Thermodynamics of Aldolization

One of the difficulties in performing aldolization is the reversibility of the reaction, which limits the equilibrium conversion. The thermodynamic equilibrium has been investigated by Craven [2] for the industrially important aldolization of acetone to diacetone alcohol (DA). Because the reaction is exothermic, the yield of aldol obtained from pure acetone decreases with temperature 23.1 % at 273 K, 16.9 % at 283 K, 12.1 % at 293 K and 9.1 % at 303 K. The same conclusions can be drawn from the work of Guthrie [3-6] who reported equilibrium constants in the aqueous phase, in relation to the of the substrates, for a series of aldol condensations at room temperature. For the aldolization of aeetaldehyde at 298 K the values relative to the reaction are AG° = -2.4 kcal mor , A77 = -9.84 kcal mol , and equilibrium constant K = 51 m . The results for a few specific reactions performed in the aqueous phase are reported in Table 1 where Ky and K2 represent the equilibrium constants for the formation of the aldol and for its dehydration, respectively. 

According to these thermodynamic data, significant conversion can be obtained for the condensation of acetone with formaldehyde. Diacetone alcohol can be formed in low yield only and has a lower tendency to dehydrate than the aldols formed from benzaldehyde. The energy balance of the process is so displaced by the formation of water that the unsaturated ketone is usually obtained. The 

2 Structure and Surface Properties of Hydrotalcites and Calcined Hydrotalcites 

Hydrotalcite (HDT) is a natural mineral of ideal formula Mg6Al2(OH)i6 (C03 ).4H20. Solids with the same structure, denoted by extension hydrotalcites (HDT), have been synthesized with Al/(Mg + Al) ratio between 0.2 and 0.33 

Fe and Cr in Mg(OH)2 or the same trivalent cations in the lattice of the divalent hydroxides of Ni, Cu, Zn, Co, and Mn [8-10]. The structure then consists of a positively charged layer, compensated by anions which retain some mobility and can be exchanged. The OH form of MgAl HDT is also a natural mineral called meixnerite. 

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