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Alkaline earth metal oxides are well-known solid bases, among which MgO is a typical solid base catalyst1)~4). The structures of the base sites and applications to many base-catalyzed reactions have been widely investigated. Solid bases are used to catalyze many C_C bond formation reactions, such as aldol reactions, which are important for synthesizing fine chemicals. In most cases, the activation of alkaline earth metal oxides and base-catalyzed reactions is carried out under a vacuum or in an inert gas because CO2 and H2O in the atmosphere poison the catalyst1),4). This requirement is an obstacle to the use of these solid base catalysts in organic synthesis reactions. Therefore, methods for the preparation of a solid base usable in air are very important. Active MgO has been prepared by thermal decomposition of hydroxide or carbonate species at higher temperatures under a high vacuum or in a high-purity inert atmosphere. The most active MgO for base-catalyzed reactions was prepared by heat treatment at 973 K in a vacuum1),4). Base sites on the MgO surface are poisoned by H2O and CO2 in the atmosphere1),3), and the resultant hydroxides and carbonates of alkaline earth metals have much greater thermal stability. Therefore, high-temperature treatment above 773 K is required to remove these species from the active sites of the surface O2 groups. Deactivation caused by H2O or CO2 results in changes in the bulk structure from oxide to hydroxide or carbonate, respectively. Elimination of H2O or CO2 at higher temperatures is accompanied by exchange of oxygen atoms between the surface and these molecules5)~8). In addition, alkaline earth metal oxides dissolve in several polar solvents, such as methanol9). Therefore, a base catalyst with high tolerance against H2O and CO2 and insoluble in a polar solvent is strongly desirable. Our previous investigation covered the surface of an MgO base catalyst with SiO2, Al2O3, TiO2, and ZrO2 to overcome the two disadvantages of poisoning by H2O and CO2 and solubility in a polar solvent10). The appropriate metal alkoxide was dissolved in ethyl acetate and reacted with the OH groups in a double hydroxide layer of Mg(OH)2. The materials obtained were covered with a monolayer or bilayer of the metal oxide. Among the base catalysts tested, MgO covered with 7 mol% or more aluminum oxide showed excellent tolerance to H2O and good reusability for the aldol reaction of acetone. The base strength of Al2O3-covered MgO containing 10 mol% Al3+ ions was >26.5 on the Hammett function scale. Data from Al K-edge X-ray absorption near-edge structure (XANES) spectroscopy indicate that octahedral Al3+ ions, which are present in the surface region, form the principal active structure11). Base sites are likely to consist of O2 ions bonded to 24 Journal of the Japan Petroleum Institute, 59, (1), 24-30 (2016)

Synthesis of Highly Active Solid Base Catalyst of Calcium Oxide Covered with Thin Layer of Titanium Oxide