Reaction mechanism and kinetic modeling of the transformation of pyrolytic limonene into cymene over bifunctional Pd/TiO2-ZrO2 catalysts

Abstract

[EN] Pyrolysis of mining truck waste tires generates limonene-rich vapors that can be selectively upgraded into high value monoaromatics such as p-cymene. This study investigates the role of bifunctional Pd/ZrO2-TiO2 catalysts on the mechanisms and kinetics of the vapor-phase conversion of limonene generated from pyrolysis. A two-stage microreactor configuration was employed, enabling the sequential pyrolysis of natural rubber, used as a representative solid model of mining truck tires, followed by ex-situ catalytic upgrading of the resulting vapors. Supported Pd (1 wt%) catalysts with tunable Lewis's acidity were synthesized, exhibiting an increasing proportion of weak Lewis's acid sites in the following order: TiO2 < Pd/TiO2 < ZrO2-TiO2 < Pd/ZrO2-TiO2. Product analysis by Py-GC-MS/FID indicates that limonene conversion proceeds through three competing pathways: (1) isomerization of limonene followed by dehydrogenation of terpinenes to p-cymene; (2) direct dehydrogenation of limonene to p-cymenene; and (3) side reactions that produce alkenes and cycloalkenes. A Langmuir-Hinshelwood kinetic model, incorporating competitive adsorption between pyrolytic limonene and co-generated byproducts successfully describes the observed conversion and selectivity trends. The model identifies the dehydrogenation of terpinenes and the initial hydrogen abstraction of limonene as the kinetically relevant steps for routes (1) and (2), respectively. Overall, this work provides a mechanistic and kinetic framework for understanding the catalytic upgrading of pyrolytic limonene over bifunctional acid-metal catalysts, offering quantitative insights into how Lewis's acidity and Pd sites cooperatively control p-cymene formation from waste-derived vapors.