JP7263184B2 - Method for producing fluorinated methane - Google Patents

Description

The present invention relates to a method for producing fluorinated methane useful as a dry etching gas.

Hydrofluorocarbons are useful as etching gases for microfabrication of semiconductors, liquid crystals, etc. In particular, methane fluoride (CH ₃ F) has attracted attention as an etching gas for forming state-of-the-art microstructures.

As a method for producing fluorinated methane, a method of thermally decomposing fluorine-containing methyl ether in the presence of a catalyst is known (Patent Documents 1 and 2).

WO2011/102268 JP 2014-114277 A

An object of the present invention is to extend the life of a catalyst in a conventionally known method for producing fluorinated methane by thermally decomposing fluorine-containing methyl ether in the presence of a catalyst.

The inventors of the present invention have made intensive studies in order to solve the above problems. As a result, they have found that the life of the catalyst can be extended by thermal decomposition in the presence of at least one gas selected from the group consisting of hydrogen fluoride, chlorine, hydrogen chloride and air. The present invention was completed as a result of further studies based on these findings.

That is, the present invention includes the following aspects.
Section 1. A method for producing fluorinated methane by vapor-phase thermal decomposition of a fluorine-containing methyl ether represented by general formula (1) in the presence of a catalyst, comprising:
A method characterized by thermal decomposition in the presence of at least one gas selected from the group consisting of hydrogen fluoride, chlorine, hydrogen chloride and air

(Wherein, R ¹ and R ² are the same or different and may be substituted linear or branched monovalent aliphatic hydrocarbon group, monovalent aromatic hydrocarbon group or monovalent is a hydrogen atom or a halogen atom).
Section 2. Item 2. The method according to Item 1, wherein thermal decomposition is performed in the presence of the gas in a volume ratio of 0.03 or more to 1 part of the fluorine-containing methyl ether.
Item 3. Item 3. The method according to item 1 or 2, comprising adding at least one gas selected from the group consisting of hydrogen fluoride, chlorine, hydrogen chloride and air to the reaction system before or simultaneously with the pyrolysis.
Section 4. Item 4. The method according to Item 3, wherein the step is a step of adding the gas in a volume ratio of 0.03 or more to 1 part of the fluorine-containing methyl ether.
Item 5. Item 5. The method according to any one of Items 1 to 4, wherein the catalyst is at least one selected from the group consisting of metal oxides, fluorinated metal oxides, and metal fluorides.
Item 6. the catalyst is alumina, chromium oxide, titanium oxide, zinc oxide, fluorinated alumina, fluorinated chromium oxide, fluorinated titanium oxide, fluorinated zinc oxide,
Item 6. The method according to any one of items 1 to 5, wherein the at least one is selected from the group consisting of AlF ₃ , TiF ₄ , CrF ₃ and ZnF ₂ .
Item 7. Item 6. The method according to any one of Items 1 to 5, wherein the catalyst is alumina.
Item 8. Item 8. The method according to Item 6 or Item 7, wherein the alumina is γ-alumina.
Item 9. Item 9. The method according to any one of Items 5 to 8, wherein the catalyst has a pore volume of 0.5 ml/g or more.
Item 10. Item 10. The method according to any one of items 1 to 9, wherein the thermal decomposition reaction has a reaction temperature of 100 to 400°C.
Item 11. Item 11. The method according to any one of Items 1 to 10, wherein the pressure during the thermal decomposition reaction is 0.05 to 1 MPa.
Item 12. Item 12. The method according to any one of Items 1 to 11, wherein the thermal decomposition is performed at a water concentration of 100 ppm or less.
Item 13. Item 12. The method according to any one of items 1 to 11, comprising a step of removing water from the reaction system before or simultaneously with the thermal decomposition.

According to the present invention, the life of the catalyst can be extended in the method of producing fluorinated methane by thermally decomposing fluorine-containing methyl ether in the presence of the catalyst. In other words, the same amount of fluorinated methane can be obtained with less catalyst usage, which is advantageous compared to conventional methods.

1. Raw material compound
In the present invention, a fluorine-containing methyl ether represented by the general formula (1) is used as a raw material.

(Wherein, R ¹ and R ² are the same or different and may be substituted linear or branched monovalent aliphatic hydrocarbon group, monovalent aromatic hydrocarbon group or monovalent is a hydrogen atom or a halogen atom).

The production method of the fluorine-containing methyl ether used as a raw material is not particularly limited, and a compound obtained by any method can be used.

In the above general formula (1), preferably R ¹ and R ² are the same or different, optionally substituted linear or branched monovalent aliphatic hydrocarbon groups having 1 to 30 carbon atoms , a monovalent aromatic hydrocarbon group having 6 to 12 carbon atoms or a monovalent cycloaliphatic hydrocarbon group having 6 to 12 carbon atoms. More preferably, R ¹ and R ² are the same or different and optionally substituted,
A linear or branched monovalent aliphatic hydrocarbon group having 1 to 10 carbon atoms, a monovalent aromatic hydrocarbon group having 6 to 10 carbon atoms, or a monovalent cyclic aliphatic hydrocarbon group having 6 to 10 carbon atoms It is a hydrogen group.

In the above, the linear or branched monovalent aliphatic hydrocarbon group having 1 to 10 carbon atoms is not particularly limited, and examples thereof include alkyl groups having 1 to 10 carbon atoms.

Specific examples of alkyl groups having 1 to 10 carbon atoms include methyl group, ethyl group, trimethyl group, propyl group, 2-methylethyl group, hexyl group and octyl group.

Among the alkyl groups having 1 to 10 carbon atoms, those having 1 to 6 carbon atoms are preferred, those having 1 to 4 carbon atoms are more preferred, and those having 1 to 3 carbon atoms are even more preferred.

Examples of monovalent aromatic hydrocarbon groups having 6 to 10 carbon atoms include, but are not limited to, phenyl, methylphenyl and ethylphenyl groups.

Examples of the monovalent cycloaliphatic hydrocarbon group having 6 to 10 carbon atoms include, but are not limited to, cyclohexyl group, methylcyclohexyl group and ethylcyclohexyl group.

In the above, the monovalent aliphatic hydrocarbon group, the monovalent aromatic hydrocarbon group or the monovalent cycloaliphatic hydrocarbon group is at least one heteroatom selected from the group consisting of a fluorine atom, a chlorine atom and a bromine atom. The atom may replace at least one of the hydrogen atoms, or may replace all hydrogen atoms.

In the above, the halogen atom is preferably fluorine atom, chlorine atom or bromine atom, more preferably fluorine atom.

Although not bound by any particular theory, the results of studies by the present inventors have suggested that the deterioration of the catalyst may be progressing due to the by-products produced when the fluorine-containing methyl ether is thermally decomposed. . Specifically, it has been suggested that this by-product may be carbonized on the surface of the catalyst, leading to a decrease in catalytic activity. It is believed that the thermal decomposition in the presence of at least one gas selected from the group consisting of hydrogen fluoride, chlorine, hydrogen chloride and air suppresses the formation of this by-product. Such a by-product is a specific unsaturated fluoride, and is considered to be commonly produced as long as the fluorine-containing methyl ether represented by the general formula (1) is used as a starting material.

Specific examples of compounds that can be used as raw materials include, but are not limited to, 1, 1,
3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether and the like.

In particular, perfluoroisobutylene ((CF ₃ ) ₂ C=CF ₂ ), which is a by-product of the production of hexafluoropropene, which is used as a raw material for fluororesins, has been discarded as an unnecessary material. 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether can be obtained by reacting with It is possible to obtain the desired product at low cost using low-cost raw materials. In the present invention, 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether as a raw material is
When it says "obtained by reacting perfluoroisobutylene with methanol," it means that 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether is obtained by such a reaction. It means that it is limited to the product and not obtained by other reactions. 1,1,3,3, by reacting perfluoroisobutylene with methanol
A method for obtaining 3-pentafluoro-2-trifluoromethylpropyl methyl ether is a known method, and known reaction conditions may be followed. For example, special table 20011-506261
The reaction may be carried out according to the method described in the publication.
2. Pyrolysis reaction method
The method of the present invention is a method of carrying out a thermal decomposition reaction in the gas phase in the presence of a catalyst using the fluorine-containing methyl ether as a starting material.

(1) Catalyst Any catalyst can be used without particular limitation as long as it has activity against the thermal decomposition reaction in the gas phase. Examples of such catalysts include metal oxides, fluorinated metal oxides, metal fluorides, and the like, and these can be used singly or in combination of two or more.

Among these, as metal oxides, alumina, chromium oxide, titanium oxide, zinc oxide,
etc. are preferred. Fluorinated metal oxides obtained by partially fluorinating these metal oxides can also be used. The fluorinated metal oxide catalyst may be obtained by fluorinating a metal oxide catalyst in advance using hydrogen fluoride or the like. Metal oxide catalysts may also be used. AlF ₃ , TiF ₄ , CrF ₃ , ZnF ₂ and the like are preferable as metal fluorides.

Among metal oxides, alumina is preferred, and α-alumina and activated alumina can be used. As activated alumina, ρ-alumina, χ-alumina, κ-alumina, η-alumina, pseudo-γ-alumina, γ-alumina, δ-alumina and θ-alumina are used. Among these, γ-alumina and η-alumina are preferred, and γ-alumina is particularly preferred. again,
Silica alumina (SiO ₂ /Al ₂ O ₃ ) as a composite oxide can also be used as a catalyst. The composition of silica SiO ₂ of silica alumina is preferably 20% to 90% by weight,
50% to 80% by weight is more preferred.

The larger the pore volume of the catalyst, the higher the activity, preferably 0.4 ml / g or more,
0.5 ml/g or more is particularly preferred. Although the upper limit of the pore volume of the catalyst is not particularly limited, it is usually 5 ml/g or less, and preferably 2 ml/g in terms of reaction rate and catalyst strength.
g or less. The pore volume can be measured by a gas adsorption method, a mercury intrusion method, or the like.

The catalyst may also support alkali metal and alkaline earth metal fluorides such as KF, NaF and _MgF2 .

Although there is no particular limitation on the method for obtaining the above-mentioned fluorinated metal oxide, for example,
By contacting the metal oxide described above with anhydrous hydrogen fluoride or flon under heating, the fluorination reaction proceeds and a fluorinated metal oxide can be obtained. The method of contacting the metal oxide and hydrogen fluoride is not particularly limited, and may be a continuous method in which hydrogen fluoride is circulated in a reaction tube filled with a catalyst, or hydrogen fluoride or Freon is added to a container containing a catalyst. It may be a batch type that encloses. In particular, the distribution method is preferable in that the processing time is short.
Freon preferably has a large number of fluorine atoms and a small number of carbon atoms. Examples include trifluoromethane, difluorochloromethane, octafluoroethane and the like.

The degree of fluorination of the metal oxide is not particularly limited, but the fluorine content is preferably about 5 to 50% by weight based on the total weight of the fluorinated metal oxide.

The temperature of the metal oxide fluorination treatment is preferably higher than the thermal decomposition reaction described later, for example, about 150 to 500°C is preferable, about 200 to 400°C is more preferable, and about 250 to 350°C. is more preferred. If the temperature of the fluorination treatment is too low, the fluorination will be insufficient and the effect of the catalyst will be small.

(2) Thermal decomposition reaction conditions The thermal decomposition reaction of fluorine-containing methyl ether can be carried out by contacting the fluorine-containing methyl ether in the gaseous state with the catalyst in the presence of the above catalyst. is not particularly limited, for example, using a tubular flow-type reactor, filling the reactor with the catalyst described above, introducing fluorine-containing methyl ether used as a raw material into the reactor, and producing a gas phase state A method of contacting the catalyst can be mentioned.

As for the temperature of the thermal decomposition reaction, if the temperature is too low, the conversion rate of the raw material tends to decrease, and if it is too high, impurities tend to increase. For this reason, it is preferable to set it to about 100 ° C. to 400 ° C.
C. to 300.degree. C. is more preferable, and 120.degree. C. to 200.degree. C. is particularly preferable.

If the pressure inside the reaction tube during the pyrolysis reaction is too low, there is a possibility that air will be mixed in, making the operation complicated. . From these points, it is preferable to set it to about 0.05 to 1 MPa, and 0.1 to 0.5 MPa
A pressure of about a is preferable, and a pressure of about atmospheric pressure (about 0.1 MPa) is particularly preferable in terms of reaction operation.

The contact time for the reaction is not particularly limited, but the flow rate F of the fluorine-containing methyl ether, which is the raw material gas supplied to the reaction tube (at 0 degrees and 1 atmosphere (about 0.1 MPa): cc/ The ratio of the catalyst filling amount W (g) to sec): The contact time represented by W/F (g·sec/cc) is preferably about 1 to 100 g·sec/cc, more preferably 1 to 50 g·
About sec/cc is more preferable, and about 5 to 30 g·sec/cc is even more preferable. If the contact time is too long, it takes a long time to obtain the product, so it is preferable to shorten the contact time in order to increase the production amount, but if the contact time is too short, the conversion rate tends to decrease. . Therefore, the contact time that maximizes the productivity in terms of the conversion rate of the raw material and the selectivity of the target product should be selected depending on the type of catalyst used, the amount of the catalyst, the reaction conditions, and the like. Generally, it is desirable to carry out the reaction by selecting a contact time at which the conversion rate becomes 100%, depending on the type of catalyst used, the amount of catalyst, the reaction conditions, and the like.

(3) at least one gas selected from the group consisting of hydrogen fluoride, chlorine, hydrogen chloride, and air; It is necessary to thermally decompose in the presence of at least one kind of gas.

The hydrogen fluoride used for this purpose is preferably anhydrous hydrogen fluoride.

Although it is not particularly limited as long as the effects of the present invention can be obtained, the volume ratio to 1 fluorine-containing methyl ether is preferably 0.03 or more, more preferably 0.05 or more, and still more preferably 0.10 or more. It is thermally decomposed in the presence of said gas.

Further, the effects of the present invention tend to improve as the volume ratio of the gas to 1 of the fluorine-containing methyl ether increases under certain requirements. Therefore, it can be considered that the effect of the invention can be obtained by adding the gas to the reaction system before or at the same time as the thermal decomposition, regardless of the final volume ratio described above. At this time, the method of the present invention can be defined as a method including the step of adding the gas to the reaction system before or simultaneously with pyrolysis. In this step, the volume ratio to 1 part of the fluorine-containing methyl ether is preferably 0.03 or more, more preferably 0.03 or more.
05 or more, more preferably 0.10 or more.

In the above, strictly speaking, the reaction system is a closed space, meaning a reaction vessel or the like.
(4) Moisture Concentration or Moisture Removal Step In the present invention, thermal decomposition may be further carried out under conditions of low moisture concentration, specifically at a moisture concentration of 100 ppm or less. This can also extend the life of the catalyst.

Although it is not particularly limited as long as the effect of extending the life of the catalyst is obtained, thermal decomposition is preferably performed at a water concentration of 50 ppm or less, more preferably 30 ppm or less, and still more preferably 10 ppm or less.

Further, the effect of prolonging the life of the catalyst tends to improve as the water concentration becomes lower under certain conditions. Therefore, it can be considered that the effect of the invention can be obtained by removing the water from the reaction system before or at the same time as the thermal decomposition, regardless of the final water concentration described above. At this time, the method of the present invention can be defined as a method including a step of removing water from the reaction system before or simultaneously with pyrolysis. This step preferably removes 90% or more of water from the reaction system,
More preferably, it is a step of removing 95% or more, still more preferably 99% or more.

In the above, the method of suppressing the water concentration to within the above range or the method of removing water is not particularly limited, but for example, a method of using zeolite as a dehydrating agent, and a method of using rectification to remove water methods and the like. Alternatively, in order to keep the water concentration within the above range, a raw material compound whose water concentration has been adjusted to within a predetermined range (that is, dehydrated) from the beginning may be used.

In the above, strictly speaking, the reaction system is a closed space, meaning a reaction vessel or the like.

(5) Catalyst Regeneration Treatment In the method of the present invention, the catalyst activity may decrease as the reaction time elapses. In this case, the raw material organic matter may be carbonized on the surface of the catalyst. When the catalytic activity is lowered, a gas containing oxygen is circulated through the reaction tube while the catalyst is heated, and the carbon adhering to the catalyst surface reacts with the oxygen to produce a gas such as carbon dioxide or carbon monoxide. The catalyst can be regenerated by removing it in a state. The temperature inside the reaction tube during catalyst regeneration is 200°C to 500°C
about 300° C. to 400° C. is more preferable. As the oxygen-containing gas, it is efficient to use a gas with high purity, but if it contains oxygen, the same effect can be obtained, so it is economically preferable to use air.

The time for regeneration of the catalyst varies depending on the type of catalyst and the duration of use, and may be set to a time during which sufficient catalytic activity can be reproduced.
3. product
By the above-described method, a thermal decomposition reaction of the fluorine-containing methyl ether occurs to obtain the intended fluorinated methane and fluorine-containing compound.

The method for separating fluorinated methane and fluorine-containing compounds contained in the obtained product is not particularly limited, but for example, by cooling the generated gas after the thermal decomposition reaction, fluorinated methane (boiling point -79 ° C.) as a main component and a liquid component consisting of a high boiling point component whose main component is the fluorine-containing compound and may further contain unreacted raw materials. In this case, the cooling temperature may be appropriately set according to the difference in boiling points between the two.

Thereby, a component containing fluorinated methane can be separated as a gas component. The gas components include propene (boiling point −47.7° C.), pentafluoropropene (boiling point −21.0° C.) as impurities.
1°C), propane (boiling point -1.4°C), etc., but these impurities can be easily separated by distillation due to the large difference in boiling point from methane fluoride.

In addition, even when unreacted raw materials and the like are contained in the high-boiling-point component obtained as the liquid component, the unreacted raw materials and the like can be easily separated by distillation.

As a method for selectively obtaining fluorinated methane, the product after the thermal decomposition reaction may be brought into contact with water or an alkaline aqueous solution or the like to dissolve and remove the fluorine-containing compound in the aqueous phase.
Thereby, fluorinated methane can be selectively obtained.

In the above, alcohol may be used instead of water and alkaline aqueous solution. Inexpensive alcohols are preferable in terms of cost, and for example, methanol, ethanol and propanol can be used. Among these, methanol is particularly preferred. Combustion treatment is facilitated by contacting with alcohol to generate an ester.

As a method for selectively obtaining the fluorine-containing compound from the thermal decomposition product, the thermal decomposition product may be directly subjected to a distillation operation to remove fluorinated methane as a column top component. As a result, the fluorine-containing compound can sometimes be obtained as a column bottom component.

EXAMPLES The present invention will be described in more detail below with reference to examples.
<Example 1>
As a catalyst, non-fluorinated γ-alumina (Al ₂ O ₃ ) was used and packed in a metallic tubular reactor. This reaction tube is heated to 150° C., and raw materials 1, 1, 3,
3,3-pentafluoro-2-trifluoromethylpropyl methyl ether (OIME)
was supplied to the reaction tube at a flow rate of 15 ccm/min, and hydrogen fluoride was supplied at a flow rate of 3.9 ccm/min (OIME/HF molar ratio=0.26).

Table 1 shows the change in the amount of OIME treated per 1 g of catalyst and the conversion rate.

Table 1 shows the results of analyzing the outflow gas from the reaction tube by gas chromatography. The numerical values shown in Table 1 are component ratios (%) obtained by multiplying the area ratio of each peak obtained by gas chromatography by a coefficient for correcting the sensitivity of each gas.

Each symbol represents the following compound.
_CH3F : Methane Fluoride:
_{C3H6 :} Propene
HFC-1225zc: _CF2 =CHCF
HFC _{- 236fa} : _CF3CH2CF3
OIME: 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether
fluoride: 3,3,3-trifluoro-2-(trifluoromethyl)propanoyl fluoride In Table 1, the analytical results are obtained by separately analyzing low boiling point components including CH ₃ F and high boiling point components including fluoride. is expressed as a percentage. In Table 1, g-OIME/g-cat. is the amount of OIME treated per gram of catalyst.
conv. represents a conversion rate, respectively.

<Example 2>
In the same manner as in Example 1, the raw material 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether was added at a flow rate of 15 ccm/min, and hydrogen fluoride was added at 1.04 ccm/min.
It was supplied to the reaction tube at a flow rate of ccm/min (OIME/HF molar ratio=0.07). The gas effluent from the reaction tube was analyzed by gas chromatography in the same manner as in Example 1. Table 2 shows the results.
shown in

<Comparative Example 1>
In the same manner as in Example 1, the raw material 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether was fed at a flow rate of 15 ccm/min without supplying hydrogen fluoride to the reaction tube. In addition, only 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether was supplied to the reaction tube. Table 3 shows the results of analyzing the gas discharged from the reaction tube by gas chromatography in the same manner as in Example 1.

As described above, when 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether was entrained with HF (Tables 1 and 2), and when HF was not entrained (Table 3), In comparison, it was found that the decrease in the conversion rate became moderate. From this result, 1,1,3,3,3-
It has been found that the entrainment of HF in pentafluoro-2-trifluoromethylpropyl methyl ether can prolong the life of the catalyst.
<Example 3>
1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether (
OIME) was added with 500 g of molecular sieve 4A and stirred to dehydrate. OI as appropriate
The water content in the ME was measured by Karl Fischer, and dehydration was carried out until the water content decreased to 100 ppm.

As a catalyst, γ-alumina (Al ₂ O ₃ ) A not subjected to fluorination treatment was used, and this was packed in a metal tubular reactor. This reaction tube was heated to 150° C., and OIME dehydrated to 100 ppm was supplied to the reaction tube. Table 1 shows the change in the amount of OIME treated per 1 g of catalyst and the conversion rate.

Table 3 shows the results of gas chromatography analysis of the outflowing gas from the reaction tube. The numerical values shown in Table 3 are component ratios (%) obtained by multiplying the area ratio of each peak obtained by gas chromatography by a coefficient for correcting the sensitivity of each gas. Also, in Table 3, "
"g-OIME/g-cat." is the amount of OIME treated per gram of catalyst, "con
v.” represents the conversion rate, respectively.

In Table 4, the analysis results are obtained by separately analyzing the low boiling point component including CH ₃ F and the high boiling point component including fluoride, and expressing the ratio of these components to the total components as a percentage.

<Example 4>
OIME was dehydrated to 5 ppm in the same manner as in Example 3. This dehydrated OIME was supplied to the reaction tube in the same manner as in Example 3.

Table 5 below shows the results of gas chromatography analysis of the gas discharged from the reaction tube.

<Comparative Example 2>
OIME was dehydrated to 500 ppm in the same manner as in Example 3. This dehydrated OIME was supplied to the reaction tube in the same manner as in Example 3.

Table 6 below shows the results of gas chromatography analysis of the gas discharged from the reaction tube.

From the above results, it was found that the lower the water content in the raw material OIME, the slower the deterioration rate of the catalyst.

Claims (11)

Gas-phase pyrolysis of 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether (OIME) in the presence of hydrogen fluoride in the presence of a catalyst (excluding alumina) A method for producing fluorinated methane by the method. 2. The method of claim 1, wherein thermal decomposition is performed in the presence of said hydrogen fluoride in a volume ratio of 0.03 or more to said OIME 1. 3. The method according to claim 1 or 2, comprising the step of adding hydrogen fluoride to the reaction system before or simultaneously with pyrolysis. 4. The method according to claim 3, wherein said step is a step of adding said hydrogen fluoride to said OIME 1 in a volume ratio of 0.03 or more. The method according to any one of claims 1 to 4, wherein the catalyst is at least one selected from the group consisting of metal oxides, fluorinated metal oxides, and metal fluorides. the catalyst is selected from the group consisting of chromium oxide, titanium oxide, zinc oxide, fluorinated chromium oxide, fluorinated titanium oxide, fluorinated zinc oxide, _AlF3 , _TiF4 , _CrF3 and _ZnF2 The method according to any one of claims 1 to 5, wherein at least one. 7. The method according to claim 5 or 6, wherein the catalyst has a pore volume of 0.5 ml/g or more. The method according to any one of claims 1 to 7, wherein the thermal decomposition reaction has a reaction temperature of 100 to 400°C. The method according to any one of claims 1 to 8, wherein the pressure during the thermal decomposition reaction is 0.05 to 1 MPa. The method according to any one of claims 1 to 9, wherein the pyrolysis is performed at a water concentration of 100 ppm or less. The method according to any one of claims 1 to 9, comprising a step of removing water from the reaction system before or simultaneously with said pyrolysis.