A novel design for mass production of multi-walled carbon nanotubes using Co-Mo/MgO catalyst via pyrolysis of polypropylene waste: effect of operating conditions

Abstract

A two-stage process was employed for high yield production of multi-walled carbon nanotubes (MWCNTs) via pyrolysis of polypropylene (PP) waste. In the first stage, a new design was used for the pyrolysis of PP waste at 500 °C to form a mixture of condensable hydrocarbons (≥C6) and non-condensable gases (C1–C5) inside a vertical reactor closed from the bottom and connected directly to a vertical condenser at the top. This pyrolysis technique permitted to form a large amount of non-condensable hydrocarbon gases, which were used in the second stage as a carbon source for the production of MWCNTs over Co-Mo/MgO catalyst. The influences of growth temperature (700–850 °C) and carrier gas flow rate of N2 (50–110 sccm) on the yield and morphology of as-deposited MWCNTs have been investigated. The fresh Co-Mo/MgO catalyst and the as-deposited carbon were characterized by XRD, FTIR, TPR, BET surface properties, TEM, Raman spectroscopy and TGA. The results demonstrated that the adjustment of growth temperature and N2 flow-rate caused a marked impact on the yield, type and quality of as-grown MWCNTs. The optimum MWCNTs yields of 32.6 and 38.3 g/gcatalyst have been achieved at the growth temperature of 800 °C and the carrier gas flow rate of 90 sccm, respectively. TEM images illustrated the formation of pure MWCNTs at the growth temperature range of 700–800 °C, whereas mixed materials of MWCNTs and graphene nanosheets (GNSs) were obtained at the growth temperature of 850 °C. Raman spectroscopy illustrated that highly graphitized and crystalline MWCNTs were produced at all operating conditions. TGA proved that all MWCNTs samples exhibited higher thermal stability.