Re-Tooling Recycling: Liquid Metal Catalysis of Polycarbonate Pyrolysis

Polycarbonates (PC) are widely used polymeric materials with many desirable physical properties including high-impact resistance and a transparent optical grade. However, PC also account for a significant portion of the growing plastic waste crisis that is disrupting the livelihood and environmental well-being of communities around the globe. The vast majority of waste polymers are released into the environment as a result of booming demand for plastics and the inability of current recycling methods to keep up with consumption rates. The most prominent of such recycling methods are mechanical processes that degrade the physical qualities of recycled material, while more robust chemical recycling methods have yet to become viable alternatives. Pyrolysis, or thermal decomposition in anoxic conditions, is one chemical recycling process that converts polymer waste into a spectrum of products; the most valuable of which being the liquid products (oil) that can be regenerated into fresh polymer or used for other industrial applications. However, the main disadvantage of pyrolysis is the vast energy input required to drive the highly endothermic reactions. This study investigates the pyrolysis of PC in a liquid metal (LM) catalytic media using thermogravimetric analysis (TGA) and gas chromatography-mass spectroscopy (GC-MS) to assess the performance of LM catalysts compared to the non-catalytic reaction. LM’s were evaluated as catalysts due to key advantages over conventional, solid catalytic materials. Bismuth and indium were chosen for their compatibility with pyrolysis conditions. The low-melting nature of these metals (relative to PC pyrolysis reaction temperature) allows for an analysis of their catalytic activities in the liquid phase. Both LM catalysts showed promise in reducing the energy penalty of pyrolysis by significantly reducing the reaction onset temperature when compared to the non-catalytic reaction. Each catalyst influenced the selectivity of products in the oil collected from the experiment; illustrating the different possible mechanistic pathways documented in the literature. Bismuth promoted the formation of heterocyclic compounds, and the indium catalyzed reaction favored the formation of phenol and large-branched phenol derivatives. The further development of this process stands to improve the efficiency of PC pyrolysis and the circular lifecycle of plastics.