| dc.description.abstract | In recent times, plastics have become indispensable to human life. The availability of raw materials, ease of production, and versatility have made plastics ubiquitous. The pollution, however, caused by improper disposal has become a major global concern. Most plastics used today are single-use and end up in landfills and water bodies. Only about 9% of them are recycled. To remedy plastic pollution's environmental consequences and move toward creating a circular plastic economy, effectively upcycling and recycling plastic waste has gathered significant attention. One avenue toward chemically repurposing polyolefins is via hydrogenolysis.
Hydrogenolysis is a depolymerization reaction wherein hydrogen molecules break the relatively inert C-C backbone of the polyolefin. Ru is regarded as the most active hydrogenolysis catalyst. This work studied the hydrogenolysis of polyethylene using earth-abundant cobalt immobilized on a neutral support, silica (Co/SiO2). Co showed ~30% n-octadecane (n-C18) conversion into the full range of C1-C17 alkanes, compared to Ru (100% conversion to methane), Pt, Pd, and Fe (~3%, 0%, and 0% conversion, respectively). It was shown that Co/SiO2 is an excellent catalyst that selectively converts polyethylene into liquid range hydrocarbons (C5-C30) at mild conditions (250-275 °C, 20-30 bar H2, and 4-8 h). At optimized conditions (275 °C, 30 bar H2, 8 h), ~19.5% gaseous and ~58.2% liquid products were yielded with an average liquid carbon of 22 (diesel-motor oil range). The catalyst further successfully converted end-of-use polyethylenes despite additives and impurities. The catalyst's active phase was CoO, and it showed exceptional regeneration between runs, giving nearly identical product distributions. Mechanistically, it was identified that Co favors the non-terminal C-C bond cleavage route, selectivity producing oligomers over gases at low conversions, which subsequently hydrogenolyze into lower hydrocarbons, eventually forming methane in secondary and tertiary events. | |
