Rare earth element-containing cationic catalyst, and preparation method and application thereof

Abstract

This invention discloses a rare-earth element-containing cationic catalyst, its preparation method, and its application, belonging to the field of catalyst technology. Using 732 polystyrene sulfonic acid resin as a support, the target catalyst is prepared in four steps: pretreatment, zirconium coordination modification, aminobisphosphonic acid ligand grafting, and rare-earth ion chelation loading. This invention constructs a Brønsted-Lewis dual-acid synergistic catalytic system through directional structural design, achieving both ultra-stable rare-earth loading and retention of strong acidic sites, significantly improving the catalyst's water resistance, catalytic activity, and cycle stability. This catalyst can efficiently catalyze the synthesis of polyoxymethylene dimethyl ether from industrial formaldehyde aqueous solution and methyl acetal, exhibiting excellent formaldehyde conversion and target product selectivity. It is suitable for both batch and continuous production processes and has good prospects for industrial application.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology, and in particular to a rare earth element-containing cationic catalyst, its preparation method, and its application. Background Technology

[0002] Polyoxymethylene dimethyl ether (DMMn, n=3-8), as an environmentally friendly diesel additive with high cetane number and low sulfur content, can significantly improve fuel combustion performance and reduce exhaust pollutant emissions. Among its synthetic routes, the acid-catalyzed acetalization reaction using formaldehyde aqueous solution and methyl acetal as raw materials has attracted much attention due to its wide availability of raw materials and simple process. However, this reaction system faces two major challenges: Firstly, industrial formaldehyde usually exists in aqueous solution, primarily as methylene glycol and its oligomers, requiring strong acid sites to catalyze its dehydration to generate hydroxymethyl carbocations. This step is the rate-controlling step in the overall reaction kinetics, placing extremely high demands on the acid strength and water resistance of the catalyst. Secondly, the target product DMM... 3-8 The chain growth reaction requires multiple stages of acetalization. Excessively strong acid sites can trigger side reactions such as methyl acetal hydrolysis, formaldehyde cannizzaro disproportionation, and excessive polymerization of the product. Conversely, excessively weak acid sites cannot guarantee the chain growth reaction rate. Therefore, the catalyst needs to have a distribution of acid centers with matching strength and synergistic types.

[0003] To address the aforementioned reaction characteristics, existing technologies primarily employ sulfonic acid-type cation exchange resins (such as Amberlyst-15 and 732 resins) as catalysts. These resins, with their abundant Brønsted acid sites provided by sulfonic acid groups, can catalyze acetalization reactions. However, traditional sulfonic acid resins exhibit significant drawbacks in high-water systems: water molecules form strong hydrogen bonds with sulfonic acid groups, competing for proton-donating sites and poisoning the Brønsted acid sites, resulting in a substantial decrease in proton-donating capacity and consequently, a low single-pass formaldehyde conversion rate. Furthermore, the single Brønsted acid center cannot achieve precise control over the chain growth reaction rate or effectively suppress side reactions, hindering the production of the target product, DMM. 3-8 The selectivity is generally low, and high-value DMMs are also limited. 3-6The fractional fraction is even lower. To improve the activity and selectivity of the catalyst in aqueous systems, researchers have attempted to modify the resin, for example, by loading transition metals or rare earth ions through impregnation, trying to introduce Lewis acid sites to form a dual-acid synergistic catalysis. However, these conventional modification schemes have irreconcilable contradictions: if rare earth ions are directly loaded onto the sulfonic acid resin through ion exchange, the rare earth ions will exchange with the hydrogen ions of the sulfonic acid groups, neutralizing the core Brønsted acid active sites, resulting in a significant decrease in catalytic activity; if ligands containing basic chelating groups such as amino and carboxyl groups are first grafted onto the resin before loading rare earths, these basic groups will also neutralize the sulfonic acid groups, causing irreversible loss of Brønsted acid. In addition, existing modified catalysts generally suffer from severe metal ion dissolution, rapid activity decay, and short cycle life in high-acidic reaction environments, failing to meet the requirements of long-term operational stability for catalysts in continuous industrial production. Therefore, developing a novel cationic catalyst that can stably support rare earth ions to construct a gradient Lewis acid synergistic catalytic system, fully retain or even enhance the intrinsic Brønsted acid strength of the resin, and possess excellent water resistance and resistance to metal leaching is of great significance for promoting the industrial production of DMMn. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing a rare earth element-containing cationic catalyst, its preparation method, and its application.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing a cationic catalyst containing rare earth elements, comprising the following steps: (1) Rinse 732 polystyrene sulfonic acid resin with deionized water until the washing solution is no longer cloudy. Then soak it in 95% ethanol for 3-5 hours. After taking it out, rinse off the surface ethanol with deionized water. Then soak it in dilute hydrochloric acid solution for 12-24 hours. After taking it out, wash it with deionized water until the washing solution is neutral. After drying, the pretreated resin is obtained. (2) Under nitrogen protection, the pretreated resin was added to anhydrous 1,2-dichloroethane and stirred at room temperature for 12-24 h. Then zirconium tetrachloride was added and stirred for 10-30 min. The temperature was raised to 50-60 °C and stirred for 4-8 h. The mixture was filtered while hot and washed with anhydrous ethanol at 50 °C until no free chloride ions were detected in the washing liquid. After drying, Zr coordination-modified resin was obtained. The chemical reaction is shown in the diagram below: ; (3) Under nitrogen protection, (1-aminoethane-1,1-diyl)bisphosphonic acid and anhydrous triethylamine were added to anhydrous DMF and stirred at room temperature for 10-20 min. Then, Zr coordination-modified resin was added and stirring was continued for 20-40 min. The temperature was raised to 70-80℃ and the reaction was stirred for 8-15 h. The mixture was filtered while hot and then washed three times alternately with ethanol, deionized water, and ethanol. After drying, phosphonic acid grafted modified resin was obtained. The chemical reaction is shown in the diagram below: ; (4) Add the phosphonic acid grafted modified resin to the rare earth salt solution, stir at room temperature for 20-40 min, then heat to 60-70℃, stir for 10-15 h, filter to separate the resin, wash the resin repeatedly with deionized water until no free rare earth ions are detected in the washing filtrate, and dry to obtain a cationic catalyst containing rare earth elements.

[0006] Preferably, in (1), the volume ratio of 732 polystyrene sulfonic acid resin, 95% ethanol and dilute hydrochloric acid solution is 1:2-3:4-7; the concentration of dilute hydrochloric acid solution is 3-10wt%.

[0007] Preferably, in step (2), the pretreatment resin, anhydrous 1,2-dichloroethane and zirconium tetrachloride are in a weight ratio of 1:8-12:0.08-0.15.

[0008] Preferably, the free chloride ions in the washing liquid in (2) are detected by adding silver nitrate solution and observing whether precipitation occurs. If no precipitation occurs, it proves that there are no free chloride ions in the washing liquid.

[0009] Preferably, in (3), the weight ratio of (1-aminoethane-1,1-diyl) bisphosphonic acid, anhydrous triethylamine, anhydrous DMF and Zr coordination modified resin is 0.2-0.35:0.08-0.12:6-10:1.

[0010] Preferably, in step (4), the phosphonic acid grafted modified resin and rare earth salt solution are in a weight ratio of 1:4-6.

[0011] Preferably, the concentration of the rare earth salt solution in (4) is 0.03-0.08 mol / L.

[0012] Preferably, the pH of the rare earth salt solution in (4) is adjusted to 3.5 ± 0.1 by using a 3-5 wt% dilute nitric acid solution.

[0013] Preferably, the rare earth salt solution in (4) is one of the following: lanthanum nitrate hexahydrate aqueous solution, cerium nitrate hexahydrate aqueous solution, praseodymium nitrate hexahydrate aqueous solution, or neodymium nitrate hexahydrate aqueous solution, or a mixed aqueous solution of lanthanum nitrate hexahydrate / cerium nitrate hexahydrate or a mixed aqueous solution of praseodymium nitrate hexahydrate / neodymium nitrate hexahydrate.

[0014] More preferably, the rare earth salt solution in (4) is an aqueous solution of cerium nitrate hexahydrate.

[0015] Preferably, in step (4), the free rare earth ions in the washing filtrate are detected by ICP-OES. When the concentration of rare earth elements in the filtrate is ≤0.1ppm, it is considered to be free of free rare earth ions.

[0016] Furthermore, the present invention also provides a cationic catalyst containing rare earth elements, which is prepared by the above-described method.

[0017] Furthermore, the present invention also provides an application of a rare earth element-containing cationic catalyst for catalyzing the synthesis of polyoxymethylene dimethyl ether (DMMn) from liquid formaldehyde and methyl acetal using a batch reactor reaction or a continuous fixed-bed reaction.

[0018] Preferably, the liquid formaldehyde is a formaldehyde aqueous solution with a concentration of 37-90 wt%; the molar ratio of formaldehyde to methyl acetal is 1-4:1 during the synthesis of polyoxymethylene dimethyl ether; and n=3-8 in the synthesized polyoxymethylene dimethyl ether DMMn.

[0019] Preferably, the specific synthesis method of the batch reactor reaction is as follows: A 37-90 wt% formaldehyde aqueous solution and methyl acetal are added to a high-pressure reactor at a formaldehyde to methyl acetal molar ratio of 1-4:1. After stirring and mixing evenly, a cationic catalyst containing rare earth elements is added. The amount of catalyst is 4-10% of the total mass of formaldehyde and methyl acetal raw materials. After sealing the reactor, the air inside the reactor is replaced with nitrogen three times, and the pressure is increased to 0.3-0.8 MPa. Stirring and heating are started, and the stirring speed is controlled at 300-600 rpm. The temperature is raised to 60-90℃ and then kept at that temperature for 2-5 hours. After the reaction is completed, the reactor is cooled to room temperature. After depressurization, the catalyst and reaction liquid are separated by filtration. The recovered catalyst can be directly recycled after being washed with anhydrous ethanol and vacuum dried. The filtrate is the crude DMMn product, which can be directly entered into the subsequent distillation separation process.

[0020] The preferred synthesis method for the continuous fixed-bed reaction is as follows: A cationic catalyst containing rare earth elements is loaded into a tubular fixed-bed reactor. The catalyst loading amount is matched according to the reactor specifications and the design space velocity. After loading, the reactor is purged with nitrogen for 30 minutes to replace the air in the system. A 37-90 wt% formaldehyde aqueous solution and methylal are mixed evenly at a formaldehyde to methylal molar ratio of 1-4:1. The mixture is then continuously fed into the fixed-bed reactor through a metering pump, controlling the raw material volume hourly space velocity to be 0.5-1.5 h⁻¹. -1 The reaction temperature inside the reactor is 65-85℃, the system pressure is 0.4-0.6MPa, and the reactants continuously flow out of the reactor. After cooling and gas-liquid separation, crude DMMn product is obtained, which can be directly entered into the subsequent distillation and separation process.

[0021] Preferably, the distillation separation process is as follows: the crude DMMn product is first distilled at atmospheric pressure to remove light components such as methyl acetal and methanol. The light components collected from the top of the light component removal tower can be returned to the reaction system for recycling. The material after light component removal is sent to an azeotropic distillation dehydration tower, where cyclohexane is added as an azeotropic agent, and the water in the system is removed by atmospheric pressure distillation. The final DMMn product is obtained from the bottom of the tower, where n=3-8.

[0022] Preferably, the preparation mechanism of the rare earth element-containing cationic catalyst of the present invention is explained as follows: First, using 732 polystyrene sulfonic acid resin as a carrier, which contains abundant sulfonic acid groups, impurities were removed and the resin was converted to the hydrogen form through pretreatment steps such as acid washing and alcohol soaking. Then, under nitrogen protection, the pretreated resin was dispersed in anhydrous 1,2-dichloroethane, and zirconium tetrachloride (ZrCl4) was added. 4+ Exhibiting strong Lewis acidity, it can coordinate or exchange cations with sulfonic acid groups on the resin, forming stable Zr-O-S bonds, thereby anchoring zirconium ions to the resin framework surface, resulting in Zr-coordination modified resin. This step introduces transition metal centers and provides active sites for subsequent grafting reactions. Then, the Zr-coordination modified resin is reacted with (1-aminoethane-1,1-diyl)bisphosphonic acid (AEDP) in anhydrous DMF. Triethylamine is used as an acid-binding agent to neutralize the active hydrogen on the phosphonic acid group of AEDP, preventing the amino group from being protonated and deactivated, ensuring that the lone pair of electrons of the primary amino group in the AEDP molecule has strong coordination activity. Anhydrous DMF, as a polar aprotic solvent, can completely dissolve AEDP while maintaining the swollen state of the resin, allowing AEDP molecules to smoothly enter the resin channels and fully contact the zirconium centers. The primary amino group with lone pairs of electrons in the AEDP molecule acts as a strong nucleophile ligand, attacking the electron-deficient zirconium ion metal center, thus achieving the desired effect. The weakly bound chloride ion ligands on zirconium are replaced to form Zr-N coordination bonds with extremely high bond energy, achieving permanent covalent grafting of AEDP ligands onto the zirconium center. During the reaction, the hydrogen chloride generated is rapidly neutralized by triethylamine in the system to form triethylamine hydrochloride, completely removing the reaction byproducts and driving the ligand substitution reaction to proceed to completion. Finally, a modified resin with uniformly grafted phosphonic acid ligands is obtained. The phosphonic acid grafted modified resin is then immersed in a rare earth salt solution, and the pH is adjusted to 3.5 to partially deprotonate the phosphonic acid groups, making them negatively charged. The rare earth ions, as Lewis acid centers, strongly coordinate with the oxygen atoms in the phosphonic acid groups to form stable rare earth-phosphonic acid complexes. After repeated washing to remove free rare earth ions, the target catalyst is obtained. In this catalyst, rare earth ions are firmly fixed on the resin surface, while retaining the framework structure of the cation exchange resin. Overall, it is a heterogeneous catalyst with a large specific surface area, high dispersion of active sites, and good thermal stability.

[0023] Preferably, the catalytic mechanism of the rare earth element-containing cationic catalyst of the present invention in the synthesis of polyoxymethylene dimethyl ether from liquid formaldehyde and methyl acetal is explained as follows: In aqueous formaldehyde solution, formaldehyde does not exist as free formaldehyde molecules, but rather primarily exists as methanediol (HOCH2OH, hydrated formaldehyde) and a small amount of oligomethyl aldehyde diol. The synthesis of DMMn is essentially an acid-catalyzed transacetalization-stepwise acetalization tandem reaction, with the reaction pathway consisting of two steps, entirely catalyzed by acid sites: Rate-determining step of the reaction: Methylene glycol undergoes protonation at a strong Brønsted acid site, followed by dehydration to generate a highly reactive hydroxymethyl carbocation (HMCC). + (CH2OH) This step requires a very high proton-donating ability of the Brønsted acid, while the L acid site can synergistically activate the hydroxyl group of the glycol, significantly reducing the activation energy barrier of the dehydration reaction. It is the core link that determines the overall reaction rate. The main chain growth reaction involves the electrophilic addition of hydroxymethyl carbocation to methyl acetal (CH3OCH2OCH3, DMM1), followed by the removal of one molecule of methanol to generate DMM2. DMM2 then repeats the addition process with the hydroxymethyl carbocation, ultimately producing the target products DMM3-DMM8. This step requires a Brønsted acid site with matching strength for catalysis. Excessive acidity can easily trigger side reactions such as methyl acetal hydrolysis and excessive polymerization of the product, while insufficient acidity results in an inadequate chain growth rate. Traditional polystyrene sulfonic acid resins can only provide a single sulfonic acid group Brønsted acid site. In high-water systems, water molecules form strong hydrogen bonds with the sulfonic acid group, competing for the proton-donating site, causing Brønsted acid site poisoning and a significant decrease in proton-donating capacity. This leads to a high energy barrier in the rate-controlling step and a low single-pass formaldehyde conversion rate. Furthermore, a single Brønsted acid site cannot control the side reactions, hindering the development of the target product DMM1. 3-8 The selectivity is generally low.

[0024] This invention first utilizes a coordination reaction in an anhydrous system to form a stable bidentate coordination structure between the resin sulfonic acid groups and zirconium tetrachloride. This step is fundamental to the overall improvement of catalyst performance. Zr 4+ It is d 0 The strongly electron-deficient Lewis acid with this configuration, after undergoing bidentate coordination with the sulfonic acid group, forms a strongly electron-withdrawing conjugated system through the Zr-OS bond. This strongly pulls the electron cloud of the oxygen atom of the sulfonic acid group toward the Zr center, causing a sharp increase in the polarity of the SOH bond and a significant decrease in the dissociation energy of the OH bond. Ultimately, this elevates the Hammett acid strength of the sulfonic acid group to the level of a solid superacid. Even in a high-water system, under the competitive action of water molecules forming hydrogen bonds with the sulfonic acid group, this super-strong Brønsted acid site can still maintain a very strong proton-donating ability, efficiently completing the protonation and dehydration of methanediol and breaking the rate-controlling bottleneck of the reaction. Anchored Zr 4+The center itself is a highly dispersed, non-agglomerated strong L-acid site, which can specifically coordinate with the hydroxyl oxygen of methane glycol and the ether bond oxygen of the reaction intermediate. On the one hand, it weakens the OH bond of methane glycol through electron-withdrawing effect, and synergistically lowers the reaction energy barrier of the rate-determining step with the strong Brønsted acid site, thereby increasing the reaction rate of the rate-determining step. On the other hand, it can stabilize the carbocation intermediate in the electrophilic addition process, accelerate the main reaction rate of chain growth, and at the same time avoid side reactions such as rearrangement and decomposition of the intermediate. It achieves dual-center synergistic catalysis of "Brønsted acid catalytic protonation, L-acid synergistic activation and intermediate stabilization", which is a performance breakthrough that traditional single Brønsted acid resins cannot achieve.

[0025] This invention utilizes a ligand substitution reaction to form a stable Zr-N covalent bond between the primary amine group of AEDP and the Zr center, achieving directional grafting of the ligand. This step is crucial for resolving the inherent limitations of existing rare earth modification methods, and further optimizes the catalytic performance and water resistance of the catalyst. This approach achieves functional separation between rare earth loading sites and Brønsted acid catalytic sites, resolving an irreconcilable contradiction in existing rare earth modification schemes: if rare earth ions directly bind to sulfonic acid groups, they will neutralize the H+ of the sulfonic acid groups. + This leads to a significant decrease in Brønsted acid activity. If basic chelating ligands such as amino and carboxyl groups are used to support rare earth elements, the sulfonic acid groups will also be neutralized, resulting in the loss of strong Brønsted acid sites in the catalytic core. The design of this invention is to covalently graft the primary amino group of AEDP onto the Zr center, deliberately retaining the bisphosphonic acid group on the ligand as a rare earth-specific chelating site, without occupying or neutralizing the super strong Brønsted acid sites of the sulfonic acid group in the resin body. At the same time, the bisphosphonic acid group of AEDP itself is a strong acid group, and after grafting, it can also supplement the system with additional Brønsted acid sites, achieving a complete balance between "stable rare earth loading" and "retention of strong Brønsted acid activity". After the primary amine of AEDP forms a Zr-N coordination bond with the Zr center, it further saturates Zr. 44+ The coordination number significantly reduces the chance of water molecules attacking Zr centers, improving the hydrolytic stability of Zr-OS bonds from the perspectives of steric hindrance and coordination saturation. This prevents Zr centers from falling off and sulfonic acid groups from hydrolyzing and losing in high-water systems, thus extending the catalyst's cycle life compared to traditional resins.

[0026] This invention achieves ultra-stable loading of rare earth ions by forming a multidentate chelate structure between the bisphosphonic acid group of the AEDP ligand and light rare earth ions. This modification step is not a simple functional superposition, but rather a design aimed at controlling side reactions and optimizing the activity gradient in the synthesis of DMMn. The core catalytic effect is as follows: Constructing a dual L-acid gradient synergistic system to achieve highly efficient catalysis of light rare earth ions (La) throughout the entire reaction process. 3+ Ce 3+(etc.) are highly adaptable medium-strength L-acid sites, whose L-acid strength is highly matched with the requirements of chain growth reaction, and can form a gradient synergy with the strong L-acid sites in Zr centers: the strong L-acid sites in Zr centers are responsible for cooperating with Brønsted acid to complete the protonation activation of methanediol, and the medium-strength L-acid sites of rare earth ions are responsible for catalyzing the electrophilic addition reaction of chain growth. The two work together to cover the entire reaction process from reactant activation to product formation, further improving the formaldehyde conversion rate and reaction rate.

[0027] The three major side reactions in DMMn synthesis (methylal hydrolysis, formaldehyde cannizzaro disproportionation, and product overpolymerization) are all caused by an imbalance in acid strength and ratio: excessive Brønsted acid easily triggers methylal hydrolysis, while excessive Lewis acid easily triggers product overpolymerization, generating long-chain polymers with n>8. This invention reduces the occurrence of side reactions at the source by controlling the rare earth loading; simultaneously, this invention completely preserves the macroporous framework structure of the resin, whose pore size is highly matched to the DMMn molecule, further restricting the formation of long-chain polymers through the pore confinement effect, ultimately resulting in the target product DMMn. 3-8 The selectivity has been greatly improved, especially for Ce. 3+ With unique Ce 3+ / Ce 4+ Its variable valence property can quench free radicals in the reaction system, inhibit the Cannizzaro disproportionation reaction of formaldehyde from the source, and significantly reduce the generation of impurities such as methanol and formic acid. The reduction in formic acid content further avoids the corrosion of the resin skeleton by the acidic environment and the acid desorption and shedding of sulfonic acid groups, forming a virtuous cycle in the reaction system.

[0028] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention improves the Brønsted acid strength of the resin matrix by coordinating the sulfonic acid group with the zirconium center, and at the same time constructs a gradient-matched Brønsted-Lewis dual-acid synergistic catalytic system, which is adapted to the reaction characteristics of DMMn synthesis, effectively breaks through the reaction rate control bottleneck, significantly improves the formaldehyde conversion efficiency, and can still maintain excellent proton supply capacity in the high water reaction system, avoiding the poisoning of acid active sites by water molecules. 2. This invention achieves functional separation of the Brønsted acid catalytic site and the rare earth chelation site through the directional grafting of aminobisphosphonic acid ligands, solving the industry pain point that existing rare earth modification schemes cannot simultaneously achieve stable rare earth loading and acid activity retention; the multidentate chelate structure formed by the bisphosphonic acid group and rare earth ions significantly improves the stability of rare earth loading and fundamentally prevents the dissolution and loss of rare earth ions during the reaction process.

[0029] 3. The multi-level stable structure formed by coordination grafting modification in this invention significantly improves the hydrolysis stability of the catalyst in a high-acidic system. The strong coordination bond formed between the zirconium center and the ligand not only prevents the hydrolysis and shedding of the metal center, but also effectively inhibits the hydrolysis and loss of sulfonic acid groups in the resin matrix. At the same time, a hydrophobic microenvironment is constructed around the active site, which significantly extends the catalyst's cycle life and long-term continuous operation stability. 4. The preparation process of this invention is carried out under mild conditions throughout, without the need for harsh high temperature and high pressure environments. The operation process is simple and controllable, and the raw materials are all industrially common reagents that are readily available and cost-controllable. The catalyst is compatible with both batch reactor and continuous fixed bed synthesis processes and has good compatibility with industrial formaldehyde raw materials of different concentrations. There is no need to concentrate and purify the raw materials at high cost, and it has excellent feasibility for industrial scale-up. 5. By controlling the acid strength and acid ratio of the catalyst and preserving the pore structure of the resin matrix, this invention achieves the regulation of the DMMn synthesis reaction, effectively suppresses the occurrence of side reactions such as methyl acetal hydrolysis, formaldehyde disproportionation, and excessive polymerization of products, significantly improves the selectivity of the target product and the proportion of high-value fractions, and greatly reduces the energy consumption and production cost of subsequent product distillation and separation. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0031] Example 1: A specific preparation method of a cationic catalyst containing rare earth elements, comprising the following steps: (1) Rinse 732 polystyrene sulfonic acid resin with deionized water until the washing solution is no longer cloudy. Then soak it in 95% ethanol with a volume of 2 times the resin for 3 hours. After taking it out, rinse off the surface ethanol with deionized water. Then soak it in dilute hydrochloric acid solution with a concentration of 3wt% with a volume of 4 times the resin for 12 hours. After taking it out, wash it with deionized water until the washing solution is neutral. After drying, the pretreated resin is obtained. (2) Under nitrogen protection, 100g of pretreated resin was added to 800g of anhydrous 1,2-dichloroethane and stirred at room temperature for 12h. Then 8g of zirconium tetrachloride was added and stirred for 10min. The temperature was raised to 50℃ and stirred for 4h. The mixture was filtered while hot and washed with 50℃ anhydrous ethanol until no free chloride ions were detected in the washing liquid. After drying, Zr coordination modified resin was obtained. (3) Under nitrogen protection, 20g of (1-aminoethane-1,1-diyl) bisphosphonic acid and 8g of anhydrous triethylamine were added to 600g of anhydrous DMF and stirred at room temperature for 10min. Then, 100g of Zr coordination modified resin was added and stirred for another 20min. The temperature was raised to 70℃ and stirred for 8h. The mixture was filtered while hot and then washed three times with ethanol, deionized water and ethanol alternately. After drying, phosphonic acid grafted modified resin was obtained. (4) 100g of phosphonic acid grafted modified resin was added to 400g of 0.03mol / L cerium nitrate hexahydrate aqueous solution (the pH was adjusted to 3.5±0.1 by dilute nitric acid with a concentration of 3wt%), stirred at room temperature for 20min, then heated to 60℃ and stirred for 10h. The resin was separated by filtration and washed repeatedly with deionized water until no free rare earth ions were detected in the washing filtrate. After drying, a cationic catalyst containing rare earth elements was obtained.

[0032] Example 2: A specific preparation method of a cationic catalyst containing rare earth elements, comprising the following steps: (1) Rinse 732 polystyrene sulfonic acid resin with deionized water until the washing solution is no longer cloudy. Then soak it in 95% ethanol with a volume of 2.5 times that of the resin for 4 hours. After taking it out, rinse off the surface ethanol with deionized water. Then soak it in dilute hydrochloric acid solution with a concentration of 5wt% with a volume of 5 times that of the resin for 18 hours. After taking it out, wash it with deionized water until the washing solution is neutral. After drying, the pretreated resin is obtained. (2) Under nitrogen protection, 100g of pretreated resin was added to 1kg of anhydrous 1,2-dichloroethane and stirred at room temperature for 18h. Then 12g of zirconium tetrachloride was added and stirred for 20min. The temperature was raised to 55℃ and stirred for 6h. The mixture was filtered while hot and washed with 50℃ anhydrous ethanol until no free chloride ions were detected in the washing liquid. After drying, Zr coordination modified resin was obtained. (3) Under nitrogen protection, 30g of (1-aminoethane-1,1-diyl) bisphosphonic acid and 10g of anhydrous triethylamine were added to 800g of anhydrous DMF and stirred at room temperature for 15min. Then, 100g of Zr coordination modified resin was added and stirred for another 30min. The temperature was raised to 75℃ and stirred for 12h. The mixture was filtered while hot and then washed three times with ethanol, deionized water and ethanol alternately. After drying, phosphonic acid grafted modified resin was obtained. (4) 100g of phosphonic acid grafted modified resin was added to 500g of 0.05mol / L cerium nitrate hexahydrate aqueous solution (the pH was adjusted to 3.5±0.1 by dilute nitric acid with a concentration of 4wt%), stirred at room temperature for 30min, then heated to 65℃ and stirred for 12h. The resin was separated by filtration and washed repeatedly with deionized water until no free rare earth ions were detected in the washing filtrate. After drying, a cationic catalyst containing rare earth elements was obtained.

[0033] Example 3: A specific preparation method of a cationic catalyst containing rare earth elements, comprising the following steps: (1) Rinse 732 polystyrene sulfonic acid resin with deionized water until the washing solution is no longer cloudy. Then soak it in 95% ethanol with a volume of 3 times the resin for 5 hours. After taking it out, rinse off the surface ethanol with deionized water. Then soak it in dilute hydrochloric acid solution with a concentration of 10wt% with a volume of 7 times the resin for 24 hours. After taking it out, wash it with deionized water until the washing solution is neutral. After drying, the pretreated resin is obtained. (2) Under nitrogen protection, 100g of pretreated resin was added to 1.2kg of anhydrous 1,2-dichloroethane and stirred at room temperature for 24h. Then 15g of zirconium tetrachloride was added and stirred for 30min. The temperature was raised to 60℃ and stirred for 8h. The mixture was filtered while hot and washed with 50℃ anhydrous ethanol until no free chloride ions were detected in the washing liquid. After drying, Zr coordination modified resin was obtained. (3) Under nitrogen protection, 35g of (1-aminoethane-1,1-diyl) bisphosphonic acid and 12g of anhydrous triethylamine were added to 1kg of anhydrous DMF and stirred at room temperature for 20min. Then 100g of Zr coordination modified resin was added and stirred for another 40min. The temperature was raised to 80℃ and stirred for 15h. The mixture was filtered while hot and then washed three times with ethanol, deionized water and ethanol alternately. After drying, phosphonic acid grafted modified resin was obtained. (4) 100g of phosphonic acid grafted modified resin was added to a 0.08mol / L aqueous solution of cerium nitrate hexahydrate (the pH was adjusted to 3.5±0.1 by dilute nitric acid with a concentration of 5wt%), stirred at room temperature for 40min, then heated to 70℃ and stirred for 15h. The resin was separated by filtration and washed repeatedly with deionized water until no free rare earth ions were detected in the washing filtrate. After drying, a cationic catalyst containing rare earth elements was obtained.

[0034] Example 4: The difference between Example 4 and Example 2 is that the aqueous solution of cerium nitrate hexahydrate is replaced with an aqueous solution of lanthanum nitrate hexahydrate.

[0035] Example 5: The difference between Example 5 and Example 2 is that the aqueous solution of cerium nitrate hexahydrate is replaced with an aqueous solution of praseodymium nitrate hexahydrate.

[0036] Example 6: The difference between Example 6 and Example 2 is that the aqueous solution of cerium nitrate hexahydrate is replaced with an aqueous solution of neodymium nitrate hexahydrate.

[0037] Example 7: The difference between Example 7 and Example 2 is that the aqueous solution of cerium nitrate hexahydrate is replaced with a mixed aqueous solution of cerium nitrate hexahydrate / lanthanum nitrate hexahydrate (Ce 3+ and La 3+ The molar ratio is 1:1.

[0038] Example 8: The difference between Example 8 and Example 2 is that the aqueous solution of cerium nitrate hexahydrate is replaced with a mixed aqueous solution of praseodymium nitrate hexahydrate / neodymium nitrate hexahydrate (Pr 3+ and Nd 3+ The molar ratio is 1:1.

[0039] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that 732 polystyrene sulfonic acid resin was used and, after the same pretreatment as in Example 2 (1), it was used directly as a catalyst.

[0040] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that 732 polystyrene sulfonic acid resin was used. After the same pretreatment as in Example 2 (1), 100g of the pretreated resin was added to 500g of 0.05mol / L cerium nitrate hexahydrate aqueous solution (the pH was adjusted to 3.5±0.1 by dilute nitric acid with a concentration of 4wt%). The solution was stirred at room temperature for 30min, then heated to 65℃ and stirred for 12h. The resin was separated by filtration and repeatedly washed with deionized water until no free rare earth ions were detected in the washing filtrate. After drying, the catalyst was obtained.

[0041] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that steps (3)-(4) are omitted, and the Zr coordination modified resin obtained in step (2) is used as a catalyst.

[0042] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that step (3) is omitted, and the Zr coordination modified resin obtained in step (2) is directly loaded with rare earth according to step (4).

[0043] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that step (4) is omitted, and the phosphonic acid grafted modified resin obtained in step (3) is used as a catalyst.

[0044] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that zirconium tetrachloride is replaced with tin tetrachloride, and the other steps are the same as in Example 2.

[0045] Performance testing: The catalysts of all examples and comparative examples were tested for catalytic performance under the same reaction conditions and detection methods. The batch reactor reaction performance, cycle stability, and continuous fixed bed long-term operation performance were tested respectively.

[0046] Testing instruments: Agilent 7890A gas chromatograph (equipped with FID detector, FFAP capillary column), Agilent 5110 ICP-OES inductively coupled plasma atomic emission spectrometer, high-pressure reactor (1L, with temperature control and stirring system), tubular fixed-bed reactor (25mm inner diameter, 1000mm tube length, with temperature control system).

[0047] Reaction materials: 37wt% industrial grade formaldehyde aqueous solution, 98% pure methylal; high-purity nitrogen (99.999%), high-purity hydrogen (99.999%).

[0048] Key performance indicator (KPI) calculation method: 1. Formaldehyde conversion rate: Formaldehyde conversion rate (%) = ; 2.DMM 3-8 Selectivity: DMM 3-8 Selectivity (%) = ; 3.DMM 3-6 Selectivity: DMM 3-6 Selectivity (%) = ; 4. Activity retention rate: Activity retention rate (%) = ; 5. Cumulative metal dissolution rate: Metal cumulative dissolution rate (%) = .

[0049] Intermittent reactor catalytic performance and cycle stability testing: 1. Standard test conditions A 37wt% industrial formaldehyde aqueous solution and 98% pure methylal were mixed at a formaldehyde to methylal molar ratio of 2.5:1 to prepare a total raw material solution of 500g. A catalyst was added, with the catalyst amount being 7% of the total raw material mass. The high-pressure reactor was sealed, and the air inside the reactor was replaced with nitrogen three times. The pressure was increased to 0.5MPa, and stirring was started (400rpm). The temperature was raised to 75℃ and maintained for 3 hours. After the reaction was completed, the reactor was rapidly cooled to room temperature in an ice-water bath. After depressurization, samples were taken for testing.

[0050] 2. Cyclic stability test method After a single reaction, the catalyst was recovered by filtration, washed three times with anhydrous ethanol, and dried under vacuum at 60°C for 12 hours. The reaction was then repeated under the above standard test conditions for 10 consecutive cycles. The formaldehyde conversion rate of the 10th reaction was detected, and the activity retention rate was calculated. The rare earth element and Zr / Sn element content in the combined reaction solution after 10 cycles was detected by ICP-OES, and the cumulative metal dissolution rate was calculated.

[0051] The test results are shown in Table 1.

[0052] Table 1. Results of intermittent batch catalyst performance and cycle stability tests Long-cycle operation performance test of continuous fixed bed: 1. Standard test conditions The catalyst was loaded at a rate of 100 mL into a tubular fixed-bed reactor, and the system was purged with nitrogen for 30 min to replace the air. The reactants were 37 wt% industrial formaldehyde aqueous solution and 98% pure methylal, with a formaldehyde to methylal molar ratio of 2.5:1 and a feed volume hourly space velocity of 1.0 h⁻¹. -1 The reaction temperature was 75℃, and the system pressure was 0.5MPa. The system operated continuously and stably for 500 hours, with samples taken every 24 hours to analyze the composition of the reaction solution. The average formaldehyde conversion rate and average DMM were calculated. 3-8 Selective, average DMM 3-6- Percentage; after operation, the catalyst activity retention rate and cumulative metal dissolution rate are measured.

[0053] The experimental results are shown in Table 2.

[0054] Table 2. Performance test results of continuous fixed bed during long-cycle operation Data Analysis: Overall, the rare earth-containing cationic catalysts prepared in all embodiments of this invention exhibit good formaldehyde conversion efficiency and DMM (dimethylformaldehyde) performance. 3-8 Target product selectivity, high value DMM 3-6 In terms of fraction ratio, cycle stability, and metal leaching resistance, the present invention is significantly superior to the comparative samples, which fully demonstrates that the preparation process of the present invention, namely "resin pretreatment activation - sulfonic acid Zr coordination anchoring - aminobisphosphonic acid ligand directional grafting - rare earth ion multidentate chelation loading", achieves simultaneous improvement in catalyst activity, product selectivity, cycle life and structural stability, and has excellent comprehensive catalytic performance and industrial application value. The catalyst in this invention exhibits excellent overall performance, likely due to the targeted design of the preparation process and molecular structure, achieving performance breakthroughs from both the perspectives of catalytic reaction nature and material stability. From the viewpoint of enhancing catalytic activity and product selectivity, this invention utilizes sulfonic acid groups and Zr... 4+ Coordination, using Zr 4+ The strong electron-deficient properties of the Zr significantly enhance the Brønsted acid strength of the sulfonic acid group, making it reach the level of a solid superacid. Even in the high-water system of DMMn synthesis, it can still maintain a very strong proton-donating ability, efficiently catalyzing the rate-controlling step of the protonation dehydration of methane glycol, and significantly improving the conversion efficiency of formaldehyde. At the same time, the anchored Zr center and the chelated rare earth ions form a gradient-matched Lewis acid dual center. The strong Lewis acid site of the Zr center works with the Brønsted acid to complete the activation of the reactants, while the moderate-strength Lewis acid site of the rare earth ions precisely regulates the chain growth reaction. The two work together to cover the entire reaction process, which not only accelerates the main reaction rate, but also avoids side reactions caused by excessively strong or weak acidity through precise matching of acid strength, significantly improving the efficiency of DMMn synthesis. 3-8 Selectivity of target products and high-value DMM 3-6 The fractional proportion. From the perspective of structural stability and improved cycle performance, this invention, through a directional ligand substitution reaction, grafts aminobisphosphonic acid ligands onto the Zr center via Zr-N covalent bonds, thereby saturating Zr. 4+ The high coordination number enhances the hydrolytic stability of the Zr-OS bond from both steric hindrance and coordination saturation perspectives, preventing the loss of metal centers and sulfonic acid group hydrolysis in high-water systems. Furthermore, the bisphosphonic acid groups on the ligands form a multidentate chelate structure with rare earth ions, resulting in a coordination stability constant far exceeding that of conventional electrostatic adsorption. This prevents the dissolution and loss of rare earth ions at the molecular level, ensuring the catalyst maintains extremely high activity retention even during multiple cycles and long-term continuous operation, significantly extending its lifespan. More importantly, this invention achieves functional separation between Brønsted acid catalytic sites and rare earth chelate sites. Rare earth ions, supported by dedicated bisphosphonic acid ligands, do not occupy or neutralize the sulfonic acid group active sites on the resin substrate. Simultaneously, the strong acidity of the bisphosphonic acid itself replenishes the Brønsted acid sites in the system, resolving the core contradiction in existing rare earth modification schemes where "rare earth stable loading" and "Brønsted acid activity retention" are mutually exclusive. This is why the overall performance of the catalyst in this invention is far superior to conventional modification schemes. Comparative Example 1 used unmodified blank 732 polystyrene sulfonic acid resin, and its catalytic performance and cycle stability were significantly lower than those of the embodiments of the present invention. This sample could only provide a single-strength Brønsted acid site, without Lewis acid sites to form a synergistic catalytic effect, and could not effectively reduce the activation energy barrier of the rate-controlling step of the reaction. Therefore, the formaldehyde conversion efficiency was much lower than that of the embodiments of the present invention. At the same time, in the high-water reaction system, water molecules easily form hydrogen bonds with sulfonic acid groups, causing poisoning of active sites. Moreover, sulfonic acid groups are prone to hydrolysis and shedding during long-term cycling, resulting in rapid decay of catalyst activity and cycle stability that was much worse than that of the embodiments of the present invention. In addition, a single acid center cannot achieve precise control of side reactions, and side reactions such as over-polymerization of products and hydrolysis of methyl acetal occur frequently. Therefore, the selectivity of the target product and the proportion of high-value fractions are also significantly lower than those of the embodiments of the present invention. Comparative Example 2 employs a conventional rare earth impregnation modification process, which exhibits significant deficiencies in both cycle stability and catalytic activity compared to the embodiments of the present invention. In this approach, rare earth ions bind only to the resin sulfonic acid groups through electrostatic interactions, lacking dedicated chelation sites and stable coordination structures. In the acidic aqueous reaction system, they are readily eluted by H+ exchange, resulting in a large dissolution of rare earth ions during cycling and a rapid decline in catalytic activity. The activity retention rate is far lower than that of the embodiments of the present invention. Simultaneously, the binding of rare earth ions to the sulfonic acid groups neutralizes the active hydrogen of the sulfonic acid groups, causing significant loss of the core Brønsted acid sites. This leads to a decrease in the catalyst's main reaction catalytic ability, with both formaldehyde conversion and target product selectivity significantly lower than those of the embodiments of the present invention. Comparative Example 3 only underwent Zr coordination modification without ligand grafting and rare earth loading. Compared with the embodiments of the present invention, its cycling stability and product selectivity are significantly insufficient. Although this sample improved the acid strength of the sulfonic acid group and introduced Lewis acid sites through Zr coordination, resulting in better initial catalytic performance than the blank resin, the Zr center lacked ligand protection and was prone to hydrolysis and breakage in a high-water system. This led to a large amount of Zr ion dissolution and subsequent loss of the sulfonic acid group. The activity decayed rapidly during long-term cycling and continuous operation, and the activity retention rate was much lower than that of the embodiments of the present invention. At the same time, the absence of a gradient Lewis acid dual center formed by rare earth ions made it impossible to precisely control the chain growth reaction, resulting in insufficient control of side reactions. Therefore, the selectivity of the target product and the proportion of high-value fractions were also significantly lower than those of the embodiments of the present invention. Comparative Example 4 directly impregnated and loaded rare earth elements after Zr coordination without introducing aminobisphosphonic acid chelating ligands. Compared with the embodiments of the present invention, its metal anti-dissolution performance and catalytic activity were significantly inferior. In this scheme, rare earth ions can only bind through weak interactions with the residual sulfonic acid groups in the resin and the hydroxyl groups on the Zr surface, without a stable multidentate chelate structure. During the reaction, there is still a serious problem of rare earth dissolution, and the cycle stability is much lower than that of the embodiments of the present invention. At the same time, this scheme does not solve the problem of hydrolytic stability of Zr centers. During the cycle, a large amount of Zr ions are still lost, leading to a rapid decline in catalyst activity. Furthermore, it cannot achieve functional separation between acid catalytic sites and rare earth loading sites, and the problem of neutralization and loss of sulfonic acid group active sites still exists. Therefore, the overall catalytic performance is significantly lower than that of the embodiments of the present invention. Comparative Example 5 involved Zr coordination and ligand grafting but lacked rare earth ion chelation loading. Compared to the embodiments of this invention, its catalytic activity and product selectivity were significantly insufficient. Although this sample saturated the coordination number of the Zr center through ligand grafting, improving the hydrolytic stability of the Zr center, and exhibiting better metal dissolution rate and cycling stability than the sample with only Zr coordination, it lacked a gradient Lewis acid synergistic catalytic system formed by rare earth ions. This prevented precise catalysis of the entire reaction process, resulting in significant deficiencies in both the main reaction rate and the control of side reactions. Consequently, the formaldehyde conversion rate and target product selectivity were lower than those of the fully modified embodiments of this invention. Comparative Example 6, which uses tin tetrachloride instead of zirconium tetrachloride for modification, exhibits fatal defects in structural stability, cycling performance, and product selectivity compared to the embodiments of the present invention. Because the bond energy of the Sn-O bond is lower than that of the Zr-O bond, it is highly susceptible to hydrolysis and breakage in the high-acidity system synthesized in DMMn. This leads to the massive dissolution of the Sn center, grafted ligands, and chelated rare earth ions, causing rapid collapse of the catalyst structure. During cycling and long-term operation, the activity is almost completely lost, with an activity retention rate far lower than that of the Zr-modified embodiments of the present invention. Simultaneously, the excessively strong Lewis acid at the Sn center easily triggers side reactions such as excessive polymerization of the product and deep hydrolysis of methyl acetal. Therefore, the selectivity of the target product is also significantly lower than that of the embodiments of the present invention.

[0055] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A process for the preparation of a rare earth element-containing cationic catalyst, characterized in that, Includes the following steps: (1) Rinse 732 polystyrene sulfonic acid resin with deionized water until the washing solution is no longer cloudy. Then soak it in 95% ethanol for 3-5 hours. After taking it out, rinse off the surface ethanol with deionized water. Then soak it in dilute hydrochloric acid solution for 12-24 hours. After taking it out, wash it with deionized water until the washing solution is neutral. After drying, the pretreated resin is obtained. (2) Under nitrogen protection, the pretreated resin was added to anhydrous 1,2-dichloroethane and stirred at room temperature for 12-24 h. Then zirconium tetrachloride was added and stirred for 10-30 min. The temperature was raised to 50-60℃ and stirred for 4-8 h. The mixture was filtered while hot and washed with anhydrous ethanol at 50℃ until no free chloride ions were detected in the washing liquid. After drying, Zr coordination modified resin was obtained. (3) Under nitrogen protection, (1-aminoethane-1,1-diyl) bisphosphonic acid and anhydrous triethylamine were added to anhydrous DMF and stirred at room temperature for 10-20 min. Then Zr coordination modified resin was added and stirred for another 20-40 min. The temperature was raised to 70-80℃ and stirred for 8-15 h. The mixture was filtered while hot and then washed three times with ethanol, deionized water and ethanol alternately. After drying, phosphonic acid grafted modified resin was obtained. (4) Add the phosphonic acid grafted modified resin to the rare earth salt solution, stir at room temperature for 20-40 min, then heat to 60-70℃, stir for 10-15 h, filter to separate the resin, wash the resin repeatedly with deionized water until no free rare earth ions are detected in the washing filtrate, and dry to obtain a cationic catalyst containing rare earth elements.

2. The method for preparing the rare earth element-containing cationic catalyst according to claim 1, characterized in that, In (1), the volume ratio of 732 polystyrene sulfonic acid resin, 95% ethanol and dilute hydrochloric acid solution is 1:2-3:4-7; the concentration of dilute hydrochloric acid solution is 3-10wt%.

3. The method for preparing the rare earth element-containing cationic catalyst according to claim 1, characterized in that, In step (2), the pretreated resin, anhydrous 1,2-dichloroethane, and zirconium tetrachloride are in a weight ratio of 1:8-12:0.08-0.

15.

4. The method for preparing the rare earth element-containing cationic catalyst according to claim 1, characterized in that, In (3), the weight ratio of (1-aminoethane-1,1-diyl) bisphosphonic acid, anhydrous triethylamine, anhydrous DMF and Zr coordination modified resin is 0.2-0.35:0.08-0.12:6-10:

1.

5. The method for preparing the rare earth element-containing cationic catalyst according to claim 1, characterized in that, In step (4), the phosphonic acid grafted modified resin and rare earth salt solution are in a weight ratio of 1:4-6; the concentration of the rare earth salt solution is 0.03-0.08 mol / L; and the pH of the rare earth salt solution is adjusted to 3.5±0.1 by using a 3-5 wt% dilute nitric acid solution.

6. The method for preparing the rare earth element-containing cationic catalyst according to claim 1, characterized in that, The rare earth salt solution in (4) is one of the following: lanthanum nitrate hexahydrate aqueous solution, cerium nitrate hexahydrate aqueous solution, praseodymium nitrate hexahydrate aqueous solution, or neodymium nitrate hexahydrate aqueous solution, or a mixed aqueous solution of lanthanum nitrate hexahydrate / cerium nitrate hexahydrate or a mixed aqueous solution of praseodymium nitrate hexahydrate / neodymium nitrate hexahydrate.

7. A cationic catalyst containing rare earth elements, characterized in that, It is prepared by the preparation method described in any one of claims 1-6.

8. The application of the rare earth element-containing cationic catalyst according to claim 7, characterized in that, The rare earth element-containing cationic catalyst is used to catalyze the synthesis of polyoxymethylene dimethyl ether (DMMn) from liquid formaldehyde and methyl acetal using either a batch reactor reaction or a continuous fixed-bed reaction.

9. The application of the rare earth element-containing cationic catalyst according to claim 8, characterized in that, The liquid formaldehyde is a formaldehyde aqueous solution with a concentration of 37-90 wt%; the molar ratio of formaldehyde to methyl acetal is 1-4:1 during the synthesis of polyoxymethylene dimethyl ether; and n=3-8 in the synthesized polyoxymethylene dimethyl ether DMMn.

10. The application of the rare earth element-containing cationic catalyst according to claim 8, characterized in that, The process parameters of the intermittent kettle reaction are as follows: catalyst dosage is 4%-10% of the total mass of raw materials, reaction temperature is 60-90℃, reaction pressure is 0.3-0.8MPa, stirring speed is 300-600rpm, and reaction time is 2-5h; the process parameters of the continuous fixed bed reaction are as follows: raw material volume space velocity is 0.5-1.5h -1 , reaction temperature is 65-85℃, and reaction pressure is 0.4-0.6MPa.

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