Preparation, Characterization And Catalytic Applications Of Solid Superbases KOH/MgO-ZrO₂and K₂O/MgO-Al₂O₃

Solid superbases are materials that possess basic sites with strength (H_) higher than26(H_corresponding to the pKa value of the indicator). It is a kind of environment-benign catalyst and conforms to the demand of the development of green chemistry.Therefore, solid superbase catalysts are widely used in the organic reactions such as olefins isomerization reactions, alkylation reaction, Michael addition reactions, Knoevenagel condensation reactions and transesterification reactions. Compared to the conventional alkali catalyst, there are some advantages of using solid superbase materials as catalysts such as high activity and selectivity, mild reaction conditions, easy separation from the reaction system and recycling process, few corrosion to the equipment, hindrance of tar formation as by-product. It has been regarded that solid superbase catalyst has a broad development prospects in the green industry.We obtained two novel superbases of KOH/MgO-ZrO2and K2O/MgO-Al2O3by modifying composite oxides (MgO-ZrO2, MgO-Al2O3) with alkali metal compounds. The structure and surface properties of these superbase catalysts were studied in detail using the characterization techniques of EDXS, XRD, N2adsorption-desorption method, FTIR, Hammett indicators method and CO2-TPD. They were examined for the Knoevenagel condensation reaction, the Michael addition reaction and transesterification reaction. Main achievements are abtained as follow:(1) Magnesium-zirconium composite oxide support (MgO-ZrO2) with Mg/Zr molar ratio of2:1was prepared by reflux-digestion with KOH solution. Then, a series of solid superbase material KOH/MgO-ZrO2was obtained by loading different amount of KOH onto MgO-ZrO2, followed by thermal treatment at500℃under a flow of high-purity N2. We found that the material has the most superbasic sites, up to0.59mmol/g, when the KOH loading was21%. The obtained material showed excellent catalytic activity and selectivity in Knoevenagel condensation reaction, Michael addition reaction and transesterification reaction under mild reaction conditions. Since no basic sites were formed on these composites in ambient conditions, CO2and H2O contamination from the atmosphere could be avoided. Those strong basic sites as active centers were only formed in the activation prior to reaction. Therefore, they keep high efficiency in catalytic process. In addition the relationship between superbasic properties and catalytic activity were reveded:the higher superbasic strength and the more superbasic sites, the better catalytic activity. The present results offer a new type of support for the synthesis of superbase materials and other kinds of superbase catalysts.(2) Potassium-functionalized magnesium-alumina (molar ratio of3:1) was prepared by one-pot reflux-digestion with KOH solution. Compared to the conventional two-step laoding and impregnation methods, the synthesis and modification were conducted in this one-pot process. The KOH plays a double role in this synthetic process by adjusting the pH value of the system, and at the same time produces the base precursor KNO3. KNO3is decomposed to form the strongly basic species K2O on the MgO-Al2O3that isalso formed in situ during the same calcination process. It avoids frame-work damage during post-treatment, and saves time and energy. Its base strength was in the range of26.5-33.0with superbasic amount up to0.411mmol/g. The obtained material showed excellent catalytic activity and selectivity in Knoevenagel condensation reaction under mild reaction conditions. Also, we reveal the relationship between superbasic properties and catalytic activity:the higher superbasic strength and the more superbasic sites, the better catalytic activity.This strategy may open up a route for the design and synthesis of new functional superbase materials. It is envisaged that this superbase will find potential applications in the field of catalysis and fine chemical industry.