Synergistic solvent engineering and microwave radiation for efficient low-temperature catalytic depolymerization of high-crystallinity PET

doi:10.1016/S1872-2067(26)65048-6

Abstract

Abstract:

Conventional thermocatalytic recycling of plastics is typically constrained by high energy input requirements, leading to marginal gains in energy efficiency and yield. We herein report an innovative strategy that combines π-π interaction-guided solvent engineering with microwave irradiation to achieve rapid glycolysis of highly crystalline waste poly(ethylene terephthalate) (PET) under mild conditions. In spite of the lack of solubility of PET, the depolymerization pathway is shifted from a solid-liquid interfacial process to a pseudo-houmogeneous reaction through judicious cosolvent selection. Furthermore, π-π interactions between the cosolvent and PET modulate the electron density and nucleophilic character of the ester bonds, facilitating their activation by Lewis acid catalysts. Complementarily, microwave irradiation enables rapid and uniform heating via the synergistic dielectric response of ethylene glycol (high dielectric loss tangent) and a bifunctional ZnO catalyst (serving as both microwave absorber and substrate catalyst). The synergistic cosolvent-microwave system lowers the apparent activation energy for PET glycolysis from 185 to 45 kJ·mol^-1, attaining 100% PET conversion and 97.5% bis(2-hydroxyethyl) terephthalate (BHET) yield at 150 °C within 20 min. This represents a 142-fold enhancement over the system devoid of cosolvent and microwave assistance. The integrated approach also exhibits excellent cycling stability, broad applicability across various catalysts, and selective PET depolymerization from mixed plastics, highlighting its potential as a sustainable platform for plastic waste valorization.

Key words: PET glycolysis, Process intensification, Cosolvent, Microwave irradiation, Selective depolymerization

Zhifeng Ao, Wenxuan He, Zhixue Teng, Xiongwei Liu, Zhigang Shen. Synergistic solvent engineering and microwave radiation for efficient low-temperature catalytic depolymerization of high-crystallinity PET[J]. Chinese Journal of Catalysis, 2026, 86: 125-136.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65048-6

https://www.cjcatal.com/EN/Y2026/V86/I7/125

Figures/Tables 9

Scheme 1. Outline of this work.

Table 1 HSPs of specific substances and the Ra index of cosolvents from PET.

Substance	δ_d (MPa)^1/2	δ_p (MPa)^1/2	δ_h (MPa)^1/2	δ_t (MPa)^1/2	R_a from PET	R_a from EG
N,N-dimethyl aniline	18.0	4.7	5.1	19.3	2.3	21.9
N-methyl aniline	19.5	6.0	7.8	21.8	2.9	19.5
Aniline	20.1	5.8	11.2	23.7	6.0	16.9
Anisole	17.8	4.4	6.9	19.6	2.8	20.3
PET	18.2	6.4	6.6	20.4	—	20.1
EG	17	11	26	33.0	20.1	—

Fig. 1. The interaction structures between cosolvents and BHET: N,N-dimethylaniline (a), N-methylaniline (b), aniline (c), and anisole (d). (e) PET glycolysis in different solvents (150 °C, 60 min, mEG:mPET = 4, b-ZnO: PET = 0.76 wt%, mcosolvent:mPET = 4). (f) PET conversion of pretreated PET (150 °C, 60 min, mEG:mPET = 4, b-ZnO: PET = 0.76 wt%).

Fig. 2. 1H NMR spectra of mixtures of different cosolvents and BHET: N,N-dimethyl aniline (a), N-methyl aniline (b), aniline (c), and anisole (d).

Fig. 3. 1H NMR spectra of mixed solutions of different cosolvents and EG: N,N-dimethyl aniline (a), N-methyl aniline (b), aniline (c), and anisole (d).

Fig. 4. Kinetic of PET glycolysis in cosolvent system: effect of the temperature on the rate of PET glycolysis (a), and Arrhenius plots of the rate constant of PET glycolysis (b) (mEG:mPET = 4, b-ZnO: PET = 0.76 wt%, mN,N-dimethylaniline:mPET = 4). Kinetic of PET glycolysis in cosolvent-free system: effect of the temperature on the rate of PET glycolysis (c), and Arrhenius plots of the rate constant of PET glycolysis (d) (mEG:mPET = 4, b-ZnO: PET = 0.76 wt%).

Fig. 5. (a) Comparative performance of cosolvent-assisted PET glycolysis under conventional versus microwave (MW) heating. (b) Effect of temperature on the rate of PET glycolysis. (c) Arrhenius plots of the rate constant for PET glycolysis. (d) Proposed mechanism for the rapid low-temperature depolymerization of high-crystallinity PET.

Table 2 Comparison of the PET conversion catalyzed by different catalysts between the cosolvent-microwave system and convention system.

Catalyst	PET conversion/BHET yield (%) solvent-microwave	PET conversion/BHET yield (%) convention
Zn(OAc)₂·2H₂O	90.5±1.8/84.5±1.6	1.8±0.6/0
Zn(NO₃)₂·6H₂O	88.6±2.5/78.4±2.2	1.2±0.4/0
[Bmim]Cl	80.5±1.4/68.2±2.4	0.7±0.2/0
[Bmim]Br	86.3±2.7/75.9±1.8	0.9±0.4/0
1,3-Dimethylurea/Zn(OAc)₂	98.4±2.9/90.6±2.1	2.8±1.1/0
urea/ZnCl₂	93.7±1.5/79.7±2.0	2.4±0.6/0
Fe₃O₄	71.2±1.9/58.2±1.9	0.4±0.2/0
CeO₂	78.8±2.2/68.5±2.6	0.5±0.3/0

Fig. 6. Selective depolymerization of PET in plastic blends (a) and mixed textiles (b) (20 min, mEG:mPET = 4, b-ZnO:PET = 0.76 wt%, mcosolvent:mPET = 4).

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