From Waste to Worth: Investigating Encapsulated vs. Surface Metal Sites in Zeolite-Catalyzed Plastic Upcycling

Abstract

Plastic waste buildup, especially polyolefins such as polyethylene (PE), poses major environmental challenges due to their chemical inertness and poor recyclability. Catalytic upcycling through hydrocracking on Brønsted acidic zeolite-based catalysts with added metal functionality offers a promising approach to transforming waste plastics into valuable hydrocarbons. This thesis examines how the spatial arrangement of these metal sites (here, platinum (Pt)) within zeolite frameworks influences catalytic performance during PE upcycling by comparing Pt encapsulated within zeolite micropores with Pt on external surfaces. A variety of bifunctional metal–acid zeolite catalysts were synthesized using incipient-wetness impregnation, ligand-assisted incorporation, and interzeolite transformation techniques to control the location of Pt within the BEA, FAU, and MFI frameworks. Comprehensive characterization methods—including X-ray diffraction (XRD), scanning transmission electron microscopy (S/TEM), CO pulse chemisorption, n-propylamine thermogravimetric analysis (NPA-TGA), and N₂ physisorption—were employed to evaluate crystallinity, metal dispersion, acid site density, and pore structure. Findings indicate that the synthesis method significantly affects Pt dispersion and nanoparticle size, with encapsulation methods yielding smaller, more uniformly distributed metal particles compared to surface-deposited catalysts. Thermogravimetric analysis-based catalytic tests show that PE conversion depends not only on active-site density but also on accessibility and diffusion constraints. FAU-based catalysts exhibit higher overall conversion due to their more accessible pore architecture and lower diffusion resistance, while BEA-based catalysts benefit more from Pt addition, emphasizing stronger metal–acid synergy under diffusion-limited conditions.