A method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics

Abstract

The application discloses a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, and belongs to the technical fields of waste plastic resource utilization, ionic liquid catalysis and microwave chemical conversion. The application uses polyolefin plastics as raw materials, uses an ionic liquid system composed of a chlorine-containing ionic liquid and a Lewis acidic metal chloride salt as a microwave absorption medium, a reaction medium and a catalytic activity system, and places the low-temperature feeding in a microwave reactor, so that the ionic liquid rapidly couples microwave energy through ionic conduction loss and dielectric loss at a low temperature of 30-80 DEG C, and a main chain C-C bond continuous breaking reaction zone is formed at the polyolefin / ionic liquid interface, polyolefin segments are softened, activated, beta-broken, hydrogen transferred and chain transferred, and the product is mainly light olefins, and is accompanied by a small amount of aromatic hydrocarbons and C8-C 16 liquid fractions. By adjusting the types of raw materials and reaction parameters, the polyolefin cracking depth, light olefin selectivity and liquid fraction distribution can be flexibly controlled as required.

Description

Technical Field

[0001] This invention relates to the fields of waste plastic resource utilization, ionic liquid catalysis and microwave chemical conversion technology, and in particular to a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization. Background Technology

[0002] Polyolefins such as polyethylene and polypropylene are widely used in packaging, construction, automobiles, and daily necessities due to their wide availability, large production volume, and high chemical stability. However, the main chain of polyolefins is composed primarily of stable carbon-carbon (CC) and carbon-hydrogen (CH) bonds, making them difficult to degrade under natural conditions. Traditional landfill and incineration methods not only waste carbon resources but may also lead to greenhouse gas emissions and microplastic pollution. Therefore, converting waste polyolefins into lighter olefins such as ethylene, propylene, and butene is an important direction for achieving high-value utilization of plastic resources.

[0003] Existing chemical recycling methods for polyolefins mainly include thermal cracking, catalytic cracking, hydrogenolysis, and hydrocracking. Thermal cracking and catalytic cracking typically require high reaction temperatures, and polyethylene and polypropylene have poor thermal conductivity, making them prone to problems such as slow heating, localized overheating, excessive cracking, carbon buildup, and wide product distribution. When the target product is light olefins, higher temperatures and longer residence times may also lead to further aromatization, polymerization, or conversion into liquid hydrocarbons, thereby reducing the selectivity for light olefins.

[0004] Microwave conversion offers a new energy input method for the resource utilization of polyolefins. Currently, microwave conversion of polyethylene and polypropylene mainly focuses on microwave-assisted pyrolysis, microwave-assisted catalytic cracking, or synergistic cracking of microwaves and absorbing fillers. This typically requires the use of absorbing materials such as carbon black, activated carbon, silicon carbide, metals, or metal oxides to convert microwave energy into heat energy. While these methods can increase the heating rate and mitigate some heat transfer limitations, they still primarily rely on high-temperature pyrolysis or high-temperature catalytic cracking. The absorbing materials mainly provide localized heat sources, with limited effectiveness in low-temperature activation of polyolefin segments, directional breakage of C / C bonds, and selective regulation of light olefins.

[0005] Therefore, how to reduce the depolymerization temperature of polyolefins in a microwave reactor and enable microwave energy to act directly on the reaction medium and the interface between polyolefins is a key problem that needs to be solved in the microwave conversion of polyolefins to prepare light olefins.

[0006] The research article in this field (Wei Zhang et al., Low-temperature upcycling of polyolefins into liquid alkanes via tandem cracking-alkylation. Science 379, 807-811 (2023). DOI: 10.1126 / science.ade7485) utilizes a Lewis acidic ionic liquid of chloroaluminate at 70°C and under an inert atmosphere at ambient pressure to achieve C / C bond cleavage of polyethylene and polypropylene via a carbocation mechanism. The reaction includes β-cleavage, hydrogen transfer, and isomerization processes. Isopentane is added to the reaction system as a co-reactant, causing the olefin intermediates generated by cracking to undergo immediate alkylation, with the final product being liquid isoalkanes from gasoline fractions. It is generally believed in this field that olefin intermediates in Lewis acidic ionic liquid systems are prone to polymerization and coking, requiring the addition of alkane co-reactants such as isopentane for stable reaction, and the final product is mainly alkanes, making it difficult to obtain products mainly composed of olefins.

[0007] Chinese invention patent CN119855644A discloses a microwave-assisted catalytic pyrolysis method and reactor for selectively converting plastics into olefins. This method employs a microwave-assisted slurry reactor pyrolysis system, using a solid acid as a catalyst to crack polyolefins to prepare low-carbon olefins. However, the microwave polyolefin cracking route relies on a two-component separation system of a solid microwave absorber and a solid acid catalyst. There is a spatial misalignment between the microwave absorbing medium and the catalytic active sites, and energy needs to be transferred through heat conduction. The reaction depends on local high temperatures (>300 °C) on the catalyst surface to break C-C bonds. This results in severe side reactions, poor olefin selectivity, and fails to address the technical problems of weak microwave absorption and poor thermal conductivity inherent in polyolefins.

[0008] In summary, this invention provides a microwave-driven method for the low-temperature depolymerization of polyolefin plastics to prepare olefins. Existing microwave conversion methods for polyolefins still mainly rely on high-temperature pyrolysis, have weak microwave absorption capabilities, face difficulties in low-temperature breakage of main chain C-C bonds, and suffer from insufficient selectivity for light olefins. This invention is of great significance to the field of waste plastic resource utilization. Summary of the Invention

[0009] In view of the above-mentioned defects of the prior art, the purpose of the present invention is to provide a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which solves the problems that the microwave conversion of polyolefins still mainly relies on high-temperature pyrolysis, the weak microwave absorption capacity of polyolefins themselves, the difficulty of low-temperature breaking of main chain C-C bonds, and the insufficient selectivity of light olefins.

[0010] To achieve the above objectives, the technical solution provided by the present invention is as follows: This invention provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, comprising the following steps: (1) Process the polyolefin raw material into a form suitable for contact with ionic liquids; (2) A chloride-containing liquid or chloride-containing liquid precursor is mixed with a Lewis acid metal chloride salt under an inert gas atmosphere to form an ionic liquid system with microwave absorption capability and Lewis acidity; (3) Mix the polyolefin raw material with the ionic liquid system at a certain mass ratio, place it in a microwave reaction device, seal it under certain temperature conditions, and replace the air in the reaction device with an inert gas; (4) After feeding, microwave irradiation is applied, and the reaction temperature, microwave power and reaction time are controlled so that the ionic liquid absorbs microwave energy and forms a carbon-carbon bond continuous breaking reaction zone at the interface between polyolefin and ionic liquid, which promotes polyolefin chain segment activation, β-breakage, hydrogen transfer, isomerization and lightening. (5) After the reaction is completed, the mixture is cooled and gas-liquid separation and liquid-liquid phase separation are performed. The gas phase or low-boiling point fraction is collected to obtain light olefin products. The liquid phase is extracted and fractionated to obtain liquid olefin fraction. The ionic liquid phase is dehydrated, filtered and replenished with metal chloride salts and then recycled.

[0011] In this invention, the sources of polyolefin raw materials are diverse, including low-density polyethylene, high-density polyethylene, linear low-density polyethylene, homopolymer polypropylene, copolymer polypropylene, polyvinyl chloride, and polyethylene / polypropylene / polyvinyl chloride blends, as well as waste polyolefin plastics obtained from packaging films, plastic bags, woven bags, bottle caps, pipes, containers, automotive plastic parts, and industrial scraps. Those skilled in the art can select appropriate types of raw materials according to actual conditions.

[0012] Preferably, in step (1), the polyolefin raw materials are sorted, cleaned, dried, crushed or sheared to obtain polyolefin particles, flakes or pulverized materials suitable for contact with ionic liquids.

[0013] Preferably, in step (2), the chloride-containing liquid or chloride-containing liquid precursor includes one or more of 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-hexyl-3-methylimidazolium chloride, 1-octyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, 4-butylaminochloride, N-ethylpyridine chloride, N-butylpyridine chloride, and N-benzylpyridine chloride.

[0014] Preferably, in step (2), the Lewis acidic metal chloride salt includes one or more of anhydrous aluminum chloride, anhydrous ferric chloride, anhydrous zinc chloride, anhydrous tin tetrachloride, and anhydrous titanium tetrachloride.

[0015] Preferably, in step (3), the mass ratio of the ionic liquid system to the polyolefin raw material is 0.5:1 to 20:1, and more preferably 2:1 to 10:1.

[0016] When the amount of ionic liquid is too low, the surface of polyolefins is difficult to be adequately wetted, and microwave energy and acidic sites are difficult to be uniformly transferred; when the amount of ionic liquid is too high, the viscosity of the system and the post-processing burden increase, and the processing efficiency per unit mass of polyolefin decreases. By controlling the amount of ionic liquid, a balance can be achieved between polyolefin segment contact, microwave absorption, and product separation.

[0017] Chloride-containing ionic liquids and metal chloride salts can form ionic liquid systems such as chloroaluminate, chloroferrate, or chlorozincate. The proportion of metal chloride salts is used to adjust the acidity of the ionic liquid and the depth of C-C bond breaking in polyolefins.

[0018] Preferably, in step (4), the initial feeding temperature is 15~35 ℃.

[0019] Preferably, in step (4), the reaction temperature under microwave irradiation is 30~80 ℃.

[0020] The temperature mentioned is the controlled temperature of the reaction system or ionic liquid phase during microwave irradiation, which is different from the high-temperature overall preheating required for traditional thermal pyrolysis.

[0021] Preferably, in step (4), the microwave power is 100~1200 W.

[0022] Those skilled in the art can adaptively adjust the microwave power according to the reaction system temperature and the equipment. The above parameter setting range is the maximum heating power allowed by the equipment. When the system reaches the reaction temperature, the basic power, such as 25 W, can be maintained based on the equipment's heat dissipation and other conditions to keep the system at the expected reaction temperature.

[0023] Preferably, in step (4), the reaction time is 1 to 8 hours.

[0024] Preferably, the inert gas includes one or both of nitrogen and argon.

[0025] An inert atmosphere is used to remove oxygen, reducing the risk of side reactions in polyolefin oxidation and the hydrolysis or oxidative deactivation of acidic species in ionic liquids. For waste plastics with high moisture content, it is preferable to dry them at 60-100 °C before the reaction to avoid moisture consuming the acidic sites of metal chloride salts and to reduce the risk of hydrogen chloride release and equipment corrosion during the reaction.

[0026] Based on the above technical solutions, the design concept and principle of this invention are as follows: Ionic liquids possess ionic conductivity, tunable acidity, and strong microwave responsiveness, and can simultaneously serve as reaction solvents, microwave absorbing media, and acidic active components. This invention utilizes the microwave absorption effect and acidic activation of ionic liquids, using an ionic liquid system as the reaction solvent, microwave absorbing medium, and acidic active system. After low-temperature feeding, the temperature of the reaction solvent is controlled by microwaves, concentrating microwave energy coupled to the ionic liquid phase and the polyolefin / ionic liquid interface. This achieves continuous breaking of the C-C bonds in polyolefins at low temperatures of 30–80 °C, and directionally modulates the product into a light olefin.

[0027] Existing methods using Lewis acidic ionic liquids to catalyze the cracking of polyolefins produce highly reactive olefin intermediates that rapidly undergo alkylation, polymerization, cyclization, and even coking in acidic environments. Therefore, current technologies require the addition of alkane co-reactants as hydrogen donors or alkylation acceptors to stabilize the intermediates and prevent coking. The final products are primarily liquid alkanes, making it difficult to obtain light olefins with high selectivity.

[0028] The fundamental innovation of this invention lies in the fact that microwaves do not simply replace traditional heating, but rather effectively reduce the apparent activation energy of CC bond breaking through the selective polar activation of acidic reactive species and carbocation reaction intermediates in the ionic liquid. Furthermore, the high dielectric loss characteristics of the ionic liquid allow it to directly absorb microwave energy, precisely coupling the energy to the polyolefin / ionic liquid interface to form a continuous CC bond breaking reaction zone in the main chain. Combined with the interface energy concentration effect, continuous β-breaking of the main chain CC bond can be achieved under mild macroscopic conditions of 30–80 °C, without relying on local ultra-high temperatures or introducing additional co-reactants. By controlling the reaction temperature, microwave power, and reaction time, the generated light olefins rapidly vaporize and escape from the acidic ionic liquid phase in the interface reaction zone, effectively avoiding secondary side reactions. Highly selective and directional generation of light olefins can be achieved without adding alkane co-reactants. Compared to traditional thermocatalytic systems, this approach allows for adjustment of interface reaction activity without changing the macroscopic temperature, achieving more flexible and precise product control.

[0029] Based on the above mechanism, compared with the traditional high-temperature pyrolysis route, this scheme reduces the reaction temperature from 400~600 ℃ to 30~80 ℃ and the reaction pressure to near atmospheric pressure, significantly reducing process energy consumption and simultaneously lowering the requirements for high temperature and high pressure resistance of equipment, thus significantly improving process safety and reducing equipment investment and operating costs. Furthermore, this scheme overcomes the limitation that the main product of low-temperature ionic liquid systems is alkanes, achieving the directional generation of high-value-added light olefins, and possesses excellent selectivity and economic value.

[0030] Compared with the prior art, the present invention has the following advantages and beneficial effects: This invention provides a microwave-driven, low-temperature depolymerization method for preparing olefins from polyolefin plastics. This method utilizes ionic liquids to directly absorb microwave energy, overcoming the problems of weak microwave absorption and poor thermal conductivity inherent in polyolefins. The ionic liquids simultaneously provide acidic active sites and a reaction medium environment, enabling the continuous breaking of C-C bonds in the polyolefin backbone at the interface. This method offers excellent flexibility; the distribution of olefin and aromatic fractions can be synergistically adjusted by modifying the ionic liquid composition, the proportion of metal chlorides, microwave power, reaction temperature, and reaction time to meet different application requirements. Furthermore, after the reaction, the olefin products, liquid fractions, and ionic liquid phase are easily separated, and the ionic liquid can be regenerated and recycled, effectively improving process stability and operating costs, and significantly enhancing environmental friendliness. Attached Figure Description

[0031] Figure 1 This is a schematic flowchart of a microwave-driven low-temperature depolymerization method for preparing olefins from polyolefin plastics according to the present invention. Figure 2 This is a photograph of the physical product after the reaction in the microwave-driven low-temperature depolymerization of polyolefin plastics to prepare olefins in Example 1. Figure 3 This is a gas chromatogram of the liquid product from the microwave-driven low-temperature depolymerization of polyolefin plastics to prepare olefins in Example 1. Detailed Implementation

[0032] The present invention will be further described below with reference to embodiments. It should be understood that the following embodiments are only for explaining the present invention and are not intended to limit the scope of protection of the present invention. Adjustments made by those skilled in the art to the type of ionic liquid, the composition of the metal chloride salt, the plastic raw materials, the microwave power, the reaction temperature, the reaction time, and the post-treatment method without departing from the concept of the present invention shall all fall within the scope of protection of the present invention.

[0033] Unless otherwise stated, the product distribution in the following examples was evaluated using gas chromatography-mass spectrometry with a combination of external and internal standards to assess product yield and selectivity. The carbon number distributions of the olefins and aromatics are all in the range of 8–16. The data in the tables below are used to illustrate the effect of different reaction conditions on product distribution and should not be construed as limiting the scope of protection of this invention.

[0034] In microwave ionic liquid reactions, the microwave absorption capacity of the ionic liquid, the acidity of the metal chloride salt, the reaction temperature, the reaction time, and the polyolefin chain structure collectively determine the degree of C / C bond breaking. A high olefin content and a low aromatic content indicate that microwave-driven continuous breaking of C / C bonds at the ionic liquid interface is more conducive to the formation of light olefins.

[0035] The reaction pathway and process control logic of this invention are integrated into the microwave ionic liquid interface reaction process. After polyolefins enter the microwave reactor, polyethylene or polypropylene itself has a weak microwave response. Without an ionic liquid phase, the system is difficult to heat uniformly and form effective pyrolysis. After adding the ionic liquid, the cations, chloride ions, and chloride metal anions in the ionic liquid migrate, undergo friction, and orientation relaxation under an alternating electromagnetic field. The microwave energy is rapidly converted into localized heat energy and interfacial activation, thereby significantly shortening the reaction start-up time.

[0036] In this interfacial region, Lewis acidic species formed by metal chloride salts can interact with adjacent structures of the C-C bonds in polyolefin segments, making it easier for the segments to form activated intermediates. Subsequently, the intermediates undergo β-fracture to generate shorter hydrocarbon chains, releasing light olefins through hydrogen transfer and chain transfer processes. When the reaction temperature, acid strength, and residence time are moderate, the products are predominantly light olefins; when conditions are too strong, olefins may continue to undergo aromatization, polymerization, or liquid fraction growth, leading to an increase in the proportion of aromatics.

[0037] Therefore, this invention controls product distribution through multiple parameters. Reaction temperature primarily affects the softening, chain segment migration, and degree of secondary transformation of polyolefins; reaction time primarily determines the depth of transformation of long-chain intermediates into light olefins; microwave power primarily determines the ionic liquid's microwave absorption heating rate and interfacial energy density; the ionic liquid's cationic structure affects the system's viscosity, wettability, and microwave response; and the type and amount of metal chloride salt affect Lewis acidity and the stability of activated intermediates. These factors collectively determine the continuous breaking of C / C bonds and the enrichment effect of light olefins.

[0038] In the following examples, the preparation steps of the ionic liquid system are as follows: Under a dry nitrogen atmosphere, 1-ethyl-3-methylimidazolium chloride was weighed and dried under vacuum at 80 °C for 12 h to remove moisture; anhydrous aluminum chloride was separately stored in a desiccator for later use. 1-ethyl-3-methylimidazolium chloride and anhydrous aluminum chloride were slowly mixed at a molar ratio of 1:1.5 and stirred until a homogeneous chloroaluminate ionic liquid system was formed. This system simultaneously exhibits microwave absorption capability, reaction medium activity, and Lewis acid activity. This preparation example serves as a representative illustration of the preparation method for the ionic liquid system. Those skilled in the art can replace the cation with butylimidazolium, hexylimidazolium, or pyridinium structures, and can also replace the metal chloride with ferric chloride, zinc chloride, tin tetrachloride, or titanium tetrachloride.

[0039] During the preparation of ionic liquid systems, the introduction of moisture into the system should be avoided as much as possible. Moisture can coordinate with or hydrolyze anhydrous metal chlorides, weakening the Lewis acidity of chloride metal anions and potentially causing acidity fluctuations during microwave irradiation. Therefore, chloride-containing ionic liquid precursors, metal chlorides, reactors, and stirring equipment are all preferably pre-dried. For chloroaluminate systems, the proportion of metal chlorides can affect AlCl4. - With Al2Cl7 - The distribution of species can alter the activation capacity of polyolefin segments and the degree of secondary cracking.

[0040] In practice, ionic liquid systems can be directly added to the microwave reactor as a reaction medium, or they can be premixed or impregnated with polyolefin particles. For thin-film polyolefins, shearing or crushing methods can be used to increase the contact area; for granular or block polyolefins, the premixing time can be appropriately extended to allow the ionic liquid to cover the plastic surface. This premixing step facilitates the rapid formation of a continuous interfacial reaction zone after microwave startup.

[0041] Example 1 This embodiment provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, such as... Figure 1 As shown, the steps are as follows: (1) Clean, dry and cut the low-density polyethylene packaging film into pieces, and process it into a shape suitable for contact with ionic liquids; (2) Under a dry nitrogen atmosphere, anhydrous aluminum chloride and chloride ion-containing liquid 1-ethyl-3-methylimidazolium chloride are slowly mixed and stirred in an ice-water bath for 2 h to obtain a chloroaluminate ion liquid reaction system; the mixing process is preferably carried out under anhydrous and oxygen-free conditions to reduce the influence of moisture on chloroaluminate species and Lewis acid sites. (3) Weigh the polyethylene raw material and mix it with the above chloroaluminate ionic liquid system at a mass ratio of 1:5. Then add 10 μL of carbocation initiator tert-chlorobutane and stir to make the polyethylene raw material, ionic liquid system and tert-chlorobutane fully contact. Place the resulting mixture in a microwave reactor with a condenser and gas collection interface. Seal the reactor at room temperature and replace it with nitrogen three times. (4) Apply 2.45 GHz microwave irradiation, adjust the temperature of the reaction solvent by microwave power, control the reaction temperature to 60℃, the initial maximum microwave heating power is 500 W (the initial maximum microwave heating power is 500 W, when the temperature reaches the reaction temperature, the microwave power is maintained at about 25 W, the same below), and the reaction time is 4 h; during the reaction, the chloroaluminate ionic liquid system absorbs microwave energy and forms a low-temperature acidic reaction environment, and tert-butane chloroform generates carbocation active species in the Lewis acidic chloroaluminate system, thereby inducing the continuous breakage of the C-C bond of the polyethylene main chain in the reaction zone of the ionic liquid interface; (5) After the reaction is complete, the microwave reactor is removed and placed in a refrigerator to freeze for 30 min to lower the system temperature and suppress subsequent side reactions. Then, sodium hydroxide solution is slowly added to the reactor for quenching to deactivate the carbocation active species in the system. After quenching, chloroform is added to the system for extraction. After thorough shaking or stirring, the mixture is allowed to stand for layering to obtain a chloroform phase containing light olefin products. After separating the chloroform phase, it is further dried, concentrated, and analyzed by gas chromatography or gas chromatography-mass spectrometry to determine C8~C6. 16 Olefin and aromatic yields and selectivity.

[0042] In this embodiment, the initial stage of the reaction is characterized by the rapid response of the chloroaluminate ionic liquid system to microwaves and the maintenance of a low-temperature acidic reaction environment. The polyethylene film gradually softens and forms a larger contact interface with the ionic liquid. Tert-butane chloride forms a carbocation initiator in the Lewis acidic system, promoting chain transfer, β-splitting, and continuous cleavage of polyethylene segments to generate C8~C6 bonds. 16 Olefin fraction. By controlling the reaction temperature, microwave power, and reaction time, the release of light olefins can be promoted, while further aromatization, polymerization, or heavy olefin formation can be inhibited.

[0043] A photograph of the reactor after the reaction in this embodiment is attached. Figure 2 As shown, the gas chromatogram corresponding to the liquid phase product is attached. Figure 3 As shown, the corresponding C8~C 16 The olefin yield was 55.2 wt.% with a selectivity of 92.0%; the aromatic yield was 4.8 wt.% with a selectivity of 8.0%.

[0044] Example 2 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the reaction temperature in step (4) of this embodiment is 30 °C.

[0045] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 35.6 wt.% with a selectivity of 98.3%; the aromatic yield was 0.6 wt.% with a selectivity of 1.7%.

[0046] Example 3 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the reaction temperature in step (4) of this embodiment is 50 °C.

[0047] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 44.8 wt.% with a selectivity of 96.8%; the aromatic yield was 1.5 wt.% with a selectivity of 3.2%.

[0048] Example 4 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the reaction temperature in step (4) of this embodiment is 80 °C.

[0049] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 68.2 wt.% with a selectivity of 91.7%; the aromatic yield was 6.2 wt.% with a selectivity of 8.3%.

[0050] Examples 2-4, based on Example 1, changed the microwave reaction temperature while keeping other conditions constant, and investigated the effect of temperature on microwave-driven depolymerization of polyolefins to a liquid state (C8-C1). 16 The effects of alkenes and aromatics. The reaction conditions and product distribution are shown in Table 1 below.

[0051] Table 1: Effect of reaction temperature on the product distribution obtained from microwave-driven C-bond cleavage of polyolefins

[0052] Table 1 shows that increased temperature promotes softening of polyolefin segments, interfacial migration, and C / C bond breaking. However, excessively high temperatures enhance secondary cracking and aromatization, generating aromatic components and decreasing olefin selectivity. When the reaction temperature is 60 °C, the proportion of olefins is relatively high and the proportion of aromatics is relatively low, indicating that the interaction between the low-temperature microwave field and the ionic liquid is optimal.

[0053] Example 5 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the reaction time in step (4) of this embodiment is 1 h.

[0054] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 28.4 wt.% with a selectivity of 97.3%; the aromatic yield was 0.8 wt.% with a selectivity of 2.7%.

[0055] Example 6 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the reaction time in step (4) of this embodiment is 2 h.

[0056] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 38.6 wt.% with a selectivity of 96.3%; the aromatic yield was 1.5 wt.% with a selectivity of 3.7%.

[0057] Example 7 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the reaction time in step (4) of this embodiment is 8 h.

[0058] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 50.6 wt.% with a selectivity of 84.3%; the aromatic yield was 9.4 wt.% with a selectivity of 15.7%.

[0059] Examples 5-7, based on Example 1, changed the reaction time while keeping other conditions constant, and investigated the effect of reaction time on microwave-driven depolymerization of polyolefins to a liquid C8-C8 state. 16 The effects of alkenes and aromatics. The reaction conditions and product distribution are shown in Table 2 below.

[0060] Table 2: Effect of reaction time on the product distribution obtained from microwave-driven C-bond cleavage of polyolefins

[0061] As shown in Table 2, when the reaction time is too short, the polyolefin backbone is not sufficiently broken, resulting in a low olefin yield; when the reaction time is too long, the olefins that have already been generated undergo further secondary transformation and aromatization, leading to a decrease in the proportion of light olefins.

[0062] Example 8 This embodiment provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, which is basically the same as that in Embodiment 1, except that the ionic liquid precursor used in this embodiment is 1-butyl-3-methylimidazolium chloride.

[0063] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 48.6 wt.% with a selectivity of 91.5%; the aromatic yield was 4.5 wt.% with a selectivity of 8.5%.

[0064] Example 9 This embodiment provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, which is basically the same as that in Embodiment 1, except that the ionic liquid precursor used in this embodiment is 1-hexyl-3-methylimidazolium chloride.

[0065] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 42.3 wt.% with a selectivity of 89.1%; the aromatic yield was 5.2 wt.% with a selectivity of 10.9%.

[0066] Example 10 This embodiment provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, which is basically the same as that in Embodiment 1, except that the ionic liquid precursor used in this embodiment is tetrabutylammonium chloride.

[0067] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 48.6 wt.% with a selectivity of 91.5%; the aromatic yield was 4.5 wt.% with a selectivity of 8.5%.

[0068] Example 11 This embodiment provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, which is basically the same as that in Embodiment 1, except that the ionic liquid precursor used in this embodiment is N-butylpyridine chloride.

[0069] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 42.3 wt.% with a selectivity of 89.1%; the aromatic yield was 5.2 wt.% with a selectivity of 10.9%.

[0070] Examples 8-11, based on Example 1, modified the cation structure of the ionic liquid while keeping other conditions unchanged, to investigate the effects of the microwave responsiveness and interfacial wetting ability of the ionic liquid on product distribution. The reaction conditions and product distribution are shown in Table 3 below.

[0071] Table 3: Effect of ionic liquid type on product distribution obtained from microwave-driven C-O bond breaking of polyolefins

[0072] Table 3 shows that different ionic liquid cations can form microwave-responsive systems with metal chlorides, but the chain length and cation structure affect viscosity, interfacial contact, and the distribution of acidic species. The short-chain imidazolium chloride system achieves a better balance between microwave absorption and mass transfer, which is more conducive to the release of light olefins.

[0073] Example 12 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in Embodiment 1, except that the chloride salt used in this embodiment is FeCl3.

[0074] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 37.6 wt.% with a selectivity of 89.9%; the aromatic yield was 4.2 wt.% with a selectivity of 10.1%.

[0075] Example 13 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in Embodiment 1, except that the chloride salt used in this embodiment is ZnCl2.

[0076] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 24.5 wt.% with a selectivity of 95.3%; the aromatic yield was 1.2 wt.% with a selectivity of 4.7%.

[0077] Example 14 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in Embodiment 1, except that the chloride salt used in this embodiment is SnCl4.

[0078] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 33.8 wt.% with a selectivity of 85.8%; the aromatic yield was 5.6 wt.% with a selectivity of 14.2%.

[0079] Examples 12-14, based on Example 1, changed the type of Lewis acidic metal chloride while keeping other conditions constant, and investigated the effect of the chloride on microwave-driven depolymerization of polyolefins to a liquid C8-C8 state. 16 The effects of alkenes and aromatics. The reaction conditions and product distribution are shown in Table 4 below.

[0080] Table 4: Effect of metal chlorides on the product distribution of microwave-driven C-O bond breaking in polyolefins

[0081] As shown in Table 4, metal chloride salts determine the Lewis acidity and polyolefin segment activation ability of the ionic liquid system. The chloroaluminate ionic liquid formed by AlCl3 exhibits a good balance between acid strength, microwave absorption ability, and selectivity for light olefins.

[0082] Example 15 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the maximum initial microwave heating power in step (4) of this embodiment is 300 W.

[0083] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 32.1 wt.% with a selectivity of 94.7%; the aromatic yield was 1.8 wt.% with a selectivity of 5.3%.

[0084] Example 16 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the maximum initial microwave heating power in step (4) of this embodiment is 600 W.

[0085] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 60.4 wt.% with a selectivity of 90.1%; the aromatic yield was 6.6 wt.% with a selectivity of 9.9%.

[0086] Example 17 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in embodiment 1, except that the maximum initial microwave heating power in step (4) of this embodiment is 800 W.

[0087] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 51.7 wt.% with a selectivity of 82.2%; the aromatic yield was 11.2 wt.% with a selectivity of 17.8%.

[0088] Examples 15-17, based on Example 1, changed the microwave power while keeping other conditions constant, and investigated the effect of microwave power on microwave-driven depolymerization of polyolefins to a liquid C8-C8 state. 16 The effects of alkenes and aromatics. The reaction conditions and product distribution are shown in Table 5 below.

[0089] Table 5: Effect of initial maximum microwave heating power on product distribution obtained from microwave-driven C-bond breaking in polyolefins

[0090] Table 5 shows that when the microwave power is too low, the ionic liquid interface activation is insufficient, resulting in incomplete polyolefin cleavage; when the power is too high, secondary cracking, aromatization, and the formation of liquid fractions are enhanced. Therefore, a moderate microwave power is beneficial for increasing the proportion of olefin products and controlling the olefin composition.

[0091] Example 18 This embodiment provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, which is basically the same as that in Embodiment 1, except that the polyolefin raw material in this embodiment is high-density polyethylene.

[0092] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 49.8 wt.% with a selectivity of 91.9%; the aromatic yield was 4.4 wt.% with a selectivity of 8.1%.

[0093] Example 19 This embodiment provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, which is basically the same as that in Embodiment 1, except that the polyolefin raw material in this embodiment is polypropylene.

[0094] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The yield of olefins was 58.9 wt., with a selectivity of 89.2%; the yield of aromatics was 7.1 wt., with a selectivity of 10.8%.

[0095] Example 20 This embodiment provides a method for preparing olefins by microwave-driven low-temperature depolymerization of polyolefin plastics, which is basically the same as that in Embodiment 1, except that the polyolefin raw material in this embodiment is polyvinyl chloride.

[0096] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 12.6 wt.% with a selectivity of 76.8%; the aromatic yield was 3.8 wt.% with a selectivity of 23.2%.

[0097] Example 21 This embodiment provides a method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, which is basically the same as that in Embodiment 1, except that the polyolefin raw material in this embodiment is a mixture of polyethylene / polypropylene waste plastics.

[0098] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 54.6 wt.% with a selectivity of 90.2%; the aromatic yield was 5.9 wt.% with a selectivity of 9.8%.

[0099] Examples 18-21, based on Example 1, changed the type of polyolefin raw material while keeping other conditions constant, to investigate the applicability of different polyolefin plastic sources in microwave ionic liquid systems. The reaction conditions and product distribution are shown in Table 6 below.

[0100] Table 6: Effect of polyolefin feedstock on the product distribution obtained from microwave-driven C-O bond breaking of polyolefins

[0101] Table 6 shows that different polyolefin raw materials can undergo continuous C-C bond cleavage in a microwave-driven ionic liquid system. Polypropylene segments are more likely to form branched carbocation intermediates, and the proportion of light olefins is relatively high under low-temperature conditions. This invention has good applicability to raw materials from different sources.

[0102] Comparative Example 1 This comparative example is basically the same as Example 1, except that no ionic liquid system is added in this comparative example, and only polyethylene is subjected to microwave treatment.

[0103] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 3.5 wt.% with a selectivity of 94.6%; the aromatic yield was 0.2 wt.% with a selectivity of 5.4%.

[0104] Comparative Example 2 This comparative example is basically the same as Example 1, except that the same ionic liquid is used in this comparative example but no metal chloride salt is added.

[0105] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The yield of olefins was 4.8 wt.% with a selectivity of 84.2%; the yield of aromatics was 0.9 wt.% with a selectivity of 15.8%.

[0106] Comparative Example 3 This comparative example is basically the same as Example 1, except that conventional oil bath heating is used instead of microwave heating in this comparative example.

[0107] The product characterization was performed using the same method. In this embodiment, the corresponding C8~C... 16 The olefin yield was 5.4 wt.% with a selectivity of 67.5%; the aromatic yield was 2.6 wt.% with a selectivity of 32.5%.

[0108] To illustrate the importance of microwaves, ionic liquids, and metal chlorides to the method of this invention, comparative examples 1 to 3 were set up based on Example 1, and the product distribution corresponding to their processes is shown in Table 7 below.

[0109] Table 7: Distribution of Comparative Example Products

[0110] Table 7 shows that in the absence of ionic liquids, polyolefins have difficulty effectively absorbing microwaves, resulting in insufficient C / C bond breaking; in the absence of metal chlorides, acidic activity is insufficient, leading to a decrease in the formation of light olefins; and when using conventional heating, energy input mainly relies on heat transfer through the container wall, making it difficult to form a microwave-ionic liquid interface activation effect. These results indicate a synergistic effect between the microwave field, ionic liquid, and Lewis acidic metal chlorides.

[0111] Table 8 presents the gas chromatographic distribution of the liquid product from Example 1.

[0112] Table 8: Gas Chromatography Product Distribution of Liquid Products in Example 1

[0113] As shown in the above examples and comparative results, the core of this invention lies in utilizing an ionic liquid system to concentrate microwave energy, acid activation, and polyolefin chain segment cleavage into the same interfacial region. After polyolefins enter the microwave reactor, polyethylene or polypropylene itself has a weak microwave response; without an ionic liquid phase, the system struggles to heat uniformly and achieve effective cleavage. With the addition of the ionic liquid, the cations, chloride ions, and chloride metal oxides in the ionic liquid migrate, undergo friction, and orientation relaxation under an alternating electromagnetic field. Microwave energy is rapidly converted into localized heat energy and interfacial activation, thereby significantly shortening the reaction start-up time.

[0114] In summary, this invention uses polyethylene, polypropylene, or a mixture of waste plastics as raw materials, and an ionic liquid system composed of chloride ion liquid and Lewis acidic metal chloride salts as the microwave absorption medium, reaction medium, and catalytic active system. The materials are fed at low temperature and placed in a microwave reactor. At a low temperature of 30-80 °C, the ionic liquid rapidly couples microwave energy through ion conduction loss and dielectric loss, forming a continuous C / C bond breaking reaction zone at the polyolefin / ionic liquid interface. Polyolefin segments undergo softening, activation, β-splitting, hydrogen transfer, and chain transfer within this interfacial reaction zone. The resulting product is mainly light olefins, accompanied by small amounts of aromatics and C8-C6 bonds. 16 Liquid fraction. By adjusting the type of ionic liquid, the composition of the metal chloride salt, the microwave power, the reaction temperature, and the reaction time, the depth of polyolefin pyrolysis, the selectivity of light olefins, and the distribution of liquid fraction can be controlled.

[0115] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization, characterized in that, Includes the following steps: (1) Process the polyolefin raw material into a form suitable for contact with ionic liquids; (2) A chloride-containing liquid or chloride-containing liquid precursor is mixed with a Lewis acid metal chloride salt under an inert gas atmosphere to form an ionic liquid system with microwave absorption capability and Lewis acidity; (3) Mix the polyolefin raw material with the ionic liquid system at a certain mass ratio, place it in a microwave reaction device, seal it under certain temperature conditions, and replace the air in the reaction device with an inert gas; (4) After feeding, microwave irradiation is applied, and the reaction temperature, microwave power and reaction time are controlled so that the ionic liquid absorbs microwave energy and forms a carbon-carbon bond continuous breaking reaction zone at the interface between polyolefin and ionic liquid, which promotes polyolefin chain segment activation, β-breakage, hydrogen transfer, isomerization and lightening. (5) After the reaction is completed, the mixture is cooled and gas-liquid separation and liquid-liquid phase separation are performed. The gas phase or low-boiling point fraction is collected to obtain light olefin products. The liquid phase is extracted and fractionated to obtain liquid olefin fraction. The ionic liquid phase is dehydrated, filtered and replenished with metal chloride salts and then recycled.

2. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: In step (1), the polyolefin raw materials are sorted, cleaned, dried, crushed or sheared to obtain polyolefin particles, flakes or pulverized materials suitable for contact with ionic liquids.

3. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: In step (2), the chloride-containing liquid or chloride-containing liquid precursor includes one or more of 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-hexyl-3-methylimidazolium chloride, 1-octyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, 4-butylaminochloride, N-ethylpyridine chloride, N-butylpyridine chloride, and N-benzylpyridine chloride.

4. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: In step (2), the Lewis acidic metal chloride salt includes one or more of anhydrous aluminum chloride, anhydrous ferric chloride, anhydrous zinc chloride, anhydrous tin tetrachloride, and anhydrous titanium tetrachloride.

5. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: In step (3), the mass ratio of the ionic liquid system to the polyolefin raw material is 0.5:1 to 20:

1.

6. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: In step (4), the initial feeding temperature is 15~35 ℃.

7. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: In step (4), the reaction temperature under microwave irradiation is 30~80 ℃.

8. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: In step (4), the microwave power is 100~1200 W.

9. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: In step (4), the reaction time is 1 to 8 hours.

10. The method for preparing olefins from polyolefin plastics by microwave-driven low-temperature depolymerization according to claim 1, characterized in that: The inert gas includes one or both of nitrogen and argon.

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