Photocatalytic transformations of sulphonamides on titanium dioxide

Aqueous suspensions of titanium dioxide containing sulphonamides have been irradiated with near-UV light. The transformation processes have been followed by monitoring both the organic intermediates and the final products. The disappearance of sulphonamides and the formation of organic intermediates have been pursued through an HPLC/MS instrument. The degradation of all the investigated sulphonamides (sulphamerazine, sulphadiazine, sulphadimethoxine and sulfathiazole) is achieved through the breaking of the S-N bond and through an (OH)-O-. attack on the aromatic rings. The more intriguing matter is the fate of the intermediates originated through the cleavage of the S-N bond. A structure like RNH2 is formed, whose fate is highly dependent on the nature of R. From sulphathiazole and sulphadimethoxine, this intermediates (2,6-dimethoxy-4-aminopyrimidine and 2-aminothiazole) undergo a further degradation, so bringing to the ring-opening and to the release of the nitrogen contained in these structures. In fact, in both cases the parallel release of sulphur and nitrogen from the molecule is realized. Sulphur is finally found as sulphate ions, while nitrogen is transformed into nitrate and ammonium ions. The N-containing intermediates formed from sulphadiazine and sulphamerazine (4-methyl-2-aminopyrimidine) is highly persistent and does not release the nitrogen in the investigated times.