Method for producing formic acid salt, and method for producing formic acid

Abstract

This method for producing formic acid salt uses a gas including carbon dioxide and NOx, hydrogen, a catalyst, an organic phase, and an aqueous phase to produce formic acid salt. The method for producing formic acid includes: a step for producing a formic acid salt by using the abovementioned method for producing formic acid salt; and a step for protonating at least a portion of the formic acid salt to produce formic acid.

Description

Method for producing formate salt, and method for producing formic acid

[0001] This invention relates to a method for producing formate salt and a method for producing formic acid.

[0002] As solutions to the problems of global warming and the depletion of fossil fuels, there are high expectations for technologies that convert carbon dioxide into useful compounds and technologies that utilize hydrogen as a next-generation energy source.

[0003] Formic acid is an excellent compound for storing hydrogen and carbon dioxide because it requires low energy for dehydrogenation and can be easily handled, and can be used in the above-mentioned technologies. Formic acid can be produced, for example, from formate obtained by reacting hydrogen with carbon dioxide, bicarbonates, carbonates, and other compounds. As an example, Patent Document 1 describes a method for producing formic acid by reacting carbon dioxide with hydrogen in the presence of a catalyst containing an element from group 8, 9, or 10 of the periodic table, a tertiary amine (I), and a polar solvent.

[0004] Patent No. 5734286

[0005] The present invention aims to provide a new method for producing formate salt and a method for producing formic acid.

[0006] The present invention provides a method for producing formate using a gas containing carbon dioxide and NOx, hydrogen, a catalyst, an organic phase, and an aqueous phase.

[0007] Furthermore, the present invention provides a method for producing formic acid, comprising the steps of: producing a formate salt by the above-described method for producing formate salt; and protonating at least a portion of the formate salt to produce formic acid.

[0008] According to the present invention, a new method for producing formate salt and a method for producing formic acid can be provided.

[0009] Figure 1 is a schematic diagram showing an example of a three-chamber electrodialysis machine. Figure 2 is a schematic diagram showing an example of a formic acid production system.

[0010] A method for producing formate according to a first aspect of the present invention involves producing formate using a gas containing carbon dioxide and NOx, hydrogen, a catalyst, an organic phase, and an aqueous phase.

[0011] In a second embodiment of the present invention, for example, the method for producing the formate of the catalyst according to the first embodiment includes a gas contact step of contacting the aqueous phase with the gas.

[0012] In a third aspect of the present invention, for example, in the method for producing formate according to the second aspect, compound C, which is at least one selected from the group consisting of carbonates and bicarbonates, is produced by the gas contact step.

[0013] In a fourth aspect of the present invention, for example, the method for producing formate according to the third aspect includes a hydrogenation reaction step in which the catalyst is used to react the compound C with hydrogen in a two-phase system in which the organic phase and the aqueous phase are separated to produce formate.

[0014] In a fifth embodiment of the present invention, for example, in the method for producing formate according to the fourth embodiment, the aqueous phase contains compound C.

[0015] In a sixth aspect of the present invention, for example, in the method for producing formate according to the fourth or fifth aspect, the concentration of nitrite ions in the aqueous phase during the hydrogenation reaction step is less than 100 ppm.

[0016] In a seventh aspect of the present invention, for example, in a method for producing formate according to any one of the fourth to sixth aspects, the concentration of nitrite ions in the aqueous phase during the hydrogenation reaction step is 50 ppm or less.

[0017] In the eighth aspect of the present invention, for example, in a method for producing formate according to any one of the first to seventh aspects, the organic phase includes the catalyst.

[0018] In the ninth aspect of the present invention, for example, in a method for producing formate according to any one of the first to eighth aspects, the catalyst is at least one selected from the group consisting of complexes containing metal M and salt compounds thereof.

[0019] In the tenth aspect of the present invention, for example, in a method for producing formate according to any one of the first to ninth aspects, the catalyst is at least one selected from the group consisting of a metal complex represented by the following general formula (1A), its tautomers, stereoisomers, and salts thereof. (In general formula (1A), X represents an atomic group containing typical elements of groups 13 to 15 that can coordinate to M; Q independently represents a bridging structure containing typical elements of groups 14 to 16 that connects Y and X; Y independently represents an atomic group containing typical elements of groups 14 to 16 that can coordinate to M; M represents a metal atom; Z represents an anionic ligand; n represents 0 to 3; and L, if multiple are present, independently represents a neutral or anionic ligand.)

[0020] In the eleventh aspect of the present invention, for example, in the method for producing formate according to the tenth aspect, the metal complex represented by the general formula (1A) is the metal complex represented by the following general formula (2A). (In general formula (2A), X1 represents a heteroaromatic ring formed with two carbon atoms and a nitrogen atom, which may have substituents or be bonded with other substituents to form a ring; Q1 independently represents CH2, NH, or O, with CH2 and NH further having substituents; Y1 independently represents a phosphorus atom or a nitrogen atom; R independently represents an alkyl group, an aryl group, or an aralkyl group, which further have substituents; M represents a metal atom; Z represents an anionic ligand; n represents 0 to 3; and L, if multiple are present, independently represents a neutral or anionic ligand.)

[0021] In a twelfth aspect of the present invention, for example, in the method for producing formate according to the eleventh aspect, the metal complex represented by the general formula (2A) is the metal complex represented by the following general formula (3A). (In general formula (3A), R0 represents a hydrogen atom or an alkyl group, A independently represents CH, CR5, or N, R5 represents an alkyl group, aryl group, aralkyl group, amino group, hydroxyl group, or alkoxy group, Q1 independently represents CH2, NH, or O, and CH2 and NH may have further substituents, Y1 represents a phosphorus atom or a nitrogen atom, R independently represents an alkyl group, aryl group, or aralkyl group, which may have further substituents, M represents a metal atom, Z represents an anionic ligand, n represents 0 to 3, and L, if there are multiple, independently represents a neutral or anionic ligand.)

[0022] In the thirteenth aspect of the present invention, for example, in the method for producing formate according to any one of the ninth to twelfth aspects, the metal M is ruthenium.

[0023] A method for producing formic acid according to the 14th aspect of the present invention includes the steps of: producing a formate salt by a method for producing a formate salt according to any one of the 1st to 13th aspects; and protonating at least a portion of the formate salt to produce formic acid.

[0024] The details of the present invention will be described below, but the following description is not intended to limit the present invention to any particular embodiment.

[0025] [Method for Producing Formate] The method for producing formate according to the first embodiment of the present invention is a method for producing formate using a gas containing carbon dioxide and NOx, hydrogen, a catalyst, an organic phase and an aqueous phase. The method for producing formate according to the first embodiment of the present invention is a method for producing formate by converting the carbon dioxide contained in the above gas into formate. The organic phase contains an organic solvent, and the aqueous phase contains an aqueous solvent. The organic phase may mainly contain an organic solvent. The aqueous phase may mainly contain an aqueous solvent. In this specification, "main component" means the component that is present in the largest amount by mass ratio. For example, the organic phase and the aqueous phase are separated. Formate is typically produced by a hydrogenation reaction. The method for producing formate according to the first embodiment of the present invention typically includes a step of producing formate by a hydrogenation reaction of a starting compound using a catalyst in a two-phase system in which the organic phase and the aqueous phase are separated.

[0026] In this specification, "NOx" means nitrogen oxides, such as NO, NO2, N2O, N2O3, etc.

[0027] The method for producing formate according to the first embodiment of the present invention uses a catalytic reaction in a two-phase system including an organic phase and an aqueous phase, so that even when using a gas containing NOx, the catalytic turnover number (TON) and the yield of formate can be maintained and the reaction can be repeated multiple times. This is because NOx is more soluble in an aqueous solvent than in an organic solvent and is difficult to contact the catalyst in the organic phase, thereby reducing the decrease in catalytic activity. Further, when formate is produced by hydrogenating carbon dioxide via a carbonate and / or a bicarbonate as described later, first, a gas containing carbon dioxide is brought into contact with only the aqueous phase to include the carbonate and / or the bicarbonate in the aqueous phase, and then a hydrogenation reaction of the carbonate and / or the bicarbonate is carried out using the aqueous phase, the organic phase containing the catalyst, and hydrogen. Therefore, even if the gas first brought into contact with the aqueous phase contains NOx, at the stage of bringing the aqueous phase into contact with the organic phase for the hydrogenation reaction, the NOx concentration in the aqueous phase has decreased to some extent, so that the influence on the catalyst in the organic phase is reduced, and it is presumed that the decrease in catalytic activity can be reduced. By being able to use a gas containing NOx, an exhaust gas or a gas derived from an exhaust gas can be used as a carbon dioxide source. That is, according to the present invention, carbon dioxide contained in an exhaust gas or a gas derived from an exhaust gas can be converted into formate. Hereinafter, a gas containing carbon dioxide and NOx may be referred to as gas G.

[0028] Formate can be produced by hydrogenating carbon dioxide via a carbonate and / or a bicarbonate. Therefore, the starting compound for the hydrogenation reaction is at least one selected from the group consisting of, for example, carbon dioxide, a carbonate, and a bicarbonate, and preferably at least one selected from the group consisting of a carbonate and a bicarbonate.

[0029] Carbonates and bicarbonates can be produced by the reaction of carbon dioxide with a base. For example, by introducing carbon dioxide into an aqueous phase which is a basic solution, carbon dioxide dissolves in the aqueous phase while producing bicarbonate or carbonate. The method for producing formate according to the first embodiment of the present invention preferably includes a gas contact step of bringing gas G into contact with an aqueous phase. At this time, gas G preferably contacts an aqueous phase which is a basic solution containing a base. Thereby, a compound C which is at least one selected from the group consisting of carbonate and bicarbonate is produced in the gas contact step. The produced compound C dissolves in the aqueous phase.

[0030] The method for producing formate according to the first embodiment of the present invention preferably produces formate by the hydrogenation reaction of the compound C produced as described above. That is, the method for producing formate according to the first embodiment of the present invention preferably includes a hydrogenation reaction step of reacting compound C with hydrogen using a catalyst to produce formate in a two-phase system in which an organic phase and an aqueous phase are separated.

[0031] Hereinafter, each material and each step used in the method for producing formate according to the first embodiment of the present invention will be described in detail.

[0032] (Solvent) In the method for producing formate according to the first embodiment of the present invention, it is preferable to use a solvent capable of forming a two-phase system in which an organic phase containing an organic solvent and an aqueous phase containing an aqueous solvent are separated. The solvent preferably includes a solvent in which the catalyst dissolves to become uniform, and more preferably includes an aqueous solvent and an organic solvent in which the catalyst dissolves to become uniform.

[0033] Examples of the aqueous solvent include water, methanol, ethanol, ethylene glycol, glycerin, and a mixed solvent thereof, and water is preferable from the viewpoint of low environmental impact.

[0034] Examples of organic solvents include toluene, benzene, xylene, propylene carbonate, dioxane, dimethyl sulfoxide, tetrahydrofuran, ethyl acetate, methylcyclohexane, cyclopentyl methyl ether, and mixed solvents thereof. From the viewpoint of separation from aqueous solvents, it is preferable to include toluene or dioxane, and more preferable to include toluene. The organic phase may contain only toluene as the organic solvent.

[0035] (Catalyst) The catalyst is preferably at least one selected from the group consisting of complexes containing metal M and salt compounds thereof.

[0036] Metal M may include elements from groups 7 to 11 of the periodic table, such as manganese, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, and gold, or it may include elements from groups 8 to 11 of the periodic table, such as iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, and gold. Among these, manganese, iron, ruthenium, cobalt, rhodium, iridium, nickel, palladium, or copper are preferred, manganese, ruthenium, rhodium, iridium, nickel, or palladium are more preferred, manganese, ruthenium, rhodium, iridium, or palladium are even more preferred, ruthenium (Ru) or manganese (Mn) are particularly preferred, and ruthenium (Ru) is the most preferred.

[0037] The catalyst is preferably at least one selected from the group consisting of metal complexes containing a metal M and a ligand represented by the following general formula (1B), and salts thereof. (In general formula (1B), X represents an atomic group containing typical elements of groups 13 to 15 that can coordinate to metal M, Y independently represents an atomic group containing typical elements of groups 14 to 16 that can coordinate to metal M, and Q independently represents a bridging structure containing typical elements of groups 14 to 16 that connects Y and X.)

[0038] In general formula (1B), it is preferable that X and the two Ys are coordinated to the metal M contained in the catalyst.

[0039] Typical elements of groups 13 to 15 of the periodic table in X include boron, carbon, silicon, germanium, tin, nitrogen, phosphorus, arsenic, oxygen, sulfur, and selenium. Boron, carbon, silicon, germanium, tin, nitrogen, phosphorus, arsenic, and sulfur atoms are preferred, carbon, nitrogen, phosphorus, and sulfur atoms are more preferred, and carbon or nitrogen atoms are even more preferred.

[0040] X may be an atomic group with a valency of 0 to 1. Examples of atomic groups represented by X include alkyl groups, alkenyl groups, alkoxy groups, aromatic rings, and heterocycles, which may have substituents or be bonded with other substituents to form a ring.

[0041] Examples of alkyl groups in X include linear, branched, cyclic substituted, or unsubstituted alkyl groups. Preferably, alkyl groups in X include alkyl groups having 1 to 30 carbon atoms, such as methyl, ethyl, n-propyl, i-propyl, t-butyl, n-octyl, eicosyl, and 2-ethylhexyl groups, and it is preferable that the alkyl group has 6 or fewer carbon atoms, and that it is a methyl group.

[0042] Examples of alkenyl groups in X include linear, branched, cyclic substituted, or unsubstituted alkenyl groups. Preferably, the alkenyl group in X is a C2 to C30 alkenyl group, such as a vinyl group, n-propenyl group, i-propenyl group, t-butenyl group, n-octenyl group, etc., and it is preferable that the alkenyl group has 6 or fewer carbon atoms.

[0043] Examples of alkoxy groups in X include linear, branched, cyclic substituted or unsubstituted alkyloxy groups. Preferably, examples of alkoxy groups in X include substituted or unsubstituted alkoxy groups having 1 to 30 carbon atoms, such as methoxy, ethoxy, isopropoxy, t-butoxy, n-octyloxy, and 2-methoxyethoxy groups.

[0044] Examples of aromatic rings in X include phenyl rings and naphthyl rings.

[0045] Examples of heterocyclic rings in X include pyrrolidine rings, piperidine rings, pyrroline rings, imidazoline rings, imidazolidine rings, pyrrole rings, imidazole rings, pyridine rings, pyrimidine rings, triazine rings, quinoline rings, and quinazoline rings.

[0046] The group of atoms represented by X, which has a valency of 0 to 1, preferably represents a group of atoms that includes a heteroaromatic ring formed with two carbon atoms and a nitrogen atom. These groups may have substituents, or they may bond with other substituents to form a ring.

[0047] The 0-1 valent atomic group represented by X is preferably a pyrroline ring, pyridine ring, imidazoline ring, pyrimidine ring, or triazine ring, more preferably a pyridine ring or triazine ring, and even more preferably a pyridine ring.

[0048] When the 0- to 1-valent atomic group represented by X has substituents, examples of substituents include substituent group A, where alkyl groups are preferred and methyl groups are more preferred.

[0049] X may be an atomic group in which a hydrogen atom or an alkyl group is bonded to a nitrogen atom. The alkyl group is as described above, and is preferably a methyl group.

[0050] The bridging structure between Y and X, represented by Q, which includes typical elements from groups 14 to 16 of the periodic table, may have a double bond, a monocyclic structure, a fused ring structure, or substituents.

[0051] Q can be made to have various structures, but for example, the number of atoms in the portion between Y and X is preferably 1 to 5, more preferably 1 to 4, even more preferably 1 to 3, and particularly preferably 1 to 2.

[0052] The atoms in the portion between Y and X described above are not particularly limited, but carbon atoms, nitrogen atoms, phosphorus atoms, oxygen atoms, and sulfur atoms are preferred, carbon atoms, nitrogen atoms, and oxygen atoms are more preferred, carbon atoms and oxygen atoms are even more preferred, and carbon atoms are particularly preferred.

[0053] Q may have a monocyclic structure. In other words, the bridging structure represented by Q may include a cyclic structure.

[0054] If Q has a monocyclic structure, the monocyclic structure may be directly bonded to Y and X in general formula (1B), or a divalent substituent may be sandwiched between the monocyclic structure and Y and / or Z in general formula (1B). Examples of the divalent substituent include alkylene groups having 1 to 5 carbon atoms, alkenylene groups having 2 to 5 carbon atoms, heteroatoms such as oxygen atoms and sulfur atoms, or these being bonded in series.

[0055] Q preferably represents CH2, NH, or O independently, and CH2 and NH may have further substituents, with CH2 or NH being more preferable.

[0056] Q may have a fused ring structure. In other words, the cross-linked structure represented by Q may include a fused ring structure.

[0057] If Q has a fused ring structure, the fused ring structure may be directly bonded to Y and X in general formula (1B), or a divalent substituent may be sandwiched between the fused ring structure and Y and / or X in general formula (1B). The divalent substituent is the same as that described above as a divalent substituent sandwiched between the monocyclic structure and Y and / or X in general formula (1A).

[0058] Q may have substituents.

[0059] The number of carbon atoms in Q is preferably 12 or less, more preferably 10 or less, and even more preferably 8 or less.

[0060] Y may be a group of atoms with a valency of 0 to 1. Each Y independently represents a group of atoms with a valency of 0 to 1, containing typical elements from groups 14 to 16 of the periodic table that can coordinate to the atoms of metal M, and may further have substituents. Preferred typical elements from groups 14 to 16 of the periodic table are carbon atoms, nitrogen atoms, phosphorus atoms, arsenic atoms, oxygen atoms, sulfur atoms, and selenium atoms; more preferably carbon atoms, nitrogen atoms, phosphorus atoms, and arsenic atoms; even more preferably nitrogen atoms and phosphorus atoms; and particularly preferably phosphorus atoms.

[0061] In general formula (1B), it is preferable that both Y atoms represent either a nitrogen atom or a phosphorus atom, or that one Y atom represents a phosphorus atom and the other Y atom represents a nitrogen atom.

[0062] When the 0- to 1-valent atomic group represented by Y has substituents, examples of substituents include substituent group A, which are preferably alkyl groups or aryl groups, and more preferably ethyl groups, t-butyl groups, or phenyl groups.

[0063] In the catalyst, X and the two Y atoms in general formula (1B) may be coordinated to the metal M. That is, the catalyst may be a metal complex containing the metal M and a tridentate ligand.

[0064] The catalyst is preferably a metal complex that forms a hydride complex within the reaction system.

[0065] The catalyst is preferably at least one selected from the group consisting of a metal complex represented by the following general formula (1A), its tautomers, stereoisomers, and salts thereof.

[0066]

[0067] (In general formula (1A), X represents an atomic group containing typical elements of groups 13 to 15 that can coordinate to M; Q independently represents a bridging structure containing typical elements of groups 14 to 16 that connects Y and X; Y independently represents an atomic group containing typical elements of groups 14 to 16 that can coordinate to M; M represents a metal atom; Z represents an anionic ligand; n represents 0 to 3; and L, if multiple are present, independently represents a neutral or anionic ligand.)

[0068] In this specification, "group n" means "group n of the periodic table."

[0069] In general formula (1A), X, Y, and Q are equivalent to X, Y, and Q in general formula (1B), respectively, and the preferred ranges are also the same.

[0070] Furthermore, if Q has substituents and does not have either a monocyclic or fused ring structure, the substituent is a substituent on the portion of Q in the ring structure composed of Q, Y, X, and M in general formula (1A).

[0071] If Q has a monocyclic or fused ring structure, the substituent is a substituent of the monocyclic or fused ring structure, or a substituent of the portion of Q in a ring structure composed of Q, Y, X, and M in general formula (1A).

[0072] The substituents that Q may have may, for example, have heteroatoms, or may be other atoms or groups of atoms.

[0073] Examples of substituents having the heteroatom include alkoxy groups having 1 to 18 carbon atoms, arylalkoxy groups having 7 to 18 carbon atoms, aryloxy groups having 6 to 18 carbon atoms, acyl groups having 2 to 18 carbon atoms, alloyl groups having 7 to 18 carbon atoms, dialkylamino groups having 2 to 18 carbon atoms, oxygen atoms, sulfur atoms, and the like.

[0074] Examples of other atoms or groups of atoms include aromatic groups having 3 to 18 carbon atoms, alkyl groups having 1 to 18 carbon atoms, halogen atoms, and the like. Examples of aromatic groups include aryl groups having 6 to 20 carbon atoms, such as phenyl, xylyl, naphthyl, and biphenyl.

[0075] M represents a metal atom. The metal M described above is preferred.

[0076] Examples of anionic ligands represented by Z include halide ions (halogen atoms), hydride ions (hydrogen atoms), nitrate ions, and cyanide ions. Z preferably represents a halogen atom or a hydrogen atom, and more preferably a halogen atom. Z is even more preferably a chlorine atom or a bromine atom, and particularly preferably a chlorine atom.

[0077] n represents an integer between 0 and 3, and M represents the number of ligands coordinating to the metal atom. From the viewpoint of catalyst stability, n is preferably 2 or 3.

[0078] If multiple Ls exist, each L independently represents a neutral or anionic ligand.

[0079] Examples of neutral ligands represented by L include ammonia, carbon monoxide, phosphines (e.g., triphenylphosphine, tris(4-methoxyphenyl)phosphine), phosphine oxides (e.g., triphenylphosphine oxide), sulfides (e.g., dimethyl sulfide), sulfoxides (e.g., dimethyl sulfoxide), ethers (e.g., diethyl ether), nitriles (e.g., p-methylbenzonitrile), heterocyclic compounds (e.g., pyridine, N,N-dimethyl-4-aminopyridine, tetrahydrothiophene, tetrahydrofuran), and the like. Triphenylphosphine and carbon monoxide are preferred, and carbon monoxide is more preferred.

[0080] Examples of anionic ligands represented by L include hydride ions (hydrogen atoms), nitrate ions, cyanide ions, etc., with hydride ions (hydrogen atoms) being preferred.

[0081] In general formula (1A), it is preferable that X represents a heterocycle, Q represents CH2, NH, or O, Y represents a phosphorus atom, and M represents ruthenium.

[0082] Furthermore, it is preferable that Z represents a chlorine atom, n represents 1 to 3, and L independently represents a hydrogen atom, carbon monoxide, or triphenylphosphine.

[0083] Alternatively, in general formula (1A), it is preferable that X represents an atomic group in which a hydrogen atom or alkyl group is bonded to a nitrogen atom, Q represents CH2, NH, or O, Y represents a phosphorus atom, M represents manganese, Z represents a bromine atom, n represents 2 to 3, and L independently represents a hydrogen atom, carbon monoxide, or triphenylphosphine.

[0084] It is preferable that the metal complex represented by general formula (1A) is the metal complex represented by the following general formula (2A).

[0085]

[0086] (In general formula (2A), X1 represents a heteroaromatic ring formed with two carbon atoms and a nitrogen atom, which may have substituents or be bonded with other substituents to form a ring; Q1 independently represents CH2, NH, or O, with CH2 and NH further having substituents; Y1 independently represents a phosphorus atom or a nitrogen atom; R independently represents an alkyl group, an aryl group, or an aralkyl group, which further have substituents; M represents a metal atom; Z represents an anionic ligand; n represents 0 to 3; and L, if multiple are present, independently represents a neutral or anionic ligand.)

[0087] In general formula (2A), M, Q1, Z, n, and L are equivalent to M, Q, Z, n, and L in general formula (1A), respectively, and the preferred ranges are also the same.

[0088] The heteroaromatic ring formed with the two carbon atoms and nitrogen atom represented by X1 is preferably a pyrroline ring, a pyridine ring, an imidazoline ring, a pyrimidine ring, or a triazine ring, more preferably a pyridine ring or a triazine ring, and even more preferably a pyridine ring.

[0089] Examples of substituents that X1 may have include substituent group A, where alkyl groups are preferred and methyl groups are more preferred.

[0090] Y1 represents a phosphorus atom or a nitrogen atom, preferably a phosphorus atom. Both Y1s may represent either a nitrogen atom or a phosphorus atom, or one Y1 may represent a phosphorus atom and the other Y1 may represent a nitrogen atom. Both Y1s may represent either a nitrogen atom or a phosphorus atom.

[0091] The alkyl group represented by R can be a linear, branched, cyclic substituted, or unsubstituted alkyl group. Preferably, the alkyl group represented by R is an alkyl group having 1 to 30 carbon atoms, such as a methyl group, ethyl group, n-propyl group, i-propyl group, t-butyl group, n-octyl group, eicosyl group, or 2-ethylhexyl group. From the viewpoint of catalytic activity, it is preferable that the alkyl group has 12 carbon atoms or less, preferably an ethyl group or a t-butyl group, and more preferably a t-butyl group.

[0092] The aryl group represented by R can be a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, such as a phenyl group, a p-tolyl group, a naphthyl group, an m-chlorophenyl group, an o-hexadecanoylaminophenyl group, and so on. Preferably, it is an aryl group having 12 carbon atoms or less, and more preferably a phenyl group.

[0093] If R has further substituents, examples of substituents include substituent group A, which preferably consists of a methyl group, an ethyl group, an i-propyl group, a t-butyl group, or a phenyl group, and more preferably an ethyl group, an i-propyl group, or a t-butyl group.

[0094] In general formula (2A), it is preferable that X1 represents a pyridine ring or a triazine ring, Q1 represents CH2, NH, or O, Y1 represents a phosphorus atom, R represents an ethyl group, a t-butyl group, or a phenyl group, and M represents ruthenium.

[0095] Furthermore, it is preferable that Z represents a chlorine atom, n represents 1 to 3, and L independently represents a hydrogen atom, carbon monoxide, or triphenylphosphine.

[0096] It is preferable that the metal complex represented by general formula (2A) is the metal complex represented by the following general formula (3A).

[0097]

[0098] (In general formula (3A), R0 represents a hydrogen atom or an alkyl group, A independently represents CH, CR5, or N, R5 represents an alkyl group, aryl group, aralkyl group, amino group, hydroxyl group, or alkoxy group, Q1 independently represents CH2, NH, or O, and CH2 and NH may have further substituents, Y1 represents a phosphorus atom or a nitrogen atom, R independently represents an alkyl group, aryl group, or aralkyl group, which may have further substituents, M represents a metal atom, Z represents an anionic ligand, n represents 0 to 3, and L, if there are multiple, independently represents a neutral or anionic ligand.)

[0099] In general formula (3A), Y1, R, Q1, M, Z, n, and L are equivalent to Y1, R, Q1, M, Z, n, and L in general formula (2A), respectively, and the preferred ranges are also the same.

[0100] In general formula (3A), R0 represents a hydrogen atom or an alkyl group. Examples of alkyl groups represented by R0 include linear, branched, cyclic, substituted, or unsubstituted alkyl groups. Preferably, the alkyl group represented by R0 is an alkyl group having 1 to 30 carbon atoms, such as a methyl group, ethyl group, n-propyl group, i-propyl group, t-butyl group, n-octyl group, eicosyl group, or 2-ethylhexyl group. From the viewpoint of ease of raw material procurement, it is preferable that the alkyl group has 6 carbon atoms or less, and is preferably a methyl group.

[0101] In general formula (3A), R0 is preferably a hydrogen atom or a methyl group.

[0102] A independently represents CH, CR5, or N, and R5 represents an alkyl group, aryl group, aralkyl group, amino group, hydroxyl group, or alkoxy group.

[0103] The alkyl group represented by R5 can be a linear, branched, cyclic substituted, or unsubstituted alkyl group. Preferably, the alkyl group represented by R5 is an alkyl group having 1 to 30 carbon atoms, such as a methyl group, ethyl group, n-propyl group, i-propyl group, t-butyl group, n-octyl group, eicosyl group, or 2-ethylhexyl group. From the viewpoint of ease of raw material procurement, it is preferable that the alkyl group has 12 carbon atoms or less, and is preferably a methyl group.

[0104] The aryl group represented by R5 can be a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, such as a phenyl group, a p-tolyl group, a naphthyl group, an m-chlorophenyl group, an o-hexadecanoylaminophenyl group, and so on. Preferably, it is an aryl group having 12 carbon atoms or less, and more preferably a phenyl group.

[0105] Examples of aralkyl groups represented by R5 include substituted or unsubstituted aralkyl groups having 30 or fewer carbon atoms, such as trityl, benzyl, phenethyl, tritylmethyl, diphenylmethyl, and naphthylmethyl groups, and preferably aralkyl groups having 12 or fewer carbon atoms.

[0106] The alkoxy group represented by R5 is preferably a substituted or unsubstituted alkoxy group having 1 to 30 carbon atoms, such as a methoxy group, ethoxy group, isopropoxy group, t-butoxy group, n-octyloxy group, or 2-methoxyethoxy group.

[0107] In general formula (3A), it is preferable that X1 represents a pyridine ring or a triazine ring, Q1 represents CH2, NH, or O, Y1 represents a phosphorus atom, R represents an ethyl group, a t-butyl group, or a phenyl group, and M represents ruthenium.

[0108] Furthermore, it is preferable that Z represents a chlorine atom, n represents 1 to 3, and L independently represents a hydrogen atom, carbon monoxide, or triphenylphosphine.

[0109] The metal complex represented by general formula (3A) is preferably a ruthenium complex represented by the following general formula (4A).

[0110] The ruthenium complex represented by general formula (4A) is soluble in organic solvents and insoluble in water, making it suitable as a catalyst in the method for producing formate according to the first embodiment of the present invention. Since the formate produced by the reaction is readily soluble in water, separation of the catalyst and formate is facilitated in the two-phase reaction, making it easier to separate and recover the catalyst and formate from the reaction system, enabling the production of formate in high yield and facilitating the reuse of expensive catalysts.

[0111]

[0112] (In general formula (4A), R0 represents a hydrogen atom or an alkyl group, Q1 independently represents CH2, NH, or O, and CH2 and NH may have further substituents, R1 independently represents an alkyl group or an aryl group (however, if Q1 represents NH or O, at least one of R1 represents an aryl group), A independently represents CH, CR5, or N, R5 represents an alkyl group, an aryl group, an aralkyl group, an amino group, a hydroxyl group, or an alkoxy group, Z represents an anionic ligand, n represents 0 to 3, and L, if there are multiple, independently represents a neutral or anionic ligand.)

[0113] In general formula (4A), R0, A, Q1, Z, L, and n are equivalent to R0, A, Q1, Z, L, and n in general formula (3A), respectively, and the preferred ranges are also the same.

[0114] The alkyl group and aryl group represented by R1 are the same as the alkyl group and aryl group represented by R in general formula (3A), respectively, and the preferred ranges are also the same.

[0115] Metal complexes represented by general formulas (1A) to (4A) may produce stereoisomers depending on the coordination mode and conformation of the ligands, but they may also be a mixture of these stereoisomers or a single pure isomer.

[0116] Metal complexes represented by general formulas (1A) to (4A) can also be used if they are manufactured by known methods. Known methods include, for example, the method described in E. Pidko et al., ChemCatChem 2014, 6, 1526-1530.

[0117] Specific examples of ruthenium complexes represented by general formula (4A) include the compounds listed below. In the compounds listed below, Et represents an ethyl group, tBu represents a tert-butyl group, and Ph represents a phenyl group.

[0118]

[0119]

[0120]

[0121] The amount of catalyst (preferably a ruthenium complex) used is not particularly limited. From the viewpoint of fully exhibiting the function of the catalyst, the amount of catalyst used is preferably 0.1 μmol or more, more preferably 0.5 μmol or more, and even more preferably 1 μmol or more, per liter of the organic phase solvent (organic solvent). From the viewpoint of cost, it is preferably 1 mol or less, more preferably 10 mmol or less, and even more preferably 1 mmol or less, per liter of the organic solvent. Furthermore, from the viewpoint of suppressing a decrease in catalytic efficiency, it may be 100 μmol or less, or 10 μmol or less, per liter of the organic solvent. When two or more types of catalysts are used, the total amount used is sufficient as long as it is within the above ranges.

[0122] (Phase Transfer Catalyst) In the method for producing formate according to the first embodiment, since the reaction to produce formate must be carried out in a two-phase system, a phase transfer catalyst that facilitates the transfer of substances between the two phases may be used. Examples of phase transfer catalysts include quaternary ammonium salts, quaternary phosphates, macrocyclic polyethers such as crown ethers, nitrogen-containing macrocyclic polyethers such as cryptands, nitrogen-containing linear polyethers, polyethylene glycol and its alkyl ethers. Among these, quaternary ammonium salts are preferred from the viewpoint that the transfer of substances between the aqueous solvent and the organic solvent is easy even under mild reaction conditions.

[0123] Examples of quaternary ammonium salts include methyltrioctylammonium chloride, benzyltrimethylammonium chloride, trimethylphenylammonium bromide, tributylammonium tribromide, tetrahexylammonium bisulfate, decyltrimethylammonium bromide, diallyldimethylammonium chloride, dodecyltrimethylammonium bromide, dimethyldioctadecylammonium bromide, tetraethylammonium tetrafluoroborane, ethyltrimethylammonium iodide, tris(2-hydroxyethyl)methylammonium hydroxide, tetramethylammonium acetate, tetramethylammonium bromide, and tetraethylammonium iodide, with methyltrioctylammonium chloride being preferred.

[0124] The amount of phase transfer catalyst used is not particularly limited. Preferably, the amount of phase transfer catalyst used is 0.1 mmol or more, more preferably 0.5 mmol or more, and even more preferably 1 mmol or more, per liter of the organic and aqueous solvents. From a cost viewpoint, it is preferably 1 mol or less, more preferably 500 mmol or less, and even more preferably 100 mmol or less, per liter of the organic and aqueous solvents. When using two or more types of phase transfer catalysts, the total amount used should not exceed the above range.

[0125] (Hydrogen) In the first embodiment, either gaseous hydrogen from a gas cylinder or liquid hydrogen can be used as the hydrogen. As a hydrogen supply source, for example, hydrogen generated in the iron smelting process or hydrogen generated in the soda production process can be used. Hydrogen generated from the electrolysis of water can also be utilized.

[0126] When using gaseous hydrogen from a gas cylinder as the hydrogen source, the pressure should be, for example, 0.1 MPa or higher, but may also be 0.2 MPa or higher, 0.5 MPa or higher, 1 MPa or higher, 4 MPa or higher, 4.5 MPa or higher, or even 5 MPa or higher, from the viewpoint of ensuring sufficient reactivity. Furthermore, in order to prevent the equipment from becoming too large, the pressure should preferably be 50 MPa or lower, more preferably 20 MPa or lower, and even more preferably 10 MPa or lower.

[0127] (Gas G) The gas G used in the first embodiment may consist only of carbon dioxide and NOx, or it may be a mixture of carbon dioxide and NOx with other components. Other components include inert gases such as nitrogen and argon, water vapor, and any other components contained in exhaust gas, etc. Gas G may be a pre-mixed mixture, or it may be prepared by introducing carbon dioxide and NOx into the system separately. Gas G may be exhaust gas or exhaust gas-derived gas. Exhaust gas-derived gas is, for example, exhaust gas that has been treated with washing, denitrification, desulfurization, etc. Gas G may be concentrated exhaust gas or exhaust gas-derived gas.

[0128] The carbon dioxide concentration in gas G is, for example, 75% or more, preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, and less than 100%. The NOx concentration in gas G is greater than 0 ppm, preferably 0.1 ppm or more, and more preferably 0.5 ppm or more. The NOx concentration in gas G is preferably 500 ppm or less, more preferably 100 ppm or less, and even more preferably 50 ppm or less. The method for producing formate of the first embodiment may further include a step of adjusting the NOx concentration in gas G. For example, the NOx concentration in gas G may be adjusted to a desired range by denitrifying the exhaust gas.

[0129] There are no particular restrictions on the method of introducing the gas G, hydrogen, catalyst, solvent, etc. used in the reaction into the reaction vessel. All raw materials may be introduced at once, some or all of the raw materials may be introduced in stages, or some or all of the raw materials may be introduced continuously. A combination of these methods may also be used.

[0130] <Gas Contact Step> The method for producing formate according to the first embodiment may include a gas contact step in which gas G is brought into contact with the aqueous phase. The method for bringing gas G into contact with the aqueous phase is not particularly limited and may be carried out by introducing gas G into the aqueous phase or into a solvent containing the aqueous phase by bubbling (blowing). Alternatively, gas G may be introduced into a container containing the aqueous phase or a solvent containing the aqueous phase, and then the aqueous phase and gas G may be brought into contact by stirring with a stirring device or by rotating the container. Alternatively, gas-liquid contact may be brought into contact by passing the aqueous phase and gas G through a plug flow reactor.

[0131] The introduction of gas G may be carried out continuously or intermittently.

[0132] In the gas contact step, only gas G may be brought into contact with the aqueous phase, or gas G and hydrogen gas may be brought into contact with the aqueous phase. Furthermore, gas G may be brought into contact with the organic phase as long as it is in contact with the aqueous phase. For example, gas G may be brought into contact with a solvent containing both the aqueous phase and the organic phase.

[0133] The aqueous phase into which gas G comes into contact is preferably a basic solution containing a base. By contacting gas G with a basic solution, a reaction between carbon dioxide and the base produces a bicarbonate and / or carbonate (compound C). Compound C dissolves in the aqueous phase. Therefore, an aqueous solution of compound C may be obtained as a result of the gas contact step.

[0134] Examples of the bicarbonates and carbonates produced include alkali metal and alkaline earth metal carbonates or bicarbonates. Examples of bicarbonates include sodium bicarbonate and potassium bicarbonate, with potassium bicarbonate being preferred from the viewpoint of high solubility in water. That is, in the first embodiment, it is preferable that compound C containing potassium bicarbonate is produced as a bicarbonate as a result of the gas contact step. Examples of carbonates include sodium carbonate, potassium carbonate, sodium potassium carbonate, and sodium sesquicarbonate.

[0135] The solvent in the basic solution for the formation of compound C preferably contains water, and more preferably is water. In some cases, a water-soluble solvent such as methanol or ethanol may be added to the water. The base used in the basic solution is not particularly limited as long as it can react with carbon dioxide to produce a bicarbonate or carbonate, and examples include lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, cesium bicarbonate, potassium hydroxide, sodium hydroxide, diazabicycloundecene, and triethylamine. Among the above, hydroxides are preferred, potassium hydroxide and sodium hydroxide are more preferred, and potassium hydroxide is even more preferred.

[0136] The base content in the basic solution is not particularly limited, as long as compound C can be produced. The base content is preferably 0.1 mol or more, more preferably 0.5 mol or more, and even more preferably 1 mol or more per liter of aqueous solvent. Furthermore, from the viewpoint of reaction efficiency, it is preferably 30 mol or less, more preferably 20 mol or less, and even more preferably 15 mol or less. However, if the amount exceeds the solubility of the aqueous phase, the solution will be suspended.

[0137] The ratio of carbon dioxide to base used in the reaction between carbon dioxide and a base is preferably 0.1 or higher in molar ratio, more preferably 0.5 or higher, and even more preferably 1.0 or higher, from the viewpoint of producing carbonate from carbon dioxide. Furthermore, from the viewpoint of carbon dioxide utilization efficiency, it is preferably 8.0 or lower, more preferably 5.0 or lower, and even more preferably 3.0 or lower. Note that the ratio of carbon dioxide to base used is the ratio of the molar amount of carbon dioxide introduced into the container to contact with the aqueous phase in the gas contact step to the molar amount of base used, which is molar amount of CO2 (mol) / molar amount of base (mol). By setting the ratio of carbon dioxide to base used within the above range, the excessive input of carbon dioxide into the reaction vessel can be suppressed, and the amount of unreacted carbon dioxide can be minimized. This makes it easier to improve, for example, the conversion efficiency of the final formic acid. In addition, within the same container, carbon dioxide can be hydrogenated by a reaction with hydrogen, via a reaction between carbon dioxide and a base to produce a bicarbonate or carbonate, and then formate. Unreacted carbon dioxide can be recovered from the reaction vessel and reused.

[0138] The temperature of the aqueous phase in the gas contact process is not particularly limited, but considering the reaction that produces compound C containing a bicarbonate or carbonate through the reaction of carbon dioxide with a base, and the dissolution of carbon dioxide and compound C in the aqueous phase, it is preferably 0°C or higher, more preferably 10°C or higher, and even more preferably 20°C or higher. Furthermore, it is preferably 100°C or lower, more preferably 80°C or lower, and even more preferably 40°C or lower.

[0139] The reaction time in the reaction that produces compound C by the reaction of carbon dioxide and a base is not particularly limited, but for example, from the viewpoint of ensuring a sufficient amount of compound C produced, it is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 2 hours or more. Also, from the viewpoint of cost, it is preferably 24 hours or less, more preferably 12 hours or less, and even more preferably 6 hours or less. For example, in the gas contact step, it is preferable to bubble gas G into the aqueous phase for the above time.

[0140] In the gas contact process, gas G is brought into contact with the aqueous phase, causing NOx in gas G to be released into the aqueous phase as nitrite ions (NO2). - ) and nitrate ions (NO3 - ) generates.

[0141] If the method for producing formate according to the first embodiment further includes a step of adjusting the NOx concentration in gas G, a gas contact step is performed after that step. Depending on the step of adjusting the NOx concentration in gas G, the nitrite ions (NO2) in the aqueous phase after the gas contact step, i.e., the aqueous phase used in the hydrogenation reaction step described later, may be affected. - ) and nitrate ions (NO3 - The concentration of ) can be adjusted.

[0142] <Hydrogenation reaction step> The method for producing formate according to the first embodiment typically includes a step of producing formate by a hydrogenation reaction of a starting compound using a catalyst in a two-phase system in which the organic phase and the aqueous phase are separated. Preferably, the method for producing formate according to the first embodiment includes a hydrogenation reaction step in which compound C and hydrogen are reacted using a catalyst in a two-phase system in which the organic phase and the aqueous phase are separated to produce formate.

[0143] In the hydrogenation reaction step, it is preferable that the organic phase contains a catalyst and the aqueous phase contains the starting compound (compound C). The catalyst is, for example, dissolved in an organic solvent. Hereinafter, the organic phase and the aqueous phase may be collectively referred to as the reaction solution. As the aqueous phase, the aqueous phase containing compound C obtained in the gas contact step described above may be used directly in the hydrogenation reaction step.

[0144] In the first embodiment, the use of gas G may cause NOx to generate ions in the aqueous phase. For example, the aqueous phase may contain nitrite ions (NO2 - ) may contain NO2 in the aqueous phase during the hydrogenation reaction step. - The concentration is preferably less than 100 ppm. NO2 in the aqueous phase - By keeping the concentration of less than 100 ppm, the decrease in catalytic activity can be suppressed. This allows the reaction to be repeated multiple times while maintaining TON and yield. From the viewpoint of improving TON and yield, NO2 -The concentration is more preferably 50 ppm or less, still more preferably 20 ppm or less, and particularly preferably less than 5.0 ppm. NO2 in the aqueous phase - The concentration may be 1 ppm or less, or may be 0.5 ppm or less in some cases. NO2 in the aqueous phase in the hydrogenation reaction step - The concentration is, for example, 0 ppm or more and less than 100 ppm, may be more than 0 ppm and less than 100 ppm, may be more than 0 ppm and 50 ppm or less, or may be 0.5 ppm or more and 50 ppm or less.

[0145] The NO2 in the aqueous phase - The concentration can be measured by ion chromatography.

[0146] The aqueous phase may contain nitrate ions (NO3 - ). Even if NO3 is contained in the aqueous phase, it is considered that, unlike NO2, it does not have a significant influence on the catalytic activity depending on its concentration. The concentration of NO3 in the aqueous phase in the hydrogenation reaction step - can be, for example, less than 100 ppm, 50 ppm or less, or even less than 50 ppm. - -

[0147] In the hydrogenation reaction step, for example, the reaction solution is stirred. The stirring conditions of the reaction solution are not particularly limited, but the stirring power is preferably 0.2 kW / m 3 [[ID=2(5]]or more, and more preferably 0.5 kW / m 3 or more. The greater the stirring power, the more likely the gas dispersion in the aqueous and organic phases is to improve. When the reaction solution is stirred, a gas (e.g., hydrogen gas) is entrained into the reaction solution from above the liquid surface of the reaction solution, whereby the aqueous and organic phases are filled with the gas. However, the method of filling the aqueous and organic phases with gas is not limited to the above, and a sparger may be used.

[0148] ​​The shape of the impeller used to stir the reaction liquid is not particularly limited. Examples of impellers include anchor impellers, turbine impellers, paddle impellers, and large impellers collectively known as "large impellers," such as Fullzone® impellers (Shinko Environmental Solutions Co., Ltd.) and Maxblend® impellers (Sumitomo Heavy Industries Process Equipment Co., Ltd.).

[0149] In the reaction between hydrogen and compound C (hydrogenation reaction), it is preferable to introduce compound C into the reaction vessel first, followed by the introduction of hydrogen. The introduction of hydrogen and compound C may be carried out continuously or intermittently, either one or both at a time.

[0150] The reaction temperature in the reaction between hydrogen and compound C is not particularly limited, but to ensure the reaction proceeds efficiently, it is preferably 30°C or higher, more preferably 40°C or higher, and even more preferably 50°C or higher. From the viewpoint of energy efficiency, it is preferably 200°C or lower, more preferably 150°C or lower, and even more preferably 100°C or lower. The reaction temperature can be adjusted by heating or cooling, and heating is preferred. For example, it is preferable to introduce (or generate) compound C into the reaction vessel, raise the temperature, and then introduce hydrogen.

[0151] The reaction time in the reaction between hydrogen and compound C is not particularly limited, but from the viewpoint of ensuring a sufficient amount of formate produced and improving the catalyst's TON, it is, for example, 0.5 hours or more, and may be 1 hour or more, 2 hours or more, 6 hours or more, 12 hours or more, 24 hours or more, 36 hours or more, 48 hours or more, or even 60 hours or more. The upper limit of the reaction time is not particularly limited, but for example, 500 hours.

[0152] The reaction pressure (gas pressure in the reaction vessel) in the reaction between hydrogen and compound C is not particularly limited, but from the viewpoint of improving the catalyst TON, it may be, for example, 0.1 MPa or higher, 0.2 MPa or higher, 0.5 MPa or higher, 1 MPa or higher, 4 MPa or higher, 4.5 MPa or higher, or even 5 MPa or higher. The upper limit of the reaction pressure is not particularly limited, and may be, for example, 50 MPa, 20 MPa or 10 MPa.

[0153] The gas contact step and the hydrogenation reaction step may be carried out simultaneously. For example, gas G and hydrogen may be introduced into a reaction solution consisting of an organic phase containing a catalyst and an aqueous phase containing a base, and the reaction solution may be stirred. In this way, the reaction in which carbon dioxide is introduced into the aqueous phase to produce compound C containing at least one selected from the group consisting of carbonates and bicarbonates, and the reaction in which compound C and hydrogen produce formate may proceed simultaneously.

[0154] The formate produced by the reaction dissolves in the aqueous phase. This prevents the formate formation reaction from stopping due to equilibrium, allowing for high yield production of formate. Furthermore, since the aqueous phase and the organic phase can be separated by a simple method, expensive catalysts tend to be reused without losing their catalytic activity. By reusing the catalyst, high productivity can be achieved.

[0155] According to the method for producing formate according to the first embodiment of the present invention, hydrogen and carbon dioxide, for example, carbon dioxide contained in exhaust gas or gas derived from exhaust gas, can be stored as formate (e.g., alkali metal formate). Formate has the advantage of being easy to handle because it has a high hydrogen storage density, is safe, and is stable as a chemical substance, and allows for long-term storage of hydrogen and carbon dioxide. Formate has high solubility in aqueous solvents and can be separated as a highly concentrated aqueous solution of formate. The aqueous solution of formate can be used in the method for producing formic acid described later, after adjusting the concentration of formate as needed.

[0156] The concentration of the formate produced by the method for producing formate according to the first embodiment (concentration of formate in the aqueous phase) is preferably 0.5 mol / L or higher, more preferably 1 mol / L or higher, even more preferably 2.5 mol / L or higher, and particularly preferably 5 mol / L or higher, in order to produce formate with high yield and excellent productivity. Furthermore, in order to simplify the production process by producing the formate in a dissolved state, the concentration is preferably 30 mol / L or lower, more preferably 25 mol / L or lower, even more preferably 20 mol / L or lower, and particularly preferably 10 mol / L or lower.

[0157] The method for producing formate according to the first embodiment is suitable for suppressing the deterioration of catalyst efficiency even when using a gas containing NOx along with carbon dioxide, such as exhaust gas. The suppression of catalyst efficiency deterioration can be evaluated, for example, by the change in TON when the reaction of the starting compound (compound C) is repeatedly carried out using the catalyst. TON is calculated by the formula: (amount of formate produced) / (amount of catalyst used in the reaction). For example, the TON in the first reaction may be 500,000 or more, 700,000 or more, or 900,000 or more. The TON in the second reaction when the reaction of the starting compound is repeatedly carried out using the catalyst may be 500,000 or more, 700,000 or more, or 900,000 or more. The TON in the third reaction may be 500,000 or more, 700,000 or more, or 900,000 or more. There is no particular upper limit to TON; for example, it can be 5,000,000 or less.

[0158] In the method for producing formate according to the first embodiment, it is preferable to have a practically sufficient yield in the reaction of the starting compound. For example, the yield in the first reaction is preferably 40% or more, more preferably 45% or more, and even more preferably 50% or more. When the reaction of the starting compound is repeated using a catalyst, the yield in the second reaction is preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. The yield in the third reaction is preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. The upper limit of the yield is not particularly limited, and is, for example, 99% or less.

[0159] (Other components) In the hydrogenation reaction step, antioxidants may be added to the reaction solution as needed. For example, antioxidants may be added to the organic phase. Examples of antioxidants include phosphorus-based antioxidants, amine-based antioxidants, phenol-based antioxidants, and sulfur-based antioxidants, and it is preferable to include at least one selected from the group consisting of phosphorus-based antioxidants, amine-based antioxidants, and phenol-based antioxidants. With the above configuration, the degradation of the catalyst can be reduced in particular. The organic phase may also contain antioxidants.

[0160] Phosphorus-based antioxidants are preferably those that are highly stable and resistant to degradation such as hydrolysis, and are preferably compounds with a relatively bulky structure. Phosphorus-based antioxidants are, for example, phosphorus compounds. Phosphorus compounds may be organophosphorus compounds, and may be phosphites, hypophosphites, or phosphonites. Examples of phosphites include trialkyl phosphites, triaryl phosphites, alkylaryl phosphites, and thiophosphites.

[0161] Phosphorus-based antioxidants are, for example, compounds having an aryl group. Examples of aryl groups include substituted or unsubstituted aryl groups having 6 to 30 carbon atoms, such as substituted or unsubstituted phenyl groups, and preferably phenyl groups having a t-butyl group.

[0162] Examples of phosphorus-based antioxidants include triphenyl phosphite, diisooctyl phosphite, heptakis triphosphite, triisodecyl phosphite, diphenylisooctyl phosphite, diisooctylphenyl phosphite, diphenyltridecyl phosphite, triisooctyl phosphite, trilauryl phosphite, diphenyl phosphite, tris(dipropylene glycol) phosphite, diisodecylpentaerythritol diphosphite, dioleylhydrogen phosphite, and trila Uryl trithiophosphite, bis(tridecyl) phosphite, tris(isodecyl) phosphite, tris(tridecyl) phosphite, diphenyldecyl phosphite, dinonylphenylbis(nonylphenyl) phosphite, poly(dipropylene glycol)phenyl phosphite, tetraphenyldipropyl glycol diphosphite, trisnonylphenyl phosphite, tris(2,4-di-tert-butylphenyl) phosphite, tris(2,4-di-tert-butyl-5-methylphenyl) Phosphite, Tris[2-tert-butyl-4-(3-tert-butyl-4-hydroxy-5-methylphenylthio)-5-methylphenyl] phosphite, Tridecyl phosphite, Octyl diphenyl phosphite, Di(decyl) monophenyl phosphite, Distearyl pentaerythritol diphosphite, Mixture of distearyl pentaerythritol and calcium stearate, Alkyl (C10) bisphenol A phosphite, Di(tridecyl) pentaerythritol diphosphite, Di( Nonylphenyl) pentaerythritol diphosphite, bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, bis(2,6-di-tert-butyl-4-methylphenyl) pentaerythritol diphosphite, bis(2,4,6-tri-tert-butylphenyl) pentaerythritol diphosphite, bis(2,4-dicumylphenyl) pentaerythritol diphosphite, tetraphenyl-tetra(tridecyl) pentaerythritol tetraphosphite, bis(2,4-di-tert-butyl-6-methylphenyl) ethyl phosphite, tetra(tridecyl)isopropylidene diphenol diphosphite, tetra(tridecyl)-4,4'-n-butylidenebis(2-tert-butyl-5-methylphenol) diphosphite, hexa(tridecyl)-1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane triphosphite, tetrakis(2,4-di-tert-butylphenyl) biphenylene Diphosphonite, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, [(1-methyl-1-propanyl-3-ylidene)tris(1,1-dimethylethyl)-5-methyl-4,1-phenylene]hexatridecyl phosphite, 2,2'-methylenebis(4,6-di-tert-butylphenyl)-2-ethylhexyl phosphite, 2,2'-methylenebis(4,6-di-tert-butylphenyl)-octadecyl phosphite, 2, 2'-Ethylidenebis(4,6-di-tert-butylphenyl)fluorophosphite, 4,4'-Butylidenebis(3-methyl-6-tert-butylphenylditridecyl)phosphite, Tris(2-[(2,4,8,10-tetrakis-tert-butyldibenzo[d,f][1,3,2]dioxaphosphine-6-yl)oxy]ethyl)amine, 3,9-bis(4-nonylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5 Examples include undecane, 3,9-bis(2,4-di-tert-butylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, 2,4,6-tri-tert-butylphenyl-2-butyl-2-ethyl-1,3-propanediol phosphite, poly-4,4'-isopropylidenediphenol C12-15 alcohol phosphite, tetraalkyl(C12-15)-4,4'-isopropylidenediphenyl diphosphite, etc.

[0163] The phosphorus-based antioxidants are preferably tris(2,4-di-tert-butylphenyl) phosphite, triphenyl phosphite, triisodecyl phosphite, tetraalkyl(C12-15)-4,4′-isopropylidenediphenyl diphosphite, or 3,9-bis(2,4-di-tert-butylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, with tris(2,4-di-tert-butylphenyl) phosphite or triphenyl phosphite being particularly preferred.

[0164] Amine-based antioxidants are antioxidants that have an amino group. Examples of amine-based antioxidants include 6-ethoxy-2,2,4-trimethyl-1,2-dihydroquinoline, N-phenyl-1-naphthylamine, p,p'-dioctyldiphenylamine, phenothiazine, 4,4'-bis(α,α-dimethylbenzyl)diphenylamine, phenyl-α-naphthylamine, phenyl-β-naphthylamine, N,N'-diphenyl-p-phenylenediamine, N,N'-di-β-naphthyl-p-phenylenediamine, N-cyclohexyl-N'-phenyl-p-phenylenediamine, and N-phenyl-N'-isopropyl- Examples include p-phenylenediamine, aldol-α-naphthylamine, bis(1,2,2,6,6-pentamethyl-4-piperidyl) butyl(3,5-di-tert-butyl-4-hydroxybenzyl)malonate, N1,N3-bis(2,2,6,6-tetramethylpiperidine-4-yl)isophthalamide, bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, and bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate. The amine-based antioxidant is preferably a hindered amine-based antioxidant such as butyl(3,5-di-tert-butyl-4-hydroxybenzyl)malonate bis(1,2,2,6,6-pentamethyl-4-piperidyl), N1,N3-bis(2,2,6,6-tetramethylpiperidine-4-yl) isophthalamide, bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, or bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate.

[0165] Phenolic antioxidants are antioxidants having a phenol group. Examples of phenolic antioxidants include hindered phenolic antioxidants. Examples of hindered phenolic antioxidants include 2,6-di-tert-butyl-p-cresol, 2,6-di-tert-butyl-4-methoxyphenol, 2-tert-butyl-4-methoxyphenol, 3-tert-butyl-4-methoxyphenol, 3,5-di-tert-butyl-4-hydroxybenzoate hexadecyl, 2,4,6-tris(3',5'-di-tert-butyl-4'-hydroxybenzyl)mesitylene, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, and the like.

[0166] Phenolic antioxidants are not limited to the hindered phenolic antioxidants described above. Other phenolic antioxidants besides the hindered phenolic antioxidants described above include, for example, 2,6-di-tert-butyl-4-ethylphenol, 2-tert-butyl-4,6-dimethylphenol, styrene phenol, 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 2,2'-thiobis-(6-tert-butyl-4-methylphenol), and 2,2'-thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]. , 2-methyl-4,6-bis(octylsulfanylmethyl)phenol, 2,2'-isobutylidenebis(4,6-dimethylphenol), isooctyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, N,N'-hexane-1,6-diylbis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionamide], 2,2'-oxamide-bis[ethyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2-ethyl Ihexyl-3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate, 2,2'-ethylenebis(4,6-di-tert-butylphenol), 3,5-bis(1,1-dimethylethyl)-4-hydroxybenzenepropanoic acid and C13-15 alkyl ester, 2,5-di-tert-amylhydroquinone, polymer of hindered phenol (brand name AO.OH998, Adeka Palmarol), 2,2'-methylenebis[6-(1-methylcyclohexyl)-p-crezo 2-tert-butyl-6-(3-tert-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate, 2-[1-(2-hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-tert-pentylphenyl acrylate, 6-[3-(3-tert-butyl-4-hydroxy-5-methyl)propoxy]-2,4,8,10-tetra-tert-butylbenz[d,f][1,3,2]-dioxaphosfepine, hexamethylenebis[3-(3,[5-di-tert-butyl-4-hydroxyphenyl)propionate], bis[monoethyl(3,5-di-tert-butyl-4-hydroxybenzyl)phosphonate]calcium salt, reaction product of 5,7-bis(1,1-dimethylethyl)-3-hydroxy-2(3H)-benzofuranone and o-xylene, 2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazine-2-ylamino)phenol, DL-α-tocopherol (vitamin E), 2,6-bis(α-methylbenzyl)-4-methylphenol Lu, bis[3,3-bis-(4'-hydroxy-3'-tert-butyl-phenyl)butanoic acid] glycol ester, 2,6-diphenyl-4-octadecyloxyphenol, stearyl(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, distearyl(3,5-di-tert-butyl-4-hydroxybenzyl)phosphonate, tridecyl-3,5-tert-butyl-4-hydroxybenzylthioacetate, thiodiethylenebis[(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] ,4,4'-thiobis(6-tert-butyl-m-cresol),2-octylthio-4,6-di(3,5-di-tert-butyl-4-hydroxyphenoxy)-s-triazine,2,2'-methylenebis(4-methyl-6-tert-butylphenol),bis[3,3-bis(4-hydroxy-3-tert-butylphenyl)butyric acid]glycol ester,4,4'-butylidenebis(2,6-di-tert-butylphenol),4,4'-butylidenebis(6-tert-butyl-3-methylphenol),2,2'- Ethylidenebis(4,6-di-tert-butylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, bis[2-tert-butyl-4-methyl-6-(2-hydroxy-3-tert-butyl-5-methylbenzyl)phenyl]terephthalate, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene, 1,3,5-tris[(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxyethyl]isocyanurate, tetrakis[methylene-3-(3',5'-tert-tributyl-4'-hydroxyphenyl)propionate]methane, 2-tert-butyl-4-methyl-6-(2-acryloyloxy-3-tert-butyl-5-methylbenzyl)phenol, 3,9-bis[2-(3-tert-butyl-4-hydroxy-5-methylhydrocinnamoyloxy)-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[ Examples of 3-(3,5-dialkyl-4-hydroxyphenyl)propionic acid derivatives such as undecane, triethylene glycol bis[β-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate], stearyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid amide, palmityl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid amide, myristyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid amide, lauryl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid amide, phenols, etc.

[0167] Sulfur-based antioxidants are antioxidants that contain a sulfur atom (S). Examples of sulfur-based antioxidants include didodecyl 3,3'-thiodipropionate, pentaerythritol tetrakis[3-laurylthiopropionate], dimyristyl 3,3'-thiodipropionate, and distearyl 3,3'-thiodipropionate.

[0168] When an antioxidant is used, the amount used is adjusted, for example, to a range in which the antioxidant dissolves in the reaction solution. From the viewpoint of allowing the antioxidant to fully exert its function, it is preferable that the amount of antioxidant used is 1 mmol or more per liter of solvent. From the viewpoint of reducing the cost of the antioxidant, it is preferable that the amount of antioxidant used is 100 mmol or less per liter of solvent. One type of antioxidant may be used alone, or two or more types may be used in combination. In this invention, the use of an antioxidant is not required.

[0169] Furthermore, the reaction solution may contain additives such as ultraviolet absorbers and light stabilizers in place of or along with the antioxidant. Examples of these additives include those disclosed in Japanese Patent Application Publication No. 2016-44190.

[0170] <Other Steps> The method for producing formate according to the first embodiment of the present invention may include other steps in addition to the steps described above. For example, after the hydrogenation reaction step, it may include a separation step in which the organic phase containing the catalyst is separated from the reaction solution obtained. In the separation step, the reaction solution is separated into an aqueous phase and an organic phase. At this time, the aqueous phase may contain the formate obtained in the hydrogenation reaction step. Since the aqueous phase and the organic phase can be separated by a simple method, the product can be easily recovered.

[0171] The separation process is preferably carried out under an inert atmosphere. An inert atmosphere is, for example, a nitrogen atmosphere or an argon atmosphere. The separation process can be carried out using known separation methods and apparatus.

[0172] [Method for Producing Formic Acid] The method for producing formic acid according to the second embodiment of the present invention includes the steps of producing a formate salt by the above-described method for producing formate salt, and protonating at least a portion of the formate salt to produce formic acid. Hereinafter, in this specification, the step of producing a formate salt may be referred to as the first step. The step of protonating at least a portion of the formate salt to produce formic acid may be referred to as the second step. The method for producing formic acid according to this embodiment includes, for example, the step of producing a formate salt by the above-described method for producing formate salt (first step) and the second step.

[0173] In the first step, the generated formate dissolves into the aqueous phase, and an aqueous solution of formate is obtained by separating the aqueous phase. It is preferable to separate the aqueous phase in the first step and treat the obtained aqueous solution in the second step, for example, using an electrodialysis apparatus, to produce formic acid. The aqueous phase to be separated is the aqueous phase after the completion of the first step.

[0174] In the second step, the aqueous solution of formate obtained in the first step may be used as is, or the concentration of formate in the aqueous solution may be adjusted by concentration or dilution as needed. Methods for diluting the aqueous solution of formate include adding pure water. Methods for concentrating the aqueous solution of formate include removing water by distillation, and concentrating the aqueous solution using a separation membrane unit equipped with a reverse osmosis membrane. When processing using an electrodialysis apparatus, loss of formate due to concentration diffusion may occur in high-concentration aqueous solutions of formate. From the viewpoint of suppressing this, it is preferable to separate the aqueous phase in the first step and use the aqueous solution after adjusting the concentration of formate by dilution in the second step. By preparing a high-concentration aqueous solution of formate in the first step and adjusting the concentration of this aqueous solution by dilution before using it in the second step, formic acid can be produced in higher yield and with better productivity.

[0175] The degree to which the concentration of the aqueous solution of formate obtained in the first step is adjusted (preferably diluted) is not particularly limited. The concentration of formate in the aqueous solution after concentration adjustment is preferably a concentration suitable for electrodialysis, preferably 2.5 mol / L or more, more preferably 3 mol / L or more, 4.75 mol / L or more, and even more preferably 5 mol / L or more. Furthermore, when processing using an electrodialysis apparatus, from the viewpoint of suppressing the loss of formate due to concentration diffusion, the concentration of formate is preferably 20 mol / L or less, more preferably 15 mol / L or less, and even more preferably 10 mol / L or less.

[0176] Pure water can be used for dilution. Alternatively, the water produced in the second step may be used for dilution. Reusing the water produced in the second step for dilution is preferable because it has advantages such as reducing wastewater treatment costs and environmental impact.

[0177] In the method for producing formic acid according to this embodiment, an acid may be added to the aqueous solution of formate obtained in the first step, and a decarboxylation treatment may be performed before using the aqueous solution in the second step. That is, the aqueous phase in the first step may be separated, an acid may be added, and a decarboxylation treatment may be performed before using it in the second step. The aqueous solution of formate obtained in the first step may contain unreacted carbonates or bicarbonates produced by side reactions, and electrodialysis of a solution containing carbonates or bicarbonates may generate carbon dioxide, reducing the dialysis efficiency. Therefore, by adding an acid to the aqueous solution of formate obtained in the first step, performing a decarboxylation treatment, and then performing electrodialysis, formic acid can be produced in a higher yield and with better productivity.

[0178] Examples of acids used in the decarboxylation treatment include formic acid, citric acid, acetic acid, malic acid, lactic acid, succinic acid, tartaric acid, butyric acid, fumaric acid, propionic acid, hydrochloric acid, nitric acid, and sulfuric acid, with formic acid being preferred.

[0179] From the viewpoint of suppressing the amount of carbon dioxide generated during electrodialysis, the amount of acid used relative to the amount of carbon dioxide present in the solution is preferably 50% or more, and more preferably 80% or more. Furthermore, by keeping the pH of the formate solution near neutral during electrodialysis, deterioration of the electrodialysis apparatus can be suppressed, so the amount of acid used relative to the amount of carbon dioxide present in the solution is preferably 150% or less, and more preferably 120% or less.

[0180] In this embodiment, the proportion of formate protonated in the second step is preferably 10% or more, more preferably 20% or more, and even more preferably 30% or more, relative to the initial molar amount of formate in the aqueous solution of formate, from the viewpoint of increasing the purity of the recovered aqueous solution of formic acid.

[0181] Examples of electrodialysis machines used in the second step include two-chamber electrodialysis machines using a bipolar membrane and an anion exchange membrane or a cation exchange membrane, and three-chamber electrodialysis machines using a bipolar membrane, an anion exchange membrane, and a cation exchange membrane.

[0182] Figure 1 is a schematic diagram showing an example of a three-chamber electrodialysis apparatus. The electrodialysis apparatus shown in Figure 1 is equipped with multiple bipolar membranes, anion exchange membranes, and cation exchange membranes. By arranging these bipolar membranes, anion exchange membranes, and cation exchange membranes between the anode and cathode, a base cell, a sample cell (salt cell), and an acid cell are formed. By circulating an aqueous solution of formate salt into the sample cell while the electrodialysis apparatus is energized, the formate salt is converted to formic acid, formic acid can be recovered from the acid cell, water can be recovered from the sample cell, and hydroxide can be recovered from the base cell.

[0183] A two-chamber electrodialysis apparatus, for example, comprises multiple bipolar membranes and cation exchange membranes in each chamber. By alternately arranging these bipolar membranes and cation exchange membranes between the anode and cathode, salt chambers are formed between each bipolar membrane and the cation exchange membrane located on its cathode side, and base cells are formed between each bipolar membrane and the cation exchange membrane located on its anode side. By circulating an aqueous solution of formate salt into the salt chambers while the electrodialysis apparatus is energized, hydroxide is generated in the base cells, and the formate salt circulating in the salt chambers is converted to formic acid.

[0184] In the second step, formate can be protonated in a simple manner using an electrodialysis machine to obtain a formic acid solution.

[0185] [Formic Acid Production System] As shown in Figure 2, the formic acid production system 100 of this embodiment includes, for example, a formate salt production apparatus 10 and an electrodialysis apparatus 30. The production system 100 may further include a dilution apparatus 20 and a dilution water storage unit 40, and may further include a carbon dioxide cylinder 60 for introducing carbon dioxide into the production apparatus 10 and a hydrogen cylinder 50 for introducing hydrogen into the production apparatus 10. The concentration and pressure of carbon dioxide and hydrogen can be adjusted by valves 1 and 2 provided in piping L1 and piping L2.

[0186] The formate produced in the manufacturing apparatus 10 is supplied to the electrodialysis apparatus 30 as an aqueous solution of formate by separating the aqueous phase. At this time, as shown in Figure 2, the aqueous solution of formate may be pre-delivered to the dilution apparatus 20 via the flow path L3, and the concentration of formate in the aqueous solution may be adjusted by dilution in the dilution apparatus 20.

[0187] The aqueous solution, whose formate concentration has been adjusted by the dilution device 20, is sent to the electrodialysis device 30 via channel L4, where at least a portion of the formate is protonated. This produces formic acid and water from the formate. The produced formic acid can be removed via channel L5. Alternatively, the produced water may be sent to the storage unit 40 via channel L7.

[0188] A portion of the formic acid produced by the electrodialysis apparatus 30 may be supplied to the storage unit 40 through the flow path L6. The storage unit 40 may further include a water supply unit 70 and a formic acid supply unit 80. The aqueous solution of formic acid prepared in the storage unit 40 may be supplied to the dilution device 20 through the flow path L9 to perform decarboxylation treatment on the aqueous solution of formate. Each flow path of the manufacturing system 100 may be equipped with a valve (for example, valves 3 and 5 in Figure 2) to adjust the pressure and supply amount.

[0189] According to the manufacturing system 100 of this embodiment, formic acid can be produced with high yield and excellent productivity.

[0190] The formate salts and formic acid obtained in this way have a wide range of applications in various fields, such as silage additives, feed preservatives, leather tanning agents, textile dyes, rubber coagulants, antifreeze agents, cleaning agents and neutralizing agents for precision machinery, heavy metal precipitants, de-icing agents, cutting fluids, heat conduction fluids, lubricants, hydride ion sources, and hydrogen sources.

[0191] The present invention will be described in more detail below with reference to examples and reference examples, but the present invention is not limited thereto.

[0192] [Synthesis of Catalyst] (Catalyst 1) Catalyst 1 was synthesized by the following procedure. Under an inert atmosphere, 95.3 mg (0.1 mmol) of [RuHCl(PPh3)3(CO)] was added to a suspension of tetrahydrofuran (THF) (5 ml) with 40 mg (0.1 mmol) of ligand A shown below. The mixture was stirred and heated at 65°C for 3 hours to carry out the reaction. After that, it was cooled to room temperature (25°C). The resulting yellow solution was filtered, and the filtrate was evaporated to dryness under vacuum. The resulting yellow residual oil was dissolved in a very small amount of THF (1 mL), and hexane (10 mL) was slowly added to precipitate a yellow solid, which was then filtered. The filtrate was dried under vacuum to obtain catalyst 1 shown below (55 mg, yield 97%), which is a yellow crystal. In catalyst 1 and ligand A shown below, tBu represents a tertiary butyl group.

[0193]

[0194] 31 P { 1 H}(C6D6):90.8(s), 1 H (C6D6): -14.54 (t, 1H, J = 20.0Hz), 1.11 (t, 18H, J = 8.0Hz), 1.51 (t, 18H, J = 8.0Hz), 2.88 (dt, 2H, J = 16.0Hz, J=4.0Hz), 3.76 (dt, 2H, J=16.0Hz, J=4.0Hz), 6.45 (d, 2H, J=8.0Hz), 6.79 (t, 1H, J=8.0Hz). 13 C { 1 ​H}NMR (C6D6): 29.8 (s), 30.7 (s), 35.2 (t, J = 9.5 Hz), 37.7 (t, J = 6.0 Hz), 37.9 (t, J=6.5Hz), 119.5 (t, J=4.5Hz), 136.4 (s), 163.4 (t, J=5.0Hz), 209.8 (s).

[0195] [Calculation of TON and Yield] In the following examples, the TON of the catalyst and the yield of formate (potassium formate) were calculated by the following method.

[0196] First, the amount of formate contained in the aqueous phase was quantified as follows. Dimethyl sulfoxide in a mass ratio of 1 / 10 of the aqueous phase was added to the aqueous phase obtained after the reaction (either partially or entirely) to obtain a solution. 250 μL of this solution was mixed with 250 μL of heavy water. The measurement sample was prepared in this manner. Regarding this measurement data... 1 ¹H NMR measurements were performed, and from the obtained spectra, the integral value Ia of the peak originating from potassium formate and the integral value Ib of the peak originating from dimethyl sulfoxide were identified. The amount of potassium formate X (mol) contained in the aqueous phase obtained after the reaction was calculated using the following formula (1): X = (W / M) × {Ia / (Ib / R)} × (A / B) (1) (In formula (1), W is the weight (g) of dimethyl sulfoxide used to quantify potassium formate, M is the molecular weight of dimethyl sulfoxide, R is the ratio of the number of protons per molecule of dimethyl sulfoxide to the number of protons per molecule of potassium formate, Ia is the integral value of the NMR peak originating from potassium formate, Ib is the integral value of the NMR peak originating from dimethyl sulfoxide, A is the mass (g) of the aqueous phase obtained after the reaction, and B is the mass (g) of the aqueous solution used to quantify potassium formate.)

[0197] Then, based on the amount of formate produced (X mol) and the amount of catalyst used in the reaction (Y mol), the catalyst TON was calculated using the following equation (2): Catalyst TON = X / Y (2)

[0198] Furthermore, the yield (%) of formate was calculated using the following formula (3) based on the amount of formate produced (X mol) and the total amount of carbon dioxide, bicarbonate, and carbonate used in the catalytic reaction (the amount of potassium bicarbonate in the following example) (Z mol). Formate yield = 100 × X / Z (3)

[0199] [Example 1] As a gas containing carbon dioxide and NOx, a mixed gas containing 90 vol% carbon dioxide and 10 vol% nitrogen mixed with 5 vol ppm nitric oxide was used under atmospheric pressure conditions. That is, the nitric oxide concentration in the mixed gas was 0.5 vol ppm under atmospheric pressure conditions. 200 mL of 5 M potassium hydroxide aqueous solution was stirred at 800 rpm for 1 hour while the above mixed gas was bubbling at 0.9 MPa. In this way, an aqueous potassium bicarbonate solution was obtained.

[0200] The nitrite ion concentration was quantified by ion chromatography of the obtained aqueous solution under the following conditions. Analytical instrument: Dionex ICS-6000 (Thermo Fisher Scientific) Separation column: Dionex Ion Pac AS18 (2 mm × 250 mm) Guard column: Dionex Ion Pac AG18 (2 mm × 50 mm) Removal system: AERS-500 (external mode) Detector: Conductivity detector Eluent: Potassium hydroxide aqueous solution Eluent flow rate: 0.25 mL / min Injection volume: 25 μL The detection limit of nitrite ion concentration in the above analysis was 5.0 ppm.

[0201] Non-decoupling 13 The potassium bicarbonate concentration in the obtained aqueous solution was quantified by 13C NMR measurement (measurement device: Bruker, AVANCE NEO 300N).

[0202] Using the potassium bicarbonate aqueous solution obtained above as the aqueous phase, the hydrogenation experiment was carried out as follows. 50 mL of potassium bicarbonate aqueous solution (aqueous phase) was added to a reactor equipped with a U-shaped paddle agitator. The aqueous phase was bubbled with carbon dioxide at 0.9 MPa for 30 minutes to allow the carbonate formation reaction, i.e., bicarbonate production, to proceed completely. The atmosphere inside the reactor was replaced with nitrogen gas, and 50 mL of toluene, 0.125 μmol of catalyst 1, 2.7 mmol of methyltrioctylammonium chloride, and 0.3 mmol of tris(2,4-di-tert-butylphenyl) phosphite as an antioxidant were added. Then, as the first reaction, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, it was cooled to room temperature, and the pressure was carefully released after cooling. The inside of the reactor was replaced with nitrogen gas.

[0203] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Using the separated aqueous phase, the TON of the catalyst and the yield of formate from the first reaction were calculated using the method described above.

[0204] Under a nitrogen gas atmosphere, toluene was added to the separated organic phase to make up 50 mL, and the organic phase (op2) was obtained.

[0205] 50 mL of the potassium bicarbonate aqueous solution obtained above was added to an empty reactor, and the reaction of complete carbonate formation, i.e., bicarbonate production, was carried out in the same manner as in the first reaction. Then, the atmosphere in the reactor was replaced with nitrogen gas, and 50 mL of organic phase (op2) was added. Subsequently, for the second reaction, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, it was cooled to room temperature, and the pressure was carefully released after cooling. The reactor was then replaced with nitrogen gas.

[0206] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Using this separated aqueous phase, the TON of the catalyst and the yield of the formate salt for the second reaction were calculated using the method described above, similar to the first reaction.

[0207] Under a nitrogen gas atmosphere, toluene was added to the separated organic phase to make up 50 mL, and the organic phase (op3) was obtained.

[0208] 50 mL of potassium bicarbonate aqueous solution was added to an empty reactor, and the reaction proceeded completely to carbonation, i.e., bicarbonate formation, as in the first reaction. Then, the atmosphere in the reactor was replaced with nitrogen gas, and 50 mL of organic phase (op3) was added. Subsequently, for the third reaction, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, it was cooled to room temperature, and the pressure was carefully released after cooling. The reactor was then replaced with nitrogen gas.

[0209] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Using this separated aqueous phase, the TON of the catalyst and the yield of the formate salt for the third reaction were calculated using the method described above, similar to the first reaction.

[0210] [Example 2] An aqueous potassium bicarbonate solution of Example 2 was obtained in the same manner as in Example 1, except that the gas containing carbon dioxide and NOx was changed to a mixed gas containing 90 vol% carbon dioxide and 10 vol% nitrogen mixed with 50 vol ppm nitric oxide under atmospheric pressure conditions (the nitric oxide concentration relative to the entire mixed gas was 5 ppm).

[0211] The first to third reactions were carried out in the same manner as in Example 1, except that the aqueous phase was changed to the potassium bicarbonate aqueous solution of Example 2. The TON of the catalyst and the yield of the formate salt for each of the first to third reactions were calculated in the same manner as in Example 1.

[0212] [Example 3] An aqueous potassium bicarbonate solution of Example 3 was obtained in the same manner as in Example 1, except that the gas containing carbon dioxide and NOx was changed to a mixed gas containing 90 vol% carbon dioxide and 10 vol% nitrogen mixed with 300 vol ppm nitric oxide under atmospheric pressure conditions (the nitric oxide concentration relative to the entire mixed gas was 30 ppm).

[0213] The first to third reactions were carried out in the same manner as in Example 1, except that the aqueous phase was changed to the potassium bicarbonate aqueous solution of Example 3. The TON of the catalyst and the yield of the formate salt for each of the first to third reactions were calculated in the same manner as in Example 1.

[0214] [Example 4] As the gas containing carbon dioxide and NOx, a gas obtained by concentrating carbon dioxide from factory exhaust gas using a separation membrane device was used. This gas was a mixed gas containing 91.2 vol% carbon dioxide, 1.2 vol% oxygen, and 35.8 vol ppm NOx under atmospheric pressure conditions. 200 mL of 5 M potassium hydroxide aqueous solution was bubbling with the above mixed gas for 2 hours under atmospheric pressure conditions. After that, the recovered aqueous solution was bubbling with carbon dioxide at 0.9 MPa for 1 hour while stirring. In this way, the potassium bicarbonate aqueous solution of Example 4 was obtained.

[0215] The first to third reactions were carried out in the same manner as in Example 1, except that the aqueous phase was changed to the potassium bicarbonate aqueous solution of Example 4. The TON of the catalyst and the yield of the formate salt for each of the first to third reactions were calculated in the same manner as in Example 1.

[0216] Table 1 shows the nitrite ion concentrations in the potassium bicarbonate aqueous solutions of Examples 1 to 4, i.e., the nitrite ion concentrations (NO2) in the aqueous phase used in the hydrogenation reaction. - Table 1 shows the concentration, the TON of the catalyst for the reactions in Examples 1 to 4, and the yield of formate. The total-TON shown in Table 1 represents the ratio of the total amount of formate produced in the first to third reactions to the amount of catalyst used.

[0217]

[0218] As can be seen from Table 1, the TON was maintained in Examples 1 to 4 even after repeated hydrogenation reactions. Therefore, the decrease in catalytic activity was reduced in Examples 1 to 4. In particular, in Examples 1 to 3, the TON was maintained at a high level even after three hydrogenation reactions. The yields of Examples 1 to 4 were practically sufficient.

[0219] NO2 in the aqueous phase of Examples 1 to 3 -The value was below the detection limit. From the above, it was shown that the bicarbonate formation reaction proceeded without the nitric oxide in the gas being concentrated in the aqueous phase. On the other hand, in Example 4, NO2 was obtained from the aqueous phase. - Although it has been detected, it is presumed that this is influenced by differences in gas species, the environment in which the experiment was conducted, the bubbling time, or the storage conditions of the aqueous phase.

[0220] [Example 5] As a gas containing carbon dioxide and NOx, a mixed gas containing 60 vol% carbon dioxide, 7 vol% oxygen, and 0.04 vol ppm nitric oxide was used under atmospheric pressure. 50 mL of a 5 M potassium hydroxide aqueous solution was bubbling with the above mixed gas at 0.1 MPa while stirring at 800 rpm for 6 hours. Then, the recovered aqueous solution was bubbling with carbon dioxide at 0.9 MPa for 1 hour while stirring at 800 rpm. In this way, the potassium bicarbonate aqueous solution of Example 5 was obtained.

[0221] The nitrite ion concentration and potassium bisulfide concentration of the obtained potassium bicarbonate aqueous solution were quantified using the method described above.

[0222] Using the potassium bicarbonate aqueous solution obtained above as the aqueous phase, the hydrogenation experiment was carried out as follows. 50 mL of potassium bicarbonate aqueous solution (aqueous phase) was added to a reactor equipped with a U-shaped paddle stirring blade. The atmosphere inside the reactor was replaced with nitrogen gas, and 50 mL of toluene, 3 μmol of catalyst 1, 2.7 mmol of methyltrioctylammonium chloride, and 0.3 mmol of tris(2,4-di-tert-butylphenyl) phosphite as an antioxidant were added. At this time, the concentration of catalyst 1 in the organic phase was 60 μmol / L, the concentration of methyltrioctylammonium chloride was 54 mmol / L, and the concentration of the antioxidant was 6 mmol / L. After that, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the reaction was carried out by stirring at 800 rpm for 2.5 hours. After stirring, it was cooled to room temperature, and the pressure was carefully released after cooling. The inside of the reactor was replaced with nitrogen gas.

[0223] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Using the separated aqueous phase, the yields of catalyst TON and formate were calculated by the method described above.

[0224] [Example 6] The reaction was carried out in the same manner as in Example 5, except that the amount of catalyst 1 in the organic phase was changed to 0.125 μmol, i.e., to 2.5 μmol / L, and the reaction time was changed to 20 hours. The yields of catalyst TON and formate were calculated.

[0225] [Example 7] An aqueous potassium bicarbonate solution of Example 7 was obtained in the same manner as in Example 5, except that the gas containing carbon dioxide and NOx was changed to a mixed gas containing 60 vol% carbon dioxide, 6 vol% oxygen, and 10.4 vol ppm nitric oxide under atmospheric pressure conditions.

[0226] The reaction was carried out in the same manner as in Example 5, except that the aqueous phase was changed to the potassium bicarbonate aqueous solution of Example 7, and the yields of the catalyst TON and formate were calculated.

[0227] [Example 8] The reaction was carried out in the same manner as in Example 7, except that the amount of catalyst 1 in the organic phase was changed to 0.125 μmol, i.e., to 2.5 μmol / L, and the reaction time was changed to 20 hours. The yields of catalyst TON and formate were calculated.

[0228] Table 2 shows the nitrite ion concentrations in the potassium bicarbonate aqueous solutions of Examples 5 to 8, i.e., the nitrite ion concentrations (NO2) in the aqueous phase used in the hydrogenation reaction. - The concentrations, TON of the catalyst used in the reactions in Examples 5 to 8, and the yield of the formate are shown.

[0229]

[0230] The following describes NO2 in the aqueous phase used in the hydrogenation reaction. - In order to find a preferred range of concentration, experiments in Reference Examples 1 to 5 were conducted as simulations of the manufacturing method of the present invention. In Reference Examples 1 to 5, NO2 - Potassium nitrite was added to the aqueous phase as a source.

[0231] [Reference Example 1] Under a nitrogen gas atmosphere, 50 mL of water, 0.25 mol of potassium bicarbonate, 100 ppm of potassium nitrite relative to the aqueous phase, 50 mL of toluene, 0.125 μmol of catalyst 1, 2.7 mmol of methyltrioctylammonium chloride, and 0.3 mmol of tris(2,4-di-tert-butylphenyl) phosphite as an antioxidant were added to a reactor equipped with a U-shaped paddle stirring blade. After that, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, the mixture was cooled to room temperature, and the pressure was carefully released after cooling. The reactor was then purged with nitrogen gas.

[0232] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Using this aqueous phase, the yields of TON and formate were calculated by the method described above.

[0233] [Reference Example 2] Under a nitrogen gas atmosphere, 50 mL of water, 0.25 mol of potassium bicarbonate, 50 ppm of potassium nitrite relative to the aqueous phase, 50 mL of toluene, 0.125 μmol of catalyst 1, 2.7 mmol of methyltrioctylammonium chloride, and 0.3 mmol of tris(2,4-di-tert-butylphenyl) phosphite as an antioxidant were added to a reactor equipped with a U-shaped paddle stirring blade. Then, as the first reaction, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, the mixture was cooled to room temperature, and the pressure was carefully released after cooling. The reactor was then purged with nitrogen gas.

[0234] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Using this aqueous phase, the TON of the catalyst and the yield of formate from the first reaction were calculated using the method described above.

[0235] Under a nitrogen gas atmosphere, toluene was added to the separated organic phase to make up 50 mL. Under a nitrogen gas atmosphere, the entire volume of the made-up organic phase (50 mL), 50 mL of water, 0.25 mol of potassium bicarbonate, and 50 ppm of potassium nitrite relative to the aqueous phase were added to a reactor equipped with a U-shaped paddle agitator. Subsequently, in the second reaction, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, the mixture was cooled to room temperature, and the pressure was carefully released after cooling. The reactor was then purged with nitrogen gas.

[0236] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Similar to the first reaction, the TON of the catalyst and the yield of the formate salt from the second reaction were calculated using the method described above.

[0237] [Reference Example 3] Under a nitrogen gas atmosphere, 50 mL of water, 0.25 mol of potassium bicarbonate, 20 ppm of potassium nitrite relative to the aqueous phase, 50 mL of toluene, 0.125 μmol of catalyst 1, 2.7 mmol of methyltrioctylammonium chloride, and 0.3 mmol of tris(2,4-di-tert-butylphenyl) phosphite as an antioxidant were added to a reactor equipped with a U-shaped paddle stirring blade. Then, as the first reaction, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, the mixture was cooled to room temperature, and the pressure was carefully released after cooling. The reactor was then purged with nitrogen gas.

[0238] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Using this aqueous phase, the TON of the catalyst and the yield of formate from the first reaction were calculated using the method described above.

[0239] Under a nitrogen gas atmosphere, toluene was added to the separated organic phase to make up 50 mL. Under a nitrogen gas atmosphere, the entire volume of the made-up organic phase (50 mL), 50 mL of water, 0.25 mol of potassium bicarbonate, and 20 ppm of potassium nitrite relative to the aqueous phase were added to a reactor equipped with a U-shaped paddle agitator. Subsequently, in the second reaction, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, the mixture was cooled to room temperature, and the pressure was carefully released after cooling. The reactor was then purged with nitrogen gas.

[0240] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Similar to the first reaction, the TON of the catalyst and the yield of the formate salt from the second reaction were calculated using the method described above.

[0241] Under a nitrogen gas atmosphere, toluene was added to the separated organic phase to make up 50 mL. Under a nitrogen gas atmosphere, the entire volume of the made-up organic phase (50 mL), 50 mL of water, 0.25 mol of potassium bicarbonate, and 20 ppm of potassium nitrite relative to the aqueous phase were added to a reactor equipped with a U-shaped paddle agitator. Subsequently, as a third reaction, hydrogen gas was added up to 5 MPa, the temperature was raised to 90°C, and the mixture was stirred at 800 rpm for 20 hours. After stirring, the mixture was cooled to room temperature, and the pressure was carefully released after cooling. The reactor was then purged with nitrogen gas.

[0242] Under a nitrogen gas atmosphere, the reaction mixture was removed from the reactor, and the organic phase and aqueous phase were separated. Similar to the first and second reactions, the TON of the catalyst and the yield of the formate salt from the third reaction were calculated using the method described above.

[0243] [Reference Example 4] The reaction was carried out in the same manner as in Reference Example 3, except that the amount of potassium nitrite added was changed to 1 ppm relative to the aqueous phase. In the same manner as in Reference Example 3, the TON of the catalyst and the yield of formate for each of the first to third reactions were calculated.

[0244] [Reference Example 5] The reaction was carried out in the same manner as in Reference Example 3, except that the amount of potassium nitrite added was changed to 0.5 ppm relative to the aqueous phase. The TON of the catalyst and the yield of the formate salt for each of the first to third reactions were calculated in the same manner as in Reference Example 3.

[0245] Table 3 shows the yield of formate and the catalyst TON for the reactions in Reference Examples 1 to 5. The total TON shown in Table 3 represents the ratio of the total amount of formate produced in all reactions (up to 3 times) to the amount of catalyst used.

[0246]

[0247] As can be seen from Table 3, in Reference Example 1, the yield was less than 40% from the first reaction. In Reference Example 2, sufficient TON and yield were observed in the first reaction, but the TON and yield decreased significantly in the second reaction. In Reference Examples 3 to 5, the TON was maintained even after repeated reactions. Therefore, the decrease in catalytic activity was mitigated in Reference Examples 3 to 5. In particular, in Reference Examples 4 to 5, a high level of TON was maintained even after three reactions. The yields in Reference Examples 3 to 5 were practically sufficient.

[0248] According to the method for producing formate salt of this embodiment, formate salt, which is a precursor of formic acid, can be efficiently produced, for example, using exhaust gas.

Claims

1. A method for producing formate using a gas containing carbon dioxide and NOx, hydrogen, a catalyst, an organic phase, and an aqueous phase.

2. A method for producing formate according to claim 1, comprising a gas contact step of bringing the gas into contact with the aqueous phase.

3. The method for producing a formate salt according to claim 2, wherein the gas contact step produces compound C, which is at least one selected from the group consisting of carbonates and bicarbonates.

4. The method for producing a formate salt according to claim 3, comprising a hydrogenation reaction step in which the compound C and hydrogen are reacted using the catalyst in a two-phase system in which the organic phase and the aqueous phase are separated to produce a formate salt.

5. The method for producing formate according to claim 4, wherein the aqueous phase contains compound C.

6. The method for producing formate according to claim 4, wherein the concentration of nitrite ions in the aqueous phase during the hydrogenation reaction step is less than 100 ppm.

7. The method for producing formate according to claim 6, wherein the concentration of nitrite ions in the aqueous phase during the hydrogenation reaction step is 50 ppm or less.

8. The method for producing formate according to claim 1, wherein the organic phase includes the catalyst.

9. The method for producing a formate salt according to claim 1, wherein the catalyst is at least one selected from the group consisting of a complex containing a metal M and a salt compound thereof.

10. The method for producing a formate salt according to claim 9, wherein the catalyst is at least one selected from the group consisting of a metal complex represented by the following general formula (1A), its tautomers, stereoisomers, and salts thereof. (In general formula (1A), X represents an atomic group containing typical elements of groups 13 to 15 that can coordinate to M; Q independently represents a bridging structure containing typical elements of groups 14 to 16 that connects Y and X; Y independently represents an atomic group containing typical elements of groups 14 to 16 that can coordinate to M; M represents a metal atom; Z represents an anionic ligand; n represents 0 to 3; and L, if multiple Ls exist, independently represents a neutral or anionic ligand.) 11. The method for producing formate according to claim 10, wherein the metal complex represented by the general formula (1A) is the metal complex represented by the following general formula (2A). (In general formula (2A), X1 represents a heteroaromatic ring formed with two carbon atoms and a nitrogen atom, which may have substituents or be bonded with other substituents to form a ring; Q1 independently represents CH2, NH, or O, with CH2 and NH further having substituents; Y1 independently represents a phosphorus atom or a nitrogen atom; R independently represents an alkyl group, an aryl group, or an aralkyl group, which further have substituents; M represents a metal atom; Z represents an anionic ligand; n represents 0 to 3; and L, if multiple are present, independently represents a neutral or anionic ligand.) 12. The method for producing formate according to claim 11, wherein the metal complex represented by the general formula (2A) is a metal complex represented by the following general formula (3A). (In general formula (3A), R0 represents a hydrogen atom or an alkyl group, A independently represents CH, CR5, or N, R5 represents an alkyl group, aryl group, aralkyl group, amino group, hydroxyl group, or alkoxy group, Q1 independently represents CH2, NH, or O, and CH2 and NH may have further substituents, Y1 represents a phosphorus atom or a nitrogen atom, R independently represents an alkyl group, aryl group, or aralkyl group, which may have further substituents, M represents a metal atom, Z represents an anionic ligand, n represents 0 to 3, and L, if there are multiple, independently represents a neutral or anionic ligand.) 13. The method for producing formate according to claim 9, wherein the metal M is ruthenium.

14. A method for producing formic acid, comprising the steps of: producing a formate salt by the method for producing a formate salt according to any one of claims 1 to 13; and protonating at least a portion of the formate salt to produce formic acid.

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