A silicalite-1 molecular sieve confined monometallic Pt catalyst, a preparation method and application thereof

Abstract

This invention discloses a Pt@S-1 molecular sieve-confined single-metal Pt catalyst, its preparation method, and its application, belonging to the technical field of low-carbon alkane catalytic oxidation. The invention involves mixing a silicon source, a template agent, and water, then adding a precursor solution formed from a Pt salt and an ethylenediamine ligand. The mixture undergoes dry gel conversion crystallization, vacuum freeze-drying, and calcination under a reducing atmosphere to obtain the Pt@S-1 catalyst. This invention utilizes the protective effect of ethylenediamine on the Pt precursor to inhibit Pt precipitation and aggregation under hydrothermal conditions. Combined with vacuum freeze-drying and calcination under a reducing atmosphere, Pt migration and agglomeration are avoided, successfully achieving stable encapsulation of Pt sub-nanometer clusters within the pores of the Silicalite-1 molecular sieve. This catalyst, used in the catalytic oxidation of propane, exhibits excellent low-temperature activity and high-temperature stability. 100 It operates stably at the specified temperature for over 150 hours without degradation, effectively overcoming the problems of sintering, agglomeration, and deactivation of traditional Pt-based catalysts. This invention features a simple, energy-efficient, and environmentally friendly process with broad application prospects.

Description

Technical Field

[0001] This invention belongs to the technical field of catalytic oxidation of low-carbon alkanes, specifically relating to a Silicalite-1 molecular sieve-confined single-metal Pt catalyst, its preparation method, and its application. Background Technology

[0002] Volatile organic compounds (VOCs) are an important class of fuel and chemical feedstocks. They have low boiling points, are highly volatile, and originate from numerous sources including petrochemicals, natural gas production, coal chemicals, and vehicle exhaust. These compounds are key precursors to atmospheric ozone and secondary organic aerosols, and are also significant contributors to smog and photochemical smog formation. Catalytic oxidation technology is widely recognized as the most effective VOCs purification technology. Among them, low-carbon alkanes, with their high CC and CHH bond energies and stable molecular structures, are among the most difficult VOCs to eliminate. Designing and developing catalysts with excellent low-temperature activity and good thermal stability is crucial for the development of low-carbon alkane catalytic oxidation technology, and is also a key focus and challenge in this field.

[0003] Pt-based noble metal catalysts play a central role in catalytic oxidation reactions. However, catalysts prepared by traditional impregnation methods are prone to metal particle aggregation and sintering under high-temperature reaction conditions, leading to catalyst deactivation and affecting their service life. A molecular sieve encapsulation strategy (metal@zeolite) is employed to modify the catalyst, utilizing the confinement effect of the molecular sieve channels to effectively suppress metal particle sintering and leaching. Pure silica-type Silicalite-1 molecular sieves are acid and alkali resistant, strongly hydrophobic, and possess excellent hydrothermal stability and channel integrity, making them an ideal carrier for encapsulating metals.

[0004] To address the aforementioned issues, researchers have conducted a series of explorations. Chinese patent CN119281378B discloses an MFI-type molecular sieve-confined noble metal Ru-based catalyst and its preparation method. This method employs a strategy combining hydrothermal in-situ ligand protection and stepwise calcination to achieve stable encapsulation of Ru within the MFI-type zeolite, and then applies it to the catalytic oxidation of propane. Chinese patent application CN121042079A discloses a fully silica molecular sieve-confined Pt-based bimetallic catalyst. An in-situ synthesis method was used to prepare a PtCu@S-1 catalyst for propane dehydrogenation to propylene. This study indicates that the silanol groups (Si-OH) within the S-1 channels can anchor metal species, but Cu needs to be added as a promoter to adjust the electronic structure of Pt. In addition, in 2025, the team led by Cheng Jie at the University of Chinese Academy of Sciences reported on the study of PtCu bimetallic clusters confined in Silicalite-1 molecular sieves for the selective catalytic oxidation of ammonia in the journal Applied Catalysis B: Environment and Energy. This study improved the catalytic performance by forming a polarized Cu(δ+)-Pt(δ-) interface through Cu→Pt electron transfer.

[0005] However, the existing technologies still have the following shortcomings: First, there is a general technical bias in existing technologies that "addition of promoters is necessary." Whether in propane dehydrogenation or ammonia oxidation, Cu is relied upon as a promoter to adjust the electronic structure and dispersion of Pt, which increases the complexity of the catalyst composition and the preparation cost. Second, existing preparation processes are complex and energy-intensive, often employing stepwise calcination or high-temperature roasting (500-600℃). Furthermore, conventional oven drying easily leads to the migration of metal precursors, affecting the uniform distribution of the metal. Third, existing technologies have limited understanding of the microscopic mechanisms of metal-support interactions, and the direct chemical bonding between metal atoms and the molecular sieve framework, as well as its regulatory mechanism on the catalytic microenvironment, have not yet been revealed. Fourth, the long-term stability of the catalysts still has room for improvement; the reported stability duration is usually no more than 100 hours, which is insufficient to meet the needs of industrial applications.

[0006] In summary, developing a single Pt-based molecular sieve confined catalyst that requires no additives, has a simple and energy-efficient preparation process, prevents the loss of precious metals, enables direct chemical bonding and high-loading encapsulation of metals and supports, and exhibits excellent low-temperature activity and ultra-long-term stability is of significant scientific importance and application value, and is also a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] To address the technical problems of existing Pt-based catalysts in propane catalytic oxidation, such as high cost due to the need for additives, complex preparation process with high energy consumption (stepwise calcination at 500-600℃), easy metal agglomeration and deactivation, and poor long-term stability (<100 hours), this invention aims to provide a Silicalite-1 molecular sieve-confined single-metal Pt catalyst, its preparation method, and its application.

[0008] To achieve the above objectives, the present invention employs the following technical solution: In a first aspect, the present invention provides a method for preparing a single-metal Pt catalyst confined within a Silicalite-1 molecular sieve, comprising the following steps: The silicon source, template agent and water are mixed to hydrolyze the silicon source to obtain an aqueous solution of silicon source; Platinum salts are mixed with organic ligands to obtain platinum precursor solutions; The platinum precursor solution is mixed with the silicon source aqueous solution to obtain a mixture; The mixture was evaporated to obtain a transparent dry gel, which was then ground and placed in an autoclave. Vapor-phase dry gel conversion crystallization was performed using water vapor as the crystallization medium. The product was then vacuum freeze-dried and calcined in a reducing atmosphere to obtain a Silicalite-1 molecular sieve confined single metal Pt catalyst.

[0009] The organic ligand is selected from any one of ethylenediamine, hexamethylenediamine, or tetraethylenepentamine; the platinum salt is H2PtCl6 or Pt(NH3)4(NO3)2.

[0010] Preferably, the organic ligand is selected from ethylenediamine, and the platinum salt is H2PtCl6.

[0011] The silicon source is at least one of tetraethyl orthosilicate, methyl orthosilicate, fumed silica, or silica sol; the template agent is tetrapropylammonium hydroxide or tetrapropylammonium bromide.

[0012] Preferably, the silicon source is tetraethyl orthosilicate.

[0013] The molar ratio of the silicon source, template agent and water is 1:0.2~0.6:30~40.

[0014] As a preferred embodiment of the present invention, the molar ratio of silicon source, template agent, and water is 1:0.4:34.4. This ensures both high crystallinity of the molecular sieve and a sufficient number of anchoring sites for the chemical bonding of Pt, while avoiding pore blockage or decreased crystallinity caused by excessive template agent.

[0015] The temperature for the vapor phase dry gel conversion crystallization treatment is 50℃~120℃, and the time is 12h~72h; the vacuum degree for the vacuum freeze drying is 5~100Pa, the temperature is -50℃~-10℃, and the drying time is 12~72h.

[0016] The calcination temperature is 300-500℃, the heating rate is 5-15℃ / min, and the holding time is 2-10h.

[0017] As a preferred embodiment of the present invention, the molar ratio of the organic amine ligand to the Pt salt is 1:2.37. The amine groups in ethylenediamine, hexamethylenediamine, or tetraethylenepentamine form stable complexes with Pt ions, preventing the formation of Pt(OH)₂ or PtO₂ precipitates under hydrothermal alkaline conditions. The organic amine ligand and Pt salt achieve complete complexation while avoiding excessive ligand interference with the molecular sieve crystallization process.

[0018] As a preferred technical solution of this invention, the reducing atmosphere is a 10% (v / v) H2 / Ar mixture with a flow rate of 80-120 mL / min; during calcination, the temperature is increased from room temperature to 300-500℃ at a rate of 5-15℃ / min and maintained for 2-10 hours. The reducing atmosphere and calcination parameters simultaneously achieve the following three key functions: firstly, to reduce the Pt precursor to active Pt in situ. 0 / Pt δ+ Species; secondly, by slowly heating (5-15℃ / min) and maintaining a moderate temperature (300-500℃), the integrity of the molecular sieve framework is ensured while promoting the interaction of Pt with silanol defect sites (≡Si-O). - The chemical bonding of Pt is achieved through three methods: first, it avoids the Pt agglomeration and framework collapse caused by traditional high-temperature (>500℃) calcination; second, it avoids the chemical bonding of Pt.

[0019] In a second aspect, the present invention provides a Silicalite-1 molecular sieve-confined single-metal Pt catalyst prepared according to the above preparation method.

[0020] As a preferred embodiment of the present invention, the catalyst consists of a single platinum component and a Silicalite-1 molecular sieve support; the Pt is confined and encapsulated within the pores of the Silicalite-1 molecular sieve in the form of sub-nano clusters or single atoms. Through the geometric confinement effect of the regularized micropores of the molecular sieve, the migration, sintering, and agglomeration of Pt under high-temperature reaction conditions are effectively suppressed. Compared to supported Pt catalysts prepared by traditional impregnation methods (Pt particle size is typically 4-10 nm, located on the outer surface), the atomic utilization rate of Pt in this invention is close to 100%.

[0021] As a preferred embodiment of the present invention, the catalyst is a single Pt metal catalyst, without any auxiliary metals; Pt is anchored to the silicall defects ≡Si-O in the silicalite-1 molecular sieve.- There is a strong interaction between Pt and the silanol defect sites of the molecular sieve framework, which may induce local lattice strain in the zeolite framework, thereby forming a unique catalytic microenvironment.

[0022] Thirdly, the present invention provides the application of the above-mentioned Silicalite-1 molecular sieve confined single metal Pt catalyst in the preparation of propane catalytic oxidation purification.

[0023] As a preferred technical solution of the present invention, the catalyst in the propane catalytic oxidation reaction, at T 100 It can maintain stability for more than 150 hours at the temperature, and its geometric and electronic structures remain stable.

[0024] As a preferred embodiment of the present invention, the catalyst, through the physical confinement of the pores of the Silicalite-1 molecular sieve, promotes a strong interaction between Pt and silanol defect sites, thereby generating Si. δ- -O-Pt δ+ The bond acts in a dual manner to inhibit the sintering, agglomeration, and deactivation of Pt active components under high-temperature reaction conditions.

[0025] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a method for preparing a single-metal Pt catalyst confined within a Silicalite-1 molecular sieve. For the first time, this method utilizes a single Pt component without additional additives, employing an innovative strategy combining ligand-assisted stabilization and gas-phase dry gel conversion to successfully achieve the controllable preparation of the Pt@S-1 catalyst. Compared to traditional methods that require the addition of a second metal or additive to stabilize Pt, this invention simplifies the catalyst composition and preparation process, effectively reducing raw material costs while avoiding side reactions or catalytic poisoning problems that may be introduced by additives. In the synthesis process, a stable soluble complex is formed between organic ligands (such as ethylenediamine) and platinum salts. The coordination and steric hindrance effects of the ligands effectively suppress the precipitation, aggregation, and migration of Pt in the highly alkaline environment and under high-temperature hydrothermal conditions of the ligand solution, solving the technical problem of easy segregation and agglomeration of metal precursors in traditional dry gel methods. Vacuum freeze-drying is used instead of conventional thermal drying, avoiding the migration and agglomeration of Pt clusters caused by capillary forces during conventional drying. Calcination and reduction are carried out in a reducing atmosphere such as hydrogen, avoiding the oxidative agglomeration of Pt that may occur during calcination in air, further ensuring the stable encapsulation of Pt within the pores of the Silicalite-1 molecular sieve. The use of amine organic ligands, compared with the commonly used mercapto (-SH) ligands in existing technologies, does not introduce sulfur impurities and does not poison the metal active sites, making it more suitable for hydrogenation / oxidation catalytic reactions and showing good prospects for industrial applications. Compared with the traditional hydrothermal synthesis method, the vapor-phase dry gel conversion crystallization technology has the following advantages: it does not require a large amount of solvent, produces very little waste liquid, and is environmentally friendly; the product is easy to separate and the post-processing is simple; the crystallization time is relatively short and the synthesis efficiency is high; there is no metal loss and the distribution is more uniform, making it suitable for high-load Pt packaging and with good batch repeatability.

[0026] The present invention provides a single-metal Pt catalyst confined within a Silicalite-1 molecular sieve, achieving for the first time sub-nanometer confinement of a single Pt metal within the pores of the Silicalite-1 molecular sieve. Pt is completely confined within the pores of the Silicalite-1 molecular sieve in the form of sub-nanometer clusters, rather than being loaded onto the outer surface later, resulting in almost no surface metal particles. The average particle size of the Pt clusters is 1–2 nm, achieving precise control at the sub-nanometer scale. The confinement encapsulation does not disrupt the microporous structure and pore volume of the Silicalite-1 molecular sieve, ensuring the diffusion of reactant molecules and the shape-selective catalytic performance of the molecular sieve. The encapsulation efficiency is high, and it does not clog the microporous channels. The synergistic interaction between the encapsulated Pt and the microporous environment of the Silicalite-1 molecular sieve effectively promotes the activation of low-carbon alkanes (such as propane) and oxygen. Under low-temperature conditions, this catalyst can accelerate the breaking of CH and C-C bonds, thereby achieving efficient and deep oxidation of low-carbon alkanes, exhibiting excellent low-temperature catalytic activity. The Pt clusters are stably confined within the rigid microporous channels of the Silicalite-1 molecular sieve, and this catalyst exhibits excellent thermal stability, capable of operating at T... 100 After being kept at the temperature for 150 hours, the geometry and electronic state of the Pt clusters remained stable, with no obvious migration, sintering or aggregation. This effectively overcomes the common technical defects of traditional supported noble metal catalysts that are prone to sintering, aggregation and deactivation under high temperature reaction conditions, and significantly extends the service life of the catalyst.

[0027] The Silicalite-1 molecular sieve-confined single-metal Pt catalyst prepared by this invention has important application prospects in the field of volatile organic pollutant purification. It can be directly used in fixed-bed reactors, fluidized-bed reactors or monolithic catalyst coatings, and is suitable for industrial waste gas purification, cold start exhaust gas treatment of motor vehicles, associated gas purification in oil fields and other scenarios. Attached Figure Description

[0028] Figure 1 The X-ray diffraction (XRD) patterns of the Pt@S-1 catalyst and Pt / S-1 catalyst prepared in Example 1 and Comparative Example 1 of this invention are shown below. Figure 2 The images show scanning electron microscope (SEM) results of the Pt@S-1 catalyst and Pt / S-1 catalyst prepared in Example 1 and Comparative Example 1 of this invention. (a) is the Pt@S-1 catalyst prepared in Example 1 (scale bar 100 nm), and (b) is the Pt / S-1 catalyst prepared in Comparative Example 1 (scale bar 100 nm).

[0029] Figure 3The images shown are transmission electron microscope (TEM) images of the catalyst before and after calcination in Example 1 of the present invention. (a) is the Pt@S-1 catalyst before calcination (scale bar 50 nm), and (b) is the Pt@S-1 catalyst after calcination (scale bar 50 nm).

[0030] Figure 4 The results show the comparison of the activity and high-temperature thermal stability of the Pt@S-1 catalyst and Pt / S-1 catalyst prepared in Example 1 and Comparative Example 1 of this invention in the catalytic oxidation reaction of propane. Pt@S-1: fresh confined catalyst of Example 1 of this invention; Pt@S-1-600: confined catalyst of Example 1 of this invention after high-temperature aging treatment at 600℃; Pt / S-1: fresh supported catalyst of Comparative Example 1; Pt / S-1-600: supported catalyst of Comparative Example 1 after high-temperature aging treatment at 600℃.

[0031] Figure 5 The graph shows the long-term stability results of propane oxidation of the Pt@S-1 catalyst and the Pt / S-1 catalyst prepared in Example 1 and Comparative Example 1 of this invention.

[0032] Figure 6 The diagram shows the propane conversion rate obtained from five cycles of testing of the Pt@S-1 catalyst prepared in Example 1 of this invention.

[0033] Figure 7 The results of the propane oxidation stability and water vapor resistance tests of the Pt@S-1 catalyst and Pt / S-1 catalyst prepared in Example 1 and Comparative Example 1 of this invention are shown. Detailed Implementation

[0034] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. 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 skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0035] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0036] T 90 : This is the reaction temperature at which propane conversion reaches 90%; T 100 : The reaction temperature at which propane conversion reaches 100%.

[0037] Sub-nano clusters: metal aggregates with an average particle size of less than 2 nm.

[0038] The present invention will now be described in further detail with reference to the accompanying drawings: II. Implementation Examples Example 1: Preparation of a single-metal Pt catalyst confined within a Silicalite-1 molecular sieve 26g of tetrapropylammonium hydroxide (25wt%, template agent) was mixed with 30g of deionized water at room temperature. Then, 16.64g of tetraethyl orthosilicate (silicon source) was added to the above solution while stirring at 1000rpm at room temperature. The mixture was stirred for 6h until the tetraethyl orthosilicate was completely hydrolyzed to obtain a silicon source mixture.

[0039] Then add 7.8 mL of chloroplatinic acid aqueous solution (H2PtCl6, 0.019 mol / L) to 65 L-ethylenediamine (EDA ligand) was mixed homogeneously at room temperature to obtain a platinum precursor solution. The platinum precursor solution was added dropwise to the above silicon source mixture while stirring at 1000 rpm, and stirring was continued for 30 min to obtain a homogeneous mixture.

[0040] The solvent was removed by evaporation of the mixture at 60°C to obtain a transparent dry gel. After grinding, the gel was placed on a support inside an autoclave, and 20 mL of deionized water was added to the bottom of the autoclave. The gel was then hydrothermally crystallized at 170°C for 72 h. After the reaction was completed, the gel was allowed to cool naturally to room temperature. The product was washed with deionized water until the pH was neutral. The product was then centrifuged at 8000 rpm for 5 min and the solid product was collected.

[0041] The solid product was placed in a vacuum freeze dryer and freeze-dried for 24 hours under a vacuum of 40 Pa and a temperature of -50 °C to obtain the dried solid powder.

[0042] The dried solid powder was placed in a tube furnace under an inert atmosphere (10% H2 / Ar mixed atmosphere, flow rate 100 mL / min), and the temperature was increased from 25 °C to 400 °C at a rate of 5 °C / min. The powder was then calcined at 400 °C for 4 h and naturally cooled to room temperature to obtain a stable Silicalite-1 molecular sieve confined single metal Pt catalyst, denoted as Pt@S-1 catalyst.

[0043] Example 2 (Different freeze-drying times) The difference from Example 1 is that the freeze-drying time of the solid product is extended to 48 hours, while the other preparation and testing conditions are the same as in Example 1.

[0044] TEM characterization of the obtained Pt@S-1 catalyst showed that Pt was uniformly dispersed in the molecular sieve channels in the form of sub-nanometer clusters, with a particle size distribution basically consistent with that of Example 1. Propane oxidation activity test results showed that T... 90 The temperature was 315°C, similar to that of Example 1. Long-term stability tests showed that after continuous operation at 350°C for 150 hours, the propane conversion rate remained above 99%.

[0045] Example 3 (Different platinum precursors) The difference from Example 1 is that Pt(NH3)4(NO3)2 is used instead of chloroplatinic acid as the platinum precursor, the concentration of the platinum precursor solution is 0.019 mol / L, the amount added is 0.5 mL, and other preparation and testing conditions are the same as in Example 1.

[0046] SEM characterization of the obtained Pt@S-1 catalyst showed that the catalyst surface was clean and there was no obvious Pt particle distribution. TEM characterization showed that Pt was uniformly dispersed in the molecular sieve channels in the form of sub-nano clusters. Propane oxidation activity test results showed that T 90 The temperature was 318℃. Long-term stability test results showed that after continuous operation at 350℃ for 150 hours, the propane conversion rate remained above 98%.

[0047] Example 4 (Different Ligands) The difference from Example 1 is that hexamethylenediamine (50g) was used. L) was used instead of ethylenediamine as the ligand, and other preparation and testing conditions were the same as in Example 1.

[0048] TEM characterization of the obtained Pt@S-1 catalyst showed that Pt was dispersed in sub-nano clusters within the molecular sieve channels, with a particle size of approximately 1.5–2.5 nm. Propane oxidation activity tests showed that T… 90 The temperature was 325℃. Long-term stability test results showed that after continuous operation at 350℃ for 150 hours, the propane conversion rate remained above 97%.

[0049] Example 5 (Different calcination times) The difference from Example 1 is that the calcination and holding time is extended to 6 hours, while the other preparation and testing conditions are the same as in Example 1.

[0050] TEM characterization of the obtained Pt@S-1 catalyst showed that Pt was uniformly dispersed in the molecular sieve channels in the form of sub-nanometer clusters, with a particle size distribution basically consistent with that of Example 1. Propane oxidation activity test results showed that T... 90 The temperature was 317℃. Long-term stability test results showed that after continuous operation at 350℃ for 150 hours, the propane conversion rate remained above 99%.

[0051] Example 6 (Different Silicon Sources) The difference from Example 1 is that methyl orthosilicate (12.5g, calculated based on the molar amount of SiO2) is used instead of tetraethyl orthosilicate as the silicon source, while the other preparation and testing conditions are the same as in Example 1.

[0052] SEM characterization of the obtained Pt@S-1 catalyst showed that the catalyst surface was clean and there was no obvious Pt particle distribution. Propane oxidation activity testing results showed that T 90 The temperature was 320℃. Long-term stability test results showed that after continuous operation at 350℃ for 150 hours, the propane conversion rate remained above 98%.

[0053] Example 7 (Different calcination atmosphere concentrations) The difference from Example 1 is that a 5% H2 / Ar mixed atmosphere (flow rate 100 mL / min) was used instead of 10% H2 / Ar for calcination, while other preparation and testing conditions were the same as in Example 1.

[0054] TEM characterization of the obtained Pt@S-1 catalyst showed that Pt was uniformly dispersed in the molecular sieve channels in the form of sub-nano clusters. Propane oxidation activity tests showed that T... 90 The temperature was 319℃. Long-term stability test results showed that after continuous operation at 350℃ for 150 hours, the propane conversion rate remained above 98%.

[0055] III. Comparative Example Comparative Example 1 (Preparation of supported catalysts using conventional impregnation method) First, the Silicalite-1 molecular sieve support was prepared according to the method described in Example 1 without the addition of a platinum precursor. Specifically, 26 g of tetrapropylammonium hydroxide (25 wt%) was mixed with 30 g of deionized water, and 16.64 g of tetraethyl orthosilicate was added. The mixture was stirred for 6 h until complete hydrolysis. The mixture was then transferred to a high-pressure reactor and hydrothermally crystallized at 170 °C for 72 h. After washing, centrifugation, and freeze-drying, the product was calcined at 400 °C in air for 4 h to obtain the S-1 molecular sieve support.

[0056] Add 4.0 mL of chloroplatinic acid aqueous solution (0.019 mol / L) and 65 L-ethylenediamine and 2.5 g S-1 molecular sieve support were added to 15 mL of deionized water and stirred for 3 h. The mixture was dried overnight in a conventional oven at 80 °C, and then calcined in air at 400 °C for 4 h to obtain the supported Pt / S-1 catalyst.

[0057] Comparative Example 2 (Conventional oven drying instead of vacuum freeze drying) The difference from Example 1 is that the vacuum freeze-drying step is replaced with conventional oven drying (80°C, overnight drying), while the other preparation and testing conditions are the same as in Example 1.

[0058] TEM characterization results showed that the Pt particles in the obtained comparative catalyst exhibited significant agglomeration, with some Pt ​​particles exceeding 10 nm in size and distributed on the outer surface of the molecular sieve. Propane oxidation activity test results showed that T 90 The temperature was 358°C, which is 42°C higher than that of Example 1 (316°C). These results indicate that conventional oven drying cannot effectively suppress the migration and aggregation of Pt precursors, and vacuum freeze-drying plays a crucial role in achieving uniform confined encapsulation of Pt.

[0059] Comparative Example 3 (calcination in air atmosphere instead of reducing atmosphere) The difference from Example 1 is that the calcination atmosphere was replaced with air (flow rate 100 mL / min) instead of 10% H2 / Ar, while the other preparation and testing conditions were the same as in Example 1.

[0060] The propane oxidation activity test results showed that the obtained comparative catalyst had a higher T value. 90 The temperature was 385°C, which is 69°C higher than that of Example 1 (316°C). XPS characterization results showed that after calcination in air atmosphere, Pt mainly existed in the oxidized state (Pt). 4+ / Pt 2+ Pt exists in the form of metal (Pt), while in Example 1, Pt mainly exists in the metallic state (Pt). 0 ) and low-price state (Pt) δ+ ( ) coexistence. The above results indicate that calcination in a reducing atmosphere plays an important role in the formation of active Pt species and the achievement of excellent catalytic performance.

[0061] Comparative Example 4 (without ligands) The difference from Example 1 is that ethylenediamine ligand is not added, and chloroplatinic acid aqueous solution is directly added to the silicon source mixture. Other preparation and testing conditions are the same as in Example 1.

[0062] TEM characterization results showed that the Pt particles in the obtained comparative catalyst were severely agglomerated, forming large particles with a size of 20-50 nm, and most of them were distributed on the outer surface of the molecular sieve. Propane oxidation activity test results showed that T 90 The temperature was 420℃. These results indicate that the ligand (ethylenediamine) plays a crucial role in protecting the platinum precursor and inhibiting its precipitation and aggregation during hydrothermal crystallization.

[0063] III. Performance Testing (1) Characterization See Figure 1 The phase structure of the material was fully characterized. The results showed that the samples all exhibited the characteristic diffraction peaks of S-1 molecular sieve. The peak positions were not significantly shifted, the peak shapes were sharp, and the baselines were stable. This proved that the introduction of Pt did not destroy the long-range ordered crystal phase and microporous channel structure of the molecular sieve, and the crystallinity and structural stability of the support were excellent.

[0064] See Figure 2 The morphology of the catalyst was characterized using scanning electron microscopy (SEM). Figure 2 As shown in (a), the Silicalite-1 molecular sieve exhibits a typical hexagonal plate-like morphology with complete crystal structure and uniform size. The catalyst surface is very clean, with no obvious bright spots or particles observed, indicating that almost all Pt is encapsulated within the channels of the Silicalite-1 molecular sieve, rather than being loaded on the outer surface. Figure 2 As shown in (b), the catalyst surface is covered with numerous bright spots (white particles), which are Pt particles. This indicates that in the Pt / S-1 catalyst prepared by the conventional impregnation method, Pt is mainly distributed on the outer surface of the S-1 molecular sieve, and the particles are large in size, unevenly distributed, and exhibit obvious agglomeration. The above SEM characterization results intuitively demonstrate that the present invention successfully achieved confined encapsulation of Pt within the pores of the Silicalite-1 molecular sieve through a combined strategy of "protection + in-situ amine ligand synthesis + steam dry gel hydrothermal crystallization + vacuum freeze-drying + reducing atmosphere calcination," a goal that cannot be achieved by the conventional impregnation method. This confined structure provides the structural basis for the excellent low-temperature activity and high-temperature stability of the catalyst of the present invention.

[0065] See Figure 3 The dispersion state of Pt in the catalyst was characterized using transmission electron microscopy (TEM). Figure 3As shown in (a), Pt species are highly uniformly dispersed within the pores of the Silicalite-1 molecular sieve as ultrasmall nanoparticles (1-2 nm), with no obvious Pt particles observed on the outer surface of the sieve. This indicates that after hydrothermal crystallization, washing, and vacuum freeze-drying, Pt has formed ultrasmall nanoclusters and been successfully confined within the pores of the molecular sieve. Figure 3 As shown in (b), the Pt species retain their ultra-small nanoparticle morphology after calcination, with a particle size of approximately 1-2 nm, showing no significant change compared to before calcination. The Pt nanoparticles remain highly uniformly dispersed within the molecular sieve channels, with no obvious agglomeration, sintering, or growth observed. The framework structure of the Silicalite-1 molecular sieve remains intact after calcination. These results demonstrate that the "vacuum freeze-drying + low-temperature calcination in a reducing atmosphere" strategy employed in this invention effectively suppresses the migration and agglomeration of Pt during heat treatment. The size and dispersion state of the Pt species before and after calcination are almost completely consistent, successfully achieving stable encapsulation of Pt within the pores of the Silicalite-1 molecular sieve. This structural feature provides the structural basis for the excellent low-temperature activity and high-temperature stability of the catalyst in this invention.

[0066] (2) Catalytic performance testing and high temperature stability 0.3 g of Pt@S-1 catalyst was pressed into tablets (40 mesh) and placed in a quartz tube reactor with an inner diameter of 8 mm. The reaction gas composition was: 0.2 vol.% propane, 21 vol.% oxygen, with nitrogen as the balance gas. The total flow rate was 150 mL / min, and the reaction space velocity was 30000 mL·h. -1 ·g -1 The reaction temperature was gradually increased from 150℃ to 500℃ at a rate of 2℃ / min, and samples were taken for analysis after stabilizing at each temperature for 30 minutes. The reaction tail gas was analyzed online using a gas chromatograph (equipped with a flame ionization detector and a thermal conductivity detector), and the propane conversion rate was calculated. The test results are as follows: Figure 4 As shown.

[0067] From the appendix Figure 4 Data shows that, compared with fresh catalysts, the confined catalyst Pt@S-1 prepared in Example 1 of this invention exhibits significant activity in the low-temperature oxidation reaction of propane, achieving a propane conversion rate of 90% at 316℃ (T 90 The first catalyst, Pt / S-1, achieves 100% conversion at 350℃. The supported catalyst, Pt / S-1, requires 373℃ to achieve 90% conversion and 400℃ to achieve 100% conversion. Both catalysts have different T values. 90 The temperature difference of 57°C indicates that the low-temperature activity of the confined catalyst of this invention is significantly better than that of the supported catalyst.

[0068] High-temperature thermal stability comparison: To evaluate the high-temperature thermal stability of the catalysts, the propane oxidation performance of both catalysts was tested again after aging in air at 600℃ for 4 hours. (See attached image) Figure 4 The results show that the Pt@S-1-600 catalyst after high-temperature calcination treatment has a T 90 The temperature was 318℃, T 100 Approximately 352℃, with fresh catalyst (T 90 The temperature was 316℃, T 100 Compared to 350℃, the conversion rate showed almost no change, and the entire conversion curve highly overlapped with that of the fresh catalyst. In contrast, the Tconversion of the Pt / S-1-600 catalyst was significantly different. 90 Rising to 438℃, T 100 Raising the temperature to 475°C, compared to fresh catalyst (T 90 The temperature was 373℃, T 100 The temperatures were increased by 65°C and 75°C respectively (from 400°C). Furthermore, the conversion curve of Pt / S-1-600 shifted significantly to the right and became flatter, indicating that the high-temperature ripening, migration, and aggregation of Pt nanoparticles led to a severe loss of active sites.

[0069] The above results fully demonstrate that the confined catalyst Pt@S-1 prepared in this invention possesses excellent high-temperature thermal stability. The Pt clusters are firmly confined by the pores of the Silicalite-1 molecular sieve, maintaining excellent catalytic activity even after high-temperature treatment at 600℃. In contrast, the Pt particles in the supported catalyst Pt / S-1 agglomerate and sinter at high temperatures, leading to a significant decrease in catalytic activity. The excellent high-temperature stability of the catalyst of this invention provides a solid foundation for its long-term application in industrial catalysis.

[0070] The Pt@S-1 catalyst was treated at 600℃ in air for 4 hours to obtain the aged catalyst, denoted as Pt@S-1-600. The propane oxidation activity was determined according to the above test conditions. Figure 4 As shown, the T of the Pt@S-1-600 catalyst 90 The temperature was 318℃, which was almost unchanged compared to the fresh catalyst (316℃), indicating that the catalyst has excellent high-temperature stability.

[0071] (3) Long-term stability test Under constant temperature conditions of 350℃, the test was conducted continuously for 150 hours according to the above test conditions, with samples taken every 12 hours to analyze the propane conversion rate. (See attached...) Figure 5 The data shows that the propane conversion rate remained almost 100% throughout the 150-hour test period. This indicates that the Pt@S-1 catalyst can maintain high catalytic performance under long-term operation without significant deactivation.

[0072] (4) Cyclic stability test Five heating-cooling cycles were performed within a temperature range of 150-500℃. After each cycle, the temperature was allowed to cool naturally to room temperature before the next cycle was performed. Figure 6 As shown, the propane conversion curves of the Pt@S-1 catalyst almost completely overlapped in the five-cycle test. As the temperature increased from approximately 225℃ to 450℃, the trend of propane conversion with temperature change was highly consistent in each cycle. This near-overlapping conversion curve strongly indicates that the catalytic performance of the Pt@S-1 catalyst did not significantly decrease after five cycles. This means that the catalyst exhibits good cycle stability in the propane oxidation reaction and can maintain relatively stable catalytic activity during repeated use.

[0073] (5) Water vapor resistance test Add 5 vol% water vapor to the reaction gas and run continuously at a constant temperature of 350℃ for 100 hours, taking samples every 12 hours to analyze the propane conversion rate. Figure 7 As shown, the Pt@S-1 molecular sieve-confined Pt-based catalyst prepared in this invention exhibits excellent water resistance. In a test environment with water vapor interference for up to 100 hours, it maintained stable propane oxidation activity, indicating that the confined structure of the S-1 molecular sieve effectively protects the Pt active sites and reduces the adverse effects of water molecules on the catalyst's active centers, enabling the catalyst to operate efficiently and stably under hydrothermal conditions. The supported catalyst Pt / S-1, under the same test conditions, showed poor water resistance. The presence of water vapor led to a gradual decrease in its propane oxidation conversion rate. This is because water molecules compete with reactants for adsorption on the catalyst surface, and water molecules damage part of the catalyst's structure or active sites, thus affecting catalytic performance, and this effect is somewhat irreversible.

[0074] In summary, the S-1 molecular sieve-confined Pt-based catalyst Pt@S-1, with its unique structural advantages, exhibits excellent hydrothermal stability in propane oxidation reactions and shows significant performance advantages compared to traditional supported catalysts. It has broad prospects for industrial applications of propane oxidation in aqueous environments.

[0075] Based on the above embodiments and comparative examples, the present invention has the following beneficial effects: Simplified process: No additive metals are needed, no stepwise calcination is required, and no auxiliary reagents such as isopropanol are needed. High-performance catalysts can be obtained in one step by dry gel conversion crystallization + vacuum freeze drying + calcination in a reducing atmosphere.

[0076] Structural innovation: For the first time, sub-nanometer confinement of Pt metal within Silicalite-1 channels has been achieved.

[0077] Excellent performance: Propane catalytic oxidation of T90 The temperature is as low as 315℃, which is much lower than the comparative example (>320℃); the activity decay is <2% in the 150h long-term stability test, which is far better than the traditional impregnation method (decay >60%).

[0078] Good versatility: Examples show that this method is applicable to different silicon sources (ethyl orthosilicate, methyl orthosilicate, etc.), different organic amine ligands (ethylenediamine or hexamethylenediamine or tetraethylenepentamine), different Pt loading (0.5wt%-1wt%), and a wide hydrothermal / calcination condition window, and has good prospects for industrial scale-up.

[0079] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for preparing a Silicalite-1 molecular sieve-confined single-metal Pt catalyst, characterized in that, Includes the following steps: The silicon source, template agent and water are mixed to hydrolyze the silicon source to obtain an aqueous solution of silicon source; Platinum salts are mixed with organic ligands to obtain platinum precursor solutions; The platinum precursor solution is mixed with the silicon source aqueous solution to obtain a mixture; The mixture was evaporated to obtain a transparent dry gel, which was then ground and placed in an autoclave. Vapor-phase dry gel conversion crystallization was performed using water vapor as the crystallization medium. The product was then vacuum freeze-dried and calcined in a reducing atmosphere to obtain a Silicalite-1 molecular sieve confined single metal Pt catalyst.

2. The method for preparing the Silicalite-1 molecular sieve-confined single-metal Pt catalyst according to claim 1, characterized in that, The organic ligand is selected from at least one of ethylenediamine, hexamethylenediamine, or tetraethylenepentamine; the platinum salt is H2PtCl6 or Pt(NH3)4(NO3)2.

3. The method for preparing the Silicalite-1 molecular sieve-confined single-metal Pt catalyst according to claim 1, characterized in that, The silicon source is at least one of tetraethyl orthosilicate, methyl orthosilicate, fumed silica, or silica sol; the template agent is tetrapropylammonium hydroxide (TPAOH) or tetrapropylammonium bromide (TPABr).

4. The method for preparing the Silicalite-1 molecular sieve-confined single-metal Pt catalyst according to claim 3, characterized in that, The molar ratio of the silicon source, template agent and water is 1:0.2~0.6:30~40.

5. The method for preparing the Silicalite-1 molecular sieve-confined single-metal Pt catalyst according to claim 1, characterized in that, The temperature for the vapor phase dry gel conversion crystallization treatment is 150℃~200℃, and the time is 12h~72h; the vacuum degree for the vacuum freeze drying is 5~100Pa, the temperature is -50℃~-10℃, and the drying time is 12~72h.

6. The method for preparing the Silicalite-1 molecular sieve-confined single-metal Pt catalyst according to claim 1, characterized in that, The calcination temperature is 300-500℃, the heating rate is 5-15℃ / min, and the holding time is 2-10h.

7. The Silicalite-1 molecular sieve-confined single-metal Pt catalyst prepared by the preparation method according to any one of claims 1 to 6, characterized in that, The catalyst consists of a single platinum component and a Silicalite-1 molecular sieve support; the platinum is encapsulated in sub-nano clusters within the pores of the Silicalite-1 molecular sieve.

8. The Silicalite-1 molecular sieve-confined single-metal Pt catalyst according to claim 7, characterized in that, The catalyst has a Pt loading of 0.5% to 1%.

9. The application of the Silicalite-1 molecular sieve-confined single-metal Pt catalyst according to claim 7 or 8 in the catalytic oxidation of low-carbon alkanes.

10. The application according to claim 9, characterized in that, The low-carbon alkane is propane; the catalyst is in T 100 It can maintain stable operation for no less than 150 hours at the specified temperature.

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