Effect of MgAl2O4 Surface Area on the Structure of Supported Fe and Catalytic Performance in Fischer-Tropsch Synthesis

Two MgAl2O4 spinels with low and high surface area are compared as a support for iron Fischer-Tropsch synthesis catalysts. The textural properties of spinels have a great impact on the formation of the active phase and catalytic performance. A well-crystallized hematite phase is formed on the low surface area spinel (S-BET = 15 m(2) g(-1)), whereas high surface area support (S-BET = 163 m(2) g(-1)) provides highly dispersed or amorphous Fe2O3. Reducibility of the catalysts in H-2, CO, and CO/H-2 is investigated by temperature-programmed reduction (TPR), magnetometry, and diffuse reflectance Fourier-transform infrared (FTIR) spectroscopy. Adding potassium to the catalyst formulation suppresses the reduction of hematite in magnetite. In synthesis gas atmosphere, the final product of hematite reduction is Hagg carbide, and its amount is higher for the low surface area support. Average carbide particle size correlates with pore diameter of the support. Spinel-based catalysts possess low selectivity to CO2 contrary to common iron catalysts. The low surface area spinel affords a very active catalyst with high selectivity to C5+ hydrocarbons. Hydrogenation of CO over unpromoted iron catalysts seems to be a structure-insensitive reaction.