Application of calcined Mg-Al hydrotalcites for Michael additions: an investigation of catalytic activity and acid-base properties

Mg-Al hydrotalcite catalysts with different Mg/Al molar ratios (0.6, 1.4, 2.2, 3.0) were calcined and tested in liquid-phase Michael additions at room temperature. The catalysts and pure oxides (MgO and Al2O3) were characterized by XRD, XPS, N2 adsorption experiments, 27Al MAS NMR, and TG-DTA. The acid-base properties of the samples were investigated by (a) FTIR spectroscopy after pyridine and CO2 adsorption and (b) microcalorimetry with CO2 and benzoic acid. Both acid sites (Lewis) and base sites (Lewis and Brønsted) are present on the surface of calcined hydrotalcites. The calcined hydrotalcites (CHT) catalyzed the Michael additions of CH-acid compounds to methyl vinyl ketone to give high product yields with 100% selectivity; the Al-rich sample exhibited the highest activity.