Aldolizations on Calcined Hydrotalcites

Most results relate to this catalyst, after the pioneering work of Reichle [14-17] on the use of calcined hydrotalcites in the conversion of acetone to isophorone above 523 K. Indeed in a 1-inch pilot-plant reactor operated at 573 K conversion reached 23 %. Because mesityl oxide can be converted to isophorone the efficiency, as the sum of both, was 77 mol%. A further refinement of catalyst preparation [18] increased the conversion to 31 % with the same efficiency, or the efficiency to 85 % with a lower conversion of 20 %. Similar results have also been described by Ishino et al. [19], who also reported 5.2% conversion of cyclohexanone into (mainly) 2-cyclohexylidenecyclohexanone at 573 K. 

The aldol condensation of formaldehyde and acetone to methylvinylketone has been investigated by Suzuki and Ono [20] on a series of solids MgAl hydrotalcite was found to be the best catalyst, leading to 21 % conversion of acetone at 673 K, with 96 % selectivity for acetone and 64 % for formaldehyde. The carbonate form was the most suitable. Exchange by sulfates or chromates resulted in poor activity. 

The aldolization of acetaldehyde on calcined hydrotalcites of different composition has been studied by Kagunya and Jones [21] in a flow reactor at 10 bar and LHSV = 4 at temperatures between 363 and 403 K. MgAl carbonate was the best catalyst conversion reached ca 40 % at 110 °C with a broad range of products with a larger selectivity to Cg isomers. 

The condensation of formaldehyde and acetaldehyde has been studied by Dimi-triu et al. [22] in the vapor phase at 453-598 K using a pulse type microreactor. The main product was acrolein, obtained with a selectivity of ca 82% with conversion of 45 % acetaldehyde at 573 K. With such reactors it is difficult to make a good material balance because the products retained by the solid are not detected. Because of this the technique probably gives too optimistic a view of the catalytic results (selectivity and stability to coke deactivation). 

The direct conversion of butyraldehyde to 2-ethylhexanal-l has also been reported to proceed on calcined HDT of low Mg/Al ratios in the range 373-473 K, in a flow reactor. [23,24]. At 373 K both conversion and selectivity increased with reaction temperature. At 463 K conversion reached 71 % and selectivity 81 %. At constant temperature the conversion decreased slowly with time on stream. Addition of palladium to HDT, and operation in hydrogen enabled the performing of a bifunctional process in which butyraldehyde was converted to 

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