Characterization of Lewis Acid Sites in Metal−Organic Frameworks Using Density Functional Theory

The properties of Lewis acid sites in two MOFs, Cu−BTC and Cu−MIPT, were studied by using density functional theory (DFT). The strengths of these sites were investigated through exploring the geometry parameters, the natural bond orbital (NBO) charge, and the vibrational frequency as well as the adsorption energy of the probe CO molecule. The results show that both MOFs have Lewis acid sites, and the strength of the Lewis acid sites in Cu−BTC is stronger than that in Cu−MIPT. In addition, proposals for enhancing the acid strength of MOFs are discussed; increasing the number of atoms with high electronegativity in organic linkers and modification of the metal cation by the one with more empty d orbits are suggested as the two possible ways.