Polyethylene Terephthalate (PET) Recycled by Catalytic Glycolysis: A Bridge toward Circular Economy Principles

Abstract

Plastic pollution has escalated into a critical global issue, with production soaring from 2 million metric tons in 1950 to 400.3 million metric tons in 2022. The packaging industry alone accounts for nearly 44% of this production, predominantly utilizing polyethylene terephthalate (PET). Alarmingly, over 90% of the approximately 1 million PET bottles sold every minute end up in landfills or oceans, where they can persist for centuries. This highlights the urgent need for sustainable management and recycling solutions to mitigate the environmental impact of PET waste. To better understand PET's behavior and promote its management within a circular economy, we examined its chemical and physical properties, current strategies in the circular economy, and the most effective recycling methods available today. Advancing PET management within a circular economy framework by closing industrial loops has demonstrated benefits such as reduced landfill waste, minimized energy consumption, and conserved raw resources. To this end, we identified and examined various strategies based on R-imperatives (ranging from 3R to 10R), focusing on the latest approaches aimed at significantly reducing PET waste by 2040. Additionally, a comparison of PET recycling methods (including primary, secondary, tertiary, and quaternary recycling, along with the concepts of "zero-order" and biological recycling techniques) was envisaged. Particular attention was paid to the heterogeneous catalytic glycolysis, which stands out for its rapid reaction time (20-60 min), high monomer yields (>90%), ease of catalyst recovery and reuse, lower costs, and enhanced durability. Accordingly, the use of highly efficient oxide-based catalysts for PET glycolytic degradation is underscored as a promising solution for large-scale industrial applications.

Keywords: R-imperatives; chemical recycling; heterogeneous catalysts; monomer yield; packaging; plastics.

Figure 1

(a) Worldwide production of plastics from 1950 to 2022, with projections (*) for the period 2025–2050 (in million metric tons) [3,4,5]. (b) Distribution of the global plastics use in 2021 by sector of application (numeric data from [9]).

Figure 2

The identification codes (recycling symbols) of the main thermoplastics: PET—poly(ethylene terephthalate); HDPE—high-density polyethylene; PVC—poly(vinyl chloride); LDPE—low-density polyethylene; PP—polypropylene; PS—polystyrene; and O—other plastics; worldwide PET packaging consumption in 2019 by categories (numeric data from [23]).

Figure 3

Worldwide market volume of PET (in million metric tons) from 2015 with predictions till 2030 (numeric data from [26]).

Figure 4

Scientific papers published from 1967 to 2023 (Scopus search on 23 January 2024) containing “polyethylene terephthalate”, “recycling”, “glycolysis”, and/or “oxides” in title, abstract, or keywords.

Figure 5

Synthesis routes for PET by esterification and trans-esterification and representation of repeating structural unit in the polymer chain (according to [54,55]).

Figure 6

Distinct states of PET present in a stress-blow molded bottle (adaptation after [53]) and main PET properties (according to [55,56,57]).

Figure 7

Schematic representation of 4R strategy for PET management in a circular economy framework.

Figure 8

Different methods for PET recycling based on mechanical or chemical processes.

Figure 9

PET chemical recycling processes (methanolysis, hydrolysis, glycolysis, alcoholysis, aminolysis, and ammonolysis) and their corresponding chemical reactions.

Figure 10

Possible glycolysis mechanism (adapted after [142,143]).

Figure 11

Classification of catalysts and their role in PET glycolysis process.