Composite metal oxide-molecular sieve catalysts, their preparation and use

Abstract

The application discloses a composite metal oxide-molecular sieve catalyst and a preparation method and application thereof. The catalyst is mainly composed of two parts of M and Z, wherein M is a fluorine-doped modified composite metal oxide, and Z is SAPO-34 molecular sieve; the mass ratio of M to Z is 1:2-2:1. The fluorine-doped modified composite metal oxide is a composite oxide of zirconium dioxide and zinc oxide modified by a soluble fluorine salt. The preparation method is characterized in that a low-pressure rotary evaporation device is used to improve a sol-gel method for preparing the composite metal oxide. The catalyst of the application is applied to a CO2 hydrogenation reaction for preparing low-carbon olefins, can effectively reduce the selectivity of by-product CO under conventional reaction conditions, improves the yield of low-carbon olefins, and has a low-cost industrial application prospect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of catalyst preparation and application, and specifically relates to a fluorine-doped modified composite metal oxide-molecular sieve catalyst. In addition, the present application also relates to a preparation method of the catalyst, and application of the catalyst in a CO2 hydrogenation direct low-carbon olefin reaction. BACKGROUND

[0002] The CO2 hydrogenation direct low-carbon olefin technology is an effective way to realize carbon neutralization and carbon emission reduction in China under the background of double carbon. Among them, the methanol / olefin ketone intermediate path with metal oxide-molecular sieve as a composite catalyst (OX-ZEO) has the highest selectivity of low-carbon olefin of more than 90%, which has a significant advantage compared with another CO2-Fischer-Tropsch path (the selectivity of low-carbon olefin is less than 58%).

[0003] For the CO2 hydrogenation direct low-carbon olefin reaction, high temperature and high pressure can improve the yield of olefin, but the harsh reaction conditions have high investment cost, great safety hidden danger and high process requirement. The conventional reaction conditions (reaction temperature: 350-400℃, reaction pressure: 3-4MPa, H2 / CO2=3 / 1-4 / 1, GHSV=1800-3600mL / (g·h)) are more suitable for the actual industrial application scene. However, there is a prominent problem of high selectivity of by-product CO under the conventional reaction conditions.

[0004] For example, GaZrO XThe SAPO-34 composite catalyst has a low-carbon olefin selectivity of 88.8% under conventional reaction conditions, a single-pass CO2 conversion rate of only 26.7%, and a selectivity of a byproduct CO higher than 50%; the ZnZrOx / Zn-SAPO-34 composite catalyst reported in the literature (ACS catalysis, 2017, 7, 8544-8548) has a low-carbon olefin selectivity of 80-90% under conventional reaction conditions, but also has a selectivity of a byproduct CO higher than 50%. Some researchers have modified and optimized such composite metal oxide-molecular sieve catalysts, such as the metal cation-modified composite metal oxide-molecular sieve composite catalyst disclosed in Chinese Patent CN114433221A for synthesizing syngas into low-carbon olefins, which only reduces the selectivity of a byproduct alkane and does not solve the problem of high selectivity of a byproduct CO; the N-doped metal oxide-molecular sieve composite catalyst disclosed in Chinese Patent CN110327969 for synthesizing syngas into low-carbon olefins has a selectivity of a byproduct CO lower than 30%, but the reaction conditions are harsh, and the problem of high selectivity of a byproduct CO under conventional reaction conditions is also not solved. Therefore, developing a new type of metal oxide-molecular sieve catalyst with low selectivity of a byproduct CO under conventional reaction conditions is a technical problem that needs to be solved in the field of synthesizing syngas into low-carbon olefins. SUMMARY

[0005] To solve the technical problems in the prior art that the selectivity of a byproduct CO is high when a metal oxide-molecular sieve composite catalyst is used under conventional reaction conditions, and the reaction conditions are harsh when the selectivity of a byproduct CO is low, the present application provides a new type of composite metal oxide-molecular sieve composite catalyst and a preparation method thereof. The new catalyst provided by the present application is applied to the reaction of synthesizing syngas into low-carbon olefins, and under conventional reaction conditions, the selectivity of a byproduct CO can be effectively reduced while maintaining the CO2 conversion rate.

[0006] To solve the above technical problems, the technical solution provided by the present application is as follows.

[0007] The preparation method of the composite metal oxide-molecular sieve catalyst provided by the present application comprises the following steps:

[0008] S1: Dissolve a certain mass ratio of a fluorine source, a zinc salt and a zirconium salt in water to obtain a composite metal salt solution containing fluorine ions;

[0009] S2: Dissolve a proper amount of a complexing agent in water to prepare a complexing agent solution with a certain molar concentration;

[0010] S3, add the metal salt solution obtained in step S1 dropwise to the complexing agent solution obtained in S2 until the metal ions in the metal salt solution are completely complexed;

[0011] S4: The solution obtained in step 3 is heated at a constant temperature in a low-pressure rotary evaporator until a gel is formed;

[0012] S5: The gel obtained in step 4 is dried at a constant temperature of 150-200℃ for 2-5h to obtain a solid intermediate;

[0013] S6: The solid intermediate obtained in step 5 is calcined at 400-600℃ for 3-8h to obtain component M;

[0014] S7: The two components M and Z are mechanically mixed to obtain the fluorine-doped modified composite metal oxide-molecular sieve catalyst.

[0015] Preferably, the fluorine source is one or more of ammonium fluoride and zinc fluoride; the water-soluble zinc salt is one or more of zinc nitrate, zinc chloride and zinc acetate; and the water-soluble zirconium salt is one or more of zirconium nitrate and zirconium tetrachloride.

[0016] Preferably, in step S1, the mass ratio of the fluorine source, the zinc salt and the zirconium salt is 0.04-0.6:4-10:2-4.5.

[0017] Preferably, in step S3, the molar concentration ratio of the sum of the metal ions in the metal salt solution to the complexing agent solution is 1:1.5-2.

[0018] Preferably, in step 4, the constant rotation speed of the solution in the low-pressure constant temperature device is 30-40r / min, the constant temperature heating temperature is 30-40℃, and the heating time is 3-6h.

[0019] A composite metal oxide-molecular sieve catalyst, which is composed of two parts M and Z, wherein M is a fluorine-doped modified composite metal oxide, and Z is SAPO-34 molecular sieve, and the mass ratio of M to Z is 1:2-2:1; the M is a composite oxide of zirconium dioxide and zinc oxide modified by a soluble fluorine salt.

[0020] Further, the M is represented by mX-A a B b , wherein X represents a fluorine ion, A represents zirconium oxide, B represents zinc oxide, a and b respectively represent the mole number of zirconium oxide and zinc oxide (before doping), a is 0.25, b is 0.75, and m represents the mole fraction of X relative to A a B b , and the range of m is 2%-20%, preferably 5%.

[0021] The application also provides the use of the fluorine-doped composite metal oxide-molecular sieve catalyst described above in the preparation of low-carbon olefins from CO2 hydrogenation.

[0022] Preferably, the CO2 hydrogenation reaction for preparing low-carbon olefins is carried out in a pressurized fixed-bed continuous flow reactor, using CO2 and H2 as raw materials, the reaction pressure is 3-4 MPa, the reaction temperature is 350-400 DEG C, the reaction space velocity is 1800-3600 mL / (g.h), and the hydrogen-carbon ratio is 3:1-4:1.

[0023] Compared with the prior art, the technical scheme provided by the application has the following advantages or significant technical progress:

[0024] (1) The catalyst preparation method provided by the application uses an improved sol-gel method to prepare a composite metal oxide, that is, a metal salt solution completely complexed by a complexing agent is heated at a constant temperature in a low-pressure rotary evaporation device until a gel is formed. Compared with the traditional sol-gel method, the unique structure of the low-pressure rotary evaporation device used in the application can make the solution rotate more uniformly during evaporation, which is conducive to obtaining a highly dispersed composite metal oxide, the interaction between components is stronger, and the catalyst activity is further promoted; the composite metal oxide prepared by the method has a smaller nanoparticle size, which is more conducive to the exposure of active sites. In addition, the method further reduces the required temperature and time in the solvent removal process by using the principle of low pressure and low boiling point, which can reduce energy consumption and production cost and shorten the production cycle compared with the traditional sol-gel method for preparing a composite metal oxide.

[0025] (2) The application provides a new type of fluorine-doped modified composite metal oxide-molecular sieve catalyst. Compared with the unmodified catalyst, the fluorine doping changes the crystal phase of zirconium dioxide, increases the density of strong basic sites on the surface of the catalyst, and further optimizes the number and properties of active sites of the catalyst, so that the catalyst of the application can promote the conversion of formate to methanol intermediates in the application process; compared with the existing nitrogen-doped metal oxide-molecular sieve catalyst, fluorine is the most electronegative element, and the doping of fluorine not only changes the structure of the catalyst, but also significantly adjusts the electronic properties of the elements in the catalyst, thereby changing the properties of the adsorbed species on the catalytic surface, and effectively reducing the selectivity of the byproduct CO compared with the existing metal cation modified metal oxide-molecular sieve.

[0026] (3) The catalyst provided by the application is applied to the CO2 hydrogenation reaction for preparing low-carbon olefins, and the selectivity of the byproduct CO is as low as 28% under conventional reaction conditions, which is much lower than the selectivity of the byproduct CO reported in the prior art (see literature: ACS catalysis, 2017, 7, 8544-8548). At the same time, compared with the prior art, under conventional reaction conditions, a higher selectivity of low-carbon olefins and a single-pass CO2 conversion rate can be obtained at the same time, and the selectivity of the CO byproduct is significantly reduced, and the olefin yield is significantly improved.

[0027] (4) The catalyst provided by the present application can effectively solve the problem of high selectivity of by-product CO under conventional reaction conditions, thereby reducing the requirements for process conditions of industrialization of the reaction, reducing investment cost and safety hazards, and ensuring high olefin yield at low cost. At the same time, due to the strong interaction between fluorine and the zinc-zirconium solid solution formed in the composite metal oxide, the catalyst prepared by the present application directly forms active sites after calcination, without the need for further pretreatment. Compared with the existing application mode, the application steps are simplified, and the running time and cost of the CO2 hydrogenation reaction to produce low-carbon olefins are further reduced. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is the XRD pattern of 5% F-ZrZnOx (top) composite metal oxide prepared in Example 1 and undoped ZrZnOx (bottom) composite metal oxide prepared in Comparative Example 1, wherein t-ZrO2 (88-1007) and ZnO (89-1397) represent the XRD patterns of standard tetragonal zirconium dioxide and wurtzite zinc oxide, respectively.

[0029] Figure 2 is the reaction evaluation result of Example 1 and Comparative Example 1, wherein the left side is the reaction evaluation result of the undoped ZrZnOx / SAPO-34 composite metal oxide-molecular sieve catalyst, and the right side is the reaction evaluation result of the 5% F-ZrZnOx / SAPO-34 composite metal oxide-molecular sieve catalyst. DETAILED DESCRIPTION

[0030] The present application will be described in more detail below with reference to examples. It should be noted that the examples are only a part of the preferred embodiments of the present application, but not all the embodiments. Therefore, the following examples are only to better explain and understand the inventive concept and spirit of the present application, and should not be understood as limiting the scope of protection of the present application.

[0031]

Example 1

[0032] Dissolve 0.078 g NH4F, 8.90 g Zn(NO3)2-6H2O and 4.20 g Zr(NO3)4-5H2O in 50 mL deionized water, and ultrasonic treat at room temperature until the solution is clear and transparent. Dissolve 16.80 g citric acid in 50 mL deionized water. Add the metal salt solution dropwise into the citric acid, and after stirring at room temperature for 30 min, rotate the solution at a constant temperature (30-40°C) on a low-pressure rotating device at a constant speed until a gel is formed. Dry the wet gel at 150-200°C for 2-5 h, and calcine the dried solid in a muffle furnace at 400-600°C for 3-8 h to obtain the composite metal oxide 5% F-ZrZnOx. Then, grind the 5% F-ZrZnOx composite metal oxide and SAPO-34 molecular sieve (silicon-aluminum ratio = 0.01) in a ratio of 1:2 by mass in an agate mortar until they are uniformly mixed, and press, crush and sieve the obtained composite at 15 MPa to obtain the 5% F-ZrZnOx / SAPO-34 catalyst.

[0033] The XRD spectrum of the 5% F-ZrZnOx composite metal oxide prepared in this example is shown in Figure 1 Figure 1 It can be seen that a solid solution structure is formed in the 5% F-ZrZnOx composite metal oxide, and F enters the interior of the ZnO crystal lattice.

[0034]

Example 2

[0035] Dissolve 0.030 g NH4F, 8.90 g Zn(NO3)2-6H2O and 4.20 g Zr(NO3)4-5H2O in 50 mL deionized water, and ultrasonic treat at room temperature until the solution is clear and transparent. Dissolve 16.80 g citric acid in 50 mL deionized water. Add the metal salt solution dropwise into the citric acid, and after stirring at room temperature for 30 min, rotate the solution at a constant temperature (30-40°C) on a low-pressure rotating device at a constant speed until a gel is formed. Dry the wet gel at 150-200°C for 2-5 h, and calcine the dried solid in a muffle furnace at 400-600°C for 3-8 h to obtain the composite metal oxide 2% F-ZrZnOx. Then, grind the 2% F-ZrZnOx composite metal oxide and SAPO-34 molecular sieve (silicon-aluminum ratio = 0.01) in a ratio of 1:2 by mass in an agate mortar until they are uniformly mixed, and press, crush and sieve the obtained composite at 15 MPa to obtain the 2% F-ZrZnOx / SAPO-34 catalyst. The XRD spectrum of the 2% F-ZrZnOx composite metal oxide obtained in this example is basically the same as in Example 1, and is therefore omitted.

[0036]

Example 3

[0037] ​Dissolve 0.148 g NH4F, 8.90 g Zn(NO3)2-6H2O and 4.20 g Zr(NO3)4-5H2O in 50 mL deionized water, and ultrasonic treat at room temperature until the solution is clear and transparent. Dissolve 16.80 g citric acid in 50 mL deionized water. Add the metal salt solution dropwise into the citric acid, and after stirring at room temperature for 30 min, rotate the solution at a constant temperature (30-40°C) on a low-pressure rotating device at a constant speed until a gel is formed. Dry the wet gel at 150-200°C for 2-5 h, and calcine the dried solid in a muffle furnace at 400-600°C for 3-8 h to obtain the composite metal oxide 10% F-ZrZnOx. Then, grind the 10% F-ZrZnOx composite metal oxide and SAPO-34 molecular sieve (silicon-aluminum ratio = 0.01) in a ratio of 1:2 by mass in an agate mortar until they are uniformly mixed, press the obtained mixture into a tablet at 15 MPa, crush it, and sieve it to obtain the 10% F-ZrZnOx / SAPO-34 catalyst. The XRD spectrum of the 10% F-ZrZnOx composite metal oxide obtained in this example is basically the same as that of Example 1, and is therefore omitted.

[0038] [Example 4]

[0039] Dissolve 0.148 g NH4F, 8.90 g Zn(NO3)2-6H2O and 4.20 g Zr(NO3)4-5H2O in 50 mL deionized water, and ultrasonic treat at room temperature until the solution is clear and transparent. Dissolve 16.80 g citric acid in 50 mL deionized water. Add the metal salt solution dropwise into the citric acid, and after stirring at room temperature for 30 min, rotate the solution at a constant temperature (30-40°C) on a low-pressure rotating device at a constant speed until a gel is formed. Dry the wet gel at 150-200°C for 2-5 h, and calcine the dried solid in a muffle furnace at 400-600°C for 3-8 h to obtain the composite metal oxide 10% F-ZrZnOx. Then, grind the 10% F-ZrZnOx composite metal oxide and SAPO-34 molecular sieve (silicon-aluminum ratio = 0.01) in a ratio of 1:2 by mass in an agate mortar until they are uniformly mixed, press the obtained mixture into a tablet at 15 MPa, crush it, and sieve it to obtain the 10% F-ZrZnOx / SAPO-34 catalyst. The XRD spectrum of the 10% F-ZrZnOx composite metal oxide obtained in this example is basically the same as that of Example 1, and is therefore omitted.

[0040] The application effect of the catalyst of the present application is further illustrated by the following application examples.

[0041] [Application Examples 1-4]

[0042] The application of the catalysts obtained in Examples 1-4 corresponds to Application Examples 1-4, respectively.

[0043] The fluorine-doped modified composite metal oxide-molecular sieve catalyst particles prepared in Example 1 were selected, and the particle size was 20-40 mesh. The particles were loaded into a fixed bed reactor, and reaction gas was introduced for reaction. The reaction pressure was 3 MPa, the reaction temperature was 370°C, GHSV = 3600 mL / (g·h), and the reaction gas was H2:CO2= 3:1. The reaction evaluation results are shown in Table 1.

[0044] [Application Example 5]

[0045] The fluorine-doped modified composite metal oxide-molecular sieve catalyst particles prepared in Example 1 were selected, and the particle size was 20-40 mesh. The particles were loaded into a fixed bed reactor, and reaction gas was introduced for reaction. The reaction pressure was 4 MPa, the reaction temperature was 370°C, GHSV = 3600 mL / (g·h), and the reaction gas was H2:CO2= 3:1. The reaction evaluation results are shown in Table 1.

[0046] [Application Example 6]

[0047] The fluorine-doped modified composite metal oxide-molecular sieve catalyst particles prepared in Example 1 were selected, and the particle size was 20-40 mesh. The particles were loaded into a fixed bed reactor, and reaction gas was introduced for reaction. The reaction pressure was 4 MPa, the reaction temperature was 370°C, GHSV = 3600 mL / (g·h), and the reaction gas was H2:CO2= 4:1. The reaction evaluation results are shown in Table 1.

[0048] [Comparative Example 1]

[0049] 8.90 g of Zn(NO3)2·6H2O and 4.20 g of Zr(NO3)4·5H2O were dissolved in 50 mL of deionized water, and ultrasonic treatment was performed at room temperature until the solution was clear and transparent. 16.80 g of citric acid was dissolved in 50 mL of deionized water. The metal salt solution was added dropwise to the citric acid, and after stirring at room temperature for 30 min, the precursor was heated to 70-100°C in an oil bath until a gel was formed. The wet gel was dried at 150-200°C for 2-5 h, and the dried solid was calcined in a muffle furnace at 400-600°C for 3-8 h to obtain a composite metal oxide ZrZnOx. Then, the ZrZnOx composite metal oxide was uniformly mixed with SAPO-34 molecular sieve (silicon aluminum ratio = 0.01) in a mortar at a mass ratio of 1:2, and the obtained composite was tableted at 15 MPa, crushed, and sieved to obtain a ZrZnOx / SAPO-34 catalyst.

[0050] The XRD spectrum of the ZrZnOx composite metal oxide prepared in this comparative example is shown in Figure 1 From the XRD spectrum, it can be seen that the ZrZnOx composite metal oxide exists in the structure of a solid solution. Figure 1

[0051] ​The composite metal oxide-molecular sieve catalyst particles prepared in Comparative Example 1 were selected, the particle size was 20-40 mesh, and the particles were loaded into a fixed bed reactor and reacted with a reaction gas. The reaction pressure was 3 MPa, the reaction temperature was 370°C, GHSV = 3600 mL / (g h), and the reaction gas was H2:CO2= 3:1. The reaction evaluation results are shown in Table 1.

[0052]

Comparative Example 2

[0053] A solution of 0.502 g of Mn(NO3)2·4H2O, 8.90 g of Zn(NO3)2·6H2O, and 4.20 g of Zr(NO3)4·5H2O was prepared by dissolving the metal salts in 50 mL of deionized water and ultrasonically treating the solution at room temperature until it was clear and transparent. A solution of 16.80 g of citric acid was prepared by dissolving the citric acid in 50 mL of deionized water. The metal salt solution was added dropwise to the citric acid solution, and after stirring at room temperature for 30 min, the precursor was heated to 70-100°C in an oil bath until a gel was formed. The wet gel was dried at 150-200°C for 2-5 h, and the dried solid was calcined in a muffle furnace at 400-600°C for 3-8 h to obtain a metal oxide 5% Mn-ZrZnOx. Then, the 5% Mn-ZrZnOx metal oxide was uniformly mixed with SAPO-34 molecular sieve (silicon aluminum ratio = 0.01) in a mortar at a mass ratio of 1:2, and the resulting composite was tableted at 15 MPa, crushed, and sieved to obtain a 5% Mn-ZrZnOx / SAPO-34 catalyst.

[0054] The composite metal oxide-molecular sieve catalyst particles prepared in Comparative Example 2 were selected, the particle size was 20-40 mesh, and the particles were loaded into a fixed bed reactor and reacted with a reaction gas. The reaction pressure was 3 MPa, the reaction temperature was 370°C, GHSV = 3600 mL / (g h), and the reaction gas was H2:CO2= 3:1. The reaction evaluation results are shown in Table 1.

[0055] Table 1. Reaction evaluation results

[0056]

[0057]

[0058] C2-C4 in Table 1 ＝ -C4 ＝ represents low-carbon olefin products having 2-4 carbon atoms, and the C2-C4 selectivity is calculated without including the CO byproduct. ＝ -C4 ＝ selectivity.

[0059] Analysis of the data in Table 1 shows that when the fluorine-doped modified composite metal oxide-molecular sieve catalyst of this invention is applied to the CO2 hydrogenation to low-carbon olefins reaction, under conventional reaction conditions, the selectivity for low-carbon olefins can reach over 80%, but the CO selectivity is generally below 40%, with the lowest CO selectivity of 28% in Application Example 6; while in Comparative Examples 3 and 4, the CO selectivity is as high as 45.3% and 65.6%, respectively. Therefore, the fluorine-doped modified composite metal oxide-molecular sieve catalyst of this invention exhibits excellent performance in the CO2 hydrogenation to low-carbon olefins reaction.

[0060] Figure 2 C2 ＝ -C4 ＝ This refers to low-carbon olefin products with 2 to 4 carbon atoms, C2 0 -C4 0 This refers to low-carbon alkane products with 2 to 4 carbon atoms, C5. + This refers to hydrocarbon products with 5 or more carbon atoms.

[0061] Depend on Figure 2 The data shows that, compared with the undoped ZnZrOx / SAPO-34 catalyst, the 5% F-ZrZnOx / SAPO-34 catalyst exhibits similar CO2 conversion and low-carbon olefin selectivity, but the CO byproduct selectivity is significantly reduced. Figure 2 As shown in Table 1, compared with the undoped composite metal oxide molecular sieve catalyst (Comparative Example 1) and the manganese-doped composite metal oxide-molecular sieve catalyst (Comparative Example 2), the catalyst of the present invention, under conventional reaction conditions, showed no significant change in the selectivity of low-carbon olefins and CO2 conversion rate, while the selectivity of the byproduct CO decreased significantly. In Application Example 1, the CO selectivity was as low as 35%, and the olefin selectivity reached 86%, effectively solving the problem of high byproduct CO selectivity under conventional reaction conditions, and has the prospect of low-cost industrial application.

[0062] The above description is only a preferred embodiment of the present invention, and the scope of protection of the present invention is not limited thereto. Any modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A method for preparing a composite metal oxide-molecular sieve catalyst, characterized in that, Includes the following steps: S1. Dissolve a fluoride source, zinc salt, and zirconium salt in water at a mass ratio of 0.04~0.6 : 4~10 : 2~4.5 to obtain a composite metal salt solution containing fluoride ions; S2. Dissolve an appropriate amount of complexing agent in water to prepare a complexing agent solution with a certain molar concentration; S3. The metal salt solution obtained in step S1 is added dropwise to the complexing agent solution obtained in S2 until the metal ions in the metal salt solution are completely complexed; the ratio of the total molar concentration of metal ions in the metal salt solution to the molar concentration of the complexing agent solution is 1:1.5~2. S4. The solution obtained in step S3 is heated at a constant temperature in a low-pressure rotary evaporator until a gel is formed; the temperature of the constant temperature rotary heating is 30~40℃, and the heating time is 3~6 h; S5. Dry the gel obtained in step S4 at a constant temperature of 150℃~200℃ for 2h~5h to obtain a solid intermediate. S6. The solid intermediate obtained in step S5 is calcined at 400℃~600℃ for 3h~8h to obtain fluorine-doped modified composite metal oxide. S7. The fluorine-doped modified composite metal oxide and SAPO-34 molecular sieve are mechanically mixed evenly in a certain proportion to obtain the fluorine-doped modified composite metal oxide-molecular sieve catalyst.

2. The preparation method according to claim 1, characterized in that, The fluorine source is one or more of ammonium fluoride and zinc fluoride; the zinc salt is one or more of zinc nitrate, zinc chloride, and zinc acetate; and the zirconium salt is one or more of zirconium nitrate and zirconium tetrachloride.

3. The preparation method according to claim 1, characterized in that, In step S4, the solution rotates at a constant speed of 30-40 r / min on the low-pressure rotary evaporator.

4. A composite metal oxide-molecular sieve catalyst prepared by the method according to any one of claims 1 to 3, characterized in that, It consists of two components, M and Z; M is a fluorine-doped modified composite metal oxide; Z is SAPO-34 molecular sieve; the mass ratio of M to Z is 1:2 to 2:1; the fluorine-doped modified composite metal oxide is a composite oxide of zirconium dioxide and zinc oxide modified with soluble fluoride salts.

5. The composite metal oxide-molecular sieve catalyst according to claim 4, characterized in that, The M mentioned is mX-A a B b The expression indicates that X represents fluoride ions, A represents zirconium oxide, and B represents zinc oxide; m represents X relative to the amount of composite metal oxide A per unit mass. a B b The mole fraction of doping, m is 2%-20%; a and b represent the number of moles of zirconium oxide and zinc oxide per unit amount of composite metal oxide before doping, respectively, and a is 0.25 and b is 0.

75.

6. The application of the catalyst according to claim 4 or 5 in the hydrogenation of CO2 to produce low-carbon olefins, characterized in that, The catalyst is used in a pressurized fixed-bed continuous flow reactor to carry out the CO2 hydrogenation reaction to produce low-carbon olefins.

7. The application according to claim 6, characterized in that, The catalyst does not require pretreatment before the reaction.

8. The application according to claim 6, characterized in that, Using CO2 and H2 as raw materials, the reaction conditions are: reaction temperature 350-400 ℃, reaction pressure 3-4 MPa, hydrogen-carbon ratio 3:1~4:1, and space velocity 1800 mL / (g·h)~3600 mL / (g·h).

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