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Effect of Ge/Si substitutions on the local geometry of Si framework sites in zeolites: A combined high resolution 29Si MAS NMR and DFT/MM study on zeolite Beta polymorph C (BEC)
Microporous and Mesoporous Materials ( IF 5.2 ) Pub Date : 2018-03-16
Sarah R. Whittleton, Aurelie Vicente, Christian Fernandez, Somayeh F. Rastegar, Anna V. Fishchuk, Stepan Sklenak

We employed density functional theory/molecular mechanics (DFT/MM) calculations and 29Si magic-angle spinning (MAS) NMR spectroscopy to investigate the effect of single and multiple Ge/Si substitutions on the 29Si NMR parameters as well as the local geometry of SiO4 tetrahedra of the nearest (Ge-O-Si) and next-nearest (Ge-O-Si-O-Si) neighboring Si atoms. The influences of the Ge/Si substitutions are compared with the effects of the corresponding Al/Si substitutions (i.e., Al-O-Si and Al-O-Si-O-Si, respectively). Zeolite Beta polymorph C (BEC), containing double four-membered rings (D4Rs) and exhibiting three distinguishable T sites in the framework, was chosen for this study as a model of germanium containing zeolites. Our computations give a systematic downshift of the 29Si chemical shift of Si by 1–6 ppm for Ge-O-Si sequences. Furthermore, the contributions of two, three, and four Ge atoms as the nearest neighbors to the downshift of Si are not additive and the calculated downshifts lie in the intervals from 2 to 6 ppm, from 1 to 9 ppm, and from 5 to 11 ppm, respectively. Conversely, the contributions of two, three, and four Al atoms as the nearest neighbors are approximately additive. The downshifts caused by Ge nearest neighbors are less than half compared with the corresponding downshifts caused by Al. Moreover, our calculations show that there are no systematic contributions of Ge and Al as next-nearest neighbors (i.e., Ge-O-Si-O-Si and Al-O-Si-O-Si, respectively) to the 29Si chemical shift of Si, and not even the direction (sign) can be predicted without calculating the corresponding sequence.



中文翻译:

Ge / Si取代对分子筛中Si骨架位点局部几何形状的影响:高分辨率29 Si MAS NMR和DFT / MM组合研究分子筛Beta多晶型物C(BEC)

我们采用密度泛函理论/分子力学(DFT / MM)计算和29 Si魔角旋转(MAS)NMR光谱研究了单个和多个Ge / Si取代对29 Si NMR参数以及局部几何形状的影响最接近(Ge-O- Si)和最接近最近(Ge-O-Si-O- Si)的Si原子的Si O 4四面体的原子数。的锗/硅取代的影响与相应的Al / Si的取代的效应(即,AL-O-相比的Si和Al-O-Si系O-, 分别)。含双四元环(D4Rs)且在骨架中显示三个可区分的T位置的沸石Beta多晶型物C(BEC)被选作含锗沸石的模型用于本研究。我们的计算给出了Ge-O- Si序列的Si29 Si化学位移的系统下移1-6 ppm 。此外,两个,三个和四个Ge原子作为Si降档的最接近邻居的贡献降级不是相加的,并且计算出的降档分别位于2至6 ppm,1至9 ppm和5至11 ppm的区间内。相反,作为最邻近原子的两个,三个和四个Al原子的贡献是近似加和的。与由Al引起的相应降档相比,由Ge最近邻造成的降档小于一半。此外,我们的计算表明,有锗和铝的没有系统的贡献在明年近邻(即葛-O-的Si-O-和Al-O-的Si-O-,分别)为29硅化学如果不计算相应的序列,就无法预测Si的位移,甚至无法预测方向(符号)。

更新日期:2018-03-17
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