New substance, method for producing hexanuclear copper complex, method for producing dinuclear copper complex, and method for oxidizing methane to methanol

Abstract

To provide an oxidation catalyst which is a novel substance that can directly oxidize gaseous alkane such as methane more efficiently and convert it to alcohol.SOLUTION: A novel substance hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpaC0)2](ClO4)6 is used as an oxidation catalyst. The hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpaC0)2](ClO4)6 is produced by performing nitrogen substitution reaction in which 3 equivalents of copper perchlorate is added to 6,6'-bis[[bis(2-pyridylmethyl)amino]methyl]-2,2'-bipyridine (6-hpaC0 ligand). The oxidation catalyst can also be used for cyclohexane oxidation or the like other than methane.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an oxidation catalyst that can carry out an oxidation reaction of alkanes directly and highly selectively to alcohols. [Background technology]

[0002] Methane hydrate is stable at high pressure and low temperature (over 2.6 MPa at 0°C), so in nature it exists in permafrost areas on land, such as northern Canada, where temperatures are low, and in high-pressure sedimentary layers hundreds of meters below the seafloor at the continental margins. The amount of methane hydrate worldwide is estimated to be 20 to 14 quadrillion m3 in methane equivalent. 3 , and the original resource amount is several hundred trillion m 3 It has been reported that there are large amounts of methane hydrate in the waters off the coast of Japan, and the existence of concentrated zones that could be a promising resource has also been confirmed.

[0003] The C—H bond dissociation energy of gaseous alkanes is very large, making them stable gaseous substrates. The oxidation products, alcohols and aldehydes, are easily oxidized, resulting in overoxidation. Therefore, the direct and highly selective production of alcohols and aldehydes from gaseous alkanes is a very difficult reaction process. Therefore, the development of a method for directly and highly selectively converting (oxidizing) gaseous alkanes to alcohols and aldehydes is highly desirable.

[0004] In nature, the oxidation of methane to methanol is carried out by methane monooxygenase contained in methanotrophs (Non-Patent Document 1). Meanwhile, currently known methods for artificially producing methanol from methane include oxidizing methane with hydrogen peroxide in an acetonitrile solvent in the presence of an iron catalyst (Non-Patent Document 2). However, the method in Document 2 uses acetonitrile as a solvent, and because acetonitrile is more easily oxidized than methane, it is highly likely that methanol is produced by the oxidation of acetonitrile. To confirm this, it would be necessary to use methane gas composed of the 13C isotope, but no such experiments have been conducted.

[0005] Therefore, up to now, there have been no reported examples of catalysts that can directly convert gaseous alkanes into alcohols and aldehydes with high efficiency and high selectivity.

[0006] The present inventors have reported a method for directly producing phenol from benzene with high efficiency and selectivity at a very fast reaction rate using a specific binuclear copper complex as a catalyst (Non-Patent Documents 3 to 8). However, from the perspective of practical application, it is desirable to extend this method to the production of more inert gaseous alkanes. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] SJ Lippard, JMBerg, "Principles of Bioinorganic Chemistry," University Science Books, California (1994). [Non-patent document 2] Georg Suss-Fink and co-workers Adv.Synth.Catal.2004,346,317 [Non-patent document 3] Angew. Chem. Int. Ed., 2017, 56, 7779-7782. [Non-patent document 4] Abstracts of the 65th Symposium of the Japan Society of Coordination Chemistry [Non-Patent Document 5] Abstracts of the 66th Symposium of the Japanese Society of Coordination Chemistry [Non-patent document 6] Abstracts of the 67th Symposium of the Japan Society of Coordination Chemistry [Non-Patent Document 7] Abstracts of the 49th Oxidation Reaction Symposium [Non-patent document 8] Abstracts of the 50th Oxidation Reaction Symposium Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above problems, and an object of the present invention is to provide an oxidation catalyst that can more efficiently directly oxidize alkanes such as methane to convert them into alcohols. [Means for solving the problem]

[0009] The novel oxidation catalyst of the present invention is a hexanuclear copper complex [Cu(μ-OH)(μ-OH)(6-hpa C0 )2](ClO4)6(1).

[0010] The novel oxidation catalyst of the present invention is a binuclear copper complex [Cu2(ClO4)2(MeCN)2(6-hpa C0 )](ClO4)2(2). [Effects of the Invention]

[0011] According to the present invention, alkanes such as methane can be more efficiently converted into alcohols by direct oxidation. [Brief explanation of the drawings]

[0012] [Figure 1] Diagram showing the structure of the hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpaC0)2](ClO4)6(1), where (a) is the crystal structure and (b) are the geometric parameters. [Figure 2] Figure 1 shows the crystal structure of the dinuclear copper complex [Cu2(ClO4)2(MeCN)2(6-hpaC0)](ClO4)2(2). [Figure 3] Diagram showing the oxidation of methane with H2O2 catalyzed by the hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpaC0)2](ClO4)6(1), where (a) is the reaction in CH3CN / H2O and (b) is the reaction in CD3CN / H2O. [Figure 4] Figure 1 shows the oxidation of cyclohexane with H2O2 catalyzed by the hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpaC0)2](ClO4)6(1). DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, these embodiments are intended to facilitate understanding of the principles of the present invention, and the scope of the present invention is not limited to the following embodiments. Other embodiments in which a person skilled in the art appropriately replaces the configuration of the following embodiments are also included in the scope of the present invention.

[0014] The present inventors have synthesized a hexanuclear copper complex [Cu(μ-OH)(μ-OH)(6-hpa C0 We have succeeded in producing methanol from methane with high selectivity by using a novel oxidation catalyst, the hexanuclear copper complex [Cu(μ-OH)(μ-OH)(6-hpa C0 The structure of )2](ClO4)6(1) is shown in Figure 1.

[0015] The present inventors have also synthesized a dinuclear copper complex [Cu2(ClO4)2(MeCN)2(6-hpa C0 By using a novel oxidation catalyst, the dinuclear copper complex [Cu(ClO)(MeCN)(6-hpa C0 The structure of )](ClO4)2(2) is shown in Figure 2.

[0016] The 6-hpa forming the hexanuclear copper complex and the dinuclear copper complex C0 The structure of the ligand is shown below. The hexanuclear copper complex is 6-hpa C0 Three copper(II) ions are coordinated to one molecule of the ligand, and then this dimerizes. C0 It has a structure in which two equivalents of copper have reacted with the ligand.

[0017] [ka]

[0018] The copper complexes of the present invention (the aforementioned hexanuclear copper complex and binuclear copper complex) can catalyze oxidation reactions. Because the oxidation catalytic activity of the copper complexes of the present invention is so high, they can produce alcohols by oxidation of stable, unreactive gaseous alkanes such as methane. To date, no alkane oxidation catalysts have been found that possess such high reactivity and selectivity.

[0019] The oxidation reaction of the present invention is preferably carried out in a polar organic solvent such as acetonitrile or in water. Completely in water is particularly preferred. When hydrogen peroxide is used as an oxidizing agent, hydrogen peroxide is generally used in the form of an aqueous solution, and water is generated during the reaction, resulting in the incorporation of water into the polar organic solvent. However, the oxidation reaction of the present invention proceeds satisfactorily even in a mixture of water and a polar organic solvent. The ratio of the volume of water (volume at the start of the reaction) to the volume of the polar organic solvent can be any desired ratio.

[0020] In the oxidation reaction of the present invention, the substrate (gaseous alkane) for the copper complex may be appropriately selected depending on the type of raw material used and the oxidation conditions. Since the substrate (gaseous alkane) used is poorly soluble in polar solvents, the reaction is carried out under high pressure using a high-pressure experimental device to dissolve it and carry out the reaction. For example, the concentration of the dissolved substrate can be controlled by changing the applied pressure. A pressure of 5.0 MPa to 15 MPa is particularly preferred.

[0021] The amount of oxidizing agent (preferably hydrogen peroxide) added may be adjusted appropriately depending on the type of substrate and oxidation conditions, but it is particularly preferable to add 1,000 to 12,500 moles per mole of copper complex.

[0022] The concentrations of the catalyst, substrate, and oxidizing agent in the solvent used for the reaction can be appropriately selected depending on the scale of the reaction, the reaction temperature, and the substances used. For example, the catalyst concentration can be about 10 μmol / L to 500 μmol / L.

[0023] The temperature for carrying out the oxidation reaction is preferably 25 to 70° C. The oxidation reaction is preferably carried out under high pressure of 5.0 MPa to 15 MPa. A preferred oxidizing agent for use in the present invention is hydrogen peroxide. Hydrogen peroxide is an easily available and inexpensive oxidizing agent, and since it produces only water as a by-product in the reaction, it has the highest atom economy and is the best oxidizing agent other than molecular oxygen.

[0024] An example of a method using the catalyst of the present invention is a method for directly converting (oxidizing) gaseous alkanes into alcohols. Using the catalyst of the present invention, alcohols can be produced from gaseous alkanes under high pressure (10 MPa) at a temperature of about 50°C.

[0025] Substrates that can be used in the present invention include not only gaseous alkanes but also liquid alkanes such as cyclohexane and n-hexane. Also, not only compounds with one reactive site such as methane but also compounds with multiple reactive sites are included. [Example]

[0026] (1) New substance hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpa C0 )2](ClO4)6(1) (1-1)6-hpa C0 Ligand synthesis (1-1-1) Synthesis of 2,2'-bipyridine-1,1'-dioxide

[0027] [ka]

[0028] A 50 mL recovery flask was fitted with a magnetic stir bar, and 3.028 g (19.39 mmol) of 2,2'-bipyridine and 15 mL of acetic acid were added and stirred to dissolve. 6.73 mL (3.5 equivalents) of aqueous hydrogen peroxide (11 M) was added, and the mixture was immersed in an oil bath at 70 °C and stirred for 8 hours. The reaction vessel was then removed from the oil bath and stirred at room temperature for 12 hours. The reaction was carried out in an open system, taking care to avoid impact. 300 mL of acetone was then added to a 500 mL trinuclear flask, and the reaction mixture was poured into this, resulting in the formation of a white precipitate. The precipitate was collected by suction filtration and dried under vacuum to obtain 3.42 g of the desired product as a white solid (yield 94%).

[0029] (1-1-2) Synthesis of 6,6'-bis[bis(2-cyano)]-2,2'-bipyridine

[0030] [ka]

[0031] A 500 mL three-necked reaction vessel was fitted with a magnetic rotor, a ball stopper, a reflux condenser, and a pressure-equalizing dropping funnel, and dried under vacuum. 0.900 g (4.78 mmol) of 2,2'-bipyridine-1,1'-dioxide, 1.79 mL (14.35 mmol, 3 eq.), and 200 mL of dichloromethane were added and stirred to dissolve. A solution of benzoyl chloride (1.67 mL, 14.35 mmol, 3 eq.) in 10 mL of dichloromethane was added to the dropping funnel, and the mixture was refluxed for 12 hours with stirring in an oil bath at 30 °C. After cooling to room temperature, 10% aqueous potassium carbonate solution (100 mL) was added, and the mixture was extracted with three 100 mL portions of dichloromethane. The organic layer was then concentrated on an evaporator, resulting in the precipitation of a yellow-white solid. The solid was collected by suction filtration, washed with methanol, and dried in vacuo to give 0.419 g of the desired product as a white solid (yield 91%).

[0032] (1-1-3) Synthesis of 6,6'-bis[(2-aminomethyl)]-2,2'-bipyridine

[0033] [ka]

[0034] A 500 mL three-necked reaction vessel was fitted with a magnetic rotor, a three-way stopcock, a balloon, and a ball stopper, and vacuum-dried. Under nitrogen flow, 300 mL of dry THF was added to the reaction vessel using a syringe. The reaction vessel was immersed in an ice bath and, after confirming that it was sufficiently cooled, LiAlH4 (1.12 g, 28.6 mmol) was added. 6,6'-bis[bis(2-cyano)]-2,2-bipyridine (0.944 g, 4.4 mmol) was then added and stirred in the ice bath for 2 hours. The solution changed from gray to dark purple. After 2 hours, 20% aqueous NaOH was slowly added while stirring the reaction solution in the ice bath until hydrogen evolution ceased. The solution changed color from brown to yellow, and a yellow precipitate formed. Anhydrous Na2SO4 was added to the mixture, and the mixture was dehydrated. After filtration through Celite, washing with dichloromethane, the filtrate was washed with dichloromethane until it became clear. The filtrate was collected and concentrated to a certain extent using an evaporator. A three-necked reactor was fitted with a pressure-equalising dropping funnel, a hose with a vacuum valve and a Pasteur valve on both ends, and a ball stopper. H2SO4aq was placed in this three-necked reactor, and an aqueous NaCl solution was placed in the dropping funnel, which was slowly added dropwise to generate HCl gas. This was then added to the concentrated dichloromethane solution via the Pasteur valve. The precipitate that formed was collected by filtration to give 0.497 g of a reddish-brown solid. The filtrate was then concentrated in an evaporator and dried under vacuum, yielding 0.636 g of a reddish-brown solid (yield 69%).

[0035] (1-1-4)6,6'-bis[[bis(2-pyridylmethyl)amino]methyl]-2,2'-bipyridine (6-hpa C0 Synthesis of ligand

[0036] [ka]

[0037] A 100 mL two-necked reaction vessel was fitted with a magnetic rotor, a three-way stopcock, and a balloon. 0.500 g (1.39 mmol) of 6,6'-bis[(2-aminomethyl)]-2,2'-bipyridine 4HCl was added to the vessel, and 10 mL of H2O was added to dissolve the solution. 1.07 g (6.53 mmol) of 2-(chloromethyl)pyridine HCl was then added. The vessel was sealed with a ball stopper, degassed, and purged with nitrogen. The vessel was then immersed in an ice bath and stirred. After 5 minutes, 3 mL of 30 wt% aqueous NaOH was added, degassed, purged with nitrogen, and then removed from the ice bath and stirred. After 7 days, a reddish-brown solid was formed. This solid was extracted with 50 mL of dichloromethane four times. The organic layer was dehydrated with anhydrous Na2SO4 and concentrated on an evaporator, resulting in a reddish-brown oil. This oily substance was separated and purified by alumina column chromatography (developing solvent: CHCl3), and the fraction containing the target product was concentrated using an evaporator and washed with diethyl ether to obtain 0.240 g of the target product as a white solid (yield 30%).

[0038] (1-2)6-hpa C0 Synthesis of a hexanuclear copper(II) complex (1) using the ligand

[0039] [ka]

[0040] A magnetic rotor was placed in a 50 mL two-necked reaction vessel, and a three-way stopcock and balloon were attached to one end. A pale blue solution of 0.0447 g (3.3 equivalents) of copper(II) perchlorate hexahydrate dissolved in 5 mL of MeOH was added to the reaction vessel. Then, a mixture of 2 mL of MeOH and 1 mL of MilliQ water was added to 6-hPa. C00.0220 g (37.4 mmol) of the ligand was dissolved in a two-necked reaction vessel and added to the vessel. The vessel was degassed and purged with nitrogen. The solution turned blue-green. After 30 minutes, 0.0194 g (5.0 eq) of Et3N was added to the vessel using a nitrogen flow, and the vessel was immediately degassed and purged with nitrogen. After 10 minutes, the magnetic rotor was removed, the mixture was concentrated using an evaporator, and dried in a vacuum line for approximately 1 hour. The remaining dark green solid was dissolved in MeOH, collected by suction filtration using a membrane filter, and dried in vacuum (0.023 g). This solid was recrystallized from MeCN / CH2Cl2 / C6H6 to give [Cu II 6(μ-OH)6(6-hpa C0 )2](ClO4)6(1) single crystals were obtained.

[0041] (2) New substance dinuclear copper complex [Cu II 2(ClO4)2(MeCN)2(6-hpa C0 )](ClO4)2(2) Synthesis Method

[0042] [ka]

[0043] A magnetic rotor was placed in a 50 mL two-necked reaction vessel, and a three-way stopcock and balloon were attached to one end. A solution of 0.0419 g (2.2 equivalents) of copper(II) perchlorate hexahydrate dissolved in 5 mL of MeCN was added. C0 A solution of 0.0200 g of the ligand in 5 mL of MeCN was added, and the mixture was degassed, purged with nitrogen, and stirred. A deep blue solution was formed. After 30 minutes, this solution was added to 30 mL of Et2O through a Millipore filter, resulting in the formation of a whitish-blue precipitate. This precipitate was collected by filtration through a membrane filter and dried under vacuum to obtain 0.034 mg of a solid. This was recrystallized from MeCN / CH2Cl2 / C6H6 to give [Cu II 2(MeCN)2(ClO4)2(6-hpa C0 )](ClO4)2(2) single crystals were obtained.

[0044] (3) Hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpa C0 Selective methanol production by methane oxidation with hydrogen peroxide using [ClO4](1)2 as catalyst 400 μL of solution was placed in a Peek pressure reactor, and methane gas was pumped into the reactor using a high-pressure reactor to a pressure of 8 MPa. At this point, the total volume of the reaction solution was 500 μL. The details of the experiment are shown below.

[0045] A Peek pressure-resistant reactor was placed with a hexanuclear copper(II) complex (1) in MeCN (0.015 μmol, 400 μL) and a magnetic rotor. The reactor was connected to a high-pressure reactor while blowing N2 gas through it. The sample loop was filled with 100 μL of an aqueous solution (500 μmol of H2O2, 0.5 μmol of Et3N) containing H2O2 (5 M) and Et3N (5 mM), and the reactor was filled with methane gas. The pressure in the reactor was pumped until the pressure reached 8 MPa. The H2O2 and Et3N in the sample loop and the methane gas in the reactor were pumped into the pressure-resistant reactor. At this time, the reactor contained (1) (0.015 μmol, 0.3 mM), H2O2 (300 μmol, 0.6 M), Et3N (0.3 μmol, 6 mM), and methane (8 MPa, 200 μmol). The reaction vessel was immersed in a 50°C thermostatic bath and stirred with a magnetic stirrer. The reaction was allowed to proceed for a certain period of time while being shaken using a shaker. After the reaction was complete, the reaction vessel was cooled by immersing it in an ice bath for 5 minutes, after which the solution was removed from the pressure-resistant reaction vessel. 100 μL of the reaction solution was removed, and 20 μL of a 2.0 mM MeCN solution was added as a reference substance. The amount of MeOH was quantified by GC-MS. PPh3 (50 mg) was added to the remaining 400 μL of the reaction solution, the lid was closed, and the mixture was stirred for 30 minutes. The insoluble solids were removed by filtration through a Millipore filter. 100 μL of the filtrate was added to 20 μL of a 2.0 mM MeCN solution, and the amount of MeOH was quantified by GC-MS. The following procedure was also performed to quantify formaldehyde using the Nash method. 100 μL of the remaining filtrate was taken, to which 2 mL of acetic acid / ammonium acetate solution and then 2 mL of acetylacetone solution were added. The mixture was stirred in a 60°C water bath for 15 minutes, then cooled in an ice bath for 5 minutes, and stirred at room temperature for 20 minutes. The UV-vis spectrum of this solution was measured, and the amount of formaldehyde produced was calculated from the absorbance at 410 nm. A previously prepared calibration curve was used for quantification by GC-MS and UV-vis spectrum.

[0046] Hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpa C0Figure 3 shows the methane oxidation with H2O2 catalyzed by [ClO4](1). (a) shows the reaction in CH3CN / H2O, and (b) shows the reaction in CD3CN / H2O. In Figure 3, the vertical axis represents the amount of methanol and formaldehyde produced (μmol), and the horizontal axis represents the reaction time. When CH3CN / H2O was used as the reaction solvent, the amount of product saturated in approximately 1.5 hours, and the catalytic turnover number (TON) at this time was 51. On the other hand, when deuterated CD3CN / H2O was used, the amount of product saturated in approximately 2 hours, and the TON increased to 118. This is thought to be due to the solvent isotope effect, which prevented solvent oxidation from occurring, allowing methane oxidation to proceed.

[0047] (4) Hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpa C0 Selective cyclohexenol production by oxidation of cyclohexane with hydrogen peroxide using [)2](ClO4)6(1) as catalyst A two-necked recovery flask was equipped with a reflux condenser, a three-way stopcock, and a balloon on one end, and a ball stopper on the other end. A magnetic rotor was placed inside, and the flask was then vacuum dried. MeCN (19 mL), purified water (3 mL), nitrobenzene (40 μL), and cyclohexane (1.684 g, 20 mmol) were added to the reaction vessel, and the aluminum block was set to 55°C and heated for 30 minutes. The reaction solution reached a temperature of 50°C. A 0.5 mM solution of complex (1) in MeCN (1 mL, 0.5 μmol), a 0.2 M solution of EtN in MeCN (50 μL, 10 μmol), and a 10 M aqueous solution of HO (1 mL, 10 mmol) were added under N2 flow. The atmosphere was purged with nitrogen gas to remove oxygen, and the reaction was then carried out. After any time from the start of the reaction, 0.2 mL of the reaction solution was removed under N2 flow, and triphenylphosphine PPh3 (30-35 mg) was added. After 30 minutes of reaction, the insoluble solids were removed using a Millipore column, and the oxidation product was quantified by GC. A calibration curve was prepared in advance.

[0048] Hexanuclear copper complex [Cu6(μ3-OH)2(μ-OH)4(6-hpa C0The oxidation of cyclohexane with H2O2 catalyzed by [(ClO4)6](1) is shown in Figure 4. As shown in Figure 4, the catalytic turnover frequency (TOF) (h -1 ) was 185 and the catalytic turnover number TON was 1422, indicating high oxidation activity. [Industrial Applicability]

[0049] It can be used as an oxidation catalyst in the oxidation of alkanes.

Claims

1. Hexanuclear copper complex [Cu 6 (μ 3 -OH) 2 (μ-OH) 4 (6-hpa C0 ) 2 ](ClO 4 ) 6 であるNew regulated substances.

2. As shown in the following reaction scheme, 6,6'-bis[[bis(2-pyridylmethyl)amino]methyl]-2,2'-bipyridine (6-hpa C0 The hexanuclear copper complex [Cu 6 (μ 3 -OH) 2 (μ-OH) 4 (6-hpa C0 ) 2 ](ClO 4 ) 6 Manufacturing method. 【Chemical 1】

3. Hexanuclear copper complex [Cu 6 (μ 3 -OH) 2 (μ-OH) 4 (6-hpa C0 ) 2 ](ClO 4 ) 6 A method for oxidizing alkanes to alcohols by using as an oxidation catalyst.

4. 4. The method for oxidizing an alkane to an alcohol according to claim 3, wherein the alkane is methane.

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