaction conversion rate than the reaction using 2,4-pentanediol. Furthermore, in the case of 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose, it has been confirmed that the catalytic reaction does not proceed sufficiently in water when catalysts 1 and 2 are used.

[0066] [Hydrogen production (dehydrogenation): No solvent used] Secondary alcohol compounds were dehydrogenated under solvent-free conditions using a metal complex catalyst (see scheme S5 below). The experimental procedure is as follows. The results are shown in Table 4.

[0067] (Example 15) Under air, 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose (10 mmol) as a secondary alcohol compound and catalyst 1 (10 molars of catalyst 1 per 100 molars of secondary alcohol compound) were added to a flask. The reaction mixture was then heated and stirred under reflux at 240°C for 9 hours to dehydrogenate the secondary alcohol compound, yielding dihydrolevoglucocenone and hydrogen.

[0068] (Examples 16 and 17) Except for using the compounds shown in Table 4 as secondary alcohol compounds in the amounts indicated in Table 4, and changing the molar amount of catalyst, reaction temperature, and reaction time as shown in Table 4, the dehydrogenation reaction was carried out in the same manner as in Example 15, yielding the ketone compounds and hydrogen shown in Table 4, respectively.

[0069] [ka]

[0070] [Table 4]

[0071] Examples 15-17 show that 1,6-anhydro-3,4-dideoxy-β-D-threo-hexopyranose, 2,4-pentanediol, and 2,5-hexanediol can all be dehydrogenated by a dehydrogenation reaction in the presence of an iridium complex catalyst, even without the use of a solvent. Furthermore, Examples 1 to 17 show that hydrogen can be obtained by electrolyzing a ketone compound in water containing an electrolyte in the presence of a metal complex catalyst, and then subjecting the resulting secondary alcohol compound to a dehydrogenation reaction in the presence of a metal complex catalyst.

Claims

1. A method for producing a hydrogen storage compound, comprising electrolytically hydrogenating a ketone compound in water containing an electrolyte in the presence of a metal complex catalyst to obtain a secondary alcohol compound, A method for producing a hydrogen storage compound, wherein the ketone compound is at least one of a bicyclomonoketone compound and a polyvalent ketone compound.

2. The method for producing a hydrogen storage compound according to claim 1, wherein the ketone compound is at least one of a bicyclomonoketone compound and a diketone compound.

3. The method for producing a hydrogen storage compound according to claim 1, wherein the ketone compound is at least one of a bicyclomonoketone compound and a β-diketone compound.

4. The method for producing a hydrogen storage compound according to claim 1, wherein the ketone compound is at least one of dihydrolevoglucocenone, acetylacetone, 2,5-hexanedione, and 1,4-cyclohexanedione.

5. The method for producing a hydrogen storage compound according to claim 1, wherein the metal complex catalyst is an iridium complex catalyst.

6. The method for producing a hydrogen storage compound according to claim 5, wherein the metal complex catalyst is an iridium complex catalyst containing a bipyridine ligand as a ligand.

7. The method for producing a hydrogen storage compound according to claim 5, wherein the metal complex catalyst is an iridium complex catalyst containing a bipyridine ligand and a cyclopentadiene ligand as ligands.

8. The method for producing a hydrogen storage compound according to claim 1, wherein the electrolyte is at least one of sodium sulfate, potassium sulfate, potassium nitrate, and sodium chloride.

9. The method for producing a hydrogen storage compound according to claim 1, wherein the electrolytic hydrogenation is carried out at a potential that reduces the metal complex catalyst and does not reduce the ketone compound.

10. A method for producing hydrogen, comprising subjecting a secondary alcohol compound obtained by electrolytic hydrogenation of a ketone compound in water containing an electrolyte in the presence of a metal complex catalyst to a dehydrogenation reaction in the presence of a metal complex catalyst to obtain hydrogen, A method for producing hydrogen, wherein the ketone compound is at least one of a bicyclomonoketone compound and a polyvalent ketone compound.

11. A method for producing hydrogen according to claim 10, comprising repeating a cycle multiple times that includes the steps of electrolytically hydrogenating a ketone compound obtained by the dehydrogenation reaction in water containing an electrolyte in the presence of a metal complex catalyst, and subjecting a secondary alcohol compound obtained by the electrolytic hydrogenation to a dehydrogenation reaction in the presence of a metal complex catalyst to obtain hydrogen.

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