(166e) Role of Surface Hydroxyl Groups As Hydrogenating Species during Fischer-Tropsch Synthesis

Role of Surface Hydroxyl Groups as Hydrogenating Species during Fischer-Tropsch Synthesis

G. T. Kasun Kalhara Gunasooriya, Mark Saeys

Laboratory for Chemical Technology, Ghent University

Fischer-Tropsch synthesis transforms synthesis gas, a mixture of CO and H₂, to long-chain hydrocarbons and water. In recent years Fischer-Tropsch synthesis has surged as an attractive route to convert natural gas, coal and biomass to clean transportation fuels. Supported cobalt catalysts are often preferred due to their high activity, selectivity towards long-chain hydrocarbons, and low CO₂ selectivity. The detailed reaction mechanism, a complex combination of C-C bond formation, C-O scission and hydrogenation steps, remains intensely debated [1, 2]. In this study we compare different C-O activation pathways, and analyze the experimentally observed kinetic role of water [3]. The activation of C-O bonds can follow various reaction pathways. In addition to direct C-O dissociation, CO can be activated by insertion into growing hydrocarbon chains (CO insertion mechanism [2]), or through hydrogenation (hydrogen-assisted CO activation). Direct hydrogenation of the oxygen atom in CO and RCO however has a high barrier, 195 kJ/mol, while hydrogenation of the carbon atom leads to oxygenates. In addition to the direct hydrogenation pathway, a proton shuttling mechanism has been proposed [4]. In this mechanism, a water molecule acts as a proton shuttle and assists by bridging the distance between the surface hydrogen and the oxygen atom. However, the entropy cost to introduce a water molecule in the transition state increases the free energy barrier for this pathway to 157 kJ/mol under FT conditions. In this study, we propose an alternative pathway where surface OH groups act as the source of hydrogen. The formation of OH groups from gas phase H₂O is slightly favorable under FT conditions with a free energy of -3 kJ/mol, leading to a significant OH surface coverage, in addition to CO [5]. The surface OH groups are very active hydrogenating species, and the free energy barriers to hydrogenate the oxygen atoms in CO and in RCO are only 95 and 37 kJ/mol, respectively, opening a novel pathway to activate C-O bonds. The role of surface OH groups as hydrogenating species is likely general and involved in several oxygenate transformation reactions.

References

[1] van Santen, R.A., Markvoort, A.J., Filot, I.A.W., Ghouri, M.M., Hensen, E.J.M. Phys. Chem. Chem. Phys., 2013, 15, 17038

[2] Zhuo, M., Tan, K.F., Borgna, A., Saeys, M., J. Phys. Chem. C 2009, 113, 8357

[3] Hibbitts, D.D., Loveless B.T., Neurock M., Iglesia E., Angew. Chem. 2013, 52, 12273

[4] Zhao, Y.F., Yang, Y., Mims, C., Peden, C.H.F., Li, J., Mei, D., J. Catal. 2011, 281, 199   

[5] Gunasooriya, G.T.K.K., van Bavel, A.P., Kuipers, H.P.C.E, Saeys, M., submitted