Decomposition Pathways of Hydrotalcite-like Compounds Mg 1-x Al x (OH) 2 (NO 3 ) x ‚nH 2 O as a Continuous Function of Nitrate Anions Z. P. Xu and H. C. Zeng* Department of Chemical and Environmental Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received April 23, 2001. Revised Manuscript Received August 8, 2001 Thermal decomposition pathways of our recently prepared hydrotalcite-like compounds Mg 1-x Al x (OH) 2 (NO 3 ) x ‚nH 2 O in x ) 0.20-0.34 (J. Phys. Chem. B 2001, 105, 1743-1749) have been investigated with XRD, DTA, TGA, FTIR, and combined TGA/FTIR techniques. It has been found that, unlike those in carbonated hydrotalcites, the dehydroxylation and decomposition of anions in low nitrate-content (x) hydrotalcites are separated, while the two processes in the high x samples are overlapped. In line with our recent structural models, the dehydroxylation process in the samples with high x value can be further differentiated into steps, depending on chemical nature of hydroxyl group and nitrate content. The layered structure of these hydrotalcite compounds becomes thermally more stable when more nitrate ions are intercalated. The depletion of nitrate anions (decomposed into NO 2 and O 2 ) in the low x compounds is a continuous process, whereas that in the high x compounds is a discrete one. At 400 °C, most nitrate anions are still retained in the interlayer space with both D 3h and C 2v symmetries, although the dehydroxylation reaction in the low x samples is largely completed. At 500 °C, intercalated nitrate anions are mostly decomposed and the remaining ones are mainly in C 2v symmetry with a standing configuration between two dehydroxylated brucite-like layers. The nearest distance between two oxygen octahedrons changes from 3.088 to 3.040 Å to 2.99-2.97 Å when the hydrotalcite-like phase is topotactically transformed to a rock-salt-like phase. Introduction Hydrotalcite-like compounds (HTlcs), also known as layered double hydroxides (LDHs), have received ex- tensive research in recent years. 1-8 Interests in such materials rest not only in their potential catalytic applications in many chemical reactions but also in many separation, transport, and materials applications, such as anion adsorbents, anion exchangers, scavengers, gene delivery vectors, and medicine and polymer stabilizers. 1-8 In the brucite-like compounds, a divalent metal cation is located in the center of oxygen octahedron constructed by six hydroxyl groups. The resultant octahedrons are connected with one another by edge-sharing to form two- dimensionally infinite layers, which is similar to the basic structure of brucite Mg(OH) 2 . 1 The brucite-like layers can stack upon one another owing to various chemical interactions between the layers. Substitution of trivalent cations for some divalent ones in the brucite layers causes organic or inorganic anions to be inter- calated into the space between brucite-like layers (interlayer space) thus leading to the formation of hy- drotalcite-like structure that is restricted to particular M 2+ /M 3+ combinations as in mineral Mg 6 Al 2 (OH) 16 CO 3 ‚ 4H 2 O. 1b It is well-known that using the coprecipitation method trivalent and divalent cations are very evenly distrib- uted in the brucite-like layers, thermal decomposition of which will lead to the formation of well-mixed multimetal oxide materials. 9 Since physicochemical properties of ultimate calcined HTlcs are greatly influ- enced by the thermal decomposition procedure used, numerous studies of the thermal stability of various carbonated Mg 1-x Al x -CO 3 -HTlcs have been reported. 1,10-20 For example, thermal decomposition of Mg 1-x Al x -CO 3 - * To whom correspondence should be addressed. Tel: +65 874 2896. 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