手性STW沸石结构类型中的硅锗取代系列
The Si-Ge substitutional series in the chiral STW Zeolite Structure Type
摘要：
本文采用实验-理论相结合的方法，合成并研究了整个组成范围（Ge $_f$ = Ge/（Ge+Si）= 0 ~ 1）的STW分子筛。沸石的产率经历最大值，然后在系列的GeO $_2$侧下降，遵循计算的自由能曲线的倒数。随着Ge $_f$的增加，晶胞通常大致线性地膨胀，但是在高二氧化硅侧观察到显著的膨胀弹性。这可以归因于SiO $_2$的更刚性的特性和Ge单元变形的能力。密度泛函理论计算提供了一个新的分配以前有争议的$^{19}$ F MAS NMR共振闭塞的氟化物，这不仅是基于在双4-环单元中的Ge原子的数量，但也对他们的方式相关联（即，没有Ge，孤立的Ge，Ge对或封闭的Ge集群）。虽然我们发现了一个整体良好的协议之间的实验和理论趋势的优先占领Ge的不同的晶体学网站，理论模型显示出更尖锐和突然的趋势，可能是由于限制的方法和动力学因素，使亚稳态配置实际存在。
英文摘要：
The whole compositional range (Ge$_f$ = Ge/(Ge+Si)= 0 to 1) of zeolite STW has been synthesized and studied by a comprehensive combined experimental--theoretical approach. The yield of zeolite goes through a maximum and then drops at the GeO$_2$ side of the series, following the inverse of the calculated free energy curve. The unit cell generally expands, roughly linearly, as the Ge$_f$ increases, but a notable resilience to expansion is observed at the high silica side. This can be attributed to the more rigid character of SiO$_2$ and the ability of Ge units to deform. Density functional theory calculations provide a new assignment of the previously controversial $^{19}$F MAS NMR resonances for occluded fluoride, which is based not only in the number of Ge atoms in the double-4-ring units but also on the way they are associated (namely, no Ge, isolated Ge, Ge pairs or closed Ge clusters). While we found an overall good agreement between the experimental and theoretical trends in preferential occupation by Ge of different crystallographic sites, the theoretical models show more sharp and abrupt tendencies, likely due both to limitations of the approach and to kinetic factors that allow metastable configurations to actually exist.
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