Photocatalyst for preparing ethanol through methane conversion as well as preparation method and application of photocatalyst

Abstract

The invention belongs to the technical field of methane photocatalytic oxidation, and particularly relates to a photocatalyst for preparing ethanol through methane conversion and a preparation method and application thereof. The photocatalyst for conversion of methane into ethanol is zinc ferrite composite zinc oxide nanoparticles, and is synthesized by using a urea, iron and zinc metal salt in-situ one-step calcination method; the photocatalyst is used for preparing ethanol through methane conversion, methanol, peroxymethanol and ethanol are obtained through reaction under illumination by adjusting the FeZn ratio, the water amount, the reaction time, the reaction temperature, the total gas pressure and the like, and meanwhile side reactions such as generation of carbon monoxide are inhibited. A traditional methane dry reforming method is improved, water molecules serve as an oxidizing agent, and high selectivity of high-value multi-carbon liquid phase products is achieved on the same catalyst interface through photochemistry. The raw materials are wide in source, and the preparation method is simple, environmentally friendly, low in price and beneficial to application and popularization.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalytic oxidation of methane, and particularly relates to a photocatalyst for coupling methane with carbon dioxide to prepare ethanol, a preparation method thereof, and an application thereof. Background Art

[0002] Methane, as a by-product of oil extraction, is widely distributed in natural gas, shale gas, and combustible ice. Due to its rich reserves and low cost, methane is gradually becoming an important alternative to coal and oil as a chemical raw material and fuel, providing the possibility for the decarbonization transformation of the petroleum industry. However, methane usually exists in a gaseous state and has the characteristics of being flammable and explosive, which makes its long-distance transportation difficult and thus limits its wide application. Therefore, converting methane into high-value-added and easily transportable liquid fuels (such as ethanol, acetic acid, or methanol, etc.) has become an ideal solution.

[0003] The methane molecule has a tetrahedral structure, showing a high degree of symmetry and low polarity, and is a very stable inert molecule. In addition, the dissociation energy of the carbon-hydrogen bond in methane is relatively high. In contrast, the dissociation energy of the carbon-hydrogen bond in its valuable conversion products (such as ethanol, methanol, etc.) is relatively low. Therefore, achieving the selective oxidation of methane remains a major challenge. Currently, industrially, methane is mainly converted through the syngas route, but this method requires a large amount of energy and capital investment, and needs to be carried out under harsh conditions of high temperature (>900K) and high pressure (>3MPa), and at the same time, a large amount of carbon dioxide will be generated, further exacerbating the global warming problem.

[0004] In order to reduce equipment maintenance costs and carbon dioxide emissions, in recent years, people have begun to explore methods for directly converting methane into oxidation products at relatively low temperatures (<500K). However, these methods usually require the use of expensive oxidants, such as hydrogen peroxide (H2O2), nitrogen oxides (NO x ), and fuming sulfuric acid. In recent years, photocatalytic methane conversion technology has been considered a promising solution. Introducing mild oxidant water (H2O) in the photochemical process can not only consume the greenhouse gas methane but also efficiently prepare ethanol, which is not only a clean alternative but also can produce multi-carbon chemicals with high energy density, having important environmental and economic significance. Summary of the Invention

[0005] The first object of the present invention is to provide a photocatalyst for methane conversion to prepare ethanol with high activity and low price;

[0006] The second object of the present invention is to provide a preparation method of the photocatalyst.

[0007] The third object of the present invention is to provide an application of the photocatalyst in coupling methane to prepare ethanol

[0008] The medium-spin photocatalyst for the preparation of ethanol by methane conversion provided by the present invention is a zinc ferrite composite zinc oxide nanoparticle; it is prepared by grinding and mixing an iron salt and a zinc salt and then reacting at a high temperature of 500 - 600 °C for 1 - 3 h;

[0009] Among them, the molar ratio of the iron salt to the zinc salt is: 1:(1 - 25). The preferred molar ratio is: 1:(5 - 25); more preferably 1:20.

[0010] Preferably, the iron salt is one or more of ferric chloride, ferric sulfate, and ferric nitrate; the zinc salt is one or more of zinc chloride, zinc acetate, zinc sulfate, and zinc nitrate.

[0011] The present invention also provides a method for preparing the photocatalyst, and the specific steps are as follows:

[0012] (1) At 20 - 25 °C, mechanically mix the zinc salt, the iron salt, and urea in a mortar. The total amount of the zinc salt and the iron salt is 1.2 - 3.2 mmol, and the molar ratio of the iron salt to the zinc salt is: 1:(1 - 25); 83 - 167 mmol of urea, and stir for more than 1 h (for example, 1 - 2 hours).

[0013] (2) Put the mixture into a covered porcelain crucible, and heat it to 520 - 600 °C at a rate of 2 - 6 °C·min -1 and react for 1 - 3 h; after the reaction is completed, cool it naturally to obtain a brownish-yellow powder, which is the zinc ferrite composite zinc oxide nanoparticle photocatalyst. Store it in a glove box filled with pure Ar gas for further use.

[0014] The proportion of the iron-zinc oxide active component is within a reasonable range, and the yield and selectivity of the target product obtained are relatively high; the photocatalytic effect is good; if the proportion of zinc ferrite is too low, the components for activating methane are insufficient, resulting in a low yield of multi-carbon liquid products; if the proportion is too high, it will lead methane to follow the over-oxidation rather than the coupling path, resulting in a decrease in the yield of multi-carbon liquid products.

[0015] The application of the above photocatalyst in the preparation of ethanol by methane conversion provided by the present invention is to promote the photocatalytic coupling of methane to generate ethanol by utilizing the close connection between the medium-spin activation sites on the catalyst surface and the active intermediates of methane under light. The specific steps are as follows:

[0016] Ultrasonically disperse the zinc ferrite composite zinc oxide nanoparticle photocatalyst in deionized water, and then introduce methane (CH4), and react under light to obtain multi-carbon products mainly composed of ethanol; the reaction is carried out in a high-pressure reaction kettle, and the top of the high-pressure reaction kettle is provided with a light-transmitting window.

[0017] Preferably, the solid-liquid ratio of the catalyst to deionized water is (5 - 25) mg:15 ml.

[0018] The dosage of the photocatalyst described needs to be controlled within a reasonable range to efficiently photocatalytically convert methane to ethanol. If the dosage is too small, the photocatalytic reaction is slow, resulting in a decrease in the product yield; if the dosage is too large, the catalytic effect on the photocatalytic reaction no longer improves, causing waste of resources.

[0019] Preferably, the high-pressure reactor is placed in a cold water pool for reaction, and the air in the reactor is replaced with argon.

[0020] Preferably, the total gas pressure of the introduced gas raw material CH4 is 1 - 2 MPa.

[0021] Preferably, the reaction time is 0.5 - 4 h.

[0022] Preferably, the reaction temperature is 25 - 50 °C.

[0023] Preferably, the light source used for illumination includes a xenon lamp, and the wavelength of the xenon lamp is 350 nm - 1000 nm.

[0024] In the present invention, the multi-carbon products mainly composed of ethanol are specifically one or more of methanol, peroxymethanol, and ethanol; among them, the main product is ethanol.

[0025] In the present invention, methane is used as a raw material, water molecules are used as an oxidant, and methane is converted into ethanol under the combined action of light and a photocatalyst. By controlling the appropriate photocatalyst composition and dosage, the main product ethanol has a high yield and selectivity, the reaction conditions are mild, the catalytic reaction process is simple and green; using zinc oxide supported zinc ferrite as a photocatalyst, the loading of an appropriate amount of zinc ferrite can construct active iron sites with intermediate spin, generating an appropriate amount of reactive oxygen species OH and reactive intermediates CH3 and CH2OH; zinc oxide serves as a light-absorbing carrier, providing sufficient holes to transfer photogenerated charges; finally, CH3 and CH2OH are selectively coupled to achieve the efficient conversion of methane to ethanol. Its schematic diagram is as Figure 1 shown.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] In the present invention, methane is used as a raw material, water molecules are used as an oxidant, and methane is coupled to ethanol under the combined action of light and zinc ferrite composite zinc oxide nanoparticle photocatalyst. By controlling the ratio and combination form of the iron-zinc binary components and the dosage of the photocatalyst, the yield of the obtained ethanol is as high as 372 μmol·g -1 ·h -1 , having good chemical stability and photocatalytic activity; the reaction conditions in the photocatalytic system for coupling methane and carbon dioxide to ethanol are mild, the process is simple and green, and it is suitable for large-scale industrial production. Description of the Drawings

[0028] Figure 1 This is a schematic diagram of the principle of the method of the present invention.

[0029] Figure 2 This is the morphology of the zinc ferrite composite zinc oxide nanophotocatalyst provided in Example 1 of the present invention. Among them, a is the electron microscope photograph of the zinc ferrite composite zinc oxide nanoparticles; b is the distribution of zinc, iron and oxygen elements.

[0030] Figure 3 This is the X-ray diffraction image before and after the reaction provided in Example 2 of the present invention.

[0031] Figure 4 This is the high-resolution lattice structure of the zinc ferrite composite zinc oxide nanophotocatalyst provided in Example 3 of the present invention.

[0032] Figure 5 This is the iron site spin degree of the zinc ferrite composite zinc oxide nanophotocatalyst provided in Examples 1, 3, and 4 of the present invention. From top to bottom, it represents the zinc ferrite composite zinc oxide nanophotocatalyst provided in Example 3 (orange curve), the zinc ferrite composite zinc oxide nanophotocatalyst provided in Example 1 (red curve), and the zinc ferrite composite zinc oxide nanophotocatalyst provided in Example 4 (blue curve). Detailed implementation manners

[0033] Next, the present invention will be further described through examples in conjunction with the accompanying drawings. It will help to understand the present invention, but does not limit the content of the present invention.

[0034] Example 1, a zinc-iron oxide photocatalyst for photocatalytic coupling of methane to produce ethanol and its application;

[0035] (1) Preparation of the photocatalyst, the specific steps are as follows:

[0036] Synthesize zinc-iron mixed oxide nanoparticles with a ratio of 1:20 by the calcination method. Mechanically mix 83 mmol of CO(NH2)2, 0.1 mmol of FeCl3 and 2.0 mmol of Zn(NO3)2·6H2O at 25 °C for 1.5 h. Then heat the mixture in air at a rate of 4 °C·min -1 to 520 °C and keep it for 2 h.

[0037] The TEM image of the photocatalyst is as shown in Figure 2 a. It can be seen from the figure that the composite oxide particles are 10 - 20 nm; the elemental analysis image of the photocatalyst is as shown in Figure 2 b. It can be clearly seen that the distributions of iron, oxygen and zinc elements are uniform.

[0038] (2) Photocatalytic coupling of methane to produce ethanol, the specific steps are as follows:

[0039] Under continuous stirring at 700 rpm, 5 mg of the above-prepared photocatalyst was dispersed in 15 mL of DI water. After replacing the air in the high-pressure reactor with Ar, 2 MPa of CH4 was introduced and continuously stirred in the dark for 20 minutes. Then, a 300 W xenon lamp was applied for full-spectrum irradiation reaction at 25 °C in the high-pressure reactor for 1 hour, and the ethanol yield could reach 370 μmol g−1 h−1.

[0040] The XRD patterns of the photocatalyst before and after the reaction are as Figure 3 shown. Zinc oxide and zinc ferrite can be well matched with the hexagonal zinc oxide standard card (JCPDS No. 36-1451) and the spinel-structured zinc ferrite standard card (JCPDS No. 22-1012), respectively. It can be seen that the structure of the catalyst remains stable before and after the reaction.

[0041] Example 2, an iron-zinc oxide photocatalyst for photocatalytic coupling of methane to produce ethanol and its application;

[0042] (I) Preparation of the photocatalyst, the specific steps are as follows:

[0043] Iron-zinc mixed oxide nanoparticles with a ratio of 1:25 were synthesized by the calcination method. 83 mmol of CO(NH2)2, 0.08 mmol of FeCl3 and 2.0 mmol of Zn(NO3)2·6H2O were mechanically mixed at 25 °C for 1.5 h. Then the mixture was heated to 520 °C in air at a rate of 2 °C·min -1 and kept at this temperature for 2 h.

[0044] The HRTEM image of the photocatalyst is as Figure 4 shown. The lattice fringes of zinc ferrite and zinc oxide can be seen from the figure.

[0045] (II) Photocatalytic coupling of methane to produce ethanol, the specific steps are as follows:

[0046] Under continuous stirring at 700 rpm, 5 mg of the above-prepared photocatalyst was dispersed in 15 mL of DI water. After replacing the air in the high-pressure reactor with Ar, 2 MPa of CH4 was introduced and continuously stirred in the dark for 20 minutes. Then, a 300 W xenon lamp was applied for full-spectrum irradiation reaction at 25 °C in the high-pressure reactor for 1 hour, and the ethanol yield could reach 150 μmol g−1 h−1.

[0047] Example 3, an iron-zinc oxide photocatalyst for photocatalytic coupling of methane to produce ethanol and its application;

[0048] (I) Preparation of the photocatalyst, the specific steps are as follows:

[0049] Synthesize iron-zinc mixed oxide nanoparticles with a 1:10 ratio by the calcination method. Mechanically mix 83 mmol of CO(NH2)2, 0.2 mmol of FeCl3, and 2.0 mmol of Zn(NO3)2·6H2O at 25 °C for 1.5 h. Then heat the mixture in air at a rate of 2 °C·min -1 to 520 °C and hold for 2 h.

[0050] The 1 / χ m -T curves of the photocatalyst (blue curve) and the photocatalysts of Example 1 (red curve) and Example 3 (orange curve) are as Figure 5 shown. It can be seen from the figure that as the Fe / Zn ratio increases, the spin degree of Fe sites in the composite catalyst increases accordingly.

[0051] (II) Preparation of ethanol by photocatalytic coupling of methane, the specific steps are as follows:

[0052] Disperse 5 mg of the above-prepared photocatalyst in 15 mL of DI water under continuous stirring at 700 rpm. After replacing the air in the high-pressure reactor with Ar, introduce a total of 2 MPa of CH4 and continuously stir for 20 minutes in the dark. Then, carry out a full-spectrum irradiation reaction for 1 hour at 25 °C in the high-pressure reactor using a 300 W xenon lamp, and the ethanol yield can reach 280 micromoles per gram per hour.

Claims

1. A preparation method of a photocatalyst, characterized in that, A zinc ferrite composite zinc oxide nanoparticle photocatalyst is prepared by grinding and mixing an iron salt and a zinc salt followed by a high-temperature reaction; wherein, the molar ratio of the iron salt to the zinc salt is 1:(1 - 25); the specific preparation steps are as follows: (1) At 20 - 25 °C, the zinc salt, the iron salt and urea are mechanically mixed in a mortar. The total amount of the zinc salt and the iron salt is 1.2 - 3.2 mmol, and the urea is 83 - 167 mmol. Stir for more than 1 h; (2) Put the mixture into a porcelain crucible with a lid and heat it up to 500 - 600 °C at a rate of 2 - 6 °C·min -1 -1, and react for 1 - 3 h; after the reaction, cool it naturally to obtain a brownish-yellow powder, which is the zinc ferrite composite zinc oxide nanoparticle photocatalyst.

2. The preparation method according to claim 1, characterized in that, The iron salt is selected from ferric chloride, ferric sulfate, ferric nitrate; the zinc salt is selected from zinc chloride, zinc acetate, zinc sulfate, zinc nitrate.

3. A photocatalyst obtained by the preparation method according to claim 1 or 2.

4. Use of a photocatalyst as described in claim 3 in the preparation of ethanol by methane conversion, characterized in that, It is to promote the photocatalytic coupling of methane to prepare ethanol by utilizing the close connection between the adsorption and activation sites of different intermediates of methane on the surface of the catalyst under light. The specific steps are as follows: The zinc ferrite composite zinc oxide nanoparticle photocatalyst is ultrasonically dispersed in deionized water, and then CH4 is introduced. A multi-carbon product mainly composed of ethanol is obtained by reacting under light; the reaction is carried out in a high-pressure reaction kettle, and the top of the high-pressure reaction kettle is provided with a light-transmitting window.

5. The application according to claim 4, characterized in that, The solid-liquid ratio of the catalyst to deionized water is (5 - 25) mg:15 ml.

6. The application according to claim 4, characterized in that The total air pressure of the introduced CH4 is 1 - 2 MPa.

7. The application according to claim 4, wherein The reaction time is 0.5 - 4 h; the reaction temperature is 25 - 50 °C.

8. The application according to claim 4, characterized in that, The light source used for illumination is a xenon lamp, and the wavelength of the xenon lamp is 350 nm - 1000 nm.

9. The application according to claim 4, wherein The multi-carbon product mainly composed of ethanol means that the main product is ethanol, and the rest is one or more of methanol, peroxy methanol, and ethanol.

Images