Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology

The adsorption of H₂ in ZIF-76, a zeolitic imidazolate framework (ZIF) with lta topology, was investigated in a combined experimental and theoretical study. Each Zn²⁺ ion in the structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains a large amount of structural disorder, which makes this a challenging material for modeling. We therefore chose to parametrize and simulate H₂ adsorption in two distinct crystal structure configurations of ZIF-76 that differ by only the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. The simulated H₂ adsorption isotherms for both structures are in satisfactory agreement with the newly reported experimental data for the ZIF, especially at low pressures. The experimental initial isosteric heat of adsorption (Q_st) value for H₂ in ZIF-76 was determined to be 7.7 kJ mol^-1, which is comparable to that for other ZIFs and is fairly high for a material that does not contain open-metal sites. Simulations of H₂ adsorption in one of these structures resulted in Q_st values that are in very good agreement with experiment within the loading range considered. Two notable H₂ binding sites were discovered from simulations in both structures of ZIF-76; however, the preferential regions of H₂ occupancy are reversed for the two structures. The inelastic neutron scattering (INS) spectra for H₂ adsorbed in ZIF-76 contain several peaks that arise from transitions of the hindered H₂ rotor, with the lowest energy peak occurring in the range of 6.0-7.2 meV. Two-dimensional quantum rotation calculations for H₂ adsorbed at the considered sites in both structures yielded rotational transitions that are in good agreement with the peaks that appear in the INS spectra. Despite the large degree of disorder in the ZIF-76 crystal structure, the overall environment in the ZIF still gives rise to interconnected INS features as discerned from our calculations. This study demonstrates how important details of the H₂ adsorption mechanism in a ZIF with structural disorder can be obtained from a combination of experimental measurements and theoretical calculations.