Synthesis method of monolayer polyacid-hydrotalcite nanosheet and application of monolayer polyacid-hydrotalcite nanosheet in catalysis of thioether oxidation reaction

Abstract

The invention discloses a synthesis method of a single-layer polyacid-hydrotalcite nanosheet and application of the single-layer polyacid-hydrotalcite nanosheet in catalyzing thioether oxidation reaction. A liquid-liquid interface assembling method is adopted, the assembling process of polyacid and hydrotalcite is limited in a horizontal interface layer, the problem that isotropic electrostatic force drives vertical stacking assembling is solved, and the polyacid-hydrotalcite two-dimensional single-layer nanosheet is obtained through directional assembling. Due to the unique single-layer structure of the single-layer polyacid-hydrotalcite nanosheet, the single-layer polyacid-hydrotalcite nanosheet has a relatively high specific surface area, so that a polyacid catalytic active site is fully exposed, and a reactant is promoted to be in contact with the active site, so that the reaction rate is accelerated, and excellent thioether oxidation catalytic performance is shown. In addition, due to the existence of the cationic surface active agent, the nanosheet has amphipathicity, the mass transfer resistance of a thioether substrate and hydrogen peroxide in the nanosheet is reduced, and the catalytic efficiency is further improved.

Description

Technical Field

[0001] This invention relates to the field of inorganic composite material synthesis, specifically to a method for synthesizing monolayer polyacid-hydrotalcite nanosheets and its application in catalyzing thioether oxidation reactions. Background Technology

[0002] Two-dimensional layered nanomaterials, due to their unique electronic structure and high surface chemical activity, have shown great potential in interdisciplinary fields such as energy storage and catalytic conversion, becoming a research hotspot for novel functional materials. Since the successful exfoliation of monolayer graphene, atomically thin two-dimensional monolayer nanosheets (2D SNs) have demonstrated broad application prospects in sensors, electronic devices, optical devices, energy devices, and catalysis due to their excellent flexibility, large lateral size, and high specific surface area.

[0003] Polyacid-hydrotalcite composites (POM-LDHs) are a classic class of two-dimensional layered materials. The inherent catalytic activity of POMs and the co-catalytic effect of LDHs make them potentially valuable in reactions such as esterification (Hao Wang, et al., Appl. Clay Sci., 2017, 150, 34.), Knoevenagel condensation (Yu-Fei Song, et al., Dalton Trans., 2018, 47, 3059), enol epoxidation (Yu-Fei Song, et al., ACS Appl. Mater. Interfaces, 2020, 12, 36389), and selective sulfur oxidation (Yu-Fei Song, et al., ChemCatChem, 2018, 10, 188). Currently reported synthetic strategies include ion exchange, calcination reconstruction, coprecipitation, host layer modification, and exfoliation-reassembly. However, the strong electrostatic attraction between the positively charged LDH layers and the negatively charged POM anions often leads to multilayer stacking, resulting in insufficient exposure of the catalytic active sites of POMs and limiting the improvement of the catalytic performance of POM-LDH two-dimensional materials. Therefore, how to regulate the strong electrostatic attraction between the LDH cation layers and POM anions to achieve the controllable assembly of monolayer POM-LDH two-dimensional nanosheets is of significant scientific importance. Summary of the Invention

[0004] To address the aforementioned issues, this invention provides a self-assembly method using polyacids, cationic surfactants, and hydrotalcite as building blocks to prepare two-dimensional polyacid-hydrotalcite nanosheets with a monolayer structure, which are then applied to catalyze the oxidation of thioethers.

[0005] The monolayer polyacid-hydrotalcite nanosheets are two-dimensional nanosheets assembled sequentially by electrostatic forces from a layer of cationic surfactant, a layer of polyacid, and a monolayer of hydrotalcite, and the monolayer structure remains stable and does not agglomerate.

[0006] The method for synthesizing the aforementioned monolayer polyacid-hydrotalcite nanosheets is as follows: S1. Dissolve the polyacid in deionized water and the cationic surfactant in an organic solvent. Mix the two solutions, stir at room temperature, separate the layers, take the organic phase, dry it with anhydrous sodium sulfate, filter to remove the solid, and obtain a complex solution of polyacid and cationic surfactant. S2. Under nitrogen protection, nitrate-intercalated hydrotalcite is dispersed at 0.5-1.5 mg / mL in formamide or a mixed solvent of formamide and water, and stirred and exfoliated for 2-7 days to obtain a single-layer hydrotalcite suspension with Tyndall effect. S3. Add the monolayer hydrotalcite suspension to the upper layer of the complex solution of polyacid and cationic surfactant, let it stand at room temperature for 48-72 hours to perform interfacial self-assembly, collect the product by centrifugation, wash it with chloroform and methanol in sequence, and dry it to obtain monolayer polyacid-hydrotalcite nanosheets.

[0007] The polyacids mentioned are selected from K3PW 12 O 40 K4SiW 12 O 40 K6[MnMo9O 32 ]、K7[ α -PW 11 O 39 ·13H2O, K 10 [P2W 17 O 61 ·12H2O, K 11 [Er(PW 11 O 39 One or more of )2]·13H2O.

[0008] The divalent metals on the nitrate-intercalated hydrotalcite plates are one or more of Mg, Zn, Co, Cu, and Ni, and the trivalent metals are one or more of Fe, Al, and Cr.

[0009] The cationic surfactant is one or more of the following: dioctadecyl dimethyl ammonium bromide, dihexadecyl dimethyl ammonium bromide, hexadecyl trimethyl ammonium bromide, octadecyl trimethyl ammonium chloride, cationic guar gum, and tetrabutyl ammonium bromide.

[0010] The organic solvent is one or more of chloroform, dichloromethane, diethyl ether, chloropropane, carbon tetrachloride, and carbon disulfide.

[0011] The volume ratio of formamide to water in the mixed solvent of formamide and water is 10-100:1.

[0012] The monolayer polyacid-hydrotalcite nanosheets prepared above were applied to the catalytic oxidation reaction of thioethers. The specific operation was as follows: the monolayer polyacid-hydrotalcite nanosheets, hydrogen peroxide and 2-(diphenylmethylthio)acetic acid were mixed and reacted at 30-60℃ for 0.5-6 h with stirring. Finally, the product 2-diphenylmethyl sulfoxide acetic acid was separated by column chromatography.

[0013] This invention employs a liquid-liquid interface assembly method, confining the assembly process of polyacids and hydrotalcite within a horizontal interface layer. This solves the problem of isotropic electrostatic force-driven vertical stacking assembly, resulting in directional assembly of two-dimensional monolayer nanosheets of polyacids and hydrotalcite. On one side of the monolayer hydrotalcite, the complex of polyacids and cationic surfactants in the organic phase binds to the hydrotalcite at the liquid-liquid interface through electrostatic interactions driven by electrostatic forces. To balance the overall charge, nitrate ions act as neutralizers on the other side of the hydrotalcite. Due to the unique monolayer structure of the polyacid-hydrotalcite nanosheets, they possess a high specific surface area, fully exposing the catalytically active sites of the polyacids, promoting contact between reactants and active sites, thereby accelerating the reaction rate and exhibiting excellent thioether oxidation catalytic performance. Furthermore, the presence of cationic surfactants gives the nanosheets amphiphilic properties, reducing the mass transfer resistance between thioether substrates and hydrogen peroxide within the nanosheets, further improving catalytic efficiency. Attached Figure Description

[0014] Figure 1 This is a flowchart of Example 1; Figure 2 TEM and AFM images of Example 1; Figure 3 MgAl-NO3 and K7PW from Example 1 11 O 39 m-MgAl-PW 11 and s-MgAl-PW 11 Zeta potential diagram; Figure 4 s-MgAl-PW as described in Example 1 11 m-MgAl-PW 11 MgAl-NO3 and DODA-PW 11 FT-IR plot; Figure 5 s-MgAl-PW as described in Example 1 11 m-MgAl-PW 11 XRD patterns of MgAl-NO3; Figure 6 s-MgAl-PW as described in Example 111 Performance diagram for thioether oxidation reaction. Detailed Implementation

[0015] Example 1 s-MgAl-PW 11 Self-assembly synthesis of two-dimensional monolayer nanosheets: (1) Add 1 mmol K7[ α -PW 11 O 39 DODA-PW was dissolved in 20 mL of deionized water, and 7 mmol of dioctadecyldimethylammonium bromide was dissolved in 50 mL of chloroform. The two phases were mixed and stirred at room temperature for 10 min. After separation, the chloroform layer was collected, dried over anhydrous sodium sulfate, and filtered to remove solid impurities, yielding DODA-PW. 11 Complex solution; (2) Under nitrogen protection, 300 mg of MgAl-NO3 hydrotalcite was dispersed in 300 mL of formamide and stirred slowly for 48 hours to form a single-layer MgAl-LDHs nanosheet suspension with Tyndall effect.

[0016] (3) Take 10 mL of solution from (2) with a concentration of 1 mg·mL⁻¹. -1 The MgAl-NO3 formamide suspension was slowly added to 10 mL of a solution with a concentration of 5 mg·mL⁻¹. -1 DODA-PW 11 The upper layer of the chloroform solution, after standing at room temperature for 48 hours, forms s-MgAl-PW at the liquid-liquid interface. 11 Assembled product; collected by centrifugation, washed three times each with chloroform and methanol, and dried under vacuum at 60°C for 24 h to obtain monolayer polyacid-hydrotalcite nanosheets, denoted as s-MgAl-PW 11 .

[0017] Application in catalytic sulfide oxidation reaction: 10 mg of s-MgAl-PW 11 The catalyst, 0.3 mmol (30 wt%) of H₂O₂, and 0.25 mmol of 2-(diphenylmethylthio)acetic acid were mixed and reacted at 40 °C with stirring for 6 h, with real-time monitoring by thin-layer chromatography (TLC). After the reaction was completed, the conversion rate was calculated to be 94% and the selectivity >99% by high-performance liquid chromatography (HPLC) combined with external standard method. Comparative Example 1

[0018] m-MgAl-PW 11 Self-assembly synthesis of two-dimensional multilayer nanosheets: 0.33 mmol K7[ α -PW 11 O 39·13H2O was dissolved in deionized water and added dropwise to the monolayer MgAl-LDHs nanosheet suspension from step (2) of Example 1. After stirring for 5 min, a white flocculent precipitate was formed. The precipitate was centrifuged at high speed, washed three times each with deionized water and ethanol, and dried under vacuum at 60 °C for 24 h to obtain m-MgAl-PW with a multilayer stacked structure. 11 catalyst.

[0019] Application in catalytic sulfide oxidation reaction: 10 mg of multilayer m-MgAl-PW 11 The catalyst, 0.3 mmol (30 wt%) of H₂O₂, and 0.25 mmol of 2-(diphenylmethylthio)acetic acid were mixed and reacted at 40 °C with stirring for 6 h, with real-time monitoring by thin-layer chromatography (TLC). After the reaction was completed, quantification was performed by high-performance liquid chromatography (HPLC) combined with external standard method, and the conversion rate was found to be only 40%, with no significant change in selectivity.

[0020] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A single-layer multi-acid-hydrotalcite nanosheet, characterized in that, The nanosheet is a two-dimensional nanosheet assembled sequentially by electrostatic force from a layer of cationic surfactant, a layer of polyacid, and a single layer of hydrotalcite, and the single-layer structure remains stable and does not agglomerate.

2. A method for synthesizing monolayer multiacid-hydrotalcite nanosheets, characterized in that, The specific steps of the synthesis method are as follows: S1. Dissolve the polyacid in deionized water and the cationic surfactant in an organic solvent. Mix the two solutions, stir at room temperature, separate the layers, take the organic phase, dry it with anhydrous sodium sulfate, filter to remove the solid, and obtain a complex solution of polyacid and cationic surfactant. S2. Under nitrogen protection, nitrate-intercalated hydrotalcite is dispersed at 0.5-1.5 mg / mL in formamide or a mixed solvent of formamide and water, and stirred and exfoliated for 2-7 days to obtain a single-layer hydrotalcite suspension with Tyndall effect. S3. Add the monolayer hydrotalcite suspension to the upper layer of the complex solution of polyacid and cationic surfactant, let it stand at room temperature for 48-72 hours to perform interfacial self-assembly, collect the product by centrifugation, wash it with chloroform and methanol in sequence, and dry it to obtain monolayer polyacid-hydrotalcite nanosheets.

3. The synthesis method according to claim 2, characterized in that, The polyacids mentioned are selected from K3PW 12 O 40 K4SiW 12 O 40 K6[MnMo9O 32 ]、K7[ α -PW 11 O 39 ·13H2O, K 10 [P2W 17 O 61 ·12H2O, K 11 [Er(PW 11 O 39 One or more of )2]·13H2O.

4. The synthesis method according to claim 2, characterized in that, The divalent metals on the nitrate-intercalated hydrotalcite plates are one or more of Mg, Zn, Co, Cu, and Ni, and the trivalent metals are one or more of Fe, Al, and Cr.

5. The synthesis method according to claim 2, characterized in that, The cationic surfactant is one or more of the following: dioctadecyl dimethyl ammonium bromide, dihexadecyl dimethyl ammonium bromide, hexadecyl trimethyl ammonium bromide, octadecyl trimethyl ammonium chloride, cationic guar gum, and tetrabutyl ammonium bromide.

6. The synthesis method according to claim 2, characterized in that, The organic solvent is one or more of chloroform, dichloromethane, diethyl ether, chloropropane, carbon tetrachloride, and carbon disulfide.

7. The synthesis method according to claim 2, characterized in that, The volume ratio of formamide to water in the mixed solvent of formamide and water is 10-100:

1.

8. The application of the monolayer polyacid-hydrotalcite nanosheets synthesized according to any one of claims 2-7 in the catalytic oxidation reaction of thioethers.

9. The application according to claim 8, characterized in that, The specific operation of the catalytic sulfide oxidation reaction is as follows: a single layer of polyacid-hydrotalcite nanosheets, hydrogen peroxide and 2-(diphenylmethylthio)acetic acid are mixed and stirred at 30-60℃ for 0.5-6 h. Finally, the product 2-diphenylmethyl sulfoxide acetic acid is separated by column chromatography.

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