CONICET | Buscador de Institutos y Recursos Humanos

The liquid-phase synthesis of pseudoionones by cross-aldol condensation of citral with acetone was studied on alkali-promoted MgO catalysts. Alkaline metals (A) such as Li, Na, K and Cs were added to a high-surface area MgO in A/Mg molar ratios of up to 0.01. Promoters of bigger ionic radius than that of Li blocked the catalyst pores of MgO causing a decrease of both surface area and catalyst activity. In contrast, Li addition enhanced the pseudoionone yield of parent MgO. This beneficial effect of Li was further investigated by preparing, characterizing and testing several Li/MgO catalysts with different Li loadings. Results were interpreted in terms of the structural, textural and basic properties of the resulting oxides. Li loadings of up to 0.5 wt. % increased the total base site density of parent MgO by mainly increasing the density of very active strong base sites (low coordination oxygen anions) and thereby promoted the pseudoionone formation rate. Increasing the Li concentration further caused particle agglomeration and formation of unreactive carbonates that block the active sites. The citral/acetone aldol condensation mechanism on Li-MgO catalysts was also investigated and a Langmuir-Hinshelwood-Hougen-Watson kinetic expression was developed to account for the initial pseudoionone formation rate and to interpret experimental data. It was found that the rate-determining step is the abstraction of the a-proton from the acetone molecule that takes place on strong Brönsted base sites. A pseudoionone yield of 93 % was obtained at 353 K with a catalyst/citral weight ratio of 0.2 for the 0.5 wt. % Li/MgO catalyst at the end of the 6-hour catalytic run.