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

RELATIVE HUMIDITY-INDUCED REVERSIBLE HYDRATION OF SULFATEINTERCALATED LAYERED DOUBLE HYDROXIDES

Layered double hydroxides (LDH) are extremely important materials for industrial processes and in the environment, and their physical-chemical behavior depends in large part on their hydration state, but the characterization of these hydration effects on their properties are incomplete. The present study was designed to explore the interpolytype transitions induced by variation in the ambient humidity among LDHs. The cooperative behavior of intercalated water molecules resulted in a sudden, single-step, reversible dehydration of the [Zn-Cr-SO 4 ] LDH. The [Zn-Al-SO 4 ] LDH provided an interesting contrast with (1) the coexistence of the end members of the hydration cycle over the 40À20% relative humidity range during the dehydration cycle, and (2) a random interstratified intermediate in the hydration cycle. These observations showed that the [Zn-Al-SO 4 ] LDH offered sites having a range of hydration enthalpies, whereby, at critical levels of hydration (20À40%), the non-uniform swelling of the structure resulted in an interstratified phase. The variation in domain size during reversible hydration was also responsible for the differences observed in the hydration vs. the dehydration pathways. This behavior was attributed to the distortion in the array of hydroxyl ions which departs from hexagonal symmetry on account of cation ordering as shown by structure refinement by the Rietveld method. This distortion was much less in the [Zn-Cr-SO 4 ] LDH, whereby the nearly hexagonal array of hydroxyl ions offered sites of uniform hydration enthalpy for the intercalated water molecules. In this case, all the water molecules experienced the same force of attraction and dehydrated reversibly in a single step. The changes in basal spacing were also accompanied by interpolytype transitions, involving the rigid translations of the metal hydroxide layers relative to one another.

Layered double hydroxides: A review

Combination of two-dimensional layered materials and intercalation technique offers a new area for developing nanohybrids with desired functionality. Layered double hydroxides (LDHs) are mineral and synthetic materials with positively charged brucite type layers of mixed metal hydroxides. Exchangeable anions located in interlayer spaces compensate for positive charge of brucite type layer. Since most biomolecules are negatively charged, can be incorporated between LDHs. A number of cardiovascular, anti-inflammatory agents are either carboxylic acids or carboxylic derivatives and could be ion exchanged with LDHs to have controlled release. LDHs have technological importance in catalysis, separation technology, medical science and nanocomposite material engineering.

Nitrate-Intercalated Layered Double Hydroxides – Structure Model, Order, and Disorder

Layered double hydroxides with a high layer charge (+0.33 per empirical formula unit) intercalate nitrate ions with the molecular plane of the NO 3 ion inclined at ca. 70°to the metal hydroxide layer, which results in a basal spacing of 8.8 Å. Three different N-O bond lengths are observed and yield C s coordination symmetry. At lower charge (0.165 Յ x Յ 0.2), a basal spacing of 8.0 Å is observed, which indicates that the nitrate ion is intercalated with its molecular plane parallel to the metal hydroxide layer (coordination symmetry D 3h ) in a manner isostructural with carbonate-intercalated layered double hydroxides. Consequently, crystal chemical

Hydration, expansion, structure, and dynamics of layered double hydroxides

Water-vapor sorption isotherms, relative humidity (RH) controlled powder X-ray diffraction (XRD) data, and new and previously published multi-nuclear NMR spectroscopic data for a wide range of layered double hydroxides (LDHs) provide greatly increased understanding of the effects of hydration state on the structure and dynamical behavior of interlayer and surface anions and the factors controlling the expansion behavior of this group of minerals. Li,Al and Mg,Al LDH phases containing SO 4 2-, SeO 4 2-, PO 4 3-, HPO 4 2-, MoO 4 2-, ClO 4 -, SeO 3 2-, CO 3 2-, F -, Cl -, Br -, I -, OH -, and NO 3were examined. The phases studied can be grouped into three types based on basal spacing expansion, water sorption, and interlayer anion dynamics: Type 1, significantly expandable (1.5-3.0 Å); Type 2, slightly expandable (expansion <0.5 Å) and with significant interlayer water exchange; and Type 3, essentially non-expandable (0-0.2 Å) and with little interlayer water exchange. For Type 1, the fully expanded phases have a two-water layer structure, and the phase transition from one layer to two layers as determined by XRD consistently correlates with a significant step in the water sorption isotherm and with changes in the interlayer structure and dynamics as observed by NMR spectroscopy. For Type-2 phases, only one-water layer structures form, and the interlayer anions may undergo dynamical disordering with increasing RH, as observed by NMR. For both Types 1 and 2, the first water layer does not cause significant basal spacing expansion due to occupancy of vacant interstitial sites between the anions by the water molecules. For Type-3 phases, there is little interlayer water sorption because the interlayers are essentially closed due to the small size or planar shape of the anions and their strong electrostatic and hydrogen bonding interaction with the hydroxyl layers. RH has no effect on the structural environments and dynamics of the interlayer anions in this group.