Hydrotalcite-type anionic clays: Preparation, properties and applications.

Ion-exchange isotherms between hydrotalcite-like (HT) of the NO3-, CI-, and SO4- forms and F , 0% Br , I , OH-, SO42-, CO3 z-, and Naphthol Yellow S (NYS 2-) ions were determined, and the spacing and the width of the 003 reflection were measured as a function of HT composition The ion-exchange equilibrium constant for HTs of monovalent anions are in the sequence OH- > F > Cl > Br- > NO3- > I , those for divalent anions are in the sequence CO3 ~- > NYS z- > SO42- The ion-exchange equilibrium constants tend to increase as the diameters of the anions decrease, and the crystallite size in the 001 direction tends to increase with anions having higher selectivity The OH-form of HT has the smallest basal spacing and the largest crystallite size in the 001 direction

Anion-exchange properties of hydrotalcite-like compounds

https://dr.ntu.edu.sg/bitstream/10356/96770/1/30.%20Application%20of%20Layered%20Double%20Hydroxides%20for%20Removal%20of%20Oxyanions.pdf

Application of layered double hydroxides for removal of oxyanions: A review

This paper describes a systematic study on the synthesis, anion exchange, and delamination of Co-Al layered double hydroxide (LDH), with the aim of achieving fabrication and clarifying the properties of LDH nanosheet/polyanion composite films. Co-Al-CO3 LDH hexagonal platelets of 4 mum in lateral size were synthesized by the urea method under optimized reaction conditions. The as-prepared CO3(2-)-LDH was converted to Cl- -LDH by treating with a NaCl-HCl mixed solution, retaining its high crystallinity and hexagonal platelike morphology. LDHs intercalated with a variety of anions (such as NO3-, ClO4-, acetate, lactate, dodecyl sulfate, and oleate) were further prepared from Cl- -LDH via an anion-exchange process employing corresponding salts. Exchanged products in various anion forms were found to show different delamination behaviors in formamide. Among them, best results were observed for NO3- -LDH in terms of the exfoliating degree and the quality of the exfoliated nanosheets. The delamination gave a pink transparent suspension containing well-defined nanosheets with lateral sizes of up to 2 microm. The resulting nanosheets were assembled layer-by-layer with an anionic polymer, poly(sodium styrene 4-sulfonate) (PSS), onto quartz glass substrates to produce composite films. Magnetic circular dichroism (MCD) measurements revealed that the assembled multilayer films exhibited an interesting magneto-optical response.

Synthesis, anion exchange, and delamination of Co-Al layered double hydroxide: assembly of the exfoliated nanosheet/polyanion composite films and magneto-optical studies.

Layered double hydroxides (LDHs) have been investigated for many years as host materials for a range of anion exchange intercalation reactions. In this role they have been used extensively as ion-exchange materials, catalysts, sorbents and halogen absorbers. More recently, there have been a tremendous number of new developments using these materials to store and deliver biologically active materials in vivo. Significant advances have been made recently on the characterisation of these materials, including structural studies and on the mechanism of intercalation using in situ techniques.

Intercalation chemistry of layered double hydroxides: recent developments and applications

The basic salts of this system were prepared and their structures and physico-chemical properties were studies by electron microscopy, chemical analysis, X-ray powder diffraction, thermal analysis, i.r. absorption spectra, BET absorption, and acidity-basicity measurements. The salts were found to be new compounds analogous to hydrotalcite. They can be expressed by the formula; [M
 2+
 M
 3+
 (OH)2(x+y))]y+ [A
 1
 −
 , Az2- · mH2O]-(z1 + 2z2) where M2+ and M3+ denote di- and trivalent cations, A− and A2− denote mono- and divalent anions, respectively, and y = z1 + 2z2; z1 ≫ z2. The structures consist of positively charged Cd(OH)2-like basic layers and intermediate layers formed from anions and water molecules with the solid solution of divalent cation (M2+) and trivalent cation (M3+) being formed in the range of 0.6 > x/(x + y) > 0.9. The anions of Cl−, NO
 3
 −
 and ClO
 4
 −
 are easily substituted by CO
 3
 2−
 . A large part of the NO
 3
 −
 makes a monodentate-type bond and the ClO
 4
 −
 a bridge-type bond.

The Syntheses of Hydrotalcite-Like Compounds and Their Structures and Physico-Chemical Properties I: The Systems Mg2+-Al3+-NO3-, Mg2+-Al3+-Cl-, Mg2+-Al3+-ClO4-, Ni2+-Al3+-Cl- and Zn2+-Al3+-Cl-

Ahstraet--Hydrotalcite solid solutions were prepared by coprecipitation followed by hydrothermal treatment between 150 ~ and 250~C. Based on the structural formula [Mgl_xAlx(OH)~lX+l(COz)x/2 �9 mH20] x-, pure solid solutions were formed in the range Al/(Al + Mg) = 0.2 to 0.33, where m = (1 - 3x/2). Maximum crystallite size was achieved by hydrothermal treatment between 180 ~ and 200~ with x = 0.337 to 0.429. Crystal strain was also minimized at these values of x. The adsorption c~pacity for Naphthol Yellow S increased as x increased and reached a maximum (1.56 x 10 6 moles/m 2) when x = 0.287, a value eight times larger than that of Mg(OH)~. A weak endothermic DTA peak at about 350~ is probably due to the loss of structural water in the main layer of the structure. On calcination between 400 ~ and 700~ only periclase was detected, probably containing A1 in solid solution. Hydration of the calcined product resulted in the reconstruction of the original hydrotalcite structure.

Physico-chemical properties of synthetic hydrotalcites in relation to composition

Hydrotalcite-like compounds (Mgl- xAlx(OH)2) x+ (A~/~.mH20) x-, where A 2- is SOl- or CrO)- and x ~ 0.25, were prepared by a coprecipitation method and their physico-chemical properties were studied by X-ray diffraction, thermal analysis, i.r. absorption spectra and acidity-basicity measure- ment. The compounds including SO4 z- and CrO4 z- were analogous to an orthorhombic hydrotalcite and the lattice constants, ao and Co were 3.05 and 25.97 A, and 3.05 and 26.48 A, respectively. The crystallite size and strain in the 003 direction were "(27A and 4.77 x 10 -2 for the SO)- system or 83 A and 6.60 x 10 -2 for the CrO42 system, indicating a largely distorted microcrystallite. Two endothermic peaks observed at 240 and 455~ for the SOl- system and at 230 and 460~ for the CrO4 z- system are ascribed to the eliminations of interlayer water (mH20) and structural water, respectively. The compound including SO~- formed MgO by calcination at 500~ and MgSO 4 and MgAI204 by calcina- tion at 800~ while the compound including CrO~- formed MgO at 400~ and MgA12_ xCrxO4 at 800~ I.r. study of the SOl compound indicated that the bonding of SOl- was a bridge type. The highest acid and base strengths of the SO )- compound were H 0 12.2, the acidity and basicity being 0.3 and 0.1 mmol/g, respectively.

Synthesis of Hydrotalcite-Like Compounds and their Physico-Chemical Properties—The Systems Mg2+-Al3+-SO42- and Mg2+-Al3+-CrO42-

OF THE DISCLOSUREA novel magnesium hydroxide of the formula Mg(OH)2 having (i) a strain in the (101) direction of not more than 3.0 x 10-3. (ii) a crystallite size in the (101) direction of more than 800 .ANG., and (iii) a specific surface area of less than 20 m2/g. The magnesium hydroxide, if desired as coated with anionic surfactants, is useful, for example, as fire retardants for thermoplastic synthetic resins and aqueous paints. The novel magnesium hydroxide is prepared by heating in an aqueous medium at an elevated pressure a novel basic magnesium chloride or nitrate of this invention expressed by the formula Mg(OH)2-xA?mH2O wherein A is Cl or NO3. X is a number of more than 0 but less than 0.2, and m is a number of 0 to 6.

Shigeo Miyata

Papers

Anion-exchange properties of hydrotalcite-like compounds

The Syntheses of Hydrotalcite-Like Compounds and Their Structures and Physico-Chemical Properties I: The Systems Mg2+-Al3+-NO3-, Mg2+-Al3+-Cl-, Mg2+-Al3+-ClO4-, Ni2+-Al3+-Cl- and Zn2+-Al3+-Cl-

Physico-chemical properties of synthetic hydrotalcites in relation to composition

Synthesis of Hydrotalcite-Like Compounds and their Physico-Chemical Properties—The Systems Mg2+-Al3+-SO42- and Mg2+-Al3+-CrO42-

Magnesium hydroxides having novel structure, process for production thereof, and resin compositions containing them