Theoretical DFT study on metal–organic frameworks for hydrogen storage

Abstract

The cluster approach has been applied to mimic metal–organic framework (MOF) structures formed by 1,4-benzenecarboxylate linkers and metal cation connectors. The geometry optimizations are carried out at B3LYP/6-31G* level followed by frequency calculations. It was found that the replacement of OBe46+ connectors with OMg46+ (or OZn46+) ones leads to an increase in both the unit-cell sizes and basicity of oxide sites making them more potential for molecular hydrogen storage applications. Within the selected secondary building units (larger cluster models), the above replacement does not lead to an alteration on conduction and valence bands as well as in the resulting bandgap values. Bottleneck point of these structures is due to the presence of the so-called “central bulk” fourfold coordinated tetrahedral oxide anion site. The latter can be efficiently removed via replacement to “distorted cubic core” by introducing ZnTi3O46+ or MgTi3O46+ connectors which can be further replaced by MgSi3O46+ units. The all threefold coordinated O3C sites formed act as potential adsorption sites for incoming hydrogen molecules. Two-layer ONIOM (MP2/6-31G*: HF/3-21G) calculations have shown that the binding energies of molecular hydrogen lie within 5.5 and 5.3 kJ/mol for the MOF structures with MgTi3O46+ and MgSi3O46+ connectors, respectively.