Mn promoted Co catalysts for Fischer-Tropsch production of light olefins – An experimental and theoretical study

Abstract

Three different CoMn/γ-Al2O3 catalysts were prepared by the incipient wetness impregnation route and compared to a Co/γ-Al2O3 catalyst. The effect of co-impregnation vs. sequential impregnation as well as the order of component addition was investigated. All catalysts were characterised by TPR, H2-chemisorption, XRD and XPS and their activity and selectivity in the Fischer-Tropsch reaction was investigated. Complementary, self-consistent DFT calculations were performed to further address the observed promotion effects. All Mn promoted catalysts displayed heightened intrinsic activity, heightened selectivity to light olefins and C5+ species and lowered selectivity to CH4 compared to Co. The promotion effects on selectivity and intrinsic activity were found to be independent on catalyst preparation method. The catalysts undergo a restructuring during operation, in which an excess of Mn saturates the catalytically relevant sites causing the similar behaviour. The Co-specific activity differed between the Mn promoted catalysts. This was attributed to varying degrees of Mn incorporation in the Co3O4 particles, causing different degrees of reduction limiting the available metallic Co surface area. The DFT calculations suggested that the binding energy for all investigated species increases on Co in the presence of Mn, facilitating CO dissociation which can explain the higher intrinsic activity. The affected selectivities for olefins, C5+ and CH4 can all be attributed to an inhibited hydrogenation activity demonstrated by the increased barriers for CH3 and CH4 formation.

Short-channel mesoporous SBA-15 silica modified by aluminum grafting as a support for CoRu Fischer–Tropsch synthesis catalysts

“…The FTIR spectrum in the pyridine region is presented in Figure 6 along with that of the ALPS support previously discussed. [49][50][51][52][53][54][55][56][57]. This is in agreement with XRD analysis that identified this spinel oxide as the only cobalt crystalline phase in the calcined material.…”

Promoted cobalt metal catalysts suitable for the production of lower olefins from natural gas

“…Please note that these calculations are of a preliminary nature and further work is needed to arrive at full reaction pathway analysis which is outside the scope of this work. Pederson et al recently performed DFT calculations on CoMnO systems for the production of light olefins and they found that selectivities can be attributed to an inhibited hydrogenation activity demonstrated by the increased barriers for CH 3 and CH 4 formation 22 . Strømsheim et al recently showed that the restructuring of Co surface under CO exposure with K pre-adsorbed proceeded on the terraces rather than from the step edges 38 .…”

“…Till now, Co-based catalysts for the direct conversion of syngas to lower olefins focused on MnO as promoter, but the product spectrum was still dictated by the ASF distribution 18–22 . Adding alkali promoters to Co/MnO catalysts stimulates formation of Co-carbide, which inhibits methane, but promotes CO 2 production 23 .…”

Microstructure Evolution of a Co/MnO Catalyst for Fischer‐Tropsch Synthesis Revealed by In Situ XAFS Studies

“…Several groups claimed that the presence of small amounts of Mn can improve the dispersion of the cobalt active phases and favour the formation of bridged‐type absorbed CO, resulting in improvement in the FT reaction rate and C 5+ selectivity . Mn has also been shown to have major impacts on selectivity to olefins and alcohols ,. On the other hand, Bezemer et al.…”