Fe-based Fischer Tropsch Catalysts Research Articles

Effect of Precursors of Fe-Based Fischer–Tropsch Catalysts Supported on Expanded Graphite for CO2 Hydrogenation

Expanded graphite as a type of hydrophobic functional carbon material was innovatively selected as support for Fe-based Fischer–Tropsch catalysts. Through catalytic activity evaluation, ferric ammonium citrate was selected as the most suitable iron precursor for expanded graphite among ferric nitrate, ferric ammonium oxalate, ferric citrate, and ferric ammonium citrate. The catalyst with ferric ammonium citrate as the precursor and expanded graphite as support achieved the best CO2 hydrogenation performance with a CO2 conversion rate of 42.4%, a C2═–C4═ yield of 14.8%, and a CO selectivity of 8.3%. Furthermore, it was observed that the precursor that had better wettability and formed hydrogen bonds with support tended to generate smaller iron nanoparticles. In addition, nitrogen doping during catalyst calcination would cause differences in electronic properties and then influence the reducibility and carbonization of iron species.

Consolidated Co- and Fe-based Fischer-Tropsch catalysts supported on jellyfish-like graphene nanoflake framework

Different carbon nanomaterials are considered as promising supports for Fischer-Tropsch synthesis (FTS) catalysts of syngas transformation into hydrocarbons. At the same time, consolidation of carbon nanoparticles into a dense bulk material is one of the most important challenges faced by the catalytic industry. In this work consolidated Co and Fe catalysts supported on graphene nanoflakes (GNFs) were for the first time synthesized by spark plasma sintering (SPS). The synthesized catalysts contained reduced carbon-encapsulated metal nanoparticles embedded into the GNF framework, which ensured their activity in FTS without pre-activation. SPS significantly increased both the activity (from 77 to 91 µmolCO×gCo−1 ×s−1) and TOF (from 0.024 to 0.073 s−1) of the Co/GNF catalyst compared to the unsintered sample. In contrast, the activity of the Fe catalysts decreased after SPS, but their C5+ selectivity and olefins to paraffins ratio increased. These enhancements were attributed to the unique core-shell structure of the nanoparticles, highly active surface cobalt atoms, and close contact between carbon support and iron particles. SPS was proved to be a powerful approach to activate carbon-supported FTS catalysts, increase their density and modify their catalytic performance.

Design of Cobalt Fischer-Tropsch Catalysts for the Combined Production of Liquid Fuels and Olefin Chemicals from Hydrogen-Rich Syngas.

Adjusting hydrocarbon product distributions in the Fischer–Tropsch (FT) synthesis is of notable significance in the context of so-called X-to-liquids (XTL) technologies. While cobalt catalysts are selective to long-chain paraffin precursors for synthetic jet- and diesel-fuels, lighter (C10–) alkane condensates are less valuable for fuel production. Alternatively, iron carbide-based catalysts are suitable for the coproduction of paraffinic waxes alongside liquid (and gaseous) olefin chemicals; however, their activity for the water–gas-shift reaction (WGSR) is notoriously detrimental when hydrogen-rich syngas feeds, for example, derived from (unconventional) natural gas, are to be converted. Herein the roles of pore architecture and oxide promoters of Lewis basic character on CoRu/Al2O3 FT catalysts are systematically addressed, targeting the development of catalysts with unusually high selectivity to liquid olefins. Both alkali and lanthanide oxides lead to a decrease in turnover frequency. The latter, particularly PrOx, prove effective to boost the selectivity to liquid (C5–10) olefins without undesired WGSR activity. In situ CO-FTIR spectroscopy suggests a dual promotion via both electronic modification of surface Co sites and the inhibition of Lewis acidity on the support, which has direct implications for double-bond isomerization reactivity and thus the regioisomery of liquid olefin products. Density functional theory calculations ascribe oxide promotion to an enhanced competitive adsorption of molecular CO versus hydrogen and olefins on oxide-decorated cobalt surfaces, dampening (secondary) olefin hydrogenation, and suggest an exacerbated metal surface carbophilicity to underlie the undesired induction of WGSR activity by strongly electron-donating alkali oxide promoters. Enhanced pore molecular transport within a multimodal meso-macroporous architecture in combination with PrOx as promoter, at an optimal surface loading of 1 Prat nm–2, results in an unconventional product distribution, reconciling benefits intrinsic to Co- and Fe-based FT catalysts, respectively. A chain-growth probability of 0.75, and thus >70 C% selectivity to C5+ products, is achieved alongside lighter hydrocarbon (C5–10) condensates that are significantly enriched in added-value chemicals (67 C%), predominantly α-olefins but also linear alcohols, remarkably with essentially no CO2 side-production (<1%). Such unusual product distributions, integrating precursors for synthetic fuels and liquid platform chemicals, might be desired to diversify the scope and improve the economics of small-scale gas- and biomass-to-liquid processes.

Improving thermal diffusivity of supported Fe-based Fischer–Tropsch catalysts to enhance long-chain hydrocarbon production

Fischer Tropsch synthesis (FTS) is highly exothermic so heat removal remains crucial. In this study, a rational procedure is examined to remove heat in the FTS by improving the thermal diffusivity on a series of Fe-based catalysts.

First-Principles Investigation of CO Adsorption on h-Fe7C3 Catalyst

h-Fe7C3 is considered as the main active phase of medium-temperature Fe-based Fischer–Tropsch catalysts. Basic theoretical guidance for the design and preparation of Fe-based Fischer–Tropsch catalysts can be obtained by studying the adsorption and activation behavior of CO on h-Fe7C3. In this paper, the first-principles method based on density functional theory is used to study the crystal structure properties of h-Fe7C3 and the adsorption and activation CO on its low Miller index surfaces ( 1 1 ¯ 0 ) , ( 1 1 ¯ 1 ) , ( 101 ) , ( 1 1 ¯ 1 ¯ ) and ( 001 ) . It was found that the low Miller index crystal plane of h-Fe7C3 crystal has multiple equivalent crystal planes and that the maximum adsorption energy of CO at the 3F2 point of the ( 1 1 ¯ 1 ) plane is −2.50 eV, indicating that h-Fe7C3 has a better CO adsorption performance. In addition, the defects generated at the truncated position of the h-Fe7C3 crystal plane have a great impact on the adsorption energy of CO on its surface, that is, the adsorption energy of CO on Fe atoms with C vacancies is higher. The activity of CO after adsorption is greatly affected by the adsorption configuration and less affected by the adsorption energy. The higher the coordination number of Fe atoms after adsorption, the higher the CO activity. At the same time, it was found that the bonding of O and Fe atoms is conducive to the activation of CO.

Experimental Investigation on the Two-Sided Effect of Acidic HZSM-5 on the Catalytic Performance of Composite Fe-Based Fischer–Tropsch Catalysts and HZSM-5 Zeolite in the Production of Aromatics from CO2/H2

A negative effect of an HZSM-5 zeolite was observed for CO2 hydrogenation toward the production of aromatics when combined with Fe-based Fischer–Tropsch synthesis (FTS) catalysts in an improper manner. The conversions of CO2 and CO were seriously hindered due to suppression in the formation of Fe carbides. With various especially designed experiments, it is confirmed that the Brønsted acid sites in the zeolite serving as an electron acceptor should be the major factor influencing the electron transfer from the Fe phase to CO molecules, resulting in the suppression of CO dissociation. Additionally, we suggested a practical way to eliminate the negative effect of the acidic HZSM-5 zeolite by coating an inert layer on either the Fe-based catalyst or the zeolite. With these observations, it is beneficial to design a multifunctional catalyst system that can be scaled up by paying more attention toward the two-sided effect of zeolites.

Effect of the promoters on oxidation behavior of Fe-based Fischer-Tropsch catalyst: Deciphering the role of H2O

The effect of carburization and reduction degree on H2O oxidation behaviour for the iron carbides in Fe-based FTS catalyst were firstly investigated using a combination method including X-ray diffraction (XRD), Raman and temperature-programmed-hydrogenation (TPH). The relationship between carbon species transformation and H2O oxidation behaviour of iron carbides was investigated simultaneously. Based on these observations, the influence of typical promoters like K and SiO2 on the structure and H2O oxidation behaviour of Fe-based FTS catalysts was further studied. The results indicated that, for the iron catalyst, the stability of iron carbides against H2O oxidation was increased with the increase of iron carbides content, and the H2O oxidation process led to the formation of more graphitic carbon. The carburization ability was effectively enhanced when certain amount of K promoter was incorporated. Addition of K into Fe-based FTS catalyst increased the number of graphitic carbons, which increased the stability of iron carbides toward H2O oxidation ultimately. It was also found that promotion of SiO2 greatly degraded the carburization degree of the catalyst, while their tendency to be oxidized to form Fe3O4 in the H2O atmosphere was obviously hindered.

The promotional effect of Mn on Fe-based Fischer–Tropsch catalysts for the synthesis of C5+hydrocarbons

Fe–Mn oxides were prepared for FTS. The Mn amount has a great influence on the morphology and product selectivity.

From hydrophilic to hydrophobic: A promising approach to tackle high CO2 selectivity of Fe-based Fischer-Tropsch microcapsule catalysts

Zeolite supported Fischer-Tropsch (FT) catalysis has attracted extensive attentions in past few years. Capsule catalysts have unique characteristics of tuning product selectivity more precisely and enhancing stability of catalysts during reaction, which makes them to stand out of the crowed. Fe-based FT catalysts have been facing limitations for syngas conversion to hydrocarbons with their associated problems of high CO2 selectivity and rapid catalyst deactivation. A series of ZSM-5 supported Fe-based microcapsule catalysts, composed of Fe/ZSM-5 core and silicalite-1 shell, was prepared and employed for direct synthesis of gasoline range hydrocarbons from syngas. Tuning silicalite-1 shell thickness significantly affected water-gas shift reaction with increasing hydrophobicity of catalyst surface, and greatly decreased CO2 selectivity. The microcapsule catalysts significantly suppressed the CO2 selectivity as compared with supported Fe-based catalyst. Moreover, all zeolite supported Fe-based catalysts, showed significantly high gasoline range hydrocarbons selectivity (about 60%). The proposed catalyst design suggests a promising approach to overcome high CO2 selectivity challenge associated with Fe-based FT catalysts.

Conversion of syngas toward aromatics over hybrid Fe-based Fischer-Tropsch catalysts and HZSM-5 zeolites

Direct conversion of syngas (CO + H2) toward aromatics has been demonstrated over a hybrid catalyst system composed of Fe-based FTS catalyst and HZSM-5 zeolite. Over the catalyst system, the aromatics were highly selective in the liquid hydrocarbon products with a content greater than 95%, and 70–90% of the aromatics were concentered on the toluene, xylene and 3C-branched alkylbenzenes. In this study, we systematically investigated the effects of reaction conditions (temperature, pressure, space velocity and H2/CO ratio) and catalyst combination (Si/Al ratio of zeolite, weight ratio of HZSM-5 to Fe-based catalyst, metal modification of HZSM-5, and kind of Fe-based catalyst) on the CO conversion, selectivity to aromatics, aromatic distribution, and catalyst deactivation, combining with some characterization technologies (XRD, BET, NH3-TPD, H2-TPR, TG and 27Al NMR). The obtained results show that HZSM-5 zeolite was the crucial component for aromatic formation. Excessive loading of the zeolite in bed would greatly suppress the carburization of reduced Fe into Fe carbides as well as CO dissociation on the Fe carbides because of the increasingly unfavorable direct contact between the acidic HZSM-5 and Fe-based catalyst, resulting in a remarkably lower CO conversion. HZSM-5 zeolite having high Brønsted acidity favored the formation of aromatics with a high content in liquid phase. However, as HZSM-5 had originally lower Brønsted acidity or the Brønsted acidity decreased during the aromatic synthesis due to more coke deposition, the selectivity to hydrocarbons in liquid phase gradually shifted to iso-paraffins rather than aromatics. Reactions operated over FeMn-HZSM-5 catalyst system at lower temperature or higher pressure led to an easier deactivation of HZSM-5 mainly from coke deposition. Compared with the FeMn-HZSM-5, FeK-HZSM-5 deactivated seriously at same conditions probably due to heavier and olefinic primary hydrocarbons over the FeK catalyst. In addition, 27Al NMR showed that extraction of framework Al by in situ formed H2O and CO2 could be ignorable for the HZSM-5 deactivation. More detailed results and discussion from abundant experimental work on clarifying their effects on the aromatic synthesis can be found in the article.

Hydrogenation of CO2 into hydrocarbons: enhanced catalytic activity over Fe-based Fischer–Tropsch catalysts

The conversion efficiency of CO2 in CO2-FTS over Fe-based catalysts is significantly enhanced by driving the conversion of the CO intermediate via the FTS reaction over a second kind of FT component, Co or Ru, without WGS activity.

First Principle Study on the Adsorption of Hydrocarbon Chains Involved in Fischer–Tropsch Synthesis over Iron Carbides

The adsorption of n-alkyls, 1-alkenes, and n-alkanes has been studied on the χ-Fe5C2(010), Fe7C3(001), θ-Fe3C(001), and η-Fe2C(011) surfaces which have been reported as active phases for Fe-based Fischer–Tropsch catalysts. Spin-polarized density functional theory calculations with the vdW-DF2 functional were performed. The most stable adsorption configuration for n-alkyls is a bridge position for χ-Fe5C2(010), Fe7C3(001), and θ-Fe3C(001), whereas for η-Fe2C(011) the most stable configuration is on-top. The surfaces with the larger surface carbon content present a higher affinity for the adsorption of n-alkyls. For adsorption of 1-alkenes, the χ-Fe5C2(010) surface shows the strongest adsorption, with a di-σ adsorption mode. For n-alkanes, the strongest physisorption was found for the smoothest surface, i.e., the θ-Fe3C(001). The adsorption energies are found to be independent of the length of the hydrocarbon chain from a chain length of three carbon atoms for n-alkyl species, five carbon atoms for 1-alkene...

Investigating the Stability of Ru-promoted Fe-based Fischer-tropsch Catalyst at high Synthesis Gas Conversion

The productivity of an iron-based Fischer Tropsch (FT) process strongly depends on the synthesis gas conversion. However, iron is known to oxidize significantly at high CO conversion due to the high reactor water partial pressures. In commercial iron-based FT plant this necessitate operation at low single pass conversion, resulting in high separation and recycling cost of the tail gas and the overall economics of the process. Ruthenium has been reported to demonstrate significant stability in the presence of water, hence the employment of FT catalyst promoted with Ru is expected to permit high CO conversion. In this study the stability of an industrial Fe-based FT catalyst impregnated with ruthenium is investigated. The reactions were carried out at industrially relevant conditions in a 1 dm3 slurry reactor at 250°C and 20 bar. It was noted that the Fe promoted with Ru was more stable at high CO conversion implying a significant resistance to deactivation in the presence of water. An increase in CO2 and CH4 selectivity and a fall in C5+ selectivity were also noted.

CO2 hydrogenation to hydrocarbons over Co and Fe-based Fischer-Tropsch catalysts

The performances of representative Co-based and Fe-based Fischer-Tropsch catalysts have been comparatively investigated in the hydrogenation of CO and CO2. Over an un-promoted Co/γ-Al2O3 catalyst, CO2 is easily hydrogenated and its conversion rate is even faster than that of CO; however, the selectivities of the two processes are extremely different, with methane largely dominating the product distribution in the case of CO2 hydrogenation and long-chain hydrocarbons dominating the products pool during CO hydrogenation. As opposite to cobalt, CO2 hydrogenation rate over K-promoted 100Fe/10Zn/1Cu (at/at) catalysts is slower than that of CO, but the products are dominated by middle distillates when CO2 replaces CO in the feed. Such behaviors depend on the different adsorption strengths of CO and CO2, which affect the H/C atomic ratio on the catalyst surface. In the case of Fe-based catalyst, we have also found that the catalytic sites active in the chain growth process (iron carbides) are transformed into sites active in the hydrogenation reactions (iron oxides/reduced iron centers) at low CO partial pressures. Potassium has a key role in promoting the stability of chain growth sites, thus decreasing the secondary reactions of olefins.

Fe2O3 nanoparticles encapsulated in TiO2 nanotubes for Fischer–Tropsch synthesis: The confinement effect of nanotubes on the catalytic performance

The catalysts with high activity and selectivity, especially the latter, play a key role in Fischer–Tropsch synthesis technology. Herein, a novel Fe-based confinement catalyst, Fe2O3 encapsulated in TiO2 nanotubes, was prepared by a method of vacuum-assisted impregnation. The catalyst presented an excellent yield of oil phase hydrocarbons and C5+ selectivity due to the confinement effect. This interesting phenomenon was discussed from the view of the restrained molecular movement and enhanced readsorption of short-chain α-olefins in the confined space of TiO2 nanotube channels. The results over this novel confinement catalyst revealed a promising research prospect on adjusting the product distribution to long-chain hydrocarbons for Fe-based Fischer–Tropsch catalyst.

The role of zeolites in the deactivation of multifunctional fischer-tropsch synthesis catalysts: the interaction between HZSM-5 and Fe-based Ft-catalysts

In order to produce gasoline directly from syngas, HZSM-5 can be added to the Fischer-Tropsch catalyst. However, this catalytic system shows an important deactivation rate. Aiming at describing this phenomenon, Fe-based catalysts and physical mixtures containing these catalysts and HZSM-5 were employed in this reaction. All these systems were characterized using the following techniques: XRD, XPS, TPR and TPD of CO. This work shows that HZSM-5 interacts with the Fe-based Fischer-Tropsch catalyst during the reduction step, decreasing the Fe concentration on the catalytic surface and thus lowering the activity of the catalytic system in the Fischer-Tropsch Synthesis.

Enhanced Activity via Surface Modification of Fe-Based Fischer–Tropsch Catalyst Precursor with Titanium Butoxide

The surface of a model iron catalyst precursor was modified with titanium butoxide to introduce Fe–O–Ti interactions in a controlled manner and to investigate the role of these interactions in the catalyst. The reduction of the model catalyst precursors in hydrogen at 350 °C for 16 h leads to the formation of α-Fe and an iron–titanium mixed oxide, due to the incorporation of Ti into the iron oxide structure. The α-Fe phase is transformed into χ-Fe5C2 during the Fischer–Tropsch synthesis at 250 °C, whilst the Fe–Ti mixed oxide phase is preserved. A higher reaction temperature of 300 °C is required to transform some the oxide phase into a carbide phase under Fischer–Tropsch conditions. The intrinsic activity of the iron carbide phase in samples also containing the Fe–Ti mixed oxide phase is at a reaction temperature of 250 °C ca. 20 % more active than in the sample, which does not contain the mixed oxide.

Copper ferrites: A model for investigating the role of copper in the dynamic iron-based Fischer–Tropsch catalyst

Copper is a well-known reduction promoter for Fe-based Fischer–Tropsch catalysts. The role of copper is investigated using spinel copper ferrite, CuFe2O4, and delafossite, CuFeO2, as model compounds and compared with hematite. The incorporation of copper within the crystal structure results in a facile, initial reductive decomposition of the structure into magnetite and metallic copper during the catalyst pre-treatment in either hydrogen or carbon monoxide. Copper facilitates the consecutive conversion of magnetite to α-Fe in hydrogen, but not its conversion to predominantly χ-Fe5C2 in CO. The amount of carbide in the catalyst under Fischer–Tropsch conditions is dependent on the presence of copper in close proximity to the iron phases. The observed increase in catalytic activity in the catalysts ex copper ferrites is primarily due to the increased carbide surface area within the catalyst. The copper-containing model compounds show a higher CO2-selectivity, which is ascribed to the presence of a larger amount of small magnetite crystallites present during the Fischer–Tropsch synthesis. Furthermore, copper seems to facilitate secondary olefin hydrogenation.

Comparing silver and copper as promoters in Fe-based Fischer–Tropsch catalysts using delafossite as a model compound

The role of the group 11 metals copper and silver is investigated using delafossite as a model compound (and maghemite and hematite for comparison). The presence of group 11 metals in the crystal structure facilitates the reduction of trivalent iron into magnetite during the catalyst pretreatment in either hydrogen or carbon monoxide as evidenced using an in situ XRD study. Furthermore, the consecutive conversion of magnetite to α-Fe under H2-pretreatment is facilitated, implying the ability of these metals to spillover hydrogen to magnetite. The conversion of magnetite to predominantly χ-Fe5C2 under CO-treatment is not facilitated by the presence of the promoter element. The model catalysts were tested for their activity and selectivity in the Fischer–Tropsch synthesis at 250°C, 20bar, H2/CO=2.1. The samples ex AgFeO2 were the most active with a good selectivity toward the formation of liquid products, but at the expense of the CO2-selectivity in the Fischer–Tropsch synthesis. Mössbauer adsorption spectroscopy showed that the samples containing group 11 elements contained large amount of magnetite in small crystalline domains (superparamagnetic iron in the analysis at room temperature). The CO2-selectivity obtained over the samples containing the group 11 elements can be correlated with the amount of superparamagnetic iron in the spent catalyst samples.

Industrial Feasiblity of Direct Methane Conversion to Hydrocarbons over Fe-Based Fischer Tropsch Catalyst

Recently, as a direct consequence of the dwindling world oil reserves and the growing awareness of the environmental problems associated with the use of coal as energy source, there is growing interest in cheaper, abundant and cleaner burning methane. The Gas-to-Liquid technology offers perhaps the most attractive routes for the exploitation of the world huge and growing natural gas resources. Using this process the erstwhile stranded gas is converted to premium grade liquid fuels and chemicals that are easily transported. However, a widespread application of the GTL process is being hampered by economical and technical challenges. The high cost of synthesis gas, for instance, weighs heavily on the economics and competitiveness of the process limiting its wider application. This work presented a modified Gas-to-Liquid process that eliminates the costly synthesis gas production step. The proposed process utilized an alternative pathway for methane activation via the production of chloromethane derivatives which are then converted to hydrocarbons. It established that hydrocarbons mainly olefins can be economically produced from di- and tri-chloro- methanes over a typical iron-based Fischer Tropsch catalysts in a moving bed reactor at industrially relevant conditions. Some of the attractions of the proposed process include a) the elimination of the costly air separation plant requirement b) high process selectivity and c) significant reduction of carbon dioxide emissions thereby saving on feedstock loss and the costly CO2 removal and isolation processes.