Layered double hydroxides anionic clays

Depending on the final purpose of the material, the appropriate pillar can be chosen. This flexibifity in the PILC synthesis is one of the main advantages compared to other porous substrates, such as zeofites, which have one definite pore size. The technique not only focuses on clays, but other layered structures serve as host materials as well. Examples are layered double hydroxides (anionic clays), metal(IV)- phosphates and phosphonates,layered silicic acids, etc. [4,5]. 

Anionic clays, such as hydrotalchite, manasseite, stichtite, etc. are layered double hydroxides (Mg/Al Mg/Fe Mg/Cr2 Ni/Al Ni/Fe, etc.). Anionic clays exhibit poor acidic properties. The thermal decomposition of anionic clays gives rise to mixed oxides of industrial importance as catalysts. 

The structure of the layered double hydroxides is the reverse of that of the clays. They are anionic materials in which the sheets are intercalated with anions instead of cations. These ions can be exchanged with several different anions. The cationic nature of the layers lends itself to pillaring by large Keg-gin anions. 

Layered double hydroxides (LDH), also referred to as anionic clays, are very useful materials due to their anion exchange properties. LDH display a layered structure built on a stacking of positive layers ([MII1 MIII (OH)2] +), separated by inter-lamellar domains constituted of anions and water molecules ([X e nH20]x ) [117]. 

The dispersion and solid-state ion exchange of ZnCl2 on to the surface of NaY zeolite by use of microwave irradiation [17] and modification of the surface of active carbon as catalyst support by means of microwave induced treatment have also been reported [18]. The ion-exchange reactions of both cationic (montmorillonites) and anionic clays (layered double hydroxides) were greatly accelerated under conditions of microwave heating compared with other techniques currently available [19.]... 

In all previous cases, V was incorporated in a monomeric form. There are also methods to introduce oligomeric V into inorganic structures. Choudary et al. (48) advocated the use of a montmorillonite, pillared with V oligomers (V-PILC). V-PILC catalyzes the epoxidation of allylic alcohols with i-BuOOH. Oligomeric V is also used to pillar anionic clays such as layered double hydroxides (LDHs) with decavanadate anions (VioOjg) (49). 

New developments in the use of silicates to improve flame retardancy have arisen from the use of synthetic anionic clays that correspond to the family of lamellar mixed metal hydroxides, commonly named layered double hydroxides (LDH) or hydrotalcite-like compounds.17... 

Layered double hydroxides (LDHs), also known as anionic clays and hydro-talcite-like materials, are layered solids that are of increasing interest [5-8]. They consist of stacks of positively charged hydroxide layers with interlayer, charge-balancing, anions and are available as naturally occurring minerals [9] and as synthetic materials. They were prepared in the laboratory in 1942 when Feitknecht reacted dilute aqueous metal salt solutions with base [10, 11], although the first detailed structural analyses of LDHs were not performed until the late 1960s by Allmann and Taylor and their co-workers [12-15]. 

Trihro, F. and Vaccari, A. (1996). Hydrotalcite-like anionic clays (layered double hydroxides) in Comprehensive Supramolecular Chemistry (G. Alberti and T. Bein, Eds.). Pergamon, Oxford. 

Photoisomerization of indolinespirobenzopyran in anionic clay matrices of layered double hydroxides. J. Mater. Chem. 4, 1907. 

Mousty, C., Therias, S., Forano, C. and Besse, J. P. (1994). Anion-exchanging clay-modified electrodes - synthetic layered double hydroxides intercalated with electroactive organic-anions. J. Electro anal. Chem. 374, 63. 

Anionic clays are natural or synthetic lamellar mixed hydroxides with interlayer spaces containing exchangeable anions [10, 104]. The generic terms, layered double hydroxides (LDHs) or hydrotalcites are widely used, the latter because exten-... 

Another important group of catalytic compounds contain the layered double hydroxides (sometimes called anionic clays or hydrotalcites ). Presently, many researchers study these materials because of their adsorptive and catalytic properties. 

Layered double hydroxides, also known as anionic clays, belong to a class of lamellar hydroxides with a structure related to that of brucite. A large number of these hydroxides are naturally available and many more have been synthesized in laboratories. The structure of brucite, Mg(OH)2, consists of sheets of edge-connected Mg(OH)6 octahedra, which are held together by means of Van der Waals forces. The sheets have the composition Mg(OH)2, and are electrically neutral. 

Layered double hydroxides (LDHs) are a unique type of clay with anion exchange capacity. In nature, they are found in the carbonated hydrotalcite-like compounds... 

Unlike the smectites, the layered double hydroxides (LDH) are anionic clays in which brucite-like sheets are intercalated with anions instead of the cations found as charge balancing species in the smectites. The chemical compositions of the layered double hydroxides are provided in Table 5 . 

Hydrotalcites (HT) belongs to the class of anionic clay minerals, also known as layered double hydroxides (LDHs). They are probably one of the best known and used solid basic catalyst [212[. Their properties in catalytic organic reactions have been discussed extensively by Jacobs et al. [256[. More recent aspects have been analyzed briefly also... 

To set up a more environmentally friendly process, it is mandatory to replace the caustic soda by solid bases. Many catalysts such as zeolites [2], alumina [3,4], sepiolite [5] and hydrotalcites [6,7] have been regarded as promising solid basic materials. Hydrotalcites, which are anionic clays, are the subject of this investigation. They are layered double hydroxides with the general formula [M(II)i.xM(III)x(0H)2].[A" x/n].mH20. The host layers are charged positively by replacement of divalent metal cations usually Mg ", Ni and... 

Glucose oxidase Anionic clay layered double hydroxides [Zn3-Al-Cl] Amperometric detection of glucose [39]... 

Hydrotalcites are synthetic basic clays, so-called layered double hydroxides (LDHs) of magnesium and aluminum consisting of Brucite-like layers with an overall positive charge with anions (usually OH or in the interlamellar... 

The layered double hydroxides have been of interest as anion exchangers, basic catalysts, and catalyst precursors. Pillared derivatives of the type discussed earlier for the clays have been synthesized by ion exchange with polyoxometallate anions but have lower thermal stability. Several authors have reported synthesis of layered double hydroxides pillared with the Vio028 ion by direct ion exchange or by techniques that involve preswelling the layer structure with an organic anion. For example, the terephthalate anion was used in the synthesis ofVio028 and Mo7 024 intercalated phases. 

Trifiro, K. and Vaccari, A. (1996). Hydrotalcite-like Anionic Clays (Layered double Hydroxides). In J.L. Atwood, J.E.D. Davies, D.D. MacNicol, F. Vogtie. J-M. Lchn, G. Alberti, T. Bein, (Eds.), Comprehensive Supramolecular Chemistry Solid State Supramolecular Chemistry Two and Three-dimensional Inorganic Networks (VoL7, pp. 251-291). Oxford, UK Pergamon. 

Layered double hydroxides (LDHs) are complementary to smectite clays insofar as the charge on the layers and the gallery ions is reversed that is, the host layers of an LDH are 2D polyhydroxy cations and the gallery species hydrated anions. The compositions of LDHs are represented by the general formula [M j.jM j(OH)2][A"]jjy, z H2O, where A" is the gallery anion, and... 

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