High H₂ Sorption Energetics in Zeolitic Imidazolate Frameworks

A combined experimental and theoretical study of H 2 sorption was carried out on two isostructural zeolitic imidazolate frameworks (ZIFs), namely ZIF-68 and ZIF-69. The former consists of Zn 2+ ions that are coordinated to two 2-nitroimidazolate and two benzimidazolate linkers in a tetrahedral fashion, while 5-chlorobenzimidazolate is used in place of benzimidazolate in the latter compound. H 2 sorption measurements showed that the two ZIFs display similar isotherms and isosteric heats of adsorption ( Q st ). The experimental initial H 2 Q st value for both ZIFs was determined to be 8.1 kJ mol –1, which is quite high for materials that do not contain exposed metal centers. Molecular simulations of H 2 sorption in ZIF-68 and ZIF-69 confirmed the similar H 2 sorption properties between the two ZIFs, but also suggest that H 2 sorption is slightly favored in ZIF-68 with regards to uptake at 77 K/1.0 atm. This work also presents inelastic neutron scattering (INS) spectra for H 2 sorbed in ZIFs for the first time. The spectra for ZIF-68 and ZIF-69 show a broad range of intensities starting from about 4 meV. The most favorable H 2 sorption site in both ZIFs corresponds to a confined region between two adjacent 2-nitroimidazolate linkers. Two-dimensional quantum rotation calculations for H 2 sorbed at this site in ZIF-68 and ZIF-69 produced rotational transitions that are in accord with the lowest energy peak observed in the INS spectrum for the respective ZIFs. We found that the primary binding site for H 2 in the two ZIFs generates high barriers to rotation for the adsorbed H 2, which are greater than those in several metal–organic frameworks (MOFs) which possess open-metal sites. H 2 sorption was also observed for both ZIFs in the vicinity of the nitro groups of the 2-nitroimidazolate linkers. This study highlights the constructive interplay of experiment and theory to elucidate critical details of the H 2 sorption mechanism in these two isostructural ZIFs.