Design, synthesis, and catalytic properties of silica-supported, Pt-promoted iron Fischer-Tropsch catalysts

Silica-supported iron catalysts (Fe/SiO2, FePt/SiO2, and FePtK/SiO2) were prepared using a novel nonaqueous ( acetone) evaporative deposition technique. This preparation leads to relatively well-dispersed iron phases at modest (10%) metal loadings. Moreover, catalytic activities of these catalysts for Fischer - Tropsch synthesis are high and comparable to industrially relevant precipitated iron catalysts. Catalyst activities were tested following a nonregular L18 orthogonal array that enabled the number of 150-h activity tests to be reduced from 54 to 18; this statistical design was augmented with five additional runs to provide replication. Primary independent variables affecting catalysts' activity were promoter type, pretreatment gas composition (H-2, H-2/CO, or CO), pretreatment temperature (250, 280, or 320 degreesC), and reaction temperature (250 or 265 degreesC); iron carbide level measured from Mossbauer spectroscopy was correlated with activity in a separate analysis. Activity was found to increase in the order Fe/SiO2, FePt/SiO2, and FePtK/SiO2. For a given catalyst composition, activity increases to a maximum with increasing pretreatment temperature and increasing time. Catalyst activity was also positively correlated with increasing chi-carbide content for Fe/SiO2 and FePt/SiO2 catalysts but not for FePtK/SiO2. While pretreatment atmosphere greatly influences initial activity time behavior, activity is less dependent on pretreatment after about 150 h of reaction. Steady-state methane and C2+ hydrocarbon selectivities (CO2-free basis) for the FePtK/SiO2 catalyst at 250 - 265 degreesC, 10 atm, and H-2/CO = 1 are 7 - 9 and 91 - 93%, respectively, while its hydrocarbon productivity at 250 degreesC (normalized to 15 atm, H-2/CO = 0.7) of 0.27 g HC/g(cat)/h is comparable to those reported for unsupported precipitated iron catalysts of high activity and selectivity. These results indicate that preparation of an active, selective, stable, attrition-resistant supported iron catalyst for Fischer - Tropsch synthesis is feasible. Promise for additional improvements in catalyst performance through application of advanced preparation methods and optimization of catalyst chemical and physical properties is also indicated.