The prevalence of plastic waste demands solutions that can drastically improve recovery and recycling rates of these wastes. By reimagining plastic wastes as valuable carbon feedstocks for production of chemicals, fuels, or plastic monomers, the collection and sorting of plastic waste can be incentivized, ultimately reducing the amount of mismanaged plastic waste. Addition of catalysts (i.e., zeolites) to plastic deconstruction processes can improve the technoeconomic feasibility of such processes through lowering the necessary operating temperature and improving desired product selectivity. 
 

This dissertation focuses on obtaining structure-activity correlations and elucidating the mechanism for polyethylene (PE) deconstruction on zeolites through tailored catalyst design, rigorous catalyst characterization, and reaction analysis. Zeolites with varying identity of loaded metals (Pt and Ni), framework types (MFI and FAU), mesopore volumes and connectivities, and acid site densities and locations were prepared to determine the impact of these properties on plastic conversion rate, product selectivity, and catalyst stability. 

Brønsted acid sites (BAS) on metal-free MFI zeolites were found to be active for plastic deconstruction even at relatively mild reaction temperatures (<523 K), with high selectivity to C3- C7 hydrocarbons due to the shape selectivity provided by microporous voids of MFI. The presence of Pt and Ni nanoparticles, which can dehydrogenate saturated hydrocarbons to form more reactive alkenes, were found to stymie plastic deconstruction due to rapid hydrogenation of the alkenes back to alkanes under a H2 environment. The location of BAS within zeolites also plays an important role, with higher densities of BAS on catalyst surfaces facilitating higher PE cracking rates. 

The limited diffusion of PE within microporous channels of zeolites can be alleviated by creating mesopores connected by micropores, which are called hierarchical zeolites. In particular, hierarchical MFI consistently yield higher PE cracking rates, higher selectivities to branched hydrocarbons, and better catalyst stabilities than parent MFI. Hierarchical FAU materials, however, have conversion rates and product selectivities similar to parent FAU, because the formation of mesopores compromise the confinement effect provided by micropores within FAU. 

Overall, the proposed mechanisms and the structure-function relationships developed in this dissertation can aid in designing more efficient zeolites for plastic upcycling processes.