Composite catalyst for directly converting synthesis gas into p-xylene and preparation method and application thereof

Abstract

The present application relates to the technical fields of synthesis gas conversion and p-xylene synthesis, and particularly relates to a composite catalyst for directly converting synthesis gas into p-xylene, a preparation method and application thereof. The composite catalyst is mainly composed of a perovskite structure metal oxide and a molecular sieve with an outer surface modification. The preparation method of the composite catalyst comprises the following steps: mixing a soluble A salt, a soluble B salt, a complexing agent and water at room temperature, and stirring to obtain a first mixed solution; increasing the temperature of the first mixed solution to 60-90 DEG C and maintaining for 2-12 hours until a gel appears; performing CO2 supercritical drying and calcination on the gel to obtain a perovskite structure metal oxide; and mixing the perovskite structure metal oxide and the molecular sieve with the outer surface modification to obtain the composite catalyst. The composite catalyst has excellent catalytic activity and good stability, can realize direct directional conversion of synthesis gas into p-xylene, and has high p-xylene selectivity.

Description

Technical Field

[0001] This invention relates to the fields of syngas conversion and para-xylene synthesis technology, and particularly to a composite catalyst for directly converting syngas into para-xylene, its preparation method, and its application. Background Technology

[0002] Converting non-petroleum resources into syngas, which is mainly composed of CO and H2, and then converting the syngas into high-value-added chemicals are the key research directions in the field of syngas conversion.

[0003] p-Xylene is an important chemical raw material, mainly used in the production of polyester. Currently, p-xylene is primarily obtained through the catalytic reforming of naphtha. Due to thermodynamic factors, p-xylene accounts for approximately 23% of xylene. Directly preparing high-purity p-xylene requires an adsorption separation process, which incurs significant equipment and operating costs.

[0004] In recent years, many new routes for producing para-xylene have been developed both domestically and internationally. Among them, the direct catalytic conversion of syngas to para-xylene has attracted widespread attention. Generally speaking, the production of para-xylene from syngas falls into two routes: the Fischer-Tropsch synthesis route and the methanol route. For the Fischer-Tropsch synthesis route, iron-based / molecular sieve composite catalysts are commonly used. In this route, CO is first converted to para-xylene via Fe... x C y The methanol-to-methanol (MTO) process generates low-carbon olefins, which then undergo oligomerization and aromatization reactions via molecular sieves to produce aromatic products such as para-xylene. For the methanol route, a commonly used catalyst is a metal oxide / molecular sieve composite catalyst. In this process, CO first reacts with oxygen vacancies on the surface of the metal oxide to generate methanol, which then passes through a molecular sieve to produce aromatics. While the Fischer-Tropsch synthesis route exhibits high CO conversion rates on iron-based catalysts, the products are typically straight-chain alkanes, with very low yields of low-carbon olefins. Although molecular sieve catalysts can convert low-carbon olefins to aromatics, the small amount of low-carbon olefins directly limits the selectivity of aromatics. Methanol conversion on molecular sieve catalysts follows the traditional methanol-to-aromatics process, achieving high aromatic selectivity. However, the CO conversion rate on metal oxides is very low, accompanied by high CO2 selectivity, which directly limits the industrial application of the methanol route. Therefore, developing a new route to achieve high-yield production of para-xylene remains a significant challenge. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a composite catalyst for the direct conversion of syngas into p-xylene, its preparation method and application, aiming to solve the problem of low p-xylene yield in the existing catalyst-catalyzed syngas to p-xylene production route.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a method for preparing a composite catalyst for directly converting syngas into paraxylene, wherein the composite catalyst is mainly composed of a perovskite-structured metal oxide and a molecular sieve modified with an outer surface, the perovskite-structured metal oxide having the chemical formula ABO3, wherein A is at least one of La, Ce, Cr, and Ca, and B is at least one of Ni, Co, Fe, Mn, Cr, and Ga, and A and B are different;

[0008] The preparation method of the composite catalyst includes the following steps:

[0009] Step 1: At room temperature, dissolve soluble salt A and soluble salt B in water, stir, add complexing agent, and continue stirring to obtain the first mixed solution;

[0010] Step 2: Raise the temperature of the first mixed solution to 60°C. o C-90 o C, and maintain for 2-12 hours until gel appears;

[0011] Step 3: After supercritical CO2 drying and calcination, the gel is subjected to obtain a perovskite-structured metal oxide.

[0012] Step 4: Provide molecular sieves with modified outer surfaces;

[0013] Step 5: Mix the perovskite-structured metal oxide with the surface-modified molecular sieve to obtain the composite catalyst.

[0014] Optionally, in step 1, the complexing agent is selected from at least one of citric acid and ethanolamine.

[0015] Optionally, the ABO3 includes at least one of LaNiO3, LaCoO3, LaFeO3, LaMnO3, LaCrO3, CeGaO3, CrFeO3, and CaFeO3.

[0016] Optionally, in step 3, the supercritical CO2 drying temperature is 240°C. o C-280 o C, the supercritical drying time for CO2 is 1h-12h, and the supercritical drying pressure for CO2 is 7.5MPa-9.5MPa;

[0017] The firing temperature is 400. o C-600 o C, roasting time is 2h-12h.

[0018] Optionally, the molecular sieve includes at least one of ZSM-5 and ZSM-11.

[0019] Optionally, the molecular sieve with modified outer surface is prepared by the following method:

[0020] Step 41: Disperse ZSM-5 molecular sieve in a solvent to obtain a suspension. After stirring, add tetraethyl orthosilicate dropwise and continue stirring to obtain a second mixed solution.

[0021] Step 42: Heat the second mixed solution to 70°C. o C-90 o C, to remove the solvent;

[0022] Step 43: After the solvent is removed, the product is dried and calcined to obtain ZSM-5@SiO2 capsule structure molecular sieve, which is the molecular sieve with modified outer surface.

[0023] Optionally, in step 41, the mass of ZSM-5 molecular sieve is 1g-10g, the mass of solvent is 7g-40g, the stirring time is 10min each time, and the mass of tetraethyl orthosilicate is 3g-15g.

[0024] In step 43, the drying temperature is 100°C. o C-120 o C, drying time is 12h-24h; calcination temperature is 550℃. o C-650 o C, roasting time is 3h-12h.

[0025] Optionally, in step 5, the mass ratio of the perovskite-structured metal oxide to the surface-modified molecular sieve is 5:1 to 1:5; the mixing method is physical mixing, powder mixing, or dual-bed mixing.

[0026] In a second aspect, the present invention provides a composite catalyst for the direct conversion of syngas to p-xylene, wherein the catalyst is prepared by the method described in the present invention.

[0027] A third aspect of the present invention provides the application of the composite catalyst described herein in the catalytic synthesis of p-xylene from syngas.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] This invention features a scientifically designed composite catalyst with a simple preparation method, resulting in a composite catalyst with high activity and long lifespan. The composite catalyst prepared using this method exhibits an ultra-high yield of p-xylene.

[0030] This invention utilizes supercritical CO2 drying to prepare perovskite-structured metal oxides with high specific surface area. It allows for the preparation of perovskite-structured catalysts from any soluble metal salt, and the method is simple and efficient. Compared to traditional oven drying, in supercritical CO2 drying, the solvent in the gel can leave the sample being dried along with the flow of the supercritical fluid. The pores of the sample do not collapse as the solvent is removed, thus maintaining a high specific surface area.

[0031] The molecular sieve described in this invention, after surface modification (such as silica modification), can significantly improve the selectivity of p-xylene, avoid the secondary separation and purification process of p-xylene, save costs, and improve efficiency. Attached Figure Description

[0032] Figure 1 A schematic diagram of the preparation of ZSM-5@SiO2 capsule molecular sieve.

[0033] Figure 2 The image shows the XRD pattern of the catalyst prepared in Example 1. Detailed Implementation

[0034] This invention provides a composite catalyst for the direct conversion of syngas to p-xylene, its preparation method, and its application. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0035] This invention provides a method for preparing a composite catalyst, wherein the composite catalyst is mainly composed of a perovskite-structured metal oxide and a molecular sieve modified on its outer surface. The chemical formula of the perovskite-structured metal oxide is ABO3, wherein A is at least one of La, Ce, Cr, and Ca, and B is at least one of Ni, Co, Fe, Mn, Cr, and Ga, and A and B are different.

[0036] The preparation method of the composite catalyst includes the following steps:

[0037] Step 1: At room temperature (i.e., 15℃-25℃), dissolve soluble salt A and soluble salt B in water, stir, add complexing agent, and continue stirring to obtain the first mixed solution;

[0038] Step 2: Raise the temperature of the first mixed solution to 60°C. o C-90 o C (e.g., 70) o C), and maintain for 2h-12h (e.g., 2h, 4h, 5h, 7h, 8h, 10h, 11h, 12h, etc.) until gel appears;

[0039] Step 3: After supercritical CO2 drying and calcination, the gel is subjected to obtain a perovskite-structured metal oxide.

[0040] Step 4: Provide molecular sieves with modified outer surfaces;

[0041] Step 5: Mix the perovskite-structured metal oxide with the surface-modified molecular sieve to obtain the composite catalyst.

[0042] This invention provides a composite catalyst for the direct conversion of syngas to p-xylene. The composite catalyst is composed of a perovskite-structured metal oxide with a high specific surface area and a molecular sieve with an externally modified surface. The composite catalyst exhibits excellent catalytic activity and good stability. The preparation process of the composite catalyst is simple, efficient, practical, and highly reproducible. The composite catalyst can achieve the direct conversion of syngas to p-xylene.

[0043] This invention utilizes a supercritical CO2 drying method to prepare a series of perovskite-structured metal oxide catalysts (i.e., ABO3, such as LaNiO3, LaCrO3, CeGaO3, CaFeO3, etc.) with high specific surface areas, pioneering a new method for preparing catalysts for syngas conversion to para-xylene. Compared with the traditional co-precipitation method, the supercritical CO2 drying method used in this invention not only successfully prepares perovskite-structured metal oxides but also significantly increases the specific surface area of ​​these metal oxides, exposing more active sites and enhancing their catalytic activity, thereby improving CO conversion. After being combined with a molecular sieve catalyst, the intermediates (HCOO*, HCOOH*, H2CO*) generated by the perovskite-structured metal oxides are converted into hydrocarbons within the molecular sieve. The generation of hydrocarbons promotes further CO reaction intermediate formation on the perovskite-structured metal oxide catalyst, rather than conversion into CO2. Therefore, the CO2 selectivity is significantly reduced after using the composite catalyst.

[0044] In addition, compared with other methods, the CO2 supercritical drying method is universal and can be used for the drying process of any catalyst without damaging the structure of the catalyst.

[0045] Furthermore, after the molecular sieve undergoes external surface modification (e.g., ZSM-5 molecular sieve is modified with silica, and the silica completely coats the ZSM-5 molecular sieve, resulting in a ZSM-5 molecular sieve@SiO2 capsule structure), the acidity of the ZSM-5 molecular sieve's outer surface is reduced, inhibiting the alkylation and isomerization reactions of p-xylene on the molecular sieve's outer surface, and significantly improving p-xylene selectivity. Therefore, the composite catalyst of this embodiment ensures high CO conversion and low CO2 selectivity while also guaranteeing high p-xylene selectivity.

[0046] In one embodiment, A includes at least one of La, Ce, Cr, and Ca; B includes at least one of Ni, Co, Fe, Mn, Cr, and Ga, and A and B are different.

[0047] In one embodiment, the ABO3 includes at least one of LaNiO3, LaCoO3, LaFeO3, LaMnO3, LaCrO3, CeGaO3, CrFeO3, and CaFeO3.

[0048] In one embodiment, in step 1, the complexing agent is selected from at least one of citric acid and ethanolamine.

[0049] In one embodiment, in step 3, the supercritical drying temperature of CO2 is 240°C. o C-280 o C (e.g., 240) o C, 250 o C, 260 o C, 270 o C, 280 o (C, etc.), the supercritical drying time for CO2 is 1h-12h (e.g., 2h, 4h, 5h, 8h, 10h, etc.), and the supercritical drying pressure for CO2 is 7.5MPa-9.5MPa (e.g., 7.5MPa, 8MPa, 8.5MPa, 9MPa, 9.5MPa, etc.).

[0050] The firing temperature is 400. o C-600 o C, roasting time is 2h-12h.

[0051] In one embodiment, the molecular sieve includes at least one of ZSM-5 and ZSM-11.

[0052] In one implementation, combined with Figure 1 As shown, the molecular sieve with modified outer surface is prepared by the following method:

[0053] Step 41: Disperse ZSM-5 molecular sieve in a solvent (such as n-hexane) to obtain a suspension. After stirring, add tetraethyl orthosilicate dropwise and continue stirring to obtain a second mixed solution.

[0054] Step 42: Heat the second mixed solution to 70°C. o C-90 o C, to remove the solvent;

[0055] Step 43: After the solvent is removed, the product is dried and calcined to obtain ZSM-5@SiO2 capsule structure molecular sieve, which is the molecular sieve with modified outer surface.

[0056] The composite catalyst obtained by combining the perovskite-structured metal oxide catalyst with the ZSM-5@SiO2 capsule-structured molecular sieve of this invention can directly convert syngas into p-xylene with ultra-high selectivity. This is because p-xylene is mainly generated within the micropores of the ZSM-5 molecular sieve. When p-xylene diffuses to the outer surface of the ZSM-5 molecular sieve, it isomerizes into other xylenes or alkylates into C9 or higher heavy aromatics at acidic sites on the outer surface. The ZSM-5@SiO2 capsule-structured molecular sieve used in this invention can effectively shield the acidic sites on the outer surface of the ZSM-5 molecular sieve, reducing the alkylation or isomerization reactions of p-xylene, thereby significantly improving the selectivity of p-xylene.

[0057] In one embodiment, in step 41, the mass of ZSM-5 molecular sieve is 1g-10g, the mass of solvent is 7g-40g, the stirring time is 10min each time, and the mass of tetraethyl orthosilicate is 3g-15g.

[0058] In step 43, the drying temperature is 100°C. o C-120 o C, drying time is 12h-24h; calcination temperature is 550℃. o C-650 o C, roasting time is 3h-12h.

[0059] In one embodiment, in step 5, the mass ratio of the perovskite-structured metal oxide to the surface-modified molecular sieve is 5:1 to 1:5; the mixing method is physical mixing, powder mixing, or dual-bed mixing.

[0060] This invention provides a composite catalyst, which is prepared using the method described in this invention.

[0061] The composite catalyst described in this invention is composed of a perovskite-structured metal oxide with a high specific surface area and a molecular sieve with an externally modified surface. The composite catalyst exhibits excellent catalytic activity and good stability. Its preparation process is simple, efficient, practical, and highly reproducible. The composite catalyst can directly convert syngas into p-xylene.

[0062] This invention provides an application of the composite catalyst described above in the catalytic synthesis of p-xylene from syngas.

[0063] The composite catalyst used in this invention can directly convert syngas into p-xylene, and has an ultra-high p-xylene yield.

[0064] The present invention will be further described in detail below through several specific embodiments.

[0065] Example 1

[0066] Weigh out 7.39 g of Cr(NO3)3·9H2O and 7.46 g of Fe(NO3)3·9H2O (Cr / Fe molar ratio of 1 / 1) and dissolve them in 10 mL of deionized water, stirring until completely dissolved. Then add 23 g of citric acid and stir for 30 min to obtain the first mixed solution. Heat the first mixed solution to 70°C. o The gel was then formed at 240°C and maintained for 4 hours. The gel was then subjected to supercritical CO2 drying at 240°C. o C, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 400 C / min o The temperature was set at C and maintained for 4 hours. After calcination, a perovskite-structured CrFeO3 metal oxide was obtained.

[0067] 1 g of ZSM-5 molecular sieve was dispersed in 7 g of n-hexane and stirred for 10 min. Then, 3 g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was then heated to 70 °C. o C. Remove n-hexane. After all n-hexane has been removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0068] 1g of CrFeO3 and 1g of ZSM-5@SiO2 capsule-structured molecular sieve were mixed as powder and granulated to obtain a composite catalyst.

[0069] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0070] Example 2

[0071] Weigh out 8g of La(NO3)3·6H2O and 7.4g of Cr(NO3)3·9H2O (La / Cr molar ratio 1 / 1) and dissolve them in 10mL of deionized water, stirring until completely dissolved. Then add 23g of citric acid and stir for 30 minutes to obtain the first mixed solution. Heat the first mixed solution to 80°C. o The gel was then formed at 240°C and maintained for 6 hours. The gel was then subjected to supercritical CO2 drying at 240°C. o C, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 450°C / min o The temperature was set at C and maintained for 5 hours. After calcination, a perovskite-structured LaCrO3 metal oxide was obtained.

[0072] 2g of ZSM-5 molecular sieve was dispersed in 15g of n-hexane and stirred for 10 min. Then, 5g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was then heated to 70°C. o C. Remove n-hexane. After all n-hexane has been removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0073] 1g of LaCrO3 and 1g of ZSM-5@SiO2 capsule-structured molecular sieve were mixed as powder and granulated to obtain a composite catalyst.

[0074] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0075] Example 3

[0076] Weigh 8g of Ce(NO3)3·6H2O and 4.7g of Ga(NO3)3 (Ce / Ga molar ratio 1 / 1) and dissolve them in 10mL of deionized water, stirring until completely dissolved. Then add 23g of ethanolamine and stir for 30 minutes to obtain the first mixed solution. Heat the first mixed solution to 90°C. o The gel was then formed at 240°C and maintained at 12 h. The gel was then subjected to supercritical CO2 drying at 240°C. o C, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 500 C / min oThe temperature was set at C and maintained for 8 hours. After calcination, a perovskite-structured CeGaO3 metal oxide was obtained.

[0077] 5g of ZSM-5 molecular sieve was dispersed in 30g of n-hexane and stirred for 10 min. Then, 12g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was then heated to 70°C. o C. Remove n-hexane. After all n-hexane has been removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0078] 1g of CeGaO3 and 1g of ZSM-5@SiO2 capsule-structured molecular sieve were mixed as powder and granulated to obtain a composite catalyst.

[0079] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0080] Comparative Example 1

[0081] Weigh out 7.39 g of Cr(NO3)3·9H2O and 7.46 g of Fe(NO3)3·9H2O (Cr / Fe molar ratio of 1 / 1) and dissolve them in 10 mL of deionized water, stirring until completely dissolved. Then add 23 g of citric acid and stir for 30 min to obtain the first mixed solution. Heat the first mixed solution to 70°C. o The gel was then formed at 240°C and maintained for 4 hours. The gel was then subjected to supercritical CO2 drying at 240°C. o C, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 400 C / min o The temperature was set at C and maintained for 4 hours. After calcination, a perovskite-structured CrFeO3 metal oxide was obtained.

[0082] 1 g of ZSM-5 molecular sieve was dispersed in 7 g of n-hexane and stirred for 10 min. Then, 3 g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was then heated to 70 °C. o C. Remove n-hexane. After all n-hexane has been removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / mino The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0083] 1g of CrFeO3 and 1g of ZSM-5@SiO2 capsule-structured molecular sieve were mixed as particles and granulated to obtain a composite catalyst.

[0084] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0085] Comparative Example 2

[0086] Weigh out 7.39 g of Cr(NO3)3·9H2O and 7.46 g of Fe(NO3)3·9H2O (Cr / Fe molar ratio of 1 / 1) and dissolve them in 10 mL of deionized water, stirring until completely dissolved. Then add 23 g of citric acid and stir for 30 min to obtain the first mixed solution. Heat the first mixed solution to 70°C. o The gel was then formed at 240°C and maintained for 4 hours. The gel was then subjected to supercritical CO2 drying at 240°C. o C, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 400 C / min o The temperature was set at C and maintained for 4 hours. After calcination, a perovskite-structured CrFeO3 metal oxide was obtained.

[0087] 1 g of ZSM-5 molecular sieve was dispersed in 7 g of n-hexane and stirred for 10 min. Then, 3 g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was then heated to 70 °C. o C. Remove n-hexane. After all n-hexane has been removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0088] 1g of CrFeO3 and 1g of ZSM-5@SiO2 capsule molecular sieve were granulated separately, and then mixed with quartz sand in a reaction tube to obtain a composite catalyst.

[0089] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0090] Comparative Example 3

[0091] Weigh out 7.39 g of Cr(NO3)3·9H2O and 7.46 g of Fe(NO3)3·9H2O (Cr / Fe molar ratio of 1 / 1) and dissolve them in 10 mL of deionized water, stirring until completely dissolved. Then add 23 g of citric acid and stir for 30 min to obtain the first mixed solution. Heat the first mixed solution to 70°C. o The gel was then formed at 240°C and maintained for 4 hours. The gel was then subjected to supercritical CO2 drying at 240°C. o C, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 400 C / min o The temperature was set at C and maintained for 4 hours. After calcination, a perovskite-structured CrFeO3 metal oxide was obtained.

[0092] 1 g of ZSM-5 molecular sieve was dispersed in 7 g of n-hexane and stirred for 10 min. Then, 3 g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was then heated to 70 °C. o C. Remove n-hexane. After all n-hexane has been removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0093] 1g of CrFeO3 and 1g of ZSM-5@SiO2 capsule molecular sieve were granulated separately and mixed in a double-bed manner in a reaction tube to obtain a composite catalyst.

[0094] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0095] Comparative Example 4

[0096] Weigh out 7.39 g of Cr(NO3)3·9H2O and 7.46 g of Fe(NO3)3·9H2O (Cr / Fe molar ratio of 1 / 1) and dissolve them in 10 mL of deionized water, stirring until completely dissolved. Then add 23 g of citric acid and stir for 30 min to obtain the first mixed solution. Heat the first mixed solution to 70°C. o The gel was then formed at 240°C and maintained for 4 hours. The gel was then subjected to supercritical CO2 drying at 240°C. oC, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 400 C / min o The temperature was set at C and maintained for 4 hours. After calcination, a perovskite-structured CrFeO3 metal oxide was obtained.

[0097] 1 g of ZSM-5 molecular sieve was dispersed in 7 g of n-hexane and stirred for 10 min. Then, 3 g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was then heated to 70 °C. o C removes n-hexane. After all n-hexane has been removed, the sample is placed at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0098] 5g of CrFeO3 and 1g of ZSM-5@SiO2 capsule molecular sieve were granulated separately, mixed as powder and granulated in a reaction tube to obtain a composite catalyst.

[0099] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0100] Comparative Example 5

[0101] Weigh out 7.39 g of Cr(NO3)3·9H2O and 7.46 g of Fe(NO3)3·9H2O (Cr / Fe molar ratio of 1 / 1) and dissolve them in 10 mL of deionized water, stirring until completely dissolved. Then add 23 g of citric acid and stir for 30 min to obtain the first mixed solution. Heat the first mixed solution to 70°C. o The gel was then formed at 240°C and maintained for 4 hours. The gel was then subjected to supercritical CO2 drying at 240°C. o C, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 400 C / min o The temperature was set at C and maintained for 4 hours. After calcination, a perovskite-structured CrFeO3 metal oxide was obtained.

[0102] 1 g of ZSM-5 molecular sieve was dispersed in 7 g of n-hexane and stirred for 10 min. Then, 3 g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was then heated to 70 °C. oC. Remove n-hexane. After all n-hexane has been removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0103] 1g of CrFeO3 and 5g of ZSM-5@SiO2 capsule-structured molecular sieve were granulated separately, and then mixed and granulated in a reaction tube to obtain a composite catalyst.

[0104] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0105] Comparative Example 6

[0106] Weigh out 7.39 g of Cr(NO3)3·9H2O and 7.46 g of Fe(NO3)3·9H2O (Cr / Fe molar ratio of 1 / 1) and dissolve them in 10 mL of deionized water, stirring until completely dissolved. Then add 23 g of citric acid and stir for 30 min to obtain the first mixed solution. Heat the first mixed solution to 70°C. o The gel was then formed at 240°C and maintained for 4 hours. The gel was then subjected to supercritical CO2 drying at 240°C. o C, time 2 hours, pressure 8 MPa. The sample was then placed in a muffle furnace for calcination at 5... o Heating to 400 C / min o The temperature was set at C and maintained for 4 hours. After calcination, a perovskite-structured CrFeO3 metal oxide was obtained.

[0107] 1 g of ZSM-11 molecular sieve was dispersed in 7 g of n-hexane and stirred for 10 min. Then, 3 g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, a second mixed solution was obtained. The second mixed solution was heated to 70 °C. o C. After the hexane has been completely removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-11@SiO2 capsule-structured molecular sieve was obtained.

[0108] 1g of CrFeO3 and 5g of ZSM-11@SiO2 capsule molecular sieve were granulated separately, mixed as powder and granulated in a reaction tube to obtain a composite catalyst.

[0109] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0110] Comparative Example 7

[0111] Weigh out 7.39 g of Cr(NO3)3·9H2O and 7.46 g of Fe(NO3)3·9H2O (Cr / Fe molar ratio 1 / 1) and dissolve them in 10 mL of deionized water, denoted as salt solution A. Separately prepare a 1 mol / L NaOH solution, denoted as alkali solution B. Heat a beaker containing 100 mL of deionized water in a water bath until the temperature reaches 70 °C. o After step C, salt solution A and alkali solution B are added dropwise simultaneously, and stirring begins. During this process, the pH is controlled between 7 and 9 by adjusting the feed rates of salt solution A and alkali solution B, while the water bath temperature is maintained at 70°C. o C. After salt solution A has been completely added dropwise, stop adding alkali solution B and stop stirring. The precipitate will settle at 70°C. o After aging at a C water bath temperature for 2 hours, the precipitate was filtered and washed, and then placed in a 100°C water bath. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 400 C / min o C and maintained for 4 hours. After calcination, CrFeO was obtained. x (Preparation by co-precipitation) metal oxides.

[0112] 1 g of ZSM-5 molecular sieve was dispersed in 7 g of n-hexane and stirred for 10 min. Then, 3 g of tetraethyl orthosilicate was added dropwise. After stirring for 4 h, the temperature was raised to 70 °C. o C. After the hexane has been completely removed, place the sample at 100°C. o The sample was dried in a C oven for 12 hours. It was then calcined in a muffle furnace at 5°C. o Heating to 550 C / min o The temperature was maintained at C for 5 hours. After calcination, ZSM-5@SiO2 capsule-structured molecular sieve was obtained.

[0113] 1g CrFeO x (Prepared by co-precipitation method) The composite catalyst was obtained by mixing 1g of ZSM-5@SiO2 capsule-structured molecular sieve with powder and granulating.

[0114] The reduction process of the composite catalyst is an online reduction. Specifically, the temperature is increased to 400°C over 100 minutes in a 100% H₂ atmosphere. o C, and keep for 2 hours.

[0115] Figure 2 The image shows the XRD pattern of the catalyst prepared in Example 1. As shown, CrFeO3 exhibits a typical perovskite structure, indicating that perovskite-structured metal oxides can be successfully prepared using the method of this invention. Furthermore, both ZSM-5 and ZSM-5@SiO2 capsule molecular sieves exhibit typical MFI structures, indicating that the structure of the ZSM-5 molecular sieve remains unchanged after SiO2 coating.

[0116] The activity of ten composite catalysts prepared in Examples 1-3 and Comparative Examples 1-7 was tested.

[0117] The activity of the composite catalyst was tested using a fixed-bed reactor. The reaction conditions for the synthesis of p-xylene from syngas were: 1 g of composite catalyst; 5 MPa reaction pressure; and 350 °C reaction temperature. o C; reaction time: 12 h; space velocity (GHSV): 2400 mL / g cat / h; the syngas volume ratio is CO / CO2 / Ar / H2 = 33 / 5.27 / 3.02 / 58.71. The results of the composite catalyst activity test are shown in Table 1.

[0118] Table 1. Results of composite catalyst activity tests

[0119]

[0120] As can be seen from the data in Table 1, the composite catalyst prepared by the method of this invention exhibits high CO conversion, low CO2 selectivity, and high p-xylene selectivity. Furthermore, the composite catalyst prepared by supercritical CO2 drying outperforms the composite catalyst prepared by the traditional co-precipitation method (Comparative Example 7), indicating that the perovskite-structured composite catalyst with high specific surface area possesses higher catalytic activity.

[0121] In summary, this invention provides a composite catalyst for the direct conversion of syngas to p-xylene, its preparation method, and its application. This composite catalyst is composed of a perovskite-structured metal oxide with a high specific surface area and a molecular sieve with an externally modified surface. The composite catalyst exhibits excellent catalytic activity and good stability. The preparation process of this composite catalyst is simple, efficient, practical, and highly reproducible. This composite catalyst can achieve the direct and directional conversion of syngas to p-xylene.

[0122] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A method for preparing a composite catalyst for the direct conversion of syngas to p-xylene, characterized in that, The composite catalyst is mainly composed of a perovskite-structured metal oxide and a molecular sieve with an externally modified surface. The chemical formula of the perovskite-structured metal oxide is ABO3, and ABO3 includes at least one of LaNiO3, LaCoO3, LaFeO3, LaMnO3, LaCrO3, CeGaO3, CrFeO3, and CaFeO3. The molecular sieve is ZSM-5. The preparation method of the composite catalyst includes the following steps: Step 1: At room temperature, dissolve soluble salt A and soluble salt B in water, stir, add complexing agent, and continue stirring to obtain the first mixed solution; Step 2: Raise the temperature of the first mixed solution to 60℃-90℃ and maintain it for 2h-12h until gelation occurs; Step 3: After supercritical CO2 drying and calcination, the gel is obtained as a perovskite-structured metal oxide. Step 4: Provide molecular sieves with modified outer surfaces; Step 5: Mix the perovskite-structured metal oxide with the surface-modified molecular sieve to obtain the composite catalyst; The molecular sieve with modified outer surface was prepared by the following method: Step 41: Disperse ZSM-5 molecular sieve in a solvent to obtain a suspension. After stirring, add tetraethyl orthosilicate dropwise and continue stirring to obtain a second mixed solution. Step 42: Heat the second mixed solution to 70℃-90℃ to remove the solvent; Step 43: After the solvent is removed, the product is dried and calcined to obtain ZSM-5@SiO2 capsule structure molecular sieve, which is the molecular sieve with external surface modification. In step 5, the mass ratio of the perovskite-structured metal oxide to the surface-modified molecular sieve is 5:1 to 1:5; the mixing method is powder mixing or dual-bed mixing.

2. The method for preparing the composite catalyst for direct conversion of syngas to p-xylene according to claim 1, characterized in that, In step 1, the complexing agent is selected from at least one of citric acid and ethanolamine.

3. The method for preparing the composite catalyst for directly converting syngas to p-xylene according to claim 1, characterized in that, In step 3, the supercritical drying temperature of CO2 is 240℃-280℃, the supercritical drying time of CO2 is 1h-12h, and the supercritical drying pressure of CO2 is 7.5MPa-9.5MPa. The roasting temperature is 400℃-600℃, and the roasting time is 2h-12h.

4. The method for preparing the composite catalyst for direct conversion of syngas to p-xylene according to claim 1, characterized in that, In step 41, the mass of ZSM-5 molecular sieve is 1g-10g, the mass of solvent is 7g-40g, the stirring time is 10min each time, and the mass of tetraethyl orthosilicate is 3g-15g. In step 43, the drying temperature is 100℃-120℃ and the drying time is 12h-24h; the calcination temperature is 550℃-650℃ and the calcination time is 3h-12h.

5. A composite catalyst for directly converting syngas into p-xylene, characterized in that, The composite catalyst for direct conversion of syngas to p-xylene, as described in any one of claims 1-4, was prepared using this method.

6. The application of the composite catalyst according to claim 5 in the catalytic synthesis of p-xylene from syngas.

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