Crystallographic configurations of water molecules and exchangeable cations in a hydrated natural CHA-zeolite (chabazite)

Microporous and Mesoporous Materials 102 (2007) 188–195 www.elsevier.com/locate/micromeso

Crystallographic configurations of water molecules and exchangeable cations in a hydrated natural CHA-zeolite (chabazite) Akihiko Nakatsuka *, Hironao Okada, Keiko Fujiwara, Noriaki Nakayama, Tadato Mizota Graduate School of Science and Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan Received 7 March 2006; received in revised form 15 December 2006; accepted 18 December 2006 Available online 23 December 2006

Abstract Crystallographic configurations of water molecules and exchangeable cations in a hydrated natural CHA-zeolite (chabazite) with a composition of (Ca1.57Na0.49)(Al3.39Si8.55)O24 Æ 12.47H2O have been examined from the structure analyses based on single-crystal X-ray diffraction data. The structure analyses have revealed the presence of five partially occupied water sites (OW1–OW5) and four partially occupied exchangeable-cation sites (Ca1–Ca4), and the final structure refinement converged to R = 0.0283 and Rw = 0.0276 for 1255 reflections. Of these sites, the Ca4 and OW2 sites are located at the center of the [4662]-ring and the center of the 8-ring window in the [4126286]-cavity, respectively, and the remaining sites are inside the [4126286]-cavity. The Ca1 atoms can be coordinated only by water molecules, and fully coordinated Ca1 atoms can adopt an octahedral coordination or a triangular-prismatic coordination depending on configurations of water molecules. The Ca2 and Ca3 atoms can be coordinated by both framework oxygen atoms and water molecules, and fully coordinated Ca2 and Ca3 atoms can adopt a sevenfold coordination and an octahedral coordination, respectively. The Ca4 atoms are coordinated octahedrally by six framework oxygen atoms. Judging from OW  O and OW  OW distances, the hydrogen bonds formed between water molecules and framework oxygen atoms are expected to be very weak, whereas strong hydrogen bonds can exist between water molecules.  2007 Elsevier Inc. All rights reserved. Keywords: Zeolite; Chabazite; Crystal structure; Single-crystal X-ray diffraction; Hydrogen bond

1. Introduction Zeolites are widely used in various applications because of their excellent abilities as molecular sieve, catalysts, absorbents, ion-exchangers and heat-exchangers. In particular, zeolites with high water content and high hydration enthalpy, such as Mg exchanged Na-A-type zeolite [1–4] and CHA-zeolite (chabazite) [5,6], are promising heat absorbents for the zeolite–water heat pump system. The heat-exchange abilities of zeolites are closely related to their thermodynamic hydration–dehydration process. Therefore, the determination of locations and states of water molecules in zeolites with high heat-exchange abili-

*

Corresponding author. Tel.: +81 836 85 9651; fax: +81 836 85 9601. E-mail address: [email protected] (A. Nakatsuka).

1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.12.032

ties, such as chabazite, is quite important for elucidating their properties as heat absorbents for the heat pump system. The idealized chemical formula of chabazite is expressed as Ca2(Al4Si8)O24 Æ 12H2O and its aluminosilicate-framework structure was established by Dent and Smith [7]. In naturally occurring chabazites, some of Ca2+ ions are often substituted mainly by Na+ ions, which results in Si/Al ratio more than 2 to maintain charge-balance. The structure of aluminosilicate framework in chabazite is built up of double 6-rings ([4662]-rings), and consequently one [4126286]cavity per unit cell is formed (Fig. 1). Each cavity is connected to six neighboring [4126286]-cavities by sharing 8-ring windows and filled with water molecules and exchangeable cations such as Ca2+ and Na+. The occupancies of water sites and exchangeable-cation sites are usually less than unity and these are statistically distributed on

A. Nakatsuka et al. / Microporous and Mesoporous Materials 102 (2007) 188–195

Fig. 1. Framework structure drawn by linking (Si, Al) positions denoted with circles. Shaded quadrilaterals represent the 4-rings attaching the [4662]-rings, four of which are shown in the figure.

189

Table 1 Crystallographic data, data-collection and refinement parameters ˚) a (A 9.3943(8) a () 94.369(7) ˚ 3) 821.5(1) V (A Crystal system Rhombohedral Space group R3m Crystal size (mm, diameter) 0.160 Radiation used MoKa Diffractometer Rigaku AFC-7R Monochromator Graphite Scan type x–2h 2h range () 2–80 Number of measured reflections 10,574 Number of observed independent reflections with 1255 jFoj P 3r(jFoj) after averaging equivalent reflections Rint 0.0302 R 0.0283 Rw 0.0276 Weighting scheme 1/r2(jFoj)

2. Experimental

AFC-7R) with the graphite-monochromatized Mo Ka ˚ ) radiation, and the operating conditions (k = 0.71069 A were 50 kV and 200 mA. The unit-cell parameters were determined by the least-squares method from a set of 26 reflections within the range of 40 6 2h 6 52. Intensity data of reflections within 2 6 2h 6 80, corresponding to 0.02 6 sin h/k 6 0.904, with 16 6 h, l 6 16 and 0 6 k 6 16 were collected by continuous x–2h scan mode and corrected for Lorentz-polarization factors and spherical absorption effects (lr = 0.064 for Mo Ka). A total of 10,574 reflections were measured and averaged in Laue symmetry 3m to give 1922 independent reflections, of which 1255 observed independent reflections with jFoj P 3r(jFoj) were used in the structure refinements. Internal residual of the equivalent reflections (Rint in Table 1) was 0.0302.

2.1. Chemical analysis

3. Structure analyses

Single crystals of a natural chabazite from Komuro, Ohhito, Shizuoka Prefecture, Japan, were used in the present study. The cationic contents in the crystals were analyzed using an atomic absorption spectrophotometer (Hitachi Z-5310), and the water content was determined by TG analysis with heating to 800 C (MAC Science TGDTA2000S). The analytical results gave a composition of 8.64 wt% CaO, 1.50 wt% Na2O, 17.00 wt% Al2O3, 50.52 wt% SiO2 and 22.10 wt% H2O with a total of 99.76 wt%, and thus the chemical formula was calculated to be (Ca1.57Na0.49)(Al3.39Si8.55)O24 Æ 12.47H2O.

The structure refinements were carried out by minimizP ing the residual factor w(jFoj  jFcj)2 using a full-matrix least-squares program RADY [23], where w = 1/r2(jFoj). In the structure refinements, scattering factors of Al3+ and Si4+ [24] were used for cations in the aluminosilicate framework and those of O2 [25] were used for oxygen atoms in the framework and water molecules. Scattering factors for Ca2+ and Na+ were also taken from Ref. [24], but the averaged scattering factors based on the Ca2+/ Na+ ratio determined by the chemical analysis, 0.762f (Ca2+) + 0.238f(Na+), were used for the exchangeable cations. The correction terms for anomalous dispersion of each element were taken from Ref. [24] as well. The positions of water molecules and exchangeable cations were found from difference Fourier syntheses, but positions of hydrogen atoms in water molecules were not determined. In the structure refinements, the occupancies of Si4+ and Al3+, belonging to the aluminosilicate framework, were fixed at the values obtained from the chemical analysis.

each site. Such an occupation mode makes site assignments of water molecules and exchangeable cations difficult. Because of this, a consensus on the positions of water molecules and exchangeable cations in chabazites has not been obtained to date, although attempts to determine their positions have been made intensively (e.g. [8–22]). In the present study, we try to reliably determine sites of water molecules and exchangeable cations present in a hydrated natural chabazite by single-crystal X-ray diffraction, and discuss their crystallographic configurations on the basis of the resulting data.

2.2. Measurements of single-crystal X-ray diffraction intensity For X-ray diffraction intensity measurements, a selected single crystal was ground into a sphere of 0.16 mm in diameter. The measurements were carried out at room temperature using a four circle diffractometer (Rigaku

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For all the atomic sites included in the refinements, an anisotropic thermal motion model was applied to the displacement parameters. A correction for isotropic extinction effects (Type I) [26,27] was performed during the structure refinements. The detailed procedure of the structure analyses is as follows: Step I: First, only the structure parameters of atoms belonging to the aluminosilicate framework were refined. The initial values of their atomic positions used here were those reported by Ref. [17]. The difference Fourier synthesis was carried out on the basis of Fc calculated by this refinement. At this stage, crystallographically independent seven electron-density peaks corresponding to exchangeable cations or water molecules were found on the difference Fourier map. Step II: The least-squares refinements were carried out by preliminary assigning exchangeable cations or water molecules to each of the electron-density peaks found in step I. The difference Fourier synthesis was performed again on the basis of Fc calculated by this refinement. Consequently, crystallographically independent two new electron-density peaks corresponding to exchangeable cations or water molecules were found. Step III: To correctly assign exchangeable cations or water molecules to each of nine electron-density peaks found in steps I and II, the least-squares refinements were repeated with modifying the atomic assignments to these peaks until the chemical composition obtained from the refined occupancies agreed with that from the chemical analysis. Consequently, our structure analyses revealed the presence of four sites partially occupied by Ca2+ and Na+ (Ca1–Ca4) and five sites partially occupied by water molecules (OW1–OW5). The resulting occupancies of

these nine sites provided a composition of (Ca1.50Na0.47)(Al3.39Si8.55)O24 Æ 12.46H2O, which agrees excellently with the composition, (Ca1.57Na0.49)(Al3.39Si8.55)O24 Æ 12.47H2O, from the chemical analysis. Finally, the occupancies of the water sites and exchangeable-cation sites were constrained on the basis of the composition from the chemical analysis. The final refinement converged to R = 0.0283 and Rw = 0.0276, which are the lowest of reliability indices reported on chabazites, for 1255 reflections. The crystallographic data, data-collection and refinement parameters are given in Table 1. The refined structural parameters are given in Tables 2 and 3. The selected interatomic distances are listed in Table 4. 4. Results and discussion 4.1. Locations of water molecules The water sites OW1, OW3, OW4 and OW5 are inside the [4126286]-cavity. The OW1 site has 12 equivalent positions per unit cell, and they form the centrosymmetric ˚ (Fig. 2a). In each 6-ring two 6-rings separated by 4.98(4) A of OW1, there are two kinds of too short OW1  OW1 dis˚ and 1.88(5) A ˚ ] as shown in Fig. 2b, and tances [2.13(5) A simultaneous incorporation of water molecules into the equivalent positions separated by these distances is not likely because of electrostatic repulsion between water molecules. When OW1 site is occupied by water molecules to the maximum, only the configuration of the occupied posi˚ such as in Fig. 2b is possible. tions separated by 3.48(4) A The similar situation is also observed in OW5 site (Fig. 2c). Thus, the maximum allowance for occupancies of OW1 and OW5 sites should be 0.5; indeed, the occupancies of both sites obtained in the present study are lower than this maximum allowance (Table 2). Moreover, each OW5

Table 2 Refined positional, occupancy, and equivalent isotropic displacement parameters Atoms

Occupancy

x

y

z

˚ 2) Beq (A

Si/Al (12i) O1 (6f) O2 (6g) O3 (6h) O4 (6h) OW1 (12i) OW2 (3e) OW3 (2c) OW4 (6h) OW5 (6g) Ca1 (2c) Ca2 (6h) Ca3 (2c) Ca4 (1a)

0.713/0.283 1.0 1.0 1.0 1.0 0.345(16) 0.759(15) 0.555(27) 0.418(32) 0.406a 0.385a (Ca: 0.293, Na: 0.092) 0.146(4) (Ca: 0.111, Na: 0.035) 0.173(15) (Ca: 0.132, Na: 0.041) 0.068(11) (Ca: 0.052, Na: 0.016)

0.10460(4) 0.26125(12) 0.15197(11) 0.25347(13) 0.02591(14) 0.809(2) 0.5 0.2538(10) 0.311(4) 0.3643(10) 0.4070(3) 0.5831(3) 0.2089(17) 0

0.33312(3) 0.26125 0.15197 0.25347 0.02591 0.672(3) 0.5 0.2538 0.311 0.3643 0.4070 0.5831 0.2089 0

0.87654(3) 0 0.5 0.89240(21) 0.32385(18) 0.517(3) 0 0.2538 0.561(3) 0.5 0.4070 0.2311(6) 0.2089 0

0.968(3) 2.842(9) 2.245(7) 2.731(10) 2.606(9) 25.8(5) 9.96(12) 5.83(3) 28.5(4) 31.3(5) 6.477(9) 3.84(4) 4.64(4) 13.4(3)

a These values were calculated under the following constraints: P(OW5) = {12.47  12P(OW1)  3P(OW2)  2P(OW3)  6P(OW4)}/6 and P(Ca1) = {2.06  6P(Ca2)  2P(Ca3)  P(Ca4)}/2, where P(i) is the occupancy of the atom i.

A. Nakatsuka et al. / Microporous and Mesoporous Materials 102 (2007) 188–195

191

Table 3 ˚ 2) Refined anisotropic displacement parameters (A Atoms

U11

U22

U33

U12

U13

U23

Si/Al O1 O2 O3 O4 OW1 OW2 OW3 OW4 OW5 Ca1 Ca2 Ca3 Ca4

0.01118(12) 0.0306(5) 0.0356(5) 0.0227(4) 0.0313(5) 0.23(2) 0.102(10) 0.070(4) 0.51(4) 0.42(4) 0.079(1) 0.040(2) 0.061(6) 0.16(4)

0.00953(12) 0.0306 0.0356 0.0227 0.0313 0.42(3) 0.133(8) 0.070 0.51 0.42 0.079 0.040 0.061 0.16

0.01586(14) 0.0417(9) 0.0149(5) 0.0594(11) 0.0388(9) 0.29(2) 0.133 0.070 0.10(1) 0.34(3) 0.079 0.064(4) 0.061 0.16

0.00102(8) 0.0029(6) 0.0076(6) 0.0084(5) 0.0131(6) 0.24(2) 0.044(5) 0.018(4) 0.01(5) 0.36(4) 0.015(1) 0.017(2) 0.022(5) 0.04(2)

0.00117(9) 0.0107(5) 0.0022(4) 0.0034(5) 0.0057(5) 0.06(1) 0.044 0.018 0.13(2) 0.19(3) 0.015 0.008(1) 0.022 0.04

0.00056(8) 0.0107 0.0022 0.0034 0.0057 0.01(2) 0.044(8) 0.018 0.13 0.19 0.015 0.008 0.022 0.04

Table 4 ˚ ) less than 3.20 A ˚ Interatomic distances (A (Si, (Si, (Si, (Si,

Al)–O1 Al)–O2 Al)–O3 Al)–O4

1.6337(10) 1.6442(5) 1.6379(13) 1.6401(15)

Ca1–OW1 Ca1–OW3 Ca1–OW4 Ca1–OW5

2.29(2) · 6 2.295(9) 2.00(3)a · 3 2.332(10) · 6

Ca2–O2 Ca2–O3 Ca2–OW1 Ca2–OW2 Ca2–OW4 Ca2–OW4 Ca2–OW5

2.801(4) · 2 2.499(5) 2.50(2) · 2 2.307(6) 2.25(3) 2.72(4) · 2 1.592(10)a · 2

Ca3–O3 Ca3–O4 Ca3–OW1 Ca3–OW3

3.089(16) · 3 2.682(16) · 3 2.75(3) · 6 0.673(17)a

Ca4–O4

3.0231(17) · 6

a

OW1  O1 OW1  OW1 OW1  OW1 OW1  OW2 OW1  OW3 OW1  OW4 OW1  OW4 OW1  OW5 OW1  OW5 OW1  OW5

3.18(2) 1.88(5)a 2.13(5)a 2.51(2) 2.34(3)a 1.32(4)a 3.02(4) 3.20(3) 2.49(2) 3.12(2)

OW2  O3 OW2  OW1

3.1967(12) · 2 2.51(2) · 4

OW3  O4 OW3  OW1 OW3  OW4

3.055(9) · 3 2.34(3)a · 6 2.89(3) · 3

OW4  OW1 OW4  OW1 OW4  OW3 OW4  OW5 OW4  OW5

1.32(4)a · 2 3.02(4) · 2 2.89(3) 1.88(4)a · 2 3.16(4) · 2

OW5  O2 OW5  OW1 OW5  OW1 OW5  OW1 OW5  OW4 OW5  OW4 OW5  OW5

2.927(10) 2.49(2) · 2 3.12(2) · 2 3.20(3) · 2 1.88(4)a · 2 3.16(4) · 2 1.870(14)a · 2

These short distances are due to partial occupations of water molecules and exchangeable cations on each site.

˚ away from the two nearest position is located 1.592(10) A Ca2 positions. Because of this too short distance, if an OW5 position is occupied, its nearest Ca2 positions must be unoccupied. Consequently, when OW5 site is occupied to the maximum allowance in a [4126286]-cavity, Ca2 site must be completely vacant in its cavity. The OW3 site is located between the 6-ring window, corresponding to the shared face between [4126286]-cavity and [4662]-ring, and the 6-ring of OW1 positions (Fig. 2a). This site has two equivalent positions per unit cell. Each of these positions is separated from the six nearest OW1 positions ˚ , much shorter than the minimum allowed by only 2.34(3) A ˚ [28]) involved in hydrogen bonding, O  O distance (2.39 A and furthermore is present in close proximity to the nearest ˚ ]. Therefore, if an Ca3 position [Ca3–OW3 = 0.673(17) A OW3 position is occupied, its nearest OW1 and Ca3 posi-

tions must be unoccupied. Consequently, when OW3 site is fully occupied in a [4126286]-cavity, both OW1 and Ca3 sites must be completely vacant in its cavity. The OW4 site is located between the 6-ring of OW1 positions and that of OW5 positions. This site has six equivalent positions per unit cell, and they form the centro˚ (Fig. 2a). symmetric two 3-rings separated by 3.74(6) A Each OW4 position is located at 1.32(4), 1.88(4) and ˚ from the two nearest OW1, the two nearest 2.00(3) A OW5 and the one nearest Ca1 positions (Table 4), respectively. It is evident from these too short distances that if an OW4 position is occupied, the occupation of its nearest OW1, OW5 and Ca1 positions must be ruled out. Consequently, when OW4 site is fully occupied in a [4126286]-cavity, OW1, OW5 and Ca1 sites must be completely vacant in its cavity.

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A. Nakatsuka et al. / Microporous and Mesoporous Materials 102 (2007) 188–195

these distances are enough long, the incorporation of water molecules into OW2 site cannot be restricted by crystallographic configurations of water molecules and exchangeable cations in other sites. Indeed, the OW2 site has the highest occupancy of 0.759(15) among the water sites (Table 2). Such relatively high occupancies in this site were also observed in the previous studies (e.g. [10–17]) and, in particular, some of these studies reported chabazites with fully occupied OW2 sites (e.g. [11–13]).

OW3 OW2

OW1

OW4 OW5

OW4

4.2. Coordination environments of exchangeable cations OW1

OW2

The exchangeable-cation sites Ca1, Ca2 and Ca3 are inside the [4126286]-cavity. The Ca1 site is located at the center of polyhedron formed by OW1 and OW5 watermolecules. When Ca1 atom is fully coordinated by these

OW3

1.88 A

3.48 A

A

2A

3.12

8A

3.20

A

3.1

12

20

3.

2.13 A

0A

Ca1

3.

1.8

8A

OW1

3.2

3.48 A

8A

1.8

8A

3.4

3.4

3.48 A

A

8A

3.48 A

2.1

3.4

3.48 A

3A

3.4

2.1

3A

OW1

8A

A 3.24 A

3.24 A

1.87 A

3.24 A

3.48 A

A

OW1

A 7A

7A

4A

OW5

3.48 A

1.8

1.8

3.2

4 3.2

OW5

3.48

4A

3.24 A

1.8

3.2

7A

4A

3.2

1.8

7A

3.24 A

3.24 A

The remaining one water site (OW2) is located at the center of the 8-ring window of [4126286]-cavity. Each equivalent position in this site is located at 2.51(2), 4.153(9), 3.56(4), 3.560(10), 4.129(3), 2.307(6), 4.394(16) and ˚ from the nearest OW1, OW3, OW4, OW5, 6.3847(6) A Ca1, Ca2, Ca3 and Ca4 positions, respectively. Because

3.24 A

2.49 A

2.49 A

Ca1

3.24 A

Fig. 2. (a) Arrangements of equivalent positions of water sites (OW1– OW5), and possible occupied positions on (b) OW1 (black circles) and (c) OW5 (gray circles) sites in the case that these sites are occupied to the maximum. Open circles with broken lines represent vacant positions.

2.49 A

1.87 A

OW5 Fig. 3. Possible coordination polyhedra of Ca1 atom: (a) octahedron; (b) triangular prism. Open circles, black circles and gray circles represent Ca1, OW1 and OW5, respectively.

A. Nakatsuka et al. / Microporous and Mesoporous Materials 102 (2007) 188–195

water-molecules, it can be coordinated by three OW1 ˚ and three water-molecules with Ca1–OW1 = 2.29(2) A ˚ . In OW5 water-molecules with Ca1–OW5 = 2.332(10) A this case, the formation of two types of coordination polyhedra, i.e. an octahedron (Fig. 3a) or a triangular prism (Fig. 3b), is possible depending on configurations of OW1 and OW5 water-molecules. The Ca2 site is located near the 8-ring window of [4126286]-cavity. Each equivalent position in this site is surrounded by two O2, one O3, two OW1, one OW2 and three OW4 positions with the acceptable distances for (Ca, Na)– O bonding (Table 4), but there are too short OW1  OW1 ˚ ] and OW1  OW4 [= 1.32(4) A ˚ ] distances as [= 2.13(5) A shown in Fig. 4. Simultaneous incorporation of water molecules into the positions separated by these distances must be ruled out as described above. Consequently, fully coordinated Ca2 atom can be sevenfold coordinated by two ˚ , or, OW4 water-molecules with Ca2–OW4 = 2.72(4) A ˚ and one OW1 water-molecule with Ca2–OW1 = 2.50(2) A ˚ , in one OW4 water-molecule with Ca2–OW4 = 2.72(4) A ˚, addition to two O2 atoms with Ca2–O2 = 2.801(4) A ˚ , one OW2 one O3 atom with Ca2–O3 = 2.499(5) A ˚ and one water-molecule with Ca2–OW2 = 2.307(6) A ˚. OW4 water-molecule with Ca2–OW4 = 2.25(3) A The Ca3 site is located between the 6-ring window and the 6-ring of OW1 positions. This site was also reported in some dehydrated chabazites (e.g. [15–17,19,20,22]). When we consider the possible configurations of OW1 water-molecules discussed above, fully coordinated Ca3 atom can be coordinate octahedrally by three O4 atoms in the 6-ring ˚ and three OW1 window with Ca3–O4 = 2.682(16) A ˚ (Table 4). water-molecules with Ca3–OW1 = 2.75(3) A

OW4

O3 O2 O2

Ca2

OW4

OW4

2A

OW1

3.0

A

2A

32

1.3

1.

3.44 A 3.0 2A

OW1 2.13 A

OW2

Fig. 4. Configuration of water positions and framework oxygen positions surrounding Ca2 atom. Water molecules cannot be simultaneously ˚ incorporated into the positions separated by OW1  OW1 = 2.13(5) A ˚. and OW1  OW4 = 1.32(4) A

193

The remaining one exchangeable-cation site (Ca4) is located at the center of [4662]-ring. The previous studies (e.g. [13–15]) have already interpreted this site in hydrated chabazites as an exchangeable-cation site from the following reasons: (1) it is octahedrally coordinated by six framework oxygen atoms (O4); (2) it was also found in some fully dehydrated chabazites (e.g. [15–22]). The mean Ca4– ˚ (Table O4 distance in the present sample is 3.0231(17) A 4), much longer than the expected Ca–O or Na–O distance ˚ ) [29]. However, this distance is probably apparent (2.4 A because Ca4 site has a considerably low occupancy of 0.068(11) (Table 2), and actual Ca4–O4 distance, i.e. the distance between occupied Ca4 position and O4 atom, is probably closer to the expected value. From the coordination environments of the exchangeable cations described above, we can now gain an insight into the ion-exchange properties of chabazite. Bonds between exchangeable cations and water molecules are based on ion–dipole interaction, and the attractive forces between both are thus much weaker than those between exchangeable cations and framework oxygen atoms. Moreover, the size of the 8-ring window in [4126286]-cavity (max˚ ) is much larger than that of the 6-ring imum 8.12 A ˚ ). From these conwindow in [4662]-ring (maximum 6.29 A siderations, it is inferred that the ion-exchange ability of Ca1 cations, coordinated only by water molecules and located inside the [4126286]-cavity, is the highest and that of Ca4 cations, coordinated only by framework oxygen atoms and located inside the [4662]-ring, is the lowest. 4.3. Displacement parameters of water sites Of the water sites, OW1, OW4 and OW5 have the remarkably large displacement parameters of Beq = 25.8(5), 28.5(4) and 31.3(5) (Table 2), respectively. We now give an interpretation for these remarkably large displacement parameters on the basis of the crystallographic configurations of water molecules and exchangeable cations. When fully coordinated Ca1 and Ca3 atoms coexist in a [4126286]-cavity, a Ca1-octahedron or -triangular prism shares the face consisting of OW1  OW1 edges with the adjacent Ca3-octahedron and/or the face consisting of OW5  OW5 edges with the other Ca1-octahedron or -triangular prism in its cavity (Fig. 5a and b). When fully coordinated Ca2 atoms occupy the nearest equivalent positions of Ca2 site, Ca2 polyhedra share the faces consisting of one OW4  OW4 and two OW4  O2 edges with each other (Fig. 5c and d). These shared-edge lengths were deter˚ , OW5  OW5 = mined as OW1  OW1 = 3.48(4) A ˚ ˚ 3.239(17) A, OW4  OW4 = 3.74(6) A and OW4  O2 = ˚ by the present structure analysis. In fact, when 3.60(3) A adjacent polyhedra occupied by the exchangeable cations exist in a [4126286]-cavity, the shared-edge lengths should be much shorter than the corresponding edge-lengths when not, in order to shield cation–cation repulsions between adjacent polyhedra. Consequently, actual positions of the OW1, OW4 and OW5 water-molecules will differ largely

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A. Nakatsuka et al. / Microporous and Mesoporous Materials 102 (2007) 188–195 O4

O4

O4

O4 O4

O4

Ca4

Ca4

O4

O4 O4

O4

Ca3

Ca3

OW1

O1

O4

O4

O1

OW1

Ca1

O1

OW1

O1 OW1 OW1

OW1 O1

Ca1

O1

O2 OW5

O2

OW5 Ca1

O2

OW5

O2

OW5

O2

OW5

OW5

Ca1

O2

O1

OW1

O1

O1 OW1

OW1

O1

OW1

O1

OW1

O4 O4

O4

O4

O4

O4

O4

Ca4

O4

O4

O4

O4

O4

O3

O3

OW3

OW3

OW2

OW2

OW4 O3

O2

O2

Ca2

O3

O3 O2

O3

O2

O2

Ca2

Ca2

OW4

O2

Ca2

O2

O3

Ca2

Ca2

OW4 OW4

OW2

OW2

OW4

OW2

OW2

OW3

O3

O3 O4

OW1

OW1

OW3

O4

OW4

OW4

O3 O2

O4

O4

Ca4

O4

OW4

O1

O4

O4

O4

OW1

Ca3

Ca3

O4

O3

O3 O4

O4

O4

Fig. 5. Locations of the water molecules and the exchangeable cations in the crystal structure of the present chabazite: (a) a possible configuration of Ca1-, Ca3- and Ca4-polyhedra when Ca1 atom is octahedrally coordinated; (b) a possible configuration of Ca1-, Ca3- and Ca4-polyhedra when Ca1 atom is triangular-prismatically coordinated; (c) a possible configuration of Ca2- and Ca4-polyhedra when Ca2 atom is not coordinated by OW1 water-molecule; (d) a possible configuration of Ca2- and Ca4-polyhedra when Ca2 atom is coordinated by OW1 water-molecule. In (c) and (d), three of six Ca2-polyhedra in the unit cell are drawn. Atoms are represented as displacement ellipsoids, drawn at 10% probability level. Broken lines represent OW  OW and ˚ , which are acceptable O  O distances for hydrogen bonding. OW  O separations within 2.4–3.2 A

between both situations. Such positional disorders of OW1, OW4 and OW5 water-molecules are suggested as one of reasons for the remarkably large displacement parameters

of these water sites. Indeed, the OW2 and OW3 sites, which are not under the circumstances, have relatively small Beq values (Table 2).

A. Nakatsuka et al. / Microporous and Mesoporous Materials 102 (2007) 188–195

4.4. Possible hydrogen bonds In general, O  O distances related to hydrogen bonding ˚ . As shown in are in the range of approximately 2.4–3.2 A Table 4 and Fig. 5, the separations between the water sites and the framework oxygen atoms within this range are ˚ ], OW2  O3 [= 3.1967(12) A ˚ ], OW1  O1 [= 3.18(2) A ˚ ˚ OW3  O4 [= 3.055(9) A] and OW5  O2 [= 2.927(10) A]. These distances are close to the upper limit to form hydrogen bonding, which suggests that all the hydrogen bonds formed between the water molecules and the framework oxygen atoms are very weak. On the other hand, of OW  OW separations with the acceptable distances (2.4– ˚ ) for hydrogen bonding (Table 4), strong hydrogen 3.2 A ˚] bonds are likely to exist in OW1  OW2 [= 2.51(2) A ˚ and OW1  OW5 [= 2.49(2) A] separations. However, the presence of these separations is limited to the case that the nearest OW2 and OW5 positions to OW1 water-molecules are occupied as in Fig. 5b and d. We reported that the chabazite sample used in the present study has the high heat-exchange capacities at low temperatures (e.g. 623 kJ/kg at a dehydration temperature of 200 C) [5]. A large amount of dehydration, i.e. easiness of dehydration, at low temperatures is one of the causes of such high heat-exchange abilities. Therefore, the very weak hydrogen bonding between the framework oxygen atoms and the water molecules will be one of the reasons for the high heat-exchange abilities at low temperatures. The interactions between the water molecules and the exchangeable cations and between the water molecules should also influence the heat-exchange abilities, but it is difficult to discuss these effects only from the interatomic distances in the fully hydrated sample. To further examine the effects of the interactions among the water molecules, the exchangeable cations and the framework oxygen atoms on the heat-exchange abilities, we are planning to investigate the dehydration behaviors of each water site in the present chabazite. Acknowledgments We would like to thank Prof. H. Nishido of Okayama University of science for donation of the chabazite sample. Our thanks are also given to Dr. O. Ohtaka of Osaka University for critical reading of the manuscript. This work was partly supported by Grant-in-Aid for Scientific

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