Ristanović Zoran, Chowdhury Abhishek Dutta, Brogaard Rasmus Y, Houben Klaartje, Baldus Marc, Hofkens Johan, Roeffaers Maarten B J, Weckhuysen Bert M
Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science , Utrecht University , Universiteitsweg 99 , 3584 CG Utrecht , The Netherlands.
Department of Chemistry , University of Oslo , Postboks 1126 Blindern, 0318 Oslo , Norway.
J Am Chem Soc. 2018 Oct 31;140(43):14195-14205. doi: 10.1021/jacs.8b08041. Epub 2018 Oct 19.
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.
沸石的活性和选择性通常由潜在的质子和氢转移反应途径决定。我们首次使用单分子荧光显微镜直接跟踪在ZSM-5沸石中形成的单个苯乙烯衍生碳正离子物种的实时行为。我们发现，碳正离子中间体的间歇性荧光和显著的光稳定性强烈依赖于沸石骨架和客体溶剂分子所施加的局部化学环境。如密度泛函理论计算所示，通过改变苯乙烯部分的对位取代基，可以进一步改变碳正离子的稳定性。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有显著差异，这取决于它们在沸石骨架中的精确位置。通过改变MFI骨架中二聚碳正离子与溶剂化布朗斯特酸位点之间的相互作用，可以进一步改变荧光态的寿命和该过程的可逆性。先进的多维魔角旋转固体核磁共振光谱已被用于在沸石催化苯乙烯二聚化过程中对反应产物进行精确的结构解析，以证实单分子荧光显微镜数据。单分子荧光显微镜、核磁共振和密度泛函理论的这种互补方法共同表明，碳正离子和中性状态的相对稳定性在很大程度上取决于取代基以及沸石骨架内布朗斯特酸位点的局部位置。因此，在沸石与芳烃之间的主客体化学方面，就其表面迁移率和反应性而言，已经获得了新的见解。
