Metal organic frameworks MOF-5 as carbon dioxide storage studied by computational chemistry method

Abstract

The excess emission of carbon dioxide (CO₂), e.g., releasing from manufactory and power plant generator, is the major cause of global warming. Metal-Organic Frameworks (MOFs) become a primary target for CO₂ storage production. MOFs consist of zinc oxo acetate clusters and various organic aromatic linkers providing a series of Isoreticular MOFs (IRMOFs). In this work, three model clusters, (Zn₄O)₂(COOCH₃ )₁₀(COO)₂C₆H₄, (Zn₄O)₃ (COOCH₃ )₁₄(COO)₄(C₆H₄)₂ and (Zn₄O)₄(COOCH₃ )₁₈(COO)₆(C₆H₄)₃ were used to represent the ligand, then, BSSE corrected ONIOM method (MP2/6-31G(d,p) : HF/6-31G(d,p)) were applied in order to determine the adsorption site of CO₂ in MOFs. The results indicate that the cluster (Zn₄O)₄(COOCH₃ )₁₈(COO)₆(C₆H₄)₃ is sufficient to describe the adsorption and the MOF-5 corner is the preferential binding site of CO₂ molecule. In the second part, MD simulations of 2x2x2 lattice units of MOF-5 with the loading of 8, 64, 128, 256, 512 molecules of CO₂ per simulation cube (MPC) were performed at 300 K. The obtained results are in agreement with those found from ONIOM calculations that CO₂ prefers to absorb at the MOF-5 corner site. Interestingly, conclusion at MPC=512, the CO₂ was found to form second layer coordinated weakly to fill the rest space around the MOF-5 corner. The average coordination numbers of CO₂ at the corner and the linker sites are 2.9 and 3.1 molecules, respectively.