Effect of SiO(2) amount on heterogeneous base catalysis of SiO(2)@Mg–Al layered double hydroxide

The effects of SiO(2) 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(2) spheres, denoted as SiO(2)@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)(−1) h(−1). 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(base) were decreased although the total base amount was increased. Results of TEM-EDS and (29)Si CP-MAS NMR suggest that the co-existing SiO(2) causes advantages for dispersion and reduction of the LDH crystallite to improve the base catalysis of SiO(2)@Mg–Al LDH, whereas the excess SiO(2) 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(2) seeds in the co-precipitation method is an important factor to increase the base catalysis of SiO(2)@LDHs; i.e. the control of Si/(Mg + Al) atomic ratio is necessary to avoid the poisoning of highly active base sites on the LDH crystal.