Catalytic pyrolysis of used tires on noble-metal-based catalysts to obtain high-value chemicals: Reaction pathways

Valorization of Waste Tires via Catalytic Fast Pyrolysis Using Palladium Supported on Natural Halloysite

“…Both mechanisms would occur on the PdHin catalyst, in which the metal particles were deposited on the most acidic support (Al­(OH) 3 ).The maximum p -cymene formation on PdHin (twice that on PdHout) could be related to PdHin’s to this higher total acidity and acidic strength (Table ), properties that are known to enhance dehydrogenation/hydrogenation reactions. Moreover, this behavior may be due to the bifunctionality established between the support’s acidity and the Pd metal sites, as has been shown in previous studies . On the other hand, when Pd NPs are outside the tubular structure (PdHout), there was less contribution to p -cymene formation, whereas the percentage of p -cymenene was higher than the other cases.…”

“…16−20 Therefore, as a major product in TPO, limonene could be used as precursor to produce p-cymene in situ from WTP. Tavera-Ruiz et al 10 envisaged this opportunity and reported the catalytic production of cymenes from waste tires using three different heteropolyacids (H 12,13 demonstrated that Pd supported on SiO 2 (dust) and titanate nanotubes promoted the formation of benzene, toluene, xylene, and ethylbenzene, (BTXE) and p-cymene during WTP. They argued that the metallic sites, the acidity, and the pore channel structure are key for controlling product distribution; thus, the correct metal−support pair should be selected to optimize the selectivity to cymene.…”

“…It also contains some alkanes, alkenes, cycloalkenes, monoaromatic compounds (benzene, toluene, mixed xylene, and ethylbenzene (BTXE)), as well as polycyclic aromatic hydrocarbons. d , l -Limonene and BTXE are considered the most marketable components of TPO. , The aromatic terpenes, such as p -cymene, also have potential for valorization; however, their low concentrations ,− make recovery difficult through conventional technologies viz . distillation or liquid–liquid extraction.…”

“…The catalysts with the highest Lewis/Brønsted site ratio (the Mo-based catalysts) produced the highest yield of cymenes. More recently, Osorio-Vargas et al , demonstrated that Pd supported on SiO 2 (dust) and titanate nanotubes promoted the formation of benzene, toluene, xylene, and ethylbenzene, (BTXE) and p -cymene during WTP. They argued that the metallic sites, the acidity, and the pore channel structure are key for controlling product distribution; thus, the correct metal–support pair should be selected to optimize the selectivity to cymene.…”

“…Moreover, this behavior may be due to the bifunctionality established between the support's acidity and the Pd metal sites, as has been shown in previous studies. 13 On the other hand, when Pd NPs are outside the tubular structure (PdHout), there was less contribution to p-cymene formation, whereas the percentage of p-cymenene was higher than the other cases. This aromatic terpene has been reported before as a byproduct in p-cymene formation from limonene, 15 and it was also detected in D,Llimonene pyrolysis performed in this study (Table 3).…”