Reconstitution effect of Mg/Ni/Al layered double hydroxide
Introduction
Layered double hydroxides (LDH) or hydrotalcite (HT) type materials are widely applied in catalysis, ion-exchange, adsorption and pharmaceutics [1]. The general formula for the LDH materials is [MII1−x MIIIx (OH)2]x+ (Am−x/m]∙nH2O, where M are bivalent or trivalent cations with similar radii, and A is an interlayer anion, usually CO32−. The nature of the layer cations can be changed using a wide range of main group (e.g., Mg, Al) or transition metal (e.g., V, Cr, Mn, Fe, Co, and Ni) cations [2]. All these materials having various compositions share typical common characteristics: layered structure and the formation of mixed-metal oxides. When a LDH is calcined, it progressively loses physicosorbed water, then interlamellar water molecules, and finally water from the dehydroxylation of the layers, along with the charge compensating anions, leading to the collapse of the layered structure. The temperature at which these phenomena occur depends on the chemical composition. However, already above 400 °C a mixed-metal oxide usually is forming [3], [4]. The calcination sometimes could be used as an intermediate treatment to functionalize the clay by intercalation of anions in the interlayer. Such functionalization makes the memory effect especially useful. The memory effect is a unique property by which the oxide is retrotopotactically transformed into the original hydrotalcite structure in aqueous solutions or humid air atmosphere [5]. This reconstitution effect has been applied for the removal of anions [6] and to improve the catalytic properties of the specimens [7]. Detailed reconstitution effect was investigated only for classical Mg/Al systems [8], although other compositions of LDH exhibit the property to reconstruct, but not always recover the original structure [9], [10]. The aim of the present work was to study the influence of a reconstitution medium for the memory effect of the ternary Mg/Ni/Al system. The important task was to investigate the change of the cationic composition of LDH when in reconstitution media the magnesium cation is used.
Section snippets
Experimental
LDH was prepared by the coprecipitation under a low supersaturation method from the solution of the appropriate metal nitrates with a molar ratio of (Mg + Ni):Al = 3:1 and a solution of NaHCO3:NaOH with a molar ratio of 1:2. During the preparation the 15% of the 1 M Mg(NO3)2 solution was replaced by a 1 M Ni(NO3)2 solution. The solution of metal nitrates was added to the solution of NaHCO3 + NaOH (pH ≈ 12) very slowly and under vigorous stirring. After mixing the obtained gel was aged at 353 K for 6 h. The
Results and discussion
The characteristic hydrotalcite type structure of the synthesized sample was confirmed by the XRD analysis data (Fig. 2) [2]. It was previously shown that Mg/Al hydrotalcite decomposes in three steps. Firstly, evaporation of adsorbed water occurs. Secondly, the elimination of the interlayer structural water (up to 523 K) proceeds. Finally, dehydroxylation and decarbonation (up to 873 K) take place [11]. In this study, TG analysis was used to investigate thermal stability of the Mg/Ni/Al sample
Conclusions
There are only few studies where the influence of the third metal cation for the reconstitution process in LDH was analyzed. In this study, Mg/Ni/Al layered double hydroxide was successfully synthesized by the low supersaturation method, thermally decomposed and, for the first time to our knowledge, reconstituted in magnesium nitrate media. The partial substitution of the magnesium by nickel shows changes on the layered double hydroxide behaviour during the cycle: synthesis – thermal
Acknowledgements
The financial support of the company Norta Ltd. is gratefully acknowledged. The authors would like to thank Antonio Eduardo Palomares (Universidad Politécnica de Valencia) for BET measurements and Arturas Baltusnikas (Kaunas University of Technology) for helpful discussions and technical assistance.
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2013, Applied Clay ScienceCitation Excerpt :Many techniques can be used to modify the structural and surface properties of LDHs to improve their potential industrial applications. The treatment methods include: (a) calcination at elevated temperatures (Basile et al., 2001; Fleutot et al., 2012; Spratt et al., 2008); (b) intercalation by inorganic and organic species (Costa et al., 2009; Kovanda et al., 2010; Zhang et al., 2010); (c) delamination and restacking (Ma and Sasaki, 2010; Panda et al., 2008; San Román et al., 2008); (d) grafting (Tao et al., 2009); (e) structure reconstruction (Klemkaite et al., 2011; Mokhtar et al., 2010); and (f) amorphization by mechanochemical treatment. Various treatment processes result in different structural and microstructural evolution behaviors of LDHs.