A method for preparing a composite alcohol base catalyst suitable for transesterification reaction

Abstract

This invention discloses a method for preparing a composite alcohol-base catalyst suitable for transesterification reactions, relating to the fields of catalyst preparation technology and organic synthesis technology. This invention uses anhydrous alcohol phase in-situ alcoholization reaction treatment to make the catalyst surface cleaner, reduce the impact of impurities on catalyst activity, and enhance the structural stability of the catalyst. This catalyst can maintain high catalytic activity after multiple uses, extending the catalyst's service life and reducing production costs.

Description

Technical Field

[0001] This invention relates to the fields of catalyst preparation technology and organic synthesis technology, specifically to a method for preparing a composite alcohol-base catalyst suitable for transesterification reactions. Background Technology

[0002] In the field of organic chemical synthesis, transesterification is an important method for carbon-carbon bond formation and functional group conversion, widely used in biodiesel production, polymer synthesis, and the preparation of fragrance and pharmaceutical intermediates. As a core element of transesterification, the performance of the catalyst directly determines the efficiency, selectivity, and cost-effectiveness of the reaction.

[0003] Traditional transesterification catalysts, such as homogeneous base catalysts, while exhibiting high initial catalytic activity, suffer from drawbacks including difficulty in recycling, easy corrosion of equipment, and rapid deactivation due to sensitivity to moisture in the reaction system. Furthermore, while heterogeneous catalysts, such as solid base catalysts, address the recycling issue to some extent, their activity and selectivity are often limited by the catalyst's preparation method and structural characteristics, such as insufficient specific surface area and uneven distribution of active sites, resulting in low catalytic efficiency. Moreover, they are prone to deactivation after repeated cycles due to carbon buildup or loss of active components, increasing production costs and operational complexity.

[0004] In view of the above-mentioned problems of traditional transesterification catalysts, the present invention proposes a method for preparing a composite alcohol-base catalyst suitable for transesterification, which is of particular importance. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing a composite alcohol-base catalyst suitable for transesterification reactions. By constructing a support with a porous core-shell structure and using gradient impregnation technology to load weak and strong base components respectively, it not only significantly improves the activity and selectivity of the catalyst, but also enhances its structural stability and durability. It effectively solves the problems of catalyst recycling and deactivation, and provides strong support for the efficient, green and sustainable development of transesterification reactions.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing a composite alcohol-base catalyst suitable for transesterification reactions, the specific steps of which are as follows:

[0007] Step 1: Preparation of hydrophobic silica microsphere core: Monodisperse silica microspheres were prepared using the modified Stöber method. The silica microspheres were then surface-modified to be hydrophobic. After solid-liquid separation, washing, and drying, the hydrophobic silica microsphere core was obtained.

[0008] Step 2, Preparation of porous core-shell structured carrier: The core of the hydrophobic silica microspheres obtained in Step 1 is dispersed in anhydrous ethanol, and an aluminum source, mesoporous template agent and alkaline regulator are added. After room temperature stirring pre-reaction, hydrothermal treatment and calcination to remove the template agent, a porous core-shell structured carrier with a hydrophobic silica microsphere core and a hydrophilic mesoporous aluminoxane shell is obtained.

[0009] Step 3, Gradient impregnation and preloading of weak base components: The weak base co-catalytic component is dissolved in anhydrous low-carbon alcohol to prepare a low-concentration impregnation solution. The porous core-shell structured support obtained in Step 2 is placed in the low-concentration impregnation solution and subjected to vacuum impregnation treatment to allow the weak base co-catalytic component to be directionally diffused and loaded at the interface between the core and shell of the core-shell support. After solid-liquid separation and vacuum drying, a preloaded support is obtained.

[0010] Step 4, Gradient impregnation and loading of strong base components: The strong base active components are dissolved in anhydrous low carbon alcohol to prepare a high-concentration impregnation solution. The pre-loaded support obtained in step 3 is placed in the high-concentration impregnation solution and subjected to atmospheric pressure impregnation treatment to uniformly load the strong base active components onto the mesoporous aluminoxane outer shell layer of the core-shell support. After solid-liquid separation and vacuum drying, the catalyst precursor is obtained.

[0011] Step 5, in-situ alcoholization reaction in anhydrous alcohol phase: The catalyst precursor obtained in step 4 is placed in anhydrous low-carbon alcohol and an in-situ alcoholization reaction is carried out under inert gas protection, heating and stirring conditions. After the reaction is completed, the catalyst is subjected to solid-liquid separation, anhydrous alcohol washing and vacuum drying under an inert atmosphere to obtain a composite alcohol-base catalyst suitable for transesterification reaction.

[0012] Further, in step 1, the particle size of the monodisperse silica microspheres is 100nm~500nm; the surface hydrophobic modification uses at least one of hexamethyldisilazane, methyltriethoxysilane, and dimethyldichlorosilane, the modification temperature is 40℃~80℃, and the modification time is 2h~6h.

[0013] Furthermore, in the porous core-shell structured carrier obtained in step 2, the thickness of the hydrophilic mesoporous aluminoxane shell is 50nm~200nm, the mesopore diameter of the shell is 2nm~20nm, and the specific surface area of ​​the carrier is 300m² / g~600m² / g.

[0014] Furthermore, in step 2, the aluminum source is at least one of aluminum isopropoxide, aluminum sec-butoxide, and aluminum nitrate; the mesoporous template agent is at least one of P123, F127, and CTAB; the alkalinity regulator is at least one of ammonia and urea; the hydrothermal treatment temperature is 100℃~160℃, and the treatment time is 12h~48h; the calcination temperature is 450℃~600℃, and the calcination time is 3h~6h.

[0015] Furthermore, in step 3, the weak base co-catalyst component is at least one of potassium carbonate, sodium carbonate, potassium acetate, sodium acetate, magnesium hydroxide, and calcium hydroxide; the mass concentration of the low-concentration impregnation solution is 2% to 8%.

[0016] Furthermore, in step 3, the vacuum degree of the vacuum impregnation is -0.08MPa to -0.1MPa, the impregnation temperature is 30℃ to 50℃, and the impregnation time is 2h to 6h; the final loading of the weak base co-catalytic component is 1% to 5% of the dry weight of the porous core-shell structure carrier.

[0017] Furthermore, in step 4, the strong alkaline active component is at least one of sodium hydroxide, potassium hydroxide, and lithium hydroxide; and the mass concentration of the high-concentration impregnation solution is 10% to 25%.

[0018] Furthermore, in step 4, the temperature of the atmospheric pressure impregnation is 25℃~40℃, and the impregnation time is 4h~12h; the final loading of the strong alkali active component is 10%~20% of the dry weight of the porous core-shell structure carrier.

[0019] Furthermore, the mass concentration of the high-concentration impregnation solution in step 4 is 2.5 to 10 times that of the low-concentration impregnation solution in step 3; and the final loading of the strong base active component in step 4 is 3 to 15 times that of the final loading of the weak base co-catalytic component in step 3.

[0020] Furthermore, in step 5, the anhydrous low-carbon alcohol is at least one of anhydrous methanol, anhydrous ethanol, and anhydrous isopropanol; the inert gas is at least one of nitrogen and argon; the temperature of the in-situ alcoholization reaction is 60℃~120℃, and the reaction time is 4h~12h; the temperature of the vacuum drying is 60℃~100℃, and the drying time is 6h~12h.

[0021] Compared with existing technologies, this method for preparing a composite alcohol-base catalyst suitable for transesterification reactions has the following advantages:

[0022] I. This invention prepares a support with a porous core-shell structure, with a hydrophobic silica microsphere core and a hydrophilic mesoporous aluminoxane shell. This unique structure enables the catalyst to exhibit higher activity and selectivity in transesterification reactions. The weak base component and the strong base component are directionally loaded onto different parts of the support through gradient impregnation, optimizing the distribution of active sites in the catalyst and thus improving catalytic efficiency. Verification by X-ray photoelectron spectroscopy and scanning electron microscopy-energy dispersive spectroscopy shows that the catalyst prepared by this invention achieves a precise gradient distribution of the weak base component at the core-shell interface and the strong base component in the shell layer. The dispersion of active sites is improved by more than 40% compared with solid base catalysts prepared by the traditional equal-volume impregnation method.

[0023] Second, the composite alcohol-base catalyst prepared by this invention is treated with anhydrous alcohol phase in-situ alcoholization reaction, which makes the catalyst surface cleaner, reduces the impact of impurities on the catalyst activity, and enhances the structural stability of the catalyst. This catalyst can still maintain high catalytic activity after multiple uses, prolonging the service life of the catalyst and reducing production costs. The cyclic catalytic performance test verifies that after being cycled 8 times in a typical transesterification reaction system, the catalyst of this invention can still maintain more than 90% of the initial activity, which is far superior to the industry standard level of less than 60% activity of traditional solid base catalysts after 5 cycles.

[0024] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0026] Figure 1 This is an overall process flow diagram of a method for preparing a composite alcohol-base catalyst suitable for transesterification reactions;

[0027] Figure 2 A detailed process flow diagram for the preparation of a hydrophobic core and porous core-shell structured support for a composite alcohol-base catalyst suitable for transesterification reactions.

[0028] Figure 3 This paper presents a process flow for the preparation of a composite alcohol-base catalyst suitable for transesterification reactions, including a weak-base-strong-base two-component gradient impregnation and loading refinement process. Detailed Implementation

[0029] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0030] Example 1

[0031] The composite alcohol-base catalyst prepared in this embodiment is specifically designed for the application scenario of transesterification reaction of high-acid-value waste cooking oil with methanol to produce biodiesel. It can effectively reduce saponification side reactions during the transesterification process and improve the conversion rate of waste cooking oil and the yield of biodiesel. The specific preparation steps are as follows:

[0032] Step 1: Preparation of the hydrophobic silica microsphere core: Monodisperse silica microspheres with a particle size of 200 nm were prepared using a modified Stöber method. This method can precisely control the particle size uniformity and spherical integrity of the microspheres, providing a uniform and structurally stable core substrate for the subsequent construction of the core-shell structure. The prepared monodisperse silica microspheres were then treated with hexamethyldisilazane as a surface modifier, and surface hydrophobic modification was performed at 60 °C for 4 hours. This hydrophobic modification treatment can graft hydrophobic groups onto the surface of the silica microspheres, giving the microsphere core strong hydrophobicity. This creates a hydrophilic-hydrophobic gradient difference between the core and shell, providing structural driving force for the subsequent directional gradient loading of active components. At the same time, it can prevent the agglomeration of polar alkaline catalytic components on the surface of the microsphere core. After modification, solid-liquid separation, washing, and drying were performed sequentially to finally obtain a hydrophobic silica microsphere core with stable surface hydrophobic properties and uniform particle size.

[0033] Step 2: Preparation of the porous core-shell structure carrier: The hydrophobic silica microsphere cores obtained in Step 1 are uniformly dispersed in anhydrous ethanol. Anhydrous ethanol, as the dispersion medium, ensures uniform dispersion of the hydrophobic silica microsphere cores at the single-particle level, avoiding particle agglomeration and uneven shell thickness during subsequent shell growth. Aluminum isopropoxide as the aluminum source, P123 as the mesoporous template agent, and ammonia as the alkalinity regulator are added sequentially to the dispersion system. A pre-reaction is carried out by stirring at room temperature. The pre-reaction process allows the aluminum source to undergo preliminary hydrolysis and condensation in an alkaline environment, forming a uniform aluminum oxane precursor coating layer on the surface of the hydrophobic silica core, laying the foundation for stable shell growth. After the pre-reaction, the substrate is transferred to a hydrothermal reactor and subjected to hydrothermal treatment at 120°C for 24 hours. Hydrothermal treatment promotes further crystallization and cross-linking of the aluminum oxane precursor, making the shell structure more dense and stable. Simultaneously, the addition of anhydrous ethanol further enhances the stability of the core structure. The template agent forms an ordered morphology of mesoporous channels. After hydrothermal treatment, the solid product is collected and calcined at 550℃ for 4 hours to remove the template agent. High-temperature calcination can completely decompose and remove the template agent, fully opening up the mesoporous channels in the shell, while further improving the mechanical strength and structural stability of the shell. Finally, a porous core-shell structure carrier with a hydrophobic silica microsphere core and a hydrophilic mesoporous aluminoxane shell is obtained. The thickness of the hydrophilic mesoporous aluminoxane shell of this carrier is 100 nm, the pore size of the shell is 10 nm, and the specific surface area of ​​the carrier is 450 m² / g. The hydrophilic mesoporous shell is perfectly adapted to the alcohol phase reaction system of transesterification reaction, which greatly improves the dispersion and contact efficiency of the catalyst in the reactants. The rich mesoporous structure can provide sufficient binding sites for the subsequent loading of catalytic active components, while the confinement effect of the mesoporous channels can effectively inhibit the aggregation and loss of active components during the reaction.

[0034] Step 3: Gradient Impregnation Pre-loading of Weak Base Components: Potassium carbonate was used as a weak base co-catalyst component and dissolved in anhydrous low-carbon alcohol to prepare a low-concentration impregnation solution with a mass concentration of 5%. Using anhydrous low-carbon alcohol as a solvent avoids the damage of water to the porous core-shell support structure and ensures good wettability of the impregnation solution within the support pores due to its compatibility with the alcohol phase of the support. The low concentration of the impregnation solution ensures precise directional diffusion of the weak base co-catalyst component during subsequent impregnation, preventing excessive penetration of the component to the outer shell surface and thus avoiding damage to the gradient loading structure design. The porous core-shell support structure obtained in Step 2 was placed in the prepared low-concentration impregnation solution and vacuum impregnated under a vacuum of -0.09 MPa. The impregnation temperature was controlled at 40℃, and the impregnation time was 4 hours. The vacuum impregnation process completely removes air from the mesopores of the support, allowing the impregnation solution to flow smoothly under negative pressure. The weak base co-catalytic component diffuses to the core-shell interface of the core-shell support, achieving site-specific pre-loading of the weak base co-catalytic component at the interface site. The weak base co-catalytic component loaded at the core-shell interface can form a gradient alkalinity distribution inside the catalyst, which can not only play a co-catalytic role in the transesterification reaction and synergistically improve the catalytic efficiency of the main active center, but also effectively inhibit the migration and agglomeration of the strong base active component in the shell layer to the core. At the same time, it can reduce the dissolution loss of the strong base active component during the reaction process and improve the recycling performance of the catalyst. After impregnation, solid-liquid separation and vacuum drying are performed sequentially to obtain the pre-loaded support. The final loading of potassium carbonate is 3% of the dry weight of the porous core-shell structure support. Vacuum drying can stably anchor the pre-loaded weak base co-catalytic component at the interface binding site under mild and oxygen-free conditions, avoiding the migration and agglomeration of components during the drying process and the loss of the structural advantage of gradient distribution.

[0035] Step 4: Gradient impregnation and loading of strong base components: Potassium hydroxide is used as the strong base active component and dissolved in anhydrous low-carbon alcohol to prepare a high-concentration impregnation solution with a mass concentration of 15%. Using anhydrous low-carbon alcohol as the solvent ensures consistency with the previous impregnation system, avoiding the dissolution of the already loaded weak base co-catalytic component due to changes in the solvent system. The high-concentration impregnation solution ensures that the strong base active component can fully adhere to the mesoporous channels and surface of the outer shell layer, providing sufficient core catalytic active centers for the transesterification reaction. The pre-loaded support obtained in Step 3 is then placed in the solution... The prepared high-concentration impregnation solution was impregnated under normal pressure at a controlled temperature of 30°C for 8 hours. This method allows the impregnation solution to preferentially adhere to and load onto the mesoporous aluminoxane outer shell layer of the core-shell carrier through natural diffusion, preventing excessive migration towards the core. This precisely distinguishes the loading region from the vacuum impregnation in step 3, perfectly achieving a gradient loading structure design where the weak base component is at the core-shell interface and the strong base component is in the outer shell layer. This allows the strong base active component, serving as the core active center for the transesterification reaction, to be effectively utilized. After being uniformly loaded onto the hydrophilic mesoporous outer shell layer, it can fully contact reactants such as waste cooking oil and methanol during the reaction process, significantly reducing the activation energy of the transesterification reaction and improving the reaction rate and raw material conversion rate. At the same time, the confinement effect of the mesoporous channels can effectively inhibit the aggregation of strong base active components during the reaction process and reduce the occurrence of side reactions such as saponification. After impregnation, solid-liquid separation and vacuum drying are performed sequentially to obtain the catalyst precursor. The final loading of potassium hydroxide is 15% of the dry weight of the porous core-shell structure carrier. The mass concentration of the high-concentration impregnation solution used in this step is 3 times that of the low-concentration impregnation solution in step 3. The final loading of the strong base active component in this step is 5 times that of the final loading of the weak base co-catalytic component in step 3. This gradient design of concentration and loading can further enhance the basicity gradient distribution of the catalyst, ensuring high catalytic activity while significantly reducing the risk of dissolution and loss of strong base active components. Vacuum drying can remove excess solvent under mild conditions and stably anchor the strong base active components on the active sites of the mesoporous outer shell layer, avoiding aggregation and deactivation of the active centers.

[0036] Step 5: In-situ alcoholization reaction of anhydrous alcohol phase: The catalyst precursor obtained in step 4 is placed in anhydrous ethanol. Anhydrous ethanol serves as the reaction medium, providing an alcohol source for the in-situ alcoholization reaction and ensuring compatibility with the alcohol phase system of the target transesterification reaction. This guarantees good compatibility of the final catalyst in the reaction system. Under a protective atmosphere of nitrogen as an inert gas, the in-situ alcoholization reaction is carried out by heating and continuous stirring. The reaction temperature is controlled at 80℃, and the reaction time is 8 hours. The inert gas protection completely isolates carbon dioxide and moisture from the air, preventing the strong base active component from reacting with carbon dioxide to form carbonates and losing catalytic activity. It also prevents moisture from causing hydrolysis and deactivation of the alcohol-base active centers. The in-situ alcoholization reaction under heating and stirring conditions allows the strong and weak base components supported on the support to undergo a full in-situ alcoholization reaction with anhydrous ethanol, generating alcohol-base active centers with higher transesterification catalytic activity. At the same time, the alcoholization reaction further integrates the active components with the mesoporous... Chemical anchoring with an aluminoxane support significantly reduces the dissolution and loss of active components during the transesterification reaction, improving the catalyst's stability for recycling. After the reaction, solid-liquid separation, anhydrous alcohol washing, and vacuum drying are performed sequentially under a nitrogen inert atmosphere. The vacuum drying temperature is 80℃, and the drying time is 8 hours. The post-treatment under an inert atmosphere throughout the process prevents newly generated highly active alcohol-base centers from being deactivated by contact with air. Anhydrous alcohol washing removes unanchored free base components from the catalyst surface, reducing the occurrence of side reactions during the reaction. Vacuum drying thoroughly removes residual alcohol solvents and volatile impurities from inside the catalyst, ultimately yielding a composite alcohol-base catalyst with stable structure, precise gradient distribution of active centers, high catalytic activity, and strong anti-leakage performance. This catalyst is perfectly suited for the transesterification reaction system of preparing biodiesel from waste cooking oil, achieving efficient conversion of waste cooking oil under mild reaction conditions while effectively inhibiting saponification side reactions and improving the yield and purity of biodiesel.

[0037] 1. Catalyst structure characterization and verification data

[0038] The composite alcohol-base catalyst was characterized using a laser particle size analyzer and a nitrogen adsorption-desorption analyzer. The results showed that the catalyst particles were monodisperse spherical with an overall particle size distribution coefficient of only 3.2%, exhibiting excellent spherical integrity. The catalyst had a specific surface area of ​​426 m² / g, with mesopore sizes concentrated in the range of 9–11 nm and a pore volume of 0.48 cm³ / g, showing no significant changes compared to the support structural parameters, proving that the porous core-shell structure of the support was not destroyed during the preparation process. Inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis revealed that the actual loading of potassium carbonate in the catalyst was 2.97%, and the actual loading of potassium hydroxide was 14.89%, with deviations from the designed loadings of less than 1%, achieving precise and controllable loading of the active components. SEM-EDS surface scanning analysis showed that potassium was uniformly enriched in the outer shell layer, and characteristic peaks of weakly basic elements appeared at the core-shell interface, verifying the successful construction of the gradient loading structure.

[0039] 2. Catalytic performance evaluation data for transesterification reaction

[0040] Using high-acid-value waste cooking oil and methanol as reactants, with a methanol-to-oil molar ratio of 9:1, and a catalyst addition of 3% of the waste cooking oil mass, the reaction was conducted at 65℃ for 2 hours under normal pressure. The transesterification reaction was evaluated. Results showed that the transesterification conversion rate of the waste cooking oil reached 98.7%, the yield of crude biodiesel reached 97.2%, and the content of saponification byproducts was less than 0.3%. Under the same reaction conditions, the biodiesel yield of the traditional homogeneous KOH catalyst was 92.5%, with a saponification byproduct content of 2.1%, while the biodiesel yield of the SiO2-supported KOH solid alkali catalyst prepared by the traditional impregnation method was 86.3%. This demonstrates that the catalyst prepared in this invention has superior catalytic activity and reaction selectivity.

[0041] 3. Catalyst Cyclic Stability Test Data

[0042] After the reaction, the catalyst was separated by centrifugation, washed with anhydrous methanol, and vacuum dried, and then reused in the above-mentioned transesterification reaction of waste catering oil. After being recycled 8 times, the transesterification conversion rate of waste catering oil could still reach 90.2%, and the biodiesel yield remained at 88.5%. ICP-OES test showed that the dissolution rate of the active component of the catalyst after a single cycle was less than 0.8%, which was much lower than the dissolution rate of more than 3% of traditional solid base catalysts after a single cycle, verifying the excellent structural stability and cycle life of the catalyst of this invention.

[0043] Example 2

[0044] The composite alcohol-base catalyst prepared in this embodiment is specifically designed for the application scenario of synthesizing polycarbonate diols via transesterification of dimethyl carbonate and 1,4-butanediol. It can precisely control the transesterification reaction process and the molecular weight distribution of the products, significantly improve reaction selectivity, reduce side reactions, and ensure the batch stability and application compatibility of the polycarbonate diol products. The specific preparation steps are as follows:

[0045] Step 1: Preparation of the hydrophobic silica microsphere core: Monodisperse silica microspheres with a particle size of 300 nm were prepared using a modified Stöber method. This method can precisely control the particle size uniformity and spherical integrity of the microspheres, providing a uniform and structurally stable core substrate for the subsequent construction of the core-shell structure, and avoiding problems such as particle agglomeration and uneven shell coating during subsequent shell growth. The prepared monodisperse silica microspheres were then subjected to surface hydrophobic modification with methyltriethoxysilane as a surface modifier at 50°C for 5 hours. Hydrophobic alkyl groups can be grafted onto the surface of silica microspheres to give the microsphere core stable strong hydrophobicity. This creates a hydrophilic-hydrophobic gradient difference between the core and the subsequent shell, providing structural driving force for the directional interfacial loading of the weak base co-catalytic component. At the same time, it can avoid the aggregation of polar base catalytic components on the core surface, ensuring the accuracy of the subsequent gradient loading structure. After modification, solid-liquid separation, washing, and drying are performed sequentially to completely remove unreacted modifiers and impurities, finally obtaining a hydrophobic silica microsphere core with stable hydrophobic properties, uniform particle size, and excellent sphericity.

[0046] Step 2: Preparation of the porous core-shell structure carrier: The hydrophobic silica microsphere cores obtained in Step 1 are uniformly dispersed in anhydrous ethanol. Anhydrous ethanol, as the dispersion medium, ensures uniform dispersion of the hydrophobic silica microsphere cores at the single-particle level, avoiding problems such as multi-core coating, particle agglomeration, and uneven shell thickness during subsequent shell growth. Aluminum sec-butoxide (APT) is added sequentially to the dispersion system as the aluminum source, CTAB as the mesoporous template agent, and urea as the alkaline regulator. A pre-reaction is carried out by stirring at room temperature. The pre-reaction process relies on the alkaline environment released by the slow decomposition of urea to allow the aluminum source to undergo a mild and controllable hydrolysis-condensation reaction. A uniform and continuous aluminoxane precursor coating layer is formed on the surface of the silicon core, laying the foundation for the stable and orderly growth of the subsequent shell layer and avoiding shell detachment and cracking during hydrothermal treatment. After the pre-reaction, the core is transferred to a hydrothermal reactor and subjected to hydrothermal treatment at 140°C for 36 hours. Hydrothermal treatment promotes deep cross-linking and crystallization of the aluminoxane precursor, significantly improving the mechanical strength and chemical structural stability of the shell layer. Simultaneously, the CTAB mesoporous template agent helps form ordered, uniformly sized mesoporous channels within the shell layer. After hydrothermal treatment, the solid product is collected and calcined at 500°C to remove the mesoporous template agent. The calcination time was 5 hours. High-temperature calcination completely decomposed and removed the CTAB mesoporous template agent, fully opening the mesoporous channels within the shell layer. Simultaneously, it further solidified the shell layer's skeletal structure, improving the thermal stability and alcoholysis resistance of the carrier. The final product was a porous core-shell carrier with a hydrophobic silica microsphere core and a hydrophilic mesoporous aluminoxane shell. The hydrophilic mesoporous aluminoxane shell of this carrier had a thickness of 150 nm, a mesoporous pore size of 15 nm, and a specific surface area of ​​500 m² / g. The hydrophilic mesoporous aluminoxane shell is perfectly suited for the alcohol phase reaction system of dimethyl carbonate and 1,4-butanediol ester exchange, significantly improving the catalyst's performance in the reactants. The wettability and dispersibility of the material allow for full contact between the catalytic active center and the reactants. The 150nm shell thickness provides ample space for loading the active components and also controls the contact path of the reactants through the confinement effect of the shell, thereby improving the selectivity of the transesterification reaction. The 15nm mesoporous pore size ensures rapid diffusion of reactants and products, avoids pore blockage affecting reaction efficiency, and effectively inhibits the agglomeration and loss of active components during preparation and reaction. The high specific surface area of ​​500m² / g provides a large number of binding sites for the uniform loading of subsequent strong base active components, ensuring high dispersion of active centers and avoiding side reactions caused by excessive local alkalinity.

[0047] Step 3: Gradient impregnation and preloading of weak base components: Potassium acetate is used as a weak base co-catalyst component and dissolved in anhydrous low-carbon alcohol to prepare a low-concentration impregnation solution with a mass concentration of 4%. Using anhydrous low-carbon alcohol as a solvent can avoid the damage of water to the porous core-shell support framework structure. At the same time, the alcohol affinity of the alcohol is highly matched with that of the support, ensuring that the impregnation solution fully wets the mesoporous channels. The low-concentration impregnation solution can precisely control the diffusion rate and diffusion depth of the weak base co-catalyst component, avoiding excessive penetration of the component to the outer shell surface, which would damage the structure of subsequent gradient loading. The porous core-shell structured support obtained in step 2 is placed in a prepared low-concentration impregnation solution and subjected to vacuum impregnation at a vacuum level of -0.085 MPa. The impregnation temperature is controlled at 35°C, and the impregnation time is 5 hours. The vacuum impregnation process thoroughly removes residual air from the mesopores of the support. Under the action of negative pressure, the low-concentration weakly alkaline impregnation solution diffuses directionally to the interface between the core and shell of the core-shell support, achieving targeted pre-loading of the weakly alkaline co-catalytic component at the interface sites and preventing its excessive accumulation on the shell surface. The potassium acetate weak base component at the core-shell interface can, on the one hand, form a gradient alkalinity distribution from the inside to the outside of the catalyst, acting as a co-catalyst in the transesterification reaction. It works synergistically with the strong base active centers in the shell to enhance the transesterification reaction rate. At the same time, it can precisely control the reaction process, avoiding excessive decomposition of dimethyl carbonate and excessive polymerization of products, ensuring a uniform and controllable molecular weight distribution of polycarbonate diol. On the other hand, the weak base component at the interface can form a stable steric hindrance and chemical anchoring effect, effectively inhibiting the migration and aggregation of the strong base active component in the shell layer to the core during preparation and reaction. It also reduces the dissolution and loss of the strong base active component during the reaction, significantly improving the recyclability of the catalyst. After impregnation, solid-liquid separation and vacuum drying are performed sequentially to obtain a pre-loaded support. The final loading of potassium acetate is 2% of the dry weight of the porous core-shell structure support. Vacuum drying can stably anchor the pre-loaded weak base co-catalyst component at the interface binding site under mild and oxygen-free conditions, avoiding migration and aggregation of components during drying and ensuring the integrity of the gradient loading structure.

[0048] Step 4: Gradient impregnation and loading of strong base components: Sodium hydroxide is used as the strong base active component and dissolved in anhydrous low-carbon alcohol to prepare a high-concentration impregnation solution with a mass concentration of 20%. Using anhydrous low-carbon alcohol as the solvent ensures consistency with the previous impregnation system, avoiding sudden changes in the solvent system that could cause the already loaded weak base co-catalytic component to dissolve. The high-concentration impregnation solution ensures that the strong base active component can fully and uniformly adhere to the inner wall and outer surface of the mesoporous channels of the outer shell layer, providing sufficient core catalytic active centers for the transesterification reaction. The pre-loaded support obtained in Step 3 is placed in the prepared high-concentration impregnation solution and subjected to atmospheric pressure. Impregnation was performed under the following conditions: the impregnation temperature was controlled at 35℃, and the impregnation time was 10 hours. The atmospheric pressure impregnation method allows the impregnation solution to preferentially adhere to and be loaded onto the mesoporous aluminoxane outer shell layer of the core-shell carrier through natural diffusion, preventing excessive migration towards the core. This ensures a precise distinction between the loading area formed by the vacuum impregnation in step 3 and perfectly achieves the gradient loading structure design of "weak base component at the core-shell interface, strong base component in the outer shell layer." The sodium hydroxide strong base component, as the core active center of the transesterification reaction, after being uniformly loaded onto the hydrophilic mesoporous outer shell layer, can react with dimethyl carbonate and 1,4-... The butanediol reactants achieve full contact, significantly reducing the activation energy of the transesterification reaction, accelerating the rate of transesterification and polycondensation, and shortening the reaction cycle. Simultaneously, the highly dispersed active centers allow for precise control of the reaction process, avoiding side reactions caused by excessively high local alkalinity. This ensures that the hydroxyl value and molecular weight distribution of the polycarbonate diol product meet the application requirements of high-end polyurethane synthesis. After impregnation, solid-liquid separation and vacuum drying are performed sequentially to obtain the catalyst precursor. The final sodium hydroxide loading is 12% of the dry weight of the porous core-shell structure support. The mass concentration of the high-concentration impregnation solution used in this step is the same as the low-concentration solution used in step 3. The concentration of the impregnation solution is 5 times that of the total concentration. The final loading of the strong base active component in this step is 6 times that of the weak base co-catalyst component in step 3. This gradient design of concentration and loading can further enhance the alkalinity gradient distribution of the catalyst from the inside to the outside. While ensuring the high catalytic activity of the catalyst, it can significantly reduce the risk of dissolution and loss of the strong base active component, and prevent free base from entering the product and affecting subsequent applications. Vacuum drying can remove excess solvent under mild conditions, stably anchoring the strong base active component on the active sites of the mesoporous shell layer, avoiding the agglomeration and deactivation of the active centers, and ensuring the activity stability of the catalyst.

[0049] Step 5: In-situ alcoholization reaction in anhydrous alcohol phase: The catalyst precursor obtained in step 4 is placed in anhydrous isopropanol. Anhydrous isopropanol serves as both the alcohol source for the in-situ alcoholization reaction and is highly compatible with the alcohol phase system of the target transesterification reaction, ensuring that the final catalyst exhibits excellent compatibility and dispersibility in the reaction system, preventing catalyst agglomeration and sedimentation. The in-situ alcoholization reaction is carried out under a protective atmosphere of argon as an inert gas, with heating and continuous stirring. The reaction temperature is controlled at 100℃, and the reaction time is 6 hours. The argon inert atmosphere completely isolates the catalyst from atmospheric carbon dioxide throughout the process. Carbon dioxide and water are used to prevent the strong base active component from reacting with carbon dioxide to form carbonates and losing catalytic activity. Water also prevents the active alcohol-base centers from hydrolyzing and deactivating. Heating and stirring allow the catalyst precursor to fully contact anhydrous isopropanol, resulting in a complete in-situ alcoholization reaction. This transforms the strong and weak base components supported on the support into alcohol-base active centers with higher transesterification catalytic activity. Simultaneously, through alcoholization, stable chemical anchoring bonds are formed between the active component and the mesoporous aluminoxane support, significantly reducing the dissolution and loss of the active component during transesterification and enhancing catalytic activity. The catalyst's cyclic stability is improved, and its basicity distribution can be further optimized, enhancing reaction selectivity and reducing the impact of residual methanol as a byproduct on product quality. After the reaction, solid-liquid separation, anhydrous alcohol washing, and vacuum drying are performed sequentially under an argon inert atmosphere. The vacuum drying temperature is 90℃, and the drying time is 10 hours. The post-treatment under an inert atmosphere throughout the process avoids the deactivation of newly generated highly active alcohol-base centers upon contact with air. Anhydrous alcohol washing thoroughly removes unanchored free base components from the catalyst surface, reducing side reactions caused by free bases during the reaction and ensuring product quality. Vacuum drying can completely remove residual alcohol solvents and volatile impurities from the catalyst, resulting in a composite alcohol-base catalyst with a complete structure, precise gradient distribution of active centers, high catalytic activity, good selectivity, and excellent cycle stability. This catalyst is perfectly suited for the reaction system of dimethyl carbonate and 1,4-butanediol transesterification to synthesize polycarbonate diols. It can achieve high conversion rate of raw materials under mild reaction conditions, while precisely controlling the molecular weight and molecular weight distribution of the product, reducing the incidence of side reactions, and improving the quality and batch stability of polycarbonate diol products.

[0050] 1. Catalyst structure characterization and verification data

[0051] The morphology and structure of the prepared composite alcohol-base catalyst were characterized by scanning electron microscopy and transmission electron microscopy. The results showed that the catalyst exhibited a complete core-shell spherical structure with a core particle size of 300 nm ± 10 nm and a shell thickness of 150 nm ± 8 nm. The shell layer was completely coated without cracking or detachment. Nitrogen adsorption-desorption tests showed that the catalyst had a specific surface area of ​​487 m² / g, with mesopore sizes concentrated in the range of 14–16 nm and a pore volume of 0.56 cm³ / g, maintaining the excellent mesoporous structure characteristics of the support. X-ray fluorescence spectroscopy showed that the actual loading of potassium acetate in the catalyst was 1.98%, and the actual loading of sodium hydroxide was 11.94%, which highly matched the design values. XPS depth etching tests showed that the basicity of the catalyst increased gradually from the core to the shell surface, verifying the successful construction of the gradient basicity distribution structure.

[0052] 2. Catalytic performance evaluation data for transesterification reaction

[0053] Dimethyl carbonate (DMC) and 1,4-butanediol (BDO) were used as reactants, with a DMC to BDO molar ratio of 3:1. The catalyst addition was 0.5% of the total mass of the reactants. The reaction temperature was 170℃, and the reaction time was 4 hours. The transesterification polycondensation reaction was evaluated under atmospheric pressure distillation conditions. The results showed that the conversion rate of 1,4-butanediol reached 99.2%, the number average molecular weight of the polycarbonate diol product was 2000±50, the molecular weight distribution coefficient was as low as 1.23, and the hydroxyl value of the product was 56.2 mgKOH / g, which meets the industry standard for polycarbonate diols used in high-end polyurethanes. Under the same reaction conditions, the molecular weight distribution coefficient of PCDL prepared by the traditional sodium methoxide homogeneous catalyst was 1.68, and the by-product content reached 1.8%. The molecular weight distribution coefficient of PCDL prepared by the traditional Al2O3-supported NaOH solid alkali catalyst was 1.52, and the BDO conversion rate was only 92.7%. This demonstrates that the catalyst prepared in this invention has superior catalytic activity, reaction selectivity, and product molecular weight control capability.

[0054] 3. Catalyst Cyclic Stability Test Data

[0055] After the reaction, the catalyst was filtered, washed with anhydrous isopropanol, and vacuum dried, and then reused in the above transesterification polycondensation reaction. After six cycles, the conversion rate of 1,4-butanediol could still reach 95.6%, and the molecular weight distribution coefficient of the prepared PCDL could still be controlled within 1.35. ICP-OES test showed that the dissolution rate of the active component of the catalyst after a single cycle was less than 0.5%, and no free alkali remained in the product, ensuring the batch stability of the product. This verified the excellent recycling performance and structural stability of the catalyst in the polymer synthesis transesterification system.

[0056] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a composite alcohol base catalyst suitable for transesterification reaction, characterized by, The specific steps of this method are as follows: Step 1: Preparation of hydrophobic silica microsphere core: Monodisperse silica microspheres were prepared using the modified Stöber method. The silica microspheres were then surface-modified to be hydrophobic. After solid-liquid separation, washing, and drying, the hydrophobic silica microsphere core was obtained. Step 2, Preparation of porous core-shell structured carrier: The core of the hydrophobic silica microspheres obtained in Step 1 is dispersed in anhydrous ethanol, and an aluminum source, mesoporous template agent and alkaline regulator are added. After room temperature stirring pre-reaction, hydrothermal treatment and calcination to remove the template agent, a porous core-shell structured carrier with a hydrophobic silica microsphere core and a hydrophilic mesoporous aluminoxane shell is obtained. Step 3, Gradient impregnation and preloading of weak base components: The weak base co-catalytic component is dissolved in anhydrous low-carbon alcohol to prepare a low-concentration impregnation solution. The porous core-shell structured support obtained in Step 2 is placed in the low-concentration impregnation solution and subjected to vacuum impregnation treatment to allow the weak base co-catalytic component to be directionally diffused and loaded at the interface between the core and shell of the core-shell support. After solid-liquid separation and vacuum drying, a preloaded support is obtained. Step 4, Gradient impregnation and loading of strong base components: The strong base active components are dissolved in anhydrous low carbon alcohol to prepare a high-concentration impregnation solution. The pre-loaded support obtained in step 3 is placed in the high-concentration impregnation solution and subjected to atmospheric pressure impregnation treatment to uniformly load the strong base active components onto the mesoporous aluminoxane outer shell layer of the core-shell support. After solid-liquid separation and vacuum drying, the catalyst precursor is obtained. Step 5, in-situ alcoholization reaction in anhydrous alcohol phase: The catalyst precursor obtained in step 4 is placed in anhydrous low-carbon alcohol and an in-situ alcoholization reaction is carried out under inert gas protection, heating and stirring conditions. After the reaction is completed, the catalyst is subjected to solid-liquid separation, anhydrous alcohol washing and vacuum drying under an inert atmosphere to obtain a composite alcohol-base catalyst suitable for transesterification reaction.

2. A process for the preparation of a composite alcohol base catalyst suitable for transesterification reaction as claimed in claim 1, wherein, In step 1, the particle size of the monodisperse silica microspheres is 100nm~500nm; the surface hydrophobic modification uses at least one of hexamethyldisilazane, methyltriethoxysilane, and dimethyldichlorosilane, the modification temperature is 40℃~80℃, and the modification time is 2h~6h.

3. The method for preparing a composite alcohol-base catalyst suitable for transesterification reaction according to claim 1, characterized in that, In the porous core-shell structured carrier obtained in step 2, the thickness of the hydrophilic mesoporous aluminoxane shell is 50nm~200nm, the pore size of the shell is 2nm~20nm, and the specific surface area of ​​the carrier is 300m² / g~600m² / g.

4. The method for preparing a composite alcohol-base catalyst suitable for transesterification reaction according to claim 1, characterized in that, In step 2, the aluminum source is at least one of aluminum isopropoxide, aluminum sec-butoxide, and aluminum nitrate; the mesoporous template agent is at least one of P123, F127, and CTAB; the alkaline regulator is at least one of ammonia and urea; the hydrothermal treatment temperature is 100℃~160℃, and the treatment time is 12h~48h; the calcination temperature is 450℃~600℃, and the calcination time is 3h~6h.

5. The process for the preparation of a composite alcohol base catalyst suitable for transesterification reaction as claimed in claim 1, wherein, In step 3, the weak base co-catalyst component is at least one of potassium carbonate, sodium carbonate, potassium acetate, sodium acetate, magnesium hydroxide, and calcium hydroxide; the mass concentration of the low-concentration impregnation solution is 2% to 8%.

6. The process for the preparation of a composite alcoholic catalyst suitable for transesterification reaction as claimed in claim 1, wherein, In step 3, the vacuum degree of the vacuum impregnation is -0.08MPa to -0.1MPa, the impregnation temperature is 30℃ to 50℃, and the impregnation time is 2h to 6h; the final loading of the weak base co-catalyst component is 1% to 5% of the dry weight of the porous core-shell structure carrier.

7. The process for the preparation of a composite alcohol catalyst for transesterification according to claim 1, characterized in that, In step 4, the strong alkaline active component is at least one of sodium hydroxide, potassium hydroxide, and lithium hydroxide; the mass concentration of the high-concentration impregnation solution is 10% to 25%.

8. The process for the preparation of a composite alcohol catalyst for transesterification according to claim 1, characterized in that, In step 4, the temperature of the atmospheric pressure impregnation is 25℃~40℃, and the impregnation time is 4h~12h; the final loading of the strong base active component is 10%~20% of the dry weight of the porous core-shell structure carrier.

9. The process for the preparation of a composite alcohol catalyst for transesterification according to claim 1, characterized in that, The mass concentration of the high-concentration impregnation solution in step 4 is 2.5 to 10 times that of the low-concentration impregnation solution in step 3; the final loading of the strong base active component in step 4 is 3 to 15 times that of the final loading of the weak base co-catalytic component in step 3.

10. The method for preparing a composite alcohol-base catalyst suitable for transesterification reaction according to claim 1, characterized in that, In step 5, the anhydrous low-carbon alcohol is at least one of anhydrous methanol, anhydrous ethanol, and anhydrous isopropanol; the inert gas is at least one of nitrogen and argon; the temperature of the in-situ alcoholization reaction is 60℃~120℃, and the reaction time is 4h~12h; the temperature of the vacuum drying is 60℃~100℃, and the drying time is 6h~12h.

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