Controlled Synthesis of Different Morphologies of MgO and Their Use as Solid Base Catalysts

摘要

MgO is one of the most promising solid base catalysts and has attracted much attention because of its superior performance. The extent of catalytic properties of MgO is highly controlled by its morphology, particle size, crystalinity, and surface area. Here, the synthesis of MgO with different morphologies, such as random nanoflakes, arranged nanoflakes toward flower and house of card structure spheres, cubes, and hexagonal plates, through the calcination of magnesium carbonate hydrates (MCH) intermediate is presented. The intermediate MCH has been synthesized under hydrothermal or supercritical hydrothermal as well as solvothermal treatment of clear solution of Mg(NO)3, (NH4)2CO3, or nesquehonite rods in different pH's and amounts of free carbonate ions. A probable reduction mechanism is proposed to explain the formation of the MCH morphologies. The amount of carbonate ion has crucial role in the formation of different morphologies in hydrothermal condition. On calcinations of the synthesized MCH morphologies resulted in MgO with almost identical morphologies as parental MCH. The microstructures of calcined MgO are porous and made of MgO nanoparticle building blocks of the size 4-6 nm. The formed MgO nanoparticles consists of large number of edges and corners, step edges and step corners and numerous base sites of various strength (surface hydroxyl groups, low coordinate O2- sites) which are recognized as active basic sites in heterogeneous catalysis. The calcined MgO microstructures function as a strong solid base catalyst for the solvent-free Claisen-Schmidt condensation of benzaldehyde with acetophenone giving 99% conversion in 4 h. The MgO catalysts are easily recyclable with no significant loss in catalytic activity in the subsequent cycles.