PHOTOREDUCTIVE DEHALOGENATION OF BROMOFORM WITH TITANIUM-DIOXIDE COBALT MACROCYCLE HYBRID CATALYSTS

The hybrid semiconductor-macrocycle catalyst, TiO2-Co-tetrasulfophthalocyanine, (TiO2-CoTSP) effectively enhances the solar-promoted reductive dehalogenation of bromoform (CHBr3) under anaerobic conditions. Reaction rates are 4-10 times faster than those obtained using silanized TiO2, unmodified TiO2, or direct (uncatalyzed) solar photolysis under the same conditions. CHBr3 is reduced to its lower homologues, dibromomethane (CH2-Br2), and bromomethane (CH3Br). HBr is also produced. No other major dehalogenation products are observed, although methane is found in trace amounts after prolonged photolysis. 2-Propanol, the sacrificial electron donor, is oxidized stoichiometrically to acetone. The catalyst is stable in sunlight for at least 30 h without loss of activity. The reaction mechanism is postulated to involve nucleophilic attack of Co(I)TSP, generated by the semiconductor, on CHBr3. The organometallic complex that is formed, TiO2-CoTSP-CHBr2, is postulated to subsequently undergo photolysis via homolytic Co-C bond cleavage to regenerate the catalyst. 2-Propanol is oxidized by valence band holes.