Hydrotalcites and related compounds

3 HYDROTALCITES AND RELATED COMPOUNDS Preparation and Catalytic Properties 

It must also be noticed that the rehydrated Mg/Al-LDH, in spite of their moderate basic strength/1081 due to the Brpnsted nature of the active sites have been able to catalyse Michael additions normally requiring basic Lewis sites of high strength/301 

Rehydrated Mg/Al LDH have also been used successfully for the synthesis of several fine chemicals of industrial or pharmacological interest, i.e. jasminaldehyde (a-M-amylcinnamaldehyde), chalcones such as vesidryl (2, 4,4 -trimethoxychalcone) and pseudoionones (6,7-dimethyl-3,5,9-undecatrien-2-one) by aldol and Claisen-Schmidt condensations/13,1091 

In some few cases, the acid-base bifunctional Mg(Al)0 mixed oxides have been more active and selective than the rehydrated Mg/Al-LDH (Mg/Al-OH) (Table 9.1). This has been observed in the condensation reaction of heptanal and acetaldehyde yielding 2-nonenal[55] and in MPV reductions/1151 In the former reaction, obtention of 2-nonenal requires in the first step the a-hydrogen abstraction from acetaldehyde. Lewis-type basic sites of moderate strength appear more efficient than Brpnsted- 

Rehydrated Mg/Al-OH catalysts have been found to be inactive in the MPV reduction of 4-ferf-butylcyclohexanone by isopropanol, whereas the bifunctional character of the Mg(Al)0 mixed oxides made them highly active in this reaction. The aluminium alkoxidc intermediate of the MPV reaction indeed involves a cooperation between basic and acidic sites. On mixed oxides, the abstraction of a proton from isopropanol on O2 sites gives isopropoxide anions, which are then stabilized on Al3+ and form intermediates with the aldehyde. The high activity of mixed oxide comes from the synergetic effect of strong Lewis basicity and mild acidity. 

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Realizing the importance and effectiveness of the magnesium species in the sorption of SO, in the late 1980s, Akzo Nobel patented the use of hydrotalcite and related compounds for nse in an FCC to reduce SO emissions [8,9,10]. Hydrotalcite contains more active Mg species than spinel. Hydrotalcite-based compounds typically contain 3-4 moles of Mg per mole of Al, while spinels contain only 1 mole of Mg per 2 moles of Al (Figure 16.4). 

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