SBA-15-supported iron catalysts for Fischer-Tropsch production of diesel fuel

Iron supported on SBA-15, a mesoporous structured silica, has been developed as a catalyst for the Fischer-Tropsch synthesis of hydrocarbons. The catalysts retain the high surface area of the support, similar to 500 m(2)/g, average pore size, and pore volume. Inclusion of aluminum into the SBA-15 did not significantly alter these parameters. XRD, XAFS, and Mossbauer spectroscopies were used to characterize the catalyst before and after being subjected to the reaction conditions. Prior to reaction, the iron was distributed among alpha-Fe2O3, ferrihydrite, and minor gamma-Fe2O3. After reaction, the iron phases detected were nonmagnetic iron oxides, iron carbide, and metallic iron. The length of the induction period typically seen with iron-based F-T catalysts was strongly dependent on the amount of aluminum present in the catalyst. With no aluminum, the induction period lasted about 25 h, whereas the induction period decreased to less than 5 h with an Al:Si mass ratio of 0.010. A further increase in aluminum content lengthened the induction period, but always remained less than that without aluminum. Catalyst activity and product selectivity were also strongly dependent on aluminum content with the maximum diesel fuel fraction, C11+, occurring with the Al: Si ratio of 0.010 and a CO conversion of 37%. The small concentration of aluminum may serve to increase the rate of iron carbide formation, whereas higher concentrations may begin to inhibit the rate.