Layered double oxide

Memory effect The partial dehydroxylation of LDHs under moderate calcination leads to amorphous mixed oxides usually named layered double oxides (LDOs), which can be regenerated back to the original structure by contacting with solutions containing various anions. 

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. 

A number of papers have appeared on the use of layered double hydroxides (e.g. Mg and Al containing oxides). A meixnerite-like catalyst has been reported to give 100% selectivity for diacetone alcohol from acetone at 0 C at close to thermodynamic equilibrium conversion of 23% (Tichit and Fajula, 1999). The side-chain alkylation of toluene with propylene to give isobutyl benzene (for ibuprofen) is a well-known example where Na/K alloy on Na2C03/K2C03 is used as the catalyst. 

The control experiment in pure supporting electrolyte (dotted lines in Fig. 13.2) shows a sharp faradaic current spike, which is mainly due to pseudocapacitive contributions (adsorption of (bi)sulfate and rearrangement of the double layer) plus oxidation of adsorbed Hupd (dotted lines in Fig. 13.2a), but no measurable increase in the CO2 partial pressure (m/z = 44 current) above the background level (dotted lines in Fig. 13.2b). Therefore, a measurable adsorption of trace impurities from the base electrolyte can be ruled out on the time scale of our experiments. Moreover, this experiment also demonstrates the advantage of mass spectrometric transient measurements compared with faradaic current measurements, since the initial reaction signal is not obscured by pseudocapacitive effects and the related faradaic current spike. 

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

Lyklema, J., The electrical double layer on oxides, Croatica Chem. Actay 43, 249 (1971). 

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... 

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). 

The catalyst effectiveness decreased upon increasing the concentration of the intercalated complexes via aggregation of the closely associated complexes.164 Sulfides were also oxidized by heterogeneous Co(II) complexes.163 Catalytic oxidation of thiols was mediated by Mo complex intercalated in a layered double hydroxide.166... 

We are studying the glycerol transformation in the presence of different catalysts of porous structure such as layered double hydroxides (hydrotalcites, HT and mixed oxides, MO), modified zeolites (ZSM5) and new type of Raney-Ni (RNi) to find optimal conditions for producing valuable components. 

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. 

Uniformity of the electrical double layer on oxides plot of -gj vs -ApH master curves for rutile, ruthenium dioxide and hematite. The concentration of KNO3 is indicated. 

Lyklema, J. (1987), "Electrical Double Layers on Oxides Disparate Observations and Unifying Principles", Chemistry and Industry, 741 -747. 

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]. 

Figure 18. Schematic model of the structure of Pt particles on an voltammogram in relation to the electrode potential. The hydrogen, double layer, and oxide regions are based on cyclic voltammetry. The lattice disorder decreases in the order D>A>C>B." (Reproduced with permission from ref 40. Copyright 1993 ElsevierSequoia S.A., Lausanne.)...

Lyklema, J. (1987) Electrical double layers on oxides disparate observations and unifying principles. Chemistry Industry, 741-747... 

The oxidation of sulfides to sulfones has been the subject of extensive studies, since sulfones are useful synthons for the construction of various chemically and biologically significant moleculesJ Recently, a new catalytic system has been developed by exchanging potassium osmate onto chloride-saturated layered double hydroxides (Figure 9.1), which we have shown to be an efficient catalyst for the direct oxidation of sulfides to sulfones, using molecular oxygen as the stoichiometric oxidant and with delivery of two oxygen atoms simultaneously to the sulfide, reminiscent of olefin dihydroxylation reactions. 

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... 

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... 

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

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