(148d) Understanding PET Fast Hydrolysis Via Reactive Molecular Dynamics Simulation and Experimental Investigation

Polyethylene terephthalate (PET) is among the most widely utilized plastics for consumer packaging but remains a substantial source of environmental concern due to its resistance to biodegradation. In this study, we address these challenges by integrating reactive molecular dynamics (MD) simulations with experimental hydrolysis to unravel the detailed reaction pathways governing PET depolymerization in subcritical water. The simulations employ a ReaxFF force field to capture both hydrolytic and thermal mechanisms at the atomic level, revealing how ester bond cleavage, radical formation, and secondary rearrangements occur in parallel under high-temperature conditions. Through a series of temperature‐ramped and isothermal simulations, the MD trajectories track the temporal evolution of species such as bis(2‐hydroxyethyl) terephthalate (BHET), mono(2‐hydroxyethyl) terephthalate (MHET), terephthalic acid (TPA), ethylene glycol (EG), and smaller organic fragments. These atomic‐scale insights provide clarity on several long‐standing questions, including the origins of byproducts such as isophthalic acid (IPA) and benzoic acid via ring‐rearrangement or ring‐opening steps, the transitions from hydrolysis‐dominant conditions to pyrolytic processes at elevated temperatures, and the fundamental role of water‐derived species (e.g., hydroxyl radicals and hydronium ions) in catalyzing bond cleavage.

To validate and contextualize these mechanisms, batch hydrolysis experiments were conducted on post‐consumer PET at temperatures ranging from moderate (200–250 °C) to higher‐severity regimes (>300 °C). Analysis of the recovered solid residues and solution products via High‐Performance Liquid Chromatography (HPLC) revealed how hydrolysis rate, TPA yield, and intermediate formation scaled with both temperature and residence time. The measured activation energy of PET hydrolysis aligns with our simulation‐based estimates (90–120 kJ mol⁻¹ range), further supporting the accuracy of the ReaxFF framework. In addition, we analyzed how temperature and time jointly affect product distributions by adopting a severity index (SI) that consolidates key kinetic parameters. Comparing simulated and experimental trends in TPA and EG yields under various SI values demonstrated that hydrolytic depolymerization dominates at lower severities while thermal degradation pathways intensify at higher severities, generating small molecule oxygenates (e.g., formaldehyde, glyoxal) and reducing overall monomer recovery.

By coupling atomistic simulations with experimental validation, we construct a robust kinetic picture of PET depolymerization that captures both macroscopic reaction trends and the molecular origins of intermediates and byproducts. This dual approach highlights not only the primary routes toward TPA and EG but also the subtler pathways that lead to ring‐substituted isomers, C1–C3 carbonyl compounds, and gaseous products like CO₂. Such an integrated framework furnishes the reaction‐engineering tools needed to rationally optimize PET chemical recycling processes, including reaction temperature, residence time, and process design. Ultimately, our findings underscore the pivotal role of molecular simulations in revealing intricate bond‐scission events and intermediate speciation, thereby guiding experimental strategy and informing broader life‐cycle assessments aimed at advancing sustainable plastics recycling.