Plastics have become ubiquitous in modern society. Unfortunately, the current management of plastic waste (PW), based mainly on landfill disposal, is unsustainable. Chemical recycling of PW, particularly of polyolefins (PO), to liquid fuels and lubricants, is promising for generating value while diverting it from landfills. Hydrocracking is widely exploited in refineries to valorize waxes and heavy oils (C20 - C50+)and is attractive for the chemical recycling of PO. Hydrocracking catalysis is well understood for small alkanes (C6 - C12). However, it is unclear if these model compounds' mechanistic concepts can be directly applied to PO systems. Platinum on select oxides has been demonstrated to be active and selective hydrocracking catalyst. Moreover, its physicochemical properties are well established, making it an ideal candidate for fundamental studies. The current work establishes a mechanistic framework for PO hydrocracking and identifies reactivity descriptors to close the gap between model alkanes and PO. We assessed the effects of each catalyst component loading on low-density polyethylene (LDPE) as a function of reaction conditions. Our results show that the catalyst composition and structure play a significant role in LDPE hydrocracking. We demonstrate that we can control the distribution to larger alkanes and enhance the branching in the residual polymer. Moreover, we identify key similarities and differences between the hydrocracking behavior of small alkanes and LDPE and propose a new macromolecular hydrocracking mechanism that departs from small alkanes. We perform extensive characterization of the catalyst, the products, and the residual polymer to provide structure-reactivity relations.

Bridging the gap between model alkanes and polyethylene waste hydrocracking

ACS Fall 2021

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