Langmuir 2000, 16, 7421-7428
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Modification of the Hydroxyl Surface in Potassium-Acetate-Intercalated Kaolinite between 25 and 300 °C Ray L. Frost,*,† Janos Kristof,‡ Eva Mako,§ and J. Theo Kloprogge† Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia, Department of Analytical Chemistry, University of Veszprem, H-8201 Veszprem, P.O. Box 158, Hungary, and Department of Silicate and Materials Engineering, University of Veszprem, P.O. Box 158, H-8201 Veszprem, Hungary Received November 23, 1999. In Final Form: June 12, 2000 Changes in the hydroxyl surfaces of potassium-acetate-intercalated kaolinite have been studied over the ambient to predehydroxylation temperature range using a combination of X-ray diffraction (XRD) and Raman spectroscopy. Upon intercalation, the kaolinite expanded along the c-axis direction to 13.88 Å. When the intercalation complex is heated over the 50 to 300 °C range, X-ray diffraction shows the existence of three additional intercalation phases, with d spacings of 9.09, 9.60, and 11.47 Å. The amount of each phase is temperature-dependent. Thermal analysis shows three phase changes, at 88, 293, and 364 °C. These expansions are reversible, and upon cooling, the intercalation complex returned to its original spacing. The 13.88 Å phase existed only in the presence of water. It is proposed that the expanded kaolinite intercalation phases result from the orientation of the acetate within the intercalation complex. The completely intercalated kaolinite showed a single Raman band at 3607 cm-1 attributed to the innersurface hydroxyl hydrogen bonded to the acetate ion. This band corresponds to the 13.88 Å phase. Mild heating of the intercalated complex at 50 °C caused a rearrangement of the surface structure, with Raman hydroxyl-stretching bands being observed at 3594, 3604, and 3624 cm-1. Further thermal treatment at 100 °C caused these bands to shift to 3599, 3605, and 3624 cm-1. At the predehydroxylation temperature for potassium-acetate-intercalated kaolinite (250 °C), two bands were observed, at 3601 and 3633 cm-1. At this temperature, only the 11.62 Å phase existed, suggesting that the 3601 cm-1 band is related to the 11.62 Å phase. Above this temperature, no hydroxyls are spectroscopically evident. When the complex is cooled to room temperature, the Raman spectra of the hydroxyl surfaces are identical to those of the initial intercalation complex, showing that the thermal modification of the kaolinite surfaces is reversible.
Introduction Kaolinite, Al4[Si4O10](OH)8, is a naturally occurring inorganic polymer with a layer structure consisting of siloxane and gibbsite-like layers. The siloxane layer is composed of SiO4 tetrahedra linked in a hexagonal array. The bases of the tetrahedra are approximately coplanar and the apical oxygen atoms are linked to a second layer containing aluminum ions and OH groups (the gibbsitetype layer). Kaolinite has been described as a nonexpandable clay. However, when treated with aqueous solutions containing molecules such as urea, formamide, or potassium acetate, kaolinite undergoes expansion through the insertion of the molecules between the kaolinite layers. The chemical process of insertion of molecules between clay layers is known as intercalation.1-4 The reactive guest molecules enter the interlayer spaces by chemically modifying the kaolinite internal surfaces.4,5 * To whom correspondence should be addressed (r.frost@ qut.edu.au). † Queensland University of Technology. ‡ Department of Analytical Chemistry, University of Veszprem. § Department of Silicate and Materials Engineering, University of Veszprem. (1) Lagaly, G. Philos. Trans. R. Soc. London, Ser. A 1984, A311, 315. (2) Weiss, W.; Thielepape, H. O. Intercalation into Kaolinite Minerals. In Proceedings of the International Clay Conference; Heller, L., Weiss, A., Eds.; Israel University Press: Jerusalem, 1966; p 277. (3) Costanzo, P. M.; Giese, R. F. Clays Clay Miner. 1986, 34, 105. (4) Costanzo, P. M.; Giese, R. F. Clays Clay Miner. 1990, 38, 160. (5) Churchman, G. J. Clays Clay Miner. 1990, 38, 591.
The types of molecules that can be inserted into the kaolinite layers are many and varied and depend on the method used for intercalation.6-9 Potassium acetate is most useful for intercalating kaolinite and determining the changes to the hydroxyl surface of the kaolinite. Kaolinites are intercalated with potassium acetate with ease, and intercalation often approaches 100%. Changes in the kaolinite hydroxyl surface structure occur as a result of the hydrogen bonding between the acetate ions and the inner surface hydroxyls. Changes are readily followed by X-ray diffraction and vibrational spectroscopy.10-13 Previous studies have shown the appearance of additional bands in both the Raman and the infrared spectra of the potassium-acetateintercalated kaolinite.14,15 The preparation method of the (6) Heller-Kallai, L.; Huard, E.; Prost, R. Clay Miner. 1991, 26, 245. (7) Lapides, I.; Lahav, N.; Michaelian, K. H.; Yariv, S. J. Therm. Anal. 1997, 49, 1423. (8) Lahav, N. Clays Clay Miner. 1990, 38, 219. (9) Frost, R. L.; Kristof, J.; Paroz, G. N.; Tran, T. H. T.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 208, 216. (10) Frost, R. L.; Kristof, J.; Paroz, G. N.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 208, 478. (11) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Colloid Interface Sci. 1999, 214, 109. (12) Frost, R. L.; Kristof, J.; Horvath, E.; Kloprogge, J. T. J. Colloid Interface Sci. 1999, 214, 380. (13) Kristof, J.; Frost, R. L.; Felinger, A.; Mink, J. J. Mol. Struct. 1997, 41, 119. (14) Kristof, J.; Toth, M.; Gabor, M.; Szabo P.; Frost, R. L. J. Therm. Anal. 1997, 49, 1441. (15) Frost, R. L.; Kristof, J. Clays Clay Miner. 1997, 45, 551.
10.1021/la9915318 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/16/2000
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intercalated kaolinite also determined the molecular structure of the intercalation complex, resulting in different infrared and Raman spectra.16,17 Previous work by the authors has shown that additional bands at 3590, 3603, and 3610 cm-1 may be observed upon the intercalation of kaolinite with potassium acetate.11,12 The kinetics of deintercalation of potassium-acetate-intercalated kaolinite have been reported.18 Here deintercalation was found to be rapid, and 80% deintercalation occurred in the first minute.17 Previous studies have shown that the potassium-acetate-intercalated kaolinite dehydroxylates at ∼325 °C, which is 150 °C degrees less than the normal dehydroxylation temperature. A recent study reported the effect of cesium acetate on the modification of kaolinite surfaces.18 No studies of the effect of temperature on the spectroscopy of the potassium-acetate-intercalated kaolinite have been undertaken. In this paper, we report the changes to the hydroxyl surfaces of kaolinite as the potassium acetate intercalation complex is thermally treated up to the dehydroxylation temperature. Experimental Methods Potassium-Acetate-Intercalated Kaolinite. The kaolinite used in this study is the Kiralyhegy kaolinite from Hungary. Samples were analyzed for phase purity using the X-ray diffraction technique before Raman microprobe spectroscopic analysis was performed. The kaolinite is an excellent example of a highly ordered kaolinite. It was used after purification by sedimentation and size fractionation to
