Acetone rate function properties, 20

In Fig. 2 we have represented both the r Acetone values and the total site density (nj) as a function of catalyst composition. Qualitatively, the variation of r Acetone with increasing A1 content is similar to that followed by nj thereby suggesting that acetone conversion depends on both acid and base sites. Pure MgO was the most active catalyst whereas AI2O3 showed the lowest activity. This is because Al-0 pairs are much less active than Mg-0 pairs for promoting the proton abstraction and carbanion stabilization steps involved in aldol condensation reactions. We have showed [1] that the acetone aldolization rate is controlled on basic catalysts by the number of metal cation-oxygen anion surface pairs. Mg-rich Mg AlOx oxides are less active than MgO because they exhibit a lower base site density and also poor acidic properties. In contrast, Al-rich Mg AlOx oxides are more active than AI2O3 due to a proper combination of acid and base sites. 

In Fig. 7, the do values obtained for MgO, AI2O3 and MgyAlOx oxides were plotted as a function of the density of basic sites, nb (Table 1). It is observed that the initial deactivation correlates linearly with the density of basic sites. No correlation was found between the NH3 site density (n,. Table 1) and do. These results show that although Mg AlOx oxides promote the selfcondensation of acetone by both acid- and base-catalyzed pathways (Fig. 2), the initial deactivation rate is essentially related to the surface basic properties. This dependence of do with nb for MgyAlOx oxides probably reflects the fact that the acetone oligomerization rate decline is significantly higher when coke poisons very active basic Mg-0 pairs than when eliminates moderately active acidic Al-0 pairs. 

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