Computational Exploration of the Dynamics of

Article pubs.acs.org/JPCC

A Joint Experimental/Computational Exploration of the Dynamics of Confined Water/Zr-Based MOFs Systems Arnaud Planchais,† Sabine Devautour-Vinot,*,† Fabrice Salles,† Florence Ragon,‡ Thomas Devic,‡ Christian Serre,‡ and Guillaume Maurin† †

Institut Charles Gerhardt Montpellier − UMR CNRS 5253, UM2, ENSCM-Université Montpellier 2, Place E. Bataillon, 34095 Montpellier Cedex 05, France ‡ Institut Lavoisier, UMR CNRS 8180, Université de Versailles Saint-Quentin-en-Yvelines, 45 avenue des Etats-Unis, 78035 Versailles cedex, France S Supporting Information *

ABSTRACT: A joint modeling (molecular dynamics simulations)/experimental (broadband dielectric spectroscopy) approach was conducted to investigate the water adsorption in the UiO-66(Zr) MOF, and its functionalized versions bearing acidic polar groups (−COOH or 2-COOH per linker). It was first pointed out that the proton conduction measured at room temperature increases with (i) the water uptake and (ii) the concentration of the free acidic carboxylic functions. This trend was further analyzed in light of the preferential arrangements of water within the pores of each MOF as elucidated by molecular dynamics simulations. Indeed, it was revealed that the guest molecules preferentially (i) form interconnected clusters within the UiO66(Zr)s cages and generate a H-bond network responsible for the proton propagation and (ii) strongly interact with the −COOH grafted functions, resulting in the creation of additional charge carriers in the case of the hydrated functionalized solids. Broadband dielectric spectroscopy shed light on how these water configurations impact the local dynamics of both the water molecules and the MOF frameworks. The dielectric relaxation investigation evidenced the existence of one or two relaxation processes, depending on the nature of the UiO-66(Zr) framework and its hydration level. Compared to the dielectric behavior of water confined in a large variety of media, it was thus concluded that the fastest process corresponds to the dynamics of the water molecules forming clusters, while the slowest process is due to the concerted local motion of water/ligand entities.

1. INTRODUCTION Porous metal−organic frameworks (MOFs) are a relatively new class of hybrid crystalline solids built up from inorganic subunits and organic moieties. The modulation of the nature of the metal center and the organic linker allows the design of a large variety of architectures with various and uniform pore sizes, shapes, and chemical functionalities. This unprecedented versatility makes MOFs very promising for a wide range of societal-relevant applications (energy, environment, health,) in gas separation/storage, chemical sensing, drug delivery, and catalysis, among others.1−10 However, before promoting MOFs to the industrial level for certain applications, besides their required stability under moisture, their water sorption behavior is another key point that needs to be carefully considered. In this context, MOFs with either hydrophobic or hydrophilic features are required depending on the target applications. Typically, the capture of CO2 from power plant or natural gas streams is envisaged by physisorption based processes involving MOFs as adsorbent with high separation performances.4,11,12 To reduce the number of steps in such processes and thus their cost, a hydrophobic MOF adsorbing preferentially CO2 toward water would be highly desired,13 a feature hardly achieved with © 2014 American Chemical Society

the zeolites usually employed as adsorbents in the existing technology. Similarly, catalytic reactions can be hindered due to poisoning effects originating from water irreversibly adsorbed on the active sites present at the surface of some MOFs.14 This also motivates the design and engineering of catalytic MOFs with hydrophobic features. On the opposite side, the efficiency of the adsorptive heat-transformation process (chiller and heat pump) using water as working fluids is mainly driven by the water exchange capacity and a pronounced hydrophilicity of the adsorbent. Indeed, looking for a MOF with such criteria is of high importance.15,16 Regarding the design of water-mediated proton conductor based MOFs, high H+ conductivity is achieved when (i) a large concentration of water is retained by the porous frameworks and (ii) these highly mobile water molecules are arranged in such a way to form chains or clusters.17 In this latter case, a porous MOF combining hydrophobic and hydrophilic features is required.18,19 Compared to purely inorganic porous solids, most MOFs exhibit Received: April 22, 2014 Revised: June 3, 2014 Published: June 12, 2014 14441

dx.doi.org/10.1021/jp5039267 | J. Phys. Chem. C 2014, 118, 14441−14448

The Journal of Physical Chemistry C

Article

measurements were carried out at different fixed frequencies in the 10°−106 Hz domain, under nonisothermal conditions from 173 to 373 K with a constant and slow heating rate (0.5 K·min−1). The temperature of the sample was controlled by the Quatro Novocontrol system. We checked that, during the measurements performed at a given frequency and under linear heating conditions, the fluctuation of the temperature was very limited so that the isothermal conditions can be considered to be attained. Prior to electrical measurements, the samples were exposed to an atmosphere of saturated water for 24 h at 293 K. The hydrated samples were then introduced into a specific cell similar to that generally used for liquids. About 75 mg of the powder sample was placed between two gold-coated electrodes in a parallel plate capacitor configuration with an annular Teflon spacer for insulation. Two sets of water loadings were considered by heating the series of the hydrated UiO-66(Zr)s at different treatment temperatures: TT = 293, 298, 303, or 313 K and TT = 373 K for the high and low levels of hydration, respectively. The treatment was performed in situ for 30 min before cooling prior to the electrical measurements. UiO66(Zr) was fully dehydrated by a heating at 423 K for 4 h. For the mono- and difunctionalized solids, TT did not exceed 373 K to prevent any formation of anhydrides between −COOH functions.31 In many cases, the real part of the ac conductivity (σac(ω,T)) results from the superposition of three contributions: the Maxwell−Wagner−Sillars one, σMWS(ω,T), the dc conductivity, σdc(T), and the polarization conductivity, σpol(ω,T) (eq 1)

typically rather low hydrophilicity, due to the moderate polarity of the organic backbone of the ligands constituting the framework. However, the introduction of organic functionalities decorating the pores through direct or postsynthesis treatments is an efficient way to modulate the chemical and physical properties of MOFs without altering their topologies.20 In particular, while grafting apolar functions leads to an enhancement of the hydrophobicity of the material, the incorporation of polar groups tends to render the porous solid more hydrophilic. There is thus a crucial need to finely grasp the water adsorption behavior of hydrophobic/hydrophilic MOFs at the microscopic level. This is particularly true for some existing MOFs that already show promise in the applications previously discussed. As an illustration, a Zr-terephthalate based MOF, namely, the UiO-66(Zr), in its original version and grafted by polar/apolar functions is an alternative candidate to existing porous solids (zeolites, activated carbons,...) for not only the selective capture of CO2 from different gas mixtures21−23 but also for specific catalytic reactions.24 Although the water adsorption in this series of MOFs has been characterized essentially by collecting water adsorption isotherms,25 complementary to this, a microscopic picture of the adsorption phenomena is still required to understand the water arrangement within the pores and the response of the host framework to this chemical stimulus. To address this point, here, broadband dielectric spectroscopy is coupled with molecular simulations to investigate the hydrophobic UiO-66(Zr) solid as well as the mono/disubstituted acidic UiO-66(Zr)-COOH and UiO-66(Zr)-(COOH)2 versions for their hydrophilic nature23,26 and their plausible H+ conductor properties.27 Molecular dynamics simulations allow a deeper exploration of the water arrangements within the pores of this series of MOFs and their resulting interactions with the pore wall. These conclusions are further used to analyze the proton conductivities experimentally evidenced for each MOF. A further step consists of probing via the dual approach the local dynamics of the host framework and the guest molecules with the aim at elucidating the presence of concerted reorientational motions resulting from the H2O/MOF and/or the H2O/H2O interactions.

σac(ω , T ) = σMWS(ω , T ) + σdc(T ) + σpol(ω , T )

(1)

where ω is the electrical field angular frequency and T the temperature. The dc conductivity corresponds to long-range redistribution of charges, i.e., ionic or electron transport, while the polarization contribution arises from local rearrangement of charges or dipoles causing dipolar reorientation and thus resulting in the intrinsic bulk polarization. Maxwell−Wagner− Sillars (MWS) polarization is due to the accumulation of charges at the sample/electrodes interface and also depends on extrinsic parameters, such as the sample shape. According to Maxwell’s equations for a sinusoidal electrical field, the complex electric conductivity σ* and the complex dielectric function ε* are related to each other by eq (2)

2. EXPERIMENTAL SECTION 2.1. Materials. The UiO-66(Zr) solid was solvothermally synthesized according to the previously described procedure25,28 from an equimolar solution of ZrCl4 and terephthalic acid in DMF (0.2 M) placed in a Teflon-lined autoclave and heated at 493 K. The resulting solid was recovered by filtration and washed with DMF and acetone. The activation was further carried out to remove the remaining free linker and/or solvent into the pores by exchange. The functionalized −COOH and −(COOH)2 analogues were prepared from a similar reactant in water at 373 K under reflux.23,26 The solids were further washed with water at 373 K and dried in air. The series of UiO-66(Zr)s exhibits a structure based on Zr6O4(OH)4 oxoclusters bounded to 12 terephthalate anions, which leads to a porous cubic structure with two types of microporous cages interconnected through narrow triangular windows.29,30 2.2. Methods. 2.2.1. Broadband Dielectric Spectroscopy (BDS). Electrical properties of the samples were recorded by using a broadband dielectric spectrometer, a Novocontrol alpha analyzer. The conductivity was measured at 298 K, in the frequency range of 10−2−106 Hz, whereas the dielectric

σ *(ω) = iωε0ε*(ω)

(2)

with the real and imaginary parts of σ*(ω) expressed by eq (3) σ ′(ω) = ωε0ε″(ω) and σ ″(ω) = ωε0ε′(ω)

(3)

where ε0 is the dielectric permittivity of vacuum (ε0 = 8.854 × 10−12 A s V−1 m−1). Basically, dielectric measurements give access to the characteristic parameters of the dielectric relaxation response in terms of the relaxation time τ. Assuming that the dielectric relaxation or the reorientation mechanism is thermally activated, τ is expected to follow an Arrhenius type behavior (eq 4) ⎛ ΔE ⎞ ⎟ τ = τ0 exp⎜ ⎝ kT ⎠

(4)

where k corresponds to the Boltzmann’s constant, T the temperature, ΔE the energy barrier characterizing the relaxation process, and τ0 the pre-exponential factor. The inverse of this later parameter is known as the “frequency factor” or “attempt 14442



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frequency” and represents the highest relaxation frequency that could be obtained in a hypothetical structure that does not involve any energy barrier to cross. According to eq 4, an Arrhenius plot issued from the temperature and frequency data recorded at the experimental peak maximum characterizing the dielectric process for which ωτ = 1 allows the estimation of the values for ΔE and τ0. 2.2.2. Modeling. Monte Carlo (MC) Simulations. The MC simulations were performed in the grand canonical ensemble at 303 K by means of the CADSS software32 at P/Po = 1 in order to estimate the saturation capacity (SC) for each UiO-66(Zr) structure. The approximation of a rigid framework was considered with a simulation box consisting of 8 conventional unit cells; 5.0 × 108 Monte Carlo steps following 107 steps for equilibration were employed to ensure the convergence of each calculation. The periodic structure models, the atomic partial charges, and the 12-6 Lennard-Jones (LJ) atomic parameters for each MOF were taken from our previous studies.21,23 The H2O molecule was modeled by the four-site charged LJ TIP4P/ 2005 model.33 The adsorbate/adsorbent LJ interatomic potential parameters were then calculated using the Lorentz− Berthelot mixing rules. A cutoff radius of 12 Å was applied to all the LJ interactions, and the long-range electrostatic interactions were handled using the Ewald summation technique. The simulated SCs reach 35.9, 25.4, and 19.6 molecules/u.c. for UiO-66(Zr), UiO-66(Zr)-COOH, and UiO-66(Zr)-(COOH)2, respectively. Note that these calculated values match quite well the experimental saturation capacities deduced from the previously reported adsorption isotherms (33.3 and 19.7 molecules/u.c. for UiO-66(Zr) and UiO-66(Zr)-(COOH)2, respectively).23,34 Complementary Monte Carlo simulations were performed in the canonical ensemble for a loading of SC/ 10 in order to generate the initial configurations for the molecular dynamics runs considered for the low level of hydration experimentally explored. Molecular Dynamics (MD) Simulations. The MD simulations were performed by means of the DL_POLY package35 implementing the fully flexible force field we previously derived for each UiO-66(Zr) framework.23,36,37 The calculations were run at 300 K in the canonical ensemble coupled with the Berendsen thermostat starting with the initial configurations generated by the preliminary MC simulations. Two water loadings were investigated for each structure: the saturation capacity (SC) estimated by the preliminary MC simulations and SC/10 to mimic the high and low levels of hydration experimentally explored. A time step of 1 fs was used with a production run of 1 × 107 MD steps (i.e., 10 ns) following an equilibration of 2 × 106 steps. Similarly to the MC simulations, a cutoff radius of 12 Å was applied to all the LJ interactions, and the long-range electrostatic interactions were estimated using the Ewald summation technique. The radial distribution functions (RDFs) between the different water/water and water/host, the size of the water clusters, and the number of hydrogen bonds per water molecules (following the criteria defined by Feng et al.38) were estimated by averaging over 5 independent trajectories of each investigated system.

Figure 1. Real part of the global conductivity σac recorded at 298 K versus frequency f, for the hydrated (solid line) UiO-66(Zr) (black), UiO-66(Zr)-COOH (red), and UiO-66(Zr)-(COOH)2 (blue) and dehydrated (dotted line) UiO-66(Zr). The stars represent the region for which the dc conductivity values were considered.

isothermal conductivity profile strongly deviates from the behavior of the dehydrated UiO-66(Zr), which does not show any dc conductivity in the whole explored domain of frequency as this solid is an extremely poor conductor, i.e., an insulator. Indeed, this deviation emphasizes that the water molecules play a key role in the conduction process, similarly to what was already evidenced for a series of MOFs proton conductors in the presence of water.17,19,39−42 MD simulations were conducted for water loading corresponding to the saturation capacity of the solid. They evidence the formation of clusters with relatively strong hydrogen bonds (2.82 per water molecule with a mean distance dO−H = 1.81 Å; see Figure 2a and Figure S1, Supporting Information), implying up to 10 H2O molecules, an amount that is only slightly smaller than the one observed in the liquid state (14 molecules).43 The lower dimension of the embedded cluster relates to the bulky/ confined environment of the narrow cages that host the guest molecules. In addition, these clusters are interconnected, leading to a 3D hydrogen-bond network (Figure S1). Such an arrangement is at the basis of both the Grotthus and vehicular mechanisms usually evoked for the description of the proton propagation in hydrated protonic conductors.44,45 Noteworthy in our case, as the UiO-66(Zr) solid is rather hydrophobic,25 the interactions between the water molecules and the pore walls are relatively weak (the H2O/μ3-OH and H2O/phenyl distances are above 3 Å; see Figure 2a and Figure S3, Supporting Information). This implies a low probability for the creation of a charge carrier in this solid, resulting in a low dc conductivity value as experimentally evidenced at room temperature (8 × 10−9 S·cm−1). For both hydrated UiO-66(Zr)-(COOH) and UiO-66(Zr)(COOH)2 solids, in addition to the σdc signal, the Maxwell− Wagner−Sillars (MWS) contribution σMWS(ω) is also observed, as revealed by the conductivity decrease at low frequency. It confirms that σac results from the transport of ionic charges, i.e., the proton species. In contrast to the hydrated UiO-66(Zr), the MWS contribution is clearly detected for the functionalized materials, because of their higher values of the dc conductivity, i.e., 9 × 10−7 and 5 × 10−6 S·cm−1 for UiO-66(Zr)-(COOH) and UiO-66(Zr)-(COOH)2 respectively. Moreover, the conductivity increases with the concentration of the grafted functions, which suggests that the acidic functional groups mainly participate as charge carrier sources to the proton

3. RESULTS AND DISCUSSION Figure 1 reports the real part of the ac conductivity (σac(ω)) recorded at T = 298 K as a function of the frequency, for all the fully hydrated UiO-66(Zr)s. One first observes that, for the nonfunctionalized solid, the signal exhibits a σpol(ω) contribution, accompanied by a dc conductivity plateau. This 14443



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respectively), their concentration within the porosity is much lower in UiO-66(Zr)-(COOH)2, and (ii) the probability to disrupt the long-range hydrogen-bond network is higher in UiO-66(Zr)-(COOH)2, as illustrated by the snapshot provided in Figure S1. These different arrangements are consistent with the concentration of water adsorbed at saturation, which decreases from UiO-66(Zr) to UiO-66(Zr)-(COOH)2, as predicted by our MC simulations (see above). A further step consisted of exploring how the local dynamics of both the host and the guests are influenced by both the water arrangements within the pore and the resulting host/guest interactions. Although studies on the rotational motions of water confined in various porous media have been largely explored by BDS, they primarily focused on the different structural and dynamical properties of confined water compared to that of the bulk.46−54 In addition, the studies reported so far only rarely investigated the dynamical response of the host in which the water is trapped, and none of them were related to porous MOFs. Here, we propose to concomitantly address the dynamics of water and of the UiO-66(Zr) frameworks. To clearly discriminate both contributions, the dielectric properties of the series of the hydrated UiO-66(Zr)s at two levels of hydration were compared (TT ≤ 313 K and TT ≥ 323 K for the high and the low hydration levels, respectively). Figure 3a thus shows the isochronal dielectric loss spectrum for the UiO-66(Zr) sample preliminarily saturated with water vapor and then treated at different TT values, in comparison with the response for the dehydrated solid. In this latter case, the dielectric loss signal is relatively flat and the resulting values are very low or even negligible, which evidence the dielectric inactive nature of the material, in relation with the absence of relaxing dipoles.28 This further indicates that the μ3-OH groups do not substantially contribute to the dielectric response in the relevant temperature/frequency domain, as already reported in the case of silica based solids.50,55,56 By contrast, for the most hydrated UiO-66(Zr) (TT = 293 K), the spectrum is characterized in the low-temperature domain by a peak illustrating the existence of a dielectric relaxation process, labeled as P1, whereas the sharp signal increase in the hightemperature range results from the conduction-related phenomena. P1 is still observed up to TT = 313 K, although its intensity drastically decreases, and finally vanishes at TT = 323 K. Since P1 is only observed for the low values of TT, i.e., for the higher levels of hydration, one assumes that the guest molecules are likely to be involved in this relaxation process. For the monoacid functionalized hydrated solid (Figure 3b), in addition to the P1 process occurring at the same temperature domain than the one reported for the bare solid, a second and slower relaxation process (P2) is observed at TT = 293 K. Note that P1 and P2 appear as shoulders due to their overlapping since they both occur in a similar restricted range of temperatures. In addition, the P2 process is further perturbed by the conduction-related phenomena occurring at high temperature. Both P1 and P2 remain present from TT = 293 K to TT = 313 K. For the lower level of hydration (TT = 373 K), P1 completely disappears, whereas P2 is still observed in the dielectric spectrum. By contrast, for the hydrated diacid functionalized solid (Figure 3c), the dielectric response for TT = 293 K is dominated by the conduction response, as shown by the sharp signal increase even at low temperature. This agrees with the higher value of the dc conductivity recorded for UiO66(Zr)-(COOH)2 (cf. Figure 1). Noteworthy, this trend

Figure 2. Typical arrangement of the H2O molecules in the UiO66(Zr) (a), UiO-66(Zr)-COOH (b), and UiO-66(Zr)-(COOH)2 (c) at water saturation and room temperature. The mean water/MOF and water/water distances are reported in Å: the interactions between water and (i) μ3-OH (Owater−Hμ3-OH) (in magenta), (ii) the center of mass of phenyl groups (in white), (iii) water (in green), and (iv) carboxylic acid functions (Hwater−Ocarboxylic) (in blue). In orange are reported the mean intraframework distances between the carboxylic acid functions themselves. The mean distances were obtained from the RDF plots (Figures S3−S7, Supporting Information). For the sake of clarity, hydrogen atoms of the aromatic rings have been omitted.

conduction process. This assumption is supported by the MD simulations performed on the water saturated functionalized materials (Figure 2b,c, respectively) that evidence that the guest molecules strongly interact with the polar groups as attested by the relatively short COOH/water distances (1.76 and 1.72 Å for the mono- and difunctionalized solids, respectively). The characteristic distances between the water molecules and the μ3-OH group are within the same range or slightly below the values observed for the nonfunctionalized solid (3.34 and 2.80 Å for the mono- and difunctionalized solids, respectively). These interactions are thus significantly weaker than those involving the acidic groups, consistent with a lower acidity of the μ3-OH groups compared to the one of the protons from the −COOH groups.27 This observation supports that, in the functionalized solids, the charge carriers mainly originate from the acidic functions. In addition, the proton pathway can be roughly drawn for the UiO-66(Zr)-COOH from the low energy region occupied by the guests. It defines clusters constituted by an average of 7.4 H2O, connected all together or bridged via a water molecule located in the vicinity of the triangular windows, giving rise to an average of 3.33 hydrogen bonds per water molecule (Figure S1, Supporting Information). An even bulkier environment of the cages for UiO-66(Zr)-(COOH)2 as attested by a decrease of their pore sizes (Figure S2, Supporting Information) impacts the water organization: (i) while the clusters are constituted by an almost similar number of both water molecules (7.4 and 6.2 for UiO-66(Zr)-COOH and UiO-66(Zr)-(COOH)2, respectively) and resulting hydrogen bonds (3.33 and 3.10 per water molecule for UiO-66(Zr)-COOH and UiO-66(Zr)-(COOH)2 14444



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dielectric parameters extracted from the Arrhenius plots of P1 and P2 (see Table 1 and Figure S8, Supporting Information). Table 1. Dielectric Parameters Obtained from the Arrhenius Plots of the P1 and P2 Processes for the Hydrated UiO66(Zr) and UiO-66(Zr)-COOH, Treated at Different Temperatures TTb P1 process material

TT (K)

UiO-66(Zr)

UiO-66(Zr)-COOH

293 303 313 373 293 298 303 313 373

ΔE (eV)

P2 process

τ0 (s)

ΔE (eV)

τ0 (s)

0.6a 0.7a 0.7a 0.7a 0.5

10−18a 10−19a 10−19a 10−19a 10−14

−23

0.7 0.8 0.7

10 10−26 10−23

0.7 0.8 0.8 0.8

10−24 10−27 10−27 10−26

a Values determined from the first derivative of the signal. bSee Figure S8 in the Supporting Information.

The values of the activation energies and the pre-exponential factors for P1 agree very well with those characterizing the rotational motion of water clusters confined in various media.46,47,49,52,53,57−59 In that case, the so-obtained preexponential factor (10−24 s) is several orders of magnitude below any physical meaning value, which indicates that the water relaxation does not fit Starkweather’s definition of a simple relaxation.60 This suggests that a deviation from the Arrhenius behavior would occur at higher temperature. These whole findings support that the water molecules do not distribute uniformly; i.e., the formation of clusters within the porous solid is preferred to that of uniform layers.54 This relates to the 3D structure of the UiO-66(Zr)s, characterized by small microporous cages interconnected through narrow windows (Figure S2, Supporting Information). Such a resulting topology excludes the formation of water layers but favors the creation of clusters with an average size smaller than those in the liquid state. These statements hold true for the parent and the monofunctionalized solid, as evidenced above by the MD simulations (Figure 2a,b and Figure S1, Supporting Information). Finally, to confirm that P1 is associated with the dynamics of the confined water clusters, the impact of the water loading was studied. In the case of UiO-66(Zr)-COOH, P1 is not present anymore for the lowest hydration level (TT = 373 K), which suggests the absence of water clusters within the material. This behavior is consistent with the hydrophilic nature of this solid,31 which favors at the initial stage the interactions between the adsorbed water and the host rather than those between the water molecules themselves. Regarding P2, to support its association with the cooperative motion of the water molecules bonded to the host pore wall, the response of the less hydrated UiO-66(Zr)-COOH (TT = 373 K) was first considered as a starting point. The values of the dielectric parameters in terms of energy and pre-exponential factor match those reported for the ligand rotational motion in a series of dehydrated MOFs (cf. Table 1). Especially, the prefactor τ0 (τ0 = 10−14 s) converges toward typical vibrational times, which suggests that P2 corresponds to an individual/independent relaxation process. The activation energy value (ΔE = 0.51 eV) fairly agrees with those characterizing the flip of the ligands in

Figure 3. Dielectric loss versus temperature at fixed frequency ( f = 1522 Hz) for the hydrated UiO-66(Zr) (a), UiO-66(Zr)-COOH (b), and UiO-66(Zr)-(COOH)2 (c), treated at different temperatures TT for 30 min.

remains true for the whole hydration range of the solid, which prevents any data interpretation in terms of the dielectric relaxation process. Regarding the monoacid-functionalized material saturated with water (TT = 293 K), its dielectric response looks like the one reported in the case of the hydration of other families of solids, including silica nanoparticles, polymers, or graphite oxides.56−58 Indeed, these materials exhibit two dielectric signatures at the same frequency/temperature domain than the one explored here. This suggests that the same molecular dipole reorientations are probed; i.e., P1 could be ascribed to the reorientational motion of the bonded water molecules forming clusters confined within the porous solid, whereas P2 would relate to the dynamics of the water/solid surface complex. This is consistent with the examination of the 14445



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4. CONCLUSION This work addressed the microscopic adsorption mechanism of water in a series of hydrated narrow microporous cages type MOFs, namely, the UiO-66(Zr)s, whose hydrophilic features are modulated through the ligand functionalization by polar −COOH functions. Molecular dynamics simulations evidenced that the water molecules are arranged in such a way to form clusters within the cages of the pristine UiO-66(Zr). These clusters tend to establish preferential interactions with the acidic grafted functions in the functionalized solids, and their size and their concentration decrease when one passes from the mono- to the dicarboxylic versions, following the same trend than that of the water content present in these solids. This fundamental knowledge allowed the interpretation of the proton conduction properties evidenced for all the hydrated solids by broadband dielectric spectroscopy. A further step consisted of experimentally characterizing the cooperative dynamics of the host frameworks and the water molecules. This whole set of conclusions gained on a family of narrow cages type MOFs needs to be completed by a systematic exploration of hydrated MOFs with different pore sizes/ topologies. This will be the subject of our next investigation.

dehydrated terephthalate based MOFs, which ranges from 0.20 to 0.65 eV.62−70 Noteworthy, this energy value is within the upper limit, emphasizing a slightly hindered rotation of the ligands in UiO-66(Zr)-COOH. As already established in the case of the UiO-66(Zr)-NH2,70 this may be due to a steric effect resulting from the ligand functionalization and/or the existence of interactions involving the functional groups. This latter point is confirmed by the MD simulations (Figure S9, Supporting Information), which show that, at low level of hydration, the −COOH functions still interact with the nearest functional groups, as revealed by the relatively short distances (d−COOH/−COOH = 2.60 Å) similar to those present in the dried structure.21 We then concluded that, for low TT, P2 relates to the network dynamics through the ligand rotation, which is slightly perturbed by intraframework interactions. In a second step, the impact of the water content on the P2 dielectric parameters was examined (Table 1). At high level of hydration (TT = 313 K), ΔE slightly increases concomitantly with a sharp decrease of τ0 compared to data determined at TT = 373 K and both parameters remain roughly unchanged when TT decreases to 393 K. The opposite trend observed for ΔE (increase) and τ0 (decrease) results in a slightly faster process, as shown by the shift of P2 toward lower temperatures when one goes from TT = 373 K to TT = 313 K. This means that the ligand dynamics is enhanced despite the presence of sterically hindered water molecules at high level of hydration. For TT ≤ 313 K, τ0 deviates from physical meaning values, supporting that the dynamics of the ligand has changed from an individual toward an assisted/correlated movement with the water uptake, similarly to what has been previously observed for other guest/MOFs systems.61,71−73 Finally, we also noted that the sudden change of ΔE and τ0 values occurs simultaneously with (i) the sharp increase of the dielectric strength of the system, as shown by the higher intensity of the signal associated with the P2 process (1 order of magnitude), and (ii) the appearance of the P1 process (Figure 3b). This indicates that the water molecules might be also involved in the P2 process, which prompted us to assume that P2 recorded for TT ≤ 313 K corresponds to local movements of the ligands in interaction with water embedded in clusters. The resulting motion of the ligand/water entities is faster than that of the individual ligand due to faster dynamics of the water molecules constituting the clusters, consistent with the fact that P1 occurs at lower temperature than P2. These statements are corroborated by the MD simulations, showing that, at saturation, the functionalized ligand interacts with the water cluster through the acidic functions (cf. Figure 2b, dH2O/COOH = 1.76 Å), which supports that both dynamics are inevitably correlated. It also follows that the polarity of the resulting ligand/water entities is higher than that of the individual ligand, in agreement with the more intense P2 signal when one compares the dielectric response for the low and the high levels of hydration. On the basis of purely steric effects, the creation of such water/ligand entities would lead to the slowing-down of the correlated water/ligand rotational motion compared to that of the individual ligand. In contrast, one observes the opposite behavior, which can be explained by (i) a speeding up of the ligand assisted motion induced by water with a faster intrinsic dynamics and (ii) a weakening of the intraframework interactions shown by the shift of the (COOH−COOH) RDF to longer distances (Figure S11, Supporting Information), induced by water molecules that lie between two neighboring acidic functions.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing the arrangement of water molecules confined in the UiO-66(Zr)s and UiO-66(Zr)-COOH, pore size distribution of the UiO-66(Zr)s, radial distribution functions, and Arrhenius plots. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.D.-V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.M. thanks the Region Languedoc Roussillon for its funding through the Award “Chercheur d’Avenir” 2009 and the Institut Universitaire de France for its support. A.P., G.M., and S.D.-V. acknowledge the Scientific Council of Université Montpellier 2. The research leading to these results has received funding from the European Community’s Seventh Framework Program (FP7/2007-2013) under grant agreement no. 228862.

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REFERENCES

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