PNNL

The Science

Plastic wastes represent a significant source of trapped, but potentially valuable, chemical feedstocks. Converting plastics into usable chemicals remains a challenge given their different chemistries, particularly mixtures that include chlorinated plastics, such as PVC. Researchers developed and demonstrated a method that can take contaminated post-consumer mixed plastics of PVC and polyolefins and convert them into fuel-range alkanes in a single-stage catalytic process at low temperatures and pressures. The process requires no dichlorination pre-treatment and produces hydrochloric acid as a byproduct. The approach couples bond breaking and bond making to reduce the overall operating temperature.

The Impact

Managing mixed plastic wastes of PVC and polyolefins poses significant challenges, as traditional disposal methods generate toxic chlorine-containing byproducts without extensive pre-treatment of the PVC. This new method can efficiently convert these wastes into usable chemicals in a single-stage catalytic process. This process enhances efficiency and cost-effectiveness by reducing energy use, equipment needs, and intermediate steps, leading to significant savings. Removing the need for pre-processing or intensive waste sorting represents an important step to developing streamlined industrial processes that transform waste into economically valuable products.

Summary

Polyolefins and their chlorinated derivatives are among the most prevalent plastics used and discarded around the world. Traditional waste-to-energy methods and most chemical upcycling methods for using PVC require thorough, high-temperature dechlorination to prevent the release of toxic chlorinated compounds. Researchers developed a strategy for upgrading discarded PVC into chlorine-free fuel range hydrocarbons and hydrochloric acid in a single-stage process, catalyzed by chloroaluminate ionic liquids. This approach offsets the endothermic dechlorination and C-C bond cleavage reactions with exothermic alkylation and hydrogen transfer by a light isoalkane in a low-temperature (≤70 °C) tandem process, achieving complete mixed polymer conversion. It predominantly produces saturated, chlorine-free liquid hydrocarbons, mostly C₆–C₁₀ isoalkanes (50–60 wt.%), with 30–40 wt.% of light isoalkanes (C₄ or C₅) that can be recycled as alkylating agents. The team combined experimental and computational studies to probe the mechanism of the overall process. The results showed that while dehydrochlorination to produce polyenes can occur without the light isoalkanes, the light isoalkanes are crucial for breaking C-C bonds, transferring hydrogen, and forming C-C bonds through alkylation. This alkylation process helps prevent the formation of undesired unsaturated compounds that deactivate catalysts. The team demonstrated that the combined process is effective at co-processing real-world mixed and contaminated post-consumer PVC and polyolefin plastics.

Contact

Sungmin Kim, Pacific Northwest National Laboratory, sungmin.kim@pnnl.gov 

Zdenek Dohnalek, Pacific Northwest National Laboratory, zdenek.Dohnalek@pnnl.gov 

Funding

W. Z. thanks the financial support from East China Normal University. We also acknowledge funding from the Department of Energy, Office of Science, Basic Energy Sciences program, Chemical Sciences, Geosciences, and Biosciences Division, under the projects “Towards a Polyolefin-Based Refinery: Understanding and Controlling the Critical Reaction Steps” (FWP 78459) and “Advancing Key Catalytic Reaction Steps for Achieving Carbon Neutrality” (FWP 47319).