Congratulations to Dr. Sebastian Rejman on a successful PhD defense, during which he defended his PhD thesis, supervised by Prof. Bert Weckhuysen and Dr. Ina Vollmer.

In his PhD thesis, Sebastian addressed the growing plastic waste problem by investigating catalytic cracking as a promising approach to transform polyolefins, the most abundant type of plastic waste, into valuable hydrocarbon feedstocks. His work aimed to establish quantitative structure-performance relationships for this emerging process, bridging fundamental understanding and applied aspects.

Sebastian made several key discoveries about the catalytic cracking mechanism. He identified macroscopic contact problems caused by the high viscosity of high-molecular-weight polymers, demonstrating that the decomposition temperature is primarily dictated by catalyst-polymer contact, which occurs only after significant molecular weight reduction. He found that mesopore size plays little role in catalyst performance, as polymer intrusion into catalyst pores is determined by capillary forces completed before reaction onset. The activity trends of zeolite catalysts were better described by external Brønsted acid site density rather than bulk density, though this still failed to capture all performance variations.

Remarkably, Sebastian observed that zeolite materials with comparable external acid site densities displayed dramatically different responses to catalyst loading, with rate increases varying from two-fold to five-fold upon doubling the catalyst amount. He proposed that this unusual behavior stems from polymer macromolecules simultaneously interacting with multiple acid sites, introducing a concept termed “fragmentation selectivity.”

On the applied side, Sebastian identified critical impurities in post-consumer plastics detrimental to catalyst performance, with sodium emerging as the most problematic. He also discovered a strong correlation between vacuum gas oil cracking performance and polypropylene cracking activity in equilibrium fluid catalytic cracking catalysts, offering an actionable criterion for industrial catalyst selection.

Sebastian’s full thesis can be read here.