"Microwave assisted modification of TiO2 by phosphonic acids: synthesis and in-depth characterization"
Hybrid materials represent one of the most growing new material classes since these are applicable in many industrial areas such as coatings, gas storage, separation processes, catalysis and optical device production. In recent years, research on hybrid materials formed by transition metal oxides modified with organic groups has increased. One of the most used metal oxides is TiO2 since this is particularly resistant towards corrosive environments and possesses enhanced photocatalytic activity.
The modification of TiO2 is usually carried out by surface grafting which can be described as a reaction between the Ti-OH groups and specific coupling functions introducing the organic part. These coupling agents usually compose a head group of a compound which then becomes grafted entirely at the TiO2 surface and further consists of a tail group that contains the organic group providing the added hybrid property. The most applied grafting method is the organosilylation which consists in the surface modification of TiO2 with organosilanes. To a minor extent, the surface grafting of phosphonic acids onto TiO2 has been applied. The latter method is based on the reaction between the P-OH groups of the phosphonic acid and the Ti-OH groups at the TiO2 surfaces resulting in the formation of stable P-O-Ti bonds. Literature on surface grafting of phosphonic acids clarified that the reaction between P25 TiO2 (the most used TiO2 nanopowder) and phosphonic acids is strongly influenced by the reaction temperature. In particular, it was demonstrated that the application of enhanced reaction temperatures (i.e. 100°C) leads to the dissolution of the TiO2 matrix and results in the formation of titaniumphosphonate structures (crystalline structures independent from the nanocrystals). The mechanism leading to the formation of titaniumphosphonate structures from P25 TiO2 has never been clarified and requires additional studies. The qualitative determination of P-O-Ti bonds for hybrid materials based on TiO2 modified with phosphonic acids is usually carried out by CP/MAS 31P solid state NMR and FTIR while lack of information concerning the quantitative determination of this class of hybrid materials emerges from literature screening. In recent years, an increased interest on the modification of TiO2 membranes with aromatic or alkyl phosphonic acids has been observed since this modification resulted in a valid solution to improve the performances for the membranes in the separation of solvents. While a noticeable amount of data regarding the performance of the modified membranes can be found in literature, few data concerning the description of the P-O-Ti bonds can be found.
In this thesis a detailed investigation concerning the modification of TiO2 with a various range of phosphonic acids is provided. Firstly, the influence of reaction conditions (temperature, solvent, phosphonic acid concentration) on the modification of P25 TiO2 (standard TiO2 for this kind of investigations) with phenylphosphonic acid (PPA) was investigated in detail. Surface grafting of PPA onto P25 TiO2 was carried out at different reaction temperature (between 20°C and 150°C) in water. Reactions were carried out in a microwave reactor which ensures the selective heating of the reactants combined with the important advantage of the superheating of the solvent (i.e. a reaction temperatures highly above the boiling point of the solvent under the enhanced pressure in the reactor).