J. Phys. Chem. 1996, 100, 12261-12264
12261
Computational Modeling of the Nonframework Cation Location and Distribution in Microporous Titanosilicate ETS-10 M. E. Grillo* and J. Carrazza INTEVEP, S. A., Research and Technological Support Center of Petro´ leos de Venezuela, Apartado 76343, Caracas 1070A, Venezuela ReceiVed: NoVember 2, 1995; In Final Form: April 30, 1996X
The nonframework cation sites for K-ETS-10 and Na-ETS-10 have been modeled using molecular simulation techniques. This work combines a Monte Carlo packing procedure with lattice energy calculations, to simulate the charge-balancing cation sites in a titanosilicate, starting from only the framework structural data. One hundred trial packing arrangements for each cation type were randomly generated with the Monte Carlo procedure, and the lattice energy for each of these structures was calculated, considering electrostatic and repulsive short-range terms. For both K+ and Na+ cations, most of the initial arrangements converged to an identical minimum energy structure, involving four different cation sites. The relative stability of these sites depends on their framework environment and the kind of cation that occupies them.
Introduction The first synthetic incorporation of Ti in a microporous crystalline framework was reported for the MFI type zeolite.1 In this material, named TS-1, the Ti(IV) atoms are tetrahedrally coordinated. The catalytic properties of TS-1 in oxidation reactions2 have stimulated investigations on the Ti environment structure,3-6 identification of preferential substitution sites,7 and the search for different materials. A new class of titanosilicate, containing a large proportion of 6-fold-coordinated Ti sites in a microporous crystalline array, was recently discovered.8-11 One of the members of this family, ETS-4, is an analogue of the mineral zorite,12 while the other, ETS-10, is comparable topologically to zeolite β.13 Because of their high degree of disorder, the structure determination of these materials represents a challenging task. The ETS-4 structure has not yet been solved unambiguously, and the detailed structural solution of ETS-10 has only been recently reported by Anderson et al.13-15 The disorder in ETS10 is explained in terms of different stacking sequences of the same titanosilicate unit Si40Ti8O10416-, giving rise to several possible polymorphs. The most commonly described in the literature are polymorphs A and B. Both of them involve a three-dimensional 12-ring pore system. In polymorph A, the 12-ring pores are arranged in a zigzag manner, forming a spiral channel and a tetragonal lattice of symmetry P41 or P43. The stacking in polymorph B leads to a diagonal arrangement of the 12-ring pores with a monoclinic lattice of C2/c symmetry. The titanium(IV) atoms are found to be octahedrally coordinated by oxygen atoms to four silicon tetrahedra and linked to each other by O-Ti-O chains. Therefore, in the anionic structure there is a -2 charge associated to each Ti site. This charge is compensated by extraframework Na+ and K+ ions, when the material is prepared following the procedure reported by Kuznicki.8 A detailed description of the ETS-10 structure is given in ref 13. The position of the extraframework cations generally determines ion-exchange, sorptive, and catalytic properties. Furthermore, knowledge of the cation location is an important requirement to model additional structural (e.g., framework * Corresponding author. X Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(95)03256-4 CCC: $12.00
substitutions) and dynamics properties of the material. Thus far, there is little experimental data concerning the cation sites in ETS-10. Recently, Anderson et al. reported 23Na NMR experiments indicating at least two different Na+ sites in the ETS-10 sample used in their structure-solution work.13 Besides this information, no further experimental input is available on the cation location in this material. The present work describes the use of computer simulation techniques to identify the extraframework cation sites in ETS-10 and to examine the distribution of Na+ and K+ ions within these sites. These are initial results of a more extensive project, aimed at studying the structural and physicochemical properties of microporous titanosilicates, combining both experimental and molecular modeling techniques. The theoretical method used is a combination of the lattice energy minimization (LEM) technique and the Monte Carlo packing algorithm of the Catalysis software of MSI. The latter algorithm generates a large number of starting extraframework cation configurations, which are then optimized according to the minimization of their lattice energy. The encouraging results of this novel packing procedure in determining the location of nonframework cation in dehydrated zeolite Li-ABW and in zeolite 4A, based exclusively on framework structure data,16 motivated its use in the present work. Moreover, the LEM method has been extensively applied to zeolite structural problems.17 Therefore, the combination of these two techniques should provide a reliable strategy for modeling extraframework cation site location and energy, specially for cases in which this information is difficult to obtain experimentally, as in ETS10. The methodology used in the present calculations is briefly summarized in section 2. Section 3 presents the results for the nonframework site location and energy for both K+ and Na+ cations. Methodology In a first stage of the calculation, the Monte Carlo packing procedure is used to successively introduce the cations into the simulation system. This algorithm searches automatically for low-energy cation arrangements according to an energy threshold, taking into account solely short-range nonbonded interactions between the cations and the host structure. For both K-ETS-10 and Na-ETS-10, 100 initial cation configurations © 1996 American Chemical Society
12262 J. Phys. Chem., Vol. 100, No. 30, 1996
Grillo and Carrazza
TABLE 1: Interatomic Potential Parameters and Charges Useda,b i
Ai
Bi
qi
O Si Ti K Na
388611.3727 103.8039 6915.3989 12886.4561 67423.6364
0.1893 0.0007 3262.4997 0.0001 0.0005
-1.2 +2.4 +1.6 +1.0 +1.0
a The off-diagonal potential parameters take the form: A ) (A × ij i Aj)1/2; Bij ) (Bi × Bj)1/2. b qi refers to the atomic partial charge. The potential parameters, Ai and Bi, are given in (kcal mol-1 Å12) and (kcal mol-1 Å6), respectively.
were generated. The lattice energy of the generated trial structures is calculated and optimized using the program DiscoVer of MSI.18 A modified “consistent valence force field”, based on partial charges, is employed. The potential energy function describing the interaction between ions includes Coulombic and short-range repulsive energy contributions. The short-range effects are calculated with a Lennard-Jones (126) potential:
qiqj Aij Bij V(rij) ) + 12 - 6 rij r r ij
(1)
ij
Here, i and j refer to the interacting ions and qi and qj to their charges, and Aij and Bij are short-range potential parameters. The off-diagonal heteronuclear interactions are calculated as geometric averages of the form: Aij ) (Ai × Aj)1/2 and Bij ) (Bi × Bj)1/2. These quantities are related to the potential well depth ij and the interatomic distance r* ij at which the minimum 12 and Bij ) occurs in a straightforward manner: Aij ) ijr* ij 6 2ijr*ij . The short-range contributions are evaluated to a cutoff of 13.5 Å. The electrostatic term is calculated exactly by the Ewald method.19 The parameters and charges used are presented in Table 1. The optimization strategy described hereinafter has been applied to both K+ and Na+ as countercations in the ETS-10 structure. First, the coordinates of solely the nonframework cations are adjusted, until a minimum has been found. For both systems, K- and Na-ETS-10, most of the starting configurations converged to a common set of cation positions, corresponding to a minimum energy. In a second step of the structure refinement, the whole atomic positions (framework and countercations) are adjusted until a minimum lattice energy (EVlattice) is obtained. For both types of counterions, a unique set of sites results from the full ETS-10 structure minimizations, out of a significant number of initial configurations. In all the partial- or full-structure optimizations mentioned above, the unit cell parameters are held fixed (constant volume minimization). In this type of calculation, the equilibrium structure is constrained to the crystallographic volume. The site (Esite) and lattice (EPlattice) energies for each of the considered charge-balancing cations (Na+ and K+) have been examined by calculating the electrostatic site potentials. This has been carried out using the general utility lattice program (GULP), developed by Gale,20 and also provided by MSI. The optimizations with GULP were performed at constant pressure, varying the atomic coordinates, constrained to the observed framework geometry. This form of minimization allows the cell parameters (cell volume) to relax. The interatomic potential parameters and charges employed were the same as those implemented in the DiscoVer code and compiled in Table 1.
TABLE 2: Experimental and Calculated Cell Parameters for Polymorph B of ETS-10a a b c R (deg) β (deg) γ (deg)
experimental
K-ETS-10
Na-ETS-10
15.8500 15.8500 14.5100 75.2391 104.7609 90.0000
15.5612 15.5612 14.8911 74.5667 105.4333 89.9901
15.4536 15.4536 14.9754 75.0822 104.9178 89.9586
a K-ETS-10 and Na-ETS-10 refer to the calculated values of the ETS10 lattice, obtained at constant pressure, with K+ and Na+ as counterions. The distances are given in Å.
TABLE 3: Position and Electrostatic Site Energies, Esite, of Na and K Cations over the Four Crystallographic Distinct Charge-Balancing Ion Sites in the ETS-10 Structure site
coordinates (Å)
cation
Esite (kJ mol-1)
I
Na K
0.1488 0.1589
0.8120 0.7999
0.8322 0.8282
-943.33 -943.82
II
Na K
0.3921 0.3783
0.5663 0.5794
0.8237 0.8240
-926.63 -979.27
III
Na K
0.2673 0.2766
0.6854 0.6880
0.8332 0.8545
-960.37 -1010.97
IV
Na K
0.5216 0.5281
0.4339 0.4390
0.8483 0.8626
-890.61 -1063.58
Results and Discussion The simulation system consists of a periodically replicated unit cell of ETS-10 (polymorph B) and 32 monovalent chargebalancing Na+ or K+ cations. Since the disorder in ETS-10 has been described in terms of different stacking sequences of a basic titanosilicate unit, the results should be equivalent for all polymorphs. We have, thus, simulated the cation sites for only one case, polymorph B. The framework coordinates and unit cell parameters for polymorph B of ETS-10 reported by Anderson et al.14 were taken as input to the energy minimizations for the cation sites location. In this work, the lattice energy is reported per mole of the periodically repeating building unit, Ti16Si80O208M32 (M ) Na+ or K+). For both K+ and Na+ as counterions, most of the trial structure converged to a minimum energy configuration of symmetry C2h. Hence, the relaxation leads to a deviation from the original space group C2/c. The final space group has been determined within a precision of 0.2 Å in the atomic coordinates. The energy-minimized structures for the K-ETS-10 and NaETS-10 lattices show only small deviations from the observed model proposed by Anderson et al.14 The experimental and calculated cell dimensions are summarized in Table 2. The main differences are a small contraction of the calculated unit cell along the a and b axes and a small expansion along the c axis. In the case of the structure with Na+ as counterions, the experimentally determined cell volume is 2.5% larger along a and b and by 3.2% smaller along c. The fractional coordinates of the framework atoms are also very close to the observed values. Framework structural features will be addressed in subsequent efforts, involving structure and thermal stability predictions for the models proposed by Anderson et al.13 The resulting set of four distinct sites for K+ and Na+ in the ETS-10 structure are listed in Table 3. They 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. Figure 1 displays the final K-ETS-10 structure obtained after a constant pressure minimization with GULP. Sites I and II are close to the apical Si of each five-ring Si(4Si,0Ti). Site III is adjacent to the 12-ring channel. Site IV is adjacent to the orthogonal
Modeling of Nonframework Cation Sites
J. Phys. Chem., Vol. 100, No. 30, 1996 12263
Figure 1. K-ETS-10 structure obtained after a constant pressure minimization. Four crystallographic sites are identified. The 32 charge-balancing cations per repeating building unit are equally distributed over all sites (8 per site).
O-Ti-O chains. The 32 charge-balancing cations per unit cell are equally distributed over the crystallographic distinct site (8 cations per site). From a qualitative inspection of the cation sites in the structure proposed in Figure 1, at least two distinct types of chemical environment for the counterion sites might be distinguished. Namely, the local environment of the sites located adjacent to the apical silicon of the five-rings (sites I and II) and the environment of sites III and IV, opposed to those of Si. This is consistent with 23Na MAS NMR findings, which suggest at least two different Na sites in ETS-10.13 Since our calculations suggest that all the cations are located within framework volumes substantially larger than their sizes, van der Waals steric repulsions are not considered critical, and thus, electrostatic interactions should have a stronger influence in the total cation site energy than short-term interactions. The electrostatic energy per cation site as calculated by GULP are listed in Table 3, for both K+ and Na+ cations. These calculations suggest that different relative energies among these sites are obtained depending on the counterion. While K+ preferentially lowers the energy of sites III and IV, Na+ favors sites I and III. These results should only be considered as qualitative estimates of the relative site stability. The accuracy of present calculations is limited by factors such as the transferability of the potential parameters used and temperature effects (included only implicitly). Furthermore, the simulation system does not include occluded water in the structure, which should influence significantly the relative site energies for the considered ions. In ion-exchange experiments, it has been observed that H+ substitutes Na+ more effectively than K+ in ETS-10.21 Since the mobility of K+ in water is 50% higher than that of Na+,22 this difference is probably associated to energetic rather than
TABLE 4: Averaged Electrostatic Site Energies of Na, ENa, and K, EK, Cations over Different Site Types in a (Na,K)-ETS-10 Structure with Na/K ) 2.2 site I II III IV
EK (kJ mol-1)
-1056.79 -1013.04
ENa (kJ mol-1) -931.71 -915.33 -969.44
kinetic factors. The results summarized in Table 3 suggest that differences in Esite between these two cations contribute to the observed experimental trend, since for sites II, III, and IV its value is lower when K+ is present while in the case of site I they are similar for both counterions. Furthermore, calculations for an ETS-10 structure containing a mixture of Na+ and K+, in the same proportion as in the sample employed to carry out the ion-exchange experiments (Na/K ) 2.2), indicate that sites occupied by K+ are lower in energy than those occupied by Na+ by at least 87 kJ mol-1 (see Table 4), which cooperates with the difference in hydration energies (84 kJ mol-1)22 to favor the Na+ exchange. These calculations also indicate that the relative site energies for Na+ and K+ in the (Na,K)-ETS-10 lattice are different from those obtained for the Na- and K-ETS10 cases. (See Tables 3 and 4). This can be ascribed to changes in the cation-cation repulsions with the introduction of a second type of ion in the lattice. Table 5 shows the lattice energies for the equilibrated structures of Na-, K-, and (Na,K)-ETS-10 lattices, calculated at constant volume and pressure. The lowest lattice energies were obtained by placing the K+ ions on the nonframework sites, which is consistent with the site energy values reported in Tables 3 and 4.
12264 J. Phys. Chem., Vol. 100, No. 30, 1996 TABLE 5: Lattice Energies of the Energy-Minimized ETS-10 Structures, Containing K, Na, or a Mixture of These Cations as Counterionsa K-ETS-10 (K,Na)-ETS-10 Na-ETS-10
P Elattice (kJ mol-1)
V Elattice (kJ mol-1)
-475 696.0 -474 568.0 -473 732.0
-474 492.0 -473 564.4 -472 424.0
V a EP lattice and Elattice refer to the lattice energies of the structure relaxed at constant pressure and constant volume, respectively.
Conclusions Four crystallographically different sites result from the modeling of extraframework Na+ or K+ in ETS-10. Their positions are dictated by the balancing of net negative charges associated to the framework titanium atoms. The lowest lattice energy obtained corresponds to the K-ETS10 structure. This is a consequence of the more favored electrostatic interaction of the K+ ions in the ETS-10 lattice, as indicated by the calculated site energies. Different relative energies among these cation sites are obtained depending on the counterion. While K+ preferentially lowers the energy of sites III and IV, Na+ favors sites I and III. The relative site energies for the separated Na+ and K+ ions in the ETS-10 lattice are different from those obtained for the mixture of both ions in the same structure. This can be ascribed to changes in the cation-cation repulsions with the introduction of a second type of ion in the lattice. Acknowledgment. The authors thank INTEVEP for permission to publish this paper. Technical support from MSI in San Diego, CA is gratefully acknowledged. References and Notes (1) Taramasso, M.; Perego, G.; Notari, B. U.S. Patent 4 410 501, 1983. (2) Romano, U.; Esposito, A.; Maspero, F.; Neri, C. In New DeVelopments in SelectiVe Oxidation; Centi, G., Trifiro´, F., Eds.; Elsevier: Amsterdam, 1990; p 33.
Grillo and Carrazza (3) Perego, G.; Bellussi, G.; Corno, C.; Taramasso, M.; Buonomo, F.; Esposito, A. In Proceedings of the 7th International Conference on Zeolites, Tokyo, 1986; Murakami, Y., Lijima, A., Ward, J. W., Eds.; Elsevier: Amsterdam, 1987; p 129. (4) Millini, R.; Previde, E.; Perego, G.; Bellussi, G. J. Catal. 1992, 137, 497. (5) Boccuti, M. R.; Rao, K. M.; Zecchina, A.; Leofanti, G.; Petrini, G. Stud. Surf. Sci. Catal. 1998, 48, 133. (6) Carati, A.; Contarini, S.; Millini, R.; Bellussi, G. Abstract of Papers, Material Research Society Meeting; Lugar, Ed.; 1990; Extended Abstract EA24, p 47. (7) Millini, R.; Perego, G.; Seiti, K. In Zeolites and related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Ho¨lderich, W., Eds.; Elsevier: Amsterdam, 1994; Vol. 84, p 2123. (8) Kuznicki, S. M. U.S. Patent 4 853 202, 1989. (9) Kuznicki, S. M.; Thrush, A. K. European Patent 0405978A1, 1990. (10) Chapman, D. M.; Roe, A. L. Zeolites 1990, 10, 730. (11) Haushalter, R. C.; Mundi, L. A. Chem. Mater. 1992, 4, 31. (12) Sandomirskii, P. A.; Belov, N. V. Kristallografiya 1979, 24, 1198. (13) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Malley, P. J. O.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Phil. Mag. B 1995, 71, 813. (14) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347. (15) Ohsuna, T.; Terasaki, O.; Watanabe, D.; Anderson, M. W.; Lidin, S. In Zeolites and related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeifer, H., Ho¨lderich, W., Eds.; Elsevier: Amsterdam, 1994; Vol. 84, p 413. (16) Newsam, J.; Freeman, C.; Gorman, A.; Vessal, B. Abstracts of Papers, 14th North American Meeting of the Catalysis Society; Snowbird, UT, 1995; Abstract T-120. (17) Jackson, R. A.; Parker, S. C.; Tschaufeser, P. In Modelling of Structure and ReactiVity in Zeolites; Catlow, C. R. A., Ed.; Academic Press: London, U.K., 1992; pp 43-61. (18) DiscoVer Molecular Simulations Program, Version 94.1; Biosym Technologies Inc.: San Diego, CA, 1994. (19) Ewald, P. P. Ann. Phys. 1921, 64, 253. (20) Gale, J. D. GULP (the General Utility Lattice Program); Royal Institution Imperial College: London, U.K., 1992-1994. (21) Grillo, M. E.; Lujano, J.; Carrazza, J. Submitted for publication in The Proceedings of the 11th International Zeolite Conference, Seoul, Korea, 1996; Chon, H., Ihm, S.-K., Uh, Y. S., Eds.; Elsevier: Amsterdam. (22) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry. A ComprehensiVe Text, 4th ed.; John Wiley & Sons: New York, 1980; p 255.
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