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            Dibenzothiophenes (DBTs) are sulfur-containing aromatic compounds found in diesel and other fuel stocks. The traditional hydrodesulfurization (HDS) treatment process does not remove these compounds efficiently. Therefore, HDS is not suitable for producing fuels that meet the more stringent standards for Ultra-Low Sulfur Diesel. Oxidative desulfurization (ODS) is currently receiving significant attention as an alternative to HDS. ODS is a two-step process where chemical oxidation of the sulfur compounds is followed by extraction of the oxidized products from the fuel. The reaction of the DBTs with oxidants typically requires the use of a catalyst. The goal of this project is a detailed understanding of the metalloporphyrin catalyzed oxidation of DBTs.

            During the 2008-2009 funding period, we found that reactions performed in a CH₂Cl₂/CH₃CN/methanol mixture with 50 mM substrate, 5 equivalents of hydrogen peroxide and 1% catalyst gave conversions of >99%, 98%, 97% and 46% for BT, DBT, 4-MDBT and 4,6-DMDBT, respectively. The sulfoxide and sulfone derivatives of DBT were identified as the products of the reaction by GC-MS. For all substrates, both the sulfoxide and sulfone products were observed by GC-MS, and minimal conversion was observed in the absence of the catalyst. These results demonstrated that Fe(PFPP)Cl catalyzes for the oxidation of DBTs. 

            The reaction conditions were modified to investigate how well the reaction proceeds when octane replaced CH₂Cl₂/CH₃CN as the main solvent. High conversions were observed using only a minor excess of oxidant when the reactions were performed with substrate concentrations of 50 mM. Since concentrations around 200 ppm for DBT, BT, 4-MDBT and 4,6-DMDBT are more typical for real fuels, we modeled fuels using an octane solution containing all four substrates at a concentration of 200 ppm. Preliminary reactions with the model fuel showed that larger excesses of oxidant are required to achieve good conversions in reasonable time periods.

            During 2009-2010, we continued the model fuel studies. Oxidation of the sulfur compounds was achieved using 10 equivalents (relative to total sulfur) of hydrogen peroxide in the presence of 1% Fe(TPFPP)Cl. GC-MS of the oxidized model fuel revealed a mixture of sulfoxide and sulfone products.  The observed conversions of BT, DBT, 4-MDBT and DMDBT were 70%, 85%, 65% and 24%, respectively. The model fuel was subjected to a simple oxidation-extraction scheme. For comparison purposes, samples of reacted and unreacted model fuel were extracted with methanol. Extraction of the unoxidized model fuel reduced the BT, DBT, 4-MDBT and 4,6-DMDBT concentrations in the model fuel by 68%, 55%, 45% and 28%, which corresponds to a reduction of the total S content in the model fuel of 50%. Oxidation-extraction of the model fuel dropped the BT, DBT, 4-MDBT and 4,6-DMDBT concentrations in the model fuel by 98%, 95%, 94% and 77%, respectively. Therefore, oxidation-reduction treatment decreased the total sulfur content of the model fuel by approximately 91%.  These model fuel experiments showed that complete conversion of the DBT derivatives to either the sulfoxide or sulfone is sufficient to facilitate their extraction from a hydrocarbon phase.

            Most published ODS schemes involve oxidation of DBT and related compounds to the corresponding sulfones prior to extraction. The reported reactions are typically optimized to maximize the yield of sulfones, presumably because the sulfones are more polar than the corresponding sulfoxides. As a result, the literature suggests that high conversion to the sulfones is necessary for successful extraction. To provide further support for our contradictory observations, experiments to determine the extraction efficiencies of both the sulfoxides and sulfones were initiated. Since only DBT sulfone is commercially available, these experiments required synthesis of the desired compounds. Sufficient quantities of DBT sulfoxide, 4-MDBT sulfoxide, and 4,6-DMDBT sulfoxide were synthesized from the sulfides using a slight excess of hydrogen peroxide and Fe(TDCPP)Cl as the catalyst. The products were isolated and purified using chromatography on silica gel. We observed yields and selectivities for the sulfoxides that matched or exceeded those reported previously. 4-MDBT sulfone and 4,6-DMDBT sulfone could not be obtained simply by increasing the amount of hydrogen peroxide used in the reaction.  Therefore, the sulfones were obtained by reacting the sulfides with excess MCPBA.

            20 ppm solutions of DBT, DBT sulfoxide and DBT sulfone were extracted with methanol. The octane layers were analyzed by HPLC before and after treatment with methanol.  Based on qualitative comparisons of the peak areas, both the DBT sulfoxide and DBT sulfone were more efficiently removed than DBT.  Furthermore, while some DBT sulfone remained in the octane layer, DBT sulfoxide was not detected in the octane layer following methanol extraction. Therefore, although we have not precisely determined the partition coefficients, these preliminary results suggest that the sulfoxide is extracted at least as efficiently as the sulfone. Since complete conversion of sulfide to sulfone requires at least 100% more oxidant than conversion to the sulfoxide, ODS treatment processes would be more economical if the sulfoxides can be efficiently extracted from fuels. To more rigorously support our hypothesis, octanol-water partition coefficients for these compounds will be measured using standard protocols.

            Funding of this project is extremely important for the PIs overall research program and the undergraduates working on the project. To date, four UWEC students have worked on this project and three students are currently being supported.  Another undergraduate will start work on the project in the Spring of 2011. Dan Swedien and Erin Stuckert presented their work at the National ACS meeting in San Francisco using travel funds provided by this grant. These students will graduate at the end of this academic year and both will continue their education. Dan is applying to Medical and MD/Ph. D programs and Erin is applying to chemistry Ph. D. programs.

                This project provided a suitable research experience for Charles Thurber (UW-Fox Valley) for the 2010 summer with additional funding provided by the PIs NSF-REU grant. Mr. Thurber is applying for admission to UWEC as a transfer student. Once admitted, it is expected that he will continue to work on this project.