31P solid-state NMR and ATR-FTIR characterization of TiO2 functionalized with phosphonic acids for separation and purification applications

Abstract

Transition metal oxides, as titanium dioxide, are gaining interest because of their intrinsic chemical and corrosive stability in a wide variety of solvents and pH-ranges1. The modification of these materials is usually accomplished to incorporate chemical groups on the surface, introducing new properties in view of applications such as chromatography, adsorbents, sensors and membranes2. The reaction between hydrophilic titanium dioxide (TiO2) and phosphonic acids, as phenylphosphonic acid (PPA) or octylphosphonic acid (OPA), results in the formation of an hydrophobic surface layer. Aromatic or alkyl phosphonic acids can be bonded to the surface by physical adsorption or chemically through covalent bonds. Generally there are two reaction mechanisms involved in the formation of Ti-O-P bonds: i.e. heterocondensation or Ti-O-Ti cleavage. The first mechanism is based on the esterification between the phosphonic group and the hydroxyl groups at the titanium surface3 while the second mechanism is based on the protonation of oxo groups (Ti-O-Ti) of the metal oxide tetragonal structure4. Ti-O-P bonds are characterized by ATR-FTIR and 31P-CP/MAS NMR techniques. Modified titanium dioxide materials with enhanced hydrophobicity, obtained by dedicated synthesis, are tested in solid phase extraction experiments to verify their applicability in separation processes.

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