Abstract
Hydrotalcites have many important applications in catalysis, wastewater treatment, gene delivery and polymer stabilization, all depending on preparation history and treatment scenarios. In catalysis and polymer stabilization, thermal decomposition is of great importance. Hydrotalcites form easily with atmospheric carbon dioxide and often interfere with the study of other anion containing systems, particularly if formed at room temperature. The dehydroxylation and decomposition of carbonate occurs simultaneously, making it difficult to distinguish the dehydroxylation mechanisms directly. To date, the majority of work on understanding the decomposition mechanism has utilized hydrotalcite precipitated at room temperature. In this study, evolved gas analysis combined with thermal analysis has been used to show that CO2 contamination is problematic in materials being formed at RT that are poorly crystalline. This has led to some dispute as to the nature of the dehydroxylation mechanism. In this paper, data for the thermal decomposition of the chloride form of hydrotalcite are reported. In addition, carbonate-free hydrotalcites have been synthesized with different charge densities and at different growth temperatures. This combination of parameters has allowed a better understanding of the mechanism of dehydroxylation and the role that isomorphous substitution plays in these mechanisms to be delineated. In addition, the effect of anion type on thermal stability is also reported. A stepwise dehydroxylation model is proposed that is mediated by the level of aluminum substitution.
Highlights
Hydrotalcite (HT) is a layered double hydroxide, LDH
In most commercially available hydrotalcites and materials reported in the literature, the compounds tend to be of a formula [MII1−xMIIIx * (OH)2]An−, where x is close to 0.3 and the anion A is carbonate
The ability to exfoliate plate-like materials is affected by the charge density, and the hydrotalcites in this study have been further synthesized with nominal charge densities of
Summary
Hydrotalcite (HT) is a layered double hydroxide, LDH. LDHs are an interesting class of anionic layered minerals that can readily be intercalated with a large variety of anionic materials, including large biomolecules, such as DNA. In most past studies on the thermal decomposition of HTs, a complicating factor has been the simultaneous dehydroxylation and anion decomposition of the interlayers [8,9,10,11,12]. This results in overlapping of decomposition curves from thermal analysis and makes it difficult to determine the contributions due to dehydroxylation and anion decomposition This is acute in the carbonate form of HTs. The situation is further exacerbated by the fact that most of the studies have utilized HTs that have been prepared at room temperature. In order to circumvent this problem, the thermal decomposition of hydrotalcites containing chromate and molybdate anions were studied [13]. The proposed mechanism is strongly supported by the level of aluminum substitution in the lattice and similar results for the decomposition of Gibbsite, where the formation of intermediate aluminum oxyhydroxides are reported [15]
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