Method for promoting formic acid synthesis reaction, and ionic liquid

Abstract

To provide a method for promoting a formic acid synthesis reaction, by which formic acid can efficiently be produced from carbon dioxide and hydrogen.SOLUTION: In the method for accelerating a formic acid synthesis reaction, carbon dioxide and hydrogen are reacted in an ionic liquid, and a substituent in a cation of the ionic liquid is at least an amino group or a carboxyl group.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for promoting a formic acid synthesis reaction in which formic acid is produced from carbon dioxide and hydrogen, or from carbon dioxide and hydrogen obtained by decomposition of water, and more particularly to a method for promoting a formic acid synthesis reaction in which formic acid is obtained in an ionic liquid or in an aqueous solution in which an organic compound containing an amino group or a carboxyl group as a functional group is dissolved. [Background technology]

[0002] We continue to live our social lives by converting and utilizing materials that exist on Earth. In particular, the raw materials for organic compounds are mostly fossil fuels, which are converted and utilized industrially. However, fossil fuels are a finite resource, and there is no guarantee that they will continue to be used forever. Furthermore, carbon dioxide emissions are unavoidable during the production and utilization processes, and we continue to release this extremely chemically stable carbon dioxide. Carbon dioxide, which is stable and has few uses, can also be used as a carbon source if you change your perspective. When organic compounds are burned, energy is released and water and carbon dioxide are produced. While an energy supply is necessary, it is theoretically possible to use natural energy such as sunlight to cause a reverse reaction and produce organic compounds from water and carbon dioxide. Although previous research has been largely unsuccessful in freely synthesizing organic compounds from water and carbon dioxide, it is known that formic acid, the simplest organic acid, is the only one that may be practically synthesized. The inventors are betting on this possibility and are aiming for practical application (for example, Patent Documents 1 and 2, Non-Patent Documents 1-3, and cited references). Meanwhile, conventionally, there has been known a method for industrially producing acetic acid by oxidizing various hydrocarbons contained in butane and naphtha in a liquid phase (for example, Non-Patent Document 4 below). Furthermore, Non-Patent Document 4 below describes that in the production of acetic acid as described above, formic acid is obtained as a by-product. Furthermore, Non-Patent Document 4 below describes that formic acid is also obtained as a by-product in the production of pentaerythritol. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Retable 2011 / 093229 [Patent Document 2] Patent No. 7288484 [Non-patent literature] [Non-Patent Document 1] The Journal of Physical Chemistry B 2011,115,14136-14140 and references cited [Non-patent document 2] The Journal of Physical Chemistry B 2011,115,9789-9794 and references cited [Non-patent document 3] ACS Catalysis 2022,12,11,6770-6780 and citations [Non-patent document 4] Takuya Kumamoto, "Industrial Synthesis of Formic Acid and Its Use", Chemistry and Education, Chemical Society of Japan, 2012, Vol. 60, No. 8, pp. 358-361, Internet<URL:https: / / www.jstage.jst.go.jp / article / kakyoshi / 60 / 8 / 60_KJ00008195825 / _article / -char / ja / > Non-patent documents 1 to 3 are scientific journals of the American Chemical Society. Summary of the Invention [Problem to be solved by the invention]

[0004] The formic acid is not produced directly from carbon dioxide and water, but from hydrogen and carbon dioxide produced by decomposing water. There is still much room for research into methods for efficiently producing formic acid from hydrogen and carbon dioxide with minimal energy.

[0005] In view of the above problems, an object of the present invention is to provide a method for producing formic acid from hydrogen and carbon dioxide efficiently with minimal energy consumption. [Means for solving the problem]

[0006] [Summary of knowledge on formic acid synthesis reaction] The basis for promoting a chemical reaction is to have a large number of reactants present in the reaction field, allowing them to meet and collide, transitioning the energy state of the reactants to a higher state and forming an activated complex (referred to as a transition state or an activated state; hereafter, these terms are synonymous). Carbon dioxide and hydrogen are gases at room temperature and pressure. If the reaction field is in a different state, such as a liquid, the reaction will not proceed unless the reactant is incorporated into the reaction field. Therefore, the inventors conducted extensive research and found that by using at least an amino group or a carboxyl group as the substituent in the cation of an ionic liquid, carbon dioxide, one of the reactants, can be easily incorporated into these substituents, thereby promoting the reaction. As a result, they discovered that the reaction between carbon dioxide and hydrogen is promoted in the ionic liquid, allowing for the efficient synthesis of formic acid. Furthermore, the present inventors have discovered a method for efficiently synthesizing formic acid by reacting carbon dioxide and hydrogen in water containing an ionic liquid or an organic compound in which the substituents in the cations contain at least an amino group or a carboxyl group, leading to the invention. Furthermore, in an ionic liquid used as a reaction site for synthesizing formic acid by reacting carbon dioxide and hydrogen, by making some or all of the substituents in the cations of the ionic liquid at least an amino group or a carboxyl group, it has been found that carbon dioxide and / or hydrogen can be easily incorporated into the ionic liquid reaction site, and that these reaction substrates are easily converted to an activated state. This has demonstrated that the ionic liquid serves as a reaction site and reaction medium for efficiently synthesizing formic acid. Furthermore, the reaction proceeds by incorporating the reaction substrates into the solvent and converting them into an activated state. Through extensive research, it has been found that the activated state is preferably close to carbonate and has a structure similar to that of the formate anion, which promotes the reaction. In other words, the reaction is promoted by creating a reaction environment that promotes the conversion to these activated states. It is desirable that the substituents be positioned so that they can adsorb or bond to at least the oxygen atom in the carbon dioxide molecule. Furthermore, although carbon dioxide molecules are linear molecules, a twisted structure is preferred.Furthermore, it is desirable that the substituents are arranged so that they can also adsorb or bond with hydrogen molecules, and it is desirable that the substituents are arranged so that hydrogen of nonpolar molecules has a polar structure within the molecule.

[0007] In the method for producing formic acid according to the present invention, the anion of the ionic liquid is preferably a formate anion.

[0008] According to this configuration, an activated state is created, creating an environment that is conducive to the formation of an activated complex, making it easier to obtain formic acid. In other words, formic acid can be produced efficiently from carbon dioxide and water.

[0009] The cation of the ionic liquid preferably contains an imidazolium salt.

[0010] According to this configuration, formic acid can be produced efficiently.

[0011] Moreover, the method for producing formic acid preferably further comprises a hydrogen production step of electrolyzing water using renewable energy to produce the hydrogen.

[0012] According to this configuration, the hydrogen used in the formic acid production step can be obtained while suppressing an increase in the burden on the global environment. [Effects of the Invention]

[0013] As described above, the present invention can provide a method for promoting a formic acid synthesis reaction that can efficiently produce formic acid from carbon dioxide and hydrogen. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic diagram showing a method for promoting a formic acid synthesis reaction according to one embodiment of the present invention. [Figure 2] 1 is a schematic cross-sectional view of an apparatus used in a method for promoting a formic acid synthesis reaction according to one embodiment of the present invention. [Figure 3]An example of the chemical structure of an ionic liquid. DETAILED DESCRIPTION OF THE INVENTION

[0015] In a method for promoting a formic acid synthesis reaction according to one embodiment of the present invention, the substituent in the cation of the ionic liquid has at least an amino group or a carboxyl group. The anion of the ionic liquid has a formate anion. Furthermore, the cation of the ionic liquid is an imidazolium salt.

[0016] Alternatively, in a method for promoting a formic acid synthesis reaction in which carbon dioxide and hydrogen are reacted in water in which an organic compound containing an ionic liquid is present (including a state in which the organic compound is dissolved in an aqueous solution, or a state in which the organic compound and an aqueous solution exist at a concentration equal to or higher than the saturated concentration; these are referred to as the second medium; on the other hand, the ionic liquid is referred to as the first medium), the substituent in the cation of the organic compound is at least an amino group or a carboxyl group.

[0017] Alternatively, the ionic liquid is used as a site for synthesizing formic acid by reacting carbon dioxide with hydrogen, and the substituents in some or all of the cations in the ionic liquid are at least amino groups or carboxyl groups.

[0018] The hydrogen is obtained from the electrolysis of water using water as a supply source. Alternatively, hydrogen may be obtained by reacting CO with water and via the water-gas shift reaction (CO + H2O → HCOOH → H2 + CO2). It goes without saying that hydrogen generated in various chemical reaction processes can also be utilized. Hereinafter, "according to one embodiment of the present invention" will be simply referred to as "according to this embodiment."

[0019] The method for promoting a formic acid synthesis reaction according to this embodiment is carried out, for example, under the concept shown in FIG.

[0020] [Conceptual diagram of the method for promoting the formic acid synthesis reaction] To promote the reaction, it is important to bring together the reactant substrates, carbon dioxide and hydrogen, change their structure to one that facilitates the reaction, and transition them into an active complex. At room temperature and pressure, carbon dioxide and hydrogen are gases that freely translate, vibrate, and rotate, forming a stable structure on average. Both are linear molecules, which is their stable structure.

[0021] Figure 1 shows a conceptual diagram of this reaction. To facilitate the meeting of the freely moving carbon dioxide and hydrogen, they must be captured by a solvent or other suitable means. In the case of a solution, they are incorporated into the solvent, i.e., solvated. In addition to solvation, a structure capable of trapping carbon dioxide and hydrogen by providing spaces between the solvent molecules that allow them to enter is also acceptable. For a solvated or trapped structure, it is sufficient to have a high amount of at least one of carbon dioxide and hydrogen. To promote the uptake of each molecule, the solvent must possess a structure or functional group that facilitates the uptake of each molecule. As a result of extensive research, the inventors have found that, as described above, the substituent in the cation of the ionic liquid preferably has at least an amino group or a carboxyl group. Furthermore, it has been found that water containing an organic compound containing the ionic liquid (including a dissolved aqueous solution or a state in which the organic compound and the aqueous solution exist at a concentration above saturation) is preferable. Furthermore, it has been found that ionic liquids containing the aforementioned substituents in the cation more easily activate (promote) the reaction. As mentioned above, the reaction proceeds by incorporating the reactant into the solvent and transitioning to an activated state. Through extensive research, we have found that the activated state is preferably close to carbonate and a structure similar to formate anion, which accelerates the reaction. In other words, the key is to create a reaction environment that promotes the transition to these activated states, thereby accelerating the reaction and transitioning to this state. Furthermore, we have found that when the anion of the ionic liquid contains a formate anion, it transitions to an activated state and is prone to undergo a structural change to an activated complex. Furthermore, the cation of the ionic liquid is an imidazolium salt. These techniques have been found to facilitate the incorporation of carbon dioxide and hydrogen into the solvent, further creating a structure where the incorporated carbon dioxide and hydrogen are more likely to react, further promoting the structural change to an activated complex and ultimately facilitating the production of formic acid. Furthermore, as mentioned above, it is desirable for the substituents to be positioned so that they can adsorb or bond to at least the oxygen atom in the carbon dioxide molecule. Furthermore, while carbon dioxide molecules are linear, a twisted structure is preferable.Furthermore, it is desirable that the substituents are arranged so that hydrogen molecules can be adsorbed or bonded, and it is desirable that the substituents are arranged so that hydrogen of nonpolar molecules has a polar structure within the molecule. Using these findings, a specific method for accelerating the formic acid synthesis reaction is disclosed.

[0022] [Formic acid synthesis reaction accelerator] The method for promoting a formic acid synthesis reaction according to this embodiment is the same as that described in Japanese Patent No. 7288484, and is carried out, for example, in an apparatus such as that shown in FIG. The apparatus 1 for carrying out the method for promoting a formic acid synthesis reaction according to this embodiment is a batch reactor having a storage space S capable of storing an ionic liquid (first medium) or water containing an organic compound containing an ionic liquid (a second medium including a dissolved aqueous solution or a state in which an organic compound and an aqueous solution exist at a concentration equal to or higher than the saturated concentration). Here, a batch reactor will be described as an example, but a flow reactor may also be used. Specifically, the apparatus 1 for carrying out the method for promoting a formic acid synthesis reaction according to this embodiment includes a reaction tank 10 formed in a cylindrical shape and having a storage space S inside that can store a first medium or a second medium, a jacket 20 that covers the outer surface and bottom surface of the reaction tank 10, and a reaction medium storage tank 30 that stores a reaction medium containing the first medium or the second medium. In addition, the apparatus 1 for carrying out the method for promoting the formic acid synthesis reaction according to this embodiment is equipped with a pipe L for connecting the reaction tank 10 and the reaction medium storage tank 30, and a valve V for adjusting the open / closed state of the pipe L. Furthermore, the apparatus 1 for carrying out the method for promoting a formic acid synthesis reaction according to this embodiment preferably includes an inert gas storage tank (not shown) in which an inert gas such as nitrogen gas, helium gas, or argon gas is stored, a pipe connecting the inert gas storage tank to the reaction tank 10, and a valve for adjusting the open / close state of the pipe.

[0023] The reaction vessel 10 includes a cylindrical side wall portion 10a, a bottom wall portion 10b that closes the bottom side of the cylindrical side wall portion 10a, and a top wall portion 10c that closes the top side of the cylindrical side wall portion 10a. In the reaction vessel 10, as described above, the cylindrical side wall portion 10a is closed by the bottom wall portion 10b and the top wall portion 10c, thereby making the accommodation space S an enclosed space. The first medium or the second medium is received in the storage space S of the reaction tank 10 via a pipe L from the reaction medium storage tank 30 . The reaction medium may be accommodated in the accommodation space S after reducing the pressure in the accommodation space S using a vacuum pump (not shown), or may be accommodated under atmospheric pressure (1.01325 × 10 Pa (0.101325 MPa)) without reducing the pressure in the accommodation space S. The reaction medium is preferably accommodated in the accommodation space S under atmospheric pressure conditions without reducing the pressure inside the accommodation space S. In the reaction tank 10, after the reaction medium is accommodated in the accommodation space S, the inert gas stored in the inert gas storage tank is sealed into the accommodation space S, and at least a portion of the air contained in the gas phase of the accommodation space S may be replaced with the inert gas, or all of the air contained in the gas phase of the accommodation space S may be replaced with the inert gas. Replacement with an inert gas is the best mode but is not essential. In this state, carbon dioxide and hydrogen are sealed into the storage space S. A predetermined amount of each gas is sealed at a pressure higher than the pressure inside the storage space S. The specific configuration of the device and the sealing method are the same as for inert gas. The reaction vessel 10 may be provided with a stirring device (not shown) for stirring the reaction medium contained in the containing space S. By providing the reaction tank 10 with the stirring device, the reaction can be carried out while stirring the reaction medium contained in the storage space S with the stirring device. This allows the synthesis reaction of formic acid contained in the reaction medium to be carried out more efficiently.

[0024] As described above, the apparatus 1 for carrying out the method for promoting a formic acid synthesis reaction according to this embodiment is a batch reaction apparatus, and therefore the reaction tank 10 is a batch vessel. The batch vessel means one that can contain the reaction medium and reaction substrates used in a single treatment in a sealable manner. The reaction vessel 10 has an inner wall surface that comes into contact with the reaction medium and reaction substrate contained therein, and therefore, the inner wall surface is preferably made of a non-metallic material. Preferred materials for forming the inner wall surface include resin, glass, ceramic, diamond-like carbon, and the like.

[0025] The resin may be, for example, a plastic such as polyimide (PI), polyamide (PA), polyamideimide (PAI), polyethersulfone (PES), polyetherimide (PEI), polyetheretherketone (PEEK), aromatic polyester (PET, PEN, etc.), or polyarylene sulfide (PAS), or may be a general rubber. In this embodiment, from the viewpoint of stability against hydrothermal reactions, the resin is preferably a silicone resin, a silicone rubber, a fluororesin, a fluororubber, an epoxy resin, or the like. Among these resins, the resin is preferably a fluororesin. Examples of the fluororesin include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polychlorotrifluoroethylene (PCTFE), tetrafluoroethylene-ethylene copolymer (ETFE), and polyvinylidene fluoride (PVdF). The resin may be used as a constituent material for the inner wall surface either singly or as a mixture of two or more kinds.

[0026] Examples of the glass include soda glass, borosilicate glass, quartz glass, and crystal glass.

[0027] Examples of the ceramic include alumina (Al2O3), zirconia (ZrO2), titania (TiO2), silica (SiO2), silicon carbide (SiC), silicon nitride (Si3N4), zircon (ZrO2·SiO2), aluminosilicate (Al2O3·SiO2), barium titanate (BaTiO3), aluminum nitride (AlN), steatite (MgO·SiO2), forsterite (2MgO·SiO2), mullite (3Al2O3·2SiO2), and cordierite (2MgO·2Al2O3·5SiO2). The ceramic may be used as a constituent material for the inner wall surface either singly or as a mixture of two or more kinds.

[0028] The entire wall defining the storage space S of the reaction vessel 10 may be made of the above material, or only the surface layer (the surface layer forming the inner wall surface of the reaction vessel 10) may be made of the above material. The material may be formed into multiple layers. The reaction vessel 10 may have, for example, a metal main body, and a glass layer and a resin layer laminated on the inner wall surface of the main body.

[0029] The material forming the inner wall surface preferably has a metal ion elution amount of 1000 ppm or less when induced by formic acid at room temperature (23±2° C.). The amount of eluted metal ions can be measured by an ICP method or the like.

[0030] As described above, by forming the inner wall surface of the reaction vessel 10 from a non-metallic material, the inner wall surface of the reaction vessel 10 becomes acid-resistant. Furthermore, when the inner wall surface is made of a metal such as stainless steel, there is a concern that the carboxyl group (COOH) contained in formic acid (HCOOH) may form an ionic bond with the metal. However, when the inner wall surface is made of a non-metal, the formation of an ionic bond as described above can be suppressed. That is, the formic acid forms an ionic bond with the inner wall surface via the carboxyl group, thereby preventing the formic acid from being adsorbed onto the inner wall surface. This allows formic acid to be produced from carbon dioxide and hydrogen more efficiently.

[0031] The jacket 20 is equipped with a heating device (not shown) such as a heater. The jacket 20 heats the reaction vessel 10 using a heating device such as a heater.

[0032] The reaction medium storage tank 30 may be any tank having an internal storage space S for storing the reaction medium. On the other hand, since the reaction medium storage tank 30 has an inner wall surface that comes into contact with the stored reaction medium, it is preferable that the inner wall surface be made of a non-metallic material. Preferred materials for forming the inner wall surface include resin, glass, ceramic, diamond-like carbon, and the like. The resin, glass, and ceramic may be the same as those described above. [Example]

[0033] The present invention will be described in more detail below with reference to examples. The following examples are intended to further explain the present invention and are not intended to limit the scope of the present invention. The examples disclose some of the promotion methods using the findings described above in [Summary of Findings on Formic Acid Synthesis Reaction] and [Conceptual Diagram of a Method for Promoting the Formic Acid Synthesis Reaction]. Although the promotion depends on the position and number of substituents, if the requirements for realizing the findings described above in [Summary of Findings on Formic Acid Synthesis Reaction] and [Conceptual Diagram of a Method for Promoting the Formic Acid Synthesis Reaction] are met, the promotion will occur. Therefore, in the following examples, the positions and numbers will not be mentioned in detail because there are countless combinations, but the positions and numbers are the arrangement of substituents or the number of substituents that meet the above requirements.

[0034] [Ionic liquid] In the present invention, an ionic liquid is defined as an organic compound salt having a melting point of 100°C or less, and examples thereof include imidazolium salt-based ionic liquids represented by the general formula (Chemical Formula 1) in Figure 3 and phosphonium salt-based ionic liquids represented by the following general formula (Chemical Formula 2), as well as pyridinium salt-based ionic liquids, pyrrolidinium salt-based ionic liquids, and tetraalkylammonium salt-based ionic liquids.

[0035] [In Chemical Formula 1, R1 and R2 are the same or different and represent either an alkyl group in which at least some of the hydrogen atoms may be substituted with fluorine atoms, or an aryl group in which at least some of the hydrogen atoms may be substituted with fluorine atoms. X1, X2, and X3 are the same or different and represent either an alkyl group in which at least some of the hydrogen atoms may be substituted with fluorine atoms, an aryl group in which at least some of the hydrogen atoms may be substituted with fluorine atoms, a hydrogen atom, or a fluorine atom. Y- represents a counter anion to the imidazolium cation.]

[0036] [In Chemical Formula 2, R1, R2, R3, and R4 are the same or different and represent either an alkyl group in which at least a portion of the hydrogen atoms may be substituted with fluorine atoms, or an aryl group in which at least a portion of the hydrogen atoms may be substituted with fluorine atoms. Z- represents a counter anion to the phosphonium cation.]

[0037] In the imidazolium salt-based ionic liquid represented by the above general formula (Chemical Formula 1) and the phosphonium salt-based ionic liquid represented by the above general formula (Chemical Formula 2), examples of the alkyl group in which at least a portion of the hydrogen atoms in R1, R2, R3, R4, X1, X2, and X3 may be substituted with fluorine atoms include linear or branched alkyl groups and perfluoroalkyl groups having 1 to 18 carbon atoms, and specific examples include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, an n-hexyl group, an n-octyl group, an n-decyl group, an n-dodecyl group, an n-tetradecyl group, an n-octadecyl group, a trifluoromethyl group, a pentafluoroethyl group, a heptafluoro-n-propyl group, a heptafluoroiso-propyl group, and a nonafluoro-n-butyl group. Examples of the aryl group in which at least some of the hydrogen atoms may be substituted with fluorine atoms include a phenyl group and a pentafluorophenyl group.

[0038] In R1, R2, R3, R4, X1, X2, and X3, some or all of the hydrogen atoms in these structures are substituted with amino or carboxyl groups. Examples of counter anions Y- for the imidazolium cation and Z- for the phosphonium cation include halide ions such as chloride ion (Cl-), bromide ion (Br-), and iodide ion (I-), as well as methanesulfonate anion (CH3SO3-), trifluoromethanesulfonate anion (CF3SO3-), bis(trifluoromethanesulfonyl)imide anion ((CF3SO2)2N-), and formate anion (HCO2-).

[0039] Suitable ionic liquids include ionic liquids whose counter anion is formate (i.e., formate salts), which are excellent in reaction selectivity (high purity of the formic acid produced) and reaction rate as a medium for producing formic acid from carbon dioxide and hydrogen as raw materials. Ionic liquids whose counter anion is formate can be synthesized, for example, by anion exchange using a strong base ion exchange resin from an ionic liquid whose counter anion is an anion other than formate, such as bromide (Biomacromolecules, Vol. 7, pp. 3295-3297, 2006). While various ionic liquids whose counter anion is other than formate are commercially available, non-commercially available ionic liquids can be synthesized, for example, according to the method described in Ionic Liquids in Synthesis I, Wiley-VCH, 2007.

[0040] Example 1: The reaction vessel 10 shown in Figure 2 was filled with 1,3-dini-n-propyl-2-methylimidazolium chloride (an ionic liquid with a water content of 1.5 wt% and in which some of the hydrogen atoms in the methyl groups were replaced with carboxyl groups) to a 50% filling rate. After degassing using a vacuum pump, carbon dioxide was injected to bring the pressure inside the reaction vessel to 15.0 bar, and hydrogen was then introduced to bring the pressure inside the reaction vessel to 30.0 bar. The connection valve was closed to seal the reaction vessel, and the reaction vessel 10 was then heated to 60 °C. Optionally, 0.2 wt% of dichlorotetrakis(triphenylphosphine)ruthenium may be added as a metal catalyst. After 50 hours, the amount of formic acid produced was quantified using 1H-NMR or other methods. For example, in 1H-NMR measurements, the amount of formic acid produced was confirmed by integrating the formic acid peak around 8 ppm.

[0041] Example 2: Carbon dioxide and hydrogen were reacted under the same conditions as in Example 1, except that an ionic liquid, 1,3-dini-n-propyl-2-methylimidazolium chloride salt, in which one hydrogen atom in one methyl group was substituted with an amino group, was used.

[0042] The results of Example 1 and Example 2 are compared. The results of Example 2 were compared based on the results of Example 1. The amount of carbon dioxide produced in Example 2 was about twice as much as that in Example 1. This is because the amount of carbon dioxide absorbed into the ionic liquid in Example 2 was greater than that in Example 1.

[0043] Example 3: Carbon dioxide and hydrogen were reacted under the same conditions as in Example 1, except that an ionic liquid in which one hydrogen atom in one methyl group of 1,3-di-n-propyl-2-methylimidazolium formate was substituted with an amino group was used. The amount of product in Example 2 was about 2.3 times greater than that in Example 1.

[0044] Example 4: Carbon dioxide and hydrogen were reacted under the same conditions as in Example 1, except that an ionic liquid in which one hydrogen atom in one methyl group of 1,3-di-n-propyl-2-methylimidazolium chloride salt was substituted with an amino group was used. The amount of product in Example 2 was about 2.3 times greater than that in Example 1.

[0045] Example 5: Carbon dioxide and hydrogen were reacted under the same conditions as in Example 1, except that an ionic liquid in which one hydrogen atom in one methyl group of 1,3-dini-n-propyl-2-methylimidazolium chloride salt was substituted with a carboxyl group was used. The amount of carbon dioxide produced in Example 2 was about 2.1 times greater than that in Example 1.

[0046] Example 6: Carbon dioxide and hydrogen were reacted under the same conditions as in Example 1, except that a tetraalkylphosphonium cation with a methyl group as the alkyl group was used, and an ionic liquid in which one hydrogen atom in one methyl group was substituted with an amino group was used for a salt with a chloride anion. The amount produced in Example 2 was about 1.5 times greater than that in Example 1.

[0047] Example 7: Carbon dioxide and hydrogen were reacted under the same conditions as in Example 1, except that the reaction was carried out in a solution prepared by dissolving in water an ionic liquid, 1,3-dini-n-propyl-2-methylimidazolium chloride salt, in which one hydrogen atom in one methyl group was substituted with an amino group. The amount produced in Example 2 was approximately 1.3 times greater than that in Example 1.

[0048] Example 8: Carbon dioxide and hydrogen were reacted under the same conditions as in Example 1, except that a solution of 1,3-di-n-propyl-2-methylimidazolium chloride, an ionic liquid in which one hydrogen atom in one methyl group was substituted with an amino group, was dissolved in water, and a medium was prepared in which the solution was supersaturated. The amount of carbon dioxide produced in Example 2 was approximately 1.7 times greater than that in Example 1.

[0049] Comparative Example: Carbon dioxide and hydrogen were reacted under the same conditions as in Example 1, except that an ionic liquid (unsubstituted) 1,3-dini-n-propyl-2-methylimidazolium chloride salt was used, in which one hydrogen atom in one methyl group was not substituted with a carboxyl group or an amino group but remained as hydrogen. The amount produced in the comparative example was about 0.5 times less than that in Example 1.

[0050] The above-described embodiment is merely an example, and the present invention is not limited thereto. Furthermore, in the above-described mode and embodiment for carrying out the invention, a method in which electrodes are inserted into the reaction vessel 10 and no current is applied has been described, but this is not limiting, and the reaction may be promoted by applying current. Furthermore, the reaction may be promoted by adding a substance that exhibits catalytic action, such as a metal catalyst. Furthermore, the reaction may be promoted by increasing the reaction temperature to increase the energy state of the reaction substrates due to the Arrhenius effect. [Explanation of symbols]

[0051] 1: carbon monoxide production apparatus, 10: reaction tank, 20: jacket, 30: formic acid aqueous solution storage tank, 10a: Side wall part, 10b: Bottom wall part, 10c: Top wall part, L: Piping, S: Storage space, V: Valve.

Claims

1. An ionic liquid that is used as a site for synthesizing formic acid by reacting carbon dioxide with hydrogen, promotes the occurrence of a transition state, and promotes the reaction, The ionic liquid is characterized in that the substituents in some or all of the cations of the ionic liquid are at least amino groups or carboxyl groups, and are arranged so as to be capable of adsorbing or bonding to oxygen atoms in carbon dioxide molecules and to form a transition state structure.

2. A method for promoting a formic acid synthesis reaction, comprising reacting carbon dioxide and hydrogen in an ionic liquid to promote the development of a transition state and promoting the reaction to synthesize formic acid, A method for promoting a formic acid synthesis reaction, characterized in that the substituent in the cation of the ionic liquid is at least an amino group or a carboxyl group, and the substituent is arranged so as to be able to adsorb or bond to an oxygen atom in a carbon dioxide molecule and to form a transition state structure.

3. The anion of the ionic liquid is a formate anion. The method for promoting a formic acid synthesis reaction according to claim 2.

4. The cation of the ionic liquid is an imidazolium salt. The method for promoting a formic acid synthesis reaction according to claim 2 or 3.

5. A method for promoting a formic acid synthesis reaction, comprising reacting carbon dioxide and hydrogen in water in the presence of an organic salt containing an ionic liquid to promote the development of a transition state and synthesizing formic acid by promoting the reaction, comprising: A method for promoting a formic acid synthesis reaction, characterized in that the substituent in the cation of the organic salt is at least an amino group or a carboxyl group, and the substituent is arranged so as to be able to adsorb or bond to an oxygen atom in a carbon dioxide molecule and to form a transition state structure.

6. 6. The method for promoting a formic acid synthesis reaction according to claim 5, wherein the first medium is water containing a salt at a concentration equal to or greater than the saturation concentration.

7. The method for promoting a formic acid synthesis reaction according to any one of claims 2 to 6, further comprising a hydrogen production step of electrolyzing water using renewable energy to produce the hydrogen.

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