Molecular simulation of adsorption sites of light gases in the metal-organic framework IRMOF-1

Molecular simulations are known to be able to reproduce many experimental isotherms of adsorbates in the metal-organic framework IRMOF-1. Here, we show that also the positions and occupations of the adsorption sites of argon and nitrogen match well with experiments. At 30 K the molecules are localized around their crystallographic sites, while at room temperature the molecules are spread throughout the pore volume. The orientations found for individual nitrogen molecules differ from those found experimentally, although the average orientations agree well. We elucidate the pore filling mechanism for both argon and nitrogen. In addition we compute the binding energy at sites for individual argon, nitrogen, carbon dioxide, hydrogen, methane, ethane, and propane molecules and show that for all molecules the preferred site is near the zinc–oxygen cluster in the cavities where the linkers point outward. The second site, higher in energy, is near the zinc–oxygen cluster in the cavities where the linkers point inward, and the least energetically favorable site is above and beneath the linker molecule. We explain why the site in the smaller cavities is only filled up at high loadings.