Computational description of key spectroscopic features of zeolite SSZ-13
Computational description of key spectroscopic features of zeolite SSZ-13
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DOI：
10.1039/c9cp03146d
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发表时间：
2019-09-21
影响因子：
3.3
通讯作者：
Hermans, Ive
中科院分区：
文献类型：
--
作者：
Goeltl, Florian;Love, Alyssa M.;Hermans, Ive
分子筛的催化性能与Al原子的分布和相对位置以及孔网络中的缺陷密切相关。然而，确定这种分布的特征是具有挑战性的，特别是在考虑到不同的地方铝安排时。在这一贡献中，我们使用第一性原理计算和实验测量相结合的方法，建立了质子化SSZ-13中铝分布的模型。此外，我们还应用该模型来理解OH-IR、Al-27-核磁共振和Si-29-核磁共振谱的趋势。我们使用Boltzmann分布来预测给定局域Al组态的质子位置，结果表明，对于每个组态，都占据了几个H位置。因此，在所有Al组态的OH-IR振动光谱中观察到一个多峰光谱，这与不同硅铝比沸石的实验测量光谱相一致。从核磁共振谱中我们发现，由于局域应变的改变，质子位置导致了Al-27-核磁共振和Si-29-核磁共振谱的显著移动，当引入均匀的背景电荷时，局域应变消失了。这些发现得到了实验测量的支持。最后，从单胞尺寸和相邻单胞的影响两个方面讨论了该模型的不足之处。
The catalytic properties of zeolites are intimately linked to the distribution and relative positions of Al atoms and defects in the pore network. However, characterizing this distribution is challenging, in particular when different local Al arrangements are considered. In this contribution we use a combination of first principles calculations and experimental measurements to develop a model for the Al-distribution in protonated SSZ-13. We furthermore apply this model to understand trends in OH-IR, Al-27-NMR and Si-29-NMR spectra. We use a Boltzmann distribution to predict the proton position for a given local Al configuration and show that for each configuration several H positions are occupied. Therefore a multi-peak spectrum in OH-IR vibrational spectroscopy is observed for all Al configurations, which is in line with experimentally measured spectra for zeolites at different Si/Al ratios. From NMR spectroscopy we find that the proton position leads to significant shifts in Al-27-NMR and Si-29-NMR spectra due to the modification of the local strain, which is lost when a uniform background charge is introduced. These findings are supported by experimental measurements. Finally we discuss the shortcomings of the presented model in terms of unit cell size and the impact of adjacent unit cells.