Inorganic Chemistry and Catalysis Group, Debye Institute for Nanomaterials Science, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands.
Chemistry. 2010 Aug 16;16(31):9340-8. doi: 10.1002/chem.201000995.
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.
大沸石晶体已被用作微孔主体材料中扩散和催化反应性现象研究的模型体系至少二十年。然而，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。在此，我们表明，振动和光学光谱的互补使用结合理论计算允许在沸石 H-ZSM-5 中酸催化的苯乙烯衍生物的低聚反应中生成的瞬态碳阳离子物种的明确识别。由于反应物在大 H-ZSM-5 晶体中的介导扩散和最小的外部表面，反应中间体可以在沸石微孔中积累到足以通过基于同步加速器的 IR 微光谱检测到的足够浓度。UV/Vis 和 IR 光谱显示出二聚苯乙烯碳阳离子的分子排列的强烈偏振依赖性，这是由沸石通道和笼施加的，这可以根据沸石内碳阳离子的电子和振动跃迁来合理化。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像。
