Synthesis and structural characterisation of solid titanium(IV) phosphate materials by means of X-ray absorption and NMR spectroscopy

Solid titanium phosphate, TiP, materials hold great promise for wastewater treatment for removal of metal ions and complexes. A series of TiP materials, synthesised at mild conditions and short reaction times, have been structurally characterised using solid-state X-ray absorption spectroscopy, phosphorus and titanium K edge XANES and EXAFS, and P-31 and Ti-47/49 NMR spectroscopy. The titanium K edge EXAFS data of alpha-Ti(HPO4)(2)center dot H2O (alpha-TiP) revealed octahedral coordination of oxygens around titanium. Repeated washing of primary beta-/gamma-TiP with hydrochloric acid results in formation of a weakly ordered solid, TiO(OH)(H2PO4)center dot H2O, TiP1-H. The structure of TiP1-H is shown by Ti EXAFS to be a titanyl compound, containing a short Ti=O bond. The analogous data for linked titanium phosphate compounds (LTP) disclosed that inter-linkage occurs between alpha-TiP and titanyl phosphate units, supported by P-31-P-31 NOESY NMR data. Ti-47/49 NMR and Ti pre-edge XANES show evidence of two different titanium environments in LTP, one very similar to that observed in TiP1-H and a second more symmetric octahedral environment. Data are discussed in terms of induced acidic hydrolyses of titanium(IV) and phosphate counterpart during washings with hydrochloric acid and water. A straightforward relation between synthesis parameters/post synthetic treatment and structural re-arrangement in the materials is established.