Selectivity In Fischer-Tropsch Synthesis Research Articles

Tuning selectivity in low-temperature Fischer-Tropsch synthesis by applying gas recycle mode

While hydrocarbons production by Fischer-Tropsch synthesis (FTS) is widely recognized, commercial interest in FTS products is mainly constrained by oil prices. The FTS-derived fuels, although higher quality and greener, cannot currently compete with cheaper fossil fuels, motivating to shift the FTS products distribution to higher value-added chemicals, such as olefins-C5+. The high olefin selectivity in FTS over Co-based catalysts is usually ascribed to Co2C on the catalyst surface with its inherent deficiencies. This study demonstrates a novel approach to alter low-temperature FTS selectivity by applying off-gas recycling. At slow gas recycling rates, C19+ selectivity increases due to re-adsorption and re-incorporation into the growing chain of gaseous olefins from the off-gas. High-intensity gas recycling facilitates C5+ olefin vapors removal from the reactor by enhanced gas flow with their subsequent cut-off in the cold trap, thus reducing residence time of condensed olefins and probability of their secondary reactions.

A novel machine learning framework for designing high-performance catalysts for production of clean liquid fuels through Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (FTS) attracts great interest as a sustainable route for production of sustainable transportation fuels. Catalyst design and operational conditions tune the selectivity of FTS to liquid fuels, which can be predicted with machine learning models (ML). Herein, a comprehensive dataset including 27 key input features related to the catalyst structure (with the focus on carbon supports), preparation, activation, and FTS operating conditions were investigated to predict the CO conversion and C5+ selectivity using three ML algorithms. Feature engineering and selection were implemented to identify the significant catalyst formulation descriptors. Principal component analysis was used to explore the information space and decrease the dimension of the dataset before ML prediction. In addition to the well-known effects of the operational conditions on FTS, roles of physico-chemical properties of the carbon materials were also extensively analyzed. Random forest (RF) including 4 principal components, indicated the highest accuracy for prediction of the CO conversion and C5+ selectivity with R2 of 0.91 and 0.97, respectively. The proposed framework can provide a reliable strategy in designing efficient carbon supported catalysts for FTS and guide experiments by identifying the key descriptors in the catalyst formulation and operating conditions to enhance the selectivity of liquid fuels.

Enhancement of higher alcohols selectivity in Fischer-Tropsch synthesis using Walnut and Almond shell derived biochars as Copper/Cobalt catalyst support

The production of higher alcohols from synthesis gas using biochar-supported copper-cobalt catalysts is investigated. Biochars were prepared via pyrolysis of Walnut shells and Almond shells and were subsequently activated through physical and chemical methods. A 15 wt% Copper and 5 wt% Cobalt was loaded on the activated biochars via the impregnation technique. The chemical and physical properties of the catalysts were studied using various methods. The catalytic performance of the catalysts was assessed in a fixed-bed microreactor at 20 bars, 320 °C, and, an H2/CO ratio of 2, and the results were compared with the γ-Alumina supported catalyst. Strong interaction between cobalt and copper particles and the formation of bimetallic CuCo2O4 species were observed. The catalysts' reducibility was enhanced by a factor of 1.42. The catalytic studies revealed a significant enhancement in the rate of carbon monoxide hydrogenation. The catalyst supported on low-cost Almond shell-derived biochar demonstrated the best performance. The total alcohol selectivity increased from 22.2 wt% to 34.2 wt%, and the C2+ alcohol selectivity rose from 78.3 wt% to 91.1 wt%, as compared to the industrial catalyst supported on alumina. However, the selectivity for C5+ liquid hydrocarbons decreased to 16.8 wt%. Notably, biochar-supported catalysts also exhibited good stability.

Surface modification of Fe5C2 by binding silica-based ligand: A theoretical explanation of enhanced C2 oxygenate selectivity

Elucidating the role(s) of support-like ligands remains a challenge in catalytic reaction like the Fischer-Tropsch Synthesis (FTS) catalyzed by the iron catalyst supported on silica. We herein theoretically investigated surface modification of Fe5C2 by silica-based ligand and its influence on C2 oxygenate selectivity in FTS, by carrying out DFT calculations on dissociation of CO and formations of CH4 and C2 on ha-SiO2/Fe5C2(510). To mimic the structure of surface modification, the ha-SiO2/Fe5C2(510) model was built up by binding the silica cluster (the ha-SiO2 ligand) to Fe5C2(510). DFT calculations elucidated that the C + CH coupling with the lowest activation barrier among all the possible routes of C2 formation on Fe5C2(510) is suppressed after modification with the ha-SiO2 ligand because the binding ha-SiO2 ligand limits the geometry relaxation caused by the C+CH coupling. However, CO molecule is anchored by the ha-SiO2 ligand via hydrogen bond, suppressing the CO cleavage because the d-valence band center of Fe5C2(510) lowers in energy by surface modification with the ha-SiO2 ligand, but facilitating the C + CO coupling with the lowest activation barrier among all the possible routes of C2 formation. Also, CH4 formation in the ha-SiO2/Fe5C2(510) case is not so easy as that on Fe5C2(510). Therefore, C2 oxygenate is formed more easily in the ha-SiO2/Fe5C2(510) case than in the Fe5C2(510) case. The result agrees with the experimental observation that C2 oxygenate selectivity became high for iron-based FTS catalyst after surface modification of by silica-based ligand.

Tailoring Carbon Deposits on a Single-Crystal Cobalt Catalyst for Fischer–Tropsch Synthesis without Further Reduction: The Role of Surface Carbon and Penetrating Carbon

The preparation of Fischer–Tropsch synthesis (FTS) catalysts that exhibit excellent catalytic performance and are suitable for direct use in a fixed-bed reactor without further reduction is crucial in industrial applications. However, only marginal progress has been made with respect to the preparation of cobalt-based FTS catalysts owing to tendency of metallic cobalt to oxidize easily; this tendency necessitates the activation of these catalysts via reduction despite having undergone reduction treatment during their preparation. Herein, this problem has been addressed by tuning the carbon deposits (surface C and penetrating C) on the surface of a single-crystal cobalt catalyst. Screen-like surface C on a catalyst pretreated with 5% CO (p-Co–CO) exhibits diffusion suppression and chemical inertness to oxidizing gas, thus preventing the oxidation of metallic cobalt and resulting simultaneously in high activity and low CH4 selectivity (7.2%) without requiring further reduction. Moreover, the exact role of surface C and penetrating C deposited on cobalt catalysts in the FTS performance is explored. Both surface C and penetrating C enhance the activity of the cobalt catalyst but with opposite effects on the FTS selectivity. Surface C improves the adsorption ability of bridged-type CO and the formation of long-chain hydrocarbons, whereas penetrating C is conducive to adsorbing linear CO and increases undesired CH4 selectivity. This study clarifies the effect of deposited carbon on the FTS reaction and provides insights into the design of high-performance nonconventional FTS catalysts that do not require further reduction.

Cobalt Supported and Polyfunctional Hybrid Catalysts for Selective Fischer–Tropsch Synthesis: A Review

Cobalt Supported and Polyfunctional Hybrid Catalysts for Selective Fischer–Tropsch Synthesis: A Review

Highly selective formation of linear α-olefins over layered and hydrophilic Fe3O4/MAG catalysts in CO hydrogenation

Controlling the product distribution of Fischer–Tropsch synthesis (FTS) reaction has always been a challenge. Fe-based catalysts can produce linear α-olefins with high selectivity in FTS, but it is significantly affected by the secondary reactions of primary olefins. To enhance the olefin selectivity in CO hydrogenation, layered magadiite (MAG) supported Fe3O4 (Fe3O4/MAG) catalysts with hydrophilic property was prepared by solvent (ethanol and water) dispersion methods. The selectivity of C2–C4 olefins can reach 46% (O/P = 6.15) over Fe3O4/MAG (ethanol) catalyst, which is much higher than that of 14% over unsupported Fe3O4 under the same reaction conditions. The particle size of Fe3O4 in Fe3O4/MAG has a great influence on the product distribution. The Fe3O4 with particle size of about 20 nm affords high olefin selectivity to C2–C4 (52%) with super high O/P value of 15 (olefin selectivity of 93.8% in C2–C3 fraction) and low CH4 selectivity of 7%, while the one with particle size about 400 nm affords extremely high linear α-olefin selectivity of 77.9% in C2+ hydrocarbons. In situ DRIFTS and C2H4/H2-TPSR/MS showed that the layered structure and hydrophilic groups can reduce the possibility of secondary hydrogenation reactions. Also, the complex formed by ethanol and surface hydroxyls induces the chain propagation, and promotes more linear α-olefin formation.

Advances in Selectivity Control for Fischer–Tropsch Synthesis to Fuels and Chemicals with High Carbon Efficiency

Fischer–Tropsch synthesis (FTS) is a versatile technology to produce high-quality fuels and key building-block chemicals from syngas derived from nonpetroleum carbon resources such as coal, natural gas, shale gas, biomass, solid waste, and even CO2. However, the product selectivity of FTS is always limited by the Anderson–Schulz–Flory (ASF) distribution, and the key scientific problems including selectivity control, energy saving, and CO2 emission reduction still challenge the current FTS technology. Herein, we review recent significant progress in the field of FTS to obtain specific target products including fuels, olefins, aromatics, and higher alcohols with high selectivity. These achievements are enabled by developing highly efficient catalysts and a controlled reaction pathway based on an integrated process. The structural nature of catalytic active sites and established structure–performance relationships are clarified. Moreover, we specially focus on the carbon utilization efficiency, and the efforts to tune the preferential formation of value-added chemicals and strategies to reduce CO2 selectivity are summarized. The challenges and the perspectives for future FTS technology development with high carbon efficiency are also discussed.

Highly Efficient ZIF-67-Derived PtCo Alloy–CN Interface for Low-Temperature Aqueous-Phase Fischer–Tropsch Synthesis

Designing new materials for selective Fischer–Tropsch synthesis (FTS) is an elegant way to enhance local feedstock utilization like biomass and waste. In this approach, we have designed a thermally and chemically stable bimetallic PtCo/NC hybrid nanocomposite catalyst derived from a zeolitic imidazolate framework (ZIF-67, which contains cobalt as a metal center) through carbonization for low-temperature (413–473 K) aqueous-phase Fischer–Tropsch synthesis (AFTS). The selectivity of the desired range of hydrocarbons is adjusted using a highly dispersed PtCo bimetallic alloy, which facilitates extraordinary reduction of a metal oxide to active species by the synergic effect under the AFTS reaction conditions. The ZIF-derived catalyst tested in this study exhibited the highest activity to date for very low temperatures (433 K) in aqueous-phase Fischer–Tropsch synthesis with CO conversion rates between 0.61 and 1.20 molCO·molCo–1·h–1. Insights of the remarkable catalyst activity were examined by in situ X-ray photoelectron spectroscopy (XPS) studies corroborated by density functional theory (DFT) calculation. The bimetallic Co3Pt (111) surface was found to be highly active for the C–C coupling reaction between surface-adsorbed C and CO, forming a CCO intermediate with a very low activation barrier (Ea = 0.37 eV), in comparison to the C–C coupling activation barrier obtained over the Co (111) surface (Ea = 0.87 eV). This unique approach and observations create a new path for developing next-generation advanced catalyst systems and processes for selective low-temperature FTS.

Mechanistic Insights into the Effect of Sulfur on the Selectivity of Cobalt-Catalyzed Fischer–Tropsch Synthesis: A DFT Study

Sulfur is a common poison for cobalt-catalyzed Fischer–Tropsch Synthesis (FTS). Although its effects on catalytic activity are well documented, its effects on selectivity are controversial. Here, we investigated the effects of sulfur-covered cobalt surfaces on the selectivity of FTS using density functional theory (DFT) calculations. Our results indicated that sulfur on the surface of Co(111) resulted in a significant decrease in the adsorption energies of CO, HCO and acetylene, while the binding of H and CH species were not significantly affected. These findings indicate that sulfur increased the surface H/CO coverage ratio while inhibiting the adsorption of carbon chains. The elementary reactions of H-assisted CO dissociation, carbon and oxygen hydrogenation and CH coupling were also investigated on both clean and sulfur-covered Co(111). The results indicated that sulfur decreased the activation barriers for carbon and oxygen hydrogenation, while increasing the barriers for CO dissociation and CH coupling. Combining the results on elementary reactions with the modification of adsorption energies, we concluded that the intrinsic effect of sulfur on the selectivity of cobalt-catalyzed FTS is to increase the selectivity to methane and saturated short-chain hydrocarbons, while decreasing the selectivity to olefins and long-chain hydrocarbons.

Highly Dispersed CoO Embedded on Graphitized Ordered Mesoporous Carbon as an Effective Catalyst for Selective Fischer-Tropsch Synthesis of C5+ Hydrocarbons.

Herein, we report the high Fischer–Tropsch synthesis performance of the Co-based catalysts supported on graphitized ordered mesoporous carbon (GMC-900) by using a facile strategy. Compared with CMK-3 and active carbon (AC), the obtained GMC-900 by using pollution-free soybean oil as a carbon source exhibited enhanced catalytic performance after loading Co species due to its highly crystallized graphitic structure and uniform dispersion of CoO. As a result, Co/GMC-900 was an effective catalyst with the maximum C5+ selectivity of 52.6%, which much outperformed Co/CMK-3 and Co/AC. This research provides an approach to produce advanced Co-based catalysts with satisfactory performance for efficient Fischer–Tropsch synthesis.

A Modeling Study of Operating Conditions and Different Supports on Fe-Co-Ce Nanocatalyst and Optimizing of Light Olefins Selectivity in the Fischer-Tropsch Synthesis

This study demonstrates the effect of operating conditions (Red-GHSV, inlet H2/CO, Oprat-GHSV) and the effect of Fe-Co-Ce nanocatalyst support. A statistical model using the response surface methodology (RSM) was applied with the target of achieving higher olefins selectivity in Fischer-Tropsch synthesis, which indicates the interaction effects of factors. The conditions under which three objectives optimization for maximizing olefins and minimizing paraffins and methane were determined. Synthesized nanocatalysts with various supports were characterized by XRD, SEM and TPR techniques

Kinetics and Selectivity Study of Fischer–Tropsch Synthesis to C5+ Hydrocarbons: A Review

Fischer–Tropsch synthesis (FTS) is considered as one of the non-oil-based alternatives for liquid fuel production. This gas-to-liquid (GTL) technology converts syngas to a wide range of hydrocarbons using metal (Fe and Co) unsupported and supported catalysts. Effective design of the catalyst plays a significant role in enhancing syngas conversion, selectivity towards C5+ hydrocarbons, and decreasing selectivity towards methane. This work presents a review on catalyst design and the most employed support materials in FTS to synthesize heavier hydrocarbons. Furthermore, in this report, the recent achievements on mechanisms of this reaction will be discussed. Catalyst deactivation is one of the most important challenges during FTS, which will be covered in this work. The selectivity of FTS can be tuned by operational conditions, nature of the catalyst, support, and reactor configuration. The effects of all these parameters will be analyzed within this report. Moreover, zeolites can be employed as a support material of an FTS-based catalyst to direct synthesis of liquid fuels, and the specific character of zeolites will be elaborated further. Furthermore, this paper also includes a review of some of the most employed characterization techniques for Fe- and Co-based FTS catalysts. Kinetic study plays an important role in optimization and simulation of this industrial process. In this review, the recent developed reaction rate models are critically discussed.

Photocatalytic conversion of CO to fuels with water by B-doped graphene/g-C3N4 heterostructure

Photocatalytic reduction of carbon monoxide (CO) is a promising route to the production of high-value chemicals and fuels, as a supplement to high energy-input Fischer-Tropsch synthesis (FTS) and a key step in direct photo/electro-reduction CO2 to multi-carbon products. However, many current research efforts for high-efficiency FTS/CO2 reduction mainly focus on the metal-based catalysts, while metal-free and solar-driven photocatalysts are rarely explored. Here, by means of Lewis acid sites, a metal-free composite photocatalyst for CO reduction, namely boron (B) doped-graphene/g-C3N4 heterostructure, is proposed. First-principles calculations show that the dopants (B) as catalytic sites can effectively capture and activate CO molecules and reduce CO to CH3OH and CH4 in different doping content. It is worth noting that C2 products, i.e., C2H5OH, can be produced with low free energy barriers on para-doped graphene/g-C3N4. Meanwhile, the competitive hydrogen evolution reaction (HER) can be greatly suppressed, leading to the high selectivity of CO reduction. Moreover, the formation of a built-in electric field in heterostructure enhances the separation of photogenerated electrons and holes, which further accelerates the transmission of photogenerated electrons to the catalytic sites and improves the reaction efficiency. Overall, this work not only proposes a new strategy from a new perspective to solve problems of high energy consumption and low selectivity of FTS, but also provides a tandem strategy to solve problems of CO2 to multi-carbon products.

Identifying correlations in Fischer-Tropsch synthesis and CO2 hydrogenation over Fe-based ZSM-5 catalysts

Correlations in Fischer-Tropsch synthesis (FTS) and CO2 hydrogenation are investigated over Fe supported on the acidic (H) and sodium (Na) forms of ZSM-5 (Si/Al = 50). FTS reactor studies indicate the selectivity toward olefins increases from 11% over Fe/H-ZSM-5 to 29.4% over Fe/Na-ZSM-5 because of Na increasing the surface basicity of the catalyst. Reactor studies are extended to CO2 hydrogenation, where reverse water-gas shift is the dominant reaction, with Fe/Na-ZSM-5 displaying enhanced CO2 adsorption, and in turn, higher CO selectivity (∼80%) versus Fe/H-ZSM-5 (∼60%). The catalysts are characterized by a variety of analytical techniques including Mössbauer spectroscopy, Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) of NH3 and CO2 to correlate acid-base properties with catalytic performance. The findings of this study clearly show the selectivity of FTS and CO2 hydrogenation can be attenuated toward the desired products by modifying the acid-base properties of the catalyst with sodium. These results are an important step toward designing high-performance catalysts for light olefin synthesis from CO and CO2.

Fischer–Tropsch synthesis, the effect of promoters, catalyst support, and reaction conditions selection

This work brings the overview of Fischer–Tropsch synthesis (FTS) from catalysis point of view. The role of promoters’ type and their amount loaded into the catalyst are described and discussed for both, iron- (Cu, K, Na, S, Zr, Ni) and cobalt (Ru, Pt, Re, Zr, Ag, Rh, Ir, Au)-based catalysts, respectively, as same as the role of catalyst supports and reaction conditions. Catalyst supports discussed in this work are from the group of oxides, mesoporous silicas and zeolites, carbon-based materials, and carbides. Reaction conditions studied and discussed are reaction temperature, gas hourly space velocity of syngas and hydrogen to carbon monoxide ratio in syngas and reaction pressure. For iron-based catalysts, reaction temperature is discussed in the range of 508–613 K, while for cobalt types in the range of 468–513 K. Reaction pressure is discussed for both catalyst types up to 3 MPa. All the parameters are presented and discussed using FTS selectivity to main groups of products, such as CH4, CO2, C2–C4 hydrocarbons, and C5+ hydrocarbons together with syngas conversion degree. Detailed review of data published in articles studying FTS over iron and cobalt catalysts supplied large data set for presenting and discussion of trends and effects of aimed parameters and for their visualization in form of set of charts. This data analysis brings complex overview of basic trends of catalytic properties of cobalt and iron catalysts.

Controllable synthesis of core-shell Co@C@SiO2 catalysts for enhancing product selectivity in Fischer-Tropsch synthesis by tuning the mass transfer resistance

Fischer-Tropsch synthesis (FTS) is the key step in converting syngas into clean fuels. Traditional supported catalysts for FTS are problematic because the active metal crystalline size is positively related to metal loading. Therefore, increasing active metal loading may reduce the cobalt time yield (CTY) since a high CTY is usually obtained when the Co size is 8 nm. Here, a ZIF-67 (Zeolitic imidazolate framework-67) with a MOF (Metal organic framework) structure is used as a precursor to prepare the [email protected] catalyst with not only high cobalt loading (55.6 wt%) but also with a small cobalt crystal size (as small as 8.6 nm). Core-shell [email protected]@SiO2-X catalysts with different SiO2 shell thicknesses were successfully prepared by coating different amounts of TEOS on the outer surface of [email protected] to modify product selectivity. Compared with 40 wt% Co/SiO2 catalyst, core-shell [email protected]@SiO2-X catalysts exhibited improved FTS performance. Furthermore, different gaseous hourly space velocities (GHSVs) were used to obtain CO conversion at similar levels to compare CTY and the turnover frequency (TOF). Among the catalysts, the [email protected]@SiO2-1 catalyst, with its better mass transfer ability and suitable hydrophilic property, presented the highest TOF (9.75 × 10−3 s−1) and lowest CH4 selectivity (9.75%). In addition, heavy hydrocarbons were effectively suppressed with the increase in shell thickness due to the increased mass transfer resistance.

C–C coupling reactions promoted by CNT-supported bimetallic center in Fischer–Tropsch synthesis

C–C coupling reactions on M1M2/N6h surfaces for Fischer–Tropsch synthesis (FTS).

Selective Fischer–Tropsch synthesis for jet fuel production over Y3+ modified Co/H-β catalysts

Y3+, exchanged with the H protons in zeolites, decreased the acid strength of Co/Y-β-x (x = 1, 2, 3, 4) catalysts, which reduced the selectivity of gaseous hydrocarbons (C1–C4) and promoted the generation of JFRHs.

Selective Fischer-Tropsch synthesis for gasoline production over Y, Ce, or La-modified Co/H-β

Selective Fischer-Tropsch synthesis has been carried out over a series of Co/H-β catalysts to investigate the influence of rare-earth metal ions promotion on the synthesis of gasoline-range hydrocarbons (GRHs, C5–C12 hydrocarbons). H-β zeolite was doped with rare-earth metal ions, specifically Y3+, Ce3+ and La3+, by an ion-exchange method. The resulting catalysts were characterized by different methods, namely ICP-OES, BET, XRD, TEM, STEM-EDS, H2-TPR, NH3-TPD, CO-TPD and TG. It was found that the addition of Y3+ improved the dispersion of active components over the Co/H-β catalyst, while doping with Ce3+, having the ability to change its valence state as well as the number of oxygen vacancies of the support, enhanced the catalyst reducibility. However, the addition of La3+ tended to cause agglomeration of the active particles due to its large ionic radius. Performance tests showed that the Co/Y-β and Co/Ce-β catalysts achieved higher CO conversions. Due to the moderate acidity and the enhanced CO-metal bonding, the Co/Y-β catalyst exhibited optimal GRHs selectivity of 49% with a yield of 41.4% at 260 °C, 2 MPa, H2/CO = 2, and 1064 h−1. More light hydrocarbons were produced after ion-exchange with La3+ because new strongly acidic sites were formed in the zeolite, leading to low GRHs production.