Solid State NMR of Acid Sites in Non-Classical Zeolites

沸石分子筛中的布朗斯特酸中心具有位于硅和铝之间的桥氧原子上的质子，它们在广泛的催化反应中具有高度的活性。它们对不同地点的检测和调查是基础和应用研究中的一个长期课题。在这里，在这个项目的初始资金期引入了一个新的几何概念，即氧对齐模型，使我们能够预测Brnsted酸中心的氢键可能性。结构预测被一系列实验实例(Y沸石、ZSM-5、Beta、SSZ-42、SSZ-55和SSZ-59)所验证，其中预测的氢键数量与核磁共振光谱观察到的氢键数量之间有很好的一致性。通过对1H-27Al异核偶极相互作用的分析，得到了氢键酸中心存在的核磁共振波谱证明。它们的大小可以清楚地区分铝原子附近的酸性中心和其他质子物种，如H2O、NH4+或SiOH，这些物质也可以形成氢键，并表现出与酸性中心相似的1H化学位移。通过不同的~(27)Al四极相互作用，分子筛骨架内和骨架外Al_(OH)酸中心的区别是可能的。未来的工作将深化和拓宽氧排列模型对分子筛酸中心的酸性和动态性质的影响，并利用现代~(27)Al核磁共振技术(静态宽线和MAS方法)研究骨架外Al部分。邻近氧原子之间的局域质子跳跃将根据相互的氧排列角度进行研究。DFT计算将被用来探索氧对齐，也与局部质子跃迁的过渡态结构有关。在现代静态宽线实验中，利用不同的27Al{1H}交叉极化时间提供的编辑可能性，将研究骨架外Al物种的无序性及其丰富的多样性。还将特别强调进一步发展用各种双共振核磁共振脉冲技术分析1H-27Al偶极相互作用。这些对距离敏感的方法将潜在地区分孤立的单位Al催化剂，其中额外的第二个Al原子距离Brnsted质子很远(&gt；5？)，以及局部Al团簇(两个或更多的Al原子在质子5？内)。单位和多位催化剂之间的这种区别应该引起特别的兴趣，因为高(局部)铝密度可以改变催化反应的过渡态能量。

Brønsted acid sites in zeolites have protons located on a bridging oxygen atom between Si and Al, and they are highly active in a wide range of catalytic reactions. Their detection and investigation with respect to the variety of distinct sites is a long-standing subject in fundamental and applied research. Here, a novel geometrical concept was introduced in the initial funding period of this project in terms of an oxygen alignment model, enabling us to predict hydrogen bonding possibilities for Brønsted acid sites. The structural prediction was verified by a range of experimental examples (zeolite Y, ZSM-5, Beta, SSZ-42, SSZ-55, and SSZ-59), where an excellent agreement between the amount of predicted and NMR spectroscopically observed hydrogen bonds was found. The NMR spectroscopic proof of the presence of hydrogen-bonded acid sites is achieved by the analyses of the 1H-27Al heteronuclear dipolar interactions. Their magnitude can clearly distinguish between acid sites near Al atoms and other protic species, such as H2O, NH4+ or SiOH, which can also form hydrogen bonds and show similar 1H chemical shifts as acid sites. The differentiation of Brønsted acid sites within the zeolite framework and extra-framework AlOH is possible by their different 27Al quadrupolar interactions.Future work will deepen and broaden the understanding of the oxygen alignment model regarding its impact on acidic as well as dynamic properties of zeolite acid sites and investigate extra-framework Al moieties by modern 27Al NMR techniques (static wideline and MAS methods). Local proton hopping between neighbored oxygen atoms will be investigated in dependence of mutual oxygen alignment angles. DFT calculations will be used to explore oxygen alignments also with respect to the transition state structure of local proton hopping. Extra-framework Al species are less ordered and their rich diversity will be investigated by using the editing possibilities offered by different 27Al{1H} cross-polarization times in modern, static wideline experiments. Special emphasis will also be given to the further development of the analyses of 1H-27Al dipolar interactions by various double-resonance NMR pulse techniques. These distance sensitive methods will potentially allow to distinguish between isolated, single-site Al catalysts, where additional second Al atoms are distant (> 5Å) from the Brønsted proton, and local Al clustering (two or more Al atoms within 5 Å to the proton). Such a differentiation between single-site and multi-site catalysts should be of particular interest, because a high (local) Al density can alter the transition state energies of catalytic reactions.