VOLUME 102, NUMBER 21, MAY 21, 1998
© Copyright 1998 by the American Chemical Society
LETTERS Grand Canonical Monte Carlo Investigation of Water Adsorption in Heulandite-type Zeolites Yvonne M. Channon*,† and C. Richard A. Catlow The DaVy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1 X 4BS, U.K.
Alan M. Gorman Molecular Simulations Inc., 9685 Scranton Road, San Diego, California 92121
Robert A. Jackson The Department of Chemistry, Keele UniVersity, Keele, Staffordshire ST5 5BG, U.K. ReceiVed: December 10, 1997; In Final Form: April 1, 1998
The grand canonical Monte Carlo technique has been employed to investigate the sorption sites of water molecules in heulandite-type zeolites. The results obtained reveal low-energy regions for the adsorption of water and provide detailed models for the interactions between the water molecules, extraframework cations, and the framework.
Investigation of the loading and location of water molecules in zeolites is of importance owing to the influence they exert on the structural and chemical properties of these systems, in particular, their interaction with extraframework cations. Although the location of water and the strength with which it is bound to the structure have been studied experimentally,1,2 there is considerable uncertainty regarding the location of cations and water molecules, as well as the interaction of these species with each other within the structure. Moreover, the application of computational techniques, which have enjoyed considerable success in many aspects of zeolite science,3 have been limited in the case of hydrated systems,3 although molecular dynamics studies are reported.4,5 This study concerns heulandite-type zeolites, which have attracted considerable interest owing to their widespread availability and potential application in ion exchange and gas † Current address: Department of Chemistry, University of Wales, Cardiff CF1 3TB, U.K. Email:
[email protected].
separation.6 With a two-dimensional microporous channel system, two channels run parallel to each other and the c-axis as shown in Figure 1: channel A, a 10-member ring, and channel B, an 8-member ring. Channel C lies along the a-axis and intersects the A and B channels. As in other zeolites, we have little knowledge of the distribution of water within the pores of these zeolites or of the interaction of water molecules with each other or the framework. In this article we exploit the potential of modern atomistic simulation techniques to provide detailed information concerning these important questions. The grand canonical Monte Carlo technique (GCMC) provides an ideal computational tool for analysis of these problems. It has previously been applied to studying singlecomponent gas adsorption in zeolites and has mainly concentrated on adsorption of simple adsorbates such as the rare gases or small hydrocarbon molecules,7,8 although there are examples of GCMC technique being used to simulate adsorption of longer
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Figure 1. Structure of heulandite showing the channels A, B, and C.
chain hydrocarbons9,10 and of polar and nonpolar molecules such as p and m-xylene.11 GCMC simulations allow the simulation of a solid sorbant phase and a liquid or gas phase at equilibrium with a specified chemical potential.13 The system modeled is represented by an ensemble of particles (to which periodic boundary conditions are normally applied) and for which displacement, creation, and destruction of particles are permitted,12 the choice between these moves being made randomly. After the system has equilibrated (when the create/destroy ratio is close to unity), statistics at the defined pressure, volume, and temperature are accumulated in a Markov chain until a statistically significant number of iterations has been achieved (typically 107, although this figure is dependent on the number of iterations required in the equilibration step). The average number of guest molecules and the average interaction energy can then be calculated from the Markov chain. We have simulated water sorption in both purely siliceous heulandite and a more realistic model of the zeolite, containing framework aluminum and charge-compensating sodium ions. The consistent-valence force field (CVFF),14 which has been extended to include zeolitic species,15 has been used to calculate the short-range interactions. This force field has been employed as we need to include potentials for zeolite, water, and cation interactions. Although a different set of potentials would clearly alter the results, this force field models adequately the equation of state of the gas and is, we consider, appropriate for investigating this problem at the current time; the main qualitative findings of our study are not critically dependent on details of the potential model employed. The Coulombic interactions were calculated via an Ewald summation. The simulation box comprised 864 framework atoms (i.e., eight unit cells) (to which periodic boundary conditions were applied) into which water molecules were loaded. For the aluminum-containing system, one aluminum ion per unit cell was placed at a T(II) position that is known from crystallographic studies16 to have a higher than random aluminum concentration. This procedure is an approximation as the site has only partial occupation by aluminium ions, the modeling of which, however, would require the use of large supercells. The use of the approximation is unlikely to have a major effect on our results. The net charge on the framework was balanced by placing a sodium ion at the M(2) position in the unit cell. The M(2) position can be occupied by both sodium and calcium ions, hence providing a basis for comparisons in future studies. We report only the results of the sodium ion in this Letter. The unit cell then underwent lattice energy minimization from which a 2 × 2 × 2 supercell was generated (Figure 2). The simulation box thus contains eight aluminum ions, with a charge-compensating complement of eight Na+ ions. All simulations were undertaken using a constant µVT
Figure 2. Starting positions of the sodium ions in the zeolite. Large spheres are sodium ions; small spheres are aluminum ions.
Figure 3. Adsorption isotherm for water molecules in siliceous heulandite.
ensemble, across a range of water partial pressures. During the calculations, the framework is held fixed at the optimized geometry. Although ideally framework dynamics should be taken into account, it is unlikely that small framework relaxations would significantly alter the calculated distribution of water molecules or the interactions with extraframework cations. A unique feature of these calculations is that the sodium ions are also involved in the Monte Carlo displacements, though unlike the water molecules their numbers remain constant. All the calculations have been carried out using the Sorption module from the Catalysis suite of software from MSI.17 Calculations were first carried out on the siliceous heulandite. Water molecules were loaded at 300 K and at every 0.2 atm of pressure, ranging from 0.1 to 2.0 atm. The adsorption isotherm obtained from these calculations is shown in Figure 3. The low number of sorbed water molecules highlights the hydrophobicity of siliceous zeolites. Examination of the population density of accepted sorption sites of the water molecules at 0.5 atm shows preferential sorption at the center of channels A and B, although channel B has more sorption sites than channel A. This observation confirms previous results18 that the 8-ring channel has more favorable sorption sites than the 10-ring channel. Next we studied the aluminum-containing zeolite. Water molecules were loaded at 300 K and at pressures of 0.1, 0.3, and 0.5 atm. The adsorption isotherm obtained from these calculations is shown in Figure 4. From these results we see that the effect of the aluminum and sodium ions on the sorption of water molecules is, as expected, substantial. Figure 5 reveals that the sodium ions have moved extensively from their starting positions in the B channels to positions in the larger A channels, which are close to the positions of the
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Figure 4. Adsorption isotherm for sorbed water molecules in heulandite, with extraframework cations included in the system.
Figure 6. Water molecules sorbed at 0.5 atm viewed from above the supercell of heulandite, showing channels A and B running left to right and channel C running top to bottom. Formation of small water clusters (as predicted by GCMC calculations) is evident.
Figure 5. Water molecules sorbed at 0.5 atm in heulandite containing extraframework sodium ions (large spheres), as predicted by GCMC calculations.
sodium ions obtained from X-ray diffraction16 on hydrated systems. Inclusion of cation displacements in the simulation is clearly essential. Owing to the interactions between water molecules and cations, and the formation of hydrated ions, we expect to observe sorption of water molecules along the channels containing the sodium ions (channel A). The configuration shown in Figure 5 illustrates this feature and also shows that the water molecules are sorbing along channel B. We consider that the long-range electrostatic forces between the sodium ion and the water, and between the framework and water, and hydrogen bond interactions between the water molecules lead to this behavior. These results moreover support our suggestion that channel B has more favorable sorption sites for water than channel A. The configuration illustrated in Figure 6 is, in effect, a slice through the supercell. We look down from above the supercell at the A and B channels, which run left to right, in the plane of the C channel, which runs top to bottom. Examination of this configuration shows clearly that the hydration of the sodium ions does not compare to that of sodium ions in solution. The water molecules do not appear to cluster closely around the sodium ion. Indeed a plot of the pair correlation function (see Figure 7) between sodium and oxygen of the water molecule at 0.5 atm has a maximum at 3.1 Å. This distance refers to the first sphere of water molecules interacting with the sodium ion and is considerably larger than that obtained for sodium ion hydration in aqueous solution predicted by both molecular
Figure 7. Top: Pair correlation diagram between sodium ions and the oxygen of the water molecule. Bottom: Pair correlation diagram between sodium ions and the oxygen in the framework, for this system at 0.5 atm.
dynamics (of 2.36 Å)19 and Monte Carlo techniques (of 2.35 Å).20 Although it is not fully understood why the results obtained for water and sodium ion interactions in a zeolite should be so different from the same water and sodium ion interactions in an aqueous solution, it is clear that the interactions with the zeolite have a considerable influence on the position of water molecule sorption and hence subsequent sorption of water molecules to form large networks of hydrogen-bonded water molecules through the channels. The zeolite interactions also have a considerable influence on the positions of the sodium ions that could also affect the clustering of water molecules around the sodium ion. The pair correlation function for sodium ions and framework oxygen ions (see Figure 7), which has a first maximum at 3.1 Å, is the same distance as that obtained between sodium ions and the oxygen
4048 J. Phys. Chem. B, Vol. 102, No. 21, 1998 of the water. These results show that the oxygens in the framework may also loosely coordinate the sodium ions in the zeolite. The zeolite framework itself can form hydrogen bonds with the water molecules, and the water molecules may sorb at positions in the zeolite which interact with the framework via hydrogen bonds. Other water molecules can then also sorb at positions in which they interact with each other via hydrogen bonds and form clusters, as reported previously.18 In summary, our calculations have shown that, as expected, the siliceous zeolite is hydrophobic, with hydrophilicity being introduced when aluminum and sodium ions are included in the system. In both the siliceous zeolite and the aluminum (and sodium) containing system, we observe that channel B has more sorption sites for water molecules than channel A. We find that the sodium cations are loosely solvated and remain partially coordinated to the framework oxygen atoms, while hydrogenbonded clusters form among the sorbed water molecules. Our results also show that hydration results in substantial cation displacements. Inclusion of cation displacements is clearly essential in simulations of zeolite systems containing polar molecules. Acknowledgment. We thank BNFL for their provision of a studentship. We are grateful to EPSRC for provision of local computer resources at the Royal Institution, and we thank MSI Catalysis and Sorption consortium project for the provision of software. Useful discussions with Dr. Scott Owens are gratefully acknowledged.
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