NITROGEN-DOPED CATALYST FOR OXIDATIVE COUPLING REACTION OF METHANE, MANUFACTURING METHOD OF NITROGEN-DOPED CATALYST FOR OXIDATIVE COUPLING REACTION OF METHANE THEREBY, AND METHOD FOR OXIDATIVE COUPLING REACTION OF METHANE USING THE SAME

Abstract

A nitrogen-doped catalyst for oxidative coupling of methane, which is a catalyst for obtaining a C2 hydrocarbon product with high yield, and a method for manufacturing the catalyst are provided. An embodiment of the present inventive concept relates to a nitrogen-doped catalyst for oxidative coupling of methane having a silica support; and sodium tungstate and manganese supported on the support.

Claims

1. A method for manufacturing a catalyst for oxidative coupling of methane, the method comprising: a first step of preparing amorphous silica (amorphous SiO.sub.2) as a support; a second step of adding and mixing an aqueous solution of manganese into the silica of the first step and then drying the mixture to produce a catalyst having manganese (Mn) oxide supported on the silica; a third step of adding and mixing a pyridine solution into the catalyst of the second step and then drying the mixture to produce a catalyst having pyridine and manganese supported thereon; a fourth step of adding and mixing an aqueous solution of sodium tungstate into the catalyst of the third step and then drying the mixture to produce a catalyst having sodium tungstate, pyridine, and manganese supported thereon; and a fifth step of calcining the catalyst of the fourth step and obtaining a nitrogen-doped catalyst for oxidative coupling of methane.

2. The method for manufacturing a catalyst for oxidative coupling of methane according to claim 1, wherein the aqueous solution of manganese of the second step is produced by dissolving a manganese precursor in distilled water and mixing the manganese precursor such that the mass ratio of manganese oxide in the catalyst for oxidative coupling of methane is 0.5 wt % to 5 wt %.

3. The method for manufacturing a catalyst for oxidative coupling of methane according to claim 1, wherein the aqueous solution of pyridine of the third step is produced by mixing ethanol and pyridine at a volume ratio of 5 to 7:0.2 to 2.5.

4. The method for manufacturing a catalyst for oxidative coupling of methane according to claim 1, wherein the aqueous solution of sodium tungstate of the fourth step is produced by dissolving a sodium tungstate precursor in distilled water and mixing the sodium tungstate precursor such that the mass ratio of sodium tungstate in the catalyst for oxidative coupling of methane is 2 wt % to 10 wt %.

5. The method for manufacturing a catalyst for oxidative coupling of methane according to claim 1, wherein the drying in the second step to the fourth step is carried out for 12 to 24 hours at 100° C. to 120° C.

6. The method for manufacturing a catalyst for oxidative coupling of methane according to claim 1, wherein the calcination of the fifth step is carried out by raising temperature to 750° C. to 900° C. at a rate of temperature increase of 10° C./min in an air or oxygen atmosphere and then maintaining the temperature for 4 to 6 hours.

7. A nitrogen-doped catalyst for oxidative coupling of methane comprising a silica support; and sodium tungstate and manganese supported on the silica support.

8. The nitrogen-doped catalyst for oxidative coupling of methane according to claim 7, wherein the nitrogen is derived from pyridine.

9. A method for performing an oxidative coupling reaction of methane, the method comprising: a first step of packing the nitrogen-doped catalyst for oxidative coupling of methane in a reactor; and a second step of introducing a mixed gas including methane, oxygen, and an inert gas into the reactor, raising the temperature to the reaction temperature, and performing an oxidative coupling reaction of methane.

10. The method for performing an oxidative coupling reaction of methane according to claim 9, wherein the internal temperature of the reactor is maintained at 600° C. to 800° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a chart showing the XRD analysis results for a Mn/SiO.sub.2 catalyst ((a) in FIG. 1), a Na.sub.2WO.sub.4/SiO.sub.2 catalyst ((b)), a Na.sub.2WO.sub.4/Mn/SiO.sub.2 catalyst ((c)), and a Na.sub.2WO.sub.4-(PYD)-Mn/SiO.sub.2 catalyst ((d)) according to Experimental Example 1.

[0029] FIG. 2A to FIG. 2E are charts showing the results obtained by subjecting catalysts of different component to a pyridine treatment and then performing an XPS analysis according to Experimental Example 2. FIG. 2A shows the O 1s binding energies of various catalysts; FIG. 2B shows the Mn 2p binding energy; FIG. 2C shows the Na 1s binding energy; FIG. 2D shows the W 4f binding energy; and FIG. 2E shows the N 1s spectra of a (PYD*)/SiO.sub.2 catalyst, a (PYD*)-Mn/SiO.sub.2 catalyst (Comparative Example 4), a Na.sub.2WO.sub.4-(PYD*)/SiO.sub.2 catalyst (Comparative Example 5), and a Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 catalyst (Example).

[0030] FIG. 3A shows a TEM image and the results of HAADF and EDS mapping of a Na.sub.2WO.sub.4-Mn/SiO.sub.2 catalyst; and FIG. 3B shows a TEM image and the results of HAADF and EDS mapping of a nitrogen-doped Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 catalyst.

[0031] FIG. 4 is a chart showing the results of a temperature programmed reduction (TPR) analysis of the various catalysts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Hereinafter, the present inventive concept will be described in more detail by way of Examples. These Examples are intended only for illustrating the present inventive concept, and therefore, the scope of the present inventive concept is not construed to be limited by these Examples.

EXAMPLE: PRODUCTION OF NITROGEN-DOPED CATALYST FOR OXIDATIVE COUPLING OF METHANE

[0033] A catalyst for oxidative coupling of methane was produced by doping (adding) nitrogen into a catalyst for oxidative coupling of methane using pyridine. Specific production steps and methods are as follows.

Step (1) Step of Preparing Amorphous Silica

[0034] In order to pulverize amorphous silica (amorphous SiO.sub.2) uniformly and finely for the production of a uniform catalyst, amorphous silica was physically pulverized using a mortar, and a powder having a particle size of several ten nanometers (nm) to several micrometers (μm) was obtained.

Step (2) Step of Producing Mn/SiO.SUB.2 .Catalyst

[0035] Manganese was added to the silica (SiO.sub.2) that had been treated in step (1), and a Mn/SiO.sub.2 catalyst was produced. Specifically, the silica (SiO.sub.2) that had been uniformly pulverized in step (1) was transferred into an alumina crucible. Subsequently, in order to add manganese thereto, an aqueous solution of manganese was prepared by mixing 0.94 g of a manganese precursor (Mn(NO.sub.3).sub.2.4H.sub.2O) with 7.5 ml of distilled water while considering that the silica (SiO.sub.2) pore volume was 0.75 cm.sup.2/g, and then the mixture was subjected to a sonication treatment for 5 minutes to allow the precursor to be thoroughly dissolved in the aqueous solution. The aqueous solution of the manganese precursor produced as described above was added dropwise to 10 g of SiO.sub.2 that had been uniformly pulverized by the process of step (1), and then the mixture was thoroughly mixed with a glass rod. The entire amount of the aqueous solution of manganese was added and mixed by the method described above, and then the resulting mixture was dried for 12 hours in an oven at 120° C. so as to remove water.

Step (3) Step of Producing PYD-Mn/SiO.SUB.2 .Catalyst

[0036] Pyridine (PYD) was added to the Mn/SiO.sub.2 catalyst, and thus a PYD-Mn/SiO.sub.2 catalyst was produced. Specifically, in order to add pyridine to the Mn/SiO.sub.2 catalyst that had been dried in step (2), 0.5 ml of pyridine and 7 ml of ethanol were mixed, and the mixture was subjected to a sonication treatment for 5 minutes. The pyridine solution thus produced was added dropwise to the Mn/SiO.sub.2 catalyst dried in step (2), and the mixture was thoroughly mixed with a glass rod. The entire amount of the pyridine solution was added and mixed by the method described above, and then the resulting mixture was dried for 12 hours in an oven at 120° C. so as to remove water.

Step (4) Step of Adding Na.SUB.2.WO.SUB.4 .to PYD-Mn/SiO.SUB.2 .Catalyst

[0037] In order to add Na.sub.2WO.sub.4 to the pyridine-added Mn/SiO.sub.2 catalyst (PYD-Mn/SiO.sub.2) produced in step (3), 0.61 g of a Na.sub.2WO.sub.4.2H.sub.2O precursor was dissolved in 7.5 ml of distilled water, and then the mixture was subjected to a sonication treatment for 5 minutes to produce an aqueous solution of Na.sub.2WO.sub.4. The aqueous solution of Na.sub.2WO.sub.4 thus produced was added dropwise to the PYD-Mn/SiO.sub.2 catalyst, and then the mixture was mixed by stirring the mixture with a glass rod. The entire amount of the aqueous solution of Na.sub.2WO.sub.4 was added and mixed by the method described above, and then the resulting mixture was dried for 12 hours in an oven at 120° C. so as to remove water.

Step (5) Step of Calcining Nitrogen-Doped Catalyst for Oxidative Coupling of Methane at High Temperature

[0038] The catalyst for oxidative coupling of methane produced by performing the processes from step (1) to step (4) in sequence was subjected to calcining for 5 hours at 800° C. in an air atmosphere, and thus the catalyst was completed (rate of temperature increase 10° C./min).

Comparative Example 1: Production of Catalyst for Oxidative Coupling of Methane that was not Doped with Nitrogen

[0039] A Na.sub.2WO.sub.4/Mn/SiO.sub.2 catalyst that was not doped with nitrogen, which is conventionally used as a catalyst for an oxidative coupling reaction of methane, was produced. Specific production steps and methods are as follows.

Step (1) Step of Producing Mn/SiO.SUB.2 .Catalyst

[0040] Manganese was added to silica (SiO.sub.2), and a Mn/SiO.sub.2 catalyst was produced. Specifically, silica (SiO.sub.2) that had been finely pulverized using a mortar in step (1) described above was transferred into an alumina crucible, and then the following aqueous solution of manganese was produced in order to add manganese to the silica. The aqueous solution of manganese was prepared by mixing 0.94 g of a manganese precursor (Mn(NO.sub.3).sub.2.4H.sub.2O) in 7.5 ml of distilled water while considering that the silica (SiO.sub.2) pore volume was 0.75 cm.sup.2/g, and then the mixture was subjected to a sonication treatment for 5 minutes. The aqueous solution of manganese thus produced was added dropwise to 10 g of SiO.sub.2 of step (1), and then the mixture was stirred with a glass rod. The entire amount of the aqueous solution of manganese was added and mixed by the method described above, and then the resulting mixture was dried for 12 hours in an oven at 120° C. so as to remove water.

Step (2) Step of Adding Na.SUB.2.WO.SUB.4 .to Mn/SiO.SUB.2 .Catalyst

[0041] In order to add Na.sub.2WO.sub.4 to the Mn/SiO.sub.2 catalyst that had been dried in step (2), 0.61 g of a Na.sub.2WO.sub.4.2H.sub.2O precursor was mixed with 7.5 ml of distilled water, and then the mixture was subjected to a sonication treatment for 5 minutes. The aqueous solution of Na.sub.2WO.sub.4 thus produced was added dropwise to the Mn/SiO.sub.2 catalyst, and then the mixture was mixed by stirring with a glass rod. The entire amount of the aqueous solution of Na.sub.2WO.sub.4 thus prepared was added and mixed by the method described above, and then the resulting mixture was dried for 12 hours in an oven at 120° C. in order to remove water.

Step (3) Step of Calcining Na.SUB.2.WO.SUB.4./Mn/SiO.SUB.2 .Catalyst

[0042] The catalyst produced by performing the above-described steps in sequence was subjected to calcination for 5 hours at 800° C. in air (rate of temperature increase 10° C./min).

Comparative Example 2: Production of Mn/SiO.SUB.2 .Catalyst

[0043] A Mn/SiO.sub.2 catalyst was produced in order to compare the respective effects of adding Mn and NaW to a catalyst. Silica (SiO.sub.2) was transferred into an alumina crucible, and then an aqueous solution of manganese was produced in order to add manganese to the silica. The subsequent processes were carried out in the same manner as in step (2) of Comparative Example 1, and after the catalyst was dried for 12 hours in an oven at 120° C. in order to remove water, the catalyst was treated for calcining for 5 hours at 800° C. in air (rate of temperature increase 10° C./min).

Comparative Example 3: Production of Na.SUB.2.WO.SUB.4./SiO.SUB.2 .Catalyst

[0044] A NaW/SiO.sub.2 catalyst was produced in order to compare the respective effects of adding Mn and NaW to a catalyst. For the production of a sodium tungstate precursor solution, 0.61 g of a Na.sub.2WO.sub.4.2H.sub.2O precursor was mixed with 7.5 ml of distilled water, and then the mixture was subjected to a sonication treatment for 5 minutes. The aqueous solution of Na.sub.2WO.sub.4 thus produced was added dropwise to a porous silica (SiO.sub.2) powder, and then the mixture was mixed by stirring with a glass rod. The entire amount of the precursor solution thus prepared was added to be supported on the catalyst, subsequently the resulting mixture was dried for 12 hours in an oven at 120° C. in order to remove water, and a calcination process was carried out for 5 hours at 800° C. in air (rate of temperature increase 10° C./min).

Comparative Example 4: Production of (PYD)-Mn/SiO.SUB.2 .Catalyst

[0045] In order to investigate the effect of adding pyridine on a Mn/SiO.sub.2 catalyst, pyridine was supported on the surface of a Mn/SiO.sub.2 catalyst. The subsequent procedure of the production process was conducted similarly to that of Comparative Example 2, provided that a pyridine treatment process was carried out after drying of the Mn/SiO.sub.2 catalyst. A pyridine solution was prepared by mixing 0.5 ml of pyridine and 7 ml of ethanol and then subjecting the mixture to a sonication treatment for 5 minutes. The pyridine solution thus produced was added dropwise to the dried Mn/SiO.sub.2 catalyst, and then the mixture was mixed by stirring with a glass rod. The entire amount of the pyridine solution thus prepared was added and mixed by the method described above, and then the mixture was dried for 12 hours in an oven at 120° C. so as to remove water. The dried catalyst was subjected to a calcination process for 5 hours at 800° C. in air (rate of temperature increase 10° C./min).

Comparative Example 5: Production of Na.SUB.2.WO.SUB.4.—(PYD)/SiO.SUB.2 .Catalyst

[0046] A catalyst was produced by a method similar to that of Comparative Example 3 described above, provided that in order to add pyridine to a dried Na.sub.2WO.sub.4/SiO.sub.2 catalyst, 0.5 ml of pyridine and 7 ml of ethanol were mixed, and the mixture was subjected to a sonication treatment for 5 minutes. The pyridine solution thus produced was added dropwise to the dried Na.sub.2WO.sub.4/SiO.sub.2 catalyst, and then the mixture was mixed by stirring with a glass rod. The entire amount of the pyridine solution was added and mixed by the method described above, subsequently the mixture was dried for 12 hours in an oven at 120° C. so as to remove water, and then a calcination process was carried out for 5 hours at 800° C. in an air atmosphere (rate of temperature increase 10° C./min).

[0047] The properties, dispersibility, interaction, and the like of the catalysts doped with nitrogen using pyridine, which had been produced in the above-described Examples, were analyzed, and an oxidative coupling reaction of methane was carried out using the nitrogen-added catalysts. Specific experimental methods are as follows.

Test Example 1: Structural Change in Catalyst Caused by Nitrogen Addition

[0048] In order to check the structures of the catalysts produced in Examples and Comparative Examples of the present inventive concept, an XRD analysis was carried out.

[0049] FIG. 1(a) shows the XRD results for a Mn/SiO.sub.2 catalyst. According to FIG. 1(a), broad peaks are seen, and the shape of the peaks is not clearly defined because the catalyst is supported on the surface of SiO.sub.2 in an amorphous state. On the other hand, in the spectrum of FIG. 1(b) showing the XRD results of a Na.sub.2WO.sub.4/SiO.sub.2 catalyst, a peak of α-cristobalite (2θ=22.0°) is recognized together with a Na.sub.2WO.sub.4 peak. This is because since a strong interaction between Na and Si occurs, amorphous silica (amorphous SiO.sub.2) undergoes a phase change to the α-cristobalite phase during the calcination process at 800° C. In the XRD results for a Na.sub.2WO.sub.4/Mn/SiO.sub.2 model catalyst (FIG. 1(c)), the α-cristobalite phase can be recognized similarly to the case of the Na.sub.2WO.sub.4/SiO.sub.2 catalyst, and a peak for manganese supported on the surface of the catalyst can also be clearly recognized. In the case of a Na.sub.2WO.sub.4-(PYD)-Mn/SiO.sub.2 catalyst obtained by subjecting a model catalyst to a pyridine treatment (FIG. 1(d)), unlike the case of the Na.sub.2WO.sub.4/Mn/SiO.sub.2 catalyst, the appearance of a Na.sub.4WO.sub.5 peak can be seen at 2θ=17.5°. From these results, it is verified that a pyridine treatment can cause a change in the oxygen species in the metal oxide used as a catalyst.

Test Example 2: XPS Analysis for Catalyst After Nitrogen Addition

[0050] An XPS analysis was carried out for a silica (SiO.sub.2) support, a pyridine-treated support ((PYD*)-SiO.sub.2), a Mn-supported catalyst (Comparative Example 2; Mn/SiO.sub.2), a pyridine-treated Mn catalyst (Comparative Example 4; (PYD*)-Mn/SiO.sub.2), a NaW catalyst (Comparative Example 3; Na.sub.2WO.sub.4/SiO.sub.2), a pyridine-treated NaW catalyst (Comparative Example 5; Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2), a model catalyst (Comparative Example 1; Na.sub.2WO.sub.4-Mn/SiO.sub.2), and a pyridine-treated model catalyst (Example; Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2). The catalysts were treated in the same manner as in the production process for the model catalyst, that is, after supporting the catalytic substance, calcination was carried out for 5 hours at 800° C. in an air atmosphere, and then the analysis was performed.

[0051] FIG. 2A shows the O 1s binding energies of the various catalysts. The O 1s spectrum of untreated SiO.sub.2 appeared at 533.2 eV, and the O 1s spectrum of (PYD*)-SiO.sub.2 was also obtained similarly at 533.2 eV. The Si 2p binding energies of the Mn/SiO.sub.2 catalyst and the (PYD*)-Mn/SiO.sub.2 catalyst also appeared similarly at 533.2 eV. These results suggest that a pyridine treatment on an amorphous silica (amorphous SiO.sub.2) support did not affect any change in the O 1s binding energy. On the other hand, the O 1s spectrum of the Na.sub.2WO.sub.4/SiO.sub.2 catalyst appeared at 532.3 eV, and when this was compared with untreated SiO.sub.2 (533.2 eV), a peak shift of −0.9 eV appeared. In this regard, it can be said that when Na.sub.2WO.sub.4 is added to amorphous silica (amorphous SiO.sub.2), amorphous silica becomes crystalline with the α-cristobalite phase due to a strong interaction between Na and Si, and thus a shift in the O 1s binding energy occurs.

[0052] On the other hand, the O 1s spectrum of the pyridine-treated Na.sub.2WO.sub.4-(PYD*)/SiO.sub.2 catalyst is at 532.7 eV, and when this was compared with the Na.sub.2WO.sub.4/SiO.sub.2 catalyst that had not been treated with pyridine, a peak shift of +0.4 eV occurs. This is considered to be because while pyridine adsorbs to the OH— group at the SiO.sub.2 surface, and the catalyst undergoes a heat treatment process, amorphous SiO.sub.2 undergoes a phase change to the α-cristobalite phase due to an interaction between Na and Si, and the added pyridine has additional influence.

[0053] The O 1s spectrum of Na.sub.2WO.sub.4/Mn/SiO.sub.2 is 532.2 eV, and when this is compared with simple SiO.sub.2 (533.2 eV), a low shift of −1.0 eV appears. On the other hand, the O 1s spectrum of the pyridine-treated Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 catalyst is 532.5 eV, and when this is compared with the O 1s spectrum of the Na.sub.2WO.sub.4/Mn/SiO.sub.2 catalyst (532.2 eV), a peak shift of +0.3 eV occurs. These results suggest that as nitrogen is doped by a pyridine treatment, a change occurs in the O 1s binding energy.

[0054] FIG. 2B shows the Mn 2p binding energy of Mn/SiO.sub.2 (Comparative Example 2), (PYD*)-Mn/SiO.sub.2 (Comparative Example 4), Na.sub.2WO.sub.4-Mn/SiO.sub.2 (Comparative Example 1), and Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 (Example). FIG. 2C shows the Na 1s binding energy of Na.sub.2WO.sub.4/SiO.sub.2 (Comparative Example 3), Na.sub.2WO.sub.4-(PYD*)/SiO.sub.2 (Comparative Example 5), Na.sub.2WO.sub.4-Mn/SiO.sub.2 (Comparative Example 1), and Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 (Example), and FIG. 2D shows the W 4f binding energy.

[0055] When the intensities of the Mn 2p spectra are compared in FIG. 2B, it can be seen that the intensity of the Mn 2p spectrum of the (PYD*)-Mn/SiO.sub.2 catalyst is lower than that of the Mn/SiO.sub.2 catalyst. However, in the cases of the catalysts having Na.sub.2WO.sub.4 supported thereon in FIG. 2B to FIG. 2D, it can be verified that the SiO.sub.2 support acquires the α-cristobalite phase, and the intensities of Mn 2p (FIG. 2B), Na 1s (FIG. 2C), and W 4f (FIG. 2D) are all commonly increased by a pyridine treatment. This can be considered to be because the degrees of contribution of Mn, Na, and W on the catalyst surface is increased by a pyridine treatment. Since Mn, Na, W constitute a catalytically active phase in the oxidative coupling reaction of methane, when the densities of these elements at the surface are high, the elements serve as important factors for increasing the methane conversion ratio and the C2 selectivity in the oxidative coupling reaction of methane. Therefore, it is considered that the pyridine pretreatment process exerts an advantageous effect on the oxidative coupling reaction of methane.

[0056] FIG. 2E shows the N 1s spectra of (PYD*)/SiO.sub.2, (PYD*)-Mn/SiO.sub.2 (Comparative Example 4), Na.sub.2WO.sub.4-(PYD*)/SiO.sub.2 (Comparative Example 5), and Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 (Example) catalysts. The results of an XPS analysis shows that the N 1s peaks of the (PYD*)/SiO.sub.2 and (PYD*)-Mn/SiO.sub.2 catalyst do not appear. However, in the cases of the Na.sub.2WO.sub.4-(PYD*)/SiO.sub.2 and Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 catalysts, it can be seen that the N 1s peak appears at 399.9 eV. This is considered to be because as pyridine is adsorbed to the amorphous silica (SiO.sub.2) surface, and as the amorphous silica undergoes a phase change into the α-cristobalite phase due to an interaction between Na and Si, the nitrogen at the SiO.sub.2 surface is substituted into the interior of the SiO.sub.2 lattice.

Test Example 3: TEM and STEM/EDS Analysis of Catalyst After Nitrogen Addition

[0057] In order to check the surface state and the particle size of the catalyst and the dispersibility of the catalyst particles, TEM and STEM/EDS analyses were carried out. FIG. 3A shows a TEM image, HAADF, and EDS mapping of the Na.sub.2WO.sub.4-Mn/SiO.sub.2 catalyst, and FIG. 3B shows a TEM image, HAADF, and EDS mapping of the pyridine-doped Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 catalyst. When the results of FIG. 3B are compared with the results of FIG. 3A, it is implied that the concentration at the surface of the catalyst particles increased significantly. It could be verified from these results that the surface concentration of the metal oxides of the existing catalyst for oxidative coupling of methane is increased by a pyridine treatment, and the activity and long-term stability of the catalyst are increased.

Test Example 4: TPR Analysis of Catalyst After Nitrogen Addition

[0058] FIG. 4 shows the results of a temperature programmed reduction (TPR) analysis of the Na.sub.2WO.sub.4/Mn/SiO.sub.2 and Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 catalysts. From the TPR analysis results of the Na.sub.2WO.sub.4-Mn/SiO.sub.2 catalyst (FIG. 4a), it was confirmed that a reduction peak appeared at 719° C. However, in the case of the pyridine-treated Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 catalyst (FIG. 4b), it was confirmed that a reduction peak appeared at 709° C. It can be considered that these results were obtained because the pyridine-treated Na.sub.2WO.sub.4-(PYD*)-Mn/SiO.sub.2 exhibited a higher degree of contribution and superior interaction of metal oxides as compared to the model catalyst Na.sub.2WO.sub.4-Mn-SiO.sub.2.