CN119657225A - Cu-POM catalyst and preparation method and application thereof - Google Patents

Abstract

The present invention belongs to the field of catalyst technology, and specifically relates to a Cu-POM type catalyst and its preparation method and application. The Cu-POM type catalyst provided by the present invention is an Anderson type polyoxometalate centered on Cu, which is prepared by connecting the metal element copper with the polyoxometalate, and then undergoing oil bath, filtration and crystallization and other steps. The Cu-POM prepared by the present invention has the advantages of low cost, simple synthesis conditions and low toxicity as a catalyst for esterification reaction; the catalyst is used to catalyze the synthesis of esters with high yield, can achieve high catalytic efficiency, and conforms to the principle of green environmental protection. At the same time, Cu-POM has high stability and high activity, and can achieve multi-batch recycling.

Description

Cu-POM catalyst and preparation method and application thereof

Technical Field

The invention belongs to the technical field of catalysts, and particularly relates to a Cu-POM type catalyst, and a preparation method and application thereof.

Background

The ester compound is taken as one of important chemical substances in the nature, is widely used in foods, fragrances and medicines, and has important significance especially in the existence of esters in antibiotics medicines. Ester compounds are widely used in various chemical and biological substances, and are synthesized by various ways, wherein carboxylic acid or derivatives thereof such as acyl chloride, anhydride and the like are most commonly reacted with corresponding alcohols. The traditional method for synthesizing the esters cannot directly synthesize the esters by one step, unnecessary byproducts can be generated in the process to interfere with the synthesis of target products, and certain influence can be caused on the separation of the target products, so that the development of the method for efficiently preparing the esters has important significance.

The catalyst is used as a core element of catalytic reaction, and has the effects of saving reaction time, optimizing reaction conditions, reducing reaction cost, increasing reaction yield and the like. Many related catalytic systems for the preparation of esters have been reported, including both homogeneous and heterogeneous catalytic systems. The active catalyst formed by combining metal elements and organic ligands is mainly composed of gold, palladium, ruthenium, copper and the like, and halogen elements harmful to the environment are adopted as the catalysts for the reaction in some reports, and the active catalyst is high in efficiency but has great harm to the environment and the like. Under the cooperation of organic ligands, the metal-based catalyst can always exert high-efficiency catalytic action, but organic ligand components in the catalyst are often complicated in structure and high in price, and certain organic ligands have strict requirements on synthesis conditions, are unstable in reaction and become limiting factors for large-scale application of homogeneous catalysts. In the case of heterogeneous catalysts, the synthesis process is often complicated and requires stringent conditions. The transition metal catalytic system has higher selectivity and reactivity. The transition metal complex is supported and coordinated by an organic ligand and the like, and the organic ligand has high cost, complex preparation process and easy influence of external conditions. The search for new ligands to support coordinated transition metals to form new catalytic systems is of great interest for industrial applications.

Therefore, there is a need to develop a new catalyst to effectively avoid the drawbacks of the above-described methods.

Disclosure of Invention

Polyacids, also known as polyoxometallate POMs, are a subset of discrete metal oxyanion clusters, a cluster-type compound composed of heteroatoms and multiple atoms through oxygen bridges and having a specific structure and composition, and are varied in size. And it has the functional characteristics of a plurality of different physical and chemical fields. The polyanions of polyacids are somewhat centered on main group heteroatoms, which have a great influence on their properties, including silicon, phosphorus, sulfur, etc., and even some transition metal elements such as iron, cobalt may be present in the polyacid structure in the form of heteroatoms.

Aiming at the defects of the prior art, the invention aims to provide the Anderson type polyoxometallate with Cu as the center, namely Cu-POM, which is prepared by connecting metal Cu with polyoxometallate, and then researching the reaction of benzaldehyde and methanol as templates by using Cu-POM as a catalyst, and after the optimal reaction condition is determined, carrying out a substrate applicability experiment, and the result shows that the obtained product has the characteristics of better yield and multiple substrate applicability, the synthesized product is effectively utilized, accords with the green environmental protection principle, and has higher application value for industrialized application of ester synthesis.

The invention aims at realizing the following technical scheme:

in a first aspect, the present invention provides a Cu-POM-type catalyst, wherein the Cu-POM-type catalyst is an Anderson-type polyoxometalate centered on Cu, and the Cu-POM has a structural formula of (NH ₄)₂[CuMo₆O₁₈(OH)₆), and the structural formula is as follows:

in a second aspect, the invention also provides a preparation method of the Cu-POM type catalyst, which comprises the steps of connecting metal element ketone with the polyoxometallate, and preparing the Cu-POM type catalyst into a crystalline state through oil bath, filtration and crystallization.

As a preferred embodiment, the preparation method of the Cu-POM type catalyst according to the present invention comprises the steps of:

Step S1, firstly, accurately weighing a proper amount of polyoxometallate crystals, dissolving the polyoxometallate crystals in a proper amount of distilled water, and stirring the mixture under an oil bath to prepare a polyoxometallate solution;

S2, accurately weighing a proper amount of CuSO ₄, and dissolving in a proper amount of distilled water to prepare a CuSO ₄ solution;

Step S3, dropwise adding the CuSO ₄ solution into the polyoxometallate solution;

step S4, heating and stirring are continued, and insoluble substances in the finally mixed solution are filtered;

and S5, adding acetonitrile into the clear liquid, standing, separating out crystals, and collecting the product through the steps of recrystallization, suction filtration, drying and the like.

Further preferably, in step S1, the mass-to-volume ratio of the polyoxometalate crystals to distilled water is 5.25g:105mL, and the oil bath temperature is 100 ℃.

Further preferably, in step S2, the concentration of the CuSO ₄ solution is 0.0408g/mL.

Further preferably, in step S3, the drop acceleration is 20 to 30 drops/min.

Further preferably, in step S4, the temperature of the heating and stirring is 100 ℃, and the stirring time is 1.5 hours. The filtering operation should be performed while hot, and insoluble substances should be filtered off.

Further preferably, in step S5, the condition is room temperature 25±2 ℃, and the standing time is 2 to 3 days.

In a third aspect, the invention also provides the use of the Cu-POM type catalyst in promoting the formation of esters, i.e. the use of the Cu-POM type catalyst as an esterification catalyst.

Preferably, in addition to the Cu-POM catalyst as reaction catalyst, the use of the third aspect may incorporate small amounts of potassium chloride to promote the formation of esters, for example to increase the yield of methyl benzoate.

The method comprises the steps of connecting metallic element copper with polyoxometallate, collecting products through steps of oil bath, filtration and crystallization and the like, and preparing the Anderson polyoxometallate catalyst Cu-POM taking Cu as a center. Anderson type polyoxometallate is used as one of polyoxometallate types, and application is mainly focused on the fields of photocatalysis, magnetism, electrochemistry and the like. The molecular structure is expressed in a simple formula of [ Hm (XO ₆)M₆O₁₈ ] n-, m=0-6, n=2-8, wherein M represents a multi-atom, mo and V are more common, X represents a central atom, the central atom comprises more elements such as early transition metal iron, copper, zirconium and the like, 6 (Mo 6) are combined in a public edge form and simultaneously form an octahedral structure around a central group { XO6} to finally form a planar structure, the three-dimensional structure can be divided into an alpha type three-dimensional structure and a beta type three-dimensional structure, wherein the alpha type three-dimensional structure is more common, the beta type three-dimensional structure is a non-planar curved structure, the alpha type three-dimensional structure can be further divided into an A type three-dimensional structure and a B type three-dimensional structure by the difference of proton numbers connected with the central atom, wherein the A type three-dimensional structure is non-protonic, the central atom is directly connected with 6 oxygen atoms, the oxygen atoms often have different coordination types in Anderson type polyacid materials, wherein the oxygen atoms coordinated with two multiple atoms and one central heteroatom are called "three-bridged oxygen (mu 3-O)", 6 in total, distributed over and under the crystal structure, "two-bridged oxygen (mu 2-O)", which means oxygen atoms coordinated with two multiple atoms, 6 in total, distributed among 6 octahedra of planar structure, "terminal oxygen (Ot)", which means independent oxygen atoms coordinated with multiple atoms, each multiple atom having two terminal oxygen groups connected thereto, in total, 12, type A/B is distinguished by whether or not μ3-O is protonated. Because of the protonated central atoms, conformational changes of the polyacids are possible, and many scholars have linked organic ligands to the Anderson polyacids in order to prepare novel polyoxometallates, the organic ligands being bound by substitution of terminal oxygen atoms present in the polyacid structure. The method not only utilizes the catalytic effect of polyacid, but also utilizes the affinity of the organic ligand and the substrate, thereby realizing the possibility of 1+1> 2.

Compared with noble metal catalyst, the Anderson type polyoxometallate catalyst with Cu as the center has lower cost, easy separation and purification of the product, good environment and easy large-scale application, and the Cu-POM prepared by the invention has the advantages of low cost, simple synthesis condition, low toxicity and the like when being used as the catalyst for esterification reaction, and the catalyst has high yield when being used for catalyzing ester synthesis, can realize high catalytic efficiency, accords with the principle of green environmental protection, and successfully realizes the application of the catalyst in catalyzing ester compounds.

The Cu-POM prepared by the method is stable in performance as a catalyst for esterification reaction, good in catalytic activity, simple in research performance, and capable of keeping good reaction activity after cyclic utilization, for example, compared with Anderson type polyoxometallate taking Fe as a center, the catalytic performance of oxidation esterification reaction of the polyoxometallate prepared by adding metal Cu is stronger, and the yield is higher.

Drawings

FIG. 1 is an infrared spectrum of Cu-centered Anderson-type polyacid prepared in example 1;

FIG. 2 is an infrared spectrum of the Fe-centered Anderson-type polyacid prepared in example 2;

FIG. 3 is a nuclear magnetic resonance ¹ H spectrum of the compound methyl benzoate;

FIG. 4 is a chart showing nuclear magnetic resonance ¹³ C of the compound methyl benzoate;

FIG. 5 is a nuclear magnetic resonance ¹ H spectrum of the compound methyl 4-methylbenzoate

FIG. 6 is a chart showing nuclear magnetic resonance ¹³ C of the compound methyl 4-methylbenzoate;

FIG. 7 is a nuclear magnetic resonance ¹ H spectrum of the compound methyl 4-methoxybenzoate

FIG. 8 is a chart showing nuclear magnetic resonance ¹³ C of the compound methyl 4-methoxybenzoate;

FIG. 9 is a nuclear magnetic resonance ¹ H spectrum of the compound methyl 4-chlorobenzoate

FIG. 10 is a chart showing nuclear magnetic resonance ¹³ C of the compound methyl 4-chlorobenzoate;

FIG. 11 is a nuclear magnetic resonance ¹ H spectrum of the compound methyl 4-bromobenzoate

FIG. 12 is a chart showing nuclear magnetic resonance ¹³ C of the compound methyl 4-bromobenzoate;

FIG. 13 is a comparison of infrared spectra before and after a catalyst cycling experiment;

FIG. 14 is a catalyst recycling experiment.

Detailed Description

The technical scheme of the present invention is described in detail below through specific examples, but the scope of the present invention is not limited to the examples. The specific techniques or conditions are not identified in the examples and are described in the literature in this field or are carried out in accordance with the product specifications. The reagents or equipment used were conventional products available for purchase through regular channels, with no manufacturer noted. Ammonium molybdate tetrahydrate, shanghai Yi En chemical technologies Co., ltd;

Copper sulfate, chemical company of Shanghai Lingfeng chemical agent, inc.;

ethyl acetate and petroleum ether are all purchased from the company of the sciences of the ridge, potassium chloride is obtained from the company of the chemical reagent of Nanjing, and p-bromobenzaldehyde, p-chlorobenzaldehyde, p-fluorobenzaldehyde and cyclohexane-based formaldehyde are all purchased from the company of the biochemical technology of Shanghai Michael;

Benzaldehyde and p-nitrobenzaldehyde are all purchased from Shanghai Bi to get medical science and technology Co., ltd;

The reagents used are all analytically pure, and the term room temperature is defined as having the meaning known in the art and generally means 25.+ -. 2 ℃.

EXAMPLE 1 preparation of Cu-centered Anderson polyacids

The specific implementation steps are that 5.25g (NH ₄)₆Mo₇O₂₄·4H₂ O is dissolved in 105mL of deionized water, stirring and heating are carried out in an oil bath, the temperature is 100 ℃, the solution A is called solution A, 1.02gCuSO ₄ is dissolved in 25mL of deionized water, the solution B is called solution B, solution B is added into solution A, the dropping speed cannot be too fast, the dropping speed is controlled to be 25+/-1 drops/min, the solution is continuously heated and stirred for 1.5 hours under the condition of 100 ℃ after the dropping is finished, the finally mixed solution is filtered while the solution is hot, insoluble matters are filtered, clear liquid is taken, excessive acetonitrile is added, the solution is kept stand at room temperature for 48 hours, blue crystals are separated out, and the product is collected through the steps of recrystallization, suction filtration, drying and the like.

EXAMPLE 2 preparation of Fe-centered Anderson-type polyacids

The specific implementation steps are that 5.25g (NH ₄)₆Mo₇O₂₄·4H₂ O is dissolved in 105mL of deionized water, the temperature is 100 ℃ under stirring and heating in an oil bath, the solution A is called solution A, 1.3g of Fe ₂(SO₄)₃ is dissolved in 25mL of deionized water, the solution B is gradually added into the solution A, the dropping process is slow, the pH value is controlled to be 4.5-6, the solution is continuously heated and stirred for 1.5 hours under the condition of 100 ℃ after the dropping is finished, the finally mixed solution is filtered while hot, insoluble matters are filtered, clear liquid is taken and kept stand at room temperature for 48 hours, white crystals are separated out, filtered crystals are recrystallized, filtered, dried and the like, and the product is collected.

EXAMPLE 3 preparation of methyl benzoate

The steps of the specific implementation were that 152. Mu.L of benzaldehyde and 60. Mu.L of methanol were weighed, 24mg of the crystallized product of the Cu-POM catalyst prepared in example 1 and 22.2mg of KCl, 137. Mu.LH ₂O₂ and 2.0mL of acetonitrile as a solvent were simultaneously added, sealed, stirred at 80℃for 24 hours, and the progress of the reaction was monitored by thin layer chromatography. After the reaction, saturated brine and ethyl acetate were added to extract, and the extraction was repeated three times, followed by combining the organic layers. And filtering and drying insoluble matters in the water phase to recover the catalyst, adding a proper amount of anhydrous sodium sulfate for drying, adding silica gel for sand making, and separating the product by using a column chromatography technology to obtain the required target product. The results are shown in fig. 3 and 4.

EXAMPLE 4 preparation of methyl 4-methylbenzoate

The preparation method comprises the steps of weighing 1.77mL of p-methylbenzaldehyde and 606 mu L of methanol, simultaneously adding 240mg of the crystallization product of the Cu-POM catalyst prepared in the example 1 and 222mg of the additive, 1.37mL of 30% H ₂O₂ solution, stirring for 24 hours at 80 ℃ with 20.0mL of acetonitrile as a solvent, monitoring the reaction progress through thin layer chromatography, extracting the reaction mixture, concentrating an organic layer to prepare sand, separating and purifying through column chromatography, and obtaining the target product, namely 4-methyl benzoate in 77% yield, wherein the results are shown in fig. 5 and 6.

EXAMPLE 5 preparation of methyl 4-methoxybenzoate

The steps of the specific implementation were that 1.82mL of p-methoxybenzaldehyde and 606. Mu.L of methanol were weighed, 240mg of the crystallization product of the Cu-POM catalyst prepared in example 1 and 222mg of potassium chloride as an additive, 1.37mL of 30% H ₂O₂ solution as a solvent, 20.0mL of acetonitrile were simultaneously added, and the mixture was stirred at 80℃for 24 hours, and the progress of the reaction was monitored by thin layer chromatography. Extracting the reaction mixture, concentrating the extracted organic layer to prepare sand, and separating and purifying by column chromatography to obtain the target product, namely the methyl 4-methoxybenzoate, with a yield of 77%. The results are shown in fig. 7 and 8.

EXAMPLE 6 preparation of methyl 3-fluorobenzoate

The procedure was followed by weighing 1.58mL of 3-fluorobenzaldehyde and 606. Mu.l of methanol, adding 240mg of the crystallized product of the Cu-POM catalyst prepared in example 1 and 222mg of potassium chloride as an additive, 1.37mL of 30% H ₂O₂ solution in 20.0mL of acetonitrile as a solvent, stirring at 80℃for 24 hours, and monitoring the progress of the reaction by thin layer chromatography. Extracting the reaction mixture, concentrating an extracted organic layer to prepare sand, and separating and purifying by column chromatography to obtain the target product, namely the 3-fluorobenzoic acid methyl ester with 71 percent of yield.

EXAMPLE 7 preparation of methyl 4-chlorobenzoate

The process was carried out by weighing 1.77mL of 4-chlorobenzaldehyde and 606. Mu.L of methanol, adding 240mg of the crystallized Cu-POM catalyst prepared in example 1 and 222mg of potassium chloride as an additive, and 1.37mL of 30% H ₂O₂ solution in 20.0mL of acetonitrile as a solvent, stirring at 80℃for 24 hours, and monitoring the progress of the reaction by thin layer chromatography. Extracting the reaction mixture, concentrating the extracted organic layer to prepare sand, and separating and purifying by column chromatography to obtain the target product, namely the methyl 4-chlorobenzoate with a yield of 74 percent. The results are shown in fig. 9 and 10.

EXAMPLE 8 preparation of methyl 4-bromobenzoate

The procedure was followed by weighing 1.75mL of 4-bromobenzaldehyde and 606. Mu.l of methanol, simultaneously adding 240mg of the crystallized product of Cu-POM catalyst prepared in example 1 and 222mg of potassium chloride as an additive, 1.37mL of 30% H ₂O₂ solution in 20.0mL of acetonitrile as a solvent, stirring at 80℃for 24 hours, and monitoring the progress of the reaction by thin layer chromatography. Extracting the reaction mixture, concentrating an extracted organic layer to prepare sand, and separating and purifying by column chromatography to obtain the target product, namely the methyl 4-bromobenzoate with 80 percent of yield. The results are shown in fig. 11 and 12.

EXAMPLE 9 preparation of methyl 4-trifluoromethyl benzoate

The procedure was followed by adding (15.0 mmol,2.04 mL) 3-fluorobenzaldehyde and (606. Mu.L) methanol to a clean round bottom flask, and simultaneously adding corresponding amounts of the crystalline product of the Cu-POM catalyst prepared in example 1 (1.5 mol%,240 mg) and an additive potassium chloride (1.0 eq,222 mg), 30% H2O2 solution (1.37 mL), 20.0mL acetonitrile as a solvent, and stirring the reaction mixture at 80℃for 24 hours. During the period, the reaction was monitored by thin layer chromatography spot plate to ensure complete consumption of the starting material and stopping stirring. The reaction mixture was extracted, and the extracted organic layer was concentrated to give sand, which was separated and purified by column chromatography to give methyl 4-trifluoromethylbenzoate (2.26 g) as a target product in 74% yield.

EXAMPLE 10 preparation of methyl 4-nitrobenzoate

The procedure was followed by adding (15.0 mmol,1.51 mL) 4-nitrobenzaldehyde and (15.0 mmol, 606. Mu.L) methanol to a clean round bottom flask, and simultaneously adding the crystallized product of the Cu-POM catalyst prepared in example 1 (1.5 mol%,240 mg) and the additive potassium chloride (1.0 eq,222 mg), 30% H2O2 solution (1.37 mL), 20.0mL acetonitrile as a solvent, and stirring the reaction mixture at 80℃for 24 hours. During the period, the reaction was monitored by thin layer chromatography spot plate to ensure complete consumption of the starting material and stopping stirring. The reaction mixture was extracted, and the extracted organic layer was concentrated to give sand, which was separated and purified by column chromatography to give methyl 4-nitrobenzoate (2.06 g) as a target product in 76% yield.

Test example 1 reaction condition optimization comparative test

In order to verify the influence of the catalyst, additive amount, catalyst amount, temperature and time of the different metal element centers on the catalytic activity of the esterification reaction, the test was performed using methyl benzoate as a substrate in example 3 and the preparation as in example 3, and the control single variable experiments of the catalyst, additive amount, catalyst amount, reaction temperature and reaction time of the different metal element centers were respectively performed, and were set as follows:

the catalyst of the centers of different metal elements comprises Cu-POM prepared in example 1 and Fe-POM prepared in example 2, the catalyst comprises 0.1mol percent, 0.5mol percent and 3.0mol percent, the mol percent of the catalyst is expressed as the percentage of the substrate, the different additives comprise sodium sulfate, sodium sulfite, sodium chloride, sodium carbonate, sodium bicarbonate, potassium fluoride, potassium bromide, zinc chloride, calcium chloride, magnesium chloride and potassium chloride, the additive comprises 0.1eq and 0.3eq, the reaction temperature is 60, 70, 80 and 90 ℃, and the reaction time is 12, 24, 36 and 48 hours.

The result shows that at the temperature of 80 ℃,152 mu L of benzaldehyde and 60 mu L of methanol, the reaction time is 24 hours, and simultaneously 137 mu LH ₂O₂, 22.2mg KCl,24mg Cu-POM and 2.0mL of acetonitrile are added, the yield of the prepared ester compound is higher, and the catalytic reaction efficiency is higher.

Test example 2 Activity of Cu-POM catalyst

The specific implementation steps are as follows, after the esterification reaction of the embodiment 3, the water phase is filtered and dried to obtain a solid crude product of the Cu-POM catalyst, and the crude product is washed with ethanol for multiple times and dried for standby. The solid catalyst is characterized by a Fourier transform infrared spectrometer and a powder diffraction spectrum, and the result shows that the structure of the recovered solid catalyst is consistent with that of the catalyst before reaction. The results are shown in FIG. 13.

The yield of the reaction was evaluated by adjusting the amount of the reactant in the subsequent experiments in combination with the actual amount of the catalyst to ensure that the ratio of the amount of the reactant to the amount of the catalyst was unchanged. The result shows that the Cu-POM can still keep good reaction activity after 6 times of recycling, and the stability and high activity of the Anderson type polyoxometallate catalyst are also proved. The results are shown in FIG. 14.

Test example 3:

As a result of the verification test of substances which may exert a catalytic effect during the reaction, in Table 1 below, reference conditions were set, and the present test specifically referred to condition 8 in Table 1 using the optimum conditions verified in test example 1 above as reference conditions, namely, the reference reaction conditions were benzaldehyde (1.5 mmol, 152. Mu.L), methanol (1.5 mmol, 60. Mu.L), H2O2 (3.0 eq, 137. Mu.L), KCl (0.2 eq,22.2 mg), cu-POM (1.5 mol%,24 mg), acetonitrile (2.0 mL), 80℃and 24 hours. Firstly, the catalyst is replaced by ammonium molybdate which is a raw material for synthesizing Cu-POM, the condition 1 in the following table is not changed, the yield is remarkably reduced compared with the yield under the standard condition, in the condition 2, the copper sulfate which is a raw material for synthesizing Cu-POM is used for replacing the catalyst, the result shows that the yield is obviously reduced, but the yield is slightly higher than that in the condition 1 compared with the condition 1, and the activity of metal elements in catalyzing the reaction of benzaldehyde and methanol is also higher. The combination of two starting materials, ammonium molybdate and copper sulfate, in condition 3, instead of the catalyst, showed a significant decrease in the yield, but was higher than those of ammonium molybdate and copper sulfate used alone in conditions 1 and 2, which also indicated that the interaction between the metal element and the ligand was a key factor in promoting the reaction. In the condition 4, the potassium chloride is removed, other conditions are unchanged, the yield shows a certain decreasing trend, and the fact that the potassium chloride plays a certain promoting role in the oxidation and esterification reaction of benzaldehyde and methanol is also indicated. Whereas conditions 5 and 6 were only ammonium molybdate or copper sulfate added, the reaction could not proceed. Meanwhile, in condition 7, the oxidizing agent was changed to oxygen, and the result indicated that the reaction could not proceed. By combining all experimental results, the combination of the Cu-POM polyoxometallate material, potassium chloride and hydrogen peroxide in the oxidation esterification reaction of benzaldehyde and methanol is crucial for the generation of corresponding esters.

TABLE 1 yields under different conditions

Claims (10)

1. A Cu-POM catalyst, wherein the Cu-POM catalyst is an Anderson-type polyoxometalate centered on Cu, and the structural formula of the Cu-POM is: (NH ₄ ) ₂ [CuMo ₆ O ₁₈ (OH) ₆ ]. 2. The preparation method of the Cu-POM type catalyst according to claim 1, characterized in that the preparation method comprises the steps of: connecting the metal element ketone with the polyoxometalate, and preparing the crystalline state of the Cu-POM type catalyst by oil bath, filtration and crystallization. 3. The preparation method according to claim 2 is characterized in that the preparation method comprises the following steps: step S1, first accurately weighing an appropriate amount of polyoxometalate crystals and dissolving them in an appropriate amount of distilled water, stirring them under an oil bath to prepare a polyoxometalate solution; step S2, accurately weighing an appropriate amount of _CuSO4 and dissolving them in an appropriate amount of distilled water to prepare a _CuSO4 solution; step S3, adding the _CuSO4 solution dropwise to the polyoxometalate solution; step S4, continuing heating and stirring, and filtering the final mixed solution to remove insoluble matter; step S5, taking the clear solution, adding acetonitrile, standing, precipitating crystals, and collecting the product through steps such as recrystallization, suction filtration, and drying. 4. The preparation method according to claim 3, characterized in that in step S1, the mass volume ratio of the polyoxometalate crystals to distilled water is 5.25 g:105 mL, and the oil bath temperature is 100°C. 5. The preparation method according to claim 3, characterized in that in step S2, the concentration of the CuSO ₄ solution is 0.0408 g/mL. 6. The preparation method according to claim 3, characterized in that in step S3, the dropping speed is 20 to 30 drops/min. 7. The preparation method according to claim 3, characterized in that in step S4, the temperature of the heating and stirring is 100° C. and the stirring time is 1.5 hours. 8. Use of the Cu-POM catalyst according to claim 1 in promoting the production of esters. 9. The use as claimed in claim 8, characterized in that the esters include: methyl benzoate, methyl 4-methylbenzoate, methyl 4-methoxybenzoate, methyl 3-fluorobenzoate, methyl 4-chlorobenzoate, methyl 4-bromobenzoate, methyl 4-trifluoromethylbenzoate, and methyl 4-nitrobenzoate. 10. The use as claimed in claim 8, characterized in that potassium chloride is added while Cu-POM type catalyst is used as a reaction catalyst to promote the formation of esters.