Bifunctional catalyst for one-step preparation of p-xylene through carbon dioxide hydrogenation, preparation method and application

Abstract

The invention discloses a bifunctional catalyst for one-step preparation of p-xylene through hydrogenation of carbon dioxide, a preparation method and application, relates to the technical field of catalysts for preparation of p-xylene through hydrogenation of carbon dioxide, and opens up a new way for generation of high-value p-xylene through selective hydrogenation of carbon dioxide. The catalyst comprises an alkali metal modified FeMn oxide and ZSM-5 capsule zeolite, and is used for converting CO2 into p-xylene through one-step hydrogenation; wherein in the alkali metal modified FeMn oxide, Mn is dispersed on an oxide phase of Fe; the ZSM-5 capsule zeolite is subjected to hydro-thermal treatment by a TPAOH solution, and the external passivated Bronsted acid site can inhibit the xylene structuring reaction; according to the bifunctional catalyst, carbon dioxide can be converted into p-xylene in one step, the conversion rate of carbon dioxide is high, the space time yield of p-xylene is high, and the service life of the catalyst is long.

Description

Technical Field

[0001] The invention relates to the technical field of catalysts for preparing p-xylene by hydrogenating carbon dioxide, and in particular to a bifunctional catalyst for preparing p-xylene by hydrogenating carbon dioxide in one step, a preparation method and application thereof. Background Art

[0002] Aromatic hydrocarbons are the most critical platform molecules in the chemical production process and play a vital role in human production and life. Benzene, toluene and xylene are important chemical raw materials or solvents and are widely used in the dye industry, pesticide production, fragrance manufacturing, paint making, spray painting, pharmaceuticals, shoemaking, furniture manufacturing and other industries. As an important monomer, it can also produce a series of important chemicals and polymers, including phenol, aniline, styrene, resins, polyesters and nylon. Purified terephthalic acid (PTA) derived from p-xylene can be polymerized to synthesize polyethylene terephthalate (PET), which is closely related to people's lives. However, most of the aromatic hydrocarbons currently used in chemical production are produced by catalytic reforming of naphtha and high-energy chemicals. In addition, the separation of benzene, toluene and xylene isomers is also an extremely energy-consuming process. Therefore, it is urgent to develop a non-petroleum direct synthesis of p-xylene to alleviate the huge pressure on resources caused by the decline in crude oil prices.

[0003] For some time, researchers have proposed several pathways to synthesize p-xylene from non-petroleum molecules, including CO / CO2 hydrogenation, methanol aromatization, benzene / toluene methylation, and alkane aromatization. Direct hydrogenation of a carbon dioxide molecule containing one carbon atom to produce p-xylene containing eight carbon atoms is a highly green carbon conversion method. However, the direct, efficient, and green synthesis of p-xylene is challenging. Coupling a methanol synthesis catalyst with a specific zeolite to construct a bifunctional catalyst allows carbon dioxide to be directly converted into aromatics through a methanol-mediated pathway. However, the methanol-mediated pathway generally has a low CO2 conversion rate (5%-30%) and a high CO selectivity (20%-80%). This is due to the limitations of thermodynamic equilibrium, which makes it difficult to meet the requirements of industrial production. It is also possible to construct a bifunctional catalyst by coupling an iron-based catalyst with a zeolite, and it is also possible to efficiently produce aromatics through an improved Fischer-Tropsch synthesis (FTS) pathway. Typically, the FTS pathway is to generate C through reverse water gas shift (RWGS) and FTS reactions on an iron-based catalyst. n H 2n The intermediates are then subjected to olefin oligomerization and aromatization reactions on the acidic zeolite to produce aromatic hydrocarbons. The key to this process is the formation of C n H 2nThe intermediates and grafting conversions are carried out, which has a great influence on the aromatic selectivity of the final product. Due to the control of the Anderson-Schulz-Flory (ASF) law, the FTS reaction usually exhibits a broader product distribution, so the selectivity of light olefins is usually low.

[0004] In order to improve the hydrogenation activity and the proportion of light olefins, a series of additives are often added to iron-based catalysts to activate CO2 and optimize olefin selectivity. In the process of producing p-xylene by hydrogenation of carbon dioxide, p-xylene is formed in the micropores of ZSM-5 zeolite, but the p-xylene in the product will be isomerized into other isomers during the diffusion process, or alkylated to heavy aromatics externally, and the product selectivity and space-time yield of p-xylene are not satisfactory. On the other hand, the intercrystalline medium diffusion in the zeolite micropores limits the medium diffusion of reactant and product molecules, resulting in inevitable carbon deposition, which in turn leads to rapid deactivation of the catalyst. Therefore, it is a major challenge to design and develop a new bifunctional catalyst with high CO2 conversion rate and p-xylene selectivity as well as excellent p-xylene space-time yield and stability. Summary of the invention

[0005] The purpose of the present invention is to provide a bifunctional catalyst, preparation method and application for one-step preparation of p-xylene by hydrogenation of carbon dioxide, so as to open up a new way for selective hydrogenation of carbon dioxide to produce high-value p-xylene.

[0006] To achieve the above-mentioned purpose, the present invention provides the following technical scheme: a bifunctional catalyst for one-step production of p-xylene by hydrogenation of carbon dioxide, comprising an alkali metal-modified FeMn oxide and a ZSM-5 capsule zeolite, for converting CO2 into p-xylene by one-step hydrogenation; wherein in the alkali metal-modified FeMn oxide, Mn is dispersed on the oxide phase of Fe; and the ZSM-5 capsule zeolite is hydrothermally treated with a TPAOH solution, and its externally passivated Brønsted acid sites can inhibit the xylene structuring reaction.

[0007] Preferably, the mass ratio of ZSM-5 capsule zeolite to alkali metal modified FeMn oxide is 1:(0.5-1.5).

[0008] Preferably, the alkali metal is one or more of lithium, sodium and potassium.

[0009] A method for preparing a bifunctional catalyst for preparing p-xylene by hydrogenating carbon dioxide in one step, comprising the following specific contents:

[0010] According to the designed Fe:Mn ratio, Fe(NO3)3·9H2O and Mn(NO3)2·6H2O were dissolved in deionized water to obtain solution A, and the alkaline solution for preparing co-precipitation was recorded as solution B. Under stirring conditions, the two aqueous solutions A and B were simultaneously added dropwise into a beaker, and the reaction temperature and pH value were controlled. The co-precipitation product was recorded as FeMn; an alkali metal salt solution was added dropwise to the obtained FeMn, and the solution was continuously ultrasonically treated to be fully mixed to obtain alkali metal-modified FeMn oxide;

[0011] Prepare reactants, i.e., dissolve tetraethoxysilane (TEOS), aluminum nitrate, tetrapropylammonium hydroxide (TPAOH) and ethanol (EtOH) in distilled water, transfer the obtained clear solution into a reactor after stirring, react to obtain ZSM-5 zeolite with a hollow hierarchical porous structure, add TPAOH aqueous solution to the zeolite for hydrothermal treatment, filter, wash and then calcine to obtain ZSM-5 capsule zeolite;

[0012] The target catalyst product is prepared by mixing alkali metal modified FeMn oxide and ZSM-5 capsule zeolite in proportion.

[0013] Preferably, among the reactants used in the preparation process of ZSM-5 capsule zeolite, tetraethoxysilane and aluminum nitrate are converted to SiO2 and Al2O3 respectively, then the molar ratio of SiO2:Al2O3:H2O:TPAOH:EtOH is =1:X:50:0.24:4, wherein X is adjusted according to the required Si:Al ratio, and Si / Al is (10-1500):1.

[0014] Preferably, in the preparation process of ZSM-5 capsule zeolite, the clarified solution obtained by stirring after the reactants are dissolved is transferred to a polytetrafluoroethylene sealed autoclave and reacted in an oven at 150-180°C for 12-48 hours; the process of adding TPAOH aqueous solution for hydrothermal treatment includes: treating ZSM-5 zeolite with TPAOH aqueous solution at 150-180°C, the concentration of TPAOH aqueous solution is 0.2M, the amount per gram of ZSM-5 zeolite is 5mL, and the treatment time is 12-48 hours; after filtering and washing, drying at 80-120°C for 5-10 hours and then calcining, and the calcination conditions are calcining at 450-600°C in static air for 2-6 hours.

[0015] Preferably, the designed Fe:Mn ratio is 10:1, 9:1 or 8:1 in molar ratio; solution B uses one or more of ammonium nitrate, ammonium carbonate, sodium hydroxide or potassium hydroxide solution with a concentration of 0.1-2M.

[0016] Preferably, during the preparation of the alkali metal-modified FeMn oxide, the reaction temperature and pH are controlled at 50-100° C. and 6.0-9.0, respectively; the alkali metal modification is carried out by an impregnation method, the alkali metal salt solution used is an alkali metal carbonate solution, and the ultrasonic treatment is continued for more than 1 hour after the addition of the alkali metal salt solution;

[0017] In the preferred embodiment, the reaction temperature and pH are 70-80°C and 7.0-8.0 respectively, the alkali metal salt solution is K2CO3 solution, and the weight of potassium loaded in the alkali metal modified FeMn oxide does not exceed 2wt%.

[0018] A bifunctional catalyst for preparing p-xylene by hydrogenation of carbon dioxide in one step is used for catalyzing the hydrogenation of carbon dioxide to prepare p-xylene. After the catalyst is pretreated with hydrogen, it contacts with a raw gas containing carbon dioxide, and completes the hydrogenation of carbon dioxide to prepare p-xylene in one step under reaction conditions. In the carbon dioxide hydrogenation reaction, the one-step conversion rate of carbon dioxide exceeds 40%, and the CO2 hydrogenation STY of p-xylene can reach 41.7 g·kg cat -1 ·h -1 .

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The bifunctional catalyst, preparation method and application of the one-step preparation of p-xylene by hydrogenation of carbon dioxide. A novel bifunctional catalyst is constructed by combining ZSM-5 capsule zeolite with alkali metal-modified FeMn oxide, which is used to hydrogenate carbon dioxide into p-xylene in one step. When the one-step conversion rate of carbon dioxide exceeds 40%, the space-time yield of p-xylene is significantly improved, which can be as high as 41.7 g·kg cat -1 ·h -1 , alkali metal-modified FeMn oxides are responsible for the conversion of CO2 via the RWGS reaction and the generation of C via the FTS reaction. n H 2n Intermediate product, C n H 2n It is fed into ZSM-5 capsule zeolite, converted into p-xylene, and the isomerization of the product is suppressed during the diffusion process. The bifunctional catalyst of the present invention opens up the practical industrial application prospect of selectively converting carbon dioxide into p-xylene.

[0021] 2. In the bifunctional catalyst, preparation method and application of one-step production of p-xylene by hydrogenation of carbon dioxide, the alkalinity of the surface of the alkali metal-modified FeMn oxide catalyst increases, which significantly enhances the CO2 adsorption capacity, thereby affecting the carbonization rate of iron and improving the catalytic performance of the intermediate low-carbon olefin content.

[0022] 3. The bifunctional catalyst, preparation method and application of the one-step preparation of p-xylene by hydrogenation of carbon dioxide, wherein the ZSM-5 capsule zeolite is obtained by treating the HZSM-5 zeolite molecular sieve, and the TPA + Internal dissolution is more likely to occur when zeolite is treated with an alkaline solution. The continuous dissolution of internal Si and Al substances to the outside of the zeolite gradually leads to the formation of a hollow hierarchical porous structure. The present invention utilizes the amorphous SiO2 dissolved from the inside of the zeolite to cover part of the acidic sites, thereby affecting the number of acidic sites on the surface of the ZSM-5 zeolite molecular sieve and improving the catalytic performance.

[0023] 4. The bifunctional catalyst, preparation method and application of carbon dioxide hydrogenation to produce p-xylene in one step, with an average STY of more than 20 g kg after continuous catalytic reaction for 50 hours cat -1 ·h -1 , which can maintain a high space-time yield for a long time. Compared with the rapid deactivation of existing zeolite catalysts, it significantly extends the catalyst life at high space-time yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The XRD comparison patterns of FeMn and Fe2O3 with different iron-manganese ratios are shown below;

[0025] Figure 2 HRTEM image of HZSM-5 zeolite obtained in Example 2;

[0026] Figure 3 HRTEM image of the ZSM-5 capsule zeolite obtained in Example 2;

[0027] Figure 4 The nitrogen adsorption and desorption curves of the HZSM-5 zeolite and ZSM-5 capsule zeolite obtained in Example 2;

[0028] Figure 5 Al2p XPS precision spectra of the surface of HZSM-5 zeolite and ZSM-5 capsule zeolite obtained in Example 2;

[0029] Figure 6 The Fourier transform infrared spectra of di-tert-butylpyridine adsorbed on the HZSM-5 zeolite and ZSM-5 capsule zeolite obtained in Example 2;

[0030] Figure 7 XRD patterns of HZSM-5 zeolite and ZSM-5 capsule zeolite. DETAILED DESCRIPTION

[0031] A bifunctional catalyst for one-step preparation of p-xylene by hydrogenation of carbon dioxide comprises an alkali metal-modified FeMn oxide and a ZSM-5 capsule zeolite, and is used for one-step hydrogenation to convert CO2 into p-xylene. For reference, the mass ratio of the ZSM-5 capsule zeolite to the alkali metal-modified FeMn oxide is preferably 1:(0.5-1.5); in the alkali metal-modified FeMn oxide, Mn is dispersed on the oxide phase of Fe; the ZSM-5 capsule zeolite is hydrothermally treated with a TPAOH solution, and its externally passivated Bronsted acid sites can inhibit the xylene structuring reaction.

[0032] The alkali metal is one or more of lithium, sodium and potassium.

[0033] The preparation method of the above catalyst includes the following specific contents:

[0034] 1) Preparation of alkali metal modified FeMn oxide:

[0035] According to the designed Fe:Mn ratio, Fe(NO3)3·9H2O and Mn(NO3)2·6H2O are dissolved in deionized water to obtain solution A. The alkaline solution used for preparing coprecipitation is recorded as solution B. Specifically, one or more of ammonium nitrate, ammonium carbonate, sodium hydroxide, potassium hydroxide and the like can be selected. The concentration can be 1M, as long as coprecipitation can be achieved. Under stirring conditions, the two aqueous solutions A and B are added dropwise to the beaker at the same time. The reaction temperature and pH value are controlled. The coprecipitation product is recorded as F. eMn, for reference, the reaction temperature and pH can be controlled at 50-100°C and 6.0-9.0, respectively, and more preferably, can be further controlled at 70-80°C and 7.0-8.0; an alkali metal salt solution is added dropwise to the prepared FeMn, and ultrasonic treatment is continued to be performed to obtain alkali metal-modified FeMn oxide by mixing. Specifically, ultrasonic treatment can be continued for more than 1 hour after the addition of the alkali metal salt solution. This step is to perform alkali metal modification by impregnation method, and the alkali metal salt solution can be an alkali metal carbonate solution;

[0036] In a preferred embodiment, the alkali metal salt solution is a K2CO3 solution. To obtain better catalytic performance, the weight of potassium loaded in the alkali metal modified FeMn oxide can be controlled below 2 wt%.

[0037] After modification with alkali metals, the surface alkalinity of the product can be increased, and the CO2 adsorption capacity can be significantly enhanced, thereby affecting the carbonization rate of iron and improving the catalytic performance of the intermediate light olefin content;

[0038] 2) Preparation of ZSM-5 capsule zeolite:

[0039] Prepare reactants, i.e., dissolve tetraethoxysilane (TEOS), aluminum nitrate, tetrapropylammonium hydroxide (TPAOH) and ethanol (EtOH) in distilled water, transfer the obtained clear solution to a reactor (polytetrafluoroethylene sealed autoclave can be used) after stirring, and react to obtain ZSM-5 zeolite with a hollow hierarchical porous structure. For reference, the reaction conditions can be controlled to react in an oven at 150-180°C for 12-48h, and add TPAOH aqueous solution to the obtained ZSM-5 zeolite for hydrothermal treatment. Optionally, during the treatment process, ZSM-5 zeolite is treated with a TPAOH aqueous solution at 150-180°C, the TPAOH aqueous solution concentration is 0.2M, the amount per gram of ZSM-5 zeolite is 5mL, and the treatment time is 12-48 hours; after filtering and washing, calcining to remove the template agent, and finally obtaining ZSM-5 capsule zeolite, optionally, after filtering and washing, drying at 80-120°C for 5-10 hours and then calcining, and the calcination conditions can be calcined at 450-600°C in static air for 2-6 hours;

[0040] For reference, tetraethoxysilane and aluminum nitrate are converted to SiO2 and Al2O3 respectively, and the molar ratio of SiO2:Al2O3:H2O:TPAOH:EtOH is preferably =1:X:50:0.24:4, wherein X is adjusted according to the required Si, Al ratio, and Si / Al may be further preferably (10-1500):1, (the Si / Al recorded in the present invention is a conventional indicator describing the chemical composition of the molecular sieve, which refers to the molar ratio of silicon element to aluminum element).

[0041] Finally, the alkali metal modified FeMn oxide and ZSM-5 capsule zeolite are mixed in proportion to obtain the target catalyst product.

[0042] In a preferred embodiment, when preparing alkali metal modified FeMn oxide, the molar ratio of Fe to Mn can be 10:1, 9:1 or 8:1, which can be adjusted according to the needs, see Figure 1 After the introduction of manganese, no peak belonging to manganese oxide was detected due to the good dispersion of manganese in the Fe oxide phase.

[0043] The catalyst can be used to catalyze the hydrogenation of carbon dioxide to prepare p-xylene. Specifically, after the catalyst is pretreated with hydrogen, it is contacted with a raw gas containing carbon dioxide. Under the reaction conditions, the carbon dioxide hydrogenation can be completed in one step to prepare p-xylene. The principle is that the intermediate olefin produced by the catalysis of the alkali metal modified FeMn oxide is aromatized by the ZSM-5 capsule zeolite. In addition, the catalyst has a one-time conversion rate of carbon dioxide exceeding 40% in the carbon dioxide hydrogenation reaction, and the CO2 hydrogenation STY of p-xylene can reach 41.7 g kg cat -1 ·h -1 .

[0044] Embodiment 1:

[0045] This embodiment is the preparation of alkali metal modified FeMn oxide, and the specific preparation method is as follows:

[0046] Alkali metal modified FeMn oxide (Fe / Mn molar ratio of 9:1) was prepared by coprecipitation. Specifically, a certain amount of Fe(NO3)3·9H2O and Mn(NO3)2·6H2O were dissolved in deionized water to obtain solution A, and solution B was 1M ammonium nitrate solution. Under stirring, the two aqueous solutions A and B were added dropwise to a beaker at the same time, and the temperature and pH value were controlled at 60-70°C and about 6.0-7.0, respectively. The obtained catalyst was recorded as FeMn.

[0047] FeMn was ultrasonically treated by dropping K2CO3 solution, fully mixed and ultrasonicated for more than 1 hour, and the concentration and amount of K2CO3 solution were controlled to obtain an alkali metal-modified FeMn oxide with a potassium load of 1wt%, recorded as 1wt% K-FeMn.

[0048] Embodiment 2:

[0049] This embodiment is the preparation of ZSM-5 capsule zeolite, and the specific preparation method is as follows:

[0050] HZSM-5 zeolite with Si / Al ratio of 100 was prepared by hydrothermal method. The preparation method is as follows: tetraethoxysilane (TEOS), aluminum nitrate, 25% tetrapropylammonium hydroxide (TPAOH) and ethanol (EtOH) were dissolved in distilled water, wherein TEOS and aluminum nitrate were converted into SiO2 and Al2O3, and the molar ratio of SiO2:Al2O3:H2O:TPAOH:EtOH was 1:0.005:50:0.24:4. After magnetic stirring, the obtained clear solution was transferred to a sealed autoclave and placed in an oven at 180°C for 24 hours. After filtering and washing, it was dried at 100°C for 5 hours, and the obtained intermediate zeolite was recorded as HZSM-5 (such as Figures 2 to 7 (denoted as HZSM-5 in Chinese).

[0051] Next, 5 g of HZSM-5 zeolite was placed in 25 mL of 0.2 M TPAOH aqueous solution in an oven at 180° C. for hydrothermal treatment for 48 hours. Finally, the ZSM-5 capsule zeolite (eg, Figures 3 to 7 (referred to as Hollow ZSM-5 in Chinese).

[0052] As an important component of the catalyst, the structure of zeolite directly affects the distribution of the final products of carbon dioxide hydrogenation. Figure 2 and 3As shown in the figure, HZSM-5 zeolite is an oval particle of about 3 μm. The ZSM-5 capsule zeolite treated with TPAOH is a rounded rectangle with a small amount of amorphous SiO2 on the surface and a cavity formed inside the zeolite. This indicates that the TPAOH treatment greatly shortens the pore length and forms a large hollow structure inside the HZSM-5. Figure 4 The N2 adsorption-desorption isotherms of HZSM-5 and ZSM-5 capsule zeolites show that there is an H2 hysteresis loop in the desorption branch of the zeolite, which mutates at around P / P0=0.45. + When zeolite is treated with alkaline solution, internal dissolution is more likely to occur. The continuous dissolution of internal Si and Al substances to the outside of the zeolite gradually leads to the formation of a hollow structure. The Al2p XPS spectra of HZSM-5 and ZSM-5 capsule zeolites are shown in Figure 2. Figure 5 As shown in the figure, the signal at 75 eV indicates the presence of aluminum on the surface of the zeolite. It is worth mentioning that the aluminum signal on the surface of the ZSM-5 capsule zeolite is very low, which indicates that the aluminum content on the surface of the ZSM-5 capsule zeolite has decreased. The adsorbed di-tert-butylpyridine (DTBPy) was analyzed by Fourier transform infrared spectroscopy ( Figure 6 ), further clarifying the number of external Brønsted acid sites. Larger probe molecules cannot enter the pores of ZSM-5 capsule zeolite. The low signal intensity of ZSM-5 capsule zeolite indicates that TPAOH treatment inhibits the external Brønsted acid sites. Weakening the external acid sites is beneficial to improving the selectivity of para-xylene. In addition, Figure 7 The XRD results showed that the treated ZSM-5 capsule zeolite catalyst was still stable.

[0053] Embodiment 3:

[0054] 1wt% K-FeMn of Example 1 was mixed with HZSM-5 and Hollow ZSM-5 of Example 2 in a mass ratio of 1:1 to prepare two catalysts. The CO2 hydrogenation performance was tested respectively. Before the reaction, each catalyst was reduced and pretreated at 300-400°C and hydrogen for 6-8 hours. After cooling to room temperature, the raw gas (H2 / CO2=3) was introduced into the reactor. The typical reaction pressure and temperature were 3.0-4.0MPa and 300-400°C, respectively. 0.3g catalyst, GHSV=8000 (mLg·kg cat -1 ·h -1 ), the catalytic performance results of the two catalysts for converting carbon dioxide are shown in Tables 1 and 2 below:

[0055] Table 1. Distribution of hydrocarbon products from CO2 hydrogenation of catalysts

[0056]

[0057] Table 2. Distribution of aromatic products from CO2 hydrogenation of catalysts

[0058]

[0059] B, T, OX, MX, PX and A(C 9+ ) represents the corresponding selectivity among all products, where B = benzene, T = toluene, E = ethylbenzene, OX = o-xylene, MX = m-xylene, PX = p-xylene. PX / X is the molar ratio of PX to all xylenes.

[0060] As shown in Tables 1 and 2, although 1wt% K-FeMn / HZSM-5 exhibits good aromatics selectivity, C 9+ The proportion of aromatic products is the highest, and m-xylene is dominant among xylenes. The STY of p-xylene is 17.8 g·kg over 1 wt% K-FeMn / HZSM-5 catalyst. cat -1 ·h -1 In contrast, when 1wt% K-FeMn / ZSM-5 capsule zeolite was reacted, the p-xylene content in the product increased significantly, and the STY of p-xylene could reach 41.7g·kg cat -1 ·h -1 Therefore, the use of ZSM-5 capsule zeolite treated with TPAOH for an appropriate time in the composite catalyst is crucial for the synthesis of p-xylene in the process.

[0061] In addition, this embodiment also carried out an experiment on the life of 1wt% K-FeMn / ZSM-5 capsule zeolite catalyst. The experimental results showed that the average STY of the catalyst exceeded 20g·kg after continuous catalytic reaction for 50 hours. cat -1 ·h -1 .

[0062] Embodiment 4:

[0063] This example studies the effect of Si / Al on the target catalyst product during the preparation of ZSM-5 capsule zeolite. Based on Example 2, the amount of aluminum nitrate was adjusted to prepare different ZSM-5 capsule zeolites, which were mixed with 1wt% K-FeMn prepared in Example 1 in a ratio of 1:1 to obtain catalysts with different Si / Al. The test results are shown in Tables 3 and 4. For convenience, FeMn is used to represent 1wt% K-FeMn, and ZSM-5 capsule is used to represent ZSM-5 capsule zeolite.

[0064] Table 3. Distribution of hydrocarbon products from CO2 hydrogenation of catalysts

[0065]

[0066] Table 4. Distribution of aromatic products from CO2 hydrogenation of catalysts

[0067]

[0068] B, T, OX, MX, PX and A(C 9+ ) represents the corresponding selectivity among all products, where B = benzene, T = toluene, E = ethylbenzene, OX = o-xylene, MX = m-xylene, PX = p-xylene. PX / X is the molar ratio of PX to all xylenes.

[0069] It can be seen from the table that the FeMn / ZSM-5 capsule (Si / Al=25) catalyst has the highest aromatics selectivity, but the product is mainly C 9+ Aromatics are the main components, with the highest content of xylene among the xylenes, and the proportion of p-xylene is only 2.9%. After increasing Si / Al, the CO2 conversion rate and CO selectivity do not change much, but the selectivity of light alkanes drops sharply, while the selectivity of CH4 increases, the selectivity of light olefins increases, and the selectivity of aromatics decreases directly. In fact, the HZSM-5 zeolite with high Si / Al does not have enough acid sites for aromatization of olefins, so the corresponding catalyst shows lower selectivity for aromatics and p-xylene. More obviously, the trends of light olefins and aromatics selectivity are opposite, which indicates that aromatics are generated from light olefins. On the other hand, in terms of detailed aromatic distribution, when Si / Al increases from 25 to 1500, the p-xylene selectivity first increases and then decreases. Similarly, the pX / X ratio (the molar ratio of p-xylene to all xylenes) first increases and then decreases, and C 9+ The selectivity of aromatics also decreases with the increase of Si / Al; as mentioned above, light olefins can start arylation reactions at the acid sites of zeolites, and the acidity of zeolites has a great influence on the catalytic performance. When a hollow HZSM-5 zeolite with a Si / Al ratio of 100 is used, the aromatization selectivity reaches 39.7%, the para-xylene selectivity reaches 8.6%, and the pX / X ratio reaches 62.7%, at which the para-xylene space-time yield is the highest.

[0070] The above are only preferred embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope defined by the claims.

[0071] The matters not described in detail in the present invention are all known technologies to those skilled in the art.

Claims

1. A bifunctional catalyst for one-step preparation of p-xylene by hydrogenation of carbon dioxide, characterized in that: It includes alkali metal-modified FeMn oxide and ZSM-5 capsule zeolite, which are used for one-step hydrogenation to convert CO2 into paraxylene; in the alkali metal-modified FeMn oxide, Mn is dispersed on the oxide phase of Fe; the ZSM-5 capsule zeolite is hydrothermally treated with TPAOH solution, and its externally passivated Bronsted acid sites can inhibit the xylene structuring reaction.

2. The bifunctional catalyst for preparing p-xylene by hydrogenation of carbon dioxide in one step according to claim 1, characterized in that: The mass ratio of the ZSM-5 capsule zeolite to the alkali metal modified FeMn oxide is 1:(0.5-1.5).

3. The bifunctional catalyst for preparing p-xylene by hydrogenation of carbon dioxide in one step according to claim 1, characterized in that: The alkali metal is one or more of lithium, sodium and potassium.

4. A method for preparing a bifunctional catalyst for preparing p-xylene by hydrogenating carbon dioxide in one step as claimed in any one of claims 1 to 3, characterized in that: The specific contents include: According to the designed Fe:Mn ratio, Fe(NO3)3·9H2O and Mn(NO3)2·6H2O were dissolved in deionized water to obtain solution A, and the alkaline solution for preparing co-precipitation was recorded as solution B. Under stirring conditions, the two aqueous solutions A and B were simultaneously added dropwise into a beaker, and the reaction temperature and pH value were controlled. The co-precipitation product was recorded as FeMn; an alkali metal salt solution was added dropwise to the obtained FeMn, and the solution was continuously ultrasonically treated to be fully mixed to obtain alkali metal-modified FeMn oxide; Prepare reactants, i.e., dissolve tetraethoxysilane (TEOS), aluminum nitrate, tetrapropylammonium hydroxide (TPAOH) and ethanol (EtOH) in distilled water, transfer the obtained clear solution into a reactor after stirring, react to obtain ZSM-5 zeolite with a hollow hierarchical porous structure, add TPAOH aqueous solution to the zeolite for hydrothermal treatment, filter, wash and then calcine to obtain ZSM-5 capsule zeolite; The target catalyst product is prepared by mixing alkali metal modified FeMn oxide and ZSM-5 capsule zeolite in proportion.

5. The preparation method according to claim 4, characterized in that: Among the reactants used in the preparation process of the ZSM-5 capsule zeolite, tetraethoxysilane and aluminum nitrate are converted into SiO2 and Al2O3 respectively, and the molar ratio of SiO2:Al2O3:H2O:TPAOH:EtOH is =1:X:50:0.24:4, wherein X is adjusted according to the required Si:Al ratio, and Si / Al is (10-1500):

1.

6. The preparation method according to claim 4, characterized in that: In the preparation process of the ZSM-5 capsule zeolite, the clarified solution obtained by stirring after the reactants are dissolved is transferred to a polytetrafluoroethylene sealed autoclave and reacted in an oven at 150-180° C. for 12-48 hours; the process of adding a TPAOH aqueous solution for hydrothermal treatment includes: treating the ZSM-5 zeolite with a TPAOH aqueous solution at 150-180° C., the concentration of the TPAOH aqueous solution is 0.2M, the amount per gram of ZSM-5 zeolite is 5 mL, and the treatment time is 12-48 hours; after filtering and washing, drying at 80-120° C. for 5-10 hours and then calcining, and the calcination conditions are calcining at 450-600° C. in static air for 2-6 hours.

7. The preparation method according to claim 4, characterized in that: The designed Fe:Mn ratio is a molar ratio of Fe:Mn of 10:1, 9:1 or 8:1; solution B uses one or more of ammonium nitrate, ammonium carbonate, sodium hydroxide or potassium hydroxide solution, with a concentration of 1M.

8. The preparation method according to claim 4, characterized in that: During the preparation of the alkali metal modified FeMn oxide, the reaction temperature and pH are controlled at 50-100° C. and 6.0-9.0 respectively; the alkali metal modification is carried out by an impregnation method, the alkali metal salt solution used is an alkali metal carbonate solution, and ultrasonic treatment is continued for more than 1 hour after the alkali metal salt solution is added.

9. The preparation method according to claim 8, characterized in that: The reaction temperature and pH are 70-80° C. and 7.0-8.0 respectively, the alkali metal salt solution is K2CO3 solution, and the weight of potassium loaded in the alkali metal modified FeMn oxide does not exceed 2wt%.

10. A bifunctional catalyst for preparing p-xylene by hydrogenating carbon dioxide in one step as claimed in any one of claims 1 to 3, or a target catalyst product obtained by the preparation method according to any one of claims 4 to 9, used for catalytic hydrogenation of carbon dioxide to prepare p-xylene, characterized in that: After the catalyst is pretreated with hydrogen, it contacts with the raw gas containing carbon dioxide, and completes the carbon dioxide hydrogenation in the first step under the reaction conditions to produce p-xylene. In the carbon dioxide hydrogenation reaction, the carbon dioxide conversion rate exceeds 40%, and the CO2 hydrogenation STY of p-xylene can reach 41.7 g·kg cat -1 ·h -1 .

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