Cation Redistribution Upon Water Adsorption in Titanosilicate ETS-10

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1 Cation Redistribution Upon Water Adsorption in Titanosilicate ETS-10 Anjaiah Nalaparaju, George X. S. Zhao and Jianwen Jiang* Department of Chemical and Bimolecular Engineering National University of Singapore, Singapore * chejj@nus.edu.sg Abstract: Nonframework Na cations in titanosilicate ETS-10 have been analyzed from atomistic simulation. Based on the binding energies, five types of Na cations are classified. Type V cations located in the main channels are substantially mobile compared with type I-IV cations. Redistribution of Na ions is observed upon H O adsorption, in which the number of type-v ions increases while other types slightly decreases. The simulated adsorption isotherm of H O is in good agreement with experimental data, and three adsorption sites are observed for H O. As loading increases, the diffusion of H O initially rises and then drops as largely determined by H O in the main channels. Key-Words: Cation, Water, ETS-10, Adsorption, Diffusion, Atomistic simulation 1. Introduction As a typical titanosilicate, ETS-10 has a uniform pore structure and high thermal stability. The pore network in ETS-10 is mainly formed by three-dimensionally connected 1- ring and 7-ring channels. A wide-pore channel of approximately 8 Å allows small molecules to access the intracrystalline space and thus provides a large scope in separation and storage applications. The octahedral coordinated tetravalent Ti atoms induce a strong anionic nature of the framework and distinct active sites for base catalysis [1]. Neutrality of the framework is maintained by the loosely bound counterbalancing cations. The nonframework cations are readily exchangeable with others ions, well suited for ion exchange []. Knowledge of cation siting and energetics in a porous material is critical to adsorption, diffusion and catalytic properties. From the lattice energy minimization and EXAFS experiment, Sankar et al found four cation sites distributed along the Ti-O chains in polymorph B [3]. Using packing algorithm implemented with CVFF force field, Grillo et al mimicked the cation locations in both K-ETS-10 and Na-ETS- 10 [4]. Identical cation sites were observed for Na and K, but with relatively different energies. It was also inferred that the electrostatic interaction of K with ETS-10 is more favored than Na. Ching et al performed the first-principles density-functional theory calculations to determine the cation sites and electronic structure in ETS-10 [5]. They concluded that the cations are unlikely to sit in either 1-ring pore (due to the long distance from Ti-O chain) or 3-ring pore (due to the spatial restriction). All four possible cation sites from their work are in the 7-ring channel at varying distances from Ti atoms, in which the intermediate distance is the most favorable. Their study also demonstrated the mechanical stability and rigid nature of ETS-10 from the bond-order calculations, which supports the rigid framework model used in the present work. To the best of our knowledge, no information is currently available on the intriguing interplay of nonframework cations and sorbate molecules in ETS-10. This information, however, is essential to exploit adsorption, diffusive an catalytic properties and ISBN: ISSN:

2 industrial applications of ETS-10. The present study aims to fill this gap through the state-ofthe-art molecular simulations for H O in Na exchanged ETS-10 (Na-ETS-10). H O is always present in ion exchange process and also in many vapor mixtures. Our study for H O in ETS-10 is helpful to elucidate the fundamental behavior of molecules nanoconfined in ETS-10.. Model and Method The polymorph-a is considered here as a host structure for Na-ETS-10. The crystalline structure of ETS-10 has lattice parameters a = b = Å and c = 7.08 Å [6]. A unit cell of Na-ETS-10 consists of 16 Ti atoms, 80 Si atoms, 08 O atoms, and 3 Na nonframework cations for electroneutrality. The coordinates of the framework Ti, Si and O atoms were adapted from the experimental crystallographic data of Anderson et al [6]. To estimate the partial charges of the framework atoms, the densityfunctional theory calculation was performed with the DNP basis set. The interactions between Na and framework were modeled by the Lennard-Jones (LJ) and Coulombic potentials. The LJ parameters were taken from the Universal force field [7]. The three point transferable interaction potential was used for H O, which is a rigid model with O-H bond length of Å and HOH angle of [8]. In our simulations, the framework contained four ( 1) unit cells with the periodic boundary conditions in all the three directions. The framework atoms were fixed during simulation, however, the nonframework Na ions were allowed to move. The unit cell was divided into three-dimensional grids and the potential energy maps for cation and sorbate molecules were tabulated in priori and then used for interpolation during simulation. NVT MC simulation was performed at 300 K to predict the binding sites and energies of Na ions. For the Coulombic interactions, the Ewald sum with a tin-foil boundary condition were used [9]. The real/reciprocal space partition parameter and the number of reciprocal lattice vectors were chosen to be 0. and 8, respectively, to ensure the convergence. The adsorption of H O at 300 K was simulated using gibbs ensemble MC (GEMC) method. In GEMC simulation, there were two boxes, one was the Na-ETS-10 framework with fixed volume and the other contained the bulk sorbate molecules with varying volume. The beauty of GEMC is to obtain adsorption information directly at a desired pressure. The number of trial moves in our GEMC simulation was 10 7, though additional trial moves were used at high loadings. The first 10 7 moves were used for equilibration, and the second 10 7 moves to obtain averages. The mobility of cations and H O molecules was examined from MD simulations using DL_POLY suite of program [10]. The temperature was 300 K and maintained by the Noose-Hover thermostat. The initial configurations were from NVT or GEMC simulations as mentioned above. A typical MD simulation was performed for 5 ns, including 3 ns equilibration and ns production. The potential and kinetic energies were monitored during the simulation to ensure equilibration. The time step was 1 fs to maintain a good energy conservation, and the history file was saved every 1 ps for further analysis. 3. Results The electrostatic interaction was found to contribute largely to the binding energy. Based on the difference in binding energy, Na ions were classified into five types. Figure 1 illustrates the fives types of ions in ETS-10 on the (100) plane. The observed distribution of Na ions is similar to that in the polymorph B form of ETS-10 [11]. This infers that the chemical environment in polymorph A and B is largely identical and the difference is in the stacking disorder of Ti 8 S 40 O unit. From the ISBN: ISSN:

IV II III V I 0 0 00 400 600 800 1000 t (ps) Fig. 1. Five types of Na cations in Na-ETS-10 and their mean-square displacements.

3 IV II III V I t (ps) Fig. 1. Five types of Na cations in Na-ETS-10 and their mean-square displacements. MSD (A ) o I II III & IV V two orthogonal projections, it is clear that the 1-ring channels interconnect with the 7- and 1-ring channels from other directions. Sites I to IV are located on both sides of the Ti-O-Ti chain and parallel to the chain. On one projection, all these sites appear to locate in the 7-ring channel, but the view on another projection reveals they are located at the intersection of 7-ring channel and other channels. In principle, Na-ETS-10 should have 1:1 site occupancy, i.e. only four sites should present. However, an extra site is observed in the big 1-ring channel because of the large energy difference generated therein. Na ion at site IV is located between two orthogonal Ti-O-Ti chains and electrostaitcally interacts with a large number of Ti atoms, therefore, its binding energy is the highest among the five sites. The 7-ring channel is favorable for small polar molecule because of the strong electrostatic filed. Sites I and II appear to be identical from Figure 1 and also from the close binding energies. Nevertheless, they are two distinct sites based on their crystallographic positions. Na ions at sites I and II have access to guest molecule in the 1- ring channel. Due to the shielding effects by O atoms in the 5-member silica rings, however, these ions only interact weakly with guest molecules and thus can be regarded as weak sorption sites. Site III is open to the 1-ring channel, has less coordination with O atoms and less interaction energy as compared with sites I, II and IV. Nevertheless, Na ion at site III has relatively strong interaction with guest species in the 1-ring channel. For instance, Zecchina et al reported from molecular modeling that Na ion at site III forms adducts with diatomic guest molecules [1]. Site V is located in the 1-ring channel and has the lowest binding energy as a - result of the long distance from TiO 6 octahedral units. It is usually assumed that O atoms away from cations have large electron density and hence behaves as strong basic site. From the knowledge on the cation distribution, the intersection of 1-channels could be highly hydrophobic in ETS-10. This is consistent with the observation that ETS-10 framework atoms facing the 1-ring channel are potentially basic adsorption sites [13]. The mobility for each type of Na ion was examined by calculating the mean-square displacement (MSD) MSD ( t) = Δr( t ) from MD simulation, where t denotes time and Δr() t is the displacement from its initial location. The multiple timeorigin method was used to evaluate MSD for a better statistical accuracy. Figure 1 shows the MSDs of five types of Na ions in ETS-10. As a function of time, the MSD of type-v Na ions ( Na V ) in the 1-ring channels increases sharply and then tends to be flat. This is a result of the weak interaction between Na V ions with framework and the large void space in a HO (mol/kg) 1 experiment simulation N Na P (kpa) Fig.. Adsorption isotherm of H O in Na-ETS- 10. The insets are the number of Na V as a function of H O loading. N HO ISBN: ISSN:

the 1-ring channels. The ions located at the initial equilibrium positions can escape due to thermal motion, and then continuously move to the next equilibrium positions.

Figure shows the adsorption isotherm of H O at 300 K in Na-ETS-10. Despite slight overestimation, the simulation results generally agree well with the experimental data [14].

Figure 3 shows the simulation snapshots of H O molecules adsorbed in Na-ETS-10 with loading N HO = 0, 6 and 64, respectively.

4 the 1-ring channels. The ions located at the initial equilibrium positions can escape due to thermal motion, and then continuously move to the next equilibrium positions. It is obvious that only Na V ions have appreciable mobility, whereas all other Na ions have negligible vibrations around the binding sites. Figure shows the adsorption isotherm of H O at 300 K in Na-ETS-10. Despite slight overestimation, the simulation results generally agree well with the experimental data [14]. This implies that the Universal force field is accurate to model the interactions for this system. Figure 3 shows the simulation snapshots of H O molecules adsorbed in Na-ETS-10 with loading N HO = 0, 6 and 64, respectively. There are three adsorption sites for H O; OW1: the 7-ring channel, OW: the intersection of the 7-ring channels, and OW3: the 1-ring channels away from the Ti-O-Ti chains. OW1 site is highly polar in the framework and hence preferential for H O, followed by OW and OW3. The mobility of H O in each type of site increases in the order of OW1, OW and OW3. H O molecules in OW1 sites orient in such a way that the OW (oxygen in H O) atoms are toward the Na ions therein, because of the strong electrostatic interaction between H O and ions. With increasing loading, H O molecules first occupy the highly polar OW1 sites, and then OW and OW3 sites. However, OW1 site is very small and gets saturated even at low loadings. At further increased loading, the number of H O molecules at OW1 remains nearly constant, but the number at OW3 increases largely. Due to the strong interactions between H O and Na ions, upon adsorption of H O a distinct redistribution of Na ions is observed in the framework. Analyzed from the trajectory file, Na ions at sites I, II and III move into site V (the main channel), subsequently type IV ions move into sites I and II to fill some of the vacant sites. With increased loading, the number of type-i and II ions decreases marginally, the number of type III and IV ions decreases to some extent. In contrast, the number of type-v Na ions increases, as shown in the inset of Figure. Ion redistribution upon sorbate sorption has substantial influence on the properties of porous material. N H O = 0 N H O = 6 N H O = 64 Fig. 3. Snapshots at N H O = 0, 6, and 64 in Na-ETS-10. Three adsorption sites for H O are indicated as OW1, OW and OW3. H O: sticks, Na : purple spheres, Na V : cyan spheres. 4. Conclusions OW1 OW3 OW The structural and dynamic properties of cations and water molecules in Na-ETS-10 have been investigated using MC and MD simulations. The binding sites and energies of Na ions are identified and based on the binding energies, five types of ions are classified. Type- V ions in the 1-ring channels have the lowest binding energy and largest mobility, while all other types remain essentially static. Knowledge on cation distribution and energetics provides an instructive insight towards the basic and hydrophobic sites in the framework. There are three adsorption sites for H O, including OW1 in the 7-ring channel, OW in the intersection of the 7-ring channels and OW3 in the 1-ring channels. H O first adsorbs at site OW1, then at site OW and finally at site OW3 particularly at high loadings. H O molecules vibrate at site OW1 and the mobility is negligible. At site OW, the mobility first decreases and then increases as a consequence of the formation of Na -H O complex and the cooperative interaction between H O molecules. ISBN: ISSN:

5 At site OW3, the mobility is the largest and increases with loading except close to saturation. The overall mobility is determined largely by H O at site OW3. Strikingly, Na ions redistribute upon the adsorption of H O, as a result of the strong electrostatic interactions between ions and polar molecules. Specifically, the type-v Na ions in the 1-ring channels increases in number upon H O adsorption, while other types decreases slightly or moderately. Acknowledgment The authors are grateful for the support from the National University of Singapore. References: [1] Philippou A, Rocha J, Anderson MW. Catalysis letters 57 (1999) 151. [] Zhao XS, Jia LL, Phai AC. Langmuir 19 (003) [3] Sankar G, Bell RG, Thomas JM, Anderson MW, Wright PA, Rocha J. Journal of Physical Chemistry 100 (1996) 449. [4] Grillo ME, Carrazza J. Journal of Physical Chemistry 100 (1996) 161. [5] Ching WY, Xu YN, Gu ZQ. Physical Review B 54 (1996) [6] Anderson MW, Terasaki O, Ohsuna T, Malley PJO, Philippou A, et al. Philosophical Magazine B 71 (1995) 813. [7] Rappe Ak, C.J.Casewit, K.S.Colwell, III WAG, W.M.Skiff. J.Am.Chem.Soc. 114 (199) [8] Jorgenson WL, Chandrasekhar J, D.Madura J. J.Chem.Phys 79 (1983) 96. [9] Heyes DM. Phys. Rev. B 49 (1994) 755. [10] Smith W, Forester TR. J.Mol.Graph 14 (1996) 136. [11] Anderson MW, Agger JR, Luigi DP, Baggaley AK, Rocha J. Physical Chemistry Chemical Physics 1 (1999) 87. [1] Riechiardi G, Vitillo JG, Cocina D, Gribov EN, Zecchina A. Physical Chemistry Chemical Physics 9 (007) 753. [13] Kishima M, Okubo T. Journal of Physical Chemistry B 107 (003) 846. [14] Kuznicki SM, Thrush KA, Allen FM, Levine SM, Hamil MM, et al In Synthesis of microporous materials, pp. 47 ISBN: ISSN:

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