Fischer–Tropsch synthesis with promoted iron catalyst: Reaction pathways for acetic acid, glycol, 2-ethoxyethanol and 1,2-diethoxyethane Original Research Article

Reaction pathways for oxygenates, acetic acid, ethylene glycol (EG), 2-ethoxyethanol (2-EE) and 1,2-diethoxyethane (1,2-DEE) added during Fischer–Tropsch synthesis (FTS) over a doubly promoted fused iron catalysts were studied. The addition of acetic acid, EG and 2-EE affected only slightly the CO conversion but resulted in a significant reduction in H2 conversion while addition of 1,2-DEE results in slight increase in both H2 and CO conversion. Addition of these oxygenates 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 oxygenate molecules. The 1-alkene/2-alkene fraction was found to increase significantly when these oxygenates were added and then return to the original value once the addition is terminated, indicating inhibition of secondary reactions of 1-alkene by added oxygenates. Added acetic acid reversibly increased the CO2 production rate while EG, 2-EE and 1,2-DEE reversibly decreased the CO2 selectivity. Addition of these oxygenates reduced the production rate of methane. Addition of acetic acid and 1,2-DEE decreased methanol selectivity significantly while added EG results in a significant increase in methanol production. In the case of 2-EE addition, methanol selectivity was nearly constant. Reaction of acetic acid during FTS was found to produce products such as ethyl butanoate, ethylene glycol and its ether, 1,2-diethoxyethane, which are not generally observed in the normal FTS product spectrum. Addition of EG results in a significant increase in the production rate of 1,2-DEE only and no measurable amount of 2-EE was found. While addition of 2-EE caused a significant increase in the production rate of glycol, the addition of 1,2-DEE indicated a significant increase in 2-EE production rate without any measurable change in EG selectivity. The results suggest that acetic acid undergoes some Csingle bondC bond rupture while 2-EE and 1,2-DEE undergoes cleavage of the ether linkage (Csingle bondOsingle bondC bond). On the contrary, EG undergoes fast and equally probable Csingle bondOsingle bondC chain growth in both terminal positions. The results indicate that neither of these oxygenates is a significant intermediate in FTS with an iron catalyst. Product distribution in most of the oxygenate compounds are consistent with hydrogenation of the added oxygenate to acetaldehyde and/or ethanol as primary products followed by secondary reaction of these two primary oxygenate products.