Crystallography Group, Department of Geosciences, University of Bremen, Klagenfurter Straße 2-4, 28359, Bremen, Germany.
MAPEX Center for Materials and Processes, University of Bremen, 28359, Bremen, Germany.
Chemistry. 2019 Oct 22;25(59):13579-13590. doi: 10.1002/chem.201902945. Epub 2019 Sep 20.
The silicoaluminophosphate zeotype ECR-40 contains linkages of AlO tetrahedra via a common oxygen atom, thereby violating the famous "Löwenstein's rule". In this work, a combination of static density functional theory (DFT) calculations and DFT-based ab-initio molecular dynamics (AIMD) simulations were employed to study the acidity and mobility of protons associated with such unusual linkages. It was found that the Al-O-Al linkages are preferentially protonated, as deprotonation causes a local accumulation of negative charge. The protons at these linkages possess a somewhat lower Brønsted acidity than those at Si-O-Al links. AIMD simulations for fully hydrated ECR-40 predicted a partial deprotonation of the Al-O-Al linkages, whereas Si-O-Al linkages were fully deprotonated. Frequently, a coordination of water molecules to framework Al atoms was observed in the vicinity of the Al-O-Al links. Hence, these linkages appear prone to break upon dehydration, potentially explaining why Löwenstein's rule is mostly obeyed in materials formed in aqueous media.
硅铝磷酸沸石 ECR-40 包含通过共用氧原子连接的 AlO 四面体，从而违反了著名的“Löwenstein 规则”。在这项工作中，组合使用静态密度泛函理论 (DFT) 计算和基于 DFT 的从头算分子动力学 (AIMD) 模拟来研究与这种不寻常连接相关的质子的酸度和迁移率。研究发现，Al-O-Al 键优先质子化，因为去质子化会导致局部积累负电荷。这些键上的质子的 Brønsted 酸度略低于 Si-O-Al 键上的质子。对完全水合的 ECR-40 的 AIMD 模拟预测，Al-O-Al 键会部分去质子化，而 Si-O-Al 键会完全去质子化。在 Al-O-Al 键附近，经常观察到水分子与骨架 Al 原子的配位。因此，这些键在脱水时似乎容易断裂，这可能解释了 Löwenstein 规则在水相介质中形成的材料中为何大多得到遵守。
