Method for producing formic acid, catalyst, and reaction liquid

Abstract

To provide a highly practical method for producing formic acid by reducing carbon dioxide using a stable and inexpensive raw material.SOLUTION: A method for producing formic acid, including reducing carbon dioxide with a reaction liquid containing silicon, the following component (A), the following component (B), and the following component (C) in a solvent, and optionally containing the following component (D): The component (A) is one or more fluoride salts selected from potassium fluoride, sodium fluoride, and aluminum potassium hexafluoride. The component (B) is at least one tertiary amine. The component (C) is at least one acidic compound selected from sulfuric acid, sodium hydrogen sulfate, and phosphoric acid. Component (D): water.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing formic acid, a catalyst, and a reaction liquid. [Background technology]

[0002] Carbon dioxide (CO2) is emitted in various industrial processes (power generation, steel, manufacturing, etc.). Toward carbon neutrality by 2050, reducing carbon dioxide emissions and utilizing it effectively are urgent issues. Therefore, producing useful substances using carbon dioxide as a raw material is an important development theme for achieving this goal. In particular, obtaining substances such as formic acid and methanol, which have conventionally been synthesized using fossil resources, from carbon dioxide is preferable from the viewpoint of resource conservation.

[0003] On the other hand, the disposal method of solar panels used for photovoltaic power generation after their useful life has come to an issue. Regarding the recycling of solar panels, recycling methods have been developed for glass and metals such as aluminum, but the silicon contained in the panels has not been recycled, and the reality is that they are simply disposed of as waste.

[0004] Silicon has reducing power because it is composed of zero-valent silicon. The reduction reaction of carbon dioxide by silicon is thermodynamically favorable, so overheating during the reaction can be minimized. Silicon is currently produced by reducing silica using large amounts of electricity. Using silicon as a reducing agent to reduce carbon dioxide to produce useful substances is desirable because it not only contributes to the recycling of silicon discarded from solar panels, but also leads to the recovery of the energy input during silicon production.

[0005] Non-Patent Document 1 discloses a method for producing carbon monoxide by the reduction of carbon dioxide using proprietary silicon nanoparticles, using stoichiometric amounts of HF (3700%) and light. Non-Patent Document 2 discloses a method for producing methanol by the reduction reaction of carbon dioxide using proprietary silicon nanoparticles, using a stoichiometric amount of HF (220%).

[0006] Non-Patent Document 3 discloses a method for synthesizing formic acid by reducing carbon dioxide with silicon powder using tetrabutylammonium fluoride (TBAF) as a catalyst. Formic acid is an extremely important organic compound because it is used as a preservative and disinfectant, and can be reduced to produce methanol, a key industrial raw material used in the production of various chemical products. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] A. Ozin et al. Nat. Commun. 2016, 7, 125 [Non-patent document 2] M. Dasog et al. Chem. Commun. 2017, 53, 3114 [Non-patent document 3] K. Motokura et al. Energy Adv. 2022, 1, 385-390 Summary of the Invention [Problem to be solved by the invention]

[0008] The techniques described in Non-Patent Documents 1 and 2 relate to a method of reducing carbon dioxide using silicon as a reducing agent, but require the use of a large amount of toxic HF. In the technology described in Non-Patent Document 3, the TBAF used as the catalyst is an unstable and very expensive substance, making it difficult to use in large quantities and making it difficult to put into practical use. The present invention has been made in view of the above circumstances, and provides a highly practical method for producing formic acid, in which formic acid is obtained by reducing carbon dioxide using stable and inexpensive raw materials. [Means for solving the problem]

[0009] In order to achieve the above object, the present invention employs the following configuration. [1] A method for producing formic acid, comprising reducing carbon dioxide using a reaction solution containing silicon, the following component (A), the following component (B), and the following component (C) in a solvent, and optionally containing the following component (D): Component (A): One or more fluoride salts selected from potassium fluoride, sodium fluoride, potassium hydrogen fluoride, sodium hydrogen fluoride, sodium aluminum hexafluoride, and potassium aluminum hexafluoride. (B) Component: one or more tertiary amines. Component (C): One or more acidic compounds selected from sulfuric acid, sodium hydrogen sulfate, phosphorous acid, and phosphoric acid. (D) Ingredient: Water. [2] The method for producing formic acid according to [1], wherein the component (B) is one or more tertiary amines selected from a trialkylamine, a compound represented by the following formula (1), a compound represented by the following formula (2), a compound represented by the following formula (3), and a polymer-supported amine having a tertiary amine as a side chain functional group: [3] The method for producing formic acid according to [1] or [2], wherein the ratio of the number of moles of the component (B) to the number of moles of the component (A) in the reaction solution [B / A] is 0.1 to 11. [4] The method for producing formic acid according to any one of [1] to [3], wherein the ratio [C / A] of the number of moles of the component (C) to the number of moles of the component (A) in the reaction liquid is 0.1 to 10. [5] The method for producing formic acid according to any one of [1] to [4], wherein the ratio [D / A] of the number of moles of the component (D) to the number of moles of the component (A) in the reaction liquid is 0 to 300. [6] The method for producing formic acid according to any one of [1] to [5], wherein the ratio of the number of moles of silicon to the number of moles of the component (A) in the reaction liquid [Si / A] is 1 to 200. [7] The method for producing formic acid according to any one of [1] to [6], wherein the ratio [B / C] of the number of moles of the component (B) to the number of moles of the component (C) in the reaction liquid is 1.0 to 10. [8] The method for producing formic acid according to any one of [1] to [7], wherein the temperature of the reaction solution during the reduction is 20 to 200°C. [9] The method for producing formic acid according to any one of [1] to [8], wherein the carbon dioxide pressure in the vessel containing the reaction solution during the reduction is 10 to 100,000 hPa.

[10] A catalyst used in producing formic acid by reducing carbon dioxide, comprising the component (A), the component (B), and the component (C) according to any one of [1] to [9].

[11] The catalyst according to

[10] , which is represented by the following formula (4), and is formed by forming a hydrogen bond between a fluorine-containing ion generated from the component (A), a nitrogen cation generated by bonding a hydrogen atom to a tertiary nitrogen atom of the component (B), and a hydroxyl group possessed by the component (C).

[12] A reaction liquid used in producing formic acid by reducing carbon dioxide, the reaction liquid containing the catalyst according to

[10] or

[11] . [Effects of the Invention]

[0010] The method for producing formic acid of the present invention is a highly practical method for producing formic acid, which produces formic acid by reducing carbon dioxide using stable and inexpensive raw materials. [Brief explanation of the drawings]

[0011] [Figure 1] 1 shows the results of 1H NMR measurement of the catalyst according to the present invention produced in the examples. [Figure 2] FIG. 2 is an enlarged view of the chemical shift values ​​δ (ppm) in FIG. 1, where the values ​​are from 1.09 to 1.22. [Figure 3] 1 shows the results of 19F NMR measurement of the catalyst according to the present invention produced in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0012] In this specification and claims, a numerical range expressed by "to" means a numerical range in which the numbers before and after "to" are the lower and upper limits. The amounts of component (A), component (B), and component (C) are each the pure amount excluding the amounts of water and solvent.

[0013] <Method for producing formic acid> The method for producing formic acid, which is a first aspect of the present invention, is a method for reducing carbon dioxide using a reaction solution containing silicon, the following component (A), the following component (B), and the following component (C) in a solvent, and which may optionally contain the following component (D): Component (A): One or more fluoride salts selected from potassium fluoride, sodium fluoride, potassium hydrogen fluoride, sodium hydrogen fluoride, sodium aluminum hexafluoride, and potassium aluminum hexafluoride (KAlF6). (B) Component: one or more tertiary amines. Component (C): One or more acidic compounds selected from sulfuric acid, sodium hydrogen sulfate, phosphorous acid, and phosphoric acid. (D) Ingredient: Water.

[0014] The purity of silicon is not particularly limited, but from the viewpoint of improving the yield of formic acid, a higher purity is preferable. The purity of silicon is preferably 2N (99% by mass) or more, more preferably 3N (99.9% by mass) or more, and even more preferably 4N (99.99% by mass) or more.

[0015] Although the reaction can proceed even if silicon is used as a wafer without being crushed, it is preferable to crush it and use it as silicon powder for efficient reaction. The particle size of the silicon powder is not particularly limited, but it may be 300 μm or less. Furthermore, a particle size of 20 μm or less is preferable because it has the effect of improving reactivity due to an increase in specific surface area. The particle size of the silicon powder is preferably 4 μm or more, and more preferably 10 μm or more. The particle size of the silicon powder is the average particle size measured by a scanning electron microscope (SEM). The silicon powder can be adjusted to an appropriate particle size by separating it using sieves of various sizes in a sieving shaker.

[0016] The silicon is not particularly limited, and examples thereof include high-purity silicon raw materials, general silicon used in refractories, ceramics, etc., and silicon sludge, which is scraps from cutting silicon wafers used in semiconductors and solar panels. The silicon may be obtained from silicon wafers recovered from discarded solar panels. In this case, it is preferable to treat the obtained silicon with hydrochloric acid. The hydrochloric acid treatment involves mixing silicon with dilute hydrochloric acid. By performing the hydrochloric acid treatment, the amount of aluminum on the silicon surface is reduced.

[0017] Silicon wafers are manufactured by adding trace amounts of additive elements (dopants) such as B, P, and Sb to metallic silicon. However, the amount of additive elements is generally very small, so they do not affect the carbon dioxide reduction reaction. The surface of the silicon may be oxidized to some extent.

[0018] Component (B) is a nitrogen compound, and is preferably a tertiary amine. In general, a tertiary amine has three bonds of a nitrogen atom connected to an organic group R 1 ~R 3 It is described as having a covalently bonded structure with R 1 ~R 3 The carbon atom of the nitrogen atom forms a single or double bond with the nitrogen atom, and the unshared electron pair of the nitrogen atom can bond with a proton in the reaction solution to form a nitrogen cation. Here, the nitrogen cation may be referred to as an aminium ion. The tertiary amine of this embodiment is not particularly limited as long as it can form the nitrogen cation, and known tertiary amines can be used. Examples of suitable tertiary amines of this embodiment are shown below.

[0019] The tertiary amine of this embodiment may be a trialkylamine. 1 ~R 3are three independent straight-chain or branched-chain alkyl groups. The number of carbon atoms in each alkyl group is not particularly limited, and is, for example, preferably 1 to 30, more preferably 1 to 20, even more preferably 1 to 10, and particularly preferably 1 to 8 or 1 to 6. When the tertiary amine of this embodiment is a trialkylamine, triethylamine, tripropylamine, trihexylamine, and trioctylamine are preferred because they exhibit strong basicity.

[0020] In the tertiary amine of this embodiment, the organic group R 1 ~R 3 Among them, R 1 and R 2 are two independent linear or branched alkyl groups, and R 3 may be any monovalent organic group. The number of carbon atoms in each alkyl group is not particularly limited, and from the viewpoints of easily forming the nitrogen cation and easily dispersing in the reaction solution, for example, it is preferably 1 to 10, more preferably 1 to 5, and even more preferably 1 to 3. R 3 The monovalent organic group R is not particularly limited as long as it can form the nitrogen cation, and may be, for example, a polymer. 3 The polymer constituting the polymer is not particularly limited and can be appropriately selected from conventionally known polymers within the scope of the present invention, for example, polystyrene. In this specification, a polymer having a tertiary amine as one of the side chain functional groups such as those exemplified here is referred to as a "polymer-supported amine."

[0021] In the tertiary amine of this embodiment, the organic group R 1 ~R 3 Among them, R 1 and R 2 are two independent linear or branched alkyl groups, and R 3 In the above case where R is any monovalent organic group, 3 may have an optional substituent, and may be an alkyl group having 1 to 10 carbon atoms. One or more of the optional methylene groups (-CH2-) constituting this alkyl group may be, for example, -C(=NH)-, -N(-R 4It may be substituted with a divalent group containing a nitrogen atom, such as -, -NH-, etc. 4 represents an alkyl group having 1 to 4 carbon atoms. Specific examples include tertiary amines represented by the following formula (1). N,N,N',N'-tetramethylethylenediamine and 1,1,3,3-tetramethylguanidine used in the examples described below are tertiary amines corresponding to the following formula (1).

[0022] [ka] [In formula (1), a plurality of R 21 are each independently a linear or branched alkyl group having 1 to 10 carbon atoms, and R 22 is an alkylene group having 1 to 10 carbon atoms, preferably 1 to 4 carbon atoms, and one or more of the methylene groups constituting the alkylene group is -C(=NH)-, -N(-R 24 )- or -NH-, and the R 24 represents an alkyl group having 1 to 4 carbon atoms.]

[0023] In the tertiary amine of this embodiment, the organic group R 1 ~R 3 Among them, R 1 and R 2 are two independent linear or branched alkyl groups, and R 1 and R 2 The carbon atoms of the divalent methylene group obtained by removing one hydrogen atom from the terminal methyl group of the alkyl group may be bonded to each other to form a ring. Here, the number of methylene groups constituting the ring is preferably 4 to 8. One or more of the methylene groups constituting the ring may be substituted with a heteroatom such as an oxygen atom, a nitrogen atom, or a sulfur atom, except when the oxygen atoms are adjacent to each other. R bonded to a nitrogen atom constituting the ring 3 can be any monovalent organic group, for example, an alkyl group having 1 to 4 carbon atoms. Specific examples include tertiary amines represented by the following formula (2). 4-Methylmorpholine used in the examples described below is a tertiary amine represented by the following formula (2).

[0024] [ka] [In formula (2), ring X is a nitrogen-containing heterocycle formed by bonding carbon atoms at both ends of an alkylene group having 4 to 8 carbon atoms to nitrogen atoms, and any one or more methylene groups constituting the alkylene group may be substituted with an oxygen atom, a nitrogen atom, or a sulfur atom; R 31 represents an alkyl group having 1 to 4 carbon atoms.]

[0025] In the tertiary amine of this embodiment, the organic group R 1 ~R 3 Among them, R 1 and R 2 represent any identical monovalent organic group, and R 3 represents any monovalent organic group, and carbon atoms from each organic group from which one or more hydrogen atoms have been removed may be bonded to each other to form a ring. The ring may be monocyclic or polycyclic. In either case, the ring may be aromatic or non-aromatic. The number of carbon atoms constituting the ring is preferably 4 to 20, more preferably 4 to 14, and more preferably 4 to 9. One or more of the carbon atoms constituting the ring may be substituted with a heteroatom such as an oxygen atom, a nitrogen atom, or a sulfur atom, except when the oxygen atoms are adjacent to each other. Specific examples include tertiary amines represented by the following formula (3). Pyridine and diazabicycloundecene used in the examples described below are tertiary amines corresponding to the following formula (3).

[0026] [ka] [In formula (3), ring Y is a nitrogen-containing heterocycle having a C═N—C bonding structure in the ring structure, and the nitrogen-containing heterocycle may be monocyclic or polycyclic, and may be an aromatic or non-aromatic ring. The number of carbon atoms constituting the ring structure of ring Y is 4 to 20, preferably 4 to 10, and any one or more of the carbon atoms may be substituted with an oxygen atom, a nitrogen atom, or a sulfur atom (excluding cases where oxygen atoms are adjacent to each other). The carbon atoms on both ends of the C═N—C form a single bond or a double bond with the optionally substituted carbon atoms constituting the ring structure. n represents 1 or 2. Any hydrogen atom bonded to a carbon atom constituting the ring structure may be substituted with any monovalent substituent. Examples of the substituent include an alkyl group having 1 to 4 carbon atoms.]

[0027] Specific examples of the component (B) of this embodiment include, for example, one or more selected from trialkylamines having an alkyl group with 1 to 10 carbon atoms, 1,1,3,3-tetramethylguanidine, N,N,N',N'-tetramethylethylenediamine, polymer-supported amines having a tertiary amine as a side chain functional group, diazabicycloundecene, 4-methylmorpholine, and pyridine.

[0028] The solvent is not particularly limited as long as it does not interfere with the reaction of reducing carbon dioxide to obtain formic acid, but an aprotic solvent is preferred. Specific examples of suitable solvents include dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMA), dimethylformamide (DFM), acetonitrile, and acetone.

[0029] The ratio of the number of moles of component (B) to the number of moles of component (A) in the reaction liquid [B / A] is preferably 0.1 to 11, more preferably 0.5 to 5.0, even more preferably 0.9 to 3.0, and particularly preferably 1.1 to 2.2. When the ratio [B / A] is equal to or greater than the preferred lower limit, the generation of active species is promoted, and when it is equal to or less than the preferred upper limit, excessive suppression of proton reactivity is avoided.

[0030] The ratio of the number of moles of component (C) to the number of moles of component (A) in the reaction liquid [C / A] is preferably 0.1 to 10, more preferably 0.3 to 5.0, and even more preferably 0.5 to 2.0. When the ratio [C / A] is equal to or greater than the preferred lower limit, the generation of active species is promoted, and when it is equal to or less than the preferred upper limit, the decrease in reactivity due to a decrease in the concentration of active species is suppressed.

[0031] The ratio of the number of moles of component (D) to the number of moles of component (A) in the reaction liquid [D / A] is preferably 0 to 300, more preferably from 0 to 250, even more preferably 10 to 200, and particularly preferably 50 to 150. When the ratio [D / A] is equal to or greater than the preferred lower limit, the production of formic acid by protonation is accelerated, and when it is equal to or less than the preferred upper limit, the decrease in reactivity due to a decrease in the concentration of active species is suppressed.

[0032] The ratio of the number of moles of silicon to the number of moles of component (A) in the reaction liquid [Si / A] is preferably 1-200, more preferably 2-100, even more preferably 3-50, and particularly preferably 5-25. When the ratio [Si / A] is equal to or greater than the preferred lower limit, the reaction of silicon is accelerated, and the production of formic acid is also accelerated. When the ratio is equal to or less than the preferred upper limit, good stirring of the slurry-like reaction liquid is maintained.

[0033] The ratio of the number of moles of component (B) to the number of moles of component (C) in the reaction liquid [B / C] is preferably 1.0 to 10, more preferably 1.0 to 5.0, even more preferably 1.0 to 3.0, and particularly preferably 1.1 to 2.2. By keeping the ratio [B / C] at or above the preferred lower limit, it is possible to prevent toxic HF from volatilizing, while by keeping it at or below the preferred upper limit, it is possible to avoid excessive suppression of proton reactivity due to excessive enhancement of basicity.

[0034] The number of moles of silicon contained per liter of solvent in the reaction solution is preferably 0.1 to 10 moles, more preferably 0.3 to 5.0 moles, and even more preferably 0.5 to 2.0 moles. When the number is equal to or greater than the lower limit of the above range, the concentration of the reaction reagent is improved, thereby improving the reaction rate. When the number is equal to or less than the upper limit of the above range, the efficiency of stirring the reaction solution is improved.

[0035] The temperature of the reaction solution during reduction is preferably 20 to 200°C, more preferably 100 to 120°C. By keeping the reduction temperature at or above the preferred lower limit, the reaction rate is improved, and by keeping the temperature at or below the preferred upper limit, the loss of formic acid due to excessive reaction can be suppressed.

[0036] The carbon dioxide pressure in the vessel containing the reaction solution during reduction is preferably 10 to 100,000 hPa, more preferably 2,000 to 20,000 hPa, still more preferably 5,000 to 15,000 hPa, and particularly preferably 7,000 to 10,000 hPa. The reaction rate is improved by keeping the carbon dioxide pressure in the vessel containing the reaction solution at or above the preferred lower limit during reduction.

[0037] The reaction time during reduction varies depending on the temperature and pressure, but is preferably 1 to 72 hours, more preferably 16 to 24 hours. During the reduction, appropriate means may be employed to promote contact between the reaction solution and carbon dioxide, such as stirring, shaking, bubbling, etc. The formic acid obtained in the reaction solution by the reduction reaction can be separated from the solvent and silicon by common operations in the chemical industry.

[0038] The formic acid obtained in this embodiment may be further reduced to produce methanol. Conventionally, methanol has been synthesized using natural gas or coal gas as a raw material, requiring fossil resources. In contrast, by reducing the formic acid obtained in this embodiment, which uses carbon dioxide as a raw material, it is possible to produce methanol without using fossil resources, thereby contributing to resource conservation.

[0039] Catalyst A second aspect of the present invention is a catalyst used in producing formic acid by reducing carbon dioxide, which contains the components (A), (B), and (C) described in the first aspect. Each component may consist of one type or two or more types. The catalyst of this aspect may also contain the component (D) as an optional component. The catalyst of this aspect can be used in the method for producing formic acid of the first aspect.

[0040] The catalyst of this embodiment preferably has a chemical structure represented by the following formula (4): As will be explained in the Examples below, this chemical structure is 1 H NMR and 19 The catalyst can be identified by F NMR. In the catalyst represented by the following formula (4), a hydrogen bond is formed between a fluorine-containing ion generated from the component (A), a nitrogen cation generated when a hydrogen atom bonds to a tertiary nitrogen atom of the component (B), and a hydroxyl group possessed by the component (C).

[0041] [ka] [In formula (4), F - represents the entire anion moiety of the fluorine-containing ion contained in the component (A), and A + R represents the entire counter cation of the component (A). 1 ~R 3 and the nitrogen atom to which they are bonded represent the entire component (B). Z represents an atomic group other than the hydroxyl group in component (C). The dashed line represents the hydrogen bond.]

[0042] The anion portion of the fluorine-containing ion may consist of one fluoride ion (fluorine ion) or a polyvalent anion containing multiple fluorine atoms, such as HF2 containing two fluorine atoms. - It may be an ion, containing six fluorine atoms, such as AlF6 3- The former applies when the component (A) is potassium fluoride, sodium fluoride, or potassium hydrogen fluoride, and the latter applies when the component (A) is potassium hydrogen fluoride or potassium aluminum hexafluoride. + is not necessarily limited to one cation, and may be a plurality of cations. For example, when the component (A) is potassium fluoride, sodium fluoride, or potassium hydrogen fluoride, A + represents one potassium ion or sodium ion, and when the component (A) is potassium aluminum hexafluoride, A + represents three potassium ions.

[0043] The entire component (B) is one or more tertiary amines described in the first embodiment. 1 R 2 R 3 can be replaced with any one of formulas (1) to (3): All of the nitrogen atoms in formulas (1) to (3) are tertiary nitrogen atoms.

[0044] The component (C) as a whole is one or more acidic compounds selected from sulfuric acid, sodium hydrogen sulfate, phosphorous acid, and phosphoric acid, as described in the first embodiment. Z in formula (4) represents the remaining portion of the molecular structure of these oxoacids after removing one hydroxyl group.

[0045] The catalyst of this embodiment can be obtained by mixing the components (A), (B), and (C) with, if necessary, an optional component (D) and / or a solvent. The preferred blending ratio of each component is the same as the preferred blending ratio of each component in the reaction liquid described in the first embodiment.

[0046] By adding carbon dioxide to a reaction solution containing the catalyst of this embodiment, formic acid can be produced by reducing the carbon dioxide, as described in the first embodiment. Protons for reducing carbon dioxide are supplied by component (C) or optional component (D) contained in the reaction solution. It is believed that the protons and carbon dioxide molecules are present in the vicinity of the catalyst of this embodiment during the reaction. Furthermore, in formula (4), the hydrogen atom (proton) bonded to the tertiary nitrogen atom is supplied by component (C) or optional component (D).

[0047] The catalyst of this embodiment having the above-described configuration is a catalyst that can reduce carbon dioxide to obtain formic acid using stable and inexpensive raw materials.

[0048] <Reaction solution> A third aspect of the present invention is a reaction liquid used in producing formic acid by reducing carbon dioxide, the reaction liquid containing the catalyst of the second aspect. The reaction liquid of this aspect preferably contains the silicon described above in addition to the catalyst of the second aspect. By using the reaction liquid of this aspect, the method for producing formic acid of the first aspect can be carried out. In other words, the reaction liquid of this aspect is the reaction liquid described in the first aspect.

[0049] The catalyst concentration in the reaction solution of this embodiment is not particularly limited. When a solvent is included to facilitate stirring of the reaction solution, the preferred content of silicon contained per liter of solvent is as described in the first embodiment. The preferred amount of component (A) relative to silicon, the preferred amounts of components (C) and (D) relative to component (A), and the preferred amount of component (B) relative to component (C) are also as described in the first embodiment. Based on these preferred amounts, the preferred content of the catalyst in the reaction solution can be appropriately set. Note that the components (A) to (C) that make up the catalyst in the reaction solution are thought to be in an equilibrium state where they repeatedly undergo reversible bonding and dissociation, and when the amount of component (A) is relatively high, they tend to be more likely to be in a bonded state.

[0050] The reaction liquid of this embodiment may contain any optional components other than the catalyst as long as the effects of the present invention are not impaired.

[0051] By using the reaction solution of this embodiment having the above-mentioned constitution, it is possible to reduce carbon dioxide to obtain formic acid using stable and inexpensive raw materials. [Example]

[0052] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0053] [silicon] In each example, the following waste or new silicon was used: Waste silicon: Silicon recovered from used solar panels is ground into a 40μm powder using an alumina mortar and then treated with hydrochloric acid. Purity is 1N (90%) or higher. New silicon: High-purity silicon of solar panel grade is ground into a 40μm powder using an alumina mortar. Purity is 4N (99.99%) or higher.

[0054] [Experimental Example 1] In each example, a reaction solution was obtained by adding silicon (2.0 mmol), NMP solvent (2 mL), and the amounts of components (A) to (D) shown in Table 1 to a stainless steel autoclave (internal volume approximately 20 mL). The gas inside the autoclave containing this reaction solution was replaced with carbon dioxide at 9 atm (9119 hPa), and then the reaction solution was heated at 100°C for 24 hours while stirring. In Table 1, "-" means that the component in question was not blended.

[0055] In all of the examples shown in Table 1, potassium fluoride (KF) was used as component (A), triethylamine as component (B), sulfuric acid (H2SO4) as component (C), and distilled water (H2O) as component (D). After the reaction is complete, the formic acid produced is 1 The results of quantification by H-NMR are shown in Table 1. Examples 1-1 to 1-5 are working examples, and Examples 1-6 to 1-10 are comparative examples.

[0056] [Table 1]

[0057] As shown in Table 1, in Examples 1-1 to 1-5, in which the reaction solution of the present invention was used, formic acid could be obtained without using TBAF. In contrast, in Examples 1-6 to 1-10, in which any one of Components (A) to (C) was lacking, formic acid could not be obtained.

[0058] [Experimental Example 2] In each example, new silicon (2.0 mmol), NMP solvent (2 mL), component (A) or a fluoride salt other than component (A), and components (B) to (D) were added to a stainless steel autoclave (internal volume approximately 20 mL) to obtain a reaction solution. The gas inside the autoclave containing this reaction solution was replaced with carbon dioxide at 9 atm (9119 hPa), and the reaction solution was heated at 100°C for 24 hours while stirring.

[0059] The fluoride salt used in each example was 0.20 mmol of the fluoride salt shown in Table 2. In addition, in all examples shown in Table 2, 0.22 mmol of triethylamine was used as component (B), 0.20 mmol of sulfuric acid (H2SO4) was used as component (C), and 10 mmol of distilled water (H2O) was used as component (D). After the reaction is complete, the formic acid produced is 1 The results of quantification by H-NMR are shown in Table 2 together with the results of Examples 1-5. Examples 2-1 to 2-3 are working examples, and Examples 2-4 to 2-6 are comparative examples.

[0060] [Table 2]

[0061] As shown in Table 2, in Examples 2-1 and 2-2, which used reaction solutions containing a fluoride salt corresponding to component (A), formic acid was obtained, as in Example 1-5. In Example 2-3, formic acid was obtained, as in Example 1-1. In contrast, in Examples 2-4 to 2-6, which used reaction solutions containing a fluoride salt not corresponding to component (A), formic acid was not obtained.

[0062] [Experimental Example 3] Fresh silicon (2.0 mmol), NMP solvent (2 mL), and components (A) to (D) were added to a stainless steel autoclave (internal volume approximately 20 mL) to obtain a reaction solution. The gas inside the autoclave containing this reaction solution was replaced with carbon dioxide at 9 atm (9119 hPa), and the reaction solution was then heated at 100°C for 24 hours while stirring.

[0063] In each example, 0.20 mmol of a fluoride salt shown in Table 3 was used as component (A), and 0.22 mmol of a nitrogen compound shown in Table 3 was used as component (B). In addition, in each example shown in Table 3, 0.20 mmol of sulfuric acid (H2SO4) was used as component (C), and 10 mmol of distilled water (H2O) was used as component (D). After the reaction is complete, the formic acid produced is 1 The results of quantification by H-NMR are shown in Table 3 together with the results of Examples 1-5, 2-1, and 2-2. Examples 3-1 to 3-12 are all examples of working examples.

[0064] [Table 3]

[0065] As shown in Table 3, in Examples 3-1 to 3-12, in which a reaction solution containing a fluoride salt corresponding to component (A) and a nitrogen compound corresponding to component (B) was used, formic acid could be obtained, as in Examples 1-5, 2-1, and 2-2.

[0066] [Experimental Example 4] In each example, new silicon (2.0 mmol), NMP solvent (2 mL), component (A), component (B), an acidic compound corresponding to component (C) or an acidic compound not corresponding to component (C), and component (D) were added to a stainless steel autoclave (internal volume approximately 20 mL) to obtain a reaction solution. The gas inside the autoclave containing this reaction solution was replaced with carbon dioxide at 9 atm (9119 hPa), and the reaction solution was heated at 100°C for 24 hours while stirring.

[0067] The acidic compound used in each example was 0.20 mmol of the acidic compound shown in Table 4. In addition, in each example shown in Table 4, 0.20 mmol of potassium fluoride (KF) was used as component (A), 0.22 mmol of triethylamine was used as component (B), and 10 mmol of distilled water (HO) was used as component (D). After the reaction is complete, the formic acid produced is 1 The results of quantification by H-NMR are shown in Table 4 together with the results of Examples 1-5. Examples 4-1 and 4-2 are working examples, and Examples 4-3 and 4-4 are comparative examples.

[0068] [Table 4]

[0069] As shown in Table 4, in Examples 4-1 and 4-2, in which a reaction solution containing an acidic compound corresponding to component (C) was used, formic acid was obtained, as in Example 1-5. In contrast, in Example 4-3, in which a reaction solution containing an acidic compound not corresponding to component (C) was used, only a small amount of formic acid was obtained, and in Example 4-4, no formic acid was obtained.

[0070] [Experimental Example 5] In each example, new silicon (2.0 mmol), NMP solvent (2 mL), and components (A) to (D) were added to a stainless steel autoclave (internal volume approximately 20 mL) to obtain a reaction solution. The gas inside the autoclave containing this reaction solution was replaced with carbon dioxide at 9 atm (9119 hPa), and the reaction solution was then heated at 100°C for 24 hours while stirring.

[0071] In all of the examples shown in Table 5, potassium fluoride (KF) was used as component (A), triethylamine as component (B), sulfuric acid (H2SO4) as component (C), and distilled water (H2O) as component (D). The amounts of each component were as shown in Table 5. After the reaction is complete, the formic acid produced is 1 The results of quantification by H-NMR are shown in Table 5 together with the results of Examples 1-5. Examples 5-1 to 5-3 are all examples of working examples.

[0072] [Table 5]

[0073] As shown in Table 5, the amount of formic acid produced was the highest in Example 1-5, in which the amounts of component (B) and component (C) were each approximately equimolar to the amount of component (A). Note that experiments in which the amount of component (B) was set to an equimolar amount or less of the amount of component (C) were not carried out due to concerns that toxic HF would be generated.

[0074] [Experimental Example 6] In each example, new silicon (2.0 mmol), NMP solvent (2 mL), and components (A) to (D) were added to a stainless steel autoclave (internal volume approximately 20 mL) to obtain a reaction solution. The gas inside the autoclave containing this reaction solution was replaced with carbon dioxide at 9 atm (9119 hPa), and the reaction solution was then heated at 100°C for 24 hours while stirring.

[0075] In each example, 0.20 mmol of a fluoride salt shown in Table 6 was used as component (A), and 0.22 mmol of a nitrogen compound shown in Table 6 was used as component (B). In addition, in each example shown in Table 6, 0.20 mmol of sulfuric acid (H2SO4) was used as component (C), and 10 mmol of distilled water (H2O) was used as component (D). After the reaction is complete, the formic acid produced is 1The results of quantification by 1 H-NMR are shown in Table 6 together with the results of Examples 1-5, 2-1, and 3-1 to 3-8. Examples 6-1 to 6-10 are all examples of the present invention.

[0076] [Table 6]

[0077] As shown in Table 6, in Examples 6-1 to 6-10, which used a reaction solution containing a fluoride salt corresponding to component (A) and a nitrogen compound corresponding to component (B), formic acid was obtained, similarly to Examples 1-5 and 3-1 to 3-8. In particular, a large amount of formic acid was obtained when KF was used as component (A) and triethylamine, trihexylamine, diazabicycloundecene, tri-n-octylamine, and 1,1,3,3-tetramethylguanidine were used as component (B).In particular, a large amount of formic acid was obtained when NaF was used as component (A) and triethylamine, tripropylamine, diazabicycloundecene, tri-n-octylamine, and 1,1,3,3-tetramethylguanidine were used as component (B).

[0078] The functional group possessed by the polymer-supported amine (component (B)) in Table 6) is a monovalent group in which one of the terminal methyl groups of N,N,N',N'-tetramethylethylenediamine is replaced with a methylene group, and the monovalent group is bonded to polystyrene via a phenylene group. This polymer-supported amine is sold by Merck Co., Ltd. under product number 656836.

[0079] [Catalyst Identification] In order to analyze the chemical structure of the catalyst contained in the reaction solution prepared in the above-mentioned experimental examples, the following reaction solution examples 7-1 to 7-7, which were prepared by mixing the following components (A) to (D), were used: 1 H NMR and 19 Measurements were performed using F NMR in the usual manner. The results are shown in Figures 1 to 3. In the figures, the numbers in parentheses immediately after KF indicate the amount of KF.

[0080] Figures 1 and 2 1 Each reaction solution used for H NMR measurement was prepared by mixing KF at a concentration of 0 to 0.15 mmol as component (A), 0.20 mmol of triethylamine (TEA) as component (B), 0.20 mmol of sulfuric acid (HSO) as component (C), and 10 mmol of distilled water (HO) as component (D) in heavy water (DO). Among Examples 7-1 to 7-5, Example 7-4, which did not contain KF, and Example 7-5, which did not contain KF or sulfuric acid, are comparative examples, and the others are working examples.

[0081] See Figures 1 and 2. Comparing Example 7-5 and Example 7-4, it can be seen that the reaction between TEA and H2SO4 shifted the signal of the hydrogen atom of the ethyl group contained in TEA to a lower magnetic field. Furthermore, in Examples 7-3 to 7-1, in which KF was added, it can be seen that the signal gradually shifted to a lower magnetic field as the KF concentration increased (Figure 2 is a partially enlarged view of Figure 1). From this, it is thought that components (A) to (C) in the reaction solution are in equilibrium between aggregates and dissociated forms, and that the amount of aggregate increases as the concentration of component (A) increases.

[0082] Figure 3 19 Each reaction solution used for F NMR measurement was prepared by mixing 0.20 mmol of KF as component (A), 0.20 mmol of triethylamine (TEA) as component (B), 0.20 mmol of sulfuric acid (HSO) as component (C), and 10 mmol of distilled water (HO) as component (D) in heavy water (DO). Example 7-7, which does not contain TEA or sulfuric acid, is a comparative example, and Example 7-6 is an embodiment.

[0083] See Figure 3. Compared to Example 7-7, which contains only KF, signals shifted to the higher magnetic field (particularly -160 to -170 ppm in Example 7-6) were observed. This is thought to indicate the formation of a strong interaction between hydrogen and fluorine, i.e., a hydrogen bond.

[0084] From the above, it was confirmed that in the reaction solution of each Example, components (A) to (C) were associated to form a catalyst having the chemical structure represented by formula (4).

Claims

1. A method for producing formic acid, comprising reducing carbon dioxide using a reaction solution containing silicon, the following component (A), the following component (B), and the following component (C) in a solvent, and optionally containing the following component (D): Component (A): One or more fluoride salts selected from potassium fluoride, sodium fluoride, potassium hydrogen fluoride, sodium hydrogen fluoride, sodium aluminum hexafluoride, and potassium aluminum hexafluoride. Component (B): one or more tertiary amines. Component (C): One or more acidic compounds selected from sulfuric acid, sodium hydrogen sulfate, phosphorous acid, and phosphoric acid. (D) Component: Water.

2. 2. The method for producing formic acid according to claim 1, wherein the component (B) is one or more tertiary amines selected from a trialkylamine, a compound represented by the following formula (1), a compound represented by the following formula (2), a compound represented by the following formula (3), and a polymer-supported amine having a tertiary amine as a side chain functional group: 【Chemistry 1】 [In formula (1), a plurality of R ２１ are each independently a linear or branched alkyl group having 1 to 10 carbon atoms, and R ２２ is an alkylene group having 1 to 10 carbon atoms, preferably 1 to 4 carbon atoms, and one or more of the methylene groups constituting the alkylene group is -C(=NH)-, -N(-R ２４ )- or -NH-, and the R ２４ represents an alkyl group having 1 to 4 carbon atoms. 【Chemistry 2】 [In formula (2), ring X is a nitrogen-containing heterocycle formed by bonding carbon atoms at both ends of an alkylene group having 4 to 8 carbon atoms to nitrogen atoms, and any one or more methylene groups constituting the alkylene group may be substituted with an oxygen atom, a nitrogen atom, or a sulfur atom; R ３１ represents an alkyl group having 1 to 4 carbon atoms. 【Transformation 3】 [In formula (3), ring Y is a nitrogen-containing heterocycle having a C═N—C bond in the ring structure, and the nitrogen-containing heterocycle may be a monocycle or a polycycle, and may be an aromatic ring or a non-aromatic ring. The ring structure of ring Y has 4 to 20 carbon atoms, and any one or more of the carbon atoms may be substituted with an oxygen atom, a nitrogen atom, or a sulfur atom (excluding the case where oxygen atoms are adjacent to each other). The carbon atoms at both ends of the C═N—C form a single bond or a double bond with the optionally substituted carbon atoms constituting the ring structure. n represents 1 or 2.]

3. 2. The method for producing formic acid according to claim 1, wherein the ratio [B / A] of the number of moles of the component (B) to the number of moles of the component (A) in the reaction solution is 0.1 to 11.

4. 2. The method for producing formic acid according to claim 1, wherein the ratio [C / A] of the number of moles of the component (C) to the number of moles of the component (A) in the reaction solution is 0.1 to 10.

5. 2. The method for producing formic acid according to claim 1, wherein the ratio [D / A] of the number of moles of the component (D) to the number of moles of the component (A) in the reaction solution is 0 to 300.

6. 2. The method for producing formic acid according to claim 1, wherein the ratio of the number of moles of silicon to the number of moles of component (A) in the reaction liquid [Si / A] is 1 to 200.

7. 2. The method for producing formic acid according to claim 1, wherein the ratio [B / C] of the number of moles of the component (B) to the number of moles of the component (C) in the reaction solution is 1.0 to 10.

8. 2. The method for producing formic acid according to claim 1, wherein the temperature of the reaction solution during the reduction is 20 to 200°C.

9. The method for producing formic acid according to any one of claims 1 to 8, wherein the pressure of carbon dioxide in a vessel containing the reaction solution during the reduction is 10 to 100,000 hPa.

10. A catalyst used in producing formic acid by reducing carbon dioxide, A catalyst comprising the component (A), the component (B), and the component (C) according to claim 1.

11. Represented by the following general formula (4): fluorine-containing ions generated from the component (A); a nitrogen cation generated by bonding a hydrogen atom to a tertiary nitrogen atom of the component (B); a hydroxyl group contained in the component (C); The catalyst according to claim 10, wherein 【Chemistry 4】 [In formula (4), F － represents the entire anion moiety of the fluorine-containing ion contained in the component (A), and A ＋ represents the entire counter cation of the component (A). １ ~R ３ and the nitrogen atom to which they are bonded represent the entire component (B). Z represents an atomic group other than the hydroxyl group in the component (C). The dashed line represents the hydrogen bond.]

12. A reaction liquid used in producing formic acid by reducing carbon dioxide, A reaction liquid comprising the catalyst according to claim 10 or 11.

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