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Crystal structures of fully dehydrated fully Sr2+-exchanged zeolite X and of its ammonia sorption complex
Microporous and Mesoporous Materials 30 (1999) 233–241
Crystal structures of fully dehydrated fully Sr2+-exchanged zeolite X and of its ammonia sorption complex Mi Jung Kim a, Mi Suk Jeong a, Yang Kim a, Karl Seff b, * a Department of Chemistry, Pusan National University, Pusan 609-735, South Korea b Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA Received 9 June 1998; received in revised form 17 December 1998; accepted 5 January 1999
Abstract ˚ ] and of its ammonia sorption The crystal structures of fully dehydrated Sr –X [Sr Si Al O ; a=25.214(7) A 46 46 100 92 384 ˚ ], have been determined by single-crystal X-ray complex, Sr –X · 102NH [Sr Si Al O · 102NH ; a=25.127(7) A 46 3 46 100 92 384 3 diffraction techniques in the cubic space group Fd3: at 21(1)°C. The Sr –X crystal was prepared by ion exchange in a 46 flowing stream of aqueous 0.05 M Sr(ClO ) for 5 days followed by dehydration at 360°C and 2×10−6 Torr for 2 days. 42 To prepare the ammonia sorption complex, another dehydrated Sr –X crystal was exposed to 230 Torr of zeolitically 46 dried ammonia gas for 1 h followed by evacuation for 12 h at 21(1)°C and 5×10−4 Torr. The structures were refined to the final error indices, R =0.043 and R =0.039 with 466 reflections, and R =0.049 and R =0.044 with 382 1 w 1 w reflections, for which I>3s(I ). In dehydrated Sr –X, all Sr2+ ions are located at two crystallographic sites. 16 Sr2+ 46 ˚ ). The remaining 30 Sr2+ ions ions are at the centers of the double six-rings, filling that site (site I, Sr–O=2.592(6) A ˚ into the supercage from the plane of its three nearest oxygen atoms are in the supercage (site II ); each extends 0.56 A ˚ ). In the structure of Sr –X · 102NH , the Sr2+ ions are located at three crystallographic sites: 12 (Sr–O=2.469(6) A 46 3 ˚ ]; four are found at site I [Sr–O=2.652(10) A in the sodalite units (site I∞) each coordinated to three framework oxygen ˚ and also to three ammonia molecules at 2.76(8) A ˚ . The remaining 30 Sr2+ ions lie at site II. Each atoms at 2.654(9) A ˚ into the supercage where it coordinates to three framework oxygen atoms at 2.584(7) A ˚ and also to extends 1.12 A ˚ three ammonia molecules at 2.774(24) A. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Ammonia sorption complex; Crystal structure; Strontium; Zeolite X
1. Introduction The first experimental observations of the sorption of gases on zeolites and their behavior as molecular sieves were conducted on zeolite minerals. Grandjean [1] reported and extended the work of G. Friedel on ammonia sorption by dehydrated zeolites [2]. * Corresponding author. Fax: +1-808-956-5908. E-mail address: [email protected] ( K. Seff )
Electronic reflectance spectra of dehydrated Co2+-exchanged zeolite A indicated that six-ring Co2+ ions form complexes with N O, 2 C H , H O, and NH [3]. In the spectrochemical 3 6 2 3 series of these molecules, NH has the highest 3 ligand strength; NH forms a tetrahedral complex 3 with Co2+. The ammonia molecule is small enough to access the sodalite cavities of the zeolite [4]: sorption by sodalite, although slow, occurs [5]. In the crystal structure of an ammonia sorption complex
1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 01 2 - 8
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of zeolite A [5], 32 NH molecules are found per 3 unit cell. 12 NH molecules are found at two 3 equipoints in the sodalite unit. Of the remaining 20 NH molecules in the large cavity, eight coordi3 nate to threefold-axis Na+ ions. The presence of sorbed ammonia in zeolite X has a pronounced stabilization effect because it lowers the chemical potential of the zeolite crystal for the mechanically strained dehy[6 ]. G 9 zeolite drated form is higher than that of the framework and cations in the NH complex whose framework/ 3 cation structure is more like that of the relatively relaxed hydrated zeolite. In the case of a large-pore zeolite such as zeolite X, the effective pore size may be controlled by the formation of a stable inorganic complex. The strong interaction between the zeolite cation and the dipole moment of ammonia or water produces a diffusion block by clustering ammonia or water molecules about the cation in the channels. When small amounts of ammonia or water are presorbed on a dehydrated zeolite, the sorption of a second less polar sorbate such as oxygen is drastically reduced [7,8]. ESR spectra of a dehydrated copper-exchanged zeolite X suggested that Cu2+ ions occupy sites I∞ and II∞ in the sodalite cages. Only those at the latter site were observed to interact by complexation with sorbed NH or pyridine [9]. 3 In partially dehydrated Sr –X, the Sr2+ ions 44.8 are located at four sites: 6.1 at site I, 12.0 at I∞, 6.4 at II∞, and 20.3 at II [10]. Partridge and coworkers [11] calculated theoretical bond lengths and angles for selected monoand di-positive M(NH ) and M(H O) ions, 3n 2 n where M=Mg, Ca, and Sr. These included calculations on NH complexes of Sr+, but, unfortu3 nately, not of Sr2+. Martyna and Klein [12] calculated the Sr–N distances in Sr+(NH ) (n= 3n 1–6) using pseudopotentials parameterized to ab initio calculations. This work was initiated to learn the positions of Sr2+ ions for comparison with those of the other alkaline-earth cations, Mg2+, Ca2+, and Ba2+ in zeolite X [13]. It was expected that fully Sr2+-exchanged zeolite X could be readily prepared. It would also be interesting to see the changes that ammonia sorption would cause to
the distribution and placement of Sr2+ ions in the zeolite, and to observe the geometry of the resulting complexes.
2. Experimental section Large colorless crystals of sodium zeolite X, Na Si Al O per unit cell, were prepared in 92 100 92 384 St. Petersburg, Russia [14]. Each of two single crystals, an octahedron about 0.20 mm in crosssection, was lodged in a fine Pyrex capillary. Complete ion exchange was accomplished by the flow method using aqueous 0.05 M (pH=6.5) Sr(ClO ) (Johnson Matthey, 99.9%). The solution 42 was allowed to flow past each crystal at a velocity of approximately 15 mm s−1 for 5 days at 21(1)°C. Each crystal was dehydrated by slowly increasing the temperature by 25°C h−1 to 360°C under vacuum, followed by 48 h at 360°C and 2× 10−6 Torr. After cooling to room temperature, the first crystal (crystal 1), colorless, still under vacuum, was sealed in its Pyrex capillary by torch. To prepare an ammonia sorption complex of Sr2+-exchanged zeolite X, the second crystal (crystal 2) was treated with 230 Torr of zeolitically dried ammonia gas ( Union Carbide, 99.995%) for 1 h at 21(1)°C, and then evacuated for 12 h at this temperature and 5×10−4 Torr. It was then sealed in its capillary by torch. Microscopic examination showed that crystal 2 was bright red. The cubic space group Fd3: was used throughout this work. This choice is supported by: (a) the low Si/Al ratio, which in turn requires, at least in the short range, alternation of Si and Al; (b) the observation that this crystal, like all other crystals from the same batch, does not have intensity symmetry across (110) and therefore lacks that mirror plane. Diffraction data were collected with an automated Enraf–Nonius four-circle computercontrolled CAD-4 diffractometer equipped with a pulse-height analyzer and graphite monochroma˚; tor, using Mo radiation ( Ka , l=0.709 30 A 1 ˚ Ka , l=0.713 59 A). The unit cell constants at 2 21(1)°C, each determined by least-squares refinement of 25 intense reflections for which ˚ for crystal 1 and 14<2h<24°, are a=25.214(7) A ˚ a=25.127(7) A for crystal 2. All unique reflections
M.J. Kim et al. / Microporous and Mesoporous Materials 30 (1999) 233–241
in the positive octant of an F-centered unit cell for which 2h<50°, l>h, and k>h were recorded. Of the 1268 and 1257 reflections for crystal 1 and crystal 2 respectively, only the 466 and 382 reflections respectively, for which I >3s(I ), were used in subsequent structure determination.
3. Structure determination 3.1. Crystal 1, Sr –X 46 Full-matrix least-squares refinement was initiated with the atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3) and O(4)] of dehydrated Ca –X [13]. Isotropic refinement con46 verged to an R index, |F −|F ||/F , of 0.44 and a 1 o c o weighted R index, [w(F −|F |)2/wF2 ]1/2 of 0.49. w o c o The initial difference Fourier function revealed two large peaks at (0.0, 0.0, 0.0) and (0.22, ˚ −3 and 12 e A ˚ −3 0.22, 0.22) with heights of 16 e A respectively. These peaks were stable in leastsquares refinement as Sr(1) and Sr(2) respectively. Anisotropic refinement including these Sr2+ ions converged to R =0.042 and R =0.038. 1 w Occupancy refinement converged at 16.2(1) and 29.2(1) respectively. These values indicate complete and stoichiometric exchange with no suggestion of retention of excess salt. The occupancies were reset and fixed at 16 and 30 Sr2+ ions at Sr(1) and Sr(2) respectively, because the maximum number of Sr2+ ions at Sr(1) is 16, and because the cationic charge should sum to 92+ per unit cell. Anisotropic refinement of the framework atoms and cations at Sr(1) and Sr(2) converged to R =0.043 and R =0.039. 1 w In the final cycle of least-squares refinement, all shifts in atomic parameters were less than 0.01% of their corresponding standard deviations. The final difference function was featureless. 3.2. Crystal 2, Sr –X · 102NH 46 3 Full-matrix least-squares refinement was initiated with atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3) and O(4)] of crystal 1. Isotropic refinement of the framework atoms converged to R =0.48 and R =0.56. 1 w
235
A difference Fourier function showed the positions of the Sr2+ ions at Sr(1), (0.0, 0.0, 0.0) with ˚ −3, at Sr(2), (0.239, 0.239, peak height 12 e A ˚ −3, and at Sr(3), 0.239) with peak height 12 e A (0.0696, 0.0696, 0.0696) with peak height ˚ −3. These peaks were stable in least-squares 4.9 e A refinement. Isotropic refinement of framework atoms and anisotropic refinement of Sr(1), Sr(2), and Sr(3) converged to R =0.093 and 1 R =0.142 with occupancies of 11.8(10), 30.0(1), w and 4.4(2) respectively. From a subsequent difference Fourier function, ˚ −3 and 2.2 e A ˚ −3 were two peaks of height 2.4 e A found at (0.151, 0.161, 0.169) and (0.221, 0.312, 0.322). These were refined as N(3) and N(2) to occupancies of 15(1) and 101(3) respectively. Full anisotropic refinement of the framework and Sr2+ ions, with positional, occupancy and isotropic thermal parameters varying at N(2) and N(3), converged to R =0.049 and R =0.044. 1 w The occupancy numbers at Sr(1), Sr(2), Sr(3), N(2), and N(3) were fixed at the values shown in Table 1 using the considerations mentioned above for strontium ions, and allowing the maximum number of NH molecules (three) to coordinate to 3 each Sr2+ ion. The final error indices were R =0.050 and R =0.045. The shifts in the final 1 w cycle of least-squares refinement were less than 0.01% of their corresponding standard deviations. The final difference function was featureless. Crystallographic calculations were done using MolEN [15]. The full-matrix least-squares program used minimized w(F −|F |)2; the weight w of o c an observation was the reciprocal square of s(F ), its standard deviation. Atomic scattering o factors for Si, Al, O−, Sr2+ and N were used. All scattering factors [16 ] were modified to account for anomalous dispersion [17]. The final structural parameters, and selected interatomic distances and angles, are presented in Table 1 and Table 2 respectively.
4. Discussion 4.1. Description of zeolite X Zeolite X is a synthetic counterpart of the naturally occurring mineral faujasite (see Fig. 1).
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Table 1 Positional,a thermal, and occupancy parameters Atom Wyckoff Site x position
y
z
U ,b U 11 iso
U
22
U
33
U
12
U
13
U
23
Occupancyc varied
(a) Sr –X (crystal 1) 46 Si 96(g) Al 96(g) O(1) 96(g) O(2) 96(g) O(3) 96(g) O(4) 96(g) Sr(1) 16(c) I Sr(2) 32(e) II
−563(1) −571(1) −1117(3) −50(3) −394(2) −602(2) 0 2288(1)
(b) Sr –X2102NH (crystal 2) 46 3 Si 96(g) −549(1) Al 96(g) −562(2) O(1) 96(g) −1118(4) O(2) 96(g) −36(4) O(3) 96(g) −371(4) O(4) 96(g) −648(3) Sr(1) 16(c) I 0 Sr(2) 32(e) II 2419(1) Sr(3) 32(e) I∞ 685(6) N(2) 96(g) 2204(9) N(3) 96(g) 1563(37)
fixed
1237(1) 353(1) 0(3) −46(5) 680(3) 733(3) 0 2288(1)
343(1) 1235(1) 1102(3) 1380(2) 663(3) 1771(3) 0 2288(1)
148(14) 99(14) 161(42) 66(35) 113(39) 137(39) 165(6) 275(5)
90(13) 68(15) 184(42) 89(36) 116(40) 203(40) 165(6) 275(5)
117(15) 43(14) 183(42) 250(41) 166(41) 139(38) 165(6) 275(5)
−22(15) 19(14) −36(36) 37(32) 30(34) −8(36) 36(7) 153(7)
1(15) −40(16) 96 8(16) −15(16) 96 33(30) 3(35) 96 −34(33) −31(34) 96 −30(34) −56(35) 96 −25(36) −144(34) 96 36(7) 36(7) 16.2(1) 16 153(7) 153(7) 29.2(1) 30
1248(2) 366(1) 8(4) −43(4) 718(4) 728(4) 0 2419(1) 685(6) 3203(10) 1730(31)
338(1) 1236(2) 1074(3) 1405(3) 679(4) 1801(4) 0 2419(1) 685(6) 3166(10) 1583(40)
94(17) 91(18) 282(66) 154(48) 209(60) 161(54) 214(11) 287(6) 398(68) 1241(101) 569d
84(17) 77(18) 175(51) 179(48) 320(66) 149(55) 214(11) 287(6) 398(68)
112(17) 97(18) 197(57) 129(54) 274(65) 250(61) 214(11) 287(6) 398(68)
−4(27) −6(18) −94(53) 54(47) 87(58) −29(52) 61(14) 160(9) 118(83)
−7(17) 5(29) 8(42) −60(46) 15(57) 14(56) 61(14) 160(9) 118(83)
5(26) −21(27) −54(50) −82(47) 144(52) −99(47) 61(14) 160(9) 118(83)
96 96 96 96 96 96 11.8(1) 12 30.0(1) 30 4.4(2) 4 101(3) 90 15(1) 12
a Positional and anisotropic thermal parameters are given ×104. Numbers in parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b Anisotropic temperature factor: exp[−2p2a−2(h2U +k2U +l2U +2hkU +2hlU +2klU )]. 11 22 33 12 13 23 c Occupancy factors are given as the number of atoms or ions per unit cell. d This thermal parameter was fixed in least-squares refinement.
The 14-hedron with 24 vertices known as the sodalite cavity or b cage may be viewed as its principal building block. These b cages are connected tetrahedrally at six-rings by bridging oxygen atoms to give double six-rings (D6Rs, hexagonal prisms), and concomitantly to give an interconnected set of even larger cavities (supercages) accessible in three dimensions through 12-ring (24-membered ) windows. The Si and Al atoms occupy the vertices of these polyhedra. The oxygen atoms lie approximately midway between each pair of Si and Al atoms, but are displaced from those points to give near tetrahedral angles about Si and Al. Single six-rings (S6Rs) are shared by sodalite and supercages, and may be viewed as the entrances to the sodalite units. Each unit cell has eight sodalite units, eight supercages, 16 D6Rs, 16 12-rings, and 32 S6Rs. Exchangeable cations that balance the negative
change of the aluminosilicate framework are found within the zeolite’s cavities. They are usually found at the following general sites shown in Fig. 1: I at the center of a D6R, I∞ in the sodalite cavity on the opposite side of one of the D6Rs six-rings from site I, II∞ inside the sodalite cavity near an S6R, II in the supercage adjacent to an S6R, III in the supercage opposite a four-ring between two 12-rings, and III∞, somewhat or substantially distant from III but otherwise near the inner walls of the supercage or the edges of 12-rings. 4.2. Crystal 1, Sr –X 46 In this crystal structure, 46 Sr2+ ions are located at just two crystallographic sites with high occupancy. The 16 Sr2+ ions at Sr(1) fill site I at the center of the D6Rs. See Fig. 2. The octahedral ˚ , is somewhat Sr(1)–O(3) distance, 2.592(6) A
M.J. Kim et al. / Microporous and Mesoporous Materials 30 (1999) 233–241
237
Table 2 ˚ ) and angles (deg)a Selected interatomic distances (A
Si–O(1) Si–O(2) Si–O(3) Si–O(4) mean Al–O(1) Al–O(2) Al–O(3) Al–O(4) mean Sr(1)–O(3) Sr(2)–O(2) Sr(3)–O(3) Sr(2)–N(2) Sr(3)–N(3) N(3)–O(2) N(2)–O(1) N(2)–O(2) N(2)–N(2) N(3)–N(3) O(1)–Si–O(2) O(1)–Si–O(3 O(1)–Si–O(4) O(2)–Si–O(3) O(2)–Si–O(4) O(3)–Si–O(4) O(1)–Al–O(2) O(1)–Al–O(3) O(1)–Al–O(4) O(2)–Al–O(3) O(2)–Al–O(4) O(3)–Al–O(4) Si–O(1)–Al Si–O(2)–Al Si–O(3)–Al Si–O(4)–Al O(3)–Sr(1)–O(3) O(2)–Sr(2)–O(2) O(3)–Sr(3)–O(3) N(2)–Sr(2)–O(2) N(3)–Sr(3)–O(3) N(2)–Sr(2)–N(2) N(3)–Sr(3)–N(3)
Sr –X 46 (crystal 1)
Sr –X · 102NH 46 3 (crystal 2)
1.639(7) 1.678(7) 1.675(7) 1.655(8) 1.662 1.673(7) 1.695(7) 1.717(7) 1.659(8) 1.686 2.592(6) 2.469(6)
1.613(9) 1.632(10) 1.646(11) 1.618(11) 1.627 1.712(10) 1.726(10) 1.722(11) 1.700(11) 1.715 2.652(10) 2.584(7) 2.654(9) 2.774(24) 2.76(8) 3.17(9) 3.40(2) 3.05(2) 3.48(3) 2.88(12) 112.0(5) 109.9(5) 110.0(5) 106.4(5) 104.3(5) 114.1(5) 111.7(5) 107.8(5) 112.0(5) 107.0(5) 102.2(5) 116.1(5) 130.0(6) 147.8(6) 139.6(6) 152.1(7) 88.5(3)/91.5(3) 102.6(3) 91.5(3) 69.3(5)/130.2(5) 95(2)/162(2) 77.8(7) 70(3)
111.8(4) 108.1(4) 114.2(4) 105.5(3) 102.7(3) 114.2(4) 111.4(3) 107.6(3) 115.6(4) 105.3(3) 101.6(3) 114.8(4) 126.7(4) 150.8(4) 137.7(4) 160.3(5) 85.8(2)/94.2(2) 114.9(2)
a Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value.
longer than the sum of the corresponding ionic ˚ [18]. For comparison, radii, 1.12+1.32=2.44 A the Mg–O distance in Mg (H O) –X [13] and the 46 2 4
Fig. 1. A stylized drawing of the framework structure of zeolite X. Near the center of the each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1 to 4. Silicon and aluminum alternate at the tetrahedral intersections, except that Si substitutes for about 4% of the Al. Extraframework cation sites are labeled with Roman numerals.
Fig. 2. A stereoview of a sodalite cavity with attached D6R of dehydrated Sr –X. One Sr2+ ion at Sr(1) (site I ) and four 46 Sr2+ ions at Sr(2) (site II ) are shown. About 75% of the sodalite units have this arrangement. Only three Sr(2) ions surround the remaining 25%. These fractions of 25% and uncertainties would likely vanish in this and subsequent captions if this structure could be extrapolated to Si/Al=1. Ellipsoids of 20% probability are shown.
Ca–O distance in Ca –X [13] are similarly longer 46 than the sum of the corresponding ionic radii, indicating a reasonably good fit. The 30 Sr2+ ions at Sr(2) are located at site II in the supercage. See Figs. 2 and 3. Each coordi˚ , nearly the sum of nates trigonally at 2.469(6) A
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Fig. 3. A stereoview of a supercage of dehydrated Sr –X. Four Sr2+ ions at Sr(2) (site II ) are shown. About 75% (or more) of the 46 supercages have this arrangement. The remaining 25% of the supercages may have only three Sr2+ ions at Sr(2). Ellipsoids of 20% probability are shown.
the ionic radii, to three O(2) framework oxygen ˚ into the supercage from atoms and extends 0.56 A their plane. The O(2)–Sr(2)–O(2) bond angle is 114.9(2)°, not too far from trigonal planar. Clearly, site I, because it is filled, is preferred over site II. Smolin et al. [19] investigated the placements of cations in fully Ca2+-exchanged zeolite X at various stages of dehydration by heating a crystal in a stream of hot N gas. With increasing dehydra2 tion, the occupancies at sites I and II increase, consistent with the results presented here. Recently the crystal structures of Ca –X [13] 46 and Cd –X [20] have been determined by single46 crystal X-ray diffraction methods. As in Sr –X, 46 the Ca2+ and Cd2+ ions fill site I, with the remainder going to site II in the supercage. Only about 13 Ba2+ ions (relatively large ions) in Ba –X [13] and 14 Mg2+ (relatively small ions) 46 in Mg (H O) –X [13] occupy the 16-fold site I 46 2 4 position (see Tables 3 and 4). It appears that site I (at the center of the D6R) is generally the lowest energy site for cations of intermediate size. Little explanation is offered for the lack of differentiation between the mean Al–O and Si–O bond lengths in crystal 1 (see Table 2); Al–O is ˚ longer than Si–O, as usually seen to be ~0.10 A is seen in the results for crystal 2. Efforts to escape from a possible false minimum in least-squares refinement were not successful. Perhaps crystal 1 is twinned with respect to Si/Al ordering. This is often the result of ion-exchange with pH too low.
Table 3 ˚ ) of cations from six-ring planes Deviations(A
At O(3) At O(2)
Sr(1)a Sr(3)a Sr(2)b
Sr –X 46 (crystal 1)
Sr –X · 102NH 46 3 (crystal 2)
−1.40
−1.49 1.50 1.12
0.56
a A negative deviation indicates that the ion lies in the D6R (in these structures, at its center at site I ); a positive deviation, in the sodalite cavity at site I∞. b A positive deviation indicates that the ion lies in the supercage.
4.3. Crystal 2, Sr –X · 102NH 46 3 The sorption of ammonia has caused the distribution of Sr2+ ions among sites to change. The Sr2+ ions are now found at three crystallographic sites, none of which is full. See Table 4. 12 of the 16 D6Rs per unit cell are occupied by Sr(1) ions as shown in Fig. 4. The remaining four D6Rs are empty as shown in Fig. 5. The octahedral ˚ , is similar to Sr(1)–O(3) distance, 2.652(10) A that in crystal 1. The four Sr2+ ions at Sr(3) lie at site I∞, on a threefold axis in the sodalite cavity. See Fig. 5. ˚ ] for simultaEach is too close to site I [2.98(1) A neous occupancy (due to electrostatic repulsion), so if a site I is occupied, the two adjacent I∞ sites cannot be. Of the eight remaining I∞ sites per unit cell, two just outside each empty D6R, only four
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M.J. Kim et al. / Microporous and Mesoporous Materials 30 (1999) 233–241 Table 4 Distribution of alkaline-earth cations over sites in zeolite Xa Cation site
Order
Mg –X · 4H Ob 46 2
Ca –Xb 46
Ca –X · 145NHc 46 3
Sr –Xd 46
Sr –X · 102NHd 46 3
Ba –Xb 46
I I∞ II
16 32 32
14 4 28
16
16
16
30
30
30
12 4 30
14 2 30
a All entries are single-crystal diffraction results using crystals of framework composition (Si Al O )92− per unit cell prepared 100 92 384 by Bogomolov and Petranovskii [14]. All crystals are fully exchanged; all are entirely empty (anhydrous) except as indicated. b Ref. [13]. Single crystals of Mg –X · 4H O did not survive attempts at further dehydration. 46 2 c Ref. [22]. d This work.
Fig. 4. A stereoview of half of the sodalite cavities, with attached D6R, of dehydrated Sr –X · 102NH . One Sr2+ ion at Sr(1) 46 3 (site I ) and four Sr2+ ions at Sr(2) (site II ) are shown. Each of the four Sr(2) ions associates with three ammonia molecules at N(2). About 75% (or more) of the D6Rs have this arrangement. Ellipsoids of 20% probability are shown.
Fig. 5. A stereoview of the remaining half of the sodalite cavities, with attached D6R, of dehydrated Sr –X · 102NH . One 46 3 Sr2+ ion at Sr(3) (site I∞) associated with three ammonia molecules at N(3), and four Sr2+ ions at Sr(2) (site II ) are shown. Each of these four associates with three ammonia molecules at N(2). 25% of the D6Rs have this arrangement. Ellipsoids of 20% probability are shown.
are occupied; perhaps only one of each pair is used by an Sr(3) ion. In this way, Sr(1) and Sr(3) ‘fill’ the D6Rs.
Each of the remaining 30 Sr2+ ions at Sr(2) lies at site II, on a threefold axis in the supercage; ˚ to three O(2) each coordinates at 2.584(7) A ˚ oxygen atoms and is 1.12 A from their plane. See ˚ further from its O(2) Figs. 4–6. Sr(2) is 0.56 A plane than it was in empty Sr –X. Each coordi˚ 46 nates further at 2.774(24) A to three ammonia molecules deeper in the supercage. The O(2)– Sr(2)–O(2) bond angle has decreased sharply from 114.9(2)° in crystal 1 to 102.6(3)° in this threefold-axially distorted octahedral complex. To coordinate to three N(3) ammonia mole˚ along cules, each Sr(3) ion at site I∞ extends 1.50 A its threefold axis toward the center of its sodalite cavity, away from its triad of three O(3) oxygen atoms to a more octahedral coordination environ˚ , agrees ment. The Sr(3)–N(3) distance, 2.76(8) A with the sum of the corresponding ionic and ˚ ) [18,21]. van der Waals radii (1.12+1.5=2.6 A N(3)–Sr(3)–N(3) is 70(3)°. Perhaps those D6Rs that contain only five aluminum ions (instead of six; there can be four such per unit cell ) have been unable to retain their site-I Sr2+ ions upon sorption of NH , and have lost them to site I∞ where they 3 can coordinate to a ring with three aluminum ions and to three ammonia molecules. In the supercage complex, the Sr(2)–N(2) dis˚ , and the tance is nearly the same, 2.774(24) A N(2)–Sr(2)–N(2) angle is 77.8(7)°. This distance can be compared with the theoretical results of Partridge and coworkers [11] and Martyna and Klein [12] who found, respectively, that the Sr–N ˚ and distances in Sr(NH )+ complexes are 2.757 A 33 ˚ 2.60 A. Partridge and coworkers [11] calculated the N–Sr–N angle to be 88.4°.
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Fig. 6. A stereoview of a supercage of dehydrated Sr –X · 102NH . Four Sr2+ ions at Sr(2) (site II ), each of which is associated 46 3 with three ammonia molecules at N(2), are shown. About 75% of the supercages have this arrangement. The remaining 25% of the supercages may have only three Sr(NH )2+ ions. Ellipsoids of 20% probability are shown. 33
In this structure, the mean values of the Si–O ˚ and and Al–O bond lengths are normal, 1.627 A ˚ respectively. The individual bond lengths, 1.715 A however, show small variations: Si–O from ˚ and Al–O from 1.700(11) 1.613(9) to 1.646(11) A ˚ . Sr2+ ions coordinate only to O(2) to 1.726(10) A and O(3). As a consequence of these (substantially ionic) interactions, the Si–O(2), Si–O(3), Al– O(2), and Al–O(3) bonds are somewhat lengthened (see Table 2). This effect is more pronounced in crystal 1 where the Sr–O distances are shorter. Perhaps the bright red color of this complex involves an electronic transition similar to that responsible for the brilliant red flame test for Sr; applications are pyrotechnics and flares. The unusual coordination geometry about Sr(2) (three NH molecules crowded to one side) may greatly 3 affect transition probabilities. Recently the structure of Ca –X · ~135NH 46 3 was determined by single-crystal diffraction techniques [22]. In that work, the dehydrated crystal was exposed to 300 Torr of ammonia gas at 21(1)°C, but, unlike this report, ammonia gas was not evacuated in the final stage of crystal preparation, so the structure was determined in that atmosphere. Similar to the present structure, each ˚ into the of 30 site II Ca2+ ions extends 0.87 A ˚ supercage (an increase of 0.57 A upon NH sorp3 tion). Each coordinates octahedrally to three ˚ and to three framework oxygen atoms 2.383(7) A ˚ ammonia molecules at 2.75(2) A. Unlike the present structure, no Ca(NH )2+ complexes are found 33 in the sodalite units, apparently because Ca2+ ions
fit six-rings better than Sr2+. However, an additional 45 (perhaps 48) NH molecules are found 3 per unit cell, four per 12-ring. Each hydrogen bonds via its lone pair to two coordinating NH 3 molecules and is in position to associate weakly with two or more framework oxygen atoms. These 12-ring NH molecules do not coordinate to cat3 ions, and would presumably have been removed if the crystal had been evacuated at ambient temperature before the diffraction data were gathered.
5. Summary In dehydrated Sr –X, Sr2+ ions are found at 46 two sites: 16 at site I and 30 at site II. In its ammonia sorption complex, Sr2+ ions are located at three sites: 12 remain at site I; four have relocated from site I to I∞, where each coordinates to three framework oxygen atoms and also to three ammonia molecules; the remaining 30 Sr2+ ˚ further into the at site II have each shifted 0.56 A supercage to coordinate to three ammonia molecules. These coordinated molecules cannot be removed by evacuation at 21°C 5.1. Supporting material available Tables of observed and calculated structure factors (29 pages). Ordering information is given on any current masthead page.
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Acknowledgement This work was supported in part by the Basic Research Institute Program, Ministry of Education, Korea, 1996, Project No. BSRI-96-3409.
References [1] F. Grandjean, C.R. Hebd. Seances Acad. Sci. 149 (1909) 866. [2] W. Eitel, Physical Chemistry of Silicates, University of Chicago Press, Chicago, 1954, p. 994. [3] K. Klier, Molecular Sieve Zeolites vol. I , Am. Chem. Soc., Washington DC, 1971, p. 480. [4] L.E. William, O.F. Paul, A.V.G. Atholl, H.L. Jack, J. Phys. Chem. 91 (1987) 2091. [5] R.Y. Yanagida, K. Seff, J. Phys. Chem. 76 (1972) 2597. [6 ] R.M. Barrer, G.C. Bratt, J. Phys. Chem. Solids 12 (1959) 130. [7] D.W. Breck, W.G. Eversole, R.M. Milton, T.B. Reed, T.L. Thomas, J. Am. Chem. Soc. 78 (1956) 5963. [8] L.V.C. Rees, T. Berry, Molecular Sieves, Society of Chemical Industry, London, 1968, p. 149.
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[9] I.R. Leith, H.F. Leach, Proc. R. Soc. London Ser. A: 330 (1972) 247. [10] E. Dempsey, D.H. Olson, J. Phys. Chem. 74 (1970) 305. [11] C.W. Bauschlicher Jr, M. Sodupe, H. Partridge, J. Chem. Phys. 96 (1992) 4453. [12] G.J. Martyna, M.L. Klein, J. Phys. Chem. 95 (1991) 515. [13] Y.H. Yeom, S.B. Jang, S.H. Song, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 6914. [14] V.N. Bogomolov, V.P. Petranovskii, Zeolites 6 (1986) 418. [15] MolEN, Enraf–Nonius, The Netherlands, 1990. [16 ] International Tables for X-Ray Crystallography vol. IV, Kynoch Press, Birmingham, UK, 1984, p. 86. [17] International Tables for X-Ray Crystallography vol. II, Kynoch Press, Birmingham, UK, 1974, p. 149. [18] Handbook of Chemistry and Physics, 70th edition (1989/ 1990) Chemical Rubber Company, Cleveland, OH, p. F187. [19] Y. Smolin, Y.F. Shepelev, A.A. Anderson, Acta Crystallogr. Sect. B: 45 (1989) 124. [20] J.H. Kwon, S.B. Jang, Y. Kim, K. Seff, J. Phys. Chem. 100 (1996) 13720. [21] Handbook of Chemistry and Physics, 70th edition (1989/ 1990) Chemical Rubber Company, Cleveland, OH, p. D190. [22] S.B. Jang, M.S. Jeong, Y. Kim, S.H. Song, K. Seff, Micro. Meso. Mater 28 (1999) 173.
Crystal structures of fully dehydrated fully Sr2+-exchanged zeolite X and of its ammonia sorption complex Mi Jung Kim a, Mi Suk Jeong a, Yang Kim a, Karl Seff b, * a Department of Chemistry, Pusan National University, Pusan 609-735, South Korea b Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA Received 9 June 1998; received in revised form 17 December 1998; accepted 5 January 1999
Abstract ˚ ] and of its ammonia sorption The crystal structures of fully dehydrated Sr –X [Sr Si Al O ; a=25.214(7) A 46 46 100 92 384 ˚ ], have been determined by single-crystal X-ray complex, Sr –X · 102NH [Sr Si Al O · 102NH ; a=25.127(7) A 46 3 46 100 92 384 3 diffraction techniques in the cubic space group Fd3: at 21(1)°C. The Sr –X crystal was prepared by ion exchange in a 46 flowing stream of aqueous 0.05 M Sr(ClO ) for 5 days followed by dehydration at 360°C and 2×10−6 Torr for 2 days. 42 To prepare the ammonia sorption complex, another dehydrated Sr –X crystal was exposed to 230 Torr of zeolitically 46 dried ammonia gas for 1 h followed by evacuation for 12 h at 21(1)°C and 5×10−4 Torr. The structures were refined to the final error indices, R =0.043 and R =0.039 with 466 reflections, and R =0.049 and R =0.044 with 382 1 w 1 w reflections, for which I>3s(I ). In dehydrated Sr –X, all Sr2+ ions are located at two crystallographic sites. 16 Sr2+ 46 ˚ ). The remaining 30 Sr2+ ions ions are at the centers of the double six-rings, filling that site (site I, Sr–O=2.592(6) A ˚ into the supercage from the plane of its three nearest oxygen atoms are in the supercage (site II ); each extends 0.56 A ˚ ). In the structure of Sr –X · 102NH , the Sr2+ ions are located at three crystallographic sites: 12 (Sr–O=2.469(6) A 46 3 ˚ ]; four are found at site I [Sr–O=2.652(10) A in the sodalite units (site I∞) each coordinated to three framework oxygen ˚ and also to three ammonia molecules at 2.76(8) A ˚ . The remaining 30 Sr2+ ions lie at site II. Each atoms at 2.654(9) A ˚ into the supercage where it coordinates to three framework oxygen atoms at 2.584(7) A ˚ and also to extends 1.12 A ˚ three ammonia molecules at 2.774(24) A. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Ammonia sorption complex; Crystal structure; Strontium; Zeolite X
1. Introduction The first experimental observations of the sorption of gases on zeolites and their behavior as molecular sieves were conducted on zeolite minerals. Grandjean [1] reported and extended the work of G. Friedel on ammonia sorption by dehydrated zeolites [2]. * Corresponding author. Fax: +1-808-956-5908. E-mail address: [email protected] ( K. Seff )
Electronic reflectance spectra of dehydrated Co2+-exchanged zeolite A indicated that six-ring Co2+ ions form complexes with N O, 2 C H , H O, and NH [3]. In the spectrochemical 3 6 2 3 series of these molecules, NH has the highest 3 ligand strength; NH forms a tetrahedral complex 3 with Co2+. The ammonia molecule is small enough to access the sodalite cavities of the zeolite [4]: sorption by sodalite, although slow, occurs [5]. In the crystal structure of an ammonia sorption complex
1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 01 2 - 8
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M.J. Kim et al. / Microporous and Mesoporous Materials 30 (1999) 233–241
of zeolite A [5], 32 NH molecules are found per 3 unit cell. 12 NH molecules are found at two 3 equipoints in the sodalite unit. Of the remaining 20 NH molecules in the large cavity, eight coordi3 nate to threefold-axis Na+ ions. The presence of sorbed ammonia in zeolite X has a pronounced stabilization effect because it lowers the chemical potential of the zeolite crystal for the mechanically strained dehy[6 ]. G 9 zeolite drated form is higher than that of the framework and cations in the NH complex whose framework/ 3 cation structure is more like that of the relatively relaxed hydrated zeolite. In the case of a large-pore zeolite such as zeolite X, the effective pore size may be controlled by the formation of a stable inorganic complex. The strong interaction between the zeolite cation and the dipole moment of ammonia or water produces a diffusion block by clustering ammonia or water molecules about the cation in the channels. When small amounts of ammonia or water are presorbed on a dehydrated zeolite, the sorption of a second less polar sorbate such as oxygen is drastically reduced [7,8]. ESR spectra of a dehydrated copper-exchanged zeolite X suggested that Cu2+ ions occupy sites I∞ and II∞ in the sodalite cages. Only those at the latter site were observed to interact by complexation with sorbed NH or pyridine [9]. 3 In partially dehydrated Sr –X, the Sr2+ ions 44.8 are located at four sites: 6.1 at site I, 12.0 at I∞, 6.4 at II∞, and 20.3 at II [10]. Partridge and coworkers [11] calculated theoretical bond lengths and angles for selected monoand di-positive M(NH ) and M(H O) ions, 3n 2 n where M=Mg, Ca, and Sr. These included calculations on NH complexes of Sr+, but, unfortu3 nately, not of Sr2+. Martyna and Klein [12] calculated the Sr–N distances in Sr+(NH ) (n= 3n 1–6) using pseudopotentials parameterized to ab initio calculations. This work was initiated to learn the positions of Sr2+ ions for comparison with those of the other alkaline-earth cations, Mg2+, Ca2+, and Ba2+ in zeolite X [13]. It was expected that fully Sr2+-exchanged zeolite X could be readily prepared. It would also be interesting to see the changes that ammonia sorption would cause to
the distribution and placement of Sr2+ ions in the zeolite, and to observe the geometry of the resulting complexes.
2. Experimental section Large colorless crystals of sodium zeolite X, Na Si Al O per unit cell, were prepared in 92 100 92 384 St. Petersburg, Russia [14]. Each of two single crystals, an octahedron about 0.20 mm in crosssection, was lodged in a fine Pyrex capillary. Complete ion exchange was accomplished by the flow method using aqueous 0.05 M (pH=6.5) Sr(ClO ) (Johnson Matthey, 99.9%). The solution 42 was allowed to flow past each crystal at a velocity of approximately 15 mm s−1 for 5 days at 21(1)°C. Each crystal was dehydrated by slowly increasing the temperature by 25°C h−1 to 360°C under vacuum, followed by 48 h at 360°C and 2× 10−6 Torr. After cooling to room temperature, the first crystal (crystal 1), colorless, still under vacuum, was sealed in its Pyrex capillary by torch. To prepare an ammonia sorption complex of Sr2+-exchanged zeolite X, the second crystal (crystal 2) was treated with 230 Torr of zeolitically dried ammonia gas ( Union Carbide, 99.995%) for 1 h at 21(1)°C, and then evacuated for 12 h at this temperature and 5×10−4 Torr. It was then sealed in its capillary by torch. Microscopic examination showed that crystal 2 was bright red. The cubic space group Fd3: was used throughout this work. This choice is supported by: (a) the low Si/Al ratio, which in turn requires, at least in the short range, alternation of Si and Al; (b) the observation that this crystal, like all other crystals from the same batch, does not have intensity symmetry across (110) and therefore lacks that mirror plane. Diffraction data were collected with an automated Enraf–Nonius four-circle computercontrolled CAD-4 diffractometer equipped with a pulse-height analyzer and graphite monochroma˚; tor, using Mo radiation ( Ka , l=0.709 30 A 1 ˚ Ka , l=0.713 59 A). The unit cell constants at 2 21(1)°C, each determined by least-squares refinement of 25 intense reflections for which ˚ for crystal 1 and 14<2h<24°, are a=25.214(7) A ˚ a=25.127(7) A for crystal 2. All unique reflections
M.J. Kim et al. / Microporous and Mesoporous Materials 30 (1999) 233–241
in the positive octant of an F-centered unit cell for which 2h<50°, l>h, and k>h were recorded. Of the 1268 and 1257 reflections for crystal 1 and crystal 2 respectively, only the 466 and 382 reflections respectively, for which I >3s(I ), were used in subsequent structure determination.
3. Structure determination 3.1. Crystal 1, Sr –X 46 Full-matrix least-squares refinement was initiated with the atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3) and O(4)] of dehydrated Ca –X [13]. Isotropic refinement con46 verged to an R index, |F −|F ||/F , of 0.44 and a 1 o c o weighted R index, [w(F −|F |)2/wF2 ]1/2 of 0.49. w o c o The initial difference Fourier function revealed two large peaks at (0.0, 0.0, 0.0) and (0.22, ˚ −3 and 12 e A ˚ −3 0.22, 0.22) with heights of 16 e A respectively. These peaks were stable in leastsquares refinement as Sr(1) and Sr(2) respectively. Anisotropic refinement including these Sr2+ ions converged to R =0.042 and R =0.038. 1 w Occupancy refinement converged at 16.2(1) and 29.2(1) respectively. These values indicate complete and stoichiometric exchange with no suggestion of retention of excess salt. The occupancies were reset and fixed at 16 and 30 Sr2+ ions at Sr(1) and Sr(2) respectively, because the maximum number of Sr2+ ions at Sr(1) is 16, and because the cationic charge should sum to 92+ per unit cell. Anisotropic refinement of the framework atoms and cations at Sr(1) and Sr(2) converged to R =0.043 and R =0.039. 1 w In the final cycle of least-squares refinement, all shifts in atomic parameters were less than 0.01% of their corresponding standard deviations. The final difference function was featureless. 3.2. Crystal 2, Sr –X · 102NH 46 3 Full-matrix least-squares refinement was initiated with atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3) and O(4)] of crystal 1. Isotropic refinement of the framework atoms converged to R =0.48 and R =0.56. 1 w
235
A difference Fourier function showed the positions of the Sr2+ ions at Sr(1), (0.0, 0.0, 0.0) with ˚ −3, at Sr(2), (0.239, 0.239, peak height 12 e A ˚ −3, and at Sr(3), 0.239) with peak height 12 e A (0.0696, 0.0696, 0.0696) with peak height ˚ −3. These peaks were stable in least-squares 4.9 e A refinement. Isotropic refinement of framework atoms and anisotropic refinement of Sr(1), Sr(2), and Sr(3) converged to R =0.093 and 1 R =0.142 with occupancies of 11.8(10), 30.0(1), w and 4.4(2) respectively. From a subsequent difference Fourier function, ˚ −3 and 2.2 e A ˚ −3 were two peaks of height 2.4 e A found at (0.151, 0.161, 0.169) and (0.221, 0.312, 0.322). These were refined as N(3) and N(2) to occupancies of 15(1) and 101(3) respectively. Full anisotropic refinement of the framework and Sr2+ ions, with positional, occupancy and isotropic thermal parameters varying at N(2) and N(3), converged to R =0.049 and R =0.044. 1 w The occupancy numbers at Sr(1), Sr(2), Sr(3), N(2), and N(3) were fixed at the values shown in Table 1 using the considerations mentioned above for strontium ions, and allowing the maximum number of NH molecules (three) to coordinate to 3 each Sr2+ ion. The final error indices were R =0.050 and R =0.045. The shifts in the final 1 w cycle of least-squares refinement were less than 0.01% of their corresponding standard deviations. The final difference function was featureless. Crystallographic calculations were done using MolEN [15]. The full-matrix least-squares program used minimized w(F −|F |)2; the weight w of o c an observation was the reciprocal square of s(F ), its standard deviation. Atomic scattering o factors for Si, Al, O−, Sr2+ and N were used. All scattering factors [16 ] were modified to account for anomalous dispersion [17]. The final structural parameters, and selected interatomic distances and angles, are presented in Table 1 and Table 2 respectively.
4. Discussion 4.1. Description of zeolite X Zeolite X is a synthetic counterpart of the naturally occurring mineral faujasite (see Fig. 1).
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Table 1 Positional,a thermal, and occupancy parameters Atom Wyckoff Site x position
y
z
U ,b U 11 iso
U
22
U
33
U
12
U
13
U
23
Occupancyc varied
(a) Sr –X (crystal 1) 46 Si 96(g) Al 96(g) O(1) 96(g) O(2) 96(g) O(3) 96(g) O(4) 96(g) Sr(1) 16(c) I Sr(2) 32(e) II
−563(1) −571(1) −1117(3) −50(3) −394(2) −602(2) 0 2288(1)
(b) Sr –X2102NH (crystal 2) 46 3 Si 96(g) −549(1) Al 96(g) −562(2) O(1) 96(g) −1118(4) O(2) 96(g) −36(4) O(3) 96(g) −371(4) O(4) 96(g) −648(3) Sr(1) 16(c) I 0 Sr(2) 32(e) II 2419(1) Sr(3) 32(e) I∞ 685(6) N(2) 96(g) 2204(9) N(3) 96(g) 1563(37)
fixed
1237(1) 353(1) 0(3) −46(5) 680(3) 733(3) 0 2288(1)
343(1) 1235(1) 1102(3) 1380(2) 663(3) 1771(3) 0 2288(1)
148(14) 99(14) 161(42) 66(35) 113(39) 137(39) 165(6) 275(5)
90(13) 68(15) 184(42) 89(36) 116(40) 203(40) 165(6) 275(5)
117(15) 43(14) 183(42) 250(41) 166(41) 139(38) 165(6) 275(5)
−22(15) 19(14) −36(36) 37(32) 30(34) −8(36) 36(7) 153(7)
1(15) −40(16) 96 8(16) −15(16) 96 33(30) 3(35) 96 −34(33) −31(34) 96 −30(34) −56(35) 96 −25(36) −144(34) 96 36(7) 36(7) 16.2(1) 16 153(7) 153(7) 29.2(1) 30
1248(2) 366(1) 8(4) −43(4) 718(4) 728(4) 0 2419(1) 685(6) 3203(10) 1730(31)
338(1) 1236(2) 1074(3) 1405(3) 679(4) 1801(4) 0 2419(1) 685(6) 3166(10) 1583(40)
94(17) 91(18) 282(66) 154(48) 209(60) 161(54) 214(11) 287(6) 398(68) 1241(101) 569d
84(17) 77(18) 175(51) 179(48) 320(66) 149(55) 214(11) 287(6) 398(68)
112(17) 97(18) 197(57) 129(54) 274(65) 250(61) 214(11) 287(6) 398(68)
−4(27) −6(18) −94(53) 54(47) 87(58) −29(52) 61(14) 160(9) 118(83)
−7(17) 5(29) 8(42) −60(46) 15(57) 14(56) 61(14) 160(9) 118(83)
5(26) −21(27) −54(50) −82(47) 144(52) −99(47) 61(14) 160(9) 118(83)
96 96 96 96 96 96 11.8(1) 12 30.0(1) 30 4.4(2) 4 101(3) 90 15(1) 12
a Positional and anisotropic thermal parameters are given ×104. Numbers in parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b Anisotropic temperature factor: exp[−2p2a−2(h2U +k2U +l2U +2hkU +2hlU +2klU )]. 11 22 33 12 13 23 c Occupancy factors are given as the number of atoms or ions per unit cell. d This thermal parameter was fixed in least-squares refinement.
The 14-hedron with 24 vertices known as the sodalite cavity or b cage may be viewed as its principal building block. These b cages are connected tetrahedrally at six-rings by bridging oxygen atoms to give double six-rings (D6Rs, hexagonal prisms), and concomitantly to give an interconnected set of even larger cavities (supercages) accessible in three dimensions through 12-ring (24-membered ) windows. The Si and Al atoms occupy the vertices of these polyhedra. The oxygen atoms lie approximately midway between each pair of Si and Al atoms, but are displaced from those points to give near tetrahedral angles about Si and Al. Single six-rings (S6Rs) are shared by sodalite and supercages, and may be viewed as the entrances to the sodalite units. Each unit cell has eight sodalite units, eight supercages, 16 D6Rs, 16 12-rings, and 32 S6Rs. Exchangeable cations that balance the negative
change of the aluminosilicate framework are found within the zeolite’s cavities. They are usually found at the following general sites shown in Fig. 1: I at the center of a D6R, I∞ in the sodalite cavity on the opposite side of one of the D6Rs six-rings from site I, II∞ inside the sodalite cavity near an S6R, II in the supercage adjacent to an S6R, III in the supercage opposite a four-ring between two 12-rings, and III∞, somewhat or substantially distant from III but otherwise near the inner walls of the supercage or the edges of 12-rings. 4.2. Crystal 1, Sr –X 46 In this crystal structure, 46 Sr2+ ions are located at just two crystallographic sites with high occupancy. The 16 Sr2+ ions at Sr(1) fill site I at the center of the D6Rs. See Fig. 2. The octahedral ˚ , is somewhat Sr(1)–O(3) distance, 2.592(6) A
M.J. Kim et al. / Microporous and Mesoporous Materials 30 (1999) 233–241
237
Table 2 ˚ ) and angles (deg)a Selected interatomic distances (A
Si–O(1) Si–O(2) Si–O(3) Si–O(4) mean Al–O(1) Al–O(2) Al–O(3) Al–O(4) mean Sr(1)–O(3) Sr(2)–O(2) Sr(3)–O(3) Sr(2)–N(2) Sr(3)–N(3) N(3)–O(2) N(2)–O(1) N(2)–O(2) N(2)–N(2) N(3)–N(3) O(1)–Si–O(2) O(1)–Si–O(3 O(1)–Si–O(4) O(2)–Si–O(3) O(2)–Si–O(4) O(3)–Si–O(4) O(1)–Al–O(2) O(1)–Al–O(3) O(1)–Al–O(4) O(2)–Al–O(3) O(2)–Al–O(4) O(3)–Al–O(4) Si–O(1)–Al Si–O(2)–Al Si–O(3)–Al Si–O(4)–Al O(3)–Sr(1)–O(3) O(2)–Sr(2)–O(2) O(3)–Sr(3)–O(3) N(2)–Sr(2)–O(2) N(3)–Sr(3)–O(3) N(2)–Sr(2)–N(2) N(3)–Sr(3)–N(3)
Sr –X 46 (crystal 1)
Sr –X · 102NH 46 3 (crystal 2)
1.639(7) 1.678(7) 1.675(7) 1.655(8) 1.662 1.673(7) 1.695(7) 1.717(7) 1.659(8) 1.686 2.592(6) 2.469(6)
1.613(9) 1.632(10) 1.646(11) 1.618(11) 1.627 1.712(10) 1.726(10) 1.722(11) 1.700(11) 1.715 2.652(10) 2.584(7) 2.654(9) 2.774(24) 2.76(8) 3.17(9) 3.40(2) 3.05(2) 3.48(3) 2.88(12) 112.0(5) 109.9(5) 110.0(5) 106.4(5) 104.3(5) 114.1(5) 111.7(5) 107.8(5) 112.0(5) 107.0(5) 102.2(5) 116.1(5) 130.0(6) 147.8(6) 139.6(6) 152.1(7) 88.5(3)/91.5(3) 102.6(3) 91.5(3) 69.3(5)/130.2(5) 95(2)/162(2) 77.8(7) 70(3)
111.8(4) 108.1(4) 114.2(4) 105.5(3) 102.7(3) 114.2(4) 111.4(3) 107.6(3) 115.6(4) 105.3(3) 101.6(3) 114.8(4) 126.7(4) 150.8(4) 137.7(4) 160.3(5) 85.8(2)/94.2(2) 114.9(2)
a Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value.
longer than the sum of the corresponding ionic ˚ [18]. For comparison, radii, 1.12+1.32=2.44 A the Mg–O distance in Mg (H O) –X [13] and the 46 2 4
Fig. 1. A stylized drawing of the framework structure of zeolite X. Near the center of the each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1 to 4. Silicon and aluminum alternate at the tetrahedral intersections, except that Si substitutes for about 4% of the Al. Extraframework cation sites are labeled with Roman numerals.
Fig. 2. A stereoview of a sodalite cavity with attached D6R of dehydrated Sr –X. One Sr2+ ion at Sr(1) (site I ) and four 46 Sr2+ ions at Sr(2) (site II ) are shown. About 75% of the sodalite units have this arrangement. Only three Sr(2) ions surround the remaining 25%. These fractions of 25% and uncertainties would likely vanish in this and subsequent captions if this structure could be extrapolated to Si/Al=1. Ellipsoids of 20% probability are shown.
Ca–O distance in Ca –X [13] are similarly longer 46 than the sum of the corresponding ionic radii, indicating a reasonably good fit. The 30 Sr2+ ions at Sr(2) are located at site II in the supercage. See Figs. 2 and 3. Each coordi˚ , nearly the sum of nates trigonally at 2.469(6) A
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Fig. 3. A stereoview of a supercage of dehydrated Sr –X. Four Sr2+ ions at Sr(2) (site II ) are shown. About 75% (or more) of the 46 supercages have this arrangement. The remaining 25% of the supercages may have only three Sr2+ ions at Sr(2). Ellipsoids of 20% probability are shown.
the ionic radii, to three O(2) framework oxygen ˚ into the supercage from atoms and extends 0.56 A their plane. The O(2)–Sr(2)–O(2) bond angle is 114.9(2)°, not too far from trigonal planar. Clearly, site I, because it is filled, is preferred over site II. Smolin et al. [19] investigated the placements of cations in fully Ca2+-exchanged zeolite X at various stages of dehydration by heating a crystal in a stream of hot N gas. With increasing dehydra2 tion, the occupancies at sites I and II increase, consistent with the results presented here. Recently the crystal structures of Ca –X [13] 46 and Cd –X [20] have been determined by single46 crystal X-ray diffraction methods. As in Sr –X, 46 the Ca2+ and Cd2+ ions fill site I, with the remainder going to site II in the supercage. Only about 13 Ba2+ ions (relatively large ions) in Ba –X [13] and 14 Mg2+ (relatively small ions) 46 in Mg (H O) –X [13] occupy the 16-fold site I 46 2 4 position (see Tables 3 and 4). It appears that site I (at the center of the D6R) is generally the lowest energy site for cations of intermediate size. Little explanation is offered for the lack of differentiation between the mean Al–O and Si–O bond lengths in crystal 1 (see Table 2); Al–O is ˚ longer than Si–O, as usually seen to be ~0.10 A is seen in the results for crystal 2. Efforts to escape from a possible false minimum in least-squares refinement were not successful. Perhaps crystal 1 is twinned with respect to Si/Al ordering. This is often the result of ion-exchange with pH too low.
Table 3 ˚ ) of cations from six-ring planes Deviations(A
At O(3) At O(2)
Sr(1)a Sr(3)a Sr(2)b
Sr –X 46 (crystal 1)
Sr –X · 102NH 46 3 (crystal 2)
−1.40
−1.49 1.50 1.12
0.56
a A negative deviation indicates that the ion lies in the D6R (in these structures, at its center at site I ); a positive deviation, in the sodalite cavity at site I∞. b A positive deviation indicates that the ion lies in the supercage.
4.3. Crystal 2, Sr –X · 102NH 46 3 The sorption of ammonia has caused the distribution of Sr2+ ions among sites to change. The Sr2+ ions are now found at three crystallographic sites, none of which is full. See Table 4. 12 of the 16 D6Rs per unit cell are occupied by Sr(1) ions as shown in Fig. 4. The remaining four D6Rs are empty as shown in Fig. 5. The octahedral ˚ , is similar to Sr(1)–O(3) distance, 2.652(10) A that in crystal 1. The four Sr2+ ions at Sr(3) lie at site I∞, on a threefold axis in the sodalite cavity. See Fig. 5. ˚ ] for simultaEach is too close to site I [2.98(1) A neous occupancy (due to electrostatic repulsion), so if a site I is occupied, the two adjacent I∞ sites cannot be. Of the eight remaining I∞ sites per unit cell, two just outside each empty D6R, only four
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M.J. Kim et al. / Microporous and Mesoporous Materials 30 (1999) 233–241 Table 4 Distribution of alkaline-earth cations over sites in zeolite Xa Cation site
Order
Mg –X · 4H Ob 46 2
Ca –Xb 46
Ca –X · 145NHc 46 3
Sr –Xd 46
Sr –X · 102NHd 46 3
Ba –Xb 46
I I∞ II
16 32 32
14 4 28
16
16
16
30
30
30
12 4 30
14 2 30
a All entries are single-crystal diffraction results using crystals of framework composition (Si Al O )92− per unit cell prepared 100 92 384 by Bogomolov and Petranovskii [14]. All crystals are fully exchanged; all are entirely empty (anhydrous) except as indicated. b Ref. [13]. Single crystals of Mg –X · 4H O did not survive attempts at further dehydration. 46 2 c Ref. [22]. d This work.
Fig. 4. A stereoview of half of the sodalite cavities, with attached D6R, of dehydrated Sr –X · 102NH . One Sr2+ ion at Sr(1) 46 3 (site I ) and four Sr2+ ions at Sr(2) (site II ) are shown. Each of the four Sr(2) ions associates with three ammonia molecules at N(2). About 75% (or more) of the D6Rs have this arrangement. Ellipsoids of 20% probability are shown.
Fig. 5. A stereoview of the remaining half of the sodalite cavities, with attached D6R, of dehydrated Sr –X · 102NH . One 46 3 Sr2+ ion at Sr(3) (site I∞) associated with three ammonia molecules at N(3), and four Sr2+ ions at Sr(2) (site II ) are shown. Each of these four associates with three ammonia molecules at N(2). 25% of the D6Rs have this arrangement. Ellipsoids of 20% probability are shown.
are occupied; perhaps only one of each pair is used by an Sr(3) ion. In this way, Sr(1) and Sr(3) ‘fill’ the D6Rs.
Each of the remaining 30 Sr2+ ions at Sr(2) lies at site II, on a threefold axis in the supercage; ˚ to three O(2) each coordinates at 2.584(7) A ˚ oxygen atoms and is 1.12 A from their plane. See ˚ further from its O(2) Figs. 4–6. Sr(2) is 0.56 A plane than it was in empty Sr –X. Each coordi˚ 46 nates further at 2.774(24) A to three ammonia molecules deeper in the supercage. The O(2)– Sr(2)–O(2) bond angle has decreased sharply from 114.9(2)° in crystal 1 to 102.6(3)° in this threefold-axially distorted octahedral complex. To coordinate to three N(3) ammonia mole˚ along cules, each Sr(3) ion at site I∞ extends 1.50 A its threefold axis toward the center of its sodalite cavity, away from its triad of three O(3) oxygen atoms to a more octahedral coordination environ˚ , agrees ment. The Sr(3)–N(3) distance, 2.76(8) A with the sum of the corresponding ionic and ˚ ) [18,21]. van der Waals radii (1.12+1.5=2.6 A N(3)–Sr(3)–N(3) is 70(3)°. Perhaps those D6Rs that contain only five aluminum ions (instead of six; there can be four such per unit cell ) have been unable to retain their site-I Sr2+ ions upon sorption of NH , and have lost them to site I∞ where they 3 can coordinate to a ring with three aluminum ions and to three ammonia molecules. In the supercage complex, the Sr(2)–N(2) dis˚ , and the tance is nearly the same, 2.774(24) A N(2)–Sr(2)–N(2) angle is 77.8(7)°. This distance can be compared with the theoretical results of Partridge and coworkers [11] and Martyna and Klein [12] who found, respectively, that the Sr–N ˚ and distances in Sr(NH )+ complexes are 2.757 A 33 ˚ 2.60 A. Partridge and coworkers [11] calculated the N–Sr–N angle to be 88.4°.
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Fig. 6. A stereoview of a supercage of dehydrated Sr –X · 102NH . Four Sr2+ ions at Sr(2) (site II ), each of which is associated 46 3 with three ammonia molecules at N(2), are shown. About 75% of the supercages have this arrangement. The remaining 25% of the supercages may have only three Sr(NH )2+ ions. Ellipsoids of 20% probability are shown. 33
In this structure, the mean values of the Si–O ˚ and and Al–O bond lengths are normal, 1.627 A ˚ respectively. The individual bond lengths, 1.715 A however, show small variations: Si–O from ˚ and Al–O from 1.700(11) 1.613(9) to 1.646(11) A ˚ . Sr2+ ions coordinate only to O(2) to 1.726(10) A and O(3). As a consequence of these (substantially ionic) interactions, the Si–O(2), Si–O(3), Al– O(2), and Al–O(3) bonds are somewhat lengthened (see Table 2). This effect is more pronounced in crystal 1 where the Sr–O distances are shorter. Perhaps the bright red color of this complex involves an electronic transition similar to that responsible for the brilliant red flame test for Sr; applications are pyrotechnics and flares. The unusual coordination geometry about Sr(2) (three NH molecules crowded to one side) may greatly 3 affect transition probabilities. Recently the structure of Ca –X · ~135NH 46 3 was determined by single-crystal diffraction techniques [22]. In that work, the dehydrated crystal was exposed to 300 Torr of ammonia gas at 21(1)°C, but, unlike this report, ammonia gas was not evacuated in the final stage of crystal preparation, so the structure was determined in that atmosphere. Similar to the present structure, each ˚ into the of 30 site II Ca2+ ions extends 0.87 A ˚ supercage (an increase of 0.57 A upon NH sorp3 tion). Each coordinates octahedrally to three ˚ and to three framework oxygen atoms 2.383(7) A ˚ ammonia molecules at 2.75(2) A. Unlike the present structure, no Ca(NH )2+ complexes are found 33 in the sodalite units, apparently because Ca2+ ions
fit six-rings better than Sr2+. However, an additional 45 (perhaps 48) NH molecules are found 3 per unit cell, four per 12-ring. Each hydrogen bonds via its lone pair to two coordinating NH 3 molecules and is in position to associate weakly with two or more framework oxygen atoms. These 12-ring NH molecules do not coordinate to cat3 ions, and would presumably have been removed if the crystal had been evacuated at ambient temperature before the diffraction data were gathered.
5. Summary In dehydrated Sr –X, Sr2+ ions are found at 46 two sites: 16 at site I and 30 at site II. In its ammonia sorption complex, Sr2+ ions are located at three sites: 12 remain at site I; four have relocated from site I to I∞, where each coordinates to three framework oxygen atoms and also to three ammonia molecules; the remaining 30 Sr2+ ˚ further into the at site II have each shifted 0.56 A supercage to coordinate to three ammonia molecules. These coordinated molecules cannot be removed by evacuation at 21°C 5.1. Supporting material available Tables of observed and calculated structure factors (29 pages). Ordering information is given on any current masthead page.
M.J. Kim et al. / Microporous and Mesoporous Materials 30 (1999) 233–241
Acknowledgement This work was supported in part by the Basic Research Institute Program, Ministry of Education, Korea, 1996, Project No. BSRI-96-3409.
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