ASSESSING ROLES OF ZEOLITIC SURFACE AND MICROPOROUS ACID SITES FOR UPCYCLING OF BULKY POLYMERS

Abstract

The growing global plastics production has increased concern over the abundant plastic waste, leading to increased research in moving from a linear to a circular economy. Plastic upcycling, which involves turning plastic waste into monomers or fuel intermediaries, is of interest as a route to tackle plastic waste. Catalytic cracking and hydrocracking are promising methods to deconstruct plastic waste at mild working temperatures and with tunable product selectivity owing to catalyst design. Bifunctional (metal-loaded) zeolites and metal-free zeolites have shown activity for catalytic cracking and hydrocracking of polyethylene (PE). However, the mechanism of polymer cleavage on metal loaded or metal-free zeolites is still not understood. PE upcycling reactions performed at 523 K on a suit of zeolite catalysts— synthesized with varying pore architecture and hierarchy, crystallite size, acid site densities and location—showed higher conversion rates for zeolites with evenly distributed acid sites than those with acid sites concentrated on crystallite surfaces. This distribution was determined via a combination of XPS, EDX, and alcohol (benzyl alcohol and 1-pyrenemethanol) probe reactions. Higher conversions and conversion rates are also observed for zeolites with smaller crystallite sizes, given an even distribution of acid sites. These results show that concentration of acid sites on crystallite surfaces does not directly translate to improved zeolite performance, and that smaller crystallite sizes have a strong impact on PE conversion rates by mitigating diffusion limitations.