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

Yizhi Xiang^1,3, 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/MnO_x 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 P_H2/P_CO ratios.

Co₄Mn₁K_0.1 (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 P_H2/P_CO ratio is increased or decreased. The CO conversion shows clockwise hysteresis, i.e. a low reactivity state (LRS) for P_H2/P_CO 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 P_H2/P_CO = 30. The bifurcation between LRS and HRS occurs at P_H2/P_CO ~ 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 p_H2/p_CO ratios, increases significantly for smaller such ratios. Under partial pressure conditions typically applied in FT synthesis (1≤P_H2/P_CO≤3), oxygenate selectivities are between 45 and 55%. For under-stoichiometric reaction conditions, P_H2/P_CO=0.5, up to 65% of oxygenates are detected mostly in form of long-chain aldehydes. When increasing the P_H2/P_CO 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 P_H2/P_CO 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).