Preparation and application of a highly dispersed tungsten-based catalyst

Abstract

This invention discloses the preparation and application of a highly dispersed tungsten-based catalyst, belonging to the field of fine chemical technology. Using ordered mesoporous SiO2 as a support, a polydopamine coating layer is formed through in-situ self-polymerization of dopamine, strongly coordinating and anchoring tungsten species. Following pyrolysis, an atomically / sub-nanometer dispersed core-shell structure of highly dispersed tungsten-based catalyst with amorphous WO3 is obtained. This catalyst is used in the liquid-phase dehydration of methacrylamide to prepare methacrylonitrile, exhibiting advantages such as high selectivity and strong stability. It is environmentally friendly, suitable for industrial production, and has excellent application prospects.

Description

Technical Field

[0001] This invention belongs to the field of fine chemical technology, specifically relating to a method for preparing a highly dispersed tungsten-based catalyst and its application in the liquid-phase dehydration of methacrylamide (MAM) to prepare methacrylonitrile (MAN). Background Technology

[0002] Methacrylonitrile is a key monomer for synthesizing high-performance polymer polymethacrylimide foam. The methacrylamide dehydration method is one of the more promising methods for producing methacrylonitrile, and its key lies in obtaining an efficient and stable dehydration catalyst. This dehydration reaction requires the catalyst to possess suitable acidity and porous structure to activate the amide groups and promote the removal of water molecules, while simultaneously suppressing side reactions such as polymerization and deep dehydration.

[0003] Traditionally used W-Al2O3 or Zr-Al2O3 catalysts are mostly prepared by impregnation or coprecipitation. The catalyst prepared by impregnation has poor dispersion of active components and is prone to forming bulk crystalline WO3 or inactive aluminotungstate during calcination, resulting in fewer active sites and lower utilization. Although coprecipitation can improve dispersion to some extent, it is more sensitive to conditions such as pH, temperature, and aging time, and has poor batch-to-batch repeatability, making it difficult to achieve uniform dispersion at the atomic scale.

[0004] Polydopamine contains abundant catechol and amino functional groups, which can strongly coordinate with metal ions to achieve efficient anchoring of metal species. Through inert atmosphere pyrolysis, it can be converted into a nitrogen-doped carbon layer, which confines and stabilizes the active species, significantly improving dispersion and catalytic stability. Currently, there are no reports on the technology of combining polydopamine coordination anchoring with inert pyrolysis to prepare highly dispersed tungsten-based catalysts for the liquid-phase dehydration reaction of methacrylamide. Summary of the Invention

[0005] To address the problems of poor dispersibility, easy agglomeration, low selectivity, and insufficient stability of existing tungsten-based catalysts, the present invention aims to provide a highly dispersed tungsten-based catalyst and its preparation method, achieving atomic / sub-nanometer dispersion of tungsten species, and applying it to the liquid-phase dehydration of methacrylamide to prepare methacrylonitrile, significantly improving its conversion rate and selectivity.

[0006] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a highly dispersed tungsten-based catalyst, wherein the highly dispersed tungsten-based catalyst has a core-shell structure, with mesoporous SiO2 as the core and a nitrogen-doped carbon layer of uniformly dispersed tungsten species as the shell.

[0007] Based on the above technical solution, furthermore, the tungsten species in the highly dispersed tungsten-based catalyst are dispersed at the atomic or sub-nanometer level, and there is no crystalline WO3 phase.

[0008] Secondly, the present invention provides a method for preparing the above-mentioned highly dispersed tungsten-based catalyst, comprising the following steps: (1) Disperse the ordered mesoporous SiO2 support in a buffer solution, add dopamine hydrochloride, and allow dopamine to self-polymerize in situ on the support surface to form a polydopamine coating layer, thus obtaining the PDA@SiO2 support; (2) Dissolve the tungsten source in a solvent, add the PDA@SiO2 support prepared in step (1), stir and impregnate for 12-24 hours, and anchor the tungsten species in the PDA layer by utilizing the coordination effect of the catechol and amino functional groups in the polydopamine layer with the tungsten species. After separation and drying, the precursor W-PDA@SiO2 is obtained. (3) The precursor W-PDA@SiO2 obtained in step (2) is kept at 600~900℃ for 1~10 hours in an inert atmosphere to convert the polydopamine layer into a nitrogen-doped carbon layer, and the tungsten species are uniformly dispersed and confined in the nitrogen-doped carbon layer at the atomic or sub-nanometer level to obtain a highly dispersed tungsten-based catalyst.

[0009] Based on the above technical solution, further, the ordered mesoporous SiO2 support mentioned in step (1) includes SBA-15 with a specific surface area of ​​500~1000 m². 2 / g, pore size 5~10 nm.

[0010] Based on the above technical solution, further, the buffer solution mentioned in step (1) includes Tris-HCl buffer solution, the concentration of the buffer solution is 0.01~0.5 mol / L, and the pH is controlled at 7.5~9.5; the concentration of the ordered mesoporous SiO2 support is 0.01~0.1 g / mL.

[0011] Based on the above technical solution, further, in step (1), the mass ratio of dopamine hydrochloride to ordered mesoporous SiO2 is 0.1:1~0.5:1.

[0012] Based on the above technical solution, further, the reaction conditions for in-situ self-polymerization in step (1) are: stirring reaction at room temperature for 12 to 24 hours.

[0013] Based on the above technical solution, further, the tungsten source mentioned in step (2) is at least one of phosphotungstic acid, ammonium metatungstate, tungsten hexachloride, and ammonium tungstate; the solvent is at least one of water, methanol, and ethanol; and the loading amount of WO3 relative to the ordered mesoporous SiO2 support is 5~30 wt%.

[0014] Based on the above technical solution, further, the drying in step (2) is atmospheric pressure drying or vacuum drying, and the drying temperature is 60~120℃.

[0015] Based on the above technical solution, further, the inert atmosphere mentioned in step (3) is nitrogen, argon, helium or neon; the heating rate is 1~5℃ / min, and the holding time is 2~6 hours.

[0016] Thirdly, the present invention provides the application of the above-mentioned highly dispersed tungsten-based catalyst or the highly dispersed tungsten-based catalyst prepared by the above-mentioned preparation method in the reaction of catalyzing the liquid-phase dehydration of methacrylamide to prepare methacrylonitrile.

[0017] Fourthly, the present invention provides a method for preparing methacrylonitrile by dehydrating methacrylamide, wherein the highly dispersed tungsten-based catalyst is loaded into a reactor, and the reaction is carried out at a reaction temperature of 220~380℃, a reaction pressure of 1.0~5.0 MPa, and a methacrylamide mass hourly space velocity of 0.1~3.0 h⁻¹. -1 Under conditions where an inert gas is used as a carrier gas, a solution or melt containing methacrylamide is brought into contact with a catalyst and a liquid-phase dehydration reaction occurs to obtain methacrylonitrile.

[0018] Based on the above technical solution, the reactor is further selected from one of a fixed bed reactor, a fluidized bed reactor, a moving bed reactor, or a slurry bed reactor.

[0019] Based on the above technical solution, the solvent of the solution containing methacrylamide is at least one of water, methanol, acetonitrile, N,N-dimethylformamide, and toluene, and the mass concentration of methacrylamide in the solution is 10-50%.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. High dispersion: Polydopamine strong coordination anchoring + nitrogen doping carbon confinement achieves tungsten atomic / sub-nanometer dispersion, no crystalline phase WO3, and significantly improves the number and utilization of active sites; 2. High stability: The core-shell structure and carbon layer protection inhibit the migration, aggregation, and sintering of active species, significantly extending the catalyst lifespan; 3. Excellent catalytic performance: When used for liquid-phase dehydration of methacrylamide, the conversion rate is higher than 40%, the selectivity is higher than 90%, there are few side reactions, and the product purity is high; 4. Green and controllable process: The reaction conditions are mild, the reproducibility is good, and there are no highly corrosive reagents, making it suitable for industrial scale-up. Attached Figure Description

[0021] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0022] Figure 1 The image shows a TEM image of the catalyst prepared in Example 1. Detailed Implementation

[0023] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0024] All tests in the examples were performed in a fixed-bed microreactor with an inner diameter of 10 mm and a catalyst loading of 2.0 g (20-40 mesh). After the reaction stabilized for 24 hours, samples were taken online, and the product composition was analyzed by gas chromatography to calculate the conversion rate of methacrylamide and the selectivity of methacrylonitrile.

[0025] Example 1 Catalyst preparation: (1) 5.0 g of ordered mesoporous SiO2 (SBA-15, specific surface area 650 m²) was added. 2 PDA@SiO2 was dispersed in 150 mL Tris-HCl buffer (0.1 M, pH=8.5) with 1.5 g dopamine hydrochloride (mass ratio 0.3:1), stirred at room temperature for 18 h, filtered, washed with water, and dried under vacuum at 60 °C. (2) Dissolve ammonium metatungstate in deionized water, add the above PDA@SiO2 with a loading of 15 wt% based on WO3, stir and impregnate at room temperature for 18 h, filter, and vacuum dry at 80℃ for 12 h to obtain W-PDA@SiO2; (3) Under N2 atmosphere, the temperature was increased to 700℃ at 2℃ / min, kept at the temperature for 4 h, and then cooled naturally to obtain a highly dispersed tungsten-based catalyst.

[0026] Methacrylamide dehydration reaction: fixed-bed reactor, catalyst loading 2.0 g (20-40 mesh), methacrylamide (MAM) mass hourly space velocity 1.0 h⁻¹ -1 The carrier gas was N2, and the raw material was a 20 wt% MAM / acetonitrile solution. The reaction conditions were 280℃, 3.0MPa, and the sample was taken for gas chromatography analysis after 24 h of reaction.

[0027] Reaction results: MAM conversion rate 56.2%, methacrylonitrile (MAN) selectivity 95.7%.

[0028] Example 2 Same as Example 1, except that the pH of the Tris-HCl buffer was adjusted to 7.5.

[0029] Reaction results: MAM conversion rate 48.8%, methacrylonitrile selectivity 94.3%.

[0030] Example 3 Same as Example 1, except that the pH of the Tris-HCl buffer was adjusted to 9.5.

[0031] Reaction results: MAM conversion rate 55.1%, methacrylonitrile selectivity 94.6%.

[0032] Example 4 Same as Example 1, except that the amount of dopamine hydrochloride is changed to 0.5 g (mass ratio 0.1:1).

[0033] Reaction results: MAM conversion rate 54.2%, methacrylonitrile selectivity 94.0%.

[0034] Example 5 Same as Example 1, except that the amount of dopamine hydrochloride is changed to 2.5 g (mass ratio 0.5:1).

[0035] Reaction results: MAM conversion rate 56.9%, methacrylonitrile selectivity 95.3%.

[0036] Example 6 Same as Example 1, except that the stirring reaction time in step (1) is changed to 12 h.

[0037] Reaction results: MAM conversion rate 53.5%, methacrylonitrile selectivity 94.1%.

[0038] Example 7 Same as Example 1, except that the stirring reaction time in step (1) is changed to 24 h.

[0039] Reaction results: MAM conversion rate 55.5%, methacrylonitrile selectivity 95.0%.

[0040] Example 8 Same as Example 1, except that ammonium metatungstate is replaced with phosphotungstic acid of equal mass to WO3.

[0041] Reaction results: MAM conversion rate 56.3%, methacrylonitrile selectivity 94.8%.

[0042] Example 9 Same as Example 1, except that the tungsten source is replaced with tungsten hexachloride (WCl6) and the solvent is replaced with anhydrous ethanol, the rest are the same (tungsten hexachloride is dissolved in anhydrous ethanol, and the impregnation process is strictly anhydrous).

[0043] Reaction results: MAM conversion rate 50.7%, methacrylonitrile selectivity 94.2%.

[0044] Example 10 Same as Example 1, except that ammonium metatungstate is replaced with ammonium tungstate of equal mass to WO3.

[0045] Reaction results: MAM conversion rate 48.9%, methacrylonitrile selectivity 94.5%.

[0046] Example 11 Same as Example 1, except that the load is 5 wt% in terms of WO3.

[0047] Reaction results: MAM conversion rate 41.1%, methacrylonitrile selectivity 95.4%.

[0048] Example 12 Same as Example 1, except that the load is 30 wt% in terms of WO3.

[0049] Reaction results: MAM conversion rate 62.8%, methacrylonitrile selectivity 91.9%.

[0050] Example 13 Same as Example 1, except that the pyrolysis temperature is 600℃, the heating rate is 2℃ / min, and the holding time is 6 h.

[0051] Reaction results: MAM conversion rate 57.3%, methacrylonitrile selectivity 94.5%.

[0052] Example 14 Same as Example 1, except that the pyrolysis temperature is 900℃, the heating rate is 2℃ / min, and the holding time is 2 h.

[0053] Reaction results: MAM conversion rate 47.6%, methacrylonitrile selectivity 95.1%.

[0054] Example 15 Catalyst preparation: Same as in Example 1.

[0055] Methacrylamide dehydration reaction: fixed-bed reactor, catalyst loading 2.0 g (20-40 mesh), reaction temperature 220℃, pressure 5.0 MPa, MAM mass hourly space velocity 0.2 h⁻¹ -1 The carrier gas is N2; the raw material is a 10 wt% MAM aqueous solution.

[0056] Reaction results: MAM conversion rate 40.3%, methacrylonitrile selectivity 96.6%.

[0057] Example 16 Stability test: The catalyst was prepared according to Example 1 and continuously run for 200 h under the reaction conditions of the methacrylamide dehydration reaction in Example 1.

[0058] Reaction results: MAM conversion remained at 55.1%~56.5%, MAN selectivity remained at 94.8%~95.9%, and the catalyst showed no significant deactivation.

[0059] Comparative Example 1 5.0 g of ordered mesoporous SiO2 (SBA-15, specific surface area 650 m²) was added. 2 / g, pore size 7.5 nm) directly impregnated with ammonium metatungstate aqueous solution (15 wt% WO3), stirred at room temperature for 18 h, filtered, dried and calcined at 700℃ in air atmosphere for 4 h.

[0060] The dehydration reaction of methacrylamide is the same as in Example 1; Reaction results: MAM conversion rate 28.3%, methacrylonitrile selectivity 72.1%.

[0061] Comparative Example 2 W-PDA@SiO2 was prepared according to steps (1) and (2) of Example 1, without pyrolysis in step (3), and was used directly as a catalyst.

[0062] The dehydration reaction of methacrylamide is the same as in Example 1; Reaction results: MAM conversion rate was 22.7%, selectivity for methacrylonitrile was 65.4%, and the catalyst was severely coked after 2 h of reaction.

[0063] Comparative Example 3 Same as Example 1, except that the pyrolysis atmosphere in step (3) is changed to air.

[0064] Reaction results: The carbon layer was ablated, tungsten species agglomerated, the conversion rate of MAM was 21.2%, and the selectivity of methacrylonitrile was 69.8%.

[0065] Comparative Example 4 Same as Example 1, except that the pyrolysis temperature in step (3) is changed to 500°C and kept at that temperature for 4 hours.

[0066] Reaction results: Incomplete carbonization of polydopamine, poor tungsten dispersion, MAM conversion rate of 27.5%, and selectivity of methacrylonitrile of 82.3%.

[0067] Comparative Example 5 Same as Example 1, except that the pyrolysis temperature in step (3) is changed to 1000℃ and kept at that temperature for 4 hours.

[0068] Reaction results: Tungsten species were severely sintered, MAM conversion rate was 22.8%, and methacrylonitrile selectivity was 78.5%.

[0069] Comparative Example 6 Same as Example 1, except that the load amount in terms of WO3 is changed to 40 wt%.

[0070] Reaction results: Tungsten species agglomeration was severe, the conversion rate of MAM was 31.3%, and the selectivity of methacrylonitrile was 83.6%.

[0071] Comparative Example 7 Ordered mesoporous SiO2 was replaced with ordinary SiO2 (specific surface area 200 m²). 2 / g, random holes), the rest is the same as in Example 1.

[0072] Reaction results: MAM conversion rate 35.1%, methacrylonitrile selectivity 77.2%.

[0073] Comparative Example 8 Same as Example 1, except that the solvent in step (2) is changed to acetone.

[0074] Reaction results: The polydopamine layer had poor swelling properties in acetone, low coordination anchoring efficiency, uneven tungsten loading, a MAM conversion rate of 26.2%, and a methacrylonitrile selectivity of 80.5%.

[0075] Comparative Example 9 Same as Example 1, except that the pH of the buffer solution is adjusted to 6.5.

[0076] Reaction results: Dopamine self-polymerization rate was slow, coating layer was thin and uneven, tungsten anchor quantity was insufficient, MAM conversion rate was 29.5%, and methacrylonitrile selectivity was 83.1%.

[0077] Comparative Example 10 After preparing W-PDA@SiO2 according to Example 1, it was treated with steam at 700°C (flow rate 50 mL / min) for 4 hours.

[0078] The dehydration reaction of methacrylamide is the same as in Example 1; Reaction results: The nitrogen-doped carbon layer was destroyed, tungsten species hydrolyzed and agglomerated, the conversion rate of MAM was 18.3%, and the selectivity of methacrylonitrile was 66.7%.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A highly dispersed tungsten-based catalyst, characterized in that, The highly dispersed tungsten-based catalyst has a core-shell structure, with mesoporous SiO2 as the core and a nitrogen-doped carbon layer with uniformly dispersed tungsten species as the shell.

2. The method for preparing the highly dispersed tungsten-based catalyst according to claim 1, characterized in that, Includes the following steps: (1) Disperse the ordered mesoporous SiO2 support in a buffer solution, add dopamine hydrochloride, and allow dopamine to self-polymerize in situ on the support surface to form a polydopamine coating layer, thus obtaining the PDA@SiO2 support; (2) Dissolve the tungsten source in a solvent, add the PDA@SiO2 support prepared in step (1), stir and impregnate for 12 to 24 hours to anchor the tungsten species in the PDA layer, and obtain the precursor W-PDA@SiO2 after separation and drying. (3) The precursor W-PDA@SiO2 obtained in step (2) is kept at 600~900℃ for 1~10 hours in an inert atmosphere to convert the polydopamine layer into a nitrogen-doped carbon layer, and the tungsten species are uniformly dispersed and confined in the nitrogen-doped carbon layer at the atomic or sub-nanometer level to obtain a highly dispersed tungsten-based catalyst.

3. The preparation method according to claim 2, characterized in that, The ordered mesoporous SiO2 support mentioned in step (1) includes SBA-15 with a specific surface area of ​​500~1000 m². 2 / g, pore size 5~10 nm; the buffer solution includes Tris-HCl buffer solution, the concentration of the buffer solution is 0.01~0.5 mol / L, and the pH is controlled at 7.5~9.5; the concentration of the ordered mesoporous SiO2 support is 0.01~0.1g / mL.

4. The preparation method according to claim 2, characterized in that, In step (1), the mass ratio of dopamine hydrochloride to ordered mesoporous SiO2 is 0.1:1 to 0.5:1; the reaction conditions for the in-situ self-polymerization are: stirring at room temperature for 12 to 24 hours.

5. The preparation method according to claim 2, characterized in that, The tungsten source mentioned in step (2) is at least one of phosphotungstic acid, ammonium metatungstate, tungsten hexachloride, and ammonium tungstate; the solvent is at least one of water, methanol, and ethanol; the loading amount of WO3 relative to the ordered mesoporous SiO2 support is 5~30 wt%; the drying is atmospheric pressure drying or vacuum drying, and the drying temperature is 60~120℃.

6. The preparation method according to claim 2, characterized in that, The inert atmosphere mentioned in step (3) is nitrogen, argon, helium or neon; the heating rate is 1~5℃ / min and the holding time is 2~6 hours.

7. The application of the highly dispersed tungsten-based catalyst according to claim 1 or the highly dispersed tungsten-based catalyst prepared by the preparation method according to any one of claims 2-6 in the catalytic liquid-phase dehydration reaction of methacrylamide to prepare methacrylonitrile.

8. A method for preparing methacrylonitrile by dehydrating methacrylamide, characterized in that, The highly dispersed tungsten-based catalyst according to claim 1 or the highly dispersed tungsten-based catalyst prepared by any one of claims 2-6 is loaded into a reactor, and the reaction is carried out at a reaction temperature of 220-380℃, a reaction pressure of 1.0-5.0 MPa, and a methacrylamide mass hourly space velocity of 0.1-3.0 h⁻¹. -1 Under conditions where an inert gas is used as the carrier gas, a solution or melt containing methacrylamide is brought into contact with a catalyst and a liquid-phase dehydration reaction occurs to obtain methacrylonitrile.

9. The method according to claim 8, characterized in that, The reactor is selected from one of the following: a fixed-bed reactor, a fluidized-bed reactor, a moving-bed reactor, or a slurry-bed reactor.

10. The method according to claim 8, characterized in that, The solvent of the solution containing methacrylamide is at least one of water, methanol, acetonitrile, N,N-dimethylformamide, and toluene, and the mass concentration of methacrylamide in the solution is 10-50%.

Images