Jinlin Li

Fischer–Tropsch synthesis: support, loading, and promoter effects on the reducibility of cobalt catalysts

Temperature programmed reduction (TPR) and hydrogen chemisorption combined with reoxidation measurements were used to define the reducibility of supported cobalt catalysts. Different supports (e.g. Al2O3, TiO2, SiO2, and ZrO2 modified SiO2 or Al2O3) and a variety of promoters, including noble metals and metal cations, were examined. Significant support interactions on the reduction of cobalt oxide species were observed in the order Al2O3>TiO2>SiO2. Addition of Ru and Pt exhibited a similar catalytic effect by decreasing the reduction temperature of cobalt oxide species, and for Co species where a significant surface interaction with the support was present, while Re impacted mainly the reduction of Co species interacting with the support. For catalysts reduced at the same temperature, a slight decrease in cluster size was observed in H2 chemisorption/pulse reoxidation with noble metal promotion, indicating that the promoter aided in reducing smaller Co species that interacted with the support. On the other hand, addition of non-reducible metal oxides such as B, La, Zr, and K was found to cause the reduction temperature of Co species to shift to higher temperatures, resulting in a decrease in the percentage reduction. For both Al2O3 and SiO2, modifying the support with Zr was found to enhance the dispersion. Increasing the cobalt loading, and therefore the average Co cluster size, resulted in improvements to the percentage reduction. Finally, a slurry phase impregnation method led to improvements in the reduction profile of Co/Al2O3.

FISCHER-TROPSCH SYNTHESIS: DEACTIVATION OF NOBLE METAL-PROMOTED CO/AL2O3 CATALYSTS

Abstract Fresh and used, unpromoted and noble metal-promoted 15% Co/Al 2 O 3 catalysts were analyzed by XANES and EXAFS to provide insight into catalyst deactivation. XANES analysis of the catalysts gave evidence of oxidation of a fraction of the cobalt clusters by water produced during the reaction. Comparison of XANES derivative spectra to those of reference materials, as well as linear combination fitting with the reference data, suggest that some form of cobalt aluminate species was formed. Because bulk oxidation of cobalt by water is not permitted thermodynamically under normal Fischer–Tropsch synthesis (FTS) conditions, it is concluded that the smaller clusters interacting with the support deviate from bulk-like cobalt metal behavior and these may undergo oxidation in the presence of water. However, in addition to the evidence for reoxidation, EXAFS indicated that significant cobalt cluster growth took place during the initial deactivation period. Promotion with Ru or Pt allowed for the reduction of cobalt species interacting with the support, yielding a greater number of active sites and, therefore, a higher initial catalyst activity on a per gram catalyst basis. However, these additional smaller cobalt clusters that were reduced in the presence of the noble metal promoter, deviated more from bulk-like cobalt, and were therefore, more unstable and susceptible to both sintering and reoxidation processes. The latter process was likely in part due to the higher water partial pressures produced from the enhanced activity. The rate of deactivation was therefore faster for these promoted catalysts.

Fischer–Tropsch synthesis: characterization and catalytic properties of rhenium promoted cobalt alumina catalysts☆ ☆

Abstract The unpromoted and promoted Fischer–Tropsch synthesis (FTS) catalysts were characterized using techniques such as X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray absorption spectroscopy (XAS), Brunauer–Emmett–Teller surface area (BET SA), hydrogen chemisorption and catalytic activity using a continuously stirred tank reactor (CSTR). The addition of small amounts of rhenium to a 15% Co/Al2O3 catalyst decreased the reduction temperature of cobalt oxide but the percent dispersion and cluster size, based on the amount of reduced cobalt, did not change significantly. Samples of the catalyst were withdrawn at increasing time-on-stream from the reactor along with the wax and cooled to become embedded in the solid wax for XAS investigation. Extended X-ray absorption fine structure (EXAFS) data indicate significant cluster growth with time-on-stream suggesting a sintering process as a major source of the deactivation. Addition of rhenium increased the synthesis gas conversion, based on catalyst weight, but turnover frequencies calculated using sites from hydrogen adsorption and initial activity were similar. A wide range of synthesis gas conversion has been obtained by varying the space velocities over the catalysts.

Fischer–Tropsch synthesis XAFS: XAFS studies of the effect of water on a Pt-promoted Co/Al2O3 catalyst

Abstract The impact of water on the deactivation of a 0.5% Pt-promoted 15% Co/Al 2 O 3 catalyst was studied by XAFS. Catalyst samples were withdrawn from the reactor during synthesis at different partial pressures of added water and cooled in the wax product under an inert gas blanket. Synthesis operating conditions were maintained constant while differing amounts of argon were replaced by added water. Below 25% added water (H 2 O/CO=1.2; H 2 O/H 2 =0.6), the slight negative effect on activity was reversible, and no changes were observed in the EXAFS or XANES spectra. This indicates that the effect of water in this range is most likely kinetic. However, XAFS results strongly suggest that, above 25% water addition, the sudden irreversible loss in activity is due to reaction of the cobalt clusters with the support, forming cobalt aluminate-like species. The XAFS and previously reported activity data indicate that there are two regions for the water effect: at lower H 2 O/CO ratios water influences CO conversion by reversible kinetic effects while at higher H 2 O/CO ratios irreversible oxidation of cobalt occurs.

Fischer-Tropsch synthesis: effect of small amounts of boron, ruthenium and rhenium on Co/TiO2 catalysts

The effect of the addition of small amounts of boron, ruthenium and rhenium on the Fischer–Tropsch (F–T) catalyst activity and selectivity of a 10 wt.% Co/TiO2 catalyst has been investigated in a continuously stirred tank reactor (CSTR). A wide range of synthesis gas conversions has been obtained by varying space velocities over the catalysts. The addition of a small amount of boron (0.05 wt.%) onto Co/TiO2 does not change the activity of the catalyst at lower space times and slightly increases synthesis gas conversion at higher space times. The product selectivity is not significantly influenced by boron addition for all space velocities investigated. Ruthenium addition (0.20 wt.%) onto Co/TiO 2 and CoB/TiO2 catalysts improves the catalyst activity and selectivity. At a space time of 0.5 h-g cat./NL, synthesis gas conversion increases from 50–54 to 68–71% range and methane selectivity decreases from 9.5 to 5.5% (molar carbon basis) for the promoted catalyst. Among the five promoted and non-promoted catalysts, the rhenium promoted Co/TiO 2 catalyst (0.34 wt.% Re) exhibited the highest synthesis gas conversion, and at a space time of 0.5 h-g cat./NL, synthesis gas conversion was 73.4%. In comparison with the results obtained in a fixed bed reactor, the catalysts displayed a higher F–T catalytic activity in the CSTR.

Fischer–Tropsch synthesis. Effect of CO pretreatment on a ruthenium promoted Co/TiO2

The effect of pretreatment, using hydrogen or carbon monoxide, on the activity and selectivity of a ruthenium promoted cobalt catalyst (Ru(0.20 wt%)/Co(10 wt%)/TiO2) during Fischer–Tropsch (FT) synthesis was studied in a continuous-stirred tank reactor (CSTR). The hydrogen reduced catalyst exhibited a high initial synthesis gas conversion (72.5%) and reached steady state after 40 h on stream, after which the catalyst deactivated slightly with time on stream. The carbon monoxide reduced catalyst reached steady state quickly and showed a lower activity and a good stability. Methane selectivity on the carbon monoxide reduced catalyst was 15–20% (carbon base), much higher than that on the hydrogen reduced catalyst (5–10%). Carbon monoxide regeneration increased the activity on the hydrogen reduced catalyst; however, it did not have significant effect on the carbon monoxide reduced catalyst.

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.

Fischer-Tropsch Synthesis: Kinetics and Effect of Water for a Co/Al2O3 Catalyst

The reason that cobalt catalysts display little activity for the water-gas-shift reaction, the amount of water present in the reactor during the Fischer-Tropsch (FT) synthesis reaction will be directly related to the CO conversion. It is therefore important to determine the impact of water partial pressure on the kinetics of the reaction, as well as the chemical and structural changes of the catalyst. This chapter reviews that the rate expression for FT synthesis has been obtained using a 25 wt.% Co/Al2O3 catalyst in a 1 liter continuously stirred tank reactor (CSTR) operated at 493K, 1.99 MPa (19.7 atm), H2/CO feed ratios of 1.0-2.4 with varying space velocities to produce 14-63% CO conversion. Adjusting the ratios of inert gas and added water permitted the impact of added water to be made at the same total flow rate and H2 and CO partial pressures. The addition of water at low levels during FT synthesis did not impact CO conversion but at higher levels it decreased CO conversion relative to the same conditions without water addition. The catalytic activity recovered after water addition was terminated. The temporary reversible decline in CO conversion when water was added may be due to the kinetic effect of water by inhibition of CO and/or H2 adsorption.

D2O tracer studies in Co catalyzed Fischer–Tropsch reaction

Abstract The data show that the deuterium added in water together with synthesis gas provides hydrogen for the Fischer–Tropsch synthesis. In fact, the deuterium initially present in water nearly equilibrates with the hydrogen present in synthesis gas. Thus, water, once formed, is not inert but adsorbs competitively on the cobalt–titania catalyst to activate hydrogen. The data do not permit a definition of whether the exchange occurs on cobalt or the alumina support. The H/D ratio in the paraffin products is 4.4 and is very close to the H/D ratio in the feed of D 2 O/H 2 (4.1).

Fischer-Tropsch synthesis: effect of water on activity and selectivity for a cobalt catalyst

The addition of water to the syngas (H2O/CO = 0.74) feed for an unsupported cobalt catalyst utilized in a CSTR, where all of the catalyst is exposed to a common feed composition, resulted in an increase in CO conversion. Conversion remains constant during a three-day period, showing that the promotional effect was not of a transitory nature. At the same time the CO hydrogenation increased, the hydrogenation to produce methane decreased as did the secondary hydrogenation of ethene. The addition of higher fractions of H2O/CO led to permanent deactivation of the catalyst for CO conversion but the decrease in hydrogenation to methane and of alkene products remained.