Effect of the Confinement and Presence of Cations on Hydrogen

Article pubs.acs.org/JPCC

Effect of the Confinement and Presence of Cations on Hydrogen Bonding of Water in LTA-Type Zeolite S. Calero and P. Gómez-Á lvarez* Department of Physical, Chemical and Natural Systems, University Pablo de Olavide, Ctra. Utrera km. 1, 41013, Seville, Spain S Supporting Information *

ABSTRACT: The hydrogen bond is one of the most important intra- or intermolecular interactions for molecular systems, especially for those involving water, which have been systematically investigated. Although we deal with water in nanoporous materials in a number of applications, elucidating to what extent its structure is perturbed from the bulk is a virtually unexplored issue. We performed Monte Carlo simulations to compute the adsorption of water in pure silica and cation-containing LTA-type zeolites, namely, ITQ-29, LTA-4A, and LTA-5A, and evaluated the effect of the confinement and presence of cations on its structural properties. The high water stability and industrial impact of zeolites, with either hydrophobic or hydrophilic character, make them of great interest for the targeted purpose. The analysis was carried out in terms of the radial distribution functions and hydrogen bond statistics. Specifically, the percentages of water molecules engaged in i hydrogen bonds and the average number of hydrogen bonds per water molecule were computed on the basis of a geometric criterion for hydrogen-bonding definition. The water clustering was likewise evaluated in terms of the number and size of the aggregates. Results reveal a notable structural transformation with respect to bulk water and contribute insight into this problem.

1. INTRODUCTION Liquids that display hydrogen bonding1,2 have a singular intermolecular structure that affects considerably their macroscopic behavior. Among them, water3,4 is unquestionably the most significant case. Its extraordinary and not completely understood behavior arises mainly from the complicated hydrogen-bonding (HB) network. A considerable body of work exists on bulk water, both experimental5−12 and computational.13−28 Because of the significant impacts on a diversity of chemical and biological processes, widespread fundamental and industrial interest exists in the structure of water when it is confined in small cavities. Most of the reported studies on this matter are focused on water within pores of simple geometries, such as carbon nanotubes29−32 and cylindrical silica pores,33−39 or slit-shaped pores40−48 formed by two parallel surfaces. Such work has been carried out for a variety of pore diameters, hydration levels, and thermodynamic conditions and showed that confined water differs significantly from bulk water. Likewise, other authors dealt with water in a realistic Vycor-like silica mesoporous system.49−52 However, our present understanding of interfacial or confined water at the molecular level is still very limited. The hydration of zeolites is an important process such as catalysis, transport phenomena, or purification of wastewater. In many cases, highly hydrophilic zeolites are required, and so aluminosilicates with low Si/Al ratio containing charge-balanced cations are especially suitable. To gain insight into the association at the molecular level of water molecules confined in the nanometer-scale channels and pores of this kind of system is an essential point. However, the studies on this issue are scarce.53−55 Most of the experimental © 2014 American Chemical Society

techniques are difficult to carry out in nanometer environments. Besides, the quantitative interpretations of hydrogen bond formation of an associated system are not precise and unambiguous because there is no direct physical observable for the H-bond. Theoretical investigation is of great help for a better understanding of confined fluid properties. In particular, molecular simulation56,57 represents a useful alternative to the experiments. This technique provides direct information about the molecular-level structure of the fluid on the basis of a geometric definition of H-bonds applied over the microscopic configurations of the system produced along the simulation. This work is aimed at analyzing the effect of the confinement as well as the presence of extra-framework cations on the structure of water in LTA-type zeolite. In particular, we considered the all-silica structure ITQ-29 and the Na+ and Na+/ Ca2+ aluminosilicate forms of the zeolite, LTA-4A and LTA-5A, respectively. Monte Carlo molecular simulations in the grand canonical ensemble were conducted to compute the adsorption of water in the three structures at room temperature using previously validated potentials. A proper description of its hydrogen bonding along the isotherms has been achieved using a specific criterion of H-bond formation in terms of the radial distribution functions (RDFs) and HB statistics. These calculations involved the fraction of molecules in the monomer or associated state, the fraction of molecules with i hydrogen bonds f i, and the average number of hydrogen bonds per Received: February 11, 2014 Revised: March 26, 2014 Published: April 7, 2014 9056

dx.doi.org/10.1021/jp5014847 | J. Phys. Chem. C 2014, 118, 9056−9065

The Journal of Physical Chemistry C

Article

molecule nHB. In addition, we computed the average number of formed aggregates Nagg and their mean s and maximum smax size (number of molecules belonging to them). To compare the results at high degree of hydration with those for bulk liquid, molecular simulations of water in the isothermal−isobaric ensemble were also carried out. The remainder of this paper is organized as follows. In Methodology, we briefly describe the models and force fields used together with some computational details. In Results and Discussion, we report in particular on the simulated radial distribution functions (subsection 3.1) and hydrogen-bonding analysis (subsection 3.2). Finally, the main results are contained in Conclusions.

composition is Na96Si96Al96O384. LTA-5A is obtained by replacing 64 monovalent sodium cations from the LTA-4A by 32 bivalent calcium cations in an exchange after synthesis; therefore, the unit cell composition is Ca32Na32Si96Al96O384. All the structures were considered rigid, and framework atoms were placed at the experimental crystallographic positions. The LTA-4A and LTA-5A zeolites obey the generally accepted Löwenstein rule,60 which states that Al−O−Al linkages are energetically forbidden. The nonframework cations in these structures were described as charged single-interacting centers able to move freely. Their positions are adjusted depending on their interactions with the framework atoms, other cations, and water molecules. 2.2. Water Model. Numerous models exist in the literature to describe water, each capable of capturing some experimentally observed characteristics. As yet, no universal model has been developed for this important molecule. For this study, water has been represented by the TIP5P/Ew model.61 This is a rigid five-site model with a tetrahedral geometry generated by two dummy atoms whose properties have been refitted using Ewald sums. It has a single dispersive center at the oxygen atom with ε/kB =89.516 K and σ =3.0970 Å. Point charges are located at the hydrogen atoms (qH = 0.241 e−) and at the two dummy atoms, which accounts for the negative partial charge of the oxygen atom. We chose this particular model because the adsorption isotherms can be efficiently computed.62 In addition, only a five-site model was found able to reproduce the maximum of densities at about 4 °C.62 Other models, particularly TIP4P,63 TIP4P/Ew,64 and TIP4P/2005,65 have also been considered in the hydrogen-bonding calculations of bulk water in order to compare results. 2.3. Simulation Details. Monte Carlo simulations of water at room temperature were performed using the RASPA code developed by Dubbeldam and co-workers.66,67 On the one hand, the adsorption isotherms of water in the three structures were computed in the grand canonical ensemble (μVT) where volume, temperature, and chemical potential are held constant and the number of molecules fluctuates. The equilibrium conditions are equal temperature and equal chemical potential of the fluid inside and outside the zeolite. The chemical potential is determined from the fugacity, which is the effective thermodynamic pressure. We converted the imposed fugacity to the corresponding pressure using the Peng−Robinson equation of state.68 The interactions between guest molecules (water and cations) and with the zeolite host framework were modeled by Lennard-Jones and Coulombic potentials. The parameters characterizing these L-J cross interactions are summarized in Table 1S of the Supporting Information, and were taken from Castillo et al.69 except those concerning calcium, which are not defined in that work. They were calculated from the values corresponding to sodium taking into account the relation between the parameters of both cations with oxygen framework atoms reported in previous works of our group.70,71 The reliability of the resulting cross-potential parameters involving calcium was proven for comparison with experimental data72 on adsorption of water in LTA-5A. Because dispersive forces are dominated by oxygen atoms, interactions of Si atoms were not taken into account. Dispersive interactions of cations between each other were also neglected because of the strong electrostatics. The Coulombic interactions in the system were calculated using the Ewald summation56,57 by assigning partial charges to extra-framework cations (qCa = 2qNa = 0.7668 e−) and to every atom of the zeolite. The model used

2. METHODOLOGY 2.1. Structures. Zeolites are porous, crystalline aluminosilicates that possess a three-dimensional structure. The zeolite framework consists of an assemblage of TO4 tetrahedra (T = Si, Al) joined together in various regular arrangements through shared oxygen atoms to form a crystal structure containing pores of molecular dimensions into which guest molecules can penetrate. The negative charge created by the substitution of an AlO4 tetrahedron for a SiO4 tetrahedron is balanced by nonframework cations, which are located throughout the structure. A brief description of the zeolites considered in this work, namely ITQ-29, LTA-4A, and LTA-5A follows. LTA-type zeolites are crystal structures consisting of a three-dimensional cubic array (α = β = γ = 90°) of supercages (or α-cages) with approximate diameter of 11.2 Å interconnected through eightmember oxygen windows of about 4.2 Å and sodalite cages (or β-cages) with an average diameter of 6.6 Å alternating with the α-cages and separated by 2.2 Å openings. The experimental stable all-silica LTA-type structure is called ITQ-29.58 The unit cell can be described as a single α-cage with dimensions a = b = c = 11.867 Å. On the other hand, the Na+ and Na+/Ca2+ forms of the LTA-type structure, LTA-4A and LTA-5A, respectively, have an alternating silicon and aluminum arrangement that can only be periodically described in terms of eight cages. Therefore, their crystallographic unit cell is approximately twice larger in each direction than ITQ-29, specifically a = b = c = 24.555 Å. Figure 1 illustrates the unit cell. Zeolite LTA-4A was synthesized for the first time by Breck et al.59 The unit cell

Figure 1. Unit cell of the LTA-type zeolite. 9057



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explicitly distinguishes silicon from aluminum with a difference of 0.3 e−: qSi = 0.78598 e− and qAl = 0.48598 e−. On the other hand, to take the polarization of framework oxygen atoms by nearby cations into account, different charges were assigned to oxygen atoms bridging two silicon atoms, qOSi = −0.39299 e−, and those bridging one silicon and one aluminum atom, qOAl = −0.41384 e−.73 The simulation box was composed of a single unit cell for the simulations in LTA-4A and LTA-5A and eight unit cells (2 × 2 × 2) for ITQ-29 with periodic boundary conditions. The number of unit cells for each structure was chosen to get a simulation box larger than twice the LennardJones cutoff radius, which was fixed to 12 Å. Simulations were arranged in cycles of trial moves including molecular translation and rotation, and insertion or deletion of a molecule. The maximum translational and rotational displacements are adjusted to achieve an acceptance probability of 50%. The total number of cations remains constant during simulations, and only translation movements are considered for this type of particle. It must be noted that no artificial blocks were required in our simulations because water can penetrate throughout the structures, including sodalite cages.74 As for bulk water, simulations of 500 molecules were conducted in the isothermal−isobaric ensemble (NpT) in a cubic box with periodic boundary conditions56,57 employing a cutoff radius of 12 Å. Usual long-range corrections56,57 were used for LennardJones interactions and the Ewald summations56,57 for the electrostatic interactions. In this case, moves in each cycle cover molecular translations and rotations as well as volume changes. In all the simulations, a large number of cycles were needed both for equilibrating the system and for the production run. The choice of a specific criterion to determine when a HB is established remains to some extent arbitrary. The two approaches commonly used in computer simulations are based on energetic13 and geometric14,17 conditions. In the geometric definition, two molecules are assumed to be hydrogen-bonded if they satisfy some criteria on their position and relative orientation. In energetic criteria, one uses a cutoff on the pair interaction energy between two molecules to decide whether they are hydrogen-bonded. Sometimes, additional configurational criteria such as a cutoff on the O−O distance is employed in the energetic definition to make it more strict.25 In this work, we adopted the geometric criterion, which was applied to every pair of molecules in a considerable number of computer-generated configurations. According to this, two molecules are engaged if the following conditions are fulfilled: (1) the distance between the oxygen atoms of both molecules rOO is smaller than a threshold value rTOO, (2) the distance between the oxygen of the acceptor molecule and the hydrogen of the donor rOH is less than rTOH, and (3) the angle α between the O−O direction and the molecular O−H direction of the donor, where H is the hydrogen which forms the bond, has to be less than a threshold value αT. For the sake of clarity, the defined magnitudes are represented in Figure 2. We note that the critical distances are essentially the positions of the first minima in the O−O and O−H radial distribution functions. The angular criterion17 reflects the directional character of hydrogen bonds, where the bond angle O−H···O is close to linear (180°). The threshold values are well-established in the case of bulk water at various thermodynamic conditions,19 but they are not defined for water in confinement. Thus, an analysis of the RDFs and the α angle distribution corresponding to our systems has been carried out to account for the hydrogenbonding properties.

Figure 2. Distances and angle involved in the geometric criterion of H bond formation.

3. RESULTS AND DISCUSSION The thermodynamics of confined water is depicted in Figure 3. Figure 3a shows the water adsorption isotherms for the three structures at 298.15 K. The curve for the all-silica ITQ-29 is typical of hydrophobic nanoporous solids.75,76 Negligible water uptake exists up to about atmospheric pressure, after which rapid pore filling occurs. This abrupt condensation transition is not observed for aluminosilicate zeolites, consistent with previous studies.55 The water adsorption in LTA-4A and LTA-5A is shifted to quite lower values of fugacity. This hydrophilic character is attributable to the field gradient−dipole interaction energy due to the presence of cations. As can be seen from the comparison of water coverages in both Na+/Ca2+ and Na+ forms of the zeolite, the interaction for divalent cations is stronger than that for monovalent cations; LTA-5A adsorbs water earlier and exhibits the highest loading throughout the pressure range. Note that at the highest values of fugacity, the water loading is virtually the same, slightly greater than 20 mol/ kg, for all the structures, which is an indicator of nearly full hydration. Although being qualitatively the same as Figure 3a, the density of the confined water states is shown in Figure 3b to give an idea of the molecular packing. This magnitude has been defined as the relation between the adsorbed water molecules and the effective volume of the framework. As the water coverage increases, the density becomes closer to that in the bulk liquid and even exceeds it at the highest values of fugacity, as occurs in supercritical water. In the following, the structure of water along the adsorption isotherms is presented and comprehensively analyzed. 3.1. Radial Distribution Functions. Figures 4 and 5 show the gOO(r) and gOH(r) functions, respectively, for water in the bulk and in confinement in the different structures for moderate and high loadings. The results reveal the existence of hydrogen bonds in all the systems, but a redistribution of nearest neighbors is observed. On the one hand, the gOO(r) function for bulk liquid water is characterized by a pronounced first maximum at 2.8 Å arising mainly from the hydrogenbonded O−O interactions and two broad peaks at 4.5 and 6.6 Å, which are assigned to the interactions of the second and the third neighbors. This reflects the tetrahedral ordering of water molecules due to hydrogen bonding. The first minimum is located at 3.6 Å. Note that this behavior is virtually invariant with pressure. The O−O RDFs corresponding to water confined in the zeolites exhibit clear differences from that of bulk water. For pure silica ITQ-29, the first peak and the first minimum are basically kept in the same position, but the second hydration shell is absent, which is connected to the loss of the tridimensional tetrahedral structure typical of bulk liquid water. Likewise, the higher the fugacity and so the degree of 9058



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Figure 3. Computed (a) adsorption isotherms and (b) density versus fugacity for water in ITQ-29 (○), LTA-4A (□), and LTA-5A (△) zeolites at 298.15 K on logarithmic scale. Lines are guides to the eye.

Figure 4. O−O RDFs for water (a) in the bulk and confined in (b) ITQ-29, (c) LTA-4A, and (d) LTA-5A zeolites at 298.15 K for various values of fugacity.

These functions exhibit sharp peaks, placed at about 2.15 and 2 Å, respectively. This indicates that the water molecule has a greater affinity for Ca2+ than for Na+, which favors the adsorption in LTA-5A, consistent with Figure 3. Note that the peaks of gNaO(r) in LTA-4A and LTA-5A follow opposite tendencies with increasing fugacity, which is probably due to the presence of calcium cations in the latter. To provide insight into the hydration degree of cations, a number of related properties have been likewise computed and included in the Supporting Information. Table 2S collects the minimum cation−cation and water−cation distances, both absolute and average values for the generated configurations. Among the cations, the respective values follow the order Ca2+−Ca2+ > Na+−Ca2+ > Na+−Na+, consistent with the repulsive electrostatic forces. While these minimum distances generally decrease with increasing water coverage, the minimum water−cation separation is virtually independent of the fugacity. This point proves that cations act as preferential sites of adsorption. Note

hydration, the sharper the maximum. Unlike what occurs for ITQ-29, while the position of the first peak for water in LTA4A and LTA-5A remains almost invariant, the following minimum is shifted to a greater distance slightly dependent on the fugacity that ranges from 4.5 to 5 Å. This broadening of the first maximum is an indicator of a weakening of hydrogen bonding. In addition, a small second peak can be recognized at about 6 Å. On the other hand, the common feature of the gOH(r) function in bulk liquid water is the existence of a strongly sharp first maximum at 1.8 Å, related with hydrogen bonding, followed by a distinct maximum at 3.2 Å. The deep minimum following the first peak is located at 2.4 Å. These characteristic extrema remain virtually unchanged for water confined in all the structures. It is worth noting that the second maximum is significantly broader in the case of water in cationic zeolites. Figure S1 of the Supporting Information illustrates the water oxygen-nonframework cations RDFs, gNaO(r) and gCaO(r). 9059



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Figure 5. O−H RDFs for water (a) in the bulk and confined in (b) ITQ-29, (c) LTA-4A, and (d) LTA-5A zeolites at 298.15 K for various values of fugacity.

Figure 6. Number of total (red), associated (blue), and monomer (green) water molecules in (a) ITQ-29, (b) LTA-4A, and (c) LTA-5A zeolites as a function of the fugacity at 298.15 K. Lines are guides to the eye.

Supporting Information. In the case of sodium cations, this magnitude is 2−3 except at low coverage in both structures. The second hydration shell in the RDFs at approximately 4 Å corresponds to about 7 water molecules around the cation, either Na+ or Ca2+, for high water uptake. 3.2. Hydrogen Bond Statistics and Cluster Analysis. The radial distribution functions are only a weakly sensitive

that water molecules get slightly closer to calcium than to sodium cations, in agreement with previous comments. Figure S2 of the Supporting Information shows the average number of neighboring water molecules as a function of the distance from the cation. As can be seen, the average coordination corresponding to the first solvation shell is between 3 and 4 for Ca2+ cations, which is illustrated in Figure S3 of the 9060



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Table 1. Distribution of the Adsorbed Water Molecules in α and β Cages and Degree of Association in Each Cage for LTA-4A and LTA-5A Zeolites at 298.15 K and Various Values of Fugacity

measure of the collective structure of water, but their characteristic extrema are the basis of a geometric definition of a hydrogen bond. Specifically, the cutoff distances of the criterion are identified with the position of the first minima of these functions, as previously commented. These positions were found to be very stable; thus, a geometric criterion seems to be adequate. On the one hand, according to previous results for gOO(r), we assumed rTOO = 3.6 Å for ITQ-29 and rTOO = 4.6 Å for cationic zeolites, which coincide with those of water at normal and supercritical19,20,28 conditions, respectively. On the other hand, the O−H threshold distance can be unambiguously determined for water confined in all the structures from the minima of gOH(r): rTHO =2.4 Å. Finally, we have computed the α angular distribution for water pairs fulfilling the two distance conditions both in the bulk and in the structures. The results are depicted in Figure S4 of the Supporting Information as a histogram representing the percentage of angles between the molecule pairs ranging from 0° to 5°, from 5° to 10°, and so on. As shown in the figure, all distributions are virtually equal. Most angles are lower than 10°, and those higher than 30° are quite unlikely. Whereas this distribution is invariant with fugacity for bulk liquid water and water in ITQ-29, slight differences are observed in LTA-4A and LTA-5A. The results are in complete agreement with previously reported works on bulk water,19 where the maximum of the distribution was shown at about 9°. In the definition of the hydrogen bond for bulk water, one establishes the cutoff at 30° to allow some fluctuations from the most favorable configuration.17 A somewhat larger value of the threshold angle would account for nonlinear hydrogen bonds. In light of these results, the assumption that αT =30° can be extended here to confined water regardless of the structure. In short, two water molecules in this work are considered to be hydrogen-bonded if (1) the O−O separation is less than 3.6 and 4.6 Å for pure silica and cation-containing zeolites, respectively; (2) the O−H separation is less than 2.4 Å; and (3) the α angle is less than 30°. The degree of association can be observed in Figure 6, where the average number of monomers and associated molecules together with the total number of adsorbed water molecules in the simulation cell (s.c.) is presented as a function of the fugacity. Overall, our calculations provide a significant percentage of associated molecules in the structures. The rapid pore-filling phenomenon (Figure 3) characteristic of hydrophobic zeolites such as ITQ-29 implies the absence of monomers in this structure; as occurs in bulk liquid water, virtually all molecules are associated. Conversely, the presence of cations and hydrophilic character of LTA-4A and LTA-5A modify considerably the water−water interactions. At the lowest values of fugacity, most of the adsorbed molecules are monomers. As fugacity increases, and so the water loading, the hydrogen bond formation gains importance and associated molecules prevail over monomers from 100 Pa, approximately. Although association dominates at the highest coverages, the number of monomers is not negligible at all, especially in LTA4A. To get a deep understanding of the relation between the hydrogen bonding of water and the pores of the structure, a more detailed analysis has been carried out for the cationcontaining zeolites. In particular, we computed the distribution of the adsorbed water molecules in α and β cages and the percentage of associated molecules in each cage. Results are reported in Table 1 for the various values of fugacity. As can be seen, a great percentage of water molecules in sodalite cages

percentage of molecules in β-cages LTA-4A

LTA-5A

fugacity (Pa) 10−2 10−1 1 10 102 103 104 105 106 107

40 38 29 9 7 6 6 5 6 6

percentage of associated molecules in each cage

10 11 11 11 9 7 6 6 6 6

LTA4-A

LTA-5A

α-cage

β-cage

α-cage

β-cage

0 0 1 20 42 65 73 80 83 86

0 0 1 1 1 0 1 3 3 13

8 13 23 31 54 71 80 84 85 89

0 1 0 0 1 2 2 18 17 43

exist at low hydration in LTA-4A. This fact is in complete agreement with the well-known performance of small cavities as preferential sites of adsorption in the low-coverage regime. The guest molecules tend to occupy the positions where the adsorbate−adsorbate interactions are weaker than the adsorbate−pore interactions, corresponding to the energetically more favorable positions. However, this phenomenon is not observed in the case of LTA-5A because of the notable number of molecules adsorbed in this structure from the lowest values of fugacity. To illustrate this, a snapshot from the simulations at the specific fugacity of 1 Pa is included in the Supporting Information (Figure S5). Broadly speaking, the degree of clustering increases with cage capacity because the propensity to form clusters increases with the number of neighboring molecules inside a cage. This is clearly apparent from our values, which show a percentage of associated molecules at each state in α-cages that is significantly higher than that in β cages for both LTA-4A and LTA-5A. Note that although the percentage of water molecules in β-cages at high hydration is the same low quantity for both structures, the association among these molecules is more important in LTA5A. Next, we analyze the structural changes through the distribution f i of water molecules, with i = 1, 2, 3, 4, and 5 hydrogen bonds, and the average number of bonds per molecule nHB. As for bulk liquid water, these magnitudes have been computed using not only the TIP5P/Ew model but also other models reported in the literature. The results are collected in Table 2. A good agreement with previously reported data18−21 was found. Four-site based models are consistent among them, especially TIP4P/2005 and TIP4P/ Ew. The vast majority of molecules are engaged in 4 hydrogen bonds, and the fraction of molecules with 3 is also significant, Table 2. Hydrogen Bond Statistics of Bulk Liquid Water for Various Models 61

TIP5P/Ew TIP4P63 TIP4P/Ew64 TIP4P/200565 9061

f1

f2

f3

f4

f5

nHB

3.9 1.9 1.2 1.0

17.4 12.1 9.3 8.6

38.3 36.0 33.2 32.2

38.0 45.4 51.6 53.3

2.4 4.6 4.7 4.9

3.18 3.39 3.49 3.52



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resulting in a nHB value of about 3.5. When considering the fivesite TIP5P/Ew model, our simulations reveal a decrease of the fraction of molecules involved in 4 hydrogen bonds f4 in favor of f 3 and mainly f 2, which leads to a decrease of nHB to approximately 3.2. This breaking of some hydrogen bonds is probably due to the geometric constraints on the packing of HB partners. The hydrogen and dummy atoms located at tetrahedrally directed positions in this model tightly constrain hydrogen-bonded neighbors to the well-defined tetrahedral directions. Although the differences in energy parameters and charge distributions result in slight variations in these magnitudes, the molecular geometry is found to be the most important factor governing the hydrogen-bond structure in liquid water. Figure 7 shows the average number of bonds per molecule nHB for water in the structures at the different hydration levels.

Table 3. Percentages of Water Molecules Forming i Hydrogen Bonds and Average Number of Hydrogen Bonds per Molecule nHB for the Studied States fugacity (Pa)

f1

f2

f3

f4

f5

nHB

2.4

3.18

0.0 0.3 0.4 0.4 0.5 0.5 0.5 0.8

1.34 2.30 2.44 2.50 2.55 2.58 2.59 2.65

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1

0.00 1.00 1.00 1.07 1.18 1.46 1.61 1.72 1.81 1.88

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.2 0.2

1.02 1.05 1.10 1.24 1.39 1.59 1.77 1.85 1.91 1.93

bulk 3.9

17.4

69.9 18.7 14.8 12.9 11.9 10.9 10.6 11.2

26.5 41.0 37.6 36.2 34.7 34.0 33.7 30.1

10−2 10−1 1 10 102 103 104 105 106 107

0.0 100.0 99.8 93.5 83.3 62.4 51.6 44.3 39.7 36.7

0.0 0.0 0.2 6.4 15.5 30.1 36.4 40.2 41.2 42.0

10−2 10−1 1 10 102 103 104 105 106 107

98.5 94.7 90.2 77.8 66.9 53.1 43.5 39.4 36.7 35.9

1.5 5.3 9.4 20.3 27.9 35.3 38.3 38.9 39.5 39.0

105 1.5 × 2.0 × 2.5 × 5.0 × 7.5 × 106 107

Figure 7. Average number of hydrogen bonds per molecule nHB of water in (a) ITQ-29 (○), (b) LTA-4A (□), and (c) LTA-5A (△) zeolites as a function of the fugacity at 298.15 K.

An enhancement in the hydrogen-bonding network is observed with increasing loading. The nHB value for water in ITQ-29 at the highest values of fugacity is about 2.6, lower than that for the bulk counterpart. For water in the cationic zeolites, this magnitude is approximately 1 at the lowest values of fugacity, which indicates basically only formation of dimers, and nearly 2 at high coverage. This notable decrease of the hydrogen bonding can be explained in terms of the strong interactions between the local electrostatic fields created by cations and a highly polar molecule such as water. The space occupied by the cations is also an important factor and probably the reason why nHB is slightly larger for water in LTA-5A than in LTA-4A. The data obtained for this property are displayed in Table 3 along with the HB populations. The estimated error is always lower than 5%. The f i percentages at the various values of fugacity provide information on the variation of the hydrogen-bonding clustering as water uptake increases. While bulk liquid water is dominated by molecules forming 3 or 4 hydrogen bonds, the statistics relative to confinement at high hydration show that most water molecules appear with 2 and 3 hydrogen bonds in ITQ-29 and with 1 and 2 hydrogen bonds when we move to the cationic zeolites. The fraction of molecules engaged in 4 hydrogen bonds in the latter structures is virtually zero. Hence, both the confinement itself and the interaction forces in the aluminosilicate zeolites prevent the formation of the typical tetrahedral structure characterizing bulk liquid, and molecules tend to form aggregates. This water clustering has been

105 105 105 105 105

38.3 38.0 ITQ-29 3.6 0.0 32.5 7.6 37.3 10.0 39.2 11.2 40.7 12.2 41.6 13.0 42.1 13.1 42.2 15.6 LTA-4A 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 1.1 0.0 7.2 0.3 11.3 0.7 14.3 1.2 17.0 2.0 18.4 2.8 LTA-5A 0.0 0.0 0.0 0.0 0.4 0.0 1.9 0.0 4.8 0.4 10.7 0.8 16.5 1.7 19.3 2.3 19.9 3.7 21.9 3.0

characterized in terms of the average number of monomers Nmon and of aggregates Nagg and the mean s and maximum smax size of the latter. Results for various states are given in Table 4. This information together with the example of aggregate depicted in Figure 8 allows one to construct a picture of the HB network of water during the adsorption process in the zeolite frameworks. The rapid pore filling of water in ITQ-29 implies not only the virtual absence of monomers but also the presence of few large aggregates. Thus, molecules form an extended HB network similarly to the bulk state. The difference lies in the loss of the tetrahedral order. It is worth noting that the increase of hydration above 1 MPa involves the formation of more but shorter, on average, clusters. In regard to the cationic zeolites, low-sized aggregates appear at low values of the fugacity. As fugacity is increased, the cluster sizes increase, especially in LTA-5A. The number of aggregates increases with water loading up to 103−104 Pa and then takes lower values because of the formation of higher-order aggregates. However, for similar water loadings, the clustering in these structures is unquestionably simpler than that in ITQ-29 zeolite. 9062



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Table 4. Average Number of Monomers Nmon and Aggregates Nagg Together with the Mean s and Maximum smax Cluster Sizes for Water in the Three Structures at Various Values of Fugacity

implies the nucleation of water molecules, the degree of association gradually increases with coverage in LTA-4A and LTA-5A zeolites. Although water molecules remain highly structured at complete filling, a clear modification from the structure of bulk liquid was observed; the water−water hydrogen bonds diminish. This weakening of the hydrogen bonding is especially notable in the cationic zeolites. Specifically, the average number of hydrogen bonds in confined water with bulk-like densities was found to be about 2.6 and 2 in all-silica and cationic zeolites, respectively, which are far from the value of 3−3.5 in the bulk liquid. The more complex network of water in ITQ-29 than in the cationic zeolites is likewise clearly apparent from the results of the cluster analysis. Both the mean and maximum sizes of water aggregates in the pure silica structure far exceed those in LTA-4A and LTA-5A.

ITQ-29 fugacity (Pa) 1.5 × 2.0 × 2.5 × 5.0 × 7.5 × 106 107 108

105 105 105 105 105

fugacity (Pa) 10 102 103 104 105 106 107 108

Nmon 2 3 2 2 2 2 5 3 Nmon 76 80 69 64 55 53 46 42

Nagg 2.4 2.5 2.1 1.7 1.6 1.5 5.5 9.5 LTA-4A

s

smax

46 78 96 122 128 134 40 25

75 116 142 160 166 170 192 209

s

smax

Nagg 8 21 30 30 29 24 25 24 LTA-5A

2 2 4 5 6 8 9 10

■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

* Supporting Information S

3 5 11 18 29 51 66 76

Force field parameters, analysis of the hydration of cations, angle distribution for water molecules fulfilling the geometric distance criteria, and a snapshot from the simulation of water in both types of framework cages at low coverage. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

fugacity (Pa)

Nmon

Nagg

s

smax

10 102 103 104 105 106 107 108

90 80 64 55 49 48 40 27

14 24 28 25 24 21 24 26

3 3 5 7 9 11 11 11

6 9 16 34 44 76 84 123

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the European Research Council through an ERC Starting Grant (ERC2011-StG-279520RASPA).

■

REFERENCES

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Figure 8. Water cluster formed in the aluminosilicate LTA zeolite at high hydration.

4. CONCLUSIONS We performed Monte Carlo simulations to investigate the thermodynamic and structural properties at room temperature of water confined in LTA-type zeolite, both all-silica and aluminosilicate forms. The computed adsorption isotherms reflect the respective hydrophobic and hydrophilic characters of the frameworks, in agreement with previous works. According to the radial distribution functions, we assumed the geometrical definition of the hydrogen bonds suitable for the study. In the light of the results for HB statistics, water confined in the zeolites is still H-bonded. While the rapid intrusion in ITQ-29 9063



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Effect of the Confinement and Presence of Cations on Hydrogen

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