Abstract: 1.Introduction MIL-53 and MIL-101 are interesting metal−organic frameworks (MOFs) with a “breathing” framework or possessing coordinatively unsaturated metal sites (CUS) capable to coordinate organic substrates for catalytic transformations. It is suggested that the processes of hydrocarbon adsorption, diffusion, separation and catalysis by these materials are strongly influenced by flexibility of the framework and fastly moving phenylene fragments of terephthalate (TF) linkers. To govern the mentioned processes we need to learn how to monitor the flexibility of the framework and identify the specific effects of particular hydrocarbons adsorbed in the pores of MOFs or coordinated to CUS on the rotational motion of phenylene fragments. Here we demonstrate that 2H NMR can be used to effectively monitor framework flexibility as well as to identify the effect of guest loading in the pores and the selective coordination of donor molecules to CUS on rotational motion of TF linkers. 2.Experimental Deuterated analogues of MIL-53 and MIL-101 were prepared similar to earlier well known procedures.1-2 The sampes of the MOFs materials were activated under vacuum at 373-523 K and further guest molecules were adsorbed. The activated samples were sealed under vacuum in 5 mm diameter glass tubes for NMR measurements. 2H NMR spectra were recorded on a Bruker Avance 400 spectrometer with the use of an Exorcycled quadrupole-echo sequence (90X–τ1–90φ – τ2–acq–t).3 The line shape simulation protocol for investigation of phenylene mobility in MOFs with paramagnetic centers was previously developed in our group and reported in Ref. 3. 3.Results and Discussion Analysis of evolution of 2H NMR line shape with temperature shows that phenylene fragmentsof TF linkers in MIL-53 (Cr) performs movements by π-flip jumping mechanism with the rate k= 20-8000 kHz at 320-500 K and activation energy ECr= 41 kJmol-1. 3 We have established that theflexibility of the MIL-53(Al) framework can be detected by monitoring the evolution of the dynamicsof TF phenylenes of the MOF with temperature.4 This dynamics is very sensitive to the crystallinestate (LP or NP state) of MIL- 53(Al) and loading of xylene and follows guest molecule packingdensity. The rate is lower and rotational barrier is higher for the NP state compared to the sameparameters in the LP state. The data obtained offer an opportunity to control the dynamics ofterephthalate phenylenes in MIL-53 by guest molecule loading and monitor framework flexibility ofthis MOF by analyzing the dynamics of the framework fragments. 2H NMR spin alignment echo technique allowed us to probe ultraslow structural mobility (0.1−1 kHz) in MIL-53(Al) saturated with ortho or para isomers of xylene. The phenylenes’ rotation performs 1 order of magnitude slower in the presence of o-xylene (kortho = 70 Hz) compared to the same rotation in the presence of the other isomer, p-xylene (kpara = 800 Hz) at T = 373 K. This is rationalized by a stronger interaction of ortho isomer with the linker than the para isomer. The finding offers an understanding of the molecular mechanism of p- and o-xylene separation by MIL-53: stronger interaction of o-xylene with organic linker compared to p-xylene provides separation of these isomers on MIL-53.5 We have characterized the effect of electron-donor guest molecules on structural mobility in MIL-101(Cr). In a guest free material the mobile phenylene fragments of the framework populated two fractions with notably different kinetic parameters, exhibiting fast and slow rotations. Two fractions are accounted for by non equivalent disposal of TF linkers around Cr3O- trimer. Coordination of the guest donor molecule to CUS dramatically decreases the linkers torsional barriers for both rotational modes and enhances the rotation rate. Kinetic parameters of the rotation represent a sensor of interaction of the particular donor guest with CUS in the MOFs. We have evidenced that if water occupies CUS, the other guests like tert-butanol or cyclohexanone are not capable to effectively compete with water for CUS. This finding has to be taken into account when designing the catalytic process in MIL-101 with a potential water presence in the system. 4.Conclusions 2H NMR can be effectively used to characterize the rate and the mechanism of phenylenefragments motion of TF linkers in MIL-53 and MIL-101 MOFs. The flexibility of the framework, i.e., large pore (LP) and narrow pore (NP) crystalline state interconversions, can be monitored byfollowing the evolution of phenylene fragments dynamics with temperature by 2H solid-state NMR.2H NMR spin alignment echo technique applied to MIL-53(Al) saturated with ortho or para isomersof xylene allowed to probe ultraslow structural mobility in this material and clarify molecularmechanism of p- and o-xylene separation by MIL-53. The found effects of the electron-donor guestcoordinated to CUS in MIL-101 on TF linker rotational motion represents the basis for developingthe strategy for ruling and controlling the linkers rotation in MOFs with CUS. Acknowledgement. This work was supported by Russian Foundation for Basic Research, project no. 18-29-04009.

Cite: Stepanov A.G. , Khudozhitkov A.E. , Kolokolov D.I.
Framework Mobility in MIL-101 and MIL-53 Investigated by 2H solid-state NMR
19th International Zeolite Conference 07-12 Jul 2019