Fischer–Tropsch Synthesis Reaction Conditions Research Articles

Effect of pretreatment conditions on a benchmark iron catalyst for CO2 hydrogenation to light olefins

To improve the CO2-to-light olefin process in terms of economics, efficient catalysts are essential to maximize the yield. Our research was carried out on FeAlCuK, which is still considered as a benchmark for olefin formation via CO2 Fischer-Tropsch synthesis (FTS). Nevertheless, the effect of various activation conditions on the formation of desired active species and final activity of this catalyst has not been described in literature to date. We studied the pretreatment by varying both the reduction gas and temperature. The catalyst was pretreated with pure H2 or synthesis gas (H2/CO = 3) to identify the bulk and surface species formed in each case. We demonstrate that the FeCuAlK catalyst should be activated in a H2/CO atmosphere at 250 °C for 4 h before the test at typical FTS reaction conditions to reach maximum productivity. The reduction with H2/CO led to the formation of active Fe5C2 and Fe3O4 phases producing a high fraction of light olefins at higher CO2 conversion. The role of promoters K and Cu was also investigated. Selectivity for desired C2-C4 olefins increased by 10 % compared to catalysts which were used directly in FTS without preliminary activation. Characterization with ex and in situ powder X-ray diffraction, Raman spectroscopy, and XPS after each activation treatment as well as Mössbauer spectroscopy revealed a strong impact of gas nature on the phase composition which was correlated with the performance.

Atomic-scale understanding of the impact of CoO phase in cobalt-catalyzed Fischer-Tropsch synthesis: A combined computational and experimental study

In cobalt-catalyzed Fischer-Tropsch synthesis (FTS), the active metallic cobalt phase always suffers from oxidation under FTS reaction conditions, leading to the coexistence of metallic cobalt and cobalt oxide (CoO) phases. However, there still exist contradictory views on the impact of CoO phase in FTS. Some researchers proposed that the oxidation of metallic Co to CoO causes the deactivation of cobalt-based FTS catalysts, while some held the view that CoO phase could act as the potential active phase in FTS. In this study, by combining theoretical calculations and experiments, we systematically investigated the fundamental FTS reactions including CO adsorption and activation, C-C coupling, and CxHy hydrogenation on CoO phase, and compared them with those on metallic cobalt phase, aiming at identifying the exact role of CoO, understanding how CoO influences the activity and selectivity, and unraveling the intrinsic mechanism that determines the catalytic behaviors of CoO phase in FTS. The results reveal that the CoO(200) crystal facet is the dominantly exposed surface structure for CoO phase under FTS reaction conditions. The d-band center of CoO(200) surface shifts downward in comparison with that of active metallic Co surface, which causes difficulties in adsorbing and activating CO and gives rise to high energy barriers for the dissociation of C-O bond. This determines the inactive nature of CoO phase in FTS reaction. As for the product selectivity, the CoO(200) surface restricts the C-C chain growth due to its higher barriers for C-C coupling, facile desorption ability for light olefins, and lower energy barrier for CH4 formation compared with metallic cobalt, and consequently leads to the loss of selectivity to C5+ hydrocarbons and enhancement of selectivity to light olefins and undesired methane. Moreover, a strong linear relationship between the experimental observed selectivity change and the DFT-calculated energy difference of CoO and metallic cobalt phases was found, validating the DFT-calculated results. This study provides a comprehensive understanding of the nature and intrinsic catalytic behaviors of CoO phase in cobalt-catalyzed FTS.

Revealing the activity of different iron carbides for Fischer-Tropsch synthesis

Exploring the structure of iron-based catalysts on the catalytic performance of Fischer-Tropsch synthesis (FTS) reaction has attracted much attention. With this in mind, the mixture of SiO2 or Al2O3 powder and non-porous iron oxide powder (α-Fe2O3) have been investigated for understanding the nature role of different iron carbides in the FTS reaction by adjusting the formation of iron carbides. Under the typical FTS reaction conditions, the CO conversion of Al2O3/α-Fe2O3 = 1 catalyst could reach up to 61.6 %, which is about 3.3 times that of the pure α-Fe2O3 catalyst. Based on the characterization results, including in situ XPS and CO-DRIFTS as well as Mössbauer spectra, it is found that the electronic state of iron atoms is affected by the existence of SiO2 or Al2O3, and the interactions of Fe-Si or Fe-Al are formed on the surface of iron powder, which plays an important role in formation of C-rich iron carbide active phase (ε-Fe2C). Although χ-Fe5C2 is usually as the active phase in the FTS reaction, we have found that the content of ε-Fe2C is more positive to activity.

Unravelling the structure-performance relationship over iron-based Fischer-Tropsch synthesis by depositing the iron carbonyl in syngas on SiO2 in a fixed-bed reactor

A new process, in situ deposition method, was developed to unveil Fe phase evolution behavior in iron-based Fischer-Tropsch synthesis (FTS). In detail, Fe(CO)x-containing syngas (CO/H2) was fed into a fixed-bed reactor where Fe(CO)x directly decomposes to metallic Fe with subsequent favorable carburization to form active χ-Fe5C2 on SiO2 under FTS reaction conditions. This facile process gives a high χ-Fe5C2 content (80–90%) without Fe3O4 phase in spent catalyst. In contrast, the 5.3Fe/SiO2 prepared by the conventional impregnation method results in very low χ-Fe5C2 content (14.5%) and high Fe3O4 content (47%) in spent catalyst, which is ascribed to the strong Fe oxide-SiO2 interaction hindering its deep reduction into metallic Fe but terminating as FeO. The reduced FeO could not be efficiently carburized into χ-Fe5C2 and tends to oxidize into Fe3O4 during the FTS reaction. Furthermore, the factors determining CO2 selectivity are systematically investigated based on abundant experiments. Results show that CO2 selectivity is closely related to CO conversion, Fe phase composition, reaction conditions, and the presence of K promoter. In brief, when the factor enhances H*/C* ratio on χ-Fe5C2, the reaction of dissociative O* from CO shifts towards hydrogenation to H2O despite that its reaction with CO* to form CO2via Boudouard mechanism is kinetically favorable. In addition, CO2 selectivity could be reduced via reverse water-gas shift (RWGS) reaction on Fe3O4 with H2O generation.

Highly Ordered Mesoporous Fe2O3–ZrO2 Bimetal Oxides for an Enhanced CO Hydrogenation Activity to Hydrocarbons with Their Structural Stability

Highly ordered mesoporous Fe2O3–ZrO2 mixed bimetal oxides (FeZr) without any additional chemical promoters were first applied to produce the value-added hydrocarbons by CO hydrogenation through Fischer–Tropsch synthesis (FTS) reaction of syngas. To enhance a catalytic activity and structural stability, an irreducible ZrO2 as a structural promoter was incorporated in the ordered mesoporous Fe2O3 structures with a different Zr/Fe molar ratio from 0 to 1 prepared by using a hard template of KIT-6. When an optimal amount of zirconia (Zr/Fe molar ratio = 0.25) was incorporated in the ordered mesoporous Fe2O3 frameworks, the catalytic activity was significantly improved and almost 10 times higher than the mesoporous monometallic Fe2O3. The highly ordered mesoporous structures were stably preserved even under reductive FTS reaction conditions. The ordered mesoporous FeZr catalysts showed a higher C5+ selectivity even at a higher CO conversion above 80%. This improved catalytic activity and stability on the optim...

Fischer–Tropsch synthesis over a 3D foamed MCF silica support: Toward a more open porous network of cobalt catalysts

Three-dimensional mesoporous cellular silica foams (MCFs) were prepared and used as supports for cobalt Fischer–Tropsch synthesis (FTS) catalysts (Co/MCF). The silica foam MCFs have a 3D mesostructure with large spherical cell pores interconnected with small window pores, which provides an open porous network for Co FTS catalysts. The effects of catalyst structure parameters (cell pore diameter and cobalt particle size) on the performance of the Co/MCF catalysts were investigated. It was found that the Co/MCF catalyst with average pore size 7–12nm (window and cell pores) showed the highest reactivity. The MCF not only provided a cavity to suppress the growth of cobalt particles, but also enhanced the reducibility and cobalt dispersion. The Co/MCF catalysts having a large and open pore structure are favorable for the transport of reactants and the occurrence of secondary reactions, leading to high C5+ hydrocarbon selectivity. A Co/MCF catalyst with Co particle size 6.9nm presented the highest activity in the FTS reaction due to its having the highest dispersion and an appropriate degree of reduction. Compared with the Co catalysts supported on other ordered silicas (SBA-16, KIT-6, and SBA-15), the Co/MCF catalyst showed higher activity and C5+ hydrocarbon selectivity, which was attributed to the 3D open porous structure of MCF. The Co/MCF catalysts are stable under low-temperature FTS conditions (210°C). Under harsh FTS reaction conditions (250°C, 1.0MPa), the Co/MCF catalyst underwent rapid deactivation due to cobalt sintering and the formation of interacting Co–silica compound, while the stability of the Co/MCF catalyst can be improved by functionalizing the MCF using carbon coating and Al doping.

Effect of sulfur on α-Al2O3-supported iron catalyst for Fischer–Tropsch synthesis

The effects of sulfur modification (S/Fe=0.001∼0.05, molar ratio) on the α-Al2O3-supported iron catalysts for the Fischer–Tropsch synthesis (FTS) were investigated. It was found that the presence of sulfur promoted the reduction of FeO to metallic Fe during the H2 reduction process, but decreased the rates of reduction, carburization and carbon deposition of the catalysts when CO is used as the reducing agent. Both the sulfide (S2−) and sulfate species (SO42-) are found in the SxFe/α-Al2O3 catalysts after H2 reduction and FTS reaction. The sulfide species in the H2-reduced SxFe/α-Al2O3 catalysts are located on the surface of metallic Fe particles. The intensity of H2 desorption peak around 500K is enhanced by a small amount of sulfur, while an excess of sulfur suppresses the H2 adsorption. The dissociative adsorption of CO on H2-reduced catalysts was also found to be inhibited by the presence of sulfur. The activity of the Fe/α-Al2O3 catalyst can be decreased significantly by the addition of sulfur due to the inhibition of CO dissociation and thus reduce the formation of iron carbide phases under FTS reaction conditions. Resistance to sulfur poisoning of the Fe/α-Al2O3 catalyst was found to improve with increasing the reaction temperature. The presence of sulfur also suppressed the formation of C5+ hydrocarbons and shifted the products to C2–C4 hydrocarbons. At the same time, the olefin to paraffin (O/P) ratio of the C2–C4 hydrocarbons decreased with increasing S/Fe molar ratio. These may have resulted from the increasing of the H/C ratio on the surface of sulfur modified catalyst under FTS conditions.

Effective control of α-olefin selectivity during Fischer–Tropsch synthesis over polyethylene-glycol enwrapped porous catalyst

Effective control of α-olefin selectivity during Fischer–Tropsch synthesis (FTS) was achieved through continuously adding a polar solvent, polyethylene glycol (PEG), in a fixed-bed reactor over an industrial Fe-based catalyst under typical FTS reaction conditions. The α-olefin content increased drastically and was independent of carbon number in C6+ range, whereas the product carbon-number-distribution changed unobviously. Similar trend was also observed in the results obtained from a continuously stirred tank reactor using PEG as reaction medium. This phenomenon can be explained by the suppressed α-olefin secondary reactions due to the existence of a PEG-membrane on the catalyst surface.

Water-gas-shift kinetics over a Fe/Cu/La/Si catalyst in Fischer–Tropsch synthesis

The kinetic modeling of the water-gas-shift (WGS) reaction over a Fe/Cu/La/Si catalyst under Fischer–Tropsch synthesis (FTS) reaction condition was investigated. A number of Langmuir–Hinshelwood–Hougen–Watson type rate expression based on possible reaction sets, originated from the formate and direct oxidation mechanisms, were derived. Four models were used to fit the experimental data over a wide range of reaction condition. WGS rate expressions based on the formate mechanism were found to provide an improved description for WGS kinetic data. Experimental kinetics results showed that the WGS reaction under FTS reaction conditions was far from equilibrium.

Deactivation by Filamentous Carbon Formation on Co/Aluminum Phosphate during Fischer−Tropsch Synthesis

Amorphous aluminum phosphate (AlPO4)-supported cobalt catalysts, with and without Ru promotion, were evaluated to study CO hydrogenation activity during Fischer−Tropsch synthesis (FTS). The Ru-promoted catalyst (RuCo/AlPO4) exhibited higher CO conversion and C8+ selectivity than that of the unpromoted catalyst (Co/AlPO4) and Co/Al2O3 catalyst. Interestingly, filamentous carbon was formed, even at the mild FTS reaction conditions (T = 220−240 °C and P = 2.0 MPa), on Co/AlPO4, and it eventually accelerated catalyst deactivation. The presence of an Ru promoter restricted the formation of filamentous carbon and enhanced catalyst stability.

Oxygenate kinetics in Fischer–Tropsch synthesis over an industrial Fe–Mn catalyst

The kinetics models of oxygenate formation in Fischer–Tropsch synthesis (FTS) over an industrial Fe–Mn catalyst are studied in a continuous spinning basket reactor. Detailed kinetics models on the basis of possible oxygenate formation mechanisms, namely adsorbed CO or CH 2 insertion mechanisms, are derived. The calculated alcohol and acid distributions in FTS reaction fit the experimental data well with considering the esterification reactions between alcohols and acids. The alcohol formation via successive hydrogenation of intermediate [RCO-s] is more energetically favorable than the acid formation via the reaction between the [RCO-s] and [OH-s]. The alcohol formation is not via the successive hydrogenation of acid intermediates over the Fe–Mn catalyst under FTS reaction conditions.

Water gas shift reaction kinetics in Fischer–Tropsch synthesis over an industrial Fe–Mn catalyst

The kinetics of water gas shift (WGS) reaction over an Fe–Mn catalyst under Fischer–Tropsch synthesis (FTS) reaction conditions is studied in a spinning basket reactor. Experimental conditions are varied as follows: temperature of 533–573 K, reactor pressure of 10.0–26.5 bar, H 2/CO feed ratio of 0.66–2.0 and space velocity of 0.66–2.65×10 −3 Nm 3 kg cat −1 s −1. By separately fitting WGS kinetics parameters with experimental data, which is possible in the spinning basket reactor with neglecting concentration and temperature gradients, different kinetics models of WGS are derived and discriminated on the basis of four sets of WGS elementary reactions. Kinetics experimental results show that the WGS reaction under FTS reaction conditions is far from equilibrium. Two types of WGS mechanisms are investigated. One is the formate mechanism, and the other is the direct oxidation mechanism. It is found that the formate mechanism is better in fitting experimental data than the direct oxidation mechanism over the Fe–Mn catalyst under the FTS reaction conditions. The optimized kinetics model with formate intermediate dissociation as the rate-determining step (RDS) can fit the WGS experimental results well. The simplified WGS kinetics model can easily be used for industrial modeling applications.

Iron chloride-graphite intercalated compounds: Structure and selectivity measurements under Fischer-Tropsch synthesis reaction conditions

The structure and selectivity of the I- and V-stage FeCl 3-graphite compounds under reduction (at 400°C) and Fischer-Tropsch synthesis reaction (at 300°C) conditions have been studied by high-resolution electron microscopy, electron diffraction, mass spectrometric and Mossbauer spectroscopic techniques. The two compounds have been found to possess almost similar selectivities. The electron microscopic and electron diffraction techniques reveal that in the I-stage compound, the intercalation occurs in (11̄0) plane, whereas in the V-stage compound it occurs in (11̄0) and (01̄0) planes. The analyses of the electron diffraction pictures of the small particles provided a means of identification of the phases present in the catalysts. Various phases observed by electron microscopy were also confirmed by in situ Mossbauer spectroscopy. No evidence of the formation of any kind of iron carbides was found either by electron microscopy or Mossbauer spectroscopy.