期刊
RSC ADVANCES
卷 8, 期 49, 页码 28024-28031出版社
ROYAL SOC CHEMISTRY
DOI: 10.1039/c8ra04925d
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资金
- Japan Society for the Promotion of Science (JSPS) [15J10050, 17H04966]
- Grants-in-Aid for Scientific Research [15J10050] Funding Source: KAKEN
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 Si-29 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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