A review on catalytic pyrolysis of municipal plastic waste

Plastic pollution is a global issue that severely threatens the environment if not managed properly. Plastic waste is widely treated in unsustainable ways such as landfill and incineration that generally do not contribute to the circular economy or to the principles of the United Nation's sustainable development goals. Catalytic pyrolysis of plastic waste is considered an alternative solution with its potential of recovering fuels and chemicals from the plastic waste that is not recyclable. Here, we described for the first time the main steps required for running an operational pyrolysis plant from a whole system perspective. The recent advancement of plastic pyrolysis technologies is also described to guide the selection of relevant catalysts. The practical applications of products from the plastic pyrolysis are succinctly reviewed. This review will facilitate the development of the capacity to make better decisions upon the design and analysis of plastic pyrolysis processes and systems.


| INTRODUCTION
The generation of municipal solid waste (MSW) is continuously accelerating due to economic and population growth and urbanization worldwide (Kaza et al., 2018).It is estimated that the world population was 7.7 billion in 2019 and it would likely reach 8.5 billion in 2030, 9.7 billion in 2050, and 10.9 billion by the end of this century under the so-called medium population growth trajectory (United Nations (Department of Economic and Social Affairs Population Division), 2019).This means more resources will be needed to meet the world population's demand and, consequently, more waste will be potentially generated if not properly managed.
Plastic is a cheap and ubiquitous material due to its versatility, durability, and adaptability, and 368 million tons of plastic were produced globally in 2019 (Plastics Europe, 2020).One-half of the plastics currently produced are singleuse plastics (Giacovelli, 2018), and only 2% of these single-use packaging plastics flow in closed-loop recycling, despite the recycling symbol having appeared on plastic items for more than 40 years (Ellen MacArthur Foundation, 2016).During the Covid-19 pandemic, the generation of single-use plastics that do not flow in closed-loop recycling rises due to the increasing usage of PPE (Yuan et al., 2021).The production of plastics has increased 200-fold since the middle of the last century (Geyer, 2020); currently, around 6% of annual oil demand is used for plastic production, which is expected to reach 20% by 2050 (Ellen MacArthur Foundation, 2016).
Plastic waste is one of the main causes of three intertwined world disasters, that is, environmental pollution, climate change, and natural resource scarcity.The greenhouse gas (GHG) emitted from the fossil fuel-based plastics produced in 2015 is 1.8 GtCO 2 -eq for a whole life cycle perspective (excluding recycling), and the biggest share (60%) of the emissions came from the production of polymers (Zheng & Suh, 2019).For example, the production of 1 kg of PET needs 84 MJ energy, which is higher than the heating value of crude oil (44 MJ/kg; Gervet, 2007).End-of-life plastic is usually disposed of unsustainably, which poses environmental pollution for the terrestrial and marine ecosystem.Jambeck et al. (2015) calculated that 4.8-12.7 million metric tons of plastic debris entered the ocean.Lack of information due to the complexity of tracking plastic pollution systems worldwide through transmission pathways (terrestrial, aquatic, and atmospheric pathways) makes the plastic problem challenging to resolve (Bank et al., 2021).For example, there are no standardized methods to quantify and extract plastic particles in a soil (Dissanayake et al., 2022).
Worldwide, various campaigns have attempted to address these issues.More than 500 organizations including >200 businesses responsible for more than 20% of global packaging plastics, and 27 financial institutions with overall $4 trillion worth of assets have set the target of keeping plastics within a circular economy and out of the environment at their sources by 2025 (Ellen MacArthur Foundation, 2020).The European Union (EU) also developed the Plastics in the Circular Economy legislation and the Circular Economy Action Plan, to drive sustainable plastic waste management (European Commission, 2018).It was estimated that shifting five key industries (i.e., cement, aluminum, steel, plastics, and food) to the circular economy could reduce GHG emissions by 40% by 2050 (Ellen MacArthur Foundation, Material Economics, 2019).
Except for its economic burdens on mankind, plastic waste also has a profound footprint on the environment and living species on terrestrial and marine systems (Ok, 2020).In total, 60-99 million tons of plastic waste was disposed of in an unsustainable manner and ended up in the environment, while annual mismanaged plastic waste could reach 155-265 million tons by 2060 under the business-as-usual scenario (Lebreton & Andrady, 2019).Annually, 11% (19-23 million tons) of plastic waste ended up in the ocean, and this figure could well exceed 90 million tons per year by 2030 if the business-as-usual scenario is continued (Borrelle et al., 2020).Approximately 150 million tons of plastic debris was floating in the oceans (McKinsey Center for Business and Environment, 2015), and plastic waste usually reaches world's oceans through rivers, which is referred to as one of the major plastic waste transportation systems (van Emmerik & Schwarz, 2020).The annual world economic burden due to plastic debris reaching the oceanic system is $8 billion (Kershaw, 2016).
Significantly concerns have been raised about the adverse impacts of plastic pollution on marine ecosystems and beyond.The plastic debris is a cause of feeding impairment (Savinelli et al., 2020) and entanglement of marine species (Jepsen & de Bruyn, 2019;Nisanth & Kumar, 2019), and disturbs natural carbon dioxide circulation (Shen et al., 2020).In a recent study, two-thirds of marine and estuarine fish species were found to have ingested plastics; indeed, the last decade's records suggest that the average frequency of microplastic occurrence in marine species has doubled since 2010 (Savoca et al., 2021).Microplastics are plastic particles with sizes of a few microns up to 5 mm (Tirkey & Upadhyay, 2021).Microplastic particles vary in shape from irregular to spherical (Rosal, 2021), but older particles have smooth edges or are more spherical in shape due to the mechanical shear, thermal oxidation, and solar exposure (Chubarenko et al., 2016).
It has been proven that microplastics have adverse impacts on corals' physical condition, energy, growth, and health through active ingestion, passive surface adhesion, and due to the fact that microplastics are carriers of toxic chemicals and disease pathogens (Huang et al., 2020).The plastics were defined as a vector for toxic microelements as it has an increasing surface area, and organic matters quickly deposit on their surface (Bradney et al., 2019).Moreover, it was found that a high surface area of microplastics could enable the bacteria with antibiotic resistance to grow (Bank et al., 2020).For example, the microplastics in the Mediterranean Sea result in feeding impairment and negatively affects the well-being of the coral, Astroides calycularis (Savinelli et al., 2020).Moreover, there are knowledge gaps regarding coral's exposure to nanoplastics due to a lack of advanced analytical methods (Huang et al., 2020) and, more severe impacts of nanoplastics on the corals will be expected with the development of new analytical methods.A recent study estimated that the entire population of sea turtles, 41.46% of all marine mammals, and 44% of all sea birds have plastics in their stomachs (Kühn & Van Franeker, 2020).Many of the deaths among marine species are associated with film-like plastics, fishing nets, and latex/balloons (Roman et al., 2020).Seabirds play a transfer role for plastics received through marine foraging to terrestrial zones, which contributes to their spread (Grant et al., 2021).Furthermore, microplastics can enter the food chain, posing a potential threat to human health (De-la-Torre, 2020).Despite the various and considerable evidence about the severe ecological effect of plastic waste, more studies are needed to fully understand problems related to plastics (Bucci et al., 2020), especially their effect on the terrestrial system (de Souza Machado et al., 2018;Kumar, Xiong, et al., 2020).
There are a few factors that make plastic waste more problematic than other types of waste.For example, the plastic recycling rate is the lowest among the three most-used materials (i.e., plastic, paper, and glass) for packaging.For example, in the UK, the recycling rates of plastic, paper, and glass materials were reported to be 44.9%,81.9%, and 67.1%, respectively (DEFRA (Department of Economic and Social Affairs Population Division), 2018).Municipal solid plastic waste (MSPW) is usually managed in unsustainable ways such as incineration, landfill, and so forth, and the associated GHG could triple by 2030 (Advisors et al., 2019).The world needs to take bold and urgent action to curb what appears to be a mounting plastic-related disaster (Sarkar et al., 2021).
Ideally, the waste should be treated within the framework of the waste management hierarchy, which is considered as a sustainable way of achieving this goal.According to the Waste Framework Directive 2008/98/EC, the principle of the waste management hierarchy is prevention-reusing-recycling-recovering-disposing (European Commission, 2018), where the prevention of plastic waste arising, the reuse of plastics, and plastic waste recycling need to be prioritized.Reuse of plastics has potential economic and environmental benefits over single-use plastics, but the use of plastics for the same purposes on a large scale is legislatively and technically limited (Coelho et al., 2020).In the case of plastic recycling, even in European countries with advanced waste management technologies, only 32.5% of a total of 29.1 million tons of postconsumer plastics was recycled in 2018, while 42.6% were used as resources for energy recovery, and 24.9% ended up in landfill (Plastics Europe, 2020).Unrecycled and unrecyclable postconsumer plastics should be treated for energy recovery purposes rather than being sent to landfill in order to gain maximum environmental, economic, and social benefits in terms of their contributions to the circular economy (van Caneghem et al., 2019).
There are two types of plastic recycling technologies: the mechanical and chemical.For the former, plastic waste is sorted, washed, shredded, melted, and granulated into pellets which can be used as some ready raw materials for plastic goods production; the latter requires the use of various technologies such as chemolysis, pyrolysis, fluid catalytic cracking (FCC), gasification, and so forth (Ragaert et al., 2017;Singh et al., 2017).The main disadvantages of the mechanical recycling method include thermal-mechanical degradation causing random chain scission and crosslinking (Ragaert et al., 2017), and limited effectiveness for heterogeneous plastics due to varying melting parameters (Singh et al., 2017).Plastics rejected from the mechanical recycling method due to contamination could be treated by the chemical recycling technologies, for example, pyrolysis, which is more tolerant to higher levels of plastic contaminations (Holger et al., 2019;Ragaert et al., 2017).Furthermore, the impact of climate change due to pyrolysis is less than the other widely used waste management technologies such as incineration (Gear et al., 2018;Somoza-Tornos et al., 2020).Hence, pyrolysis of MSPW should be prioritized over incineration and landfill which lead to losses of valuable resources, a linear economy principle, and greater environmental concerns (Davidson et al., 2021).
Pyrolysis is the process whereby waste undergoes thermal treatment in the absence of oxygen/air breaking down polymers and monomers into smaller hydrocarbon molecules in the form of three main products (i.e., oil, gas, and solid residue) (Chen, Jin, & Chi, 2014;Davidson et al., 2021;Ragaert et al., 2017).This review will guide researchers in understanding how catalytic pyrolysis plants work, catalysts that can be used to treat MSPW, and the practical applications of pyrolysis production toward managing municipal plastic waste.
The main steps of pyrolysis will be described to facilitate the understanding of the technology from a whole system perspective.The gathered information and data can be used in studies such as the life cycle assessment (LCA) of MSPW.The types of plastic pyrolysis catalysts will be reviewed and their suitability for pyrolysis will be systematically evaluated.This will facilitate the selection of catalysts for plastic waste pyrolysis.Finally, the applications of the products derived from MSPW pyrolysis will be described in detail.

| CATALYTIC PYROLYSIS OF POLYMERS
Depending on the use of catalyst, pyrolysis could be classified into noncatalytic and catalytic, where the latter is more desirable for the treatment of plastic waste (Grause et al., 2011).Catalytic pyrolysis of plastic has some various advantages over noncatalytic pyrolysis (Chen, Feng, et al., 2020): • First, plastic pyrolysis is an endothermic process, while catalytic pyrolysis, as the name suggests, needs a lower activation energy (i.e., the minimum energy required to start the thermal degradation of feedstock); thus, catalytic pyrolysis consumes less energy and speeds up the conversion of plastic to products such as oil, gas, and char (Al-Salem et al., 2017;Sharuddin et al., 2016).For example, Kremer et al. (2022) conducted a kinetic study of MPW pyrolysis, and the results showed that using catalysts reduced the activation energy by around 10%-50%.• Second, catalytic pyrolysis has a positive impact on the purity and quality of products (L opez, De Marco, Caballero, Adrados, & Laresgoiti, 2011;Manos et al., 2002), and the product distribution under its lower temperature environment than noncatalytic pyrolysis (Miskolczi et al., 2009).Miskolczi et al. (2009) studied the pyrolysis of the mixture of high-density polyethylene (HDPE) and polypropylene (PP) at 520 C. The pyrolysis process without any catalyst, produced 5.1% gases, 18.2% gasoline, and 17.9% light oil, while the pyrolysis process with the catalyst ZSM-5 produced higher-value products under the same conditions (12.2% gases, 34.5% gasoline, and 24.1% light oil).
Despite the fact that pyrolysis is one of the most heavily researched technologies for resource recovery from plastic waste (Davidson et al., 2021) and is a quite commercially mature technology (Jeswani et al., 2021), there are few plastic pyrolysis plants worldwide (Jeswani et al., 2021).However, it is worth noting that the cost of the products derived from the chemical recycling of MPW (pyrolysis, gasification, etc.) is higher than petroleum products, and is the main barrier against the commercialization of MPW pyrolysis (Villalobos et al., 2006).For example, Pacheco-Lopez et al. (2021) conducted an economic assessment of ethanol production from plastic pyrolysis used as a gasoline alternative, and its production cost is 130% higher than fossil-based gasoline.Many countries have ambitious zero-carbon targets to mitigate climate change and are on the way to systematically shifting toward more sustainable waste management, where chemical recycling will be prioritized (Bauer et al., 2022).This means that climate change will be the driver to making pyrolysis a more economically viable technology.
To understand the whole process of plastic pyrolysis plants, a number of LCA studies have been carried out to define the main steps or system boundaries of plastic pyrolysis, as illustrated in Figure 1 (Benavides et al., 2017;Jeswani et al., 2021).Typically, the plastic pyrolysis process comprises collection and transportation of waste, sorting, pretreatment of feedstock, pyrolysis, and purification of received products (Yuan et al., 2022).
A sorting step is necessary to eliminate unwanted materials that can affect the quality of the production, and to ensure the optimal performance of the pyrolysis process (Jeswani et al., 2021).MSPW is usually contaminated with organic matter such as soil and plants, and nonorganic materials such as glue, dye, and labels that can adversely affect the pyrolysis process and product quality.Borsodi et al. (2011) compared the pyrolysis of clear HDPE and HDPE contaminated with motor oil in a tubular reactor, applying 500 C temperature in the absence and presence of catalyst, Y-zeolite.The effect of the use of catalyst on product distribution was significant, if clean plastic was fed into the reactor.However, the oil received from the pyrolysis of contaminated plastic in the presence of catalyst (Y-zeolite) had high sulfur, chlorine, and nitrogen content as the oil from the pyrolysis of contaminated plastic in the absence of the catalyst (Borsodi et al., 2011).This suggested that even some catalyst is not able to deal with the plastic contamination and consequently, the products had poor quality.
MSPW normally goes through a washing process to remove contamination.For example, MSPW of the province of Granada in Spain contains 10-14 wt% of dirt (Calero et al., 2018); contamination of organic origins can be effectively removed by a washing process with water at room temperature, while, to remove nonorganic materials such as glue, paint, and fat, the washing process needs hot water (Calero et al., 2018).Calero et al. (2018) in their study used 10 L of water to wash a kilogram of plastics for 30 min, and wastewater from the washing process needed to be treated before discharging into the sewage system to meet local legislation requirements: wastewater to be discharged into the sewage network of the Granada City Council has to show the chemical oxygen demand at not more than 1400 mgO 2 /L (Calero et al., 2018).Sometimes, wastewater that has not been treated can be directly discharged into sewage if the dirt in the wastewater does not exceed certain limits (Calero et al., 2018).Recommended temperature of water for hot washing of plastics was 60 C (Al-Sabagh et al., 2016;Awaja & Pavel, 2005).
Thermal pretreatment as a drying process is an important step to control the moisture content of feedstock for the pyrolysis process.Increasing moisture content in feedstock reduces the reaction temperature in the pyrolysis reactor, and consequently, the process of cracking of feedstock into lighter molecules can be incomplete or prolonged (Chen, Jin, & Chi, 2014;Kaewluan & Pipatmanomai, 2011;Karamarkovic & Karamarkovic, 2010;Li et al., 1999).For instance, Li et al. (1999) studied how moisture content affected pyrolysis time and found increasing moisture content of the wood-chips from 5.25% to 14.83% doubled pyrolysis time from 6 to 12 min.This revealed that the overall efficiency of the pyrolysis process was reduced with increasing moisture content of the feedstock.The moisture content of polymers in MSPW can vary depending on location, weather patterns, and polymer type.The moisture content of PE, PET, PP, PS, and EPS in MSW sampled in the province of Granada, Spain, has been found to be 11.78, 8.9, 1.58, 20.98, and 16.1 wt%, respectively (Calero et al., 2018).The moisture content of MSPW needs to be analyzed to define the optimal parameters for the drying process.
In addition to the contaminants that can be removed via the washing and drying processes, polymers can contain chemical additives that are used to improve certain physical characteristics (e.g., tensile strength, plasticity, fireresistance, etc.) of plastics and the chemical contaminations of such can negatively affect the pyrolysis process.One of the widely used additives is chlorine, which is mainly added to produce fire-resistant plastics.The PVC polymers used for packaging have a high chlorine content (Wang, Xian, et al., 2020).The pyrolysis of plastics containing chlorine produces chlorinated hydrocarbon molecules, which causes problems with corrosion and making oil more halogenated (Qureshi et al., 2020).It is recommended that MPW feedstock contains no more than 7% PVC for pyrolysis plants (Haig et al., 2018).Thermal pretreatment of polymers at low temperature before feeding into the pyrolysis reactor can be used to de-chlorinate the feedstock (Fukushima et al., 2009;Wu & Williams, 2013).In the pretreatment process, plastic waste is melted in a low-temperature treatment reactor operating within a temperature range of 300-330 C, from which chlorine-containing gases are subsequently evaporated (Fukushima et al., 2009).The bond between C and Cl is weaker than the C C bond, and PVC pyrolysis at temperatures of 225-507 C produces free radical Cl which readily reacts with hydrogen to form HCl (Mortezaeikia et al., 2021).Hydrocarbon gases containing hydrochloric gases separated by evaporation at the low temperature are pumped into the de-HCl gas incinerator for combustion to reclaim hydrochloric acid (Fukushima et al., 2009).
After the pretreatment steps to remove unwanted contaminants, the plastic waste feedstock is fed into the pyrolysis reactor, where the feedstock is thermally decomposed into highly saturated hydrocarbon vapor in the absence of air/oxygen and in the presence of catalyst (Chen, Yin, et al., 2014;Jeswani et al., 2021).The condensable gases contained in vapor are collected in the form of oil via condensation while the remaining incondensable gas is collected as syngas (Jeswani et al., 2021).Inside the pyrolysis reactor, nitrogen is used as an inert gas to purge and to replace air/oxygen to avoid feedstock oxidation (Anene et al., 2018;Maniscalco et al., 2021).The oil and gas produced do need some conditioning to reach the desired quality by various forms of purification, for example, distillation (Fukushima et al., 2009).Heavy oil with unwanted matter content can be upgraded by thermal treatment with catalysts to gasoline and diesel-like products with low chloride and high alkane and aromatic contents (Lopez-Urionabarrenechea et al., 2015).
One of the main factors that could affect the quality of products and their yields is reactor type.There are various types of reactors that can be used to pyrolyze MPW, namely fixed bed, fluidized bed, rotary kiln, microwave, batch or semi-batch, plasma pyrolysis reactors, and so forth.Fluidized bed, a fixed-bed, batch, the rotary kiln, and semi-batch reactors should be highlighted as they are widely used on a commercial scale due to either their very simple designs or their efficiencies.Fixed-bed reactors have a simple design and are usually operated at lower temperatures, which is more suited to the production of liquids, but they have the disadvantage of poor heat transfer.Fluidized bed reactors are relatively suitable to deal with materials such as MPW, which is a poor heat conductor, as the fluidization serves to improve heat transfer between plastic particles (Kaminsky et al., 2004).This contributes to improved homogenization of the temperature inside the reactor and allows for a rapid heating rate, which allows for reduced residence time and, consequently, a reduction in the formation of residue and increased production of lighter hydrocarbons (Singh et al., 2019).Despite fluidized bed reactors having numerous advantages, they do have issues about defluidization as a result of the agglomeration of molten MPW feedstock (Dai et al., 2022).This reduces both the quality of products and the efficiency of thermal conversion process.Conical spouted bed reactors, which notably have a more complex design, can be used to minimize the defluidization issue.
For batch reactors, MPW is fed into the reactor prior to the pyrolysis process, and products are removed after complete thermal conversion (Serra et al., 2022).Semi-batch reactors are not completely closed systems, as are the batch reactors, as reagents or products can be added or removed while the pyrolysis process is ongoing (Serra et al., 2022).The main disadvantages of the reactors are that (a) feedstock cannot be continuously fed into the reactors, and there is an issue about system scale-up, and (b) poor heat transfer (Dai et al., 2022;Inayat et al., 2021;Lopez et al., 2017).However, the issue with poor heat transfer can be minimized by installing a stirrer inside the reactors.
Rotary kiln reactors also see widespread use due to its easily adjustable parameters such as residence time, temperature, and so forth.However, heating energy is usually transferred from the reactor walls to the plastic particles inside the reactor, resulting in issues regarding the homogenization of the temperature inside the reactor.This can cause overcracking or incomplete cracking of hydrocarbon chains, which negatively affects the product quality.Metal or ceramic balls can be loaded inside the reactor to minimize this issue (Dai et al., 2022).
It is worth noting that the main difference of catalytic pyrolysis from noncatalytic pyrolysis is adding catalysts.The purpose of that is to improve the efficiency of the conversion process to produce oil, gas, and char.What catalysts can be used in the pyrolysis of MSPW is described in detail in the section about catalysts.

| CATALYSTS
The main purpose of the use of catalysts in MSPW pyrolysis is to speed up the chemical reactions that occur during the cracking of polymer chains.Choosing an optimal catalyst for MPW pyrolysis on an industrial scale is essential as the MPW pyrolysis plant requires a large amount of catalyst.To date, many types of catalysts have been developed to deal with plastic pyrolysis such as ZSM-5 (Mangesh et al., 2020), Ziegler-Natta (Kumagai & Yoshioka, 2016), HZSM-5, FCC, HY zeolite (Wang, Wang, et al., 2021), and MCM-41.In particular, mesoporous molecular sieve catalysts are widely used for polymer pyrolysis.The main difference between mesoporous molecular sieve catalysts and other forms of catalysts is in their topology, as mesoporous molecular sieve catalysts have an ordered pore structure.Also, it is worth noting existing catalysts have been modified by, for example, the impregnation of transition metals into catalyst structures to improve their catalytic performance.
The principal factors that define the effectiveness of catalysts used in the pyrolysis process are as follows: (1) The acidity of catalysts.pH plays a major role in boosting the cracking activity of polymers, and especially wax (Budsaereechai et al., 2019;Pan et al., 2021;White, 2006).High acidity catalysts such as zeolite-based catalysts enhance the production of gases and reduce oils, while mild and low acidity catalysts such as clay-based catalysts tend to show the opposite (Manos et al., 2001).
(2) Pore structure of catalysts.Micropores increase oil yields, while mesopores increase gas yields in MPW pyrolysis (Miskolczi et al., 2006;Pan et al., 2021).Higher hydrocarbons are not able to enter the micropores due to their size, and the quality of the oil is thus improved (Ratnasari et al., 2017).

| The main advantages and disadvantages of catalytic pyrolysis
There are three main advantages of catalytic MSW pyrolysis.
(1) Adjustment of the yields of products: Selecting catalysts based on their pore structure and pH for the pyrolysis process helps to control the yields and distribution of pyrolysis products (Pan et al., 2021).For example, using certain acid catalysts such as HY, Hβ, HZSM-5, and HUSY zeolites enhances the quality of the resultant oil (Aguado et al., 2008;Chen, Zhang, et al., 2020;Elordi et al., 2009;Lopez et al., 2017;L opez, De Marco, Caballero, Laresgoiti, Adrados, & Aranzabal, 2011;Marcilla et al., 2009;Serrano et al., 2012).Anene et al. (2018) compared oils received from the noncatalytic and catalytic pyrolysis of HDPE, LDPE, PP, and a mixture of LDPE and HDPE in the presence of zeolite-type catalysts in a laboratory-scale batch reactor at 460 C. The oil produced from the pyrolysis in the presence of zeolite-type catalyst mainly contained gasoline fraction carbons (C 7 -C 12 ), and no diesel fraction (C 13 -C 20 ), or heavy fraction (C 21 -C 40 ) as would otherwise be gained from noncatalytic pyrolysis (Anene et al., 2018).
(2) The formation of undesirable substances during polymer pyrolysis can be inhibited or reduced through the choice of catalyst: For example, ZSM-5 is effective at reducing the amount of solid residue, sulfur, nitrogen, and phosphorous in the resultant oil (Miskolczi et al., 2009).
(3) Increased efficiency of the pyrolysis process: The presence of a catalyst during the pyrolysis process accelerates chemical reactions and the decomposition of polymers as the participation of the catalyst reduces the activation energy of the associated reactions.For example, Miskolczi et al. (2006) found that the presence of FCC, ZSM-5, or clinoptilolite in the pyrolysis of polymers can reduce the activation energy by 40 kJ/mol.This inevitably increases the conversion rate of MSW, which usually proceeds at a lower temperature than noncatalytic pyrolysis.
However, there are also some disadvantages of catalytic pyrolysis.
(1) Deactivation of catalysts.After the pyrolysis process, deactivation occurs due to the deposition of coke on the surface of the catalysts (L opez, de Marco, Caballero, Laresgoiti, Adrados, & Torres, 2011).The catalysts with the strong acid sites and micropores promote greater deactivation process (Chen et al., 2021;Huang et al., 2009).For example, zeolites, which are considered highly acidic catalysts, are mainly deactivated due to acid-site poisoning (Argyle & Bartholomew, 2015;Bibby et al., 1992;Magnoux et al., 1987;Nakasaka et al., 2015).Another way to minimize the problem of deactivation of the catalyst caused by deposition of coke is to reduce its formation during pyrolysis.Typically, deactivated catalysts need to be regenerated or replaced by fresh catalyst, which in both cases will clearly result in additional costs (L opez, De Marco, Caballero, Adrados, & Laresgoiti, 2011).Another way to minimize the deactivation issue is to select the catalyst based on its topology (shape of selectivity; Castano et al., 2011); for example, HZSM-5 has poreslike channels, which prevent the entry of bulky coke molecules into pores, maintaining its catalytic activity for longer (Schirmer et al., 2001).
(2) Additional expenditure for catalysts.Despite benefits such as better product selectivity, lowering the activation energy of polymers, and saving thermal energy, additional costs are associated with the purchase, regeneration, and disposal of catalysts.To make catalytic pyrolysis more attractive, widely available, and cheap, alternative catalysts such as clay or fly ash have been studied.

| Catalysts used for the pyrolysis of municipal plastic waste
Some catalysts could favor liquid formation, and some could be used to produce more gas or char.For example, zeolite or clay-based catalysts recommended themselves in producing liquid, but zeolite catalysts promote the production of a high-quality liquid containing diesel or gasoline range carbon such as lighter aliphatic hydrocarbons (Serra et al., 2022).Also, zeolite catalysts such as FCC or ZSM-5 have been widely used in oil and gas industry since the last century, and currently, they are popular catalysts to pyrolyze plastic waste (Fadillah et al., 2021).It is worth noting that despite both, zeolite and clay-based catalysts being desirable to receive liquid, the use of zeolite catalysts results in a formation of a higher coke and lower oil than clay-based catalysts due to their acidic nature (Hafeez et al., 2019;Manos et al., 2001;Serra et al., 2022).Clay-based catalysts usually need to be modified to produce a high-quality oil (Serra et al., 2022).
Review papers published more focused on catalysts that are desirable to use for the production of liquid products rather than metal-based catalysts which are favorable to produce more gas products (Miandad et al., 2016).

| The clay-based catalyst
Clay-based catalysts are inexpensive (Budsaereechai et al., 2019;Patil et al., 2018), and are made from an abundant raw materials.There are various clay-based catalysts used in the pyrolysis of MSPW, namely bentonite clay (Budsaereechai et al., 2019), red clay (Patil et al., 2018), halloysite clay (Kanattukara et al., 2023), and so forth.These catalysts contain hydrated SiO 2 (silica), alkali metal and alkaline earth oxides, and other chemical species (Budsaereechai et al., 2019;Pan et al., 2021;Patil et al., 2018).Clay-based catalysts are usually cleaned of organic impurities and calcinated before use (Patil et al., 2018).Patil et al. (2018) used red clay for the pyrolysis of dealkaline lignin, LDPE, and PS, where the clay was dried at 105 C for a day and was then sieved to gain red clay with a particle size of <105 μm.The final step before its use is calcination at 500 C for 4 h.
Clay-based catalysts can play a significant role in improving the oil yield of the process.The ratio of SiO 2 /Al 2 O 3 is usually used to define the acidity of clay-based catalysts (Budsaereechai et al., 2019), where a low ratio means they are more acidic in nature.In general, clay-based catalysts are considered mild acidic catalysts, therefore, it is preferable to use them when the production of increased amount of oil are desirable, as the catalyst prevents excessive decomposition of polymer chains (Chen, Yu, et al., 2020;Manos et al., 2001).Manos et al. (2001) compared the catalytic performance of clay-based catalysts with Y Zeolite catalyst in the pyrolysis of PE in a semi-batch reactor at a final temperature of 500 C. The yield of oil produced from the pyrolysis in the presence of clay catalysts was found to be around 70%, while the Y Zeolite catalyst resulted in a less than 50% yield (Manos et al., 2001).
The surface area of clay-based catalysts is lower than widely used zeolite and metal-based catalysts.It is well known that increasing the surface area increases the production of gas (Miandad et al., 2016).In the study by Kanattukara et al. (2023), activated alumina, ZSM-5, FCC, and halloysite clay were compared about their catalytic performances in the pyrolyzation of polyolefin polymers.The surface area of halloysite clay (28 m 2 /g) was the lowest among the above catalysts (activated alumina = 260 m 2 /g, ZSM-5 = 341 m 2 /g, and FCC = 282 m 2 /g).The gas volume produced from ZSM-5 was twice as halloysite clay showing the better catalytic performance of ZSM-5.
Even though clay-based catalysts are cheap, their performance in the pyrolysis process is lower than commercially available metal-based catalysts.To improve its catalytic performance, possible technologies to modify them have been widely studied.Li et al. (2017) modified pillared clay by adding metallic elements such as Al, Fe, Zr, and Ti into the inter-layered structure of the bentonite clay.Using Fe-modified pillared clay catalyst in the pyrolysis of simulated mixed plastics wastes (42 wt% HDPE, 35 wt% PP, 18 wt% PS, and 5 wt% PET) at a heating rate 40 C/min and a final temperature 500 C for 30 min produced an oil that was largely diesel range carbon fractions in a 80.5% proportion and gas containing a large volume of hydrogens at 47.7% (Li et al., 2017).This new modified clay-based catalyst had the advantage of very low cost over more widely used catalysts such as zeolites and mesoporous catalysts.Thus, it has the potential to be used on industrial scales.
ZSM is one of the widely studied catalysts in the pyrolysis of polymers.ZSM-5 needs lower temperatures than many other catalysts.L opez, de Marco, Caballero, Laresgoiti, Adrados, and Torres (2011) compared ZSM-5 and Red Mud in the pyrolysis of a mixture of plastics (40 wt% PE, 35 wt% PP, 18 wt% PS, 4 wt% PET, and 3 wt% PVC) in a semi-batch reactor at 440 C and 500 C. The pyrolysis of plastics with Red Mud as the catalyst produced 76.2 wt% oil and 21.6 wt% gas at 440 C, and 57 wt% oil and 41.3 wt% gas at 500 C, while ZSM-5 produced 56.9 wt% oil and 40.4 wt% gas at 440 C, and 39.8 wt% oil and 58.4 wt% gas at 500 C. ZSM-5 showed better catalytic performance at both temperatures than Red Mud due to its highly acidic nature and porosity.This positive effect of ZSM-5 on production has been observed in various other studies (de Marco et al., 2009;Miskolczi et al., 2006;Williams & Bagri, 2004).
HZSM-5 can be produced by the treatment of ZSM-5 with ammonium ion exchange reactions (Pan et al., 2021).HZSM-5 is also considered to have as a high performance as ZSM-5.The use of HZSM-5 in the pyrolysis of LDPE in continuous-stirred microwave pyrolysis and batch microwave pyrolysis reactors improved the efficiencies of the associated pyrolysis processes (Fan et al., 2021).This was reflected in the net energy gains (energy production after subtracting energy consumption), which increased from 9.85 MJ/kg and À42.86 MJ/kg (noncatalytic pyrolysis) to 34.16 MJ/kg and À38.86 MJ/kg, for the continuous-stirred and batch microwave reactors, respectively.
Even though ZSM-5 has a high catalytic performance, it gradually loses its activity along the pyrolysis process, which negatively affects the overall production.L opez, de Marco, Caballero, Laresgoiti, Adrados, and Torres (2011) studied the catalytic performance of fresh and spent ZSM-5 in the pyrolysis of a mixture of polymers and compared the results with noncatalytic pyrolysis.Deactivated ZSM-5 was found to have a reduced catalytic performance compared to fresh ZSM-5 as oil and gas yields were almost identical to those of noncatalytic pyrolysis.Thereafter, deactivated ZSM-5 was regenerated and added to the pyrolysis process, resulting in significantly improved yields of oil and gas, to the same level as pyrolysis with fresh ZSM-5.This indicates that, after the pyrolysis process, ZSM-5 needs to be regenerated or replaced with fresh catalyst, which would incur additional expenditure.HZSM-5 has a longer life cycle than ZSM-5 as it has the ability to reduce coke formation (Garforth et al., 1997).It is well known that the coke results in the blockage of catalysts' pores and, consequently, leads to their deactivation.HZSM-5 also has channel-like structures, which makes it less affected by the coke formed as compared to various zeolite-based catalysts (Schirmer et al., 2001).Bulky char molecules can also enter pores deeply to become trapped.HZSM-5 is able to maintain its activity longer than zeolite catalysts with regard to carbon fouling (Castano et al., 2011).
There are a number of technologies that have the potential to improve the efficiency of HZSM-5.Desilication of HZSM-5 is a technology that has been developed to improve its catalytic performance.Desilicated HZSM-5 has larger pore sizes and greater acidity than raw HZSM-5, which decreases the pyrolysis temperature required and affects the production of high-value compounds (Rac et al., 2013).In Jung et al. (2021), pyrolysis of PS in the presence of desilicated HZSM-5 produced more high-value compounds (benzene, toluene, ethylbenzene, and xylene) than ZSM-5.Also, in Ma, Yu, Yan, et al. (2017), modified Fe/HZSM-5 and Ni/HZSM-5 catalysts showed greater catalytic performance than HZSM-5 in the pyrolysis of brominated high-impact polystyrene.The use of Fe-and Ni-modified HZSM-5 decreased the yield of oil from 64.4 wt% to 63.2 wt% and 61.2 wt%, respectively (corresponding to an increase in the yield of gas), compared to HZSM-5.Also, it was observed that, in the presence of the modified catalysts, plastic pyrolysis produces an oil that contains an increased proportion of singlering aromatic compounds.This suggests that the use of Fe-and Ni-modified catalysts promotes the production of lighter molecules via cracking.
There are various studies related to the co-pyrolysis of polymers with other materials of nonpolymer origin, from which there appeared to be a good potential to gain high-value products (Lin et al., 2020(Lin et al., , 2021;;Nandakumar et al., 2023;Sekyere et al., 2023;Zhao et al., 2020).For example, in the study by Lin et al. (2020), a mixture of waste corn stover and HDPE was used as a feedstock for catalytic pyrolysis where a higher yield of mono-aromatics were produced (>70%).Moreover, in the co-pyrolysis of cellulose and PE, HZSM-5 promoted the production of aromatic hydrocarbons (Zhao et al., 2020).

| Y zeolite
Y zeolite mainly favors the formation of liquid hydrocarbon fractions, which corresponds to the reduced production of gas product (Chen et al., 2021).Onwudili et al. (2019) in their study compared zeolite catalysts (FCC, Y zeolite, and ZSM-5) in the pyrolysis of a mixture of PE (62%) (HDPE 19% and LDPE 43%), PP (8%), PS (15%), and PET (15%) at 500 C and 600 C.This resulted in the pyrolysis of polymers in the presence of Y zeolite producing the highest yield of aromatic hydrocarbons in oil at both temperatures compared to other zeolite catalysts.The oil produced by pyrolysis at 600 C contains aromatics with more than 90 wt% benzene and toluene.This means that Y zeolite can be used in the pyrolysis of a mixture of polymers to gain high-quality oil.However, Y zeolite is deactivated faster than other zeolite catalysts due to coke deposition.Y zeolite has cages in its pore topology where coke is easily trapped, which speeds up the deactivation process.Deactivated Y zeolite contributes to the production of less oil, corresponding to an increase in the production of wax (Chen et al., 2021).This means that Y Zeolite needs to be regenerated or changed by fresh catalysts more often than other catalysts.
As other zeolite catalysts, Y zeolite can be modified to get a better catalytic activity.Akubo et al. (2019) modified Y zeolite by impregnating transition metals (Ni, Fe, Mo, Ga, Ru, and Co) into the catalyst at 1 and 5 wt% metal concentrations and used it in the two-stage pyrolysis of HDPE at 600 C. Loading of metals into the Y zeolite at 1 wt% metal concentrations slightly increased the yield of aromatic compounds of oils.The catalyst with 5 wt% metal concentrations showed different results.The Ni, Fe, and Ru-Y zeolite catalysts decreased the aromatic hydrocarbon content of the oil, while Mo and Co-Y zeolite catalysts significantly reduced the yield of aromatic compounds by 15%-30% in the oil and increased coke formation.It is known that the deposition of coke on the catalysts negatively affects their activity, which means that the metals, and the concentrations of such, loaded into Y zeolite catalyst need to be carefully selected and controlled as even this can reduce the catalytic performance of Y zeolite.

| MCM-41
MCM-41 is a new type of mesoporous zeolite catalyst that has a large pore size.Thus, its specific surface area can be up to 1000 m 2 /kg (Chi et al., 2018).Also, it is able to significantly reduce the activation of energy of polymers, and contributes to the gain in value-added products.In the pyrolysis of PP, adding MCM-41 catalyst reduced the activation energy from 191-276 to 58-104 kJ/mol, and increased low carbon olefins (C4-C8) corresponding to a decrease of C8 and higher olefins.Among the zeolite catalysts, MCM-41 and ZSM-5 can selected as catalysts for the polymer pyrolysis to obtain are most appropriate to gaining a high yield of oil (Ma, Yu, Wang, et al., 2017).However, MCM-41 has an advantage over ZSM-5 in that it promotes the production of oil containing more gasolinerange hydrocarbons.
The catalytic activity of MCM-41 is usually increased by loading transition metals into the catalyst (Pan et al., 2021).In Chi et al. (2018), aluminum was incorporated into MCM-41 and then used as a catalyst for the pyrolysis of PP.The Al-MCM-41 catalyst produced oil containing more olefins than oil received from the pyrolysis with unmodified MCM-41.This means that AL-MCM-41 allows for improved cracking activity of heavy molecules than MCM-41.In Ma, Yu, Yan, et al. (2017), Fe and Ni were impregnated into ZSM-5 and MCM-41, with four different catalysts so obtained (Fe-ZSM-5, Ni-ZSM-5, Fe-MCM-41, and Ni-MCM-41).Fe-ZSM-5 and Ni-ZSM-5 showed good catalytic activity, and oil yields were decreased to 63.2 and 61.2 wt%, respectively.Fe-MCM-41 and Ni-MCM-41, compared to ZSM-type catalysts, preserved the oil yields at 65.9 and 65.3 wt%, respectively, but those produced contained more single-ring aromatics.Fe/Ni-ZSM-4 catalysts promoted the formation of two-ring aromatics, compromising the formation of single-ring aromatics.This study also proved that the MCM-41 catalyst favors the production of value-added oil from polymer pyrolysis.

| Fluid catalytic cracking
To obtain an FCC catalyst, zeolite catalysts (Y zeolites as the major component) are combined with nonzeolite acid (alumina-silica) by binders (Degnan, 2000).FCC is one of the most widely used catalyst due to its catalytic activity.FCC, as with many other catalysts, decreases oil yield compared to noncatalytic pyrolysis, which corresponds to an increase in gas yield.Despite decreasing the oil yield, the oil that is obtained contains more valuable components.For example, adding an FCC catalyst with a catalyst-to-plastic ratio of 0.1 to the pyrolysis of PP plastic at 300 C decreased oil yield from 74 to 64.7 wt% and increased gas yield from 23.7 to 33.1 wt% (Aisien et al., 2021).Also, the calorific value of oil was 43.435 MJ/kg, which is in the range of gasoline, diesel, and kerosine.
FCC, as with other zeolite catalysts, has an issue related to its deactivation.Deactivation of FCC is reversible with regard to coke deposition on the catalyst, but irreversible with regard to contaminations come from polymer feedstock.Except for coke, deactivation can be caused by impurities contained in the FCC catalyst.For example, FCC can contain oxygen (0-2%), sulfur (0-7.5%), and nitrogen (0-0.4%), which usually cause poisoning of the catalyst (Cerqueira et al., 2008).Also, the thermal treatment of FCC to regenerate it can result in its dealumination, which disrupts the balance of aluminum-to-silica ratio.The dealumination issue of FCC can be resolved simply, by adding fresh FCC to deactivated FCC.

| Fly ash catalysts
Fly ash is the by-product of burning pulverized coal, and it can be found in almost all developed and developing countries (Ram & Masto, 2014).Nearly half of fly ash ends up in landfills.However, fly ash can be used as a catalyst due to its unique composition as it contains 80-90 wt% of SiO 2 , Al 2 O 3 , and Fe 2 O 3 (Gaurh & Pramanik, 2018).Fly ash cannot be sent directly to the pyrolysis process without pretreatment.To use it as a catalyst for the pyrolysis of polymers, it needs to be synthesized.Then, synthesized fly ash is calcined at a high temperature (600-900 C) to increase the surface area of the catalyst (Gaurh & Pramanik, 2018).In Gaurh and Pramanik (2018), untreated fly ash was determined to have a low surface area at 1.74 m 2 /g, while calcination of fly ash at 800 C resulted in a surface area of 310.4 m 2 /g.The analysis of particle size and surface morphology of catalysts showed that the calcined catalyst has spherical particles and pores, positively affecting the surface area (Gaurh & Pramanik, 2018).This means that the pore size and shape are important as they affect the surface area that is closely related to the catalytic performance of catalyst.
Calcined synthesized fly ash catalyst produced at 800 C was used as a catalyst in the pyrolysis of PE at 700 C, which doubled the valuable aromatic content, such as benzene, toluene, ethyl benzene, and xylene in oil compared to noncatalytic pyrolysis of PE (from 10.92% to 21%-22%; Gaurh & Pramanik, 2018).Singh, Ruj, et al. (2020) found that the oil produced by the pyrolysis of LDPE at 500 ± 30 C in the presence of fly ash catalyst was lighter in color than the oil received from pyrolysis in the absence of the catalyst.This means that the LDPE pyrolytic oil with the catalyst was less dense and had more light hydrocarbon molecules than the oil without catalyst.However, the oil produced with the fly ash catalyst had a smaller number of valuable components compared to the oil received from the pyrolysis of LDPE with a zeolite catalyst.

| Red mud
Red Mud is an industrial waste from alumina production through the Bayer process that is widely used in hydrogen production due to its principal component, Fe 2 O 3 ( Álvarez et al., 1999;Eamsiri et al., 1992;Llano et al., 1994).It also contains SiO 2 , Al 2 O 3 , or TiO 2 , which has a positive influence on MPW pyrolysis (L opez, De Marco, Caballero, Adrados, & Laresgoiti, 2011).The BET surface of the catalyst is about 20-30 m 2 /g (Sushil & Batra, 2008;White, 2006), and the pH is in a range between 10 and 12 (Pradhan et al., 1998).These parameters can be adjusted to improve the catalytic performance of Red Mud.For example, surface area can be increased by activated Red Mud treatment on an industrial scale (Pradhan et al., 1998).
There are very few studies related to MPW pyrolysis with a Red Mud catalyst.Rezvanipour et al. (2014) studied the pyrolysis of PS in a semi-batch unstirred stainless-steel reactor at 600 C with red mud.Overall, 90 wt% of products are oil there.de Marco et al. (2009) compared different catalysts (HZSM-5, Red Mud, and AlCl 3 ) based on their catalytic performance in the pyrolysis of a mixture of polymers at 500 C in a stainless-steel unstirred autoclave.Red Mud produced the maximum oil yield at around 65 wt% and minimum gas yield at around 30 wt%, while the other two catalysts produced around 50 wt% oil and 40 wt% gas.Based on these studies, it can be stated that Red Mud is a more suitable catalyst for deriving more liquid and fewer gas products from polymers.
There are other studies, that compared the catalytic performance of Red Mud to noncatalytic pyrolysis of polymers.Adrados et al. (2012) compared noncatalytic and catalytic pyrolysis (using Red Mud) of a mixture of plastics (40 wt% PE, 35 wt% PP, 18 wt% PS, 4 wt% PET, and 3 wt% PVC) in a stainless-steel unstirred reactor at 500 C.
Noncatalytic was found to produce 65.2 wt% oil and 34 wt% gas, whilst the pyrolysis in the presence of Red Mud gives 57 wt% oil and 42.4%wt gas.Yanik et al. (2001) and de Marco et al. (2009) observed the same tendency, proving the catalytic performance of Red Mud in the pyrolysis of polymers.

| Metal-based catalysts
Metal-based catalysts are usually used in a two-stage pyrolysis method when the production of gas is desirable instead of liquid (Acomb et al., 2016;He et al., 2021;Prabu & Chiang, 2022).In the first stage, the plastic feedstock is pyrolyzed at around 500-600 C to produce evaporated hydrocarbon molecules.Then, the gaseous products are pumped to the second reactor, where they are converted into hydrogen-rich gas and carbon nanotubes in the presence of metal-based catalysts at temperatures higher than 700 C.However, under the high temperature required for plastic pyrolysis, metalbased catalysts can potentially get deactivated due to the deposition of carbon on metal particles.For example, it was observed that Ni-and Co-based catalysts were deactivated when the pyrolysis temperature was higher than 650 C (Karimi et al., 2021;Prabu & Chiang, 2022;Wang, Jiang, et al., 2021).It is worth noting that more hydrogen production normally corresponds to increased production of carbon due to the decomposition of methane and other light hydrocarbon gases and the carbon can be deposited on the catalysts used (Acomb et al., 2016).
Metal-based catalysts are usually prepared by impregnating the metals (Ni, Fe, Co, etc.) onto catalyst supports such as zeolites (Li et al., 2023), Al 2 O 3 (Yang, Chuang, & Wey, 2015), MgO (Dong et al., 2022), and so forth.Bimetals, for example, Ni-Fe, can be impregnated onto the support material to improve the conversion process and the quality of products (Wang, Shen, et al., 2020).In the study by Chen, Zhang, et al. (2020), Ni and Fe were loaded onto MCM-41 resulting in increased acidity, which allowed multi-ring aromatics to be broken into single-ring compounds.It is worth noting that Ni promotes the production of gases, while Fe facilitates debromination (Fadillah et al., 2021).
The selectivity of metal-based catalysts affects the composition and yield of the gas and carbon nanotubes produced.Ni-based catalysts are widely used in plastic pyrolysis, and are particularly suitable for producing hydrogen (Acomb et al., 2016;Aupretre et al., 2002;Liu & He, 2012).Aside from Ni, there are other metal-based catalysts, namely, Fe, Co and Cu (Acomb et al., 2016).Co catalysts are as effective as Ni catalysts in producing hydrogen-rich gas from plastic waste.Hydrogen-rich gas usually contains hydrogen, methane, and carbon monoxide (Cai et al., 2021).Gas yield and its hydrogen content typically increase with increasing temperature (Al-Fatesh et al., 2023).For example, Al-Asadi and Miskolczi (2018) using Ni/zeolite catalysts, obtained around a 30% gas yield with 15%-25% hydrogen content at 600 C, while increasing temperature to the 900 C produced a 60%-70% gas yield with >25% hydrogen content.Also, metal-based catalysts can be selected to change the yield of carbon nanotubes and their quality (Acomb et al., 2016).The quality of carbon nanotubes depends on the plastic types used.Polyolefin plastics are preferred for producing purer and cleaner carbon nanotubes that have increased graphite structure content.Pyrolysis of polyolefin plastics favors the production of lighter hydrocarbons by random and beta scissions, for example, ethylene, propylene, and methane, which are easily absorbed onto the surface of metal catalysts (Hernadi et al., 2000).They are then broken down into hydrogen and carbon in the presence of metal-based catalysts, and the carbon dissociated is dissolved into the metal particles (Zhou et al., 2017).Finally, the carbon dissolved into the particles becomes saturated, and forms carbon nanotubes (Cai et al., 2021).An additional separation step is necessary to obtain pure hydrogen from the hydrogenrich gas.
There are many other factors that affect the catalytic performance of metal catalysts, such as acidity, calcination temperature, metal solubility for carbon, metal-support interaction, and so forth.(Acomb et al., 2016).It is well known that catalyst acidity has a positive impact on breaking hydrocarbon chains.However, the effect of the calcination temperature of catalysts or metal solubility for carbon are less discussed.Metal carbon solubility is important as it affects the interaction between catalyst particles and hydrocarbon molecules.A high metal-carbon solubility enhances the production of carbon nanotubes.Liu et al. (2013) compared Fe, Co, and Ni catalysts; of the three, Fe was found to demonstrate better carbon solubility and the production of higher-quality carbon nanotubes.In the study by Acomb et al. (2016), Fe and Ni catalysts were found to demonstrate greater carbon nanotube yields than Co and Cu catalysts due to their improved metal-support interaction.Too strong or weak a metal-support interaction can lead to issues about the formation of metal particles that are not easily detached from the catalyst's surface, reducing the formation of carbon nanotubes thereon.
Metal catalysts are typically calcinated prior to being fed into the reactor, and the calcination temperature affects the extent of the metal-support interaction (Acomb et al., 2016).Chai et al. (2007) applied different calcination temperatures to a Co catalyst which resulted in different levels of metal-support interaction.They also concluded that the extent of the metal-support interaction needed to be at an "intermediate" level to allow for the effective growth of carbon nanotubes.
Some studies also considered the effects of adding manganese content to metal-based catalysts for promoting the yield of carbon nanotubes (He et al., 2021;Liu et al., 2013).For example, in the study by He et al. (2021), the addition of 10% manganese content to an Fe-based catalyst increased the yield of carbon nanotubes from 23.4 to 32.9 wt%.

| MPW PYROLYSIS PRODUCT APPLICATIONS
Pyrolysis of MSPW produces oil, gas, and char (Benavides et al., 2017).The quality or distribution of products from the pyrolysis of plastic can be adjusted by the appropriate selection of a pyrolysis reactor, catalyst, and other parameters (Qureshi et al., 2020).For example, oil with similar quality to diesel can be received (Santaweesuk & Janyalertadun, 2017), but there are still certain limitations to completely replacing conventional diesel.The way in which pyrolysis product can be applied depends on their quality.In this section, the application of products received from the pyrolysis of MSPW will be discussed, and Figure 2 is used to help our understanding of such.
Oil and gas are defined as the main products of MSPW pyrolysis and the ways in which they can be applied play a key role in considering their cost benefits and environmental footprint.The oil produced from the pyrolysis of MSPW usually needs to be upgraded as it contains impurities, and its quality is usually lower than the quality of conventional gasoline and diesel (Haig et al., 2018).Also, the oil can go through chemical recycling to produce plastics (Jung et al., 2023).For example, the oil can be cracked to produce ethylene, which is then polymerized to produce LDPE pellets (Li et al., 2022).
The gas obtained from the pyrolysis of MSPW is usually combusted to generate power and heat energy (Haig et al., 2018;Kanattukara et al., 2023).The heat energy can be used for the thermal decomposition of feedstock in a pyrolysis reactor.However, pyrolysis gases can contain certain impurities, and consequently, an upgrade or cleaning process needs to be used to obtain high-quality gas (Huang et al., 2022).Another way to obtain upcycled and more valuable products from pyrolysis gas is the synthesis of carbon nanotubes and hydrogen-rich gas from pyrolysis gas (Jiang et al., 2022;Wang et al., 2023).
Among all three products of pyrolysis of MSPW, the char was usually disposed of or landfilled.However, increasing studies show that char can act as a resource that allows for power and heat energy generation due to its high carbon, hydrogen, and nitrogen content (Jamradloedluk & Lertsatitthanakorn, 2014).It is preferable to use the char produced from lower-temperature pyrolysis as its carbon, hydrogen, and nitrogen content is generally higher than the char received at higher-temperature pyrolysis.The char received from high-temperature pyrolysis is good for use as an adsorption material.Also, char can be used as a soil conditioner to adjust soil characteristics such as moisture content, acidity, and so on.However, based on our best knowledge, there have not been any studies to define concerns regarding F I G U R E 2 Applications of products received from the pyrolysis of MSPW.
the use of the char derived from the plastic pyrolysis as a soil conditioner.It is worth noting that many new advanced technologies have been developed to produce char-based sensors, supercapacitors, construction materials, and so forth.
In this section, the potential applications of the products received from the pyrolysis of polymers will be discussed.
The quality of oil, gas, and char produced and their quantities from pyrolysis are usually assessed based on their relationships with such parameters as temperature, heating rate, retention time, reactor type, and so forth (Guedes et al., 2018).As shown in Table 1, the catalysts that can be used in the pyrolysis of MPW can be categorized into three groups, namely oil product favored that contains low content of lighter hydrocarbons (ash, clay, and red mud catalysts), oil product favored that contains high content of lighter hydrocarbons (zeolite catalysts), and gas product favored that contains hydrogen content catalysts (metal-based catalysts).In Table 2, product yields and their characteristics received from noncatalytic and catalytic plastic pyrolysis with the above-mentioned three groups of catalysts can be seen.Pyrolysis processes with the same catalysts can have different yields of products with different compositions as the other factors also affect them.Pyrolysis temperature and catalysts have a more significant impact on products than other factors (Miandad et al., 2016;Yansaneh & Zein, 2022).
A widely applied temperature for noncatalytic plastic pyrolysis is around 500 C, which normally favors a high yield of oil and biochar.The presence of catalysts that are oil product favored increases the yield of gas and oil with a higher content lighter hydrocarbons compared to noncatalytic pyrolysis.It is worth noting that the zeolite catalysts produce the oil with more gasoline (C4-C12)-and diesel (C9-C20)-range hydrocarbons than noncatalytic pyrolysis (Almeida & Marques, 2016;Wexler et al., 2005).It is desirable to use catalysts that are favored to produce oil with high content of lighter hydrocarbons which can be mixed with conventional gasoline or diesel fuels after upgrading processes.
T A B L E 1 Main catalysts used in MPW pyrolysis.

Product(s) favored
Catalyst Key findings They are preferable to produce an increased amount of oil.

Y Zeolite
• Deactivated faster than other zeolite catalysts due to active coke formation (Onwudili et al., 2019).• Oil produced contains increased proportions of aromatics such as benzene and toluene (Onwudili et al., 2019).
They are preferable to produce an increased amount of gas.

Metal-based catalysts
• Ni-based catalysts are common metal-based catalysts to produce hydrogen-rich gas due to their high catalytic performance compared to Fe, Co, and Cu-based catalysts (Acomb et al., 2016;Aupretre et al., 2002).• Main issue being deactivation due to coke deposition (Karimi et al., 2021;Prabu & Chiang, 2022;Wang et al., 2023).• Fe-based catalysts produce higher yields and quality of carbon nanotubes than other widely used metal catalysts (e.g., Ni, Co, and Cu;Acomb et al., 2016;Liu et al., 2013).• Polyolefin plastics are preferred to produce purer and cleaner carbon nanotubes and higher yields of hydrogen (Hernadi et al., 2000).
T A B L E 2 Product yields and features of noncatalytic and catalytic plastic pyrolysis.A two-step pyrolysis of plastics in the presence of a catalyst with metal-based catalysts at a temperature of 500-800 C has been designed for high yields of gas with a high hydrogen content (Cai et al., 2021;Li et al., 2023).Moreover, around 80% of the oil produced is in the range of C6-C16 hydrocarbons.Despite catalysts with high catalytic performance producing oil with a high content of diesel-or gasoline-range hydrocarbons, its main product is hydrogen-rich gas.Waste-to-hydrogen systems are considered a potential contribution to zero-emission transport and could be techno-economically feasible with reasonable governmental support (Lui et al., 2022).

Reference
The use of char depends on its quality, and catalytic performance plays a crucial role in improving it.Noncatalytic or catalytic pyrolysis in the presence of catalysts such as zeolites, ash, clay, and red mud catalysts, produce char which can be combusted due to its high carbon content, around the same as coal (Haig et al., 2018).Char derived from MPW also contains H, N, and Cl, and Ultimate or proximate analysis is usually applied to define the composition of char and its energy value (Saptoadi et al., 2016).Char derived from plastic pyrolysis needs to be upgraded as volatile matters such as benzene, toluene, ethyl benzene, xylene (BTEX) and alkyl phenol compounds might be captured into the char (Belbessai et al., 2022).Also, the char may contain heavy metals (Cd, Pb, Zn, Cu, Hg, and As), and it is typically demineralized by using an HCl solvent (Belbessai et al., 2022).
MPW pyrolysis in the presence of metal-based catalysts can produce a solid product with a high carbon nanotube content which can be used to detect pancreas and liver cancer due to its excellent properties such as biocompatibility, and thermodynamic and optical features (Ahmadian et al., 2022), and in the production of solar cells (Muchuweni et al., 2022) and electronic items (Azara et al., 2022;Jain et al., 2022).More complex analysis including scanning electron microscopy, transmission electron microscopy (TEM), fast Fourier transform of high-resolution TEM, Raman spectroscopy, reflectance, and thermogravimetric analysis for the carbon nanotubes produced from MPW pyrolysis is necessary to define its quality (Lehman et al., 2011).The structure of the carbon deposited on metal-based catalysts can be filamentous and amorphous depending on pyrolysis temperature, catalyst, and plastic composition (Wang, Xian, et al., 2020).Also, other parameters such as carbon nanotube size and diameter, the number of layers they form, and degree of graphitization can be used as metrics of the quality of the associated nanotubes.The nanotubes produced will need to undergo further upgrading processes due to their impurities, morphological defects, and so forth, which are additional parameters that can be used to define the quality of nanotubes.The purpose of use of upgraded carbon nanotubes depends on their characteristics such as thermal conductivity, electrical resistivity, and tensile strength and modulus (Azara et al., 2022).

| Oil
Liquid oil and wax can be stored more effectively than other energy resources, which gives the freedom to use it as an energy resource (Ikäheimo et al., 2019).Also, it has a higher heating value than other fuels, at 42.1-49.4MJ/kg (Kunwar et al., 2016).The main application of oil produced from the pyrolysis of polymers is as a fuel for engines, which is usually blended with diesel and gasoline.In general, the oil contains important valuable aromatics such as benzene, toluene, xylene, and styrene (Kumagai & Yoshioka, 2016), which positively affect the performance of such engines.The oil received from the pyrolysis of polymers has almost the same properties as diesel produced from petrochemical feedstocks (Miskolczi et al., 2009), but additional steps are necessary to upgrade it (Das & Tiwari, 2018;Joo & Guin, 1998) to meet the requirements of automotive diesel standards, for example, EN 590:2014 + A1:2017 (Deutsches Institut für Normung (DIN), 2014).The oil from the pyrolysis of polymers has a high paraffin content and the same boiling point range as conventional diesel, making its characteristics close to the diesel (Gala et al., 2020).Various studies have observed ways to positively affect the quality of the oil.The distillation process converts heavy oil from the pyrolysis of polymers into light oil that is rich in aromatic and alkanes, the quality of which resembles gasoline and diesel-like products (Lopez-Urionabarrenechea et al., 2015).Adding more heat carriers to the pyrolysis reactor increases heat transfer between particles inside the reactor and results in the production of oil with a maximum proportion of the gasolinerange light carbon fractions C 5 -C 12 (Zhang et al., 2020).Overall, the oil received from the pyrolysis of polymers has lower aromatic content than conventional diesel and, consequently, the oil blended with diesel can be used as a fuel for engines, not alone (Mangesh et al., 2020).Furthermore, density, distillation curve, kinematic viscosity, flash point, and cold filter plugging point of pyrolysis oil are not within the frame of conventional diesel (Gala et al., 2020).However, Gala et al. (2020) demonstrated that diesel blended with polymer pyrolysis oil in a 50%:50% ratio is able to meet the 21 requirements of the automotive diesel fuel standard, EN 590:2014 + A1:2017.Despite this, studies have shown the possibility of the use of diesel blended with the oil produced from the pyrolysis of polymers, but there are still limitations to blend the oil at a high ratio with diesel.Mangesh et al. (2020) studied how suitable the oils received from the pyrolysis of different polymers (HDPE, LDPE, PP, and styrene) were for use in blending with diesel.The study showed that the oil from the pyrolysis of PP was the most suitable and this particular oil contained the largest proportion of low carbon fractions and had the lowest viscosity compared to all oils received from pyrolysis of HDPE, LDPE, and styrene.Thus, blending the oil from the pyrolysis of PP with diesel was preferable for use with diesel engines than other polymer oils (Mangesh et al., 2020).The peak cylinder pressures of the engine worked on the mixture of the oil from the pyrolysis of PP and diesel with ratios 5%/95%, 10%/90%, and 15%/85% are 3%, 5%, and 7% higher than using conventional diesel alone, respectively (Mangesh et al., 2020).This means that adding PP oil to conventional diesel increases diesel engine performance.In practice, MSPW fed to a pyrolysis reactor mainly contains a mixture of PP, PS, PE, PET, and PVC, and consequently, producing ideal pure oil to blend with diesel is quite a challenging task.
Another main challenge of the use of the oil as a fuel for engines is regarding the associated environmental impacts and operational issues.One of the main components in the oil received from the pyrolysis of polymers is low-sulfur diesel and low-sulfur naphtha and, thus, it could reduce GHG emissions of engines by 1%-14% by replacing conventional diesel (Benavides et al., 2017).However, some studies found that the use of oil from the pyrolysis of polymers blended with diesel actually results in increased harm to the environment.Kalargaris et al. (2017a) showed that the emissions of NO X , unburned hydrocarbon, CO, and CO 2 were increased with increasing oil share in the fuel used for engines.Also, the use of blending pyrolysis oil and diesel cannot fully provide for engine load in the same that conventional diesel does.The long-term operational effect of the engine working on a blend of 75% pyrolysis oil received from plastics and 25% diesel has been tested (Kalargaris et al., 2017b).After 36 working hours, the operation of the engine failed and a piston cracked due to a deposit on the piston from excessive emissions (Kalargaris et al., 2017b).Kalargaris et al. (2017a) suggested that blending plastic pyrolysis oil and diesel with a maximum ratio of 60%-70%/40%-30% has the potential to reach 80%-90% engine load under optimum engine performance.However, Panda et al. (2016) found that blending the oil from the pyrolysis of plastics with diesel in ratio of 30%/70% has the potential to improve engine performance.They also concluded that blending the oil from the pyrolysis of plastics with diesel at a ratio of 50%/50% represents the upper limit at which the engine would be able to work, and that further studies in this regard are desperately needed.
Also, plastic pyrolysis oil could be used as a feedstock for the production of plastic (Jeswani et al., 2021).For example, the pyrolysis oil goes through the cracking process to produce ethylene, which is then polymerized to produce LDPE (Jeswani et al., 2021).Jeswani et al. (2021) conducted the life cycle assessment (LCA) study to compare the environmental footprint of the chemical recycling of plastic waste (the pyrolysis of plastic waste to receive oil and synthesizing it to produce plastic pellets), energy recovery technology (incineration), and RDF (refuse-derived fuel technology).They found that for plastics, the chemical recycling of plastic waste has obvious advantages with regard to a number of parameters over the other two technologies (Jeswani et al., 2021).The climate change impact of chemical recycling is the lowest at 736 kg CO 2 -eq/t or 1162 kg CO 2 -eq/t with system credit, while the climate change impacts for energy recovery and RDF are three times greater than pyrolysis without credit (Jeswani et al., 2021).

| Gas
Gas can be a resource for power and heat generation, or otherwise, some part of it can be returned to the pyrolysis system to produce the electricity and heating energy necessary for the pyrolysis of the polymers (Benavides et al., 2017).The calorific value of gas is relatively higher than other fuels, for example, Kumagai and Yoshioka (2016) noted it was 50 MJ/kg.The most ideal polymers for the production of high-quality gas with high calorific value are PE and PP (Honus, Kumagai, Fedorko, et al., 2018).The flammability limits of the gas produced from the pyrolysis of PE, PP, PS, and PVC are the same as the flammability limit of natural gas; PET, however, has high upper flammability limits (Honus, Kumagai, Moln ar, et al., 2018).
Recently, technologies have been developed to receive nanomaterials with superior characteristics and hydrogen-rich gas from gas produced from plastic pyrolysis.This usually has two stages: (1) pyrolysis of polymers to produce oil, char, and syngas; and (2) reforming produced syngas to hydrogen-rich gas and carbon nanotubes (Williams, 2020).The application of hydrogen is wide-ranging from producing ammonia for fertilizer to hydrogen fuel for vehicles (Williams, 2020).Currently, around 96% of world hydrogen production comes from fossil fuels, especially from natural gases (Williams, 2020).Hydrogen production from syngas received pyrolysis of polymers has the potential to partly replace hydrogen production from natural gas.The main factors greatly affecting the production of hydrogen from pyrolysis gas are feedstock, temperature, steam, and catalyst of pyrolysis (Williams, 2020).Increasing the temperature in the pyrolysis of polymers increases the gas production yield, an increase that is correlated to hydrogen-rich gas production (Williams, 2020).Adding catalysts to the pyrolysis of polymers promotes increased gas yield and decreased formation of oil and, thus, the impacts of catalysis are to increase the volume of hydrogen production (Williams, 2020).
Also, carbon nanotubes can be integrated into other materials to improve their properties (Borsodi et al., 2016;Wu et al., 2016).Wu et al. (2016) co-produced hydrogen and high-value carbon nanotubes from incondensable gases received from the pyrolysis of PP.Syngas produced from pyrolysis undergoes the steam-reforming process in the presence of the catalyst, NiMnAl, after which carbon nanotubes are recovered from the catalyst (Wu et al., 2016).Then, carbon nanotubes from the pyrolysis-catalytic steam reforming of waste plastic are added to LDPE in a ratio of 2 wt%/98 wt% (Wu et al., 2016).This results in an increase of the tensile and flexural strength of LDPE by 15%-19%, and Charpy impact strength by 10% (Wu et al., 2016).The same trend was detected in another study conducted by Borsodi et al. (2016), where 0.5 wt% carbon nanotubes added to 99.5 wt% of LDPE increase its Charpy impact and tensile strength.
The yield of hydrogen and quality of carbon nanotubes can be affected by the catalyst composition (Yao et al., 2018), pyrolysis temperature (Bajad et al., 2017), and feedstock composition (Bajad et al., 2017).For example, Yao et al. (2018) tested various different catalyst types (Ni/γ-Al 2 O 3 , Ni/α-Al 2 O 3 , Fe/γ-Al 2 O 3 , Fe/α-Al 2 O 3 , and Ni-Fe/γ-Al 2 O 3 ) to produce hydrogen and carbon nanotubes.They found that catalysts with Fe are more favorable than those with Ni due to higher hydrogen yield and higher purity and graphitization of carbons (Yao et al., 2018).Ahamed et al. (2020) tested Fe (sol-gel), Ni (SG), FeNi (sol-gel), Fe (impregnation), Ni (impregnation), and FeNi (impregnation) as catalysts to gain a high production of pure hydrogen and carbon nanotubes.The catalyst, FeNi (SG) shows a relative optimal yield in terms of hydrogen production at 25.14 mg/g plastic and carbon nanotubes at 360 mg/g plastic .Both studies show that catalysts with Fe are optimal for the production of hydrogen-rich syngas and carbon nanotubes.Bajad et al. (2017) tested the effect of temperature of pyrolysis and synthesizing process on the associated carbon nanotube yield; their maximum yield at 6.033 g/30 g PE was achieved at a pyrolysis temperature of 700 C and synthesizing temperature of carbon nanotubes at 800 C (Bajad et al., 2017).
Some polymers, which are good feedstock for pyrolysis to produce fuels can be less favorable for producing carbon nanotubes.Synthesizing the incondensable gas received from the pyrolysis of certain plastics, such as PS and PET cannot form carbon (Bajad et al., 2017).This means that these plastics are not suitable for the production of carbon nanotubes.Carbon formation from the incondensable gas received from the pyrolysis of LDPE and PP is detected (Bajad et al., 2017).However, some studies claim that PS and PET can be used as a feedstock for synthesizing carbon nanotubes if the entire gas received from the pyrolysis of plastics is directly pumped into the reactor for synthesizing carbon nanotubes without separation of condensable gas from incondensable gas (Acomb et al., 2014;Zhuo et al., 2012).
It is well-known that the presence of PVC in polymer feedstock for the pyrolysis has adverse impacts, for example, corroding of metals and halogenation of oil (Qureshi et al., 2020).Furthermore, it has been found that MSPW composed of PVC has a negative effect on the formation of carbon nanotubes (Borsodi et al., 2016).Borsodi et al. (2016) studied the conversion efficiency of gas received from the pyrolysis of different polymer feedstocks to carbon nanotubes.The study showed that 1% PVC and 99% HDPE feedstock has the lowest conversion efficiency at 36.5% compared to other polymers, 100% HDPE at 47.1%, 50% HDPE+50% PP (waste) at 48.1%, and MPW at 60.7%.This means that even a small amount of PVC in polymer feedstock can negatively affect the production of carbon nanotubes.
The LCA study of the production of carbon nanotubes from pyrolysis shows that it has good potential to reduce environmental impact (Ahamed et al., 2020;Trompeta et al., 2016).However, the full scale of the environmental footprint of the production of carbon nanotubes from gas received from the pyrolysis of plastics has not been defined due to limitations to data gathering from industry and carbon nanotube production plants (Ahamed et al., 2020).Further LCA studies are necessary to fill this gap.The energy demand for a larger-scale plant could be lower than a lab-scale one, and it could positively affect general economic viability and environmental footprint (Gavankar et al., 2015).Gavankar et al. (2015) conducted a cradle-to-gate LCA of the production of carbon nanotubes, concluding that scaling up the production of carbon nanotubes can achieve an 84%-94% reduction.Nevertheless, the technologies used to produce carbon nanotubes from pyrolysis gas need considerable further study to reduce production costs and consider environmental, economic, and social impacts.

| Char
There are a few ways that the char derived from the pyrolysis of polymers can be used, for example, in the production of activated carbon.Also, it can be used as fuel to generate energy due to its high carbon and hydrogen content (Jamradloedluk & Lertsatitthanakorn, 2014).How the char might be used mainly depends on its properties (e.g., composition, porosity, etc.), which can be adjusted via the pyrolysis parameters.In this subsection, the applications of char received from the pyrolysis of MSPW will be discussed.
The char defined as a by-product of the pyrolysis of polymers is mainly disposed of in unsustainable ways, for example, the most common way is through landfill (Benavides et al., 2017).The pyrolysis of certain polymers together can be positively correlated with the production of char.The pyrolysis mixture of polymers PP/PE/PS with PVC, for instance, increases the production of char (Yu et al., 2016).Most residues are derived from the pyrolysis of PET as cracking it produces an increased amount of polyaromatics, which is considered as a precursor of char formation (Singh, Verma, & Singh, 2020).This means that the pyrolysis of polymer feedstock that contains high proportions of PET produces more char and less oil and gas.
It is worth noting that the highest yield of char with high carbon, hydrogen, and nitrogen content is gained at lower pyrolysis temperature conditions (Martín-Lara et al., 2021).The yield of char negatively correlates with increasing pyrolysis temperature and a high temperature in the reactor produces char with higher ash and lower volatile content (Martín-Lara et al., 2021).Volatilization reactions become predominant with increasing temperature, and thus char gets less volatile content and the yield of oil is increased (Liu et al., 2016).Martín-Lara et al. (2021) investigated yields of char received from the pyrolysis of polymer mixtures at 450 C, 500 C, and 550 C, finding that the highest yield of char was received at the lowest temperature.The same tendency was observed in another study, whereas the temperature was increased from 400 C to 700 C, the yield of char reduced from 49.8% to 32.3% (Buah et al., 2007).Also, the higher heating value of char reduces with increasing pyrolysis temperature.In Martín-Lara et al. (2021) also found the higher heating value is reduced from 23.94 MJ/kg at 400 C to 3.69 MJ/kg at 550 C.This could be explained by the loss carbon, hydrogen, and nitrogen with volatile matters, which are the main elements of the source of energy (Tian et al., 2021).Buah et al. (2007) also showed that proportions of C/H/N in char were decreased from 55.13%/4.33%/1.02%at 400 C to 49.91%/0.78%/0.93% at 700 C. it has been further demonstrated by a number of studies the functional group (C, H, N) of char to be used as a fuel is decreased with increasing pyrolysis temperature (Cafiero et al., 2015;Jamradloedluk & Lertsatitthanakorn, 2014;L opez et al., 2010).To summarize, the char received from the pyrolysis of polymers at a lower temperature is more preferable for use as a fuel rather than high-temperature pyrolysis because of its higher carbon, hydrogen, and nitrogen content, and higher heating value.For example, Jamradloedluk and Lertsatitthanakorn (2014) noted that char was powdered and then briquetted for use as a combustion fuel.
The structure of char has external macroscopic morphology and internal microstructural morphology.The former is more important for the absorption ability of the char as it defines the effectiveness of the diffusion and reaction processes of the adsorbent (Qin et al., 2020).Pyrolysis char has an ideal surface area or porosity for use as a heavy metal adsorbent (Martín-Lara et al., 2021).Pyrolysis temperature has a significant influence on the structure of char and, consequently, its absorption properties (Singh, Kumar, et al., 2020).For example, the char received from the pyrolysis of polymers at 550 C had a greater ability to remove arsenic from aqueous media compared to the char produced from the pyrolysis of polymers at 450 C (Singh, Ruj, et al., 2020).Martín-Lara et al. (2021) tested the lead adsorption of char produced at different temperatures (450, 500, and 500 C), showing that the lead removal proportions are 37.79%, 44.41%, and 73.48%, respectively.This kind of tendency has been observed in a number of studies, namely that char produced by higher temperature pyrolysis is more favorable for heavy metal adsorption compared to that produced from lowtemperature pyrolysis (Fang et al., 2016;Singh, Verma, & Singh, 2020).
There are also other certain that are considered to influence the absorption properties of char, namely feedstock material for pyrolysis, a dose of char on the sorption process, pH, and contact time (Singh, Verma, & Singh, 2020).The absorption yield of char could be further improved by the pyrolysis of a mix of different feedstocks rather than individual ones (Bernardo et al., 2013).Bernardo et al. (2013) studied the ability of char received from the co-pyrolysis of pine, used tires, and plastic wastes to remove lead from the aqueous media, showing that the co-pyrolysis of a mix of different feedstocks results in better absorption performance with regard to lead than the char received from the pyrolysis of these same materials individually.
The char from the pyrolysis of plastic can be used as a soil amendment (Manyà, 2012).There are multiple associated benefits related to the environment, economy, and society.The use of char as soil remediation contributes to battling against climate change because the char acts as a means of sequestration of GHG in the soil (Lehmann et al., 2006).In general, char is alkaline in nature and thus, plays a positive role in retaining nutrients in the soil (Carter et al., 2013;Lehmann et al., 2006).Furthermore, it can be effective at increasing soil pH, making it less acidic, and consequently, metal ion mobility is increased (Chen et al., 2011).For instance, Kumar et al. (2021), found that the pH of char received from the pyrolysis of PVC, PE, and PET is equal to 10.07, confirming its alkaline nature.Some heavy metals in the soil such as Cr, Ni, Pb, Cr, B, Co, and Ba can be neutralized with the action of char, where they are adsorbed (Kumar et al., 2021).Ghosh et al. (2012) studied the effects of char on the carbon content and fertility of soil.Char was blended with soil in ratios, 25%/75%, 50%/50%, and 75%/25%.Blending char and soil in ratios 50%/50%, and 75%/25% resulted in a significant increase in carbon content and nutrients in the soil, such as nitrogen, phosphorus, potassium, calcium, and magnesium (Ghosh et al., 2012).
A comprehensive assessment of the addition of char received from the pyrolysis of PVC, PE, and PET to soil as an amendment shows positive influences on general soil quality and plant growth (Kumar et al., 2021).It is notable that char increases properties of soil like electrical conductivity, moisture content, and available phosphorus in the soil, whilst the bulk density of the soil is lowered (Kumar et al., 2021).The increasing electrical conductivity can be positively correlated with plant growth.Plant height is greater in soil with added char than soil without, which makes it an ideal soil amendment for the agricultural industry (Kumar et al., 2021).It unintentionally helps to combat problems such as food security, soil degradation, poverty, and so forth.There are many studies about the use of biochar as a soil amendment, but studies related to the use of char derived from the pyrolysis of MSPW for that purpose are scarce.More studies are needed to explore the influences of the use of char produced from MPW pyrolysis in greater depth.
Finally, there are many other ways that char can be applied, namely char-based sensors (Chaudhary et al., 2021;Spanu et al., 2020), supercapacitors (Pandey et al., 2021;Vivekanandhan, 2018), construction material (Kumar, Xiong, et al., 2020), and so forth.Char can be used in the production of electrochemical, photoluminescence-based optical, electrogenerated chemiluminescence, and humidity sensors (Spanu et al., 2020).These sensors are usually made from widely used synthetic carbonaceous nanomaterials, which can be replaced by sensors made from char due to environmental benefits.Also, In the construction industry, the use of char derived from plastic pyrolysis shows an excellent potential for use as an asphalt binder as the char increases the stiffness and elasticity of the asphalt binders (Kumar, Choudhary, & Kumar, 2020).
Activated carbon can be produced from the char received from the pyrolysis of MSPW, but there is a lack of appropriate commercially available products.Activated carbon is usually produced via the thermal treatment of char at higher temperatures (Jamradloedluk & Lertsatitthanakorn, 2014).For example, Jamradloedluk and Lertsatitthanakorn (2014) received activated carbon from the thermal treatment of char at 900 C for 3 h.

| CONCLUSIONS
The production of plastic materials and generation of MSPW has increased tremendously since the middle of the last century.It is predicted that, in the future, these tendencies will continuously accelerate, and the global plastic crisis will become greater.To solve this global issue, or at least mitigate it, mankind needs to take bold and urgent action to ensure a truly sustainable circular economy.The current technologies used in waste management such as landfilling, mechanical recycling, and incineration are ineffective due to the associated resource losses, environmental burdens, and linear economy principle that is dominated by the extract-use-dispose principle.Mechanical recycling cannot provide the same quality of recycled materials as those initially produced, and nor can it upcycle plastics to more valuable materials.Catalytic pyrolysis is considered a potential contributor to a more sustainable approach to combating the global plastic crisis.
In this review, the main steps in MPW pyrolysis are described, which consist of the following: • Feedstock preparation: MPW usually goes through a pretreatment process to receive clean and granulated feedstock for the pyrolysis process.First, MSW is collected and transported to the plant.MSW is composed of various waste types, for example, cartons, glass, plastics, etc. MSPW needs to be separated from MSW and then cleaned of the dirt and contamination that can directly affect the operational process and process product distribution.Also, if the PVC content in the MSPW feedstock is high, dichlorination is necessary to eliminate chlorine fractions.• Pyrolysis of MSPW in the reactor is the main step, which results in the production of oil, gas, and char.
• The products received from the pyrolysis of MSPW should go through purification and upgrading processes to remove impurities, especially oil.
Also, the plastics that comprise the catalysts for the pyrolysis of MSPW are described in this review.Zeolite catalysts are an ideal for the production of oil with high quality.The inexpensive catalysts, such as ash fly catalysts, red mud, or clay-based catalysts, have lower catalytic activity than zeolite catalysts and they usually promote the production of more oil and less gas.
In this review, practical applications of products received from the pyrolysis of MSPW, oil, gas, and char are discussed.The oil can be used as a fuel, which is usually blended with diesel and gasoline.However, it needs to go through an oil upgrading process to gain oil of an appropriate quality for blending with diesel or gasoline.Also, the oil can be used as a raw material for the polymerization of plastics, which is defined as a technology to upcycle materials.In the case of gas, it is usually burnt to produce power and heat energy.There is another advanced way to produce high-value products; the gas can go through a synthesizing process to produce carbon nanotubes and hydrogen-rich gas.The char is usually disposed of or landfilled in commercially available plants.There are many more sustainable ways to use the char.For example, char can be used as a fuel for power and heat energy generation due to its high carbon, hydrogen, and nitrogen content or as a soil amendment to improve soil quality.Also, it is an effective adsorption material to remove heavy and toxic metals.There are other innovative applications for char, though these need further improvement and development to become commercially available, for instance, char-based sensors, supercapacitors, construction materials, and so forth.
DATA AVAILABILITY STATEMENT Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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I G U R E 1 An illustration of pyrolysis-based plastic waste treatment.