Systematic study of low temperature cracking of low-density polyethylene with ZSM-5

Abstract

The viability of cracking low-density polyethylene (LDPE) over H-ZSM-5 catalysts at reaction temperatures below 300 °C was demonstrated using ZSM-5 catalysts with varying textual properties and acid site content. While ZSM-5 made from standard syntheses could catalyze LDPE cracking it was found that increasing the external surface area of H-ZSM-5 from 24 m2/g to 325 m2/g resulted in a dramatic increase in LDPE conversion. For all catalysts the products were permanent hydrocarbon gas products in the C1–C5 range. H-ZSM-5 prepared in nanosheet and nanosphere morphologies achieved 64 wt% and 58 wt% LDPE conversion, respectively, compared to a standard H-ZSM-5 sample with micron-sized crystals that achieved 28 wt% conversion after 8 h at 250 °C under inert conditions. It was also found that strong Brønsted acid sites were necessary for high conversion of LDPE in this temperature regime as shown by the relatively weaker solid acid catalysts Al-MCM-41 and B-MFI converting negligible LDPE under the same reaction conditions. Lastly, a low molecular weight model molecule (n-tetracosane) was tested to gain insight into the cracking mechanism of LDPE over MFI catalysts. The findings in this work point to new approaches for catalyst design to achieve low temperature LDPE depolymerization.