Incorporation of Lithium and Sodium in MIL-74 Super-Sodalites: A₂M₇(PO₄)₁₂·4tren·A(H₂O)₁₆ (with A = Li or Na and M = Al or Ga)

Incorporation of lithium and sodium in MIL-74 aluminum and gallium phosphates has been successfully achieved by hydrothermal synthesis resulting in A2Al7(PO4)12·4tren·A(H2O)16 (A = Li, Na) and Li2Ga7(PO4)12·4tren·Li(H2O)16. The structure-directing agent is the tris(2-aminoethyl)amine (tren), and the synthesis was carried during 36 h at a temperature of 453 K under autogenous pressure. Single-crystal X-ray diffraction and solid-state MAS NMR spectroscopies (6Li, 7Li, 23Na, 27Al, 31P) were used to establish the structure. Their crystal structures are cubic, and are described in I4̄3m space group. The open-framework is built up from a supersquare enneameric unit, T5P4, with five TO4 tetrahedra (T = {Li or Na, Al} or {Li, Ga}) and four triconnected PO4 tetrahedra. The supersquare substitutes the square units of the sodalite cage. The center of the supersquare is occupied by one TO4 connected by its four corners to four PO4 groups located in the middle of each of the sides of the supersquare. At each corner of the supersquare a TO4 unit connects four PO4 tetrahedra belonging to two supersquare units. The corner connected T5P4 supersquares generate a three-dimensional framework of super-sodalite topology. Its resulting cavities of 10 Å in diameter are limited by 12-ring windows closed by tren species attached to the PO4 by their terminal P−O, through the hydrogen bond. The multinuclear NMR analysis shows that the alkaline cations are incorporated into the aluminum phosphate network substituting partially into TO4. It also indicates that one alkaline cation is trapped at the center of the super-sodalite cage, surrounded by a double shell of water forming a cluster of 16 molecules (Li(H2O)16 or Na(H2O)16). The cationic (T = Al or Ga) occupancy is measured indirectly by 31P NMR, indicating some cationic order in the I4̄3m space group, getting closer to a description in the P4̄3n space group.