Coupling thermal and catalytic cracking of polymer wastes to boost carbon nanotubes production: Effects of HZSM-5 zeolites

The efficient synthesis of carbon nanotubes from polymer wastes has been considered one promising technology for the effective management of current environment crisis. In the two-stage reaction system, HZSM-5 zeolites were introduced into the two-stage reaction system between the upper and lower stages, which was expected to upgrade the pyrolysis gas from polymer wastes and boost the growth of carbon nanotubes on Fe-Al2O3 catalyst. The physicochemical properties of HZSM-5 zeolites, Fe-Al2O3 catalyst and the as-obtained carbon materials were systematically characterized by XRD, N2 physisorption, 27Al MAS NMR, NH3-TPD, H2-TPR, SEM, TEM, TG-DSC and Raman. It was found that the presence of HZSM-5 zeolites increased the length and yield, and narrowed the diameter of carbon nanotubes. In the absence of HZSM-5 zeolites, the yield of carbon nanotubes was 34 mg/g with an average diameter of 19 nm. The introduction of HZSM-5 and meso-HZSM-5 increased the yield of carbon nanotubes to 109 mg/g and 90 mg/g, and recduced the average diameter to 15 nm and 13 nm, respectively. The improvement of HZSM-5 zeolites on the synthesis of carbon nanotubes was observed for divrese polymer wastes inlcluding LDPE, HDPE and PP. It was attributed to the unique acidity and porosity of HZSM-5 zeolites, which enhanced the secondary cracking of the pyrolysis gas and the formation of light hydrocarbons.