(208f) Bistability of the Fischer Tropsch Reaction over Promoted Co/MnOx Catalysts

Authors: 
Kruse, N. - Presenter, Washington State University
Bistability of the Fischer Tropsch Reaction over promoted Co/MnOx Catalysts

Yizhi Xiang1,3, Libor Kovarik2 and Norbert Kruse1

1Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA

2 Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA

3 Dave C. Swalm School of Chemical Engineering, Mississippi State University, MS State, Mississippi, 39762, USA

Co/MnOx catalysts have recently enjoyed considerable interest in studies of the Fischer-Tropsch (FT) synthesis because of their capability to form olefins and oxygenates of varying chain length. In our previous work we showed that potassium-promoted are particularly active in this regard.1 We suggested that such unique product distribution was due to the formation of cobalt carbide during the reaction process. Bulk Co2C particles with prismatic morphologies were also advocated to be responsible for short-chain olefin formation.2 In the present study, we will demonstrate for the first time the occurrence of kinetic hysteresis effects along with chemical restructuring while cycling the syngas composition between high and low PH2/PCO ratios.

Co4Mn1K0.1 (indices indicating atomic ratios) catalyst precursors are prepared by fast oxalate co-precipitation from metal nitrates using acetone and water as solvents. Oxalate precursors are thermally decomposed under H2 to provide the active catalyst. Activation results in the occurrence of nanosized metal particles and metal oxide phases. High-pressure catalytic tests at 40 bar are performed in a fixed-bed flow reactor. Steady-state activity and selectivity are determined for every H2/CO ratio after 12 h time-on-stream.

Both the conversion of CO and the selectivity of paraffin/oxygenate formation depend on whether the PH2/PCO ratio is increased or decreased. The CO conversion shows clockwise hysteresis, i.e. a low reactivity state (LRS) for PH2/PCO ratios decreasing from initially high (30/1) to low and, vice versa, a high reactivity state (HRS) for these ratios increasing from low to high again. On the other hand, both clockwise and counter-clockwise behavior is observed for oxygenate and paraffin formation, respectively. More specifically, while the CO conversion is up to 20% at the most on the LRS branch, it increases significantly on the HRS branch and is up to 90% at PH2/PCO = 30. The bifurcation between LRS and HRS occurs at PH2/PCO ~ 3. The hysteresis loop finally closes by running the catalyst under very lean conditions or in pure H2. The total oxygenate selectivity, starting with almost 0% at high pH2/pCO ratios, increases significantly for smaller such ratios. Under partial pressure conditions typically applied in FT synthesis (1≤PH2/PCO≤3), oxygenate selectivities are between 45 and 55%. For under-stoichiometric reaction conditions, PH2/PCO=0.5, up to 65% of oxygenates are detected mostly in form of long-chain aldehydes. When increasing the PH2/PCO ratios from low back to high, oxygenate selectivities remain considerably higher than those of the forward run, i.e. clockwise hysteresis is observed. This is in striking contrast to paraffin formation which demonstrates counter-clockwise hysteresis behavior. Catalytic hysteresis in terms of activity and selectivity is driven by a reversible Co - Co2C bulk phase transformation as will be shown by in-situ XRD and HRTEM. Once being formed at low PH2/PCO ratios, Co2C has proven to be rather perseverant with regard to its reduction to metallic Co. The slowness of the process is one of the reasons that rate/selectivity oscillations have not yet been observed in our studies.

The observation of a reaction-induced reversible Co-Co2C reconstruction opens new opportunities for tuning the selectivity of the Fischer Tropsch reaction to O-functionalized long-chain hydrocarbons.

References

  1. Xiang, Y., Kruse, N., Nature Commun. 7, 13058 (2016).
  2. Zhong, L., Yu, F., An, Y., Zhao, Y., Sun, Y., Li, Z., Lin, T., Lin, Y., Qi, X., Dai, Y., Gu, L., Hu, J., Jin, S., Shen, Q., Wang, H., Nature 538, 84 (2016).
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