Open-framework fluorinated gallium and aluminium phosphates: an in situ study of the hydrothermal synthesis by X-ray diffraction using synchrotron radiation

The formation mechanisms of microporous materials synthesized under mild hydrothermal conditions are still not elucidated and it is necessary to utilize in situ characterizations for gaining information and new insights concerning the chemical processes which take place within the cell. The hydrothermal crystallizations of the open-framework gallium and aluminium fluorinated phosphates with the ULM-3 and ULM-4 structural types have been investigated by time-resolved energy-dispersive X-ray diffraction. It has been shown that the nature of the phosphorus source (H3PO4, polyphosphoric acid or P2O5) could drastically affect the crystallization behaviour. For example, in the gallium system, an intermediate appears at short reaction times before the formation of the ULM-3 structure when P2O5 is used instead of H3PO4, whereas no other phase is observed during the crystallization of ULM-4. However, with aluminium, an intermediate phase is systematically formed before the crystallization of the ULM-3 solid. Kinetics data for each system have been modelled using the Avrami–Erofe’ev equation and these calculations indicate the reactions to be diffusion controlled.