Versatile Energetic Behavior of ZIF-8 upon High Pressure Intrusion

Article pubs.acs.org/JPCC

Versatile Energetic Behavior of ZIF‑8 upon High Pressure Intrusion− Extrusion of Aqueous Electrolyte Solutions Guillaume Ortiz, Habiba Nouali, Claire Marichal, Gérald Chaplais,* and Joel̈ Patarin* Univ de Haute Alsace (UHA), CNRS, Equipe Matériaux à Porosité Contrôlée (MPC), Institut de Science des Matériaux de Mulhouse (IS2M), UMR 7361, F-68093 Mulhouse, France

ABSTRACT: Metal−organic materials (MOFs) constitute very attractive materials for storage, separation, catalysis, and drug delivery because of their crystalline hybrid organic−inorganic structures and their large porous volume. Here, we report the energetic behavior, in term of storage/restoration of mechanical energy, of ZIF-8 upon high pressure intrusion−extrusion of aqueous KCl, LiCl, and NaCl solutions of variable concentration. Comparison with the energetic performances of the “ZIF-8− water” system is performed. Whatever the nature of the electrolyte (KCl, LiCl, NaCl), an increase of the intrusion pressures and thereby of the stored energy are observed with the increasing of salt concentration. However, the three studied systems differ, at least for the highest concentrations, by behaving as a shock-absorber for KCl and as a bumper for NaCl and LiCl. A combination of several characterization techniques used before and after intrusion−extrusion experiments, i.e., X-ray diffraction, solid state NMR, and N2 adsorption−desorption experiments, allows us to establish that the ZIF-8 network is preserved after such a treatment.

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INTRODUCTION In the actual economic and environmental contexts, with global awareness of the crisis of conventional fossil fuels and their detrimental impact on environment, the development of new route for clean and renewable alternative energy solutions with the possibility to store and restore the unused energy has stimulated worldwide attention.1,2 In this field of new energetic applications, the intrusion−extrusion of a nonwetting liquid into a lyophobic porous material has been widely explored by Eroshenko.3−5 In 2001, our group applied this process to strongly hydrophobic zeolites (so-called zeosils) using water as a nonwetting liquid, and thus opening a new domain of applications for this type of porous solids with a nonpolluting and very accessible liquid at low cost.6,7 The principle of this application is based on thermodynamic effect between water and hydrophobic porous zeosil.8 The mechanical energy produced corresponds to a work (W), more exactly to a displacement force, which can be expressed as V W = ∫0 − PdV where P is the pressure, and V is the intruded volume. Since then, the energetic performances of numerous “zeosil− water” systems have been widely studied.8−13 Depending on © 2014 American Chemical Society

various physical parameters of material, such as the pore system (cavities or channels), its dimensionality (1, 2, or 3-D) or the pore size, the “zeosil−water” system is able to restore, dissipate or absorb the supplied mechanical energy during the compression step (intrusion), and hence behaves as a spring, shock-absorber or bumper, respectively when the pressure is released (extrusion). In order to increase the stored energy, a first approach was to increase the pore volume with materials more porous than zeolites such as grafted mesoporous materials.14,15 On zeosils (pure silica zeolites), experiments were performed to create additional micro-, meso-, and macropores using porogen (carbon black) or templating agents (surfactants), respectively.16 However, the formation of few additional micropores led only to a slight increase of the stored energy (+20%), the presence of meso- and macropores having no real contribution. Hydrophobic metal−organic framework materials (MOFs), which are characterized by a larger microporous volume than zeolites appear also to be promising solids for such an Received: December 17, 2013 Revised: March 13, 2014 Published: March 13, 2014 7321

dx.doi.org/10.1021/jp412354f | J. Phys. Chem. C 2014, 118, 7321−7328

The Journal of Physical Chemistry C

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mercury porosimeter (Micromeritics Model Autopore IV). ZIF-8 powder was directly introduced in the cell. This latter, which contains the “MOF−aqueous medium” system, consists in a polypropylene cylinder of 2 cm3 sealed by a mobile piston. Then, this cell is introduced in a 15 cm3 penetrometer of the porosimeter which is filled with mercury. The volume variation is determined through a capacity measurement which depends on the mercury height in the capillary tube of the penetrometer. The value of the intrusion (Pint) and extrusion (Pext) pressures correspond to that of the half volume total variation. Pressure is expressed in MPa, and volume variation in mL per gram of sample. The experimental error is estimated to 1% on the pressure and on the volume. After intrusion−extrusion experiments, the ZIF-8 samples were filtered and washed with 20 mL of distilled water to remove salts. Powder X-ray Diffraction. The powder XRD patterns of the different samples were collected between 5 and 50° (2θ) (step 0.04°) on a STOE STADI-P diffractometer in Debye− Scherrer geometry, equipped with a linear position-sensitive detector and employing Ge monochromated Cu Kα1 radiation (λ = 1.5406 Å). Nitrogen Adsorption−Desorption Measurements. N2 adsorption−desorption isotherms were carried out using a Micromeritics ASAP 2420 apparatus at 77 K. Prior to the adsorption measurements, samples were outgassed at 90 °C overnight under vacuum. BET surface (SBET) areas for ZIF-8, before and after intrusion−extrusion experiments, were calculated according to the criteria given in the literature,34,35 namely in the 0.001 ≤ p/p° ≤ 0.025 range. Langmuir surface (SLang) areas were calculated thanks to the Langmuir equation assuming a monolayer coverage of N2 and a cross-sectional area of 16.2 Å2 per molecule in the same range as BET surface. The microporous volume (Vμ) was determined by t-plot method. NMR spectroscopy. All NMR experiments were performed on a Bruker Avance II 400WB spectrometer (B0 = 9.4 T). For 1 H−13C CPMAS NMR experiments, samples were packed in a 4 mm diameter cylindrical zirconia rotor and spun at a spinning frequency of 12 kHz. A proton π/2-pulse duration of 4 μs, a contact time of 1 ms, and a recycle delay of 4 s were used. 7 Li and 23Na MAS NMR experiments were recorded with a standard 2.5 mm Bruker MAS probehead, the rotor was spinning at 30 kHz. A pulse duration of 1 μs corresponding to a flip angle of π/8 and a recycle delay of 1 s were used for both experiments. 7Li, 23Na, and 13C chemical shifts are relative to 4 M LiCl and NaCl aqueous solutions and tetramethylsilane (TMS), respectively.

application. It was the case for instance for, ZIF-8 [Zn(MeIm)2, where MeIm = 2-methylimidazolate] which presents a SOD framework topology (cubic symmetry, space group I4̅3m and a ≈ 17.0 Å), with cages diameter of 11.6 and a 3.4 Å pore aperture delimited by 6 membered-rings.17−21 Above all, this material possesses a microporous volume of around 0.6 cm3 g−1,17,21,22 and a stability in water which is commonly recognized,19,20,23,24 even if this is still a matter of debate in the literature.25 Thus, very recently, we extended the water intrusion−extrusion process to this MOF by evidencing its stability under water pressure.26 We showed in addition that the “ZIF-8−water system” displays a shock-absorber behavior at a rather low pressure (around 27 MPa) with a stored energy (13 J g−1) close to that observed for the best “zeosil−water” systems.8 Another way to increase the stored energy consists in increasing the intrusion pressure. This aim can be reached by replacing pure water by aqueous electrolyte solutions, the solid−liquid interfacial tension being usually higher compared to pure water.27 Therefore, according to the Laplace-Washburn relation, higher intrusion pressures are expected.28 The influence of additive electrolytes in water was studied by Liu et al. on zeolite Y29 and Zhao et al. on ZSM-5.30 For both cases, the higher the concentration of the aqueous electrolyte solution, the higher the intrusion pressure.30,31 According to these authors, this might be due to the increase of the effective liquid−vapor interfacial tension and/or an electrostatic repulsion which contribute to a higher transport resistant force. In such media, water molecules are in strong interaction with ions. Ion-dipole interactions, which depend on the size of the ion, its charge, its polarizability and the dilution of the medium, involve interactions energies of hundreds of kJ mol−1. The increase of applied pressure observed in the presence of aqueous electrolyte solutions might be explained by the additional force required to break these solvation bonds.32,33 Recently, Tzanis et al. have highlighted the effect of aqueous electrolyte solutions on the intrusion pressure on Silicalite-1, a pure-silica zeolite. This nanoporous material differs from zeolite Y and ZSM-5 because no charge compensating cations are present in the pores of this pure-silica zeolite. Compared to the “Silicalite-1−water” system (intrusion pressure = 96 MPa, stored energy = 11 J g−1), the intrusion pressures are considerably higher and reach for instance 280 MPa for the “Silicalite-1−LiCl, 3H2O” system leading to a stored energy almost tripled.32 In this work, the energetic performances of “ZIF-8−aqueous electrolyte solutions” systems are described. The influence of both the cation size (K+, Na+, Li+) and the salt solution concentration (KCl, NaCl, LiCl) on the intrusion pressure are studied. The ZIF-8 samples are fully characterized by XRD, N2 adsorption−desorption measurements, and 7Li, 13C, and 23Na MAS NMR spectroscopy before and after intrusion−extrusion experiments.

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RESULTS AND DISCUSSION Intrusion−Extrusion Experiments. For these experiments, three different salts (KCl, LiCl, NaCl) at three different concentrations (1, 2.5, and 4 M) were used. The latter concentration corresponds almost to that of the maximum solubility of KCl at room temperature. All the results were compared to the “ZIF-8−water” system.26 The pressure− volume diagrams after 1, 2, and 3 intrusion−extrusion cycles are reported in Figures 1, 2, and 3 for the “ZIF-8−KCl aqueous solutions”, “ZIF-8−LiCl aqueous solutions” and “ZIF-8−NaCl aqueous solutions” systems, respectively. The corresponding characteristics data are listed in Table 1. On these diagrams, the low pressure range (P < 10 MPa) is not reported. Indeed, as explained for the “ZIF-8−water” system,26 the volume variations observed below correspond to

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EXPERIMENTAL SECTION Salt Aqueous Solutions and ZIF-8. KCl, NaCl, and LiCl aqueous solutions of various concentrations (1, 2.5, and 4 M) were prepared by dissolving the corresponding salts in distilled water. ZIF-8 (BasoliteTM Z1200) was purchased from SigmaAldrich and used as received. Intrusion−Extrusion Experiments. The intrusion−extrusion experiments of aqueous electrolyte solutions in ZIF-8 samples were performed at room temperature using a modified 7322



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Figure 3. Pressure−volume diagrams of the “ZIF-8−NaCl aqueous solutions” system. For clarity the diagrams for each salt concentration are shifted by 0.7 mL g−1. For comparison the diagram of the “ZIF-8− water” system is also reported.26 (A) intrusion curve of first cycle; (B) intrusion curves of second and third cycles; (C) extrusion curves of first, second, and third cycles.

Figure 1. Pressure−volume diagrams of the “ZIF-8−KCl aqueous solutions” system. For clarity the diagrams for each salt concentration are shifted by 0.9 mL g−1. For comparison the diagram of the “ZIF-8− water” system is also reported.26

volume is slightly lower than the microporous volume obtained from nitrogen adsorption−desorption isotherms (i.e., 0.58 cm3 g−1). Such a difference was already observed for the “ZIF-8− water” but also for numerous “zeosil−water” systems and explained by a bulk water density lower than 1.9 The energetic performances of such a system are summarized in Table 1. Compared to the “ZIF-8−water” system, the stored energy (Eint) is almost doubled (23.6 instead of 13.3 J per gram of solid). “ZIF-8−LiCl Aqueous Solutions” System. In the case of “ZIF-8−LiCl aqueous solutions” system, the first intrusion− extrusion cycle (arrow A, Figure 2) exhibits one main steep step with an intrusion pressure around 30, 40, and 52 MPa for LiCl concentrations of 1, 2.5, and 4 M, respectively. In all cases, the intruded volume (Vint) is around 0.5 mL g−1 (Table 1). Like for the “ZIF-8−KCl aqueous solutions” system, at a low salt concentration (LiCl 1 M), a shock-absorber behavior is observed and the phenomenon is reproducible over the second and third cycles. All the liquid is expelled from the solid at a lower pressure close to 22 MPa. At higher salt concentrations, the behavior is fairly different. After the extrusion step of the first cycle (arrow C, Figure 2), the extruded volume is lower, meaning that a part of the liquid is still present in the porous solid (around 0.05 mL g−1 for LiCl 2.5 M and 0.15 mL g−1 for LiCl 4 M). Thus, the corresponding systems act mainly as a bumper, and for these salt concentrations the intruded volume for the next cycles is lower. Indeed, for the second cycle, the intruded volume is close to 0.35 mL g−1 for LiCl 4 M (Table 1), and surprisingly, the intrusion−extrusion process is completely reversible for the second and third cycles (arrows B and C, Figure 2), the extruded volumes being close to the intruded ones and a shock-absorber behavior is observed. However, the shape of the second and third intrusion curves is less steep. Moreover, compared to the second and third cycles of the “ZIF-8−KCl aqueous solutions” system, mainly due to a lower intruded volume, the energetic performances of such a system are lower (see Table 1). “ZIF-8−NaCl Aqueous Solutions” System. An intermediate behavior between the “ZIF-8−KCl aqueous solutions” system and the “ZIF-8−LiCl aqueous solutions” system is observed for the pressure−olume diagrams of the “ZIF-8−NaCl aqueous

Figure 2. Pressure−volume diagrams of the “ZIF-8−LiCl aqueous solutions” system. For clarity the diagrams for each salt concentration are shifted by 0.9 mL g−1. For comparison the diagram of the “ZIF-8− water” system is also reported.26 (A) intrusion curve of first cycle; (B) intrusion curves of second and third cycles; (C) extrusion curves of first, second, and third cycles.

the compression of the powder and to the filling of the interparticle porosity. Each “ZIF-8−salt aqueous solutions” is discussed separately within next three paragraphs. “ZIF-8−KCl Aqueous Solutions” System. In the case of “ZIF-8−KCl aqueous solutions” systems, the three intrusion− extrusion cycles (Figure 1) exhibit one main step with an intruded pressure (Pint) higher than that for pure water and close to 31, 39, and 47 MPa for KCl concentrations of 1, 2.5, and 4 M, respectively. This step corresponds to the intrusion of the liquid into the pores of ZIF-8. Whatever the KCl solution concentration, the intrusion−extrusion process is reversible with a hysteresis. Like for the “ZIF-8−water” system, the extrusion curves are superimposable and, to a less extent, a similar behavior is observed for the intrusion curves. Depending on the salt concentration, the extrusion pressure is in the range 42−27 MPa (see Table 1). Therefore, the “ZIF-8−KCl aqueous solutions” system exhibits a shock-absorber behavior. The intruded volume (Vint) is around 0.5 mL g−1 (Table 1). This 7323



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Table 1. Characteristics of the “ZIF-8−Aqueous Electrolyte Solutions” Systems, Intrusion (Pint) and Extrusion (Pext) Pressures, Intruded (Vint) and Extruded (Vext) Volumes, and Stored (Eint) and Restored (Eext) Energies for Three Different LiCl, NaCl, and KCl Salt Concentrations (1, 2.5, and 4 M) aqueous solution

concn (M)

water

KCl

1

2.5

4

LiCl

1

2.5

4

NaCl

1

2.5

4

cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle cycle

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

Pint (MPa)

Pext (MPa)

Vint (mL g−1)

Vext (mL g−1)

Eint (J g−1)

Eext (J g−1)

26.6 26.6 26.6 31.5 30.5 30.5 39.3 38.5 38.5 47.2 47.2 47.2 30.5 29.2 29.2 40.0 37.2 37.2 52.1 44.8 44.8 31.6 30.8 30.8 39.3 38.5 38.5 52.0 51.7 51.7

22.3 22.3 22.3 27.5 27.5 27.5 34.7 34.7 34.7 42.3 42.3 42.3 25.6 25.6 25.6 30.3 30.3 30.3 33.7 33.7 33.7 28.2 28.2 28.2 34.7 34.7 34.7 44.0 44.0 44.0

0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.45 0.45 0.50 0.35 0.35 0.50 0.50 0.50 0.50 0.48 0.48 0.50 0.45 0.45

0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.45 0.45 0.45 0.35 0.35 0.35 0.50 0.50 0.50 0.48 0.48 0.48 0.45 0.45 0.45

13.3 13.3 13.3 15.8 15.3 15.3 19.7 19.3 19.3 23.6 23.6 23.6 15.3 14.6 14.6 20.0 16.7 16.7 26.0 15.7 15.7 15.8 15.4 15.4 19.7 18.5 18.5 26.0 23.3 23.3

11.2 11.2 11.2 13.7 13.7 13.7 17.4 17.4 17.4 21.2 21.2 21.2 12.8 12.8 12.8 13.6 13.6 13.6 11.8 11.8 11.8 14.1 14.1 14.1 16.7 16.7 16.7 19.8 19.8 19.8

first hydration sphere evolve in the same sense (dH2O(Li+) = 7.6 Å, dH2O (Na+) = 7.2 Å, dH2O (K+) = 6.6 Å).36 Besides, for the three studied “ZIF-8−electrolyte aqueous solution” a significant difference is observed. For the higher concentrations and at the first cycle, the systems evolve from a shock-absorber to a bumper behavior in the following order: KCl, NaCl, and then LiCl. For the last two salts a nonextruded volume of 0.05 and 0.15 mL g−1 is measured, respectively, and seems to be related to the diameter of the first hydration sphere. In addition, the fact that an amount of the liquid is not expelled from the porosity when the systems return to the atmospheric pressure could be explained also by the calculated binding energies (E) of alkali metal cations (E(Li+) > E(Na+) > E(K+)) to the imidazolate linkers.37 For the second and third cycle whatever the nature of salts a shock-absorber behavior is observed. XRD Characterizations. The ZIF-8 samples before and after intrusion−extrusion experiments with the higher salt concentration (4 M) were characterized by XRD analysis and the XRD patterns are reported in Figure 5. For comparison, the XRD pattern of ZIF-8 after intrusion−extrusion of water is also reported. As a main observation, whatever the electrolyte solution, the ZIF-8 framework is preserved. On the XRD pattern of the intruded−extruded ZIF-8 sample with KCl, two additional peaks corresponding to traces of potassium chloride can also be observed at 2θ = 28.2 and 40.5° indicating that the washing for this sample was not perfect.

solutions” system (Figure 3). As described above for LiCl, for the highest salt concentration (NaCl 4 M) after the first intrusion−extrusion cycle, a part of liquid (about 0.05 mL g−1) is not expelled from the solid and the stored energy for the next cycles is lower (Table 1: Eint = 26.0 J g−1 for the first cycle and 23.3 J g−1 for the second and third cycles). “ZIF-8−Electrolyte Aqueous Solutions” Behaviors. As a general trend and in agreement with the results published by Han and co-workers on Y zeolite,29,31 whatever the “ZIF-8− aqueous electrolyte solution” system, the intrusion pressure increases with the salt concentration, which de facto leads to an increase of the stored energy as shown in Figure 4. One can observed, at least for the highest salt concentration (4 M) and for the first cycle, that the intrusion pressure seems to decrease slightly with the increase of the ionic radius (ri) of the cation (Pint(LiCl) = 52.1 MPa, Pint(NaCl) = 52.0 and Pint(KCl) = 47.2 MPa for ri(Li+) = 0.76 Å, ri(Na+) = 1.02 Å, ri(K+) = 1.38 Å considering a coordination number equals to 6). In other words, a higher intrusion pressure is obtained for the smallest cation (Li+). Such an evolution is still under discussion in the literature. Indeed, two groups, i.e. Zhao et al. who have studied the electrolyte intrusion−extrusion process on zeolite ZSM-530 and Liu et al. who have investigated the behavior of zeolite Y,29 have found opposite results. Moreover, if one takes into account the hydrodynamic diameter of hydrated cationic species (dH2O) instead of ionic radius of single cation, it can be found that the intrusion pressure and the diameter of the 7324



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Figure 6. 1H−13C CPMAS NMR spectra of ZIF-8 before (a), after three intrusion−extrusion cycles with water (b), and with NaCl 4 M (c) (∗, spinning side bands).

ppm correspond to the carbon atom bridging the two nitrogen atoms in the imidazole-ring and to the C−H-groups of the ligand, respectively. The signal at 12 ppm is assigned to the methyl group of the 2-methylimidazolate moieties. No significant differences are observed between the three spectra. Therefore, at a local order the ZIF-8 structure is still preserved. 23 Na MAS and 7Li MAS NMR spectroscopy. The ZIF-8 samples after NaCl 4 M and LiCl 4 M intrusion−extrusion experiments were characterized by 23Na (I = 3/2) MAS and 7Li (I = 3/2) MAS NMR spectroscopy. 39K (I = 3/2) MAS NMR was not performed on the ZIF-8 samples after KCl 4 M intrusion−extrusion experiments. Indeed, in this first approach, 39 K is a much more challenging nucleus to study than 23Na and 7 Li because 39K possesses a significant quadrupolar coupling constant leading to very large resonances. Moreover, its low gyromagnetic ratio often produces acoustic ringing that distorts the spectrum. The 23Na MAS NMR spectra of ZIF-8 after NaCl 4 M intrusion−extrusion experiment before and after washing with distilled water are reported in Figure 7. In both cases, two signals are observed; one intense and narrow at 6.7 ppm and a second broader one at 1 ppm. These two resonances suggest the presence of two types of sodium atoms with different environment. The intensity of the sharp signal, contrary to the broader one, decreases significantly after the washing step. Therefore, this component may correspond to sodium atoms weakly interacting with its environment. Moreover, this resonance presents a width at half height of only 35 Hz, without quadrupolar line shape, suggesting a regular and symmetric environment that could correspond to the hydrated cations. The broader component (166 Hz of width at half height) without quadrupolar line shape might be assigned to Na+ cations with a larger distribution of environment or with a greater quadrupolar coupling constant suggesting an interaction with the framework. Those Na+ cations may be trapped inside the pores of the ZIF-8 structure. A similar behavior is observed for Li+ cations (see Figure 8). N2 Adsorption−Desorption Isotherms at 77 K. Figure 9 illustrates the N2 adsorption−desorption isotherms of the ZIF8 samples before, after three water intrusion−extrusion cycles and after three intrusion−extrusion cycles with the 4 M

Figure 4. Evolution of the intrusion parameters of first cycle of the “ZIF-8−aqueous electrolyte solution” system: intrusion pressure (a) and corresponding stored energy (b) versus salt solution concentration.

Figure 5. XRD patterns of ZIF-8 samples, before (a) and after three intrusion−extrusion cycles with water (b) and with LiCl 4 M (c), NaCl 4 M (d), and KCl 4 M (e) aqueous solutions (∗, traces of KCl).

NMR Spectroscopy. 1H−13C CPMAS NMR spectroscopy. The 1H−13C CPMAS NMR spectra of ZIF-8 samples before and after water or NaCl 4 M intrusion−extrusion experiments are reported in Figure 6. The resonances at about 150 and 123 7325



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are reported in Table 2. In all cases, the isotherms are mainly of type I characteristic of microporous solids. Table 2. N2 Adsorption−Desorption Data (Microporous Volume (Vμ), BET Surface (SBET) and Langmuir Surface (SLang)) of the ZIF-8 Samples before and after Three Intrusion−Extrusion Cycles with Water and with LiCl, NaCl, and KCl 4 M Aqueous Solutions salt aqueous solution ZIF-8 before intrusion− extrusion experiment ZIF-8 after water intrusion− extrusion experiments ZIF-8 after aqueous electrolyte solution intrusion−extrusion experiments LiCl NaCl KCl

Figure 7. 23Na MAS NMR spectra of ZIF-8 after three intrusion− extrusion cycles with NaCl 4 M without washing (a) and after washing with distilled water (b).

concn (M)

4 4 4

Vμ (cm3 g−1)

SBET (m2 g−1)

SLang (m2 g−1)

0.66

1860

1890

0.66

1840

1870

0.51 0.56 0.59

1377 1490 1572

1405 1611 1688

After intrusion−extrusion with the salt aqueous solutions compared to water, a decrease of the pore volume is observed. The decrease is close to 10% for KCl 4 M but reaches 22% for LiCl 4 M. An intermediate value (close to 15%) is found for NaCl 4 M. Concerning the “ZIF-8−KCl system”, this decrease may be due to the presence of traces of KCl (see XRD results). For the other systems, the results are in good agreement with the nonextruded volumes of the corresponding intrusion− extrusion curves. Indeed, the larger the nonextruded volume, the larger the decrease of the microporous volume. Thus, for the “ZIF-8−LiCl 4 M aqueous solution” system, according to the nonextruded volume (close to 0.15 mL g−1), the expected decrease of the pore volume is 30%.

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CONCLUSION In this paper, high pressure intrusion−extrusion experiments of variable concentration aqueous electrolyte solutions in a metal−organic framework, namely ZIF-8, are reported. Characterizations were performed before and after intrusion− extrusion experiments. Whatever the electrolyte (KCl, NaCl, or LiCl) used, the ZIF-8 framework is preserved, the intrusion pressure increases with the salt concentration thereby leading to an increase of the stored energy. This latter can reached twice that measured when using water. Nevertheless, the behavior of the system depends on the salt nature. Indeed, by considering the highest concentration (4 M) of the aqueous electrolyte solution and the first intrusion−extrusion cycle, the “ZIF-8−aqueous electrolyte solution” system behaves, as a shock-absorber for KCl, and as a bumper for NaCl and LiCl. In this latter case, an amount of liquid (highest for LiCl) is not expelled from the solid. This versatile behavior could be explained by interactions between lithium cations and imidazolate groups that are stronger than those implying sodium and especially potassium cations. At last, even if the nature of the intruded species (water, cation, hydrated cation) is not completely well established, it remains, at least for aqueous electrolyte solutions involving KCl, NaCl, or LiCl, that the intrusion pressure appears to reduce slightly when the ionic radius of the cation grows or when the hydrodynamic diameter of the hydrated cation decreases. This study provides promising results and other “MOFs−(electrolyte) aqueous systems” are going to be investigated in order to shed the light on the host−

Figure 8. 7Li MAS NMR spectra of ZIF-8 after three intrusion− extrusion cycles with LiCl 4 M without washing (a) and after washing with distilled water (b).

Figure 9. N2 adsorption−desorption isotherms of ZIF-8 samples at 77 K: before (a) and after three intrusion−extrusion cycles with water (b) and with KCl (c), NaCl (d), and LiCl (e) 4 M aqueous solutions (filled symbols, adsorption curves; empty symbols, desorption curves).

electrolyte solutions. The corresponding textural characteristics (BET and Langmuir surface areas and microporous volumes) 7326



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guest interaction implied in those intrusion−extrusion experiments and to study the stability of MOFs after such a hydromechanical treatment.

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AUTHOR INFORMATION

Corresponding Authors

*(J.P.) E-mail: [email protected]. Telephone: 33-3 89 33 68 80. *(G.C.) E-mail: [email protected]. Telephone: 33-3 89 33 68 87. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding from the Agence Nationale de la Recherche under the project ‘‘SOFT-CRYSTAB’’ (ANR2010-BLAN-0822).

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REFERENCES

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Versatile Energetic Behavior of ZIF-8 upon High Pressure Intrusion

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