Design and engineering of tandem reaction systems for monomer synthesis and polymer deconstruction

Abstract

Due to their advantageous material properties and low cost, plastics have become an integral and ubiquitous part of modern-day society. The failure to develop effective waste management strategies for plastics, however, has meant that plastics are typically single-use materials with more than 80% of plastics ending up in landfills or the environment as waste where they linger for decades. This waste management failure has led to serious environmental and public health concerns. Efforts to mechanically recycle plastic continue to be largely ineffectual because of the inferior properties of recycled plastics compared to virgin materials and the complexities of dealing with mixed plastic waste. To tackle the plastic waste crisis, multiple approaches are needed that tackle plastic waste both at the beginning and end of the polymer life cycle. This dissertation showcases several strategies to address these issues that rely on tandem reaction systems to overcome thermodynamic and kinetic challenges. ☐ Significant efforts have focused on developing more sustainable and biodegradable polymers, of which polylactic acid is one of the most promising. Although already manufactured industrially for select applications, widespread adoption is hampered by the higher cost relative to existing polymers. A majority of that expense comes from the cost of the monomer, lactic acid, which is produced industrially via microbial fermentation. Recent efforts to produce lactic acid from synthetic routes have been promising, but have suffered from high catalyst cost, or low yields of pure lactic acid. As an alternative to these processes, we have developed a novel lactic acid synthesis pathway starting from acetone in a tandem, two-step process. Acetone is first oxidized to methyl glyoxal using selenium dioxide. Then, the methyl glyoxal undergoes a hydride shift reaction to lactic acid via a stannosilicate catalyst. The reaction is performed at mild conditions (<100 °C and autogenous pressure) and results in pure lactic acid instead of lactate salts or alkyl lactates. Through optimization of reaction parameters including solvent choice, catalyst identity, catalyst loading, selenium dioxide to acetone ratios, and temperature we achieved the highest reported yield of pure lactic acid from a synthetic pathway of nearly 95%. The role of solvent-catalyst interactions is examined as well as the critical role that water plays in suppressing side reactions. ☐ In contrast to mechanical recycling, chemical recycling strategies show promise as potential avenues for generating value-added products from plastic waste. In particular, oxidation strategies can lead to production of high-value products such as esters and acids. However, polyolefins such as polyethylene and polypropylene present a unique challenge for chemical recycling due to their lack of heteroatoms and chemical inertness. We have thus developed a tandem reaction strategy for polyolefin deconstruction via oxidative pathways. First, the hydrocarbon chain is oxyfunctionalized via an autoxidation radical reaction mechanism. The kinetics of this reaction are studied and by using a chemical looping strategy, we have been able to reduce the induction period of the reaction by over 75%. Next, two different reaction strategies are examined as potential options for the secondary reaction in the tandem system. A VOx/TiO2-catalyzed ketone cleavage reaction initially showed promising activity towards chain scission, but the catalyst rapidly deactivates under reaction conditions due to reduction of the vanadia species. The other strategy pursued was Baeyer-Villiger oxidation to transform the ketones into esters, followed by hydrolysis. Using NHPI as an alternative to more costly peroxyacids, we developed an effective reaction system that transformed the alcohol and ketone oxygenates from the autoxidation reaction into ketones and esters, respectively. The high activity of the system, even at ambient temperatures, and its high selectivity towards the ester products demonstrates its promise as a part of the tandem reaction strategy. ☐ Another polyolefin deconstruction strategy that has shown great promise is a cracking-alkylation tandem reaction system that efficiently converts polyolefins to light alkanes at temperatures under 80 °C. The drawback to this system, however, is the need for a hydrocarbon co-reactant that results in increased costs and low carbon efficiency. Building off this work, we report a tandem cracking-hydrogenation system that also efficiently converts polyolefins into light alkanes at mild temperatures. Through optimization of reaction parameters, we achieved 80% conversion of LDPE at 70 °C in three hours, with over 90% selectivity towards gasoline-range (C4-C12) branched alkanes. By using hydrogenation instead of alkylation, the carbon efficiency is greatly improved without dramatically altering the conversion of polymer or distribution of products. Increasing hydrogen pressure results in increased conversion of polymer and the loading of the Pd/C hydrogenation catalyst can be minimized without negatively impacting the activity. At very high loadings of catalyst, the activity of the system decreases because of chloride adsorbing to the surface of the carbon support. The Pd/C catalyst shows no sign of deactivation over repeated use. The system is effective across multiple substrate types with similar conversions and product distributions for a variety of polyolefins.