Effect of the Position of Amine Groups on the CO2, CH4, and H2 Adsorption Performance of Graphene Nanoflakes

CO2 separation from CO2/CH4 and CO2/H2 mixtures is essential in H2 production from biogas. Although amine-functionalized carbon materials are excellent candidates for selective CO2 adsorption, their adsorption performance depends on the amine edge position, which requires further clarification. Herein, 19 graphene nanoflakes (GNFs) bearing two amines at various positions on the zigzag, corner, and armchair edges were evaluated. All structures exhibited better CO2 adsorption compared to CH4 and H2. In the GNFs with isolated amines, only one main bond (H2N-CO2) was involved in CO2 adsorption, which was influenced by charge transfer from the GNFs to CO2. GNFs with adjacent amines had an additional hydrogen bond (H2N-CO2) that became stronger with decreasing steric hindrance. The CO2 adsorption performance of amines decreased as the number of methyl groups on the nitrogen increased, which interfered with CO2 adsorption. The presence of H2O hindered the interaction of amines and CO2 owing to the strong hydrogen bond between H2O and amines.