Upcycling of Polystyrene, Polyolefin, and Polyvinyl Chloride Waste into Chemicals, Surfactants, and Lubricants

Abstract

Large volume thermoplastics made from alkene monomers such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS), account for >60% of annual plastic production owing to their excellent physicochemical and mechanical performance in many applications. As a consequence of widespread use, addition polymerization plastics contribute significantly to plastic pollution. Unfortunately, existing remediation approaches (i.e., mechanical recycling) have not meaningfully decreased the amount of plastic waste that is mismanaged, primarily due to low profitability and difficulties in collecting and sorting the waste. To address these challenges, this dissertation presents energy-efficient methods to convert plastic waste into value-added chemicals and materials. PS utilized in single-use plastic cutlery, packaging, and construction is rarely collected for recycling due to its bulkiness and ease of contamination. In this regard, a new low temperature process was developed to convert PS waste into value-added aromatic commodity chemicals with high tolerance of contaminants. This approach, which relies on a two-step process, begins with thermochemical degradation carried out with AlCl3 generating quantitative yields of benzene by dephenylation. Subsequently, structurally distinct alkyl chlorides, sulfinyl chlorides, and acyl chlorides are introduced in the reaction to afford industrially and pharmaceutically relevant aromatic ketones and sulfides. Notably, the same "degradation and upcycling" approach is applicable to polyolefins and can be employed to design a synthetic strategy to produce surfactants from PE and PP waste, thus helping to improve the circularity of the polymers. In the first reaction, the polyolefins are degraded into olefin-rich intermediates and upcycled utilizing different oxidation protocols. To target hard soaps and soft detergents, it is critical to control the molar mass and carbon distribution of hydrocarbon intermediates in thermolysis. Control over the carbon distribution was achieved by regulating the temperature gradient which enables the selective production of oil or wax. In a subsequent reaction, the wax products are subjected to manganese-catalyzed oxidation to furnish fatty acids. Separately, the oil is oxidized to alkyl hydrogen sulfates and neutralized to generate ionic detergents. All waste-derived surfactants demonstrated excellent foamability, emulsifying power, and low critical micellization concentration compared to commercial benchmarks. In the later part of the dissertation, Cl-free hydrocarbons were produced from PVC, including polyalphaolefin lubricants. This was achieved by harnessing the ability of chloroaluminate ionic liquids and select Lewis acids to weaken and cleave the C-Cl bond. The working hypothesis is that introducing α-olefins in the reaction suppresses elimination reactions that would otherwise produce low-value polyenes. The results reveal that PVC undergoes dechlorination, chain scissions, and alkylation in the presence of AlCl3 at 70 °C, furnishing polyalphaolefin mimics of controlled viscosity and no polyene byproduct. The rheological and tribological properties of the lubricants are fine-tuned by judiciously selecting proper α-olefin chain length, solvent type, and catalyst loading, which altogether govern the degree of branching. Ultimately, turning PVC plastics into high-value lubricants without the need for stoichiometric amount of chlorine scavengers presents a scalable and practical avenue for reclaiming this hard-to-recycle polymer. In the long run, the integration of waste-based renewable resources in the manufacturing of surfactants, commodity chemicals, and lubricants will reduce the carbon footprint associated with their production and curb the plastic pollution crisis.