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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 2008-2010 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. 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. The reaction conditions were modified to investigate how well the reaction proceeds when octane replaced CH₂Cl₂/CH₃CN as the main solvent. 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. 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. 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 observation that complete conversion to the sulfone is not required for a successful ODS process, extraction experiments on authentic samples of DBT sulfoxide and sulfone were performed. Importantly, DBT sulfoxide was not detected in the octane layer following methanol extraction.

During the 2010-2011 funding period, we optimized the synthetic procedures for metalloporpyrin catalyzed sulfoxidation of dibenzothiophene, 4-MDBT and 4-DMDBT. All syntheses were performed using 1% Fe(TPFPP)Cl as the catalyst, and the desired sulfoxides were purified on silica gel using a mixture of hexane and ethyl acetate. The highest isolated yields of sulfoxide were obtained when 1.1, 1.25 and 10 eq of hydrogen peroxide were employed for the oxidations of DBT, 4-MDBT and 4,6-DMDBT, respectively. The isolated yields for the DBT reaction were 93% sulfoxide and 4% sulfone. For isolated yields for the 4-MDBT reaction were 89% sulfoxide and 8.5% sulfone. For 4.6-DMDBT, the isolated yield of the sulfoxide was 35% for the reaction with hydrogen peroxide. If MCPBA was employed as the oxidant in place of hydrogen peroxide, the 4,6-DMDBT sulfoxide was isolated in 72% yield and the sulfone in 15% yield. These observed yields and selectivities for the sulfoxides matched or exceeded those reported previously.

Our data showed that DBT sulfoxide is efficiently extracted from octane by methanol, and that that the less polar sulfoxides interact more strongly (longer HPLC retention times) with silica than the sulfones. We sought a theoretical explanation for these observations, and we approached the problem both experimentally and computationally. Using biphenyl sulfoxide and sulfone as models for the DBT compounds, the interaction between the sulfur compounds and methanol were modeled. Both the sulfoxide and sulfone could form hydrogen bonds with methanol. The computations showed that the methanol H to sulfoxide O distance was shorter than methanol H to sulfone O distance. Experiments were performed to determine the enthalpies of mixing for diphenyl sulfoxide and diphenyl sulfone with methanol. The measured enthalpies of mixing for the sulfoxide and the sulfone were -0.39 ± 0.0039 cal/mol and -0.34 ± 0.0034 cal/mol, respectively. Therefore, despite their lower dipole moments, we believe that hydrogen bond type interactions are responsible for the efficient removal of sulfoxides from octane when the extraction solvent is methanol. The optimized sulfoxidation procedures described above will allow us to synthesize the sulfoxides and sulfones on the scale required to repeat the heat of solution experiments for the dibenzothiophenes and its alkyl derivatives.

Funding of this project is extremely important for the PIs overall research program and the undergraduates working on the project. Two UWEC students were supported during this grant period. Charles Thurber and Mariah Dorner presented their work at a Regional ACS meeting in Indianapolis using travel funds provided by this grant. Mariah has started her junior year and she is still considering graduate school as an option. Charles is committed to applying to graduate schools next fall. To date, a total of six UWEC students and three NSF-REU have worked on this project. Three students who were previously supported by the grant have now graduated from UWEC. Of those students, Dan Swedien is now in medical school, Erin Stuckert is attending graduate school at Colorado State and Mike Williams is working.