Abstract : Microporous aluminophosphates AlPO4-n have drawn considerable attention due to their potential to act as heterogeneous catalysts and molecular sieves. Hydration of these compounds usually modifies the coordination of framework aluminium species and causes a reversible structure deformation. Here, A simultaneous use of experimental tools and theoretical approach based on the Density Functional Theory (DFT) brings a better knowledge of the interactions between water molecule and AlPO4-n frameworks.
Information on the behaviour of water in the pores has been obtained from the study of a model compound, AlPO4-34. A dehydration / rehydration mechanisms has been proposed as well as a partially hydrated phase, in agreement with solid-state NMR and X-ray diffraction results. A synergy is proposed between diffraction and molecular dynamics. A description of the stability domains of the hydrated phases is also developed.
New structures are then studied with the same theoretical approach. First, we have described the unit cell of dehydrated AlPO4-36 by combining X-ray diffraction data and DFT calculations. Then, a complete experimental study (Infrared spectroscopy, X-ray diffraction, thermal analysis, NMR) has been supplemented by a static and dynamic theoretical approach to get information on calcined rehydrated AlPO4-18 phase.
To enlarge the possibilities of DFT studies to more complex structures with shorter computer time, a comparison of structural data obtained by plane waves and localised basis has been performed. Finally, a linear scaling approach has been successfully applied to study the energy and the geometry of two crystalline aluminophosphates: AlPO4-34 and AlPO4-18.