Abstract

1
Influence of Cobalt Catalyst Characteristics on
Fischer-Tropsch Synthesis in Conventional
Gas-Phase Media and Supercritical Fluid Phase
Media
Nimir O. Elbashir, Deborah B. Boroughs,
Christopher B. Roberts Department of Chemical
Engineering, Auburn University, Alabama USA
P. Dutta, A. Manivannan, M. S. Seehra Physics
Department, West Virginia University, Morgantown,
USA
Abstract
XRD and Magnetic Characterizations
This study covers the performance of several
cobalt-based catalytic systems of different
surface characteristics in both conventional
gas-phase and supercritical hexane (SCH) phase
Fischer-Tropsch synthesis (FTS). The reaction was
conducted in a high-pressure FTS reactor setup at
reaction temperature of 230-250 ?C, syngas (H2/CO
ratio of 2) flowrate of 50 sccm/gcat, and total
pressures ranging from 20-65 bar. The surface
characteristics of these catalysts were measured
by N2 physisorption using a TriStar 3000 gas
adsorption analyzer. Room temperature x-ray
diffraction (XRD), temperature and magnetic field
(H) variation of the magnetization (M), and
low-temperature (5 K) electron magnetic resonance
were used for determining the electronic states
(Co0, CoO, Co3O4, Co2 ) of cobalt. The catalyst
samples included calcined, reduced (before the
reaction), and used samples (after the reaction).
Correlation of the catalyst activity and
selectivity with the catalysts surface
characteristics reveal that pore radius of the
catalyst has a direct influence on both syngas
and CO conversions in gas-phase FTS, however, no
such correlation was observed in the case of
SCH-FTS. The XRD and magnetic characterizations
of the used catalysts reveal that in situ
reducibility of the Co3O4 to hcp Co0 or fcc Co0
is taking place during the FTS reaction. However,
minimal in situ reduction was observed in the
case of gas-phase FTS, whereby significant
changes in the reduced cobalt electronic state
and support (alumina, and silica) phase were
detected. As a result, both the activity and
selectivity of the cobalt catalyst were found to
be very stable and recoverable during SCH-FTS for
relatively long time-on-stream (15 days).
Silica-supported cobalt catalysts
Silica Supported Catalyst
Alumina-supported cobalt catalysts
Room temperature XRD patterns. The expected line
positions for fcc Co0, hcp Co0 CoO, and Co3O4.
Alumina Supported Catalyst
Silica-supported cobalt catalysts
Alumina-supported cobalt catalysts
Supercritical Phase FTS vs. Conventional FTS Media
Gas Phase Reaction
Liquid Phase Reaction
Gas Phase
Liquid Phase
aDetailed descriptions of the characterizations
is given in Duta, Elbashir, et al, Catalysis
Letters (2004) in press. bCatalyst reduced in CO
environment at 280 ?C and 150 psi for 18 hr.
Wax
Active site
Temperature variations of the magnetic
susceptibility (?) under FC (field-cooled) and
ZFC (zero-field-cooled) conditions.
Catalyst Active Surface
Active site
Hot Spots
Supercritical Phase Reaction
Syngas

• Liquid-like density and
• heat capacity
• Gas-like diffusivity and
• transport properties

Stability of the Activity and Structure of the
15 Co/Al2O3
Solvent Propane Pentane Hexane
hcp Co0 ?-Al2O3 ?-Al2O3
Co3O4, Co2 ?-Al2O3
Fixed Bed Reactor
High Pressure FTS Reactor Setup
SCH-FTS In Situ Reduction Co3O4 CoO
hcpCo0 ?-Al2O3 ? ? Al2O3
Static Mixer
SV
MFC
H2
H2/CO 1, Syngas FR 50sccm/gcat Hexane FR
1mL/min, Ptotal 60-65 bar
Reactor
FID
Catalyst Bed
Co3O4, Co2 ?-Al2O3
6-Way Valve
GC
Co3O4, fccCo0, ?-Al2O3
CO
Furnace
Hot Trap
6-Way Valve
He
Back Press. Regulator
Hexanes
One way Valve
Temperature Monitoring
Gas-Phase FTS In Situ Reduction Co3O4 CoO
Co3O4, fccCo0 ?-Al2O3 ? Al2O3
Cold Trap
Ice Bath
HPLC
Pump
Surface Characterizations of the Cobalt-Based
Catalysts
H2/CO 1, Syngas FR 50sccm/gcat Ptotal 20 bar
Summary and Conclusions

• Catalyst pore characteristics have no influence
  on either the activity or selectivity in SCH-FTS.
  This can be attributed to the high solubility of
  heavy hydrocarbons in the dense solvent medium
  and the elimination of mass transfer barriers.
• In situ reduction of the catalyst takes place
  during FTS reaction. The in situ reduction of
  Co3O4 in SCH-FTS gave active crystal-lines of hcp
  and fcc Co0.
• The formation of hexagonal and cubic Co0 phases
  during the SCH-FTS reaction has resulted in very
  stable (for more than 13 days) and recoverable
  catalyst activity and selectivity.
• Our results suggest that the in situ reduction of
  the silica-supported catalyst in the gas-phase
  FTS follows this pathway
• Co3O4 H2 3CoO H2O
• 2 CoO SiO2 Co2SiO4
  (in gas-phase FTS)
• 3CoO 3 H2 3Co0 3H2O
  (in SCH-FTS)

a Prepared by impregnation of an aqueous cobalt
nitrate solution in low surface area (LSA) (200
m2/g) fumed silica, and high surface area (HSA)
(380 m2/g) fumed silica. b Purchased from United
Catalyst Co.
Influence of the Catalyst Surface Characteristics
ScHexane-FTS
Gas-Phase FTS
Acknowledgments
The authors would like to acknowledge the
financial support of the Consortium for Fossil
Fuel Science (under the Department of Energy
Grant DE-FC26-02NT41594) and by Nippon Oil
Corporation, Japan.
?SCF Group, Auburn University. 2004 AIChE Annual
Meeting, Austin, TX.