(293d) Zeolite Supported Pt for Depolymerization of Low-Density Polyethylene By Induction Heating

The development of plastics over the past half century has led to rapid technological advances. Nonetheless, long-term negative environmental impacts of these materials compel new upcycling strategies. Worldwide production of polymers in 2015 reached 380 MMT. Despite the substantial amounts of polymers potentially available for reutilization, it has been estimated that of all synthetic polymers produced since 1950, only 7% have been recycled, compared to 60% which have been discarded (lifetimes > 20 yr). Currently, reconstituting these polymers into chemical products often requires high-temperature pyrolysis yielding unattractive product distributions. This work aims to address this issue by converting plastic waste into useful alkanes/alkenes oligomers, using rationally designed heterogeneous catalysts suitable for use in radio frequency (RF)-based induction heating.

Pt(0), either alone or in alloy form, is widely used for hydrocracking catalysis due to its high dehydrogenation activity leading to high rates of carbon-carbon cleavage of adsorbed alkene moieties. Four types of zeolites, decorated with Pt, were chosen to elucidate the role of crystal size, pore structure and acidity (as quantified, e.g., as Si/Al molar ratio). Zeolite-supported Pt catalysts mixed with Fe₃O₄ were inductively heated under inert atmospheres (no H₂) to depolymerize low density polyethylene. The depolymerization showed high concentrations of alkene/alkane hydrocarbons with a narrow product distribution that could be tuned between light gases to C14 hydrocarbons based on the zeolite structure. The addition of Sn as a promoter was found to increase activity with a preference for heavier products, and to reduce the coke formation. LDPE conversions up to 95% with minimal coke and light gas formation were obtained with Pt/LTL or SSZ-13 at 375°C estimated surface temperature and 2 h reaction time. Reaction kinetics show that the depolymerization process is diffusion limited across moderate temperature ranges (~280-500 C).