Environmentally Benign Synthesis Using Nanostructure-Controlled Base Catalysts

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Basic organic group-immobilized silicate materials have been utilized as the heterogeneous catalysts for C-C bond formation reactions such as Knoevenagel condensation, aldol condensation, and Michael addition. When using the amine-immobilized mesoporous silicas as base catalysts, the real effects of amine site and silica support have not been well investigated. Here we examined the effect of amine itself on the reaction as well as the conjugate effect of silicate additives such as SBA-15, and other ordered porous silicates. As a result, the combination of some secondary amine (as a base) and periodical mesoporous silicate (as an additive) drastically enhanced the reactivity in the reaction of aromatic aldehydes with acetone leading to the selective formation of β-hydroxyketones. The yield was low in the absence of the mesoporous silicate and the mesoposous silica itself was totally inert. It was suggested that the prominent effect of the silicate is not only due to the increase in the reactant concentration on the silicate surface but also to synergistic effect of base with acid. Activity of amine-immobilized silicate (organic-functionalized molecular sieves ; OFMSs) is higher than that of the simple mixture of amine and silicate. Very often, the OFMSs were effective when the supports were ordered mesoporous material and the reaction was carried out in polar solvents ; however, the yield was low in a non-polar solvent. In contrast, silicate-organic composite materials (SOCMs) were remarkably active in a non-polar solvent. The mesoporous silicate was more active than the microporous silicate. The active sites of the SOCM catalysts were considered to be SiO^-(^+NR_4) moieties located inside the pore near the pore-entrance. Although the high activity was not due to the leached species, a gradual loss of activity was observed after repeated use.