Effect Of SiO2 Amount On Heterogeneous Base Catalysis Of SiO2 ...

Effect of SiO2 amount on heterogeneous base catalysis of SiO2@Mg–Al layered double hydroxide†

Mahiro Shirotori,a Shun Nishimura *ab and Kohki Ebitani*ab Author affiliations

* Corresponding authors

a School of Materials Science, Japan Advances Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa, Japan E-mail: [email protected] Fax: +81-761-51-1149 Tel: +81-761-51-1613

b Graduate School of Advanced Science and Technology, Japan Advances Institute of Science and Technology, Japan

Abstract

The effects of SiO2 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 SiO2 spheres, denoted as SiO2@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 TOFbase were decreased although the total base amount was increased. Results of TEM-EDS and 29Si CP-MAS NMR suggest that the co-existing SiO2 causes advantages for dispersion and reduction of the LDH crystallite to improve the base catalysis of SiO2@Mg–Al LDH, whereas the excess SiO2 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 SiO2 seeds in the co-precipitation method is an important factor to increase the base catalysis of SiO2@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.

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