Sustainable Catalytic Screens for Plastic Waste Upcycling into Chemicals

Thesis Abstract

Plastic waste accumulation presents a pressing environmental and economic challenge, necessitating efficient transformation routes that convert heterogeneous polymers into value-added chemicals under sustainable conditions. This study addresses the gap in scalable catalytic strategies that simultaneously enable selective depolymerization and valorization of mixed plastic streams into useful chemical feedstocks, reducing energy input and waste generation. The aims are to design, synthesize, and evaluate sustainable catalytic screens capable of promoting upcycling reactions under mild to moderate temperatures using abundant, earth-abundant metals and recyclable supports; to optimize reaction conditions for selectivity toward targeted chemical families (monomeric hydrocarbons, oxygenates, and aromatics); and to assess lifecycle impacts relative to conventional disposal pathways. Specific objectives include (i) synthesizing a library of heterogeneous catalytic screens incorporating transition metal oxides (e.g., MnOx, Co3O4, NiO) immobilized on mesoporous carbon/nitrogen-doped carbon supports; (ii) evaluating catalytic performance in the depolymerization and downstream upgrading of representative polyolefins and PET in batch and flow reactor configurations; (iii) identifying structure–activity relationships through in situ spectroscopic probes and ex situ characterization to determine active sites, leaching behavior, and catalyst recyclability; (iv) developing a kinetic model and conducting a life cycle assessment to compare environmental and economic metrics against conventional recycling and disposal routes; and (v) proposing a scalable process design incorporating separation and catalyst recovery steps. The research adopts a design, implementation, and evaluation methodology combining materials synthesis, catalytic testing, and systems analysis. A design-first phase will generate a catalyst library (n = 40–60 screens) via impregnation and templating methods, followed by a screening campaign using representative feedstocks post-consumer PET, HDPE, and LDPE blends (quantities collected from municipal streams totaling ~1200 kg). Data collection employs gas chromatography–mass spectrometry (GC-MS) for product composition, gas chromatography with thermal conductivity detection (GC-TCD) for light hydrocarbon outlines, Fourier-transform infrared spectroscopy (FT-IR) for functional group evolution, and X-ray photoelectron spectroscopy (XPS) for oxidation state and surface chemistry. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and operando X-ray absorption near-edge structure (XANES) measurements will elucidate active-site dynamics under reaction conditions. A factorial experimental design will be used to identify optimum temperature (220–380°C), pressure (1–5 MPa, inert or mild hydrogen co-feeding), contact times (5–120 min), and catalyst loading (0.5–5 wt%). Recyclability will be tested over at least ten consecutive cycles with regeneration steps. A parallel life cycle assessment (LCA) will use a cradle-to-grave approach, quantifying global warming potential, energy consumption, and water use, employing SimaPro software and the ReCiPe impact method. Data analysis integrates quantitative and qualitative approaches. Regression and nonlinear kinetic modeling will extract rate constants and activation energies; analysis of variance (ANOVA) will determine the significance of synthesis variables on conversion and selectivity. Multivariate principal component analysis (PCA) will reveal correlations between catalyst features (surface area, acidic/basic site density, metal valence) and performance. Thematic analysis will be applied to process flow observations and regeneration efficiency data to identify practical bottlenecks. Theoretical framing incorporates green chemistry principles and the theory of heterogeneous catalysis with insights from the Sabatier principle to rationalize activity–stability trade-offs, alongside mechanisms of polymer chain scission and repolymerization pathways. Expected findings include identification of at least three catalytic screens that demonstrate >60% conversion of mixed plastics to platform chemicals with selectivity toward oxygenates and light hydrocarbons, high catalyst recyclability (retaining >90% initial activity after five cycles), and minimal metal leaching (<0.5 wt%). The study is anticipated to reveal robust structure–activity relationships linking metal–oxide interfaces, carbon supports, and polymerization/depolymerization pathways, thereby informing scalable reactor designs and process integration. The contribution to knowledge lies in providing a validated, sustainable catalyst platform for upcycling diverse plastic wastes into commercially relevant chemical streams, coupled with a decision framework for process optimization and environmental assessment. Practical recommendations include guidelines for feedstock preprocessing, catalyst regeneration protocols, and reactor configuration choices to maximize selectivity while minimizing energy input and environmental burden. The work concludes that sustainable catalytic screens can offer a viable pathway for continuous plastic waste valorization, with policy-relevant implications for circular economy strategies and industrial adoption.