Experimental and theoretical H2 adsorption studies were performed in two isostructural zeolitic imidazolate frameworks (ZIFs) with gme (gmelinite) topology, namely ZIF-68 and ZIF-69. The former consists of Zn2+ ions coordinated to two 2-nitroimidazolate (nIM) and two benzimidazolate (bIM) linkers in a tetrahedral fashion, while 5-chlorobenzimidazolate (cbIM) is used in place of bIM in the latter ZIF. Experimental H2 adsorption measurements showed that the two ZIFs display similar isotherms and isosteric heat of adsorption (Qst). The zero-coverage Qst value for H2 in both ZIFs was determined to be 8.1 kJ mol−1, which is quite high for materials that do not contain open-metal sites. Molecular simulations of H2 adsorption in ZIF-68 and ZIF-69 confirmed the similar H2 adsorption properties between the two ZIFs. Inelastic neutron scattering (INS) studies of H2 adsorbed in both ZIFs revealed a broad range of intensities starting from about 4 meV in the corresponding spectra. The most favorable H2 adsorption site in both ZIFs corresponds to a confined region between two adjacent nIM linkers. Two-dimensional quantum rotation calculations for H2 bound at this site in ZIF-68 and ZIF-69 produced rotational transitions that are in close agreement with the lowest energy peak observed in the INS spectra for the respective ZIFs. Moreover, it was discovered that the primary binding site for H2 in 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.

High H2 adsorption energetics in zeolitic imidazolate frameworks

ACS Spring 2022

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