A Survey on the Chemical Recycling of Polyolefins into Monomers

Abstract

The growing global concern over plastic waste accumulation has brought this issue to the forefront of environmental discussions. The increasing demand for plastic materials has led to the widespread production of plastic resins. However, the low cost of plastics, combined with high supply and consumption rates, has resulted in a troubling surge in post-consumer plastic waste. At the same time, the essential role plastics play in ensuring quality, convenience, and modern living has made them indispensable. In this context, the concept of circularity introduces a transformative shift in consumption habits, product design, and the management of raw materials and waste. A central strategy for promoting circularity in the plastics economy is the development of chemical recycling technologies. These processes aim to convert plastic waste into higher-value materials for the chemical industry, often generating liquid and gaseous products that can serve as feedstocks—ideally leading to the recovery of the original monomers. As polyolefins are the most widely used plastics worldwide, efficient recovery of their corresponding monomers is crucial to advancing circular strategies. This review explores current methods for the chemical depolymerization of polyolefins and critically analyzes efforts focused on the direct recovery of olefinic monomers.

1. Introduction

The global accumulation of plastic waste has become an increasingly pressing issue, driven by the significant growth in plastic consumption over the past 50 years due to the exceptional versatility of these materials [1,2,3]. In 2023, global plastic production surpassed 413.8 million tons, reflecting both the high demand and a continuing upward trend in production and consumption. In 2017, China was the world’s largest plastic producer, accounting for 29% of global output—a share that increased to 33.3% by 2023. In contrast, Europe, which held the second-largest share in 2017 (19%), saw its relative market participation decline to 12.3% by 2023 [4,5].

In parallel with increasing production, plastic waste is being generated at an even faster rate, as many plastic products are designed for short-term or single-use applications. Packaging alone accounts for 44% of global plastic production and becomes waste rapidly. As a result, plastic waste has emerged as a major environmental concern, particularly due to its frequent improper disposal. This leads to the widespread pollution of marine and freshwater ecosystems and contributes to the long-term occupation of landfill space, given plastics’ low degradability and low density [1,4,5].

Polyolefins—primarily polyethylene (PE) and polypropylene (PP)—are the most produced polymers globally, representing approximately 45.2% of the total plastic production [4]. Their success stems from their light weight, low cost, high malleability, ductility, and resistance to environmental degradation and various cleaning solvents [3,4,6]. Used extensively in packaging, PE and PP have experienced increased demand due to their durability and excellent barrier and optical properties. These materials protect goods during transport and allow for a visual inspection of packaged products, making them valuable in increasingly decentralized manufacturing and distribution systems [7].

According to Brazil’s National Solid Waste Policy (Law No. 12,305/2010), recycling is defined as a transformation process that modifies the physical, physicochemical, or biological properties of solid waste aiming to convert it into raw materials or new products, in accordance with the standards established by competent environmental and health authorities [8]. The characteristics of the waste material must be carefully considered when selecting a recycling technique, as different materials require different methods to maximize value recovery [9,10,11].

Global surveys indicate that only a small fraction of plastic waste is recycled. While 15% of plastic waste is collected for recycling, only 9% is effectively recycled, with 19% incinerated, 50% landfilled, and 22% escaping formal waste management systems. This mismanaged fraction is often openly burned or released into terrestrial and aquatic environments, particularly in low-income countries. Recycling rates in less developed regions tend to be even lower, underscoring the urgent need to establish a circular economy for plastics. Achieving this requires raising awareness among governments, producers, and consumers about the environmental consequences of improper plastic disposal [4,12].

Recycling processes are generally categorized as primary, secondary, tertiary, and quaternary [13]. Primary recycling refers to the in-plant reprocessing of clean manufacturing residues such as trimmings and defective parts, and is unsuitable for urban solid waste [14,15]. Secondary or mechanical recycling is more applicable to post-consumer waste and includes pre-treatment steps like sorting, grinding, washing, and drying [11,15,16]. It is the most widely used method globally due to its simplicity and low cost. However, not all plastic materials can be efficiently recycled by this method. Many plastics undergo degradation due to exposure to heat, light, or mechanical stress throughout their life cycle, which compromises their properties upon remelting. Additionally, contamination and improper sorting can make the process economically and environmentally unfeasible [14,15].

Tertiary recycling—also called chemical recycling—involves modifying plastic waste chemically to break it down into smaller molecules for reuse and constitutes the main subject of the review, as described in the subsequent sections [15]. Quaternary recycling, or energy recovery, incinerates plastic waste to generate energy but leads to the complete destruction of polymer structures and potential environmental and health impacts from toxic emissions. Although useful for reducing waste volume, quaternary recycling is controversial and should be minimized in favor of more sustainable options [11,15].

Chemical recycling has gained increasing prominence as a promising strategy for enabling a true circular economy in plastics. In this approach, plastic residues are broken down through various chemical reactions into smaller molecules that can be reintegrated into the chemical value chain. For instance, these molecules can serve as raw materials in processes typically associated with petroleum refining, where they are upgraded into higher-value compounds—including valuable monomers for the production of new polymers—thus potentially closing the plastic loop [14]. Chemical recycling is especially advantageous for processing mixed or contaminated plastic waste streams that cannot be efficiently treated by mechanical means [11,15]. Nevertheless, the success of chemical recycling depends on prior physical treatment steps such as sorting, cleaning, and shredding, which help prevent catalyst deactivation, equipment fouling, and the contamination of the final products. These preparatory steps are essential for achieving acceptable efficiency and product quality in chemical recycling processes [13,17,18].

This review focuses on chemical recycling strategies for PE and PP, including their commercial variants—HDPE, LDPE, and LLDPE. The incorporation of comonomers like butene, hexene, or octene into ethylene polymerization is standard practice for producing linear low-density polyethylene (LLDPE), which contains a high density of short-chain branches [19]. PP typically degrades at lower temperatures than PE due to its tertiary carbon atoms, which are more prone to chain scission [20]. While the thermal degradation mechanisms of different PE types are similar, LLDPE tends to degrade faster due to its branched structure, which increases reactivity, especially near reactor walls where localized overheating can accelerate reactions.

According to the literature, there are no significant differences in the thermal degradation mechanisms among the various PE types, although LLDPE generally degrades faster than HDPE. LLDPE tends to exhibit a higher reactivity due to its branched molecular structure, which increases its sensitivity to internal temperature gradients during thermal processes. This effect is particularly pronounced near reactor walls, where localized temperature elevations can accelerate reaction rates and lead to non-uniform degradation behavior. Although there are many studies dedicated to the recovery of ethylene and propylene, no studies were identified that specifically investigate the recovery of higher molar mass monomers, perhaps reflecting the lower commercial importance of these compounds [21].

A truly circular plastics economy requires, in principle, that residual plastics be efficiently converted back into their original monomer units—a goal that is not yet realized in practice. While polyolefins can be successfully transformed into gaseous and liquid products, these streams typically contain a wide variety of compounds that are predominantly used as fuels rather than as monomer precursors. Even when liquid fractions are employed as feedstocks in established cracking processes to produce monomers, it is estimated that less than 50% of the resulting naphtha pyrolysis oil is effectively reintegrated into the chemical production chain as monomers. Consequently, the chemical recycling of polyolefins does not inherently establish a circular loop, prompting considerable criticism from environmental groups. This highlights the urgent need to develop advanced recycling technologies that can significantly improve monomer recovery—specifically through plastic-to-monomer, or more precisely, polyolefin-to-monomer pathways [22].

According to the available literature, the chemical recycling of polyolefins into monomers is still at an early stage of development. In light of this, the present work conducts a comprehensive review of chemical recycling strategies applied to polyolefin waste, with a particular focus on approaches aimed at improving monomer recovery efficiency. These strategies are evaluated as promising candidates for advancing toward a truly circular polyolefin economy. In the following sections, various techniques are described and critically assessed with respect to the types of products generated and the effectiveness of monomer recovery—particularly ethylene and propylene.

To support this review, references were selected using a systematic methodology. Recent and comprehensive review articles were first identified for each technology discussed, including microwave-assisted pyrolysis, plasma-assisted pyrolysis, supercritical fluids, catalytic pyrolysis, oxidative and electrochemical degradation, ionic liquids, and tandem catalysis. From these sources, key experimental studies were selected based on their technical quality, relevance to monomer recovery from polyolefins, and the level of detail provided. This approach ensured a consistent and well-structured literature foundation for the critical analysis presented in this work.

2. Microwave-Assisted Pyrolysis

2.1. Fundamentals of Microwave-Assisted Pyrolysis

Microwaves are a type of electromagnetic radiation with characteristic frequencies ranging from 300 MHz to 300 GHz, corresponding to wavelengths between 1 and 300 mm. These waves are capable of inducing rapid and highly selective heating of materials through electromagnetic interaction, without requiring direct physical contact. Microwaves have been widely employed to reduce reaction times, potentially leading to significant cost savings. According to Putra et al. [16], the most commonly used commercial frequencies are 2.450 GHz (λ ≈ 12 cm), primarily associated with the excitation of water molecules and commonly used in household appliances, and 0.95 GHz (λ ≈ 33 cm), typically selected for its efficiency in signal transmission and applied at industrial scale and in mobile technologies. Like other forms of electromagnetic radiation, microwaves can be transmitted, absorbed, or reflected by materials depending on their dielectric properties [23,24].

Microwave-assisted pyrolysis has emerged as a promising alternative for plastic waste management due to its ability to deliver rapid, energy-efficient heating through in situ volumetric energy transfer. In this process, microwave radiation serves as the heat source for the thermal degradation of plastic waste, with the potential to maximize the production of light gaseous products—including monomers. Putra et al. [23] identified several factors that influence microwave-assisted pyrolysis, including the type of plastic used, the nature of microwave absorbents, reaction temperatures and times, and the presence of catalysts to promote conversion into light hydrocarbons [23,25].

In conventional heating processes, energy is transferred gradually from the outer surfaces of the reactor to its interior, where most of the material is located [23,25]. Since plastic mixtures are typically poor thermal conductors and often highly viscous under processing conditions, these systems face significant heat transfer limitations that can hinder performance [26]. In contrast, microwave heating enables a more efficient and direct energy delivery throughout the material volume. This occurs through the interaction between the polymer molecules and the electromagnetic field, allowing for a more uniform heating and increased internal temperatures relative to the surrounding environment [23,24,27].

One potential advantage of microwave-assisted pyrolysis lies in its applicability to recycled materials with large particle sizes and a high moisture content, without the need for costly pre-treatment steps such as drying, grinding, or size reduction. This can streamline the process, reduce operational complexity, and lower overall costs [23]. Additionally, the technique is capable of producing organic oils that are considered valuable chemical resources [24]. When microwave energy is applied in conjunction with an oxidizing agent or a catalyst, the conversion of plastics into higher-value chemicals can be significantly enhanced. Several studies have demonstrated the production of high-quality end products through the careful control of key process parameters—particularly when catalysts are employed. Among the most critical factors for achieving effective conversion are the operating temperature of the catalyst and the relative quantities of catalyst or oxidizing agents used with respect to the plastic feedstock [24].

On the other hand, several inherent limitations help explain why microwave-assisted pyrolysis has not yet achieved widespread commercial adoption. Firstly, the generation of microwaves and the associated emitting equipment can be prohibitively expensive—especially for large-scale reactors. To enable the use of smaller microwave devices while ensuring an effective exposure of the reacting materials, the emitters may need to be installed within the recycling streams themselves, a configuration that is often impractical or even unfeasible in industrial-scale operations [23].

Moreover, conventional temperature measurement instruments used in chemical processing are generally inadequate for this application, due to the localized and non-uniform heat dissipation characteristic of microwave heating. This makes accurate temperature control within the reactor particularly challenging [23]. Another critical limitation involves the frequency—or wavelength—of the microwaves, which must be well matched to the dielectric properties of the specific plastic being treated. Because these properties vary significantly among different polymers and waste mixtures, a single microwave system may not be universally effective.

As a result, microwave systems may need to be tailored for each plastic type, which is commercially impractical. Alternatively, microwave susceptors—materials such as carbon or silicon carbide that efficiently absorb microwave energy and convert it to heat—must be added to the plastic feedstock [23,24,25]. While this approach enhances heating, it also undermines some of the key benefits of microwave irradiation by reintroducing conventional heat transfer limitations. Furthermore, the recovery and separation of susceptors from the solid residue adds complexity to the process. As is evident from these challenges, the technology still requires significant development before it can be scaled up for industrial implementation.

Figure 1 illustrates a typical experimental setup used in microwave-assisted catalytic pyrolysis.

2.2. Previous Works of Microwave-Assisted Pyrolysis

Undri et al. [28] published an article about the chemical recycling of high-density polyethylene (HDPE) and PP using microwave-assisted pyrolysis. In this case, the authors opted to add tire scraps and carbon to the polymer waste as absorbents. The main goal of the authors was to obtain a liquid mixture of linear hydrocarbons for posterior processing or use as fuel, not necessarily monomers. Indeed, HDPE conversion resulted in linear alkanes and 1-alkenes, while PP conversion resulted mainly in a mixture of methyl-branched alkanes and alkenes [28]. All experiments were conducted in batch in the presence of absorbents, with a power input ranging between 1.2 and 6 kW, polymeric load of 120 g (therefore, 10 to 50 kW/kg), residence times from 35 to 260 min, and temperatures between 700 and 900 K. Under these conditions, HDPE was partially decomposed, while PP samples were completely decomposed. At the end of the process, the most predominant fractions were the liquid and solid streams, with average yield values of 57 wt% and 23 wt%, respectively. The average yield of the obtained gaseous products was equal to 20 wt%; however, as this fraction was not analyzed, it was not possible to assert whether monomers had been recovered. It was concluded that waste HDPE and PP could be recycled with the microwave technique, making possible the generation of high-quality liquids for use as fuels, not monomer production [28].

Zhang et al. [27] studied the microwave-assisted pyrolysis of low-density polyethylene (LDPE) using the zeolite ZSM-5 as a catalyst, seeking to maximize the conversion of plastic into liquid yields (mainly gasoline, not monomers). Operation parameters, such as the catalytic reaction temperature (300 to 500 °C) and the plastic/catalyst ratio (1 to 4.68 in weight), were manipulated in order to increase the resulting liquid fraction [27]. It must be observed that the microwave-assisted reaction system and the catalytic bed were arranged in two distinct stages, so that the catalyst bed was not irradiated with the electromagnetic waves. Particularly, the temperature of the microwave-assisted pyrolysis reaction was kept equal to 480 °C in all experiments, while the residence time was kept equal to 10 min [27]. The final products comprised averages of 27 wt% of liquids, 69 wt% of gases, 2 wt% of char, and 1 wt% of coke. The best operation condition selected by the authors to operate the catalytic reaction included a temperature of 450 °C and plastic/catalyst ratio of 2, resulting in about 34 wt% of liquids [27]. The gaseous stream contained hydrogen (>30 wt%), methane, ethane (>55 wt%), ethylene, and smaller amounts of other components in the absence of a catalyst, while the major gaseous products obtained in the presence of the catalyst were ethylene (55–70 wt%) and ethane (20–35 wt%) [27], allowing for the recovery of about 35 wt% of the monomer ethylene.

Suriapparao et al. [25] investigated the influence of operation parameters on the microwave-assisted pyrolysis of PP in the presence of a susceptor, including graphite, aluminum, silicon carbide, activated carbon, lignin, and fly ash. In the experiments, in addition, the power input was varied between 180 and 800 W using domestic microwave equipment, while the polymer mass ranged from 5 to 50 g, (3.6 to 160 kW/kg) and the polymer/susceptor ratio ranged from 5:1 to 100:1 wt/wt. Tests were also conducted for LDPE, LLDPE, and polyisoprene (PIP). The authors reported in particular that the susceptors also acted as catalysts for the reactions; however, the main pursued goal was the maximization of the liquid products, not the monomers. The obtained gas fraction contained maximum amounts of C₃, C₄, C₅, and C₆ of 12, 34, 44, and 26 wt%, respectively [25].

Jing et al. [29] studied the microwave-assisted co-pyrolysis of bamboo and PP using the zeolite HZSM-5 as a catalyst to produce bio-oil (not monomers). The experimental conditions (catalytic temperature, feedstock/catalyst ratio, and bamboo/PP ratio) were varied to maximize the yield of the liquid fraction, which was around 62 wt%. According to the obtained results, the reaction temperature of 350 °C, ratio of bamboo/PP of 1:1, and feedstock/HZSM-5 ratio of 2:1 was the operation condition that resulted in the highest gaseous fraction of 65 wt%, although the detailed compositions were not presented. According to the authors, increasing the amount of PP in the feed caused the increase of aromatic and naphthenic contents in the bio-oil [29].

Suriapparao et al. [30] studied the microwave-assisted pyrolysis of mixtures of algae biomass with PP, PE, and expanded PS separately. Experiments were performed in the presence and absence of ZSM-5 as a catalyst and the char obtained as a pyrolysis product was also used as a susceptor in all experiments. The authors aimed at enhancing the quality of the produced liquid oil (not producing monomers), although the amounts of the gaseous fraction were close to 55 wt% in the catalytic co-pyrolysis [30]. Despite that, the detailed compositions of the obtained gas fractions were not reported. It was observed that the use of the catalyst increased the gas and coke fractions of the final products [30].

Chen et al. [31] also used ZSM-5 to perform the microwave-assisted pyrolysis of LDPE at 450 °C. In this case, the employed catalyst was a core-shell SiC foam zeolite, which acted simultaneously as a microwave absorber and a catalyst, added in the plastic mixture. This configuration effectively allowed the heat generation directly onto the ZSM-5 zeolite shell, facilitating the catalytic reaction. The authors investigated the effects of the SiO₂/Al₂O₃ ratio and alkaline treatment on the performances of the SiC foam. The findings revealed that the zeolite acidity and pore structure played vital roles in enhancing the quality of both gas and liquid products during the upgrading process. The authors hypothesized that the introduction of a mesoporous structure into the ZSM-5 zeolite through alkaline treatment could enhance the diffusion of larger molecules and products, enhancing the selectivity towards high-valued light olefins and aromatics, while simultaneously suppressing the formation of undesirable alkanes [31]. The concentration of gaseous compounds (H₂, C₁–C₅) in the product was higher than 70 wt% regardless of the SiO₂/Al₂O₃ fraction. Nevertheless, the recovery of monomers was always smaller than 30 wt% [31].

Cao et al. [32] proposed the use of MAX (Ti₃AlC₂) as a susceptor and catalyst for the microwave-assisted pyrolysis of LDPE, HDPE, and PP aiming to produce hydrogen and nanofibers for Na-ion batteries (not monomers). The authors obtained the highest gas fractions at a MAX/HDPE ratio of 1, with gas yields of approximately 70 wt%, containing 40 wt% of hydrogen, 25 wt% of methane, 25 wt% of ethylene, and 10 wt% of other compounds, with a maximum monomer (ethylene) recovery of 25 wt%. Based on the obtained results, the authors showed that the proposed catalyst system is promising for microwave-assisted pyrolysis reactions intended to produce hydrogen (not monomers) [32].

Jing et al. [29] investigated the microwave-assisted pyrolysis of heavy hydrocarbon liquids and their blends, obtained from PE and PP pyrolysis reactions, in the presence of steam, as a second pyrolysis stage for the manufacture of light compounds. As a result, gaseous fraction yields ranged between 52 and 93 wt%, depending on the selected experimental conditions, containing mostly C₂H₄ (around 45 wt%, allowing for the recovery of about 40 wt% of ethylene). It was observed in particular that the increase of the reaction temperature and shortening of the residence time allowed the increase of the light olefin contents of the gaseous phase [29].

Other authors also performed microwave-assisted pyrolysis of polyolefins to increase the liquid fraction of the products (not monomers) and, consequently, did not report sufficiently high gaseous fraction yields (above 40 wt%). For example, Arshad et al. [33] performed microwave-assisted pyrolysis of PS, PP, and LDPE [33]; Jing et al. [34] recycled PP through microwave-assisted pyrolysis and reported that the obtained small amounts of gaseous fractions were composed mostly of C₃H₆ [34]; Dai et al. [35] converted HDPE, LDPE, and PP mostly into low aromatic naphtha by combining the microwave-assisted technique with the tandem catalyst Zn/SBA-15 [35].

Other studies employed the microwave-assisted pyrolysis of polyolefins to investigate different aspects of the reaction technology. For example, Arshad et al. [36] investigated the effect of reaction time and microwave power on PP, LDPE, and PS reactions [36]; Ramzan et al. [37] studied the production of hydrogen and carbon nanotubes through PP, HDPE, and PS microwave-assisted reactions, using NiFe₂O₄, Al₂O₃, and Fe₂O₃ as catalysts, and reporting yields of 93 wt% of hydrogen and up to 73 wt% of solid carbon [37]; Wang et al. [38] used biochar catalysts derived from corn stover and Douglas fir to perform the microwave-assisted catalytic pyrolysis of LDPE and real waste plastics, reporting yields of 84 wt% of gases, which contained mainly hydrogen (70 vol%) and methane (25 vol%) [38].

Table 1. Selected research works in the area of microwave pyrolysis.

Authors	Feedstock	Catalyst	Temperature (°C)	Maximum Gaseous Yield (wt%)	Gas Composition
Undri et al. (2014) [28]	HDPE	-	25–250	~34	-
Zhang et al. (2015) [27]	LDPE	ZSM-5	300–500	~70	H2, methane, ethane, ethylene, and others
Suriapparao et al. (2015) [25]	PP	(Susceptors) graphite, aluminum, silicon carbide, activated carbon, lignin and fly ash	260	~87	0–12, 68 wt% of C3
Suriapparao et al. (2022) [30]	PP, PE, EPS	ZSM-5	600	~50	-
Zhao et al. (2018) [39]	PP	ZSM-5	550	~30	-
Chen et al. (2022) [31]	LDPE	ZSM-5	450	~75	~50 wt% of olefins C2, C3, and C4
Cao et al. (2022) [32]	LDPE, HDPE	MAX (Ti3AlC2)	500–1000	>60	High purity hydrogen, and value-added graphitic carbon nanofibers
Jing et al. (2017) [29]	LDPE, HDPE, PP	-	zone 1:1000 zone 2:550	~85	~55 wt% of olefins C2, C3 and C4

summarizes some characteristics of published reports that investigated the microwave-assisted pyrolysis technology.

2.3. Perspectives on Microwave-Assisted Pyrolysis

The microwave-assisted pyrolysis of polyolefins offers several advantages, including faster heating rates of the feedstock, more uniform energy distribution throughout the reactor volume, and the absence of direct contact between the heat source and the reacting material. However, significant drawbacks remain. Polyolefin wastes typically exhibit low microwave absorptivity and poor thermal conductivity within the commercially used frequency ranges, necessitating the use of susceptors. In addition, microwave generators and associated illumination systems remain prohibitively expensive for full-scale industrial applications.

Furthermore, based on the existing literature, the technology has not yet demonstrated high selectivity for monomer production. While studies by Chen et al. [31], Zhang et al. [27], and Jing et al. [29] reported olefin recoveries exceeding 50 wt%, these results are not consistently replicated and remain limited to laboratory-scale setups. For this reason, Solis et al. [40] classified microwave-assisted pyrolysis as having a Technology Readiness Level (TRL) of 4, indicating that it is still in the early experimental phase [40].

We believe that this field will continue to attract significant research interest in the near future. However, substantial technological advances will be required before microwave-assisted processes can be viably implemented at the industrial level. In particular, more rapid progress is expected in microwave-assisted catalytic cracking of pyrolyzed streams, as fixed catalyst beds are more easily irradiated. Even in this scenario, however, the success of the process remains highly dependent on the development of catalysts with an improved selectivity for ethylene and propylene.

3. Plasma-Assisted Pyrolysis

3.1. Fundamentals of Plasma-Assisted Pyrolysis

An ionized gas generated by an electrical discharge is commonly referred to as plasma. Plasma is often considered the fourth state of matter, as it consists primarily of molecular fragments and free atoms that collectively form an electrically neutral system, despite containing both charged and neutral species [41]. Technologies that utilize or produce plasma are generally referred to as plasma technologies. These technologies are being extensively explored for a variety of industrial applications, including the recycling of plastic waste. Under the highly energetic conditions created by plasma, complex (or simple) molecules can be converted into simpler (or more complex) compounds. For this reason, plasma processes are typically associated with high temperatures [42].

Plasma technologies are not new; they were first applied in the 19th century for metallurgical processing and, by the early 20th century, were being used in the chemical industry for acetylene extraction from natural gas [41,42].

A significant portion of the observable matter in the universe exists in the plasma state. This is because stars, along with most visible interstellar matter, are composed of plasma. In addition to these omnipresent astrophysical plasmas, plasma can generally be classified into two main types: high-temperature (or fusion) plasmas and low-temperature plasmas, also referred to as gas discharges. More broadly, plasmas can also be categorized based on whether or not they are in thermal equilibrium. In a plasma at thermal equilibrium, all constituent species—including electrons, ions, and neutral atoms—share the same temperature [43].

High-temperature, or fusion plasmas, are typically in thermal equilibrium and are commonly found in stellar environments, with temperatures ranging from 4000 K to 20,000 K. Low-temperature plasmas, by contrast, can be further categorized into three main types: thermal plasma, cold plasma, and warm (or intermediate) plasma. Thermal plasmas are characterized by high temperatures—although lower than those of fusion plasmas—and are considered to be in thermal equilibrium when the energy transfer from electrons to gas-phase species is sufficiently rapid for the gas temperature (T₀) to match the electron temperature (T_e).

Cold plasma, on the other hand, is a type of non-equilibrium plasma in which the energy transfer from electrons to neutral species is inefficient, resulting in a low overall energy content. In such systems, the plasma species rapidly equilibrate with their surroundings, often reaching ambient temperatures. Finally, warm plasmas exhibit translational temperatures of approximately 2000 K—higher than cold plasmas but still considerably lower than thermal plasmas. Energy dissipation in warm plasmas typically occurs via non-equilibrium discharges into the surrounding medium [42].

In recent years, plasma technology has been employed in the treatment of municipal solid waste, particularly hazardous waste, due to its ability to eliminate toxic and harmful components that pose risks to both the environment and public health [44]. Thermal plasma treatment is generally applied in two main forms: plasma-assisted pyrolysis and plasma-assisted gasification. In both approaches, the plasma generator—typically an electrical discharge device—serves as the sole heat source.

Plasma-assisted pyrolysis is conducted in the absence of oxygen, whereas plasma-assisted gasification occurs in the presence of oxygen or oxygen-containing compounds. In pyrolysis, complex molecules are typically broken down into simpler hydrocarbons. In contrast, gasification leads to the formation of gas mixtures rich in carbon monoxide (CO) and carbon dioxide (CO₂). Within this framework, plasma-assisted pyrolysis emerges as a potential technology for the recovery of monomers from polyolefin waste, while plasma-assisted gasification can be used to produce synthesis gas (syngas) for integration into the chemical value chain [42].

Plasma-assisted pyrolysis combines electrical discharges with thermochemical pyrolysis, conducted in the absence of oxygen, to depolymerize the long-chain molecules that constitute polyolefin materials. Typically, the process involves two main stages: firstly, a conventional pyrolysis step generates a stream of gaseous products, then, these pyrolysis vapors are exposed to plasma conditions for further transformation [45]. In some configurations, a third catalytic stage may be added to steer the reaction toward specific target products by enhancing selectivity [42]. However, in certain cases, plasma is used directly as the heat source in a single-stage pyrolysis process [46,47].

This type of plasma-assisted pyrolysis often operates at extremely high temperatures—ranging from 2000 to 10,000 K—generated by a plasma arc, which enables the complete breakdown of waste materials into simpler molecules [41]. According to Gabbar et al. [46], thermal plasma technology enhances the performance of pyrolysis reactors by producing free radicals that accelerate chemical reactions and boost thermal cracking rates, thereby improving reaction kinetics [46].

The distribution of gaseous, liquid, and solid products resulting from plasma-assisted pyrolysis depends heavily on operating parameters, particularly the plasma input power. Notably, gas yields from this process can reach 70–80 wt% of the feedstock—significantly higher than the 10–20 wt% typically achieved through the conventional pyrolysis of polyolefins at 400–500 °C [48].

Figure 2 shows a representative experimental setup commonly used in plasma-assisted pyrolysis studies.

3.2. Previous Works of Plasma-Assisted Pyrolysis

Tang et al. [49] investigated the plasma-assisted pyrolysis of PP to convert waste plastics into gaseous fuel and useful chemicals. The results of a series of experiments indicated that optimum operation conditions employed a power input of 35.2 kVA and feed rate of 60 g/min (35,200 kJ/kg), leading to gaseous yields of 96 wt%, containing 54 mol% of hydrogen, 17 mol% of acetylene, 5 mol% of methane, 5 mol% of CO, and 19 mol% of other hydrocarbons [49].

In order to prevent the undesired conversion of monomer molecules into other compounds, the desired species can be separated from the gaseous effluent mixture that is fed into the plasma section through a suitable separation technique before, which also makes the flowsheet more complex and can increase the operation costs significantly [48]. For example, Guddeti et al. [50] investigated the plasma-assisted pyrolysis of PP in an ICP (Induction-Coupled Plasma) reactor that could expose the gaseous mixture to very high temperatures (3000–8000 K) and very high heating rates (106 K/s). PP powder was fed into the clean ICP reactor for 45 min in a powder feed of 1.5 g/min. The input plasma power varied in the range between 10 and 20 kVA, and was combined with a very rapid quenching of the effluents (1000 K/s), resulting in smaller amounts of undesired species produced by the decomposition of the desired products. The PP solid conversion was around 78 wt%, and the gaseous products were composed mostly of propylene (93.7 mol%), methane (2.6 mol%), ethylene (1.7 mol%), and butanes and butenes (1.3 mol%) [50]. In this case, the monomer recovery reached the impressive value of 72 wt%, although it is doubtful whether similar conditions might be employed in a large-scale facility in an economically feasible manner.

Mohsenian et al. [51] carried out experiments with PE, PP, and other polymers in a lab-scale thermal plasma reactor equipped with a twin DC (direct current) plasma torch, a stainless-steel vacuum chamber, a high current DC power supply, a high voltage AC (alternate current) power supply, a plasma working-gas supply, and a water-cooling device. The temperatures studied varied from 11,000 K to 16,000 K and it was shown that gas products consisted essentially of hydrogen (in PE, up to 67.4 mol%; in PP, up to 62.8 mol%) and a mixture of hydrocarbons (in PE, up to 46.5%; in PP, up to 47.4%) without any significant amount of carbon monoxide. It was found that the increase of temperature caused the increase of hydrogen concentrations and the reduction of hydrocarbon concentrations in the gas products [51].

Yao et al. [52] reported the utilization of nonthermal H₂ plasma for initiating the hydrogenolysis of HDPE at ambient conditions, leading to the generation of light hydrocarbons. By employing plasma-activated H₂, the hydrogenolysis process of PE can efficiently occur at room temperature, resulting in the remarkably selective production (95 wt%) of light alkanes (C₁–C₃), although CH₄ accounted for most of the gas products (>70 mol%). The experiments of non-thermal plasma-assisted HDPE hydrogenolysis were carried out in a quartz tube reactor equipped with a dielectric barrier discharge (DBD) plasma generator under standard reaction conditions of 101 kPa H₂ partial pressure, 100 mL/min H₂ flow rate, and 90 W [52].

Several studies have also explored the combination of plasma-assisted pyrolysis with catalysts. Xiao et al. [45] conducted experiments with 2 g of PP, combining the use of plasma-assisted pyrolysis and the ZSM-5 catalyst, placed in a fixed bed reactor equipped with a coaxial dielectric barrier discharge (DBD) plasma generator. The authors investigated the effect of plasma on the pyrolysis process, the stability of the plasma-catalytic system, and the effects of reaction temperatures (200 to 500 °C) and input plasma power (60–120 W) on the system performance. In the absence of plasma, the catalytic pyrolysis of PP generated a significant amount of oil (54 wt%), smaller amounts of gas (20 wt%), and a large quantity of wax (26 wt%). However, when the plasma was turned on (60 W), a rapid increase of the gas yield and significant decrease of the wax yield could be observed. By increasing the input plasma power, the yields of gas products also increased. These findings suggest that plasma can indeed promote the cracking of heavy hydrocarbons to light hydrocarbons. Nevertheless, H₂ and CH₄ were the main products, while monomers were recovered in small quantities. This implies that the plasma conditions facilitated the cleavage of C–H and C–C bonds. With the increase of ZSM-5 (3:1), the amount of C₃H₈ reached 40 mol% in the gas phase, meaning that the combination of plasma and catalysis technologies can allow the significant increase of rates of monomer recovery in chemical recycling processes. On the other hand, in the liquid phase, the use of plasma caused the increase of benzene and toluene contents (and the reduction of the xylene contents), which can constitute an environmental drawback [45].

Gabbar et al. [46] studied the plasma-assisted pyrolysis of 15 g of LDPE, reporting the production of 93 wt% of light oil and 7 wt% of gas, using an input plasma source of 270 W under vacuum (−0.95 bar) and a temperature of 550 °C. The authors did not report the composition of the gas phase, so the monomer recovery was not reported [46].

Diaz-Silvarrey et al. [47] investigated the recovery of ethylene from PE through plasma-assisted pyrolysis using cold plasma catalysis (without additional heating) and showed that monomer recovery was 55 times greater than observed in conventional pyrolysis. The HDPE samples were pyrolyzed under nitrogen atmosphere at 500–700 °C, with heating rates of 30–75 °C/min for 15 min. Cold plasma was generated using an AC power supply with energy inputs ranging from 90 J/mL to 180 J/mL. The maximum gas yield was equal to 60 wt% with an ethylene recovery of 20 wt% at 700 °C, with a heating rate of 30 °C/min and energy input of 180 J/mL [47].

Table 2. Selected research works in the area of plasma pyrolysis.

Authors	Feedstock	Power Input	Temperature (°C)	Maximum Gaseous Yield (wt%)	Gas Composition (mol%)
Tang et al. (2003) [49]	PP	35.2 kVA	Not informed	~96	H2 (~54%), acetylene (~17%), methane (~5.0%), CO (~2%), and CxHy and unknown (~19%)
Guddeti et al. (2000) [50]	PP	10–20 kVA	2727–7727	~78	Propylene (93.7%,) methane (2.6%), ethylene (1.7%), and butanes and butenes (1.3%)
Mohsenian et al. (2016) [51]	PE and PP	Not informed	10,727–15,727	-	H2 (up to 67.4%) and hydrocarbons (up to 47.4%)
Yao et al. (2021) [52]	HDPE	60–90 W	Room temp.	~95	Methane (70%) and other hydrocarbons
Xiao et al. (2022) [45]	PP	60–120 W	200–500	~45	H2 and hydrocarbons in different quantities
Gabbar et al. (2017) [46]	LDPE	270 W	550	~7	-
Diaz-Silvarrey et al. (2018) [47]	HDPE	30–60 W	500–700	~60	H2 and hydrocarbons in different quantities

summarizes some characteristics of published reports that investigated the plasma-assisted pyrolysis technology.

3.3. Perspectives on Plasma-Assisted Pyrolysis

Plasma-assisted pyrolysis of plastic waste presents several potentially competitive advantages for the chemical recycling of polyolefins. These include the highly efficient degradation of long macromolecules due to the inherently high energy density of plasma, accelerated reaction rates driven by free-radical mechanisms, and rapid startup and shutdown capabilities. In addition, the process offers high thermal efficiency—approximately 70%—and employs plasma electrodes with long operational lifespans, which can help justify the relatively high initial capital investment. The energy input is also highly controllable and can be tailored to match the enthalpy requirements of the feedstock. Furthermore, plasma-assisted pyrolysis is capable of destroying hazardous substances while generating end products that are considered safe for both human health and the environment [46,48].

On the other hand, plasma-assisted pyrolysis faces several critical limitations that hinder its industrial deployment. The extremely high temperatures required by thermal plasma reactors lead to substantial energy consumption, rendering the process inherently energy-inefficient and economically unappealing. In addition, the technology exhibits a low selectivity for monomer recovery due to the stochastic and largely uncontrollable nature of plasma-induced reactions. Consequently, achieving high yields of valuable monomers often demands the use of advanced catalytic systems or intricate purification schemes, both of which increase operational costs and process complexity. These combined technical and economic barriers significantly constrain the scalability of plasma-assisted pyrolysis, which, to date, remains restricted to laboratory-scale investigations [48].

Due to its high energy requirements, plasma-assisted pyrolysis is currently best suited for large-scale applications such as the neutralization of hazardous medical waste, where the energy input can be partially recovered in the form of syngas for subsequent use in chemical processes [53]. In fact, among the studies reviewed, only one reported high monomer recovery rates—achieved at temperatures exceeding 2000 °C. At more practical temperatures (ranging from room temperature to 700 °C), although substantial gas yields were sometimes observed, these gas streams were typically rich in hydrogen and methane, with only low concentrations of monomers.

We believe that this field will continue to attract research interest in the near future. However, plasma-assisted technologies will need to undergo significant development before they become viable at the industrial scale. As previously noted in the context of microwave-assisted pyrolysis, progress in plasma-based systems will likely depend on the development of catalysts that, when combined with plasma conditions, can deliver a significantly higher selectivity for ethylene and propylene than is currently achievable.

4. Chemical Recycling Through Supercritical Technology

4.1. Fundamentals of Chemical Recycling Through Supercritical Technology

A chemical compound is said to be in a supercritical state when its temperature and pressure exceed the compound’s critical temperature and critical pressure, respectively. Under these conditions, the substance is referred to as a supercritical fluid (SCF). The critical point marks the boundary beyond which vapor–liquid equilibrium no longer exists. Above this point, it is not possible to distinguish between liquid and gas phases, regardless of variations in pressure or temperature.

One of the most intriguing features of supercritical fluids is that their properties can be continuously tuned between those of liquids and gases, without the abrupt changes typically associated with phase transitions. For instance, a supercritical fluid can exhibit a liquid-like density while simultaneously displaying gas-like transport properties such as diffusivity and viscosity [54]. These unique characteristics have opened the door to a wide range of applications that are not feasible under subcritical conditions.

As a result, supercritical fluids have been widely studied across multiple scientific and engineering disciplines. Notably, progress has also been made in applying SCF technology to the treatment of plastic waste.

Table 3. The critical temperature and pressure of some materials.

Solvent	Critical Temperature (°C)	Critical Pressure (MPa)	Refs.
Water	374.15	22.05	[56]
Ethanol	240.75	6.14	[57]
Acetone	234.95	4.70	[58]
CO2	30.98	7.38	[58]
Methanol	239.45	8.10	[58]

presents the most commonly used supercritical fluids in chemical recycling applications, along with their respective critical temperatures and pressures [55].

For the purpose of monomer recovery, the use of solvents in their supercritical state enables the establishment of reaction conditions that are not achievable in conventional pyrolysis processes. Moreover, combining thermal cracking—typically conducted at high temperatures exceeding the critical points of common solvents (as shown in Table 3)—with supercritical fluids can open alternative reaction pathways, potentially altering the composition and properties of the resulting product streams [54].

Among the various supercritical fluid candidates, water stands out as particularly attractive due to its low cost, non-toxicity, and widespread availability. As reported by Moriya and Enomoto [59], supercritical water is capable of dissolving nonpolar organic compounds that are normally insoluble under ambient conditions. Consequently, supercritical water has been widely investigated for its effectiveness in cracking and reforming high molecular weight hydrocarbons [59].

Supercritical water can exhibit solvent properties comparable to those of conventional organic solvents. Additionally, it can act as an effective catalyst in organic reactions due to the abundance of hydrogen and hydroxyl ions in the medium. The homogeneous reaction environment provided by supercritical water also facilitates favorable conditions for chemical recycling processes. However, it is important to note that water has a relatively high critical point, which can pose operational challenges.

As an alternative, supercritical carbon dioxide (scCO₂) offers several competitive advantages. Like water, scCO₂ is inexpensive, non-toxic, and non-hazardous. Moreover, it exhibits excellent mass transfer capabilities, full miscibility with gaseous reactants, and allows for easy separation from the final product via simple depressurization [55].

Hydrolysis is an effective method for depolymerizing condensation polymers, as it selectively breaks down polymer chains into their constituent monomers. This approach is particularly suitable for typical polycondensation materials. For example, Mitsubishi has successfully developed a chemical recycling process using supercritical methanol to depolymerize post-consumer PET bottles [60]. This innovative method converts PET into its monomers, yielding pure streams of terephthalic acid (TPA) and ethylene glycol (EG).

Supercritical fluid (SCF) technologies have also been investigated for the chemical recycling of other condensation polymers, including nylon 6, polycarbonates, and polyurethanes, achieving high monomer recovery efficiencies [60]. Notably, Li and Xu [55] explored the use of SCF technology for the recycling of electronic waste. Compared to conventional recycling approaches, SCF processing enabled a more effective removal of hazardous substances, simultaneous recovery of valuable materials without secondary pollution, and chemical recycling of constituent polymers—such as epoxy resins, phenolic resins, and polyurethanes—into compounds that were either dissolved in the aqueous phase or recovered in the form of oils [55].

On the other hand, it is not immediately evident that the use of supercritical fluids (SCFs) offers clear advantages for the chemical recycling of polyaddition materials such as polyolefins, since the cleavage of their polymer chains is not driven by a specific functional group reaction. As a result, it is also unclear whether SCFs can improve the efficiency of monomer recovery in the recycling of these materials. In light of this uncertainty, the following section reviews previous studies on the chemical recycling of polyolefins under supercritical conditions. Figure 3 provides a schematic overview of the key variables influencing supercritical fluid-based recycling, along with the typical products obtained through this process.

4.2. Previous Works of Chemical Recycling Through Supercritical Technology

Watanabe et al. [61] investigated the pyrolysis of HDPE in supercritical water, in a batch reactor at 420 °C for 30 min. Increasing the density of water was found to enhance the rate of the cracking of HDPE samples. The use of supercritical water resulted in higher yields of shorter chain hydrocarbons, an increased ratio of 1-alkene to n-alkane, and higher PE conversions. Approximately 25 mol% of C₁ to C₄ molecules were obtained; however, over 60 mol% of the product consisted of carbon monoxide. Small amounts of carbon dioxide and hydrogen were also formed [61]. Therefore, despite the higher gas content in the product, the overall monomer recovery was low.

PE cracking was performed through hydrothermal experiments in supercritical water and compared to conventional thermal cracking by Moriya et al. [59]. Reaction was carried out at 425 °C for up to 180 min with 2.52 g of HDPE, with a water/HDPE mass ratio of 5/1 and pressure of 42 MPa. When the thermal cracking was performed for 120 min, the yields were equal to 71.8 wt% of oil and 12 wt% of gas, while the hydrothermal process performed in supercritical water yielded 90.2 wt% of oil and 6.5 wt% of gas at the same reaction conditions, which does not indicate the significant impact of supercritical hydrocracking on the increase of the rates of HDPE degradation. When the reaction time in supercritical water was extended to 180 min, the yields were equal to 77.7 wt% of oil and 13.2 wt% of gas, surprisingly smaller than the previous ones and probably reflecting the experimental variability. The composition of the gas phase produced after 180 min of reaction consisted of 34.3 mol% of methane, 28.5 mol% of ethane, 14.6 mol% of propane, and small amounts of CO, CO₂, H₂, and other hydrocarbons. Notably, significant monomer recovery was not reported [59].

Čolnik et al. [62] studied the hydrothermal cracking of polypropylene at sub- and supercritical conditions, with temperatures ranging from 425 to 450 °C and pressures ranging from 29 to 40 MPa. The liquid yields reached up to 90 wt% at several operation conditions, while the reaction performed at 450 °C for 240 min provided the highest amounts of gaseous products, around 20 wt%. The gas compositions comprised approximately 70 to 80 mol% of C₂ to C₄ species, with lower amounts of CO₂, C₁, and other hydrocarbons [62].

The conversion of virgin polypropylene into high-valued light oil under supercritical conditions was studied by Chen et al. [63]. The experiments were conducted in a 35 mL stainless steel high-pressure reactor, placed inside a tubular furnace with 1 g of polymer sample. The reaction temperatures ranged from 380 to 500 °C, while the reaction time ranged from 0.5 to 6 h and the system pressure was kept at 23 MPa. Increasing the temperature and reaction time resulted in higher conversions, as one might already expect, with a nearly 100% conversion in liquids and gaseous products achieved at temperatures of 425 and 450 °C. After 4 h of reaction at 450 °C, the amount of liquid products generated was over 80% (primarily composed of naphtha), while the gaseous product reached approximately 20%, containing more than 45 wt% of C₃ molecules. The condition that produced the highest amount of gas was at 500 °C, which was tested for only 30 min, generating approximately 30 wt% of gas; however, the gas composition was not reported [63]. The same authors continued studying the hydrothermal processing (HTP) of virgin and waste polyethylene under supercritical conditions in a subsequent work with the same reactor system and conditions. The major products consisted of light oils in the gasoline and diesel ranges, with properties that were very similar to the ones of commercial products. For virgin PE, the conversions were nearly 100% in gaseous and liquid products, while for the waste samples, the total conversions ranged from 80% to 98%. Approximately 20 wt% of gases formed in virgin HDPE HTP were reported to be formed at 425 and 450 °C: ethene accounted for 5.8 wt%, propene for 38.3 wt%, C₄ olefins for 19.3 wt%, and other olefinic hydrocarbons for 5.5 wt%. Small amounts of alkanes were also formed. One point that deserves attention is related to the analysis of the gaseous phase. The formation of CO, CO₂, and H₂ was not analyzed, as the employed analytical method could not detect these analytes. Therefore, the described olefin concentrations might be different if the authors had considered the total gas composition [63].

Although the previous studies did not directly focus on the formation of monomers, as the main objective was to maximize the yields of light oil fractions, there was a clear tendency to produce higher amounts of gaseous products with the increasing reaction temperature and reaction time. This suggests that the use of higher reaction temperatures and longer reaction times might lead to higher contents of gaseous olefins.

The hydrothermal liquefaction of virgin polypropylene under supercritical conditions was also studied by Seshasayee et al. [64] in a 4 mL reactor vessel, using 0.12 g of polymer and 0.34 to 2.07 mL of water at temperatures ranging from 350 to 450 °C, at a pressure of 25 MPa. The article focused on the formation of oil and its characterization, which reached approximately 30 wt% for PP. The reactions performed at 450 °C for 30 and 60 min resulted in approximately 80 wt% of gaseous products, which composition were not analyzed. Other polymers were also tested, such as poly(ethylene terephthalate) (PET), polycarbonates (PC), and polystyrene (PS). PC and PS yielded the highest amounts of oil. It is important to note that this study used very small reactor vessels and a highly diluted system [64].

Su et al. [65] studied the degradation of high-density polyethylene under supercritical water conditions for oil formation. Experiments were conducted in a reaction vessel with a volume of 125 mL and operated at temperatures between 450 and 480 °C. The operating pressures were not provided. The polymer conversions in oil and gaseous products reached approximately 100%, and the reaction performed at 480 °C yielded the highest amounts of gaseous products (around 30 wt%), resulting in 74.4 wt% of C₂–C₄ molecules, with higher amounts of alkanes than alkenes. Once again, it was observed that the amount of gas increased with the reaction temperature [65].

Lu et al. [66] studied the supercritical water hydrothermal liquefaction of 1 g of recycled mechanical HDPE pellets. A batch tubular reactor with a capacity of 37 mL was connected to a valve and heated by a tubular furnace at temperatures of 425, 450, and 475 °C, reaching the pressure of 23 MPa. The reaction times ranged from 0.5 to 4 h. Conversions attained in experiments performed for 2 h were approximately equal to 100%, with a majoritarian formation of light oil in the naphtha range and a gas formation of up to 20 wt% at 475 °C. It was observed once more that higher amounts of gaseous products were formed as the reaction temperature and time increased. On the other hand, with increasing temperature, the amount of olefins in the gas decreased, while the amount of paraffins increased. In the reaction performed at 450 °C, the amount of olefins in the gas stream ranged from 100 wt% (0.5 h) to approximately 30 wt% (4 h) [66].

The hydrothermal and solvothermal liquefaction under supercritical conditions were investigated by Liu et al. [67] with samples of 5 g of e-waste linear low-density polyethylene (LLDPE), crosslinked polyethylene (XLPE), and carbon-doped crosslinked polyethylene. The temperature ranged from 300 to 375 °C under pressures between 9 to 23 MPa, with reaction times ranging from 30 to 120 min. The reactor had a capacity of 100 mL and operated with 30 mL of solvent. The used solvents included water, methanol, acetone, and 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM [PF6]), with methanol leading to the highest formation of gaseous products (~55 wt%). The gases were analyzed through GC-TCD, resulting in 6 wt% of H₂ and 94 wt% of CH₄, while other hydrocarbons were not analyzed, so it is unknown if monomer recovery indeed occurred [67,68].

Zhang et al. [68] investigated the degradation of HDPE powder under supercritical water conditions at temperatures ranging from 500 to 550 °C, with a pressure of 25 MPaI. The reaction system comprised a continuous reactor with a capacity of 180 mL with residence times (rt) ranging from 80 to 240 s. At 520 °C and the longest residence time (240 s), the highest formation of gaseous products (~40 wt%) was observed, with a production of hydrocarbon molecules, mainly in the C₁ to C₃ range. Higher temperatures resulted in increased amounts of olefins (C⁼/C = 4.20% at 550 °C and rt = 120 s), while longer residence times led to higher amounts of paraffins (C⁼/C = 0.59% at 520 °C and rt = 240 s).

Table 4. Selected research works in the area of recycling under supercritical conditions.

Authors	Feedstock	Pressure	Temperature (°C)	Maximum Gaseous Production	Gas Composition (mol%)
Watanabe et al. (2001) [61]	PE	Not informed	420	-	C1 to C4 molecules (~30%), 60% of CO, CO2, and H2
Moriya et al. (1999) [59]	HDPE	42 MPa	420	6.5–13.2%	Methane (34.3%), ethane (28.5%), propane (14.6%), CO, CO2, H2, and other hydrocarbons (small amounts)
Čolnik et al. (2022) [62]	PP	40 MPa	425 and 450	~20%	C2 to C4 (70 to 80%), CO2, C1, and other hydrocarbons (small amounts)
Chen et al. (2019) [63]	PP	23 MPa	380 to 500	~20–30%	C3 (45%)
Jin et al. (2020) [69]	PE	23 MPa	380 to 500	~20%	Ethane (5.8%), propene (38.3%), C4 olefins (19.3%), other olefinic hydrocarbons for (5.5%), and alkanes (30.6%)
Seshasayee et al. (2020) [64]	PP	25 MPa	350 to 450	~80%	-
Su et al. (2004) [65]	HDPE	Not informed	450 and 480	~30%	C2–C4 (74.40%)
Lu et al. (2022) [66]	HDPE	23 MPa	425 to 475	~20%	Paraffins and olefins
Liu et al. (2022) [67]	PE	9 to 23 MPa	300 to 375	~55%	H2 (6%), CH4 (94%)
Zhang et al. (2007) [68]	HDPE	25 MPa	500 to 550	~40%	C1 to >C5 (most C2 and C3), H2, and others

summarizes characteristics of the reports presented previously and involving the chemical recycling of polyolefins under supercritical conditions [68].

4.3. Perspectives on Chemical Recycling Through Supercritical Technology

Based on the studies reviewed, it is clear that the liquefaction and degradation of various polymers—such as polypropylene, high-density polyethylene, and polyethylene waste—under supercritical water conditions can lead to the formation of both oils and gaseous products. This outcome is expected, given the relatively high temperatures employed in most experiments. Available data indicate that reaction temperature and residence time are the key parameters influencing both polymer conversion and product distribution. In particular, higher temperatures and longer reaction times tend to increase gas yields—a trend that aligns with thermochemical behavior. Notably, extended reaction times appear to favor the formation of paraffinic compounds. However, it is important to highlight that many studies did not quantify key gaseous species such as CO and CO₂, primarily due to limitations in the analytical techniques employed.

Since hydrothermal liquefaction (HTL) can process wet feedstocks containing organic contaminants without the need for pre-treatment steps such as washing or drying, it offers significant operational advantages for the treatment of contaminated polyolefin residues. Moreover, the hydrogen-donating properties of subcritical and supercritical water have been effectively leveraged in the upgrading of heavy crude oils. In the context of polyolefins, the liquid products derived from HTL typically exhibit a low oxygen content and high calorific value, making them suitable as energy-dense fuels or valuable chemical intermediates [62].

The application of supercritical conditions offers several advantages over conventional chemical recycling methods for polyolefins, including an improved process performance, greater economic feasibility, and a potentially lower environmental impact. This approach is particularly suitable for processing challenging waste streams, such as mixed plastics or plastics contaminated with organic residues. Additionally, the ability to produce high-purity oil products at relatively lower temperatures further increases the attractiveness of this technology [59].

However, it is important to note that most studies on supercritical chemical recycling have been conducted in batch mode, whereas pyrolysis processes are typically carried out in semi-batch configurations. This discrepancy complicates direct comparisons between the two approaches. For instance, semi-batch processes often yield higher molecular weight products—and therefore fewer light gases—since volatile components are continuously removed from the system as they form. In contrast, batch processes retain short-chain molecules within the reactor throughout the entire reaction time, increasing the likelihood of further chain scission and lighter product formation.

Consequently, based on the current body of data, it cannot be conclusively stated that supercritical operation inherently leads to lighter product fractions. Moreover, the impacts of solvent purification and recycling—both technically and economically—remain unclear and must be carefully evaluated to assess the full viability of these processes.

Moreover, supercritical recycling processes require operation under elevated temperatures and pressures, which inherently increases the risk of hazardous conditions. These extreme operating parameters demand rigorous safety protocols and specialized equipment to maintain process stability and control. As a result, safety considerations constitute a critical factor when assessing the industrial feasibility of supercritical depolymerization technologies [55].

It is worth noting that, although most of the studies discussed thus far were conducted using small quantities of polymers in highly diluted systems, a few industrial-scale initiatives have also been reported in the literature—potentially signaling a higher Technology Readiness Level (TRL) for supercritical recycling technologies. For example, Mura Technology, in collaboration with Dow and its subsidiary Renew ELP, is constructing an industrial-scale plant in Teesside, Northeast England, with an initial processing capacity of 20,000 tons of plastic waste per year, and plans to expand that capacity to 80,000 tons annually [70,71].

Similarly, Stopford, in partnership with the University of Birmingham, has announced the development of a novel hydrothermal recycling process known as CircuPlast. This technology employs a continuous high-temperature, high-pressure system to process waste plastics and produce chemical precursors such as naphtha, gas oil, and wax [72,73]. However, it should be emphasized that no independent confirmation of the plant’s construction or preliminary performance results for the Stopford technology were found in the open literature.

In summary, chemical recycling under supercritical conditions demonstrates promising potential for the liquefaction and degradation of polymers, enabling the production of oils and gases with properties comparable to commercial products. However, further research is required to clearly establish the technical and economic benefits, elucidate reaction mechanisms, and optimize process conditions for achieving target products with maximum efficiency.

A review of recent studies reveals a strong focus on maximizing liquid-phase yields, with particularly encouraging results for the production of light oils in the naphtha range at relatively low temperatures. Nevertheless, the high pressures and solvent volumes required may compromise some of these advantages. Notably, the existing literature does not extensively explore more extreme temperature conditions or longer residence times to favor gas formation—an area that presents clear opportunities for future research.

5. Chemical Recycling Through Catalytic Pyrolysis

5.1. Fundamentals of Catalytic Pyrolysis

Pyrolysis is a thermal decomposition process in which organic materials are heated in the absence of oxygen, resulting in the breakdown of complex molecules into simpler compounds such as gases, liquids, and solid residues [13,74]. It is a specific form of thermal degradation (or thermolysis) that typically occurs at temperatures above 400 °C. Pyrolysis can be applied to convert a wide range of materials—including plastics, biomass, and waste—into valuable products such as biofuels, syngas, and carbon black.

Within this context, catalytic pyrolysis involves the use of catalysts to enhance the quality and selectivity of the products obtained [74,75]. The primary goal of incorporating a catalyst is to exert greater control over the composition of the resulting compounds while simultaneously reducing the required reaction temperature and residence time. In this sense, catalytic pyrolysis resembles conventional catalytic cracking processes employed in oil refineries to convert high molecular weight petroleum fractions into lighter, higher-value products [74,75].

The incorporation of a catalyst in pyrolysis aims to enhance both the quality and quantity of the resulting product fractions by facilitating reactions such as decarboxylation, decarbonylation, hydrogenation, and cracking, among others [76]. However, when the primary objective is the direct production of monomers, the catalyst must be specifically designed to promote rapid carbon–carbon bond scission while suppressing the formation of aromatics, naphthenes, and saturated compounds.

Catalysts can be introduced either directly into the feedstock (in situ) or applied downstream to the gaseous or liquid products exiting the primary pyrolysis reactor (ex situ), typically using a fixed-bed tubular reactor [77]. It is important to note, however, that in situ catalytic pyrolysis presents several challenges—particularly when processing real plastic waste streams. One major issue is the difficulty of separating the catalyst from the resulting solid residue, which can hinder the recovery and reuse of both the catalyst and the feed-derived materials, thereby compromising the overall economic viability of the process [78].

It is important to highlight that both in situ and ex situ catalytic pyrolysis can be effectively employed to convert plastic residues into stable and valuable chemical products. In in situ configurations, the catalyst is mixed directly with the feed and operates at the same temperature as the pyrolysis reactor. This setup typically promotes an immediate cracking of the vapors as they are released, favoring the formation of lighter fractions, reducing the risk of a secondary polymerization of primary products, and increasing the yield of aromatic compounds due to the short vapor residence time [79]. In contrast, ex situ catalytic pyrolysis offers a greater operational flexibility, as the pyrolysis and catalytic stages can be conducted at different temperatures and under distinct chemical environments. Additionally, catalyst contamination tends to occur at lower rates, since most solid impurities are retained in the primary pyrolysis reactor. A further advantage of the ex situ approach is the potential to incorporate multiple catalytic reactors in series, enabling the sequential execution of distinct transformations tailored to optimize the product stream [80].

The issue of agglomeration between the catalyst and viscous pyrolysis residues is notably more severe in in situ configurations, where catalyst separation and regeneration can be particularly challenging. In contrast, ex situ systems generally offer easier catalyst handling and regeneration, closely resembling established procedures used in conventional refinery operations. However, it is important to emphasize that these specific aspects have not been thoroughly addressed in the available literature, representing a technological gap that warrants further investigation [79].

A wide range of catalysts has been evaluated in the literature for various roles in catalytic pyrolysis, including both homogeneous and heterogeneous systems—such as standard fluid catalytic cracking (FCC) catalysts, metal oxides, and mesoporous materials. The structural and mechanistic resemblance between plastic pyrolysis and FCC processes likely explains the frequent use of zeolites in studies focused on the catalytic pyrolysis of plastic waste [81,82]. The incorporation of solid catalysts in pyrolysis offers several advantages: it can lower the reaction temperature, reduce the formation of solid residues, decrease the average molecular weight of the products, shorten residence times, improve product selectivity, and allow for a fine-tuning of the product stream composition [82]. Despite these benefits, maintaining catalyst activity and efficiency over extended operation periods remains a significant challenge that has yet to be fully addressed [80].

The study conducted by Harussani et al. [82] provided a comprehensive overview of key aspects of the catalytic pyrolysis process and the main parameters influencing product yields, with a specific focus on the chemical recycling of polypropylene (PP). A variety of catalysts were examined, and the authors highlighted acidity and product selectivity as the most critical chemical properties affecting catalytic performance. The catalysts used for plastic pyrolysis were grouped into three main categories: clay-based materials (e.g., calcium bentonite), iron-based and mesoporous materials (e.g., MCM-41), and zeolites (e.g., HZSM-5 and Y-zeolites) [81,83]. The primary advantages of these catalysts include their high density of micro- and mesopores, low cost, selective cracking capabilities, and thermal stability. In particular, acidic microporous catalysts such as HZSM-5 have been extensively applied to reduce coke formation and enhance the yield of liquid products [81].

Several reactor configurations have been proposed for conducting pyrolysis reactions; however, fluidized bed reactors remain the most commonly employed for both in situ and ex situ catalytic processes, largely due to their close resemblance to the fluid catalytic cracking (FCC) systems used in petroleum refining. These reactors offer notable advantages, including ease of operation, scalability, and widespread support in the literature [84,85,86]. Additionally, fluidized beds exhibit excellent heat and mass transfer properties, a reduced susceptibility to clogging from molten polymers, and the ability to maintain a nearly uniform temperature profile throughout the reactor [87,88].

Despite these benefits, many studies report the use of large quantities of inert solids—up to 95 wt%—to ensure proper fluidization in the presence of molten plastics. This practice significantly increases capital costs and decreases volumetric productivity. Furthermore, most studies fail to detail the procedures for catalyst regeneration and reuse in such systems. This may explain why industrial-scale operations often prefer rotating drum or series-configured fixed-bed reactors for catalytic pyrolysis. This preference is particularly evident when the objective is to convert plastic waste directly into monomers or light gaseous products, since the broader residence time distribution typical of fluidized bed reactors tends to favor the formation of a wide range of molecular weight compounds [89]. Figure 4 presents a schematic diagram of the main stages involved in catalytic pyrolysis. It includes representations of both in situ configurations, in which the catalyst is physically mixed with the feedstock, and ex situ configurations, where the catalyst is located in a downstream bed following the pyrolysis zone.

The main objective of the present manuscript is not reviewing the field of the pyrolysis of plastic wastes, which has been reviewed by other researchers both in terms of reaction mechanisms and process technologies [13,74,75,76,77], but to identify which techniques show promise in selectively yielding valuable monomers such as ethylene and propylene, thereby contributing to the circularity of the plastic chain.

5.2. Previous Works of Catalytic Pyrolysis

Miandad et al. [90] described the progress and challenges of catalytic plastic waste pyrolysis and the future perspectives in comparison to thermal pyrolysis. The factors that affect the catalytic pyrolysis process mostly were identified, including the reaction temperature, retention time, feedstock composition, and the type of catalyst. It was emphasized that thermal pyrolysis usually produces low-quality liquid oil, demanding both high temperatures and retention times. In order to overcome these issues, it was claimed that catalytic pyrolysis can potentially convert 70 to 80 wt% of plastic wastes into liquid oil with characteristics that are similar to the ones of conventional diesel fuel, such as the high combustion heat of 38–46 MJ/kg, density of 0.77–0.84 g/cm³, viscosity of 1.74–2.50 mm²/s, kinematic viscosity of 1.1–2.3 cSt, pour point of (−9)–(−67) °C, boiling point of 68–352 °C, and flash point of 26–48 °C. Thus, the quality of the produced liquid oil is higher when catalytic pyrolysis is performed. Moreover, process by-products such as char can be potentially used as adsorbent materials for the removal of heavy metals, pollutants, and odor from wastewater and polluted air, while the produced gases can be potentially used to produce energy in situ in industrial facilities. Despite all the potential advantages of catalytic pyrolysis, some challenges still remain to be solved, such as the high process energy input, the catalyst costs, and the catalyst regeneration and reuse. The possible solutions for these challenges demand the investigation of cheaper alternative catalysts, catalyst regeneration, and the overall process optimization [90].

Aguado et al. [79] investigated the conversion of low-density polyethylene (LDPE) into high-value hydrocarbons using a two-step reaction system consisting of an initial pyrolytic furnace followed by an independent reactor containing a nanosized n-HZSM-5 zeolite, where the catalytic reforming of the pyrolytic vapors took place. The system was run at temperatures between 425 and 475 °C and the results compared with those obtained in the absence of a catalyst. Temperatures higher than 450 °C were required to reach conversion values above 90 wt%. At that temperature and in the absence of catalysts, thermal cracking of LDPE generated almost exclusively α-olefins and n-paraffins over a wide range of molecular weights, most of which (74.7 wt%) were collected as liquid products. Catalytic reforming over n-HZSM-5 was effective at 425 °C and caused a significant increase in the amount of gaseous hydrocarbons (73.5 wt% selectivity at 450 °C) that consisted primarily of olefins [79].

Marcilla et al. [91] studied the evolution of the gas compositions during thermal and catalytic pyrolysis of vacuum gas oil (VGO), polyethylene (PE), and vacuum gas oil–polyethylene blends (VGO–PE). A vertical batch reactor was used to perform the pyrolysis reactions. The reactor configuration and the use of a carrier gas facilitated the generation of gas. The outlet reactor stream was heated at 300 °C to prevent the condensation of the less volatile compounds. The reactor was heated by an electric furnace, which was connected to a programmable temperature controller. The sample was placed in a crucible which was leaned against a rod in the middle of the reactor. A thermocouple in direct contact with the sample was used to monitor the actual process temperature. In the catalytic experiments, the ratio of catalyst/sample was equal to 7:1 and samples of 300 mg (VGO, PE, or VGO–PE) were used. Prior to the experiments, the system was purged with a nitrogen stream at room temperature for 30 min to ensure an inert atmosphere. The nitrogen flow rate used was 150 mL/min. Experiments were carried out from 30 to 550 °C at 5 °C/min [91]. According to the authors, the obtained results indicated that VGO was not degraded in thermal pyrolysis. However, during the thermal decomposition of polyethylene, gases were generated in the whole studied temperature interval. In the thermal decomposition of the VGO–PE blend, non-condensed products were only collected in the interval of decomposition associated with the degradation of the PE, where n-alkenes constituted the major products. For the PE samples, thermal pyrolysis produced 18.7 wt% of gases and 80.6 wt% of condensable products, whereas catalytic pyrolysis yielded 31.7 wt% and 71.9 wt% of gases and liquids, respectively, with 4.9 wt% of coke. The predominant products obtained were isobutane and isopentane [87,91].

According to Lin et al. [87], mixtures of post-consumer polyethylene wastes (HDPE/LDPE) were pyrolyzed over various catalysts using a fluidized-bed reactor operating in the 290–430 °C range under atmospheric pressure. The yields of volatile hydrocarbons for zeolitic catalysts (HZSM-5 > HUSY > HMOR) were higher than for nonzeolitic catalysts (SAHA MCM-41). Observed differences in product yields and product distributions were attributed to the nature of the catalyst and the reaction temperature. Product distributions with HZSM-5 contained more olefinic materials with about 60 wt% in the range of C₃–C₅ [87].

It has been shown that both the product yields and product distributions can be controlled if appropriate reaction temperatures and suitable catalysts are used to perform the polymer pyrolysis, potentially leading to a cheaper process operation and more valuable products. For example, Wei et al. [74] showed that the conversion of post-consumption LDPE/HDPE/PP polymer wastes into volatile hydrocarbons was higher than 80 wt% in 20 min in the presence of cracking catalysts in a fluidized bed reactor. In general, the use of acidic zeolite catalysts, ZSM-5, MOR, and USY, provided higher amounts of volatile hydrocarbons than non-zeolitic catalysts (MCM-41 and ASA). ASA and MCM-41 catalysts provided the lowest conversion values and generated olefin-rich products with broad molar mass distributions. Higher product selectivities were observed with ZSM-5, which provided 60 wt% of products in the C₃–C₅ range, and MOR, which generated the highest i-C4 yields [85].

Elordi et al. [92] investigated the use of HZSM-5, HY, and Hβ zeolite-based catalysts in the pyrolysis of high-density polyethylene (HDPE), continuously fed into a conical spouted bed reactor (CSBR) operated at 500 °C and atmospheric pressure. Product streams were grouped into seven lumps: light olefins and light alkanes in the gas fraction; non-aromatic C–C compounds, single-ring aromatics, and C hydrocarbons in the liquid fraction; wax and coke. The results were compared with those obtained in thermal pyrolysis performed in a CSBR and with those obtained with catalysts in bubbling fluidized beds. It was shown that the HZSM-5 zeolite-based catalyst was very selective (58 wt%) to light olefins, whereas high yields of non-aromatic C–C products (45 wt%) were obtained with the Hβ and HY zeolite-based catalysts. Wax yields increased as reactions proceeded, especially with HY and Hβ zeolite-based catalysts, due to catalyst deactivation by coke formation. Product distributions obtained with the different catalysts and their evolution during the continuous operation depended on the employed zeolites [92].

Cai et al. [93] investigated the catalytic pyrolysis of HDPE, LDPE, and PP using Fe/Al₂O₃ catalysts in a two-stage fixed-bed reactor. The first pyrolysis stage was operated at 500 °C, while the catalytic upgrading stage reached 800 °C. The study focused on the effects of catalyst acidity and steam introduction on product distribution. The pyrolysis processes generated approximately 40% gaseous products and between 30–35% liquid products. The gaseous products were mainly composed of H₂ and CH₄, while ethylene accounted for only 3.7% [93].

Li et al. [94] investigated the catalytic pyrolysis of polyethylene using a hierarchical Ni/ZSM-5 catalyst in a two-stage fixed-bed reactor. The first stage, dedicated to pyrolysis, was operated at 500 °C, while the catalytic upgrading stage reached 800 °C. The study aimed to enhance hydrogen and carbon nanotube (CNT) production by modifying the catalyst through alkali treatment, improving metal dispersion and catalytic activity. The process yielded 48.7 wt% gaseous products and 6.35 wt% liquid products, with the remaining fraction as solid carbon deposits (43.8 wt%). The gas phase composition was dominated by H₂ (55.76 vol%) and CH₄, while ethylene accounted for a minor fraction. The results demonstrated that the modified Ni/ZSM-5 catalyst significantly improved hydrogen production (26.3 mmol/g PE) and promoted CNT formation with a high purity (66%), highlighting its potential for plastic waste valorization [94].

More recently, the increased production of gaseous products through the catalytic pyrolysis of plastic wastes using different catalysts FCC [95], USY [85,86,96], ZSM-5 [27,85,86,96], SAHA [86], Silicalite [85,86,96], TiCl4/MgCl2 [84], silica–alumina [97], Fe/Al₂O₃ [98], and Ni/ZSM-5 [94], have been reported by many researchers. Normally, polymer wastes were decomposed thermally with the aid of these catalysts to enhance the product yields.

Table 5. Selected research works in the area of the catalytic pyrolysis of polyolefins.

Authors	Feedstock	Reactor	Catalyst	Temperature (°C)	Time	Monomer (wt%)
Lin et al. (2008) [87]	HPDE/LDPE	Fluidized-bed	HUSY, HZSM-5, HMOR, SAHA, MCM-41	290, 330, 360, 390, 430	1–20 min	4% C2; 23% C3
Aguado et al. (2007) [79]	LDPE	Batch reactor—fixed bed reactor	HZSM-5, Al-MCM-41	425, 450, 475	120 min	>50% (C1–C4)
Marcilla et al. (2008) [91]	LDPE	Batch reactor—fixed bed reactor	FCC	350–550	52 min	4% C2; 15% C3, 20% C4
Zhang et al. (2015) [27]	LDPE	Microwave + packed-bed reactor	ZSM-5	249–450	-	80% ethylene (375 °C)
Sharratt et al. (1997) [88]	HDPE	Fluidized-bed reactor	HZSM-5	290, 330, 360, 390, 430	30, 20, 15 min	26.5% propene
Lin et al. (2005) [86]	PP	Fluidized-bed reactor	USY	290, 330, 360, 390, 430	15 min	C1–C4
Lin et al. (2007) [95]	PE/PP	Fluidized-bed reactor	RCat-c1 (FCC), USY, ZSM-5, SAHA, Silicalite	330, 360, 390, 420, 450	30 min	C1–C4
Wei et al. (2010) [85]	LDPE/HDPE/PP	Fluidized-bed reactor	USY	290, 330, 360, 390, 420	20 min	C1–C4
Donaj et al. (2012) [84]	LDPE/HDPE/PP	Fluidized quartz-bed reactor	TiCl4/MgCl2	500, 650	1.67, 2.5 h	12% methane, 6% ethane, 13% ethene, 12% propane, and 12% propene
Jung et al. (2010) [99]	PE/PP	Fluidized-bed reactor	Quartz sand	650, 750	-	34% methane, 7% ethane, 12% ethene, 1% propane, and 5% propene
Park et al. (2019) [100]	PP	Fluidized-bed reactors connected in series	Sand	400	-	52% of ethene, propene, 1,3-butadiene, and butenes
Elordi et al. (2009) [92]	HDPE	Conical spouted bed reactor (CSBR)	HZSM-5, HY and Hβ zeolite	500	-	70% (HZSM-5), 25% (HY), and 40% (Hβ)
Elordi et al. (2011) [101]	HDPE	Conical spouted bed reactor (CSBR)	HZSM-5, HY and Hβ zeolite	500	15 h	55% C2–C4 (HZSM-5), 20% C2–C4 (HY), and 25% C2–C4 (Hβ)
Lin et al. (2008) [87]	HDPE/LDPE	Fluidized-bed reactor	HZSM-5	290, 330, 360, 390, 430	20 min	C1–C4
Ali et al. (2002) [96]	HDPE	Fluidized-bed reactor	ZSM-5, US-Y, ASA, Cat-A (FCC), E-Cat	360, 450	-	72.6 (C1–C4) 68.6 (C1–C4)
Kodera et al. (2006) [97]	PP	Moving-bed reactor	Silica–alumina	700	10 min	methane, 18.7 %; ethylene, 19.5 %, ethane, 9.7 %; propylene, 24.2 %; propane, 3.4 %
Cai et al. (2021) [93]	PP, HDPE and LDPE	Two stages fixed-bed reactor	Fe/Al2O3	500–800	30 min	Ethylene 3.7%
Li et al. (2023) [94]	PE	Two stages fixed-bed reactor	Ni/ZSM-5	500–800	40 min	Traces

shows some of the works found in the literature that are related to the catalytic pyrolysis of polyolefins.

5.3. Perspectives on Catalytic Pyrolysis

According to the literature, catalytic pyrolysis presents several competitive advantages for the chemical recycling of polyolefin waste, including lower reaction temperatures, accelerated polymer degradation, shorter residence times, and the production of lighter products [82,90]. Among the catalysts investigated, zeolites have been the most extensively studied—primarily due to their strong acidity and well-defined microporous structures, which facilitate efficient C–C bond cleavage via β-scission mechanisms. These properties, along with their similarity to catalysts used in conventional fluid catalytic cracking (FCC) processes, make zeolites particularly attractive for promoting the formation and recovery of valuable hydrocarbons, especially light olefins such as ethylene and propylene [27,83,87,92].

Zeolites—particularly HZSM-5, HY, and Hβ—exhibit strong acidity and shape-selective properties that facilitate the efficient cracking of polymer chains and enhance selectivity toward olefins. Studies have reported light olefin yields as high as 58 wt% using HZSM-5, along with significant improvements in product quality compared to thermal pyrolysis. Additionally, zeolite catalysts enable lower reaction temperatures and shorter residence times, making the process more energy efficient. Despite these advantages, challenges such as catalyst deactivation due to coke formation, limited monomer selectivity, and the need for effective catalyst regeneration remain critical barriers to industrial-scale implementation.

Nonetheless, the selectivity toward monomeric olefins remains relatively low in most studies, often requiring additional processing steps or catalyst modifications to achieve yields compatible with circular plastic production chains. Moreover, the commercial viability of catalytic pyrolysis continues to face substantial technical and economic hurdles, including the need for more efficient and cost-effective catalysts, improved reactor and process designs, and robust strategies for catalyst regeneration and reuse. Therefore, while zeolite-catalyzed pyrolysis shows considerable promise for converting polyethylene waste into olefins, further research and technological advancement are essential to fully unlock its industrial potential.

6. Chemical Recycling Through Electrochemical and Oxidative Degradation

6.1. Fundamentals of Electrochemical and Oxidative Degradation

Electrochemical processes have been extensively studied for applications such as water splitting (for hydrogen and oxygen production), CO₂ reduction, oxygen reduction, organic synthesis, and biomass conversion. Notably, the electrochemical reduction of CO₂ has been widely explored as a route for producing storable fuels and valuable chemical feedstocks [80]. More recently, electrochemical technologies have gained attention as promising tools for the sustainable recycling of plastic waste.

These processes allow for the precise control of electron energy through the manipulation of potential differences, which can be particularly advantageous for selectively recycling multilayer polymer materials or complex plastic mixtures. The basic setup involves two electrodes—a cathode and an anode—between which an electric current is applied. This current may be supplied externally (e.g., via a power source) or generated internally through spontaneous redox reactions, enabling controlled polymer chain degradation and chemical transformation [102]. In particular, the use of segregated electrochemical cells—separated by semi-permeable membranes—can enhance product selectivity by isolating the species formed at each electrode compartment [103].

However, since plastic materials are inherently poor electrical conductors, one of the primary challenges in electrochemical recycling lies in developing a reaction medium capable of facilitating electrical conductivity between the electrodes. Additionally, it is crucial to design electrochemical systems that enable effective interaction between the electrode surfaces and the plastic material, ensuring efficient electron transfer for the desired chemical transformations [104].

In electrochemical recycling systems, waste plastics are typically converted into forms suitable for anodic oxidation. For polyesters or biomass-derived materials, this often involves a preliminary hydrolysis step to produce soluble intermediates. In the case of polyolefins such as polyethylene (PE) and polypropylene (PP), a common strategy is to subject the material to thermal pyrolysis to generate pyrolytic oils, which are more reactive and compatible with electrochemical oxidation processes. Additionally, some studies have reported the solubilization of these polymers in strong acids or appropriate organic solvents to facilitate their electrochemical processing [98,102].

Additionally, electrochemical processes use electrons as reagents, which produce no waste and are increasingly derived from renewable energy sources. This reduces the reliance on potentially harmful chemical reducing and oxidizing agents that often generate undesirable by-products. Another key advantage is the inherent separation of half-reactions, which allows for a greater flexibility compared to conventional thermocatalytic approaches. For instance, electrochemical systems can enable combined catalysis, where a single electrochemical reaction yields two valuable products simultaneously [102].

Petersen et al. [98] conducted a literature review on electrochemical methods with potential applications in the chemical recycling of various materials. The authors identified several electrochemical techniques frequently explored for recycling purposes: (i) electrostatic separation, which applies a constant electric current across two electrodes to exploit differences in electrical properties, leading to material separation and agglomeration around the electrodes; (ii) electroplating, where an electric current is used to deposit solid metal onto the cathode; (iii) electrocoagulation, involving the generation of ions via electric current to induce coagulation of suspended solids, droplets, and flakes; (iv) electrodialysis, which uses ion exchange membranes and electric fields to transport ions between solutions; (v) electrochemical ion exchange, a combination of electrodialysis and ion exchange; (vi) electroelectrodialysis, which integrates electrolysis and electrodialysis to separate ions from solution; (vii) electrodeionization, a variation of electrochemical ion exchange used for ion removal from solutions; (viii) electroflotation, in which in-situ gas bubbles (typically from water splitting) promote the flotation of suspended particles; (ix) sludge electrolysis, which dissolves solid, metal-containing particles via electric current and recovers them by electrodeposition.

As noted by the authors, none of these methods are directly applicable to the chemical recycling of plastic waste for monomer recovery [98]. Therefore, this field requires more focused investigation based on the currently available literature. Figure 5 shows a diagram of typical chemical recycling through electrochemical and oxidative degradation.

6.2. Previous Works of Electrochemical and Oxidative Degradation

Wang et al. [105] described the chemical recycling of PET in the form of a hydrolysate, which was prepared through alkaline hydrolysis in aqueous solutions. The final hydrolysate contained the terephthalate and ethyleneglycol monomers in the aqueous solution. Then, an electrocatalytic reaction was performed, using NiCo₂O₄ as a catalyst and tin dioxide nanosheets (SnO₂) as a cathodic catalyst for the electroreduction of CO₂ to formic acid [105]. Therefore, the proposed chemical recycling scheme was not intended for the manufacture of the monomers, as they were produced in fact during the first reaction stage.

Zhou et al. [104] published a review on recent innovations and unconventional processes for plastic recycling and upcycling, with emphasis on multidisciplinary catalytic techniques to achieve cost-effective and/or sustainable approaches. The authors focused on chemical, electrochemical, photochemical, and biological approaches for plastics recycling and upcycling. Particularly, Zhou et al. [104] described some published works that made use of electrochemical approaches to promote the depolymerization of PET and PS, but did not describe any previous work that performed the depolymerization of polyethylene or polypropylene through electrochemical techniques [104]. One of the methodologies described by Zhou et al. [104] involved the hydrolysis of PET before the electrolysis in alkaline water to produce more valuable oxygenated products and H₂. The authors also provided a techno-economic analysis (TEA) for a chemical–electrochemical tandem process intended for the upcycling of PET to TPA, potassium diformate (KDF), and H₂ [104]. In order to facilitate the interaction between substrates and electrodes, Zhou et al. [104] reported additionally that the plastic material should be dissolved in a suitable organic solvent to allow the preparation of a homogeneous electrolyte medium. For example, PS should be dissolved in tetrahydrofuran (THF) to allow the electrochemical functionalization of the benzenoid ring and to provide a pathway for PS waste recycling. Furthermore, the electroreductive dearomatization of PS dissolved in THF can occur to generate olefinic products and increase the value of the product. It is important to point out that the authors carried out the electrocatalytic oxidation reactions with help of a CoNi_0.25P catalyst [104].

Pichler et al. [106] performed the oxidative depolymerization of PE in diluted nitric acid to convert PE into organic acids and then to produce hydrocarbons, H₂, and CO₂ through photo or electrocatalytic decarboxylation reactions. The authors showed that it is possible to carry out a tandem process that integrates the previous processes, allowing the direct conversion of polyethylene into gaseous hydrocarbon products with a total hydrocarbon yield of 1.0% for the oxidative/photocatalytic route and 7.6% for the electrolytic route. Using TiO₂ or carbon nitride as catalysts, it was possible to produce ethane and propane through photocatalysis and ethylene and propylene through electrocatalysis. This two-step plastics recycling process can use sunlight or renewable electricity to convert polyethylene into valuable and easily separable gaseous platform chemicals [106]. The authors performed the electrocatalytic procedure with the help of electrodes of carbon paper, graphite rod, and glass coated with fluorine-doped tin oxide (FTO) as working electrodes, Pt sheets as counter electrodes, and a single Ag/AgCl junction saturated with NaCl. A higher ethylene productivity was observed at higher pH values, giving a faradaic yield of approximately 30% for carbon paper electrodes at pH 10. Alkaline conditions were shown to be beneficial, as acid deprotonation facilitates the decarboxylation step, which gives access to a good faradaic yield for this reaction. Acetylene and CO₂ were produced as gaseous by-products, while adipic acid was detected in the liquid phase [106].

Partenheimer [107] carried out PE oxidation experiments in solutions and in the presence of a catalyst (Co/Mn/Zr/Br). The reactor was pressurized at 70 bar with air and heated for 15 min at different temperatures. The author showed that the product stream contained succinic acid and glutaric acid at different concentrations, depending on the amounts of catalyst and solvent and on the temperature. For example, to produce 62 wt% of succinic acid and 29 wt% of glutaric acid, a 2:4 PE/catalyst ratio was used with 8 wt% of water at a temperature of 180 °C. Gas formation was not reported [107].

Pifer and Sen [108] carried out the oxidative degradation of PE, PP, PMMA, PS, and PAM in the presence of oxides and oxygen at 170 °C, and reported the production of short-chain diacids (pimelic acid, adipic acid, glutaric acid, and succinic acid) with good yields, which were as high as 73 wt% [108].

Jiao et al. [109] performed the conversion of plastic wastes into acetic acid through the catalytic photoinduced sequential C–C cleavage of PE, PP, PVC bags, disposable food containers, and food wrapping films at room temperature and atmospheric pressure, in the presence of air atmosphere, pure water, and simulated irradiation from the sun. For example, it was shown that PE can be completely photodegraded to CO₂ in 40 h using Nb₂O₅ as a catalyst. In these reactions, OH radicals trigger the oxidative C–C cleavage of PE to form CO₂, while the produced acetic acid is derived from the photoreduction of CO₂ via intermediate C–C coupling [109].

Jiang et al. [110] designed a thermo-coupled solar electrochemical system to perform the depolymerization of plastics and produce fuels, promoting the use of solar energy for the conversion of polymers. For example, PP could be converted into light fuel and hydrogen through solar-powered electrolysis coupled with pyrolysis. The results indicated that the depolymerization temperature could be significantly reduced in respect to conventional pyrolysis. Furthermore, obtained conversions were high and reached 68.48% at 350 °C, resulting in the solid (31.52%), liquid (7.85%), and gaseous materials (60.63%), while conversions of 28.39% were obtained at similar conditions for standard pyrolysis experiments [110].

Bäckström et al. [111] used nitric acid (0–0.50 g/mL aqueous solutions) as a catalyst to carry out the electro-oxidative degradation of LDPE. The tests were performed in a Milestone UltraWAVE single reaction chamber microwave for 2 h at 180 °C and 40 bar. After the reaction, the solution was neutralized with 3 M sodium hydroxide solution and lyophilized. The product streams contained mostly organic acids (malonic, succinic, glutaric, adipic, and pimelic acids) [111]. After that, Bäckström et al. [112] presented an effective route to recycle HDPE through a similar microwave-assisted process, which allowed the selective degradation of HDPE into succinic, glutaric, and adipic acids [112].

Table 6. Selected research works in the areas of the electrochemical oxidation of polyolefins.

Authors	Feedstock	Catalyst	Temperature (°C)	Maximum Gaseous Production	Gas Composition
Wang et al. (2022) [105]	PET	NiCo2O4	-	-	-
Pichler et al. (2021) [106]	PE	TiO2, carbon nitride	180 °C	20%	H2, CO2, ethane, ethene, propane, and propylene
Partenheimer et al. (2003) [107]	PVC, PS, PP, PE, PET, PBT, PEN	Co/Mn/Br/Zr, Co, Co/Zr, Co/Mn, Co/Ce, Co/Ni, Ni, Co/NHPI	150–220 °C	-	-
Pifer et al. (1998) [108]	PE, PP, PMMA, PS PAM	-	170 °C	-	-
Jiao et al. (2020) [109]	PE, PP	Nb2O5	25 °C	-	-
Jiang et al. (2020) [110]	PP	-	350 °C	60.63%	C1–C5, H2
Bäckström et al. (2017) [111]	LDPE	Nitric acid	180 °C	-	-
Bäckström et al. (2019) [112]	HDPE	Nitric acid and crotonic acid	180 °C	-	-

shows some works found in the literature related to electrochemical degradation and oxidation for polyolefins.

6.3. Perspectives on Electrochemical and Oxidative Degradation

Research on the use of electrochemical methods for monomer recovery has primarily focused on oxygenated compounds such as PET, poly(ethylene glycol), and biomass-derived materials (mainly cellulose and lignin), as reported by Wang et al. [105], Zhou et al. [104], and Partenheimer [107]. Although a few studies have investigated the electrochemical treatment of polyolefins (e.g., PE and PP), their goal has generally been the production of intermediate organic compounds—such as carboxylic acids—rather than the recovery of original monomers [107,108,109,111,112].

It is noteworthy that only a small number of studies have reported gas production as a final product from electrochemical or oxidative degradation processes. Some works that combine electrochemistry with photocatalysis have demonstrated the generation of minor amounts of gases, but not in quantities sufficient to indicate effective monomer recovery, as observed in studies by Jiang et al. [110] and Pichler et al. [106]. Despite the fact that the electrochemical and oxidative degradation of polyolefins has been explored since the 1990s, these technologies remain at low Technology Readiness Levels (TRLs), typically limited to small-scale laboratory experiments involving low polymer quantities.

Common polymers are chemically stable and generally resistant to direct electrochemical modification, often requiring a preliminary thermal depolymerization step—such as thermolysis or solvolysis—prior to electrochemical processing. The resulting electroactive depolymerization products are typically subjected to oxidation, either to generate higher-value compounds or to support water electrolysis for hydrogen production [103]. Although these processes may not yield high levels of monomer recovery, the resulting products—particularly carboxylic acids and hydrogen—are valuable for the chemical industry and offer a foundation for new polymer synthesis routes. A major challenge in advancing electrochemical oxidation for breaking nonpolar C–C bonds lies in avoiding overoxidation, which can lead to the complete degradation of the polymer into CO₂ [102].

7. Chemical Recycling with Ionic Liquids

7.1. Fundamentals of Chemical Recycling with Ionic Liquids

Ionic liquids are a class of salts that exist in the liquid state at or near room temperature. Composed entirely of ions—charged particles—they differ from conventional salts, which are typically crystalline solids with high melting points. Ionic liquids exhibit several distinctive properties, including a low vapor pressure, high thermal stability, and non-flammability. These characteristics make them attractive for a wide range of applications in chemistry, materials science, and engineering [98,99].

Structurally, ionic liquids consist of a combination of positively charged cations and negatively charged anions. The specific choice of these ionic species determines the physical and chemical properties of the resulting liquid. Given the vast number of possible cation–anion combinations, a large variety of ionic liquids can be synthesized, each with tailored characteristics. This tunability is one of their key advantages: by selecting appropriate ion pairs, researchers can adjust parameters such as viscosity, conductivity, solubility, and even chemical reactivity [102,103,104]. As a result, ionic liquids have found applications as solvents, electrolytes, and catalysts in areas including organic synthesis, electrochemistry, extraction processes, energy storage systems, and as environmentally friendly alternatives to traditional organic solvents [113,114].

It is essential to consider the potential toxicity and environmental impact of ionic liquids when evaluating their applications. While many ionic liquids are regarded as relatively safe, concerns remain regarding their long-term ecological effects. As a result, research efforts have increasingly focused on the development of greener alternatives—such as ionic liquids derived from natural sources or specifically engineered to exhibit reduced toxicity [113,114]. Indeed, research in this field continues to expand, with ongoing studies exploring the synthesis, properties, and diverse applications of ionic liquids. As scientific understanding of these materials deepens, their potential contributions to technological innovation across various sectors become increasingly evident [98,99]. It is therefore unsurprising that several studies have examined the use of ionic liquids in chemical recycling processes. These investigations include applications involving polymers such as polycarbonate, lignin, and general plastic waste, where ionic liquids function as solvents or catalysts.

Notably, some ionic liquids have demonstrated the ability to cleave carbon–carbon bonds in polyolefin waste. However, most studies focus on enhancing the yield of liquid products rather than gaseous fractions [115]. An illustrative example of a chemical recycling process employing ionic liquids is presented in Figure 6.

7.2. Previous Works of Chemical Recycling with Ionic Liquids

Adams et al. [116] used ionic liquids to recycle LDPE and HDPE wastes. Experiments were carried out in temperatures between 120 and 250 °C in the presence of HCl or H₂SO₄ for 24 to 72 h. The used ionic liquids were 1-Ethyl-3-methylimidazolium (Emim), 1-Butyl-3-methylimidazolium (Bmim), and 1-butylpyridinium (C4Py), in the presence of AlCl₃. Polymer conversions reached 95% and resulted in gaseous products, mostly C₃–C₅ alkanes, such as propane, 2-methyl-propane, 2-methyl-butane, and n-butane. In spite of that, the gaseous products did not contain monomers [116].

Zhang et al. [115] reported a novel technique to convert untreated post-consumer materials into high-quality liquid alkanes with significant yields. This method involved the synergistic combination of two reactions: the endothermic cleavage of polymer C–C bonds and the exothermic alkylation reactions of the resulting cracking products. By employing this innovative approach, the complete conversion of PE and PP into liquid isoalkanes (ranging from C₆ to C₁₀) was achieved at temperatures below 100 °C. The key catalyst utilized in this process was a Lewis acidic species generated within a chloroaluminate ionic liquid, which played a critical role in enhancing the reaction efficiency. Furthermore, the resulting alkylate product exhibited distinct phase separation, facilitating easy separation from the catalyst–reactant mixture. These findings underscore the potential of this method for a sustainable transformation of polyolefin waste into valuable liquid alkane products [115].

7.3. Perspectives on Chemical Recycling with Ionic Liquids

The use of ionic liquids for the controlled degradation of biomass and PET has been widely documented in the literature. In contrast, their application to the controlled degradation of polyolefins remains in its early stages. Only a limited number of studies have been identified, all of which involve the combination of ionic liquids with the Lewis acid salt aluminum chloride. Notably, the study by Adams et al. [116] reports substantial gas production with some degree of monomer recovery; however, these findings require further validation, as no follow-up publications or confirmations of the results have been found [116].

More recently, Zhang et al. [115] reported the successful degradation of polyolefins into naphtha-range molecules using extremely mild temperature conditions [116]. This promising study may pave the way for the development of new technologies and further exploration of different ionic liquid formulations for polyolefin depolymerization. Based on the current evidence, it is plausible that ionic liquids could be used for monomer recovery from polyolefins, although further investigation and validation are still required.

8. Chemical Recycling Through Tandem Catalysis

8.1. Fundamentals of Tandem Catalysis

Tandem catalysts are highly efficient systems capable of promoting the metathesis of linear alkanes selectively and at moderate temperatures by combining two distinct catalytic functions: one for alkane dehydrogenation and another for olefin metathesis. This approach involves the sequential execution of two or more catalytic reactions within the same system to convert raw materials into target products in a synergistic manner [117,118]. In such configurations, the intermediate generated in the first catalytic step is immediately consumed in the subsequent reaction, eliminating the need for isolation or purification. This integration enhances process efficiency by reducing material and energy losses typically associated with intermediate handling and separation [117].

Goldman et al. [118] described a fundamental tandem catalytic process for the chemical recycling of polyolefins. In this system, a dehydrogenation catalyst (denoted as M) reacts with an alkane to produce the corresponding terminal alkene (C_n) and a metal hydride species (MH₂). The terminal alkene then undergoes olefin metathesis to yield an internal alkene (C_2n−2) and ethylene. Subsequently, the resulting alkenes act as hydrogen acceptors and react with MH₂ to form two new alkanes, simultaneously regenerating the catalyst M and closing the catalytic cycle. The authors also investigated two heterogeneous catalytic systems capable of mediating this alkane interconversion [118].

By integrating consecutive catalytic steps into pyrolysis, tandem catalysis aims to enhance process efficiency, improve the selectivity of target products, and minimize the formation of undesired by-products. The effectiveness of this approach depends heavily on the selection of appropriate catalysts for each step and the careful optimization of reaction conditions to fully realize the benefits of tandem catalysis in thermal pyrolysis [119].

8.2. Previous Works of Tandem Catalysis

The use of tandem catalysis for the controlled polyethylene degradation can allow the efficient breaking of polymer bonds, improving the selectivity and yields of desired products. Furthermore, the use of different consecutive catalytic steps helps to control the reactions and to minimize the formation of undesired by-products.

Jia et al. [117] carried out the PE degradation based on a tandem catalytic cross-metathesis process, which involves a catalyst for alkane dehydrogenation and another catalyst for olefin metathesis. Firstly, the dehydrogenation catalyst removes hydrogen from the PE. Then, the olefin metathesis catalyst modifies the alkenes generated in the breakage of the PE chains. The authors used the cross-metathesis of PE (120 mg) with n-octane (2.5 or 4.0 mL) and various g-Al₂O₃—supported iridium catalysts (20.1 mmol Ir) and Re₂O₇/g-Al₂O₃ (57 mmol Re₂O₇). The conversion of PE to oil and wax products and the distribution of soluble n-alkane products occurred after heating the mixture for 4 days at 175 °C. The amounts of soluble products were determined by GC analyzes with mesitylene (20 mL) as an internal standard. Various types of PEs, including HDPE, LDPE, and LLDPE, can be completely degraded to low-MW oils and waxes within 1 day at 175 °C. The tandem system uses a highly efficient alkane dehydrogenation catalyst, the molecular pincer-type iridium complex, to perform the dehydrogenation of PE and of the light alkanes. The resulting unsaturated PE and light alkene undergo rhenium-catalyzed cross-metathesis to form two new olefins, which are hydrogenated by the iridium catalyst to produce saturated alkanes [117].

Zhang et al. [120] investigated how tandem catalytic processes could be deployed to convert waste polyethylene, and also performed solvent-free depolymerization of two different commercial grades of PE: low molecular-weight polyethylene, LDPE plastic bag, and HDPE bottle cap. The stability of the Pt/g-Al₂O₃ catalyst was investigated by conducting three consecutive 6-h reactions, with the regeneration of the recovered catalyst between each experiment. The results showed that liquid/wax yields decreased by 15 wt% in the second run but stabilized in the third run. The higher-molecular-weight polymers behaved similarly to the low-molecular-weight polyethylene and liquid wax products (~C₃₀). After 24 h at 280 °C, the overall liquid yields were equal to 69 and 55 wt% for LDPE and HDPE, respectively. However, the results were similar for three different plastics (including two commercial grade samples of LDPE and HDPE). Therefore, the authors showed that waste polyolefins can constitute a viable feedstock for the generation of molecular hydrocarbon products [120].

Dai et al. [121] studied tandem catalytic pyrolysis of LDPE into highly valuable naphtha (Al₂O₃ followed by ZSM-5 zeolite). The Al₂O₃ showed excellent performance for catalytic reforming of LDPE pyrolysis vapors, mainly producing C₅–C₂₃ olefins that are the important precursors to form aromatics via Diels–Alder, aromatization, and polymerization reactions. Experimental results also showed that the selectivity of monoaromatics and C₅–C₁₂ alkanes/olefins could reach 100% over Al₂O₃ followed by ZSM-5 tandem catalysis at the temperature of 550 °C, with a catalyst to plastic ratio of 4:1, and Al₂O₃ to ZSM-5 ratio of 1:1. The authors reported the product (monoaromatics and C₅–C₁₂ alkanes/olefins) could be a renewable feedstock for new plastic production in the chemical industry, so that this finding might provide a new insight for a circular economy [35,121]. A representative chemical recycling process employing tandem catalysts is shown in Figure 7.

8.3. Perspectives on Tandem Catalysis

In general, tandem catalysis refers to a strategy in which two or more sequential reactions are carried out synergistically, with intermediates generated in one step serving as reactants for the next. This approach facilitates the efficient and sustainable synthesis of desired products. As such, tandem catalysts have found applications in areas such as organic synthesis, energy conversion, and the chemical recycling of plastic waste [103,104]. A critical requirement for successful tandem catalysis is the mutual compatibility of the catalysts involved in each step. Achieving this compatibility can be challenging, as some catalyst combinations may lead to undesirable side reactions or a reduced overall process efficiency.

Studies investigating the depolymerization of polyolefins using tandem catalysis have thus far been limited to laboratory-scale experiments, typically involving small polymer quantities (on the order of grams or milligrams), which corresponds to a low Technology Readiness Level (TRL). These investigations generally employed low reaction temperatures and long residence times, resulting in the formation of liquid and gaseous hydrocarbons rather than direct monomer recovery. Nevertheless, the approach shows promise—particularly due to its lower energy requirements compared to conventional thermal degradation. To advance this technology toward practical application, future research must focus on significantly reducing reaction times while maintaining product yield and selectivity.

9. Some Environmental Aspects

In addition to technical efficiency and selectivity, the environmental implications of emerging chemical recycling technologies must be considered. Although this review primarily addresses the mechanisms and monomer recovery potential of each advanced technique, it is also important to assess their environmental impact. However, it is very important to observe that this constitutes a significant gap in this field, as the vast majority of the published material has not considered or discussed the environmental aspects of the investigated technologies. For this reason, some important environmental effects of these technologies are highlighted below based on the personal assessment of the authors.

Microwave-assisted and plasma-assisted pyrolysis, for instance, enable rapid reactions with a lower thermal input, which may reduce energy consumption compared to conventional pyrolysis. However, plasma generation and energy sourcing require evaluation, particularly in large-scale applications. Supercritical fluid processes (e.g., with water or CO₂) eliminate the need for solvents or extreme thermal cracking, but typically demand high pressures and precise control systems, potentially increasing energy usage and operational complexity. Electrochemical and oxidative degradation techniques operate under mild conditions and offer cleaner transformation pathways. Their sustainability, however, largely depends on the origin of the electricity used and the handling of oxidation by-products. Still, these technologies may offer significant environmental advantages in the context of the global energy transition, marked by a growing adoption of renewable energy sources in both developed and developing regions. The use of ionic liquids and tandem catalysts presents opportunities for achieving high selectivity and efficiency. Nevertheless, their environmental performance must be critically assessed, especially regarding solvent or catalyst synthesis, recyclability, and end-of-life disposal. As many of these technologies are still in early development stages, comprehensive life cycle assessments (LCAs) are limited. Further studies are essential to better quantify their environmental impact in terms of energy demand, greenhouse gas emissions, and potential toxicity of auxiliary materials.

10. Conclusions

The present study investigated different technologies that can be potentially used for the recycling of polyolefins and the production of olefin monomers. Although most of them could be used to produce high amounts of gases as a product, none of the analyzed processes have demonstrated the ability to consistently generate product streams that are rich in monomers. In general, the proposed technologies still present low TRL values for chemical recycling, although some of them are already widely used in the chemical industry. Nevertheless, it is important to highlight that some catalytic and supercritical pyrolysis technologies are being validated in pilot plants, although it is not yet clear that these processes will allow the recovery of large amounts of olefin monomers. Among the investigated processes, catalytic pyrolysis, especially when performed with zeolites, can be regarded as the one that comes the closest to the concept of “plastic to monomer” due to its ability to generate gas streams with a relatively high olefin content, as reported in several studies. However, it must be highlighted that the long-term performances of catalysts should still be characterized under conditions representative of industrial operation, which is of fundamental importance for the economic feasibility of the proposed process. Particularly, studies that combined zeolites with the microwave-assisted technology achieved very good results. Finally, while the objective of producing olefinic gases with high selectivities has not been achieved yet in many of the analyzed technologies, excellent results were obtained in terms of producing light oil in the range of naphtha, gasoline, and diesel, which can be considered promising routes toward contributing to the circular economy of plastic waste.