Investigation of the Evolution of Intermediate Phases of AlPO₄-18 Molecular Sieve Synthesis

In the present work, we examined the intermediate phases formed during the hydrothermal synthesis of microporous material, AlPO 4 -18. The evolution of the long-range ordering of the gel samples as a function of crystallization time was followed by powder X-ray diffraction. The development of the local environments of P and Al atoms was monitored by 31 P and 27 Al magic angle spinning (MAS) NMR. Several representative intermediate phases were further characterized by 27 Al multiple quantum MAS and 27 Al → 31 P cross-polarization experiments. Our results show that the formation of AlPO 4 -18 undergoes several stages. Mixing phosphorus and aluminum sources together with a template at room temperature yielded a mixture containing an ordered aluminophosphate (AlPO) phase and a phosphate material. These materials were converted to an amorphous AlPO phase under hydrothermal conditions. Continuous heating resulted in the transformation of the amorphous phase into a crystalline microporous material, AlPO 4 -5. A further increase in heating time led to the disappearance of AlPO 4 -5 and the formation of AlPO 4 -18.