Kinetically Resolved Measurements of Polyolefin Upcycling over Zeolite Catalysts

Abstract

Plastic waste is one of the most pressing environmental challenges of the modern era, yet current waste management strategies are unable to effectively address the rapidly increasing plastic production at scale. Catalytic upcycling of polyolefins over zeolite catalysts provides a promising pathway to mitigate this problem and contribute to a circular economy, but a quantitative understanding of early-stage reaction kinetics is underexplored in the literature. This thesis employs a microreactor-based methodology to investigate the initial kinetics of polyethylene upcycling over zeolites BEA-12.5, FAU-15 and MFI-40 across polymer lengths of 0.5 kDa, 3 kDa, and 4 kDa. A first-order kinetic model with exponential decay was fit to time-resolved solid conversion data to allow for the extraction of rate constants k and deactivation constants kd. Across molecular weights, BEA12.5 achieved the highest raw rate constant, followed by FAU-15 and MFI-40, consistent with the relative framework pore accessibility. Rate constants decreased monotonically with increasing polymer length across frameworks, confirming that polymer chain length is an intrinsic kinetic variable. Normalization by surface Brønsted acid site (SBAS) density inverted framework ordering, with MFI-40 achieving the highest per-site rate constant, suggesting its surface sites are more productive than the other frameworks on a per-site basis. Temperature-dependent experiments with BEA-12.5 and 4 kDa polyethylene at 423 K, 473 K and 523 K yielded an apparent activation energy of Ea = 40.4 kJ / mol. Together, these results offer a kinetically grounded understanding of structure-activity relationships in polyolefin upcycling and provide a foundation for further kinetic analysis, and, ultimately, the rational design of more efficient upcycling systems.