Catalytic carbonization of polypropylene into cup-stacked carbon nanotubes with high performances in adsorption of heavy metallic ions and organic dyes

• Catalytic carbonization of PP into cup-stacked carbon nanotubes (CS-CNTs).
• Yield and morphology of CS-CNTs were modulated by halogenated compound content.
• Halogenated compound significantly influenced the degradation products of PP.
• Coalescence and reconstruction of NiO were affected by PP degradation products.
• Acid-treated CS-CNT showed high performances in adsorption of metallic ions and dyes.

A one-pot approach was demonstrated to effectively synthesize cup-stacked carbon nanotubes (CS-CNTs) through carbonization of polypropylene (PP) under the combined catalysis of halogenated compound and NiO at 700 °C. The effect of halogenated compound on the morphology, microstructure, phase structure and thermal stability of the resultant CS-CNTs was studied by scanning electron microscope, transmission electron microscope (TEM), high-resolution TEM, X-ray diffraction, Raman spectroscopy and thermal gravimetric analysis. Long, straight and surface-smooth CS-CNTs with graphene nanosheets oblique to CS-CNT axis at the angle of 21–25° were obtained when a low content of halogenated compound was added. The synergetic catalytic efficiency of combined catalysts in the carbonization of PP into CS-CNTs followed in the sequence: fluorinated compound/NiO ≪ chlorinated compound/NiO < bromated compound/NiO < iodated compound/NiO. Furthermore, the effect of halogenated compound on the degradation products of PP was analyzed. A low content of halogenated compound facilitated the formation of a large amount of light hydrocarbons with a small amount of aromatics, which promoted the coalescence and reconstruction of NiO catalyst into rhombic shape. The rhombic NiO then catalyzed the light hydrocarbons and aromatics into CS-CNTs. Finally, the resultant CS-CNTs were oxidized to prepare acid-treated CS-CNTs, which showed high performances in adsorption of heavy metallic ions and organic dyes. The adsorption kinetic and isotherms by acid-treated CS-CNTs were also studied.

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