Cobalt hydrotalcites, formation

Subsequently, in 1999 the same group showed that the activity of the ruthenium hydrotalcite was significantly enhanced by the introduction of cobalt(II), in addition to ruthenium(III),in the Brucite layer [115]. For example, cinnamyl alcohol underwent complete conversion in 40 min in toluene at 60 °C, in the presence of ruthenium/cobalt hydrotalcite, compared with 31% conversion under the same conditions with ruthenium hydrotalcite. A secondary aliphatic alcohol, 2-octanol, was smoothly converted into the corresponding ketone but primary aliphatic alcohols, for example, 1-octanol, exhibited extremely low activity. The authors suggested that the introduction of cobalt induced the formation of higher oxidation states of ruthenium, for example, Ru(IV) to Ru( VI), leading to a more active oxidation catalyst. However, on the basis of the reported results it is not possible to rule out low-valent ruthenium species as the active catalyst in a hydridometal pathway. The results obtained in the oxidation of representative alcohols with ruthenium hydrotalcite and ruthe-nium-cobalt-hydrotalcite are compared in Table 5. 

Hydrotalcite-like compounds (HTlcs) have attracted much attention in recent years as catalyst precursors and catalyst support. This is due to (i) their ability to accommodate a large variety of bivalent and trivalent cations (ii) the homogeneous mixture of the cations on an atomic scale and (iii) the formation of thermostable mixed oxides, often denoted as ex-HTlcs, with high surface area upon decomposition. The first two properties are a result of the precursor while the last property appears to be related to the decomposition mechanism. The transitions in the structural properties of Co-based hydrotalcites upon high temperature treatments have been extensively studied in our group.15,16 In the first decomposition step, water is removed from the structure. This transition is followed by dehy-droxylation and decarbonation, as well as carbonate reorganization in the interlayer. Thermal treatment in air finally leads to a solid solution of cobalt spinels (Co(Co, A1)2C>4). Mixtures of CoO and C0AI2O4 are formed upon treatment in inert. 

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... 

In this work the formation of nickel, cobalt or zinc aluminum hydrotalcite-type coprecipitates upon impregnation of y-alumina at near neutral pH and ambient temperature was confirmed by EXAFS and X-ray dif action. The role of the metal ion concentration in solution on the composition of the supported coprecipitate was studied as well as the influence of the specific surface area of the Y-alumina. The deposition of Co(II), Ni(II) and Zn(II) ions onto a commercial almnina was first investigated. Since the coprecipitation mechanism is likely to be affected both by the impurity level and the thermal pretreatment of the carriers before impregnation, supports of high purity were prepared by hydrolysis of aluminum alkoxides and were submitted to identical pretratments immediately before impregnation. The deposition of Ni(ll) onto these supports was then examined. 

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