Method for preparing ethylene and propylene by adopting catalyst for selectively passivating acid site

Abstract

The invention relates to the technical field of low-carbon olefin preparation, and discloses a method for preparing ethylene and propylene by adopting a catalyst for selectively passivating an acid site, which comprises the following steps: S1, synthesizing by adopting a hydrothermal method to obtain molecular sieve raw powder; the molecular sieve raw powder is added into a metal salt aqueous solution, ion exchange is carried out, drying and aerobic roasting are carried out, the metal salt modified molecular sieve is obtained, the molecular sieve is SAPO-17, SAPO-34 or SAPO-35, and the metal salt is sodium salt and / or potassium salt; s2, mechanically mixing the metal salt modified molecular sieve with a metal oxide to obtain a catalyst; and S3, contacting the mixed gas of hydrogen and carbon dioxide with a catalyst, and reacting to prepare ethylene and propylene. By adopting the strategy of selectively shielding the acid site on the outer surface of the molecular sieve by using the metal salt and compounding and coupling by using the metal oxide, the conversion rate of carbon dioxide and the selectivity of ethylene and propylene in the product are greatly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of low-carbon olefin preparation, and in particular to a method for preparing ethylene propylene by using a catalyst with selectively passivated acid sites. BACKGROUND

[0002] Currently, the technical route for preparing low-carbon olefins from CO2 hydrogenation mainly includes two categories: one is the methanol intermediate route, that is, CO2 is first hydrogenated to synthesize methanol, and then the methanol is converted into low-carbon olefins through a methanol-to-olefin process; the other is the improved Fischer-Tropsch synthesis route, that is, CO2 is converted into carbon monoxide (CO) through a reverse water gas shift reaction (RWGS), and then the carbon monoxide is subjected to Fischer-Tropsch synthesis (FTS) to realize C-C chain growth to generate olefins. Among them, the bifunctional catalyst is the core of the methanol intermediate route, and its performance directly determines the reaction efficiency and product selectivity. In current research, molecular sieves are widely used to construct a bifunctional catalytic system due to their adjustable pore structure, shape-selective selectivity and surface acidity, forming a synergistic catalytic mode of “metal active center-molecular sieve”.

[0003] SAPO series molecular sieves show potential in inhibiting methane by-product and improving the selectivity of low-carbon olefins due to their weak surface acidity and various topological structures. The team of Professor Sun Yuhang and Professor Gao Peng of the Shanghai Institute of Higher Research of the Chinese Academy of Sciences designed an In2O3 / SAPO-34 bifunctional catalyst, which can realize the high-selectivity synthesis of low-carbon olefins (CO2 conversion rate is 34.1%, and the olefin selectivity in total hydrocarbons is 76.9%) (Direct conversion of CO2 into liquid fuels with high selectivity over a bifunctional catalyst, Nature Chemistry, 2017, 9, 1019). However, the selectivity of ethylene and propylene in the olefin product is usually 55-65%. Compared with butene, ethylene and propylene are important basic raw materials in the petrochemical industry, and have higher economic value and practical application significance. Therefore, improving the selectivity of ethylene and propylene in low-carbon olefins has more application value and practical significance. SUMMARY

[0004] In order to solve the above technical problems, the present application provides a method for preparing ethylene propylene by using a catalyst with selectively passivated acid sites. By adopting the strategy of selectively shielding the acid sites on the outer surface of the molecular sieve with metal salts, the secondary conversion of ethylene and propylene into low-carbon alkanes and C 4+ hydrocarbons can be effectively inhibited, thereby greatly improving the selectivity of ethylene and propylene in the product; at the same time, the molecular sieve with selectively passivated acid sites is compounded and coupled with metal oxides, which is more conducive to the catalytic reaction, promotes the activation and conversion of carbon dioxide, and further improves the conversion rate.

[0005] The object of the present application is achieved by the following technical solutions. The present application provides a method for preparing ethylene propylene by using a catalyst for selectively passivating acidic sites, comprising the following steps: S1, using a hydrothermal method to synthesize a molecular sieve raw powder; adding the molecular sieve raw powder into an aqueous solution of a metal salt, performing ion exchange, and then drying and oxygen roasting to obtain a metal salt modified molecular sieve with a content of medium-strong acid sites of 0.05-0.3 mol / kg, wherein the molecular sieve is SAPO-17, SAPO-34 or SAPO-35, and the metal salt is a sodium salt and / or a potassium salt; S2, mechanically mixing the metal salt modified molecular sieve with a metal oxide, wherein the metal oxide is at least one of InZr x O (1+1.5x) , GaZr x O (1+1.5x) , ZnCr x O (1+1.5x) , ZnAl x O (1+1.5x) , ZnGa x O (1+1.5x) and ZnZr x O (1+1.5x) , wherein the value of x is 0.1-10, to obtain a catalyst; S3, contacting a mixed gas of hydrogen and carbon dioxide with the catalyst to perform a reaction to prepare ethylene and propylene.

[0006] In the carbon dioxide hydrogenation reaction, while the reaction products diffuse out of the pores of the molecular sieve of the bifunctional catalyst, ethylene and propylene are prone to secondary conversion at the acid sites on the outer surface to obtain butene and C 4+ hydrocarbons, thereby resulting in a low ethylene propylene selectivity. Therefore, the present application selectively passivates the outer surface of the molecular sieve by using a metal salt solution. Since the template agent added during the synthesis of the molecular sieve has not been removed by roasting, the template agent still exists in the pores of the molecular sieve raw powder, and thus the metal salt can selectively passivate the outer surface of the molecular sieve. Then, the molecular sieve is roasted to remove the template agent, and sufficient acid sites still exist in the pores of the molecular sieve. By controlling the content of the acid sites in the passivated molecular sieve and the type of the metal salt, a higher CO2 conversion rate and ethylene propylene selectivity can be achieved.

[0007] Therefore, the application can realize selective passivation of the outer surface of the molecular sieve, on the one hand, the acid sites inside the molecular sieve channel are reserved, the catalyst still has high catalytic activity and CO2 conversion rate, on the other hand, the content of the acid sites on the outer surface of the molecular sieve is reduced, the secondary conversion of ethylene propylene on the outer surface is reduced in a targeted manner, and the ethylene propylene selectivity is improved. At the same time, the metal oxide is coupled with the molecular sieve after selective passivation of the acid sites, the metal oxide has a high specific surface area and more active sites, which is more conducive to the catalytic reaction, promotes the activation and conversion of carbon dioxide and thus improves the conversion rate.

[0008] Preferably, in S1, the molar ratio of SiO2, Al2O3 and P2O5 in the raw material of the hydrothermal synthesis method is 0.1:1.0:1.0-1.2.

[0009] Preferably, in S1, the hydrothermal synthesis method specifically comprises: mixing a silicon source, an aluminum source, a phosphorus source and water, adding a template agent and aging, then performing temperature rising crystallization, and drying to obtain a molecular sieve raw powder.

[0010] Preferably, the template agent comprises cyclohexylamine, cyclohexyl imine or tetraethylammonium hydroxide; the aging is performed at room temperature for 1-3 hours; and the temperature rising crystallization is performed by rising the temperature to 150-200 DEG C for 60-80 hours.

[0011] Preferably, in S1, the sodium salt is sodium nitrate or sodium chloride; and the potassium salt is potassium nitrate or potassium chloride.

[0012] Preferably, in S1, the concentration of the metal salt aqueous solution is 0.05-3 mol / L, more preferably 0.5-3 mol / L; and the ion exchange is performed by rising the temperature to 60-90 DEG C for 5-15 hours.

[0013] Preferably, in S1, the aerobic calcination is performed by rising the temperature to 500-700 DEG C for 5-7 hours.

[0014] Preferably, in S1, the content of the medium-strong acid sites in the metal salt modified molecular sieve is 0.1-0.3 mol / kg, more preferably 0.2-0.3 mol / kg.

[0015] The acid strength is defined by NH3-TPD peaks, including weak acid, medium-strong acid and strong acid.

[0016] The NH3-TPD is according to the desorption peak position of NH3, the position of the desorption peak refers to that under the standard test conditions, under the test conditions of the ratio (w / f) of the sample mass w to the carrier gas flow rate f = 100 g•h / L, a TCD records the thermal conductivity signal of desorbed NH3, a desorption curve is drawn, and the inorganic solid is divided into three acid strengths according to the peak position vertex of the curve. Weak acid refers to an acid site with a NH3 desorption temperature less than 245 DEG C; medium-strong acid refers to an acid site with a NH3 desorption temperature of 245-500 DEG C; and strong acid refers to an acid site with a NH3 desorption temperature greater than 500 DEG C.

[0017] Preferably, in S2, the specific surface area of the metal oxide is 5-150 m 2 / g.

[0018] Preferably, in S2, the mass ratio of the metal salt modified molecular sieve to the metal oxide is 0.1-20:1, more preferably 0.3-5:1.

[0019] Preferably, in S3, the reaction conditions are as follows: the temperature is 300-500 DEG C, more preferably 300-450 DEG C; the pressure of the mixed gas is 0.5-10 MPa, more preferably 1-8 MPa; the space velocity of the mixed gas is 300-10000 mL•g -1 •h -1 , more preferably 500-9000 mL•g -1 •h -1 , further preferably 500-6000 mL•g -1 •h -1 .

[0020] Preferably, in S3, the molar ratio of H2 / CO2 in the mixed gas is 0.2-4.5, more preferably 0.3-3.5.

[0021] The present application directly converts carbon dioxide and hydrogen into ethylene and propylene in one step, and the selectivity of low-carbon olefins can reach 82-90%, the selectivity of ethylene and propylene can reach 78-86%, the ratio of ethylene and propylene is between 0.8 and 1.7, and the selectivity of byproduct methane is low (<10%), which has good application prospect.

[0022] Preferably, the process for directly converting carbon dioxide and hydrogen into ethylene and propylene is carried out under the preferred reaction conditions, when the reaction temperature is 300-450 DEG C, the space velocity is 1000-6000 mL•g -1 •h -1 , and the molar ratio of H2 / CO2 is 0.3-3.5, the selectivity of ethylene and propylene is 82-86%, and the selectivity of byproduct methane is <8%.

[0023] Compared with the prior art, the present invention has the following beneficial effects: (1) By adopting a strategy of selectively shielding the acidic sites on the outer surface of molecular sieves with metal salts, the secondary conversion of ethylene and propylene into low-carbon alkanes and C can be effectively suppressed. 4+ Hydrocarbons, thereby greatly improving the selectivity of ethylene and propylene in the product; (2) By combining and coupling metal oxides with molecular sieves that have been selectively passivated by acidic sites, metal oxides have a higher specific surface area and more active sites, which is more conducive to the catalytic reaction, promotes the activation and conversion of carbon dioxide, and thus improves the conversion rate. (3) The catalyst preparation process is simple and the conditions are mild, and the selectivity of ethylene and propylene can reach 78-86%, which greatly improves the technical economy; and the ratio of ethylene to propylene can be changed between 0.8 and 1.5 by adjusting the reaction conditions, which is beneficial to cope with changes in product price. Attached Figure Description

[0024] Figure 1 The NH3-TPD diagrams are for the molecular sieves prepared in Example 2 and Comparative Example 2.

[0025] Figure 2 The images show the pyridine infrared spectra of the molecular sieves prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0026] The technical solution of the present invention will be illustrated below with specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0027] I. Catalyst Component I (Metal Oxide) (1) The metal oxides CuAl2O4, ZnO, and FeO were all commercially available. The 10% FeO+InZr2O4 catalyst was obtained by mixing FeO with InZr2O4, with FeO accounting for 10% of the total mass of FeO+InZr2O4.

[0028] (2) The preparation method of metal oxide includes the following steps: two of zinc nitrate, chromium nitrate, aluminum nitrate, zirconium nitrate, indium nitrate and gallium nitrate are used as precursors. The precursors are mixed with ammonium carbonate in water at room temperature, wherein ammonium carbonate is used as a precipitant and the molar ratio of ammonium ions to metal ions is 1:1 (or, ammonium carbonate can be in excess). The resulting mixture is aged, and after aging, it is taken out and washed, filtered and dried in sequence. The resulting solid is calcined in air atmosphere to obtain metal oxide.

[0029] In this invention, the metal oxide was prepared using the precipitation method described above. The specific parameters used in the preparation process are shown in Table 1, and all other conditions were the same. The metal oxide was InZr.x O (1+1.5x) ,GaZr x O (1+1.5x) ZnCr x O (1+1.5x) ZnAl x O (1+1.5x) ZnGa x O (1+1.5x) and ZnZr x O (1+1.5x) One or more of the following, where x ranges from 0.1 to 10. The specific surface area of ​​metal oxides is 5-150 m². 2 / g.

[0030] Table 1 Preparation parameters of metal oxides

[0031] II. Catalyst Component II (Molecular Sieve) SAPO-17, SAPO-34, and SAPO-35 molecular sieves are all 8-ring microporous molecular sieves with pore sizes of 3.6 × 5.1 nm, 3.6 × 4.8 nm, and 3.8 × 3.8 nm, respectively. The SAPO-17, SAPO-34, and SAPO-35 molecular sieves in component II of this invention can be commercially available products or self-prepared molecular sieves; hydrothermal synthesis is used as an example here.

[0032] The moderately strong acids described in this invention can be tested using methods such as solid-state NMR spectroscopy (H-N), NH3-TPD, infrared spectroscopy, and chemical titration. However, the methods for testing acidity are not limited to those described above.

[0033] (1) SAPO-17 molecular sieve According to the molar ratio of SiO2:Al2O3:P2O5:CHA:H2O = 0.1:1.0:1.0:1.38:80, silica sol (mass concentration 30%), pseudoboehmite, phosphoric acid, and deionized water were weighed and mixed at room temperature. Cyclohexylamine (CHA) was then added dropwise as a template agent. After stirring and aging at 30ºC for 2 h, the mixture was transferred to a stainless steel hydrothermal reactor and crystallized at 2ºC / min to 200ºC for 96 h. After crystallization, the mixture was rapidly cooled to room temperature in a water bath and repeatedly centrifuged and washed until the pH of the supernatant was 7 at the end of the washing process. The precipitate was dried at 90ºC for 12 h to obtain molecular sieve powder. A portion of the molecular sieve powder was calcined in air at 600ºC for 6 h to obtain SAPO-17 molecular sieve.

[0034] (2) SAPO-34 molecular sieve According to the molar ratio of SiO2:Al2O3:P2O5:TEAOH:H2O=0.1:1.0:1.2:2.0:40, silica sol (mass concentration of 30%), pseudoboehmite, phosphoric acid, and deionized water were weighed and mixed at room temperature. Then, tetraethylammonium hydroxide (TEAOH) was added dropwise as a template agent. After stirring and aging at 30ºC for 2 h, the mixture was transferred to a stainless steel hydrothermal reactor and crystallized at 180ºC for 50 h at a heating rate of 2ºC / min. After crystallization, the mixture was rapidly cooled to room temperature in a water bath and repeatedly centrifuged and washed until the pH of the supernatant was 7 at the end of the washing process. The precipitate was dried at 90ºC for 12 h to obtain molecular sieve powder. A portion of the molecular sieve powder was calcined in air at 600ºC for 6 h to obtain SAPO-34 molecular sieve.

[0035] (3) SAPO-35 molecular sieve According to the molar ratio of SiO2:Al2O3:P2O5:HMI:H2O=0.1:1.0:1.0:1.2:50, silica sol (mass concentration of 30%), aluminum isopropoxide, phosphoric acid, and deionized water were weighed and mixed at room temperature. Cycloheximine (HMI), a template agent, was added dropwise. After aging at 30ºC for 2 h with stirring, the mixture was transferred to a stainless steel hydrothermal reactor and crystallized at 2ºC / min to 200ºC for 48 h. After crystallization, the mixture was rapidly cooled to room temperature in a water bath and repeatedly centrifuged and washed until the pH of the supernatant was 7 at the end of the washing process. The precipitate was dried at 90ºC for 12 h to obtain molecular sieve powder. A portion of the molecular sieve powder was calcined in air at 600ºC for 6 h to obtain SAPO-35 molecular sieve.

[0036] (4) Metal salt modified molecular sieves Taking sodium salt as an example, sodium nitrate or sodium chloride is dissolved in deionized water to prepare a sodium salt aqueous solution of a certain concentration. The above-mentioned molecular sieve raw powder (SAPO-17, SAPO-34, or SAPO-35) is dried in a vacuum oven at 120ºC for 12 hours or more. Then, 10 g of molecular sieve raw powder is added to the prepared sodium salt aqueous solution, and ion exchange is carried out at a certain temperature. Afterwards, the solid product is centrifuged and washed three times with deionized water. Finally, the obtained solid is dried at 110ºC overnight and calcined in air at 600ºC for 6 hours to obtain sodium salt modified molecular sieve.

[0037] Other metal salt modified molecular sieves were prepared using the same preparation method as described above, and the specific preparation parameters are shown in Table 2.

[0038] Table 2. Preparation and performance parameters of metal salt modified molecular sieves

[0039] III. Catalyst The desired proportions of metal oxides and molecular sieves are added to a container and mechanically mixed to obtain a catalyst. Separation, crushing, and mixing are achieved by utilizing one or more of the forces such as extrusion, impact, shearing, and friction generated by the high-speed movement of the material and / or container. The conversion of mechanical energy, thermal energy, and chemical energy is achieved by adjusting the temperature and carrier gas atmosphere, further regulating the interactions between different components.

[0040] In a specific embodiment of the present invention, during the mechanical mixing process, the mixing temperature can be set to 20-100°C, and the mixing can be carried out in an atmosphere or directly in air. The atmosphere is selected from any of the following gases: a) nitrogen and / or an inert gas; b) a mixture of hydrogen and nitrogen and / or an inert gas, wherein the volume of hydrogen in the mixture is 5-50%; c) a mixture of CO and nitrogen and / or an inert gas, wherein the volume of CO in the mixture is 5-20%; d) a mixture of O2 and nitrogen and / or an inert gas, wherein the volume of O2 in the mixture is 5-20%. The inert gas is one or more of helium, argon, and neon.

[0041] In a specific embodiment of the present invention, mechanical mixing may be performed using one or more of the following methods: mechanical stirring, ball milling, shaking table mixing, and mechanical grinding, as detailed below: a) Mechanical stirring: In a stirring tank, metal oxides and molecular sieves are mixed using a stirring rod. By controlling the stirring time (5-120 min) and speed (30-300 rpm), the degree of mixing and relative distance between the metal oxides and molecular sieves can be adjusted. b) Ball milling: The abrasive and metal oxides and molecular sieves are rolled at high speed in the grinding tank, which generates strong impact and crushing on the catalyst, thereby dispersing and mixing the metal oxides and molecular sieves; the particle size of the catalyst can be adjusted by controlling the mass ratio of abrasive (material can be stainless steel, agate, quartz, size range: 5-15 mm) to catalyst to 20-100:1. c) Shaking: The metal oxide and molecular sieve are premixed and loaded into a container; the metal oxide and molecular sieve are mixed by controlling the reciprocating or circular oscillation of the shaking table; uniform mixing and adjustment of their relative distance are achieved by adjusting the oscillation speed (range: 1-70 rpm) and time (range: 5-120 min); d) Mechanical grinding: The metal oxide and molecular sieve are premixed and loaded into a container; under a certain pressure (range: 5-20 kg), the mixed catalyst is subjected to relative motion through a grinding tool (speed range: 30-300 rpm) to adjust the particle size and relative distance of the catalyst and achieve uniform mixing.

[0042] 4. Catalytic reaction The catalytic reaction is carried out in a fixed-bed or moving-bed reactor equipped with a gas mass flow meter and an online product analysis chromatograph (the reactor exhaust gas is directly connected to the chromatograph's quantitative valve for periodic real-time sampling and analysis). The air in the reactor is replaced with an inert gas (one or more of helium, argon, and neon). A mixture of carbon dioxide and hydrogen is used as the feedstock, contacting the catalyst for the catalytic reaction. The molar ratio of carbon dioxide to hydrogen is 1:0.2–4.5, the reaction temperature is 300–500 °C, the pressure of the mixed gas is 0.5–10 MPa, and the gas hourly space velocity (GHSV) is 500–10000 mL•g. -1 •h -1 .

[0043] Examples 1-13 and Comparative Examples 1-11 S1. Metal oxides are prepared by precipitation method, and the specific preparation parameters are shown in Table 1; or, commercially available metal oxides are used. S2. Molecular sieves or metal salt modified molecular sieves are prepared using the above preparation method, and the specific preparation parameters are shown in Table 2; or, commercially available molecular sieves are used. S3. The metal salt modified molecular sieve is mechanically mixed with the metal oxide to obtain the catalyst. The specific catalyst preparation and its parameter characteristics are shown in Table 3 and Table 4. S4. Catalytic reaction is carried out using carbon dioxide and hydrogen as raw materials in contact with the catalyst. 2 g of catalyst is placed in a fixed-bed reactor, and the air in the reactor is replaced with argon. Then, the temperature is raised to 300°C in an H2 atmosphere. A mixture of carbon dioxide and hydrogen (H2 / CO2 molar ratio = 0.2~4.5) is switched, with a mixed gas pressure of 0.5~10 MPa. The temperature is raised to the reaction temperature of 300~500°C, and the space velocity of the mixed gas is adjusted to 500~10000 mL•g. -1 •h -1 The specific parameters of the catalytic reaction are shown in Table 5, and the products were analyzed by online chromatography.

[0044] Table 3. Preparation parameters of catalysts in Examples 1-13

[0045] Table 4. Preparation parameters of catalysts in Comparative Examples 1-11

[0046] Table 5 Catalytic reaction parameters used in Examples 1-13 and Comparative Examples 1-11

[0047] Table 6. Application effect data of catalysts in Examples 1-13 and Comparative Examples 1-11 (10 h)

[0048] As shown in Table 6, tests were conducted after 10 hours of continuous reaction in a fixed-bed reactor. Examples 1-13 demonstrate that the final catalytic effect varies depending on the temperature, pressure, space velocity, and the molar ratio of H2 / CO2 in the mixed gas. However, the overall catalytic activity and ethylene-propylene selectivity are high, while methane selectivity is low. Specifically, the CO2 conversion rate reaches 30-50%, the selectivity of low-carbon olefins (one or more of ethylene, propylene, and butene) in hydrocarbon products can reach 82-90%, the selectivity of ethylene and propylene in hydrocarbon products can reach 78-86%, the molar ratio of ethylene to propylene is between 0.8 and 1.7, and the methane selectivity is <10%.

[0049] like Figure 1 The figures shown are NH3-TPD diagrams of the molecular sieves prepared in Example 2 (part 5) and Comparative Example 2 (part 2), indicating that the number of medium-strong acid sites decreased after surface passivation treatment. Figure 2 The pyridine infrared spectra of the molecular sieves prepared in Example 1 (part 4) and Comparative Example 1 (part 1) are shown. The characteristic peaks pointing to Brønsted acid are significantly reduced, which proves that the number of acid sites on the outer surface is significantly reduced after passivation treatment.

[0050] As shown in Table 6, the molecular sieves used in Comparative Example 1 (compared to Example 1), Comparative Example 2 (compared to Example 2), and Comparative Example 3 (compared to Example 3) were SAPO-17, SAPO-34, and SAPO-35, respectively, with unpassivated acidic sites on their outer surfaces. The reaction results of Comparative Examples 1-3 indicate that the molecular sieves with unpassivated acidic sites on their outer surfaces exhibit lower selectivity for low-carbon olefins in the reaction products, low overall selectivity for ethylene and propylene, and higher selectivity for methane. This is because ethylene undergoes a secondary transformation at the acidic sites on its outer surface, resulting in a higher C content in the products. 4+ The selection is relatively high.

[0051] Compared to Example 1, the molecular sieve in Comparative Example 4 used a significantly lower concentration of sodium nitrate aqueous solution during ion exchange, yet the acid site content of the molecular sieve remained high. The reaction results showed a significantly reduced selectivity for ethylene-propylene. This is because low-concentration metal salts cannot completely exchange the acid sites on the outer surface; therefore, ethylene-propylene will further convert to lower-carbon alkanes and C2O4. 4+ Hydrocarbons. In Example 13, compared to Example 1, the concentration of sodium nitrate aqueous solution was increased during ion exchange. The ethylene-propylene selectivity was comparable to that in Example 1. This is because the metal salt concentration in Example 1 was sufficient to completely passivate the acid sites on the outer surface, keeping the acid sites at a low level; further increasing the metal salt concentration had little effect on the reaction performance.

[0052] In Comparative Examples 5 and 6, rubidium nitrate and cesium chloride were used, respectively. The reaction results showed that, compared to Example 1, the CO2 conversion rate gradually decreased in Comparative Examples 5-6, and the selectivity for ethylene and propylene also declined. This is because the ionic radii of rubidium and cesium ions are larger than those of sodium ions. Larger metal ions can clog the pores, thus limiting diffusion and preventing the achievement of the technical effects of this invention. The reaction results of Comparative Examples 4-6 indicate that controlling the content of acid sites and the type of metal salt in the passivated molecular sieve is crucial for achieving higher CO2 conversion rates and ethylene and propylene selectivity.

[0053] Compared to Example 4, Comparative Examples 7 and 8 used catalysts containing only metal oxides and no molecular sieves. The CO2 conversion rate was very low, and the products were mainly byproducts such as dimethyl ether and methane, with very low selectivity for ethylene and propylene. Comparative Example 8 used catalysts containing only molecular sieves and no metal oxides. Since molecular sieves lack the ability to activate CO2, the catalytic reaction had almost no activity. The reaction results of Comparative Examples 7 and 8 show that the reaction effect is poor when the catalyst contains only component I or component II, and it completely lacks the excellent reaction performance described in this invention.

[0054] Compared to Example 4, Comparative Example 9 used a Cu-based oxide catalyst, resulting in a very low conversion rate and primarily producing byproducts such as methane, with very low selectivity for C2-C4 hydrocarbons. Compared to Example 5, Comparative Example 10 used a single-component ZnO catalyst. ZnO has a large grain size and a low specific surface area (<1 m²). 2 The catalyst in Comparative Example 11, which uses the same catalyst as in Example 1 but with an additional 10% FeO component in the oxide composition, produced mainly methane. The reaction products were predominantly methane, with very low selectivity for low-carbon olefins, ethylene, and propylene. This was because the FeO in the catalyst was reduced during the reaction to form iron carbide, thus transforming the reaction into a traditional Fischer-Tropsch synthesis route, with the product distribution following an ASF distribution. The results of Comparative Examples 9-11 demonstrate that the oxide component in the catalyst is crucial for the preparation of highly selective low-carbon olefins.

[0055] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing ethylene propylene using a catalyst that selectively passivates acidic sites, characterized in that, Includes the following steps: S1. Molecular sieve raw powder is synthesized by hydrothermal method; the molecular sieve raw powder is added to a metal salt aqueous solution for ion exchange, and then dried and calcined with oxygen to obtain a metal salt modified molecular sieve with a medium-strong acid site content of 0.05~0.3 mol / kg, wherein the molecular sieve is SAPO-17, SAPO-34 or SAPO-35, and the metal salt is sodium salt and / or potassium salt; S2. Mechanically mix the metal salt-modified molecular sieve with a metal oxide, wherein the metal oxide is InZr. x O (1+1.5x) GaZr x O (1+1.5x) ZnCr x O (1+1.5x) ZnAl x O (1+1.5x) ZnGa x O (1+1.5x) and ZnZr x O (1+1.5x) At least one of the following, wherein x takes the value of 0.1 to 10, is used to obtain a catalyst; S3. A mixture of hydrogen and carbon dioxide is brought into contact with a catalyst to react and produce ethylene and propylene.

2. The method for preparing ethylene propylene using a catalyst that selectively passivates acidic sites according to claim 1, characterized in that, In S1, the hydrothermal synthesis method specifically includes: mixing silicon source, aluminum source, phosphorus source and water, adding template agent and aging, then heating and crystallizing, and drying to obtain molecular sieve raw powder.

3. The method for preparing ethylene propylene using a catalyst that selectively passivates acidic sites according to claim 2, characterized in that, The template agent includes cyclohexylamine, cyclohexylimine, or tetraethylammonium hydroxide; the aging is carried out at room temperature for 1-3 hours.

4. The method for preparing ethylene propylene using a catalyst with selectively passivated acidic sites according to claim 2 or 3, characterized in that, The temperature-raising crystallization involves raising the temperature to 150~200℃ and crystallizing for 60~80 hours.

5. The method for preparing ethylene propylene using a catalyst that selectively passivates acidic sites according to claim 1, characterized in that, In S1, the sodium salt is sodium nitrate or sodium chloride; the potassium salt is potassium nitrate or potassium chloride.

6. The method for preparing ethylene propylene using a catalyst that selectively passivates acidic sites according to claim 1, characterized in that, In S1, the concentration of the metal salt aqueous solution is 0.05~3 mol / L; the ion exchange is carried out by heating to 60~90℃ and holding for 5~15 h.

7. The method for preparing ethylene propylene using a catalyst with selectively passivated acidic sites according to claim 1, 5, or 6, characterized in that, In S1, the aerobic roasting involves heating to 500~700℃ and holding at that temperature for 5~7 hours.

8. The method for preparing ethylene propylene using a catalyst with selectively passivated acidic sites according to claim 1, 5, or 6, characterized in that, In S2, the mass ratio of the metal salt modified molecular sieve to the metal oxide is 0.1~20:

1.

9. The method for preparing ethylene propylene using a catalyst that selectively passivates acidic sites according to claim 1, characterized in that, In S3, the reaction conditions are: temperature 300~500℃, mixed gas pressure 0.5~10 MPa, and mixed gas space velocity 300~10000 mL•g. -1 •h -1 .

10. The method for preparing ethylene propylene using a catalyst with selectively passivated acidic sites according to claim 1 or 9, characterized in that, In S3, the molar ratio of H2 / CO2 in the mixed gas is 0.2~4.5.

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