J. Phys. Chem. C 2007, 111, 4503-4511
4503
In Situ X-ray Single-Crystal Study on the Dehydration Mechanism in the Monoclinic Polytype of Tschernichite, the Mineral Analog of Zeolite Beta Alberto Alberti,*,† Giuseppe Cruciani,† Ermanno Galli,§ Roberto Millini,‡ and Stefano Zanardi‡ Dipartimento di Scienze della Terra, UniVersita` di Ferrara, Via G. Saragat, 1, I-44100 Ferrara, Italy, EniTecnologie, Via Maritano 26, 20097 San Donato Milanese, Milano, Italy, and Dipartimento di Scienze della Terra, UniVersita` di Modena e Reggio Emilia, Via S. Eufemia, 19, I-41100 Modena, Italy ReceiVed: August 9, 2006; In Final Form: January 30, 2007
Tschernichite, a very rare pentasil zeolite, is the natural aluminum-rich analog of zeolite beta, a large pore aluminosilicate, the peculiar structure and acidity of which makes it one of the most important acid catalysts. Tschernichite, like zeolite beta, is a disordered structure consisting of two distinct polytypes with monoclinic and tetragonal symmetry, respectively. The monoclinic polytype crystallizes in the C2/c space group, with cell parameters a ) 17.982(1), b ) 17.985(1), c ) 14.619(1) Å, β ) 114.33 (1)°, and V ) 4308 Å3 at 25 °C, and its structure is characterized by a three-dimensional channel system of 12-membered rings of tetrahedra. The dehydration process of the monoclinic polytype |Na0.8K0.3Mg0.4Ca8.0(H20)67| [Al18.0Si46.0O128]-BEA was studied by single-crystal X-ray data diffraction collected at room temperature, at 80, 150, and 250 °C in a hot nitrogen stream. During the dehydration process, the variation of the unit-cell volume was always less than 1.3%. In room conditions, monoclinic tschernichite is characterized by disorder in cation sites and water molecule distribution. At 80 °C almost 65% of H2O is lost, and this involves a reorganization of extraframework cations. At 250 °C, all H2O is lost and six extraframework sites were localized. Only one of these displays a coordination number greater than four, and two are only coordinated to three framework oxygens. As a result, about 90% of Ca cations are four- or three-coordinated. The structural collapse of the monoclinic polytype of tschernichite occurs at a temperature below 350 °C. The combination of a large frequency of silanols, associated with the stacking faults, together with the high Ca content, probably explains the relatively low temperature of the structural collapse.
Introduction Over the last 10 years, the Antarctic region has been the focus of several mineralogical papers concerning the discovery of a number of new and very rare natural zeolites in the Jurassic Ferrar Dolerites of Mt. Adamson.1-3 The attention of zeolite scientists was particularly attracted by the discovery of the natural counterparts of the synthetic zeolites ZSM-5 and NU87, named mutinaite and gottardiite, respectively,4,5 and for the occurrence of tschernichite,6 a Ca-rich zeolite the framework topology of which has been suggested to resemble that of the synthetic zeolite beta.7 Zeolite beta, first reported in 1967 by Mobil Oil Corporation,8 is a large-pore, high-silica zeolite with a three-dimensional channel system. The peculiar pore structure and high acidity make zeolite beta a very active and selective catalyst for a wide spectrum of reactions.9-19 The framework structure of zeolite beta was solved independently by Newsam et al.20 and Higgins et al.21 through a clever combination of various techniques, from model building to DLS refinement, high-resolution electron microscopy imaging, electron diffraction, X-ray powder diffraction, and X-ray powder pattern simulation. The peculiar diffraction pattern of this material was characterized by a set of sharp reflections at h ) 3n and k ) 3n, and a set of diffuse maxima for h * 3n or k * * Dipartimento di Scienze della Terra, Sezione di Mineralogia, Petrografia e Geofisica, Via G. Saragat, 1, I-44100 Ferrara, Italy. Tel: +39-532-974732. Fax: +39-532-974767. E-mail:
[email protected]. † Universita ` di Ferrara. ‡ EniTecnologie. § Universita ` di Modena e Reggio Emilia.
3n, frequently superimposed to continuous streaks parallel to c*, pointing to a structure disordered in the direction normal to (001), with disorder due to (a/3 and (b/3 displacements in the (001) plane. Both groups of authors agreed that the structure of zeolite beta could be described as a disordered sequence of different polytypes with frequent planar faults: polytype A, tetragonal with space group P4122 (or P4322) and cell parameters a ) b ≈ 12.5 Å and c ≈ 26.4 Å, and polytype B, monoclinic with space group C2/c and cell parameters a ≈ b ≈ 12.5xx2 Å, c ≈ 14.4 Å, and β ≈ 114°.20,21 Both polytypes can be described as consisting of tetragonal layer-like building units. According to the OD theory, these two structures represent the two maximum degrees of order (MDO) topologies. Newsam et al.20 also suggested that the high density of hydroxyl groups (silanols) often observed in zeolite beta materials was associated with the stacking faults as a way of compensating the unsatisfied bonds. The discovery at Goble, Oregon, of tschernichite, the natural counterpart of synthetic zeolite beta, was described by Smith et al.7 and Boggs et al.22 The authors observed that “the tschernichite patterns match best with computed X-ray patterns for an approximately equal amount of the A and B arrangements in a random sequence”,22 where A and B refer to the polytypic forms described above. More recently, tschernichite has also been found at Mt. Adamson, Northern Victoria Land, Antarctica.6 In both occurrences, the mineral occurs either as large, colorless, steep tetragonal dipyramids terminating in a basal pinacoid, or as radiating hemispherical groups of small crystals.
10.1021/jp065145s CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007
4504 J. Phys. Chem. C, Vol. 111, No. 12, 2007 Microprobe chemical analyses of small and large crystals revealed a silica content higher in the small crystals (Si/Al ∼ 4) than in the large ones (Si/Al ∼ 3).6 The discovery of this mineral in nature is important because it implies that an organic template may not be necessary for synthesis and that a zeolite beta with an Si/Al ratio lower than that achieved so far may be synthesized.7 Alberti et al. were able to isolate the tetragonal and monoclinic polytypes of tschernichite from samples of Mt. Adamson (Antarctica)23 and to demonstrate, using single-crystal X-ray diffraction data, that large crystals are characterized by a great prevalence of polytype B (monoclinic), whereas small crystals are characterized by a dominant presence of the tetragonal polytype A. Both crystal structures were refined, and cation sites were located.23 The aim of this work is to study the thermal behavior of tschernichite. In particular, the opportunity of having large crystals of this zeolite, suitable for X-ray diffraction analysis, allowed us to perform an in situ single-crystal study of the monoclinic polytype. It was not possible to collect the diffraction intensities for an analogous single crystal of the tetragonal polytype A because of the excessively small dimensions of the crystals of this polytype: a program to collect in situ X-ray diffraction data from the small crystal (i.e., tetragonal polytype) using synchrotron light radiation is therefore in progress. The physical and hydrothermal properties of the NH4 exchanged form of the mineral from Goble (Oregon) were examined previously by Szostak and co-workers.24 According to the authors, the ammonium form of tschernichite “exhibits excellent thermal stability”; indeed, the series of X-ray powder diffraction patterns reported did not show amorphization until 900 °C. These results, however, were obtained by analyzing the whole sample of tschernichite from Goble, without discriminating between large and small crystals, that is, between monoclinic and tetragonal polytypes. In this paper, the thermal behavior of the monoclinic polytype of the Ca-rich mineral tschernichite will be compared to that reported for the NH4 form.24 Experimental Section A fragment of a crystal of tschernichite from Antarctica, measuring 0.36 × 0.29 × 0.06 mm3, was selected and glued to a quartz capillary with a very small amount of refractory cement (M-BOND GA-100) composed mainly of silica. This capillary fragment was in turn attached to a steel tube and inserted into an ENRAF NONIUS FR 559 goniometer head, made up according to the design proposed by Tuinstra and Fraase Storm.25 The heater is based on the principle of double gas streams flowing parallel to the goniometer head axis, which provides a cylindrical cool-gas stream that coaxially encloses the hot-gas jet. The crystal was heated with a hot nitrogen flow, and the temperature was measured with a thermocouple placed about 4 mm below the capillary. Single-crystal data collections were performed using a Nonius Kappa CCD diffractometer equipped with a CCD detector and Mo KR radiation. Four data collections were carried out at the following temperatures: room temperature (TSH-RT), 80 °C, (TSH-80), 150 °C (TSH-150), and 250 °C (TSH-250). The fifth data collection, scheduled at the temperature of 350 °C, was not completed because, despite the visual integrity of the crystal, only a few, rather intense, reflections persisted on the diffraction pattern images, indicating that at least a partial amorphization (or structural collapse) of the structure without apparent damage of the crystal had occurred. The same diffraction pattern was
Alberti et al. TABLE 1: Data Collection and Cell Determination Parameters sample N° of images: cell determination data collection exposure time: cell determination data collection frame rotation width crystal-to detector distance
TSH-RT
TSH-80
TSH-150
TSH-250
10 phi 10 phi 10 phi 10 phi 181 phi 224 omega 224 omega 224 omega 387 omega 60 s 60 s 2° 40 mm
60 s 100 s 2° 50 mm
60 s 100 s 2° 50 mm
60 s 100 s 2° 50 mm
also observed after the crystal was taken back to RT, suggesting that the dehydration process in tschernichite is not readily reversible on the short term. These findings will be discussed later. To ensure that the sample reached a status as close as possible to its thermodynamic equilibrium at each temperature step, and to prevent loss of crystallinity, the heating profile was as follows: ramp from room temperature to 80 °C with a 2 °C/ min heating rate, maintained at this temperature for about 14 h prior to data collection, and then repeating this procedure up to 150, 250, and 350 °C. More details concerning the data collection parameters are given in Table 1. It must be pointed out that a complete data collection was performed at room temperature with a standard goniometer head using an optimized setup (TSH-RT), while the high temperature measurements were carried out using a less-optimal setup, with lower resolution and counting statistics in order to avoid technical problems with the crystal heater device and to reduce the counting times. The DENZO-SMN package26 was used for the refinement of the unit cell parameters and data reduction. The SHELXL93 program27 was employed in all of the crystal structure refinements. Systematic extinctions were always consistent with the monoclinic symmetry C2/c, and the starting parameters used for refinement were taken from the structure of the monoclinic polytype of tschernichite.23 The scattering amplitude for the T site is taken into account for actual Si/Al ratios, whose value were fixed as analytical values obtained by the chemical analysis (reported in Table 2). Scattering amplitudes of Ca and Mg were used for extraframework cation sites. The extraframework sites were assigned on the basis of the electron densities localized in the Fourier maps. To reduce the effect of the correlation between temperature factors and occupancy of the extraframework sites, almost always present when partial occupancies occur as in the case of this mineral, care was taken to refine these variables alternately. Crystallographic R1 (Fo-based) factors for these structures, over the investigated temperature range, varied between 12.8 and 15.1% indicating that, as expected, some degree of polytypic disorder is present in our sample. This was confirmed by the reconstructed precession images of reciprocal lattice planes showing a set of sharp reflections and continuous streaks elongated along c* for all data collections. The sharp reflections, with h (and k) ) 3n, are related to the family structure, whereas the diffuse peaks, with h (and k) ) 3n ( 1 are due to stacking disorder in the layer sequence. It clearly implies that the data quality of our present refinement was not optimal and justifies the relatively poor agreement factors. The highest value of the reliability index of the refinement was obtained for TSH-250, probably due to the onset of structural degrading of tschernichite. Refinement parameters are listed in Table 2. Table 3S reports the atomic coordinates, occupancy, and temperature factors, Table 4S reports the T-O distances, Table 5S reports the
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TABLE 2: Crystal Structure Refinement Parameters |Na0.80K0.34Mg0.38 Ca7.97Ba0.04. (H20)66.8| [Al18.01Si46.01O128]
composition crystal size (mm) sample name temperature a (Å) b (Å) c (Å) β (deg) V (Å3) space group maximum 2θ measured reflections unique reflections observed reflection >4s rint (%) R1 (%) no. of parameters largest diffraction peak and hole (e/Å3) N° of e- in extra framework cation sites chemical analysis structure refinement N° of cations from structure refinement Ca Mg N° of water molecules from structure refinement
TSH-RT 25 °C 17.982(1) 17.985(1) 14.619(1) 114.33(1) 4308.0 90.0 23782 6348 3990 15.4 13.8 324 4.15/-1.14
0.36 × 0.29 × 0.06 TSH-80 TSH-150 80 °C 150 °C 17.879(1) 17.863(1) 17.888(1) 17.857(1) 14.635(1) 14.636(1) 114.34(1) 114.35(1) 4264.5 4253.3 C2/c 54.8 54.9 6857 7546 4163 4299 2286 2003 12.2 14.4 13.0 12.8 292 274 1.67/-1.37 1.28/-1.13
TSH-250 250 °C 17.829(2) 17.828(3) 14.671(2) 114.22(1) 4252.8 55.7 9251 4615 2006 15.8 15.1 254 1.70/-0.68
181 161
181 160
181 152
181 157
7.61 0.70 48.4
7.60 0.70 17.8
7.34 0.44 7.9
7.42 0.71 0.0
T-O-T angles, and Table 3 reports the coordination distances of the extraframework cations (Tables 3S, 4S, and 5S are available in the Supporting Information). The chemical composition was determined on the same crystal used for the data collection and was obtained by an ARL-SEMQ microprobe in wavelength dispersive mode operating at 15 kV, 10 mA, and using a beam size of 45 µm. A defocused beam and low beam current were chosen to minimize the Na loss. Three point analyses were performed, and their low variability indicated a good compositional homogeneity of the crystal. The water content was taken from the value6 for the large crystals from Mt. Adamson (Antarctica). The thermal curves reported by Galli et al.6 show that the major water loss occurs in the temperature range from RT to 300 °C and that a further minor weight loss occurs at higher temperature (up to about 500 °C); a further loss, associated with an exothermal peak, occurs at about 550 °C. Results Unit Cell Variations. The analysis of the unit cell parameters, reported in Table 2, shows that the cell volume of the monoclinic polytype of natural zeolite tschernichite is characterized by a modest but significant decrease during the dehydration process; at 250 °C the cell volume is about 1.3% smaller than that at room temperature; this result is a consequence of a slight increase of the cell parameter c and a significantly larger decrease of a, b, and β (Figure 1). About 80% of the unit cell volume decrease occurs when the temperature increases to 80 °C; at this temperature most of the H2O is also lost, and at 150 °C only a very small quantity of water remains in the channel of the mineral. Above this temperature, the cell volume does not change appreciably (see Table 2 and Figure 1). On the basis of the very low decrease in volume as a consequence of heating and dehydration, the monoclinic polytype of tschernichite can be classified as belonging to the first group of the classification of Alberti and Vezzalini28 or as a noncollapsible framework according to Baur.29 Framework. The crystal structure refinement of the monoclinic polytype carried out on the data obtained at room
temperature is in remarkable agreement with the one reported previously.23 In particular, the T-O distances confirm a substantial disorder in the Si/Al distribution (see Table 4S). Concerning the dimensions and shape of the 12-membered ring channel system and channel modifications with temperature, the straight ones, running parallel to [110] and [11h0], respectively, are decidedly elliptic with pores opening at room temperature (sensu Baerlocher et al.30) 5.86 × 8.22 Å, assuming an effective ionic radius for framework oxygens of 1.35 Å (see Table 4 and Figure 2). These values differ markedly from those given by Newsam et al.20 for monoclinic polymorph (6.8 × 7.3 Å, based on DLS refinement). The pore opening of straight channels of monoclinic tschernichite does not change remarkably with temperature, as can be seen in Table 4. The free apertures of the tortuous 12-MR channels (5.86 × 5.86 Å) are quite a lot larger than those reported20 (5.5 × 5.5 Å) and with a “diagonal” aperture of 6.60 Å (Table 4 and Figure 2). When the temperature increases the short distances become slightly larger, whereas the “diagonal” ones become slightly shorter so that the shape of the ring becomes more circular (see Table 4). These modifications will be discussed in detail later. Concluding, the modest variations in dimensions and shape of the channel system confirm that the framework of the monoclinic polytype of tschernichite behaves as a rigid type. Extraframework. The distribution of the extraframework sites found in TSH-RT partly resembles that of the refinement reported previously. Three extraframework cation sites occupy the same positions as the three sites attributed to Ca atoms by Alberti et al.23 in their refinement of the monoclinic phase of tschernichite.23 They are all at bond distance from four framework oxygens; one of these (Ca1 in Table 3S), highly occupied, also coordinates six water molecules, whereas the other two (Ca2 and Ca3), weakly occupied, coordinate three water molecules; two H2O are at a large coordination distance (around 3 Å) from the cations (see Table 3). These cation sites are distributed along the 12-ring channels parallel to the (001) plane and form two chains of polyhedra on the opposite sides of the channels.
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TABLE 3: Selected Interatomic Extraframework Distances (Å) in Monoclinic Tschernichite at Different Temperatures .
TSH-RT
TSH-80
TSH-150
TSH-250
Ca1-O7 Ca1-O12 Ca1-O14 Ca1-O10 Ca1-W1 Ca1-W2 Ca1-W3 Ca1-W4 Ca1-W12 Ca1-W12
2.57(1) 2.61(1) 2.59(1) 2.59(1) 2.68(3) 2.38(2) 2.45(2) 2.78(2) 2.97(2) 3.09(2)
2.47(1) 2.52(1) 2.49(1) 2.49(1) 2.66(7) 2.30(4) 2.50(6) 2.57(7)
2.41(1) 2.47(1) 2.42(1) 2.46(1) 3.11(13) 2.32(7) 2.52(6)
2.39(1) 2.46(1) 2.38(1) 2.44(1)
Ca2-O1 Ca2-O4 Ca2-O7 Ca2-O8 Ca2-W5 Ca2-W4 Ca2-W11
2.20(3) 2.61(2) 2.63(2) 2.69(2) 2.66(6) 3.04(3) 3.06(3)
2.27(4) 2.58(4) 2.68(4) 2.73(4)
2.38(6) 2.47(5) 2.71(5) 2.58(5)
2.29(6) 2.46(5) 2.60(6) 2.51(5)
Ca3-O15 Ca3-O4 Ca3-O14 Ca3-O13 Ca3-W6 Ca3-W1 Ca3-W14
2.21(2) 2.59(2) 2.68(2) 2.65(2) 2.45(6) 3.11(3) 2.92(5)
2.31(3) 2.45(3) 2.69(3) 2.48(3)
2.07(3) 2.32(3) 2.58(3) 2.61(3)
Ca4-W7 Ca4-W11 Ca4-W9 Ca4-W10 Ca4-W6 Ca4-W7 Ca4-W10 Ca4-Ca4
2.09(4) 2.21(3) 2.28(4) 2.32(3) 2.82(5) 2.84(4) 3.04(4) 1.39(4)
Ca5-W5 Ca5-W3 Ca5-W6 Ca5-W6 Ca5-W13 Ca5-W12 Ca5-W2 Ca5-W4 Ca5-O16 Ca5-O15 Ca5-O9 Ca5-Mg
2.05(8) 2.22(6) 2.17(7) 2.28(8) 2.53(8) 2.90(6)
Mg-O15 Mg-O16 Mg-W2 Mg-W3 Mg-W4 Mg-W6 Mg-O9 Mg-Ca5 Ca6-O7 Ca6-O10 Ca6-O14 Ca6-O11 Ca6-O11 Ca6-O6 Ca6-O12
2.92(6) 3.28(6) 1.87(6) 1.79(3) 2.13(2) 2.27(3) 2.57(3)
2.84(6) 2.32(3) 2.48(3) 2.78(3) 2.57(3) 2.92(7)
2.25(10)
1.98(7)
2.35(11) 2.67(9) 2.32(11) 2.85(10) 1.25(10)
2.45(8) 2.39(10) 2.76(4) 2.38(5) 2.71(4) 0.94(6)
1.73(3) 2.12(3) 2.24(5) 2.16(7) 2.93(7)
1.82(6) 2.21(5) 2.42(10) 2.20(8) 2.02(11)
1.78(4) 2.28(4)
2.76(3) 1.25(10)
2.73(6) 0.94(6)
2.64(4) 0.87(5)
2.46(11) 2.98(14) 2.77(14) 2.47(15) 2.91(15) 2.99(13) 2.76(14)
2.52(9) 2.11(8) 2.51(8) 2.43(9)
2.55(4) 2.75(5) 2.42(4) 2.79(5) 2.83(5) 2.92(4) 2.77(5)
2.69(4) 2.38(4) 2.69(4) 0.87(5)
2.68(5) 1.87(6)
2.75(8)
Another site (labeled Mg) is attributed to Mg on the basis of its very short distance from framework oxygens. It is interesting that this site displays a coordination environment and distances very similar to those attributed to Mg in the tetragonal polytype of tschernichite.23 Moreover, its location inside the channel system in the monoclinic phase strongly resembles the Mg location in the channel system of the tetragonal form. This site is bonded with two framework oxygens and two water molecules
Figure 1. Relative variations in unit cell parameters at different temperatures.
if we accept a value less than 2.6 Å as the coordination distance of an Mg cation. The site labeled W8 by Alberti et al.23 and attributed to a water molecule is now attributed to a cation site (Ca4 in Tables 3S and 3). This site is coordinated only to water molecules and is located at the intersection of the channel system. Its distance from the surrounding water molecules (four of these are in the range 2.1-2.3 Å) supports this assumption. The atomic displacement factor of this site is strongly anisotropic (with principal mean square displacement 0.22, 0.06, and 0.05 Å, respectively) so that we cannot establish whether the site is characterized by static or dynamic disorder. Its location strongly resembles that attributed to a cation in the tetragonal polytype of tschernichite (site Ca323), which is at the intersection of the channel system and is only coordinated to water molecules. It is important to note that in zeolites with a framework that contains 12-ring channels, cations are frequently located along the axis of the channels and display strongly anisotropic atomic displacement factors elongated along the channel axis (e.g., zeolite omega31) or alternately occupy a number of very near sites (e.g., offretite32 and mordenite33); in both cases, these are surrounded by a “tube” of water molecules. Another extraframework site bonded to five water molecules with distances in the range of 2.0-2.5 Å and to one water molecule and one framework oxygen at quite a long distance (∼2.9 Å) was attributed to a cation site (Ca5 in Tables 3S and 3). The reasons for this decision will be discussed later. A number of other extraframework sites were localized in the wide channel system and attributed to water molecules on the basis of their coordination distances from cations, their large distance from framework oxygens (g2.8 Å), and because they disappear when the temperature increases. Overall, the crystal structure refinement of TSH-RT indicates 161 electrons for the extraframework cations, which favorably compares with the 181 e- provided by the chemical analysis (Table 2). About 48 water molecules were located, compared with the 67 water molecules given by the chemical analysis (∼70% of the total); this discrepancy is, however, not surprising because zeolites with marked disorder in the extraframework sites, as in this case, usually give a total number of water molecules lower than the “true” figure. It is easy to explain this result assuming that water molecules spread over the entire free volume of the channels or cages of the framework rather than a number of well-defined crystallographic positions. The strong correlation between occupancy and thermal displacement is a trivial consequence of this situation, which can only partly be overcome by refining these variables alternately.
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TABLE 4: Selected Distances (Å) and Areas (Å2) Across Straight and Sinusoidal 12-Ring Channels oxygens
O7-O14
O8-O16
O5-O16
O1-O15
O2-O9
O13-O5
C.F.Aa
TSCH-RT TSCH-80 TSCH-150 TSCH-250
5.86 5.81 5.83 5.84
6.84 6.80 6.78 6.73
7.41 7.44 7.54 7.60
8.22 8.19 8.21 8.28
7.59 7.58 7.53 7.67
6.87 6.78 6.72 6.71
39.9 39.6 39.6 40.0
oxygens
O15-O1
O9-O5
O2-O16
C.F.A.
TSCH-RT TSCH-80 TSCH-150 TSCH-250
5.77 5.74 5.90 5.94
6.60 6.51 6.57 6.51
6.59 6.50 6.48 6.47
31.4 30.7 31.3 31.2
a Crystallographic free area (sensu Baerlocher et al.30) calculated using for the diameter of the ring the mean of the O-O distances reported in the table.
Figure 2. Si and O atoms delimiting the straight (left) and sinusoidal (right) 12-ring channels of monoclinic tschernichite.
When the temperature increases to 80 °C (TSH-80) almost 65% of the water molecules found at RT are lost. The four water molecules bonded to Ca1 (W1, W2,W3, and W4), which is by far the most occupied cation site, are still present, whereas the greatest part of the other water sites found in TSH-RT are now empty and two new positions, weakly occupied, have been localized. It is easy to assume that the latter are due to the migration of water molecules to more energetically favored positions at 80 °C. The most evident modification induced by dehydration is the migration of the cations located in the Ca4 sites to new positions (labeled Ca6 in Tables 3S and 3) as a consequence of the loss of almost all water molecules coordinating the site. The new site is located inside the polyhedral unit 425462 (the 1,4-stellated hexagonal prism or mtw unit according to the labeling of Smith34) and is bonded to seven framework oxygens with distances in the range of 2.46-2.99 Å. It is interesting that the four oxygen atoms bonded to Ca1 are also bonded to Ca6 (see Table 3) so that these oxygens are, as in a sandwich, at coordination distances from the Ca1 and Ca6 sites. Ca6 is slightly shifted from the center of the 425462 unit (the distance from the two positions symmetrically equivalent inside the cage is 1.03 Å). The environment around the 1,4-stellated hexagonal prism is shown in Figure 3. The occupancy of Ca6 (∼10%) does not completely account for the amount of Ca present at room conditions in the Ca4 site. At the same time, the occupancy of Ca1 increases at 80 °C by about 10% with respect to its value at TSH-RT. Therefore, we can infer that the cations in Ca4 at room temperature disperse at 80 °C to the Ca1 and the new Ca6 positions. Concerning Ca5, almost all of the coordinated water molecules are lost, only W2 and W3 are bonded to Ca5, so that cations which occupy this site move toward the walls of the channels approaching the Mg position (see Tables 3S and 3); therefore, Ca5 is now coordinated to the same framework oxygens coordinating Mg.
No significant variations either in occupancies or in coordinates were observed for the Ca2, Ca3, and Mg sites. When the crystal is heated to 150 °C (TSH-150), almost all water molecules are expelled. Only the W1, W2, W3, and W4 sites, bonded to Ca1, are still occupied and their occupancy accounts for only 10% of the water content given by the chemical formula (see Table 2). This means that Ca1 must be surrounded, on average, by less than four water molecules. At the same time, the occupancy of Ca1 decreases significantly whereas that of Ca6 increases. We can infer that there is a partial migration of cations from Ca1 to these positions. In this temperature range, the cations occupying Ca5 continue to move toward the Mg position, maintaining their coordination with the water molecules. It is interesting that although the occupancy of W1 and W4 is strongly reduced, that of W2 and W3 does not change dramatically. This result can be explained by considering that W2 and W3 are not only at coordination distance from Ca1 but also from Ca5 and Mg. When the temperature increases up to 250 °C (TSH-250), all water is completely lost. Consequently, Ca1, Ca2, and Ca3 are fourfold-coordinated to framework oxygens, and Ca6 maintains its coordination with seven oxygens (Table 3). Moreover, the occupancy of Ca1 is further lowered whereas that of Ca5 is markedly increased, suggesting that the migration of cations from Ca1 to other sites, already observed at 150 °C, continues. When dehydration is complete, both Mg and Ca5 are only coordinated to the same three framework oxygens (see Table 3). It is worth noting that, despite the different locations of cations and their migrations as a function of the heating temperature, the number of electrons afforded by the structural refinements does not vary remarkably and is in close agreement with that provided by the chemical analysis (see Table 2).
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Figure 3. Coordination of Ca1 and Ca6 cations with respect to the 425462 cage (mtw unit according to Smith34) in dehydrated monoclinic tschernichite.
Figure 4. Framework structure of monoclinic tschernichite at 250 °C. Extraframework cations are also shown.
Discussion One of the most interesting features found in the dehydration process of the monoclinic polytype of tschernichite is the low coordination number of the majority of the extraframework cation sites. In fact, if a value as high as 3.2 Å is assumed as the maximum coordination distance for cations, three Ca sites (Ca1, Ca2, and Ca3 in Table 3) are four-coordinated whereas Ca5 and Mg appear to be bonded only to three framework
oxygens. Temperature-resolved investigations of Ca-rich zeolites, such as mesolite,35 scolecite,35 laumontite,36 gismondine,37 garronite,38 epistilbite,39 and yugawaralite40 have shown that increased interaction of the Ca cations with framework oxygens when the Ca coordination is below six can be regarded as triggering the collapse of the structure. In all of these zeolites, characterized by a Ca coordination not less than six, dehydration is accompanied by a strong distortion of the framework so that they must be classified in group II of the Alberti and Vezzalini scheme.28 However, a fourfold coordination of calcium cations was reported recently in medium/large pore Ca-rich zeolites like boggsite,41 mordenite,42 and rho43 and in small-pore Ca-rich zeolites like wairakite.44 In these materials, dehydration occurs without remarkable changes in the framework and in the cell volume. Indeed, the largest decrease in cell volume during dehydration occurs in the Ca-exchanged form of zeolite rho (∼10% in volume and 4% in cell parameter a); this ∆V value is quite low considering the exceptional flexibility of this zeolite and much lower than that found in the collapsible Ca-zeolites. Consequently boggsite, mordenite, rho, and wairakite must be classified in group I of the above-cited scheme. Moreover, their crystal structures are stable up to 500 °C and above, and the rehydration process is rapid and completely reversible. Therefore, we can infer that a Ca coordination below six triggers the collapse of the structure in zeolites with frameworks subject to strong distortion during dehydration (i.e., classified in group II of the Alberti and Vezzalini scheme28 or collapsible in the sense of Baur29); vice versa, a Ca coordination of less than six (five, four, or even three) can be borne by zeolites with frameworks that do not change remarkably during dehydration (i.e., classified in group I of the Alberti and Vezzalini scheme28 or noncollapsible in the sense of Baur29). As pointed out before, the framework of the monoclinic polytype of tschernichite is also accompanied by weak distortions of the framework and a very small decrease in the cell volume and for these reasons it must be classified in group I of the above-mentioned scheme. However, this zeolite differs from the others of the same group because it loses its crystallinity at low temperatures (down 350 °C). This low thermal stability can be attributed to the following: I. the very low Si/Al ratio of this polytype;
Temperature-Induced Transformations II. the high calcium content and the low coordination of Ca ions; III. the high frequency of silanol groups associated to stacking faults in tschernichite. I. It is well known that, as a general rule, the thermal stability of a framework in natural zeolites rises with the increase of the Si/Al ratio. This ratio is, in monoclinic tschernichite, the lowest found to date in natural pentasil zeolites so that this peculiarity is a good candidate to explain the low thermal stability. In fact, the Si/Al ratio in Ca-mordenite and in boggsite is among the highest found in natural zeolites (5.1 and 4.5, respectively), but it is 3.2 and even 2.0 in the zeolites rho and wairakite, respectively. Consequently, the Si/Al ratio alone cannot be the cause of the loss of crystallinity at low temperatures of the monoclinic polytype of tschernichite. II. It can be inferred that a large quantity of Ca cations with coordination numbers less than six may be an important factor in destabilizing the framework and consequently explaining the low thermal stability of tschernichite. In Ca-mordenite the Si/ Al ratio is 5.1 so that about 16 out of 100 tetrahedra are occupied by Al; in dehydrated form42 about 50% of the Ca is coordinated to less than six framework oxygens, and, consequently, four Ca atoms every 100 tetrahedra (Si or Al centered) have a coordination less than six. In dehydrated boggsite the Si/Al ratio is 4.5 and almost all of the extraframework cations are calcium, which means nine Ca atoms every 100 tetrahedra. In this zeolite about two-thirds of Ca atoms are fourfold-coordinated,41 and, consequently, six Ca every 100 tetrahedra display this coordination. Using the same procedure, we found that about 12, 12, and 16 Ca atoms every 100 tetrahedra in tschernichite (this work), zeolite rho,43 and wairakite,44 respectively, are characterized by a coordination number less than six. These figures show clearly that the number of Ca atoms with low coordination cannot by itself be considered the sole cause for the loss of crystallinity at low temperatures of monoclinic tschernichite. Furthermore, tschernichite is already completely dehydrated at 250 °C, while the structural degradation occurs at a higher temperature. III. The high-resolution transmission electron microscopy (HR-TEM) study by Szostak et al.45 on natural tschernichite from Goble revealed the occurrence of an extremely high stacking fault frequency, comparable with that of its synthetic counterpart. As mentioned previously, Newsam et al.20 associated the high density of silanol hydroxyls in zeolite beta with the occurrence of stacking faults and this was further confirmed by HR-TEM observations and defect site modeling by Wright et al.46 Assuming a similar occurrence of silanol hydroxyl groups both in zeolite beta and in tschernichite, it might be suggested that the loss of these hydroxyls could be responsible for the onset of the destabilization of the tschernichite structure, as we found for T > 250 °C. Therefore, it is inferred that the occurrence of silanol hydroxyl groups associated with stacking faults, in combination with a high Ca content, can be regarded as the key causes for the collapsing of the zeolite tschernichite at low temperatures. Many terms have been used to describe the effects of dehydration in zeolites: phase transition, phase transformation, structural collapse, structural breakdown, amorphization, polyamorphism, and so forth. According to Baur29 the term collapsible is appropriate for frameworks in which, upon a change in cell dimensions, all T-O-T hinges corotate, that is, vary in the same direction, while the framework distorts. Bish and Carey47 distinguish a structural collapse in which T-O-T bonds are broken but the structure is still recognizable using
J. Phys. Chem. C, Vol. 111, No. 12, 2007 4509 diffraction data “similar” to the original zeolite (collapsed structures also typically retain some sorption capacity) and structural breakdown resulting in the complete loss of the zeolite structure (usually with the loss of all sorption capacity). In such cases the breakdown does not necessarily mean amorphization but can generate a new structure in which it is extremely difficult to recognize the initial structure. Greaves et al.48 find that a low-density material like a zeolite will collapse on heating, to form a rigid amorphous solid, with a density comparable to that of a zeolite, without the system ever becoming molten or fluid. Their results suggest that zeolite collapse follows a route involving polyamorphism, with a more ordered and lower density amorphous (LDA) phase defining the onset of zeolite collapse, and a high-density more disordered amorphous phase (HDA) forming later, on further heating. As pointed out by Navrotsky,49 the identification of LDA and HDA phases is significant because it implies the existence of two noncrystalline structural states with different properties. The LDA phase is the one with stronger bonding and a more open framework than the denser HDA phase. In a likely scenario, the removal of a hydroxyl group would trigger local amorphization leaving the ordered arrangement of zeolite pores and channels temporarily unaffected. This would explain the persistence in the diffraction images at 350 °C of intense basal diffraction. The only persistence of these diffractions at temperatures above 350 °C was also observed in a timeresolved synchrotron-based diffraction experiment on a tschernichite sample from Mt. Adamson (work in preparation), and in an X-ray powder diffraction pattern of calcined beta.46 Our hypothesis is the following: after the complete dehydroxylation of tschernichite, a low-density phase is formed where the tetrahedral framework is amorphized but the channel system does not collapse. This structure can be compared to that of an ordered mesoporous material where the surfactant has been removed. As for an ordered mesoporous material, the powder X-ray diffraction pattern of the amorphous phase of tschernichite has only a few reflections (110, 330) that characterize the pore dimensions. This new phase can be considered the low-density amorphous (LDA) phase of Greaves et al.48 or the collapsed phase of Bish and Carey.47 According to the DTG curve reported by Galli et al.6 for the large crystals of tschernichite, an exothermic reaction occurs around 550 °C. This reaction may indicate the formation of the high-density more-disordered amorphous phase (HDA) proposed by Greaves and co-workers48 or the structural breakdown of Bish and Carey.47 Whatever is the cause of the thermal instability of tschernichite, these results seem to be an insurmountable obstacle for utilization in catalysis of the synthetic counterpart of this natural zeolite with analogue Si/Al ratio. However, the study on NH4 exchanged tschernichite24 indicates that this material dealuminates readily by steaming up to a Si/Al ratio of around 10 (from 29Si NMR) and in its acid form is thermally stable to temperatures as high as 900 °C. This high thermal stability is comparable with that of as-synthesized beta with an Si/Al ratio slightly higher (about 13).50 Moreover, the steam treatment gives rise to strong acid sites. It should be noted that the tschernichite sample studied by Szostak et al.24 was probably a mixture of monoclinic and tetragonal polytypes of tschernichite, as confirmed by its Si/Al ratio (3.3), intermediate between the 2.95 and 3.70 values of the Si/Al ratios22 that characterize the large crystals, where the monoclinic polytype is dominant, and the small crystals, where the tetragonal polytype prevails, respectively, whereas in our sample the monoclinic polytype dominates.
4510 J. Phys. Chem. C, Vol. 111, No. 12, 2007 As pointed out in the Results section, the loss of water molecules causes some changes in the framework, evidenced by modifications in the shape and dimensions of the channels. Changes in interaction of cations with framework oxygens is probably responsible for these modifications. In particular, the Ca5 site, which at room temperature is far from the framework oxygens, moves by 1.24 Å toward the walls of the channel system, in particular toward O9, O15, and O16 (see Table 3). These oxygens delimitate both the straight and the sinusoidal 12-rings. As a result, the shape of the sinusoidal channel, which at room temperature has an almost square shape (see Figure 2), tends to become more circular as the O1-O15 distance increases, while O9-O5 and O2-O16 decrease (see Table 4). O15 and O1 are also the atoms of the major axis of the strongly elliptical straight channel, with O7 and O14 those of the minor axis. These distances do not change significantly, whereas the distances O8-O16 and O5-O16, as well as O2O9 and O13-O5, vary remarkably in the opposite sense. The oxygen atom O5 seems to play an important role in these modifications because it not only concurs to delimitate both rings of the channel system but also because it is one of the vertexes of the Si2 tetrahedron. This tetrahedron seems to be the keystone of the structural modifications induced by heating in monoclinic tschernichite. In fact it strongly rotates around the tetrahedral cation as evidenced by the strong shifts of the oxygen atoms during the heating process (O1, O5, O6, O7 moving by 0.12, 0.17, 0.25, 0.09 Å, respectively). It is observed that three of the oxygens of the tetrahedron (O1, O5, O7) delimitate the channel system, whereas the fourth (O6), which is by far the most shifted among the framework oxygens, coordinates the cation site Ca6, which, empty at RT, is occupied when the cations located in the Ca4 site migrate to the new C6 position as a consequence of the loss of almost all water molecules coordinating the site. It is interesting to follow the increasing interaction of the Ca cations in Ca1 (by far the most occupied site) with the framework oxygens, when the bonded water molecules are lost. In fact, as the W sites become less occupied, and empty at 250 °C, the average distance of the Ca1 site from its coordinated oxygens decreases (2.59, 2.49, 2.44, and 2.42 Å at RT, 80, 150, and 250 °C, respectively). Conclusions The in situ single-crystal X-ray diffraction analysis, performed at room temperature and after heating at 80, 150, and 250 °C, showed that the framework of the monoclinic polytype of tschernichite is a noncollapsible type. During the heating process, the framework behaved as a very rigid structure, with amorphization starting after heating to above 250 °C. Only small variations were noted in the dimensions and shapes of the channels; moreover, the cell volume contraction was less than 1.3%, one of the lowest found so far in natural zeolites. The dehydration process of the monoclinic polytype of tschernichite may be divided into two main stages. During the first stage, occurring below 100 °C, the water molecules not bonded to the most populated cation site Ca1 are lost. As a consequence, a reorganization of extraframework cations occurs. In the second step, from about 100 °C to approximately 200 °C, all water molecules are expelled and the majority of extraframework cations assume a fourfold or even threefold coordination. No remarkable variation of the unit cell volume was noted in this temperature range. The loss of crystallinity of the monoclinic polytype of tschernichite occurs below 350 °C, indicating a low thermal stability of natural Ca-rich tschernichite, but its dealuminated NH4 exchanged form is stable
Alberti et al. up to about 900 °C.24 The latter result is associated with the ability of a beta-like material to crystallize with a very low Si/ Al ratio (less than 3.0) and in the absence of organic cations makes tschernichite an interesting material, with potentially useful catalytic properties. Moreover, as pointed out by Szostak and co-workers,24 “the existence of such a material offers an intriguing starting point and challenge for the synthetic zeolite chemist to produce the synthetic analogue of tschernichite”. Acknowledgment. We thank the Centro di Strutturistica Diffrattometrica of the University of Ferrara for X-ray data collection. Italian PNRA and MIUR (“Stability of zeolites under non-ambient physicochemical conditions: from structural behaViour to atomistic modeling.” PRIN 2004) are acknowledged for financial support. The “Consiglio Nazionale delle Ricerche” of Italy is acknowledged for funding the electron microprobe laboratory at the Dipartimento di Scienze della Terra of the Universita` di Modena e Reggio Emilia. We thank Prof. David Bish for the critical reading of the manuscript. Supporting Information Available: Tables 3S, 4S, and 5S and cif files. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Vezzalini, G.; Quartieri, S.; Rossi, A.; Alberti, A. Terra Antartica 1994, 1, 96. (2) Alberti, A.; Cruciani, G.; Galli, E.; Merlino, S.; Millini, R.; Quartieri, S.; Vezzalini, G.; Zanardi, S. Stud. Surf. Sci. Catal. 2001, 135, 83. (3) Galli, E.; Quartieri, S.; Vezzalini, G.; Alberti, A.; Franzini, M. Am. Mineral. 1997, 82, 423. (4) Vezzalini, G.; Quartieri, S.; Galli, E.; Alberti, A.; Cruciani, G.; Kvick, Å. Zeolites 1997, 19, 323. (5) Alberti, A.; Vezzalini, G.; Galli, E.; Quartieri, S. Eur. J. Mineral. 1996, 8, 69. (6) Galli, E.; Quartieri, S.; Vezzalini, G.; Alberti, A. Eur. J. Mineral. 1995, 7, 1029. (7) Smith, J. V.; Pluth, J. J.; Boggs, R. C.; Howard, D. G. J. Chem. Soc., Chem. Commun. 1991, 363. (8) Wadlinger, R. L.; Kerr, G. T.; Rosinski, E. J. U.S. Patent 3,308,069, 1967. (9) Bellussi, G.; Pazzuconi, G.; Perego, C.; Girotti, G.; Terzoni, G. J. Catal. 1995, 157, 227. (10) Perego, C.; Amarilli, S.; Millini, R.; Bellussi, G.; Girotti, G.; Terzoni, G. Microporous Mater. 1996, 6, 395. (11) Hoefnagel, A. J.; van Bekkum, H. Appl. Catal., A 1993, 97, 87. (12) Kouwenhoven, H. W.; Gunnewegh, E. A.; van Bekkum, H. Tagunsber. 1996, 9601, 9. (13) Das, J.; Bath, Y. S.; Halgeri, A. B. Catal. Lett. 1994, 23, 161. (14) Wang, I.; Tsai, T. C.; Huang, S. T. Ind. Eng. Chem. Res. 1990, 29, 2005. (15) De Jong, K. P.; Mesters, C. M. A. M.; Peferoen, D. R. G.; van Brugge, P. T. M.; de Groot, C. Chem. Eng. Sci. 1996, 51, 2053. (16) Nivarthy, G. S.; Feller, A.; Seshan, K.; Lercher, J. A. Microporous Mesoporous Mater. 2000, 35-36, 75. (17) Boretto, L.; Camblor, M. A.; Corma, A.; Perez-Pariente, J. J. Appl. Catal. 1992, 82, 37. (18) Lee, J. K.; Rhee, H. K. Catal Today 1997, 38, 235. (19) Wang, Z. B.; Kamo, A.; Youeda, T.; Komatsu, T.; Yashima, T. Appl. Catal. 1997, 159, 119. (20) Newsam, J. M.; Treacy, M. M. J.; Koetsier, W. T.; De Gruyter, C. B. Proc. R. Soc. London 1988, A20, 375. (21) Higgins, J. B.; LaPierre, R. B.; Schlenker, J. L.; Rohrman, A. C.; Wood, J. D.; Kerr, G. T.; Rohrbaugh, W. J. Zeolites 1988, 8, 446. (22) Boggs, R. C.; Howard, D. G.; Smith, J. V.; Klein, G. L. Am. Mineral. 1993, 78, 822. (23) Alberti, A.; Cruciani, G.; Galli, E.; Merlino, S.; Millini, R.; Quartieri, S.; Vezzalini, G.; Zanardi, S. J. Phys. Chem. B 2002, 106, 10277. (24) Szostak, R.; Lillerud, K. P.; Stoker, M. J. Catal. 1995, 148, 91. (25) Tuinstra, F.; Fraase Storm, G. M. J. Appl. Crystallogr. 1978, 11, 257. (26) Otwinowski, Z.; Minor, W. In Methods in Enzymology: Macromolecular Crystallography, Part A: Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997; p 307.
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