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 = k₀ exp (-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.

References`

[1] H. Schulz, M. Claeys, S. Harms, Stud. Surf. Sci. Catal. 107 (1997) 193.

[2] [4] E. Iglesia, S.C. Reyes, R.J. Madon, S.L. Soled, Adv. Catal. 39 (1993) 221.

[3] Ø. Borg, S. Eri, E.A. Blekkan, S. Storsæter, H. Wigum, E. Rytter, A. Holmen, J. Catal. 248 (2007) 89.

 [4] C.H. Bartholomew, W. Lee, Stud. Surf. Sci. Catal. 130 (2000) 1151.

[5] R.A. Dictor, A.T. Bell, J. Catal. 97 (1986) 121.

[6] J.L. Li, X.D. Zhan, G. Jacobs, B. H. Davis, Appl. Catal. 208 (2002) 203.

[7] T.K. Das, W.A. Conner, J. L. Li, G. Jacobs, M.E. Dry, B.H. Davis, Energy & Fuels 19 (2005) 1430.

[8]  W. Ma, G. Jacobs, M.K. Gnanamani, U.M. Graham, in Advances in Fischer Tropsch Synthesis, Catalysts, and Catalysis, eds. Burt H. Davis and Mario L. Occelli. Taylor & Francis Group, 2009, p.31.

[9] W. Ma, G. Jacobs, D. Sparks, M.K. Gnanamani, R. Pendyala, B.H. Davis, C.H. Yen, J.L.S. Klettlinger, T.M. Tomsik, Fuel, accepted, in press.

[10] W.H. Lee, C.H. Bartholomew, J. Catal. 120 (1989) 256.

[11] K.B. Kester, J.L. Falconer, J. Catal.  89 (1984) 380.

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