Pretreated multiwalled carbon nanotube adsorbents with amine-grafting for removal of carbon dioxide in confined spaces

Abstract

Three different methods, including thermal treatment, treatment with HNO₃ and O₂ oxidation, were used to pretreat multiwalled carbon nanotubes (MWCNTs) before grafting with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPS). The types and contents of the O-containing groups generated by the various pretreatment methods were quantified and the corresponding amine-grafting reactions investigated. The alkoxyl groups of AEAPS react with the O-containing groups on the pretreated MWCNTs through silylation reactions in which there is no carboxyl acids induced by O₂ gas oxidation. The grafted primary amino groups can be accessible to capture the maximum amount of CO₂. A dynamic fixed-bed system was used to characterize the adsorption behavior of low concentrations of CO₂. The adsorption/desorption operations of 10 repeated cycles were investigated to verify the sustained excellent performance. The highest CO₂ adsorption capacity of 0.64 mmol g⁻¹ was achieved by the O₂-oxidized MWCNTs with AEAPS-grafting, which is almost 7.1 times the CO₂ adsorption capacity of the oxidized MWCNTs without amine-grafting. This indicates that O₂ gas oxidation is simple to operate and highly efficient for the pretreatment of MWCNTs. The adsorbent obtained has a high capacity, high thermal stability, high tolerance to moisture and low regeneration cost and shows promise for the direct capture of low concentrations of CO₂ in confined spaces.