Enthalpies of formation of microporous titanosilicates ETS-4 and ETS-10

Microporous and Mesoporous Materials 47 (2001) 285±291

www.elsevier.com/locate/micromeso

Enthalpies of formation of microporous titanosilicates ETS-4 and ETS-10 Hongwu Xu, Yiping Zhang, Alexandra Navrotsky * Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, One Shields Avenue, Davis, CA 95616-8779, USA Received 20 March 2001; accepted 20 May 2001

Abstract The energetics of microporous titanosilicates ETS-4 (K1:13 Na3:92 Ti3:07 Si8:17 O25
8:64H2 O) and ETS-10 (K0:61 Na1:09 Ti1:10 Si4:98 O13
2:89H2 O) has been investigated by high-temperature drop solution calorimetry using lead borate as the solvent at 974 K. Combining the measured heats of drop solution with the published enthalpies of 0 drop solution and formation for the constituent oxides, the standard enthalpies of formation from the oxides (DHf;ox ) 0 and from the elements (DHf;el ) for both phases were derived for the ®rst time. The obtained values (in kJ/mol) are 0 0 0 as follows: DHf;ox …ETS-4† ˆ 818:5  13:1, DHf;el …ETS-4† ˆ 14, 642:8  15:9, DHf;ox …ETS-10† ˆ 262:2  3:1, and 0 0 0 DHf;el …ETS-10† ˆ 6995:2  6:1. Comparison between the DHf;ox (or DHf;el ) values of the two phases suggests that ETS-4 is thermodynamically more stable than ETS-10 with respect to the oxides (or the elements) at 298 K and 1 atm. This behavior can largely be attributed to the higher degree of hydration of ETS-4 than that of ETS-10. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Energetics; Enthalpy of formation; Titanosilicate; ETS-4; ETS-10

1. Introduction Microporous titanosilicates comprise a new family of molecular sieve materials that possess zeolite-like properties such as catalysis, separation, absorption and ion exchange [1±3]. Unlike zeolites whose framework is built of tetrahedral [AlO4 ] and [SiO4 ] units, however, titanosilicates, especially those having higher Ti/Si ratios (>0.025) [4], consist of [TiO6 ] octahedra (or possibly [TiO5 ] square-

* Corresponding author. Tel.: +1-530-752-9289; fax: +1-530752-9307. E-mail address: [email protected] (A. Navrotsky).

pyramids [5]) and [SiO4 ] tetrahedra. The di€erent coordinations of Ti4‡ and Si4‡ produce unique framework topologies, with more new structures still being discovered. The novel pore and channel structures in titanosilicates and the new chemistry associated with Ti make this group of materials have some unique properties and thus potentially ®nd novel applications such as for manufacturing chloro¯uorocarbon (CFC)-free air conditioners based on evaporative and desiccant cooling [6±9]. ETS (Engelhard titanium silicate)-4 and ETS-10 represent two of the earliest discovered microporous titanosilicates [1,10±12]. ETS-4 has an approximate formula of M6 Ti3 Si8 O25
nH2 O (M ˆ K or Na) [13] and is structurally similar to the rare

1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 3 8 8 - 2

286

H. Xu et al. / Microporous and Mesoporous Materials 47 (2001) 285±291

titanosilicate mineral zorite (space group Cmmm) [1,14±16]. The framework of ETS-4 consists of chains of corner-sharing [TiO6 ] octahedra along the b axis, with neighboring octahedra in a chain being laterally linked by [SiO4 ] tetrahedra. Pairs of [SiO4 ] tetrahedra connected to the adjacent [TiO6 ] chains are further joined via a structural unit containing one [TiO6 ] surrounded by four [SiO4 ] tetrahedra [15]. This topology ideally produces two orthogonal sets of channels that are de®ned by 12-membered rings perpendicular to the c axis and eight-membered rings perpendicular to b. However, like zorite, ETS-4 commonly exhibits substantial stacking faults along the (1 0 0) and (0 0 1) planes, and the faulted structure can be described as an intergrowth of four hypothetical ordered polymorphs [5]. As a result, the 12-ring channels along c become segmented, whereas the eight-ring channel along b is not a€ected. Correspondingly, the e€ective pore size of ETS-4 determined by  [1], absorption measurements is only about 3.7 A which is comparable to the pore size for a smallpore zeolite. ETS-10 has a tetragonal structure with the ideal composition M2 TiSi5 O13
nH2 O (M ˆ K, Na) [2,17,18]. In this structure, chains of [TiO6 ] octahedra are linked to two-folded chains of [SiO4 ] tetrahedra, forming [TiSi4 O13 ] columns [18]. These columns are packed into layers parallel to (0 0 1), with the columns in adjacent layers perpendicular to each other. The layers are further connected by [SiO4 ] tetrahedra into a three-dimensional framework. This framework contains three orthogonal sets of channels, de®ned by 12-membered rings, along the a, b and c axes. As in ETS-4, considerable degree of stacking disorder occurs in ETS-10 along its (0 0 1) plane; the disordered structure can be described as an intergrowth of two hypothetical ordered polymorphs [2]. Nevertheless, the stacking faults in ETS-10, unlike those in ETS-4, do not appear to cause blockage of the 12-ring channel systems. Thus, ETS-10 has a much higher adsorption capacity over ETS-4. Absorption measurements show that its e€ective pore size is about  [1], which is comparable to the pore size for a 8A large-pore zeolite. It is noted that both ETS-4 and ETS-10 were successfully synthesized more than a decade ago

[10±12], yet their structures had not been solved until recently, with some details still being matters of debate [2,15,16]. The diculties for the structural solution largely resulted from the high degree of stacking disorder associated with the titanosilicate frameworks. The full characterization of the highly faulted structures needed to employ a combination of several experimental techniques such as X-ray di€raction (XRD), high-resolution electron microscopy, and 29 Si nuclear resonance spectroscopy as well as theoretical modeling [2,16, 19±23]. Although much e€ort has been devoted to the structural characterization of ETS-4 and ETS-10, their thermodynamics (and its relation to their crystal chemistry) is essentially an untouched area. Studying the thermodynamics of these materials is important in the assessment of their utilization under various conditions. In this study, we synthesized the ETS-4 and ETS-10 samples by the hydrothermal methods of Kuznicki [10] and Kuznicki and Thrush [11]. High-temperature reaction calorimetry was used to determine their standard enthalpies of formation from the oxides and from the elements. Implications for the thermodynamic stability, hydration energetics, and the apparent thermal stability are discussed. 2. Experimental methods 2.1. Sample synthesis The ETS-4 and ETS-10 samples were prepared using the modi®ed hydrothermal methods of Kuznicki [10] and Kuznicki and Thrush [11]. The synthesis procedure for ETS-4 is as follows: 19.95 g of sodium disilicate (Aldrich, 27% SiO2 ), 3.52 g of NaOH, 1.79 g of KF and 80 ml of deionized H2 O were mixed in a 200-ml beaker. While being stirred the mixture (clear solution) was slowly added with 10 ml of TiCl3 solution, resulting in a thick purple gel. The gel was charged into 23-ml Te¯on-lined autoclaves and then the loaded autoclaves were placed in an oven and heated at 200°C for 170 h. The ®nal product, a white polycrystalline solid, was isolated by vacuum ®ltration, washed with deionized water, and dried in air.

H. Xu et al. / Microporous and Mesoporous Materials 47 (2001) 285±291

The synthesis procedure for ETS-10 is similar to that for ETS-4 except that the pH of the solution must be controlled at appropriate values. Our synthesis proceeded as follows: 10.20 g of sodium disilicate (Aldrich, 27% SiO2 ), 0.89 g of NaOH, 1.29 g of KF and 50 ml of deionized H2 O were mixed in a 200-ml beaker. While being stirred, the solution was slowly added with 7.0 g of TiCl3 solution, resulting in a thick purple gel that has a pH of 11.4. Dilute HCl was slowly added to this gel until its pH reached 10.4. The gel was charged into 23-ml Te¯on-lined autoclaves and then the loaded autoclaves were heated at 200°C for 170 h. The ®nal product, a white polycrystalline solid, was isolated by vacuum ®ltration, washed with deionized water, and dried in air. Unlike the previous synthesis of ETS-10 [10], our synthesis did not use any ETS-4 powders as seed crystals. 2.2. Characterization To identify the synthesized phases, powder XRD experiments were carried out with a Scintag PAD-V di€ractometer using CuKa radiation and a solid-state scintillation detector. A thin layer of the sample powder was mounted on a zero-background quartz sample holder, and the data were collected from 5° to 70° 2h with a scanning rate of 1°/min. The compositions of dehydrated samples were determined by electron microprobe analysis using CAMECA SX-50 with a voltage of 20 kV and a beam current of 10 nA. Water contents were measured using a Netzsch Thermal Analyzer STA 409 system with a heating rate of 10°C/min in static air.

¯ow of Ar gas with a rate of 40 ml/min was passed through the calorimeter [25]. The measured enthalpy of drop solution includes the heat e€ect associated with heating the sample from room temperature to 701°C plus the enthalpy of solution of the sample. All water is evolved into the gas phase [25]. The calorimeter was calibrated using the known heat contents of corundum pellets weighing 15 mg. Six pellets were dropped for each sample.

3. Results and discussion The XRD patterns (Fig. 1) are in good agreement with the published ETS-4 and ETS-10 data [1], indicating the presence of a single phase in both samples. Speci®cally, all the peaks of ETS-4 can be indexed based on an orthorhombic unit cell, and the re®ned cell parameters are: a ˆ  b ˆ 7:183…1† A,  and c ˆ 6:961…1† 23:218…4† A,  A. For ETS-10, all the re¯ections belong to a

2.3. High-temperature drop solution calorimetry High-temperature calorimetric measurements were performed using a Tian±Calvet microcalorimeter operating at 701°C with molten lead borate (2PbO
B2 O3 ) as the solvent. The equipment and experimental procedure have been described in detail by Navrotsky [24]. A sample pellet weighing 15 mg was dropped from room temperature into the solvent in the hot calorimeter. To remove the water vapor evolved by dehydration, a

287

Fig. 1. XRD patterns of (a) ETS-4, and (b) ETS-10.

288

H. Xu et al. / Microporous and Mesoporous Materials 47 (2001) 285±291

 and c ˆ tetragonal structure with a ˆ 7:460…2† A  27:336…7† A. These parameters agree reasonably well with the values published in the literature  b ˆ 7:1848…1† (e.g., ETS-4 [15]: a ˆ 23:2248…4† A,   A, and c ˆ 6:9616…1† A; ETS-10 [18]: a ˆ 7:481…1†  and c ˆ 27:407…5† A);  the small di€erences are A presumably due to the slight di€erences in composition among the samples. As also noted in the previous studies [1], the XRD patterns of ETS-4 and especially ETS-10 contain both broad and narrow peaks (Fig. 1), implying that extensive disorder occurs in the otherwise highly crystalline structures. Microprobe analysis (Table 1) shows that the actual compositions of our samples are close to the ideal stoichiometries [12,17] and the samples are homogeneous. Water contents determined by thermogravimetry (TG) indicate that ETS-4 is more hydrated than ETS-10 (Table 1). We therefore adopt the following chemical formula as

K1:13 Na3:92 Ti3:07 Si8:17 O25
8:64H2 O for ETS-4 and K0:61 Na1:09 Ti1:10 Si4:98 O13
2:89H2 O for ETS-10. The heats of drop solution (DHds ) of the ETS-4 and ETS-10 samples in lead borate at 974 K were measured to be 1572:3  12:5 and 596:7  2:5 kJ/ mol, respectively (Table 2). Using these two values and the previously determined heats of drop solution for K2 O, Na2 O, TiO2 , SiO2 , and H2 O (Table 3), the standard molar enthalpies of formation of ETS-4 and ETS-10 from the oxides 0 (DHf;ox ) were calculated through thermochemical cycles. For example, the cycle used for calculation 0 of the DHf;ox of ETS-4 is as follows: …1:13  1=2†K2 O…s; 298 K† ! …1:13  1=2†K2 O…soln; 974 K†

…1†

…3:92  1=2†Na2 O…s; 298 K† ! …3:92  1=2†Na2 O…soln; 974 K†

…2†

Table 1 Chemical compositions (wt.%) of ETS-4 and ETS-10 a

ETS-4 (K1:13 Na3:92 Ti3:07 Si8:17 O25
8:64H2 O) ETS-10 (K0:61 Na1:09 Ti1:10 Si4:98 O13
2:89H2 O) a

K2 O

Na2 O

SiO2

TiO2

H2 O

5.00 5.69

11.39 6.76

46.04 59.63

22.97 17.52

14.6 10.4

The mole fraction of H2 O accounts for both water and OH .

Table 2 Enthalpies of drop solution in lead borate at 974 K and enthalpies of formation from the oxides and from the elements at 298 K for ETS-4 and ETS-10 DHds (kJ/mol)a ETS-4 ETS-10 a

0 DHf;ox (kJ/mol)

1572:3  12:5…6† 569:7  2:5…6†

0 DHf;el (kJ/mol)

818:5  13:1 262:2  3:1

14642:8  15:9 6995:2  6:1

Uncertainty is two standard deviations of the mean (value in parentheses is the number of experiments).

Table 3 Enthalpies of drop solution in lead borate at 974 K and enthalpies of formation from elements at 298 K of several oxides used in calculations of the enthalpies of formation of ETS-4 and ETS-10 Oxide DHds (kJ/mol) 0 DHf;el (kJ/mol)d a

K2 O 193:7  1:1a 363:2  2:1

From Ref. [26]. From Ref. [27]. c Heat content from 298 to 974 K. d From Ref. [28]. b

Na2 O 113:1  0:8a 414:8  0:3

SiO2

TiO2

H2 O

39:1  0:3a 910:7  1:0

55:4  0:8b 944:0  0:8

68.9c;d 285:8  0:1

H. Xu et al. / Microporous and Mesoporous Materials 47 (2001) 285±291

3:07TiO2 …s; 298 K† ! 3:07TiO2 …soln; 974 K†

…3†

8:17SiO2 …s; 298 K† ! 8:17SiO2 …soln; 974 K†

…4†

8:64H2 O…l; 298 K† ! 8:64H2 O…g; 974 K†

…5†

…1:13  1=2†K2 O …soln; 974 K† ‡ …3:92  1=2† Na2 O …soln; 974 K† ‡ 3:07TiO2 …soln; 974 K† ‡ 8:17SiO2 …soln; 974 K† ‡ 8:64H2 O …g; 974 K† ! K1:13 Na3:92 Ti3:07 Si8:17 O25
8:64H2 O …s; 298 K† …6†

…1:13  1=2†K2 O …s; 298 K† ‡ …3:92  1=2† Na2 O …s; 298 K† ‡ 3:07TiO2 …s; 298 K† ‡ 8:17SiO2 …s; 298 K† ‡ 8:64H2 O …l; 298 K† ! K1:13 Na3:92 Ti3:07 Si8:17 O25
8:64H2 O…s; 298 K† …7† from which the enthalpy of formation of ETS-4 is 0 computed by: DHf;ox ˆ DH1 ‡ DH2 ‡ DH3 ‡ DH4 ‡ DH5 ‡ DH6 , where DHi (i ˆ 1±6) is the enthalpy of the reaction i. Similarly, the enthalpies of formation of ETS-4 0 and ETS-10 from the elements (DHf;el ) can be de0 0 rived from their DHf;ox values and the DHf;el values of the binary oxides (Table 3) by using appropriate reaction cycles. The enthalpies of formation thus obtained are presented in Table 2. The enthalpies of formation of ETS-4 and ETS10 from oxides are 818:5  13:1 and 262:2  3:1 kJ/mol, respectively. For ease of comparison, 0 the DHf;ox for ETS-4 can be expressed on a 13oxygen basis as for ETS-10, and the calculated value is 425:6  6:8 kJ/mol. This value is signi®0 cantly more negative than the DHf;ox of ETS-10 ( 262:2  3:1 kJ/mol), indicating that at the standard condition (298 K, 1 atm) ETS-4 is more stable than ETS-10 with respect to their constituent oxides. As for most condensed phase reactions, the entropic term T DS is expected to be small and thus DG  DH , making the enthalpy a good indicator of the thermodynamic stability. The more exothermic enthalpy of formation for ETS-4 compared with ETS-10 can be correlated to their di€erences in framework type, extra-framework species, and degree of hydration. One major

289

di€erence between these two phases is that ETS-4 is much more hydrated than ETS-10: 4.49 H2 O per formula for ETS-4 as compared with 2.89 H2 O for ETS-10 on the 13-oxygen basis. As seen in zeolites [29,30], hydration of extra-framework cations is an exothermic process, and thus it may serve as the driving force for stabilizing hydrated microporous phases at low temperatures [31]. In fact, the enthalpies of formation of ETS-4 and ETS-10 are very similar when calculated per mole of H2 O: 94:7  1:5 kJ/mol for ETS-4 and 90:7  1:1 kJ/ mol for ETS-10. Thus, the higher degree of hydration of ETS-4 is possibly the major factor responsible for its higher thermodynamic stability relative to the oxides. The closeness of the formation enthalpies per mole of water between ETS-4 and ETS-10 indicates that the e€ect of titanosilicate framework and that of extra-framework cations on their thermodynamic stability may cancel each other. Our recent studies on two series of microporous silicotitanate phases suggest that increasing Si/Ti ratio of the framework and decreasing ionic potential (charge/radius, Z=r) of the extra-framework cations increase the phase stability [31]. As shown in Fig. 2, while the enthalpy of formation per mole of water for each series becomes more exothermic with increasing alkali content, the value for the

Fig. 2. Enthalpies of formation from the oxides per mole of water for (a) the cubic (Na1 x Csx )3 Ti4 Si3 O15 (OH)
nH2 O phases and (b) the tetragonal (Na1 x Csx )3 Ti4 Si2 O13 (OH)
nH2 O phases as a function of Cs/(Cs‡Na) ratio. Data are taken from Ref. [31].

290

H. Xu et al. / Microporous and Mesoporous Materials 47 (2001) 285±291

tetragonal series (Si/Ti ˆ 0:5) is more endothermic than that of the cubic series (Si/Ti ˆ 0:75) at the same Cs/(Cs‡Na) ratio. ETS-10 (Si/Ti  4:53) is more siliceous than ETS-4 (Si/Ti  2:66), and thus it might be expected to have the more exothermic formation enthalpy. However, the number of alkali atoms per two-oxygen mole for ETS-10 (0.26) is smaller than that of ETS-4 (0.40). Hence ETS-10 would be expected to have a more endothermic enthalpy based on its higher alkali content, which, however, is probably canceled out by the exothermic e€ect due to its larger Si/Ti ratio. The e€ects of the stacking disorder in the ETS framework, described earlier, on the energetics are expected to be small. As demonstrated by Fialips et al. [32] and de Ligny and Navrotsky [33], natural and synthetic kaolinite samples with various defect densities have essentially the same formation enthalpy within experimental error. Likewise, the polytypism in phlogopite appears not to contribute signi®cantly to its enthalpy of formation [34]. These observations suggest the absence of a strong thermodynamic driving force for the ordering of disordered stacking sequences, and the transformations to the ordered polymorphs are governed by kinetics. Similar behavior is likely to occur in the ETS phases, as is re¯ected by the common occurrence of extensive stacking disorder in these structures. Although its thermodynamic stability at room temperature is higher than that of ETS-10, ETS-4 appears thermally less stable (decomposing at lower temperature on heating) than ETS-10. Previous XRD experiments show that ETS-4 starts amorphization at 523 K [16,35], whereas ETS-10 can maintain its framework up to 923 K [36]. This disparity in thermal stability is related both to the higher water content of ETS-4 and to di€erent structural behavior of water in the two structures. As revealed by di€erential thermal analysis (DTA), ETS-10 displays a broad endothermic peak with its maximum at 378 K, indicating the presence of loosely bound, zeolite-type water [37]. In contrast, the DTA curve of ETS-4 contains not only an endothermic maximum at 413 K due to the removal of zeolite-type water, but also a peak at 523 K that results from the desorption of more tightly bound water [37]. Structure re®nements of

ETS-4 have demonstrated that a portion of its water molecules participate in forming the titanosilicate chains in the structure [15,16]. When this type of water is removed on heating above 523 K, the framework tends to collapse and the phase starts to amorphize. On the other hand, all the water in ETS-10 belongs to the zeolite-type, and thus its removal does not a€ect the integrity of the ETS-10 framework so strongly. In addition, the larger water content of ETS-4 may itself lead to thermal instability and ``cracking'' the structure because larger amounts of water try to emerge on heating, locally disrupting the structure. Furthermore, once the boiling point of water is exceeded, the entropy of dehydration becomes strongly positive (to form H2 O gas with a large positive DV ). Thus the higher the water content, the more steeply the free energy of formation changes with temperature. Therefore a more hydrated framework, though thermodynamically more stable at room temperature, would become thermodynamically less stable at high temperature. Thus in discussing the ``stability'' of these materials, one must distinguish carefully among thermodynamic stability at room temperature, thermodynamic stability at high temperature and kinetic stability (the latter related to the temperature at which the rate of decomposition becomes signi®cant). The opposite trends in the thermodynamic stability (in terms of enthalpy of formation) and the thermal stability have also been observed in Na-faujasites as a function of Al/Si ratio [38], although the structural mechanisms responsible for the phenomenon are completely di€erent.

4. Conclusions High-temperature drop solution calorimetry has been used to study the energetics of microporous titanosilicates ETS-4 and ETS-10. Using the measured heats of drop solution and the known enthalpies for the constituent oxides, the standard enthalpies of formation of both phases from the oxides and from the elements were derived for the ®rst time. The results suggest that at the standard condition (298 K, 1 atm) ETS-4 is

H. Xu et al. / Microporous and Mesoporous Materials 47 (2001) 285±291

more stable than ETS-10 with respect to the constituent oxides or the elements. The higher thermodynamic stability for ETS-4 is interpreted as a result of its higher degree of hydration, as is consistent with the trend seen in aluminosilicate zeolites. Acknowledgements We thank Bob Haushalter for allowing us to use his synthesis laboratory at the NEC Research Institute and Martin Wilding for performing the microprobe analysis. This work was supported by the National Science Foundation (grant no. DMR-97-31782) and the Department of Energy Environmental Management Science Program (grant no. FG07-97ER45674). References [1] S.M. Kuznicki, A. Thrush, F.M. Allen, S.M. Levine, M.M. Hamil, D.T. Hayhurst, M. Mansour, in: M.L. Occelli, H. Robson (Eds.), Synthesis of Microporous Materials, Van Nostrand Reinhold, New York, 1992. p. 427. [2] M.W. Anderson, O. Terasaki, T. Ohsuna, P.J.O. Malley, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Philos. Mag. B 71 (1995) 813. [3] M. Nyman, B.X. Gu, L.M. Wang, R.C. Ewing, T.M. Neno€, Micropor. Mesopor. Mater. 40 (2000) 115. [4] A. Jentys, C.R.A. Catlow, Catal. Lett. 22 (1993) 251. [5] C. Braunbarth, H.W. Hillhouse, S. Nair, M. Tsapatsis, Chem. Mater. 12 (2000) 1857. [6] P.S. Zurer, Looming Ban on Production of CFCs, Halons Spurs Switch to Substitutes, (special review) Chem. Eng. News 71 (46) (1993) 12. [7] R. Carli, C.L. Bianchi, V. Ragaini, Catal. Lett. 33 (1995) 49. [8] R.J. Davis, Z. Liu, J.E. Tabora, W.S. Wieland, Catal. Lett. 34 (1995) 101. [9] R. Robert, P.R. Rajamohanan, S.G. Hegde, A.J. Chandwadkar, P.J. Ratnasamy, J. Catal. 155 (1995) 345. [10] S.M. Kuznicki, US Pat. 4, 853, 202 (1989). [11] S.M. Kuznicki, A.K. Thrush, Eur. Pat. 0405978A1, 1990. [12] D.M. Chapman, A.L. Roe, Zeolites 10 (1990) 730. [13] M.L. Balmer, B.C. Bunker, L.Q. Wang, C.H.F. Peden, Y. Su, J. Phys. Chem. B 101 (1997) 9170.

291

[14] P.A. Sandomirskii, N.V. Belov, Sov. Phys. Crystallogr. 24 (1979) 686. [15] G. Cruciani, P. De Luca, A. Nastro, P. Pattison, Micropor. Mesopor. Mater. 21 (1998) 143. [16] A. Philippou, M.W. Anderson, Zeolites 16 (1996) 98. [17] M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Nature 367 (1994) 347. [18] X. Wang, A.J. Jacobson, Chem. Commun. (1999) 973. [19] T. Armaroli, G. Busca, F. Milella, F. Bregani, G.P. Toledo, A. Nastro, P. De Luca, G. Bagnasco, M. Turco, J. Mater. Chem. 10 (2000) 1699. [20] G. Sankar, R.G. Bell, J.M. Thomas, M.W. Anderson, P.A. Wright, J. Rocha, J. Phys. Chem. 100 (1996) 449. [21] X. Yang, R.E. Truitt, J. Phys. Chem. 100 (1996) 3713. [22] M.E. Grillo, J. Garrazza, J. Phys. Chem. 100 (1996) 12261. [23] S. Ganapathy, T.K. Das, R. Vetrivel, S.S. Ray, T. Sen, S. Sivasanker, L. Delevoye, C. Fernandez, J.P. Amoureux, J. Am. Chem. Soc. 120 (1998) 4752. [24] A. Navrotsky, Phys. Chem. Miner. 24 (1997) 222. [25] A. Navrotsky, R.P. Rapp, E. Smelik, P. Burnly, S. Circone, L. Chai, K. Bose, H.R. Westrich, Am. Mineral. 79 (1994) 1099. [26] I. Kiseleva, A. Navrotsky, I.A. Belitsky, B.A. Fursenko, Am. Mineral. 81 (1996) 668. [27] R.L. Putnam, A. Navrotsky, B.F. Wood®eld, J. BoerioGoates, J.L. Shapiro, J. Chem. Thermodynamics 31 (1999) 229. [28] R.A. Robie, B.S. Hemingway, Thermodynamic Properties of Minerals and Related Substances at 298.15 K and 1 Bar (105 Pascals) Pressure and at Higher Temperatures, Geological Survey Bulletin no. 2131, 1995. [29] I. Petrovic, A. Navrotsky, M.E. Davis, S.I. Zones, Chem. Mater. 5 (1993) 1805. [30] Y. Hu, A. Navrotsky, C.-Y. Chen, M.E. Davis, Chem. Mater. 7 (1995) 1816. [31] H. Xu, A. Navrotsky, M.D. Nyman, T.M. Neno€, J. Mater. Res. 15 (2000) 815. [32] C.-I. Fialips, A. Navrotsky, S. Petit, Am. Mineral. 86 (2001) 304. [33] D. de Ligny, A. Navrotsky, Am. Mineral. 84 (1999) 506. [34] J.D. Clemens, S. Circone, A. Navrotsky, P.F. McMillan, B.K. Smith, V.J. Wall, Geochim. Cosmochim. Acta 51 (1987) 2569. [35] V. Valtchev, S. Mintova, B. Mihailova, L. Konstantinov, Mater. Res. Bull. 31 (1996) 163. [36] J. Rocha, L.D. Carlos, J.P. Rainho, Z. Lin, P. Ferreira, R.M. Almeida, J. Mater. Chem. 10 (2000) 1371. [37] B. Mihailova, V. Valtchev, S. Mintova, L. Konstantinov, J. Mater. Sci. Lett. 16 (1997) 1303. [38] I. Petrovic, A. Navrotsky, Microporous Mater. 9 (1997) 1.