Fischer-Tropsch synthesis: activity and selectivity for Group I alkali promoted iron-based catalysts

The impact of the Group I alkali metals upon the activity of iron catalysts has been obtained at medium pressure synthesis conditions and at the same conversion levels. The relative impact of the alkali metal depends upon the conversion level with potassium being the promoter that impacts the highest activity at all conversion levels. At low conversions, Li is nearly as effective as potassium in improving the catalytic activity but is the poorest promoter at high conversion levels. In fact, three alkalis (Li, Cs and Rb) should be viewed as inhibitors since they decrease the catalytic activity for CO conversion below that of the unpromoted iron catalyst. The differences in the impact of the various Group I alkali metals at lower (less than or similar to 40%) conversions are slight but become much greater at higher CO conversion levels. The major differences of the alkali metals at higher conversion levels is due to the impact of the promoter upon the water-gas-shift (WGS) reaction. At higher conversion levels, with a synthesis gas or "syngas" of H-2/CO = 0.7, the WGS reaction becomes rate controlling because hydrogen production becomes the rate limiting factor in the Fischer-Tropsch synthesis (FTS). The basicity of the promoter appears to be the determining factor for the rate of catalyst deactivation and on the secondary hydrogenation of ethene. (C) 2002 Elsevier Science B.V. All rights reserved.