UTSA-74: A MOF-74 Isomer with Two Accessible Binding Sites per Metal Center for Highly Selective Gas Separation

A new
metal–organic framework Zn2(H2O)­(dobdc)·0.5­(H2O) (UTSA-74, H4dobdc
= 2,5-dioxido-1,4-benzenedicarboxylic acid), Zn-MOF-74/CPO-27-Zn isomer,
has been synthesized and structurally characterized. It has a novel
four coordinated fgl topology with one-dimensional channels
of about 8.0 Å. Unlike metal sites in the well-established MOF-74
with a rod-packing structure in which each of them is in a five coordinate
square pyramidal coordination geometry, there are two different Zn2+ sites within the binuclear secondary building units in UTSA-74 in which one of them (Zn1) is in a tetrahedral while
another (Zn2) in an octahedral coordination geometry. After activation,
the two axial water molecules on Zn2 sites can be removed, generating UTSA-74a with two accessible gas binding sites per Zn2 ion.
Accordingly, UTSA-74a takes up a moderately high and
comparable amount of acetylene (145 cm3/cm3)
to Zn-MOF-74. Interestingly, the accessible Zn2+ sites
in UTSA-74a are bridged by carbon dioxide molecules instead
of being terminally bound in Zn-MOF-74, so UTSA-74a adsorbs
a much smaller amount of carbon dioxide (90 cm3/cm3) than Zn-MOF-74 (146 cm3/cm3) at room
temperature and 1 bar, leading to a superior MOF material for highly
selective C2H2/CO2 separation. X-ray
crystal structures, gas sorption isotherms, molecular modeling, and
simulated and experimental breakthroughs comprehensively support this
result.