Fischer-Tropsch Synthesis: Reaction Pathways for 14 C-Labeled Acetic Acid
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Fischer-Tropsch Synthesis: Reaction Pathways for 14C-Labeled Acetic Acid Amitava Sarkar Æ Robert A. Keogh Æ Shiqi Bao Æ Burtron H. Davis
Received: 13 July 2007 / Accepted: 29 August 2007 / Published online: 13 September 2007 Springer Science+Business Media, LLC 2007
Abstract Reaction pathways for 14C labeled acetic acid (at both carboxylic and methyl positions) added during Fischer-Tropsch synthesis over a doubly promoted fused iron catalysts were studied in a CSTR at 100 psig, 270 C and ca. 90% CO conversion. The addition of acetic acid slightly affected the CO conversion but results in a significant reduction in H2 conversion. Both the unlabeled and labeled acetic acid addition caused a large decrease in the alkene ratio for C2 hydrocarbons as compared to an increase for the C3 and C4 hydrocarbons suggesting a direct formation pathway of ethane from added acetic acid. The 1-alkene/2-alkene fraction was found to increase significantly when acetic acid was added and returned to the original value once the addition is terminated, indicating inhibition of hydrogenation activity of the catalyst by acetic acid. Distribution of 14C suggests some C–C bond rupture and direct formation of ethane from labeled acetic acid. In the case of 1-14C labeled acetic acid addition, the gradual increase of the relative molar activity (14C content per mole) with carbon number reveals that 14C containing part of acetic acid initiate chain growth and also participates in the chain propagation for hydrocarbon formation in FTS. When acetic acid labeled at methyl position was added, the results indicated that 14C containing part of acetic acid participates in chain initiation only. The addition of acetic acid decreases methane and methanol selectivity while it increases ethanol, acetaldehyde and acetone selectivity in FTS. Reaction of acetic acid during
A. Sarkar R. A. Keogh S. Bao B. H. Davis (&) Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511-8479, USA e-mail: [email protected]
FTS was found to produce products like ethyl butanoate, ethylene glycol, and its ether, 1,2-diethoxyethane which are not generally observed in the normal FTS product spectrum. The results indicate that acetic acid is not a significant intermediate in FTS with an iron catalyst. 14 C-distribution in most of the labeled oxygenate compounds are consistent with the hydrogenation of the acetic acid to acetaldehyde and/or ethanol as primary products followed by secondary reaction of these two primary oxygenate products. Keywords Fischer-Tropsch synthesis Iron catalyst Tracer Labeled acetic acid Reaction pathways Oxygenates
1 Introduction Among the products of the Fischer-Tropsch synthesis (FTS) with an iron catalyst are oxygenates, especially at lower CO conversions. The CO insertion mechanism to produce oxygenates was an accidental discovery during addition of high concentrations of ethene and this reaction has become known as the oxo or hydroformylation reaction [1]. Both iron and cobalt ca
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