Non-noble metal catalyst, preparation method thereof and application of non-noble metal catalyst in preparation of acetic acid through methane oxidation

Abstract

The invention discloses a non-noble metal catalyst, a preparation method thereof and application of the non-noble metal catalyst in preparation of acetic acid through methane oxidation, and belongs to the field of direct conversion and utilization of methane. The active metal of the catalyst is transition non-noble metal MnOx, and the carrier is an MFI type molecular sieve (ZSM-5). The catalyst can directly react methane and oxygen at low temperature to prepare acetic acid without adding a co-reducing agent and carbonylation raw materials (such as H2 and CO). The nanometer MnOx catalyst has the advantages of mild reaction conditions, high acetic acid selectivity and the like, the acetic acid selectivity can reach 81.8%, and the nanometer MnOx catalyst shows good recycling stability.

Description

Technical Field

[0001] This application relates to a non-precious metal catalyst and its preparation method, and its application in the oxidation of methane to acetic acid, belonging to the field of direct methane conversion and utilization. Background Technology

[0002] Methane, a major component of natural gas, shale gas, and methane hydrate, is abundant. The direct conversion of methane into easily transportable liquid oxygenates has attracted widespread attention [Appl. Catal. B Envir. 2016, 198, 525 547; Chem. Rev. 2020, 120, 1438 1511; Chin. J. Chem. Eng. 2021, 38, 18 29]. However, the direct and selective oxidation of methane to oxygen-containing compounds using molecular oxygen is challenging. Due to the nonpolarity and high symmetry of CH4, the CH bond is difficult to activate; furthermore, hyperoxidation via combustion to produce CO2 and H2O is thermodynamically preferred. Therefore, the activation conversion of methane under mild conditions has long been considered the "holy grail" reaction in chemical research [Chem. 2019, 5, 2296 2325; Chin. J. Chem. Eng. 2021, 38, 18 29].

[0003] Currently, there are two main pathways for methane conversion: indirect and direct conversion. For the indirect conversion pathway, methane is first reformed at high temperatures (700–1100°C) using steam to produce syngas, which is then used for Fischer-Tropsch synthesis to obtain basic chemicals. This conversion route is lengthy, energy-intensive, and emits large amounts of the greenhouse gas CO2, resulting in significant economic costs and environmental pressure [Appl. Catal. A Gen. 1999, 186, 3 12; Science. 2017, 356, 523 527; CatalToday. 2017, 285, 147 158]. The direct methane conversion pathway avoids the energy-intensive steam reforming process, but still faces challenges such as high energy input and uncontrollable product selectivity (prone to over-oxidation). Early studies on the direct conversion of methane focused on chemical looping gas-solid phase reactions at moderate temperatures (400-500 °C) [Acc. Chem. Res, 2017, 50, 418-425]. Oxidation products are easily oxidized by oxidants such as N₂O or O₂, and the process is complex. Conversely, a gas-solid-liquid phase model using water as a solvent is more efficient because it releases oxygen-containing compounds at lower reaction temperatures (<100 °C), but typically requires expensive hydrogen peroxide as an oxidant. Catalysts using oxygen as an oxidant under mild conditions (<150 °C) have been developed [Nature, 2017, 551, 605-608; Angew. Chem. Int. Ed. 2022, 61, e202204116], but require the addition of CO or H₂ as a reducing agent, complex, or reactant to assist oxygen in the oxidation of methane, and necessitate noble metals such as Rh and Ir as active components. Recent research using O2 as the sole oxidant has attracted attention, but the active metal components used are still mainly noble metals, such as Au-based catalysts [Nat. Catal. 2022, 5, 45-54]. To date, no reports have been published on the development of a highly efficient catalyst system for methane conversion using only oxygen and non-noble metals as active components.

[0004] Currently, the main products of thermocatalytic methane conversion are C1 oxygen-containing compounds. Regarding carbon-carbon coupling products such as ethanol, acetic acid, and acetaldehyde, etc., these are C1 oxygen-containing compounds. 2+Reports on oxygen-containing products are scarce, and the challenge lies in simultaneously activating the C-H bonds and coupling the carbon-carbon bonds in methane. This typically requires a strong oxidant such as hydrogen peroxide, and necessitates the introduction of CO as a carbonylation feedstock for carbon-carbon coupling [Angew. Chem. Int. Ed. 2022, 61, e202204116]. This faces challenges such as expensive oxidants and competitive oxidation by CO. Utilizing only oxygen as the oxidant is more economical and can mitigate over-oxidation to some extent. Designing and controlling the synthesis of catalysts with multi-metal active sites to achieve the low-temperature conversion of methane to high-value-added C2 products using only oxygen is of great significance. Summary of the Invention

[0005] Based on this, this application provides a nanoscale dispersed transition metal catalyst and its application in the low-temperature direct selective oxidation of methane and oxygen to acetic acid. The active metal of the catalyst is the transition non-noble metal MnOx, and the support is an MFI-type molecular sieve (ZSM-5). The catalyst can directly react methane and oxygen to produce acetic acid at low temperatures without the need for a reducing agent or carbonylation feedstock (e.g., H2, CO). The nanoscale MnOx of this invention... x The catalyst has the advantages of mild reaction conditions and high selectivity for acetic acid, with an acetic acid selectivity of up to 81.8%, and exhibits good stability for recycling.

[0006] According to a first aspect of this application, a non-noble metal catalyst is provided. The Mn catalyst supported on an MFI-type molecular sieve (ZSM-5) has Mn metal present in the form of nanoparticles.

[0007] A non-precious metal catalyst, the non-precious metal catalyst comprising a ZSM-5 support and metal Mn;

[0008] Among them, metallic Mn exists in the form of MnOx nanoparticles and is dispersed on the ZSM-5 support;

[0009] The mass content of metallic Mn is 0.1 to 4 wt% of the catalyst mass.

[0010] Optionally, the size of the MnOx nanoparticles is 1–5 nm.

[0011] Optionally, the silicon-aluminum molecular ratio of the ZSM-5 is 21 to 180.

[0012] Preferably, the silicon-aluminum molecular ratio of the ZSM-5 is 25 to 38.

[0013] Optionally, the mass content of metallic Mn is 0.25 to 2.2 wt% of the catalyst mass.

[0014] Optionally, the non-precious metal catalyst is composed of a ZSM-5 support and metal Mn, wherein the metal Mn exists in the form of MnOx nanoparticles and is dispersed on the ZSM-5 support.

[0015] Optionally, the ZSM-5 support is 96 to 99.9 wt% of the catalyst mass, preferably 97.8 to 99.75 wt%.

[0016] According to a second aspect of this application, a method for preparing a non-precious metal catalyst is provided.

[0017] The preparation method of the above-mentioned non-noble metal catalyst includes:

[0018] A solution containing a manganese precursor was added dropwise to a ZSM-5 support, mixed thoroughly, allowed to stand, dried, and calcined to obtain the non-precious metal catalyst.

[0019] Optionally, the manganese precursor is selected from at least one of manganese chloride, manganese nitrate, and manganese acetate. Manganese chloride is preferred.

[0020] Optionally, the calcination is carried out in an oxygen-containing atmosphere at a temperature of 400–550°C for 2–4 hours.

[0021] Preferably, the roasting is carried out in an air atmosphere.

[0022] In a preferred embodiment, the non-precious metal catalyst is prepared by impregnation, in which a manganese precursor solution is slowly dripped into a ZSM-5 support and allowed to stand at room temperature to 100°C for 2 to 48 hours; the crude product is dried to remove solvent and impurity molecules from the pores; and then calcined at 400 to 550°C to obtain the non-precious metal catalyst, denoted as MnOx / ZSM-5 catalyst. The feed amount is calculated based on the composition of the non-precious metal catalyst described above.

[0023] According to a third aspect of this application, an application of a non-precious metal catalyst is provided. The main products of this catalyst in the low-temperature direct selective oxidation of methane are methanol (CH3OH) and acetic acid (CH3COOH). The optimal catalyst of this invention can convert methane to acetic acid with a selectivity of 81.8% under mild conditions and exhibits good cycle stability.

[0024] The above-mentioned application of the non-precious metal catalyst in the oxidation of methane to acetic acid involves reacting a mixture containing methane, oxygen, the non-precious metal catalyst, and water in a closed container to obtain acetic acid.

[0025] Optionally, the amount of non-precious metal catalyst used is 1×10⁻⁶ of the reaction solution. 4 ~2.5×10 3 wt%.

[0026] Preferably, the amount of non-precious metal catalyst used is 1×10⁻⁶ of the reaction solution. 3 ~2.5×10 3 wt%.

[0027] Optionally, the volume ratio of methane to oxygen is 6:1 to 30:1.

[0028] Optionally, the reaction temperature is 25–180℃; the reaction pressure is 0.1–5.0 MPa; and the reaction time is 0.1–10 h.

[0029] Preferably, the reaction temperature is 150–180°C; the reaction pressure is 1.1–3.5 MPa; and the reaction time is 0.25–5 h.

[0030] The non-precious metal catalyst described above enables the direct and selective oxidation of methane and oxygen to acetic acid and methanol at low temperature. The reaction is carried out in a high-pressure autoclave reactor under the following conditions: a high-pressure autoclave with an effective volume of 30–80 mL; and a catalyst dosage of 1 × 10⁻⁶ m³ / s of the added reaction solution. 4 ~2.5×10 3 wt%, preferably 1×10 3 ~2.5×10 3 wt%; temperature 25–180℃, preferably 150–180℃; reaction pressure 0.1–5.0 MPa, preferably 1.1–3.5 MPa; reaction time 0.1–10 h, preferably 0.25–5 h; reactants are a mixture of methane and oxygen, wherein the volume ratio of methane to oxygen is 6:1–30:1; water volume is 10–50 mL, and preferably the amount of 2.2 wt% MnOx / ZSM-5 catalyst is 2.5 × 10⁻⁶ of the added reaction solution. 3 Under reaction conditions of wt%, 180℃, 3MPaCH4, 0.5MPa O2, 1200rpm, and 3h, 8.1μmol / g of acetic acid can be obtained while maintaining 81.8% selectivity. cat The yield was good, and it also showed good cycling stability in the application test.

[0031] The beneficial effects that this application can produce include:

[0032] 1) The molecular sieve-supported MnOx nanoparticle material prepared by the present invention has the advantages of well-defined structure and uniform distribution of active components.

[0033] 2) This invention establishes a clear structure-activity relationship. The reaction pathway depends on the size of the metal oxide loaded on the surface of the molecular sieve. Multi-metal sites (clusters, nanoparticles) can simultaneously achieve the activation of methane and the coupling of carbon-carbon bonds, while the carbon-carbon coupling pathway of isolated metal sites is blocked.

[0034] The catalyst preparation method of this invention is simple, efficient, and exhibits excellent performance. The high-pressure reactor system used in its application is easy to operate, and the reaction conditions are relatively mild. Specifically, the 2.2 wt% MnOx / ZSM-5 catalyst can oxidize methane using only oxygen, achieving a yield of 8.1 μmol / g while maintaining 81.8% acetic acid selectivity. cat The catalyst exhibits high yield and good stability. The catalyst preparation method is simple and the reaction is economical. Attached Figure Description

[0035] Figure 1 The X-ray diffraction (XRD) spectra of the samples and supports of Examples 1, 4 and Comparative Examples 2-3 are shown.

[0036] Figure 2 Transmission electron microscopy (TEM) image and EDS mapping image of the 2.2 wt% MnOx / ZSM-5 sample prepared in Example 1.

[0037] Figure 3 Transmission electron microscopy (TEM) image and EDS mapping image of the 0.29 wt% Mn1 / ZSM-5 sample prepared in Example 3.

[0038] Figure 4 The image shows a transmission electron microscope (TEM) image of the 4 wt% Mn / ZSM-5 sample prepared in Example 4.

[0039] Figure 5 The graphs show the low-temperature direct selective oxidation performance of methane by Mn / ZSM-5 catalysts with different loadings prepared in Examples 1-4 and samples prepared in Comparative Examples 1-3.

[0040] Figure 6 The cyclic stability test result is shown for the 2.2 wt% MnOx / ZSM-5 catalyst prepared in Example 1. (Example 6)

[0041] Figure 7 The image shows the in-situ diffuse reflectance infrared spectrum of the 2.2 wt% MnOx / ZSM-5 catalyst prepared in Example 1. (Example 7) Detailed Implementation

[0042] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0043] Unless otherwise specified, all raw materials used in the embodiments of this application were purchased through commercial channels.

[0044] Unless otherwise specified, all test methods are standard and all instrument settings are those recommended by the manufacturer.

[0045] The silicon-aluminum molecular ratio of ZSM-5 molecular sieve is 25.

[0046] The analysis method in the embodiments of this application is as follows:

[0047] The instrument used for X-ray diffraction (XRD) testing was a PANalytical X'Pert-Pro-1 powder X-ray diffractometer;

[0048] The instrument used for testing the transmission electron microscope (TEM) images and EDS mapping images is a JEM-2100F.

[0049] The instrument used for the ICP test was Thermal Scientific-IRIS Intrepid II XSP;

[0050] The instrument used for in-situ diffuse reflectance infrared spectroscopy was a Bruker VERTEX 70.

[0051] Example 1: Preparation of 2.2 wt% MnOx / ZSM-5

[0052] 7.2 g of MnCl2·4H2O was dissolved in deionized water to prepare a 50 mL solution. 500 μL of this solution was taken and mixed with 170 μL of deionized water to obtain an impregnation solution. The impregnation solution was added dropwise to 1 g of ZSM-5 molecular sieve support, followed by sonication and stirring. The mixture was allowed to stand overnight at room temperature, then placed at 80 °C. The dried sample was placed in a muffle furnace and calcined at 400 °C for 4 hours in static air to obtain the 2.2 wt% MnOx / ZSM-5 catalyst. ICP analysis showed that the weight content of metallic Mn was 2.2 wt%, with the remainder being ZSM-5 support.

[0053] Example 2: Preparation of 1.1 wt% Mn / ZSM-5

[0054] The catalyst preparation method in this embodiment is the same as in Example 1, but the amount of manganese chloride (MnCl2·4H2O) solution used is 250 μL. The weight content of metallic Mn was determined by ICP to be 1.1 wt%, with the remainder being ZSM-5 support.

[0055] Example 3: Preparation of 0.29wt% Mn1 / ZSM-5

[0056] The catalyst preparation method in this embodiment is the same as in Example 1, but the amount of manganese chloride (MnCl2·4H2O) solution used is 62.5 μL. The weight content of metallic Mn was determined by ICP to be 0.29 wt%, with the remainder being ZSM-5 support.

[0057] Example 4: Preparation of 4wt% Mn / ZSM-5

[0058] The catalyst preparation method in this embodiment is the same as in Example 1, but the amount of manganese chloride (MnCl2·4H2O) used is changed to 0.144 g, which is dissolved to form a 670 μL solution. ICP analysis showed that the weight content of metallic Mn was 4 wt%, with the remainder being a ZSM-5 support.

[0059] Example 5: Activity Evaluation Test of Low-Temperature Direct Oxidation Catalyst for Methane

[0060] The catalysts prepared in the embodiments and comparative examples of this invention were all tested using this method. The reaction was carried out in a high-pressure reactor lined with polytetrafluoroethylene. First, 50 mg of catalyst and 20 mL of water were added to a reactor with a liner volume of 38 mL at room temperature. After purging with 0.5 MPa O2 five times, the pressure was increased to 0.5 MPa, then CH4 was added, and the pressure was increased to 3.5 MPa. The temperature was then raised to 180 °C under vigorous stirring at 1200 rpm, and the reaction was stopped after 3 h. The temperature was then lowered to 10 °C under ice-water bath conditions, and the gaseous products of the reaction were collected using a gas bag, and the liquid products of the reaction were filtered out. Quantitative analysis was performed by gas chromatography in a nickel-containing conversion furnace and high-resolution liquid NMR. Among them, the acetic acid selectivity = 2*n(acetic acid) / n(methanol + acetic acid*2 + carbon dioxide) × 100%, and the oxygen-containing compound yield = n(oxygen-containing compound) / m(catalyst) × reaction time.

[0061] Example 6: Stability test of 2.2% wt% MnOx / ZSM-5 catalyst

[0062] In this embodiment, the activity evaluation test conditions are the same as those in Example 5. The difference is that the catalyst used in each test is the same as that used in the previous reaction, and multiple parallel tests are performed for each test to ensure that the quality of the catalyst used in the next test is consistent. A total of 4 cycles of repeated tests are performed.

[0063] Example 7: In-situ diffuse reflectance infrared radiation on 2.2% wt% MnOx / ZSM-5 catalyst

[0064] The sample was washed with a He stream at 380 °C for 2 h, and the background spectrum was recorded after cooling to 150 °C. The sample was exposed to a gas stream of 5 vol.% CH4 / He and 5 vol% O2 / He. Water was introduced by gas bubbling at room temperature, and the reaction spectrum was recorded every 30 minutes during the experiment. After 2 h of reaction, the sample was purged with He for 20 min, and the spectrum was recorded again.

[0065] Comparative Example 1: Preparation of MnO catalyst

[0066] 2 g of MnCl2·4H2O was dissolved in deionized water to prepare a 50 mL solution A; 0.8 g of ammonium bicarbonate was dissolved in deionized water to prepare a 20 mL solution as precipitant B. Precipitant B was added dropwise to solution A at room temperature. After precipitation, the suspension was aged for 30 min, washed, filtered, and dried overnight at 80 °C. This yielded MnCO3. MnO was then calcined at 400 °C for 6 h at a volume ratio of 20% H2 / Ar.

[0067] Comparative Example 2: Preparation of Mn2O3 catalyst

[0068] In this embodiment, the catalyst is prepared in the same way as in Comparative Example 1, but MnCO3 is calcined in air at 520°C for 6 hours, while the rest remains unchanged, to obtain Mn2O3.

[0069] Comparative Example 3: Preparation of MnO2 catalyst

[0070] In this embodiment, the catalyst is prepared in the same way as in Comparative Example 1, but MnCO3 is calcined in air at 400°C for 6 hours, while the rest remains unchanged, to obtain MnO2.

[0071] Comparative Example 4: Preparation of 2wt% Fe / ZSM-5 catalyst

[0072] In this embodiment, the catalyst is prepared in the same way as in Example 1, but the metal source is replaced with FeCl3·6H2O, while the rest remains unchanged, to obtain the 2wt% Fe / ZSM-5.

[0073] Comparative Example 5: Preparation of 2wt% Co / ZSM-5 catalyst

[0074] In this embodiment, the catalyst is prepared in the same way as in Example 1, but the metal source is replaced with CoCl3·6H2O, while the rest remains unchanged, to obtain the 2wt% Co / ZSM-5.

[0075] Comparative Example 6: Preparation of 2wt% Zn / ZSM-5 catalyst

[0076] The catalyst in this embodiment is prepared in the same way as in Example 1, but the metal source is replaced with ZnCl3·6H2O, while the rest remains unchanged, to obtain the 2wt% Zn / ZSM-5.

[0077] result

[0078] like Figure 1 The XRD patterns of the catalysts prepared in Examples 1 and 4 and Comparative Examples 2-3 of this invention are shown. The results show that loading Mn does not change the crystal form of the support ZSM-5. No diffraction peaks of metal Mn or MnOx were detected in the catalysts prepared in Examples 1 and 4, indicating that the metal is highly dispersed.

[0079] like Figure 2As shown, the transmission electron microscope (TEM) image and EDS mapping image of the catalyst prepared in Example 1 of this invention show that Mn in the 2.2 wt% MnOx / ZSM-5 catalyst is uniformly dispersed in the form of nanoparticles on the ZSM-5 surface with a statistical size of 3 nm.

[0080] like Figure 3 As shown, the transmission electron microscope (TEM) image and EDS mapping image of the catalyst prepared in Example 3 of this invention show that there are no metal clusters and nanoparticles in the 0.29wt% Mn1 / ZSM-5 catalyst.

[0081] like Figure 4 The image shows a transmission electron microscope (TEM) image of the catalyst prepared in Example 4 of this invention. The results show that the 4wt% Mn / ZSM-5 catalyst exhibits severe metal agglomeration and presents as blocky MnOx.

[0082] like Figure 5 The figures show the activity evaluation test results of the catalysts prepared in Examples 1-4 and Comparative Examples 1-3 of this invention for the low-temperature direct oxidation of methane. The results indicate that the 2.2 wt% MnOx / ZSM-5 catalyst can utilize oxygen to oxidize methane to produce acetic acid and methanol. The acetic acid selectivity is as high as 81.8%, and the yield and selectivity of acetic acid decrease with decreasing metal loading. No acetic acid products were detected in the monodisperse 0.29 wt% Mn1 / ZSM-5, only C1 products (CH3OH, HCOOH) were produced. No acetic acid products were detected on the severely agglomerated 4 wt% Mn / ZSM-5 and on the single-phase MnOx.

[0083] like Figure 6 As shown in the figure, the catalyst stability test diagram in Example 6 of the present invention shows that the 2.2wt% MnOx / ZSM-5 catalyst prepared in Example 1 did not show a significant decrease in acetic acid yield and selectivity in 4 cycles of reuse test, indicating that the prepared catalyst has good stability.

[0084] like Figure 7 As shown in the in-situ diffuse reflectance infrared spectrum of Example 7 of this invention, the results indicate that signals of intermediates *CH3, *CH2, and ethylene (*C2H4) were detected on the 2.2wt% MnOx / ZSM-5 catalyst. This indicates that methane was activated on the catalyst to produce methyl and methylene groups, which further coupled to produce ethylene intermediates, and then oxidized to produce acetic acid products.

[0085] Table 1. Evaluation results of catalytic reaction performance of samples from each embodiment and comparative example.

[0086]

[0087] Note: The reaction conditions were: 3 MPa methane, 0.5 MPa oxygen, 20 mL aqueous solution, and reaction at 180 °C for 3 h.

[0088] Table 1 shows the activity evaluation results of the catalysts prepared in Examples 1-4 and Comparative Example 16 for the low-temperature direct oxidation of methane. Sequence 14 shows that the yield and selectivity of acetic acid exhibit a volcano-shaped curve with increasing metal loading. Sequences 4-7 show that no acetic acid products were produced under higher loading and single-phase MnO. Combined with electron microscopy characterization, this indicates that the MnOx nanoparticles on the ZSM-5 surface can simultaneously activate methane and undergo C-C coupling, while the carbon-carbon coupling pathway is blocked on monodisperse Mn. Furthermore, no acetic acid products were produced on 2wt% Fe / ZSM-5, 2wt% Co / ZSM-5, and 2wt% Zn / ZSM-5; only C1 products were produced, indicating that the combination of the ZSM-5 support and MnOx nanoparticles has unique properties.

[0089] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A non-noble metal catalyst characterized in that, The non-noble metal catalyst comprises a ZSM-5 carrier and metal Mn; The metal Mn exists in the form of MnOx nanoparticles and is dispersed on the ZSM-5 carrier. The mass content of the metal Mn is 0.1-4 wt% of the mass of the catalyst.

2. The non-noble metal catalyst of claim 1, wherein, The size of the MnOx nanoparticles is 1-5 nm.

3. The non-noble metal catalyst of claim 1, wherein, The ZSM-5 has a silicon-aluminum molecular ratio of 21-180. Preferably, the ZSM-5 has a silicon-aluminum molecular ratio of 25-38.

4. The non-noble metal catalyst of claim 1, wherein, The mass content of the metal Mn is 0.25-2.2 wt% of the mass of the catalyst.

5. The method for producing a non-noble metal catalyst according to any one of claims 1 to 4, characterized by, The preparation method comprises: The solution containing a manganese precursor is dropped into the ZSM-5 carrier, mixed uniformly, left to stand, dried, and calcined to obtain the non-noble metal catalyst.

6. The production method according to claim 5, wherein The manganese precursor is at least one selected from manganese chloride, manganese nitrate, and manganese acetate. Preferably, the calcination is performed in an oxygen-containing atmosphere, the calcination temperature is 400-550 ℃, and the calcination time is 2-4 h.

7. Use of the non-noble metal catalyst according to any one of claims 1 to 4 for the oxidation of methane to acetic acid, characterized in that, The mixture containing methane, oxygen, the non-noble metal catalyst, and water is reacted in a closed container to obtain acetic acid.

8. Use according to claim 7, characterized in that, The amount of non-noble metal catalyst is 1 x 10 4 ~ 2.5 x 10 3 wt% of the reaction liquid; Preferably, the amount of non-noble metal catalyst used is from 1 x 10 3 to 2.5 x 10 3 wt%.

9. Use according to claim 7, characterized in that, The volume ratio of methane to oxygen is 6:1-30:

1.

10. Use according to claim 7, characterized in that, The reaction temperature is 25-180 ℃, the reaction pressure is 0.1-5.0 MPa, and the reaction time is 0.1-10 h. Preferably, the reaction temperature is 150-180 ℃, the reaction pressure is 1.1-3.5 MPa, and the reaction time is 0.25-5 h.

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