Catalysis of Organic..

58Hydrotalcite-like Catalystssample in regard to MgAl3, however the activity of the former one is higher than inthe last. This implies that the activity of the solids on the acetone self-condensationreaction is probably driven by the adsorption-desorption equilibrium between theintermediate species and the basic sites. Thus, it is suggested that a high interactionoccurs between strong basic sites and the reaction intermediates, leading to a slowintermediate desorption to form the final products. On the other hand, the interactionof the reaction intermediates with weaker basic sites leads to an easier desorption ofthe products which is reflected in higher reaction rates.Table 2. Results of self-condensation of acetone over various calcined hydrotalcitelikecompounds.Catalyst%Se %Se Mesityl Reaction rate % ConversionAldol Oxide (x10 7 mol/g s)MgAl3 11.9 88.1 8.1 2.3MgZnAl-7 12.9 87.1 6.8 1.9MgZnAl-12 12.3 87.7 10.4 2.9ZnAl3 11.5 88.5 19.5 5.6Experimental SectionCatalyst PreparationThe HTs were prepared by coprecipitation at 60 °C and constant pH according toprocedures described earlier (10). For the MgAl3 HT for instance, an aqueoussolutions containing 0.75 mol/L of Mg (NO 3 ) 2 .6H 2 O and 0.25 mol/L of Al(NO 3 ) 3 .9H 2 O, and the second one 1.5 mol/L of KOH and 0.5 mol/L of K 2 CO 3 , wereintroduced by two electric pumps to a 4 L flask and mixed under vigorous stirring.The mixture was aged at 60 °C for 18 h under stirring. The precipitate was washedseveral times until the solution was free of excess ions, and then dried at 100 °Cfresh HTs. For the synthesis of MgZnAl HTs, Zn(NO 3 ) 2 .6H 2 O was used as Znsource. During the synthesis the percent of zinc was varied keeping the ratioM 2+ /M 3+ =3. Hereinafter, the solids thus obtained will be referred to as MgZnAlwt.%Zn.Characterization of SolidsChemical composition of fresh HTs was determined in a Perkin Elmer Mod.OPTIMA 3200 Dual Vision by inductively coupled plasma atomic emissionspectrometry (ICP-AES). The crystalline structure of the solids was studied by X-raydiffraction (XRD) using a Siemens D-500 diffractometer equipped with a CuKαradiation source. The average crystal sizes were calculated from the (003) and (110)reflections employing the Debye-Scherrer equation. Textural properties of calcinedHTs (at 500°C/4h) were analyzed by N 2 adsorption-desorption isotherms on anAUTOSORB-I, prior to analysis the samples were outgassed in vacuum (10 -5 Torr)at 300°C for 5 h. The specific surface areas were calculated by using the Brunauer-