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 Zn2+ 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.