Impact of the Flexible Character of MIL-88 Iron(III) Dicarboxylates on the Adsorption of n-Alkanes

Abstract

Adsorption of n-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 (2CF3, 4CH3) makes the adsorption of n-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.