Physico-Chemical Studies on the Catalytic Cracking of Polyolefins

The growing plastic waste problem demands scalable solutions to convert waste polymers back into valuable chemical building blocks. Catalytic cracking offers a promising approach to transform polyolefins, the most abundant type of plastic waste, into hydrocarbon feedstocks. However, fundamental understanding of the underlying chemistry remains limited. This PhD thesis compiles a set of studies aiming at shedding light on both fundamental and applied aspects of this emerging process, aiming at establishing quantitative structure-performance relationships. A primary discovery concerns macroscopic contact problems inherent to the high viscosity of high molecular weight polymers. For commercial polymers, the decomposition temperature is primarily dictated by the contact between catalyst and polymer, which in turn is only enabled after significant reduction in molecular weight. Secondly, the mesopore size of the catalyst was found to not play a significant role in determining catalyst performance. This can be explained by the fact that polymer entering into the catalyst pores is determined by capillary intrusion, which is completed before reaction onset in experiments with a ramped temperature. However, polymer does not enter into micropores. The activity trends of a set of zeolite catalysts were significantly better described by the density of external Brønsted acid sites (BAS), rather than the bulk BAS density. And yet, external BAS density fails at capturing all trends in activity. Zeolite materials with comparable external BAS density displayed dramatically different responses to catalyst loading, with rate increases varying from two-fold to five-fold upon doubling catalyst amount. This unusual behavior likely stems from polymer macromolecules (>100 nm length) simultaneously interacting with multiple acid sites. Concentrated site distributions may favor end-chain scission over middle-chain scission from homogeneous distributions, introducing a concept termed "fragmentation selectivity." On the applied side of this work, key impurities in post-consumer plastic detrimental to catalyst performance were identified. This was made possible by preparation of model polymer samples with controlled levels of various contaminants such as calcium or titanium dioxide. The most critical impurities were identified as typical zeolite poisons, such as sodium. To enable long-term catalyst stability, these impurities will need to be removed to the highest possible degree. A broad range of equilibrium fluid catalytic cracking catalysts (ECAT) was tested in PP cracking. This revealed a strong correlation between the vacuum gas oil cracking performance and PP cracking activity, offering an actionable criterion for the selection of plastic cracking catalysts from industrial catalyst libraries.