Reducing gas detection material, method for detecting presence of reducing gas using the same, and method for producing reducing gas detection material

Abstract

To provide a detection material of a reducing gas capable of detecting the reducing gas with good sensitivity and capable of intuitively perceiving the detection data even by visual observation in the form of a change in hue.SOLUTION: Solution to Problem The present invention is a reducing gas detection material comprising a compound that is a salt of a dye compound that forms a salt with a basic compound and exhibits redox activity, and a hydroxide of a cation having one or more substituents in which the total number of carbon atoms contained in each substituent is 10 to 40. Since the detection material sharply generates a color tone change in the presence of the reducing gas, the presence of the reducing gas can be notified in a visually discriminable manner.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a reducing gas sensing material, a method for detecting the presence of a reducing gas using the same, and a method for producing the reducing gas sensing material. [Background technology]

[0002] To prevent accidents, means of detecting hydrogen gas leaks are needed in the chemical industry, including oil refineries, and in hydrogen gas stations, the number of which has been increasing in recent years. Catalytic combustion hydrogen gas sensors (see, for example, Patent Documents 1 to 3) are known as hydrogen gas sensors used for such purposes. However, while this type of gas sensor is highly sensitive, it has a high operating temperature and is a combustion-type sensor, which means that it poses a risk of explosion at high concentrations.

[0003] Furthermore, hydrocarbon gases such as hydrogen sulfide gas, formic acid gas, and hexane gas, and carbon monoxide gas may cause health problems and environmental pollution, and their emissions are regulated by laws and regulations such as the Offensive Odor Control Act and the Air Pollution Control Act. All of the gases exemplified here have reducing properties, and hydrogen gas is one of these reducing gases. Various gas sensors for detecting these reducing gases are also known. Examples of such sensors include those relating to hydrogen sulfide gas and those relating to hydrocarbon gases, which are described in Patent Document 4 and Patent Document 5, respectively.

[0004] These gas sensors are based on electrochemical methods using metal oxides or semiconductors, and although highly sensitive, they tend to have problems with power sources and installation locations.Moreover, many of these gas sensors indicate the presence of a target substance in the form of numerical data, making it difficult to intuitively determine its presence.

[0005] On the other hand, the present inventors have proposed the use of conjugated compounds that are liquid at room temperature as sensors for acidic and basic gases, although they do not detect reducing gases (see Patent Documents 6 to 8). These conjugated compounds have fluorescence and change the color and intensity of their fluorescence when exposed to acidic gases or ammonia. Therefore, unlike semiconductor sensors that detect gases using numerical data, they are useful for intuitively determining the presence of a target gas by visual inspection. The conjugated compound described in Patent Document 6 is capable of detecting both acidic and basic substances, while the conjugated compound described in Patent Document 7 detects ammonia, a basic gas, and the conjugated compound having a pyridine ring described in Patent Document 8 changes its color and fluorescence emission in the presence of an acid or base. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-114434 [Patent Document 2] Japanese Patent Application Publication No. 2019-113423 [Patent Document 3] Japanese Patent Application Publication No. 2020-176991 [Patent Document 4] Patent No. 6364356 [Patent Document 5] Special Publication No. 2015-525361 [Patent Document 6] Japanese Patent Application Publication No. 2018-076251 [Patent Document 7] Japanese Patent Publication No. 2021-143140 [Patent Document 8] Japanese Patent Application Publication No. 2023-110935 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made in consideration of the above circumstances, and aims to provide a reducing gas detection material that can detect reducing gases with high sensitivity and whose detection information can be intuitively perceived visually in the form of a change in color tone. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above problems and have found that an ionic liquid of a dye, which is a salt of a dye compound that forms a salt with a basic compound and exhibits redox activity, and a hydroxide of a cation having one or more substituents, the total number of carbon atoms in each substituent being 10 to 40, undergoes a sharp change in color tone in the presence of a reducing gas, thereby completing the present invention. Specifically, the present invention provides the following.

[0009] (1) The present invention provides a reducing gas sensing material comprising an ionic liquid that is a salt of a dye compound that forms a salt with a basic compound and exhibits redox activity, and a hydroxide of a cation having one or more substituents, the total number of carbon atoms contained in each substituent being 10 to 40.

[0010] (2) The present invention also provides the reducing gas sensing material according to (1), wherein the dye compound is indigo carmine.

[0011] (3) The present invention also provides the reducing gas sensing material according to item (1) or (2), wherein the cation is a tetraalkylphosphonium having four alkyl groups each having 10 to 40 carbon atoms in total.

[0012] (4) The present invention also provides a reducing gas sensing material according to (3), wherein the tetraalkylphosphonium is trihexyltetradecylphosphonium.

[0013] (5) The present invention also provides a method for detecting the presence of a reducing gas in a gas to be measured by contacting the reducing gas detection material described in any one of (1) to (4) itself or a substrate impregnated with the reducing gas detection material with the gas to be measured, and observing the change in color tone or absorption wavelength.

[0014] (6) The present invention also provides the method according to (5), wherein the reducing gas is hexane, formic acid, sulfurous acid, hydrogen, formaldehyde, hydrazine, carbon monoxide or hydrogen sulfide.

[0015] (7) The present invention also provides a method for producing a reducing gas detection material, comprising: an ion exchange step of converting a salt of a cation having one or more substituents, the total number of carbon atoms of which is 10 to 40, into a hydroxide of the cation by an ion exchange method; and a reaction step of preparing an ionic liquid dye by reacting the hydroxide of the cation with a dye compound that forms a salt with a basic compound and exhibits redox activity.

[0016] (8) The present invention also provides a method for producing a reducing gas sensing material according to claim 7, wherein the dye compound is indigo carmine.

[0017] (9) The present invention also provides a method for producing a reducing gas sensing material according to item (7) or (8), wherein the cation is a tetraalkylphosphonium having four alkyl groups with a total of 10 to 40 carbon atoms.

[0018] (10) The present invention also provides a method for producing a reducing gas sensing material according to (9), wherein the tetraalkylphosphonium is trihexyltetradecylphosphonium.

[0019] (11) The present invention also provides a method for producing a reducing gas sensing material according to any one of (7) to (10), wherein the ion exchange step is carried out using an anion exchange resin. [Effects of the Invention]

[0020] According to the present invention, there is provided a reducing gas detection material that can detect reducing gases with high sensitivity and allows the detection information to be intuitively perceived visually in the form of a change in color tone. [Brief explanation of the drawings]

[0021] [Figure 1] Figure 1 is a chart showing the change in absorption spectrum when a thin film of IC (indigo carmine) ionic liquid is exposed to hexane gas (Hex). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure to hexane gas. [Figure 2] Figure 2 is a chart showing the change in absorption spectrum when a thin film of IC ionic liquid is exposed to formic acid gas (FA). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure to formic acid gas. [Figure 3] Figure 3 is a chart showing the change in absorption spectrum when a thin film of IC ionic liquid is exposed to sulfur dioxide gas (SO2). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure to sulfur dioxide gas. [Figure 4] Figure 4 is a chart showing the change in absorption spectrum when a thin film of IC ionic liquid is exposed to hydrogen gas (H2). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure to hydrogen gas. [Figure 5] Figure 5 is a chart showing the change in absorption spectrum when a thin film of IC ionic liquid is exposed to carbon monoxide gas (CO). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure to carbon monoxide gas. [Figure 6] Figure 6 is a chart showing the change in absorption spectrum when a thin film of IC ionic liquid is exposed to hydrogen sulfide gas (HS). The lowest solid line represents the absorption spectrum before exposure, the dotted line represents the absorption spectrum after exposure at a concentration of 100 ppm, and the highest solid line represents the absorption spectrum after exposure at a concentration of 300 ppm. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, one embodiment of the reducing gas sensing material of the present invention, one embodiment of the method for detecting the presence of a reducing gas of the present invention, and one embodiment of the method for manufacturing a reducing gas sensing material of the present invention will be described. Note that the present invention is not limited to the following embodiment and can be practiced with appropriate modifications within the scope of the present invention.

[0023] <Reducing gas detection material> First, one embodiment of the reducing gas detection material of the present invention will be described. The reducing gas detection material of the present invention is composed of a dye ionic liquid, and changes color when exposed to a reducing gas. This allows the user to perceive the presence of a reducing gas through visual information, i.e., a change in color. In addition to such visual information, the reducing gas detection material of the present invention may be combined with an instrument that detects the change in color as a change in absorbance, thereby notifying the user of the presence of a reducing gas by emitting an alarm or displaying some kind of numerical change.

[0024] The reducing gas to be detected by the reducing gas sensing material of the present invention is not particularly limited as long as it has some reducing properties. Examples include hydrocarbons such as hexane, carbon monoxide, hydrogen sulfide, formic acid, sulfurous acid, hydrogen, formaldehyde, and hydrazine. The reducing "gas" referred to in the present invention does not necessarily have to have a boiling point lower than room temperature; it may be any gas that has a certain vapor pressure at room temperature and exists as a gas. The term "room temperature" used in the present invention refers to the temperature range in which the reducing gas sensing material of the present invention is used. For example, if the reducing gas sensing material of the present invention is used in an animal's living environment, the "room temperature" range would be approximately -30°C to 50°C. However, if the reducing gas sensing material of the present invention is used in a high-temperature environment such as a chemical plant, the "room temperature" range may be 100°C or higher.

[0025] The reducing gas sensing material of the present invention is characterized by comprising an ionic liquid that is a salt of a dye compound that forms a salt with a basic compound and exhibits redox activity, and a hydroxide of a cation having one or more substituents, the total number of carbon atoms contained in each substituent being 10 to 40. These will be explained below.

[0026] [Pigment compounds] The dye compounds used are those that form salts with basic compounds and exhibit redox activity. A "redox-active" dye compound is one that is reduced by the action of a reducing compound (i.e., a reducing agent) and changes color tone. Dye compounds that "form salts with basic compounds" include compounds that have an acidic group, such as a sulfo group, a phenolic hydroxyl group, or a carboxyl group, and that can undergo an acid-base reaction with a basic compound to form a salt. They also include compounds that are used as dye compounds in the form of a salt with a basic compound, such as indigo carmine, represented by the chemical formula below. Furthermore, dye compounds that are already salts can also undergo a cation-exchange reaction with the hydroxide of a cation, a basic compound, to produce a cation-exchanged salt, as described below. In this respect, they can also be said to "form salts with basic compounds."

[0027] [ka]

[0028] Examples of such dye compounds include indigo carmine, methyl viologen, tetrathiafulvalene, luciferin, and methylene blue, and among these, indigo carmine is preferred.

[0029] [Cation hydroxide] The hydroxide of the cation is A n+ OH n- A is a compound represented by the formula: and is used to form a salt with the dye compound to form an ionic liquid. n+The cation represented by the formula (I) is one having a bulky substituent, and forms a salt with the dye compound to give an ionic liquid. That is, if a compound having an acidic substituent that does not form a salt is used as the dye compound, this dye compound will form a salt with the hydroxide of the cation through an acid-base reaction to become an ionic liquid. Also, if a dye compound that is already in the form of a salt is used as the dye compound, the cation of this dye compound (sodium ion in the case of indigo carmine) will be A n+ The cation is exchanged with the cation represented by

[0030] A n+ The cation represented by the formula (I) has one or more substituents with a total carbon number of 10 to 40 in each substituent. These substituents make the cation bulky and contribute to converting the dye compound into an ionic liquid. Examples of the substituent include an alkyl group having about 6 to 20 carbon atoms and which may have a branch, and an alkylaryl group or aralkyl group having such an alkyl group. One or more of these substituents are contained in the cation. For example, when the cation is a quaternary ammonium or quaternary phosphonium, preferred embodiments include an embodiment in which four of these substituents are bonded to the nitrogen atom or phosphorus atom, which is the central element of the cation.

[0031] Examples of the cation include phosphonium cations, ammonium cations, pyrazolium cations, pyrrolidinium cations, imidazolium cations, pyridinium cations, sulfonium cations, and piperidinium cations. Among these, phosphonium cations or ammonium cations are preferred, and phosphonium cations are more preferred. For example, the phosphonium cation includes tetraalkylphosphonium cations, and although not particularly limited, a preferred example of such a tetraalkylphosphonium cation is trihexyltetradecylphosphonium cation represented by the following chemical formula:

[0032] [ka]

[0033] The anion of the above dye compound and A n+ The reducing gas detection material of the present invention is an ionic liquid composed of a cation represented by the formula (I) and a cation represented by the formula (II). This ionic liquid can also be called an ionic liquid dye compound. The dye compound contained therein changes color when reduced as described above, and therefore changes color when exposed to a reducing gas. Therefore, this ionic liquid can notify an observer of the presence of a reducing gas in the gas to be detected in the form of a visible color change.

[0034] The pH of the ionic liquid is desirably set within a range that produces an appropriate color change upon reduction of the dye compound. Specifically, the acidic substituents contained in the dye compound, which is an ionic liquid, form salts, and the color changes when this salt is dissolved. Therefore, it is necessary to adjust the pH of the ionic liquid within an appropriate range. For example, when using an ionic liquid that is a salt of an indigo carmine anion and a trihexyltetradecylphosphonium cation, a preferred pH range is 11.4 to 13.0. The pH of the ionic liquid can be adjusted appropriately using an acid or a base.

[0035] The reducing gas sensing material of the present invention, which is an ionic liquid, can be used in a state where it is applied to a non-absorbent material such as glass or plastic, or in a state where it is absorbed into a liquid-absorbent substrate. Examples of liquid-absorbent substrates include paper, wood, nonwoven fabric, etc. Among these, paper is preferred as the substrate, and among papers, filter paper is particularly preferred.

[0036] <Method for manufacturing reducing gas sensing material> The present invention also includes a method for producing the reducing gas sensing material of the present invention. The method for producing the reducing gas sensing material of the present invention is characterized by comprising an ion exchange step of converting a salt of a cation having one or more substituents, each of which has a total carbon number of 10 to 40, into a hydroxide of the cation by ion exchange, and a reaction step of preparing an ionic liquid dye by reacting the hydroxide of the cation with a dye compound that forms a salt with a basic compound and exhibits redox activity. These points will be explained below, but explanations that overlap with the explanation of the reducing gas sensing material of the present invention will be omitted as appropriate.

[0037] [Ion exchange process] The ion exchange step is a step of converting a salt of a cation having one or more substituents, the total number of carbon atoms of which is 10 to 40, into a hydroxide of the cation by an ion exchange method. As described above in the description of the reducing gas sensing material of the present invention, the ionic liquid constituting this sensing material is A n+ OH n- The ion exchange step is carried out to prepare the hydroxide of the cation.

[0038] The cation salt used in this step has one or more substituents, each of which has a total of 10 to 40 carbon atoms. As already mentioned, these substituents make the cation bulky, contributing to converting the dye compound into an ionic liquid. Examples of the substituent include an alkyl group having about 6 to 20 carbon atoms, which may have a branch, and an alkylaryl group or aralkyl group having such an alkyl group. One or more of these substituents are contained in the cation. For example, when the cation is a quaternary ammonium or quaternary phosphonium, preferred embodiments include an embodiment in which four of these substituents are bonded to the nitrogen atom or phosphorus atom, which is the central element of the cation.

[0039] Examples of the cation include phosphonium cations, ammonium cations, pyrazolium cations, pyrrolidinium cations, imidazolium cations, pyridinium cations, sulfonium cations, and piperidinium cations. Among these, phosphonium cations or ammonium cations are preferred, and phosphonium cations are more preferred. For example, examples of the phosphonium cation include tetraalkylphosphonium cations, and examples of such tetraalkylphosphonium cations include, but are not limited to, trihexyltetradecylphosphonium cations represented by the following chemical formula:

[0040] [ka]

[0041] The anions that make up the salts of cations include Cl - , Br - , I - , NCO3 - , CH3COO - , BF4 - , PF6 - , AlCl4 - , Al2Cl7 - , ClO4 - , NO3 - , CF3COO - , CH3SO3 - , CF3SO3 - , (CF3SO2)2N - , (CF3SO2)3C - , AsF6 - , SbF6 - , NbF6 - , C4F9SO3 - , (C2F5SO2)2N - , (CF3SO2)(CF3CO)N - Among these, Cl - or Br - For example, if the cation is the trihexyltetradecylphosphonium cation, its chloride (i.e., Cl- salts of Br - and salts) are commercially available.

[0042] The salt of the cation is anion-exchanged by an ion exchange method and converted into the hydroxide of the cation. The ion exchange method can be any known method without any particular limitation, but among these, an ion exchange method using an anion exchange resin is preferred.

[0043] Anion exchange resins are resins made of polymers that have positively charged fixed ions in their molecules and negatively charged ions that electrically neutralize them. In the present invention, the negative charges are OH - An anion exchange resin with the following structure is used. Such anion exchange resin can absorb Cl contained in water. - and Br - and other anions, and instead - The anion exchange resin is a styrene or acrylic base material containing quaternary ammonium (-N(CH3)3 + Examples of such anion exchange resins include those into which a hydroxy group such as hydroxyl group or hydroxyl group has been introduced. Various types of such anion exchange resins are commercially available, and such commercially available products may be obtained and used.

[0044] When ion exchange is performed using an anion exchange resin, a sufficient amount of anion exchange resin is added to an aqueous solution of a cation salt and stirred under heating. A preferred temperature is about 50°C, and a preferred stirring time is about 2 to 3 hours. After stirring is completed, the anion exchange resin is removed by filtration, and the water solvent is distilled off from the resulting filtrate. This yields a hydroxide of the cation, in which the cation salt has been exchanged for hydroxide ions.

[0045] The hydroxide of the cation obtained in this step is subjected to a reaction step.

[0046] [Reaction process] The reaction step is a step of preparing an ionic liquid dye by reacting a dye compound that forms a salt with a basic compound and exhibits redox activity with the hydroxide of the cation.

[0047] As already explained, the dye compound used is one that forms a salt with a basic compound and exhibits redox activity. This dye compound is a compound that is reduced by the action of a compound that exhibits reducing properties (i.e., a reducing agent) and changes its color tone. Furthermore, this dye compound includes not only compounds that can form salts through an acid-base reaction with a basic compound, but also compounds that are used as dye compounds in the form of salts, such as the above-mentioned indigo carmine, i.e., dye compounds that are already salts.

[0048] Examples of such dye compounds include indigo carmine, methyl viologen, tetrathiafulvalene, luciferin, and methylene blue, and among these, indigo carmine is preferred.

[0049] When a dye compound that forms a salt by acid-base reaction with a basic compound is reacted with a cation hydroxide, the reaction is an acid-base reaction. To carry out this reaction, the dye compound and the cation hydroxide are dissolved in an organic solvent and stirred. An amount of cation hydroxide corresponding to the number of equivalents of acidic groups contained in the dye compound is used. For example, if a dye compound has two acidic groups per molecule, two moles of cation hydroxide are used per mole of the dye compound. A preferred organic solvent used in the reaction is a mixed solvent of dichloromethane and ethanol. The stirring time can be approximately 12 to 16 hours, and the stirring temperature can be approximately 50°C.

[0050] When a dye compound that is already a salt is reacted with a cation hydroxide, the reaction is a cation exchange reaction. That is, the countercation of the anionic dye compound is exchanged for the cation bearing a bulky substituent. To carry out this reaction, the dye compound and the cation hydroxide are dissolved in deionized or distilled water and stirred. An amount of cation hydroxide equivalent to the number of equivalents of the anions contained in the dye compound is used. For example, if one molecule of the dye compound contains two anionic groups, two moles of cation hydroxide are used per mole of the dye compound. The stirring time can be approximately 12 to 16 hours, and the stirring temperature can be approximately 50°C.

[0051] By stirring the dye compound and the hydroxide of the cation in the solvent as described above, a neutralization reaction or a cation exchange reaction occurs between them, resulting in an ionic liquid dye compound, i.e., the reducing gas sensing material of the present invention. As already mentioned, since there is an appropriate pH range depending on the type of dye compound used, it is preferable to adjust the pH of the solvent during stirring as needed so that it falls within an appropriate range. After the reaction is complete, the solvent is removed, for example, by distillation under reduced pressure, to obtain the reducing gas sensing material of the present invention. [Example]

[0052] The reducing gas sensing material of the present invention will be explained in more detail below by showing examples, but the present invention is not limited to the following examples in any way.

[0053] Preparation of cation hydroxides [ka]

[0054] Commercially available trihexyltetradecylphosphonium bromide (2.50 g, 4.43 mmol) was added to deionized water (30 mL) and stirred. 10 g of a strongly basic anion exchange resin (Fujifilm Wako Pure Chemical Industries, Ltd., No. 8) was added and stirred at 50°C for 3 hours. The mixture was then cooled to room temperature, and the anion exchange resin was removed by suction filtration. The filtrate was concentrated under reduced pressure and washed twice with ethanol to obtain the hydroxide of the cation as a pale yellow liquid (yield: 2.15 g, 94%). Hereinafter, trihexyltetradecylphosphonium cation will be referred to as P 66614 + The hydroxide of the cation obtained above is called P 66614 + OH - Also called.

[0055] Indigo carmine ion exchange [ka]

[0056] Indigo carmine (1.00 g, 2.16 mmol) was dissolved in deionized water (10 mL). 66614 + OH - (2.15 g, 4.29 mmol), chloroform (10 mL), and ethanol (10 mL) were added and stirred at 50° C. for 14 hours. The reaction mixture was then extracted with chloroform, washed three times with saturated brine (100 mL), and the solvent was evaporated under reduced pressure to obtain the target indigo carmine ionic liquid (IC ionic liquid) as a deep blue liquid (yield: 1.60 g, 57%). 1 H NMR(CDCl3):δ(ppm)=10.2(s,2H,-NH),7.80(m,4H,Ph),7.01(s,2H,Ph),2.03(m,4H ,-PCH2-),1.81(m,4H,-CH2-),1.25(m,40H,-CH2-),0.90(t,12H,J=6.00Hz,-CH3). FT-IR (KBr, cm -1 ):2935(-CH2-) and 1654(-CO-).

[0057] ·Reducing gas detection test of IC ionic liquid 10.0 mg of the IC ionic liquid obtained by the above procedure was weighed and dissolved in dichloromethane (2.0 mL). This was dropped onto a glass substrate and spin-coated at 3000 rpm for 30 seconds to form a thin film. The absorption spectra of this thin film were measured before and after exposure to various reducing gases to test its recognizing ability. The reducing gases used in this test were hexane gas (Hex), formic acid gas (FA), sulfur dioxide gas (SO2), hydrogen gas (H2), carbon monoxide gas (CO), and hydrogen sulfide gas (HS). Of these reducing gases, hydrogen sulfide gas was exposed at a concentration of 100 ppm or 300 ppm, while the other gases were exposed at a concentration of 100%. The changes in the absorption spectra before and after exposure to each reducing gas are shown in Figures 1 to 6, respectively. Figure 1 shows the change in absorption spectrum when a thin film of IC ionic liquid is exposed to hexane gas (Hex). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure. Figure 2 shows the change in absorption spectrum when a thin film of IC ionic liquid is exposed to formic acid gas (FA). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure. Figure 3 shows the change in absorption spectrum when a thin film of IC ionic liquid is exposed to sulfur dioxide gas (SO2). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure. Figure 4 shows the change in absorption spectrum when a thin film of IC ionic liquid is exposed to hydrogen gas (H2). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure to hydrogen gas. Figure 5 shows the change in absorption spectrum when a thin film of IC ionic liquid is exposed to carbon monoxide gas (CO). The dotted line represents the absorption spectrum before exposure, and the solid line represents the absorption spectrum after exposure. Figure 6 shows the change in absorption spectrum when a thin film of IC ionic liquid is exposed to hydrogen sulfide gas (HS). The lowest solid line represents the absorption spectrum before exposure, the dotted line represents the absorption spectrum after exposure at a concentration of 100 ppm, and the highest solid line represents the absorption spectrum after exposure at a concentration of 300 ppm.

[0058] When various reducing gases were exposed to the IC ionic liquid, the absorption spectrum changed as shown in Figures 1 to 6. Hydrogen sulfide gas changed from purple to pink, and other gases changed from purple to blue or indigo. From these results, it can be seen that the reducing gas sensing material of the present invention has the ability to distinguish reducing gases that can be distinguished visually.

Claims

1. A reducing gas detection material characterized by comprising an ionic liquid which is a salt of a dye compound which forms a salt with a basic compound and exhibits redox activity, and a hydroxide of a cation having one or more substituents, the total number of carbon atoms contained in each substituent being 10 to 40.

2. 2. The reducing gas sensing material according to claim 1, wherein the dye compound is indigo carmine.

3. 2. The reducing gas sensing material according to claim 1, wherein the cation is a tetraalkylphosphonium having four alkyl groups each having 10 to 40 carbon atoms in total.

4. 4. The reducing gas sensing material according to claim 3, wherein the tetraalkylphosphonium is trihexyltetradecylphosphonium.

5. A method for detecting the presence of a reducing gas in a gas to be measured by contacting the reducing gas detection material according to any one of claims 1 to 4 itself or a substrate impregnated with the reducing gas detection material with the gas to be measured, and observing a change in color tone or absorption wavelength.

6. 6. The method of claim 5, wherein the reducing gas is hexane, formic acid, sulfurous acid, hydrogen, formaldehyde, hydrazine, carbon monoxide, or hydrogen sulfide.

7. an ion exchange step of converting a salt of a cation having one or more substituents, the total number of carbon atoms contained in the substituents being 10 to 40, into a hydroxide of the cation by an ion exchange method; a reaction step of preparing an ionic liquid dye by reacting a dye compound that forms a salt with a basic compound and exhibits redox activity with a hydroxide of the cation.

8. 8. The method for producing a reducing gas sensing material according to claim 7, wherein the dye compound is indigo carmine.

9. 8. The method for producing a reducing gas sensing material according to claim 7, wherein the cation is a tetraalkylphosphonium having four alkyl groups each having 10 to 40 carbon atoms in total.

10. 10. The method for producing a reducing gas sensing material according to claim 9, wherein the tetraalkylphosphonium is trihexyltetradecylphosphonium.

11. 8. The method for producing a reducing gas sensing material according to claim 7, wherein the ion exchange step is carried out using an anion exchange resin.

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