JP7633651B2 - Method for producing esters - Google Patents

Description

The present invention relates to a method for producing esters that can effectively utilize alkanes, particularly gaseous alkanes.

Currently, in order to establish a future industrial base that can utilize a variety of natural carbon resources in a balanced way, there is a demand for technology to convert methane and other alkane gas resources into useful chemical products and energy in unprecedented ways.

In particular, the main components of natural gas and shale gas are said to be alkanes that are gaseous at room temperature. In particular, methane released in agriculture and livestock farming has a global warming potential 25 times that of carbon dioxide, and utilizing it plays an important role in aiming for a decarbonized society.

For this reason, if it becomes possible to easily convert alkane gas resources, such as methane contained in abundant reserves of natural gas, into chemical products and energy using unprecedented technology, it will be possible to break away from the problem of oil dependency facing modern society and reduce carbon dioxide emissions, making this technology a major contribution to mitigating global warming.

However, the process of directly converting alkane gas resources such as methane into chemical products is highly difficult, and currently indirect processes such as via synthesis gas (CO+H ₂ ) produced by reforming methane are used.

As part of the utilization of alkane gas resources, there is a demand for methods to oxidize alkanes and convert them into esters. Known methods for synthesizing esters include Fischer's esterification, which uses sulfuric acid as a catalyst to esterify acetic acid and ethanol through dehydration and condensation, a method of converting acetaldehyde into ethyl acetate using a base catalyst, and a direct synthesis method from ethylene and acetic acid. However, no methods were known that use alkanes, which are the main components of natural gas.

When introducing functional groups into alkanes, methods of introducing functional groups by oxidation or the like have been considered, but problems such as combustion of raw materials and products at high temperature conditions have been pointed out, and reactions under milder conditions are being sought. For example, a method is known in which the C-H groups of alkanes are activated and oxidized under milder temperature conditions using a catalyst, and although this method can activate and oxidize secondary and tertiary C-H groups and C-H groups at benzyl positions, it is known that it cannot activate and oxidize the C-H groups of alkyl groups in primary alkanes, which are useful when using resources such as natural gas. (Non-Patent Document 1).

As for alcohols and ketones obtained by oxidation of alkanes, Patent Document 1 discloses a method for oxidizing alkanes with oxygen, in which an alkane is reacted in the presence of an aldehyde, a copper-based catalyst, and a nitrogen-containing compound. Patent Document 1 discloses a wide variety of alkanes, and the nitrogen-containing compounds disclosed are nitriles, aromatic amines, and tertiary amines. However, there is no suggestion about producing esters, and there is no suggestion at all about synthesizing esters using lower alkanes such as methane or ethane.

JP 2002-30008 A

Yukiko Hayashi, Naruyoshi Komiya, Ken Suzuki, Shun-Ichi Murahashi, Tetrahedron Letters, 54 (2013), 2706-2709.

The objective of the present invention is to produce esters that can effectively utilize alkanes, particularly gaseous alkanes. In particular, the objective of the present invention is to provide a method for constructing esters by oxidizing alkanes having primary alkyl groups.

Means for Solving the Problems The present inventors have conducted intensive research to solve the above problems, and as a result have found that esters can be efficiently produced by using a specific catalyst in combination with specific reaction conditions, thereby completing the present invention.
[1] An alkane represented by the general formula R ¹ H,
and an aldehyde represented by the general formula R ² CHO,
Providing an oxygen source to produce an ester represented by the general formula ^R2COOR1 in the presence of a metal salt (M(L) _m/n ) ^;
Method for producing esters.
( ^R1 is a C1-C20 linear or cyclic hydrocarbon group, and ^R2 is a C1-C30 linear or cyclic hydrocarbon group. M represents a metal element of Groups 8-12. L constitutes a counter ion for the metal element ion ^and is at least one selected from a carboxylate ion having a carbon number of C1-C20, a halogen ion, a nitrate ion, a ^sulfate ion, _ClO4- , ^BPh4- , and _{PF6- .} m represents the valence of the metal element ion, and n represents the valence of the counter ion.)
[2] The method for producing an ester according to [1], wherein the alkane is methane, ethane, propane, or butane.
[3] The method for producing an ester according to [1] or [2], wherein the ester is produced in the presence of water and/or one or more organic compounds selected from a nitrogen-containing compound and a halogenated hydrocarbon as a solvent.
[4] The oxidation method according to any one of [1] to [3], wherein in the metal salt (M(L) _m/n ), M is at least one selected from iron, nickel and ruthenium.
[5] A method for producing an ester according to any one of [1 ^] to [4], wherein in the metal salt (M(L) _m/n ), L is a metal salt selected from a C1 to C20 carboxylate ion, a halogen ion, a nitrate ion, a sulfate ion, a perchlorate ion, BP ₄ - , and PF ₆ ^- .
[6] The method for producing an ester according to any one of [1] to [5], wherein the organic compound is a nitrile or a chloroalkane.
[7] The method for producing an ester according to any one of [1] to [6], wherein when water and the organic compound are used together as a solvent, the mass ratio of water:organic compound is 0.01:100 to 100:0.01.
[8] The method for producing an ester according to any one of [1] to [7] above, wherein the alkane is methane or ethane.
[9] The method for producing an ester according to any one of [1] to [8] above, wherein R ² of R ² CHO is a linear or cyclic C1-C30 hydrocarbon group.
[10] The method for producing an ester according to any one of [1] to [9] above, wherein the alkane is represented by R ³ CH ₃ , and R ³ is a linear, chain, or cyclic aliphatic hydrocarbon having a carbon number of C0 to C19.

According to the present invention, gaseous alkanes, which are the main components of natural gas and shale gas, can be converted into esters. The reaction conditions can also be milder than those of conventional ester synthesis methods. Gaseous alkanes are greenhouse gases generated in agriculture and the livestock industry, but according to the present invention, they can be effectively utilized as various chemical products, which can be effective in preventing global warming. In addition, because the reaction conditions are mild, it is possible to install the plant in a small to medium-sized facility.

FIG. 1 is a conceptual diagram showing an example of a reaction mechanism of the present invention. FIG. 1 shows a gas chromatographic mass spectrometry chart of the product obtained when an ester was produced using ethane in Example 1.

Hereinafter, the embodiment of the invention will be specifically described.
First, the raw materials used in the present invention will be described.
Reactant
(1) Alkane In the present invention, an alkane represented by the general formula R ¹ H (R ¹ is a C1-C20 linear or cyclic hydrocarbon group) is used. The alkane of the present invention can be represented as R ³ CH _3. R ³ is an aliphatic hydrocarbon having a carbon number of C0-C19, which may be linear, branched, or cyclic, and is characterized by having one or more functional groups that are recognized as methyl groups, ethyl groups, n-propyl groups, or n-butyl groups.

Examples of chain alkanes include methane, ethane, propane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-undecane, and n-dodecane. Examples of branched alkanes include 2-methylpropane, 2-methylheptane, 2,3-dimethylhexane, and 2,3,4-trimethylpentane.

Examples of cyclic alkanes include cyclohexane, cyclopentane, cyclooctane, cycloheptane, and cyclododecane.
In the present invention, it is preferable to use alkanes that are gaseous at room temperature, such as methane, ethane, propane, and butane.

These alkanes are known to be contained in natural resources such as natural gas, and in gases produced in the agricultural and livestock industries, but according to the present invention, these gases can be used in their gaseous state.
Of these, methane or ethane is more preferable for exerting the effects of the present invention.

(2) Aldehyde As the aldehyde, one represented by the general formula ^R2CHO ( ^R2 is a C1-C30 linear or cyclic aliphatic hydrocarbon group or aromatic hydrocarbon group, which may further have a ring structure) is used.

The aldehydes used in the present invention may be aliphatic aldehydes and aromatic aldehydes, and specific examples include aliphatic aldehydes such as acetaldehyde, propionaldehyde, n-butyraldehyde, isobutyraldehyde, n-valeraldehyde, isovaleraldehyde, pivalaldehyde, n-hexylaldehyde, n-heptylaldehyde, n-octylaldehyde, n-nonylaldehyde, and n-decylaldehyde, as well as substituted or unsubstituted aromatic aldehydes such as benzaldehyde, p-chlorobenzaldehyde, m-chlorobenzaldehyde, m-cyanobenzaldehyde, p-tolualdehyde, and p-methoxybenzaldehyde, and mixtures thereof.

Of these, C2 to C30 aldehydes are preferred, and of these, propionaldehyde is most preferred.
The amount of aldehyde used is appropriately selected depending on the amount of ester to be produced, but if the amount is an equivalent amount, an equivalent amount of ester can be produced. Usually, the amount is in the range of 0.1 to 1000 mol %, preferably 1 to 400 mol %, based on the alkane, but is not particularly limited.

(3) Metal Salt The metal salt used in the present invention is used as a catalyst in the reaction for synthesizing an ester from an alkane and an aldehyde.

The metal salt used is expressed as M(L) _m/n , where M represents a metal element of Groups 8 to 12, L constitutes a counter ion for the metal element ion and is at least one selected from a carboxylate ion having carbon numbers of C1 to C20, a halogen ion, a nitrate ion, a sulfate ion, _ClO4- , _BPh4- , ^and _PF6- , m represents the ^valence of the metal element ion, and n represents the valence of the counter ion.

M represents a metal element of groups 8 to 12. Specific examples include Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, and Au, with Fe, Ni, Cu, and Ru being preferred.

In order for the metal salts used in the present invention to exhibit catalytic activity, differences in the metal valence, coordination number, coordination structure, counter anion, etc. have a significant effect on catalytic activity. In addition, activity can change significantly depending on the combination of the metal and its counter ion and the structure of the counter ion.

In the present invention, since an ester is synthesized by adding an aldehyde to an alkane and using oxygen as an oxidizing agent, a redox reaction occurs between the substrate or raw material and the metal of the metal salt, so a metal whose valence changes easily in the reaction system is preferable. For example, in the case of copper element, known copper metal salts or copper powder can be used as copper metal salts, but there are three types of valence, II-valent, I-valent, and 0-valent, and they are appropriately selected depending on the type of alkane or aldehyde used in the reaction and the solvent system. Depending on the solvent system, the stability and existence state change depending on the presence or absence of solvation, so they are selected taking these into consideration. L may become a radical and move during the reaction. As L, for example, carboxylic acids having aliphatic or aromatic hydrocarbons with carbon numbers C1 to C20 can be mentioned, but among them, Cu(OAc) _{2.nH 2} O, which is easy to obtain and handle and has high reactivity, is preferably used. As the copper metal salt, the following known compounds can be used. Specific examples include Cu(OAc) ₂ ·nH ₂ O, Cu(OAc), CuCl, CuCl ₂ ·nH ₂ O, CuBr, CuBr ₂ , CuSO ₄ ·nH ₂ O, Cu(NO ₃ ) ₂ ·nH ₂ O, Cu(ClO ₄ ) ₂ and the like.
The amount of the metal salt used is not particularly limited, but is usually in the range of 0.000001 to 200 mol %, preferably 0.00001 to 5 mol %, based on the alkane.

(4) Oxygen source The oxygen used in this reaction may be any of (a) oxygen gas, (b) air (composed of 20% oxygen, 78% nitrogen, 1% carbon dioxide, etc.), and (c) a mixture of oxygen gas and an inert gas such as nitrogen. The gas pressure may be reduced or increased, but an oxygen pressure of 1 atmosphere is also possible.

(5) Solvent In the reaction used in the present invention, it is preferable to use water and/or an organic compound as a solvent. As the organic compound, it is preferable to use a nitrogen-containing compound and a halogenated alkane in order to synthesize an ester.

As the organic solvent for the nitrogen-containing compound, nitriles can be used. As the nitriles, conventionally known nitriles can be used regardless of whether they are aliphatic or aromatic nitriles. Specific examples of aliphatic nitriles include acetonitrile, propionitrile, butyronitrile, isobutyronitrile, valeronitrile, isovaleronitrile, trimethylacetonitrile, hexanenitrile, 4-methylvaleronitrile, heptanenitrile, octanenitrile, undecanenitrile, decanenitrile, stearonitrile, cyclohexanecarbonitrile, cyclopentanecarbonitrile, etc., and aliphatic dinitriles include malononitrile, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, azelanitrile, etc. Examples of aromatic nitriles include benzonitrile, o-, m-, and p-phthalonitrile, etc.

The halogenated alkane may be any halide in which the hydrogen of the alkane is replaced with a halogen, such as fluoride, chloride, bromide, or iodide. Of these, methylene chloride, chloroform, dichloroethane, etc. are preferably used in the present invention.

Water and organic compounds may be added as solvents for the aldehydes and metal salts added.
The amount of these to be used is not particularly limited, but it is usually 0.01 to 100 mol %, preferably 0.1 to 10 mol %, based on the alkane.

When using both nitriles and halogenated alkanes, water is not required, but if water is not present, the reaction yield may be low, and the nitriles may decompose as the aldehydes are consumed, or the oxidation of the aldehydes or the halogenated alkanes may progress.

Furthermore, if there is too much water, the oxidation reaction will hardly proceed. For this reason, it is desirable to add water to the nitriles in a molar ratio (nitrile/water) of 0.01/100 to 1000/0.01, preferably 0.1/10 to 10/0.1.

In the present invention , an oxygen source is supplied to the alkane and aldehyde, and they are reacted in the presence of the metal salt to produce an ester represented by the general formula R ² COOR ^1. At this time, water or an organic compound may be introduced, if necessary.

Although the reaction mechanism is not clear, one possible mechanism is shown in FIG. 1 when, for example, methane is used as the alkane, acetaldehyde is used as the aldehyde, and Cu(OAc) ₂ ·nH ₂ O is used as the catalyst.

For example, acetaldehyde generates acyl radicals in the presence of divalent copper salt (Cu(OAc) ₂ ). First, acetaldehyde, which is also used as a solvent in the reaction system, reduces copper(II) salt to copper(I) salt, and acetic acid, which was the counter ion, is released into the solvent. However, hydrogen is extracted from the aldehyde generated during oxidation, and the acetaldehyde with the hydrogen extracted becomes a radical aldehyde. For copper(I) salt, the oxygen fed into the reaction system is oxidized by molecular oxygen to generate so-called oxyradical active species. This oxyradical active species further reacts with radical aldehyde to generate peracetic acid, and this reaction occurs instantly. At this time, the monovalent copper is oxidized to divalent, and copper active species of peracetic acid are generated on the copper. This copper peracetic acid active species extracts hydrogen atoms from alkanes (methane in the figure, but ethane is similar), and it is thought that this stage is a concerted process of OO cleavage and CH cleavage. Next, Cu(II)-OR (R is a methyl or ethyl group derived from the substrate) exchanges ligands with the acetic acid that was removed from the copper in the first step, regenerating the alcohol product and copper acetate. This is followed by recombination of the ligands to produce methyl alcohol, which then reacts with acetaldehyde in the presence of oxygen to produce a hemiacetal, which is then oxidized to produce an ester. This mechanism is an example of the case where a copper metal salt is used, but the inventors believe that metallic oxyradical active species are one of the important active species for carrying out this reaction.

The reaction temperature is usually in the range of 0° C. to 150° C., and preferably in the range of about 25° C. to 80° C. The reaction proceeds even at room temperature, such as about 20° C.
In the present invention, it is preferable to carry out the reaction under pressurized conditions, and it is preferable to use a pressurized reaction apparatus such as an autoclave. The pressurized conditions may be from normal pressure to 10 atmospheres (1 MPa), and for example, a method can be adopted in which the autoclave is connected to an apparatus that performs nitrogen replacement at 8 atmospheres and constantly supplies oxygen at 1 atmosphere from the start of the reaction throughout the reaction.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples in any way.

(High pressure reaction apparatus)
The reactor used was a high-pressure reactor TS-31H (manufactured by Taiyo Systems Co., Ltd.) equipped with a Shimadzu LC-20AT liquid delivery unit.
(Gas Chromatography Mass Spectrometry: GC-MS)
The products were sampled and analyzed using a Shimadzu GCMS-QP2010 Plus. A RESTEK Rtx-VMS column (30 m × 0.25 mm) was used for chromatography. After sampling with an autosampler, the products were automatically injected. Detection was performed using a mass spectrometer.

Example 1
Copper acetate (20 μmol, 3.64 mg) was dissolved in 40 mL of acetonitrile, and 100 μL of the solution (Cu(OAc) ₂ = 0.050 μmol, 9.1 μg), acetonitrile (5.0 mL), and dichloromethane (6.0 mL) were added to a high-pressure reactor, and the vessel was pressurized to 4.0 MPa with a mixed gas of ethane/oxygen (= 3/1). After the vessel was pressurized, propionaldehyde (2.0 mmol, 144 μL) dissolved in acetonitrile (4 mL) was added, and the reaction was started at 25°C. The reaction solution was sampled at regular intervals, processed using a silica gel column, and the reaction products were identified by GC-MS. The GC-MS chart is shown in Figure 2. In addition, a calibration curve for each product was created and quantified. The results are shown in Table 1.
In Example 1, it was found that the following reaction occurred to obtain the ester.

Example 2
An ester was produced in the same manner as in Example 1, except that methane was used instead of ethane. The results are shown in Table 1.

(Reference Example 1)
The same procedure as in Example 1 was carried out except that ethanol was used instead of ethane. The results are shown in Table 3.

(Reference Example 2)
The same procedure as in Comparative Example 1 was carried out except that acetic acid was used instead of propynaldehyde in Reference Example 1. The results are shown in Table 3.

In the above example, when ethanol was used as a substrate, an ester was produced, but when a carboxylic acid was used, an ester was not produced. This can be considered to mean that ethanol may be one of the reaction intermediates in the present invention, but that the carboxylic acid is not present in the reaction pathway.

(Comparative Example 1)
The same procedure as in Example 1 was carried out except that cyclohexane was used instead of ethane, but no ester was produced.

Claims (5)

A chain alkane represented by the general formula R ¹ H;
and an aldehyde represented by the general formula R ² CHO,
Providing an oxygen source to produce an ester represented by the general formula ^R2COOR1 in the presence of copper acetate ^;
Method for producing esters.
( ^R1 is a C1 to C20 chain hydrocarbon group, and ^R2 is a C1 to C30 chain, cyclic hydrocarbon group . ) The method for producing an ester according to claim 1, wherein the chain alkane is methane, ethane, propane or butane. The method for producing an ester according to claim 1 or 2, characterized in that the ester is produced in the presence of water and/or one or more organic compounds selected from nitrogen-containing compounds and halogenated hydrocarbons as a solvent. 4. The method for producing an ester according to claim 3 , wherein the organic compounds are nitriles and chloroalkanes. The method for producing an ester according to claim 3 or 4 , wherein when water and the organic compound are used together as a solvent, the mass ratio of water:organic compound is 0.01:100 to 100:0.01.