Synthesis and Characterization of New Lamellar Templated Titanium(IV) Phosphates with Perforated Layers: MIL-28n or Ti3O2X2(HPO4)x(PO4)y·(N2CnH2n+6)z·(H2O)2 (n = 2, 3; x = 0, 2; y = 4, 2; z = 3, 2; X = F, OH)

Abstract

The title compounds (MIL-28 n ) were prepared hydrothermally (3-4 days, 453-473 K, autogenous pressure) in the presence of organic bases (NH 2 -(CH 2 ) n -NH 2 (n = 2, 3)). The structure of MIL-28 3 or [Ti 3 O 2 (OH) 2 (HPO 4 ) 2 (PO 4 ) 2 ].(NH 3 -(CH 2 ) 3 -NH 3 ) 2 .(H 2 O) 2 has been determined ab initio from powder diffraction data and was refined in the orthorhombic space group Fm2m (no. 42). Its unit cell is a = 18.331(2) A, b = 18.943(3) A, c = 7.111(1) A, V = 2469.2(2) A 3 , z = 4. MIL-28 3 is two-dimensional. Its inorganic layers are made of trans-linked corner-sharing TiO 6 octahedra chains connected with tetrahedral phosphate groups and related via isolated TiO 4 (OH) 2 octahedra. This creates 10-member-ring apertures within the inorganic layers in which diamine molecules are present. Both water molecules, located in the interlayer space, and organic agents interact with terminal OH groups and ensure the stability of the structure. MIL-28 2 or [Ti 3 O 2 F 2 (PO 4 ) 4 ].(NH3-(CH 2 ) 2 -NH 3 ) 3 .(H 2 O) 2 crystallizes in a triclinic space group (P1 (no. 1)) with the following parameters: a = 10.071(5) A, b = 18.920(2) A, c = 7.107(4), a = 89.88(2), β = 106.23(2)°, y = 100.45(1)°, and V = 1273.5-(3) A 3 . Although the structure of compound MIL-28 2 has not been determined, a brief study will show that it shares strong features with those of MIL-28 3 .