Catalyst for catalyzing methanol hydrolysis of polyethylene glycol terephthalate as well as preparation method and application of catalyst

Abstract

The invention belongs to the technical field of catalysis of alcoholysis of polyethylene glycol terephthalate, and discloses a catalyst for catalyzing alcoholysis of polyethylene glycol terephthalate as well as a preparation method and application of the catalyst. The catalyst comprises a carrier and an active component loaded on the carrier, wherein the carrier is a Beta molecular sieve, and the active component is magnesium oxide. The catalyst disclosed by the invention takes the widely available Beta molecular sieve as the carrier, and the active component is cheap magnesium, so that the problem of strong corrosivity of concentrated sulfuric acid is completely avoided, and the catalyst does not need to depend on noble metal. The design not only reduces the corrosion damage to equipment, but also enables the cost of raw materials for preparing the catalyst to be low, and is more suitable for large-scale industrial application.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of catalytic polyethylene terephthalate alcoholysis, and particularly relates to a catalyst for catalytic polyethylene terephthalate methanolysis and a preparation method and application thereof. BACKGROUND

[0002] Plastic pollution has become a global environmental challenge, and polyethylene terephthalate (PET) as a polyester material widely used in beverage packaging and textile fields constitutes a significant part of solid waste. According to statistics, as early as 2019, the global annual output of PET has exceeded 310 million tons, and these materials are difficult to degrade in the natural environment and can last for hundreds of years. Traditional incineration or landfill treatment methods not only fail to fully utilize their intrinsic value, but also may cause secondary pollution. Under the current background of advocating circular economy, there is an urgent need to develop efficient PET chemical recycling technology. Among various chemical recycling methods, methanolysis technology has attracted much attention because it can directly depolymerize waste PET into high-value dimethyl terephthalate (DMT). DMT is not only an important monomer for the synthesis of recycled PET, but also a key raw material for the production of plasticizers, resins and coatings, and has broad market prospects. Compared with the hydrolysis product terephthalic acid (PTA) and the glycolysis product bis-hydroxyethyl terephthalate (BHET), DMT has the characteristics of easy separation and purification, making it an ideal target product for PET chemical recycling. Through methanolysis technology, the conversion from waste PET to DMT can effectively close the key link of plastic circular economy and provide technical support for solving the problem of plastic pollution.

[0003] Currently, the industrial production of DMT in China mainly follows two process routes using purified terephthalic acid (PTA) as raw material: one is the concentrated sulfuric acid catalysis method, that is, PTA reacts with methanol to esterify at 170-180℃ under normal pressure. The patent document with publication number CN104072374A discloses a "synthesis method and process of dimethyl terephthalate", which has mild reaction conditions, but concentrated sulfuric acid causes serious corrosion of equipment and easily causes environmental pollution, and the process has poor sustainability; the other is the non-catalytic high-temperature and high-pressure method, which requires harsh conditions of 250℃ or above and 8-12 MPa, which avoids the corrosion problem, but has the significant defects of large equipment investment and high energy consumption. Compared with this, direct preparation of DMT from waste PET not only can greatly reduce energy consumption, but also provides an effective way for resource utilization and environmental pollution control of waste plastics.

[0004] PET methanolysis process usually needs to be carried out at high temperature (180-280℃) and high pressure (20-40 atm), and even needs supercritical conditions (300℃, more than 80 atm) in early non-catalytic methanolysis, which has problems of high energy consumption and strict equipment requirements. In order to reduce the reaction severity and improve the reaction efficiency, the development of catalysts has become a research focus. Homogeneous catalysts such as zinc acetate, magnesium acetate, cobalt acetate and lead dioxide have been widely used. Although these catalysts have high activity, they face the core challenge of being difficult to separate from the product, leading to product contamination and inability to recycle. In the prior art, the patent document with publication number CN110938231A reports a method for preparing DMT by methanolysis of waste PET using a urea-based deep eutectic solvent as a catalyst; another document with publication number CN113735705A discloses a PET methanolysis process using polyionic liquid (such as PIL-Zn 2+ , PIL-Co 2+ , etc.) as a catalyst. Although the above-mentioned catalytic systems can achieve complete degradation of PET, there are still some obvious limitations: some catalysts such as specific deep eutectic solvents or metal-based polyionic liquids have high synthesis cost, and may cause environmental burden due to metal ion residues or degradation products; in addition, the introduction of cosolvents in some methods may also bring additional environmental safety problems and recovery costs, such as the patent document CN119059903A discloses a PET methanolysis process using phenolic substances as an aid. Therefore, developing a new type of catalytic process that can efficiently depolymerize waste PET under mild reaction conditions, while achieving high yield and high selectivity of DMT, has become an important development direction in this field. This not only helps to reduce energy consumption and pollution, and improve process economy, but also provides key technical support for promoting waste plastic resource utilization and green recycling.

[0005] Therefore, it is urgent to develop a non-homogeneous catalyst for waste PET methanolysis that is simple, efficient and low in cost. SUMMARY

[0006] The present application aims to overcome the deficiencies of the prior art and provides a catalyst for catalyzing the methanolysis of polyethylene terephthalate, as well as a preparation method and application thereof.

[0007] In order to achieve the purpose of the present application, the technical solutions adopted by the present application are as follows: The present application provides a catalyst for catalyzing the methanolysis of polyethylene terephthalate, which comprises a carrier and an active component loaded on the carrier; wherein the carrier is a Beta molecular sieve, and the active component is magnesium oxide.

[0008] Preferably, the magnesium oxide loading in the catalyst is 0.33wt% to 33.13wt% of the support mass.

[0009] A second aspect of the present invention provides a method for preparing the catalyst described in the first aspect for catalyzing the methanol hydrolysis of polyethylene terephthalate, comprising the following steps: (1) Dissolve the soluble magnesium salt and template agent in water to obtain a mixed solution; (2) Disperse the Beta molecular sieve in water and heat until a suspension is formed; (3) Add the mixed solution described in step (1) to the suspension in step (2), age it at 100-180°C for 7-8 h, wash and dry the aged product, and calcine it in air at 300-600°C for 2-6 h to obtain a catalyst for catalyzing the methanol hydrolysis of polyethylene terephthalate.

[0010] Preferably, the ratio of the total mass of soluble magnesium salt and Beta molecular sieve to the mass of template agent is (0.5-4):1.

[0011] Preferably, the template agent in step (1) is one of cetyltrimethylammonium bromide, dodecyltrimethylammonium bromide, triethylamine, polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer, polyethylene glycol-polypropylene glycol-polyethylene glycol triblock copolymer, polyethylene glycol, and polyethylene glycol octylphenyl ether.

[0012] More preferably, the template agent in step (1) is a polyethylene glycol-polypropylene glycol-polyethylene glycol triblock copolymer (P123).

[0013] Preferably, the soluble magnesium salt in step (1) is at least one of magnesium nitrate, magnesium acetate, and basic magnesium carbonate.

[0014] More preferably, the soluble magnesium salt in step (1) is magnesium nitrate.

[0015] Preferably, the preparation process of Beta molecular sieve in step (2) is as follows: Tetraethylammonium hydroxide is dissolved in water to obtain a tetraethylammonium hydroxide solution. A silicon source and an aluminum source are added sequentially to the tetraethylammonium hydroxide solution, mixed and aged to obtain a gel. The gel is crystallized to obtain a crystallized product. The crystallized product is washed, dried and then calcined in air at 300-600°C for 2-6 hours to obtain a Beta molecular sieve.

[0016] More preferably, the molar ratio of silicon source, aluminum source, tetraethylammonium hydroxide and water is 1:(0.02-0.08):(0.3-0.6):(8-22).

[0017] More preferably, the crystallization is carried out by holding the crystal at a temperature of 130–220°C for 3–4 days.

[0018] The third aspect of the present invention provides the application of the catalyst described in the first aspect in the methanol hydrolysis of polyethylene terephthalate.

[0019] Preferably, the specific operation of catalytic methanol hydrolysis of polyethylene terephthalate is as follows: polyethylene terephthalate, catalyst and methanol are reacted at 150-220°C and 0.5 MPa for 0.5-2.5 h to generate dimethyl terephthalate (DMT).

[0020] Preferably, the mass ratio of polyethylene terephthalate, catalyst and methanol is 1:(0.005-0.1):(2-7).

[0021] Preferably, after the reaction is completed, the temperature is lowered to room temperature to obtain the reactants. The reactants are centrifuged and the solid precipitate is collected. The solid precipitate is then removed by rotary evaporation or vacuum filtration to obtain white needle-like crystals, which are dimethyl terephthalate.

[0022] Compared with the prior art, the present invention has the following advantages: (1) This invention avoids the use of corrosive and precious metal components, resulting in low cost. The catalyst uses widely available Beta molecular sieves as a support, and the active component is inexpensive magnesium, completely avoiding the strong corrosiveness of concentrated sulfuric acid and eliminating the need for precious metals. This design not only reduces corrosion damage to equipment but also makes the raw materials for catalyst preparation inexpensive, making it more suitable for large-scale industrial applications.

[0023] (2) The catalyst is a solid heterogeneous catalyst, which can be separated from the product liquid by simple operations such as filtration and centrifugation after the reaction, thereby realizing recycling and reuse. This completely solves the industry pain points of homogeneous catalysts such as concentrated sulfuric acid and metal acetate being difficult to separate, polluting the product, and unable to be reused, which is in line with the principles of green chemical industry.

[0024] (3) The catalyst prepared in this invention is used to catalyze the methanol hydrolysis of polyethylene terephthalate, which can achieve the complete conversion of waste polyethylene terephthalate (PET) and the yield of dimethyl terephthalate (DMT) can reach 95.11%. This yield level is significantly higher than most existing homogeneous or heterogeneous catalytic systems, which demonstrates the excellent performance of this catalyst in terms of reaction efficiency and product selectivity.

[0025] (4) The catalyst prepared in this invention is used to catalyze the methanol hydrolysis of polyethylene terephthalate, achieving a highly efficient reaction at 190°C and a relatively low initial pressure (0.5 MPa). This effectively reduces the pressure and temperature resistance requirements of the equipment in the production process, reduces energy consumption, and has significant cost advantages for industrial applications.

[0026] (5) Environmentally friendly and green: The entire catalytic system does not require the use of toxic or environmentally unfriendly cosolvents. The main reaction medium is methanol, and the solid-phase catalyst is easy to recover and reuse. This process reduces the generation and emission of pollutants such as acidic wastewater and heavy metal ion residues from the source, and is a green technology path that meets the requirements of sustainable development. Attached Figure Description

[0027] Figure 1 XRD patterns of catalysts in Examples 1 to 3 and Example 5. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer and more explicit, the invention is further described in detail below through embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0029] Example 1 A catalyst for the methanol hydrolysis of polyethylene terephthalate (PET) is prepared by the following steps: (1) Dissolve 34.32g of tetraethylammonium hydroxide in 40mL of pure water and stir at 40℃ until completely dissolved to obtain the first solution; add 35.21g of tetraethyl orthosilicate to the first solution and stir until the solution becomes clear, add 1.36g of aluminum isopropoxide and continue stirring until it is completely dissolved to obtain a clear solution again; age the solution at 140℃ for 7-8 h to obtain gel A; transfer the gel A to a stainless steel hydrothermal synthesis reactor with a polytetrafluoroethylene liner and keep it at 160℃ for 3-4 days to complete crystallization, take out the product and wash it with deionized water by suction filtration until the pH value is close to neutral, dry it at 90℃ to obtain Beta molecular sieve raw powder, and calcine it at 550℃ in air atmosphere for 4 h to obtain Beta molecular sieve B; (2) Dissolve 0.844 g magnesium nitrate hexahydrate and 0.856 g polyethylene glycol-polypropylene glycol-polyethylene glycol triblock copolymer (P123) in 10 mL of pure water to form a second solution; (3) Weigh 0.867 g of the Beta molecular sieve B prepared in step (1) and disperse it in 10 mL of pure water. Stir at 60 °C until a suspension C is obtained. (4) The second solution prepared in step (2) is added dropwise to the suspension C prepared in step (3) to form suspension D. The suspension is aged at 140°C for 7-8 h to obtain suspension E. The suspension is washed with deionized water until the pH value is 7.6-8.5, dried at 90°C, and then calcined at 550°C in air atmosphere for 4 h to obtain a catalyst for catalyzing the methanolysis of polyethylene terephthalate. Based on Beta molecular sieve, the magnesium oxide loading in the catalyst is 13.27 wt%.

[0030] Examples 2 to 11 A catalyst for catalytic methanol hydrolysis of polyester. The preparation methods of Examples 2 to 11 are basically the same as those of Example 1, except that the masses of magnesium nitrate hexahydrate, P123 and Beta molecular sieve are different in steps (2) and (3). The masses of magnesium nitrate hexahydrate, P123 and Beta molecular sieve in Examples 3 to 12 are shown in Table 1 below.

[0031] Table 1. Masses of magnesium nitrate hexahydrate, P123, and Beta molecular sieves in Examples 2–11 Comparative Example 1 A catalyst for catalytic methanol hydrolysis of polyester, with Comparative Example 1 being the Beta molecular sieve prepared in step (1) of Example 1.

[0032] Comparative Example 2 A catalyst for catalytic methanol hydrolysis of polyester, Comparative Example 2, is prepared by calcining magnesium nitrate hexahydrate in air at 550°C for 4 h.

[0033] Performance testing: (1) XRD characterization: The catalysts of Examples 1, 2, 3, and 5 were characterized by XRD, and the results are as follows: Figure 1 As shown, by Figure 1 Multiple characteristic diffraction peaks of the Beta molecular sieve (PDF: 47-0183) can be observed. With the introduction of MgO, the peak intensity weakens; the peak intensity in Example 1 is significantly weaker than that in Examples 2, 3, and 5. This is attributed to the dilution of the crystalline Beta molecular sieve caused by the addition of MgO. When the MgO loading is below the monolayer dispersion threshold, MgO is highly dispersed in a monolayer form on the Beta molecular sieve, therefore, the characteristic crystalline phase peaks of MgO cannot be observed.

[0034] Application Example 1 The catalysts prepared in Examples 1-11, Comparative Example 1, and Comparative Example 2 of this invention for the methanol hydrolysis of polyethylene terephthalate were used in the methanol hydrolysis reaction of waste polyester. The specific process is as follows: 3g of waste polyethylene terephthalate (PET) powder, 0.1g of catalyst, and 15mL of methanol were added to a high-pressure reactor equipped with a mechanical stirrer. The reactor was purged three times with nitrogen gas at 0.5MPa. The stirring was turned on and the temperature was raised to 180°C for 1 hour for depolymerization reaction. After the reaction was completed, the temperature was lowered to room temperature to obtain the reactants. The reactants were diluted to 100mL with acetonitrile. The yield of the solution was determined by HPLC, and the experimental results are shown in Table 2 below. In Table 2, the conversion rate is the conversion rate of waste PET to other substances, and the yield is the yield of the target product DMT.

[0035] Table 2 Catalytic performance of catalysts in Examples 1 to 11 and Comparative Example 1 Table 2 shows that the catalytic activity of PET methanololysis depends on the methanol activation ability of the catalyst. Pure Beta molecular sieve (Comparative Example 1) exhibits poor catalytic activity in the PET methanololysis reaction, making it difficult to convert PET feedstock and resulting in low selectivity for DMT. Pure MgO (Comparative Example 2) can achieve a PET conversion rate of 99.6%, and the DMT yield is more than four times that of the catalyst in Comparative Example 1. The catalytic performance of MgO in methanololysis is far superior to that of Beta molecular sieve. With the increase of MgO loading on the Beta molecular sieve support, the catalyst activity improves, and both the PET conversion rate and DMT yield increase. When the MgO loading increases to 13.27 wt% (Example 1), the PET methanololysis achieves the highest reactivity, with all PET converted. With further increases in MgO loading, the DMT yield gradually decreases. This is because MgO is dispersed on the surface of the Beta molecular sieve, and the high MgO loading blocks the molecular sieve channels, inhibiting effective methanol mass transfer and intensifying side reactions.

[0036] Application Example 2 As can be seen from Application Example 1, the catalyst prepared in Example 1 has the highest PET conversion rate and DMT yield in the catalytic reaction. Therefore, the effect of different amounts of catalyst in Example 1 on catalytic performance was investigated. The reaction process was the same as in Application Example 1. The amount of catalyst and catalytic performance are shown in Table 3 below.

[0037] Table 3 Catalytic performance of catalysts at different dosages in Example 1 Table 3 shows that the DMT yield gradually increases with increasing catalyst dosage; however, the improvement in reaction activity is not significant after the catalyst mass fraction reaches 1 wt%. The DMT yield increases to 94.81% when the catalyst mass fraction is 10 wt%, but this is too high a catalyst dosage. Considering all factors, a catalyst mass fraction of 1 wt% yields better catalytic performance, with almost complete PET conversion and a DMT yield of 89.93%.

[0038] Application Example 3 As can be seen from Application Example 2, a good catalytic effect can be obtained when the amount of catalyst used in Example 1 is 0.03g. Therefore, the amount of catalyst used was selected as 0.03g to investigate the effect of different amounts of methanol on catalytic performance. The reaction process is the same as in Application Example 1. The amount of methanol and catalytic performance are shown in Table 4 below.

[0039] Table 4 Catalytic performance at different methanol dosages As shown in Table 4, the DMT yield gradually increased with the increase of methanol dosage. However, further increasing the methanol dosage after reaching 15 mL did not significantly improve the reaction activity. Considering all factors, a methanol dosage of 15 mL yielded better catalytic performance, with almost complete PET conversion and a DMT yield of 89.93%.

[0040] Application Example 4 As can be seen from Application Example 3, a good catalytic effect can be obtained when the methanol volume is 15 mL. Therefore, the catalyst dosage was selected as 0.03 g and the methanol volume was 15 mL to investigate the effect of different reaction temperatures on catalytic performance. The reaction process was the same as in Application Example 1. The reaction temperature and catalytic performance are shown in Table 5 below.

[0041] Table 5 Catalytic performance at different reaction temperatures As shown in Table 5, the DMT yield gradually increases with increasing reaction temperature. However, further increases in reaction temperature beyond 190℃ do not significantly improve reaction activity. Considering all factors, a reaction temperature of 190℃ yields better catalytic performance, with almost complete conversion of PET and a DMT yield of 92.09%.

[0042] Application Example 5 As can be seen from Application Example 4, a good catalytic effect can be obtained at a reaction temperature of 190℃. Therefore, the catalyst dosage was selected as 0.03g, the methanol volume as 15mL, and the reaction temperature as 190℃ to investigate the effect of different reaction times on catalytic performance. The reaction process was the same as in Application Example 1, and the reaction time and catalytic performance are shown in Table 6 below.

[0043] Table 6 Catalytic performance at different reaction times As shown in Table 6, the DMT yield gradually increased with increasing reaction time. However, after 1.5 h, further increasing the reaction time did not significantly improve the reaction activity. Considering all factors, a reaction time of 1.5 h yielded better catalytic performance, with almost complete conversion of PET and a DMT yield of 95.11%.

[0044] From Application Examples 1 to 5, it can be seen that the catalyst in Example 1 has the best catalytic performance. The parameters selected for the application of this catalyst are: catalyst dosage of 0.03g, methanol volume of 15mL, reaction temperature of 190℃, and reaction time of 1.5h.

[0045] Application Example 6 Using the catalyst prepared in Example 1, methanol hydrolysis catalytic reactions of PET from different plastic sources were carried out at a catalyst dosage of 0.03 g, a methanol volume of 15 mL, a reaction temperature of 190 °C, and a reaction time of 1.5 h. The reaction process was the same as in Application Example 1. The plastic sources and catalytic performance are shown in Table 7 below.

[0046] Table 7 Catalytic properties of PET from different plastic sources As shown in Table 7, the catalyst in Example 1 has good methanol hydrolysis performance for high-purity waste PET. The presence of organic dyes and some inorganic dyes will inhibit the methanol hydrolysis reaction.

Claims

1. A catalyst for catalyzing the methanol hydrolysis of polyethylene terephthalate, characterized in that, The catalyst includes a support and an active component supported on the support; wherein the support is a Beta molecular sieve and the active component is magnesium oxide.

2. The catalyst according to claim 1, characterized in that, The magnesium oxide loading in the catalyst is 0.33wt% to 33.13wt% of the support mass.

3. A method for preparing a catalyst for the methanol hydrolysis of polyethylene terephthalate as described in any one of claims 1 to 2, characterized in that, Includes the following steps: (1) Dissolve the soluble magnesium salt and template agent in water to obtain a mixed solution; (2) Disperse the Beta molecular sieve in water and heat until a suspension is formed; (3) Add the mixed solution described in step (1) to the suspension in step (2), age it at 100-180°C for 7-8 h, wash and dry the aged product, and calcine it in air at 300-600°C for 2-6 h to obtain a catalyst for catalyzing the methanol hydrolysis of polyethylene terephthalate.

4. The preparation method according to claim 3, characterized in that, The ratio of the total mass of soluble magnesium salt and Beta molecular sieve to the mass of template agent is (0.5-4):

1.

5. The preparation method according to claim 4, characterized in that, Step (1) The template agent is any one of hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, triethylamine, polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer, polyethylene glycol-polypropylene glycol-polyethylene glycol triblock copolymer, polyethylene glycol, and polyethylene glycol octylphenyl ether; the soluble magnesium salt is at least one of magnesium nitrate, magnesium acetate, and basic magnesium carbonate.

6. The preparation method according to claim 4, characterized in that, Step (2) The preparation process of Beta molecular sieve is as follows: Tetraethylammonium hydroxide is dissolved in water to obtain a tetraethylammonium hydroxide solution. A silicon source and an aluminum source are added to the tetraethylammonium hydroxide solution in sequence. After mixing, the solution is aged to obtain a gel. The gel is crystallized to obtain a crystallized product. After washing and drying, the crystallized product is calcined in an air atmosphere at 300-600℃ for 2-6 h to obtain Beta molecular sieve.

7. The preparation method according to claim 6, characterized in that, The molar ratio of silicon source, aluminum source, tetraethylammonium hydroxide and water is 1:(0.02~0.08):(0.3~0.6):(8~22).

8. The application of the catalyst according to any one of claims 1 to 2 in the methanol hydrolysis of polyethylene terephthalate.

9. The application according to claim 8, characterized in that, The specific operation of catalytic methanol hydrolysis of polyethylene terephthalate is as follows: polyethylene terephthalate, catalyst and methanol are reacted at 150-220℃ and 0.5MPa for 0.5-2.5h to generate dimethyl terephthalate.

10. The application according to claim 9, characterized in that, The mass ratio of polyethylene terephthalate, catalyst and methanol is 1:(0.005~0.1):(2~7).

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