Anisotropic Chemical Shielding, M-Site Ordering, and Characterization of Extraframework Cations in ETS-10 Studied through MAS/MQ-MAS NMR and Molecular Modeling Techniques
Anisotropic Chemical Shielding, M-Site Ordering, and Characterization of Extraframework Cations in ETS-10 Studied through MAS/MQ-MAS NMR and Molecular Modeling Techniques
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DOI：
10.1021/ja972714o
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发表时间：
1998-05
影响因子：
15
通讯作者：
S. Ganapathy;T. Das;R. Vetrivel;S. S. Ray-S.;T. Sen;S. Sivasanker;L. Delevoye;C. Fernandez;J. Amoureux
中科院分区：
文献类型：
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作者：
S. Ganapathy;T. Das;R. Vetrivel;S. S. Ray-S.;T. Sen;S. Sivasanker;L. Delevoye;C. Fernandez;J. Amoureux
本文报道了利用高分辨率魔角旋转（MAS）核磁共振光谱和分子模型研究相结合得出的ETS-10中Si和Ti位点的局部结构特征。合成了纯高结晶的ETS-10和铝取代ETS-10 (ETAS-10)，不含杂质ETS-4相，并通过MAS和多量子魔角自旋（MQ-MAS） NMR对其进行了充分研究。实验中观察到的29Si共振更精确地分配到晶体学上不等效的Si位，并与t位几何结构建立了相关性。29Si慢MAS NMR被证明是一种非常通用和强大的方法，可以明确地在沸石晶格中建立杂原子取代，并用于探测ETS-10和ETAS-10中不同Si位点的局部对称性和化学屏蔽。利用ETAS-10的29Si和27Al的MAS NMR谱分析证实，铝取代只发生在Si[4Si，0Ti]硅位上。这反过来又被用来为计算机图形技术生成聚类模型。这些簇模型的电子结构和计算的铝替代能值精确地确定了铝在ETAS-10中的地形位置。在量子化学簇模型计算的基础上，预测了ETAS-10的酸度。本文还报道了首次应用MQ-MAS NMR研究分子筛中的阳离子环境，并在本研究中用于研究ETS-10和ETAS-10中钠阳离子的局部结构特征。
The local structural characteristics of Si and Ti sites in ETS-10 as derived from a combination of high-resolution magic angle spinning (MAS) NMR spectroscopic and molecular modeling studies are reported. Pure and highly crystalline ETS-10 and aluminum-substituted ETS-10 (ETAS-10), devoid of impurity ETS-4 phase, were synthesized and fully studied by MAS and multiple-quantum magic angle spinning (MQ-MAS) NMR. More accurate assignments of the experimentally observed 29Si resonances to the crystallographically nonequivalent Si sites are made, and a correlation with T-site geometry is established. 29Si slow MAS NMR is shown to be a very general and powerful methodology to unequivocally establish heteroatom substitution in the zeolite lattice, and this was used to probe the local symmetry and chemical shielding at different Si sites in ETS-10 and ETAS-10. 29Si and 27Al MAS NMR spectral analysis of ETAS-10 is used to confirm that the aluminum substitution occurs only in Si[4Si,0Ti] silicon sites. This, in turn, was used to generate cluster models for computer graphics techniques. The electronic structure of such cluster models and the calculated aluminum substitution energy values pinpoint the topographical location of aluminum in ETAS-10. The acidity of ETAS-10 is predicted on the basis of the quantum chemical cluster model calculations. The first application of MQ-MAS NMR to study cation environments in molecular sieves is also reported and is used in the present study to investigate the local structural characteristics of sodium cations in ETS-10 and ETAS-10.