Mechano-Catalytic Depolymerization of Polyolefins

Plastic waste pollution endangers the environment and human health. Most current end-of-life strategies, such as incineration and landfilling, are not sustainable. Instead, plastic recycling can reintroduce plastics into the value chain, but the predominantly used mechanical recycling downgrades product quality. Chemical recycling as an alternative transforms plastic waste into small molecules that can be used in the chemical industry. For polyethylene and polypropylene, however, state-of-the-art pyrolysis is unselective and only yields a low-value hydrocarbon stream. The low selectivity is rooted in the high temperatures which are needed to activate the inert plastics but do not allow for control over reactive intermediates. Therefore, low-temperature alternatives such as mechano-chemical activation are desirable. This thesis investigates the mechano-chemical depolymerization of polyolefins and shows that they can be converted into small hydrocarbons by ambient ball milling. The first experimental chapter describes initial findings on the mechano-chemical depolymerization of polypropylene, where mechano-chemical reactivity is initiated by homolytic backbone cleavage. Resulting radicals can undergo subsequent reactions forming small hydrocarbons, but their yields suffer from the low stability of radical intermediates. This can be addressed by catalytic activation of grinding spheres, producing a new class of direct mechano-catalytic spheres termed surface-activated mechano-catalysts (SAM catalysts). The use of these spheres increases small hydrocarbon formation rates due to the interaction of paramagnetic surface species with radical intermediates. Ball milling of polypropylene with SAM catalysts can produce up to 45% C1–C10 hydrocarbons within 1 h. The second experimental chapter systematically investigates the influence of ball milling parameters during the mechano-chemical depolymerization of polypropylene. Yields of small hydrocarbons are positively correlated with high impact forces in the shaker mill achieved by, for example, heavier grinding spheres and higher milling frequencies. While other mechano-chemical processes can be rationalized based on the dose of kinetic energy supplied to the system, small hydrocarbon yields from mechano-chemical polypropylene depolymerization depend disproportionately on the applied forces. Reactivity can instead be rationalized by adapting the thermo-mechanical Zhurkov equation which relates chain cleavage in polymers to an applied macroscopic stress that lowers the activation energy for bond cleavage. The third experimental chapter shows that inorganic crystalline materials such as sand and quartz can enhance depolymerization yields during ball milling of polypropylene. Crystal fracture generates surface radical species through homolytic Si–O bond cleavage in the crystal lattice, and these highly reactive species initiate plastic degradation by abstracting hydrogen atoms from the backbone of polypropylene. Milling with sand for 1 h results in an increase in small hydrocarbon yields by a factor of 25 compared to milling without sand. The fourth experimental chapter discusses the effect of zeolite-based catalysts on the mechano-chemical conversion of polypropylene. While zeolite-based materials are initially highly active during ball milling, they quickly deactivate by structural collapse due to forceful mechanical impacts. Catalyst deactivation can be countered by immobilizing zeolite species on surface-roughened grinding spheres which protects catalytic sites against harsh impacts and yields another class of SAM catalysts.