Exceptional H₂ sorption characteristics in a Mg²⁺-based metal-organic framework with small pores: Insights from experimental and theoretical studies

Experimental sorption measurements, inelastic neutron scattering (INS), and theoretical studies of H₂ sorption were performed in α-[Mg₃(O₂CH)₆], a metal-organic framework (MOF) that consists of a network of Mg²⁺ ions coordinated to formate ligands. The experimental H₂ uptake at 77 K and 1.0 atm was observed to be 0.96 wt%, which is quite impressive for a Mg²⁺-based MOF that has a BET surface area of only 150 m² 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 H₂ in α-[Mg₃(O₂CH)₆] is very unusual for a porous material, as there exist several different peaks that occur below 10 meV. Simulations of H₂ sorption in α-[Mg₃(O₂CH)₆] revealed that the H₂ 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 H₂ 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 H₂ sorbed at the most favorable site in α-[Mg₃(O₂CH)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 H₂ 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 H₂ in a MOF with small pore sizes.