Oleophobic nanopore in graphene membrane enhances CO2 capture and separation after spontaneous hydrocarbon adsorption

Abstract

Context

Membrane separation technology is a great candidate for capturing and separating CO₂ from air and flue gas, aiming at combating global warming. In particular, numerous experimental and theoretical investigations have revealed the outstanding performance of porous graphene membrane in sieving CO₂ due to its high-selectivity and energy-efficiency. Some experimental studies have confirmed that the graphene can be spontaneously contaminated by the hydrocarbons in ambient air, due to its large surface energy. However, how the covered hydrocarbons on porous graphene membrane affect the CO₂ capture and separation remains elusive. In this study, we employed molecular dynamics (MD) simulation approach to investigate CO₂/N₂ separation capacity of the oleophobic N24 nanopores and the oleophilic C24 nanopores in graphene membrane after covering the oleaginous hydrocarbon, C₈H₁₈, films. Interestingly, our MD simulations demonstrate that the oleophobic N24 nanopore shows higher CO₂ transport rate and CO₂/N₂ selectivity compared with the oleophilic C24 nanopore after the membrane adsorbed by C₈H₁₈ films, indicating that the oleophobic N24 graphene nanopore can ameliorate CO₂ capture and separation upon C₈H₁₈ films adsorption. Mechanically, on the one hand, C₈H₁₈ can more likely block the C24 nanopore, due to the stronger affinity of C₈H₁₈ to the C24 pore, which results in the reduced gas transport rate. On the other hand, the quadrupole interaction between CO₂ and N24 nanopore helps the favorable capture and separation of CO₂ by N24 nanopore. The combined effect thus determines the better CO₂ separation performance of hydrocarbon covered N24 nanopore. Therefore, our findings not only reveal the carbon capture and separation performance of porous graphene membrane upon spontaneously adsorbing hydrocarbon from the ambient air or the flue gas for the first time, but also exploit the oleophobic nanopore capable of ameliorating the membrane capacity, which is beneficial to future practical application of porous graphene membrane in CO₂ separation.

Methods

We conducted all MD simulations with GROMACS software package. VMD software package was used to visualizing the simulation conformations and trajectories. Periodic boundary conditions were applied in all directions (x, y, and z). Temperature was constrained at 350 K using the v-rescale thermostat. Long-range electrostatic interactions were computed using the PME method, and van der Waals (vdW) interactions were calculated with a cutoff distance of 12 Å. Bonds involving hydrogen atoms were constrained to their equilibrium values using the LINCS algorithm.