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Water adsorption in hydrophobic nanopores: Monte Carlo simulations of water in silicalite
Microporous and Mesoporous Materials 90 (2006) 293–298 www.elsevier.com/locate/micromeso
Water adsorption in hydrophobic nanopores: Monte Carlo simulations of water in silicalite Chen E. Ramachandran, Shaji Chempath, Linda J. Broadbelt, Randall Q. Snurr
*
Department of Chemical and Biological Engineering, Institute for Environmental Catalysis, Northwestern University, Evanston, IL 60208, USA Received 15 August 2005; received in revised form 8 October 2005; accepted 14 October 2005 Available online 15 December 2005 Dedicated to the late Denise Bartomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science
Abstract Grand canonical Monte Carlo simulations have been carried out to investigate the adsorption of water from the vapor phase into the zeolite silicalite. For truly hydrophobic micropores, the simulations predict essentially no adsorption of water at low pressures, followed by rapid pore filling as pressure is increased. The effect of silanol defects in real silicalite samples was explored through simulations using ‘‘seeded’’ water molecules to represent hydrophilic defects. These defects promote adsorption of some water at low pressures, as molecules form hydrogen-bonded clusters around the defects. The defects also shift the pore filling to a lower pressure than in the completely hydrophobic material. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Water; Silicalite; Defect; Hydrophobicity; Adsorption
1. Introduction Understanding the behavior of water in hydrophobic micropores is important in a wide variety of fields, ranging from biological membrane transport to applications of carbon nanotubes, as well as water purification using zeolites and activated carbons. It is also an interesting scientific question whether water will adsorb in a truly hydrophobic pore. In the literature, molecular simulations and thermodynamic models predict very dramatic water isotherms in hydrophobic micropores [1–4]. At low pressures, hardly any water molecules exist in the pores. This persists until a specific pressure is reached, after which rapid pore filling occurs [3,4]. This has been observed experimentally for water in activated carbons [3]. A similar steep transition between empty and filled pores has been observed in a molecular dynamics study of hydrophobic pores [5], in *
Corresponding author. Tel.: +1 847 467 2977; fax: +1 847 491 3728. E-mail address: [email protected] (R.Q. Snurr).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.10.021
which the system oscillates between pores devoid of water and pores completely filled with water. Hummer et al. [6] observed similar behavior for MD simulations of water in hydrophobic carbon nanotubes, in which small changes in the interaction between pore walls and water molecules profoundly changed water hydration of pores, leading to sharp transitions between filled and empty states on a nanosecond timescale. McCallum et al. [7] studied the adsorption of water on activated carbon experimentally as well as through grand canonical Monte Carlo (GCMC) simulations and found that the density of active surface sites contributed significantly to the loading before pore filling occurred. Silicalite is a zeolite comprised of SiO2 tetrahedra with the MFI crystal structure [8,9]. It is widely regarded as hydrophobic, and the earliest studies on silicalite showed that it readily adsorbs organic molecules over water [8]. These studies, as well as later ones [8,10–12], however, show that silicalite does adsorb a small amount of water. MFI zeolites can also be prepared with various Si/Al ratios, and a direct relationship has been established
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experimentally between increased Al content and increased water loading [12]. In this work, we present results of a GCMC study of water adsorption in silicalite. The curious behavior observed in simulations of water confined in hydrophobic nanopores such as carbon nanotubes was also observed in our simulations of silicalite. This behavior contrasts with the experimentally observed water adsorption isotherms in silicalite, which do not show any pore filling1 but which do show low levels of adsorbed water even at low pressures. We will show that the water adsorbed at low pressures is likely associated with adsorption on hydrophilic defects [14]. 2. Simulation methods 2.1. GCMC simulations Grand canonical Monte Carlo simulations are well suited for investigating adsorption in zeolites and related materials on the molecular level. In the grand canonical ensemble, the volume, temperature, and chemical potential are held constant while the number of molecules fluctuates. Simulations were run using the simulation code Music [15,16], which was written in-house. At the low pressures used in our simulations, it was assumed that water vapor is an ideal gas and its fugacity is equal to the gas-phase pressure. The simulation box was composed of eight silicalite unit cells with periodic boundary conditions. All simulations reported here were at a temperature of 291 K. Properties of interest were calculated based on averaged values over the equilibrated regime for each simulation. Even with biasing of insertion moves, the required number of Monte Carlo steps for each isotherm point ranged from 24 million to 360 million. Other researchers have also found that unusually long simulations are needed to equilibrate water in hydrophobic pores [7]. The van der Waals interactions between atoms were represented by a Lennard-Jones 12-6 function, and electrostatic interactions between point charges placed on the atoms were calculated using CoulombÕs law. For the silicalite/water interactions, Ewald summation was used for the Coulombic interactions and the Lorentz–Berthelot mixing rules were used to obtain the Lennard-Jones parameters. For water/water interactions, a spherical cut-off based on ˚ for both the molecular center of mass was enforced at 13 A Lennard-Jones and Coulombic interactions.
2.2. Silicalite model The locations of the silicon and oxygen atoms of the zeolite framework were taken from XRD data [17], and the framework was considered to be rigid. The LennardJones parameters for the zeolite oxygen atoms were obtained from previous work [15] and are listed in Table 1. Since silicon atoms are recessed behind oxygen atoms in the pore walls, they are not readily accessible to sorbates. Therefore, van der Waals interactions between silicon atoms and the sorbate molecules were not considered. However, Coulombic interactions between sorbates and both oxygen and silicon atoms were modeled explicitly. The silicon atoms carried a + 1.4 partial charge, whereas oxygen atoms carried a 0.7 partial charge [15]. Interpolation from pre-tabulated Lennard-Jones and Coulombic interaction grids was used to speed up the calculation of zeolite/sorbate interactions. Three different types of sites can be identified in silicalite: zig-zag channels, straight channels, and intersections. The process of defining sites using a methane probe molecule is described elsewhere [18]. In our simulations, the regions that were inaccessible to the methane probe but accessible by water molecules were categorized into the nearest, by distance, defined site. In some simulations, we introduced defect sites within silicalite in a simple way by randomly placing ‘‘seed’’ water molecules within either zig-zag channels, straight channels, or intersections. Each defect in a unit cell was initially replicated in the other seven unit cells of the simulation box. These seeded water molecules are meant to mimic intracrystalline defect silanol sites. They were allowed to rotate during the GCMC simulation but were not allowed to be removed or translated. The resulting silicalite structure with seeds was then used to generate a water adsorption isotherm. Seeded water molecules were not counted in the reported water loadings. 2.3. Water model Numerous models exist in the literature for bulk water, each capable of capturing some experimentally observed characteristics such as the vapor pressure or radial
Table 1 Potential parameters ˚) ri (A Water (SPC/E)
1
As this manuscript was being completed, an experimental report appeared showing pore filling in silicalite at pressures considerably above the bulk saturation pressure [13]. It is interesting to note that Desbiens et al. used GCMC simulations to predict pore filling above the bulk saturation pressure, using the TIP4P water model, the same silicalite partial charges as in Table 1, and Lennard-Jones parameters for the ˚ and e/k = 93.53 K. silicalite oxygens of r = 3.0 A
O H O H e O Si
3.16 – 3.15 – – 2.80 –
ei/kb (K)
qi
References
78.212 0.848 [23] – 0.424 Water (TIP4P) 77.941 – [24] – +0.52 – 1.04 Silicalite 107.5 0.7 [15] – +1.4 ˚ , h = 104.5°, and Psat = 2.2 kPa at For the SPC/E model: rO–H = 1 A ˚ , h = 104.5° rO–e = 300 K [33]. For the TIP4P model: rO–H = 0.9572 A ˚ , and Psat = 4.5 kPa at 300 K [27]. 0.15 A
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Fig. 1. Comparison of water models SPC/E (d) and TIP4P (j) for adsorption in silicalite at 291 K versus experimental data (solid line) from Olson et al. [12] at 298 K. For the experimental curve, the experimental value of Psat was used, and for the simulation results, the Psat values of 2.2 kPa and 4.5 kPa were used for the SPC/E and TIP4P models, respectively.
distribution function. As of yet, no universal model has been developed for this important molecule. Several potential models have been used in the past to simulate water in silicalite [19–22], mostly to elucidate the diffusion behavior. For this study, the SPC/E model [23] was chosen to represent water, and the Lennard-Jones parameters and partial charges are listed in Table 1. The SPC/E model has been used extensively in the literature for a wide variety of different systems since it was first introduced. Several other water models were also investigated for comparison purposes (i.e., TIP3P, TIP4P, and TIP5P) [24–28]. Results are presented for the SPC/E model unless otherwise indicated.
3. Results and discussion The simulated adsorption isotherms of water in silicalite using the SPC/E and TIP4P models are shown in Fig. 1. Both isotherms show qualitatively the same behavior: practically no water molecules are adsorbed at low pressure and then the pores fill with water. The onset pressure of the pore filling is different for the two different models, and it appears that this value is very sensitive to the potential parameters used [13]. Fleys and Thompson [29] also found extremely low loadings of water in silicalite from GCMC simulations with somewhat different potential parameters.
Fig. 2. Water adsorption isotherms with the SPC/E model at 291 K with and without seeds to mimic defects: 0.5 seeds per unit cell (n), 4 seeds per unit cell (m), base case with no seed molecules (d and dashed line). The seeds were placed in the channel intersections. Experimental data from Olson et al. [12] are also shown (solid line).
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They did not report pore filling, possibly because they did not simulate high enough pressures to observe this behavior with their model. The maximum loading from our simulations is 53 molecules per unit cell, which is similar to the expected pore filling value of 57 molecules per unit cell [12]. Experimental isotherms for water in silicalite have been reported by several groups [8,10–12]. There are some differences, but all show low levels of water adsorption (less than 10 molecules per unit cell), starting gradually at low pressures. The experimental results of Olson et al. [12] are shown in Fig. 1 for comparison with the simulation results. None of the vapor phase experimental isotherms indicate pore filling [13], although pore filling has been observed for adsorption of water in aluminophosphate molecular sieves [30]. Clearly, the simulated and experimental isotherms in Fig. 1 show qualitatively different behavior. One possible reason for this is the presence of silanol and other defects in the experimental silicalite samples. It is believed that silanol defects arise from the use of templating agents in
the synthesis process. Estimates of as many as four defects per unit cell have been suggested [12]. We reasoned that silanol defects could serve as hydrophilic sites that might affect adsorption of water at low loadings. To test this hypothesis, we determined the water isotherms with various numbers of seeded water molecules used to mimic these defects, as described in the previous section. Fig. 2 shows the results with 0.5 seeds per unit cell and 4 seeds per unit cell. The isotherm with only 0.5 seeds per unit cell is essentially identical to the base case. It should be noted that the seeded water molecules, which occupy some pore volume, are not included in the loading quantified on the y-axis. Pore blockage by the seed molecules might explain some of the differences with the base case observed at the higher loadings, but the differences are also indicative of the difficulties in equilibrating the system when the pores are completely full. Using 4 seeds per unit cell produces qualitative changes in the isotherm. The loading at low pressures is non-zero
Fig. 3. Snapshot of water clustering around defect seed molecules in silicalite. Conditions: 1 seed molecule (located in a straight channel) in 8 unit cells, 0.34P/Psat, and 291 K. Oxygen atoms of seed molecules are represented by the large blue spheres. Adsorbed water moleculesÕ oxygen atoms are represented by large red spheres, and the hydrogen atoms are represented by the smaller white spheres. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Snapshot of water clustering around defect seed molecules in silicalite. Conditions: 4 seed molecules per unit cell (located in channel intersections), 0.136P/Psat, and 291 K. Oxygen atoms of seed molecules are represented by the large blue spheres. Adsorbed water moleculesÕ oxygen atoms are represented by large red spheres, and the hydrogen atoms are represented by the smaller white spheres. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
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and is more consistent with the experimental data. This supports the suggestion that defect sites within the pores influence the adsorption of water at low loading. A truly hydrophobic pore shows essentially zero loading of water, but real silicalite samples show finite loadings due to hydrophilic defect sites. Introduction of seeds also shifts the pressure at which pore filling begins to a lower value. A snapshot from a simulation with 1 seed molecule in the simulation box (eight unit cells) is shown in Fig. 3. The adsorbed water molecules clearly form clusters that seem to grow around the seed molecule (note that the clusters at the top/bottom and left/right are connected due to the periodic boundary conditions). Fig. 4 shows a snapshot from a simulation with 4 seed molecules per unit cell at low pressure. Again, the adsorbed water molecules are located preferentially near the seed molecules. Fig. 5 shows a snapshot from a simulation with 4 seed molecules per unit cell at a higher pressure. Here, it can be seen that the pores are almost full of water molecules. Pair distribution functions were also calculated. The pair distribution function based on the centers of mass between the seeded molecules and non-seeded water molecules, g[rCOM], showed a very structured configuration of water around the defects at low loading, evident from a ˚ [31]. The distance and intensity sharp peak at 2.758 A indicate hydrogen bonding occurs between the seeded and non-seeded water molecules. Using g[rCOM], the number of nearest neighbors around the seeded water molecules could be calculated [32]. For the case with 4 seed molecules per unit cell, results showed on average, approximately one water molecule surrounding each seed within hydrogen bonding distance at low pressures (corresponding to Fig. 4) and up to four water molecules surrounding each seed at high pressures (Fig. 5).
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Fig. 5. Snapshot of water adsorption in silicalite near saturation. (Note: For better visualization of seed molecules, this figure is shown as viewed from a different angle compared to Figs. 3 and 4.) Conditions: 4 seed molecules per unit cell (located in channel intersections), 0.45P/Psat, and 291 K. Oxygen atoms of seed molecules are represented by the large blue spheres. Adsorbed water moleculesÕ oxygen atoms are represented by large red spheres, and the hydrogen atoms are represented by the smaller white spheres. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
The radial distribution function between the oxygen of a seeded molecule and the hydrogen of non-seeded water molecules, g[rO–H], is shown in Fig. 6 for the case
Fig. 6. Radial distribution function between the oxygen atom of seeded water molecules and the hydrogen atom of adsorbed water molecules. Conditions: 0.5 seed molecules per unit cell (located in channel intersections), 0.14P/Psat, and 291 K.
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corresponding to 0.5 seed molecules per unit cell and low ˚ and the pressure. The first g[rO–H] peak occurs at 1.765 A ˚ second at 3.2 A, again consistent with hydrogen bonding. 4. Conclusions The molecular simulations presented in this work predict that in a perfect silicalite crystal, the pore system is hydrophobic and essentially no water adsorbs at low pressure. At some intermediate pressure, a sharp transition occurs where the pores become saturated with water molecules. The presence of hydrophilic defects, such as silanol sites, in experimental samples leads to adsorption of small amounts of water at low pressures. Water molecules preferentially form hydrogen-bonded clusters around the defects. The presence of hydrophilic defects also leads to a decrease in the pressure where pore filling occurs. Acknowledgements This research is supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy, Grant No. DE-FG02-03ER15457. Additional support from the National Science Foundation (CTS-0302428) is also gratefully acknowledged. The authors thank Prof. Alain Fuchs for helpful discussions. References [1] S. Vaitheeswaran, H. Yin, J.C. Rasaiah, G. Hummer, Proc. Nat. Acad. Sci. 101 (49) (2004) 17002. [2] N. Floquet, J.P. Coulomb, N. Dufau, R. Kahn, Stud. Surf. Sci. Catal. 154 (2004) 1804–1811. [3] T. Ohba, H. Kanoh, K. Kaneko, J. Phys. Chem. B 108 (39) (2004) 14964. [4] A. Striolo, K.E. Gubbins, A. Chialvo, P.T. Cummings, Mol. Phys. 102 (2004) 243–251. [5] O. Beckstein, M.S.P. Sansom, Proc. Nat. Acad. Sci. 100 (12) (2003) 7063–7068. [6] G. Hummer, J.C. Rasaiah, J.P. Noworyta, Nature 414 (2001) 188– 190. [7] C.L. McCallum, T.J. Bandosz, S.C. McCrother, E.A. Muller, K.E. Gubbins, Langmuir 15 (1999) 533–544. [8] E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner, J.V. Smith, Nature 271 (1978) 512–516.
[9] International Zeolite Association. Available from:. [10] B.V. Kuznetsov, T.A. Rakhmanova, Russ. J. Phys. Chem. 75 (6) (2001) 933–938. [11] A. Giaya, R.W. Thompson, Micropor. Mesopor. Mater. 55 (2002) 265–274. [12] D.H. Olson, W.O. Haag, W.S. Borghard, Micropor. Mesopor. Mater. 35–36 (2000) 435–446. [13] N. Desbiens, I. Demachy, A.H. Fuchs, H. Kirsch-Rodeschini, M. Soulard, J. Patarin, Angew. Chem. Int. Ed. 44 (2005) 5310–5313. [14] R.R. Cole, M.S. Gruszkiewicz, J.M. Simonson, A.A. Chialvo, Y.B. Melnichenko, G.D. Wignall, G.W. Lynn, J.S. Lin, A. Habenschuss, B. Gu, K.L. More, T.D. Burchell, A. Striolo, Y. Leng, P.T. Cummings, W.T. Cooper, M. Schilling, K.E. Gubbins, H. Trielinghaus, in: R. Wanty, R. Seal (Eds.), Influence of Nanoscale Porosity on Fluid Behavior, Water/Rock Interaction, vol. 1, 2004, pp. 735– 739. [15] A. Gupta, L.A. Clark, R.Q. Snurr, Langmuir 16 (2000) 3910– 3919. [16] A. Gupta, S. Chempath, M.J. Sanborn, L.A. Clark, R.Q. Snurr, Mol. Simul. 29 (2003) 29–46. [17] D.H. Olson, G.T. Kokotailo, S.L. Lawton, W.M. Meier, J. Phys. Chem. 85 (15) (1981) 2238. [18] R.L. June, A.T. Bell, D.N. Theodorou, J. Phys. Chem. 95 (22) (1991) 8866. [19] C. Bussai, S. Hannongbua, R. Haberlandt, J. Phys. Chem. B 105 (2001) 3409–3414. [20] C. Bussai, S. Fritzsche, R. Haberlandt, S. Hannongbua, J. Phys. Chem. B 107 (2003) 12444–12450. [21] P. Demontis, G. Stara, G.B. Suffritti, J. Phys. Chem. B 107 (2003) 4426–4436. [22] K.S. Smirnov, D. Bougeard, Chem. Phys. 292 (2003) 53–70. [23] H.J.C. Berendsen, J.R. Grigera, T.P. Straatsma, J. Phys. Chem. 91 (1987) 6269–6271. [24] W.L. Jorgensen, J. Chandrasekhar, J.D. Madura, R.W. Impey, M.L. Klein, J. Phys. Chem. 79 (1983) 926. [25] W.L. Jorgensen, J.D. Madura, Mol. Phys. 56 (1985) 1381. [26] M.W. Mahoney, W.L. Jorgensen, J. Chem. Phys. 112 (20) (2000) 8910–8922. [27] R.F. Cracknell, D. Nicholson, N.G. Parsonage, H. Evans, Mol. Phys. 71 (5) (1990) 931. [28] M.W. Mahoney, W.L. Jorgensen, J. Chem. Phys. 115 (23) (2001) 10758–10768. [29] M. Fleys, R.W. Thompson, J. Chem. Theory Comput. 1 (2005) 453– 458. [30] B.L. Newalkar, R.V. Jasra, V. Kamath, S.G.T. Bhat, Micropor. Mesopor. Mater. 20 (1998) 129–137. [31] C.E. Ramachandran, Ph.D. Thesis, Northwestern University, Evanston, IL, 2005. [32] R.Q. Snurr, J. Karger, J. Phys. Chem. B 101 (1997) 6469–6473. [33] G.C. Boulougouris, I.G. Economou, D.N. Theodorou, J. Phys. Chem. B 102 (6) (1998) 1029.
Water adsorption in hydrophobic nanopores: Monte Carlo simulations of water in silicalite Chen E. Ramachandran, Shaji Chempath, Linda J. Broadbelt, Randall Q. Snurr
*
Department of Chemical and Biological Engineering, Institute for Environmental Catalysis, Northwestern University, Evanston, IL 60208, USA Received 15 August 2005; received in revised form 8 October 2005; accepted 14 October 2005 Available online 15 December 2005 Dedicated to the late Denise Bartomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science
Abstract Grand canonical Monte Carlo simulations have been carried out to investigate the adsorption of water from the vapor phase into the zeolite silicalite. For truly hydrophobic micropores, the simulations predict essentially no adsorption of water at low pressures, followed by rapid pore filling as pressure is increased. The effect of silanol defects in real silicalite samples was explored through simulations using ‘‘seeded’’ water molecules to represent hydrophilic defects. These defects promote adsorption of some water at low pressures, as molecules form hydrogen-bonded clusters around the defects. The defects also shift the pore filling to a lower pressure than in the completely hydrophobic material. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Water; Silicalite; Defect; Hydrophobicity; Adsorption
1. Introduction Understanding the behavior of water in hydrophobic micropores is important in a wide variety of fields, ranging from biological membrane transport to applications of carbon nanotubes, as well as water purification using zeolites and activated carbons. It is also an interesting scientific question whether water will adsorb in a truly hydrophobic pore. In the literature, molecular simulations and thermodynamic models predict very dramatic water isotherms in hydrophobic micropores [1–4]. At low pressures, hardly any water molecules exist in the pores. This persists until a specific pressure is reached, after which rapid pore filling occurs [3,4]. This has been observed experimentally for water in activated carbons [3]. A similar steep transition between empty and filled pores has been observed in a molecular dynamics study of hydrophobic pores [5], in *
Corresponding author. Tel.: +1 847 467 2977; fax: +1 847 491 3728. E-mail address: [email protected] (R.Q. Snurr).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.10.021
which the system oscillates between pores devoid of water and pores completely filled with water. Hummer et al. [6] observed similar behavior for MD simulations of water in hydrophobic carbon nanotubes, in which small changes in the interaction between pore walls and water molecules profoundly changed water hydration of pores, leading to sharp transitions between filled and empty states on a nanosecond timescale. McCallum et al. [7] studied the adsorption of water on activated carbon experimentally as well as through grand canonical Monte Carlo (GCMC) simulations and found that the density of active surface sites contributed significantly to the loading before pore filling occurred. Silicalite is a zeolite comprised of SiO2 tetrahedra with the MFI crystal structure [8,9]. It is widely regarded as hydrophobic, and the earliest studies on silicalite showed that it readily adsorbs organic molecules over water [8]. These studies, as well as later ones [8,10–12], however, show that silicalite does adsorb a small amount of water. MFI zeolites can also be prepared with various Si/Al ratios, and a direct relationship has been established
294
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experimentally between increased Al content and increased water loading [12]. In this work, we present results of a GCMC study of water adsorption in silicalite. The curious behavior observed in simulations of water confined in hydrophobic nanopores such as carbon nanotubes was also observed in our simulations of silicalite. This behavior contrasts with the experimentally observed water adsorption isotherms in silicalite, which do not show any pore filling1 but which do show low levels of adsorbed water even at low pressures. We will show that the water adsorbed at low pressures is likely associated with adsorption on hydrophilic defects [14]. 2. Simulation methods 2.1. GCMC simulations Grand canonical Monte Carlo simulations are well suited for investigating adsorption in zeolites and related materials on the molecular level. In the grand canonical ensemble, the volume, temperature, and chemical potential are held constant while the number of molecules fluctuates. Simulations were run using the simulation code Music [15,16], which was written in-house. At the low pressures used in our simulations, it was assumed that water vapor is an ideal gas and its fugacity is equal to the gas-phase pressure. The simulation box was composed of eight silicalite unit cells with periodic boundary conditions. All simulations reported here were at a temperature of 291 K. Properties of interest were calculated based on averaged values over the equilibrated regime for each simulation. Even with biasing of insertion moves, the required number of Monte Carlo steps for each isotherm point ranged from 24 million to 360 million. Other researchers have also found that unusually long simulations are needed to equilibrate water in hydrophobic pores [7]. The van der Waals interactions between atoms were represented by a Lennard-Jones 12-6 function, and electrostatic interactions between point charges placed on the atoms were calculated using CoulombÕs law. For the silicalite/water interactions, Ewald summation was used for the Coulombic interactions and the Lorentz–Berthelot mixing rules were used to obtain the Lennard-Jones parameters. For water/water interactions, a spherical cut-off based on ˚ for both the molecular center of mass was enforced at 13 A Lennard-Jones and Coulombic interactions.
2.2. Silicalite model The locations of the silicon and oxygen atoms of the zeolite framework were taken from XRD data [17], and the framework was considered to be rigid. The LennardJones parameters for the zeolite oxygen atoms were obtained from previous work [15] and are listed in Table 1. Since silicon atoms are recessed behind oxygen atoms in the pore walls, they are not readily accessible to sorbates. Therefore, van der Waals interactions between silicon atoms and the sorbate molecules were not considered. However, Coulombic interactions between sorbates and both oxygen and silicon atoms were modeled explicitly. The silicon atoms carried a + 1.4 partial charge, whereas oxygen atoms carried a 0.7 partial charge [15]. Interpolation from pre-tabulated Lennard-Jones and Coulombic interaction grids was used to speed up the calculation of zeolite/sorbate interactions. Three different types of sites can be identified in silicalite: zig-zag channels, straight channels, and intersections. The process of defining sites using a methane probe molecule is described elsewhere [18]. In our simulations, the regions that were inaccessible to the methane probe but accessible by water molecules were categorized into the nearest, by distance, defined site. In some simulations, we introduced defect sites within silicalite in a simple way by randomly placing ‘‘seed’’ water molecules within either zig-zag channels, straight channels, or intersections. Each defect in a unit cell was initially replicated in the other seven unit cells of the simulation box. These seeded water molecules are meant to mimic intracrystalline defect silanol sites. They were allowed to rotate during the GCMC simulation but were not allowed to be removed or translated. The resulting silicalite structure with seeds was then used to generate a water adsorption isotherm. Seeded water molecules were not counted in the reported water loadings. 2.3. Water model Numerous models exist in the literature for bulk water, each capable of capturing some experimentally observed characteristics such as the vapor pressure or radial
Table 1 Potential parameters ˚) ri (A Water (SPC/E)
1
As this manuscript was being completed, an experimental report appeared showing pore filling in silicalite at pressures considerably above the bulk saturation pressure [13]. It is interesting to note that Desbiens et al. used GCMC simulations to predict pore filling above the bulk saturation pressure, using the TIP4P water model, the same silicalite partial charges as in Table 1, and Lennard-Jones parameters for the ˚ and e/k = 93.53 K. silicalite oxygens of r = 3.0 A
O H O H e O Si
3.16 – 3.15 – – 2.80 –
ei/kb (K)
qi
References
78.212 0.848 [23] – 0.424 Water (TIP4P) 77.941 – [24] – +0.52 – 1.04 Silicalite 107.5 0.7 [15] – +1.4 ˚ , h = 104.5°, and Psat = 2.2 kPa at For the SPC/E model: rO–H = 1 A ˚ , h = 104.5° rO–e = 300 K [33]. For the TIP4P model: rO–H = 0.9572 A ˚ , and Psat = 4.5 kPa at 300 K [27]. 0.15 A
C.E. Ramachandran et al. / Microporous and Mesoporous Materials 90 (2006) 293–298
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Fig. 1. Comparison of water models SPC/E (d) and TIP4P (j) for adsorption in silicalite at 291 K versus experimental data (solid line) from Olson et al. [12] at 298 K. For the experimental curve, the experimental value of Psat was used, and for the simulation results, the Psat values of 2.2 kPa and 4.5 kPa were used for the SPC/E and TIP4P models, respectively.
distribution function. As of yet, no universal model has been developed for this important molecule. Several potential models have been used in the past to simulate water in silicalite [19–22], mostly to elucidate the diffusion behavior. For this study, the SPC/E model [23] was chosen to represent water, and the Lennard-Jones parameters and partial charges are listed in Table 1. The SPC/E model has been used extensively in the literature for a wide variety of different systems since it was first introduced. Several other water models were also investigated for comparison purposes (i.e., TIP3P, TIP4P, and TIP5P) [24–28]. Results are presented for the SPC/E model unless otherwise indicated.
3. Results and discussion The simulated adsorption isotherms of water in silicalite using the SPC/E and TIP4P models are shown in Fig. 1. Both isotherms show qualitatively the same behavior: practically no water molecules are adsorbed at low pressure and then the pores fill with water. The onset pressure of the pore filling is different for the two different models, and it appears that this value is very sensitive to the potential parameters used [13]. Fleys and Thompson [29] also found extremely low loadings of water in silicalite from GCMC simulations with somewhat different potential parameters.
Fig. 2. Water adsorption isotherms with the SPC/E model at 291 K with and without seeds to mimic defects: 0.5 seeds per unit cell (n), 4 seeds per unit cell (m), base case with no seed molecules (d and dashed line). The seeds were placed in the channel intersections. Experimental data from Olson et al. [12] are also shown (solid line).
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They did not report pore filling, possibly because they did not simulate high enough pressures to observe this behavior with their model. The maximum loading from our simulations is 53 molecules per unit cell, which is similar to the expected pore filling value of 57 molecules per unit cell [12]. Experimental isotherms for water in silicalite have been reported by several groups [8,10–12]. There are some differences, but all show low levels of water adsorption (less than 10 molecules per unit cell), starting gradually at low pressures. The experimental results of Olson et al. [12] are shown in Fig. 1 for comparison with the simulation results. None of the vapor phase experimental isotherms indicate pore filling [13], although pore filling has been observed for adsorption of water in aluminophosphate molecular sieves [30]. Clearly, the simulated and experimental isotherms in Fig. 1 show qualitatively different behavior. One possible reason for this is the presence of silanol and other defects in the experimental silicalite samples. It is believed that silanol defects arise from the use of templating agents in
the synthesis process. Estimates of as many as four defects per unit cell have been suggested [12]. We reasoned that silanol defects could serve as hydrophilic sites that might affect adsorption of water at low loadings. To test this hypothesis, we determined the water isotherms with various numbers of seeded water molecules used to mimic these defects, as described in the previous section. Fig. 2 shows the results with 0.5 seeds per unit cell and 4 seeds per unit cell. The isotherm with only 0.5 seeds per unit cell is essentially identical to the base case. It should be noted that the seeded water molecules, which occupy some pore volume, are not included in the loading quantified on the y-axis. Pore blockage by the seed molecules might explain some of the differences with the base case observed at the higher loadings, but the differences are also indicative of the difficulties in equilibrating the system when the pores are completely full. Using 4 seeds per unit cell produces qualitative changes in the isotherm. The loading at low pressures is non-zero
Fig. 3. Snapshot of water clustering around defect seed molecules in silicalite. Conditions: 1 seed molecule (located in a straight channel) in 8 unit cells, 0.34P/Psat, and 291 K. Oxygen atoms of seed molecules are represented by the large blue spheres. Adsorbed water moleculesÕ oxygen atoms are represented by large red spheres, and the hydrogen atoms are represented by the smaller white spheres. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Snapshot of water clustering around defect seed molecules in silicalite. Conditions: 4 seed molecules per unit cell (located in channel intersections), 0.136P/Psat, and 291 K. Oxygen atoms of seed molecules are represented by the large blue spheres. Adsorbed water moleculesÕ oxygen atoms are represented by large red spheres, and the hydrogen atoms are represented by the smaller white spheres. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
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and is more consistent with the experimental data. This supports the suggestion that defect sites within the pores influence the adsorption of water at low loading. A truly hydrophobic pore shows essentially zero loading of water, but real silicalite samples show finite loadings due to hydrophilic defect sites. Introduction of seeds also shifts the pressure at which pore filling begins to a lower value. A snapshot from a simulation with 1 seed molecule in the simulation box (eight unit cells) is shown in Fig. 3. The adsorbed water molecules clearly form clusters that seem to grow around the seed molecule (note that the clusters at the top/bottom and left/right are connected due to the periodic boundary conditions). Fig. 4 shows a snapshot from a simulation with 4 seed molecules per unit cell at low pressure. Again, the adsorbed water molecules are located preferentially near the seed molecules. Fig. 5 shows a snapshot from a simulation with 4 seed molecules per unit cell at a higher pressure. Here, it can be seen that the pores are almost full of water molecules. Pair distribution functions were also calculated. The pair distribution function based on the centers of mass between the seeded molecules and non-seeded water molecules, g[rCOM], showed a very structured configuration of water around the defects at low loading, evident from a ˚ [31]. The distance and intensity sharp peak at 2.758 A indicate hydrogen bonding occurs between the seeded and non-seeded water molecules. Using g[rCOM], the number of nearest neighbors around the seeded water molecules could be calculated [32]. For the case with 4 seed molecules per unit cell, results showed on average, approximately one water molecule surrounding each seed within hydrogen bonding distance at low pressures (corresponding to Fig. 4) and up to four water molecules surrounding each seed at high pressures (Fig. 5).
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Fig. 5. Snapshot of water adsorption in silicalite near saturation. (Note: For better visualization of seed molecules, this figure is shown as viewed from a different angle compared to Figs. 3 and 4.) Conditions: 4 seed molecules per unit cell (located in channel intersections), 0.45P/Psat, and 291 K. Oxygen atoms of seed molecules are represented by the large blue spheres. Adsorbed water moleculesÕ oxygen atoms are represented by large red spheres, and the hydrogen atoms are represented by the smaller white spheres. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
The radial distribution function between the oxygen of a seeded molecule and the hydrogen of non-seeded water molecules, g[rO–H], is shown in Fig. 6 for the case
Fig. 6. Radial distribution function between the oxygen atom of seeded water molecules and the hydrogen atom of adsorbed water molecules. Conditions: 0.5 seed molecules per unit cell (located in channel intersections), 0.14P/Psat, and 291 K.
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corresponding to 0.5 seed molecules per unit cell and low ˚ and the pressure. The first g[rO–H] peak occurs at 1.765 A ˚ second at 3.2 A, again consistent with hydrogen bonding. 4. Conclusions The molecular simulations presented in this work predict that in a perfect silicalite crystal, the pore system is hydrophobic and essentially no water adsorbs at low pressure. At some intermediate pressure, a sharp transition occurs where the pores become saturated with water molecules. The presence of hydrophilic defects, such as silanol sites, in experimental samples leads to adsorption of small amounts of water at low pressures. Water molecules preferentially form hydrogen-bonded clusters around the defects. The presence of hydrophilic defects also leads to a decrease in the pressure where pore filling occurs. Acknowledgements This research is supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy, Grant No. DE-FG02-03ER15457. Additional support from the National Science Foundation (CTS-0302428) is also gratefully acknowledged. The authors thank Prof. Alain Fuchs for helpful discussions. References [1] S. Vaitheeswaran, H. Yin, J.C. Rasaiah, G. Hummer, Proc. Nat. Acad. Sci. 101 (49) (2004) 17002. [2] N. Floquet, J.P. Coulomb, N. Dufau, R. Kahn, Stud. Surf. Sci. Catal. 154 (2004) 1804–1811. [3] T. Ohba, H. Kanoh, K. Kaneko, J. Phys. Chem. B 108 (39) (2004) 14964. [4] A. Striolo, K.E. Gubbins, A. Chialvo, P.T. Cummings, Mol. Phys. 102 (2004) 243–251. [5] O. Beckstein, M.S.P. Sansom, Proc. Nat. Acad. Sci. 100 (12) (2003) 7063–7068. [6] G. Hummer, J.C. Rasaiah, J.P. Noworyta, Nature 414 (2001) 188– 190. [7] C.L. McCallum, T.J. Bandosz, S.C. McCrother, E.A. Muller, K.E. Gubbins, Langmuir 15 (1999) 533–544. [8] E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner, J.V. Smith, Nature 271 (1978) 512–516.
[9] International Zeolite Association. Available from: