Article pubs.acs.org/JPCC
Probing the Guest-Mediated Structural Mobility in the UiO-66(Zr) Framework by 2H NMR Spectroscopy Alexander E. Khudozhitkov,†,‡ Hervé Jobic,§ Daniil I. Kolokolov,*,†,‡ Dieter Freude,∥ Jürgen Haase,∥ and Alexander G. Stepanov*,†,‡ †
Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia ‡ Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090, Russia § Institut de Recherches sur la Catalyse et l’Environnement de Lyon, CNRS, Université de Lyon, 2. Av. A. Einstein, 69626 Villeurbanne, France ∥ Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnéstrasse 5, 04103 Leipzig, Germany S Supporting Information *
ABSTRACT: The solid-state 2H NMR technique (analysis of both the spectrum line shape and the spin−lattice relaxation) was used to probe both slow and fast dynamical modes of the phenylene fragments of terephthalate linkers of the UiO-66(Zr) framework affected by the presence of benzene guest in the pores of the material. Such approach allowed us to probe different motions within a broad range of time scale, 10−3−10−11 s. The internal dynamics in the UiO-66(Zr) framework is represented by torsional motions of the phenylene fragment of the linker including 2-site 180° flips (πflips) of the plane of the phenylene ring and its restricted librations. In the presence of benzene loaded in the MOF pores the rate of π-flips decreases essentially and the activation barrier for this motion increases. The activation barrier has been found to increase almost in a linear fashion on benzene loading. Such observation is surprisingly unique among other MOFs with mobile linkers, like MIL-53(Al) or MOF-5. The fast librational motion occurs on a scale of ∼1010 Hz and shows no notable dependence on the guest loading. It has been established that anisotropy of T1 relaxation of the 2H NMR powder pattern of the phenylene fragments is especially sensitive to the librational motion when this motion is in a range of 107−1011 Hz. Within this range of libration frequencies, analysis of the anisotropic spin−lattice (T1) relaxation allows quantitative estimation of the rate of librational motion.
1. INTRODUCTION
new materials and the search for potential applications of the existing ones. There are several strategies to control the structural dynamics in MOFs, including synthetic modification of the linkers or the frameworks itself,14,15 temperature regulation, and finally guests inclusion.16 All three strategies are currently applied to control the structural dynamics. However, if we are interested in particular features of already available systems only the last two options are in the focus. Among the last two, guest introduction offers more options in controlling the internal dynamics by varying either the chemical nature of the guest or the guest loading. There are a number of investigations of guest-mediated internal dynamics in MOFs and related materials.16−20 It was shown that the chemical nature of the guest and the guest loading could influence the linker rotational dynamics in different ways. For example, it was shown for the ordered porous molecular crystal with phenylenes as the mobile
Porous metal−organic frameworks (MOFs) constitute a class of solid materials formed by inorganic nodes (metal cations or metal oxide clusters) bridged together by organic linkers in a regular manner. Such composition makes MOFs unique among other crystalline porous solids because they combine the ordered nature of a crystal and the flexibility of a polymer framework, exhibiting a pronounced local mobility of its building units. This internal structural mobility mediates many fascinating MOFs properties such as optical1−3 and dielectrical response,4 the conductivity,5−7 the adsorption, and the molecular transport of chemical species confined inside their pores. Since the walls of the MOFs pores and the windows interconnecting them are constituted by the linkers, their potential mobility, including rotational and librational motions, could strongly affect both the morphology and the effective size of the frameworks inner space. Indeed, recent studies of the guests mobility in different MOFs have already confirmed the essential role of the framework flexibility in the molecular transport.8−13 Hence, characterization and control of the structural dynamics in MOFs is key for the rational design of © XXXX American Chemical Society
Received: April 6, 2017 Revised: May 4, 2017 Published: May 5, 2017 A
DOI: 10.1021/acs.jpcc.7b03259 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
altered notably only when the guest occupies the window between the pores, resulting in the change of the linker librational dynamics in a step-like manner.21 A different example is MIL-53(Al) (see Figure 1b), where the strong effect of xylene guest on the rotation of the phenylene fragment of terephthalate linker was observed only after saturation with guest molecules.20 Further systematic studies are needed to clarify the general regularities of the effect of the guest molecules on the rotational motion of the MOFs linkers. The study of the effect of progressive guest loading could be of particular interest. The framework of UiO-66(Zr) consists of tetrahedral- and octahedral-type cages of 8 and 11 Å diameter, interconnected by windows, with dimensions similar to those in ZIF-8 (see Figure 1a and 1c). However, since the phenylene fragments of terephthalate linkers that form these windows are turned out of the windows plane (like in MIL-53(Al), see Figure 1b and 1c) the aperture is larger, about 5−7 Å.22 Such topology allows UiO-66(Zr) to show remarkable adsorption/separation properties8,23 and the ability to host very large molecules like xylenes or certain drugs (caffeine, ibuprofen).15,24 In fact, the phenylene fragments in UiO-66(Zr) are remarkably mobile,15,25 capable to perform 180° 2-site flips (π-flips) around its C2 symmetry axis (Figure 1d). Additionally, during such rotations the phenylene rings are crossing the window plane, shrinking its effective aperture to a great extent. If fast enough, this process could effectively control the migration of the guests within the framework. In this regard, the UiO-66(Zr) could be considered as a convenient model to study the effects of controllable guest adsorption on the structural mobility in MOFs. As a guest molecule, benzene appears to be a good model candidate as it is relatively large with a size comparable to the window’s aperture. It is the simplest aromatic compound capable of having π−π interactions with the aromatic ring of the phenylene fragment of the terephthalate linker. In the present work we investigate the dynamics of the UiO66(Zr) phenylene fragments in the presence of benzene with solid-state 2H NMR in a broad range of characteristic times.17,18,26,27 The low natural abundance of the 2H isotope requires substitution of protium for deuterium in phenylene fragments. This does not affect the torsional dynamics of the phenylenes, because the electronic structure of the linker or its effective size is not notably changed. We report on the experimental observation of the slow and fast dynamical modes of the UiO-66(Zr) framework affected by benzene guest with different loading. The slow rotational modes are probed by the line shape analysis of the 2H NMR spectrum. The fast libration modes are investigated by analysis of the 2H NMR spin−lattice relaxation time.
fragments that the exposure of the material to a relatively low quantity of I2 vapor decreased the rotation rate by 104 times.18 In the case of ZIF-8 (see Figure 1a) the linker’s mobility is
2. EXPERIMENTAL SECTION 2.1. Materials. The synthesis and activation of deuterated UiO-66(Zr) was performed as reported earlier,22 with deuterated terephthalic acid as a reactant. 2.2. 2H NMR Experiment. 2H NMR experiments were performed at the Larmor frequency ω0/2π = 61.424 MHz on a Bruker Avance-400 spectrometer using a high-power probe with a 5 mm horizontal solenoid coil. All 2H NMR spectra were obtained by a Fourier transform of a quadrature-detected and phase-cycled quadrupole echo after two phase-alternating 90° pulses in the pulse sequence (90°x − τ − 90°y − τ − acquisition − t), where τ = 20 μs and t is a repetition time of the sequence during accumulation of the NMR signal.28 The duration of the
Figure 1. (a) ZIF-8 framework pores are connected by very narrow windows (∼3.4 Å) composed by 2-methylimidazolate linkers that normally lie in the window plane, almost closing the window. (b) Pores of MIL-53 (Al) represent 1D channels composed by terephthalate linkers orthogonal to the channels cross section. (c) Pores of UiO-66(Zr) are connected by windows composed by terephthalate linkers with the phenylene fragment partially turned out of the windows plane. (d) Plane of the phenylene fragment in UiO66(Zr) exhibits relatively slow 2-site 180° flips and much faster local restricted librations about the C2 symmetry axis. B
DOI: 10.1021/acs.jpcc.7b03259 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C π/2 pulse was 1.6 μs. Spectra were typically obtained with 32− 1000 scans with a repetition time ranging from 1 to 5 s. Inversion−recovery experiments for measurements of spin− lattice relaxation times (T1) were carried out using the pulse sequence 180°x − tv − 90°x − τ − 90° y − τ − acquisition − t (1)
where tv was the variable delay between the 180° and the 90° pulses. The repetition time t was always longer than 5-fold of the obtained relaxation time T1. The temperature of the samples was controlled with a flow of nitrogen gas by a variable-temperature unit BVT-3000 with a precision of about 1 K. The sample was allowed to equilibrate at least 15 min at the temperature of the experiment before the NMR signal was acquired. Modeling of spectra line shape and spin−lattice relaxation was performed with a homemade FORTRAN program based on the standard formalism.21,29−31 2.3. 2H NMR Sample Preparation. In order to prepare the sample for the NMR experiments, 0.08 g of UiO-66(Zr) powder was loaded in a 5 mm (o.d.) glass tube and connected to a vacuum line. The sample was then heated up to 503 K and kept at this temperature for 24 h under vacuum to a final pressure above the sample of 10−2 Pa. After cooling back to room temperature, the material was exposed to the previously degassed benzene vapors in the calibrated volume. The adsorbate was condensed on the sample by cooling it at liquid nitrogen temperature. The quantity of adsorbed benzene corresponded to 12 molecules per unit cell (molecules/uc) (∼1 molecules per UiO-66(Zr) cage) and 22 molecules/uc (maximum loading capacity). After adsorption, the neck of the tube was sealed off, while the lower part of the glass tube was held in liquid nitrogen in order to prevent sample heating by the flame. Finally, the sample was heated for 20 h at 373 K to get a uniform distribution of molecules over the pore void of the studied material.
Figure 2. 2H NMR experimental (left) and simulated (right) spectra of UiO-66(Zr) loaded with 12 molecules of benzene per unit cell. T (in Kelvin) is the measuring temperature, and kfm is the mean flipping rate constant.
3. RESULTS AND DISCUSSION We first consider the motion occurring in the slower time domain (10−3−10−7 s) for UiO-66(Zr) phenylene fragments by monitoring the evolution with the temperature of the 2H NMR line shape. The experimental results for evolution of the 2H NMR line shape for different benzene loadings are given in Figures 2 and 3. In both cases the experimental results show the evolution of the spectra line shapes from an axially symmetric Pake-powder pattern with quadrupole constant Q0 = 176 ± 2 kHz and asymmetry parameter η = 0, typical for almost immobile phenylenes (τC ≫ Q0−1 ≈ 5 × 10−6 s at T < 253 K), up to an averaged anisotropic pattern with effective quadrupole constant Q1 ≈ 80 ± 2 kHz and remarkably high asymmetry parameter η ≈ 0.8 at T > 381 K. Such pattern is characteristics of the 2-site 180° flips (π-flips) of the plane of the phenylene ring about the C2 symmetry axis in the fast motional regime (τC ≪ Q0−1 ≈ 5 × 10−6 s). The decrease of the effective quadrupolar constant Q1 by 10% compared to the value expected for π-flips within the phenylene geometry is indicative of the presence of additional fast librations of the plane of the phenylene ring about the same symmetry axis.25,32 Qualitatively the observed mobility is fully consistent with the previously reported dynamics in the guest-free UiO66(Zr).25
Figure 3. 2H NMR experimental (left) and simulated (right) spectra of UiO-66(Zr) loaded with 22 molecules of benzene per unit cell. T (in Kelvin) is the measuring temperature, and kfm is the mean flipping rate constant.
The progressive increase of the π-flips rate (characterized by the flipping rate constant kfm) upon increasing the temperature is reflected in increasing intensity of the central part of the spectrum. This simple observation can be used to estimate the guest’s influence on the phenylene dynamics. Figure 4 shows that an increase of the loading of benzene in the MOF pores results in a notable decrease of the phenylene fragment flipping rate compared to the guest-free case under similar conditions. C
DOI: 10.1021/acs.jpcc.7b03259 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
comparison of the phenylene dynamics for different loadings of benzene and to quantify their hindering we can consider only the mean flipping rate constant kfm. For instance, at 315 K (Figure 4), the loading of 12 molecules/uc results in a decrease of the flipping rate kfm by 11 times and the saturation of the MOF pores with 22 molecules/uc slows down the rotation additionally by 4 times, i.e., we observe indeed a progressive and notable dependence of the flipping rate constant kfm on the concentration of the loaded benzene. More details of the effect of benzene loading on the phenylene flipping rate are shown in Figure 5. Figure 4. Effect of benzene loading on the phenylene fragment line shape and the rate of its π-flips at 315 K. Unloaded guest-free UiO66(Zr) (a); benzene loading of 12 (b) and 22 molecules/uc (c). Corresponding rate constants kfm (in Hz units) are shown above each spectrum.
Similarly to the guest-free case, the motional model used to fit the experimental patterns includes two motions of the phenylene fragment about the C2 axis: the π-flips and the local librations restricted within a sector 2φlib (see Figure 1d). The librational motion characterized by the rate constant klib is assumed to be very fast (klib ≫ Q0), and thus, its effect on the line shape is parametrized only by the libration angle φlib. In contrast, the π-flips have a fixed geometry of the motion but somewhat more complex behavior of the rate factor: at each temperature the π-flips are characterized by a distribution of the flipping rate constant k.25,33 This distribution in the flipping rates was accounted for by the presence of the defects in the UiO-66(Zr) framework, namely, by the absence of each 1/12 of the carboxylic linkers at each Zr cluster.22,34 As a result, at a given temperature the π-flips are defined by the mean flipping rate constant kfm and the distribution width for the rate constants σ. Our recent investigation of the guest-free material25 showed that a log-normal33 distribution for the rate constants was an appropriate model for description of the π-flipping motion. The simulated patterns presented in Figures 2 and 3 show that such model fits very well with UiO-66(Zr) loaded with benzene guests as well. The main fitting results are given in Table 1. Let us first consider π-flips. Similar to the guest-free material25 the rate constant distribution width σ is equal to 5. This is expected as it is mainly related to the inhomogeneity inside the framework itself and should be identical for the material prepared under exactly the same conditions. For a
Figure 5. Arrhenius plots for the mean rate constants kfm of π-flipping motion of UiO-66(Zr). Guest-free material (data of ref 25) (○). Material loaded with benzene: 12 (□) and 22 molecules/uc (Δ).
Figure 5 clearly shows that in the presence of benzene the flipping motion is slowing down with increasing benzene loading. Arrhenius plots give the following parameters for the rate constants kfm: E12 = 38.5 ± 1.5 kJ mol−1 and k012 = (2 ± 1) × 1012 Hz for phenylene π-flips with benzene loading of 12 molecules/uc; E22 = 48.0 ± 1.5 kJ mol−1 and k022 = (6 ± 3) × 1012 Hz for the flips with benzene loading of 22 molecules/uc. For guest-free material the following parameters were found earlier in ref 25; E = 30 ± 2 kJ mol−1 and k0 = (0.5 ± 0.8) × 1012 s−1. A comparison of these parameters shows that the main effect of increasing benzene loading consists in a slow down of the rotation rate and an increase of the activation energy for phenylene π-flips. The pre-exponential factor does not change strongly. We introduce the pore occupancy factor α = n/Nmax, where n is the actual loading and Nmax is the maximum loading for a given molecular species. Then we can compare the guest loading dependence of the rotation barrier for different systems. In the present case the dependence is almost linear (see Figure 6a). This is in fact not a trivial observation, as for the recently studied MIL-53(Al) filled with xylene20 the dependence is almost step-like (see Figure 6b), i.e., a notable change in the activation barrier occurs only when the pores are fully saturated with the xylene guest. Thus, the UiO-66(Zr) MOF offers a framework topology with an example of an almost linear relationship between the activation barrier of the phenylene fragment rotation and the benzene−guest loading: the more benzene loaded the higher the activation barrier for phenylene π-flips. We further performed a more detailed analysis of the fast librational modes. As mentioned above (see Table 1) the
Table 1. Effect of Different Benzene Loading on the Flipping Rate Constant kfm and the Libration Angle Amplitude φlib for Phenylene Fragments of UiO-66(Zr) 12 C6H6 molecules/uc
22 C6H6 molecules/uc
T, K
kfm × 10−3, Hz
φlib
kfm × 10−3, Hz
φlib
253 273 298 315 337 359 381
8 50 150 300 800 2500 5400
