Synthesis of transition metal doped lamellar double hydroxides as base catalysts for acetone aldol condensation

Transition metals TM (TM=Fe⁺², Zn⁺², Ni⁺² and Cu⁺²) doped lamellar double hydroxides (LDHs) were prepared by coprecipitation method and characterized with N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and temperature-programmed desorption of CO₂ (TPD-CO₂). XRD patterns of the dried samples showed that these solids contained the crystallized lamellar double hydroxides (LDHs) after calcination. TPD-CO₂ experiments revealed that these LDHs contain both moderate and strong basic sites varying with transition metal dopants. It was found that the gas-phase acetone aldol condensation was a surface basicity and texture-sensitive reaction. The basicity of the catalysts followed the sequence MgZnAlO>MgFeAlO>MgCuAlO>MgNiAlO, which agreed well with the variation of the catalytic activity in the acetone aldol condensation. The selectivity to isophorone and phorone could be correlated to the textural properties of the catalysts. Catalyst with large surface area and bigger pore diameter favored the formation of isophorone.