Layered Double Hydroxides LDHs

The present paper will describe the results of adsoi ption experiments toward layered double hydroxides (LDH). 

One problem associated with the peroxotungstate-catalyzed epoxidation system described above is the separation of the catalyst after the completed reaction. To overcome this obstacle, efforts to prepare heterogeneous tungstate catalysts have been conducted. De Vos and coworkers employed W catalysts derived from sodium tungstate and layered double hydroxides (LDH - coprecipitated MgCU, AICI3, and NaOH) for the epoxidation of simple olefins and allyl alcohols with... 

The results obtained by Kuila et al. and Acharya et al. [63,64] from the EVA elastomer blended with lamellar-like Mg-Al layered double hydroxide (LDH) nanoparticles demonstrate that MH nanocrystals possess higher flame-retardant efficiency and mechanical reinforcing effect by comparison with common micrometer grade MH particles. Kar and Bhowmick [65] have developed MgO nanoparticles and have investigated their effect as cure activator for halogenated mbber. The results as shown in Table 4.2 are promising. 

An information science research group devised a new model which could explain information storage in the prebiotic phase of the biogenesis process. They assume that layered double hydroxide (LDH) minerals acted as proto-RNA molecules on the young Earth about 4 billion years ago. This hypothesis relates to Cairns-Smith s genetic takeover thesis, which thus again became the subject of discussion. 

Layered metal hydroxides can be categorized into several classes according to their structure (Figure 13.1). Layered double hydroxides (LDHs), which have a hydrotalcite-like structure [27-34], can be expressed as [M2+1 xM3+x(0H)2] (Am )x/m,nH20,... 

A) Layered double hydroxide (LDH), (B) hydroxy double salt (HDS) and (C) hydrocalumite. 

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]. 

Under microwave heating, the Heck olefinations were achieved in 30-60 min, as opposed to 10-40 h by conventional heating. The recyclable heterogeneous LDH-Pd(0) catalytic system circumvents the need to use expensive and air-sensitive basic phosphines as ligands in the palladium-catalyzed coupling of chloroarenes. This novel Mg-Al layered double-hydroxide (LDH) support in the catalytic system stabilizes the nanopalladium particles and also supplies adequate electron density to the anchored palladium(O) species and facilitates the oxidative addition of the deactivated electron-rich chloroarenes. 

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. 

Abstract This chapter is intended to provide a state-of-the-art review of nanocomposite materials prepared by the assembly of layered double hydroxides (LDH) and polymers, including their synthesis and characterization, and point out their potential applications. 

Iu search for efficieut aud greeuer processes over the past few years various heterogeneous catalysts such as titanium incorporated mesoporous molecular sieves [45,46], Schiff-base complexes supported on zeolite [47] and Zn(II)-Al(III) layered double hydroxide (LDH) [48], oxomolybdenum(VI) complexes supported on MCM-41 and MCM-48 [49], polyoxometallate supported materials [50], Co and Mn-AlPO s [51] etc. have been developed and studied for the catalytic epoxidatiou of a-pinene. Many of these processes suffer from drawbacks and limited applicability due to the poor conversion and also the selectivities. Sacrificial aldehydes are often used as an oxygen acceptor in such processes to achieve reasonable yield and selectivities but, this procedure leads to an increase in the E-factors and decrease in the atom economy [51]. 

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. 

In the example in Figure 2.24, a clay (a layered double hydroxide [LDH]) was intercalated with a transition metal complex (NH4)2MnBr4. The EXAFS data in Figure 2.24(a) shows the Mn K-edge EXAFS of the pure complex, and we see one coordination sphere of four Br atoms at a distance of 2.49 A, corresponding well to the tetrahedral coordination found in the X-ray crystal structure. However, after intercalation, the complex reacts with the layers in the clay, and the coordination changes to distorted octahedral where Mn is now surrounded by four 0 atoms at a distance of 1.92 A and two Br atoms at a distance of 2.25 A. 

SYNTHESIS OF OSMATE EXCHANGED Mg-Al LAYERED DOUBLE HYDROXIDES (LDH-OSO4)... 

Synthesis and characterization of a new Sn-incorporated CoAl-layered double hydroxide (LDH) and catalytic performance of Co-spinel microcrystallites in the partial oxidation of methanol... 

Layered double hydroxides (LDHs), with a hydrotalcite-like structure, are a class of materials which have received considerable attention in the last decade. The structure of LDHs is based on the stacking of metal cation hydroxide (brucite-like) layers, with a positive charge on the layers resulting from the isomorphous substitution of some of the bivalent... 

Tungstate and Molybdate exchanged Layered Double Hydroxides (LDHs) as catalysts for selective oxidation of organics and for bleaching... 

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