Zeolitic Imidazolate Frameworks as H2 Adsorbents: Ab Initio Based Grand Canonical Monte Carlo Simulation

Abstract

We report the H2 uptake behavior of 10 zeolitic−imidazolate frameworks (ZIFs), based on grand canonical Monte Carlo (GCMC) simulations. The force fields (FFs) describing the interactions between H2 and ZIF in the GCMC were based on ab initio quantum mechanical (QM) calculations (MP2) aimed at correctly describing London dispersion (van der Waals attraction). Thus these predictions of H2 uptake are based on first principles (non empirical) and hence applicable to new framework materials for which there is no empirical data. For each of these 10 ZIFs we report the total and excess H2 adsorption isotherms up to 100 bar at both 77 and 300 K. We report the hydrogen adsorption sites in the ZIFs and the relationships between H2 uptake amount, isosteric heat of adsorption (Qst), surface area, and free volume. Our simulation shows that various ZIFs lead to a variety of H2 adsorption behaviors in contrast to the metal−organic frameworks (MOFs). This is because ZIFs leads to greater diversity in the adsorption sites (depending on both organic linkers and zeolite topologies) than in MOFs. In particular, the ZIFs uptake larger amounts of H2 at low pressure because of the high H2 adsorption energy, and ZIFs have a variety of H2 adsorption sites. For example, ZIF-11 has an initial Qst value of ∼15 kJ/mol, which is higher than observed for MOFs. Moreover, the preferential H2 adsorption site in ZIFs is onto the organic linker, not nearby the metallic joint as is the case for MOFs.