Catalysis of Organic..

Tzompantzi, Valente, Cantú and Gómez 57The unit cell parameters were calculated assuming a 3R stacking sequence,therefore a = 2d 110 and c = 3d 003 where c is the interlayer distance regulated by thesize and charge of anion placed between the layers and the cell parameter a is theaverage metal-metal distance inside the layers. A way to verify the isomorphicsubstitution of Mg 2+ by Zn 2+ is analyzing the change of cell parameter a, the a valuevaried from 3.054 Å for MgAl3 to 3.072 Å for ZnAl3, this variance can be explainedby the differences of cation sizes, since the ionic radius of Zn 2+ is bigger than Mg 2+ ,0.88 and 0.86Å respectively. The a values for MgZnAl samples rely on theirdifferent chemical composition and are between the range > 3.054 and < 3.072 Å.These results confirm the isomorphic substitution of Zn in the HTs layers.All samples showed type IV isotherms, which correspond to mesoporoussolids. The specific surface area (SSA) of the calcined solids were 62 and 254 m 2 /gfor ZnAl3 and MgAl3, both with pore volumes of 0.478 and 1.148 cc/g, and an avg.pore diameter of 225 and 181 Å, respectively. The SSA of the MgZnAl samplesdecreased as the Zn content rose, but they displayed remarkable bigger SSA than theZnAl3 sample, for instance, the MgZnAl-12 sample presented a SSA of 197 m 2 /g, apore volume of 0.664 cc/g, and an avg. pore diameter of 135 Å.Figure 2 shows illustrated mechanism for acetone self-condensation overcalcined hydrotalcites, where the enolate ion is formed in a first step followed by twopossible kinetic pathways: 1) In the first case the subtracted proton is attracted by thebasic sites and transferred to the oxygen of the enolate ion to form an enol inequilibrium. 2) In the second case the enolate ion reacts with an acetone molecule inthe carbonyl group, to produce the aldol (diacetone alcohol). Finally, the β carbon isdeprotonated to form a ternary carbon and then loses an OH - group to obtain the finalproducts.H 3 COCCH 3+H 3 COCCH 3H 3 CH 3 COHCCHHOCCH 3H 3 CH 3 CCCHOCCH 3AcetoneAldolMesityl OxideFigure 2. Pathway proposed for acetone self-condensation.Table 2 presents the aldolization reaction results, where a selectivity around12% to aldol and 88% to mesityl oxide was obtained. It is noticed that when Mg 2+was partially or totally substituted by Zn 2+ , the reaction rate was improved. Forinstance, the reaction rates in MgZnAl-12 and ZnAl3 were 1.3 and 2.4 times higherthan MgAl3, respectively. It is stated that the addition of Zn 2+ inside the layers ofcalcined hydrotalcite improves the aldolization process, increasing theoligomerization conversion of acetone to mesityl oxide, due to charge defectsgenerated by the Zn 2+ insertion, thus changing the basic sites strength. It has beenreported (10) that when Mg 2+ is replaced by Zn 2+ a substantial decrease in basicstrength is observed, so a lower basic strength is expected in the ZnAl3 calcined