Molecular sieve confined metal catalysts having different axial dimensions and methods of making and using the same

Abstract

The application belongs to the technical field of catalyst preparation, and particularly relates to a molecular sieve confined metal catalyst with different axial sizes, a preparation method thereof and application. The preparation method of the molecular sieve confined metal catalyst with different axial sizes comprises the following steps: mixing a water-soluble metal source, an organic amine ligand, an additive, a silicon source, a template agent, an alkali metal compound and water, performing a hydrothermal crystallization reaction, and obtaining a solid-phase product; the additive comprises one or more of urea, ammonium fluoride, lysine, hydrofluoric acid, methanol, ethanol, propanol, butanol, benzyl alcohol, benzaldehyde, P123 and F127; the solid-phase product is calcined to obtain a molecular sieve confined metal precursor; and the molecular sieve confined metal precursor is subjected to a reduction reaction to obtain the molecular sieve confined metal catalyst with different axial sizes. The molecular sieve confined metal catalyst with different axial sizes provided by the application has high catalytic activity and good thermal stability under high-temperature (600-650 DEG C) conditions.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation technology, specifically relating to a molecular sieve confined metal catalyst with different axial dimensions, its preparation method, and its application. Background Technology

[0002] Low-carbon olefins are a crucial cornerstone of the modern petrochemical industry. With the development and application of shale gas, the direct dehydrogenation of low-carbon alkanes to produce low-carbon olefins has become an important technological process. In the process of producing low-carbon olefins from low-carbon alkanes through dehydrogenation, high temperature is a necessary condition for the CH bonds in the low-carbon alkane molecule, which are difficult to activate. Therefore, the catalysts for the dehydrogenation of low-carbon alkanes need to possess high-temperature stability and activity.

[0003] Currently, existing technologies typically employ an impregnation method to prepare dehydrogenation catalysts for low-carbon alkanes. This method involves first preparing a molecular sieve as a support, then impregnating the sieve in a metal precursor solution for sequential crystallization, calcination, and reduction treatments. The metal particles are then loaded at the pore openings of the molecular sieve using a siphon effect. However, in low-carbon alkane dehydrogenation catalysts prepared using this method, the metal particles are only distributed near the pore openings of the molecular sieve. Under high-temperature conditions (550–650 °C), these metal particles spontaneously migrate, agglomerate, and grow, thus reducing the activity and stability of the catalyst. For example, the industrially commonly used PtSn / Al₂O₃ catalyst deactivates rapidly and requires continuous regeneration, resulting in significant resource waste. Furthermore, existing technologies cannot control the particle morphology of the molecular sieve; they cannot influence the structure and particle size of the metal particles by controlling the pore structure of the molecular sieve. In summary, low-carbon alkane dehydrogenation catalysts prepared using existing technologies exhibit low catalytic activity and poor thermal stability under high-temperature conditions. Summary of the Invention

[0004] The purpose of this invention is to provide a molecular sieve-confined metal catalyst with different axial dimensions, its preparation method, and its application. The molecular sieve-confined metal catalyst with different axial dimensions provided by this invention exhibits high catalytic activity and good thermal stability under high temperature (600~650℃) conditions.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing molecular sieve-confined metal catalysts with different axial dimensions includes the following steps: A water-soluble metal source, an organic amine ligand, an additive, a silicon source, a template agent, an alkali metal compound, and water are mixed and subjected to a hydrothermal crystallization reaction to obtain a solid-phase product. The additive includes one or more of urea, ammonium fluoride, lysine, hydrofluoric acid, methanol, ethanol, propanol, butanol, benzyl alcohol, benzaldehyde, P123, and F127. The water-soluble metal source includes noble metals and auxiliary metals. The solid product was calcined to obtain a molecular sieve confined metal precursor. The molecular sieve-confined metal precursor was subjected to a reduction reaction to obtain molecular sieve-confined metal catalysts with different axial dimensions.

[0006] Preferably, the precious metal includes one or more of platinum, rhodium, ruthenium, and iridium; the auxiliary metal includes one or more of tin, gallium, cerium, cobalt, manganese, indium, and zinc; and the molar ratio of the precious metal to the auxiliary metal is 1:2 to 2000.

[0007] Preferably, the organic amine ligand comprises a first organic amine ligand and a second organic amine ligand, wherein the first organic amine ligand and the second organic amine ligand independently comprise one or more of ethylamine, ethylenediamine, propylenediamine, butylamine, cyclohexylamine, cyclohexylimine, hexamethylenediamine, and cyclohexanediamine; the molar ratio of the noble metal to the first organic amine ligand in the water-soluble noble metal source is 1:20~400; and the molar ratio of the auxiliary metal to the second organic amine ligand in the water-soluble auxiliary metal source is 1:20~400.

[0008] Preferably, the molar ratio of the noble metal to the silicon source in the water-soluble noble metal source is 0.0004~0.030:1, the molar ratio of the auxiliary metal to the silicon source in the water-soluble auxiliary metal source is 0.0004~0.030:1, the molar ratio of the silicon source to the additive is 1:0.1~10, the molar ratio of the silicon source to the template agent is 1:0.1~0.6, the molar ratio of the silicon source to the alkali metal compound is 1:0.1~0.5, and the molar ratio of the silicon source to water is 1:10~200.

[0009] Preferably, the hydrothermal crystallization reaction is carried out at a temperature of 90-200°C for 1-5 days.

[0010] Preferably, the calcination temperature is 300~800℃, and the time is 2~24h; the surface area of ​​the molecular sieve-confined metal precursor is 250~400m². 2 / g, with a particle size of 100nm~10μm, and morphology including hexagonal prism, octahedron, spherical, flake, strip, planar or layered.

[0011] Preferably, the temperature of the reduction reaction is 150~700℃ and the time is 1~8h; the reducing atmosphere of the reduction reaction is carbon monoxide or hydrogen.

[0012] The present invention provides molecular sieve confined metal catalysts with different axial dimensions prepared by the preparation method described above, including all-silica MFI molecular sieves and noble metals and auxiliary metals supported in the channels of the all-silica MFI molecular sieves; the loading amount of the noble metals is 0.001~4wt%; the loading amount of the auxiliary metals is 0.01~5wt%.

[0013] This invention provides the application of molecular sieve-confined metal catalysts with different axial dimensions as described above in the dehydrogenation of low-carbon alkanes.

[0014] Preferably, the method for catalyzing the dehydrogenation of low-carbon alkanes using molecular sieve-confined metal catalysts with different axial dimensions includes the following steps: In an auxiliary atmosphere, low-carbon alkanes are dehydrogenated in the presence of molecular sieve-confined metal catalysts with different axial dimensions to obtain low-carbon olefins. The auxiliary atmosphere includes one or more of nitrogen, hydrogen, and carbon dioxide, and the molar ratio of the auxiliary atmosphere to the low-carbon alkane is 1~100:1; the mass hourly space velocity of the low-carbon alkane is 1~200 h⁻¹. -1 The dehydrogenation reaction is carried out at a temperature of 450~700℃.

[0015] This invention provides a method for preparing molecular sieve-confined metal catalysts with different axial dimensions. By introducing different types of additives, the molecular sieve is induced to form molecular sieve particles of specific sizes along a specific axis. Simultaneously, the auxiliary metal and noble metal are confined within the molecular sieve channels. The preparation method provided by this invention simultaneously forms the active metal component in situ while forming the molecular sieve. The metal can be loaded within the channels inside the molecular sieve, and the specific molecular sieve morphology restricts the position and structure of the metal within the molecular sieve channels, ensuring that the metal is uniformly dispersed within the molecular sieve channels and does not agglomerate due to migration.

[0016] Compared with the traditional impregnation method, the present invention causes the auxiliary metal and the noble metal to have different positions within the molecular sieve channels due to the difference in the morphology of the molecular sieve particles. During the reduction reaction, the auxiliary metal migrates within the channels when it is reduced to zero-valent metal. Due to the specific axial length change of the molecular sieve, the migration rate and position of the auxiliary metal change, thereby precisely controlling the interaction between the auxiliary metal and the noble metal at similar positions to form specific alloy clusters. Due to the confinement effect of the molecular sieve channels themselves, these alloy clusters cannot grow further within the channels. At this time, the alloy particles exist stably at specific channel positions.

[0017] This invention utilizes molecular sieve-confined metal catalysts with different axial dimensions to catalyze the dehydrogenation of low-carbon alkanes. After the dehydrogenation catalytic reaction of low-carbon alkanes, the promoter metal and noble metal are still uniformly and highly dispersed in the pores of the molecular sieve due to the confinement effect of the pores, thereby improving the dehydrogenation activity and thermal stability of the catalyst. Attached Figure Description

[0018] Figure 1Transmission electron microscope image of the sheet-like MFI molecular sieve confined metal precursor prepared in Example 1; Figure 2 Scanning electron microscope image of the chain-shaped MFI molecular sieve confined metal precursor prepared in Example 2; Figure 3 Scanning electron microscope image of the planar MFI molecular sieve confined metal precursor prepared in Example 3; Figure 4 This is a scanning electron microscope image of the layered MFI molecular sieve confined metal precursor prepared in Example 4; Figure 5 The molecular sieve-confined metal catalyst prepared in Example 1 was subjected to oxidation at 600°C and a mass hourly space velocity (H₂S) of 4 h⁻¹. -1 Catalytic performance diagram of ethane dehydrogenation under certain conditions; Figure 6 The molecular sieve confined metal catalyst prepared in Example 1 was subjected to oxidation at 620°C and a mass hourly space velocity (H₂S) of 10 h⁻¹. -1 Catalytic performance diagram of ethane dehydrogenation under certain conditions; Figure 7 The molecular sieve-confined metal catalyst prepared in Example 1 was subjected to oxidation at 600°C and a mass hourly space velocity (H₂S) of 4 h⁻¹. -1 Catalytic performance of ethane dehydrogenation under the given conditions of 300 h; Figure 8 The molecular sieve-confined metal catalyst prepared in Example 2 was subjected to oxidation at 560 °C and a mass hourly space velocity (H₂S) of 3.6 h⁻¹. -1 Catalytic performance diagram of propane dehydrogenation under certain conditions; Figure 9 The molecular sieve-confined metal catalyst prepared in Example 3 was subjected to oxidation at 580 °C and a mass hourly space velocity (H₂Sv) of 3.6 h⁻¹. -1 Catalytic performance diagram of propane dehydrogenation under certain conditions; Figure 10 The molecular sieve-confined metal catalyst prepared in Example 4 was subjected to oxidation at 600 °C and a mass hourly space velocity (H₂S) of 3.6 h⁻¹. -1 Catalytic performance diagram of propane dehydrogenation under certain conditions; Figure 11 The molecular sieve-supported metal catalyst prepared for Comparative Example 1 was subjected to oxidation at 550 °C and a mass hourly space velocity (H₂S) of 3.6 h⁻¹. -1 Catalytic performance of propane dehydrogenation under certain conditions. Detailed Implementation

[0019] This invention provides a method for preparing molecular sieve-confined metal catalysts with different axial dimensions, comprising the following steps: A water-soluble metal source, an organic amine ligand, an additive, a silicon source, a template agent, an alkali metal compound, and water are mixed and subjected to a hydrothermal crystallization reaction to obtain a solid-phase product. The additive includes one or more of urea, ammonium fluoride, lysine, hydrofluoric acid, methanol, ethanol, propanol, butanol, benzyl alcohol, benzaldehyde, P123, and F127. The water-soluble metal source includes noble metals and auxiliary metals. The solid product was calcined to obtain a molecular sieve confined metal precursor. The molecular sieve-confined metal precursor was subjected to a reduction reaction to obtain molecular sieve-confined metal catalysts with different axial dimensions.

[0020] In this invention, unless otherwise specified, all raw materials / components used in the preparation are commercially available products well known to those skilled in the art.

[0021] This invention involves mixing a water-soluble metal source, an organic amine ligand, additives, a silicon source, a template agent, an alkali metal compound, and water, and then performing a hydrothermal crystallization reaction to obtain a solid-phase product. The additives include one or more of urea, ammonium fluoride, lysine, hydrofluoric acid, methanol, ethanol, propanol, butanol, benzyl alcohol, benzaldehyde, P123, and F127. During crystallization, the additives adsorb onto specific crystal faces of the molecular sieve, restricting the growth of those faces. This results in shorter molecular sieve particle sizes along the axial direction perpendicular to the crystal face, while other axial directions, unaffected by adsorbed molecules, gradually grow during crystallization. Using different additives alters the adsorption onto the molecular sieve crystal faces, leading to different growth rates along the molecular sieve axial directions and resulting in molecular sieve particles of different sizes along specific axial directions.

[0022] As one embodiment of the present invention, mixing water-soluble metal source, organic amine ligand, additive, silicon source, template agent, alkali metal compound and water includes: first mixing silicon source, template agent, alkali metal compound and water, and aging treatment to obtain molecular sieve synthesis liquid; second mixing the molecular sieve synthesis liquid, additive, water-soluble metal source, organic amine ligand and water to obtain precursor mixture.

[0023] In one embodiment of the present invention, the silicon source includes tetraethyl orthosilicate, tetramethyl orthosilicate, tetrapropyl orthosilicate, tetrabutyl orthosilicate, fumed silica, or silica sol.

[0024] In one embodiment of the present invention, the template agent includes one or more of tetrapropylammonium hydroxide, tetraethylammonium hydroxide, tetramethylammonium hydroxide, and tetrabutylammonium hydroxide. The molar ratio of the silicon source to the template agent is 1:0.1~0.6, specifically 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, or 1:0.6.

[0025] In one embodiment of the present invention, the alkali metal compound includes one or more of alkali metal hydroxides, alkali metal nitrates, alkali metal carbonates, and alkali metal chlorides; the alkali metal hydroxide includes one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide; the alkali metal nitrate includes one or more of lithium nitrate, sodium nitrate, potassium nitrate, and cesium nitrate; the alkali metal carbonate includes one or more of lithium carbonate, sodium carbonate, potassium carbonate, and cesium carbonate; the alkali metal chloride includes one or more of lithium chloride, sodium chloride, potassium chloride, and cesium chloride; the molar ratio of the silicon source to the alkali metal compound is 1:0.1~0.5, specifically 1:0.1, 1:0.2, 1:0.3, 1:0.4, or 1:0.5. The present invention regulates the dissolution rate of the silicon source by limiting the molar ratio of the silicon source to the alkali metal compound and the molar ratio of the silicon source to the template agent. When the content of the alkali metal compound and the template agent is high, the dissolution rate of the silicon source is faster, and vice versa.

[0026] In one embodiment of the present invention, the molar ratio of the silicon source to water is 1:10 to 200, specifically 1:10, 1:20, 1:100, or 1:200. The present invention ensures the complete dissolution of the silicon source, template agent, and alkali metal compound by limiting the molar ratio of the silicon source to water.

[0027] In one embodiment of the present invention, the first mixing includes mixing a template agent, an alkali metal compound, and water, followed by the dropwise addition of a silicon source; the template agent can be prepared as an aqueous solution, the mass fraction of which can be 25%, and the mass fraction of which in the molecular sieve synthesis solution can be 0.05-7%; the first mixing can be carried out under stirring, the stirring rate being 50-500 rpm, further being 200-400 rpm, specifically 250 rpm, 300 rpm, or 400 rpm; the time being 1-360 min, specifically 1 min, 5 min, 10 min, 60 min, 100 min, 120 min, 240 min, or 360 min; and the dropping rate of the silicon source being 0.5 mL / min.

[0028] In one embodiment of the present invention, the aging treatment can be carried out under stirring. The stirring rate can be 50-500 rpm, more specifically 200-400 rpm, and more specifically 250 rpm, 300 rpm, or 400 rpm. The aging treatment temperature can be 25-60℃, more specifically 25-35℃, and more specifically 25℃, 30℃, or 35℃. The time can be 5-720 min, specifically 5 min, 60 min, 100 min, 240 min, 360 min, 480 min, 600 min, or 720 min. The present invention can control the degree of hydrolysis of the silicon source by limiting the temperature and time of the aging treatment, thereby affecting its interaction with additives, determining the growth rate of molecular sieve crystals along different axes, and ultimately causing changes in the morphology of the molecular sieve.

[0029] After aging treatment to obtain the molecular sieve synthesis liquid, the molecular sieve synthesis liquid, additives, water-soluble metal source, organic amine ligand and water are mixed for a second time to obtain a precursor mixture; the water-soluble metal source includes noble metals and auxiliary metals. In one embodiment of the present invention, the water-soluble metal source includes a water-soluble noble metal source and a water-soluble auxiliary metal source. The noble metal in the water-soluble noble metal source includes one or more of platinum, rhodium, ruthenium, and iridium. The water-soluble noble metal source includes noble metal chlorocomplexic acid or noble metal chloride, specifically chloroplatinic acid, iridium trichloride, rhodium trichloride, or platinum trichloride. The auxiliary metal in the water-soluble auxiliary metal source includes one or more of tin, gallium, cerium, cobalt, manganese, indium, and zinc. The water-soluble auxiliary metal source includes auxiliary metal nitrates, auxiliary metal halide salts, auxiliary metal carbonates, auxiliary metal sulfates, or auxiliary metal organic salts, specifically zinc nitrate, indium nitrate, gallium nitrate, or cobalt chloride. The molar ratio of the noble metal in the water-soluble noble metal source to the auxiliary metal in the water-soluble auxiliary metal source is 1:2 to 2000, and more preferably 1:4 to 1000. This invention ensures that the metal can form single metal ions or complex ions and be uniformly distributed in the molecular sieve by limiting the types of metals and salts of water-soluble metal sources.

[0030] In one embodiment of the present invention, the molar ratio of the noble metal to the silicon source in the water-soluble noble metal source is 0.0004~0.030:1, specifically 0.0005:1, 0.009:1, 0.012:1, 0.016:1, 0.022:1, or 0.028:1. The present invention ensures sufficient substitution sites for the noble metal in the molecular sieve by limiting the molar ratio of the noble metal ions to the silicon source in the water-soluble noble metal source. The molar ratio of the auxiliary metal to the silicon source in the water-soluble auxiliary metal source is also 0.0004~0.030:1; specifically 0.0005:1, 0.009:1, 0.012:1, 0.016:1, 0.022:1, or 0.028:1. The present invention also ensures sufficient substitution sites for the auxiliary metal in the molecular sieve by limiting the molar ratio of the auxiliary metal ions to the silicon source in the water-soluble auxiliary metal source.

[0031] In one embodiment of the present invention, the organic amine ligand includes a first organic amine ligand and a second organic amine ligand. The first organic amine ligand includes one or more of ethylamine, ethylenediamine, propylenediamine, butylamine, cyclohexylamine, cyclohexylimine, hexamethylenediamine, and cyclohexanediamine. The present invention ensures the anchoring of noble metals through coordination by limiting the type of the first organic amine ligand, thus avoiding metal agglomeration. The molar ratio of the noble metal to the first organic amine ligand in the water-soluble noble metal source is 1:20~400, specifically 1:20, 1:50, 1:100, 1:200, 1:300, or 1:400. By limiting the molar ratio of noble metal ions to the first organic amine ligand in the water-soluble noble metal source, the present invention ensures coordination between the first organic amine ligand and the noble metal ions, thereby facilitating the introduction of the noble metal into the channels of the molecular sieve and improving the dispersibility and stability of the noble metal.

[0032] In one embodiment of the present invention, the second organic amine ligand includes one or more of ethylamine, ethylenediamine, propylenediamine, butylamine, cyclohexylamine, cyclohexylimine, hexamethylenediamine, and cyclohexanediamine. The present invention limits the type of the second organic amine ligand to ensure that the auxiliary metal can be anchored through coordination, thus avoiding metal agglomeration. The molar ratio of the auxiliary metal to the second organic amine ligand in the water-soluble auxiliary metal source is 1:20~400, specifically 1:20, 1:50, 1:100, 1:200, 1:300, or 1:400. By limiting the molar ratio of the auxiliary metal ion to the second organic amine ligand in the water-soluble auxiliary metal source, the present invention ensures coordination between the second organic amine ligand and the auxiliary metal ion. This not only facilitates the introduction of the auxiliary metal into the pores of the molecular sieve but also promotes interaction with noble metals, improving the dispersibility and stability of the auxiliary metal. Furthermore, it further promotes interaction with noble metals, increasing their dispersion and forming alloys.

[0033] In one embodiment of the present invention, the additive includes one or more of urea, ammonium fluoride, lysine, hydrofluoric acid, methanol, ethanol, propanol, butanol, benzyl alcohol, benzaldehyde, P123, and F127; the molar ratio of the silicon source to the additive is 1:0.1~10, further 1:0.5~7, specifically 1:0.5, 1:1, 1:3, 1:5, or 1:7. The present invention controls the morphology of molecular sieve particles by limiting the molar ratio of the additive to the silicon source, thereby obtaining molecular sieve particles with different axial dimensions. Furthermore, the morphology of the molecular sieve also affects the dispersion and structure formation of metal particles. By directly adding the additive to the molecular sieve synthesis solution, the present invention not only helps to induce the formation of molecular sieve particles with specific axial lengths, but also confines the metal particles to specific positions within the molecular sieve particles with specific morphologies, improving the anchoring degree of the metal particles at specific positions within the molecular sieve particles. Simultaneously, the auxiliary metal can undergo electronic effects with the noble metal to form an alloy phase, further inhibiting the migration and growth of noble metal particles and improving the thermal stability of the low-carbon alkane dehydrogenation catalyst. It can also regulate the diffusion behavior of molecules, enabling molecules to rapidly diffuse into the active sites of the pores and diffuse out of the pores, thereby enhancing the catalytic activity and stability of the catalyst.

[0034] The present invention does not have a special limitation on the amount of water used, as long as it is sufficient to dissolve the water-soluble metal source.

[0035] In one embodiment of the present invention, the second mixing includes first mixing the molecular sieve synthesis liquid, the water-soluble metal source, the organic amine ligand, and water, and then adding the additive; the second mixing can be carried out under stirring, the stirring rate can be 50~500 rpm, and the second mixing time is 0.5~6 h, specifically 0.5 h, 1 h, or 6 h; after the additive is added, stirring is carried out for 10~60 min, specifically 10 min, 30 min, or 60 min.

[0036] The molar ratio of silica, template agent, alkali metal compound, water and additives in the precursor mixture is 1:0.3:0.1:40:0.0017~4, specifically 1:0.3:0.1:40:0.0017, 1:0.3:0.1:40:1 or 1:0.3:0.1:40:4.

[0037] The resulting precursor mixture is subjected to a hydrothermal crystallization reaction to obtain a solid-phase product. In one embodiment of the invention, the temperature of the hydrothermal crystallization reaction is 90~200℃, specifically 100℃, 120℃, 150℃, 170℃, 190℃, or 200℃; the time is 1~5 days, specifically 1 day, 2 days, 3 days, 4 days, or 5 days. This invention, by limiting the temperature and time of the hydrothermal crystallization reaction, ensures the formation of crystal nuclei and the perfection of particle morphology for silicalite-1 molecular sieves, thereby regulating grain growth and assembly to drive the formation of molecular sieve particles with specific axial lengths.

[0038] In this invention, metal is introduced in situ during the hydrothermal crystallization reaction, simultaneously forming molecular sieve particles with different axial dimensions. This simplifies the preparation process and introduces metal while controlling the morphology, avoiding the adhesion of metal introduced in post-processing to the particle surface. This simultaneous anchoring of metal particles and changes in the molecular sieve morphology better confines the metal particles to specific locations. By regulating the diffusion behavior of molecules, the reaction is synergistically controlled, resulting in improved catalytic selectivity and stability.

[0039] In one embodiment of the present invention, after the hydrothermal crystallization reaction is completed, the process further includes: sequentially subjecting the hydrothermal crystallization product to solid-liquid separation, washing, and drying to obtain a solid product. The present invention does not specifically limit the method of solid-liquid separation; any solid-liquid separation method well-known in the art can be used. The washing includes washing the obtained solid until it is neutral; the present invention does not specifically limit the washing method. The drying temperature can be 60~150℃, more specifically 60~100℃, and the drying time can be 4~48h, more specifically 5~20h, and the drying time can be 6h, 10h, or 12h.

[0040] After obtaining the solid product, the present invention calcines the solid product to obtain a molecular sieve-confined metal precursor. In one embodiment of the present invention, the calcination temperature is 300~800℃, specifically 300℃, 400℃, 500℃, 600℃, 700℃, or 800℃; the time is 2~24h, specifically 4h, 8h, 12h, 16h, 20h, or 24h. The present invention, by limiting the calcination temperature and time, removes template agents, additives, activates molecular sieve channels and auxiliary metal active sites, while simultaneously enhancing the stability of the framework structure.

[0041] In one embodiment of the present invention, the surface area of ​​the molecular sieve-confined metal precursor is 250~400 m². 2 / g, specifically 260m 2 / g、270m 2 / g、310m 2 / g、340m 2 / g or 390m 2 / g. This invention provides more metal loading sites by limiting the surface area of ​​the confined metal precursor in the molecular sieve, allowing the active metal to be uniformly dispersed and avoiding aggregation, thereby improving catalytic activity.

[0042] The particle size of the molecular sieve-confined metal precursor is 100 nm to 10 μm, specifically 100 nm, 200 nm, 500 nm, 1 μm, 3 μm, 5 μm, 7 μm, or 9 μm. This invention controls the axial length of the molecular sieve by limiting the particle size of the molecular sieve-confined metal precursor, thereby regulating the position of metal particles in molecular sieve particles of different axial sizes. This specifically anchors the metal particles between the desired molecular sieve channels or crystals, preventing metal particle aggregation and improving metal dispersion. The morphology of the molecular sieve-confined metal precursor can be hexagonal prism, octahedral, spherical, sheet-like, elongated, planar, or layered.

[0043] After obtaining the molecular sieve-confined metal precursor, this invention performs a reduction reaction on the molecular sieve-confined metal precursor to obtain molecular sieve-confined metal catalysts with different axial dimensions. In one embodiment of this invention, the reduction reaction temperature is 150~700℃, specifically 180℃, 250℃, 350℃, 450℃, 500℃, 550℃, or 600℃; the time is 1~8h, specifically 2h, 3h, 4h, 5h, or 6h; the reduction atmosphere is carbon monoxide or hydrogen. This invention reduces noble metals and auxiliary metals to elemental metals by limiting the temperature and time of the reduction reaction. During the reduction process, the two interact, causing electrons to transfer from the auxiliary metal to the noble metal, forming active sites in the alloy phase, promoting the activation of CH bonds, and thus enhancing the catalytic activity of the catalyst.

[0044] This invention provides molecular sieve-confined metal catalysts with different axial dimensions prepared by the preparation method described above, comprising an all-silica MFI molecular sieve and noble metals and auxiliary metals supported within the pores of the all-silica MFI molecular sieve; the loading amount of the noble metal is 0.001~4 wt%; the loading amount of the auxiliary metal is 0.01~5 wt%. In one embodiment of this invention, the all-silica MFI molecular sieve is a silicalite-1 molecular sieve; the noble metal can be iridium, rhodium, or platinum, and the loading amount of the noble metal element is 0.001~4 wt%, specifically 0.001 wt%, 0.04 wt%, 0.1 wt%, 0.3 wt%, 0.5 wt%, 0.7 wt%, 1 wt%, 2 wt%, 3 wt%, or 4 wt%. This invention improves dispersibility and thus catalytic performance by limiting the loading amount of the noble metal element to ensure uniform dispersion at specific positions within the molecular sieve pores. The additive metal can be one or more selected from tin, gallium, cerium, cobalt, manganese, indium, and zinc. The loading amount of the additive metal element is 0.01~5wt%, specifically 0.01wt%, 0.05wt%, 0.1wt%, 0.8wt%, 1wt%, 2wt%, 3wt%, 4wt%, or 5wt%. This invention ensures uniform dispersion within the molecular sieve channels by limiting the loading amount of the additive metal element, preventing agglomeration and damage to the molecular sieve channels, and improving catalytic performance.

[0045] This invention improves catalytic performance by defining specific metal types to create effective active sites. The noble metal and the promoter metal can partially form an alloy phase. In this invention, the promoter metal can interact electronically with the noble metal to form an alloy phase, inhibiting the migration and growth of noble metal particles and improving the thermal stability of the low-carbon alkane dehydrogenation catalyst.

[0046] This invention provides the application of molecular sieve-confined metal catalysts with different axial dimensions as described above in the dehydrogenation of low-carbon alkanes.

[0047] As one embodiment of the present invention, the method for catalyzing the dehydrogenation of low-carbon alkanes using molecular sieve-confined metal catalysts with different axial dimensions includes the following steps: In an auxiliary atmosphere, low-carbon alkanes are dehydrogenated in the presence of molecular sieve-confined metal catalysts with different axial dimensions to obtain low-carbon olefins. In one embodiment of the present invention, the auxiliary atmosphere includes one or more of nitrogen, hydrogen, and carbon dioxide, and the molar ratio of the auxiliary atmosphere to the low-carbon alkane is 1 to 100:1, specifically 1:1, 10:1, 20:1, 50:1, 80:1, or 100:1; the mass hourly space velocity of the low-carbon alkane is 1 to 200 h⁻¹. -1The dehydrogenation reaction is carried out at a temperature of 450~700℃.

[0048] To further illustrate the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments thereof. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0049] Example 1 12.51 g of a 25% tetrapropylammonium hydroxide aqueous solution (0.015 mol of tetrapropylammonium hydroxide) and 0.29 g of sodium chloride (0.005 mol) were added dropwise to deionized water (27.59 g, 1.53 mol). The mixture was stirred at 300 rpm for 2 h at room temperature. Then, 10.4 g of tetraethyl orthosilicate (0.05 mol) was added dropwise, and stirring was continued for 4 h to obtain the molecular sieve synthesis solution. A metal precursor was obtained by mixing 0.10 g of zinc nitrate, 0.2 mL of a 0.0051 mol / L chloroplatinic acid solution, 2 mL of ethylenediamine, and 10 mL of deionized water. The molecular sieve synthesis solution was prepared by adding the metal precursor solution to the molecular sieve synthesis solution and stirring for 0.5 h. Then, urea (3 g, 0.05 mol) was added and stirred for 1 h to obtain a precursor mixture. The molar ratio of each component in the precursor mixture was: SiO2:TPAOH:NaCl:H2O:CH4N2O=1:0.3:0.1:40:1. The precursor mixture was transferred to a high-pressure hydrothermal synthesis reactor and crystallized at 170 °C for 72 h. The resulting solid was taken out and washed until neutral, dried in an oven at 60 °C for 10 h, and then calcined at 550 °C for 10 h to obtain a plate-like MFI molecular sieve confined metal precursor. Figure 1 This is a transmission electron microscope image of the sheet-like MFI molecular sieve confined metal precursor prepared in Example 1, based on... Figure 1 The specific surface area of ​​the plate-like MFI molecular sieve confined metal precursor can be determined to be 280 m². 2 / g.

[0050] The sheet-like MFI molecular sieve confined metal precursor was sequentially compressed, pulverized, and sieved. The particles after sieving through a 20-40 mesh were subjected to a reduction reaction at 500°C for 4 hours under a hydrogen atmosphere to obtain a molecular sieve confined metal catalyst, denoted as PtZn@silicalite-1-sheet; wherein the loading of Zn was 0.1% and the loading of Pt was 0.35%.

[0051] Example 2 12.51 g of a 25% tetrapropylammonium hydroxide aqueous solution (0.015 mol of tetrapropylammonium hydroxide) and 0.20 g of sodium hydroxide (0.005 mol) were added dropwise to deionized water (27.59 g, 1.53 mol). The mixture was stirred at 250 rpm for 2 h at 35 °C. Then, 10.4 g of tetraethyl orthosilicate (0.05 mol) was added dropwise, and stirring was continued for 4 h to obtain the molecular sieve synthesis solution. 0.08 g of indium nitrate, 0.1 mL of a 0.0051 mol / L chloroplatinic acid solution, 2 mL of ethylenediamine, and 10 mL of deionized water were mixed to obtain the metal precursor solution. The solution was prepared by adding the metal precursor solution to the molecular sieve synthesis solution and stirring for 0.5 h. Then, benzyl alcohol (21.6 g, 0.2 mol) was added dropwise and stirred for 10 min to obtain a precursor mixture. The molar ratio of each component in the precursor mixture was: SiO2:TPAOH:NaOH:H2O:C7H9O=1:0.3:0.1:40:4. The precursor mixture was transferred to a high-pressure hydrothermal synthesis reactor and crystallized at 150 °C for 96 h. The resulting solid was taken out and washed until neutral, dried in an oven at 80 °C for 12 h, and then calcined at 550 °C for 8 h to obtain the chain-shaped MFI molecular sieve confined metal precursor. Figure 2 This is a scanning electron microscope image of the chain-like MFI molecular sieve-confined metal precursor prepared in Example 2, based on... Figure 2 The specific surface area of ​​the chain-shaped MFI molecular sieve confined metal precursor can be determined to be 290 m². 2 / g.

[0052] The chain-shaped MFI molecular sieve confined metal precursor was sequentially compressed, pulverized, and sieved. The particles after 20-40 mesh sieve were reduced at 600℃ for 2 hours under a hydrogen atmosphere to obtain a molecular sieve confined metal catalyst, denoted as PtIn@silicalite-1-chain; wherein the loading of In was 0.4% and the loading of Pt was 0.2%.

[0053] Example 3 12.51 g of a 25% tetrapropylammonium hydroxide aqueous solution (0.015 mol of tetrapropylammonium hydroxide) and 0.50 g of potassium nitrate (0.005 mol) were added dropwise to deionized water (27.59 g, 1.53 mol). The mixture was stirred at 400 rpm for 4 h at 30 °C. Then, 10.4 g of tetraethyl orthosilicate (0.05 mol) was added dropwise, and stirring was continued for 6 h to obtain a molecular sieve synthesis solution. A metal precursor solution was obtained by mixing 0.08 g of gallium nitrate, 0.0016 g of iridium trichloride, 2 mL of ethylenediamine, and 10 mL of deionized water. Then, the molecular sieve synthesis solution was prepared. The metal precursor solution was added to the sieve synthesis solution and stirred for 1 h; then benzaldehyde (21.19 g, 0.2 mol) was added dropwise and stirred for 0.5 h to obtain a precursor mixture. The molar ratio of each component in the precursor mixture was: SiO2:TPAOH:KNO3:H2O:C7H7O=1:0.3:0.1:40:4. The precursor mixture was transferred to a high-pressure hydrothermal synthesis reactor and crystallized at 180℃ for 120 h. The resulting solid was taken out and washed until neutral, dried in an oven at 100℃ for 12 h, and then calcined at 600℃ for 10 h to obtain a planar MFI molecular sieve confined metal precursor. Figure 3 This is a scanning electron microscope image of the planar MFI molecular sieve-confined metal precursor prepared in Example 3, based on... Figure 3 The specific surface area of ​​the planar MFI molecular sieve confined metal precursor is 300 m². 2 / g.

[0054] The planar MFI molecular sieve confined metal precursor was sequentially pressed, pulverized, and sieved. The particles after sieving through a 20-40 mesh were reduced at 500°C for 6 hours in a hydrogen atmosphere to obtain a molecular sieve confined metal catalyst, denoted as IrGa@silicalite-1-planar type; wherein the loading of Ga was 0.6% and the loading of Ir was 0.2%.

[0055] Example 4 9.07 g of a 25% tetraethylammonium hydroxide aqueous solution (0.015 mol of tetraethylammonium hydroxide) and 0.53 g of sodium carbonate (0.005 mol) were added dropwise to deionized water (27.59 g, 1.53 mol). The mixture was stirred at 300 rpm for 1 h at 25 °C. Then, 10.4 g of tetraethyl orthosilicate (0.05 mol) was added dropwise, and stirring was continued for 10 h to obtain the molecular sieve synthesis solution. A metal precursor solution was obtained by mixing 0.07 g of cobalt chloride, 0.0020 g of rhodium trichloride, 2 mL of ethylenediamine, and 10 mL of deionized water. Then, the solution was added to the molecular sieve synthesis solution. The metal precursor solution was added to the liquid and stirred for 6 hours; then P123 (0.5 g, 0.000085 mol) was added and stirred for 20 minutes to obtain a precursor mixture. The molar ratio of each component in the precursor mixture was: SiO2:TPAOH:Na2CO3:H2O:C7H7O=1:0.3:0.1:40:0.0017. The precursor mixture was transferred to a high-pressure hydrothermal synthesis reactor and crystallized at 200℃ for 24 hours. The resulting solid was taken out and washed until neutral, dried in an oven at 80℃ for 6 hours, and then calcined at 500℃ for 6 hours to obtain a layered MFI molecular sieve confined metal precursor. Figure 4 This is a scanning electron microscope image of the layered MFI molecular sieve confined metal precursor prepared in Example 4, based on... Figure 4 The specific surface area of ​​the layered MFI molecular sieve confined metal precursor can be determined to be 310 m². 2 / g.

[0056] The layered MFI molecular sieve confined metal precursor was sequentially tableted, pulverized, and sieved. The particles after 20-40 mesh sieve were reduced at 500℃ for 6 hours in a hydrogen atmosphere to obtain a molecular sieve confined metal catalyst, denoted as RhCo@silicalite-1-layered; wherein the loading of Co was 1% and the loading of Rh was 0.55%.

[0057] Comparative Example 1 12.51 g of a 25% tetrapropylammonium hydroxide aqueous solution (0.0015 mol of tetrapropylammonium hydroxide) was mixed with 27.59 g (2 mol) of deionized water and stirred at 500 rpm for 2 h at 25 °C. Then, 10.7 g (0.05 mol) of tetraethyl orthosilicate was added dropwise and the mixture was stirred and aged at room temperature for 6 h to obtain an initial gel. The molar ratio of tetrapropylammonium hydroxide: tetraethyl orthosilicate: water in the initial gel was 0.3:1:40. The initial gel was added to a 100 mL high-pressure reactor and hydrothermally crystallized at 200 °C for 120 h. After removal, the product was washed with water and centrifuged three times. The resulting solid product was dried at 100 °C for 12 h and then calcined in an air atmosphere in a muffle furnace at 550 °C for 10 h to obtain silicalite-1 spherical molecular sieves.

[0058] 2.5 mL of 0.5123 M chloroplatinic acid solution was added to a mixture of 2.5 mL deionized water and 1.5 mL ethylenediamine to obtain a platinum precursor solution. 0.08 g of indium nitrate was added to a mixture of 5 mL deionized water and 0.1 mL ethylenediamine to obtain an indium precursor solution. The platinum and indium precursor solutions were added to 20 mL ethylenediamine, and then 1 g of silicalite-1 spherical molecular sieve was added for impregnation for 10 h. The resulting solid was dried at 60 °C for 12 h, and then calcined at 550 °C for 5 h in an air atmosphere in a muffle furnace. The resulting solid product was sequentially tableted, pulverized, and sieved. Particles sieved through a 20-40 mesh were reduced at 550 °C in a hydrogen atmosphere for 2 h to obtain a molecular sieve-supported metal catalyst, denoted as PtIn@S1 catalyst, wherein the mass fraction of Pt is 0.45% and the mass fraction of In is 0.50%.

[0059] Application Examples 1-6 The molecular sieve-confined metal catalysts prepared in Examples 1-4 were placed in continuous flow fixed-bed microreactors for catalyzing the dehydrogenation reaction of low-carbon alkanes (propane or ethane). The reaction conditions for each application example are as follows: Application Example 1: The molecular sieve-confined metal catalyst prepared in Example 1 was used. The loading amount of the molecular sieve-confined metal catalyst was 0.3 g. The conditions were: atmospheric pressure, temperature 600 °C, and ethane mass hourly space velocity (WHSV) of 4 h⁻¹. -1 The volume ratio of nitrogen to ethane was 3:1, the reaction time was 51.8 h, and the catalytic results were as follows: Figure 5 As shown. Figure 5 The molecular sieve-confined metal catalyst prepared in Example 1 was subjected to oxidation at 600°C and a mass hourly space velocity (H₂S) of 4 h⁻¹. -1 Catalytic performance diagram of ethane dehydrogenation under certain conditions; based on Figure 5It can be seen that, under the conditions of Example 1, the ethane conversion rate of the molecular sieve confined metal catalyst prepared in Example 1 is 25%, and the ethylene selectivity is 99%, indicating that the molecular sieve confined metal catalyst prepared in Example 1 of the present invention has good thermal stability.

[0060] Application Example 2: The molecular sieve-confined metal catalyst prepared in Example 1 was used. The loading amount of the molecular sieve-confined metal catalyst was 0.3 g. The conditions were: atmospheric pressure, temperature 620 °C, and ethane mass hourly space velocity (WHSV) of 10 h⁻¹. -1 The volume ratio of nitrogen to ethane was 3:1, the reaction time was 47.1 h, and the catalytic results were as follows: Figure 6 As shown. Figure 6 The molecular sieve confined metal catalyst prepared in Example 1 was subjected to oxidation at 620°C and a mass hourly space velocity (H₂S) of 10 h⁻¹. -1 Catalytic performance diagram of ethane dehydrogenation under certain conditions; based on Figure 6 It can be seen that, under the conditions of Example 2, the ethane conversion rate of the molecular sieve confined metal catalyst prepared in Example 1 is 29%, and the ethylene selectivity is 99%, indicating that the molecular sieve confined metal catalyst prepared in Example 1 of the present invention has good thermal stability.

[0061] Application Example 3: The molecular sieve-confined metal catalyst prepared in Example 1 was used. The loading amount of the molecular sieve-confined metal catalyst was 0.3 g. The conditions were: atmospheric pressure, temperature 600 °C, and ethane mass hourly space velocity (WHSV) of 4 h⁻¹. -1 The volume ratio of nitrogen to ethane was 3:1, the reaction time was 300 h, and the catalytic results were as follows: Figure 7 As shown. Figure 7 The molecular sieve-confined metal catalyst prepared in Example 1 was subjected to oxidation at 600°C and a mass hourly space velocity (H₂S) of 4 h⁻¹. -1 Catalytic performance graph of ethane dehydrogenation under the given conditions of 300 h; based on Figure 7 It can be seen that, under the conditions of Example 3, the ethane conversion rate of the molecular sieve confined metal catalyst prepared in Example 1 is 25%, and the ethylene selectivity is 99%, indicating that the molecular sieve confined metal catalyst prepared in Example 1 of the present invention has good thermal stability.

[0062] Application Example 4: The molecular sieve-confined metal catalyst prepared in Example 2 was used. The loading amount of the molecular sieve-confined metal catalyst was 0.3 g. Under normal pressure conditions, the temperature was 560 °C, and the mass hourly space velocity of propane was 3.6 h⁻¹. -1 The volume ratio of nitrogen to propane was 3:1, the reaction time was 32.1 h, and the catalytic results were as follows: Figure 8 As shown. Figure 8 The molecular sieve-confined metal catalyst prepared in Example 2 was subjected to oxidation at 560 °C and a mass hourly space velocity (H₂S) of 3.6 h⁻¹. -1 Catalytic performance diagram of propane dehydrogenation under certain conditions; based on Figure 8It can be seen that, under the conditions of Example 4, the propane conversion rate of the molecular sieve confined metal catalyst prepared in Example 2 is 46%, and the propylene selectivity is 99%, indicating that the molecular sieve confined metal catalyst prepared in Example 2 of the present invention has good catalytic performance and thermal stability.

[0063] Application Example 5: The molecular sieve-confined metal catalyst prepared in Example 3 was used. The loading amount of the molecular sieve-confined metal catalyst was 0.3 g. Under normal pressure conditions, the temperature was 580 °C, and the mass hourly space velocity of propane was 3.6 h⁻¹. -1 The volume ratio of nitrogen to propane was 3:1, the reaction time was 44.3 h, and the catalytic results were as follows: Figure 9 As shown. Figure 9 The molecular sieve-confined metal catalyst prepared in Example 3 was subjected to oxidation at 580 °C and a mass hourly space velocity (H₂Sv) of 3.6 h⁻¹. -1 Catalytic performance diagram of propane dehydrogenation under certain conditions; based on Figure 9 It can be seen that, under the conditions of Example 5, the propane conversion rate of the molecular sieve confined metal catalyst prepared in Example 3 is 45%, and the propylene selectivity is 99%, indicating that the molecular sieve confined metal catalyst prepared in Example 3 of the present invention has good catalytic performance and thermal stability.

[0064] Application Example 6: The molecular sieve-confined metal catalyst prepared in Example 4 was used. The loading amount of the molecular sieve-confined metal catalyst was 0.3 g. Under normal pressure conditions, the temperature was 600 °C, and the mass hourly space velocity of propane was 3.6 h⁻¹. -1 The volume ratio of nitrogen to propane was 3:1, the reaction time was 23.1 h, and the catalytic results were as follows: Figure 10 As shown. Figure 10 The molecular sieve-confined metal catalyst prepared in Example 4 was subjected to oxidation at 600 °C and a mass hourly space velocity (H₂S) of 3.6 h⁻¹. -1 Catalytic performance diagram of propane dehydrogenation under certain conditions; based on Figure 10 It can be seen that, under the conditions of Example 6, the propane conversion rate of the molecular sieve confined metal catalyst prepared in Example 4 is 43%, and the propylene selectivity is 99%, indicating that the molecular sieve confined metal catalyst prepared in Example 4 of the present invention has good catalytic performance and thermal stability.

[0065] Comparative Application Example 1 The molecular sieve-supported metal catalyst prepared in Comparative Example 1 was placed in a continuous flow fixed-bed microreactor for catalyzing the dehydrogenation reaction of low-carbon alkanes (propane or ethane). The reaction conditions were: 0.3 g of molecular sieve-supported metal catalyst, atmospheric pressure, temperature of 550 °C, and a propane mass hourly space velocity of 3.6 h⁻¹. -1 The volume ratio of nitrogen to propane was 3:1, the reaction time was 11 hours, and the catalytic results were as follows: Figure 11 As shown.

[0066] Figure 11The molecular sieve-supported metal catalyst prepared for Comparative Example 1 was subjected to oxidation at 550 °C and a mass hourly space velocity (H₂S) of 3.6 h⁻¹. -1 Catalytic performance diagram of propane dehydrogenation under certain conditions; based on Figure 11 It can be seen that, compared with the propane conversion rate of the molecular sieve supported metal catalyst prepared in Comparative Example 1 under the conditions of Application Example 1, the selectivity of propylene is 99%, indicating that the catalytic performance and thermal stability of the molecular sieve supported metal catalyst prepared in Comparative Example 1 are poor.

[0067] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for preparing molecular sieve-confined metal catalysts with different axial dimensions, comprising the following steps: A water-soluble metal source, an organic amine ligand, an additive, a silicon source, a template agent, an alkali metal compound, and water are mixed and subjected to a hydrothermal crystallization reaction to obtain a solid-phase product. The additive includes one or more of urea, ammonium fluoride, lysine, hydrofluoric acid, methanol, ethanol, propanol, butanol, benzyl alcohol, benzaldehyde, P123, and F127. The water-soluble metal source includes noble metals and auxiliary metals. The solid product was calcined to obtain a molecular sieve confined metal precursor. The molecular sieve-confined metal precursor was subjected to a reduction reaction to obtain molecular sieve-confined metal catalysts with different axial dimensions.

2. The preparation method according to claim 1, characterized in that, The precious metal includes one or more of platinum, rhodium, ruthenium, and iridium; the auxiliary metal includes one or more of tin, gallium, cerium, cobalt, manganese, indium, and zinc; the molar ratio of the precious metal to the auxiliary metal is 1:2 to 2000.

3. The preparation method according to claim 1, characterized in that, The organic amine ligand includes a first organic amine ligand and a second organic amine ligand, wherein the first organic amine ligand and the second organic amine ligand independently include one or more of ethylamine, ethylenediamine, propylenediamine, butylamine, cyclohexylamine, cyclohexylimine, hexamethylenediamine, and cyclohexanediamine; the molar ratio of the noble metal to the first organic amine ligand in the water-soluble noble metal source is 1:20~400; the molar ratio of the auxiliary metal to the second organic amine ligand in the water-soluble auxiliary metal source is 1:20~400.

4. The preparation method according to claim 1, characterized in that, The molar ratio of the noble metal to the silicon source in the water-soluble noble metal source is 0.0004~0.030:1; the molar ratio of the auxiliary metal to the silicon source in the water-soluble auxiliary metal source is 0.0004~0.030:1; the molar ratio of the silicon source to the additive is 1:0.1~10; the molar ratio of the silicon source to the template agent is 1:0.1~0.6; the molar ratio of the silicon source to the alkali metal compound is 1:0.1~0.5; and the molar ratio of the silicon source to water is 1:10~200.

5. The preparation method according to claim 1, characterized in that, The hydrothermal crystallization reaction is carried out at a temperature of 90~200℃ for 1~5 days.

6. The preparation method according to claim 1, characterized in that, The calcination temperature is 300~800℃, and the time is 2~24h; the surface area of ​​the molecular sieve-confined metal precursor is 250~400m². 2 / g, with a particle size of 100nm~10μm, and morphology including hexagonal prism, octahedron, spherical, flake, strip, planar or layered.

7. The preparation method according to claim 1, characterized in that, The reduction reaction is carried out at a temperature of 150~700℃ for 1~8 hours; the reducing atmosphere is carbon monoxide or hydrogen.

8. The molecular sieve confined metal catalyst with different axial dimensions prepared by the preparation method according to any one of claims 1 to 7 includes an all-silica MFI molecular sieve and a noble metal and an auxiliary metal supported in the pores of the all-silica MFI molecular sieve; the loading of the noble metal is 0.001 to 4 wt%; and the loading of the auxiliary metal is 0.01 to 5 wt%.

9. The application of the molecular sieve confined metal catalyst with different axial dimensions as described in claim 8 in the dehydrogenation of low-carbon alkanes.

10. The application according to claim 9, characterized in that, The method for catalyzing the dehydrogenation of low-carbon alkanes using molecular sieve-confined metal catalysts with different axial dimensions includes the following steps: In an auxiliary atmosphere, low-carbon alkanes are dehydrogenated in the presence of molecular sieve-confined metal catalysts with different axial dimensions to obtain low-carbon olefins. The auxiliary atmosphere includes one or more of nitrogen, hydrogen, and carbon dioxide, and the molar ratio of the auxiliary atmosphere to the low-carbon alkane is 1~100:1; the mass hourly space velocity of the low-carbon alkane is 1~200 h⁻¹. -1 The dehydrogenation reaction is carried out at a temperature of 450~700℃.

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