Fischer-Tropsch Synthesis: Reaction Pathways for 14C-Labeled Acetic Acid

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 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. ¹⁴C-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.