Collapse and Recrystallization Processes in Zinc-Exchanged Zeolite-A

but the precipitation of NaAlSiO4 (nepheline) is blocked by the nucleation of ZnAl2O4 (gahnite), which persists to at least 1100 °C. Within the hydra...
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J. Phys. Chem. B 1997, 101, 10105-10114

10105

Collapse and Recrystallization Processes in Zinc-Exchanged Zeolite-A: A Combined X-ray Diffraction, XAFS, and NMR Study L. M. Colyer,† G. N. Greaves,*,‡ S. W. Carr,§ and K. K. Fox§ Department of Chemistry, UniVersity of Keele, Staffordshire, U.K. ST5 5BG, Department of Physics, UniVersity of Wales, Aberystwyth, Ceredigion, U.K. SY23 2BZ, and UnileVer Research, Quarry Bank Road, Bebington, Wirral, U.K. L63 3JN ReceiVed: June 3, 1997; In Final Form: September 21, 1997X

The collapse and recrystallization of zinc-exchanged sodium zeolite-A has been observed in situ using combined X-ray spectroscopy and diffraction. The temperature at which this occurs is reduced by the presence of zinc, but the precipitation of NaAlSiO4 (nepheline) is blocked by the nucleation of ZnAl2O4 (gahnite), which persists to at least 1100 °C. Within the hydrated zeolite phase zinc occupies disordered octahedral sites that become less disordered with calcination. When zeolite-A collapses, zinc adopts a tetrahedral but strongly disordered geometry, suggesting nonhomogeneity in the amorphous phase from which ZnAl2O4 precipitates. Combined X-ray methods have been complemented by solid-state 27Al and 29Si NMR, which have enabled estimates to be made of the extent of disorder in the starting zinc-exchanged zeolite-A structure as well as the degree of crystallinity in the gahnite aluminosilicate glass ceramic that develops after collapse.

Introduction Establishing the structural reasons behind thermal stability in zeolites is a critical step in understanding not only the fragility of their low-density architecture at high temperatures but also the propensity of these materials to collapse and recrystallize to form high-density ceramics at relatively low temperatures. Zeolite materials in fact exhibit a wide range of thermal stability, stretching from paulingite and phillipsite that collapse at around 250 °C to mordenite and offretite that maintain their microporous structures to temperatures in excess of 800 °C.1 Furthermore, the ease with which the composition of a given zeolite can be modified through cationic exchange can serve both as a means of altering the stability of the microporous phase and as a means of providing variety in the ceramics generated through subsequent recrystallization.2-5 Compared to the traditional processes of glass annealing or powder sintering, ceramic formation from zeolitic precursors has the advantage of being accomplished at comparatively low temperatures. In almost all cases the recrystallization of zeolites incorporates an intermediate phase. All the indications are that this phase is amorphous but is generated at temperatures well below the melting point of the corresponding aluminosilicate glass. The nature of what amounts to an order-disorder transition is unclear. Work on sodium zeolite-A (zeolite Na-A), which is the zeolite best studied in this regard to date and the subject of this paper, has revealed the following empirical picture. The temperature at which the microstructure collapses and recrystallization is established can occur anywhere between 820 and 1100 °Csthe lower temperatures prevailing when the rate of heating is slow6 and vice versa. The transformation apparently shares some phenomenology with the glass transition. An additional feature concerns the inclusion of water during heat treatment, which can dramatically reduce the temperature of zeolite collapse, particularly under pressure, presumably through the hydrolytic severance of bridging oxygens. Under 100 kPa †

The University of Keele. The University of Wales, Aberystwyth. § Unilever Research. X Abstract published in AdVance ACS Abstracts, November 1, 1997. ‡

S1089-5647(97)01800-2 CCC: $14.00

of steam, zeolite Na-A collapses to an amorphous phase at 350 °C.7 The breaking of oxygen bridges, however, does not appear to be unilateral. In their high-resolution electron microscopy studies of zeolite Na-A Thomas and Bursill8 found evidence for the retention of sodalite and supercage fragments throughout the process of collapse, signifying the preservation of a high degree of short-range order. Lutz and co-workers6 have argued that an intermediate phase furnished with these units provides a low-temperature route to recrytsallisation in zeolite Na-A. After collapse, zeolite Na-A recrystallizes first to β-carnegieite and then to nepheline (NaAlSiO4).9,10 The temperature of recrystallization is very dependent on the temperature of collapse so that with extended heating at intermediate temperatures recrystallization can be complete even by 900 °C.6 Both β-carnegieite and nepheline are polymorphs of NaSiAlO4 and are isomorphous with the SiO2 phases β-crystobalite and tridymite, respectively. As in the structure of these SiO2 isomorphs, the β-carnegieite and nepheline networks contain 6-fold aluminosilicate rings charge compensated by contained, nearby sodiums. Both of these feldspar phases along with the starting zeolite Na-A exhibit complete silicon-aluminum ordering. Since the soldalite cages that are the major polyhedral unit in the structure of zeolite-A are also constructed from ordered 6-fold rings, if these are a feature of the intermediate phase,6,8 then it is clear that some degree of silicon-aluminum ordering must be retained throughout the transformation from zeolite Na-A to nepheline. This would explain the lower temperatures for precipitation of nepheline from zeolite Na-A compared to that from nepheline glass where silicon-aluminum ordering is much reduced. To date X-ray diffraction (XRD) has been the main structural tool by which zeolite collapse and recrystallization have been followed.6,10 Adsorption capacity can also give a measure of the fraction of intermediate phase that is formed.9 More recently, though, significant advances have been made in tracing the structural stability of microporous solids by Thomas and colleagues through the combination of XRD with X-ray absorption fine structure (XAFS) spectroscopy.11,12 In conjunction with specially designed furnaces,13 it has been possible to © 1997 American Chemical Society

10106 J. Phys. Chem. B, Vol. 101, No. 48, 1997 correlate the substantial changes in long-range order obtainable from XRD with alterations in short-range order deduced from XAFS. One of the initial demonstrations of this technique was the collapse of magnesium-exchanged sodium zeolite-B (zeolite Mg/Na-B). This and its recrystallization to Mg2Al4Si5O18 (cordierite) was found to be catalyzed by ZnO, which appeared to compensate for a slight excess of aluminum in the starting material. The complete transformation from zeolite to aluminosilicate ceramic was recorded in situ for the first time using combined XAFS-XRD.12 Collapse (detected by XRD) was seen to coincide with the dissolution of ZnO (detected by XAFS), and the subsequent devitrification of cordierite glass was found to be pre-empted by an intermediate-stuffed quartz phase. An extension of this methodology has been to combine XRD with small-angle X-ray scattering (SAXS), and as a result, it has been possible to directly correlate changes in crystallinity with alterations in the microstructure as they occur during heat treatment.14,15 In this paper we report the application of combined XAFSXRD to directly observe the collapse and recrystallization of zinc-exchanged zeolite Na-A (zeolite Zn/Na-A). The in situ XAFS-XRD experiments described here were complemented with ex situ solid-state 27Al and 29Si NMR measurements to highlight changes in the aluminosilicate network with heat treatment as the microporous structure of zeolite-A is exchanged for a dense glass ceramic. Experimental Section Preparation of Materials. Zeolite Na-A was supplied by Unilever. It was washed in distilled water for 170 h to remove the initial alkalinity and then dried at 120 °C for 5 h. Ion exchange with zinc was conducted at 60 °C for 130 h with regular stirring (15 g being added to 200 mL of 0.1 M zinc nitrate). After zeolite Zn/Na-A was washed and dried, its composition determined by analytical atomic spectroscopy was Zn3.1Na5.8Si12Al12O48‚26H2O. The cyrstalline model compounds for XAFS and XRD analysis were ZnO and ZnAl2O4. ZnO was supplied by BDH Chemicals, and ZnAl2O4 was prepared by sintering ZnO and Al2O3. Specimens for in situ XAFS-XRD experiments were prepared as self-supporting thin disks in the case of zeolite Zn/Na-A. Samples of ZnO were made by diluting the powder in fumed silica prior to pelletizing. ZnAl2O4 powder, which was measured at room temperature, was supported on sellotape. In all cases sample thickness was chosen so that specimens were approximately 10% transmitting. Specimens for solid-state NMR, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were prepared from loose powders in the usual fashion. Combined XAFS-XRD. The experimental arrangements for XAFS-XRD and their application to in situ high-temperature measurements have been well-described elsewhere.11-13 XAFS spectra in this study were obtained in transmission geometry using ion chambers, and XRD powder patterns were collected in reflection as shown in Figure 1 using an INEL curved position-sensitive detector. A miniature annular furnace was used (Linkam Scientific Instruments), and this was inclined at a fixed angle with respect to the incoming X-rays to accommodate both sets of detectors. Measurements made in air and furnace temperatures were controlled by a thermocouple attached to the annular heater. The specimen temperatures quoted below are accurate to (10 °C and refer to values that were separately calibrated from an on-axis thermocouple with respect to the furnace element control thermocouple.

Colyer et al.

Figure 1. Schematic of experimental arrangement for combined XAFS-XRD experiments. See refs 12, 16, and 17 for further details.

The combined XAFS-XRD experiments described in this paper were performed on the superconducting wiggler station 9.3 at the CLRC Synchrotron Radiation Source at Daresbury Laboratory. The SRS was operating at 2 GeV and with a circulating current of ∼200 mA. A vertically focusing mirror was used in conjunction with a rapidly scanning order-sorting monochromator.17 The furnace heating schedule began with rapid heating (40 °C min-1) to 590 °C, which was then followed by a gradual ramp (1.7 °C min-1) to 816 °C. Each XRD powder pattern was collected over 380 s at 10.3 keV (1.2 Å) to avoid Zn fluorescence. The XAFS spectrum that followed lasted for 180 s and covered the energy range 9.5-10.1 keV. With a 40 s dead time for slewing the monochromator, the total XAFSXRD cycle time was 600 s. Data collection began at 590 °C and ended at 816 °C. Solid-State NMR. Ex situ solid-state NMR 29Si and 27Al spectra were obtained on a Bruker MSL 400 at the EPSRC solidstate NMR facility at UMIST. 29Si data was recorded at 79.5 MHz and a spinning speed of 4.5 kHz. For 27Al spectra the spectrometer frequency was 104.6 MHz and the spinning speed 10 kHz. Thermal Analysis. In situ TGA and DSC were performed in flowing nitrogen from ambient temperature to 1050 °C at a heating rate of 10 °C min-1 using alumina as a standard. Results Combined XAFS-XRD. In situ combined XAFS-XRD results that follow the heat treatment of zeolite Zn/Na-A described above are displayed in Figure 2. Each Zn K-edge XAFS spectrum was individually background subtracted using the Daresbury EXBACK code, and the corresponding Fourier transforms are plotted sequentially from 590 to 816 °C in Figure 2a. The height of the main peak in the radial distribution function falls as collapse approaches but then sharpens again as recrystallization takes place, despite the increasing temperatures. There are also significant changes in the structure around zinc beyond nearest-neighbor distances that coincide with collapse and recrystallization. The Fourier transform of the XAFS spectrum collected at 726 °C marks the collapse to an amorphous intermediate, as judged from the environments of zinc atoms. The drop in the height of the first peak in Figure 2a is greatest, and this is also accompanied by the virtual disappearance of subsequent peaks characteristic of the crystalline phases. The XRD powder patterns that were obtained in tandem, normalized to allow for the decay of the beam current of the SRS, are shown in Figure 2b and cover the temperature range 595-811 °C. This sequence reveals even more dramatically the point of collapse of zeolite Zn/Na-A. The diffuse scatter of the 714 °C profile contrasts sharply with the DebyeScherrer patterns of the starting zeolite and the new crystalline phase that subsequently precipitates out. From the combined XAFS-XRD results presented in Figure 2 this transformation

Zinc-Exchanged Zeolite-A

Figure 2. In situ combined XAFS-XRD experiments following the heat treatment of zeolite Zn/Na-A: (a) Fourier transforms of Zn K-edge XAFS from 590 to 816 °C; (b) beam decay-corrected XRD powder patterns from 595 to 811 °C. After rapid heating at 40 °C min-1 from ambient to 590 °C, the temperature was raised at 1.7 °C min-1 to 816 °C.

takes place at 720 °C. It can be seen more clearly in Figure 3 where the changing diffraction peak areas characteristic of the two crystalline phases are plotted. In situ combined XAFS-XRD experiments were also performed on ZnO, and the results have already been published.12 They have been used in this study of zeolite Zn/Na-A both to provide a model compound spectrum for zinc XAFS and also to provide a baseline for the thermal component to the Zn-O XAFS Debye-Waller factor. The room-temperature XAFS spectrum of ZnO is shown in Figure 4a. The spectrum of ZnAl2O4 also recorded at room temperature is included in Figure 4b. Solid-State NMR. Ex situ solid-state 29Si and 27Al NMR spectra recorded at room temperature for zeolite Zn/Na-A are plotted in parts a and b of Figure 5, respectively. Data were obtained both from the as-prepared material and for a specimen that had been held at 1100 °C for 2 h in air. The narrow 29Si resonance at -89.1 ppm in as-prepared zeolite Zn/Na-A clearly

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Figure 3. Changes in crystallinity and local order during the in situ heat treatment of zeolite Zn/Na-A: (a) integrals of the zeolite-A 210 and gahnite 311 reflections from Figure 2 plotted against temperature and showing the respective decay and growth of these two crystalline phases; (b) interatomic distance, RZn-O; (c) the static Debye-Waller factor, 2σZn-O2, deduced from analysis of in situ Zn XAFS of heattreated zeolite Zn/Na-A. Errors are given in the text. (b) and (c) show the effect on the zinc environment of the emergence of the amorphous intermediate from zeolite Zn/Na-A followed by the gradual precipitation of the gahnite phase.

broadens after heat treatment but also shifts to -92.5 ppm. By contrast, 27Al exhibits a narrow line at 59.1 ppm plus a broader feature centered around 52 ppm in as-prepared zeolite Zn/NaA. After heat treatment, though, the narrow resonance disappears, the broad distribution narrows a little, and a new sharp doublet feature emerges close to 9 ppm. Thermal Analysis. In situ DSC traces from 20 to 1050 °C are presented for zeolite Zn/Na-A in the upper half of Figure 6a. The corresponding traces from 20 to 1300 °C for the untreated zeolite Na-A are shown in Figure 6b for comparison. Parallel TGA scans indicate a total weight loss of 20.7% for zeolite Zn/Na-A and 23.9% for zeolite Na-A, which is entirely due to the removal of water from the starting material. The primary endotherm for dehydration is at 217 °C for zeolite Zn/ Na-A compared to the higher figure of 224 °C for zeolite NaA. The temperatures of collapse and recrystallization are also marked in Figure 6. In particular the collapse temperature of

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Figure 4. Room-temperature Zn K-edge XAFS data for the model compounds ZnO (a) and ZnAl2O4 (b). In each case the weighted fine structure is shown above and the Fourier transform below. The solid curves refer to experiment and the dotted curves to least-squares fitting using EXCURVE with E0 ) 15 eV, VPI ) -2.5, and AFAC ) 0.8. The local structures for zinc obtained from XAFS are compared to crystallographic values for both models in Table 1.

Figure 5. Ex situ solid-state 29Si and 27Al NMR for zeolite Zn/Na-A. 29Si spectra are shown in (a) with 27Al in (b). The as-prepared material is shown above and the material after heat treatment at 1100 °C below. Structure in the 9 ppm 27Al peak is due to quadrupolar effects.18

825 °C for zeolite Zn/Na-A is significantly lower than the figure of 900 °C for zeolite Na-A. Moreover, it is clear from Figures

6a and 3a that the temperature of collapse of zeolite Zn/Na-A from in situ DSC compared to in situ XAFS-XRD is even

Zinc-Exchanged Zeolite-A

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TABLE 1: Environments of Zinc in ZnO 19 and ZnAl2O4 20 Crystalline Models: X-ray Diffraction and Room-Temperature XAFS Compared crystallographical determined environment model ZnO

ZnAl2O4

a

XAFS determined environment

coordination number

atom type

interatomic distance, Å

coordination number

atom type

interatomic distance, Å

Debye-Waller factor, Å2

4 6 1 6 3 6 6 6 3 6 2 12 6 4 12 12 4 12 12 16

O Zn O Zn O O Zn O O O Zn Zn Zn O Al O Zn O O Al

1.97 3.20 3.21 3.24 3.79 3.80 4.56 4.56 4.95 4.99 5.20 5.59 5.62 1.95 3.35 3.39 3.50 4.28 5.10 5.25

4

O

1.96

0.009

12

Zn

3.21

0.021

9 6 6

O Zn O

3.72 4.54 4.43

0.030 0.022 0.026

9 2

O Zn

5.08 5.28

0.046 0.013

18 4 12 12 4

Zn O Al O Zn

5.58 1.93a 3.35 3.28 3.71a

0.037 0.010 0.014 0.022 0.032

12 4

Al Al

5.29 5.48a

0.011 0.024

4 12

O Zn

5.45 5.72

12

Zn

5.65

0.019

Multiple scattering was included for these shells, without which interatomic distances were reduced by 0.02 Å.

Analysis and Discussion

Figure 6. In situ DSC scans for zeolite Zn/Na-A (a) and zeolite Na-A (b). Data were collected against an alumina standard in flowing nitrogen at a heating rate of 10 °C min-1. The temperatures of the primary endotherms for hydration and the exotherms for zeolite collapse and for recrystallization are marked. Note that the temperatures of dehydration and collapse are significantly lower for zeolite Zn/Na-A than for zeolite Na-A.

further depressed from 825 to 720 °C. This reflects the reduction in heating rate from 10 to 1.7 °C min-1 and demonstrates the sensitivity of both XAFS and XRD in detecting phase transitions over protracted heating schedules.

X-ray Diffraction of As-Prepared and of Material Heat Treated at 1200 °C. X-ray powder diffraction patterns for the starting material zeolite Na-A and for the as-prepared zeolite Zn/Na-A were determined in separate ex situ measurements. The XRD pattern for zeolite Na-A was completely consistent with the cubic Pm3m structure21 with a lattice parameter of 12.297 Å. The lattice parameter refined for zeolite Zn/Na-A was a ) 12.167 Å, the powder pattern matching that of hydrated partially zinc-exchanged zeolite Na-A.22 The in situ XRD pattern at room temperature was indexed with respect to the latter, and as the temperature rose, the diffraction peaks, although reduced in intensity, survived temperatures up to 697 °C. The ex situ room-temperature pattern of material heat treated at 1200 °C for 20 h was identified as the Fd3m spinel gahnite ZnAl2O4 with a refined lattice parameter a of 8.091 Å. This pattern was used to index the in situ XRD structure emerging at 731 °C and growing consistently through to 811 °C. As noted above, neither zeolite Zn/Na-A nor gahnite XRD lines were present at 714 °C (Figure 3a). XAFS Model Compounds. The dotted lines in Figure 4 correspond to least-squares refinement of the Zn K-edge k3weighted XAFS for the model compounds ZnO (Figure 4a) and ZnAl2O4 ( Figure 4b). The fine structure is shown above and the Fourier transform below. The crystallographically determined environments for zinc in ZnO 19 and ZnAl2O4 20 are listed in Table 1, and these were used as starting structures for XAFS refinement using the Daresbury analysis program EXCURVE. The local structures of zinc obtained from K-edge XAFS analysis are also listed in Table 1. These demonstrate quite close agreement, with nearest-neighbor oxygen distances within 0.02 Å. Multiple scattering was incorporated into the refinement of the ZnAl2O4 spectrum for fourth and sixth shells. Although fitting was extended as far as 5.5 Å, analysis is mainly reliable out to 3.5 Å (i.e., for nearest-neighbor oxygens and cations). The calculated O, Zn, and Al phases shifts and the refined values for E0 (15 eV), VPI (-2.5 eV), and AFAC (0.8) were carried over for the analysis of in situ zinc K-edge XAFS at elevated

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Figure 7. Increase in the first-shell Debye-Waller factor in ZnO, 2σZn-O2, due to thermal disorder at temperatures from 25 to 820 °C. These values were analyzed from in situ XAFS measurements of ZnO, using the starting structure shown in Figure 4a and the parameters listed in Table 1.

Figure 9. k3-weighted Zn K-edge XAFS spectra and Fourier transforms for zeolite Zn/Na-A heat treated and measured at 726 °C (solid lines). Least-squares refinement results obtained using EXCURVE are shown with dotted lines. The model structure comprises one shell of 4.(4) oxygens at 1.91 Å and with a Debye-Waller factor of 0.046 Å2 (E0 ) 19 eV, VPI ) -2.5, and AFAC ) 0.8). Correlations between the coordination number and Debye-Waller factor are given by the marked contour in Figure 11.

Figure 8. k3-weighted Zn K-edge XAFS spectra and Fourier transforms for hydrated zeolite Zn/Na-A obtained at room temperature (solid lines). Least-squares refinement results obtained using EXCURVE are shown with dotted lines, and the model structure is listed in columns 1 and 5 of Table 2 (E0 ) 18 eV, VPI ) -2.5, and AFAC ) 0.8). Correlations between first-shell coordination number and Debye-Waller factor are given in Figure 11.

temperatures. To establish the Debye-Waller factor for zincoxygen correlations, 2σZn-O2, at high temperatures in the absence of static disorder, separate in situ measurements of crystalline ZnO were made.12 2σZn-O2 values were derived from in situ spectra using as a starting point the zinc environment analysis obtained from the room-temperature XAFS shown in Figure 4a with the parameters listed in Table 1. 2σZn-O2 was observed

to rise linearly with temperature as shown in Figure 7, more than doubling in size between 25 and 820 °C. The temperaturedependent Debye-Waller factors obtained from ZnO have been used to estimate the static disorder present in zinc environments during the heat treatment of zeolite Zn/Na-A. Local Environments versus Crystallography for Zeolite Zn/Na-A at Ambient Temperature and at 658 °C. Selected in situ zinc K-edge XAFS data for zeolite Zn/Na-A from the heat treatment sequence illustrated by the Fourier transforms in Figure 2a were analyzed in detail using the photoelectron parameters obtained from fitting the crystalline models. Spectra at 25, 658, 726, 777, and 811 °C were chosen, and the comparison between experimental data (solid curves) and leastsquares fitting (dotted curves) are shown for 25 °C in Figure 8, for 726 °C in Figure 9, and for 811 °C in Figure 10. The model zinc environments obtained from this analysis are listed in Tables 2 and 3. XAFS analysis by default incurs some correlation between derived coordination numbers and DebyeWaller Factors 2σ2. These correlations are shown in Figure 11 where contours of 95% significance are drawn for first-shell oxygen numbers and bond length variances. Tables 2 and 3 include crystallographic zinc environments for comparison with the data analyzed from XAFS. Detailed crystallographic studies have been made of hydrated22 and dehydrated23 zinc-exchanged zeolite Na-A. The degree of ion exchange is different in each case, though, and also different from the composition of zeolite Zn/Na-A used in this study.

Zinc-Exchanged Zeolite-A

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TABLE 2: Averaged Crystallographic Environments for Zinc in Hydrated22 and Dehydrated23 Zeolite Zn/Na-A Compared to XAFSa hydrated zeolite Zn/Na-A interatomic distance, Å 1.93 1.87 1.98 1.99 2.00 2.11 2.135 2.19 2.20 2.30

Zn1 (8g)

XAFS room temp, CN D-W/Å2

Zn1 (8g)

Zn2 (8g)

XAFS 658 °C, CN D-W/Å2 5.1 O, 0.032 6.2 O, 0.037

3O 7.0 O, 0.025 6 O (H2O)

4 O (O/H2O) 6 O (H2O) 1.0 O, 0.045 1.1 O, 0.034

7 O (O/H2O) 6 O (H2O)

4 Na

6 O (O/H2O) 1 O, (H2O) 3O

1 O 0.031

1.8 Si, 0.020

1.8 Si, 0.036

7 Si/Al

3.35 3.54 3.65 3.85 3.92

Zn3 (6f)

6.0 O, 0.022

2.80 2.98 3.04 3.16 3.08 3.21

Zn2 (1a)

dehydrated zeolite Zn/Na-A

6 Si/Al 8 O (H2O)

1.6 Zn, 0.029

0.5 Zn, 0.021 6 O, (H2O) 3 Zn

3 O, (H2O)

8 O, (H2O)

12 O, (O/H2O)

3.0 O, (0.068)

3.5 O, 0.053

a Italic characters refer to the split oxygen shell employed in the analysis to mimic the crystallographic arrangement. Errors are given in the text. Note that XAFS for the hydrated material was measured at room temperature, whereas the dehydrated material was measured in situ at 658 °C.

TABLE 3: Environment of Zinc in ZnAl2O4 20 Compared to Heat-Treated Zeolite Zn/Na-A at 811 °C crystallographic environment CN 4 12 12 4 12 12 16 4 12

atom interatomic type distance, Å O Al O Zn O O Al O Zn

1.95 3.35 3.39 3.50 4.28 5.10 5.25 5.45 5.72

XAFS-determined environment CN

atom interatomic Debye-Waller type distance, Å factor, Å2

3.7 2.9

O Al

1.90a 3.28

0.035 0.083

3.2 2.3

Zn O

3.47 4.51

0.035 0.055

8.3 16.5 1.84

Al O Zn

5.17 5.44 5.66

0.090 0.056 0.034

a Compared to the room-temperature XAFS analysis of ZnAl O 2 4 given in Table 1, multiple scattering corrections have not been included.

Figure 10. k3-weighted Zn K-edge XAFS spectra and Fourier transforms zeolite Zn/Na-A heat treated and measured at 811 °C (solid lines). Least-squares refinement results obtained using EXCURVE are shown with dotted lines, and the model structure is listed in the righthand columns of Table 3 (E0 ) 18 eV, VPI ) -2.5, and AFAC ) 0.8). Correlations between first-shell coordination number and DebyeWaller factor are given in Figure 11.

Nevertheless, they offer a guide here for the XAFS analysis of zinc environments measured in situ. In their XRD study of hydrated partially zinc-exchanged zeolite Na-A (Na2Zn5Si12Al12O48‚24H2O) Kim and Seff22 identified three distinct crystallographic cation sites occupied by Zn2+ in the special positions 8(g), 1(a), and 6(f) with occupancies 3, 1, and 1, respectively. The first of these, Zn1, is located just inside the large cavity in the structure of zeolite Na-A and comprises a distorted shell of seven oxygens at an average distance of 2.30 Å. Zn2 is octahedrally coordinated to water molecules at 2.11 Å at the center of the sodalite unit. Zn3 is also coordinated to water molecules but deep in the large cavity with an average nearest-neighbor distance of 2.13 Å. Mean values for these three sites are listed in Table 2, together with four additional cation shells and oxygen neighbors. The zinc environment refined from the XAFS of the asprepared zeolite Zn/Na-A is listed in Table 2 alongside the crystallographically determined local structure.22 XAFS analysis was based on a five-shell model that included a split oxygen shell to mimic the Zn-O distances at 2.1 and 2.3 Å (Table 2). Comparison with experimental data is given in Figure 8. The

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Colyer et al. 27Al

Figure 11. Contours of 95% significance in least-squares fitting of the first oxygen neighbor shell of zinc from in situ XAFS for the heat treatment of zeolite Zn/Na-A according to the schedule given in the caption to Figure 2. The temperatures quoted in the figure refer to the temperatures of measurement.

XAFS data indicate a predominantly octahedral environment for zinc but with an average oxygen distance at 2.00 Å, significantly shorter than the Zn1, Zn2, and Zn3 oxygen distances obtained from crystallography, which average out at 2.23 Å. A similar discrepancy between XAFS and XRD was reported for nickel sites in an earlier study of zeolite Ni/Na-Y.24 Now as then the shorter oxygen shell distance is attributed to the fact that XAFS probes only occupied sites, whereas values obtained from XRD reflect the average of occupied and unoccupied sites. Mean nearest-neighbor oxygen distances at ambient and higher temperatures are plotted in Figure 3b. The average Debye-Waller factor 2σZn-O2 for zinc sites in zeolite Zn/Na-A at 25 °C is 0.025 Å2 (Table 2). This is made up of a thermal contribution with the remainder reflecting the degree of site disorder and site variability. Taking the thermal contribution at ambient temperature from the Debye-Waller factor for tetrahedral sites in ZnO given in Table 1, 0.009 Å2, we can estimate a static component of 0.016 Å2 for zeolite Zn/Na-A at 25 °C. This can be compared with static DebyeWaller factor values at higher temperatures in Figure 3c. With a fitting error in of (0.002 Å2 (Figure 11), 0.016 Å2 is in close agreement with 0.017 Å,2 the static zinc-oxygen Debye-Waller factor across the Zn1, Zn2, and Zn3 sites from the structure reported by Kim and Seff.22 The subsequent shells of silicon/aluminum and oxygen are less well-reproduced in this XAFS determination, but our data are certainly consistent with silicon and oxygen shells at the approximate crystallographic interatomic distances.22 Because of the differences in composition between the material of ref 22 and the present material, coordination numbers are necessarily speculative. However, we identify a zinc shell at 3.35 Å that contrasts with a closest Zn-Zn distance of 4.43 Å in the Kim and Seff structure. A zinc shell at approximately this distance persists through to high temperatures in our heat treatment of zeolite Zn/Na-A and may reflect some clustering of zinc in the total structure generated in the ion-exchange and drying procedures employed. We have found separate evidence for the existence of disorder in the macroscopic structure in the

solid-state NMR spectrum for the hydrated material. Plotted in Figure 5b there is a sharp crystalline line at 59.1 ppm, but this is accompanied by a broader line around 50 ppm, which is indicative of aluminums in disordered nonframework environments. The slightly acidic conditions that accompany ion exchange could have generated this25 as too could have the subsequent drying conditions at 120 °C. The production of steam is known to cause some leaching of aluminum out of the zeolite framework.26 Since zinc charge compensates for tetrahedral aluminum, it is likely to be associated with such nonframework aluminums. The presence of a disordered component involving zinc and aluminum would have been difficult to detect with crystallographic methods but is more obvious using spectroscopic probes of XAFS and solid-state NMR. The structure of fully zinc-exchanged (Zn6Si12Al12O48) but partially dehydrated zeolite-A obtained crystallographically23 is characterized by two zinc sites. Two out of the six zinc ions occupy 8(g) positions in the sodalite cage, which comprise a distorted tetrahedral site with an average oxygen distance of 2.11 Å. The remaining four zinc ions are almost in the plane of the 6-fold rings, coordinated to three framework oxygens at 1.99 Å in a trigonal planar manner. The average crystallographic zinc oxygen distance in this partially dehydrated structure is therefore 2.03 Å, significantly shorter than the 2.23 Å distance in the fully hydrated structure22 described above. Zinc K-edge XAFS at 658 °C was analyzed as described above with a split oxygen shell and four additional shells and also reveals a slight shortening of zinc-oxygen bond lengths. The zinc environments obtained from XAFS for the as-prepared and 658 °C treated material are compared and contrasted with the results of crystallography in Table 2. At 658 °C the thermal Debye-Waller factor obtained from ZnO, 2σZn-O2, has risen from 0.009 Å2 at room temperature to 0.025 Å2 (Figure 7). Compared to the Debye-Waller factor of 0.037 ( 0.002 Å2 analyzed from the in situ data for zeolite Zn/Na-A (Table 2), this points to a static component at 658 °C of 0.012 Å2s30% less than that deduced in as-prepared zeolite Zn/Na-A at room temperature (0.016 Å2, Figure 3c). In addition to the reduction in bond length and in the static Debye-Waller factor in zeolite Zn/Na-A from room temperature to 658 °C, there is also a small but significant reduction in the oxygen coordination number, which can be clearly seen in Figure 11. The changes in all three structural parameters reflect the removal of water from the material. However, the fact that the coordination number does not drop to four during the initial rapid temperature rise to 658 °C at 40 °C min-1 suggests that dehydration is not complete at that stage. Of the outer shells also included in the XAFS analysis of zeolite Zn/Na-A at 658 °C and listed in Table 2, in addition to silicon and oxygen, a zinc shell at 3.36 Å was detected as in the as-prepared material. It is interesting to note that in the fully zinc-exchanged zeolite-A structure reported by McCusker and Seff23 there are Zn-Zn correlations as close as 3.65 Å. Zinc Environments for Zeolite Zn/Na-A at Collapse and during Devitrification. At 720 °C the diffraction pattern of zeolite Zn/Na-A disappears and the accompanying zinc K-edge XAFS sharply alters (Figure 2). The XAFS spectrum and Fourier transform at this stage are plotted in Figure 9 where the zinc environment refines to a single shell of 4.4 oxygens at 1.91 Å, indicating that zinc occupies predominantly 4-fold sites but in a vitreous matrix. The shortening of the zinc-oxygen distance in the vicinity of collapse plotted in Figure 3b is commensurate with a change from octahedral to tetrahedral geometry. At the point of collapse the total zinc-oxygen

Zinc-Exchanged Zeolite-A Debye-Waller factor, 2σZn-O2, has risen significantly to 0.046 Å2 (Figure 11), which, with a corresponding thermal contribution of 0.027 Å2 (from ZnO, Figure 7), points to an underlying static disorder of 0.019 Å2. This is slightly higher than static disorder in the hydrated zeolite structure at 25 °C (0.016 Å2) but significantly greater than that prior to collapse at 658 °C (0.012 Å2). The increase in static disorder on collapse can clearly be seen in Figure 3c. The magnitude of the Debye-Waller factor is untypical of a homogeneous glass, where “intermediate cations” like zinc will often adopt well-defined regular networkforming sites.27 Such a degree of static disorder in this noncrystalline structure, however, would support the observation that nanostructural elements from zeolite-A are retained on collapse8sdespite the obvious absence of crystalline order from Figure 2. At the same time, the additional presence of zinc occupying tetrahedral sites at this point will offer different nucleating sites to the zeolite framework remnants in zeolite Na-A proposed by Lutz et al.6 This would explain why we find that gahnite is precipitated in zeolite Zn/Na-A rather than carnegieite or nepheline. As the temperature continues to rise, the diffraction pattern of gahnite begins to emerge from the heat-treated zeolite Zn/ Na-A and outer shells in the local structure of zinc begin to develop in the XAFS Fourier transforms. These changes can be clearly seen in Figure 2 and also in Figure 3. From the 777 °C XAFS spectrum zinc nearest-neighbor oxygens refine to the disordered tetrahedral geometry observed on collapse at 720 °C. In addition, though, further shells are starting to develop, reminiscent of the spinel structure of ZnAl2O4 given earlier in Figure 4 and in Table 1. Figure 10 shows the XAFS and Fourier transform for data collected at 811 °C. Despite the much larger total Debye-Waller factors and the reduced wave vector range, similarities between Figure 4b and Figure 10 can clearly be seen. Accordingly, by use of the ZnAl2O4 zinc environment given in Table 1 as the initial structure, the 811 °C XAFS data were refined to give the best fit shown in Figure 10 with the refined structural parameters listed in Table 3. Compared to the XAFS spectrum of ZnAl2O4 (Figure 4b), the effective k-range of the 811 °C spectrum (Figure 10) is reduced from 10 to 6 Å-1. This will lead in turn to a reduction in intershell resolution, ∆R ()π/ ∆k), from 0.3 to 0.5 Å, limiting the significance of shells analyzed with a closer separation. We note too a small reduction in the anlaysed zinc-oxygen distance from 1.93 Å at room temperature (Table 1) to 1.90 Å (Table 3). This can be attributed to thermal asymmetry in the distribution of oxygens at 811 °C. Although this effect can be corrected through the use of cumulants in the XAFS analysis,28 this has not been pursued here in order to maintain consistency between all the spectra analyzed. Notwithstanding the various limitations inherent in analyzing high-temperature XAFS spectra, the 811 °C spectrum (Figure 11 and Table 3) indicates a significant drop in the total DebyeWaller factor for first shell, 2σZn-O2, from 0.048 Å2 at 777 °C to 0.035 Å2 at 811 °C. By use of the ZnO reference (Figure 7), this represents a huge fall in the static disorder around zinc from 0.019 to 0.006 Å2. Illustrated in Figure 3c, this is indicative of the incorporation of zinc into a substantial crystalline fraction from the amorphous collapsed phase generated at 726 °C. Assuming a simple mixture of collapsed material (with a static Debye-Waller factor of 0.019 Å2) and ZnAl2O4 (with little or no static disorderssee Table 1), the fraction of crystalline zinc at 811 °C in heat-treated zeolite Zn/ Na-A is ∼0.68. For this high-temperature spectrum the refined coordination numbers of more distant shells are fairly unreliable, but we note that the occupancy of the first two of theses

J. Phys. Chem. B, Vol. 101, No. 48, 1997 10113 aluminum (3.3 Å) and zinc (3.5 Å)sare consistent with this level of crystallinity. If two-thirds of the collapsed zeolite Zn/ Na-A (Zn3.1Na5.8Si12Al12O48) recrystallizes as ZnAl2O4, the composition of the amorphous material remaining at 811 °C will be ZnNa5.8Si12Al7.9O39.7. With an Al/Si ratio of 0.7 the average Q4(nAl) species for silicon in this amorphous aluminosilicate will be Q4(3Al). At temperatures higher than 811 °C we might expect more zinc to precipitate as ZnAl2O4, leaving an amorphous residue with a lower Al/Si ratio. Turning therefore to the ex situ solid-state NMR spectra shown in Figure 5 obtained from 1100 °C treated material, we have already discussed the structured resonance of 27Al in the as-prepared material as being indicative of tetrahedral sitessmainly in the frameworksbut with some disordered nonframework locations. This is replaced after heat treatment at the higher temperature of 1100 °C with two quite separate features: a broad resonance at 53.6 ppm and a narrow one at 9.0 ppm. The latter indicates the presence of aluminum in a well-ordered octahedral environment,29 which we can clearly associate with the octahedral sites of aluminum in ZnAl2O4.20 The broad resonance at 53.6 ppm, however, is typical of tetrahedral aluminum sites in oxide glasses,30 which is consistent with aluminum being retained in the amorphous collapsed material after devitrification. Before and after heat treatment the 29Si solid-state NMR changes from a narrow resonance at -89.1 ppm consistent with Q4(Al) species in zeolite-A 31 to a broad resonance reflecting a range of Q4 species in an aluminosilicate glass.32 In particular, the chemical shift at -92.5 ppm is indicative of Q4(2Al) with a Al/Si ratio of 0.5. This suggests that at 1100 °C half of the aluminum is still present in the amorphous collapsed form, the remaining half having precipitated as ZnAl2O4. Given the overall stoichiometry of the starting zeolite material (Zn3.1Na5.8Si12Al12O48), this corresponds to all of the zinc present being converted to ZnAl2O4 at 1100 °C and leaving an amorphous sodium aluminosilicate of composition Na5.8Si12Al5.8O35.6 with an Al/Si ratio of 0.48. Summary Using combined XAFS-XRD, we have been able to follow directly the solid-state metamorphosis of zeolite Zn/Na-A from a microporous crystal to a high-density ceramic. Detailed analysis reveals how the precipitation of the β-carnegieite and nepheline phases characteristic of zeolite Na-A is blocked in zeolite Zn/Na-A at temperatures as high as 1100 °C by the formation of a zinc spinel glass ceramic. This is despite the fact that the presence of ion-exchanged zinc in zeolite Na-A substantially reduces the temperature of collapse and of recrystallization. Zinc in the starting hydrated material occupies distorted octahedral sites that exhibit some reduction in disorder upon calcination. At the point of collapse though, zinc switches to tetrahedral coordination, but this is accompanied by substantial disorder in nearest-neighbor oxygens, indicative of the formation of a nonhomogeneous glass. Zinc maintains its tetrahedral geometry but gradually crystallizes out of the collapsed amorphous phase as ZnAl2O4 (gahnite)sthe crystalline fraction increasing with heat treatment temperature. We have found that solid-state NMR and X-ray techniques can be used to considerable advantage in elucidating both the degree of crystalline perfection in the starting zeolite and the approximate compositions of the final crystalline and amorphous components. Finally, the propensity of zinc to nucleate as gahnite from collapsed zeolitic structures necessarily serves to reduce the Al/ Si ratio in the remaining noncrystalline material. This is undoubtedly a factor that will influence the efficacy of small amounts of ZnO in the synthesis of Mg2Al4Si5O18 (cordierite)

10114 J. Phys. Chem. B, Vol. 101, No. 48, 1997 from magnesium-exchanged zeolite-B12 by restoring the stoichiometry of cordierite when the precursor material is slightly aluminum-rich. Acknowledgment. This paper is dedicated to Professor Sir John Meurig Thomas FRS on the occasion of his 65th birthday. His insatiable enthusiasm, scientific intuition, and abiding optimism are unique qualities that we and many others continue to benefit fromsmany happy returns! With regard to this particular work we also acknowledge technical contributions from A. J. Dent, G. E. D. Derbyshire, B. R. Dobson, and C. A. Ramsdale from the SRS, G. Sankar from the Royal Institution, R. H. Jones from the University of Keele, B. Gore from UMIST, and W. Whitby from Unilever Research. Finally, we thank EPSRC for the funding of synchrotron radiation beam time at the SRS at Daresbury Laboratory and for the provision of a CASE in conjunction with Unilever Research. References and Notes (1) Dyer, A. An Introduction to Zeolite Molecular SieVes; John Wiley & Sons: Chichester, New York, Brisbane, Toronto, Singapore, 1988; p 108. (2) Chowdhry, U.; Corbin, D. R.; Subramanian, M. A. US Patent, No. 4,814,303, 1989. (3) Subramanian, M. A.; Corbin, D. R.; Chowdhry, U. AdV. Ceram. 1989, 26, 239. (4) Subramanian, M. A.; Corbin, D. R.; Chowdhry, U. Bull. Mater. Sci. 1993, 16, 665. (5) Corbin, D. R; Parise, J. B.; Chowdhry, U.; Subramanian, M. A. Mater. Res. Soc. Symp. Proc. 1991, 233, 213. (6) Lutz, W.; Engelhardt, G.; Fichter-Schmitter, H.; Peuker, Ch.; Lo¨ffler, E.; Siegel, H. Cryst. Res. Technol. 1985, 20, 1217. (7) Lutz, W.; Fahlke, B.; Lohse, U.; Seidel, R. Chem. Technol. 1983, 35, 250. (8) Thomas, J. M.; Bursill, L. A. Angew. Chem., Int. Ed. Engl. 1980, 19, 745. (9) Thomas, J. L.; Mange, M.; Eyraud, C. Molecular SieVe Zeolites-I; Advances in Chemistry 101; American Chemistry Society: Washington, DC, 1971; p 443.

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