Low-temperature methane oxidative coupling series catalyst and preparation method thereof

Abstract

The invention discloses a low-temperature methane oxidative coupling series catalyst and a preparation method thereof, and belongs to the field of catalysts. The invention aims to solve the problem that methyl free radicals generated in the existing methane oxidative coupling mainly depend on gas phase coupling to generate a C2 product, so that the low-temperature catalytic activity of the methyl free radicals is seriously limited. The catalyst is formed by connecting an oxide Eu2MxO3 with an acidic molecular sieve b-KZSM-5 in series, and is applied to methane oxidative coupling reaction, oxygen is activated by using a rare earth oxide rich in oxygen vacancies so as to activate methane, and generated methyl free radicals are subjected to C-C coupling at acidic sites of the acidic molecular sieve so as to generate a C2 product. The C-C coupling efficiency is improved in a manner of connecting molecular sieves in series, so that the low-temperature reaction activity of the catalyst is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of catalysts, specifically, it relates to a low-temperature methane oxidation coupling tandem catalyst and its preparation method. Background Technology

[0002] With the ongoing transformation of the global energy structure and the increasing demand for clean chemicals, the efficient and high-value utilization of abundant natural gas (mainly composed of methane), as an important fossil energy source and chemical feedstock, has attracted widespread attention. Methane molecules possess extremely high CH bond energies (~435 kJ / mol) and a symmetrical tetrahedral structure, posing significant challenges to their direct activation and directional conversion. Traditional indirect methane conversion pathways (such as the production of methanol or hydrocarbons from syngas) are typically energy-intensive, lengthy, and require substantial investment. In contrast, oxidative coupling (OCM) of methane can directly convert methane into more valuable ethylene and ethane (C2 hydrocarbons) under relatively mild conditions in a single step, providing a highly attractive alternative for the efficient utilization of natural gas. The core of the OCM reaction lies in the selective activation of methane molecules to generate methyl radicals (•CH3) through the action of a catalyst, promoting their gas-phase coupling to generate C2 products (C2H6, C2H4), while simultaneously inhibiting the deep oxidation of the target products on the catalyst surface (generating CO and CO2). Highly efficient OCM catalysts typically consist of oxides of alkali metals (such as Li, Na), alkaline earth metals (such as Mg, Ca, Sr, Ba), or rare earth metals (such as La, Sm) as the main active component or support, such as the classic Li / MgO and La2O3 systems. However, this reaction faces two key bottlenecks in practical applications: first, the reaction needs to be carried out at high temperatures (usually >750℃) to achieve a considerable methane conversion rate; second, the single-pass yield of C2 products is insufficient, and deep oxidation side reactions are difficult to completely suppress. Therefore, how to improve the low-temperature yield is a major challenge currently facing the field of methane oxidative coupling. Current research on low-temperature activity mainly focuses on regulating the activation ability of oxygen species and surface basicity. For example, researchers (ScienceAdvances, 3, 2017, e1603180) have successfully reduced the OCM reaction temperature from the traditional 800-900℃ to 650℃ by innovatively introducing titanium dioxide as a promoter to modify the manganese-based catalyst. The core mechanism of this breakthrough lies in the fact that titanium dioxide can react with Mn at low temperatures. x O y The formation of specific compounds promotes the "low-temperature chemical cycle" process, significantly reducing the activation energy barrier of oxygen molecules. However, the catalysts are complex in composition, cumbersome in preparation, and have low production capacity. Therefore, developing novel catalytic materials to lower the catalytic reaction temperature is of great promise.

[0003] The core bottleneck in improving the low-temperature activity of the methane oxidative coupling (OCM) reaction lies in the insufficient activation capacity of oxygen and the low coupling efficiency of methyl radicals at low temperatures. Patent application CN202311507831 addresses this issue by mixing lanthanum salt with an alkaline solution, followed by solid-liquid separation to obtain a solid product, and then calcining the solid product to develop a columnar lanthanum oxide with a bimodal pore structure. This lanthanum oxide significantly improves the selectivity for C2 and higher hydrocarbons in the methane oxidative coupling reaction. However, the bimodal lanthanum oxide requires 100 hours of low-temperature settling, which is insufficient for practical production capacity requirements. For example, patent CN202311509089 describes how the active component forms single-atom active sites on a support, significantly improving the dispersibility and atom utilization of the active component, thus enhancing the selectivity and yield of C2 hydrocarbons. However, the single-atom system relies on precise ligand control (such as phthalocyanine) and ultrasonic dispersion, making operation complex and increasing the difficulty of practical application. Therefore, designing novel materials to effectively reduce the activation temperature of oxygen molecules is crucial in the design of methane oxidative coupling catalysts.

[0004] Currently, methyl radicals generated in methane oxidative coupling mainly rely on gas-phase coupling to form C2 products, severely limiting their low-temperature catalytic activity. For example, patent CN116966900A modifies silica composite oxide into fumed silica or pure silicon molecular sieves modified with sodium tungstate. This catalyst achieves selectivity of over 70% for the direct production of ethane and ethylene from methane oxidative coupling. However, its low low-temperature activity limits its practical application. For instance, patent CN116283472A utilizes the synergistic coupling effect of oxygen carrier and non-equilibrium plasma to alter its catalytic performance, promoting the directional coupling of methyl radicals to olefins. Furthermore, changes in the catalytic properties of the oxygen carrier, such as its morphology, affect the electric field distribution and electron energy distribution of the non-equilibrium plasma, altering the relative flux of plasma particles reaching the oxygen carrier and enhancing its catalytic performance, thus solving the problem of low-temperature methane activation difficulties during methane oxidative coupling. However, plasma catalysis suffers from high equipment costs, poor plasma stability, and secondary pollution, hindering large-scale application. Summary of the Invention

[0005] To address the aforementioned issues, this invention employs an oxide-tandem acidic molecular sieve method for the oxidative coupling reaction of methane. A rare-earth composite oxide rich in oxygen vacancies activates oxygen, thereby activating methane. The resulting methyl radicals undergo C-C coupling at the acidic sites of the acidic molecular sieve to generate C2 products. By using a tandem molecular sieve approach, the C-C coupling efficiency is improved, the formation of the byproduct carbon dioxide is reduced, and the reaction selectivity is enhanced, thus significantly increasing the low-temperature reaction activity of the catalyst.

[0006] To address the aforementioned technical problems, the present invention adopts the following technical solution: The purpose of this invention is to provide a low-temperature methane oxidation coupling tandem catalyst, characterized in that it consists of individually packaged Eu... 2-x M x Composed of O3 and b-KZSM-5, Eu 2-x M x In O3, M is Mg, Ca, Sr or Ba, x < 2, and the molar ratio of Eu to M is 2:(0-2). Furthermore, the silicon-to-aluminum ratio of b-KZSM-5 is specified to be 20-200.

[0007] Further specifying, during use, Eu2M will be... x O3 and b-KZSM-5 are mixed at a mass ratio of (0.5~2):(0.2~5), or Eu2M x O3 and b-KZSM-5 layers are stacked, with Eu at the reactant gas inlet. 2-x M x O3, the reactant outlet is b-KZSM-5, Eu 2-x M x The thickness of the O3 layer is controlled between 0.5cm and 2cm, and the thickness of the b-KZSM-5 layer is controlled between 0.2cm and 5cm.

[0008] Another object of the present invention is to provide a low-temperature methane oxidation coupling tandem catalyst, Eu2M. x The method for preparing O3 includes the following steps: Eu(NO3)3·6H2O deionized aqueous solution and M(NO3)2·xH2O deionized aqueous solution are mixed evenly, and then evaporated using an ultrasonic atomizer. The resulting spray is delivered through a nozzle and carrier gas through a tube furnace at a temperature of 800℃-1200℃. The deposited solid particles are collected at the outlet, and the collected solid particles are calcined in air at 500℃-1000℃.

[0009] Furthermore, the carrier gas in the nozzle is air.

[0010] Another object of the present invention is to provide a method for preparing the low-temperature methane oxidative coupling tandem catalyst b-KZSM-5, characterized by comprising the following steps: Step 1: ZSM-5 zeolite (b-ZSM-5) with a flake-like morphology was synthesized using a urea-assisted method. Step 2: Add b-ZSM-5 to potassium nitrate solution, stir in a water bath, filter, wash with water, dry in air, and calcine to obtain b-KZSM-5.

[0011] To further specify, step 1 involves the following steps: Dissolve 4.0g of 25% tetrapropylammonium hydroxide aqueous solution in 5.1g of deionized water with stirring, and then add 2.8g of tetraethyl orthosilicate with stirring. Then add 0.07g aluminum isopropoxide and 0.65g urea, and continue stirring at room temperature until a gel forms; Then crystallize at 160℃-220℃ for two days, filter, wash with water, dry in air, and calcine at 450℃-650℃.

[0012] Further specifying, 300 mg of b-ZSM-5 was added to 20 ml of 0.1 M potassium nitrate solution.

[0013] Further specified, stirring is performed in a water bath at 70℃-120℃.

[0014] Further specified, calcination at 300℃-500℃.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The low-temperature activity of the tandem catalyst in this invention is significantly improved. Conventional methane oxidative coupling reactions only exhibit significant activity at temperatures above 800°C, while the tandem molecular sieve system achieves a 10% yield at 650°C. This greatly reduces the required reaction temperature, significantly reduces energy consumption, and saves energy.

[0016] The methane oxidative coupling catalytic system of oxide tandem molecular sieves of the present invention can maintain good activity without the need for additional doping with Ce or La or coating with Li, which can effectively simplify the complexity of catalyst synthesis and easily achieve 10g-100g scale-up production.

[0017] The present invention utilizes b-KZSM-5 molecular sieve to improve reaction yield through a tandem catalyst, which is simple to operate, inexpensive, and reduces production costs.

[0018] In this invention, the oxide-zeolite catalyst (OX-ZeO) is widely used in the synthesis of hydrocarbons from syngas. High performance is achieved by separating the key reaction steps of CO activation and CC coupling onto two different types of active centers with complementary properties. Methyl radical coupling is also a key reaction step in the methane oxidative coupling reaction, and the application of oxide-zeolite catalysts to the methane oxidative coupling reaction represents the development trend of catalysts for this reaction. Attached Figure Description

[0019] Figure 1 It is Eu 2-x Ba x XRD pattern of O3; Figure 2 This is the XRD pattern of b-KZSM-5 molecular sieve; Figure 3 It is Eu 2-x Ba x TEM image of O3; Figure 4 This is a TEM image of b-KZSM-5 molecular sieve; Figure 5 This describes the catalytic performance of the Eu2O3 sample for methane oxidative coupling. Figure 6 The catalytic performance of Eu2O3 tandem b-KZSM-5 molecular sieve samples for methane oxidative coupling is shown. Figure 7 It is the catalytic ability of Eu2O3 in series with different molecular sieves; Figure 8 It is the catalytic ability of Eu2O3 and b-KZSM-5 molecular sieve in different series connections. Detailed Implementation

[0020] The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0021] Example 1 In this embodiment, the low-temperature methane oxidation coupling tandem catalyst is a separately packaged Eu catalyst. 2-x Ba x Composed of O3 and b-KZSM-5, x=0.2, with a molar ratio of Eu to M of 2:0.2. The silicon-to-aluminum ratio of b-KZSM-5 is 140.

[0022] Eu 2-x M x O3 synthesis method: Using Eu(NO3)3·6H2O and M(NO3)2·xH2O as precursors, 0.4 mol / L deionized aqueous solutions of Eu(NO3)3·6H2O and M(NO3)2·xH2O were uniformly mixed in different proportions. The mixed solution was evaporated using an ultrasonic nebulizer, and the resulting spray was delivered through a nozzle carrier gas (air) into a tube furnace at a temperature of 1000℃. The deposited sample was collected at the outlet. The collected sample was then calcined in air at 850℃ in a muffle furnace for 4 hours.

[0023] Synthesis of b-ZSM-5: ZSM-5 zeolite with a flake-like morphology (denoted as b-ZSM-5, where 'b' represents flake-like) was synthesized using a urea-assisted method. As a typical procedure, tetrapropylammonium hydroxide (TPAOH; 4.0 g, 25% aqueous solution) was dissolved in deionized water (5.1 g) with stirring for 0.5 hours, followed by the addition of tetraethyl orthosilicate (TEOS; 2.8 g) with stirring. After the addition of aluminum isopropoxide (0.07 g) and urea (0.65 g), the mixture was stirred at room temperature for 12 hours. The resulting gel was transferred to an autoclave and crystallized at 180 °C for two days, yielding a SiO2:Al2O3:TPAOH:H2O:urea molar composition of 1:0.0128:0.3684:33.8346:0.812. The synthesized product was collected by filtration, washed with water, dried in air, and calcined at 550 °C.

[0024] Synthesis of b-KZSM-5: 300 mg of b-ZSM-5 was added to 20 ml of 0.1 M potassium nitrate solution and stirred in a water bath at 80 °C for 12 h. The synthesized product was collected by filtration, washed with water, dried in air, and calcined at 350 °C for 4 h.

[0025] OCM reaction test conditions: The methane oxidative coupling reaction was carried out at atmospheric pressure in a fixed-bed plug flow silica glass reactor. Catalytic performance and stability were tested using 100 mg of catalyst at a mixed gas flow rate of 16.7 mL / min within a temperature range of 550–700 °C, corresponding to a gas space velocity (GHSV) of 20,000 mL / h / g. cat The reaction gas consisted of CH4, O2, and the equilibrium gas N2 in a volume ratio of 3 / 1 / 6. Each temperature point was held for 25 minutes to ensure sample temperature stability. The effluent gas was analyzed using an online gas chromatograph (GC9790 plus).

[0026] During use, Eu2M x O3 and b-KZSM-5 layers are stacked, and the inlet of the reactor is Eu. 2-x M x O3, the outlet of the reaction device is b-KZSM-5, Eu 2-x M x The O3 layer is 0.7cm thick, and the b-KZSM-5 layer is 1.5cm thick, with one layer of each being laid.

[0027] Figure 1 Eu was displayed 2-x Ba x The XRD pattern of Eu₂O₃, with Ba doping, conforms to the standard card (PDF# 34-0392), belonging to la. No diffraction peaks belonging to the Ba species were detected.

[0028] Figure 2 The XRD pattern of b-KZSM-5 molecular sieve is shown.

[0029] Figure 3 Eu was displayed 2-x Ba x TEM images of O3 show products that are regular spherical polyhedra with diameters of 0.5–3 μm. The striking contrast between the observed center and edges suggests that the primary grains have a hollow structure. The formation of this unique hollow structure can be attributed to the rapid evaporation of water from the droplet surface within the tube furnace, accompanied by the rapid deposition of nitrates onto the particle surface, ultimately forming hollow oxide particles.

[0030] Figure 4 This is a TEM image of b-KZSM-5 molecular sieve, which exhibits a uniform hexagonal columnar shape and has a smaller b-axis thickness compared to ordinary ZSM5.

[0031] Figure 5 This demonstrates the catalytic performance of the Eu2O3 sample for methane oxidative coupling.

[0032] Figure 6 This section describes the catalytic performance of Eu₂O₃ tandem with b-KZSM-5 molecular sieve samples for methane oxidative coupling. Compared to Eu₂O₃, the EuO-bZSM5 catalyst exhibits better C₂ content in the temperature range of 550°C to 650°C. 2+ The yield increased significantly at 650°C. 2+ Selectivity increased from 27.8% to 35.0%, with ethylene selectivity increasing from 5.3% to 14.5%.

[0033] Figure 7 The catalytic activity of Eu₂O₃ in series with different molecular sieves was observed. When Eu₂O₃ was connected in series with pure silica neutral molecular sieves S₁ and bS₁, the yield decreased significantly. Except for the Eu₂O₃-MgAPO catalyst, where the yield remained relatively stable, the yield increased when Eu₂O₃ was connected in series with other acidic molecular sieves with different pore structures. Compared to the Eu₂O₃ catalyst, the conversion rates of each catalyst were relatively constant, but the reaction selectivity differed significantly, especially the ethylene selectivity.

[0034] Figure 8 The catalytic activity of Eu2O3 and b-KZSM-5 molecular sieves in different series configurations is compared. Since the reaction involves the transport of intermediates in the gas phase, the assembly method between the two components also has a significant impact on the performance of the composite catalyst. Experiments show that stacking molecular sieve layers has the best reaction activity.

[0035] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. A low-temperature methane oxidation coupling tandem catalyst, characterized in that, From individually packaged Eu 2-x M x The composition consists of O3 and b-KZSM-5, where M is Mg, Ca, Sr or Ba, x < 2, and the molar ratio of Eu to M is 2:(0-2).

2. The catalyst according to claim 1, characterized in that... The silicon-to-aluminum ratio of b-KZSM-5 is 20-200.

3. The catalyst according to claim 1, characterized in that, During use, Eu2M x O3 and b-KZSM-5 are mixed at a mass ratio of (0.5~2):(0.2~5), or Eu2M x O3 and b-KZSM-5 layers are stacked, and the inlet of the reactor is Eu. 2-x M x O3, the outlet of the reaction device is b-KZSM-5, Eu 2-x M x The thickness of the O3 layer is controlled between 0.5cm and 2cm, and the thickness of the b-KZSM-5 layer is controlled between 0.2cm and 5cm.

4. Low-temperature methane oxidation coupling tandem catalyst Eu2M x The method for preparing O3 is characterized by, The process includes the following steps: mixing Eu(NO3)3·6H2O deionized water solution and M(NO3)2·xH2O deionized water solution evenly, then evaporating using an ultrasonic atomizer, and the resulting spray is delivered through a nozzle carrier gas through a tube furnace at a temperature of 800℃-1200℃, collecting the deposited solid particles at the outlet, and calcining the collected solid particles in air at 500℃-1000℃.

5. The method according to claim 4, characterized in that, The nozzle carries air.

6. A method for preparing low-temperature methane oxidation coupling tandem catalyst b-KZSM-5, characterized in that, Includes the following steps: Step 1: ZSM-5 zeolite (b-ZSM-5) with a flake-like morphology was synthesized using a urea-assisted method. Step 2: Add b-ZSM-5 to potassium nitrate solution, stir in a water bath, filter, wash with water, dry in air, and calcine to obtain b-KZSM-5.

7. The method according to claim 6, characterized in that, Step 1: Detailed instructions Dissolve 4.0g of 25% tetrapropylammonium hydroxide aqueous solution in 5.1g of deionized water with stirring, and then add 2.8g of tetraethyl orthosilicate with stirring. Then add 0.07g aluminum isopropoxide and 0.65g urea, and continue stirring at room temperature until a gel forms; Then crystallize at 160℃-220℃ for two days, filter, wash with water, dry in air, and calcine at 450℃-650℃.

8. The method according to claim 6, characterized in that, Add 300 mg of b-ZSM-5 to 20 ml of 0.1 M potassium nitrate solution.

9. The method according to claim 6, characterized in that, Stirring in a water bath at 70℃-120℃.

10. The method according to claim 6, characterized in that, Calcination at 300℃-500℃.

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