Nanostructured Catalyst for Fischer–Tropsch Synthesis

Fischer–Tropsch synthesis (FTS) is a heterogeneous catalytic process for the production of fuels or chemicals from synthesis gas (CO + H₂), which can be derived from nonpetroleum feedstocks such as natural gas, coal, or biomass. Co, Ru, Fe and Ni are all active in FTS, but only cheaper Fe and Co based catalysts are used in industry because the price of Ru is relatively high. However, the industrial Fe‐ and Co‐ based FTS catalysts normally work at a relatively high temperature range of 493—623 K in order to get a reasonable space time yield. Moreover, the product selectivity of FTS is governed by the law of polymerization, i.e., a so‐called Anderson‐Schulz‐Flory distribution holds, which restricts its industrial application. In this account, we highlight some of our progress toward the design/fabrication of nanostructured Fe, Co and Ru catalysts to improve FTS activity at the low temperature and to change the product selectivity and confine the product distribution into a certain range.     

The Optimally Performing Fischer–Tropsch Catalyst

Microkinetics simulations are presented based on DFT‐determined elementary reaction steps of the Fischer–Tropsch (FT) reaction. The formation of long‐chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal–carbon and metal–oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain‐growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain‐growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal.     

hcp‐Co Nanowires Grown on Metallic Foams as Catalysts for Fischer–Tropsch Synthesis

The Fischer–Tropsch synthesis (FTS) is a structure‐sensitive exothermic reaction that enables catalytic transformation of syngas to high quality liquid fuels. Now, monolithic cobalt‐based heterogeneous catalysts were elaborated through a wet chemistry approach that allows control over nanocrystal shape and crystallographic phase, while at the same time enables heat management. Copper and nickel foams have been employed as supports for the epitaxial growth of hcp‐Co nanowires directly from a solution containing a coordination compound of cobalt and stabilizing ligands. The Co/Cu_foam catalyst was tested for Fischer–Tropsch synthesis in a fixed‐bed reactor, showing stability and significantly superior activity and selectivity towards C5+ compared to a Co/SiO₂‐Al₂O₃ reference catalyst under the same conditions.     

Gas‐Phase Thermodynamics as a Validation of Computational Catalysis on Surfaces: A Case Study of Fischer–Tropsch Synthesis

Density functional theory has become a valuable tool to study surface catalysis. However, due to the scarcity of clean and reliable experimental data on surfaces, the theoretical methods employed to explore heterogeneous catalytic mechanisms are usually less well validated than those for gas‐phase reactions. We argue herein that gas‐phase reactions and the corresponding surface reactions are related through the Born–Haber cycle and computational catalysis on surfaces will be less meaningful if gas‐phase behavior cannot first be suitably determined. In this contribution, we have constructed a set of gas‐phase reactions relevant to the Fischer–Tropsch synthesis as a case study. With this set, we have tested the validity of the widely used PBE and B3LYP functionals and found that neither of them are capable of describing all kinds of gas‐phase reactions properly, such that some surface reactions may be biased falsely against the others. Significantly, XYG3, which is a double‐hybrid functional that includes Hartree–Fock‐like exchange and many‐body perturbation correlation effects, presents a significant improvement for all of the gas‐phase reactions, holding promise for further development for surface catalysis.     

Selective Transformation of Syngas into Gasoline‐Range Hydrocarbons over Mesoporous H‐ZSM‐5‐Supported Cobalt Nanoparticles

Bifunctional Fischer–Tropsch (FT) catalysts that couple uniform‐sized Co nanoparticles for CO hydrogenation and mesoporous zeolites for hydrocracking/isomerization reactions were found to be promising for the direct production of gasoline‐range (C_5–11) hydrocarbons from syngas. The Brønsted acidity results in hydrocracking/isomerization of the heavier hydrocarbons formed on Co nanoparticles, while the mesoporosity contributes to suppressing the formation of lighter (C_1–4) hydrocarbons. The selectivity for C_5–11 hydrocarbons could reach about 70 % with a ratio of isoparaffins to n‐paraffins of approximately 2.3 over this catalyst, and the former is markedly higher than the maximum value (ca. 45 %) expected from the Anderson–Schulz–Flory distribution. By using n‐hexadecane as a model compound, it was clarified that both the acidity and mesoporosity play key roles in controlling the hydrocracking reactions and thus contribute to the improved product selectivity in FT synthesis.     

Effects of titanium silicalite and TiO2 nanocomposites on supported Co‐based catalysts for Fischer–Tropsch synthesis

Titanium silicalite (TS) and TiO₂ nanocomposites were prepared by mixing TS and TiO₂ with different ratios in ethanol. They were impregnated with 15 wt% Co loading to afford Co‐based catalysts. Fischer–Tropsch synthesis (FTS) performance of these TS–TiO₂ nanocomposite‐supported Co‐based catalysts was studied in a fixed‐bed tubular reactor. The results reveal that the Co/TS–TiO₂ catalysts have better catalytic performance than Co/TS or Co/TiO₂ each with a single support, showing the synergistic effect of the binary TS–TiO₂ support. Among the TS–TiO₂ nanocomposite‐supported Co‐based catalysts, Co/TS–TiO₂‐1 presents the highest activity. These catalysts were characterized using N₂ adsorption–desorption measurements, X‐ray diffraction, X‐ray photoelectron spectroscopy, H₂ temperature‐programmed reduction, H₂ temperature‐programmed desorption and transmission electron microscopy. It was found that the position of the active component has a significant effect on the catalytic activity. In the TS–TiO₂ nanocomposites, cobalt oxides located at the new pores developed between TS and TiO₂ can exhibit better catalytic activity. Also, a positive relationship is observed between Co dispersion and FTS catalytic performance for all catalysts. The catalytic activity is improved on increasing the dispersion of Co.     

Revisiting alternative pathways in the Fischer–Tropsch process: Accurate density functional theory calculations on “magic” Ru12 clusters

Models of the Fischer–Tropsch reaction typically focus on two proposed mechanisms for the initial carbon monoxide dissociation: unassisted dissociation (carbide mechanism), and hydrogen‐assisted dissociation via an adsorbed oxymethylidene (HCO*) intermediate. Much evidence for hydrogen‐assisted dissociation comes from density functional theory calculations modeling ruthenium nanoparticle catalysts as infinite, periodic metal slabs. However, the generalized gradient approximations (GGAs) used in these calculations can make significant errors in reaction barrier heights. How these errors affect the predicted selectivity to unassisted vs. hydrogen‐assisted dissociation is not well understood. We address the problem by considering a different regime, applying GGA and beyond‐GGA approximations to CO dissociation on a “magic” nonmagnetic Ru₁₂ cluster modeling supported nanoparticle catalysts. Both approximations concur that hydrogen‐assisted dissociation is facile on this cluster, providing additional support for its potential role in real catalysts.     

Ab‐Initio calculations of the CO adsorption and dissociation on substitutional Fe–Cu surface alloys relevant to Fischer–Tropsch Synthesis: bcc‐(Cu)Fe(100) and fcc‐(Fe)Cu(100)

Direct CO dissociation is seen the main path of the first step in the Fischer–Tropsch Synthesis (FTS) on the reactive iron surfaces. Cu/Fe alloy film is addressed with various applications over face‐centered‐cubic (fcc)‐Cu and body‐centered‐cubic (bcc)‐Fe in the FTS, i.e. preventing iron carbide formation (through direct CO dissociation) by moderating the surface reactivity and facilitating the reduction of iron surfaces, respectively. In this study by density functional theory, the stable configurations of CO molecule on various Cu/Fe alloys over fcc‐Cu(100) and bcc‐Fe(100) surfaces with different CO coverage (25% and 50%) have been evaluated. Our results showed that the ensemble effect plays a fundamental role to CO adsorption energy on the surface alloys over bcc‐Fe(100); on the other hand, the ligand effect determines the CO stability on the fcc‐Cu(100) surface alloys. CO dissociation barrier was also calculated on the surface alloys that showed although the CO dissociation process is thermodynamically possible on the more reactive surface alloys, but according to their high barrier, CO dissociation does not occur directly on these surfaces. Copyright © 2013 John Wiley & Sons, Ltd.     

One Hundred Years of the Max‐Planck‐Institut für Kohlenforschung

This Essay is an account of the institutional and scientific development of the Max‐Planck‐Institut für Kohlenforschung in Mülheim an der Ruhr (Germany), which is the successor to the Kaiser‐Wilhelm‐Institut für Kohlenforschung founded in 1914. The Essay is divided into four main parts, corresponding to the four major periods which are closely associated with the respective Directors of the Institute from 1914 to 2014: 1) Franz Fischer; 2) Karl Ziegler; 3) Günther Wilke; and 4) the period beginning with Manfred T. Reetz, who established a directorate comprising five Directors of equal status, each heading a different research department under the banner of catalysis. Along with key historical events associated with the Institute, research highlights of the four periods are featured.     

Impact of Hydrogenolysis on the Selectivity of the Fischer–Tropsch Synthesis: Diesel Fuel Production over Mesoporous Zeolite‐Y‐Supported Cobalt Nanoparticles

Selectivity control is a challenging goal in Fischer–Tropsch (FT) synthesis. Hydrogenolysis is known to occur during FT synthesis, but its impact on product selectivity has been overlooked. Demonstrated herein is that effective control of hydrogenolysis by using mesoporous zeolite Y‐supported cobalt nanoparticles can enhance the diesel fuel selectivity while keeping methane selectivity low. The sizes of the cobalt particles and mesopores are key factors which determine the selectivity both in FT synthesis and in hydrogenolysis of n‐hexadecane, a model compound of heavier hydrocarbons. The diesel fuel selectivity in FT synthesis can reach 60 % with a CH₄ selectivity of 5 % over a Na‐type mesoporous Y‐supported cobalt catalyst with medium mean sizes of 8.4 nm (Co particles) and 15 nm (mesopores). These findings offer a new strategy to tune the product selectivity and possible interpretations of the effect of cobalt particle size and the effect of support pore size in FT synthesis.     

Study of an iron-based Fischer–Tropsch synthesis catalyst incorporated with SiO2

The effect of adding SiO₂ to a precipitated iron-based Fischer–Tropsch synthesis (FTS) catalyst was investigated using N₂ physical adsorption, H₂ differential thermogravimetric analysis, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer spectroscopy. The FTS performances of the catalysts with or without SiO₂ were compared in a fixed bed reactor. The characterization results indicated that SiO₂ facilitates the high dispersion of Fe₂O₃ and significantly influences the Fe/Cu and Fe/K contacts, which play an important role in the surface basicity, reduction and carburization behaviors, as well as the FTS performances. The incorporation of SiO₂ enhances the Fe/Cu contact, further enlarges the H₂ adsorption and promotes the reduction of Fe₂O₃ → FeO_x, while the transformation of FeO_x → Fe is suppressed probably due to the strong Fe–SiO₂ interaction. SiO₂ indirectly weakens the surface basicity and severely suppresses the carburization and CO adsorption of the catalyst. In the FTS reaction, it was found that SiO₂ decreases the FTS initial activity but improves the catalyst stability. Due to the lower surface basicity than the catalyst without SiO₂, the catalyst incorporated with SiO₂ has higher selectivity to light hydrocarbons and methane and decreased selectivity to the olefins and heavy hydrocarbons.