Synthesis and X-Ray Powder Structures of Three Novel Titanium Phosphate Compounds

Abstract

Three titanium phosphate porous compounds were prepared hydrothermally and their structures solved from their X-ray powder diffraction patterns collected on a laboratory X-ray source. The compound, [Ti3(PO4)4(H2O)2]·NH3(1), crystallizes in triclinic symmetry witha=8.2506(4),b=8.7879(4),c=5.1022(2) Å;α=90.703(1),β=91.083(1),γ=110.158(1)°; space groupP1, andZ=1. The structure was solvedab initioby direct methods and refined by Rietveld methods. The structure consists of two octahedrally coordinated titanium atoms, one of which is bound only by phosphate oxygens and the other by phosphate oxygens and water molecules. The bridging of titanium atoms by phosphate oxygens creates a framework consisting of one-dimensional channels oriented along thec-axis. The ammonia molecules are located in these channels. Compound (2), [Ti2O(PO4)2(H2O)2], also crystallizes in the triclinic space groupP1witha=8.818(1),b=9.654(1),c=5.109(1) Å;α=93.818(2),β=93.665(3),γ=73.313(3)°; andZ=2. The complete structure was solved by direct methods. The two independent titanium atoms are bridged by an oxygen atom. The remaining coordination sites of the octahedra are completed by phosphate oxygens and water oxygen atoms. As in the case of compound (1) the water molecules are coordinated to only one type of Ti atom. The structure consists of one-dimensional channels into which the water oxygens are projected. The compound (NH4)2[(Ti3O2) (HPO4)2(PO4)2] (3) crystallizes in the noncentric space groupP21, witha=8.5165(3),b=16.7331(5),c=5.1813(2) Å;β=91.173(2)°; andZ=2. A partial structural model was obtained from direct methods procedures and the structure was completed by Fourier methods following Rietveld refinement of the full pattern. In this case there are three independent titanium atoms and they are bridged by oxygen atoms of the Ti3O2group. All the Ti atoms are octahedrally coordinated. As in the case of compounds (1) and (2), compound (3) forms a framework structure containing one-dimensional channels. The hydroxyl groups of the monohydrogen phosphate and the ammonium cations are located in the channels. In the channel the Ti–PO4framework and the hydroxyl groups create cavities around the ammonium cations which block its exchange with other cationic species.