Enhancing hydrogen storage by metal substitution in MIL-88A metal-organic framework

MIL-88A metal-organic framework with the unsaturated Fe metal coordination sites has demonstrated to be a promising material for gas storage and capture. However, the hydrogen storage capacity of MIL-88A has to be improved to meet the practical level at the ambient conditions. In this research, we elucidated the effects of transition metal substitution on the hydrogen storage capability of MIL-88A. The trivalent transition metals including Sc, Ti, V, Cr, Mn, and Ni have been selected to substitute for Fe in MIL-88A. Using the van der Waals dispersion-corrected density functional theory calculations, we explored the most favorable adsorption configurations of the hydrogen molecule in M-MIL-88A (M = Sc, Ti, V, Cr, Mn, Ni). We found that the V-MIL-88A has the strongest binding energy of 17 kJ mol(-1) with the hydrogen molecule in the side-on configuration on the metal site. Besides, the grand canonical Monte Carlo simulations showed that the metal substitution greatly influences not only the favorable adsorption configuration and energy but also the hydrogen uptake due to the modification of the H-2@MIL-88A interaction. Sc-MIL-88A was found to offer the highest gravimetric H-2 uptake compared to the other M-MIL-88A. The value is 5.13 wt% at (cryogenic temperature 77 K, 50 bar) and 0.72 wt% at (room temperature 298 K, 100 bar) for the absolute; 4.63 wt% at (77 K, 10 bar) and 0.29 wt% at (298 K, 100 bar) for the excess capacity. Furthermore, Sc-MIL-88A also exhibited the highest volumetric uptake up to 52 g L-1 at 77 K and 7.1 g L-1 at 298 K for the absolute; 46 g L-1 at 77 K and 2.8 g L-1 at 298 K for the excess loading.