Residue catalyst support removal and purification of carbon nanotubes by NaOH leaching and froth flotation

The synthesis of single-walled carbon nanotubes (SWNTs) by the catalytic decomposition of gaseous carbon-containing molecules has been considered to be the most promising process for large-scale production with a relatively low cost. However, the as-prepared SWNTs from this process always contain significant amounts of impurities such as metallic catalysts, catalyst support, and other unwanted forms of carbon. For a variety of potential applications, the purification of SWNTs is considered to be an important step. In this research, NaOH solution was utilized to dissolve the silica support, a major impurity, from the SWNTs produced by the CO disproportionation over a CoMo/SiO2 catalyst. Subsequently, froth flotation was used to further recover and concentrate the total carbon after the NaOH pretreatment. To achieve a high purity of total carbon by the proposed purification techniques, the effects of operating parameters were investigated in order to minimize the entrainment of undissolved silica with the froth. At the optimum operating conditions: 3 h of sonication time, 30 mg/l of surfactant dosage, 1.0 g/l of pulp density, 100 ml/min of air flow rate, and 22 cm of froth height, the purity of total carbon was found to increase to 30% after the first step of NaOH treatment and to 78% after the second step of froth flotation as compared to the initial carbon content of only 4% of the as-prepared SWNTs. From the temperature-programmed oxidation (TPO) results, the Raman spectra and the scanning electron microscope (SEM) images, the physical and chemical structures of SWNTs are not damaged by NaOH treatment and froth flotation.