OXYFLUORINATED MICROPOROUS COMPOUNDS ULM-N - CHEMICAL-PARAMETERS, STRUCTURES AND A PROPOSED MECHANISM FOR THEIR MOLECULAR TECTONICS

ULM phases appear in systems of the type M(2)O(3) (M=Al, Ga)/P2O5/AF (A=H+, NH+)/amine/H2O under hydrothermal conditions (453 K, autogeneous pressure, 24 h). The nature of the phases is influenced by the source of fluorine (HF or NH4F), the pH and, finally, the amine concentration and shape. The presence of NH4+ or large amounts of amine suppress the templating role, while pH controls the hydration and protonation of the template, the coordination polyhedra around M, their type of connection and the kind of secondary building units (SBU) which lead to framework formation. The geometry of the cavity depends on the spherical or linear shape of the amines. Only a limited number of SBU types are necessary for describing all the structures and allow a mechanism to be proposed explaining the formation of the ULM-n phases. It proceeds in three steps: an oligomeric condensation in the solution governed by the charge density of the species; the formation of zero-charge ammonium-SBU (MBU) associations; and infinite condensation of these MBU to give the resulting solid.