MCM molecular sieve encapsulated with nano-metal atoms, preparation method thereof, and application in gas-phase direct epoxidation of propylene

Abstract

The present invention discloses an MCM molecular sieve encapsulated with nano-metal atoms, a preparation method thereof, and its application in the direct vapor-phase epoxidation of propylene. The preparation method of the nano-metal atoms encapsulated in the MCM molecular sieve comprises the following steps: 1) mixing the MCM molecular sieve raw powder with a passivating agent for a first reaction, then mixing with an alkaline solution for a second reaction, filtering, washing, and mixing with a metal complex solution to obtain solution A, wherein the metal atoms are at least one of gold atoms, platinum atoms, and palladium atoms; 2) mixing an organic template, water, a silicon source, and an alkaline source into a gel to obtain solution B; 3) mixing the solution A and the solution B and performing hydrothermal crystallization, filtering, washing, drying, and calcining. In the molecular sieve prepared by this method, the nanoparticles are all in the molecular sieve pores, thereby improving the catalytic performance of the catalyst. When performing the direct vapor-phase epoxidation of propylene, the catalyst exhibits excellent propylene oxide selectivity.

Description

Technical Field

[0001] The invention relates to a preparation method of nano-metal atoms encapsulated in MCM molecular sieve, the obtained MCM molecular sieve encapsulating nano-metal atoms, and application of the MCM molecular sieve in gas-phase direct epoxidation of propylene. Background Art

[0002] Due to their large surface area, pore volume, and uniform pore size, MCM-41 and other M41S series molecular sieves have attracted significant interest. The emergence of M41S mesoporous materials has not only broadened the application range of molecular sieves but also addressed the shortcomings of microporous materials. However, the low acidity of pure silicon MCM-41 has made it difficult to use as a catalyst in industrial production. In recent years, the introduction of heteroatoms into mesoporous molecular sieves to enhance their acidity and stability has attracted increasing attention. Nearly all transition metals and main-group elements can be incorporated into molecular sieves through hydrothermal synthesis and impregnation methods, serving as catalytic active centers to enhance their catalytic activity. Among these heteroatoms, noble metals exhibit excellent catalytic performance in reactions, with the state and particle size of the noble metal in the molecular sieve directly determining the catalytic performance.

[0003] Metal nanoparticle catalysts can be used to produce fine chemicals, improve fuels, produce hydrogen, utilize solar energy, and eliminate pollutants. However, in certain high-temperature reactions, metal particles tend to aggregate or become inactivated due to metal leaching. In order to solve the problem of metal nanoparticle stability, people have made great efforts, including strengthening the interaction between metal and support, adding promoters, and adjusting the diameter or morphology of metal particles. The coating of metal nanoparticles in molecular sieves can be mainly divided into two strategies: post-synthesis method and in-situ confinement method. The post-synthesis strategy refers to the introduction of metal nanoparticles after the zeolite structure is constructed. In contrast, the in-situ confinement method requires the co-crystallization of zeolite and metal precursor, and obtains metal nanoparticles by in situ reduction.

[0004] The in-situ confinement method involves introducing metal nanoparticles or precursors into the interior of molecular sieve crystals through a one-step hydrothermal synthesis process. This method first involves mixing synthesized metal nanoparticles or a soluble metal precursor with a molecular sieve synthesis gel, followed by high-temperature crystallization. The resulting product is further calcined to remove organic matter and then reduced in the presence of a reducing agent to yield metal nanoparticles. This method is simple and effective, but because metal particles are larger than the molecular sieve pores, the reduced metal expands the molecular sieve's pore structure during the reduction process, disrupting the molecular sieve's structure and reducing the catalyst's effectiveness. The post-synthesis method is widely used because it offers the advantage of not being restricted to the type of molecular sieve framework. By impregnating the molecular sieve support with a metal colloid or soluble metal precursor, metal nanoparticles can be incorporated into the molecular sieve's internal pores. However, this method results in the majority of the metal nanoparticles remaining on the molecular sieve's surface, with only a small fraction penetrating the pores, hindering their catalytic activity. Summary of the Invention

[0005] In view of the above-mentioned problems existing in the prior art, the present invention provides a new method for preparing nano-metal atoms encapsulated in MCM molecular sieves. In the molecular sieve prepared by this method, the nanoparticles are all in the molecular sieve pores, which can improve the catalytic performance of the catalyst and have a better selectivity for propylene oxide when performing direct epoxidation of propylene in gas phase. Specifically, the method uses a passivating agent to passivate the silanol groups on the outer surface of the MCM-41 molecular sieve. These passivated outer surfaces will not undergo desiliconization under the action of alkaline substances, thereby protecting the outer surface of the MCM-41 molecular sieve. Then, the alkaline substance desiliconizes the interior of the molecular sieve pores, forming new mesopores and macropores. Then, the metal particles are embedded in the pores of the molecular sieve, and the size of the metal nanoparticles is regulated at any time according to the size of the molecular sieve pores. Finally, the outer pores etched by the alkaline substance are encapsulated by secondary crystallization, thereby utilizing the confinement effect in the molecular sieve pores without agglomeration.

[0006] The first aspect of the present invention provides a method for preparing nano-metal atoms (M) encapsulated in MCM molecular sieves, comprising the following steps:

[0007] 1) mixing MCM molecular sieve raw powder with a passivating agent for a first reaction, then mixing with an alkaline solution for a second reaction, filtering, washing, and mixing with a metal complex to obtain a solution A, wherein the metal complex is selected from at least one of haloauric acid, haloauric acid salt, haloplatinic acid, haloplatinate, sodium chloropalladate, palladium trifluoroacetate, and sodium tetrabromopalladate;

[0008] 2) mixing an organic template, water, a silicon source, and an alkali source into a gel to obtain a solution B;

[0009] 3) mixing the solution A and the solution B and performing hydrothermal crystallization, filtering, washing, drying and calcining.

[0010] According to some embodiments of the preparation method of the present invention, preferably, the MCM molecular sieve is MCM-41 molecular sieve.

[0011] According to some embodiments of the preparation method of the present invention, preferably, the passivating agent comprises a general formula R a R b R c SiR d The organosilane shown, wherein R a 、R b 、R c and R d the same or different, each independently selected from hydrogen, halogen, C1-C 20 Alkyl, C1-C 20 Alkoxy, C3-C 20 Cycloalkyl, C6-C 20 Aryl and C1-C 20 Any one of the halogenated alkyl groups, and R a 、R b 、R c and R d are not hydrogen and / or halogen at the same time; preferably, R d is halogen, R a 、R b and R c It is not hydrogen and / or halogen at the same time; the passivating agent is further preferably selected from at least one of diphenyldichlorosilane, trimethylchlorosilane and dimethyldichlorosilane.

[0012] According to some embodiments of the preparation method of the present invention, preferably, the weight ratio of the passivating agent to the molecular sieve raw powder is 4:5 to 1:10, preferably 3:5 to 1:10, and more preferably 3:7 to 1:9.

[0013] According to some embodiments of the preparation method of the present invention, preferably, in the alkaline solution, the content of alkali is 1 to 5 weight %.

[0014] According to some embodiments of the preparation method of the present invention, preferably, the alkaline solution is a sodium hydroxide solution.

[0015] According to some embodiments of the preparation method of the present invention, preferably, the weight ratio of the alkaline solution to the MCM molecular sieve raw powder is 1:20 to 1:10, preferably 3:50 to 2:25.

[0016] According to some embodiments of the preparation method of the present invention, preferably, the metal complex is formed by a metal atom source and a solution capable of complexing with the metal atom source. More preferably, the metal complex is selected from at least one of haloauric acid (HAuX4, X = F, Cl, Br or I), haloauric acid salts, haloplatinic acid (HAuX4, X = F, Cl or Br), haloplatinic acid salts, sodium chloropalladate (Na2PdCl4), palladium trifluoroacetate, and sodium tetrabromopalladate.

[0017] According to some embodiments of the preparation method of the present invention, preferably, the weight ratio of the metal complex solution to the molecular sieve raw powder is 0.5 to 1:5.

[0018] According to some embodiments of the preparation method of the present invention, preferably, the conditions of the first reaction include: temperature of 50-80° C., preferably 60-70° C.; time of 2-6 h, preferably 3-5 h.

[0019] According to some embodiments of the preparation method of the present invention, preferably, the conditions of the second reaction include: temperature of 20-40° C., and time of 1-2 h.

[0020] According to some embodiments of the preparation method of the present invention, preferably, the organic template is selected from at least one of cetyltrimethylammonium bromide, cetyltrimethylammonium chloride and cetyltriethylammonium bromide, preferably cetyltrimethylammonium chloride.

[0021] According to some embodiments of the preparation method of the present invention, preferably, the silicon source is selected from at least one of white carbon black, tetraethyl orthosilicate, sodium silicate and silica sol, preferably tetraethyl orthosilicate.

[0022] According to some embodiments of the preparation method of the present invention, preferably, the alkali source is selected from at least one of sodium hydroxide, tetramethylammonium hydroxide and ammonia water, preferably ammonia water.

[0023] According to some embodiments of the preparation method of the present invention, preferably, the solution B satisfies, on a molar basis, SiO2: a H2O: b R: c OH - , wherein R is an organic template, a is 80 to 160, preferably 100 to 140; b is 0.1 to 0.7, preferably 0.2 to 0.5; c is 2 to 7, preferably 4 to 5. In the present invention, the silicon source is calculated as SiO2, the gold source is calculated as Au, the platinum source is calculated as Pt, the palladium source is calculated as Pd, and the base is calculated as OH - The solvent is calculated as H2O, and the organic template is calculated as R.

[0024] According to some embodiments of the preparation method of the present invention, preferably, the conditions of the hydrothermal crystallization include: a temperature of 110 to 140° C., preferably 120 to 130° C.; and a time of 72 to 108 hours, preferably 84 to 100 hours.

[0025] According to some embodiments of the preparation method of the present invention, preferably, the drying conditions include: a temperature of 100-140° C., preferably 110-130° C., and a time of 4-6 hours. The drying removes moisture and organic templates on the surface of the molecular sieve.

[0026] According to some embodiments of the preparation method of the present invention, preferably, the calcination conditions include: a temperature of 400-700°C, preferably 500-600°C, and a calcination time of 5-8 hours. The calcination removes organic templates and moisture from the molecular sieve pores, thereby increasing the molecular sieve framework strength.

[0027] According to some specific embodiments of the preparation method of the present invention, preferably, the preparation method of encapsulating nano-metal atoms (M) in MCM molecular sieve includes but is not limited to the following steps:

[0028] (1) MCM molecular sieve powder and a passivating agent are stirred at a certain temperature for a period of time, and then an alkaline solution is added to the resulting product. After reacting at room temperature for a period of time, the product is filtered and washed, and then the washed product is mixed with a metal complex solution to obtain A;

[0029] (2) The organic template, water, silicon source and alkali are uniformly mixed to form a gel. The molar ratio of the obtained reaction mixture is SiO2: a H2O: b R: c OH - , wherein R is an organic template, the value of a is 80 to 160, the value of b is 0.1 to 0.7, and the value of c is 2 to 7. The mixed solution is B;

[0030] (3) Add B to A and hydrothermally crystallize at a certain crystallization temperature for a period of time. The product is filtered, washed, dried and calcined to obtain M@MCM-41 molecular sieve.

[0031] The second aspect of the present invention provides an MCM molecular sieve encapsulated with nano-metal atoms, which is prepared according to the above preparation method, wherein the metal atom M is at least one of a gold atom, a platinum atom and a palladium atom.

[0032] According to some embodiments of the molecular sieve of the present invention, preferably, the metal atoms exist in the MCM molecular sieve in the form of a single substance.

[0033] In the present invention, for example, but not limited to, can be measured by XPS, the XPS of platinum atom is 71eV, the XPS of palladium atom is 335eV, and the XPS of gold atom is 84eV, can prove that metal atom exists in MCM molecular sieve only in simple substance form, and does not exist in oxide form.And it exists only inside MCM molecular sieve, and does not exist on the surface of MCM molecular sieve. For example, if platinum exists in oxide form, PtO is 72.4eV, PtO2 is 74.9eV, if it is surface is generally PtO2. For another example, if palladium exists in oxide form or exists on the surface of MCM molecular sieve, the XPS of palladium atom is 336.7eV. Using H2-TPR test, if there is gold oxide, reduction peak will appear with hydrogen reduction, if there is no reduction peak, then it is gold simple substance. When the MCM molecular sieve encapsulated with nano metal atoms (gold atoms) of the present invention is tested by H2-TPR, no reduction peak occurs, indicating that gold atoms exist in MCM molecular sieve in simple substance form.

[0034] According to some embodiments of the molecular sieve of the present invention, preferably, when the metal atom is a platinum atom, the XPS of the platinum atom is 71 eV.

[0035] According to some embodiments of the molecular sieve of the present invention, preferably, when the metal atom is a palladium atom, the XPS of the palladium atom is 335 eV.

[0036] According to some embodiments of the molecular sieve of the present invention, preferably, when the metal atom is a gold atom, the XPS of the gold atom is 84 eV

[0037] According to some embodiments of the molecular sieve of the present invention, preferably, the MCM is MCM-41, and the MCM molecular sieve encapsulating nano-metal atoms is denoted as M@MCM-41.

[0038] According to some embodiments of the molecular sieve of the present invention, preferably, the specific surface area of the M@MCM-41 is 910-990 m 2 The M@MCM-41 of the present invention has a larger specific surface area, thereby improving conversion rate and selectivity.

[0039] According to some embodiments of the molecular sieve of the present invention, preferably, the size of the metal nanoparticles in the M@MCM-41 can be controlled to be 5 to 25 nm.

[0040] The third aspect of the present invention provides the use of the above-mentioned preparation method or the above-mentioned MCM molecular sieve encapsulated with nano-metal atoms in the gas-phase direct epoxidation of propylene.

[0041] Beneficial effects of the present invention:

[0042] The present invention first utilizes a passivating agent to protect the silanol group on the outside of the molecular sieve, an alkaline substance can enter the molecular sieve pores and etch from the inside without etching the outer surface of the molecular sieve, two sets of independent pore structures of the internal molecular sieve are interconnected to form new macropores and mesoporous structures due to the etching of the alkaline substance, the size of the internal pore structure of the molecular sieve is regulated according to the concentration and amount of the alkaline substance, then metal nanoparticles are introduced into the molecular sieve pores, due to the confinement effect of the molecular sieve pore structure, the metal nanoparticles will not exceed the size of the molecular sieve pores and destroy the internal structure of the molecular sieve, finally secondary crystallization is utilized to repackage the internal etched pores, and the metal nanoparticles are thoroughly encapsulated in the molecular sieve pores. Compared with conventional post-processing methods, the post-processing method can only load most of the metal nanoparticles on the outer surface of the molecular sieve, and simultaneously because the metal nanoparticles are exposed to the outside, after treatment such as high temperature, the metal nanoparticles will continue to agglomerate or even lose, thereby causing a decline in catalytic effect. Compared to conventional one-step hydrothermal synthesis, the one-step method is simpler, but it requires reduction to convert the nanoparticles to a metallic state. However, the pores of the reduced metal nanoparticles are much larger than those of the molecular sieve itself, which inevitably enlarges and destroys the internal structure of the molecular sieve, thereby reducing the catalytic performance of the catalyst. Therefore, the method of the present invention effectively addresses the problems of existing methods, improves the catalytic performance of the catalyst, and has significant effects in the direct vapor-phase epoxidation of propylene. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is the HRTEM image of the Au@MCM-41 molecular sieve obtained in Example 1 of the present invention;

[0044] Figure 2 This is the low-angle XRD pattern of Au@MCM-41 molecular sieve obtained in Example 3 of the present invention;

[0045] Figure 3 This is the HRTEM image of the Au@MCM-41 molecular sieve obtained in Example 3 of the present invention;

[0046] Figure 4 This is the XPS graph of Au@MCM-41 molecular sieve obtained in Example 3 of the present invention;

[0047] Figure 5 This is the HRTEM image of the Au@MCM-41 molecular sieve obtained in Example 4 of the present invention;

[0048] Figure 6 This is the XPS graph of the Pd@MCM-41 molecular sieve obtained in Example 5 of the present invention;

[0049] Figure 7 This is the XPS graph of the Pt@MCM-41 molecular sieve obtained in Example 6 of the present invention. DETAILED DESCRIPTION

[0050] In order to make the present invention easier to understand, the present invention will be described in detail below with reference to embodiments. These embodiments are only for illustration and do not limit the scope of application of the present invention.

[0051] The test method of the present invention and the equipment used in the test are as follows:

[0052] (1) XRD uses Philips X-Pert series X-ray diffractometer to determine the structure of molecular sieves.

[0053] (2) HRTEM uses the Jem-3010 high-resolution transmission electron microscope of Rigku Company to determine the regularity of the molecular sieve.

[0054] (3) XPS uses Thermo Scientific's ESCALAB 250 spectrometer to measure the bonding of metal particles.

[0055] (4) BET was measured using a Micromeritics ASAP2020 fully automatic surface area analyzer.

[0056] The silicon source of the present invention is calculated as SiO2, the gold source is calculated as Au, the platinum source is calculated as Pt, the palladium source is calculated as Pd, and the alkali is calculated as OH - The solvent is calculated as H2O, and the organic template is calculated as R.

[0057] [Example 1]

[0058] 5.0 g of MCM-41 molecular sieve powder was stirred with 2.1 g of trimethylsilyl chloride at 50 ° C for 2 h, and then the product was mixed evenly with 0.4 g of 1 wt% sodium hydroxide solution and stirred at room temperature for 1 h. The product was then filtered and washed to obtain solution D. 2.0 g of chloroauric acid was added thereto to obtain solution A.

[0059] 4.4 g of hexadecyltrimethylammonium bromide (CTAB) and 38.9 g of deionized water were added to the reactor in sequence and stirred evenly. 5 g of tetraethyl orthosilicate (TEOS) was slowly added dropwise. Finally, 4.8 g of NaOH was added to adjust the pH of the solution to 11-13, which was Solution B. The molar ratio of the reaction mixture was SiO2: 90 H2O: 0.5R: 5OH. - , B and A were mixed and transferred to a crystallization reactor, heated to 110 ° C, and crystallized at a constant temperature for 72 hours. After the crystallization was complete, the temperature was lowered to room temperature, the reaction mixture was separated, washed, and dried at 110 ° C, and finally calcined at 400 ° C for 8 hours to obtain Au@MCM-41 molecular sieve. The high-magnification transmission electron microscopy image of the sample is shown in Figure 1The specific surface area of the product obtained by BET analysis is shown in Table 1.

[0060] [Example 2]

[0061] The difference from Example 1 is that the passivator is changed to dimethyldichlorosilane, the amount is changed to 0.56 g, the passivation temperature is changed to 60 ° C, the passivation time is changed to 3 h, the mass fraction of the sodium hydroxide solution is changed to 2 wt %, the amount is 0.3 g, the template agent is changed to hexadecyltrimethylammonium chloride (CTAC), the amount is 1.8 g, the amount of water is changed to 20.3 g, the silicon source is changed to sodium silicate, the amount is 4 g, the amount of NaOH is changed to 3.4 g, the Au source complex is changed to sodium chloroaurate, the amount is 1.5 g, the crystallization temperature is changed to 120 ° C, the crystallization time is changed to 80 h, the drying temperature is changed to 120 ° C, the calcination temperature is changed to 500 ° C, the calcination time is changed to 6 h, the other components and synthesis conditions remain unchanged, and the molar ratio of the reaction mixture obtained is SiO2: 80H2O: 0.4R: 6OH - The specific surface area of the product obtained by BET analysis is shown in Table 1.

[0062] [Example 3]

[0063] The difference from Example 1 is that the passivator is changed to diphenyldichlorosilane, the amount is changed to 1.25 g, the passivation temperature is changed to 70 ° C, the passivation time is changed to 4 h, the mass fraction of the sodium hydroxide solution is changed to 3 wt %, the amount is 0.35 g, the reaction time is changed to 2 h, the template agent is changed to hexadecyltrimethylammonium chloride (CTAC), the amount is 1.4 g, the amount of water is changed to 25.9 g, the amount of TEOS is changed to 3 g, the alkali source is changed to ammonia water, the amount is 2.0 g, the amount of Au source complex is changed to 1.0 g, the crystallization temperature is changed to 130 ° C, the crystallization time is changed to 90 h, the drying temperature is changed to 130 ° C, the calcination temperature is changed to 550 ° C, the calcination time is changed to 7 h, the other components and synthesis conditions remain unchanged, and the molar ratio of the reaction mixture obtained is SiO2: 100H2O: 0.3R: 4OH - The small angle powder XRD diffraction of the sample is shown in Figure 2 , high magnification transmission electron microscopy images are shown in Figure 3 , the state of gold atoms in the molecular sieve is analyzed by XPS. Figure 4 The specific surface area of the product obtained by BET analysis is shown in Table 1.

[0064] [Example 4]

[0065] The difference from Example 3 is that the weight fraction of the sodium hydroxide solution is changed to 4 weight percent, and the other components and synthesis conditions remain unchanged. Figure 5 The specific surface area of the product obtained by BET analysis is shown in Table 1.

[0066] [Example 5]

[0067] 5.0 g of MCM-41 molecular sieve powder was stirred with 2.1 g of trimethylsilyl chloride at 50 ° C for 2 h, and then the product was mixed evenly with 0.4 g of 1 wt% sodium hydroxide solution and stirred at room temperature for 1 h. The product was then filtered and washed to obtain solution D. 2.0 g of sodium chloropalladate was added to solution D to obtain solution A.

[0068] 4.4 g of hexadecyltrimethylammonium bromide (CTAB) and 38.9 g of deionized water were added to the reactor in sequence, stirred evenly, and 5 g of tetraethyl orthosilicate (TEOS) was slowly added dropwise. Finally, 4.8 g of NaOH was added to adjust the pH of the solution to 11-13, which is solution B. The molar ratio of the reaction mixture obtained is SiO2:90 H2O:0.5R:5OH - , B and A were mixed and transferred to a crystallization kettle, heated to 110 ° C, and crystallized at a constant temperature for 72 hours. After the crystallization was complete, the temperature was lowered to room temperature, the reaction mixture was separated, washed and dried at 110 ° C, and finally calcined at 400 ° C for 8 hours to obtain Pd@MCM-41 molecular sieve. The specific surface area of the product obtained by BET analysis of the sample is shown in Table 1. The state analysis of Pd atoms in the molecular sieve by XPS is shown in Figure 6 .

[0069] [Example 6]

[0070] 5.0 g of MCM-41 molecular sieve powder and 2.1 g of trimethylchlorosilane were stirred at 50° C. for 2 h. The product was then mixed evenly with 0.4 g of a 1 wt% sodium hydroxide solution and stirred at room temperature for 1 h. The product was then filtered and washed to obtain solution D. 2.0 g of chloroplatinic acid was added to solution D to obtain solution A.

[0071] 4.4 g of hexadecyltrimethylammonium bromide (CTAB) and 38.9 g of deionized water were added to the reactor in sequence, stirred evenly, and 5 g of tetraethyl orthosilicate (TEOS) was slowly added dropwise. Finally, 4.8 g of NaOH was added to adjust the pH of the solution to 11-13, which is solution B. The molar ratio of the reaction mixture obtained is SiO2:90 H2O:0.5R:5OH - , B and A were mixed and transferred to a crystallization kettle, heated to 110 ° C, and crystallized at a constant temperature for 72 hours. After the crystallization was complete, the temperature was lowered to room temperature, the reaction mixture was separated, washed and dried at 110 ° C, and finally calcined at 400 ° C for 8 hours to obtain Pt@MCM-41 molecular sieve. The specific surface area of the product obtained by BET analysis of the sample is shown in Table 1. The state analysis of Pt atoms in the molecular sieve by XPS is shown in Figure 7 .

[0072] [Comparative Example 1]

[0073] 4.4 g of hexadecyltrimethylammonium bromide (CTAB) and 38.9 g of deionized water were added to the reactor in sequence and stirred evenly. 2.0 g of chloroauric acid was added and stirring was continued. 5 g of tetraethyl orthosilicate (TEOS) was slowly added dropwise. Finally, 4.8 g of NaOH was added to adjust the pH of the solution to 11-13. The molar ratio of the reaction mixture was SiO2: 90 H2O: 0.5 R: 5 OH. - The mixed solution was transferred to a crystallization kettle, heated to 110°C, and crystallized at this constant temperature for 72 hours. After complete crystallization, the temperature was cooled to room temperature. The reaction mixture was separated, washed, and dried at 110°C. Finally, it was calcined at 400°C for 8 hours to obtain the Au-MCM-41 molecular sieve raw powder. The Au-MCM-41 molecular sieve was placed at the bottom of a quartz tube and hydrogen was introduced. The temperature was raised to 400°C at a rate of 3°C / min and maintained for 2 hours. The final product was the Au@MCM-41 molecular sieve. The specific surface area of the product obtained by BET analysis is shown in Table 1.

[0074] [Comparative Example 2]

[0075] 4.4 g of hexadecyltrimethylammonium bromide (CTAB) and 38.9 g of deionized water were added to the reactor in sequence and stirred evenly. 5 g of tetraethyl orthosilicate (TEOS) was slowly added dropwise. Finally, 4.8 g of NaOH was added to adjust the pH of the solution to 11-13. The molar ratio of the reaction mixture was SiO2: 90 H2O: 0.5 R: 5 OH. - The mixed solution was transferred to a crystallization kettle, heated to 110°C, and crystallized at a constant temperature for 72 hours. After complete crystallization, the temperature was lowered to room temperature, and the reaction mixture was separated, washed, and dried at 110°C. Finally, it was calcined at 400°C for 8 hours to obtain the MCM-41 molecular sieve raw powder. MCM-41 molecular sieve, 2.0g chloroauric acid, and 50mL deionized water were mixed uniformly. Subsequently, the reaction mixture was separated, washed, and dried at 110°C. Finally, it was calcined at 400°C for 8 hours to obtain Au / MCM-41 molecular sieve. The specific surface area of the product obtained by BET analysis is shown in Table 1.

[0076] [Comparative Example 3]

[0077] 5.0 g of MCM-41 molecular sieve raw powder was mixed evenly with 0.4 g of 1 wt% sodium hydroxide solution and stirred at room temperature for 1 h. The product was then filtered and washed to obtain solution D. 2.0 g of chloroauric acid was added thereto to obtain solution A.

[0078] 4.4 g of hexadecyltrimethylammonium bromide (CTAB) and 38.9 g of deionized water were added to the reactor in sequence and stirred evenly. 5 g of tetraethyl orthosilicate (TEOS) was slowly added dropwise. Finally, 4.8 g of NaOH was added to adjust the pH of the solution to 11-13, which was Solution B. The molar ratio of the reaction mixture was SiO2: 90 H2O: 0.5R: 5OH. - Mix B and A and transfer them to a crystallization reactor. The temperature was raised to 110°C and crystallized at this constant temperature for 72 hours. After complete crystallization, the temperature was lowered to room temperature. The reaction mixture was separated, washed, and dried at 110°C. Finally, it was calcined at 400°C for 8 hours to obtain Au / MCM-41 molecular sieve. The specific surface area of the product obtained by BET analysis is shown in Table 1.

[0079] Test Example

[0080] The samples obtained in Examples 1-6 and Comparative Examples 1-3 were respectively used in gas-phase direct epoxidation experiments of propylene. The results are shown in Table 1.

[0081] The catalyst activity evaluation of the direct epoxidation of propylene in the gas phase was carried out in a fixed-bed microreactor at atmospheric pressure. The catalyst dosage was 0.30 g. The reaction gas composition was regulated by a mass flow meter to a volume ratio of C3H6:H2:O2:N2 of 1:1:1:7. After mixing, the gas passed through the catalyst bed at a space velocity of 7000 mL·(g·h). -1 Before the reaction, the catalyst was pretreated as follows: 10% (volume ratio, the same below) H2 / N2 and 10% O2 / N2, heat treated for 0.5 h in succession, and then under N2 atmosphere, the reaction temperature was lowered to 423 K, all reaction gases were turned on, and after the reaction was carried out for 0.5 h, the reaction products were qualitatively and quantitatively analyzed using gas chromatography.

[0082] Table 1. Results of direct vapor phase epoxidation of propylene

[0083]

[0084]

[0085] From Comparative Examples 1-2 and Example 1, Table 1 shows that in Comparative Example 1, Au@MCM-41 is prepared by an in-situ constraint method. The process of this method is quite simple and convenient. It is only necessary to use H2 reduction to pull the metal out of the skeleton. However, this method will force the metal atoms in the skeleton to be pulled out of the skeleton, thereby destroying the skeleton structure of the molecular sieve. Secondly, the metal atoms precipitated from the skeleton will be larger than the pores, which will also have a certain destructive effect on the structure of the pores. Therefore, the overall structure of the molecular sieve will be greatly changed, so the catalytic performance of the catalyst obtained by this method is low; while Comparative Example 2 adopts a post-synthesis strategy method, which is basically the same as the ordinary loading method. The metal atoms are ultimately loaded on the surface of the molecular sieve in the form of oxides, which will cause the overall loss of metal atoms during the reaction, thereby affecting the performance of the catalyst; in Comparative Example 3, since no passivating agent is used, the added alkali will directly etch the skeleton structure of the molecular sieve surface, and the molecular sieve structure will be damaged to a certain extent. Subsequently, when a metal source is added, the metal atoms cannot be encapsulated inside the pores of the molecular sieve, so that the reaction performance of the catalyst is poor.

[0086] Depend on Figure 2 It can be seen that the Au@MCM-41 molecular sieve obtained by the method provided by the present invention still has the characteristic peak of high regularity of MCM-41 molecular sieve in small-angle XRD, indicating that the modification of metal atoms does not destroy the structure of the molecular sieve itself; Figure 3 and Figure 4 It can be seen that the iron atoms in the Au@MCM-41 molecular sieve obtained by the method provided by the present invention are in the form of single substance ( Figure 4 It exists in the form of gold elemental 4f7 / 2 / orbital with a wavelength of 84eV, and it can be clearly seen in the electron microscope image that the size of the metal element is about 5nm.

[0087] Depend on Figure 1 、 Figure 3 and Figure 5 It can be seen that by changing the amount of alkali, the size of the metal nanoparticles can be controlled at any time, and the size of the metal nanoparticles varies from 5 to 25 nm.

[0088] As can be seen from Table 1, as the size of metal nanoparticles increases, the catalytic activity becomes better and better. However, when the metal nanoparticles are too large, although the structure of the molecular sieve itself will not be damaged, the alkali etching will increase the interconnectivity of the pores inside the molecular sieve, causing the strength of the molecular sieve itself to decrease, and thus the catalytic performance will decrease to a certain extent.

[0089] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, under the technical enlightenment provided by the present invention, as common knowledge in the art, other equivalent variations and improvements can be made, which should also be considered as the scope of protection of the present invention.

Claims

1. A method for preparing nano-metal atoms encapsulated in MCM molecular sieves, comprising the following steps: 1) mixing MCM molecular sieve raw powder with a passivating agent for a first reaction, then mixing with an alkaline solution for a second reaction, filtering, washing, and mixing with a metal complex to obtain a solution A, wherein the metal complex is selected from at least one of haloauric acid, haloauric acid salt, haloplatinic acid, haloplatinic acid salt, sodium chloropalladate, palladium trifluoroacetate, and sodium tetrabromopalladate; 2) Mixing the organic template, water, silicon source and alkali source into a gel to obtain solution B; 3) mixing the solution A and the solution B and performing hydrothermal crystallization, filtering, washing, drying and calcining; The passivating agent comprises the general formula R a R b R c SiR d The organosilane shown, wherein R a 、R b 、R c and R d the same or different, each independently selected from hydrogen, halogen, C1-C 20 Alkyl, C1-C 20 Alkoxy, C3-C 20 Cycloalkyl, C6-C 20 Aryl and C1-C 20 Any one of the halogenated alkyl groups, and R a 、R b 、R c and R d It is not hydrogen and / or halogen at the same time.

2. The preparation method according to claim 1, characterized in that The MCM molecular sieve is MCM-41 molecular sieve.

3. The preparation method according to claim 1, characterized in that R d is halogen, R a 、R b and R c are not hydrogen and / or halogen at the same time; and / or, The weight ratio of the passivating agent to the molecular sieve raw powder is 4:5 to 1:

10.

4. The preparation method according to claim 3, characterized in that The passivating agent is selected from at least one of diphenyldichlorosilane, trimethylchlorosilane and dimethyldichlorosilane; and / or The weight ratio of the passivating agent to the molecular sieve raw powder is 3:5 to 1:

10.

5. The preparation method according to claim 4, characterized in that The weight ratio of the passivating agent to the molecular sieve raw powder is 3:7 to 1:

9.

6. The preparation method according to any one of claims 1 to 5, characterized in that In the alkaline solution, the content of alkali is 1 to 5% by weight; and / or, The weight ratio of the alkaline solution to the MCM molecular sieve raw powder is 1:20 to 1:

10.

7. The preparation method according to claim 6, characterized in that The alkaline solution is a sodium hydroxide solution; and / or the weight ratio of the alkaline solution to the MCM molecular sieve raw powder is 3:50 to 2:

25.

8. The preparation method according to any one of claims 1 to 5, characterized in that The weight ratio of the metal complex to the molecular sieve raw powder is 0.5 to 1:

5.

9. The preparation method according to any one of claims 1 to 5, characterized in that The conditions of the first reaction include: temperature of 50-80°C; time of 2-6 h; and / or, The conditions of the second reaction include: temperature of 20-40° C. and time of 1-2 h.

10. The preparation method according to claim 9, wherein The conditions of the first reaction include: temperature of 60-70° C.; time of 3-5 h.

11. The preparation method according to any one of claims 1 to 5, characterized in that The organic template is at least one selected from cetyltrimethylammonium bromide, cetyltrimethylammonium chloride and cetyltriethylammonium bromide; and / or, The silicon source is selected from at least one of white carbon black, ethyl orthosilicate, sodium silicate and silica sol; and / or, The alkaline source is selected from at least one of sodium hydroxide, tetramethylammonium hydroxide and ammonia water; and / or, The solution B satisfies, in moles, SiO2:a H2O:b R:c OH - , wherein R is an organic template, the value of a is 80~160; the value of b is 0.1~0.7; and the value of c is 2~7.

12. The preparation method according to claim 11, characterized in that In the solution B, the value of a is 100-140; the value of b is 0.2-0.5; and the value of c is 4-5.

13. The preparation method according to any one of claims 1 to 5, characterized in that The hydrothermal crystallization conditions include: temperature of 110-140° C.; time of 72-108 h; and / or, The drying conditions include: a temperature of 100-140°C; a time of 4-6 hours; and / or, The calcination conditions include: a temperature of 400-700° C.; and a calcination time of 5-8 h.

14. The preparation method according to claim 13, wherein The hydrothermal crystallization conditions include: a temperature of 120-130° C.; a time of 84-100 h; and / or, The drying conditions include: a temperature of 110-130° C.; and / or, The calcination conditions include: a temperature of 500-600°C. 15 . The MCM molecular sieve encapsulated with nano-metal atoms obtained by the preparation method according to claim 1 , wherein the metal atom M is at least one of a gold atom, a platinum atom and a palladium atom.

16. The molecular sieve according to claim 15, characterized in that Metal atoms exist in the MCM molecular sieve in the form of a single substance.

17. The molecular sieve according to claim 15, characterized in that When the metal atom is platinum, the XPS of the platinum atom is 71 eV; when the metal atom is palladium, the XPS of the palladium atom is 335 eV; when the metal atom is gold, the XPS of the gold atom is 84 eV.

18. The molecular sieve according to claim 15, characterized in that The MCM is MCM-41, and the MCM molecular sieve encapsulating nano-metal atoms is denoted as M@MCM-41.

19. The molecular sieve according to claim 18, wherein The specific surface area of M@MCM-41 is 910~990m 2 / g.

20. The molecular sieve according to claim 18, wherein The size of the metal nanoparticles in the M@MCM-41 can be controlled to be 5-25 nm.

21. Use of the MCM molecular sieve encapsulating nano-metal atoms prepared by the preparation method according to any one of claims 1 to 14 or the MCM molecular sieve encapsulating nano-metal atoms according to any one of claims 15 to 20 in gas-phase direct epoxidation of propylene.

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