Controlling catalyst deactivation: temperature regulation for the directed synthesis of easily regenerable and refractory tar in the pyrolysis of waste films

The pollution caused by waste plastic mulch film (WPF) poses a severe environmental issue. This study investigated the decisive role of catalytic bed temperature (300, 350, and 400 °C) in governing product selectivity and catalyst deactivation during the ex-situ catalytic pyrolysis of WPF using phosphoric acid-activated walnut shell biochar. By correlating pyrolysis oil composition with the chemical properties and pyrolysis kinetics of deposited tar, the temperature-driven evolution of reaction pathways was investigated. The results demonstrated that at 350 °C, the highest olefin selectivity (69%) in pyrolysis oil was achieved, but this condition promotes in-situ esterification, generating oxygen-containing tar that induces chemical deactivation. In contrast, at 400 °C, aromatization was enhanced (aromatics content reaches 18%), while carbon deposits evolve into a highly reactive O-doped structure. Kinetics analysis revealed that the tar formed at 300 °C exhibits the highest apparent activation energy (close to 60 kJ/mol), and that at 400 °C, the lowest (40–50 kJ/mol), indicating improved regenerability at higher temperature. This work highlights a temperature-controlled trade-off: while 350 °C maximizes target olefin production, 400 °C facilitates the formation of more readily regenerable coke. These findings provide critical guidance for designing selective and regenerable catalysts for plastic upcycling.