Hot Pickering emulsion interfacial catalysis accelerates polyethylene terephthalate (PET) glycolysis†

Qinan Chen ^ac, Shuyao Wu *^ab, Po Zhang ^a, Xi-Ming Song ^a and Zhining Song *^a
aLiaoning Key Lab for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China. E-mail: sywu@lnu.edu.cn; songzhining@lnu.edu.cn
^bCenter for Analysis and Testing, College of Chemistry, Liaoning University, Shenyang 110036, China
^cNorth Huajin Chemical Industries Group Corporation, Panjin 124021, China

First published on 10th October 2023

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1. Introduction

Plastic pollution is one of the most serious environmental challenges in the past half-century. In stark contrast to skyrocketing consumption, only 33% of waste plastics were collected for recycling, and 25% of them still ended up in landfills or the natural environment, which is predicted to double by 2025 (about 12 billion tons).¹ Polyethylene terephthalate (PET) accounts for 39% of total plastic production and is widely used in synthetic fibers, insulating materials, films, and packaging, making it the largest proportion of plastic pollutants.¹ Even worse, PET could remain in nature for decades to centuries with little degradation due to its stable properties.² Hence, new and effective strategies are urgently needed to reduce the amount of discarded PET while achieving a circular economy for polyester.

Although mechanical recycling is commonly used in industry to dispose of waste PET, it is a degrading process as the reprocessed PET has deteriorated properties and can only be used to manufacture low-grade products. In contrast, the chemical recycling of PET for depolymerization and repolymerization is the most fundamental and effective solution to realize a closed cycle of carbon and hydrogen resources in polyesters,^3–7 which can alleviate the environmental pollution and energy crisis caused by waste PET. Recently, the main chemical methods for depolymerizing PET are hydrolysis, pyrolysis, alcoholysis, and ammonolysis.³ Among them, glycolysis has been widely studied for its advantages such as low volatility, the feasibility of continuous production, and relatively mild reaction conditions.^8–27 Nevertheless, the reaction conditions of glycolysis (e.g. high temperature) are still demanding, the reaction efficiency is generally low, and some additives are difficult to separate from the reactant of ethylene glycol (EG).⁹ Therefore, novel catalytic technology is essential to reduce the reaction temperature, improve the catalytic efficiency, and simultaneously facilitate the separation of components.

Pickering emulsion interfacial catalysis (PEIC) has been proven to be an efficient catalytic system,^28–36 which can overcome the problem of low reaction efficiency caused by the insolubility and incompatibility of hydrophilic and hydrophobic reactants in heterogeneous reactions. The huge interface area of Pickering emulsions can effectively promote heat and mass transfer of reactants and products, and significantly increase the reaction rate.^37–41 In addition, PEIC has advantages that traditional emulsions stabilized by surfactants do not have, for example, low emulsifier dosage, low cost, high stability, good recoverability, and non-toxicity.^42–44 However, the stability of emulsions has been limiting the application of PEIC at high temperatures. To stabilize Pickering emulsions at high temperatures, medium pressure is usually required.^45,46 To date, there are few reports of PEICs at high temperatures under atmospheric pressure.

Herein, as shown in Scheme 1, we constructed a hot Pickering emulsion interfacial catalysis (HPEIC) system composed of biphenyl (BP) with dissolved PET as the dispersed phase, EG with dissolved catalyst Zn(OAc)₂ as the continuous phase and modified asymmetric nanonets (M-ANNs) as the super-performance emulsifier,⁴⁷ where M-ANNs could stabilize the Pickering emulsion even at a high temperature of 190 °C under atmospheric pressure. In the HPEIC system, the targeted and partitioned dissolution of PET and Zn(OAc)₂ can significantly improve the glycolysis efficiency of PET and reduce the reaction temperature. When using 0.11 wt% of M-ANNs, an ultra-low dosage for emulsifiers, the HPEIC system achieved 100% depolymerization of PET and over 90% PET monomer yield within 5 min at 170 °C. Whereas, the homogeneous catalytic system with 1-methyl-2-pyrrolidinone (NMP) as a co-solvent, in the same reaction time, only achieved 78% PET monomer yield at 190 °C, which decreased sharply when the temperature was lowered to 170 °C. Furthermore, NMP is difficult to be separated from EG.

2. Experimental

2.1 Materials

Tetraethyl orthosilicate (TEOS), (3-aminopropyl)trimethoxysilane (APTMS), and n-octyltrimethoxysilane (OTMS) were purchased from Acros Organics. Poly(ethylene terephthalate) (PET), hexadecyltrimethylammonium bromide (CTAB), ethanol, toluene, ammonia (25 wt% in water), tetrahydrofuran (THF), ethylene glycol (EG), zinc acetate (Zn(OAc)₂), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), zinc sulfate monohydrate (ZnSO₄·7H₂O), manganese acetate (Mn(OAc)₂), cobalt acetate (Co(OAc)₂), 1-methyl-2-pyrrolidinone (NMP) and biphenyl (BP) were purchased from Adamas-beta. The number-average molecular weight (M_n) and weight-average molecular weight (M_w) of PET measured by gel permeation chromatography (GPC) were 4.18 × 10⁴ g mol⁻¹ and 8.13 × 10⁴ g mol⁻¹, respectively, and the polymer dispersity index (PDI) was 1.94. Silica nanospheres with a diameter of about 35 nm were purchased from XF-Nano. All chemicals and reagents were used as received without further purification.

2.2 Synthesis of M-ANNs

The synthesis of M-ANNs refers to our previous work.⁴⁷ A certain amount of CTAB was dissolved into 50 g of distilled water, and the pH of the solution was adjusted to 10 by NH₃·H₂O. 1.65 g of TEOS and 0.25 g of APTMS were dissolved into 30 g of toluene. After that, the aqueous solution and the toluene solution were emulsified with a homogenizer for 5 min. Then, the emulsion was transferred to a flask and heated at 70 °C under stirring for 12 hours. After the reaction, the emulsion was washed and centrifuged with ethanol for demulsification. The obtained precipitate was collected, washed, and filtered with hot ethanol and distilled water several times. 50 mg of the precipitate above was dispersed into 50 g of ethanol by ultrasonic, the dispersion was transferred to a flask, and then 40 mg of OTMS was added, and the mixture was heated under stirring at reflux for 12 hours. The mixture was centrifuged and the obtained precipitate was collected, washed several times with ethanol and distilled water, then filtered and dried, finally, the modified asymmetric nanonets (M-ANNs) were obtained.

The synthesis was conducted by changing the CTAB/silane ratio (g/g) as 0.9 × 10⁻², 1.1 × 10⁻², 1.3 × 10⁻², 1.5 × 10⁻², 2.0 × 10⁻², and 2.5 × 10⁻², where silane = TEOS + APTMS (Table S1†). The synthesis was also conducted by changing the toluene/water ratio (g/g) as 0.2, 0.6, 1.0, 1.4, 1.8, and 2.2 (Table S2†).

The synthesis parameters of the M-ANN used in part 2.6 are as follows. 38 mg of CTAB, 1.65 g of TEOS, 0.25 g of APTMS, 50 g of water and 30 g of toluene. The CTAB/silane ratio (g/g) is 2.0 × 10⁻², and the toluene/water ratio (g/g) is 0.6.

2.3 Treatment of PET before depolymerization

The PET particles were crushed with a crusher for 1 min and then passed through a 100 mesh sieve. The obtained powder was dried to constant weight for the depolymerization experiments.

2.4 Heterogeneous catalytic depolymerization of PET

1.5 g of PET was dispersed into 60 g of EG by ultrasonic, and then heated to a specified temperature between 150 °C–190 °C. The depolymerization reaction was started by adding 0.5 g of Zn(OAc)₂ into the EG suspension under stirring. After the reaction, the mixture was poured into 500 mL of water and the PET was filtered out. The filtrate was concentrated by evaporation and left at a low temperature overnight to precipitate a large number of white needle-like bis(2-hydroxyethyl)terephthalate (BHET) crystals, which were filtered and dried to constant weight. The PET residue was dispersed in THF and filtered again, and then the unreacted PET was separated and dried to constant weight.

2.5 Homogeneous catalytic depolymerization of PET

1.5 g of PET was dispersed into 30 g of EG and 30 g of NMP by ultrasonic, and then heated to a specified temperature between 150 °C–190 °C. After dissolving PET with stirring, 0.5 g Zn(OAc)₂ was added under stirring to start the depolymerization reaction. After the reaction, the solution was poured into 500 mL of water and the PET was filtered out. The filtrate was concentrated by evaporation and left at a low temperature overnight to precipitate a large number of BHET crystals, which were filtered and dried to constant weight. The PET residue was dispersed in THF and filtered again, and then the unreacted PET was separated and dried to constant weight.

NMP with a boiling point above 190 °C was chosen as the co-solvent, which was used for homogeneous catalysis of PET glycolysis.⁹

2.6 Hot Pickering emulsion interfacial catalysis (HPEIC) depolymerization of PET

1.5 g of PET was added to 30 g of BP and then heated to a specified temperature between 150 °C–190 °C to disperse PET. 0.5 g of Zn(OAc)₂ and 30 mg of M-ANN were dissolved or dispersed into 30 g of EG by ultrasonic and then heated to the same temperature mentioned above. The above two components were mixed rapidly and vigorously with a vortex mixer to form an emulsion, and then quickly transferred to a flask and the depolymerization reaction started. The temperature of the system is also maintained at the same one mentioned above. After the reaction, the unreacted PET and M-ANNs were filtered out by quickly dropping the temperature to 100 °C. The filtrate was filtered again by quickly dropping the temperature to 50 °C for filtering out BP. Then, the filtrate was poured into 500 mL of water, concentrated by evaporation, and left at a low temperature overnight to precipitate a large number of BHET crystals, which were filtered and dried to constant weight. The water in the filtrate was then removed by evaporation to recycle EG and Zn(OAc)₂. The PET residue with M-ANNs was dispersed in THF, and filtered again, and the unreacted PET with M-ANNs was separated and dried to constant weight.

The BP, EG with Zn(OAc)₂, and unreacted PET with M-ANNs recovered after the depolymerization reaction (mentioned above) were reused for the next depolymerization cycle of PET. Before each cycle depolymerization reaction, additional EG was replenished to make the total weight of EG and Zn(OAc)₂ equal to its initial weight, and additional PET was replenished to make the total weight of PET and M-ANNs equal to its initial weight as well.

2.7 Characterization

The morphology of M-ANNs was observed with a Hitachi SU-8010 scanning electron microscopy (SEM) operating at 10 kV. The M-ANNs were dispersed in ethanol, dropped onto a silicon wafer support and dried at ambient temperature, and then vacuum-sputtered with Pt. Transmission electron microscope (TEM) measurements were performed with a JEOL JEM-2100F operating at 200 kV. The samples were dispersed in ethanol, dropped onto holey carbon-coated copper grids, and then dried at ambient temperature. Atomic Force Microscopy (AFM) measurements were performed with a Bruker Multimode 8 with NanoScope V under ambient conditions and ScanAsyst mode. The contact angle was measured with water in an air environment, with a Kruss DSA100 automatic video contact angle measurement instrument. FT-IR measurements were performed with a Bruker Equinox 55. Nuclear magnetic resonance (NMR) analysis was performed with a Bruker Avance NEO 600M, in the d₆-DMSO solvent. High-resolution mass spectrum (HRMS) measurements were performed with an Agilent 6550 Qtof & Thermo Fisher-QE operating at 4 kV in the electrospray ionization (ESI) model under the conditions of a scan range of 100 m/z–3200 m/z, a solvent of acetonitrile, a flow rate of 0.3 mL min⁻¹, carrier gas temperature of 150 °C and jacket gas temperature of 350 °C. X-ray diffraction (XRD) measurements were performed with a Rigaku SmartLab SE using Cu Kα radiation (λ = 1.54060 Å) and operating at 45 kV and 30 mA under the conditions of a scan range of 5°–90° and a scan rate of 5° min⁻¹. Differential scanning calorimeter (DSC) measurements were performed with a Waters Q100 in nitrogen, heating from 20 °C to 200 °C at a rate of 10 °C min⁻¹. High-performance liquid chromatography (HPLC) measurement was performed with an Agilent 1100 equipped with Waters 2487 TUV detector and an Agilent 5 HC-C18 (2) 250 × 4.6 mm column under the conditions of a solvent of methanol/water (60:40 v/v) and a flow rate of 0.8 mL min⁻¹. Morphology of the emulsions was observed with a Nikon 80I optical microscope. ICP-OES measurements were performed with a PerkinElmer Avio 200 at a dilution ratio of 100.

The conversion of PET and yield of BHET are defined as,

	(1)

	(2)

where W₀ is the initial weight of PET, W′ is the weight of undepolymerized PET, W_B is the yield weight of BHET, M_B is the molar mass of BHET and M_P is the molar mass of PET (per repeating unit).

The average diameter (d) of emulsion droplets was measured with at least 50 droplets. The number (n) of emulsion droplets was measured with at least five 600 μm × 300 μm regions in the emulsion micrographs, and the average number was taken to represent the entire number of emulsion droplets.

3. Results and discussion

3.1 Morphology of M-ANNs

M-ANNs with controllable morphologies were synthesized via a simple interfacial anisotropic self-assembly approach integrated with hole-forming techniques according to our previous study.⁴⁷ As shown in the SEM image of Fig. 1a, net-like morphology with the hollow structure of M-ANNs was obtained and their contact angle with water is about 110° (Fig. 1a inset). The TEM image in Fig. 1b further reveals the hollow structure of M-ANNs. The AFM images in Fig. 1c and d also show the hollow structure of M-ANNs, and the average thickness of M-ANNs is about 18 nm. The AFM data is consistent with the SEM and TEM results.

We also found that the hole size of the hollow structure of M-ANNs could be regulated by simply adjusting the ratio of CTAB/silane (silane = TEOS + APTMS). As shown in Fig. S1 (ESI†) and Fig. 2a, as the weight ratio of CTAB/silane increased from 0.9 × 10⁻² to 2.5 × 10⁻², the hole diameter of M-ANNs increased from 4 nm to 26 nm and then decreased to 24 nm. The maximum hole diameter was achieved when the CTAB/silane ratio was 2.0 × 10⁻². Besides, the toluene/water ratio also affects the hole size. As shown in Fig. 2b, neither a higher nor lower toluene/water ratio is favorable for relatively large hole formation, and the maximum hole is achieved when the weight ratio of toluene/water is 0.6. By adjusting the above two factors, M-ANNs with a maximum hole diameter of 26 nm were finally obtained. The previous study has shown that growth in the hole diameter could increase the free energy of particle detachment, and thus can enhance the stability of Pickering emulsions.⁴⁷ Therefore, in the following, M-ANNs with a hole diameter of 26 nm were used for constructing the HPEIC system.

3.2 Construction of HPEIC system by M-ANNs

M-ANNs were used as a super-performance emulsifier to construct the BP/EG HPEIC system for PET glycolysis. For comparison, the OTMS-modified silica nanospheres (M-NSPs)⁴⁸ with a diameter of 35 nm and contact angle of about 110° were also prepared (Fig. S2†) as a commonly used particulate emulsifier to construct the same system.

As shown in the microscopic photograph and optical photograph of Fig. 3a, M-ANNs could stabilize the emulsion of BP in EG, which can disperse PET and dissolve catalyst Zn(OAc)₂, respectively, with an ultra-low usage amount of only 0.05 wt% at 190 °C for at least 60 min. However, the emulsion stabilized by M-NSPs broke at 190 °C in 1 min (Fig. 3b), even though the usage amount of M-NSPs was 1 wt%, which was about 20 times higher than that of M-ANNs. Furthermore, the emulsion stabilized by M-ANNs at 190 °C was stable enough for at least 10 hours (Fig. S3†). Thus, the HPEIC system with PET in its internal phase of BP and catalyst Zn(OAc)₂ in its external phase of EG, as a stable platform for PET glycolysis, can be constructed by M-ANNs at high temperatures.

3.3 Depolymerization of PET by the HPEIC system

As shown in Scheme 1, in the HPEIC system of BP/EG constructed by M-ANNs at a specified temperature between 150 °C–190 °C, PET dispersed in BP was depolymerized at the emulsion interface by EG in the presence of catalyst Zn(OAc)₂ which dissolved in EG. The product was identified by NMR, HRMS, FT-IR, HPLC, DSC and XRD as shown in Fig. S4 and Table S3.†

As shown in the ¹H-NMR spectrum of Fig. S4a,† the chemical shift at δ = 8.13 ppm is the proton peak of the benzene ring, δ = 4.33 ppm corresponds to the methylene group of –COO–CH₂–, δ = 3.73 ppm and δ = 4.98 ppm correspond to the methylene group and hydroxyl group of –CH₂OH, respectively. The ¹H-NMR result is consistent with the ¹H-NMR of BHET.²⁰ The HRMS spectrum (ESI model) of Fig. S4b† indicates that the peak at m/z = 255.0773 is the ion peak of M + H and the peak at m/z = 277.0691 is the ion peak of M + Na. It can be determined that the relative molecular mass of the product is about 254 g mol⁻¹, which is as same as that of BHET. As shown in the FT-IR spectrum of Fig. S4c,† the absorption peaks at 3446 cm⁻¹ and 1135 cm⁻¹ indicate the presence of the –OH group. The absorption peaks at 2964 cm⁻¹ and 2881 cm⁻¹ are signed to the C–H band. The absorption peaks at 1716 cm⁻¹ and 1689 cm⁻¹ correspond to the CO band. The absorption peaks at 1282 cm⁻¹ and 1074 cm⁻¹ are signed to C–O and C–O–C bands. The absorption peak at 1412 cm⁻¹ corresponds to the aromatic CC band. The absorption peaks at 1018 cm⁻¹ and 875 cm⁻¹ are attributed to the aromatic C–H band. The product FT-IR spectrum is highly consistent with the FT-IR spectrum of pure BHET. The HPLC of Fig. S4d† shows the product BHET has a sharp endothermic peak at 5.56 min, indicating that the product BHET is pure.

The DSC and XRD results are also provided as shown in Fig. S4e and S4f (the detailed analysis is shown in ESI†). The PET conversion and the yield of BHET are 100% and 83%, respectively. These results suggest that PET can be completely depolymerized to high-purity monomer BHET in the HPEIC system at a high temperature for a short time. The mechanism of PET glycolysis catalyzed by Zn(OAc)₂ is shown in Scheme S1.†

3.4 Effect of reaction temperature on PET depolymerization

After successfully depolymerizing PET to monomer BHET in the HPEIC system constructed by M-ANNs, influence factors of depolymerization efficiency in the HPEIC system were investigated. For comparison, PET glycolysis in heterogeneous and homogeneous catalytic systems was also investigated (details see Fig. S5†). Temperature, as it is known, is a key factor affecting the rate of chemical reactions, and thus the effect of reaction temperature on PET glycolysis in the HPEIC system was first investigated.

As shown in Fig. 4a, in the heterogeneous catalytic system, the yield of BHET was 20% and 77% after 1 min and 40 min at 190 °C (Fig. 4a black line), respectively. While in the homogeneous catalytic system, the yield of BHET could reach 70% after 1 min and 81% after 20 min (Fig. 4a red line). Compared with the heterogeneous and homogeneous catalytic systems, the yield of BHET reached 78% after 1 min and achieved the highest yield of 83% only after 5 min in the HPEIC system (Fig. 4a blue line). Although the yield of BHET all decreased with the reaction temperature reducing from 190 °C to 170 °C for the three catalytic systems (Fig. 4b), the reaction efficiency of the HPEIC system is better than that of the other two systems in the same reaction temperature condition, especially much better at relatively lower temperatures (Fig. S5†). For example, as shown in Fig. S5e,† for the HPEIC system, the yield of BHET achieved its highest yield of 80% at 150 °C after about 400 min of reaction, but the yields of BHET were 56% and 40% for the other two systems, respectively, at the same reaction temperature and time. The BHET yields after reaction for 5 min at different temperatures in the three catalytic systems are compared as shown in Fig. 4c. The yield of BHET in the HPEIC system at 150 °C was 40%, extremely higher than that of homogeneous and heterogeneous catalytic systems (7% and 2%). Furthermore, at 170 °C, the yield of BHET in the HPEIC system (77%) was also much higher than that of homogeneous and heterogeneous catalytic systems (44% and 20%), close to that of the homogeneous system at 190 °C (78%), and far exceeded that of the heterogeneous system at 190 °C (49%). On the other hand, it can be seen from Fig. 4d that the HPEIC system requires the least reaction time in the three systems to reach the same BHET yield of about 76%, that the lower the temperature is, the greater the time difference is. Because the yield of BHET at 170 °C could achieve its zenith (77%) after 5 min in the HPEIC system, 170 °C is considered to be an economical reaction temperature for PET glycolysis for the HPEIC system, lowering 20 °C compared to the traditional PET depolymerization methods (basically about 190 °C).

Several other catalysts were also used for the PET glycolysis in the HPEIC system at 170 °C, and the corresponding results are shown in Table 1. The glycolysis efficiencies in the HPEIC system are all significantly higher than those in both homogeneous and heterogeneous systems, proving the high performance of the HPEIC system for PET glycolysis. The yield of BHET using catalyst Zn(OAc)₂ in the HPEIC system is the highest one.

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3.5 Effect of M-ANNs and catalyst Zn(OAc)₂ amount on the PET depolymerization

Besides temperature, the effect of emulsifier M-ANNs and the catalyst Zn(OAc)₂ contents in the HPEIC system on the glycolysis efficiency was also investigated. Firstly, as shown in Fig. 5a, b, Fig. S6, S7, and Table S4,† when the dosage of M-ANNs raised from 30 mg to 70 mg, the average diameter of emulsion droplets decreased from 46.8 μm to 42.0 μm, and the corresponding surface area of emulsion droplets (A_s) increased compared to that constructed by 30 mg M-ANNs (A_s0). The relationship of PET conversion and BHET yield with the M-ANNs amount and the ratio of A_s/A_s0 are shown in Fig. 5c. The ratio of emulsion droplet surface area A_s/A_s0 increased from 1.00 to 1.18, and both the PET conversion and the BHET yield increased as the dosage of M-ANNs raised from 30 mg to 70 mg. When the amount of M-ANNs was 50 mg, the conversion of PET was close to 100% after 5 min at 170 °C. The increase of BHET yield with the increase of A_s/A_s0 in the HPEIC system is attributed to that the larger interface area can effectively promote heat and mass transfer of reactants and products.^37–41

Furthermore, the effect of catalyst Zn(OAc)₂ amount on the glycolysis efficiency in the HPEIC system was investigated as well. As shown in Fig. S8,† when the amount of Zn(OAc)₂ was raised from 0 g to 0.5 g, the PET conversion and BHET yield surged to the highest level of 100% and 90%, respectively, which was essentially unchanged with more Zn(OAc)₂.

3.6 Mechanism of PET depolymerization in the HPEIC system

In the HPEIC system at 170 °C, when the amount of M-ANNs and Zn(OAc)₂ was 70 mg (0.11 wt% of the total weight of the system) and 0.5 g, respectively, the yield of BHET could reach 90% after 5 min, which exceeds the yields at 180 °C and 190 °C in other depolymerization systems reported in the literature, shown in Table 2, demonstrating the superiority of the HPEIC system in the reaction rate, temperature, and efficiency. The HPEIC system can largely save energy consumption for PET depolymerization by greatly shortening reaction time and lowering the reaction temperature.

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The dissolved state of the reactants in solvents and its access to the catalyst as well as reaction activation energy are key factors affecting the traditional catalytic reaction efficiency. Higher temperatures can accelerate the reaction by providing enough activation energy and promoting the dissolution of the reactants, which is vital for the glycolysis of PET in common ways. However, as the solubility of PET, EG, and Zn(OAc)₂ in solvents is extremely different, it is difficult to find a suitable solvent to well dissolve all of the reactants and catalysts. As shown in Fig. 4, in the heterogeneous catalytic system, its lower reaction efficiency is due to the insolubilization of PET in EG. For the homogeneous catalytic system using NMP as the co-solvent, although PET was ostensibly dissolved, its dissolved state may not be efficient enough for glycolysis or access to the catalyst, owing to the vast gap between the polarity of PET, EG, and Zn(OAc)₂, resulting in lower reaction efficiency than that of the HPEIC system. The highly efficient depolymerization of PET in the HPEIC system at relatively low temperatures is attributed to the suitable dissolution of all the components in respective phases, as well as an efficient reaction at the interface of emulsion droplets. As shown in Scheme 1, in the HPEIC system, BP as the dispersed phase of the emulsion can disperse and swell PET, making the PET chain largely unfolded to expose more active sites for the glycolysis; meanwhile, EG as the continuous phase can well dissolve the catalyst Zn(OAc)₂. Furthermore, the catalyst Zn(OAc)₂ dissolved in EG can be enriched onto M-ANNs (223 mg g⁻¹, see Table S5†) located at the emulsion droplets interface, where glycolysis reaction occurs, resulting in the full and effective access of PET to the catalyst and the reduction of the reaction activation energy. As the catalytic reaction progresses, the produced BHET dissolves into EG, hence preventing the accumulation of BHET on the emulsion interface from affecting the reaction efficiency.

3.7 Recyclability of HPEIC system for PET depolymerization

From the perspective of environmental protection and economic benefits, it is important to recycle the remaining reactants, catalysts, and additives after PET depolymerization. As shown in Fig. S9† and Fig. 6, after six cycles, the emulsion constructed by M-ANNs was still stable, and the conversion of PET and the yield of BHET were maintained at a high level without any significant decrease. Compared with the homogeneous reaction system, in which it is difficult to separate EG and co-solvent NMP by distillation since their boiling points are overlapped, in the HPEIC system, BP and EG can be easily separated and recycled by filtration and distillation, reflecting the great advantages of HPEIC system in the recycling economy.

4. Conclusions

In summary, a HPEIC system for PET glycolysis was successfully constructed by using M-ANNs, a kind of high-performance and ultra-low-usage emulsifier. Compared with homogeneous and heterogeneous catalytic systems, the HPEIC system significantly increased the reaction rate of PET glycolysis and reduced the reaction temperature. When using 0.11 wt% of M-ANNs, the HPEIC system achieved 100% depolymerization of PET at 170 °C and a yield of BHET up to 90% in a remarkably short time (5 min). The HPEIC system achieved lower reaction temperature with higher reaction efficiency compared to conventional reaction systems, demonstrating the superiority of the HPEIC system with energy conservation and high efficiency. The excellent performance of the HPEIC system is attributed to the well-targeted and partitioned dissolution of all components, catalyst enrichment, and decrease of reaction activation energy at the huge droplet interface. The universality of the HPEIC system has been verified, and the recyclability was confirmed to be excellent after six cycles. As a feasible strategy, HPEIC systems would improve the efficiency of catalytic reactions, and its intensive development has important practical significance in industrial production. However, many organic solvents can be highly toxic, especially when PET monomer is used for the food industry. It is of great significance to find a green solvent instead of biphenyl used in HPEIC systems, such as ionic liquids or deep eutectic solvents. Another challenge is to reduce the amount of EG in the HPEIC system and improve its utilization.

Author contributions

Qinan Chen: investigation, sample preparation, experiment design, data analysis, writing – original draft. Shuyao Wu: data analysis, writing – review & revision. Po Zhang: sample preparation. Xi-Ming Song: supervision, writing – review & revision. Zhining Song: project administration, supervision, data analysis, writing – review & revision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51773085).

References

1. K. Hu, Y. Yang, Y. Wang, X. Duan and S. Wang, Chem. Catal., 2022, 2, 724–761 CrossRef CAS .
2. S. Kakadellis and G. Rosetto, Science, 2021, 373, 49–50 CrossRef CAS PubMed .
3. I. A. Ignatyev, W. Thielemans and B. Vander Beke, ChemSusChem, 2014, 7, 1579–1593 CrossRef CAS PubMed .
4. H. Sardon and A. P. Dove, Science, 2018, 360, 380–381 CrossRef CAS PubMed .
5. E. Barnard, J. J. R. Arias and W. Thielemans, Green Chem., 2021, 23, 3765–3789 RSC .
6. M. Hong and E. Y. X. Chen, Green Chem., 2017, 19, 3692–3706 RSC .
7. N. George and T. Kurian, Ind. Eng. Chem. Res., 2014, 53, 14185–14198 CrossRef CAS .
8. J. Xin, Q. Zhang, J. Huang, R. Huang, Q. Z. Jaffery, D. Yan, Q. Zhou, J. Xu and X. Lu, J. Environ. Manage., 2021, 296, 113267 CrossRef CAS PubMed .
9. B. Liu, X. Lu, Z. Ju, P. Sun, J. Xin, X. Yao, Q. Zhou and S. Zhang, Ind. Eng. Chem. Res., 2018, 57, 16239–16245 CrossRef CAS .
10. Z. Wang, Y. Wang, S. Xu, Y. Jin, Z. Tang, G. Xiao and H. Su, Polym. Degrad. Stab., 2021, 190, 109638 CrossRef CAS .
11. F. R. Veregue, C. T. Pereira da Silva, M. P. Moisés, J. G. Meneguin, M. R. Guilherme, P. A. Arroyo, S. L. Favaro, E. Radovanovic, E. M. Girotto and A. W. Rinaldi, ACS Sustainable Chem. Eng., 2018, 6, 12017–12024 CrossRef CAS .
12. P. A. Krisbiantoro, Y.-W. Chiao, W. Liao, J.-P. Sun, D. Tsutsumi, H. Yamamoto, Y. Kamiya and K. C.-W. Wu, Chem. Eng. J., 2022, 450, 137926 CrossRef .
13. S. Shirazimoghaddam, I. Amin, J. A. F. Albanese and N. R. Shiju, ACS Eng. Au, 2023, 3, 37–44 CrossRef CAS PubMed .
14. N. H. Le, T. T. Ngoc Van, B. Shong and J. Cho, ACS Sustainable Chem. Eng., 2022, 10, 17261–17273 CrossRef CAS .
15. L. Deng, R. Li, Y. Chen, J. Wang and H. Song, J. Mol. Liq., 2021, 334, 116419 CrossRef CAS .
16. C. Zhu, C. Fan, Z. Hao, W. Jiang, L. Zhang, G. Zeng, P. Sun and Q. Zhang, Appl. Catal., A, 2022, 641, 118681 CrossRef CAS .
17. B. Liu, W. Fu, X. Lu, Q. Zhou and S. Zhang, ACS Sustainable Chem. Eng., 2019, 7, 3292–3300 CrossRef CAS .
18. Y. Geng, T. Dong, P. Fang, Q. Zhou, X. Lu and S. Zhang, Polym. Degrad. Stab., 2015, 117, 30–36 CrossRef CAS .
19. P. Fang, B. Liu, J. Xu, Q. Zhou, S. Zhang, J. Ma and X. Lu, Polym. Degrad. Stab., 2018, 156, 22–31 CrossRef CAS .
20. Y. Liu, X. Yao, H. Yao, Q. Zhou, J. Xin, X. Lu and S. Zhang, Green Chem., 2020, 22, 3122–3131 RSC .
21. W. Zheng, C. Liu, X. Wei, W. Sun and L. Zhao, Chem. Eng. Sci., 2023, 267, 118329 CrossRef CAS .
22. S. Marullo, C. Rizzo, N. T. Dintcheva and F. D'Anna, ACS Sustainable Chem. Eng., 2021, 9, 15157–15165 CrossRef CAS .
23. A. M. Al-Sabagh, F. Z. Yehia, A. M. F. Eissa, M. E. Moustafa, G. Eshaq, A. M. Rabie and A. E. ElMetwally, Polym. Degrad. Stab., 2014, 110, 364–377 CrossRef CAS .
24. T. Wang, C. Shen, G. Yu and X. Chen, Polym. Degrad. Stab., 2022, 203, 110050 CrossRef CAS .
25. T. Wang, C. Shen, G. Yu and X. Chen, Polym. Degrad. Stab., 2021, 194, 109751 CrossRef CAS .
26. M. Imran, B.-K. Kim, M. Han, B. G. Cho and D. H. Kim, Polym. Degrad. Stab., 2010, 95, 1686–1693 CrossRef CAS .
27. F. Chen, G. Wang, C. Shi, Y. Zhang, L. Zhang, W. Li and F. Yang, J. Appl. Polym. Sci., 2013, 127, 2809–2815 CrossRef CAS .
28. L. Ni, C. Yu, Q. Wei, D. Liu and J. Qiu, Angew. Chem., Int. Ed., 2022, 61, e202115885 CrossRef CAS PubMed .
29. Z. Ming, E. Rammile, D. Lianlian, L. Xiaolong, L. Ting, Z. Xiaoming, P. B. Bernard and Y. Hengquan, Nat. Commun., 2022, 13, 475 CrossRef PubMed .
30. W. Hua, D. Xuanlin, M. Xiaohui, Q. Dong and Q. Yan, Nat. Commun., 2021, 12, 6113 CrossRef PubMed .
31. Y. Qu, D. Sun and Y. Yu, Chem. Eng. J., 2022, 438, 135655 CrossRef CAS .
32. S. Liu, Z. Lin, Z. Cai, J. Long, Z. Li and X. Li, Bioresour. Technol., 2018, 264, 382–386 CrossRef CAS PubMed .
33. Y. Shan, C. Yu, J. Yang, Q. Dong, X. Fan and J. Qiu, ACS Appl. Mater. Interfaces, 2015, 7, 12203–12209 CrossRef CAS PubMed .
34. M. Pera-Titus, L. Leclercq, J.-M. Clacens, F. De Campo and V. Nardello-Rataj, Angew. Chem., Int. Ed., 2015, 54, 2006–2021 CrossRef CAS PubMed .
35. Z. Chen, L. Zhou, W. Bing, Z. Zhang, Z. Li, J. Ren and X. Qu, J. Am. Chem. Soc., 2014, 136, 7498–7504 CrossRef CAS PubMed .
36. L. Tao, M. Zhong, J. Chen, S. Jayakumar, L. Liu, H. Li and Q. Yang, Green Chem., 2018, 20, 188–196 RSC .
37. X. Zhang, Y. Hou, R. Ettelaie, R. Guan, M. Zhang, Y. Zhang and H. Yang, J. Am. Chem. Soc., 2019, 141, 5220–5230 CrossRef CAS PubMed .
38. S. Yan, H. Zou, S. Chen, N. Xue and H. Yang, Chem. Commun., 2018, 54, 10455–10458 RSC .
39. H. Chen, H. Zou, Y. Hao and H. Yang, ChemSusChem, 2017, 10, 1989–1995 CrossRef CAS PubMed .
40. L. Wei, M. Zhang, X. Zhang, H. Xin and H. Yang, ACS Sustainable Chem. Eng., 2016, 4, 6838–6843 CrossRef CAS .
41. W. Zhang, L. Fu and H. Yang, ChemSusChem, 2014, 7, 391–396 CrossRef CAS PubMed .
42. R. Aveyard, B. P. Binks and J. H. Clint, Adv. Colloid Interface Sci., 2003, 100–102, 503–546 CrossRef CAS .
43. A. Böker, J. He, T. Emrick and T. P. Russell, Soft Matter, 2007, 3, 1231–1248 RSC .
44. J. Wu and G.-H. Ma, Small, 2016, 12, 4633–4648 CrossRef CAS PubMed .
45. S. Crossley, J. Faria, M. Shen and D. E. Resasco, Science, 2010, 327, 68–72 CrossRef CAS PubMed .
46. P. A. Zapata, J. Faria, M. P. Ruiz, R. E. Jentoft and D. E. Resasco, J. Am. Chem. Soc., 2012, 134, 8570–8578 CrossRef CAS PubMed .
47. Q. Chen, F. Liang, T. Yang, Q. Li, S. Wu and X.-M. Song, J. Colloid Interface Sci., 2022, 628, 109–120 CrossRef CAS PubMed .
48. Notes: Silica nanospheres were purchased from a commercial source, and their modification method is as follows. 50 mg of silica nanospheres was dispersed into 50 g of ethanol by ultrasonic, and then the dispersion was transferred to a flask, then 32 mg of OTMS was added, and the mixture was heated under stirring at reflux for 12 hours. The mixture was centrifuged and the obtained precipitate was collected, washed several times with ethanol and distilled water, and then filtered and dried, finally, M-NSPs were obtained.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3gc03125j

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