Monoatomically substituted polyacid cluster-based two-dimensional nanosheet, method for preparing same, and catalyst

Abstract

The application relates to the technical field of materials, in particular to a single-atom substituted polyacid cluster-based two-dimensional nanosheet, a preparation method thereof and a catalyst, the polyacid cluster-based two-dimensional nanosheet comprising: a plurality of Keggin-type polyacid clusters [PW 11 MO 39 ] connected through intermolecular forces and / or coordination bonds between adjacent Keggin-type polyacid clusters [PW 11 MO 39 ]; and surface ligands electrostatically adsorbed on the surface of the Keggin-type polyacid clusters [PW 11 MO 39 ]; wherein M comprises at least one of rare earth elements and transition metal elements. The two-dimensional nanosheet can be effectively used as a catalyst in photocatalysis, olefin epoxidation and other reactions, has high reactivity, is low in price, does not contain heavy metals and can be recycled and reused.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of materials, in particular to a single-atom substituted polyoxometalate cluster-based two-dimensional nanosheet and a preparation method and catalyst thereof. BACKGROUND

[0002] Clusters with atomic precision structure are a unique aggregation state of matter between atoms and nanocrystals. Due to the similarity of the chemical and electronic properties of clusters, clusters can be used as the "superatom" unit to construct cluster-based assemblies. However, in addition to the atomic / molecular building blocks with a clear connection mode, the isotropic surface of the cluster can greatly limit the synthesis of cluster assembly superstructures with programmable structures and customized properties. SUMMARY

[0003] The present application aims to at least partially solve one of the technical problems in the related art. To this end, the present application provides a single-atom substituted polyoxometalate cluster-based two-dimensional nanosheet with low cost, no heavy metals or recyclable and reusable, and a preparation method and catalyst thereof.

[0004] In a first aspect, the present application provides a single-atom substituted polyoxometalate cluster-based two-dimensional nanosheet. According to an embodiment of the present application, the polyoxometalate cluster-based two-dimensional nanosheet comprises: a plurality of Keggin-type polyoxometalate clusters [PW 11 MO 39 ] connected by intermolecular forces and / or coordination bonds between adjacent Keggin-type polyoxometalate clusters [PW 11 MO 39 ]; surface ligands, the surface ligands being adsorbed on the surface of the Keggin-type polyoxometalate clusters [PW 11 MO 39 ] by electrostatic adsorption; wherein M comprises at least one of a rare earth element and a transition metal element. In other words, the polyoxometalate cluster-based two-dimensional nanosheet is a cluster ordered assembly with cationic surface ligands on the surface. By introducing rare earth and / or transition metal single-atom substitution sites, the present application successfully controls the programmable interaction between and inside the polyoxometalate cluster building blocks, and obtains an ultrathin polyoxometalate cluster nanosheet superstructure, which can be used as a catalyst for photocatalysis and olefin epoxidation, has high catalytic efficiency, is recyclable and reusable, does not contain heavy metals, is low in price, and is environmentally friendly.

[0005] According to an embodiment of the present application, the polyoxometalate cluster-based two-dimensional nanosheet has a strip shape, and adjacent Keggin-type polyoxometalate clusters [PW 11 MO 39The groups are connected by coordination bonds. Specifically, metal M can form coordination bonds with O, and multiple Keggin-type polyacid clusters are then linked together by MO bonds. As an example, multiple Keggin-type polyacid clusters can connect to each other and spread out to form two-dimensional nanosheets. These two-dimensional nanosheets can be effectively used as catalysts in photocatalysis, olefin epoxidation catalysis, and other reactions, exhibiting high catalytic activity.

[0006] In other embodiments, the polyacid cluster-based two-dimensional nanosheets are square in shape and adjacent to the Keggin-type polyacid clusters [PW 11 MO 39 The polyacid clusters are connected by intermolecular forces. In this nanosheet, the polyacid clusters are basically in a monolayer flat state, which can be effectively used as catalysts in reactions such as photocatalysis and olefin epoxidation catalysis, and has high catalytic activity.

[0007] According to embodiments of this application, the Keggin-type polyacid cluster [PW 11 MO 39 The size is 0.7–0.9 nm.

[0008] According to embodiments of this application, the thickness of the polyacid cluster-based two-dimensional nanosheets is 2.5–4 nm. Within this thickness range, the polyacid clusters can achieve a monolayer spreading state, resulting in a larger effective contact area and higher catalytic activity when used for catalysis.

[0009] According to embodiments of this application, the Keggin-type polyacid cluster [PW 11 MO 39 The mass ratio / molar ratio of the surface ligand to the surface ligand is 100:20-30.

[0010] According to embodiments of this application, the M element includes at least one of Y, Pr, Nd, Gd, Dy, Er, Fe, Mn, and Ni.

[0011] According to embodiments of this application, the surface ligand includes at least one selected from hexadecyltrimethylammonium bromide, tetrabutylammonium bromide, and oleylamine. The aforementioned surface ligand can be effectively adsorbed onto the surface of polyacid clusters via electrostatic interactions, protecting the structure of the polyacid clusters.

[0012] A second aspect of this application provides a method for preparing the aforementioned polyacid cluster-based two-dimensional nanosheets. According to embodiments of this application, the method includes: reacting an inorganic salt containing element M with a Keggin-type polyacid cluster H3PW... 12 O 40xH2O is mixed with water to obtain an aqueous solution; the surface ligand is mixed with an organic solvent to obtain an organic solution; the aqueous solution and the organic solution are mixed and reacted to obtain the mono-atom substituted polyacid cluster-based two-dimensional nanosheet.

[0013] According to an embodiment of the present application, the inorganic salt containing the M element includes at least one of a nitrate of the M element and a chloride of the M element.

[0014] According to an embodiment of the present application, the inorganic salt containing the M element, the Keggin-type polyacid cluster H3PW 12 O 40 xH2O can be mixed at a molar ratio of 140-160:40-60. In this way, the two-dimensional nanosheet can be assembled at a suitable ratio.

[0015] According to an embodiment of the present application, the surface ligand includes at least one selected from cetyltrimethylammonium bromide, tetrabutylammonium bromide, and oleylamine. The surface ligand can be effectively adsorbed on the surface of the polyacid cluster by electrostatic action to protect the structure of the polyacid cluster.

[0016] According to an embodiment of the present application, the organic solvent includes at least one of chloroform, toluene, and octadecene. In this way, the surface ligand can be better dissolved, which is conducive to subsequent reactions.

[0017] In some embodiments, the aqueous solution and the organic solution are reacted at a temperature of 50-70°C for 2-5 hours. Within the above temperature range and time range, the polyacid clusters can form a structure connected by coordination bonds, and then assemble to form two-dimensional nanosheets.

[0018] In some embodiments, the aqueous solution and the organic solution are reacted at a temperature of 100-150°C for 2-5 hours. Within the above temperature range and time range, the polyacid clusters can form a structure connected by intermolecular forces, and then assemble to form two-dimensional nanosheets.

[0019] In a third aspect, the present application provides a catalyst. According to an embodiment of the present application, the catalyst includes the polyacid cluster-based two-dimensional nanosheet described above or the polyacid cluster-based two-dimensional nanosheet prepared by the method described above. The catalyst can be effectively used in photocatalysis, olefin epoxidation catalysis, and other reactions, has high catalytic activity, and the polyacid cluster and the surface ligand in the polyacid cluster-based two-dimensional nanosheet superstructure are inexpensive and do not contain heavy metals. Moreover, the catalyst after the catalytic reaction can be recovered by centrifugation to obtain a pure product for repeated use. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 FIG. 1 is a transmission electron microscope (TEM) image of a mono-atom substituted polyacid cluster-based two-dimensional nanosheet prepared in Example 1 of the present application.

[0021] Figure 2 is a transmission electron microscope photo of a monosubstituted polyoxometalate cluster-based two-dimensional nanosheet prepared in Embodiment 7 of the present application.

[0022] Figure 3 is a graph of the yield and turnover frequency test results of monosubstituted polyoxometalate cluster-based two-dimensional nanosheets prepared in Embodiments 1 and 7 of the present application for olefin epoxidation reactions. DETAILED DESCRIPTION

[0023] Embodiments of the present application are described in detail below with reference to examples illustrated in the accompanying drawings. The embodiments described below by reference to the accompanying drawings are exemplary and are intended to explain the present application, and are not to be understood as limiting the present application.

[0024] The present application is made based on the discovery and realization of the inventors on the following facts and problems:

[0025] In order to overcome the limitation of the isotropic surface of the cluster on the synthesis of cluster assembly superstructure, the inventors propose that a single metal atom can be introduced into a specific position of the polyoxometalate cluster, which can serve as an anisotropic interaction site of directional coordination bond in modular synthesis; at the same time, the polyoxometalate cluster with a high negative charged surface can be easily modified by cationic surfactants with different alkyl chain lengths, so that the programmable adjustment of the coordination site, number and orientation can be guided by adjustable non-covalent interaction. Based on these anisotropic clusters, the inventors successfully realized the synthesis of monolayer two-dimensional nanosheet superstructure.

[0026] Therefore, in a first aspect, the present application provides a monosubstituted polyoxometalate cluster-based two-dimensional nanosheet. According to embodiments of the present application, the monosubstituted polyoxometalate cluster-based two-dimensional nanosheet comprises: a plurality of Keggin-type polyoxometalate clusters [PW 11 MO 39 ] connected by intermolecular forces and / or coordination bonds between adjacent Keggin-type polyoxometalate clusters [PW 11 MO 39 ]; and a surface ligand, the surface ligand being adsorbed on the Keggin-type polyoxometalate cluster [PW 11 MO 39surface; wherein M comprises at least one of rare earth elements, transition metal elements. In other words, the polyacid cluster-based two-dimensional nanosheet is a cluster ordered assembly with surface cationic surface ligand coating. By introducing rare earth and / or transition metal single atom substitution sites, the programmable interaction between and inside the polyacid cluster building blocks is controlled, and an ultra-thin polyacid cluster nanosheet superstructure is successfully obtained, which can be used as a catalyst in photocatalysis and olefin epoxidation catalytic reactions, has high catalytic efficiency, can be recycled and reused, does not contain heavy metals, is low in price, and is environmentally friendly.

[0027] According to the embodiments of the present application, the shape of the polyacid cluster-based two-dimensional nanosheet is not particularly limited, and in some embodiments, the shape of the polyacid cluster-based two-dimensional nanosheet is strip-shaped, and adjacent Keggin-type polyacid clusters [PW 11 MO 39 ] are connected by coordination bonds. Specifically, the metal M can form a coordination bond with O, and then the plurality of Keggin-type polyacid clusters are connected together by M-O bonds. As an example, the plurality of Keggin-type polyacid clusters can be connected to each other and laid out to form a two-dimensional nanosheet. The two-dimensional nanosheet can be effectively used in photocatalysis, olefin epoxidation catalysis and the like, and has high catalytic activity.

[0028] In other embodiments, the shape of the polyacid cluster-based two-dimensional nanosheet is square-shaped, and adjacent Keggin-type polyacid clusters [PW 11 MO 39 ] are connected by intermolecular forces. In the nanosheet, the polyacid clusters are basically in a single-layer laid-out state, and can be effectively used in photocatalysis, olefin epoxidation catalysis and the like, and has high catalytic activity.

[0029] It can be understood that "square" herein includes square, rectangular, approximately square, and approximately rectangular shapes. The intermolecular force refers to van der Waals force, which refers to relatively weak interaction between molecules, including but not limited to hydrogen bond, dipole-dipole interaction, London dispersion force, etc.

[0030] According to the embodiments of the present application, the size of the Keggin-type polyacid cluster [PW 11 MO 39 ] is 0.7-0.9 nm, specifically 0.7 nm, 0.72 nm, 0.74 nm, 0.76 nm, 0.78 nm, 0.8 nm, 0.82 nm, 0.84 nm, 0.86 nm, 0.88 nm, 0.9 nm, etc. Within the above size range, the cluster can be used as an atomically precise assembly building block model to explore the assembly behavior of sub-nanometer materials.

[0031] Herein, the Keggin-type polyacid cluster [PW11 MO 39 The size of the MO

[0032] According to an embodiment of the present application, the polyoxometalate cluster-based two-dimensional nanosheet has a thickness of 2.5-4 nm, specifically, 2.5 nm, 2.8 nm, 3 nm, 3.2 nm, 3.5 nm, 3.8 nm, 4 nm, etc. Within the above thickness range, the polyoxometalate cluster group can achieve a single-layer spreading state, and when used for catalysis, the effective contact area is larger, and higher catalytic activity can be achieved.

[0033] In this document, the thickness of the polyoxometalate cluster-based two-dimensional nanosheet can be detected by an atomic force microscope test method.

[0034] According to an embodiment of the present application, the Keggin-type polyoxometalate cluster [PW 11 MO 39 The mass ratio of the MO

[0035] According to an embodiment of the present application, the M element includes at least one of Y, Pr, Nd, Gd, Dy, Er, Fe, Mn, Ni, etc.

[0036] According to an embodiment of the present application, the surface ligand includes at least one selected from cetyltrimethylammonium bromide, tetrabutylammonium bromide, and oleylamine. The above surface ligand can be effectively adsorbed on the surface of the polyoxometalate cluster by electrostatic action to protect the structure of the polyoxometalate cluster.

[0037] The second aspect of the present application provides a method for preparing the polyoxometalate cluster-based two-dimensional nanosheet described above. According to an embodiment of the present application, the method comprises:

[0038] S1: mixing an inorganic salt containing an M element, a Keggin-type polyoxometalate cluster H3PW 12 O 40 ·xH2O with water to obtain an aqueous solution, wherein x is an uncertain value.

[0039] According to an embodiment of the present application, the inorganic salt containing the M element includes at least one of a nitrate of the M element and a chloride of the M element. As an example, the inorganic salt containing the M element can be yttrium nitrate, praseodymium nitrate, neodymium nitrate, gadolinium nitrate, dysprosium nitrate, erbium chloride, iron nitrate, manganese nitrate, nickel nitrate, etc.

[0040] According to an embodiment of the present application, the inorganic salt containing M element, the Keggin-type polyacid cluster H3PW 12 O 40 The xH2O can be mixed in a molar ratio of 140-160:40-60 (specifically, 140:40, 145:40, 150:40, 155:40, 160:40, 140:45, 140:50, 140:55, 140:60, etc.). In this way, the two-dimensional nanosheet can be assembled in a suitable ratio.

[0041] It should be noted that the H3PW 12 O 40 In the xH2O, x refers to the number of crystal water, which can be 0 or not 0. The H3PW 12 O 40 The xH2O can be directly purchased, and the x value is not explicitly marked in the directly purchased product, so it is not limited in this paper, and the directly purchased product can be used.

[0042] S2: mixing the surface ligand with the organic solvent to obtain an organic phase solution.

[0043] According to an embodiment of the present application, the surface ligand includes at least one selected from cetyltrimethylammonium bromide, tetrabutylammonium bromide, and oleylamine. The above-mentioned surface ligand can be effectively adsorbed on the surface of the polyacid cluster by electrostatic action, and protect the structure of the polyacid cluster.

[0044] According to an embodiment of the present application, the organic solvent includes at least one of chloroform, toluene, and octadecene. In this way, the surface ligand can be better dissolved, which is beneficial to subsequent reactions.

[0045] S3: mixing and reacting the aqueous phase solution and the organic phase solution to obtain the mono-atom substituted polyacid cluster-based two-dimensional nanosheet.

[0046] In this step, after the aqueous phase solution and the organic phase solution are mixed, phase separation occurs. The inorganic salt containing M element, the Keggin-type polyacid cluster H3PW 12 O 40 The surface ligand in the organic phase solution reacts with the xH2O at the interface to generate a monolayer of polyacid cluster-based two-dimensional nanosheet.

[0047] According to an embodiment of the present application, by setting different reaction conditions, the polyacid clusters can be connected by different forces to form two-dimensional nanosheets. In other words, by gradually adjusting the reaction time, reaction temperature and ligand content to balance the interaction, two-dimensional nanosheets of two different phases can be obtained.

[0048] In some embodiments, the temperature at which the aqueous solution and the organic solution are reacted is 50-70°C (e.g., 50°C, 52°C, 54°C, 56°C, 58°C, 60°C, 62°C, 64°C, 66°C, 68°C, 70°C, etc.), and the reaction time is 2-5h (e.g., 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, etc.). Within the above temperature range and time range, the polyacid clusters can form a structure connected by coordination bonds, and then assemble to form two-dimensional nanosheets.

[0049] In some embodiments, the temperature at which the aqueous solution and the organic solution are reacted is 100-150°C (e.g., 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, etc.), and the reaction time is 2-5h (e.g., 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, etc.). Within the above temperature range and time range, the polyacid clusters can form a structure connected by intermolecular forces, and then assemble to form two-dimensional nanosheets.

[0050] It can be understood that, after the reaction of the organic phase solution and the aqueous phase solution is completed, the obtained solid-liquid mixed system can be directly subjected to a step of separating the polyacid cluster-based two-dimensional nanosheets. For example, the solution obtained after the reaction can be subjected to centrifugation and washing to obtain the polyacid cluster-based two-dimensional nanosheets.

[0051] In some embodiments, non-polar and / or weakly polar solvents and polar solvent solutions can be used for washing and centrifugation. For example, the solvents that can be used include, but are not limited to, chloroform, dichloromethane, methane, ethanol, isopropanol, acetone, and methanol.

[0052] In a third aspect, the present application provides a catalyst. According to embodiments of the present application, the catalyst comprises the polyacid cluster-based two-dimensional nanosheets described above or the polyacid cluster-based two-dimensional nanosheets prepared by the method described above. The catalyst can be effectively used in photocatalysis, olefin epoxidation catalysis, and other reactions, and has high catalytic activity. The polyacid clusters in the superstructure of the polyacid cluster-based two-dimensional nanosheets are inexpensive and do not contain heavy metals. Moreover, the catalyst after the catalytic reaction can be recovered by centrifugation to obtain a pure product for repeated use.

[0053] Embodiments of the present application are described in detail below.

[0054] Embodiment 1

[0055] Synthesis of polyacid cluster-based belt-shaped nanosheets (Nanosheet-1, according to the results of synchrotron radiation test analysis, the clusters are directly connected by coordination bonds)

[0056] 50 μmol H3PW 12 O40 • xH2O (Aladdin, P100467) and 150 pmol Nd(N03)3were dissolved in 4 mL deionized water with stirring for 48 h to obtain the aqueous phase. CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL chloroform to obtain the organic phase. Then, the organic phase was added dropwise into the aqueous phase with vigorous stirring for 10-30 min. Then, the mixture was transferred into a 10 mL Teflon high-pressure reactor. After heating to 60 °C for 3 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product. The TEM image is shown in Figure 2. Figure 1 .

[0057] Example 2

[0058] 50 pmol H3PW 12 O 40 • xH2O and 140 pmol Er(N03)3were dissolved in 4 mL deionized water with stirring for 48 h, and CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL chloroform to obtain the organic phase. Then, the organic phase was added dropwise into the aqueous phase with vigorous stirring for 10-30 min. Then, the mixture was transferred into a 10 mL Teflon high-pressure reactor. After heating to 60 °C for 3 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product.

[0059] Example 3

[0060] 50 pmol H3PW 12 O 40 • xH2O and 150 pmol Fe(N03)3were dissolved in 4 mL deionized water with stirring for 48 h, and CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL chloroform to obtain the organic phase. Then, the organic phase was added dropwise into the aqueous phase with vigorous stirring for 10-30 min. Then, the mixture was transferred into a 10 mL Teflon high-pressure reactor. After heating to 55 °C for 3 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product.

[0061] Example 4

[0062] 50 pmol H3PW 12 O 40 • xH2O and 140 pmol Dy(N03)3were dissolved in 4 mL deionized water with stirring for 48 h, and CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL chloroform to obtain the organic phase. Then, the organic phase was added dropwise into the aqueous phase with vigorous stirring for 10-30 min. Then, the mixture was transferred into a 10 mL Teflon high-pressure reactor. After heating to 60 °C for 2 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product.

[0063] Example 5

[0064] 50 μmol H3PW 12 O 40 ·xH2O and 150 μmol Y(NO3)3were dissolved in 4 mL deionized water with stirring for 48 h, CTAB (15 mg) was dissolved in 6 mL chloroform to obtain the organic phase. Then, the organic phase was dropped into the water phase with strong stirring for 10-30 min. Then the mixture was transferred into a 10 mL Teflon high-pressure reactor. After heating to 60 °C, the reaction was carried out for 3 h, the organic phase was separated, washed with chloroform and centrifuged to obtain the white product.

[0065] Example 6

[0066] 50 μmol H3PW 12 O 40 ·xH2O and 150 μmol Mn(NO3)2were dissolved in 4 mL deionized water with stirring for 48 h, CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL chloroform to obtain the organic phase. Then, the organic phase was dropped into the water phase with strong stirring for 10-30 min. Then the mixture was transferred into a 10 mL Teflon high-pressure reactor. After heating to 60 °C, the reaction was carried out for 3 h, the organic phase was separated, washed with chloroform and centrifuged to obtain the white product.

[0067] Example 7

[0068] Synthesis of polyacid cluster-based square nanosheets (nanosheet-2, according to the results of synchrotron radiation test, the clusters are indirectly connected by intermolecular forces)

[0069] 50 μmol H3PW 12 O 40 ·xH2O and 150 μmol Nd(NO3)3were dissolved in 4 mL deionized water with stirring for 50 h to obtain the water phase. CTAB (9 mg) and TBAB (7 mg) were dissolved in 8 mL chloroform to obtain the organic phase. Then, the organic phase was dropped into the water phase with strong stirring for 10-30 min. Then the mixture was transferred into a 10 mL Teflon high-pressure reactor. After heating to 120 °C, the reaction was carried out for 3 h, the organic phase was separated, washed with chloroform and centrifuged to obtain the white product, and the transmission electron microscope photo is shown in Figure 2 .

[0070] Example 8

[0071] 50 μmol H3PW 12 O 40xH₂O and 145 μmol Dy(NO₃)₃ were dissolved in 4 mL of deionized water and stirred for 48 h. CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL of chloroform to obtain the organic phase. Then, the organic phase was added dropwise to the aqueous phase, and the mixture was stirred vigorously for 10–30 min. The mixture was then transferred to a 10 mL Teflon high-pressure reactor. After heating to 120 °C and reacting for 2 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product.

[0072] Example 9

[0073] 50 μmol H3PW 12 O 40 xH₂O and 150 μmol Ni(NO₃)₃ were dissolved in 4 mL of deionized water and stirred for 48 h. CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL of chloroform to obtain the organic phase. The organic phase was then added dropwise to the aqueous phase, and the mixture was stirred vigorously for 10–30 min. The mixture was then transferred to a 10 mL Teflon high-pressure reactor. After heating to 110 °C and reacting for 3 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product.

[0074] Example 10

[0075] 50 μmol H3PW 12 O 40 xH₂O and 140 μmol Er(NO₃)₃ were dissolved in 4 mL of deionized water and stirred for 48 h. CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL of chloroform to obtain the organic phase. The organic phase was then added dropwise to the aqueous phase, and the mixture was stirred vigorously for 10–30 min. The mixture was then transferred to a 10 mL Teflon high-pressure reactor. After heating to 60 °C and reacting for 2 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product.

[0076] Example 11

[0077] 50 μmol H3PW 12 O 40 xH₂O and 150 μmol Y(NO₃)₃ were dissolved in 4 mL of deionized water and stirred for 48 h. CTAB (7 mg), TBAB (6 mg), and OAM (2 mg) were dissolved in 6 mL of chloroform to obtain the organic phase. The organic phase was then added dropwise to the aqueous phase, and the mixture was stirred vigorously for 10–30 min. The mixture was then transferred to a 10 mL Teflon high-pressure reactor. After heating to 60 °C and reacting for 3 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product.

[0078] Example 12

[0079] 50 μmol H3PW12 O 40 • xH2O and 150 μmol Fe(NO3)3 were dissolved in 4 mL deionized water with stirring for 48 h, and CTAB (7 mg) and TBAB (6 mg) were dissolved in 6 mL chloroform to obtain an organic phase. Then, the organic phase was added dropwise into the water phase with strong stirring for 10-30 min. Then, the mixture was transferred into a 10 mL Teflon high-pressure reaction kettle. After heating to 120 °C and reaction for 3 h, the organic phase was separated, washed with chloroform, and centrifuged to obtain a white product.

[0080] Performance test:

[0081] 1. Catalytic epoxidation of olefins by polyacid cluster-based two-dimensional nanosheets: 350 μmol of an olefin (cyclooctene), no less than 10 mg of the two-dimensional nanosheet catalyst prepared in the examples, and 200 μL of H2O2 were added in 10 mL of CHCl3. The reaction was continuously magnetically stirred at 25 °C. After the reaction was completed, the supernatant after centrifugation was quantitatively dispersed in ethanol, and GC-MS was used for detection and analysis. The internal standard method (toluene) was used for quantitative analysis of the product yield. The yield and turnover frequency test results are shown in Table 1, and the yield and turnover frequency results of Example 1 and Example 7 are shown in Figure 3 .

[0082] 2. Stability test: The precipitate (i.e., two-dimensional nanosheets) obtained by centrifugation in the above 1 was washed with ethanol, which can be centrifuged and washed, and then the washed precipitate was added to chloroform and cyclooctene, and the catalytic epoxidation of olefins in the above 1 was repeated. The turnover frequency test results of the 10th catalytic reaction are shown in Table 1.

[0083] 3. Cluster size: The size of the cluster was obtained by testing the actual sample by atomic force microscopy.

[0084] 4. Thickness of two-dimensional nanosheets: The thickness was tested by atomic force microscopy.

[0085] Table 1

[0086]

[0087] From the test results in the above Table 1, it can be seen that the two-dimensional nanosheets of the present application can be effectively used to catalyze the epoxidation of olefins, have high catalytic activity, high product yield, and can be recovered and reused after simple centrifugal washing.

[0088] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are contained in at least one embodiment or example of the present application. In the specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any suitable manner in any one or more embodiments or examples. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the specification and the features of the different embodiments or examples without contradiction.

[0089] Although the embodiments of the present application have been shown and described above, it is understood that the above-described embodiments are exemplary and are not to be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A method of preparing monoatomically substituted polyacid cluster-based two-dimensional nanosheets, characterized by, The method comprises the following steps: An inorganic salt containing M element, a Keggin type polyacid cluster H3PW 12 O 40 and water to obtain an aqueous solution; wherein M includes at least one of rare earth elements and transition metal elements; a molar ratio of the Keggin type polyacid cluster and the inorganic salt containing M element is 40-60: 140-160. mixing a surface ligand with an organic solvent to obtain an organic phase solution; wherein the surface ligand comprises at least one selected from cetyltrimethylammonium bromide, tetrabutylammonium bromide and oleylamine; mixing and reacting the aqueous phase solution and the organic phase solution to obtain the mono-atom substituted polyacid cluster-based two-dimensional nanosheet; wherein the aqueous phase solution and the organic phase solution are reacted at a temperature of 50-70 DEG C for 2-5 hours, or the aqueous phase solution and the organic phase solution are reacted at a temperature of 100-150 DEG C for 2-5 hours; The mono-atom substituted polyacid cluster-based two-dimensional nanosheet comprises: a plurality of Keggin-type polyoxometalate clusters [PW 11 MO 39 ] connected by intermolecular forces and / or coordination bonds between adjacent said Keggin-type polyoxometalate clusters [PW 11 MO 39 ] The surface ligands are electrostatically adsorbed on the Keggin-type polyoxometalate cluster [PW 11 MO 39 ] surface.

2. The method of claim 1, wherein, at least one of the following conditions is met: the inorganic salt containing the M element comprises at least one of a nitrate of the M element and a chloride of the M element; the organic solvent comprises at least one of chloroform, toluene and octadecene.

3. The method of claim 1, wherein, The polyacid cluster-based two-dimensional nanosheet has a strip shape, and adjacent Keggin-type polyacid clusters [PW 11 MO 39 ] are connected by coordination bonds.

4. The method of claim 1, wherein, The polyacid cluster-based two-dimensional nanosheet has a square shape, and adjacent Keggin-type polyacid clusters [PW 11 MO 39 ] are connected by intermolecular forces.

5. The method of claim 1, wherein, at least one of the following conditions is met: The Keggin-type polyoxometalate cluster [PW 11 MO 39 ] has a size of 0.7-0.9 nm; the thickness of the polyacid cluster-based two-dimensional nanosheet is 2.5-4 nm; The mass ratio of the Keggin-type polyoxometalate cluster [PW 11 MO 39 ] to the surface ligand is 100:20~30; the M comprises at least one of Y, Pr, Nd, Gd, Dy, Er, Fe, Mn and Ni.

6. A catalyst characterized by, The polyacid cluster-based two-dimensional nanosheet prepared by the method in any one of claims 1-5.

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