Structural and proton conductivity studies of fibrous π-Ti2O(PO4)2·2H2O: application in chitosan-based composite membranes

Abstract

Although the fibrous polymorphic modification of titanium phosphate, π-Ti₂O(PO₄)₂·2H₂O (π-TiP) has been known for decades, its crystal structure has remained unsolved. Herewith, we report the crystal structure of π-TiP at room temperature, as determined from synchrotron radiation powder X-ray diffraction, and corroborated by ³¹P solid state NMR and accurate density functional theory calculations. In contrast to the previously reported ρ-TiP polymorph, the as-synthesized hydrated phase crystallizes in the monoclinic system (P2₁/c, a = 5.1121(2) Å, b = 14.4921(9) Å, c = 12.0450(11), β = 115.31(1)°, Z = 4), and is composed of corner-sharing titanium octahedra and phosphate units arranged in a pattern that is unique to the ρ-TiP polymorph. The unit cell was confirmed by electron diffraction, while the formation of planar packing imperfections and stacking faults along the [101] plane was revealed by HRTEM analysis. An in situ dehydration study of π-TiP, monitored by high-temperature powder X-ray diffraction, led to a new anhydrous monoclinic (P2₁/c, a = 5.1187(13) Å, b = 11.0600(21) Å, c = 14.4556(26), β = 107.65(2)°, Z = 4) phase that crystallizes at 500 °C. The latter resembles the packing fashion of the parental π-TiP, albeit titanium atoms are present in both distorted tetrahedral and octahedral coordination environments. Anhydrous π-TiP was found to partially rehydrate at room temperature, reversibly adopting the structure of the initial phase. The studies carried out under different conditions of leaching and impregnation with H₃PO₄ showed that π-TiP exhibits an extrinsic proton conductivity (1.3 × 10⁻³ S cm⁻¹ at 90 °C and 95% RH) due to the presence of the protonated phosphate species bound on the particles surface, as revealed by ³¹P MAS-NMR spectroscopy data. The composite membranes of Chitosan (CS) matrices filled with H₃PO₄-impregnated π-TiP solid show an increment of proton conductivity up to 4.5 × 10⁻³ S cm⁻¹, at 80 °C and 95% RH, which is 1.8-fold higher than those of the bare CS membranes.