Neutral hydrolysis usually takes place in a continuous flow reactor or batch reactors in series at 200-300 ℃ and 1-4 MPa, with a water/PET mass ratio generally between 2 and 12^[22]. Neutral hydrolysis does not use strong acids or strong alkalis, making it environmentally friendly, but the high hydrophobicity of PET significantly reduces the reaction rate. Onwucha et al.^[23] investigated the non-catalytic neutral hydrolysis of PET at 200 ℃. The study found that as the reaction time increased from 6 h to 24 h, the yield of TPA increased from 86% to 98%. Therefore, suitable reaction rates require high temperature and pressure conditions or the introduction of catalysts, further increasing production costs and safety management costs. The main reaction equation for PET neutral hydrolysis is shown in Figure 3.

To enhance the reaction rate of neutral hydrolysis under milder conditions, Gao et al^[24] used ZSM-5 as a catalyst and employed supercritical CO₂ to intensify the neutral hydrolysis of PET. At 205 ℃, with a water/PET mass ratio of 10 for 5 hours, introducing an appropriate amount of CO₂ can swell PET, increase the activity of PET chains, and improve the depolymerization rate of PET by 38.4% (from 35.8% to 74.2%); however, excessive CO₂ will plasticize PET, induce PET crystallization, and hinder hydrolysis. Stanica-Ezeanu et al^[25] utilized a combination of NaHCO₃ + KHCO₃ salts instead of traditional acetate (zinc, cobalt, copper, cadmium, etc.) catalysts, solving the problem of reduced TPA purity caused by the incorporation of metal ions into the reaction system. At 195 ℃, with a catalyst dosage of 40 wt% for 2 hours, the yield of TPA increased by 16.2% (from 79.5% to 95.7%), but the final products of this system are TPA-Na₂ or TPA-K₂, which are highly soluble in water and remain in the liquid phase, requiring additional strong acid treatment to precipitate and separate TPA.

The reaction conditions for acidic hydrolysis are generally milder. Initially, hydrolysis is carried out in a sulfuric acid aqueous solution with a concentration higher than 14 mol/L at 85~90 ℃ for 1~5 hours. After purification by alkali dissolution and acid precipitation, EG and TPA with a purity of over 99% can be obtained^[26]. However, issues such as strong acids corroding equipment, high recycling costs, and the generation of large amounts of wastewater significantly restrict its practical application, severely hindering the industrialization process. The main reaction equation for PET acidic hydrolysis is shown in Figure 4.

In recent years, the research direction of acidic hydrolysis has mainly focused on the development of green and efficient new acid catalysts that are easy to recycle. Yang et al^[27] used p-toluenesulfonic acid (PTSA), which is inexpensive, to catalyze the hydrolysis of PET. The study found that PTSA has high reactivity. At 150°C, with a PTSA concentration of 0.8 mol/L for 90 minutes, PET was completely depolymerized, and the yield of TPA was 96.2%. Yang et al^[28] used TPA as a catalyst for the acidic hydrolysis of PET. At 220°C, with a TPA concentration of 0.1 g/mL, an H₂O/PET mass ratio of 8, and a reaction time of 3 hours, PET was completely depolymerized, and the TPA yield was 95.5%. Compared with traditional processes, this method avoids complicated purification processes and the generation of large amounts of saline wastewater. Kang et al^[29] altered the concentrations of Brønsted and Lewis acid sites in ZSM-5 and applied it to microwave-assisted PET hydrolysis. The results showed that at 230°C for 20 minutes, the TPA yield could reach 98%, and the contribution of Brønsted acid sites to PET hydrolysis was greater. Compared with no catalyst, the reaction time was halved, and the activation energy was reduced to about one-sixteenth of that without a catalyst.

Alkaline hydrolysis generally uses NaOH or KOH aqueous solutions with concentrations of 4 wt% to 20 wt%, at 200~250 ℃ and 1.4~2.0 MPa, the reaction takes 3~5 hours, and EG and high-purity TPA can be obtained after acidification and filtration^[30]. Alkaline hydrolysis depolymerization is thorough and highly tolerant to raw material contamination. However, issues such as high-temperature reaction conditions, large consumption of alkali catalysts which are difficult to recover, severe equipment corrosion, and high waste alkali solution treatment costs limit the industrial application of alkaline hydrolysis. The main reaction equation for PET alkaline hydrolysis is shown in Figure 5.

To make the reaction proceed under milder conditions, Ügdüler et al^[31] used a NaOH-ethanol-water ternary depolymerization agent to optimize the traditional two-step alkaline hydrolysis method. At 80 ℃, with an ethanol-water volume ratio of 3:2, 5 wt% NaOH, and a duration of 20 minutes, the TPA yield was 95%. They believed that ethanol increased the solubility of the product in the depolymerization agent, thereby promoting the hydrolysis reaction to proceed efficiently at low temperature and pressure.

In addition, studies^[32-33] have shown that adding phase transfer catalysts or surfactants such as quaternary ammonium salts (e.g., trioctylmethylammonium bromide) can also accelerate the reaction rate, allowing the reaction to proceed under milder conditions, but does not alter the reaction mechanism. Wang et al.^[34] combined alkyl quaternary ammonium units with heteropoly acid anions to develop a pH-responsive [CTA]₃PW phase transfer catalyst. After reacting at 110 ℃ for 5 hours, the TPA yield reached 94%. The separation and recovery of the product and the catalyst can be achieved by adjusting the pH of the system, and the change in phase state does not affect the recovery of the catalyst structure.

The methanol alcoholysis reaction usually takes place at 180-280 ℃ and 2-4 MPa. After cooling and crystallization, refined dimethyl terephthalate (DMT) and EG are obtained. Metal salts (such as zinc acetate), metal oxides (such as zinc oxide), and other catalysts need to be added to increase the reaction rate^[36-37]. The methanol alcoholysis process is simple, has a high tolerance for raw material contamination, and its products are easy to separate, which has been industrialized by large PET production companies such as Dupont, Eastman, and Hoechst. However, this method has high energy consumption, and the market share of the product DMT is low, making it difficult to integrate into the existing PET synthesis process. The main reaction formula for PET methanol alcoholysis is shown in Figure 6.

To make the reaction proceed under milder conditions, Ye et al^[38] developed a Ti_0.5Si_0.5O₂ mesoporous solid acid catalyst with strong Lewis acid sites and high specific surface area using a simple hydrolysis method, which exhibited excellent catalytic activity and stability for the methanolysis of PET. At 160 ℃, with 5 wt% Ti_0.5Si_0.5O₂, and within 2 hours, PET was completely depolymerized, with a DMT yield of 98.2%, and no significant loss of activity was found after five cycles. Tang et al^[39] prepared MgO/NaY catalysts by the incipient wetness impregnation method and applied them to the methanolysis of PET. At 200 ℃, with a methanol/PET mass ratio of 6, a catalyst dosage of 4 wt%, and within 30 minutes, the PET conversion rate was 99%, and the DMT yield was 91%. After six cycles, it still maintained good catalytic activity. Tang et al^[40] selected acetonitrile as a co-solvent for the methanolysis of PET, successfully reducing the reaction temperature to below 120 ℃ and significantly shortening the reaction time (from 10 h to 2 h). The study found that the addition of acetonitrile could promote the rupture of the PET surface, increase its specific surface area, thereby allowing better contact with methanol, which is beneficial to the chain scission and degradation of PET.

Ethylene glycol alcoholysis typically reacts at 180-250 ℃ and 0.1-0.6 MPa, producing bis(2-hydroxyethyl) terephthalate (BHET) monomer and its oligomers. The commonly used catalysts are mainly metal acetates, such as zinc acetate, manganese acetate, cobalt acetate, and lead acetate, etc.^[41]. The ethylene glycol alcoholysis method has mild conditions and a short process flow. The product BHET has a wide range of uses and high added value, and can be used to prepare polyurethane, unsaturated polyester, coatings, plasticizers, etc. It has been applied in small-scale industrial production by companies such as Eastman in the United States, Hoechst in Germany, and TORAY in Japan. However, this process has higher requirements for raw materials, and the reaction often accompanies the formation of oligomers, making the purification process of BHET complex. In addition, during the depolymerization process, if the catalyst activity is too high or it contains impurity groups, it will also lead to side reactions and poor BHET quality, which restricts the further expansion of the process. The main reaction equation for PET ethylene glycol alcoholysis is shown in Figure 7.

Ghaemy et al^[42] completely glycolyzed PET with ethylene glycol at 198 ℃ for 10 hours, and analyzed the glycolysis products, finding that the main components of the depolymerization products were BHET (>75%), a small amount of dimer, and trimer. To further improve the depolymerization efficiency, Yu et al^[43] developed a Ti-Si-ethylene glycolate catalyst by combining Si with low catalytic activity for oligoesters and Ti with high catalytic activity, which was used for PET glycolysis. At 203℃, with an EG/PET mass ratio of 4, a catalyst dosage of 0.56 wt%, and a reaction time of 3.8 hours, the yield of product BHET was 90.1%. Moreover, as an ethylene glycolate, this catalyst avoided the introduction of impurity groups during the depolymerization process. Huang et al^[44] used acetonitrile as a co-solvent for PET glycolysis. The swelling effect of acetonitrile caused the PET surface to rupture, thereby promoting PET depolymerization and significantly accelerating the degradation of oligomers to generate BHET, successfully reducing the reaction temperature to below 90°C. At 90°C, with an acetonitrile/PET mass ratio of 4, an EG/PET mass ratio of 4, a catalyst dosage of 4 wt%, and a reaction time of 12 hours, the PET conversion rate was 96% and the BHET yield was 90%.

The conditions for methanol alcoholysis are relatively harsh, and the energy consumption is high. Ethylene glycol alcoholysis is relatively mild, but the reaction often accompanies the formation of oligomers. Based on this, Japan's TEIJIN company has developed a PET ethylene glycol alcoholysis-methanol transesterification process, which has achieved commercial production at a scale of 30,000 tons/year. The process flow is shown in Figure 8. Waste PET is subjected to ethylene glycol alcoholysis after crushing and cleaning, and the alcoholysis product undergoes transesterification with methanol at 60 ℃ and atmospheric pressure. Through methanol transesterification, the oligomers that were not completely depolymerized during the alcoholysis process can be effectively and uniformly converted into DMT, thereby increasing the monomer yield. The crude DMT is melted and vacuum distilled to obtain stable quality with a purity of over 99%. This process effectively utilizes the mild depolymerization conditions of ethylene glycol alcoholysis and the ease of purifying DMT, with controllable reactions and high product purity. However, the disadvantages are the overly long process flow, high production energy consumption, and further optimization and adjustment are still needed.

The aminolysis method usually involves reactions carried out at 70-180 ℃ under medium and low-pressure conditions. After filtration, drying, and purification, terephthalamide with higher added value can be obtained^[46]. However, the purification process of the aminolysis product is complex and the reaction speed is relatively slow, often requiring the use of catalysts to increase the degradation rate. The main reaction equation for PET aminolysis is shown in Figure 9. Liang et al.^[47] selected anhydrous ammonia for the aminolysis reaction of PET. This method does not require a catalyst or solvent and can aminolyze PET into terephthalamide under mild conditions. At 120 ℃ for 2 hours, the yield of terephthalamide was 90.6%, with a purity of 96.2%. Mittal et al.^[48] studied the effect of quaternary ammonium salts on PET aminolysis and found that at 40 ℃ for 25 days, quaternary ammonium salts could accelerate the degradation rate of PET (from 31% to 40%). Jain et al.^[49] investigated the effect of zinc acetate on PET aminolysis at room temperature and pressure and analyzed the products of PET aminolysis with and without zinc acetate catalysis. The results showed that the introduction of zinc acetate reduced the reaction time to one-third of the original (from 45 days to 15 days), and all depolymerization products were terephthalamide.

Compared with ammonolysis, the reaction conditions for aminolysis are milder, usually carried out in an aqueous solution of primary amine at 20-100 ℃ and normal pressure, commonly using methylamine, ethylamine, and ethanolamine. Research^[50] shows that partial aminolysis of PET can improve its performance for manufacturing fibers or adhesives with specific properties. However, aminolysis has more side reactions and low product recovery rates, and most of the required organic amine solvents are toxic or expensive, which limits its further development and large-scale application. The main reaction equation for PET aminolysis is shown in Figure 10.

Radadiya et al^[51] selected ethanolamine to react with PET for 2 h at 160 ℃ without a catalyst, and the yield of N,N'-bis(2-hydroxyethyl) terephthalamide (BHETA) was only 77%. To further enhance the reaction rate, Gopal et al^[52] synthesized kaolin and bentonite-supported catalysts containing 5 wt% phosphotungstic acid (PWA) using the wet impregnation method and applied them to catalyze the ethanolamine degradation reaction of PET. The study found that bentonite had better acid loading properties compared to kaolin. At 160 ℃ and 5 h, the yield of the product BHETA was 96%. Zhang et al^[53] developed a green and efficient aminolysis process where PET was swollen with acetic acid and then degraded by ethanolamine to BHETA under mild conditions. The study found that the aminolysis rate was related to the swelling rate. When the swelling rate was 75%, the degradation rate of PET in ethanolamine solution at 70 ℃ and atmospheric pressure for 4 h could reach 99%, and the yield of the product BHETA was 73%. Singh et al^[54] used sulfuric acid polyborate to catalyze the aminolysis reaction of PET. The study found that at 160 ℃ and an ethanolamine/PET mass ratio of 4 for 4 h, the yield of BHETA from the depolymerization products of PET bottles, films, and fibers was higher than 95%. To develop higher value-added functionalized downstream aminolysis products, Chan et al^[55] performed aminolysis on PET with diethylenetriamine, and the depolymerization product could be crosslinked with ethylene glycol diglycidyl ether in water to obtain mechanically stable hydrogels without purification, which exhibited excellent adsorption capacity for dyes.

Under high temperature and pressure, the ionic product of water increases and it functions as an acid catalyst. Its polarity causes ester bonds, ether bonds, and amide bonds to break easily. Therefore, super/subcritical water has a high dissolving capacity for most organic substances. The main reaction equation for PET super/subcritical hydrolysis is shown in Figure 11. Čolnik et al.^[56] studied the depolymerization of PET bottles in super/subcritical water and found that when the water/PET mass ratio was 10 at 300 ℃ for 30 minutes, the TPA yield was 90% with a purity of 97%. When the temperature exceeded 300 ℃, TPA underwent decarboxylation to produce benzoic acid, resulting in a decreased yield. Meanwhile, EG undergoes secondary reactions catalyzed by TPA, dehydrating to form acetaldehyde or polymerizing into diethylene glycol, thus decreasing its yield^[57]. Jaime-Azuara et al.^[58] also demonstrated this point. At a water/PET mass ratio of about 12.3, 310 ℃, and 12 minutes, PET completely depolymerized, yielding 94.2% TPA and 77.5% EG. However, due to the relatively harsh critical conditions of water (Tc=374.3 ℃, Pc=22.13 MPa), the high equipment requirements hinder its further scale-up and continuous application.

Compared with supercritical water, the critical conditions of methanol (Tc=240 ℃, Pc=7.95 MPa) and ethanol (Tc=243 ℃, Pc=6.38 MPa) are relatively mild, resulting in less corrosion to equipment and an increased yield of EG in the supercritical alcoholysis of PET; thus, supercritical alcoholysis has gained increasing attention. The main reaction formula for supercritical alcoholysis of PET is shown in Figure 12. Since the 1990s, the technology of supercritical alcoholysis has developed rapidly. Yan et al.^[59] investigated the effects of temperature, time, and the ethanol/PET mass ratio on the supercritical ethanolysis of PET. At 310 ℃, 60 minutes, and an ethanol/PET mass ratio of 10, PET was completely depolymerized, with a diethyl terephthalate (DET) yield of 98% and an EG yield of 89.8%. When the recovered ethanol was reused for depolymerization, the yields of DET and EG decreased by only about 7% and 4%, respectively.

To reduce the reaction energy consumption and decrease the amount of solvent alcohol, Liu et al^[60-61] adopted CO₂ to enhance the supercritical methanol/ethanolysis of PET. The study found that at 270 ℃, with an alcohol/PET mass ratio of 6 and a duration of 40 minutes, the introduction of an appropriate amount of CO₂ could effectively increase the yield of supercritical alcoholysis products. The DMT yield from methanolysis increased by 18% (from 77% to 95%), and the DET yield from ethanolysis increased by 37% (from 53% to 90%). Therefore, it is believed that in the depolymerization process dominated by methanol/ethanol, CO₂ promotes depolymerization by reducing the interaction between PET molecular chains and providing a weakly acidic environment. Yang et al^[62] developed a ZnO/γ-Al₂O₃ catalyst by loading ZnO on acidic γ-Al₂O₃, and applied it to the supercritical ethanolysis of PET. At 270 ℃, 60 minutes, an ethanol/PET mass ratio of 10, and a catalyst dosage of 5 wt%, PET was completely depolymerized, and the DET yield increased by 23.9% (from 68.3% to 92.2%). Moreover, after being reused five times, it still exhibited excellent PET alcoholysis activity.

In summary, among several processes for the chemical depolymerization and recycling of PET, the hydrolysis method is relatively mature, with inexpensive and readily available solvents, and the product TPA has a high added value, which can be used to prepare fine chemical products such as polyester fibers, resins, coatings, and plasticizers. Among them, neutral hydrolysis is environmentally friendly, but the reaction rate and product purity are lower; acidic and alkaline hydrolysis have milder reaction conditions compared to neutral hydrolysis, with faster reaction rates and higher product purity, but there are drawbacks such as easy equipment corrosion, high waste liquid treatment costs, large consumption of acid-base catalysts, and difficulty in recovery. At present, the direct esterification method of TPA and EG is mainly used in industrial PET production, and the closed-loop cycle of PET can be achieved by obtaining TPA and EG through the hydrolysis method. Therefore, the hydrolysis method still has important research significance, and it is necessary to develop new green, efficient, and easily recyclable catalysts to promote the efficient and green production of the hydrolysis method. Table 1 summarizes the recent research on PET chemical depolymerization methods.

The production costs of methanol alcoholysis and EG-methanol transesterification are high, and the product DMT is difficult to integrate into the existing PET synthesis process, which has gradually been phased out by the market. However, DMT still has exploratory significance in the preparation of functionalized, high value-added polyester products (such as resins, polyester paints, and engineering plastics, etc.) through transesterification, and it is the key to further industrializing the alcoholysis method and efficiently utilizing renewable resources. Ethylene glycol alcoholysis is conducted under mild conditions with a short process flow, and its product BHET has a wide range of applications and higher added value, which can be used to prepare polyurethane, unsaturated polyester, coatings, plasticizers, etc., and is theoretically the optimal choice for achieving efficient utilization of renewable resources. Therefore, further enhancing the depolymerization efficiency and focusing on the development of decolorization and purification technologies for BHET are the keys to further industrializing the ethylene glycol alcoholysis method.

The products of ammonolysis/aminolysis have a wide range of applications and can be used to prepare functional products such as coatings, inks, adhesives, and engineering plastics. However, the reaction rate is slow, with many by-products and complex product purification processes, which make it difficult to be valued and currently still remain in the laboratory research stage. Therefore, exploring a more economical and environmentally friendly new route is the key to changing the current situation of ammonolysis/aminolysis.

The super/subcritical fluid depolymerization technology has received more attention in recent years due to its advantages such as excellent depolymerization rate, high product purity, and easy separation. However, the harsh reaction conditions require higher demands on equipment, making it difficult to be applied to industrial continuous production. Developing a relatively mild supercritical depolymerization process suitable for continuous production is the future research direction.