Quasi-Elastic Neutron Scattering to probe the mechanism of catalytic plastic recycling

Currently over 350 million metric tonnes of plastic are produced globally each year, with 55 % being discarded, leading to urban, and aquatic pollution. Low-Density Polyethylene (LDPE), one of the most common polymers, takes centuries to decompose, meaning there is a sprawling glut of polymer waste in the world. In the last 10 years processes such as polymer cracking with solid acid catalysts have been explored to form olefins, and aromatics,[2] which would normally be synthesised from fossil fuels, thus establishing a “circular economy”, limiting waste, and lowering fossil fuel demand. Zeolites (microporous solid-acid catalysts) have been widely used for this process, as many different frameworks exist, allowing delicate control over catalytic reaction pathways. HZSM-5 (MFI framework, pore sizes 5.1 x 5.5 Å, and 5.3 x 5.6 Å) is widely used for LDPE cracking, due to its powerful acid sites, lowering the reaction temperature (Figure 1). However, little is known about the catalytic mechanism of this process, particularly whether the reaction occurs on the surface, or within the pores. In this work we will perform QENS measurements on both pure LDPE, and a physically mixed LDPE-zeolite mixture, to observe changes in the polymers motion, linked with pore infiltration and surface interactions. Measurements will be performed above the melting point of LDPE, but below reaction temperatures, allowing the motions to be captured by the spectrometers.