Understanding limonene synthesis from waste tire pyrolysis through a kinetics-based perspective

Abstract

The pyrolysis of mining truck waste tires (MTWT) produces a multicomponent liquid abundant in limonene and its derivatives, including cymene and single-ring aromatics, among others. Nonetheless, although crucial for enhancing the chemical valorization of MTWT, the reaction pathways for the synthesis and subsequent transformation of pyrolytic limonene remain insufficiently comprehended. This study introduces an innovative kinetic methodology for analyzing limonene production during MTWT pyrolysis. The study combines thermogravimetric analysis (TGA) and analytical pyrolysis (Py-GC-MS) to analyze the pyrolytic behavior of MTWT and its constituent polymers, natural rubber (NR), butadiene rubber (BR), and styrene-butadiene rubber (SBR). The results of Py-GC-MS experiments showed that limonene is generated from natural rubber after C-C cleavage of polymeric chains followed by (i) an intramolecular cyclization or (ii) a Diels-Alder reaction of two isoprene units. Thereafter, limonene undergoes secondary reactions to generate cycloalkenes and aromatics. Our findings show that the formation of isoprene from the cleavage reaction of NR has the lowest activation energy (53.3 kJ/mol) within the reaction mechanism, while limonene conversion into tertiary products like aromatics or cycloalkenes requires higher activation energies (219 kJ/mol). This result suggests that catalytic materials, higher residence times or lower reaction temperatures are required to control product distribution. The kinetic model presented here can be used in future ex-situ catalytic pyrolysis studies to unravel limonene conversion into higher-value chemicals.