Theoretical Investigations on MIL-100(M) (M=Cr, Sc, Fe) with High Adsorption Selectivity for Nitrogen and Carbon Dioxide over Methane.

The removal of impurity gases (N2 , CO2 ) in natural gas is critical to the efficient use of natural gas. In this work, the selective adsorption for N2 and CO2 over CH4 on MIL-100 (M) (M=4 Cr, 10 Cr, 6 Fe, 1 In, 1 Sc, 3 V) is studied by density functional theory (DFT) calculations. The calculated adsorption energy of the large-size cluster model (LC) of MIL-100 (M) shows that the 4 MIL-100 (4 Cr) is the best at the refinement of natural gas due to the lower adsorption energy of CH4 (-2.58 kJ/mol) in comparison with that of N2 (-21.49 kJ/mol) and CO2 (-23.82 kJ/mol). 1 MIL-100 (1 Sc) and 1 MIL-100 (6 Fe) can also achieve selective adsorption and follows the order 4 MIL-100 (4 Cr)>1 MIL-100 (1 Sc)>1 MIL-100 (6 Fe). In the research of the selective adsorption mechanism of MIL-100 (M) (M=4 Cr, 1 Sc, 6 Fe), the independent gradient model (IGM) indicates that these outstanding adsorbents interact with CO2 and N2 mainly through the electrostatic attractive interaction, while the van der Walls interaction dominates in the interaction with CH4. The atomic Projected Density of State (PDOS) further confirms that CH4 contributes least to the intermolecular interaction than that of CO2 and N2 . Through the scrutiny of molecular orbitals, it is found that electrons transfer from the gas molecule to the metal site in the adsorption of CO2 and N2 . Not only does the type of the metallic orbitals, but also the delocalization of the involved orbitals determines the selective adsorption performance of MIL-100. Both Cr and Sc share their d z 2 ${{d}_{{z}^{2}}}$ orbitals with the gases, making 1 MIL-100 (1 Sc) another potential effective separator for CH4 . Additionally, the comparison of adsorption energy and PDOS shows that the introduction of ligands such as benzene impedes the electron donation from gas molecules (CO2 , N2 ) to the metal site, indicating electron-withdrawing ligands will further favor the adsorption.