Plastics, as one of the most ubiquitous products of the linear economy, exemplify these challenges. Their widespread use, low recovery rates, and environmental persistence have made them a focal concern. Among them, polyethylene terephthalate (PET) stands out due to its extensive use in packaging, textiles, and consumer goods. Global PET production exceeds millions of tons annually, contributing substantially to plastic waste accumulation in landfills and natural ecosystems [3]. Its production from virgin fossil feedstocks also produces considerable greenhouse gas emissions, further compounding its environmental footprint. The proposed research project on improving chemical recycling of PET within a circular economy framework has the potential to significantly reduce these environmental impacts and create a more sustainable and efficient economy.
Although PET is technically recyclable, global recycling rates remain low. Mechanical recycling, the most prevalent method, faces significant limitations such as polymer degradation, contamination sensitivity, and downcycling, as highlighted in recent evaluations of industrial PET systems [4]. As a result, chemical recycling is gaining attention due to its potential to enable closed-loop recycling while preserving material quality. By depolymerizing PET into its constituent monomers or valuable intermediates, chemical recycling offers a promising alternative. However, the current processes often yield complex product mixtures, including monomers like terephthalic acid (TPA), ethylene glycol (EG), and dimethyl terephthalate (DMT), as well as intermediates such as bis(2-hydroxyethyl) terephthalate (BHET) and terephthalamide [5]. To improve the efficiency, selectivity, and economic viability of chemical recycling, there is an urgent and critical need to design more integrated and targeted reaction networks that maximize the recovery of desirable monomers while minimizing byproduct formation. This research addresses that need by developing novel circular economy network configurations tailored for the efficient chemical recycling of PET.
This research builds upon established methodologies for reaction network development, notably the double-direction search [6] and double-ended synthesis planning [7], which have proven effective in constructing extended gate-to-gate pathways. However, these prior approaches are limited in scope and do not consider end-of-life scenarios. In contrast, our novel methodology [8] introduces a cradle-to-cradle circular reaction network by explicitly incorporating the entire life cycle, from raw material supply to end-of-life management. To build this network, we develop an enhanced double-direction method consisting of (i) a forward search for feedstock-to-product transformations using literature review, (ii) a forward search for product-to-intermediate degradation pathways relevant to end-of-life options, and (iii) a retrosynthetic search for product-to-intermediate routes using the ASKCOS tool [9]. These pathways are screened through chemical similarity evaluation, allowing feasible connections between forward- and backward-derived reactions. Subsequently, reaction data availability is verified using databases such as Reaxys to ensure practical implementability. A hierarchical screening process is then applied to identify promising strategies. Each pathway is benchmarked against business-as-usual (BAU) metrics, including the conventional process's market price and life cycle emissions. Pathways that outperform BAU references in both economic and environmental terms are retained. Finally, a case study analysis is conducted to provide scenario-based insights for stakeholders across the product value chain.
To demonstrate the applicability of the proposed methodology, a circular reaction network is constructed for polyethylene terephthalate (PET), incorporating both upstream feedstocks and downstream end-of-life scenarios. The feedstocks include biomass sources (corn stover, wheat straw, and switchgrass) and fossil-based resources (benzene, toluene, xylene, and natural gas). Forward reaction searches map feedstock-to-product conversions based on conventional and literature-reported processes. In contrast, a second forward search captures product-to-intermediate pathways representing key end-of-life technologies, including mechanical recycling, chemical recycling (thermal pyrolysis, hydrolysis, methanolysis, glycolysis, ammonolysis, and aminolysis), thermal recycling, and landfilling. In parallel, a retrosynthetic search using ASKCOS identifies over 3,000 candidate chemicals and 100,000 reactions. These are filtered through chemical similarity evaluations, reducing the network to approximately 700 chemicals and 28,000 reactions. After verifying data availability and feasibility, a final circular reaction network comprising 100 chemicals and 221 reactions is established, highlighting the potential impact of our approach on advancing PET recycling. Optimization and hierarchical screening identify promising strategies from both economic and environmental perspectives. Among the evaluated pathways, aminolysis emerges as an up-and-coming option despite relying on a non-commercialized reaction with low technology maturity. A detailed case study is conducted to generate actionable insights for various stakeholders. These include guidance on prioritizing recycling technologies when upstream modifications (e.g., to virgin PET production) are limited, recommendations for future research and investment to support emerging reactions and concrete steps for implementing the identified strategies.
References
[1] Zuin Zeidler, V. G. (2023). Defining sustainable chemistry—an opportune exercise?. Science, 382(6667), eadk7430.
[2] Savini, F. (2023). Futures of the social metabolism: Degrowth, circular economy and the value of waste. Futures, 150, 103180.
[3] Soong, Y. H. V., Sobkowicz, M. J., & Xie, D. (2022). Recent advances in biological recycling of polyethylene terephthalate (PET) plastic wastes. Bioengineering, 9(3), 98.
[4] Bezeraj, E., Debrie, S., Arraez, F. J., Reyes, P., Van Steenberge, P. H., D'hooge, D. R., & Edeleva, M. (2025). State-of-the-art of industrial PET mechanical recycling: technologies, impact of contamination and guidelines for decision-making. RSC Sustainability.
[5] Umdagas, L., Orozco, R., Heeley, K., Thom, W., & Al-Duri, B. (2025). Advances in Chemical Recycling of Polyethylene Terephthalate (PET) via Hydrolysis: A Comprehensive Review. Polymer Degradation and Stability, 111246.
[6] Xu, Z., & Mahadevan, R. (2022). Efficient Enumeration of Branched Novel Biochemical Pathways Using a Probabilistic Technique. Industrial & Engineering Chemistry Research, 61(25), 8645-8657.
[7] Weber, J. M., Guo, Z., Zhang, C., Schweidtmann, A. M., & Lapkin, A. A. (2021). Chemical data intelligence for sustainable chemistry. Chemical Society Reviews, 50(21), 12013-12036.
[8] Kim, S., and Bakshi, B. R. (2025). Discovering Net-Zero Chemical Processes and Pathways by Developing Circular Reaction Networks and their Hierarchical Screening. Industrial & Engineering Chemistry Research, under review.
[9] ASKCOS, https://askcos.mit.edu/ (Accessed 10 May 2023)