The dehydration and rehydration processes in the natural zeolite mesolite studied by conventional and synchrotron X-ray powder diffraction

The dehydration and rehydration processes in the natural zeolite mesolite studied by conventional and synchrotron X-ray powder diffraction

The dehydration and rehydration processes in the natural zeolite mesolite studied by conventional and synchrotron X-ray powder diffraction Kenny St~hl...

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The dehydration and rehydration processes in the natural zeolite mesolite studied by conventional and synchrotron X-ray powder diffraction Kenny St~hl and Ronnie Thomasson

Inorganic Chemistry 2, University of Lund, Lund, Sweden The dehydration and rehydration process in the natural zeolite mesolite, Nas.33Cas.a3AIlsSi24Oso-nH20 [structure type NAT, space group Fdd2, Z = 3, n(H20) = 21.33, a = 18.4071(4), b = 56.668(1), c = 6.5464(1) A] has been studied by powder diffraction using CuK~I and synchrotron radiation [k = 1.1999(1) A] and Rietveid analysis. The samples were dehydrated for 1 h at 458, 473, 523, 573, and 598 K, respectively, and sealed in capillaries prior to data collection at room temperature. After partial loss of OW(4), equatorially coordinated to Ca, after dehydration at 458 K, mesolite goes through an order/disorder transition on dehydration at 473 K. The cations become randomly distributed over the Ca and Na sites and OW(4) is expelled completely. Reflections having k 4= 3n are lost and the resulting crystal structure, metamesolite, is very close to that of natrolite, the Na site being equally occupied by Na, Ca, and vacancies, and n(H20) varying between 14.7(3) (T = 473 K) and 11.1(2) (T = 573 K). T = 523 K: Metamesolite; Fdd2, Z = 1, n(H20) = 11.3(1), a = 18.11287(8), b = 18.63331(8), c = 6.56618(3) ~. Dehydration at 598 K destroys the crystalline structure and the material becomes amorphous. Rehydration of metamesolite restores the original water content, but the random Na/Ca distribution is retained and a new, partially occupied OW(4') site coordinated to Ca appears: cation-disordered mesolite; Fdd2, Z = 1, n(H20) = 23.2(4), a = 18.6180(9), b = 19.0312(9), c = 6.5421(3) A. Keywords: Zeolite; mesolite; metamesolite; cation-disordered mesolite; dehydration; rehydration; X-ray synchrotron powder diffraction

INTRODUCTION The natural zeolite mesolite (structure type code NAT) belongs to the group of fibrous zeolites with the same framework topology and Si/AI ordering as in natrolite and scolecite. The Si]A1-O4 tetrahedra are connected in 4 = 1 units, forming chains and boat-shaped channels along the c-direction. In natrolite, a Na coordinates four framework oxygens and one water 0 from above and one from below the framework O plane, in a distorted octahedron (cf. Figure la). Each water is shared between two Na in zigzaging - N a - O W - N a - O W - chains along the channels with two Na and two waters per c-repetition. In scolecite, 2 the Ca-O coordination is similar to the Na-O coordination in natrolite, but with an additional water in the framework O plane, in a pentagonal bi.pyramid (cf. Figure Ib). Each scolecite channel contams one Ca and three waters per c-repetition and each water is coordinated by one Ca only. The mesolite crystal structure s can be described as layers Address reprint requests to Dr. St~hl at Inorganic Chemistry 2, University of Lund, PO Box 124, S 221 00 Lund, Sweden. Received 20 January 1993; accepted 12 July 1993 © 1994 Butterworth-Heinemann 12

ZEOLITES, 1994, Vol 14, January

of the natrolite (* 1) and scolecite (*2) structures along the b-axis (Figure 2). The composition is close to the ideal: Nas.3~Cas.33A116Si24080-21.33 H20, Z = 3, in space group Fdd2, and with braes ~ 3bnat. Previous dehydration studies of mesolite4'5 have established a stepwise behavior and approximate changes in the unit cell. The aim of the present work, by means of Rietveld analyses of powder diffraction data, was to determine the order in which the water molecules leave the zeolite and the structural changes induced by the dehydration.

EXPERIMENTAL Prismatic, twinned .rods of mesolite from Poona, India, were ground with a mortar and pestle. The powder was sifted (< 50 lxm), and placed in the funnel of 0.3 mm Lindemann capillaries and heated in a furnace for 1 h. After heating, the capillaries were sealed and the samples packed through vibration. Six samples were prepared: one unheated (denoted M293 in the following) and four heated to 458(1), 473(1), 523(1), and 573(1) K (M458, M473, M523, and M573, respectively, in the following). After data collection, the M523 sample was opened

Dehydration

a

03 02

Figure 1 coordination metamesolite disordered

03

02

(a) Na-0 coordination in mesolite (M293; (b) Ca-0 in mesolite (M293); (c) NalCa-0 coordination in (M523); (d) Na/Ca-0 coordination in cationmesolite (R523).

and rehydrated at room temperature (R523 in the following). One additional sample, dehydrated at 598(l) K, showed mainly amorphous scattering with weak diffraction peaks. The M293, M458, M473, and R523 data sets were accumulated for 16 h using an INEL powder diffractometer equipped with a CPS 120 position sensitive detector covering 120” and CuK~li radiation. The CPS120 detector is divided into 4096 steps (approx. O.OS”/step) and was calibrated with a quartz standard.6 The background from air and capillary scattering was recorded separately and subtracted from the sample spectra.‘j The M523 and M573 data sets were collected at beam line 9.1 at SRS, Daresbury, England, in 28 steps of 0.01”. The wavelength, 1.1999( 1) A, was calibrated with a Si standard. The M533 data set was collected for 4.5 s/step between 10.3 and 80” and for 6.8 s/step between 80 and 105”. The M573 data set was collected for 4s/step between 10 and 80” and for 7slstep between 80 and 110”. The number of counts recorded at each step was normalized against the number of recorded monitor counts. All seven data sets were collected at room temperature with spinning samples.

and

rehydration

of mesolite:

K. Stahl

and

R. Thomasson

and M573) half-widths. In addition, the 20 zero-point and 10 or 12 (M523) background parameters (Chebyshev type I) were refined. In all refinements, the structural parameters included one scale factor, three unit cell parameters, and isotropic temperature factor coefficients: one each for %/Al, framework oxygens, Ca/Na, and waters. Refinement strategies for coordinates and occupancy factors were according to the following. M293 and M458: The refinements were started from the mesolite parameters given by Artioli et al3 Independent refinements of all coordinates converged successfully. However, the small improvements in Rvalues and the resulting large e.s.d.‘s of %/Al-O distances, 0.04-0.07 A, could not justify the large increase in the number of parameters. Instead, all coordinates for M293 and all framework coordinates for M458 were fixed. Water occupancy factors showed significant deviations from unity only for OW(4) in M458, and the others were fixed to unity. M473, M523, M573, and R523: The refinements were started from the natrolite parameters given by Artioli et al.’ Fractional coordinates were refined for all atoms. Initial refinements of M473 starting from the original mesolite framework did not converge. The additional water site in R523, OW(4’), was found in a difference Fourier map and the hydrogen positions were derived from the H(41) and H(42) positions of the original mesolite structure. Hydrogen parameters were in all refinements coupled to the corresponding water oxygen. Initial refinements of the CuKoli data sets in the range 8” I 26 5 116” resulted in severe profile misfit for the first reflections. Excluding the five (M293 and M458) or two (M473 and R523) reflections with 28 < 16” markedly improved the overall fit. The refinements were considered converged when all parameter shifts were less than 0.2 parameter e.s.d. Final profile agreement values [R(p), R(wp), and GOF] and extracted Bragg intensity agreement values [R(B)] are given in Table 1. Scattering factors for neutral atoms and anomalous scattering corrections (interpolated

REFINEMENTS The Rietveld analysis program used in this work is essentially the LHMPl program’ modified to fit the locally used I/O formats, to allow for variable step sizes6 and to use Chebyshev polynomials for backram minimizes the quantity l/Y:,, and Y:, being the recorded intensities prior to background subtraction. Pseudo-Voigt functions with five-peak asymmetry correction were used in all refinements with a Lorentzian component y = yl + ~~(28) + ys(20)* and f.w.h.m. defined as (U * tan* 8 + V * tan 8 + W)“*. yi, y2, y3, U, V, W, and one asymmetry parameter were refined with peaks widths limited to 12 or 16 (M523

a

b/2 Figure 2 Ca, large

Projection of the mesolite circles; Na, small circles,

ZEOLITES,

structure on the waters omitted.

ab-plane.

1994, Vol 14, January

13

Dehydration and rehydration of mesolite: K. St&hl and R. Thomasson Table 1 Data collection and refinement data for mesolite, metamesolite, and cation-disordered mesolite

Sample ~. (~) 26 range (°) R(p) (%) R(wp) (%) GOF R(B) (%) Parameters Observations Bragg reflection a (~) b (,~) c (A) n(H20)

M293

M458

M473

1.5406 16-116 9.89 12.55 10.92 3.78 48 3,259 1,305 18.3586(8) 56.537(2) 6.5373(3) 19.8(1 )

1.5406 16-116 11.13 12.58 4.73 3.05 55 3,269 430 18.2807(4) 18.7242(4) 6.5484(1 ) 14.7(3)

M523

M573

R523

1.1999 10.3-105 5.70 7.39 2.82 2.08 58 9,471 727 18.11287(8) 18.63331 (8) 6.56618(3) 11.3( 1)

1.1999 10-110 7.00 9.06 2.76 2.24 56 10,001 794 18.0812(2) 18.5758(2) 6.56504(6) 11.1 (2)

1.5406 16-116 8.03 10.07 5.00 1.78 59 3,265 449 18.6180(9) 19.0312(9) 6.5421 (3) 23.2(4)

1.5406 16-116 7.84 9.98 4.69 2.56 28 3,259 1,313 18.4071(4) 56.668(1 ) 6.5464(1 ) 21.33

Sample ~. (/~) 28 range (°) R(p) (%) R(wp) (%) GOF R(B) (%) Parameters Observations Bragg reflection a (A) b (]k) c (~) n(H20)

for ~. = 1.1999 ~) were taken from the International Tables of X-ray Crystallography. s The general crystallographic programs used in this work have been described by Lundgren. 9 Structural parameters of M523 and R523 are given in Table 2. Diffraction patterns and final difference patterns are shown in

Figure 3.* DISCUSSION The dehydration of mesolite starts with a reduction of the OW(4) occupancy, g = 0.71(3) at 458 K. OW(4) is the equatorial water in the Ca coordination (cf. Figure lb). After dehydration at 473 K, the diffraction pattern showed a remarkable change. Diffraction peaks having k ¢: 3n were strongly reduced and broadened and disappeared completely after dehydration at 523 K (Figure 4). It can be shown with simple pattern calculations, based on the original mesolite structure, that it is only the extraframework atoms, and specifically OW(4), that significantly contributes to these peaks. T h e transformation will therefore essentially be restricted to those atoms and may be achieved in two ways: (1) Accidentally: Without OW(4), the Ca ion and the remaining Ca coordinated waters, OW(2) and OW(3), may assume positions corresponding to the Na and water sites in the Na

*Complete lists of refined parameters and step intensity data are held by the authors.

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ZEOLITES, 1994, Vol 14, January

Table 2 Fractional coordinates (x 104), refined occupancy factors (g), and isotropic temperature factor coefficients for metamesolite (M523) and cation disordered mesolite (R523)

M523

x/a

y/b

z/c

Si(1) Si(2) AI O(1) 0(2) 0(3) 0(4) 0(5) NC OW(1) H(11) H(12)

0 1530(1) 371(1) 157(2) 693(2) 1001(2) 2087(2) 1784(2) 2268(1) 444(4) 394 910

0 2127(1) 932(1) 695(2) 1824(2) 328(2) 1529(2) 2336(2) 285(1) 1914(3) 1462 1910

0 6215(5) 6157(5) 8647(8) 6091 (8) 5171(7) 7107(7) 3910(7) 6105(6) 907(12) 175 1593

g

B(A 2)

0.326(2) 0.707(7) 0.707 0.707

0.82(2) 0.82 0.82 1.25(4) 1.25 1.25 1.25 1.25 1.73(8) 5.7(3) 6.70 6.70

0.310(7) 1.0 1.0 1.0 0.46(2) 0.46 0.463

R523 Si(1)

Si(2) AI O(1) 0(2) 0(3) 0(4) 0(5) NC OW(1 ) H(11) H(12) OW(4') H(41 ') H(42')

0

0

0

1.4(1)

1566(3) 416(3) 182(5) 757(6) 1018(5) 2161(5) 1813(5) 2273(4) 460(7) 410 926 1881(13) 1962 1351

2076(3) 890(3) 669(5) 1727(5) 273(5) 1561(7) 2316(5) 241 (4) 2024(7) 1571 2020 591(11) 1079 732

6261(15) 6132(16) 8653(20) 6073(24) 5154(20) 7092(18) 3916(20) 6152(18) 905(31 ) 176 1590 767(40) 945 655

1.4 1.4 1.8(1) 1.8 1.8 1.8 1.8 2.9(3) 7.9(4) 8.9 8.9 4(1) 5 5

Dehydration and rehydration of mesolite: K. SMhl and R. Thomasson

24

33 M293

M523

16

22 (---~

cO r._) -~

0

8

r

T

T

30

10

T

50

2Th

70

30

10

30

50

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70

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M458 2O

H573

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i

i

50

70

Figure 3 Diffraction patterns and final difference pattern, 20 < 70 °, for mesolite (M293 and M458), metamesolite (M473, M523, and M573), and cation-disordered mesolite (R523).

channels. The scattering power of one Ca occupying every second, or two Na in every cation site is very similar and may not be easily detected; (2) with a completely random cation occupancy, corresponding to a natrolite structure with equal numbers of Ca, Na, and vacancies in the natrolite Na site. To distinguish between the two models, the sample dehydrated at 523 K was rehydrated at room temper-

ature. Reentering o f OW(4) in model 1 would be restricted to the original Ca-containing channels, and the reflections with k 4: 3n would reappear. In model 2, the OW(4) would follow the randomly distributed Ca ions and only give rise to reflections having k 4: 3n. The resulting diffraction pattern, R523 (Figure 3), was found to be completely without reflections having k 4: 3n, thus in favor of model 2. Refinements,

ZEOLITES, 1994, Vol 14, January

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Dehydration and rehydration of mesolite: K. St&hl and R. Thomasson

I

I I

II

I I

I

III

I

III

effect of correlations between occupancy and thermal parameters. A comparison of the most flexible parameters o f the framework, the T - O - T angles, of natrolite, scolecite, a n d m e s o l i t e have s h o w n v e r y d i s t i n c t differences. 3 T h e M473, M523, and M573 structures, in the following denoted metamesolite, are in this respect very close to natrolite. With the additional water present in R523, the Si(2)-O(2)-AI angle, connecting neighboring framework chains, is opened up, causing an increase in the a- and b-axes and bringing the framework closer to that of scolecite. 2 T h e extraframework structures of M473, M523, M573, and R523 are all superpositions of Na and Ca coordinations that could not be resolved with the present data. This structural disorder shows up as increased refined temperature factors, especially for the waters. In general, when two water positions, each with ideal cation distances, are refined as one average position, the observed distance between cation and average water position will be too short. This bondshortening effect is to some extent observed in all o f these structures (Table 3), but is most obvious in R523: Na/Ca - OW(1) = 2.06(2) /~. In cation-disordered mesolite (R523) (Figure ld), the two superimposed structures (Figures la and b) are the most different, which enhances the disorder effect. T h e collective broadening of k ~ 3n peaks (Figure 3), which could not be fitted with the Rietveld program used, provides, nevertheless, a clue to a part o f the dehydration process: T h e extent of broadening, f.w.h.m. = 0.12 ° for reflections having k = 3n and 0.30 ° for those having k :~ 3n, can be related to a (particle) size 1° of approximately 500 • o f regions having the original mesolite cation and water structure. Most likely, these regions correspond to the

I II

0 QD

_~L 4-

_

?

2Th

2?

28

Figure 4 Enlargement of the low-angle part of the diffraction pattern (CuK~I data) for mesolite (top), mesolite dehydrated at 473 K (middle), and at 523 K (bottom). Bragg positions having k 4= 3n are marked for mesolite.

starting from the structural parameters of natrolite, rapidly converged assuming equal numbers of Na, Ca, and vacancies in the natrolite Na site. T h e M473, M523, and M573 data sets showed no trace of OW(4). T h e refined water occupancy, in molecules per formula unit (f.u.), varies between 14.7(3) (M473), close to full natrolite water occupancy, and 11.1(2) (M573). Assuming a minimum oxygen coordination num ber of six for Ca, and Ca never being next neighbors in a channel, the lower limit will be 10.67 HzO/f.u. In R523, rehydrated metamesolite or cation-disordered mesolite, a difference Fourier map revealed a reentered, partially occupied OW(4') site. T h e refined OW(4') occupancy appears somewhat too high to correspond to one OW(4') per Ca, but it may be an

Table 3 A selection of bonding distances ()~) in samples with fully refined atomic coordinates Sample

M473

M523

Si(1 )-0(1 ) *2 -0(5) *2

1.600(8) 1.611(8)

Si(2)-0(2) -O(3) -0(4) -0(5) AI-O(1 ) -0(2) -0(3) -0(4) Si(1 }-O(1 )-AI Si(2)-O(2)-AI Si(2)-O(3)-AI Si(2)-O(4)-AI Si(1 )-O(5)-Si(2) Na/Ca-OW(1 ) -OW(1)' -OW(4') -0(2) -O(2)' -0(3) -0(4)

16

M573

R523

1.597(4) 1.623(4)

1.583(6) 1.635(6)

1.59(1 ) 1.62(1)

1.610(9) 1.59(1 ) 1.608(9) 1.65(2)

1.619(4) 1.620(5) 1.613(4) 1.629(6)

1.612(7) 1.611 (7) 1.589(7) 1.615(8)

1.65(1) 1.62(1 ) 1.58(1 ) 1.67(2)

1.73(1 ) 1.73(1 ) 1.782(9) 1.77(1 )

1.737(6) 1.762(4) 1.729(4) 1.749(5)

1.762(9) 1.764(7) 1.753(7) 1.750(7)

1.76(2) 1.72(1 ) 1.75(1 ) 1.73(1 )

139.9(6) 132.6(5) 134.3(5) 131.8(6) 143.4(6) 2.19(1 ) 2.19(1) 2.53(1 ) 2.66(1 ) 2.36(1) 2.38(1 )

ZEOLITES, 1994, Vol 14, January

140.3(3) 129.6(3) 136.3(3) 133.7(3) 143.2(3) 2.225(8) 2,252(8) 2,450(6) 2.575(6) 2,377(5) 2,432(5)

139.8(4) 129.4(4) 135.9(4) 133.6(4) 142.6(4) 2.182(14) 2.332(13) 2.440(9) 2.568(9) 2.377(8) 2.418(8)

140.1 (7) 135.3(7) 134.1 (7) 137.5(9) 143.6(8) 2.06(2) 2.25(2) 2.25(2) 2.63(2) 2.75(2) 2.43(1) 2.59(2)

Dehydration and rehydration of mesolite: K. St~hl and R. Thomasson

interior of the powder crystallites and their appearance is due to a highly diffusion-controlled expulsion of water. It should in this context be noted that the temperature limits given here are valid only under the conditions for which the dehydrations were carried out. During a continuous heating experiment, e.g., t.g.a. or d.t.a., the recorded transition temperatures will be higher due to the diffusion control. The weak broad peaks with k ~ 3n present after dehydration at 573 K (M573) are also caused by small regions with the original mesolite structure. The regions are trapped inside crystallites by occasional collapses of the crystal structure at this high dehydration temperature. The decrease in the peak-to-background ratio between M523 and M573 confirms a certain transformation to an amorphous form at the higher temperature. On heating to 598 K, the material becomes almost completely amorphous: The powder pattern showed a large increase in the background intensity and only a few weak and broad diffraction peaks. The dehydration behavior of mesolite can thus be summarized in the following steps given with approximate transition temperatures: • •



T < 473 K. Initial, minor loss of OW(4), coordinated equatorially to Ca. T = 473 K. The partial loss of OW(4) and the increased temperature enables an order-disorder transition and the remaining OW(4) is expelled. The resulting crystal structure, which constitutes metamesolite, is isostructural to natrolite, with a random mixture of Na, Ca, and vacancies in the natrolite Na site. The remaining water gives an essentially fully occupied natrolite water site. Small regions, approximately 500 /~ wide at 473 K, still contain the original mesolite structure. 473 K < T 598 K. The water content is continuously reduced from n(H20) = 16 to 10.67 [11.1(2) for M573].



T > 598 K. The crystal structure is destroyed and the material becomes amorphous.

Finally, a word of caution: The cation-disordered mesolite, although here artificially obtained, may also be naturally occurring. Its powder diffraction pattern appears very similar to that of natrolite, and without at least a unit cell determination and chemical analysis, it may very well be passed as (Ca-rich) natrolite. I d e n t i f i c a t i o n o f a n a t u r a l - o c c u r r i n g cationdisordered mesolite will provide an essential piece of information on the history of rock formation.

ACKNOWLEDGEMENTS Technical assistance from Dr. R. Cernik, Daresbury, and financial support from the Swedish Natural Science Research Council, is gratefully acknowledged.

REFERENCES 1 Artioli, G., Smith, J.V. and Kvick, ~. Acta Crystallogr. 1984, C40, 1658 2 Kvick, ~, St&hi, K. and Smith, J.V.Z. Kristallogr. 1985, 171, 141 3 Artioli, G., Smith, J.V. and Pluth, J.J. Acta Crystallogr. 1986, C42, 937 4 Peng, C.J. Am. Mineral. 1955, 40, 834 5 van Reeuwijk, L.P. Am. Mineral. 1972, 57, 499 6 St~hl, K. and Thomasson, R. J. Appl. Crystallogr. 1992, 25, 251 7 Howard, C.J. and Hill, R.J. A Computer Program for Rietveld Analysis of Fixed Wavelength X-ray and Neutron Powder Diffraction Patterns. Australian Atomic Energy Commission Report Ml12, Lucas Hights Research Laboratory, New South Wales, Australia, 1986 8 International Tables for X-ray Crystallography, Kynoch Press, Birmingham (Present distributor: Kluwer, Dordrecht), 1974, Vol. 4 9 Lundgren, J.-O. Crystallographic Computer Programs. Report No. UUIC-B13-4-05, University of Uppsala, Sweden, 1982 10 Klug, H.P. and Alexander, L.E. X-ray Diffraction Procedures, Wiley, New York, 1962, p. 530

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