The Rigaku Journal 14

THE RIGAKU JOURNAL VOL. 12 / NO. 2 / 1995

CRYSTAL STRUCTURE OF ZEOLITE Y AS A FUNCTION OF ION EXCHANGE

JAMES A. KADUK AND JOHN FABER

Amoco Corporation, P.O. Box 3011, 150 W. Warrenville Rd., Naperville, IL 60566, U.S.A.

The crystal structures of a series of hydrated Na-, NH4-, Caand La-exchanged zeolite Y have been de-termined as a function of exchange conditions using synchrotron powder diffraction data. Four sieves were also studied in the dehydrated state in situ at 300°C. Only small differences in the framework structures were observed; the largest differences involve 02 and 03, to which the cations are coordinated. In hydrated NaY (LZ-Y52), sites I', II, and II' are occupied. In ammonium-exchanged sieves, the Na at sites I' and II are re-placed by NH4 ions, with the residual sodium displaced to sites 11'. In uncalcined Ca-FAU, the Ca are dis-tributed over sites I', II', and II. At 300°C, the Ca occupy sites I or I', depending on the cation concentration. On returning to room temperature and the hydrated state, the Ca return to their original positions. The La in uncalcined La-FAU preferentially occupy site V, near the center of the 12-ring windows. At 300°C, La ex-hibits a strong preference for site I', and remains at this site on return to room temperature. Ca and La occupy different sites, both at room temperature and at 300°C, and exhibit different mobility. The cation contents calculated from the refined site occupancies agree well with those determined by chemical analysis. The crys-tal structures of NH4and H-Y were studied as a function of framework aluminum concentration. The frame-work structure becomes more uniform as the Al concentration decreases. Significant changes in structure and property trends occur at cell dimensions of approximately 24.45 Å. Faujasite crystallinity decreases markedly with dealumination.

Introduction Zeolites are crystalline aluminosilicates, com-

posed of corner-sharing AlO4 and SiO4 tetrahedra joined into 3-dimensional frameworks having pores of molecular dimensions. They are a subset of the larger class of molecular sieves. A pure sil-ica framework is neutral. The presence of alumi-num in the framework results in a negative framework charge, which is balanced by cations. These cations are relatively mobile, and can be replaced using standard ion exchange techniques.

Faujasite (International Zeolite Association structure type FAU [1], Fig. 1) crystallizes in the cubic space group Fd 3 m, with a lattice constant ranging from about 24.2-25.1Å, depending on the framework aluminum concentration, cations, and state of hydration. There are 192 tetrahedral sites per unit cell. This zeolite is most conveniently visualized (Fig. 2) as being formed from 24-tetrahedra cuboctahedral units (sodalite cages), joined through hexagonal prisms (also known as double 6-rings), The structure can be viewed as the diamond structure, with the sodalite cages playing the role of carbon atoms, and the double 6-rings the role of C-C bonds.

The pore structure is characterized by super-cages approximately 12Å in diameter, which are

linked through windows about 8Å in diameter composed of rings of 12 linked tetrahedra (12-rings). These cages and pores permit access to quite large molecules, making this structure useful in catalytic applications. It is not much of an ex-aggeration to say that each drop of gasoline passes through this zeolite at least once.

The most commonly-encountered faujasites are zeolites X (higher Al) and zeolite Y (lower

Fig. 1 The faujasite structure, viewed down [110]. The larger hatched circles represent the tetrahedral “T” sites (Al or Si), and th smaller open circles indi-cate the oxygen positions.

Vol. 12. No. 2 1995 15

Al). Zeolite Y is the most important catalytic zeo-lite, and is generally synthesized in the Na form. Most of the catalysis of interest is acid catalysis, which requires replacing the Na cations by pro-tons, converting the sieve into the H-form. This cannot be done by direct ion exchange, since most H-faujasites are not acid stable. An indirect strat-egy is therefore used. An ammonium exchange is carried out, followed by a calcination to decom-pose the NH4

+ cations into ammonia and protons. In this way, highlycrystalline H-FAU can be pre-pared. In some catalytic applications, it is desir-able to incorporate metal cations into the sieve, to modify the number and nature of the acid sites, and to affect the diffusion of reactants and prod-ucts.

Several extra framework sites are commonly populated in cation-exchanged faujasites (Fig. 2), and a standard nomenclature has developed [2-4]. Five sites will be found to be occupied in this work:

I at the center of the double 6-rings I' in the sodalite cage, adjacent to a hexagonal

ring shared by the sodalite cage and a dou-ble 6-ring

II in the supercage, adjacent to an unshared hexagonal face of a sodalite cage

II' in the sodalite cage, adjacent to an un-shared hexagonal face

V near the center of the 12-ring apertures be-tween supercages

Cations located at different sites would be ex-pected to affect catalytic properties in different ways. We have studied the crystal structure of faujasite at many stages in the preparation of a series of Caand La-exchanged faujasites as part of a multitechnique (NMR, XRD, adsorption tech-niques, and catalytic testing) project with the aims of obtaining a detailed understanding of the lon-gand short-range structure of these sieves, and correlating changes in structure to catalytic prop-erties. We have studied Caand La-exchanged sieves because they can yield useful catalysts, are diamagnetic (permitting NMR studies), and have similar ionic radii (permitting some insight into charge/radius effects on cation locations).

The framework aluminum concentration of zeolite Y can be reduced by treatments with steam at high temperatures. Changing the framework aluminum concentration provides an additional way of controlling the number, strength, and dis-tribution of the acid sites, and thus the catalytic activity. We have also studied the crystal struc-tures of a series of NH4and H-FAU having frame-work Al concentrations ranging from 56/cell to essentially zero, to develop additional struc-ture/activity correlations.

Although there is considerable literature on the location of cations in faujasite [4], it is not always clear if the materials studied are directly comparable to the ones of our interest. In addition, we can find no reference to the positions of am-monium cations in zeolite Y. To calibrate the re-sults of other analytical techniques, it was impor-tant to characterize the average structures of these particular materials. The availability of synchro-tron radiation and the Rietveld refinement tech-nique permit much more information to be ex-tracted from powder diffraction patterns, leading to more precise structural characterizations than those obtainable in older studies, which generally used individual intensities extracted from the powder diffraction patterns.

Experimental Commercial LZ-Y52 (hydrated NaY, sample

097) was used as the starting material. Three room-temperature exchanges with ammonium

Fig. 2 Diagram of the faujasite structure, illustrating the oxygen positions and cation site designations (from Refer-ence 2).

The Rigaku Journal 16

nitrate solution reduce the Na content to approxi-mately 2.5 wt%, corresponding to 12 Na/ cell (sample 185-F). Additional exchanges at 80°C reduce the Na content even further, to about 3 Na/Cell (sample 011-F). Additional exchanges at even more severe conditions can reduce the Na content to below 1 Na/cell. These 10w-Na ammo-nium forms were steamed at progressively more-severe conditions to produce the series of dealu-minated materials. An aliquot of 185-F was cal-cined at 300°C, to produce the HNa-FAU 58-4. The relationships among the sieves are illustrated in Fig. 3.

The NH4Na-FAU was exchanged at room temperature with aqueous Ca2+ or La3+ solution. (It is worth noting that this procedure differs from the direct exchange of Na-FAU generally used to prepare metal-exchanged faujasites.) Two materi-als, 70-1 (Ca) and 69-3 (La), were air-dried. These air-dried materials were calcined at 300°C to pro-duce 71-4 (Ca) and 61-1 (La). To study the effects of calcination temperature on site occupancy, the LaNa sieve 61-1 was calcined further at 500°, to produce 70-2. The calcined MNa sieves 71-4 and 61-1 were exchanged with ammonium nitrate so-lution, and calcined at 300°C to produce the HM-forms 64-1 (Ca) and 64-4 (La). Separate aliquots of 71-4 and 61-1 were subjected to a second metal

exchange, and then calcined at 300°C, to produce the sieves 63-3 (Ca) and 64-2 (La).

Chemical analysis of these sieves (Table 1) permits calculation of the cation contents. LZY52 is known to have 56 framework Al per unit cell distributed among the 192 tetrahedral sites. The observed Al concentration was assumed to corre-spond to 56 Al/cell, and the cation concentrations calculated relative to this value. None of the ion exchange procedures used in this study would be expected to dealuminate the sieve. For the hydro-gen forms, the H concentration was calculated as the difference between the Al content and the ob-served equivalents of cations. The La sieve 64-2 was found to contain an excess of La. The other sieves have cation concentrations close to the val-ues representing complete exchange.

The samples were mixed with known concen-trations (target 15 wt%) of micronised quartz in-ternal standard in a Spex 8000 mixer/mill. X-ray powder diffraction patterns were measured from flat plate samples at ambient conditions on beam-line X3B1 at the National Synchrotron Light Source at Brookhaven National Laboratory, using a calibrated wavelength of approximately 1.15Å.

Patterns were measured from low angles to 60° 2θ in 0.01° steps. This step size represents a compromise between data collection time and

Fig. 3 A flowchart illustrating the preparation of the ion-exchanged sieves, and their relationships

Vol. 12. No. 2 1995 17

Table 1 Chemical analysis of ion-exchanged faujasites

Sieve/Description Al, wt% Na, wt % La, wt% Ca, wt% Cations/Al56Si136O384 097, NaY, LZ-Y52 11.3 9.50 - - Na56 185-F, NH4NaY 12.1 2.26 - - (NH4)42Na14 011-F, NH4Y 13.2 0.69 - - (NH4)53Na3 44-3, NH4Y 11.89 0 - - (NH4)52Al52 46-5, NH4Y (NH4)61Al51 46-1, NH4Y (NH4)32Al32 46-3, NH4Y (NH4)14Al14 46-2, NH4Y (NH4)3Al3 46-4, NH4Y (NH4)0Al0 66-1, HY H34Al34 17-3, HY 6.82 0 - - H28Al28 20-4, HY 6.00 0 - - H28Al28 20-6, HY 3.91 0 - - H14Al14 20-7, HY 2.76 0 - - H3Al3 20-8, HY 0.05 0 - - H0Al0 70-1, Ca(Na)Y, air dried 11.4 1.8 - 6.1 Ca20Na10 71-4, Ca(Na)Y, calcined @ 300°C, at room temp

70-1, Ca(Na)Y, in situ, 300°C 11.4 1.8 - 6.1 Ca20Na10 69-3, La(Na)Y, air dried 9.9 1.9 14.3 - La15Na13 61-1, La(Na)Y, calcined @ 300°C, at room temp

10.3 2.1 13.5 - La14Na13

70-2, La(Na)Y, calcined @ 500°C, at room temp

10.2 2.2 13.1 - La14Na14

69-31, la(Na)Y, in situ, 300°C 9.9 1.9 14.3 - La15Na13 58-4, H(Na)Y 12.7 2.6 - - Na13H43 64-1, H(Ca)Y 11.6 0.2 - 1.5 Ca5Na1H45 64-4, H(La)Y 12.4 0.4 12.4 - La11Na2H21 63-3, CaY, calcined @ 300°C, at room temp

10.9 0.6 - 7.4 Ca26Na4

63-3, CaY, in situ, 300°C 10.9 0.6 - 7.4 Ca26Na4 64-2, LaY, calcined @ 300°C, at room temp

9.6 0.9 18.4 - La22Na5 excess

64-2, LaY, in situ, 300°C 9.6 0.9 18.4 - La22Na5 excess

precise definition of the low-angle quartz peaks. The counting time was 4 sec/step. The samples were rocked from ω-1 to ω+1° during each data point. An ω-scan of a faujasite peak demonstrated acceptable powder averaging.

Samples 70-1, 69-3, 63-3, and 64-2 were stud-ied at ambient conditions, and also in situ at 300°C. This temperature was chosen because TGA weight loss indicates that the sieves are completely dehydrated at this temperature. It is also a catalytically-relevant temperature. The sieves (containing the quartz internal standard)

were loaded into 1 mm fused silica ("quartz") cap-illaries, only one end of which were sealed. The samples were first heated to 100°C, and the pow-der patterns measured.

Successful refinements of these 100°C pat-terns were not obtained. The low-angle and high-angle portions corresponded to significantly dif-ferent lattice parameters. Dehydration of zeolites results in significant increases in lattice parame-ters [5]. The time necessary for equilibration of the dehydrated sieves at these conditions in this apparatus is apparently of the order of hours. The

The Rigaku Journal 18

samples changed significantly during the 5-8 hours of data collection.

After the 100°C scans, the samples were heated to 300°C, left to equilibrate for several hours, and the diffraction patterns measured again. These high-temperature patterns yielded very good refinements.

All data processing was carried out using GSAS [6]. Even though the low-angle region may contain information about the extra-framework species, only the 15-60° 2θ portions of the pat-terns were included in the refinements, to mini-mize the effects of surface roughness, microab-sorption, and peak asymmetry at low angles. The 19.6-19.9° 2θ region containing the quartz (101) peak was also excluded, as the peak definition was relatively poor.

The faujasite framework and quartz structural models were refined. The framework atoms were refined subject to a soft constraint, corresponding to the weighted average of the Si-O and Al-O dis-tances for the framework composition. The soft constraint contributions to the final χ2 were negli-gible. All atoms were refined isotropically; the framework oxygens were assigned a common dis-placement coefficient, and a separate coefficient was refined for the extra-framework atoms.

After initial refinement of scale factors, lattice parameters, and the background using fixed struc-tural models, difference Fourier maps were calcu-lated. The principal difference peaks were identi-fied as cations, and added to the structural model. Additional cations were added as the frameworks were refined. Individual cation site occupancies were refined, as well as the positions. In certain refinements, the Na site occupancies were fixed to correspond to the composition derived from chemical analysis.

The scale factors for faujasite and quartz, as well as the lattice parameters for both phases, were refined. The peak profiles were described by a pseudo-Voigt function. The gaussian U (strain), cauchy X (size) and asymmetry terms were re-fined for each phase. The remaining profile terms were fixed at the instrumental values. It was sometimes necessary to include the faujasite cauchy Y (strain) profile term. The background was described using a real space pair correlation function. The characteristic distances and the number of terms varied among the samples.

The final refinements yield residuals higher than would normally be considered acceptable. Typical observed, calculated, and difference pat-terns are illustrated in Figs. 4-6. Reflecting the compromise in step size, the largest errors occur at the quartz peaks, particularly the (100) peak at 15.53° 2θ. -

The best refinements are obtained for the de-hydrated samples measured in situ at 300°C. The poorest refinements are obtained for highAl hy-drated sieves. An X-ray diffraction experiment is sensitive to the long-range ordered average elec-tron density. In these refinements, the sieve is de-scribed using a crystalline model, having long-range order. Such a model seems to work well for the low-Al sieves, in which there are few cations and little water. The electron density present is either well ordered (and thus detected) or com-pletely disordered (and thus undetectable).

At higher framework Al concentrations, there are more cations and water molecules. It seems highly unlikely that these Al and cations are dis-tributed completely at random. Such regions of local order may be large enough to scatter coher-ently, and thus affect the diffraction intensities. Such local variations are not included in the struc-tural model, which thus represents more of an ap-proximation for high-Al sieves than for the highly-dealuminated samples. We believe that local partial ordering is the most likely source of the residuals in the patterns of the high-Al sieves. In no case are ordered water molecules detected. The slopes of the final Wilson plots suggest that the ESDs are underestimated by a factor of 2. The final faujasite structural parameters for the NH4-FAU 44-3 are reported in Table 2, and represent typical values for all of these sieves. The coordi-nates, occupancies, and isotropic displacement coefficients of the cations are reported in Table 3. The cation site occupancies are summarized in Table 4.

Results and Discussion

Framework Structures All tetrahedral sites in the faujasite framework

are crystallographically-equivalent. The oxygen and cation site labels are illustrated in Fig. 2 [2]. O1 represents the “side: of the double 6-ring, O2 the edge shared by two adjacent 6-ring of the so-dalite cage, O3 the edge a hexagonal face of a

Vol. 12. No. 2 1995 19

double 6-ring shares with the 4-ring of the sodalite cage, and O4 the edge shared by the sodalite cage 4-ring and the 6-ring exposed to the supercage.

The changes in framework structure among all of these sieves are in general small. There are no significant framework structural differ-ences between NH4- and H-form sieves of

Fig. 4 Observed, calculated, and difference patterns of the ammonium-exchanged faujasite 44-3, containing 14.87% quartz internal standard. The crosses represent the observed data points, and the solid line the calculated pattern. The difference pattern is plotted at the same vertical scale. The lower row of tick marks represents the faujasite peak positions and the upper row the quartz peaks.

Fig. 5 Observed, calculated, and difference patterns of the La-exchanged faujasite 64-2, measured at room tem-perature. The crosses represent the observed data points, and the solid line the calculated pattern. The difference pattern is plotted at the same vertical scale. The lower row of tick marks represents the faujasite peak positions and the upper row the quartz peaks.

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comparable lattice dimensions. Framework structural trends with composition can be dis-

cussed independently of the exchanged form. The average T-O distance decreases smoothly

Fig. 6 Observed, calculated, and difference patterns of the La-exchanged faujasite 64-2, measured in situ at 300°C. The crosses represent the observed data points, and the solid line the calculated pattern. The difference pattern is plotted at the same vertical scale. The lower row of tick marks represents the faujasite peak positions and the upper row the quartz peaks.

Table 2 44-3 (NH4)52Al52Si140O384·191H2O with 14.87% quartz, X3B1 16 Jul 94, plate

Vol. 12. No. 2 1995 21

Table 3 Selected structural parameters for the cations in exchanged faujasites.

Sieve Site I’, x=y=z frac

Site II’, x=y=z frac

Site II, x=y=z frac

Site V, x=y=z frac

Site I, 0,0,0 frac

Uiso, Ų

097 0.0692(5) 0.036(2) Na

0.1677(3) 0.56(2) Na

0.2580(5) 0.33(1) Na

- 0.03(2) Na

0/044(6)

185-F 0.0741(7) 0.47(2) NH4

0.1683(5) 0.38(2) Na

0.2659(4) 0.73(2) NH4

- - 0.019(7)

011-F 0.0834(5) 0.73(2) NH4

0.1697(16) 0.14(2) Na

0.2642(4) 0.84(2) NH4

- - 0.041(7)

44-3 0.0839(4)

0.70(2) NH4 - 0.2647(3)

0.87(2) NH4 - - 0.032(6)

46-5 0.0973(6) 0.59(3) NH4

- 0.2650(6) 0.68(3) NH4

- - 0.057(13)

46-1 0.1077(16) 0.35(2) NH4

- 0.2708(10) 0.48(3) NH4

- - 0.166(30)

70-1 0.0706(4)

0.23(1) Ca 0.1682(3) 0.312 Na

0.20(1) Ca

0.2708(5) 0.21(1) Ca

0.4960(25) 0.07(1) Ca

- 0.050(5)

71-4 0.0687(5) 0.37(1) Ca

0.1667(3) 0.312 Na

0.20(1) Ca

0.2709(5) 0.198(7) Ca

- - 0.038(5)

70-1, 300°C 0.0611(4) 0.156 Na

0.20(1) Ca

- 0.2319(4) 0.156 Na

0.20(1) Ca

- 0.63(2) Ca 0.045(5)

69-3 0.0702(6)

0.069(2) La 0.1669(3) 0.406 Na

0.034(3) La

0.2652(4) 0.083(2) La

0.4900(1) 0.250(3) La

- 0.045(3)

61-1 0.0656(1) 0.354(3) La

0.1683(3) 0.406 Na

0.046(3) La

0.2792(5) 0.059(2) La

0.4871(5) 0.050(2) La

0.098(3) La 0.031(2)

70-2 0.0658(1) 0.351(3) La

0.1678(3) 0.437 Na

0.032(3) La

0.2782(6) 0.056(2) La

0.4873(1) 0.022(2) La

0.095(3) La 0.029(2)

69-3, 300°C 0.067(1) 0.434(3) La

0.1653(4) 0.36(1) Na

0.2368(5) 0.059(2) La

- - 0.031(1)

58-4 0.0727(6)

0.26(1) Na? 0.1665(3) 0.80(3) O?

0.2706(4) 0.35(1) Na?

- - 0.032(6)

64-1 0.0780(6) 0.062 Ca

0.24(2) O?

0.1629(4) 0.047 Ca

0.40(3) O?

0.2634(4) 0.047 Ca

0.47(2) O?

- - 0.046(8)

64-4 0.0653(1) 0.275(3) La

0.1687(5) 0.062 Na

0.2735(6) 0.046(2) La

0.4856(6) 0.036(2) La

0.104(3) La 0.013(2)

63-3 0.0709(2)

0.40(1) Ca 0.1670(2) 0.125 Na

0.32(1) Ca

0.2731(5) 0.174(6) Ca

0.4921(13) 0.071(5) Ca

- 0.032(4)

63-3, 300°C 0.0678(2) 0.53(1) Ca

0.1702(13) 0.12 Na

0.2356(4) 0.24(1) Ca

- - 0.021(3)

64-2 0.0649(1)

0.470(3) La 0.1682(3) 0.156 Na 0.089(3)

0.2828(6) 0.052(3) La

0.4854(3) 0.091(2) La

0.125(3) La 0.032(1)

64-2, 300°C 0.0672(1) 0.627(3) La

0.1659(3) 0.156 Na

0.061(2) La

0.2366(10) 0.027(2) La

- - 0.030(1)

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Table 4 Site occupancies (cations/cell) in exchanged faujasites.

Sieve/Description I’ II’ II V I 097, NaY, LZ-Y52 12 Na 18 Na 11 Na - - 185-F, NH4NaY, 2.5% Na 16 NH4 13 Na 23 NH4 - - 011-F, NH4Y, 0.7% Na 23 NH4 4 Na 27 NH4 - - 44-3, NH4Y 22 NH4 - 29 NH4 - - 46-5, NH4Y 19 NH4 - 22 NH4 - - 46-1, NH4Y 11 NH4 - 15 NH4 - - 70-1, Ca(Na)Y, air dried 7 Ca 10 Na

7 Ca 7 Ca 2 Ca -

71-4, Ca(Na)Y, calcined @ 300°C, at room temp.

12 Ca 10 Na 6 Ca

6 Ca Trace Ca?

70-1, Ca(Na)Y, in situ, @ 300°C 6 Ca 5 Na

- 6 Ca 5 Na

- 10 Ca

69-3, La(Na)Y, air dried 2 La 13 NA

1 La 3 La 8 La -

61-1, La(Na)Y, calcined @ 300°C, at room temp

11 La 13 Na 1 La

2 La <1 La 2 La

70-2, La(Na)Y, calcined @ 500°C, at room temp

11 La 13 Na 1 La

2 La <1 La 2 La

69-3, La(Na)Y, in situ, @ 300°C 14 La 12 Na 2 La - - 58-4, H(Na)Y 8 Na? 25 O? 11 Na? - - 64-1, H(Ca)Y 2 Ca?

8 O? 1.5 Ca? 13 O?

1.5 Ca? 15 O?

- -

64-4, H(La)Y 9 La 2 Na 1 La 1 La 2 La 63-3, CaY, calcined @ 300°C, room temp

13 Ca 10 Ca 4 Na

5 Ca 2 Ca -

63-3, CaY, in situ, @ 300°C 20 Ca 6 Ca 4Na

7 Ca - 1 Ca

64-2, LaY, calcined @ 300°C, room temp

15 La 5 Na 3 La

2 La 3 La 2 La

64-2, LaY, in situ, @ 300°C 20 La 5 Na 2 La

1 La - -

with the lattice parameter (Fig. 7), as might be expected for gradually decreasing framework aluminum concentration. With the exception of the most highly dealuminated NH4-Y and H-Y, the average T-O distances exhibit a good linear correlation with unit cell dimension:

T -Oavg, Å=0.340+0.04395a The correlation coefficient is 0.99. At the lowest Al concentrations, the deviations of the individual T-O distances from the average become smaller. At low Al concentrations, the framework structure becomes more uniform, consistent with the nar-rowing of the 29Si MASNMR line widths in highly dealuminated sieves.

For sieves having unit cells less than about 24.45 Å, the individual T-O distances do not dif-

fer significantly from the average distance. In the sieves having larger unit cells, however, the T-O3 distance becomes significantly smaller than the average, and the T-O4 distance is significantly larger than the average (Fig. 7). This occurs not just for the NH4 sieves, but also for the H-forms. The trend is thus not caused by the cations. Both O3 and O4 are part of the sodalite cage, but only O3 is "near" cations (when present). The signifi-cance of these structural differences is unclear, and computational studies will be needed to un-derstand them more fully.

Conventional experience is that the properties of faujasite (especially the catalytic activity, water content, and stability of the hydrogen forms) change significantly at cell dimensions about

Vol. 12. No. 2 1995 23

24.45 Å. Perhaps the changes in properties reflect the structural changes which take place for cells about this size.

In the Ca- and La-containing sieves, the T-Ol distances are in general slightly shorter than the average, although there is significant scatter. The T-O2 distances are in general slightly longer than the average, with those in the La-containing sieves 69-3, 64-4, and 64-2 being very long (1.66-1.68Å). These long distances decrease at elevated temperature. The T-O3 distances are in general shorter than average, but increase in the presence of La. At 300°C, this distance lengthens to about 1.66 Å. The T-O4 distances are near the average at room temperature, but decrease to about 1.615 Å at 300°C.

The deviations of individual O-T-O angles from the ideal tetrahedral value are very small. It is reasonable to consider the framework to be composed of rigid T-O4 units connected by flexi-ble "ball joints". The T-O-T angles do not change greatly among these sieves.

The structures of the CaNa-FAU 70-1 at room temperature and 300°C illustrate the largest struc-tural variations observed in this series (Fig. 8). When the T site and its four second nearest

neighbors are superimposed, the weighted RMS deviation in positions is only 0.02 Å. The maxi-mum difference in T-O-T angles is about 10°. The differences in the O positions are 0.09, 0.04, 0.26, and 0.15Å for O1, O2, O3, and O4, respectively. The largest structural changes involve O3, which is involved in the shortest cation-oxygen dis-tances. Despite the large variations in chemical composition, cation locations, and temperatures, the changes in framework structure are relatively subtle.

Extraframework Species In these sieves, cations were observed at vary-

ing combinations of sites I', II', II, V, and I. In all cases, the shortest cation-oxygen distances in-volve O2 and O3; cations are not coordinated to the framework oxygens O1 or O4.

NH4-Exchange of Na-FAU There are remarkably few structure reports for

hydrated faujasites corresponding to these sieves; most attention has centered on the dehydrated forms. Although we can find no references to the crystal structure of hydrated Na/NH4-FAU, the structure of hydrated Na-Y [7-10] and Na-X [11] have been reported. The site occupancies were:

Fig. 7 Individual and average T-O distances in NH4- and H-faujasites as a function of framework aluminum con-centration.

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Site/Sieve Na-Y [7]

Na-Y [8] Na-Y [10]

Na-X [11]

I, cations/cell 8Na 3Na 2.6Na 9Na I’ 1Na +

13H2O 12Na 11Na 8Na +

12H2O II None 1Na 13Na 24Na +

8H2O II’ 11H2O 8H2O 25H2O 26H2O III 25Na 34Na none none

The site III is located in the supercage, near

the 3 linked 4-rings (Fig. 2). In the hydrated Na-FAU 097, the occupancy

of site I' is 0.36(2); this corresponds to 12 Na/cell, in good agreement with Marti, Soria and Cano [7, 9] and Mortier, Van den Bossche, and Uytterho-even [MVU, 10]. The occupancy of site II corre-sponds to 11 Na/cell, and agrees with that ob-served by MVU. Our observed occupancy of site II' is consistent with that previously reported [10, 11]. This site is generally described as occupied by water molecules. For reasons described below, we believe that there is significant occupancy of this site by Na cations.

Site I' is 2.57 Å from three framework O3. It is 2.47 A from three sites II'; this is the source of the usual identification of site II' as water oxygen. The identification as Na or O cannot be confirmed from only a single diffraction experiment, though slightly better residuals were obtained when mod-eling the site as Na rather than O. Even modeling

as Na, not all of the cations have been located in LZ-Y52. A significant fraction of the Na cations must be disordered among supercage sites.

In the ammonium-exchanged sieves, the site I' moves significantly toward the center of the so-dalite cage, and away from the framework atoms. The occupancy increases with the degree of NH4 exchange. Both of these observations suggest the replacement of Na by NH4 at this site. Likewise, the occupancy of site II and the site-framework distance increase with increasing ammonium ex-change.

The position of site II' remains constant with ion exchange. The occupancy drops when the Na content is reduced from 54 to 12/cell, but de-creases even more when the Na content is lowered from 12 to 3/cell. This last decrease accounts completely for the overall change in Na content between 185-F and 011-F. The changes in occu-pancy and the constancy of position suggest that this is the last Na site to be exchanged. It may very well be partially-occupied by oxygen in LZ-Y52, but seems to represent a hydrated Na in the ammonium-exchanged materials. In the ex-changed samples, the cation content calculated from the refined occupancies agrees remarkably well with the stoichiometry determined by chemi-cal analysis. Additional evidence for interpreting site II' as Na is provided by a study of dehydrated NaY at 20 K [12].

NH4- and H-FAU In these sieves of varying framework Al con-

centration, ammonium ions are observed only at sites I' and II. Site II' inside the sodalite cage is vacant in all of these sieves; this provides addi-tional evidence to support our identification of this as a sodium site. The low sodium content of these sieves corresponds to <1 Na/cell, which would not be detectable.

The site I' nitrogen is 3.04 Å from 3 frame-work oxygens 03 in 44-3; this is a reasonable dis-tance for a non-hydrogen-bonded NH4-O. The cation interaction with the framework appears nondirectional. The N-N distance inside the so-dalite cage is 2.85 Å, this is also a reasonable value. In 46-5 and 46-1, the N1-O distance in-creases to 3.37 Å, with a decrease in the N-N dis-tance to 1.93 Å. Site I' has a multiplicity of 32/cell or 4/sodalite cage. As the occupancy of this site decreases below about 50% (2 ammonium/cage), the N-N distance decreases, and the N-framework

Fig. 8 An overlay of the first and second coordination spheres of a T atom in the Ca(Na)-FAU 70-1 at room temperature (dashed) and at 300° (solid), illustrating the maximum structural variation in these sieves.

Vol. 12. No. 2 1995 25

distance increases. This trend suggests that cation-cation interactions, in addition to cation-framework interactions, are important in deter-mining the actual cation locations.

The site II nitrogen is 3.08 Å from 3 frame-work oxygens O2, and does not move with de-creasing site occupancy. There appear to be no steric constraints on this cation, and the cation-framework interaction appears nondirectional. The cation concentrations calculated from the site occupancies agree remarkably well with those expected from the lattice parameters. No cations could be detected at concentrations lower than 26/cell. At low concentrations, they appear to be too delocalized. Even at 26/cell, the isotropic dis-placement coefficient is very large, indicating considerable disorder.

In the two highest-Al H-FAU (66-1 and 17-3), some extraframework density was detected at ap-proximately -.11, -.11, -.11-near site U. This den-sity was modeled as N in 66-1 and O in 173. The average occupancy is 0.34, and the thermal pa-rameter is large. This blob is 3.78 Å from O3, and the O-O distance is 1.05 Å. It represents a small tetrahedral cluster of electron density at the center of the sodalite cage. It perhaps represents residual ammonium N – a continuation of the trend ob-served in the NH4-FAU (even farther from the framework). Another possibility is that it repre-sents an extraframework Al species, such as the AlO4 observed in a neutron diffraction study [13], and which we sometimes observe in dealuminated faujasites.

Calcium The CaNa-FAU 70-1 was not calcined, and

can be considered completely hydrated. The elec-tron density at sites I' and II was attributed com-pletely to Ca. There was too much electron den-sity at site II' to represent only sodium. The Na occupancy of this site was fixed to reproduce the Na content determined by chemical analysis, and the Ca occupancy of this site was refined. The Ca cations are approximately equally-distributed over sites I', II' and II. The I'-O3 distance of 2.60(1) Å is reasonable for direct coordination, but the long II'-O and II-O distances of 3.04 and 3.24(1) Å in-dicate that these cations are hydrated, and interact with the framework indirectly.

A small additional Ca population was de-tected at site V, more than 5 Å away from the

nearest framework oxygens. The Ca cations lo-cated here are likely completely hydrated. The Ca-O distances are reasonable for hydrogen bonding contacts between coordinated water molecules and framework oxygen atoms. The cation content calculated from the refined occupancies is Ca23Na10, which corresponds well to the Ca20Na10 determined by chemical analysis.

Sample 71-4 was prepared by calcining 70-1 at 300°C in air. The powder pattern was measured at ambient conditions, so the material is also a hydrated NaCa-FAU. The Na occupancy of site II' was fixed at 10 per cell. Compared to 70-1, the Ca population of site I' is higher, while those of sites II', II, and V have decreased. The largest peak in the difference map is near site V, but did not re-fine to significant occupation. The I'-O3 distance (2.54(1) Å) and the II-O2 distance (3.05 Å) have decreased, indicating a stronger interaction with the framework.

The sieve 70-1 was also studied in situ in its dehydrated form at 300°C. Since site II' was com-pletely vacant, the Na content was distributed equally over sites I' and II using fixed occupan-cies, and only the Ca site occupancies were re-fined. The cation-framework distances decreased considerably, to 2.41(1) Å (I'-O3) and 2.37(1) Å (II-O2).

A large occupancy of site I, at the center of the double 6-ring, was observed, with the Ca-O3 distance of 2.55(1) Å. This is the only sieve for which such a high occupation of this site was ob-served, consistent with the large structural devia-tions compared to the other sieves.

The shorter cation-oxygen distances indicate a stronger interaction with the framework. The strength can be quantified approximately by bond valence calculations [14]. The cation valences, calculated from the sums of bond valences, are:

Sieve Site I', Ca/Na Site II, Ca/Na 71-4 0.64/0.41 0.16/0.10 70-1, 300° 0.90/0.57 1.01/0.65

The lower values than the expected 2.0 for Ca and 1.0 for Na point out how much of the coordination is accounted for by water molecules at room tem-perature, and how coordinativelyunsaturated the cations are at reaction temperatures. It is therefore not surprising that the cations might act as Lewis acids.

The structure of this CaNaY is considerably different at 300°C and when cooled to room tem-

The Rigaku Journal 26

perature (71-4). During cooling and rehydration, the cations seem to move to positions not greatly different than those they occupied during the original exchange.

After a second Ca exchange and calcination, the sample 63-3, with 4 Na/cell, was produced. The Na occupancy of site II' was fixed at 4 Na/cell. The structure is actually very similar to that of 71-4, with Ca primarily occupying sites I' and II', with smaller occupancy of sites II and V. The I'-O3 distance of 2.56(1) Å and II-O2 dis-tance of 3.04(1) Å are also very similar to those of 71-4. The cation content calculated from the re-fined site occupancies is Ca24Na4, in excellent agreement with the Ca26Na4 determined by chemi-cal analysis.

This CaY 63-3 was also studied in situ at 300°C. Site II' is populated (in contrast to 70-1 at 300°), but much less populated than at room tem-perature, and was described as Na with a fixed occupancy. Ca move from sites II', II, and V at room temperature to site I' at 300°C. The I'-O3 distance of 2.50(1) Å and the II-O2 distance of 2.39(1) Å are very similar to those in 70-1 at 300°C. The cation content calculated from the refined occupancies is Ca25Na4, in excellent agreement with the Ca26Na4 derived from chemi-cal analysis.

Site I is occupied in 70-1 but not in 63-3 at 300°C. Accepting this observation, it is possible to rationalize the cation populations at sites I and I' (The Ca occupancy at site II is similar for the two sieves.). The occupancy of site I in 70-1 is 0.63( 1). This site is only 2.62 Å from site I' too close for both sites to be occupied simultaneously by Ca and/or Na. It therefore seems likely that site I or I' will be occupied across any 6-ring separat-ing them, but not both. The sum of the site I and I' occupancies in 70-1 is 0.98, suggesting that occu-pation of one site does indeed preclude occupation of the other. There are few enough Ca and Na that the Na can occupy the more-favorable sites I' and II, rather than being displaced to site II'.

In 63-3, there are more Ca ions-too many to be accommodated at site I. They therefore prefer-entially occupy site I'. It is uncommon that this site is more than half-occupied. Cation-cation re-pulsions playa role in this, as the Ca-Ca distance in 63-3 is 4.00 Å, compared to 4.47 Å in 70-1. The Ca occupy site I' up to about half occupancy, and the remaining Ca reside at II. The small num-

ber of Na cations end up at II'. Computational studies, even at the molecular mechanics level, should shed light on the factors influencing cation site occupancies in these materials.

The cation site occupancies, and the move-ment of the sites with changes in temperature, are conveniently illustrated in graphical form in Fig. 9.

Lanthanum The LaNa-FAU 69-3 was not calcined, and

can be considered completely hydrated. Site II' was described primarily as Na, but a small occu-pancy by La was necessary to account for the ob-served electron density. Significant La was de-tected at sites I' and II, with La-O distances of 2.62(2) and 3.06(2) Å, respectively. The majority of the La occupies site V. As with the Ca-sieve 70-1, these La are >5 Å away from the nearest framework oxygens, and certainly represent com-pletely-hydrated cations.

Calcination of 69-3 at 300°C produced 61-1. In this sieve, lanthanum exhibits a strong "prefer-ence" for site I', with only small concentrations detected at II', II, V, and I. Site II' was described primarily as Na, with a fixed Na occupancy and a refined La occupation. The La-O distances de-creased to 2.56(1) and 3.00(1) Å for sites I' and II, respectively. Calcination of 61-1 for a second time, but at 500°C, yielded 70-2. The crystal structure is essentially identical to that of 61-1.

The LaNa-FAU 69-3 was also studied in its dehydrated state in situ at 300°C. Site II' seems to be occupied only by Na, as it was not necessary to fix the occupancy to obtain a reasonable chemical composition. The great majority of the La occu-pies site I', with a La-O3 distance of 2.524(6) Å. The site II-O2 distance decreases to 2.49(1) Å. These distances are much smaller than those in the hydrated samples. The bond valence valences of La at site I' and II are 1.43 and 1.69. At reaction temperature, the La are coordinatively unsatu-rated. The cation content calculated from the re-fined site occupancies is La16Na12, in excellent agreement with the La15Na13 derived from the chemical analysis.

Like the Ca sieves, the La cations move when the sieve is heated to 300°C. Unlike the Ca, the La remain at their high-temperature locations when the sieve is cooled to room temperature and hy-drated.

Vol. 12. No. 2 1995 27

A second La exchange of 69-3 and calcination at 300°C resulted in the LaY 64-2. This sieve con-tains an approximately 40% excess of cations, and thus the nature of some of the extraframework species may be different than in the rest of the sieves. The majority of the La occurs at site I' at room temperature (La-O3= 2.58(1) Å), with smaller concentrations at sites II', II, V, and I. This is the same pattern as 69-3, and the cation-oxygen distances are very similar. The composi-tion calculated from the refined site occupancies is La22Na5, in good agreement with the La22Na5 derived from chemical analysis.

This LaY was also studied in situ in its dehy-drated state at 300°C. The general pattern of site occupancies is the same as for 69-3. The La move from sites II, V, and I to occupy preferentially site I'. The I'-O3 and II-O2 distances of 2.54(1) and 2.53(1) Å are very similar to those observed in other La-containing sieves. In this refinement, the site I' occupancy is significantly greater than 1/2, perhaps indicating that the La occur as La-O-La dimers inside the sodalite cage [33]. Site II' seems

mostly occupied by Na, with a minority La con-centration.

In the presence of La, especially at 300°, the T-O2 and T-O3 distances become significantly longer. These oxygens interact most strongly with the cations at sites I' and II. The extension of the T -O bonds would then be expected from simple bond valence arguments.

The cation site occupancies, and their motion with changes in temperature, are illustrated in Fig. 10.

H(M) Sieves The HNa-FAU 58-4, HCa-FAU 64-1, and

HLaFAU 64-4 share similar features. One is that the refinements were difficult. For two of these, 584 and 64-1, much more electron density was observed at sites I', II', and II than could be ac-counted for by the cation contents. This extra den-sity was modeled as oxygen, representing water molecules. The isotropic displacement coeffi-cients of the framework atoms in all three of these sieves were significantly higher than in the other sieves, perhaps indicating the presence of defects and/or disorder. There is no chemical evidence for

Fig. 9 Cation site occupancies and locations in Ca-exchanged faujasites. All of the sites I, I’, II’, II and V can be transformed to equivalent sites on the 3-fold axis having fractional coordinates x=y=z. This 3-fold axis passes through the center of the thin section of the FAU structure superimposed on this diagram. The horizontal position of the vertical bars represents the position of the cation site on this 3-fold axis, and the height the population at this site. When two bars are present at a site, the bar on the left represents the Ca occupancy, and the bar on the right indicates the Na occupancy; a single bar at a site represents Ca occupation. Each row of bars represents the site populations for a particular sieve.

The Rigaku Journal 28

residual N from incompletely-decomposed am-monium ions.

In 58-4, sites I' and II were described as Na, and site II' as O. All three occupancies were re-fined. Since Ca seems to occur at all three sites I', II', and II, the Ca occupancies in 64-1 were fixed to equally distribute the cations over these three sites at the concentration calculated from the chemical analysis. The remaining electron density was described as oxygens. For 64-4, the small Na concentration was located at site II' with a fixed occupancy. Describing the remaining sites as La yielded a reasonable refinement. The cation-framework distances:

Sieve I'-O3, Å II-O2, Å I-O3, Å 58-4HNaY 2.64(1) 3.26 64-1HCaY 2.79(2) 3.19(2) 64-4HLaY 2.58(1) 2.98(1) 2.85(1)

are in general longer than those observed in the more cation-rich sieves. The cation content calcu-lated from the refined occupancies in 64-4 is La13Na2, in reasonable agreement with the chemi-cal analysis of La11Na2H21.

Calcium vs. Lanthanum The structures of both hydrated [4, 15-26] and

dehydrated [4, 21, 26-28] Ca-exchanged faujasites have been studied by powder and single crystal diffraction techniques. In the hydrated materials, the Ca are observed predominantly at sites I and I', with population of site II reported less-frequently. Site II' is almost always described as occupied by water molecules. There is considerable variation in the reported Ca site occupancies, reflecting dif-ferences in the preparations and thermal histories. Since the preparation of these materials varies, it is not clear how directly comparable the literature results are to our sieves. In dehydrated Ca fau-jasites, the Ca are observed predominantly at sites I and II, with lesser occupation of site I'.

In a recent single crystal study [29] of the de-hydration of CaX having the composition Ca46Al92S100O384·200H2O, Ca ions were located at sites I', II, and III in the hydrated form, with site II' occupied both by Ca and H2O. At 573 K, the Ca were located primarily at sites I and II, with small populations at sites I' and III. The cations migrated on dehydration, and on rehydration an inverse migration was observed, bring the site occupancies close to their original in the hydrated form. A very recent single crystal study of dehy-drated Ca48X at 360°C [30] indicated fully-

Fig. 10 Cation site occupancies and locations in La-exchanged faujasites. All of the sites I, I’, II’, II and V can be transformed to equivalent sites on the 3-fold axis having fractional coordinates x=y=z. This 3-fold axis passes through the center of the thin section of the FAU structure superimposed on this diagram. The horizontal position of the vertical bars represents the position of the cation site on this 3-fold axis, and the height the population at this site. When two bars are present at a site, the bar on the left represents the La occupancy, and the bar on the right indicates the Na occupancy; a single bar at a site represents La occupation. Each row of bars represents the site populations for a particular sieve.

Vol. 12. No. 2 1995 29

occupied sites I, with the remaining Ca located at site II.

Hydrated [4, 31-34] and dehydrated [4, 24-25, 34-43] La-FAU have also been studied exten-sively. In the hydrated materials, La is generally concentrated at site I', but is also reported at sites II and V (or delocalized in the supercage) in some samples. Site II' is always described as a water molecule. In dehydrated materials, La is observed concentrated at site I or I', depending on the mate-rial. A more-recent study of a highly-La ex-changed Y in the presence of steam at 660°C demonstrates that there are two La per sodalite cage (at site I'), and that these La are bridged by two hydroxyl groups to form T-O-La-(OH)2-La-O-T linkages stretching across the sodalite cages [44]. The cation locations and their migration in faujasites less highly-exchanged with La than ours have been studied by 23Na, 139La, 27Al, and 29Si NMR in a very recent (and very elegant) study [45].

With the exception of our interpretation of site II', the cation site occupancies generally parallel those previously reported. Since the reported cation locations vary, it was important to deter-mine them experimentally for these particular ma-terials. Our materials were prepared in a different manner, and have lower framework Al contents (and thus lower cation concentrations) than many of the materials studied previously. The overall cation density may affect cation locations, as cation-cation repulsions seem to playa role in de-termining site occupancies.

Calcium is distributed over sites I', II', and II at room temperature, with a preference for site I' as the Ca concentration is increased. At 300°C, site I is occupied significantly in sieve 70-1, but is vacant in 63-3. In 70-1, site II' is vacant, but lightly occupied in 63-3 at 300°C. Although the Ca move to different positions at high tempera-ture, they appear to move to their original loca-tions when the material is cooled and rehydrated; the structures of 70-1 (which has never been cal-cined) and 71-4 (which was calcined and rehy-drated) are very similar.

Before calcination, most of the La occurs at site V, far from the framework. Both at high tem-peratures, and after calcination, La exhibits a strong preference for site I', although small con-centrations occur at other sites. As the sieve is heated and dehydrated, the La move to new posi-

tions, which are retained as the sieve is cooled and rehydrated.

Ca and La exhibit different site preferences at high temperatures, and different mobility as the sieves are cooled and rehydrated. The interpreta-tion of site II' may not be important for correla-tions to catalytic performance, since cations lo-cated there are still inside sodalite cages, and in-accessible to organic molecule reactants and products.

Conclusions about the population inside the sodalite cages and double 6-rings vs. in the super-cages seem firm. Sites I', II', and I represent cations located inside the sodalite cages and dou-ble 6-rings, and thus inaccessible to organic mole-cules in a catalytic reaction. Sites II and V are in the supercages, and thus accessible. Arguments about accessibility of cations must be made with caution, since the presence of organic molecules may alter the relative stability of the cation sites. The most stable adsorption site for aromatic molecules is near site II [2].

The original intent behind this series of ex-changes was that the initial exchange would re-place the accessible ammonium ions with Ca or La, leaving the cations inside the sodalite cages intact. Calcination was thought to cause the Ca or La to move inside the sodalite cages, displacing the remaining Na to accessible locations, where they could be removed by a second Ca or La ex-change.

These thoughts were only partially correct. In the (NH4)Na-sieves, approximately 60% of the ammonium ions occupy accessible site II. In the uncalcined CaNa-FAU 70-1, only 40% of the Ca occupy accessible sites. Even at room tempera-ture, the Ca have found their way to inaccessible sites. The Na did indeed remain inside the sodalite cages. After calcination, only about 25-30% of the Ca occupy accessible sites, and some of the Na remains inside the sodalite cages.

In the uncalcined LaNa-FAU 69-3, 78% of the La occupy accessible sites. Once the La-containing sieves have been calcined, only about 10% of the La occupy accessible sites. The Na remaining in the sieves occurs in the sodalite cages. Such differences in cation accessibility should be important in rationalizing catalytic per-formance.

The Rigaku Journal 30

Global Parameters In addition to the usual "single crystal" struc-

tural parameters, a Rietveld refinement requires several other "global" parameters. Useful chemi-cal information can be obtained from their refined values.

Lattice Constants The average value for a tetrahedral Al-O bond

is 1.74 Å, and the average value for a tetrahedral Si-O bond is 1.60 Å. As the framework aluminum concentration in a zeolite increases, one would expect the lattice parameters to increase also.

Three correlations between the cubic unit cell parameter of faujasite and the framework alumi-num concentration are in common use: Breck-Flanigen [46], Kerr-Dempsey [47-49], and Ficht-ner-Schmittler [50]. Sohn [51] and Beyer [52] have reported correlations very similar to those of Kerr-Dempsey and Fichtner-Schmittier.

These three correlations relate the number of framework Al per unit cell (out of 192 tetrahedral sites/cell), NAl, to the lattice parameter, a:

Correlation Equation, NAl= Breck-Flanigen 115.2 (a-24.191) Kerr-Dempsey 112.1 (a-24.222) Fichtner- Schmittler 107.1 (a-24.238)

These correlations follow the same general trend, but are not coincident. Considering the quality of the fits and the uncertainly in the framework Al concentrations used to derive the correlations, we can expect an accuracy of 1-2 Al/unit cell from them.

The Breck-Flanigen correlation was derived for samples of hydrated NaX and NaY. Dempsey and Kerr used samples with a wide range of alu-minum contents, carefully prepared to contain only framework Al. Most of the sieves were in the ammonium form, but some H-faujasites were in-cluded. Sohn's correlation involved NH4/Na, H, and NH4 exchanged forms, and the samples con-tained extraframework aluminum. The Beyer sieves were dealuminated by the SiCl4 technique, and were studied in the hydrated ammonium form.

Since we know that ammonium exchange of Na-FAU can result in an increased lattice parame-ter, and that the cell dimension increases as the sieve is dehydrated [5], it is not clear which corre-lation is most appropriate to use for any individual sieve. If we accept the common consensus that the framework Al content of the NaFAU LZ-Y52 is

56Al/cell, this material falls only on the Breck-Flanigen correlation. In this study, structural trends have been analyzed as function of lattice parameter because the choice of the appropriate correlation is not clear.

We observe a reasonable and typical value of 24.6773(2) Å for the lattice parameter of hydrated NaY. Only small changes are observed for ammo-niumor Ca-exchanged materials at room tempera-ture. The unit cell of the never-calcined LaNaY 69-3 is also identical to that of the starting mate-rial. La-containing sieves which have been sub-jected to a calcination exhibit increased cell di-mensions. These differences in lattice parameter reflect the strength of the cation-framework inter-actions and cation mobility.

All four of the sieves measured at 300°C ex-hibit increased unit cell sizes compared to room temperature. This increase could result merely from dehydration [5], or reflect changes caused by the cations. At 300°C, the cell of 70-1 is 0.075 Å larger than at room temperature, while that of 63-3 increases only by 0.037 Å. Since the Ca occupy different sites in these sieves at high temperature, there does seem to be an effect of the cations on the cell dimension.

The unit cell of 61-1 is 0.046 Å larger than that of 69-3 at room temperature. The calcination and rehydration, which result in changed La posi-tions, increase the cell size. At 300°C, the cell of 69-3 is larger than that of 61-1 by 0.075 Å, and the cell dimension of 64-2 increases by 0.069 Å.

This 0.07 Å change seems to be attributable to dehydration, since the La occupy about the same positions in all of these sieves.

Both cation locations and the state of hydra-tion can affect the lattice parameters. The ob-served cell variations correspond to framework Al concentrations of 54-72 Al/cell, using the Breck-Flanigen correlation. It is clear that caution must be used when using the reported correlations to calculate framework Al concentration. Since the average T-O distance remains constant among these Ca- and La-sieves, we have no evidence for real changes in framework Al concentration.

Scale Factors-Crystallinity In GSAS, the scale factor of a phase is pro-

portional to the number of unit cells of that phase present in the sample. In general, knowledge of the number of unit cells is sufficient to permit the

Vol. 12. No. 2 1995 31

straightforward calculation of the quantitative phase analysis [53, 54], since the unit cell contents are unambiguous.

For hydrated zeolites, this is not the case. From the refinement, we know the number of unit cells present in the sample, but not their complete contents. All of the water molecules are omitted from the structural model, since they are too-disordered to be located.

Knowledge of the cell contents is important because the samples were prepared by adding a known mass of quartz internal standard to a known mass of hydrated zeolite. From the refined scale factors, we can easily calculate the relative concentrations of faujasite and quartz on a dry basis. To convert these relative concentrations into absolute concentrations, and thus determine the concentration of amorphous material in the samples, the true concentration of quartz must be known, and it is known only for the hydrated samples.

The consensus description of the water con-tent of hydrated NaY (LZ-Y52) in the literature is Na56Al56Si136O384·220H2O, which corresponds to 23.7 wt% water. We measure (by TGA) the water content of LZ-Y52 to be 24.36%. Use of this value results in a faujasite content of 97(1) wt% in the sieve 097 (LZ-Y52).

The water concentrations in the Ca- and La-exchanged sieves were determined by TGA weight loss at 300°C, and average about 24 wt% (with a range of 21.7-26.6%). Replicate measure-ments of certain sieves indicate a reproducibility of about 0.7% absolute.

The water content of ammonium-exchanged sieves is generally somewhat lower, about 22.0%. Using this value, we derive a sieve content of 95(1) wt% for the sieve 185-F. It proves difficult to determine the water content of NH4-sieves, since there is no temperature range in which water desorption takes place without ammonium ion decomposition. Implicit in this discussion is the assumption that the vast majority of the water in a partially-crystalline sieve resides in the sieve, and not in the amorphous component.

In a study of a sieve as a function of alumi-num concentration, an additional complication can be anticipated. It is well known that the hydro-philicity of a sieve decreases as the framework aluminum concentration decreases. Thus the water

content of these sieves would be expected to vary with aluminum concentration.

The water contents of the three H-FAU 17-3, 20-6, and 20-7, and the NH4-FAU 44-3 have been determined by thermogravimetric analysis:

Sieve 20-7 20-6 17-3 44-3

H2O, wt% 3.95 10.1 21.6 22.38

The water content does indeed decrease as the framework aluminum concentration, and thus the lattice parameter, decreases (Fig. 11). For cells smaller than about 24.45 Å, good correlation is observed:

wt% H2O in FAU=78.73a-1905.6 For larger cells, the water concentration is ap-proximately constant at 22.0 wt%. For the calcula-tion of quantitative phase analysis, these relations were used to calculate the water content in un-measured sieves. The framework (and thus cation) contents were calculated using the Kerr-Dempsey correlation for NH4-FAU in which cations not detected.

The crystallinities (=faujasite concentrations) calculated from the refined scale factors are re-ported in Table 5. These crystallinities are in good agreement with those calculated from the micro-pore volume, especially for sieve concentrations <70%. Apparent large differences between the micropore volume results and the traditional pow-der diffraction analysis result entirely from the effects of structural variation on the diffracted intensities. As the degree of dealumination in-creases (the severity of the steam treatment in-creases), the crystallinity drops markedly. It is important to realize this when assessing the cata-lytic performance of such materials.

The measured crystallinity of the NH4-sieve 44-3 is 108(2)%, and the calculated crystallinities of several of the Ca- and La-exchanged sieves are slightly greater than 100%. While these anoma-lously-high values could be explained by an actual water content lower than that assumed in the cal-culation, the water contents were determined ex-perimentally. The difference is only 4 ESD from 100% (and we know that the ESDs are underesti-mated),

While we do not have a good explanation for these anomalously high results, we can suggest two possible contributing factors. Microabsorp-tion effects would be expected to yield high fau-

The Rigaku Journal 32

jasite crystallinities, since the sieve is the least-highly-absorbing phase in these samples. Such effects would be larger as the quartz concentration is increased.

We have assumed that all of the TGA weight loss represents water present within the crystalline lattice of the sieve. If some of the water is the small concentration of amorphous component, anomalously high sieve concentrations would be calculated. The value of the micropore volume (0.348cc/gm) we use to represent 100% crystal-line faujasite corresponds to 222 H2O/unit cell of liquid water for a framework of this composition. Since some of the cations occupy sites in the su-percages, they occupy some of this micropore volume. Some measured water contents corre-spond to over 240 H2O/cell, and indicate that some water resides inside the sodalite cages

and/or external to the sieve. It is clear that we do not yet understand everything about calculating the crystallinity of a zeolite.

Profiles The GSAS pseudo-Voigt instrumental profile

function for the X381 instrument, determined us-ing SRM1976 alumina plate, has the coefficients: U=6.427, V=-1.067, W=O, X=0.6102, Y=0.6796, and asym=0.6733; all the other coefficients are zero.

Like most faujasites we have examined, the line profiles of these sieves are dominated by strain rather than size broadening, but the exact interpretation of the strain broadening is unclear. Most of the samples exhibit only a small amount of size broadening, corresponding to an average crystallite size of 2500-3400 Å. The two lowest-

Fig. 11 The measured water contents of NH4- and H-faujasites as a function of framework aluminum concentration.

Table 5 Composition and crystallinity of NH4- and H-FAU NH4-FAU H-FAU

46-4 22.3(3) % Si192O384·2H2O 20-8 22.3(3) % Si192O384·2H2O 46-2 48.3(4) % (NH4)2Al2Si190O384·20H2O 20-7 49.8(5) % H3Al3Si189O384·24H2O 46-3 60.6(7) % (NH4)14Al14Si178O384·81H2O 20-6 62.3(7) % H13Al13Si179O384·75H2O 46-1 85.1(9) % (NH4)26Al25Si166O384·189H2O 20-4 69.4(9) % H26Al26Si166O384·167H2O

17-3 74.2(9) % H28Al28Si164O384·174H2O 66-1 92.0(17) % H34Al34Si158O384·181H2O

46-5 96.9(19) % (NH4)41Al41Si151O384·189H2O 44-3 108(2) % (NH4)50Al50Si142O384·191H2O

Vol. 12. No. 2 1995 33

Al sieves, 46-4 and 20-8, exhibit significant size broadening, corresponding to average domain sizes of 1600 and 1100 Å, respectively.

The ammonium-forms generally display slightly higher strain broadening than the H-forms of similar framework Al concentration. For both forms, the degree of strain increases monotoni-cally with the degree of dealumination. The func-tional form of the strain broadening varies with framework Al concentration; at high [Al] (includ-ing the cation-exchanged sieves), the broadening is primarily gaussian, while in the most highly-dealuminated sieves it is cauchy (Lorentzian). We don't know what the differences in profile shapes mean. The profile U terms are significantly larger at 300°C that at room temperature. There is a gen-eral increase in U as the number of calcinations increases.

The profile U terms of the H(M)-FAU 58-4, 64-1, and 64-4 are significantly larger than the others, indicating larger amounts of strain.

Conclusions This combination of synchrotron powder dif-

fraction, Rietveld analysis, and in situ measure-ments on a series of systematically-varied materi-als has led to a much more detailed understanding of the changes in cation site occupancies during the ion exchange process. This study represents an attempt to extract the maximum information pos-sible from powder diffraction patterns. As power-ful as these techniques are, they cannot provide the complete answer. Confirmation of our inter-pretation of the occupancies of mixed sites re-quires application of other analytical techniques, particularly magic angle spinning nuclear mag-netic resonance spectroscopy, resonant X-ray dif-fraction, and neutron powder diffraction.

Acknowledgments We thank J. T. Miller and P. D. Hopkins for

the samples and their interest in this problem. G. J. Ray, B. L. Meyers, and S. Pei collaborated in this work, and provided the NMR and adsorption data against which the diffraction results are com-pared. Ying-Mei Chen provided technical support. We thank Peter W. Stephens (SUNY Stony Brook) and Robert Dinnebier (NSLS X-3) for their assistance in data collection and hospitality.

This work represents research carried out in part at the National Synchrotron Light Source at

Brookhaven National Laboratory, which is sup-ported by the U.S. Department of Energy, Divi-sion of Materials Sciences and Division of Chemical Sciences. The Suny X3 beamline at NSLS is supported by the Division of Basic En-ergy Sciences of the U.S. Department of Energy (DE-FGO2-86-ER45231 ).

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