Layered double hydroxides properties

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. 

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]. 

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. 

The selective intercalation of guests into solid hosts offers the potential for application in catalysis and separation science. An excellent case in point is zeolites, which exhibit shape and size selective inclusion properties and are used for an enormous variety of processes [44,45]. Additionally, a munber of layered materials have been reported to possess selective intercalation properties, including layered metal phosphonates [46,47], montmorUlonite [48], magnesium aluminum oxide [49], and layered double hydroxides [50-59]. 

In order to produce high-performance elastomeric materials, the incorporations of different types of nanoparticles such as layered silicates, layered double hydroxides, carbon nanotubes, and nanosilica into the elastomer matrix are now growing areas of rubber research. However, the reflection of the nano effect on the properties and performance can be realized only through a uniform and homogeneous good dispersion of filler particles in the rubber matrix. 

Recently, the cobalt(II)-tetrasulfonatophthalocyanine system was reinvestigated for its catalytic activity while intercalated into a Mg5Al2 -layered double hydroxide. The intercalate exhibited catalytic properties in the activation of atmospheric dioxygen for the oxidation of a thiolate to a disulfide (97a) and for the oxidation of 2,6-di-tert-butylbenzene to (nearly exclusively) the 2,6,2, 6 -tetra-tert-butyldiphe-noquinone (97b). In marked contrast to the results reported for the homogeneous catalyst, this intercalated catalyst remained active for... 

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. 

Shi, L. Li, D., Wang, J., Li, S., Evans, D.G., and Duan, X. 2005. Synthesis, flame retardant and smoke-suppressant properties of a borate-intercalated layered double hydroxide. Clays Clay Miner., 53(3), 294-300. 

Similarly, numerous different nanoparticles, including organomodified clays,3 nanoparticles of silica,4 layered double hydroxides (LDH),5 or polyhedral silsesquioxanes (POSS),6 have been combined with intumescent formulations in polymeric materials to create large synergistic effects (see Chapter 12 for more details) the nanoparticles acting as char reinforcer or char expander that result in differences in terms of FR properties. 

Provided in this chapter is an overview on the fundamentals of polymer nanocomposites, including structure, properties, and surface treatment of the nanoadditives, design of the modifiers, modification of the nanoadditives and structure of modified nanoadditives, synthesis and struc-ture/morphology of the polymer nanocomposites, and the effect of nanoadditives on thermal and fire performance of the matrix polymers and mechanism. Trends for the study of polymer nanocomposites are also provided. This covers all kinds of inorganic nanoadditives, but the primary focus is on clays (particularly on the silicate clays and the layered double hydroxides) and carbon nanotubes. The reader who needs to have more detailed information and/or a better picture about nanoadditives and their influence on the matrix polymers, particularly on the thermal and fire performance, may peruse some key reviews, books, and papers in this area, which are listed at the end of the chapter. 

Manzi-Nshuti, C., Wang, D., Hossenlopp, J. M., and Wilkie, C. A. Aluminum-containing layered double hydroxides The thermal, mechanical, and fire properties of (nano)composites of poly(methyl methacrylate), J. Mater. Chem. (2008), 18, 3091-3102. 

C.X. Zhao, Y. Liu, D.Y. Wang, D.L. Wang, and Y.Z. Wang, Synergistic effect of ammonium polyphosphate and layered double hydroxide on flame retardant properties of poly(vinyl alcohol), Polym. Degrad. Stabil., 2008, 93 1323-1331. 

Chisem, I. C. and Jones, W. (1994). Ion-exchange properties of lithium aluminum layered double hydroxides. J. Mater. Chem. 4, 1737. 

Fudala, A., Palinko, I. and Kiricsi, I. (1999). Preparation and characterization of hybrid organic-inorganic composite materials using the amphoteric property of amino acids amino acid intercalated layered double hydroxide and montmorillonite. Inorg. Chem. 38, 4653. 

Intercalation compounds involving layered double-hydroxides (LDHs), illustrated by the structure shown in Figure 19 (Park et al., 2002), were synthesized to facilitate the measurement of the uniaxial stress (i.e., along the direction perpendicular to the layers), that host layers exert on intercalated species. Below a certain thickness of -A 00 mm, the intercalated structures become more transparent to visible light, so that the information obtained by PL represents the properties of the bulk, not of the surface. Because it is sensitive to the deformation of the intercalants, the PL from samarium complexes was employed to measure the uniaxial stress, by using PL peak positions as a function of pressure (Park et al., 2002 Sapelkin et al., 2000). 

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 are typically synthesized from coprecipitation of mixed metal cations by base titration either with or withont hydrothermal treatment which nsnally enhances crystalline properties." " Becanse contamination with airborne carbonate ions freqnently occurs during synthesis procedures, special caution is needed to prepare carbonate-free LDHs. On the other hand. 

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... 

Leroux. E, Raymundo-Pinero, E., Fedelec. J.-M.. and Beguin, F. 2006. Textural and electrochemical properties of carbon replica obtained from styryl organo-modified layered double hydroxide. Journal of Materials Chemistry 16, 2074-2081. 

Catalytic oxidations by in situ generated peroxotungsten complexes immobilized on layered double hydroxides (LDH) Relation between catalytic properties and peroxotungstate micro-environment... 

Hydrogenations of acetonitrile and pentylnitrile (valeronitrile) in gas and liquid phases respectively were carried out on catalysts obtained from Ni/Mg/Al layered double hydroxides (LDHs) precursors of various Mg/Ni molar ratios. Their catalytic properties were compared with those of a commercial Ni/Al203 catalyst. Selectlvities to primary amines, higher than 90% were obtained on catalysts with Mg/Ni molar ratios in the range 0.3-1. This behaviour was correlated with the acido-basic properties of the solids characterized by TPD of NH3 and microcalorimetry of monoethylamine adsorption. Both studies show that upon Mg addition, the surface acidity, which is responsible for secondary amine formation decreases. 

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