Co-production of hydrogen and carbon nanotubes via catalytic pyrolysis of polyethylene over Fe/ZSM-5 catalysts: Effect of Fe loading on the catalytic activity

Thermochemical conversion is a promising waste plastic recycling technology from both an economic and environmental point of view, as it can convert plastics into high-value chemicals. In this work, the co-production of hydrogen and carbon nanotubes (CNTs) by catalytic pyrolysis of polyethylene over Fe/ZSM-5 catalysts with different Fe loading (5, 10, 20 and 30 wt%) was investigated using a two-stage fixed bed reactor. The results show that the yield of CNTs and hydrogen increases first and then decreases with the increase of Fe loading. The 20Fe/ZSM-5 catalyst generated the highest CNTs yield of 262.24 mg/gPE and the maximum hydrogen production of 31.72 mmol/gPE. The fresh and spent catalysts were characterized by XRD, H2-TPR, SEM, TEM, etc., in order to explore the correlation between the catalytic activity and the Fe active metal load of the catalysts. The results showed that, within a certain range, the number of active sites increased with the increase of Fe loading, which promoted the conversion of hydrogen and CNTs. However, when the Fe loading reached 30%, the active particles agglomerated and inhibited the growth of CNTs. 30Fe/ZSM-5 catalyst produced many carbon nanoonions (CNOs) and carbon nanofibers (CNFs). Overall, in terms of the yield of the produced hydrogen and CNTs, the 20Fe/ZSM-5 catalyst shows the best catalytic performance.