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.

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a b s t r a c tAcetate-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 mm.

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Partial cation exchange of Mg 2+ ions with Fe 3+ 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 R3m, a = 3.108 Å, c = 23.08 Å) 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 IntroductionThe layered double hydroxide (LDH) of Mg and Fe has potential applications in catalysis, [1] water purification, [2,3] safe storage of vitamins, [4] anion exchange, [5] as well as cation sorption.[6] The oxide residue obtained by the thermal decomposition of the Mg/Fe LDH is a bifunctional catalyst with both acidic and basic surface sites.[7] The Fe 0 /Fe 2+ / Fe 3+ composition of the catalyst can be finely tuned by controlled reduction, whereby catalysts with high olefin selectivity can be generated for use in the Fischer-Tropsch reaction. Given the magnetic properties of the oxides of Fe III , the Mg/Fe LDH is also used as a precursor for the synthesis of single-phase magnetic ferrites [8] and magnetic nanocomposites.[9] Furthermore, the Mg/Fe LDH is less toxic than many combinations of di-and trivalent cations and is proposed as a model medium for drug delivery. [10] The Mg/Fe LDH is obtained by the isomorphous substitution of a fraction x of Mg 2+ ions by Fe 3+ ions in Mg-(OH) 2 to yield positively charged layers with the composi-x+ . [11] Anions are incorporated into the interlayer. Many pairs of di-and trivalent cations can be combined with a variety of anions [12][13][14] to obtain a large and diverse family of layered double hydroxides. The Mg/ Fe LDH in many ways stands apart from other LDHs.(1) There is limited literature on the anion-exchange and -uptake properties of the pristine and calcined Mg/Fe LDH. Evidence for the anion-exchange of Mg/Fe LDHs is 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 Ͼ 350 nm, a characteristic of oxide-hydroxides of Fe 3+ . 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.at best equivocal and there are some definitive reports, [4] including this work, of the absence of anion-exchange reactions in Fe-containing LDHs.(2) In general, the a parameter of LDHs decreases with increasing trivalent metal content.[ (3) If the composition is fixed at x = 0.25, as suggested by some authors, [17] or at x = 0.2, as suggested by others, [5,18] does the Mg/Fe LDH crystallize in a cation-ord...