Exceptional H 2 sorption characteristics in a Mg 2+ -based metal–organic framework with small pores: insights from experimental and theoretical studies

Experimental sorption measurements, inelastic neutron scattering (INS), and theoretical studies of H 2 sorption were performed in α-[Mg 3 (O 2 CH) 6 ], a metal–organic framework (MOF) that consists of a network of Mg 2+ ions coordinated to formate ligands. The experimental H 2 uptake at 77 K and 1.0 atm was observed to be 0.96 wt%, which is quite impressive for a Mg 2+ -based MOF that has a BET surface area of only 150 m 2 g −1 . Due to the presence of small pore sizes in the MOF, the isosteric heat of adsorption ( Q st ) value was observed to be reasonably high for a material with no open-metal sites ( ca. 7.0 kJ mol −1 ). The INS spectra for H 2 in α-[Mg 3 (O 2 CH) 6 ] is very unusual for a porous material, as there exist several different peaks that occur below 10 meV. Simulations of H 2 sorption in α-[Mg 3 (O 2 CH) 6 ] revealed that the H 2 molecules sorbed at three principal locations within the small pores of the framework. It was discovered through the simulations and two-dimensional quantum rotation calculations that different groups of peaks correspond to particular sorption sites in the material. However, for H 2 sorbed at a specific site, it was observed that differences in the positions and angular orientations led to distinctions in the rotational tunnelling transitions; this led to a total of eight identified sites. An extremely high rotational barrier was calculated for H 2 sorbed at the most favorable site in α-[Mg 3 (O 2 CH) 6 ] (81.59 meV); this value is in close agreement to that determined using an empirical phenomenological model (75.71 meV). This rotational barrier for H 2 exceeds those for various MOFs that contain open-metal sites and is currently the highest yet for a neutral MOF. This study highlights the synergy between experiment and theory to extract useful and important atomic level details on the remarkable sorption mechanism for H 2 in a MOF with small pore sizes.