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

The hybrid semiconductor-macrocycle catalyst, TiO2-Co-tetrasulfophthal ocyanine, (TiO2-CoTSP) effectively enhances the solar-promoted reducti ve dehalogenation of bromoform (CHBr3) under anaerobic conditions. Rea ction 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, dibromo methane (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 sac rificial electron donor, is oxidized stoichiometrically to acetone. Th e catalyst is stable in sunlight for at least 30 h without loss of act ivity. The reaction mechanism is postulated to involve nucleophilic at tack of Co(I)TSP, generated by the semiconductor, on CHBr3. The organo metallic complex that is formed, TiO2-CoTSP-CHBr2, is postulated to su bsequently undergo photolysis via homolytic Co-C bond cleavage to rege nerate the catalyst. 2-Propanol is oxidized by valence band holes.