Mingsheng Luo

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

Abstract 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 (≲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 H2/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.

Fischer-Tropsch synthesis: Group II alkali-earth metal promoted catalysts

The effects of four Group II alkali-earth metals (barium, beryllium, calcium and magnesium) and potassium promoters on iron Fischer–Tropsch synthesis (FTS) catalysts product selectivity, syngas conversion and productivity are compared to the unpromoted iron catalyst. Iron FTS catalysts promoted with Group II alkali-earth metals have lower overall FTS activities and lower alpha values than a potassium promoted iron catalyst, but higher values than an unpromoted catalyst. All Group II metal promoters yielded similar carbon utilization as the unpromoted iron catalyst, but higher than a potassium promoted catalyst regardless of the space time. Carbon dioxide rates indicate that a potassium promoted iron catalyst possesses better water-gas shift (WGS) activity than any Group II metal promoted catalyst when both are operated at high CO conversions. Ca and Mg can even suppress the WGS activity below that of unpromoted iron catalysts. Among the Group II metals, a Ba promoted catalyst has the highest WGS activity while Mg and Ca promoted catalysts showed the lowest WGS activity. A potassium promoted catalyst generated the highest yield of CO2 and hydrocarbon, but the lowest methane and total liquid product (C5+) rate. All alkali promoted catalysts yielded higher total liquid rate, but lower gas fraction than unpromoted catalysts. Alkali-earth promoted catalysts produced a slightly higher C2–C4 olefin ratio, but a K promoted catalyst produced a notably higher C2 olefin fraction than Group II metal promoted catalysts.

Fischer–Tropsch synthesis: induction and steady-state activity of high-alpha potassium promoted iron catalysts

Abstract Iron Fischer–Tropsch synthesis (FTS) catalysts with different potassium loadings showed different induction periods during which the conversion increased from a low initial level to a peak value before declining to attain a lower stable activity at the same reaction conditions. A lower K loading produced a slightly higher peak conversion and a shorter induction period. Although, the induction period and the peak conversion were slightly dependent on the K loading for the iron catalyst, the stabilized conversions and the stabilization periods were independent of potassium content. The C 2 C 4 olefin to paraffin ratio of the gaseous products and the CO 2 selectivity did not change significantly as the potassium content increased from 5 to 10%. An increase in reaction temperature produced a new induction period and a higher conversion than was obtained before the reaction temperature was increased. The H 2 /CO ratio also had an important influence on FTS conversions. Increasing the H 2 /CO ratio in the feed gas lowered the H 2 utilization. A higher H 2 /CO ratio feedstock gas produced lower FTS catalyst activity compared to a low H 2 /CO ratio gas.

Fischer–Tropsch synthesis: activation of low-alpha potassium promoted iron catalysts

Abstract The purpose of this study is to investigate the influence of activation gas type (CO, syngas and H 2 ), activation temperature and pressure on FTS activity and selectivity. A series of Fischer–Tropsch synthesis (FTS) reactions in a continuously stirred tank reactor (CSTR) for 400–1000 h were carried using a Fe/K/Si=100/1.25/5.1 (atomic ratio) catalyst. The highest CO conversion was obtained for a CO activated catalyst while hydrogen activation yielded the lowest, regardless of the activation temperature. A higher activation temperature yielded a higher peak CO conversion during the induction period. FTS activity for syngas pretreatment showed a dependency on the activation pressure. In addition to the FTS activity, CO activation showed the best overall hydrocarbon productivity. The lowest hydrocarbon production rate was obtained with the H 2 activated catalyst. Carbon monoxide activation also yielded the lowest methane selectivity and activation temperature did not affect the CH 4 selectivity in the range of 230–270 °C. Activation temperature had little effect on the formation of olefin (C 2 –C 20 ) and the olefin fraction decreased in the order of H 2 >syngas>CO activation. The induction period was present following each of the three pretreatments: CO, H 2 , or syngas with the same H 2 /CO ratio as used during the synthesis period.

Fischer-Tropsch synthesis : influence of support on the impact of co-fed water for cobalt-based catalysts

Co catalysts were prepared with variable cobalt oxide-support interactions through judicious selection of the cobalt loading, the type of support utilized, and the promoter employed, if any, along with its loading. For a comparable Co loading range, while a positive effect of water was found for catalysts identified to have supports that only weakly interacted with the cobalt clusters, an adverse impact of water was recorded when cobalt was supported on more strongly interacting supports, such as TiO 2 and especially, Al 2 O 3 . However, alumina supported cobalt catalysts were found to have much higher active site densities in the cobalt loading range explored, due to a smaller average crystallite size. More robust Co/Al 2 O 3 catalysts, less sensitive to the negative effect of water, were obtained at higher Co loadings, where the average cluster size was > 10 nm.

Deactivation and Regeneration of Alkali Metal Promoted Iron Fischer-Tropsch Synthesis Catalysts

Abstract Potassium promoted iron Fischer-Tropsch synthesis (FTS) catalyst showed a low deactivation rate of 0.083% per day after an initial conditioning period of 300 hours. Beryllium promoted iron catalyst produced an even more stable activity than potassium promotion, with a deactivation rate as low as 0.0062% per day. Higher temperature generated a shorter conditioning period for potassium promoted catalyst. For potassium promoted catalyst, FTS activity, CO 2 selectivity, hydrocarbon productivity and water gas shift activity showed the same conditioning period of 300 hours of time on stream after passing a peak value at about 120 hours of reaction time. A higher water gas shift activity, higher CO 2 and CH 4 selectivity was obtained for the beryllium promoted catalyst than by potassium. Beryllium promoted catalyst also showed a superior regenerability when either H 2 or CO was utilized. When rejuvenated with CO a better activity recovery was achieved than with H 2 .