Elucidation of Catalytic and Kinetic Aspects of Polyolefin Pyrolysis for Hydrogen and Nanocarbon Production

PhD synopsis Mr. Abdul Rafey (2022CHZ8183)
MS Team Link: https://teams.microsoft.com/meet/431556101104653?p=PkVJkkph0s4KJzppOY

Title : Elucidation of Catalytic and Kinetic Aspects of Polyolefin Pyrolysis for Hydrogen and Nanocarbon Production
Abstract :
The rapid generation of plastic waste, particularly polyolefin-rich waste such as polyethylene and polypropylene, has created significant environmental and resource-management challenges. Polyolefins account for more than 60% of total plastic production, creating a strong need for their sustainable valorization. Catalytic pyrolysis offers a promising route for converting waste polyolefins into fuels and value-added products, particularly hydrogen (H2) and nanocarbon. However, effective implementation requires an understanding of catalyst composition, metal–support interactions, reaction kinetics, carbon deposition, catalyst deactivation, and process sustainability. This study, therefore, presents an integrated experimental, mechanistic, kinetic, and sustainability-oriented investigation of catalytic pyrolysis of waste polyolefins for the simultaneous production of H2 and nanocarbons. Initially, Al2O3-supported Fe, Ni and Fe–Ni catalysts were investigated for the pyrolysis of LDPE, HDPE and PP. Catalysts were prepared by wet impregnation method and characterized using advanced analytical techniques. The pyrolysis process was carried out in a two-stage reactor, at an operating temperature of 500 °C followed by catalytic reforming at 800 °C. The results demonstrated that catalyst composition strongly influenced H2 production, gas composition and carbon deposition. H2 was the dominant gaseous product, while filamentous and tubular nanocarbon was simultaneously deposited on the catalyst surface. After developing the experimental setup, the application of coal fly ash as a catalyst support was studied in the pyrolysis process. Through this study, a sustainable approach for simultaneous valorization of plastic and industrial solid waste was developed. Ni incorporation increased the fly ash surface area from 2.8 to 19.5 m2 g-1 and substantially enhanced H2 production. Raw fly ash produced approximately 25–58 vol% H2, whereas Ni/fly ash generated approximately 60–75 vol% H2, depending on the feedstock. PP showed the highest H2 production, reaching approximately 11.2 g H2 per 100 g plastic. Carbon yields over Ni/fly ash were approximately 32.68, 36.76 and 37.55 g per 100 g of LDPE, HDPE and PP, respectively. The characterization of deposited solids confirmed the presence of predominantly filamentous and graphitic carbon, including MWCNT structures. Further, the variation in Fe/Ni ratios of 1:1, 1:2, 1:3, 2:1 and 3:1 were studied. Ni-rich catalysts exhibited favorable reducibility and promoted higher H2 fractions, whereas increasing Fe content favored CH4 formation. LDPE generally produced approximately 265–70 vol% H2, while PP achieved approximately 80 vol% H2 with the Fe/Ni (2:1) catalyst. Carbon deposition was predominantly filamentous, with Ni-rich catalysts, particularly Fe/Ni ratios of 1:2 and 1:3, producing comparatively uniform CNT-like structures. The reaction kinetics of LDPE, HDPE and PP were investigated using thermogravimetric analysis and FWO, KAS and Starink model-free isoconversional methods at heating rates of 10–40 °C min-1. Catalyst addition shifted degradation toward lower temperatures, confirming catalytic promotion of polymer decomposition. Activation energy varied significantly with conversion, demonstrating the complex nature of polyolefin degradation. For LDPE, average activation energy decreased from 182.8 kJ mol-1 without catalyst to 157.2 and 169.9 kJ mol-1 with Ni/Al2O3 and Ni/fly ash, respectively. For PP, it decreased from 179.1 to 139.3 and 142.1 kJ mol-1. The observed behavior was associated with sequential polymer-chain activation, C–C bond cleavage, radical formation, β-scission, hydrogen transfer, C–H activation and dehydrogenation. Catalyst deactivation and carbon recovery were further addressed using an aniline-assisted treatment, which recovered approximately 40–50 wt% of deposited carbon. Raman and FESEM analyses confirmed retention of graphitic and tubular/filamentous structures, while the absence of visibly attached metal particles at nanotube tips indicated effective separation of carbon from the catalyst phase. Finally, cradle-to-gate LCA was performed for H2 production using Ni/fly ash catalyst. The baseline process produced 0.09 kg H2 and 0.32 kg nanocarbon per kilogram of LDPE, with a climate-change impact of 15.34 kg CO₂-eq kg-1 H2. Catalyst preparation accounted for approximately 73.6% of this impact, while catalyst reuse over three cycles reduced the impact to 9.4 kg CO₂-eq kg-1 H2. The work combines catalyst development, waste-derived catalyst supports, catalyst–polymer structure–activity relationships, conversion-dependent reaction kinetics, carbon recovery and catalyst regeneration with environmental assessment. Collectively, the study provides a comprehensive scientific framework for advancing waste-polyolefin-to-hydrogen and nanocarbon conversion toward a more resource-efficient and circular waste-valorization process.