Core-shell catalysts for methanol dehydrogenation to polyoxymethylene dimethyl ether, their preparation and application

Abstract

This invention discloses a core-shell catalyst for the dehydrogenation of methanol to polyoxymethylene dimethyl ether (POMDE), its preparation, and its application. The catalyst comprises an acidic molecular sieve, a metal active component, and aluminum-containing mesoporous silica. The acidic molecular sieve serves as the core structure, with the metal active component coating the surface of the acidic molecular sieve to form a first coating layer, and the aluminum-containing mesoporous silica coating the surface of the metal active component to form a second coating layer. The prepared core-shell catalyst uses the middle acidic molecular sieve and the outermost aluminum-containing mesoporous silica as acidic sites, and the middle metal active component as a dehydrogenation site. By integrating these two reaction sites, a bifunctional catalyst is constructed, achieving in-situ coupling of the two-step reactions of methanol dehydrogenation to formaldehyde and methanol-formaldehyde condensation to POMDE. This not only simplifies the reaction steps but also improves the selectivity of the reaction.

Description

Technical Field

[0001] This invention relates to the field of catalyst materials technology, and in particular to core-shell catalysts for the dehydrogenation of methanol to polyoxymethylene dimethyl ether, their preparation and application. Background Technology

[0002] Polyoxymethylene dimethyl ether (PODE) n and DMM n Its general formula is CH3O(CH2O). n CH3 (where n is an integer ≥ 1, generally less than 8), typically n is 3–6 or 3–8, is a polyether oligomer. At room temperature, it is a colorless or pale yellow volatile flammable liquid with a slight ether odor. Polyoxymethylene dimethyl ether has advantages such as high cetane number, high oxygen content, high flash point, low pour point, and similar physicochemical properties to diesel fuel. It can be used as a blending component for clean diesel fuel that is free of sulfur and aromatics, and is stably miscible with diesel fuel.

[0003] The traditional synthetic route for polyoxymethylene dimethyl ether (POD) is a two-step process. Methyl-terminated feedstocks such as methylal and methanol are reacted with oxymethylene feedstocks such as gaseous formaldehyde, formaldehyde aqueous solution, trioxymethylene, and paraoxymethylene under the catalysis of an acidic catalyst via a condensation reaction. Chinese patent CN111574340A discloses a system and method for synthesizing POD from methanol. The system includes a methanol dehydrogenation unit, a formaldehyde absorption unit, an etherification reaction unit, and a purification and separation unit connected in sequence. Formaldehyde is prepared in the methanol dehydrogenation unit, and then the formaldehyde and methanol are mixed in the etherification reaction unit to synthesize the product, POD. Chinese patent CN1 10054550A discloses a process system and method for producing polyoxymethylene dimethyl ether (PODE) using gaseous formaldehyde. This method uses methanol as raw material, obtaining methanol gas through a methanol gasification device. The methanol gas is then passed into a methanol dehydrogenation reactor, where formaldehyde gas and hydrogen gas are generated under the action of a catalyst. The mixture of formaldehyde and hydrogen gas is then passed through a cooling device to obtain formaldehyde gas at a lower temperature. Methyl acetal is introduced into a methyl acetal preheating device to obtain methyl acetal gas. The methyl acetal gas and formaldehyde gas are then introduced into a gas mixer, and the mixture is introduced into a PODEN reaction tower. n The reaction inside the reaction tower, under the action of a catalyst, produces polyoxymethylene dimethyl ether.

[0004] The above methods all employ a two-step synthetic route: first, formaldehyde is prepared by dehydrogenation of methanol in the presence of a catalyst; then, polyoxymethylene dimethyl ether is prepared by condensation of formaldehyde and methanol in the presence of a catalyst. This two-step synthetic route for preparing polyoxymethylene dimethyl ether has the following disadvantages:

[0005] (1) Complex process: The two-step method needs to be carried out in stages. First, methanol is dehydrogenated to formaldehyde, and then the formaldehyde is further condensed with methanol to generate the target product, which increases the complexity of equipment and operation.

[0006] (2) High energy consumption: Each step requires specific reaction conditions, such as temperature and pressure, which usually leads to high energy consumption. For example, methanol dehydrogenation is an endothermic reaction, which requires additional energy to drive the reaction; and the subsequent condensation reaction also requires different temperature conditions.

[0007] To address the shortcomings of the aforementioned two-step synthesis of polyoxymethylene dimethyl ether (PODE), researchers further discovered that each step requires a catalyst to catalyze the dehydrogenation or condensation reaction. Based on this, integrating the catalysts used in both steps allows for a one-step coupling of the two reactions, thereby reducing process complexity and energy consumption. Under the action of a bifunctional catalyst (possessing both metallic and acidic sites), methanol undergoes partial dehydrogenation to formaldehyde at the metallic active site, and PODEn is synthesized from methanol and formaldehyde at the acidic active site. This route does not involve a formaldehyde separation step; formaldehyde reacts directly with methanol in situ to generate PODEn. Therefore, it has the advantages of a short reaction route, simple process, and low energy consumption. Furthermore, hydrogen is released as a high-quality byproduct instead of being oxidized to water. Therefore, this route is superior to the in-situ oxidation of methanol to PODEn (where methanol is oxidized in air or an oxygen atmosphere to formaldehyde and water, and then PODEn is generated in situ). n Compared to the direct route, this method offers high atom economy. The literature "Hydrogen-efficient non-oxidative transformation of methanol into dimethoxymethane over a tailored bifunctional Cu catalyst, Sustainable Energy Fuels, 2021, 5, 117" discloses a bifunctional copper / zeolite catalyst that can simultaneously catalyze the dehydrogenation of methanol to formaldehyde, followed by the condensation of formaldehyde with methanol to obtain polyoxymethylene dimethyl ether. This is a catalyst for the continuous production of polyoxymethylene dimethyl ether directly from methanol in the gas phase. The copper sites primarily handle methanol dehydrogenation, while the zeolite mainly provides acidic sites for the condensation of methanol and formaldehyde to form polyoxymethylene dimethyl ether. Although the above method of integrating two functional catalysts can couple two reactions into a single step, the above catalyst still has the following drawbacks in the one-step preparation of polyoxymethylene dimethyl ether:

[0008] (1) Bifunctional catalysts require simultaneous optimization of two different types of reactions, namely dehydrogenation and condensation, which increases the requirements for catalyst selectivity. If the catalyst is not designed properly, it may lead to an increase in side reactions, such as methanol dehydration to form dimethyl ether, excessive dehydrogenation of methanol to form CO or CO2, or methanol reacting with formaldehyde to form methyl formate. These will reduce the selectivity and yield of the target product.

[0009] (2) The mass transfer efficiency within the bifunctional catalyst is a key issue. Since formaldehyde is a gaseous product, it needs to be rapidly transferred from the dehydrogenation metal site to the acidic site for condensation reaction. If the catalyst structure is poorly designed, it may cause unnecessary side reactions of formaldehyde during the transfer process, or it may fail to reach the acidic site in time, thereby affecting the reaction rate and selectivity.

[0010] (3) Polyoxymethylene dimethyl ether is divided into PODEs with a degree of polymerization n=1. n PODEs with a degree of polymerization n = 3–6 or 3–8 n The preparation process of high-polymerization-degree polyoxymethylene dimethyl ether (PODE) involves a continuous condensation reaction. First, formaldehyde and methanol are used to prepare methyl acetal (DMM). Then, DMM reacts with formaldehyde to yield PODE2, and so on, to produce PODE2. n-1 Condensation with formaldehyde yields poly(methoxydimethyl ether) (PODE) with high polymer content. n In practical applications, polyoxymethylene dimethyl ethers with a high degree of polymerization have advantages such as higher energy density and better combustion characteristics. However, the bifunctional catalysts mentioned above are obtained by combining metallic copper and zeolite in a simple manner. After formaldehyde gas is obtained from the methanol dehydrogenation reaction, some formaldehyde desorbs and diffuses, reducing the formaldehyde gas concentration. This makes it easier for the preparation of methyl acetal to continue on the catalyst surface, hindering the formation of a continuous condensation reaction. Consequently, only methyl acetal with a degree of polymerization of 1 is detected in the reaction products, and products with a high degree of polymerization are not detected. Summary of the Invention

[0011] The purpose of this invention is to provide a core-shell catalyst for the dehydrogenation of methanol to polyoxymethylene dimethyl ether, its preparation and application, in order to solve the problems of low reaction selectivity and low yield of high-polymerization products of the above-mentioned bifunctional catalysts.

[0012] To achieve the above objectives, the first aspect of the present invention provides a core-shell catalyst for the dehydrogenation of methanol to produce polyoxymethylene dimethyl ether. The core-shell catalyst comprises an acidic molecular sieve, a metal active component, and aluminum-containing mesoporous silica. The acidic molecular sieve serves as the core structure, the metal active component is coated on the surface of the acidic molecular sieve to form a first coating layer, and the aluminum-containing mesoporous silica is coated on the surface of the metal active component to form a second coating layer.

[0013] To address the problem of unreasonable structural design in existing bifunctional catalysts for methanol dehydrogenation and condensation, this invention redesigns the catalyst structure. First, an acidic molecular sieve serves as the core structure. Then, a metal active component and an aluminous mesoporous silica layer are sequentially coated onto its surface. After coating, the entire catalyst exhibits a core-shell structure. The middle acidic molecular sieve and the outermost aluminous mesoporous silica layer primarily serve as catalytic sites for the condensation reaction of formaldehyde and methanol, while the metal active component between the acidic molecular sieve and the aluminous mesoporous silica primarily serves as the reaction site for methanol dehydrogenation.

[0014] In the actual reaction process, methanol undergoes methanol dehydrogenation with the metal active component in the intermediate layer to produce formaldehyde. The formaldehyde is located between the acidic molecular sieve and the aluminum-containing mesoporous silica, forming a confined space. This confined space increases the density of formaldehyde per unit space, which is beneficial for PODE. n-1 The accessibility to formaldehyde is significantly enhanced by regulating the relative ratio of dehydrogenation active sites (metallic active sites) to condensation active sites (acidic active sites), thereby significantly promoting the chain growth reaction of PODEn and improving the selectivity of high-polymerization-degree PODEn products. In contrast, ordinary bifunctional catalysts with both acidic and metallic active sites simply combine substances with acidic and metallic active sites, lacking the three-layer core-shell catalyst structure design of this invention. In practical applications, methanol undergoes dehydrogenation at the metallic active sites to generate formaldehyde. Since formaldehyde exists in gaseous form, it easily desorbs from the catalyst and diffuses, reducing its concentration. This forces the reaction to formaldehyde and methanol to produce methyl acetal to continue, making it difficult to obtain high-polymer polyoxymethylene dimethyl ether in subsequent continuous condensation reactions. This invention utilizes a three-layer core-shell catalyst design to confine formaldehyde generated from methanol catalyzed by the metal active component between an acidic molecular sieve and aluminum-containing mesoporous silica. This restricts formaldehyde gas diffusion to a certain extent, increasing the formaldehyde concentration within the confined space. This allows the methyl acetal prepared from formaldehyde and methanol to further react with formaldehyde to produce highly polymerizable polyoxymethylene dimethyl ether (POMME). The core-shell catalyst of this invention exhibits good selectivity for highly polymerizable POMME, which is beneficial for the application of POMME in diesel blending components.

[0015] Preferably, the dehydrogenated metal active component is at least one of noble metals and non-noble metals.

[0016] It should be noted that there are many metal catalysts that can be used in the methanol dehydrogenation reaction to prepare formaldehyde. In principle, all commonly used methanol dehydrogenation catalysts can be used as the metal active component of this invention.

[0017] Preferably, in step (1), the acidic molecular sieve is one of ZSM-5 molecular sieve, BETA molecular sieve, Al-MCM-41 molecular sieve, Al-SBA-15 molecular sieve, and Al-MCM-22 molecular sieve.

[0018] Among them, the silicon-to-aluminum ratio (Si / A ratio) of ZSM-5 molecular sieve ranges from 25 to 360, that of BETA molecular sieve ranges from 25 to 300, that of Al-MCM-41 molecular sieve ranges from 25 to 200, that of Al-SBA-15 molecular sieve ranges from 25 to 200, and that of Al-MCM-22 molecular sieve ranges from 25 to 200.

[0019] A second aspect of this invention provides a method for preparing a core-shell catalyst, comprising the following steps:

[0020] (1) A metal active component is coated on the surface of an acidic molecular sieve to obtain a molecular sieve composite material;

[0021] (2) The molecular sieve composite material was dispersed in a mixed solution, and then tetraethyl orthosilicate and aluminum nitrate were added in sequence. The mixture was stirred and reacted. After centrifugation, washing, drying and calcination, the nanocomposite material was obtained.

[0022] (3) The nanocomposite material was heat-treated in a reducing atmosphere to obtain a core-shell catalyst.

[0023] The main function of step (1) in this invention is to load the metal active component onto the acidic molecular sieve. Methods for loading the metal active component that are well known to those skilled in the art, such as impregnation, hydrothermal synthesis, ion exchange, precipitation, physical mixing, and chemical vapor deposition, can all be used in this invention.

[0024] Preferably, in step (1), when the active metal component is a non-precious metal, the raw material of the non-precious metal includes at least one of divalent soluble metal salts and at least one of trivalent soluble metal salts.

[0025] The specific process of coating the surface of acidic molecular sieves with metal active components is as follows:

[0026] Acidic molecular sieves are dispersed in deionized water and stirred thoroughly by ultrasonication to obtain solution A; divalent soluble metal salt, trivalent soluble metal salt and urea are dissolved in deionized water and stirred thoroughly to obtain solution B; solution B is added dropwise to solution A and stirring is continued to obtain a mixed solution.

[0027] The mixture was subjected to a hydrothermal reaction. After the reaction was completed, it was centrifuged, washed and dried to obtain molecular sieves / M1M2-LDHs.

[0028] Calcining molecular sieves / M1M2-LDHs yields molecular sieves / M1M2-LDO, which is a molecular sieve composite material. Here, M1 and M2 represent divalent and trivalent soluble metals, respectively.

[0029] When the active metal component is a non-precious metal, the non-precious metal includes divalent and trivalent soluble metal salts. This is mainly to grow layered double hydroxides (LDHs) in situ on the surface of acidic molecular sieves. After high-temperature calcination, the LDHs form complex metal oxides (LDOs), which are then reduced to form metal-metal oxides. Based on the formation characteristics of LDHs, M... 3+ / (M 2+ +M 3+ The molar ratio of ) is between 0.17 and 0.33.

[0030] Preferably, in step (1), when the active metal component is a non-precious metal and a precious metal, the raw material of the non-precious metal includes at least one of divalent soluble metal salts and at least one of trivalent soluble metal salts, and the raw material of the precious metal is at least one of soluble precious metal salts.

[0031] The specific process of coating the surface of acidic molecular sieves with metal active components is as follows:

[0032] Acidic molecular sieves are dispersed in deionized water and stirred thoroughly by ultrasonication to obtain solution A; divalent soluble metal salt, trivalent soluble metal salt and urea are dissolved in deionized water and stirred thoroughly to obtain solution B; solution B is added dropwise to solution A and stirring is continued to obtain a mixed solution.

[0033] The mixture was subjected to a hydrothermal reaction. After the reaction was completed, it was centrifuged, washed and dried to obtain molecular sieves / M1M2-LDHs.

[0034] Molecular sieves / M1M2-LDHs were impregnated in a soluble noble metal salt solution to obtain molecular sieves / M3-M1M2-LDHs;

[0035] Calcining molecular sieves / M3-M1M2-LDHs yields molecular sieves / M3-M1M2-LDO, which is a molecular sieve composite material. Here, M3 represents a soluble noble metal.

[0036] In this invention, divalent and trivalent soluble metal salts can generate LDHs in situ on the surface of acidic molecular sieves through hydrothermal reaction. However, it is difficult to coat noble metal soluble metal salts through the above reaction. Therefore, when loading noble metal active components, an impregnation method is used to impregnate the prepared nanocomposite material in a solution of noble metal soluble metal salts, thereby adsorbing the noble metal onto the nanocomposite material and achieving noble metal coating.

[0037] As mentioned above, there are many known methods for loading metal active components onto the surface of acidic molecular sieves. However, each method has different effects on the distribution of metal active components on the acidic molecular sieve and the subsequent aggregation of metal active components. Based on this, the present invention specifically loads metal active components onto the surface of acidic molecular sieves in the form of LDHs. This increases the uniformity of the distribution of metal active components. After subsequent calcination and roasting, it can further reduce the aggregation of metal active components. This is particularly important for improving the catalytic activity of the entire catalyst and can further improve the reaction selectivity.

[0038] It should also be noted that LDHs are coated on the surface of acidic molecular sieves in their own layered form. This results in an empty shell structure in the middle layer after being coated with aluminum-containing mesoporous silica, which is the confined space mentioned above.

[0039] Preferably, the hydrothermal reaction temperature is 100–140°C and the time is 12–24 h.

[0040] Preferably, the roasting temperature is 400-500℃ and the time is 2-6 hours.

[0041] Preferably, the divalent soluble metal salt is at least one selected from nickel, copper, and cobalt salts; the trivalent soluble metal salt is at least one selected from aluminum, iron, manganese, and chromium salts; and the soluble noble metal salt is at least one selected from platinum, palladium, ruthenium, and rhodium salts. It should be noted that when coating the surface of acidic molecular sieves with non-noble metal active components, this invention primarily utilizes the reaction of divalent and trivalent metal ions on the surface of the acidic molecular sieve to form LDHs. Therefore, any divalent or trivalent metal ions capable of catalyzing methanol dehydrogenation and generating LDHs can be used in this invention, and are not limited to the divalent and trivalent soluble metal salts listed above.

[0042] More preferably, the nickel salt is nickel nitrate or nickel chloride, the aluminum salt is aluminum nitrate or aluminum chloride, the copper salt is copper nitrate or copper chloride, the cobalt salt is cobalt nitrate or cobalt chloride, the iron salt is ferric nitrate, the chromium salt is chromium nitrate, the platinum salt is chloroplatinic acid, sodium chloroplatinate or potassium chloroplatinate, the ruthenium salt is ruthenium trichloride, the palladium salt is palladium nitrate or palladium acetate, and the rhodium salt is rhodium trichloride.

[0043] Preferably, the mass ratio of acidic molecular sieve, dehydrogenation active component and aluminum-containing mesoporous silica is 1:0.1-0.4:0.1-0.4, based on the mass of the metal reduced state in the metal active component.

[0044] Preferably, in step (2), the mixed solution includes deionized water, ethanol, hexadecyltrimethylammonium bromide and 25wt% NH3·H2O.

[0045] Preferably, in step (2), the calcination temperature is 400-550℃ and the time is 2-6h.

[0046] Preferably, in step (2), the silicon-to-aluminum ratio of the aluminum-containing mesoporous silica is in the range of 25 to 300.

[0047] In step (2) of this invention, the corresponding tetraethyl orthosilicate and aluminum nitrate can be added according to the silicon-to-aluminum ratio of aluminum-containing mesoporous silica (Al-MS).

[0048] Preferably, in step (3), the reducing atmosphere is pure hydrogen or a mixture of hydrogen and argon. When the reducing atmosphere is a mixture of hydrogen and argon, the volume ratio of hydrogen to argon is (1-3):(7-9).

[0049] Preferably, in step (3), the heat treatment temperature for the reduction reaction is 400-600℃ and the time is 2-4h.

[0050] The third aspect of the present invention provides an application of a core-shell catalyst in the in-situ coupling of methanol dehydrogenation to produce polyoxymethylene dimethyl ether.

[0051] The reaction process for the in-situ coupling of methanol dehydrogenation to polyoxymethylene dimethyl ether catalyzed by a core-shell catalyst in this invention is as follows:

[0052]

[0053] The first step is a methanol dehydrogenation reaction, where methanol is converted to polyoxymethylene dimethyl ether (POD) under the catalysis of a metal active site (metal active component). The second step is a condensation reaction, where methanol and formaldehyde react at acidic active sites (acidic molecular sieves and aluminum-containing mesoporous silica) to form POD. These two steps can be coupled in situ using a bifunctional catalyst (a composite catalyst of metal and acidic active sites), ultimately resulting in a one-step reaction of methanol to POD.

[0054] Preferably, the continuous reaction process for the in-situ coupling of methanol dehydrogenation to produce polyoxymethylene dimethyl ether is as follows:

[0055] The reaction is carried out in a fixed-bed reactor. After molding and tableting, the core-shell catalyst is sieved to obtain particles with a diameter of 20-40 mesh and loaded into the reaction tube. The core-shell catalyst bed is heated to 150-260°C under a flowing N2 atmosphere. Subsequently, methanol is metered in by a high-pressure constant flow pump, with a methanol volume hourly space velocity of 0.2-2 h⁻¹ (LCHSV) based on liquid phase feed. -1 After being vaporized, methanol is thoroughly mixed with N2 and introduced into the reaction tube. The reaction temperature is 120–240°C and the reaction pressure is atmospheric pressure.

[0056] Preferably, the batch reaction process for the in-situ coupling of methanol dehydrogenation to produce polyoxymethylene dimethyl ether is as follows:

[0057] The reaction is carried out in a condenser-equipped reactor or an atmospheric pressure glass reactor. A highly stable, high-boiling-point solvent is added during the reaction to ensure that the reaction temperature reaches above the boiling point of methanol. Under argon protection, the reaction temperature is 120–220°C, the reaction time is 0.5–6 hours, the reaction pressure is atmospheric pressure, and the amount of core-shell catalyst is 1.5 wt% of the mass of methanol.

[0058] Therefore, the present invention employs the above-mentioned core-shell catalyst for methanol dehydrogenation to polyoxymethylene dimethyl ether, its preparation and application, and has the following beneficial effects:

[0059] (1) The core-shell catalyst prepared by the present invention is a bifunctional catalyst with acidic sites and metal active sites, which can realize the in-situ coupling of methanol dehydrogenation and methanol-formaldehyde condensation reaction. This not only simplifies the reaction steps, but also improves the selectivity and yield of the reaction.

[0060] (2) The core-shell catalyst of the present invention uses the middle acidic molecular sieve and the outermost aluminum-containing mesoporous silica as acidic sites and the dehydrogenation metal active component in the middle layer as metal active sites. Both have synergistic effects, which can better control the reaction path, reduce the generation of by-products, and improve the selectivity of the target product.

[0061] (3) The core-shell catalyst of the present invention has a three-layer core-shell structure, with an acidic molecular sieve as the core, a metal-metal oxide (metal active component) as the middle layer, and an outermost shell structure of aluminum-containing mesoporous silica. The surrounding structure of the intermediate metal active site by the bilayer acid catalyst promotes the reaction of the generated intermediate product formaldehyde with methanol / PODE in a confined space environment. n-1 Condensation reactions occur at acidic sites, thereby effectively promoting chain growth reactions and improving the selectivity of high-polymerization-degree products.

[0062] (4) By controlling the composition of the metal active sites, the present invention can effectively control the dehydrogenation temperature and dehydrogenation reaction rate of methanol as well as the selectivity of dehydrogenation products; by controlling the type of molecular sieve core and the silicon-aluminum ratio and the silicon-aluminum ratio of the Al-MS shell, the product selectivity of the reaction between methanol and formaldehyde can be controlled, effectively avoiding the generation of by-products such as methyl formate; by controlling the relative ratio and distance between the metal active sites and the acid active sites, the composition and selectivity of the products can be effectively controlled, thereby achieving highly selective synthesis of PODEn.

[0063] (5) This invention uses a core-shell catalyst to prepare polyoxymethylene dimethyl ether by in-situ coupling of methanol dehydrogenation, realizing a one-step method to replace the traditional two-step method, reducing energy consumption and equipment investment, and lowering production costs.

[0064] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0065] Figure 1 This is a schematic diagram of the structure of the catalyst prepared in this invention. Detailed Implementation

[0066] The present invention will be further described below. It should be noted that this embodiment is based on the present technical solution and provides detailed implementation methods and specific operation processes, but the present invention is not limited to this embodiment.

[0067] Example 1

[0068] A method for preparing a core-shell catalyst for the dehydrogenation of methanol to polyoxymethylene dimethyl ether includes the following steps:

[0069] (1) Take 1g of ZSM-5 molecular sieve (silicon-to-aluminum ratio 300), disperse it in 30ml of deionized water, sonicate and stir thoroughly, and record it as solution A; weigh 0.4955g of nickel nitrate hexahydrate, 0.2131g of aluminum nitrate nonahydrate and 0.3070g of urea and dissolve them in 30ml of deionized water, stir thoroughly, and record it as solution B; slowly add solution B dropwise to solution A, and continue stirring for 2h to obtain a mixed solution;

[0070] (2) Place the mixture obtained in step (1) in a hydrothermal reactor and hydrothermally heat it at 120°C for 12 hours;

[0071] (3) After centrifuging the product obtained in step (2), take the solid phase and wash it repeatedly with deionized water until the filtrate is neutral.

[0072] (4) The solid phase obtained after washing in step (3) is dried at 80°C for 12 hours to obtain ZSM-5 molecular sieve / NiAl-LDHs. It is then calcined at 500°C for 3 hours to obtain ZSM-5 molecular sieve / NiAl-LDO.

[0073] (5) The ZSM-5 molecular sieve / NiAl-LDO obtained in step (4) was dispersed in a mixed solution containing 40 ml deionized water, 20 ml ethanol, 0.12 g cetyltrimethylammonium bromide and 0.40 ml 25 wt% NH3·H2O. The mixture was subjected to ultrasonic treatment for 2 h. Then, 0.3353 g tetraethyl orthosilicate and 0.0242 g aluminum nitrate nonahydrate were added sequentially, and the mixture was stirred for another 12 h. Finally, after the reaction was completed, the solid was centrifuged from the reaction mixture, washed sequentially with deionized water and ethanol, vacuum dried, and then calcined at 500 °C for 3 h to obtain the ZSM-5 molecular sieve / NiAl-LDO@Al-MS nanocomposite material.

[0074] (6) The molecular sieve / NiAl-LDO@Al-MS nanocomposite material from step (5) was reduced at 450℃ for 3 h in a reducing atmosphere of H2 / Ar = 1:9 to obtain ZSM-5 molecular sieve / Ni-Ab2O3@Al-MS nanocomposite material with exposed metal active sites. The ZSM-5 molecular sieve / Ni-Al2O3@Al-MS nanocomposite material is the three-layer core-shell catalyst designed in this invention, and its structural schematic diagram is shown below. Figure 1 As shown.

[0075] In the in-situ coupling of methanol dehydrogenation to prepare polyoxymethylene dimethyl ether, the above-mentioned ZSM-5 molecular sieve / Ni-Al2O3@Al-MS nanocomposite material was used as a catalyst. The catalytic reaction was carried out in a fixed-bed reactor and was continuous. After molding and tableting, the catalyst was sieved to obtain particles with a particle size of 20-40 mesh and then packed into a quartz reaction tube. The catalyst bed was heated to a preset temperature of 150°C under a flowing N2 atmosphere. Subsequently, anhydrous methanol was quantitatively fed through a high-pressure constant flow pump at a methanol volume hourly space velocity of 0.2 h⁻¹. -1 Methanol is vaporized at 150°C and then thoroughly mixed with N2 before being introduced into a reaction tube for catalytic reaction. The reaction temperature is 120°C and the reaction pressure is atmospheric pressure.

[0076] Calculations showed that the methanol conversion rate was 6.5%, the polyoxymethylene dimethyl ether yield was 5.1025%, and the polyoxymethylene dimethyl ether selectivity was 78.5%, of which the DMM selectivity was 42.7%, and the PODE... 2-6 The selectivity rate was 35.8%.

[0077] Example 2

[0078] A method for preparing a core-shell catalyst for the dehydrogenation of methanol to polyoxymethylene dimethyl ether includes the following steps:

[0079] (1) Take 1g of BETA molecular sieve (silicon-to-aluminum ratio 25), disperse it in 30ml of deionized water, sonicate and stir thoroughly, and record it as solution A; weigh 1.7285g of cobalt nitrate hexahydrate, 1.1140g of aluminum nitrate nonahydrate and 1.6052g of urea and dissolve them in 30ml of deionized water, stir thoroughly, and record it as solution B; slowly add solution B dropwise to solution A, and continue stirring for 2h to obtain a mixed solution;

[0080] (2) Place the mixture obtained in step (1) in a hydrothermal reactor and hydrothermally heat it at 120°C for 12 hours;

[0081] (3) After centrifuging the product obtained in step (2), take the solid phase and wash it repeatedly with deionized water until the filtrate is neutral.

[0082] (4) The solid phase obtained after washing in step (3) was dried at 80°C for 12 h to prepare BETA molecular sieve / CoAl-LDHs. 0.1279 g of rhodium trichloride trihydrate was dissolved in 15 ml of water and then impregnated onto BETA molecular sieve / CoAl-LDHs. After vacuum drying, BETA molecular sieve / Rh-CoAl-LDHs was obtained. Then, it was calcined at 500°C for 4 h to obtain BETA molecular sieve / Rh-CoAl-LDO nanocomposite material.

[0083] (5) The BETA molecular sieve / Rh-CoAl-LDO nanocomposite obtained in step (4) was dispersed in a mixed solution containing 100 ml deionized water, 50 ml ethanol, 0.30 g cetyltrimethylammonium bromide and 1.0 ml 25 wt% NH3·H2O. The mixture was subjected to ultrasonic treatment for 2 h. Then, 0.8595 g tetraethyl orthosilicate and 0.0155 g aluminum nitrate nonahydrate were added sequentially, and the mixture was stirred for another 12 h. Finally, after the reaction was completed, the solid was centrifuged from the reaction mixture, washed sequentially with deionized water and ethanol, vacuum dried, and then calcined at 500 °C for 4 h to obtain the BETA molecular sieve / Rh-CoAl-LDO@Al-MS nanocomposite.

[0084] (6) The BETA molecular sieve / Rh-CoAl-LDO@Al-MS nanocomposite material from step (5) was reduced at 500℃ for 3 hours in a reducing atmosphere of H2 / Ar = 1:9 to obtain the BETA molecular sieve / RhCo-Al2O3@Al-MS nanocomposite material with exposed metal active sites. The BETA molecular sieve / RhCo-Al2O3@Al-MS nanocomposite material is the three-layer core-shell catalyst designed in this invention, and its structural schematic diagram is shown below. Figure 1 As shown.

[0085] The above-mentioned BETA molecular sieve / RhCo-Al2O3@Al-MS nanocomposite material was used as a catalyst in the in-situ coupling of methanol dehydrogenation to prepare polyoxymethylene dimethyl ether. The catalytic reaction was carried out in a reaction vessel with condensation function and was a batch reaction. Hexadecane was added during the reaction to ensure that the reaction temperature reached above the boiling point of methanol. Under argon protection, the reaction temperature was 220℃, the reaction time was 6h, the reaction pressure was atmospheric pressure, and the amount of catalyst used was 5wt% of the mass of methanol.

[0086] Calculations showed that the methanol conversion rate was 18.6%, the polyoxymethylene dimethyl ether yield was 8.6118%, and the polyoxymethylene dimethyl ether selectivity was 46.3%, of which the DMM selectivity was 24.5% and the PODE2-6 selectivity was 21.8%.

[0087] Example 3

[0088] A method for preparing a core-shell catalyst for the dehydrogenation of methanol to polyoxymethylene dimethyl ether includes the following steps:

[0089] (1) Take 1g of Al-SBA-15 molecular sieve (silicon-to-aluminum ratio 200), disperse it in 30ml of deionized water, sonicate and stir thoroughly, and record it as solution A; weigh 0.7128g of cobalt nitrate hexahydrate, 0.3935g of aluminum nitrate nonahydrate and 0.5316g of urea, dissolve them in 30ml of deionized water and stir thoroughly, and record it as solution B; slowly add solution B dropwise to solution A, and continue stirring for 2h to obtain a mixed solution;

[0090] (2) Place the mixture obtained in step (1) in a hydrothermal reactor and hydrothermally heat it at 120°C for 24 hours;

[0091] (3) After centrifuging the product obtained in step (2), take the solid phase and wash it repeatedly with deionized water until the filtrate is neutral.

[0092] (4) The solid phase obtained after washing in step (3) was dried at 80°C for 12 h to prepare Al-SBA-15 molecular sieve / CuCr-LDHs. Using the impregnation method, 0.0313 g of palladium nitrate dihydrate was dissolved in 15 ml of water and then impregnated onto Al-SBA-15 molecular sieve / CuCr-LDHs. After vacuum drying, Al-SBA-15 molecular sieve / Pd-CuCr-LDHs was obtained. Then, it was calcined at 500°C for 5 h to obtain Al-SBA-15 molecular sieve / Pd-CuCr-LDO nanocomposite material.

[0093] (5) The Al-SBA-15 molecular sieve / Pd-CuCr-LDO nanocomposite obtained in step (4) was dispersed in a mixed solution containing 160 ml deionized water, 80 ml ethanol, 0.48 g cetyltrimethylammonium bromide and 1.60 ml 25 wt% NH3·H2O. The mixture was subjected to ultrasonic treatment for 2 h. Then, 1.3791 g tetraethyl orthosilicate and 0.0166 g aluminum nitrate nonahydrate were added sequentially, and the mixture was stirred for another 12 h. Finally, after the reaction was completed, the solid was centrifuged from the reaction mixture, washed sequentially with deionized water and ethanol, vacuum dried, and then calcined at 500 °C for 5 h to obtain the Al-SBA-15 molecular sieve / Pd-CuCr-LDO@Al-MS nanocomposite.

[0094] (6) The Al-SBA-15 molecular sieve / Pd-CuCr-LDO@Al-MS nanocomposite material from step (5) was reduced at 500℃ for 3 h in a reducing atmosphere of H2 / Ar = 1:9 to obtain an Al-SBA-15 molecular sieve / PdCu-Cr2O3@Al-MS nanocomposite material with exposed metal active sites. The Al-SBA-15 molecular sieve / PdCu-Cr2O3@Al-MS nanocomposite material is the three-layer core-shell catalyst designed in this invention, and its structural schematic diagram is shown below. Figure 1 As shown.

[0095] The Al-SBA-15 molecular sieve / PdCu-Cr2O3@Al-MS nanocomposite material was used as a catalyst in the in-situ coupling of methanol dehydrogenation to prepare polyoxymethylene dimethyl ether. The catalytic reaction was carried out in a glass reactor under atmospheric pressure and was a batch reaction. Dodecane was added during the reaction to allow the reaction temperature to reach above the boiling point of methanol. Under argon protection, the reaction temperature was 120°C, the reaction time was 0.5 h, the reaction pressure was atmospheric pressure, and the amount of catalyst used was 1 wt% of the mass of methanol.

[0096] Calculations showed that the methanol conversion rate was 4.5%, the polyoxymethylene dimethyl ether (POM) yield was 3.159%, and the POM selectivity was 70.2%, of which the DMM selectivity was 36.6%. 2-6 The selectivity rate was 33.6%.

[0097] Example 4

[0098] A method for preparing a core-shell catalyst for the dehydrogenation of methanol to polyoxymethylene dimethyl ether includes the following steps:

[0099] (1) Take 1g of Al-MCM-41 molecular sieve (silicon-to-aluminum ratio 150), disperse it in 30ml of deionized water, sonicate and stir thoroughly, and record it as solution A; weigh 1.2388g of nickel nitrate hexahydrate, 0.1901g of copper nitrate trihydrate, 0.5097g of ferric nitrate nonahydrate and 0.6820g of urea, dissolve them in 30ml of deionized water, stir thoroughly, and record it as solution B; slowly add solution B dropwise to solution A, and continue stirring for 2h to obtain a mixed solution;

[0100] (2) Place the mixture obtained in step (1) in a hydrothermal reactor and hydrothermally heat it at 120°C for 12 hours;

[0101] (3) After centrifuging the product obtained in step (2), take the solid phase and wash it repeatedly with deionized water until the filtrate is neutral.

[0102] (4) The solid phase obtained after washing in step (3) is dried at 80°C for 12 h to prepare Al-MCM-41 molecular sieve / NiCuFe-LDHs. It is then calcined at 500°C for 3 h to obtain Al-MCM-41 molecular sieve / NiCuFe-LDO nanocomposite material.

[0103] (5) The Al-MCM-41 molecular sieve / NiCuFe-LDO nanocomposite obtained in step (4) was dispersed in a mixed solution containing 80 ml deionized water, 40 ml ethanol, 0.24 g cetyltrimethylammonium bromide and 0.80 ml 25 wt% NH3·H2O. The mixture was subjected to ultrasonic treatment for 2 h. Then, 0.6905 g tetraethyl orthosilicate and 0.0062 g aluminum nitrate nonahydrate were added sequentially, and the mixture was stirred for another 12 h. Finally, after the reaction was completed, the solid was centrifuged from the reaction mixture, washed sequentially with deionized water and ethanol, vacuum dried, and then calcined at 500 °C for 3 h to obtain the Al-MCM-41 molecular sieve / NiCuFe-LDO@Al-MS nanocomposite.

[0104] (6) The Al-MCM-41 molecular sieve / NiCuFe-LDO@Al-MS nanocomposite material from step (5) was reduced at 450℃ for 3 h in a reducing atmosphere of H2 / Ar = 1:9 to obtain an Al-MCM-41 molecular sieve / NiCu-Fe3O4@Al-MS nanocomposite material with exposed metal active sites. The Al-MCM-41 molecular sieve / NiCu-Fe3O4@Al-MS nanocomposite material is the three-layer core-shell catalyst designed in this invention, and its structural schematic diagram is shown below. Figure 1 As shown.

[0105] The Al-MCM-41 molecular sieve / NiCu-Fe3O4@Al-MS nanocomposite material was used as a catalyst in the in-situ coupling of methanol dehydrogenation to prepare polyoxymethylene dimethyl ether. The catalytic reaction was carried out in a fixed-bed reactor as a continuous process. After molding and tableting, the catalyst was sieved to obtain particles with a diameter of 20–40 mesh and then loaded into a quartz reaction tube. The catalyst bed was heated to a preset temperature of 150°C under a flowing N2 atmosphere. Subsequently, anhydrous methanol was quantitatively fed through a high-pressure constant-flow pump at a methanol volume hourly space velocity (MHSV) of 2 h⁻¹. 1 Methanol is vaporized at 150°C and then thoroughly mixed with N2 before being introduced into a reaction tube for catalytic reaction. The reaction temperature is 240°C and the reaction pressure is atmospheric pressure.

[0106] Calculations showed that the methanol conversion rate was 20.5%, the polyoxymethylene dimethyl ether (POM) yield was 7.6465%, and the POM selectivity was 37.3%, of which the DMM selectivity was 20.9% and the PODE selectivity was [not specified]. 2-6 The selectivity rate was 16.4%.

[0107] Example 5

[0108] A method for preparing a core-shell catalyst for the dehydrogenation of methanol to polyoxymethylene dimethyl ether includes the following steps:

[0109] (1) Take 1g of Al-MCM-22 molecular sieve (silicon-to-aluminum ratio 100), disperse it in 30ml of deionized water, sonicate and stir thoroughly, and record it as solution A; weigh 0.6178g of cobalt nitrate hexahydrate, 0.6194g of nickel nitrate hexahydrate, 0.4285g of ferric nitrate nonahydrate and 0.5733g of urea, dissolve them in 30ml of deionized water, stir thoroughly, and record it as solution B; slowly add solution B dropwise to solution A, and continue stirring for 2h to obtain a mixed solution;

[0110] (2) Place the mixture obtained in step (1) in a hydrothermal reactor and hydrothermally heat it at 120°C for 12 hours;

[0111] (3) After centrifuging the product obtained in step (2), take the solid phase and wash it repeatedly with deionized water until the filtrate is neutral.

[0112] (4) The solid phase obtained after washing in step (3) is dried at 80°C for 12 h to prepare Al-MCM-22 molecular sieve / CoNiFe-LDHs. It is then calcined at 500°C for 3 h to obtain Al-MCM-22 molecular sieve / CoNiFe-LDO nanocomposite material.

[0113] (5) The Al-MCM-22 molecular sieve / CoNiFe-LDO nanocomposite obtained in step (4) was dispersed in a mixed solution containing 120 ml of deionized water, 60 ml of ethanol, 0.36 g of cetyltrimethylammonium bromide and 1.2 ml of 25 wt% NH3·H2O. The mixture was subjected to ultrasonic treatment for 2 h, and then 1.0373 g of tetraethyl orthosilicate and 0.0062 g of aluminum nitrate nonahydrate were added sequentially, and the reaction was continued for 12 h. Finally, after the reaction was completed, the solid was centrifuged from the reaction mixture, washed sequentially with deionized water and ethanol, vacuum dried, and then calcined at 500 °C for 3 h to obtain the Al-MCM-22 molecular sieve / CoNiFe-LDO@Al-MS nanocomposite.

[0114] (6) The Al-MCM-22 molecular sieve / CoNiFe-LDO@Al-MS nanocomposite material from step (5) was reduced at 500℃ for 3 hours in a reducing atmosphere of H2 / Ar = 1:9 to obtain an Al-MCM-22 molecular sieve / CoNi-Fe3O4@Al-MS nanocomposite material with exposed metal active sites. The Al-MCM-22 molecular sieve / CoNi-Fe3O4@Al-MS nanocomposite material is the three-layer core-shell catalyst designed in this invention, and its structural schematic diagram is shown below. Figure 1 As shown.

[0115] The Al-MCM-22 molecular sieve / CoNi-Fe3O4@Al-MS nanocomposite material was used as a catalyst in the in-situ coupling of methanol dehydrogenation to prepare polyoxymethylene dimethyl ether. The catalytic reaction was carried out in a reaction vessel with condensation function and was a batch reaction. Dodecane was added during the reaction to allow the reaction temperature to reach above the boiling point of methanol. Under argon protection, the reaction temperature was 150°C, the reaction time was 3 h, the reaction pressure was atmospheric pressure, and the amount of catalyst used was 2w1% of the mass of methanol.

[0116] Calculations showed that the methanol conversion rate was 12.1%, the polyoxymethylene dimethyl ether (POM) yield was 7.5988%, and the POM selectivity was 62.8%, of which the DMM selectivity was 32.1%. 2-6 The selectivity rate was 30.7%.

[0117] Comparative Example 1

[0118] The difference between this comparative example and Example 1 is that the outermost layer of aluminum-containing mesoporous silica coating was not performed. The preparation method of the core-shell catalyst for methanol dehydrogenation to polyoxymethylene dimethyl ether includes the following steps:

[0119] (1) Take 1g of ZSM-5 molecular sieve (silicon-to-aluminum ratio 300), disperse it in 30ml of deionized water, sonicate and stir thoroughly, and record it as solution A; weigh 0.4955g of nickel nitrate hexahydrate, 0.2131g of aluminum nitrate nonahydrate and 0.3070g of urea, dissolve them in 30ml of deionized water and stir thoroughly, and record it as solution B; slowly add solution B dropwise to solution A, and continue stirring for 2h to obtain a mixed solution;

[0120] (2) Place the mixture obtained in step (1) in a hydrothermal reactor and hydrothermally heat it at 120°C for 12 hours;

[0121] (3) After centrifuging the product obtained in step (2), take the solid phase and wash it repeatedly with deionized water until the filtrate is neutral.

[0122] (4) The solid phase obtained after washing in step (3) is dried at 80°C for 12 hours to obtain ZSM-5 molecular sieve / NiAl-LDHs. It is then calcined at 500°C for 3 hours to obtain ZSM-5 molecular sieve / NiAl-LDO nanocomposite material.

[0123] (5) The ZSM-5 molecular sieve / NiAl-LDO nanocomposite material in step (4) was reduced at 450℃ for 3h in a reducing atmosphere of H2 / Ar=1:9 to obtain the ZSM-5 molecular sieve / Ni-Al2O3 nanocomposite material.

[0124] In the process of using the above-mentioned ZSM-5 molecular sieve / Ni-Al2O3 nanocomposite material as a catalyst in the in-situ coupling of methanol dehydrogenation to prepare polyoxymethylene dimethyl ether: the catalytic reaction is carried out in a fixed-bed reactor and is a continuous reaction. After the above-mentioned catalyst is shaped, pressed into tablets, and sieved to obtain particles with a particle size of 20-40 mesh, it is loaded into a quartz reaction tube. The catalyst bed is heated to a preset temperature of 150°C under a flowing N2 atmosphere. Subsequently, anhydrous methanol is quantitatively fed through a high-pressure constant flow pump. The methanol volume hourly space velocity is 0.2 h⁻¹. After the methanol is vaporized at 150°C, it is thoroughly mixed with N2 and then introduced into the reaction tube for catalytic reaction. The reaction temperature is 120°C and the reaction pressure is atmospheric pressure.

[0125] Calculations showed that the methanol conversion rate was 5.9%, the polyoxymethylene dimethyl ether (POM) yield was 4.12%, and the POM selectivity was 69.8%, with DMM selectivity at 63.6% and PODE at [missing value]. 2-6 The selectivity rate was 6.2%.

[0126] Therefore, this invention employs the aforementioned core-shell catalyst for methanol dehydrogenation to polyoxymethylene dimethyl ether, as well as its preparation and application. The prepared core-shell catalyst uses the middle acidic molecular sieve and the outermost aluminum-containing mesoporous silica as acidic sites, and the dehydrogenation metal active component in the middle layer as the metal active site. By integrating the two reaction sites, a bifunctional catalyst is constructed, realizing the in-situ coupling of methanol dehydrogenation and methanol-formaldehyde condensation reaction. This not only simplifies the reaction steps but also improves the selectivity and yield of the reaction.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a core-shell catalyst for the dehydrogenation of methanol to polyoxymethylene dimethyl ether, characterized in that: Includes the following steps: (1) A metal active component is coated on the surface of an acidic molecular sieve to obtain a molecular sieve composite material; The acidic molecular sieve is one of ZSM-5 molecular sieve, BETA molecular sieve, Al-MCM-41 molecular sieve, Al-SBA-15 molecular sieve, and Al-MCM-22 molecular sieve; When the active metal component is a non-precious metal, the non-precious metal raw materials include at least one divalent soluble metal salt and at least one trivalent soluble metal salt. The specific process of coating the surface of acidic molecular sieves with metal active components is as follows: Acidic molecular sieves are dispersed in deionized water and stirred thoroughly by ultrasonication to obtain solution A; divalent soluble metal salt, trivalent soluble metal salt and urea are dissolved in deionized water and stirred thoroughly to obtain solution B; solution B is added dropwise to solution A and stirring is continued to obtain a mixed solution. The mixture was subjected to a hydrothermal reaction. After the reaction was completed, it was centrifuged, washed and dried to obtain molecular sieves / M1M2-LDHs. Divalent and trivalent soluble metal salts generate LDHs in situ on the surface of acidic molecular sieves through hydrothermal reaction. The LDHs are coated on the surface of acidic molecular sieves in a layered form. Molecular sieves / M1M2-LDHs are calcined to obtain molecular sieves / M1M2-LDO, which is a molecular sieve composite material. (2) The molecular sieve composite material was dispersed in a mixed solution, and then tetraethyl orthosilicate and aluminum nitrate were added in sequence. The mixture was stirred and reacted. After centrifugation, washing, drying and calcination, the nanocomposite material was obtained. (3) The nanocomposite material was heat-treated in a reducing atmosphere to obtain a core-shell catalyst.

2. The preparation method according to claim 1, characterized in that: In step (1), when the active metal component is a non-precious metal and a precious metal, the raw material of the precious metal is at least one of the soluble precious metal salts; The specific process of coating the surface of acidic molecular sieves with metal active components is as follows: Acidic molecular sieves are dispersed in deionized water and stirred thoroughly by ultrasonication to obtain solution A; divalent soluble metal salt, trivalent soluble metal salt and urea are dissolved in deionized water and stirred thoroughly to obtain solution B; solution B is added dropwise to solution A and stirring is continued to obtain a mixed solution. The mixture was subjected to a hydrothermal reaction. After the reaction was completed, it was centrifuged, washed and dried to obtain molecular sieves / M1M2-LDHs. Molecular sieves / M1M2-LDHs were impregnated in a soluble noble metal salt solution to obtain molecular sieves / M3-M1M2-LDHs; Molecular sieves / M3-M1M2-LDHs are calcined to obtain molecular sieves / M3-M1M2-LDO, which is a molecular sieve composite material.

3. The preparation method according to claim 1, characterized in that: Based on the mass of the reduced metal in the active metal component, the mass ratio of acidic molecular sieve, active metal component, and aluminum-containing mesoporous silica is 1:0.1~0.4:0.1~0.

4.

4. The core-shell catalyst prepared by the method according to any one of claims 1 to 3, characterized in that: The core-shell catalyst comprises an acidic molecular sieve, a metal active component, and aluminum-containing mesoporous silica. The acidic molecular sieve serves as the core structure, the metal active component coats the surface of the acidic molecular sieve to form a first coating layer, and the aluminum-containing mesoporous silica coats the surface of the metal active component to form a second coating layer.

5. The application of the core-shell catalyst according to claim 4, characterized in that: Application of core-shell catalysts in the in-situ coupling of methanol dehydrogenation to polyoxymethylene dimethyl ether.

6. The application of the core-shell catalyst according to claim 5, characterized in that: The continuous reaction process for the in-situ coupling of methanol dehydrogenation to produce polyoxymethylene dimethyl ether is as follows: The reaction is carried out in a fixed-bed reactor. After the core-shell catalyst is shaped and pressed into tablets, particles with a diameter of 20-40 mesh are sieved and loaded into the reaction tube. The core-shell catalyst bed is heated to 120-260°C under a flowing N2 atmosphere. Subsequently, methanol is metered in by a high-pressure constant flow pump, with a methanol volume hourly space velocity of 0.2-2 h⁻¹ (LCHSV) based on liquid phase feed. -1 After being vaporized, methanol is thoroughly mixed with N2 and introduced into the reaction tube. The reaction temperature is 120~240℃ and the reaction pressure is atmospheric pressure.

7. The application of the core-shell catalyst according to claim 5, characterized in that: The batch reaction process for the in-situ coupling of methanol dehydrogenation to produce polyoxymethylene dimethyl ether is as follows: The reaction is carried out in a reaction vessel with condensation function or an atmospheric pressure glass reactor. A highly stable, high-boiling-point solvent is added during the reaction to ensure that the reaction temperature reaches above the boiling point of methanol. Under argon protection, the reaction temperature is 120~220℃, the reaction time is 0.5~6h, the reaction pressure is atmospheric pressure, and the amount of core-shell catalyst is 1~5wt% of the mass of methanol.

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