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.

Automated summary

Solid titanium phosphate materials are promising for wastewater treatment. Structural characterizations reveal complex structures and different titanium environments. The treatment with hydrochloric acid and water significantly affects the material structure.