(68d) Fischer-Tropsch Synthesis: Influence of CO Conversion on Selectivities, H2/CO Usage Ratios, and Catalyst Stability Over 0.27% Ru 25%Co/Al2O3 Using a Slurry-Phase Reactor

During the Fischer-Tropsch synthesis (FTS) reaction, selectivities of hydrocarbons (HCs) including CH₄, and C₅₊, 1-olefins and 2-olefins exhibit dependencies on the CO conversion (X_CO).  In order to compare the selectivities over different catalysts, experiments must be conducted at similar conversion levels [1,2].  A majority of FTS investigations available in the open literature over supported Co catalysts were conducted below or at moderate conversion levels – in the range of 40-55%- and do not  provide information about the dependencies between catalyst conversion and selectivity.  For instance, there are uncertainties about how CO conversion influences the extent of secondary reaction of olefins at high CO conversion levels.  In addition, a complete set of fundamental data relating catalyst selectivities (HCs, CO₂), H₂/CO usage ratios, and catalyst stability as a function of the CO conversion level over a wide range (10 to 90%) on cobalt-based catalysts is lacking.  Recently, Borg et al. [2] conducted experiments over Re promoted Co/Al₂O₃ catalysts for CO conversions up to 72% and found that C₅₊ selectivities increased monotonically with increasing CO conversion levels, however, formation of methane, CO₂  by the water-gas shift (WGS) reaction  and catalyst stability were not discussed.

In the present work, we have explored the effect of CO conversion on hydrocarbon selectivities (i.e., CH₄ and C₅₊), olefin and paraffin selectivities, H₂/CO usage ratios, CO₂ selectivity, and catalyst stability over a wide range of CO conversion – from 8 to 94% on 0.27%Ru-25%Co/Al₂O₃ catalyst using a 1-L continuously stirred tank reactor (CSTR). Experiments were conducted at reactor pressures of 1.5 MPa and 2.5 MPa, temperatures of 205°C and 220°C, H₂/CO feed ratios of 1.4 and 2.1 and gas space velocities ranging from 0.3-15 Nl/gcat/h.  Four separate tests over the catalyst were conducted and they were reproducible within their experimental errors.  It was observed that with the increase in CO conversion, CH₄ selectivity decreased until it reached a minima at X_CO about 66%, after which CH₄ selectivity increased dramatically with increase in X_CO. However, the corresponding C₅₊ selectivity did not change as markedly as CH₄ with CO conversion; only a slight increase of C₅₊ selectivity with increasing CO conversion was observed below 66% CO conversion.  This may be due to higher partial pressures of H₂O being produced at the higher CO conversion levels, thereby promoting heavier hydrocarbon formation by water inhibiting secondary hydrogenation of primary olefins [3-5] and maintained high C₅₊ selectivity even through CH₄ selectivity was increased greatly between the CO conversions of 66 and 94%.  It is interesting that a significant increase in CO₂ selectivity was observed when CO conversions were greater than 66%, suggesting that the WGS reaction was enhanced greatly at high CO conversions over the cobalt catalyst.  This result implies that the metallic phase of cobalt – the surface of which is suggested to be responsible for providing FT catalytic activity - could be transformed to an oxidized species (e.g., CoO or a cobalt-support compound) which catalyzes the WGS reaction at CO conversion levels greater than 66%.  It can be safely concluded that, to avoid the formation of excessive amounts of CH₄ and CO₂, CO conversion levels should be maintained below 70% over cobalt based catalysts.  In addition, it was found that deactivation rates for 0.27%Ru-25%Co/Al₂O₃ catalyst were significantly high at high CO conversion levels then at lower CO conversions.  Finally, it was also observed that increasing CO conversions up to 94% resulted in decreased H₂/CO usage ratios and olefin contents at both the temperatures.

References

[1] R.J. O’Brien, L. Xu, R.L. Spicer, S. Bao, D.R. Milburn, B.H. Davis, Catal. Today  36 (1997) 325.

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

[3] C. Aaserud, A.-M. Hilmen, E. Bergene, S. Eri, D. Schanke, A. Holmen, Catal. Lett. 94 (2004) 171.

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

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

Acknowledgement

The financial support from Qatar National Research Fund under grant (NPRP 08-173-2-050) for this study is greatly acknowledged.

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