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Ind. Eng. Chem. Res. 2006, 45, 518-524
Oxidation of Aliphatic and Aromatic Sulfides Using Sulfuric Acid James Nehlsen, Jay Benziger,* and Ioannis Kevrekidis Department of Chemical Engineering, Princeton UniVersity, Princeton, New Jersey 08544
Sulfuric acid is proposed as an inexpensive oxidant for use in the oxidative desulfurization (ODS) of petroleum. The oxidative extraction of thiophene, tetrahydrothiophene, and dibutyl sulfide by concentrated sulfuric acid in a two-liquid-phase reactor is demonstrated. The reaction proceeds rapidly at room temperature at sulfide concentrations greater than ∼100 ppm S and yields sulfoxides, sulfonates, and other oxidized sulfur species as products. The oxidized sulfur species are effectively extracted into the acid phase. The reactions are firstorder in sulfide at high acid concentrations and first-order in acid at lower acid concentrations. This technique might be able to reduce the load on hydrodesulfurization (HDS) or ODS reactors by pretreating high-sulfur feed streams. I. Introduction The removal of sulfur from petroleum-derived fuels is a critical part of modern refining. Sulfur compounds inhibit the operation of catalytic converters in automobiles and form SO2 when combusted, leading to acid rain and other environmental problems. The removal of sulfur to low levels in diesel fuel has recently become required by law.1 The traditional method of removing sulfur, hydrodesulfurization (HDS), has limited effectiveness at removing thiophene derivatives, such as dibenzothiophene, from heavier diesel fuel.2 Oxidative desulfurization (ODS) is a promising new method for the removal of these heavier sulfur compounds.2,3 ODS selectively oxidizes sulfur compounds to form sulfoxides or sulfones; the sulfoxides and sulfones can be extracted with a polar solvent. Sulfur has a strong affinity for oxygen, and oxygen can be added to sulfides and thiophene derivatives without breaking any carbon-carbon bonds. This process has been shown to effectively remove virtually all of the sulfur from a diesel fuel stream4 and is effective on the polynuclear aromatic family of sulfur compounds including substituted dibenzothiophene.5,6 ODS typically uses hydrogen peroxide as the oxidizing agent, often in conjunction with acetic or formic acid (forming a peroxy acid).6-8 The cost of hydrogen peroxide is one of the limiting economic factor in ODS. At the current cost of hydrogen peroxide, ODS is commercially viable only on streams with low starting levels of sulfur (99%), (2) 2000 ppmw sulfur (∼0.047 M) as tetrahydrothiophene in tetrahydrofuran (Alfa Aesar, >99%), (3) 2050 ppmw sulfur (0.056 M) as thiophene (Aldrich, >99%) in toluene (Sigma-Aldrich, 99.5%), and (4) dibutyl sulfide (Aldrich, 96%) in toluene. A small amount of decane (Aldrich, >99%) was added to each stock solution as an internal standard for GC analysis. Ten milliliters of the organic solution and 10 mL of sulfuric acid (dilutions of 96% H2SO4, Fisher) were added to the reaction vessel, forming two liquid phases. The reaction temperature was 22 °C. The two-phase mixture was vigorously stirred for 1 h (unless otherwise indicated). After reaction, the phases were allowed to separate, and the organic layer was removed by syringe to prevent further reaction. The concentration of the sulfide species in the organic layer was measured by gas chromatography (30-m methyl silicone capillary column, flame ionization detector). The concentration of unreacted sulfide species was determined by normalizing all results using the internal standard (decane). We were unable to
10.1021/ie050901m CCC: $33.50 © 2006 American Chemical Society Published on Web 12/17/2005
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Figure 2. Reduction of sulfuric acid to elemental sulfur by tetrahydrothiophene. The top layer is THT in toluene; the bottom layer is 16 M sulfuric acid. The reduction produces the Tyndall effect with formation of colloidal sulfur.
accurately analyze the composition of the aqueous layer by GC because of the high acid content. Compositions of organic species in the aqueous phases are semiquantitative. To analyze the aqueous phase by GC, the solutions were diluted to reduce the acid concentration. However, the acid either adsorbed or reacted with the GC column packing, and the integrated GC peaks varied by 10-25%. Elution times were reproducible, so we could identify products but not the product yields. Quantitative analysis was limited to the organic phase. Product identification was carried out with GC/MS analysis. Tetrahydrothiophene (THT) and thiophene were reacted in a homogeneous solution of tetrahydrofuran and sulfuric acid. Ten milliliters of a of 10 vol % solution of THT or thiophene in tetrahydrofuran (THF) was mixed with 10 mL of sulfuric acid. A single-phase solution was formed and permitted to react for ∼1 h at 22 °C. The mixtures were diluted with water to reduce the acid concentration, and samples were analyzed by gas chromatography/mass spectroscopy in the analytic chemistry laboratories at Princeton University. The mass fragmentation patterns were used for product identification. III. Results All three sulfide compounds were successfully removed from the hydrocarbon solvent by reaction with sulfuric acid and extraction into the aqueous (acid) phase. No oxidized reaction products were detected in the organic phase, and only trace amounts (
