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首页> 外文期刊>Chemistry: A European journal >Visualizing the crystal structure and locating the catalytic activity of micro- and mesoporous ZSM-5 zeolite crystals by using in situ optical and fluorescence microscopy
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Visualizing the crystal structure and locating the catalytic activity of micro- and mesoporous ZSM-5 zeolite crystals by using in situ optical and fluorescence microscopy

机译：通过使用原位光学和荧光显微镜可视化晶体结构并确定微孔和中孔ZSM-5沸石晶体的催化活性

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摘要

A combination of optical and fluorescence microscopy was used to study the morphology of micro- and mesoporous H-ZSM-5 zeolite crystals (17 x 4 x 4 mu m) and to evaluate, in a spatially resolved manner, the effect of mesoporosity. introduced via desilication, on catalytic performance. For this purpose, the oligomerization of various styrene molecules was used as a model reaction. in which the carbocation intermediates formed in the zeolite pores act as reporter molecules. In situ confocal fluorescence measurements after the template removal process showed that the crystals generally consist of three different subunits that have pyramidal boundaries with each other. Examination of these crystals during styrene oligomerization revealed differences in the catalytic activity between the purely microporous and the combined micro- and mesoporous crystals. The introduction of intracrystalline mesoporosity limits the formation to dimeric carbocation intermediates and facilitates the transport of styrene molecules inside the zeolite volume. This leads to a more uniform coloration and fluorescence pattern of the crystals. Moreover, the oligomerization of various styrene compounds, which differ in their reactivity, provides a good way of estimating the Bronsted acid strength in a spatially resolved manner, showing a nonhomogeneously distributed Bronsted acidity over the volume of the crystals. More detailed information on the structure of the ZSM-5 crystals was revealed for mesoporous crystals during the oligomerization of 4-methoxystyrene. This reaction induced an "explosion" of the crystal leading to the formation of a complex system with at least eight different subunits. Finally, polarized-light microscopy was used to unravel the pore geometry in these individual building blocks. The observed differences in catalytic behavior between micro- and mesoporous ZSM-5 crystals are strengthened by the microspectroscopic techniques employed, which show that upon desilication the crystal morphology is affected, the product distribution is changed towards less conjugated carbocation intermediates, and that a gradient in Bronsted acid strength appears to be present.
机译：光学和荧光显微镜的组合用于研究微孔和中孔H-ZSM-5沸石晶体（17 x 4 x 4微米）的形态，并以空间分辨的方式评估中孔的影响。通过脱硅引入催化性能。为此，将各种苯乙烯分子的低聚用作模型反应。其中在沸石孔中形成的碳正离子中间体充当报告分子。模板去除过程后的原位共聚焦荧光测量表明，晶体通常由三个不同的亚单元组成，彼此之间具有金字塔边界。在苯乙烯低聚过程中对这些晶体的检查表明，纯微孔晶体与组合的微孔和中孔晶体之间的催化活性存在差异。晶内介孔的引入限制了二聚碳阳离子中间体的形成，并促进了苯乙烯分子在沸石体积内的运输。这导致晶体的更均匀的着色和荧光图案。此外，反应性不同的各种苯乙烯化合物的低聚提供了一种以空间分辨的方式估算布朗斯台德酸强度的好方法，显示出在晶体体积上布朗斯台德酸度的分布不均匀。对于4-甲氧基苯乙烯低聚期间的中孔晶体，揭示了有关ZSM-5晶体结构的更多详细信息。该反应引起晶体的“爆炸”，从而导致形成具有至少八个不同亚基的复杂系统。最后，使用偏振光显微镜揭示了这些独立构件中的孔几何形状。所观察到的微孔和中孔ZSM-5晶体之间在催化行为上的差异通过所采用的显微技术得到了加强，这表明，在脱硅作用下，晶体形态受到影响，产物分布朝着较少共轭的碳正离子中间体方向变化，并且似乎存在布朗斯台德酸强度。

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