Catalysis by Hydrotalcite Materials

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Clays are one of the important natural materials, which have been examined for catalytic applications. Clays are characterized by a layered structure and hence they are slippery when wet. They are broadly classified as cationic or smectite type (having layer lattice structure in which two-dimensional oxyanions are separated by layers of hydrated cations) and anionic or brucite type (in which the charge on the layer and the gallery ion is reversed complimentary to smectite type). Anionic clays are rare in nature but they are relatively easy and inexpensive to prepare in the laboratory. Hydrotalcite belongs to the class of anionic clays wherein the positively charged contiguous layers formed by the edge sharing of Mg*(OH)6 and Al(OH)6 octahedra are separated by charge balancing anions and water, these occupying the interlayer spaces. A large variety of compounds with hydrotalcite like structure represented by the general formula [M(II)1-x M(III)x (OH)2] [An+x/n]m.H2O

Where 0Co-AL>Ni-Al
Among the Mg-Al LDHs the order was found to be
Mg-Al CO32->Mg-Al CO32-,SO42->M-Al CO32-,Cl-
Further the selectivity to acrolen was found to be higher on Mg-Al LDH than on MgO. Thi anamalous behaviour is explained by considering the acid- base cooperation – the basic sites facilitating the carbanion formationfrom acetaldehyde and the acid sites increasing the electrophilicity of the carbonyl carbon. Such a coopertive behaviour is further established by the fact theat the Mg-Al LDH calcined at 573 K show higher activity as compared to the 673 K calcined sample. The sample calcined at 573 K has a shigher surface Al content as compared to the sample calcined at 673 K. This behaviour is assuemed to exist in many of the formaldehyde involved condernsations. Chaudhary et al [] have been successful in carrying out the aldol condensation reactions with uncalcined Mg-Al O TBu catalyst which gave excellent yield of aldols with no further dehydration at a temperature as low as 273 K. Tichit et al [] have investigated the Claisen-Schmidt condensation of benzaldehyde with acetone over Mg Al carbonate LDH. The initial rate of the reaction was found to depend on the calcination temperature of the sample. Maximum rate was observed for the sample calcined at 823 K. The results obtained are in agreement with the observed decomposition patterns indicating the formation of high surface area mixed oxide and at higher temperatures, an MgAl2O4 spinel []. However, the suggestion that the active sites for the reaction involve hydroxyl groups rather than O2- species seems to be artificial. The Claisen-Schmidt condensation of benzaldehyde with substituted acetophenones was carried out by Climent et al over a series of basic catalysts like Cs exchanged zeolite and sepiolite and calcined hydrotalcites with varying Mg/Al ratio. Maximum activity was observed for Mg-Al LDH with Al/(Al+Mg) ratio between 0.25 and0.30. The increase in Al content upto certain extent causes the segregation of Al on the surface resulting in the generation of cation defect sites and hence more low coordinated oxide ions[]. A further increase in Al content may result in lowering in strength of these sites due to Al Lewis acidity or in decrease in number of such sites due to the decrease in Alsurface/Albulk ratio. Further, the crystallite size of the catalyst was also found to affect the activity. Samples with smaller crystallites were found to be more active. In such samples, a higher population of lower coordinated oxide ions is expected and hence these sites may be the active sits for the Claisen-Schmidt condensation. To our knowledge, the cannizaro reaction had not been successful with uncalcined or calcined LDHs, which indicate the requirement of stronger basic sites for the reaction. Hydrogenation-Dehydrogention Reactions

Hydrogenation reactions over solid bases are unique in several aspectsinclding faster hydrogentionof conjugated dienses as compared to monoenes, preference for 1,4...
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