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A porous framework titanium(IV) N,N‘-piperazinebis(methylenephosphonate) (MIL-91(Ti)) and its aluminum analogue have been prepared under hydrothermal conditions (MIL = Material Institut Lavoisier). The structure of the aluminum analogue, AlOH(H2L)·nH2O (n ∼ 3, L = O3P−CH2−NC4H8N−CH2−PO3) was solved from a small single crystal and refined against laboratory powder X-ray diffraction data. The structure of the titanium form (TiO(H2L)·nH2O (n ∼ 4.5)) was determined using the structure of the aluminum form as a starting model and refining it against laboratory X-ray data. Their structures are built up from trans corner-sharing chains of TiO6 or AlO6 octahedra linked together in two directions via the diphosphonate groups. In each case this gives rise to a three-dimensional hybrid network with small channels along the b axis, filled with free water molecules. Thermogravimetric analysis and X-ray thermodiffractometry of the samples reveal that water is lost reversibly below 423 K and that both structures are stable up to 463 K. Dehydration gives porous solids (pore size ∼ 3.5 × 4.0 Å2) which adsorb nitrogen at 77 K to give Langmuir surface areas close to 500 m2·g-1. Crystal data for MIL-91(Ti) are as follows: space group C2/m (No. 12) with a = 19.415(2) Å, b = 7.071(1) Å, c = 11.483(1) Å, β = 92.78(1)°, V = 1574.70(1) Å3, and Z = 2. Crystal Data for MIL-91(Al) are as follows: space group C2/m (No. 12) with a = 18.947(2) Å, b = 6.915(1) Å, c = 11.295(1) Å, β = 90.45(1)°, and V = 1479.90(1)Å3.
Serre et al. (Sat,) studied this question.