Hydrotalcite-like materials

Thus, for example, for a system of benzyl chloride, butyl bromide, and the hydrotalcite-like material, we expect the following reactions to occur. Butyl bromide would undergo a halide substitution by the interlayer Cl anions, leaving Br anions in the interlayer space the intercalated Br anions would, in turn, attack benzyl chloride to yield benzyl bromide leaving Cl anions in the interlayer space, the interlayer Cl anions being cycled as follows ... 

Here, H.T.-Cl and H.T.-Br denote hydrotalcite-like materials containing Cl and Br interlayer anions, respectively. 

Since the hydrotalcite-like material contains exchangeable interlayer anions, it can be used as a catalyst for reactions for which anion-exchange resins have been used as catalyst. The disproportionation of trimethoxysilane (eq. 2) is... 

For the halide-exchange reactions, into a SO-cm flask equipped with a condenser, 30 cm of toluene or DMF as a solvent and 1.0 g portion of the hydrotalcite-like material (3.3 mmol of interlayer Cl anions) were introduced. A 33 mmol of benzyl chloride was added to the mixture and the temperature was kept at 343 or 373 K with stirring, followed by the addition of a given amount of an alkyl bromide (or iodide) (33-195 mmol). Reactions were conducted under a nitrogen atmosphere. The liquid phase was withdrawn periodically and analyzed by a gas chromatograph equipped with a 2-m long SE-30 column and a... 

Because of the size of the alkyl halides, the halide exchange probably occurs at the external edge surfaces of the hydrotalcite-like material, not in the interlayer space. This is supported by the fact that no expansion of the interlayer space was observed after the material was used in the halide-exchange reaction. The diffusion of the halide ions in the interlayer space is not the rate-determining step, since the rate of the reaction greatly depends on the kind of alkyl bromide. 

Figure 3 shows the change in trimethoxysilane conversion with reaction time for the reactions in the presence of the hydrotalcite-like material containing interlayer Cl anions. Here, the conversion was calculated from the amount of unreacted (CH30)3SiH. The conversion increased with reaction time and was 70% at 6 h. The selectivity of the reaction is defined as follows ... 

Reaction conditions (CH30)3 iH 41 mmol, hydrotalcite-like material (H.T.-Cl )= 0,62 g (interlayer Cl" anion= 2.0 mmol), and reaction temperature 323 K. 

The interlayer Cl" anions in a synthetic hydrotalcite-like material, Mg6Al2(0H)i5Cl2 4H20, react with butyl bromide or benzyl bromide in a non-polar solvent of toluene. Almost all the chloride ions in the interlayer space... 

Behrens M, et al. Phase-pure Cu,Zn,Al hydrotalcite-like materials as precursors for copper rich Cu/Zn0/Al203 catalysts. Chem Mater. 2009 22(2) 386-97. 

Synthesis and Characterization of Layered Precursors. Hydrotalcite-like materials of Mg-Al, Zn-Cr, Ca-Al, and Ni-Al were prepared from the nitrate salts. A mixed solution of the appropriate divalent and the trivalent cation salts in the required molar ratios was co-precipitated with a base, typically NaOH, at room temperature. The pH of the suspension was maintained between -8 and 10 during the addition, which required approximately 1 hour. After complete addition, the precipitate was allowed to age for 16 hours and later washed with deionized water. The purity and crystallinity of the material was verified by X-ray diffraction. DTA/TGA analysis was used to study the thermal stability as well as the accompanying weight loss of the materials. 

Some other materials are AIPO4 [293, 302], AIP04-Ti0 [294], magnesium phosphates [26], alkali-exchanged zeolites [148, 286], hydrotalcite-like materials [295], lithium aluminaies and HAIO2 [202]... 

Electronic state of iron complexes in the interlayer of hydrotalcite like materials. 664 ... 

The method for preparation of hydrotalcite-like materials was similar to that previously described by Miyata (1975) and Drezdzon (1988). The compounds with Mg/Al mole ratios of 2 and 3 woe synthesized. For the compound of formula Mg4Al2(0Hi2(N03)2.xH20, the M Al mole ratio is 2. A solution containing 35.84 g of Mg(N03)2.6H20 and 26.23 g of A10 O3)3.9H2O in 200 mL deionized water was stirred in a flask. A base solution containing 20 g NaOH in 300 mL deionized wato was prepared. The base solution was added dropwise from a sq>aration funnel to the mixture of metal nitrate solution. Afto complete addition of the base solution, the mixture with a final pH of 9.5 was heated at 323 K for 2 h. The precipitate was filtered, washed with lionized wato and dried at 323 K. 

The reactions were performed in a plug-flow type reactor operating at one atomospheric pressure. Saturated 2-butanol vapor (293 K) was fed with a nitrogen-gas carrier to the catalyst bed. About 0.3 g of hydrotalcite-like materials were heated under a nitrogen stream at a given temperature for ca. 2h before starting the reactions. The products from the outlet of the reactor were analyzed by a HP 5890 gas chromatography with FID detector. 

Table 8-1 The catalytic activities of hydrotalcite-like materials in 2-butanol decomposition. 

C. A. Antonyraj, S. Kannan, Appl. Catal., A 2008, 338, 121-129. Hantzsch pyridine synthesis using hydrotalcites or hydrotalcite-like materials as solid base catalysts. 

LDHs, also known as hydrotalcite-like materials, are a class of host-guest materials consisting of positively charged metal oxide or hydroxide sheets with intercalated anions and water molecules. Generally, their chemical structure is represented by the formula [M i xM + t(OH)2 2H2O], where M + is a divalent metal ion (such as... 

In ion exchange, ions are exchanged between two different environments so, zeolites, hydrotalcite-like materials, and clays can exchange nonframework ions when suspended in aqueous solutions of other ions in some cases, the process can also be performed in molten salts at moderate temperatures. Sodium/silver substitution in p-alumina takes place at 300 °C according to the reaction ... 

In Figure 20.10, XRD patterns of the synthesized ZnAl-LDH and their derived mixed oxides (C-ZnAl) are presented. As previously explained, XRD peaks for ZnAl-LDH sample correspond to the characteristic diffractograms for the layered structure of the hydrotalcite-like materials and the symmetrical peaks indicate satisfactory level of order in the stacks [2, 9, 39,40]. After thermal treatment due to the collapse of the layered structure new dififaction peaks typical for mixed oxides are observed that correspond to the presence of the zincite, ZnO phase and the spinel phase (ZnAl O ) [15, 40]. The broadening of the peaks with lower intensities indicates lower crystallinity of these phases. 

Basic supports such as calcined hydrotalcite-like materials are also employed for this purpose. Particularly, Sun et al. proposed Mg(Al)0 type hy-drotalcites to be the ideal catalysts. The Al cations present in the support stabilize the dispersed metal particles against sintering. Most of the reports in this direction suggest Sn as the best element for promotion. The size of Pt ensembles and the bond strength of chemisorbed hydrocarbons are modified by the addition of Sn. Thus, Pt-Sn combination performs better than Pt alone. Sn forms an alloy with platimun. The performance of the catalyst was foimd to be dependent on the composition of the Pt and Sn precursors and the catalyst preparation procedure. PtGa bimetallic nanoparticles were foimd to exhibit not only high activity and selectivity for ethane dehydrogenation, they also... 

V-Mo-Nb-W-containing hydrotalcite-like materials as precursors of catalysts for oxidative dehydrogenation of hydrocarbons and alcohols... 

Keywords hydrotalcite-like materials (LDHs), oxidative dehydrogenation (ODH)... 

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