The Key Role of Hydride Transfer in Chloride Catalyzed Upcycling of Polyethylene

The Science

Polyolefin materials are challenging to recycle, leading to environmental and resource concerns. One strategy for managing this waste is plastic upcycling, where discarded or used plastics are converted into other carbon products. Researchers have developed a single-step upcycling route that couples cracking and alkylation, using the heat released in alkylation to enable carbon-carbon bond breakage. Using Lewis acid chlorides, such as aluminum and gallium chloride, polyolefins were converted into gasoline-range hydrocarbons at mild reaction temperatures. The Lewis acids and solvent molecules create a polar environment that stabilizes the ionic intermediates and facilitates hydrogen transfer.

The Impact

The largest group of plastics produced globally is polyolefins. Their low cost, versatility, and durability have resulted in their widespread adoption in single-use applications ranging from plastic bags to surgical masks. However, this extensive use presents significant environmental challenges, as polyolefins are highly resistant to degradation. By developing reactions capable of converting these ubiquitous plastics to other forms of usable carbon, scientists can effectively recycle carbon and keep valuable resources active.

Summary

Waste from single-use plastics is a growing challenge for sustainability. Much of this waste is polyolefin materials, which are highly degradation resistant and currently challenging to recycle. Researchers are exploring polymer upcycling, where discarded plastic is converted into valuable commodity chemicals or commercial feedstocks, as an option for mitigating waste. Researchers developed an approach that couples reactions, one exothermic and one endothermic, into a single step to drive both cracking and alkylation processes. They found that Lewis acid chlorides, particularly aluminum and gallium chloride, facilitated the conversion of low-density polyethylene to alkanes at low temperatures (<100 ^oC). The reactivity is controlled by the ability of the Lewis acid to perform hydride transfer and stabilize the carbenium ion in solution, rather than acid strength. The Lewis acid and dichloromethane solvent create a highly polar environment that leads to carbenium formation and stabilization. The overall efficiency of conversion is significantly affected by the accessibility of the polymer to the catalyst, as determined by polymer crystallinity and branching degree. This work demonstrates a proof-of-concept for polyolefin upcycling with straightforward reagents, with potential applicability to other polymers and Lewis acid systems.

Contact

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

Johannes Lercher
Pacific Northwest National Laboratory
johannes.lercher@pnnl.gov

Funding

We thank G. L. Haller (Yale University) for his discussion of the manuscript and helpful suggestions. J.A.L., J.G.C., M. L. S., W.Z., H.W., S.K., J.H., W.H., J.M., B.Y., D.M.C., and M.-S.L. thank the Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) program, Division of Chemical Sciences, Geosciences and Biosciences (Towards a polyolefin-based refinery: understanding and controlling the critical reaction steps, FWP 78459) for funding support. J.H also thanks DOE BES, Division of Chemical Sciences, Geosciences and Biosciences (Multifunctional Catalysis to Synthesize and Utilize Energy Carrier, FWP 47319) for funding support of in situ NMR work.