MOF-based polyester polycondensation catalyst as well as preparation method and application thereof

Abstract

The invention relates to the technical field of catalyst materials, in particular to an MOF-based polyester polycondensation catalyst and a preparation method and application thereof. The preparation method comprises the following steps: weighing MOF-5, adding into DMF (Dimethyl Formamide), and carrying out ultrasonic dispersion to obtain a suspension; weighing a titanium source, adding the titanium source into DMF, stirring and dissolving to obtain a titanium solution; and adding a titanium solution into the suspension liquid under stirring, stirring for 6-24 hours at the temperature of 60-120 DEG C and the speed of 300-1000 rpm, filtering, and drying the obtained solid product to obtain the Ti-MOF-5 polyester polycondensation catalyst. Ti < 4 + > is introduced into an MOF-5 framework, the Lewis acidity of the MOF-based polyester polycondensation catalyst is enhanced, Ti < 4 + > serving as a Lewis acid center can effectively activate ester groups and hydroxyl groups in a PET polycondensation reaction, so that reaction activation energy is reduced, the polycondensation reaction is accelerated, the Ti-MOF-5 catalyst has relatively low toxicity and relatively high biocompatibility, and compared with a traditional antimony catalyst, the Ti-MOF-5 catalyst has the advantages that the molecular weight of the Ti-MOF-5 catalyst is reduced, and the molecular weight of the Ti-MOF-5 catalyst is reduced. The Ti-MOF-5 not only can improve the reaction efficiency, but also meets the requirements of environmental protection and sustainable development.

Description

Technical Field

[0001] The present invention relates to the technical field of catalyst materials, in particular to an MOF-based polyester polycondensation catalyst, a preparation method thereof, and applications thereof. Background Art

[0002] Polyethylene terephthalate (PET) is one of the thermoplastic polymers with the highest production and consumption globally and is widely used in fields such as textiles, packaging, and beverage bottles. The synthesis of PET mainly proceeds through the polycondensation reaction of terephthalic acid (PTA) and ethylene glycol (EG) at high temperatures, and a polyester polycondensation catalyst is required in this process to increase the polymerization rate and control the molecular weight to ensure the physicochemical properties of the polymer.

[0003] In traditional PET synthesis, antimony-based catalysts (such as antimony trioxide, antimony acetate, and antimony glycolate) are widely used due to their high catalytic activity and low by-product formation (for example, Patent CN99122632.1 Preparation method of antimony glycolate catalyst for polyester). However, antimony, as a heavy metal, the toxic substances that may be released during the production process will cause non-negligible pollution to the ecological environment, especially in the production of food-grade PET, and this problem is particularly prominent. Therefore, the widespread application of antimony-based catalysts has gradually been restricted by environmental regulations, and there is an urgent need to develop more green and non-toxic alternative catalysts. Germanium-based catalysts and aluminum-based catalysts are two other options. Germanium-based catalysts have low toxicity and basically conform to the development trend of green environmental protection, but their catalytic activity is poor and their stability is poor in a complex reaction system, making it difficult to meet the requirements of industrial production. Aluminum-based catalysts have received more attention in recent years due to their low cost, low toxicity, and no obvious negative impact on PET products (such as Patent CN101962437 A A new aluminum catalyst for synthesizing PET by esterification polycondensation method). However, aluminum-based catalysts also face the problem of low catalytic activity, which will greatly increase the energy consumption during the production process.

[0004] In recent years, titanium-based catalysts have become another alternative to antimony-based catalysts due to their low toxicity and good catalytic performance. Titanium, as a transition metal, its Ti 4+Ions have strong Lewis acidity in the catalytic reaction and can efficiently activate the ester groups and hydroxyl groups in the reaction, thereby accelerating the PET polycondensation reaction. Patent CN106832241, a method for preparing polyethylene terephthalate using a composite catalyst, proposes a PET synthesis method based on a composite catalyst of tetrabutyl titanate, tetraethyl orthosilicate, and phenylphosphonic acid. This method can obtain PET products with high molecular weight, narrow molecular weight distribution, and excellent mechanical properties in a relatively short reaction time. However, titanium-based catalysts also have their inherent defects. High catalytic activity is often accompanied by overly violent reactions, which easily lead to the generation of a large number of by-products, thereby affecting the appearance quality of PET, especially the generation of an undesirable yellow color during the reaction, which affects the quality of the final product. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide an MOF-based polyester polycondensation catalyst, its preparation method and application, so as to at least solve the problems that the existing catalysts for synthesizing polyethylene terephthalate have low catalytic activity and are prone to generating a large number of by-products.

[0006] The present invention solves the above technical problems through the following technical means:

[0007] In the first aspect, an embodiment of the present invention provides a preparation method of an MOF-based polyester polycondensation catalyst, including the following steps:

[0008] Weigh MOF-5 and add it to DMF for ultrasonic dispersion to obtain a suspension;

[0009] Weigh a titanium source and add it to DMF for stirring and dissolving to obtain a titanium solution;

[0010] While stirring, add the titanium solution to the suspension, and stir at 60 - 120 °C for 6 - 24 h at 300 - 1000 rpm, then filter, and dry the obtained solid product to obtain a Ti-MOF-5 polyester polycondensation catalyst.

[0011] In some embodiments, the molar ratio of MOF-5 to the titanium source is (0.5 - 1.5):1.

[0012] In some embodiments, the titanium source is at least one of isopropyl titanate, butyl titanate, titanium chloride, and titanium nitrate.

[0013] In some embodiments, the preparation method of MOF-5 is as follows:

[0014] Weigh a zinc salt and dissolve it in an organic solvent under magnetic stirring to obtain a metal salt solution;

[0015] Weigh terephthalic acid and dissolve it in an organic solvent under magnetic stirring to obtain a ligand solution;

[0016] The ligand solution was added to the metal salt solution under stirring at a rotation speed of 500 - 800 rpm, then a nucleating agent was added, and the temperature was raised to 60 - 120 °C for reaction for 4 - 12 h. The solid product was collected by centrifugation and dried to obtain white powdery MOF-5.

[0017] In some embodiments, the molar volume ratio of the zinc salt, terephthalic acid, and organic solvent is 1 mmol:(1 - 6) mmol:(60 - 200) mL, and the dosage of the nucleating agent is 1 - 10% of the mass of the zinc salt.

[0018] In some embodiments, the zinc salt is at least one of Zn(NO3)2·6H2O, ZnCl2, ZnSO4·H2O, and Zn(CH3COO)2·2H2O.

[0019] In some embodiments, the organic solvent is one of methanol, ethanol, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0020] In some embodiments, the nucleating agent is at least one of polyethylene glycol, cetyltrimethylammonium bromide, polyvinylpyrrolidone, Tween-80, sodium dodecyl sulfate, and triblock copolymer Pluronic F127.

[0021] In a second aspect, an MOF-based polyester polycondensation catalyst provided by an embodiment of the present invention is prepared by using the preparation method described in the first aspect above.

[0022] In a third aspect, an embodiment of the present invention further provides an application of the MOF-based polyester polycondensation catalyst in the second aspect above in the synthesis of polyethylene terephthalate.

[0023] The MOF-based polyester polycondensation catalyst and its preparation method of the present invention use terephthalic acid, which is a raw material for synthesizing polyethylene terephthalate, as an organic ligand and zinc as a metal node, and a stable metal-organic framework MOF-5 is synthesized by a solvothermal method. Then, catalytic active sites are introduced into the MOF-5 framework through a post-synthesis ion exchange (PSIM) strategy to obtain a novel polyester catalyst Ti-MOF-5. 4+ A novel polyester catalyst Ti-MOF-5 is obtained.

[0024] The MOF-based polyester polycondensation catalyst of the present invention has the following advantages:

[0025] (1) By introducing Ti 4+ into the MOF-5 framework, the Lewis acidity of the MOF-based polyester polycondensation catalyst is enhanced. As a Lewis acid center, Ti 4+ can effectively activate the ester group and hydroxyl group in the PET polycondensation reaction, thereby reducing the reaction activation energy and accelerating the polycondensation reaction.

[0026] (2) MOF-5 has good compatibility with the reaction system, enabling the catalyst to be uniformly dispersed in the reaction system. Moreover, Ti-MOF-5 has a high specific surface area and a regular pore structure, which not only enables Ti-MOF-5 to provide more catalytic active sites, but also can effectively transfer reactant molecules through its pores. The intermediates in the PET polycondensation reaction can diffuse and react efficiently within the pores of MOF-5, effectively promoting the transesterification reaction and avoiding the occurrence of cross-reactions or oxidation reactions, thereby reducing the occurrence of side reactions and improving the selectivity and efficiency of the reaction.

[0027] (3) Ti-MOF-5 can maintain the stability of the framework structure under the conditions of high-temperature polycondensation reaction, preventing the catalyst from deactivating due to pyrolysis or agglomeration, ensuring that the catalyst has persistent catalytic activity throughout the reaction process, and extending the service life of the catalyst.

[0028] (4) The Ti-MOF-5 catalyst has low toxicity and higher biocompatibility. Compared with traditional antimony-based catalysts, Ti-MOF-5 can not only improve the reaction efficiency, but also meet the requirements of environmental protection and sustainable development. Description of the Drawings

[0029] Figure 1 is the XRD diffraction pattern of the Ti-MOF-5 polyester polycondensation catalyst prepared in Examples 1-3. Detailed Embodiments

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Apparently, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts shall fall within the protection scope of the present application.

[0031] In the embodiments of the present application, words such as "exemplary" or "for example" are used to indicate examples, illustrations, or explanations. Any embodiment or design solution described as "exemplary" or "for example" in the embodiments of the present application should not be construed as being more preferred or having more advantages than other embodiments or design solutions. Rather, the use of words such as "exemplary" or "for example" is intended to present related concepts in a specific manner.

[0032] The English letter interpretations in the present application are as follows:

[0033] PET: polyethylene terephthalate; PTA: terephthalic acid; EG: ethylene glycol; DMF: N,N-dimethylformamide; DMAc: N,N-dimethylacetamide; PEG: polyethylene glycol; CTAB: cetyltrimethylammonium bromide; PVP: polyvinylpyrrolidone.

[0034] In this application, terephthalic acid, the raw material for synthesizing polyethylene terephthalate, is used as the organic ligand, and zinc is used as the metal node. A stable metal-organic framework MOF-5 is synthesized by the solvothermal method. Then, catalytic active sites are introduced into the MOF-5 framework through a post-synthesis ion exchange (PSIM) strategy. Since Ti 4+ has a higher charge density and has a stronger affinity for oxygen atoms in the hard acid-hard base interaction. When Ti 4+ contacts the oxygen atoms in the MOF-5 structure, it can form stronger coordination bonds, thus replacing Zn 4+ and forming a new MOF-5 derivative Ti-MOF-5 containing Ti 2+ . 4+ Specifically, the preparation method of the MOF-based polyester polycondensation catalyst of this application includes the following steps:

[0035] Weigh MOF-5 and add it to DMF for ultrasonic dispersion to obtain a suspension; weigh the titanium source and add it to DMF for stirring and dissolution to obtain a titanium solution; add the titanium solution to the suspension under stirring, and stir at 60-120 °C at 300-1000 rpm for 6-24 h, filter, and dry the obtained solid product to obtain the Ti-MOF-5 polyester polycondensation catalyst.

[0036] Among them, the molar ratio of MOF-5 to the titanium source is (0.5-1.5):1, and the titanium source is at least one of isopropyl titanate, butyl titanate, titanium tetrachloride, and titanium nitrate. The concentration of MOF-5 in the suspension is 5-30 mg / mL, and the concentration of the titanium source in the titanium solution is 2-15 mg / mL.

[0037]

[0038] ​In this application, MOF-5 is synthesized by the solvothermal method. The preparation method of MOF-5 is as follows: Weigh a zinc salt and dissolve it in an organic solvent under magnetic stirring to obtain a metal salt solution; weigh terephthalic acid and dissolve it in an organic solvent under magnetic stirring to obtain a ligand solution; add the ligand solution to the metal salt solution under stirring at a speed of 500 - 800 rpm, then add a nucleating agent, and raise the temperature to 60 - 120 °C for reaction for 4 - 12 h to ensure sufficient ion exchange. Centrifuge to collect the solid product, and dry the solid product to obtain white powdery MOF-5. The molar volume ratio of the zinc salt, terephthalic acid, and the organic solvent is 1 mmol : (1 - 6) mmol : (60 - 200) mL, and the dosage of the nucleating agent is 1 - 10% of the mass of the zinc salt. The zinc salt is at least one of Zn(NO3)2·6H2O, ZnCl2, ZnSO4·H2O, and Zn(CH3COO)2·2H2O. The organic solvent is one of methanol, ethanol, N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc), and the nucleating agent is at least one of polyethylene glycol (PEG), cetyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), Tween-80, sodium dodecyl sulfate (SDS), and triblock copolymer Pluronic F127.

[0039] To better understand the above technical solution, the above technical solution will be described in detail below in combination with the accompanying drawings of the specification and specific embodiments.

[0040] Example 1

[0041] The preparation method of the MOF-based polyester polycondensation catalyst in this example is as follows:

[0042] (1) Synthesize MOF-5:

[0043] Weigh 1.19 g of Zn(NO3)2·6H2O and dissolve it in 120 mL of methanol under magnetic stirring to obtain a metal salt solution. Weigh 1.33 g of terephthalic acid and dissolve it in 120 mL of methanol under magnetic stirring to obtain a ligand solution. Slowly add the metal salt solution to the ligand solution under magnetic stirring at a speed of 600 rpm, then add 0.12 g of PVP, and raise the temperature to 80 °C for reaction for 10 h. Finally, collect the product by centrifugation, and dry the collected product in an oven to obtain white powdery MOF-5.

[0044] (2) Synthesize the catalyst Ti-MOF-5:

[0045] Weigh 0.760 g of MOF-5 and ultrasonically disperse it in 60 mL of DMF to obtain a suspension. Weigh 0.421 g of Ti(OC3H7)4 and dissolve it in 60 mL of DMF to obtain a titanium solution. Then, add the titanium solution to the suspension under magnetic stirring and stir at 800 rpm at 100 °C for 20 h. Filter the obtained solid product and dry it at 80 °C for 12 h to obtain the Ti-MOF-5 polyester polycondensation catalyst, denoted as Ti-MOF-5(1:1.5).

[0046] Example 2 The difference between this example and Example 1 is that the amount of Ti(OC3H7)4 used is 0.280 g, and other conditions remain unchanged. Finally, the Ti-MOF-5 polyester polycondensation catalyst is prepared, denoted as Ti-MOF-5(1:1).

[0047] Example 3

[0048] The difference between this example and Example 1 is that the amount of Ti(OC3H7)4 used is 0.140 g, and other conditions remain unchanged. Finally, the Ti-MOF-5 polyester polycondensation catalyst is prepared, denoted as Ti-MOF-5(1:0.5).

[0049] Example 4

[0050] The preparation method of the MOF-based polyester polycondensation catalyst in this example is as follows:

[0051] (1) Synthesize MOF-5:

[0052] Weigh 1.485 g of Zn(NO3)2·6H2O and dissolve it in 250 mL of methanol under magnetic stirring to obtain a metal salt solution. Weigh 0.83 g of terephthalic acid and dissolve it in 500 mL of methanol under magnetic stirring to obtain a ligand solution. Slowly add the metal salt solution to the ligand solution under magnetic stirring at 500 rpm, then add 0.015 g of sodium dodecyl sulfate, and raise the temperature to 60 °C and react for 4 h. Finally, collect the product by centrifugation and dry the collected product in an oven to obtain white powdery MOF-5.

[0053] (2) Synthesize the catalyst Ti-MOF-5:

[0054] Weigh 0.760 g of MOF-5 and ultrasonically disperse it in 152 mL of DMF to obtain a suspension. Weigh 0.421 g of Ti(OC3H7)4 and dissolve it in 28 mL of DMF to obtain a titanium solution. Then, add the titanium solution to the suspension under magnetic stirring and stir at 100 rpm at 60 °C for 6 h. Filter the obtained solid product and dry it at 80 °C for 12 h to obtain the Ti-MOF-5 polyester polycondensation catalyst.

[0055] Example 5

[0056] The preparation method of the MOF-based polyester polycondensation catalyst in this example is as follows:

[0057] (1) Synthesize MOF-5:

[0058] Weigh 1.485 g of Zn(NO3)2·6H2O and dissolve it in 250 mL of methanol under magnetic stirring to obtain a metal salt solution. Weigh 4.98 g of terephthalic acid and dissolve it in 750 mL of methanol under magnetic stirring to obtain a ligand solution. Under the action of magnetic stirring at a speed of 800 rpm, slowly add the metal salt solution to the ligand solution, then add 0.075 g of Tween-80, and raise the temperature to 120 °C and react for 12 h. Finally, collect the product by centrifugation, and dry the collected product in an oven to obtain white powdery MOF-5.

[0059] (2) Synthesize the catalyst Ti-MOF-5:

[0060] Weigh 0.760 g of MOF-5 and ultrasonically disperse it in 25 mL of DMF to obtain a suspension. Weigh 0.421 g of Ti(OC3H7)4 and dissolve it in 210 mL of DMF to obtain a titanium solution. Then, under magnetic stirring, add the titanium solution to the suspension, and stir at 300 rpm at 120 °C for 24 h. Filter the obtained solid product and dry it at 80 °C for 12 h to obtain the Ti-MOF-5 polyester polycondensation catalyst.

[0061] Example 6

[0062] The preparation method of the MOF-based polyester polycondensation catalyst in this example is as follows:

[0063] (1) Synthesize MOF-5:

[0064] Weigh 1.077 g of ZnSO4·H2O and dissolve it in 250 mL of N,N-dimethylformamide under magnetic stirring to obtain a metal salt solution. Weigh 4.98 g of terephthalic acid and dissolve it in 650 mL of N,N-dimethylformamide under magnetic stirring to obtain a ligand solution. Under the action of magnetic stirring at a speed of 800 rpm, slowly add the metal salt solution to the ligand solution, then add 0.054 g of polyethylene glycol, and raise the temperature to 120 °C and react for 12 h. Finally, collect the product by centrifugation, and dry the collected product in an oven to obtain white powdery MOF-5.

[0065] (2) Synthesize the catalyst Ti-MOF-5:

[0066] Weigh 0.760 g of MOF-5 and ultrasonically disperse it in 25 mL of DMF to obtain a suspension. Weigh 0.340 g of tetrabutyl titanate and dissolve it in 100 mL of DMF to obtain a titanium solution. Then, add the titanium solution to the suspension under magnetic stirring and stir at 300 rpm at 120 °C for 24 h. Filter the obtained solid product and dry it at 80 °C for 12 h to obtain the Ti-MOF-5 polyester polycondensation catalyst.

[0067] Performance testing:

[0068] (1) XRD test characterization

[0069] Perform XRD tests on the Ti-MOF-5 polyester polycondensation catalysts prepared in Examples 1-3 and the MOF-5 prepared in Example 1 respectively. The results are shown in Figure 1 .

[0070] Figure 1 The data show that MOF-5 and different Ti-MOF-5 samples exhibit obvious diffraction peaks in the low-angle (about 5° - 15°) region, indicating that the introduction of Ti 4+ does not destroy the basic crystal structure of MOF-5. With the increase in the content of Ti 4+ , the peak intensities of the Ti-MOF-5 samples change to varying degrees. Especially at a relatively high Ti 4+ ratio (such as 1:1.5), the peak intensity weakens, indicating that the incorporation of Ti 4+ has a certain impact on the crystallinity of the MOF-5 framework. This may be due to the partial exchange of Ti 4+ with Zn 2+ in the MOF-5 framework or its entry into the lattice interior, resulting in microscopic deformation or disorder of the local crystal structure. However, overall, the doping of Ti 4+ causes some lattice distortions locally or slight changes in the XRD diffraction peaks, without affecting the stability of the MOF-5 crystal structure.

[0071] (2) Specific surface area test

[0072] Take the Ti-MOF-5 polyester polycondensation catalysts prepared in Examples 1-3 and the MOF-5 prepared in Example 1 as samples, use a Belsorp MAXII analyzer, perform N2 adsorption and desorption, and test the BET surface area and pore structure gas adsorption of the materials at -195 °C. The total surface area is determined using the Brunauer-Emmett-Teller (BET) equation, and the micropore width and micropore volume are calculated by applying the Dubinin-Radushkevich (DR) equation. The test results are shown in Table 1:

[0073] Sample <![CDATA[BET(m 2 / g)]]> MOF-5 1572 Ti-MOF-5(1:0.5) 1538 Ti-MOF-5(1:1) 1496 Ti-MOF-5(1:1.5) 1404

[0074] Table 1

[0075] The data in Table 1 show that appropriate doping with Ti 4+ has little effect on the pore structure of MOF-5, and the specific surface area remains near the value of the original MOF-5.

[0076] (3) Catalytic activity test

[0077] Before the test, first, the Ti-MOF-5 polyester polycondensation catalysts prepared in Examples 1-3 were used in the synthesis of polyethylene terephthalate. Specifically, polyethylene terephthalate was synthesized according to the following synthesis method:

[0078] 1.0 kg of terephthalic acid (PTA) and 1.0 kg of ethylene glycol (EG) were added to a high-pressure reactor. After stirring evenly, the system was sealed and filled with nitrogen to 0.2 MPa, and then heated to about 250 °C. After reacting for 4 h, the esterification product was obtained. 0.1 g of the Ti-MOF-5 catalysts prepared in Examples 1-3 were respectively added to the esterification product. After evacuating and heating to 275 °C, after continuously reacting for 3 h, nitrogen was filled to normal pressure, and then the product was discharged into cooling water. The product was cut into polyethylene terephthalate cylindrical pellets with a diameter of about 3 mm using a pelletizer.

[0079] In addition, three groups of comparative examples were designed as follows:

[0080] Comparative Example 1

[0081] The catalyst in the above polyethylene terephthalate synthesis method was replaced with 0.1 g of Ti(OC3H7)4, and other conditions remained unchanged for comparative verification of catalytic activity.

[0082] Comparative Example 2

[0083] The catalyst in the polyethylene terephthalate synthesis method of Comparative Example 1 was replaced with MOF-5, and other conditions remained unchanged for comparative verification of catalytic activity.

[0084] Comparative Example 3

[0085] The catalyst in the polyethylene terephthalate synthesis method of Comparative Example 2 was replaced with antimony glycolate, and other conditions remained unchanged for comparative verification of catalytic activity.

[0086] Subsequently, the catalytic activity test was carried out. Under the same polycondensation conditions, the higher the intrinsic viscosity of the polyester obtained, the higher the catalytic activity of the catalyst. The activity of the catalyst can be evaluated by measuring the viscosity of the polyester with an Ubbelohde viscometer. The empirical formula for the viscosity-average molecular weight:

[0087]

[0088] where K = 2.4×10 -4 and a = 0.82

[0089] Generally, the viscosity-average molecular weight of the polyethylene terephthalate stock solution is controlled at 18,000 - 30,000 g / mol. An excessively low viscosity-average molecular weight (less than 18,000 g / mol) will result in shorter molecular chains of PET, thereby reducing the strength, toughness, and thermal stability of the polymer and affecting the mechanical properties of the product; while an excessively high viscosity-average molecular weight (exceeding 30,000 g / mol) will lead to difficult melt processing of PET, increase energy consumption, and easily generate bubbles and uneven flow during molding, affecting the quality of the finished product.

[0090] The test results of the catalytic activities of the Ti-MOF-5 polyester polycondensation catalysts prepared in Examples 1 - 3 and used in the synthesis of polyethylene terephthalate, as well as the catalysts in Comparative Examples 1 - 3, are shown in Table 2:

[0091]

[0092] Table 2

[0093] The data in Table 2 show that the Ti-MOF-5 catalyst has high catalytic activity, and the intrinsic viscosity and viscosity-average molecular weight of the polyester stock solution both gradually increase with the increase of the Ti 4+ content, indicating that the catalytic activity of the Ti-MOF-5 catalyst is positively correlated with the Ti 4+ content. However, in order to balance catalytic activity and product performance, the ratio of titanium and zinc contents should be appropriately adjusted, and the molar ratio of MOF-5 to Ti(OC3H7)4 should be controlled within the range of 1:0.5 - 1.0, which can enable the catalyst to have high catalytic activity and ensure the smooth progress of the melt processing technology. Through the analysis of Comparative Examples 1 and 2, under the same reaction conditions, using Ti(OC3H7)4 as the catalyst, the viscosity-average molecular weight (6.12×10 4 g / mol) is much larger than that of other examples. It can be seen that using pure Ti(OC3H7)4 as the catalyst will make the reaction too intense, making it difficult to control the reaction process and subsequent processing, while using MOF-5 alone as the catalyst has too low catalytic efficiency to meet the production requirements. Compared with Comparative Example 3, by regulating the Ti 4+ loading amount in Ti-MOF-5, it is expected to completely replace the traditional antimony-based catalyst to reduce the limitation of heavy metal antimony on the application of PET products.

[0094] (4) Mechanical property test

[0095] The Ti-MOF-5 polyester polycondensation catalysts prepared in Examples 1-3 and the catalysts in Comparative Examples 1-3 were used to participate in the synthesis of polyethylene terephthalate as samples, and the tensile strength and flexural strength of each polyethylene terephthalate sample were tested respectively. The results are shown in Table 3. The tensile strength and flexural strength were tested by standard tensile tests (ISO527) and flexural tests (ISO178), and the stress values of the materials at the maximum load were recorded.

[0096] (5) Ash content test

[0097] The Ti-MOF-5 polyester polycondensation catalysts prepared in Examples 1-3 and the catalysts in Comparative Examples 1-3 were used to participate in the synthesis of polyethylene terephthalate as samples, and ash content tests were carried out respectively. The test results are shown in Table 3. According to the ISO3451 test method, the sample was burned at high temperature until a constant weight was obtained, and the ratio of the weight of the remaining solid to the initial weight was used to reflect the content of inorganic substances in the material.

[0098] Sample No. Tensile Strength (MPa) Flexural Strength (MPa) Flexural Modulus (MPa) Ash Content (%) Example 1 140 185 9000 0.03 Example 2 155 200 10400 0.01 Example 3 130 175 8200 0.02 Comparative Example 1 95 130 5200 0.55 Comparative Example 2 105 145 6500 0.04 Comparative Example 3 140 190 9100 0.02

[0099] Table 3

[0100] Mechanical strength and ash content are important indicators for evaluating the quality of PET. The occurrence of side reactions, such as hydrolysis, transesterification, etc., may not only cause the molecular chain of PET to break, reducing the mechanical strength, but also increase the ash content, affecting the purity of the material. In the present invention, all indicators of Example 2 perform best, and both the mechanical strength and the product purity are higher than those of the products synthesized by the existing catalyst (Comparative Example 3). By analyzing Comparative Example 1, it can be found that a reaction that is too intense will ultimately lead to a decrease in the mechanical properties and purity of the product.

[0101] The above examples are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention. The technologies, shapes, and structures not described in detail in the present invention are all well-known technologies.

Claims

1. Preparation method of MOF-based polyester polycondensation catalyst, characterized in that It includes the following steps: Weigh MOF-5 and add it to DMF for ultrasonic dispersion to obtain a suspension; Weigh a titanium source and add it to DMF for stirring and dissolution to obtain a titanium solution; Under stirring, add the titanium solution to the suspension, and stir at 300 - 1000 rpm for 6 - 24 h at a temperature of 60 - 120 °C, then filter. Dry the obtained solid product to obtain the Ti-MOF-5 polyester polycondensation catalyst.

2. The preparation method of the MOF-based polyester polycondensation catalyst according to claim 1, characterized in that, The molar ratio of the MOF-5 to the titanium source is (0.5 - 1.5):

1.

3. The preparation method of the MOF-based polyester polycondensation catalyst according to claim 1, wherein, The titanium source is at least one of isopropyl titanate, butyl titanate, titanium oxychloride, and titanium nitrate.

4. The preparation method of the MOF-based polyester polycondensation catalyst according to claim 1, characterized in that The preparation method of the MOF-5 is as follows: Weigh a zinc salt and dissolve it in an organic solvent under magnetic stirring to obtain a metal salt solution; Weigh terephthalic acid and dissolve it in an organic solvent under magnetic stirring to obtain a ligand solution; Under stirring at a speed of 500 - 800 rpm, add the ligand solution to the metal salt solution, then add a nucleating agent, heat up to 60 - 120 °C and react for 4 - 12 h. Centrifuge to collect the solid product, and dry the solid product to obtain white powdery MOF-5.

5. The preparation method of the MOF-based polyester polycondensation catalyst according to claim 4, characterized in that, The molar volume ratio of the zinc salt, terephthalic acid, and organic solvent is 1 mmol:(1 - 6) mmol:(60 - 200) mL, and the dosage of the nucleating agent is 1 - 10% of the mass of the zinc salt.

6. The preparation method of the MOF-based polyester polycondensation catalyst according to claim 5, wherein, The zinc salt is at least one of Zn(NO3)2·6H2O, ZnCl2, ZnSO4·H2O, and Zn(CH3COO)2·2H2O.

7. The preparation method of the MOF-based polyester polycondensation catalyst according to claim 5, characterized in that, The organic solvent is one of methanol, ethanol, N,N-dimethylformamide, and N,N-dimethylacetamide.

8. The preparation method of the MOF-based polyester polycondensation catalyst according to claim 5, characterized in that, The nucleating agent is at least one of polyethylene glycol, cetyltrimethylammonium bromide, polyvinylpyrrolidone, Tween-80, sodium dodecyl sulfate, and triblock copolymer Pluronic F127.

9. MOF-based polyester polycondensation catalyst, characterized in that, It is prepared by using the preparation method described in any one of claims 1 - 8.

10. Application of the MOF-based polyester polycondensation catalyst according to claim 9 in the synthesis of polyethylene terephthalate.

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