On the selectivity of cobalt-based Fischer-Tropsch catalysts : Evidence for a common precursor for methane and long-chain hydrocarbons

• 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.

Ämnesord

NATURVETENSKAP  -- Kemi -- Fysikalisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Physical Chemistry (hsv//eng)

Nyckelord

Fischer-Tropsch

Cobalt

Selectivity

ASF

Alpha value

Methane

Water

H-2/CO ratio

Physical chemistry

Fysikalisk kemi

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