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

Inspired by a recently reported metal-organic framework (MOF), V2Cl2.8(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 M2Clx(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 V2Cl2.8(btdd) as one of the best performing hydrogen adsorbents and predict that Ti2Cl2.8(btdd) is equally promising while Sc2Cl2(btdd) and Ti2Cl2(btdd) may be even better. Our analysis indicates that an empty dx2-y2 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 dx2-y2 orbital in Cr(II) and later M(II) greatly weakens H2 binding. Our findings will be useful in designing MOFs to enhance H2 adsorption.