Hydrotalcite sheets

In the present work we presume a parallel orientation of the cobalt phtalocyanine between the hydrotalcite sheets. This hypothesis was confirmed by a direct synthesis in which we voluntary omitted the addition of the cobalt phtalocyanine tetrasulfonate. The solid obtained, noted as DSo, was not an MgAl hydrotalcite but NaNOa (Fig. 2a). This result indicates that CoPcTs contributes certainly to the charge compensation in the hydrotalcite samples (DS3 to DSe). A parallel orientation of a metal phtalocyanine in the observed interlayer spacing is conceivable due to its planar geometry [20], moreover a parallel orientation of a nickel phtalocyanine between a LiAl hydrotalcite layers was already described by Dutta et al. [25]. 

Taking this one step further, perhaps even an inorganic gene may have been provided by clay mineral sources. Earliest clay samples are of a mineral called montmorillonite that consists of sheets of aluminosilicates in which Fe2+, Fe3+ and Mg2+ are substituted for some of the Al3+, and Al3+ is substituted for Si4+. The oxygen content of the layers does not change and the alternative valencies allow the production of positive and negatively charged layers. Dramatically, Paecht-Horowitz and co-workers showed that the amino acid adenylate could be polymerised with up to 50 units on the montmorillonite surface in aqueous solution. Similar condensation reactions for carbohydrates on hydrotalcite surfaces have... 

Hydrotalcite is a natural mineral of ideal formula Mg6Ab.(()H)1, iCO . 4H20, having a structure similar to brucite, Mg(OH)2. In hydrotalcite the Mg cations are partially replaced with Al3+ and the resulting positive charge is compensated by anions, typically carbonate, in the interlamellar space between the brucite-like sheets. When hydrotalcite is calcined at ca. 500 °C it is decarbonated and dehydrated to afford a strongly basic mixed Mg/Al oxide. Rehydration restores the original hydrotalcite structure and creates Bronsted base sites (OH ) in the interlamellar space. 

Layered Cobalt Hydroxides. The above results demonstrate the importance of the interlayer Co rather than the framework cobalt, in forming the required metallic Co catalyst. A structure with optimal catalytic activity would consist of one where there were only Co/Al hydroxide sheets, a structure closely approximated to by the hydrotalcite series of double layered hydroxides. 

The results in Table II suggest that molybdate- and vanadate-pillared hydrotalcites contain both acidic and basic sites, the basic sites located on the metal hydroxide sheets, and the acidic sites located on the polyoxometalate pillars. 

Hydrotalcites, or layered double hydroxides (LDH), are a class of material related to the mineral hydrotalcite which has the formula [Mg6Al2(0H)i6]C03-4H20. They are layered double hydroxides with the general formula M M]l ,(0H)2,A zH20 which have the trivalent and divalent cations contained within the octahedral sites of posihvely charged hydroxide sheets, in a stmcture akin to that of brucite, Mg(OH)2. The anions, A , and water are contained between the layers of sheets and their intercalation and removal can be monitored by the position of the (003) refiection in their powder diffraction patterns [56]. A schematic of the structure is shown in Figure 21.15 [56]. 

Fig. 7-3. Representative hydrotalcite structure. The charge imparted by Al3+ incorporation into the edgesharing octahedra of the brucite sheets is compensated by charge balancing anions in the interlayer.

Hitherto the most successful pillaring has been carried out on smectite type clays. The original research on pillaring included several series of unsuccessful experiments on mica, vermiculite and the sheet silicic acid minerals magadiite, silhydrite and kenyaite. Recently renewed attention has focused on these and other alternative sheet structures. They include rectorite, zirconium phosphates, tetrasilicic micas, hydrotalcites and silicic acids, the general structural features of which are shown in Figure 4. 

If we call ABC, the three fold axis of the hydroxyl group, the stack may have the sequence BC-CA-AB-BC, thus constituting three sheets in the unit cell vide supra rhombohedral 3R symmetry or BC-CB-BC with two sheets in the unit cell vide supra hexagonal 2H symmetry. Pyroaurite (MgFc hydoxycarbonatc) and hydrotalcite crystallize in... 

Phenol hydroxylation over CoNiAl ternary fresh hydrotalcites yielded catechol and hydroquinionc with a preference to catechol. No other products were observed by GC indicating no transitory formation of p-bcnzoquinonc. This is in contrast to the results we have earlier reported for copper containing hydrotalcites (87, 105] where we observed both catechol and hydroquinone to significant levels, suggesting the nature of cations in the brucite-likc sheets in influencing the course of the reaction. Although both nickel and cobalt separately in a binary hydrotalcite with aluminum as trivalent cation showed no conversion of... 

Hydrotalcites are a class of two-dimensional materials consisting of mixed metal hydroxide sheets interspersed with interlayer anions. It is known in the chemistry of hydrotalcites, that it is difficult to synthesize single-phase copper containing hydrotalcites while copper is an important clement explored for various selective oxidation reactions. However, it is also known that Cu-containing hydrotalcites can be synthesized with an addition of another bivalent metal ion. which by itself can independently form hydrotalcites. 

The hydroxycarbonate precursors showed dif action patterns consistent with the layered double hydroxide structure proposed for hydrotalcite-like compounds, i.e., brucite-like sheets formed by OH groups and cations (Mg and AP ) in octahedral coordination. Water molecules and charge-compensating COs anions are located in the interlayers. The hydrotalcite-like precursor composition is [Mg,, .Al,(0H)2] (C03), . mH20 with r = Al/(A1+Mg) the stoichiometric hydrotalcite compound, Mg6Al2(0H)i6C03.4H20, is obtained for r = 0.25. Upon thermal decomposition at 673 K, the hydroxycarbonate precursors lose... 

PVC sheet stabilized with hydrotalcite. [Data from Gilbert, m. Ho, k c, Hitt, d j,... 

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