Hydrodechlorination of model molecules in pyrolysis oil from plastic wastes: understanding of kinetics and influence of operating conditions

Due to their low recycling rates and harmful disposal methods, the environmental challenge posed by plastics drives the need for sustainable solutions. Pyrolysis is identified as a promising method for chemical recycling, capable of breaking down plastic waste into smaller molecules that can be further processed into fuels and chemicals. However, pyrolysis oils are often contaminated with chlorine-containing compounds, which can lead to corrosion, catalyst deactivation, and lower-quality products in subsequent refining processes. Hydrodechlorination process appears to be the best and the most efficient way for upgrading plastic pyrolysis oil (PyOil), allowing the deep removal of chlorine. Hence, PyOil becomes an interesting feedstock for the petrochemical industry, facilitating the integration of plastic into an open-loop recycling system. In this study, a commercial NiMo/Al₂O₃ hydrotreatment catalyst was employed to examine the hydrodechlorination reaction for specific chlorinated compounds, including 1-chlorohexane and 2-chlorotoluene, 4-chlorotoluene and 2,4-dichlorotoluene. The experiments were conducted in a stirred batch reactor and the influence of temperature, pressure and initial chlorine concentration on the reactivity of the mentioned compounds are investigated. Furthermore, a possible reaction mechanism for 2,4-dichlorotoluene is proposed. Results state that the increase of both pressure and temperature leads to a better performance of hydrodechlorination process. Nevertheless, the parameter that exerts the most significant influence on hydrodechlorination performance was the chlorine initial concentration of the reactive solution. A comparative analysis was conducted to evaluate the results obtained for model molecules and for pyrolytic plastic oil hydrodechlorination. This suggests that the compounds under investigation in this study may not be representative of the actual mixture. It is therefore evident that further research is required in order to improve comprehension of the hydrodechlorination process of PyOil.