Elevation of pyrolysis from a random to a product-selective process through the selection of reaction conditions

Abstract

This review redefines waste plastic pyrolysis as a controllable process governed by underlying reaction mechanisms rather than a purely thermal decomposition route, positioning it as a tunable platform for chemical production. It demonstrates that product distribution is governed by the interplay of reaction conditions, reactor design, catalyst chemistry, and vapor-phase transformations, rather than the polymer structure alone. A key transition from radical-dominated thermal cracking to catalyst-mediated ionic pathways is established. In this transition, catalysts suppress uncontrolled free-radical reactions and promote selective carbocation-driven mechanisms. Importantly, catalysts are reconceptualized as post-cracking molecular architects that primarily act on pyrolysis vapors. In addition, their direct secondary transformations, such as β-scission, isomerization, aromatization, and hydrogen transfer, are directed toward targeted hydrocarbons. Acidic catalysts favor aromatic formation, basic catalysts promote olefin generation via hydrogen abstraction, and metal catalysts regulate hydrogenation-dehydrogenation reactions, improving product stability and selectivity. Reaction engineering parameters, including temperature, heating rate, and reactor configuration, critically control heat- and mass-transfer, vapor residence time, and the extent of secondary cracking. Catalyst morphology further influences diffusion and reaction pathways, where microporous structures enhance gas formation, while mesoporous and hierarchical catalysts enable higher liquid yields with reduced coking. Overall, this review highlights that the selective production of fuels and chemicals from plastic waste is achieved through coordinated control of kinetics, reaction mechanisms, and transport phenomena. This establishes pyrolysis as an engineered and scalable route for sustainable resource recovery and circular chemical manufacturing.