(178b) A Heterogeneous System for Alkane Rearrangement: A Means to Depolymerize Polyethylene at Low Temperature (< 200°C)

Over eight billion metric tons of plastic has been created between 1950-2015, and this figure is expected to double in the next three decades. Accumulation of plastic in our landfills, waterways and oceans, whether in the form of plastic bags, micro-plastics, or other waste, is becoming a critical issue. The current status of our recycling technology is not capable of solving this problem. New technologies, developed through fundamental research, are thus required to develop robust and cost-competitive recycling strategies, or upcycling, to further the circular materials economy. One recent approach leverages a dual catalytic system, dehydrogenation and olefin metathesis, for low temperature depolymerization of waste plastics such as polyethylene (PE) to liquid alkanes. Previous work has demonstrated this technology as proof of concept, though relies on difficult to recycle and costly homogeneous catalysts, like iridium pincers.

In this work, we sought to develop and explore a variety of robust heterogeneous dehydrogenation and olefin cross metathesis catalytic systems to maximize reactivity in the depolymerization of PEs of various molecular weights. We explored the use of noble metal dehydrogenation catalysts in conjunction with rhenium and molybdenum based olefin cross metathesis catalysts on a model system, alkane rearrangement of n-eicosane and n-pentane. Our best catalytic system is capable of >70% conversion of 1 g of n-eicosane in 15 hr at 200°C, resulting in a distribution of alkane products (C₆, C₇, C₈, etc.). We have applied these systems to PE substrates resulting in a >75% reduction in molecular weight of the original polymer, while producing a tunable distribution of linear alkane products. The role of support, promotors, process conditions, and catalyst pretreatment will be presented.