Preparation method of modified catalyst and application thereof in regeneration of methanol in methane conversion

Abstract

The application relates to the technical field of catalyst preparation, in particular to a preparation method of a modified catalyst and application of the modified catalyst in regeneration of methanol in methane conversion, and the preparation method comprises the following steps: S1, zinc nitrate, urea, a mesoporous regulator and deionized water are mixed and stirred under water bath conditions; S2, the mixed solution obtained in the step S1 is aged at 68-70 DEG C for 2-3h, after centrifugal separation, the solid is washed to be neutral with deionized water, then washed with anhydrous ethanol, and precursor powder is obtained through drying; S3, the precursor powder, urea and a loading modifier are ball-mixed for 20-30min, calcined in a muffle furnace under air atmosphere, and then ground through a 200-mesh screen after natural cooling, so that a nitrogen-doped modified catalyst is obtained. The application is aimed at the field of photocatalytic methane preparation of high-value chemicals, solves the problem of methane peroxidation to generate carbon dioxide, proposes a method of inhibiting methane peroxidation by constructing chemical bonds, and provides a new method for solving the problem of methane peroxidation.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of catalyst preparation, in particular to a modified catalyst preparation method and its application in regenerating methanol in methane conversion. BACKGROUND

[0002] Methane selective oxidation to methanol is a major challenge in the field of energy and chemical industry. Although photocatalytic technology can activate methane under mild conditions, existing catalysts (such as ZnO, TiO2) lack precise control over the reaction path, resulting in the target product being easily over-oxidized to CO2. The traditional ZnO catalyst has a CO2 selectivity as high as 67% in the CH4 photo-oxidation process, while the methanol product is almost zero. The fundamental reason is that: (1) the active sites on the catalyst surface (such as Zn-O) have insufficient stability for the intermediate CH3OH*, leading to continuous dehydrogenation; (2) the photo-generated holes directly attack the C-H bond, resulting in an uncontrollable reaction path. This will lead to a large amount of carbon loss (more than 50% in most systems), seriously polluting the environment, causing huge economic losses, and exacerbating the greenhouse effect. The root cause of this problem lies in the thermodynamic properties of the reaction: the extremely strong C-H bond in the methane molecule (bond energy as high as 435 kJ / mol) requires a high-activity catalyst site to activate, but these active sites often further oxidize the product methanol (C-H bond energy 402 kJ / mol) to CO2 during the catalytic process. In response to this challenge, researchers have proposed various control strategies, but each method has obvious limitations. From the perspective of thermodynamics, increasing the reaction pressure can inhibit the generation of CO2, but additional energy is required for gas compression, which increases overall energy consumption. In terms of kinetic control, reducing the reaction temperature can effectively inhibit over-oxidation, but it also significantly reduces the reaction rate, resulting in a significant decrease in production efficiency. The time control strategy avoids further oxidation of methanol by shortening the reaction time, but this method artificially sets an upper limit on the yield, making it difficult to achieve the high yield required for industrial production. In addition, the use of membrane reactors and other spatial restriction methods can separate the product in time, but the complex device design and high equipment investment bring new economic burdens. These control methods are essentially compromises and trade-offs in reaction conditions, and do not fundamentally solve the core contradiction in the catalytic process: the balance between the high-activity oxygen species required for methane activation and the stability of methanol. The existence of this key scientific problem has seriously restricted the practical application and industrialization process of methane photocatalytic oxidation to methanol technology.

[0003] The strategy of only focusing on inhibiting over-oxidation by traditional methods cannot solve the inherent thermodynamic limitations, which prompts an innovative catalyst design strategy: by constructing hydrogen bond traps (such as N-H bonds) to preferentially capture electrons and inhibit C-H bond breaking, while ensuring the activation performance of the material. The present application is a synergistic design of nitrogen-doped zinc oxide (N-ZnO): on the one hand, the substrate material ZnO is used to ensure the activation performance of the catalyst, and on the other hand, the over-oxidation of methane is inhibited to reduce the generation of CO2. SUMMARY

[0004] In view of the deficiencies of the prior art, the purpose of the present application is to provide a preparation method of a modified catalyst and its application in regenerating methanol in methane conversion. In view of the problems existing in the prior art of methane photocatalytic conversion, the selectivity of high-value chemicals in methane conversion is low, and the present application realizes the generation of more than 95% of high-value chemicals by inhibiting the continuous dehydrogenation of methane, and a method for preparing high-performance photocatalytic materials. In the process of catalyzing methane conversion, the nitrogen-doped catalytic material is designed as a trap to preferentially break by constructing a chemical bond with a bond energy less than the "C-H" bond, thereby inhibiting the breaking of the "C-H" bond of methane to achieve nearly 100% generation of methanol and formaldehyde. In addition, nitrogen doping successfully improves the activation performance of the base material and increases the methane conversion rate. This technology successfully modifies the catalytic performance of the base material and can be widely used in natural gas conversion, solar fuel preparation and other clean energy fields.

[0005] To achieve the above purpose, the present application provides the following technical solutions:

[0006] The preparation method of the nitrogen-doped modified catalyst comprises the following preparation steps:

[0007] S1. Mix 36-40 parts of zinc nitrate, 22-25 parts of urea, 1-3 parts of mesoporous regulator and 300-350 parts of deionized water by mass fraction, and stir at a speed of 550-600 r / min under the condition of water bath at 68-70℃ for 10-15 min;

[0008] S2. Age the mixed solution obtained in step S1 at 68-70℃ for 2-3h, and after centrifugal separation, wash the solid to neutral with deionized water, and then wash with anhydrous ethanol for 3-4 times, dry for 10-12h to obtain a precursor powder;

[0009] S3. Ball mill mix the precursor powder, 3-4 parts of urea and 1-3 parts of loading modifier for 20-30 min, calcine in a muffle furnace under air atmosphere, grind through a 200 mesh sieve after natural cooling, and obtain a nitrogen-doped modified catalyst;

[0010] The preparation of the mesoporous regulator comprises the following steps:

[0011] S11. 0.5-1 parts of cetyltrimethylammonium bromide and 0.5-1 parts of P123 were added into 20-25 parts of deionized water, and stirred at a speed of 350-400 r / min for 15-20 min under the condition of water bath at 60-65 °C;

[0012] S12. 0.2-0.4 parts of mesitylene and 0.5-0.8 parts of the stabilizing enhancer were added into the solution obtained in step S11, and the stirring was continued for 10-13 min;

[0013] S13. The mixture obtained in step S12 was left to stand and age at 55-60 °C for 3-4 h, and the solid was obtained by centrifugation, washed with ethanol for 3-4 times, and vacuum freeze-dried to obtain the mesoporous regulator.

[0014] Preferably, the preparation of the modified support includes the following steps:

[0015] S31. 0.1-0.3 parts of chloroplatinic acid was dissolved in 15-20 parts of anhydrous ethanol, and stirred at a speed of 350-400 r / min for 8-10 min under the condition of water bath at 38-40 °C to obtain solution A;

[0016] S32. 2-3 parts of hexamethylenetetramine and 4-6 parts of ammonium borate were added into 15-20 parts of anhydrous ethanol, and ultrasonically dispersed at a frequency of 40 kHz for 15-20 min to obtain solution B;

[0017] S33. Solution A was added into solution B at a dropping speed of 1-2 ml / min under the condition of 58-60 °C, and then the temperature was increased to 70-75 °C, and the stirring was continued for 1-2 h at a speed of 350-400 r / min. The precipitate was separated by centrifugation, washed with anhydrous ethanol for 3-4 times, and vacuum dried at 78-80 °C to obtain the modified support.

[0018] Preferably, the preparation of the stabilizing enhancer includes the following steps:

[0019] S121. 0.2-0.3 parts of graphene oxide was added into 4-6 parts of anhydrous ethanol, and ultrasonically dispersed for 20-30 min to obtain a preliminary mixture;

[0020] S122. 0.2-0.3 parts of tetraethyl orthosilicate, 0.1-0.2 parts of boric acid, and 0.05-0.1 parts of polyethylene glycol were added into the preliminary mixture, and stirred at a speed of 350-400 r / min for 10-15 min under the condition of water bath at 58-60 °C to obtain a secondary mixture;

[0021] S123.0.1-0.2 parts of 50% mass fraction hydrochloric acid is added dropwise into the secondary step mixed solution, and after reaction for 1-2 h, the solution is aged at 35-40 DEG C for 5-6 h, and then vacuum dried at 65-70 DEG C for 10-12 h, and after grinding, the stable reinforcing agent is obtained through 200 mesh screen.

[0022] Preferably, the calcination in step S3 is heated to 300 DEG C at a heating rate of 5 DEG C / min and kept for 1 h.

[0023] Preferably, the drying temperature in step S2 is 78-80 DEG C.

[0024] Preferably, the temperature of vacuum freeze drying in step S13 is -50~-40 DEG C.

[0025] Preferably, the stirring speed in step S31 is 300-350 r / min.

[0026] Preferably, the ultrasonic dispersion frequency in step S121 is 40 kHz.

[0027] Application of a modified catalyst in the regeneration of methanol in methane conversion.

[0028] Compared with the prior art, the application has the beneficial effects that:

[0029] 1. The application is directed to the field of photocatalytic methane to high-value chemicals, solves the problem of methane peroxidation to generate carbon dioxide, proposes a method of inhibiting methane peroxidation by constructing chemical bonds, and proposes a new method to solve the problem of methane peroxidation. At the same time, the activation performance of the base material is optimized, the activation performance of the original catalyst is improved, the yield and selectivity of methanol are improved, and the directional generation of methanol is promoted.

[0030] 2. The application improves the photo-generated carrier separation efficiency of the nitrogen-doped modified catalyst by loading a modifier, inhibits methane peroxidation, forms a deep synergy of electron-hole separation and reaction microenvironment regulation with a mesoporous regulator, improves the thermal stability of the catalyst, inhibits C-H bond rupture, and blocks the over-oxidation path from the root, solving the economic and environmental contradiction of photocatalytic methane to methanol. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The preparation process flow chart of the modified catalyst of the application;

[0032] Figure 2 The XRD spectrum of the nitrogen-doped modified catalyst obtained in Example 1 and Comparative Examples 1-3 of the application;

[0033] Figure 3 The TEM image of the nitrogen-doped modified catalyst obtained in Comparative Example 1 of the application;

[0034] Figure 4 TEM image of the nitrogen-doped modified catalyst obtained in Example 1 of the present application;

[0035] Figure 5 EPR spectrum of the nitrogen-doped modified catalyst obtained in Comparative Example 1 of the present application;

[0036] Figure 6 EPR spectrum of the nitrogen-doped modified catalyst obtained in Example 1 of the present application;

[0037] Figure 7 Column chart of CH3OH production over time of the nitrogen-doped modified catalyst obtained in Example 1 and Comparative Examples 1-3 of the present application;

[0038] Figure 8 Column chart of CO2 production over time of the nitrogen-doped modified catalyst obtained in Example 1 and Comparative Examples 1-3 of the present application;

[0039] Figure 9 Column chart of CO production over time of the nitrogen-doped modified catalyst obtained in Example 1 and Comparative Examples 1-3 of the present application;

[0040] Figure 10 Comparison chart of total product amount of the nitrogen-doped modified catalyst obtained in Example 1 and Comparative Examples 1-3 of the present application at 40 min;

[0041] Figure 11 Comparison chart of product proportion of the nitrogen-doped modified catalyst obtained in Example 1 and Comparative Example 1 of the present application;

[0042] Figure 12 In-situ infrared spectrum of the nitrogen-doped modified catalyst obtained in Example 1 of the present application;

[0043] Figure 13 In-situ infrared spectrum of the nitrogen-doped modified catalyst obtained in Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0044] The present application will be described in detail below with reference to embodiments of the present application. Obviously, the described embodiments are only a part but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present application.

[0045] Please refer to Figures 1-13 The present application provides a technical solution:

[0046] In order to solve the problem of photocatalytic methane peroxidation, the application provides a nitrogen-doped modified catalyst for photocatalytic preparation of methanol from methane under mild conditions without additional oxidants. Through synergistic effect, the activation performance of the base material is improved, and the activation of methane is promoted, and the chemical bond of preferential 'C-H' bond breaking is constructed as a trap to inhibit the continuous dehydrogenation of methane, on the one hand, high-value chemical methanol and formaldehyde are generated close to 100%, and on the other hand, the generation of CO2 is inhibited. The modified catalyst is in the form of micro-particle platinum supported on boron nitride carrier, and after calcination, it does not enter the ZnO crystal lattice, so that the main phase of the catalyst is still N-ZnO, and the increase of ZnO oxygen vacancies in the nitrogen doping process further enhances the activation performance of the catalyst, and the reaction of methane is efficiently converted (methanol selectivity > 78%, which is 2.1 times higher than that of the undoped ZnO) in the same time, and the nitrogen-doped modified catalyst is successfully modified. On the other hand, the formaldehyde generated in the catalytic process of the nitrogen-doped modified catalyst is combined with protons (H*) to generate methanol again, and the formaldehyde generated in the catalytic process of ZnO is further dehydrogenated to CO2.

[0047] Example 1

[0048] Preparation method of the modified catalyst

[0049] Before preparing the nitrogen-doped modified catalyst, the preparation of the stability enhancer, the mesoporous regulator and the supported modifier is carried out:

[0050] The preparation of the stability enhancer comprises the following steps:

[0051] S121. 0.2g of graphene oxide is added to 4ml of anhydrous ethanol, and ultrasonic dispersion is carried out at a frequency of 40kHz for 20min to obtain a preliminary mixture;

[0052] S122. 0.2g of tetraethyl orthosilicate, 0.1g of boric acid and 0.05g of polyethylene glycol are added to the preliminary mixture, and stirring is carried out at a speed of 350r / min under the condition of a water bath at 58℃ for 10min to obtain a secondary mixture;

[0053] S123. 0.1ml of 50% hydrochloric acid is added dropwise to the secondary mixture, and after reaction for 1h, the mixture is placed at 35℃ for aging for 5h, and then vacuum drying is carried out at 65℃ for 10h, and after grinding, the stability enhancer is obtained by sieving through a 200 mesh sieve;

[0054] The preparation of the mesoporous regulator comprises the following steps:

[0055] S11. 0.5g of cetyltrimethylammonium bromide and 0.5g of P123 are added to 20ml of deionized water, and stirring is carried out at a speed of 350r / min under the condition of a water bath at 60℃ for 15min;

[0056] S12. To the solution obtained in step S11, 0.2 g of mesitylene and 0.5 g of the stabilizing enhancer were added, and stirring was continued for 10 min;

[0057] S13. The mixture obtained in step S12 was aged at 55°C for 3 h, and the solid was obtained by centrifugation, washed with ethanol for 3 times, and vacuum freeze-dried (-50°C) to obtain the mesoporous regulator;

[0058] The preparation of the modified catalyst comprises the following steps:

[0059] S31. 0.1 g of chloroplatinic acid was dissolved in 15 ml of anhydrous ethanol, and stirring was carried out at 38°C water bath condition at a speed of 300 r / min for 8 min to obtain solution A;

[0060] S32. 2 g of hexamethylenetetramine and 4 g of ammonium borate were added to 15 ml of anhydrous ethanol, and ultrasonic dispersion was carried out at a frequency of 40 kHz for 15 min to obtain solution B;

[0061] S33. Solution A was added to solution B at a dropping speed of 1 ml / min at 58°C, and then the temperature was increased to 70°C, and stirring was carried out at a speed of 350 r / min for 1 h. The precipitate was separated by centrifugation, washed with anhydrous ethanol for 3 times, and dried at 78°C under vacuum to obtain the modified catalyst support;

[0062] S1. 36 g of zinc nitrate, 22 g of urea, 1 g of the mesoporous regulator, and 300 g of deionized water were mixed, and stirring was carried out at 68°C water bath condition at a speed of 550 r / min for 10 min;

[0063] S2. The mixture obtained in step S1 was aged at 68°C for 2 h, and the solid was washed with deionized water until neutral, washed with anhydrous ethanol for 3 times, and dried at 78°C for 10 h to obtain the precursor powder;

[0064] S3. The precursor powder, 3 g of urea, and 1 g of the modified catalyst support were ball-milled and mixed for 20 min, calcined in a muffle furnace under air atmosphere (heated to 300°C at a heating speed of 5°C / min and kept for 1 h), and ground through a 200 mesh sieve after natural cooling to obtain the nitrogen-doped modified catalyst. The nitrogen doping amount in this embodiment was about 0.5, and this embodiment was further named as 0.5N-ZnO and N-ZnO.

[0065] Example 2

[0066] The preparation method of the modified catalyst comprises the following steps:

[0067] Before the preparation of the nitrogen-doped modified catalyst, the preparation of the stabilizing enhancer, the mesoporous regulator, and the modified catalyst support was carried out:

[0068] The preparation of the stabilizing enhancer comprises the following steps:

[0069] S121. 0.3 g of graphene oxide was added to 6 ml of anhydrous ethanol, and ultrasonic dispersion was performed at a frequency of 40 kHz for 30 min to obtain a primary mixture;

[0070] S122. 0.3 g of tetraethyl orthosilicate, 0.2 g of boric acid, and 0.1 g of polyethylene glycol were added to the primary mixture, and stirring was performed at a speed of 400 r / min under the condition of a water bath at 60°C for 15 min to obtain a secondary mixture;

[0071] S123. 0.2 ml of 50% hydrochloric acid was added dropwise to the secondary mixture, and reaction was performed for 2 h. After that, the mixture was left to stand and age at 40°C for 6 h, and then vacuum drying was performed at 70°C for 12 h. After grinding, the mixture was sieved through a 200-mesh screen to obtain a stable reinforcing agent;

[0072] The preparation of the mesopore regulator includes the following steps:

[0073] S11. 1 g of cetyltrimethylammonium bromide and 1 g of P123 were added to 25 ml of deionized water, and stirring was performed at a speed of 400 r / min under the condition of a water bath at 65°C for 20 min;

[0074] S12. 0.4 g of mesitylene and 0.8 g of the stable reinforcing agent were added to the solution obtained in step S11, and stirring was continued for 13 min;

[0075] S13. The mixture obtained in step S12 was left to stand and age at 60°C for 4 h, and then centrifugation was performed to obtain a solid. The solid was washed with ethanol for 4 times and vacuum freeze-dried at -40°C to obtain a mesopore regulator;

[0076] The preparation of the supported modifier includes the following steps:

[0077] S31. 0.3 g of chloroplatinic acid was dissolved in 20 ml of anhydrous ethanol, and stirring was performed at a speed of 350 r / min under the condition of a water bath at 40°C for 10 min to obtain solution A;

[0078] S32. 3 g of hexamethylenetetramine and 6 g of ammonium borate were added to 20 ml of anhydrous ethanol, and ultrasonic dispersion was performed at a frequency of 40 kHz for 20 min to obtain solution B;

[0079] S33. Solution A was added to solution B at a dropping rate of 2 ml / min under the condition of a water bath at 60°C, and then the temperature was increased to 75°C. Stirring was performed at a speed of 400 r / min for 2 h, and then centrifugation was performed to separate the precipitate. The precipitate was washed with anhydrous ethanol for 4 times, and then vacuum drying was performed at 80°C to obtain a supported modifier;

[0080] S1. 40 g of zinc nitrate, 25 g of urea, 3 g of mesoporous regulator were mixed with 350 g of deionized water, and stirred at 600 r / min under the condition of 70℃ water bath for 15 min;

[0081] S2. The mixed solution obtained in step S1 was aged at 70℃ for 3 h, and after centrifugal separation, the solid was washed to neutral with deionized water, and then washed with anhydrous ethanol for 4 times, and dried at 80℃ for 12 h to obtain a precursor powder;

[0082] S3. The precursor powder, 4 g of urea and 3 g of loading modifier were ball-milled for 30 min, calcined in a muffle furnace under air atmosphere (heated to 300℃ at a heating rate of 5℃ / min and kept for 1 h), and then ground through a 200 mesh sieve to obtain a nitrogen-doped modified catalyst.

[0083] Example 3

[0084] Preparation method of modified catalyst:

[0085] Before preparing the nitrogen-doped modified catalyst, the preparation of the stability enhancer, mesoporous regulator and loading modifier was carried out:

[0086] The preparation of the stability enhancer includes the following steps:

[0087] S121. 0.22 g of graphene oxide was added to 5 ml of anhydrous ethanol, and ultrasonic dispersion was carried out at a frequency of 40 kHz for 22 min to obtain a preliminary mixed solution;

[0088] S122. 0.22 g of tetraethyl orthosilicate, 0.12 g of boric acid and 0.06 g of polyethylene glycol were added to the preliminary mixed solution, and stirred at 370 r / min under the condition of 59℃ water bath for 11 min to obtain a secondary mixed solution;

[0089] S123. 0.12 ml of 50% mass fraction hydrochloric acid was added dropwise to the secondary mixed solution, and reacted for 1.5 h, then aged at 36℃ for 5.5 h, and then vacuum dried at 66℃ for 11 h. After grinding, a 200 mesh sieve was obtained to obtain a stability enhancer;

[0090] The preparation of the mesoporous regulator includes the following steps:

[0091] S11. 0.6 g of cetyltrimethylammonium bromide and 0.6 g of P123 were added to 21 ml of deionized water, and stirred at 370 r / min under the condition of 61℃ water bath for 16 min;

[0092] S12. 0.3 g of mesitylene and 0.6 g of the stability enhancer were added to the solution obtained in step S11, and continued to stir for 11 min;

[0093] S13. The mixed solution obtained in step S12 is aged at 56°C for 3.5 h, and then centrifuged to obtain a solid, which is washed with ethanol for 3 times and vacuum freeze-dried (-42°C) to obtain the mesoporous regulator;

[0094] The preparation of the support modifier comprises the following steps:

[0095] S31. 0.2 g of chloroplatinic acid is dissolved in 16 ml of anhydrous ethanol, and stirred at a speed of 320 r / min for 9 min under the condition of a 39°C water bath to obtain solution A;

[0096] S32. 2.2 g of hexamethylenetetramine and 5 g of ammonium borate are added to 16 ml of anhydrous ethanol, and ultrasonically dispersed at a frequency of 40 kHz for 16 min to obtain solution B;

[0097] S33. Solution A is added to solution B at a dropping speed of 1 ml / min under the condition of 59°C, and then heated to 71°C, and stirred at a speed of 370 r / min for 1 h, and then centrifuged to separate the precipitate, which is washed with anhydrous ethanol for 3 times and dried at 79°C to obtain the support modifier;

[0098] S1. 37 g of zinc nitrate, 23 g of urea, 2 g of mesoporous regulator and 320 g of deionized water are mixed, and stirred at a speed of 570 r / min for 11 min under the condition of a 69°C water bath;

[0099] S2. The mixed solution obtained in step S1 is aged at 69°C for 2.5 h, and then centrifuged to separate the solid, which is washed with deionized water until neutral, and then washed with anhydrous ethanol for 3 times and dried at 79°C for 11 h to obtain a precursor powder;

[0100] S3. The precursor powder, 3.2 g of urea and 2.2 g of the support modifier are ball-mixed for 22 min, and then calcined in a muffle furnace under the condition of air atmosphere (heated to 300°C at a heating speed of 5°C / min and then kept for 1 h), and then ground through a 200-mesh sieve to obtain a nitrogen-doped modified catalyst.

[0101] Example 4

[0102] The preparation method of the modified catalyst comprises the following steps:

[0103] Before the preparation of the nitrogen-doped modified catalyst, the preparation of the stability enhancer, the mesoporous regulator and the support modifier is performed:

[0104] The preparation of the stability enhancer comprises the following steps:

[0105] S121. 0.28 g of graphene oxide is added to 5.5 ml of anhydrous ethanol, and ultrasonically dispersed at a frequency of 40 kHz for 28 min to obtain a preliminary mixed solution;

[0106] S122.0.28g tetraethyl orthosilicate, 0.18g boric acid, 0.07g polyethylene glycol were added into the preliminary mixed solution, and stirred at 380r / min for 14min under the condition of 59℃ water bath to obtain a secondary mixed solution;

[0107] S123.0.16ml 50% hydrochloric acid was added dropwise into the secondary mixed solution, and reacted for 1.5h. After standing and aging at 38℃ for 5.5h, vacuum drying was carried out at 68℃ for 11.5h. After grinding, the stable enhancer was obtained by passing through a 200 mesh sieve;

[0108] The preparation of the mesoporous regulator includes the following steps:

[0109] S11.0.8g cetyltrimethylammonium bromide and 0.8g P123 were added into 24ml deionized water, and stirred at 380r / min for 18min under the condition of 64℃ water bath;

[0110] S12.0.35g 1,3,5-trimethylbenzene and 0.7g stable enhancer were added into the solution obtained in step S11, and continued to stir for 12min;

[0111] S13. The mixed solution obtained in step S12 was placed and aged at 58℃ for 3.5h, and the solid was obtained by centrifugation. After washing with ethanol for 4 times, vacuum freeze drying was carried out at-48℃ to obtain the mesoporous regulator;

[0112] The preparation of the supported modifier includes the following steps:

[0113] S31.0.25g chloroplatinic acid was dissolved in 18ml anhydrous ethanol, and stirred at 340r / min for 9min under the condition of 39℃ water bath to obtain solution A;

[0114] S32.2.8g hexamethylenetetramine and 5.8g ammonium borate were added into 17ml anhydrous ethanol, and ultrasonically dispersed at a frequency of 40kHz for 17min to obtain solution B;

[0115] S33.Solution A was added to solution B at a dropping rate of 2ml / min under the condition of 59℃, and then the temperature was increased to 74℃. After stirring at 380r / min for 1.5h, the precipitate was separated by centrifugation, washed with anhydrous ethanol for 4 times, and dried at 79℃ under vacuum to obtain the supported modifier;

[0116] S1.39g zinc nitrate, 24g urea, 2.5g mesoporous regulator and 340g deionized water were mixed, and stirred at 580r / min for 14min under the condition of 69℃ water bath;

[0117] S2. The mixture obtained in step S1 was aged at 69℃ for 2.5h, and after centrifugal separation, the solid was washed to neutral with deionized water, and then washed with anhydrous ethanol for 4 times, and dried at 79℃ for 11.5h to obtain the precursor powder;

[0118] S3. The precursor powder, 3.8g of urea and 2.5g of the loaded modifier were mixed by ball milling for 27min, and then calcined in a muffle furnace under air atmosphere (heated to 300℃ at a heating rate of 5℃ / min and then kept for 1h), and after natural cooling, the mixture was ground through a 200 mesh sieve to obtain the nitrogen-doped modified catalyst.

[0119] Comparative Example 1

[0120] Comparative Example 1 and Example 1 have the following differences, the only difference is that no urea is added in the present comparative example, and the remaining steps are completely the same in Comparative Example 1 and Example 1, and further the present comparative example is named as ZnO.

[0121] Comparative Example 2

[0122] Comparative Example 2 and Example 1 have the following differences, the only difference is that the nitrogen doping amount is controlled to be 0.1 in the present comparative example, i.e. the addition amount of urea in step S1 is 3.8g, and the remaining steps are completely the same in Comparative Example 2 and Example 1, and further the present comparative example is named as 0.1N-ZnO.

[0123] Comparative Example 3

[0124] Comparative Example 3 and Example 1 have the following differences, the only difference is that the nitrogen doping amount is controlled to be 0.9 in the present comparative example, i.e. the addition amount of urea in step S1 is 34.2g, and the remaining steps are completely the same in Comparative Example 3 and Example 1, and further the present comparative example is named as 0.9N-ZnO.

[0125] Performance test:

[0126] Figure 1 Figure 2 Figure 1 is the XRD pattern of the nitrogen-doped modified catalyst obtained in Example 1 and Comparative Examples 1-3 of the present application. It can be seen from Figure 1 that no impurity phase is generated in Example 1, which indicates that the addition of urea, the loading of the modifier and the mesopore regulator in the present application does not destroy the ZnO crystal framework. Figure 2 Figure 2 is the TEM image of the nitrogen-doped modified catalyst obtained in Comparative Example 1 and Example 1 of the present application. It can be seen from the figure that Comparative Example 1 presents a uniform nanorod structure with a smooth surface. The nanorod surface of Example 1 is roughened and has a hole or defect structure, which directly proves the increase of oxygen vacancies. Figures 3-4 Figure 2 is the TEM image of the nitrogen-doped modified catalyst obtained in Comparative Example 1 and Example 1 of the present application. It can be seen from the figure that Comparative Example 1 presents a uniform nanorod structure with a smooth surface. The nanorod surface of Example 1 is roughened and has a hole or defect structure, which directly proves the increase of oxygen vacancies. Figures 5-6The EPR spectra of the nitrogen-doped modified catalysts obtained in Comparative Example 1 and Example 1 of this invention are shown respectively. Among them, Comparative Example 1 shows only a weak ∙OH signal (g=2.003), indicating that the photogenerated hole oxidation ability is limited. In Example 1, the ∙OH signal intensity is significantly improved, proving that nitrogen doping enhances the separation efficiency of photogenerated carriers and promotes ∙OH generation.

[0127] The nitrogen-doped modified catalysts prepared in Examples 1 and Comparative Examples 1-3 were used to achieve 100% selective photocatalytic conversion of methane into high-value chemicals. The method was carried out in a 400 mL glass reactor (PQ-256, Beijing Pofila Technology Co., Ltd., Beijing, China) with a quartz window for light irradiation. A 300W xenon lamp (full-spectrum light, 600mW cm⁻¹) was used. -2 The CEL-HXF300 (CEAULIGHT Technology Co., Ltd., Beijing, China) was used as the light source. A cooling water bath was used to control the reaction system temperature at 60±1°C. First, 5g of nitrogen-doped modified catalyst was dispersed in 20mL of deionized water, then added to the reactor. The reactor was sealed and purged with oxygen gas in nitrogen for 10 minutes. Afterward, 5mL of CH4 (99.99%) was injected into the reactor along with the light irradiation. During this time, as the reaction continued, methane was continuously converted into products such as methanol, formaldehyde, and carbon dioxide. After 40 minutes, the yield of carbon dioxide dropped to its lowest point, and the products methanol and formaldehyde accounted for more than 92% of the system.

[0128] Appendix Figure 7 The bar chart shows the change in CH3OH yield over time for the nitrogen-doped modified catalysts obtained in Example 1 and Comparative Examples 1-3 of this invention. Comparative Example 1 showed a methanol yield close to 0 μmol, indicating that Example 1 exhibited the best performance. This demonstrates that the modified catalyst of this invention yields the best methanol, and the optimal nitrogen doping ratio is 0.5. (See attached image.) Figure 8 The bar chart shows the CO2 yield of the nitrogen-doped modified catalysts obtained in Example 1 and Comparative Examples 1-3 of this invention as a function of time. In Comparative Example 1, the CO2 yield reached as high as 14 μmol, while in Example 1, the CO2 yield was only 1.8 μmol (a decrease of 87%), confirming that the "hydrogen bond trap" effectively blocks the continuous dehydrogenation pathway. (See attached image.) Figure 9 The bar chart shows the CO production over time of the nitrogen-doped modified catalysts obtained in Example 1 and Comparative Examples 1-3 of this invention. CO production was continuous in Comparative Example 1, while the CO production in Example 1 approached zero, indicating that the modified catalyst of this invention almost completely suppressed some oxidation side reactions. (See attached image.) Figure 10This is a comparison chart of the total product output of the nitrogen-doped modified catalysts obtained in Example 1 and Comparative Examples 1-3 over 40 minutes. In Example 1, methanol accounted for the highest proportion of the total product, while the total CO2 and CO content was less than 2 μmol. In Example 1, the methanol selectivity increased from 0% in Comparative Example 1 to >78%, while the CO2 selectivity decreased from 67% to <10%. (See attached image) Figure 11 The diagram shows the product proportions of the nitrogen-doped modified catalysts obtained in Example 1 and Comparative Example 1 of this invention. In Comparative Example 1, CO2 is dominant (85%), with methanol absent. In Example 1, methanol + formaldehyde account for 92%, while CO2 accounts for only 8%, achieving the targeted conversion of high-value chemicals. Compared with the traditional metal oxide catalyst ZnO, which produces a large amount of carbon dioxide (over 85%) and zero methanol yield when used as a catalyst, the methanol yield of the product of this invention is greatly increased, and the selectivity of methanol and formaldehyde is close to 100%.

[0129] Appendix Figures 12-13 The in-situ infrared spectra of the nitrogen-doped modified catalysts obtained in Example 1 and Comparative Example 1 of this invention are shown respectively. It can be observed that ZnO and N-ZnO exhibit significantly different intermediate evolution behaviors during the photocatalytic oxidation of methane. For ZnO, at 1471 cm⁻¹... -1 and 1583cm -1 Signal enhancements of CH2* and CO2 species were detected at 1645 cm⁻¹, indicating that methane underwent a continuous dehydrogenation and peroxidation process on the ZnO surface, ultimately producing CO2. Meanwhile, the infrared spectrum of N-ZnO showed enhancement at 1645 cm⁻¹. -1 OH*, 1071cm -1 CH3* and 956cm -1 CH3O* has different intermediates: nitrogen doping not only promotes the adsorption and dissociation of water molecules OH*, but also effectively regulates the activation and selective conversion of methane by stabilizing the CH3* and CH3O* intermediates. In addition, CH2O* adds H* to regenerate methanol.

[0130] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a modified catalyst, characterized by, The preparation steps include the following: S1. 36-40 parts of zinc nitrate, 22-25 parts of urea, 1-3 parts of mesoporous regulator and 300-350 parts of deionized water are mixed, stirred at a speed of 550-600 r / min under the condition of 68-70℃ water bath for 10-15 min; S2. The mixed solution obtained in step S1 is aged at 68-70℃ for 2-3h, and after centrifugal separation, the solid is washed to neutral with deionized water, and then washed with anhydrous ethanol for 3-4 times, dried for 10-12h to obtain the precursor powder; S3. The precursor powder, 3-4 parts of urea and 1-3 parts of loading modifier are ball-mixed for 20-30 min, calcined in a muffle furnace under air atmosphere, ground through a 200 mesh sieve after natural cooling to obtain a nitrogen-doped modified catalyst; The preparation of the mesoporous regulator includes the following steps: S11. 0.5-1 parts of cetyltrimethylammonium bromide and 0.5-1 parts of P123 are added to 20-25 parts of deionized water, stirred at a speed of 350-400 r / min under the condition of 60-65℃ water bath for 15-20 min; S12. 0.2-0.4 parts of 1,3,5-trimethylbenzene and 0.5-0.8 parts of a stabilizing enhancer are added to the solution obtained in step S11, and stirring is continued for 10-13 min; S13. The mixed solution obtained in step S12 is aged at 55-60℃ for 3-4h, and the solid is obtained by centrifugation, washed with ethanol for 3-4 times and vacuum freeze-dried to obtain the mesoporous regulator; The preparation of the loading modifier includes the following steps: S31. 0.1-0.3 parts of chloroplatinic acid is dissolved in 15-20 parts of anhydrous ethanol, stirred at 38-40℃ water bath for 8-10 min to obtain solution A; S32. 2-3 parts of hexamethylenetetramine and 4-6 parts of ammonium borate are added to 15-20 parts of anhydrous ethanol, ultrasonically dispersed at a frequency of 40 kHz for 15-20 min to obtain solution B; S33. Solution A is added to solution B at a dropping speed of 1-2 mL / min at 58-60℃, and then heated to 70-75℃, stirred at a speed of 350-400 r / min for 1-2h, the precipitate is separated by centrifugation, washed with anhydrous ethanol for 3-4 times, and dried at 78-80℃ under vacuum to obtain the loading modifier; The preparation of the stabilizing enhancer includes the following steps: S121. 0.2-0.3 parts of graphene oxide is added to 4-6 parts of anhydrous ethanol, ultrasonically dispersed for 20-30 min to obtain a preliminary mixed solution; S122. 0.2-0.3 parts of tetraethyl orthosilicate, 0.1-0.2 parts of boric acid and 0.05-0.1 parts of polyethylene glycol are added to the preliminary mixed solution, stirred at a speed of 350-400 r / min under the condition of 58-60℃ water bath for 10-15 min to obtain a secondary mixed solution; S123. To the mixture of the previous step, 0.1-0.2 parts of 50% hydrochloric acid by mass fraction was added dropwise, and after reaction for 1-2 h, the mixture was aged at 35-40 °C for 5-6 h, and then vacuum dried at 65-70 °C for 10-12 h. After grinding, the product was passed through a 200 mesh sieve to obtain the stable enhancer.

2. The method of claim 1, wherein the modified catalyst is prepared by the steps of: The calcination in step S3 was performed at a temperature increasing rate of 5 °C / min to 300 °C and then held for 1 h.

3. The method of claim 1, wherein the modified catalyst is prepared by the steps of: The drying temperature in step S2 was 78-80 °C.

4. The method of claim 1, wherein the modified catalyst is prepared by the steps of: The temperature for vacuum freeze drying in step S13 was -50 ~ -40 °C.

5. The method of claim 1, wherein the modified catalyst is prepared by the steps of: The stirring speed in step S31 was 300-350 r / min.

6. The method of claim 1, wherein the modified catalyst is prepared by the steps of: The frequency for ultrasonic dispersion in step S121 was 40 kHz.

7. The use of a modified catalyst prepared by the method of any one of claims 1-6 in the regeneration of methanol in methane conversion.

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