Proton Acidity and Proton Mobility in ECR-40, a Silicoaluminophosphate that Violates Löwenstein's Rule.

Abstract

The silicoaluminophosphate zeotype ECR‐40 contains linkages of AlO4 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.