Thermal Behavior and Decomposition of Intercalated Kaolinite

-Intercalation complexes of a Hungarian kaolinite were prepared with hydrazine and potassium acetate. The thermal behavior and decomposition of the kaolinite-potassium acetate complex was studied by simultaneous TA-EGA, XRD, and FTIR methods. The intercalation complex is stable up to 300"C, and decomposition takes place in two stages after melting of potassium acetate intercalated in the interlayer spaces. Dehydroxylation occurred, in the presence of a molten phase, at a lower temperature than for the pure kaolinite. FTIR studies revealed that there is a sequence of dehydroxylation for the various OH groups of intercalated kaolinite. The reaction mechanism was followed up to 1000"(2 via identification of the gaseous and solid decomposition products formed: H20, CO2, CO, C3H60, intercalated phases with basal spacings of 14.1 /~,, 11.5 /~, and 8.5 /~ as well as elemental carbon, I~H2(CO3)3" 1.5H20, K2CO31.5H20, and KAISiO4. Key Words--Infrared spectroscopy, Intercalation, Kaolinite, Thermal Analysis, X-ray powder diffraction. I N T R O D U C T I O N Clay minerals can interact with organic and inorganic materials by adsorption, intercalation, and cation exchange. The basic principles of intercalation reactions were reported earlier for kaolinite (Lagaly, 1984). The reactive guest molecules enter the interlayer spaces and expand the silicate layers. The reactive molecules were classified by Weiss et al. (1966) as follows: t) compounds forming strong hydrogen bonds to the silicate layers, e.g., urea, formamide, acetamide, and hydrazine; 2) compounds with pronounced betaine-like character with the possibility of strong dipole interactions with the silicate layers, e.g., dimethyl sulphoxide; and 3) alkali salts of short-chain fatty acids, in particular acetic and propionic acids. Non-reacting guest compounds are entrained between the layers by reactive guest molecules. Formation of intercalation compounds has been used as a sensitive method for distinction of different types of kaolinites according to Fernandez-Gonzales et al. (1976), Jackson and Abdel-Kader (1978), and Theng et aL (1984). Range et aL (1969) and Keller and Haenni (1978) showed kaolins to be commonly and intimately intermixed, containing microdomains with different chemical reactivity. It is difficult to detect such mixing by XRD and DTA methods. The maximum degree of reaction is not always 100% even after very long reaction times, as this depends on the type of kaolin, degree of structural order, and particle size (Wiewiora and Brindley, 1969; Weiss et aL, 1963a). The intercalation reactivity appears to be one of the important Copyright 9 1995, The Clay Minerals Society factors that control technical applicability. The aim of the present work is to reveal the intercalation reactivity of a Hungarian kaolin that formed in a sandstone by kaolinitization (probably with a hydrothermal aftereffect), to collect some data about the ordered-disordered structure of this kaolin by intercalation, and to study the thermal behavior and decomposition reactions of the kaolin-potassium acetate complex via the continuous monitoring of the reaction products. Investigations were started by intercalation of hydrazine. It is known from different authors that hydrazine reacts readily with several types of kaolinite, being special among the amines in that the complex formed is particularly strong (van Olphen, 1963; Weiss et aL, 1963b; Thompson, 1985). The intercalation reactivities of the investigated kaolin towards hydrazinc and potassium acetate were compared as well. EXPERIMENTAL METHODS