Hydrophilicity effect on CO2/CH4 separation using carbon nanotube membranes: insights from molecular simulation

Abstract

Molecular simulation methods were applied to study the effect of hydrophilicity on CO2/CH4 separation using carbon nanotube (CNT) membranes. CNTs with a diameter of ~1 nm were functionalised by varying amounts of carbonyl groups, in order to achieve various hydrophilicity. The presence of –CO groups inside the CNT allow a significant gain in the diffusion selectivity of CO2, while in contrast the adsorption selectivity is hardly changed. The corresponding permeation selectivity increases as the hydrophilicity of the CNT-based membrane increases. However, the permeability of CO2 decreases due to a combination of the intermolecular interactions between the gas and functional groups and the steric effects of the added functional groups. Considering both the permeation selectivity and permeability, it was found that the maximum separation performance is achieved in a certain hydrophilic CNT membrane. Moreover, the separation performance of hydrophilic CNTs for CO2/CH4 mixtures breaks the Robeson upper bound.