High H2Sorption Energetics in Zeolitic Imidazolate Frameworks

A combined experimental and theoretical study of H2sorption 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. H2sorption measurements showed that the two ZIFs display similar isotherms and isosteric heats of adsorption (Qst). The experimental initial H2Qstvalue 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 H2sorption in ZIF-68 and ZIF-69 confirmed the similar H2sorption properties between the two ZIFs, but also suggest that H2sorption 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 H2sorbed 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 H2sorption site in both ZIFs corresponds to a confined region between two adjacent 2-nitroimidazolate linkers. Two-dimensional quantum rotation calculations for H2sorbed 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 H2in the two ZIFs generates high barriers to rotation for the adsorbed H2, which are greater than those in several metal–organic frameworks (MOFs) which possess open-metal sites. H2sorption 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 H2sorption mechanism in these two isostructural ZIFs.