TiO2 photocatalytic degradation of haloquinolines in water:: Aromatic products GM-MS identification.: Role of electron transfer and superoxide

To explain the differences between the product distributions obtained when quinoline (Q) in water is degraded by TiO2 photocatalysis and by the photo-Fenton process, we have proposed a mechanism based on the reaction between superoxide and quinoline radical-cation (Q(.+)), both species resulting from electron transfers at the TiO2 surface (J. Phys. Chem. B, 101, 2650(1997)). Here is reported a GC-MS identification of the products obtained by TiO2 photocatalytic degradation of 3-bromoquinoline and of 2-, 4-, and 6-chloroquinolines to determine the substituent effects and verify this mechanism. In all cases, besides halomonohydroxyquinolines, the main products resulted, as in the case of Q, from the oxidative cleavage of the heterocycle and were accounted for by the substituted-Q(.+) + O2(.-) reaction. With the halogen substituent on the pyridine ring, the latter reaction also took place on the benzene ring; the steric hindrance to the O-2(.-) attack onto the pyridine ring of substituted-Q(.+) supports the cycloaddition mechanism for this reaction. To account for the formation of 2-aminobenzaldehyde from 2-chloro and 3-bromoquinoline, initial reductive dehalogenation is assumed to occur to a small extent. The removal rates for 6-chloroquinoline and Q were similar and those for the other haloquinolines were about twice greater, i.e. ring deactivation for electrophilic substitution had no negative effect on these rates.