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

Authors: 
Ma, W., University of Kentucky
Jacobs, G., University of Kentucky
Das, T. K., University of Kentucky
Davis, B. H., University of Kentucky, Center for Applied Energy Research


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 (-Ea/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.

References`

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

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