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Achieving large volumetric gas storage capacity in metal-organic frameworks by kinetic trapping: A case study of xenon loading in MFU-4 Hana Bunzen, Felicitas Kolbe, Andreas Kalytta-Mewes, German Sastre, Eike Brunner, and Dirk Volkmer J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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Journal of the American Chemical Society

Achieving large volumetric gas storage capacity in metal-organic frameworks by kinetic trapping: A case study of xenon loading in MFU-4 Hana Bunzen,†,* Felicitas Kolbe,‡ Andreas Kalytta-Mewes,† German Sastre,§ Eike Brunner,‡ and Dirk Volkmer†,* †

Chair of Solid State and Materials Chemistry, Institute of Physics, University of Augsburg, Universitätsstraße 1, 86159 Augsburg, Germany ‡ Department of Bioanalytical Chemistry, Technische Universität Dresden, Bergstraße 66, 01062 Dresden, Germany § Instituto de Tecnologia Quimica CSIC-UPV, Universidad Politecnica de Valencia, Av. Los Naranjos s/n, 46022 Valencia, Spain ABSTRACT: One of the main problems of gas storage in porous materials is that many molecules of interest adsorb too weakly to be retained effectively. To enhance gas storage in metal-organic frameworks (MOFs), we propose to use kinetic trapping, i.e. a process where the guest gas is captured in the voids at loading conditions and not released immediately at normal conditions. In this approach, the diffusion-limiting pore size and the framework flexibility have to be matched to the gas, requiring flexible pore apertures to be smaller than the van der Waals diameter of the trapped guest. We selected the Metal-Organic Framework Ulm University-4 (MFU-4) with a pore aperture of 2.52 Å as a model coordination framework and used it for storage of xenon (with van der Waals diameter of 4.4 Å). Although xenon atoms are substantially larger than the MOF pore aperture, MFU-4 could be loaded with xenon by applying moderately high gas pressures. This is demonstrated to be due to the pore flexibility as confirmed by computational studies. The xenon loading could be tuned (from 0 wt% to more than 44.5 wt%) by changing the loading parameters such as pressure, temperature and time, and the xenon atoms remained inside the pores upon exposing the material to air atmosphere at room temperature. To understand the material behavior, TGA, XRPD, 129Xe NMR spectroscopy and computational studies were carried out.

■ INTRODUCTION

Metal-organic frameworks (MOFs), introduced in the mid1990s,1,2 are crystalline porous materials with extremely high surface areas (up to around 7000 m2/g).3 This makes them attractive materials for adsorption-based applications.4-6 In these processes, the adsorption phenomena depend on the material properties and the character of the guest molecules. Despite the possibility of tailoring the MOF pore size, shape and functionality,7,8 many of the frameworks interact with gas molecules too weakly to become retained efficiently. Herein we propose kinetic trapping of guest molecules as an alternative approach to overcome this problem. Kinetic trapping occurs when the molecular guest is captured in the voids of a porous material at high pressure and temperature and from which it cannot be released at normal conditions.9 This is due to the activation energy barrier for gas diffusion of the entrapped sorbate within the ultra-narrow pores. To make use of this concept for gas storage applications, the diffusion-limiting pore size and the flexibility/rigidity of the porous framework compound have to be matched to the gas, requiring pore apertures smaller than the van der Waals diameter of the trapped guest. To demonstrate kinetic trapping as an efficient approach for gas storage in MOFs, we selected a MOF called MFU-410 to trap xenon gas. MFU-4 has been previously reported to pro-

vide exceptional uptake selectivity for small molecules, e.g. H2/D2 separation11,12 and CO2/N2 separation13. One potential application, namely the utility of the size-selective properties of MFU-4 for gas sensing applications has been demonstrated very recently.14 Such behavior can be related to some basic structural properties of the framework: MFU-4 consists of benzobistriazolate ligands and Zn(II) ions and contains two types of cavities with diameters of 3.88 Å and 11.94 Å, which are connected by narrow (only 2.52 Å) pore apertures (Figure 1a). Despite the very narrow pore windows, MFU-4 exhibits high loading capacity due to the larger voids. The total potentially accessible void volume per unit cell was calculated to be as high as 5372 Å3 which is 53.1 % of the unit cell volume.10 Although the MOF can be classified as microporous based on the larger void diameter, the very narrow pore apertures makes it ultramicroporous15 (i.e. pores < 0.7 nm). This makes MFU-4 and MOFs with similar framework parameters perfect candidates for host materials for gas storage enhanced by kinetic trapping. As a guest, we selected xenon. This was not only due to its suitable van der Waals diameter (4.4 Å)16 which is substantially larger than the MOF pore aperture, but mainly because it is an attractive gas with a number of applications, including commercial lighting and its use in medicine.17,18 Furthermore, its adsorption processes and host-guest interactions in porous materials can be followed and analyzed by 129Xe NMR spec-

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troscopy.19-21 Although there have been numerous reports on gas capture and separation using MOFs,4-6 only a few publications have addressed xenon capture and separation (see references in ref. 22-24 and Table S3). However, none of them has reported a material capable of adsorbing a large amount of xenon and storing it at ambient temperature in air over a long period of time, as we show here. ■ RESULTS AND DISCUSSION

Xenon loading experiments. Despite the very narrow pore aperture (2.52 Å), the xenon guests filled the voids when samples of MFU-4 were exposed to xenon gas at elevated pressure with the framework remaining intact. The xenon loading could be influenced by changing parameters such as pressure, temperature and time (Table 1). At 30 bar, xenon atoms entered the voids even at room temperature (Sample 1 in Table 1). Upon raising the temperature, the amount of loaded xenon increased significantly (Samples 2-4 in Table 1) up to above 40 wt% for temperatures above 100 °C. A loading of 40 wt% means that 1 g of the material contains 0.4 g of xenon, i.e. 1.835·1021 atoms corresponding to 69 mL of xenon gas at normal conditions. By prolonging the loading time from 18 h to 48 and 72 h, the amount adsorbed did not change dramatically (Samples 3, 6 and 7 in Table 1) and also increasing the gas pressure (from 30 to 50 bar) did not influence the loading significantly (Samples 3 and 8 in Table 1). Moreover, increasing the temperature from 100 °C to 150 °C resulted in a decrease of the amount adsorbed (Sample 3 and 4 in Table 1). The highest loading achieved was 44.5 wt% (Sample 6 in Table 1) which represented a xenon density in the voids of 1.852 g/cm3. This is close to the density of liquid xenon at 273.2 K under its saturated vapor pressure (1.912 g/cm3) and much larger than the density of gaseous xenon at standard conditions (0.0058 g/cm3).17,25 Considering the time, energy and gas costs, the loading conditions of Sample 3 (Table 1) were selected as optimal and this sample was further analyzed using TG-MS (Figure S1c) and XRPD analyses (Figure S2b)

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and used for xenon release experiments. Xenon release experiments. Release of the xenon gas as a function of temperature was followed by thermogravimetric analysis (TGA) under nitrogen atmosphere for six different temperatures (Figure 1d) and the data were used to calculate an activation energy of 31.6 kJ/mol assuming Arrhenius-type behavior of activated transport (see Supporting Information for details), which requires the Xe atoms to surmount a welldefined energy maximum on the energy hypersurface for intracrystalline diffusion. The isothermal plots showed that the rate of xenon release was fast in the initial period and then became slower with time, indicating that the process was thermally activated with the interior atoms traveling a longer diffusion path length than those near the surface of the MOF particles. The measurements also confirmed that desorption from the MOF crystal was rather limited at room temperature and much easier at elevated ones. At room temperature in air, a gradual release over more than 42 days was observed (Figure 1c, and Figures S3 and S4). Additionally, if the sample was stored in a sealed container, the xenon release could be slowed down even more, for instance, from 20.0 wt% (i.e. 14.3 Xe/unit cell, Figure S4a) to 31.7 wt% (i.e. 26.7 Xe/unit cell, Figure S5) after 30 days, which offers good storage possibilities for the system. On the other hand, if a rapid xenon release is desired, it can be achieved by heating the guest loaded material to elevated temperatures, for instance 300 °C. Since the temperature range required for the guest release lay below the MOF decomposition temperature (Figure 1b), the framework structure was not affected by the heating and the material could be re-used for the xenon storage, as we demonstrated by repeating the xenon loading-release cycle three times (Figure S7). For comparison, if MFU-4 was loaded with krypton instead of xenon (Sample 11 in Table S2), the guest was completely released at room temperature within few hours (Figure S6) which is in good agreement with its smaller van der Waals diameter compared to xenon.

Table 1. Xenon loading into MFU-4 under various conditions. Sample name

Xenon pressure (bar)

Temp. (° C)

Time (h)

Xenon loaded (wt%)a

Number of Xe atoms per unit cell of MFU-4

Number of Xe atoms per voidb

Xenon loaded in 1 g of MFU-4 (mmol)

Xenon density in the voids (g/cm3)c

1

30

25

18

14.5

9.7

2.4

1.29

0.392

2

30

50

18

33.6

29.1

7.3

3.85

1.169.

3d

30

100

18

42.8±0.7

42.9±1.2

10.7±0.3

5.69±0.16

1.726±0.047

4

30

150

18

40.5

39.1

9.8

5.18

1.572

5

30

100

5

39.2

37.1

9.3

4.91

1.489

6

30

100

48

44.5

46.1

11.5

6.11

1.852

7

30

100

72

43.7

44.6

11.2

5.91

1.793

44.2

11.1

5.86

1.779

8

50

100

72

43.5

e

50

40

144

-

-

-

-

-

10e

36

25

144

-

-

-

-

-

9 a

Determined by TGA (Figure S1). Assuming all Xe atoms are located in the larger void (with four larger voids per unit cell of MFU-4). c Calculated for the total potentially accessible void volume of 5372 Å3 which corresponds to 53.1 % of the unit cell.10 d Average data from five loading experiments (Figure S1b). e Loading conditions used in the 129Xe NMR studies. b


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Figure 1. Overview of xenon release from MFU-4: (a) unit cell of MFU-4 with voids displayed; (b) thermogravimetric analysis of MFU-4 loaded with xenon (Sample 3) carried out under a nitrogen atmosphere at a heating rate of 10 K min−1; (c) xenon release from MFU-4 (Sample 3) stored at room temperature exposed to air atmosphere determined by TGA (for details see Figure S4 and Table S1); (d) xenon release from MFU-4 (Sample 3) at various temperatures followed by gravimetric analysis under nitrogen atmosphere. 129

Xe NMR experiments. To know more about the xenon occupancy of the voids, the gas loading and release were studied in situ by high pressure 129Xe NMR spectroscopy. The chemical shift of adsorbed xenon is structure- and surfacesensitive and depends on xenon-wall as well as xenon-xenon interactions.19,26 Hyperpolarized xenon produced by spinexchange optical pumping provides a sensitivity gain of several orders of magnitude and is particularly useful for low surface area materials and at low xenon densities.27-31 Therefore, 129 Xe NMR spectroscopy has developed into a widely used method to study various porous materials like zeolites, MOFs, nanochannels and similar structures and many others.19,20,32-41 In porous materials with well-interconnected voids, xenon atoms exchange rapidly between the different voids. A single, averaged signal is then usually observed. This is perceived for most zeolites and MOFs studied so far. In contrast, different lines are found for voids which are not interconnected by sufficiently large windows, thus preventing exchange. This behavior is already known for special zeolites such as zeolite NaA, in which the existence of different Xen-clusters causing separate lines in the NMR spectra was observed.42 The same behavior was later demonstrated in zeolite AgA too.43 The recorded spectra of xenon loaded MFU-4 indeed showed not only one averaged signal for adsorbed xenon but separate lines. This indicates the presence of different Xenclusters inside the voids and the absence of fast inter-cage

exchange processes. The spectrum of a sample prepared by loading MFU-4 with xenon at 50 bar and 40 °C for six days (Sample 9 in Table 1, Figure 2a) showed a signal for free xenon gas at 48.4 ppm which would correspond to a pressure of 54 bar at 298 K (the chemical shift of the gas-phase signal increases with increasing density, i.e. with increasing pressure).44-46 Several lines were resolved between 100 – 200 ppm due to different occupations of the voids, i.e. different Xenclusters (n: number of xenon atoms per void) were observed.42,43 The signal at the lowest chemical shift (126.9 ppm) can be assigned to a smaller Xen-cluster, since the chemical shift increases with the number of xenon atoms per cluster. After exposing the MOF to air, the signal for free xenon gas disappeared. Xenon slowly desorbed and the population distribution changed. The signals at lower chemical shifts became more intense, i.e. smaller Xen-clusters increasingly occurred inside the voids (Figure S8). Comparing the 129Xe spectra of Sample 9 and Sample 10, one could identify at least ten clearly resolved signals in the chemical shift range between 99.2 – 179.7 ppm (Figure 2b), i.e. at least ten different Xen-clusters. The shift difference ∆δ between the neighboring signals varied from 5.2 – 16.1 ppm and increased with increasing n (Figure 2c). Additionally, the spectrum measured ten days after air exposure showed a pronounced “shoulder” to lower chemical shifts, between 70.0 – 95.0 ppm. This indicated the presence of non-resolved signals



Figure 2. Xenon loading and release from MFU-4 followed by 129Xe NMR spectroscopy: (a) 129Xe NMR spectra of MFU-4 loaded with xenon (black = Sample 9, measured at 50 bar; blue = Sample 9, measured 24 h after air exposure) measured at RT; (b) 129Xe NMR spectra of MFU-4 with different population distributions because of different xenon loading states (black = Sample 9, measured at 50 bar; dark blue = Sample 9, measured 24 h after air exposure; blue = Sample 10, measured 22 h after air exposure; light blue = Sample 10, measured 10 days after air exposure) measured at RT. Ten separate lines for different degrees of filling are resolved. Each line represents one Xen-cluster; (c) 129Xe chemical shifts δn of the different Xen-clusters (± 0.5 ppm). ∆δ denotes the chemical shift difference δn – δn-1.

due to even smaller clusters. The maximum loading predicted by simulated sorption analysis was 15 xenon atoms per void of MFU-4 (Figure S2a). Therefore, one could expect a maximum number of 15 different signals representing Xen-clusters with n = 1 – 15. Assuming that the chemical shift difference between small clusters was about 5 ppm, then the spectrum after ten days of air exposure in Figure 2b could only be fitted if at least four additional signals at decreasing chemical shift were assumed, i.e. that in total at least 14 different signals of Xenclusters could be detected. It should be emphasized that the aforementioned previous adsorption experiments were performed on zeolite A which exhibits a much wider pore aperture of about 4 Å compared to MFU-4.42,43 The exact value depends on the cations (e.g. Na, Ag) contained within the voids of the zeolite. These experiments were carried out in sealed tubes at various xenon pressures, but not under air atmosphere. In contrast, it is here demonstrated that xenon even passes through the extremely small pore apertures of MFU-4 (2.52 Å). Xenon is then really entrapped inside the cavities and remains there even after pressure release and under air exposure. Thus, we were able to record 129Xe NMR spectra of xenon adsorbed on MFU-4 at ambient conditions. Computational chemistry. Two different calculations were performed using classic molecular dynamics and periodic DFT. In molecular dynamics, the objective is to see the type of trajectories (hence the extent of diffusion of gas molecules exchanging from one large void in MFU-4 to another , as well as to calculate the diffusion coefficient of Xe in MFU-4 at different temperatures, from which the energetic barrier for diffusion can be obtained. The diffusion was found to be very

slow, as shown by the almost flat (nearly zero slope) mean square displacement (MSD) plots (Figure S9a). These slopes are related to the diffusion coefficient. Only the MSD plot at the highest temperature (448 K) shows linearity and a diffusion coefficient of 7·10-8 cm2/s can be obtained from the fit. On the other hand, the trajectory plots (Figures S9b and S9c) show little, albeit some, Xe jumps from void to void, going throughout the small pore. This indicates, in agreement with the experiments, that diffusion is possible although very slow. With the periodic DFT, an energy-scan calculation was performed with the Xe atom (Figure 3a) going from the center of the large void (x=0.0) towards the center of the small pore (x=0.5) and finally to the center of the next large void (x=1.0). This was calculated with PBESol functional including dispersion corrections according to the Tkatchenko and Scheffler (TS) scheme. The results (Figure 3b) indicate 66 kJ/mol as the activation barrier and 51 kJ/mol apparent activation barrier (measured from the 15 kJ/mol level in Figure 3b). It is likely that the apparent activation barrier decreases with an increase in the energy level above 15 kJ/mol due to the presence of more than one xenon in the large void. Selected snapshots of the energy scan (Figure 3c) show: the most stable location of Xe (Figure 3c, left), where the small pore is geometrically unaffected, with d(Cl-Cl) = 4.23 Å; the transition state of diffusion (Figure 3c, middle), where the left 4Cl-window is expanded, with d(Cl-Cl) = 4.8 Å, while the 8Cl-gate shrinks in the x-direction, with d(Cl-Cl) ≈ 3.9 Å; and a stabilized intermediate (Figure 3c, right), where the Xe is inside the small pore and the small pore returns to the equilibrium geometry, d(Cl-Cl) = 4.23 Å, in spite of the large size of Xe. A periodic DFT calculation was also performed using Kr instead of Xe. The results (Figure S12) show an activation barrier of 46


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Figure 3. Computational chemistry studies of the xenon diffusion path in MFU-4: (a) large void and small pore (depicted as yellow spheres) in the unit cell of MFU-4, and x-fractional coordinates of the Xe atom along the diffusion path used to calculate the energetic barrier. The 8 chloride ligands forming the small pore (8Cl-gate) are highlighted in green; (b) energy scan calculated with CASTEP/PBESol using the shown model; (c) optimized geometry of the small pore (8Cl-gate, green balls) of MFU-4 in the energy scan corresponding to the Xe (pink ball) coordinates x = 0.3 Å (left), 0.4 Å (middle), and 0.5 Å (right). Cl-Cl non-bonding distances are 4.23 Å (left and right), and 4.8 Å in the gate opening (middle).

kJ/mol, considerably smaller than that for Xe (66 kJ/mol, Figure 3b), which helps to explain the lack of trapping when using Kr (see krypton release in Figure S6). Furthermore the adsorption energy of Xe in the large void of MFU-4 has been calculated where the Xe location has been fully optimized instead of constraining its path to crossing the small pore. All atoms of MFU-4 have also been optimized while the unit cell parameters of the cubic cell were kept fixed at a = 21.57 Å. An adsorption energy of 24.4 kJ/mol has been obtained, with the corresponding geometry depicted in Figure S10, showing Xe close to an octahedral Zn at a distance of 4.86 Å. This value is in agreement with values commonly found for heat of adsorption of xenon in MOFs (Table S3). An equivalent calculation using a cluster model (Figure S11) gives similar results. ■ EXPERIMENTAL SECTION Materials and methods. The thermogravimetric analysis (TGA) of xenon loaded samples was performed with a TGA Q500 analyser in the temperature range of 25–600 °C under a nitrogen atmosphere at a heating rate of 10 K min−1. The TG-MS analysis was performed with a NETZSCH STA 409C analyzer coupled to a mass spectrometer via Skimmer® in a temperature range of 25– 300 °C under a nitrogen atmosphere at a heating rate of 10 °C min−1. The xenon release kinetics was studied with a NETZSCH

STA 409 PC Luxx analyzer at a constant temperature (25, 38, 55, 65, 74 °C and 93 °C) for 24 h under a nitrogen atmosphere. X-ray powder diffraction data (XRPD) were collected in the 5–50° 2θ range using a Seifert XRD 3003 TT – powder diffractometer with a Meteor1D detector operating at room temperature using Cu Kα1 radiation (λ=1.54187). For the 129Xe NMR measurements, 100 150 mg of MFU-4 were transferred into a single crystal sapphire tube, evacuated, and heated up to 130 °C for activation overnight. For loading the sample, 36 bar or 50 bar of xenon gas were applied to the MOF by freezing a defined amount of xenon in the tube (liquid nitrogen, -196 °C). The NMR spectra were recorded at room temperature (25 °C) on an Avance 300 (Bruker, Karlsruhe, Germany) NMR spectrometer equipped with an HR 10 mm probe. The measurements were performed at a resonance frequency of 83.04 MHz using a pulse length of 11 µs and a relaxation delay of 15 s. The 129Xe chemical shifts were referenced to xenon gas at zero pressure. MFU-4 synthesis. The ligand, benzobistriazole, was synthetized according to the literature.47 The MOF material was prepared by microwave-assisted synthesis following the procedure published previously.10 Prior to the xenon loading experiments, the sample was kept under vacuum at 320 °C for 24 h to remove any solvent molecules from the voids. Xenon loading. For each experiment, 50-100 mg of MFU-4 were placed into a steel vessel constructed from metal tubing attached to a manometer. The vessel was filled with xenon gas and kept at the desired pressure and temperature for a desired



period of time. Upon cooling down, the gas pressure was released and the sample (ca. 5-10 mg) was immediately analyzed with TGA and XRPD methods. Xenon release. The xenon loaded sample was kept in a container opened to air (if not otherwise stated) and after a certain period of time a small amount (ca. 5-10 mg) was taken and analyzed by TGA and XRPD methods. Computational chemistry. MFU-4 force field. Following a procedure from a previous publication,13 we found that the Universal Force Field48 (UFF) is not able to properly describe some aspects of our system, such as the flexibility of the MOF, the Cl ligands and the H atoms, so we undertook a re-parameterization of the corresponding terms, with the resulting UFF-modified values being used. In our model, we exclude the Lennard-Jones and Coulombic interactions in 1-2 and 1-3 bonded atoms and include it in 1-4 bonded ones. This is taken into account by the directive 'special_bonds lj/cut/coul/long 0 0 1' within the LAMMPS code. Molecular dynamics. With the force field explained above, molecular dynamics (MD) was performed within the NVT ensemble for 30 ns, with the explicit relaxation of all the atoms of the system using LAMMPS49 software. The NVT ensemble was employed using the experimental unit cell parameters of MFU-4 (a = 21.697 Å). One unit cell of MFU-4 contains 408 atoms comprising 4 large and 4 small (8Cl-gate) cavities, which is enough for a correct sampling without replicated neighbors too nearby. The time step selected was 0.1 fs. The sequence of calculations in each run was: 600 steps of energy minimization at 0 K starting from the initial configuration and using conjugate gradients (300 cycles) as well as Hessian-free truncated Newton (300 cycles) algorithms, since the combination of two algorithms provides a good choice to reach absolute minima and avoid premature convergence. Then, this is followed by a short equilibration of 2000 steps at 20 K and then the production run consisting of 300 million steps of molecular dynamics at each of the following temperatures: 198, 248, 298, 348, 398 and 448 K. The MFU-4 unit cell contained 17 atoms of Xe, all of them initially located in the same large void. Assuming a saturation capacity of 60 Xe atoms per unit cell containing 4 large voids, this corresponds to 15 Xe per large void. Hence, our initial loading is slightly larger; but it has to be taken into account that, as the simulation time increases and with the central void occupied and the adjacent voids empty, an average loading of 17/4 (4.25) Xe/large void is to be expected at equilibrium. The mean square displacements (MSD) obtained are shown in Figure S9a. The expression used to calculate the MSD is the following, < X2(t) > = 1/(Nm . Nto) ∑i ∑t [Xi(t+to) - (Xi(to)]2 (1) o where ‘Nm’ is the number of diffusing molecules, ‘Nto’ the number of time origins used in calculating the average and ‘Xi’ the coordinate of the center of mass of molecule ‘i’. < X2(t) > = 6.D.t + B; (2) where ‘t’ is the simulation time and B the thermal factor arising from atomic vibrations. And the activation energy (Ea) for the diffusion can be calculated according to: D = D0.exp(-Ea/RT) (3) The trajectories of the diffusing molecules (Figures S8b and S8c) show in all the runs that few atoms diffuse across several unit cells and most of them stay in the same large void. Each atom is represented in a different color. The geometric particularities of the micropores of MFU-4 allow to differentiate visually the regions corresponding to the large cavity (squares of ca. 10×10 Å)

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from those corresponding to the small (8Cl-gate) cavity where the trajectories become narrower. Periodic DFT calculations. The large size of the Xe atom is expected to give a considerable overlap of its electronic cloud with those of the Cl ligands, forming the small pore of MFU-4, leading to considerable charge polarization and dispersion when Xe diffuses throughout this small pore. Taking into account that the above force field would not be realistic to yield an accurate value for the activation barrier for the diffusion, periodic DFT calculations were performed by moving a Xe atom along the diffusion reaction coordinate in order to obtain an energetic profile of the activation barrier. The periodic-DFT calculations were performed with the CASTEP (version 8.0) code50,51 which uses plane waves and pseudopotentials. The calculations used on-the-fly generated ultrasoft pseudopotentials and an energy cut-off of 500 eV, which was found to give well-converged total energies. All the atomic coordinates, except the reaction path coordinate and the cell parameters, were optimized using a Broyden-Fletcher-GoldfarbShanno (BFGS) optimization algorithm using the PBESol52 functional. Convergence thresholds for the electronic energy and for the geometry optimization were set to 2.0×10-6 eV and 1.0×10-4 eV per atom for the total energy and the fine grid scale was set to 2.5 in order to increase the accuracy. The size of the FFT grid used to represent wavefunctions, charge density and potentials is, in this way, multiplied by 2.5 times the default value, providing a greater accuracy and faster convergence of the wavefunction. The Brillouin zone was sampled in the k-space using the Monkhorst– Pack scheme within 1×1×1 mesh points. Dispersion corrections were considered throughout the TS scheme, introduced by Tkatchenko and Scheffler53 which employs a pairwise dispersion correction method (equation 4), as introduced by Al-Saidi et al.54 who found an optimum s6 value of 1.06, which we used in the present work. The PBESol-TS has been found to yield excellent geometries in microporous non-ionic silicate crystals.55 = − ⋅ ∑

,

⋅ ( )

(4)

■ CONCLUSIONS

In this work we reported on a strategy of using MOFs having pore apertures smaller than the size of the guest as a possible approach to enhance gas storage in MOFs at ambient conditions. To demonstrate this concept, we used MFU-4, an ultramicroporous MOF with 2.5 Å large pore apertures, for xenon storage. We showed that upon exposure of the MOF material to xenon atmosphere at high pressure, large amounts of the guest were adsorbed but only slowly released at decreasing pressure, as observed by 129Xe NMR, XRPD, TG measurements. The amount of the loaded gas could be controlled from 0 wt% to more than 44.5 wt% by changing parameters such as gas pressure, temperature and time. The rate of release of the retained guest molecules was shown to be thermally activated, which offers the possibility of controlled release through thermal programming. First principle calculation demonstrate that apart from matching the diffusion-limiting diameter of the pores inside of the framework to the kinetic diameter of the sorbate, a sufficient flexibility of the pores and (to a lesser extent) the framework itself is required to achieve a reversible high loading capacity. Hence, here we present not only a material having an exceptionally high xenon storage capacity at normal conditions (above 40 wt%), which upon kinetic trapping in the framework comes close to the volumetric density of liquid xenon, but we would also like to point out the more general


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Journal of the American Chemical Society utility of the concept of kinetic trapping that could be applied to other MOFs with flexible ultra-narrow pores. This unveils a new concept toward gas densification and efficient storage of energy carriers such as methane (kinetic diameter 3.8 Å) and the efficient separation of small molecules having very similar dimensions (e.g. Kr/Xe, or small hydrocarbon mixtures) via kinetic trapping, which we are currently investigating. A particularly interesting aspect might be to control the kinetic trapping of gases via the dynamic response of the porous framework toward an external (switchable) physical stimulus such as an electric field, which has been demonstrated, at least partially, very recently.56

ASSOCIATED CONTENT Supporting Information. Additional results of xenon loading and release experiments, 129Xe NMR measurements and computational chemistry details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT H.B. is grateful to the program “Chancengleichheit für Frauen in Forschung und Lehre” from the University of Augsburg for financial support via a fellowship. D.V. and E.B. are grateful for financial support from DFG, Priority Programme „Coordination Networks: Building Blocks for Functional Systems” (SPP 1928). Financial support from the DFG (FOR 2433 “MOF Switches”) is also gratefully acknowledged.

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