(603c) Manganese Promotion of Silica-Supported Cobalt Fischer-Tropsch Synthesis Catalysts

Fischer-Tropsch synthesis (FTS) has long been investigated as a chemical process for producing transportation fuels using non-petroleum feedstocks such as natural gas, coal, or biomass.  While modern cobalt FTS catalysts are highly selective toward long-chain hydrocarbons used in liquid fuels, methane is an unwanted byproduct from these catalysts [1].  Consequently, a large body of contemporary FTS research concerns methods for shifting the product distribution in favor of higher molecular weight hydrocarbons.  Certain non-catalytically active elements, referred to as promoters, are known to improve catalyst performance when added to FTS catalysts in small amounts.  Promoting cobalt FTS catalysts with manganese has repeatedly been found to shift the product distribution to heavier, more olefinic products and enhance catalyst activity in some circumstances [2, 3].

This presentation will discuss our ongoing investigations into the role of manganese promoters in cobalt FTS catalysts supported on silica.  The catalysts were characterized by XRD, TEM, and STEM-EELS to assess particle size distribution and composition.  The STEM-EELS elemental maps showed a preferential association between cobalt and manganese.  Kinetics experiments in a fixed bed micro-reactor were conducted to assess apparent activation energies and partial order dependencies on reactants.  Activity and selectivity data at different space velocities and long-term stability data were collected.

Using a direct hydrogenation synthesis procedure, we observed strong sensitivity of methane selectivity reduction on manganese loading for Mn/Co atomic ratios of less than 0.05.  Variation of catalyst synthesis procedure and manganese loading were used to identify an optimal manganese loading for activity and selectivity enhancement.  The possibility of promotion effects arising from cobalt-manganese surface alloy formation will be discussed.

References

[1] Zhang, Q. et al. ChemCatChem. 2 (2010) 1030.

[2] Bezemer, G.L. et al. J. Catal. 237 (2006) 152.

[3] Dinse, A. et al. J. Catal. 288 (2012) 104.