Hydrogen storage in a prototypical zeolitic imidazolate framework-8.

Zeolitic imidazolate frameworks (ZIFs) are a new class of nanoporous compounds which consist of tetrahedral clusters of MN4 (M ) Co, Cu, Zn, etc.) linked by simple imidazolate ligands.1,2 As a subfamily of metal-organic frameworks (MOFs), ZIFs exhibit the tunable pore size and chemical functionality of classical MOFs. At the same time, they possess the exceptional chemical stability and rich structural diversity of zeolites.2 Because of these combined features, ZIFs show great promise for hydrogen storage applications. However, in contrast to a large number of extensive studies for other MOFs,3-5 no experimental data concerning the nature of H2ZIF interactions and the manner in which hydrogen molecules are adsorbed have been reported yet. Such fundamental studies hold the key to optimizing this new class of ZIF materials for practical hydrogen storage applications. In particular, the major adsorption sites and their binding energies are the key features of a system that determine its adsorption properties at a given temperature and pressure. ZIF8 is a prototypical ZIF compound (Zn(MeIM)2, MeIM ) 2-methylimidazolate) with a SOD (sodalite) zeolite-type structure, exhibiting an interesting nanopore topology formed by four-ring and six-ring ZnN4 clusters as shown in Figure 1. Since the nanopores are only accessible through narrow six-ring funnel-like channels (Figure 1a), one wonders how H2 molecules are adsorbed, where the major binding sites are, and what the binding energies are. Herein, using the difference Fourier analysis of neutron powder diffraction data along with first-principles calculations, we provide answers to these questions for the first time. Surprisingly, the strongest adsorption sites that we identified (see Figure 1b) are directly associated with the organic linkers, instead of the triangular faces of the ZnN4 tetrahedra (i.e., metal sites), in strong contrast to other MOFs, where the faces of the metal-oxide tetrahedra are typically the primary adsorption sites. At high H2 loading, the ZIF8 structure is capable of holding up to 28 H2 molecules (i.e., 4.2 wt %) in the form of highly symmetric novel three-dimensional (3D) interlinked H2-nanoclusters. ZIF8 was synthesized using a solvothermal method as described in ref 2. Neutron powder diffraction data were collected on the high-resolution neutron powder diffractometer (BT-1) at NIST Center for Neutron Research. Because of the large incoherent cross section of H2, adsorption was studied as a function of D2 concentration per ZIF8 molecular formula (Zn6N24C48H60). Target amounts of D2, that is, 3, 16, and 28 D2 per 6 Zn, were loaded into the ZIF8 sample at 70 K. One H2/6 Zn corresponds to ≈0.15 wt % hydrogen uptake. The sample was then cooled to 30 K at which point the D2 was completely adsorbed. Once the system was equilibrated at 30 K, the sample was further cooled to 3.5 K before the diffraction measurement. No evidence of solid deuterium was observed on the structural refinement of the D2 loaded samples, indicating all D2 was adsorbed into the ZIF8. The top panel of Figure 2 shows the neutron diffraction data from the ZIF8 bare material measured at 3.5 K. The refined lattice parameters and atomic positions of Zn, N, and C agree well with previously reported room-temperature X-ray diffraction results.2 The high-resolution neutron diffraction data also enabled us to unambiguously determine the orientation (i.e., the hydrogen positions) of the methyl group, which was not possible in the X-ray measurement. We note that the methyl group orientation and associated tunnel-splitting is a very sensitive probe for determination of the guest-host interactions.6 For comparison, the neutron diffraction patterns from ZIF8 with the following D2 concentrations: n D2 ) 3, 16, 28 per 6 Zn are also shown in Figure 2 (also see Supporting Information). Using the model of the refined ZIF8 host structure, we performed Rietveld † NIST Center for Neutron Research. ‡ University of Maryland. § University of Pennsylvania. Figure 1. (a) (001) view of refined crystal structure of ZIF8 host lattice from neutron powder diffraction along with the available free space (pore structure) for H2 occupation, based on van der Waals interactions. (b) A (111) view of the real-space Fourier-difference scattering-length density superimposed with six-ring pore aperture of the ZIF8 structure, indicating the location of the first adsorption sites (red-yellow regions).