(521eu) Catalytic Upcycling of Polyolefin Wastes Using W-Based Polyoxometalates

Polyolefins including polyethylene and polypropylene are produced at a scale of over 150 million tons per year globally, which are mostly single-use plastics and end up in landfills, incinerators, or oceans. Their degradation and embrittlement produce microplastics, which may concentrate toxic organic compounds and release toxic additives including Cr (up to hundreds of ppm). Such additives may also pose challenges during polyolefin upcycling. For example, the trace metals in the plastics can gradually accumulate within the plastic upcycling catalysts impacting their performance and recyclability. They may also be undesired contaminants in the product streams. These adverse events can negatively impact both the economics and environmental sustainability aspects of the overall process. Yet, the role of trace metals in plastics during its upcycling has received scant attention. We have investigated the catalytic cracking of post-consumer high- density polyethylene (HDPE) samples, using W-based polyoxometalates that possess several advantages. For example, the high proton mobility can enhance the accessibility of acid sites during melt phase processing. Further, polyoxometalates are highly soluble in protic solvents which can be harnessed to separate the catalysts from coke and recycle them. Additionally, metals such as Cr present in plastics could potentially be reduced to a cationic state and removed by cation exchange, while the tungstate units remain in solution as anionic polyoxometalates. Harnessing these advantages, melt phase HDPE upcycling was successfully demonstrated in a fixed bed reactor wherein the cracked products were carried away by a flowing nitrogen stream. The products are mainly paraffins and coke. These results along with coke characterization and catalyst recovery will be presented. Cr in the HDPE was confirmed to accumulate in the recovered catalyst and affects the product distribution after several cycles. Possible roles of Cr in the recovered catalyst will also be discussed.