Comprehensive DFT investigation of small-molecule adsorption on the paradigm M-MOF-74 family of metal-organic frameworks

The capture of toxic chemicals such as NH3, H2S, NO2 and SO2 is essential due to the tremendous threats they pose to human health and the environment. The M-MOF-74 family of metal-organic frameworks has recently gained attention as a promising category of sorbent materials for the capture of toxic chemicals; however, no clear and comprehensive relationships have been established between the capability of the M-MOF-74 to capture all target toxic chemicals and their properties such as the nature and magnetic state of the metal sites. Density-functional theory (DFT) is employed to investigate the binding energy of target molecules on M-MOF-74 with different metals including Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. The Hubbard U correction is employed to properly treat d electrons of transition metals and its effect explored on the bandgap of M-MOF-74. The magnetic properties of M-MOF-74 are investigated in detail along with their impact on the target molecule adsorption. Our calculations with DFT+U lead to good agreement with available experimentally determined bandgaps and structural properties. M-MOF-74 (M = Ti, V, Cr, Mn, Fe, Co and Cu) exhibit antiferromagnetic behavior, while ferromagnetic behavior prevails for Ni-MOF-74. Not surprisingly, the coordinatively unsaturated metals (M = Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) are the most likely sites for chemical adsorption of the target adsorbates, and V-MOF-74 and Ti-MOF-74 predicted to be efficient adsorbents for the target molecules, which can be rationalized on the basis of the metal d-band features. The spin configuration of transition metals in M-MOF-74 is found to have a negligible effect on adsorbate binding energies, which suggests that common DFT calculations without careful consideration of the material magnetic states can indeed be used to rapidly screen binding energies of adsorbates on such MOFs, with some notable exceptions; for instance, V-MOF-74 shows potential for magnetic sensing of NO2. This study provides further insight into the role of different unsaturated metals and their magnetic state for the removal of target toxic molecules by metal-organic frameworks.