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Reversible and Site-Dependent Proton-Transfer in Zeolites Uncovered at the Single-Molecule Level
Journal of the American Chemical Society ( IF 15.0 ) Pub Date : 2018-10-03 , DOI: 10.1021/jacs.8b08041
Zoran Ristanović 1 , Abhishek Dutta Chowdhury 1 , Rasmus Y Brogaard 2 , Klaartje Houben 3 , Marc Baldus 3 , Johan Hofkens 4 , Maarten B J Roeffaers 5 , Bert M Weckhuysen 1
Affiliation  

Zeolite activity and selectivity is often determined by the underlying proton and hydrogen-transfer reaction pathways. For the first time, we use single-molecule fluorescence microscopy to directly follow the real-time behavior of individual styrene-derived carbocationic species formed within zeolite ZSM-5. We find that intermittent fluorescence and remarkable photostability of carbocationic intermediates strongly depend on the local chemical environment imposed by zeolite framework and guest solvent molecules. The carbocationic stability can be additionally altered by changing para-substituent on the styrene moiety, as suggested by DFT calculations. Thermodynamically unstable carbocations are more likely to switch between fluorescent (carbocationic) and dark (neutral) states. However, the rate constants of this reversible change can significantly differ among individual carbocations, depending on their exact location in the zeolite framework. The lifetimes of fluorescent states and reversibility of the process can be additionally altered by changing the interaction between dimeric carbocations and solvated Brønsted acid sites in the MFI framework. Advanced multidimensional magic angle spinning solid-state NMR spectroscopy has been employed for the accurate structural elucidation of the reaction products during the zeolite-catalyzed dimerization of styrene in order to corroborate the single-molecule fluorescence microscopy data. This complementary approach of single-molecule fluorescence microscopy, NMR, and DFT collectively indicates that the relative stability of the carbocationic and the neutral states largely depends on the substituent and the local position of the Brønsted acid site within the zeolite framework. As a consequence, new insights into the host–guest chemistry between the zeolite and aromatics, in terms of their surface mobility and reactivity, have been obtained.

中文翻译:

在单分子水平上发现的沸石中的可逆和位点相关质子转移

沸石活性和选择性通常由潜在的质子和氢转移反应途径决定。我们第一次使用单分子荧光显微镜直接跟踪沸石 ZSM-5 中形成的单个苯乙烯衍生碳阳离子物种的实时行为。我们发现碳阳离子中间体的间歇性荧光和显着的光稳定性在很大程度上取决于沸石骨架和客体溶剂分子施加的局部化学环境。如 DFT 计算所示,通过改变苯乙烯部分上的对位取代基,还可以改变碳阳离子稳定性。热力学不稳定的碳正离子更有可能在荧光(碳正离子)和暗(中性)状态之间切换。然而,这种可逆变化的速率常数在各个碳正离子之间可能存在显着差异,这取决于它们在沸石骨架中的确切位置。通过改变 MFI 框架中二聚碳阳离子和溶剂化布朗斯台德酸位点之间的相互作用,可以另外改变荧光状态的寿命和过程的可逆性。先进的多维魔角旋转固态核磁共振光谱已被用于在沸石催化苯乙烯二聚过程中对反应产物进行准确的结构解析,以证实单分子荧光显微镜数据。这种单分子荧光显微镜、核磁共振、和 DFT 共同表明碳正离子和中性状态的相对稳定性在很大程度上取决于沸石骨架内布朗斯台德酸位点的取代基和局部位置。因此,就沸石和芳烃的表面迁移率和反应性而言,对它们之间的主客体化学有了新的认识。
更新日期:2018-10-03
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