Impact of the Flexible
Character of MIL-88 Iron(III)
Dicarboxylates on the Adsorption of <i>n</i>‑Alkanes
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Abstract
Adsorption of <i>n</i>-alkane vapors was performed
to
probe the unusual highly flexible character of a series of iron(III)
dicarboxylate materials of the MIL-88 structure type. In agreement
with the presence of strong intraframework interactions within the
dried closed pores form of MIL-88, it appears first that an increase
of the size and aromaticity of the spacer makes it more difficult
to adsorb alkanes at room temperature. Thus, this led to a high level
of adsorption in the iron fumarate MIL-88A and poor levels in the
terephthalate and naphthalenedicarboxylate based MIL-88(B and C, respectively).
Second, upon increase in the length of the alkane, diffusion limitations
of the guest occur within the very narrow pores, also illustrated
through kinetics of adsorption measurements, which result in an overall
decrease in the adsorption capacity. Noteworthy, the swelling of the
flexible non modified MIL-88 solids occurs only for the MIL-88A sample,
because of the number and orientation of aromatic rings that are arranged
in trimers within the MIL-88 structures and, therefore, making those
more difficult to open than those where the rings are arranged in
dimers such as the metal terephthalate MIL-53 structures. Interestingly,
modification of the organic linkers by grafting several bulky functional
groups (2CF<sub>3</sub>, 4CH<sub>3</sub>) makes the adsorption of <i>n-</i>alkanes easier because of a strong decrease in interactions
within these trimers associated with a lower pore contraction upon
drying, while the substitution of a hydrogen atom by a bromine one
on the spacer proved to be not sufficient for an improvement of the
adsorbed amounts
