Cation Behavior in Faujasite Zeolites upon Water Adsorption: A

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Cation Behavior in Faujasite Zeolites upon Water Adsorption: A Combination of Monte Carlo and Molecular Dynamics Simulations Cyril Abrioux,† Benoit Coasne,† Guillaume Maurin,† François Henn,*,† Marie Jeffroy,‡,§ and Anne Boutin‡ Institut Charles Gerhardt Montpellier, UMR 5253 CNRS, UniVersite´ Montpellier II, Place E. Bataillon, 34095 Montpellier cedex 05, France, Laboratoire de Chimie Physique, UMR 8000 CNRS, UniVersite´ Paris-Sud, 91405 Orsay, France, and Institut Français du Pe´trole, 1-4 AVenue de Bois Preáu, F-92852 Rueil-Malmaison Cedex, France ReceiVed: March 13, 2009; ReVised Manuscript ReceiVed: April 28, 2009

Grand Canonical Monte Carlo and molecular dynamics simulations are employed to investigate the influence of water adsorption on the arrangements and the dynamics of the sodium cations in faujasites Na56Y and Na96X. The water adsorption provokes significant cation redistributions in Na56Y, while the partition of the cations among the different crystallographic sites is not affected upon the whole adsorption process in Na96X. The first water molecules in Na56Y are adsorbed both in the sodalite cage and in the supercage, that is, interacting with cations in SI′ and SIII′, respectively. In contrast, the first water molecules in Na96X are located within the supercage interacting with cations in SIII′ only. The cation dynamics are then explored. In Na56Y, only very local motion is observed for cations in sites SI′ whatever the water loading. At low and intermediate water loadings, the cations initially in SII and SIII′ present local displacements around their initial sites only whereas they move over much longer distances at high loading. Finally, due to a strong steric repulsion between cations in Na96X, the average cation mean square displacement for this system is always smaller than for Na56Y. 1. Introduction Microporous zeolite materials attract a great deal of attention because of their use in industry for catalysis, phase separation, ionic exchange, and so forth.1,2 The significant impact of these systems results from their large surface area, the nanoscopic size of their pores, and the large variety of chemical compositions, that is, Si/Al ratio and the nature of the extra-framework cations. From a fundamental point of view, zeolites are model systems that can be used to investigate the effect of nanoconfinement on both thermodynamic and dynamic properties of fluids.3,4 The substitution of Si with Al atoms in the aluminosilicate zeolites induces a net negative charge on the framework that is compensated by the introduction of extra-framework cations (Li+, Na+, K+, Ca2+, Ba2+, Mg2+, and so forth). Many experimental and theoretical studies have shown that the location of these cations plays a crucial role on the thermodynamics, dynamics, and catalysis of various adsorbates/zeolite systems.5-10 Because of the hydrophilic character of these materials (arising from the strong electrostatic cation-water interaction), the presence of water molecules in the porosity of aluminosilicate zeolites cannot be avoided in many applications operating at ambient temperature. This byproduct can have a beneficial or detrimental role on the adsorption/separation properties of zeolites depending on the degree of hydration necessary to observe significant cation displacements. As a typical illustration, in the separation of para- and meta-xylene, it is known that 3 wt % of water can improve up to 50% the separation ability of barium faujasite BaX by favoring the displacements of the * To whom correspondence should be addressed. E-mail: francois.henn@ univ-montp2.fr. † Institut Charles Gerhardt Montpellier. ‡ Laboratoire de Chimie Physique Orsay. § Institut Français du Pe´trole.

compensating extra-framework cations.11 As a result, understanding the effect of water adsorption on the distribution of cations in zeolites such as faujasite is of crucial interest in various applications. Such studies, which aim at investigating the effect of polar solvents on the mobility of extra-framework cations, are also relevant to other applied or fundamental topics such as the confinement of electrolytes in nanomembranes12 and ionic transport in biological channels.13,14 In the case of zeolites and other nanoporous solids, this work is also of interest for the purpose of the ionic exchange in which some cations such as Na+ are substituted with other alkali or alkaline earth ions in order to improve or orientate the properties of the material toward a specific application such as detergent.15 Among this class of materials, significant effort has been devoted to the study of faujasite zeolites, due to their particular importance in applications in the field of catalysis and phase separation including alkylation, transalkylation, isomerization of m-xylene into p-xylene and cracking catalysis.16-19 The aim of the present paper is to probe the effect of water adsorption on the thermodynamics and dynamics of cations in faujasite by means of complementary Monte Carlo and molecular dynamics simulations. We select an explicit (Si,Al) model8-10,20,21 to describe the microscopic structure of the faujasite in which the Si and Al atoms are explicitly taken into account as it corresponds to a more realistic situation from both chemical and structural point of views. Two faujasite systems with different degrees of Al/Si substitution leading to Si/Al ratios of 1.0 and 2.4 and corresponding to 96 and 56 Na+ cations per unit cell, respectively, are considered. These two structures will be referred as Na96X and Na56Y, respectively. We have recently investigated the adsorption of water in these two zeolites using grand canonical Monte Carlo (GCMC) simulations.22 The simulated macroscopic data including adsorption enthalpies and

10.1021/jp902274t CCC: $40.75  2009 American Chemical Society Published on Web 05/22/2009

Cation Behavior in Faujasite Zeolites upon Water Adsorption isotherms obtained for each faujasite were then favorably compared to those experimentally reported in the literature.22 In the present paper, we first explore the microscopic redistribution of the cations upon the whole adsorption process using grand canonical Monte Carlo simulations. We also address the local arrangement of the water molecules around the cations corresponding to the solvation process. Further, from the configurations generated by our GCMC calculations, molecular dynamics (MD) simulations are then performed to probe the dynamics of the cations as a function of the adsorbed amount of water. The originality of this contribution is to combine complementary GCMC and MD simulations to get a complete picture of the cations behavior upon water adsorption and to compare the role of the cation concentration into the nanoporosity of the material.

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10697 TABLE 1: Interatomic Potential Parameters and Atomic Partial Charges Used to Describe the Interactions between the Framework, the Extraframework Cations and the Watera (a) atom charges species

charge (e)

Si Al Oz Na

2.4 1.4 -1.2 1.0 (b) Buckingham potential

ion pair

A (eV)

B (Å)

C (eV · Å6)

Na-Oz

3542.21

0.24186

0.00

(c) Lennard-Jones potential

2. Computational Details 2.1. Faujasite Zeolite. Faujasite is an aluminosilicate zeolite with the general chemical formula Mx/m, Alx, Si192-x, O384, where M is a cation of charge +m.23 The number x of cations in the unit cell can vary from 0 to 96, depending on the Si/Al ratio. The crystalline structure of faujasite as well as the position of the extra-framework cations is already known from X-ray or neutron diffraction.24,25 The faujasite structure belongs to the Fd3m space group of symmetry with a unit cell of 24.85 Å that contains 192 TO4 tetrahedra (with T ) Si or Al).24 The alkali extra-framework cations in faujasite are mainly located in different crystallographic sites that are usually labeled as follows: (i) the sites I (SI) are located in the center of the hexagonal prism, while the sites I′ (SI′) are present in the sodalite cage toward the hexagonal prism, (ii) the sites II (SII) and sites III′ (SIII′) are located in the 6-ring and 12-ring windows of the supercage respectively, (iii) the sites III (SIII) are in the center of the 4-ring windows of the supercage. 2.2. Interatomic Potentials. The set of interatomic potentials considered in this work to model the interactions between the sodium cations and the rigid zeolite framework is that derived by Kramer et al.26 based on a combination of ab initio calculations and adjustment on experimental structural data for R-quartz.27 This interatomic potential has been validated by a favorable comparison between the energy optimized and experimental structures as well as the elastic properties of several materials (Berlinite, Zeolite X, and so forth). This interatomic potential consists of a Buckingham interaction term and a Coulombic contribution reported in Equation 1

Uij ) Aij exp(-Bijrij) -

Cij rij6

+

qiqj rij

(1)

where i and j are labels for the cation Na+ and the oxygen atom of the framework (Oz), Aij, Bij, and Cij correspond to the Buckingham potential parameters for the Na+-Oz interaction and qi, qj are the charges carried out by each interacting atom of the system summarized in Table 1. The cation/cation, cation/ Si, and cation/Al interactions were treated only via a Coulombic contribution. All the Coulombic interactions in the present work were computed using the Ewald summation technique. The parameters selected for the Ewald summation were R ) 2.5 and kmax ) 10. The TIP4P model was employed to represent the water molecule.28 This model is based on a description of a rigid water molecule using 4 sites, 3 electrostatic centers (the 2 hydrogen atoms with a charge of 0.52e and 1 pseudoatom with a charge

ion pair

εij (eV)

σij (Å)

Ow-Ow Ow-Oz Na-Ow

0.00672 0.00736 0.00540

3.1536 3.0768 2.8688

a Oz is the oxygen of the framework and Ow is the oxygen of the water.

of -1.04e) and one repulsion-dispersion center located on the oxygen atom. The interaction of the water molecules with the zeolite framework is described as the sum of the Coulombic contribution and a Lennard-Jones term between the oxygen atoms of the water molecule (Ow) and the oxygen atoms of the zeolite (Oz). Following the work by Di Lella et al.,29 the Lennard-Jones cross parameters for the water-zeolite interaction were derived from the Lennard-Jones parameters determined by Pascual et al.30 for oxygen and the Lorentz-Berthelot combining rules. Table 1 lists all the interatomic potential parameters used in this work. 2.3. Grand Canonical Monte Carlo Simulations. The different configurations of the faujasite Na56Y and Na96X upon various pressure of water were generated using GCMC. The GCMC technique is a stochastic method that simulates a system having a constant volume V (the pore with the adsorbed phase) in equilibrium with an infinite reservoir of particles imposing its chemical potential µ and temperature T.27,31,32 These simulations were carried out using the GIBBS code developed by the IFP and the Laboratoire de Chimie Physique (CNRS and Universite´ de Paris Sud - 11). Random translations were attempted with a maximum displacement of 0.3 Å in order to obtain an acceptance probability of 40-50%. We also used MC steps that consist of simultaneous deletion/insertion attempts of a particle as originally proposed by Di Lella et al.29 This particle move can be seen as a superdisplacement of a randomly selected particle through the simulation box, which significantly improves the sampling of the phase space. Such a simultaneous deletion/insertion of particles has proved to be very efficient in simulating the effect of water adsorption on the distribution of cation in zeolites.29 Starting with a random initial configuration with x cations, the system is allowed to equilibrate for 80 × 106 MC Steps. Then the data “at equilibrium” were averaged for another 80 × 106 MC steps. In these GCMC calculations, we considered a rigid zeolite framework. This assumption is expected to be reasonable as thermodynamic and structural properties are not too sensitive to the flexibility of the host material when the size of the adsorbate significantly differs from those of the windows. This is supported by several molecular simulation studies with a rigid framework which reasonably reproduce the experimental data.33,34

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TABLE 2: Pair Potential Parameters Used to Describe the Interactions within the Zeolite Framework during the Molecular Dynamics Simulationsa (a) Buckingham potential ion pair

A (eV)

F (Å)

C (eV.Å6)

Si-Oz Al-Oz Oz-Oz

30023.00 26998.00 894.60

0.1621 0.1622 0.3244

12.84 12.84 0.00

(b) three-body angle potential angle Oz-Si-Oz Oz-Al-Oz a

k (eV · rad-2) 12.10 2.20

θ0 (deg) 109.47 109.47

Oz is the oxygen of the framework.

Our Monte Carlo simulations were averaged over five (Si,Al) different configurations to get results that are independent of any particular realization of the zeolite framework, that is, distribution of the Al atoms upon the 192 T sites. For Na56Y, the five configurations were built by randomly distributing the 56 Al atoms among the 192 possible T-sites. Only configurations obeying the Loewenstein rule,35 which prohibits Al-O-Al sequence, were selected. For Na96X, there is only one possible distribution for the Al atoms, if one wants to verify the Loewenstein rule as there are 96 Al atoms and 96 Si atoms, that is, the Al atoms occupy necessarily one site every two T-sites. 2.4. Molecular Dynamics Simulations. The dynamics of the cations was investigated as a function of the adsorbed amount of water in the zeolite using MD. In contrast to the thermodynamics and structural properties investigated using GCMC simulations, the dynamic properties are known to depend on the flexibility of the host framework (dynamics of the guest species are often coupled to that of the host matrix).36,37 As a result, we considered a flexible zeolite framework in the MD simulations.Thesamecation-zeolite,water-water,water-zeolite, and water-cation interatomic potential functions were used in the MD codes as those used in the MC simulations. The zeolite-zeolite interatomic potentials were described using Buckingham potentials, including explicit Si-Oz, Al-Oz, and Oz-Oz terms, and additional harmonic three-body terms for the Oz-Si-Oz and Oz-Al-Oz intratetrahedral angles. The corresponding parameters, which are taken from the paper of Ramsahye et al.,38 are reported in Table 2. Again, the electrostatic interactions were evaluated using the Ewald summation method. The MD simulations were performed using the DLPOLY MD package39 in the NVT ensemble using the Berendsen thermostat32 at 400 K. This temperature is higher than in the MC study; this is because it is more complex to observe significant motion of the cations at 300 K on a reasonable time scale. As a result, we arbitrarily increased the temperature up to 400 K in order to enhance the rate of cation displacements, assuming that it does not affect the microscopic mechanisms of the cation motions. A similar approach was used by Jaramillo and Auerbach20 who performed MD simulation runs at temperature up to1000 K to estimate the cation dynamics in faujasites. In contrast to the GCMC simulations, we used a single Si/Al configuration for representing the Na56Y in these MD simulations (we recall that there is only one possible Al distribution for the Na96X so that no averaging of the data is needed for this sample). The typical Na56Y configuration was selected as the one leading to a simulated cation distribution as close as possible from the average one obtained in the GCMC

Figure 1. Distribution of the sodium cations calculated at 300 K in the four sites of the faujasite Na56Y as a function of the number of adsorbed water molecules per unit cell: (squares) SI, (diamonds) SI′, (triangles) SII, (circles) SIII′, and (stars) nonlocalized cations. A cation is considered on a given site if it is located at a distance shorter than 2.0 Å from the crystallographic site position; otherwise it is considered as nonlocalized. Dashed lines are guided for eyes.

simulations. A time step of 1 fs was used, and we first performed 50 ps of equilibration followed by 2 ns of production. The mean square displacements 〈∆r2j (t)〉 (MSD)32 for the different type of cations (cations in the sites SI, SI′, SII, and SIII′) were evaluated by means of eq 2 N

MSD(t) ) 〈∆rj2(t)〉 )

∑

N

∑

1 1 ∆r2(t) ) (r (t) - rj(0))2 N j)1 j N j)1 j (2)

where N corresponds to the number of extraframework cations, rj(t) corresponds to the position of the atom j at a time t and rj(0) corresponds to the position of the atom j at the initial time. We also used multiple time origins in order to improve the statistics of the calculation. To investigate the effect of water adsorption on the mobility of the cations, we calculated the MSD of the cations for several water loadings. Throughout this paper, we emphasize that the labels SI′, SII, and SIII′ for the cations refer to their location in the starting configuration and not to their position at any subsequent time. We are aware that this choice constitutes an arbitrary choice as the cations can move upon water adsorption toward a site of a different type from that of its initial location. Nevertheless, although keeping track of the current location of the cation is feasible, we did not attempt to do so as it is not necessary for the purpose of this paper. Moreover, following the site location of a cation rises the nontrivial question of labeling a cation in a site A which would occupy for a very brief laps of time a different site B and then moving back to a site A. Should we consider this cation as A or B? How long should the cation stay in B to “officially” consider it as cation A or B? As a result, to keep things as simple as possible, we assumed in the MD simulations that a cation keeps the label corresponding to its initial position, that is, SI, SI′, SII, and SIII′. 3. Results and Discussion 3.1. Faujasite Na56Y. Figure 1 reports the distribution of the sodium cations in Na56Y as a function of the number of adsorbed water molecules. The cations are considered in a given site if their positions are at a distance shorter than 2.0 Å from the crystallographic coordinates of the corresponding site determined from X-ray or neutron diffraction; the cations located at a larger distance from any site are considered as “nonlocalized” in the

Cation Behavior in Faujasite Zeolites upon Water Adsorption

Figure 2. Number of water molecules solvating cations in the faujasite Na56Y as a function of the total number of adsorbed water molecules per unit cell: (squares) SI, (diamonds) SI′, (triangles) SII, (circles) SIII′, and (stars) nonlocalized cations. A water molecule solvates a cation if the distance between the cations and the water oxygen (Ow) is shorter than 3.5 Å. Dashed lines are guided for eyes.

present work. Using this assumption, we found that the cations that are labeled nonlocalized correspond only to cations present in the supercage. Figure 1 clearly shows that the distribution of the cations in the dry state is significantly modified upon water adsorption. This result is consistent with previous works on water adsorption in faujasites34,40 and other zeolites such as Na+-Mordenite8 (see also refs 29 and 41 for reviews on water physisorption in zeolites). Here, our results suggest that the populations of sites SI and SI′ are correlated, that is, the sum of the cations in these two sites remains almost constant whatever the loading (NSI + NSI′ ∼ 20 Na+), which corresponds in average to 2.5 per sodalite cage). This outcome was emphasized by Mortier et al. from experimental consideration42 and by Beauvais et al.34 from numerical simulation. As soon as few water molecules are adsorbed, some cations initially in SI tend to move to SI′. A similar correlation is also observed for the sites SII, SIII′, and nonlocalized cations. Again, the sum of the cations in these three sites is almost unchanged whatever the loading (NSII + NSIII′ + Nnonlocalized ∼ 36 Na+). The population of the cations in SII first decreases from 26 to 19 Na+ as the loading increases up to 210 H2O/uc. Then for larger loadings, the number of cations comes back to 25 Na+. In contrast, the number of cations in SIII′ first increases from 8 up to 12 Na+ in the range of water loading [0, 70] H2O/uc and then decreases down to 6 Na+. The number of nonlocalized cations increases from 0 up to 6 Na+ in the range of water loading [0, 290] H2O/uc. These observations highlight that the adsorption of water molecules induces the redistribution of cations in the supercages among the SII, SIII′ and nonlocalized ones. This result is because the solvation of cations in SIII′ is more favorable than that of cations in SII, as the SIII′ is a more accessible position for the adsorbed water molecules. On the other hand, as the number of adsorbed water molecules becomes larger than 210 H2O/uc, the number of cations in SII increases due to steric effects; that is, moving some cations from SIII′ back to SII allows gaining some space in the supercages. The increase of nonlocalized cations upon adsorption shows that the water molecules detrap cations out of their site and hence take part in the cation redistribution. The arrangement of the water molecules and the cations within the zeolite were analyzed from the pair correlation functions g(r), which are related to the probability of finding an atom at a distance r from another atom.43 One can obtain the number of solvating water molecules around a cation from the integration of the first peak of the pair correlation function. Figure 2 shows the number of water molecules solvating cations

J. Phys. Chem. C, Vol. 113, No. 24, 2009 10699 in the faujasite Na56Y as a function of the water loading obtained from our GCMC simulations. We considered in our calculations that a water molecule solvates a cation if the distance between the oxygen atom of the water molecule and the cation is shorter than 3.5 Å, which corresponds to the minimum of the corresponding pair correlation function. We note that cations in SI are not solvated as the water molecules cannot enter the hexagonal prism where these cations are located. In contrast, even at low loading (∼50 adsorbed water molecules), the cations in SI′ are solvated in average by 1.5 water molecules, which shows that the adsorbate penetrates the sodalite cages at the beginning of the adsorption process. We show in Figure 3a a typical configuration of water molecules solvating two cations in SI′ at low loading. At higher loading close to the saturation, the cations in SI′ are solvated by up to 3 water molecules. In a bulk aqueous solution of sodium chloride, the number of water molecules solvating a sodium cation is about 5.4.44 The smaller value obtained in our case is due to the steric effect of the pore walls of the sodalite cage which hinders the “liquidlike” behavior of the cation/water molecule system. We report in Figure 3b a typical configuration at high pressure (3000 Pa) where we can observe that 3 water molecules located in the same sodalite cage solvate the same cation. In addition, we observe that the first adsorbed water molecules do not solvate cations in SII, which is in agreement with the experimental data previously reported in the literature.45 Solvation of cations in SII first occurs as the number of adsorbed water molecules becomes larger than 32 H2O/uc. For a loading close to the saturation capacity, cations in SII can be solvated on average by 3.5 water molecules. This value is still smaller than the solvation sphere of the sodium cation in a bulk solution.43 A typical illustration is provided in Figure 3c where we can observe four water molecules solvating a cation in SII at high loading. Regarding cations in SIII′, they are already solvated at low loading. At full loading, these cations are solvated by up to 5.5 water molecules which is close to the bulk behavior.43 Meanwhile, the important difference with the bulk state appears for the secondary solvation sphere where water molecules solvate more than one cation (SII and/or SIII′). That is not the case in dilute solutions but that can be observed for highly concentrated case as in the faujasite supercage in which 56 or 96 cations are mixed with about 100 water molecules. As far as SIII′ cation is concerned, this result means that the zeolite framework does not influence, that is, when no steric hindrance applies, the first cation solvation sphere very much. This result confirms that the cations in SIII′ correspond to both steric and energetic preferential adsorption sites for the water molecules.22 Another finding of the present work is that solvation of the nonlocalized cations does not differ from that of cations in SIII′. Again, this is because these types of cations are located in open spaces of the faujasite framework where the water molecules can easily access (in a similar way to what was observed for cations in SIII′). Furthermore, Figure 4 reports the mean square displacements (MSD) obtained from our MD simulations performed at 400 K for the sodium cations initially located in SI′, SII, and SIII′ as a function of the water loading. The MSD of the cations in SI is not shown because as we have already observed in Figure 1, the number of cations in SI estimated from our GCMC calculations is too small to obtain good statistics for their displacements. Figure 4a shows that, whatever the considered water loading, the MSD of the cations initially in SI′ are rather small (between 0.2 and 1.6 Å2 for 2 ns), which correspond to only local displacements of the cations around their mean

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Figure 4. MSD calculated at 400 K for cations in the faujasite Na56Y for different water loadings: (a) cations in SI′, (b) cations in SII, and (c) cations in SIII′.

Figure 3. Typical arrangement of water molecules solvating cations at 300 K in the faujasite Na56Y: (a) cations in SI′ (in the sodalite cage) solvated by water at low loading (30 H2O/uc), (b) cations in SI′ (in the sodalite cage) solvated by water at high loading (290 H2O/uc), and (c) a cation in SII (in the supercage) solvated by water at high loading (290 H2O/uc). The atoms of the rigid framework are represented using the following colors: yellow (Si), pink (Al), and red (Oz). The cations are the purple spheres, and the water molecules are the white (H) and red (Ow) segments. The distances are reported in angstroms.

positions. An illustration of such behavior is provided in Figure 5a, which represents a typical series of positions occupied by a cation in SI′ upon adsorption of 250 H2O/uc as detected during the MD simulation run. This observation suggests that these types of cations are strongly trapped in their initially crystallographic sites within the whole range of loading.

Furthermore, the MSD of the cations initially in SII are rather small up to 96 H2O/uc (∼10 Å2 for 2 ns). Such a low mobility of the cations is also due to a strong localization of the cations around their initial site as illustrated in the trajectory shown in Figure 5b. This MSD becomes much important at high loading (∼60 Å2 for 2 ns), which indicates that in such case the cation SII are able to move over longer distances. Such a possible “long-range” trajectory is illustrated in Figure 5c where we can observe a typical series of positions occupied by a cation initially in SII for a loading of 250 H2O/uc along the MD trajectory. In this figure, we can observe the following redistribution of the cation: SII f SIII′ f SIII′; the cation first moves after 360 ps to a SIII′ and further shifts to another SIII′. While, we did not focus in this work on the correlation between cation and water dynamics, it can be readily assumed from the highly concentrated configuration of cations in the supercage that when a cation moves, all the cations and water molecules have to


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Figure 6. Distribution of the sodium cations calculated at 300 K in the four sites of the faujasite Na96X as a function of the number of adsorbed water molecules per unit cell: (squares) SI, (diamonds) SI′, (triangles) SII, (circles) SIII′, and (stars) nonlocalized cations. A cation is considered on a given site if it is located at a distance shorter than 2.0 Å from the crystallographic site position; otherwise it is considered as nonlocalized. Dashed lines are guided for eyes.

Figure 5. A typical series of positions occupied by a cation in the faujasite Na56Y at 400 K as recorded in the MD simulation runs: (a) cation in the sodalite cage at high coverage (250 H2O/uc), (b) cation in the supercage at intermediate coverage (96 H2O/uc), and (c) cation initially in SII in the supercage at high coverage (250 H2O/uc). The atoms of the framework are represented using the following colors: yellow (Si), pink (Al), and red (O). The successive positions of the extra-framework cation are the spheres blue, cyan, purple, and green (cations in sites SI′, SII, SIII′, and nonlocalized, respectively). The other cations and the water molecules have been removed for the sake of clarity.

accommodate their positions, leading to collective motions, as it was previously claimed from conductivity and dielectric measurements.46 Figure 4c reports the MSD of the cations initially in SIII′. They exhibit very similar behavior than those pointed out for the cations in SII at both low and intermediate loadings. This observation indicates that cations in SIII′ remain close to their initial sites. In contrast, the corresponding MSD is about 120 Å2 after 2 ns at higher loading for 250 H2O/uc. Such a value is significantly larger than the MSD for the cations initially located in SII. This result is because cations in SIII′ are less bounded to the zeolite framework than the cations in SII. This observation corroborates with the positions occupied by the cation in the dehydrated faujasite, which shows that the sites SII are energetically favored for the cations.22 3.2. Faujasite Na96X. Figure 6 reports the distribution of the sodium cations in the faujasite Na96X as a function of the number of adsorbed water molecules per unit cell obtained from our GCMC simulations. Except at high loading where nonnegligible fluctuations are observed, the distribution of the cations obtained for the dehydrated faujasite is almost unchanged upon water adsorption, 32 cations in SI′, SII, and SIII′. It is worth noting that there are up to two cations in SI at high water loading. This observation suggests that cation triplets SI′-SI-SI′ are stable at high water loading. Such triplets, which do not correspond to stable states at low and intermediate loadings because of the strong cation-cation repulsion, seem to be stable at high loading. Because of the screening exerted by the water molecules at high loading, we also observed up to four nonlocalized cations, which come from the redistribution of cations initially in SIII′ upon the adsorption of a large number of water molecules. Figure 7 reports the number of water molecules solvating the cations in the faujasite Na96X as a function of the number of adsorbed water molecules per unit cell. As for Na56Y, the cations in SI are not solvated because the water molecules cannot access the hexagonal prism. At low loading, the water molecules solvate only the cations in sites SIII′ and the nonlocalized ones. From 1.6 up to 2 water molecules solvate on average each of these types of cations when about 60 water molecules are adsorbed. A typical illustration of such behavior is provided in Figure 8a where we can distinguish two water molecules solvating the same cation in SIII′. The solvation of the cations in SI′ and SII occurs when the water molecules start being adsorbed in the sodalite cage (∼84 adsorbed water molecules).

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Figure 7. Number of water molecules solvating the cations in the faujasite Na96X as a function of the total number of adsorbed water molecules per unit cell: (squares) SI, (diamonds) SI′, (triangles) SII, (circles) SIII′, and (stars) nonlocalized cations. A water molecule solvates a cation if the distance between the cations and the water oxygen (Ow) is shorter than 3.5 Å. Dashed lines are guided for eyes.

This result suggests that above such a loading, the water molecules interact with both cations in SI′ and SII. These observations are in qualitative good agreement with the interpretation reported by Dzhigit et al.47 who proposed from the shape of the adsorption isotherm that the water molecules are first adsorbed in the supercage before going through the sodalite cages above 64 H2O/uc. Such a loading corresponds to a situation where two water molecules solvate each cation in SIII′. For larger numbers of adsorbed water molecules, cations in SI′ and SII start being solvated. At full loading, the cations in SI′ and SII are solvated in average by 2.5 and 3.5 water molecules, respectively. Again, these values, which are lower than those obtained for the bulk (∼5.4),43 are due again to the steric hindrance caused by the zeolite framework. We show in Figure 8b a typical configuration where 3 water molecules solvate one cation in SI′ at high loading. For this loading, the number of water molecules solvating the cations in SIII′ and the nonlocalized cations are 4.5 and 5.5, respectively. The difference between these two values is due to the full occupation of the SII. For Na96X, all of the SII are occupied so that there is no extra-space available for water molecules to solvate the cations in SIII′. This result is different with what was observed for Na56Y where a larger number of solvating molecules in SIII′ was found (5.5) because some of the SII were not occupied. In contrast to the cations in SIII′, the nonlocalized ones are fully solvated (as much as in the bulk solution), which indicates that there is no strong steric repulsion from the zeolite framework that would prevent the water molecules from interacting with these cations. As a typical illustration, Figure 8c shows a configuration where 5 water molecules solvate a cation in SIII′. Figure 9 exhibits the mean square displacement (MSD) at 400 K of the cations in SI′, SII, and SIII′ for different water loadings. The number of cations in SI is too small to obtain good statistics for their displacements. So, the resulting MSD is not reported. Such a trend is very similar to what was found for the same type of cations in the case of the faujasite Na56Y. We also show in Figure 10a a typical series of positions occupied by a cation in SI′ as seen in the course of a MD simulation run for a water loading of 250 H2O/uc. In contrast to Na56Y (Figure 5a), we observe a displacement from SI′ to SI through the hexagonal window. According to the site population (Figure 6), which shows that SI is almost unoccupied, it may be concluded that this motion is highly reversible, that is, back and forth motion, so that SI′ remains the most probable site. Remarkably, this trajectory is the only one that passes

Figure 8. Typical arrangement of water molecules solvating cations at 300 K in the faujasite Na96X (a) cations in SIII′ (in the supercage) solvated by water at low loading (60 H2O/uc), (b) a cation in SI′ (in the sodalite cage) solvated by water at high loading (290 H2O/uc), and (c) cations in SIII′ (in the dodecahedral window) solvated by water at high loading (290 H2O/uc). The atoms of the rigid framework are represented using the following colors: yellow (Si), pink (Al) and red (Oz). The extra-framework cations are the purple spheres, and the water molecules are the white (H) and red (Ow) segments. The distances are reported in angstroms.

through a hexagonal window, thus showing that the energy barrier between the sodalite cage and the hexagonal prism is low enough to allow this motion during the simulation time scale, that is, 2 ns. Regardless, it can be considered as a local motion since the maximum distance from the crystallographic site is about 2 Å. Figure 9b shows the MSD of the cations



Figure 9. Mean square displacements (MSD) calculated at 400 K for cations in the faujasite Na96X for different water loadings: (a) cations in SI′, (b) cations in SII, and (c) cations in SIII′.

initially in SII. At intermediate loading [96, 200] H2O/uc, the MSD is rather small (less than 10 Å2 after 2 ns). An illustration of the possible trajectory is shown in Figure 10b for a loading of 96 H2O/uc. At higher loading for 250 H2O/uc, the MSD becomes much larger (∼45 Å2 in 2 ns). Figure 10c illustrates a possible “long range” trajectory of a cation. Figure 9c shows the MSD of the cations initially in SIII′. This MSD is small (from 10 up to 20 Å2 in 2 ns) for both low and intermediate loadings [96, 150] H2O/uc. In contrast, at higher loading for 250 H2O/uc the MSD for cations in SIII′ reaches a value of about 70 Å2 in 2 ns. These results for the Na96X are quite similar to that obtained for the Na56Y. The only difference occurs for the MSD values. Indeed, for the Na96X the amplitude of the MSD is slightly smaller than the MSD for the Na56Y. This result suggests that the larger number of cations in Na96X hinders the dynamic of the cations compared to that in Na56Y. 4. Conclusion Our GCMC simulation shows that the cations present in Na56Y and Na96X behave in a different way upon the adsorption of water; the adsorbate provokes significant cation redistribution in the case of Na56Y while the populations of the cation site

Figure 10. A typical series of positions occupied by a cation in the faujasite Na96X at 400K as recorded in the MD simulation runs: (a) cation in the sodalite cage initially in SI′ at high coverage (250 H2O/uc), (b) cation in the supercage at intermediate coverage (96 H2O/uc), and (c) cation in the supercage at high coverage (250 H2O/uc). The atoms of the framework are represented using the following colors: yellow (Si), pink (Al), and red (O). The successive positions of the extra-framework cation are the spheres orange, blue, cyan, purple, and green (cations in sites SI, SI′, SII, SIII′, and nonlocalized, respectively). The other cations and the water molecules have been removed from the graphs for the sake of clarity.

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are not affected in the case of Na96X. Further, we emphasize that in Na56Y, the first water molecules are adsorbed in both the sodalite cage and the supercage, interacting with cations in SI′ and SIII′ respectively. In contrast, the first water molecules in Na96X are located within the supercage interacting with cations in SIII′ only. Despite these different behaviors upon water adsorption for Na56Y and Na96X, comparable cation dynamics are observed in both faujasites. For instance, only local motion is detected for cations in sites SI′ whatever the water loading. Cations initially in SII and SIII′ also exhibits only local displacements around their initial site at low and intermediate water loadings. In contrast, cations in SII and SIII′ move over much longer distances at high loading. Finally, due to the strong steric repulsion present in Na96X (which contains the largest number of sodium cations), the average cation mean square displacement for this sample is always smaller than for Na56Y. It is worth noting that we did not observe any cation transfer between the supercages and the sodalite cages in the molecular dynamics runs. This result can be explained by the diameter of the hexagonal window (∼5 Å) and the diameter of the cation with its solvation sphere (∼7 Å). Indeed, a cation has to get rid of its solvation sphere in order to pass through this window. This event is characterized by a high energy barrier so that it is a rare event. To detect such reorganization, a more appropriate technique such as Kinetic Monte Carlo32 or blue moon MD48,49 technique should be considered. However, a SI′SI-SI′ trajectory through the hexagonal window between the sodalite cage and the hexagonal prism appears to be possible in the Na96X at high loading during the simulation time scale. This event can be due to the following three factors: (i) the strong cationic repulsion, that is, four cations into the sodalite cage, (ii) the steric hindrance induced by the water molecules, that is, four water molecules into the sodalite cage, and (iii) the smaller solvation sphere around the cation, that is, about 2.5 water molecules per cations (Figure 7). Noteworthy, all our results agree with the previous experimental data reported by Mortier42 and Schoonheydt46 who emphasized the complex behavior of the cations and water molecules subsystems and their resulting correlation in different zeolites. Further, it is interesting to compare the results obtained in the present work with the predictions made by Shirono et al. using molecular dynamics simulations.21 As far as Na96X is concerned, the distribution of cations in the “dry” state, observed by Shirono et al. is identical to that obtained in this work. In addition, these authors also found that (i) there is no cation redistribution for this system upon water adsorption and (ii) the water molecules first interact with cations in SIII′ before those located in SII. Unfortunately, it was not possible to compare in a quantitative way their results for the NaY systems with ours as we did not consider the same concentration of extraframework cations (Na64Y21 vs Na56Y here). However, these authors emphasized that surprisingly no cation redistribution occurs for NaY upon water adsorption, which disagrees with what we observed. As pointed out by Shirono et al.,21 the sodium cations do not move appreciably during the MD simulation run starting with initial configurations randomly generated for each water loading; this explains the lack of cation redistribution in their simulations as the cations remain located in the sites where they were initially placed. In contrast, a different approach was used in the present work as we first performed a Monte Carlo simulation run to equilibrate the system, thus allowing us for instance not to suffer from large energy barriers associated with passing the hexagonal windows. Further, using higher temper-

Abrioux et al. ature (400 K here instead of 300 K in ref 21) and larger simulation run time (2 ns here instead of 400 ps in ref 21) could also explain the deviation between both studies. We are also aware that the quality of the predictions obtained from molecular simulations depends on the selection of both the models to describe the guest molecule and the adsorbent and the interatomic potentials. We recently provided an illustration of the effect of different forcefields with explicit and T-atom models in terms of thermodynamic data (adsorption isotherm, isosteric heat of adsorption, and cation distribution).22 It is thus rather difficult to compare the conclusions drawn here to that obtained by Shirono et al., as the models for representing the water and the interatomic potentials significantly differ. Finally, the flexibility of the framework that was not considered by Shirono et al. could also play a role in the dynamics of the cation. References and Notes (1) Corma, A. Chem. ReV. 1997, 97, 2373. (2) Soler-Illia, G. J. D.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (3) Karger, J.; Vasenkov, S.; Auerbach, S. M.; Diffusion in Zeolites; In Handbook of Zeolite Science and Technology; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker, Inc.: New York 2003. (4) Demontis, P.; Suffritti, G. B. Chem. ReV. 1997, 97, 2845. (5) Maurin, G.; Devautour, S.; Henn, F.; Giuntini, J. C.; Senet, P. J. Chem. Phys. 2002, 117 (4), 1405. (6) Devautour, S.; Abdoulaye, A.; Giuntini, J. C.; Henn, F. J. Phys. Chem. B 2001, 105, 9297. (7) Moı¨se, J. C.; Bellat, J. P.; Me´thivier, A. Microporous Mesoporous Mater. 2001, 43, 91. (8) Maurin, G.; Bell, R. G.; Devautour, S.; Henn, F.; Giuntini, J. C. J. Phys. Chem. 2004, 108, 3739. (9) Maurin, G.; Llewellyn, P. L.; Poyet, Th.; Kuchta, B. J. Phys. Chem. B 2005, 109, 125. (10) Plant, D. F.; Maurin, G.; Jobic, H.; Llewellyn, P. L. J. Phys. Chem. B 2006, 110, 14372. (11) Moise, J. C.; Bellat, J. P.; Cotier, V.; Paulin, C.; Methivier, A. Sep. Sci. Technol. 1998, 33, 2335. (12) Kim, E.; Xiong, H.; Striemer, C. C.; Fang, D. Z.; Fauchet, P. M.; McGrath, J. L.; Amemiya, S. J. Am. Chem. Soc. 2008, 130, 4230. (13) Long, S. B.; Tao, X.; Campbell, E. B.; McKinnon, R. Nature 2007, 450, 376. (14) Lockless, S. W.; Zhou, M.; McKinnon, R. PLoS Biol. 2007, 5, e121. (15) Coker, E. N. Zeolites to Porous MOF materials - the 40th AnniVersary of International Zeolite Conference; Xu, R.; Gao, Z.; Chen, J.; Yan, W., Eds.; Elsevier: NewYork, 2007; Vol. 170a, p 110. (16) Zhu, Z.; Chen, Q.; Zhu, W.; Kong, D.; Li, C. Catal. Today 2004, 93-95, 321. (17) Borgna, A.; Sepu´lveda, J.; Magni, S. I.; Apesteguá, C. R. Appl. Catal., A 2004, 276, 207. (18) Davis, R. J. J. Catal. 2003, 216, 396. (19) Zaidi, H. A.; Pant, K. K. Catal. Today 2004, 96, 155. (20) Jaramillo, E.; Auerbach, S. M. J. Phys. Chem. B 1999, 103, 9589. (21) Shirono, K.; Endo, A.; Daiguji, H. J. Phys. Chem. B 2005, 109, 3446. (22) Abrioux, C.; Coasne, B.; Maurin, G.; Henn, F.; Boutin, A.; Di Lella, A.; Nieto-Draghi, C.; Fuchs, A. H. Adsorption 2008, 14, 743. (23) Meier, W.; Olson, M. D. H. Structure Commission of the International Zeolite Association; Elsevier: Amsterdam,1978. (24) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311. (25) Vitale, G.; Mellot, C. F.; Bull, L. M.; Cheetham, A. K. J. Phys. Chem. 1997, 101, 4559. (26) Kramer, G. J.; Farragher, N. P.; van Beest, B. W. H.; van Santen, R. A. Phys. ReV. B 1991, 43, 5068. (27) Nicholson, D.; Parsonage, N. G. Computer Simulation and the Statistical Mechanics of Adsorption; Academic Press: New York, 1982. (28) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926. (29) Di Lella, A.; Desbiens, N.; Boutin, A.; Demachy, I.; Ungerer, P.; Bellat, J. P.; Fuchs, A. H. Phys. Chem. Chem. Phys. 2006, 8, 5396. (30) Pascual, P.; Ungerer, P.; Tavitian, B.; Pernot, P.; Boutin, A. Phys. Chem. Chem. Phys. 2003, 5, 3684. (31) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford: Clarendon, 1987. (32) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From Algorithms to Applications, 2nd ed.; Academic Press: London, 2002.



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Cation Behavior in Faujasite Zeolites upon Water Adsorption: A

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