Catalytic Pyrolysis of Plastics in a Staged Free-Fall Reactor

Polyolefins like polyethylene (PE), polypropylene (PP), and polystyrene (PS) are significant constituents of plastic waste streams that can be converted to new base chemical feeds by chemical recycling via pyrolysis. Catalysts like zeolites have already been widely applied to direct the product distribution toward more favorable chemicals or to reduce the pyrolysis temperature, but they are relatively expensive. This work considers activated bauxite residue (ARM) as a cheap and abundant cracking catalyst for catalytic pyrolysis. Low-density polyethylene (LDPE), PP, and PS are pyrolyzed in a staged free-fall reactor (1 h, 500 °C, multiple batches at 10 g/batch). Compared to thermal pyrolysis, using ARM as a catalyst produced a lighter oil (boiling up to 66 °C lower at 80% volume distilled) under reaction conditions similar to thermal pyrolysis, with a similar liquid yield. PS pyrolysis with ARM showed a slightly enhanced oil yield (90–95 wt%), with the primary product from pyrolysis being styrene. ARM also induced enhanced aromatic isomerization, yielding more substituted aromatics than thermal pyrolysis of PS. Compared to the commonly applied zeolites HY and H-ZSM-5, ARM showed less cracking, as HY produced less liquid products for PE (51.6%) and PP (63.0%) pyrolysis and similar levels of isomerization in PS pyrolysis. H-ZSM-5 further cracked the polyolefins and formed the highest levels of benzene, toluene, and xylenes (BTX), up to 7.8%. ARM as a catalyst provided cracking performance between thermal pyrolysis and HY zeolite-catalyzed performance, highlighting the potential of a disposable cracking catalyst in the pyrolysis of hard-to-recycle plastic waste.