Influence of Mg–Al Mixed Oxide Compositions on Their Properties and Performance in Aldol Condensation

The influence of chemical composition of Mg–Al mixed oxides on their properties and catalytic performance in aldol condensation of furfural and acetone was studied. Pure alumina, Mg–Al layered double hydroxides with a wide range of Mg/Al molar ratios from 0.5 to 15, and pure magnesia were prepared by the same synthesis method, and corresponding oxides were produced by calcination of the as-prepared samples at T = 450 °C. The physicochemical properties of the samples were investigated by XRD, N2 physisorption, SEM, CO2-TPD, and NH3-TPD, and their catalytic performance was evaluated in aldol condensation of furfural and acetone. SEM images revealed that the Al content in the samples influenced the size and the morphology of both large agglomerates and plate-like crystals. The CO2-TPD measurements proved that the concentration of basic sites and the distribution of their strengths was determined by the Mg/Al molar ratio in the Mg–Al mixed oxides. However, over the whole range of Mg/Al compositions studied here, no clear correlation between the chemical composition and the number and strength of basic sites was found. Nonetheless, the change in the strength and in the density of basic sites affected the catalytic performance of calcined Mg–Al mixed oxides. Additionally, morphology and crystal size also influenced the performance of the catalysts. Finally, the composition of reaction products was affected by acid–base and textural properties of the prepared Mg–Al-mixed oxides.