Effect of Pyrolysis Conditions on the MOFs-Derived Zinc-Based Catalysts in Acetylene Acetoxylation

Abstract

The preparation method and calcination temperature of metal-organic framework (MOFs)-derived materials are critical factors affecting catalytic performance. In this work, the preparation conditions of MOFS precursors were optimized, and zinc-based catalysts with different activities (MOF5-700, MOF5-750, and MOF5-800) were obtained by pyrolysis of MOFS precursors under nitrogen, which were then applied to an acetylene acetoxylation reaction system. According to the results, the conversion rate of acetic acid under catalysis was significantly different. (MOF5-700 (48%), MOF5-750 (62%), and MOF5-800 (22%)). Comparing the activity of the catalyst with the industrial catalyst Zn(OAc)₂/AC (20%), MOF5-750 showed higher activity, and the acetic acid conversion rate remained around 60% after 50 h of stability testing. By characterization analysis, MOFs-derived materials were obtained after proper temperature pyrolysis. They have high mesoporous content, defects, and oxygen-containing functional groups and can maintain a good crystal structure, greatly reducing the loss of active components. This is the main reason for the good performance of the MOF5-750 catalyst in acetylene acetoxylation. Thus, the preparation conditions and favorable pyrolysis temperature of MOF derivative catalysts play a key role in the catalytic performance of acetylene acetoxylation.

1. Introduction

Vinyl acetate (VAc) is an important raw material in the chemical industry. It is mainly used in the production of important downstream chemicals, such as polyvinyl acetate (PVAc), polyvinyl alcohol (PVA), and vinyl acetate-ethylene copolymer emulsion (VAE), and is also widely used in coatings, adhesives, vinylon, and other fields [1,2,3,4]. There are two main synthesis methods for vinyl acetate: the first is the acetylene process, which uses acetylene and acetic acid as raw materials to synthesize vinyl acetate, as shown in Formula (1). The other is the ethylene process, which synthesizes ethylene acetate from ethylene, oxygen, and acetic acid, as shown in Equation (2) [5,6]. Although oil reserves and natural gas are relatively low in China, relatively abundant coal resources have been discovered [7]. Therefore, the synthesis of VAc via the calcium carbide acetylene process has great potential [8].

$${\mathrm{C}}_2{\mathrm{H}}_2 + \mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H} \stackrel{Catalyst}{\to } \mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}{\mathrm{H}}_3$$

(1)

$${\mathrm{C}}_2{\mathrm{H}}_4 + \mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H} + 0.5{\mathrm{O}}_2\stackrel{Catalyst}{\to } \mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}{\mathrm{H}}_3 + {\mathrm{H}}_2\mathrm{O}$$

(2)

Currently, most acetylene acetoxylation catalysts used on a large scale in industries are mainly prepared with zinc as the active component and activated carbon as the carrier. Although this catalyst has high economic efficiency, it has disadvantages such as poor mechanical strength, poor stability, a low conversion rate, and easy carbon accumulation [4]. To solve these problems, He et al. improved the activity and stability of the catalyst by doping it with bimetallic Zn-Ni and Zn-Co [9]. Li et al. prepared Zn-O-C, Zn-O/N-C, and Zn-N-C catalysts using different MOF materials and applied them to the acetylene acetoxylation reaction to elucidate the structure and electronic changes between Zn, O, and N during MOF pyrolysis [10]. The structure between Zn, O, and N during MOF pyrolysis was elucidated, and the electronic changes influenced the adsorption capacity and catalytic reaction activity of the catalysts. In addition, activated carbon carriers were modified using nitrogen, oxygen, and boron to alter the electron cloud density around the zinc by changing the degree of defects in the carrier, thereby increasing the catalytic activity. However, there are many views as to what influences catalyst performance [11,12,13,14].

MOF materials can be used in gas separation, sensors, catalytic reactions, and other industries due to their structural and pore diversity, large specific surface area, and surface defects [15,16,17,18,19]. They can also be used to prepare templates or precursors for porous carbon materials or metal composites. MOFs are directly converted into porous carbon materials using pyrolysis, and the pyrolysis temperature and atmosphere are the key factors that change their properties [20,21,22,23]. Zhang [24] et al. investigated the effect of pyrolysis conditions (temperature, ramp-up rate, and two-part ramp-up) on the redox (ORR) performance of ZIF-derived porous carbon catalysts using co-doped ZIF-8 material. Mehar et al. modulated C₅₊ selectivity for Fischer-Tropsch synthesis by tuning the pyrolysis temperature of MOFs derived from Fe-based catalysts. Furthermore, the Fe@C catalysts were successfully synthesized through direct pyrolysis of Fe-MIL-88B with MOFs structure at high temperatures (600, 700, and 800 °C). The results indicated that the catalyst Fe@C-800 pyrolyzed at 800 °C achieved an exceptionally high C₅₊ hydrocarbon selectivity near 90% with good reaction stability [25]. Thus, this study shows that MOF materials can be applied to catalytic acetylene acetoxylation after pyrolysis.

In this study, MOF-5 was mainly used as catalyst precursors, and the appropriate preparation conditions were first screened before being calcined under inert gas to obtain zinc-based catalysts at different calcination temperatures, and the effects of the pyrolysis temperature on the catalyst structure and the catalytic reaction were investigated.

2. Results and Discussion

2.1. Effect of Different Preparation Conditions on MOFs-Derived Zinc-Based

We first applied Zn(NO₃)₂·6H₂O and H₂BDC at mass ratios of 1:1, 1:0.5, and 1:0.3 and a hydrothermal temperature of 140 °C, and after calcination at 750 °C, it was applied to the catalytic reaction against acetylene acetoxylation, and the evaluation results are shown in Figure 1. The catalyst showed more than 60% good activity when the raw material mass ratio for the preparation of the catalyst was 1:0.5 and 1:0.3, but the activity dropped abruptly when the raw material mass ratio was 1:1, and the stable activity was only approximately 10%.

The synthesis size or morphological control of MOF crystals usually depends on the precise control of the nucleation and growth of MOF, and MOFs of different sizes can be obtained by adjusting the solvent concentration, polarity, solubility of the solvent, and reaction time [26].

Figure 2 shows the SEM images of MOF-5 prepared at a hydrothermal temperature of 140 °C at 300 times magnification when the mass ratios of Zn(NO₃)₂·6H₂O/H₂BDC are 1:1, 1:0.5, and 1:0.3. Through comparison, it was found that the mass ratio of the raw materials would directly affect the size and shape of MOF-5 crystals. When the mass ratio was 1:1, the shape of the particles tended to be lamellar, probably because of the high-temperature pyrolysis, which was not conducive to inhibiting the flow of the active components, resulting in a lower activity of the catalyst for the application of the reaction. When the mass ratio was 1:0.5, although there was good activity, it can be seen from Figure 1 that the activity fluctuated greatly and was not stable enough, while the mass ratio of 1:0.3 showed good activity and stability. For the subsequent study, we used the 1:0.3 feedstock mass ratio.

Figure 3 shows MOF-5 materials prepared at different hydrothermal temperatures for TG analysis. In the first stage, within the temperature range of 30~200 °C, the sample began to lose weight slowly, and the mass loss was about 5%, which was caused by the evaporation of the crystal water in the sample and the solvent molecules on the surface. In the second stage, in the temperature range of about 200~280 °C, the sample weight loss rate is relatively fast, and the final mass loss is about 25%, which may be caused by the volatilization of solvent molecules in the structure of the sample pore and the decomposition of carboxylic acid on the organic ligand. In the third stage, each sample has almost no obvious mass loss at 280~420 °C. In the fourth stage, at about 420~520 °C, the sample lost weight rapidly, which was caused by further decomposition of the organic ligand and preliminary decomposition of the MOF-5 material. At this time, the skeleton began to collapse. Finally, above 600 °C, the organic ligand decomposition is almost complete. Although the TG curves of different hydrothermal temperatures are slightly different, the trend is generally the same [27].

Figure 4 shows the microscopic morphology of MOF-5 prepared for hydrothermal conditions of 120 °C, 140 °C, 160 °C, and 180 °C. The hydrothermal temperature affects the microscopic morphology, but the overall shape still shows square particles. The presence of rice-like material on the sample particles at 120 °C is due to the formation of smaller crystal particles. The surface of the morphology formed at 140 °C is relatively smooth, but cracks start to appear at 160 °C, and the cracks become very obvious at 180 °C. The solvent DMF, at 153 °C, decomposes to formic acid at high temperatures because of its boiling point [28]. Cracks may be due to the thermal decomposition of the solvent into gas in the structure, resulting in morphological fractures [29]. In summary, when using DMF as a solvent, controlling the heating temperature is critical, so the optimum hydrothermal temperature is 140 °C. The cracking is due to the breakdown of gas from the solvent in the structure, resulting in the breakage of the morphology. Therefore, when using DMF as a solvent, the heating temperature should preferably not be higher than its boiling point temperature.

Figure 5 shows the catalysts obtained after calcination of the MOF-5 precursor at 750 °C using different hydrolysis temperatures. The acetic acid conversion rate of the four sets of data did not change much overall, indicating that changes in specific surface area, pore volume, and pore size of the catalysts due to the hydrothermal temperature did not have a significant effect on the activity. The data on the specific surface area, pore volume, and pore size of each sample are shown in

Table 1. Specific surface area, pore volume and pore size data of each sample.

Samples	BET Surface Area m2/g	Pore Volume cm3/g	Pore Size nm
MOF5-750 HT (120 °C)	306.89	0.19	4.67
MOF5-750 HT (140 °C)	1149.44	0.87	6.36
MOF5-750 HT (160 °C)	606.61	0.41	7.42
MOF5-750 HT (180 °C)	508.23	0.34	4.73

. Based on the above results, the optimum hydrothermal temperature is 140 °C.

The pyrolysis temperature of MOF-derived materials is an important factor affecting the catalyst activity [30]. Thus, we conducted high-temperature calcination of MOF-5 precursors at 700 °C, 750 °C, and 800 °C. The micromorphology of the catalyst obtained after treatment is shown in Figure 6. Figure 7 shows the TEM-mapping images of MOF5-750. It can be seen from the figure that the elements were uniformly dispersed with no agglomeration, thus providing a high number of active sites.

The treated catalysts were evaluated for acetylene acetoxylation performance; moreover, the activities of the catalysts (MOF-5 and 10% Zn(OAc)₂/AC) [9,10] were compared in the literature, and the evaluation results were shown in Figure 8a. The data on the number of samples (n = 10), mean (M) and standard deviation (SD) for each sample are shown in

Table 2. Means (M) (n = 10) and standard deviations (SD) of the five sets of catalyst variables.

Testing Materials	Mean(M)	Standard Deviation (SD)
10% Zn (OAc)2/AC	19.95	1.65
MOF-5	31.95	2.40
MOF5-700	47.78	0.38
MOF5-750	62.43	5.32
MOF5-800	22	0.99

. Figure 8b shows the sample means as well as the error bars for the various catalysts in the form of a bar chart. It can be seen that there were some errors in the individual catalyst tests, but MOF5-750 was still able to maintain an activity of over 60%. Among them, MOF5-750 demonstrated better reaction activity, which could be due to the larger specific surface area providing more active sites, whereas MOF5-800 demonstrated poor reactivity.

To date, doping of elements N and O has been proven to be able to modify the electronic structure of carbon materials, and further enhance the performance of catalysts for catalytic reactions [31,32,33]. Our team has previously synthesized three different MOF precursors, and after the pyrolysis process at 450 °C, the acetic acid conversion to the obtained catalysts was found to differ significantly, namely, Zn-O-C (33%), Zn-O/N-C (27%), and Zn-N-C (12%) [10]. Thus, in order to investigate the impact of N, O, and other elements doping on the performance of the catalyst prepared from the same MOF precursors, the organic ligands H2BDC (containing C and O elements) that were used to synthesize the MOF5 precursor were swapped with H₂BDC(NO₂) (containing C, N, and O elements) and H₂BDC(NH₂) (containing C and N elements), respectively, and the corresponding MOF-5 precursor was prepared at 140 °C under hydrothermal conditions. Figure 9 displays the SEM of MOF-5 synthesized by different organic ligands under hydrothermal conditions at 140 °C. After comparison, it was observed that the organic ligands with varied functional groups had an impact on the particle size and surface morphology of MOF-5. Specifically, the MOF-5 prepared for H₂BDC(NH₂) has the largest particle shape, while the MOF-5 particles obtained from H₂BDC(NO₂) had the most significant number of surface cracks. The catalysts obtained by calcination of the above three MOF-5 synthesized from organic ligands containing different functional groups at 450 °C were applied to the acetylene acetoxylation, and the acetic acid conversion is demonstrated in Figure 10.

The three catalytic properties are not significantly different, indicating that the size of MOF-5 particles and the number of cracks on the surface of the particles did not have a significant effect on the reaction activity. Therefore, changing the organic ligands containing different functional groups in this catalytic reaction slightly affects the activity of the catalytic reaction. We still choose H₂BDC as the raw material to prepare MOF5 precursor.

Therefore, we obtained better catalyst preparation conditions. Under the condition that the mass ratio of Zn (NO₃)₂·6H₂O to H₂BDC is 1:0.3, the precursor of MOF-5 can be obtained through hydrothermal treatment at 140 °C, and then an ideal zinc-based catalyst derived from MOFs can be obtained by calcination at 750 °C. We tested the MOF5-750 zinc-based catalyst for 50 h of activity, and the results are shown in Figure 11. It can be seen that MOF5-750 can basically ensure that the activity is maintained at about 60% under the 50-h activity test, and the stability is good.

Compared with the CH₃COOH conversion rate, the calcination temperature significantly affects the reactivity. Therefore, we investigated how the calcination temperature affects the catalytic reaction using relevant characterization.

2.2. Analysis of the Effect of Different Pyrolysis Conditions on MOFs-Derived Zinc-Based

The specific surface area, pore volume, and pore size data of each sample are shown in

Table 3. Specific surface area pore volume and pore size data of each sample.

Samples	BET Surface Area m2/g	Pore Volume cm3/g	Pore Size nm
MOF-5	301.40	0.33	6.26
MOF5-700	443.95	0.33	6.44
MOF5-750	1149.44	0.87	6.36
MOF5-800	1709.15	1.06	5.34

(where the raw data of sample MOF-5 was referenced to previous work [10]), and the N₂ adsorption/desorption isothermal curves and pore size distribution of the samples are shown in Figure 12a. The catalysts obtained after high-temperature calcination have a multilayer structure of microporous (<2 nm), mesoporous (2–50 nm), and macro-porous (<50 nm) at the same time. According to IUPAC classification, it can be classified as H4 type hysteresis loop. H4 type hysteresis loop is usually found in some micro-mesoporous carbon materials and mesoporous zeolite molecular sieves, which is typical of AC type with narrow cleavage pore samples and also indicates three catalysts with narrow cleavage pore characteristics. Combined with the pore volume pore size distribution curves, it can be observed that the pore sizes of MOF5-700, MOF5-750, and MOF5-800 are concentrated at 3–4 nm, and their adsorbed nitrogen volumes gradually increase, which is consistent with the trend of pore volume data size (0.33, 0.87, and 1.06 cm³g⁻¹). We analyzed the data on the ratio of mesopore pore volume to total pore volume and found (Figure 12b) that MOF5-750 has the highest mesopore content, indicating that the higher mesopore content facilitates the catalyst reaction.

All the samples were collected and measured with XRD, and the results are presented in Figure 13. The calcined sample showed the characteristic diffraction peaks of ZnO. The MOFs samples (MOF5-700 and MOF5-750) under the pyrolysis conditions of 700 °C and 750 °C can be observed to have obvious diffraction peaks at 31°, 34°, 36°, 47°, 56°, 62°, and 67°. This corresponds to the reflection of (100), (002), (102), (110), (103), and (112) crystal planes of the ZnO standard diffraction peak (JCPDS No. 41-1487). The results show that the zinc in the MOF5-700 and MOF5-750 samples is mainly in the form of ZnO. The results of XRD characterization showed that compared with the diffraction peaks of MOF5-700 and MOF5-750, MOF5-800 did not observe the obvious characteristic diffraction peaks of ZnO, and two diffraction peaks at 24.41° and 43.71° were amorphous peaks, which were consistent with the (002) and (101) crystal planes of AC [34]. It shows that a large amount of zinc element is lost after the sample is calcined at high temperature [35]. In addition, only a very small amount of Zn content was detected by testing the MOF5-800 sample. For example, according to the ICP test, the Zn content in MOF5-800 was only 0.6%, and the result measured by XPS was 0.38%. Both measurement results can prove that MOF5-800 has a very low Zn content compared with the other two samples.

To deeply understand the roles of the pyrolysis temperature on Zn content and oxygen-containing functional groups. We performed XPS analysis on three catalysts, as shown in Figure 14. The XPS spectra (Figure 14a) clearly show Zn 2p, O 1s, and C 1s signals of the catalyst samples; it can be clearly seen that there are Zn 2p, O 1s, and C 1s signals in MOF5-700 and MOF5-750 catalyst samples, while Zn 2p and O 1s signals in MOF5-800 catalyst samples cannot be significantly observed. It indicates that the higher temperature calcination results in the loss of Zn and O elements in the catalyst to a certain extent. At the same time, we used wide survey XPS spectra and ICP to measure the content of the Zn element in three different samples, and the results are shown in Figure 14d. The results show that there is a certain difference in the content of Zn in the samples tested by the two methods. The main reason for this difference is that wide survey XPS spectra can only measure the content of Zn on the surface of the catalyst and cannot detect the actual content of Zn in the samples or even in the whole. ICP can detect the content of Zn in the whole catalyst. Through the comparison of the two tests, it can be concluded that with high temperature calcination, the active component on the surface of the catalyst is most likely to lose, while the active component inside the catalyst is covered by the carbon skeleton, which can play a certain role in inhibiting the loss of the active component.

The peak positions of Zn 2p_1/2 and Zn 2p_3/2 for MOF5-700, MOF5-750, and MOF5-800 catalysts can be clearly observed at 1022.33 eV and 1045.38 eV, respectively, as shown in Figure 14b. Therefore, the binding energy of Zn 2p with different catalysts has no significant deviation, indicating that the sample has not significantly changed the electron cloud density of zinc through calcination. The O 1s spectrum shown in Figure 14c can be divided into three peaks: O (531.0 eV) [36], -C-OH (531.7 eV) [37], and -C=O (533.9 eV) [38] adsorbed at the defect. MOF5-800 has no child peak at the position of 531.0eV, which may be caused by the high temperature at which O adsorbed at the defect is lost in the form of CO₂.

The results of the catalyst evaluation showed that the activities of MOF5-750 and MOF5-700 were higher than those of MOF5-800. In summary, catalyst samples calcined at appropriate calcination temperatures are beneficial to prevent the loss of active components, and higher -OH, -C=O content in the samples is conducive to improving the catalytic performance.

Raman spectroscopy can characterize the degree of defecting in the sample. As shown in Figure 15. We performed Raman characterization of MOF5-700, MOF5-750, and MOF5-800. For MOF5-700, MOF5-750, and MOF5-800, they all exhibit two characteristic peaks of carbon material: the D band at 1362 cm⁻¹ and the G band at 1588 cm⁻¹ correspond to the in-plane vibrations of disordered graphitic carbon and sp₂ carbon atoms, respectively. From the D/G band intensity ratio (I_D/I_G), the I_D/I_G of MOF5-700, MOF5-750, and MOF5-800 were 2.77, 2.33, and 2.12, respectively, showing an increasing and then decreasing trend, and MOF5-750 had the most surface defects, indicating that the carbonization process from MOFs to porous carbon was successful. With the increase in calcination temperature, the graphitization of the catalyst gradually increases. This indicates that the higher calcination temperature destroys the graphitized structure, resulting in fewer structural defects. In addition, the decrease of the I_D/I_G value also indicates that the graphitic structure becomes less stable, less able to maintain its unique morphology, and less able to transfer electrons, thus showing poor reaction performance in the catalytic reaction.

3. Materials and Methods

3.1. Materials

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, Tianjin Guangfu,98%), terephthalic acid (H₂BDC, Aladdin Shanghai China, 99.0%), N, N-dimethylformamide (DMF, Macklin Shanghai China, 99.0%), CH₃COOH (Tianjin Beilian, 99.8%), 5-Aminoisophthalic acid (H₂BDC (NH₂), Macklin Shanghai China, 98.0%), C₂H₂ (gas, 99.0%), AC (pH = 6–8, coconut carbon) and 5-Nitroisophthalic acid (H₂BDC (NO₂), Macklin Shanghai China, 98.0%), all reagents, were analytical grade; no further purification was required.

3.2. Catalysts’ Preparation

The MOF-5 precursor was prepared using Zn(NO₃)₂·6H₂O and H₂BDC as raw materials. The MOF-5 precursor is shown in Figure 16.

3.2.1. Preparation of MOF5 Precursor

Firstly, 4.72g of Zn(NO₃)₂·6H₂O (15.8 mmol) and 4.72g H₂BDC (28.4 mmol) were weighed, with a mass ratio of 1:1 and a molar ratio of 1:1.8. They were dissolved in 100mL of DMF solution, and then the solution containing Zn(NO₃)₂·6H₂O was slowly poured into the solution containing H₂BDC. After the two solutions were completely mixed, the solution was transparent. The solution was placed on a magnetic stirrer, and stirred at a speed of 800 r min⁻¹ for 1 h. The solution was then poured into a 200 mL hydrothermal reactor and kept at 140 °C for 24 h. After cooling to room temperature, the solution was placed in a centrifuge and collected at a speed of 8000 r min⁻¹. In order to replace the unreacted solvent molecules, the precursor of MOF-5 was obtained by activation of CH₂Cl₂ for 12 h and vacuum drying at 80 °C for 10 h. Using the same method, we weigh 4.72 g of Zn(NO₃)₂·6H₂O (15.8 mmol) and 2.36 g of H₂BDC (14.2 mmol), and the mass ratio is 1:0.5. The molar ratio was 1:0.9, 4.72 g Zn(NO₃) ₂·6H₂O (15.8 mmol) and 1.46 g H₂BDC (14.2 mmol), and their mass ratio was 1:0.3 and molar ratio was 1:0.54. The precursors containing different mass ratios of MOF-5 were prepared by the same method.

3.2.2. Preparation of MOF5-700, MOF5-750 and MOF5-800 Catalysts

(1) The prepared MOF-5 precursor with a mass ratio of 1:0.3 was calcined in an N₂ tube furnace at a heating rate of 5 °C/min, and the roasting temperature was set at 700 °C. After reaching the target temperature, the precursor was held for 4 h. After heat preservation, it was naturally cooled to room temperature to obtain the required catalyst, named MOF5-700.

(2) The prepared MOF-5 precursor with a mass ratio of 1:0.3 was calcined in an N₂ tube furnace at a heating rate of 5 °C/min, and the roasting temperature was set at 750 °C. After reaching the target temperature, the precursor was held for 4 h. After heat preservation, it was naturally cooled to room temperature to obtain the required catalyst, named MOF5-750.

(3) The prepared MOF-5 precursor with a mass ratio of 1:0.3 was calcined in N₂ tube furnace at the heating rate of 5 °C/min, and the roasting temperature was set at 800 °C. After reaching the target temperature, the precursor was held for 4 h. After heat preservation, it was cooled to room temperature naturally to obtain the required catalyst, named MOF5-800. The preparation process is shown in Figure 17.

3.3. Catalysts Characterization

The specific surface area, pore volume, and pore size of the sample were tested by an automated fast physical adsorption analyzer (BET, Micromeritics, ASAP 2460, Atlanta, GA, USA). High-resolution TEM (HRTEM) data were performed by using a Tecnai G2 F20 instrument (FEI Company, Hillsboro, Oregon, USA). The scanning electron microscope (SEM, JEOL, JSM-6490LV, Tokyo, Japan) instrument was used to image the material. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCAlAB 250Xi, Waltham, MA, USA) was used to analyze the chemical states of elements. The thermogravimetric analyzer (TG, NETZSCH, STA 449F3, Selb, Germany) was used to test the thermal stability of the samples. An x-ray diffractometer (XRD, Bruker, D8 ADVANCE, Karlsruhe, GRE) was used to test and analyze the composition, crystal structure, and phase of the sample. The crystal particle size could be roughly calculated by the Bragg equation.

3.4. Catalyst Evaluation

A self-assembled fixed-bed reactor was used to test catalyst activity. The reaction tube is a stainless-steel tube with an inner diameter of 10 mm. Before loading the catalyst, the appropriate amount of quartz sand was filled to the bottom of the reaction tube, and then the appropriate amount of quartz cotton was loaded to prevent the infiltration of the catalyst, and then a certain amount of catalyst was added. Reaction conditions: the heating temperature was set at 220 °C, the gas hourly space velocity of C₂H₂ (GHSV) was 500 h⁻¹, and the molar ratio of C₂H₂/CH₃COOH was 1:3. Before the reaction started, CH₃COOH was used to activate the catalysts for 0.5 h, and C₂H₂ was introduced to start the reaction. Furthermore, the reaction product is condensed through the condensing tube, and the sample to be tested is collected every hour. GC-9A (Shimadzu, Tokyo, Japan) gas chromatography was used to analyze the percentage content of each substance in the samples to be tested. We used a metal tube filled column with an inner diameter of 4 mm and a length of 2 m. The model is PEG/20000 (Zhonghuida, Dalian, China). The temperature of the filling column inlet was set at 150 °C, and the hydrogen flame ionization detector was used. The temperature of the detector was set at 220 °C, and the temperature of the gasification chamber was set at 200 °C. When sample collection is complete, samples are taken using a 1μL microsampler and then injected into the injection port at the top of the chromatography to begin sample testing. Evaluation criteria: conversion rate of CH₃COOH. The formula is as follows:

$$X_A=\frac{m_{A0}-m_A}{m_{A0}}\times 100\%$$

(3)

Including m_A0 is the mass fraction of CH₃COOH of the reactants, X_A is CH₃COOH conversion, and m_A is the remaining CH₃COOH mass fraction.

4. Conclusions

In this study, the mass ratio of the raw materials, Zn(NO₃)₂·6H₂O/H₂BDC, hydrothermal temperature, and modification of organic ligands with different functional groups affect the microscopic morphology and particle size. MOF5-700, MOF5-750, and MOF5-800 catalysts obtained after calcination at 700 °C, 750 °C, and 800 °C, the microscopic particles, respectively, can still maintain their original cubic structure. When MOF5-700, MOF5-750, and MOF5-800 catalysts were applied to the acetylene acetoxylation reaction system, the CH₃COOH conversion rate varied significantly. Compared with MOF5-700 (48%) and MOF5-800 (22%), MOF5-750 can reach more than 60% activity and shows good stability. Furthermore, MOF5-750 shows higher activity compared with the MOF-5 (31%) catalyst previously developed by our team and the conventional industrial catalyst Zn(OAc)₂/AC (20%). By analysis of XRD and XPS, MOFs-derived materials were obtained after proper temperature pyrolysis. They have high mesoporous content, defects, and oxygen-containing functional groups and can maintain a good crystal structure, greatly reducing the loss of active components. This is the main reason for the good performance of the MOF5-750 catalyst in acetylene acetoxylation. In this work, the preparation and pyrolysis conditions of MOFS derivative catalysts were studied. Such research will attract more and more attention, which will further promote the application of industrial catalysis and provide a new strategy for the design and synthesis of efficient catalysts.