CO2-assisted low-carbon alkane oxidative dehydrogenation catalyst and application thereof

Abstract

The invention discloses a CO2-assisted low-carbon alkane oxidative dehydrogenation catalyst and application thereof, and belongs to the field of low-carbon alkane application. The prepared catalyst is characterized in that the catalyst has a core-shell structure and is synthesized step by step through a hydrothermal method and a precipitation deposition method, and the core structure is composed of a metal active component, a non-metal auxiliary agent and a molecular sieve with an MFI structure; and the shell structure consists of a metal active component and an oxide carrier. The catalyst with the core-shell structure is applied to a CO2-assisted alkane oxidative dehydrogenation olefin preparation reaction, the reaction can be sequentially and synergistically carried out in a nano space, H2 is efficiently removed, thermodynamic equilibrium is remarkably broken, and the catalyst has extremely high stability and high olefin selectivity. According to the catalyst, through innovation of a physical structure, synergy of chemical reaction on energy and dynamics is achieved, a thought is provided for development of a next-generation efficient, stable and energy-saving low-carbon catalysis process, and the catalyst is the leading edge direction of the current heterogeneous catalysis field.

Description

Technical Field

[0001] This invention relates to a CO2-assisted oxidative dehydrogenation catalyst for low-carbon alkanes and its application, belonging to the field of low-carbon alkane chemical utilization. Background Technology

[0002] Currently, the main sources of low-carbon olefins are naphtha cracking and the catalytic reforming of its cracking products. However, with the over-exploitation of oil leading to rising crude oil prices and the increasing demand for low-carbon olefins in production and daily life, traditional petroleum routes can no longer meet the industrial needs for low-carbon olefins. Therefore, more and more researchers are focusing on new pathways that can replace traditional petroleum routes, such as syngas to olefins, methanol to olefins, and dehydrogenation of low-carbon alkanes to olefins. These new pathways have good prospects for industrial development due to the abundance, availability, and low cost of raw materials.

[0003] The dehydrogenation of low-carbon alkanes to produce low-carbon olefins includes both anaerobic and aerobic dehydrogenation. However, due to thermodynamic limitations, direct dehydrogenation of low-carbon alkanes can only achieve high yields of low-carbon olefins at sufficiently high temperatures, which easily leads to catalyst coking and cracking. When O2 is used as an oxidant, the oxidative dehydrogenation reaction of low-carbon alkanes has a low temperature and high conversion rate, but it is prone to deep oxidation, resulting in reduced olefin selectivity. To address these issues, CO2 can be introduced as a weak oxidant, consuming hydrogen through a reverse water-gas reaction (RGWS) to promote the continued conversion of low-carbon alkanes and overcome thermodynamic limitations. Furthermore, during the high-temperature reaction, CO2 can eliminate carbon deposits on the catalyst surface through a reverse Boudouard reaction, thereby improving the stability of CO2-assisted oxidative dehydrogenation of low-carbon alkanes. Although the selectivity of low-carbon olefins can usually be significantly improved with the help of CO2, the competitive reaction between CO2 and low-carbon alkanes often leads to a decrease in the conversion rate of low-carbon alkanes. Therefore, developing more efficient catalytic systems to simultaneously accelerate the reaction of low-carbon alkanes in dehydrogenation and CO2 in RWGS reaction is the most effective approach.

[0004] However, under CO2-assisted alkane dehydrogenation conditions, the activity of catalysts such as Fe, Co, and Ni is inhibited, causing their alkane conversion rates to deviate significantly from the equilibrium conversion rate. Furthermore, simple physical coupling or impregnation of Fe, Co, and Ni catalysts with Cu, Fe, and Cr countercurrent gas catalysts results in only a small amount of hydrogen being captured by the active sites of the countercurrent gas catalyst, leading to poor catalytic performance. Therefore, developing a catalyst that can both eliminate the negative effects of CO2 and effectively capture hydrogen for the RGWS reaction to significantly overcome thermodynamic limitations is crucial. The catalyst designed in this invention achieves both of these effects: firstly, the addition of a small amount of non-metallic promoter to the core catalyst eliminates the activity inhibition effect caused by CO2 introduction; secondly, its unique core-shell structure forces hydrogen to leave the catalyst through the shell structure, thus more efficiently removing most of the hydrogen instead of allowing it to leave directly. This results in an alkane conversion rate under CO2 conditions that significantly exceeds the thermodynamic equilibrium conversion rate, exhibiting extremely high stability and olefin selectivity. Summary of the Invention

[0005] [Technical Issues] To overcome the CO2 inhibition activity problem of catalysts such as Fe, Co, and Ni, this invention addresses this issue by adding a small amount of non-metallic promoters. Furthermore, to efficiently remove hydrogen and achieve true pull equilibrium, this invention develops a core-shell structure catalyst. This structure significantly improves catalytic performance by driving synergistic effects on the energy and kinetics of the chemical reaction, providing a key technological solution for developing efficient, stable, and energy-saving next-generation low-carbon catalytic processes, and representing the cutting-edge development direction in the field of heterogeneous catalysis.

[0006] [Technical Solution] This invention provides a catalyst for CO2-assisted dehydrogenation of low-carbon alkanes to olefins. The catalyst has a core-shell structure, wherein the core structure comprises a metal active component A, a non-metallic promoter B, and a molecular sieve; and the shell structure is composed of a metal active component C and an oxide support D. Among them, the metal active component A is one or more of iron, cobalt, nickel, and zinc; the non-metallic auxiliary B is one or more of fluorine, chlorine, bromine, and iodine; the metal active component C is one or more of chromium, copper, and iron; and the oxide carrier D is one or more of cerium dioxide, aluminum oxide, magnesium oxide, indium oxide, and lanthanum oxide.

[0007] In one embodiment of the present invention, the molecular sieve type is an MFI type molecular sieve.

[0008] In one embodiment of the present invention, the active metal component in the core structure accounts for 0.1 wt% to 30 wt% of the weight of the core structure portion. More preferably, it accounts for 2 wt% to 30 wt%.

[0009] In one embodiment of the present invention, the non-metallic additive in the core structure accounts for 0.01 wt% to 10 wt% of the weight of the core structure portion. More preferably, it is 0.5 wt% to 10 wt%.

[0010] In one embodiment of the present invention, the active metallic component in the shell structure accounts for 0.1 wt% to 30 wt% of the weight of the shell structure portion. More preferably, it is 3 wt% to 30 wt%.

[0011] The present invention also provides a method for preparing the above-mentioned catalyst, comprising the following steps: (1) The soluble metal precursor I, silicon source, non-metallic auxiliary precursor, template agent and water source are mixed evenly in a certain proportion and placed in a hydrothermal reactor for hydrothermal reaction for a period of time; then cooled to room temperature, filtered and washed, dried and calcined to obtain the core structure part of the catalyst. (2) The obtained catalyst core structure is partially dispersed in deionized water to form a dispersion solution; the oxide support precursor and precipitant are added to the dispersion solution by precipitation deposition method, and the precipitate formed coats the outside of the catalyst core structure. The sample is then obtained by filtration, washing, drying and calcination. (3) The sample obtained in step (2) is dispersed again in deionized water, and the soluble metal precursor II and precipitant are added to the solution by precipitation deposition method. Then, after filtration, washing, drying and calcination, a catalyst with a core-shell structure is finally formed.

[0012] In one embodiment of the present invention, the silicon source mentioned in step (1) is one or more of silicon dioxide, sodium silicate, propyl orthosilicate, hexamethyldisiloxane, ethyl orthosilicate, and isopropyl orthosilicate.

[0013] In one embodiment of the present invention, the template agent mentioned in step (1) is one or more of tetrapropylammonium hydroxide, tetramethylammonium hydroxide, and hexadecyltrimethylammonium bromide.

[0014] In one embodiment of the present invention, the soluble metal precursor I in step (1) is one or more of the following: nitrate, acetate, sulfate, and chloride of a soluble metal.

[0015] In one embodiment of the present invention, the non-metallic additive precursor mentioned in step (1) is one or more of lithium salt, sodium salt, potassium salt and ammonium salt containing halogen elements.

[0016] In one embodiment of the present invention, in step (1), the mass ratio of soluble metal precursor I to non-metallic auxiliary precursor is 1:(0.1-1.5).

[0017] In one embodiment of the present invention, in step (1), the mass ratio of soluble metal precursor I to silicon source is 1:(5-30).

[0018] In one embodiment of the present invention, in step (1), the mass ratio of soluble metal precursor I to template agent is 1:(10-40).

[0019] In one embodiment of the present invention, the hydrothermal reaction conditions in step (1) are hydrothermal crystallization at 100-300°C for 1-15 days.

[0020] In one embodiment of the present invention, the calcination conditions in step (1) are calcination at 200-700°C for 1-24 hours. Specifically, calcination at 500°C for 6 hours is optional.

[0021] In one embodiment of the present invention, the oxide carrier precursor in step (2) is one or more of nitrate, acetate, sulfate and chloride.

[0022] In one embodiment of the present invention, in step (2), the mass ratio of the oxide support precursor to the catalyst core structure is 1:(1-3).

[0023] In one embodiment of the present invention, in step (2), after the oxide support precursor and precipitant are added to the dispersion solution, the mixture is aged at room temperature for 2-5 hours, and the resulting precipitate coats the outside of the catalyst core structure. Specifically, aging for 3 hours is optional.

[0024] In one embodiment of the present invention, the soluble metal precursor II in step (3) is one or more of the following: nitrate, acetate, sulfate, and chloride of a soluble metal.

[0025] In one embodiment of the present invention, in step (3), the mass ratio of soluble metal precursor II to the sample obtained in step (2) is 1:(0.05-0.5).

[0026] In one embodiment of the present invention, in step (3), soluble metal precursor II and precipitant are added to the solution and aged at room temperature for 1-3 h, followed by filtration, washing, drying and calcination.

[0027] In one embodiment of the present invention, the precipitant mentioned in step (2) or step (3) is one or more of ammonia, urea, sodium carbonate, and sodium hydroxide.

[0028] In one embodiment of the present invention, the drying conditions in steps (1), (2) or (3) are drying at 50-150°C for 1-24 hours.

[0029] In one embodiment of the present invention, the calcination conditions in step (2) are calcination at 200-700°C for 1-24 hours. Specifically, calcination at 400°C for 4 hours is optional.

[0030] In one embodiment of the present invention, the calcination conditions in step (3) are calcination at 200-700°C for 1-24 hours. Specifically, calcination at 600°C for 4 hours is optional.

[0031] This invention provides a catalyst for CO2-assisted dehydrogenation of low-carbon alkanes to olefins prepared by the above-described method.

[0032] The unique structure of the core-shell catalyst obtained in this invention enables the synergistic occurrence of CO2-assisted alkane oxidative dehydrogenation within a nanoscale confinement space, efficiently removing the byproduct hydrogen gas and thus significantly breaking the thermodynamic equilibrium constraint. This catalyst not only achieves olefin selectivity far exceeding that of traditional methods but also exhibits extremely high stability.

[0033] This invention provides the application of the CO2-assisted dehydrogenation catalyst for producing olefins from low-carbon alkanes prepared by the above method in the catalytic dehydrogenation reaction of low-carbon alkanes to olefins.

[0034] In one embodiment of the present invention, the catalyst is used in the following reaction conditions for CO2-assisted dehydrogenation of low-carbon alkanes to olefins: reaction temperature of 300-700℃, reaction pressure of 0.1-2.0 MPa, reaction space velocity of 1000-40000 mL / g / h, CO2 volume fraction in the reaction feedstock of 5-50%, and reaction mode of fixed bed reactor.

[0035] In one embodiment of the present invention, the volume fraction of CO2 in the reaction raw materials is preferably 20%.

[0036] In one embodiment of the present invention, the reaction raw materials include low-carbon alkanes, CO2, and N2. The volume ratio of the three is 1:1 (0.5-3).

[0037] In one embodiment of the present invention, the low-carbon alkane is a C2-C4 saturated alkane, including ethane, propane, etc.

[0038] [Technical Effects] This invention provides a catalyst for CO2-assisted dehydrogenation of low-carbon alkanes to olefins and its preparation method. In the reaction of catalyzing the dehydrogenation of low-carbon alkanes to olefins, it exhibits an ethane conversion rate that breaks through thermodynamic equilibrium and an ethylene selectivity of up to 98%. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the structure of catalyst A prepared in Example 1. Detailed Implementation

[0040] The present invention will be further illustrated below with reference to specific embodiments. It should be noted that the listed embodiments are only for further illustration of the present invention and not for limiting the scope of the present invention.

[0041] 1. The process of catalytic CO2-assisted dehydrogenation of low-carbon alkanes to olefins: The catalyst performance evaluations in the following examples and comparative examples were all conducted in a straight fixed-bed reactor. The specific steps were as follows: 0.3 g of pre-dried and compressed catalyst particles (20-40 mesh) were placed in a reaction tube and heated from room temperature to the target temperature of 300-700 °C at a heating rate of 10 °C / min in an inert gas (Ar). After stabilizing for 15 min, the reaction was carried out using feed gas (30% ethane / 30% CO2) at a reaction pressure of 0.1-2.0 MPa. The reaction tail gas was analyzed online using a GC-7820 gas chromatograph, and the curves of alkane conversion and olefin selectivity over time could be calculated.

[0042] Alkane conversion rate = (moles of imported alkane - moles of exported alkane) / moles of imported alkane × 100% Product selectivity = (moles of exported product) × (number of carbon atoms in product molecule) / (number of carbon atoms in alkane) / (moles of imported alkane - moles of exported alkane) × 100% 2. Preparation of catalysts for CO2-assisted dehydrogenation of low-carbon alkanes to olefins: Example 1 (1) Weigh 0.52 g of cobalt nitrate and 0.17 g of ammonium fluoride and dissolve them in 16.84 g of deionized water. Then weigh 7.0 g of tetraethyl orthosilicate and 10.6 g of tetrapropylammonium hydroxide and add them to the cobalt nitrate solution. After stirring at room temperature for 10 hours, transfer the solution to a 100 mL hydrothermal reactor and place the reactor in an oven at 160 °C for hydrothermal reaction for 72 hours. After hydrothermal reaction, allow the solution to cool naturally to room temperature. After filtration and washing until the filtrate is neutral, dry it at 80 °C for 12 hours and calcine it at 500 °C for 6 hours to obtain the core structure of catalyst A.

[0043] (2) Next, 2g of the catalyst core structure was uniformly dispersed in 50mL of deionized water. Then, 2.78g of cerium nitrate was weighed and added to the catalyst core structure solution and stirred for 10min to dissolve it completely. Then, a certain amount of 1.0mol / L ammonia solution was slowly added dropwise to the solution to make the pH=9. The solution was aged at room temperature for 3h, then filtered and washed, dried at 80℃ for 8h, and the obtained solid was ground into powder and calcined at 400℃ for 4h to obtain the solid product.

[0044] (3) The 3.2g solid product obtained in the above steps was dispersed again in 180mL of deionized water. Then, 0.63g of copper nitrate was weighed and added to the solution, and stirred for 10min to ensure complete dissolution. Next, a certain amount of 0.5mol / L sodium carbonate solution was slowly added dropwise to adjust the pH to 9. The solution was aged at room temperature for 1h, then filtered and washed. It was dried at 75℃ for 12h, and the resulting solid was ground into powder and calcined at 600℃ for 4h. ICP analysis showed that the cobalt content was 5.1 wt%, the copper content was 10.3 wt%, and the fluorine content was 1.4 wt%. Its structural schematic diagram is shown below. Figure 1 As shown.

[0045] Example 2 Similar to Example 1, but with cobalt nitrate replaced by 0.75g of ferrous sulfate in step (1), and all other processes remaining unchanged, catalyst A2 was prepared. ICP analysis showed that the iron content was 6.4 wt%, the copper content was 9.7 wt%, and the fluorine content was 1.4 wt%.

[0046] Example 3 Similar to Example 1, but with cobalt nitrate replaced by 0.31g of nickel nitrate in step (1), and all other processes remaining unchanged, catalyst A3 was prepared. ICP analysis showed that the nickel content was 2.8 wt%, the copper content was 9.9 wt%, and the fluorine content was 1.4 wt%.

[0047] Example 4 Similar to Example 1, but with ammonium fluoride replaced by 0.12g sodium chloride in step (1), and all other processes remaining unchanged, catalyst B1 was prepared. ICP analysis showed that the cobalt content was 5.3 wt%, the copper content was 10.4 wt%, and the chlorine content was 1.2 wt%.

[0048] Example 5 Similar to Example 1, but with ammonium fluoride replaced by 0.09 g potassium bromide in step (1), and all other processes remaining unchanged, catalyst B2 was prepared. ICP analysis showed that the cobalt content was 4.8 wt%, the copper content was 9.6 wt%, and the bromine content was 0.79 wt%.

[0049] Example 6 Similar to Example 1, but with tetrapropylammonium hydroxide replaced by 8.7g tetramethylammonium hydroxide in step (1), and all other processes remaining unchanged, catalyst C was prepared. ICP analysis showed that the cobalt content was 4.5 wt%, the copper content was 9.5 wt%, and the fluorine content was 1.2 wt%.

[0050] Example 7 Similar to Example 1, but with tetraethyl orthosilicate replaced by 7.5g of silicon dioxide in step (1), and all other processes remaining unchanged, catalyst D was prepared. ICP analysis showed that the cobalt content was 4.6 wt%, the copper content was 10.2 wt%, and the fluorine content was 1.1 wt%.

[0051] Example 8 Similar to Example 1, but with 3.19 g of indium nitrate replaced in step (2), and all other processes remaining unchanged, catalyst E was prepared. ICP analysis showed that the cobalt content was 5.1 wt%, the copper content was 10.4 wt%, and the fluorine content was 0.9 wt%.

[0052] Example 9 Similar to Example 1, the ammonia solution in step (2) was replaced with sodium hydroxide solution, while other processes remained unchanged, and catalyst F was prepared. ICP analysis showed that the cobalt content was 5.1 wt%, the copper content was 10.2 wt%, and the fluorine content was 0.9 wt%.

[0053] Example 10 Similar to Example 1, but with copper nitrate replaced by 0.49g of chromium nitrate in step (3), and all other processes remaining unchanged, catalyst G was prepared. ICP analysis showed that the cobalt content was 5.2 wt%, the chromium content was 3.3 wt%, and the fluorine content was 1.2 wt%.

[0054] 3. Catalytic CO2-assisted oxidative dehydrogenation of alkanes to olefins: Example 11 Catalyst AG was applied to the CO2-assisted oxidative dehydrogenation of ethane to ethylene. The reaction conditions were: feed gas composition of 40 vol% C2H6 / 40 vol% CO2 / 20 vol% N2, reaction temperature of 600℃, reaction space velocity of 9000 mL / g / h, reaction pressure of 0.1 MPa, and an evaluation period of 500 hours. Performance data for CO2-assisted oxidative dehydrogenation of ethane to ethylene are shown in Table 1.

[0055] Example 12 Catalyst AG was applied to the CO2-assisted oxidative dehydrogenation of ethane to ethylene. The reaction conditions were: feed gas composition of 20 vol% C2H6 / 20 vol% CO2 / 60 vol% N2, reaction temperature of 600℃, reaction space velocity of 9000 mL / g / h, reaction pressure of 0.1 MPa, and an evaluation period of 500 hours. Performance data for CO2-assisted oxidative dehydrogenation of ethane to ethylene are shown in Table 1.

[0056] Example 13 Catalyst AG was applied to the CO2-assisted oxidative dehydrogenation of ethane to ethylene. The reaction conditions were: feed gas composition of 40 vol% C2H6 / 40 vol% CO2 / 20 vol% N2, reaction temperature of 600℃, reaction space velocity of 20000 mL / g / h, reaction pressure of 0.1 MPa, and an evaluation period of 500 hours. Performance data for CO2-assisted oxidative dehydrogenation of ethane to ethylene are shown in Table 1.

[0057] Example 14 Catalyst AG was applied to the CO2-assisted oxidative dehydrogenation of propane to propylene. The reaction conditions were: feed gas composition of 20 vol% C3H8 / 20 vol% CO2 / 60 vol% N2, reaction temperature of 550℃, reaction space velocity of 4500 mL / g / h, reaction pressure of 0.1 MPa, and an evaluation period of 500 hours. Performance data for CO2-assisted oxidative dehydrogenation of propane to propylene are shown in Table 2.

[0058] Table 1. Performance of catalyst AG in catalyzing CO2-assisted ethane oxidative dehydrogenation to ethylene.

[0059] Table 2 Performance of catalyst AF in catalytic CO2-assisted propane oxidative dehydrogenation to propylene

[0060] As can be seen from the data in Tables 1 and 2, the alkane dehydrogenation catalyst prepared in this invention exhibits extremely high catalytic activity, selectivity, and stability for both CO2-assisted ethane oxidative dehydrogenation and CO2-assisted propane oxidative dehydrogenation. At various reaction temperatures and alkane concentrations, the alkane conversion rates significantly exceeded the thermodynamic equilibrium conversion rates, with ethylene selectivity exceeding 98% and propylene selectivity around 97%. In particular, the catalyst prepared in this invention showed no significant deactivation within 500 hours under the reaction conditions.

[0061] Comparative Example 1 Similar to Example 1, steps (2) and (3) are omitted, while other processes remain unchanged, to obtain catalyst H without a shell structure. The specific preparation process is as follows: 0.52 g of cobalt nitrate and 0.17 g of ammonium fluoride were weighed and dissolved in 16.84 g of deionized water. Then, 7.0 g of tetraethyl orthosilicate and 10.6 g of tetrapropylammonium hydroxide were weighed and added to the cobalt nitrate solution. After stirring at room temperature for 10 hours, the mixture was transferred to a 100 mL hydrothermal reactor and placed in an oven at 160 °C for hydrothermal reaction for 72 hours. After hydrothermal reaction, the mixture was allowed to cool naturally to room temperature, filtered, washed until the filtrate was neutral, dried at 80 °C for 12 hours, and calcined at 500 °C for 6 hours to obtain catalyst H.

[0062] The catalyst samples were evaluated using CO2-assisted ethane oxidative dehydrogenation and CO2-assisted propane oxidative dehydrogenation, respectively. Evaluation conditions were as follows: Ethane dehydrogenation: 20 vol% C2H6 / 20 vol% CO2 / 60 vol% N2, reaction temperature 600℃, reaction space velocity 9000 mL / g / h, reaction pressure 0.1 MPa, reaction time 15 h; Propane dehydrogenation: 20 vol% C3H8 / 20 vol% CO2 / 60 vol% N2, reaction temperature 550℃, reaction space velocity 4500 mL / g / h, reaction pressure 0.1 MPa, reaction time 15 h. The results are shown in Table 3.

[0063] Comparative Example 2 Same as in Example 1, except that the ammonium fluoride added in step (1) is removed, and the other processes remain unchanged, to obtain core-shell catalyst I without additives.

[0064] The samples were evaluated using CO2-assisted ethane oxidative dehydrogenation and CO2-assisted propane oxidative dehydrogenation, respectively, under the same evaluation conditions as Comparative Example 1. The results are shown in Table 3.

[0065] Comparative Example 3 Catalyst J was prepared by simultaneously impregnating a shell-free catalyst H with 5 wt% Ce and Cu using an impregnation method.

[0066] The samples were evaluated using CO2-assisted ethane oxidative dehydrogenation and CO2-assisted propane oxidative dehydrogenation, respectively, under the same evaluation conditions as Comparative Example 1. The results are shown in Table 3.

[0067] Comparative Example 4 Similar to Example 1, the added core structure portion in steps (1) and (2) is removed, while other processes remain unchanged, to obtain catalyst K without a core structure. The specific preparation process is as follows: Weigh 2.78 g of cerium nitrate and add it to 50 mL of deionized water and stir for 10 min to dissolve it completely. Then slowly add a certain amount of 1.0 mol / L ammonia solution to the solution to make the pH=9. Aging at room temperature for 3 h, then filter, collect the solid and wash it. Dry it at 80 °C for 8 h and then calcine it at 400 °C for 4 h to obtain the solid product. Weigh 3.2g of the solid product obtained in the above steps and disperse it again in 180mL of deionized water. Then weigh 0.63g of copper nitrate and add it to the solution and stir for 10min to dissolve it completely. Then slowly add a certain amount of 0.5mol / L sodium carbonate solution to the solution to make the pH=9. Aging at room temperature for 1h, then filter and wash, dry at 75℃ for 12h, grind the obtained solid into powder, and calcine at 600℃ for 4h to obtain catalyst K.

[0068] Catalyst H from Comparative Example 1 and catalyst K were mixed in a 1:1 mass ratio using both particulate and powder mixing methods to obtain catalysts L (particulate mixture) and M (powder mixture). The samples were evaluated using CO2-assisted ethane oxidative dehydrogenation and CO2-assisted propane oxidative dehydrogenation, respectively, under the same evaluation conditions as Comparative Example 1. The results are shown in Table 3.

[0069] Comparative Example 5 Similar to Example 1, but with cobalt nitrate replaced by an equal amount of chromium nitrate in step (1), and other processes remaining unchanged, catalyst N was prepared.

[0070] Comparative Example 6 Similar to Example 1, in step (3), copper nitrate was replaced with an equal amount of nickel nitrate, while other processes remained unchanged, to prepare catalyst O.

[0071] Table 3 Performance of catalyst HK in catalytic CO2-assisted oxidative dehydrogenation of ethane and propane to propylene

[0072] As shown in Table 3, the alkane conversion of catalysts without a shell structure (Comparative Examples 1, 3, and 4) can reach or exceed the thermodynamic equilibrium value of the dehydrogenation reaction. However, the olefin selectivity decreases significantly, and partial carbon deposition covers the active sites of the catalyst, causing continuous deactivation within 15 hours, exhibiting poor stability and low olefin selectivity. Compared with the data in Tables 1 and 2, the catalyst of this invention shows excellent CO2-assisted alkane oxidative dehydrogenation performance.

[0073] Furthermore, it can be seen from Comparative Example 2 and Example 1 that if no additive is added to the core portion of the core-shell catalyst, even with a core-shell structure, the conversion rate cannot be significantly improved; on the contrary, it deviates far from the equilibrium conversion rate. This indicates that in this invention, the additive also plays an important role in the CO2-assisted alkane oxidative dehydrogenation performance of the catalyst.

[0074] Furthermore, by combining Comparative Examples 5-6 and Example 1, it can be further seen that even when a core-shell structure is constructed, the selection of the active metal components in the core and shell portions has a significant and important impact on the overall CO2-assisted alkane oxidative dehydrogenation performance of the catalyst.

[0075] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A catalyst for CO2-assisted oxidative dehydrogenation of low-carbon alkanes to olefins, characterized in that, It has a core-shell structure, wherein the core structure is composed of a metal active component A, a non-metallic auxiliary agent B and a molecular sieve; and the shell structure is composed of a metal active component C and an oxide support D. Metal active component A is one or more of iron, cobalt, nickel, and zinc; non-metallic auxiliary agent B is one or more of fluorine, chlorine, bromine, and iodine; metal active component C is one or more of chromium, copper, and iron; oxide carrier D is one or more of cerium dioxide, aluminum oxide, magnesium oxide, indium oxide, and lanthanum oxide.

2. The catalyst according to claim 1, characterized in that, In the core structure, the active metal component A accounts for 0.1 wt% to 30 wt% of the core structure; the non-metallic auxiliary agent B accounts for 0.1 wt% to 10 wt% of the core structure; and the molecular sieve type is MFI.

3. The catalyst according to claim 1, characterized in that, In the shell structure, the active metal component C accounts for 0.1 wt% to 30 wt% of the weight of the shell structure portion.

4. A method for preparing the catalyst according to any one of claims 1 to 3, characterized in that, Includes the following steps: (1) Mix the soluble metal precursor I, silicon source, non-metallic auxiliary precursor, template agent and water evenly, and place them in a hydrothermal reactor for hydrothermal reaction for a period of time; The catalyst was then cooled to room temperature, filtered, washed, dried, and calcined to obtain the core structure portion of the catalyst. (2) The obtained catalyst core structure is dispersed in deionized water to form a dispersion solution; The oxide support precursor and precipitant were added to the dispersion solution, and the precipitate formed coated the outside of the catalyst core structure. The sample was then obtained by filtration, washing, drying and calcination. (3) The sample obtained above is dispersed again in deionized water, and soluble metal precursor II and precipitant are added to the solution. Then, the sample is filtered, washed, dried and calcined to form a catalyst with a core-shell structure.

5. The preparation method according to claim 4, characterized in that, In step (1), the soluble metal precursor I is one or more of the following: nitrate, acetate, sulfate, and chloride of a soluble metal; the non-metallic auxiliary precursor is one or more of the following: lithium salt, sodium salt, potassium salt, and ammonium salt containing halogen elements; the silicon source is one or more of the following: silicon dioxide, sodium silicate, propyl orthosilicate, hexamethyldisiloxane, ethyl orthosilicate, and isopropyl orthosilicate; and the template agent is one or more of the following: tetrapropylammonium hydroxide, tetramethylammonium hydroxide, and hexadecyltrimethylammonium bromide.

6. The preparation method according to claim 4, characterized in that, The oxide carrier precursor mentioned in step (2) is one or more of nitrate, acetate, sulfate, and chloride; the soluble metal precursor II mentioned in step (3) is one or more of soluble metal nitrate, acetate, sulfate, and chloride; the precipitant mentioned in step (2) or step (3) is one or more of ammonia, urea, sodium carbonate, and sodium hydroxide; the precipitant is one or more of ammonia, urea, sodium carbonate, and sodium hydroxide.

7. The preparation method according to claim 4, characterized in that, In steps (1), (2) or (3), the drying conditions are drying at 50-150℃ for 1-24 hours; the calcination conditions are calcination at 200-700℃ for 1-24 hours.

8. A method for CO2-assisted oxidative dehydrogenation of low-carbon alkanes to produce olefins, characterized in that, The method utilizes the catalyst described in any one of claims 1 to 3 for catalysis.

9. The method according to claim 8, characterized in that, The low-carbon alkane mentioned is a saturated alkane with 2-4 carbon atoms.

10. The method according to claim 8 or 9, characterized in that, The reaction conditions are: temperature 300-700℃, reaction pressure 0.1-2.0 MPa, reaction space velocity 1000-40000 mL / g / h; the volume fraction of CO2 in the reaction feedstock is 5-50%.

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