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Computational study of structural and thermal properties of the microporous titanosilicate ETS-10
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
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Computational Study of Structural and Thermal Properties of the Microporous Titanosilicate ETS- 10 M. E. Grillo, J. Lujano and J. Carrazza INTEVEP S. A., Research and Technological Support Center of Petr61eos de Venezuela. Apartado 76343, Caracas 1070A, Venezuela. A theoretical approach to evaluate structural and physical chemical properties of ETS-10 has been chosen, employing calculation strategies that have proven to be accurate in the case of zeolites. The non-framework cation sites for K § and Na § ions have been modelled, combining a Monte Carlo Packing (MCP) procedure with the Lattice Energy Minimization (LEM) technique. A set of four distinct extra-framework cation sites distributed along the Ti--O-Ti rods has been obtained. They are surrounded on both sides by silicon five rings, and located between two Ti atoms of the rods. The lowest lattice energy has been obtained by placing K § cations on the non-framework sites. This is consistent with cation exchange properties of ETS- 10. Na § ions are easier to exchange by H § than K § despite the fact that mobility of hydrated K § is 50% higher than that of hydrated Na+ ions. In the case of the K-ETS-10 minimum-energy structure, the T i - O bond length along the T i - O - T i rod (1.97/~) increases in about 3.5 %, with respect to the proposed structure. Average Ti---O distances of 2.13 A along the Ti--O-Si linkages have been obtained. The T i - O - T i bond angle (163 ~ is about 5% larger than in the proposed starting structure. Lattice energy minimizations were performed for A1 substitution into two d i f f e r e n t Si c h e m i c a l e n v i r o n m e n t s in the E T S - 1 0 structure; Si(4Si, 0Ti) and Si(3Si, 1Ti). The calculated stability difference for the two studied A1 insertion positions is striking, indicating a clear preference for the sites connected to tetrahedral silicon, as opposed to those connected to octahedral Ti sites. This result is in good agreement with the AI, Ti avoidance in the ETAS- 10 structure, indicated by zgsi and 27A1MAS N M R spectra. Molecular dynamic simulations suggest a significantly higher thermal stability for the polymorph B of ETS-10, compared to the experimentally observed for the disordered material. 1. INTRODUCTION ETS-10 is a recently discovered titanosilicate [ 1], whose structure has been solved, based on a combination ofHREM, MAS NMR, XRD and ED results [2]. It contains TiO6 octahedra, of which two of the oxygen corners are shared with other titanium, forming a < T i - O - T i > straight chain, while the other four are shared with SiO4 tetrahedra. Sets of periodically spaced chains are joined in orthogonal directions. Stacking of layers with this array gives rise to a three-dimensional network of interconnecting 12-membered ring channels.
2324 Very few zeolite structure types have such an open pore system, the most important being zeolite Y (FAU) and zeolite [3 (BETA). Each Ti atom in this structure is associated with a - 2 charge, that is compensated by non-framework cations. The cation density in ETS-10 is approximately equivalent to that in a zeolite with Si/AI = 2.5. Because of the interconnecting network of wide pore straight channels, and the high extra-framework cation density, ETS-10 is a potentially useful material for applications in catalysis. Furthermore, aluminum can be incorporated into ETS-10, by substituting tetrahedral silicon, thus generating acidic sites. In ETS- 10, the stacking pattern of these layers of orthogonally set < T i - O - T i > chains is not regular, giving rise to defects and faults in the structure. This disorder can be modelled by a random combination of two polymorphs, resulting from different stacking sequences. One of them, polymorph A, corresponds to a ABCD stacking and has a monoclinic lattice with space group C2/c, while the other corresponds to a ABAB stacking, and has a tetragonal lattice with a space group P41 or P43. Not much is known about the physical chemical and catalytic attributes of ETS-10. Furthermore, it is not clear how they are affected by the disorder in the structure. In this work, we have chosen a theoretical approach to evaluate these properties, employing calculation strategies that have proven to be accurate in the case of zeolites. Some inferences from these results have been compared to experirnentally determined characteristics of the material.
2. EXPERIMENTAL 2.1 Computational determination of structural properties Non-framework cation location: The non-framework cation sites for K + and Na + ions within the ETS- 10 structure have been modelled, combining a Monte Carlo Packing (MCP) procedure with the Lattice Energy Minimization (LEM) technique. The MCP algorithm developed by Biosym/MSl [3] enables a large number of starting extraframework cation configurations to be generated, which are then optimized based on the minimization of their lattice energy. In the MCP procedure, the ions are s u c c e s s i v e l y i n t r o d u c e d into the structure, so as to create low energy Table 1 arrangements according to an energy Interatomic potenctial parameters used. threshold for the short-range interactions ch refers to the atomic partial charge. between the cations and the host lattice. The potential parameters, A i and Bi, are In this way, one hundred trial packing given in kcal mol-I/~ n and kcal mo1-1 ./k6, configurations for each cation type were respectively. randomly generated. The present lattice simulations were i Ai Bi q~ carded out using the Discover code of O 388611.3727 0.1893 -1.2 Biosym/MSI [4]. The lattice energy c a l c u l a t i o n r e l i e s on c l a s s i c a l Si 103.8039 103.8039 +2.4 interatomic potential functions (force field). A modified "Consistent Valence Ti 6915.3989 3262.4997 +1.6 Force Field"-based on partial chargesK 12886.4561 0.0001 +1.0 is used. The short-range interactions are calculated with a Lennard-Jones (12-6) Na 67423.6364 0.0005 +1.0 potential function to a cut-off of 13.5 tl,. i
2325 The long-range coulombic interactions are calculated using periodic boundary conditions by the Ewald summation method to an accuracy of 0.1 kJ/mol. The partial charges and parameters used in the short-range energy terms are presented in Table 1. The optimization strategy applied for both K + and Na § as counter ions was first to minimize the lattice energy solely with respect to the non-framework cation coordinates, and then allow to relax the whole (framework and counter ions) atomic positions. The simulation system consists of a unit cell of the polymorph B containing 336 atoms, (16 T i , 80 Si, 208 O and 32 charge-balancing ions). The disorder in ETS-10 has been described in terms of different stacking sequences of the same titanosilicate unit. We have, thus, modelled the cation sites for only one case, polymorph B. The framework atomic coordinates and unit cell parameters reported by Anderson et al. [5] were taken as input to the lattice energy minimizations. AI incorporation: Lattice energy minimization has proven to be an appropriate technique to investigate the relative stabilities of aluminum sites within zeolite framework structures [6]. In the present work, we apply this technique to evaluate the relative stabilities of the two possible Si environments for A1 substitution, Si(3Si, 1Ti) and Si(4Si, 0Ti). In these calculations the point at issue is whether in the titanosilicate structure an A1-O-Ti linkage is possible, or if only A1 sites avoiding titanium neighbors are found. A measure of the stability of a given Al T-site, (T=Tetrahedral), is obtained from the minimum lattice energy of the Al-substituted (ETAS-10) structure. The A1 substitution energy, A, is calculated relative to the lattice energy of the siliceous ETS- 10 structure. In these calculations, K § cations were taken as charge balancing ions in the ETAS-10 lattice. Although the synthesized structure contains a mixture of Na § and K § ions, this approximation avoids the problems of considering the different permutations o f N a § and K § cations around the Al site. The additional negative charge introduced by the A1 incorporation in the tetrahedral framework site is neutralized in the calculation with an extra K § cation. This additional cation was introduced in the structure by the MCP procedure described before. For each of the considered Al-substituted structures, thirty different packing configurations of this additional K § ion in the K-ETAS- 10 lattice were generated. The lattice energy of each trial structure was first optimized with respect to the 33 K § counter ions, while the framework was held rigid. These structures were then fully relaxed, with respect to the whole framework plus counter ions atomic coordinates.
2.2 Computational determination of thermal stability Molecular dynamics simulations have been performed employing the Discover code of Biosym/MSI in the temperature range of 773 - 1273 K [4]. The equations of motion were integrated using the velocity form of the Verlet algorithm. The temperature range for the crystalline to amorphous phase transition ofpolymorph B ofETS- 10 is determined from the change in the Ti--O bond radial distribution functions (RDF), when the sample is heated. The temperatures were controlled by scaling particles velocities, and the pressure was maintained at 1 arm throughout the molecular dynamics, by the ParrineiloRahman method of pressure control. The simulation starts from a previously relaxed (K, Na)-ETS-10 structure. First, the system is equilibrated to the desired temperature for 70,000 steps in a time step of 1 fs (0.1 x 10-15 s) in the constant-pressure, constant-temperature ensemble (NPT). The dynamics is run for further 20,000 steps in the same ensemble, period over which the time averaged data collection starts.
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2.3 Synthesis and characterization of ETS-10 ETS-10 was synthesized following the procedure described by Kuznicki [1]. The XRD pattern and 29Si MAS NMR spectra of the obtained material agrees with thos~ reported by other authors [2]. The sodium and potassium composition of the prepared material was equal to 8.66 wt% and 6.73 wt% respectively, for aNa/K molar ratio of2.18. Ionic exchange properties were determined by eluding 500 mL of a 10 mM HNO3 over 1 g of material. Samples of the eluted solution were collected and analyzed for sodium and potassium by atomic absorption. To determine their thermal stability, samples of ETS-10 were heated in static air between 573 and 873 K for 1 hr. XRD patterns of these treated samples were obtained at room temperature with a PHILIPS diffractometer, model 1730-10, using Cu kcx radiation. 3. RESULTS AND DISCUSSION 3.1 Structural properties Non-framework cation location: The structure optimizations were initially performed at constant volume with the Discover code. For both K + and Na + as counter ions, a unique set of sites results from the full structure minimizations, as shown in Figure 1. The resulting set of four distinct extra-framework cation sites are distributed along the Ti--O-Ti rods, surrounded on both sides by silicon five rings, and located between two Ti atoms of the rods. The unit cell vector and angles were then allowed to change by optimizing the lattice energy at constant pressure, using the lattice simulation program GULP [7]. Most of the initial configurations for K + and Na + converged to a minimum energy structure of symmetry C2h. The experimental and calculated cell dimensions are summarized in Table 2. There are no significant deviations of the cell volume and atomic coordinates for the equilibrated polymorph B, from the proposed structure of Anderson et al. [5]. The main c h a n g e upon r e l a x a t i o n is a contraction of the unit cell along Table 2 the a and b axis and an expansion Experimental and calculated cell parameters for along the c axis. The fractional polymorph B of ETS- 10. K-ETS- 10 and coordinates are close to those of Na-ETS-10 refer to the calculated values for the the starting structure. lattice with K and Na as counter ions. The E l e c t r o s t a t i c e n e r g i e s per distances are given in/I,. cation site are listed in Table 3, for both K+ and Na+ as counter-ions. Experimental K-ETS- 10 Na-ETS- 10 These calculations indicate that different relative energies among a 15.8500 15.5612 15.4536 these sites are obtained depending b 15.8500 15.5612 15.4536 on the counter-ion. While K + preferentially lowers the energy c 14.5100 14.8911 14.9754 of sites III and IV, Na + favors sites I and III. 0~ (o) 75.2391 74.5667 75.0822 The calculated relative energies [~ (o) 104.7609 105.4333 104.9178 of Na § and K + ions over the determined sites is consistent with T (o) 90.0000 89.990I 89.9586 cation exchange experiments, I
I I
I
III
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Figure 1. Schematic diagram of the structure of ETS-10. The dark atoms in the f r a m e w o r k represent Ti, and the light ones Si and O. The black spheres represent the n o n - f r a m e w o r k compensation cations. The roman numbers indicate the four crystallographic sites where the counterions are located. The two black atoms in the structure, labelled Ti and T2, represent the sites where a l u m i n u m substitution was modelled. The numbers indicated the calculated T i - O bond distances, in A . carried out as part of this work. W h e n an acid solution is eluted over a (Na,K)-ETS-10 sample with a Na/K molar ratio of 2.2, this cation ratio in the eluted solution raises to 5.2, despite the fact that mobility of hydrated K § is 50% higher than that of hydrated N a + ions [8]. This result indicates that N a + ions are easier to exchange by H + than K +, and since this goes against kinetic trends, energetic considerations must be involved; that is, N a + must be easier to elude, in part because they are less tightly bound to the structure than K +, in accordance with our theoretical calculations. Local structure o f the Ti environment: In the refined structure proposed by Anderson et al. [2], the T i - O bonds in the TiO6 octahedra has almost the same length, 1.90 A along the Ti--O-Ti rods and 1.87 A along the T i - O - S i linkages, respectively. In order to check the structural changes of the local Ti environment on relaxation, we examined the optimum energy configurations of the Table 3 Electrostatic site energies, E '~', of Na and K K-ETS- 10 and (Na, K)-ETS- 10 lattices. W e h a v e used a N a / K ratio o f 2.2, cations over the four crystallographic consistently with our ETS- 10 sample. The distinct sites in the ETS-10 structure. The ten K + ions have been distributed over the tabulated energies are in kJ/mol sites IV (8 ions) and III (2 ions), since site E TM Er these are found to be the most energetically favored sites in the K - E T S - 1 0 structure I - 943.33 - 943.82 (see Table 3). In the case of the K-ETS- 10 minimumII - 926.63 - 979.27 energy structure, the T i - O bond length III - 960.37 - 1010.97 along the T i - O - T i rod (1.97 A) increases in about 3.5 %, with respect to the proposed IV - 890.61 - 1063.58 structure. For the m i n i m u m energy (Na,
2328 K)-ETS-10 configuration, a ight corrugation of about 0.01 in the Ti-O bond length alternating along the chain has been determined. EXAFS spectrum has been successfully interpreted by previous lattice simulations with a larger variation in the T i - O distances (2.11 and 1.71 /~) K-ETAS-IO E1 E2 A along the T i - O - T i chain [9]. Obtaining these structural features with present lattice T1 - 4697.6 - 11823.6 47.2 s i m u l a t i o n s s h o u l d not be expected, as the force field used does not include three-body T2 - 3849.0 t e r m s for the T i - O - T i interactions, which are essential to reproduce this effect. Average T i - O distances of 2.13 and 2.19 A along the Ti--O-Si linkages have been obtained for the minimum energy K-ETS- 10 and (K, Na)-ETS- 10 structures, respectively. The Ti--O-Ti bond angle in the (K, Na)-ETS-10 structure increases upon relaxation by only about 1.3 %, to 157 ~ In the minimized K-ETS- 10 structure, the T i - O - T i bond angle (163 ~ is about 5% larger than in the proposed starting structure, and similar to the value obtained by the lattice simulation that fits the EXAFS results (165 ~ [9]. Table 4 Lattice energies (LE) per SiO2 unit (kJ tool 1) of the minimized ETAS-10 structures for the two tetrahedral (T) sites considered. E 1is the minimized LE for the relaxed counterions and fixed structure. E 2 is the LE for the full minimized structure. A is the A1 substitution energy calculated relative to the LE of the K-ETS 10 structure.
3.2
Aluminum substitution Lattice energy minimizations were performed for A1 substitution into two different Si chemical environments in the ETS-10 structure; Si(4Si, 0Ti) and Si(3Si, 1Ti). These sites are identified in Figure 1 as T1 and T2, respectively. The framework position involving a A 1 - O - S i - O - T i linkage (T1) is found as the energetically preferred site for A1 incorporation. (See Table 4.) For this site, more than half of the thirty starting packing configurations of the additional K + ion in the K-ETAS-10 structure converged to the same energy minimum. In contrast, the thirty initial trial structures substituted at the framework position involving an A I - O - T i linkage (T2), could only be partially minimized with respect to the non-framework cation positions. Further relaxations of these structures with respect to the whole atomic coordinates did not converge to a local energy minimum. This result might be expected, based on the electrostatic repulsion of neighboring negative charges on the aluminum and titanium sites. Furthermore, the T2 tetrahedra is connected to the titanium octahedra, forming a three-member ring, which is critically unfavorable, due to the additional strain introduced by the larger A1 ion. In summary, the calculated stability difference for the two studied AI insertion positions is striking, indicating a clear preference for the sites connected to tetrahedral silicon, as opposed to those connected to octahedral Ti sites. This result is in good agreement with the A1, Ti avoidance in the ETAS-10 structure, suggested by 29Si and 27A1 MAS N M R spectra [10].
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3.3 Thermal stability The simulations were performed on a minimized (K,Na)-ETS-10 1273 K structure, with a N a ~ ratio of 2.2, as in our ETS-10 sample. The radial distribution functions (RDF) for the Ti-O bond, averaged over the last 10,000 iterations (10 ps) of the dynamics run, were calculated in the 873 K temperature range of 773 - 1273 K. Figure 2 displays the RDF for the Ti-O bond in several coordination s p h e r e s o b t a i n e d f r o m these calculations, compared to those for the proposed ETS- 10 structure. The present MD simulations suggest a significantly higher thermal n0 773 K stability for the p o l y m o r p h B, compared to the experimentally observed for the disordered material. From the temperature dependence of the experimental x-ray diffraction pattern, a departure from crystallinity I | I Proposed is observed in the temperature range I II I structure of 773 to 873 K, for the disordered material. Whereas, the calculated R D F for p o l y m o r p h B in this temperature range still presents a sharp peak around 2 A for the first T i - O coordination sphere and a significant proportion of T i - O bonds 0 2 4 6 8 10 around 3.5 and 4.5/~, corresponding Ti-O Radial Distance (A) to the second coordination sphere. At 1273 K, a nearly continuous Figure 2. Radial distribution functions of the distribution of T i - O bond lengths is Ti-O bond for the proposed ETS-10 obtained, indicating a significant loss structure, compared to the obtained by MD of crystallinity. The MD simulations simulations at different temperatures. As also s h o w a p o s i t i v e t h e r m a l displayed, the distribution tends to a expansion coefficient, obtained from continium with increasing temperature. the change in cell volume with temperature. This variance in the predicted and calculated thermal stabilities is probably due to the fact that the simulations were performed on a specific ordered polytype of the ETS-10 structure (polymorph B), whereas the real sample consists of a random distribution of layer stacking sequences. In this way, the disordered material presents a random arrangement of pores and faulted layers, which have been observed by high-resolution electron microscopy [11]. The faults in the ETS- 10 structure can result in larger double or quadruple micropores formed by coalescence of several pores or even larger formed by two defect micropores. In fact, Anderson et al. [2] proposed the disordered material composed of different domains of faulted layers. The presence of such defects and boundaries between faulted domains in ETS-10 should influence significantly the thermal stability of this material. L_
2330 4. CONCLUSIONS The present work illustrates the ability of atomistic simulation techniques based on classical potential energy functions (force fields) to simulate and predict structural and physical properties of the microporous titanosilicate ETS-10. Using a force field potential that combines coulombic and short-range interactions, the calculations are able to model structural properties, starting from only the framework structural data. A combination of the Monte Carlo Packing procedure with the Lattice Simulation technique proved to be efficient in modelling the extra-framework cation sites. The calculated relative energies for Na + and K + cations at these sites are consistent with observed ion-exchange properties. The present lattice energy optimization predicts a distortion of the TiO6 octahedra. However, in order to simulate variations in the Ti-O bond distances along the Ti--O chains, it is necessary to include extra terms in the potential energy function, to take into account the directionality of the T i - O - T i bond. The present evaluation of the relative stabilities of two different Al-substitution sites in the ETS-10 framework confirms an A1,Ti avoidance in the ETS-10 structure, analogous to the Lowenstein's rule for zeolites, suggested by the 29Si and 27A1 MAS NMR experiments. The present MD simulations predict a higher temperature for the crystallineamorphous phase transition of the polymorph B, compared to the experimentally observed range for the disordered material.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
S.M. Kuznicki, US Patent 4,853,202 (1989). M . W . Anderson, O. Terasaki, T. Ohsuna, P. J. O. Malley, A. Philippou, S. P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Philosophical Magazine B , 71 (1995) 813. Catalysis Package, version 2.3.7, Biosym/MSI, San Diego, California. Discover Molecular Simulations Program, version 94.1, Biosym/MSI, San Diego, California. M . W . Anderson, O. Terasaki, T. Ohsuna, A. Phillippou, S. P. Mackay, A. Ferreira, J. Rocha and S. Lidin, Nature, 367 (1994) 347. C . R . A . Catlow, Modelling of Structure and Reactivity in Zeolites, Academic Press, London, 1992. GULP (The General Utility Lattice Program), J.D. Gale, Royal Institution Imperial College, UK 1992-1994. F . A . Cotton and G. Wilkinson, Advanced Inorganic Chemistry, p. 255, Fourth Edition, John Wiley, New York, 1980. G. Sankar, R. G. Bell, J. M. Thomas, M. W. Anderson, P. A. Wright and J. Rocha, J. Phys. Chem., 100 (1996) 449. M . W . Anderson, A. Philippou, Z. Lin, A. Ferreira, and J. Rocha, Angew. Chem. Int. Ed. Engl., 34 (1995) 1003. T. Ohsuna, O. Terasaki, D. Watanabe, M. W. Anderson and S. Lidin, in Stud. Surf. Sci. Catal., v. 84, p. 413, J. Weitl~mp, H. G. Karge, H. Pfeifer and W. Htilderich Eds., Elsevier Science B. V., Amsterdam, 1994. The authors thank INTEVEP for permission to publish this article. Technical support from Biosym/MSI is gratefully acknowledged.
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Computational Study of Structural and Thermal Properties of the Microporous Titanosilicate ETS- 10 M. E. Grillo, J. Lujano and J. Carrazza INTEVEP S. A., Research and Technological Support Center of Petr61eos de Venezuela. Apartado 76343, Caracas 1070A, Venezuela. A theoretical approach to evaluate structural and physical chemical properties of ETS-10 has been chosen, employing calculation strategies that have proven to be accurate in the case of zeolites. The non-framework cation sites for K § and Na § ions have been modelled, combining a Monte Carlo Packing (MCP) procedure with the Lattice Energy Minimization (LEM) technique. A set of four distinct extra-framework cation sites distributed along the Ti--O-Ti rods has been obtained. They are surrounded on both sides by silicon five rings, and located between two Ti atoms of the rods. The lowest lattice energy has been obtained by placing K § cations on the non-framework sites. This is consistent with cation exchange properties of ETS- 10. Na § ions are easier to exchange by H § than K § despite the fact that mobility of hydrated K § is 50% higher than that of hydrated Na+ ions. In the case of the K-ETS-10 minimum-energy structure, the T i - O bond length along the T i - O - T i rod (1.97/~) increases in about 3.5 %, with respect to the proposed structure. Average Ti---O distances of 2.13 A along the Ti--O-Si linkages have been obtained. The T i - O - T i bond angle (163 ~ is about 5% larger than in the proposed starting structure. Lattice energy minimizations were performed for A1 substitution into two d i f f e r e n t Si c h e m i c a l e n v i r o n m e n t s in the E T S - 1 0 structure; Si(4Si, 0Ti) and Si(3Si, 1Ti). The calculated stability difference for the two studied A1 insertion positions is striking, indicating a clear preference for the sites connected to tetrahedral silicon, as opposed to those connected to octahedral Ti sites. This result is in good agreement with the AI, Ti avoidance in the ETAS- 10 structure, indicated by zgsi and 27A1MAS N M R spectra. Molecular dynamic simulations suggest a significantly higher thermal stability for the polymorph B of ETS-10, compared to the experimentally observed for the disordered material. 1. INTRODUCTION ETS-10 is a recently discovered titanosilicate [ 1], whose structure has been solved, based on a combination ofHREM, MAS NMR, XRD and ED results [2]. It contains TiO6 octahedra, of which two of the oxygen corners are shared with other titanium, forming a < T i - O - T i > straight chain, while the other four are shared with SiO4 tetrahedra. Sets of periodically spaced chains are joined in orthogonal directions. Stacking of layers with this array gives rise to a three-dimensional network of interconnecting 12-membered ring channels.
2324 Very few zeolite structure types have such an open pore system, the most important being zeolite Y (FAU) and zeolite [3 (BETA). Each Ti atom in this structure is associated with a - 2 charge, that is compensated by non-framework cations. The cation density in ETS-10 is approximately equivalent to that in a zeolite with Si/AI = 2.5. Because of the interconnecting network of wide pore straight channels, and the high extra-framework cation density, ETS-10 is a potentially useful material for applications in catalysis. Furthermore, aluminum can be incorporated into ETS-10, by substituting tetrahedral silicon, thus generating acidic sites. In ETS- 10, the stacking pattern of these layers of orthogonally set < T i - O - T i > chains is not regular, giving rise to defects and faults in the structure. This disorder can be modelled by a random combination of two polymorphs, resulting from different stacking sequences. One of them, polymorph A, corresponds to a ABCD stacking and has a monoclinic lattice with space group C2/c, while the other corresponds to a ABAB stacking, and has a tetragonal lattice with a space group P41 or P43. Not much is known about the physical chemical and catalytic attributes of ETS-10. Furthermore, it is not clear how they are affected by the disorder in the structure. In this work, we have chosen a theoretical approach to evaluate these properties, employing calculation strategies that have proven to be accurate in the case of zeolites. Some inferences from these results have been compared to experirnentally determined characteristics of the material.
2. EXPERIMENTAL 2.1 Computational determination of structural properties Non-framework cation location: The non-framework cation sites for K + and Na + ions within the ETS- 10 structure have been modelled, combining a Monte Carlo Packing (MCP) procedure with the Lattice Energy Minimization (LEM) technique. The MCP algorithm developed by Biosym/MSl [3] enables a large number of starting extraframework cation configurations to be generated, which are then optimized based on the minimization of their lattice energy. In the MCP procedure, the ions are s u c c e s s i v e l y i n t r o d u c e d into the structure, so as to create low energy Table 1 arrangements according to an energy Interatomic potenctial parameters used. threshold for the short-range interactions ch refers to the atomic partial charge. between the cations and the host lattice. The potential parameters, A i and Bi, are In this way, one hundred trial packing given in kcal mol-I/~ n and kcal mo1-1 ./k6, configurations for each cation type were respectively. randomly generated. The present lattice simulations were i Ai Bi q~ carded out using the Discover code of O 388611.3727 0.1893 -1.2 Biosym/MSI [4]. The lattice energy c a l c u l a t i o n r e l i e s on c l a s s i c a l Si 103.8039 103.8039 +2.4 interatomic potential functions (force field). A modified "Consistent Valence Ti 6915.3989 3262.4997 +1.6 Force Field"-based on partial chargesK 12886.4561 0.0001 +1.0 is used. The short-range interactions are calculated with a Lennard-Jones (12-6) Na 67423.6364 0.0005 +1.0 potential function to a cut-off of 13.5 tl,. i
2325 The long-range coulombic interactions are calculated using periodic boundary conditions by the Ewald summation method to an accuracy of 0.1 kJ/mol. The partial charges and parameters used in the short-range energy terms are presented in Table 1. The optimization strategy applied for both K + and Na § as counter ions was first to minimize the lattice energy solely with respect to the non-framework cation coordinates, and then allow to relax the whole (framework and counter ions) atomic positions. The simulation system consists of a unit cell of the polymorph B containing 336 atoms, (16 T i , 80 Si, 208 O and 32 charge-balancing ions). The disorder in ETS-10 has been described in terms of different stacking sequences of the same titanosilicate unit. We have, thus, modelled the cation sites for only one case, polymorph B. The framework atomic coordinates and unit cell parameters reported by Anderson et al. [5] were taken as input to the lattice energy minimizations. AI incorporation: Lattice energy minimization has proven to be an appropriate technique to investigate the relative stabilities of aluminum sites within zeolite framework structures [6]. In the present work, we apply this technique to evaluate the relative stabilities of the two possible Si environments for A1 substitution, Si(3Si, 1Ti) and Si(4Si, 0Ti). In these calculations the point at issue is whether in the titanosilicate structure an A1-O-Ti linkage is possible, or if only A1 sites avoiding titanium neighbors are found. A measure of the stability of a given Al T-site, (T=Tetrahedral), is obtained from the minimum lattice energy of the Al-substituted (ETAS-10) structure. The A1 substitution energy, A, is calculated relative to the lattice energy of the siliceous ETS- 10 structure. In these calculations, K § cations were taken as charge balancing ions in the ETAS-10 lattice. Although the synthesized structure contains a mixture of Na § and K § ions, this approximation avoids the problems of considering the different permutations o f N a § and K § cations around the Al site. The additional negative charge introduced by the A1 incorporation in the tetrahedral framework site is neutralized in the calculation with an extra K § cation. This additional cation was introduced in the structure by the MCP procedure described before. For each of the considered Al-substituted structures, thirty different packing configurations of this additional K § ion in the K-ETAS- 10 lattice were generated. The lattice energy of each trial structure was first optimized with respect to the 33 K § counter ions, while the framework was held rigid. These structures were then fully relaxed, with respect to the whole framework plus counter ions atomic coordinates.
2.2 Computational determination of thermal stability Molecular dynamics simulations have been performed employing the Discover code of Biosym/MSI in the temperature range of 773 - 1273 K [4]. The equations of motion were integrated using the velocity form of the Verlet algorithm. The temperature range for the crystalline to amorphous phase transition ofpolymorph B ofETS- 10 is determined from the change in the Ti--O bond radial distribution functions (RDF), when the sample is heated. The temperatures were controlled by scaling particles velocities, and the pressure was maintained at 1 arm throughout the molecular dynamics, by the ParrineiloRahman method of pressure control. The simulation starts from a previously relaxed (K, Na)-ETS-10 structure. First, the system is equilibrated to the desired temperature for 70,000 steps in a time step of 1 fs (0.1 x 10-15 s) in the constant-pressure, constant-temperature ensemble (NPT). The dynamics is run for further 20,000 steps in the same ensemble, period over which the time averaged data collection starts.
2326
2.3 Synthesis and characterization of ETS-10 ETS-10 was synthesized following the procedure described by Kuznicki [1]. The XRD pattern and 29Si MAS NMR spectra of the obtained material agrees with thos~ reported by other authors [2]. The sodium and potassium composition of the prepared material was equal to 8.66 wt% and 6.73 wt% respectively, for aNa/K molar ratio of2.18. Ionic exchange properties were determined by eluding 500 mL of a 10 mM HNO3 over 1 g of material. Samples of the eluted solution were collected and analyzed for sodium and potassium by atomic absorption. To determine their thermal stability, samples of ETS-10 were heated in static air between 573 and 873 K for 1 hr. XRD patterns of these treated samples were obtained at room temperature with a PHILIPS diffractometer, model 1730-10, using Cu kcx radiation. 3. RESULTS AND DISCUSSION 3.1 Structural properties Non-framework cation location: The structure optimizations were initially performed at constant volume with the Discover code. For both K + and Na + as counter ions, a unique set of sites results from the full structure minimizations, as shown in Figure 1. The resulting set of four distinct extra-framework cation sites are distributed along the Ti--O-Ti rods, surrounded on both sides by silicon five rings, and located between two Ti atoms of the rods. The unit cell vector and angles were then allowed to change by optimizing the lattice energy at constant pressure, using the lattice simulation program GULP [7]. Most of the initial configurations for K + and Na + converged to a minimum energy structure of symmetry C2h. The experimental and calculated cell dimensions are summarized in Table 2. There are no significant deviations of the cell volume and atomic coordinates for the equilibrated polymorph B, from the proposed structure of Anderson et al. [5]. The main c h a n g e upon r e l a x a t i o n is a contraction of the unit cell along Table 2 the a and b axis and an expansion Experimental and calculated cell parameters for along the c axis. The fractional polymorph B of ETS- 10. K-ETS- 10 and coordinates are close to those of Na-ETS-10 refer to the calculated values for the the starting structure. lattice with K and Na as counter ions. The E l e c t r o s t a t i c e n e r g i e s per distances are given in/I,. cation site are listed in Table 3, for both K+ and Na+ as counter-ions. Experimental K-ETS- 10 Na-ETS- 10 These calculations indicate that different relative energies among a 15.8500 15.5612 15.4536 these sites are obtained depending b 15.8500 15.5612 15.4536 on the counter-ion. While K + preferentially lowers the energy c 14.5100 14.8911 14.9754 of sites III and IV, Na + favors sites I and III. 0~ (o) 75.2391 74.5667 75.0822 The calculated relative energies [~ (o) 104.7609 105.4333 104.9178 of Na § and K + ions over the determined sites is consistent with T (o) 90.0000 89.990I 89.9586 cation exchange experiments, I
I I
I
III
2327
Figure 1. Schematic diagram of the structure of ETS-10. The dark atoms in the f r a m e w o r k represent Ti, and the light ones Si and O. The black spheres represent the n o n - f r a m e w o r k compensation cations. The roman numbers indicate the four crystallographic sites where the counterions are located. The two black atoms in the structure, labelled Ti and T2, represent the sites where a l u m i n u m substitution was modelled. The numbers indicated the calculated T i - O bond distances, in A . carried out as part of this work. W h e n an acid solution is eluted over a (Na,K)-ETS-10 sample with a Na/K molar ratio of 2.2, this cation ratio in the eluted solution raises to 5.2, despite the fact that mobility of hydrated K § is 50% higher than that of hydrated N a + ions [8]. This result indicates that N a + ions are easier to exchange by H + than K +, and since this goes against kinetic trends, energetic considerations must be involved; that is, N a + must be easier to elude, in part because they are less tightly bound to the structure than K +, in accordance with our theoretical calculations. Local structure o f the Ti environment: In the refined structure proposed by Anderson et al. [2], the T i - O bonds in the TiO6 octahedra has almost the same length, 1.90 A along the Ti--O-Ti rods and 1.87 A along the T i - O - S i linkages, respectively. In order to check the structural changes of the local Ti environment on relaxation, we examined the optimum energy configurations of the Table 3 Electrostatic site energies, E '~', of Na and K K-ETS- 10 and (Na, K)-ETS- 10 lattices. W e h a v e used a N a / K ratio o f 2.2, cations over the four crystallographic consistently with our ETS- 10 sample. The distinct sites in the ETS-10 structure. The ten K + ions have been distributed over the tabulated energies are in kJ/mol sites IV (8 ions) and III (2 ions), since site E TM Er these are found to be the most energetically favored sites in the K - E T S - 1 0 structure I - 943.33 - 943.82 (see Table 3). In the case of the K-ETS- 10 minimumII - 926.63 - 979.27 energy structure, the T i - O bond length III - 960.37 - 1010.97 along the T i - O - T i rod (1.97 A) increases in about 3.5 %, with respect to the proposed IV - 890.61 - 1063.58 structure. For the m i n i m u m energy (Na,
2328 K)-ETS-10 configuration, a ight corrugation of about 0.01 in the Ti-O bond length alternating along the chain has been determined. EXAFS spectrum has been successfully interpreted by previous lattice simulations with a larger variation in the T i - O distances (2.11 and 1.71 /~) K-ETAS-IO E1 E2 A along the T i - O - T i chain [9]. Obtaining these structural features with present lattice T1 - 4697.6 - 11823.6 47.2 s i m u l a t i o n s s h o u l d not be expected, as the force field used does not include three-body T2 - 3849.0 t e r m s for the T i - O - T i interactions, which are essential to reproduce this effect. Average T i - O distances of 2.13 and 2.19 A along the Ti--O-Si linkages have been obtained for the minimum energy K-ETS- 10 and (K, Na)-ETS- 10 structures, respectively. The Ti--O-Ti bond angle in the (K, Na)-ETS-10 structure increases upon relaxation by only about 1.3 %, to 157 ~ In the minimized K-ETS- 10 structure, the T i - O - T i bond angle (163 ~ is about 5% larger than in the proposed starting structure, and similar to the value obtained by the lattice simulation that fits the EXAFS results (165 ~ [9]. Table 4 Lattice energies (LE) per SiO2 unit (kJ tool 1) of the minimized ETAS-10 structures for the two tetrahedral (T) sites considered. E 1is the minimized LE for the relaxed counterions and fixed structure. E 2 is the LE for the full minimized structure. A is the A1 substitution energy calculated relative to the LE of the K-ETS 10 structure.
3.2
Aluminum substitution Lattice energy minimizations were performed for A1 substitution into two different Si chemical environments in the ETS-10 structure; Si(4Si, 0Ti) and Si(3Si, 1Ti). These sites are identified in Figure 1 as T1 and T2, respectively. The framework position involving a A 1 - O - S i - O - T i linkage (T1) is found as the energetically preferred site for A1 incorporation. (See Table 4.) For this site, more than half of the thirty starting packing configurations of the additional K + ion in the K-ETAS-10 structure converged to the same energy minimum. In contrast, the thirty initial trial structures substituted at the framework position involving an A I - O - T i linkage (T2), could only be partially minimized with respect to the non-framework cation positions. Further relaxations of these structures with respect to the whole atomic coordinates did not converge to a local energy minimum. This result might be expected, based on the electrostatic repulsion of neighboring negative charges on the aluminum and titanium sites. Furthermore, the T2 tetrahedra is connected to the titanium octahedra, forming a three-member ring, which is critically unfavorable, due to the additional strain introduced by the larger A1 ion. In summary, the calculated stability difference for the two studied AI insertion positions is striking, indicating a clear preference for the sites connected to tetrahedral silicon, as opposed to those connected to octahedral Ti sites. This result is in good agreement with the A1, Ti avoidance in the ETAS-10 structure, suggested by 29Si and 27A1 MAS N M R spectra [10].
2329
3.3 Thermal stability The simulations were performed on a minimized (K,Na)-ETS-10 1273 K structure, with a N a ~ ratio of 2.2, as in our ETS-10 sample. The radial distribution functions (RDF) for the Ti-O bond, averaged over the last 10,000 iterations (10 ps) of the dynamics run, were calculated in the 873 K temperature range of 773 - 1273 K. Figure 2 displays the RDF for the Ti-O bond in several coordination s p h e r e s o b t a i n e d f r o m these calculations, compared to those for the proposed ETS- 10 structure. The present MD simulations suggest a significantly higher thermal n0 773 K stability for the p o l y m o r p h B, compared to the experimentally observed for the disordered material. From the temperature dependence of the experimental x-ray diffraction pattern, a departure from crystallinity I | I Proposed is observed in the temperature range I II I structure of 773 to 873 K, for the disordered material. Whereas, the calculated R D F for p o l y m o r p h B in this temperature range still presents a sharp peak around 2 A for the first T i - O coordination sphere and a significant proportion of T i - O bonds 0 2 4 6 8 10 around 3.5 and 4.5/~, corresponding Ti-O Radial Distance (A) to the second coordination sphere. At 1273 K, a nearly continuous Figure 2. Radial distribution functions of the distribution of T i - O bond lengths is Ti-O bond for the proposed ETS-10 obtained, indicating a significant loss structure, compared to the obtained by MD of crystallinity. The MD simulations simulations at different temperatures. As also s h o w a p o s i t i v e t h e r m a l displayed, the distribution tends to a expansion coefficient, obtained from continium with increasing temperature. the change in cell volume with temperature. This variance in the predicted and calculated thermal stabilities is probably due to the fact that the simulations were performed on a specific ordered polytype of the ETS-10 structure (polymorph B), whereas the real sample consists of a random distribution of layer stacking sequences. In this way, the disordered material presents a random arrangement of pores and faulted layers, which have been observed by high-resolution electron microscopy [11]. The faults in the ETS- 10 structure can result in larger double or quadruple micropores formed by coalescence of several pores or even larger formed by two defect micropores. In fact, Anderson et al. [2] proposed the disordered material composed of different domains of faulted layers. The presence of such defects and boundaries between faulted domains in ETS-10 should influence significantly the thermal stability of this material. L_
2330 4. CONCLUSIONS The present work illustrates the ability of atomistic simulation techniques based on classical potential energy functions (force fields) to simulate and predict structural and physical properties of the microporous titanosilicate ETS-10. Using a force field potential that combines coulombic and short-range interactions, the calculations are able to model structural properties, starting from only the framework structural data. A combination of the Monte Carlo Packing procedure with the Lattice Simulation technique proved to be efficient in modelling the extra-framework cation sites. The calculated relative energies for Na + and K + cations at these sites are consistent with observed ion-exchange properties. The present lattice energy optimization predicts a distortion of the TiO6 octahedra. However, in order to simulate variations in the Ti-O bond distances along the Ti--O chains, it is necessary to include extra terms in the potential energy function, to take into account the directionality of the T i - O - T i bond. The present evaluation of the relative stabilities of two different Al-substitution sites in the ETS-10 framework confirms an A1,Ti avoidance in the ETS-10 structure, analogous to the Lowenstein's rule for zeolites, suggested by the 29Si and 27A1 MAS NMR experiments. The present MD simulations predict a higher temperature for the crystallineamorphous phase transition of the polymorph B, compared to the experimentally observed range for the disordered material.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
S.M. Kuznicki, US Patent 4,853,202 (1989). M . W . Anderson, O. Terasaki, T. Ohsuna, P. J. O. Malley, A. Philippou, S. P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Philosophical Magazine B , 71 (1995) 813. Catalysis Package, version 2.3.7, Biosym/MSI, San Diego, California. Discover Molecular Simulations Program, version 94.1, Biosym/MSI, San Diego, California. M . W . Anderson, O. Terasaki, T. Ohsuna, A. Phillippou, S. P. Mackay, A. Ferreira, J. Rocha and S. Lidin, Nature, 367 (1994) 347. C . R . A . Catlow, Modelling of Structure and Reactivity in Zeolites, Academic Press, London, 1992. GULP (The General Utility Lattice Program), J.D. Gale, Royal Institution Imperial College, UK 1992-1994. F . A . Cotton and G. Wilkinson, Advanced Inorganic Chemistry, p. 255, Fourth Edition, John Wiley, New York, 1980. G. Sankar, R. G. Bell, J. M. Thomas, M. W. Anderson, P. A. Wright and J. Rocha, J. Phys. Chem., 100 (1996) 449. M . W . Anderson, A. Philippou, Z. Lin, A. Ferreira, and J. Rocha, Angew. Chem. Int. Ed. Engl., 34 (1995) 1003. T. Ohsuna, O. Terasaki, D. Watanabe, M. W. Anderson and S. Lidin, in Stud. Surf. Sci. Catal., v. 84, p. 413, J. Weitl~mp, H. G. Karge, H. Pfeifer and W. Htilderich Eds., Elsevier Science B. V., Amsterdam, 1994. The authors thank INTEVEP for permission to publish this article. Technical support from Biosym/MSI is gratefully acknowledged.