Synthesis method of a polyacid MOF hydrotalcite ternary multi-level structure composite material

Abstract

This invention discloses a method for synthesizing a multi-acid MOF-hydrotalcite ternary hierarchical composite material. Through an in-situ growth strategy, this invention successfully constructs a highly efficient acid-base catalyst composite material with an acidic polyacid supported within the MOF cavity and a basic hydrotalcite supported on the outer surface of the MOF crystals, all three components maintaining a stable flower-like octahedral structure. Through assembly and design, this invention ensures that the acid and base active sites are distributed respectively inside and on the outer surface of the MOF, enabling precise control of the sequence of acid-base cascade reactions. The shortened reaction path improves mass transfer efficiency. Because this composite material retains the structure of the MOF, it utilizes the MOF to achieve spatial separation of the acid and base active sites, allowing acid-catalyzed and base-catalyzed reactions to proceed independently without interference or mutual influence, significantly improving reaction efficiency.

Description

Technical Field

[0001] This invention belongs to the field of inorganic composite material synthesis technology, specifically relating to a method for synthesizing a multi-acid MOF hydrotalcite ternary multi-level structure composite material. Background Technology

[0002] Multi-level nanocomposite materials compensate for the limitations of single-functional materials, providing more possibilities for expanding their application fields. Polyoxometalates (POMs) are nanoscale polynuclear inorganic functional metal oxides formed by the coordination of a series of transition metal ions (such as W, Mo, V, etc.) in higher valence states with oxygen atom clusters. Their well-defined composition and structure, tunable acidity and redox capabilities, high thermal stability, and oxidation performance greatly promote their application in catalysis. Layered double hydroxides (LDHs) are two-dimensional anionic clay materials composed of positively charged lamellar cations and interlayer anions. Due to their tunable lamellar chemical composition and interlayer anions, large specific surface area, and abundant basic sites, they have broad application prospects in heterogeneous catalysis. POM-LDH composite materials possess multiple active sites and can be used for dehydrogenation reactions (Gabriela Carja, et al., J. Catal., 2002, 209, 16), oxidative desulfurization (Yu-Fei Song, et al., Inorg. Chem. Front., 2016, 3, 1007), dye adsorption (Mohand Said Ouali, et al., J. Hazard. Mater., 2008, 153, 911), fluorescent materials (Yu-Fei Song, et al., Inorg. Chem. Front., 2021, 8, 1324), benzyl alcohol oxidation-Nevonhal condensation tandem reaction (Yu-Fei Song, et al., ChemCatChem, 2016, 8, 929), etc. Serial reactions refer to the process of converting reactants into complex products through two or more consecutive catalytic reactions in a single reactor. This process simplifies operation and reduces energy consumption, and has become an increasingly important research area.

[0003] However, if the catalyst cannot achieve spatial separation of different types of active sites, mutual interference and influence can easily occur in continuous catalytic reactions. For POM and LDH, which have acidic and basic active sites respectively, simple composite or intercalation methods cannot solve the problem of acid-base neutralization. Summary of the Invention

[0004] The purpose of this invention is to provide a method for synthesizing a multi-acid MOF hydrotalcite ternary multi-level composite material, which solves the technical problem of existing POM-LDH tandem catalysts being prone to neutralization due to the inability to achieve spatial separation of acid and base sites.

[0005] The structure of the polyacid MOF hydrotalcite ternary multi-level composite material is that the acidic polyacid is loaded in the MOF cavity, and the alkaline hydrotalcite is loaded on the outer surface of the MOF crystal. All three components maintain a stable structure.

[0006] The synthesis method of the aforementioned multi-acid MOF hydrotalcite ternary hierarchical composite material is as follows:

[0007] S1. The metal-organic framework material containing polyacids is uniformly dispersed in methanol and denoted as solution A.

[0008] S2. Dissolve the transition metal salt, copper salt, and alkaline source in methanol, and denote the solution as solution B;

[0009] S3. Mix solution A and solution B, transfer to a hydrothermal reactor for hydrothermal reaction, separate the products after the reaction is complete, and dry to obtain polyacid MOF hydrotalcite ternary multi-level structure composite material.

[0010] The polyacid in the internally encapsulated metal-organic framework material is of the Keggin type; the metal-organic framework material is Cu. 2+ The three-dimensional framework structure coordinated with trimesic acid is commonly known as HKUST-1. The structural characteristics of the metal-organic framework material encapsulating polyacids are as follows: the polyacid molecules are located within the cavities of the metal-organic framework and do not interact with Cu. 2+ It is coordinated and its morphological characteristics are octahedral nanocrystals.

[0011] The method for synthesizing the metal-organic framework material containing polyacids is as follows: copper nitrate and Keggin-type polyacids are mixed evenly in water, and then added dropwise to a methanol solution of pyromellitic acid to produce a green precipitate, which is then separated by centrifugation.

[0012] The Keggin-type polyacid mentioned is H3PW 12 O 40 H3PMo 12 O 40 H4SiW 12 O 40 One or more of them.

[0013] The metal cation of the transition metal salt is Fe. 2+ Mn 2+ Co 2+ Ni 2+ Mg 2+ Zn2+ Fe 3+ Al 3+ Cr 3+ One or two of them.

[0014] The anions of the transition metal salt are one or more of sulfate, acetate, nitrate, carbonate, chloride, perchlorate, and hexafluorophosphate.

[0015] The copper salt is one or more of copper sulfate, copper chloride, copper perchlorate, copper acetate, copper nitrate, and basic copper carbonate.

[0016] The alkaline source is one or more of NaOH, KOH, Na2CO3, urea, and hexamethylenetetramine.

[0017] The mass ratio of the internally encapsulated multi-acid metal-organic framework material, transition metal salt, copper salt, and alkali source is 3-9: 2-6: 1-5: 4-9.

[0018] The temperature of the hydrothermal reaction is 60-180 degrees Celsius.

[0019] The hydrothermal reaction time is 2-72 hours.

[0020] The specific operation for separating the product is as follows: after the reaction is completed, remove the supernatant in the reaction vessel, then add methanol, stir, and after the nanocrystals settle, pour off the supernatant liquid. Repeat the operation 1-5 times.

[0021] This invention successfully constructed a highly efficient acid-base catalyst with a flower-like octahedral structure of POM@MOF@LDH through an in-situ growth strategy. During the in-situ growth of LDH on the outer surface of MOF, its regular octahedral structure was preserved, and its advantages of high porosity and large specific surface area were utilized. The resulting ternary composite material has several advantages in the application of catalytic acid-base tandem reactions: (1) Through assembly and design, the acid and base active sites are distributed in the interior and outer surface of MOF, respectively, which can precisely control the order of acid-base tandem reactions, and the shortening of the reaction path improves the mass transfer efficiency. (2) Since the structure of MOF is preserved, the spatial separation of acid and base active sites is achieved by using MOF, and acid catalysis and base catalysis can be carried out independently without mutual interference or influence, which greatly improves the reaction efficiency. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the reaction process in Example 1.

[0023] Figure 2 The images show SEM and TEM images of NENU-5@CuCo-LDH during the synthesis process of Example 1.

[0024] Figure 3 The image shows the XRD pattern of NENU-5@CuCo-LDH from Example 1.

[0025] Figure 4 The image shows the FT-IR plot of NENU-5@CuCo-LDH from Example 1. Detailed Implementation

[0026] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention. Example

[0027] (1) Mix 0.24 g of copper nitrate trihydrate and 0.21 g of phosphomolybdic acid hydrate (H3PMo) 12 O 40 PMo (abbreviated as PMo) 12 Dissolve the compound in 10 mL of water to obtain solution A. Then adjust the pH of solution A to 2.5 with 1M NaOH solution.

[0028] (2) Dissolve 0.14 g of pyromellitic acid in 10 mL of methanol to obtain solution B;

[0029] (3) Under continuous stirring at room temperature, solution B was added dropwise to solution A, and a green precipitate gradually appeared. After the addition was complete, the product was collected by centrifugation and washing to obtain the precursor NENU-5;

[0030] (4) Dissolve 0.25 g of cobalt nitrate tetrahydrate, 0.20 g of copper nitrate and 0.72 g of urea in 60 mL of methanol to obtain solution C; take 0.06 g of NENU-5 and sonicate it in 10 mL of methanol to make reaction solution D.

[0031] (5) Mix reaction solutions C and D and transfer them to a hydrothermal reactor. Place the reactor in an oven at 120 °C and react for 24 h. After the reaction is complete, remove the supernatant in the reactor, add methanol, stir, and after the nanocrystals settle, discard the supernatant. Repeat the operation 3 times. After drying, the POM@MOF@LDH ternary multi-level composite material can be obtained.

[0032] Material characterization reveals the following: 1. The polyacids are uniformly dispersed within the MOF channels, preserving their molecular structure and acidity characteristics in the composite structure; 2. The MOF spatially separates the acidic POM molecules and the basic LDH molecules while retaining their porous properties; 3. The LDH molecules are uniformly interwoven on the outer surface, forming an ultrathin nanosheet morphology, and are tightly bound to the MOF through coordination bonds, possessing basic sites.

[0033] 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 multi-acid MOF hydrotalcite ternary hierarchical composite material, characterized in that, The composite material has an acidic polyacid loaded inside the MOF cavity and an alkaline hydrotalcite loaded on the outer surface of the MOF crystal, with all three components maintaining a stable structure.

2. A method for synthesizing a multi-acid MOF hydrotalcite ternary hierarchical composite material, characterized in that, The specific steps of the synthesis method are as follows: S1. The metal-organic framework material containing polyacids is uniformly dispersed in methanol and denoted as solution A. S2. Dissolve the transition metal salt, copper salt, and alkaline source in methanol, and denote the solution as solution B; S3. Mix solution A and solution B, transfer to a hydrothermal reactor for hydrothermal reaction, separate the products after the reaction is complete, and dry to obtain polyacid MOF hydrotalcite ternary multi-level structure composite material.

3. The synthesis method according to claim 2, characterized in that, The polyacid in the internally encapsulated metal-organic framework material is of the Keggin type, and the metal-organic framework material is Cu. 2+ A three-dimensional framework structure coordinated with pyromellitic acid.

4. The synthesis method according to claim 2, characterized in that, The metal cation of the transition metal salt is Fe. 2 + Mn 2+ Co 2+ Ni 2+ Mg 2+ Zn 2+ Fe 3+ Al 3+ Cr 3+ One or two of them.

5. The synthesis method according to claim 2, characterized in that, The anions of the transition metal salt are one or more of sulfate, acetate, nitrate, carbonate, chloride, perchlorate, and hexafluorophosphate.

6. The synthesis method according to claim 2, characterized in that, The copper salt is one or more of copper sulfate, copper chloride, copper perchlorate, copper acetate, copper nitrate, and basic copper carbonate.

7. The synthesis method according to claim 2, characterized in that, The alkaline source is one or more of NaOH, KOH, Na2CO3, urea, and hexamethylenetetramine.

8. The synthesis method according to claim 2, characterized in that, The mass ratio of the internally encapsulated multi-acid metal-organic framework material, transition metal salt, copper salt, and alkali source is 3-9: 2-6: 1-5: 4-9.

9. The synthesis method according to claim 2, characterized in that, The hydrothermal reaction temperature is 60-180 degrees Celsius, and the hydrothermal reaction time is 2-72 hours.

10. The synthesis method according to claim 2, characterized in that, The specific operation for separating the product is as follows: after the reaction is completed, remove the supernatant in the reaction vessel, then add methanol, stir, and after the nanocrystals settle, pour off the supernatant liquid. Repeat the operation 1-5 times.

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