Fischer–Tropsch Synthesis: Characterization Rb Promoted Iron Catalyst

Abstract

Rubidium promoted iron Fischer–Tropsch synthesis (FTS) catalysts were prepared with two Rb/Fe atomic ratios (1.44/100 and 5/100) using rubidium nitrate and rubidium carbonate as rubidium precursors. Results of catalytic activity and deactivation studies in a CSTR revealed that rubidium promoted catalysts result in a steady conversion with a lower deactivation rate than that of the corresponding unpromoted catalyst although the initial activity of the promoted catalyst was almost half that of the unpromoted catalyst. Rubidium promotion results in lower methane production, and higher CO₂, alkene and 1-alkene fraction in FTS products. Mössbauer spectroscopic measurements of CO activated and working catalyst samples indicated that the composition of the iron carbide phase formed after carbidization was χ-Fe₅ C₂ for both promoted and unpromoted catalysts. However, in the case of the rubidium promoted catalyst, ɛ′-Fe_2.2C became the predominant carbidic phase as FTS continued and the overall catalyst composition remained carbidic in nature. In contrast, the carbide content of the unpromoted catalyst was found to decline very quickly as a function of synthesis time. Results of XANES and EXAFS measurements suggested that rubidium was present in the oxidized state and that the compound most prevalent in the active catalyst samples closely resembled that of rubidium carbonate.