Hydrogen adsorption simulations in isomorphous borohydride and imidazolate frameworks: Evaluations using interpolation

Abstract

We simulate hydrogen adsorption on two topologically identical crystalline solids using the grand canonical Monte Carlo (GCMC) method. One solid is the γ - magnesium borohydride, γ-Mg(BH4)2, the first borohydride crystal with a permanent porosity. The other solid is ZIF-72, Zn(dcIm)2, a zinc imidazolate framework synthesized with dichloroimidazole (dcIm) ligands. Both solids maintain narrow sized pore networks, capable of storing molecular hydrogen. We introduce an interpolation scheme for the temperature dependence of adsorption isotherms. The interpolation employs a special control function based on the adsorption enthalpy. We compare the hydrogen capacities of the two samples at variable temperatures and pressures and attribute the discrepancies to the implicit surface texture and size of the confinements. Notably, the porous Mg(BH4)2 can physically adsorb 3.44 wt% H2 at cryogenic temperatures that is cumulative to the already high content of 14.9 wt% of atomic hydrogen bound on the boron atoms. This content makes the γ-Mg(BH4)2 one of the most hydrogen rich solids reported to date.