Preparation method and application of catalyst modified by magnesium oxide and titanium dioxide supported Au and Br

Abstract

The application belongs to the technical field of methane coupling photocatalysts, and particularly discloses a preparation method and application of a catalyst modified by MgO and Au / TiO2(Br). The Au / MgO / TiO2(Br) catalyst prepared by the method is used for methane coupling catalysis, the catalytic reaction conditions are optimized, high catalytic efficiency is ensured, the selectivity of the target product C 2+ is up to 90% at a reaction temperature of 120 DEG C, and the yield is as high as 38.7 mmol / g / h. In addition, the process for preparing the Au / MgO / TiO2(Br) catalyst is simple, the preparation conditions of the catalyst are optimized, and no corrosive substance needs to be added in the catalyst reaction process, and no acid-base substance and toxic substance by-product are generated in the whole process, the harm of the reaction to the reaction operator is small, the environmental pollution is negligible.

Description

Technical Field

[0001] This invention belongs to the field of methane coupling photocatalyst technology, specifically relating to the preparation method and application of catalysts modified with magnesium oxide and supported on titanium dioxide with Au and then modified with Br. Background Technology

[0002] Methane is one of the most stable molecules, widely found in natural gas, shale gas, and coalbed methane. It possesses advantages such as high calorific value, low cost, and safety (non-toxicity), making it a convenient and clean fuel. Furthermore, as an important C1 feedstock, methane is also used to manufacture high-value-added chemical products. Common methods for methane conversion include direct oxidation to CH3OH, oxidative coupling to ethane, and direct aromatization to aromatics. All these conversion methods begin with the activation of the inert carbon-hydrogen bonds in methane.

[0003] A methane molecule is composed of four equivalent sp... 3 Methane has a tetrahedral structure composed of hybrid CH bonds and a central carbon atom. The CH bond length is 1.087 Å, and the HCH bond angle is 109.5°. Methane has relatively low polarizability (2.84 × 10⁻⁶). -40 C 2 ·m 2 ·J -1 It requires a relatively high local electric field to be polarized, and its carbon-hydrogen bond dissociation energy is very high (439.3 kJ·mol⁻¹). -1 Methane has an ionization potential (IP) as high as 12.6 eV and an electron affinity (EA) of approximately -1.9 eV, meaning that the methane anion (CH4) has a high ionization potential (IP) of 12.6 eV and an electron affinity (EA) of approximately -1.9 eV. - It is less stable than neutral methane. Methane is a weak acid, has a very high pKa (approximately 48), and moderate proton affinity (543.9 kJ·mol⁻¹). -1 Methane possesses both a lower-energy highest occupied molecular orbital (HOMO) and a higher-energy lowest unoccupied molecular orbital (LUMO). Therefore, the energy required to remove electrons from the HOMO for oxidation or to donate electrons to the LUMO for reduction is relatively high. Methane activation typically requires harsh conditions such as high temperature, high pressure, strong oxidants, or strong acids / bases, and presents two significant challenges. First, the target products are more reactive than methane and are readily further oxidized to CO and CO2. Second, under such stringent conditions, rapid coke buildup and sintering can easily lead to catalyst deactivation. Therefore, selective conversion of methane under mild conditions is of great significance and challenge, often referred to as the "holy grail" of catalysis.

[0004] Photocatalysis is a promising method for methane conversion because photogenerated holes produced at room temperature can activate the CH bonds of methane. Photocatalysis involves three steps. Under light irradiation, a semiconductor is excited by photons with energy greater than its band gap, generating holes and electrons. These photogenerated holes and electrons are then separated and migrate to the semiconductor surface. Finally, the electrons and holes on the surface undergo reduction and oxidation reactions, respectively, to generate the product. For photocatalytic methane conversion, light-generated holes directly oxidize the CH bonds of methane or induce the formation of reactive oxygen species to oxidize the CH bonds, generating free radicals that ultimately convert to the product. Photocatalysis can achieve various methane conversion methods, including C2C coupling, partial oxidation, dry reforming, and fluid reforming. Among these, direct C2C coupling (DCCM) of methane enables carbon chain growth to directly obtain C2 from methane. + An efficient pathway for compounds. Based on the presence or absence of oxidants such as O2, CO2, and H2O in the DCCM process, DCCM can be divided into non-oxidative coupling and oxidative coupling. Although significant progress has been made in recent years, the lack of efficient or suitable photocatalysts means that photocatalytic DCCM still suffers from low product yields, poor product selectivity, and low quantum yields. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a method for preparing and applying a catalyst modified with magnesium oxide and supported on titanium dioxide (Au) and then modified with Br, in order to achieve high catalytic efficiency, high selectivity, and high yield in the catalysis of methane coupling.

[0006] The method for preparing the catalyst of the present invention, which is magnesium oxide-modified titanium dioxide supported on Au and then modified with Br, includes the following steps: S1. Add titanium dioxide and magnesium nitrate hexahydrate to the solvent in sequence, stir and react, then evaporate to dryness to obtain solid A. Grind solid A into powder A, and calcine powder A to form MgO / TiO2 support. S2. Add the MgO / TiO2 support and reducing agent to a mixed solution of soluble gold salt and water, stir and react at room temperature, wash and filter to obtain solid B, and grind solid B into powder B. S3. Powder B and bromobenzene are added to a solvent and mixed and stirred. After the reaction is complete, the mixture is washed by centrifugation with the solvent, dried under vacuum, and then ground to obtain the catalyst Au / MgO / TiO2(Br).

[0007] Preferably, the solvent in S1 is an aqueous solution, and 1g of titanium dioxide and 0.0477g of magnesium nitrate hexahydrate are added sequentially. After stirring for more than 4 hours, the mixture is stirred and evaporated to dryness in an 80°C water bath to obtain solid A.

[0008] Preferably, in step S1, powder A is placed in a tube furnace and calcined at 300°C for 3 hours to form a carrier.

[0009] Preferably, the mass fraction of MgO loading in the MgO / TiO2 support in S1 is 0.75%.

[0010] Preferably, the mass ratio of the carrier to the reducing agent in S2 is 10:1.

[0011] Preferably, in the S2 mixture of soluble gold salt and water, the volume ratio of the soluble gold salt solution to the aqueous solution is 1:300.

[0012] Preferably, the soluble gold salt in S2 is a HAuCl4·3H2O solution with a concentration of 5 mg / ml, and the reducing agent is NaBH4; the stirring reaction time is 2 h.

[0013] Preferably, the mass fraction of Au loading in solid B in S2 is 0.5%.

[0014] Preferably, the solvent in S3 is tetrahydrofuran, and 20 mg of powder B and 3 mg of bromobenzene are added to tetrahydrofuran respectively, and the mixture is stirred and reacted for 15 min.

[0015] This invention also provides the application of the above-mentioned magnesium oxide-modified titanium dioxide supported Au and Br-modified catalyst in the photocatalytic methane coupling process.

[0016] The beneficial effects of this invention are: First, the Au / MgO / TiO2(Br) catalyst prepared in this invention was used for the catalysis of methane coupling. The catalytic reaction conditions were optimized, and high catalytic efficiency was ensured. At a reaction temperature of 120℃, the target product C... 2+ The selectivity reached 90%, and the yield was as high as 38.7 mmol / g / h. Secondly, the preparation process of Au / MgO / TiO2(Br) catalyst in this invention is simple, the preparation conditions of the catalyst are optimized, and no corrosive substances are added during the catalyst reaction process. No acidic or alkaline substances or toxic byproducts are generated throughout the process. The reaction poses little harm to the reaction operator and the environmental pollution is negligible. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below.

[0018] Figure 1 This is a transmission electron microscope image of the Au / MgO / TiO2(Br) catalyst. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0020] Example 1 This embodiment provides a method for preparing a catalyst modified with magnesium oxide and supported on titanium dioxide, then modified with Br, including the following steps: S1. Add 80ml of water to a 100ml beaker, then add 1g of titanium dioxide and 0.0477g of magnesium nitrate hexahydrate in sequence. After stirring for at least 4 hours, stir and evaporate to dryness in an 80℃ water bath until a dry solid A is obtained. Grind solid A into powder A, and place powder A in a tube furnace for calcination at 300℃ for 3 hours to form a MgO / TiO2 support. S2. Take another 100ml beaker, add 30ml of water, and then add 0.1ml of HAuCl4·3H2O (5mg / ml) solution to form a mixed solution of HAuCl4·3H2O (5mg / ml) and water. Then add the MgO / TiO2 support and reducing agent to the mixed solution and stir the reaction at room temperature for 2 hours. After the reaction, wash and filter to obtain solid B. Grind solid B into powder B. S3. In a separate 20ml beaker, add 5g of tetrahydrofuran, then add 200mg of powder B and 3mg of bromobenzene, mix and stir. After the reaction is complete, wash three times with tetrahydrofuran by centrifugation, dry overnight in a vacuum drying oven, and then grind to obtain the catalyst Au / MgO / TiO2(Br). The mass fraction of Au loading in the catalyst Au / MgO / TiO2(Br) is 0.5%, and the mass fraction of MgO loading is 0.75%. Its transmission electron microscope image is shown below. Figure 1 As shown.

[0021] Example 2 The Au / MgO / TiO2(Br) catalyst prepared in Example 1 with a Br-modified metal Au loading of 0.5% by mass and a MgO loading of 0.75% by mass was used in the photocatalytic methane coupling process: the catalyst was loaded onto a glass fiber filter membrane by suction filtration and placed in a photoreactor. CH4 and O2 (5 vol%) / N2 were introduced at a flow rate of 48 ml / min and 2 ml / min, respectively. The temperature was heated to 120 °C and the reaction was carried out for 10 minutes. Gas chromatography analysis was then performed.

[0022] Example 3 The effect of Au loading on the catalytic performance of the catalyst is shown in Table 1.

[0023] Table 1. Catalytic performance of Au / MgO / TiO2 with different Au loadings (CH4 / O2 = 19.5:0.5, room temperature, optical power density = 1000 mW / cm²) 2 ) ; The results showed that the catalytic performance was optimal when the Au loading was 0.5 wt%.

[0024] Example 4 The effect of MgO loading on the catalytic performance of the catalyst is shown in Table 2.

[0025] Table 2 Catalytic performance of Au / MgO / TiO2 with different MgO loadings (CH4 / O2 = 19.5:0.5, room temperature, optical power density = 1000 mW / cm²) 2 ) ; Preliminary screening revealed that higher product yields and selectivity were achieved with Au loading of 0.5% (wt.)% and MgO loading of 0.75% (wt.)%. C2 + The yield was as high as 7477.7 umol / g / h, and the selectivity reached 80.5%.

[0026] Example 5 The effect of test flow rate on the catalytic performance of the catalyst is shown in Table 3.

[0027] Table 3 Catalytic performance of Au / MgO / TiO2(Br) at different flow rates and its comparison with Au / TiO2 ; The results showed that the catalytic performance was optimal when CH4:O2 / N2 = 39:1 and the total flow rate was 50 ml / min.

[0028] Example 6 The effect of the test gas ratio on the catalytic performance of the catalyst is shown in Table 4.

[0029] Table 4 Catalytic performance of Au / MgO / TiO2(Br) under different CH4:O2 / N2 conditions ; The results showed that the catalytic performance was optimal when CH4:O2 / N2 = 48:2.

[0030] Example 7 The effect of test temperature on the catalytic performance of the catalyst is shown in Table 5.

[0031] Table 5 Catalytic performance of Au / MgO / TiO2(Br) under different temperatures ; The results showed that the catalytic performance was optimal when the temperature was controlled at 120℃.

[0032] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not describe all details exhaustively, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification.

Claims

1. A method for preparing a catalyst modified with magnesium oxide and supported on Au and then modified with Br, characterized in that, Includes the following steps: S1. Add titanium dioxide and magnesium nitrate hexahydrate to the solvent in sequence, stir and react, then evaporate to dryness to obtain solid A. Grind solid A into powder A, and calcine powder A to form MgO / TiO2 support. S2. Add the MgO / TiO2 support and reducing agent to a mixed solution of soluble gold salt and water, stir and react at room temperature, wash and filter to obtain solid B, and grind solid B into powder B. S3. Powder B and bromobenzene are added to a solvent and mixed and stirred. After the reaction is complete, the mixture is washed by centrifugation with the solvent, dried under vacuum, and then ground to obtain the catalyst Au / MgO / TiO2(Br).

2. The method for preparing the catalyst modified with magnesium oxide and supported on titanium dioxide and then modified with Br according to claim 1, characterized in that, The solvent in S1 is an aqueous solution. 1g of titanium dioxide and 0.0477g of magnesium nitrate hexahydrate are added sequentially, and the mixture is stirred for more than 4 hours. Then, it is stirred and evaporated to dryness in an 80°C water bath to obtain solid A.

3. The method for preparing the catalyst modified with magnesium oxide and supported on titanium dioxide and then modified with Br according to claim 2, characterized in that, In step S1, powder A is placed in a tube furnace and calcined at 300°C for 3 hours to form a carrier.

4. The method for preparing the catalyst modified with magnesium oxide and supported on titanium dioxide and then modified with Br according to claim 3, characterized in that, The MgO loading in the MgO / TiO2 support in S1 is 0.75% by mass.

5. The method for preparing the catalyst modified with magnesium oxide and supported on titanium dioxide and then modified with Br according to claim 1, characterized in that, The mass ratio of the carrier to the reducing agent in S2 is 10:

1.

6. The method for preparing the catalyst modified with magnesium oxide and supported on titanium dioxide and then modified with Br according to claim 5, characterized in that, In the S2 mixture of soluble gold salt and water, the volume ratio of the soluble gold salt solution to the aqueous solution is 1:

300.

7. The method for preparing the catalyst modified by Br after supporting Au with magnesium oxide-modified titanium dioxide according to claim 6, characterized in that, The soluble gold salt in S2 is HAuCl4·3H2O solution with a concentration of 5 mg / ml, and the reducing agent is NaBH4; the stirring reaction time is 2 h.

8. The method for preparing the catalyst modified with magnesium oxide and supported on titanium dioxide and then modified with Br according to claim 7, characterized in that, The mass fraction of Au loading in solid B in S2 is 0.5%.

9. The method for preparing the catalyst modified with magnesium oxide and supported on Au and then modified with Br according to claim 8, characterized in that, The solvent in S3 is tetrahydrofuran. 20 mg of powder B and 3 mg of bromobenzene are added to tetrahydrofuran and the mixture is stirred for 15 min.

10. The application of the magnesium oxide-modified titanium dioxide catalyst supported on Au and modified with Br according to any one of claims 1-9 in the photocatalytic methane coupling process.

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