A method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant

Abstract

This invention discloses a method for the selective oxidation of methane to oxygen-containing compounds using water as the oxidant. The method employs orthorhombic molybdenum trioxide as the sole catalyst and water as the sole oxidant, allowing methane to react with water in a closed reaction system, resulting in the directional conversion to oxygen-containing compounds. This method eliminates the need for added precious metal catalysts, hydrogen peroxide, or oxygen as external oxidants, and avoids the use of co-catalysts, co-reactants, or reducing agents such as carbon monoxide or hydrogen. The selective oxidation of methane is achieved solely with water and orthorhombic molybdenum trioxide, simplifying the reaction system composition. Furthermore, the preparation process is simple and inexpensive, reducing catalyst costs and oxidant safety risks, while simultaneously achieving the directional conversion of methane to oxygen-containing compounds. This invention solves the problems of high oxidant costs and complex processes caused by co-reactants in existing technologies, and eliminates the generation of excessive CO or CO2 oxidation products.

Description

Technical Field

[0001] This invention belongs to the field of methane catalytic conversion technology, specifically relating to a method for the selective oxidation of methane to oxygen-containing compounds using water as an oxidant. Background Technology

[0002] Methane, a major component of natural gas and a core component of unconventional oil and gas resources such as shale gas and combustible ice, is a low-carbon alkane resource with abundant reserves and wide distribution, occupying an important position in the energy structure. However, methane is also a typical strong greenhouse gas, with a greenhouse effect per molecule far exceeding that of carbon dioxide; its uncontrolled emissions will exacerbate global warming. Therefore, the efficient and targeted conversion of methane into high-value-added chemicals has significant strategic value. The direct conversion of methane into oxygen-containing compounds, such as methanol, formic acid, acetic acid, acetaldehyde, and ethanol, is a technological route with high atom utilization and a simple reaction pathway. Among these, acetic acid, acetaldehyde, and ethanol, being C2 oxygen-containing compounds, have high added value and strong market demand, making related research of outstanding strategic value. Therefore, achieving the selective oxidation of methane to prepare high-value oxygen-containing compounds can not only realize the efficient utilization and value enhancement of low-carbon resources but also reduce environmental pollution caused by methane venting and combustion, alleviating greenhouse gas emission pressure. This has significant scientific significance and application prospects for promoting the clean and efficient utilization of fossil resources and contributing to green and low-carbon development.

[0003] Existing systems for the oxidation of methane to prepare oxygen-containing compounds suffer from several drawbacks. First, the catalysts rely on multiple components or noble metals. For example, current technologies often employ noble metals such as Rh, Ir, and Au (CN112892588A, CN120268444A), or multi-metal composite systems such as Ce-Fe and Pd-Mo (CN120515484A, CN120695816A), resulting in complex and costly processes, and difficulty in simultaneously achieving metal dispersibility and stability. Second, existing reaction systems often require harsh reaction conditions or co-reactants. Some systems require the addition of CO as a carbonylation feedstock (e.g., CN120515484A), or rely on strong acid media and high-temperature, high-pressure conditions, leading to poor process environmental friendliness and high energy consumption. Furthermore, existing systems suffer from insufficient selectivity for C2 products and inadequate catalyst recyclability. Single-metal or non-noble-metal systems primarily produce C1 products, such as MnO disclosed in CN121222477A. x The ZSM-5 catalyst exhibits low selectivity for C2 products and is prone to agglomeration and deactivation.

[0004] In the direct oxidation of methane, the selection of the oxidant is one of the core challenges. While hydrogen peroxide boasts high oxidation efficiency, its high cost hinders large-scale application. Molecular oxygen, though widely available, is difficult to activate under mild conditions and easily leads to over-oxidation of the product, generating CO and CO2, thus reducing the selectivity of the target product. Water, as an environmentally friendly, low-cost, and mild oxidant, can effectively inhibit over-oxidation during methane conversion, making it an ideal oxygen source for achieving selective methane oxidation. However, to date, no research has been reported on the direct selective oxidation of methane using water as the oxidant to prepare oxygen-containing compounds. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant, which addresses the shortcomings of the prior art. This method simplifies the composition of the reaction system, and the preparation process is simple and inexpensive, reducing the cost of catalysts and the safety risks of oxidants, while realizing the directional conversion of methane to oxygen-containing compounds.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: a method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant, using orthorhombic molybdenum trioxide as the sole catalyst and water as the sole oxidant, in which methane reacts with water in a closed reaction system to be directionally converted into oxygen-containing compounds.

[0007] This invention employs orthorhombic molybdenum trioxide as the sole catalyst and water as the sole oxidant, enabling the directional conversion of methane into oxygen-containing compounds through a closed reaction system. This method eliminates the need for precious metal catalysts, external oxidants such as hydrogen peroxide or oxygen, and avoids the use of co-catalysts, co-reactants, or co-reducing agents such as carbon monoxide or hydrogen. The selective oxidation of methane is achieved solely with water and orthorhombic molybdenum trioxide, simplifying the reaction system composition. Furthermore, the preparation process is simple and inexpensive, reducing catalyst costs and oxidant safety risks, while simultaneously achieving the directional conversion of methane into oxygen-containing compounds. This invention solves the problems of high oxidant costs and complex processes caused by co-reactants in existing technologies, and eliminates the generation of excessive CO or CO2 oxidation products.

[0008] As a preferred embodiment, the specific process of the method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant according to the present invention is as follows: Orthorhombic molybdenum trioxide and deionized water are added to a high-pressure reactor lined with polytetrafluoroethylene (PTFE). After sealing, methane is introduced into the reactor until the initial pressure is 0.1–2.4 MPa. The reactor is then placed in a heating device and heated to 140–220°C, and the reaction is maintained at this temperature for 2–8 hours, with magnetic stirring performed during the reaction. After the reaction is completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds is obtained. This method defines specific reaction conditions and parameters, allowing the reaction to proceed under mild conditions, avoiding the stringent requirements of high temperature and high pressure on the equipment. Simultaneously, magnetic stirring ensures sufficient contact between the catalyst and the reactants, achieving efficient preparation of oxygen-containing compounds under mild conditions.

[0009] As a further preferred option, the obtained reaction mixture is subjected to solid-liquid separation to obtain an aqueous solution containing oxygen-containing compounds and a solid catalyst. Solid-liquid separation enables the preliminary separation of the product and the catalyst, facilitating subsequent product collection and quantitative analysis, and also providing a basis for catalyst recovery and reuse.

[0010] As a further preferred embodiment, the method of the present invention further includes a catalyst recovery step: the obtained solid catalyst is washed with deionized water, dried, and then a recyclable orthorhombic molybdenum trioxide catalyst is obtained. The washing liquid obtained is combined with the aqueous solution containing oxygen-containing compounds. Through the above catalyst recovery step, the catalyst can be recycled, reducing catalyst consumption, while improving product yield and enhancing the economics of the process.

[0011] As a further preferred embodiment, the amount of the orthorhombic molybdenum trioxide catalyst is 0.01~0.1g, and the amount of deionized water is 10~30mL. By limiting the ratio of the reactants, the amount of catalyst is controlled while ensuring catalytic efficiency, thus avoiding material waste.

[0012] Preferably, the orthorhombic molybdenum trioxide is prepared by a hydrothermal method. The preparation process is as follows: ammonium heptamolybdate is dissolved in deionized water, concentrated nitric acid is added, and after thorough mixing, the mixture is transferred to a reaction vessel lined with polytetrafluoroethylene (PTFE). Crystallization is carried out at a hydrothermal reaction temperature of 160–200°C for 12–36 hours. After cooling, the mixture is centrifuged and dried at 60–80°C for 12–24 hours to obtain the orthorhombic molybdenum trioxide catalyst. The hydrothermal method for preparing orthorhombic molybdenum trioxide is simple to operate and uses mild preparation conditions.

[0013] Preferably, the oxygen-containing compound includes at least one selected from methanol, formic acid, ethanol, acetaldehyde, and acetic acid. The method of this invention can directionally convert methane into oxygen-containing compounds with higher added value, rather than deeply oxidizing it to carbon dioxide, thus offering the advantage of good product selectivity.

[0014] Compared with existing technologies, this invention has the following advantages: The method of this invention uses orthorhombic molybdenum trioxide as the sole catalyst and water as the sole oxidant. In a closed reaction system, it activates the CH bonds in methane and reacts with water molecules, achieving the directional conversion of methane into oxygen-containing compounds, primarily C2 oxygen-containing compounds such as acetic acid, acetaldehyde, and ethanol. This method does not add precious metal catalysts, hydrogen peroxide, or oxygen as external oxidants, nor does it use carbon monoxide, hydrogen, or other co-catalysts, co-reactants, or co-reducing agents. The selective oxidation of methane can be achieved using only water and orthorhombic molybdenum trioxide, simplifying the reaction system composition. Furthermore, the preparation process is simple and low-cost, reducing catalyst costs and the safety risks of oxidants, while simultaneously achieving the directional conversion of methane into oxygen-containing compounds. This invention solves the problems of high oxidant costs and complex processes caused by co-reactants in existing technologies, and it does not generate excessive CO or CO2 oxidation products. Attached Figure Description

[0015] Figure 1 The XRD powder diffraction patterns of the orthorhombic molybdenum trioxide catalysts used in Examples 1, 10, and 11 are shown below. Figure 2 XPS spectra of the orthorhombic molybdenum trioxide catalysts used in Examples 1 and 10; Figure 3 The images show SEM images of the orthorhombic molybdenum trioxide catalysts used in Examples 1 and 10. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. All raw materials used in the following embodiments are commercially available products.

[0017] Example 1: 0.05 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 2.4 MPa. The reactor was placed in a heating device and heated to 220 °C. The reaction was carried out at a constant temperature for 4 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0018] Example 2: 0.025 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 2.4 MPa. The reactor was placed in a heating device and heated to 220 °C. The reaction was carried out at a constant temperature for 4 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0019] Example 3: 0.01 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 2.4 MPa. The reactor was placed in a heating device and heated to 220 °C. The reaction was carried out at a constant temperature for 4 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0020] Example 4: 0.05 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 1.2 MPa. The reactor was placed in a heating device and heated to 220 °C. The reaction was carried out at a constant temperature for 4 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0021] Example 5: 0.05 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 0.1 MPa. The reactor was placed in a heating device and heated to 220 °C. The reaction was carried out at a constant temperature for 4 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0022] Example 6: 0.05 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 2.4 MPa. The reactor was placed in a heating device and heated to 180 °C. The reaction was carried out at a constant temperature for 4 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0023] Example 7: 0.05 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 2.4 MPa. The reactor was placed in a heating device and heated to 140 °C. The reaction was carried out at a constant temperature for 4 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0024] Example 8: 0.05 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 2.4 MPa. The reactor was placed in a heating device and heated to 220 °C. The reaction was carried out at a constant temperature for 2 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0025] Example 9: 0.05 g of orthorhombic molybdenum trioxide and 15 mL of deionized water were added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane was introduced into the reactor until the initial pressure was 2.4 MPa. The reactor was placed in a heating device and heated to 220 °C. The reaction was carried out at a constant temperature for 8 hours with magnetic stirring during the reaction. After the reaction was completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds was obtained.

[0026] Take appropriate amounts of the reaction mixtures obtained in Examples 1 to 9, and divide each into 3 equal parts. Take 2 parts of the reaction mixture and use gas chromatography by headspace injection to test the yields of methanol, ethanol, acetaldehyde, and acetone. Dilute the remaining 1 part of the reaction mixture ten times and use ion chromatography to test the yields of formic acid and acetic acid. The results are shown in Table 1.

[0027] The orthorhombic molybdenum trioxide catalysts used in Examples 1 to 9 above were prepared by a hydrothermal method. The preparation process was as follows: 1 g of ammonium heptamolybdate was dissolved in 50 mL of deionized water, 6 mL of concentrated nitric acid with a concentration of 65-68% was added, and after mixing evenly, the mixture was transferred to a reaction vessel equipped with a polytetrafluoroethylene liner. The mixture was crystallized at a hydrothermal reaction temperature of 180°C for 24 hours, cooled, centrifuged, and dried at 60°C for 12 hours to obtain the orthorhombic molybdenum trioxide catalyst.

[0028] Example 10: Take an appropriate amount of the reaction mixture obtained in Example 1, wash it three times with deionized water, dry it overnight at 60°C, and recover the orthorhombic molybdenum trioxide catalyst; mix the orthorhombic molybdenum trioxide recovered from the reaction mixture of Example 1 and deionized water at a mass-volume ratio of (0.01~0.1) g: (10~30) mL, add it to a high-pressure reactor equipped with a polytetrafluoroethylene liner, seal it, and then charge the reactor with methane until the initial pressure is 2.4 MPa; place the reactor in a heating device, raise the temperature to 220°C, and react at a constant temperature for 4 hours, with magnetic stirring during the reaction; after the reaction is completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds is obtained.

[0029] Example 11: Take an appropriate amount of the reaction mixture obtained in Example 10, wash it three times with deionized water, dry it overnight at 60°C, and recover the orthorhombic molybdenum trioxide catalyst; mix the orthorhombic molybdenum trioxide recovered from the reaction mixture of Example 10 and deionized water at a mass-volume ratio of (0.01~0.1) g: (10~30) mL, add it to a high-pressure reactor equipped with a polytetrafluoroethylene liner, seal it, and then charge the reactor with methane until the initial pressure is 2.4 MPa; place the reactor in a heating device, raise the temperature to 220°C, and react at a constant temperature for 4 hours, with magnetic stirring during the reaction; after the reaction is completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds is obtained.

[0030] Take appropriate amounts of the reaction mixtures obtained in Examples 10 and 11, and divide each into 3 equal parts. Take 2 parts of the reaction mixture and use gas chromatography by headspace injection to test the yields of methanol, ethanol, acetaldehyde, and acetone. Dilute the remaining 1 part of the reaction mixture ten times and use ion chromatography to test the yields of formic acid and acetic acid. The results are shown in Table 1.

[0031] Table 1

[0032] Comparing the catalyst dosages in Examples 1, 2, and 3, it was found that reducing the catalyst dosage increased the yield of C2 oxygen-containing products (ethanol, acetaldehyde, and acetic acid) per unit mass of catalyst. When the catalyst dosage was 0.01 g, the yields of ethanol, acetaldehyde, and acetic acid were 34.5 μmol / g. cat 205.0 μmol / g cat 294.0 μmol / g cat The main product is C2.

[0033] Comparing the methane pressures in Examples 1, 4, and 5, it can be found that increasing the methane pressure can increase the yield of C2 oxygen-containing products (ethanol, acetaldehyde, and acetic acid). When the methane pressure reaches 2.4 MPa, the yields of ethanol, acetaldehyde, and acetic acid are 20.1 μmol / g. cat 48.9 μmol / g cat 83.0 μmol / g cat .

[0034] Comparing the reaction temperatures of Examples 1, 6, and 7, it can be found that the catalyst exhibits the best catalytic activity at a reaction temperature of 220°C; while lowering the reaction temperature significantly reduces the reaction activity.

[0035] Comparing the reaction times of Examples 1, 8, and 9, it can be found that when the reaction time is 2 hours, the yield of total organic matter is 144.8 μmol / g. cat The total organic matter yield reached its highest level of 204.4 μmol / g after 4 hours of reaction. cat Extending the reaction time to 8 hours further reduced the total organic matter yield.

[0036] Figure 1The XRD powder diffraction patterns are those of the orthorhombic molybdenum trioxide catalysts used in Examples 1, 10, and 11. The characteristic diffraction peaks of the catalyst in Example 1 are at 12.76°, 23.36°, 25.71°, 27.35°, 33.15°, 33.77°, 35.50°, 38.98°, 45.79°, 46.33°, 49.28°, 52.82°, 54.16°, and 55.23°.

[0037] Figure 2 The XPS spectra of the orthorhombic molybdenum trioxide catalysts used in Examples 1 and 10 are shown below. Figure 2 (A) and Figure 2 (B) High-resolution spectra of Mo (3d) and O (1s), respectively. XPS analysis results show that molybdenum in the catalyst of Example 1 is mainly in the form of Mo. 6+ Oxidation states exist, and oxygen species are composed of lattice oxygen and surface hydroxyl groups.

[0038] Figure 3 The images shown are SEM images of the orthorhombic molybdenum trioxide catalysts used in Examples 1 and 10. Figure 3 (A) is a SEM image of the catalyst in Example 1. Figure 3 (B) is a SEM image of the catalyst in Example 10. As can be seen from the SEM image, the orthorhombic molybdenum trioxide catalyst exhibits a short rod-shaped morphology with a grain size of approximately 5 μm.

[0039] Examples 10 and 11 investigated the catalytic activity of the recovered catalyst. Compared with Example 1, it can be seen that the recovered catalyst still maintains high catalytic activity, and the activity retention rate is good after two cycles. Figure 1 The XRD patterns of the recovered catalyst were basically consistent with those of the fresh catalyst, indicating that the catalyst has good structural stability. Figure 2 This indicates that the Mo element in the recovered catalyst of Example 10 was partially reduced to Mo. 5+ .Depend on Figure 3 The SEM images show that the morphology of the recovered catalyst remains basically unchanged.

Claims

1. A method for the selective oxidation of methane to oxygen-containing compounds using water as an oxidant, characterized in that, Using orthorhombic molybdenum trioxide as the sole catalyst and water as the sole oxidant, methane is directionally converted into oxygen-containing compounds by reacting with water in a closed reaction system.

2. The method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant according to claim 1, characterized in that, The specific process is as follows: Orthorhombic molybdenum trioxide and deionized water are added to a high-pressure reactor lined with polytetrafluoroethylene. After sealing, methane is introduced into the reactor until the initial pressure is 0.1~2.4 MPa. The reactor is placed in a heating device and heated to 140~220℃. The reaction is carried out at a constant temperature for 2~8 hours, and magnetic stirring is performed during the reaction. After the reaction is completed, a reaction mixture containing a solid catalyst and oxygen-containing compounds is obtained.

3. The method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant according to claim 2, characterized in that, The resulting reaction mixture was subjected to solid-liquid separation to obtain an aqueous solution containing oxygen-containing compounds and a solid catalyst.

4. The method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant according to claim 3, characterized in that, It also includes a catalyst recovery step: the obtained solid catalyst is washed with deionized water, dried, and then a recyclable orthorhombic molybdenum trioxide catalyst is obtained. The washing liquid obtained is combined with the aqueous solution containing oxygen-containing compounds.

5. The method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant according to claim 2, characterized in that, The amount of the orthorhombic molybdenum trioxide catalyst used is 0.01~0.1g, and the amount of deionized water used is 10~30mL.

6. The method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant according to claim 1, characterized in that, The orthorhombic molybdenum trioxide was prepared by a hydrothermal method. The preparation process was as follows: ammonium heptamolybdate was dissolved in deionized water, concentrated nitric acid was added, and after mixing evenly, it was transferred to a reaction vessel equipped with a polytetrafluoroethylene liner. It was crystallized at a hydrothermal reaction temperature of 160~200℃ for 12~36 hours, cooled, centrifuged, and dried at 60~80℃ for 12~24 hours to obtain the orthorhombic molybdenum trioxide catalyst.

7. The method for selective oxidation of methane to oxygen-containing compounds using water as an oxidant according to claim 1, characterized in that, The oxygen-containing compound includes at least one of methanol, formic acid, ethanol, acetaldehyde, and acetic acid.

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