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Vapor-Phase Oxidesulfurization _ODS_ of Organosulfur Compounds


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									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

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

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
  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.

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.

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