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On the selectivity of cobalt-based Fischer-Tropsch catalysts: Evidence for a common precursor for methane and long-chain hydrocarbons
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
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2010 (English)In: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694, Vol. 274, no 1, 84-98 p.Article in journal (Refereed) Published
Abstract [en]

A total of 36 cobalt-based supported catalysts were investigated in the Fischer-Tropsch reaction at industrially relevant process conditions: 483 K, 20 bar, molar H-2/CO ratio = 2.1, pellet size: 53-90 mu m. The effect of adding water vapour to the feed was investigated for 20 of the catalysts, and a H-2/CO ratio of 1.0 was used for a few catalysts. The catalysts differed in support material, Co loading, promoters, Cl content, Co particle size (larger than similar to 6 nm), morphology, degree of reduction and preparation technique and showed a large variation in selectivity. For each set of process conditions, a linear relationship seems to exist between the selectivity to methane (and other light products) and C5+ indicating a common precursor, i.e. a common monomer pool, for all hydrocarbon products. A high selectivity to C5+ is mainly an effect of a high intrinsic chain-growth probability and unlikely to be a result of an enhanced cc-olefin readsorption. The universal effect of external water addition on the hydrocarbon selectivities is limited to a decrease in the methane selectivity. A small proportion of the catalysts developed "pure methanation" sites upon exposure to high partial pressures of water.

Place, publisher, year, edition, pages
2010. Vol. 274, no 1, 84-98 p.
Keyword [en]
Fischer-Tropsch, Cobalt, Selectivity, ASF, Alpha value, Methane, Water, H-2/CO ratio
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-26827DOI: 10.1016/j.jcat.2010.06.007ISI: 000281320700008Scopus ID: 2-s2.0-77955585587OAI: oai:DiVA.org:kth-26827DiVA: diva2:375159
Note

QC 20101207

Available from: 2010-12-07 Created: 2010-11-29 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Fischer-Tropsch Synthesis over Cobalt-based Catalysts for BTL applications
Open this publication in new window or tab >>Fischer-Tropsch Synthesis over Cobalt-based Catalysts for BTL applications
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fischer-Tropsch synthesis is a commercial technology that allows converting synthesis gas, a mixture of CO and H2, into fuels and chemicals. This process could be one of the actors in the reduction of oil dependency of the transportation sector. In fact, it has great potential for producing synthetic fuels also from renewable sources, such as biomass, after its thermochemical conversion (gasification) into synthesis gas. Concerning the quality of a diesel fuel produced with this technology, it has a lower local environmental impact than conventional diesel, since it is practically free of sulphur and nitrogen compounds and yields lower exhaust emissions of hydrocarbons, CO and particulates. The present study focuses on the use of cobalt-based catalysts for the production of diesel. In particular, it looks upon correlation between product selectivities when varying the catalyst properties and the effect of process parameters, such as a low H2/CO ratio, typical of a biomass-derived synthesis gas, and the water partial pressure.

Different cobalt-based catalysts, with different properties, such as conventional 3-dimensional porous network supports (γ-Al2O3, α-Al2O3, TiO2, SiO2), Co-loading, preparation technique, etc., were investigated in the Fischer–Tropsch reaction at industrially relevant process conditions. For a set of process conditions, a linear relationship seems to exist between the selectivity to methane (and other light products) and higher hydrocarbons (identified by the industrially relevant parameter SC5+, selectivity to hydrocarbons with more than 4 carbon atoms) indicating a common precursor.

Ordered mesoporous materials (SBA-15), characterized by a 1-dimensional mesoporous network, were tested as model supports and showed the possibility of occurrence of CO-diffusion limitations at diffusion distances much shorter than those required for conventional 3-dimensional porous network supports. The linear relationship mentioned above, derived for conventional supports, was shown to be an efficient tool for indicating whether measured selectivities are affected by CO-diffusion limitations. Some of the catalysts were exposed to H2-poor syngas and to external water addition and the effects on the selectivity relationships were investigated.

Furthermore, the possibility of internal water-gas shift of a H2-poor syngas with mixtures of Co/γ-Al2O3 and a Cu/ZnO/Al2O3 catalyst was investigated both as a technical solution for direct use of a model bio-syngas in the Fischer-Tropsch synthesis, and as a means to study the effect of indigenous water removal on the reaction rate to hydrocarbons. It was found that removal of indigenously produced water slows down the reaction rate significantly. Lastly, the effect of water partial pressure on the Fischer–Tropsch rate of the Co catalyst supported on narrow-pore γ-Al2O3, on its own, was studied. Inlet water partial pressure was varied by external water vapor addition at different H2/CO molar ratios ranging from 1 to 3. The effect of water showed to be positive on the rate for all the H2/CO ratios, but more significantly at H2-poor conditions. The nature of this positive effect on the rate seems to be unrelated to changes in amounts of amorphous polymeric carbon detectable by temperature-programmed hydrogenation of the spent catalyst.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. xiv, 95 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:36
Keyword
cobalt, Fischer-Tropsch, syngas, selectivity
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-102304 (URN)978-91-7501-446-3 (ISBN)
Public defence
2012-09-28, Q2, Osquldasväg 10 NB, KTH, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20120914

Available from: 2012-09-14 Created: 2012-09-13 Last updated: 2012-09-14Bibliographically approved

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