Photochemical production of formic acid

Abstract

The present invention provides a method for highly selectively producing formic acid at room temperature and atmospheric pressure using a reusable organic catalyst without using hydrogen or a metal catalyst. [Solution] One aspect of the present invention relates to a method for producing formic acid, which comprises irradiating a solution containing carbon dioxide as a carbonate source and a base or carbonate salt as a carbonate source, a hydrogen source, and a solvent with light at room temperature and atmospheric pressure in the presence of one or more hydrogen atom transfer catalysts selected from the group consisting of an N-heterocyclic carbene catalyst, an imidazolium-2-carboxylate catalyst, an imidazolinium-2-carboxylate catalyst, an imidazolium salt catalyst, an imidazolinium salt catalyst, a triazolium salt catalyst, a triazolium catalyst, and a thiazolium salt catalyst, and one or more photosensitizers selected from the group consisting of a carbazole catalyst.

Description

[Technical Field]

[0001] The present invention relates to a photochemical method for producing formic acid. [Background technology]

[0002] The reaction for producing formic acid from carbon dioxide and carbonates is attracting attention as an important reaction from the perspectives of storing hydrogen, a clean energy medium, and effectively utilizing carbon dioxide, which can cause global warming.

[0003] For example, Patent Document 1 discloses a photochemical reaction device that includes an oxidation reaction electrode that oxidizes water to generate oxygen and a reduction reaction electrode that reduces carbon dioxide to synthesize carbon compounds, and that is configured by electrically connecting these electrodes, and that the reduction reaction electrode reduces carbon dioxide in a liquid containing water by utilizing irradiated light energy, thereby synthesizing carbon compounds. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-94194 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the device of Patent Document 1 and the known reaction from carbon dioxide to formic acid have the following problems. Most photochemical carbon dioxide reductions are thermal or electrolytic reactions using metal catalysts, but the product selectivity is very poor. · Since rare metals such as iridium and rhodium are used in many photochemical carbon dioxide reduction processes that use organometallic catalysts, the country risk is very high. When using photosensitizers and hydrogen atom transfer catalysts, the activity of known catalysts is not sufficient, and there is ample room for improvement in the catalysts. Purified water is used as the reaction solvent, but the usability of tap water or seawater has not been demonstrated. -By-product formation must be controlled. The possibility of recovering and reusing the catalyst has not been demonstrated. No measures have been taken to increase the carbon dioxide concentration around the catalyst.

[0006] Therefore, an object of the present invention is to provide a means for highly selectively producing formic acid at room temperature and atmospheric pressure using a reusable organic catalyst without using hydrogen or a metal catalyst. [Means for solving the problem]

[0007] The present inventors have investigated various means for solving the above-mentioned problems. They have found that in a method for photochemically producing formic acid from carbon dioxide as a carbonate source and a base or carbonate as a carbonate source, when an N-heterocyclic carbene catalyst, imidazolium-2-carboxylate catalyst, imidazolium salt catalyst, imidazolinium salt catalyst, triazolium catalyst, or thiazolium salt catalyst is used as a hydrogen atom transfer catalyst and a carbazole catalyst is used as a photosensitizer, formic acid can be produced with high selectivity. Based on this finding, the present inventors have completed the present invention.

[0008] That is, the present invention includes the following aspects and embodiments. (Embodiment 1) A method for producing formic acid, comprising irradiating a solution containing carbon dioxide as a carbonate source and a base or carbonate salt as a carbonate source, a hydrogen source, and a solvent with light at room temperature and atmospheric pressure in the presence of one or more hydrogen atom transfer catalysts selected from the group consisting of an N-heterocyclic carbene catalyst, an imidazolium-2-carboxylate catalyst, an imidazolinium-2-carboxylate catalyst, an imidazolium salt catalyst, an imidazolinium salt catalyst, a triazolium salt catalyst, a triazolium catalyst, and a thiazolium salt catalyst, and one or more photosensitizers selected from the group consisting of a carbazole catalyst. (Embodiment 2) The hydrogen atom transfer catalyst is one or more compounds selected from the group consisting of N-heterocyclic carbene catalysts, imidazolium-2-carboxylate catalysts, imidazolinium-2-carboxylate catalysts, imidazolium salt catalysts, and imidazolinium salt catalysts, and the N-heterocyclic carbene catalyst is represented by Formula I-1' [ka] I-1' and the imidazolium-2-carboxylate catalyst or imidazolinium-2-carboxylate catalyst is represented by formula I-2' [ka] I-2' and the imidazolium salt catalyst or imidazolinium salt catalyst is represented by formula I-3' [ka] I-3' [In formula I-3', X - is a halogen anion, a tetrafluoroborate ion, or a bicarbonate ion. In formulas I-1′, I-2′, and I-3′, R 3 , R 3’ , R 4 , and R 4’ is hydrogen or R 3’ and R 4’ come together to form a bond, and R 3 and R 4 is hydrogen and R 5 ~R 10 are each independently hydrogen, alkyl having from 1 to 6 carbon atoms which may be substituted with aryl having from 6 to 18 carbon atoms or alkyl having from 1 to 6 carbon atoms, alkoxy having from 1 to 6 carbon atoms, or halogen. (Embodiment 3) In Formula I-3′, X - is Cl - , BF4 - , or HCO3 -In formulae I-1′, I-2′ and I-3′, R 5 ~R 10 are, independently of each other, hydrogen, methyl, 1-methyl-ethyl, 1-ethyl-propyl, 1-propyl-butyl, bromo, diphenylmethyl, or methoxy. (Embodiment 4) The carbazole catalyst is represented by Formula II [ka] II [In formula II, R 11 is hydrogen or aryl having 6 to 18 carbon atoms, and R 12 and R 12’ are independent of each other, NR 14 R 14’ or OH, and R 13 and R 13’ are each independently hydrogen or NR 15 R 15’ and R 14 and R 14’ are each independently an alkyl having 1 to 10 carbon atoms or an aryl having 6 to 18 carbon atoms (the aryl having 6 to 18 carbon atoms may be substituted with 1 to 3 substituents independently selected from an alkoxy having 1 to 10 carbon atoms), or R 14 and R 14’ together with the nitrogen atom to which they are attached form a 3- to 10-membered heterocycle, and R 15 and R 15’ are each independently alkyl having 1 to 10 carbon atoms. The method for producing the present invention according to any one of embodiments 1 to 3, wherein (Embodiment 5) In Formula II, R 11 is hydrogen or phenyl, and R 12 and R 12’ is NR 14 R 14’ and R 13 and R 13’ are each independently hydrogen or NR 15 R 15’ and R 14 and R14’ are, independently of each other, methyl or phenyl, the phenyl being substituted with methoxy in the para position relative to the carbon attached to the nitrogen, or R 14 and R 14’ together with the nitrogen atom to which they are attached form azetidinyl, pyrrolidinyl, or azepanyl, and R 15 and R 15’ is methyl. (Embodiment 6) The method according to any one of embodiments 1 to 5, wherein the base is a carbonate. [Effects of the Invention]

[0009] The present invention makes it possible to provide a means for highly selectively producing formic acid at room temperature and atmospheric pressure using a reusable organic catalyst without using hydrogen or a metal catalyst. [Brief explanation of the drawings]

[0010] [Figure 1] Experiment 7: A diagram showing an experimental scheme for examining catalyst reuse and photographs showing the results. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will now be described in detail.

[0012] In one embodiment of the present invention, the hydrogen atom transfer catalyst is one or more compounds selected from the group consisting of N-heterocyclic carbene catalysts, imidazolium-2-carboxylate catalysts, imidazolium-2-carboxylate catalysts, imidazolium salt catalysts, imidazolinium salt catalysts, triazolium salt catalysts, triazolium catalysts, thiazolium salt catalysts, and pyrazolinium salt catalysts.

[0013] The N-heterocyclic carbene catalyst is not limited. The N-heterocyclic carbene catalyst can be, for example, the following formula I-1: [ka] I-1 It is expressed as:

[0014] The imidazolium-2-carboxylate catalyst and the imidazolinium-2-carboxylate catalyst are not limited. The imidazolium-2-carboxylate catalyst or the imidazolinium-2-carboxylate catalyst can be, for example, a catalyst represented by the following formula I-2: [ka] I-2 In addition, in formula I-2, R 3’ and R 4’ When R are taken together to form a bond, the compound of formula I-2 is an imidazolium-2-carboxylate catalyst. 3’ and R 4’ When taken together do not form a bond, the compound of formula I-2 is an imidazolinium-2-carboxylate catalyst.

[0015] The imidazolium salt catalyst and the imidazolinium salt catalyst are not limited. The imidazolium salt catalyst or the imidazolinium salt catalyst can be, for example, a catalyst represented by the following formula I-3: [ka] I-3 In addition, in formula I-3, R 3’ and R 4’ When R are taken together to form a bond, the compound of formula I-3 is an imidazolium salt catalyst. 3’ and R 4’ When taken together do not form a bond, the compound of formula I-3 is an imidazolinium salt catalyst.

[0016] The triazolium salt catalyst is not limited. The triazolium salt catalyst may be, for example, the triazolium salt catalyst represented by the following formula I-4: [ka] I-4 It is expressed as:

[0017] The triazolium catalyst is not limited. The triazolium catalyst may be, for example, the triazolium catalyst represented by the following formula I-5: [ka] I-5 It is expressed as:

[0018] The thiazolium salt catalyst is not limited. The thiazolium salt catalyst may be, for example, a compound represented by the following formula I-6: [ka] I-6 It is expressed as:

[0019] In formula I-1, I-2, I-3, I-4, or I-5, R 1 and R 2 are each independently an alkyl having from 1 to 10 carbon atoms (e.g., an alkyl having from 1 to 6 carbon atoms), a cycloalkyl having from 3 to 10 carbon atoms (e.g., a cycloalkyl having from 3 to 6 carbon atoms), or an aryl having from 6 to 18 carbon atoms. These alkyls, cycloalkyls, and aryls may be substituted with one to three substituents independently selected from the group consisting of an unsubstituted or alkyl-substituted aryl or heteroaryl having from 1 to 6 carbon atoms and from 6 to 18 carbon atoms, an alkyl having from 1 to 10 carbon atoms (e.g., an alkyl having from 1 to 6 carbon atoms), an alkoxy having from 1 to 10 carbon atoms (e.g., an alkoxy having from 1 to 6 carbon atoms), and a halogen (e.g., fluoro (F), chloro (Cl), bromo (Br), or iodo (I)). In formula I-1, I-2, I-3, I-4, or I-5, R 1 and R 2When R is phenyl (Ph), the phenyl may be substituted at a position selected from the ortho and para positions relative to the carbon bonded to the nitrogen of the N-heterocycle, imidazolium ring, imidazolinium ring, or triazolium ring with one to three substituents independently selected from the group consisting of alkyl having from 1 to 10 carbon atoms (e.g., alkyl having from 1 to 6 carbon atoms) optionally substituted with aryl having from 6 to 18 carbon atoms or alkyl having from 1 to 6 carbon atoms, alkoxy having from 1 to 10 carbon atoms (e.g., alkoxy having from 1 to 6 carbon atoms), and halogen. For example, in Formula I-1, I-2, I-3, I-4, or I-5, R 1 and R 2 are each independently methyl, isopropyl, t-butyl, cyclohexyl, benzyl, 2,4-dimethyl-5-pyrimidylmethyl, or phenyl, and the phenyl is optionally substituted with 1 to 3 substituents, each independently selected from the group consisting of methyl, 1-methyl-ethyl, 1-ethyl-propyl, 1-propyl-butyl, bromo, diphenylmethyl, and methoxy, at a position selected from the ortho and para positions relative to the carbon bonded to the nitrogen of the N-heterocycle, imidazolium ring, imidazolinium ring, or triazolium ring. In one embodiment, in Formula I-1, I-2, I-3, I-4, or I-5, R 1 and R 2 are the same group.

[0020] In formula I-6, R 2is an alkyl having from 1 to 10 carbon atoms (e.g., an alkyl having from 1 to 6 carbon atoms), a cycloalkyl having from 3 to 10 carbon atoms (e.g., a cycloalkyl having from 3 to 6 carbon atoms), or an aryl having from 6 to 18 carbon atoms. These alkyls, cycloalkyls, and aryls may be substituted with 1 to 3 substituents independently selected from the group consisting of unsubstituted or alkyl-substituted aryl or heteroaryl having from 1 to 6 carbon atoms and from 6 to 18 carbon atoms, or alkyl having from 1 to 10 carbon atoms (e.g., alkyl having from 1 to 6 carbon atoms), which may be substituted with an alkyl having from 1 to 10 carbon atoms (e.g., alkoxy having from 1 to 10 carbon atoms (e.g., alkoxy having from 1 to 6 carbon atoms), and halogen (e.g., fluoro (F), chloro (Cl), bromo (Br), or iodo (I)). In Formula I-6, R 2 When R is phenyl (Ph), the phenyl may be substituted at a position selected from the ortho and para positions relative to the carbon bonded to the nitrogen of the thiazolium ring with one to three substituents independently selected from the group consisting of alkyl having from 1 to 10 carbon atoms (e.g., alkyl having from 1 to 6 carbon atoms) optionally substituted with aryl having from 6 to 18 carbon atoms or alkyl having from 1 to 6 carbon atoms, alkoxy having from 1 to 10 carbon atoms (e.g., alkoxy having from 1 to 6 carbon atoms), and halogen. For example, in Formula I-6, R 2 is methyl, isopropyl, t-butyl, cyclohexyl, benzyl, 2,4-dimethyl-5-pyrimidylmethyl, or phenyl, and the phenyl is optionally substituted with 1 to 3 substituents independently selected from the group consisting of methyl, 1-methyl-ethyl, 1-ethyl-propyl, 1-propyl-butyl, bromo, diphenylmethyl, and methoxy at positions selected from the ortho- and para-positions relative to the carbon bonded to the nitrogen of the thiazolium ring.

[0021] In formula I-1, I-2 or I-3, R 3 , R 3’ , R 4 , and R 4’are each independently hydrogen, alkyl having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms), alkoxy having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms), or aryl having 6 to 18 carbon atoms. For example, in Formula I-1, I-2, or I-3, R 3 , R 3’ , R 4 , and R 4’ are, independently of each other, hydrogen, methyl, ethoxy, or phenyl. For example, in formula I-1, I-2, or I-3, R 3 , R 3’ , R 4 , and R 4’ is hydrogen.

[0022] In formula I-1, I-2 or I-3, R 3’ and R 4’ may together form a bond.

[0023] In formula I-1, I-2 or I-3, R 3’ and R 4’ may be taken together to form an aryl having 6 to 18 carbon atoms. For example, in formula I-1, I-2, or I-3, R 3’ and R 4’ may be taken together to form naphthyl.

[0024] In formula I-1, I-2 or I-3, R 3’ and R 4’ may form an aryl having 6 to 18 carbon atoms together with the carbon atom to which they are attached. For example, in formula I-1, I-2, or I-3, R 3’ and R 4’ may be taken together with the carbon atom to which they are attached to form phenyl.

[0025] In formula I-6, R 3 and R 4are each independently hydrogen, alkyl having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms), alkoxy having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms), or aryl having 6 to 18 carbon atoms. For example, in Formula I-6, R 3 and R 4 are each independently hydrogen, methyl, ethoxy, or phenyl. For example, in formula I-6, R 3 and R 4 is hydrogen.

[0026] In formula I-6, R 3 and R 4 may be taken together to form an aryl having 6 to 18 carbon atoms. For example, in Formula I-6, R 3 and R 4 may be taken together to form naphthyl.

[0027] In formula I-6, R 3 and R 4 may form an aryl having 6 to 18 carbon atoms together with the carbon atom to which they are attached. For example, in Formula I-6, R 3 and R 4 may be taken together with the carbon atom to which they are attached to form phenyl.

[0028] In formula I-4, R 3 is hydrogen, alkyl having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms), alkoxy having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms), or aryl having 6 to 18 carbon atoms. For example, in Formula I-4, R 3 are, independently of each other, hydrogen, methyl, ethoxy, or phenyl.

[0029] In formula I-5, R 3 and R 3’are each independently hydrogen, alkyl having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms), alkoxy having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms), or aryl having 6 to 18 carbon atoms. 3 and R 3’ may be taken together to form an imine, such as =N-phenyl. For example, in formula I-5, R 3 and R 3’ are, independently of each other, hydrogen, methyl, ethoxy, or phenyl.

[0030] In formula I-3, I-4 or I-6, X - For example, in formula I-3, I-4, or I-6, X - is a halogen anion, tetrafluoroborate ion (BF4 - ), or bicarbonate ion (HCO3 - ). For example, a halogen anion is a fluoride ion (F - ), chloride ions (Cl - ), bromide ion (Br - ), or iodide ion (I - )

[0031] In one embodiment of the present invention, the N-heterocyclic carbene catalyst is represented by the following formula I-1' [ka] I-1' It is expressed as:

[0032] In one embodiment of the present invention, the imidazolium-2-carboxylate catalyst or imidazolinium-2-carboxylate catalyst is represented by the following formula I-2' [ka] I-2' It is expressed as:

[0033] In one embodiment of the present invention, the imidazolium salt catalyst or imidazolinium salt catalyst is represented by the following formula I-3' [ka] I-3' It is expressed as:

[0034] In formula I-1′, I-2′ or I-3′, R 3 , R 3’ , R 4 , and R 4’ is hydrogen or R 3’ and R 4’ come together to form a bond, and R 3 and R 4 In formula I-1′, I-2′, or I-3′, R 5 ~R 10 are each independently hydrogen, alkyl having 1 to 6 carbon atoms which may be substituted with aryl having 6 to 18 carbon atoms or alkyl having 1 to 6 carbon atoms, alkoxy having 1 to 6 carbon atoms, or halogen, and in one embodiment, R 5 ~R 10 are, independently of each other, hydrogen, methyl, 1-methyl-ethyl, 1-ethyl-propyl, 1-propyl-butyl, bromo, diphenylmethyl, or methoxy; in one embodiment, R 5 ~R 10 are, independently of each other, hydrogen, methyl, 1-methyl-ethyl, 1-ethyl-propyl, or 1-propyl-butyl. In one embodiment, in Formula I-1', I-2', or I-3', R 5 and R 8 are the same group, and R 6 and R 9 are the same group, and R 7 and R 10 are the same group. In one embodiment, in formula I-1′, I-2′, or I-3′, R 5 , R 7 , R 8 and R 10 are the same group, and R 6 and R 9 are the same group. In one embodiment, in formula I-1′, I-2′, or I-3′, R 5 ~R 10 are the same group.- is a halogen anion, a tetrafluoroborate ion, or a bicarbonate ion.

[0035] In one embodiment of the present invention, the N-heterocyclic carbene catalyst is the following compound: [ka]

[0036] In one embodiment of the present invention, the imidazolium-2-carboxylate catalyst is the following compound: [ka]

[0037] In one embodiment of the present invention, the imidazolium or imidazolinium salt catalyst is the following compound: [ka]

[0038] In one embodiment of the invention, the triazolium catalyst is the following compound: [ka]

[0039] In one embodiment of the present invention, the thiazolium salt catalyst is the following compound: [ka]

[0040] The hydrogen atom transfer catalyst of one embodiment of the present invention has the effect of adsorbing carbon dioxide and can increase the carbon dioxide concentration around the catalyst. Therefore, by using the hydrogen atom transfer catalyst described above in the present invention, carbon dioxide can be efficiently converted into formic acid.

[0041] In one embodiment of the present invention, the content of the hydrogen atom transfer catalyst is not limited, but is preferably in the range of 0.1 mol % to 10 mol %, more preferably 1 mol % to 8 mol %, and even more preferably 2 mol % to 6 mol %, relative to the number of moles of the hydrogen source as the raw material. By carrying out this method using the hydrogen atom transfer catalyst in an amount within the above-mentioned range, formic acid can be obtained in high yield.

[0042] In one embodiment of the invention, the photosensitizer is one or more compounds selected from the group consisting of carbazole catalysts.

[0043] The carbazole catalyst is not limited. The carbazole catalyst may be, for example, a compound represented by the following formula II: [ka] II It is expressed as:

[0044] In formula II, R 11 is hydrogen or an aryl having 6 to 18 carbon atoms. For example, in Formula II, R 11 is hydrogen or phenyl.

[0045] In formula II, R 12 and R 12’ are independent of each other, NR 14 R 14’ or OH. NR 14 R 14’ Medium, R 14 and R 14’ are each independently an alkyl having 1 to 10 carbon atoms (e.g., an alkyl having 1 to 6 carbon atoms) or an aryl having 6 to 18 carbon atoms. The aryl may be substituted with 1 to 3 substituents independently selected from an alkoxy having 1 to 10 carbon atoms (e.g., an alkoxy having 1 to 6 carbon atoms). NR 14 R 14’ Medium, R 14 and R 14’When R is phenyl, the phenyl may be substituted with 1 to 3 substituents independently selected from alkoxy having 1 to 10 carbon atoms (e.g., alkoxy having 1 to 6 carbon atoms) at positions selected from the ortho and para positions relative to the carbon bonded to the nitrogen. For example, in Formula II, R 12 and R 12’ is NR 14 R 14’ and R 14 and R 14’ are, independently of each other, methyl or phenyl, and the phenyl is substituted with methoxy at the para position relative to the carbon attached to the nitrogen. 12 and R 12’ are the same group.

[0046] In formula II, R 12 and R 12’ However, independently of each other, NR 14 R 14’ If R 14 and R 14’ may form a 3- to 10-membered heterocycle together with the nitrogen atom to which they are attached. For example, R 14 and R 14’ together with the nitrogen atom to which they are attached form an azetidinyl, pyrrolidinyl, or azepanyl, in one embodiment a pyrrolidinyl or azepanyl.

[0047] In formula II, R 13 and R 13’ are each independently hydrogen or NR 15 R 15’ NR 15 R 15’ Medium, R 15 and R 15’ are each independently an alkyl having 1 to 10 carbon atoms (e.g., an alkyl having 1 to 6 carbon atoms). For example, NR 15 R 15’ Medium, R 15 and R 15’ is methyl. In Formula II, R 13 and R 13’is preferably hydrogen.

[0048] In one embodiment of the present invention, the carbazole catalyst is the following compound: [ka]

[0049] In the carbazole catalyst of one embodiment of the present invention, by arranging cyclic amino groups with relatively large steric hindrance at the 3- and 6-positions of the carbazole, the free rotation of the bond between the carbazole C atom and the amine N atom, i.e., the C-N bond, is controlled, thereby enhancing the resonance effect. Furthermore, by arranging electron-donating groups at the 1-, 3-, 6-, and 8-positions of the carbazole, the resonance effect can be enhanced.

[0050] In one embodiment of the present invention, the content of the photosensitizer is not limited, but is preferably in the range of 0.01 mol % to 8 mol %, more preferably 0.1 mol % to 6 mol %, and even more preferably 1 mol % to 4 mol %, relative to the number of moles of the hydrogen source as a raw material. By carrying out this method using the photosensitizer in an amount within the above-mentioned range, formic acid can be obtained in high yield.

[0051] In one embodiment of the present invention, the hydrogen atom transfer catalyst and the photosensitizer can be recycled to maintain a high yield of formic acid.

[0052] In one embodiment of the present invention, when carbon dioxide is selected as the carbonate source, a base is used together with carbon dioxide. The base used is not limited. From the viewpoint of producing few by-products and highly selectively producing formic acid (salt), a carbonate is preferred. The carbonate is not limited. Examples of carbonates include alkali metal carbonates such as lithium carbonate, sodium carbonate, potassium carbonate, and cesium carbonate; alkali metal bicarbonates such as lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, and cesium bicarbonate; and alkaline earth metal salts such as magnesium carbonate and calcium carbonate. In one embodiment, the carbonate is one or more salts selected from the group consisting of alkali metal carbonates and alkali metal bicarbonates. In one embodiment, the carbonate is one or more salts selected from the group consisting of alkali metal carbonates. By carrying out this method using a carbonate as a base, formic acid can be obtained in high yield.

[0053] In one embodiment of the present invention, when a carbonate is selected as the carbonate source, the carbonate is not limited. Examples of carbonates include the same compounds as the carbonates used as the base when carbon dioxide is selected as the carbonate source. When a carbonate is selected as the carbonate source, the carbonate itself acts as a base, so that a base other than the carbonate does not need to be used. By carrying out this method using a carbonate as the carbonate source, formic acid can be obtained highly selectively and in high yield.

[0054] In one embodiment of the present invention, the hydrogen source is not limited. Examples of the hydrogen source include ascorbic acid, such as L-ascorbic acid, D-araboascorbic acid, riboflavin, and Hantzsch ester. By carrying out the present method using the compounds exemplified above as the hydrogen source, formic acid can be obtained in high yield.

[0055] In one aspect of the present invention, the solvent is not limited. Examples of the solvent include water (purified water, tap water, seawater, a mixture of two or more of these, etc.), organic solvents (for example, alcohols (ethyl alcohol, isopropyl alcohol, t-butyl alcohol, 1-methoxy-2-propyl alcohol, 2,2,2-trifluoroethyl alcohol, a mixture of two or more of these, etc.), acetonitrile, 1,4-dioxane, diglyme, DMA, DMSO, a mixture of two or more of these, etc.), and mixed solvents of water and organic solvents. In one embodiment, when the solvent is a mixed solvent of water and organic solvent, the water and organic solvent are immiscible. In one embodiment, when the solvent is a mixed solvent of water and organic solvent, the water and organic solvent are miscible. In one embodiment, when the solvent is a mixed solvent of water and organic solvent, the water and organic solvent are partially miscible (water and the organic solvent are partially miscible from the viewpoint of solubility, but exist separately). When the solvent is a mixed solvent of water and organic solvent, the blending ratio of water to organic solvent is not limited. For example, the volume ratio of the organic solvent to water (organic solvent / water) is usually 1 to 10, in one embodiment, 2 to 9, and in one embodiment, 3 to 8. By carrying out the present method using the compounds exemplified above as the solvent, formic acid can be obtained in high yield inexpensively and / or industrially safely.

[0056] In one aspect of the present invention, in "room temperature and normal pressure," room temperature usually means a temperature of 15°C to 40°C, in one embodiment 15°C to 25°C, in one embodiment 20°C to 40°C, and normal pressure usually means atmospheric pressure.

[0057] In one embodiment of the present invention, the light is not limited as long as it has a wavelength that can be absorbed by the hydrogen atom transfer catalyst and the photosensitizer, particularly the photosensitizer. Examples of the light include LED light, sunlight, and the like, which usually have a maximum emission wavelength of 350 nm to 500 nm, for example, 427 nm.

[0058] As explained in detail above, the method of this embodiment allows formic acid to be produced from carbon dioxide and a base or carbonate with high yield and high selectivity. Furthermore, when a formate salt is produced by the method of this embodiment, formic acid can be obtained by treating it with an acid. Formic acid obtained by the method of this embodiment is expected to be used as a hydrogen storage source, a raw material for useful compounds, and the like. Therefore, the method of this embodiment can provide these hydrogen storage sources and raw materials for useful compounds. [Example]

[0059] The present invention will be described in more detail below using examples, although the technical scope of the present invention is not limited to these examples.

[0060] [Experiment 1: Study of hydrogen atom transfer catalysts] In a Schlenk tube equipped with a carbon dioxide-filled balloon, a solution containing carbon dioxide (1 atm: bubbling), potassium carbonate (1.1 equivalents relative to the hydrogen source L-ascorbic acid) as a base, L-ascorbic acid (0.5 mmol) as a hydrogen source, and a mixed solvent of acetonitrile and water (acetonitrile:water = 8:2 (volume ratio), 10 mL) as a solvent was irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature (25 °C, 1 atm) in the presence of an N-heterocyclic carbene catalyst, imidazolium-2-carboxylate catalyst, imidazolium salt catalyst, imidazolinium salt catalyst, triazolium catalyst, or thiazolium salt catalyst (5 mol% relative to the hydrogen source L-ascorbic acid) shown in Table 1 below as a hydrogen atom transfer catalyst, and a carbazole catalyst (3 mol% relative to the hydrogen source L-ascorbic acid) shown in the following reaction formula as a photosensitizer. Table 1 shows the resulting formate concentrations along with the hydrogen atom transfer catalysts used.

[0061] [ka]

[0062] [Table 1] TIFF2026043328000023.tif230167

[0063] From Table 1, the counter anion (X - ) tend not to have a significant effect on catalytic activity. It was also found that reactivity tends to decrease when the hydrogen atom transfer catalyst has an electron-withdrawing group, a sterically bulky substituent, or an alkyl group in the side chain. Furthermore, it was found that reactivity tends to decrease when the hydrogen atom transfer catalyst has substituents at the 4th and 5th positions of the imidazolium skeleton. It was also found that reactivity tends to decrease when the hydrogen atom transfer catalyst is a triazolium catalyst or a thiazolium salt catalyst.

[0064] [Experiment 2: Examination of photosensitizers] In a Schlenk tube equipped with a carbon dioxide-filled balloon, a solution containing carbon dioxide (1 atm: bubbling), potassium carbonate as a base (1.1 equivalents relative to the hydrogen source L-ascorbic acid), L-ascorbic acid (0.5 mmol) as a hydrogen source, and a mixed solvent of acetonitrile and water (acetonitrile:water = 8:2 (volume ratio), 10 mL) was irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature (25°C, 1 atm) in the presence of an imidazolium salt catalyst or imidazolium-2-carboxylate catalyst shown in the following reaction formula (5 mol% relative to the hydrogen source L-ascorbic acid) as a hydrogen atom transfer catalyst and a carbazole catalyst shown in Table 2 or 3 (3 mol% relative to the hydrogen source L-ascorbic acid) as a photosensitizer.

[0065] Table 2 shows the results when an imidazolium salt catalyst was used as the hydrogen atom transfer catalyst, as the concentration of carbazole catalyst used and the resulting formate salt.

[0066] [ka]

[0067] [Table 2]

[0068] Table 3 shows the results when imidazolium-2-carboxylate catalyst was used as the hydrogen atom transfer catalyst, as well as the concentration of carbazole catalyst used and formate salt obtained.

[0069] [ka]

[0070] [Table 3]

[0071] Tables 2 and 3 show that for carbazole catalysts used as photosensitizers, a four-membered amino ring tends to decrease reactivity, while a seven-membered amino ring tends to increase reactivity. Here, for the seven-membered amino ring, the purity was less than 50%, but the amount added was 5.4 mg, equivalent to 0.015 mmol if the catalyst were a single compound. Therefore, for carbazole catalysts with a seven-membered amino ring, further improved reactivity can be expected by using a high-purity compound. At the 9th position of the carbazole skeleton of the carbazole catalyst used as a photosensitizer, Ph was found to be more reactive than H.

[0072] [Experiment 3: Study of bases (carbonates)] In a Schlenk tube equipped with a carbon dioxide-filled balloon, a solution containing carbon dioxide (1 atm: bubbling), a carbonate shown in Table 4 or 5 as a base (1.1 equivalents relative to the hydrogen source L-ascorbic acid), L-ascorbic acid (0.5 mmol) as a hydrogen source, and a mixed solvent of acetonitrile and water (acetonitrile:water = 8:2 (volume ratio), 10 mL) was irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature (25°C, 1 atm) in the presence of an imidazolium salt catalyst or imidazolium-2-carboxylate catalyst shown in the following reaction formula (5 mol% relative to the hydrogen source L-ascorbic acid) as a hydrogen atom transfer catalyst and a carbazole catalyst shown in the following reaction formula (3 mol% relative to the hydrogen source L-ascorbic acid) as a photosensitizer.

[0073] Table 4 shows the results when an imidazolium salt catalyst was used as the hydrogen atom transfer catalyst, in terms of the carbonate used as the base and the concentration of the resulting formate.

[0074] [ka]

[0075] [Table 4]

[0076] Table 5 shows the results when imidazolium-2-carboxylate catalyst was used as the hydrogen atom transfer catalyst, as a function of the carbonate used as the base and the concentration of formate obtained.

[0077] [ka]

[0078] [Table 5]

[0079] [Experiment 4: Examination of hydrogen sources] In a Schlenk tube equipped with a carbon dioxide-filled balloon, a solution containing carbon dioxide (1 atm: bubbling), potassium carbonate as a base (1.1 equivalents relative to the hydrogen source L-ascorbic acid), a compound listed in Table 6 or 7 (0.5 mmol) as a hydrogen source, and a mixed solvent of acetonitrile and water (acetonitrile:water = 8:2 (volume ratio), 10 mL) was irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature and atmospheric pressure (room temperature (25°C, 1 atm)) in the presence of an imidazolium salt catalyst or imidazolium-2-carboxylate catalyst shown in the following reaction formula (5 mol% relative to the hydrogen source L-ascorbic acid) as a hydrogen atom transfer catalyst and a carbazole catalyst shown in the following reaction formula (3 mol% relative to the hydrogen source L-ascorbic acid) as a photosensitizer.

[0080] Table 6 shows the results when using an imidazolium salt catalyst as the hydrogen atom transfer catalyst, as a function of the hydrogen source used and the concentration of formate obtained.

[0081] [ka]

[0082] [Table 6]

[0083] Table 7 shows the results when imidazolium-2-carboxylate catalyst was used as the hydrogen atom transfer catalyst, as a function of the hydrogen source used and the concentration of formate obtained.

[0084] [ka]

[0085] [Table 7]

[0086] [Experiment 5: Examination of light source] [ka] In a Schlenk tube equipped with a carbon dioxide-filled balloon, a solution containing carbon dioxide (1 atm: bubbling), potassium carbonate as a base (1.1 equivalents relative to the hydrogen source, L-ascorbic acid), L-ascorbic acid (0.5 mmol) as a hydrogen source, and a mixed solvent of acetonitrile and water (acetonitrile:water = 8:2 (volume ratio), 10 mL) was irradiated with sunlight for 4 hours at room temperature and pressure (room temperature (25°C), 1 atm) in the presence of the imidazolium salt catalyst shown in the reaction formula above (5 mol% relative to the hydrogen source, L-ascorbic acid) as a hydrogen atom transfer catalyst and the carbazole catalyst shown in the reaction formula above (3 mol% relative to the hydrogen source, L-ascorbic acid) as a photosensitizer. As a result, the production of formic acid was confirmed.

[0087] [Experiment 6: Solvent Examination] In a Schlenk flask equipped with a balloon filled with carbon dioxide, a solution containing carbon dioxide (1 atm: bubbling), potassium carbonate (1.1 equivalents relative to the hydrogen source L-ascorbic acid) as a base, L-ascorbic acid (0.5 mmol) as a hydrogen source, and a compound shown in Table 8 or 9 as a solvent was irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature and atmospheric pressure (room temperature (25°C), 1 atmosphere) in the presence of an imidazolium salt catalyst or imidazolium-2-carboxylate catalyst shown in the following reaction formula (5 mol% relative to the hydrogen source L-ascorbic acid) as a hydrogen atom transfer catalyst and a carbazole catalyst shown in the following reaction formula (3 mol% relative to the hydrogen source L-ascorbic acid) as a photosensitizer.

[0088] Table 8 shows the results when an imidazolium salt catalyst was used as the hydrogen atom transfer catalyst, as well as the compound or mixture used as the solvent and the concentration of the resulting formate salt.

[0089] [ka]

[0090] [Table 8] In the table, the ratios of the mixed solvents are all volume ratios.

[0091] Table 9 shows the results when imidazolium-2-carboxylate catalyst was used as the hydrogen atom transfer catalyst, as well as the compound or mixture used as the solvent and the concentration of formate salt obtained.

[0092] [ka]

[0093] [Table 9] In the table, the ratios of the mixed solvents are all volume ratios.

[0094] [Experiment 7: Examination of catalyst reuse] In a Schlenk tube equipped with a carbon dioxide-filled balloon, a solution containing carbon dioxide (1 atm: bubbling), potassium carbonate (0.55 mmol) as a base, L-ascorbic acid (0.5 mmol) as a hydrogen source, and a mixed solvent of acetonitrile and water (acetonitrile:water = 8:2 (volume ratio), 5 mL) was irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature (25 °C, 1 atm) in the presence of imidazolium-2-carboxylate catalyst (0.025 mmol) as a hydrogen atom transfer catalyst and carbazole catalyst (0.015 mmol) as a photosensitizer, as shown in Figure 1. After the reaction, the lower layer (aqueous layer: formate (product), L-ascorbic acid, and potassium carbonate) was removed from the reaction solution (two-layer system), and the formate (potassium formate) concentration in the lower layer was measured by NMR. Next, L-ascorbic acid (0.5 mmol) as a hydrogen source, potassium carbonate (0.55 mmol) as a base, and water (1 mL) as a solvent were added to the remaining upper layer (organic layer: hydrogen atom transfer catalyst and photosensitizer), and the mixture was irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature (25°C, 1 atm) while bubbling carbon dioxide. After the re-reaction, the lower layer (aqueous layer: formate (product), L-ascorbic acid, and potassium carbonate) was removed from the reaction solution (two-layer system), and the formate (potassium formate) concentration in the lower layer was measured by NMR. Next, L-ascorbic acid (0.5 mmol) as a hydrogen source, potassium carbonate (0.55 mmol) as a base, and water (1 mL) as a solvent were added to the remaining upper layer (organic layer: hydrogen atom transfer catalyst and photosensitizer). The mixture was then irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature (25°C, 1 atm) while bubbling carbon dioxide. Finally, the lower layer (aqueous layer: formate (product), L-ascorbic acid, and potassium carbonate) was removed from the reaction mixture (two-layer system) after the third reaction, and the formate (potassium formate) concentration in the lower layer was measured by NMR.

[0095] From FIG. 1, it was found that the hydrogen atom transfer catalyst and photosensitizer used in the present invention are reusable.

[0096] [Experiment 8: Examination of Carbonate Source]

[0097] [ka]

[0098] In a reaction vessel, a solution containing potassium carbonate (0.55 mmol) as the carbonate source, L-ascorbic acid (0.5 mmol) as the hydrogen source, and a mixed solvent of acetonitrile and water (acetonitrile:water = 8:2 (volume ratio), 10 mL) as the solvent was irradiated with LED light (λmax = 427 nm) for 4 hours at room temperature (25°C) in an argon atmosphere in the presence of the imidazolium salt catalyst (0.025 mmol) shown in the above reaction formula as the hydrogen atom transfer catalyst and the carbazole catalyst (0.015 mmol) shown in the above reaction formula as the photosensitizer. As a result, the production of formic acid was confirmed.

[0099] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to add, delete, and / or replace part of the configuration of each embodiment with other configurations.

Claims

1. A method for producing formic acid, comprising irradiating a solution containing carbon dioxide as a carbonate source and a base or a carbonate salt as a carbonate source, a hydrogen source, and a solvent with light at room temperature and atmospheric pressure in the presence of one or more hydrogen atom transfer catalysts selected from the group consisting of an N-heterocyclic carbene catalyst, an imidazolium-2-carboxylate catalyst, an imidazolinium-2-carboxylate catalyst, an imidazolium salt catalyst, an imidazolinium salt catalyst, a triazolium salt catalyst, a triazolium catalyst, and a thiazolium salt catalyst, and one or more photosensitizers selected from the group consisting of a carbazole catalyst.

2. the hydrogen atom transfer catalyst is one or more compounds selected from the group consisting of N-heterocyclic carbene catalysts, imidazolium-2-carboxylate catalysts, imidazolinium-2-carboxylate catalysts, imidazolium salt catalysts, and imidazolinium salt catalysts; The N-heterocyclic carbene catalyst is represented by the formula I-1' 【Chemistry 1】 I-1' It is expressed as The imidazolium-2-carboxylate catalyst or imidazolinium-2-carboxylate catalyst is represented by the formula I-2' 【Chemistry 2】 I-2' It is expressed as The imidazolium salt catalyst or imidazolinium salt catalyst is represented by Formula I-3' 【Transformation 3】 I-3' [In formula I-3', X － is a halogen anion, a tetrafluoroborate ion, or a bicarbonate ion. It is expressed as In formulae I-1′, I-2′ and I-3′, R ３ , R ３’ , R ４ , and R ４’ is hydrogen, or R ３’ and R ４’ together form a bond, and R ３ and R ４ is hydrogen, R ５ ~R １０ are each independently hydrogen, alkyl having 1 to 6 carbon atoms which may be substituted with aryl having 6 to 18 carbon atoms or alkyl having 1 to 6 carbon atoms, alkoxy having 1 to 6 carbon atoms, or halogen; The method of claim 1.

3. In formula I-3', X － is Cl － , B.F. ４ － , or HCO ３ － and In formulae I-1′, I-2′ and I-3′, R ５ ~R １０ are independently of each other hydrogen, methyl, 1-methyl-ethyl, 1-ethyl-propyl, 1-propyl-butyl, bromo, diphenylmethyl or methoxy, The method of claim 2.

4. The carbazole catalyst is represented by Formula II 【Chemistry 4】 II [In formula II, R １１ is hydrogen or an aryl having 6 to 18 carbon atoms, and R １２ and R １２’ are, independently of each other, NR １４ R １４’ or OH, and R １３ and R １３’ are each independently hydrogen or NR １５ R １５’ and R １４ and R １４’ are each independently an alkyl having 1 to 10 carbon atoms or an aryl having 6 to 18 carbon atoms (the aryl having 6 to 18 carbon atoms may be substituted with 1 to 3 substituents independently selected from an alkoxy having 1 to 10 carbon atoms), or R １４ and R １４’ together with the nitrogen atom to which they are attached form a 3- to 10-membered heterocycle, and R １５ and R １５’ are each independently an alkyl having 1 to 10 carbon atoms. The method according to any one of claims 1 to 3, wherein the compound is represented by the formula:

5. In formula II, R １１ is hydrogen or phenyl, R １２ and R １２’ is NR １４ R １４’ and R １３ and R １３’ are each independently hydrogen or NR １５ R １５’ and R １４ and R １４’ are, independently of each other, methyl or phenyl, the phenyl being substituted with methoxy in the para position relative to the carbon attached to the nitrogen, or R １４ and R １４’ together with the nitrogen atom to which they are attached form an azetidinyl, pyrrolidinyl, or azepanyl; R １５ and R １５’ is methyl, The method of claim 4.

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