Patent Description:
Hydrogen (H<NUM>) has been produced in an amount of about five hundred billion Nm<NUM> all over the world. The hydrogen has attracted much attention as future clean energy as well as has been applied for a variety of uses such as refinement of oil or production of ammonia. For example, a fuel cell is capable of efficiently supplying electricity when the hydrogen is supplied externally thereto. However, the hydrogen is highly reactive gas, so that it is difficult to be transported and stored. Therefore, there has been a need for a safe and inexpensive transportation and storage technology in order to stably supply the hydrogen. In the field of the fuel cell, there has been a problem that a poisoning substance is by-produced on a surface of an electrode catalyst by the action of carbon monoxide. Thus, there has been a need to supply high purity hydrogen generally containing <NUM> ppm or less of carbon monoxide.

As a hydrogen storage method, at present, a method for storing hydrogen as high pressure gas in a gas cylinder is commonly used. However, in this method, there are problems of safety upon transportation of the high pressure gas, and hydrogen brittleness of container materials. A method for storing hydrogen gas in the form of liquid hydrogen under an extremely low temperature is also used. However, there are problems that much energy is consumed in a liquefaction process and that the liquid hydrogen is lost in a percentage of <NUM>% per day to <NUM>% per day due to vaporization.

In order to solve the above described problems with regard to hydrogen transportation and storage technologies, there has been considered a method for storing hydrogen as liquid fuel (e.g., methanol and formic acid) which is obtained by hydrogenating carbon dioxide. For example, formic acid (HCOOH) has recently been attracted the attention as a hydrogen storage material since the formic acid, which is in the liquid form at normal temperature and has a relatively low toxicity, can be reversibly converted to hydrogen (H<NUM>) and carbon dioxide (CO<NUM>). However, there has been a problem that a dehydrogenation reaction of the formic acid using a conventionally known catalyst generally requires a high temperature of <NUM> or higher, and generates carbon monoxide as a by-product. Therefore, there has been a need to develop a catalyst which allows high quality hydrogen to be produced from the formic acid under a mild condition.

Recently, many reports have been made with regard to a dehydrogenation reaction of formic acid using a metal complex catalyst (PTLs <NUM>, <NUM> and <NUM> and NPLs <NUM> to <NUM>). In these reactions, although hydrogen is produced through dehydrogenation of formic acid, carbon monoxide is hardly by-produced. However, most of them need organic solvents or amine additives. On the other hand, a reaction in water free of organic additives is problematic in low catalytic activity and durability (PTLs <NUM> to <NUM>, NPLs <NUM> to <NUM>). Besides the above reports, the present inventors have been found catalysts which are extremely highly active in the dehydrogenation reaction of formic acid in water free of organic additives. <CIT> (PTL <NUM>), for example, discloses a bipyridine complex catalyst which contains a bidentate ligand including a <NUM>-membered ring skeleton. However, these catalysts are problematic in durability because they are easily decomposed in a high-concentration formic acid solution or under a high temperature reaction condition (PTLs <NUM> to <NUM>, NPLs <NUM> to <NUM>).

The present invention has an object to provide a catalyst allowing hydrogen to be produced through dehydrogenation of formic acid in a highly efficient, highly energy-efficient, highly selectively, and highly durable manner even in a high-concentration aqueous formic acid solution under a high temperature reaction condition.

The present invention also has an object to provide a method for producing hydrogen through dehydrogenation of formic acid using the catalyst in a highly efficient and inexpensive manner with simple operation.

The present inventors conducted extensive studies to solve the above-described problems and consequently have found that a metal complex as defined in claim <NUM> which contains a bidentate ligand including an aromatic heterocyclic <NUM>-membered ring having <NUM> nitrogen atoms ora bidentate ligand including an aromatic heterocyclic <NUM>-membered ring having <NUM> nitrogen atoms; and an aromatic heterocyclic <NUM>-membered ring having <NUM> or <NUM> nitrogen atoms is useful for a dehydrogenation reaction of formic acid, to thereby complete the present invention.

The catalyst used for a dehydrogenation reaction of at least one of formic acid and a formic acid salt is a salt of a complex, having a structure represented by any selected from the group consisting of the following Formulae (<NUM>), (<NUM>) to (<NUM>):
<CHM>
where.

In the catalyst used for a dehydrogenation reaction of at least one of formic acid and a formic acid salt the complex has not a structure represented by any selected from the group consisting of the following Formulae (<NUM>) to (<NUM>):
<CHM>
<CHM>
<CHM>.

Also disclosed herein (not claimed) is a method for dehydrogenating at least one of formic acid and a formic acid salt, the method including:
allowing a solution containing at least one of formic acid and a formic acid salt to react in the presence of the catalyst as disclosed herein.

The invention also relates to a method for producing hydrogen, the method including:
allowing a solution containing at least one of formic acid and a formic acid salt to react in the presence of the catalyst defined above, to thereby dehydrogenate the at least one of formic acid and a formic acid salt.

Use of a complex according to the present invention, an isomer or a salt of the complex as a catalyst enables to provide a high-pressure hydrogen gas free of carbon monoxide through dehydrogenation of formic acid or a formic acid salt in a highly efficient, highly energy-efficient, highly selectively, and highly durable manner. By the method for dehydrogenating according to the present invention hydrogen is allowed to be regenerated easily from formic acid or a formic acid salt which is liquid fuel suitable for transportation and storage.

A complex catalyst according to the present invention has extremely higher durability than those of the complex catalysts described in PTLs <NUM> to <NUM>, and has excellent catalytic performance allowing the catalyst to stably maintain high catalytic performance for a long period of time in a high-concentration formic acid solution under a high temperature reaction condition.

As used herein, the phrase "at least one of formic acid and a formic acid salt" refers to the formic acid alone, the formic acid salt alone, a mixture of the formic acid with the formic acid salt, or a mixture of the formic acid or the formic acid salt with an acid or a base.

In the present invention, a dehydrogenation reaction of at least one of formic acid and a formic acid salt represented by the following scheme allows hydrogen and carbon dioxide to be highly efficiently produced. Upon the reaction, there is a possibility that carbon monoxide and water are by-produced through a decarbonylation reaction of formic acid. However, a catalyst used for dehydrogenation of formic acid according to the present invention allows only the dehydrogenation reaction of formic acid to proceed in a highly selective and highly efficient manner, to thereby produce a hydrogen gas free of carbon monoxide.

In the complex catalyst represented by any of the Formulae (<NUM>), (<NUM>) to (<NUM>), a counter ion thereof is not particularly limited. Examples of anions serving as the counter ion include a hexafluorophosphate ion (PF<NUM>-), a tetrafluoroborate ion (BF<NUM>-), a hydroxide ion (OH-), an acetate ion, a carbonate ion, a phosphate ion, a sulfate ion, a nitrate ion, a halide ion (e.g., a fluoride ion (F-), a chloride ion (Cl-), a bromide ion (Br-), and an iodide ion (I-)), a hypohalite ion (e.g., a hypofluorite ion, a hypochlorite ion, a hypobromite ion, and a hypoiodite ion), a halite ion (e.g., a fluorite ion, a chlorite ion, a bromite ion, and an iodite ion), a halate ion (e.g., a fluorate ion, a chlorate ion, a bromate ion, and an iodate ion), a perhalate ion (e.g., a perfluorate ion, a perchlorate ion, a perbromate ion, and a periodate ion), a trifluoromethanesulfonate ion (OSO<NUM>CF<NUM>-), and a tetrakis(pentafluorophenyl)borate ion [B(C<NUM>F<NUM>)<NUM>-]. Examples of cations serving as the counter ion include, but are not limited to, various metal ions, such as a lithium ion, a magnesium ion, a sodium ion, a potassium ion, a calcium ion, a barium ion, a strontium ion, an yttrium ion, a scandium ion, and a lanthanoid ion; and a hydrogen ion. One type of these counter ions may be used alone, or two or more types may be used in combination. However, the above description is intended to only exemplify possible mechanisms, and the present invention is not limited thereto.

Notably, examples of the alkyl group in the present invention include, but are not limited to, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group and a tert-butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, and an icosyl group. The same can be applied to a group and an atomic group (e.g., an alkoxy group) derived from the alkyl group. Examples of the alcohol and the alkoxide ion include, but are not limited to, alcohols and alkoxide ions derived from the above-described alkyl groups. Moreover, as used herein, the term "halogen" refers to any halogen element. Examples thereof include fluorine, chlorine, bromine, and iodine. In the present invention, in the case where a substituent has an isomer, any isomer may be used unless otherwise restricted. For example, a "propyl group" as simply referred to herein may be an n-propyl group or an isopropyl group. Similarly, a "butyl group" as simply referred to herein may be an n-butyl group, an isobutyl group, a sec-butyl group, or a tert-butyl group. However, the above description is intended to only exemplify possible mechanisms, and the present invention is not limited thereto.

The catalyst according to the present invention is a catalyst containing, as an effective ingredient, a complex represented by any of the General Formulae (<NUM>), (<NUM>) to (<NUM>), tautomers or stereoisomers thereof, as salts of the complex, and used for a method for dehydrogenating at least one of formic acid and a formic acid salt (or a method for producing hydrogen) or a method for producing hydrogen. For example, one compound or a plurality of compounds serving as the effective ingredient may be used as-is as the complex catalyst according to the present invention, or a mixture of the above-described isomers may be used as the complex catalyst. Other ingredients may be appropriately added (preferably in an amount of less than <NUM>% by mass).

In the complex catalyst according to the present invention, any one of aromatic rings constituting the ligand includes an aromatic heterocyclic <NUM>-membered ring skeleton having <NUM> nitrogen atoms. The ligand significantly improves efficiency of the dehydrogenation reaction of formic acid. For example, a bipyridine complex which contains a bidentate ligand including a <NUM>-membered ring skeleton similar to the complex catalyst represented by the General Formula (<NUM>) to (<NUM>) exhibits only extremely low catalytic performance in the dehydrogenation reaction of formic acid (see, Comparative Example <NUM>). From the above results, it has been found that it is necessary for one or more of aromatic rings constituting the ligand to include an aromatic heterocyclic <NUM>-membered ring skeleton having <NUM> nitrogen atoms in order to accelerate the dehydrogenation reaction of formic acid.

A method for dehydrogenating formic acid according to the present disclosure (or a method for producing hydrogen according to the present invention) includes at least one step selected from the group consisting of a step of stirring a solution containing at least one of formic acid and a formic acid salt with a catalyst containing, as the effective ingredient, the complex catalyst, tautomers or stereoisomers thereof, or salts thereof according to the present invention, and a step of heating the solution. Specifically, for example, the complex catalyst according to the present invention is added to solution containing at least one of formic acid and a formic acid salt, and then stirred, optionally with heating. In the case of heating, a temperature may be for example, <NUM> to <NUM>, preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>. A method for collecting hydrogen released is not particularly limited. For example, known methods such as a downward displacement of water method or an upward displacement method may be appropriately used.

In the method for dehydrogenating formic acid according to the present disclosure (or a method for producing hydrogen according to the present invention), the formic acid or the formic acid salt can also be dehydrogenated under pressure by using a sealable reaction container. A gas pressure in the reaction container may be for example, <NUM> MPa to <NUM> MPa, preferably <NUM> MPa to <NUM> MPa. Pressure inside the reaction container is spontaneously increased, which allows high pressure hydrogen gas to be spontaneously supplied without pressurizing by means of external energy.

In the method for dehydrogenating formic acid according to the present disclosure (or a method for producing hydrogen according to the present invention), a concentration of the complex catalyst may depend on, for example, reaction velocity, solubility of the complex in the reaction solution, and economic efficiency. Appropriate concentration of the catalyst is <NUM> × <NUM>-<NUM> M to <NUM> × <NUM>-<NUM> M, preferably <NUM> × <NUM>-<NUM> M to <NUM> × <NUM>-<NUM> M.

In the method for dehydrogenating formic acid according to the present disclosure (or a method for producing hydrogen according to the present invention), a ratio of an amount of substance (the number of molecules) of catalyst molecules to that of formic acid molecules may, for example, be the ratio of the formic acid molecules to the catalyst molecules of <NUM>:<NUM> to <NUM>:<NUM>, <NUM>, <NUM> at the start of the reaction. Hydrogen can be continuously produced by additionally adding or continuously adding dropwise formic acid molecules during the reaction. As used herein, the term formic molecule includes formic acid and a formic acid salt, which may be used alone or as a mixture thereof. In the case where the mixture is used, it is generally used in a pH range of <NUM> to <NUM>, preferably <NUM> to <NUM>, but the formic acid may be dehydrogenated at a pH out of the above range by additionally adding an acid or a base. In the case where the formic acid salt is used alone, examples of a positive ion serving as a counter cation may includevarious metal ions such as a lithium ion, a magnesium ion, a sodium ion, a potassium ion, a calcium ion, a barium ion, a strontium ion, an yttrium ion, a scandium ion, or a lanthanoid ion; an ammonium ion, tetramethyl ammonium, and tetraethyl ammonium. Although these counter ions may be used alone or two or more counter ions may be used in combination.

The catalyst, a tautomer or stereoisomer thereof, or a salt thereof according to the present invention can be used as a catalyst for dehydrogenation of formic acid in, for example, a formic acid fuel cell. In the case where the catalyst is used in the fuel cell, for example, the cell only has to contain therein the catalyst for dehydrogenation of formic acid according to the present invention and include a mechanism for generating hydrogen by dehydrogenating formic acid according to the above described method. A specific configuration of the fuel cell may be for example, a configuration of a known fuel cell which can be appropriately applied thereto. Furthermore, an application of the catalyst for dehydrogenation of formic acid according to the present invention is not limited to those mentioned above, and, for example, the catalyst for dehydrogenation of formic acid according to the present invention can be used in every technical field in which hydrogen (H<NUM>) is needed to be supplied.

In the method for dehydrogenating formic acid according to the present disclosure, a reaction solvent to be used is not particularly limited. For example, the solvent may be water or an organic solvent, and one solvent may be used alone or two or more solvents may be used in combination. In the case where the complex catalyst according to the present invention is soluble in water, water is preferably used from the viewpoint of simplicity and convenience. The organic solvent is not particularly limited, but a highly polar solvent is preferable from the viewpoint of solubility of the catalyst. Examples thereof include nitriles such as acetonitrile, propionitrile, butyronitrile, or benzonitrile; primary alcohols such as methanol, ethanol, n-propyl alcohol, or n-butyl alcohol; secondary alcohols such as isopropyl alcohol or s-butyl alcohol; tertiary alcohols such as t-butyl alcohol; polyhydric alcohols such as ethylene glycol or propylene glycol; ethers such as tetrahydrofuran, dioxane, dimethoxyethane, or diethyl ether; amides such as dimethylformamide or dimethylacetamide; sulfoxides such as dimethyl sulfoxide; and esters such as ethyl acetate. Furthermore, formic acid as a raw material may be in the form of a solution or a salt.

Hereinafter, examples of the present invention will be described in more detail. However, the present invention is not limited to the following examples.

[Cp*Ir(H<NUM>O)<NUM>]SO<NUM> and a ligand corresponding to it in an equal amount were dissolved into water, followed by stirring at room temperature under an argon stream for <NUM> hours. The resultant reaction liquid was filtered. The resultant filtrate was concentrated under reduced pressure. The resultant product was dried at <NUM> under reduced pressure for <NUM> hours, to thereby the intended product.

The spectral data of the resultant complex catalysts are presented below.

The spectral data of the compound represented by the following Formula (<NUM>) is presented below.

<NUM>H NMR (<NUM>, D<NUM>O): δ = <NUM> (dt, J = <NUM>, <NUM>, <NUM>), <NUM> - <NUM> (m,<NUM>), <NUM> (ddd, J = <NUM>, <NUM>, <NUM>, <NUM>), <NUM> (dd, J = <NUM>, <NUM>, <NUM>), <NUM> (s, <NUM>); IR (KBr): <NUM>, <NUM>, <NUM>, <NUM>, <NUM>-<NUM>; UV/Vis: λmax <NUM>; ESI-MS (m/z): [M-SO<NUM>-H<NUM>O-H]+; found, <NUM>; Elemental analysis calcd. for C<NUM>H<NUM>IrN<NUM>O<NUM>S+H<NUM>O: C, <NUM>; H, <NUM>; N, <NUM>. Found: C, <NUM>; H, <NUM>; N, <NUM>.

<NUM>H NMR (<NUM>, D<NUM>O): δ = <NUM> (d, J = <NUM>, <NUM>), <NUM> - <NUM> (m,<NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (ddd, J = <NUM>, <NUM>, <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (s, <NUM>); IR (KBr):<NUM>, <NUM>, <NUM>, <NUM>, <NUM>-<NUM>; UV/Vis: λmax <NUM> (shoulder peak); ESI-MS (m/z): [M-SO<NUM>-H<NUM>O-H]+; found, <NUM>; Elemental analysis calcd. for C<NUM>H<NUM>IrN<NUM>O<NUM>S+H<NUM>O: C, <NUM>; H, <NUM>; N, <NUM>. Found: C, <NUM>; H, <NUM>; N, <NUM>.

<NUM>H NMR (<NUM>, D<NUM>O): δ = <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> - <NUM> (m,<NUM>), <NUM> - <NUM> (m,<NUM>), <NUM> (s, <NUM>); IR (KBr): <NUM>, <NUM>, <NUM>, <NUM>, <NUM>-<NUM>; UV/Vis: λmax <NUM>; ESI-MS (m/z): [M-SO<NUM>-H<NUM>O-H]+; found, <NUM>; Elemental analysis calcd. for C<NUM>H<NUM>IrN<NUM>O<NUM>S+H<NUM>O: C, <NUM>; H, <NUM>; N, <NUM>. Found: C, <NUM>; H, <NUM>; N, <NUM>.

<NUM>H NMR (<NUM>, D<NUM>O): δ = <NUM> (t, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (s, <NUM>); UV/Vis: λmax <NUM>; ESI-MS (m/z): [M-SO<NUM>-H<NUM>O-H]+; found, <NUM>.

<NUM>H NMR (<NUM>, D<NUM>O): δ = <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>); IR (KBr): <NUM>, <NUM>, <NUM>, <NUM>-<NUM>; UV/Vis: λmax <NUM>; ESI-MS(m/z): [M-SO<NUM>-H<NUM>O-H]+; found, <NUM>; Elemental analysis calcd. for C<NUM>H<NUM>IrN<NUM>O<NUM>S: C, <NUM>; H, <NUM>; N, <NUM>. Found: C, <NUM>; H, <NUM>; N, <NUM>.

<NUM>H NMR (<NUM>, D<NUM>O): δ = <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>); IR (KBr): <NUM>, <NUM>, <NUM>-<NUM>; UV/Vis: λmax <NUM>; ESI-MS (m/z): [M-SO<NUM>-H<NUM>O-H]+; found, <NUM>; Elemental analysis calcd. for C<NUM>H<NUM>RhN<NUM>O<NUM>S+<NUM><NUM>O: C, <NUM>; H, <NUM>; N, <NUM>. Found: C, <NUM>; H, <NUM>; N, <NUM>.

The spectral data of the compound represented by the Formula (<NUM>) is presented below.

<NUM>H NMR (D<NUM>O, <NUM>): <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>). <NUM>H NMR (DMSO-ds, <NUM>): <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>); ESI-MS (+): m/z <NUM> [M-H<NUM>O-H]+.

<NUM>H NMR (D<NUM>O, <NUM>):<NUM> (d, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (d, J = <NUM>, <NUM>), <NUM> (t, J = <NUM>, <NUM>), <NUM> (s, <NUM>); <NUM>C NMR (D<NUM>O, <NUM>): δ = <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. ESI-MS (+): m/z <NUM> [M-H<NUM>O-H]+.

<NUM>H NMR (DMSO-ds, <NUM>): <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>); <NUM>C NMR (DMSO-d<NUM>, <NUM>): <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>; ESI-MS: <NUM> [M-H<NUM>O-H]+. for C<NUM>H<NUM>IrN<NUM>O<NUM>S·<NUM><NUM>O: C <NUM>, H <NUM>, N <NUM>. Found: C <NUM>, H <NUM>, N <NUM>.

<NUM>H NMR (D<NUM>O, <NUM>): δ = <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>). <NUM>H NMR (d<NUM>-DMSO, <NUM>): <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>). <NUM>C NMR (d<NUM>-DMSO, <NUM>): δ = <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. ESI-MS (+): m/z <NUM> [M-H<NUM>O-H]+.

A solution of the complex catalyst as disclosed herein in water was degassed. Thus prepared catalyst solution was added to a <NUM> degassed solution of formic acid in water (<NUM>), followed by stirring with heating. An amount of gas released was measured by means of a gas meter (Shinagawa W-NK-<NUM>). A gas component released was measured by means of a gas chromatography GL SCIENCES (GC390), hydrogen was measured by means of a thermal conductivity detector (TCD), and carbon dioxide and carbon monoxide were measured by means of a methanizer and a hydrogen flame ionization detector (FID). As a result, it was found that hydrogen and carbon dioxide were released in a ratio of <NUM>:<NUM>, and that carbon monoxide was not able to be detected (equal to or lower than the detection threshold of <NUM> ppm). From the reaction results presented in Table <NUM>, all catalysts including a ligand containing a <NUM>-membered ring, which were according to the present invention, exhibited a high catalytic activity. As described in NPL <NUM>, in the dehydrogenation reaction of formic acid, it has been found that a catalyst is activated by the electron-donating action of a substituent on a ligand in the catalyst. Based on this finding, the catalyst represented by the General Formula (<NUM>), which was obtained by introducing four methyl groups to the catalyst represented by the General Formula (<NUM>), exhibited a turnover frequency (TOF: the number of substrate (i.e., formic acid) molecules on which the catalyst acts per <NUM> molecule of the catalyst per <NUM> hour) <NUM> times higher than that of the catalyst represented by the General Formula (<NUM>). Further, when a strong electron-donating hydroxyl group was introduced, the complex represented by the General Formula (<NUM>) in which the hydroxyl group had been substituted exhibited the TOF <NUM> times or more higher than that of the unsubstituted complex represented by the General Formula (<NUM>).

A solution of the complex catalyst represented by the General Formula (<NUM>) in water was degassed. Thus prepared catalyst solution (<NUM>µmol) was added to an <NUM>% degassed solution of formic acid in water (<NUM>), followed by stirring with heating. The catalytic reaction proceeded for <NUM> hours or longer. After the reaction, the formic acid was completely decomposed. A time-dependent change in an amount of a gas released is illustrated in <FIG>. This complex catalyst was able to exhibit stable catalytic performance without deteriorating for <NUM> week or longer even in a high-concentration formic acid solution.

A solution of the complex catalyst represented by the General Formula (<NUM>) in water was degassed. Thus prepared catalyst solution (<NUM>µL, <NUM>µmol) was added to a <NUM> degassed solution of formic acid/sodium formate (<NUM>/<NUM> to <NUM>/<NUM>) in water (<NUM>) of which pH had been adjusted, followed by stirring at <NUM>. A pH-dependent change in reaction velocity is illustrated in <FIG>.

An <NUM> degassed solution of formic acid in water (<NUM>) containing the complex catalyst represented by the General Formula (<NUM>) (<NUM>µmol) was placed into a glass autoclave. The reaction container was stirred with heating at <NUM>, and amounts and ratios of gases released from a back pressure valve at <NUM> MPa were measured (<FIG>). As a result, a turnover efficiency of the catalyst was extremely high, i.e., <NUM>,<NUM> times per <NUM> hour. It was confirmed that hydrogen and carbon dioxide were released in a ratio of approximately <NUM>:<NUM> and that <NUM>% or more of the formic acid was decomposed after the reaction. These results indicate that hydrogen is released without deteriorating the complex catalyst even under severe reaction conditions, that is, in a high-concentration aqueous formic acid solution, at a high temperature of <NUM> and high pressure of <NUM> MPa.

In a dehydrogenation reaction of formic acid using the unsubstituted bipyridine complex catalyst represented by the General Formula (<NUM>) (<NUM> aqueous formic acid solution, reaction temperature: <NUM>), the TOF was <NUM>. The catalyst according to the present invention exhibited significantly higher catalytic activity than that value under the same reaction conditions. The complex represented by the General Formula (<NUM>), which was an active form of the complex represented by the General Formula (<NUM>) obtained by introducing hydroxyl groups to a ligand thereof, had the TOF of <NUM>,<NUM>. Therefore, according to the present invention, it has been found that a catalyst exhibiting excellent catalytic performance can be designed only by including a <NUM>-membered ring skeleton on a ligand thereof. In fact, the complex catalyst represented by the General Formula (<NUM>) in which a hydroxyl group had been introduced to a ligand thereof exhibited the highest catalytic performance.

The catalyst for dehydrogenation of formic acid represented by the General Formula (<NUM>) where R = H or OH described in PTLs <NUM> and <NUM> exhibits extremely high reaction velocity under the optimal reaction conditions, that is, in a formic acid solution which has a concentration of <NUM> or less and of which pH has been adjusted to <NUM>, and at a reaction temperature of <NUM> or less. However, the catalytic performance is significantly low under conditions other than the above-described optimal reaction conditions. The conversion rate of formic acid after reaction is low (<NUM>%). On the other hand, under severe reaction conditions, that is, in a formic acid solution having a concentration of <NUM> or more and/or at a temperature of <NUM> or higher, the catalytic performance was significantly deteriorated within <NUM> hour.

Claim 1:
A catalyst used for a dehydrogenation reaction of at least one of formic acid and a formic acid salt, the catalyst being a salt of a complex:
having a structure represented by any selected from the group consisting of the following Formulae (<NUM>), (<NUM>) to (<NUM>):
<CHM>
where
R<NUM> to R<NUM> each independently denote an alkyl group, a hydroxy group (-OH), an alkoxy group (-OR), a nitro group, a halogen group, a carboxylic acid group, an alkylamino group, or a phenyl group, or adjacent R groups may be linked together to form a ring; and R<NUM> to R<NUM> may be substituted by one substituent or a plurality of substituents,
Z<NUM> denotes a water molecule, a hydrogen atom, an alkoxide ion, a hydroxide ion, a halide ion other than a chloride ion, a carbonate ion, a trifluoromethanesulfonate ion, a sulfate ion, a nitrate ion, a formate ion, or an acetate ion, or is absent; and
m denotes a positive integer, <NUM>, or a negative integer,
<CHM>
<CHM>
<CHM>
where
R<NUM> to R<NUM> each independently denote a hydrogen atom, an alkyl group, a hydroxy group (-OH), an alkoxy group (-OR), a nitro group, a halogen group, a carboxylic acid group, an alkylamino group, or a phenyl group, or adjacent R groups may be linked together to form a ring; and R<NUM> to R<NUM> may be substituted by one substituent or a plurality of substituents,
Z<NUM> denotes a water molecule, a hydrogen atom, an alkoxide ion, a hydroxide ion, a halide ion, a carbonate ion, a trifluoromethanesulfonate ion, a sulfate ion, a nitrate ion, a formate ion, or an acetate ion, or is absent; and
n denotes a positive integer, <NUM>, or a negative integer;
provided that the complex is not having a structure selected from the group consisting of the following formulas (<NUM>) to (<NUM>):
<CHM>
<CHM>
<CHM>