188 Chem. Mater. 2011, 23, 188–199 DOI:10.1021/cm102648n
Effect of Temperature on the Local Structure of Kaolinite Intercalated with Potassium Acetate Claire E. White,† John L. Provis,*,† Laura E. Gordon,† Daniel P. Riley,‡ Thomas Proffen,§ and Jannie S. J. van Deventer† †
Department of Chemical and Biomolecular Engineering, and ‡Department of Mechanical Engineering, University of Melbourne, Victoria 3010, Australia, and §Lujan Neutron Scattering Center, Los Alamos National Laboratory, New Mexico 87545, United States Received September 14, 2010. Revised Manuscript Received November 23, 2010
Kaolinite intercalated with potassium acetate is of great interest in the areas of environmental remediation and industrial application; however, its exact atomic structure and the changes which occur when heated have remained largely elusive. Here, neutron pair distribution function analysis is used to investigate the local structural characteristics of this complex material, revealing that hydrated potassium acetate exists as a single layer in the interlamellar spacing of kaolinite. Furthermore, the potassium ions within the intercalated complex are most likely associated with the resonance structure of the acetate molecules, and upon heating (and decomposition of the carbon containing molecules), these ions become strongly associated with the negative charge located on the oxygen atoms in the alumina layers of dehydroxylated kaolinite. Several possible orientations of hydrated potassium acetate within the interlamellar spacing of kaolinite have been proposed and investigated using density functional modeling, revealing the complex nature of this material. Nevertheless, this investigation has shown that the dehydroxylated form of the intercalated compound contains highly strained alumina and available alkali (potassium), making it a viable alternative to traditional aluminosilicates. Introduction The intercalation of different compounds within kaolinite (crystalline layered aluminosilicate) is an area of significant interest for various industrial processes and types of environmental remediation. In such intercalated complexes, the guest molecules enter the interlamellar spacing by breaking the hydrogen bond network between the oxide layers, thus expanding the characteristic interlayer distance of kaolinite.1-4 Many types of compounds are used to form these intercalated complexes, including potassium acetate, urea, and hydrazine.5 Potassium acetate (C2H3O2K) has been shown to readily intercalate within the kaolinite structure, often approaching 100% intercalation (calculated by comparing areas of the X-ray diffraction reflections for the intercalate and nonintercalate basal reflections).6 However, the exact structure of this intercalated complex (abbreviated KA-Kao here) remains largely unknown due to the disordered nature of hydrated potassium acetate within kaolinite. *Corresponding author. E-mail:
[email protected]. Phone: þ61 3 8344 8755. Fax: þ61 3 8344 4153.
(1) Wada, K. Am. Mineral. 1961, 46, 78–91. (2) Weiss, A. Angew. Chem. 1961, 73, 736. (3) Maxwell, C. B.; Malla, P. B. “Kaolin-potassium acetate intercalation complex and process of forming same.” U.S. Patent 5672555, 1997. (4) Smith, D. L.; Milford, M. H.; Zuckerman, J. J. Science 1966, 153, 741–743. (5) Lagaly, G. Philos. Trans. R. Soc., London, A 1984, 311, 315–332. (6) Frost, R.; Krist of, J.; Mako, E.; Kloprogge, J. T. Langmuir 2000, 16, 7421–7428.
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Previous investigations into the KA-Kao complex have used the experimental techniques of X-ray diffraction, vibrational spectroscopy, thermal analysis, and mass spectrometry.6-14 From this range of experimental techniques, structural models for the KA-Kao complex have been proposed. From vibrational spectroscopy, G abor et al.7 reported that a new absorption band (3604 cm-1) seen for the complex at 25 °C is proof of the presence of the intercalated compound and that the strongly negatively charged oxygens of the acetate anions should form hydrogen bonds with the inner surface hydroxyls of the kaolinite. It was also reported that, upon heating, the hydrogen-bonded OH groups disappear first (by 350 °C) and that there is a specific sequence of dehydroxylation among the various OH groups present. Frost et al.8 also assigned this band (which they observed at 3605 cm-1) to the inner surface OH groups strongly hydrogen bonded to (7) Gabor, M.; Toth, M.; Krist of, J.; Komaromi-Hiller, G. Clays Clay Miner. 1995, 43, 223–228. (8) Frost, R. L.; Krist of, J.; Horvath, E.; Kloprogge, J. T. J. Colloid Interface Sci. 1999, 214, 109–117. (9) Krist of, J.; Frost, R. L.; Horvath, E.; Kocsis, L.; Inczedy, J. J. Therm. Anal. Calorim. 1998, 53, 467–475. (10) Frost, R. L.; Krist of, J.; Horvath, E.; Kloprogge, J. T. J. Raman Spectrosc. 2001, 32, 271–277. (11) Krist of, J.; Toth, M.; Gabor, M.; Szabo, P.; Frost, R. J. Therm. Anal. 1997, 49, 1441–1448. (12) Frost, R. L.; Krist of, J.; Paroz, G. N.; Tran, T. H.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 204, 227–236. (13) Frost, R. L.; Locos, O. B.; Krist of, J.; Kloprogge, J. T. Vib. Spectrosc. 2001, 26, 33–42. (14) Frost, R. L.; Krist of, J.; Schmidt, J. M.; Kloprogge, J. T. Spectrochim. Acta Part A 2001, 57, 603–609.
Published on Web 12/21/2010
r 2010 American Chemical Society
Article
Figure 1. Models suggested by Frost et al.6 for kaolinite intercalated with potassium acetate, from the Raman spectra of the hydroxyl-stretching region of the inner-surface hydroxyls hydrogen bonded to the acetate, correlated with the basal reflections seen in X-ray diffraction results.
acetate. They also reported, using DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy, bands at 909 and 897 cm-1 which were attributed to inner surface hydroxyl groups in kaolinite hydrogen-bonded to water or acetate groups. From X-ray diffraction, G abor et al.7 reported a 14.1 A˚ reflection due to the basal plane of the KA-Kao complex and that after removal of absorbed water there is a partial collapse of the structure resulting in new basal plane spacings of 11.5 and 8.5 A˚, along with the initial 14.1 A˚ plane. Upon exposure to the atmosphere, the new basal planes of the heated complex gradually disappear, restoring the 14.1 A˚ via uptake of water by the interlayer potassium acetate. Similar X-ray diffraction results were seen by Krist of et al.11 and Frost et al.6 However, the paper of Frost et al.6 was the only investigation to suggest detailed atomistic models for the KA-Kao complex (Figure 1). These models were based on results from Raman spectra and correlated with the 001 reflections seen in X-ray diffraction. Frost et al.6 showed that removal of water upon heating led to reorientation of the intercalate (up to 300 °C, which is the melting point of potassium acetate7), as in Figure 1b,c. As it stands, these models in Figure 1 are the only detailed structures proposed in the literature. However, they do not provide full details regarding the correlations between acetate molecules and positioning of potassium cations within the structure (if present), other than to suggest that the potassium ions might fit into the ditrigonal cavities of the siloxane layer of kaolinite. Furthermore, there is no information regarding the interactions between the silica surface of kaolinite and the acetate molecules. A recent investigation by Mak o et al.15 used atomistic Monte Carlo simulations to study two structural configurations of the KA-Kao complex, corresponding to the ∼11 and ∼14 A˚ basal spacings. The structural configuration corresponding to the 11 A˚ spacing consisted of potassium acetate (anhydrous) in a single layer, oriented as (15) Mak o, E.; Rutkai, G.; Krist of, T. J. Colloid Interface Sci. 2010, 349, 442–445.
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shown in Figure 1c. The second configuration, the phase with a basal spacing at ∼14 A˚, was reported to consist of a double layer of potassium acetate with the same orientation as shown in Figure 1c. Furthermore, this phase was proposed to be anhydrous, contrary to previous investigations on this material. As will be discussed later in this paper, this structural configuration is implausible due to the lack of water, which makes this model inconsistent with thermal analysis results. It was mentioned that a small amount of water was trialed during the simulation, and it did not affect the result obtained. However, the presence of only a small amount of water still does not correlate with thermal analysis results, which point toward water forming an integral component of this structure. Also of significant interest regarding the KA-Kao complex is its thermal decomposition and the subsequent formation of a highly reactive aluminosilicate. Thermal analysis of the KA-Kao complex has been carried out, revealing that the onset of dehydroxylation occurs at a lower temperature than for nonintercalated kaolinite (∼310 °C compared with ∼450 °C for kaolinite).6 Gabor et al.7 reported that the decomposition of the KA-Kao complex begins only after melting of the potassium acetate; hence, the decomposition occurs in the presence of a molten phase. However, there are discrepancies in the literature regarding the nature of dehydroxylation. It was reported by Gabor et al.7 that there is a sequence of dehydroxylation in the KA-Kao complex, with the hydrogen-bonded OH groups of kaolinite disappearing first (by 350 °C), followed by all the outer hydroxyls by 400 °C, with dehydroxylation complete by 500 °C. Conversely, Krist of et al.9 reported that the KA-Kao complex decomposes in two steps: the first step at 376 °C and, then, a slow process over a wide temperature range between 400 and 550 °C. It was also stated that early dehydroxylation is due to the removal of inner surface hydroxyls which are hydrogenbonded to the intercalating acetate ions, while the second step is due to the removal of hydroxyls which are not hydrogen-bonded to the acetate. More recent work by some of the coauthors of that paper, namely, Frost et al.,6 reported a slightly different version of decomposition/ dehydroxylation, which starts with the first decomposition step at 364 °C, where dehydroxylation of the KAKao complex takes place, along with the liberation of a small amount of acetate decomposition products (as determined by Krist of et al.11). The second stage, at 438 °C, is mainly the decomposition of acetate, with only a small amount of water liberated from the nonintercalated kaolinite (since full intercalation is not achieved, and therefore, some nonintercalated kaolinite is present). When the temperature is increased further, solid state reactions take place between the dehydroxylated kaolinite and the potassium carbonate obtained as a decomposition product of the intercalating potassium acetate.6 Therefore, given the different interpretations of results in the literature regarding thermal decomposition of the KA-Kao complex and the lack of information regarding
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the atomic structural changes occurring during this process, pair distribution function (PDF) analysis is used here: (i) to elucidate the structure of the KA-Kao complex and (ii) to determine the local structural changes occurring due to heating. PDF analysis is a technique used to probe the structure of disordered, amorphous, and nanocrystalline materials,16 including aluminosilicates such as zeolites17 and geopolymers.18,19 This technique utilizes the sine Fourier transform of the total scattering function, which provides direct information about the local structural correlations.16 Neutron PDF analysis has been used in this investigation to study the KA-Kao complex and the effects of temperature (up to 750 °C) on the local structure of this material, providing novel structural insight regarding the formation of a highly disordered aluminosilicate. Materials and Methods KGa-1b kaolinite (Source Clays Repository, Columbia MO)20 was mixed with solid potassium acetate (Sigma-Aldrich, ReagentPlus) at a mass ratio of 1:1.75 (kaolinite/potassium acetate) using a mortar and pestle. This mixed powder was then added to a saturated solution of potassium acetate in D2O (99.8% enriched in D), at a solution/kaolinite ratio of 10 mL/g. The resulting mixture was shaken by hand for several minutes and then left to stand for 2 days. After centrifugation, the solids were redispersed in potassium acetate-D2O saturated solution and then dried at 200 °C for 1 hour in an argon environment in a tube furnace. Deuteration was used to induce the exchange of hydrogen for deuterium in kaolinite, in order to reduce incoherent scattering from hydrogen during neutron diffraction measurements. Once dried, the intercalated complex was rinsed of excess potassium acetate by being redispersed in D2O for 30 min and again dried at 200 °C for an hour. This rinsing treatment was repeated three times to provide the highest possible level of deuteration, with all processing being carried out in an argon environment. This final intercalated material is referred to as the KA-Kao complex throughout this investigation. Dehydroxylated samples were obtained by heating the KAKao complex to 450, 500, 550, 600, 650, and 750 °C at a rate of 10 °C/min in a tube furnace under an argon environment, followed by cooling under the same conditions. High-resolution time-of-flight neutron powder total scattering was carried out on the NPDF beamline at the Lujan Neutron Scattering Center, Los Alamos National Laboratory.21 Samples were measured in standard vanadium cans in a Displex cryostat at 15 K. Standard data reduction was performed using the PDFgetN software,22 including background subtractions to remove incoherent (16) Egami, T.; Billinge, S. J. L. Underneath the Bragg Peaks: Structural Analysis of Complex Materials; Pergamon: Elmsford, NY, 2003. (17) Martı´ nez-I~ nesta, M. M.; Peral, I.; Proffen, T.; Lobo, R. F. Microporous Mesoporous Mater. 2005, 77, 55–66. (18) Bell, J. L.; Sarin, P.; Driemeyer, P. E.; Haggerty, R. P.; Chupas, P. J.; Kriven, W. M. J. Mater. Chem. 2008, 18, 5974–5981. (19) White, C. E.; Provis, J. L.; Proffen, T.; van Deventer, J. S. J. J. Am. Ceram. Soc. 2010, 93, 3486–3492. (20) Pruett, R. J.; Webb, H. L. Clays Clay Miner. 1993, 41, 514–519. (21) Proffen, T.; Egami, T.; Billinge, S. J. L.; Cheetham, A. K.; Louca, D.; Parise, J. B. Appl. Phys. A: Mater. Sci. Process. 2002, 74, S163– S165. (22) Peterson, P. F.; Gutmann, M.; Proffen, T.; Billinge, S. J. L. J. Appl. Crystallogr. 2000, 33, 1192. (23) Page, K.; White, C. E.; Estell, E. G.; Llobet, A.; Proffen, T. J. Appl. Crystallogr. 2010, submitted.
White et al. scattering.23 For all samples, a Qmax value of 24 A˚-1 was used. Low Q extrapolations were applied to each total scattering function, as discussed by Egami and Billinge.16 Comparisons are made in this investigation with nonintercalated kaolinite at various stages during dehydroxylation, as presented in our previous work.24 In this current investigation, a Qmax value of 30 A˚-1 has been used for the more crystalline samples (unheated, 450 and 500 °C), and a Qmax of 24 A˚-1 has been used for all other samples (550, 600, 650, and 750 °C). The Qmax values employed for the nonintercalated samples in this study are different from those used in our previous investigation,24 so that the same Qmax value is used when comparing intercalated and nonintercalated PDFs in the Results and Discussion section in this study to ensure nonsample dependent differences are minimized. The unheated kaolinite sample was measured at the same temperature (15 K) on the HIPD instrument, Lujan Neutron Scattering Center, Los Alamos National Laboratory. Throughout this investigation, the intercalated complex samples are referred to with a prefix of “D_” followed by the temperature at which the samples were thermally treated. Likewise, nonintercalated kaolinite samples are referred to with a prefix of “N_” followed by the ex situ calcination temperature. The reference intercalated kaolinite complex is denoted D_200 as the synthesis procedure involved heating the material to 200 °C. High-resolution powder diffraction data for nonintercalated kaolinite were obtained at the Australian Synchrotron, using the Powder Diffraction beamline.25 The sample was measured in a glass capillary at 80 K with a wavelength of 0.82631 A˚. Highresolution powder diffraction data for the D_200 sample were collected on beamline 11-BM at the Advanced Photon Source.26 The sample was measured in a Kapton capillary at 100 K using a wavelength of 0.458859 A˚. Simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) measurements were performed on the D_200 sample using a Perkin-Elmer Diamond DTA/ TGA in an alumina crucible. The sample was heated from 30 to 1000 °C at a heating rate of 10 °C/min in nitrogen. Density functional modeling was carried out on several proposed structural configurations using the generalized gradient functional BLYP as implemented in the DMol3 v4.4 software,27,28 employing a Quad-core desktop workstation. The initial periodic kaolinite unit cell, as determined in our previous work,29 was expanded along the c-axis to match the basal reflection expansion seen in the Results and Discussion of this investigation (14.05 A˚). Hence, the unit cell dimensions were a = 5.149 A˚, b = 8.934 A˚, c = 14.470 A˚, R = 91.930°, β = 105.042°, and γ = 89.791°. All modeling was carried out using periodic boundary conditions with the dimensions fixed to these values. The numerical basis set used was double numerical (two atomic orbitals for each occupied atomic orbital) plus a polarization p-function on all hydrogen atoms (DNP) to account for hydrogen bonding.28 No pseudopotentials or effective core potentials were used. For Models 1 and 2, convergence thresholds were set (24) White, C. E.; Provis, J. L.; Proffen, T.; Riley, D. P.; van Deventer, J. S. J. J. Phys. Chem. A 2010, 114, 4988–4996. (25) Wallwork, K. S.; Kennedy, B. J.; Wang, D. AIP Conf. Proc. 2007, 879, 879–882. (26) Wang, J.; Toby, B. H.; Lee, P. L.; Ribaud, L.; Antao, S. M.; Kurtz, C.; Ramanathan, M.; Von Dreele, R. B.; Beno, M. A. Rev. Sci. Instrum. 2008, 79, 085105. (27) Delley, B. J. Chem. Phys. 1990, 92, 508–517. (28) Delley, B. J. Chem. Phys. 2000, 113, 7756–7764. (29) White, C. E.; Provis, J. L.; Riley, D. P.; Kearley, G. J.; van Deventer, J. S. J. J. Phys. Chem. B 2009, 113, 6756–6765.
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Figure 2. High-resolution X-ray diffraction of nonintercalated kaolinite and the KA-Kao complex (D_200). (a) Full d-spacing ranges measured, with the basal reflections marked. The largest d-spacing reflection for kaolinite is at 7.14 A˚; hence, data at larger d-spacings were not obtained for this sample. (/) indicates the 002 reflection of the 14.05 A˚ phase. (b) The range 1.5 e d e 4.6 A˚, with important reflections marked. Peaks marked with (†) are reflections characteristic of the 14.05 A˚ intercalated complex. (A) denotes the reflection of the anatase impurity in the source kaolinite.
at 1 10-4 hartree for energy, 0.02 hartree/A˚ for maximum force, and 0.05 A˚ for maximum displacement. An SCF (SelfConsistent Field) convergence of 10-4 hartree was used, along with 3 2 1 k-point sampling. For Models 3 and 4, convergence thresholds were set at 110-5 hartree for energy, 0.002 hartree/A˚ for maximum force, and 0.005 A˚ for maximum displacement. An SCF convergence of 10-6 hartree was used, along with 5 3 2 k-point sampling. A lower level of convergence was used for Models 1 and 2, as initial computations at the higher level did not converge.
Results and Discussion High-Resolution X-ray Diffraction. The high-resolution X-ray diffraction (XRD) patterns of nonintercalated kaolinite and the KA-Kao complex (D_200) are shown in Figure 2, scaled to normalize the intensities of the 020 reflection of kaolinite at 4.46 A˚ in the two samples. Figure 2a shows that the intercalation of kaolinite with potassium acetate expands the structure along the c-axis, resulting in the dominant 001 reflection appearing at 14.05 A˚, in agreement with previous X-ray diffraction
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investigations on the hydrated KA-Kao complex.6,7,11 Hence, even though the material in this investigation was synthesized in an inert environment and calcined to 200 °C during the synthesis process, water molecules (either hydrogenated or deuterated) rehydrated the KA-Kao complex some time prior to measurement, resulting in the dominant 14.05 A˚ reflection. Apart from this dominant phase, there exist reflections from additional phases at 8.59 and 9.93 A˚ (as marked in Figure 2a). The d-spacings of these reflections do vary slightly from the previous reports in the literature; however, it has been shown that these reflections depend greatly on the method and environment of synthesis.6 Furthermore, a recent investigation by Mak o et al.15 reported that the reflection seen ˚ at ∼8.9 A in some studies (but not here) is due to anhydrous crystalline potassium acetate. It should be noted that D2O has been used during synthesis of the intercalated complex here instead of H2O, and there may, therefore, be slight differences between the material presented in this investigation and that studied by Frost et al,6 as it has been shown previously that deuteration changes the hydrogen-bonding geometry in intercalated kaolinite.30 Also noticeable in Figure 2a is the presence of substantial preferred orientation in the c-axis reflections for kaolinite and associated intercalated phases. It is almost inevitable that, during filling of capillaries for X-ray studies, kaolinite and intercalated phases will align along the c-axis due to the platelike particle shapes. However, as has been reported in our previous investigations,24,31 preferred orientation is not apparent in neutron scattering studies due to the larger sample environments used (>10 mm in diameter, compared to
