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

Revisiting the Structure of Calcined and Hydrated AlPO-11 with DFT-Based Molecular Dynamics Simulations*

Published crystal structures of the AEL-type aluminophosphate AlPO-11 in its calcined form (space group I m a 2 ) show some peculiar features, such as unusually short Al-O and P-O bonds and near-linear Al-O-P angles. Although experimental evidence for the presence of dynamic disorder was presented, the nature of the associated distortions remained unresolved. In this study, ab initio molecular dynamics (AIMD) calculations in the framework of density functional theory (DFT) were employed to study the dynamic behaviour of this zeotype. At 100 K, static local distortions that break the I m a 2 symmetry are present in the time-averaged structures computed from the AIMD trajectories. At 300 and 500 K, the time-averaged structures approach I m a 2 symmetry. Although shortened Al-O and P-O bonds and near-linear Al-O-P angles were found in the average structures, an analysis of radial and angular distribution functions confirmed their absence in the instantaneous structures. This deviation is due to a precession-like motion of some oxygen atoms around the Al-P connection line, which moves their time-averaged positions closer to this line. In hydrated AlPO-11, some of the water molecules are coordinated to framework Al atoms, leading to an octahedral coordination of 1/5 of the Al sites. DFT optimisations and AIMD simulations on partially hydrated models delivered evidence for a preferential adsorption at the Al1 site. No dynamic disorder was observed for the hydrated form.

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

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.