(106c) Kinetics of Methane Formation During Fischer-Tropsch Synthesis Over Co/SiO2 and Co/Al2O3 Catalysts

Kinetics of methane
formation during Fischer-Tropsch synthesis over Co/SiO₂ and Co/Al₂O₃
catalysts

Wenping Ma, Gary Jacobs, Tapan K. Das, and
Burtron H. Davis^*

Center for
Applied Energy Research, University of Kentucky, 2540 Research Park Drive,
Lexington, Kentucky, 40511, USA.

Abstract

 
 Water produced during
Fischer-Tropsch synthesis (FTS) was found to decrease CH₄ selectivity
significantly.  This was assumed to
take place by way of water inhibiting the secondary
hydrogenation of primary olefins [1-3]. 
This implies that the power law kinetic model,
,
which was previously used to describe the rate of methane formation over cobalt-
[4] and iron-based catalysts [5] can be improved to include a parameter to
define the impact of water.  In the
present contribution, a new
empirical model incorporating a water effect term (
) in the
power law model -
,
where k = k₀exp (-E_a/RT), is the CH₄ formation rate
constant, a and b are the reaction orders for CO and H₂,
respectively, and m is a water effect constant - was used to fit the methane kinetic data obtained over 25%Co/HP14 g-Al₂O₃ and
15%Co/SiO₂ catalysts.  Therefore,
the new methane kinetic model has the same form as the CO consumption model developed
by CAER [6-9]. Kinetic experiments over the two catalysts were conducted under a range of conditions: 205-220 ^oC,
280 psig, H₂/CO = 1.0-2.5 and 3.3-22 Nl/g-cat/h in a 1-L
continuously stirred tank reactor (CSTR).  The results indicate that
water exhibits a negative effect on CH₄ formation during FTS for
both types of catalyst, suggesting that the impact of water on CH₄
formation during FTS is kinetic in nature, consistent with interpretations
reported in the literature.  Thus,
both CO conversion and CH₄ selectivity could be accurately described
by the CAER kinetic model [8,9] over the range of conditions used in the
work.  Activation energies of
methane formation obtained on the two supported Co catalysts were found to be
close (~135.5 kJ/mol), in agreement with the range of values reported in the
open (i.e., 98-145 kJ/mol).

     The kinetics equations
obtained at 220 ^oC over 25%Co/HP14 g-Al₂O₃ and 15%Co/SiO₂
catalysts are described as follows:

25%Co/HP14 g-Al₂O₃ :
  mol/g-cat/h;

15%Co/SiO₂:
, mol/g-cat/h

The values of
reaction orders a and b calculation for CH₄
formation, as well as the CH₄ formation activation energies, are
different from those of CO consumption kinetic results reported in [8] and [9].
 This may reflect that CH₄
formation on the catalysts is involved in not only normal FTS polymerization
pathways, but also another pathway, for example methanation, which may take
place on different active sites.

Acknowledgements

This work was supported by NASA
contract, #NNX07AB93A and the commonwealth of Kentucky.

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* Corresponding author:  Burtron H. Davis, davis@caer.uky.edu,
Tel: 859-257-0251.