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 Ima2) 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 Ima2 symmetry are present in the time-averaged structures computed from the AIMD trajectories. At 300 and 500 K, the time-averaged structures approach Ima2 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 the connection 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 an adsorption at the Al1 site. No dynamic disorder was observed for the hydrated form.