Synthesis and Characterization of Mesostructured Titanium(IV) Fluorophosphates with a Semicrystalline Inorganic Framework

Abstract

Hexagonal and lamellar mesostructured titanium(IV) fluorophosphates [CH3(CH2)nNR3]xTi2OF2H2-x(PO4)2·2H2O (x = 1, 2; n = 9−17; R = CH3, H) have been synthesized using aqueous solutions of titanium fluorophosphates and surfactants such as alkyltrimethylammoniumbromide or alkylamine. X-ray powder diffraction and HREM show that the hexagonal phase consists of arrays of hexagonal mesopores filled with surfactant. An extensive study of the synthesis conditions of the hexagonal CTAB phase was conducted with parameters such as the nature of the surfactant, concentrations of precursors, molar ratios, presence or absence of stirring, aging times, and temperature. Well-crystallized compounds appear only at low concentrations of surfactant and titanium, and the formation of the hexagonal phase occurs through the competitive complexation of titanium by fluorine and phosphates. It appears that the mesostructures form almost immediately at room temperature, whereas crystallization of the inorganic walls occurs only during hydrothermal aging. HREM, XRD, and solid-state NMR experiments confirm the existence of a semicrystalline order inside the inorganic framework. Removal of the surfactant by calcination of the hexagonal phase under nitrogen atmosphere leads to a mesoporous solid with semicrystalline pore walls exhibiting a surface area of over 400 m2/g. A mechanism of formation is proposed on the basis of the TEM results, followed by a brief discussion concerning the mesostructured metallophosphates synthesized to date.