Nanoscale Cu(II) MOFs Formed via Microemulsion: Vibrational
Mode Characterization Performed using a Combined FTIR, Synchrotron
Far-IR, and Periodic DFT Approach
posted on 2021-09-10, 21:43authored byCourtney Ennis, Aaron C. Y. Tay, Jonathan L. Falconer, S. J. Lee, Carla J. Meledandri
Two Cu(II) metal–organic frameworks
(MOFs) were prepared
on the nanoscale at room temperature using a microemulsion method,
namely, [Cu3(BTC)2(H2O)3] (BTC = benzene-1,3,5-tricarboxylate), known as HKUST-1 (1), and [Cu2(OH)(BTC)(H2O)]·2H2O (2). Thermochemical and gas sorption properties of
the microporous topologies were characterized by mid- and far-infrared
vibrational spectroscopy, supported by periodic density functional
theory calculations. The mid-infrared profile of 1 appeared
altered in response to gas sorption under variable temperature and
pressure conditions. Vibrational mode analysis indicated the most
sensitive infrared peaks were associated with the internal vibrations
of organic linker moieties indirectly coupled to the Cu(II)–gas
coordination site, activated by a lowered symmetry induced by guest
interactions. Synchrotron far-infrared spectroscopy was shown to be
a useful diagnostic for the microstructure of 1 and 2 where different temperature dependences were displayed in
the low-frequency region. The loss of residual water during the activation
of 2 at elevated temperature coincides with peaks indicative
of free paddle-wheel moieties emerging in the far-IR spectra. As demonstrated
for both materials 1 and 2, vibrational
mode analysis was effective in screening MOF materials for their propensity
toward gas uptake and, inversely, the diffusion of guest species such
as adsorbed water from the microporous environments.