Platinum-tungsten bimetallic catalyst for PET (Polyethylene Terephthalate) depolymerization and method for directionally depolymerizing PET by using platinum-tungsten bimetallic catalyst

Abstract

The invention discloses a platinum-tungsten bimetallic catalyst for PET depolymerization and a method for directional depolymerization of PET by the platinum-tungsten bimetallic catalyst, and belongs to the technical field of solid waste recycling, the platinum-tungsten bimetallic catalyst for PET depolymerization is prepared by a deposition-precipitation method, a mesoporous molecular sieve MCM-48 is used as a carrier to load platinum-tungsten bimetallic, the molar ratio of platinum to tungsten is 1: (0.5-2), the molecular sieve MCM-48 is a mesoporous molecular sieve MCM-48, and the mesoporous molecular sieve MCM-48 is a mesoporous molecular sieve MCM-48 carrier. And the platinum loading capacity is 4-6 wt%. By utilizing the platinum-tungsten bimetallic catalyst, one-pot depolymerization of PET can be realized to prepare p-toluic acid and p-xylene, the platinum-tungsten bimetallic catalyst can be used for multiple times through reduction regeneration, the catalytic activity is high, and the PET depolymerization method under catalysis of the platinum-tungsten bimetallic catalyst is suitable for recycling and upgrading of PET wastes in various forms and has wide application prospects. The technical problems of efficient depolymerization and directional conversion of the waste PET are solved, and a sustainable technical path is provided for resource recycling of the PET.

Description

Technical Field

[0001] The invention belongs to the technical field of solid waste recycling, and in particular relates to a platinum-tungsten bimetallic catalyst for PET depolymerization and a method for directional depolymerization of PET. Background Art

[0002] Plastic waste pollution has now become the focus of environmental governance. Nowadays, the plastic with the highest annual output is polyethylene terephthalate (PET), which is widely used to make plastic packaging boxes, fiber textiles, etc. and has a short service life. The development of refined and high-value utilization technology for waste PET is a key link in solving the environmental pollution of waste plastics. At present, the problem of resource waste in the industrial process of recycling, utilization and upgrading of waste PET needs to be solved urgently. The mechanical and physical "downgrading and recycling" method leads to the loss of plastic functions, the degree of biodegradation conversion is limited, and the application conditions are immature. Chemical upgrading and recycling to prepare important chemicals is one of the best ways to achieve refined and high-value utilization of waste PET, which can return it to petroleum products and realize the recycling of carbon resources at the molecular level.

[0003] The chemical treatment of waste PET for upgrading and recycling to prepare high-value chemicals is generally divided into two steps: ① depolymerization of waste PET into small molecule monomers; ② catalytic conversion of small molecule monomers to obtain high-value chemicals. At present, chemical depolymerization of waste PET is mainly carried out by methanolysis, hydrolysis, glycolysis, aminolysis and other methods to obtain small molecules such as dimethyl terephthalate, terephthalic acid, ethylene terephthalate, terephthalonitrile, terephthalamide, etc., and then the small molecules are further converted and upgraded into higher-value products (Chinese patent documents with publication numbers CN119118826A and CN117049962A, etc.), and most of the research in the prior art focuses on the directional depolymerization and upgrading of waste PET to prepare oxygen-free light aromatic compounds such as benzene, toluene and xylene. However, given the rich oxygen content in PET, it is also important to selectively convert PET into high-value oxygen-containing small molecules. If we want to achieve one-step targeted depolymerization of waste PET and upgrade it to produce high-value oxygen-containing chemicals, the development of high-performance catalysts is the technical key. It is also a new direction to break through the bottleneck of low-value and extensive "violent cracking" of waste PET chemical recycling in the future.

[0004] p-Toluic acid (p-TA) is an important oxygen-containing chemical raw material. Compared with other PET depolymerization and upgrading products (such as benzene, toluene, xylene), it has high value and is widely used in the manufacture of photosensitive materials, organic synthesis intermediates, and fungicides such as phosphoramide. However, industrial production still relies on the oxidation of p-xylene (PX) with concentrated nitric acid. The reaction time is as long as 30 h, but the yield is only about 58%. At the same time, concentrated nitric acid is used as a reactant, and the risk factor in the synthesis process is high, and the requirements for the corrosion resistance of equipment and maintenance costs are high. Therefore, it is urgent to develop a green, simple separation, and low-cost route to prepare p-toluic acid. Summary of the Invention

[0005] In order to solve the deficiencies existing in the above-mentioned prior art, the present invention provides a platinum-tungsten bimetallic catalyst for PET depolymerization and a method for the directional depolymerization of PET to produce p-toluic acid and p-xylene. The platinum-tungsten bimetallic catalyst is a heterogeneous solid catalyst that can be recycled and catalyzes the directional one-pot depolymerization and selective hydrogenation reaction of PET to obtain high-value oxygen-containing aromatic compound p-toluic acid and aromatic hydrocarbon p-xylene.

[0006] The specific technical solutions adopted are as follows:

[0007] A platinum-tungsten bimetallic catalyst for PET depolymerization is prepared by the following method:

[0008] Prepare a mixture containing chloroplatinic acid, sodium tungstate, mesoporous molecular sieve MCM-48, and deionized water. Naturally dry the mixture until mud cracks are formed, and then raise the temperature for further drying. Reduce the dried solid in a reducing gas atmosphere at 400-500 °C for 4-5 h to obtain a platinum-tungsten bimetallic catalyst for PET depolymerization;

[0009] In the platinum-tungsten bimetallic catalyst for PET depolymerization, the carrier is mesoporous molecular sieve MCM-48, platinum is in the form of zero-valent platinum, and tungsten is in the form of WO x (tungsten is a mixture of pentavalent and hexavalent) is loaded on mesoporous molecular sieve MCM-48, and the molar ratio of platinum to tungsten elements is 1:0.5-2, and the platinum loading is 4 wt%-6 wt% (obtained by ICP test).

[0010] Naturally drying the mixture at room temperature until mud cracks are formed is beneficial to the full loading of platinum-tungsten bimetal on the MCM-48 mesoporous molecular sieve and ensures the catalytic effect of the catalyst.

[0011] Preferably, the temperature is raised to 50-70 °C for further drying for 10-14 h.

[0012] Preferably, the reducing gas atmosphere is a hydrogen-argon mixed atmosphere, and the heating rate during the reduction process is 1-3 °C / min.

[0013] The present invention also provides a method for the directional depolymerization of PET to produce p-toluic acid and p-xylene, using the platinum-tungsten bimetallic catalyst for PET depolymerization. This platinum-tungsten bimetallic catalyst can catalyze the depolymerization of PET, promote the in-situ hydrodeoxygenation reaction of the reaction intermediate terephthalic acid, selectively generate p-toluic acid and co-produce p-xylene. This platinum-tungsten bimetallic catalyst is relatively stable in a hydrothermal acidic system and can be recycled through reduction for multiple applications, maintaining good catalytic activity.

[0014] Specifically, the method for the directional depolymerization of PET to produce p-toluic acid and p-xylene includes the following steps:

[0015] S1. Construct a reaction system using crushed PET, the platinum-tungsten bimetallic catalyst for PET depolymerization, and an acid solution, where the acid solution is an aqueous solution of phosphotungstic acid;

[0016] S2. Place the reaction system constructed in step S1 under a hydrogen atmosphere and carry out a depolymerization reduction reaction at a temperature of 240 - 280 °C for 8 - 16 hours. After the reaction is completed, quench and cool to obtain p-toluic acid and p-xylene.

[0017] The depolymerization reduction reaction equation can be expressed as:

[0018]

[0019] Preferably, the crushed PET is pre-cleaned and pre-crushed waste PET in the form of flakes, granules or powder. The waste PET includes but is not limited to PET water bottles, PET films, PET trays, PET ribbons, PET binding ropes, PET non-woven fabrics, and blended fabrics of PET with other components such as cotton and spandex. The pre-cleaning and pre-crushing operations can prevent the interference and poisoning of other pollutants to the catalyst and ensure the conversion effect of waste PET. Of course, corresponding pure PET chemicals can also be used to construct the reaction system.

[0020] The method of the present invention is applicable to the recycling and upgrading of various forms of PET waste, with high efficiency and good adaptability.

[0021] Preferably, the molar ratio of platinum to tungsten elements in the platinum-tungsten bimetallic catalyst for PET depolymerization is 1:1.5, the platinum loading is 5 wt%, and the mass ratio of crushed PET to the platinum-tungsten bimetallic catalyst for PET depolymerization is 1:0.5 - 2. Under the above condition parameters, the platinum-tungsten bimetallic catalyst for PET depolymerization has good catalytic effect and can prepare the products p-toluic acid and p-xylene in high yield, which helps to efficiently utilize the carbon resources in PET.

[0022] Preferably, the pH value of the aqueous solution of phosphotungstic acid is 1 - 5.

[0023] Further preferably, the pH value of the phosphotungstic acid aqueous solution is 2, and the dosage ratio of the pulverized PET to the phosphotungstic acid aqueous solution is 1 g: 40 - 80 mL.

[0024] Preferably, the depolymerization reduction reaction is carried out under the condition of a pressure of 1 - 5 MPa.

[0025] Preferably, the conditions of the depolymerization reduction reaction are 2 MPa, 260 °C, and 12 h.

[0026] The present invention also provides a method for treating waste PET products, and uses the method for the directional depolymerization of PET to produce p-toluic acid and p-xylene to treat waste PET.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] (1) The Pt-W bimetallic catalyst adopted in the present invention is a heterogeneous solid catalyst, which has the advantages of high catalytic efficiency, convenient recovery, and recyclability, is beneficial to improving the selectivity of intermediate hydrogenation products, and can realize the one-step directional depolymerization and selective hydrogenation of waste PET.

[0029] (2) The high-value conversion strategy of PET developed by the present invention has the advantages of short reaction time, simple steps, and high catalytic efficiency, can prepare high-value oxygen-containing aromatic chemical p-toluic acid by one step, and at the same time prepare p-xylene, breaking through the bottleneck of the thermodynamic limit of PET depolymerization. Compared with the traditional petroleum manufacturing route of chemical monomers, this green catalytic system greatly reduces the environmental impact and production cost, and alleviates the consumption of petroleum resources to a certain extent.

[0030] (3) The method of the present invention has a wide application range, the types of waste PET raw materials are variable, and various waste PET materials such as PET water bottles, PET films, PET trays, PET ribbons, PET binding ropes, PET non-woven fabrics, and blended fabrics of PET with other components such as cotton and spandex can all be efficiently upgraded and transformed directionally within a short time according to the method of the present invention. Description of the Drawings

[0031] Figure 1 It is an XRD characterization result diagram of the PtW / MCM-48 catalyst (Pt loading is 5 wt%, and the molar ratio of Pt / W changes).

[0032] Figure 2 It is the NMR spectrum diagram of p-toluic acid obtained in Example 1.

[0033] Figure 3 It is the NMR spectrum diagram of p-xylene obtained in Example 1. Detailed Embodiments

[0034] To make the objectives, features, and advantages of the present invention more apparent and understandable, the following provides a detailed description through specific embodiments. Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein. Those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined correspondingly without conflict.

[0035] For the operating methods without specific conditions noted in the following embodiments, they are generally in accordance with conventional conditions or the conditions recommended by the manufacturer. The content not described in detail in this specification belongs to the prior art well-known to those skilled in the art. The experimental materials used in the following embodiments, unless otherwise specified, can be obtained from conventional biochemical reagent companies.

[0036] The following platinum-tungsten bimetallic catalyst for PET depolymerization (PtW / MCM-48 catalyst) in the following examples was prepared by the following deposition-precipitation method:

[0037] In a 100 mL beaker, add chloroplatinic acid, sodium tungstate, 0.5 g of MCM-48 mesoporous molecular sieve, and a small amount (5 - 10 mL) of deionized water. Continuously stir for 5 min, then perform ultrasonic treatment for 15 min. The resulting mixture is naturally dried at ambient temperature until obvious mud cracks are formed, and then dried in an oven at 60 °C for 12 h. Grind the dried solid evenly and place it in a tubular furnace for reduction at 450 °C for 4 h (reduction gas: 5% H 2 / 95% Ar; heating rate: 2 °C / min; gas flow rate: 30 mL / min) to obtain the PtW / MCM-48 catalyst.

[0038] In the PtW / MCM-48 catalyst, the carrier is the mesoporous molecular sieve MCM-48. The molar ratio of Pt / W loaded and the platinum loading amount can be regulated by changing the feeding amounts of chloroplatinic acid and sodium tungstate. The Pt / W is generally controlled at 1:0.5 - 1:2, and the platinum loading amount is generally controlled at 4 wt% - 6 wt%. Experiments have shown that when the Pt / W molar ratio is 1:1.5 and the platinum loading amount is 5 wt%, the best catalytic performance for the depolymerization reaction can be achieved.

[0039] The XRD characterization results of the PtW / MCM-48 catalysts (Pt loading amount is 5 wt%, and the Pt / W molar ratios are 1:2, 1:1.8, 1:1.5, 1:1.3, 1:0.5 respectively), the mesoporous molecular sieve MCM-48, and the corresponding active metals are as Figure 1 shown, proving that platinum exists in the form of zero-valent platinum and tungsten exists as WO x(Tungsten is a mixture of pentavalent and hexavalent) is successfully loaded onto the mesoporous molecular sieve MCM-48.

[0040] Among them, the MCM-48 mesoporous molecular sieve can be synthesized according to the method described in the prior art. Specifically, cetyltrimethylammonium bromide (CTAB) can be used as a template agent and synthesized by hydrothermal method. First, add 600 g of deionized water to a beaker, then dissolve 9.1 g of NaOH under stirring to prepare an alkaline solution. Subsequently, add sodium fluoride (NaF) and CTAB, and magnetically stir the solution in a constant-temperature oil bath for 2 h. Finally, add 101.2 g of tetraethyl orthosilicate (TEOS) dropwise to the mixture. The molar ratio of the above added materials is TEOS:H 2 O:NaOH:NaF:CTAB = 1:68.608:0.468:0.093:0.466. Transfer the obtained solution to a hydrothermal reactor, crystallize at 110 °C for 48 h, perform hydrothermal exchange at 80 °C. The solid after vacuum filtration is rinsed with deionized water and ethanol 2-3 times, and then preliminarily dried at 110 °C. Grind the dried solid evenly, put it into a muffle furnace and calcine at 550 °C for 15 h to remove the CTAB template agent, and finally obtain the white powder-like mesoporous molecular sieve MCM-48.

[0041] Example 1

[0042] Add 0.05 g of PET powder, 0.025 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) into a high-pressure autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller. After sealing the high-pressure autoclave reactor, replace the gas in the high-pressure autoclave with hydrogen three times, and then pressurize and fill hydrogen at ambient temperature to make the pressure reach 2 MPa. Turn on the stirring and heating to raise the temperature of the reaction system to 260 °C and react at this temperature for 12 h. After the reaction is completed, quench the high-pressure autoclave in an ice-water bath to ambient temperature. The liquid phase is extracted three times with chloroform, using 1,4-dioxane as an internal standard. Mix the extract with deuterated chloroform, and then analyze it by nuclear magnetic resonance spectroscopy (NMR). The results show that the PET conversion rate in this example is 100%, and the yield of p-toluic acid (the NMR spectrum is as Figure 2 shown, 1 H NMR (600 MHz, CDCl 3 ) δ = 8.01 (d, J = 8.0 Hz, 2H), 7.28 (d, J = 8.0 Hz, 2H), 2.43 (s, 3H).) is 52.8%, and the yield of p-xylene (the NMR spectrum is as Figure 3 shown, 1 H NMR (600 MHz, CDCl 3) δ = 7.05 (s, 4H), 2.30 (s, 6H). The yield was 37%.

[0043] Example 2

[0044] 0.05 g of PET powder, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) were added into an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller. After sealing the autoclave reactor, the gas in the autoclave was replaced with hydrogen three times, and then hydrogen was pressurized and filled into the autoclave at ambient temperature to make the pressure reach 2 MPa. Stirring and heating were started to raise the temperature of the reaction system to 260 °C, and the reaction was carried out at this temperature for 12 h. After the reaction was completed, the autoclave was quenched to ambient temperature in an ice-water bath. The liquid phase was extracted three times with chloroform, and 1,4-dioxane was used as the internal standard. The extract was mixed with deuterated chloroform, and then analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate in this example was 100%, the yield of p-toluic acid was 53.4%, and the yield of p-xylene was 36.4%.

[0045] In addition, in order to test the recyclability of the PtW / MCM-48 catalyst, a recycling experiment was carried out on the reacted PtW / MCM-48 catalyst. The treatment method was as follows: The solid catalyst was recovered by filtration, washed with deionized water and ethanol, and dried at 60 °C. First, it was calcined in a muffle furnace at 450 °C for 4 h, and then reduced in a tubular furnace at 450 °C for 4 h (reduction gas: 5% H 2 / 95% Ar; heating rate: 2 °C / min; gas flow rate: 30 mL / min). The treated catalyst was used for the next cycle experiment. The test results showed that the reacted PtW / MCM-48 catalyst could re-enter the next cycle after being reduced by hydrogen, and the yield was almost unchanged. If it was put into use without being reduced by hydrogen, the yield would decrease. After being reused 6 times, the PtW / MCM-48 catalyst still had high catalytic efficiency, and the yields of p-toluic acid and p-xylene still reached 51.2% and 34.3% respectively.

[0046] Example 3

[0047] Pre-crush the actual single-component waste PET product (Coca-Cola plastic bottle), and then add 0.05 g of crushed PET bottle chips, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) into an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller. After sealing the autoclave reactor, replace the gas in the autoclave with hydrogen three times, and then pressurize and fill hydrogen at ambient temperature until the pressure reaches 2 MPa. Turn on the stirring and heating to raise the temperature of the reaction system to 260 °C, and react at this temperature for 12 h. After the reaction is completed, quench the autoclave in an ice-water bath to ambient temperature. Extract the liquid phase with chloroform three times, using 1,3,5-trioxane as the internal standard. Mix the extract with deuterated chloroform, and then analyze it by nuclear magnetic resonance spectroscopy (NMR). The results show that the PET conversion rate in this example is 94.1%, the yield of p-toluic acid is 53.8%, and the yield of p-xylene is 28.7%.

[0048] Example 4

[0049] Pre-crush the actual single-component waste PET product (colored plastic tape), and then add 0.05 g of crushed PET colored tape, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) into an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller. After sealing the autoclave reactor, replace the gas in the autoclave with hydrogen three times, and then pressurize and fill hydrogen at ambient temperature until the pressure reaches 2 MPa. Turn on the stirring and heating to raise the temperature of the reaction system to 260 °C, and react at this temperature for 12 h. After the reaction is completed, quench the autoclave in an ice-water bath to ambient temperature. Extract the liquid phase with chloroform three times, using 1,3,5-trioxane as the internal standard. Mix the extract with deuterated chloroform, and then analyze it by nuclear magnetic resonance spectroscopy (NMR). The results show that the PET conversion rate in this example is 92.9%, the yield of p-toluic acid is 53.9%, and the yield of p-xylene is 25.4%.

[0050] Example 5

[0051] Pre-crush the waste PET actual blended product (white polyester-cotton, 65% PET + 35% cotton), then add 0.05 g of crushed white polyester-cotton of PET, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) into an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller. After sealing the autoclave reactor, replace the gas in the autoclave with hydrogen three times, and then pressurize and fill hydrogen at ambient temperature to make the pressure reach 2 MPa. Turn on the stirring and heating to raise the temperature of the reaction system to 260 °C and react at this temperature for 12 h. After the reaction is completed, quench the autoclave in an ice-water bath to ambient temperature. Extract the liquid phase with chloroform three times, using 1,4-dioxane as an internal standard, mix the extract with deuterated chloroform, and then analyze it by nuclear magnetic resonance spectroscopy (NMR). The results show that the PET conversion rate in this example is 93.6%, the yield of p-toluic acid is 48.4%, and the yield of p-xylene is 30.4%.

[0052] Example 6

[0053] Pre-crush the waste PET actual blended product (black polyester-cotton, 85% PET + 15% cotton), then add 0.05 g of crushed black polyester-cotton of PET, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) into an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller. After sealing the autoclave reactor, replace the gas in the autoclave with hydrogen three times, and then pressurize and fill hydrogen at ambient temperature to make the pressure reach 2 MPa. Turn on the stirring and heating to raise the temperature of the reaction system to 260 °C and react at this temperature for 12 h. After the reaction is completed, quench the autoclave in an ice-water bath to ambient temperature. Extract the liquid phase with chloroform three times, using 1,4-dioxane as an internal standard, mix the extract with deuterated chloroform, and then analyze it by nuclear magnetic resonance spectroscopy (NMR). The results show that the PET conversion rate in this example is 99.9%, the yield of p-toluic acid is 52.2%, and the yield of p-xylene is 22.5%.

[0054] Example 7

[0055] In an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller, 0.05 g of PET powder, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.4), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave reactor, the gas in the autoclave was replaced with hydrogen three times, and then hydrogen was pressurized and filled at ambient temperature until the pressure reached 2 MPa. Stirring and heating were started to raise the temperature of the reaction system to 260 °C, and the reaction was carried out at this temperature for 12 h. After the reaction, the autoclave was quenched to ambient temperature in an ice-water bath. The liquid phase was extracted three times with chloroform, with 1,3,5-trioxane as the internal standard. The extract was mixed with deuterated chloroform, and then analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate in this example was 90.6%, the yield of p-toluic acid was 44.0%, and the yield of p-xylene was 20.4%.

[0056] Example 8

[0057] In an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller, 0.05 g of PET powder, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave reactor, the gas in the autoclave was replaced with hydrogen three times, and then hydrogen was pressurized and filled at ambient temperature until the pressure reached 2 MPa. Stirring and heating were started to raise the temperature of the reaction system to 250 °C, and the reaction was carried out at this temperature for 12 h. After the reaction, the autoclave was quenched to ambient temperature in an ice-water bath. The liquid phase was extracted three times with chloroform, with 1,3,5-trioxane as the internal standard. The extract was mixed with deuterated chloroform, and then analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate in this example was 91.3%, the yield of p-toluic acid was 44.7%, and the yield of p-xylene was 16.1%.

[0058] Example 9

[0059] In an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller, 0.05 g of PET powder, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 3) were added. After sealing the autoclave reactor, the gas in the autoclave was replaced with hydrogen three times, and then hydrogen was pressurized and filled at ambient temperature to make the pressure reach 2 MPa. Stirring and heating were started to raise the temperature of the reaction system to 260 °C, and the reaction was carried out at this temperature for 12 h. After the reaction, the autoclave was quenched to ambient temperature in an ice-water bath. The liquid phase was extracted three times with chloroform, 1,4-dioxane was used as the internal standard, the extract was mixed with deuterated chloroform, and then analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate in this example was 96.0%, the yield of p-toluic acid was 44.1%, and the yield of p-xylene was 8.5%.

[0060] Example 10

[0061] In an autoclave reactor equipped with an electromagnetic stirrer, a thermocouple, and a programmable temperature controller, 0.05 g of PET powder, 0.05 g of PtW / MCM-48 catalyst (Pt loading is 5 wt%, Pt / W = 1:1.5), and 3 mL of phosphotungstic acid aqueous solution (pH = 2) were added. After sealing the autoclave reactor, the gas in the autoclave was replaced with hydrogen three times, and then hydrogen was pressurized and filled at ambient temperature to make the pressure reach 2 MPa. Stirring and heating were started to raise the temperature of the reaction system to 260 °C, and the reaction was carried out at this temperature for 10 h. After the reaction, the autoclave was quenched to ambient temperature in an ice-water bath. The liquid phase was extracted three times with chloroform, 1,4-dioxane was used as the internal standard, the extract was mixed with deuterated chloroform, and then analyzed by nuclear magnetic resonance spectroscopy (NMR). The results showed that the PET conversion rate in this example was 100.0%, the yield of p-toluic acid was 50.8%, and the yield of p-xylene was 28.6%.

[0062] The above-described examples have detailed the technical solutions of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, supplements, or substitutions in a similar manner within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A platinum-tungsten bimetallic catalyst for PET depolymerization, characterized in that: Prepared by the following preparation method: A mixture comprising chloroplatinic acid, sodium tungstate, mesoporous molecular sieve MCM-48 and deionized water is prepared, the mixture is naturally dried until mud cracks are formed, and then the temperature is increased to further dry the mixture, and the dried solid is reduced at 400-500° C. for 4-5 hours in a reducing gas atmosphere to obtain a platinum-tungsten bimetallic catalyst for PET depolymerization; The carrier of the platinum-tungsten bimetallic catalyst for PET depolymerization is mesoporous molecular sieve MCM-48, the molar ratio of platinum to tungsten elements is 1:0.5-2, and the platinum loading is 4wt%-6wt%.

2. The platinum-tungsten bimetallic catalyst for PET depolymerization according to claim 1, characterized in that: The reducing gas atmosphere is a hydrogen-argon mixed atmosphere, and the heating rate during the reduction process is 1-3°C / min.

3. A method for producing p-toluic acid and p-xylene by directional depolymerization of PET, characterized in that: Utilize the platinum-tungsten bimetallic catalyst for PET depolymerization as described in claim 1 or 2.

4. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 3, characterized in that: The following steps are involved: S1. A reaction system is constructed using crushed PET, a platinum-tungsten bimetallic catalyst for PET depolymerization and an acid solution, wherein the acid solution is a phosphotungstic acid aqueous solution; S2. placing the reaction system constructed in step S1 under a hydrogen atmosphere at a temperature of 240-280° C. for 8-16 hours of depolymerization reduction reaction, and quenching and cooling after the reaction is completed to obtain p-toluic acid and p-xylene.

5. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 4, characterized in that: Shredded PET is pre-cleaned, pre-crushed waste PET in the form of flakes, granules or powder.

6. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 4, characterized in that: The molar ratio of platinum to tungsten elements in the platinum-tungsten bimetallic catalyst for PET depolymerization is 1:1.5, the platinum loading is 5wt%, and the mass ratio of crushed PET to the platinum-tungsten bimetallic catalyst for PET depolymerization is 1:0.5-2.

7. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 4, characterized in that: The pH value of the phosphotungstic acid aqueous solution is 1-5.

8. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 4, characterized in that: The dosage ratio of the crushed PET and the phosphotungstic acid aqueous solution is 1g:40-80mL.

9. The method for producing p-toluic acid and p-xylene by directional depolymerization of PET according to claim 4, characterized in that: The depolymerization reduction reaction is controlled to be carried out under a pressure of 1-5 MPa.

10. A method for treating waste PET products, characterized in that: The method for producing p-toluic acid and p-xylene by directional depolymerization of PET as described in any one of claims 3 to 9 is used to treat waste PET.

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