New V<sup>IV</sup>-Based Metal–Organic Framework Having Framework Flexibility and High CO<sub>2</sub> Adsorption Capacity
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Abstract
A vanadium based metal–organic framework (MOF),
VO(BPDC)
(BPDC<sup>2–</sup> = biphenyl-4,4′-dicarboxylate), adopting
an expanded MIL-47 structure type, has been synthesized via solvothermal
and microwave methods. Its structural and gas/vapor sorption properties
have been studied. This compound displays a distinct breathing effect
toward certain adsorptives at workable temperatures. The sorption
isotherms of CO<sub>2</sub> and CH<sub>4</sub> indicate a different
sorption behavior at specific temperatures. In situ synchrotron X-ray
powder diffraction measurements and molecular simulations have been
utilized to characterize the structural transition. The experimental
measurements clearly suggest the existence of both narrow pore and
large pore forms. A free energy profile along the pore angle was computationally
determined for the empty host framework. Apart from a regular large
pore and a regular narrow pore form, an overstretched narrow pore
form has also been found. Additionally, a variety of spectroscopic
techniques combined with N<sub>2</sub> adsorption/desorption isotherms
measured at 77 K demonstrate that the existence of the mixed oxidation
states V<sup>III</sup>/V<sup>IV</sup> in the titled MOF structure
compared to pure V<sup>IV</sup> increases the difficulty in triggering
the flexibility of the framework