An isotope-selective responsive system based on molecular recognition in porous materials has potential for the storage and purification of isotopic mixtures but is considered unachievable because of the almost identical physicochemical properties of the isotopes. Herein, a unique isotope-responsive breathing transition of the flexible metal−organic framework (MOF), MIL-53(Al), which can selectively recognize and respond to only D 2 molecules through a secondary breathing transition, is reported. This novel phenomenon is examined using in situ neutron diffraction experiments under the same conditions for H 2 and D 2 sorption experiments. This work can guide the development of a novel isotope-selective recognition system and provide opportunities to fabricate flexible MOF systems for energy-efficient purification of the isotopic mixture.
Deuterium has been recognized as an irreplaceable element
in industrial
and scientific research. However, hydrogen isotope separation still
remains a huge challenge due to the identical physicochemical properties
of the isotopes. In this paper, a partially fluorinated metal–organic
framework (MOF) with copper, a so-called FMOFCu, was investigated
to determine the separation efficiency and capacity of the framework
for deuterium extraction from a hydrogen isotope mixture. The unique
structure of this porous material consists of a trimodal pore system
with large tubular cavities connected through a smaller cavity with
bottleneck apertures with a size of 3.6 Å plus a third hidden
cavity connected by an even smaller aperture of 2.5 Å. Depending
on the temperature, these two apertures show a gate-opening effect
and the cavities get successively accessible for hydrogen with increasing
temperature. Thermal desorption spectroscopy (TDS) measurements indicate
that the locally flexible MOF can separate D2 from anisotope
mixture efficiently, with a selectivity of 14 at 25 K and 4 at 77
K.
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