Sustainable Chemical Recycling of PET: Promise and Challenges of Microwave-Assisted Solvolysis

Abstract

Polyethylene terephthalate (PET) is a common plastic widely used in food packaging, especially plastic bottles, and in fibers and textiles. The widespread use of PET and its intentional and unintentional release and disposal over the past few decades have placed significant pressure on the environment, necessitating the development of green, low-cost, and efficient recycling technologies to mitigate this impact. This article reviews recent advances in microwave-assisted catalytic depolymerization of PET. It begins by outlining the fundamental principles of PET materials science and depolymerization mechanisms. The article then reviews the historical evolution of this research and presents a benchmark comparison of different depolymerization methodologies. Finally, through a critical analysis and comparison of state-of-the-art approaches, the article identifies emerging trends and highlights promising directions for future research in the field. A comprehensive comparison of conventional and microwave heating methods is presented, indicating that catalyst-assisted microwave systems can shorten reaction times and achieve high product yields under optimized conditions. Key advantages and limitations are highlighted, and persistent challenges are discussed. Overall, this article surveys the latest progress in the chemical recycling of PET and provides critical insights for future research and further development of microwave-assisted catalytic PET depolymerization technology.

1. Introduction

Plastics are a widely used class of materials mainly composed of polymeric substances. Owing to their excellent plasticity, tunable mechanical properties, ease of processing, durability, and low cost, plastics have been widely adopted in numerous industries and daily life. Their extensive use has profoundly shaped the development of modern society [1]. However, most commercial plastics are synthetic materials derived from fossil resources. Because of their chemical stability and resistance to degradation, they decompose very slowly in natural environments. Consequently, plastic waste can accumulate persistently in the environment, causing severe and long-lasting ecological impacts [2].

According to the Organization for Economic Co-operation and Development (OECD) report “Global Plastics Outlook”, global plastic waste generation increased from 156 million tons in 2000 to 353 million tons in 2019. If current policies remain unchanged, this amount is expected to nearly triple by 2060, reaching approximately 1.014 billion tons. Figure 1 summarizes the global plastic waste management pathways in 2019. Only 9% of plastic waste was recycled, while 19% was treated through incineration with energy recovery. In contrast, 50% was disposed of in sanitary landfills, and the remaining 22% was managed through uncontrolled disposal, including uncontrolled dumping, open-air burning, or direct release into the environment. As a result, the OECD projects that annual plastic leakage into the environment will increase from 22 million tons in 2019 to 44 million tons in 2060, with the cumulative stock of plastic in rivers and oceans reaching approximately 493 million tons [3]. Consistent with this trend, approximately 8 to 12 million tons of plastic currently enter the ocean each year, further accelerating the long-term accumulation of plastics in aquatic environments [4].

Figure 1. Global Plastic Waste Management Pathways in 2019.

The global plastic pollution challenge is further complicated by the wide diversity of plastic types in use, as shown in Figure 2. Major commodity plastics include low-density polyethylene, commonly used in plastic bags and films, and its more rigid counterpart, high-density polyethylene, widely used for detergent bottles and piping. Polyethylene terephthalate (PET) is particularly prevalent in the packaging industry for plastic bottles and food containers, as well as some types of PET fibers. Other representative plastic materials include polypropylene (PP), which is heat-resistant and used in microwaveable containers; polystyrene (PS), commonly found in disposable tableware and foam packaging; and polyvinyl chloride (PVC), an essential and highly durable material used in applications ranging from building pipes to medical catheters [5]. Together, these diverse polymers occur across a vast array of consumer and industrial products, underscoring the complexity and scale of the global plastic waste problem.

Figure 2. Common plastic types.

Among major commodity plastics, PET is of particular concern because it is closely linked to three key issues in plastic waste management: large-scale packaging and textile waste, microplastic fiber pollution, and chemically accessible recycling pathways. PET is the most widely used polyester and is extensively applied in packaging and textiles. Its chemical recycling can depolymerize PET into monomers or value-added intermediates, thereby providing a pathway toward improved circularity [6]. The textile dimension is particularly relevant to microplastic pollution, as synthetic textiles, including PET fiber and recycled polyester, can release microplastic fibers during household washing [7]. Unlike polyolefins such as PE and PP, PET contains cleavable ester bonds, making selective depolymerization via solvolysis or biocatalysis more feasible under relatively mild conditions.

A potential challenge is that PET is not a uniform material, as its products range from beverage bottles to PET fibers. Although they share the same underlying chemical structure, substantial variability and diversity are observed in crystallinity levels and molecular weights. This heterogeneity even exists within a single item; for example, the neck, wall, and base of a PET bottle exhibit different degrees of crystallinity [8]. These physical differences can translate into chemical resistance, solvent accessibility, and density variations [9], which in turn profoundly impact how the material behaves during chemical recycling. Consequently, such structural variability must be accurately characterized and explicitly considered when developing and optimizing effective depolymerization strategies.

In addition to structural heterogeneity, a further source of heterogeneity is the difference between transparent and colored or opaque PET formulations. Colored PET contains organic dyes or inorganic pigments such as copper/cobalt-based organometallics, phthalocyanines, and carbon black, while opaque-white PET typically contains titanium dioxide (TiO₂) as an opacifying agent. In general, colored and opaque PET formulations provide increased opacity and enhanced ultraviolet (UV) shielding, which in turn improve the thermal and photostability of the material [10]. The presence of these additives severely hinders physical recycling, as they degrade polymer purity and reduce the degree of polymerization of the resulting recyclate. Consequently, colored and opaque PET waste are often excluded from closed-loop, bottle-to-bottle recycling systems and are instead downcycled into lower-value products [11]. Within the context of chemical recycling, this additive-induced heterogeneity makes decolorization a critical pretreatment or integrated process step.

Without effective color removal, the recovered monomers or intermediates, such as terephthalic acid (TPA) and bis(2-hydroxyethyl) terephthalate (BHET), may remain colored and contaminated, limiting their reuse potential [12]. For microwave-assisted solvolysis, colored and opaque PET are important not only because of their depolymerization behavior, but also because of their influence on product purification and catalyst recovery. Residual dyes, pigments, and opacifying agents can remain in the liquid or solid residues after depolymerization, resulting in colored BHET or TPA products and increasing the burden of filtration, washing, and downstream purification. This issue becomes especially relevant for microwave systems using heterogeneous catalysts, nanocatalysts, ionic liquids (ILs), or deep eutectic solvents (DESs), where catalyst recyclability and product purity are key advantages. Thus, for colored or opaque PET, the choice of catalyst and solvent system should be evaluated together with decolorization and separation strategies.

Several strategies have been explored to address this challenge. For instance, in 2008, Gupta et al. [13] employed oxidative bleaching with H₂O₂ to decolorize blue and green PET bottles, but this process resulted in a decrease in the product’s intrinsic viscosity. More advanced approaches have also been developed. In 2022, Li et al. [14] developed a tandem reaction-hydrolysis/glycolysis process that simultaneously depolymerizes and decolorizes PET, yielding high-purity TPA or high-yield BHET. In 2025, Wang et al. [15] further achieved efficient decolorization of recycled BHET using DESs, enabling a closed-loop recycling process. Despite these promising lab-scale advancements, the development of robust, scalable industrial processes for effective recycling of colored and opaque PET remains a challenge and an important area for further innovation.

2. Plastic Waste Treatment Methods and Strategies

2.1. Conventional Plastic Treatment Strategies

Conventional plastic waste treatment strategies, notably landfilling and incineration, have long been associated with severe environmental and public health impacts. Incineration, in particular, presents substantial risks due to the incomplete combustion of polymers and the complex thermally-mediated reactions involving plastic additives [16]. This process generates acidic gases (e.g., HCl, NO_x, SO₂) and particulate matter [17]. Even for relatively “cleaner” plastics such as PET, co-incineration with contaminants under realistic waste-treatment conditions can still result in the generation of acidic and toxic substances [18]. Although modern incineration facilities employ advanced flue gas treatment systems, emissions levels and their associated health risks remain highly sensitive to feedstock composition, operational parameters, and the effectiveness of pollution control technologies. Moreover, incineration can lead to the formation of highly toxic byproducts, notably dioxins and furans [19].

Halogenated polymers, such as PVC, along with plastics containing brominated flame retardants serve as major precursors for dioxin and furan formation during thermal treatment or uncontrolled burning. More importantly, even chlorine-free polyolefins like polyethylene (PE) and polypropylene (PP) can generate chlorinated dioxins when combusted in the presence of inorganic chlorine sources [20]. As persistent organic pollutants, dioxins are of particular concern, due to their environmental persistence, bioaccumulation potential, and severe health impacts, including carcinogenicity and endocrine-disrupting effects [21].

Landfilling, while often perceived as a less immediate environmental threat than incineration, introduces distinct and potentially severe contamination risks. Within landfill environments, plastics progressively fragment into micro- and nanoplastics, which can be mobilized through landfill leachate. This leachate is frequently contaminated with chemical additives leached from plastic debris, such as phthalates and bisphenol A, which are known for their endocrine-disrupting and ecotoxicological effects [22]. Furthermore, persistent subsurface landfill fires pose a severe hazard. Such smoldering can persist for months, releasing high levels of toxic smoke, odorous compounds, and greenhouse gases. The plastic content of the waste can further exacerbate the toxicity of these emissions and pose serious risks to public health [23]. For example, the Ghazipur landfill in Delhi has been identified through satellite observations as a significant methane super-emitter, where recurrent summer fires occur; a major fire in 2024 released large volumes of toxic smoke, posing serious health risks to nearby communities. Comparable incidents have also been reported at the Brahmapuram landfill in Kerala and the Pirana landfill in Ahmedabad. Collectively, these profound drawbacks underscore the urgent need to develop and adopt more sustainable alternatives to traditional disposal methods.

2.2. Plastic Recycling Strategies

Currently, plastic recycling methodologies are broadly categorized as either physical or chemical processes [24]. Physical recycling, often termed mechanical recycling, reprocesses plastic waste into new products without altering the polymer’s fundamental molecular structure. The process typically involves washing, shredding, melting, and re-pelletizing the waste material, which is then remolded into new goods through techniques such as injection molding, extrusion, or blow molding, including the production of plastic lumber [25].

However, this approach faces significant limitations. The thermal and mechanical stress during processing inevitably degrades the polymer chains, leading to a significant reduction in strength and mechanical properties, a phenomenon known as down-cycling [26,27]. Furthermore, the melt reprocessing stage can release toxic volatile organic compounds, such as aldehydes and aromatic hydrocarbons, posing health risks to workers and the environment [28]. Another critical drawback is the contamination of the recycled material. Recyclates often contain non-intentionally added substances, including degradation byproducts, impurities, as well as residual additives such as phthalates, flame retardants, and heavy metals [29]. These contaminants compromise final product safety, substantially restricting its suitability for high-value product applications, particularly those involving food contact materials.

To address the challenges of plastic waste, various chemical recycling methods have been developed to convert polymers back into valuable feedstocks. Among these, thermochemical processes are well-established. Pyrolysis, for instance, operates at 400–600 °C to convert mixed plastics into a combination of oil, gas, and char [30]. At even higher temperatures (>800 °C) and with oxygen input, gasification transforms plastic waste into combustible synthesis gas (syngas) [31]. Another thermochemical route, hydrocracking, decomposes plastic at temperatures of 200–450 °C to yield fuel oil and hydrocarbons under high-pressure hydrogen [32].

In contrast, there exist other approaches providing greater selectivity under milder conditions. Solvolysis employs specific solvents and catalysts at lower temperatures to break down polymers into their constituent monomers or oligomers [33]. Similarly, biocatalysis utilizes enzymes to achieve degradation at temperatures often below 60 °C [34]. A critical limitation, however, is that both solvolysis and biocatalysis are primarily effective for heteroatom-containing polymers like PET and are largely inapplicable to polyolefins such as PE and PP [35,36]. While cutting-edge methods like photocatalysis and electrolysis offer environmentally benign conversion at ambient temperatures, they remain limited in their application because of low reaction efficiency and scalability challenges [37,38]. Consequently, pyrolysis and solvolysis-based techniques remain the most extensively studied approaches, offering a compelling balance between feedstock tolerance and the potential for high-value chemical recovery [33]. Of the two, solvolysis is particularly attractive due to its versatility across catalyst–solvent systems, relatively mild operating conditions, and potential to recover well-defined monomers or oligomers. The aforementioned methods are summarized in Table 1.

Table 1. Common chemical recycling methods.

Method	Temperature	Target	Input	Output
Pyrolysis	400–$600 °\mathrm{C}$	Multiple plastics	Heat	Oil, gas, and char
Gasification	>$800 °\mathrm{C}$	Multiple plastics	Heat + ${\mathrm{O}}_2$	Mixed combustible gas
Hydrocracking	200–$450 °\mathrm{C}$	PP, PE	Heat + ${\mathrm{H}}_2$	Fuel oil and hydrocarbons
Solvolysis	70–$300 °\mathrm{C}$	PET, PU, PC	Solvent + catalysts	Monomers
Biocatalysis	<$60 °\mathrm{C}$	PET, PU, PLA	Solvent + enzymes	Monomers
Photocatalysis	RT	PLA, PVC, PE, PS	Light, e.g., LED or UV	Surface products
Electrolysis	RT	Mixed plastics	Electricity	Multiple products

Note: RT = room temperature; PU = polyurethane; PC = polycarbonate; PLA = polylactic acid.

2.3. Microwave Heating Method

Compared with conventional conductive heating, microwave irradiation has emerged as a superior energy source for solvolysis, primarily due to its unique non-contact, volumetric heating mechanism. As early as 1992, the use of microwave energy as a heat source for thermally driven chemical reactions had already attracted broad attention. Lewis and co-workers discussed that microwaves could promote polymer depolymerization mainly by accelerating the temperature ramp of reaction media, while additional non-thermal effects have also been proposed but remain debated and have not been conclusively demonstrated [39,40]. In 2006, Nikje et al. experimentally examined this idea and showed that, in the presence of heavy-metal salt catalysts, microwave irradiation can accelerate PET solvolysis depolymerization [41]. Building on these efforts, Shukla et al. in 2008 further demonstrated that microwaves can markedly shorten reaction time without sacrificing the yield of the target product BHET, while also exploring the substitution of conventional heavy-metal salts with more environmentally benign catalysts [42]. Along the same sustainability-oriented narrative of catalyst recyclability and reduced toxicity, Alnaqbi et al. in 2015 employed 1-butyl-3-methylimidazolium bromide ([BMIm]Br, C₈H₁₅BrN₂) as a catalyst to convert PET to BHET under microwave conditions, reporting high conversion and systematically discussing the temperature–time operating window; this study can be viewed as one of the logical starting points for later developments in DES/ionic liquid (IL)-catalyzed microwave glycolysis [43].

Between 2022 and 2023, Zangana et al. and Selvam et al. introduced heterogeneous-catalysis concepts into microwave glycolysis and evaluated simple metal oxides such as CaO and ZnO, achieving high BHET yields and providing a practical, greener, and recyclable catalyst option for subsequent studies [44,45]. Research on DES-based systems continued to advance in 2024: Ha et al. reported BHET yields exceeding 99% using a DES composed of choline chloride (ChCl, C₅H₁₄ClNO) and zinc acetate [Zn(OAc)₂, Zn(CH₃COO)₂] as the catalytic system. The high efficiency of Zn(OAc)₂-containing systems can be rationalized by a Lewis acid-assisted glycolysis mechanism, in which Zn²⁺ coordinates with the carbonyl oxygen of PET ester groups, increases the electrophilicity of the carbonyl carbon, and thereby facilitates nucleophilic attack by ethylene glycol and subsequent ester-bond cleavage [46]. Ricci et al. adopted a zinc chloride (ZnCl₂)–urea [CO(NH₂)₂] type-IV DES and emphasized the importance of energy input and power–time combination strategies [47,48].

Beyond glycolysis, microwave-assisted heating has also been applied to other PET depolymerization pathways. Compared with glycolysis, hydrolysis is typically more demanding, but it produces TPA, which has more direct and widespread downstream applications. In 2016, Ikenaga et al. performed microwave-assisted PET hydrolysis under high-pressure conditions, achieving complete degradation and discussing the effects of homogeneous catalysts such as HCl and NaOH on TPA yield [49]. Microwave-assisted catalytic depolymerization of PET remains an active topic to this day. In 2025, McCrea et al. reported a methanolysis process converting PET to dimethyl terephthalate (DMT) using a recyclable protic IL prepared from sulfuric acid and triethylamine; under optimized conditions, they achieved 100% conversion and approximately 98% isolated DMT yield. This work not only extends microwave-assisted ionic-liquid catalytic systems from glycolysis to methanolysis, but also demonstrates an inexpensive and recyclable protic ionic liquid catalyst that remains effective for complex PET waste, such as dyed carpet fibres with latex backing [50].

A key advantage of microwave-assisted solvolysis is the substantial reduction in the reaction’s activation energy. For PET glycolysis, activation energies of as low as 29 kJ/mol have been reported under microwave heating, in stark contrast to the >80 kJ/mol typically required using conventional methods [51]. In addition to its kinetic benefits, microwave energy accelerates the physical degradation of the polymer matrix. Microstructural evidence from scanning electron microscopy imaging reveals that microwaves promote the formation of more significant surface fissures, thereby facilitating faster solvent penetration and internal swelling [47]. This improvement in mass transfer, combined with rapid and uniform volumetric heating, enhances reaction homogeneity and substantially improves product selectivity by suppressing the formation of undesirable byproducts often associated with slow temperature ramping and thermal gradients inherent to conventional heating systems [52]. Figure 3 illustrates the schematic representation. Collectively, these advantages position microwave-assisted solvolysis as a highly promising strategy for polymer depolymerization, motivating a more detailed examination of its principles and applications.

Figure 3. Schematic diagram of temperature gradient and volumetric heating.

3. Microwave-Assisted Solvolysis Methods

3.1. Overview

Solvolysis is a chemical recycling strategy that depolymerizes PET through the cleavage of ester linkages using a solvent, such as water, alcohol, or glycol, typically under elevated temperatures and catalytic conditions [5]. This process yields constituent monomers and oligomers, like TPA or BHET, which can be purified and repolymerized to produce virgin-quality PET.

3.1.1. Development and Mechanistic Considerations of Microwave-Assisted Solvolysis

In the 1990s, before solvolysis studies were widely applied to PET, microwaves were discussed more broadly as a reaction-heating method for material and polymer processing. In an MRS Bulletin article, Lewis and co-workers [39] summarized progress in microwave processing of polymers and suggested that microwaves might influence reactions and material transformations through rapid heating (i.e., a fast temperature ramp) as well as potential non-thermal effects. Since 2000, research attention has increasingly shifted toward the PET glycolysis context. For example, Pingale and Shukla [53] explicitly applied microwave irradiation to PET glycolysis and attributed the enhanced performance to direct energy coupling into the reaction mixture via dipole rotation and ionic conduction, enabling more efficient reaction progress.

In the past decade, researchers began to examine microwave-assisted solvolysis from a more kinetics-oriented perspective. Chen et al. (2013) investigated the kinetics of PET glycolysis under microwave conditions and, using an Arrhenius analysis, reported a lower apparent activation energy compared with conventional heating [54]. Since 2020, the role of ethylene glycol (EG) as both reactant and reaction medium has been extensively explored. In particular, Zangana et al. (2022) and Selvam et al. (2023) demonstrated that high catalytic efficiency under microwave glycolysis can be achieved even with simple catalysts, and proposed that, within a microwave field, EG—as a polar medium—may couple more strongly with catalyst surfaces and interfacial regions, potentially allowing the system to reach effective reaction temperatures more rapidly and increasing the likelihood of localized reactive events [44,45]. In 2024, Ha et al. achieved quantitative depolymerization within 9 min with a 99% BHET yield, and interpreted the exceptionally high yield in terms of a hydrogen-bonding network formed among the DES components, EG, PET, and zinc acetate [47].

At the physical level, microwave heating is generally attributed to dielectric energy dissipation through dipolar polarization or rotation in polar molecules and ionic conduction in media containing mobile ions [55,56,57]. These mechanisms describe the conversion of electromagnetic energy into heat and provide the established physical basis for microwave heating. In heterogeneous systems, differences in dielectric properties among the solvent, polymer, and catalyst phases may also produce selective heating, interfacial polarization, and spatial temperature gradients. Localized hot spots have been observed or proposed in some microwave-assisted heterogeneous catalytic systems, although their formation and persistence depend strongly on the dielectric properties, field distribution, catalyst structure, and heat-transfer characteristics of the specific system [57,58,59].

For microwave-assisted PET glycolysis, interactions among polar EG, catalyst surfaces, and Lewis acidic sites have been proposed to contribute to rapid heating and enhanced catalyst–solvent contact. Studies using nano-ZnO and Mn-loaded ZnO have related the observed reaction enhancement to catalyst polarity, surface structure, hydrogen bonding, and the availability of Lewis acid sites [45,60]. However, these interpretations do not by themselves establish a universal microwave-specific mechanism for PET depolymerization. Selective heating, interfacial polarization, localized temperature gradients, and enhanced mass transfer should therefore be regarded as plausible and system-dependent explanations rather than universally demonstrated mechanisms in PET solvolysis.

The magnitude and practical importance of these effects remain uncertain because the available PET solvolysis studies generally do not provide matched thermal histories, rigorous internal or spatial temperature measurements, and controls capable of separating bulk thermal effects from localized microwave-induced phenomena. Studies in broader microwave chemistry have shown that improved internal temperature monitoring and agitation can eliminate some apparent differences between microwave and conventional heating [55,61]. Consequently, a broadly applicable non-thermal microwave effect has not been conclusively demonstrated, and its existence and practical significance remain controversial [56,61].

3.1.2. Major PET Solvolysis Pathways

Solvolysis encompasses several distinct chemical pathways for PET depolymerization, classified by the nucleophilic agent employed, namely aminolysis, ammonolysis, hydrolysis, alcoholysis and glycolysis [62,63,64].

Among these, glycolysis is the most widely utilized for microwave-assisted PET recycling systems due to its favorable reaction kinetics, solvent compatibility and suitability for efficient dielectric heating. Glycolysis relies on the use of polyols, typically EG, to cleave the ester bonds through transesterification. While the process is noted for its simplicity and rapid reaction rates, the resulting products often require substantial downstream purification, as the glycolysis mixture may contain oligomeric intermediates and other impurities, while residual EG must also be recovered and separated after the reaction [5,65,66]. Figure 4 illustrates the molecular formulas and simplified reaction scheme [62,64].

Figure 4. Simplified reaction schematic diagram of glycolysis.

Hydrolysis, based on the principle of ester hydrolysis, uses water (H₂O) under acidic or alkaline catalysis to break the ester linkages, yielding the corresponding acid and alcohol. This route can produce high-purity TPA but often requires harsh reaction conditions [67]. The molecular formulas and simplified reaction scheme are shown in Figure 5 [62,64].

Figure 5. Simplified reaction schematic diagram of hydrolysis.

In contrast, alcoholysis, particularly methanolysis, reacts PET with alcohols such as methanol or ethanol to generate their respective diesters, such as DMT, making it compatible with industrial processes that utilize DMT as a feedstock [68]. Figure 6 provides a schematic representation of the molecular formulas and reaction pathway [62,64].

Figure 6. Molecular formulas and simplified reaction schematic diagram of methanolysis.

Other related methods include ammonolysis and aminolysis, which involve reacting PET with ammonia (NH₃) or amines, respectively, to produce the corresponding diamides. However, the commercial value and market size of the products generated by these two methods are considerably more limited, making them less attractive for industrial applications [68]. Figure 7 and Figure 8 show the molecular formulas and simplified reaction schemes [62,64].

Figure 7. Molecular formulas and simplified reaction schematic diagram of ammonolysis.

Figure 8. Molecular formulas and simplified reaction schematic diagram of aminolysis.

In industrial applications, hydrolysis and glycolysis are the most prevalent methods for PET recycling when considering factors such as reaction conditions, catalysts, and final products. Between the two, glycolysis is generally favored due to its comparatively milder operational conditions.

3.2. Glycolysis of PET Pellets

The advantages of glycolysis for PET recycling are multifaceted. First, from an environmental perspective, the process is compatible with heterogeneous catalysts and can avoid the corrosive acids, strong bases, or toxic organic solvents employed in some other solvolysis routes, such as the methanol used in methanolysis [69]. Second, with optimized solvent and catalyst systems, glycolysis can achieve mild reaction conditions without compromising product yield. Indeed, laboratory-scale studies have demonstrated that the process can proceed under mild conditions to achieve sizable conversion of PET [70]. From the perspective of catalyst development, conventional PET glycolysis has evolved from soluble metal salts toward bulk and mixed metal oxides, nanostructured and magnetically recoverable catalysts, porous solid catalysts, and supported organocatalysts. This progression reflects a gradual shift in research priorities. Early studies primarily sought to accelerate PET conversion and increase BHET yield, whereas more recent work has placed increasing emphasis on catalyst recovery, product selectivity, metal leaching, reuse stability, and downstream purification. Given that reaction time is significantly influenced by temperature and catalyst selection, research in this field has predominantly focused on catalyst development and the elucidation of reaction kinetics.

The pronounced temperature dependence and sluggish kinetics of the uncatalyzed reaction were recently quantified by Gabrič et al. [71]. Under conventional heating, more than 3000 min was required to achieve a final BHET yield of approximately 95% at 190 °C. At 170 °C, the BHET yield decreased to approximately 80%, whereas at 150 °C, only a 14% yield was obtained even after 4000 min. These results demonstrated that catalysis is clearly indispensable for practical PET glycolysis, motivating extensive investigations into a wide range of catalytic systems. López-Fonseca et al. [69] demonstrated that the introduction of a catalyst dramatically accelerated PET glycolysis. Using zinc acetate as the catalyst, they observed complete PET conversion within one hour. In their comparative study, other sodium and potassium salts, including sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃), sodium sulfate (Na₂SO₄), and potassium sulfate (K₂SO₄), all exhibited lower catalytic activity than zinc acetate.

In addition to soluble metal salts, several metal oxides have been investigated as potential alternative catalysts. Imran et al. [72] found that complex oxides such as ZnMn₂O₄ exhibited superior catalytic performance and produced higher BHET yields than simple oxides such as ZnO. Building on this approach, Alzuhairi et al. [73] explored the benefits of nanostructuring and reported that nano-ZnO achieved a higher BHET yield than its bulk ZnO counterpart. To address the common challenge of nanocatalyst recovery, Du et al. [74] investigated the performance of nanoscale CoFe₂O₄. They found that this catalyst not only maintained high catalytic efficiency but also allowed easy magnetic separation, offering a more environmentally friendly alternative to nanomaterials that are difficult to recover.

Whereas mixed, nanoscale, and magnetic oxides were primarily developed to improve accessible catalytic activity and catalyst recovery, porous solids and supported catalysts represent a parallel strategy aimed at enhancing structural stability, reusability, and control over catalyst residues. Chen et al. [75] synthesized solid base catalysts (Zn/Al hydrotalcites) via calcination in 2014. While this catalyst significantly increased the reaction rate, it paradoxically led to a lower final BHET yield, highlighting the complex trade-offs between reaction kinetics and product selectivity in catalyst design. The efficacy of conventional zeolites as catalysts for PET glycolysis has been found to be limited. For instance, a study by Shukla et al. [42] reported that both $\beta$-zeolites and Y-zeolite exhibited sluggish reaction kinetics and achieved only low product yields. In 2024, based on a commercial zeolite catalyst (500-FAU) with modest catalytic activity, Yang et al. [76] prepared a nanoscale FAU-type zeolite, 100-FAU, which achieved superior catalytic efficiency due to its higher Lewis acidity and basicity.

Considering the environmental risks posed by heavy-metal-containing or hard-to-recover nanocatalysts, Fehér et al. [77], demonstrated the ecological advantages of thermally stable organocatalysts such as Si-TBD and Si-TEA in 2022. Taken together, these studies demonstrate that no catalyst class is universally superior for conventional PET glycolysis. Zinc acetate remains an effective benchmark because of its high catalytic activity, but its solubility increases the burden of catalyst separation, metal-residue control, and product purification. Mixed and nanostructured metal oxides can provide high BHET yields by increasing the number and accessibility of surface active sites, although these advantages may be offset by synthesis complexity, particle aggregation, metal leaching, and recovery difficulties. Magnetically separable catalysts offer a promising compromise between activity and recoverability, but their long-term durability and metal-release behavior remain insufficiently characterized. Porous catalysts and supported organic bases may facilitate catalyst reuse and reduce soluble metal residues, but their performance can be limited by diffusion resistance, restricted accessibility of active sites, catalyst leaching, or gradual deactivation.

The overall research trend therefore extends beyond maximizing the reported BHET yield toward balancing catalytic efficiency, selectivity, recovery, durability, and downstream processing requirements. Future studies should prioritize standardized comparisons using consistent PET feedstocks, PET-to-EG ratios, catalyst loadings, reaction and product-isolation procedures, and should report not only conversion and BHET yield, but also catalyst leaching, recovery efficiency, multi-cycle stability, and purification demand. Such information is required to determine whether the high activity observed at the laboratory scale can translate into practical and scalable glycolysis processes.

In order to estimate the thermal power associated with maintaining the reaction system at a given temperature, a simplified steady-state heat-loss model was adopted under the assumptions of constant ambient temperature ($T_0$), similar convection conditions, and a fixed external surface area (A):

$$P\left(T\right)=a\left(T-T_0\right)+b\left[{\left(T+273.15\right)}^4-{\left(T_0+273.15\right)}^4\right]$$

where $P\left(T\right)$ represents the useful thermal power (in W) required to maintain the system at temperature T; $a=hA$ (in W K⁻¹) is the coefficient associated with convective and conductive heat losses, defined as the product of the heat transfer coefficient h and the surface area A; and $b=\epsilon \sigma A$ (in W K⁻⁴) is the coefficient associated with radiative heat loss, defined as the product of the surface emissivity $\epsilon$, the Stefan–Boltzmann constant $\sigma$, and the surface area A.

For the present estimation, the following parameters were assumed: $T_0=25 °\mathrm{C}$, $h\approx 10\mathrm{W}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$, $\epsilon \approx 0.8$, and $A\approx 0.1{\mathrm{m}}^2$. Here, $h\approx 10\mathrm{W}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$ was adopted as a representative natural-convection heat transfer coefficient in air [78], while $\epsilon \approx 0.8$ was taken as a reasonable emissivity estimate for a practical, non-polished engineering surface [79]. From these assumptions, $P\left(T\right)$ was calculated.

It should be noted that this model estimates the useful thermal power required at the sample level, rather than the actual electrical input power of the heating device. A substantial fraction of the input energy in a typical oven may be absorbed by the oven structure itself or lost through the walls and ventilation. In addition, oven efficiency varies substantially depending on appliance type, loading condition, and testing method: the overall efficiency of a simple oven is often relatively low, on the order of 10% [80], whereas some heavily loaded laboratory convection ovens may reach much higher benchmark values, around 65% [81]. Therefore, in the present study, ${\eta }_{\mathrm{oven}}=0.3$ was adopted as an intermediate representative estimate rather than an exact measured value, and the corresponding oven input power was estimated as $P=P\left(T\right)/{\eta }_{\mathrm{oven}}$.

A parameter sensitivity analysis was also conducted using oven efficiencies of ${\eta }_{\mathrm{oven}}=0.2$, $0.3$, and $0.4$. Relative to the baseline value of ${\eta }_{\mathrm{oven}}=0.3$, the estimated oven input power increased by 50% at ${\eta }_{\mathrm{oven}}=0.2$ and decreased by 25% at ${\eta }_{\mathrm{oven}}=0.4$. Although the assumed efficiency substantially affects the absolute energy estimates, it does not alter the relative ranking among the conventional-heating systems because the same efficiency factor is applied consistently. The calculated values should therefore be interpreted as sensitivity-dependent, screening-level estimates rather than experimentally validated energy-consumption data.

Based on this treatment, the model was subsequently used only as a first-order, screening-level estimate to support a qualitative comparison of the reviewed catalyst-assisted glycolysis methods. The calculated values should therefore be interpreted as indicative reference values rather than validated experimental energy-consumption data.

Throughout this review, we highlight sustainability-related considerations for different PET depolymerization schemes rather than assigning categorical environmental-performance rankings. These considerations are discussed qualitatively based on reported or inferable process features, including estimated energy demand, EG and catalyst recovery, product and byproduct usability, catalyst residues, solvent or salt management, and potential waste streams [82,83]. Because the summarized studies differ in feedstock type, catalyst loading, solvent ratio, reactor configuration, reaction time, purification procedure, and system boundary, these considerations should be interpreted as qualitative screening factors rather than as a quantitative life-cycle assessment or direct environmental ranking.

Table 2 summarizes the calculated P values, the corresponding estimated energy demand, the reported product yields, and the sustainability-related considerations for the different schemes.

Table 2. Comparison of calculated $P\left(T\right)$, sustainability-related considerations, and yield across different PET pellet depolymerization schemes.

Type	Catalyst	Max Yield Dosage	Max Yield Time (min)	Max Yield Temperature (°C)	Max BHET Yield	Sustainability-Related Considerations	P (W)	${\mathit{W}}_{0.2}$ (MJ)	${\mathit{W}}_{0.3}$ (MJ)	${\mathit{W}}_{0.4}$ (MJ)
None	N/A	N/A	3000	190	95%	High energy demand; limited viability	1121.70	302.859	201.906	151.430
Acid metal salt	Zn(OAc)2	$1\%m_{\mathrm{PET}}$	60	196	75%	Zn recovery; purification burden	1178.17	6.362	4.241	3.181
Alkali metal salt	Na2CO3	$1\%m_{\mathrm{PET}}$	60	196	80%	Low toxicity; recovery optional	1178.17	6.362	4.241	3.181
Metal oxide	ZnO	$1\%m_{\mathrm{PET}}$	80	260	78%	Recoverable solid; Zn leaching risk	1876.77	13.512	9.008	6.756
Complex oxides	ZnMn2O4	$1\%m_{\mathrm{PET}}$	60	260	92%	High yield; mixed-metal leaching concern	1876.77	10.134	6.756	5.067
Nano-catalyst	nano-ZnO	$15\%m_{\mathrm{PET}}$	60	178	89%	Difficult recovery; nanoparticle release risk	1012.90	5.469	3.646	2.735
Nano-catalyst	CoFe2O4-60%	$0.7\%m_{\mathrm{PET}}$	60	210	92%	Magnetic recovery; Co leaching concern	1315.53	7.104	4.736	3.552
Solid catalyst	Zn/Al HT	$1\%m_{\mathrm{PET}}$	50	196	76.50%	Stability; synthesis-waste uncertainty	1178.17	5.301	3.534	2.651
Zeolite	Zeolite-$\beta$	$1\%m_{\mathrm{PET}}$	480	196	66%	Low efficiency; recoverable zeolite	1178.17	50.897	33.931	25.448
Zeolite	Zeolite-Y	$1\%m_{\mathrm{PET}}$	480	196	65%	Low efficiency; recoverable zeolite	1178.17	50.897	33.931	25.448
Zeolite	500-FAU	$2\%m_{\mathrm{PET}}$	60	196	5.40%	Very low yield	1178.17	6.362	4.241	3.181
Zeolite	100-FAU	$2\%m_{\mathrm{PET}}$	90	196	72%	Fine-particle separation concern	1178.17	9.543	6.362	4.772
Other catalyst	Urea	$1.25\%m_{\mathrm{PET}}$	210	170	69%	Benign; nitrogenous waste concern	943.30	17.829	11.886	8.915
DES	Urea/ZnCl2	$1.25\%m_{\mathrm{PET}}$	30	170	83%	ZnCl2 recovery; corrosion concern	943.30	2.5467	1.6978	1.2734
Ionic liquid	Formate IL	$1.25\%m_{\mathrm{PET}}$	180	180	84.5%	Recovery; biodegradability uncertainty	1030.67	16.697	11.131	8.348
Organic base	Si-TBD	$5\%m_{\mathrm{PET}}$	60	190	88%	Base leaching; regeneration waste	1121.70	6.057	4.038	3.029
Organic base	Si-TEA	$5\%m_{\mathrm{PET}}$	60	190	80%	Amine degradation; catalyst loss	1121.70	6.057	4.038	3.029

Note: P denotes calculated power, and $W_{0.2}$, $W_{0.3}$, and $W_{0.4}$ denote the estimated energy consumption calculated using oven efficiencies of 0.2, 0.3, and 0.4, respectively. OAc = acetate; HT = hydrotalcite; IL = ionic liquid. The sustainability-related considerations are qualitative screening factors rather than quantitative environmental rankings.

3.3. Glycolysis of PET Fiber

The catalyst trends discussed for bottle- and pellet-derived PET cannot be directly transferred to PET fibers because catalyst performance is also influenced by feedstock morphology and polymer structure. Although bottle-grade and fiber-grade materials share the same PET backbone, they differ in crystallinity, molecular weight, molecular orientation, surface morphology, and additive composition, all of which can affect solvent penetration, ester-bond accessibility, and the observed glycolysis performance. Owing partly to differences in isophthalic acid content, bottle-grade PET generally exhibits lower crystallinity than fiber-grade PET, resulting in different crystallization behavior during recycling [84]. This distinction is particularly relevant to chemical depolymerization because reactions in semicrystalline PET preferentially proceed through the amorphous regions, where reagents such as ethylene glycol and methanol can penetrate more readily [85]. In addition, the intrinsic viscosity (IV) of bottle-grade PET, approximately 0.80 dL g⁻¹, is generally higher than that of fiber-grade PET (0.55–0.67 dL g⁻¹). Because IV is positively correlated with molecular weight, this difference may further contribute to their distinct depolymerization behavior [86]. A qualitative comparison of the major catalyst classes used for PET glycolysis is provided in Table 3.

Table 3. Qualitative comparison of major catalyst classes for PET glycolysis.

Catalyst Class	Representative Systems	Main Catalytic Role	Advantages	Limitations	Research Priority
Soluble salts	Zn(OAc)2; Na2CO3	Lewis acid or base-assisted ester cleavage	Simple preparation; high activity or low cost	Difficult recovery; metal or salt residues	Recovery and residue control
Bulk and mixed oxides	ZnO; ZnMn2O4	Surface acid–base catalysis	Recoverable; thermally stable; high yield possible	High temperature; leaching; variable activity	Lower loading and improved durability
Nano- and magnetic oxides	nano-ZnO; CoFe2O4	High surface activity and magnetic recovery	Rapid reaction; accessible active sites; easy separation	Aggregation; nanoparticle release; metal leaching	Multi-cycle stability and safe recovery
Solid bases and zeolites	Zn/Al HT; Zeolite-$\beta$; Zeolite-Y; FAU	Surface basic sites and pore-confined catalysis	Reusable; tunable acidity, basicity, and porosity	Diffusion resistance; moderate activity; fine-particle separation	Pore accessibility and selectivity
Molecular and supported bases	Urea; Si-TBD; Si-TEA	Hydrogen-bonding or base-assisted catalysis	Reduced heavy-metal use; relatively mild systems	Moderate activity; leaching; catalyst deactivation	Activity, regeneration, and reuse
Ionic liquids and DESs	Formate IL; Urea/ZnCl2	Ionic, Lewis acid, and hydrogen-bond-assisted catalysis	Tunable composition; high yield at reduced temperature	High viscosity; corrosion; recovery and purification burden	Closed-loop recovery and process economics

Note: OAc = acetate; HT = hydrotalcite; FAU = faujasite; IL = ionic liquid; DES = deep eutectic solvent; TBD = 1,5,7-triazabicyclo[4.4.0]dec-5-ene; TEA = triethylamine.

Early research on the glycolysis of PET fibers typically employed simple salts or weak acid/base catalysts. Despite extensive investigations by Shukla et al. [42,87], a catalyst capable of significantly enhancing the BHET yield has not been identified yet. A more recent trend is the application of novel catalytic systems. For instance, Chen et al. [88] explored mixed catalysts to achieve high BHET yields in short reaction times in 2015, while Dai et al. [89] developed an advanced heterogeneous catalyst, Zn-MCM-41, based on zinc oxide to improve the final product yield in 2024.

However, overall, research focusing specifically on the catalytic glycolysis of PET fibers remains relatively scarce. Furthermore, these studies seldom provide a mechanistic analysis of the differences between PET fibers and other PET forms, such as bottle-grade pellets, which arise from their distinct crystallinities and molecular weights. There is a clear lack in the field of comparative, side-by-side studies on the glycolysis of different grades of PET.

Table 4 summarizes the calculated $P\left(T\right)$, sustainability-related considerations, and yield across different PET fiber depolymerization schemes. However, in the existing literature, it is rare to find a detailed, quantitative report of reaction time, temperature, and yield. Our recent benchmark analysis of the available data revealed an interesting trend: when lower-performing catalysts under the reported conditions are employed, such as zeolites for PET pellet or acetic acid for PET fibers, the final BHET yields for both feedstocks are comparable (around 60%). However, a clear divergence appears when representative high-performing catalysts and reaction times reported in the literature are considered. Pellet PET can achieve a BHET yield of up to 90%, whereas the yield for PET fibers reaches only 82%. This discrepancy suggests a difference in the reported attainable yield between these two grades of PET. Nevertheless, this trend should be regarded as indicative rather than conclusive, because the present cross-study comparison does not fully account for differences in catalyst chemistry, feedstock crystallinity, molecular weight, particle size, solvent ratio, and purification procedures.

Table 4. Comparison of calculated $P\left(T\right)$, sustainability-related considerations, and yield across different PET fiber depolymerization schemes.

Type	Catalyst	Max Yield Dosage	Max Yield Time (min)	Max Yield Temperature (°C)	Max BHET Yield	Sustainability-Related Considerations	P (W)	${\mathit{W}}_{0.2}$ (MJ)	${\mathit{W}}_{0.3}$ (MJ)	${\mathit{W}}_{0.4}$ (MJ)
Simple acid	Acetic acid	$0.5\%m_{\mathrm{PET}}$	480	190	62%	Acid recovery; neutralization burden	1121.70	48.458	32.305	24.229
Simple alkali	Lithium hydroxide	$0.5\%m_{\mathrm{PET}}$	480	190	64%	Caustic handling; neutralization waste	1121.70	48.458	32.305	24.229
Acid metal salt	Sodium sulfate	$0.5\%m_{\mathrm{PET}}$	480	190	66%	Low toxicity; salinity management needed	1121.70	48.458	32.305	24.229
Acid metal salt	Potassium sulfate	$0.5\%m_{\mathrm{PET}}$	480	190	64%	Low toxicity; salinity management needed	1121.70	48.458	32.305	24.229
Mixed oxides	Zn/Al mixed oxides	$1\%m_{\mathrm{PET}}$	25	196	82%	Recoverable solid; stability/leaching uncertain	1178.17	2.651	1.767	1.325
Advanced heterogeneous catalyst	Zn-MCM-41	$5\%m_{\mathrm{PET}}$	120	190	70%	Support synthesis; Zn leaching uncertain	1121.70	12.114	8.076	6.057
Metal oxides	$\mathrm{ZnO}$	$5\%m_{\mathrm{PET}}$	120	190	65%	Recoverable solid; leaching requires evaluation	1121.70	12.114	8.076	6.057
Acid metal salt	$\mathrm{Zn}{\left(\mathrm{OAc}\right)}_2$	$5\%m_{\mathrm{PET}}$	120	190	67.00%	Zn recovery; metal-residue control needed	1121.70	12.114	8.076	6.057

Note: P denotes calculated power, and W_0.2, W_0.3, and W_0.4 denote the estimated energy consumption calculated using oven efficiencies of 0.2, 0.3, and 0.4, respectively. OAc = acetate; HT = hydrotalcite. The sustainability-related considerations are qualitative screening factors rather than quantitative environmental rankings.

3.4. Microwave-Assisted PET Glycolysis

Microwave irradiation has been identified as an effective heating approach for PET solvolysis, including glycolysis and hydrolysis [90]. Research on microwave-assisted PET glycolysis can be traced back to the late 1990s [91]. Unlike conventional glycolysis, the performance of microwave-assisted systems is jointly influenced by solvent selection, catalyst chemistry, and operating conditions. Accordingly, the development of this field has progressed from the screening of solvent systems toward kinetic analysis, heterogeneous metal-oxide catalysts, nanostructured catalysts, and integrated catalyst–solvent systems.

Early studies primarily focused on solvent selection and examined methanol, diethylene glycol, propylene glycol, and other reaction media [41,53,92]. Among these solvents, ethylene glycol (EG) became the predominant reaction medium for microwave-assisted PET glycolysis because its relatively high dielectric loss tangent (tan$\delta$) enables effective microwave absorption [45]. In this system, EG functions both as the glycolysis reagent and as the polar medium that couples with microwave irradiation. The establishment of EG as the principal reaction medium subsequently shifted research attention toward catalyst development and reaction kinetics.

Soluble metal salts provided an early catalytic basis for microwave-assisted glycolysis. A kinetic framework for Zn(OAc)₂-catalyzed glycolysis under microwave conditions showed lower apparent activation energies and substantially shorter reaction times than those reported for conventional heating [54]. Subsequent studies extended catalyst development toward heterogeneous metal oxides. CaO was shown to substantially reduce the time required for PET depolymerization [44], while nanostructured and modified metal oxides were investigated to improve surface activity and catalyst–solvent interactions. In particular, the high surface area and polar facets of nano-ZnO were associated with enhanced hydrogen bonding and stronger interactions between the catalyst and solvent under microwave irradiation [45]. Mn-loaded ZnO was further used to examine the proposed intensification of contact between reactants and Lewis acidic surface sites under microwave conditions [60]. Taken together, these studies indicate a progression from soluble metal salts toward heterogeneous, nanostructured, and modified metal oxides, reflecting increasing interest in both catalytic surface activity and catalyst recoverability.

More recent research has moved beyond the optimization of an individual catalyst toward the integrated design of catalyst–solvent systems. A ChCl–Zn(OAc)₂ deep eutectic solvent (DES) system achieved a BHET yield of up to 99% after systematic optimization [47]. The same study showed that excessive reaction time, large PET particle size, and an inappropriate PET-to-EG ratio could substantially reduce the final BHET yield. These observations demonstrate that the performance of microwave-assisted glycolysis cannot be determined by catalyst identity alone. Instead, high conversion and product yield require coordinated optimization of the catalyst system, PET particle size, PET-to-EG ratio, microwave power, and irradiation time.

Table 5 summarizes the reported catalyst loading, reaction time, microwave power, BHET yield, calculated apparent energy demand, and sustainability-related considerations for representative microwave-assisted PET glycolysis systems. A qualitative comparison of the major catalyst classes used in microwave-assisted PET glycolysis is presented in Table 6.

Table 5. Comparison of calculated $P\left(T\right)$, sustainability-related considerations, and yield across different PET microwave-assisted depolymerization schemes.

Type	Catalyst	Max Yield Dosage	Max Yield Time (min)	Max Yield Power (W)	Max BHET Yield	Sustainability-Related Considerations	P (W)	W (MJ)
Acid metal salt	$\mathrm{Zn}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2$	$2\%m_{\mathrm{PET}}$	35	500	78.00%	Zn recovery; purification burden	$500.00$	1.05
Acid metal salt	${\mathrm{NaHCO}}_3$	$0.5\%m_{\mathrm{PET}}$	35	700	65%	Low toxicity; recovery optional	700.00	1.47
Acid metal salt	${\mathrm{Na}}_2{\mathrm{CO}}_3$	$0.5\%m_{\mathrm{PET}}$	35	700	64%	Low toxicity; recovery optional	700.00	1.47
Metal oxide	$\mathrm{CaO}$	$10\%m_{\mathrm{PET}}$	4	800	75.00%	High loading; caustic waste concern	800.00	0.192
Metal oxide	nano-$\mathrm{ZnO}$	$1\%m_{\mathrm{PET}}$	45	500	95.00%	Nanoparticle recovery/release risk	500.00	1.35
Loaded metal oxide	Mn/$\mathrm{ZnO}$	$10\%m_{\mathrm{PET}}$	5	200	88%	High loading; metal-leaching concern	200.00	0.06
DES	$\mathrm{ChCl}$-$\mathrm{Zn}{\left(\mathrm{OAc}\right)}_2$	$7\%m_{\mathrm{PET}}$	9	1000	99%	Zn-based DES; recovery/purification concerns	1000.00	0.54

Note: P denotes calculated power, and W denotes calculated energy consumption. DES denotes deep eutectic solvent. ChCl = choline chloride; OAc = acetate; Zn(OAc)₂ = zinc acetate. The sustainability-related considerations are qualitative screening factors rather than quantitative environmental rankings.

Table 6. Qualitative comparison of major catalyst classes for microwave-assisted PET glycolysis.

Catalyst Class	Representative Systems	Catalytic Role	Advantages	Limitations	Research Priority
Soluble zinc salts	Zn(OAc)2	Lewis acid activation	High activity; low loading	Separation; Zn residues	Recovery and reuse
Alkali salts	NaHCO3; Na2CO3	Base-assisted glycolysis	Low cost; low toxicity	Moderate yield; salt waste	Selectivity and recovery
Metal oxides	CaO	Surface acid–base catalysis	Fast reaction; solid recovery	High loading; caustic residues	Lower loading and durability
Modified metal oxides	nano-ZnO; Mn/ZnO	Enhanced surface Lewis acid activity	High yield; short reaction time	Aggregation; leaching	Recovery and cycle stability
Deep eutectic solvents	ChCl–Zn(OAc)2	Lewis acid and solvent functions	High yield; tunable system	Recovery; purification burden	Closed-loop recycling

Note: OAc = acetate; ChCl = choline chloride. The comparison is qualitative and is based on the systems reviewed in this section.

Comparison of Table 5 with Table 2 and Table 4 suggests that microwave-assisted glycolysis can achieve shorter reaction times and lower calculated apparent energy demand under the reported laboratory-scale conditions. Metal-oxide and DES-based systems, in particular, have achieved high reported BHET yields over reaction times ranging from several minutes to less than one hour. Nevertheless, the reviewed studies differ in catalyst loading, PET feedstock, particle size, solvent ratio, microwave power, and reactor configuration. The numerical comparison therefore reflects the performance reported under individual experimental conditions rather than a standardized ranking of catalyst systems.

The apparent energy comparison is also based on a simplified heat-loss model and literature-reported reaction parameters rather than measured industrial energy-consumption data. The calculated values should therefore be interpreted as screening-level estimates of apparent reaction-stage energy demand rather than conclusive evidence that microwave heating universally requires less energy than conventional heating. Further experimental and process-level measurements are required to determine whether the shorter reaction times observed at the laboratory scale translate into lower overall energy consumption during scale-up and industrial operation.

Despite the broad range of catalysts and operating conditions already investigated, several important research gaps remain. First, the microscopic mechanisms responsible for the observed enhancement under microwave irradiation have not been systematically evaluated. In particular, the relative contributions of microwave heating, catalyst activity, and changes in reactant contact or mass transfer remain insufficiently distinguished. Second, different PET feedstocks, including colored PET, bottle-grade PET, and fiber-grade PET, may exhibit different depolymerization behavior, but only a limited number of studies have compared these materials under consistent microwave conditions. Third, most reported systems remain at the laboratory scale, and their industrial relevance has not yet been fully validated despite promising laboratory-scale reaction times and yields.

Future research should establish a more systematic comparative framework for microwave-assisted PET glycolysis. This framework should include mechanistic evaluation of microwave-assisted glycolysis and the operating parameters that control its performance, together with direct comparisons of different PET feedstocks under consistent catalyst, solvent, microwave, and product-isolation conditions. Such comparisons are needed to clarify how feedstock characteristics, including crystallinity, molecular weight, morphology, particle size, and additive composition, affect PET conversion, BHET yield, product selectivity, and purification requirements.

In parallel, greater attention should be given to catalyst performance and reaction kinetics. Future catalyst development should focus not only on achieving high activity, but also on identifying systems that combine catalytic performance with low cost, limited environmental impact, facile recovery, and stable reuse. Reaction time should also be systematically evaluated because different catalysts may exhibit distinct reaction-time–yield profiles. Rather than reporting only the final yield at a single reaction time, future studies should quantify how PET conversion and BHET yield evolve throughout the reaction, identify the time required to reach the maximum yield, and assess whether prolonged irradiation promotes product degradation, oligomer formation, or other secondary reactions.

Beyond laboratory-scale optimization, the scale-up and industrial applicability of microwave-assisted glycolysis should be evaluated using process-level data on energy consumption, catalyst recovery, product yield and purity, solvent recovery, and downstream purification. Addressing these questions is necessary to determine whether the high reaction rates and BHET yields reported at the laboratory scale can be translated into practical, economically viable, and environmentally sustainable microwave-assisted PET glycolysis processes.

4. Technology Development, Scale-Up, and Future Research Directions

Recent research in microwave-assisted catalytic PET depolymerization has increasingly focused on catalyst design, reaction engineering, process control, and scale-up. These aspects are closely connected and are essential for determining whether the advantages observed in laboratory-scale experiments can be translated into practical industrial applications.

4.1. Microwave-Responsive Catalyst and Solvent Design

Regarding catalyst development, designing novel systems tailored to maximize the inherent advantages of microwave irradiation remains a critical research focus. For example, DESs and ILs can serve dual roles as both microwave-absorbing media and active catalytic components. This dual functionality may enable rapid heating and high energy density within the reaction system, thereby reducing reaction times and enhancing selectivity [93].

Concurrently, heterogeneous catalytic systems are rapidly emerging as a primary area of interest due to their tunability and ease of recovery [94]. An important direction within this domain is the structural engineering of these catalysts. By establishing more robust structure–activity relationships based on crystal facets, morphology, and defect engineering, researchers may be able to tailor catalysts to promote localized heating effects, interfacial interactions, and possible “hot-spot” formation under microwave irradiation. These localized phenomena have been discussed as important factors in microwave-assisted heterogeneous catalysis and may contribute to improved catalytic efficiency and product yields [95]. However, their direct role in PET glycolysis still requires further validation through dielectric characterization, temperature mapping, and controlled thermal comparisons. Furthermore, the development of doped or composite materials, such as mixed metal oxides, represents a promising design strategy for advanced catalyst systems. These materials are expected to provide opportunities for accelerated reaction kinetics, improved tolerance to feed impurities, and stronger resistance to deactivation [60].

4.2. Reactor Engineering and Process Control

Beyond catalyst and solvent design, continuous microwave systems and automated control strategies are being explored to mitigate issues related to microwave penetration depth, uneven heating, and temperature control during scale-up.

In academia, current research focuses on developing novel reactors and alternative strategies to overcome the inherent limitations of microwave penetration depth and uneven heating. Within reactor design, structured configurations such as silicon carbide honeycombs have emerged as a promising solution. Because of their geometrically optimized heat-transfer characteristics and microwave-absorbing properties, these structured reactors may offer advantages over conventional industrial fixed-bed systems under microwave irradiation [96].

To address the complexities of microwave reaction mechanisms, multi-scale simulation provides a compelling new perspective. Microwave multiphase reactors are governed by coupled multiphysics phenomena—specifically fluid dynamics, electromagnetics, heat transfer, and chemical kinetics—spanning dimensions from micron-sized catalyst particles to meter-scale resonant cavities. To manage this complexity, researchers have proposed advanced multi-scale modeling techniques such as homogenization. This method can reduce the computational burden at the macro-scale while preserving key micro-scale details, thereby helping to connect theoretical models with experimental data and guide the scale-up of microwave reactors [97].

However, such models require systematic experimental validation. Future studies should validate the dielectric properties of PET, solvents, catalysts, and reaction mixtures under relevant temperatures; compare predicted temperature fields with infrared thermography, fiber-optic temperature probes, or other in situ temperature-mapping methods; and benchmark simulated kinetics against measured PET conversion, BHET yield, oligomer distribution, and reaction-rate data. Matched thermal-history comparisons between microwave and conventional heating systems would also help distinguish microwave-specific effects from conventional bulk thermal effects.

4.3. Industrial Scale-Up and Technology Assessment

These developments in catalyst design, reactor configuration, and process control are beginning to support pilot-scale and planned industrial applications. However, publicly available operating, energy-consumption, and economic data remain limited.

For instance, gr3n’s microwave-assisted depolymerization technology is reportedly being scaled up to a planned 40,000 t/year industrial facility in Spain. Expected to begin operations in 2027, this plant targets the closed-loop recycling of hard-to-recycle PET plastics and related PET fibers through TPA/EG recovery and repolymerization [98,99,100]. A key engineering feature of this system is the decoupling of hardware and software within its automation framework. This open architecture allows equipment to interface across different layers, potentially facilitating dynamic feed adjustment and continuous reaction control during scale-up [101].

However, publicly available data on long-term operational performance, product quality, energy consumption, and economic feasibility remain limited. Therefore, this example is presented as evidence of growing industrial interest and planned scale-up rather than definitive proof of robust commercial scalability.

To place these developments in a broader technological context, Table 7 qualitatively compares conventional and microwave heating across reaction performance, catalyst compatibility, energy assessment, reactor requirements, scalability, and industrial readiness.

Table 7. Comparison of conventional and microwave heating for PET solvolysis.

Aspect	Conventional Heating	Microwave Heating
Heating mode	Wall heating; conduction and convection	Dielectric coupling; volumetric heating
Reaction kinetics	Slower heating; longer reaction time	Rapid heating; shorter reaction time
Product selectivity	Sensitive to prolonged heating	High BHET yield under optimized conditions
Catalyst compatibility	Broad catalyst compatibility	Favors polar and ionic systems
Energy and cost	Established energy data; mature equipment	Potential reaction-stage savings; techno-economic assessment
Reactor and scale-up	Simple control; established scale-up	Field uniformity and penetration limits
Industrial readiness	Commercially established	Pilot-scale and planned industrial use

Overall, the clearest laboratory-scale advantage of microwave heating is its rapid energy delivery, which can substantially shorten PET depolymerization times, particularly in polar or ionic catalyst–solvent systems. However, shorter reaction times do not necessarily indicate lower process-level energy consumption or operating cost, because microwave performance also depends on field uniformity, penetration depth, reactor geometry, and control requirements. Conventional heating therefore retains advantages in reactor simplicity, established scale-up, and industrial maturity. Microwave heating is better regarded as a promising process-intensification strategy whose practical benefits still require validation through continuous operation, process-level energy measurements, and techno-economic assessment.

5. Conclusions

The history of PET glycolysis is reviewed, a significant body of research in the literature is compared, and several promising directions for future research are identified. Laboratory-scale studies indicate that variations in PET grade, particularly in crystallinity and relative molecular weight, significantly influence catalyst suitability and, consequently, the optimal reaction temperature and residence time. Although systematic studies are currently lacking, this analysis could be further extended to distinct structural regions of PET bottles, which may exhibit different physicochemical responses during glycolysis.

Moreover, the reviewed studies indicate that microwave-assisted glycolysis is a promising strategy for improving reaction efficiency under laboratory-scale conditions. Under optimized conditions, microwave irradiation can accelerate PET depolymerization and improve BHET selectivity by promoting rapid heating and reducing prolonged exposure to secondary reactions. However, these advantages should be considered together with the practical limitations of microwave-assisted systems. Most reported studies remain confined to laboratory-scale reactors, and translation to industrial implementation requires further advances in reactor design, microwave penetration depth, field uniformity, catalyst recovery and reuse, feedstock tolerance, product purification, and process control. Large reaction volumes may introduce non-uniform microwave energy distribution, localized overheating, and potential hot-spot formation, which could accelerate undesirable side reactions if not properly controlled. In addition, the cost of specialized microwave reactors, long-term catalyst recyclability, energy efficiency under continuous operation, and overall economic feasibility remain insufficiently evaluated. Although significant progress has been achieved in catalyst design, reactor engineering, and pilot-scale implementation, large-scale microwave-assisted PET glycolysis still faces critical knowledge gaps. Accordingly, future research should place greater emphasis on experimentally validated scale-up strategies, techno-economic assessment, life-cycle assessment, and long-term operational studies.