Tuning Metal-Dihydrogen Interaction in Metal-Organic Frameworks for Hydrogen Storage

Inspired by a recently reported metal-organic framework (MOF), V₂Cl_2.8(btdd) [H₂btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin], that shows a greatly improved H₂ adsorption enthalpy, we employ density functional theory to probe how the number of d electrons and the mixed valences influence the M-H₂ interaction inside the M₂Cl(btdd) MOFs. We find a cliff in the H₂ adsorption energy: the interaction strength remains strong from Sc to V and then falls sharply at Cr. Our results confirm V₂Cl_2.8(btdd) as one of the best performing hydrogen adsorbents and predict that Ti₂Cl_2.8(btdd) is equally promising while Sc₂Cl₂(btdd) and Ti₂Cl₂(btdd) may be even better. Our analysis indicates that an empty d orbital is the key to the much stronger binding of H₂ at the open M(II) site (M = Sc, Ti, or V), whereas a partially filled d orbital in Cr(II) and later M(II) greatly weakens H₂ binding. Our findings will be useful in designing MOFs to enhance H₂ adsorption.