Advancements in Chemical Recycling Catalysts for Plastic Waste in South Korea

Abstract

Plastics are widely used in various industries because of their light weight, low cost, and high durability. The mass production and consumption of plastics have led to a rapid increase in plastic waste problem, necessitating the development of effective recycling technologies. The chemical recycling of plastics has emerged as a promising strategy to address these challenges, enabling the conversion of plastic waste into high-purity monomers or oils, even from contaminated or mixed plastic feedstock. This review focuses on the development of catalysts for the chemical recycling of plastics in South Korea, which has one of the highest per capita plastic consumption rates and both academic and industrial efforts in this field. We examine catalytic depolymerization processes for recovering monomers from polymers, such as polyethylene terephthalate (PET) and polycarbonate (PC), as well as catalytic pyrolysis processes for polyolefins, including polyethylene (PE), polypropylene (PP), and polystyrene (PS). By summarizing recent academic research and industrial initiatives in South Korea, this review highlights the strategic role of the country in advancing chemical recycling. Moreover, this review proposes future research directions including the development of reusable catalysts, energy-efficient recycling process, and strategies for recycling mixed or contaminated plastic waste.

1. Introduction

Petroleum-based polymers, known as plastics, are widely used in various fields, including agriculture, households, automobiles, and packing materials [1] because of their advantages such as lightness, affordability, high durability, and resistance to biodegradation [2]. Due to these advantages, plastic consumption has increased since the onset of mass production. In 2019, global plastic consumption reached 460 million tons, and it is expected to rise to 1.231 billion tons by 2060 [3].

In South Korea, plastic usage has also significantly increased from 8.0 million tons in 2017 to 11.9 million tons in 2021 [4]. Among the various types of plastics, polyethylene (PE) is the most widely produced, followed by polypropylene (PP) and other polymers, such as polyethylene terephthalate (PET), polystyrene (PS), and polyvinyl chloride (PVC). These plastics contributed to 38% of the waste generated from packaging materials and containers, 21.0% from textiles and clothing, 17.3% from buildings, and 9.1% from household goods and other products [5].

The large-scale production and consumption of plastics have led to significant environmental concerns owing to plastic waste accumulation [6]. The majority of plastic waste is either landfilled (79%) or incinerated (12%), with only 9% being recycled [7]. Because plastics are highly resistant to biological decomposition, landfill waste continues to accumulate over time. Additionally, some discarded plastics enter the ocean and break down into microplastics, which pose a severe threat to marine ecosystems and human health because of their accumulation in the food chain [8]. Furthermore, South Korea has one of the highest per capita plastic consumption rates globally, with 132.7 kg per person recorded in 2016 and 114.4 kg in 2018 [9]. Therefore, an increase in plastic consumption exacerbates environmental challenges and necessitates the development of effective recycling solutions.

To address this issue, extensive research has been conducted on plastic-recycling technologies. Global research on the chemical recycling of plastics has increased significantly, from 146 papers in 2016 to 1027 in 2024, whereas research in South Korea has grown from 5 studies in 2016 to 77 in 2024 (Figure 1). The combination of high plastic consumption and strong research activity highlights growing importance in the field of chemical recycling of plastics in South Korea. In this review, we focus on research related to catalysts for the chemical recycling of plastic waste in South Korea. This review first examines catalysts for depolymerizing plastics, such as PET and polycarbonate (PC), into their monomeric compounds. Subsequently, we explore Korean studies on catalysts for the pyrolysis of various polymers, including PE, PP, PS, and nylon.

2. Mechanisms of Chemical Recycling of Plastics

2.1. Categorization of Chemical Recycling Methodologies

Polymeric materials can be classified based on their polymerization mechanisms, namely, chain polymerization [10], and step polymerization [11]. In chain polymerization, monomers are sequentially attached to an initiator, leading to the formation of polymers such as PE, PP, and PS. In contrast, step polymerization involves the reaction of monomeric units containing functional groups to form polymer chains with PET, nylon, or PC as representative examples.

Chemical recycling methods can be broadly categorized into depolymerization and pyrolysis. Polymers synthesized via chain polymerization are predominantly recycled via pyrolysis, whereas those produced via step polymerization are primarily subjected to depolymerization (Figure 2).

Depolymerization is a recycling process that breaks polymer chains to obtain monomers, which can then be re-polymerized to synthesize new polymers. This approach offers the advantage of theoretically maintaining the physical properties of the original polymer without causing degradation. Depolymerization is primarily applied to step-polymerized plastics such as poly(ethylene terephthalate) (PET), nylon, and polycarbonate (PC). However, in the absence of a catalyst, depolymerization suffers from low reaction rates and yields, necessitating further research on catalytic systems to enhance efficiency. Pyrolysis, on the other hand, involves the random cleavage of polymer chains to produce high-value products such as oil and gas. These products are particularly valuable because they can be utilized in the chemical and fuel industries. However, pyrolysis also faces challenges because it typically requires high temperatures and results in low yields when conducted without a catalyst [12]. Therefore, both pyrolysis and depolymerization require substantial energy inputs and prolonged reaction times, underscoring the importance of catalyst development to lower reaction temperatures, and enhance the efficiency of chemical recycling processes.

2.2. Mechanisms of Depolymerization

Polymers containing heteroatoms in their main backbone, such as ester or carbonate bonds, are usually recycled through depolymerization. This process involves cleaving the bond between the carbonyl carbon and the heteroatom, as it is the weakest link in the polymer chain. The carbonyl carbon, which is highly electrophilic, is susceptible to attack by nucleophiles, leading to depolymerization. In the case of PET, water, methanol, and ethylene glycol have been used as nucleophiles to produce terephthalic acid (TPA), dimethyl terephthalate (DMT), and bis(2-hydroxyethyl) terephthalate (BHET) monomers via transesterification, respectively. Because depolymerization is an equilibrium reaction with polymerization, the nucleophilic reactant is often used in excess and serves as the solvent to drive the reaction toward monomer formation—a process known as solvolysis [13].

Catalysts are being developed to lower the activation energy of the depolymerization reaction, thereby improving process efficiency and reducing operational costs. A central strategy in catalyst design focuses on stabilizing the intermediate during the carbonyl substitution reaction [14]. One approach involves increasing the electrophilicity of the carbonyl carbon to facilitate nucleophilic attack. This is typically achieved using Lewis acid, which interacts with the carbonyl oxygen, thereby enhancing the electrophilic character of the adjacent carbon. Another complementary strategy is to increase the nucleophilicity of the nucleophilic solvent, which can be promoted by bases capable of accepting hydrogen bonds. For instance, in the PET glycolysis, the intermediate state of the transesterification reaction is stabilized through hydrogen bonding between a base catalyst and the hydroxyl hydrogen atom of ethylene glycol, thus accelerating the reaction (Figure 3). As a result, catalytic systems that possess both Lewis acidic and basic functionalities, such as metal salts, ionic liquids, and deep eutectic solvents, are commonly employed for depolymerization [14]. In addition, nanocatalysts or heterogeneous catalysts bearing Lewis acid or base sites have also demonstrated effectiveness in these reactions.

2.3. Mechanisms of Pyrolysis

PE, PP, and PS, of which main backbones are composed solely of carbon atoms, can be thermally degraded into high-value-added products through pyrolysis. Since C-C bonds possess high bond dissociation energies, substantial thermal energy is required to initiate their cleavage. Under such high-temperature conditions, the random scission of C-C bonds occurs, generating hydrocarbon radicals. These radicals undergo hydrogen transfer to form secondary radicals, which then lead to β-scission reactions, ultimately generating shorter hydrocarbon fragments. The pyrolysis of PE typically yields less-branched n-alkanes, whereas PP produces branched hydrocarbons due to the influence of methyl groups in its structure [15]. In contrast, the pyrolysis of PS results in various aromatic compounds such as styrene and toluene, through β-scission [16].

The pyrolysis temperatures follow the order PS < PP < PE, indicating that PE requires the highest energy input. This trend is attributed to differences in the stability of the hydrocarbon radicals generated during pyrolysis. PE consists of secondary carbon atoms (-CH₂-) along its backbone, whereas PS and PP contain tertiary carbon atoms (-CHR-). Hydrocarbon radicals formed at tertiary carbons are more stable than those at secondary carbon. The increased stability of radicals at tertiary carbons leads to longer lifetimes, thereby enhancing the propagation of chain scission reactions in PS and PP [17]. Furthermore, the C-C bond at a tertiary carbon has a lower bond dissociation energy compared to that at a secondary carbon, facilitating bond cleavage at lower temperatures [18]. The particularly low pyrolysis temperature of PS is further attributed to the resonance stabilization of the radical intermediates by the phenyl substituent.

To reduce the high thermal energy requirements of pyrolysis, various catalysts have been developed. Solid acid catalysts, particularly zeolites, are widely employed due to their ability to generate and stabilize radical intermediates. Zeolites, ordered porous alumina silicates, are well-established pyrolysis catalysts, featuring both Lewis and Bronsted acid sites as well as tunable pore sizes and acidity [19]. Their Bronsted acid sites facilitate proton addition reactions, while Lewis acid sites promote radical generation (Figure 4).

Pyrolysis reactions can be enhanced through hydrogenolysis, a bond cleavage process assisted by the addition of hydrogen gas (Figure 4). Metal catalysts capable of stabilizing intermediates via hydrogen adsorption are commonly used in hydrogenolysis. Additionally, the chemical recycling of PE or PP has been explored through processes such as hydrocracking and cross metathesis, which typically involve metal or metal oxide catalysts.

3. Depolymerization

3.1. Depolymerization of PET

Polyethylene terephthalate (PET) is a polyester synthesized by the condensation polymerization of terephthalic acid (TPA) and ethylene glycol (EG) [20]. Globally, PET production has reached 24 million tons. In 2018, 4.3 million tons of PET were produced as packaging materials in Europe, with bottles accounting for 79% of this amount [21]. In addition to packaging materials, PET is used in fiber form, representing 49.7% of total PET production [22]. Moreover, PET is the most widely recycled polymer, with the highest recycling rate among plastics (19.5%) [23].

The PET depolymerization methods are classified based on the kind of solvent used. Glycolysis with ethylene glycol [24], hydrolysis with water [25], and methanolysis with methanol [26] are the three representative methodologies of PET depolymerization. Glycolysis is considered to be the most promising PET depolymerization method because of its mild reaction conditions and compatibility with existing chemical processes. Glycolysis requires catalysts to decrease reaction temperature and reduce operation costs. Zinc acetate is recognized as the most effective catalyst for PET glycolysis because of the high Lewis acidity of Zn²⁺ and the hydrogen-bond-accepting properties of acetate (Figure 3) [27]. Myungwan Han and coworkers used zinc acetate to react at 230 °C for 6 h, achieving a 71% yield. The activation energy was determined using the reaction rate equation [28]. Do Hyun Kim and coworkers revealed that PET glycolysis in the presence of zinc acetate followed a nucleation and growth mechanism, rather than a simple secondary reaction. They also discovered that the crystallinity of PET significantly influenced the depolymerization process [29]. Additionally, Hyun Park and coworkers used zinc acetate to react at 250 °C for 30 min achieving a 65% yield of BHET monomers. Their experiment was designed using the Taguchi method, and the reaction was performed after microwave pretreatment [30].

In addition to zinc acetate, several homogeneous catalysts, such as metal salts, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), ionic liquid, and deep eutectic solvents have been used as homogeneous catalysts for the PET glycolysis [31,32,33]. Joungmo Cho and coworkers studied the introduction of a co-solvent for low-temperature PET glycolysis. They used an alkali metal salt as a catalyst and introduced anisole as a co-solvent for the reaction. Through this PET glycolysis process, they achieved an 86% yield of BHET at 153 °C in 2 h. They concluded that the low reaction temperature was due to anisole, which reduced the activation energy of PET glycolysis (Figure 5c) [34].

Despite the current advancements in PET glycolysis, the process still faces challenges in improving product purity, specifically, BHET monomers. One of the major impurities is the residual zinc acetate catalyst. Moreover, as zinc acetate salts are typically discarded after a single use, the use of zinc acetate as a catalyst raises additional environmental concerns.

Heterogeneous PET glycolysis catalysts have been actively investigated to mitigate the limitation of zinc acetate catalysts for separation and purification [14]. Manganese oxide catalysts supported on graphene oxide achieved a 96.4% BHET yield at 300 °C in 80 min. The catalysts were synthesized in a short time at low temperatures using ultrasonic treatment [35]. However, PET glycolysis systems using heterogeneous catalysts often require high reaction temperatures because of their low surface areas [36].

Korean research groups recently developed heterogeneous PET glycolysis catalysts that reduce the reaction temperature below the boiling temperature of ethylene glycol solvent to achieve an ambient pressure reaction. Bong Gill Choi and coworkers synthesized a metal-doped catalyst on hexagonal boron nitride (hBN), achieving a 92.1% BHET yield at 100 °C for 30 min. The catalysts were synthesized by exfoliating hBN and depositing metal nanoparticles using a Taylor–Couette flow. For comparison, they conducted PET glycolysis in both a hydrothermal reactor and a fluidic dynamic reactor. The experimental results demonstrated that the fluidic dynamic reactor achieved a higher yield (Figure 5a,b) [37].

Figure 5. (a) Schematic of the hydrothermal and fluidic dynamic reactor for PET glycolysis; (b) PET conversion using the hydrothermal reactor and fluidic dynamic reactor (reproduced with permission [37], copyright 2020, copyright Wiley-VCH); (c) the energy diagram of PET glycolysis depending on the presence of anisole (reproduced with permission [34], copyright 2020, copyright American Chemical Society).

Figure 5. (a) Schematic of the hydrothermal and fluidic dynamic reactor for PET glycolysis; (b) PET conversion using the hydrothermal reactor and fluidic dynamic reactor (reproduced with permission [37], copyright 2020, copyright Wiley-VCH); (c) the energy diagram of PET glycolysis depending on the presence of anisole (reproduced with permission [34], copyright 2020, copyright American Chemical Society).

The same group also prepared exfoliated MnO₂ nanosheets for use as glycolysis catalysts. The catalysts showed 100% BHET yield after 30 min at 200 °C, attributed to their high surface area and reactivity [38]. Chanmin Lee and coworkers synthesized a dual-porous ZIF-8 and used it as a catalyst for PET glycolysis. The catalyst showed higher reactivity than conventional ZIF-8 due to its many exposed active sites, achieving a 76.1% BHET yield after 4 h at 180 °C [39]. Do-Young Hong and coworkers synthesized a catalyst with a high surface area and thermal stability by supporting Mn₃O₄ on a spinel-structured MgAl₂O₄ support. This catalyst achieved a BHET yield of >95% at 190 °C for 3 h. The catalyst exhibited minimal deactivation even after 11 cycles and was fully regenerated upon thermal treatment at 500 °C [40]. Geon Dae Moon and coworkers utilized oyster shells as biomass-derived catalysts for PET depolymerization. Thermal treatment of the oyster shells converted the CaCO₃ content into CaO, which significantly influenced the catalytic activity. The catalyst achieved a BHET yield of 68.6% under reaction conditions of 195 °C for 1 h [41]. Hoik Lee and coworkers proposed a method for the chemical recycling of PET using finely sized MgO-doped SiO₂ catalysts via a wet impregnation method. The catalyst achieved high-yield production of high-purity BHET, with a yield of 95.1% at 196 °C for 2 h. This high efficiency is attributed to the basicity of MgO, which facilitates the activation of the carbonyl groups in PET. Larger catalyst particles are more easily removed after the reaction, leading to improved BHET purity [42].

The integration of magnetic properties into heterogeneous catalysts can improve the separation and purification processes after glycolysis. Dohyun Kim and coworkers synthesized Fe₂O₃@MoS₂ nanocomposites using a Taylor–Couette flow reactor. The deposition of Fe₂O₃ nanoparticles on the MoS₂ surface prevented their aggregation, and the resulting catalyst was readily separated through simple filtration. Products with 90% BHET yield were obtained after reacting at 225 °C for 3 h. The catalysts maintained high efficiency after seven reuse cycles [43]. Kwangjin An and coworkers synthesized Fe₃O₄ nanoparticles for magnetic PET glycolysis catalysts using various methods, including co-precipitation, thermal decomposition, and hydrothermal methods. Among the synthesized nanoparticles, those prepared via coprecipitation exhibited the largest surface area and excellent dispersibility in ethylene glycol, leading to enhanced catalytic performance. As a result, the catalyst synthesized via co-precipitation achieved a BHET yield of 93.5% at 195 °C for 2 h [44].

PET depolymerization via hydrolysis is an environmentally friendly approach that uses water as a solvent. Hyun Gil Cha and coworkers performed PET hydrolysis using ZSM-5-based zeolite catalysts. These zeolites functioned as acidic catalysts due to the presence of Brønsted and Lewis acid sites, enhancing their catalytic activity. To investigate this effect, the acidity was regulated by varying the Si/Al ratio. Experimental results showed that the optimal catalytic activity was achieved at a Si/Al ratio of 25, yielding 100% TPA at 230 °C for 30 min [45].

Methanolysis is another promising approach because the highly pure monomer, dimethyl terephthalate (DMT), can be obtained via this reaction. Conventional methanolysis typically requires high temperatures and pressures, which pose significant limitations. To overcome this, Joungmo Cho and coworkers conducted methanolysis with dichloromethane co-solvents and K₂CO₃ catalysts at room temperature. They achieved a DMT yield of 93.1% at 25 °C in 24 h. This enhanced reactivity was attributed to the relatively low activation energy (66.5 kJ/mol), which facilitated the reaction at lower temperatures [46].

3.2. Depolymerization of Polycarbonate

Polycarbonate (PC) is synthesized through the stepwise polymerization of bisphenol A (BPA) and various carbonates such as dimethyl carbonate [47]. PC is widely used in high-stability capacitors, data storage, automobiles, aircraft, bullet-proof glass, and construction materials because of their excellent optical properties, thermal resistance, mechanical strength, and stability [48]. The global production of PC increased from 1.5 million tons in 1999 to 1.7 million tons in 2001, reaching 3.3 million tons by 2008 [49].

PC depolymerization primarily aims to acquire BPA monomers. Various organic and metal-based catalysts have been employed to optimize the depolymerization process. Jeung Gon Kim and coworkers investigated the chemical depolymerization of PC using the organic catalyst, TBD. Compared with other catalysts, TBD enabled the efficient depolymerization of PC under mild reaction conditions and at a relatively low temperature. As a result, a BPA and dimethyl carbonate (DMC) yield exceeding 98% was achieved in 2-methyltetrahydrofuran co-solvent at 75 °C for 12 h. Furthermore, this study demonstrates that the use of different alcohol-based solvents can produce various high-value organic carbonates, highlighting the versatility of this approach [50]. They also conducted research on the depolymerization of waste PC, including compact disks, to produce high-value-added substances from BPA through a one-pot reaction. TBD was used as a catalyst for the depolymerization of PC, leading to the formation of BPA and hydroxamic acid, which then reacted to synthesize 1,4,2-dioxazol-5-one. The reaction was carried out at 110 °C for 5 h, achieving an 85% yield of 1,4,2-dioxazol-5-one [51].

Ji-Hun Seo and coworkers studied the ethanolysis of PCs using ionic liquid catalysts. Various ionic liquids were tested to examine the effect of alkyl chain length on PC depolymerization, with 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) demonstrating the highest depolymerization efficiency. This catalyst achieved a 99.9% BPA yield at 90 °C for 10 h, with 100% PC conversion maintained even after five reuses (Figure 6) [52].

Jechan Lee and coworkers recycled phone case waste into BPA monomers via depolymerization using incineration bottom ash, a by-product of municipal solid waste, as a catalyst. Using this catalyst, they obtained a BPA yield of 25.86 wt.% and an oil yield of 44.75 wt.% under a 600 °C N₂ atmosphere. The study suggested that metal oxides, such as CaO, in the catalyst facilitated the catalytic activity. It was also found that the CO₂ environment reduced the catalytic activity and led to a lower yield compared to reactions performed in a N₂ atmosphere because of the adhesion of CO₂ to the active sites [53].

Jeong Gon Kim and coworkers explored a chemical recycling method for polycarbonate (PC) and polyesters. They employed ball milling in a solvent- and catalyst-free environment, demonstrating that monomers or high-value-added chemicals could be obtained solely through mechanical energy transfer and direct contact between the reactants [54].

4. Pyrolysis

4.1. Pyrolysis of PE

Polyethylene (PE), which is synthesized through chain polymerization, consists of carbon and hydrogen atoms connected by C-C single bonds. Because of its stable C-C backbone, PE is hardly cleaved via solvolysis; rather, it is primarily recycled through pyrolysis at high temperatures, which enables the conversion of waste PE into high-value products such as oils and gases [55].

PE is broadly classified into two main types: low-density polyethylene (LDPE) and high-density polyethylene (HDPE). LDPE is synthesized via high-pressure polymerization and exhibits low crystallinity [56]. Because of its excellent chemical resistance, high impact strength, and high gas permeability, LDPE has been widely used in various applications [57]. In contrast, HDPE possesses high crystallinity, which imparts superior mechanical properties, rigidity, and wear resistance, making it suitable for applications in plastic pipes, containers, and bottles [58]. A comparative study of pyrolysis rates revealed that LDPE pyrolyzed at lower temperature compared with HDPE, owing to its lower packing density. The initiation temperature for PE pyrolysis followed the trend of LDPE < LLDPE < HDPE at the same heating rate [59].

The pyrolysis of PE requires high temperature and pressure because high thermal energy is required to break the strong C-C bonds. High pyrolysis temperatures can be reduced using zeolite-based catalysts, which facilitate C-C bond activation. Zeolites are ordered porous aluminosilicate solid materials with controlled Lewis acidity. The high Lewis acidity, large surface area, and thermal stability of zeolites enable their use as efficient catalysts for PE pyrolysis. Optimizing the catalytic properties by varying the type of zeolite or the addition of dopants has led to the production of high-value-added products in the desired forms [60].

Zeolite-based catalysts for PE pyrolysis have been extensively studied in Korea. Hee chul Woo and coworkers conducted the catalytic pyrolysis of PE using silica/alumina and various zeolites, including HZSM-5, natural zeolite (NZ), and Y zeolite, at temperatures ranging from 450 to 500 °C. Compared to non-catalytic pyrolysis, catalytic pyrolysis yields compounds with a narrower carbon number distribution at lower temperatures. The HZSM-5 catalyst produced a higher proportion of aromatic compounds, whereas the silica/alumina and NZ catalysts favored the formation of iso-paraffins and olefins [61]. Dae-Hyun Shin and coworkers compared ZSM-5, zeolite-Y, mordenite, and amorphous silica/alumina as catalysts for HDPE pyrolysis. Pyrolysis with zeolite-Y, mordenite, and amorphous silica/alumina yielded 71–82 wt.% oil, whereas ZSM-5 resulted in a lower oil yield of 35 wt.%, highlighting the influence of the catalytic structure on product composition [62]. Jechan Lee and coworkers employed H-ZSM-11 zeolite catalysts for pyrolyzing LDPE waste at 700 °C. Without a catalyst, 28.6 wt.% pyrolytic gas was obtained, whereas the gas yield increased to 80.8 wt.% with a catalyst. The catalytic environment promotes the dehydrogenation of propane, resulting in a higher propylene yield. By adjusting the temperature during pyrolysis, specific products were selectively obtained, showing that temperature variations significantly influenced the product selectivity in the presence of catalysts (Figure 7) [63].

The addition of hydrogen facilitates the cleavage of the C-C bond in PE via hydrogenolysis. The hydrogenolysis reaction can be enhanced using metal catalysts that stabilize the intermediates by adsorbing hydrogen atoms on metal surfaces. Byung Hyo Kim and coworkers synthesized Ni/zeolite nanocatalysts and performed pyrolysis under mild conditions at 350 °C for 2 h. The metallic Ni component of the catalysts facilitates hydrogen adsorption, whereas the zeolite component stabilizes the radicals and carbocations formed during pyrolysis. The influence of hydrogen pressure was investigated by regulating the pressure in the range of 1–20 bar. The results showed that the highest liquid fuel yield was obtained at 20 bar of hydrogen (Figure 8) [19].

Catalytic hydrocracking is another approach for reducing the pyrolysis temperatures, although the synergistic effect between hydrogenation and hydrocracking remains unclear. Insoo Ro and coworkers investigated the depolymerization of PE via hydrogenolysis and hydrocracking using Ru-based zeolite catalysts. Various support materials, including SiO₂, SBA-15, γ-Al₂O₃, TiO₂, and Zeolite-Y were tested, with the Ru/Zeolite-Y catalyst achieving the highest yield (96.9%). Interestingly, while the addition of water had either zero or a negative effect on the yield of most catalysts, the addition induced an increase in the yield from 80.6 to 96.9% for the Ru/Zeolite-Y catalysts. This enhancement was attributed to the synergistic effect between the Ru metal sites, which activate C-C bonds and H₂, and the Brønsted acid sites in Zeolite Y, which facilitate carbocation formation. Additionally, zeolites act as molecular sieves in the production of medium-chain-length liquid fuels. They concluded that achieving an optimal balance between the metal and acidic sites was crucial for maximizing the catalytic effect (Figure 9) [64].

In addition to zeolite-based catalysts, other porous materials have also been explored for PE pyrolysis. Seung-Su Kim and coworkers employed an Al-doped mesoporous silica material, Al-SBA-16, for the pyrolysis of PE. The Al-SBA-16 catalyst predominantly produced C7-C10 hydrocarbons during pyrolysis to achieve 75.8 and 20.6 wt.% oil and gas, respectively, after a 2 h reaction at 430 °C. The Al-SBA-16 catalyst possesses a higher number of acid sites, which explains the high reaction yield, leading to more active catalytic reactions. In contrast, the SBA-16 catalyst, with its lower acidity, promoted random cracking, resulting in a broader range of hydrocarbons (C7-C14) [65].

4.2. Pyrolysis of Polypropylene

Polypropylene (PP) is composed of a single carbon and hydrogen bond and is synthesized via chain polymerization [66]. PP is widely used in trays, funnels, pails, bottles, and carboys because of its excellent chemical resistance and low density [67]. Additionally, PP is lighter than other plastics, making it ideal for a wide range of applications. PP is produced in large quantities in modern industries, accounting for 16% of global plastic production. PP sales are projected to exceed USD 124.01 billion by 2019, with an annual growth rate of 6% [68].

As PP is synthesized through chain polymerization, it can be chemically recycled via pyrolysis. PP consists of strong C-C bond backbones that require significant thermal energy for pyrolysis. To reduce the energy demand, zeolite-based catalysts have been developed for PP pyrolysis in South Korea. In 2013, Sang Chai Kim and coworkers successfully pyrolyzed random PP using the silicoaluminophosphate (SAPO)-11 zeolite. SAPO-11 effectively suppressed char formation and significantly reduced both the pyrolysis temperature and activation energy [69]. Young-Kwon Park and coworkers investigated the influence of the ratio of silica to alumina in zeolite catalysts and gallium additives. Among the tested catalysts, the gallium-incorporated HZSM-5 catalysts with a silica to alumina ratio of 30 exhibited the lowest activation energy and the highest aromatic hydrocarbon production, and they demonstrated superior catalytic performance (Figure 10). Moreover, the gallium-incorporated catalysts efficiently lowered the PP pyrolysis temperature compared to the other catalysts, demonstrating their potential for energy-efficient PP depolymerization (Figure 10) [70].

4.3. Pyrolysis of Polystyrene

Polystyrene (PS) is synthesized via the chain polymerization of styrene monomers [71]. PS is a colorless, rigid, and hard plastic, widely used in food containers, cutlery, and disposable razors [72]. Various forms of PS, including expanded polystyrene (EPS), high-impact polystyrene (HIPS), and oriented polystyrene (OPS) are widely used in industrial applications [73]. Currently, a substantial amount of PS is discarded after a single use because it is predominantly designed for single-use products, thereby posing significant environmental challenges [74]. Consequently, there is a growing need for effective PS recycling methods.

PS pyrolysis is typically conducted in the temperature range of 300–900 °C under an oxygen-free atmosphere, producing oil, gas, and by-products. A key challenge in PS pyrolysis is minimizing the formation of polyaromatic solid compounds, such as char, which readily formed owing to side reactions involving the aromatic rings of PS. Thus, optimization of the pyrolysis temperature range and the development of catalysts to suppress char formation are required. At temperatures above 500 °C, gas and char are the predominant products, whereas, at a moderate temperature range between 300 and 500 °C, liquid compounds are primarily produced [75]. Sei Ki Moon and Hidehiro Kumazawa, along with coworkers, conducted pyrolysis in the temperature range of 370–400 °C using a batch stirring reactor. A kinetic study determined an activation energy of 224 kJ/mol for the reaction, via first-order reaction rate kinetics. The liquid products primarily consisted of single- and double-aromatic ring compounds, with the highest yield of styrene at 70 wt.% [76]. Jae Chun Hyun and coworkers optimized the temperature profile of the PS pyrolysis process to minimize the reaction time and energy consumption. Assuming both random and specific degradation mechanisms, they predicted changes in the molecular weight distribution using a continuous kinetic model. Their study demonstrated that maintaining the process at 465.3 °C provided the most economically viable conditions [77].

Catalysts have been used to optimize the product selectivity and reduce the reaction temperatures of PS pyrolysis. Myung Jae Choi and coworkers evaluated the catalytic performances of various oxide materials in PS pyrolysis. Among the oxide catalysts, BaO achieved the highest styrene yield of 84.29 wt.% at 350 °C, which was attributed to the strong basicity of BaO. Additionally, the Fe₂O₃-KOH/Al catalysts exhibited superior activity compared to Fe₂O₃ or Fe/Al alone. The yield decreased with repeated pyrolysis owing to the increase in particle size [78]. They also conducted PS pyrolysis using a swirling fluidized bed reactor to efficiently produce valuable chemicals. The pyrolysis was carried out in the presence of catalysts such as Fe₂O₃, BaO, and HZSM-5 (silica to alumina ratio of 30). An investigation into the effects of adjusting the ratio of the swirling gas to the primary fluidizing gas on the pyrolysis process showed that an increase in the swirling gas ratio led to enhanced styrene and oil yields [79].

Juwon Min and coworkers examined the economic feasibility of catalytic pyrolysis compared with that of thermal pyrolysis. Using a K₂O/SiO₂ catalyst, they achieved 80.2% styrene selectivity with a light impurity ratio of 45%. These findings indicate that pyrolysis with basic catalysts is more economically and environmentally favorable than pyrolysis with acidic catalysts or thermal pyrolysis. Additionally, industrial wastes, such as steel slags, have been explored as cost-effective catalysts for PS pyrolysis in a CO₂ environment. The combined effect of steel slag and CO₂ facilitates the pyrolysis of PS into syngas and C1-C2 hydrocarbons, as well as condensable aromatic compounds (Figure 11) [80].

4.4. Pyrolysis of Nylon

Nylon is a polyamide polymer that contains amide bonds, with nylon 6 and nylon-66 as the most common. Nylon 6 is synthesized by initiating the polymerization of caprolactam at high temperatures. It offers advantages such as wear resistance, self-lubrication, elasticity, and heat resistance, making it widely used in textiles, electronics, automotive products, and household goods [81]. Nylon-66 is synthesized from hexamethylenediamine and adipic acid. It has excellent properties such as chemical resistance, wear resistance, and toughness, making it suitable for various industrial applications [82]. Both nylon 6 and nylon-66 are highly resistant to biological degradation, making them difficult to biodegrade and contributing to environmental issues. Additionally, nylon recycling continues in modern industries, with only 2% of the 5.58 million tons of polyamide produced in 2019 originating from recycling sources [83]. The high crystallinity and chemical inertness of nylon make it difficult to recycle using conventional recycling methods. This issue can be addressed by developing catalysts for polyamide pyrolysis reactions.

The pyrolysis of nylon 6 can produce caprolactam monomers [84]. Jechan Lee and coworkers studied the pyrolysis of tea bag waste, composed of nylon 6, and found that the pyrolysis temperature significantly affected the composition and yield of the resulting products. At 500 °C, the highest caprolactam yield of 6.2 wt.% was achieved. During this process, gas, a pyrolytic liquid, and char are generated, all of which can be utilized as energy sources (Figure 12) [85].

They also chemically recycled discarded fishing nets to recover caprolactams using carbonized shell waste as a catalyst in N₂ and CO₂ environments. The catalyst carbonized in a CO₂ environment exhibited a high caprolactam recovery rate of 80 wt.% at 500 °C, as the CO₂ activated the amide bonds, facilitating bond cleavage [86].

4.5. Pyrolysis of Other Plastics Including Mixed Plastics

A significant proportion of manufactured plastics consists of mixed phase materials composed of different polymers. The chemical recycling of mixed plastics remains challenging owing to the differences in the pyrolysis temperatures of the constituent polymers, and the difficulty in separating the decomposed products. However, certain polymer combinations, such as PE and PP, share chemical similarities, allowing PE-PP mixed plastics to be recycled through catalytic pyrolysis. Young-Kwon Park and coworkers investigated the pyrolysis of a PE-PP mixture using two zeolite catalysts, Desilicated Beta and Al-MSU-F. The study revealed that Desilicated Beta facilitated pyrolysis at a lower temperature than Al-MSU-F, which was attributed to its higher acidity and optimal pore size. Additionally, Desilicated Beta primarily produced aromatic hydrocarbons, whereas Al-MSU-F favored branched hydrocarbon formation [87].

The pyrolysis of PE-PS mixed plastics was also explored, revealing a synergistic effect between PS and PE. Young-Hwa Seo and coworkers found that the yield from a PE-PS mixture was higher than that from PE alone, suggesting that PS enhances the pyrolysis of PE. Their results also showed that the product yield varied with the mixing ratio of both polymers, with a 1:1 PE-PS mixture at 600 °C achieving the highest yield of small aromatic compounds [88].

PET can be chemically recycled via pyrolysis, as well as depolymerization, yet several challenges remain. The aromatic rings in PET tend to be converted into low-value polycyclic hydrocarbons, biphenyl derivatives, and char. Furthermore, polycyclic compounds interfere with catalyst–polymer interactions, reducing catalytic activity. To address this issue, catalysts capable of promoting ring-opening reactions have been used in PET pyrolysis [85]. The use of Pd catalysts in the catalytic pyrolysis of PET led to a 44% reduction in the formation of 2-naphthalenecarboxylic acid, a representative polycyclic hydrocarbon, and a 79% decrease in the formation of biphenyl-4-carboxylic acid, a major biphenyl derivative [89].

The recovery of high-value-added products through the pyrolysis of polymers, such as PET and PS, has been investigated. Mixed plastics with PS and PET can be recycled by the co-pyrolysis of both polymers, rather than through the depolymerization of PET, because the chemical mechanisms of pyrolysis and depolymerization via solvolysis differ. Young-Kwon Park and coworkers studied the pyrolysis of PS and PET using Al-MSU-F catalysts. The results confirmed that the large pore size and acidity of the Al-MSU-F catalyst significantly lowered the pyrolysis temperature of PS and PET while enhancing the production of aromatic compounds [90].

5. Chemical Recycling Industry in South Korea

The chemical recycling of plastics is being implemented at industrial scales beyond laboratory research. In particular, several major chemical companies in South Korea are actively engaged in the development and commercialization of chemical recycling technologies.

SK Chemicals established a Recycling Incubation Center dedicated to the chemical recycling of PET, focusing on the production of recycled BHET [91]. This chemically recycled monomer is used in various applications, including detergents and mobile phone components. SK Geocentric partnered with Loop Industries to establish Infinite Loop^TM Ulsan, a commercial-scale PET depolymerization facility projected to supply 70,000 tons of PET annually, as a part of a broad joint venture targeting the Asian market [92]. Terracle introduced a biocatalytic PET depolymerization technology that operates at atmospheric pressure and below 100 °C, producing TPA monomers with a purity of over 99% and a recycling rate of over 99% [93]. Lotte Chemical operates a PET depolymerization plant in Ulsan, South Korea, producing BHET monomers from waste PET, which is repolymerized into recycled PET. The company aims to supply over one million tons of eco-friendly recycled materials by 2030 [94]. The company also runs a pyrolysis plant that processes 1500 tons of PE and PP per year [95]. LG Chem employs high-temperature and high-pressure supercritical steam to pyrolyze PE plastic waste, producing pyrolysis oil, which is then fed into the petrochemical processes. Through this method, 10 tons of plastic waste can yield approximately 8 tons of pyrolysis oil and 2 tons of gas [96]. These examples highlight a growing industrial momentum toward commercializing chemical recycling technologies in South Korea. Furthermore, in addition to these examples, many other companies in South Korea are actively investing in pilot plants and scaling up operations, indicating that the sector is gaining substantial traction [97,98].

Research institutes and corporations in South Korea are contributing to innovation in the chemical recycling of plastics through patent activity. The Korea Institute of Science and Technology (KIST) applied for a patent covering the use of transition metal salts for the depolymerization of cured epoxy resin products, along with the corresponding methodology [99]. Korea Electric Power Corporation (KEPCO) filed a patent for a chemical recycling method of plastics using activated carbon catalysts [100]. In addition, numerous other Korean research institutions and companies have submitted patent applications, both domestically and internationally, covering catalysts for the chemical upcycling of plastics [101,102,103]. These industrial initiatives and patent activities underscore the ongoing efforts in South Korea to commercialize and advance chemical recycling technologies.

6. Conclusions and Outlook

Addressing the global environmental crisis requires a multifaceted approach, including the regulation of CO₂ emissions and implementation of sustainable technologies. Although international efforts to achieve carbon neutrality are intensifying, only 4.5% of countries worldwide have achieved their goal of reducing CO₂ emissions. Given that a significant amount of CO₂ is emitted during the production of plastics, plastic recycling holds substantial potential to contribute toward global CO₂ emission goals. For example, PET emits 2.436 kg of CO₂ per kilogram, HDPE emits 1.676 kg per kilogram, and PVC emits 2.127 kg per kilogram, all of which contribute to high CO₂ emissions from plastic production [104].

Chemical recycling offers an effective route for achieving sustainability by recovering monomers and high-value chemicals from plastic waste. Polymers synthesized by condensation polymerization, such as PET and PC, are recycled through depolymerization. PET depolymerization is commonly conducted via glycolysis, which employs ethylene glycol as the solvent and reactant to produce BHET monomers. In addition to glycolysis, the chemical recycling of PET through hydrolysis, methanolysis, and pyrolysis has been conducted. Polymers synthesized by chain polymerization, such as PE, PP, and PS, are primarily recycled through pyrolysis. Despite the promise of these technologies, chemical recycling remains energy-intensive and currently contributes to CO₂ emissions due to the high reaction temperatures and the involved separation processes.

To address this challenge, the development of catalytic systems has become a critical focus. Catalysts can lower reaction temperatures, reduce energy consumption, and improve the overall efficiency of depolymerization and pyrolysis processes. In South Korea, research efforts have led to diverse catalytic systems, including the use of zinc acetate catalysts, molecular catalysts, and nanocatalysts for PET depolymerization. Organic or metal catalysts are primarily used to obtain BPA monomers through PC depolymerization. PE is typically decomposed using zeolite-based catalysts, yielding high-value-added products such as oil and gas. Chemical recycling catalysts for plastics developed in research groups in South Korea are summarized in

Table 1. Summary depolymerization condition and product yield of plastics.

	Catalysts	Temperature	Time	Product	Yield	Ref.
PET	Zinc acetate	230 °C	6 h	BHET	71%	28
	Zinc acetate	250 °C	0.5 h	BHET	65%	30
	Potassium acetate	153 °C	2 h	BHET	86.5%	34
	Mn3O4	300 °C	80 min	BHET	96.4%	35
	Pd/hBN	100 °C	0.5 h	BHET	92.1%	37
	MnO2 nanosheet	200 °C	0.5 h	BHET	100%	38
	ZIF-8	180 °C	4 h	BHET	76.1%	39
	Mn3O4/p-spMgAl800	190 °C	3 h	BHET	97.6%	40
	Oyster shell	195 °C	1 h	BHET	68.6%	41
	MgO-doped SiO2	196 °C	2 h	BHET	95.1%	42
	Fe2O3@MoS2	225 °C	3 h	BHET	90%	43
	Fe3O4	195 °C	2 h	BHET	93.5%	44
	ZSM-5-25	230 °C	0.5 h	TPA	100%	45
	K2CO3	25 °C	24 h	DMT	93.1%	46
PC	TBD	75 °C	12 h	BPA	>98%	50
	[EMIM][Ac]	90 °C	10 h	BPA	99.9%	52
	MSW-IBA	600 °C		BPA	25.9 wt.%	53
PE	HZSM-5	450 °C	1 h	Oil	10.9 wt.%	61
				Gas	88.4 wt.%
	NZ	450 °C	1 h	Oil	65.1 wt.%
				Gas	34.9 wt.%
	ZSM-5	450 °C	0.5 h	Oil	35 wt.%	62
				Gas	63.5 wt.%
	Zeolite Y	450 °C	0.5 h	Oil	71.5 wt.%
				Gas	27 wt.%
	H-ZSM-11	700 °C		Oil	19.2 wt.%	63
				Gas	80.8 wt.%
	Ni/Zeolite	350 °C	2 h	Oil	25 wt.%	19
				Gas	58 wt.%
	Ru/Zeolite	250 °C	3 h	Oil	88.2 wt.%	64
				Gas	1.4 wt.%
	Al-SBA-16	430 °C	2 h	Oil	75.8 wt.%	65
				Gas	20.6 wt.%
PS		400 °C	0.5 h	Styrene	71.6 wt.%	77
	BaO	350 °C	1 h	Styrene	61.7 wt.%	78
	Fe2O3	450 °C	20 min	Styrene	71.5 wt.%	79
	K2O/SiO2	375 °C	1 h	Styrene	80.2 wt.%	80
Nylon		500 °C		Caprolactam	6.2 wt.%	85
	Shell waste	500 °C		Caprolactam	80 wt.%	86

.

Despite the advancements in catalyst development, the chemical recycling of plastics is still more energy-intensive than mechanical recycling and generates more CO₂ emissions. For this reason, as of 2016, only 1% of plastic waste was chemically recycled, compared to 12% through mechanical recycling. In order to increase the proportion of waste processed through chemical recycling, significant cost reductions and process improvements are necessary. Werner and coworkers predicted that the cost of chemical recycling will decrease by 37.5% between 2019 and 2040, while the cost of mechanical recycling is expected to remain nearly constant [105]. To support this shift, global efforts, including those in South Korea, are focused on enhancing the energy efficiency of chemical recycling technologies [106].

Further progress in chemical recycling will depend on reducing process costs and minimizing environmental impacts. Key strategies include optimizing reaction conditions, improving catalyst reusability, and lowering the energy demand for post-reaction solvent recovery. High-performance catalysts with large surface areas and enhanced reactivities are essential to reduce energy consumption and environmental impact. Another critical challenge is the environmental impact of single-use catalysts. For example, zinc acetate, which is commonly used in PET depolymerization, is often discarded after a single use, causing residual metal pollution. To overcome the environmental pollution caused by the one-time use of such catalysts, there is a demand for the development of catalysts that can be easily recovered and reused without performance degradation.

Chemical recycling is especially promising for treating end-of-life plastic waste, which typically contains a wide range of additives such as plasticizers, flame retardants, antioxidants, acid scavengers, and light or heat stabilizers [107]. These additives, while essential for enhancing plastic performance and durability, often complicate recycling processes. Thus, developing a catalytic system that can tolerate these additives is crucial for the effective chemical recycling of real-world plastic waste.

It is important to consider the influence of plastic additives on chemical recycling catalysts, because certain additives can significantly reduce catalytic activity by blocking active sites or interfering catalyst–polymer chain interactions [108]. For example, during the pyrolysis of PE, phosphate and stearate additives have been shown to deactivate ZSM-5 zeolite catalysts by forming residues that deform active sites [108]. Additionally, additives have been reported to impair enzymatic degradation efficiency [109] and reduce the performance of metal-based catalysts [110]. A recent study investigated various catalysts to identify the causes of catalytic ability loss due to additives and to explore catalytic types that are more resistant to additive-related deactivation [111]. To advance chemical recycling as a viable commercial process, further research is urgently needed on catalytic systems that remain highly active and selective in the presence of complex additive mixtures commonly found in post-consumer plastics.

While South Korea has made significant strides in catalytic development and industrial applications for chemical recycling, the overall research output and scale of implementation still lag behind global leaders like the USA, EU, and China. To enhance the global competitiveness of South Korea, increased investment in long-term research and development (R&D) and interdisciplinary collaboration is essential. Strategic national initiatives should prioritize (i) the development of reusable catalysts with high selectivity and efficiency, (ii) the scale-up of pilot programs to bridge lab-to-industry gaps, and (iii) the creation of collaborative platforms that unite academic researchers and chemical industries.

In conclusion, chemical recycling holds great promise as a cornerstone technology for achieving a circular plastic economy. Realizing this potential will require sustained innovation in catalysis, process engineering, and waste sorting technologies. Through coordinated global initiatives and continued research, particularly in South Korea, chemical recycling can emerge as an environmentally responsible and economically viable solution to the growing plastic waste crisis.