ZIF-67-LDH catalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of catalyst preparation, and particularly relates to a ZIF-67-LDH catalyst and a preparation method and application thereof. The prepared ZIF-67-LDH catalyst is high in HMF conversion rate and good in selectivity, the HMF conversion rate can reach 100%, the FDCA selectivity can reach 84.0%, and a new thought and method are provided for converting HMF into FDCA. Meanwhile, the obtained catalyst has good thermal stability and can be recycled, HMF can be completely converted after 10 times of circulation, and the yield of FDCA is still kept at 82.7%.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalyst preparation, and particularly relates to a ZIF-67-LDH catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] HMF (5-hydroxymethylfurfural) is an organic compound formed by the dehydration of monosaccharides under acidic or high-temperature conditions, and can be used to convert into a variety of commercial chemicals. Among them, FDCA (2,5-furandicarboxylic acid) generated by the oxidation of HMF has received extensive attention from the industrial and academic circles because it is a green alternative to petroleum-based materials, and FDCA-based polyesters can replace PET plastics.

[0003] Common methods for preparing FDCA by oxidizing HMF mainly include the following: 1. Chemical oxidation method. Although its oxidation reaction is rapid, a large amount of waste salts and solid waste will be generated during the process, which causes great environmental harm, and the FDCA yield is low and the purity is poor; 2. Enzyme-catalyzed oxidation method. Its oxidation conditions are mild and the reaction system is environmentally friendly, but the product separation and purification are difficult, there are many by-products, and the FDCA purity is poor; 3. Noble metal-catalyzed oxidation method. Its reaction activity is high and the catalyst is easy to separate, but the noble metal cost is high, and it is difficult to achieve large-scale application.

[0004] Zeolitic imidazolate framework materials (ZIFs) are a class of metal-organic framework compounds constructed with transition metal cations as central nodes and bridged by imidazole-based organic ligands, and can maintain structural stability in both alkaline media and organic solvents. As a typical representative of ZIFs, ZIF-67 has excellent thermal stability and chemical stability. At present, there are few literature reports on preparing catalysts from ZIFs to oxidize HMF. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides a ZIF-67-LDH catalyst, a preparation method thereof, and an application thereof. A ZIF-67-LDH catalyst is prepared by compounding NiAl-LDH (nickel-aluminum layered double hydroxide) with ZIF-67. It has high conversion rate and high selectivity when oxidizing HMF, and can efficiently and selectively oxidize HMF to FDCA.

[0006] In the first aspect of the present invention, a ZIF-67-LDH catalyst is provided, and the catalyst is obtained by compounding NiAl-LDH with ZIF-67.

[0007] The preparation method of the ZIF-67 is as follows:

[0008] Dissolve Co(NO3)2·6H2O (cobalt nitrate hexahydrate) in methanol to obtain a cobalt nitrate solution. At the same time, dissolve C4H6N2 (2-methylimidazole) in methanol to obtain a methanol solution of 2-methylimidazole; then slowly add the methanol solution of 2-methylimidazole dropwise to the cobalt nitrate solution and continuously stir for 1 hour to ensure thorough mixing; let the mixed solution stand at room temperature, then centrifuge. The obtained precipitate is washed with methanol multiple times and dried at 50 °C to finally obtain the ZIF-67 material;

[0009] Preferably, the molar ratio of Co(NO3)2·6H2O to C4H6N2 is 3:8; the material ratio of Co(NO3)2·6H2O to methanol is 1.05 g:40 ml; the material ratio of C4H6N2 to methanol is 0.79 g:40 ml; the stirring time is 1 h, standing for 24 h, and centrifuging at a speed of 6000 rpm for 20 minutes.

[0010] The preparation method of the NiAl-LDH is as follows:

[0011] Take nickel chloride hexahydrate (NiCl2·6H2O), aluminum nitrate nonahydrate (Al(NO3)3·9H2O) and urea and add them to a beaker, add deionized water, stir evenly, transfer the evenly stirred mixed solution to a hydrothermal reaction kettle, place it in a blast drying oven for reaction. After the reaction terminates, let it cool to room temperature, then carry out vacuum filtration, wash with deionized water multiple times until the pH of the washing liquid is about 7, and then dry the product to obtain NiAl-LDH.

[0012] Preferably, the molar ratio of NiCl2·6H2O, Al(NO3)3·9H2O and urea is 2:1:15; use a magnetic stirrer to stir for 1 h; place it in a blast drying oven at 120 °C for reaction for 6 h; when drying, place the obtained product in an oven and dry it under the constant temperature condition of 90 °C for 10 hours.

[0013] In the second aspect of the present invention, a preparation method of the above catalyst is provided, including the following steps:

[0014] S1, prepare NiAl-LDH: Take nickel chloride hexahydrate (NiCl2·6H2O), aluminum nitrate nonahydrate (Al(NO3)3·9H2O) and urea and add them to a beaker, add deionized water, stir evenly, transfer the evenly stirred mixed solution to a hydrothermal reaction kettle, place it in a blast drying oven for crystallization reaction. After the reaction terminates, let it cool to room temperature, then carry out vacuum filtration, wash with deionized water multiple times until the pH of the filtrate is 7, and then dry the product to obtain a NiAl-LDH sample for standby;

[0015] S2, prepare the ZIF-67-LDH catalyst:

[0016] S21. Dissolve C4H6N2 in methanol to obtain a methanol solution of C4H6N2 for standby.

[0017] S22. Place the NiAl-LDH obtained in S1 and Co(NO3)2·6H2O in a beaker, add methanol to form a uniformly dispersed mixed solution, and slowly drop the methanol solution of C4H6N2 obtained in S21. Continuously stir for reaction. After the reaction, let the mixture stand, and retain the solid product after centrifugation.

[0018] S23. Wash the solid product obtained in S22 with methanol multiple times and dry it to obtain the ZIF-67-LDH catalyst.

[0019] Preferably, in S1, the molar ratio of NiCl2·6H2O, Al(NO3)3·9H2O, and urea is 2:1:15; stir using a magnetic stirrer for 1 h; react in a forced-air drying oven at 120 °C for 6 h; during drying, place the obtained product in an oven and dry it under a constant temperature condition of 90 °C for 10 hours.

[0020] Preferably, in S2, the molar ratio of Co(NO3)2·6H2O to C4H6N2 is 3:8.

[0021] Preferably, in S22, dissolve 0.79 g of 2-methylimidazole (C4H6N2) in 40 ml of methanol; stir for 1 h, let it stand for 24 h, and centrifuge at a speed of 6000 rpm for 20 minutes.

[0022] Preferably, in S22, the mass ratio of NiAl-LDH to Co(NO3)2·6H2O is 1-7:10.5.

[0023] Preferably, in S23, the drying temperature is 50 °C.

[0024] The application of the ZIF-67-LDH catalyst prepared by the above method in the catalytic oxidation of HMF is also within the protection scope of the present invention.

[0025] The beneficial effects of the present invention are as follows:

[0026] (1) The ZIF-67-LDH catalyst prepared by the present invention has a high conversion rate and good selectivity for HMF. Its HMF conversion rate can reach 100%, and the FDCA selectivity can reach 84.0%, providing a new idea and method for the conversion of HMF to FDCA.

[0027] (2) The catalyst obtained by the present invention has good thermal stability and can be recycled. After 10 cycles, the yield of FDCA still remains at 82.7%. Description of the Drawings

[0028] Figure 1 XRD patterns of NiAl-LDH, ZIF-67, and ZIF-67-LDH-3;

[0029] Figure 2 SEM images of (a) ZIF-67, (b) NiAl-LDH, (c) ZIF-67-LDH-3, and SEM-EDS image and corresponding elemental maps of ZIF-67-LDH-3;

[0030] Figure 3 High-resolution transmission electron microscopy images of ZIF-67-LDH-3 (a)(b)(c);

[0031] Figure 4 XPS spectra of ZIF-67-LDH-3, ZIF-67, and NiAl-LDH: (a) Co 2p, (b) Ni 2p, (c) Al 2p, (d) O 1s, (e) C 1s, (f) N 1s;

[0032] Figure 5 N2 adsorption-desorption isotherms of (a) ZIF-67, (b) NiAl-LDH, (c) ZIF-67-LDH-3;

[0033] Figure 6 Characterization of ZIF-67-LDH-3 catalyst: (a) DTA and TGA curves during heating of ZIF-67-LDH in N2 atmosphere, (b) EPR of NiAl-LDH and ZIF-67-LDH with various ratios, (c) FT-IR spectra;

[0034] Figure 7 Reaction mechanism of HMF oxidation to FDCA over ZIF-67-LDH catalyst;

[0035] Figure 8 Effects of different reaction conditions on the catalytic oxidation of HMF to prepare FDCA. Among them, (a) effect of different temperatures, reaction conditions: 0.05 g HMF, 0.36 ml t-BuOOH, 6 h, 10 ml CH3CN, 0.05 g catalyst; (b) effect of different reaction times, reaction conditions: 0.05 g HMF, 0.36 ml t-BuOOH, 120 °C, 10 ml CH3CN, 0.05 g catalyst; (c) effect of catalyst dosage, reaction conditions: 0.05 g HMF, 0.36 ml t-BuOOH, 10 h, 120 °C, 10 ml CH3CN; (d) effect of the amount of oxidant t-BuOOH, reaction conditions: 0.05 g HMF, 12 h, 120 °C, 0.075 g catalyst;

[0036] Figure 9It is a test diagram for the recovery of the ZIF-67-LDH-3 catalyst;

[0037] Figure 10 It is the SEM diagram of the ZIF-67-LDH-3 catalyst after the HMF oxidation reaction;

[0038] Figure 11 It is the XRD spectra of the ZIF-67-LDH-3 catalyst before and after the HMF oxidation reaction;

[0039] Figure 12 It is the core-level XPS spectra of ZIF-67-LDH-3 before and after the HMF oxidation reaction: (a) Co2p, (b) Ni2p, (c) Al 2p, (d) O1s. Detailed implementation manners

[0040] In order to enable those skilled in the art to better understand the present invention, the present invention will be further elaborated below in conjunction with the specific implementation manners.

[0041] Example 1

[0042] The preparation method of ZIF-67 includes the following steps:

[0043] First, 1.05 g of Co(NO3)2·6H2O is dissolved in 40 mL of methanol to obtain a cobalt nitrate solution. At the same time, 0.79 g of C4H6N2 is dissolved in an equal volume of methanol to obtain a methanol solution of C4H6N2. Subsequently, the methanol solution of C4H6N2 is slowly added dropwise to the cobalt nitrate solution, and continuous stirring is carried out for 1 hour to ensure sufficient mixing. The mixed solution is left to stand at room temperature for 24 hours, and then centrifuged at a speed of 6000 rpm for 20 minutes. The obtained precipitate is washed with methanol multiple times and dried at 50°C to finally obtain the ZIF-67 material.

[0044] Example 2

[0045] A preparation method of a ZIF-67-LDH catalyst includes the following steps:

[0046] (1) Accurately weigh 5.46 g of NiCl2·6H2O, 3.75 g of aluminum nitrate nonahydrate (Al(NO3)3·9H2O), and 9.00 g of urea. The above reagents are successively added to a 200 mL beaker, 100 mL of deionized water is added, and the mixture is stirred on a magnetic stirrer for 1 hour. The uniform mixed solution is transferred to a hydrothermal reaction kettle and placed in a forced-air drying oven at 120°C for reaction for 6 hours. After the reaction system terminates, it is allowed to cool naturally to the ambient temperature, and then vacuum filtration treatment is carried out. The product is washed with deionized water multiple times until the pH value of the filtrate is detected to reach about 7. The obtained product is placed in an oven and dried at a constant temperature of 90°C for 10 hours to finally obtain the NiAl-LDH sample.

[0047] (2) Place 0.3 g of the NiAl-LDH obtained in (1) and 1.05 g of Co(NO3)2·6H2O in a 100 mL beaker, add 40 mL of methanol to form a uniformly dispersed mixed solution;

[0048] Subsequently, dissolve 0.79 g of C4H6N2 in 40 ml of methanol to obtain a methanol solution of C4H6N2. Slowly add the methanol solution of C4H6N2 to the above mixed solution and continuously stir for 1 hour to ensure full reaction. After the reaction is completed, let the mixture stand for 24 hours, then centrifuge at a speed of 6000 rpm for 20 minutes. The obtained solid product is washed with methanol multiple times and dried at 50 °C to finally obtain the ZIF-67-LDH catalyst.

[0049] Example 3

[0050] The difference from Example 2 is that in (2), the added masses of NiAl-LDH are 0.1 g, 0.2 g, 0.4 g, 0.5 g, 0.6 g, and 0.7 g respectively, and other steps are the same as those in Example 2.

[0051] Name the obtained catalysts as ZIF-67-LDH-1 (added mass of NiAl-LDH is 0.1 g), ZIF-67-LDH-2 (added mass of NiAl-LDH is 0.2 g), ZIF-67-LDH-4 (added mass of NiAl-LDH is 0.4 g), ZIF-67-LDH-5 (added mass of NiAl-LDH is 0.5 g), ZIF-67-LDH-6 (added mass of NiAl-LDH is 0.6 g), and ZIF-67-LDH-7 (added mass of NiAl-LDH is 0.7 g). For easy distinction, name the catalyst obtained in Example 2 as ZIF-67-LDH-3.

[0052] Test Example 1 Catalyst Characterization

[0053] The ZIF-67-LDH catalysts involved in this test example are all ZIF-67-LDH-3 prepared in Example 2.

[0054] Characterize the NiAl-LDH, ZIF-67, and ZIF-67-LDH-3 obtained in the examples, and their X-ray diffraction patterns are as Figure 1As shown, characteristic peaks of crystal planes such as (003), (006), (012), (015), and (018) can be observed in the X-ray diffraction pattern of NiAl-LDH. Meanwhile, the diffraction peaks of ZIF-67 correspond to the (011), (112), and (212) crystal planes of its crystal, indicating that the prepared ZIF-67 has a single-phase structure. In the XRD pattern of the composite catalyst ZIF-67-LDH-3, due to the relatively low addition amount of NiAl-LDH, which is 0.3 g, the corresponding diffraction peak signal is weak. New characteristic peaks appear in the diffraction pattern of the ZIF-67-LDH-3 composite material. These peak positions are exactly the same as those of the pure-phase ZIF-67, and no other impurity peaks are detected, which fully confirms the successful synthesis of ZIF-67 in the composite material.

[0055] The SEM images of NiAl-LDH, ZIF-67, and ZIF-67-LDH-3 obtained in the examples, the SEM-EDS image of ZIF-67-LDH-3, the corresponding elements, and the energy-dispersive X-ray spectroscopy (EDS) analysis of ZIF-67-LDH-3 are as Figure 2 shown, as Figure 2 shown in b, NiAl-LDH presents a typical hexagonal sheet structure. By Figure 2 observing c clearly, a large number of crystals with a rhombic dodecahedron morphology are distributed on the surface of the loaded NiAl-LDH, and its morphological characteristics are exactly the same as those of the pure-phase ZIF-67 ( Figure 2 a). In addition Figure 2 the element distribution map in d shows that elements such as Co, Ni, and Al are evenly distributed on the surface of the ZIF-67-LDH-3 composite material, which confirms that ZIF-67 has been successfully and evenly loaded on the surface of NiAl-LDH.

[0056] The HRTEM image of the ZIF-67-LDH-3 (prepared in Example 2) composite catalyst is as Figure 3 shown. It can be clearly observed that the crystalline region of NiAl-LDH is intertwined with the amorphous region of ZIF-67. The lattice fringes with spacings of 0.25 nm and 0.20 nm correspond to the (012) and (107) crystal planes of hexagonal phase [Ni6Al2(OH) 16 CO3·4H2O, respectively.

[0057] The Co 2p fine spectra of ZIF-67 and ZIF-67-LDH-3 are as Figure 4 shown in a. Through analysis, it can be known that the characteristic peaks at 796.1 eV and 782.4 eV correspond to Co 2p 1 / 2 and Co2p 3 / 2The peaks at 801.8 eV and 787.2 eV are identified as their corresponding satellite peaks; Figure 4 b shows the Ni 2p XPS fine spectrum. As Figure 4 shown in b, four characteristic peaks can be found: 874.3 eV and 856.6 eV are attributed to Ni 2p 1 / 2 and Ni 2p 3 / 2 orbits respectively. Meanwhile, the peaks at 878.9 eV and 862.1 eV are identified as the corresponding satellite peaks, which confirms that the Ni element in NiAl-LDH exists in the +2 valence state; Figure 4 c shows that the Al 2p spectra of NiAl-LDH and ZIF-67-LDH-3 both show the existence of Al 3+ . Although Al itself may not be a direct oxidation active site, it can indirectly affect the oxidation performance of the catalyst by influencing the electron cloud density and coordination environment of surrounding metal ions (such as Ni and Co). The O 1s peak can usually be divided into different oxygen species such as surface adsorbed oxygen (O ads ), oxygen vacancy (O v ), and lattice oxygen (O latt ). The proportion and binding energy changes of different oxygen species in ZIF-67-LDH-3 reflect the activity and quantity of surface oxygen species of the catalyst, which have an important impact on its ability to oxidize HMF to produce FDCA. Compared with single ZIF-67 and NiAl-LDH, the characteristic peaks of the Co 2p, Ni 2p, and Al 2p fine spectra in the ZIF-67-LDH-3 composite material all show slight displacements, which strongly confirms the significant interaction between ZIF-67 and NiAl-LDH due to electron transfer.

[0058] Figure 5 a - c show the results of pore structure characterization of NiAl-LDH, ZIF-67, and ZIF-67-LDH-3 using N2 adsorption - desorption isotherms. Table 1 shows the N2 physical adsorption data of ZIF-67, NiAl-LDH, and ZIF-67-LDH-3. From Figure 5 and Table 1, it can be seen that the ZIF-67 sample shows a typical type I adsorption isotherm, which is consistent with its microporous structure. The specific surface area test data shows that the BET specific surface area of the ZIF-67-LDH-3 composite material is 2030.0 m 2 / g, which is greater than that of pure LDH (32.56 m 2 / g) and pure ZIF-67 (2020.0 m 2 / g). It is worth noting that the adsorption isotherm of the composite material exhibits a mixed characteristic of type I and type IV, accompanied by the appearance of an H3-type hysteresis loop, which confirms the coexistence of micropores and mesopores in the material. The existence of the micropore characteristics provides direct evidence for the ZIF-67 component.

[0059] Table 1 N2 physical adsorption data of ZIF-67, NiAl-LDH, and ZIF-67-LDH-3

[0060]

[0061] Figure 6 a shows the thermogravimetric analysis results of ZIF-67, LDH, and their composite material ZIF-67-LDH-3 at high temperature. The mass loss in the initial stage (temperature below 100 °C) can be attributed to the desorption of adsorbed water and carbon dioxide. In the temperature range of 100 °C to 330 °C, the thermal decomposition behavior of ZIF-67-LDH-3 is similar to that of LDH, showing two obvious weight loss stages: the first stage (100 - 200 °C) corresponds to the removal of interlayer water molecules in LDH; the second stage (200 - 330 °C) is related to the decomposition of NO3 - and OH - in the interlayer. It is worth noting that when the temperature exceeds 500 °C, ZIF-67-LDH-3 shows similar thermal decomposition characteristics to ZIF-67, which is due to the decomposition and carbonization of organic ligands, resulting in the collapse of the crystal structure. This thermal decomposition behavior provides strong evidence for the existence of the ZIF-67 structure in the composite material.

[0062] Figure 6 b shows the EPR spectral analysis of NiAl-LDH and various ratios of ZIF-67-LDH. As shown in the figure, the ZIF-67-LDH-3 sample exhibits an obvious characteristic signal at a G value of 2.003, which can be attributed to the capture of electrons by oxygen vacancies. Through comparative analysis of different samples, it is found that the oxygen vacancy concentration shows the following distribution law: ZIF-67-LDH-3 > ZIF-67-LDH-1 > ZIF-67-LDH-7 > NiAl-LDH. The experimental results show that when ZIF-67 grows in-situ on the surface of LDH, the oxygen vacancy concentration in the material increases significantly. In the reaction system for the oxidation of HMF to prepare FDCA, these oxygen vacancies can not only effectively adsorb HMF molecules but also promote the electron transfer between the catalyst and the reactants, thus facilitating the generation of reactive oxygen species and improving the catalytic performance.

[0063] Fourier transform infrared spectroscopy analysis reveals the structural characteristics of the samples. In the infrared spectrum of ZIF-67, 1590 cm -1The absorption peak at corresponds to the stretching vibration mode of the C=N bond in the 2-methylimidazole ligand, while the characteristic peaks at 2950 cm -1 and 3127 cm -1 are respectively attributed to the vibrations of the C-H bonds in the aromatic ring and the aliphatic chain. In addition, multiple characteristic peaks in the range of 600 - 1300 cm -1 can be ascribed to the stretching and bending vibration modes of the imidazole ring.

[0064] For the pure NiAl-LDH sample, the broad peak at 3450 cm -1 reflects the stretching vibration of the O-H bond in the lamellar structure, and the absorption peak at 1630 cm -1 is related to the O-H bending vibration of the interlayer water molecules. It is worth noting that the sharp peak at 1370 cm -1 can be attributed to the bending vibration of CO3 2- in the layered structure, while the vibration band below 800 cm -1 originates from the vibration modes of the metal-oxygen bonds (Ni-O, Al-O, and Ni-O-Al). By analyzing the infrared spectrum of the ZIF-67-LDH-3 composite material, it is found that it simultaneously has the characteristic absorption bands of ZIF-67 and NiAl-LDH, which fully confirms the successful preparation of the ZIF-67-LDH catalyst.

[0065] Experimental Example 2 Catalytic Oxidation of HMF Experiment

[0066] 2.1 Any one of the following two devices can be selected for this experimental example:

[0067] Device 1: The specific operation is as follows: Add 50 mg of HMF, 50 mg of ZIF-67-LDH catalyst, an appropriate amount of solvent (20 ml of deionized water or 10 ml of acetonitrile), and an oxidant (oxygen or tert-butyl hydroperoxide) into the inner lining of a 50 mL stainless steel autoclave with a PTFE inner lining in sequence, and then carry out the catalytic oxidation reaction under the conditions of 80 °C for 6 h.

[0068] Device 2: Use a three-necked round-bottom flask as the reaction vessel. This device is equipped with a mechanical stirring system and a reflux condenser for the catalytic oxidation experiment of 5-hydroxymethylfurfural. The specific operation is as follows: Add 50 mg of HMF, 50 mg of ZIF-67-LDH catalyst, and an appropriate amount of solvent (20 ml of deionized water or 10 ml of acetonitrile) into the reactor; after heating the reaction system to the predetermined temperature of 80 °C in an oil bath, slowly add the hydrogen peroxide solution diluted to a concentration of 3% through a constant pressure dropping funnel, and carry out the oxidation reaction under atmospheric pressure.

[0069] After the reaction is completed, centrifuge the mixed solution, collect the supernatant, and then quantitatively analyze the sample by high performance liquid chromatography.

[0070] The reaction mechanism of HMF oxidation to FDCA over ZIF-67-LDH catalyst is as Figure 7 shown.

[0071] 2.2 Experimental data analysis

[0072] High performance liquid chromatography (HPLC) was used to analyze HMF and its derivative oxidation products in the samples. High performance liquid chromatography: An ultraviolet-visible detector (Shimadzu, 254 nm) and a C-18-A chromatographic column (Diamosil, 5 μm, 4.6 mm × 250 mm) were used. A water-acetonitrile mixture (95:5, v / v) was used as the mobile phase (flow rate 0.8 mL / min, 30 °C). Under these conditions, the elution times of 2,5-furandicarboxylic acid (FDCA), 5-formyl-2-furancarboxylic acid (FFCA), 5-formyl-2-furoic acid (HMFCA), 5-hydroxymethylfurfural (HMF) and 2,5-furandicarbaldehyde (DFF) were 4.6, 5.3, 6.8, 9.4 and 11.0 min, respectively.

[0073] Calculation method of HMF conversion rate:

[0074] Y HMF : Conversion rate of HMF

[0075] m: Amount of HMF added before the catalytic reaction

[0076] mi: Remaining amount of HMF detected by liquid chromatography after the catalytic reaction

[0077] Calculation method of reaction product selectivity:

[0078] S: Selectivity of HMFCA, DFF, FFCA, FDCA

[0079] n: Total molar amount of HMFCA, DFF, FFCA, FDCA in the product after the catalytic reaction

[0080] ni: Molar amount of HMFCA, DFF, FFCA, FDCA in the product detected by liquid chromatography after the catalytic reaction

[0081] 2.2.1 The test results of HMF oxidation performance over different catalysts in System 1 are shown in Table 2. Reaction conditions: 0.05 g HMF, 0.36 ml t-BuOOH, 6 h, 80 °C, 10 mL CH3CN, 0.05 g catalyst.

[0082] Table 2 Test results of HMF oxidation performance over different catalysts

[0083]

[0084] 2.2.2 The test results of the oxidation performance of HMF on different catalysts in System 2 are shown in Table 3. Reaction conditions: 0.05 g of HMF, 1 MPa of O2, 6 h, 80 °C, 20 ml of deionized water, NaOH:HMF = 4:1, 0.05 g of catalyst.

[0085] Table 3 Test results of the oxidation performance of HMF on different catalysts

[0086]

[0087] 2.2.3 The test results of the oxidation performance of HMF on different catalysts in System 3 are shown in Table 4. Reaction conditions: 0.05 g of HMF, 20 ml of H2O2 (3%), 6 h, 80 °C, 20 ml of deionized water, NaOH:HMF = 4:1, 0.05 g of catalyst.

[0088] Table 4 Test results of the oxidation performance of HMF on different catalysts

[0089]

[0090]

[0091] 2.2.4 The test results of the oxidation performance of HMF on different catalysts in System 4 are shown in Table 5. Reaction conditions: 0.05 g of HMF, t-BuOOH (0.36 ml, 70 wt%), 6 h, 80 °C, 20 ml of deionized water, NaOH:HMF = 4:1, 0.05 g of catalyst.

[0092] Table 5 Test results of the oxidation performance of HMF on different catalysts

[0093]

[0094] 2.2.5 The test results of the oxidation performance of HMF on different catalysts in System 5 are shown in Table 6. Reaction conditions: 0.05 g of HMF, 1 MPa of O2, 6 h, 80 °C, 10 ml of CH3CN, 0.05 g of catalyst.

[0095] Table 6 Test results of the oxidation performance of HMF on different catalysts

[0096]

[0097]

[0098] 2.2.6 The test results of the oxidation performance of HMF on different catalysts are shown in Table 7. The reaction conditions are as follows: 0.05 g of HMF, 20 ml of H2O2 (3%), 6 h, 80 °C, 10 ml of CH3CN, and 0.05 g of catalyst.

[0099] Table 7 Test results of the oxidation performance of HMF on different catalysts

[0100]

[0101] As can be seen from Table 2 - Table 7, in the six different catalytic systems, there are significant differences in the reaction performance of the oxidation of HMF (5 - hydroxymethylfurfural) to FDCA (2,5 - furandicarboxylic acid). Through the comparative analysis of the catalytic effects, it is found that the first system exhibits the most excellent catalytic performance. Specifically, ZIF - 67 achieved an HMF conversion rate of 80.7%, and the FDCA selectivity reached 30.1%; in contrast, NiAl - LDH had a higher HMF conversion rate of 91.9%, but its FDCA selectivity was slightly lower, only 28.8%. It is worth noting that among a series of ZIF - 67 - LDH with different ratios, ZIF - 67 - LDH - 3 showed more prominent catalytic performance, with an HMF conversion rate of 92.1% and an FDCA selectivity of 63.3% simultaneously. Based on these experimental results, it can be concluded that ZIF - 67 - LDH - 3 has the most excellent catalytic activity in the reaction of catalyzing the oxidation of HMF to FDCA.

[0102] To improve the catalytic performance of ZIF - 67 - LDH, taking ZIF - 67 - LDH - 3 as an example, the influence law of reaction conditions on the catalytic performance was investigated. Through the analysis of experimental data, it was found that the reaction temperature has a significant influence on the selectivity of FDCA. As Figure 8 shown in a, the selectivity of FDCA shows an obvious parabolic characteristic with the change of temperature. Specifically, when the temperature rises from 60 °C to 120 °C, the selectivity of FDCA continuously increases, and at 120 °C, the selectivity of FDCA reaches 71.4%, and the conversion rate of HMF is 100%. However, when the temperature exceeds 120 °C, the FDCA selectivity starts to decline instead. Therefore, the optimal temperature for maximizing the FDCA yield is 120 °C, and this temperature was used in subsequent experiments.

[0103] This study investigated the influence law of the reaction time in the catalytic system on the HMF conversion process ( Figure 8b). The experiment was carried out at 120 °C, and the changes in the reaction process were monitored within 2 to 16 hours. The data in the initial stage (2 h) showed that the conversion rate of HMF reached 85.0%, and the selectivity of FDCA was 50.2% at this time. As the reaction proceeded to 4 - 10 hours, the selectivity of FDCA showed a significant upward trend, increasing from 66.1% to 76.1%. It is worth noting that when the reaction time reached 6 hours, HMF was completely converted. During the experiment, it was observed that the selectivities of intermediate products such as DFF, HMFCA, and FFCA gradually decreased, while the selectivity of FDCA reached a peak of 76.1% at 10 hours. However, when the reaction time was extended to 12 - 16 hours, the selectivity of FDCA showed a slight decrease due to the further degradation of FDCA. Based on the above experimental results, it can be determined that the optimal reaction time for the selective oxidation of HMF to FDCA in the ZIF-67-LDH catalytic system is 10 hours.

[0104] According to Figure 8 c's experimental data, the correlation between the catalyst dosage and the HMF conversion efficiency and FDCA selectivity was systematically studied, and it was found that HMF could be completely converted when the catalyst dosage was in the range of 25 - 75 mg. The experimental data showed that when 25 mg of the catalyst was used, the selectivity of FDCA was 70.4%; as the catalyst dosage increased to 50 mg, the selectivity of FDCA increased to 76.1%; when the dosage reached 75 mg, the selectivity of FDCA reached a peak of 80.9%. As the catalyst dosage increased to 100 mg, a significant decrease in the selectivity of the target product was observed in the experiment. Based on the comprehensive experimental results, 75 mg was determined as the optimal catalyst dosage, and the optimal FDCA selectivity could be obtained under this condition.

[0105] To further optimize the reaction conditions, this study investigated the effect of the dosage of the oxidant t-BuOOH on the reaction selectivity ( Figure 8 d). The experimental results showed that HMF could be completely converted when the dosage of the oxidant was between 0.12 - 0.6 ml, and the selectivity of FDCA was positively correlated with the dosage of the oxidant, gradually increasing with the increase in the addition amount of t-BuOOH. When the dosage of the oxidant reached 0.6 ml, the selectivity of the system for FDCA reached the best value of 84.0%. Based on this experimental result, the dosage of t-BuOOH was determined to be 0.6 ml in the subsequent research to ensure the best catalytic effect.

[0106] Through the optimization of the experimental conditions, under the best experimental conditions (120 °C, 10 h, 0.6 ml t-BuOOH, 0.075 g ZIF-67-LDH-3), the best yield of FDCA by ZIF-67-LDH-3 could reach 84.0%,

[0107] Test Example 3 Catalyst Stability

[0108] To evaluate the recycling performance of the ZIF-67-LDH catalyst, taking ZIF-67-LDH-3 as an example, 10 consecutive reaction experiments were carried out on ZIF-67-LDH-3. As Figure 9 shown, after each reaction, the catalyst was recovered by centrifugation, washed with ethanol and dried at room temperature for 12 hours before being used in the next reaction. After 10 cycles, HMF could be completely converted, and the yield of FDCA still remained at 82.7%, with only a slight decrease in selectivity. This minor performance decay might be attributed to the trace metal loss of the catalyst during the recycling process. The ICP analysis results showed that the contents of Co, Ni, and Al elements in the reaction solution were all below the detection limit, confirming that the catalyst had good structural stability during the reaction and no obvious loss of active components occurred. Further systematic analysis of the recycled catalyst was carried out by various characterization methods. The SEM image ( Figure 10 ) indicated that the morphological characteristics of the catalyst did not change significantly after 10 cycles. The XRD pattern ( Figure 11 ) showed that the main crystal phase structure of the catalyst remained intact and no obvious changes occurred in the characteristic diffraction peaks. The XPS analysis results ( Figure 12 ) confirmed that the chemical valence states of the elements in the catalyst remained stable during the reaction. These characterization results fully demonstrated that the ZIF-67-LDH catalyst could maintain stability during multiple recycling uses.

Claims

1. A ZIF-67-LDH catalyst, characterized in that, The catalyst is obtained by compounding NiAl-LDH and ZIF-67.

2. A method for preparing the catalyst according to claim 1, characterized in that, It includes the following steps: S1. Prepare NiAl-LDH: Take NiCl2•6H2O, Al(NO3)3•9H2O and urea and add them into a beaker, add deionized water, stir evenly, transfer the evenly stirred mixed solution into a hydrothermal reaction kettle, place it in a blast drying oven for reaction, after the reaction terminates, cool it to room temperature, then carry out vacuum filtration, wash it with deionized water for multiple times until the pH of the filtrate is 7, and then dry the product to obtain a NiAl-LDH sample for standby; S2. Prepare the ZIF-67-LDH catalyst: S21. Dissolve C4H6N2 in methanol to obtain a methanol solution of C4H6N2 for standby; S22. Place the NiAl-LDH obtained in S1 and Co(NO3)2•6H2O in a beaker, add methanol to form a uniformly dispersed mixed solution, and slowly drop the methanol solution of C4H6N2 obtained in S21, continuously stir for reaction, after the reaction ends, let the mixture stand, and retain the solid product after centrifugation; S23. Wash the solid product obtained in S22 with methanol for multiple times and dry it to obtain the ZIF-67-LDH catalyst.

3. The preparation method according to claim 2, wherein In S1, the molar ratio of NiCl2•6H2O, Al(NO3)3•9H2O and urea is 2:1:15; use a magnetic stirrer to stir for 1 h; place it in a blast drying oven at 120 °C for reaction for 6 h; when drying, place the obtained product in an oven and dry it under the condition of constant temperature at 90 °C for 10 hours.

4. The preparation method according to claim 2, characterized in that, In S2, the molar ratio of Co(NO3)2•6H2O and C4H6N2 is 3:

8.

5. The preparation method according to claim 2, characterized in that, In S21, the material-liquid ratio of C4H6N2 and methanol is 0.79 g:40 ml.

6. The preparation method according to claim 2, wherein, In S22, the material-liquid ratio of Co(NO3)2•6H2O and methanol is 1.05 g:40 ml; the stirring time is 1 h, let it stand for 24 h, and centrifuge at a speed of 6000 rpm for 20 minutes.

7. The preparation method according to claim 2, wherein, In S22, the mass ratio of NiAl-LDH and Co(NO3)2•6H2O is 1-7:10.

5.

8. The preparation method according to claim 2, characterized in that, In S23, the drying temperature is 50 °C.

9. Application of the catalyst described in claim 1 or the catalyst prepared by the preparation method described in any one of claims 2-8 in the catalytic oxidation of HMF.

10. The application according to claim 9, characterized in that, The catalytic oxidation of HMF is to oxidize HMF to FDCA.

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