Fischer-Tropsch synthesis: Effect of ammonia in syngas on the Fischer-Tropsch synthesis performance of a precipitated iron catalyst

The effect of ammonia in syngas on the Fischer-Tropsch synthesis (FTS) reaction over 100Fe/5.1Si/2.0Cu/3.0K catalyst was studied at 220-270 °C and 1.3 MPa using a 1-L slurry phase reactor. The ammonia added in syngas originated from adding ammonia gas, ammonium hydroxide solution, or ammonium nitrate (AN) solution. A wide range of ammonia concentrations (i.e., 0.1-400 ppm) was examined for several hundred hours. The Fe catalysts withdrawn at different times (i.e., after activation by carburization in CO, before and after co-feeding contaminants, and at the end of run) were characterized by ICP-OES, XRD, Mössbauer spectroscopy, and synchrotron methods (e.g., XANES, EXAFS) in order to explore possible changes in the chemical structure and phases of the Fe catalyst with time; in this way, the deactivation mechanism of the Fe catalyst by poisoning could be assessed. Adding up to 200 ppmw (wt. NH₃/av. Wt. feed) ammonia in syngas did not significantly deactivate the Fe catalyst or alter selectivities toward CH₄, C₅₊, CO₂, C₄-olefin, and 1-C₄ olefin, but increasing the ammonia level (in the AN form) to 400 ppm rapidly deactivated the Fe catalyst and simultaneously changed the product selectivities. The results of ICP-OES, XRD, and Mössbauer spectroscopy did not display any evidence for the retention of a nitrogen-containing compound on the used catalyst that could explain the deactivation (e.g., adsorption, site blocking). Instead, Mössbauer spectroscopy results revealed that a significant fraction of iron carbides transformed into iron magnetite during co-feeding high concentrations of AN, suggesting that oxidation of iron carbides occurred and served as a major deactivation path in that case. Oxidation of χ-Fe₅C₂ to magnetite during co-feeding high concentrations of AN was further confirmed by XRD analysis and by the application of synchrotron methods (e.g., XANES, EXAFS). It is postulated that AN oxidized χ-Fe₅C₂ during FTS via its thermal dissociation product, HNO₃. This conclusion is further supported by reaction tests with co-feeding of similar concentrations of HNO₃. Additional oxidation routes of iron carbide to magnetite by HNO₃ and/or by its thermal decomposition products are also considered: Fe₅C₂ + NO_x (and/or HNO₃) → Fe₃O₄. In this study, ion chromatography detected that 50-80% HNO₃ directly added or dissociated from AN eventually converted to ammonia during or after its oxidation of iron carbide, resulting from the reduction of NO_x (NO_x + H₂ + CO → NH₃ + CO₂ + N₂ + H₂O) by H₂ and/or CO.