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Anisotropic Thermal Expansion of SiO2 and AlPO4 Clathrasils with the AST-Type Structure Mahrez Amri, Guy J. Clarkson, and Richard I. Walton* Department of Chemistry, UniVersity of Warwick, CoVentry, CV4 7AL United Kingdom ReceiVed: January 14, 2010; ReVised Manuscript ReceiVed: February 25, 2010
The thermal expansion behavior of silica and aluminophosphate clathrasil materials that adopt the AST-type structure has been studied in both as-made and calcined forms using X-ray diffraction (single crystal or powder) over the temperature range 100-400 K. The four materials studied all adopt body-centered tetragonal variants of the ideal cubic AST-type structure. In the as-made form, where quinuclidinum cations fill the larger cages and fluoride anions occupy interstitial sites at the center of double four-ring units, both materials have net positive volume thermal expansion coefficients, larger than dense forms of SiO2, but both show highly anisotropic thermal expansivity with positive expansion in the ab plane and negative expansion parallel to c. The anisotropy becomes more pronounced upon removal of the extra-framework species, with larger magnitudes of positive (Ra and Rb) and negative (Rc) coefficients than those before calcination. This indicates a greater structural flexibility once free pore volume is available, although still net positive thermal expansion is seen. In the case of as-made SiO2-AST (that contains template and fluoride ions) and the fully calcined AlPO4 analogue, full structure refinements as a function of temperature have been performed; this reveals that whether or not template and fluoride are present, the double-four-ring composite building units show a volume contraction with temperature, whereas an expansion of linking T-O-T bonds is seen. These opposing effects are responsible for the anisotropy in thermal expansion behavior. Introduction Negative thermal expansion (NTE), the contraction of a material upon heating instead of the expected expansion, has practical applications in engineering fields, where the design of structures that are exposed considerable temperature gradients is required. In such situations, the control of the thermal expansion behavior is of key importance: this includes areas such as precision optical devices and electronics through to cements that provide solid adhesion when exposed to a wide temperature range. The occurrence and applications of NTE have been the subject of several reviews.1–3 NTE has now been reported in a number of classes of crystalline materials that have extended structures: this includes the mixed-oxide materials ZrW2O8,4 Sc2(WO4)3,5 open-framework zeolite silicates and phosphates,6 pyrophosphates MP2O7 at high temperature,7 alloys,8,9 nitrides,10 metal-organic framework materials,11 and metal cyanide networks.12 Although most of these materials are perhaps unlikely to be used in real applications because of the complexity of their synthesis, they nevertheless provide very important fundamental model systems for understanding the atomic-scale origins of unusual thermal behavior, especially since their structures can often be modified in a systematic manner, for example, by isovalent doping, or in the case of porous materials by varying the nature of extra-framework species. The occurrence of NTE in zeolite materials was first noted in 1993 for Na-zeolite X (FAU-type).13 Later, in 2001, Lightfoot et al. presented thermal expansion data for 17 calcined zeolite and zeotype materials (i.e., free of extra-framework species) and suggested that the NTE was “widespread” in such solids and rather more common than the positive thermal expansion that * To whom correspondence
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might intuitively be expected.6 This observation has been borne out by subsequent studies that not only have widened the range of zeolite structure types studied to provide new examples, but also have focused on how the magnitude and direction of the NTE are dependent on the details of the structure. In some of these cases, full structure refinement (i.e., atomic positions and thermal displacement parameters, rather than just unit cell parameters) as a function of temperature has been performed. For example, for siliceous faujasite (FAU-type), a model involving the transverse vibrations of two-coordinate, bridging oxygens was proposed to explain the occurrence of NTE from Rietveld refinement of powder X-ray data,14 for siliceous chabazite (CHA-type structure) it was observed that only certain structural units had flexibility that contributed to negative thermal expansion,15,16 for siliceous ferrierite (FER-type) a second-order phase transition at 400 K gave a switch volume thermal expansivity from positive to negative, driven by transverse vibrations of Si-O-Si linkages at high temperature,17 and recently we have shown that the structurally related phosphates AlPO-34 (CHA-type) and AlPO-18 (AEI-type) have NTE in directions that maps onto parts of their crystal structures that show similar connectivities.18 One important conclusion from these studies is that the framework density of the zeolite structure type may have an important influence on the thermal expansion properties. For example, in a study of the correlation between zeolite void space and thermal behavior, Woodcock et al. suggested that positive thermal expansion was favoured by high framework density and the presence of one-dimensional channels.6 Bearing this in mind, it would seem logical to investigate the thermal expansion properties of the clathrasils: these are related to the zeolites in that they possess threedimensional structures made from corner-shared tetrahedral units, but they have smaller windows and guest molecules reside
10.1021/jp100368e 2010 American Chemical Society Published on Web 03/12/2010
Thermal Expansion of SiO2 and AlPO4 Clathrasils
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Figure 1. Ideal AST structure showing the connectivity of double-4rings to give larger [46412] cages. The structure is drawn show the connectivity of T atoms only, which are located at the nodes of the framework.
in cages and must be decomposed into smaller fragments before they can escape from the inorganic framework.19 The framework densities of the clathrasils lie between those of the zeolites and the dense polymorphs of SiO2, so these materials should provide an interesting comparison that might shed more light on the origin of NTE in open-framework structures. In some cases, AlPO4 analogues of clathrasils are known (clathralpos according to the definitions of Liebau et al.19), and so this family also gives the opportunity for the effect of framework composition to be investigated. AlPO-16 is an example of a clathralpo. Its structure solution was reported in 1991,20 and the framework of the material was then given the structure code AST.21 This phase was obtained at 150 °C from an aluminophosphate gel in the presence of quinuclidine as structure directing agent (SDA). The introduction of the fluoride route in zeolite synthesis later allowed the synthesis of a tetragonal variant of AlPO-16.22 The pure silica form of AST was also reported in 1991, by Caullet et al.,23 from fumed silica using fluoride as mineralizing agent and quinuclidine as SDA; this material is also known as octadecasil. Figure 1 shows a representation of the ideal AST-type structure: the framework is built from 4-1 secondary building units to give double-4-rings (D4R, or [46] cages) and larger [46612] cages.21 The largest apertures are formed by 6-rings. In the as-made form, organic template molecules reside in the [46612] cages while fluoride ions are located at the center of the [46] cages. Despite the lack of channels in the AST structure, it has been reported that template removal is possible to yield neutral SiO2 and AlPO4 materials24 (as we will discuss further below). In this paper, we describe an investigation of the thermal expansion properties of SiO2 and AlPO4 materials with the AST structure type in both as-made and calcined forms: this has allowed us to study the effect of framework composition and presence of extraframework species on a relatively simple tetrahedrally coordinated structure in order to understand further the complex thermal behavior of tetrahedrally connected networks. Experimental Section Pure silica octadecasil SiO2-AST was synthesized using quinuclidine as SDA based on the method described by Caullet et al.23 In a typical synthesis, 0.234 g of quinuclidine (Aldrich, 97% wt in water) was completely dissolved in 1.051 g of
Figure 2. SEM images of (a) as-made SiO2-AST and (b) as-made AlPO4-AST.
distilled water. Then 0.252 g of precipitated silica (Cab O-Sil) was added under vigorous stirring by hand until homogeneous. Hydrofluoric acid 0.226 g (Aldrich, 40 wt % in water) was added to the previous mixture dropwise and mixed for further homogenization to give the final gel composition: SiO2:0.5 quinuclidine:0.5 HF:20 H2O. The gel was transferred into a Teflon-lined steel autoclave (∼50% fill) and heated to 170 °C under static conditions for 15 days. The final product was washed with water and dried at 70 °C in air to give highly crystalline material. The aluminophosphate analogue of octadecasil, AlPO4-AST, was synthesized using quinuclidine as structure directing agent following the method described by Schott-Darie et al.22 In a typical synthesis, a solution of 1 g of distilled water, 0.576 g of orthophosphoric acid (85 wt % in water, Fisher), and 1 g of aluminum isopropoxide (99.99%, Aldrich) was prepared and stirred until homogeneous. A homogeneous solution obtained by dissolving 0.286 g of quinuclidine (99.9%, Aldrich) in 1.537 g of deionized water was added to the previous solution under vigorous stirring. Finally, 0.129 g of hydrofluoric acid (40 wt % in water, Fluka) was added dropwise to the mixture. The final gel mixture of composition 1 P2O5:1 Al2O3:1 quinuclidine:1 HF:60 H2O was stirred until homogeneous and transferred to a Teflon-lined stainless steel autoclave for crystallization at 150 °C. The pure phase of AlPO-16 AST-type structure was reported in literature to crystallize after 24 h at 150 °C.22 Repeated synthesis using the same time of reaction reveals that the material has a very poor crystallinity with an average particle size in the order of nanometers. The crystallinity of the material was improved by increasing the time of reaction, and the pure and crystalline phase of AlPO-16 was successfully synthesized after a 15 day
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Amri et al. diffraction on beamline ID31 of the European Synchrotron Radiation Facility26 was recorded to measure the thermal expansivity of the polycrystalline samples. Samples were transferred to a thin-walled silica glass capillary and heated from 100 to 400 K in 100 K intervals using an Oxford Cryosystems 700 series nitrogen cryostream device. Temperature was measured accurately using a thermocouple situated close to the sample and calibrated using a Pt thermocouple.27 A weighting allowing an increase of data collection time at high scattering angles to minimize the low atomic scattering effect characteristic for X-ray diffraction was used. The program Fullprof28 was used to refine the unit cell parameters using the Le Bail method to extract peak intensity with a Peudo-Voigt peak shape function to describe the experimental profile. Additional corrections were made for peak asymmetry due to axial divergence for low angle peaks.29 The background was described as a set of points determined using linear interpolation of the data, and the zero shift parameter was initially fixed from the value obtained with Si standard for the calibration of the zero position of the detector in the diffractometer.
Figure 3. Cell parameters and unit cell volume versus temperature for the as-made materials: (a-c) SiO2-AST and (d-f) AlPO4-AST. Errors are smaller than the points used.
reaction. The product was washed with deionized water and filtered through coarse paper and finally dried at 70 °C in air. Laboratory characterization was performed using a variety of methods. Powder X-ray diffraction analysis was performed using a Bruker D5000 powder X-ray diffractometer operating with Cu KR radiation and fitted with an MRI TC-Basic furnace for measurements above room temperature, where alumina sample holders were used. Thermal analysis was performed on a Mettler Toledo TGA/DSC 1 instrument from 30 to 900 at 10 °C min-1 in flowing air, with ∼10 mg of sample heated in an alumina crucible. Samples were imaged using a Jeol 6100 scanning electron microscope. For one sample, single crystal X-ray diffraction was carried out in-house using an Oxford Diffraction Gemini R diffractometer equipped with CCD camera detector. The wavelength used in the experiment was 0.71073 Å (Mo KR radiation). The crystal, of 0.1 × 0.1 × 0.1 mm3 size, was glued with epoxy resin to a glass fiber, and the temperature was controlled using an Oxford Cryosystem Cobra cooler. The crystal-to-detector distance was 5.5 cm. Six data sets were collected between 100 and 350 K with a 50 K interval. The low temperature data were collected using a liquid nitrogen cryostream. The data collection nominally covered a hemisphere of reciprocal space, by a combination of four sets of exposures with different angles for the crystal. Crystal decay was proved to be negligible by repeating the initial frames at the end of data collection and analyzing the duplicate reflections. The program SHELXL-9725 was used to determine the crystal structure and unit cell parameters at each temperature. High resolution X-ray powder
Results For SiO2-AST, large, high quality single crystals were produced (Figure 2a) that were suitable for laboratory single crystal diffraction studies of the as-made material as a function of temperature (see the Supporting Information). The systematic absences in the reflected intensities indicated a tetragonal space group I4/m (no. 87), the same as that reported by Caullet et al. for a material prepared by a similar route.23 In the case of AlPO4AST, the crystals were considerably smaller (Figure 2b), and so high resolution powder X-ray diffraction was used to study the response of the as-made material to temperature. Indexing of the X-ray powder diffraction at 300 K indicated a tetragonal space group I4j (no. 82) consistent with that reported by others for the same material.22,30 The unit cell parameters at 300 K were refined from the values previously reported at room temperature and were used as a starting point for analysis at other temperatures, where the same symmetry was found (see the Supporting Information). Figure 3 shows the unit cell parameters and volume as a function of temperature for both materials, and Table 1 contains linear and volume expansion coefficients from analysis of these data. For the as-made SiO2-AST, the quinuclidinium template was found to be disordered and in fact is located in a position of higher symmetry than the cation itself. The carbon composition was fixed at 14 carbon atoms per unit cell, by choosing appropriate values for the occupancies that gave reasonable displacement parameters for the carbon atoms; this was found to give a lower R value and found to be a simpler approach to structure refinement than restraining the occupancies to a set value. The framework silicon and oxygen atoms were refined freely without constraint along with the interstitial fluoride ion located at the center of the D4R building unit. Anisotropic
TABLE 1: Thermal Expansion Coefficients of the Isostructural Analogues SiO2 and AlPO4 with the AST Structure Topology material
space group
T range (K)
Ra (×10-6 K-1)b
Rc (×10-6 K-1)b
Rv (×10-6 K-1)b
Q+F-SiO2 Q+F-AlPO4 SiO2a AlPO4
I4/m (no. 87) I4j (no. 82) I4/m (no. 87) I4/m (no. 87)
100-350 100-400 110-460 110-460
+19.6 +26.5 +19.5 +36.9
-2.54 -5.83 -10.8 -19.7
+36.7 +42.2 +28.2 +53.9
a This material is partially calcined (see text). b Ra, Rc, and Rv values are those obtained by linear fitting of the variation of cell parameters and volume with temperature and calculated using the equation RL ) (L - Lref)/Lref(T - Tref). The reference temperature Tref is the lowest temperature in the specified range.
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Figure 4. (a-c) Si-Si distances in as-made SiO2 as a function of temperature (labeled by denoting the intermediate bridging oxygen atom) and (d-e) associated bond angles. The graphics in the center represent the location of the linkages whose behavior is shown on the adjacent graphs where they are highlighted as gray (red in color version online) bonds in relation to the single, crystallographic D4R structural unit (see Figure 1).
Figure 5. Thermogravimetric data (TGA-DSC) for (a) as-made SiO2AST and (b) as-made AlPO4-AST.
atomic displacement parameters were used for all the atoms, except for the minor carbon components C3 and C4. The refined structure reveals the material to contain two quinuclidinium and two fluoride ions per unit cell, although no hydrogens were
located due to the disorder of the quinuclidinium. Plots of Si-Si interatomic distances and angle as a function of temperature are shown in Figure 4, and Si-O bond distances are recorded in the Supporting Information. Thermogravimetric analysis of the as-made SiO2- and AlPO4AST samples indicates that the breakdown of organic template occurs rather differently in each case, despite the fact that the molecule, as well as the framework topology, is the same in each case (Figure 5). For SiO2-AST, a first sharp mass loss, associated with a strong exothermic signal, occurs at 460 °C. Then, a second mass loss with a small inflection point in the curve is observable at 640 °C, associated with a broad exothermic peak. These features can be attributed to decomposition and loss of the organic species. It must be noted that the mass loss is still occurring even at the highest temperature reached (nearly 950 °C) in the thermogravimetric analysis (TGA) and suggests difficulty in calcining the SiO2 octadecasil. The total experimental mass loss of 10% (Figure 5a) was found to be significantly less than the expected mass loss of 17.9% for the composition of Si20O40 · 2(Q + F-) (Q+ ) quinuclidinium), and this suggests that at this temperature the organics were not completely removed. In contrast, thermogravimetric analysis of the as-made quinuclidinium fluoride AlPO4-AST performed under air flow (Figure 5b) shows excellent agreement between the calculated and the experimental mass loss, especially if we note that a very small mass loss (
