Development of C-C and C-O bond cleavage catalysts for the depolymerization of plastics

Chemical recycling of plastics provides more flexibility for materials circulation than landfill disposal or incineration. Intensive research efforts have been devoted to maximizing the efficiency of chemical recycling through catalyst development. The frontier of knowledge in this field has advanced significantly for individual recycling routes, however, studies either consolidating existing knowledge or providing a more universal understanding are rare. Additionally, some uncommon parameters which may significantly influence the catalytic recycling process have not been adequately explored. In this thesis, a Ru-modified zeolite catalyst was synthesized and used to transform polyolefins (polyethylene, polypropylene and polystyrene) selectively into methane under a hydrogen atmosphere via a hydrocracking/hydrogenolysis mechanism. Further investigations studied the catalytic activity and reaction mechanism of polyolefin depolymerization under a hydrogen atmosphere in the presence of silica-alumina-based catalysts. Product distributions were examined to classify carbon-carbon bond cleavage mechanisms as either hydrocracking or hydrogenolysis. An activity-mechanism map was then proposed to benchmark the activity of a given silica-alumina-based catalyst that affords specific product selectivity. The activity-mechanism map systematically facilitates the correlation of catalyst components with activity. In addition, the hydrogen atmosphere was shown to positively affect the kinetics and product selectivity of polyolefin depolymerization compared to a nitrogen atmosphere. Different applied atmospheres also influence the depolymerization rate of polyethylene terephthalate (PET) hydrolysis, though further studies are required to clarify possible mechanisms. This thesis describes the journey from an individual study of polyolefin depolymerization to a more general understanding of catalytic activity versus bond cleavage mechanisms, and the investigation of an atypical parameter for PET transformation. The rational and linear evolution approach may result in a more rapid catalyst discovering process. Investigation of understudied parameters may also lead to important insights for chemical recycling and should not be overlooked.