Selective Sorption of Organic Molecules by the Flexible Porous Hybrid Metal−Organic Framework MIL-53(Fe) Controlled by Various Host−Guest Interactions

Abstract

The structures of four iron(III) carboxylate materials with the MIL-53 structure and the guest molecules water, pyridine, or 2,6-dimethylpyridine (2,6-lutidine) have been solved and refined from high-resolution powder X-ray diffraction data. These materials have porous structures constructed from infinite chains of trans-corner-shared {FeO 4 (OH,F) 2 } octahedra cross-linked by benzenedicarboxylate ligands. With pyridine, the unit cell is partially expanded with respect to the hydrated phase, with hydrogen bonds between N donors and OH framework atoms, whereas with lutidine, water is co-sorbed to give a fully expanded version of the MIL-53(Fe) structure in which water molecules bridge the N donors of the organic molecule and the framework OH groups by hydrogen bonding. Careful dehydration of MIL-53(Fe)[2,6-lutidine,H 2 O] provides a new material, MIL-53(Fe)[2,6-lutidine 0.5 ], with an intermediate unit cell volume: in this case no host-guest hydrogen bonds are formed. Time-resolved in situ energy-dispersive X-ray diffraction shows that when exposed to an aqueous mixture of pyridine and 2,6-lutidine the hydrated MIL-53(Fe) takes up both guest molecules initially to give two distinct phases, but the ultimate product is only MIL-53(Fe)[2,6-lutidine,H 2 O].

Theoretical study of conformational disorder and selective adsorption of small organic molecules in the flexible metal-organic framework material MIL-53-Fe

“…In another work by the same authors, it was found that while hydrated MIL-53-Fe adsorbed both pyridine and 2,6-lutidine (2,6-dimethylpyridine) initially, it selectively adsorbed 2,6-lutidine over pyridine with time. 17 This example illustrates the importance of gaining a molecular level understanding of the interactions between a MOF material and adsorbed organic molecules. In this work, by using state-of-the-art density functional theory (DFT) methods, we study the adsorption of a sub-set of the organic molecules studied by Millange et al, 11 including quinone (1,4-benzoquinone), pyridine, and 2,6-lutidine (shown in Figure 1), in the MIL-53-Fe structure (depicted in Figure 2).…”

“…In the case of single component adsorption quinone and pyridine in MIL-53-Fe, for example, for quinone, two ordered phases were identified at two different temperatures, 20 and for pyridine, one ordered phase was identified at room temperature, and there was evidence of another phase between 363~423 K which could not be indexed due to poor crystallinity. 17 To account for these metastable adsorption phases, which cannot be easily identified in experiments but are still important in relation to the applications being considered for MOF materials, in addition to the ground state adsorption configuration which has already been solved by experiment for each adsorbate molecule, we also consider several other possible adsorption configurations.…”

“…17 Unlike the case of pyridine, where pyridine molecules displace occluded water molecules in MIL-53-Fe, 17 upon exposure of MIL-53-Fe[H2O] to 2,6-lutidine, the water molecules remain in the channel and act as a bridge between MIL-53-Fe and 2,6-lutidine. 17 Millange et al concluded this was due to the presence of methyl groups in 2,6-lutidine (see Figure 1) which prevents direct hydrogen bonding between MIL-53-Fe and 2,6-lutidine. 17 It remains unclear why the same situation did not occur in the adsorption of pyridine molecules in MIL-53-Fe.…”

“…17 Millange et al concluded this was due to the presence of methyl groups in 2,6-lutidine (see Figure 1) which prevents direct hydrogen bonding between MIL-53-Fe and 2,6-lutidine. 17 It remains unclear why the same situation did not occur in the adsorption of pyridine molecules in MIL-53-Fe. In this study, we seek to explain this observation by performing high-level atomistic, density functional theory simulations.…”

“…We note that ~20% of OH -inorganic linkers in the experimental sample of MIL-53-Fe were occupied by isovalent F -ions. 17 As a first order approximation, all models of MIL-53-Fe contain only OH -inorganic linkers; we assume that because the F -ions are likely to be disordered and are minority species, the influence of guest-hydroxide interactions is likely to control the observed response of the lattice. Hence possible host-guest interactions include: (1) -, (2) CH-, and (3) hydrogen bonds.…”