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

机译：SiO2含量对SiO2 @ Mg-Al层状双氢氧化物非均相碱催化的影响

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摘要

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.
机译：SiO _（2）量对通过共沉淀法与SiO _（2）球共存制备的高活性细晶Mg-Al型层状双氢氧化物的碱催化的影响，表示为SiO _（2）@研究了LDH。在Si /（Mg + Al）原子比为0-0.50的情况下，在Si /（Mg + Al）= 0.17的情况下，Knoevenagel缩合反应的活性最高，反应速率为171.1 mmol g（cat） 〜（？1）h〜（？1）。随着LDH晶粒尺寸的减小，Si /（Mg + Al）原子比达到0.17，碱活性随之增加。但是，在Si /（Mg + Al）原子比超过0.17的情况下，尽管总碱量增加，但反应速度和TOF_（碱）降低。 TEM-EDS和〜（29）Si CP-MAS NMR的结果表明，共存的SiO _（2）有利于分散和还原LDH结晶，从而改善SiO _（2）@ Mg–的碱催化作用。 Al LDH，不幸的是，过量的SiO_（2）物种使Si /（Mg + Al）原子比超过0.17的细晶LDH晶体上的高活性位点中毒。根据这些结果，我们认为共沉淀法中球形SiO _（2）晶种的数量是增加SiO _（2）@LDHs碱催化的重要因素。即，必须控制Si /（Mg + Al）原子比，以避免LDH晶体上高活性碱位中毒。

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