Synthesis of SrO–Al2O3 Solid Base Catalysts from Strontium Hydroxide and Aluminum Alkoxide by a Solid-liquid Interface Reaction

Abstract

Alkaline earth metal oxides such as MgO are well known to show activities as solid base catalysts. The preparation, active site structure, and application of MgO to numerous base-catalyzed organic reactions have been extensively investigated1)~4). Strontium oxide (SrO) has a higher base strength among the alkaline earth metal oxides, because strontium (Sr) has lower electronegativity within the group of alkaline earth metals. The base strength increases in the order MgO < CaO < SrO < BaO5),6). Therefore, SrO is expected to achieve higher catalytic activity in various organic reactions. SrO catalysts are applicable to many base-catalyzed reactions, such as aldol condensation5), nitroaldol reaction7), Michel addition8),9), Cannizzaro reaction10), transesterification11)~18), Tishchenko reaction19),20), diacetone alcohol decomposition6), and amination of alkene21). However, fewer studies have been carried out with SrO than MgO as the base catalyst, particularly the preparation and application of SrO as a base catalyst, partially because of the difficulties associated with the preparation of SrO. Alkaline earth metal oxides are usually obtained by thermal decomposition of the corresponding hydroxide or carbonate at elevated temperatures2),3). For example, Mg(OH)2 is converted to MgO by thermal decomposition around 650 K22),23). However, the melting point of Sr(OH)2 is slightly lower than the decomposition temperature of the hydroxide24). Consequently, Sr(OH)2 first melts and then decomposes as the temperature increases. The decomposition of Sr(OH)2 in the liquid phase results in solid SrO with low surface area. In contrast, thermal decomposition of SrCO3 to SrO requires very high temperatures (>1073 K) because of the high thermal stability of SrCO37),10),25)~29). Therefore, the conventional thermal decomposition method is not appropriate for SrO catalyst preparation from the hydroxide or carbonate. To overcome these problems, we investigated solid-liquid interface reaction for the preparation of SrO base catalyst. In general, a metal alkoxide with high reactivity to[Regular Paper]