Understanding hydrogen adsorption in metal-organic frameworks with open metal sites: a computational study.

Recent experimental investigations show that the open metal sites may have a favorable impact on the hydrogen adsorption capacity of metal-organic frameworks (MOFs); however, no definite evidence has been obtained to date and little is known on the interactions between hydrogen and the pore walls of this kind of MOFs. In this work, a combined grand canonical Monte Carlo simulation and density functional theory calculation is performed on the adsorption of hydrogen in MOF-505, a recently synthesized MOF with open metal sites, to provide insight into molecular-level details of the underlying mechanisms. This work shows that metal-oxygen clusters are preferential adsorption sites for hydrogen, and the strongest adsorption of hydrogen is found in the directions of coordinatively unsaturated open metal sites, providing evidence that the open metal sites have a favorable impact on the hydrogen sorption capacity of MOFs. The storage capacity of hydrogen of MOF-505 at room temperature and moderate pressures is predicted to be low, in agreement with the outcome for hydrogen physisorption in other porous materials.