Method for preparing formaldehyde through photocatalytic methane selective oxidation

Abstract

The invention discloses a method for preparing formaldehyde through photocatalytic methane selective oxidation, and relates to the technical field of formaldehyde preparation. The method comprises the following steps: ultrasonically dispersing a titanium dioxide supported copper monatomic catalyst into 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 formaldehyde and other oxidation products, in the reaction process, the dosage of the titanium dioxide supported copper monatomic catalyst is 0.1-10 mg, and the dosage of the water solvent is 0-100 mL; the catalyst is a titanium dioxide supported copper monatomic catalyst. According to the method, methane is used as a raw material, oxygen is used as an oxidizing agent, methane is selectively converted into formaldehyde under the combined action of illumination and the titanium dioxide supported copper monatomic catalyst, the obtained formaldehyde has high generation rate and selectivity, the reaction process is green and economical, and the method has wide application prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of formaldehyde preparation, and particularly relates to a method for preparing formaldehyde by photocatalytic methane selective oxidation. Background Art

[0002] As an important chemical raw material, formaldehyde is widely used in the synthesis of bulk chemicals such as phenolic resin and 1,4-butanediol. Industrial formaldehyde is produced by catalytic oxidation of methanol. The catalyst used is usually expensive metal catalysts such as silver. Methanol is mainly obtained from synthesis gas through a high temperature and high pressure process. Therefore, the traditional formaldehyde synthesis process has many disadvantages such as many steps, harsh process conditions, and severe environmental pollution.

[0003] Methane, as the main component of natural gas and shale gas, has the advantages of abundant reserves and low price. It is considered to be the most promising bulk chemical raw material. The development of a process for producing formaldehyde by one-step oxidation of methane under mild conditions has great technical advantages. However, methane has an extremely high CH bond dissociation energy (434 kJ / mol), and the activation and catalytic conversion of methane usually require harsh reaction conditions. At present, the industry mainly uses the indirect method to convert methane, which is to convert methane into synthesis gas through steam reforming at high temperature (>700 °C). The synthesis gas is then used for Fischer-Tropsch synthesis or methanol synthesis, and finally formaldehyde can be obtained through methanol oxidation. Obviously, the above reaction process has the disadvantages of many steps, high energy consumption, low total carbon utilization, high environmental pollution, and easy carbon deposition and deactivation of the catalyst at high temperature.

[0004] Photocatalysis, as an emerging green synthesis technology, has attracted widespread attention from scholars. Recently, it has been reported that photocatalytic methane selective oxidation to formaldehyde at room temperature has been achieved, which provides possibilities for the development of this process. However, under oxidative conditions, methane oxidation first produces methyl hydroperoxide and methanol, which are further converted into deep oxidation products such as formaldehyde and carbon dioxide. The products are widely distributed, and it is a huge challenge to obtain formaldehyde with high selectivity.

[0005] Jiang Haiying et al. (ACS nano, 2024, 18(20): 12994-13005) used titanium nitride as raw material and obtained a nitrogen-doped mixed crystalline phase titanium dioxide catalyst by high-temperature calcination. At room temperature and light conditions, methane was highly selectively oxidized to formaldehyde with a selectivity of nearly 90%. However, the high price of carbon nitride and the high energy consumption of the calcination process limit the application prospects of this catalyst.

[0006] Tang Zhiyong et al. (Journal of the American Chemical Society, 2022, 144(35):15977-15987) used commercially available titanium dioxide as a raw material and obtained mixed crystal titanium dioxide with a specific crystal phase ratio by ultra-high temperature calcination. The mixed crystal titanium dioxide can selectively photooxidize methane to formaldehyde. However, the preparation of the catalyst needs to be carried out at high temperature, which does not meet the requirements of green chemical industry, and the product generation rate needs to be improved.

[0007] A Chinese patent with publication number CN111701590B discloses an Au x / cubic-WO3 photocatalyst has high catalytic performance for methane oxidation to formaldehyde, but the low tungsten reserves and expensive gold limit the application prospects of this catalyst.

[0008] 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 existing technology, it is urgent to provide a cheap and efficient method for preparing formaldehyde by oxidation of methane. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide a method for preparing a catalyst for preparing formaldehyde by oxidation of methane and a method for preparing formaldehyde by selective photocatalytic oxidation of methane.

[0010] To solve the above technical problems, the present invention is achieved through the following technical solutions: The present invention is a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a titanium dioxide-supported copper single-atom catalyst, and specifically comprises the following steps: adding a hydrofluoric acid solution to tetrabutyl titanate, transferring the mixture to a crystallization kettle after stirring for a certain period of time, and placing the crystallization kettle in an oven for crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing and drying to obtain a titanium dioxide carrier; dispersing the titanium dioxide carrier in water to obtain a uniform solution, adding a certain amount of copper nitrate solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring, adding a certain concentration of sodium borohydride solution for light-proof reaction, and subjecting the solid product to centrifugation, water washing and drying to obtain a titanium dioxide-supported copper single-atom catalyst.

[0011] The present invention is further preferably 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 temperature is 100-200° C., and the crystallization time is 12-48 h.

[0012] The present invention is further preferably characterized in that the concentration of the sodium borohydride solution is 0.02-0.5 mol / L, and the mass ratio of the sodium borohydride to copper nitrate is 50:1-5:1.

[0013] The present invention also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper single-atom catalyst in water and adding the catalyst into a reactor, sequentially introducing oxygen and methane into the reactor until a desired pressure is reached, stirring the reaction under light conditions for a period of time to obtain formaldehyde and other oxidation products, wherein the amount of the titanium dioxide-supported copper single-atom catalyst used in the reaction process is 0.1-10 mg, and the amount of the water solvent used is 0-100 mL.

[0014] The present invention is further preferably that the titanium dioxide-supported copper single-atom catalyst comprises a titanium dioxide carrier and a copper active component; wherein the mass fraction of the copper active component is 0.1-2wt%.

[0015] The present invention is further preferably 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 0-10 bar and the methane partial pressure is 10-20 bar.

[0016] The present invention is further preferably that the reaction temperature is 25°C, the reaction time is 0.5-5 h, the light source used in the photocatalytic reaction is a xenon lamp, the emission wavelength is 200-1300 nm, and the light intensity is 200-800 mW / cm 2 .

[0017] The present invention has the following beneficial effects: the method for preparing formaldehyde 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 formaldehyde under the action of light and a titanium dioxide-supported copper single-atom catalyst. By controlling the catalyst preparation conditions and the copper loading amount, the methane and oxygen partial pressures, and the reaction time, the resulting formaldehyde generation rate can reach 3.7 mmol / g / h, and the selectivity exceeds 70%. The catalyst preparation process and the catalytic reaction process are green and economical, 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 This is the UV-visible absorption spectrum of the liquid phase reaction product provided in Example 1 of the present invention.

[0020] Figure 2 This is the 1H NMR 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 copper catalyst provided in Example 1 of the present invention.

[0022] Figure 4 This is a TEM image of the titanium dioxide-supported copper 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 copper 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 preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and subjecting the mixture to centrifugation, water washing and drying (80°C) to obtain a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2).

[0026] The XRD spectrum of the titanium dioxide-supported single-atom copper catalyst is as follows: Figure 3 As shown, it shows that the synthesized TiO2 has an anatase crystal structure. In addition, no diffraction peak of copper is observed, which is due to the low copper loading and its high dispersion. Figure 4 This is the TEM image of Cu / TiO2. It can be seen from the figure that the titanium dioxide carrier is a thin sheet with a length and width of about 40 nm and a thickness of about 5 nm. Figure 5 This is a spherical aberration transmission electron microscope image of Cu / TiO2. The bright spots of single copper atoms can be clearly seen, indicating that copper exists in the form of single atoms.

[0027] This embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 minutes to remove the air in the reactor, then closing the oxygen inlet valve and the reactor outlet valve, opening the methane inlet valve, introducing 20 bar methane, then closing the methane inlet valve, turning on the xenon lamp to start the photocatalytic reaction, and after reacting for 1 hour, 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, and analyzing the formaldehyde in the liquid phase product by a UV-visible spectrophotometer using a colorimetric method (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR. The UV-visible absorption spectrum of the adduct of formaldehyde and the color developer is shown in FIG. Figure 1 As shown, the 1H NMR spectra of methyl hydroperoxide and methanol are as follows Figure 2 As shown; The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 1 h.

[0028] Example 2. This example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing, and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and obtaining a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2) after centrifugation, water washing, and drying (80°C).

[0029] The present embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 1 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, analyzing the formaldehyde in the liquid phase product with a UV-visible spectrophotometer by a colorimetric method (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0030] The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 0.1 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 1 h.

[0031] Example 3. This example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and subjecting the mixture to centrifugation, water washing and drying (80°C) to obtain a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2).

[0032] The present embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 1 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, analyzing the formaldehyde in the liquid phase product with a UV-visible spectrophotometer by a colorimetric method (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0033] The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 1.0 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 1 h.

[0034] Example 4. This example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing, and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and subjecting the mixture to centrifugation, water washing, and drying (80°C) to obtain a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2).

[0035] The present embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 methane inlet valve, introducing 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 1 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, analyzing the formaldehyde in the liquid phase product by a colorimetric method using an ultraviolet-visible spectrophotometer (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0036] The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 1 h.

[0037] Example 5. This example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing, and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and subjecting the mixture to centrifugation, water washing, and drying (80°C) to obtain a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2).

[0038] The present embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 (at this time, the oxygen partial pressure is 3 bar), opening the methane inlet valve, introducing 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 1 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, analyzing the formaldehyde in the liquid phase product by a UV-visible spectrophotometer using a colorimetric method (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0039] The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 1 h.

[0040] Example 6. This example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing, and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and subjecting the mixture to centrifugation, water washing, and drying (80°C) to obtain a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2).

[0041] The present embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 (at this time, the oxygen partial pressure is 5 bar), opening the methane inlet valve, introducing 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 1 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, analyzing the formaldehyde in the liquid phase product by a UV-visible spectrophotometer using a colorimetric method (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0042] The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 1 h.

[0043] Example 7. This example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing, and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and subjecting the mixture to centrifugation, water washing, and drying (80°C) to obtain a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2).

[0044] The present embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 20 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, analyzing the formaldehyde in the liquid phase product with a UV-visible spectrophotometer by a colorimetric method (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0045] The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 2 h.

[0046] Example 8. This example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing, and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and obtaining a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2) after centrifugation, water washing, and drying (80°C).

[0047] The present embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 3 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, analyzing the formaldehyde in the liquid phase product with a UV-visible spectrophotometer by a colorimetric method (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0048] The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 3 h.

[0049] Example 9. This example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a single-atom copper catalyst supported on titanium dioxide, and specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for a 24 h crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing, and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L copper nitrate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1s, stirring the reaction for 30 min under light-shielding conditions, and subjecting the mixture to centrifugation, water washing, and drying (80°C) to obtain a single-atom copper catalyst supported on titanium dioxide (Cu / TiO2).

[0050] The present embodiment also provides a method for preparing formaldehyde by selective oxidation of methane by photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported copper 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 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 5 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, analyzing the formaldehyde in the liquid phase product with a UV-visible spectrophotometer by a colorimetric method (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0051] The titanium dioxide-supported copper catalyst includes a titanium dioxide carrier and a copper active component, the mass fraction of the copper active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 5 h.

[0052] Comparative Example 1. This comparative example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a titanium dioxide-supported gold single-atom catalyst, specifically comprising the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for 24 h of crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L aqueous solution of chloroauric acid to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, then dripping 1 mL of a 0.1 mol / L aqueous solution of sodium borohydride at a rate of 1 drop / 1 s, and stirring the reaction for 30 min under light-shielding conditions. min, and after centrifugation, water washing and drying (80 °C), the titanium dioxide-supported gold catalyst (Au / TiO2) was obtained.

[0053] The present comparative example also provides a method for preparing formaldehyde 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 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 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 1 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 contents, analyzing the formaldehyde in the liquid phase product by a colorimetric method using an ultraviolet-visible spectrophotometer (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0054] 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 solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 1 h.

[0055] Comparative Example 2. This comparative example provides a method for preparing a catalyst for preparing formaldehyde by oxidation of methane. The catalyst is a titanium dioxide-supported palladium single-atom catalyst, which specifically comprises the following steps: adding 9.6 mL of a hydrofluoric acid solution having a mass concentration of 40 wt% to 60 mL of tetrabutyl titanate, transferring the mixture to a 100 mL crystallization kettle after stirring for 30 min, and placing the crystallization kettle in an oven at 180°C for 24 h of crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing and drying to obtain a titanium dioxide carrier; dispersing 0.12 g of the titanium dioxide carrier in 50 mL of water to obtain a uniform solution, adding 47 uL of a 200 mmol / L potassium chloropalladate aqueous solution to the uniform solution, controlling the reaction temperature at 5-30°C and continuously stirring for 10 min, and then dripping 1 mL of a 0.1 mol / L sodium borohydride aqueous solution at a rate of 1 drop / 1 s, and stirring the reaction for 30 min under light-shielding conditions. min, and after centrifugation, water washing and drying (80 °C), the titanium dioxide-supported palladium catalyst (Pd / TiO2) was obtained.

[0056] The present comparative example also provides a method for preparing formaldehyde by selective oxidation of methane through photocatalysis, comprising the following steps: ultrasonically dispersing a titanium dioxide-supported palladium 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 20 bar methane, then closing the methane inlet valve, turning on a xenon lamp to start a photocatalytic reaction, and after reacting for 1 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 contents, analyzing the formaldehyde in the liquid phase product by a colorimetric method using an ultraviolet-visible spectrophotometer (selecting acetylacetone as a color developer, which can form a yellow adduct with formaldehyde under the action of acetic acid and ammonium acetate), and analyzing methyl hydroperoxide and methanol by 1H NMR.

[0057] The titanium dioxide supported palladium catalyst comprises a titanium dioxide carrier and a palladium active component, the mass fraction of the palladium active component is 0.5 wt%, the amount of catalyst in the reactor is 1 mg, the amount of water solvent is 10 mL, the wavelength of the xenon lamp light source is 200-1300 nm simulated sunlight, and the light intensity is 800 mW / cm 2 , reaction time 1 h. The photocatalytic methane oxidation reaction to prepare formaldehyde was carried out using the methods provided in Example 1-9 and Comparative Example 1-2. The results are shown in the following table: .

[0058] It can be seen from the results of Example 1, Comparative Example 1 and Comparative Example 2 in the above table that the titanium dioxide-supported copper catalyst shows the best catalytic effect, with a formaldehyde selectivity of over 70%, while the catalysts supported by gold or palladium are significantly lower than those supported by copper catalysts in terms of formaldehyde generation rate and selectivity.

[0059] From the results of Examples 1 to 3 in the above table, it can be seen that the copper active component loading has a significant effect on the performance of titanium dioxide. When the loading is too low, the active component content is insufficient, resulting in poor catalytic performance; when the loading is increased to 0.5%, the formaldehyde generation rate and selectivity are both the highest; when the loading is further increased to 1%, the activity decreases, which may be due to the agglomeration of copper atoms reducing the number of active sites.

[0060] From the results of Example 1, Example 4, Example 5 and Example 6 in the above table, it can be seen that the oxygen partial pressure also affects the reaction performance. When there is no oxygen, the formaldehyde generation rate is very low, indicating that the activation of molecular oxygen is one of the important processes of the reaction; when the oxygen partial pressure increases to 1 bar, the formaldehyde generation rate is significantly increased; further increasing the oxygen partial pressure will lead to the generation of a large amount of carbon dioxide, which is because too much oxygen generates excessive active oxygen free radicals, resulting in deep oxidation of a large number of products.

[0061] From the results of Example 1, Example 7, Example 8 and Example 9 in the above table, it can be seen that as the reaction time increases, the formaldehyde generation rate and selectivity tend to decrease. This is because the residence time of formaldehyde increases and further deep oxidation to carbon dioxide occurs.

[0062] In summary, the method for preparing formaldehyde 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. Under the action of light and a titanium dioxide-supported copper single-atom catalyst, methane is selectively converted into formaldehyde. By controlling the catalyst preparation conditions and the copper loading amount, the methane and oxygen partial pressures, and the reaction time, the resulting formaldehyde generation rate can reach 3.7 mmol / g / h, and the selectivity exceeds 70%. The catalyst preparation process and the catalytic reaction process are green and economical, and have broad application prospects.

[0063] 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.

[0064] 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 preparing a catalyst for preparing formaldehyde by oxidation of methane, wherein the catalyst is a titanium dioxide-supported copper single-atom catalyst; characterized in that: The steps include: Tetrabutyl titanate and hydrofluoric acid are mixed for crystallization reaction to obtain a titanium dioxide carrier; titanium dioxide, copper nitrate and sodium borohydride are reacted in a water solvent in the dark to obtain a titanium dioxide-supported copper single atom catalyst.

2. The method for preparing a catalyst for preparing formaldehyde by oxidation of methane according to claim 1, characterized in that: The specific steps include: Adding hydrofluoric acid solution to tetrabutyl titanate, transferring the mixture to a crystallization kettle after stirring for a certain period of time, and placing the crystallization kettle in an oven for crystallization reaction, and subjecting the solid product obtained by the crystallization reaction to centrifugation, water washing and drying to obtain a titanium dioxide carrier; The titanium dioxide carrier is dispersed in water to obtain a uniform solution, a certain amount of copper nitrate solution is added to the uniform solution, the reaction temperature is controlled at 5-30°C and continuous stirring is performed, a certain concentration of sodium borohydride solution is added to react in the dark, and the titanium dioxide-supported copper single atom catalyst is obtained after centrifugation, water washing and drying.

3. A method for preparing a catalyst for preparing formaldehyde by oxidation of methane according to claim 1 or 2, 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 temperature is 100-200 °C, and the crystallization time is 12-48 h.

4. A method for preparing a catalyst for preparing formaldehyde by oxidation of methane according to claim 1 or 2, characterized in that: The concentration of the sodium borohydride solution is 0.02-0.5 mol / L, and the mass ratio of the sodium borohydride to copper nitrate is 50:1-5:

1.

5. A method for preparing formaldehyde by selective oxidation of methane through photocatalysis, characterized in that: The steps include: The titanium dioxide-supported copper single atom 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 formaldehyde and other oxidation products. The amount of titanium dioxide-supported copper single atom catalyst used in the reaction process is 0.1~10 mg, and the amount of water solvent used is 0~100 mL.

6. The method for preparing formaldehyde by photocatalytic methane selective oxidation according to claim 5, characterized in that: The titanium dioxide-supported copper single-atom catalyst comprises a titanium dioxide carrier and a copper active component; wherein the mass fraction of the copper active component is 0.1-2 wt %.

7. The method for preparing formaldehyde by photocatalytic methane selective oxidation according to claim 5, 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 0-10 bar and the methane partial pressure is 10-20 bar.

8. The method for preparing formaldehyde by photocatalytic methane selective oxidation according to claim 5, characterized in that: The reaction temperature is 25°C, the reaction time is 0.5-5 h, and the light source used in the photocatalytic reaction is a xenon lamp with an emission wavelength of 200-1300 nm and a light intensity of 200-800 mW / cm 2 .

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