Reversible hydration and aqueous exfoliation of the acetate-intercalated layered double hydroxide of Ni and Al: Observation of an ordered interstratified phase

Acetate-intercalated layered double hydroxides (LDHs) of Ni and Al undergo reversible hydration in the solid state in response to the ambient humidity. The LDH with a high layer charge (0.33/formula unit) undergoes facile hydration in a single step, whereas the LDH with a lower layer charge (0.24/formula unit) exhibits an ordered interstratified intermediate, comprising the hydrated and dehydrated layers stacked alternatively. This phase, also known as the staged S-2 phase, coexists with the end members suggesting the existence of a solution-type equilibrium between the S-2 phase and the end members of the hydration cycle. These LDHs also undergo facile aqueous exfoliation into 2-5 nm-thick tactoids with a radial dimension of 0.2-0.5 μm. © 2012 Elsevier Inc. All rights reserved

Exfoliation of Layered Double Hydroxides (LDHs) : A New Route to Mineralize Atmospheric CO2

Layered Double Hydroxide (LDH) of Mg-Al-Acetate (Mg/Al = 3) was prepared by an anion exchange method starting from a nitrate precursor. The high hydration energy of acetate was exploited to exfoliate LDH into nanosheets having a thickness of 2-3 nm. A colloidal suspension of LDH layers was freeze dried to retain the layers in the exfoliated state. Thermal decomposition of exfoliated LDH at 400 °C resulted in the mixed metal oxide having a surface area of 275 m2 g-1 which is higher than pristine LDH oxide (100 m2 g-1). This enhanced surface area of exfoliated LDH is responsible for a large amount of CO2 sorption (1.4 mmol g-1) at room temperature as compared to pristine LDH (0.63 mmol g-1). This enhanced CO2 uptake is equal to some of the porous solids at similar conditions. The combination of aqueous exfoliation and the freeze drying technique has resulted in the successful synthesis of a novel CO2 adsorption sorbent based on LDHs

Synthesis and structure refinement of layered double hydroxides of Co, Mg and Ni with Ga

Homogeneous precipitation by urea hydrolysis results in the formation of highly ordered layered double hydroxides of divalent metal ions (Co, Mg, Ni) and Ga. Structure refinement shows that these carbonate containing layered hydroxides crystallize with rhombohedral symmetry (space group R-3m) in the structure of the 3R1 polytype. An analysis of the structure shows that, coulombic attraction between the layer and interlayer remains invariant in different layered hydroxides, whereas the strength of hydrogen bonding varies. The Ni-Ga LDH has the weakest hydrogen bonding and Co-Ga, the strongest, as reflected by the layer-interlayer oxygen-oxygen distances. The poor polarity of the OH bond in the Ni-Ga hydroxide points to the greater covalency of the (M2+/Mâ²3+)-oxygen bond in this compound as opposed to the Co-Ga hydroxide. These observations are supported by IR spectra. © Indian Academy of Sciences

Synthesis of the benzoate intercalated layered double hydroxide of nickel and aluminum: Application of Mering's rule

Homogeneous precipitation from a mixed metal nitrate solution using benzamide as a hydrolysis agent results in the formation of a benzoate-intercalated layered double hydroxide of nickel and aluminum with a basal spacing of 15.6 à . The alternation in the line widths of the basal reflections has been shown to arise by the random interstratification of a 8.8 à phase with an altered orientation of the benzoate ions. This work is an illustration of the application of Mering's rules to model disorder in layered double hydroxides. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Structure and composition of the layered double hydroxides of Mg and Fe: Implications for anion-exchange reactions

Partial cation exchange of Mg2+ ions with Fe3+ ions employing solid Mg(OH)2 as precursor yielded an ordered layered double hydroxide of Mg and Fe in the presence of carbonate anions. Structure refinement revealed that the compound adopts the polytype structure 3R 1 (space group R\bar 3m, a = 3.108 à , c = 23.08 à ) and does not show any signs of cation order. It crystallizes with a unique cation ratio of Mg/Fe = 4. At this ratio, the compound shows a single sharp absorption in its electronic spectrum at 280 nm. Attempts to prepare the LDH with a higher Fe content resulted in the phase separation of excess Fe into X-ray amorphous binary compounds, the existence of which can be discerned only by the appearance of absorptions at λ > 350 nm, a characteristic of oxide-hydroxides of Fe3+. The nitrate-containing compound also forms with a similarly low Fe content. At this composition, the compound does not exhibit any anion-exchange properties as the nitrate ions intercalated in layered hydroxides of low layer charge are not labile. This explains the paucity of information on anion-exchange reactions of layered double hydroxides of Mg and Fe. UV/Vis spectrophotometry shows that Mg/Fe LDH crystallizes with a fixed Fe content. The structure of the Mg/Fe-CO3 LDH was obtained by Rietveld refinement. The ordered Mg/Fe-CO3 LDH was prepared by cation exchange starting from Mg(OH)2. The UV/Vis absorption spectra show that the LDH composition corresponds to Mg/Fe = 4. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Homogeneous precipitation by formamide hydrolysis: Synthesis, reversible hydration, and aqueous exfoliation of the layered double hydroxide (LDH) of Ni and Al

Homogenous precipitation by formamide hydrolysis results in the formation of a formate-intercalated layered double hydroxide (LDH) of Ni(II) and Al(III). The formate-LDH is sensitive to the atmospheric humidity and reversibly exchanges its intercalated water with atmospheric moisture. The hydration/dehydration cycle is complete within a narrow range of 0-30 relative humidity with significant hysteresis and involves a randomly interstratified intermediate phase. When immersed in water, the formate ion grows its hydration sphere (osmotic swelling), eventually leading to the exfoliation of the metal hydroxide layers into lamellar particles having in-plane dimensions of 100-200 nm and a thickness of 9-12 nm. These nanoplatelets restack to thicker tactoids again upon evaporation of the dispersion. The intercalated formate ion can be exchanged with nitrate ions in solution but not with iodide ions. These observations have implications for many applications of LDHs in the area of carbon dioxide sorption and catalysis. © 2010 American Chemical Society