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=⁴Cr, ¹⁰Cr, ⁶Fe, ¹In, ¹Sc, ³V) 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 ⁴MIL-100 (⁴Cr) 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). ¹MIL-100 (¹Sc) and ¹MIL-100 (⁶Fe) can also achieve selective adsorption and follows the order ⁴MIL-100 (⁴Cr)>¹MIL-100 (¹Sc)>¹MIL-100 (⁶Fe). In the research of the selective adsorption mechanism of MIL-100 (M) (M=⁴Cr, ¹Sc, ⁶Fe), 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 CH_4. 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 orbitals with the gases, making ¹MIL-100 (¹Sc) 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.