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

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 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 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.