Layered double hydroxides anions

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. 

Anionic complexes can easily be prepared by the sulfonation of the aromatic rings in the complexes. Sulfonated cobalt phthalocyanine intercalated in a layered double hydroxide host was a stable catalyst for the oxidation of thiols162,163 and phenol derivatives.164 It was concluded that the complex has been intercalated with the plane of the phthalocyanine ring perpendicular to the sheet of the host (edge-on orientation) (Fig. 7.2). 

D layered double hydroxides [Mn1 xMIII40H)2][X 7,- nH20] + insulating anion variable [23]... 

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.]... 

Abstract Layered double hydroxides (LDHs) comprise au extensive class of materials that are very easy to synthesize in the laboratory, albeit not always as pure phases. In this chapter, we review the wide variety of methods that are available for the synthesis of LDHs and focus on the way in which the physicochemical properties of the materials (such as phase piuity, crystallinity and surface area) vary with synthesis method. The flexibility of the different methods is also discussed some methods can be used to synthesize LDHs containing a wide range of constituent cations and anions, whilst others are more limited in scope. In some cases, the potential for scale-up of a method to produce larger quantities of material is also noted. 

Fig.1 Idealized structure of a layered double hydroxide with interlayer carbonate anions. Several parameters are defined. Reprinted with permission from [6]. Copyright Elsevier...

A further example of ion-exchange of an organometallic complex onto a layered support has been provided by the anion exchange of a sulfonated ruthenium BINAP complex onto the external surface of layered double hydroxides [119]. Although achvihes and enantioselechvities for the hydrogenation of dimethyl itaconate were comparable to the homogeneous catalyst, and catalyst deactivation was not detected, with geraniol as substrate, catalyst deactivation was unavoidable. 

Jacobs, P.A. (1997) The Immobilization of Sulfonated Ru-EINAP Chloride by Anion Exchange on Layered Double Hydroxides, Studies in Surface Science Catalysis,... 

There are probably several mineral phases, particularly for the highly alkaline systems, that remain to be discovered. Mixed hydroxides may control solubility. Calcium zincate (CaZn2(OH)6), for example, is thermodynamically more stable than Zn(OH)2 above pH 11.5 and may be important in cementitious systems. Another group of minerals is that of the hydrotalcite-like minerals, the layered double hydroxides (LDH, M2+2M3+l/yXy (OH)6 where X is an anion). Cobalt, Ni and Zn can form such minerals (Johnson Glasser 2003) under neutral to alkaline conditions. For the majority of species, however, solubility-limiting phases do not appear to control dissolved concentrations. 

It has been pointed out that these pillared intercalates are intrinsically difficult to synthesize in highly crystalline form because the layered hosts are basic, whereas most heteropolyacids are acidic and tend to decompose. Narita et al. (392) tried direct synthesis of a heteropolyanion-pillared layered double hydroxide by a coprecipitation reaction of Zn2+ and A1J+ ions in the presence of a moderately acidic lacunary Keggin anion, a-SiWn039 XRD of the product showed a basal spacing of 14.6 A, which corresponds to a gallery height of 9.9 A. The surface area was found to be 97 m2 g, which is three times that of the layered double hydroxide. 

Parvulescu et al. noted an interesting change in EG telomer product selectivity upon immobilization of an Pd/TPPTS catalyst on a basic support [58]. In an attempt to address the issues associated with recovery and reuse of the telomerization catalyst, the anionic TPPTS ligand was immobilized on various layered double hydroxides by ion exchange methods (Scheme 11). The use of these catalysts in the telomerization of methanol and ethylene glycol resulted in a remarkable shift in... 

Constantino V. R. L. and Pinnavaia, T. J. Basic properties of Mgj x2+Alx3+ layered double hydroxides intercalated by carbonate, hydroxide, chloride, and sulfate anions, Inorg. Chem., 1995, 34, 883-92. 

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). 

Anion exchange Amberlite IRA 900 Polyvinylpyridinium polymer Layered double hydroxides Sulfonated porphyrins, phthalocyanines Sorokin and Meunier (112), Mirkhani et al. (113) Labat and Meunier (114) Chibwe and Pinnavaia (115)... 

Meyn, M., Beneke, K., and Legaly, G. Anionic-exchange reactions of layered double hydroxides, Inorg. 

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