Effect of SiO amount on heterogeneous base catalysis of SiO@Mg-Al layered double hydroxide.

The effects of SiO amount on the base catalysis of highly active finely crystallized Mg-Al type layered double hydroxides prepared by the co-precipitation method with coexistence of SiO spheres, denoted as SiO@LDHs, were investigated. With the Si/(Mg + Al) atomic ratios of 0-0.50, the highest activity for the Knoevenagel condensation was observed in the case of Si/(Mg + Al) = 0.17, as the reaction rate of 171.1 mmol g(cat) h. The base activity increased concomitantly with decreasing LDH crystallite size up to Si/(Mg + Al) atomic ratio of 0.17. However, above the Si/(Mg + Al) atomic ratio of 0.17, the reaction rate and TOF were decreased although the total base amount was increased. Results of TEM-EDS and Si CP-MAS NMR suggest that the co-existing SiO causes advantages for dispersion and reduction of the LDH crystallite to improve the base catalysis of SiO@Mg-Al LDH, whereas the excess SiO species unfortunately poisons the highly active sites on the finely crystallized LDH crystals above a Si/(Mg + Al) atomic ratio of 0.17. According to these results, we inferred that the amount of spherical SiO seeds in the co-precipitation method is an important factor to increase the base catalysis of SiO@LDHs; the control of Si/(Mg + Al) atomic ratio is necessary to avoid the poisoning of highly active base sites on the LDH crystal.