Tuning the Metal Binding Energy and Hydrogen Storage in Alkali Metal Decorated MOF-5 Through Boron Doping: A Theoretical Investigation

Using ab initio quantum chemical calculations, we explore the possibility of improving the light metal binding energy as well as the hydrogen adsorption energy in MOF-5 so that the clustering problem at high concentration can be overcome. To mimic the linking group (C 6 H 4 (COO) 2 2– ) in MOF-5, we have considered the simple benzene ring. Because the binding energy of light metals (Li, Na, and Mg) to benzene is poor, we have modified the carbon ring by substituting two carbon atoms with two boron atoms. The metal ion (atom) interaction with C 4 B 2 H 6 2– (C 4 B 2 H 6 ) is found to be very strong, and the interaction energies are extremely high as compared with the metal cohesive energies. Metal sites in these complexes are found to have partial ionic (positive) character and are shown to adsorb molecular hydrogen with reasonably good adsorption enthalpies. We have also studied the extended periodic system of lithium as well as sodium-decorated modified MOF-5 and their hydrogenated counterparts. In the case of lithium-decorated MOF-5, each formula unit is found to adsorb 18 hydrogen molecules with three H 2 per lithium atom at zero temperature, which corresponds to a gravimetric density of ∼4.3 wt %. However, in the case of Na-decorated MOF-5, the corresponding wt % is found to be 7.4 because each Na can adsorb six molecular hydrogen with 36 hydrogen molecules per formula unit.