Daniela Correa-Muriel

Waste tire pyrolysis offers an attractive route for converting abundant waste into valuable chemicals; however, the resulting pyrolytic oil remains a complex mixture with limited direct applications. Enriching the oil in monoaromatic hydrocarbons (MAHs) enhances its value and facilitates downstream processing. Here, tungstophosphoric acid (TPA)-TiO₂ catalysts were optimized using a central composite design to evaluate the effects of TPA loading (5.7–69 wt%) and calcination temperature (138–562 °C) on catalyst acidity and aromatic selectivity. Response surface modeling identified an optimal catalyst containing approximately 37 wt% TPA calcined at 350 °C, exhibiting a balanced distribution of Brønsted acid sites and high MAH selectivity. Structural characterization revealed that excessive TPA loading caused partial pore blockage and reduced surface area, whereas high calcination temperatures altered the supported heteropolyacid phase, negatively affecting catalyst performance. Product distribution analysis demonstrated that acidity governs aromatic selectivity, driving the transition from terpene aromatization and p-cymene accumulation toward reactions that maximize MAH formation, whereas excessive acidity favored heavier aromatic compounds. The optimized catalyst achieved MAH selectivities above 60% in a laboratory-scale reactor. These findings establish quantitative relationships among catalyst synthesis, acidity, and aromatic selectivity, providing a rational framework for designing supported heteropolyacid catalysts for pyrolysis-based waste valorization.

Pyrolysis as a waste treatment method has gained relevance because it can generate higher value-added products in addition to reducing the environment’s secondary pollution. In this study, the catalytic pyrolysis of waste tires was evaluated using NiTiO₃ and CoTiO₃ ilmenites as catalysts and precursors of metal catalysts with the aim to produce an oil enriched in high-value hydrocarbons, such as benzene, toluene, a xylenes mixture, and products less-reported, such as p-cymene and p-cymenene. The experiments were performed in an analytical pyrolyzer coupled to GC/MS. The effect of the nature of the catalysts on the product distribution was compared with the uncatalyzed reaction. The main products of uncatalyzed pyrolysis were D, L-limonene (~60%), and isoprene (~25%) due to the depolymerization of natural rubber. Meanwhile, Ni-ilmenites-based catalysts favored the formation of target compounds to expense D, L-limonene. Moreover, the presence of metal in reduced-ilmenite sharply enhanced the selectivity by ~50% concerning oxidized ilmenite and above 80% compared to the uncatalyzed reaction for p-cymene and p-cymenene. By contrast, the Co-ilmenites-based catalysts showed a marginal effect on secondary reactions. Finally, the feasibility of forming the aromatic terpenes, p-cymene, and p-cymenene from limonene in the non-catalytic pyrolysis was evaluated.