Metal organic frameworks (MOFs) consist of metal oxide cores linked by multifunctional organic linkers, forming crystal-like structures with well-defined pore size. They are highly promising materials in a large variety of applications including gas storage, solvent separation, drug delivery, and catalysis. The guest-host interaction is of key importance for all these applications and therefore its detailed understanding is fundamental for designing efficient MOF materials.
This work investigates and compares for the first time the interaction of C5 alkane and ketone in a representative MOF material, the UiO-66(Zr), at ambient temperature by solid-state NMR. NMR methods have been already successfully applied to the study of structure, dynamics, and flexibility of different MOFs but they have been employed up to date only in very few cases for characterizing their interaction with liquid organic compounds largely at small loadings.
Pentane and 2-pentanone were chosen to represent C5 alkane and ketone. Both molecules are comparable in size, yet contain different functional groups and therefore are expected to have different interactions with the MOF materials. Furthermore, the guest-host interaction can be impacted by the structure of the MOF itself. In particular, UiO-66 is composed of smaller tetrahedral pores and larger octahedral pores. The adsorbent−adsorbate interaction was investigated in detail by a combination of 1H and 13C NMR MAS spectroscopy, 1H 2D NOESY experiments, and 1H site-selective relaxometry. Moreover, various solvent concentrations corresponding to different amounts of solvent molecules per MOF unit cell were also studied.
The obtained results show that the interaction of pentane with UiO-66 is strongly concentration dependent. At very low concentrations the pentane signals are shifted to positions towards lower ppm values with respect to the positions of the signals of the free pentane. The peak to peak distances between two nearby signals are decreased compared to the corresponding distances in the free pentane. With increased pentane concentration above a well-defined amount, the appearance of a second and then a third pentane species is observed. Both species have signals at the same chemical shifts as that of free pentane. Supported by numerical analysis, relaxometry results, and computational simulation, our data indicate that pentane can be found at low concentrations only in the smaller tetrahedral pores. With increasing concentration, the pentane molecules start to occupy also the larger octahedral pores and later on the inter-grain space. To our knowledge, this is the first time that three different types of solvent species are observed in a MOF material. The findings for pentane are compared to the results for 2-pentanone. In particular it was found that the peak to peak distance between two nearby signals for the first species is changed in such a way that only three signals are observable instead of the usual four. These findings contribute to a understanding of the adsorption mechanisms of small alkane and ketone molecules inside MOF pores.
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