Method for preparing monomer dmt by mechanical-chemical piezoelectric synergistic catalytic depolymerization of pet waste plastic

Abstract

This invention belongs to the field of solid waste resource utilization, specifically disclosing a method for preparing dimethyl terephthalate (DMT) monomer from waste PET plastics through mechanochemical-piezoelectric synergistic catalytic depolymerization. By adding a piezoelectric catalyst to a mechanochemical ball milling system, the mechanochemical reaction generated during ball milling is synergistic with the piezoelectric effect of the piezoelectric material. The piezoelectric catalyst generates an enhanced local potential difference during mechanical collision, grinding, and shearing, thereby activating the polyester structure of PET. Under mild conditions of room temperature and pressure, with methanol as a nucleophile, a rapid alcoholysis reaction of the ester bonds in the PET polyester chain is achieved, resulting in the breakage and transformation of the polyester chain to obtain dimethyl terephthalate (DMT) monomer. The obtained DMT is further purified through solid-liquid separation and recrystallization to obtain high-purity DMT. This invention features low energy consumption, mild conditions, and high catalyst stability.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization, and in particular to a method for preparing monomer DMT from waste PET plastics through mechanochemical-piezoelectric synergistic catalytic depolymerization. Background Technology

[0002] Currently, a large amount of post-consumer plastics are directly landfilled or disposed of globally each year, with approximately 80% of waste plastics not receiving effective resource recovery. These plastics, remaining in the environment for extended periods, gradually decompose into microplastics, carrying plasticizers, stabilizers, and other chemical components into soil, water bodies, and marine organisms. Ultimately, they return to humans through the food chain, posing potential harm to the ecological environment and human health. Polyethylene terephthalate (PET) is a typical and widely used polyester plastic product, extensively used in beverage bottles, packaging films, and fiber materials. In recent years, my country's polyester industry has seen continuous annual output growth, exceeding 50 million tons. The resulting accumulation of large amounts of PET waste plastics has placed significant pressure on the environment and resource recycling system, becoming a major constraint on the sustainable development of the polyester industry. Therefore, developing efficient and environmentally friendly PET waste plastic recycling and reuse technologies has become a critical issue that the industry urgently needs to address.

[0003] Plastics are a class of polymeric compounds produced through monomer addition or condensation reactions. They mainly include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET) / polybutylene terephthalate (PBT), polystyrene foam (EPS), and nylon (PA). Polyesters, formed by the condensation of dicarboxylic acids (such as terephthalic acid, TPA) and diols (such as ethylene glycol), possess good stability, heat resistance, and mechanical properties. PET polyester has the largest usage volume, resulting in a correspondingly large amount of waste. In the resource recovery process of PET waste plastics, efficiently breaking down the polymer chains and converting them into reusable monomer raw materials, such as dimethyl terephthalate (DMT) and ethylene glycol, is a key step in achieving closed-loop recycling. Currently, commonly used PET depolymerization methods mainly include hydrolysis, alcoholysis, and aminolysis under solution conditions. Taking alcoholysis as an example, while traditional methanol alcoholysis can produce high-purity DMT, it typically requires high temperature (200–300 °C) and high pressure conditions, and often necessitates the addition of expensive catalysts, leading to high costs, high energy consumption, severe equipment corrosion, and long reaction times. Especially for highly crystalline or dyed PET waste plastics, conventional solution alcoholysis struggles to achieve rapid and efficient depolymerization to prepare DMT, limiting the further application of this technology in green chemistry and large-scale recycling. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing monomer DMT from waste PET plastics through a mechanochemical-piezoelectric synergistic catalytic depolymerization.

[0005] To achieve the above objectives, the present invention is implemented according to the following technical solution: A method for preparing monomer DMT from waste PET plastics through mechanochemical piezoelectric synergistic catalytic depolymerization includes the following steps: S1. Add PET waste plastic, piezoelectric catalyst and solid alkali in a mass ratio of 1:(0.1-0.5):(0.05-0.2) to a polytetrafluoroethylene grinding jar, then add methanol, the ratio of methanol to PET waste plastic is (1-5) mL:1 g, and finally load the grinding balls. S2. At room temperature, the polytetrafluoroethylene grinding jar is placed in a ball mill for ball milling for 15 min to 4 h. After ball milling, the ball milling product is washed with methanol and filtered to collect the filter cake. The collected filter cake is extracted with acetonitrile and filtered to collect the filtrate. The collected filtrate is then rotary evaporated to obtain a white solid, dimethyl terephthalate (DMT).

[0006] Furthermore, the piezoelectric catalyst is one of barium titanate, bismuth ferrite, zinc oxide, polyvinylidene fluoride, molybdenum disulfide, iron tetroxide, stannous sulfide, and cadmium sulfide.

[0007] Furthermore, the alkali is one of potassium hydroxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, and potassium carbonate.

[0008] Furthermore, the grinding balls are two polytetrafluoroethylene grinding balls with a diameter of 2 cm.

[0009] Preferably, the ball mill is an oscillating ball mill, and the frequency during ball milling is 15-30 Hz.

[0010] Preferably, the ball mill is a planetary ball mill, and the rotational speed during ball milling is 450-600 rpm.

[0011] The principle of this invention is as follows: By adding a piezoelectric catalyst to a mechanochemical ball milling system, the mechanochemical reaction generated by ball milling and the piezoelectric effect of the piezoelectric material are combined. The piezoelectric catalyst can generate an enhanced local potential difference during mechanical collision, grinding and shearing, thereby activating the polyester structure of PET. Under the condition of methanol as a nucleophile and mild conditions of room temperature and pressure, a rapid alcoholysis reaction of ester bonds in the PET polyester chain is achieved, thereby causing the polyester chain to break and transform to obtain dimethyl terephthalate (DMT) monomer. The obtained DMT is further purified by solid-liquid separation and recrystallization to obtain high-purity DMT.

[0012] Compared with the prior art, the present invention has the following advantages and technical effects: 1. The mechanochemical method of the present invention has mild reaction conditions, overcoming the harsh conditions such as solvents, high temperature, high pressure, and supercriticality required by the prior art, and the depolymerization reaction effect is good.

[0013] 2. The mechanochemical method of the present invention has high yield; the conversion rate of PET waste plastic to organic esters and organic alcohols can be achieved by reacting at 25 Hz for 30 minutes.

[0014] 3. The mechanochemical method of the present invention has the ability to provide energy, thereby increasing the rate of depolymerization reaction and achieving the goal of green and efficient depolymerization. Attached Figure Description

[0015] Figure 1 Comparison of solid morphology before and after alcoholysis of PET waste plastic: (a) PET waste plastic raw material before ball milling; (b) mixed state of catalyst and PET waste plastic after ball milling; (c) dimethyl terephthalate (DMT) crystals after purification.

[0016] Figure 2 The effects of different reaction parameters on the yield of DMT, a product of mechanochemical alcoholysis: (a) the change in DMT yield as the amount of different piezoelectric catalysts added gradually increases; (b) the effect of different types of alkali on the DMT yield; and (c) the trend of DMT yield obtained under different ball milling frequencies.

[0017] Figure 3 The DMT yields obtained by using four piezoelectric catalysts—BaTiO3, Bi2Fe4O9, ZnO, and PVDF—in three consecutive cycles under the same reaction conditions are shown. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0019] Example 1: 0.1 g of waste PET plastic, along with piezoelectric catalyst BaTiO3 (0 g, 0.01 g, 0.02 g, 0.03 g, 0.04 g, 0.05 g, and 0.06 g respectively), 0.02 g of NaOH, and 0.5 mL of methanol, were added to a 20 mL polytetrafluoroethylene (PTFE) grinding jar. Two PTFE balls with a diameter of 2 cm were then placed inside. The grinding jar was placed in a vibrating ball mill and milled for 30 min at 25 Hz. The initial morphology of the waste PET plastic was as follows: Figure 1 As shown in (a), after ball milling, it is gradually pulverized and thoroughly mixed with the catalyst as follows: Figure 1As shown in (b) above. After ball milling, methanol (10 mL × 2) was added to the grinding jar to wash the ball-milled product. The resulting mixture was transferred to a core funnel for filtration to remove soluble alkali. Subsequently, 20 mL of acetonitrile was added to the filter cake for soaking and washing again. The filtrate was collected and the acetonitrile was removed by rotary evaporation to obtain a white solid dimethyl terephthalate (DMT), as shown in (b). Figure 1 As shown in (c) above. The solid piezoelectric catalyst can be recycled for the next cycle experiment after drying.

[0020] Example 2: The difference from Example 1 is that Bi2Fe4O9 is used as the piezoelectric catalyst.

[0021] Example 3: The difference from Example 1 is that ZnO is used as the piezoelectric catalyst.

[0022] Example 4: The difference from Example 1 is that PVDF is used as the piezoelectric catalyst.

[0023] Example 5: 0.1 g of waste PET plastic, 0.05 g of piezoelectric catalyst BaTiO3, 0.02 g of solid alkali (NaOH, KOH, K2CO3, NaHCO3, Na2CO3, etc., respectively), and 0.5 mL of methanol were added to a 20 mL polytetrafluoroethylene (PTFE) grinding jar, along with two PTFE balls with a diameter of 2.0 cm. The grinding jar was placed in a planetary ball mill and milled at 500 rpm for 30 min. After milling, methanol (10 mL × 2) was added to the grinding jar to wash the milled product. The resulting mixture was transferred to a core funnel for filtration to remove soluble alkali. Subsequently, 20 mL of acetonitrile was added to the filter cake for soaking and washing again. The filtrate was collected, and the acetonitrile was removed by rotary evaporation to obtain white dimethyl terephthalate (DMT) solid. The solid piezoelectric catalyst can be recycled for the next cycle experiment after drying.

[0024] Example 6: 0.1 g of waste PET plastic, 0.05 g of piezoelectric catalyst BaTiO3, 0.02 g of NaOH, and 0.5 mL of methanol were added to a 20 mL polytetrafluoroethylene (PTFE) grinding jar, and PTFE balls with a diameter of 2 cm were placed inside. The grinding jar was placed in a vibrating ball mill and ball milled for 30 min at 15 Hz, 20 Hz, 25 Hz, and 30 Hz, respectively. After ball milling, methanol (10 mL × 2) was added to the grinding jar to wash the ball milling product. The resulting mixture was transferred to a core funnel for filtration to remove soluble alkali. Subsequently, 20 mL of acetonitrile was added to the filter cake for soaking and washing again. The filtrate was collected, and the acetonitrile was removed by rotary evaporation to obtain a white solid dimethyl terephthalate (DMT). The solid piezoelectric catalyst can be recycled for the next cycle experiment after drying.

[0025] To verify the actual depolymerization efficiency of the mechanochemical alcoholysis method of the present invention under mild ball milling conditions and the recycling performance of the piezoelectric catalyst, the DMT produced in Examples 1-4 was weighed and calculated in conjunction with the theoretical yield. Simultaneously, the residual solids after ball milling were weighed to assess the presence of unreacted PET. Figure 2 As shown in (a), the DMT yield is highest at 0.05 g when the piezoelectric catalyst (BaTiO3, Bi2Fe4O9, ZnO, and PVDF) content reaches approximately 99%, indicating the most significant piezoelectric effect within the system and achieving almost complete PET conversion. Further increasing the piezoelectric catalyst (BaTiO3, Bi2Fe4O9, ZnO, and PVDF) to 0.06 g slightly decreases the DMT yield to approximately 70%, possibly due to the excess polymer affecting mechanical energy transfer between solid particles and reducing grinding efficiency. The comparative results show that piezoelectric catalysts (BaTiO3, Bi2Fe4O9, ZnO and PVDF) can significantly enhance the efficiency of mechanochemical depolymerization reaction within a certain range. The optimal amount of piezoelectric catalysts (BaTiO3, Bi2Fe4O9, ZnO and PVDF) added is about 0.05 g, which can achieve the highest DMT yield. When the amount of catalyst is 0.05 g, no unreacted PET was detected in the experiment. By weighing and calculation, the conversion rate of PET can reach up to 100%, and the DMT yield can reach up to 99%. This indicates that the method of the present invention can achieve rapid and complete chain breaking of PET and obtain high-yield DMT under the conditions of 25 Hz and 30 min.

[0026] In the expanded study of catalyst types, in addition to piezoelectric materials BaTiO3, Bi2Fe4O9, ZnO, and PVDF, the depolymerization effects of molybdenum disulfide (MoS2), iron(III) oxide (Fe3O4), stannous sulfide (SnS), and cadmium sulfide (CdS) as catalysts were compared and tested. The experimental conditions remained consistent with the above, only the catalyst type was changed. The catalysts exhibited significantly lower performance than those in the mechanochemical alcoholysis system of this invention. Figure 3 The depolymerization ability of the four piezoelectric catalysts was compared, and the highest DMT yield was only 23.7%.

[0027] The DMT produced in Example 5 was weighed and calculated in conjunction with the theoretical yield. Simultaneously, the residual solids after ball milling were weighed to assess the presence of unreacted PET. Figure 2As shown in (b), strong bases NaOH and KOH significantly promote depolymerization in this system, achieving DMT yields of approximately 91.2% and 82.4%, respectively. The depolymerization efficiency decreases significantly with decreasing alkalinity. Potassium carbonate (K₂CO₃), while possessing some alkalinity, has limited promoting effect, yielding only 15.8% DMT. Sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃), being even weaker alkalinities, are less effective at participating in ester bond cleavage, resulting in DMT yields of 4.7% and 2.9%, respectively. This indicates that strong alkalinity dominates the ester bond cleavage process. Strong bases can more effectively participate in the ring-opening and cleavage of PET chains under mechanical action, thus significantly increasing DMT production.

[0028] The DMT produced in Example 6 was weighed and calculated in conjunction with the theoretical yield. Simultaneously, the residual solids after ball milling were weighed to assess the presence of unreacted PET. Figure 2 As shown in (c), the DMT yield increases significantly with increasing ball milling frequency. At a lower frequency of 15 Hz, the chain-breaking efficiency of PET is low due to insufficient mechanical energy input, resulting in a DMT yield of only 27.5%. When the frequency is increased to 20 Hz, the mechanical impact and friction during ball milling are enhanced, accelerating PET chain breaking and increasing the DMT yield to 46.3%. Further increasing the frequency to 25 Hz further enhances the effective transfer of mechanical energy and facilitates the activation of the piezoelectric catalytic effect, significantly increasing the DMT yield to 62.4%. When the frequency is increased to 30 Hz, the system achieves a DMT yield of approximately 85.0%, the highest yield level in this experiment. These comparative results indicate that ball milling frequency is one of the important factors affecting the efficiency of mechanochemical alcoholysis. Appropriately increasing the ball milling frequency can enhance the contact, breakage, and stress effects between solid particles, thereby accelerating the PET molecular chain breaking process and significantly increasing the amount of DMT generated.

[0029] In addition, cyclic stability tests were conducted on the piezoelectric catalysts, represented by BaTiO3, Bi2Fe4O9, ZnO, and PVDF, recovered in Examples 1-4. Figure 3 After each reaction, the catalyst was washed with methanol, dried, and recovered before being directly used in the next ball milling reaction. In three consecutive cycles, the obtained DMT was a white solid. Comparison of the actual DMT yields from the three cycles showed that the DMT yield was approximately 99% in the first reaction, approximately 95% in the second, and approximately 91% in the third. The yield decrease in each of the three experiments was less than 5%. These results indicate that the piezoelectric catalyst is structurally stable in the mechanochemical system, exhibits good reusability, and can maintain high reactivity for at least three cycles.

[0030] In summary, the mechanochemical-piezoelectric synergistic green catalytic alcoholysis system of the present invention can achieve the complete conversion of 0.1 g PET under mild conditions of 25 Hz and 30 min, and obtain a high DMT yield of about 99%. At the same time, the activity decay of the piezoelectric catalyst after three cycles is less than 5%, which fully demonstrates the significant advantages of this method in terms of high efficiency, low energy consumption and recyclability.

[0031] The technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made in accordance with the technical solutions of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing monomer DMT from waste PET plastics through mechanochemical piezoelectric synergistic catalytic depolymerization, characterized in that, Includes the following steps: S1. Add PET waste plastic, piezoelectric catalyst and solid alkali in a mass ratio of 1:(0.1-0.5):(0.05-0.2) to a polytetrafluoroethylene grinding jar, then add methanol, the ratio of methanol to PET waste plastic is (1-5) mL:1 g, and finally load the grinding balls. S2. At room temperature, the polytetrafluoroethylene grinding jar is placed in a ball mill for ball milling for 15 min to 4 h. After ball milling, the ball milling product is washed with methanol and filtered to collect the filter cake. The collected filter cake is extracted with acetonitrile and filtered to collect the filtrate. The collected filtrate is then rotary evaporated to obtain a white solid, dimethyl terephthalate (DMT).

2. The method for preparing monomer DMT from waste PET plastics through mechanochemical piezoelectric synergistic catalytic depolymerization according to claim 1, characterized in that: The piezoelectric catalyst is one of barium titanate, bismuth ferrite, zinc oxide, polyvinylidene fluoride, molybdenum disulfide, iron tetroxide, stannous sulfide, and cadmium sulfide.

3. The method for preparing monomer DMT from waste PET plastics through mechanochemical piezoelectric synergistic catalytic depolymerization according to claim 1, characterized in that: The alkali is one of potassium hydroxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, and potassium carbonate.

4. The method for preparing monomer DMT from waste PET plastics through mechanochemical piezoelectric synergistic catalytic depolymerization according to claim 1, characterized in that: The grinding balls are two polytetrafluoroethylene grinding balls with a diameter of 2 cm.

5. The method for preparing monomer DMT from waste PET plastics through mechanochemical piezoelectric synergistic catalytic depolymerization according to claim 1, characterized in that: The ball mill is an oscillating ball mill, and the frequency during ball milling is 15-30 Hz.

6. The method for preparing monomer DMT from waste PET plastics through mechanochemical piezoelectric synergistic catalytic depolymerization according to claim 1, characterized in that: The ball mill is a planetary ball mill, and the rotational speed during ball milling is 450-600 rpm.

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