Functionalisation versus mineralisation of some N-heterocyclic compounds upon UV-illumination in the presence of un-doped and iron-doped TiO2 photocatalysts

Heterogeneous photocatalytic oxidation of some N-heterocyclic compounds (4-picoline, and 6- and 8-methylquinoline) in oxygenated solvents (water or acetonitrile), containing dispersed photocatalyst (un-doped or iron-doped titanium dioxide), was investigated under UV-illumination in a photochemical reactor. This work aimed to correlate experimental parameters such as structural aspects of the substrates, photocatalyst chemical and surface properties, illumination times, and the nature of the solvent with the extent of mineralisation of the substrates and, also, possible selective methyl group functionalisation.Analysis of the products resulting from heterogeneous photocatalytic oxidation of 6- and 8-methylquinoline suspensions in oxygenated acetonitrile with illumination periods of <24 h detected, in both cases, the corresponding formyl derivatives quinoline-6- and -8-carbaldehyde, though at low levels. The presence of water appeared to inhibit heterogeneous photocatalytic functionalisation. However, the heterogeneous photocatalytic degradation of such compounds in water proceeds via polyhydroxylated intermediates which consequently undergo mineralisation, which, from a pollution control perspective is beneficial.The physicochemical properties of the photocatalyst were also shown to be influential. Particularly, differences in the affinity to, and mode of adsorption of the substrate compounds studied gave rise to differences in the extent of oxidation.Analysis of photogenerated oxidation products enabled some mechanistic insight into the course of the semiconductor-mediated reaction.The results obtained allow a useful comparison of the functionalisation of N-Heterocyclic compounds via heterogeneous photocatalytic processes in the absence of water, to those carried out in the presence of water, which gave complete mineralisation.