Observation and Reactivity of the Chainlike Species ([Al(PO4)2]3-)n during the X-ray Diffraction Investigation of the Hydrothermal Synthesis of the Super-sodalite Sodium Aluminophosphate MIL-74 (Na2Al7(PO4)12·4trenH3·Na(H2O)16)

Abstract

The hydrothermal synthesis of the aluminum phosphate Na2Al7(PO4)12·4trenH3·Na(H2O)16 (MIL-74) has been studied by X-ray diffraction (conventional source) and in-situ time-resolved energy-dispersive X-ray diffraction (synchrotron radiation). Before the crystallization of MIL-74, the reaction of the starting aqueous aluminophosphate gel containing the tren species (tris(2-aminoethyl)amine) and sodium hydroxide gives rise to the formation of a hydrated chainlike solid Al(PO4)2·trenH3·6H2O (phase 1), which transforms into the hydrate Al(PO4)2·trenH3·2.5H2O (phase 2) upon heating at 180 °C. Its structure was determined by means of single-crystal X-ray diffraction and consists of a one-dimensional inorganic chain of tetrahedral unit AlO4 alternating with the phosphate group PO4. The resulting ([Al(PO4)2]3-)n chains interact through hydrogen bonds with the trenH3 and water molecules, generating a layerlike inorganic−organic network. Phase 2 was observed to be an intermediate, which disappears when the MIL-74 solid starts to crystallize. The observation of such an intermediate phase allows for the proposition of a chemical reaction pathway for the formation of the three-dimensional network of MIL-74. It seems that the sodium cations play a crucial role and directly react with the chainlike precursor. They would participate in the construction of the inorganic framework as anchorage nodes during the condensation process.