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

Mahiro Shirotori; Shun Nishimura; Kohki Ebitani

doi:10.1039/c8ra04925d

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

RSC Adv. 2018 Aug 6;8(49):28024-28031. doi: 10.1039/c8ra04925d. eCollection 2018 Aug 2.

Authors

Mahiro Shirotori¹, Shun Nishimura^{1

2}, Kohki Ebitani^{1

2}

Affiliations

¹ School of Materials Science, Japan Advances Institute of Science and Technology 1-1 Asahidai Nomi Ishikawa 923-1292 Japan s_nishim@jaist.ac.jp +81-761-51-1149 +81-761-51-1613.
² 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 TOF_base 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.