Advances and approaches for chemical recycling of plastic waste

2.1 Recycling of poly(ethylene terephthalate) (SPI Code 1)

Poly(ethylene terephthalate), often abbreviated PET or PETE, is a semicrystalline, thermoplastic polymer that is known for its high strength. Industrially, PET is synthesized through a polycondensation reaction between ethylene glycol (EG) and terephthalic acid (TA) or through a transesterification reaction between dimethyl terephthalate (DMT) and EG (Scheme 1).^{[21 ]} Efficient recycling of PET has reached the most advanced stage of maturity among the common plastics, and a variety of methods have proven utility on a large scale.^{[21-23 ]} There have been instances where PET undergoes primary and secondary recycling, that is, the recycling of plastic bottles. However, a significant remaining problem with recycling of PET is that the mechanical properties of the nonvirgin material are greatly reduced with each reuse. The strain-at-break (the percent of the length that a sample can be stretched before the sample breaks) for virgin PET, for example, is 42%, whereas after only the fifth cycle of extrusion, the strain-at-break was only 0.7%.^{[24 ]} This downcycling process limits the ability to thermo-mechanically recycle PET. For this reason, tertiary recycling via chemical processes has been the main focus of research in the past few years.^{[21, 25 ]}

PET can undergo pyrolysis to yield its precursor monomers, TA, and EG. Kenny et al.^{[26 ]} have shown that the pyrolysis of PET at 450°C yields TA and oligomers thereof, which can be further hydrolyzed to obtain the TA monomer. Despite such promising advances, the pyrolysis of PET is seldom used as a method to depolymerize PET into its monomeric units on an industrial scale because pyrolysis generally leads to other liquid and gaseous side-products, reducing process efficiency, and necessitating costly separation steps.^{[21 ]}

Du et al.^{[27 ]} used PET from carpet waste as a source and studied the thermal and catalytic decomposition of this waste into oils. The catalytic degradation focused on using an aluminosilicate zeolite, ZSM-5, or CaO as the catalyst. They also looked at how steam would affect the final decomposition products. They found that using a catalyst, namely CaO and steam during the pyrolysis process would yield large percentages of benzene in high purity. These studies foreshadow the promise held by many decades of plastic waste to serve as the next source of what are typically viewed as petrochemicals.

Another chemical process that has shown great promise for the depolymerization of PET into its monomeric units is hydrolysis (reaction with water at elevated temperatures and/or with a catalyst).^{[28 ]} The products yielded from this method are TA (or a terephthalate salt) and EG. There are three different types of hydrolysis that have been studied in greatest detail: acidic, alkaline, or neutral hydrolysis. While acidic hydrolysis can take place using concentrated acids, such as phosphoric or nitric, the most common acid used is sulfuric acid.^{[29-31 ]} Although the yields obtained from this method are generally high, separation of the EG from the highly acidic solution is a major drawback of this method. Additionally, the amount of acid needed to industrialize this process pose economic, process, and environmental problems.

Alkaline hydrolysis generally employs aqueous solutions of 4–20 wt% NaOH.^{[30 ]} This process yields the sodium terephthalate and EG in relatively good yields, up to 100% PET conversion. However, longer reaction times (3–5 hr) and high temperatures (>200°C) than needed for acidic hydrolysis techniques are notable drawbacks of this method. Polk et al.^{[32 ]} have recently improved on the alkaline hydrolysis process, demonstrating that the addition of a phase transfer catalyst (trioctylmethylammonium bromide) facilitated the reaction at lower temperatures (70–95°C) while yielding high purity (99.6%) TA in up to 93% yield.

Neutral hydrolysis employs water or steam in the presence of catalysts.^{[30 ]} This process uses high temperatures (200–300°C) and elevated pressures (1–4 MPa). Neutral hydrolysis, without the need for stoichiometric acid or base, would be ideal, but these processes generally produce low purity monomers and have relatively slow rate of reaction.^{[30 ]} This process does not take into account for any mechanical impurities, such as sand and particulates, which can reduce the purity of the monomer for further use. Campanelli et al.^{[33 ]} found that the use of large ratios of water:PET (5:1) are needed for the complete depolymerization of PET.

Another method to depolymerize PET into its monomers is through methanolysis. In this process, methanol reacts with PET at high temperatures (180–280°C) and pressures (20–40 atm) in the presence of a catalyst, most commonly zinc acetate.^{[34, 35 ]} This reaction leads to the formation of DMT and EG, which can then be used to resynthesize PET through a transesterification reaction. A major drawback of this method, outside of the high temperatures and pressures, is again the purification process. The crude product contains not only DMT and EG but also other alcohols and phthalate derivatives.^{[21 ]} A noteworthy advance was developed by Tang et al.,^{[36 ]} who demonstrated that DMT recovered from methanolysis of PET could be exploited as a starting material for the synthesis of gasoline and jet fuel.

Aminolysis of PET is an area that has not been widely exploited, likely because this process requires an amine (which is often toxic or expensive) to depolymerize PET, yielding diamides of TA. The reaction temperatures generally range from 20 to 100°C.^{[21 ]} In a study done by Teotia et al.,^{[37 ]} four different amines—methylamine, ethylenediamine, ethanolamine, and butylamine—were reacted with PET. Unprecedented conversion of PET into lower molecular weight oligomers was achieved at ambient temperatures and pressures, but this required reaction times ranging from 10 to 85 days. Soni et al.^{[38 ]} also studied aminolysis of PET with a variety of amines and achieved complete degradation of the PET to the diamide after 45 days of reaction. Hoang et al.^{[39 ]} showed that ethelyenediamine was even more effective and was able to depolymerize PET to yield a range of oligomers after only 17 hr at 100°C. Significantly, longer reaction times were needed to achieve degradation to small molecules; however, several catalysts, such as dibutyl tin oxide, sodium acetate, and cetyltrimethylammonium bromide, are under development that show promise for shorter reaction times and improved selectivity, but these have not yet demonstrated large-scale applicability.^{[40, 41 ]}

Glycolysis of PET is an area that has been widely studied. This is a very versatile process due to the various potential applications of the products obtained. In this process, PET is depolymerized by glycols to form monomers, oligomers, and/or polyols, which can then be used for different applications. Some of the glycols that have drawn particular recent interest for this application include ethylene glycol, diethylene glycol (DEG), propylene glycol, butylene glycol, and dipropyleneglycol (DPG).^{[42, 43 ]} If EG is used to depolymerize PET, the major product formed is bis(2-hydroxyethyl) terephthalate (BHET), which can be used to synthesize PET. A challenge in these promising reactions is the immiscibility of PET with the polyols. Liu et al.^{[44 ]} sought to address this problem by conducting a careful study on the role of different solvents in the conversion of PET to BHET. DMSO proved most effective among solvents screened for effective cosolvation of EG and PET and thus showed an increase in BHET yield to 82% (when compared to 20% without the addition of DMSO) with remarkably short reaction times of 1 min at 190°C and atmospheric pressure.

Catalyst development has also been an area of active research. For many years, zinc acetate was the primary catalyst used in the glycolysis of PET. Troev et al.^{[45 ]} have recently developed a titanium (IV) phosphate catalyst for the glycolysis of PET using EG, DEG, or 1,2-propylene glycol. This catalyst achieved shorter reaction times than previous catalysts, with increased yield and selectivity for BHET formation. Titanium is advantageous because it is nontoxic, though the catalyst could not be efficiently recycled. Wang et al.^{[46 ]} have sought to exploit sustainably produced organocatalysts such as urea in place of transition metal catalysts. Urea was quite an effective catalyst, facilitating 100% conversion of PET with 74% selectivity of BHET. Additionally, urea could be recycled five times without loss of activity or selectivity for BHET.

Ionic liquids can also be used as a catalyst for the glycolysis of PET.^{[47, 48 ]} Despite their high initial cost, ionic liquids have become increasingly attractive tools for green chemistry because of their low volatility and recyclability. Consequently, Yue et al.^{[49 ]} have explored the utility of 1-butyl-3-methylimidazolium hydroxide, ([Bmim]OH) to serve as a catalyst. A 100% conversion of the PET with a selectivity of 72% of BHET was achieved, compared to only 11% conversion of PET without the addition of the ionic liquid as a catalyst. In a follow-up study,^{[50 ]} these researchers found that Lewis acidic ([Bmim]ZnCl₃) ionic liquid likewise facilitated a 100% PET conversion but with improved selectivity of 84% for BHET and minimal catalyst loading (0.16 wt%). Liu et al.^{[51 ]} reported that deep eutectic solvents could also be used to catalyze the glycolysis of PET with EG. They found that the combination of 1,3-dimethylurea (1-3-DMU) with 5 wt% Zn(OAc)₂ was able to convert 100% of the PET with 82% selectivity for BHET at 190°C in just 10 min. They associated the high selectivity with relatively mild reaction conditions due to the acid–base synergistic effects between 1-3-DMU and Zn(OAc)₂. The catalyst was also recycled up to five times without any loss in conversion efficiency. However, the zinc content was shown to decrease by 25% after the five cycles, which limits further recycling ability of the catalyst.

Whereas the foregoing discussion highlights efforts to leverage glycolysis of PET with EG to synthesize BHET as a monomer feedstock, an emerging area of interest is the use of the chemically recycled PET products as feedstocks for other polymer formulations. Mecit and Akar^{[52 ]} used different glycols to convert PET into lower-weight oligomers having hydroxyl end groups that were then reacted with toluene diisocyanate to produce urethane oils. The recycled urethane oils showed similar Koenig hardness values and touch/hard to dry times to commercially available oils. Desai et al.^{[53 ]} synthesized polyol blends by reacting PET waste with plant-derived starch. The oligomers so formed were then esterified with fatty acids—themselves primary constituents of low-value, high-volume waste products from other industries.^{[54-58 ]} The esterified materials were then used to synthesize polyurethanes. These authors also demonstrated the facile tunability of adhesion, flexibility, and chemical resistance properties of the polyurethanes as a function of PET products present in the formulation.

Amaro et al.^{[59 ]} used DEG and PET to synthesize oligomers that proved effective as secondary plasticizers in PVC formulations resulting in improved thermal stability and flexibility of the final PVC product. Furthermore, migration of PET-derived plasticizer migration was greatly decreased compared to traditional PVC plasticizers such as, di(2-ethylhexyl)phthalate (DHEP) which leach out over time. Initial PET oligomerization was achieved in this instance by action of a Ca/Zn stearate catalyst at 250°C for only 20 min. Recently, Sirohi et al.^{[60 ]} have described alcoholysis of PET to synthesize oligomers that can be used as a plasticizer in nitrile-PVC rubber blends. They used a ZnCl₂ catalyst and 1-decanol as the alcohol with reaction temperatures of 190°C for 4 hr. Once resultant depolymerized products were blended with nitrile-PVC rubber blends, the tensile properties and the aging resistance of the materials was significantly buttressed.

2.2 Polyethylene (SPI Codes 2 and 4)

Polyethylene is a lightweight and durable thermoplastic that finds use in films, tubing, packaging, plastic bags and bottles, and even automobile parts. PE is typically made by polymerization of ethylene (C₂H₄), often facilitated by a Ziegler–Natta or metallocene catalyst.^{[28 ]} The linear alkyl chains comprising the PE backbone lack polar functional groups, and are inert to many chemical reactions, including common acids and bases, under standard conditions. To complicate recycling efforts further, there are multiple types of PE: HDPE, LDPE, LLDPE, ultrahigh molecular weight polyethylene, and many other crosslinked types of PE.^{[28 ]} Each of these different types of PE is used for different purposes depending primarily on the mechanical property profile required. The most-used PEs include HDPE (SPI Code 2) and LDPE/LLDPE (together sharing SPI Code 4). HDPE is flexible, translucent, and shows good toughness at low temperatures. There is little to no branching in HDPE, thus it is said to be a linear polymer. LDPE and LLDPE differ from HDPE in that both are branched polymers. LDPE is semirigid and has branches that are both short and long throughout the polymer backbone. LLDPE differs from LDPE in that this form of PE features only shorter branches (Figure 2).

The structural variability and relative chemical inertness of PE have relegated most studies on PE recycling to variations on pyrolysis. A primary difficulty associated with efforts to recycle PE by pyrolysis is that the thermal degradation of PE usually proceeds via random scissions at CC bonds. This homolytic scission generates two radical chains that can go on to form a complex mixture of olefinic products and highly crosslinked polymeric products.^{[61 ]} Moreover, the melt flow index (a key figure of merit for processability) changes by two orders of magnitude, from 2.25 (reasonable flow) for virgin LDPE to a nearly unprocessable melt flow index of 0.02 g/10 min after 100 extrusion cycles.^{[61 ]} This changing behavior is accompanied by drastic, deleterious drift of mechanical properties of the recycled LDPE, so these approaches do not provide a long term, sustainable path forward for PE recycling. When mechanical processing of PE is no longer feasible, depolymerization of PE through pyrolysis to yield hydrocarbons for fuel/energy applications is often the final stage of use for such materials.

There are two major processes for the pyrolysis of PE: thermal or catalytic pyrolysis. Thermal pyrolysis is simply heating PE at high pressure to break down the polymer backbone to form smaller organic molecules. Catalytic pyrolysis utilizes a catalyst in an effort to reduce the temperature and reaction time and thus improve the economic viability and, in some cases, the selectivity. In a study by Ahmad et al.,^{[62 ]} the thermal pyrolysis of commercially available HDPE pellets at 350°C which yielded a liquid oil product in 81% yield. The oil consisted mainly of paraffinic hydrocarbons, most of which, contained between 6 and 16 carbon atoms. Specificity for a single chemical commodity from PE pyrolysis, however, remains elusive. An intriguing divergent strategy has been devised by Palos et al.,^{[63 ]} who eschewed the selectivity problem and instead sought to take waste HDPE and chemically transform it into a complex mixture akin to “crude” oil. As long as the complex mixture is comprised by molecules similar to those found in petroleum, such a complex mixture could hypothetically be processed by established petroleum refining and cracking techniques. This nascent approach used waste HDPE that had already been sorted, washed, and shredded. The HDPE samples so prepared were then heated in a batch autoclave reactor at 430°C with short reaction times, up to 38 min, to yield what they refer to as “plastic oil,” with impressive yields of up to 85–90 wt% oil recovered. While the oil obtained is not clean enough to be used directly in place of fuel, this plastic oil can be refined by established methodologies to yield target products. The main drawback of this creative approach is the potentially high energy required to produce the plastic oil on large scales.

While many researchers focus on pristine or near-ideal PE sources in their proof-of-principle studies, Das and Tiwari^{[64 ]} examined the important difference between utilizing virgin and waste plastic products. Additionally, a mixed waste feedstock mixture of different polymers was used. The mixture consisted of HDPE, LDPE, and PP. The virgin polymers were obtained commercially, and the waste plastics were obtained in the form primarily of packaging plastics, plastic containers, and bottles. The thermal pyrolysis of this mixed waste was undertaken at 350 or 400°C and the reaction time was 8 hr. Lower temperature reactions yielded lighter hydrocarbons (< C₂₀) while an increase in temperature yielded heavier hydrocarbons (> C₂₀). This offers the potential to tune the oil obtained by varying reaction temperature and holds promise for exploiting mixed waste streams and thereby circumventing some of the challenges associated with waste separation.

Miandad et al.^{[65 ]} likewise recognized the importance of developing strategies to deal with mixed waste. In this study, the plastic waste was collected in the form of disposable plates, grocery bags, and cups and comprised not only linear alkyl polymers (PP and PE) but also aromatic-bearing polystyrene (PS) and PET. The plastic samples were crushed into fine powders and used on their own as well as mixed together in varying ratios (as little as 20 wt% PE) with reaction conditions of 450°C for 75 min. Depending on the ratios of the plastic waste liquid oil yields ranged from 24 to 54%. The initial feedstock used was 1 kg, which shows the potential to scale the system to an industrial level. The oil obtained consisted of large amounts of aromatic compounds and showed higher heating values between 41 and 42 MJ/kg, which is comparable to that of commercially available diesel (43 MJ/kg).

The area of catalytic pyrolysis of PE has also achieved notable advances in the last few years with regard to increasing yields and lowering reaction time/temperature. Santos et al.^{[66 ]} looked at three different zeolites, HZSM-5, USY, and NH₄ZSM-5. These catalysts differ based on pore size, acidic sites, and surface area. This study garnered important insight that catalysts with larger pore sizes led to a greater yield of liquid products and that the less acidic catalysts would yield lighter fractions of gas and liquid products, thus affording flexibility in the process based on end use.

An intriguing study by Chattopadhyay et al.^{[67 ]} examined the effects of catalysts on pyrolysis of an exceedingly complex and disparate waste stream of HDPE, PP, PET and paper biomass (consisting primarily of cellulose). Cobalt complexes were used as catalysts with various chelating agent along with Al₂O₃ and/or CeO₂. The catalytic pyrolysis of the paper waste by itself yielded mainly gaseous and solid products, whereas pyrolysis of the paper/PP/HDPE/PET mixture yielded more liquid products that were rich in aromatics and olefins.

The potential for pyrolytic approaches to valorize PE waste is counterbalanced by the high energy cost. A greener avenue that has shown recent promise in the decomposition of plastic waste is the use of supercritical water.^{[68, 69 ]} An added benefit of using supercritical water is that there is no need for a catalyst. Moriya and Enomoto^{[70 ]} undertook one such study on supercritical water as a medium for the thermal cracking of PE for comparison to established pyrolysis techniques. A 5:1 ratio of H₂O:HDPE was used, and the mixture was reacted at 425°C for 2 hr, the oil conversion rate was shown to be 90%.

Encouragingly, the yield of organic oil produced was higher for supercritical water cracking, while commensurately less coke/char was formed in the process. The oil product mainly was composed of a majority of alkenes with the presence of some alkanes with very little presence of any aromatic moieties.

While most of the aforementioned approaches to recycling PE have focused on the pyrolysis to form shorter chain molecules that ultimately tend to be used in fuel applications, the pyrolysis products may also be leveraged as feedstocks to produce other polymers. Recently, Bäckström et al.^{[71 ]} depolymerized HDPE waste to synthesize a mixture of succinic, glutaric, and adipic acid. This was accomplished through a microwave-assisted hydrothermal process with the addition of HNO₃. Whereas microwave reactions have in the past been dismissed as impractical for large-scale operations, recent advances and implementation on a commercial scale have begun to dispel these preconceptions.^{[72 ]} The products of the microwave reaction were carried forward as plasticizers in a poly(lactic acid) formulation. Incorporating these PE-derivative plasticizers into the PLA formulation increased the strain at break to 144% as compared to 6% for PLA without the use of any plasticizer. This study illuminates a creative path forward to exploit postconsumer petrochemical waste for improved performance of sustainably sourced polymers.

2.3 Polypropylene (SPI Code 5)

Polypropylene (PP) has a similar backbone to PE, but the presence of an additional methyl group as a sidechain on each repeat unit has a significant impact on the properties. PP is a light weight, tough, crystalline thermoplastic polymer that finds use in reusable food containers, the automotive industry, and even the furniture market.^{[73 ]} PP is synthesized from the propylene (C₃H₆) using either a Ziegler–Natta or metallocene catalyst. PP can have three different types of structures, atactic, isotactic, and syndiotactic,^{[28 ]} depending on the relative disposition of the methyl side chains with respect to one another along the backbone (Figure 3).

The configuration of the methyl groups has an effect on the crystallinity of the PP and consequently on bulk properties. The tacticity of PP generally has little effect on recycling techniques and consequently will not be discussed further here. Very little of the PP that enters the marketplace—less than 1%—ends up being recycled, largely due to the fact that PP is generally found in mixed waste streams^{[12 ]} such as cable coverings, electronic appliances, and rugs. In each of those materials, PP is not the only polymer that is present, thus, having to wash and separate each component are major barriers to recycling efforts.^{[73 ]}

In cases where separation is effective and practical, there are still drawbacks to melt reprocessing of PP. As with the other polymers discussed herein, recursive heating cycles rapidly degrade the PP backbone. Thermal degradation of PP is more severe than in PE because the tertiary carbon atom present in the PP backbone is susceptible to thermo-oxidative and photo-oxidative degradation.^{[73 ]} While some PP plastics include stabilizer to prevent degradation from occurring, if the PP polymer has been successfully recycled into a new material, this material must then include additional stabilizers to imbue the “newly” made material the same oxidative stability, while the buildup of stabilizer and sacrificial stabilizer degradation products contributes commensurately to the deterioration of PP properties. The elongation-at-break for a virgin, unstabilized sample of PP is 65%; however, after just 10 recycling cycles the elongation-at-break has decreased to 45%.^{[74 ]} When PP can no longer be mechanically recycled through either shredding or melt reprocessing, there are some emerging strategies to convert PP into value-added feedstocks.

Interest in tertiary recycling of PP has been growing rapidly. The depolymerization of PP into propylene has even been accomplished by Guddeti et al.^{[75 ]} In this process, PP was depolymerized in an induction-coupled plasma reactor. Under these conditions, PP was converted to gaseous products (up to 78 wt%) and of the gaseous product formed 94% was identified as propylene. Other studies have used plasma as a media to depolymerize PP into other value added commodities.^{[76 ]} One major advantage of using a plasma reactor is that reaction times are very short; however, the cost to setup and operate plasma reactors on an industrial scale detracts from the practical viability of these approaches. Another drawback of using such high temperatures is that any contaminants present, as would be expected in a waste stream, tend to yield many side reactions and have profound impact on product yields.

As for PE recycling strategies, supercritical water is a privileged media for contemporary PP recycling as well. Chen et al.^{[77 ]} used supercritical water to convert PP into oil. Optimal reaction conditions were found to be at 425°C for 2–4 hr or 450 °C for 0.5–1 hr, in which case up to 91 wt% of the PP was converted into oil. The composition of the oil was found to be olefins, paraffins, cycloalkanes, and aromatics. The analysis of the oil showed that it had similar properties to that of naphtha, which can be further purified to make gasoline. The use of supercritical water to depolymerize PP is a largely sustainable step toward upgrading waste PP into value-added feedstocks.

The decomposition of PP can also yield solid carbonaceous products that can be used for various applications. Liu et al.^{[78 ]} described a two-stage reactor. In the first step of the process, PP is catalytically pyrolyzed to yield gaseous and liquid products. These products are then taken to the second stage of the process in which the products are thermally decomposed to make either carbon nanotubes or gaseous products, which mainly consisted of H₂ or CH₄. This process allows for the collection of gaseous and solid products that can be used in different applications. In a similar vein, Mishra et al.^{[79 ]} successfully synthesized carbon nanotubes and hydrogen gas from waste PP using a chemical vapor deposition method with a nickel-based catalyst. The carbon nanotubes synthesized by this process showed high transmittance (85%) of light at 550 nm, which can be exploited as transparent electrodes for optoelectronic devices. The gas evolved was mostly aliphatic molecules, with very little to no presence of aromatics.

Given the difficulties associated with obtaining near-pure PP from waste, the incorporation of PP into other waste streams for recycling has drawn interest as well (some of the PE recycling strategies have already illustrated this approach; vide infra). The annual availability of gigaton quantities of biomass (primarily lignocellulosic) make this an attractive waste stream for valorization.^{[80-82 ]} Simple pyrolysis of biomass waste alone generally leads to low-value final products. The addition of PP to biomass waste, however, can enhance the properties of the final products. Zhao et al.,^{[83 ]} for example, mixed bamboo waste with PP and the copyrolysis of the two yielded bio oils that have potential as fuels. To accomplish this transformation, a zeolite catalyst (HZSM-5) was used while the bamboo:PP ratio and catalyst loading were varied. Even under optimal conditions, bamboo alone still gives quite a poor oil yield (30 wt%) and low oil quality. The bamboo-derived oil consisted primarily of aliphatic hydrocarbons.

When a 2:1 ratio of bamboo:PP was employed, they found that oil production could be more than doubled to 62 wt% with attendant improvement in oil quality. The bamboo/PP-derived oil consisted of more aromatic and naphthenic compounds. Such compositions are ideal feedstocks for jet fuel production due to the large amounts of heavier hydrocarbons produced. Lee et al.^{[84 ]} further demonstrated the broader scope of such processes by showing that PP addition to high cellulose content agricultural waste from sources other than bamboo can likewise improve the production of oils in high yields and of more desirable composition than can be accomplished by pyrolysis of cellulose alone.

2.4 Current industrial efforts in plastic recycling

Given the contemporary importance of plastics recycling, there are significant industrial efforts to commercialize processes based on the previously discussed strategies to chemically recycle mixed waste streams comprising both hydrolysable plastics (primarily PET) and nonhydrolyzable plastics (such as PE, PP, and PS).

2.4.1 Industrial approaches to recycling PET and mixed polyester waste

There are many different companies active in the chemical recycling of PET and other polyesters on an industrial scale from real-world waste streams. Most of their recycling methods of PET employ variations of the depolymerization approaches discussed in the previous section with the aim of producing various monomers. Chemical recycling of PET has reached a more mature state due to the relative ease of its depolymerization as compared to polyolefins.

Depolymerization of polyesters to several different monomer feedstocks has been successfully commercialized. Carbios focuses on enzymatic hydrolysis of polyesters such as PET (Scheme 2) to yield TA. While the optimal enzyme varies depending on the particular polyester to be hydrolyzed, the enzymes employed by Carbios are all hydrolase enzymes produced by Thermobifida alba or Fusarium solana pisis. The use of T. alba, a thermophilic bacterial species, allows the hydrolysis to be carried out at higher temperatures and is a key to the success of this process.

Following enzyme-catalyzed hydrolysis, the product TA is purified and used to prepare virgin PET. The patented Carbios process accomplished depolymerization at 20–80°C and is operable over a wide pH range from 4 to 10.^{[85 ]} The specific pH values are selected on the basis of the particular polyester to be depolymerized. The enzyme loading varies from 0.005 to 15 wt% and reaction times can vary from 5 to 72 hr.

Another company focusing on depolymerization of PET and polyesters is Gr3n. Their process focuses on less-easily processed polyester waste streams that include colored PET bottles and polyester textiles. The process they employ is their patented DEMETO (depolymerization by microwave technology) technology, which is able to cut reaction times down to 10 min.

In the first step of the patented process, the plastic waste is ground up and mixed with a solvent.^{[86 ]} The solvent is typically ethylene glycol (Scheme 3) and an alkali base, such as NaOH, KOH, or LiOH. This heterogeneous mixture is then transferred to a reaction chamber where microwaves are used to heat the chamber to a temperature of 150–350°C with a pressure between 1 and 20 atm. After this treatment, the reaction mixture is filtered and the EG is distilled to be reused in the process. When PET waste is treated by the DEMETO process, TA is the major product and is readily obtained in pure form by simple precipitation, filtration, washing and drying. DEMETO also works as a method to recycle polyamides. In this case, the hydrolysis of amides is analogous to the hydrolysis of esters discussed in detail in the previous section. Amides, however, are somewhat less reactive to hydrolysis. For this reason, acidic media, such as HCl, H₃PO₄, or H₂SO₄, is used in the DEMETO processing of polyamides. Although DEMETO is a more energy-intensive process than the low-temperature Carbios process, the time required is much shorter.

Whereas Carbios and Gr3n focus on production of TA from PET (Scheme 2), companies such as Ioniqa Technologies and Jeplan have developed commercial variations of the depolymerization of PET into BHET (Scheme 3). In Ioniqa's patented process, an ionic catalyst complex is able to degrade PET into BHET.^{[87 ]} This ionic catalyst complex generally consists of a nanoparticle, such as magnetite or maghemite, that is magnetic in nature, a bridging group, such as triethoxysilylpropyl, and an ionic catalyst, such as (bmim)⁺ and FeCl₄⁻. This catalyst is able to depolymerize PET waste at temperatures of 200°C with only a catalyst loading of 2 wt%. Yields for this process can be up to 93% with near quantitative conversion of the PET waste.

It is notable that Ioniqa's magnetic metal particles serve a dual role of catalyst and pigment adsorbent. Magnetic particles are also easily removed by magnetic catchers in the process line to prevent their incorporation in end products. In contrast, Jeplan uses separate catalyst and activated carbon to remove inks and dyes. This approach simplifies catalyst preparation but makes separation of the catalyst and adsorbent less straightforward that in Ioniqa's approach. Jeplan' process is successful for recycling mixed colored and clear PET bottles and fibers by heated the waste materials with BHET at 220°C for 1 hr.^{[88 ]} This achieves what they term as a predecomposition product in which the PET is broken down into oligomers. Ethylene glycol and sodium methylate catalyst are added to the predecomposition solution and are heated at 200°C for 2 hr to completely decompose the PET to form BHET. The solution is cleaned with activated carbon in order to remove dyes and impurities. The BHET solution is then purified and obtained as a pure monomer in 98% yield, which is used to synthesize virgin PET.

Both the processed developed by Ioniqa Technologies and Jeplan demonstrate the additional considerations one must keep in mind when converting the knowledge learned from academic studies to practical handling of mixed waste streams. In the case of mixed PET waste processes, these considerations include recovering catalyst and removal of pigments and dyes, considerations mentioned much less frequently in academic investigations.