A Review on the Microwave-Assisted Pyrolysis of Waste Plastics

: The exhaustion of fossil energy and the severe pollution induced by using plastics has forced people to embark on the road to sustainable development. The high value of the recycling of plastics has become an important part of energy conservation. Microwave treatment, owning speciﬁc interactions between the electric ﬁeld and the molecules of treated materials, presents potential advantages in the application of plastic pyrolysis. Therefore, the research status of the microwave-assisted pyrolysis (MAP) of plastics to produce high-value-added liquid oil, gas, and solid carbon was reviewed in this paper. The effects of plastic properties, microwave treatment parameters, microwave absorbers, co-pyrolysis, catalysts, and reactor devices on the process and the products were analyzed. It is essential to optimize the experimental design by studying microwave-assisted co-pyrolysis technology and the application of catalysts, understanding the mechanism of co-pyrolysis to improve product selectivity. At the same time, the continuous MAP device for large-scale plastics treatment still needs to be developed. In addition, developing a large-scale simulation computing research platform for screening catalysts, optimizing processes, and commercial production is required to make the process more efﬁcient.


Introduction
Plastics, widely used worldwide, are the main products originating from petroleum.In 2020 alone, global plastic products reached 367 million tons [1].At the same time, the amount of plastic waste is also increasing.In 2021, the recycling of waste plastics in China was about 19 million tons, with a recovery rate of 31%, ranking first in the world.Some waste plastics that have not been recycled are burned or landfilled.The natural degradation of waste plastics is very slow [2].Plastics have been widely found in the ocean, especially in polar regions and trenches [3,4], and with the plastic particles becoming smaller, some microplastics have certain toxicity [5].These microplastics will eventually flow into humans through biological enrichment, which will affect our health [6].Therefore, plastic pollution has caused a very serious situation, increasing the recycling of plastics and realizing a sustainable recycling plastic economy are of great significance.
The chemical recovery methods for waste plastics include pyrolysis, gasification, and solvolysis [7,8].Among them, pyrolysis is considered an economical recovery method.This method can transform waste plastics from low-value to high-value chemical raw materials, which has attracted great attention.It is also one of the future exploration directions of the industrialization of plastic recycling technology; thus, plastic recycling has become the research focus of treatment technology [9].Compared with traditional incineration or landfill, the advantage of plastic pyrolysis is that all pyrolysis products can be recycled under the closed system of anoxia, which can effectively reduce the emission of toxic substances.For plastic pyrolysis, the experimental temperature is concentrated at 300-900 • C. The composition of raw materials determines the products, and their basic products include alkanes, olefins, aromatic hydrocarbons, hydrogen, carbon deposits, etc. [10][11][12][13].According to the recycled under the closed system of anoxia, which can effectively reduce the emission of toxic substances.For plastic pyrolysis, the experimental temperature is concentrated at 300-900 °C.The composition of raw materials determines the products, and their basic products include alkanes, olefins, aromatic hydrocarbons, hydrogen, carbon deposits, etc. [10][11][12][13].According to the heat source, the heating methods can be divided into conventional, microwave-assisted, solar energy, and plasma pyrolysis.
The energy of microwave-assisted pyrolysis (MAP) comes from microwave radiation, the heat transfer mode is gradually diffused from the internal heating of the substance to the external, and the heat source and raw materials are heated without contact.By contrast, conventional pyrolysis uses an electric heater or a burner, and the plastic is heated from outside to reach the desired temperature through conduction.This method has uneven temperature distribution, serious heating inertia, and high energy consumption.The disadvantage of solar pyrolysis is that it is greatly affected by the weather.Plasma pyrolysis [14] gasifies and decomposes plastics in a short time by ionizing gas to generate a high temperature as the heat source.This method has a high energy density and an extremely high working temperature, which greatly limits its popularization and application.These four types of pyrolysis schematic diagrams are shown in Figure 1.In the traditional plastic pyrolysis, due to uneven heating, the excessive local temperature will lead to excessive pyrolysis of local plastics to produce non-condensable small molecules and coking, while an insufficient internal pyrolysis temperature will lead to more long-chain hydrocarbons, resulting in lower recovery values.The MAP technology can rapidly heat the inside of materials and improve the heating efficiency.The products of MAP of plastics tend to be medium and short chain hydrocarbons, providing maximum possibility for the replacement of transport fuels with bio-oil/methanol blends in a CRDI engine.Microwave heating has the advantages of environmental protection, safety, controllability, rapidity, and strong operability in the application of plastic pyrolysis.Therefore, MAP is one of the bright ways for the green recycling of plastic and sustainable development.All of these should definitely attract the researcher's attention to use MAP as an important prospect.
Based on the systematic study of the MAP of plastics, this review provides a new perspective for improving the efficient recycling technology of waste plastics.General properties, usages, and main products were described; the theory of MAP technology was explained, and the effects of technological conditions and catalysts on the MAP of plastics were reviewed in this paper.The application progress of computational simulation was also summarized.In addition, the development orientation was predicted.

Normal Properties of Plastics
Plastics are long-chain macromolecular compounds polymerized from monomers.Common monomers are ethylene, propylene, terephthalic acid, ethylene glycol, vinyl chloride, styrene, etc. Plastic products can be seen everywhere in life.Grocery bags, food packaging bags, beverage bottles, shells of electronic equipment, basins, toys, medical In the traditional plastic pyrolysis, due to uneven heating, the excessive local temperature will lead to excessive pyrolysis of local plastics to produce non-condensable small molecules and coking, while an insufficient internal pyrolysis temperature will lead to more long-chain hydrocarbons, resulting in lower recovery values.The MAP technology can rapidly heat the inside of materials and improve the heating efficiency.The products of MAP of plastics tend to be medium and short chain hydrocarbons, providing maximum possibility for the replacement of transport fuels with bio-oil/methanol blends in a CRDI engine.Microwave heating has the advantages of environmental protection, safety, controllability, rapidity, and strong operability in the application of plastic pyrolysis.Therefore, MAP is one of the bright ways for the green recycling of plastic and sustainable development.All of these should definitely attract the researcher's attention to use MAP as an important prospect.
Based on the systematic study of the MAP of plastics, this review provides a new perspective for improving the efficient recycling technology of waste plastics.General properties, usages, and main products were described; the theory of MAP technology was explained, and the effects of technological conditions and catalysts on the MAP of plastics were reviewed in this paper.The application progress of computational simulation was also summarized.In addition, the development orientation was predicted.

Normal Properties of Plastics
Plastics are long-chain macromolecular compounds polymerized from monomers.Common monomers are ethylene, propylene, terephthalic acid, ethylene glycol, vinyl chloride, styrene, etc. Plastic products can be seen everywhere in life.Grocery bags, food packaging bags, beverage bottles, shells of electronic equipment, basins, toys, medical protective articles, and so on, are all over our living environment.The American Plastics Industry Association divides common plastic products into seven types, as shown in Table 1.

MAP Technology
Microwave heating is induced through the interactions between microwave electromagnetic fields and the molecules of treated materials.When microwave acts on raw materials, due to dipole rotation or ion conduction, the moving direction of polar molecules or ions changes with the alternating electromagnetic field.Heat is thereby generated by high-frequency friction induced by fast dipole reorientation.Thus, electromagnetic energy is converted into heat energy via the molecules located in the field.During microwave heating, the microwave absorber evenly distributed in the raw material receives microwaves and forms hot spots in the raw material after receiving microwaves, thus realizing the rapid heating of raw materials, improving the heat transfer efficiency and heating rate, and having a high energy utilization rate.Therefore, microwave heating has great application potential in plastic chemical-recycling engineering.
Substances that can convert microwave radiation energy into heat energy are usually polar molecules or ionic compounds, and their ability to absorb and convert microwave radiation energy depends on the dielectric properties of the material.The worse the dielectric properties, the smaller the interaction with microwaves and the lower the thermal conversion efficiency.The evaluation of dielectric properties of materials consists of two parts: the dielectric constant (ε ) and the dielectric loss factor (ε ).The former measures the degree of microwave radiation energy absorbed by materials, and the latter quantifies the ability to convert radiation energy into heat.The ratio of dielectric loss factor to dielectric constant is called dielectric loss tangent (tanδ = ε /ε ), which measures the overall ability of materials to utilize microwaves.Therefore, according to the value of tanδ, the microwave utilization ability of materials can be divided into three parts: low (tanδ < 0.1), medium (0.1 < tanδ < 0.5), and high (tanδ > 0.5).The dielectric properties of materials change greatly with temperature.The research of Salema et al. [15] shows that the dielectric constant of rice husks and other biomass changes slightly with a temperature between 24 and 450 • C, and increases sharply after exceeding 450 • C. Table 2 shows the dielectric properties of various plastics and common absorbers.It can be seen that for plastics, the tangent value of the dielectric loss angle is small, resulting in a poor ability to convert microwaves into heat.Therefore, mixing materials with large tanδ values as microwave absorbers is necessary to improve the absorption rate and achieve a rapid temperature rise.Moreover, it needs to be noted that metal oxides do not show uniform dielectric properties.For example, Fe 3 O 4 has high dielectric properties, while Al 2 O 3 is a transparent material at low temperatures.When the temperature is greater than 900 • C, alumina will become a high-loss material [16].Polyethylene (PE) 0.001-0.002[18] Polypropylene (PP) 0.003-0.004[18] Polyvinyl chloride (PVC) 0.0056 [16] Polypropylene (PS) 0.0002-0.0003[19] Polycarbonate (PC) 0.01 [20] Natural rubber 0.002-0.005[18] Carborundum (SiC) 0.25-0.37[16] Activated carbon (AC) 0.31-0.9[16] Fe 3 O 4 0.199 [16] Al 2 O 3 0.001 [16] Wood 0.11 [18] 3. Influencing Factors, Pyrolysis Residue, and Energy Consumption of MAP of Plastics

Plastic Types
As shown in Table 1, there are obvious differences in the pyrolysis products of different plastics.In the review of Abbas-Abadi et al. [21], the pyrolysis properties of polyolefin plastics were summarized.It is not difficult to find that their structures and densities are different due to the different types of synthetic monomers and additives.Therefore, when studying the pyrolysis process, the influence of plastic types should be considered.The products from HDPE and LDPE tend to generate wax rather than oil because of fewer branched chains.The primary cracking of free radicals is inhibited during pyrolysis.Therefore, in the experiment of the microwave pyrolysis of HDPE by Undri et al. [22], the main products of pyrolysis were short-chain hydrocarbons (C1-C4) in the gas phase, long-chain waxy hydrocarbons (>C20) in the solid phase, and a less liquid phase.However, there are many methyl branches in PP, which is easy to trigger the free radical cracking mechanism of β-elimination in the pyrolysis process.Therefore, the main product of PP pyrolysis is a low viscosity liquid mainly composed of methyl-branched hydrocarbons.In the pyrolysis process of PS, because it contains thermally unstable vinyl functional groups, it is beneficial to produce styrene monomer through the mechanism of end-chain breakage; thus, the pyrolysis of PS can often obtain higher pyrolysis oil [1].Meanwhile, PS's main products should be aromatics and low-molecular-weight olefins [23].Moreover, the pyrolysis products of PET and PVC are corrosive to the reactor, making the relevant research limited [24].In addition, researchers [25] have also studied the co-pyrolysis of waste polystyrene (PSW) and waste polypropylene (PPW), and found that the properties of the oil achieved were similar to those of gasoline, with a density and viscosity of 0.76 g/mL and 2.4 cSt, respectively.The experiments show that the microwave pyrolysis of mixed plastics presents great energy-recovery potential.

Microwave Power
Microwave power directly determines the heating rate of the sample and is one of the key parameters in the pyrolysis process.It is obvious that the higher the microwave power, the greater the heating rate, which was mainly due to the increase in energy density, beneficial to the thermal effect.In studying the influence of microwave power and the number of carbon absorbers on pyrolytic polypropylene, Suriapparao et al. [26] found that the experimental results follow these rules: when the microwave power is fixed, the heating rate decreases with the increasing amount of carbon absorbers, and the time required for heating to the target temperature is longer; when the number of carbon absorbers is fixed, the heating rate increases with the increase in the power, and the time required for heating to the target temperature is shorter.In addition, the composition analysis of pyrolysis oil shows that different microwave power has different selectivities for product formation.The selectivity of cycloalkane is the highest under a microwave power of 450 W, and the highest yield of liquid oil is 63.4%.Jing et al. [27] also found that the yield of a target product can be adjusted by microwave power.Jing et al. [28] used a commercial spherical activated carbon (SAC) with metal cations (Mg, Ca, Na, K, Ba, Sr) for the pyrolysis of polypropylene to obtain value-added products.The effect of input power on pyrolysis products was studied.It was found that the combination of low power and high power could improve the yield of light oil during pyrolysis.At the same time, the liquid yield was increased to more than 70%.Therefore, the design of the power employed in pyrolysis process is also a good strategy to improve liquid products.

Microwave Absorbers
Plastics have a low dielectric constant and weak microwave absorption capacity, being transparent to microwaves.Without a microwave absorber, these types of plastics may not reach 200 • C. Therefore, microwave absorbers are normally required to improve the system temperature.Different microwave absorbers have various properties, which may greatly influence the process of MAP of plastics.Microwave absorbers include carbonbased materials, such as activated carbon, graphite, SiC, metals and their derivatives, aluminosilicate molecular sieves, and so on [29][30][31][32][33].The addition of the microwave absorber can reduce energy loss and greatly improve heat transfer efficiency.The intrinsic structures and valence bands determine the heating rates of the reacting system, resulting in different pyrolysis degrees and product distributions.Researchers also compared the efficiency of various absorbers, which not only act as microwave absorbers but assist pyrolysis reactions.
In the research and application of biochar made from biomass as microwave absorbers, the distribution of pyrolysis products of plastics under the microwave is affected by the biomass's different pore-size structures of biochar.Rex et al. [25] used rice husk carbon (RHC), corn husk carbon (CHC), and coconut sheath carbon (CSC) as microwave absorbents to study the pyrolysis of PS.The results show that the group have the highest oil yield of 86.1 wt.% achieved by using CSC due to its good porous structure, which increases the contact area with PS.Furthermore, the pyrolysis of PS was promoted.Zou et al. [34] found that the application of biochar absorbers could effectively improve the quality of oil and gas products from LDPE pyrolysis, which was shown to be related to the porous structure of biochar and the inorganic elements in it.Suriapparao et al. [35] compared the individual effects of lignin and graphite as microwave absorbers on PP pyrolysis.They find that the heating rate is similar for the graphite and lignin groups, while the oil yield is higher under the graphite group.This indicates that graphite not only converts microwave into heat energy but also has a catalytic effect.In addition, the analysis of the pyrolysis products reveals that graphite promotes the formation of olefins, while lignin has good selectivity for cycloalkanes.
Silicon carbide has good microwave absorbent properties and is often used as a microwave absorber in the MAP of plastics.Reddy et al. [32] reported the effect of SiC particle size on the microwave pyrolysis rate of plastics.The heating rate increased significantly as the particle size increased from 0.12 mm to 3 mm.This is because the dielectric loss factor of SiC particles less than 0.25 mm will be reduced by an order of magnitude.Liu et al. [36] investigated the effect of the amount of silicon carbide (20, 25, 30, 35, 40 g) on PET (fixed amount of 30 g) pyrolysis.The pyrolysis results revealed that PET pyrolysis did not produce oil.The yield of solid phase products gradually increased from 4.33 wt.% to a maximum of 25.33 wt.% when increasing the silicon carbide loading up to 35 g.The reason is that the increase in SiC content makes the pyrolysis temperature distribution inside the sample more uniform, which is favorable to the occurrence of pyrolysis.However, the yield of each product fluctuated slightly when the SiC was 40 g, indicating that too much SiC absorber would not further affect the product yields.Therefore, when using SiC as a microwave absorber, care should be taken to select the appropriate particle size and dosage.
In a study by Bartoli et al. [37], the effects of carbon and SiC as microwave absorbers on the microwave pyrolysis of PS were separately compared.The results showed that the yield rates of both were similar, but the pyrolysis rate of polymers with carbon as microwave absorbers was much higher than that of SiC.Zhao et al. [38] compared the effects of carbon powder, Fe 3 O 4 , and CaO as microwave absorbers on the yield of PET and wood chip microwave co-pyrolysis products, respectively.The results showed that the carbon powder had high thermal conversion efficiency, formed local hot spots, intensified the pyrolysis process, and produced more benzoic acid (BA) and its derivatives in the liquid product.The CaO group reduced the yield of BA due to the strong basicity of CaO, which promoted the condensation of phenyl and benzoyl to form CO 2 and H 2 O.In addition, the neutralization reaction between acid and CaO catalyzed the cleavage to ketones and hydrocarbons.Fe 3 O 4 had the ability to promote decarboxylation and C-C bond breaking, which reduced the content of BA and its derivatives and phenols, and promoted the formation of benzene and its derivatives and esters.
In addition, in the study of metal as microwave absorbers, Hussain [39] reported good results with barbed wire as a microwave absorber.After microwave heating for 12-15 min, the temperature can reach 1200 • C, PS was converted to 80% liquid and 15% gas, where the main component of the liquid was aromatic hydrocarbons.This is due to the coupling effect of barbed wire and microwave, and the larger surface facilitates microwave reception and thermal radiation, which promotes the pyrolysis of PS.Therefore, these reports indicate that different pyrolysis results are obtained by selecting different absorbing materials as microwave absorbers.The distribution of pyrolysis products can be modulated or influenced by the choice of microwave absorbers.

Biomass Co-Pyrolysis
As a renewable resource, biomass is an important node of the carbon cycle and the only source of renewable carbon [40][41][42].Biomass consists of cellulose, hemicellulose, and lignin with C, H, O, N, and S as the main elements.Pyrolysis to make oil is one of the efficient utilizations of biomass.The application and development of biomass pyrolysis products containing oxygen are limited to some extent due to their high acidity and low calorific value [43][44][45][46].In recent years, people have had strong interests in the co-pyrolysis of plastics and biomass.Many reports have demonstrated that there is a synergistic effect in the co-pyrolysis process of plastics and biomass, which increases the yield of pyrolysis products and improves the quality of products [47][48][49].
Sridevi et al. [50] studied the synergistic effect of co-pyrolysis of rice hull (RH) and polystyrene (PS) and found that different proportions of PS and RH had different synergistic effects.Oxyfuran in RH pyrolysis can react with hydrocarbons with a high carbon-hydrogen ratio produced by PS pyrolysis to generate a Diels-Alder reaction, which improves the yield of hydrocarbons, thus obtaining a higher liquid yield.The results showed that the higher the content of rice husk, the higher the liquid yield and the more obvious the positive synergism in the co-pyrolysis process.Similarly, Zhang et al. [43] further conducted microwave-assisted co-pyrolysis of pepper straw (CS) and polypropylene (PP).The study also proved an obvious synergistic effect.When the ratio is 1:1, the oxygen-containing compounds in pyrolysis oil are reduced by 76.69% compared with CS.Compared with direct co-pyrolysis, the oxygen content in the oil products of CS and PP co-pyrolysis pretreated by microwave decreased by 4.32%.Microwave pretreatment damaged the lignin structure, cracked the CS particles, increased the contact area between PP and CS, and promoted the interaction between CS and PP during pyrolysis.
Beneš et al. [51] used a rather novel method to depolymerize the glyceride in coconut oil to produce polyols.Firstly, coconut oil was subject to transesterification with glycerol to obtain a glycerol monoester with hydroxyl terminal group.Then, the co-pyrolysis of polycarbonate and glycerol was performed at 200-220 • C under microwave-assisted heating, and the polycarbonate was completely transformed into polyol.The reason is that excessive hydroxyl groups promote the fracture of carbonate bonds, so that PC is converted to bisphenol A(BPA) and aromatic carbonate polyols, as shown in Figure 2.They also found that BPA will decompose into phenol and iso-allylphenol above 220 • C, which will affect the purity of the product polyols.
Processes 2023, 11, x FOR PEER REVIEW 8 of 23 heating, and the polycarbonate was completely transformed into polyol.The reason is that excessive hydroxyl groups promote the fracture of carbonate bonds, so that PC is converted to bisphenol A(BPA) and aromatic carbonate polyols, as shown in Figure 2.They also found that BPA will decompose into phenol and iso-allylphenol above 220 °C, which will affect the purity of the product polyols.In the study of Suriapparao Group [52], the synergistic effect of co-pyrolysis of algae (FA) with PP, PE, and EPS was investigated.In the co-pyrolysis combination of FA+PP and FA+PE, the pyrolytic volatile oxygen compounds of PA are easy to react with the volatile hydrocarbons of polymer pyrolysis, resulting in an increased gas yield in the product, showing a positive synergistic effect of the gas.However, the oil in FA+EPS has a higher yield, and the content of single aromatics is about 70%.This is due to the higher activation energy of the ring-opening reaction of aromatic compounds, which provides hydrogenfree radical deoxidation for oxygen-containing organic matter in FA pyrolysis through hydrogen transfer, thus increasing the oil yield of the product and showing positive oil production synergy, which is consistent with the research work of Mahari et al. [53].It is also found that these synergies make a lower energy demand of co-pyrolysis than that of single pyrolysis.
Zhao et al. [54] studied the difference in bamboo/polypropylene co-pyrolysis products in different proportions with microwave assistance.After testing, when the catalytic temperature is 250 °C, and the bamboo/PP ratio is 1:2, the oil yield is 61.62 wt.%.The contents of aliphatic hydrocarbons and aromatic hydrocarbons in bamboo pyrolysis products are extremely low, but the hydrocarbon content in the products is significantly increased, because PP can be used as a hydrogen donor to provide hydrogen for the dehydration and dehydrogenation of bamboo pyrolysis steam on the catalyst, reducing the formation of coke; thus, forming a synergistic effect with bamboo pyrolysis.In the study of Suriapparao Group [52], the synergistic effect of co-pyrolysis of algae (FA) with PP, PE, and EPS was investigated.In the co-pyrolysis combination of FA+PP and FA+PE, the pyrolytic volatile oxygen compounds of PA are easy to react with the volatile hydrocarbons of polymer pyrolysis, resulting in an increased gas yield in the product, showing a positive synergistic effect of the gas.However, the oil in FA+EPS has a higher yield, and the content of single aromatics is about 70%.This is due to the higher activation energy of the ring-opening reaction of aromatic compounds, which provides hydrogen-free radical deoxidation for oxygen-containing organic matter in FA pyrolysis through hydrogen transfer, thus increasing the oil yield of the product and showing positive oil production synergy, which is consistent with the research work of Mahari et al. [53].It is also found that these synergies make a lower energy demand of co-pyrolysis than that of single pyrolysis.
Zhao et al. [54] studied the difference in bamboo/polypropylene co-pyrolysis products in different proportions with microwave assistance.After testing, when the catalytic temperature is 250 • C, and the bamboo/PP ratio is 1:2, the oil yield is 61.62 wt.%.The contents of aliphatic hydrocarbons and aromatic hydrocarbons in bamboo pyrolysis products are extremely low, but the hydrocarbon content in the products is significantly increased, because PP can be used as a hydrogen donor to provide hydrogen for the dehydration and dehydrogenation of bamboo pyrolysis steam on the catalyst, reducing the formation of coke; thus, forming a synergistic effect with bamboo pyrolysis.
It is clear from the aforementioned information that the products are highly dependent on the types of the pyrolyzed materials and the operating conditions; in the same way as the pyrolysis of plastics, the application of MAP in waste tires also presents similar characters.Due to the unique compositions, the solid yield from the pyrolysis of waste tires was found to be high with high HHV carbon black [55], which can act as a good microwave absorber.The obvious synergistic effect of the co-pyrolysis of waste tires and plastics would undoubtedly benefit in improving the products' values, giving this technique a high development and utilization value.
In the case of co-pyrolysis of biomass with PE, PP, PS, etc., the different compositions and content of the organic matter in biomass have different effects on the distribution of co-pyrolysis products with plastics.In the process of co-pyrolysis, the behavior that biomass promotes the pyrolysis of plastics lies in the Diels-Alder reaction between the oxygen-containing free radicals generated by biomass pyrolysis and the hydrogen-rich hydrocarbons generated by polymer pyrolysis, which reduces the oxides in the pyrolysis oil and increases the hydrocarbon content through dehydration.In biomass co-pyrolysis, the pyrolysis products of PE or PP are mainly aliphatic hydrocarbons, partial cyclic aliphatic hydrocarbons, and polycyclic aromatic hydrocarbons, among which aliphatic hydrocarbons are mainly olefin.The monomer of PS is styrene, so the product oil in the process of pyrolysis is mainly aromatic hydrocarbons containing benzene rings.Table 3 provides the co-pyrolysis results of plastics and other substances under the MAP process and fixed bed process.It can be found that the HHV of pyrolysis oil under the MAP process is high and the residue rate is relatively low.

Catalyst
In the process of MAP of plastics, the addition of a catalyst can improve the selectivity of pyrolysis products and increase the output of certain products.Common catalysts used in the plastic catalytic pyrolysis can be divided into molecular sieve catalysts, metal compounds, etc. [70][71][72][73][74].
In the process of catalytic cracking of plastic macromolecules with catalysts, the cracking of plastics mainly includes thermal transformation and catalytic transformation.The direct pyrolysis process of plastic macromolecules generally follows the free radical mechanism, including initiation, pyrolysis propagation, and radical coupling [75].In the presence of a catalyst, the catalyst can reduce the initial cracking temperature of plastics and participate in the process of free radical coupling, thus improving the selectivity of products.In addition, the catalyst can generate more active sites under the action of the microwave [76,77], which increases the contact between raw materials and the catalyst, thus improving the pyrolysis rate of plastics.

Molecular Sieve Catalyst
Molecular sieve catalysts are a type of solid acid catalyst, and their internal pore structure can reasonably be used to improve the mass transfer process in the reaction and improve the efficiency of the catalytic reaction and product selectivity, which has become a research hotspot.The molecular sieve catalyst reported for pyrolysis of biomass includes SAPO-34, ZSM-5, HY, Hβ, and MCM-41, etc.
Zeng et al. [78] compared SAPO-34, HZSM-5, HY, and Hβ molecular sieves to study the in-situ catalytic co-pyrolysis of HDPE and waste edible oil (WCO), respectively.They found that pore size is the key factor for production composition.If the pore size is small, the pyrolysis macromolecules cannot enter the zeolite to further react with the internal active sites to form small molecular products.Therefore, plastic macromolecules can only be simply broken at the active sites over the surface to generate liquid products with large carbon numbers.Meanwhile, with a larger pore size, the macromolecular carbon chain enters the molecular sieve and can be broken over the highly acidic sites inside to form more gases.Interestingly, among the four catalysts, the HZSM-5 catalyst has the highest content of BTX in the products obtained, while SAPO-34 has the lowest content.This is related to the fact that the pore size of the SAPO-34 molecular sieve is much smaller than the kinetic diameter of light aromatic hydrocarbons, and the small pore size of the molecular sieve cannot induce the formation of BTX.Therefore, reasonable design of molecular sieve characteristic parameters can change the selectivity of plastic pyrolysis products.
Ding et al. [79] studied the influence of NiO as an in-situ catalytic pyrolysis catalyst and a HY molecular sieve as an in-situ catalyst on microwave pyrolysis of LDPE.NiO can promote the dehydrogenation and fracture of long-chain hydrocarbon molecules and reduce coke deposition.It may be that NiO produces large amounts of olefin through hydrogen extraction.Then, cycloolefin and aromatics were synthesized by Diels-Alder reaction catalyzed by HY.Therefore, the synergistic catalysis of different catalysts is a favorable way to improve the quality of pyrolysis oil.
ZSM-5 was also investigated by several researchers and it was found that the ZSM-5 catalyst can significantly improve the microwave pyrolysis of plastics as well as co-pyrolysis with a type of biomass [80,81].The combination of different molecular sieves presents synergistic effects, e.g., the strong cracking properties of the outer layer MCM-41 split the polymer macromolecules into smaller molecules, while ZSM-5 further catalyzes the isomerization of small molecules into hydrocarbons [29].Compared with ZSM-5, ZSM-5/MCM-41 can further reduce the coke formation rate to prevent pore blocking, extend catalyst life, and form more medium hydrocarbon quantities.
Further studies demonstrate that morphology is also important regarding pyrolysis.In the microwave pyrolysis LDPE studied by Chen et al. [82], core-shell SiC foam@ZSM-5 catalyst with large-medium-microporous distribution was prepared by a slurry coating method, and the influence of ZSM-5 treated with different concentrations of alkaline treated (AT) on the pyrolysis of plastics was studied, Figure 3 shows the core-shell SiC foam@ZSM-5 catalyst model and the reaction diagram.Compared with the parent zeolite, more mesoporous structures are produced after AT, and the acidity is reduced.A more mesoporous structure improves the diffusion ability of molecules in zeolite and reduces the formation of heavy aromatics.The decrease in acidity inhibits olefin's hydrogen transfer reaction and promotes olefin's formation.The total selectivity of ZSM-5(50)-0.50ATcatalyst for light olefin and aromatics during the pyrolysis of LDPE is 58.6~64.9%,which is higher than that of the parent ZSM-5 molecular sieve (53.6%).
Processes 2023, 11, x FOR PEER REVIEW 11 of 23 transfer reaction and promotes olefin's formation.The total selectivity of ZSM-5(50)-0.50ATcatalyst for light olefin and aromatics during the pyrolysis of LDPE is 58.6~64.9%,which is higher than that of the parent ZSM-5 molecular sieve (53.6%).

Metal-Based Catalyst
In MAP, the choice of catalysts greatly influences the pyrolysis products of plastics.At present, the metal-related catalysts used for microwave pyrolysis of plastics include monometallic (Mg, Al, Fe, Co, Ni, Ce, etc.), bimetallic [83,84], and strong base catalysts (KOH).Because iron promotes the breaking of the C-H bond [85], many reports about ironbased catalysts exist.In the MAP of PP, PE, and PS with iron-based catalysts, the main products are hydrogen and carbon nanotubes, while Ni-Al and Co-Al bimetallic catalysts can only catalyze their pyrolysis into liquids and solids, with a small proportion of gases [72].KOH can react chemically with activated carbon to promote activated carbon to form more pore networks [86,87], and can also react with polymers to catalytically crack heavier fractions and promote the formation of light hydrocarbon molecules [23].MgO was found efficient in decomposing long-chain volatiles and promoting converting the Diels-Alder reaction between light olefins to single aromatics [88].
Bimetallic catalysts have been attracting researchers' attention, possessing higher development potential due to the synergistic effect.One can use carbon nanotubes to provide the reaction surface and pore structure, while the other active component can act as inhibiting coke formations [89]; moreover, the synergistic effect can also attribute to the electron transfer between both active components [90,91].For example, iron-cobalt was used to investigate their synergistic effect.It found that cobalt reduces the crystallization of iron and increases the specific surface area and porosity of the catalyst.In addition, there may be a synergistic effect of electron transfer between iron and cobalt, further promoting polymers' cracking.

Metal-Based Catalyst
In MAP, the choice of catalysts greatly influences the pyrolysis products of plastics.At present, the metal-related catalysts used for microwave pyrolysis of plastics include monometallic (Mg, Al, Fe, Co, Ni, Ce, etc.), bimetallic [83,84], and strong base catalysts (KOH).Because iron promotes the breaking of the C-H bond [85], many reports about iron-based catalysts exist.In the MAP of PP, PE, and PS with iron-based catalysts, the main products are hydrogen and carbon nanotubes, while Ni-Al and Co-Al bimetallic catalysts can only catalyze their pyrolysis into liquids and solids, with a small proportion of gases [72].KOH can react chemically with activated carbon to promote activated carbon to form more pore networks [86,87], and can also react with polymers to catalytically crack heavier fractions and promote the formation of light hydrocarbon molecules [23].MgO was found efficient in decomposing long-chain volatiles and promoting converting the Diels-Alder reaction between light olefins to single aromatics [88].
Bimetallic catalysts have been attracting researchers' attention, possessing higher development potential due to the synergistic effect.One can use carbon nanotubes to provide the reaction surface and pore structure, while the other active component can act as inhibiting coke formations [89]; moreover, the synergistic effect can also attribute to the electron transfer between both active components [90,91].For example, iron-cobalt was used to investigate their synergistic effect.It found that cobalt reduces the crystallization of iron and increases the specific surface area and porosity of the catalyst.In addition, there may be a synergistic effect of electron transfer between iron and cobalt, further promoting polymers' cracking.
Wang et al. [92] prepared a heterogeneous Fe/Ni-CeO 2 @CNTs bimetallic catalyst by loading Fe/Ni on CeO 2 @CNTs as the substrate, which was used in the microwave pyrolysis of LDPE, and 91.5 vol% high-purity hydrogens were obtained with a hydrogen yield of 50.2 mmol/g plastic .The addition of nickel and CeO 2 carrier improved the performance of the catalyst.This is attributed to the promotion of H 2 generation by plasma effect discharge of Fe/Ni alloy nanoparticles under the microwave, as shown in Figure 4.The oxygen vacancy of CeO 2 reacts with carbon deposition to generate CO 2 and then reacts with methane to generate CO and H 2 , which not only reduces methane production but also increases the hydrogen concentration.The consumption of carbon deposits also alleviates the carbonization of iron and nickel.Therefore, it can be concluded that the high-quality recycling of waste plastics can be realized by adding additives to Fe-based catalysts.Cao et al. [93] reported that the MAX Ti3AlC2 catalyst was used in the process of hydrogen production by microwave pyrolysis of polyolefin, which had high hydrogen production selectivity.Ti3AlC2 not only has catalytic performance but also is a microwavesensitive material.Therefore, in the temperature effect experiment, the selectivity of hydrogen increased from 11.0 vol% at 500 °C to 74.9 vol% at 1000 °C.At the same time, highly graphitized carbon fibers which can be used as negative electrode materials of sodiumion batteries were obtained.The reason why the Ti3AlC2 catalyst has high hydrogen selectivity is that Ti3AlC2 decomposes into TiC and TiAl in the catalytic process, and there is a synergistic effect between them and CNF.
Terapalli et al. [23] used KOH as a catalyst for the microwave pyrolysis of polystyrene, and studied the effects of KOH as an addition on PS pyrolysis products.It is found that the addition of KOH changes the reaction mechanism and promotes the formation of olefins, alkanes, and cycloalkanes, which is related to the fact that the carbonyl group of KOH can promote molecular dehydrogenation.When KOH was used in co-pyrolysis, Sridevi et al. [50] found that the content of aromatic hydrocarbons in co-pyrolysis products increased, while the content of oxygenates containing carbonyl and acid decreased, indicating that KOH promoted molecular dehydration and decarbonylation and improved the quality of bio-oil.

Pyrolysis Temperature
Appropriate pyrolysis temperature can improve the selectivity of the target product.When the pyrolysis temperature is low, the plastic cannot be completely pyrolyzed or generate wax with a large carbon number.When the pyrolysis temperature is too high, the excessive pyrolysis of the plastic will generate more non-condensable small molecules.Therefore, in order to obtain the target product, researchers need to reveal the operating temperatures.
From the study by Fan et al. [94], that the above 460 °C for the pyrolysis of PS, secondary pyrolysis of volatiles into low molecular weight gaseous hydrocarbons would be enhanced.For PET, a report from Liu et al. [36] shows that PET cannot be fully pyrolyzed Cao et al. [93] reported that the MAX Ti 3 AlC 2 catalyst was used in the process of hydrogen production by microwave pyrolysis of polyolefin, which had high hydrogen production selectivity.Ti 3 AlC 2 not only has catalytic performance but also is a microwavesensitive material.Therefore, in the temperature effect experiment, the selectivity of hydrogen increased from 11.0 vol% at 500 • C to 74.9 vol% at 1000 • C. At the same time, highly graphitized carbon fibers which can be used as negative electrode materials of sodium-ion batteries were obtained.The reason why the Ti 3 AlC 2 catalyst has high hydrogen selectivity is that Ti 3 AlC 2 decomposes into TiC and TiAl in the catalytic process, and there is a synergistic effect between them and CNF.
Terapalli et al. [23] used KOH as a catalyst for the microwave pyrolysis of polystyrene, and studied the effects of KOH as an addition on PS pyrolysis products.It is found that the addition of KOH changes the reaction mechanism and promotes the formation of olefins, alkanes, and cycloalkanes, which is related to the fact that the carbonyl group of KOH can promote molecular dehydrogenation.When KOH was used in co-pyrolysis, Sridevi et al. [50] found that the content of aromatic hydrocarbons in co-pyrolysis products increased, while the content of oxygenates containing carbonyl and acid decreased, indicating that KOH promoted molecular dehydration and decarbonylation and improved the quality of bio-oil.

Pyrolysis Temperature
Appropriate pyrolysis temperature can improve the selectivity of the target product.When the pyrolysis temperature is low, the plastic cannot be completely pyrolyzed or generate wax with a large carbon number.When the pyrolysis temperature is too high, the excessive pyrolysis of the plastic will generate more non-condensable small molecules.Therefore, in order to obtain the target product, researchers need to reveal the operating temperatures.
From the study by Fan et al. [94], that the above 460 • C for the pyrolysis of PS, secondary pyrolysis of volatiles into low molecular weight gaseous hydrocarbons would be enhanced.For PET, a report from Liu et al. [36] shows that PET cannot be fully pyrolyzed below 550 • C. According to Zhang et al. [43], the pyrolysis range of PP was found to be 425-510 • C. Influenced by density, the pyrolysis range of polyethylene is wide, ranging from 300 • C to 520 • C [95,96].The optimum pyrolysis temperature range of PVC is 250-350 • C [97].In a word, the control of plastic pyrolysis temperature is influenced by the material type, and the best pyrolysis oil can be obtained by choosing the best pyrolysis temperature range.

The Device Used for MAP
The product distribution of the MAP of plastics is also affected by the design of experimental equipment.The products required by the experimental device generally include microwave ovens, reaction vessels, temperature detectors, gas condensers, liquid collection bottles, gas collection bags, and insulation materials.The successful design of the experimental device depends on the matching of each component and the tightness of the whole system.In the reported literature, most experimental devices for MAP of plastics use batch reactors [98], and there are continuous reactors [99] in the expansion devices.The biggest feature of an intermittent pyrolysis device is that the sample is added at one time, and a new sample can only be replaced after pyrolysis.The continuous reaction device can continuously feed and make the system work.Figure 5 shows the schematic diagram of four typical MAP of plastics.
Small MAP devices in the laboratory can be divided into two types: in-situ catalytic pyrolysis [49] and ex-situ catalytic pyrolysis [100].In the in-situ catalytic pyrolysis device, the reaction container is placed in the center of the microwave reactor, and the reaction container is connected to a steam condensation device, a temperature sensor, and a gas purge inlet.Terapalli et al. [23] used an in-situ catalytic device and a borosilicate flask as the reaction vessel.In the in-situ catalytic experiments, the first microwave oven is often used for the direct pyrolysis of samples, and the second microwave oven is used as the heat source for the catalytic reforming of volatiles.This way is more conducive to the regeneration of the catalyst and the separation from the reactants, and to some extent slows down the deactivation of the catalyst by carbon deposition.The ex-situ catalytic pyrolysis device designed by Suriapparao et al. [66] is an example.
Liang et al. [100] added a continuous stirring device in the microwave pyrolysis reactor.Compared with the pyrolysis experiment without stirring, it was found that the long carbon chain of C14-C20 had higher selectivity under continuous stirring, while the experiment without stirring produced more methane gas because the rotation increased the temperature uniformity of the system and prevented the long-chain molecules from overheating and cracking into non-condensable small molecules due to hot spot effect.In addition, in the experimental laboratory device, supplying energy to the pyrolysis system without interruption is called continuous heating pyrolysis, and alternately supplying energy through power supply and power failure at a fixed time interval is called intermittent heating pyrolysis.Jing et al. [101] observed that reasonable control of the size of the container and the amount of absorber is helpful to the formation of wax in continuous heating mode, while intermittent heating can obtain more liquid products.
In the pilot systems, Zhang et al. [102] developed a set of continuous microwave radiation dual-mode spiral crackers.They designed the reaction vessel to be cylindrical and horizontal and pushed the feed through the screw rod, which realized the high recovery of organic matter in waste-printed circuit boards (WPCB) (88.03-92.79%).Zhou et al. [99] developed a continuous downdraft microwave-assisted pyrolysis system (CMAP).The reaction vessel is designed to be cylindrical and vertical, with a stirring rod in the middle and raw materials supplied by the upper airtight hopper.The device realizes a material handling capacity of 10 kg/h, possessing advantages of fast material handling and small heat loss, and having great potential for commercial application.The insulation part is the key to the microwave pyrolysis device.In the process of high-temperature pyrolysis, heat loss should be prevented.The temperature in the reaction container is ensured to reach the standard evenly, and the reaction is carried out smoothly without distortion.Thermal insulation materials used for microwave pyrolysis need to have high microwave transparency at working temperature, and improper selection of materials can easily cause the thermal insulation materials to absorb microwaves and reduce the energy utilization rate.Silica cotton [23,103], glass wool [104], and ceramic fiber [53,105] are commonly used in microwave pyrolysis devices.
Accurate temperature control is very important in the pyrolysis process, and it also plays a decisive role in the success of the experiment.At present, there are still differences in temperature measurement systems, including K-type thermocouples, infrared (IR), and fiber optic (FO) temperature measurements.Kappe [106] explained the measurement of the chemical reaction temperature in the process of microwave heating in detail.Ordinary PT100 thermocouples will be coupled with a microwave to generate heat, which makes the results inaccurate.Suriapparao [35] separated the two wires by adding four insulation layers to the Cr-Al thermocouple to reduce the interference of microwave coupling and improve temperature measurement accuracy.A considerable number of microwave pyrolysis researchers use microwave-compatible K-type thermocouples, which are directly inserted into reactants to monitor the reaction temperature in real-time.This type of thermocouple is less affected by microwaves and is widely used [23,36,66,83,94].Wang et al. [27] made a blank control test with the K-type thermocouple and found that there was no obvious difference between the temperature measured continuously under microwave working conditions and the temperature measured within three seconds after the microwave working was suspended.In addition, an IR is also a means of measuring the temperature, but it obtains the apparent temperature of the reaction system and cannot reflex the real temperature of the internal reaction, so it is used cautiously [107-109].In addition, FO is considered to be the best choice because its probe posses microwave transparency and is directly inserted into the reaction system.However, for the system with high viscosity, there will still be errors in the measurement results using FO due to the hot spot effect [110].The insulation part is the key to the microwave pyrolysis device.In the process of high-temperature pyrolysis, heat loss should be prevented.The temperature in the reaction container is ensured to reach the standard evenly, and the reaction is carried out smoothly without distortion.Thermal insulation materials used for microwave pyrolysis need to have high microwave transparency at working temperature, and improper selection of materials can easily cause the thermal insulation materials to absorb microwaves and reduce the energy utilization rate.Silica cotton [23,103], glass wool [104], and ceramic fiber [53,105] are commonly used in microwave pyrolysis devices.
Accurate temperature control is very important in the pyrolysis process, and it also plays a decisive role in the success of the experiment.At present, there are still differences in temperature measurement systems, including K-type thermocouples, infrared (IR), and fiber optic (FO) temperature measurements.Kappe [106] explained the measurement of the chemical reaction temperature in the process of microwave heating in detail.Ordinary PT100 thermocouples will be coupled with a microwave to generate heat, which makes the results inaccurate.Suriapparao [35] separated the two wires by adding four insulation layers to the Cr-Al thermocouple to reduce the interference of microwave coupling and improve temperature measurement accuracy.A considerable number of microwave pyrolysis researchers use microwave-compatible K-type thermocouples, which are directly inserted into reactants to monitor the reaction temperature in real-time.This type of thermocouple is less affected by microwaves and is widely used [23,36,66,83,94].Wang et al. [27] made a blank control test with the K-type thermocouple and found that there was no obvious difference between the temperature measured continuously under microwave working conditions and the temperature measured within three seconds after the microwave working was suspended.In addition, an IR is also a means of measuring the temperature, but it obtains the apparent temperature of the reaction system and cannot reflex the real temperature of the internal reaction, so it is used cautiously [107-109].In addition, FO is considered to be the best choice because its probe posses microwave transparency and is directly inserted into the reaction system.However, for the system with high viscosity, there will still be errors in the measurement results using FO due to the hot spot effect [110].

Residues from MAP of Plastics
The pyrolysis products of plastics generally include gas, liquid, and solid, and the solid component is generally coke deposits.The quantity of the pyrolysis products would highly depend on the operating conditions, the types of plastics, as well as the catalysts types.Some unwanted products would form during the process, which may mainly refer to the polymerization of olefins and aromatics, and finally generate coke.Table 3 lists the proportion of residues produced.In the study of Potnuri et al. [111], the amount of coke produced increases with the increase in KOH, and it is thought that KOH accelerates the co-pyrolysis rate of plastics and biomass to produce coke.The research on the used frying oil (UFO) and plastic (PW) co-pyrolysis by Mahari et al. [68] found that at a high UFO/PW ratio, the hydrogen supply of PW was insufficient, which reduced the depolymerization and dehydrogenation of polymer, increased carbonization, and led to the formation of coke.Suriapparao et al. [65] found that the yield of coke in plastic pyrolysis was related to the types of co-pyrolysis biomass.It was found that higher coke was obtained in the co-pyrolysis products of RH and PS.The research of Saifuddin et al. [112] shows that the increase in plastic composition is helpful to reduce the formation of coke during the co-pyrolysis of bamboo and LDPE, since the increase in plastic provides hydrogen for the pyrolysis of polymer, which makes the pyrolysis of polymer produce hydrocarbons as much as possible instead of coke.Temperature is also an important factor affecting the pyrolysis products.In the study of PS MAP by Fan et al. [94], SiC was used as the microwave absorbent, and almost all PS converted to liquid oil at 460 • C, while 36.44 wt.% wax and 56.00 wt.% liquid oil were produced at 340 • C. In addition, selecting suitable catalysts, such as HZSM-5, HY, Hβ, SAPO-34, etc. [78,113], can obviously reduce coke formation.

Energy Consumption of MAP of Plastics
Low energy consumption and high energy efficiency are the outstanding advantages of MAP.Suriapparao et al. [65] conducted the MAP of rice husk and plastic, and reported that the highest efficiency of the microwave co-pyrolysis process could reach 68%.In the MAP study of PP by Kamireddi et al. [114], the microwave conversion efficiency calculated by experiments reached 84.7%, and the pyrolysis oil with a calorific value of 45.4 MJ/kg was obtained.In the study of MAP of HDPE, Zhou et al. [99] calculated the energy balance and found that the energy efficiency of pyrolysis of HDPE can reach 89.6%, better than that of traditional pyrolysis.Zhang et al. [115] analyzed the energy of pyrolysis of mixed plastics in a rotary kiln with a filling degree of 20%, and found that the total energy efficiency was 65.8%.In addition, Rex et al. [25] estimated the cost of PS and PP microwave pyrolysis oil and the price of commercial gasoline, and found that the price of pyrolysis oil was much lower than that of commercial gasoline (2019).Therefore, MAP has high energy conversion efficiency and high application value.

Modeling and Simulation Research
The development of computer skills makes it possible to transfer experiments to simulation, which can not only free people from time-consuming and laborious experiments but also save manpower and material costs.The simulation study can verify and predict the synergistic law between product yield and process parameters in the reaction, provide optimization scheme and guidance for experimental or commercial amplification, improve development efficiency, and help to understand the temperature distribution law in the reaction system and the mechanism of catalytic reaction under microwave pyrolysis.Machine learning (ML), based on a large number of experimental studies, can analyze the data by computer software, and predict the results of amplification experiments.In addition, there are a few reports on the catalytic process of MAP based on the FDTD model and molecular dynamics calculation.
Suriapparao's team has many applications of ML in microwave pyrolysis experiments [23,67,[116][117][118][119].They used the response surface method (RSM) of Central Composite Design (CCD) to optimize the design of the pyrolysis experiment of municipal solid waste [116].A quadratic polynomial with MATLAB fitted the experimental results, and the functions of independent variables such as average heating rate and oil yield on microwave power and pedestal quality were obtained, and a three-dimensional surface diagram was drawn.The trend of the diagram matched well with the experimental results.In another paper by Suriapparao et al. [67], the ML method was used on microwave co-pyrolysis of waste tea powder (WTP) and PS.The effects of the PS to WTP ratio on oil yield, coke, gas, and water output, average heating rate, and conversion rate were evaluated by fitting and mapping.The fitting coefficients of their experimental results and predicted values were 0.91, 0.85, 0.86, 0.93, and 0.9, respectively, which showed that the surface experimental values were in good agreement with the predicted values and can provide guidance in experiments.Terapalli et al. [23] established a theoretical model of support vector regression (SVR) based on limited experimental data in the microwave pyrolysis of styrene, and predicted the influence of process variables such as product yield on catalyst and PS.In order to ensure the correctness of the model, the reliable model equation was obtained by the leave-one-out (LOO) cross-validation method.In addition, Neha et al. [120] optimized the operating parameters of the co-pyrolysis experiment of kitchen waste and LDPE by CCD and RSM and established a credible response prediction and a verifiable mathematical model by regression analysis combined with the experimental results.Among them, the model error of liquid oil was 4.8-7.1%.There are a few reports on the application of numerical simulation in the MAP of plastics.Jing et al. [121] used the finite-difference time-domain-finite difference method (FDTD-FDM) model to simulate the temperature field of HDPE and absorber blends under microwave heating.Considering the problems of material properties, microwave source, medium, and space, the electromagnetic model and heat transfer model are established and the coupling calculation is carried out.The results show that the temperature difference of the heated material increases gradually with the extension of heating time.Mixing materials during heating can reduce the temperature difference, but it does not change the heating time, which shows that rotating materials regularly can promote uniform temperature distribution.In addition, it is also found that activated carbon has a small thermal conductivity, which will produce many "hot spots" as a microwave absorber, while SiC has good thermal conductivity and is suitable as a microwave absorber.Yao et al. [122] calculated the MAP model of polypropylene on Fe clusters by the ReaxFF method in OVITO software.Through calculation, it was found that microwaves enhanced the adsorption capacity of Fe clusters to polypropylene and reduced the bond energy of C-H bond, thus increasing the output of H 2 , shortening the bond length of the C-C bond, and inhibiting the fracture of C-C bond to generate more carbon nanotubes.This calculation successfully reveals the catalytic mechanism of iron clusters and provides cases for more model calculations.

Future Development and Challenges
Under the long-term vision of carbon neutrality in the global peak carbon-dioxide emissions, it has become a sustainable development strategy for human beings to improve the efficient utilization of plastic recycling and reduce white pollution.Further optimization in the laboratory can improve the selectivity of the target products in a regulated way, and the research results will be actively transformed into the process of commercial mass production, which will be the future research interest.
In the process of plastic pyrolysis, the traditional conduction convection heating method with external heat source or Ni-Cr alloy heating element [123] has shortcomings such as low energy utilization rate, uneven temperature distribution, and long pyrolysis time.Microwave-assisted heating can directly act on molecules, reducing reaction time, improving energy utilization efficiency, and reducing process cost.With the development of technology, microwave technology has gradually matured and high-power microwave technology has been developed into an industrial capability.At present, microwave-assisted heating has been widely used in the research area of plastic pyrolysis in the laboratory and will become the development trend for replacing conventional heating to supply energy for plastic pyrolysis.
The need for plastics with poor dielectric properties in microwave pyrolysis is to improve pyrolysis efficiency utilizing the microwave absorber or biomass co-pyrolysis with good dielectric properties.The use of microwave absorbers is beneficial, which not only improves the pyrolysis efficiency, but also has a certain catalytic effect, thus changing the product distribution.At present, most of the research focuses on the influence of using only one microwave absorber, but research on the influence of using different microwave absorbers at the same time is rare.
In the co-pyrolysis of biomass and plastics, hydrogen-rich plastics provide hydrogen atoms for the deoxidation of the biomass, which makes the co-pyrolysis of both raw materials show obvious synergy and improves the quality of biomass pyrolysis oil.Therefore, it is a trend that the deep development of co-pyrolysis has advantages.
The selectivity of products can be controlled by changing the composition, morphology, structure, and proportion of catalysts through reasonable design.Acidity is the decisive factor of catalyst activity.The higher the acidity, the easier it is to pyrolyze macromolecules into small gaseous molecules, but at the same time, it is accompanied by coking.Therefore, it is also very important to accurately regulate the acidity of the catalyst.In molecular sieve catalysts, the pore structure greatly influences the types of products, and the cooperation and utilization of different molecular sieves will be the research focus in the future.Metal catalysts can significantly pyrolyze polyolefin into hydrogen and high-quality carbon fiber because of their unique plasma effect, but its large-scale experiment remains to be carried out.
The arrangement of the pyrolysis reactor affects the distribution of products.In the in-situ catalytic pyrolysis reactor, because the catalyst is in direct contact with pyrolysis raw materials, its performance often decreases due to carbon deposition, and the regeneration of the catalyst is also challenging.However, the ex-situ catalytic pyrolysis device adopts the form of separation of materials and catalysts, which reduces the formation of carbon deposits to some extent, but the pyrolysis process is not controlled.At present, the combination of in-situ catalytic agitation pyrolysis and ectopic catalysis has great development space to improve the selectivity of target products.Although the intermittent microwave pyrolysis device is mostly used in the laboratory, it can only be used as a research platform for researchers to analyze the pyrolysis products of raw materials, develop catalysts, and adjust process-operating parameters in small experiments.The limitations of this operating platform in handling raw materials are obvious.Therefore, after the initial effect is achieved in the laboratory, it is necessary to further diversify the development of pilot plants to expand the scale of processing raw materials.At the same time, it is necessary to realize commercial applications and develop MAP devices that can continuously process plastics in large quantities.
In the research of MAP of plastics, the application of simulation is still less.In the application of machine learning technology, the mathematical model established by researchers corresponds to a specific experimental system and cannot reflect the general law.Therefore, it is of great significance to develop a generalized model that can be used to predict pyrolysis.Although the mathematical model developed by machine learning can predict unknown experiments, it is based on a certain database of experiments.In addition, the research reports using FDTD and ReaxFF methods are very limited, and numerical simulations and molecular dynamics' calculation are effective means for us to understand the pyrolysis state and mechanism of plastics, which will be very helpful for the design of pyrolysis devices and the development of catalysts.

Conclusions
In this paper, the effects of technological parameters such as plastic type, microwave power, microwave absorber, co-pyrolysis, catalyst, pyrolysis temperature, and device setting on pyrolysis products in the MAP of plastics are summarized.It shows that the MAP process is affected by many factors, and the aspects of improving product selectivity and process amplification need further exploration and development.In terms of computational simulation, machine learning is an important way to simulate the MAP of plastics, but the established computational model is only applicable to a specific research object, and cannot be applied to all or more objects.At the same time, the research of FDTD and ReaxFF methods in the MAP of plastics still needs to be further expanded.The high valueadded liquid oil or hydrogen and carbon fiber generated by the MAP of plastics have high purity, basically contain no pollution sources, and can realize ultra-low carbon recovery of waste plastics to a high degree.Compared with traditional pyrolysis, microwave pyrolysis will gradually become the mainstream method to rapidly and efficiently recover waste plastics and transform them into high-value-added products.It is hoped that the in-depth study of MAP of plastics will provide assistance for improving waste conversion and commercialization and support the development goal of peak carbon dioxide emissions and carbon neutrality strategy.

Figure 2 .
Figure 2. The carbonate linkages of PC react with the hydroxyl group of TCCO to produce BPA and aromatic carbonate-ester polyols.Adapted from Beneš[51] copyright (2017), with permissions from Elsevier.

Figure 2 .
Figure 2. The carbonate linkages of PC react with the hydroxyl group of TCCO to produce BPA and aromatic carbonate-ester polyols.Adapted from Beneš [51] copyright (2017), with permissions from Elsevier.

Figure 3 .
Figure 3. Pyrolysis behavior of plastics in core-shell silicon carbide foam@ZSM-5 catalyst, (a) Global transport in reactor scale, (b) directional heat transfer in mesoscale, and (c) internal diffusion in nanoscale.Adapted from Chen [82] copyright (2022), with permissions from Elsevier.

Figure 3 .
Figure 3. Pyrolysis behavior of plastics in core-shell silicon carbide foam@ZSM-5 catalyst, (a) Global transport in reactor scale, (b) directional heat transfer in mesoscale, and (c) internal diffusion in nanoscale.Adapted from Chen [82] copyright (2022), with permissions from Elsevier.

Processes 2023 , 23 Figure 4 .
Figure 4.The schematic diagram of the decomposition of waste plastics over Fe/Ni-CeO2@CNTs catalyst in the microwave radiation field.Adapted from Wang[92] copyright (2022), with permissions from Elsevier.

Figure 4 .
Figure 4.The schematic diagram of the decomposition of waste plastics over Fe/Ni-CeO 2 @CNTs catalyst in the microwave radiation field.Adapted from Wang [92] copyright (2022), with permissions from Elsevier.

Table 1 .
General properties, usages, and main pyrolysis products of plastics.

Table 2 .
Dielectric properties of various plastics and common absorbers (2.45 GHz).

Table 3 .
Experimental conditions, liquid yield, calorific value, and residue of co-pyrolysis of biomass and plastics in different pyrolysis processes.