Billions of tons of plastic waste have been generated since the 1950s, with a significant portion ending up in landfills. Recycling efforts are insufficient to address this massive waste problem, particularly for single-use packaging films. The mixed nature of plastic waste, containing various polymers (polyolefins (POs), polyvinyl chloride (PVC), polyethylene terephthalate (PET)) and additives, makes mechanical recycling challenging. Pyrolysis produces low-value products, while hydrogenolysis or hydrocracking of pure POs over metal-based catalysts yields valuable products like lubricants and fuels. However, contaminants like PVC deactivate these catalysts. PVC decomposition produces HCl, a corrosive and toxic compound, rendering contaminated lubricants and fuels unsuitable for use. Addressing this requires either PVC-tolerant catalysts or an efficient chlorine removal step, eliminating the need for labor-intensive PVC sorting. This paper proposes a solution to this challenge.

Several approaches to chlorine removal have been reported, including solvothermal treatment and pyrolysis over a zeolite catalyst. These methods, however, haven't fully addressed the challenges associated with real-world mixed plastic waste streams.

This research introduces a two-stage strategy: absorptive dechlorination followed by hydrogenolysis or hydrocracking. The first stage involves treating the contaminated plastic waste with magnesium-aluminum mixed oxide (Mg,AlOx) to trap chlorine as inert MgCl2. Mg3AlO4 provides basic sites to bind HCl formed from PVC. The second stage utilizes Ru/TiO2 or Pt/WO3/ZrO2 catalysts for hydrogenolysis or hydrocracking, respectively, to convert the dechlorinated polyolefins into valuable products. The effectiveness of the dechlorination step in protecting the catalysts from poisoning and eliminating HCl emissions was investigated. The study explored various aspects, including the influence of different polymers (amorphous PP, HDPE), the effect of dechlorination temperature and pressure, and the behavior of different chlorinated compounds. Catalyst and adsorbent regeneration were also examined.

The Ru/TiO2 catalyst showed high activity for pure polypropylene hydrogenolysis, yielding 66% liquid products with lubricant properties. However, with 10 wt% PVC, the catalyst showed almost no activity. The two-stage process effectively addressed this issue. The first stage, using Mg3AlO4.5 at 30 bar H2 and 250 °C, resulted in nearly complete chlorine extraction. Subsequent hydrogenolysis over Ru/TiO2 of the dechlorinated polypropylene yielded ~70% liquid products, close to the yield obtained with pure polypropylene. Analysis of liquid products revealed differences in microstructure between pure PP and PP-PVC mixture hydrogenolysis, despite both showing lubricant properties. The two-stage strategy proved effective for various polymer compositions. The mechanism of dechlorination was investigated, showing MgCl2 formation and partial hydrogenation of polyenes formed during PVC decomposition. The role of hydrogen was crucial, as dechlorination under He led to a significant decrease in liquid yield and increased molecular weight of the liquid product. High H2 pressure was shown to be essential for efficient chlorine removal and maximizing liquid yield. The study expanded to include other chlorinated compounds, showing success with TCB and TCE but not PVDC. Hydrocracking using Pt/WO3/ZrO2 also demonstrated the effectiveness of the two-stage process, with a significant decrease in solid residue compared to hydrocracking of the non-dechlorinated mixture. The regeneration of both the catalyst and the chlorine trap was demonstrated to be feasible. The Mg3AlO4.5 trap proved effective in eliminating HCl from the gas phase, mitigating the environmental impact and equipment corrosion caused by HCl.

The findings demonstrate that the two-stage dechlorination/hydrogenolysis strategy successfully addresses the key challenge of using metal catalysts for the chemical recycling of chlorine-contaminated polyolefins. The strategy enables the processing of mixed plastic waste streams containing significant amounts of PVC without catalyst poisoning and HCl emissions. The results highlight the importance of high H2 pressure and the protective role of the dechlorination step in preventing Cl from entering the liquid product and modifying catalyst activity. While noble metal catalysts were used in this study, the dechlorination strategy is applicable to Earth-abundant catalysts, opening avenues for cost-effective upcycling. The process improvement lies in achieving this at lower temperatures than previously reported analogues, saving energy and simplifying downstream purification. This work significantly advances the feasibility of chemical recycling of plastic waste into valuable products.

This study presents a viable two-stage strategy for upcycling chlorine-contaminated polyolefin plastic waste. The approach addresses the challenges of catalyst deactivation and HCl emission associated with existing methods. The use of Mg3AlO4.5 as a chlorine trap followed by hydrogenolysis or hydrocracking over noble metal catalysts efficiently converts the plastic waste into valuable lubricant-range hydrocarbons. Future research could focus on employing cheaper, earth-abundant catalysts while maintaining the effectiveness of the dechlorination step and exploring further optimization of dechlorination conditions for various types of contaminated plastics.

The study primarily focuses on polypropylene and its mixture with PVC. While other polymers were tested, a more comprehensive analysis across a wider range of polymers and additives is needed for complete generalization. The high regeneration temperature for the Mg3AlO4.5 trap is a potential limitation, as it may counterbalance the benefits of the lower dechlorination temperature. The study's applicability to large-scale industrial processes needs further investigation.