The origin of the reduced phosphorus-carbon bond stability of α-aminoalkylphosphonic acid grafted on titania

Abstract

TiO₂ materials grafted with organophosphonic acids have been reported in various application fields, owing to the desired specific interactions provided by the organic functionalities, in combination with the structural properties and mechanical and chemical stability provided by the TiO₂ supports. In this work, the thermal stability of aminoalkylphosphonic acids and alkylphosphonic acids grafted on TiO₂ P25 was investigated with infrared spectroscopy and TGA, and the results show that the thermal stability of aminomethylphosphonic acid and (S)-(+)-1-aminoethylphosphonic acid grafted on TiO₂ P25 is significantly lower than that of other aminoalkylphosphonic acids and the alkylphosphonic acids on TiO₂ P25. Moreover, there is P–H bond formation associated with the detachment of the aminomethyl and aminoethyl moieties, in contrast to the other aminoalkylphosphonic acids and the alkylphosphonic acids. The results of DFT calculations confirm that the P–C bond strengths, expressed as values of the electron density in the bond, of the α-aminoalkylphosphonic acids grafted on anatase (1 0 1) are indeed lower than those of the other aminoalkylphosphonic acids and alkylphosphonic acids and provide a possible explanation.