CO2/CH4 separation through monolayer nanoporous graphene oxide supported ionic liquid membrane

Abstract

Nanoporous graphene oxide (NPGO) with a single atomic layer thickness and good mechanical strength has been regarded as a promising candidate in gas separation. However, it is challenging to precisely control the pore size to match the target gas. In this work, the pore size was modulated by supporting ionic liquid (IL) on the monolayer NPGO membrane (NPGO-SILM). The separation performance and mechanism for CO2/CH4 in NPGO-[EMIM][BF4], NPGO-[EMIM][PF6] and NPGO-[EMIM][TF2N] were investigated by molecular dynamics simulation. It was found that the pore size of NPGO greatly affects the permeability and selectivity, and 5.6 Å is the most suitable pore size. After coating IL on NPGO, the CO2/CH4 selectivity is obviously enhanced. With the IL thickness increasing from 5 to 11 Å, the CO2 permeance decreased from ∼105 to ∼104 GPU, while the selectivity increased, and almost no CH4 passed through the membrane with 11 Å thickness. Among the studied three types of ILs, the NPGO-[EMIM][TF2N] membrane shows the best CO2/CH4 separation performance due to the strongest adsorption capacity between [TF2N] anion and CO2. The interaction energy, PMF and probability distribution results suggest that the improved selectivity is attributed to the synergistic effect of cations and anions. The anions are responsible for the adsorption of large amounts of CO2. At the same time, the cations are mainly distributed around the center of the NPGO pore because of the strong interaction between cation and NPGO, which would be favorable for dynamically adjusting pore size to allow the passage of CO2 and prevent CH4 from passing through the membrane.