Kinetic analysis of thiol oxidation to study the effects of fluorinated groups on metal phthalocyanine catalysts

The oxidation of thiol (RSH) to disulfide (RSSR) is important biologically and industrially. Corrosive and malodorous thiols exist as contaminants in wastewater discharge from mining facilities, pulp and paper mills, tanneries, and oil refineries. The elimination of thiols from petroleum products is necessary for even cleaner fuels. Thiols in gas products can also inhibit catalyst activity for some downstream processes.Experiments and mechanistic kinetic studies were conducted for the aerobic oxidation of 2-mercaptoethanol (2-ME) and 4-fluorobenzenethiol (4-FBT) catalyzed by cobalt phthalocyanines: H16PcCo, F16PcCo, and F64PcCo, each exhibiting a metal center subject to increasing Lewis acidity and steric hindrance. The experiments were performed in a reaction-limited, isothermal, bench-scale, semi-batch reactor, with thiol concentrations measured using GC/FID. Conversions of 2-ME to 2-hydroxyethyl disulfide and 4-FBT to 4-fl uorophenyl disulfide in excess of 90% are achieved.Kinetic analyses suggest that the substrate binding and electron transfer are directly related to the Lewis acidity and steric bulkiness of catalyst molecules. Radical expulsion seems to be related to steric bulkiness. Substrate binding was found to be the slow step for thiol oxidations catalyzed by H16PcCo. The rate determining step for thiol oxidations, catalyzed by F16PcCo and F64PcCo, is the expulsion of the thiyl (RS•) radical from the catalyst molecule. Catalytic models show that the radical coupling to form the disulfide (RSSR) product occurs in solution, outside the catalyst cavity.