Observation and Reactivity of the Chainlike Species ([Al(PO₄)₂]^3-)<i>_n</i> during the X-ray Diffraction Investigation of the Hydrothermal Synthesis of the Super-sodalite Sodium Aluminophosphate MIL-74 (Na₂Al₇(PO₄)₁₂·4<i>t</i><i>renH</i><i>₃</i>·Na(H₂O)₁₆)

Abstract

The hydrothermal synthesis of the aluminum phosphate Na 2 Al 7 (PO 4 ) 12 ·4 trenH 3 ·Na(H 2 O) 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(PO 4 ) 2 · trenH 3 ·6H 2 O (phase 1 ), which transforms into the hydrate Al(PO 4 ) 2 · trenH 3 ·2.5H 2 O (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 AlO 4 alternating with the phosphate group PO 4 . The resulting ([Al(PO 4 ) 2 ] 3- ) n chains interact through hydrogen bonds with the trenH 3 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.