A method for plasma-catalytic oxidation of methane to methanol

Abstract

The present invention belongs to the technical field of methane resource utilization and plasma chemical synthesis, and relates to a method for plasma-catalytic oxidation of methane to methanol. Specifically as follows: In a dielectric barrier plasma reactor, methane and oxygen are used as raw material gases. Methane and oxygen are activated through a DBD reactor, and methanol is synthesized in one step under the action of a non-metal supported catalyst. The method of the present invention has high methanol selectivity, and the catalyst used does not generate carbon dioxide. The supported boron-based catalyst adopted in the present invention enables the reaction to occur at low temperature and normal pressure, with mild conditions and simple operation; the process belongs to a one-step method, with a simple process flow, a green method, and solves the problem that the conventional catalytic oxidation of methane to methanol requires high temperature and high pressure. It is applicable to the oxidation synthesis of organic compounds from various lower hydrocarbons, and products such as formaldehyde, formic acid, ethanol, and acetic acid can be obtained in addition to methanol.

Description

Technical Field

[0001] The present invention belongs to the technical fields of methane resource utilization and plasma chemical synthesis, and relates to a method for plasma-catalytic oxidation of methane to methanol. Background Art

[0002] Methane (CH4) is a hydrocarbon with the lowest carbon content (the highest hydrogen content). It is a colorless and odorless gas under standard conditions. As the main component of natural gas, biogas, pit gas, shale gas, and natural gas hydrate resources, it has abundant reserves on the earth. Its combustion will produce a large amount of carbon dioxide (CO2). At the same time, both CH4 and CO2 are important components of greenhouse gases, and the former has a higher hazard. Therefore, methods for converting CH4 into clean and high-value energy have attracted much attention. Among the derivatives of CH4, methanol (CH3OH) has become the most attractive product. It has advantages such as good power performance, less pollution, easy pressurization into a liquid, and easy storage. It is considered a convenient, clean, and excellent energy fuel. At the same time, CH3OH is also one of the important raw materials for chemical production. It can react with isobutene to obtain MTBE (methyl tert-butyl ether), which is used as a high-octane unleaded gasoline additive or solvent. In addition, it can also be used to prepare other downstream high-value energy products such as olefins and propylene to solve the problem of resource shortage.

[0003] So far, the conversion of methane into chemicals mainly includes two parts: direct conversion and indirect conversion. The mature industrial route is achieved through indirect conversion under high temperature and pressure, mainly including steam or dry reforming to produce syngas (CO + H2), and subsequent Fischer-Tropsch synthesis. It is an energy-intensive two-stage process. These reactions usually occur between 700–800 °C, which will inevitably lead to the complete oxidation of methane to CO2 and catalyst deactivation, and it is also an expensive and energy-consuming process. Therefore, in recent years, more and more scholars have been committed to developing a process route for directly converting methane into methanol in one step.

[0004] The research on the direct oxidation of methane to methanol can be divided into two parts: homogeneous catalysis and heterogeneous catalysis. For the research on the homogeneous catalysis of methane to methanol, it is mostly carried out in a liquid phase system, and noble metal ions are used as the active centers for selective oxidation.

[0005] The published literature "Chemical Communications. 2006, 44, 4617-4619" reported a homogeneous system for the selective catalytic oxidation of methane to methanol by methyl bisulfate. The net reaction catalyzed by mercury ion Hg(II) is the oxidation of methane by concentrated sulfuric acid, producing methyl bisulfate, water, and sulfur dioxide. When the methane conversion rate is 50%, the selectivity for methyl bisulfate reaches 85% (the methanol yield is 43%; the main by-product is carbon dioxide), and the molar productivity is 10 -7mol cm -3 s -1 , the mercury(II) turnover frequency is 10 -3 s -1 。

[0006] In the process of homogeneous catalytic selective oxidation of methane to methanol, strong acids are used as reaction media and metal ions are used as catalysts, which can obtain high methane conversion rates and improved selectivity. However, strong acids themselves have strong corrosiveness, are extremely demanding on the relevant characteristics of reaction equipment, have too high catalyst costs and are not easy to recycle, and the products are also difficult to separate from the medium. The recovery of by-products (SO2) is also a difficult problem to solve. The above shortcomings limit the application of homogeneous catalytic processes in industrial production.

[0007] In recent years, heterogeneous catalytic oxidation of methane to methanol has become a research hotspot, and the focus is on developing suitable catalysts. The research on heterogeneous catalysis is mainly for gas-solid catalytic oxidation to methanol. Compared with homogeneous catalytic systems, it has the advantages of catalyst recyclability, environmental friendliness, and easy product separation. Currently, the main catalysts studied include noble metal catalysts and metal / molecular sieve catalysts.

[0008] Noble metal-based catalysts are considered to be the best catalysts for selective oxidation to methanol because the C-H activation energy barrier is much lower than that of non-noble metal-based catalysts. Currently, the noble metal catalysts studied more are mainly based on metal elements such as zirconium (Zr), ruthenium (Ru), iridium (Ir), platinum (Pt), palladium (Pd), etc.

[0009] Patent CN110639547A (application date: September 10, 2019) discloses an iridium-based heterogeneous composite oxide catalyst for methane oxidation to alcohol products and its preparation method. The catalyst consists of a copper oxide or molecular sieve support and an active component of iridium-based heterogeneous composite oxide, where the active component iridium-based composite oxide is composed of iridium oxide and at least one combination of ruthenium, gold, palladium, silver, platinum, copper, or their oxides. The reaction conditions of this patent are a methane pressure of 3 bar, an air pressure of 1 bar, a reaction temperature of 150 °C, a reaction time of 3 hours, and a stirring rate of the reaction kettle of 800 r / min. The alcohol products include methanol and ethanol, and the methanol selectivity is greater than or equal to 70%.

[0010] Patent CN111333487A (application date: April 16, 2020) discloses a method for photocatalytic oxidation of methane to methanol. Under light irradiation conditions, the Au / ZnO catalyst is added to water and stirred, and CH4 and O2 are introduced. The methane pressure is greater than or equal to 15 bar, and the methanol selectivity reaches 100%.

[0011] The open literature "Science, 2020, 367, 193 - 197" reported a "molecular fence" strategy. H2 and O2 in situ synthesized H2O2 on a catalyst with zeolite - fixed Au - Pd alloy nanoparticles, significantly improving the utilization rate of H2O2. Under mild conditions (70 °C), the methane conversion rate was 17.3%, and the methanol selectivity was 92%.

[0012] The open literature "ACS Catalysis. 2019, 9, 6938 - 6944" reported that the CuPdO2 / CuO interface in PdxCu 1-x O / C could effectively activate CH4 and convert it into CH3OH using H2O2 or O2 as oxidants under mild conditions, where the optimized Pd 0.3 Cu 0.7 O / C had a CH3OH yield of 4076.5 μmol g -1 , with a selectivity of 93.9%.

[0013] The open literature "Catalysis Today. 2020, 339, 48 - 53" reported that Pd - Au / CNTs showed excellent methanol productivity and selectivity for the direct conversion of methane to methanol under mild conditions. The strong interaction between Pd - Au nanoparticles and their carbon support inhibited the activation of methane. The strong interaction between Pd - Au nanoparticles and their carbon support inhibited the activation of methane.

[0014] Due to the limitations of noble metal catalysts, many scholars are committed to studying new inexpensive metal catalysts as alternatives to minimize material use to meet the cost - effective catalytic goal. The discovery of methane monooxygenase has made Cu - based or Fe - based zeolite molecular sieve catalysts favored by many scholars.

[0015] The open literature "Advanced Materials, 2023, 2208504" reported the correlation between the characteristic Cu size in Cu / ZSM - 5 catalysts and their methane direct oxidation to methanol reaction properties. The single - copper species stably anchored by aluminum pairs within the zeolite channels was identified as the key structure for improving the C1 oxygenate yield and methanol selectivity of methane direct oxidation.

[0016] The open literature "Journal of the American Chemical Society. 2005, 127, 1394 - 1395" reported that the specific microporous structures and Si / Al ratios of pentasil zeolites (such as ZSM - 5 and mordenite) may be particularly suitable for stabilizing bis(μ - oxo) - bridged bimetallic species, which can convert methane to methanol at low temperatures.

[0017] The open literature "ACS Central Science. 2016, 2, 424 - 429" conducted copper exchange on various commercially available zeolite molecular sieves with different topological structures at low reaction temperatures (483 - 498 K). Catalytic conversion was confirmed through isotope labeling experiments. The catalytic rate and apparent activation energy were affected by the zeolite topology. Cage-based zeolites (such as Cu-SSZ-13) had the highest catalytic rate.

[0018] The open literature "Fuel. 2023, 340, 127525" reported that at 200 °C, using O2 as the oxidant, methanol was synthesized by the photothermal catalytic oxidation of methane on Cu-MOR / g-C3N4. The optimal activation temperature, reaction temperature, and CH4 / O2 / H2O ratio were 500 °C, 200 °C, and 24 / 3 / 8, respectively. Due to the thermal catalytic interaction between Cu-MOR and g-C3N4, the methanol selectivity reached 46.71%. When visible light irradiated the catalyst, the methanol yield increased by 25.9%, and the maximum methanol yield was 3.09 μmol h -1 g cat -1 。

[0019] The open literature "Ind. Eng. Chem. Res. 1993, 584 - 587" reported the V2O5 / SiO2 catalyst. During the reaction, N2O was preferably used as the oxidant. Increasing the vanadium loading in the catalyst decreased the methanol selectivity. When the vanadium loading was 2%, the methanol selectivity reached a maximum of 57%.

[0020] The open patent CN101618327A (application date: December 21, 2007) disclosed a multi-component composite catalyst. This catalyst used mesoporous molecular sieve as the carrier, with Mo and V as the basic active components, and any two components selected from Fe, La, Co, and Ni were additionally added. In the reaction, O2 was used as the oxidant, and the maximum selectivities of the products methanol and formaldehyde were 80%, and the yields of methanol and formaldehyde were 9.6%.

[0021] The open literature "J. Catal. 2013, 300. 584 - 587" reported the Fe / ZSM-5 catalyst. Its typical feature was that it could react with the oxidant N2O at a low temperature (160 °C) to achieve the oxidation of methane to methanol. The surface α-O active sites played an important role in the cleavage of the C-H bond to form hydroxyl and methoxy groups.

[0022] According to existing research, the preparation of methanol by heterogeneous catalytic oxidation of methane has received extensive attention from scholars. However, the raw material prices of some oxidants (N2O and H2O2) are currently too high. In contrast, O2 is more economically applicable, but the reaction energy barrier is relatively high. Generally, a relatively high temperature is required to activate methane, and the oxidation products, methanol or methanol derivatives, are easily over-oxidized. Therefore, how to activate methane with O2 to prepare methanol at low temperature is of great significance for large-scale applications.

[0023] Non-thermal equilibrium plasma technology can activate CH4 and O2 through high-energy electrons, and then carry out the reaction under mild reaction conditions. The gas temperature of non-thermal plasma is generally close to room temperature. Under this mild reaction temperature, the deep oxidation of the generated target product into carbon dioxide can be effectively prevented.

[0024] The public literature "Chemical Engineering and Processing: Process Intensification. 2008, 47, 780 - 786" reported a comprehensive study on the performance of dielectric barrier discharge combined with Cu–Zn–Al (CZA)-based catalysts for partial oxidation of methane. It was found that adding copper zinc aluminum oxide (CZA) catalyst could improve the conversion rate of plasma-catalytic methane conversion to methanol, and the methanol selectivity was almost twice that of non-catalyst NTP.

[0025] The public literature "Chem. Eng. J. 2011, 166, 288 - 293" reported a method for plasma conversion of methane to synthetic fuels. At room temperature, the selectivity of liquid products was 70%, and the selectivity of methanol was less than 15%.

[0026] The public literature "Applied Catalysis B: Environmental. 2021, 296, 120384" reported that with the promotion of nickel-based catalysts, methane could be selectively oxidized to methanol in CH4 / O2 plasma with excellent catalytic stability. CH4 achieved a liquid oxygenate selectivity of 76% and a CH3OH selectivity of 42% in a single plasma, and when a 10 wt.% NiO / γ-Al2O3 catalyst was added, the selectivities were further increased to 81% and 50% respectively. Compared with a single plasma, the energy efficiency of plasma catalysis increased by 84% (from 0.76 to 1.4 mol / kWh).

[0027] The open literature "Fuel Processing Technology. 2018, 179, 32 - 41" reported that in a dielectric barrier discharge plasma reactor, the partial oxidation of CH4 to CH3OH was studied, and the effects of various parameters such as the feed gas ratio, SIE, total flow rate, and packing were investigated. Glass beads had the best selectivity for methanol, approximately 35%, while in a DBD reactor without packing, its selectivity was approximately 25%. In a similar manner, with glass beads filled, the yield of methanol increased to 5.4%, while in a DBD reactor without filling, the yield of methanol increased to 1.7%.

[0028] The open patent CN111974393A (application date: September 15, 2020) discloses a preparation method of a catalyst for low-temperature plasma-photocoupled methane to methanol and a preparation method of methanol. The plasma generates high-energy electrons to activate methane at normal temperature and pressure. Adding a Cu-C catalyst enables the light generated by the plasma to be utilized to activate H2O, further increasing the yield of methanol.

[0029] The open patent CN113713799A (application date: May 25, 2020) discloses a preparation method of a metal-supported catalyst for low-temperature plasma methane and oxygen to methanol and a method for preparing methanol. This method uses a NiO / γ-Al2O3 catalyst and combines it with a coaxial dielectric barrier discharge plasma reactor with circulating water as the ground electrode to oxidize methane to methanol with molecular oxygen as the oxidant at normal temperature and pressure.

[0030] The open patent CN113816831A (application date: September 26, 2021) discloses a preparation method of a catalyst for low-temperature plasma-thermal coupling methane and steam reforming to methanol and a method for preparing methanol. The plasma generates high-energy electrons to activate methane at normal temperature and pressure. Adding a Cu-based catalyst enables the active molecules such as free radicals generated by the plasma to combine, further increasing the yield of methanol. The methane conversion rate is greater than or equal to 3.4%, and the methanol selectivity is greater than or equal to 46.67%.

[0031] In recent years, due to the good anti-depth oxidation performance of boron-based catalysts, they have attracted much attention in the field of oxidative dehydrogenation of light alkanes to olefins, and they may show good performance in the field of methane oxidation.

[0032] The open literature "Nature Communications. 2020, 11, 5693" reported that the B2O3-based catalyst has selectivity in the direct conversion of methane to formaldehyde and carbon monoxide (selectivity of about 94%, formaldehyde / carbon monoxide ratio of about 1, conversion rate of 6%), and is highly stable (running time of more than 100 hours) in a fixed-bed reactor (550 °C, 100 kPa, space velocity 4650 mL g cat -1 h -1 ). Further development of this non-metal oxide catalyst will bring innovative strategies and catalyst systems for the efficient and selective oxidation of methane (and other alkanes) to valuable chemicals.

[0033] In summary, in the previous technical solutions for the production of methanol from methane and oxygen, the selectivity of methanol was relatively low under low-temperature and atmospheric-pressure conditions, with the selectivity of methanol being less than 45%, and the products were easily deeply oxidized to carbon dioxide, resulting in a relatively high selectivity of by-product carbon dioxide; the selectivities of several good catalysts in this patent are all greater than 45%, and no by-product carbon dioxide is generated. Summary of the Invention

[0034] To solve the above problems, the object of the present invention is to provide a method for plasma-catalytic oxidation of methane to methanol.

[0035] Technical principle: Under the reaction conditions of low temperature and atmospheric pressure, high-energy electrons (1 - 10 eV) generated by dielectric barrier discharge (DBD) are used to activate small molecules (CH4, O2). Under the collision of high-energy electrons, reactant molecules (CH4, O2) are excited and dissociated to generate active groups such as CH3·, CH2·, CH· and O2*, O·. The introduction of the catalyst further changes the reaction path of free radicals, thereby affecting the selectivity distribution of products.

[0036] Technical solution of the present invention:

[0037] A method for plasma-catalytic oxidation of methane to methanol is as follows: In a dielectric barrier plasma reactor, methane and oxygen are used as raw material gases, methane and oxygen are activated through a DBD reactor, and methanol is synthesized in one step under the action of a non-metal supported catalyst.

[0038] The non-metal supported catalyst is a supported boron-based catalyst, including an active component and a carrier. The active component is boron B, the carrier is a metal oxide or zeolite molecular sieve, and the mass percentage of the active component in the catalyst is 1 - 40%.

[0039] The molar ratio of methane to oxygen is 1:1 - 5:1, and the total gas velocity of the feed gas is 100 - 300 mL / min; the dielectric barrier discharge uses an AC power supply, the power frequency is 1 kHz - 50 kHz, the output power is 10 - 40 W, and the reaction temperature is controlled at 100 - 500 °C by changing the discharge power, and the width of the discharge region is 20 - 100 mm.

[0040] In the non-metal supported catalyst, the active component precursor is H3BO3.

[0041] The carrier is one or more mixtures of SiO2, Al2O3, ZrO2, CeO2, Fe2O3, MgO, and zeolite molecular sieve.

[0042] Preferably, the non-metal supported catalyst is one or more mixtures of B2O3 / Fe2O3, B2O3 / SAPO-34, and B2O3 / S-1.

[0043] The non-metal supported catalyst is prepared by the incipient wetness impregnation method, and the specific steps are as follows:

[0044] (1) Calcining the carrier: The temperature is 200 - 500 °C, and calcining for 2 - 5 h to remove moisture and impurities;

[0045] (2) Incipient wetness impregnation of the catalyst: Dissolve the active component precursor in deionized water; then add the carrier and stir in a constant temperature water bath to obtain a solid product and let it stand, and perform room temperature aging treatment for 10 - 12 h; wherein, according to the loading amount of the catalyst and the mass of the carrier, the mass ratio of the carrier to the active component precursor is obtained, and the mass ratio of the carrier to deionized water is 1:(2 - 50);

[0046] (3) Preliminary water removal: Dry the aged solid product at a temperature of 100 - 130 °C for 10 - 15 h;

[0047] (4) Calcining: Treat the dried solid product at a calcining temperature of 400 - 650 °C for 4 - 10 h;

[0048] (5) Tabletting: Tablet the powder obtained by calcining and sieve to obtain a non-metal supported catalyst with a mesh size of 20 - 40.

[0049] Preferably, the reaction temperature provided by the plasma catalytic reactor is 150 °C, the total gas velocity of methane and oxygen in the discharge zone is 300 mL / min; the output power of the plasma power supply is 15 W, the discharge frequency is 14.9 kHz; the width of the discharge region is 20 mm.

[0050] The described plasma catalytic reactor is a wire-cylinder type single dielectric barrier discharge reactor. The outside of the reactor is wrapped with aluminum foil 20 mm wide, and then metal wire is wound around the aluminum foil as the grounding electrode of the reactor. The upper end of the reactor is equipped with a rubber stopper with a central hole as the upper head, and a metal rod is installed through the central hole of the rubber stopper as the high-voltage electrode of the reactor. The distance between the outer wall of the metal rod and the inner wall of the cylindrical reactor is 0.3 - 30 mm. The reactor is a single-layer dielectric insulating material. The air inlet is above the discharge area. The lower end of the reactor is connected to a liquid-phase collector, and the collector is placed in a cold trap to collect the liquid phase. The catalyst is filled into the discharge area of the reactor, and the catalyst bed is supported by a quartz sand sieve plate. A temperature controller is externally connected to the discharge area to display the reaction temperature in real time.

[0051] Among them, the high-voltage electrode and the grounding electrode are made of copper, iron, tungsten, aluminum or stainless steel; the barrier medium is made of quartz glass, hard glass, alumina ceramic, polytetrafluoroethylene or non-metallic composite material.

[0052] The beneficial effects of the present invention are as follows:

[0053] The methanol selectivity of the method of the present invention is high. The methanol selectivities of the B2O3 / Fe2O3, B2O3 / SAPO-34, and B2O3 / S-1 catalysts are all greater than 45%, and no carbon dioxide is generated by the catalysts used in the present invention.

[0054] The supported boron-based catalyst adopted by the present invention enables the reaction to occur at low temperature and normal pressure, with mild conditions and simple operation. The process belongs to a one-step method, with a simple process flow and a green method, solving the problem that the conventional catalytic oxidation of methane to methanol requires high temperature and high pressure. It is applicable to the oxidation synthesis of organic compounds from various lower hydrocarbons, and products such as formaldehyde, formic acid, ethanol, and acetic acid can be obtained in addition to methanol. Description of the Drawings

[0055] Figure 1 It is the experimental device diagram of the CH4 / O2 plasma reaction of the present invention; in the figure: 1 plasma power supply; 2 high-voltage probe; 3 digital oscilloscope; 4 mass flowmeter; 5 methane; 6 oxygen; 7 temperature control device; 8 current probe; 9 capacitor; 10 low-voltage probe; 11 soap film flowmeter; 12 gas product; 13 liquid product; 14 cold trap; 15 gas chromatograph a; 16 gas chromatograph b.

[0056] Figure 2 It is the GC-MS result analysis diagram of the products of the methane selective oxidation reaction in the examples. Detailed Embodiments

[0057] The following describes the specific embodiments of the present invention in detail in combination with the technical solutions and the drawings.

[0058] The experimental device adopted in the embodiments of the present invention is as follows Figure 1 shown, including a plasma power supply 1, a high-voltage probe 2, a digital oscilloscope 3, a mass flowmeter 4, methane 5, oxygen 6, a temperature control device 7, a current probe 8, a capacitor 9, a low-voltage probe 10, a soap film flowmeter 11, a gas product 12, a liquid product 13, and a cold trap 14. Among them, the plasma power supply 1 is connected to the high-voltage probe 2, the digital oscilloscope 3 is connected to the high-voltage probe 2, the current probe 8, and the low-voltage probe 10. The current probe 8 is connected to the low-voltage probe 10 and a capacitor 9 is provided between them; methane 5 and oxygen 6 are fed into the reactor, and mass flowmeters 4 are provided for both. A temperature control device 7 is provided in the reactor. The gas product 12 in the cold trap 14 is measured for flow rate by the soap film flowmeter 11. The gas product 12 and the liquid product 13 are respectively connected to a gas chromatograph a 15 and a gas chromatograph b 16.

[0059] Comparative Example 1

[0060] The reaction pressure is 0.1 MPa. Methane and oxygen are introduced into the discharge reactor at a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). First, the reaction raw material gas is introduced to displace the air in the reaction system, and at the same time, the raw material gas is pre-mixed for 10 min. After the raw material gas is uniformly mixed, the plasma power supply is turned on to start discharging. The reactor structure is a single dielectric barrier wire-cylinder type reactor. The stainless steel rod installed in the quartz tube serves as the internal electrode, and the aluminum foil wound around the outer wall of the quartz tube serves as the grounding electrode. The diameter of the internal electrode is 2 mm, the discharge gap is 3.5 mm, and the length of the discharge area is 20 mm. A sieve plate is provided at the lowest end of the discharge area in the quartz tube.

[0061] The plasma discharge parameters are: power 15 W, frequency 14.9 kHz, and the discharge duration is 3 h. The reaction products include gas and liquid phases. The gas phase products are directly analyzed online by gas chromatography, and the liquid phase products are collected through a cold trap and qualitatively and quantitatively analyzed by gas chromatography and GC-MS, 1 1H-NMR analysis. The reaction results are: the methane conversion rate is 2.4%, the selectivity of the liquid phase product is 59.6%, the selectivity of methanol is 28.4%, and the by-products include ethane, ethylene, carbon monoxide, formaldehyde, formic acid, ethanol, propanol, acetaldehyde, acetic acid, propionaldehyde, and acetone.

[0062] Comparative Example 2

[0063] Repeat Comparative Example 1. Charge 0.6 g of the catalyst iron oxide (α-Fe2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles. Before the reaction, the catalyst is calcined at 500 °C for 5 h. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, the product analysis shows that the methane conversion rate is 2.89%, the selectivity of the liquid-phase product is 41.16%, and the selectivity of methanol is 14.88%.

[0064] Example 1:

[0065] Repeat Comparative Example 2. Charge 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / α-Fe2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles. The loading amount of the active component in terms of elemental B is 20% (by weight), and the catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, the product analysis shows that the methane conversion rate is 3.38%, the selectivity of the liquid-phase product is 79.82%, and the selectivity of methanol is 48.77%.

[0066] Example 2:

[0067] Repeat Comparative Example 2. Charge 0.6 g of boron-loaded magnesium oxide catalyst (denoted as B2O3 / MgO) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles. The loading amount of the active component in terms of elemental B is 20% (by weight), and the catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, the product analysis shows that the methane conversion rate is 3.21%, the selectivity of the liquid-phase product is 74.99%, and the selectivity of methanol is 36.66%.

[0068] Example 3:

[0069] Repeat Comparative Example 2, and charge 0.6 g of boron-loaded cerium oxide catalyst (denoted as B2O3 / CeO2) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, the loading of the active ingredient calculated as element B is 20% (by weight), and the catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After discharging for 3 h, product analysis shows that the methane conversion rate is 2.97%, the selectivity of the liquid-phase product is 75.56%, and the selectivity of methanol is 36.86%.

[0070] Example 4:

[0071] Repeat Comparative Example 2, and charge 0.6 g of boron-loaded zirconium oxide catalyst (denoted as B2O3 / ZrO2) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, the loading of the active ingredient calculated as element B is 20% (by weight), and the catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After discharging for 3 h, product analysis shows that the methane conversion rate is 2.61%, the selectivity of the liquid-phase product is 63.62%, and the selectivity of methanol is 35.56%.

[0072] Example 5:

[0073] Repeat Comparative Example 2, and charge 0.6 g of boron-loaded S-1 catalyst (denoted as B2O3 / S-1) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, the loading of the active ingredient calculated as element B is 20% (by weight), and the catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After discharging for 3 h, product analysis shows that the methane conversion rate is 3.62%, the selectivity of the liquid-phase product is 73.19%, and the selectivity of methanol is 46.73%.

[0074] Example 6:

[0075] Repeat Comparative Example 2, and charge 0.6 g of boron-loaded silica catalyst (denoted as B2O3 / SiO2) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the flow rate of methane is 200 mL / min and the flow rate of oxygen is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, the loading of the active ingredient based on element B is 20% (by weight), and the catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After discharging for 3 h, the product analysis shows that the methane conversion rate is 3.59%, the selectivity of the liquid-phase product is 67.44%, and the selectivity of methanol is 35.40%.

[0076] Example 7:

[0077] Repeat Comparative Example 2, and charge 0.6 g of boron-loaded SAPO-34 catalyst (denoted as B2O3 / ZnO) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the flow rate of methane is 200 mL / min and the flow rate of oxygen is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, the loading of the active ingredient based on element B is 20% (by weight), and the catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After discharging for 3 h, the product analysis shows that the methane conversion rate is 1.80%, the selectivity of the liquid-phase product is 69.93%, and the selectivity of methanol is 55.72%.

[0078] Example 8:

[0079] Repeat Comparative Example 2, and charge 0.6 g of boron-loaded alumina catalyst (denoted as B2O3 / Al2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the flow rate of methane is 200 mL / min and the flow rate of oxygen is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, the loading of the active ingredient based on element B is 20% (by weight), and the catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After discharging for 3 h, the product analysis shows that the methane conversion rate is 3.79%, the selectivity of the liquid-phase product is 70.84%, and the selectivity of methanol is 31.26%.

[0080] Table 1 Evaluation results of catalytic performance of boron supported on different metal oxide carriers

[0081]

[0082] None of the boron-based catalysts used in all the above embodiments produced by-products of carbon dioxide, and the methane conversion rates of the three catalysts, namely B2O3 / Fe2O3, B2O3 / SAPO-34, and B2O3 / S-1, were all greater than 3%, and the methanol selectivities were all approximately 45%. Taking B2O3 / Fe2O3 as an example, the loading amounts of the catalyst preparation, different reaction temperatures, and different methane-to-oxygen molar ratios are listed in the embodiments.

[0083] Example 9:

[0084] Repeat Example 1. Load 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) into the discharge area of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, and the loading amount of the active ingredient calculated as element B is 1% (by weight). The catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After discharging for 3 h, the product analysis shows that the methane conversion rate is 1.42%, the selectivity of the liquid-phase product is 74.20%, and the selectivity of methanol is 18.96%.

[0085] Example 10:

[0086] Repeat Example 1. Load 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) into the discharge area of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, and the loading amount of the active ingredient calculated as element B is 5% (by weight). The catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After discharging for 3 h, the product analysis shows that the methane conversion rate is 1.69%, the selectivity of the liquid-phase product is 75.55%, and the selectivity of methanol is 22.78%.

[0087] Example 11:

[0088] Repeat Example 1. Charge 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, and the loading of the active ingredient calculated as element B is 10% (by weight). The catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, analysis of the products shows that the methane conversion rate is 2.63%, the selectivity for liquid-phase products is 74.82%, and the selectivity for methanol is 29.10%.

[0089] Example 12:

[0090] Repeat Example 1. Charge 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, and the loading of the active ingredient calculated as element B is 15% (by weight). The catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, analysis of the products shows that the methane conversion rate is 2.93%, the selectivity for liquid-phase products is 75.61%, and the selectivity for methanol is 49.36%.

[0091] Example 13:

[0092] Repeat Example 1. Charge 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, and the loading of the active ingredient calculated as element B is 17% (by weight). The catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, analysis of the products shows that the methane conversion rate is 3.17%, the selectivity for liquid-phase products is 80.88%, and the selectivity for methanol is 53.96%.

[0093] Example 14:

[0094] Example 1 was repeated. 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) was loaded into the discharge zone of a dielectric barrier discharge plasma reactor. Methane and oxygen were in a molar ratio of 2:1 (where the methane flow rate was 200 mL / min and the oxygen flow rate was 100 mL / min). The catalyst was in the form of 20-40 mesh particles, with the active ingredient loading amount calculated as element B being 25% (by weight), and the catalyst calcination temperature was 600 °C. The discharge parameters were set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, analysis of the products showed that the methane conversion rate was 3.97%, the selectivity for liquid-phase products was 78.60%, and the selectivity for methanol was 43.24%.

[0095] Example 15:

[0096] Example 1 was repeated. 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) was loaded into the discharge zone of a dielectric barrier discharge plasma reactor. Methane and oxygen were in a molar ratio of 2:1 (where the methane flow rate was 200 mL / min and the oxygen flow rate was 100 mL / min). The catalyst was in the form of 20-40 mesh particles, with the active ingredient loading amount calculated as element B being 30% (by weight), and the catalyst calcination temperature was 600 °C. The discharge parameters were set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, analysis of the products showed that the methane conversion rate was 3.90%, the selectivity for liquid-phase products was 80.57%, and the selectivity for methanol was 40.19%.

[0097] Example 16:

[0098] Example 1 was repeated. 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) was loaded into the discharge zone of a dielectric barrier discharge plasma reactor. Methane and oxygen were in a molar ratio of 2:1 (where the methane flow rate was 200 mL / min and the oxygen flow rate was 100 mL / min). The catalyst was in the form of 20-40 mesh particles, with the active ingredient loading amount calculated as element B being 40% (by weight), and the catalyst calcination temperature was 600 °C. The discharge parameters were set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, analysis of the products showed that the methane conversion rate was 3.26%, the selectivity for liquid-phase products was 71.72%, and the selectivity for methanol was 37.57%.

[0099] Table 2 Evaluation results of the catalytic performance of B2O3 / Fe2O3 catalysts with different boron loadings (by weight)

[0100]

[0101] The preferred B loading amount is 17% (by weight).

[0102] Example 18:

[0103] Repeat Example 13, and charge 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, and the loading amount of the active ingredient based on element B is 17% (by weight). The catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: the reaction temperature is 200 °C, and the frequency is 14.9 kHz. After discharging for 3 h, the product analysis shows that the methane conversion rate is 3.41%, the selectivity of the liquid-phase product is 84.03%, and the selectivity of methanol is 47.25%.

[0104] Example 19:

[0105] Repeat Example 13, and charge 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, and the loading amount of the active ingredient based on element B is 17% (by weight). The catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: the reaction temperature is 250 °C, and the frequency is 14.9 kHz. After discharging for 3 h, the product analysis shows that the methane conversion rate is 3.61%, the selectivity of the liquid-phase product is 87.55%, and the selectivity of methanol is 37.85%.

[0106] Example 20:

[0107] Repeat Example 13, and charge 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen are in a molar ratio of 2:1 (where the methane flow rate is 200 mL / min and the oxygen flow rate is 100 mL / min). The catalyst is in the form of 20-40 mesh particles, and the loading amount of the active ingredient based on element B is 171% (by weight). The catalyst calcination temperature is 600 °C. The discharge parameters are set as follows: the reaction temperature is 300 °C, and the frequency is 14.9 kHz. After discharging for 3 h, the product analysis shows that the methane conversion rate is 3.73%, the selectivity of the liquid-phase product is 88.94%, and the selectivity of methanol is 29.21%.

[0108] Example 21:

[0109] Example 13 was repeated. 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) was loaded into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen were in a molar ratio of 2:1 (where the methane flow rate was 200 mL / min and the oxygen flow rate was 100 mL / min). The catalyst was in the form of 20-40 mesh particles, with the active ingredient loading of 17% (by weight) based on elemental B, and the catalyst calcination temperature was 600 °C. The discharge parameters were set as follows: reaction temperature 130 °C, frequency 14.9 kHz. After 3 h of discharge, analysis of the products showed that the methane conversion was 2.87%, the selectivity of the liquid-phase products was 81.61%, and the selectivity of methanol was 45.31%.

[0110] Table 3 Evaluation results of the catalytic performance of B2O3 / Fe2O3 catalyst at different reaction temperatures

[0111]

[0112]

[0113] The preferred reaction temperature is 150 °C.

[0114] Example 22:

[0115] Example 13 was repeated. 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) was loaded into the discharge zone of the dielectric barrier discharge plasma reactor. Methane and oxygen were in a molar ratio of 1:1 (where the methane flow rate was 150 mL / min and the oxygen flow rate was 150 mL / min). The catalyst was in the form of 20-40 mesh particles, with the active ingredient loading of 17% (by weight) based on elemental B, and the catalyst calcination temperature was 600 °C. The discharge parameters were set as follows: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, analysis of the products showed that the methane conversion was 3.57%, the selectivity of the liquid-phase products was 69.22%, and the selectivity of methanol was 45.41%.

[0116] Example 23:

[0117] Example 13 was repeated. 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) was filled in the discharge area of the dielectric barrier discharge plasma reactor. Methane and oxygen were in a molar ratio of 3:1 (where the methane flow rate was 225 mL / min and the oxygen flow rate was 75 mL / min). The catalyst was in the form of 20-40 mesh particles, with the active ingredient loading amount calculated as element B being 17% (by weight), and the catalyst calcination temperature was 600 °C. The discharge parameters were set as: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, product analysis showed that the methane conversion rate was 2.61%, the selectivity of the liquid-phase product was 81.09%, and the selectivity of methanol was 48.59%.

[0118] Example 24:

[0119] Example 13 was repeated. 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) was filled in the discharge area of the dielectric barrier discharge plasma reactor. Methane and oxygen were in a molar ratio of 4:1 (where the methane flow rate was 240 mL / min and the oxygen flow rate was 60 mL / min). The catalyst was in the form of 20-40 mesh particles, with the active ingredient loading amount calculated as element B being 17% (by weight), and the catalyst calcination temperature was 600 °C. The discharge parameters were set as: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, product analysis showed that the methane conversion rate was 2.07%, the selectivity of the liquid-phase product was 83.64%, and the selectivity of methanol was 43.56%.

[0120] Example 25:

[0121] Example 13 was repeated. 0.6 g of boron-loaded iron oxide catalyst (denoted as B2O3 / Fe2O3) was filled in the discharge area of the dielectric barrier discharge plasma reactor. Methane and oxygen were in a molar ratio of 5:1 (where the methane flow rate was 250 mL / min and the oxygen flow rate was 50 mL / min). The catalyst was in the form of 20-40 mesh particles, with the active ingredient loading amount calculated as element B being 17% (by weight), and the catalyst calcination temperature was 600 °C. The discharge parameters were set as: power 15 W (reaction temperature 150 °C), frequency 14.9 kHz. After 3 h of discharge, product analysis showed that the methane conversion rate was 1.71%, the selectivity of the liquid-phase product was 84.21%, and the selectivity of methanol was 40.45%.

[0122] Table 4 Evaluation results of the catalytic performance of B2O3 / Fe2O3 catalysts with different methane-oxygen molar ratios

[0123]

[0124] The preferred methane-oxygen molar ratio is 2:1.

Claims

1. A method for plasma-catalytic oxidation of methane to methanol, characterized in that, In a dielectric barrier plasma reactor, using methane and oxygen as raw material gases, methane and oxygen are activated through a DBD reactor, and methanol is synthesized in one step under the action of a non-metal supported catalyst. The non-metal supported catalyst is a supported boron-based catalyst, including an active component and a support. The active component is boron B, and the support is a metal oxide or zeolite molecular sieve. The mass percentage of the active component in the catalyst is 17 - 40%. The molar ratio of methane to oxygen is 1:1 - 5:1, and the total gas velocity of the raw material gas is 100 - 300 mL / min. The dielectric barrier discharge uses an alternating current power supply, the power frequency is 1 kHz - 50 kHz, the output power is 10 - 40 W, and the reaction temperature is controlled at 150 - 250 °C by changing the discharge power. The width of the discharge area is 20 - 100 mm. In the non-metal supported catalyst, the precursor of the active component is H3BO3. The support is one or more mixtures of SiO2, Al2O3, ZrO2, CeO2, Fe2O3, and MgO.

2. The method for preparing methanol by plasma-catalytic oxidation of methane according to claim 1, characterized in that, The non-metal supported catalyst is one or two mixtures of B2O3 / Fe2O3 and B2O3 / Al2O3.

3. A method for preparing methanol by plasma-catalytic oxidation of methane according to claim 1, characterized in that, The non-metal supported catalyst is prepared by the excess impregnation method. The specific steps are as follows: (1) Calcining the support: The temperature is 200 - 500 °C, and calcining for 2 - 5 h to remove moisture and impurities. (2) Excess impregnation of the catalyst: Dissolve the precursor of the active component in deionized water; then add the support and stir in a constant temperature water bath to obtain a solid product and let it stand for treatment, with room temperature aging for 10 - 12 h. Among them, according to the loading amount of the catalyst and the mass of the support, the mass ratio of the support to the precursor of the active component is obtained, and the mass ratio of the support to deionized water is 1:(2 - 50). (3) Preliminary water removal: Dry the aged solid product at a temperature of 100 - 130 °C for 10 - 15 h. (4) Calcining: Treat the dried solid product at a calcining temperature of 400 - 650 °C for 4 - 10 h. (5) Tabletting: Tablet the powder obtained by calcining and sieve it to obtain a non-metal supported catalyst with a mesh size of 20 - 40.

4. A method for preparing methanol by plasma-catalytic oxidation of methane according to claim 1, characterized in that, The reaction temperature provided by the plasma catalytic reactor is 150 °C, and the total gas velocity of methane and oxygen in the discharge area is 300 mL / min. The output power of the plasma power supply is 15 W, and the discharge frequency is 14.9 kHz. The width of the discharge area is 20 mm.

5. A method for preparing methanol by plasma-catalytic oxidation of methane according to claim 1 or 2 or 3 or 4, characterized in that, The described plasma catalytic reactor is a wire-cylinder type single dielectric barrier discharge reactor. The outside of the reactor is wrapped with aluminum foil 20 mm wide, and then metal wire is wound around the aluminum foil to serve as the grounding electrode of the reactor; a rubber stopper with a central hole is equipped at the upper end of the reactor as the upper head, and a metal rod is installed through the central hole of the rubber stopper as the high-voltage electrode of the reactor; the distance between the outer wall of the metal rod and the inner wall of the cylindrical reactor is 0.3 - 30 mm; the reactor is made of a single-layer dielectric insulating material; an air inlet is provided above the discharge area, the lower end of the reactor is connected to a liquid-phase collector, and the collector is placed in a cold trap to collect the liquid phase; the catalyst is filled into the discharge area of the reactor, and the catalyst bed is supported by a quartz sand sieve plate; a temperature controller is externally connected to the discharge area to display the reaction temperature in real time.

6. A method for preparing methanol by plasma-catalytic oxidation of methane according to claim 5, characterized in that, The materials of the described high-voltage electrode and the grounding electrode are copper, iron, tungsten, aluminum or stainless steel; the barrier medium is made of quartz glass, hard glass, alumina ceramic, polytetrafluoroethylene or non-metallic composite material.

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