A method for the chemical looping oxidative coupling of methane and carbon dioxide to produce c2-c6 olefins

Abstract

The application discloses a method for preparing C2-C6 olefins by chemical chain oxidative coupling of methane and carbon dioxide. The method utilizes the coupling synergy of an oxygen carrier and CO2, stimulates selective oxidation of methane, promotes directional recombination coupling of methyl radicals under the action of a catalyst to generate ethane, and further generates C2-C6 low-carbon olefins such as ethylene. The method improves the methane conversion rate, produces additional negative carbon benefits, and has good emission reduction effect.

Description

Technical fields:

[0001] This invention relates to the fields of catalysis and energy utilization, specifically to a method for producing C2-C6 olefins based on the chemical chain oxidative coupling of methane and carbon dioxide. Background technology:

[0002] Olefins are an important basic organic chemical raw material, but their production has always relied on petroleum cracking, which puts pressure on already depleted fossil fuel resources and causes serious environmental pollution problems. Therefore, finding new sources of olefins is an urgent issue. Methane is the main component of natural gas and an important energy source with abundant reserves. Converting the underutilized methane into high-value olefin chemicals is an effective solution to the olefin production problem.

[0003] Currently, the method for preparing olefins from methane is methane oxidative coupling technology, which involves the oxidation and coupling of methane to form olefins under the action of a catalyst. However, this method has the following drawbacks: methane is easily over-oxidized during this process, resulting in low selectivity and yield of olefins, while also producing water and carbon dioxide as byproducts.

[0004] CN 106964341 A discloses a low-temperature methane oxidative coupling catalyst, its preparation method, and its application. The catalyst, composed of three active components—Mn2O3, Na2WO4, and MnTiO3—and a SiO2 support, exhibits good low-temperature activity and selectivity. This allows the oxidative coupling of methane to olefins to achieve a methane conversion rate of up to 27% and a C2-C3 hydrocarbon selectivity of 76% at relatively low temperatures of 620-700℃ and atmospheric pressure. CN109438159A discloses a methane oxidative coupling method based on chemical chain lattice oxygen transfer technology. This method utilizes a Na2WO4 / SiO2 and MnO2 composite material to achieve the oxidative coupling of methane to olefins. At 800℃-850℃, the methane conversion rate reaches 23-26%, the C2 selectivity reaches 81-84%, and the C2 primary yield reaches 19-21%.

[0005] However, the methane oxidative coupling process described in the above patent cannot generate additional negative carbon gains, cannot reduce the CO2 content in the mixed gas, and has a low comprehensive utilization rate of carbon-containing resources. Summary of the Invention:

[0006] This invention provides a method for the chemical chain oxidative coupling of methane and carbon dioxide to produce C2-C6 olefins. By introducing carbon dioxide and utilizing the synergistic effect of the coupling between the oxygen carrier and CO2, the selective oxidation of methane is stimulated to produce olefins, thus solving the problems of low methane conversion rate and low carbon negative yield in the traditional methane oxidative coupling process.

[0007] This invention is achieved through the following technical solutions:

[0008] A method for the chemical chain oxidative coupling of methane and carbon dioxide to produce olefins, the method comprising the following steps:

[0009] (1) The reactor is set to a reaction temperature between 750-850℃, and inert gas is turned on for purging; SiO2, TiO2, or Al2O3 is selected as the inert dispersion carrier, and the metal oxide oxygen carrier has the general formula Mn x A y WO4, where A is one or two transition metals, alkali metals, or alkaline earth metals, x:y = 1:1 or 1:2, Mn x A y WO4 is loaded onto an inert dispersion carrier;

[0010] (2) After the temperature reaches the set temperature and stabilizes, a mixture of methane and carbon dioxide gas is introduced, and olefin gaseous products are obtained after the reaction.

[0011] After reacting with methane, the oxygen carrier loses lattice oxygen, and a large number of oxygen vacancies are formed on the surface and inside. At this time, air is introduced to oxidize and restore the lattice oxygen of the oxygen carrier. The reaction temperature is 750-850℃.

[0012] Preferably, the oxygen carrier that has lost lattice oxygen is in a reduced state. First, an inert gas Ar is introduced for purging at a temperature of 750-850℃ for 5-10 minutes. Then, air is introduced to oxidize the oxygen carrier and restore its lattice oxygen at a temperature of 750-850℃ for 5-10 minutes. Finally, the process returns to step (1) for a cycle.

[0013] C2-C6 olefins, more preferably C2-C4 olefins, and most preferably ethylene (C2H4).

[0014] Preferably, in step (1), the particle size of the oxygen carrier is between 20 and 40 mesh; the alkali metal element is selected from Li, K, and Na.

[0015] Preferably, in step (1), the alkaline earth metal element is selected from one of Ca, Mg, and Sr.

[0016] The transition metal is selected from one or two of Co, Zn, and Fe.

[0017] Preferably, in step (2), the methane-carbon dioxide mixture is one of biogas, landfill gas, carbon-rich natural gas, or coalbed methane.

[0018] Preferably, in step (2), the ratio of methane to CO2 in the methane-carbon dioxide mixture is 10:1 to 1:10.

[0019] Preferably, the oxygen carrier is prepared as follows: tungstate is dissolved in deionized water to prepare an aqueous tungstate solution; SiO2, TiO2, or Al2O3 is slowly added to the aqueous tungstate solution, and the mixture is continuously stirred at room temperature to form a suspension; a manganese nitrate solution with a stoichiometric amount equal to that of tungstate is weighed and added dropwise to the above suspension, and the mixture is continuously stirred rapidly at room temperature to form a milky white suspension; then, the mixture is continuously stirred and heated at 100-200°C to evaporate, forming a viscous paste; the resulting viscous paste is transferred to an oven and dried overnight at 100-120°C, then calcined at 800-900°C for 1-3 hours, and finally compressed into tablets to obtain oxygen carrier particles.

[0020] This invention uses methane and carbon dioxide as raw materials. Through a chemical chain cycle, the active oxygen inside the oxygen carrier provides the oxygen source for methane activation. Methane is catalytically oxidized by the oxygen carrier, resulting in methane cracking to produce methyl groups. CO2 is decomposed to produce CO, and oxygen atoms enter the metal oxide lattice of the oxygen carrier, replenishing the oxygen element. The oxygen carrier activates the CH bond, promoting the generation of methyl free radicals while inhibiting deep oxidation. The oxygen carrier simultaneously plays a role in in-situ dehydrogenation and catalysis during the reaction. Utilizing the synergistic coupling effect of the oxygen carrier and CO2, selective oxidation of methane is stimulated, promoting the directional recombination and coupling of methyl free radicals under the action of the catalyst to produce ethane. Ethane further undergoes in-situ dehydrogenation to produce low-carbon olefins (C2-C6) such as ethylene. After the oxidative coupling process is completed, an inert gas is introduced for purging, followed by air to restore the lattice oxygen of the oxygen carrier. This effectively alleviates the accumulation of carbon deposits from methane cracking and further inhibits the occurrence of deep oxidation reactions. The chemical chain oxidative coupling process and the air oxidation process are carried out alternately and cyclically to achieve the production of C2-C6 low-carbon olefins such as ethylene. The reaction equation is as follows:

[0021] Oxygen carrier releases oxygen: Mn x A y WO4→Mn x A y WO3+[O]

[0022] Oxidative coupling of methane: CH4+[O]→CH3·+[OH]

[0023] CH3· + CH3· → C2H6 → C2H4

[0024] Carbon dioxide replenishes lattice oxygen: Mn x A y WO3 + CO2 → Mn x A y WO4+CO

[0025] Overall reaction: CH4 + CO2 + Mn x A yWO4→CO / CO2+H2O+C2H6 / C2H4+Mn x A y WO3.

[0026] The beneficial effects of this invention are as follows:

[0027] (1) The method of the present invention has readily available oxygen sources, a wide range of raw material sources, simple operation, high resource utilization rate, and good economic benefits; it can effectively utilize CO2, generate additional negative carbon benefits, and has a good emission reduction effect.

[0028] (2) The active oxygen species of the oxygen carrier provide oxygen source for the oxidative coupling of methane and carbon dioxide, which activates the CH bond of methane and inhibits the occurrence of deep oxidation. The oxygen carrier plays the role of in-situ dehydrogenation and catalysis during the reaction. After the oxidative coupling process is completed, inert gas is introduced for purging, and then air is introduced to restore the lattice oxygen of the oxygen carrier. This can also effectively alleviate the accumulation of carbon deposits from methane cracking and further inhibit the occurrence of deep oxidation reaction.

[0029] (3) By utilizing the synergistic effect of the coupling between the oxygen carrier and carbon dioxide, methane is selectively oxidized at high temperature, which promotes the directional recombination and coupling of methyl radicals. In the catalytic oxidation reaction, CO2 is decomposed to generate CO, and oxygen atoms enter the metal oxide lattice to replenish oxygen, which helps maintain the oxygen supply capacity of the oxygen carrier. Attached image description:

[0030] Figure 1 This describes the effect of the TiO2-supported Mn / Na2WO4 oxygen carrier in Example 1 on the oxidative coupling of methane and carbon dioxide to olefins at different temperatures.

[0031] Figure 2 This is the result of the SiO2-supported Mn / ZnWO4 oxygen carrier catalyzing the oxidative coupling of methane and carbon dioxide to olefins at 800℃ in Example 2;

[0032] Figure 3 This is the result of the oxidation-coupling of methane to olefins catalyzed by the TiO2-supported Mn / CoWO4 oxygen carrier in Example 3;

[0033] Figure 4 This is the result of 20 cycles of reaction at 800°C using the Mn / Na2WO4 oxygen carrier in Example 1;

[0034] Figure 5 This is the result of the oxidation-coupling of methane to olefins catalyzed by the TiO2-supported Mn / Fe2(WO4)3 oxygen carrier in Example 4. Detailed implementation method:

[0035] The following is a further description of the invention, but not a limitation thereof.

[0036] Example 1:

[0037] Weigh 3.33g of sodium tungstate dihydrate and dissolve it in 30.0ml of deionized water to prepare an aqueous solution. Separately, weigh 15.00g of dried TiO2 in a 100ml beaker. Slowly add the prepared sodium tungstate aqueous solution to the TiO2 and stir continuously at room temperature for 1 hour to form a suspension. Weigh 2.47g of manganese nitrate tetrahydrate aqueous solution, dilute it with deionized water to 30.0mL, and add it dropwise to the suspension formed by the sodium tungstate and the carrier. Continue to stir rapidly at room temperature for 3 hours to form a milky white suspension. Then place it on a heated magnetic stirrer and heat it at 180 degrees Celsius to evaporate it into a paste-like viscous substance. Transfer the obtained paste-like viscous substance to a 105°C oven and dry it overnight. Then calcine it in a muffle furnace at 850°C for 2 hours with a heating rate of 5°C / min. Crush the calcined oxygen carrier to obtain particles with a particle size of 20-40 mesh.

[0038] Oxygen-carrier particles were placed in a reaction apparatus, and the reaction temperature was set at 800℃. An inert gas (high-purity argon, Ar > 99.9%) was continuously purged. After reaching the designated reaction temperature, a methane:carbon dioxide mixture of 1:1 was introduced into the reactor at a rate of 40 ml / min, and the reacted gas was collected. The results are as follows: Figure 1 As shown, at 800℃, the carbon dioxide conversion rate can reach 28.70%, and the C2+ olefin yield in the gaseous products can reach 24.85%, with C2 products being the main component, of which the C2 olefin yield is 23.83%. After reacting at high temperature for 5 min, Ar is introduced for purging for 5 min, followed by air introduction for oxidation for 5 min. The oxidized oxygen carrier is then recycled for the methane-carbon dioxide oxidative coupling step. Under the high-temperature heating condition of 800℃, the methane conversion rate and C2+ olefin selectivity during 20 cycles are shown in the figure. Figure 4 As shown.

[0039] Example 2

[0040] Mn was prepared using the same method as in Example 1. x A y The WO4 type metal oxide differs in that it uses SiO2 as the catalyst support and selects 5ZnO·12WO3 instead of Na2WO4.

[0041] Oxygen-carrier particles were placed in a reaction apparatus, and the reaction temperature was set at 800℃. During the heating process, an inert gas (high-purity argon, Ar > 99.9%) was continuously introduced. After reaching the designated reaction temperature, the mixture was stabilized for 10 minutes. A 1:1 mixture of methane and carbon dioxide was then introduced into the reactor at a rate of 60 ml / min, and the reacted gas was collected. The test results are as follows: Figure 2As shown, under the selected experimental conditions, a methane conversion rate of 21.28% and an olefin selectivity of 88.47% can be achieved, with a maximum carbon dioxide conversion rate of 20.90%.

[0042] Example 3

[0043] TiO2 was selected as the catalyst support. Manganese nitrate and cobalt tungstate crystals were weighed in a stoichiometric ratio of 1:1. An appropriate amount of deionized water was added and stirred at room temperature to dissolve the corresponding metal salt solution. Ethylene glycol and citric acid were added in a molar ratio of 1:1:1.5 to the metal ions and mixed. The pH was adjusted to 7.5 using ammonia and deionized water. The water bath temperature was raised to 80℃ and stirred at a constant temperature until a gel was formed. The prepared gel was placed in an oven at 105℃ for constant drying and aging until it exhibited a honeycomb structure while retaining its adhesiveness. The aged gel was then calcined at 800℃ for 3 hours. The calcined and stabilized catalyst was then crushed to obtain oxygen-carrying particles.

[0044] The oxygen carrier particles were placed in the reaction apparatus, and the reaction temperature was set at 750℃. During the heating process, an inert gas (high-purity argon, Ar>99.9%) was continuously purged. After reaching the specified reaction temperature and stabilizing for 10 minutes, a methane-carbon dioxide (6:1) mixture was introduced into the reactor at a specified flow rate of 60 ml / min. The gas after the reaction was collected, and the test results are as follows. Figure 3 As shown, under the set reaction conditions, a methane conversion rate of 18.49% and an olefin selectivity of 88.92% can be achieved, and a carbon dioxide conversion rate of 16.89% can be achieved.

[0045] Example 4

[0046] TiO2 was selected as the catalyst support. Ferrous tungstate crystals and manganese nitrate solution were weighed in a stoichiometric ratio of 1:1. An appropriate amount of deionized water was added and stirred at room temperature to dissolve the metal salt solution. Ethylene glycol and citric acid were added in a molar ratio of 1:1:1.5 to the metal ions and mixed. The pH was adjusted to 7.5 using ammonia and deionized water. The water bath temperature was raised to 80℃ and stirred at a constant temperature until a gel was formed. The prepared gel was placed in an oven at 105℃ for constant drying and aging until it exhibited a honeycomb structure while retaining its adhesiveness. The aged gel was then calcined at 900℃ for 4 hours. The resulting powder was compressed and crushed, and particles with a diameter of 20-40 mesh were screened to obtain oxygen carrier particles.

[0047] The oxygen carrier particles were placed in the reaction apparatus, and the reaction temperature was set at 800℃. During the heating process, an inert gas (high-purity argon, Ar>99.9%) was continuously purged. After reaching the specified reaction temperature, the mixture was stabilized for 10 minutes. A methane-carbon dioxide (6:1) mixture was then introduced into the reactor at a rate of 80 ml / min. The reacted gas was collected, and the test results are as follows: Figure 5 As shown, at 800℃, a methane conversion rate of 18.6% and an olefin selectivity of 86.94% can be achieved, and a carbon dioxide conversion rate of 17.97% can be achieved.

[0048] Example 5

[0049] Al₂O₃ was selected as the inert carrier. The required calcium tungstate and manganese nitrate solutions were weighed in a stoichiometric ratio of 1:1. An appropriate amount of deionized water was added and stirred at room temperature to dissolve the corresponding metal salt solutions. Ethylene glycol and citric acid were added in a molar ratio of 1:1:1.5 to the metal ions and mixed. The pH was adjusted to 7.5 using ammonia and deionized water. The water bath temperature was raised to 80℃, and the mixture was stirred at a constant temperature until a gel was formed. The prepared gel was placed in an oven at 105℃ for constant drying and aging until it exhibited a honeycomb structure while retaining its adhesiveness. The aged gel was then calcined at 900℃ for 4 hours. The resulting powder was compressed and crushed, and particles with a diameter of 20-40 mesh were screened to obtain oxygen carrier particles.

[0050] The oxygen carrier particles were placed in the reaction apparatus, and the reaction temperature was set at 750℃. During the heating process, an inert gas (high-purity argon, Ar>99.9%) was continuously purged. After reaching the specified reaction temperature, the mixture was stabilized for 10 minutes. A methane-carbon dioxide mixture of 10:1 was then introduced into the reactor at a rate of 60 ml / min. The methane conversion rate was 15.6%, the carbon dioxide conversion rate was 17.5%, and the olefin selectivity was 88.1%.

[0051] Comparative Example 1:

[0052] Referring to Example 1, the difference is that only methane gas is introduced.

[0053] Oxygen-carrier particles were placed in a reaction apparatus, and the reaction temperature was set to 800℃. An inert gas (high-purity argon, Ar > 99.9%) was continuously purged. After reaching the designated reaction temperature, methane gas was introduced into the reactor at a rate of 40 ml / min, and the reacted gas was collected. The methane conversion rate was 12.6%, and the olefin selectivity was 83.8%.

Claims

1. A method for the chemical chain oxidative coupling of methane and carbon dioxide to produce C2-C6 olefins, characterized in that, The method includes the following steps: (1) Set the reaction temperature in the reactor between 750-850℃ and purge with inert gas; use SiO2, TiO2, or Al2O3 as the inert dispersion carrier, and the metal oxide oxygen carrier has the general formula Mn. x A y WO4, where A is one or two transition metals, alkali metals, or alkaline earth metals, x:y = 1:1 or 1:2, Mn x A y WO4 is loaded onto an inert dispersion carrier; (2) After the temperature reaches the set temperature and stabilizes, a mixture of methane and carbon dioxide gas is introduced, and olefin gaseous products are obtained after the reaction.

2. The method according to claim 1, characterized in that, It also includes step (3): the oxygen carrier that has lost lattice oxygen after reacting with methane is oxidized by passing air through it to restore the lattice oxygen of the oxygen carrier. The reaction temperature is 750-850℃.

3. The method according to claim 2, characterized in that, In step (3), inert gas is first introduced for purging at a temperature of 750-850℃ for 5-10 minutes. Then, air is introduced to oxidize the oxygen carrier and restore its lattice oxygen at a temperature of 750-850℃ for 5-10 minutes. Finally, the process returns to step (1) for a cycle.

4. The method according to claim 1 or 2, characterized in that, C2-C6 olefins are replaced with C2-C4 olefins.

5. The method according to claim 1 or 2, characterized in that, In step (1), the oxygen carrier particle size is between 20 and 40 mesh.

6. The method according to claim 1 or 2, characterized in that, The alkali metal element is selected from one of Li, K, and Na; the alkaline earth metal element is selected from one of Ca, Mg, and Sr; and the transition metal is selected from one or two of Co, Zn, and Fe.

7. The method according to claim 1 or 2, characterized in that, In step (2), the methane and carbon dioxide mixed gas is one of biogas, landfill gas, carbon-rich natural gas, and coalbed methane.

8. The method according to claim 1 or 2, characterized in that, In step (2), the ratio of methane to CO2 in the methane-carbon dioxide mixture is 10:1 to 1:

10.

9. The method according to claim 1 or 2, characterized in that, The oxygen carrier used is prepared as follows: tungstate is dissolved in deionized water to prepare an aqueous tungstate solution; SiO2, TiO2, or Al2O3 is slowly added to the aqueous tungstate solution, and the mixture is continuously stirred at room temperature to form a suspension; a manganese nitrate solution with a stoichiometric amount equal to that of tungstate is weighed and added dropwise to the above suspension, and the mixture is continuously stirred rapidly at room temperature to form a milky white suspension; then, the mixture is continuously stirred and heated at 100-200℃ to evaporate, forming a viscous paste; the resulting viscous paste is transferred to an oven and dried overnight at 100-120℃, then calcined at 800-900℃ for 1-3 hours, and finally compressed into tablets to obtain oxygen carrier particles.

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