Computational Study of Hydrogen Chemisorption on a Multi-Phenyl Organic Linker as a Model of Hydrogen Spillover on Metal-Organic Frameworks#

Abstract

Hydrogenation is greatly facilitated by a positive charge (left) on the multi-phenyl linker. Hydration of the third phenyl ring is most likely when the two other phenyl rings are fully hydrogenated. Hydrogen spillover reported on SNU-3 (empirical formula: Zn3C54H60N2O18) is computationally studied to suggest viable mechanism for propagation of hydrogenation to the multi-phenyl organic linker and the proper organic linker for metal-organic frameworks (MOFs) with increased hydrogen spillover effect. Density functional theory calculations on the model compound of the linker indicate that, for the neutral organic linker, every aromaticity-breaking step to add hydrogen requires high energetic cost. For positively charged cases, the high endothermicity disappeared, facilitating hydrogen storage by spillover. Thus, hydrogen can be easily stored only for the positively charged linkers. It is also demonstrated that nitrogen-based organic linkers are more redox active than others, suggesting that MOFs that contain nitrogen-based organic linkers may be better hydrogen storage materials when the spillover is the primary mechanism in transition-metal-doped MOFs.