Method for synthesizing methanol by continuously oxidizing methane through photo-thermal catalysis

Abstract

The invention belongs to the technical field of catalytic conversion of natural gas, and particularly relates to a method for synthesizing methanol by continuously oxidizing methane through photo-thermal catalysis, which comprises the following steps: by taking a ZnO-loaded transition metal oxide as a catalyst and oxygen as an oxidant, continuously and efficiently oxidizing methane to synthesize methanol under normal pressure in cooperation with light energy and heat energy, the method comprises the following specific steps: step 1, loading a catalyst into a fixed bed reactor, introducing nitrogen under a normal pressure condition, and carrying out programmed heating to a reaction temperature; and 2, after the reaction temperature is stable, closing the nitrogen, introducing reaction gases CH4, O2 and H2O (g), and turning on the light source for reaction. According to the method, the methanol is synthesized by continuously and efficiently oxidizing methane under the normal-pressure condition through cooperation of light energy and heat energy, the technological process is simple, the method is suitable for continuous operation, and the catalyst is low in preparation cost, excellent in catalytic performance, high in methanol selectivity and high in industrial application value; the reaction is carried out under the normal pressure condition, so that the safety problem caused by high-pressure reaction is avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of natural gas catalytic conversion, and specifically relates to a method for synthesizing methanol by continuous oxidation of methane with photothermal catalysis. Background Art

[0002] The natural gas industry is based on protecting people's livelihood and promoting emission reduction. It is an important energy source to support the comprehensive green transformation of economic and social development. According to the "2019 International Energy Outlook", by 2050, global natural gas production will increase significantly from 133.1 trillion British thermal units to 191.6 trillion British thermal units. Natural gas is mainly mined in remote areas. Its main component is methane, but methane has a boiling point of -161.49 °C and is flammable and explosive, making it inconvenient for long-distance transportation. Directly converting methane into high-value-added liquid chemical methanol is conducive to long-term storage and transportation, and can also achieve efficient utilization of methane resources. However, due to the highly symmetrical tetrahedral geometry of methane molecules, it has a low polarizability (2.84×10 -40 C 2 m 2 J -1 ), high ionization energy (12.6 eV), large HOMO-LUMO distance (9.91 eV) and relatively high CH bond energy (435 kJ mol -1 ) and other characteristics, and the activation conditions are harsh. In addition, the dissociation energy of the CH bond of the product methanol is about 47 kJ / mol lower than that of the CH bond of methane, and it is easy to be further oxidized during the reaction, resulting in a decrease in methanol selectivity. Therefore, the methane oxidation reaction to methanol has always been regarded as the "Dream Reaction" in the field of catalysis.

[0003] Currently, the thermal catalytic process of methane oxidation to methanol mainly relies on zeolite materials as catalysts, but the preparation process of these zeolite-based catalysts is cumbersome and costly, limiting their economic feasibility in industrial-scale applications. In addition, photocatalysis, as an alternative method, is usually carried out in intermittent reactors, a mode of operation that is not conducive to large-scale industrial applications. Therefore, the development of catalysts and process flows that are cost-effective, easy to operate, and suitable for continuous operation is crucial to promoting the industrialization of methane oxidation to methanol technology.

[0004] In the chemical industry, supported catalysts are widely used due to their efficient catalytic performance. As a simple preparation technique, the impregnation method is to immerse the support in a solution containing metal precursors, and then obtain the catalyst by evaporating the water and calcining. However, due to capillary suction, the solvent will move in the absorption layer during the conventional evaporation and drying process, disrupting the distribution of the metal precursor, causing the active species to aggregate on the support surface, and the interaction with the support is also weakened. The metal is unevenly distributed on the support surface, reducing the reaction activity. Summary of the invention

[0005] The purpose of the present invention is to provide a method for synthesizing methanol by continuous oxidation of methane with photothermal catalysis, so as to solve the problems that the existing photocatalysts are not conducive to large-scale industrial application in intermittent reactors, the preparation of thermal catalysts is cumbersome, and the solvent of supported catalysts moves in the absorption layer during conventional evaporation and drying, disrupting the distribution of metal precursors and causing uneven distribution of metals on the carrier surface, thereby reducing the reaction activity.

[0006] In order to achieve the above object, the present invention adopts the following technical solution: A method for synthesizing methanol by continuous oxidation of methane through photothermal catalysis, using ZnO-loaded transition metal oxide as a catalyst, oxygen as an oxidant, and synergizing light energy and thermal energy to continuously and efficiently oxidize methane to synthesize methanol under normal pressure conditions, and the specific steps are as follows: Step 1: Load the catalyst into a fixed bed reactor, introduce nitrogen gas under normal pressure to perform programmed temperature increase to the reaction temperature; Step 2: After the reaction temperature stabilizes, turn off the nitrogen, introduce reaction gases CH4, O2 and H2O (g), and turn on the light source to react.

[0007] Preferably, the reaction temperature in step 1 is 150-300°C.

[0008] Preferably, the gas flow ratio of CH4, O2 and H2O (g) in step 2 is 56-80:0-21:24-60, and the reaction pressure is normal pressure.

[0009] Preferably, in step 1, the catalyst is ZnO loaded with a transition metal oxide, the transition metal oxide is any one of Fe2O3, Co3O4, and NiO, wherein the loading amount of the loaded transition metal oxide is 0.5% to 5%.

[0010] Preferably, the method for preparing the catalyst comprises the following steps: S1: Dissolve 20 mmol Zn(NO3)2•6H2O and 60 mmol CO(NH2)2 in 80 mL deionized water, and perform hydrothermal reaction at 120℃ for 3 h. After the reaction, filter, wash, dry, and calcine in air atmosphere at 400℃ for 2 h to obtain white ZnO powder. S2: Dissolve a transition metal salt in 30 mL of deionized water, wherein the transition metal salt is any one of Ni(NO3)2•6H2O, Fe(NO3)2•9H2O, Co(NO3)2•6H2O and Ni(NO3)2•6H2O, and drop it onto the ZnO powder under vigorous stirring; then pre-freeze the obtained sample and place it in a vacuum freeze dryer for freeze drying, and finally calcine it in an air atmosphere at 400 °C for 2 h to obtain a catalyst.

[0011] Preferably, in step S2, the freeze-drying temperature is -20 to -10°C, and the freeze-drying time is 5 to 20 hours.

[0012] Compared with the prior art, the present invention has the following beneficial effects: (1) The photothermal catalytic reaction is carried out in a continuous fixed-bed reactor, which can continuously oxidize methane into methanol and has strong industrial application value. The reaction is carried out under normal pressure conditions, avoiding the safety issues caused by high-pressure reactions. (2) When the catalyst of the present invention is used for the photothermal catalytic continuous oxidation of methane to synthesize methanol, there is no need to pretreat the catalyst before the reaction begins, and the operation process is simple; (3) The catalyst of the present invention is used for the first time in the photothermal catalytic continuous oxidation of methane to methanol, showing excellent catalytic activity and methanol selectivity; (4) The preparation method of the catalyst of the present invention is simple, requiring only three inorganic compounds, zinc nitrate, urea and transition metal salt, without the need to add additional templates, surfactants, surface modifiers, etc. The raw materials are widely available, the preparation method is simple and the cost is low; (5) When preparing the supported catalyst in the present invention, freeze drying is used instead of conventional evaporative drying during the impregnation process, thereby maintaining the spatial distribution of the transition metal salt on the ZnO before drying, making the transition metal oxide more evenly distributed on the ZnO surface, and enhancing the interaction between the transition metal oxide and ZnO. The resulting catalyst has good stability and high repeatability. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The XRD patterns of the catalysts of Example 1, Comparative Example 1 and Comparative Example 2 are shown in FIG. Figure 2 TEM image of the catalyst of Example 1; Figure 3 The XPS graphs of the catalysts of Example 1 and Comparative Examples 1 to 3 are shown; Figure 4 The catalytic performance diagram of the continuous oxidation of methane to methanol by embodiments 1 to 3; Figure 5 This is a catalytic performance diagram of the photothermal catalytic continuous oxidation of methane to methanol in Example 4; Figure 6 This is a catalytic performance diagram of the photothermal catalytic continuous oxidation of methane to methanol in Example 5; Figure 7 This is a catalytic performance diagram of the photothermal catalytic continuous oxidation of methane to methanol in Example 6; Figure 8 This is a catalytic performance diagram of the photothermal catalytic continuous oxidation of methane to methanol in Example 7; Fig. 9 The catalytic performance diagram of different preparation batches of the catalyst in Example 1; Fig.10 The methanol yield and selectivity of the catalyst in Example 1 within 48 hours; Fig.11 The catalytic performance diagram of the catalysts of Examples 1, 8, 9 and Comparative Examples 1 to 3; Fig.12 This is the catalytic performance diagram of the catalyst of Comparative Example 4. DETAILED DESCRIPTION

[0014] The technical solution of the present invention is described in detail below in conjunction with the embodiments and drawings, but the implementation of the present invention is not limited thereto. For process parameters not particularly noted, conventional techniques may be used. Example 1

[0015] The preparation method of NiO / ZnO catalyst is as follows: S1: Dissolve 20 mmol Zn(NO3)2•6H2O and 60 mmol CO(NH2)2 in 80 mL deionized water, and perform hydrothermal reaction at 120℃ for 3 h. After the reaction, filter, wash, dry, and calcine in air atmosphere at 400℃ for 2 h to obtain white ZnO powder. S2: Dissolve 0.1011 g Ni(NO3)2•6H2O in 30 mL deionized water and add it dropwise onto ZnO powder under vigorous stirring; then freeze-dry the obtained sample at -10 °C for 10 h and finally calcine it at 400 °C in air atmosphere for 2 h to obtain NiO / ZnO catalyst.

[0016] The method and performance test of NiO / ZnO catalyst photothermal catalytic methane continuous oxidation to methanol are as follows: Take 100 mg of catalyst (40-60 mesh) and load it into a fixed bed reactor. Use nitrogen as the protective atmosphere. Raise the temperature of the reactor from room temperature to the reaction temperature of 200 °C by programmed temperature increase. After the reaction temperature stabilizes, turn off the nitrogen and introduce reaction gases CH4, O2 and H2O (g). The gas flow ratio CH4 / O2 / H2O is 72:9:48. Turn on the xenon lamp. After the reaction is carried out for 1 hour, collect the product for analysis. The results are as follows: Figure 4 As shown, the methanol yield was 47.48 μmol g cat -1 h -1 . Example 2

[0017] The preparation method of NiO / ZnO catalyst is the same as that in Example 1.

[0018] The method and performance test of continuous oxidation of methane to methanol by thermal catalysis of NiO / ZnO catalyst are as follows: Take 100 mg of catalyst (40-60 mesh) and load it into a fixed bed reactor. Use nitrogen as the protective atmosphere. Raise the temperature of the reactor from room temperature to the reaction temperature of 200 °C by programmed temperature increase. After the reaction temperature stabilizes, turn off the nitrogen and introduce reaction gases CH4, O2 and H2O (g). The gas flow ratio CH4 / O2 / H2O=72:9:48. After the reaction is carried out for 1 hour, the product is analyzed. The results are as follows: Figure 4 As shown, the methanol yield is 0. Example 3

[0019] The preparation method of NiO / ZnO catalyst is the same as that in Example 1.

[0020] The method and performance test of NiO / ZnO catalyst photocatalytic methane continuous oxidation to methanol are as follows: Take 100 mg of catalyst (40-60 mesh) and load it into a fixed bed reactor. Pass the reaction gases CH4, O2 and H2O (g). Turn on the xenon lamp. The gas flow ratio CH4 / O2 / H2O is 72:9:48. After the reaction is carried out for 1 hour, the product is analyzed. The results are as follows: Figure 4 As shown, the methanol yield was 2.88 μmol g cat -1 h -1 .

[0021] from Figure 4 It can be seen that the NiO / ZnO catalyst exhibits excellent photothermal synergy, in which light energy is the driving force for the photothermal catalytic continuous oxidation of methane to methanol, while thermal energy makes the reactants more easily activated and converted, accelerates mass transfer, facilitates the generation of photogenerated charge carriers, and promotes carrier separation and desorption of product methanol. Compared with the single photocatalytic reaction, the photothermal catalytic efficiency is increased by 16.5 times. Example 4

[0022] The preparation method of NiO / ZnO catalyst is the same as that in Example 1.

[0023] Take 100 mg of catalyst (40-60 mesh) and load it into a fixed bed reactor. Use nitrogen as the protective atmosphere. Raise the temperature of the reactor from room temperature to the reaction temperature of 150-300 °C by programmed temperature increase. After the reaction temperature stabilizes, turn off the nitrogen and introduce reaction gases CH4, O2 and H2O (g). The gas flow ratio CH4 / O2 / H2O is 72:9:48. Turn on the xenon lamp. After the reaction is carried out for 1 hour, collect the product for analysis. The results are as follows: Figure 5 As shown in the figure, the methanol yield is higher at 200 °C. Example 5

[0024] The preparation method of NiO / ZnO catalyst is the same as that in Example 1.

[0025] Take 100 mg of catalyst (40-60 mesh) and load it into a fixed bed reactor. Use nitrogen as the protective atmosphere. Raise the temperature of the reactor from room temperature to the reaction temperature of 200 °C by programmed temperature increase. After the reaction temperature stabilizes, turn off the nitrogen and introduce reaction gases CH4, O2 and H2O (g). The gas flow ratio CH4 / O2 / H2O is 56-80:9:48. Turn on the xenon lamp. After the reaction is carried out for 1 hour, collect the product for analysis. The results are as follows: Figure 6 As shown, the methane flow rate is 72 mL min -1 The methanol yield is higher. Example 6

[0026] The preparation method of NiO / ZnO catalyst is the same as that in Example 1.

[0027] Take 100 mg of catalyst (40-60 mesh) and load it into a fixed bed reactor. Use nitrogen as the protective atmosphere. Raise the temperature of the reactor from room temperature to the reaction temperature of 150-300 °C by programmed temperature increase. After the reaction temperature stabilizes, turn off the nitrogen and introduce reaction gases CH4, O2 and H2O (g). The gas flow ratio CH4 / O2 / H2O is 72:0-21:48. Turn on the xenon lamp. After the reaction is carried out for 1 hour, collect the product for analysis. The results are as follows: Figure 7 As shown, the oxygen flow rate is 9 mL min -1 The methanol yield is higher. Example 7

[0028] The preparation method of NiO / ZnO catalyst is the same as that in Example 1.

[0029] Take 100 mg of catalyst (40-60 mesh) and load it into a fixed bed reactor. Use nitrogen as the protective atmosphere. Heat the reactor from room temperature to the reaction temperature of 150-300 °C by programmed temperature increase. After the reaction temperature stabilizes, turn off the nitrogen and introduce reaction gases CH4, O2 and H2O (g). The gas flow ratio CH4 / O2 / H2O is 72:9:24-60. Turn on the xenon lamp. After the reaction is carried out for 1 hour, collect the product for analysis. The results are as follows: Figure 8 As shown, the water vapor flow rate is 48 mL min -1 The methanol yield is higher.

[0030] Fig. 9 The catalytic performance of different preparation batches of the catalyst in Example 1; Fig. 9 It can be seen that the catalytic performance of NiO / ZnO catalysts prepared in different batches has good reproducibility, indicating that freeze-drying can maintain the consistency of the spatial distribution of the transition metal salt on the carrier, making the prepared catalyst highly reproducible.

[0031] Fig.10 The methanol yield and selectivity of the catalyst in Example 1 within 48 h; Fig.10 It can be seen that the NiO / ZnO catalyst has good stability and durability, and can efficiently and persistently perform photothermal catalytic continuous oxidation of methane to synthesize methanol. Example 8

[0032] The preparation method of Fe2O3 / ZnO catalyst is as follows: S1 is the same as in Example 1; S2: 0.2937 g Fe(NO3)2•9H2O was dissolved in 30 mL deionized water and added dropwise onto ZnO powder under vigorous stirring; the resulting sample was then freeze-dried at -10 °C for 15 h and finally calcined at 400 °C in air atmosphere for 2 h to obtain Fe2O3 / ZnO catalyst.

[0033] The Fe2O3 / ZnO catalyst was used to perform photothermal catalytic methane continuous oxidation to methanol and its performance was tested in the same manner as in Example 1. Fig.11 As shown, the methanol yield was 9.95 μmol g cat -1 h -1 . Example 9

[0034] The preparation method of Co3O4 / ZnO catalyst is as follows: S1 is the same as in Example 1; S2: Dissolve 0.1995 g Co(NO3)2•6H2O in 30 mL deionized water and add it dropwise onto ZnO powder under vigorous stirring; then freeze-dry the obtained sample at -10 °C for 12 h and finally calcine it at 400 °C in air atmosphere for 2 h to obtain Co3O4 / ZnO catalyst.

[0035] The same method as in Example 1 was used to perform photothermal catalytic methane continuous oxidation to methanol synthesis and performance testing. The results are as follows: Fig.11 As shown, the methanol yield was 26.70 μmol g cat -1 h -1 .

[0036] Comparative Example 1 The preparation process of NiO catalyst is as follows: Take an appropriate amount of Ni(NO3)2•6H2O and grind it, then calcine it at 400 ℃ in air atmosphere for 2 h to obtain NiO catalyst.

[0037] The same method as in Example 1 was used to perform photothermal catalytic methane continuous oxidation to synthesize methanol and the performance test was performed. The results are as follows: Fig.11 As shown, the methanol yield is 0.

[0038] Comparative Example 2 The preparation method of the ZnO catalyst is shown in steps S1 and S2 in Example 1.

[0039] The same method as in Example 1 was used to perform photothermal catalytic methane continuous oxidation to synthesize methanol and the performance test was performed. The results are as follows: Fig.11 As shown, the methanol yield was 8.91 μmol g cat -1 h -1 , which is lower than NiO / ZnO, Fe2O3 / ZnO and Co3O4 / ZnO, indicating that the loading of transition metal oxides (especially NiO) can significantly improve the activity of ZnO catalyst.

[0040] Comparative Example 3 The NiO / ZnO-comp catalyst was prepared by evaporation drying during the impregnation process. The specific steps are as follows: S1 is the same as in Example 1; S2: 0.1011 g Ni(NO3)2•6H2O was dissolved in 30 mL deionized water and added dropwise onto the ZnO powder under vigorous stirring. The resulting sample was then dried in an oven at 60 °C for 12 h and then calcined in an air atmosphere at 400 °C for 2 h to obtain the NiO / ZnO-comp catalyst.

[0041] The NiO / ZnO-comp catalyst was subjected to photothermal catalytic methane continuous oxidation to methanol and performance testing in the same manner as in Example 1. Fig.11 As shown, the methanol yield was 24.02 μmol g cat -1 h -1 , which is significantly lower than that of NiO / ZnO catalyst prepared by freeze drying, indicating that freeze drying has unique advantages in the process of preparing catalysts by impregnation method.

[0042] Comparative Example 4 Fe-ZnO, Co-ZnO, Ni-ZnO and Cu-ZnO catalysts were prepared by the preparation method in the patent application number 2024115146419 submitted by the applicant and the patent name was "A non-precious metal zinc oxide nanosheet photocatalyst and its preparation method and application". The above catalysts were subjected to photothermal catalytic methane continuous oxidation to methanol and performance testing in the same way as in Example 1. The results are as follows Fig.12 As shown, the methanol yields were 1.62, 2.97, 3.74, and 1.25 μmol g cat -1 h -1 , are lower than the activity of ZnO prepared by the present invention.

[0043] The structure of the catalyst prepared by the present invention was characterized.

[0044] Figure 1 is the XRD diagram of the catalysts of Example 1, Comparative Example 1 and Comparative Example 2; Figure 1 It can be seen that the catalysts prepared in Example 1 and Comparative Example 1 both show characteristic diffraction peaks of the ZnO hexagonal wurtzite structure, and there is no shift in the peak position, indicating that NiO is loaded on the ZnO surface and the ZnO structure is not changed; at the same time, no characteristic diffraction peaks attributable to NiO in Comparative Example 1 are observed, indicating that the NiO species is highly dispersed.

[0045] Figure 2 is the TEM image of the catalyst of Example 1; Figure 2 No obvious NiO nanoparticles were observed, and the Ni element was evenly distributed on ZnO.

[0046] Figure 3 is the XPS graph of the catalysts of Example 1 and Comparative Examples 1 to 3; Figure 3 It can be seen that compared with Comparative Examples 1 and 2, the Zn 2 p 2 / 3 and Ni 2 p 2 / 3The results show that the above two samples shift towards high binding energy and low binding energy, respectively, while there is almost no shift in comparative example 3, indicating that freeze-drying is beneficial to enhance the interaction between NiO and ZnO.

[0047] The above embodiments are only preferred embodiments of the present invention. All equivalent changes and modifications made within the scope of the present invention should belong to the protection scope of the present invention.

Claims

1. A method for synthesizing methanol by continuous oxidation of methane by photothermal catalysis, characterized in that: Using ZnO-loaded transition metal oxides as catalysts and oxygen as oxidant, light energy and thermal energy are coordinated to continuously and efficiently oxidize methane to synthesize methanol under normal pressure conditions. The specific steps are as follows: Step 1: The catalyst is loaded into a fixed bed reactor, and nitrogen is introduced under normal pressure to perform programmed temperature increase to the reaction temperature; Step 2: After the reaction temperature stabilizes, turn off the nitrogen, introduce reaction gases CH4, O2 and H2O (g), and turn on the light source to react.

2. The method for synthesizing methanol by continuous photothermal catalytic oxidation of methane according to claim 1, characterized in that: The reaction temperature in step 1 is 150-300°C.

3. The method for synthesizing methanol by continuous photothermal catalytic oxidation of methane according to claim 2, characterized in that: The gas flow ratio of CH4, O2 and H2O (g) in step 2 is 56-80:0-21:24-60, and the reaction pressure is normal pressure.

4. A method for synthesizing methanol by continuous photothermal catalytic oxidation of methane according to any one of claims 1 or 3, characterized in that: In the step 1, the catalyst is composed of ZnO loaded with transition metal oxide, the transition metal oxide is any one of Fe2O3, Co3O4, and NiO, wherein the loading amount of the loaded transition metal oxide is 0.5% to 5%.

5. The method for synthesizing methanol by continuous photothermal catalytic oxidation of methane according to claim 4, characterized in that: The preparation method of the catalyst comprises the following steps: S1: 20 mmol Zn(NO3)2•6H2O and 60 mmol CO(NH2)2 were dissolved in 80 mL deionized water and subjected to hydrothermal reaction at 120 °C for 3 h. After the reaction, the mixture was filtered, washed, dried, and calcined at 400 °C in air atmosphere for 2 h to obtain white ZnO powder. S2: Dissolve a transition metal salt in 30 mL of deionized water, wherein the transition metal salt is any one of Fe(NO3)2•9H2O, Co(NO3)2•6H2O and Ni(NO3)2•6H2O, and drop it onto the ZnO powder under vigorous stirring; then pre-freeze the obtained sample and place it in a vacuum freeze dryer for freeze drying, and finally calcine it in an air atmosphere at 400 °C for 2 h to obtain a catalyst.

6. The method for synthesizing methanol by continuous photothermal catalytic oxidation of methane according to claim 5, characterized in that: In step S2, the freeze-drying temperature is -20 to -10°C, and the freeze-drying time is 5 to 20 hours.

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