A vacant site anchoring all-silicon core-shell monatomic catalyst and a preparation method thereof

Abstract

The application belongs to the technical field of catalytic chemistry and porous materials, and specifically discloses a kind of vacancy anchoring full-silicon core-shell monatomic catalyst and its preparation method, the application makes Fe highly dispersed in the form of monatomic through the synergistic anchoring effect of silicon vacancy and ligand, avoids the metal oxide cluster produced by traditional impregnation method, inhibits excessive pyrolysis caused by local overheating, reduces the generation of gas impurities such as low-carbon alkane, the crystalline shell layer of the catalyst shortens the diffusion path of the product dimethyl ethylene ketone, DMK molecules can be quickly desorbed and leave the active center after generation, the content of by-product dimers is reduced, the structure of the catalyst improves the feed rate and accelerates the product diffusion, prevents the secondary reaction of reaction intermediates in the pore, greatly reduces the carbon deposition rate, forms a hydrophobic gradient layer by MTES modification, effectively repels the associated by-products isobutyric acid generated in the reaction, blocks the DMK polymerization reaction, thereby maintaining the yield of the target product DMK.

Description

Technical Field

[0001] This invention belongs to the field of catalytic chemistry and porous materials technology, and specifically discloses a vacancy-anchored all-silicon core-shell single-atom catalyst and its preparation method. Background Technology

[0002] Dimethyl ketene (DMK), a highly reactive fine chemical intermediate, has wide applications in the synthesis of high-performance polymers and in pharmaceuticals and pesticides. Currently, the gas-phase thermal decomposition of isobutyric anhydride (IBAN) is the main route for the industrial production of DMK. This reaction typically occurs at high temperatures, where a catalyst breaks the anhydride bond and removes one molecule of acid or water. Although conventional phosphate ester or supported metal oxide catalysts exhibit some activity initially, the high reaction temperature and the tendency of DMK molecules to self-polymerize limit the selectivity of traditional processes.

[0003] In existing technologies, catalysts prepared by traditional ion exchange or impregnation methods often exist in the form of metal oxide clusters or Fe-O-Fe polymers, resulting in uneven distribution of active centers and complex electronic structures. During continuous operation, active metal clusters are prone to high-temperature sintering or migration, which not only induces severe carbon deposition and coking, blocking molecular sieve channels, but also easily leads to local overheating and excessive pyrolysis, producing a large amount of impurities such as carbon monoxide, carbon dioxide, and low-carbon alkanes, which seriously shortens the service life of the catalyst.

[0004] While core-shell molecular sieves have been explored for improving catalytic stability, existing core-shell catalysts generally suffer from excessively thick shells (typically between 20-500 nm). This excessively thick shell significantly increases the mass transfer resistance between reactants entering the active site and products escaping, causing DMK molecules to remain within the sieve channels for too long. This makes them highly susceptible to addition reactions that generate byproducts such as dimers (TMCD). Furthermore, if the associated isobutyric acid generated during the reaction is not promptly removed, it can further catalyze the polymerization of DMK, leading to a decrease in the yield of the target product. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention aims to construct a core-shell molecular sieve catalyst with stable single-atom sites and excellent diffusion performance, overcoming the defects of excessively thick shells, Fe agglomeration, and by-product adsorption in existing technologies, thereby improving catalytic efficiency and selectivity.

[0006] The first aspect of the present invention proposes a vacancy-anchored all-silica core-shell single-atom catalyst, comprising a hierarchical all-silica Silicalite-1 core, iron atom active sites anchored to the all-silica Silicalite-1 core, a crystalline Silicalite-1 shell, and a hydrophobic gradient structure located on the outer surface of the shell.

[0007] In some specific embodiments of the vacancy-anchored all-silica core-shell single-atom catalyst described in the first aspect, the hierarchical all-silica Silicalite-1 nucleus is obtained by etching after a single crystallization. The etching method after the single crystallization includes preparing a single crystallization mother liquor by mixing tetraethyl orthosilicate as a silicon source, tetrapropylammonium hydroxide as a template agent, and ammonium fluoride as an etchant with water. After hydrothermal crystallization, the solid is mixed with an alkali and etched to obtain the hierarchical all-silica Silicalite-1 nucleus.

[0008] In some specific embodiments of the vacancy-anchored all-silicon core-shell single-atom catalyst described in the first aspect, the method for preparing the iron atom active sites anchored to the all-silicon Silicalite-1 core includes mixing the graded all-silicon Silicalite-1 core with an iron-ethanolamine complex precursor solution, drying, and calcining to obtain the graded all-silicon Silicalite-1 core with iron atom active sites anchored.

[0009] In some specific embodiments of the vacancy-anchored all-silica core-shell single-atom catalyst described in the first aspect, the method for preparing the crystalline Silicalite-1 shell includes mixing a hierarchical all-silica Silicalite-1 core anchored with iron atom active sites with a secondary crystallization mother liquor, and after the secondary crystallization reaction, obtaining a catalyst precursor that has undergone secondary crystallization, wherein the secondary crystallization mother liquor includes tetraethyl orthosilicate, tetrapropylammonium hydroxide, and water.

[0010] In some specific embodiments of the vacancy-anchored all-silicon core-shell single-atom catalyst described in the first aspect, the method for fabricating the hydrophobic gradient structure located on the outer surface of the shell includes mixing a catalyst precursor that has undergone secondary crystallization with an alcohol solution of methyltriethoxysilane to obtain a vacancy-anchored all-silicon core-shell single-atom catalyst.

[0011] A second aspect of this invention provides a method for preparing a vacancy-anchored all-silicon core-shell single-atom catalyst as described in any of the first aspects, comprising the following steps: Step 1: The mother liquor from the first crystallization process is subjected to hydrothermal crystallization to obtain a core sample. The core sample is then etched with an alkali to obtain a graded all-silica Silicalite-1 core. Step 2: Mix the graded all-silica Silicalite-1 core with Fe-containing... 3+ The sample was contacted with an ethanolamine solution, dried, and calcined to obtain an iron-loaded sample. Step 3: The iron-loaded sample is mixed with the secondary crystallization mother liquor. After the secondary crystallization reaction, the catalyst precursor after secondary crystallization is obtained. Step 4: The catalyst precursor that has undergone secondary crystallization is mixed with an alcoholic solution of methyltriethoxysilane and reacted to obtain a surface with a hydrophobic gradient structure, thus obtaining a vacancy-anchored all-silicon core-shell single-atom catalyst.

[0012] In some specific embodiments of the preparation method of the vacancy-anchored all-silicon core-shell single-atom catalyst described in the second aspect, the primary crystallization mother liquor in step 1 is subjected to hydrothermal crystallization at 170~180℃ for 70~80 hours to obtain a graded all-silicon Silicalite-1 core. The primary crystallization mother liquor, by molar amount, includes 0.8~1.2 parts of tetraethyl orthosilicate, 0.1~0.4 parts of tetrapropylammonium hydroxide, 0.08~0.12 parts of ammonium fluoride, and 28~32 parts of water.

[0013] In some specific embodiments of the preparation method of the vacancy-anchored all-silicon core-shell single-atom catalyst described in the second aspect, the etching of the core sample using alkali is carried out by using a 0.2 mol / L NaOH solution and etching the core sample for 25 to 35 minutes at a solid-liquid ratio of (0.8~1.2 g):(15~25 mL) to form uniform silicon vacancies. The etched sample is the graded all-silicon Silicalite-1 core.

[0014] In some specific embodiments of the preparation method of the vacancy-anchored all-silicon core-shell single-atom catalyst described in the second aspect, in step 2, Fe... 3+ The method for preparing the ethanolamine solution is as follows: dissolve ferric nitrate in water, slowly add ethanolamine dropwise, wherein the molar ratio of ethanolamine to ferric ions is 2:1 to 5:1, and the calcination method is to calcine at 300 to 400°C for 3 to 5 hours.

[0015] In some specific embodiments of the preparation method of the vacancy-anchored all-silicon core-shell single-atom catalyst described in the second aspect, the secondary crystallization reaction in step 3 is carried out by mixing the iron-supported sample with the secondary crystallization mother liquor and performing an in-situ secondary crystallization reaction at 170~180℃ for 3~5 hours. The secondary crystallization mother liquor, by molar amount, includes 0.8~1.2 parts of tetraethyl orthosilicate, 0.05~0.09 parts of tetrapropylammonium hydroxide, and 30~40 parts of water.

[0016] In some specific embodiments of the preparation method of the vacancy-anchored all-silicon core-shell single-atom catalyst described in the second aspect, the alcohol in step 4 is selected from any one or a mixture of methanol, ethanol, and isopropanol; the solid-liquid ratio of the secondary crystallized catalyst precursor to the alcohol solution of methyltriethoxysilane is (0.8~1.2g):(10~20mL).

[0017] The third aspect of the present invention provides a method for preparing dimethyl ketene by catalytic cracking of isobutyric anhydride, comprising contacting isobutyric anhydride raw material with the catalyst described in any one of the first aspects or the catalyst prepared by the method described in any one of the second aspects under catalytic cracking conditions.

[0018]

[0019] After 100 hours of reaction, unreacted isobutyric anhydride, associated isobutyric acid, and a small amount of DMK oligomers remain on the surface and in the pores of the catalyst. To eliminate the interference of these non-carbon-depositing components on the detection results, the catalyst sample recovered from the fixed-bed reactor should be immediately immersed in anhydrous ethanol and ultrasonically washed three times, and then vacuum dried at 100°C for 2 hours to completely remove the physically adsorbed light organic matter.

[0020] Thermogravimetric analysis (TGA) was used for testing in an air atmosphere: 100 mg of dried sample was placed in an alumina crucible, the air flow rate was set to 50~100 mL / min, and the temperature program was as follows: from room temperature to 800℃ at a heating rate of 10℃ / min. The mass loss between 400℃ and 700℃ in the TGA curve was taken as the total cumulative carbon deposition in mg over 100 hours.

[0021] This invention relates to the fields of catalytic chemistry and porous materials, and particularly to a core-shell molecular sieve catalyst, specifically a molecular sieve catalyst with a hierarchical all-silica Silicalite-1 core + vacancy-anchored Fe single atom + ultrathin crystalline shell (≤5 nm) + hydrophobic gradient structure on the shell surface, and its preparation method. This catalyst is suitable for highly selective oxidative cracking reactions such as the pyrolysis of isobutyric anhydride to prepare dimethyl ketene (DMK).

[0022] The hierarchical all-silica Silicalite-1 nucleus anchored with iron atom active sites in this invention is also known as an iron-loaded sample.

[0023] Advantages of this invention: This invention utilizes the synergistic anchoring effect of silicon vacancies and ligands to highly disperse Fe in single-atom form. Fe atoms are anchored at silicon vacancies via Si-O-Fe-O-Si bonds. This method stably avoids the metal oxide clusters produced by traditional impregnation methods, effectively suppressing excessive pyrolysis caused by local overheating and reducing the generation of gaseous impurities such as carbon monoxide, carbon dioxide, and low-carbon alkanes (e.g., propane and methane). The in-situ secondary crystallization reaction for 4 hours resulted in the catalyst producing no more than 5... The nm-sized crystalline shell shortens the diffusion path of the product dimethyl ketene (DMK). After DMK molecules are generated, they can quickly desorb and leave the active center, effectively avoiding the addition reaction that occurs due to excessive residence time in the molecular sieve channels. This reduces the content of the byproduct dimer (TMCD). The catalyst core has a mesoporous-microporous multi-level structure, which improves the feedstock entry rate and accelerates product diffusion. It prevents secondary reactions of reaction intermediates in the channels, alleviates carbon deposition and coking, and significantly reduces the carbon deposition rate. Through MTES modification, a hydrophobic gradient layer with a contact angle of 110-130° is formed, which effectively repels the associated byproduct isobutyric acid generated in the reaction and blocks the DMK polymerization reaction, thereby maintaining the yield of the target product DMK. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the catalyst structure in Example 1 of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1 Step 1: Using tetraethyl orthosilicate (TEOS) as the silicon source, tetrapropylammonium hydroxide (TPAOH) as the template agent, and ammonium fluoride (NH4F) as the etching agent, the etching process is carried out according to the following ratio: n(TEOS):n(TPAOH):n(F4F). - A primary crystallization mother solution was prepared with a molar ratio of n(H2O) = 1:0.3:0.1:30. Hydrothermal crystallization was carried out at 170~180℃ for 70~80 hours to obtain a nucleus sample. The sample was filtered, dried, and etched with 0.2mol / L NaOH solution at a solid-liquid ratio of 1g:20mL for 30 minutes to form silicon vacancies. The etched sample is the graded all-silica Silicalite-1 nucleus.

[0027] Step 2: Dissolve ferric nitrate in deionized water, and slowly add ethanolamine dropwise while stirring. When the final solution is reached, the molar ratio of ethanolamine to ferric ions is 2:1 to 5:1. Continue stirring to obtain a solution containing Fe. 3+ An ethanolamine solution (0.05 mol / L, with Fe) 3+ (Calculation). The etched sample was then placed in a container containing Fe. 3+ After being fully contacted with the ethanolamine solution, the sample is dried and calcined at 300-400°C for 3-5 hours under a nitrogen atmosphere to anchor iron atoms at silicon vacancies, forming iron atom active sites anchored at the silicon vacancies of the all-silicon Silicalite-1 core. These iron atom active sites are single-atom active sites, thus obtaining an iron-loaded sample.

[0028] Step 3: The above-mentioned iron-loaded sample was placed in a secondary crystallization mother liquor, which was prepared according to a molar ratio of n(TEOS):n(TPAOH):n(H2O)=1:0.07:35. The sample was subjected to an in-situ secondary crystallization reaction at 172°C for 4 hours to grow a Silicalite-1 ultrathin shell layer in a directed manner outside the nucleus, thus obtaining a catalyst precursor that had undergone secondary crystallization.

[0029] At room temperature, the secondary crystallized catalyst precursor was added to an ethanol solution containing 0.6 vol% methyltriethoxysilane (MTES) at a solid-liquid ratio of 1 g:15 mL and reacted for 1.25 hours to hydrophobically modify the shell surface, ultimately obtaining Cat-1, an all-silicon core-shell single-atom catalyst with vacancy anchoring.

[0030] Comparative Example 1 Compared to Example 1, Comparative Example 1 uses a conventional impregnation method instead of an iron-ethanolamine complex precursor solution. Steps 1 and 3 are the same. Step 2 of Comparative Example 1 is as follows: Step 2: Dissolve ferric nitrate in deionized water to obtain a ferric nitrate aqueous solution (0.05 mol / L, with Fe... 3+ The etched sample is brought into full contact with a 0.05 mol / L ferric nitrate aqueous solution, dried, and calcined at 300-400℃ for 3-5 hours to load iron species onto the etched sample, thus obtaining an iron-loaded sample.

[0031] The catalyst obtained in Comparative Example 1 was named Cat-2.

[0032] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 does not perform the NaOH etching process in step 1; the remaining steps are the same as in Example 1. Step 1 of Comparative Example 2 is specifically as follows: Step 1: Using tetraethyl orthosilicate (TEOS) as the silicon source, tetrapropylammonium hydroxide (TPAOH) as the template agent, and ammonium fluoride (NH4F) as the etching agent, the etching process is carried out according to the following ratio: n(TEOS):n(TPAOH):n(F4F). - The mother liquor for primary crystallization was prepared by mixing n(H2O) in a molar ratio of 1:0.3:0.1:30. Hydrothermal crystallization was carried out at 170~180℃ for 70~80 hours to obtain a fully silicate Silicalite-1 sample with a hierarchical pore structure as the core.

[0033] Iron was loaded onto the all-silica Silicalite-1 sample in the manner described in step 2 of Example 1, and then the loaded sample was subjected to secondary crystallization and surface hydrophobic modification in the manner described in step 3 of Example 1 to obtain the catalyst Cat-3. Comparative Example 3 The difference between Comparative Example 3 and Example 1 lies in the duration of the secondary crystallization in step 3. In Comparative Example 3, the secondary crystallization time in step 3 is 24 hours, specifically: Step 3: Place the above-mentioned iron-loaded sample in a secondary crystallization mother liquor, which is prepared according to the molar ratio of n(TEOS):n(TPAOH):n(H2O)=1:0.07:35. The sample is subjected to an in-situ secondary crystallization reaction at 172°C for 24 hours to obtain a catalyst precursor that has undergone secondary crystallization.

[0034] At room temperature, the secondary crystallized catalyst precursor was added to an ethanol solution containing 0.6 vol% methyltriethoxysilane (MTES) at a solid-liquid ratio of 1 g:15 mL and reacted for 1.25 hours to perform hydrophobic modification on the shell surface, resulting in the final catalyst Cat-4.

[0035] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that NH4F etchant is not added in step 1 of Comparative Example 4, and a Silicalite-1 core with only conventional microporous structure is prepared. The remaining steps are the same as in Example 1. Specifically, step 1 of Comparative Example 4 is as follows: Step 1: Using tetraethyl orthosilicate (TEOS) as the silicon source and tetrapropylammonium hydroxide (TPAOH) as the template agent, prepare a primary crystallization mother solution with a molar ratio of n(TEOS):n(TPAOH):n(H2O)=1:0.3:30. Perform hydrothermal crystallization at 170~180℃ for 70~80 hours to obtain a pure silicon Silicalite-1 sample as the core. Use 0.2mol / L NaOH solution to etch the core sample for 30 minutes at a solid-liquid ratio of 1g:20mL to form uniform silicon vacancies, and obtain the etched sample.

[0036] Next, the etched sample was processed using steps 2 and 3 of Example 1 to finally obtain the catalyst Cat-5.

[0037] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that step 3 of Comparative Example 5 does not involve surface hydrophobicity modification. Step 3 of Comparative Example 5 specifically involves: Step 3: The iron-loaded sample was placed in a secondary crystallization mother liquor, which was prepared according to a molar ratio of n(TEOS):n(TPAOH):n(H2O) = 1:0.07:35. The sample was subjected to an in-situ secondary crystallization reaction at 172°C for 4 hours, resulting in the directional growth of a crystalline Silicalite-1 ultrathin shell with a thickness not exceeding 5 nm outside the nucleus, thus obtaining the catalyst Cat-6. Comparative Example 6 The difference between Comparative Example 6 and Example 1 lies in the duration of the secondary crystallization in step 3. In Comparative Example 3, the secondary crystallization time in step 3 is 2 hours, specifically: Step 3: Place the above-mentioned iron-loaded sample in a secondary crystallization mother liquor, which is prepared according to the molar ratio of n(TEOS):n(TPAOH):n(H2O)=1:0.07:35. The sample is subjected to an in-situ secondary crystallization reaction at 172°C for 2 hours to obtain a catalyst precursor that has undergone secondary crystallization.

[0038] At room temperature, the secondary crystallized catalyst precursor was added to an ethanol solution containing 0.6 vol% methyltriethoxysilane (MTES) at a solid-liquid ratio of 1 g:15 mL and reacted for 1.25 hours to perform hydrophobic modification on the shell surface, finally obtaining the catalyst Cat-7.

[0039] Example 2 The core-shell molecular sieve catalyst Cat-1 prepared in Example 1 of this invention and the comparative samples Cat-2 to Cat-6 prepared in Comparative Examples 1 to 6 were respectively placed in a fixed-bed reactor and reacted at a reaction temperature of 450°C and a mass hourly space velocity (WHSV) of 1.5 h⁻¹. -1 Catalytic cracking was performed under conditions where the volume ratio of raw material isobutyric anhydride to carrier gas nitrogen was 1:10. The selectivity (%) of the target product dimethyl ketene (DMK) was determined by gas chromatography, the conversion rate reduction (%) after 100 hours of continuous operation, and the carbon deposition rate (mg·g) after 100 hours of operation was calculated. -1 ·h -1 The test results are shown in Table 1: Table 1

[0040] The Cat-1 catalyst prepared in Example 1 exhibited a DMK selectivity of up to 98.5%, and the conversion rate decreased by only 1.2% after 100 hours of continuous operation, with a carbon deposition rate of 0.08 mg·g⁻¹. -1 ·h -1 The immersion method yielded Comparative Example 1 and Comparative Example 2 (without vacancy etching), compared to Example 1, showed a significant decrease in selectivity and a surge in carbon deposition rate to 0.5 mg·g. -1 ·h -1 In Comparative Example 3, the shell thickness exceeded 5 nm due to excessive secondary crystallization time, which limited mass transfer and diffusion. In Comparative Example 6, the insufficient crystallization time prevented the formation of a complete and continuous crystalline protective shell on the core surface, directly exposing some active sites and weakening the hydrophobic modification effect. The reactants and byproducts directly contacted the active sites over a large area, leading to a large number of side reactions. As a result, the product molecules remained on the catalyst surface due to lack of guidance, turning into carbon deposits that blocked the pores. In Comparative Example 4, the absence of NH4F etchant resulted in the lack of hierarchical pore structure, leading to a single diffusion path. In Comparative Example 5, the lack of hydrophobic modification could have excluded the byproduct isobutyric acid, increased DMK selectivity, and reduced the conversion rate reduction value after 100 hours.

[0041] By designing hierarchical all-silicon cores, vacancy-anchored single atoms, ultrathin shells, and hydrophobic gradient surfaces, the selectivity of the target product is significantly improved, while the occurrence of side reactions and coking is fundamentally reduced by shortening the diffusion path and changing the surface wettability.

[0042] Example 3 Catalysts Cat-1 to Cat-7 were placed in fixed-bed reactors, respectively, and the reaction was carried out at a temperature of 480℃ and a WHSV of 2.0 h⁻¹. -1 The catalytic cracking reaction was carried out with a volume ratio of isobutyric anhydride to carrier nitrogen of 1:10. After the reaction reached steady state after 10 hours of continuous operation, online sampling was performed from the gas phase pipeline after the condenser at the outlet of the fixed-bed reactor. The composition of the tail gas was detected using an online gas chromatograph equipped with a thermal conductivity detector (TCD, TDX-01 packed column) and a flame ionization detector (FID, using a polar quartz capillary column). During the experiment, nitrogen in the inlet gas was used as an internal standard component. The volume percentage concentration (V%) of gaseous impurities such as carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and propane (C3H8) in the tail gas was calculated and quantified. The detection results are shown in Table 2. Table 2

[0043] Catalyst Cat-1 exhibits a reduction in gaseous impurities, with a total gaseous byproduct content of only 0.23 V%, representing a reduction of approximately 23 times and 17 times compared to the conventional impregnation method catalyst Cat-2 (5.31 V%) and the vacancy-free Cat-3 (3.97 V%). The CO and CH4 produced by Cat-1 are only approximately 1 / 20 and 1 / 39 of the corresponding components in Cat-2, respectively. For catalysts Cat-4 with a shell thickness exceeding 5 nm or Cat-5 lacking hierarchical pores, the total impurities increase to 1.03 V% and 1.50 V%, respectively, approximately 4.5 times and 6.5 times that of the catalyst in Example 1. Furthermore, the impurity content of Cat-6 without hydrophobic modification is nearly 10 times higher than that of the present invention, at 2.26 V%, while Cat-7, with an incomplete shell formation, has a total impurity content more than 3.5 times that of the present invention.

Claims

1. A vacancy-anchored all-silicon core-shell single-atom catalyst, characterized in that, It includes a hierarchical all-silica Silicalite-1 core, iron atom active sites anchored to the all-silica Silicalite-1 core, a crystalline Silicalite-1 shell, and a hydrophobic gradient structure located on the outer surface of the shell.

2. The vacancy-anchored all-silicon core-shell single-atom catalyst according to claim 1, characterized in that, The graded all-silica Silicalite-1 nucleus is obtained by etching after a single crystallization. The etching method after the single crystallization includes preparing a single crystallization mother solution by mixing tetraethyl orthosilicate as a silicon source, tetrapropylammonium hydroxide as a template agent, and ammonium fluoride as an etchant with water. After hydrothermal crystallization, the solid is mixed with alkali and etched to obtain the graded all-silica Silicalite-1 nucleus.

3. The vacancy-anchored all-silicon core-shell single-atom catalyst according to claim 1, characterized in that, The method for preparing iron atom active sites anchored to the all-silicon Silicalite-1 nucleus includes mixing graded all-silicon Silicalite-1 nucleus with an iron-ethanolamine complex precursor solution, drying, and calcining to obtain graded all-silicon Silicalite-1 nucleus anchored with iron atom active sites.

4. The vacancy-anchored all-silicon core-shell single-atom catalyst according to claim 1, characterized in that, The method for preparing the crystalline Silicalite-1 shell includes mixing a hierarchical all-silica Silicalite-1 core anchored with iron atom active sites with a secondary crystallization mother liquor. After the secondary crystallization reaction, a catalyst precursor that has undergone secondary crystallization is obtained. The secondary crystallization mother liquor includes tetraethyl orthosilicate, tetrapropylammonium hydroxide, and water.

5. The vacancy-anchored all-silicon core-shell single-atom catalyst according to claim 1, characterized in that, The method for fabricating the hydrophobic gradient structure located on the outer surface of the shell includes mixing a catalyst precursor that has undergone secondary crystallization with an alcohol solution of methyltriethoxysilane to obtain a vacancy-anchored all-silicon core-shell single-atom catalyst.

6. A method for preparing a vacancy-anchored all-silicon core-shell single-atom catalyst, characterized in that, Includes the following steps: Step 1: The mother liquor from the first crystallization process is subjected to hydrothermal crystallization to obtain a core sample. The core sample is then etched with an alkali to obtain a graded all-silica Silicalite-1 core. Step 2: Mix the graded all-silica Silicalite-1 core with Fe-containing... 3+ The sample was contacted with an ethanolamine solution, dried, and calcined to obtain an iron-loaded sample. Step 3: The iron-loaded sample is mixed with the secondary crystallization mother liquor. After the secondary crystallization reaction, the catalyst precursor after secondary crystallization is obtained. Step 4: The catalyst precursor that has undergone secondary crystallization is mixed with an alcoholic solution of methyltriethoxysilane and reacted to obtain a surface with a hydrophobic gradient structure, thus obtaining a vacancy-anchored all-silicon core-shell single-atom catalyst.

7. The method for preparing a vacancy-anchored all-silicon core-shell single-atom catalyst according to claim 6, characterized in that, In step 1, the primary crystallization mother liquor is subjected to hydrothermal crystallization at 170-180°C for 70-80 hours to obtain a graded all-silica Silicalite-1 nucleus. The primary crystallization mother liquor, by molar amount, comprises 0.8-1.2 parts tetraethyl orthosilicate, 0.1-0.4 parts tetrapropylammonium hydroxide, 0.08-0.12 parts ammonium fluoride, and 28-32 parts water; and / or, the etching of the nucleus sample using alkali is performed by using a 0.2 mol / L NaOH solution at a solid-liquid ratio of (0.8-1.2 g):(15-25 mL) for 25-35 minutes to form uniform silicon vacancies. The etched sample is the graded all-silica Silicalite-1 nucleus.

8. The method for preparing the vacancy-anchored all-silicon core-shell single-atom catalyst according to claim 6, characterized in that, In step 2, Fe is present. 3+ The method for preparing the ethanolamine solution is as follows: ferric nitrate is dissolved in water, and ethanolamine is slowly added dropwise. The molar ratio of ethanolamine to iron ions is 2:1 to 5:

1. The calcination method is calcination at 300 to 400°C for 3 to 5 hours. And / or, the secondary crystallization reaction in step 3 is performed by mixing the iron-loaded sample with the secondary crystallization mother liquor and performing an in-situ secondary crystallization reaction at 170 to 180°C for 3 to 5 hours. The secondary crystallization mother liquor, by molar amount, includes 0.8 to 1.2 parts of tetraethyl orthosilicate, 0.05 to 0.09 parts of tetrapropylammonium hydroxide, and 30 to 40 parts of water.

9. The method for preparing the vacancy-anchored all-silicon core-shell single-atom catalyst according to claim 6, characterized in that, In step 4, the alcohol is selected from methanol, ethanol, isopropanol or a mixture thereof; the solid-liquid ratio of the secondary crystallized catalyst precursor to the alcohol solution of methyltriethoxysilane is (0.8~1.2g):(10~20mL).

10. A method for preparing dimethyl ketene using isobutyric anhydride catalytic cracking, characterized in that, The isobutyric anhydride feedstock is contacted with the catalyst according to any one of claims 1 to 5, or the catalyst prepared by any one of claims 6 to 9, under catalytic cracking conditions.

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