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Formation and hydration enthalpies of the hydrosodalite family of materials
Microporous and Mesoporous Materials 88 (2006) 283–292 www.elsevier.com/locate/micromeso
Formation and hydration enthalpies of the hydrosodalite family of materials Eric C. Moloy, Qingyuan Liu, Alexandra Navrotsky
*
Thermochemistry Facility and NEAT ORU, University of California at Davis, Davis, CA 95616-8779, United States Received 25 August 2005; received in revised form 20 September 2005; accepted 28 September 2005 Available online 14 November 2005
Abstract The four end-member structures of the hydrosodalite family of materials (Na6+x(OH)x[Al6Si6O24](H2O)N) are synthesized via hydrothermal techniques and characterized using X-ray diffraction, thermal analysis, scanning electron microscopy, and inductively coupled plasma optical emission spectroscopy. High-temperature drop-solution calorimetry in molten 2PbO Æ B2O3 at 975 K is used to measure the formation and hydration enthalpies. We report the formation enthalpies, from both the oxides and the elements, for a total of eight samples, two for each end-member. The average formation enthalpy (from oxides) is 88.1 ± 1.0 kJ/mol-TO2 for basic sodalite {ideally Na8(OH)2[Al6Si6O24](H2O)4}, 79.6 ± 1.2 kJ/mol-TO2 for hydroxysodalite {ideally Na8(OH)2[Al6Si6O24]}, 68.0 ± 1.1 kJ/mol-TO2, for hydrosodalite {ideally Na6[Al6Si6O24](H2O)8}, and 54.9 ± 1.1 kJ/mol-TO2, for sodalite {ideally Na6[Al6Si6O24]}. The corresponding average hydration enthalpies are 76.7 ± 5.3 kJ/mol-H2O and 37.0 ± 2.4 kJ/mol-H2O for the Na8 and Na6 series, respectively. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Zeolite; Sodalite; Formation enthalpy; Hydration enthalpy; Hydrothermal synthesis
1. Introduction Zeolites are nanoporous materials, formed by both natural and synthetic processes, characterized as open framework, aluminosilicate structures comprised of corner sharing tetrahedra (tectosilicates). The structures possess extra-framework cations, as necessary, for charge balance from the incorporation of aluminum and other trivalent tetrahedral atoms. While the extra-framework cations are usually hydrated, to various extents, the role of water with respect to formation, structure, stability, and reactivity remains a largely open question. While water is not an integral component of zeolitic frameworks, about two thirds of the approximately 150 classified framework types [1] collapse upon complete, or even partial, dehydration. Additionally, pure-silica zeolites (hydrophobic, and therefore
*
Corresponding author. E-mail address: [email protected] (A. Navrotsky).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.09.020
relatively water-free) only exist in approximately 20 framework types. Thus, water plays a major role in stabilizing zeolite structure. Nitrate bearing cancrinite and sodalite form in the caustic chemical conditions of some high-level nuclear-waste treatment facilities [2,3]. These precipitated phases foul process equipment and are thought to be the primary constituents of the resilient heels in the bottom of Savannah River storage tanks. Approximately one million gallons of high-level nuclear-waste fluid may have leaked into the sediment of the US Department of Energy (DOE) Hanford Site [3]. The role of these phases with respect to radionuclide (primarily 137Cs) containment and diffusion remains an open and very important question, as only a small percentage of the total nuclear-waste has been processed. Sodalite is technically classified as a feldspathoid. However, because this family of materials utilizes a primary building unit (PBU) common to many zeolitic frameworks, the b-cage, sodalite can also be considered a zeolite. Natural sodalite (Na8Cl2[Al6Si6O24]) is a relatively abundant
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mineral that can also be synthesized in the laboratory [4,5]. Numerous structural and compositional variants can also be synthesized. The general structural formula can be r written as M qþ m X x ½Aln Si12n O24 ðH2 OÞ0–8 , where qm rx = n, and n 6 6. Formally, every silicon atom carries a charge of +4 and oxygen carries a charge of 2. Each oxygen atom bridges two [TO4] tetrahedra, and shares its charge accordingly. Each tetrahedron, therefore, is formally charge neutral. The incorporation of aluminum into the framework, however, with a formal charge of +3, introduces a 1 charge to the framework as [AlO4]. For an aluminosilicate sodalite that incorporates the greatest amount of aluminum possible (Si/Al ratio of unity) the structural formula possesses a charge of 6. This net negative charge must be compensated by extra-framework cations (Mq+). Additional cations and anions (Xr) also enter the structure, such that the sum of the extra-framework charges (cations and anions) equals n. While monovalent alkali metals predominate, a large variety of monovalent and divalent cations can reside inside the b-cages (Na+, K+, Zn2+, Ca2+, Mg2+, etc.). Similarly, halides are common anions, but a large variety of complex anion species can also occupy the b-cage (OH, 2 2 2 CO2 etc.). Finally, a high 3 ; SO4 , SCN , CrO4 ; MoO4 degree of substitution can occur in the tetrahedral (T) sites: Ge4+, Ga3+, P3+, B3+ [6–9]. In addition to the extra-framework species, water is found to various extents. Although consisting of the same PBU (b-cage), sodalite does not appear to have catalytic applications like those of the structural similar faujasites (FAU) and A-type zeolite (LTA). The PBU of the sodalite framework is the b-cage, a symmetrical cage constructed from six single 4-rings (S4Rs—four tetrahedral sites and four oxygen sites) and eight S6Rs (Fig. 1A). The b-cage is also the PBU for a variety of other zeolitic frameworks (AST, EMT, FAU, FRA, LTA, LTN, TSC) [1], and knowledge about its fundamental properties may therefore be applicable to the more complex frameworks. There is no characteristic channel system in sodalite, and the framework is characterized by ‘‘apertures’’ of single 6-rings [1]. The b-cage can be thought of as a truncated cube: the six S4Rs are situated in the center position of each of the six sides of the cube, and the eight S6Rs are formed by ‘‘chopping off’’ small tetrahedra from each of the eight corners of the cube. Each b-cage is connected to eight other b-cages by sharing one of the eight S6Rs. This arrangement forms a BCC network of cages (Fig. 1B). While various experimental techniques are being used to examine sodalite formation, including solubility experiments, computer simulations, and NMR spectroscopy, there is little information about their thermodynamic properties. Knowledge of the enthalpies and free energies of formation, dehydration, and decomposition is necessary to understand the conditions under which such phases can form. High temperature oxide melt solution calorimetry continues to evolve as a versatile tool in earth and materials
Fig. 1. (A) The unit cell of sodalite contains one fully constructed b-cage (idealized pure-silica structure, O-atoms removed, perspective view). Every corner contains 1/8 of a b-cage, however, so the volume of the unit cell is equivalent to the volume of two b-cages. (B) The sodalite framework (idealized pure-silica structure, O-atoms removed, perspective view). Any single b-cage (darkened) is surrounded by eight, octahedrally coordinated b-cages (by sharing the eight S6Rs). The framework can be thought of as a BCC arrangement of b-cages.
science [10,11]. The development of more sensitive and stable instrumentation allows the determination of the thermodynamic properties for samples with a wide range of chemical compositions (including volatile-bearing phases) over a wide temperature range and/or sample sizes. The energetics of formation/hydration of several zeolite phases has been successfully measured using this technique in our laboratory [12–14]. This paper reports the formation enthalpies of the four end-members of the hydrosodalite family of materials.
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For simplicity, [SOD] is often written to represent [Al6Si6O24], and the abbreviated nomenclature of 8:2:4, 8:2:0, 6:0:8, and 6:0:0 are used to identify the ideal end-member structures (Na:OH:H2O structural formula stoichiometries). These phases are also referred to as basic sodalite (8:2:4), hydroxysodalite (8:2:0), hydrosodalite (6:0:8), and sodalite (6:0:0), respectively. 2. Experimental methods 2.1. Synthesis The syntheses of the four hydrosodalite end-members all started with the direct hydrothermal synthesis of Na8(OH)2[SOD](H2O)4. This phase was then treated hydrothermally to leach sodium hydroxide from the structure to produce the truly zeolitic phase, Na6[SOD](H2O)8. The two hydrated phases were then dehydrated under vacuum in a tube furnace. A total of eight samples (two for each end-member) were synthesized. The recipe for Na8(OH)2[SOD](H2O)4 outlined by Campbell et al. [5] was modified slightly. These samples were synthesized by first mixing approximately 10.0 g of NaOH (solid, pellets, 97% min, Alfa Aesar) with approximately 25 mL of distilled water in a Teflon liner for a 45 mL acid digestion vessel (Parr) with a stir bar. Because the dissociation of NaOH is strongly exothermic, the pellets were dropped slowly into the aqueous solution. Approximately 2.5 g aluminum isopropoxide (solid, powder, 99.99+%, Alfa Aesar) was added slowly, allowing sufficient time for the solids to dissolve. Approximately 2.5 g of tetraethylorthosilicate (TEOS, 99+%, Alfa Aesar) was added drop-wise to the solution, allowing sufficient time for the suspension that forms after the addition of liquid TEOS to clear. Distilled water was then added to fill the liner to 90% capacity, and the sealed Parr vessel was placed in a furnace at 453 K. After 10 days in the furnace, the autoclave was removed and allowed to cool. The residual aqueous solution was removed from the liner with a pipette, taking care not to disturb the crystals on the bottom. The liner was filled again with distilled water, shaken for a minute to wash the crystals, and the slurry was poured into a centrifuge tube and spun for about 10 min to drive the crystals to the bottom. The excess aqueous solution was removed, and the process was repeated for a total of four washings. After the last cleaning, the centrifuge tube containing the crystals was placed in a 373 K drying oven overnight. The samples were then characterized by XRD and TGA/DSC. Campbell et al. [5] reported that soxhlet extraction was used to convert 8:2:4 sodalite to 6:0:8 hydrosodalite, Na6[SOD](H2O)8. This method did not work well for us, primarily because the water vapor condensed before reaching the condenser. Instead, we used hydrothermal treatment at elevated temperatures (373–398 K) for 14 days with daily changing of the water. All dehydration experiments to make Na8(OH)2[SOD] and Na6[SOD] were carried out under vacuum (104 Torr)
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in a specially built silica glass tube apparatus in a Lindberg/Blue M single-zone tube furnace. The samples were heated at 1–5 K/min to 100 K below the respective setpoints and then heated at 0.5 K/min to the final temperatures. The furnace is especially stable, and the overshoot associated with this low heating rate is less than 1 K at the temperatures of interest. This is critical because the dehydration temperatures and decomposition temperatures are close together. After heating for the prescribed time and then allowing the furnace to cool, the contents of the silica-tube were isolated from the environment with valves and moved to an argon (99.997%) environment glovebox. The samples were removed from the tube and kept under secondary containment until calorimetry. 2.2. Characterization The sodalite samples were analyzed chemically for Na, Si, and Al by Galbraith Laboratories using lithium metaborate fusion and inductively coupled plasma optical emission spectroscopy (ICP-OES). The OH content was taken as the molar difference between the Na and Al contents on the basis of charge conservation. Thermogravimetric analysis (TGA) provided total water content. The extraframework water content was determined by subtracting the OH water from the total water content. X-ray diffraction (XRD) was the primary tool for phase identification and structural study. All patterns were generated on a Scintag XDS 2000 diffractometer utilizing a ˚ ), a copper 2.2 kW, 60 kV X-ray source (k = 1.54065 A 250 mm radius goniometer, and a liquid-nitrogen cooled, germanium solid-state detector with a 0.05 mm beryllium window. The scans were run from 10° to 70° (2-theta) using a 0.02° step and either 5 or 10 s dwell times. Two Rietveld refinements [15] were generated: the first from the Jade software package by Materials Data (MDI), and the second from Materials Studio by Accelrys. The Jade refinements were run with the following options: ka present, inverse root of the intensity, negative Isotropic B, eighth order polynomial background curve, Pearson-VII peakshape function, and sample displacement. The Materials Studio XRD patterns were indexed using the TREOR90 algorithm [16]. The crystal systems were identified as cubic, ˚ for the Na8 with a starting unit cell parameter of 8.89 A ˚ for the Na6 materials. Automatic materials and 8.85 A space group identification [17] was done by carrying out Pawley refinement [18] on the above cubic cell, and the space group was identified as P 43n. Due to concerns of re-hydration reactions, Rietveld analysis was not performed on the dehydrated samples. 2.3. Thermal analysis Differential thermal analysis and thermogravimetric analysis (DTA–TGA) were run simultaneously on a Netzsch 449C Jupiter system, using 35 mg samples, heating rates of 5 K/min, argon (99.997%) flow-rates of
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40 mL/min, covered Pt crucibles, and an empty reference crucible. 2.4. Scanning electron microscopy Particle morphology was observed on a high resolution scanning electron microscopy (Philips FEI XL30). 2.5. Oxide-melt solution calorimetry High-temperature drop-solution calorimetry was performed in a Tian-Calvet twin microcalorimeter operating at 975 K in 2PbO Æ B2O3 solvent using Ar flushing gas (60 mL/min) to expel any evolved water. The operation of this instrument has been discussed previously [10–14]. Samples were pressed into 5 mg pellets and dropped from room temperature into the solvent in the hot reaction zone of the calorimeter. The dehydrated samples were pressed and weighed in an Ar environment glove box. The pellets were removed from the glove box in a vial, which was opened at the calorimeter immediately before the drop. The total time of exposure of the sample to air was a few seconds. 3. Results
The best-fit data are those with the smallest weighted agreement factors (R in Jade and RWP in Materials Studio). Phase identification for basic sodalite (Na8) was confirmed from XRD using Jade via a comparison to ICDD-PDF #41-0009 [20]. The XRD scan is shown in Fig. 3A. This structure belongs to the P 43n (218) space group with a ˚ . This reported cubic lattice parameter of 8.903 ± 0.001 A ˚ compares to 8.894 ± 0.0002 A reported here, a difference ˚ . Felsche and Luger [20] report an ideal structure of 0.009 A with ideal chemical composition. Phase identification for hydrosodalite basic sodalite (Na6) was confirmed by comparison to ICDD-PDF #400102 [20]. The XRD scan can be seen in Fig. 3B. This structure also belongs to the P 43n (218) space group, with ˚. a reported cubic lattice parameter of 8.854 ± 0.001 A Using the Materials Studio results, we report unit cell ˚ and 8.847 ± lattice parameters of 8.851 ± 0.0006 A ˚ 0.0003 A. These differences are not significant when one considers that the reported uncertainties are those of the fit, not including any effects arising from systematic errors or small differences in composition. The structural evolution during the hydrothermal NaOH extraction process is shown in Fig. 3C. This shows complete conversion to 6:0:8 hydrosodalite, Na6[SOD](H2O)8.
3.1. Synthesis and chemical analysis 3.3. Thermal analysis Eight samples were synthesized and characterized. First, four different batches of Na8(OH)2[Al6Si6O24](H2O)4 were produced. Two of the four batches were then converted to Na6[Al6Si6O24](H2O)8 via hydrothermal extraction. Then, approximately half of each of the four batches was dehydrated. Thus, there are two basic sodalite samples, two hydroxysodalite samples (dehydrated basic sodalite), two hydrosodalite samples, and two sodalite samples (dehydrated hydrosodalite). The chemical compositions and structural formulas for the four hydrated end-members are listed in Table 1. It is assumed that the dehydration removed all of the water but the hydroxide, and that it did not change the framework chemistry, as shown by Luger [19]. SEM micrographs of the products, confirming good crystallinity, are shown in Fig. 2. 3.2. X-ray diffraction Lattice and refinement parameters for both Jade and Materials Studio for all four samples are listed in Table 2.
The bulk of our knowledge about hydrosodalite dehydration and decomposition comes from the work of Felsche, Luger, and coworkers [6,19–21] and Schipper et al. [22]. The dehydrations of both end-members (8:2:4 and 6:0:8) are recognized to occur in two steps, and the characteristics of the two dehydrations are markedly different. Luger [19] indicates that basic sodalite (8:2:4) and hydrosodalite (6:0:8) can be dehydrated at 770 K under vacuum or air for one week and 12 h, respectively. Hydrosodalite is significantly easier to dehydrate, possibly because two of the eight S6Rs are vacant (no sodium). In somewhat contradictory statements, Felsche and Luger [6] also state that the same materials can be dehydrated at temperatures of 775, 863, or 920 K for 8:2:4 and 675 or 695 K for 6:0:8 sodalite. The DSC curves for the two hydrated end-members are illustrated in Fig. 4. The curves are similar in overall characteristics to those reported previously [6,19]. The 8:2:4 to 8:2:0 dehydration is a sluggish two-step process with broad
Table 1 Molar composition of the four sodalite samples examined in this study Sample ID
Ideal
Na
OH
Al
Si
O
H2O
Structural formula
Na8-1 Na8-2 Na6-1 Na6-2
8:2:4 8:2:4 6:0:8 6:0:8
7.60 7.82 6.20 6.13
1.64 1.84 0.21 0.14
5.96 5.98 5.99 5.99
6.04 6.02 6.01 6.01
24 24 24 24
3.00 3.27 8.04 7.97
Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
The two hydrated end-members are represented. The anhydrous phases are derived from these starting compounds, assuming complete dehydration, and no change in the framework structural chemistry (Na, OH, Al, Si).
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Table 2 Lattice parameters and refinement parameters for the four hydrated sodalite phases studied in this report using Jade (R) and Materials Studio (RWP) Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
a0
esd
R
8.8940 8.8940 8.8459 8.8407
0.0002 0.0002 0.0004 0.0005
12.28 11.64 19.77 23.73 RWP
Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
8.8901 8.8836 8.8514 8.8470
0.0005 0.0009 0.0006 0.0003
16.38 16.17 11.75 13.41
composition (Na2O)Na6(Al6Si6O24). This decomposition involves the hydroxyl decomposition reaction 2OH = O2 + H2O (gas), as originally reported by Schipper et al. [22]. Luger [19], who synthesized 8:2:4 sodalite via kaolinite, reported endothermic peaks at 420, 650, and 1020 K. As discussed by Schipper et al. [22], these peaks are highly sensitive to heating rates. We stress that the process is irreversible and kinetically controlled, and other factors, such as particle size, may play a role. These reactions do not represent equilibrium and no meaningful thermodynamic parameters can be derived from them. In contrast, the 6:0:8 to 6:0:0 dehydration is a much sharper, two-step process with endothermic peaks at approximately 464 and 535 K. These peaks are similar to 375 and 500 K reported by Luger [19]. The transition to Na6(Al6Si6O24) carnegieite is exothermic at about 1229 K, which compares to 1175 K reported by Luger [19]. Based upon the direct thermal analysis of the actual materials examined in this study, dehydration temperatures of 903 K and 673 K were chosen for 8:2:4 and 6:0:8 sodalite, respectively, as discussed in the synthesis procedure. 3.4. Calorimetry Calorimetric data are presented in Table 3. A thermodynamic cycle must be written to calculate the quantities of interest: enthalpies of formation, relative to both the constituent oxides (DHf-ox) and the elements (DHf-el), and hydration enthalpies (DHhyd). All reactions are written generally to accommodate the different stoichiometries that differentiate the eight different samples. The abbreviations s, d, l, and g stand for solid, dissolved (in the melt), liquid, and gas. TR stands for room temperature (298 K) and TC stands for the calorimeter temperature (975 K). The following reaction takes place when the zeolite samples are dropped into the calorimeter: Fig. 2. SEM images of (A) Na8(OH)2[Al6Si6O24](H2O)4, and (B) Na6[Al6Si6O24](H2O)8, and Na6[Al6Si6O24].
endothermic peaks at approximately 361 K and 839 K. The third endothermic peak at 990 K is associated with the decomposition of 8:2:0 sodalite to a carnegieite phase of
NaN Na ðOHÞN Na N Al ½AlN Al SiN Si O24 ðH2 OÞN H O ðsodalite; s; T R Þ 2
N Na N Al Na2 Oðd; T C Þ þ Al2 O3 ðd; T C Þ þ N Si SiO2 ðd; T C Þ ! 2 2 N OH H2 Oðg; T C Þ þ N H2 O þ DH ds ð1Þ 2
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E.C. Moloy et al. / Microporous and Mesoporous Materials 88 (2006) 283–292 100% 90% 80%
Relative Intensity
70% 60% 50% 40% 30% 20% 10% 0% 10
20
30
A
40
50
60
70
50
60
70
Two-theta 100% 90% 80%
Relative Intensity
70% 60% 50% 40% 30% 20% 10% 0% 10
20
30
B
40
Two-theta 120% 110% 100%
Relative Intensity
90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 10
C
20
30
40
50
60
70
Two-theta
Fig. 3. XRD pattern for (A) Na8(OH)2[SOD](H2O)4 sodalite, (B) Na6[SOD](H2O)8, and (C) hydrothermal structural evolution during NaOH extraction for 0, 4, 5, 6, 7, 8, 9, and 10 days.
E.C. Moloy et al. / Microporous and Mesoporous Materials 88 (2006) 283–292
289
where DH ds ¼ enthalpy of drop solution measured in this study; N Na ð113:1 0:8Þ kJ=mol [12]; DH 2 ¼ 2 N Al DH 3 ¼ ð107:9 1:0Þ kJ=mol [12]; 2 DH 4 ¼ N Si ð39:1 0:3Þ kJ=mol [12]; N OH DH 5 ¼ N H2 O þ ð68:9 0:1Þ kJ=mol [23]. 2 For the anhydrous products, DH5 is only evaluated using the hydroxide content, as the water content is assumed to be zero. Enthalpy calculations relative to the elements (DHf-el) use the DHf-ox values above and the tabulated enthalpies of formation of the oxides from the elements [23]. N Na N Na O2 ðg; T R Þ ! Na2 Oðs; T R Þ 2 2 3N Al N Al O2 ðg; T R Þ ! Al2 O3 ðs; T R Þ N Al Alðs; T R Þ þ 2 2 N Si Siðs; T R Þ þ N Si O2 ðg; T R Þ ! N Si SiO2 ðs; T R Þ N H2 O þ N OH N OH 2 O2 ðg; T R Þ N H2 O þ H2 ðg; T R Þ þ 2 2 N OH ! N H2 O þ H2 Oðl; T R Þ 2
N Na Naðs; T R Þ þ
ð7Þ ð8Þ ð9Þ
ð10Þ
DH f -el ¼ DH f -ox þ DH 7 þ DH 8 þ DH 9 þ DH 10 where
Fig. 4. DSC curves for (A) 8:2:4 and (B) 6:0:8 sodalite.
The following component reactions (2)–(5) represent the enthalpies of drop solution of the stoichiometric number of moles of the component oxides. N Na N Na Na2 Oðs; T R Þ ! Na2 Oðd; T C Þ 2 2 N Al N Al Al2 O3 ðs; T R Þ ! Al2 O3 ðd; T C Þ 2 2 N Si SiO2 ðs; T R Þ ! N Si SiO2 ðd; T C Þ N OH N H2 O þ H2 Oðl; T R Þ 2 N OH ! N H2 O þ H2 Oðg; T C Þ 2
DH 2
ð2Þ
DH 3
ð3Þ
DH 4
ð4Þ
DH 5
ð5Þ
N Na ð414:82 0:28Þ kJ=mol; 2 N Al DH 8 ¼ ð1675:7 1:3Þ kJ=mol; 2 DH 9 ¼ N Si ð910:7 1:0Þ kJ=mol; N OH DH 10 ¼ N H2 O þ ð258:83 0:042Þ kJ=mol. 2 DH 7 ¼
DHf-ox values are all significantly negative, indicating the stability of the sodalite phases. The formation enthalpies are listed in Table 3A for all eight samples. The hydration enthalpy is evaluated from the enthalpies of drop solution of hydrated and dehydrated samples as NaN Na ðOHÞN OH ½AlN Al SiN Si O24
Then, for the formation reaction from the oxides at 298 K
ðdehydrated sodalite; s; T R Þ þ NH2 Oðl; T R Þ
N Na N Al Na2 Oðs; T R Þ þ Al2 O3 ðs; T R Þ þ N Si SiO2 ðs; T R Þ 2 2 N OH þ N H2 O þ H2 Oðl; T R Þ ! NaN Na ðOHÞN OH 2
! NaN Na ðOHÞN OH ½AlN Al SiN Si O24 ðH2 OÞN H O ðhydrated sodalite; s; T R Þ 2
The hydration enthalpy is then calculated as DH hyd ¼ DH ds;hydrated sodalite þ DH ds;dehydrated sodalite þ N DH H2 O
½AlN Al SiN Si O24 ðH2 OÞN H O ðSOD; s; T R Þ DH f -ox 2 ð6Þ DH f -ox ¼ DH ds þ DH 2 þ DH 3 þ DH 4 þ DH 5
ð11Þ
where the first two terms are measured enthalpies of drop solution and the last is the heat content of water, 68.9 kJ/ mol [23]. The hydration enthalpies are listed in Table 3B.
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Table 3 (A) Enthalpies of drop-solution (DHds), formation enthalpies from oxides (DHf-ox) and from elements (DHf-el), (B) hydration enthalpies per mol of sodalite (structural formula, DHhyd-SOD) and per mole of water ðDH hyd-H2 O Þ ID
Drops
DHds
DHf-ox
DHf-el
kJ/mol-SOD
kJ/mol-TO2
kJ/mol-SOD
kJ/mol-TO2
kJ/mol-SOD
kJ/mol-TO2
A Hydrous Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
9 9 9 10
1426.4 ± 10.7 1482.3 ± 11.8 1749.1 ± 11.9 1695.5 ± 16.2
118.9 ± 0.9 123.5 ± 1.0 145.8 ± 1.0 141.3 ± 1.4
1035.4 ± 11.7 1077.8 ± 12.7 980.6 ± 12.7 929.8 ± 16.8
86.3 ± 1.0 89.8 ± 1.1 81.7 ± 1.1 77.5 ± 1.4
14093.2 ± 13.8 14275.4 ± 14.6 14866.6 ± 14.6 14774.0 ± 18.3
1174.4 ± 1.1 1189.6 ± 1.2 1238.9 ± 1.2 1231.2 ± 1.5
Anhydrous Na7.60(OH)1.64[Al5.96Si6.04O24] Na7.82(OH)1.84[Al5.98Si6.02O24] Na6.20(OH)0.21[Al5.99Si6.01O24] Na6.13(OH)0.14[Al5.99Si6.01O24]
10 10 10 8
987.6 ± 12.2 1009.3 ± 12.4 919.1 ± 12.6 829.7 ± 13.2
82.3 ± 1.0 84.1 ± 1.0 76.6 ± 1.1 69.1 ± 1.1
803.1 ± 13.1 830.0 ± 13.3 704.3 ± 13.3 613.3 ± 13.8
66.9 ± 1.1 69.2 ± 1.1 58.7 ± 1.1 51.1 ± 1.2
13085.1 ± 15.0 13181.7 ± 15.1 12510.1 ± 15.2 12394.1 ± 15.6
1090.4 ± 1.3 1098.5 ± 1.3 1042.5 ± 1.3 1032.8 ± 1.3
DHhyd B Hydration Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
kJ/mol-SOD
kJ/mol-H2O
232.3 ± 16.3 247.9 ± 17.1 276.3 ± 17.4 316.5 ± 20.9
77.5 ± 5.4 75.8 ± 5.2 34.4 ± 2.2 39.7 ± 2.6
Uncertainties of measured values are reported as two standard deviations of the mean. Uncertainty of the combined reactions was taken as the square root of the sum of the squares of uncertainty of each individual reaction.
These values fall within the range of values reported for zeolites in the literature [12,24,25]. 4. Discussion To begin an analysis of energetic data, the proper structures of, and environments around, the water molecules should be established for the two hydrated end-members. The number of guest molecules (water/hydroxide) that reside in the interior of each b-cages is determined by dividing the stoichiometric value of the particular species in the structural formula by two (two b-cage volumes per unit cell). Thus, for the ideal basic sodalite structure Na8(OH)2[SOD](H2O)4, one hydroxide and two waters reside in each b-cages, while for Na6[SOD](H2O)8, there are four water molecules. The same accounting method applies to sodium, an extra-framework species that resides in the centers of S6Rs. While there is some displacement of these ions from the S6Rs, they are essentially localized in cages and not as mobile as water and hydroxide. From the vantage point of the centers of the b-cages, an important position by which to describe the structure of the extra-framework and guest species, sodium ions appear in all eight S6Rs. As depicted in Fig. 5, however, only four of the eight sodium ions belong to any particular b-cage. Four of the eight sodium ions are displaced out of the S6Rs toward the centers of the b-cages. The other four sodium ions move toward the centers of the adjoining b-cages, producing a similar relationship of four in and four out with respect to the b-cage center. The four sodium cations then coordinate with what-
ever content of water and hydroxide molecules reside in the b-cage, forming an interwoven, ditetrahedral arrangement [6,20,26–28]. One tetrahedron is defined by the sodium ions, and the second is defined by the water/hydroxide molecules. Due to different amounts of both sodium and water/hydroxide inside the b-cages according to the different structural formulas for each end-member, the tetrahedra often have only three of the four sites occupied. XRD data [6] suggests, however, that all sites are occupied by a statistical distribution of available ions, so there is no long range ordering of the vacancies. The extent of the displacement of the sodium ions out of the S6Rs is an area of active research and beyond the scope of this paper, but such displacement varies from structure to structure according to the number of sodium ions and guest molecules. Anhydrous sodalite is thought to not have significant sodium displacement out of the S6Rs [29]. The various displacements are shown schematically in Fig. 5. There are three basic interactions to consider when evaluating the energetics: (1) hydrogen bonding between water/ water and water/hydroxide, (2) hydrogen bonding between water and framework oxygens, and (3) the reduction of Na+–Na+ charge repulsion by positioning water (oxygen) between any two sodium atoms. The Na8 materials are more ideal than the Na6 materials because of the symmetric distribution of sodium cations in all of the S6Rs, while in Na6, two of the eight S6Rs are vacant. However, Felsche and Luger [6,20] have indicated that there is no evidence of hydrogen bonding between water molecules and framework oxygen atoms in basic sodalite. Thus, the dominant interactions are water/water,
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Na
O Na
Na
O
H O
H Na
H-bonding in Na6. Hydrogen bonding, for example, is thought to produce the antagonistic behavior in the unit cell volumes of hydrosodalite: the unit cell volume decreases with increasing water content (8% for 0–8 waters). Thus, there are competing interactions; 25% less electrostatic interactions (one less sodium), no hydroxide ions (one in basic sodalite), but two more water molecules that H-bond with the framework. Therefore, the relatively similar total hydration enthalpies for Na6 and Na8 seem reasonable.
B
Na
O Na
O
Na
C
H Na
H
A
O
Na
O Na
O
Na
H
291
Na Na
Na
O Na
Na D
Fig. 5. Schematic drawings of Na, H2O, and OH configurations in the four ideal end-members studied in this report. The cubes that are drawn in dashed lines represent a cube formed by the sites that the sodium ions occupy in the b-cage. Gray molecules for Na, O and H represent additional sites that the particular atoms/molecules (with occupancies of less than one) can reside statistically. While hydroxide ions are identified explicitly, water hydrogen atoms are omitted for clarity. (A) Na8(OH)2 [SOD](H2O)4: Two water molecules and one hydroxide ion are statistically distributed on the four vacant corners not occupied by sodium. (B) Na8(OH)2[SOD]: One hydroxide occupies each b-cage, where the oxygen is in the center and the orientation varies statistically with the hydrogen pointing to the vacant corners. (C) Na6[SOD](H2O)8: three sodium cations are even distributed on four sites. (D) Na6[SOD]: same as (C) but without the water oxygens.
hydrogen bonding and electrostatic interactions. In the hydrosodalite structure (Fig. 5C), in comparison to basic sodalite, there is one less sodium cation, one less hydroxide ion, and two extra, charge neutral water molecules. It is not surprising, therefore, that significant differences between the hydration enthalpies (37.0 kJ/mol-H2O for Na6 versus 76.7 kJ/mol-H2O for Na8) are observed. The observation that the Na8 waters are more exothermically bonded than the Na6 waters is fully consistent with our structural analysis: one more sodium cation, one more hydroxide ion, fewer water molecules, and one less guest species overall in the Na8 than in Na6 structures. What may be a little curious is that the total hydration enthalpies per formula unit of the zeolite vary by only about 36% (from 232.3 to 316.5 kJ/mol) while the enthalpies of hydration per water molecule vary by about a factor of two. The material with less water per formula, Na8, holds those water molecules more tightly (more exothermic enthalpy per mole of H2O) but has the less negative overall hydration enthalpy. While there is no reported hydrogen bonding in basic sodalite, there is significant
5. Conclusions The four end-member structures of the hydrosodalite family of materials were successfully synthesized and characterized. Oxide melt solution calorimetry provided formation enthalpies and hydration enthalpies for the Na8 and Na6 series were also calculated from the calorimetric data. Both the formation enthalpies and the hydration enthalpies are comparable to values published for other sodium zeolites. A structural analysis of the two series of materials explains their differences in enthalpies. Acknowledgements We thank May Nyman for advice about synthesis and Miaojun Wang for taking the SEM photographs. This work was supported by the US Department of Energy EMSP Program, grant DE-FG07-01ER63298. References [1] C. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, fifth ed., Elsevier, International Zeolite Association, Amsterdam, 2001. [2] C. Weber, Oak Ridge National Laboratory (UT-Batelle) Report: ORNL/TM-2001/109, 2001. [3] B.R. Bickmore, K.L. Nagy, J.S. Young, J.W. Drexler, Environ. Sci. Technol. 35 (2001) 4481. [4] L. Pauling, Zeitsch. Kristallogr. 74 (1930) 213. [5] B.J. Campbell, J.M. Delgado, A.K. Cheetham, B.B. Iversen, N.P. Blake, S.R. Shannon, S. Latturner, G.D. Stucky, J. Chem. Phys. 113 (2000) 10226. [6] J. Felsche, S. Luger, Thermochim. Acta 118 (1987) 35. [7] M.E. Brenchley, M.T. Weller, Zeolites 14 (1994) 682. [8] P.J. Mead, M.T. Weller, Zeolites 15 (1995) 561. [9] N.C. Nielsen, H. Bildsøe, H.J. Jakobsen, P. Norby, Zeolites 11 (1991) 622. [10] A. Navrotsky, Phys. Chem. Miner. 2 (1977) 89. [11] A. Navrotsky, Phys. Chem. Miner. 24 (1997) 222. [12] I. Kiseleva, A. Navrotsky, I.A. Belitsky, B.A. Fursenko, Am. Miner. 81 (1996) 668. [13] S. Yang, A. Navrotsky, Micropor. Mesopor. Mater. 37 (2000) 175. [14] Q. Liu, H. Xu, A. Navrotsky, Micropor. Mesopor. Mater., doi:10. 1016/j.micromeso.2005.08.008. [15] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [16] P.E. Werner, L. Eriksson, M. Westdahl, J. Appl. Cryst. 18 (1985) 367. [17] A.J. Markvardsen, W.I.F. David, J.C. Johnson, K. Shankland, Acta Crystallogr. A 57 (2001) 47. [18] G.E. Engel, S. Wilke, O. Ko¨nig, K.D.M. Harris, F.J.J. Leusen, J. Appl. Cryst. 32 (1999) 1169.
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[19] S. Luger, Compounds of the Hydrosodalite Family, Konstanzer Dissertationen, Hartung-Gorre Verlag, 1987, p. 107. [20] J. Felsche, S. Luger, Berich. Bunsen. Phys. Chem. 90 (1986) 731. [21] G. Engelhardt, J. Felsche, P. Sieger, J. Am. Chem. Soc. 114 (1992) 1173. [22] D.J. Schipper, T.W. Lathouwers, C.Z. Van Doorn, J. Am. Chem. Soc. 56 (1973) 523. [23] R.A. Robie, B.S. Hemingway, Geological Survey Bulletin No. 2131, 1995.
[24] S.-H. Shim, A. Navrotsky, T.R. Gaffney, J.E. MacDougal, Am. Miner. 84 (1999) 1870. [25] S. Yang, A. Navrotsky, R. Wilkin, Am. Miner. 86 (2001) 438. [26] J. Felsche, S. Luger, C. Baerlocher, Zeolites 6 (1986) 367. [27] S. Luger, J. Felsche, P. Fischer, Acta Crystallogr. C 43 (1987) 1. [28] J. Felsche, S. Luger, P. Fischer, Acta Crystallogr. C 43 (1987) 809. [29] E.C. Moloy, R.T. Cygan, F. Bonhomme, D.M. Teter, A. Navrotsky, Chem. Mater. 16 (2004) 2121.
Formation and hydration enthalpies of the hydrosodalite family of materials Eric C. Moloy, Qingyuan Liu, Alexandra Navrotsky
*
Thermochemistry Facility and NEAT ORU, University of California at Davis, Davis, CA 95616-8779, United States Received 25 August 2005; received in revised form 20 September 2005; accepted 28 September 2005 Available online 14 November 2005
Abstract The four end-member structures of the hydrosodalite family of materials (Na6+x(OH)x[Al6Si6O24](H2O)N) are synthesized via hydrothermal techniques and characterized using X-ray diffraction, thermal analysis, scanning electron microscopy, and inductively coupled plasma optical emission spectroscopy. High-temperature drop-solution calorimetry in molten 2PbO Æ B2O3 at 975 K is used to measure the formation and hydration enthalpies. We report the formation enthalpies, from both the oxides and the elements, for a total of eight samples, two for each end-member. The average formation enthalpy (from oxides) is 88.1 ± 1.0 kJ/mol-TO2 for basic sodalite {ideally Na8(OH)2[Al6Si6O24](H2O)4}, 79.6 ± 1.2 kJ/mol-TO2 for hydroxysodalite {ideally Na8(OH)2[Al6Si6O24]}, 68.0 ± 1.1 kJ/mol-TO2, for hydrosodalite {ideally Na6[Al6Si6O24](H2O)8}, and 54.9 ± 1.1 kJ/mol-TO2, for sodalite {ideally Na6[Al6Si6O24]}. The corresponding average hydration enthalpies are 76.7 ± 5.3 kJ/mol-H2O and 37.0 ± 2.4 kJ/mol-H2O for the Na8 and Na6 series, respectively. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Zeolite; Sodalite; Formation enthalpy; Hydration enthalpy; Hydrothermal synthesis
1. Introduction Zeolites are nanoporous materials, formed by both natural and synthetic processes, characterized as open framework, aluminosilicate structures comprised of corner sharing tetrahedra (tectosilicates). The structures possess extra-framework cations, as necessary, for charge balance from the incorporation of aluminum and other trivalent tetrahedral atoms. While the extra-framework cations are usually hydrated, to various extents, the role of water with respect to formation, structure, stability, and reactivity remains a largely open question. While water is not an integral component of zeolitic frameworks, about two thirds of the approximately 150 classified framework types [1] collapse upon complete, or even partial, dehydration. Additionally, pure-silica zeolites (hydrophobic, and therefore
*
Corresponding author. E-mail address: [email protected] (A. Navrotsky).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.09.020
relatively water-free) only exist in approximately 20 framework types. Thus, water plays a major role in stabilizing zeolite structure. Nitrate bearing cancrinite and sodalite form in the caustic chemical conditions of some high-level nuclear-waste treatment facilities [2,3]. These precipitated phases foul process equipment and are thought to be the primary constituents of the resilient heels in the bottom of Savannah River storage tanks. Approximately one million gallons of high-level nuclear-waste fluid may have leaked into the sediment of the US Department of Energy (DOE) Hanford Site [3]. The role of these phases with respect to radionuclide (primarily 137Cs) containment and diffusion remains an open and very important question, as only a small percentage of the total nuclear-waste has been processed. Sodalite is technically classified as a feldspathoid. However, because this family of materials utilizes a primary building unit (PBU) common to many zeolitic frameworks, the b-cage, sodalite can also be considered a zeolite. Natural sodalite (Na8Cl2[Al6Si6O24]) is a relatively abundant
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mineral that can also be synthesized in the laboratory [4,5]. Numerous structural and compositional variants can also be synthesized. The general structural formula can be r written as M qþ m X x ½Aln Si12n O24 ðH2 OÞ0–8 , where qm rx = n, and n 6 6. Formally, every silicon atom carries a charge of +4 and oxygen carries a charge of 2. Each oxygen atom bridges two [TO4] tetrahedra, and shares its charge accordingly. Each tetrahedron, therefore, is formally charge neutral. The incorporation of aluminum into the framework, however, with a formal charge of +3, introduces a 1 charge to the framework as [AlO4]. For an aluminosilicate sodalite that incorporates the greatest amount of aluminum possible (Si/Al ratio of unity) the structural formula possesses a charge of 6. This net negative charge must be compensated by extra-framework cations (Mq+). Additional cations and anions (Xr) also enter the structure, such that the sum of the extra-framework charges (cations and anions) equals n. While monovalent alkali metals predominate, a large variety of monovalent and divalent cations can reside inside the b-cages (Na+, K+, Zn2+, Ca2+, Mg2+, etc.). Similarly, halides are common anions, but a large variety of complex anion species can also occupy the b-cage (OH, 2 2 2 CO2 etc.). Finally, a high 3 ; SO4 , SCN , CrO4 ; MoO4 degree of substitution can occur in the tetrahedral (T) sites: Ge4+, Ga3+, P3+, B3+ [6–9]. In addition to the extra-framework species, water is found to various extents. Although consisting of the same PBU (b-cage), sodalite does not appear to have catalytic applications like those of the structural similar faujasites (FAU) and A-type zeolite (LTA). The PBU of the sodalite framework is the b-cage, a symmetrical cage constructed from six single 4-rings (S4Rs—four tetrahedral sites and four oxygen sites) and eight S6Rs (Fig. 1A). The b-cage is also the PBU for a variety of other zeolitic frameworks (AST, EMT, FAU, FRA, LTA, LTN, TSC) [1], and knowledge about its fundamental properties may therefore be applicable to the more complex frameworks. There is no characteristic channel system in sodalite, and the framework is characterized by ‘‘apertures’’ of single 6-rings [1]. The b-cage can be thought of as a truncated cube: the six S4Rs are situated in the center position of each of the six sides of the cube, and the eight S6Rs are formed by ‘‘chopping off’’ small tetrahedra from each of the eight corners of the cube. Each b-cage is connected to eight other b-cages by sharing one of the eight S6Rs. This arrangement forms a BCC network of cages (Fig. 1B). While various experimental techniques are being used to examine sodalite formation, including solubility experiments, computer simulations, and NMR spectroscopy, there is little information about their thermodynamic properties. Knowledge of the enthalpies and free energies of formation, dehydration, and decomposition is necessary to understand the conditions under which such phases can form. High temperature oxide melt solution calorimetry continues to evolve as a versatile tool in earth and materials
Fig. 1. (A) The unit cell of sodalite contains one fully constructed b-cage (idealized pure-silica structure, O-atoms removed, perspective view). Every corner contains 1/8 of a b-cage, however, so the volume of the unit cell is equivalent to the volume of two b-cages. (B) The sodalite framework (idealized pure-silica structure, O-atoms removed, perspective view). Any single b-cage (darkened) is surrounded by eight, octahedrally coordinated b-cages (by sharing the eight S6Rs). The framework can be thought of as a BCC arrangement of b-cages.
science [10,11]. The development of more sensitive and stable instrumentation allows the determination of the thermodynamic properties for samples with a wide range of chemical compositions (including volatile-bearing phases) over a wide temperature range and/or sample sizes. The energetics of formation/hydration of several zeolite phases has been successfully measured using this technique in our laboratory [12–14]. This paper reports the formation enthalpies of the four end-members of the hydrosodalite family of materials.
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For simplicity, [SOD] is often written to represent [Al6Si6O24], and the abbreviated nomenclature of 8:2:4, 8:2:0, 6:0:8, and 6:0:0 are used to identify the ideal end-member structures (Na:OH:H2O structural formula stoichiometries). These phases are also referred to as basic sodalite (8:2:4), hydroxysodalite (8:2:0), hydrosodalite (6:0:8), and sodalite (6:0:0), respectively. 2. Experimental methods 2.1. Synthesis The syntheses of the four hydrosodalite end-members all started with the direct hydrothermal synthesis of Na8(OH)2[SOD](H2O)4. This phase was then treated hydrothermally to leach sodium hydroxide from the structure to produce the truly zeolitic phase, Na6[SOD](H2O)8. The two hydrated phases were then dehydrated under vacuum in a tube furnace. A total of eight samples (two for each end-member) were synthesized. The recipe for Na8(OH)2[SOD](H2O)4 outlined by Campbell et al. [5] was modified slightly. These samples were synthesized by first mixing approximately 10.0 g of NaOH (solid, pellets, 97% min, Alfa Aesar) with approximately 25 mL of distilled water in a Teflon liner for a 45 mL acid digestion vessel (Parr) with a stir bar. Because the dissociation of NaOH is strongly exothermic, the pellets were dropped slowly into the aqueous solution. Approximately 2.5 g aluminum isopropoxide (solid, powder, 99.99+%, Alfa Aesar) was added slowly, allowing sufficient time for the solids to dissolve. Approximately 2.5 g of tetraethylorthosilicate (TEOS, 99+%, Alfa Aesar) was added drop-wise to the solution, allowing sufficient time for the suspension that forms after the addition of liquid TEOS to clear. Distilled water was then added to fill the liner to 90% capacity, and the sealed Parr vessel was placed in a furnace at 453 K. After 10 days in the furnace, the autoclave was removed and allowed to cool. The residual aqueous solution was removed from the liner with a pipette, taking care not to disturb the crystals on the bottom. The liner was filled again with distilled water, shaken for a minute to wash the crystals, and the slurry was poured into a centrifuge tube and spun for about 10 min to drive the crystals to the bottom. The excess aqueous solution was removed, and the process was repeated for a total of four washings. After the last cleaning, the centrifuge tube containing the crystals was placed in a 373 K drying oven overnight. The samples were then characterized by XRD and TGA/DSC. Campbell et al. [5] reported that soxhlet extraction was used to convert 8:2:4 sodalite to 6:0:8 hydrosodalite, Na6[SOD](H2O)8. This method did not work well for us, primarily because the water vapor condensed before reaching the condenser. Instead, we used hydrothermal treatment at elevated temperatures (373–398 K) for 14 days with daily changing of the water. All dehydration experiments to make Na8(OH)2[SOD] and Na6[SOD] were carried out under vacuum (104 Torr)
285
in a specially built silica glass tube apparatus in a Lindberg/Blue M single-zone tube furnace. The samples were heated at 1–5 K/min to 100 K below the respective setpoints and then heated at 0.5 K/min to the final temperatures. The furnace is especially stable, and the overshoot associated with this low heating rate is less than 1 K at the temperatures of interest. This is critical because the dehydration temperatures and decomposition temperatures are close together. After heating for the prescribed time and then allowing the furnace to cool, the contents of the silica-tube were isolated from the environment with valves and moved to an argon (99.997%) environment glovebox. The samples were removed from the tube and kept under secondary containment until calorimetry. 2.2. Characterization The sodalite samples were analyzed chemically for Na, Si, and Al by Galbraith Laboratories using lithium metaborate fusion and inductively coupled plasma optical emission spectroscopy (ICP-OES). The OH content was taken as the molar difference between the Na and Al contents on the basis of charge conservation. Thermogravimetric analysis (TGA) provided total water content. The extraframework water content was determined by subtracting the OH water from the total water content. X-ray diffraction (XRD) was the primary tool for phase identification and structural study. All patterns were generated on a Scintag XDS 2000 diffractometer utilizing a ˚ ), a copper 2.2 kW, 60 kV X-ray source (k = 1.54065 A 250 mm radius goniometer, and a liquid-nitrogen cooled, germanium solid-state detector with a 0.05 mm beryllium window. The scans were run from 10° to 70° (2-theta) using a 0.02° step and either 5 or 10 s dwell times. Two Rietveld refinements [15] were generated: the first from the Jade software package by Materials Data (MDI), and the second from Materials Studio by Accelrys. The Jade refinements were run with the following options: ka present, inverse root of the intensity, negative Isotropic B, eighth order polynomial background curve, Pearson-VII peakshape function, and sample displacement. The Materials Studio XRD patterns were indexed using the TREOR90 algorithm [16]. The crystal systems were identified as cubic, ˚ for the Na8 with a starting unit cell parameter of 8.89 A ˚ for the Na6 materials. Automatic materials and 8.85 A space group identification [17] was done by carrying out Pawley refinement [18] on the above cubic cell, and the space group was identified as P 43n. Due to concerns of re-hydration reactions, Rietveld analysis was not performed on the dehydrated samples. 2.3. Thermal analysis Differential thermal analysis and thermogravimetric analysis (DTA–TGA) were run simultaneously on a Netzsch 449C Jupiter system, using 35 mg samples, heating rates of 5 K/min, argon (99.997%) flow-rates of
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40 mL/min, covered Pt crucibles, and an empty reference crucible. 2.4. Scanning electron microscopy Particle morphology was observed on a high resolution scanning electron microscopy (Philips FEI XL30). 2.5. Oxide-melt solution calorimetry High-temperature drop-solution calorimetry was performed in a Tian-Calvet twin microcalorimeter operating at 975 K in 2PbO Æ B2O3 solvent using Ar flushing gas (60 mL/min) to expel any evolved water. The operation of this instrument has been discussed previously [10–14]. Samples were pressed into 5 mg pellets and dropped from room temperature into the solvent in the hot reaction zone of the calorimeter. The dehydrated samples were pressed and weighed in an Ar environment glove box. The pellets were removed from the glove box in a vial, which was opened at the calorimeter immediately before the drop. The total time of exposure of the sample to air was a few seconds. 3. Results
The best-fit data are those with the smallest weighted agreement factors (R in Jade and RWP in Materials Studio). Phase identification for basic sodalite (Na8) was confirmed from XRD using Jade via a comparison to ICDD-PDF #41-0009 [20]. The XRD scan is shown in Fig. 3A. This structure belongs to the P 43n (218) space group with a ˚ . This reported cubic lattice parameter of 8.903 ± 0.001 A ˚ compares to 8.894 ± 0.0002 A reported here, a difference ˚ . Felsche and Luger [20] report an ideal structure of 0.009 A with ideal chemical composition. Phase identification for hydrosodalite basic sodalite (Na6) was confirmed by comparison to ICDD-PDF #400102 [20]. The XRD scan can be seen in Fig. 3B. This structure also belongs to the P 43n (218) space group, with ˚. a reported cubic lattice parameter of 8.854 ± 0.001 A Using the Materials Studio results, we report unit cell ˚ and 8.847 ± lattice parameters of 8.851 ± 0.0006 A ˚ 0.0003 A. These differences are not significant when one considers that the reported uncertainties are those of the fit, not including any effects arising from systematic errors or small differences in composition. The structural evolution during the hydrothermal NaOH extraction process is shown in Fig. 3C. This shows complete conversion to 6:0:8 hydrosodalite, Na6[SOD](H2O)8.
3.1. Synthesis and chemical analysis 3.3. Thermal analysis Eight samples were synthesized and characterized. First, four different batches of Na8(OH)2[Al6Si6O24](H2O)4 were produced. Two of the four batches were then converted to Na6[Al6Si6O24](H2O)8 via hydrothermal extraction. Then, approximately half of each of the four batches was dehydrated. Thus, there are two basic sodalite samples, two hydroxysodalite samples (dehydrated basic sodalite), two hydrosodalite samples, and two sodalite samples (dehydrated hydrosodalite). The chemical compositions and structural formulas for the four hydrated end-members are listed in Table 1. It is assumed that the dehydration removed all of the water but the hydroxide, and that it did not change the framework chemistry, as shown by Luger [19]. SEM micrographs of the products, confirming good crystallinity, are shown in Fig. 2. 3.2. X-ray diffraction Lattice and refinement parameters for both Jade and Materials Studio for all four samples are listed in Table 2.
The bulk of our knowledge about hydrosodalite dehydration and decomposition comes from the work of Felsche, Luger, and coworkers [6,19–21] and Schipper et al. [22]. The dehydrations of both end-members (8:2:4 and 6:0:8) are recognized to occur in two steps, and the characteristics of the two dehydrations are markedly different. Luger [19] indicates that basic sodalite (8:2:4) and hydrosodalite (6:0:8) can be dehydrated at 770 K under vacuum or air for one week and 12 h, respectively. Hydrosodalite is significantly easier to dehydrate, possibly because two of the eight S6Rs are vacant (no sodium). In somewhat contradictory statements, Felsche and Luger [6] also state that the same materials can be dehydrated at temperatures of 775, 863, or 920 K for 8:2:4 and 675 or 695 K for 6:0:8 sodalite. The DSC curves for the two hydrated end-members are illustrated in Fig. 4. The curves are similar in overall characteristics to those reported previously [6,19]. The 8:2:4 to 8:2:0 dehydration is a sluggish two-step process with broad
Table 1 Molar composition of the four sodalite samples examined in this study Sample ID
Ideal
Na
OH
Al
Si
O
H2O
Structural formula
Na8-1 Na8-2 Na6-1 Na6-2
8:2:4 8:2:4 6:0:8 6:0:8
7.60 7.82 6.20 6.13
1.64 1.84 0.21 0.14
5.96 5.98 5.99 5.99
6.04 6.02 6.01 6.01
24 24 24 24
3.00 3.27 8.04 7.97
Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
The two hydrated end-members are represented. The anhydrous phases are derived from these starting compounds, assuming complete dehydration, and no change in the framework structural chemistry (Na, OH, Al, Si).
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Table 2 Lattice parameters and refinement parameters for the four hydrated sodalite phases studied in this report using Jade (R) and Materials Studio (RWP) Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
a0
esd
R
8.8940 8.8940 8.8459 8.8407
0.0002 0.0002 0.0004 0.0005
12.28 11.64 19.77 23.73 RWP
Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
8.8901 8.8836 8.8514 8.8470
0.0005 0.0009 0.0006 0.0003
16.38 16.17 11.75 13.41
composition (Na2O)Na6(Al6Si6O24). This decomposition involves the hydroxyl decomposition reaction 2OH = O2 + H2O (gas), as originally reported by Schipper et al. [22]. Luger [19], who synthesized 8:2:4 sodalite via kaolinite, reported endothermic peaks at 420, 650, and 1020 K. As discussed by Schipper et al. [22], these peaks are highly sensitive to heating rates. We stress that the process is irreversible and kinetically controlled, and other factors, such as particle size, may play a role. These reactions do not represent equilibrium and no meaningful thermodynamic parameters can be derived from them. In contrast, the 6:0:8 to 6:0:0 dehydration is a much sharper, two-step process with endothermic peaks at approximately 464 and 535 K. These peaks are similar to 375 and 500 K reported by Luger [19]. The transition to Na6(Al6Si6O24) carnegieite is exothermic at about 1229 K, which compares to 1175 K reported by Luger [19]. Based upon the direct thermal analysis of the actual materials examined in this study, dehydration temperatures of 903 K and 673 K were chosen for 8:2:4 and 6:0:8 sodalite, respectively, as discussed in the synthesis procedure. 3.4. Calorimetry Calorimetric data are presented in Table 3. A thermodynamic cycle must be written to calculate the quantities of interest: enthalpies of formation, relative to both the constituent oxides (DHf-ox) and the elements (DHf-el), and hydration enthalpies (DHhyd). All reactions are written generally to accommodate the different stoichiometries that differentiate the eight different samples. The abbreviations s, d, l, and g stand for solid, dissolved (in the melt), liquid, and gas. TR stands for room temperature (298 K) and TC stands for the calorimeter temperature (975 K). The following reaction takes place when the zeolite samples are dropped into the calorimeter: Fig. 2. SEM images of (A) Na8(OH)2[Al6Si6O24](H2O)4, and (B) Na6[Al6Si6O24](H2O)8, and Na6[Al6Si6O24].
endothermic peaks at approximately 361 K and 839 K. The third endothermic peak at 990 K is associated with the decomposition of 8:2:0 sodalite to a carnegieite phase of
NaN Na ðOHÞN Na N Al ½AlN Al SiN Si O24 ðH2 OÞN H O ðsodalite; s; T R Þ 2
N Na N Al Na2 Oðd; T C Þ þ Al2 O3 ðd; T C Þ þ N Si SiO2 ðd; T C Þ ! 2 2 N OH H2 Oðg; T C Þ þ N H2 O þ DH ds ð1Þ 2
288
E.C. Moloy et al. / Microporous and Mesoporous Materials 88 (2006) 283–292 100% 90% 80%
Relative Intensity
70% 60% 50% 40% 30% 20% 10% 0% 10
20
30
A
40
50
60
70
50
60
70
Two-theta 100% 90% 80%
Relative Intensity
70% 60% 50% 40% 30% 20% 10% 0% 10
20
30
B
40
Two-theta 120% 110% 100%
Relative Intensity
90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 10
C
20
30
40
50
60
70
Two-theta
Fig. 3. XRD pattern for (A) Na8(OH)2[SOD](H2O)4 sodalite, (B) Na6[SOD](H2O)8, and (C) hydrothermal structural evolution during NaOH extraction for 0, 4, 5, 6, 7, 8, 9, and 10 days.
E.C. Moloy et al. / Microporous and Mesoporous Materials 88 (2006) 283–292
289
where DH ds ¼ enthalpy of drop solution measured in this study; N Na ð113:1 0:8Þ kJ=mol [12]; DH 2 ¼ 2 N Al DH 3 ¼ ð107:9 1:0Þ kJ=mol [12]; 2 DH 4 ¼ N Si ð39:1 0:3Þ kJ=mol [12]; N OH DH 5 ¼ N H2 O þ ð68:9 0:1Þ kJ=mol [23]. 2 For the anhydrous products, DH5 is only evaluated using the hydroxide content, as the water content is assumed to be zero. Enthalpy calculations relative to the elements (DHf-el) use the DHf-ox values above and the tabulated enthalpies of formation of the oxides from the elements [23]. N Na N Na O2 ðg; T R Þ ! Na2 Oðs; T R Þ 2 2 3N Al N Al O2 ðg; T R Þ ! Al2 O3 ðs; T R Þ N Al Alðs; T R Þ þ 2 2 N Si Siðs; T R Þ þ N Si O2 ðg; T R Þ ! N Si SiO2 ðs; T R Þ N H2 O þ N OH N OH 2 O2 ðg; T R Þ N H2 O þ H2 ðg; T R Þ þ 2 2 N OH ! N H2 O þ H2 Oðl; T R Þ 2
N Na Naðs; T R Þ þ
ð7Þ ð8Þ ð9Þ
ð10Þ
DH f -el ¼ DH f -ox þ DH 7 þ DH 8 þ DH 9 þ DH 10 where
Fig. 4. DSC curves for (A) 8:2:4 and (B) 6:0:8 sodalite.
The following component reactions (2)–(5) represent the enthalpies of drop solution of the stoichiometric number of moles of the component oxides. N Na N Na Na2 Oðs; T R Þ ! Na2 Oðd; T C Þ 2 2 N Al N Al Al2 O3 ðs; T R Þ ! Al2 O3 ðd; T C Þ 2 2 N Si SiO2 ðs; T R Þ ! N Si SiO2 ðd; T C Þ N OH N H2 O þ H2 Oðl; T R Þ 2 N OH ! N H2 O þ H2 Oðg; T C Þ 2
DH 2
ð2Þ
DH 3
ð3Þ
DH 4
ð4Þ
DH 5
ð5Þ
N Na ð414:82 0:28Þ kJ=mol; 2 N Al DH 8 ¼ ð1675:7 1:3Þ kJ=mol; 2 DH 9 ¼ N Si ð910:7 1:0Þ kJ=mol; N OH DH 10 ¼ N H2 O þ ð258:83 0:042Þ kJ=mol. 2 DH 7 ¼
DHf-ox values are all significantly negative, indicating the stability of the sodalite phases. The formation enthalpies are listed in Table 3A for all eight samples. The hydration enthalpy is evaluated from the enthalpies of drop solution of hydrated and dehydrated samples as NaN Na ðOHÞN OH ½AlN Al SiN Si O24
Then, for the formation reaction from the oxides at 298 K
ðdehydrated sodalite; s; T R Þ þ NH2 Oðl; T R Þ
N Na N Al Na2 Oðs; T R Þ þ Al2 O3 ðs; T R Þ þ N Si SiO2 ðs; T R Þ 2 2 N OH þ N H2 O þ H2 Oðl; T R Þ ! NaN Na ðOHÞN OH 2
! NaN Na ðOHÞN OH ½AlN Al SiN Si O24 ðH2 OÞN H O ðhydrated sodalite; s; T R Þ 2
The hydration enthalpy is then calculated as DH hyd ¼ DH ds;hydrated sodalite þ DH ds;dehydrated sodalite þ N DH H2 O
½AlN Al SiN Si O24 ðH2 OÞN H O ðSOD; s; T R Þ DH f -ox 2 ð6Þ DH f -ox ¼ DH ds þ DH 2 þ DH 3 þ DH 4 þ DH 5
ð11Þ
where the first two terms are measured enthalpies of drop solution and the last is the heat content of water, 68.9 kJ/ mol [23]. The hydration enthalpies are listed in Table 3B.
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Table 3 (A) Enthalpies of drop-solution (DHds), formation enthalpies from oxides (DHf-ox) and from elements (DHf-el), (B) hydration enthalpies per mol of sodalite (structural formula, DHhyd-SOD) and per mole of water ðDH hyd-H2 O Þ ID
Drops
DHds
DHf-ox
DHf-el
kJ/mol-SOD
kJ/mol-TO2
kJ/mol-SOD
kJ/mol-TO2
kJ/mol-SOD
kJ/mol-TO2
A Hydrous Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
9 9 9 10
1426.4 ± 10.7 1482.3 ± 11.8 1749.1 ± 11.9 1695.5 ± 16.2
118.9 ± 0.9 123.5 ± 1.0 145.8 ± 1.0 141.3 ± 1.4
1035.4 ± 11.7 1077.8 ± 12.7 980.6 ± 12.7 929.8 ± 16.8
86.3 ± 1.0 89.8 ± 1.1 81.7 ± 1.1 77.5 ± 1.4
14093.2 ± 13.8 14275.4 ± 14.6 14866.6 ± 14.6 14774.0 ± 18.3
1174.4 ± 1.1 1189.6 ± 1.2 1238.9 ± 1.2 1231.2 ± 1.5
Anhydrous Na7.60(OH)1.64[Al5.96Si6.04O24] Na7.82(OH)1.84[Al5.98Si6.02O24] Na6.20(OH)0.21[Al5.99Si6.01O24] Na6.13(OH)0.14[Al5.99Si6.01O24]
10 10 10 8
987.6 ± 12.2 1009.3 ± 12.4 919.1 ± 12.6 829.7 ± 13.2
82.3 ± 1.0 84.1 ± 1.0 76.6 ± 1.1 69.1 ± 1.1
803.1 ± 13.1 830.0 ± 13.3 704.3 ± 13.3 613.3 ± 13.8
66.9 ± 1.1 69.2 ± 1.1 58.7 ± 1.1 51.1 ± 1.2
13085.1 ± 15.0 13181.7 ± 15.1 12510.1 ± 15.2 12394.1 ± 15.6
1090.4 ± 1.3 1098.5 ± 1.3 1042.5 ± 1.3 1032.8 ± 1.3
DHhyd B Hydration Na7.60(OH)1.64[Al5.96Si6.04O24](H2O)3.00 Na7.82(OH)1.84[Al5.98Si6.02O24](H2O)3.27 Na6.20(OH)0.21[Al5.99Si6.01O24](H2O)8.04 Na6.13(OH)0.14[Al5.99Si6.01O24](H2O)7.97
kJ/mol-SOD
kJ/mol-H2O
232.3 ± 16.3 247.9 ± 17.1 276.3 ± 17.4 316.5 ± 20.9
77.5 ± 5.4 75.8 ± 5.2 34.4 ± 2.2 39.7 ± 2.6
Uncertainties of measured values are reported as two standard deviations of the mean. Uncertainty of the combined reactions was taken as the square root of the sum of the squares of uncertainty of each individual reaction.
These values fall within the range of values reported for zeolites in the literature [12,24,25]. 4. Discussion To begin an analysis of energetic data, the proper structures of, and environments around, the water molecules should be established for the two hydrated end-members. The number of guest molecules (water/hydroxide) that reside in the interior of each b-cages is determined by dividing the stoichiometric value of the particular species in the structural formula by two (two b-cage volumes per unit cell). Thus, for the ideal basic sodalite structure Na8(OH)2[SOD](H2O)4, one hydroxide and two waters reside in each b-cages, while for Na6[SOD](H2O)8, there are four water molecules. The same accounting method applies to sodium, an extra-framework species that resides in the centers of S6Rs. While there is some displacement of these ions from the S6Rs, they are essentially localized in cages and not as mobile as water and hydroxide. From the vantage point of the centers of the b-cages, an important position by which to describe the structure of the extra-framework and guest species, sodium ions appear in all eight S6Rs. As depicted in Fig. 5, however, only four of the eight sodium ions belong to any particular b-cage. Four of the eight sodium ions are displaced out of the S6Rs toward the centers of the b-cages. The other four sodium ions move toward the centers of the adjoining b-cages, producing a similar relationship of four in and four out with respect to the b-cage center. The four sodium cations then coordinate with what-
ever content of water and hydroxide molecules reside in the b-cage, forming an interwoven, ditetrahedral arrangement [6,20,26–28]. One tetrahedron is defined by the sodium ions, and the second is defined by the water/hydroxide molecules. Due to different amounts of both sodium and water/hydroxide inside the b-cages according to the different structural formulas for each end-member, the tetrahedra often have only three of the four sites occupied. XRD data [6] suggests, however, that all sites are occupied by a statistical distribution of available ions, so there is no long range ordering of the vacancies. The extent of the displacement of the sodium ions out of the S6Rs is an area of active research and beyond the scope of this paper, but such displacement varies from structure to structure according to the number of sodium ions and guest molecules. Anhydrous sodalite is thought to not have significant sodium displacement out of the S6Rs [29]. The various displacements are shown schematically in Fig. 5. There are three basic interactions to consider when evaluating the energetics: (1) hydrogen bonding between water/ water and water/hydroxide, (2) hydrogen bonding between water and framework oxygens, and (3) the reduction of Na+–Na+ charge repulsion by positioning water (oxygen) between any two sodium atoms. The Na8 materials are more ideal than the Na6 materials because of the symmetric distribution of sodium cations in all of the S6Rs, while in Na6, two of the eight S6Rs are vacant. However, Felsche and Luger [6,20] have indicated that there is no evidence of hydrogen bonding between water molecules and framework oxygen atoms in basic sodalite. Thus, the dominant interactions are water/water,
E.C. Moloy et al. / Microporous and Mesoporous Materials 88 (2006) 283–292
Na
O Na
Na
O
H O
H Na
H-bonding in Na6. Hydrogen bonding, for example, is thought to produce the antagonistic behavior in the unit cell volumes of hydrosodalite: the unit cell volume decreases with increasing water content (8% for 0–8 waters). Thus, there are competing interactions; 25% less electrostatic interactions (one less sodium), no hydroxide ions (one in basic sodalite), but two more water molecules that H-bond with the framework. Therefore, the relatively similar total hydration enthalpies for Na6 and Na8 seem reasonable.
B
Na
O Na
O
Na
C
H Na
H
A
O
Na
O Na
O
Na
H
291
Na Na
Na
O Na
Na D
Fig. 5. Schematic drawings of Na, H2O, and OH configurations in the four ideal end-members studied in this report. The cubes that are drawn in dashed lines represent a cube formed by the sites that the sodium ions occupy in the b-cage. Gray molecules for Na, O and H represent additional sites that the particular atoms/molecules (with occupancies of less than one) can reside statistically. While hydroxide ions are identified explicitly, water hydrogen atoms are omitted for clarity. (A) Na8(OH)2 [SOD](H2O)4: Two water molecules and one hydroxide ion are statistically distributed on the four vacant corners not occupied by sodium. (B) Na8(OH)2[SOD]: One hydroxide occupies each b-cage, where the oxygen is in the center and the orientation varies statistically with the hydrogen pointing to the vacant corners. (C) Na6[SOD](H2O)8: three sodium cations are even distributed on four sites. (D) Na6[SOD]: same as (C) but without the water oxygens.
hydrogen bonding and electrostatic interactions. In the hydrosodalite structure (Fig. 5C), in comparison to basic sodalite, there is one less sodium cation, one less hydroxide ion, and two extra, charge neutral water molecules. It is not surprising, therefore, that significant differences between the hydration enthalpies (37.0 kJ/mol-H2O for Na6 versus 76.7 kJ/mol-H2O for Na8) are observed. The observation that the Na8 waters are more exothermically bonded than the Na6 waters is fully consistent with our structural analysis: one more sodium cation, one more hydroxide ion, fewer water molecules, and one less guest species overall in the Na8 than in Na6 structures. What may be a little curious is that the total hydration enthalpies per formula unit of the zeolite vary by only about 36% (from 232.3 to 316.5 kJ/mol) while the enthalpies of hydration per water molecule vary by about a factor of two. The material with less water per formula, Na8, holds those water molecules more tightly (more exothermic enthalpy per mole of H2O) but has the less negative overall hydration enthalpy. While there is no reported hydrogen bonding in basic sodalite, there is significant
5. Conclusions The four end-member structures of the hydrosodalite family of materials were successfully synthesized and characterized. Oxide melt solution calorimetry provided formation enthalpies and hydration enthalpies for the Na8 and Na6 series were also calculated from the calorimetric data. Both the formation enthalpies and the hydration enthalpies are comparable to values published for other sodium zeolites. A structural analysis of the two series of materials explains their differences in enthalpies. Acknowledgements We thank May Nyman for advice about synthesis and Miaojun Wang for taking the SEM photographs. This work was supported by the US Department of Energy EMSP Program, grant DE-FG07-01ER63298. References [1] C. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, fifth ed., Elsevier, International Zeolite Association, Amsterdam, 2001. [2] C. Weber, Oak Ridge National Laboratory (UT-Batelle) Report: ORNL/TM-2001/109, 2001. [3] B.R. Bickmore, K.L. Nagy, J.S. Young, J.W. Drexler, Environ. Sci. Technol. 35 (2001) 4481. [4] L. Pauling, Zeitsch. Kristallogr. 74 (1930) 213. [5] B.J. Campbell, J.M. Delgado, A.K. Cheetham, B.B. Iversen, N.P. Blake, S.R. Shannon, S. Latturner, G.D. Stucky, J. Chem. Phys. 113 (2000) 10226. [6] J. Felsche, S. Luger, Thermochim. Acta 118 (1987) 35. [7] M.E. Brenchley, M.T. Weller, Zeolites 14 (1994) 682. [8] P.J. Mead, M.T. Weller, Zeolites 15 (1995) 561. [9] N.C. Nielsen, H. Bildsøe, H.J. Jakobsen, P. Norby, Zeolites 11 (1991) 622. [10] A. Navrotsky, Phys. Chem. Miner. 2 (1977) 89. [11] A. Navrotsky, Phys. Chem. Miner. 24 (1997) 222. [12] I. Kiseleva, A. Navrotsky, I.A. Belitsky, B.A. Fursenko, Am. Miner. 81 (1996) 668. [13] S. Yang, A. Navrotsky, Micropor. Mesopor. Mater. 37 (2000) 175. [14] Q. Liu, H. Xu, A. Navrotsky, Micropor. Mesopor. Mater., doi:10. 1016/j.micromeso.2005.08.008. [15] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [16] P.E. Werner, L. Eriksson, M. Westdahl, J. Appl. Cryst. 18 (1985) 367. [17] A.J. Markvardsen, W.I.F. David, J.C. Johnson, K. Shankland, Acta Crystallogr. A 57 (2001) 47. [18] G.E. Engel, S. Wilke, O. Ko¨nig, K.D.M. Harris, F.J.J. Leusen, J. Appl. Cryst. 32 (1999) 1169.
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