Facile one-pot synthesis of diverse crystalline titanium phosphate ion-exchangers via MW-assisted hydrothermal treatment

Herein, for the first time, the MW-assisted hydrothermal treatment of a titanium precursor in aqueous orthophosphoric acid, precisely controlled by varying conditions, successfully yielded a complete library of crystalline titanium phosphates (TiP), including the α-phase ( α-TiP , Ti(HPO 4 ) 2 ·H 2 O), the π-phase ( π-TiP , Ti 2 O(PO 4 ) 2 ·2 H 2 O), the ρ-phase ( ρ-TiP , Ti 2 O(PO 4 ) 2 ·2 H 2 O), and unresolved metastable phases. The synthesis conditions necessary for the formation of each crystalline phase of titanium phosphates were studied in detail. The resulting materials were thoroughly characterized using powder X-ray diffraction (PXRD), Electron microscopy, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The crystalline phase and morphology of the obtained nanomaterials were strongly correlated with the P: Ti ratio and the corresponding H 3 PO 4 concentration in the reaction mixture. The reactions that take place with relatively low reagent concentrations (≤ 1.0 M) and P: Ti molar ratios were resulted in an unindexed crystalline solids with flexible layers. The pure crystalline π-TiP, ρ-TiP , and α-TiP phases confirmed by PXRD data and exhibiting both nanofibrous and layered plate-like particles, were successfully obtained at H 3 PO 4 concentrations of 1.4 M, 1.8 M, and 3.0 M, respectively. The developed "one-pot" synthesis of crystalline titanium phosphates is superior to previously reported routes, being both rapid (4 h) and environmentally responsible. In addition, the activation energy (E a ) of thermal dehydration as compared the properties of all the obtained crystalline phases was calculated, applying the Vyazovkin isoconversional method, based on TGA analysis. The thermal decomposition of the crystalline matrices clearly occurs in two or three weight loss steps. The maximal E a values for the loss of zeolitic water were determined as 338.2 kJ/mol for π-TiP , 345.1 kJ/mol for ρ-TiP , and a substantially higher 440 kJ/mol for α-TiP . This trend clearly establishes that α-TiP exhibits the highest thermal resistance to zeolitic water removal. Experiments demonstrate that the obtained materials function as effective ion exchangers, exhibiting differential selectivity toward alkali cations.