Molecular insight into the high selectivity of double-walled carbon nanotubes

Abstract

Combining experimental knowledge with molecular simulations, we investigated the adsorption and separation properties of double-walled carbon nanotubes (DWNTs) against flue/synthetic gas mixture components (e.g.CO₂, CO, N₂, H₂, O₂, and CH₄) at 300 K. Except molecular H₂, all studied nonpolar adsorbates assemble into single-file chain structures inside DWNTs at operating pressures below 1 MPa. Molecular wires of adsorbed molecules are stabilized by the strong solid–fluid potential generated from the cylindrical carbon walls. CO₂ assembly is formed at very low operating pressures in comparison to all other studied nonpolar adsorbates. The adsorption lock-and-key mechanism results from perfect fitting of rod-shaped CO₂ molecules into the cylindrical carbon pores. The enthalpy of CO₂ adsorption in DWNTs is very high and reaches 50 kJ mol⁻¹ at 300 K and low pore concentrations. In contrast, adsorption enthalpy at zero coverage is significantly lower for all other studied nonpolar adsorbates, for instance: 35 kJ mol⁻¹ for CH₄, and 14 kJ mol⁻¹ for H₂. Applying the ideal adsorption solution theory, we predicted that the internal pores of DWNTs have unusual ability to differentiate CO₂ molecules from other flue/synthetic gas mixture components (e.g.CO, N₂, H₂, O₂, and CH₄) at ambient operating conditions. Computed equilibrium selectivity for equimolar CO₂–X binary mixtures (where X: CO, N₂, H₂, O₂, and CH₄) is very high at low mixture pressures. With an increase in binary mixture pressure, we predicted a decrease in equilibrium separation factor because of the competitive adsorption of the X binary mixture component. We showed that at 300 K and equimolar mixture pressures up to 1 MPa, the CO₂–X equilibrium separation factor is higher than 10 for all studied binary mixtures, indicating strong preference for CO₂ adsorption. The overall selective properties of DWNTs seem to be superior, which may be beneficial for potential industrial applications of these novel carbon nanostructures.