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Violation or Abidance of Löwenstein’s Rule in Zeolites Under Synthesis Conditions?

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journal contribution
posted on 24.10.2019, 19:40 by Xiaomin Tang, Zhiqiang Liu, Ling Huang, Wei Chen, Chengbin Li, Guiru Wang, Guangchao Li, Xianfeng Yi, Anmin Zheng
Löwenstein’s rule (−Al–O–Al– bond formation is forbidden) has been generally accepted for the Al distribution in zeolites. In contrast, it was illustrated recently that the violation of Löwenstein’s rule is present for several common zeolites in the protonated form (e.g., H-SSZ-13 and H-MOR) based on theoretical calculations of bare zeolite frameworks. By far, whether the direct −Al–O–Al– bond can form in zeolites has not been confirmed, and thus a more in-depth investigation on the Al distribution in zeolites is required to reconcile such conflict. In this work, we employed the state-of-the-art periodic density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulation to check the validity of Löwenstein’s rule under hydrothermal synthesis conditions (e.g., structure-directing agent (SDA), water, and reaction temperature). It is illustrated that with the synthesis conditions taken into account, Löwenstein’s rule is obeyed and the −Al–O–Al– linkage is avoided. In addition, the directing function of SDA on the Al location in the zeolite framework is quantified and visualized. Moreover, a standard operating procedure for investigation on the Al location and Brønsted acidic sites distribution in zeolites has been proposed.