Hydrogen in the metal-organic framework Cr MIL-53

Practical methods for hydrogen storage are still a major bottleneck in the realization of an energy economy based on hydrogen as an energy carrier.(1)Surface adsorption within crystalline, nanoporous, metal-organic frameworks (MOFs) provides a promising storage method that relies on sufficiently strong adsorption interactions for a large fraction of the storage capacity. Only few MOF structures were studied up to date using neutron diffraction to resolve the adsorption sites.^2-5 Here we use in situ neutron diffraction to characterize hydrogen (deuterium) adsorption sites in the MOF Cr MIL-53. The strongest adsorption interactions are present at three different sites where the hydrogen touches nearby organic linkers from two directions. Perhaps surprisingly, there is no strong direct interaction with the Cr-O cluster. Large breathing modes of the crystalline lattice are observed upon loading hydrogen reversibly to an equivalent of 5.5 wt %. Such breathing modes are known to occur for polar solvents like H₂O, alkanes, and also for the gas CO₂; however, as shown here it also occurs for weakly van der Waals adsorbed hydrogen molecules.