Hierarchical porous Beta zeolite encapsulated metal catalyst, its preparation method and application

Abstract

The present invention discloses a hierarchical pore Beta zeolite encapsulated metal catalyst, its preparation method and application, belonging to the field of catalysts. The preparation method includes: mixing an aluminum source, a silicon source, a template agent and water to obtain a first reactant, carrying out hydrothermal crystallization reaction at 110-130 °C for 20-30 hours, and obtaining Beta zeolite seeds; adding silica spheres into a solution of a cationic polyelectrolyte to obtain a second reactant, and then adding the second reactant into a solution of an anionic polyelectrolyte, and obtaining silica spheres with polyelectrolytes after drying treatment; dissolving a metal precursor in a solution of Beta zeolite seeds to obtain a third reactant, and then impregnating the silica spheres with polyelectrolytes into the third reactant, carrying out hydrothermal crystallization reaction at 110-130 °C for 2-8 days to obtain a mixed sol; treating the mixed sol to obtain a fourth reactant, calcining the fourth reactant at 400-650 °C for 2-6 hours, and then reducing it in a mixed gas of hydrogen and argon at 450-650 °C for 1-3 hours.

Description

Technical Field

[0001] The present invention relates to the field of catalysts, and particularly relates to a metal-encapsulated hollow spherical hierarchical pore Beta zeolite catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Clean air is required for human health and well-being. However, as an important source of air pollution, volatile organic compounds (VOCs) can cause complex environmental pollution problems such as photochemical smog, haze, and ozone layer depletion, posing a great threat to the ecological environment and human health. Therefore, studying effective methods for treating VOCs pollution is an urgent task to solve the air pollution problem. Summary of the Invention

[0003] Among the numerous methods for eliminating VOCs, the catalytic oxidation method has the characteristics of low combustion temperature, low energy consumption, high efficiency, and no secondary pollution. In particular, it can achieve the elimination of low-concentration (<1000 ppm) VOCs at low temperatures (<400 °C). Therefore, it has become an important technical means for effectively treating VOCs pollution. Currently, VOCs elimination catalysts mainly include three categories: transition metals and their oxides, perovskites, and supported noble metals. The first two types of catalysts have been studied for many years, but the temperatures required for their catalytic oxidation of VOCs are relatively high, the energy consumption is large, and their oxidation activities need to be further improved. Noble metal-loaded zeolites have become a research hotspot for VOCs elimination catalysts because they combine the dual advantages of zeolites and noble metals, showing high low-temperature catalytic activity, large specific surface area, and easy mass transfer of reactant molecules. Currently, the main limitation of such catalysts is that during the catalytic oxidation of VOCs, there are disadvantages such as easy aggregation of metal nanoparticles and easy sintering at high temperatures, which cannot meet the requirements for long-term use in engineering applications.

[0004] To solve at least one of the above problems and defects existing in the prior art, embodiments of the present invention provide a metal-encapsulated hollow spherical hierarchical pore Beta zeolite catalyst, a preparation method thereof, and an application thereof.

[0005] According to one aspect of the present invention, there is provided a preparation method of a metal-encapsulated hollow spherical hierarchical pore Beta zeolite catalyst, including:

[0006] Step S1: Mix an aluminum source, a silicon source, a template agent, and water and stir for a first period of time to obtain a first reactant, and subject the first reactant to hydrothermal crystallization reaction at 110 - 130 °C for 20 - 30 hours to obtain Beta zeolite seeds;

[0007] Step S2: Add silica spheres into the solution of cationic polyelectrolyte, wash them with ammonia water after ultrasonic treatment and standing to obtain a second reactant, then add the second reactant into the solution of anionic polyelectrolyte, wash them with ammonia water after ultrasonic treatment and standing, and obtain silica spheres with polyelectrolyte after drying treatment;

[0008] Step S3: Dissolve a metal precursor in the solution of Beta zeolite seeds to obtain a third reactant, then immerse the silica spheres with polyelectrolyte into the third reactant, perform ultrasonic treatment and stir for a second time, and carry out hydrothermal crystallization reaction at 110 - 130 °C for 2 - 8 days to obtain a mixed sol;

[0009] Step S4: Wash the mixed sol with ammonia water, ethanol and water in sequence until neutral, then perform vacuum drying to obtain a fourth reactant, calcine the fourth reactant at 400 - 650 °C for 2 - 6 hours, and then reduce it in a mixed gas of hydrogen and argon at 450 - 650 °C for 1 - 3 hours to obtain the encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite.

[0010] According to another aspect of the present invention, there is provided an encapsulated metal catalyst of a hollow spherical hierarchical pore Beta zeolite, wherein,

[0011] the wall thickness of the encapsulated metal catalyst is 2 - 10 nm, the metal content is 0.1 - 6.0 wt%, the metal particle size is less than 2 nm, and the metal particles in the encapsulated metal catalyst are encapsulated in the pore channels of the zeolite,

[0012] the encapsulated metal catalyst is prepared according to the preparation method of the encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite described in the foregoing embodiments.

[0013] According to still another aspect of the present invention, there is provided an application of the encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite in the removal of VOCs, wherein the encapsulated metal catalyst is the encapsulated metal catalyst described in claim 9, or is prepared according to the preparation method of the encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite described in the foregoing embodiments.

[0014] The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite according to the present invention, its preparation method and application have at least one of the following advantages:

[0015] (1) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite according to the present invention, its preparation method and application use zeolite as a carrier to encapsulate noble metal materials, utilize the rich pore structure of zeolite to reduce the diffusion resistance of reactants and products, reduce agglomeration, thereby improving the reaction rate and inhibiting sintering;

[0016] (2) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application utilize the confinement effect of the zeolite pore structure to encapsulate metal particles in the zeolite pores to form a hollow core-shell structure, controlling the metal size distribution and high-temperature agglomeration;

[0017] (3) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application utilize the unique three-dimensional twelve-membered ring topological structure (forming micropores) of the Beta zeolite, which has a linear channel of 0.66×0.67 nm and a curved channel of 0.56×0.56 nm, and can effectively improve the diffusion performance of toluene, etc.;

[0018] (4) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application utilize the synergistic effect of metal particles and zeolite to coat a thin layer of zeolite shell outside the metal nanoparticles to form a hollow nanocapsule zeolite sphere. This structure has a hierarchical pore structure of macropores-mesopores-micropores at the same time, which can effectively reduce the diffusion resistance of reactants and products, significantly improve the reaction efficiency and inhibit sintering;

[0019] (5) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application adopt an in-situ synthesis method to prepare the catalyst. Compared with the mesoporous material impregnation method, it can directly control the structure and thickness of the zeolite and the size of the encapsulated metal particles;

[0020] (6) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application synthesize a structurally stable hollow zeolite by using mesoporous silica spheres as the metal carrier and the silicon source required for the growth of zeolite seeds, solving the problem of easy collapse of hollow zeolites;

[0021] (7) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application regulate the shell thickness of the Beta zeolite by using cationic and anionic electrolytes, and at the same time introduce intracrystalline mesopores by using its flexible chains to synthesize a thin-walled (wall thickness 2-10 nm) encapsulated metal catalyst of hollow spherical hierarchical pore zeolite;

[0022] (8) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application utilize the thin-walled hollow spherical hierarchical pore zeolite encapsulated metal catalyst to carry out the VOCs elimination reaction, and at the same time has the advantages of high catalytic activity and high stability. Description of the Drawings

[0023] These and / or other aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the preferred embodiments in conjunction with the accompanying drawings, in which:

[0024] Figure 1 A flow chart of a preparation method of a metal-encapsulated catalyst of a hollow spherical hierarchical pore Beta zeolite according to an embodiment of the present invention is shown;

[0025] Figure 2 A comparative effect diagram of catalyzing the complete oxidation of toluene is shown;

[0026] Figure 3 A comparative diagram of the thermal stability of catalyzing the oxidation of toluene is shown. Detailed embodiments

[0027] The technical solution of the present invention will be further specifically described below through embodiments in conjunction with the accompanying drawings. In the specification, the same or similar reference numerals indicate the same or similar components. The following description of the embodiments of the present invention with reference to the accompanying drawings is intended to explain the overall inventive concept of the present invention and should not be construed as a limitation of the present invention.

[0028] In the embodiments of the present invention, the problems of easy aggregation and easy sintering at high temperature of metal nanoparticles can be effectively solved by improving the dispersion and stability of metal nanoparticles on the molecular sieve support. The stability and anti-poisoning performance of noble metal catalysts can be improved by adjusting the metal particle size strategy and the support structure performance strategy together. With the reduction of the metal particle size, metal nanoclusters exhibit unique catalytic properties different from bulk metals. Especially when the particle size is less than 5 nm, the catalytic reaction activity is significantly improved. However, it faces the problems of aggregation and easy sintering at high temperature.

[0029] To solve this problem, using a molecular sieve as a support to encapsulate noble metal materials becomes a candidate solution. This material can effectively reduce the diffusion resistance of reactants and products by utilizing the rich pore structure of the molecular sieve, improve the reaction rate and inhibit sintering; on the other hand, by exerting the confinement effect of the molecular sieve pore structure, the metal particles are encapsulated in the molecular sieve pores to control the metal size distribution and high-temperature aggregation.

[0030] In one example, in order to obtain a stable hollow structure, a relatively thick molecular sieve shell wall can be synthesized, which prolongs the diffusion path of reactants and products.

[0031] Combined with the kinetic diameters of aromatic hydrocarbons such as toluene and xylene in VOCs, compared with the pore structure of ZSM-5 (0.53×0.56 nm), Beta zeolite is composed of a unique three-dimensional dodecasil ring topological structure, with a linear channel of 0.66×0.67 nm and a bent channel of 0.56×0.56 nm, which can effectively improve the diffusion performance of toluene and the like. Therefore, when selecting the microporous or cage-like structure of Beta zeolite to encapsulate metals, according to different entry methods, metal nanoparticles can be confined in the microporous structure or dispersed on the mesoporous surface. The encapsulation position of the metal relative to the support directly affects the reaction sites. A thin layer of zeolite shell can be coated on the outside of the metal nanoparticles by utilizing the synergistic effect between the metal particles and the zeolite to form a hollow nanocapsule zeolite sphere. This structure has a hierarchical pore structure of macropores - mesopores - micropores, which can effectively reduce the diffusion resistance of reactants and products, and significantly improve the reaction efficiency and inhibit sintering.

[0032] See Figure 1 , which shows a method for preparing an encapsulated metal catalyst of hollow spherical hierarchical pore Beta zeolite according to an embodiment of the present invention. The preparation method includes:

[0033] Step S1: Mix an aluminum source, a silicon source, a template agent and water and stir for a first period of time (for example, 2 - 6 hours) to obtain a first reactant, subject the first reactant to hydrothermal crystallization reaction at 110 - 130 °C for 20 - 30 hours, and obtain Beta zeolite seeds;

[0034] Step S2: Add silica spheres to a solution of a cationic polyelectrolyte, wash with ammonia water after ultrasonic treatment (for example, 0.5 - 1 hour) and standing (for example, 0.5 - 1 hour) to obtain a second reactant, then add the second reactant to a solution of an anionic polyelectrolyte, wash with ammonia water after ultrasonic treatment (for example, 0.5 - 1 hour) and standing (for example, 0.5 - 1 hour), and obtain silica spheres with polyelectrolytes after drying (for example, at 70 - 90 °C);

[0035] Step S3: Dissolve a metal precursor in a solution of Beta zeolite seeds to obtain a third reactant, then immerse the silica spheres with polyelectrolytes in the third reactant, perform ultrasonic treatment (for example, 0.5 - 1 hour) and stir for a second period of time (for example, 1 - 5 hours), and perform hydrothermal crystallization reaction at 110 - 130 °C for 2 - 8 days to obtain a mixed sol;

[0036] In step S4, the mixed sol is washed with ammonia water, ethanol, and water in sequence until it is neutral (to remove the upper layer of colloid in the mixed sol), and then dried under vacuum to obtain a fourth reactant. The fourth reactant is calcined at 400 - 650 °C for 2 - 6 hours, and then reduced in a mixed gas of hydrogen and argon at 450 - 650 °C for 1 - 3 hours to obtain the encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite.

[0037] In the examples of the present invention, micron-sized mesoporous silica spheres are used as the metal carrier and the silicon source required for the growth of zeolite seeds. An anionic polyelectrolyte and a cationic polyelectrolyte are introduced to form silica spheres with polyelectrolytes. Then, the metal precursor is dissolved in the solution of Beta zeolite seeds, and the silica spheres with polyelectrolytes are impregnated into it. On the one hand, the silica spheres with polyelectrolytes provide a silicon source for the formation of Beta zeolite. Through an in-situ hydrothermal crystallization reaction, Al in the seed solution reacts with mesoporous silica (specifically, the silicon species precursor [Si x O y - ) at a temperature of 110 - 130 °C to form Beta zeolite. If the temperature is too low, the reactants will always be in a colloidal state. If the temperature is too high, other zeolites (such as ZSM-5) will be formed. Specifically, in the silica spheres with polyelectrolytes, the outermost anionic polyelectrolyte is negatively charged, and the zeolite seed solution (specifically, the template agent therein) is positively charged. Therefore, there is a strong electrostatic attraction force between the anionic polyelectrolyte and the zeolite seed solution, which can prevent the aluminum source from entering the interior of the silica spheres with polyelectrolytes, for example, reacting with the cationic polyelectrolyte, thus avoiding the collapse of the hollow structure caused by their reaction. Moreover, after the silica spheres dissolve (specifically, dissolve in the third reactant), a silicon species precursor [Si x O y - is formed. The silicon species precursor [Si x O y - gradually moves outward and forms regular [Si x Al y O] - with the aluminum source under the action of the template agent (such as an organic template agent). Furthermore, [Si x Al y O] - arranges into Beta zeolite. During the process of Beta zeolite formation, the silica spheres gradually dissolve to form a hollow structure and react with the aluminum source in the seed solution to form zeolite.

[0038] ​​​Moreover, in the preparation process, the metal precursor is first dissolved in the seed solution, and then the seed solution containing the metal precursor is reacted with the silica spheres with polyelectrolytes. In this way, not only can the utilization rate of the noble metal be improved, but more importantly, the hollow polycrystalline spheres will gradually nucleate around the metal and encapsulate the metal into the shell (specifically, the pores of Beta zeolite). That is, the metal precursor will be encapsulated into the pores of Beta zeolite, and a thin layer of zeolite crystals will coat the metal nanoparticles, synthesizing thin-walled zeolite hollow spheres. Thus, the metal nanoparticles are confined within the thin walls of the hollow polycrystalline sphere zeolite, avoiding the situation where the metal nanoparticles are only adsorbed on the surface of the zeolite, enabling the metal particles to be highly ordered and dispersed within the thin walls of the zeolite and preventing agglomeration. Therefore, the thin-walled hollow spherical hierarchical pore Beta zeolite encapsulated metal catalyst of the present invention can effectively improve the dispersion degree and utilization rate of the metal active sites, and thus has excellent high catalytic activity and product selectivity in the VOCs elimination reaction.

[0039] During the formation of the zeolite, the zeolites stack to form macroporous structures (greater than 50 nm) and mesoporous structures (2 - 50 nm), and microporous structures (less than 2 nm) are formed inside the zeolites. At the same time, the anionic polyelectrolyte and the cationic polyelectrolyte can also improve the carrier topology structure by using their flexible chains, introducing intracrystalline mesopores (2 - 50 nm) into the zeolite hollow spheres, thereby preparing a hierarchical pore hollow sphere structure with macropores - mesopores - micropores. This structure can effectively improve the mass transfer resistance of the single microporous structure to the reactants.

[0040] The wall thickness of the encapsulated metal catalyst of the present invention is 2 - 10 nm, the metal content is 0.1 - 6.0 wt% (for example, 1.0 - 3.0 wt%), and the metal particle size is less than 2 nm.

[0041] In one embodiment, the wall thickness of the hollow sphere is regulated by the electrostatic self-assembly of the charged polymer and the zeolite precursor. Specifically, during the process of obtaining the silica spheres with polyelectrolytes, the process of obtaining the second reactant is repeated 0 - 4 times. Repeating 0 times means treating with the cationic polyelectrolyte once and treating with the anionic polyelectrolyte once. Repeating 1 time means treating with the cationic electrolyte twice and treating with the anionic polyelectrolyte once. And so on, repeating 4 times means treating with the cationic electrolyte five times and treating with the anionic polyelectrolyte once.

[0042] Each time the cationic polyelectrolyte is used for treatment, since an excessive amount of silica spheres are provided during the treatment, new silicon species precursors [Si x O y - ​(Silica spheres without polyelectrolyte before), and then anionic polyelectrolyte was adsorbed outside the cationic polyelectrolyte treated last time to form a structure of (silica spheres - cationic polyelectrolyte) n -anionic polyelectrolyte, where n represents the number of repeated treatments. This substance has a negative charge on its surface and will form a layer of molecular sieve during the subsequent crystallization process. And with the dissolution of the silica spheres, a hollow spherical structure will be formed. As the number of repetitions increases, more silica spheres can be adsorbed, so the wall thickness of the hollow spherical molecular sieve structure formed later will be thicker.

[0043] In one example, in each repetition process, the concentration and volume of the solution of the cationic polyelectrolyte used are the same. The concentration of the solution of the cationic polyelectrolyte used is 0.5 wt% - 5.0 wt%, for example 1.0 wt% - 2.0 wt%.

[0044] In one example, the concentration of the solution of the anionic polyelectrolyte used is 0.5 wt% - 5.0 wt%, for example 1.0 wt% - 2.0 wt%.

[0045] The anionic polyelectrolyte includes at least one of sodium polystyrene sulfonate, sodium polyacrylate, sodium polymethacrylate, and sodium polyvinyl sulfonate.

[0046] The cationic polyelectrolyte includes at least one of poly(diallyldimethylammonium chloride) and poly(epichlorohydrin dimethylammonium).

[0047] In one example, the cationic polyelectrolyte is poly(diallyldimethylammonium chloride), the anionic polyelectrolyte is sodium polystyrene sulfonate, and the mass ratio of silica spheres, cationic polyelectrolyte, and anionic polyelectrolyte is greater than or equal to 50(n + 1) : (n + 1) : 1, where n represents the number of repeated treatments and n is a natural number. The meaning of greater than or equal to 50(n + 1) : 1 : 1 is that the mass ratio of silica spheres can be increased to ensure that there are enough silica spheres for the adsorption reaction with the cationic polymer. For example, in the case of one treatment (the number of repetitions is 0), 0.1 g of silica spheres can be used, 10 mL of poly(diallyldimethylammonium chloride) (10 wt%) and 10 mL of sodium polystyrene sulfonate (10 wt%) can be used.

[0048] In the process of obtaining silica spheres with polyelectrolyte, the purpose of washing with ammonia water: on the one hand, to remove some trace impurities, and on the other hand, to provide an alkaline environment for the synthesis of molecular sieve. For example, ammonia water can be used to wash twice.

[0049] Specifically, the process of obtaining the first reactant includes the following steps:

[0050] Adding an aluminum source to the template and stirring at room temperature until it becomes clear (e.g., 3 hours), and obtaining a first reaction solution;

[0051] Adding the silicon source to the template and water and stirring at room temperature until it becomes clear, and obtaining a second reaction solution;

[0052] The first reaction liquid is added dropwise to the second reaction liquid, and stirred at 70-90° C. for 2-6 hours to obtain a first reactant.

[0053] Since the silicon source has a lower solubility than the aluminum source, the embodiment of the present invention uses more water to form the second reaction solution containing the silicon source.

[0054] In one example, the aluminum source includes one of aluminum powder and aluminum oxide; the silicon source includes one of fumed silica, silica gel, inorganic silicate or ethyl orthosilicate; the template includes one of tetraethylammonium hydroxide and tetraethylammonium bromide. The molar ratio of the aluminum source, the silicon source, the template and water is 1:(10-50):(1-10):(5-500).

[0055] In one example, the metal precursor includes one of the precursors of Pd, Pt, Ru, Rh and Au. The preparation process of the metal precursor is as follows: a metal salt solution, hydrochloric acid, tetrahydrofuran, triphenylphosphine and sodium borohydride solution are mixed, stirred at room temperature for a period of time (e.g., 10-30 minutes), and then centrifuged, rotary evaporated (e.g., at 25-50°C), and extracted to obtain a metal precursor. The metal salt is one of the chlorinated metal salts of Pd, Pt, Ru, Rh and Au.

[0056] In one example, the ratio of the metal precursor to the Beta molecular sieve seed crystals is 1.0 g: 3.0-5.0 g.

[0057] In step S4, vacuum drying is performed at 80-110 degrees. The purpose of vacuum drying is to remove water. However, in order to avoid the formation of new molecular sieves, the highest temperature should not exceed 110 degrees.

[0058] In step S4, the fourth reactant is calcined to remove the organic template. If the calcination time is too long (longer than 6 hours), the product structure will collapse. If the calcination time is too short (shorter than 2 hours), the organic template will not react completely.

[0059] In step S4, the reduction reaction is to reduce the metal that may be oxidized to metal element or metal atom, and the reduction reaction can be achieved by using a mixed gas of hydrogen and argon at a reduced cost (compared with pure hydrogen). In one example, the volume fraction of hydrogen in the mixed gas of hydrogen and argon is 10%-30%.

[0060] Another embodiment of the present invention provides a metal-encapsulated catalyst of hollow spherical hierarchical pore Beta zeolite. The wall thickness of the metal-encapsulated catalyst is 2 - 10 nm, the metal content is 0.1 - 6.0 wt% (for example, 1.0 - 3.0 wt%), and the metal particle size is less than 2 nm. The metal particles in the metal-encapsulated catalyst are encapsulated in the zeolite pores. The metal-encapsulated catalyst is prepared according to the preparation method of the foregoing embodiment.

[0061] Another embodiment of the present invention provides an application of a metal-encapsulated catalyst of hollow spherical hierarchical pore Beta zeolite in the removal of VOCs. The metal-encapsulated catalyst is the metal-encapsulated catalyst described in the foregoing embodiment, or is prepared according to the preparation method described in the foregoing embodiment.

[0062] For example, the thin-walled metal-encapsulated catalyst of hollow spherical hierarchical pore zeolite is pressed into particles with a mesh size of 20 - 40, and then mixed with quartz sand, where the content of the catalyst in the mixture is 10 wt% - 30 wt%; standard air (21% N2 + 79% O2) is used as the carrier gas to introduce VOCs vapor into a fixed-bed reaction device at a certain flow rate, the feed space velocity is 30,000 mL / (g·h) - 60,000 mL / (g·h), the VOCs model compound is toluene, and the reaction pressure is atmospheric pressure. In this way, the removal of VOCs can be achieved by using the catalyst of the present invention.

[0063] Specific embodiments will be described in detail below in conjunction with the accompanying drawings. Those skilled in the art should understand that the present invention is not limited to the specific embodiments and can be reasonably modified after understanding the concept of the present invention.

[0064] Examples 1 - 8: Preparation of a thin-walled metal-encapsulated catalyst of hollow spherical hierarchical pore zeolite.

[0065] 0.27 g of aluminum powder was added to 19.0 g of TEAOH (tetraethylammonium hydroxide) and stirred at room temperature for 3 hours (h) until clear. 21.0 g of fumed silica was added in portions to a mixed solution of 45.8 g of water and 40.0 g of TEAOH reagent, and stirred at 80 °C for 4 h until clear. Then the aluminum-containing solution was dropped into the silicon-containing solution, stirred at 80 °C for 4 h to obtain a zeolite stock solution for standby. Subsequently, the zeolite stock solution was loaded into a polytetrafluoroethylene reaction kettle, sealed and crystallized at 115 °C for 24 h to obtain a zeolite seed solution for standby.

[0066] Weigh 0.1 g of mesoporous silica spheres and add them to 10 mL of a PDDA (poly(diallyldimethylammonium chloride)) solution (with a concentration of 10 wt%). Ultrasonicate for 0.5 h, let stand for 0.5 h, wash with ammonia water, then add to 10 mL of a PSS (sodium polystyrene sulfonate) solution (with a concentration of 10 wt%). Ultrasonicate for 0.5 h, let stand for 0.5 h, wash with ammonia water, and then dry at 80 °C to obtain mesoporous silica spheres with polyelectrolytes for standby.

[0067] Weigh an appropriate amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ and dissolve it in 20 mL of a molecular sieve seed solution (the concentration of the seed solution is 10 wt%). Then immerse the mesoporous silica spheres with polyelectrolytes into the above solution, ultrasonicate for 0.5 h, stir for 3 h, and then crystallize at 115 °C for 72 h to obtain a mixed sol. Wash the mixed sol with ammonia water, ethanol, and water in sequence until neutral, and then dry under vacuum to obtain a white powder. Wash the synthesized white powder with deionized water until neutral, dry at 80 °C, calcine at 550 °C for 4 h, and then reduce it at 550 °C for 2 h in a hydrogen-argon mixed gas with a hydrogen volume content of 10% to obtain a hollow spherical hierarchical pore molecular sieve encapsulated metal catalyst.

[0068] Among them, the amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ used in Example 1 is 0.0537 g, the amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ used in Example 2 is 0.1342 g, the amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ used in Example 3 is 0.2683 g, the amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ used in Example 4 is 0.5367 g, the amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ used in Example 5 is 0.8050 g, the amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ used in Example 6 is 1.0734 g, the amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ used in Example 7 is 1.342 g, and the amount of [Pd(NH₂CH₂CH₂NH₂)₂]Cl₂ used in Example 8 is 1.6101 g.

[0069] Examples 9 - 13: Preparation of a thin-walled hollow spherical hierarchical pore molecular sieve encapsulated metal catalyst.

[0070] 0.27 g of aluminum powder was added to 19.0 g of TEAOH and stirred at room temperature for 3 h until it became clear. 21.0 g of fumed silica was added in several portions to a mixed solution of 45.8 g of water and 40.0 g of TEAOH reagent, and stirred at 80 °C for 4 h until it became clear. Then the aluminum-containing solution was added dropwise to the silicon-containing solution, stirred at 80 °C for 4 h to obtain the zeolite stock solution for standby. Subsequently, the mixed sol was loaded into a polytetrafluoroethylene autoclave, sealed and crystallized at 115 °C for 24 h to obtain the zeolite seed solution for standby.

[0071] 0.25 g of mesoporous silica spheres were weighed and added to an appropriate amount of PDDA solution, sonicated for 0.5 h, left to stand for 0.5 h, washed with ammonia water, then added to an appropriate amount of PSS solution, sonicated for 0.5 h, left to stand for 0.5 h, washed with ammonia water, and then dried at 80 °C to obtain mesoporous silica spheres with polyelectrolytes for standby.

[0072] Among them, the PDDA and PSS used in Example 9 were 0 mL (treated zero times); the PDDA used in Example 10 was 20 mL and the PSS was 10 mL, and the PDDA treatment was carried out twice, with 10 mL used each time; the PDDA used in Example 11 was 30 mL and the PSS was 10 mL, and the PDDA treatment was carried out three times, with 10 mL used each time; the PDDA used in Example 12 was 40 mL and the PSS was 10 mL, and the PDDA treatment was carried out four times, with 10 mL used each time; the PDDA used in Example 13 was 50 mL and the PSS was 10 mL, and the PDDA treatment was carried out five times, with 10 mL used each time.

[0073] 0.1342 g of [Pd(NH2CH2CH2NH2)2]Cl2 was weighed and dissolved in 20 mL of the zeolite seed solution (the concentration of the seed solution was 10 wt%), and then the mesoporous silica spheres with polyelectrolytes were impregnated into the above solution, sonicated for 0.5 h, stirred for 3 h, and then crystallized at 115 °C for 72 h to obtain a mixed sol. The mixed sol was washed with ammonia water, ethanol and water in turn until neutral, and then dried in vacuo to obtain a white powder. The synthesized white powder was washed with deionized water until neutral, dried at 80 °C, calcined at 550 °C for 4 h, and then reduced at 550 °C for 2 h in a hydrogen-argon mixed gas with a hydrogen volume content of 10% to obtain a hollow spherical hierarchical pore molecular sieve encapsulated metal catalyst.

[0074] Examples 14 - 18: Preparation of a thin-walled hollow spherical hierarchical pore molecular sieve encapsulated metal catalyst.

[0075] Same as Example 3, with the only difference being that during the process of obtaining the molecular sieve seed solution, the crystallization time of Example 14 is 6 h, the crystallization time of Example 15 is 24 h, the crystallization time of Example 16 is 48 h, the crystallization time of Example 17 is 60 h, and the crystallization time of Example 18 is 96 h.

[0076] Example 19: Catalytic performance evaluation.

[0077] The catalysts obtained in Examples 2, 10, and 12 were used in the reaction of catalytic complete oxidation of toluene. The specific steps are as follows: Weigh 0.1 g of 20 - 40 mesh catalyst and mix it with 2.0 g of quartz sand, load it into a fixed - bed reactor, heat it to 120 - 300 °C, then pump in toluene vapor, and then increase the temperature step - by - step and introduce oxygen. The space velocity is 60,000 mL / (g·h). The obtained products are condensed and collected, and all the obtained products are carbon dioxide and water.

[0078] The effect of Example 19 is as Figure 2 shown. It can be seen from Figure 2 that the temperature at which the catalyst obtained in Example 2 has a toluene conversion rate of 100% is 181 °C, and its catalytic performance is superior to that of the catalysts obtained in Example 10 and Example 12. While using the existing catalyst, under the same conditions, the toluene conversion rate is only 70%. It can be seen from Figure 3 that the catalyst of Example 2 does not show obvious deactivation within 300 h, while using the existing catalyst, under the same conditions, deactivation occurs at 48 h.

[0079] The encapsulated metal catalyst of the hollow - spherical hierarchical - pore Beta molecular sieve according to the present invention, its preparation method and application have at least one of the following advantages:

[0080] (1) The encapsulated metal catalyst of the hollow - spherical hierarchical - pore Beta molecular sieve according to the present invention, its preparation method and application use the molecular sieve as a carrier to encapsulate the noble metal material, and utilize the rich pore structure of the molecular sieve to reduce the diffusion resistance of reactants and products, reduce agglomeration, thereby improving the reaction rate and inhibiting sintering;

[0081] (2) The encapsulated metal catalyst of the hollow - spherical hierarchical - pore Beta molecular sieve according to the present invention, its preparation method and application utilize the confinement effect of the pore structure of the molecular sieve to encapsulate metal particles in the molecular sieve pores to form a hollow core - shell structure, controlling the metal size distribution and high - temperature agglomeration;

[0082] (3) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application utilize the unique three-dimensional twelve-membered ring topological structure of the Beta zeolite, which has a linear channel of 0.66×0.67 nm and a curved channel of 0.56×0.56 nm, and can effectively improve the diffusion performance of toluene, etc.;

[0083] (4) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application utilize the synergistic effect of metal particles and zeolite to coat a thin layer of zeolite shell outside the metal nanoparticles to form a hollow nanocapsule zeolite sphere. This structure has a hierarchical pore structure of macropores - mesopores - micropores at the same time, which can effectively reduce the diffusion resistance of reactants and products, and significantly improve the reaction efficiency and inhibit sintering;

[0084] (5) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application adopt an in-situ synthesis method to prepare the catalyst. Compared with the mesoporous material impregnation method, it can directly control the structure and thickness of the zeolite and the size of the encapsulated metal particles;

[0085] (6) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application synthesize a structurally stable hollow zeolite by using mesoporous silica spheres as the metal carrier and the silicon source required for the growth of zeolite seeds, and solve the problem of easy collapse of the hollow zeolite;

[0086] (7) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application regulate the shell thickness of the Beta zeolite by using cationic and anionic electrolytes, and at the same time introduce intracrystalline mesopores by using its flexible chains to synthesize a thin-walled (wall thickness 2 - 10 nm) encapsulated metal catalyst of hollow spherical hierarchical pore zeolite;

[0087] (8) The encapsulated metal catalyst of the hollow spherical hierarchical pore Beta zeolite of the present invention, its preparation method and application utilize the thin-walled hollow spherical hierarchical pore zeolite encapsulated metal catalyst to carry out the VOCs elimination reaction, and at the same time has the advantages of high catalytic activity and high stability.

[0088] Although some embodiments of the general inventive concept of the present invention have been shown and described, those of ordinary skill in the art will understand that changes can be made to these embodiments without departing from the principles and spirit of the general inventive concept. The scope of the present invention is defined by the claims and their equivalents.

Claims

1. A preparation method of a metal-encapsulated catalyst of hollow spherical hierarchical pore Beta zeolite, comprising: Step S1: Mix an aluminum source, a silicon source, a template agent and water and stir for a first period of time to obtain a first reactant. Hydrothermally crystallize the first reactant at 110 - 130 °C for 20 - 30 hours to obtain Beta zeolite seeds. The template agent includes one of tetraethylammonium hydroxide and tetraethylammonium bromide; Step S2: Add silica spheres into a solution of a cationic polyelectrolyte, subject it to ultrasonic treatment, let it stand, and then wash it with ammonia water to obtain a second reactant. Then add the second reactant into a solution of an anionic polyelectrolyte, subject it to ultrasonic treatment, let it stand, and then wash it with ammonia water. After drying treatment, obtain silica spheres with polyelectrolytes; Step S3: Dissolve a metal precursor in a solution of Beta zeolite seeds to obtain a third reactant. Then immerse the silica spheres with polyelectrolytes into the third reactant, perform ultrasonic treatment and stir for a second period of time, and hydrothermally crystallize at 110 - 130 °C for 2 - 8 days to obtain a mixed sol; Step S4: Wash the mixed sol with ammonia water, ethanol and water in sequence until neutral, then vacuum dry to obtain a fourth reactant. Calcinate the fourth reactant at 400 - 650 °C for 2 - 6 hours, and then reduce it in a mixed gas of hydrogen and argon at 450 - 650 °C for 1 - 3 hours to obtain the metal-encapsulated catalyst of the hollow spherical hierarchical pore Beta zeolite.

2. The preparation method according to claim 1, wherein In step S3, the silica spheres with polyelectrolytes are dissolved in the third reactant to form a silicon species precursor [Si x O y - , and the silicon species precursor [Si x O y - gradually moves outwards and, under the action of the template agent in the Beta zeolite seed, forms regular [Si x Al y O] - with the aluminum source in the Beta zeolite seed. Furthermore, [Si x Al y O] - arranges into Beta zeolite. During the process of Beta zeolite formation, the silica spheres gradually dissolve to form a hollow structure, and the metal precursor dissolved in the Beta zeolite seed solution is encapsulated into the pores of the Beta zeolite during the formation of the zeolite.​​ Beta zeolites are stacked to form macropores and mesopores, and micropores are formed inside the zeolites. The macropores, mesopores and micropores form hierarchical pores.

3. The preparation method according to claim 2, wherein In step S2, in the process of obtaining silica spheres with polyelectrolytes, the process of obtaining the second reactant is repeated 0 - 4 times; The structure of the silica spheres with polyelectrolytes is (silica spheres - cationic polyelectrolyte) n - anionic polyelectrolyte structure, where n represents the number of repeated treatments.

4. The preparation method according to claim 3, wherein In each repetition process, the concentration and volume of the solution of the cationic polyelectrolyte used are the same. The concentration of the solution of the cationic polyelectrolyte used is 0.5 wt% - 5.0 wt%; The concentration of the solution of the anionic polyelectrolyte used is 0.5 wt% - 5.0 wt%; The cationic polyelectrolyte includes at least one of polydiallyldimethylammonium chloride and polyepichlorohydrin dimethylammonium; The anionic polyelectrolyte includes at least one of sodium polystyrene sulfonate, sodium polyacrylate, sodium polymethacrylate and sodium polyvinyl sulfonate.

5. The preparation method according to claim 4, wherein The cationic polyelectrolyte is polydiallyldimethylammonium chloride; The anionic polyelectrolyte is sodium polystyrene sulfonate; The mass ratio of the silica spheres, the cationic polyelectrolyte and the anionic polyelectrolyte is greater than or equal to 50(n + 1):(n + 1):

1.

6. The preparation method according to any one of claims 1 - 5, wherein In step S1, the process of obtaining the first reactant includes the following steps: Add the aluminum source into the template agent and stir at room temperature until it becomes clear to obtain a first reaction solution; Add the silicon source to the template agent and water, stir at room temperature until clear, and obtain a second reaction solution; Drop the first reaction solution into the second reaction solution, stir at 70 - 90 °C for 2 - 6 hours to obtain a first reactant, The aluminum source includes one of aluminum powder and alumina; The silicon source includes one of fumed silica, silica gel, inorganic silicate, and tetraethyl orthosilicate; The molar ratio of the aluminum source, silicon source, template agent, and water is 1:(10 - 50):(1 - 10):(5 - 500).

7. According to the preparation method described in claim 6, wherein, The ratio of the metal precursor to the Beta zeolite seed is 1.0 g:3.0 - 5.0 g, The metal precursor includes one of the precursors of Pd, Pt, Ru, Rh, and Au, The second time is 1 - 5 hours.

8. According to the preparation method described in claim 7, wherein, In step S4, in the mixed gas of hydrogen and argon, the volume fraction of hydrogen is 10% - 30%.

9. A metal-encapsulated catalyst of hollow spherical hierarchical pore Beta zeolite, wherein, The wall thickness of the metal-encapsulated catalyst is 2 - 10 nm, the metal content is 0.1 - 6.0 wt%, the metal particle size is less than 2 nm, and the metal particles in the metal-encapsulated catalyst are encapsulated in the pore channels of the zeolite, The metal-encapsulated catalyst is prepared according to the preparation method of the metal-encapsulated catalyst of hollow spherical hierarchical pore Beta zeolite described in any one of claims 1 - 8.

10. Application of a metal-encapsulated catalyst of hollow spherical hierarchical pore Beta zeolite in VOCs removal, wherein, The metal-encapsulated catalyst is According to the metal-encapsulated catalyst described in claim 9, or is prepared according to the preparation method of the metal-encapsulated catalyst of hollow spherical hierarchical pore Beta zeolite described in any one of claims 1 - 8.

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