Pd-based catalyst supported on pda modified mgal-ldh, preparation method thereof and method for hydrogenation of acetylene

Abstract

The application provides a PDA modified MgAl-LDH supported Pd-based catalyst, a preparation method thereof and an acetylene hydrogenation method. The preparation method comprises the following steps: mixing a MgAl-LDH dispersion liquid and a first modifier solution to obtain a modified carrier; mixing and reacting the modified carrier solution and a Pd precursor solution, and performing centrifugal separation and drying to obtain the PDA modified MgAl-LDH supported Pd-based catalyst; and the first modifier is selected from dopamine hydrochloride. The application also provides the PDA modified MgAl-LDH supported Pd-based catalyst prepared by the above method and an acetylene hydrogenation method based on the catalyst. The PDA modified MgAl-LDH supported Pd-based material is used as the catalyst in the application, the modified carrier significantly enhances the interaction between the active component and the carrier, and the catalyst performance is obviously improved.

Description

Technical Field

[0001] This invention relates to PDA-modified MgAl-LDH supported Pd-based catalysts, their preparation methods, and acetylene hydrogenation methods, belonging to the field of industrial catalyst technology. Background Technology

[0002] Ethylene is an important chemical raw material, generally produced using naphtha cracking. However, this method produces ethylene containing acetylene impurities, affecting its quality. Therefore, removing acetylene from ethylene is a key focus in the chemical industry. Currently, thermocatalysis is mainly used to induce an addition reaction between acetylene and hydrogen to produce ethylene, thus purifying the ethylene while increasing its yield. Palladium (Pd), due to its unique valence electron configuration, exhibits a strong tendency to break and form CH bonds in catalytic reactions. Therefore, Pd-based catalysts demonstrate high hydrogenation activity for alkenes and alkynes, attracting widespread attention in the chemical field. However, Pd-based catalysts prepared by traditional methods are prone to aggregation during growth, leading to excessive catalyst activity, triggering numerous side reactions during catalytic hydrogenation, and shortening catalyst lifespan. Therefore, new synthetic strategies are needed to reduce the aggregation of active components.

[0003] To enhance the anchoring strength between the catalyst and the support, the interaction between the active component and the support can be changed from adsorption to chelation, significantly improving the SMSI effect. This can be achieved by modifying the support to alter the way the active component is loaded. Hydrotalcite (LDH) itself has a strong adsorption capacity for polar small organic molecules; therefore, the LDH surface can be modified with small organic polymers to form a polymer coating with stronger chelating properties. Polymeric dopamine (PDA), grown from dopamine molecules, possesses all the properties of dopamine molecules due to the exposed nitrogenous bases on the outer side of the dopamine molecule. It can not only transmit bioelectrical signals but also serve as an excellent medium for electron transfer in the chemical field.

[0004] Deke Ma et al. (Deke Ma, Shanhua Qian, Shuaishuai Zhou, and Da Bian; Fabrication and Characterization of Polyelectrolyte Coatings by Polymerization and Co-Deposition of Acrylic Acid Using the Dopamine in Weak Acid Solutio; Langmuir, 2022, Vol. 38, No. 33, 10256-10264) disclosed the following: Polymeric dopamine (PDA), grown by polymerizing dopamine molecules, possesses various properties of dopamine molecules due to the nitrogenous bases exposed on the surface of the dopamine molecule. It can not only transmit bioelectrical signals but also serve as a good medium for electron transfer in the chemical field. The nitrogenous bases in PDA can chelate metal ions, enhancing the anchoring of metal ions. Furthermore, it possesses self-reducing properties, capable of reducing metal ions coordinated to its surface to a zero-valence metal state. However, the preparation method described in this paper is cumbersome and time-consuming, making industrial application difficult. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention aims to provide a PDA-modified MgAl-LDH supported Pd-based catalyst and its preparation method. This method is simple in its steps and suitable for industrial applications.

[0006] To achieve the above objectives, this invention provides a method for preparing a PDA-modified MgAl-LDH supported Pd-based catalyst, comprising the following steps:

[0007] Step 1: Mix the MgAl-LDH dispersion with the first modifier solution to obtain the PDA-modified MgAl-LDH support;

[0008] Step 2: The PDA-modified MgAl-LDH support solution and the Pd precursor solution are mixed and reacted, and then separated by centrifugation and dried to obtain the PDA-modified MgAl-LDH supported Pd-based catalyst.

[0009] The first modifier is selected from dopamine hydrochloride, and the amount of the first modifier is 2.5%-10.0% of the mass of the MgAl-LDH.

[0010] In the above preparation method, preferably, the amount of the first modifier is 2.5%, 5%, 7.5% or 10% of the mass of the MgAl-LDH.

[0011] In the above preparation method, preferably, the amount of the Pd precursor is 1.0-1.5% of the mass of the MgAl-LDH, more preferably 1.3%.

[0012] In the above preparation method, preferably, the Pd precursor is selected from Pd nitrates and / or Pd chlorides, such as Na2PdCl4.

[0013] In the above preparation method, preferably, step 1 further includes the step of preparing a self-assembled carboxylic acid coating, specifically including: mixing and reacting the PDA-modified MgAl-LDH support with a second modifier to obtain a PDA-modified MgAl-LDH support with a self-assembled carboxylic acid coating;

[0014] The second modifier is selected from 11-mercaptoundecanoic acid, and the amount of the second modifier is 2.0-3.0% of the mass of the MgAl-LDH, more preferably 2.5%.

[0015] In the above preparation method, preferably, the reaction time of step 2 is 12h-24h.

[0016] In the above preparation method, after mixing the PDA-modified MgAl-LDH carrier solution and the Pd precursor solution, the mixture is stirred and reacted. After stopping the stirring, centrifugation can be performed immediately, or the mixture can be allowed to stand for aging first, and then centrifugation can be performed. That is, step 2 can also include a standing aging step, specifically including: after mixing the PDA-modified MgAl-LDH carrier solution and the Pd precursor solution, aging is performed first, and then centrifugation is performed.

[0017] The aging temperature is 20℃-50℃, and the aging time is 6h-18h.

[0018] In the above preparation method, preferably, the first modifier solution is prepared in the following manner:

[0019] The first modifier was dissolved in an aqueous methanol solution, and then the pH value was adjusted to 7.5-10.5 to obtain the first modifier solution.

[0020] The methanol-water solution is composed of methanol and water in a volume ratio of 1:1.

[0021] In the above preparation method, preferably, the second modifier solution is prepared in the following manner:

[0022] The second modifier is dissolved in an aqueous methanol solution, and then the pH value is adjusted to 7.5-10.5 to obtain a solution of the second modifier.

[0023] The methanol-water solution is composed of methanol and water in a volume ratio of 1:1.

[0024] According to a specific embodiment of the present invention, preferably, the above preparation method includes the following specific steps:

[0025] (1) Weigh out Mg(NO3)2, Al(NO3)3 and urea respectively, and dissolve them in deionized water in order to obtain a solution; transfer the above solution to a closed Teflon reaction vessel, place it in an oven, react, centrifuge, wash and dry to obtain the carrier MgAl-LDH;

[0026] (2) Weigh the carrier MgAl-LDH, add it to deionized water, and disperse it by ultrasonication to prepare a carrier dispersion; weigh the first modifier, dissolve it in a certain amount of methanol-water solution to prepare a mixed solution, adjust the pH, mix the above solution with the MgAl-LDH carrier dispersion prepared by ultrasonication, stir, and obtain the modified carrier PDA-MgAl-LDH.

[0027] (3) Weigh the modified carrier PDA-MgAl-LDH, add methanol-water solution, adjust the pH solution, and stir the reaction under nitrogen protection; then add the prepared Na2PdCl4 aqueous solution, continue stirring the reaction, and keep the reaction under nitrogen protection throughout; after the reaction is completed, collect by centrifugation, and dry under vacuum to obtain the modified MgAl-LDH supported Pd-based material.

[0028] Furthermore, step (2) also includes:

[0029] Weigh the modified carrier PDA-MgAl-LDH, add it to deionized water, and ultrasonically disperse it to prepare a modified carrier dispersion. Weigh the second modifier, dissolve it in a certain amount of methanol-water solution to prepare a mixed solution, adjust the pH, mix the above solution with the modified carrier dispersion prepared by ultrasonic dispersion, stir, and obtain a further modified carrier, namely PDA(SAM)-MgAl-LDH.

[0030] Because carboxylic acid groups can strongly adsorb and even react with LDH, it is difficult for chain alkanes with carboxylic acid groups to directly self-assemble and grow on the LDH surface to form a modified coating with exposed carboxyl alcohol groups. However, thiol groups (-SH) can undergo an addition reaction with the pyridine groups exposed on PDA at room temperature to form disulfide bonds (CSC). This invention uses low-carbon alkanoic acids with terminal thiol groups to form a self-assembled carboxylic acid monolayer (SAM) with exposed carboxyl alcohol groups on the PDA surface. Specifically, a PDA-modified coating is first applied to the LDH surface, and then a layer of organic carboxylic acid is further self-assembled and grown on its surface using thiol alkanoic acid. This coating, with its stronger chelating properties, further reduces the aggregation that occurs during the growth of the metal catalyst, thereby improving catalyst dispersibility and ultimately increasing reaction efficiency.

[0031] The present invention also provides a PDA-modified MgAl-LDH supported Pd-based catalyst, which is prepared by the above preparation method.

[0032] The catalyst comprises a core component and a support, with the core component loaded on the support; the support is modified magnesium aluminum hydrotalcite.

[0033] This invention modifies the catalyst support, changing the loading mode and interaction of the active metal component Pd on the support surface, i.e., changing electrostatic attraction to coordination chelation, thereby reducing the particle size of Pd nanoparticles and improving their dispersibility. There is an SMSI effect between the Pd active component and the modified support, which effectively improves the growth structure of the Pd-based catalyst and enables it to exhibit high reaction efficiency in the selective hydrogenation reaction of acetylene in industry.

[0034] This invention utilizes a PDA-modified MgAl-LDH-supported Pd-based catalyst to catalytically reduce acetylene to ethylene with high conversion and selectivity. Furthermore, this invention provides a novel, green, and efficient method for preparing ethylene, characterized by good dispersibility of the metal active species, high safety, and high yield.

[0035] The present invention also provides a method for hydrogenating acetylene, which is carried out using the above-mentioned PDA-modified MgAl-LDH supported Pd-based catalyst.

[0036] According to a specific embodiment of the present invention, preferably, the acetylene hydrogenation method includes: contacting the C2 fraction feed gas with the PDA-modified MgAl-LDH supported Pd-based catalyst under a protective atmosphere.

[0037] According to a specific embodiment of the present invention, preferably, the above-mentioned acetylene hydrogenation method includes: adding a certain amount of catalyst into a microreactor, first introducing nitrogen gas as a preheating gas, then introducing a feed gas containing acetylene, starting heating, and reacting at a certain temperature.

[0038] According to a specific embodiment of the present invention, preferably, in the above-described acetylene hydrogenation method, the reaction temperature is 60°C-80°C, more preferably 70°C.

[0039] According to a specific embodiment of the present invention, preferably, in the reaction process of the above-mentioned acetylene hydrogenation method, the flow rate of the C2 fraction feed gas is controlled at 8000 NL / H-12000 NL / H (preferably 10000 NL / H), and the pressure inside the reactor is 0.1 MPa-0.4 MPa (preferably 0.2 MPa).

[0040] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0041] Beneficial effect 1: This invention uses PDA-modified MgAl-LDH supported Pd-based material as a catalyst, which is low in cost, high in activity and good in selectivity. Compared with traditional supports, the modified support significantly enhances the interaction between the active component and the support, thus significantly improving the performance of the catalyst.

[0042] Beneficial effect 2: The acetylene used in this invention is a basic raw material commonly used in industry, which is inexpensive and readily available.

[0043] Beneficial Effect 3: The present invention uses a carrier method that covers the LDH support surface with a PDA coating and further covers it with a SAM coating. This changes the loading mode and interaction of the active metal component Pd on the support surface, that is, it changes from electrostatic attraction to coordination chelation, which reduces the particle size of Pd nanoparticles, improves their dispersibility, and improves the selectivity of the acetylene semi-hydrogenation reaction of Pd-based catalysts.

[0044] Beneficial Effect 4: This invention provides new ideas and reference directions for improving the performance of novel catalyst supports and preparing high-performance supported metal catalysts. Attached Figure Description

[0045] Figure 1 Fourier transform infrared spectra of PDA-LDH prepared with different dopamine dosages;

[0046] Figure 2 The nitrogen adsorption-desorption curves (a) and pore size distribution diagram (b) are for different catalyst supports.

[0047] Figure 3 X-ray powder diffraction pattern of PDA-modified MgAl-LDH supported Pd-based catalyst;

[0048] Figure 4 This is a scanning electron microscope image of the catalyst;

[0049] Figure 5 Transmission electron microscopy (TEM) image of the Pd / PDA-LDH catalyst;

[0050] Figure 6 Transmission electron microscopy image of the Pd / PDA(SAM)-LDH catalyst;

[0051] Figure 7 Transmission electron microscopy image of the Pd / LDH catalyst prepared by impregnation method;

[0052] Figure 8 Fourier transform infrared spectra of dopamine, PDA-LDH, Pd / PDA-LDH and Pd / PDA(SAM)-LDH;

[0053] Figure 9The ultraviolet spectrum of the Pd-based catalyst grown on the surface of the PDA-LDH modified support;

[0054] Figure 10 The image shows the X-ray photoelectron spectrum of the catalyst.

[0055] Figure 11 The graph shows the changes in acetylene conversion and ethylene selectivity of the catalyst with reaction temperature. Detailed Implementation

[0056] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0057] Example 1

[0058] 1. Synthesis of MgAl-LDH support

[0059] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0060] 2. PDA modification on the surface of MgAl-LDH carrier material (dopamine dosage 2.5%, unaged)

[0061] Weigh 0.2 g of MgAl-LDH carrier and add it to 10 mL of deionized water. Disperse it by sonication for 15 min to prepare a carrier dispersion. Weigh 2.5% of the mass of MgAl-LDH and dissolve it in a certain amount of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by sonication. Stir in a sealed container at room temperature for 12 h. After stirring is stopped, immediately centrifuge, collect, wash, and vacuum dry to obtain a PDA-coated modified MgAl-LDH carrier with a dopamine content of 2.5%. The sample is labeled F1.

[0062] Example 2

[0063] 1. Synthesis of MgAl-LDH support

[0064] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0065] 2. PDA modification on the surface of MgAl-LDH carrier material (5% dopamine dosage, no aging)

[0066] Weigh 0.2g of the carrier MgAl-LDH and add it to 10mL of deionized water. Disperse it by sonication for 15min to prepare a carrier dispersion. Weigh 5% of the mass of the above MgAl-LDH dopamine hydrochloride (DA) and dissolve it in a certain amount of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by sonication. Stir in a sealed container at room temperature for 12h. After stopping stirring, immediately centrifuge, collect, wash, and vacuum dry to obtain a PDA-coated modified MgAl-LDH carrier with 5% dopamine content. The sample is labeled F2.

[0067] Example 3

[0068] 1. Synthesis of MgAl-LDH support

[0069] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0070] 2. PDA modification on the surface of MgAl-LDH carrier material (dopamine dosage 7.5%, unaged)

[0071] Weigh 0.2 g of MgAl-LDH carrier and add it to 10 mL of deionized water. Disperse the carrier using ultrasound for 15 min to prepare a carrier dispersion. Weigh 7.5% of the mass of MgAl-LDH and dissolve it in a certain amount of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by ultrasound dispersion. Stir the mixture under sealed conditions at room temperature for 12 h. After stirring is stopped, immediately centrifuge, collect, wash, and vacuum dry to obtain a PDA-coated modified MgAl-LDH carrier with a dopamine content of 7.5%. The sample is labeled as F3.

[0072] Example 4

[0073] 1. Synthesis of MgAl-LDH support

[0074] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0075] 2. PDA modification on the surface of MgAl-LDH carrier material (10% dopamine content, no aging)

[0076] Weigh 0.2g of the carrier MgAl-LDH and add it to 10mL of deionized water. Disperse it by sonication for 15min to prepare a carrier dispersion. Weigh 10% of the mass of the above MgAl-LDH dopamine hydrochloride (DA) and dissolve it in a certain amount of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by sonication. Stir in a sealed container at room temperature for 12h. After stopping stirring, immediately centrifuge, collect, wash, and vacuum dry to obtain a PDA-coated modified MgAl-LDH carrier with 10% dopamine content. The sample is labeled F4.

[0077] Example 5

[0078] 1. Synthesis of MgAl-LDH support

[0079] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0080] 2. PDA modification on the surface of MgAl-LDH carrier material (dopamine dosage 2.5%, aging)

[0081] Weigh 0.2 g of MgAl-LDH carrier and add it to 10 mL of deionized water. Disperse it by sonication for 15 min to prepare a carrier dispersion. Weigh 2.5% of the mass of MgAl-LDH and dissolve it in a certain amount of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by sonication. Stir in a sealed container at room temperature for 12 h. After stopping stirring, let it stand for aging overnight. Then collect by centrifugation, wash, and vacuum dry to obtain a PDA-coated modified MgAl-LDH carrier with a dopamine content of 2.5%. The sample is labeled G1.

[0082] Example 6

[0083] 1. Synthesis of MgAl-LDH support

[0084] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0085] 2. PDA modification on the surface of MgAl-LDH carrier material (dopamine dosage 5%, aging)

[0086] Weigh 0.2g of MgAl-LDH carrier and add it to 10mL of deionized water. Disperse it by sonication for 15min to prepare a carrier dispersion. Weigh 5% of the mass of MgAl-LDH and dissolve it in a certain amount of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by sonication. Stir in a sealed container at room temperature for 12h. After stopping stirring, let it stand for aging overnight. Then collect by centrifugation, wash, and vacuum dry to obtain a PDA-coated modified MgAl-LDH carrier with 5% dopamine content. The sample is labeled G2.

[0087] Example 7

[0088] 1. Synthesis of MgAl-LDH support

[0089] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0090] 2. PDA modification on the surface of MgAl-LDH carrier material (dopamine dosage 7.5%, aging)

[0091] Weigh 0.2 g of MgAl-LDH carrier and add it to 10 mL of deionized water. Disperse the carrier using ultrasound for 15 min to prepare a carrier dispersion. Weigh 7.5% of the mass of MgAl-LDH and dissolve it in a certain amount of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by ultrasound dispersion. Stir the mixture under sealed conditions at room temperature for 12 h. After stopping the stirring, let it stand for aging overnight. Then, collect the mixture by centrifugation, wash it, and vacuum dry it to obtain a PDA-coated modified MgAl-LDH carrier with a dopamine content of 7.5%. The sample is labeled G3.

[0092] Example 8

[0093] 1. Synthesis of MgAl-LDH support

[0094] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0095] 2. PDA modification on the surface of MgAl-LDH carrier material (dopamine dosage 10%, aging)

[0096] Weigh 0.2g of MgAl-LDH carrier and add it to 10mL of deionized water. Disperse it by sonication for 15min to prepare a carrier dispersion. Weigh 10% of the mass of MgAl-LDH and dissolve it in a certain amount of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by sonication. Stir in a sealed container at room temperature for 12h. After stopping stirring, let it stand for aging overnight. Then collect by centrifugation, wash, and vacuum dry to obtain a PDA-coated modified MgAl-LDH carrier with 10% dopamine content. The sample is labeled G4.

[0097] Fourier transform infrared spectroscopy was performed on PDA-LDH samples prepared under different reaction conditions, and the results are as follows: Figure 1 As shown. For dopamine hydrochloride, its infrared spectrum is located at 1508 cm⁻¹. -1 and 1606cm -1 absorption peak at ( Figure 1 a) in the figure is attributed to the indole group, indicating the presence of a pyridine structure, approximately 3464 cm. -1 The strong absorption peak at that location is the hydroxyl absorption peak. Without overnight aging, the FTIR of PDA-coated MgAl-LDH surface is as follows: Figure 1As shown in Figure a, the surface hydroxyl absorption peak is significantly enhanced, but with the increase of dopamine hydrochloride input, the 1508 cm⁻¹ peak corresponding to the indole group decreases. -1 and 1606cm -1 The absorption peak at that location hardly changed. However, under overnight aging, such as Figure 1 As shown in b, the absorption peak corresponding to the indole group is slightly enhanced, especially the absorption peak of the indole group in sample G2, which shows a more significant enhancement. This indicates that after overnight aging, the PDA coating produces more and more stable indole groups. This is because the catechol group of dopamine is first oxidized to dopaquinone under alkaline conditions. During stirring, an intramolecular cyclization reaction occurs, producing an unstable intermediate. After stopping stirring and standing for a period of time, due to intermolecular and intramolecular rearrangement, a stable 5,6-dihydroxyindole is finally generated. The pyridine group exposed by this group helps the Pd active component to chelate on the support surface. The support (G2) with more exposed indole groups was selected for subsequent catalyst preparation.

[0098] Example 9

[0099] 1. Synthesis of MgAl-LDH support

[0100] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0101] 2. PDA modification on the surface of MgAl-LDH carrier material

[0102] Weigh 0.2 g of MgAl-LDH carrier and add it to 10 mL of deionized water. Disperse it by ultrasonication for 15 min to prepare a carrier dispersion. Weigh 5% of the mass of MgAl-LDH and dissolve it in 15 mL of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by ultrasonication. Stir the mixture in a sealed container at room temperature for 12 h. After stopping the stirring, let it stand for aging overnight. Then, collect the mixture by centrifugation, wash it, and vacuum dry it to obtain a PDA-coated modified MgAl-LDH carrier with a 5% dopamine content. The sample is labeled G2.

[0103] 3. Preparation of Pd / PDA-LDH catalyst

[0104] Weigh 0.2 g of the optimized PDA-LDH sample G2 and place it in a three-necked flask. Disperse it in 15 mL of a 1:1 methanol-water solution under ultrasonication. Adjust the pH to 8.5 by adding NaOH solution dropwise, and stir the reaction under nitrogen protection for 24 h. Then, add the prepared Na2PdCl4 aqueous solution (0.0026 g Na2PdCl4 dissolved in 5 mL of deionized water) using a disposable sterile syringe, and continue stirring the reaction for 12 h under nitrogen protection throughout. After the reaction, collect the sample by centrifugation, wash it three times with deionized water and once with methanol, and then vacuum dry to obtain the Pd / PDA-LDH catalyst material.

[0105] Example 10

[0106] 1. Synthesis of MgAl-LDH support

[0107] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0108] 2. PDA modification on the surface of MgAl-LDH carrier material

[0109] Weigh 0.2 g of MgAl-LDH carrier and add it to 10 mL of deionized water. Disperse it by ultrasonication for 15 min to prepare a carrier dispersion. Weigh 5% of the mass of MgAl-LDH and dissolve it in 15 mL of 1:1 methanol-water solution to prepare a mixed solution. Add NaOH solution dropwise to adjust the pH to 8.5. Mix the above solution with the MgAl-LDH carrier dispersion prepared by ultrasonication. Stir the mixture in a sealed container at room temperature for 12 h. After stopping the stirring, let it stand for aging overnight. Then, collect the mixture by centrifugation, wash it, and vacuum dry it to obtain a PDA-coated modified MgAl-LDH carrier with a 5% dopamine content. The sample is labeled G2.

[0110] 3. Growth of self-assembled carboxylic acid coating (SAM) on PDA surface and preparation of Pd / PDA(SAM)-LDH catalyst

[0111] Weigh 5 mg of 11-mercapto-undecanoic acid and dissolve it in 15 mL of 1:1 methanol-water solution under ultrasonic treatment. Adjust the pH to 8.5 by adding NaOH solution dropwise and let it stand for later use.

[0112] Weigh 0.2 g of the optimized PDA-LDH sample G2 and place it in a three-necked flask. Add the prepared 11-mercapto-undecanoic acid solution using a sterile disposable syringe, and stir the reaction under nitrogen protection for 24 h. Then, add the prepared Na2PdCl4 aqueous solution (0.0026 g Na2PdCl4 dissolved in 5 mL deionized water) using a sterile disposable syringe, and continue stirring the reaction for 12 h under nitrogen protection throughout. After the reaction, collect the sample by centrifugation, wash three times with deionized water and once with methanol, and vacuum dry to obtain the self-assembled carboxylic acid coating modified Pd / PDA(SAM)-LDH catalyst material.

[0113] Comparative Example 1

[0114] 1. Synthesis of MgAl-LDH support

[0115] Weigh out 2.00 g Mg(NO3)2, 1.46 g Al(NO3)3, and 0.90 g urea, and dissolve them in 10 mL of deionized water in that order. Transfer the solution to a 15 mL sealed Teflon reactor, place it in an oven at 150 °C, and react for 48 h. Centrifuge, wash, and dry to obtain the MgAl-LDH support.

[0116] 2. Preparation of Pd / LDH catalyst

[0117] Weigh 0.2 g of the MgAl-LDH support and add it to 15 mL of deionized water, then disperse it by ultrasonication. Dissolve 0.0026 g of Na₂PdCl₄ in 5 mL of deionized water. Mix the support dispersion suspension with different amounts of Na₂PdCl₄ solution, and stir at a constant temperature of 50 °C for 6 h to ensure thorough mixing and loading. Add 2 mL of a mixed solution containing 0.05 g of sodium ascorbate and 0.01 g of sodium citrate as a reducing agent, and continue stirring at 50 °C for another 6 h to reduce the mixture. After the above reaction is complete, centrifuge, wash, and dry to obtain the Pd / LDH catalyst.

[0118] Table 1. Specific surface area and pore structure data of LDH, PDA, and SAM-modified LDH.

[0119] carrier <![CDATA[S BET (m 2 / g)]]> <![CDATA[V tot (cm 3 / g)]]> Average pore size (nm) MgAl-LDH 13.05 0.0395 12.63 PDA-LDH 14.60 0.0524 13.57 PDA(SAM)-LDH 17.11 0.0538 14.39

[0120] Low-temperature nitrogen physical adsorption-desorption (BET) tests were performed on PDA-modified supports (PDA-LDH) and SAM-modified PDA-LDH (PDA(SAM)-LDH) supports. The nitrogen adsorption-desorption curves and pore size distribution diagrams are shown below. Figure 2 As shown. Figure 2As shown, the nitrogen adsorption-desorption curves of MgAl-LDH, PDA-LDH, and PDA(SAM)-LDH supports all exhibit type IV curves, indicating capillary condensation, and all also possess an H3-type hysteresis loop. Therefore, MgAl-LDH, PDA-LDH, and PDA(SAM)-LDH supports are all mesoporous materials.

[0121] Furthermore, the capillary condensation sections of MgAl-LDH, PDA-LDH, and PDA(SAM)-LDH gradually become steeper, indicating a gradually more uniform mesopore distribution. Moreover, the hysteresis phenomenon becomes increasingly pronounced, suggesting a gradual increase in the mesopore size. Figure 2 As shown in b, it can be seen that compared with MgAl-LDH, the shoulder of the PDA-LDH curve shifts to the right, indicating that its mesopore size is increased. Compared with PDA-LDH, the shoulder of PDA(SAM)-LDH shifts to the right more obviously, indicating that the mesopore size is further increased.

[0122] Table 1 shows the specific surface area and pore structure data for MgAl-LDH, PDA-LDH, and PDA(SAM)-LDH. The data directly show that the PDA-LDH carrier exhibits significantly increased specific surface area, pore volume, and average pore diameter, while PDA(SAM)-LDH further exhibits even larger specific surface area, pore volume, and average pore diameter.

[0123] Since the large specific surface area and pore structure of the support are conducive to the anchoring of the catalyst, and the SAM coating has carboxyl alcohol groups with stronger chelating properties, growing the SAM coating on the PDA-LDH surface can not only improve the porous structure, but also help to further improve the anchoring of the active metal.

[0124] Pd active component Pd was anchored on the surface of MgAl-LDH support and modified supports PDA-LDH and PDA(SAM)-LDH to obtain Pd / LDH, Pd / PDA-LDH, and Pd / PDA(SAM)-LDH catalysts. XRD characterization results are shown below. Figure 3 As shown, compared with the supported LDH and PDA-LDH, weaker Pd characteristic peaks were observed in the spectra of Pd / LDH, Pd / PDA-LDH, and Pd / PDA(SAM)-LDH. At the same time, it can be seen that the modified support still maintains the clear characteristic diffraction peaks of MgAl-LDH, indicating that the crystal structure of the support remains basically stable after surface modification.

[0125] The prepared materials were characterized and analyzed using scanning electron microscopy. For example... Figure 4As shown in a and b, the PDA-modified MgAl-LDH retains its layered structure, but the surface of the MgAl-LDH layers on the modified support is rougher. Furthermore, SEM images of Pd / LDH, Pd / PDA-LDH, and Pd / PDA(SAM)-LDH are also provided. Figure 4 No Pd nanoparticles were found in the sample. The analysis suggests that the Pd nanoparticles loaded were either too small in size or had a relatively low loading amount.

[0126] The phase state of the modified support and its supported Pd-based catalyst was further observed and analyzed using transmission electron microscopy. Figure 5 The TEM characterization results of the Pd / PDA-LDH catalyst show that MgAl-LDH in the Pd / PDA-LDH material exhibits an ultrathin lamellar structure. Figure 5 The presence of shadows at the edges of the sheets, possibly due to the PDA coating, is observed under transmission electron microscopy. Magnified observation reveals Pd metal nanoparticles dispersed on the support surface, with a particle size of approximately 5 nm. Figure 5 The presence of d and e in the Pd nanoparticles corresponds to the difficulty in observing active metal nanoparticles on the surface of the support sheets in SEM characterization results. However, upon magnification, the lattice structure of the Pd nanoparticles becomes clearly visible. Figure 5 In f), the lattice spacing is about 0.224 nm, which corresponds to the (111) crystal plane of Pd. The uniform distribution of the active component Pd on the surface of the support indicates its good dispersibility.

[0127] The Pd / PDA(SAM)-LDH catalyst was characterized by TEM, and the results are as follows: Figure 6 As shown in ac, the roughness of the carrier surface is further increased after the PDA-LDH surface is covered with a SAM coating. Figure 6 In the image, d and e are magnified images of Pd / PDA(SAM)-LDH under a transmission electron microscope, showing that Pd nanoparticles are uniformly distributed on the surface of the sheets. Figure 6 (d) The aggregation phenomenon is not obvious, and the particle size of Pd particles is approximately 3 nm. It is worth noting that compared to Pd nanoparticles on the PDA-LDH surface, the particle size of Pd nanoparticles loaded on the PDA-LDH support surface after SAM coating modification is further reduced. This may be because the carboxyl alcohol groups on the SAM coating surface affect the Pd... 2+ The chelation effect is stronger, effectively preventing the aggregation of Pd components. The lattice fringes of the crystalline region of the Pd nanoparticles were measured. Figure 6 The lattice spacing of Pd nanoparticles was measured to be approximately 0.224 nm, corresponding to the (111) crystal plane of Pd, indicating that Pd nanoparticles were successfully loaded onto the surface of the modified support. Figure 6Figure fj shows the distribution of N, S, and Pd elements in the Pd / PDA(SAM)-LDH catalyst. The uniform distribution of N and S elements indicates the uniform coverage of the PDA and SAM modified coating, while the good distribution of the Pd component is attributed to the excellent chelating properties of the PDA and SAM modified coating. This invention, through further modification of the support surface, reduces the aggregation tendency of the Pd component to a certain extent, obtaining Pd nanoparticles loaded on its surface with smaller particle size and more uniform distribution.

[0128] Figure 7 The TEM characterization results show the Pd single-metal catalyst (Pd / LDH) prepared on the unmodified MgAl-LDH support surface. Compared with the modified support, Pd directly loaded on the unmodified MgAl-LDH surface resulted in significant agglomeration and uneven distribution of Pd nanoparticles. These results indicate that modifying the MgAl-LDH support surface before loading Pd components can significantly reduce the particle size of the Pd-based catalyst and improve its dispersibility.

[0129] Fourier transform infrared spectroscopy was used to analyze PDA-LDH, Pd / PDA-LDH, and Pd / PDA(SAM)-LDH. The results are as follows: Figure 8 As shown, at approximately 1512cm -1 and 1612cm -1 The weak characteristic absorption peak corresponds to the characteristic peak of the pyridine structure. Pd / PDA(SAM)-LDH shows a weak characteristic absorption peak at 1176 cm⁻¹. -1 and 1692cm -1 Two weak characteristic absorption peaks are observed at 1072 cm⁻¹, indicating the presence of a carboxyl group. -1 The absorption peak at 638 cm⁻¹ is attributed to a disulfide bond (CSC), formed by the addition reaction between the thiol group and the internal pyridine group. These results demonstrate the successful binding of carboxyalkyl acids to the PDA surface. Due to the interaction between PDA and SAM, the characteristic absorption peak of Pd / PDA(SAM)-LDH shows a slight shift. The FTIR spectra of Pd / PDA-LDH and Pd / PDA(SAM)-LDH are located at 638 cm⁻¹. -1 The characteristic absorption peak at the location is the stretching vibration peak of the Pd-O bond, indicating that the active Pd is anchored on the modified support.

[0130] Pd / PDA-LDH and Pd / PDA(SAM)-LDH were characterized by UV-Vis spectroscopy, and the interaction between Pd and the PDA and SAM modified coatings was analyzed. Figure 9The UV-Vis spectra of Pd / PDA-LDH and Pd / PDA(SAM)-LDH are presented. The UV spectrum of Pd / PDA-LDH exhibits two characteristic absorption peaks: a peak at 232.8 nm corresponding to the complex formed by chelation between Pd and indole groups, and a peak at 287.8 nm corresponding to the indole group itself. In the UV spectrum of Pd / PDA(SAM)-LDH, the main characteristic absorption peak at 207.8 nm corresponds to the chelation between the active component Pd and the carboxyl alcohol groups in the SAM coating. The UV-Vis characterization of Pd / PDA-LDH and Pd / PDA(SAM)-LDH demonstrates good growth of the active component Pd on the modified coating surface.

[0131] The electronic structure of the prepared product was further characterized and analyzed using X-ray photoelectron spectroscopy. Figure 10 The ac in the image represents the N1s XPS spectrum, and the peak at 400.10 eV is attributed to the standard peak of the pyridine structure in the PDA coating. Figure 10 As shown in a), compared to PDA-LDH, the N1s binding energy in Pd / PDA-LDH and Pd / PDA(SAM)-LDH undergoes a significant negative shift. Figure 10 (b, c). In the Pd 3d XPS spectrum ( Figure 10 The df in the middle, compared with Pd loaded on the unmodified MgAl-LDH surface ( Figure 10 In section f), for Pd / PDA-LDH and Pd / PDA(SAM)-LDH obtained by loading Pd onto the modified support, Pd 3d XPS spectra clearly show the presence of Pd. 2+ This indicates that electron transfer occurred between N in the modified coating and the anchored Pd. (Comparison) Figure 10 As can be seen from g and h, the S2p binding energy hardly shifted before and after Pd loading onto the PDA(SAM)-LDH surface, indicating that almost no electron transfer occurred in the S element. This suggests that the thiol group did not coordinate with Pd. In summary, both PDA and SAM have good anchoring effects on the Pd active component, and varying degrees of electron transfer exist in the interfacial interactions.

[0132] The catalysts prepared in Examples 9-10 and Comparative Example 1 were used in a six-channel microreactor for the selective hydrogenation of acetylene to ethylene. Although different catalysts were used, the process parameters for hydrogenation were the same. The operating steps were as follows: A certain amount of catalyst (the amount added could be adjusted according to the reactor specifications) was added to the microreactor. Nitrogen gas was first introduced as a preheating gas, followed by a C2 fraction feed gas, with a flow rate controlled at 10000 NL / H and a pressure controlled at approximately 0.2 MPa. Heating was initiated, and a temperature gradient was set at 5°C intervals during testing. Based on the actual reaction results and instrument operation, the results within the temperature range of 60°C–80°C were selected as samples. After stabilizing the reaction at different temperature points, the feed gas and product gas were sampled and analyzed using a gas chromatograph (Agilent 7890) to determine the gas component content and calculate the conversion rate and selectivity of the catalytic hydrogenation of acetylene. The formulas are as follows:

[0133]

[0134] Note: The contents mentioned above are gas volume percentages.

[0135] The evaluation results of the catalysts in Examples 9-10 and Comparative Example 1 are shown in Table 2 below. Figure 11 As shown, at a reaction temperature of 70℃, the acetylene conversion rate of the catalyst Pd / PDA-LDH reached 96.66%, and the ethylene selectivity reached 88.48%, while the acetylene conversion rate of the catalyst Pd / PDA(SAM)-LDH reached 97.02%, and the ethylene selectivity reached 94.96%.

[0136] Regarding reaction temperature, the optimal reaction temperatures for both Pd / PDA-LDH and Pd / PDA(SAM)-LDH catalysts are 70℃, considering both acetylene conversion and ethylene selectivity. The decrease in reaction efficiency due to excessively high temperatures is a result of increased side reactions, consistent with the findings of the previous two studies. In the comparison before and after modification, the acetylene conversion and ethylene selectivity of Pd / PDA-LDH and Pd / PDA(SAM)-LDH catalysts prepared on PDA and SAM modified coating surfaces were significantly higher than those of Pd / LDH catalysts prepared directly on unmodified LDH support surfaces. At a reaction temperature of 70℃, the acetylene conversion of the Pd / PDA-LDH catalyst reached 96.66%, and the ethylene selectivity reached 88.48%, while the acetylene conversion of the Pd / PDA(SAM)-LDH catalyst reached 97.02%, and the ethylene selectivity reached 94.96%, indicating that the acetylene conversion values ​​of the two catalysts are quite similar. Regarding ethylene selectivity, the catalyst prepared after SAM coating modification showed a significant increase in selectivity, approximately 6 percentage points higher. Comprehensive characterization analysis results demonstrate that after modification of the LDH support, the PDA coating on the surface improves the dispersibility of the Pd-based catalyst and effectively regulates the electronic structure of Pd through enhanced SMSI effect, thereby achieving the effect of improving acetylene conversion and ethylene selectivity. Further SAM modification can further improve acetylene conversion and ethylene selectivity, ultimately resulting in the Pd / PDA(SAM)-LDH catalyst exhibiting even higher reaction efficiency.

[0137] Table 2 Evaluation results of acetylene hydrogenation catalysts in Examples 9-10 and Comparative Example 1

[0138]

[0139] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a PDA-modified MgAl-LDH supported Pd-based catalyst, comprising the following steps: Step 1: Mix the MgAl-LDH dispersion with the first modifier solution to obtain the PDA-modified MgAl-LDH support; Step 2: The PDA-modified MgAl-LDH support solution and the Pd precursor solution are mixed and reacted, and then separated by centrifugation and dried to obtain the PDA-modified MgAl-LDH supported Pd-based catalyst. The first modifier is selected from dopamine hydrochloride, and the amount of the first modifier is 2.5%-10.0% of the mass of the MgAl-LDH.

2. The preparation method according to claim 1, wherein, The amount of the Pd precursor used is 1.0-1.5% of the mass of the MgAl-LDH. The Pd precursor is selected from Pd nitrates and / or Pd chlorides.

3. The preparation method according to claim 1, wherein, Step 1 further includes: The PDA-modified MgAl-LDH support is mixed and reacted with a second modifier to obtain a PDA-modified MgAl-LDH support with a self-assembled carboxylic acid coating. The second modifier is selected from 11-mercaptoundecanoic acid, and the amount of the second modifier is 2.0-3.0% of the mass of the MgAl-LDH.

4. The preparation method according to claim 1, wherein, The reaction time for step 2 is 12-24 hours.

5. The preparation method according to claim 1, wherein, Step 2 further includes: after mixing the PDA-modified MgAl-LDH carrier solution with the Pd precursor solution, aging it first, and then centrifuging it. The aging temperature is 20℃-50℃, and the aging time is 6h-18h.

6. The preparation method according to claim 1, wherein, The first modifier solution is prepared in the following manner: The first modifier was dissolved in an aqueous methanol solution, and then the pH value was adjusted to 7.5-10.5 to obtain the first modifier solution. The methanol-water solution is composed of methanol and water in a volume ratio of 1:

1.

7. The preparation method according to claim 3, wherein, The second modifier solution is prepared in the following manner: The second modifier is dissolved in an aqueous methanol solution, and then the pH value is adjusted to 7.5-10.5 to obtain a solution of the second modifier. The methanol-water solution is composed of methanol and water in a volume ratio of 1:

1.

8. A PDA-modified MgAl-LDH supported Pd-based catalyst, which is prepared by the preparation method according to any one of claims 1-7.

9. An acetylene hydrogenation method, which is carried out using the PDA-modified MgAl-LDH supported Pd-based catalyst as described in claim 8.

10. The acetylene hydrogenation method according to claim 9, wherein, The acetylene hydrogenation method includes: contacting a feed gas containing acetylene with the PDA-modified MgAl-LDH supported Pd-based catalyst under a protective atmosphere.

11. The acetylene hydrogenation method according to claim 9, wherein, The reaction temperature is 60℃-80℃.

12. The acetylene hydrogenation method according to claim 10, wherein, During the reaction, the flow rate of the feed gas is controlled at 8000 NL / H-12000 NL / H, and the pressure inside the reactor is 0.1 MPa-0.4 MPa.

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