Localization of Residual Water in Alkali-Metal Cation-Exchanged X

Jens Hunger, Ilir A. Beta, Heinz Böhlig, Chris Ling, Herve Jobic, and Bernd Hunger. The Journal of Physical Chemistry B 2006 110 (1), 342-353. Abstra...
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J. Phys. Chem. B 2000, 104, 439-448

439

Localization of Residual Water in Alkali-Metal Cation-Exchanged X and Y Type Zeolites C. E. A. Kirschhock,*,† B. Hunger,‡ J. Martens,† and P. A. Jacobs† Centrum Voor OpperVlaktechemie en Katalyse, K. U. LeuVen, Kardinaal Mercierlaan 92, B-3001 HeVerlee, Belgium, and Wilhelm-Ostwald-Institut fu¨ r Physikalische und Theoretische Chemie, UniVersita¨ t Leipzig, Linne´ strasse 2, D-04103 Leipzig, Germany ReceiVed: June 10, 1999; In Final Form: September 30, 1999

The arrangements of residual water and alkali-metal cations in zeolites Y and X have been determined by Rietveld refinement in combination with Fourier analysis of X-ray powder patterns. The sodium forms NaY and NaX as well as partially cation exchanged forms MNaY and MNaX (M ) K, Rb, Cs) were studied. It was shown that only sodium participates in the interaction with the water molecules, therewith causing the formation of distinctive sodium-water structures in the large cavity system of the hosts. Heavier alkali-metal ions that are also present affect the formation of these structures in an indirect way by blocking cation positions for sodium. The results, gleaned from X-ray diffraction, were put into relation to temperature-programmed desorption (TPD) measurements, obtained from previous investigation of these zeolites.

Introduction Zeolites offer unique features as gas adsorbents, gas separators, and catalysts. These properties are related to the distinctive interaction of molecules with the zeolitic cations in the sterically restricted environment of the tetrahedral framework. It has been known for a long time that the cation distribution in the cavity system of a given zeolite strongly affects its activity in gas sorption.1,2 Also it is common knowledge that the presence of coadsorbents such as water has large impact on the arrangement of the cations in the host3-7 and therewith on the guest-host interaction.8 These mechanisms responsible for the expression of a given cation distribution therefore should enable the further development of suitable zeolitic materials. Water has been observed to desorb from faujasite type zeolite in a stepwise manner.3,9-12 This implies that the small polar molecules partake in specific interactions with the host depending on the water content. Under consideration that atmospheric water is also present in most of the industrial processes, this observation is extremely important. Our recent investigation of the thermal desorption behavior of water from NaX zeolites, partially exchanged with heavier alkali-metal cations,12 has shown that the presence of different cation types strongly influences the course of desorption at higher temperatures (about 450 K). Especially the quantity of water desorbed in different steps was observed to be sensitive toward cation exchange, whereas the effective desorption energy for each desorption stage remained almost the same.12 To pinpoint the reason for this behavior, the determination of the structure of these materials by X-ray powder diffraction was attempted. X-ray diffraction is a versatile tool to investigate the cation distribution and the interaction between host, cations, and guest molecules. Single crystal13-15 and powder diffraction16 have both been applied in studies of the structure of faujasites. Whereas the problem of overlapping reflections16 does not occur in single crystal diffraction and a concise analysis of the structure factors * Corresponding author. Tel: +32-16-321465. Fax: +32-16-1998. E-mail: [email protected]. † K. U. Leuven. ‡ Universita ¨ t Leipzig.

is straightforward, most of the zeolites do not form single crystals of a suitable size easily.17 If single crystals are available at all, they very often are destroyed upon further manipulation like cation exchange or dehydration.15 There also remains some doubt on the homogeneity of the specimens due to limited transport properties in the cavity system. Although powder diffraction allows investigation of the zeolite material used for sorption and catalysis, it suffers the setback of the projection of the spatial scattering of the crystals into one dimension, due to a random orientation of the powder particles. Thus, reflections, which can easily be separated in single crystal diffraction, overlap or even fall together in a powder pattern.16 This is especially true for high-symmetry space groups such as the cubic groups Fd3h or Fd3hm, encountered in zeolites X and Y, respectively. Rietveld refinement in combination with Fourier chart analysis is a useful method to sidestep this problem. Rietveld refinement16,18,19 relies on the fitting of the powder pattern as a whole, using a refinable parameter set, describing the atomic structure of the sample. As for faujasites, the framework structure is known, the choice of a starting parameter set is obvious. On the basis of the parameter set, structure factors can be extracted to calculate the observed electron density at each point in the unit cell (uc). The electron density can be studied in generated Fourier charts that gain accuracy as the refinement progresses. Thus, it is possible to localize guest molecules with known geometry like benzene,20 xylenes,21 aniline,22,23 dinitrobenzene,8 pyrrole24 or, as recently reported, hydrofluorocarbons.25 Water molecules are visible in the Fourier analysis of X-ray powder patterns only as a single speck of electron density. Thus, the assignment and distinction of the cations involve further considerations such as the assessment of the local environment. The same is true of course in the presence of different cation types. Here very often the assignment of the observed electron density heavily relies on bond distances to the zeolite framework. However, with the knowledge of the chemical composition of the sample, especially the exact cation content, and with some patience it is possible to obtain the concise cation distribution in the host. Most of the time the cations in faujasites

10.1021/jp9919112 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/30/1999

440 J. Phys. Chem. B, Vol. 104, No. 3, 2000

Figure 1. Illustration of the connectivity of cages in faujasite along with the Roman numeration of cation positions encountered in these hosts. Cation sites are labeled with the Roman number with the prefix S.

(e.g., zeolites X and Y which have the same topology but different silicon to aluminum ratios, e.g., 1-1.5 for X and larger than 1.5 for Y) are found on the same positions.26,27 The tetrahedral framework forms three types of cages arranged in a diamond like structure. A repetitive unit along the cubic 3-fold axis, including cation sites is illustrated in Figure 1. Cations always have been observed to occupy sites where a maximal coordination by framework oxygen and or guest molecules is granted.3-7,28 Favored are especially sites close to six rings of the framework. This is particularly true in the absence of guest molecules. Providing guest molecules can participate in cation interaction, cation sites isolated from the cavity walls, like position SIV, have been observed.8 Former studies of the cation distribution of alkali-metal cations in faujasites mainly dealt with fully hydrated13,15,29 or fully dehydrated15,30 materials. Partially dehydrated zeolites also have been looked at but an elucidation of the water-cation structure has not been attempted in detail.28,31 Along with the observation of the desorption behavior of partially exchanged zeolites NaX and NaY, this prompted us to endeavor a comparative study of the cation and water arrangements in these hosts. Experimental Section Sample Preparation. The investigated materials were obtained from NaX (Si/Al ) 1.18) and NaY (Si/Al ) 2.43) by partial cation exchange with nitrate solutions of the respective alkali-metal ion. The same batches of parent zeolite were used for modification.12 The samples for X-ray diffraction were prepared in the TPD apparatus used for the investigation of their

Kirschhock et al. water desorption behavior. Experimental details of the temperature-programmed desorption are described in detail elsewhere.12 First all water contained in the fully hydrated zeolites was desorbed by heating the samples above the temperature where the last water previously was observed to come off (10 K/min; He stream). The samples were left to cool in the TPD setup down to 523 K in a helium flow with a very low water content (about 0.005 vol %) and kept at that temperature for 1 h. The samples then were transferred into ampules under the helium atmosphere and sealed to prevent further water intake. TPD experiments with these samples showed that water was absorbed by the zeolites under these conditions. The samples along with their composition and the water content determined by X-ray diffraction and TPD are listed in Table 1. XRD. Diffractograms were recorded in a Stoe Stadi P diffractometer (Debye Sherrer geometry, Cu KR1 radiation) between 5 and 80° 2θ in steps of 0.02°.24 The GSAS software package32 served for Rietveld refinement and Fourier analysis of the structures. Small and high angle regions (4-15° and 9-80° 2θ, respectively) simultaneously were refined with different profile parameters to circumvent problems with the asymmetric distortion of the reflection shape at low angles. Some of the refined powder patterns are depicted in Figure 2. Refined parameters are summarized in Tables 2 and 3 for each sample. To determine whether the structures of the samples change with temperature, powder patterns at elevated temperature (400 °C) were recorded. For this a Stoe oven in combination with a static position sensitive detector (7-42° 2θ) was used. Essentially the same patterns as at room temperature were observed. The refinement of these high-temperature diagrams with the same structural parameters as determined for the roomtemperature measurements is possible. Thus, it is proven that the cation-water arrangement in the studied zeolites is not so much determined by temperature but by the existing water quantity in the host. Results The known crystal structures of zeolites NaX and NaY, in the space groups Fd3h and Fd3hm respectively,13,14,20,33 served as starting parameters for the Rietveld refinement. The parameters of the tetrahedral frameworks were left free to refinement but were regularly checked and reset for consistency (Tables 2-5). Cation coordinates were inserted after inspection of the observed electron density in the vicinity of classical cation positions4 (Figure 1). The cation positions mostly also were refined without constraint. Exceptions will be discussed with the respective structure. The occupancy numbers were constrained to account for the known chemical composition. This included for the X samples the numbers per unit cell of each type of cation, for the Y samples only the sum of exchanged and unexchanged cations. The requirement for reasonable temperature factors was used to help the distinction of cation types. Wrong assignments usually resulted in either negative or extremely high Debye-Waller factors. In those cases where after these considerations the correct assignment still remained uncertain, the evaluation of distances to framework oxygen were compared to typical values of the respective alkali-metal cations. Consequently, a concise distinction between cation types and good agreement between the refined and known chemical composition (Table 1) was possible. Special attention has been given to the assignment of electron density on site SII′. Other authors reported that this site is exclusively occupied by small molecules such as water or

Water and Alkali Metals in Zeolites

J. Phys. Chem. B, Vol. 104, No. 3, 2000 441

TABLE 1: Silicon Aluminum Ratio and Composition (Numbers per Unit Cell) of Investigated Samples As Determined by Chemical Analysis (ca), Temperature-Programmed Desorption (TPD) and Rietveld Refinement (XRD) Si/Al sodium cations/uc

by ca by XRD exchanged cations/uc by ca by XRD water/uc by TPD by XRD water interacts with

NaX

KNaX

CsNaX(5)

CsNaX(30)

NaY

1.18 88.0 87.5 0.0 0.0 21.0 32.5 SII* + SIII

1.18 22.0 21.5 66.0 66.5 7.0 7.0 SII*

1.18 83.5 83.5 4.5 4.5 17.0 35.0 SII* + SIII

1.18 61.5 61.0 26.5 27.5 0.0 19.0 SII* + SIII

2.43 56.0 55.0 0.0 0.0 9.0 19.0 SII* + SIII′

Figure 2. Rietveld refined powder patterns of the samples KNaX (top), CsNaX(5) (middle), and RbY (bottom). Shown are calculated patterns and the differences between measured and calculated data. The insets show the low-angle regions, refined with a different set of profile parameters.

ammonia.7,27 However, there have been observations of the occupancy of this site with cations before.15,28,34 Attempts to interpret the presently observed electron density on this site as water failed. No satisfactory refinement was possible with this assumption. The currently observed distance of this position to oxygen atoms of the zeolitic six ring rather indicates occupancy with cations (Tables 4 and 5). The interpretation of electron density on SII′ as cations, however, concluded in good agreement of the refined number of cations with the samples’ compositions. Water was localized after a similar fashion by analysis of Fourier charts. The observed electron densities in a plane along

KNaY 2.43

RbNaY

CsNaY

2.43

2.43

19.5

27.5

24.0

36.5

27.5

15.0 SII* + SIII′

4.5 SII*

32.0 8.0 7.0 SII*

the 3-fold axis of the space group Fd3hm for the samples NaX and KNaX (Figure 3) are examples. The analysis of numbers of cations per unit cell proved again to be useful for the distinction between water and cations. The inspection of the immediate environment of the detected electron densities also helped during the assignment (Figure 3). Water was only detected in the vicinity of SII* and SIII sites occupied by sodium. Depending on the presence of sodium on SIII sites the water molecules are either placed between SIII and SII* or, lacking SIII sodium ions, directly in front of SII* sodium sites. X Zeolites. NaX. About two-thirds of the cations in the investigated NaX sample reside in the smaller cages, including position SII in the six ring window connecting the sodalite and supercage. The rest is distributed over SII* and SIII sites. Between those cations, water molecules were localized (Figure 3). Considering the occupancy factors, derived without mutual constraint, every cation on SII* (11 per uc) is surrounded by three water molecules (33 per uc). Those water molecules also are interacting with cations on SIII positions (20 per uc) (Figure 4). This and similar arrangements of cations on SII or SII* with water and cations on SIII will from now on be termed SIIwater-SIII structures. The occupancy numbers of cations in the sodalite cage indicate that occupancy of a given SII′ site by sodium prevents the occupancy of neighboring sites SII. Given the distances of 2.16 Å between SII′ and SII this is not surprising. This means that the sum of all cations on SII′ and SII cannot exceed 32 per uc, which is the number of six ring windows in the structure. The same of course is true for cations on sites SII and SII*, which are even closer than SII′ and SII. A similar situation also is encountered regarding the occupancy of the SI site in the double six rings and position SI′ in front of those windows.35 The number of SI′ cations is always smaller than the possible sites minus the occupied neighboring SI positions. SII′ and SI′ are located in a cubelike arrangement. Thus, every position SI′ is surrounded by three positions SII′ and vice versa. The relatively short distances between neighboring sites SI′ and SII′ make their simultaneous occupancy unlikely. This leaves only the sites SI′ and SII′ along one of the 3-fold axes open for simultaneous occupancy in one sodalite cage. As soon as there are more than two cations present in a given cage, the cations therefore have to assume equivalent sites (SI′ or SII′, respectively). Considering these rules leads to the conclusion that a maximum of 64 sodium ions per unit cell can be accommodated in the small cages. This would be the case when SI′ and SII positions are occupied by 100%, resulting in eight cations per each sodalite unit. Such an arrangement would effectively prevent the occupancy of SI and SII′ sites. In the present sample only 56 ions are located in sodalite cages and hexagonal prisms, a substantial part of it on position SII′. By preventing the full occupancy of sites SI′ and SII, this allows at least a partial

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TABLE 2: Refined Fractional Coordinates, Occupancies (Fractional/No. per uc), Thermal Parameters (×10 Å2), Unit Cell Parameter a0 (Å), and Reliability Values in Percent for the Investigated X Zeolitesa T1

(96g)

T2

(96g)

O1

(96g)

O2

(96g)

O3

(96g)

O4

(96g)

SI

(16c) x)y)z)0

SI′

(32e) x)y)z

SII′

(32e) x)y)z

SII

(32e) x)y)z

SII*

(32e) x)y)z

SIII

(48f) x ) y ) 3/8

Ow

a0 Rp Rwp Rf χ2 a

x y z U x y z U x y z U x y z U x y z U x y z U type U occ type x U occ type x U occ type x U occ type x U occ type z U occ x y z U occ

NaX

KNaX

CsNaX(5)

CsNaX(30)

-0.05497(6) 0.12594(9) 0.03501(7) 0.65(3) 0.12583(7) -0.05376(11) 0.03627(12) 0.72(8) 0.1060(5) -0.1050(8) -0.0007(6) 1.2(11) 0.1696(10) 0.1716(6) -0.0349(7) 1.4(3) -0.0018(9) -0.0019(7) 0.1418(8) 1.2(8) 0.1784(7) 0.1793(8) 0.3213(8) 0.9(3) Na+ 7.3 (4) 0.526(6)/8.43 Na+ 0.0624(6) 7.7(4) 0.49(2)/15.68 Na+ 0.1819(11) 6.42(10) 0.401(4)/12.83 Na+ 0.23170(6) 3.28(4) 0.602(4)/19.28 Na+ 0.2591(9) 5.3(8) 0.345(9)/11.06 Na+ 0.1617(9) 9.0(7) 0.418(11)/20.16 0.3245(11) 0.3244(10) 0.2364(13) 2.3(7) 0.340(8)/32.62 24.9443(3) 3 3.9 10.4 0.68

-0.05510(7) 0.12399(4) 0.03615(6) 0.59(6) 0.12516(6) -0.05391(9) 0.03520(8) 0.69(9) 0.1047(9) -0.1074(9) 0.0019(10) 1.41(7) 0.1746(6) 0.1732(4) -0.03849(7) 1.59(8) -0.0030(3) -0.0038(6) 0.1411(5) 1.73(6) 0.1749(7) 0.1758(8) 0.3206(9) 1.7(5) K+ 4.71(11) 0.533(11)/8.54 K+ 0.0686(12) 6.08(9) 0.352(8)/11.26 K+ 0.1781(4) 5.29(9) 0.240(12)/7.68 Na+ 0.2317(11) 4.13(9) 0.667(12)/13.76 Na+ K+ 0.25c 0.2451(7) 4.01c 4.01c 0.247(4)/7.90 0.320(11)/10.24 K+ 0.1781(8) 4.8(7) 0.601(13)/28.70 0.3098(14) 0.3098(12) 0.3098(12) 9.2(11) 0.223(12)/7.04 25.1042(7) 4.4 5.8 7.6 0.62

-0.05723(18) 0.12631(13) 0.03655(17) 1.09(4) 0.12588(13) -0.05435(11) 0.03080(12) 0.97(3) 0.1143(7) -0.1082(9) -0.0047(8) 1.79(18) 0.1745(8) 0.1733(4) -0.0417(7) 1.13(5) -0.0001(12) -0.0051(18) 0.1420(11) 2.20(8) 0.1815(9) 0.1789(10) 0.3191(8) 1.46(6) Na+ 5.4(7) 0.406(12)/6.50 Na+ Cs+ 0.0621(9) 0.0882(2) 4.1(8) 5.3b 0.284(11)/9.09 0.141(11)/4.51

-0.05649(11) 0.12713(8) 0.03687(9) 0.85(6) 0.12575(6) -0.05136(10) 0.03603(12) 0.98(9) 0.1057(7) -0.1012(4) -0.0006(8) 1.43(10) 0.1701(9) 0.1715(5) -0.0323(6) 1.54(6) -0.0050(10) -0.0082(8) 0.1446(6) 1.42(9) 0.1816(4) 0.1779(4) 0.3249(8) 1.39(6) Na+ 5.4(4) 0.630(10)/10.08 Cs+ 0.0867(17) 6.5(4) 0.169(9)/5.41 Na+ 0.1859(12) 4.0(5) 0.287(6)/9.18 Na+ 0.2294(11) 5.5(9) 0.590(6)/18.88 Na+ 0.2670(6) 6.0(9) 0.208(6)/6.66 Na+ Cs+ 0.1474(9) 0.1688(10) 5.2(11) 6.1b 0.335/16.08 0.460b)/22.08 0.338(11) 0.340(8) 0.241(6) 6.0(10) 0.195(13)/18.72 25.073(8) 5.1 6.1 8.1 0.41

Na+ 0.2293(4) 1.83(8) 0.657(9)/21.02 Na+ 0.2469(20) 3.55(11) 0.363(11)/11.62 Na+ 0.1626(11) 5.0b 0.74(4)/35.52 0.3187(10) 0.3190(12) 0.2302(12) 8.47(13) 0.363(5)/34.84 25.0640(9) 4.8 6.5 7.3 0.79

The samples were refined in the space group Fd3h. b Refined with anisotropic temperature factors. c Constrained during refinement.

occupancy of the favorable SI site.7 Here is the cation surrounded by six framework oxygen atoms in an almost perfect octahedral environment. The occupancy of SII′ instead of SII sites also leaves some SII* positions available for cations in the supercage. This position can only be assumed when the nearby site SII is empty. Indeed, does the refined number of cations on SII* match the number of obtainable sites (e.g., 32 - occ(SII)). The cation on SII′ on the other side of the six ring is still relatively close to the SII* position. This explains the rather long distance of 2.94 Å between SII* and the framework oxygen O3.

SII* cations “see” three framework oxygen atoms of the zeolite framework but are relatively open from the supercage side. Thus, it is comprehensible that sodium ions on SII* positions tend to shield themselves with present guest molecules such as, for example, benzene,20 aniline,22,23 dinitrobenzene,8 pyrrole24 or, as in the present case, with water. The refined occupancy numbers clearly indicate that each SII* ion is surrounded by three water molecules at a distance of 2.37 Å, giving rise to a distorted trigonal prismatic environment (Figure 4). The distance between these water molecules of 3.04 Å implies mutual hydrogen bonding. The water molecules are also

Water and Alkali Metals in Zeolites

J. Phys. Chem. B, Vol. 104, No. 3, 2000 443

TABLE 3: Refined Fractional Coordinates, Occupancies (Fractional/No. per uc), Thermal Parameters (×10 Å2), Unit Cell Parameter a0 (Å), and Reliability Values in Percent for the Investigated Y Zeolitesa T

(192i)

O1

(96 g) x ) 0, y ) -z (96g) x)y

O2 O3

(96g) x)y

O4

(96 g) x)y

SI

(16c) x)y)z)0

SI′

(32e) x)y)z

SII′

(32e) x)y)z

SII

(32e) x)y)z

SII*

(32e) x)y)z

SIII′

(96g) x)y

Ow

a0 Rp Rwp Rf χ2 a

x y z U y U x z U x z U x z U type U occ type x U occ type x U occ type x U occ type x U occ type x z U occ x y z U occ

NaY

KNaY

RbNaY

CsNaY

-0.05319(2) 0.12933(3) 0.03633(8) 0.45(11) 0.1054(4) 1.11(8) 0.1742(6) -0.0393(5) 1.36(6) -0.0030(4) 0.1494(5) 1.41(9) 0.1783(4) 0.3295(7) 1.05(7) Na+ 9.3 (4) 0.48(7)/7.70 Na+ 0.0660(9) 7.2(10) 0.504(8)/16.13

-0.05456(7) 0.12490(5) 0.03592(4) 0.86(7) 0.1076(4) 1.69(8) 0.1733(6) -0.0321(5) 1.54(5) -0.0051(4) 0.1419(5) 1.23(8) 0.1781(4) 0.3244(7) 1.90(6) K+ 8.12(12) 0.539(15)/9.50 K+ 0.0866(4) 4.12(10) 0.391(9)/12.51 K+ 0.1782(8) 5.19(14) 0.157(18)/5.02 K+ 0.2643(4) 4.6(9)b 0.293(14)/9.38 Na+ .2643(4)b 4.6(9)b 0.150(13)/4.80 Na+ 0.3950(9) 0.144(5) 4.0(8) 0.152(15)/14.59 0.3285(11) 0.3285(11) 0.2041(10) 5.0(9) 0.156(9)/14.98 24.6736(7) 3.6 4.7 10.3 0.56

-0.05304(9) 0.12578(6) 0.03630(6) 00.84(7) 0.1047(4) 1.03(8) 0.1744(6) -0.0331(5) 1.67(6) -0.0024(4) 0.1451(5) 1.10(2) 0.1813(4) 0.3169(7) 1.99(5) Na+ 9.4(7) 0.275(13)/4.40 Na+ 0.0638(3) 6.9(7) 0.585(4)/18.73

-0.05404(9) 0.12373(5) 0.03422(4) 0.437(8) 0.1095(4) 0.941(5) 0.1803(6) -0.0352(5) 0.816(9) -0.0028(4) 0.1468(5) 1.03(14) 0.1775(4) 0.3160(7) 1.1(2) Na+ 7.2(12) 0.421(5)/6.74 Na+ 0.0715(8) 4.3(15) 0.564(4)/18.06

Na+ 0.2390(5) 7.0(9) 0.140(17)/4.48 Rb+ 0.25719(10) 3.94(4) 0.867(9)/27.74

Na+ 0.2438(3) 5.4(10) 0.217(11)b/6.94 Cs+ 0.2640(9) 5.2(5) 0.755(9)/24.16

0.2982(4) 0.2982(4) 0.2982(4) 8.2(11) 0.135(16)/4.32 24.7861(4) 4.0 5.3 11.5 0.62

0.3007(11) 0.3007(11) 0.3007(11) 5.2(7) 0.217(11)b/6.94 24.7060(9) 2.9 3.7 9.1 0.44

Na+ 0.2345(6) 3.2(8) 0.173(5)/5.55 Na+ .2615(8) 4.0(19) 0.205(6)/6.56 Na+ 0.4005(5) 0.1501(7) 4.3(12) 0.198(7)/19.00 0.3257(9) 0.3257(9) 0.1915(11) 5.7(6) 0.194(11)/18.62 24.6429(4) 3.74 4.84 10.81 0.44

The samples were refined in the space group Fd3hm. b Constrained during refinement.

within the interaction distance (2.58 Å) to cations on yet another kind of site. These cations reside in front of four rings of the supercage. Depending on their location directly on or slightly off the mirror plane of the space group, these cations are termed SIII and SIII′, respectively (Figure 1). A cation on such a site has two possible local environments in the discussed structure. It is neighbored either by one or by two water molecules depending on the occupancy of one or two SII* sites in the vicinity. The possibility of no close water molecules can be discounted considering the observed distance of 2.58 Å, which makes interaction highly probable. Refinements of this position as the symmetrical SIII site and as the SIII′ site have been attempted. The latter proved to be unstable during refinement with regard to position and temperature factors. Best results were obtained when the cation was situated directly on the mirror plane on SIII. Observation of this highly symmetric position rather than a slightly shifted SIII′ site argues for a symmetrical environment of this cation. A sodium ion with two water

neighbors is the center of an almost ideal tetrahedron with two framework oxygens O4 at a distance of 2.95 Å. Nonetheless, the amount of localized water is smaller than two molecules per SIII cation. So at least some of the cations on this site experience an unsymmetrical environment. With the assumptions that every water molecule is situated between an SII* and a SIII cation and that every SII* sodium ion is surrounded by three water molecules, an assessment of the distribution of SIIwater-SIII structures in the eight supercages of the unit cell is possible (Figure 5). About one of the eight supercages hosts three SII* ions interlinked with three occupied SIII sites (Figure 5b). One cage is filled with two linked SII* ions (Figure 5c), and three contain isolated SII-water-SIII structures (Figure 5d). This calculates to only four SIII cations with a symmetric and 16 with an unsymmetric environment. The presence of both SIII and SIII′ cations explains the failure to localize cations directly on SIII′. The refinement of two neighboring sites (e.g., SIII and SIII′) causes them to correlate strongly, especially when

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TABLE 4: Selected Distances (Å) and Angles (deg) in the X Samples NaX T1 1.621 1.647 1.659 1.634 108.0 105.7 110.5 106.6 114.3 111.4

T-O1 T-O2 T-O3 T-O4 O4-T-O3 O4-T-O2 O4-T-O1 O3-T-O2 O3-T-O1 O2-T-O1 T-O1-T T-O2-T T-O3-T T-O4-T

KNaX T2 1.652 1.683 1.657 1.657 108.5 107.0 109.5 109.0 111.0 111.8

141.2 145.9 146.5 143.0

SI-O2 SI′-O2 SII′-O3 SII-O3 SII*-O3 SIII-O1 SIII-O4 SII*-Ow SIII-Ow SII*-Ow-SIII

Na+

2.93 2.50 2.66 2.35 2.94 3.47 2.95 2.37 2.58 147.5 SII* 3.16

Ow-Ow

T1 1.624 1.645 1.676 1.641 109.0 106.0 111.1 111.1 108.1 111.7

CsNaX(5) T2 1.663 1.624 1.658 1.673 111.4 104.7 109.5 111.3 113.6 105.9

137.7 149.8 147.6 148.1

2.40 2.75

K+ 2.87 2.70 2.83 2.62 2.73 3.44 2.59

SIII 3.57

T1 1.639 1.669 1.645 1.661 103.5 111.3 109.7 109.5 117.0 106.0

Na+ 2.89 2.65 2.50 2.34 2.63 3.30 3.05 2.59 2.61 148.9 SII* 3.14

T2 1.657 1.696 1.681 1.670 106.8 104.2 107.3 112.2 115.2 110.4 131.9 154.0 145.0 147.8

Cs+

CsNaX(30) T1 1.666 1.690 1.662 1.689 108.5 109.9 113.5 108.3 107.3 109.3

Na+ 2.92

3.28

SIII 3.9

T2 1.630 1.686 1.673 1.649 110.5 109.7 104.4 115.7 104.3 111.6 146.6 142.2 147.9 135.7

Cs+ 3.00

2.71 2.34 3.09 3.34 2.65 2.64 2.68 132.8 SII* 3.48

3.55 3.05

SIII 2.51

TABLE 5: Selected Distances (Å) and Angles (deg) in the Y Samples T-O1 T-O2 T-O3 T-O4 O4-T-O3 O4-T-O2 O4-T-O1 O3-T-O2 O3-T-O1 O2-T-O1 T-O1-T T-O2-T T-O3-T T-O4-T SI-O2 SI′-O2 SII′-O3 SII-O3 SII*-O3 SIII′-O1 SIII′-O4 SII*-Ow SIII′-Ow SII*-Ow-SIII′ Ow-O1

NaY

KNaY

RbNaY

CsNaY

1.675 1.674 1.645 1.624 113.2 110.9 104.0 115.8 110.7 101.0 145.7 150.9 143.1 129.7 2.81 2.62

1.637 1.654 1.640 1.659 110.3 110.0 105.0 108.1 109.1 114.2 135.2 139.7 148.5 139.5 2.79 2.95 2.83 3.04 3.04 2.74 2.61 2.69 2.75 179.2 3.15

1.650 1.655 1.651 1.630 101.6 114.0 112 109.5 111.4 108.4 142.0 142.6 143.1 142.0 2.78 2.44

1.648 1.664 1.662 1.657 100.1 113.2 108.4 110.6 115.1 109.3 128.2 140.0 136.1 152.1 2.58 2.63

2.37 2.78

2.42 2.93

2.53

2.44

2.21 2.78 2.69 2.70 2.83 2.80 163.8 2.92

one site is a special position on a symmetry element. As a matter of fact the rather high-temperature factor (Table 2) and the shape of the observed electron density (Figure 3) of this site indicate this possibility of a split position. KNaX. Sodium in this sample is found only in the supercages close to or on SII sites. Potassium has been located in the large cavities (SII*, SIII) as well as in the double six ring (SI) and the sodalite cage (SI′, SII′). Water is present only in low amounts (seven per uc), directly in front of all SII* sites, populated with sodium (Figure 6). The refinement of this sample required some special attention. Very early during data processing it became clear that the small cages contained exclusively potassium. Even the attempts to

Figure 3. Observed electron densities in the samples NaX (top) and KnaX (bottom) in the supercages, calculated after the final refinement cycle. The projections are along the 3-fold axis, perpendicular to a mirror plane. Assigned electron density is indicated.

assign mixed sodium-potassium populations on sites SI, SI′, and SII′ resulted in unreasonably high amounts of sodium along with meaningless negative temperature factors. The same problem was encountered when assuming water occupancy on

Water and Alkali Metals in Zeolites

Figure 4. Linked SII-water-SIII structure of a bent type in sample NaX. Here and in all following structure representations the sodiumwater structures are pronounced. Filled atoms indicate SII* sites, crossed, and doubly crossed atoms indicate symmetric and asymmetric SIII sites. Small open circles mark water molecules.

Figure 5. Ordering scheme of water and sodium in a supercage to determine the extent of linking. The numbers of SII* cations (filled), water molecules (open), and symmetric (crossed) and asymmetric (doubly crossed) SIII cations are listed.

site SII′. The distances of this position to framework oxygen atoms are both typical for water hydrogen bonding and potassium Coulombic interaction.15,29,30 Of the 66 potassium cations 27 were localized in the small cages whereas 19 were situated in the sodalite cage. This amounts to occupancy of about half of the 16 available hexagonal prisms (SI) and an average occupancy of about 2.4 per sodalite cage. The assignment of potassium on all these sites also is in accordance with the observations of other groups.15,29,30 The remaining potassium ions and the sodium ions were located in the supercage. With regard to cation numbers per unit cell and reliability factors, the best results of refinement were achieved with the assignment of only potassium on SIII (29 per uc) and both kinds of alkalimetal ions being present on SII and SII* sites, respectively. A further maximum of electron density was determined by inspection of the Fourier charts (Figure 3). This position on the 3-fold axis is quite far from the lattice and hence was assumed

J. Phys. Chem. B, Vol. 104, No. 3, 2000 445

Figure 6. Arrangement of cations and water in sample KNaX (same markings as before).

to be water. Seven molecules per unit cell (a bit less than one per supercage) could be refined on this position, indicating that not all of the eight supercages are occupied by water. To resolve the positions of the cations close to SII, it was presumed that all potassium ions not yet localized are situated on this position. Refinement of the position with a fixed occupancy factor of these SII* potassium ions left two maxima in the difference of observed and calculated electron density. Those residual electron densities are shifted along the 3-fold axis with respect to the refined potassium position. Sodium was placed on these two sites (SII and SII* respectively). The parameters of sodium on SII could be properly refined. Inspection of the observed electron density showed this position to be clearly separated from the cations on SII* (Na+ and K+, Figure 3). Approximately 14 sodium ions were refined on SII. The SII* sodium position and temperature factor could not be refined because of strong correlation with the position SII* of potassium. The occupancy number refined to eight sodium ions on SII*. It is noteworthy that like in the NaX sample, the number of SII* sodium ions relates to the number of cations localized on site SII′. These push the ions on SII into the supercage on SII*. Also, the number of water molecules matches the number of these exposed sodium ions. The distance between water and sodium on SII* (2.59 Å) supports the previous assignments. Sodium on SII* thus experiences a distorted tetrahedral environment of oxygen atoms with three oxygen atoms of the zeolite framework a bit farther away (2.75 Å) than the water molecule. Whether those eight sodium-water structures are homogeneously distributed over the eight supercages of the unit cell or occur clustered cannot be decided on the basis of the diffraction data. CsNaX(5). The water-sodium arrangement in this sample resembles the structure determined in the parent NaX sample (Figure 4). Water resides between sodium ions on SII* and SIII. All cesium in this sample (4.5 per eight sodalite cages) was located in the sodalite cages on position SI′.15,28 The exchange of a small number of sodium by cesium should on first glance have no large effect on the cation arrangement in the host. Nonetheless, considerable changes concerning the cation distribution were observed. The cesium cations reside on SI′ positions in the sodalite cage. This position was assigned to these alkali-metal ions due to the observed distances to oxygen atoms of the double six ring. The penetration of the small cage system of faujasites by cesium ions has already been reported by others,15,28 who observed Cs+ at similar coordinates.

446 J. Phys. Chem. B, Vol. 104, No. 3, 2000

Kirschhock et al.

Figure 7. Isolated SII-water-SIII structure of a bent type in sample CsNaX(30) (same markings as before).

Figure 8. Isolated SII-water-SIII structure of a stretched type in sample NaY (same markings as before).

The size of these cations prevents the further accommodation of cations in a cesium-occupied cage. Thus, more than half of the available sodalite cages (4.5 out of eight) are effectively blocked for the access of sodium ions. This increases the number of cations in the supercages where each of the 32 six ring windows has a nearby sodium ion either on site SII (21 per uc) or SII* (11 per uc) and where sites SIII are occupied with 35.5 sodium ions. The observed SII* position in this sample is rather close to the six ring window. This explains the absence of cations on site SII′ in comparison to the NaX sample where cations on the SII* site are shifted farther into the supercage. In this cesiumexchanged form all sodium ions in the sodalite cage are found on position SI′ (nine per uc). This restricts the availability of SI positions. To determine the maximally allowed number of cations on SI, the relation occmax(SI) ) 16-(occ(SI′)/2) applies.35 The derived occupancy numbers of Na+ and Cs+ on SI′ allow for only 9.2 or less sodium ions on SI sites, which nicely agrees with the localized 6.5 ions in the hexagonal cages. The remaining 35 sodium ions were localized on SIII sites. Like in the unexchanged sample, three water molecules were located in the direct vicinity of each SII* sodium ion (34 per uc). But unlike the previous situation, more SIII sodium ions are present. This increases the number of SIII ions with unsymmetrical environment. Inspection of the occupancy factors while considering the ordering scheme in Figure 5 results in approximately four supercages with two linked SII* ions each (Figure 5c), and the remaining four supercages containing isolated SIIwater-SIII structures (Figure 5d). This combines to four SIII ions with two water neighbors versus 28 SIII with unsymmetrical surroundings. It manifested itself in the temperature factor of this site during data processing. Only refinement with an anisotropic temperature factor brought about satisfactory results. The electron density in this region extended toward the position of the localized water molecules, indicating the presence of cations on SIII′. CsNaX(30). Cesium ions in this sample were refined on positions SI′ (5.5 per uc) and SIII (22 per uc). Despite the high content of cesium in the large cages, similar water-sodium arrangements as before were observed (Figure 7). About 5.5 of the eight smaller cages are occupied by cesium. Like previously described, Cs ions on SI′ cause a reduction of accessible sites for cations in the sodalite cages. Unlike the low exchanged sample sodium cations assume positions SI and SII′ instead of SI′. The SI site is occupied almost to the maximally permitted content considering the presence of 5.5 cesium ions on SI′ sites. Thus, only SII′ positions are left open for occupancy

by sodium in the sodalite cages. Therefore, in the small cages the accommodation of 25 instead of 20 cations in CsNaX(5) is possible, despite the higher number of blocked cages (5.5 vs 4.5). Obviously, this is caused by the presence of 22 cesium ions, localized on SIII sites along with 41.5 sodium ions on SII, SII*, and SIII sites. Like already noticed for the unexchanged sample, the presence of SII′ cations causes the shift of occupied SII* sites into the supercage. With the high cation content in the supercage it is not surprising that only 6.5 sodium ions occupy this place. These form, as before, water structures also including SIII sites occupied by sodium. The latter were refined by fixing the occupancy factor for cesium on SIII to the value required by the sample composition. Anisotropic temperature factors were required to describe the observed electron density in the vicinity of these SIII sites correctly. Best results were achieved with the same values for sodium as in sample CsNaX(5) and with a disklike shape for the Cs ion. The latter was chosen to resemble the results obtained by Norby et al.28 Y Zeolites. NaY. In NaY, similar water-cation structures as in the X samples were detected. As before, positions close to SII and SIII are occupied by sodium, and water molecules were localized between those positions (Figure 8). Yet, there are several differences between the cation-water arrangement in the two hosts. Unlike as in zeolite X, sodium is found on SIII′ rather than the symmetric SIII site. This causes the structure to be of a stretched kind in comparison to the bent type, encountered in the X samples. This brings about longer sodium-water distances than before (SII*-OW: 2.83 Å; SIII′OW: 2.80 Å), and the water molecules are closer to the framework (OW-O1 2.92 Å), within hydrogen bonding distance. The number of water molecules between SII* and SIII′ cations matches a ratio of OW:SII*:SIII′ to 3:1:3, indicating that each cation on SII* is surrounded by three water molecules, which are linked to one SIII′ site each. About 6.5 of these water complexes were localized in the eight supercages per unit cell. The remaining 1.5 supercages are filled with sodium ions in sites SII. The quite far shift of the cations on SII* into the supercage seemed to be caused by the water molecules and not by cations on SII′ sites. There, no cations were detected. The sodalite cages are occupied only by sodium ions on SI′. Their number relates to the number of empty hexagonal prisms. Not only do the water molecules in the sample cause a shift of SII cations toward SII*, but also the correlation of the number of water molecules with the number of localized SIII′ cations allows the conclusion that the presence of water is necessary to permit the occupancy of these sites. In the absence of water the

Water and Alkali Metals in Zeolites cations now situated on SIII′ most probably assume the adjacent SII sites, which are empty, regarding the occupancy factors. KNaY. Potassium was localized in the hexagonal prism (SI), in the sodalite cages (SI′ and SII′), and on SII positions. The latter is also occupied by sodium, which furthermore is found on SIII′ sites. A similar SII-water-SIII′ arrangement as in the unexchanged sample has been detected. As already observed before, only sodium participates in water complex formation. Again, the water content matches the number of sodium ions on SIII′. The refinement of the electron density close to SII has proven to be difficult due to the simultaneous occupancy by both types of alkali-metal cations. Only refinement with the assumption of a mixed occupancy on SII resulted in the proper number of cations per unit cell in the sample (55 per uc). However, the coordinates of K+ and Na+ on SII had to be constrained to the same values. The refined number of sodium ions on this site agrees nicely with the number of water molecules. This coincidence supports the refinement of a double population on SII. Furthermore, all remaining sodium ions participate in water complex formation. About 4.5 of the eight supercages are occupied by four sodium ions interlinked with water. The remaining supercages contain only potassium and no water. RbNaY and CsNaY. In these samples the heavy alkali-metal ions exclusively were observed on SII sites, shifted into the supercage, due to the ion size. SII positions not occupied with Rb+ or Cs+ are occupied with sodium ions. These results are in good agreement with the recently published investigations by Norby et al.28 The high exchange rates imply the presence of heavy ions in every supercage, which seem to impede the occupancy of SIII sites by sodium. This of course explains the missing water molecules on the corresponding sites. Instead, water molecules were localized on the 3-fold axis. Like in the highly potassium exchanged NaX sample (see section KNaX), the blockage of SIII sites prevents the SII-water-SIII arrangement. Comparison to Previous TPD Studies. The nonisothermal desorption of water on the presented samples has been already studied by temperature-programmed desorption experiments.12 According to the preparation conditions the water contained in the samples investigated by X-ray diffraction should correspond to water desorbed in the last step before full dehydration with an effective desorption energy of about 80 kJ mol-1 (see Table 1). A reasonable agreement, however, has only been observed for two of the six systems where results of both methods were available. In all other cases the amount of water determined by XRD has been much larger. A closer inspection of Table 1 reveals a connection between the deviation of the results of both methods with the cationwater arrangement in the samples. A large difference in water content exclusively is observed in samples where SIII-waterSII structures were encountered. The water contents determined by XRD and TPD, in samples where each water molecule is only interacting with a single SII* cation, are, on the other hand, almost identical. This may be caused by the relatively high heating rate of 2-10 K/min during TPD measurements. The SII-water-SIII arrangement depends on the availability of SIII cations neighboring occupied SII* sites (Figures 4, 7, and 8). Thus, a certain degree of order is necessary. During the heating the cations do not have sufficient time for extensive migration. Thus, fewer water molecules are detected by the off-equilibrium TPD method than by the structure investigation of the tempered sample. This effect is the strongest in sample CsNaX(30). The observation

J. Phys. Chem. B, Vol. 104, No. 3, 2000 447 of SII-water-SIII complexes in this sample is rather curious since this sample loses all water at lower temperatures than the other NaX materials12 during TPD measurements. This indicates either that the water structure is considerably destabilized by the presence of larger amounts of cesium or that the already mentioned ordering process is much slower than in the other samples, so that during the TPD measurement this structure is skipped. A possible destabilizing influence by Cs+ can only be of indirect nature, because cesium does not participate in the interaction with water molecules. But the presence of the heavy cations causes the blockage of sodalite cages and therefore crowds the supercages. This leads also to a high occupancy of the available hexagonal prisms, leaving only SII′ in the remaining sodalite cages accessible, which destabilizes the SII* cations considerably. It seems plausible that the necessary ordering of sodium ions on SII* and SIII is hindered by the simultaneous presence of Cs+ on SIII sites. Thus, the formation of SII-water-SIII arrangements can be expected to be much slower than in samples where SIII exclusively is occupied by sodium. With these arguments it becomes clear that both kinetic as well as energetic considerations can explain the failure of TPD to detect the same number of water molecules observed by X-ray diffraction when SII-water-SIII structures are involved. Further support for the critical role of SIII occupancy stems from the observation that an increase of exchange degree toward 45 Cs cations per unit cell fully prevents the formation of an SII-water-SIII structure. Cs+ now blocks about 80% of the SIII sites and no presence of sodium on these sites can be detected. Here a similar SII*-water interaction as in sample KNaX is observed. The details of this structure will be discussed in a different context.36 This explanation is further supported by the good agreement of the water content in samples KNaX and CsNaY. The content of water molecules determined by TPD measurements is very close to the water localized in the structure (Table 1). Unlike as in the SII-water-SIII arrangement no ordering of cations in the supercages is necessary so that the high heating rate does not impede the formation of these tetrahedral SII*-water arrangements (Figure 6). Summary Only sodium has been observed to interact with water, therewith forming netlike arrangements. The heavier alkali-metal ions influence by their presence the formation of these specific distributions. Comparison of the individual structures reveals the preference of four types of water-sodium structures at low water contents. Three of those structures are closely related and follow the same principles. The linked SII-water-SIII structure of a bent type is realized in the samples NaX (Figure 4) and CsNaX(5). It can only be realized when large numbers of sodium ions are present in the supercages, which have free access to SII* and SIII sites. The number of cations on site SII* determines the number of water molecules, whereas the number of occupied SIII sites determines the degree of linking of the SII* sites (Figure 5). The isolated SII-water-SIII structure, observed in the sample CsNaX(30), is a variation of the linked type (Figure 7). Here the number of accessible SIII sites for sodium is severely limited by the simultaneous occupancy of Cs+ on these sites. Therefore, only isolated SII-water-SIII structures are observed.

448 J. Phys. Chem. B, Vol. 104, No. 3, 2000 This again stresses the necessity of sodium ions on SIII to induce the formation of this structure. Very similar structures of an isolated kind are observed in Y zeolites where the supercages are open for sodium occupancy like in the samples NaY and KNaY (Figure 8). The low amount of cations in these structures, however, causes a stretching of the previously observed arrangement. Sodium is now located on SIII′ rather than SIII sites. This isolated SII-water-SIII structure of a stretched type allows a much more homogeneous charge distribution in the large cavities. The last sodium-water structure is established in those cases where the presence of other cations blocks the access to SIII sites. Either the direct occupancy of this site (KNaX, CsNaX(45)) or the direct neighborhood of the heavy alkali-metal ions on sites SII (RbNaY, CsNaY) can keep sodium from the SIII sites. Water here is located in front of those sodium ions that occupy SII* sites (Figure 6). Conclusions The comparison of the arrangement of small quantities of water in several faujasites allows insight into the mechanisms responsible for the formation of a given cation distribution. The high affinity of sodium cations toward water has a crucial impact for cation ordering. The discovered SII-water-SIII structure seems to satisfy the need of the sodium ions for shielding as well as the need of the water molecules for Coulombic interaction and hydrogen bond formation. Especially the observation of the occupancy of SIII′ sites by sodium in NaY, while SII sites in the vicinity are empty, is strong evidence for the structure directing force of the water molecules. Another striking observation is the lack of participation of other alkali-metal cations in water structure formation. Nonetheless, these cations affect the sodium-water interaction in an indirect way. By blocking cation sites in the supercage from the occupancy by sodium, they not only slow the formation of SII-water-SIII structures but can also change or even prevent this arrangement. Thus, their presence influences the arrangement of sodium ions and water molecules and should also affect the affinity of the zeolite for water or other sorbates. The properties of a zeolite as catalyst or adsorbent are often characterized by the interaction of the cations with the guest molecules.20-25 Our results show that the number of a specific kind of cation in the faujasite supercage and their arrangement can be tuned by the degree of cation exchange and the presence of a coadsorbent like water. Thus, the possibility to tailor specific environments for specific guests seems within reach. We are now studying the influence of different amounts of water on the formation of sodium-water structures in faujasites and the effect of these structures on the adsorption properties of these materials.36 Acknowledgment. We thank Prof. Fuess (TU Darmstadt, Germany) for use of his equipment. This work was sponsored by the Belgian Government in the frame of IUAP-PAI (post-

Kirschhock et al. doctoral fellowship to C.E.A.K). B.H. gratefully acknowledges the financial support of the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft, Graduate College “Physical Chemistry of Interfaces”. References and Notes (1) Coe, C. G. In Gas Separation Technology; Vansant, E. F., Dewolfs, R., Eds.; Process Technology Proceedings, Part 8; Elsevier: Amsterdam, 1990. (2) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984. (3) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry, and Use; Wiley: New York, 1974. (4) Smith, J. V.; Bennett, J. M.; Flanigen, E. Nature 1967, 215, 241. (5) Dempsey, E. J. Phys. Chem. 1969, 73, 3660. (6) Dendooven, E.; Mortier, W. J.; Uytterhoeven, J. B. J. Phys. Chem. 1984, 88, 1916. (7) Van Dun, J. J.; Mortier, W. J. J. Phys. Chem. 1988, 92, 6740. (8) Kirschhock, C.; Fuess, H. Zeolites 1996, 17, 381. (9) Kulkarni, S. J.; Kulkarni, S. B. Thermochim. Acta 1982, 54, 251. (10) Hoffmann, J.; Hunger, B.; Dombrowski, D.; Bauermeister, R. J. Thermal Anal. 1990, 36, 1487. (11) Hunger, B.; Matysik, S.; Heuchel, M.; Geidel, E.; Toufar, H. J. Thermal Anal. 1997, 49, 553. (12) Hunger, B.; Klepel, O.; Kirschhock, C.; Heuchel, M.; Toufar, H.; Fuess, H. Langmuir, in press. (13) Olson, D. H. J. Phys. Chem. 1970, 74, 2758. (14) Olson, D. H. Zeolites 1995, 15, 439. (15) Shepelev, Y. F.; Butikova, I. K.; Smolin, Y. I. Zeolites 1991, 11, 287. (16) McCusker, L. B. In Zeolites and Related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G., Pfeiffer, H., Hoelderich, W., Eds.; Studies in Surface Science and Catalysis, Vol. 84; Elsevier: Amsterdam, 1994; p 353. (17) Charnell, J. J. Cryst. Growth 1971, 8, 291. (18) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. (19) Young, R. A., Ed. The RietVeld Method; Oxford University Press: Oxford, U.K., 1993. (20) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311. (21) Czjzek, M.; Fuess, H.; Vogt, T. J. Phys. Chem. 1991, 95, 5255. (22) Czjzek, M.; Vogt, T.; Fuess, H. Zeolites 1992, 11, 237. (23) Kirschhock, C.; Fuess, H. Microporous Mater. 1997, 8, 19. (24) Foerster, H.; Fuess, H.; Geidel, E.; Hunger, B.; Jobic, H.; Kirschhock, C.; Klepel, O.; Krause, K. Phys. Chem. Chem. Phys. 1999, 1, 593. (25) Grey, C. P.; Poshni, F. I.; Gualtieri, A. F.; Norby, P.; Hanson, J. C.; Corbin, D. R. J. Am. Chem. Soc. 1997, 119, 1981. (26) Smith, J. V. AdV. Chem. Ser. 1971, 101, 171. (27) Mortier, W. J. Compilation of Extraframework Sites in Zeolites; Butterworth-Heinemann: London, 1982. (28) Norby, P.; Poshni, F. I.; Gualtieri, A. F.; Hanson, J. C.; Grey, C. P. J. Phys. Chem. B 1998, 102, 839. (29) Mortier, W. J.; Bosmans, H. J. J. Phys. Chem. 1971, 75, 21. (30) Mortier, W. J.; Bosmans, H. J.; Uytterhoven, J. B. J. Phys. Chem. 1972, 76, 652. (31) Mortier, W. J.; Van den Bossche, E.; Uytterhoeven, J. B. Zeolites 1984, 4, 41. (32) Larson, A. C.; Von Dreele, R. B. GSAS, General Structure Analysis System; Report LAUR 86; Los Alamos National Laboratory: Los Alamos, NM, 1995. (33) Meier, W. M.; Olson, D. H.; Bearlocher, Ch. Zeolites 1996, 17, 1-230. (34) Koller, H.; Burger, B.; Schneider, A. M.; Engelhard, G.; Weitkamp, J. J. Microporous Mater. 1995, 5, 219. (35) Dempsey, E.; Olson, D. H. J. Phys. Chem. 1970, 74, 305. (36) Kirschhock, C.; Hunger, B.; Martens, J.; Fuess, H. Manuscript in preparation.