Liquid-Phase Adsorption and Separation of Xylene Isomers by the Flexible Porous Metal–Organic Framework MIL-53(Fe)

We report a study of the use of the porous metal–organic
framework material MIL-53­(Fe), FeIII(OH)0.8F0.2[O2C–C6H4–CO2], for the separation of BTEX mixtures (benzene, toluene,
ethylbenzene, and the three xylene isomers). Crystal structures of
the three host:guest materials MIL-53­(Fe)­[xylene], where xylene =
the ortho, meta, or para isomer of dimethylbenzene, have been solved and refined from powder
X-ray diffraction. Each exhibits a fully expanded form with a variety
of host:guest and guest:guest interactions responsible for stabilizing
the structure. While the ortho- and meta- isomers present a similar arrangement when occluded in the MIL-53
host, the para-xylene shows a distinctly different
set of interactions with the host. Upon thermal treatment, xylenes
are partially lost to give crystalline phases MIL-53­(Fe)­[xylene]0.5, the structures of which have also been solved. The kinetics
of uptake of each xylene by MIL-53­(Fe)­[H2O], in which the
water is replaced by the organic guest, have been studied using time-resolved
energy-dispersive X-ray diffraction: this shows differences in kinetics
of the adsorption of the three isomers. Under chromatographic conditions
in heptane at 293 K, anhydrous MIL-53­(Fe) is able to separate the
three xylene isomers with elution of the para-xylene
before the other two isomers, and at 323 K the host is able to resolve
all components of the BTEX mixture.