Preparation, Characterization And Catalytic Applications Of KOH/La₂O₃-MgO And La₂O₃-ZrO₂ Solid Superbases

Solid superbases are basic materials with basic strength (H-) higher than 26. Solid superbase catalysts enjoy the supremacy such as high catalytic activity, easy separation from the products, mild reaction conditions and reduced pollutants, which make them to be environmentally friendly catalysts and promising in wide applications. However, there are numbers of disadvantages over the reported superbases such as less variety, insufficient basic sites amounts, rigorous preparation conditions and difficulty in storage. Thus, it is of great significance to develop new type of solid superbase catalysts.The present work opened up research on preparation of solid superbases derived from composite oxides. For the first time, novel superbases of KOH/La₂O₃-MgO and La₂O₃-ZrO₂ were obtained. The structure, morphology and basic properties of these two materials were studied in detail, assisted by the characterization techniques of XRD, N2 physical adsorption, SEM, Hammett indication methods and CO₂-TPD. Their catalytic apllications in the Knoevenagel condensation reaction and the Michael addition reaction were investigated as well. The main achievements are illustrated as follow:(1) Solid superbase material KOH/La₂O₃-MgO was obtained by loading KOH onto La₂O₃-MgO support, followed by thermal treatment at 500oC under a flow of high-purity N2. The as-prepared material showed base strength (H-) above 26.5. The optimum KOH loading on La₂O₃-MgO was 10wt%, with the corresponding superbasic sites amount reaching 0.57 mmol/g. The material showed high catalytic activity and selectivity in Knoevenagel condensation reaction under room temperature without solvent. What's more, discussion was also made on the relationship between the superbasicity and the catalytic activity of the superbase. It was found that the catalytic efficiency of the KOH/La₂O₃-MgO superbase is dependent on both the strength and amount of superbasic sites. Moreover, the superbase can be generated by simple thermal treatment of precursor at relative low temperature under an atmosphere of high-purity nitrogen. With in situ generation of catalytic active sites for target reactions, the storage is not a problem. The present results open up a new route for the design and synthesis of superbase materials of new functionalities using composite oxides as supports. It is envisaged that this superbase will find wide applications in the sectors of catalysis and fine chemical industry.(2) Solid superbase La₂O₃-ZrO₂ was obtained through thermal treatment of the precursor at 600oC under N2 flow, which was prepared by hydrothermal assistant co-precipitation method. The as-prepared material showed base strength in range of 26.5≤H-<33.0 with the superbasic sites amount up to 1.36 mmol/g. The superbase La₂O₃-ZrO₂ displayed morphology of square cylinder. The average pore size of the material was 14.0 nm, indicative of mesoporous material. While the relative low BET specific surface area (24.5 m²/g) of the superbase indicated high density of superbasic sites amount. High catalytic efficiency towards Michael addition at room temperature was obtained when the superbase was employed as catalyst. Furthermore, the preparartion procedure of La₂O₃-ZrO₂ superbase in present study provides a new rote for oxide superbase preparartion, where rigorous pretreatment conditions like high temperature evacuation needed for traditional oxide superbases are avoided. Meanwhile, the present study highlights the possibility of direct preparation of composite oxide superbase without using any potassium salts as modifing agents.