Synthesis, Structure and Properties of Related Microporous N,N‘-Piperazinebismethylenephosphonates of Aluminum and Titanium

Abstract

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(H 2 L)· n H 2 O ( n ∼ 3, L = O 3 P−CH 2 −NC 4 H 8 N−CH 2 −PO 3 ) was solved from a small single crystal and refined against laboratory powder X-ray diffraction data. The structure of the titanium form (TiO(H 2 L)· n H 2 O ( 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 TiO 6 or AlO 6 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 Å 2 ) which adsorb nitrogen at 77 K to give Langmuir surface areas close to 500 m 2 ·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) Å 3, 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)Å 3 .