10340
J. Phys. Chem. B 1997, 101, 10340-10346
Structure of Dehydrated Zeolite Li-LSX by Neutron Diffraction: Evidence for a Low-Temperature Orthorhombic Faujasite J. Ple´ vert, F. Di Renzo,* and F. Fajula Laboratoire de Mate´ riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM, Montpellier, France
G. Chiari Dipartimento di Scienze Mineralogiche e Petrologiche, UniVersita` degli Studi di Torino, Torino, Italy ReceiVed: April 28, 1997; In Final Form: July 8, 1997X
Li-LSX (Li-faujasite with Si/Al ) 1), cubic at room temperature, is orthorhombic (Fddd) at 10 K. In the room-temperature Fd3h phase, Li cations in the supercage are evenly distributed between 2-fold-coordinated sites III and III′, in close agreement with the cation distribution in Na-X. The cubic-orthorhombic phase transition implies extensive redistribution of strain through the lattice and displacement of Li cations. In the low-temperature Fddd phase the occupancy of sites II is reduced, and nearly half the Li cations are in the supercage, most of them in a new site III′, midway between neighboring sites II.
Introduction The space group of a given zeolite does not always correspond to the highest symmetry admitted by its aluminosilicate framework. The distribution of adsorbed molecules or chargecompensating cations can lower the overall symmetry of the lattice. The tetrahedra of the zeolite framework rotate and move to optimize interactions with guest molecules or cations located in the micropores.1-3 Well-known examples are represented by the interaction of several organic molecules with the framework of ZSM-5, whose symmetry changes from monoclinic to orthorhombic upon adsorption,4 or the different symmetry of the MTN-type clathrasils, depending on the nature of the template amine.5 A clear example of the effect of the nature and distribution of inorganic cations is represented by the zeolites of the stilbite group, in which the symmetry is lowered as the cation content increases and rotational displacements within the framework are induced.6 Temperature-induced changes of symmetry are also known in zeolites. Monoclinic ZSM-5 undergoes a reversible transition to the orthorhombic phase when heated beyond 355 K.4 A lowtemperature rhombohedral phase and a high-temperature cubic phase have been reported for dehydrated zeolites Na-A and K-A.7-9 The low-temperature phase of Na-A has been alternatively indexed as orthorhombic.10 No structure refinement of this phase has been published. According to the literature reports, the rhombohedral-cubic transition occurs between 623 and 673 K and is characterized by a strong hysteresis.9,10 The framework composition of the zeolite severely affects the phase behavior: the rhombohedral distortion decreases7 and disappears11 for Si/Al ratios of the lattice higher than 1, viz. for solids with a lower cation density and a disordered distribution of the tetrahedra Si and Al. Dehydrated zeolite A exchanged with mixed cations, for instance Li and Na, does not present any rhombohedral distortion.12 The effects of micropore filling, cation content, and temperature upon the phase transitions are related. Literature data can be sketchily summarized by stating that efficient pore-filling stabilizes the high-temperature, low-symmetry phases. In the case of ZSM-5, the as-synthesized tetrapropylammonium-filled X
Abstract published in AdVance ACS Abstracts, November 1, 1997.
S1089-5647(97)01433-8 CCC: $14.00
zeolite is orthorhombic at room temperature, and the adsorption of organic molecules within the calcined zeolite stabilizes again the high-temperature phase.4 In the case of zeolite A, the hydrated form is always cubic,7,9 as is the dehydrated form exchanged with a bulky cation like Tl8+. To date, only cubic space groups have been observed for faujasite, whatever the type of cation or the Si/Al ratio. The composition of the framework is a critical parameter for the choice of a space group: The Fd3h space group is expected for a framework with Si/Al ) 1 and ordered alternation of Si and Al tetrahedra. For a Si/Al ratio higher than 1, an ordered distribution of tetrahedra is no longer possible, and the Fd3hm space group would correspond to a fully disordered distribution. Structural refinements on dehydrated zeolite X (faujasite with Si/Al < 1.5) have been carried out in both these space groups, for solids with 1.09 e Si/Al e 1.23.13-18 Faujasite with Si/Al ) 1 was not available until 1987,19 and all structural refinements of more silica-rich samples are probably affected by the disorder in the distribution of tetrahedra. Al ordering is such a critical parameter that the existence of a trigonal faujasite has been postulated, on the basis of hypotheses on the distribution of Si and Al in zeolite Y (faujasite with Si/Al > 1.5).20 The Al atom distribution for Si/Al ratio > 1 may also affect the position of the extraframework cation.21 In fact, the cation sites depend on the distribution of surrounding Al atoms and the distortion of the framework. The possibility for the cations to be located in several sites, due to the disordered distribution of Al, makes the interpretation of the Fourier maps, obtained by diffraction techniques, more difficult. The assignments of density peaks become less certain as the fractional occupancy of the site decreases. Indeed, the positions of the cations in aluminum-rich faujasite are not unambiguously defined.13-18 Position and fractional occupation of cationic sites change with the Si/Al ratio, the type of countercations, and the hydration level of the sample under investigation. Faujasite-type zeolite LSX (low silica X)19 has a Si/Al ratio of 1. Its structure is formed of alternating SiO4 and AlO4 tetrahedra in accordance with the Loewenstein’s Al-O-Al avoidance rule. Zeolite LSX presents the highest number of charge-compensating cations among all faujasites. Cations are known to be distributed over six possible sites within the © 1997 American Chemical Society
Structure of Dehydrated Zeolite Li-LSX
J. Phys. Chem. B, Vol. 101, No. 49, 1997 10341
TABLE 1: Coordination and Occupancy of Sites III and III′ in Monocationic Dehydrated Zeolites X (Li and Na Forms) 2O4
2O1, 2O4
13
Na92X92 Na86X8614 Na88X8818 Li86X8615
23 Na
O4
O1, O4
10 Na 24 Na 11 Na
19 Na
22 Li
faujasite framework.22 Table 1 summarizes the different sites occupied by cations in monocationic dehydrated zeolite X, according to the oxygen atoms to which cations are coordinated. Three examples correspond to zeolites Na-X,13,14,18 while the fourth one is a zeolite Li-X.15 It is recognized13-18 that the cation sites lying on the 3-fold axes passing throught the 6-rings (rings formed of six tetrahedra) have well-defined positions. The sites I′ and II are generally fully occupied in dehydrated zeolites X exchanged with alkali cations, leaving sites I and II′ either poorly occupied or empty. The occupation of a site I′ prevents the location of a cation in the nearby sites I, as is the case for sites II and II′. In the case of zeolite LSX in the alkali form, 6-rings provide sites for two-thirds of the total number of cations. The remaining cations can be located in several alternative sites in the supercages (sites III and III′). These sites can be classified according to their position relative to the 4-rings RIII and RIII′.22 The assignments of density peaks become less certain for a low fractional occupancy of the site, as is the case for the sites III and III′ in the supercages. Only four coordination types of cations with O1 and O4 oxygen atoms within the supercage have been reported in the litterature (Table 1). The coordination (2O4) corresponds to the site above the ring RIII and is referred to as the “ideal” site III.22 A site (2O1, 2O4) coordinated to four O1 and O4 oxygen atoms has been reported by Forano et al.15 This position is the only one reported so far concerning lithium cations in the supercages of zeolite X. Another type of sites corresponds to cations that do not lie on the 4-rings, but are displaced toward the 6-rings RII. These cations are mainly monocoordinated to an oxygen atom O4 or coordinated to three oxygen atoms centered on O4 (2O1, O4) depending upon the coordination distances. Two-fold-coordinated (O1, O4) sites have also been reported.18 To better define the position of Li cations in faujasite, a structural refinement from neutron powder diffraction data on a Li-exchanged zeolite LSX (Si/Al ) 1) has been carried out, and results are reported in this paper. Experimental Section Lithium-exchanged zeolite LSX was obtained by ion exchange starting from a solid of composition Na77K19Si96Al96O384. At first, the solid was exchanged three times with 1 M NaCl solution at 368 K, to remove potassium cations. Three further exchanges with 1 M LiCl solution were required to remove the sodium cations from the zeolite. After each exchange, the solid was washed with a diluted LiOH solution and dried overnight at room temperature under vacuum. The LiCl solution was prepared from metallic isotope 7Li in order to prevent the presence in the final solid of the 6Li isotope in the neutron diffraction experiments. 6Li is a strong neutron absorber, its absorption cross section being σ6Li ) 940 b (b ) barn), to be compared with σ7Li ) 0.045 b. N2 adsorption/desorption at 77 K on the exchanged solid outgassed at 523 K allowed the measurement of a microporous volume of 0.342 cm3/g, indicating that no loss of crystallinity took place during cation exchange. Atomic absorption spectroscopy indicated a Si/Al ratio < 1.01 and cation contents of
Figure 1. Nomenclature of the faujasite structure: types of oxygen atoms (left), cationic sites (top right), and rings (bottom right)22 represented around a supercage.
99.5% lithium and 0.5% sodium (molar) with ppm traces of potassium and calcium. The anhydrous unit cell composition of the solid assumed for structure determination was Li95NaSi96Al96O384. Additional information was provided by a sample from the same Na,K-LSX batch, exchanged three times with NaCl and once with LiCl solution, with a cell composition Li82Na14Si96Al96O384. For the neutron diffraction experiment, a 2 g sample of zeolite was put in a cylindrical vanadium container (diameter 5 mm), which was later placed in the neutron beam. The container was held 12 h at 700 K under vacuum (2 × 10-6 Torr) for complete dehydration. Without cooling the sample to room temperature, the vanadium container was installed in a cryofurnace and kept for 1 h at 500 K in He flow. Without further manipulation the temperature was decreased and the sealed cryofurnace was installed in the neutron beam. Two data sets were collected at 10 and 300 K on the D1A multicollimator diffractometer23,24 at ILL (Grenoble, France) with λ ) 1.90562(5) Å. The determination of the position of lithium cations and the structure refinements were performed using the GSAS package.25 The neutron scattering lengths for Si, Al, O, and 7Li were taken as bSi ) 4.149, bAl ) 3.449, bO ) 5.803, and bLi ) -2.22 fm.26 Determination and Refinement of the Structure Figure 2 shows the neutron powder diffraction pattern of LiLSX at the two different temperatures of 300 and 10 K. The room-temperature powder pattern (Figure 2a) corresponds to the cubic unit cell of faujasite indexed with a ) 24.6665(1) Å. The space group is Fd3h given the strict alternance of Si and Al atoms in zeolites LSX. In contrast, the sample recorded at low temperature (Figure 2b) reveals a clearly different powder pattern. In particular, some diffraction peaks of the roomtemperature recording show a splitting at low temperature, suggesting that zeolite Li-LSX undergoes a structural transformation as the temperature is lowered. The indexation of the low-temperature pattern was not straightforward. An ab initio indexation cannot be attempted due to the broadening of the lines in neutron diffraction experiments and the large unit cell parameters of the sample under study. However, the lack of well-resolved peaks in the powder pattern can be overcome by the knowledge of the structure of the room-temperature phase. Powder patterns were generated in selected space groups with lower symmetries chosen among the subgroups of Fd3h. A trial and error procedure
10342 J. Phys. Chem. B, Vol. 101, No. 49, 1997
Ple´vert et al. TABLE 3: Atomic Positions in the Cubic Unit Cell of Li-LSX at 300 K (Thermal Factors Have the Form exp[8π2Uiso(sin θ/λ)2]) atom
x
y
z
occupancy
no. per cell
Uiso
Si Al O1 O2 O3 O4 Li1′ Li2 Li3 Li3′
-0.0503(3) -0.0499(5) -0.1043(2) -0.0015(3) -0.0216(2) -0.0738(2) 0.0457(5) 0.2226(3) 0.3756(3) 0.4411(3)
0.1246(3) 0.0371(5) 0.0027(4) 0.0004(3) 0.0734(3) 0.0824(3) 0.0457(5) 0.2226(3) 0.1250 0.3580(1)
0.0372(4) 0.1232(5) 0.0986(3) 0.1524(1) 0.0688(3) 0.1715(3) 0.0457(5) 0.2226(3) 0.1250 0.1400(2)
1 1 1 1 1 1 1.00(6) 1.00(5) 0.32(4) 0.17(4)
96 96 96 96 96 96 32.0 32.0 15.4 16.3
0.012(2) 0.037(3) 0.024(2) 0.022(2) 0.032(2) 0.028(2) 0.081(11) 0.021(6) 0.123(33) 0.041(40)
TABLE 4: Interatomic Distances and Angles in the Cubic Unit Cell of Li-LSX at 300 K
Figure 2. Neutron powder diffraction patterns of dehydrated Li-LSX (a) at 300 K and (b) 10 K. λ ) 1.905 62 Å.
TABLE 2: Details of Data Collection and Rietveld Refinement at 300 and 10 K room temperature
low temperature
chemical formula space group cell parameters (Å)
Li95NaSi96Al96O384 Fd3h a ) 24.6665(1)
cell volume (Å3) radiation type wavelength (Å) temperature (K) no. of contributing reflections Rwp ) [∑ωi(yio - yic)2/∑ωiyio2]1/2 RI ) ∑|Io - Ic|/ΣIo
V ) 15008(1) neutron 1.90562(5) 300 616 0.029
Li95NaSi96Al96O384 Fddd a ) 24.757(2) b ) 24.371(1) c ) 24.874(1) V ) 15007(2) neutron 1.90562(5) 10 1828 0.042
0.049
0.051
was used to select the proper cell parameters. The cell parameters leading to a powder pattern close to the observed one were refined keeping the framework unchanged. The powder pattern was indexed in an orthorhombic unit cell with a ) 24.757(2), b ) 24.371(1), and c ) 24.874(1) Å. Details about data collection and Rietveld refinement are given in Table 2. Despite the large changes in the cell parameters, the cell volume V ) 15007(2) Å3 is the same for the roomtemperature and the low-temperature phases. This suggests that the choice for the unit cell was correct, since the cell of zeolitetype materials is not subject to high thermal expansion at temperatures below room temperature. This fact is also confirmed by the sample partially exchanged with lithium (Li82Na14Si96Al96O384), which maintained the cubic symmetry at 10 K with a volume variation with respect to room-temperature lower than 0.1%. A unit cell transformation without volume variation suggests a displacive-type transition of second order. The space group Fddd was selected at first for the lowtemperature phase because it is a maximal subgroup of space
atoms
distance (Å)
Si-O1 Si-O2 Si-O3 Si-O4 〈Si-O〉 Al-O1 Al-O2 Al-O3 Al-O4 〈Al-O〉
1.625(9) 1.716(11) 1.645(10) 1.582(9) 1.642(10) 1.700(11) 1.661(11) 1.758(12) 1.736(11) 1.714(12)
atoms
distance (Å)
atoms
angle (deg)
Li1′-O3 1.883(5) O3-Li1′-O3 118.5(3) Li1′-O2 3.086(4) O2-Li1′-O2 119.6(1) Li2-O2 Li2-O4 Li3-O4
1.988(4) O2-Li2-O2 3.108(4) O4-Li2-O4 2.01(5) Si-O1-Al Si-O2-Al Li3′-O1 1.96(7) Si-O3-Al Li3′-O4 1.88(4) Si-O4-Al
119.0(2) 120.0(2) 143.8(4) 128.6(4) 125.6(3) 138.6(4)
group Fd3h, in which only the 3h symmetry operation is lost in the structural transformation. The determination of the lithium cation positions was carried out using both Rietveld refinement and Fourier difference methods processed iteratively. The density peaks in the Fourier maps were attributed to lithium cations on the basis of their high negative intensity and reasonable dLi-O distance criteria. In the interpretation of the Fourier-difference maps, special care was devoted to the region surrounding O1 and O4 oxygen atoms in the supercage, since these positions are particularly favorable to cation occupancy. The Rietveld refinement was carried out under slightly weighed soft constraints of expected framework T-O distances, dSi-O ) 1.62 Å and dAl-O ) 1.74 Å. Room-Temperature Cubic Phase The first lithium cation positions located on the Fourierdifference maps were the sites I′ and II, fully occupied. After refinement of the structure with the framework atoms and the first 64 lithium cations, the Fourier maps revealed a peak in “ideal” site III with fractional occupancy FLi3 ) 1/3. At this stage the remaining cations in site III′ of the supercage were detectable in the Fourier maps, although with some difficulty. Adding this new site to the model did not improve the Rwp factor, but the structure RB factor, on the contrary, was lowered. Furthermore, the refinement of the fractional occupancy of the site yielded a total number of lithium cations in the unit cell (96) close to the expected one (95). The refined atomic coordinates and thermal parameters are given in Table 3, and selected interatomic distances are reported in Table 4. Figure 3 shows the observed and calculated profiles, together with their difference curve for the final Rietveld refinement of the structure at 300 K. The mean distances T-O for the two tetrahedra, 〈dSi-O〉 ) 1.64(1) Å and 〈dAl-O〉 ) 1.71(1) Å, are in agreement with the ordering of Si and Al atoms in the cell. The scattered values of the T-O distances (Table 4) are similar to those observed by Shepelev et al.16 in partially lithium-exchanged zeolite X obtained by single-crystal X-ray diffraction. The authors pointed
Structure of Dehydrated Zeolite Li-LSX
J. Phys. Chem. B, Vol. 101, No. 49, 1997 10343
Figure 3. Results of the Rietveld refinement for Li-LSX at 300 K.
Figure 5. Cation sites in the supercage of zeolite Li-LSX at 300 K.
Figure 4. Projection along the [111] direction of the hexagonal prism, showing the tilting of tetrahedra as a function of the size of the cation for zeolites (a) Li-X and (b) Na-X.
out that in the hydrated sample the individual T-O distances are more uniform, suggesting an effect of the cations on the T-O distance variations. The angles T-O-T, reported in Table 4, are distributed in a range varying from 125° to 145°. The observed values are very close to those observed by Forano et al.15 in the case of zeolite Li-X. The structure of faujasite with sodium as countercation presents quite different angles, in particular for the T-O2-T and T-O3-T angles.13,18 Figure 4 shows the influence of the angle variations on the shape of the hexagonal prism for zeolites Li-X and Na-X. The framework adapts itself to the size of the cation, leading to remarkable distortions of the six-membered rings RI in the case of the zeolite Li-X. The tetrahedra in the rings RI are linked by O2 and O3 oxygen atoms. The T-O2-T and T-O3-T angles are close to the minimum values observed in the distribution of T-O-T angles in silicates,28,29 suggesting that the framework of zeolite LiLSX bears T-O-T bonds with maximum constraints already at room temperature. The sites I′ and II are fully occupied, whereas sites I and II′ are empty, as expected for dehydrated low-silica zeolite X. In fact, the lithium cation is too small to be located in site I, in octahedral coordination at the center of the hexagonal prism. Cations are located near the plane of their coordination oxygen atoms, with all O-Li-O angles close to 120° (Table 4). In this position, the distances dLi-O are at a minimum, in accordance with a strained framework. In the case of zeolite NaX, cations occupy more relaxed out-of-plane sites, the angles O-Na-O being in the range 113.4-117.7°.18 The distances dLi1′-O3 ) 1.883(5) Å and dLi2-O2 ) 1.988(4) Å show a difference of 0.1 Å, quite large for cations in similar environments. Identical values were obtained by Forano et al.15 in the case of zeolite Li-X. The site in the supercage showing the deepest negative peak in the Fourier-difference maps was the ideal site III, with (2O4)
Figure 6. Results of the Rietveld refinement for zeolite Li-LSX at 10 K.
coordination (distance dLi3-O4 ) 2.01(5) Å). The interaction of the lithium cations with O4 atoms corresponds to a distortion of the ring RIII, with dO4-O4 ) 3.11(1) Å much smaller than dO3-O3 ) 3.76(1) Å. A second site in the supercage (site III′) is coordinated to two oxygen atoms, O1 and O4, with shorter distances (dLi3′-O1 ) 1.96(7) Å and dLi3′-O4 ) 1.88(4) Å). The cations in the supercage are equally distributed over the two sites III and III′, shown in Figure 5. From Table 1, it can be noted that these two sites in the supercage are the most common ones for the sodium cations. Low-Temperature Orthorhombic Phase With respect to the Fd3h space group, the choice of the Fddd space group for the low-temperature phase involved three times as many independent framework atoms to make up for the loss of the 3h symmetry operation. The positions of the 18 framework atoms obtained at the end of the Rietveld refinement and the lithium cations localized in the Fourier maps are listed in Table 5. Values of Uiso were selected close to 0.008, considered as a suitable initial value for thermal motions of atoms at 10 K. Further refinement of the thermal parameters did not significantly improve the results, due to the large number of parameters already included. Significant distances and angles are reported in Table 6. Figure 6 shows the results of the Rietveld refinement. The structural RB factor obtained, RB ) 5.1%, confirms the choice of the Fddd space group to describe the low-temperature phase of Li-LSX.
10344 J. Phys. Chem. B, Vol. 101, No. 49, 1997
Ple´vert et al.
TABLE 5: Atomic Positions in the Orthorhombic Unit Cell of Li-LSX at 10 K atoms
x
y
z
occupancy
no. per cell
Si1 Si2 Si3 Al1 Al2 Al3 O11 O12 O13 O21 O22 O23 O31 O32 O33 O41 O42 O43 Li1′ Li2′ Li2 Li31 Li32 Li33 Li3′1 Li3′2 Li3′3
-0.0475(8) 0.0368(8) 0.1247(9) -0.0476(9) 0.1231(9) 0.0375(8) -0.1039(8) 0.0979(9) 0.0025(8) 0.0017(9) 0.1553(7) -0.0003(8) -0.0198(8) 0.0700(8) 0.0749(8) -0.0681(9) 0.1743(8) 0.0842(8) 0.0502(23) 0.194(8) 0.2197(33) 0.377(6) 0.125 0.125 0.0685(28) 0.390(40) -0.021(10)
0.1234(9) -0.0530(8) 0.0368(8) 0.0378(8) -0.0497(9) 0.1261(9) 0.0036(12) -0.1002(10) 0.1042(9) -0.0015(9) 0.0006(10) 0.1560(6) 0.0742(9) -0.0254(6) 0.0734(9) 0.0852(9) -0.0783(6) 0.1713(9) 0.0507(15) 0.190(8) 0.2256(28) 0.125 0.361(9) 0.125 -0.0306(32) 0.069(24) 0.396(11)
0.0380(7) 0.1254(9) -0.0452(8) 0.1211(9) 0.0384(9) -0.0498(8) 0.0941(7) -0.0027(9) -0.1063(7) 0.1510(8) 0.0003(9) 0.0027(7) 0.0687(8) 0.0764(7) -0.0195(8) 0.1676(8) 0.0775(8) -0.0740(8) 0.0500(22) 0.192(8) 0.2215(34) 0.125 0.125 0.376(7) 0.3856(33) -0.030(40) 0.076(8)
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1.00(8) 0.08(8) 0.57(8) 0.44(5) 0.23(5) 0.39(5) 0.59(5) 0.05(5) 0.17(5)
32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32.0 2.5 18.2 7.0 3.7 6.2 18.9 1.6 5.4
TABLE 6: Interatomic Distances in the Orthorhombic Unit Cell of Li-LSX at 10 K atoms
distance (Å)
atoms
distance (Å)
Si1-O11 Si1-O23 Si1-O31 Si1-O41 Si2-O13 Si2-O21 Si2-O32 Si2-O42 Si3-O11 Si3-O22 Si3-O33 Si3-O43 〈Si-O〉
1.628(17) 1.665(16) 1.578(17) 1.580(17) 1.654(16) 1.655(17) 1.618(16) 1.636(17) 1.648(17) 1.622(17) 1.651(16) 1.610(17) 1.629(17)
A1-O11 Al1-O21 Al1-O31 Al1-O41 Al2-O12 Al2-O22 Al2-O32 Al2-O42 Al3-O13 Al3-O23 Al3-O33 Al3-O43 〈Al-O〉
1.757(17) 1.720(17) 1.719(18) 1.713(17) 1.716(18) 1.743(17) 1.724(17) 1.744(18) 1.733(17) 1.764(17) 1.754(17) 1.706(18) 1.733(18)
atoms
distance (Å)
Li1′-O31 Li1′-O32 Li1′-O33 Li2′-O21 Li2′-O22 Li2′-O23 Li2-O21 Li2-O22 Li2-O23 Li31-O41 Li32-O42 Li33-O43 Li3′1-O11 Li3′2-O12 Li3′3-O13
1.88(7) 2.03(4) 1.92(7) 2.25(20) 2.25(20) 2.14(19) 1.99(10) 1.84(10) 1.97(7) 2.04(12) 1.88(10) 1.99(12) 2.28(8) 2.34(22) 2.02(18)
The observations of dT-O distances as well as the O-T-O angles close to 109° do not indicate notable deformations of the tetrahedral sites. The Si-O-Al angles shows variation up to 5° around the values of the room-temperature phase, with a corresponding tilting of the tetrahedra. This deformation is better evidenced by the distances between the Si or Al atoms at the center of tetrahedra. Table 7 reports the list of distances dSi-Al of both the low-temperature and the room-temperature phases. The mean value 〈dSi-Al〉 is identical for the two phases, but there are important variations in some individual dSi-Al distances. In the hexagonal prisms the dSi-Al distances in the direction along the prism axis are slightly larger in the low-temperature phase. Simultaneously, the two shorter distances dSi-Al correspond to bonds in two opposite sides of both the 6-ring windows RI and RII. Furthermore, the 4-ring RIII in the [100] direction is the most distorted, and its area SIII is the smallest one (SIII(100) ) 9.19 Å2, SIII(010) ) 9.41 Å2, and SIII(001) ) 9.33 Å2). The distortion of the RIII rings appears also in O4-O4
TABLE 7: Some Significative Distances in the Framework of Li-LSX at 10 and 300 K structure at 10 K atoms
distance (Å)
(Si3-Al1)O11 (Si1-Al2)O12 (Si2-Al3)O13 (Si2-Al1)O21 (Si3-Al2)O22 (Si1-Al3)O23 (Si1-Al1)O31 (Si2-Al2)O32 (Si3-Al3)O33 (Si1-Al1)O41 (Si2-Al2)O42 (Si3-Al3)O43 〈Si-Al〉
3.24(1) 3.22(1) 3.18(1) 3.05(1) 2.96(1) 3.03(1) 2.94(1) 3.04(1) 3.07(1) 3.13(1) 3.09(1) 3.04(1) 3.08(1)
structure at 300 K
atoms
distance (Å)
O11-O12 O11-O13 O11-O13 O41-O41 O42-O42 O43-O43 (O11-O43)Si (O12-O41)Si (O13-O42)Si (O11-O41)Al (O12-O42)Al (O13-O43)Al
3.28(4) 3.65(3) 3.68(3) 2.87(5) 3.40(4) 3.03(5) 2.57(3) 2.64(3) 2.62(3) 2.84(3) 2.80(3) 2.72(3)
atoms
distance (Å)
(Si-Al)O1 (Si-Al)O2 (Si-Al)O3 (Si-Al)O4 〈Si-Al〉
3.16(1) 3.04(1) 3.03(1) 3.10(1) 3.08(1)
O1-O1 O4-O4 (O1-O4)Si (O1-O4)Al
3.54(1) 3.11(1) 2.61(1) 2.77(1)
distances, the O41-O41 distance in the [100] direction being much smaller than in the two other directions. The anisotropy of the structure is also observed in the occupancy and location of the lithium cations. The sites I′ and II occupy almost identical positions in the two phases, with 〈dLi1′-O3〉 ) 1.94(6) Å and 〈dLi2-O2〉 ) 1.93(9) Å at low temperature. The sites I′ and II are close to the 3-fold axis, and no evidence of disorder of lithium cations around this axis appeared in the Fourier-difference maps. While the occupancy of the sites in the hexagonal prism (sites I′) is the expected one, the sites connected to the 6-ring windows RII show a lower occupancy factor. The main difference with respect to the roomtemperature phase is the occupation of site II, which contains only 18 cations instead of the 32 expected ones. A site II′ (〈dLi2′-O2〉 ) 2.2(2) Å) is also observed, with very low occupancy (2.5 Li/u.c.) (u.c. ) unit cell). In space group Fddd, a site III (or III′) of space group Fd3h is split into three independent sites derived from the three positions xyz, zxy, yzx, which are a circular permutation of the coordinates. For instance, the site Li3 of the cubic structure corresponds to three sites Li31, Li32, and Li33, which are seen in the direction of the axes a, b, and c, respectively, by an observer standing at the center of a sodalite cage. In the Fourier maps, the intensities of the three peaks corresponding to the ideal site III are different. In the refinement, the fractional occupation parameters were highly correlated, and the individual population in the three sites was difficult to estimate. Nevertheless, it appeared that the three fractional occupation parameters were not equivalent and their sum gave 17 cations in site III. Another site (III′) is localized in the supercages in a position coordinated to (O1). These sites Li3′ are in the plane formed by the closest oxygen atom O1 and by the two tetrahedral sites forming the TO1 bonds, as shown in Figure 7 in the case of site Li3′1. The site is located on the 12-ring, at the edge of a hexagonal prism between two supercages, midway between the sites II of neighboring sodalite cages. The distribution of cations in the three sites III′ is uneven, but also in this case occupational parameters were highly correlated. The total population of lithium in the three III′ sites corresponds to 26 cations. The number of lithium cations in the supercage (Li3+Li3′) is particularly high (43). If the anisotropy of the cation distribution is considered, a gathering of 26 cations is observed in the portion of supercage facing the 4-rings RIII with the smallest surface, in the two close sites Li31 and Li3′1. This value closely corresponds to the maximum occupation of the sites. In fact, the occupation of a site III prevents two sites III′ from being occupied. This condition sets the maximal occupation of the supercage sites in a given direction at 2nLi31 + nLi3′1
Structure of Dehydrated Zeolite Li-LSX
J. Phys. Chem. B, Vol. 101, No. 49, 1997 10345
Figure 7. Cation sites facing RIII100 rings in the supercage of zeolite Li-LSX at 10 K.
TABLE 8: Deformation of the Faujasite Structure as a Function of Temperature and the Type of Cation. T-O-T Angles and Characteristic Interatomic Distances in Zeolite Li-LSX at 10 and 300 K, Na-X, and All-Silica Faujasite Li-LSX (10 K) 〈T-O1-T〉 〈T-O2-T〉 〈T-O3-T〉 〈T-O4-T〉 〈T-O-T〉 〈cation1′-O3〉 〈cation2-O2〉 〈O3-O3〉RI′ 〈O2-O2〉RII
144.0 125.6 128.5 136.4 133.6 1.94 1.93 3.30 3.33
Li-LSX (300 K)
Angles (deg) 143.8 128.6 125.6 138.6 134.2 Distances (Å) 1.88 1.99 3.24 3.43
Na-X18
siliceous faujasite27
134.8 145.6 141.2 145.5 141.8
138.4 149.3 145.8 141.4 143.7
2.24 2.34 3.78 3.85
3.82 3.86
e 32, to be compared with an experimental value of 2(7.0) + 18.9 ) 32.9. The total number of lithium cations found in the unit cell (95.5) is in good agreement with the results of the chemical analysis. In the orthorhombic structure, the low occupation of site II and the high population in sites Li3′, which happen to be located in a position intermediate between sites II, suggest a mechanism of cation transfer from sites II to sites III with the correspondent framework modifications. The deformation of the lattice in the cubic-orthorhombic transition can be highlighted by the comparison of T-O-T angles in the structure of Li-LSX at 10 and 300 K, reported in Table 8 with literature results on Na-X18 and siliceous faujasite Y.27 The structure of siliceous faujasite contains no countercations, and the angles T-O-T cover a narrow range around 144°, a value that corresponds closely to the estimated value of strain-free bond observed in silicates.28 The zeolite Na-X shows minor variations around this value. However, in the case of the zeolite Li-LSX, the angles T-O-T are distributed in a range varying from 125° to 145°. The angles affected by the largest deformations are the T-O2-T and T-O3-T angles, which correspond to the oxygen atoms in interaction with twothirds of the total number of lithium cations in the unit cell. The influence of the type of cations on the shape of the hexagonal prism in the case of zeolite Na-X and Li-LSX is shown in Figure 4. The framework adapts itself to the size of the cation, leading to notable distortions of the six-membered rings RI in the case of the zeolite Li-LSX. The deformation
is quantified by the decrease in the distance between two O2 atoms, in the case of the ring RI, and between two O3 atoms, in the case of the ring RII, and is driven by the shorter cationoxygen distance in the Li-exchanged zeolite (Table 8). The T-O2-T and T-O3-T angles in zeolite Li-LSX are close to the minimum values observed in the distribution of T-O-T angles in silicates,28,29 suggesting that the framework bears T-O-T bonds with maximum constraints already at room temperature. It can be expected that further strain imposed by the reduced thermal motion can be relaxed only through a deformation of the structure, corresponding to the phase transition observed at low temperature. A significant difference between the cubic and low-temperature structure of zeolite Li-LSX is represented by the relative degree of deformation of the rings RI and RII. In the roomtemperature cubic phase, the rings RI, in the hexagonal prisms, are significantly more strained than the rings RII, between sodalite cages and supercages (Table 8). In the low-temperature orthorhombic phase, rings RI and RII present identical average values of Li-O and O-O distances. This corresponds to a more even sharing of deformation throughout the structure, probably related to the displacement of nearly half the Li2 cations in the Li3′ sites, midway between two RII rings. The relevance of cation ordering for the phase transition is stressed by the observation of the usual cubic symmetry at 10 K for the partially lithium-exchanged zeolite LSX with Li82Na14Si96Al96O384 composition. Conclusions Zeolite Li-LSX (faujasite with Si/Al ) 1) presents an orthorhombic Fddd low-temperature phase. As in the case of the cubic-rhombohedral transition in zeolite A, the Si-Al disorder induced by a Si/Al ratio > 1 and the presence of several types of cations prevent the formation of a noncubic lowtemperature phase of faujasite. Such a phase has never been observed in the case of samples with Si/Al > 1, and mixedcation samples present cubic symmetry also at 10 K. Notwithstanding the relevance of the nature of the chargecompensating cation, the phase transition can hardly be described as an optimization of the local interaction between Li cations and oxygen atoms of the lattice. In effect, the orthorhombic phase presents a lower occupancy of the highly coordinated sites II and a significant increase of the number of cations in the supercage, most of them in a new low-coordinated site III′. The phase transition corresponds to a global redistribution of strain throughout the lattice, probably triggered by the high strain of Li-containing hexagonal prisms already at room temperature. An energetic modelization of the transition is in process. Acknowledgment. The authors wish to acknowledge A. Hewat for supervision of the data acquisition, F. Fauth and P. Cross for experimental assistance at the ILL, Grenoble, and A. Goursot for fruitful discussion. One of the authors (G.C.) is grateful to the CNR Committee 5 for funding its participation in this research. References and Notes (1) Van Santen, R. A.; de Man, A. J. M.; Jacobs, W. P. J. H.; Teunissen, E. H.; Kramer, G. J. Catal. Lett. 1991, 9, 273. (2) Hammonds, K. D.; Dove, M. T.; Giddy, A. P.; Heine, V.; Winkler, B. Am. Mineral. 1996, 81, 1057. (3) Barthomeuf, D. Stud. Surf. Sci. Catal. 1997, 105, 1677. (4) Fyfe, C. A.; Strobl, H.; Kokotailo, G. T.; Kennedy, G. J.; Barlow, G. E. J. Am. Chem. Soc. 1988, 110, 3373. (5) Ko¨nnecke, M.; Fuess, H. Zeolites 1995, 15, 264.
10346 J. Phys. Chem. B, Vol. 101, No. 49, 1997 (6) Galli, E.; Alberti, A. Bull. Soc. Fr. Miner. Cristallogr. 1975, 98, 331. (7) Bursill, L. A.; Lodge, E. A.; Thomas, J. M.; Cheetham, A. K. J. Phys. Chem. 1981, 85, 2409. (8) Cheetham, A. K.; Fyfe, C. A.; Smith, J. V.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1982, 823. (9) Appel, W.; Ihringer, J.; Knorr, K.; Prandl, W. Zeolites 1987, 7, 423. (10) Belbeoch, B.; Roulliay, M.; Kahn, R.; Cohen de Lara, E. Zeolites 1983, 3, 99. (11) Adams, J. M.; Haselden, D. A. J. Chem. Soc., Chem. Commun. 1982, 822. (12) Jira´k, Z.; Bosacek, V.; Vratislav, S.; Herden, H.; Scho¨llner, R.; Mortier, W. J.; Gellens, L.; Uytterhoeven, J. B. Zeolites 1983, 3, 255. (13) Smolin, Yu. I.; Shepelev, Yu. F.; Butikova, I. K.; Petranovskii, I. K. Kristallografiya 1983, 28, 72. (14) Al-Ajdah, G. N. D.; Al-Rished, A. A.; Beagley, B.; Dwyer, J.; Fitch, F. R.; Ibrahim, K. J. Inclusion Phenom. 1985, 3, 135. (15) Forano, C.; Slade, R. C. T.; Krogh Andersen, E.; Krogh Andersen, I. G.; Prince, E. J. Solid State Chem. 1989, 82, 95. (16) Shepelev, Yu. F.; Anderson, A. A.; Smolin, Yu. I. Zeolites 1990, 10, 61. (17) Shepelev, Yu. F.; Butikova, I. K.; Smolin, Yu. I. Zeolites 1991, 11, 287. (18) Olson, D. H. Zeolites 1995, 15, 439. (19) Ku¨hl, G. H. Zeolites 1987, 7, 451. (20) Takaishi, T. J. Phys. Chem. 1995, 99, 10982. (21) Takaishi, T. Zeolites 1996, 17, 389. (22) The framework of faujasite consists of a diamond-like stacking of sodalite cages connected to each other by hexagonal prisms. The resulting
Ple´vert et al. structure presents large accessible supercages as shown in Figure 1. The sodalite cage is made of two types of 6-rings (rings formed of six tetrahedra), RI and RII, connected by 4-rings RIII. Four 6-ring windows RII connect the sodalite cage to the supercage. The windows RI of two different sodalite cages face each other forming a hexagonal prism. The 4-rings of these prisms are the fourth type of ring of the faujasite structure and can be referred to as ring RIII′. Cationic sites I and I′, located at the center of the hexagonal prisms or on the windows RI, respectively, are coordinated to oxygen atoms O3. Cationic sites II and II′ are located on the windows RII, in the plane or the window or inside the sodalite cage, respectively, and are coordinated to oxygen atoms O2. The TO2 and TO3 bonds (T: tetrahedral site) are directed in toward the small cages of the framework (sodalite cage and hexagonal prism), in contrast with the oxygen atoms O1 and O4, protruding into the supercage. The sites III and III′ are located inside the supercage in coordination with oxygen atoms O1 and O4. Sites III protrude from the rings RIII, and according to the nomenclature of Olson,18 sites III′, in a broader acception, are all supercage sites which do not correspond to ideal sites III. (23) Hewat, A. W. Nucl. Instrum. Methods 1975, 127, 361. (24) Hewat, A. W.; Bailey, I. Nucl. Instrum. Methods 1976, 137, 463. (25) Larson, A. C.; Von Dreele, R. Generalized Structure Analysis System; Los Alamos National Laboratory Report LAUR 86-748, 1988. (26) Sears, V. F. International Tables for Crystallography, Vol. C, Mathematical, Physical and Chemical Tables; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Dordrecht, 1992. (27) Hriljac, J. A.; Eddy, M. M.; Cheetham, A. K.; Donohue, J. A.; Ray, G. J. J. Solid State Chem. 1993, 106, 66. (28) Liebau, F. Structural Chemistry of Silicates; Springer-Verlag: Berlin, 1985. (29) Baur, W. H. Acta Crystallogr. 1980, B36, 2198.
