Yongqing Zhang

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.

CO and CO2 hydrogenation study on supported cobalt Fischer–Tropsch synthesis catalysts

Abstract The conversion of CO/H2, CO2/H2 and (CO+CO2)/H2 mixtures using cobalt catalysts under typical Fischer–Tropsch synthesis conditions has been carried out. The results show that in the presence of CO, CO2 hydrogenation is slow. For the cases of only CO or only CO2 hydrogenation, similar catalytic activities were obtained but the selectivities were very different. For CO hydrogenation, normal Fischer–Tropsch synthesis product distributions were observed with an α of about 0.80; in contrast, the CO2 hydrogenation products contained about 70% or more of methane. Thus, CO2 and CO hydrogenation appears to follow different reaction pathways. The catalyst deactivates more rapidly for the conversion of CO than for CO2 even though the H2O/H2 ratio is at least two times larger for the conversion of CO2. Since the catalyst ages more slowly in the presence of the higher H2O/H2 conditions, it is concluded that water alone does not account for the deactivation and that there is a deactivation pathway that involves the assistance of CO.

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: 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: Changes in Phase and Activity During Use

Four iron catalysts (unpromoted, K-promoted, Si-promoted and K,Si-promoted) were activated and subjected to common Fischer–Tropsch synthesis conditions. At increasing times on stream, samples were withdrawn from the continuously stirred tank reactor in the reactor wax while keeping the sample blanketed with an inert gas. Mössbauer spectra were recorded for various samples and the iron phases of the catalyst were compared to the catalytic activity. A simple model based on bulk composition of the catalyst is not related to the catalytic activity during the course of the run.