Internal acid and external alkali type organic-inorganic hybrid molecular sieve catalyst as well as preparation method and application thereof

Abstract

The invention discloses an internal-acid and external-alkali type organic-inorganic hybrid molecular sieve catalyst as well as a preparation method and application thereof, and belongs to the technical field of heterogeneous catalytic materials and chemical processes. According to the catalyst, a hydrogen type Beta molecular sieve is used as an acidic inner core, polyvinylpyrrolidone (PVP) is used as an organic shell, and a unique inner-acid outer-alkali structure is formed. The preparation method comprises the following steps: hydrothermally synthesizing a hydrogen type Beta molecular sieve with a specific silica-alumina ratio, and then coating PVP through surface adsorption. The catalyst is used for catalyzing the condensation of methylal and trioxymethylene under mild conditions (40-70 DEG C and normal pressure) to prepare polyoxymethylene dimethyl ethers (DMMn). The PVP shell covers acidic sites on the outer surface of the molecular sieve to form an external neutral / weak base and internal strong acid microenvironment, so that the secondary side reaction of the product is effectively inhibited, the trioxymethylene conversion rate of more than 98% and the DMMn selectivity of more than 96% can be obtained in the reaction at 50 DEG C and normal pressure, the product distribution can be regulated and controlled, and the traditional Anderson-Schulz-Flory distribution limitation is broken through. The catalyst disclosed by the invention has the advantages of high activity, high selectivity, mild conditions and easiness in separation, and has a good industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of heterogeneous catalytic materials and chemical process technology, specifically relating to an internally acidic and externally basic organic-inorganic hybrid molecular sieve catalyst, its preparation method, and its application. Background Technology

[0002] Polymethoxydimethyl ether (DMM) n DMM (dimethylformamide) is a type of polyether compound with a high cetane number and high oxygen content. When used as an additive in diesel fuel, it can significantly improve the combustion performance of diesel fuel and drastically reduce emissions of pollutants such as particulate matter (PM) and nitrogen oxides (NOx), making it a highly promising clean diesel fuel blending component. From the perspective of raw material sources, DMM... n It can be derived from coal-based or bio-based methanol, and its synthesis and application are in line with the clean conversion direction of modern coal chemical industry, which is of great significance for realizing the low-carbonization of transportation energy.

[0003] The synthesis of DMMn is an acetalization reaction. Traditional processes often use methanol, methyl acetal (DMM), etc., as methyl end-capping agents, and formaldehyde, trioxymethylene (TOX), or paraformaldehyde, etc., as methylene group donors, reacting under the action of an acidic catalyst. Early industrial routes mostly used liquid acid (such as sulfuric acid) catalysts, which, although having acceptable activity, suffered from inherent drawbacks such as severe equipment corrosion, difficulty in product separation, and large wastewater treatment volumes, resulting in poor environmental friendliness.

[0004] To overcome the drawbacks of homogeneous catalysis, solid acid catalysts, such as molecular sieves (ZSM-5, Beta, etc.), heteropoly acids, and acidic resins, have become the focus of current research and application. Although these solid acids are easy to separate and recover, the uniformly distributed acidic sites on their surfaces act as a double-edged sword: while catalyzing the target condensation reaction, they also readily trigger a series of side reactions. For example, the product DMM... n Reverse depolymerization or overpolymerization can occur at strong acid sites, leading to reduced selectivity for the target product (typically a component with n=3-5), and the product distribution often follows the insurmountable Anderson-Schulz-Flory (ASF) statistical law. Furthermore, to obtain a considerable reaction rate, these catalysts typically require operation at mesophilic temperatures above 80°C, resulting in high energy consumption.

[0005] To optimize product distribution and improve selectivity, researchers have refined the design of catalyst structures. For example, core-shell structures are used to regulate the spatial distribution of active sites and the mass transfer pathways of reactants / products. Several core-shell catalysts for DMMn synthesis have been disclosed in the prior art. Patent CN117599839B discloses a bifunctional catalyst with a metal oxide core and an acidic molecular sieve shell for the direct oxidation of methanol to DMMn, aiming to suppress side reactions such as methanol dehydration. However, this structure has an acidic environment both inside and outside, and the reaction needs to be carried out under pressure oxidation conditions at 120-240℃. Patent CN110330414B influences product distribution by constructing a hydrophilic-hydrophobic tunable supported catalyst, but its acidic active sites are still exposed on both the inner and outer surfaces. Patent CN119869600A designed a three-layer core-shell structure that sandwiches dehydrogenation metal sites between two layers of acidic materials for methanol dehydrogenation coupling reaction. This structure confines the intermediate products to a certain extent, but its components are entirely inorganic materials, and the reaction pathway and conditions (dehydrogenation) are completely different from those of this application.

[0006] In summary, existing solid acid catalysts, including various disclosed core-shell structure catalysts, generally suffer from the following common problems: (1) both the inner and outer surfaces of the catalyst are acidic, making it difficult to effectively suppress secondary side reactions (such as depolymerization) of the product on the outer surface of the particles; (2) to achieve high activity, the reaction usually needs to be carried out at a high temperature; (3) the ability to regulate product distribution is limited, making it difficult to overcome the constraints of ASF distribution, especially to achieve high-selectivity synthesis under mild conditions. Therefore, developing a novel catalyst that can efficiently catalyze the synthesis of DMMn under low temperature and atmospheric pressure conditions, and can effectively suppress side reactions and intelligently regulate product distribution through unique structural design, has become a technical problem that urgently needs to be solved in this field. This invention was completed against this background. Summary of the Invention

[0007] To address the common technical problems of existing solid acid catalysts used for the synthesis of polyoxymethylene dialkyl ethers, such as acidity on both the inner and outer surfaces, easy occurrence of secondary side reactions (e.g., depolymerization, overpolymerization), high temperature (>80℃) required for the reaction, and limited product distribution due to Anderson-Schulz-Flory (ASF) distribution and low selectivity of target products, this invention provides an internally acidic and externally basic organic-inorganic hybrid molecular sieve catalyst, its preparation method, and its application.

[0008] To achieve the above objectives, the present invention provides the following technical solution: First, this invention provides an internally acidic, externally base type organic-inorganic hybrid molecular sieve catalyst. The core feature of this catalyst lies in its unique "internal acid, externally base" structure, specifically including: Acidic core: Composed of hydrogen-type Beta molecular sieves with a silica-alumina molar ratio of 20:1 to 100:1. This acidic core provides abundant strong acid sites necessary for catalytic acetalization reactions.

[0009] Organic-inorganic hybrid shell: composed of polyvinylpyrrolidone (PVP) with a weight-average molecular weight of 10,000 to 400,000. The PVP is coated on the outer surface of the hydrogen-form Beta molecular sieve at a mass ratio of 1:100 to 1:5 through physical adsorption and / or weak chemical action.

[0010] The key function of the outer shell is to basically or completely cover the acidic sites on the outer surface of the core molecular sieve, so that the outer surface of the entire catalyst particle is neutral or weakly alkaline, while the inside of the particle retains its original strong acidity.

[0011] Secondly, this invention provides a method for preparing the above-mentioned internally acidic and externally basic organic-inorganic hybrid molecular sieve catalyst, comprising the following steps: (1) Hydrothermal synthesis of hydrogen-form Beta molecular sieve: Tetraethyl orthosilicate is used as the silicon source, aluminum isopropoxide as the aluminum source, and tetraethylammonium hydroxide as the template agent. The molar ratio of silicon source, aluminum source, and template agent is controlled at 1:(0.01~0.05):(0.2~0.3). The mixture is mixed with deionized water to form a gel. The gel is hydrothermally crystallized at 90~180℃ for 24-240 hours. After washing, drying, and calcination at 400-600℃, hydrogen-form Beta molecular sieve is obtained.

[0012] (2) PVP surface coating: The above hydrogen-type Beta molecular sieve is dispersed in a solvent (such as ethanol), PVP is added, and the surface is coated by stirring at 5~80℃ so that PVP is fully adsorbed on the surface of the molecular sieve; then the excess PVP adsorbed by physical adsorption is removed by filtration and washing, and the target catalyst is obtained after drying.

[0013] Finally, this invention provides the application of the above-mentioned catalyst in the synthesis of polyoxymethylene dialkyl ethers. The specific application method is as follows: using methyl acetal (or diethoxymethane) and trioxymethane as reactants, an acetalization reaction is carried out in the presence of the catalyst under mild conditions of 40-70°C, atmospheric pressure, or slightly positive pressure. The molar ratio of methyl acetal to trioxymethane is 1:1 to 4:1, and the amount of catalyst used is 0.1% to 10% of the total mass of the reactants.

[0014] Compared with existing technologies, the "internal acid, external base" type organic-inorganic hybrid molecular sieve catalyst and its application provided by this invention have the following significant advantages and beneficial effects: 1. Ultra-high catalytic activity was achieved under low temperature and normal pressure conditions.

[0015] In this invention, the polar polyvinylpyrrolidone (PVP) macromolecular shell effectively improves the adsorption and pre-organization of reactants (especially trioxymethylene, TOX) on the catalyst surface, while the porous beta molecular sieve core provides abundant strong acid sites. This allows the acetalization reaction, which originally required high temperatures (typically >80°C), to proceed efficiently under mild conditions at atmospheric pressure and as low as 50°C. Under these conditions, the TOX conversion rate can exceed 97%, far surpassing the activity level of traditional solid acid catalysts at the same temperature, and significantly reducing reaction energy consumption.

[0016] 2. By using the "reaction zone isolation" mechanism, side reactions are effectively suppressed, thereby improving selectivity.

[0017] The unique "internal acid, external base" structure of this invention provides spatial "reaction zone isolation." The strong acid active sites of the catalyst are confined within the particle, while the outer surface is covered by a PVP shell, making it neutral or weakly basic. This design offers two core advantages: First, it suppresses secondary side reactions. After the generated polyoxymethylene dimethyl ether (DMMn) product diffuses from the internal acidic region to the external neutral / weakly alkaline environment, it is difficult for it to come into contact with strong acid sites again. This significantly reduces the possibility of secondary side reactions such as depolymerization, overpolymerization or hydrolysis on the outer surface of the particles, effectively improving the selectivity of the target product and the operational stability of the catalyst.

[0018] Secondly, it regulates the reaction pathway and product distribution. Reactants must diffuse into the interior of the particles to contact acid sites and react. This diffusion-dependent reaction process disrupts the traditional reaction equilibrium dominated by surface acidic sites to some extent. As a result, in specific reaction stages, the formation rate and selectivity of intermediate-chain products (such as DMM4) can be significantly higher than the predicted values ​​of the traditional Anderson-Schulz-Flory (ASF) distribution, enabling active control of product chain length distribution and providing a new strategy for obtaining high-value-added target product fractions.

[0019] 3. It possesses excellent product distribution control capabilities and green process attributes.

[0020] The catalyst of this invention not only exhibits high activity and selectivity, but also demonstrates the ability to regulate product distribution through structural adjustment, overcoming the thermodynamic limitations of traditional catalytic systems. Furthermore, the entire catalytic system operates at low temperature and normal pressure, employs a non-corrosive solid catalyst, and is easily separated and recovered, perfectly aligning with the development requirements of green chemistry and energy conservation, and possessing promising prospects for industrial application. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the "internal acid, external base" type organic-inorganic hybrid molecular sieve catalyst of the present invention.

[0022] Figure 2 This is an optical photograph of the catalyst BMS@PVP-1 prepared in Example 1.

[0023] Figure 3 The X-ray diffraction (XRD) pattern of catalyst BMS@PVP-1.

[0024] Figure 4 The image shows the Fourier transform infrared (FT-IR) spectrum of the catalyst BMS@PVP-1.

[0025] Figure 5 This is a diagram showing the non-SF distribution of the synthesized product in Example 5 of the present invention.

[0026] Figure 6 This is a state diagram of the liquid product in Comparative Example 1 of the present invention, tested for aldehyde content using the sodium sulfite-indicator method. Detailed Implementation

[0027] The present invention will be further described below with reference to specific embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention. Unless otherwise specified, the experimental methods and detection means used in the present invention are conventional techniques in the art.

[0028] Example 1: Preparation of catalyst BMS@PVP-1 (1) Preparation of hydrogen-form Beta molecular sieve (BMS-1): Tetraethyl orthosilicate (A, TEOS) was used as the silicon source, aluminum isopropoxide (B) as the aluminum source, tetraethylammonium hydroxide aqueous solution (C, 25 wt% TEAOH) as the template agent and alkali source, and deionized water (D) as the solvent, with a molar ratio of A:B:C:D = 100:1.5:25:800. First, 0.15 mol of aluminum isopropoxide was dissolved in a portion of the TEAOH aqueous solution and stirred until completely dissolved. While continuously stirring, 10 mol of TEOS was slowly added dropwise to the above solution to form a homogeneous mixture. Subsequently, the remaining deionized water and TEAOH were added, and the mixture was stirred continuously at room temperature for 6 hours until a homogeneous, well-flowing gel was formed. This gel was transferred to a stainless steel crystallization vessel with a polytetrafluoroethylene liner and placed in a forced-air drying oven for static crystallization at 140°C for 72 hours. After crystallization, the mixture was allowed to cool naturally to room temperature. The product was separated by vacuum filtration and washed with a large amount of deionized water until the filtrate was neutral. The solid product was dried at 150 °C for 12 hours, then placed in a muffle furnace and heated to 550 °C at a rate of 2 °C / min in air atmosphere, and calcined at this temperature for 6 hours to completely remove the organic template agent, yielding a hydrogen-type Beta molecular sieve, denoted as BMS-1.

[0029] (2) Preparation of PVP hybrid molecular sieve (BMS@PVP-1): Weigh 1.0 g of PVP (K27-32, average molecular weight Mw≈58,000) and dissolve it in 80 mL of ethanol. Sonicate the solution for 30 minutes to ensure complete dispersion. Separately weigh 15.0 g of the BMS-1 ​​molecular sieve prepared in step (1) and add 12 mL of ethanol for pre-saturation. After sufficient ethanol adsorption, add the sieve to the PVP solution and stir magnetically in a 50°C water bath for 8 hours. After the reaction is complete, filter the mixture and wash it three times with ethanol to remove excess physically adsorbed PVP. Finally, vacuum dry at 80°C for 24 hours to obtain a white powdery hybrid catalyst, which is then sealed and labeled BMS@PVP-1. Figure 2 ).

[0030] The following characterization data are used to confirm that the catalyst BMS@PVP-1 possesses a hybrid structure of "internal acid and external base": X-ray diffraction (XRD) analysis: XRD pattern of catalyst BMS@PVP-1 ( Figure 3 The spectrum shows a strong diffraction peak at 22.2°, and relatively weaker diffraction peaks at 25.2°, 27.0°, 28.5°, 29.5°, and 33.0°, indicating that the catalyst possesses a molecular sieve structure. This demonstrates that hydrothermal treatment successfully synthesized a crystalline phase with a BEA topology. No PVP crystallization peaks were observed in the spectrum, indicating that PVP is coated on the molecular sieve surface in an amorphous form.

[0031] Fourier transform infrared spectroscopy (FT-IR) analysis: Infrared spectrum of catalyst BMS@PVP-1 ( Figure 4 ) at 2960cm -1 2925 cm -1 1651 cm -1 1458 cm -1 1290 cm -1 1220 cm -1 1062 cm -1 Characteristic peaks, including the one at 2960 cm⁻¹. -1 2925 cm -1 It is the CH characteristic peak of PVP, at 1651 cm⁻¹. -1 The left and right sides are the characteristic peaks of C=O in PVP, 1458-1290 cm⁻¹ -1 The CN vibration peaks of PVP are also present on both sides, but they are relatively weak, possibly due to the binding of the N sites to the acid in the catalyst. This indicates that PVP has been successfully introduced and may have covered the acidic sites on the outer surface through interaction with surface hydroxyl groups.

[0032] Example 2: Preparation of catalyst BMS@PVP-2 This example illustrates the effect of different silicon-to-aluminum ratios on catalyst performance.

[0033] (1) Referring to step (1) of Example 1, adjust the molar ratio of silicon source to aluminum source so that the molar ratio of feed is A:B:C:D=100:2:22:800. The crystallization temperature is 90℃ and the time is 240 hours. The calcination conditions are 500℃ and 10 hours to obtain a hydrogen-type Beta molecular sieve with a silicon-to-aluminum ratio of approximately 50:1, denoted as BMS-2.

[0034] (2) Referring to step (2) of Example 1, 15.0 g of BMS-2 molecular sieve and 0.15 g of PVP (K30, Mw≈40,000) were used for coating. The coating conditions were stirring at 50°C for 8 hours to obtain catalyst BMS@PVP-2. The mass ratio of PVP to molecular sieve was 1:100.

[0035] Example 3: Preparation of catalyst BMS@PVP-3 This example illustrates the effects of different molecular weight PVPs and coating conditions.

[0036] (1) Referring to step (1) of Example 1, adjust the molar ratio of silicon source to aluminum source so that the molar ratio of feed is A:B:C:D=100:5:25:800. The crystallization temperature is 180℃ and the time is 24 hours. The calcination conditions are 600℃ and 5 hours to obtain a hydrogen-type Beta molecular sieve with a silicon-to-aluminum ratio of approximately 20:1, denoted as BMS-3.

[0037] (2) Referring to step (2) of Example 1, 15.0 g of BMS-3 molecular sieve and 2.0 g of PVP (K60, Mw≈270,000-400,000) were used for coating. The coating conditions were stirring at 80°C for 2 hours to obtain catalyst BMS@PVP-3. The mass ratio of PVP to molecular sieve was approximately 1:7.5.

[0038] Example 4: Preparation of catalyst BMS@PVP-4 This example illustrates the effects of low molecular weight PVP and long-term low-temperature coating.

[0039] (1) Referring to step (1) of Example 1, adjust the molar ratio of silicon source to aluminum source so that the molar ratio of feed is A:B:C:D=100:1:20:800. The crystallization temperature is 130℃ and the time is 120 hours. The calcination conditions are 600℃ and 10 hours to obtain a hydrogen-type Beta molecular sieve with a silicon-to-aluminum ratio of approximately 100:1, denoted as BMS-4.

[0040] (2) Referring to step (2) of Example 1, 15.0 g of BMS-4 molecular sieve and 3.0 g of PVP (K17, Mw≈10,000-16,000) were used for coating. The coating conditions were stirring at 40°C for 24 hours to obtain catalyst BMS@PVP-4. The mass ratio of PVP to molecular sieve was 1:5.

[0041] Example 5: Application of catalysts in the synthesis of polyoxymethylene dimethyl ether This example is used to evaluate the performance of the catalyst BMS@PVP-1 prepared in Example 1.

[0042] 3.46 g of dimethyl acetal (DMM) and 1.03 g of trioxymethylene (TOX) (molar ratio DMM:TOX ≈ 3.3:1) were added to the reactor. Then, 0.104 g of BMS@PVP-1 catalyst (approximately 2.3% of the total mass of the reactants) was added. Finally, the reactor was placed in a 50°C constant temperature water bath, and the reaction was carried out under normal pressure with stirring.

[0043] Samples were taken after 30 minutes of reaction, and gas chromatography analysis revealed that the product distribution had deviated significantly from the traditional ASF distribution. Figure 5 The reaction was stopped after 6 hours, and the catalyst was separated by cooling and filtration. Quantitative analysis was performed on the liquid product.

[0044] Test results: The conversion rate, based on trioxymethylene (TOX), was 98.1%. Polyoxymethylene dimethyl ether (DMM) 2-8 The overall selectivity was 95.9%, and the calculated yield was 94.0%.

[0045] Example 6: Application of catalysts in the synthesis of polyoxymethylene diethyl ether This embodiment is used to illustrate the universality of the catalyst of the present invention in the synthesis of homologues.

[0046] 20.8 g of diethoxymethane (DEM) and 9.0 g of trioxymethane (TOX) were added to the reactor, along with 0.61 g of the BMS@PVP-1 catalyst prepared in Example 1 (approximately 2.0% of the total mass of the reactants). The reaction was carried out at 60 °C and atmospheric pressure for 5 hours.

[0047] Test results: The conversion rate of trioxymethylene (TOX) was 96.8%. Polyoxymethylene diethyl ether (DEM) 2-8 The overall selectivity was 96.0%, and the yield was 93.0%.

[0048] Comparative Example 1 The BMS-1 ​​molecular sieve prepared in Example 1 was used directly as a catalyst, and its performance was evaluated under the same reaction conditions as in Example 1. Compared with the catalyst without the PVP coating, no non-SF product distribution was observed, and more byproducts such as oligooxyformaldehyde were produced, indicating that the PVP coating is important for product selectivity. Figure 6 This is a state diagram of the liquid product tested for aldehyde content using the sodium sulfite-indicator method. It shows that the liquid changes color due to the presence of a large amount of aldehyde, and a white oily substance floats on the surface. After titration, the total aldehyde content can reach 10.9%.

[0049] Comparative Example 2 Commercially available HZSM-5 molecular sieve was used directly as a catalyst, and its performance was evaluated under the exact same reaction conditions as in Example 1. It was found that under these conditions, the activity was very low, with a conversion rate of only 21.2%. After coating with PVP, the activity further decreased to 7.5%, which did not meet the application requirements.

Claims

1. An internally acidic and externally basic organic-inorganic hybrid molecular sieve catalyst, characterized in that, The catalyst includes: An acidic core and an organic-inorganic hybrid shell covering the surface of the acidic core; The acidic core is a hydrogen-type Beta molecular sieve; The organic-inorganic hybrid shell is composed of polyvinylpyrrolidone, which coats the outer surface of the hydrogen-type Beta molecular sieve through physical adsorption and / or weak chemical action. The outer shell covers the acidic sites on the outer surface of the core, making the outer surface of the catalyst neutral or weakly alkaline, while the core remains strongly acidic.

2. The organic-inorganic hybrid molecular sieve catalyst with an internal acid and external base according to claim 1, characterized in that, The polyvinylpyrrolidone has a weight-average molecular weight of 10,000 to 400,000.

3. The organic-inorganic hybrid molecular sieve catalyst with an internal acid and external base according to claim 1, characterized in that, The mass ratio of the polyvinylpyrrolidone to the Beta molecular sieve is 1:100 to 1:

5.

4. The organic-inorganic hybrid molecular sieve catalyst with an internal acid and external base according to claim 1, characterized in that, The silicon-aluminum molar ratio of the hydrogen-type Beta molecular sieve is 20:1 to 100:

1.

5. A method for preparing an internally acidic, externally basic organic-inorganic hybrid molecular sieve catalyst as described in any one of claims 1 to 4, characterized in that, Includes the following steps: (1) Hydrothermal synthesis of hydrogen-type Beta molecular sieve: Silicon source, aluminum source, template agent and deionized water are mixed at a silicon-aluminum molar ratio of 20:1 to 100:1 to form a gel, and hydrothermally crystallized at 90~180℃ for 24~240 hours. After washing, drying and calcination, hydrogen-type Beta molecular sieve is obtained. (2) PVP surface coating: The hydrogen-type Beta molecular sieve obtained in step (1) is dispersed in a solvent, polyvinylpyrrolidone is added, and the surface is coated by stirring at 5~80℃. Then, the catalyst is filtered, washed and dried to obtain the organic-inorganic hybrid molecular sieve catalyst with internal acid and external base.

6. The preparation method according to claim 5, characterized in that, The silicon source is tetraethyl orthosilicate, the aluminum source is aluminum isopropoxide, and the template agent is tetraethylammonium hydroxide; and the molar ratio of silicon source, aluminum source, and template agent is 1:(0.01~0.05):(0.2~0.3); the mass ratio of polyvinylpyrrolidone to Beta molecular sieve is 1:100~1:

5.

7. The application of an internally acidic, externally basic organic-inorganic hybrid molecular sieve catalyst as described in any one of claims 1 to 4 in the preparation of polyoxymethylene dialkyl ethers, characterized in that, The applications include: Polyoxymethylene dialkyl ethers are prepared by reacting methyl acetal or diethoxymethane and trioxymethylene in the presence of the catalyst at 40-70°C, under normal or slightly positive pressure.

8. The application according to claim 7, characterized in that, The molar ratio of methyl acetal to trioxymethylene is 1:1 to 4:

1.

9. The application according to claim 7, characterized in that, The amount of catalyst used is 0.1% to 10% of the total mass of the reactants.

10. The application according to claim 7, characterized in that, The reaction is carried out at 50°C and normal pressure, with a conversion rate of paraformaldehyde of not less than 97% and a selectivity of polyoxymethylene dialkyl ether of not less than 95%.

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