The Potential of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) as Adsorbents for the Efficient Separation of CH4 from CO2 and H2S.

Carbon dioxide (CO2) and hydrogen sulfide (H2S) as harmful gases are always associated with methane (CH4) in natural gas, biogas, and landfill gas. Given that chemisorption and physisorption are the key gas separation technologies in industry, selecting appropriate adsorbents is crucial to eliminate these harmful gases. The adsorption of CH4, CO2, and H2S has been studied based on the density functional theory (DFT) in this work to evaluate the feasibility of transition metal (M = Mn, Fe, Co, Ni, Cu, Mo) porphyrin-like moieties embedded in graphene sheets (MN4-GPs) as adsorbents. It was found that the interactions between gas molecules and MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) are different. The weaker interactions between CH4 and MN4-GPs (M = Co, Ni, Cu, Mo) than those between CO2 and MN4-GPs or between H2S and MN4-GPs are beneficial to the separation of CH4 from CO2 and H2S. The maximum difference in the interactions between gas molecules and MoN4-GPs means that MoN4-GPs have the greatest potential to become adsorbents. The different interfacial interactions are related to the amount of charge transfer, which could promote the formation of bonds between gas molecules and MN4-GPs to effectively enhance the interfacial interactions.