VIEWS: 3 PAGES: 2 POSTED ON: 11/3/2011
Vapor-Phase Oxidesulfurization (ODS) of Organosulfur Compounds over Supported Metal Oxide Catalysts Sukwon Choi and Israel E. Wachs Dept. of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 Introduction In the past few years, several studies by EPA concluded that the presence of sulfur in gasoline has an adverse impact on the performance of automotive catalytic converters and on particulate matter emissions of less than 2.5 microns. Consequently, both EPA and DOE have recommended that limiting the level of sulfur in gasoline (15 ppm) and diesel fuels (30 ppm) would be essential for meeting lower vehicle emission standards in the future (by 2006). Sulfur compounds routinely found in petroleum feedstocks include mercaptans (RSH), sulfides (RSR), disulfides (RSSR), saturated /unsaturated cyclic sulfides (C2-C5 cyclic sulfur compounds), benzothiophenes and their derivatives. The conventional approach to remove sulfur from fuel is via catalytic hydrodesulfurization (HDS), which operates at elevated temperatures and extremely high pressures. During HDS, H2 is converted to H2S and subsequently reacted with O2 in the Claus process to H2O and elemental sulfur, which is disposed in special landfills or maintained on site. An overall hydrogen balance clearly shows that the very valuable H2 becomes converted to invaluable H2O. In addition, the manufacture of H2 produces significant amounts of global warming CO2, NOx, SOx and involves very energy intensive methane steam reforming and water gas-shift catalytic processes. An alternative vapor-phase oxidesulfurization (ODS) of the various organosulfur compounds typically found in petroleum feedstocks is currently under investigation at Lehigh University for removal of sulfur by air oxidation while simultaneously converting the organosulfur compounds into valuable chemical intermediates. Experimental Catalyst Preparation. The supported vanadia catalysts used in this study were prepared by the incipient wetness impregnation method employing V-isoprpoxide in isopropanol under a N2 environment to prevent hydrolysis of the precursor. This technique is described in detail elsewhere [1,2]. The oxide support materials used in this study are TiO2 (55 m2/g), ZrO2 (39 m2/g), Nb2O5 (55 m2/g), CeO2 (36 m2/g), Al2O3 (180 m2/g) and SiO2 (300 m2/g). Concentrations of the supported metal oxides were all prepared to exhibit approximately monolayer surface coverage. Characterization. In Situ Raman spectroscopy was performed with a system comprised of an Ar+ laser (Spectra Physics, model 2020-50) set at 514.5 nm, and a Spex Triplemate spectrometer (model 1877) connected to a Princeton Applied Research (model 143) OMA III optical multichannel photodiode array detector. The samples were initially dehydrated by heating in flowing O2(20%)/He to 300 °C prior to any analyses, which also achieved complete oxidation of the catalyst. The ODS activities of the catalysts were obtained from an isothermal fixed-bed reactor system operating at atmospheric pressure. The feed gas contained 1000 ppm of the reactant (CH3SH, CH3SCH3, CH3SSCH3, thiophene), 18% O2 in He balance and was introduced into the reactor at a flow rate of 150 ml/min. Sample runs were performed between 200-450 oC. Analysis of the reaction products was accomplished using a FTIR (model #101250 Midac) or an online gas chromatograph (HP 5890A) equipped with a thermal conductivity detector (TCD) and a sulfur chemiluminescence detector (SCD 355, Sievers). Temperature Programmed Surface Reaction Mass Spectrometry (TPSR-MS) was also carried out with an AMI-100 system equipped with an online mass spectrometer (Dycor DyMaxion). The adsorption was performed between 50- 100 oC using 50-200 mg of catalyst and was ramped to 500 oC at a heating rate of 10 o C/min in 5% O2/He or He at 30 mL/min. Results and Discussion The steady-state reactivity data showed that CH3SH, CH3SCH3, and CH3SSCH3 are selectively oxidized to H2CO/SO2 and that thiophene is oxidized to maleic anhydride/SO2 over various supported vanadia catalysts at moderate temperatures (<450 oC) with high selectivities (>80% range at high conversions). The supported vanadia catalysts used did not deactivate under reaction conditions and were found to be sulfur tolerant because of the rather fast oxidation of the surface sulfur intermediate. Raman spectroscopy revealed that the supported metal oxide phases were 100% dispersed on the oxide supports. Thus, the exclusive presence of surface metal oxide species allowed the determination of the number of active surface sites in the catalyst samples since dispersion was 100 %. The turnover frequencies (TOF: Activity per surface metal atom based on the yield of product) for each oxidation reaction varied about one order of magnitude with the specific oxide support. All reactions exhibited a zero-order dependence on the oxygen partial pressure (0.5-20%) and a first-order dependence on the CH3SH/CH3SCH3/CH3SSCH3/thiophene partial pressures (<1%), which suggest that the surface vanadia species is fully oxidized under the investigated reaction conditions. Temperature Programmed Surface Reaction Mass Spectroscopy (TPSR-MS) experiments revealed that for the selective oxidation reactions of CH3SH, CH3SCH3, and CH3SSCH3 followed a Mars Van Krevlen reaction mechanism where gas-phase O2 is used to rapidly replenish the oxygen in the metal oxide catalyst. In the case of thiophene, however, the oxidation reaction followed a Langmuir-Hinshelwood reaction path where gas-phase O2 is involved in the formation of the reaction products. The detailed reaction mechanism for each organosulfur compound will be presented as well as the rate-determining steps and the surface reaction intermediates. References 1. Deo, G., Wachs, I. E. J. Catal. 146 (1994) 323-334. 2. Wachs, I. E., in: Spivey(Ed.), J. J., Catalysis, vol. 13, The Royal Society of Chemistry, Cambridge, 1997, pp. 37-54.
Pages to are hidden for
"Vapor-Phase Oxidesulfurization _ODS_ of Organosulfur Compounds "Please download to view full document