Impact of the Nature of Exchangeable Cations on LTA-Type Zeolite

Article pubs.acs.org/JPCC

Impact of the Nature of Exchangeable Cations on LTA-Type Zeolite Hydration Paula Gómez-Á lvarez,* Julio Perez-Carbajo, Salvador R. G. Balestra, and Sofia Calero* Department of Physical, Chemical and Natural Systems, Universidad Pablo de Olavide, ES-41013 Seville, Spain ABSTRACT: Due to their high cation-exchange ability as well as to their molecular sieve properties, natural zeolites are widely used in industry. In this work, we used molecular simulation to comprehensively explore at the microscopic scale the hydration process in aluminum-rich (Si/Al ratio = 1) LTA-type zeolites with different compositions of charge balancing sodium and calcium cations. The results reveal the nature of the cation to be more influential than the density of the cations, which is detectable in both the adsorption isotherms, and the energetic and structural descriptions of system interactions. The higher affinity of water to calcium cations in relation to sodium cations leads to a more hydrophilic character of Ca-containing zeolites, and the water clustering proceeds differently during the adsorption process. However, the extra-framework cations negligibly influence on the hydrogenbonded network of the adsorbed water at saturation, when a dense phase is formed, as well as on cation-water interactions.

1. INTRODUCTION Zeolites are crystalline porous materials formed by covalently bonded TO4 tetrahedra, where the central T atom is usually silicon Si or aluminum Al. All-silica lattices are charge neutral, but the substitution of Si4+ by Al3+ defines a negative charge that must be balanced by extra-frameworks cations (EFCs). Although all-silica lattices have been proved hydrophobic,1 the presence of aluminum atoms and EFCs in the zeolites increases the electrostatic field in the pores leading to a hydrophilic character of variable degree depending on the Si/Al ratio and consequently, on the EFC concentration. Another influencing factor on the hydrophobicity of the zeolite is the nature of the charge-balancing cations. The process of water adsorption in zeolites is of importance in a diverse of industrial and environmental applications such as catalysis, transport phenomena, or purification of wastewater.2−4 Furthermore, it has a key role in understanding the liquid-phase adsorption process in zeolites from water solutions. The high chemical stability (resistance to water) of zeolites, as well as their high thermal stability, low cost, easy cation-exchange and molecularsieving properties, make them greatly interesting for such purposes. In most cases, structures with low Si/Al ratio are required due to their highly hydrophilic behavior. There is a considerable body of work about the influence of water on the structural stability of zeolites with cations.5−8 However, our present understanding at the molecular level of the water adsorption process in these materials is still limited.9−13 In this regard, molecular simulation techniques14 represent a valuable alternative to experiments. Due to their well-defined structure, large surface area, and high porosity, Linde type A (LTA) zeolite15 has attracted much attention in the zeolite family. This material can be synthesized with tailored adsorption properties by controlling the Al © XXXX American Chemical Society

content in the framework. In particular, it is normally synthesized with a very low Si/Al ratio (Si/Al ratio = 1, Zeolite 4A). In this work, we investigate the role of the zeolite chemical composition in regard to the present EFCs on the hydration of LTA-type zeolite with such Si/Al proportion. We conducted Monte Carlo simulations in the Grand Canonical ensemble to compute the adsorption isotherms of water in the LTA aluminosilicate lattice for a variety of possible combinations of monovalent sodium cations and divalent calcium cations. The microscopic interactions of the adsorbed water molecules among each other through hydrogen bonds and with the host structures containing cations were largely analyzed.

2. METHODS We considered x Na+/y Ca2+ forms of the LTA-type zeolite with a Si/Al ratio of 1, with general formula NaxCaySi96Al96O384. In particular, the negative charge induced by the Al content has been fully compensated with the following combinations of sodium and calcium cations: 96 Na+, 80 Na+/8 Ca2+, 64 Na+ /16 Ca2+, 48 Na+ /24 Ca2+, 32 Na+/32 Ca2+, 16 Na+/40 Ca2+, and 48 Ca2+. Note that the 96 Na+ and 32 Na+/32 Ca2+ forms correspond to the experimentally synthesized LTA-4A and LTA-5A zeolites, respectively. The LTA-type aluminosilicate framework with Si/Al = 1 presents a cubic crystal structure with the space group Fm3c and a unit cell dimension of 24.555 Å. The structure consists of an arrangement of sodalite units (or β-cages) located at the vertices of a simple cubic lattice and Received: July 11, 2016 Revised: September 19, 2016

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DOI: 10.1021/acs.jpcc.6b06916 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Radial distribution functions for a single water molecule interacting with the framework oxygen atom and with the Na+ and Ca2+ cations in the experimental LTA-5A zeolite (a, b, and c, respectively) and with Na+ cation in LTA-4A zeolite (d).

Jones cutoff radius, which was fixed to 12 Å as typical. Periodic boundary conditions14 were exerted in the three dimensions. Long-range electrostatic interactions were calculated using the Ewald summation14 because Vlugt et al.23 concluded this method to be superior to other alternatives (direct summation methods), in both accuracy and speed. They showed that for studying adsorption in zeolites, even when nonframework cations are present, the energy and forces computed with the Ewald summation are well-defined and unique. Simulations were arranged in cycles of trial moves including molecular translation and rotation, and insertion or deletion of a molecule. Only translation movements are considered for the cations. We executed 50 000 cycles for equilibrating the system and 500 000 for the production run. We also perform complementary simulations in the canonical ensemble (NVT) for a single water molecule at 298 K. In this case, moves only involve molecular translation and rotation. No artificial blockage was required in our simulations because both water and cations enter the sodalite cages.16 We have indeed checked the accessibility of the sodalite cages by diffusion calculations (molecular dynamics simulations) by using the described models and force fields and RASPA code. Water molecules can diffuse throughout the structure although sometimes the diffusion is limited by the strong interactions with cations. We characterize the hydrogen-bonded (HB) statistics of the adsorbed water molecules using a geometric definition of HB formation.24 The O···O and O···H cutoff intermolecular distances of the criterion are identified with the position of the first minima in the respective radial distribution functions (RDFs). We checked and considered 30° as threshold O···O− H angle.

linked to each other by four-membered rings. The sodalite cages are accessible via six-ring windows, which are large enough to allow the diffusion of cations and water.16 The central void cavity formed by eight sodalite cages is called a supercage (or α-cage). Supercages are linked via eightmembered rings with a diameter of about 4.2 Å. All the structures were treated as rigid, and framework atoms were placed at the experimental crystallographic positions.15 The extra-framework cations were described as charged single interacting centers. They were able to move and their positions determined by the interactions with the framework atoms, other cations, and water molecules. For the 96 Na+ and 32 Na+/ 32 Ca2+ forms of the zeolite, we have checked that there are negligible variations in adsorption data by starting from the known crystallographic cation positions of experimentally synthesized LTA-4A and LTA-5A zeolites, or from random initial positions using an equilibration period of the system. Water has been defined by the five-site TIP5P/Ew model.17 The interactions between guest molecules (water and cations) and with the zeolite framework were modeled by Lennard-Jones (L-J) and Coulombic potentials. The parameters characterizing the L-J cross interactions are taken from previous works.11,18 Because L-J forces are dominated by the framework oxygen atoms, this type of interaction with the central either Si or Al atoms was not taken into account. van der Waals forces between cations were also neglected due to the strong electrostatic interactions. Partial charges were assigned to extra-framework cations and to every atom of the zeolite.19 We conducted Grand Canonical Monte Carlo (GCMC) simulations using RASPA code20,21 to compute the adsorption isotherms of water in the distinct zeolites with cations at 298 K. In the grand canonical ensemble (μVT), the chemical potential μ, volume V, and temperature T are held constant, and the number of molecules fluctuates. The chemical potential is determined from the imposed fugacity, which is the effective thermodynamic pressure.22 The simulation box was composed of a single unit cell, because it is larger than twice the Lennard-

3. RESULTS The above-described models and force fields have been previously validated with experimental data on adsorption isotherms. However, when studying microscopic behaviors, one B


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the oxygen atom of the zeolite is located at nearly 1.9 Å as is evident from the inset plot in Figure 1a. This distance is in agreement with the literature.5 As regards interactions with cations, parts b and c of Figure 1 evidence that the single water molecule in LTA-5A is unquestionably attracted by the calcium cations. The positions of the first peak of the RDF between the water oxygen and the Ca2+ cation in LTA-5A (Figure 1c) and the Na+ cation in LTA-4A (Figure 1d) ranges between 2 and 2.5 Å, being slightly larger for the latter, as previously reported.5,13 Likewise, we evaluated the average minimum distances from the extra-framework cations to the zeolite oxygen atom for the cation containing structures in the absence of water. Results are plotted in Figure 2. As can be seen, the calcium cations are located closer to the framework oxygen than the sodium cations, except for the NaCa zeolite form with highest density of sodium cations, where both distances are almost the same. We found values of about 2.51 Å in the LTA-4A zeolite, and 2.48 and 2.56 Å in the LTA-5A for calcium and sodium cations, respectively. According to previous works,5,25,26 Ca2+ and Na+ cations in the dehydrated 5A, are at about 2.3−2.35 Å from the O atoms building the four-rings in the β-cage, O(3), and at about 2.70−2.90 Å from the O atoms building the six-rings together with O(3), O(2), the distances being slightly larger for the Na+ cation. Our average values for this system (32 Na+/32 Ca2+ zeolite form) are hence consistent, and the influence of the cation environment is evident from the figure.

Figure 2. Average minimum distance from the extra-framework cations Na+ (red) and Ca2+ (blue) to the framework oxygen atoms in all the considered structures containing cations.

should be cautious in considering solely isotherm agreement as a measure of simulation fidelity. In this sense, we first evaluate the reliability of the methods to predict structural and energetic properties characterizing the interactions between guest particles and the zeolite framework. Figure 1 shows the RDFs for a single water molecule interacting with the framework oxygen atom and with the Na+ and Ca2+ cations in the experimental LTA-5A and LTA-4A zeolites. Results were obtained from NVT calculations at 298 K. The first peak in the RDF between the hydrogen atom of the molecule of water and

Figure 3. Average occupation profiles of water in LTA-type zeolite (Si/Al ratio of 1) with 96 Na+ (top left), 32 Na+ and 32 Ca2+ (top right), and 48 Ca2+ (bottom left) for similar intermediate water loadings (9−10 mol·kg−1) at 298 K from GCMC runs of the same length. The relation between color and occupation probability density (from blue to red) is shown in the bar color ramp. Picture at bottom right shows the atomic framework and solvent surface of the structure. C


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Figure 4. Average occupation profiles from NVT simulations of single water molecule in LTA-type zeolite (Si/Al ratio of 1) with 96 Na+ at 298 K (a) and with 48 Ca2+ at 298 K (b), at 500 K (c), and at 298 K from a number of runs with different starting configurations (d). The color code is the same as in Figure 3.

molecular simulation technique and force fields used in this work are found to suitably reproduce either experimental or computational reported information on the behavior of these systems. The interaction of water molecules with the sodium or calcium EFCs leads to distinct adsorption mechanisms that are apparent along the molecular simulations. Figure 3 shows the average occupation profiles for intermediate water loadings (about 9−10 mol·kg−1) in various zeolites with cations from GCMC runs of the same length. Although water distribution within the pores is roughly uniform for the all-sodium form (LTA-4A zeolite), an evidently inhomogeneous distribution is observed in the all-calcium form. This is due to the higherenergy calcium cations that, in lower concentrations, compensate the negative charge induced by aluminum atoms. The Ca/Na = 1 form (LTA-5A zeolite) exhibits a halfway behavior. The analysis in the all-sodium and all-calcium zeolites has been extended to the low-coverage regime in Figure 4 by conducting NVT simulations of a single water molecule. As previously reported,5 and is apparent from the pictured results, the sodalite cages are the energetically most favorable adsorption site. Figure 4b suggests strong interactions with the calcium cation, which represent a potential well for the water molecule deeper than that corresponding to sodium cation. This hypothesis is supported by Figure 4c: At high temperature, the molecule of water can be occasionally displaced from one potential well to another. We have also reproduced such water−calcium ion attractive behavior using

Figure 5. Energy probability distribution between a single water molecule and framework atoms and EFCs in the all-sodium and allcalcium LTA zeolite forms from NVT simulations at 298 K. Energy E is shifted by its average value ⟨E⟩, and σ denotes the standard deviation.

From an energetic viewpoint, we found the average potential energy of a single water molecule with the cations in LTA-4A and LTA-5A of approximately −78 and −157 kJ·mol−1, respectively, which are consistent with studies of ion hydration.27 These values represent most of the total potential energy of the water molecule (that involves also the interaction with the framework atoms), and that obtained for the LTA-4A zeolite is in agreement with available heats of adsorption of water in structures with sodium cations.28,29 Overall, the D


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Figure 6. Adsorption isotherms of water (a) and average cation−water potential energy vs fugacity (b) in the different sodium/calcium forms of the LTA-type zeolite with Si/Al ratio of 1 at 298 K.

The above results are supported by Figure 5, which displays the energy probability distribution between a single water molecule and framework atoms and EFCs in the all-sodium and all-calcium zeolite forms from NVT simulations. The wider distribution for the all-calcium structure indicates that the molecule of water requires more potential energy to cross the energy barriers in this material than in the all-sodium structure, despite the larger density of cations in the latter. Figure 6 shows the adsorption isotherms of water as well as the average cation−water potential energy as fugacity increases in the different considered sodium/calcium forms of the LTAtype zeolite with Si/Al ratio of 1. As is apparent from Figure 6a, the presence of calcium cations clearly favors adsorption at lower fugacities. Indeed, the onset fugacities seem to decrease with the increasing number of Ca2+ cations in the zeolite. The greater electrostatic charge of divalent calcium cations leads to a stronger interaction with the dipole of water and hence to a more noticeable hydrophilic character of the zeolite. Although water uptakes in the structures at high fugacities are quite close, a clear tendency is also observed: Saturation uptakes increase with a decrease in the total number of cations in the zeolite. This is due to the volume occupied by the cations, which reduces the available space for water molecules. The shapes of the isotherms are similar in all cases and give information on the pore filling mechanism. Particularly, the smoothly increasing adsorption curves denote that the condensed phase develops under supersaturation conditions. This behavior is opposite to that of hydrophobic zeolites, where an abrupt condensation transition occurs at pressures larger than the bulk saturation pressure.11,31 On the basis of these results, we can state that the type of charge-balancing cations is more influential on the adsorption performance than the amount of cations; i.e., the energetic factor overcomes volume factor. The associated energetic factors are addressed in Figure 6b. The value in the low-coverage regime for all the zeolites with relatively high calcium contents is approximately the same: about −100 kJ·mol−1. With increasing fugacity and so water loading, this potential energy increases up to about −60 kJ· mol−1 at the highest fugacity as a consequence of strong water− water interactions. For zeolites with high sodium contents, this magnitude takes higher values. In particular, it varies from −75 kJ·mol−1 at low coverage to −50 kJ·mol−1 at saturation in the all-sodium form of the zeolite. Figures 7 and 8 provide structural description of the water−cation interactions.

Figure 7. Water (oxygen)−cation radial distribution functions at 298 K for various zeolites and for similar water loadings (9−10 mol·kg−1).

Figure 8. Average minimum water (oxygen)−cation distance (dONa in red and dOCa in blue) in the various zeolites for 0.1 Pa (squares), 1 Pa (circles), and 10 Pa (triangles) at 298 K.

alternative potentials.5,30 Then, we can conclude that, for comparable cation sizes as it is the case, considerably larger MC simulation runs are required to sufficiently explore a representative number of microscopic states in zeolites containing calcium cations in relation to those only compensated by sodium ions. Instead, we conduct at ambient temperature a number of single-molecule NVT simulations using different starting configurations. The average results from these simulations are displayed in Figure 4d. In this way, it is ensured that an ensemble average is taken, providing sufficient convergence. E


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Figure 9. O−O and O−H radial distribution functions of water molecules at 298 K in the highly hydrated sodium/calcium zeolite forms at 107 Pa (top) and for similar intermediate loadings, 9−10 mol·kg−1 (bottom). The dotted line represents data of bulk water and the colors of continuous lines mean the same as in Figure 6.

Figure 10. Fraction of associated molecules fass (a) and average number of hydrogen bonds per molecules nHB (b) as a function of the fugacity for water adsorbed in the sodium/calcium zeolite forms at 298 K. Line colors mean the same as in Figure 6.

Table 1. Fraction of Associated Molecules fass and Average Number of Hydrogen Bonds per Molecules nHB at 298 K in the 48 Ca2+, the 32 Na+/32 Ca2+, and the 96 Na+ Forms of the LTA Zeolite (Si/Al Ratio = 1) for Similar Intermediate Water Loadings (9−10 mol·kg−1) zeolite (Na content)

fass

nHB

0 32 96

0.22 0.30 0.40

1.12 1.20 1.20

Table 2. Fraction of Associated Molecules fass and Average Number of Hydrogen Bonds per Molecules nHB at 107 Pa and 298 K for Water Adsorbed in the Sodium/Calcium Zeolite Forms

Figure 7 shows the RDFs of the water molecules (oxygen) with sodium gNaO(r) and calcium gCaO(r) cations for similar intermediate water loadings (9−10 mol·kg−1) in various zeolites. These functions exhibit sharp peaks placed at about 2.15 and 2 Å, respectively, regardless of the host structure. Therefore, water−cation forces are hardly affected by the present EFCs. The higher intensity of the peaks corresponding to water−calcium RDFs with respect to those involving sodium cations denotes larger coordination by water molecules. The coordination numbers can be found elsewhere.5,11

zeolite (Na content)

fass

nHB

0 16 32 48 64 80 96

0.89 0.88 0.89 0.85 0.84 0.84 0.84

1.99 1.97 1.94 1.92 1.90 1.88 1.85

To gain insights, Figure 8 shows the average minimum oxygen water−cation distances in the targeted structures for several low values of fugacity, namely 0.1, 1, and 10 Pa. Several aspects can be inferred from this figure. First, in accord with previous comments, the distance values from the oxygen atom of water to the calcium cations dOCa are shorter than these to the sodium cations dONa in all structures containing calcium cations. Second, we found these values slightly affected by the F


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The Journal of Physical Chemistry C present EFCs (density and nature) in the zeolite: dONa decreases (and dOCa increases) from the all-calcium to the allsodium zeolite composition. Finally, dONa decreases with increasing fugacity (and so water loading) whereas dOCa is almost independent of fugacity. This latter finding evidences that the closeness of water molecules is unquestionably induced by energy affinity in the case of calcium cations, whereas also volume factors affect the interaction with the sodium cations. Next, we turn our attention to interactions between the water molecules adsorbed in the pores. Figure 9 displays gOO(r) and gOH(r) at 298 K of the adsorbed molecules at saturation and at comparable intermediate loadings (9−10 mol·kg−1). These for bulk liquid water are also plotted (dotted lines). The behavior of the RDFs reveals the existence of hydrogen bonds in all systems. A clear weakening of the HB network of adsorbed water at saturation in relation to bulk water is deduced from the considerable broadening of the first peak and the shift of the second hydration shell in gOO(r). These aspects have been found and comprehensively dealt in previously reported works.11 Here we focus on the variations of water−water interactions induced by the present EFCs. Though negligible changes in this regard are observed at high fugacity for the dense phase within the pores, plots for intermediate coverage reveal differences. The locations of the first peaks and the following minima in gOO(r) are variable depending on the type of zeolite: The first peak of the RDFs corresponding to zeolites with the highest density of sodium cations are located at roughly the same position of bulk water, and it is shifted to larger distances as the density of calcium cations increases. This denotes that the weaker affinity of water to monovalent sodium cations favors water−water interactions if we compare it to the affinity of water to Ca-rich structures. In the latter, the closest neighboring water molecules are at distances above 3 Å. The position of the first peak in gOH(r) remains virtually unchanged with respect to bulk water. However, it is worth noting that the second hydration shell unfolds into two distinct peaks for adsorbed water, especially in zeolites with high density of sodium cations. Also, the intensity of the first peaks is lower in the latter zeolites, indicating a less complex HB network. Overall, as occurs for the adsorption performance, the nature of the cations is found to be a stronger influence on the microscopic structure of the adsorbed water than the density of cations. Figure 10 displays the fraction of associated (linked through hydrogen bonds) water molecules fass and the average number of hydrogen bonds per molecule nHB as a function of fugacity. As can be seen, the larger amount of neighboring water molecules as the fugacity is raised leads to higher-order clusters. In particular, we found these curves smoothly increasing, in agreement with the adsorption performance (Figure 6). nHB values vary from nearly above 1 to almost 2 along the fugacity range. At a given value of fugacity, the lowest degree of association corresponds to the all-sodium structure, and the highest to the all-calcium zeolite, which is consistent with the larger uptakes in the latter due to the stronger affinity. To compare states with similar uptakes, Table 1 collects the values of these properties for about 9−10 mol·kg−1 of water in the 48 Ca2+, 32 Na+/32 Ca2+, and 96 Na+ zeolite forms (at values of fugacity of 1, 10, and 100 Pa, respectively). The nHB values characterizing the complexity of the HB network are similar and indicate that the majority of associated water molecules are linked through only one hydrogen bond. However, the percentage of associated molecules fass in the all-sodium zeolite

is almost twice this for the all-calcium zeolite, and an intermediate value corresponds to the 32 Na+/32 Ca2+ zeolite. Therefore, consistent with the RDFs of Figure 9 (bottom), the HB statistics prove a notable weakening of self-association of water with increasing the amount of calcium cations for a given water loading. On the basis of the obtained results, we can state that water clustering proceeds differently during adsorption depending on the type of cations in the aluminosilicate zeolite. However, data collected in Table 2 indicate that the hydrogen bondings of water molecules when a dense phase is formed within the pores (at the highest values of fugacity) are similar for all the structures. Specifically, fass and nHB at 107 Pa are about 0.9 and 2, respectively, for the all-calcium form and slightly decrease with increasing the density of sodium cations due to volume effects. Note that these values are far from the values obtained in bulk water, where basically all molecules are associated and nHB is about 3.211 using the same water model.17 These HB statistics are in agreement with the RDFs depicted in Figure 9 (top).

4. CONCLUSIONS The cations that compensate the negative charge of the LTAtype aluminosilicate zeolite (Si/Al ratio of 1) have a key role on the adsorption and clustering of water molecules inside the pores. Particularly, the influence of the nature of cations is much larger than the influence of the cation density. The calcium cations act as preferential adsorption sites due to the higher charge with respect to sodium cations, creating an attractive potential well for water molecules. This leads to the hydrophilic character reflected in the adsorption isotherms, and closely connected with the weaker degree of water hydrogen bonding in relation to that observed in the presence of sodium cations for comparable water uptakes. Hence, the water clustering proceeds differently during the adsorption process. In regard to the procedure, we can conclude that MC simulations involving zeolites with calcium cations require more cycles to reach equilibrium than for the zeolite compensated by sodium cations. The impact of the environment generated by the cations is, however, unsubstantial on the hydrogen-bonded network of the dense phase of water, at saturation conditions, as well as on cation−water interactions.

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AUTHOR INFORMATION

Corresponding Authors

*P. Gómez-Á lvarez. E-mail: [email protected]. Tel: +34954977594. *S. Calero. E-mail: [email protected]. Tel: +34-954977594. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. [279520]. S.R.G.B. thanks Spanish MINECO (CTQ2013-48396-P) for his predoctoral fellowship.

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Impact of the Nature of Exchangeable Cations on LTA-Type Zeolite

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