Catalysis of Organic..

Diez, Di Cosimo and Apesteguia 3630medium (n MO ) strength basic sites. The observed proportionality between rPSand n Oin Fig. 6 indicates that the rate-determining step in the reaction mechanism is effectively24r 0 PS (µmol/min m2 )201612841 2 3 4 5 6 7n O(µmol/m 2 )Figure 6. Initial PS formation rate as a function of the density of strong basic sites.controlled by the surface base strength of the sample. The reaction pathway to producepseudoionone from the aldol condensation of citral with acetone involves the initialabstraction of the α-proton from acetone forming a carbanion that consecutively attacksthe carbonyl group of the contiguously adsorbed citral molecule. Then, the resultingunstable intermediate dehydrates forming pseudoionone and regenerating an active siteon the catalyst surface. In this mechanism, the function of weak Lewis acid sites (Mg 2+and Li + cations) is to favor the adsorption of both reactants via their carbonyl groups. Ingeneral, the basis of any base-catalyzed aldol condensation is the abstraction of the α-proton, the most acidic site of the acetone molecule, with a pK a of 20 (13). In thisregard, it had to be noted that we have studied the liquid-phase self-condensation ofacetone to produce diacetone alcohol using the catalysts of Table 2 (14), and observedthat, similarly to the results reported in this work for acetone/citral reaction, the additionof small amounts of Li promotes the initial formation of diacetone alcohol. This resultsupports the assumption that the strong base sites of Li-doped MgO efficiently promotethe abstraction of proton H α from the acetone molecule that would be the rate-limitingstep for the synthesis of pseudoionones via the citral/acetone aldol condensation.Finally, we remark that precisely because of its strong basicity, low coordinationoxygen atoms O 2- are very reactive and may reconvert to surface OH - in the presence ofwater formed during the pseudoionone synthesis (see reaction scheme 1). Recent reportsconfirm that the surface of metal oxides with basic character are often restructuringduring the reaction (15). Thus, the formation of pseudoionones on Li-doped MgO maybe accompanied by the gradual modification of the catalyst surface, by reconverting