Aldol condensation of citral with acetone on MgO and alkali-promoted MgO catalysts

Abstract

The liquid-phase synthesis of pseudoionones (PSs) by cross-aldol condensation of citral with acetone was studied on alkali-promoted MgO catalysts. Alkaline metals (A), including 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 greater ionic radius than Li blocked the catalyst pores of MgO, causing a decrease in both surface area and catalyst activity. In contrast, the addition of Li enhanced the PS 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. The 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 mainly by increasing the density of very active strong base sites (low coordination oxygen anions), and thereby promoted the PS 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 PS formation rate and to interpret experimental data. It was found that the rate-determining step is the abstraction of the α-proton from the acetone molecule that occurs on strong Brönsted base sites. A PS 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-h catalytic run.