Method for synthesizing methyl hydroperoxide by photocatalytic selective oxidation of methane

Abstract

The invention discloses a method for synthesizing methyl hydroperoxide through photocatalytic methane selective oxidation, and relates to the technical field of catalytic synthesis. The method comprises the following steps: ultrasonically dispersing a titanium dioxide supported catalyst in water, adding into a reaction kettle, sequentially introducing oxygen and methane into the reaction kettle until the required pressure is reached, and stirring and reacting for a period of time under an illumination condition to obtain methyl hydroperoxide and other oxidation products, in the photocatalytic reaction process, the dosage of the titanium dioxide supported catalyst is 0.1-10 mg, and the dosage of the water solvent is 10-100 mL. According to the synthetic method of methyl hydroperoxide provided by the invention, methane is used as a raw material, oxygen is used as an oxidant, water is used as a solvent, methane is selectively converted into methyl hydroperoxide under the action of illumination and the titanium dioxide supported gold catalyst, the generation rate of the obtained methyl hydroperoxide can reach 2335.5 [mu] mol / g / h, and the selectivity reaches 73.6%; the preparation process and the catalytic reaction process of the catalyst are economical and green, and the catalyst has a wide application prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of catalytic synthesis, and particularly relates to a method for synthesizing methyl hydroperoxide by photocatalytic selective oxidation of methane. Background Art

[0002] Methane, as the main component of natural gas and shale gas, has the advantages of abundant reserves and low prices. It is considered to be the most promising bulk chemical raw material in the field of replacing increasingly depleted energy sources such as coal and oil to produce liquid fuels and basic chemicals. The development of methane catalytic conversion technology has broad development prospects.

[0003] The nonpolar tetrahedral structure of methane gives it a high CH bond dissociation energy (434 kJ / mol). The activation and directional conversion of methane is considered the "holy grail" in the field of chemistry and catalysis. At present, the industry mainly uses the indirect method to convert methane, converting methane into synthesis gas through steam reforming at high temperature (>700 °C), and then the synthesis gas is used for Fischer-Tropsch synthesis or methanol synthesis. Methanol is an important basic chemical raw material and is widely used in the synthesis of bulk chemicals such as olefins and acetic acid. However, the above two-step reaction process has high energy consumption, low total carbon utilization, high environmental load (emitting a large amount of greenhouse gas carbon dioxide), and the catalyst is prone to carbon deposition and deactivation at high temperature. Direct oxidation of methane to produce methanol can avoid the high energy consumption synthesis gas process, but the direct conversion reported so far needs to be carried out at high temperature (200~500 °C) or use expensive hydrogen peroxide (higher in price than product methanol) as an oxidant, which limits the application of these processes.

[0004] Sunlight is a clean energy source that is inexhaustible. In recent years, photocatalytic methane oxidation to methanol has been regarded as a very promising technology. However, methanol is extremely unstable and can easily undergo deep oxidation to formaldehyde, carbon dioxide and other over-oxidation products, resulting in low product selectivity and poor process economy. Developing new technical routes is an effective way to solve this problem. Most studies have shown that methyl hydroperoxide is the primary product of the above reaction, which is easily reduced to methanol. If methyl hydroperoxide can be first obtained with high selectivity through methane oxidation and then reduced to methanol, the selectivity of methanol will be greatly improved.

[0005] Pu Lingyu et al. (Nano Research, 2024, 17(4): 2473-2480) used brookite titanium dioxide as a photocatalyst and hydrogen peroxide as an oxidant at room temperature and light conditions to highly selectively oxidize methane to methyl hydroperoxide with a selectivity close to 100%. However, the higher price of hydrogen peroxide than methanol hinders the prospects for industrial application of this process.

[0006] Wu Wenting et al. (Nano Research, 2024, 17(5): 3810-3818) used zinc oxide loaded with gold nanoparticles of specific particle size and exposed crystal surfaces as a catalyst and molecular oxygen as an oxidant to achieve highly selective photocatalytic oxidation of methane to produce methyl hydroperoxide at room temperature. However, the controllable synthesis of gold requires high requirements, and improper control has a great impact on product selectivity. In addition, the high cost of zinc oxide limits its application prospects.

[0007] Therefore, it is of great significance to develop a catalyst with simple preparation process, low cost and high activity. In view of the shortcomings of the prior art, it is urgent to provide a cheap and efficient method for preparing methyl hydroperoxide by oxidation of methane. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a method for preparing a catalyst for preparing methyl hydrogen peroxide by oxidation of methane and a method for preparing methyl hydrogen peroxide by photocatalytic selective oxidation of methane in view of the deficiencies in the prior art.

[0009] To solve the above technical problems, the present invention is achieved through the following technical solutions: The present invention is a method for synthesizing methyl hydroperoxide by selective oxidation of methane through photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported catalyst in water and adding the catalyst into a reactor, sequentially introducing oxygen and methane into the reactor until the required pressure is reached, stirring the reaction under light conditions for a period of time to obtain methyl hydroperoxide and other oxidation products, wherein the amount of the titanium dioxide-supported catalyst used in the photocatalytic reaction is 0.1-10 mg, and the amount of the water solvent used is 10-100 mL.

[0010] The present invention is further preferably that the titanium dioxide-supported catalyst is a titanium dioxide-supported gold catalyst, wherein the mass fraction of the gold active component in the titanium dioxide-supported gold catalyst is 0.1-2 wt %.

[0011] It is further preferred that the photocatalytic reaction is carried out in a high-pressure reactor with a quartz window on the top, wherein the oxygen partial pressure is 1-10 bar, and the methane partial pressure is 10-20 bar.

[0012] The present invention is further preferred that the photocatalytic reaction temperature is 25°C, the reaction time is 0.5-5 h, the light source required for the reaction is a xenon lamp and the emission wavelength of the xenon lamp is 200-1300 nm, and the light intensity is 200-800 mW / cm 2 .

[0013] The present invention is further preferred, that the specific preparation method of the titanium dioxide supported gold catalyst is: when tetrabutyl titanate is in a stirring state, a hydrofluoric acid solution is added to the tetrabutyl titanate, after stirring for a certain period of time, the mixed solution is transferred to a crystallization kettle and the crystallization kettle is placed in an oven for crystallization reaction, and the solid product obtained by the crystallization reaction is subjected to centrifugation, water washing and drying to obtain a titanium dioxide carrier; the titanium dioxide carrier is dispersed in an alkaline aqueous solution, and then a certain amount of chloroauric acid aqueous solution is gradually added dropwise, and after continuous stirring for a certain period of time, it is centrifuged, water washed and dried, and finally placed in a hydrogen atmosphere for heat treatment to obtain a titanium dioxide supported gold catalyst.

[0014] The present invention is further preferably that the mass ratio of tetrabutyl titanate to hydrofluoric acid solution is 5:1-10:1, the stirring time is 0.5-2 h, the crystallization reaction temperature is 100-200° C., and the crystallization reaction time is 12-48 h.

[0015] The present invention is further preferably such that the alkaline aqueous solution is any one of a sodium hydroxide aqueous solution, an ammonium carbonate aqueous solution and an ammonia solution, and the pH value of the titanium dioxide alkaline solution is 8-11.

[0016] The present invention is further preferably that the dropping speed of the chloroauric acid solution is controlled at 5-20 mL / h, the heat treatment temperature is 200-500°C, the heat treatment time is 1-5 h, and the heating rate is 1-5°C / min.

[0017] The present invention has the following beneficial effects: the method for synthesizing methyl hydrogen peroxide by photocatalytic methane selective oxidation provided by the present invention uses methane as a raw material, oxygen as an oxidant, and water as a solvent, and selectively converts methane into methyl hydrogen peroxide under the action of light and a titanium dioxide-supported gold catalyst. By controlling catalyst preparation conditions, gold loading, methane and oxygen partial pressures, and reaction time, the generation rate of the obtained methyl hydrogen peroxide can reach 2335.5 μmol / g / h, and the selectivity reaches 73.6%. The catalyst preparation process and the catalytic reaction process are economical and green, and have broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for describing the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0019] Figure 1 The liquid phase reaction product provided by Example 1 of the present invention 1 H NMR spectrum.

[0020] Figure 2 This is the UV-visible absorption spectrum of the liquid phase reaction product provided in Example 1 of the present invention.

[0021] Figure 3 This is the XRD spectrum of the titanium dioxide-supported gold catalyst provided in Example 1 of the present invention.

[0022] Figure 4 This is a TEM image of the titanium dioxide-supported gold catalyst provided in Example 1 of the present invention.

[0023] Figure 5 This is a spherical aberration corrected HAADF-STEM image of the titanium dioxide-supported gold catalyst provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0024] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] Example 1. This example provides a method for synthesizing methyl hydrogen peroxide by selective oxidation of methane through photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported gold catalyst in water and adding the catalyst to a reactor, opening the circulating water in the reactor jacket, controlling the reactor temperature at 25°C, opening the oxygen inlet valve and the reactor outlet valve, introducing oxygen at normal pressure for 30 min to remove the air in the reactor, then closing the oxygen inlet valve and the reactor outlet valve, opening the methane inlet valve, introducing 19 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 2 h, taking a gas phase product with a sampling needle and injecting it into a gas chromatograph to analyze the carbon dioxide and carbon monoxide content. The methyl hydrogen peroxide and methanol in the liquid phase product are separated by a gas chromatography. 1 H NMR analysis, formaldehyde was analyzed by colorimetry using a UV-visible spectrophotometer (acetylacetone was selected as the color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate); methyl hydroperoxide and methanol 1 H NMR spectrum Figure 1 As shown, the UV-visible absorption spectrum of the adduct of formaldehyde and the developer is as follows Figure 2 shown.

[0026] The titanium dioxide supported gold catalyst comprises a titanium dioxide carrier and a gold active component, the mass fraction of the gold active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm, and the light intensity is 800 mW / cm 2, reaction time 2 h.

[0027] This embodiment also provides a method for preparing a titanium dioxide-supported gold catalyst, and the specific steps are as follows: add 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, stir for 30 min, transfer to a 100 mL hydrothermal kettle, place in an oven at 180 ° C for crystallization for 24 h, then centrifuge, wash with water to neutrality, and finally dry at 80 ° C overnight to obtain a titanium dioxide carrier.

[0028] 0.1 g of titanium dioxide was dispersed in 30 mL of water, and ammonium carbonate was added to adjust the pH to 10. 10 mL of chloroauric acid solution was added dropwise to the above solution. The solution was stirred for 30 min and centrifuged and washed with water until neutral, then dried at 80 °C. Finally, the solution was heat treated at 200 °C in a 5% H2 / Ar mixed gas for 2 h to obtain a titanium dioxide-supported gold catalyst.

[0029] The XRD spectrum of the titanium dioxide supported gold catalyst is as follows Figure 3 As shown, it indicates that the synthesized titanium dioxide has an anatase crystal structure. In addition, no diffraction peak of gold is observed, which is due to the low loading amount and high dispersion of gold. Figure 4 This is the TEM image of the titanium dioxide supported gold catalyst. It can be seen from the figure that the titanium dioxide carrier has a thin flake morphology with a length and width of about 40 nm and a thickness of about 5 nm. Figure 5 This is a spherical aberration-corrected transmission electron microscopy image of a titanium dioxide-supported gold catalyst. It can be seen that gold exists in the form of atomic clusters with a high active site exposure area.

[0030] Example 2. This example provides a method for synthesizing methyl hydrogen peroxide by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported gold catalyst in water and adding the catalyst to a reactor, opening the circulating water in the reactor jacket, controlling the reactor temperature at 25°C, opening the oxygen inlet valve and the reactor outlet valve, introducing oxygen at normal pressure for 30 min to remove the air in the reactor, then closing the oxygen inlet valve and the reactor outlet valve, opening the methane inlet valve, introducing 19 bar methane, then closing the methane inlet valve, turning on the xenon lamp to start the photocatalytic reaction, and after reacting for 2 h, taking the gas phase product with a sampling needle and injecting it into a gas chromatograph to analyze the carbon dioxide and carbon monoxide content. The methyl hydrogen peroxide and methanol in the liquid phase product are separated by a gas chromatography. 1H NMR analysis, formaldehyde is analyzed by colorimetry using a UV-visible spectrophotometer (acetylacetone is selected as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate). The titanium dioxide-supported gold catalyst includes a titanium dioxide carrier and a gold active component, the mass fraction of the gold active component is 0.1 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm, and the light intensity is 800 mW / cm 2 , reaction time 2 h.

[0031] This embodiment also provides a method for preparing a titanium dioxide-supported gold catalyst, and the specific steps are as follows: add 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, stir for 30 min, transfer to a 100 mL hydrothermal kettle, place in an oven at 180 ° C for crystallization for 24 h, then centrifuge, wash with water to neutrality, and finally dry at 80 ° C overnight to obtain a titanium dioxide carrier.

[0032] 0.1 g of titanium dioxide was dispersed in 30 mL of water, and ammonium carbonate was added to adjust the pH to 10. 10 mL of chloroauric acid solution was added dropwise to the above solution. The solution was stirred for 30 min and centrifuged and washed with water until neutral, then dried at 80 °C. Finally, the solution was heat treated at 200 °C in a 5% H2 / Ar mixed gas for 2 h to obtain a titanium dioxide-supported gold catalyst.

[0033] Example 3. This example provides a method for synthesizing methyl hydrogen peroxide by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported gold catalyst in water and adding the catalyst to a reactor, opening the circulating water in the reactor jacket, controlling the reactor temperature at 25°C, opening the oxygen inlet valve and the reactor outlet valve, introducing oxygen at normal pressure for 30 min to remove the air in the reactor, then closing the oxygen inlet valve and the reactor outlet valve, opening the methane inlet valve, introducing 19 bar methane, then closing the methane inlet valve, turning on the xenon lamp to start the photocatalytic reaction, and after reacting for 2 h, taking the gas phase product with a sampling needle and injecting it into a gas chromatograph to analyze the carbon dioxide and carbon monoxide content. The methyl hydrogen peroxide and methanol in the liquid phase product are separated by a gas chromatography. 1 H NMR analysis, formaldehyde is analyzed by colorimetry using a UV-visible spectrophotometer (acetylacetone is selected as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate). The titanium dioxide-supported gold catalyst includes a titanium dioxide carrier and a gold active component, the mass fraction of the gold active component is 1.0 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm, and the light intensity is 800 mW / cm 2 , reaction time 2 h.

[0034] This embodiment also provides a method for preparing a titanium dioxide-supported gold catalyst, and the specific steps are as follows: add 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, stir for 30 min, transfer to a 100 mL hydrothermal kettle, place in an oven at 180 ° C for crystallization for 24 h, then centrifuge, wash with water to neutrality, and finally dry at 80 ° C overnight to obtain a titanium dioxide carrier.

[0035] 0.1 g of titanium dioxide was dispersed in 30 mL of water, and ammonium carbonate was added to adjust the pH to 10. 10 mL of chloroauric acid solution was added dropwise to the above solution. The solution was stirred for 30 min and centrifuged and washed with water until neutral, then dried at 80 °C. Finally, the solution was heat treated at 200 °C in a 5% H2 / Ar mixed gas for 2 h to obtain a titanium dioxide-supported gold catalyst.

[0036] Example 4. This example provides a method for photocatalytic methane selective oxidation to synthesize methyl hydrogen peroxide, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported gold catalyst in water and adding the catalyst to a reactor, opening the circulating water in the reactor jacket, controlling the reactor temperature at 25°C, opening the oxygen inlet valve and the reactor outlet valve, introducing oxygen at normal pressure for 30 min to remove the air in the reactor and make the oxygen partial pressure 3 bar, then closing the oxygen inlet valve and the reactor outlet valve, opening the methane inlet valve, introducing 17 bar methane, then closing the methane inlet valve, turning on the xenon lamp to start the photocatalytic reaction, and after reacting for 2 h, taking the gas phase product with a sampling needle and injecting it into a gas chromatograph to analyze the carbon dioxide and carbon monoxide content. The methyl hydrogen peroxide and methanol in the liquid phase product are separated by a gas chromatography. 1 H NMR analysis, formaldehyde is analyzed by colorimetry using a UV-visible spectrophotometer (acetylacetone is selected as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate). The titanium dioxide-supported gold catalyst includes a titanium dioxide carrier and a gold active component, the mass fraction of the gold active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm, and the light intensity is 800 mW / cm 2 , reaction time 2 h.

[0037] This embodiment also provides a method for preparing a titanium dioxide-supported gold catalyst, and the specific steps are as follows: add 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, stir for 30 min, transfer to a 100 mL hydrothermal kettle, place in an oven at 180 ° C for crystallization for 24 h, then centrifuge, wash with water to neutrality, and finally dry at 80 ° C overnight to obtain a titanium dioxide carrier.

[0038] 0.1 g of titanium dioxide was dispersed in 30 mL of water, and ammonium carbonate was added to adjust the pH to 10. 10 mL of chloroauric acid solution was added dropwise to the above solution. The solution was stirred for 30 min and centrifuged and washed with water until neutral, then dried at 80 °C. Finally, the solution was heat treated at 200 °C in a 5% H2 / Ar mixed gas for 2 h to obtain a titanium dioxide-supported gold catalyst.

[0039] Example 5. This example provides a method for photocatalytic methane selective oxidation to synthesize methyl hydrogen peroxide, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported gold catalyst in water and adding the catalyst to a reactor, opening the circulating water in the reactor jacket, controlling the reactor temperature at 25°C, opening the oxygen inlet valve and the reactor outlet valve, introducing oxygen at normal pressure for 30 min to remove the air in the reactor and make the oxygen partial pressure 5 bar, then closing the oxygen inlet valve and the reactor outlet valve, opening the methane inlet valve, introducing 15 bar methane, then closing the methane inlet valve, turning on the xenon lamp to start the photocatalytic reaction, and after reacting for 2 h, taking the gas phase product with a sampling needle and injecting it into a gas chromatograph to analyze the carbon dioxide and carbon monoxide content. The methyl hydrogen peroxide and methanol in the liquid phase product are separated by a gas chromatography. 1 H NMR analysis, formaldehyde is analyzed by colorimetry using a UV-visible spectrophotometer (acetylacetone is selected as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate). The titanium dioxide-supported gold catalyst includes a titanium dioxide carrier and a gold active component, the mass fraction of the gold active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm, and the light intensity is 800 mW / cm 2 , reaction time 2 h.

[0040] This embodiment also provides a method for preparing a titanium dioxide-supported gold catalyst, and the specific steps are as follows: add 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, stir for 30 min, transfer to a 100 mL hydrothermal kettle, place in an oven at 180 ° C for crystallization for 24 h, then centrifuge, wash with water to neutrality, and finally dry at 80 ° C overnight to obtain a titanium dioxide carrier.

[0041] 0.1 g of titanium dioxide was dispersed in 30 mL of water, and ammonium carbonate was added to adjust the pH to 10. 10 mL of chloroauric acid solution was added dropwise to the above solution. The solution was stirred for 30 min and centrifuged and washed with water until neutral, then dried at 80 °C. Finally, the solution was heat treated at 200 °C in a 5% H2 / Ar mixed gas for 2 h to obtain a titanium dioxide-supported gold catalyst.

[0042] Example 6. This example provides a method for photocatalytic methane selective oxidation to synthesize methyl hydrogen peroxide, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported gold catalyst in water and adding the catalyst into a reactor, opening the circulating water in the reactor jacket, controlling the reactor temperature at 25°C, opening the oxygen inlet valve and the reactor outlet valve, introducing oxygen at normal pressure for 30 min to remove the air in the reactor, then closing the oxygen inlet valve and the reactor outlet valve, opening the methane inlet valve, introducing 19 bar methane, then closing the methane inlet valve, turning on the xenon lamp to start the photocatalytic reaction, and after reacting for 3 h, taking the gas phase product with a sampling needle and injecting it into a gas chromatograph to analyze the carbon dioxide and carbon monoxide content. The methyl hydrogen peroxide and methanol in the liquid phase product are separated by a gas chromatography. 1 H NMR analysis, formaldehyde is analyzed by colorimetry using a UV-visible spectrophotometer (acetylacetone is selected as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate). The titanium dioxide-supported gold catalyst includes a titanium dioxide carrier and a gold active component, the mass fraction of the gold active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm, and the light intensity is 800 mW / cm 2 , reaction time 3 h.

[0043] This embodiment also provides a method for preparing a titanium dioxide-supported gold catalyst, and the specific steps are as follows: add 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, stir for 30 min, transfer to a 100 mL hydrothermal kettle, place in an oven at 180 ° C for crystallization for 24 h, then centrifuge, wash with water to neutrality, and finally dry at 80 ° C overnight to obtain a titanium dioxide carrier.

[0044] 0.1 g of titanium dioxide was dispersed in 30 mL of water, and ammonium carbonate was added to adjust the pH to 10. 10 mL of chloroauric acid solution was added dropwise to the above solution. The solution was stirred for 30 min and centrifuged and washed with water until neutral, then dried at 80 °C. Finally, the solution was heat treated at 200 °C in a 5% H2 / Ar mixed gas for 2 h to obtain a titanium dioxide-supported gold catalyst.

[0045] Example 7. This example provides a method for synthesizing methyl hydrogen peroxide by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported gold catalyst in water and adding the catalyst to a reactor, opening the circulating water in the reactor jacket, controlling the reactor temperature at 25°C, opening the oxygen inlet valve and the reactor outlet valve, introducing oxygen at normal pressure for 30 min to remove the air in the reactor, then closing the oxygen inlet valve and the reactor outlet valve, opening the methane inlet valve, introducing 19 bar methane, then closing the methane inlet valve, turning on the xenon lamp to start the photocatalytic reaction, and after reacting for 4 h, taking the gas phase product with a sampling needle and injecting it into a gas chromatograph to analyze the carbon dioxide and carbon monoxide content. The methyl hydrogen peroxide and methanol in the liquid phase product are separated by a gas chromatography. 1 H NMR analysis, formaldehyde is analyzed by colorimetry using a UV-visible spectrophotometer (acetylacetone is selected as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate). The titanium dioxide-supported gold catalyst includes a titanium dioxide carrier and a gold active component, the mass fraction of the gold active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm, and the light intensity is 800 mW / cm 2 , reaction time 4 h.

[0046] This embodiment also provides a method for preparing a titanium dioxide-supported gold catalyst, and the specific steps are as follows: add 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, stir for 30 min, transfer to a 100 mL hydrothermal kettle, place in an oven at 180 ° C for crystallization for 24 h, then centrifuge, wash with water to neutrality, and finally dry at 80 ° C overnight to obtain a titanium dioxide carrier.

[0047] 0.1 g of titanium dioxide was dispersed in 30 mL of water, and ammonium carbonate was added to adjust the pH to 10. 10 mL of chloroauric acid solution was added dropwise to the above solution. The solution was stirred for 30 min and centrifuged and washed with water until neutral, then dried at 80 °C. Finally, the solution was heat treated at 200 °C in a 5% H2 / Ar mixed gas for 2 h to obtain a titanium dioxide-supported gold catalyst.

[0048] Comparative Example 1. This comparative example provides a method for synthesizing methyl hydroperoxide by photocatalytic selective oxidation of methane. The difference from Example 1 is that, except for adjusting the preparation method of the titanium dioxide-supported gold catalyst to prepare the titanium dioxide-supported gold catalyst by a photoreduction method, the rest is the same as Example 1.

[0049] The preparation steps of the titanium dioxide-supported gold catalyst prepared by photoreduction are as follows: 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% was added to 60 mL of tetrabutyl titanate, and the mixture was transferred to a 100 mL hydrothermal kettle after stirring for 30 min, and then crystallized in an oven at 180°C for 24 h, centrifuged, washed with water until neutral, and finally dried overnight at 80°C to obtain a titanium dioxide carrier; 0.36 g of titanium dioxide carrier was ultrasonically dispersed in a mixed solution of 40 mL of water and 10 mL of methanol, to which 261 μL of 35 mM chloroauric acid aqueous solution was added, and irradiated with a 365 nm LED lamp for 60 min. The obtained purple solid was centrifuged and washed with water, then dried at 60°C, and finally calcined at 300°C for 2 h to obtain the photoreduction-prepared titanium dioxide-loaded gold catalyst.

[0050] Comparative Example 2. This comparative example provides a method for synthesizing methyl hydroperoxide by photocatalytic selective oxidation of methane. The difference from Example 1 is that, except for adjusting the preparation method of the titanium dioxide-supported gold catalyst to prepare the titanium dioxide-supported gold catalyst by sodium borohydride reduction method, the rest is the same as Example 1.

[0051] The steps for preparing the titanium dioxide supported gold catalyst by the sodium borohydride reduction method are as follows: add 9.6 mL of hydrofluoric acid with a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, stir for 30 min, transfer to a 100 mL crystallization kettle, crystallize in an oven at 180°C for 24 h, centrifuge, wash with water to neutrality, and finally dry overnight at 80°C to obtain the titanium dioxide carrier; 90 μL of 35 mM chloroauric acid and 30 μL of 1wt% polyvinyl pyrrolidone solution were added to 20 mL of water in sequence, and then 150 μL of 0.1M sodium borohydride was quickly added. After stirring for 0.5 h, the solution was added to a dispersion containing 0.12 g of titanium dioxide carrier and 50 mL of water. After stirring for 0.5 h, the mixture was centrifuged and washed with water and dried at 80°C. Finally, it was calcined at 300°C for 2 h to obtain the titanium dioxide-supported gold catalyst prepared by sodium borohydride reduction method.

[0052] The methods provided in Examples 1-7 and Comparative Examples 1-2 were used to carry out the photocatalytic selective oxidation of methane to synthesize methyl hydroperoxide. The results are shown in the following table:

[0053] By comparing the results of Example 1, Comparative Example 1 and Comparative Example 2 in the above table, it can be seen that the preparation method of the catalyst has a significant effect on the generation rate and selectivity of methyl hydroperoxide. Under the catalytic action of the catalyst described in Example 1, the generation rate of methyl hydroperoxide reaches 2335.5 μmol / g / h, and the selectivity reaches 73.6%. The generation rate of methyl hydroperoxide on the catalyst described in Comparative Example 1 is one order of magnitude lower than that in Example 1. In addition, the selectivity of methyl hydroperoxide is less than 20%. The reason may be that under the action of photoreduction, gold is preferentially deposited on the conduction band of the titanium dioxide carrier, which is not conducive to the activity of methane; the activity of the catalyst described in Comparative Example 2 is also significantly lower than that in Example 1. The reason may be that the gold obtained by the catalyst preparation method described in Comparative Example 2 is often in the form of nanoparticles. Compared with gold atom clusters, the number of exposed active sites is relatively small, which is not conducive to the efficient reaction.

[0054] By comparing the results of Example 1, Example 2 and Example 3 in the above table, it can be seen that the gold loading has a significant effect on the performance of titanium dioxide. When the loading is too low, the content of active components is insufficient, resulting in poor catalytic performance; when the loading is increased to 0.5wt%, the generation rate and selectivity of methyl hydroperoxide are significantly improved; when the loading is further increased to 1wt%, the activity decreases instead. The reason for the decrease in activity may be that the agglomeration of gold atoms reduces the number of active sites.

[0055] By comparing the results of Example 1, Example 4 and Example 5 in the above table, it can be seen that the methane and oxygen partial pressures also affect the reaction performance. When the methane partial pressure is reduced and the oxygen partial pressure is increased, the generation rate and selectivity of methyl hydroperoxide are reduced. This is because reducing the methane partial pressure reduces its solubility in water and its contact with the catalyst, and increasing the oxygen partial pressure causes more methyl hydroperoxide to be over-oxidized and converted into carbon dioxide.

[0056] By comparing the results of Example 1, Example 6 and Example 7 in the above table, it can be seen that as the reaction time increases, the methyl hydroperoxide generation rate and selectivity decrease. This is because the residence time of methyl hydroperoxide increases and further deep oxidation to carbon dioxide occurs on the catalyst surface.

[0057] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0058] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for synthesizing methyl hydroperoxide by selective oxidation of methane through photocatalysis, characterized in that: The steps include: The titanium dioxide-supported catalyst is ultrasonically dispersed in water and added into a reactor. Oxygen and methane are introduced into the reactor in sequence until the required pressure is reached. The reaction is stirred for a period of time under light conditions to obtain methyl hydroperoxide and other oxidation products. The amount of titanium dioxide-supported catalyst used in the photocatalytic reaction is 0.1~10 mg, and the amount of water solvent used is 10~100 mL.

2. The method for synthesizing methyl hydroperoxide by selective photocatalytic oxidation of methane according to claim 1, characterized in that: The titanium dioxide-supported catalyst is a titanium dioxide-supported gold catalyst, wherein the mass fraction of the gold active component in the titanium dioxide-supported gold catalyst is 0.1-2 wt %.

3. The method for synthesizing methyl hydroperoxide by selective photocatalytic oxidation of methane according to claim 1, characterized in that: The photocatalytic reaction is carried out in a high-pressure reactor with a quartz window on the top, wherein the oxygen partial pressure is 1-10 bar, and the methane partial pressure is 10-20 bar.

4. The method for synthesizing methyl hydroperoxide by selective photocatalytic oxidation of methane according to claim 1, characterized in that: The photocatalytic reaction temperature is 25°C, the reaction time is 0.5-5 h, the light source required for the reaction is a xenon lamp, and the emission wavelength of the xenon lamp is 200-1300 nm, and the light intensity is 200-800 mW / cm 2 .

5. A method for synthesizing methyl hydroperoxide by photocatalytic selective oxidation of methane according to claim 1 or 2, characterized in that: The specific preparation method of the titanium dioxide supported gold catalyst is: When tetrabutyl titanate is in a stirring state, a hydrofluoric acid solution is added to the tetrabutyl titanate, and after stirring for a certain period of time, the mixed solution is transferred to a crystallization kettle, and the crystallization kettle is placed in an oven for a crystallization reaction, and a solid product obtained by the crystallization reaction is centrifuged, washed with water, and dried to obtain a titanium dioxide carrier; The titanium dioxide carrier is dispersed in an alkaline aqueous solution, and then a certain amount of chloroauric acid aqueous solution is gradually added. After continuous stirring for a certain period of time, the mixture is centrifuged, washed with water, and dried. Finally, the mixture is placed in a hydrogen atmosphere for heat treatment to obtain a titanium dioxide-loaded gold catalyst.

6. The method for synthesizing methyl hydroperoxide by photocatalytic selective oxidation of methane according to claim 5, characterized in that: The mass ratio of tetrabutyl titanate to hydrofluoric acid solution is 5:1-10:1, the stirring time is 0.5-2 h, the crystallization reaction temperature is 100-200° C., and the crystallization reaction time is 12-48 h.

7. The method for synthesizing methyl hydroperoxide by selective photocatalytic oxidation of methane according to claim 5, characterized in that: The alkaline aqueous solution is selected from any one of sodium hydroxide aqueous solution, ammonium carbonate aqueous solution and ammonia water, and the pH value of the titanium dioxide alkaline solution is 8-11.

8. The method for synthesizing methyl hydroperoxide by photocatalytic selective oxidation of methane according to claim 5, characterized in that: The dropping speed of the chloroauric acid solution is controlled at 5-20 mL / h, the heat treatment temperature is 200-500° C., the heat treatment time is 1-5 h, and the heating rate is 1-5° C. / min.

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