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Detection of carbocationic species in zeolites: Large crystals pave the way

机译：沸石中碳正离子物种的检测：大晶体铺平道路

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摘要

Large zeolite crystals have been used as model systems for the investigation of diffusion and catalytic reactivity phenomena in microporous host materials for at least two decades. However, their potential in assisting the detection of elusive reactive intermediates appears to have been underestimated. Herein, we show that a complementary use of vibrational and optical spectroscopy in combination with theoretical calculations allows for the unambiguous identification of transient carbocationic species generated upon the acid-catalyzed oligomerization of styrene derivatives within zeolite H-ZSM-5. Thanks to the mediated diffusion of the reactant in the large H-ZSM-5 crystals and minimal external surface the reaction intermediates can be accumulated within zeolite micropores in sufficient concentrations to allow their detection by synchrotron-based IR microspectroscopy. The UV/Vis and IR spectra display strong polarization dependence of on the molecular alignment of the dimeric styrene carbocations imposed by the zeolite channels and cages that can be rationalized in terms of the electronic and vibrational transitions of the intrazeolite carbocations. Based on these findings, a molecular-level picture of the macroscopic arrangement of the reaction intermediates confined within microporous zeolite matrices can be devised.
机译：大型沸石晶体已用作模型系统，用于研究微孔基质材料中的扩散和催化反应现象至少已有二十年了。但是，它们在协助检测难以捉摸的反应中间体方面的潜力似乎被低估了。在这里，我们表明，振动和光谱学与理论计算相结合的互补使用，可以明确鉴定在沸石H-ZSM-5内苯乙烯衍生物的酸催化低聚反应后产生的瞬态碳阳离子物质。由于反应物在大的H-ZSM-5晶体中介导的扩散和最小的外表面，反应中间体可以以足够的浓度积聚在沸石微孔中，从而可以通过基于同步加速器的红外光谱法对其进行检测。 UV / Vis和IR光谱显示出强烈的偏振依赖性，取决于沸石通道和笼子所施加的二聚苯乙烯碳正离子的分子排列，可以根据沸石内碳正离子的电子和振动跃迁合理化。基于这些发现，可以设计出限制在微孔沸石基质中的反应中间体的宏观排列的分子水平图。

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