BIOCATALYTIC TREATMENT OF ORGANOSULFUR COMPOUNDS IN EMULSIONS IN SUPERCRITICAL FLUIDS

Introduction Sulfur levels in refinery crude oil feedstocks are increasing annually, while at the same time, federal and local regulators are requiring reductions in these levels for the finished transportation fuels. Sulfur moieties of concern are typically organosulfur complexes. Production and utilization of oils with appreciable sulfur content results in increased environmental pressures and costs to producers, refiners, and end users. Environmentally, utilization of high sulfur fuel products results in production of gaseous sulfoxide chemicals that are believed to be partially responsible for “acid rain,” additionally, the sulfur in fuel has a detrimental affect on vehicle emission controls. In order to meet upcoming requirements and future ultraclean sulfur requirements for gasoline and diesel fuels, new economical sulfur removal processes must be developed. Biological removal of sulfur from oil (biodesulfurization) is a potentially attractive alternative to conventional refinery processes such as hydrotreating. Biodesulfurization processes offer the potential of lower capital and operating costs compared to traditional refinery processes and the potential to overcome many of the steric limitations of traditional catalytic methods. Biodesulfurization may occur via oxidative or reductive pathways. Oxidized products can be removed either by distillation or by heating above 300°C to eliminate sulfur dioxide. Practical limitations for aqueous phase bioprocessing of organic constituents are well recognized and include bioavailability of substrates, relatively slow reaction kinetics, catalyst recycle, expensive separation processes to recover the desired products, and maintenance of biological integrity. Proteins and whole cells have been shown to be active in organic solvents; however, organic solvents can be expensive, and separation or recycle from the reaction mixture can be difficult. Supercritical fluids (SCFs) systems offer several advantages over conventional water and liquid organic solvents as reaction media. SCFs provide high solubility of organic compounds, enhanced mass-transfer due to the elimination of liquid/liquid interfaces, enhanced reaction kinetics, control of product solubility with minor variations in temperature and pressure, and a more energy efficient separation of final products. Enzymatic activity in SCFs has been proven and well documented. Limiting factors which may affect enzymatic activity in SCF solvent systems have been identified and are well characterized. Reverse micelles and microemulsions consisting of water and SCFs can simultaneously disperse high concentrations of both hydrophilic molecules such as proteins and hydrophobic compounds. Larger amounts of water may be dispersed in SCFs by forming emulsions, allowing more hydrophilic materials to be solubilized in the fluid. Several research groups have demonstrated that stable reverse micelles, microemulsions and emulsions based on perfluoropolyetherammonium carboxylate (PFPE) (CF3O(CF2CF(CF3)O)nCF2COONH4) surfactant can be formed in supercritical carbon dioxide. These emulsions are easily broken by decreasing pressure, allowing for simple catalyst recovery and energy efficient product separation. We report the first biooxidation of organosulfur compounds in emulsions in supercritical fluids. Hemoproteins were chosen as biocatalysts for their ability to oxidize sulfides, thioanisoles, thiophenes and dibenzothiophenes (DBT) to their sulfoxide/sulfone products. DBT was chosen for the reaction studies as a compound typical of organosulfur compounds in crude oils and fuels. Oxidation of DBT is well known in aqueous buffer/organic solvents and organic solvents. Conversion up to 99% has been reported when the reaction was catalyzed by hemoglobin in aqueous buffer/organic solvents (75/25 vol.%) and 28% conversion when the reaction was performed in 99 vol.% ethanol. Protein catalysts used in the current study included horseradish peroxidase (HRP), hemoglobin (Hb), Cytochrome c (Cyt c), and soybean peroxidase (SBP). Supercritical fluids explored included carbon dioxide, methane, ethane and trifluoromethane. PFPE was selected as the surfactant for our CO2 studies. The biooxidation of DBT is depicted in Figure 1.