Recent technological developments in Fischer-Tropsch catalysis

Recent advances in Co2C-based nanocatalysts for direct production of olefins from syngas conversion

Syngas conversion provides an important platform for efficient utilization of various carbon-containing resources such as coal, natural gas, biomass, solid waste and even CO₂. Various value-added fuels and chemicals including paraffins, olefins and alcohols can be directly obtained from syngas conversion via the Fischer-Tropsch Synthesis (FTS) route. However, the product selectivity control still remains a grand challenge for FTS due to the limitation of Anderson-Schulz-Flory (ASF) distribution. Our previous works showed that, under moderate reaction conditions, Co₂C nanoprisms with exposed (101) and (020) facets can directly convert syngas to olefins with low methane and high olefin selectivity, breaking the limitation of ASF. The application of Co₂C-based nanocatalysts unlocks the potential of the Fischer-Tropsch process for producing olefins. In this feature article, we summarized the recent advances in developing highly efficient Co₂C-based nanocatalysts and reaction pathways for direct syngas conversion to olefins via the Fischer-Tropsch to olefin (FTO) reaction. We mainly focused on the following aspects: the formation mechanism of Co₂C, nanoeffects of Co₂C-based FTO catalysts, morphology control of Co₂C nanostructures, and the effects of promoters, supports and reactors on the catalytic performance. From the viewpoint of carbon utilization efficiency, we presented the recent efforts in decreasing the CO₂ selectivity for FTO reactions. In addition, the attempt to expand the target products to aromatics by coupling Co₂C-based FTO catalysts and H-ZSM-5 zeolites was also made. In the end, future prospects for Co₂C-based nanocatalysts for selective syngas conversion were proposed.

Selective Conversion of Syngas to Olefins via Novel Cu-Promoted Fe/RGO and Fe-Mn/RGO Fischer-Tropsch Catalysts: Fixed-Bed Reactor vs Slurry-Bed Reactor

Fischer-Tropsch has become an indispensable choice in the gas-to-liquid conversion reactions to produce a wide range of petrochemicals using recently emerging biomass or other types of feedstock such as coal or natural gas. Herein we report the incorporation of novel Cu nanoparticles with two Fischer-Tropsch synthesis (FTS) catalytic systems, Fe/reduced graphene oxide (rGO) and Fe-Mn/rGO, to evaluate their FTS performance and olefin productivity in two types of reactors: slurry-bed reactor (SBR) and fixed-bed reactor (FBR). Four catalysts were compared and investigated, namely Fe, FeCu7, FeMn10Cu7, and FeMn16, which were highly dispersed over reduced graphene oxide nanosheets. The catalysts were first characterized by transmission electron microscopy (TEM), nitrogen physisorption, X-ray fluorescence (XRF), X-ray diffraction (XRD), and H-TPR techniques. In the SBR, Cu enhanced olefinity only when used alone in FeCu7 without Mn promotion. When used with Mn, the olefin yield was not changed, but light olefins decreased slightly at the expense of heavier olefins. In the FBR system, Cu as a reduction promoter improved the catalyst activity. It increased the olefin yield mainly due to increased activity, even if the CO2 decreased by the action of Cu promoters. The olefinity of the product was improved by Cu promotion but it did not exceed the landmark made by FeMn16 at 320 °C. The paraffinity was also enhanced by Cu promotion especially in the presence of Mn, indicating a strong synergistic effect. Cu was found to be better than Mn in enhancing the paraffin yield, while Mn is a better olefin yield enhancer. Finally, Cu promotion was found to enhance the selectivity towards light olefins C2-4. This study gives a deep insight into the effect of different highly dispersed FTS catalyst systems on the olefin hydrocarbon productivity and selectivity in two major types of FTS reactors.

Thermodynamic Equilibrium Analysis of Product Distribution in the Fischer-Tropsch Process Under Different Operating Conditions

Thermodynamic equilibrium analysis is necessary to provide a fundamental understanding of the distribution of the products formed in the Fischer-Tropsch process. The thermodynamic equilibrium distribution of the products formed at constant temperature and pressure was studied based on the minimization of the total Gibbs free energy of the system. The effects of temperature, pressure, and feed ratio on the product distribution were investigated under typical operating conditions. The distribution of the total products obtained from the reactions of added ethylene or ethanol was also studied. The results indicated that the products formed in a state of thermodynamic equilibrium follow Anderson-Schulz-Flory's general polymerization distribution at carbon numbers greater than about three. Both olefins and paraffins are primary products and there are essentially no alcohol and water at high degrees of conversion when the conditions for thermodynamic equilibrium are satisfied. The olefins formed in the Fischer-Tropsch process consist essentially of propylene. The product distribution is very sensitive to feed composition, and to temperature and pressure to a lesser extent. The product spectrum can be described broadly by the probability of chain growth relative to chain termination. This parameter decreases with increasing temperature, the feed ratio of hydrogen to carbon monoxide, and after the addition of ethanol to the feed, but increases with increasing pressure and after the addition of ethylene to the feed. An increase in reaction temperature results in a shift in selectivity towards low carbon number hydrocarbons and more hydrogenated products.

Cobalt-Iron-Manganese Catalysts for the Conversion of End-of-Life-Tire-Derived Syngas into Light Terminal Olefins

Co‐Fe‐Mn/γ‐Al₂O₃ Fischer–Tropsch synthesis (FTS) catalysts were synthesized, characterized and tested for CO hydrogenation, mimicking end‐of‐life‐tire (ELT)‐derived syngas. It was found that an increase of C₂‐C₄ olefin selectivities to 49 % could be reached for 5 wt % Co, 5 wt % Fe, 2.5 wt % Mn/γ‐Al₂O₃ with Na at ambient pressure. Furthermore, by using a 5 wt % Co, 5 wt % Fe, 2.5 wt % Mn, 1.2 wt % Na, 0.03 wt % S/γ‐Al₂O₃ catalyst the selectivity towards the fractions of C₅₊ and CH₄ could be reduced, whereas the selectivity towards the fraction of C₄ olefins could be improved to 12.6 % at 10 bar. Moreover, the Na/S ratio influences the ratio of terminal to internal olefins observed as products, that is, a high Na loading prevents the isomerization of primary olefins, which is unwanted if 1,3‐butadiene is the target product. Thus, by fine‐tuning the addition of promoter elements the volume of waste streams that need to be recycled, treated or upgraded during ELT syngas processing could be reduced. The most promising catalyst (5 wt % Co, 5 wt % Fe, 2.5 wt % Mn, 1.2 wt % Na, 0.03 wt % S/γ‐Al₂O₃) has been investigated using operando transmission X‐ray microscopy (TXM) and X‐ray diffraction (XRD). It was found that a cobalt‐iron alloy was formed, whereas manganese remained in its oxidic phase.

Mn–Fe nanoparticles on a reduced graphene oxide catalyst for enhanced olefin production from syngas in a slurry reactor

Fe nanoparticles (NPs) supported on reduced graphene oxide (rGO) nano-sheets were promoted with Mn and used for the production of light olefins in Fischer–Tropsch reactions carried out in a slurry bed reactor (SBR). The prepared catalysts were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), transmission electron microscope (TEM), Raman spectroscopy, N₂ physisorption, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopic (XPS) methods. Mn was shown to preferentially migrate to the Fe NP surface, forming a Mn-rich shell encapsulating a core rich in Fe. The Mn shell regulated the diffusion of molecules to and from the catalyst core, and preserved the metallic Fe phase by lowering magnetite formation and carburization, so decreasing water gas shift reaction (WGSR) activity and CO conversion, respectively. Furthermore, the Mn shell reduced H₂ adsorption and increased CO dissociative adsorption which enhanced olefin selectivity by limiting hydrogenation reactions. Modification of the Mn shell thickness regulated the catalytic activity and olefin selectivity. Simultaneously the weak metal–support interaction further increased the migration ability owing to the utilization of a graphene-based support. Space velocities, pressures and operating temperatures were also tested in the reactor to further enhance light olefin production. A balanced Mn shell thickness produced with a Mn concentration of 16 mol Mn/100 mol Fe was found to give a good olefin yield of 19% with an olefin/paraffin (O/P) ratio of 0.77. Higher Mn concentrations shielded the active sites and reduced the conversion dramatically, causing a fall in olefin production. The optimum operating conditions were found to be 300 °C, 2 MPa and 4.2 L g⁻¹ h⁻¹ of 1 : 1 H₂ : CO syngas flow; these gave the olefin yield of 19%.

Mn acted as a promoter by forming a Mn-rich layer around a core rich in Fe. The outer layer hindered the formation of magnetite, and impeded H₂ adsorption whilst encouraging CO dissociative adsorption, which gave the perfect conditions for olefin production.

New size-dependent kinetic equations for hydrocarbon production rates from Fischer–Tropsch synthesis on an iron-based catalyst

The effect of catalyst nanoparticle size on hydrocarbon production rates in the Fischer–Tropsch synthesis (FTS) has been investigated on an iron-based catalyst. A series of iron oxide particles was prepared via precipitation by a microemulsion method. New size-dependent kinetic models for hydrocarbon production rates were developed using a thermodynamic analysis method. Size-dependent parameters of the models were evaluated using the experimental results with a non-linear optimisation routine by minimising the mean absolute relative residual. Because of the role of iron carbide in the FTS reaction by iron catalysts, the iron carbide particle sizes was considered as an important factor in this paper. Experimental results show that the hydrocarbon product distribution shows a slight shift to lower molecular weight hydrocarbons by decreasing catalyst particle sizes and increasing the reaction temperature. The value of the surface tension energy (σ) for paraffin and olefin production on the iron catalyst is calculated in the range 1.2–0.9 J m–2 and 0.62–0.49 J m–2 respectively. These values are lower than for metals and are related to the presence of iron carbide in the catalytic reaction. Also, σ for paraffin production is higher than that for olefin production. The size- and carbon number-independent activation energies for paraffin and olefin formation are 9.1 and 14.9 kJ mol−1, respectively.

Recent developments in the application of nanomaterials to understanding molecular level processes in cobalt catalysed Fischer–Tropsch synthesis

This perspective offers an overview of using nanomaterials for improving our understanding of the underlying mechanism of cobalt catalysed Fischer–Tropsch chemistry. This is considered in terms of enabling the rational development of improved (more selective, efficient, longer lived) catalysts.