Metal-organic frameworks as H₂ storage materials: Implications of cation doping on uptake and binding

Metal-organic frameworks (MOFs) have received considerable attention as potential H₂ storage materials. However, despite the nearly limitless structural possibilities of MOFs, none have yet succeeded in reaching relevant uptake standards. To further understand the factors necessary for strong H₂ binding within MOF materials, we have pursued cation doping and inclusion as a strategy to create highly favorable binding sites. Here we describe the first experimental studies of cation insertion into MOF materials through the ligand functionality. Chemical reduction of the ligand and inclusion of charge-balancing cations has indeed resulted in nearly doubling the H₂ uptake, though most likely attributable to dynamic structural behavior as opposed to direct H₂-cation interaction. Li⁺ and Mg²⁺ alkoxide formation at pendant hydroxyl sites within the pores has also resulted moderate increases in uptake. Notably, the H₂ binding affinity exhibits an unusual trend with loading that suggests direct H₂-cation interactions.