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On the selectivity to higher hydrocarbons in Co-based Fischer–Tropsch synthesis

Paper ID Volume ID Publish Year Pages File Format Full-Text
53378 46966 2016 14 PDF Available
Title
On the selectivity to higher hydrocarbons in Co-based Fischer–Tropsch synthesis
Abstract

•There is a common pool of monomer units (CHx; CH2) on Co in FTS.•The monomer pool generates all products, independent of chain length.•Suppressing hydrogen coverage favor generation of long hydrocarbon chains.•Consistently, carbon deposits are suggested to promote hydrogen over monomer units.•Close proximity of Co crystals on the support gives a slight positive C5+ response.

Impact of process variables on selectivity to C5+ products during cobalt Fischer–Tropsch synthesis (FTS) are summarized and discussed, comprising temperature; pressure; synthesis gas composition; transport limitations of synthesis gas and products; conversion of CO; and effect of water. Further, effect of catalyst formulation, preparation and activation is included, specifically catalyst synthesis; cobalt crystallite size; crystal structure of cobalt; distribution of crystallites; support materials; pore structure; acidity; surface modifications; reduction and pretreatment. Other topics comprise promoters with focus on reduction promoters including rhenium and nickel; impurities like alkali and sulphur; deactivation and regeneration. Discussion is based on mechanistic understanding, specifically kinetics based on hydrogen assisted CO dissociation; a linear relationship between CH4 and C5+ products; possible reinsertion of olefins; hydrogenation of olefins; and surface coverage based on SSITKA data. It is concluded that the majority of reported data on FTS can be rationalized in terms of a CHx pool mechanism on cobalt where x shifts toward monomers and high polymerization probability (α-value); or methane and light products formation, depending on synthesis conditions and catalyst properties.Selectivity responses are illustrated mainly based on cobalt catalysts on transition alumina supports. How the support in itself affects C5+ selectivity is challenging. There clearly is a change in the composition of the cobalt surface pool of reaction intermediates, but how this change originates in support properties remains to be illuminated. So far there is no direct evidence for the termination process; β-hydrogen abstraction, to play a major role in determining FTS selectivity.

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Keywords
Fischer–Tropsch Synthesis; Cobalt catalyst; Selectivity
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On the selectivity to higher hydrocarbons in Co-based Fischer–Tropsch synthesis
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Publisher
Database: Elsevier - ScienceDirect
Journal: Catalysis Today - Volume 261, 1 March 2016, Pages 3–16
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Catalysis
Get Full-Text Now
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Price was $35.95
You save - $31
Price after discount Only $4.95
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