Layered double hydroxides metal hydroxides

Zinc hydroxy double salts are layered materials similar to layered double hydroxides which show intercrystalline reactivity and incorporate organic compounds between layers.337 Hydroxy double salts of high crystallinity can be obtained by reacting ZnO with organic metal salts in water. Zinc oxide crystals could then be prepared by thermal treatment of hydroxy zinc acetate.338... 

Similar methods of encapsulation are also observed in pillared clays, which were also introduced as catalysts as long ago as the early 1980s. The field has been thoroughly reviewed up to 2000 [65], Layered double hydroxide structures have also been used for the entrapment of metal coordination compounds [66],... 

Hydrotalcitelike compounds (HTs) are layered double hydroxides with the general formula Mm2 Mm3 (OH)2(j Ax m/r V H20, where M2 and M3 are divalent and trivalent metal cations, re-... 

Hydrotalcites are layered double hydroxides with the general formula Mg6Al2(0H)i6[C03].4H20. Loading these compounds with potassium carbonate strongly increases their C02 uptake [25, 35], Notably, the hydrotalcite structure already breaks down below 400 °C [26] into a mixed metal oxide. 

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]. 

LDH LEU LIBD LAW LET LILW LIP LLNL LLW LMA LMFBR LOI LREE L/S LTA LWR Layered double hydroxide Low enriched uranium Laser-induced breakdown detection Low-activity waste Linear energy transfer Low- and intermediate-level nuclear waste Lead-iron phosphate Lawrence Livermore National Laboratory Low-level nuclear waste Law of mass action Liquid-metal-cooled fast-breeder reactor Loss on ignition Light rare earth elements (La-Sm) Liquid-to-solid ratio (leachates) Low-temperature ashing Light water reactor... 

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. 

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... 

As mentioned previously, the role of the metal is not specific. Even transition metal free layered double hydroxides are suitable catalysts for the olefin epoxidation with O2 and a sacrificial aldehyde (205). Leaching of the metal from the solid catalyst is a serious problem since organic acids that are potential metal ligands accumulate during the reaction. Leaching occurs, for example, with the polybenzimidazole-supported Ni2+ catalyst (199). 

The 0-d nanoparticles can be nano-metal oxides (such as silica,1 titania,2 alumina3), nano-metal carbide,4 and polyhedral oligomeric silsesquioxanes (POSS),5 to name just a few the 1-d nanofibers can be carbon nanofiber,6 and carbon nanotubes (CNT),7 which could be single-wall CNTs (SWCNT) or multiwall CNTs (MWCNT) etc. the 2-d nano-layers include, but are not limited to, layered silicates,8 layered double hydroxides (LDH),9 layered zirconium phosphate,10 and layered titanates,11 etc. 3-d nano-networks are rarely used and thus examples are not provided here. 

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]. 

Rives, V. and Ulibarri, M. A. (1999). Layered double hydroxides (LDH) intercalated with metal coordination compounds and oxometalates. Coord. Chem. Rev. 181, 61. 

The anionic polyoxometalates comprise a much larger class of metal oxide-based pillaring agents (18). It was proposed that hydrotaldtes (also referred to as layered double hydroxides, or LDHs) would be suitable host materials for pillaring with polyoxometalates. Subsequently, a route for synthesizing isopol3rmetalate-pillared hydrotalcites via organic-anion-pillared precursors was developed (19-20). 

Complex precipitates, such as layered double hydroxides, are frequently prepared as precursors of oxide catalysts. Hydrotalcite is a hydrated hydroxy-carbonate mineral with the formula Mg,sAl2(0H)i6C03-4H20. It represents the better known and most popular member of a family of layered double metal hydroxide com-... 

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. 

The relationship between electrochemical items and materials science can be grouped according to three main aspects as shown in Figure 1.1. It should be noted that electrosynthetic methods allow for preparing a variety of materials, from porous oxide layers in metal anodes, to MOFs (Mueller et al., 2006), layered double hydroxides (LDHs Yarger et al., 2008), and porous carbons (Kavan et al., 2004). Furthermore, porous materials can be modified, functionalized, or hybridized (yide infra) via electrochemically assisted procedures, thus resulting in the preparation of novel materials. 

Nickel batteries use P-Ni(OH)2 as electrode material. This material converts to P-NiOOH during the charging process and this rearranges to y-NiOOH when it is overcharged. This last process is accompanied by a significant expansion, because of the difference in density between P-NiOOH and y-NiOOH, which may result in poor electric contact between the current collector and P-Ni(OH)2/p-NiOOH, with concomitant decrease in the discharge capacity of the battery. Among others, layered double hydroxides of Ni and other metals, often termed stabilized a-Ni(()H), or doped Ni(0H)2, have been tested as electrode materials (Bernard et al., 1996). The effect of the interlayer anions on the electrochemical performance of layered double hydroxide electrode materials has been recently studied by Lei et al. (2008) (see Chapter 6). 

Immobilization of a sulfonated chiral manganese-salen catalyst on a fimctio-nalized Merrifield resin yielded a remarkably active epoxidation catalyst [42]. Its activity and enantioselectivity was examined by epoxidation of 6-cyanochromene, indene, styrene, 4-methylstyrene, and trans-stilbene using m-CPBA/NMO and quantitative yields were obtained in less than 5 min. Enantioselectivities were between 33% (4-methylstyrene) and 96% ee (6-cyanochromene). The same complex was also supported on silica and a layered double hydroxide (LDEI) and the catalytic performances of the systems were compared. Recycling experiments were carried out and the silica-based system showed metal leaching combined with a significant decrease in yield and ee. The layered double hydroxide- and resin-catalysts exhibited a slight decrease in activity and constant ee values in five consecutive reactions. 

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... 

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