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Structure and Dynamics of AlPO4-5 and Other Aluminophosphates:  Classical Molecular Dynamics and ab initio Calculations

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posted on 26.07.2007, 00:00 by Patrice Bordat, Johanna Kirstein, Pierre Labéguerie, Mohammadou Merawa, Ross Brown
While the topology of the molecular sieve AlPO4-5 (structure type AFI) is well established, details of the structure determined experimentally, such as the precise space group, have varied significantly since its first synthesis 25 years ago. Classical molecular dynamics simulations with several force fields, periodic ab initio energy minimization, and ab initio magnetic-shielding calculations are therefore applied here to models of calcined AlPO4-5 and other aluminophosphates and compared to experimental data to help to resolve experimental discrepancies. Empirical models with partial atomic charges are overall in better agreement with a range of diffraction and NMR data for AlPO4-5 and other aluminophosphate sieves, than is a model with formal cation charges and polarizable oxygens. An order−disorder transition is found in the model, which may explain disagreements in the literature. It is predicted to occur close to room temperature from a triclinic to an on average hexagonal space group. Periodic ab initio and density functional theory calculations confirm that the space group of the most stable structure is not P6cc.