Pyrolysis-catalysis of different waste plastics over Fe/Al2O3 catalyst: High-value hydrogen, liquid fuels, carbon nanotubes and possible reaction mechanisms

To understand the correlation between plastic structure and products, and the possible reaction mechanisms, the pyrolysis–catalysis of different types of waste plastics was investigated in the presence of an Fe/Al2O3 catalyst. Polypropylene (PP), high-density polyethylene (HDPE), low-density polyethylene (LDPE), high-impact polystyrene (HIPS), and general-purpose polystyrene (GPPS) were used as the feedstock. The results showed that PP, HDPE, and LDPE showed higher H2-enriched gaseous product yields (>40 wt%), but higher H2 concentration (over 70 vol%) was obtained from the gaseous products of HIPS and GPPS. In terms of liquid oil, the yield exhibited a relatively stable tendency, which was ~ 20 wt%, and the liquid product comprised hydrocarbons with a carbon number range mainly between 8 and 16. For solid carbon deposits, HIPS waste plastic led to a higher solid deposit yield of 49.4 wt%, followed by GPPS of 48.7 wt%. By contrast, PP, HDPE, and LDPE showed lower yields of 30.2 wt%, 36.9 wt%, and 35.9 wt%, respectively. In summary, more, purer and cleaner carbon nanotubes (CNTs) could be obtained from pyrolysis–catalysis of polyolefin (PP and PE), while more amorphous carbon and hydrogen were gathered from PS.