UC Santa Barbara

Polyethylene is a ubiquitously used commodity plastic due to its low cost, superior physical properties, and chemical inertness. Despite the wide use of polyethylene, current recycling technologies fail to recover sufficient value from waste polyethylene to promote a circular plastic economy. The world urgently needs to develop and implement new effective strategies to decrease the amount of discarded plastic while simultaneously reducing new inputs of fossil carbon feedstock and energy.Advanced recycling involves chemically deconstructing polyethylene to form useful products that can be reused. These chemical processes are often accelerated and facilitated by catalysts. The combination of specific catalysts under the right reaction allows polyethylene to be transformed in novel ways. This dissertation focuses on two catalytic transformations for recycling polyethylene. The first recycling strategy that was investigated involves converting polyethylene to propylene, the chemical precursor needed to make polypropylene, another widely used commodity plastic. The transformation combines three catalytic reactions: transfer dehydrogenation, ethenolysis, and olefin isomerization. Using MTO/Cl-Al2O3 as a bifunctional ethenolysis and isomerization catalyst with mono-unsaturated polyethylene, high selectivity to propylene (≥94%) is achieved in a semicontinuous process due to the continuous removal of propylene from the reaction mixture. For this result, the rate of propylene formation was limited by the isomerization catalyst (Cl-Al2O3, 16 times slower than the rate of ethenolysis). By adding a more active and less ethylene-inhibited isomerization catalyst (SiO2-Al2O3), the rate of propylene formation in tandem ethenolysis/isomerization of polyethylene was increased by a factor of five. A preliminary life cycle assessment of the process suggests moderate greenhouse gases emissions reductions could be achieved by adopting this technology. Second, polyethylene was upgraded to value-added α,ω-divinyl-functionalized oligomers with shorter, tunable chain lengths via a sequence of bromination, dehydrobromination, and olefin metathesis reactions. These oligomers can potentially be polymerized to form polyethylene. The three chemical transformations were conducted in series and achieved high yields each individual step. Preliminary technoeconomic analyses demonstrate that this three-step process could be economically viable on an industrial scale. The catalyst synthesis procedures and catalytic reactions were conducted using standard glovebox and Schlenk line techniques to ensure that the catalysts were not deactivated by air and moisture. High pressure batch reactions were performed in Parr reactors. The products of the chemical reactions were analyzed using a variety of techniques including GC-FID, GC-TCD, GC-MS, 1H and 13C NMR, GPC, and elemental analysis. Life cycle assessment and techno-economic analysis were performed using standard procedures.