Study of the role of the support material in the manganese promotion of cobalt-based Fischer-Tropsch synthesis catalysts

The manganese promotion of cobalt−based catalysts for the Fischer-Tropsch synthesis was investigated using four different supports. Different loadings of manganese were deposited on the catalysts using incipient wetness impregnation before the impregnation of cobalt and rhenium. All the Mn-promoted catalysts exhibited an increase in chain growth probability and olefin content in the products. This enhanced C 5+ selectivity is attributed mainly to decreased methane production. The promotion of the catalytic activity was dependent on the support material. On γ−Al 2 O 3 the catalyst activity increased with increasing manganese loading. Using a narrow pore silica support (SiO 2 −N) a maximum in the activity was found at an intermediate manganese loading. Further addition of Mn decreased the activity and worsened the product distribution with an increase in methane selectivity. The catalysts supported on TiO 2 and wide pore silica (SiO 2 −W), did not show major activity changes with Mn addition. The addition of Mn decreased the metallic surface area in most cases, indicating blocking of Co sites. The pore structure and the metal−support interactions play a role in how the Mn and Co are dispersed, yielding enhanced or inhibited activity. The origin of the promotion was investigated with CO and syngas adsorption experiments coupled with recording DRIFTS spectra. The results indicated a decrease in CO bond energy due to an increase in electron density of the Co with the Mn addition. The electronic effect appeared to be in balance with the site blocking of Mn. Therefore, choice of support and preparation conditions of the materials are important. • Mn promotion of supported Co FTS catalysts was studied. • Mn enhances C5 + and olefin selectivity. • The effect of Mn on catalyst activity depends on support properties. • Overall effect a balance between site blockage and electronic effects. • Electronic effect due to enhanced electron density on Co weakening C-O bond in adsorbed CO.