Synthesis of α,β-unsaturated ketones over thermally activated Mg–Al hydrotalcites

The synthesis of α,β-unsaturated ketones on hydrotalcite-derived Mg–Al mixed oxides was studied. Five samples with Mg–Al ratios of 0.5–9.0 were prepared by coprecipitation and characterized by infrared spectroscopy, CO2 chemisorption and X-ray photoelectron spectroscopy. The selectivity to α,β-unsaturated ketones [mesityl oxide (MO), isomesityl oxide (IMO), and isophorone (IP)] on pure MgO and calcined hydrotalcites was practically 100%. Magnesium oxide contained both the highest basic site density and the strongest basic sites; as a result, it exhibited a high activity for acetone oligomerization reactions. The addition of small amounts of Al to MgO (Mg/Al>5) diminished drastically the density of surface basic sites because of a significant Al surface enrichment, and caused the catalytic activity to drop. At higher Al content (1<Mg/Al<5) the incorporation of Al+3 cations within the MgO lattice increased the basic site density and the catalytic activity partially recovered. Al-rich HTC samples (Mg/Al<1) converted acetone at a high rate, probably reflecting an optimum concentration of acid–base sites for promoting both the acid and the base-catalyzed acetone condensations. Formation of IP increased with the Al content at expenses of mesityl oxides. It appears that the cyclization of linear trimers to yield IP is favored on samples with higher surface hydroxylation. Calcined Mg–Al hydrotalcites with high Al content are therefore suitable catalysts for producing IP from acetone oligomerization.