Synthesis, structure and properties of related microporous N,N′-piperazinebismethylenephosphonates of aluminum and titanium

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)center dot nH(2)O (eta similar to 3, L = O3P-CH2-NC4H8N-CH(2)Z, 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)-nH(2)O (n similar to 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 similar to 3.5 x 4.0 angstrom(2)) which adsorb nitrogen at 77 K to give Langmuir surface areas close to 500 m(2)center dot g(-1). Crystal data for MIL-91 (Ti) are as follows: space group C2/m. (No. 12) with a = 19.415(2) angstrom, b = 7.071 (1) angstrom, c = 11.483(1) 92.78(1)degrees, V 1574.70(1) angstrom(3), and Z = 2. Crystal Data for MIL-91 (Al) are as follows: space group C2/m (No. 12) With a = 18.947(2) angstrom, b = 6.915(1) angstrom, c = 11.295(1) angstrom, beta = 90.45(1)degrees, and V = 1479.90(1)angstrom(3).