JP2545748B2 - Method for recovering styrene from polystyrene - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a catalyst for recovering styrene from polystyrene, and more particularly to a method for recovering styrene from polystyrene using a basic catalyst.

[0002]

2. Description of the Related Art Polymer materials with high heat resistance and robustness are
It is used in automobiles, home appliances, industrial products such as various furniture and office automation equipment, and is an important basic material for modern society. However, these industrial products have a life span and will be discarded after a certain period of time. In the conventional industrial technology, the emphasis is on how to manufacture a high-performance and long-life product at a low price, and therefore, the technical development regarding the treatment after disposal is not considered at all. In the post-disposal treatment, some of expensive metals were sometimes recovered and reused, but most of the parts made of polymer materials were discarded as they were. Due to its robustness, discarded polymeric materials permanently disperse on the surface of the earth and destroy the Earth's bioenvironmental mechanism.

Recycling of waste polymer materials has been called for from the viewpoint of conservation of the global environment, and in 1991, the Ministry of International Trade and Industry and the Ministry of Transport instituted a "law on promotion of utilization of recycled resources".
Was enacted. Under this law, automobiles and household appliances are classified as Class 1 designated products. Against this background,
Development of recycling technology for waste polymer materials is desired.

Among the recycling techniques, there is also a recycling technique such as a PSP tray, which is directly recovered after use, washed, melted, and molded to be reused as a PSP tray. However, this method cannot be an essential recycling technique because the polymer product is discarded after being recycled several times. Further, in the case of polyolefin such as polyethylene or polypropylene, the polyolefin is zeolite,
By catalytic cracking using a solid acid catalyst such as silica or alumina, it is converted into gasoline-based fuel oil,
The technology to recover is almost established. On the other hand, regarding polystyrene, a method of converting the original styrene and recovering it is being studied. Most of these methods are catalytic cracking methods using a solid acid catalyst such as zeolite, and various solid acid catalysts have been studied. However, in catalytic cracking using a solid acid catalyst, the hydrocarbon decomposing power of the solid acid catalyst is too strong, and unsaturated hydrocarbons have a hydrogenation ability, so that by-products such as benzene, ethylbenzene, and indane derivatives are produced. However, there was a problem that a large amount of styrene was produced and the recovery rate of styrene was lowered.

[0005]

DISCLOSURE OF THE INVENTION The inventors of the present invention have conducted extensive studies to solve the above-mentioned problems, and have continued their studies by paying attention to solid basic catalysts. As a result, solid basic catalysts such as alkali metal oxides or alkaline earth metal oxides such as barium oxide, calcium oxide and potassium oxide, or basic transition metal oxides such as chromium oxide, cobalt oxide and iron oxide. When polystyrene is catalytically decomposed using
It was found that styrene was recovered very efficiently.

Therefore, it is an object of the present invention to provide a method for recovering styrene monomer by catalytic decomposition of waste polystyrene using a basic metal oxide catalyst.

[0007]

The above object can be achieved by the following (1) to (5).

(1) A method for recovering styrene from polystyrene by catalytic cracking using a catalyst, wherein the catalyst is a simple basic metal oxide catalyst, or a mixture of two or more basic metal oxide catalysts. Alternatively, it is a metal oxide catalyst in which these are supported on a heat-resistant oxide powder.

(2) The method according to (1) above, wherein the basic metal oxide catalyst is sodium oxide (N
a ₂ O), O potassium oxide (K _2), and wherein the alkali metal oxide catalyst selected from the group consisting of rubidium oxide (Rb ₂ O).

(3) The method according to (1), wherein the basic metal oxide catalyst is magnesium oxide (Mg
O), calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO), which is an alkaline earth metal oxide catalyst.

(4) In the method described in (1) above, the basic metal oxide catalyst is chromium (III) oxide.
(Cr ₂ O ₃ ), iron oxide (III) (Fe ₂ O ₃ ), copper oxide (II) (CuO), cobalt oxide (II, III) (Co
₃ O ₄ ), and a transition metal oxide catalyst selected from the group consisting of zinc oxide (ZnO).

(5) In the method described in (1) above, the heat-resistant oxide powder is alumina (Al ₂ O ₃ ),
A method selected from silica (SiO ₂ ) and titanium oxide (TiO ₂ ).

The present invention will be described in detail below.

The present invention is a method for catalytically decomposing polystyrene into styrene using a basic oxide catalyst.

The oxide catalyst which can be used in the present invention is a solid basic oxide catalyst, and of the alkali metal oxides, alkaline earth metal oxides and transition metal oxides, basic oxides are preferred. Can be used. Specifically, alkali metal oxide catalysts include sodium oxide (Na ₂ O), potassium oxide (K ₂ O), and rubidium oxide (Rb ₂ O).
And the alkaline earth metal oxide catalysts include magnesium oxide (MgO), calcium oxide (C
aO), strontium oxide (SrO), and barium oxide (BaO). In the present invention, it is preferable to use potassium oxide, magnesium oxide or barium oxide. Transition metal oxide catalysts include chromium oxide (III) (Cr ₂ O ₃ ), iron oxide (III) (Fe
₂ O ₃ ), copper oxide (II) (CuO), cobalt oxide (I
_{I, III) (Co 3 O} 4), and zinc oxide (ZnO), and the like as an example. Of these, potassium oxide and barium oxide are most preferably used in the present invention.

In the present invention, the above basic oxide catalysts may be used alone, or two or more of them may be mixed and used. Moreover, you may use the multiple oxide of the said oxide. Further, the above oxide is alumina (Al
₂ O ₃ ), silica (SiO ₂ ) and titanium oxide (TiO 2
_It may be supported on a heat resistant oxide powder such as ₂ ).

The above-mentioned basic oxide catalyst can be obtained, for example, by dissolving the corresponding nitrate in a solvent such as water, treating with an alkali such as aqueous ammonia, and calcining the obtained precipitate. For example, in the above alkali or alkaline earth metal oxide catalyst, the corresponding metal nitrate is dissolved in water, alkali such as aqueous ammonia is added to generate a precipitate, and the obtained precipitate is washed and dried, and then, for example, 500 ℃
It can be obtained by firing at. Similarly, transition metal oxide catalysts can be prepared. To prepare the double oxide catalyst, it is possible to obtain a metal nitrate having a desired composition ratio, for example, by dissolving it in water or the like, treating it with an alkali such as aqueous ammonia, and calcining the obtained precipitate. it can. Further, in order to support the basic oxide catalyst on the heat-resistant oxide such as silica, alumina, and titanium oxide, as described above, the heat-resistant oxide is mixed with the precipitate obtained by treating the metal nitrate with alkali. It can be prepared by firing. The calcination of the catalyst in which a basic oxide is supported on these double oxides or heat resistant oxides is performed, for example, at 50
Performed at 0 ° C.

The solid basic catalyst that can be used in the method of the present invention may be in the form of a lump, a powder or any other shape, but since it is used for catalytic cracking, it is preferable to use a powdered catalyst. .

In the method of the present invention, the amount of the catalyst used is
It is 5 to 20% by weight, preferably 5 to 15%, and particularly preferably 10% based on the weight of polystyrene.

The polystyrene that can be decomposed in the present invention may be any of those widely used as general polymer materials, and is not limited by the molecular weight and the like.

The catalytic cracking of the present invention can be carried out, for example, using the apparatus shown schematically in FIG. In FIG. 1, for example, a stainless tube is used for the reaction container 1.
The reaction vessel 1 is provided with a metal mesh 3 holding the catalyst 2, a thermocouple 4, and a nitrogen introducing pipe 5. In addition, the reaction vessel is covered with an electric furnace 6 around,
The reaction vessel can be heated. Further, the reaction vessel is equipped with a cooling pipe 7 for condensing the catalytic decomposition product and a trap 8 for collecting the decomposition product liquefied by the cooling pipe. The pipe 9 for moving the decomposition product from the reaction vessel to the cooling pipe is preferably covered with a flexible heater 10 so that the decomposition product is not condensed in the pipe. The electric furnace 6 is used for melting the polystyrene filled in the bottom of the reaction vessel and at the same time setting the reaction temperature of the catalytic decomposition reaction. Further, the present apparatus is equipped with a regulator 11, a flow meter 12, and a gas meter 13 for adjusting and monitoring the flow rate of nitrogen introduced into the reaction vessel. Further, behind the trap 8, a gas meter 14 for measuring the amount of the gas component not condensed in the cooling pipe 7 is installed, and further an analyzer (not shown) for analyzing the gas component is installed. ing. The analyzer is not particularly limited as long as it can analyze a gas component, but a gas chromatograph, GC-M
ASS etc. can be mentioned.

The nitrogen introducing pipe 5, the pipe 9, the reaction vessel 1,
The mesh 3 is preferably made of the same metal material, and for example, stainless steel can be preferably used.

Further, the mesh 3 for holding the catalyst is preferably placed within a range of about 5 mm to 50 mm from the polystyrene filled in the reaction vessel, and particularly preferably at a position of about 10 mm.

The thermocouple 4 is installed near the mesh 3 holding the catalyst powder, and measures and controls the temperature of the catalytic cracking reaction.

The tip of the nitrogen introducing pipe 5 is located near the bottom of the reaction vessel 1 so that nitrogen gas can flow out from the bottom.

Polystyrene is filled in the reactor constructed as described above and catalytically decomposed.

Polystyrene vapor melted by the nitrogen gas stream introduced from the nitrogen inlet pipe 5 is carried to the catalyst layer. As a result, polystyrene comes into contact with the solid basic catalyst, and catalytic decomposition of polystyrene occurs. The flow rate of nitrogen is adjusted so as to efficiently bring the polystyrene vapor into contact with the catalyst layer. In the present invention, the nitrogen flow rate is 5 ml / min to 500 ml / min, preferably 50 ml / min.

The temperature of the catalytic cracking reaction is 250 to 400.
℃, preferably 300-350 ℃. The reaction time is not particularly limited, but is preferably 0.5 hour to 5 hours, and particularly preferably 3 hours.

The decomposed product containing styrene is condensed in the cooling pipe 7 through the pipe 9 and collected in the trap 8. The amount of gas components not condensed in the cooling pipe is measured by the gas meter 14 and analyzed by the analyzer connected to the reactor.

The collected liquid component is quantitatively analyzed by gas chromatography analysis, mass spectrometry, etc., and styrene is recovered from the decomposition mixture by an appropriate means (eg, distillation, column chromatography, etc.).

[0031]

Function: A crushed polystyrene product is filled in the bottom of the reaction vessel, and a stainless steel mesh is placed on the top of the filled material to hold the basic oxidation catalyst. Nitrogen gas is caused to flow out from the narrow tube to the bottom of the reaction vessel, and the thermally-melted polystyrene vapor is carried to the catalyst layer to catalytically decompose polystyrene. The polystyrene vapor catalytically decomposed in the catalyst layer is guided to the cooling pipe, condensed therein, and collected in the trap.

By this, styrene is efficiently recovered from polystyrene.

[0033]

EXAMPLES The present invention will be described in more detail based on the following examples.

Example 1 Preparation of catalyst (1) Preparation of alkali and alkaline earth metal oxide catalysts K ₂ O, MgO, CaO, BaO according to the following method.
And alkaline earth metal oxides were prepared.
Here, the case of preparing K ₂ O will be described as an example.

Potassium nitrate (20 g, 0.2 mol) was dissolved in water (100 ml). To this solution, at 25 ° C, 30
Aqueous ammonia (50 ml) was added, and the mixture was reacted for 1 hour to generate a precipitate. This precipitate was separated by filtration, washed with distilled water, and then dried. The obtained solid was baked at 500 ° C. to obtain 9.5 g of an oxide. This was used as an alkali and alkaline earth metal catalyst.

(2) Preparation of transition metal oxide catalyst Cr ₂ O ₃ , Fe ₂ O ₃ , Co ₃ O ₄ , ZnO, CuO
And the like transition metal oxides were prepared by the same procedure as in (1) above.

(3) Preparation of TiO ₂ Distilled water (100 ml) was added to a solution of titanium isopropoxide (22 g, 0.2 mol) in propanol (50 ml),
It was hydrolyzed by reacting for 3 hours to obtain a gel-like precipitate. The gel precipitate obtained was filtered off and dried. The dried product was baked at 500 ° C. to obtain TiO ₂ . This was used as a catalyst.

(3) Preparation of SiO ₂ and Al ₂ O ₃ These were prepared from silicon or aluminum alkoxides. Hereinafter, the preparation of Al ₂ O ₃ will be described as an example.

Aluminum isopropoxide (17 g,
Hydrolyzed by adding distilled water (100 ml) to a propanol (100 ml) solution of 0.2 mol) and reacting for 10 hours. The gel-like product obtained was filtered off and dried. The obtained dried product was baked at 500 ° C. to obtain Al ₂ O ₃ .

(4) Preparation of mixed catalyst of SiO ₂ and Al ₂ O ₃ Mixed catalyst of SiO ₂ and Al ₂ O ₃ was mixed with respective alkoxides in a predetermined mixing ratio, and the above (3)
Prepared by hydrolysis in the same manner as. For example,
The alkoxides used in making Al ₂ O ₃ / SiO ₂ in a weight ratio of 4: 1 were aluminum isopropoxide (40 g) and ethyl silicate (7 g).

(5) Preparation of ZSM5 Na-type ZSM5 was prepared according to US Pat. No. 3,702,886 of Mobile Oil Company. This was subjected to cation exchange with aqueous ammonia, and the obtained product was calcined at 500 ° C. to obtain an H-ZSM5 catalyst.

(6) Preparation of Al ₂ O ₃ —Fe ₂ O ₃ —K ₂ O Catalyst The alumina powder (14 g) obtained in (4) above was mixed with an aqueous solution of iron nitrate (10 g) and potassium nitrate (4 g) ( Fifty
ml), the water was evaporated to dryness as it was, and the obtained product was calcined at 500 ° C. Thereby, Al ₂ O ₃ -Fe ₂
O ₃ was obtained _{-K 2 O / 70wt% -20wt%} -10wt% of the catalyst.

Example 2 Catalytic decomposition method of polystyrene For catalytic decomposition of polystyrene, an ordinary flow reactor was used. In FIG. 1, as the nitrogen introducing pipe 5 and the pipe 9, a stainless pipe having a diameter of 6 mm and a length of 300 mm was used. For the reaction vessel 1, a stainless tube having a diameter of 25 mm and a length of 300 mm was used. The bottom of the reaction vessel 1 was filled with 10 g of a crushed polystyrene product, and the stainless mesh 3 was placed at a position 10 mm above the filling. 1 in this stainless steel mesh 3
g catalyst powder 2 was retained. A thermocouple 4 was installed on the portion holding the catalyst powder, and the reaction temperature was controlled at 350 ° C. The reaction container 1 is provided with a nitrogen introducing pipe 5 extending from the upper part of the reaction container to the bottom part of the reaction container, and nitrogen gas flows out from the pipe 5. This nitrogen gas passes through the stainless mesh 3 from the bottom of the reaction vessel 1,
It flows to the pipe 9 installed at the upper part of the reaction vessel. Due to this flow of nitrogen gas, the heat-melted polystyrene vapor is carried to the catalyst layer and catalytically decomposed. The flow rate of nitrogen gas is adjusted by the regulator 11, the flow meter 12, and the gas meter 13. The flow rate of nitrogen gas was set to 50 ml / min. The polystyrene vapor catalytically decomposed in the catalyst layer passes through the pipe 9 kept at 200 ° C. by the flexible heater 10 so as not to condense, and is condensed in the cooling pipe 7 in which the water kept at 0 ° C. is circulated, and the trap 8 Captured inside.
The gas components not collected in the cooling pipe pass through the gas meter 14 connected to the trap, and are quantified by gas chromatography (not shown) directly connected to the reactor. Further, the liquid condensed and collected in the cooling pipe 7 is also quantitatively analyzed by a gas chromatograph or a mass spectrometer. The experiment is continued for 3 hours, and after the experiment is completed, the catalyst is taken out and weighed. The carbonaceous component (coke) deposited on the catalyst is quantified by comparison with the weight before the start of the experiment. In addition, since unreacted residue adheres to the vessel wall in the reaction tube after the end of the experiment, elute with hexane to quantify the residue.

The collected liquid component is produced by an appropriate means (eg, distillation, column chromatography, etc.),
Styrene is recovered from the cracked mixture.

By the above method, the weight of the gas component, the liquid component, the amount of coke, and the reaction residue due to the catalytic decomposition of polystyrene was measured, and the ratio with the weight of the polystyrene used for catalytic decomposition (10 g) was obtained as a weight ratio. The performance of each catalyst was compared.

Comparative results are shown in Examples 3 to 7 below.

Example 3 Observation of Catalytic Cracking Oilification According to the experimental method described in Example 2, the catalytic cracking of polystyrene was carried out using the various catalyst powders described in Example 1. The results of measuring the amounts of oil, coke, and reaction residue due to decomposition are shown in Tables 1 to 3. Table 1 shows the results when a solid basic catalyst was used and the results by simple thermal decomposition. The decomposition at 350 ° C. produces almost no gas component. From Table 1, oil content of about 80 wt% can be recovered even by simple thermal decomposition, but the oil content increases when a solid basic catalyst is used, 86.5 wt% for K ₂ O and 93.4 for BaO.
You can see that wt% is reached.

Table 2 shows the results using a solid acidic catalyst. With HZSM5, silica / alumina, etc., the amount of oil produced is reduced to around 79 wt%. The amount of coke adhering to the surface of the catalyst was 7 to 9 wt% for the solid acidic catalyst, but decreased to about 3 wt% for the solid basic catalyst.

Table 3 shows the results when a transition metal oxide was used as a catalyst. Here again, it can be seen that, with basic zinc oxide, iron oxide, copper oxide, etc., the amount of oil produced increases as compared with simple thermal decomposition. With alumina, the amount of oil produced was 78.5 wt% (see Table 1). However, if the surface of this alumina is coated with iron oxide or potassium oxide, the surface becomes basic and the amount of oil produced is 84.9 wt%.
Increased to%. In addition, the amount of coke deposited is 3.3 wt%
Has been reduced to.

[0050]

[Table 1]

[Table 2]

[Table 3] Example 4 Composition of Oil Produced by Catalytic Cracking The oil component produced by catalytic cracking of polystyrene contains styrene, benzene, toluene, ethyl benzene and the like. The results of the analysis of the composition of the oil components produced by various catalysts are shown in Table 4 et al. Table 4 summarizes the results of oil fractionation by the solid basic catalyst and the results of oil fractionation by simple thermal decomposition. Styrene, which was about 70 wt% in simple pyrolysis, was 77 wt% in catalytic cracking with a solid basic catalyst.
It went up and down. Particularly, in the case of BaO catalyst, the ratio of styrene content becomes 94 wt% or more when styrene dimer is included. That is, almost all of the produced oil is recovered as the styrene content. Table 5 shows the composition of oil components catalytically cracked using a solid acidic catalyst. The proportion of styrene in the oil is almost 70 wt% or less, which is smaller than the amount of styrene in the simple pyrolysis oil. Table 6 shows the results when a transition metal oxide was used as a catalyst. 76 wt% for basic zinc oxide, iron oxide, chromium oxide, etc.
From the above, styrene was recovered and it was shown that the selectivity was high.

In general, solid acid catalysts produce a large amount of by-products such as benzene and ethyl-benzene, whereas solid basic catalysts and transition metal oxides hardly produce these by-products.
It was found that a small amount of toluene tends to be a byproduct.

[0052]

[Table 4]

[Table 5]

[Table 6] Example 5 Catalytic Cracking Rate of Polystyrene According to the experiments of Examples 1 and 2, when a solid basic catalyst such as BaO or K ₂ O was used for catalytic cracking of polystyrene, a large amount of oil was produced. It was confirmed that the ratio of styrene content in the oil content was significantly improved. Therefore, the rate of catalytic decomposition of polystyrene by these catalysts was also evaluated by measuring the change over time in the amount of produced oil. The results are shown in FIGS. 2 and 3. FIG. 2 shows the results when a solid basic catalyst was used, and FIG. 3 shows the results when a solid acidic catalyst was used. The results of simple pyrolysis are shown in each figure for comparison. The solid basic catalyst has a higher oil production rate than the simple pyrolysis, and particularly with BaO or K ₂ O, the catalytic cracking reaction is completed in about 20 minutes. On the other hand, when the solid acidic catalyst is used, the catalytic cracking rate is not much different from that in the case of simple thermal cracking, and the amount of oil produced gradually increases even after 90 minutes. From the above, it can be concluded that the solid basic catalyst also has a significantly higher oil production rate by catalytic cracking than the solid acidic catalyst.

Example 6 Recovery rate and recovery rate of styrene from polystyrene In Example 5, the catalytic cracking rate was observed by measuring the amount of oil produced by catalytic cracking over time. Further, by measuring the composition of the oil component collected over time, it is possible to know the change over time in the styrene yield. FIG.
Shows the results when a solid basic catalyst was used. In the simple thermal decomposition, the yield of styrene after 90 minutes was about 50 wt%, whereas with the solid basic catalyst, especially K ₂ O, the yield of 60 wt% or more was obtained about 20 minutes after the start of the reaction. Styrene yield is 71 wt after 60 minutes with BaO catalyst.
Rose to%. That is, 7.1 grams of styrene will be recovered from 10 grams of polystyrene.
Fig. 5 shows the results when a solid acidic catalyst was used.
The styrene yield was inferior to that in the case of simple thermal decomposition, and was about 40 wt% at most.

Example 7 Recovery of Styrene Content (Including Dimer) from Polystyrene In Example 6, experimental results regarding recovery of styrene from polystyrene are described. However, as can be seen from the data described in Example 4, some catalysts may contain a considerable amount of styrene dimer in the produced oil. Then, the component which combined styrene and a styrene dimer was made into styrene content, and the time-dependent recovery of styrene content was measured. FIG. 6 shows the result when a solid basic catalyst was used. In the simple thermal decomposition, the yield of styrene content was about 60 wt% even after 90 minutes, but the solid basic catalyst, especially,
With BaO or K ₂ O, the styrene content yield was close to 80 wt% 20 minutes after the reaction started. With the BaO catalyst, the styrene content increased to 87 wt% after 60 minutes. That is, 8.7 grams of styrene can be recovered from 10 grams of polystyrene. FIG. 7 shows the results when the solid acidic catalyst was used. In this case, the yield of the styrene content was around 45 wt%, which was lower than that in the case of simple thermal decomposition.

[0055]

As described above, when the basic oxidation catalyst or basic transition metal oxide catalyst of the present invention is used, polystyrene is efficiently decomposed into styrene and styrene is recovered from polystyrene in good yield. Further, the method of the present invention is an efficient method for recovering styrene with almost no formation of benzene, ethylbenzene, or indane derivatives. Furthermore,
Since the method of the present invention uses a solid basic catalyst, styrene can be easily recovered, and a simple polystyrene recycling method can be obtained. The method of the present invention can greatly contribute to the recycling of waste polymeric materials.

[Brief description of drawings]

FIG. 1 is a schematic view of an example of a reaction apparatus used for catalytic decomposition of polystyrene of the present invention.

FIG. 2 is a graph showing an oil production rate in catalytic cracking of polystyrene of the present invention for various basic catalysts.

FIG. 3 is a graph showing the oil production rate in the catalytic cracking of polystyrene of the present invention for various acidic catalysts.

FIG. 4 is a graph showing changes over time in styrene yield during catalytic decomposition of polystyrene of the present invention for various basic catalysts.

FIG. 5 is a graph showing changes with time of styrene yield in catalytic decomposition of polystyrene of the present invention for various acidic catalysts.

FIG. 6 is a graph showing the time-dependent changes in the combined yield of styrene and styrene dimer in the catalytic decomposition of polystyrene of the present invention for various basic catalysts.

FIG. 6 is a graph showing changes over time in the combined yield of styrene and styrene dimer in the catalytic decomposition of polystyrene of the present invention for various acidic catalysts.

[Explanation of symbols]

1 ... Reaction vessel, 2 ... Catalyst, 3 ... Mesh, 4 ... Thermocouple,
5 ... Nitrogen introduction pipe, 6 ... Electric furnace, 7 ... Cooling pipe, 8 ... Trap, 9 ... Piping, 10 ... Flexible heater, 11 ... Regulator, 12 ... Flow meter, 13, 14 ... Gas meter.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location // B29K 25:00 C08L 25:06 (56) References JP-A-2-29492 (JP, A ) JP-A-49-93326 (JP, A)

Claims (5)

(57) [Claims] 1. A method for recovering styrene from polystyrene by catalytic cracking using a catalyst, wherein the catalyst is a simple basic metal oxide catalyst, a mixture of two or more basic metal oxide catalysts, or A method comprising a metal oxide catalyst in which these are supported on a heat-resistant oxide powder. 2. The method according to claim 1, wherein the basic metal oxide catalyst is sodium oxide (Na ₂ O),
Potassium oxide (K ₂ O) and rubidium oxide (Rb ₂
O) an alkali metal oxide catalyst selected from the group consisting of O). 3. The method according to claim 1, wherein the basic metal oxide catalyst is magnesium oxide (MgO),
Calcium oxide (CaO), strontium oxide (Sr
O), and an alkaline earth metal oxide catalyst selected from the group consisting of barium oxide (BaO). 4. The method according to claim 1, wherein the basic metal oxide catalyst is chromium (III) oxide (Cr ₂ O).
₃ ), iron (III) oxide (Fe ₂ O ₃ ), copper (II) oxide
(CuO), cobalt oxide (II, III) (Co
₃ O ₄ ), and a transition metal oxide catalyst selected from the group consisting of zinc oxide (ZnO). 5. The method according to claim 1, wherein the refractory oxide powder is alumina (Al ₂ O ₃ ), silica (S
iO ₂ ) and titanium oxide (TiO ₂ ).