Study of Manganese Promoter on a Precipitated Iron-Based Catalyst for Fischer-Tropsch Synthesis

摘要

The effects of Manganese (Mn) incorporation on a precipitated iron-based Fischer-Tropsch synthesis (FTS) catalyst were investigated using N_2 physical adsorption, air differential thermal analysis (DTA), H_2 temperature-programmed reduction (TPR), and Mossbauer spectroscopy. The FTS performances of the catalysts were tested in a slurry phase reactor. The characterization results indicated that Mn increased the surface area of the catalyst, and improved the dispersion of α-Fe_2O_3 and reduced its crystallite size as a result of the high dispersion effect of Mn and the Fe-Mn interaction. The Fe-Mn interaction also suppressed the reduction of α-Fe_2O_3 to Fe_3O_4, stabilized the FeO phase, and (or) decreased the carburization degree of the catalysts in the H_2 and syngas reduction processes. In addition, incorporated Mn decreased the initial catalyst activity, but improved the catalyst stability because Mn restrained the reoxidation of iron carbides to Fe_3O_4, and improved further carburization of the catalysts. Manganese suppressed the formation of CH_4 and increased the selectivity to light olefins (C_(2-4)~=), but it had little effect on the selectivities to heavy (C_5+) hydrocarbons. All these results indicated that the strong Fe-Mn interaction suppressed the chemisorptive effect of the Mn as an electronic promoter, to some extent, in the precipitated iron-manganese catalyst system.