Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology

The
adsorption of H2 in ZIF-76, a zeolitic imidazolate
framework (ZIF) with lta topology, was investigated in a combined
experimental and theoretical study. Each Zn2+ ion in the
structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate
linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains
a large amount of structural disorder, which makes this a challenging
material for modeling. We therefore chose to parametrize and simulate
H2 adsorption in two distinct crystal structure configurations
of ZIF-76 that differ by only the relative positions of one imidazolate
and one 5-chlorobenzimidazolate linker. The simulated H2 adsorption isotherms for both structures are in satisfactory agreement
with the newly reported experimental data for the ZIF, especially
at low pressures. The experimental initial isosteric heat of adsorption
(Qst) value for H2 in ZIF-76
was determined to be 7.7 kJ mol–1, which is comparable
to that for other ZIFs and is fairly high for a material that does
not contain open-metal sites. Simulations of H2 adsorption
in one of these structures resulted in Qst values that are in very good agreement with experiment within the
loading range considered. Two notable H2 binding sites
were discovered from simulations in both structures of ZIF-76; however,
the preferential regions of H2 occupancy are reversed for
the two structures. The inelastic neutron scattering (INS) spectra
for H2 adsorbed in ZIF-76 contain several peaks that arise
from transitions of the hindered H2 rotor, with the lowest
energy peak occurring in the range of 6.0–7.2 meV. Two-dimensional
quantum rotation calculations for H2 adsorbed at the considered
sites in both structures yielded rotational transitions that are in
good agreement with the peaks that appear in the INS spectra. Despite
the large degree of disorder in the ZIF-76 crystal structure, the
overall environment in the ZIF still gives rise to interconnected
INS features as discerned from our calculations. This study demonstrates
how important details of the H2 adsorption mechanism in
a ZIF with structural disorder can be obtained from a combination
of experimental measurements and theoretical calculations.