As active phases in low-temperature Fischer–Tropsch synthesis for liquid fuel production, epsilon iron carbides are critically important industrial materials. However, the precise atomic structure of epsilon iron carbides remains unclear, leading to a half-century of debate on the phase assignment of the ε-Fe2C and ε′-Fe2.2C. Here, we resolve this decades-long question by a combined theoretical and experimental investigation to assign the phases unambiguously. First, we have investigated the equilibrium structures and thermal stabilities of ε-FexC (x = 1, 2, 2.2, 3, 4, 6, 8) by first-principles calculations. We have also acquired X-ray diffraction patterns and Mössbauer spectra for these epsilon iron carbides and compared them with the simulated results. These analyses indicate that the unit cell of ε-Fe2C contains only one type of chemical environment for Fe atoms, while ε′-Fe2.2C has six sets of chemically distinct Fe atoms.

The adsorption and activation of CO, H2O, CO2 and H2 on the clean as well as O, OH and H precovered Fe(100) surface at 0.25 ML coverage and Fe(111) surface at 0.33 ML coverage were computed on the basis of density functional theory (GGA-PBE) to investigate the catalytic activity of metallic iron. On the clean Fe(100) and Fe(111) surfaces, the most favorable reactions are H2O and CO dissociation, and the surface intermediates are the coadsorbed C + O + 2H; CO hydrogenation is kinetically favored and thermodynamically disfavored, whereas CO direct oxidation and COOH formation are kinetically much unfavorable. The O- and OH-precovered Fe(100) surfaces at 0.25 ML coverage and Fe(111) surfaces at 0.33 ML coverage do not promote CO2 formation from the CO direct oxidation and COOH formation from CO and OH coupling, whereas the 0.33 ML O-precovered Fe(111) surface suppresses CO dissociation. In contrast to the 0.25 ML H precovered Fe(110) surface, which does not show a significant effect in the surface reactions, the 0.33 ML H precovered Fe(111) surface can lower the barriers for CO oxidation and hydrogenation, whereas the barrier of COOH formation is increased. It was concluded that the metallic iron surfaces, in particular, the O and OH precovered iron surfaces are not appropriate catalysts for the water-gas shift reaction; however, CO2 dissociation is kinetically and thermodynamically much favorable on all these surfaces. For a given reaction on different surfaces, there is no linear Brønsted–Evans–Polanyi (BEP) correlation between the barriers and reaction energies.

Comprehensive spin-polarized density functional theory (DFT) combined with ab initio molecular dynamic (AIMD) simulations have been performed to explore the structures, energies, and diffusion behavior of platinum on Fe5C2 surfaces with importance in Fischer–Tropsch (F–T) catalysis. The results indicate that the morphology of the Pt promoter on Hägg carbide highly depends on the surface structure of the Fe5C2. The Fe5C2(111) surface shows a stronger Pt affinity than the Fe5C2(100) surface, while the Fe5C2(100) surface has stronger Ptn aggregation ability. Under the realistic iron-based F–T reaction conditions, the adsorbed Pt atoms can diffuse easily, and tend to aggregate following the line-band-layer mode on Fe5C2(100). While on the Fe5C2(111) surface, the adsorbed Pt atoms prefer sitting on the most stable sites, and form dispersed structures rather than aggregated structures until the coverage of Pt is above 2/7 (the surface atom ratio of Pt to Fe). In addition, the three-dimensional structures are not favored on both surfaces because of the stronger Fe–Pt interaction over the Pt–Pt bonding. The adsorbed Pt atoms possess negative charges and the stronger electron transfer is observed on the Fe5C2(100) surface, especially at the low coverage.

Three density functional approximations (DFAs), PBE, PBE+U, and Heyd–Scuseria–Ernzerhof screened hybrid functional (HSE), were employed to investigate the geometric, electronic, magnetic, and thermodynamic properties of four iron oxides, namely, α-FeOOH, α-Fe2O3, Fe3O4, and FeO. Comparing our calculated results with available experimental data, we found that HSE (a = 0.15) (containing 15% “screened” Hartree–Fock exchange) can provide reliable values of lattice constants, Fe magnetic moments, band gaps, and formation energies of all four iron oxides, while standard HSE (a = 0.25) seriously overestimates the band gaps and formation energies. For PBE+U, a suitable U value can give quite good results for the electronic properties of each iron oxide, but it is challenging to accurately get other properties of the four iron oxides using the same U value. Subsequently, we calculated the Gibbs free energies of transformation reactions among iron oxides using the HSE (a = 0.15) functional and plotted the equilibrium phase diagrams of the iron oxide system under various conditions, which provide reliable theoretical insight into the phase transformations of iron oxides.

•Particulate heat transfer model used in pyrolysis downer reactor•Two analysis method for mixing degree of binary granular materials•Effect of internals on particulate mixing and heat transfer in downer reactor•Pyrolysis terminal temperature and increasing rate of particle temperature•Contribution of three forms of heat transfer on particle temperature increasingTo investigate the effect of internals in downer reactor on the particulate mixing and heat transfer, a model of heat transfer based on discrete element method (DEM) has been developed and validated by experiments. The agreement between the simulated results and the experiment phenomena is able to prove the reasonability and correctness of the model mechanism of the particulate heat transfer using DEM. The effect of internals in downer reactor on the temperature increasing rate of particles has been predicted by the particulate heat transfer model based on DEM, which is important for the fast pyrolysis process. Two types of internals, the tube group internals and the baffle internals, have been designed to improve the mixing and heat transfer between fuel particles and solid heat carriers in downer reactor. The internals not only increase the particle residence time in downer reactor, but also enhance the mixing degree of binary particulate materials. Most importantly, the internals affect the temperature increasing rate of fuel particles controlled by the mixing degree of fuel particles and heat carriers. The terminal temperature fuel particle at the exit of downer reactor is determined by the average temperature increasing rate and the mean residence time of the fuel particle. As shown in the simulated results, the mixing degree of binary particulate materials in the baffle type downer reactor is remarkably better than that in the tube type downer reactor. At the same time, the temperature increasing rate of fuel particles in the baffle type downer reactor is also larger than that in the tube type downer reactor. It can be concluded that the baffle internals not only restrict the increase of particle residence time, but also mix the fuel particles and solid heat carriers sufficiently, which make the fuel particles heated rapidly. A shorter resistance time, a perfect mixing degree of two types of granular materials and a higher temperature increasing rate can confirm that the internal of baffle group is an excellent choice for the fast pyrolysis process in downer reactor.

Surface O removal by H and CO on Co(0001) has been studied using periodic density functional method (revised Perdew–Burke–Ernzerhof ; RPBE) and ab initio atomistic thermodynamics. On the basis of the quantitative agreement in the H2O formation barrier between experiment (1.34 ± 0.07 eV) and theory (1.32 eV), H2O formation undergoes a consecutive hydrogenation process [O + 2H → OH + H → H2O], while the barrier of H2O formation from OH disproportionation [2OH → H2O + O] is much lower (0.72 eV). The computed desorption temperatures of H2 and H2O under ultrahigh vacuum conditions agree perfectly with the experiment. Surface O removal by CO has a high barrier (1.41 eV) and is strongly endothermic (0.94 eV). Precovered O and OH species do not significantly affect the barriers of H2O and CO2 formation. All of these results indicate that the present RPBE method and the larger surface model are more suitable for studying cobalt systems.

The reactions of CO and H2O on the clean Fe(110) surface as well as surfaces with 0.25 monolayer O, OH, and H precoverage have been computed on the basis of density functional theory (GGA-PBE). Under the considerations of the reductive nature of CO as reactant and H2 as product as well as the oxidative nature of CO2 and H2O, we have studied the potential activity of metallic iron in the water-gas shift reaction. On the clean surface, CO oxidation following the redox mechanism has a similar barrier as CO dissociation; however, CO dissociation is much more favorable thermodynamically. Furthermore, surfaces with 0.25 monolayer O, OH, and H precoverage promote CO hydrogenation, while they suppress CO oxidation and dissociation. On the surfaces with different CO and H2O ratios, CO hydrogenation is promoted. On all of these surfaces, COOH formation is not favorable. Considering the reverse reaction, CO2 dissociation is much favorable kinetically and thermodynamically on all of these surfaces, and CO2 hydrogenation should be favorable. Finally, metallic iron is not an appropriate catalyst for the water-gas shift reaction.

Experimental product distribution of Fischer–Tropsch synthesis frequently presents notable deviations from the typical double-α Anderson–Schulz–Flory pattern: bump or dip around the breaking carbon number, positive or negative deviation for heavy hydrocarbons. These irregularities were studied experimentally in a fixed-bed reactor over an industrial Fe/Mn catalyst, and theoretically by a product separation model based on Aspen Plus software. First, it was found that the unsteady state of reaction condition or improper gas chromatograph procedure could lead to deviation for heavy hydrocarbon distribution. Second, the bump near the breaking carbon number could be attributed to the accumulation of water in hot trap, which leads to an inaccurate measurement of the wax amount. This irregularity can be eliminated by selecting either a higher temperature or a lower pressure of the hot trap. Third, vaporization or flash loss of the oil sample during product collection could result in dip in light hydrocarbon distribution. High syngas conversion levels should be avoided for accurate data acquirement.

The Fischer−Tropsch synthesis kinetic rates were studied by discussing the hydrogenation steps of the favored hydrogen-assisted CO dissociation mechanism and the surface site balance over an industrial iron-based catalyst. Their mathematical expressions are the two proposed models −rFT = (kFTPCOPH2α)/(kCOPCO + ∑i=1ixi) and −rFT = (kFTPCOPH2α)/((kCOPCO + ∑i=1ixi)2) in the literature. Unresolved relationships and differences between the two models are addressed here and analyzed specifically. When the significance of each of the terms in the two rival models is critically evaluated, both surface site balances of the two could be simplified to two involved terms: an adsorbed CO term and a vacant site term. Although the two models could hardly be distinguished from each other at the typical range of experimental conditions, further theoretical analyses and extrapolated experimental data favor the latter [its simplified form is −rFT = (kFTPCOPH20.5)/((1 + kCOPCO)2)]. These studies are helpful to understand the CO activation processes, and some contradictory conclusions and experimental phenomena in the literature can be explained consistently and reasonably.

Recently, 5-chloromethylfurfural (CMF) was proposed as a central intermediate in the conversion of carbohydrate-based material into useful organic commodities. In the present work, we have calculated the thermochemistry using the highly accurate G4 theory and several state-of-art density functional theory (DFT) methods (e.g., X1, M06-2X, B2PLYP-D, and XYG3) for the conversion from CMF to 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) in water, and that to biofuels 5-ethoxymethylfurfural (EMF) and ethyllevulinate (EL) in alcohol. New reaction mechanisms have been proposed, which complement the well-recognized Horvat mechanisms. The assessment of DFT methods suggested that XYG3 be a viable method for biomass related thermochemistry calculations.

In this paper, an empirical regularity has been proposed for dense fluids, namely, that the thermal pressure coefficient is a near-parabola function of pressure. The regularity has been tested with experimental data for both associating and nonassociating compounds. The applicable ranges have also been investigated widely. It is found that the regularity holds well from the freezing temperature to critical temperature, and no obvious limits were found for pressure and compound type. Moreover, parameters of the thermal pressure coefficient expression were regressed from experimental data for n-alkanols, and the statistical results show it is an accurate correlation equation. Further, on the basis of the Lennard-Jones (12−6) potential function, the theoretical analysis was given to confirm the existence and uniqueness of the peak point for Lennard-Jones fluids.

The ground-state potential energy surface of the 1-hexyl system, including the main decomposition and isomerization processes, has been calculated with the MPW1K, BB1K, MPWB1K, MPW1B95, BMK, M05-2X and CBS-QB3 methods. On the basis of these data, thermal rate coefficients of different reaction channels and branching ratios were then calculated using the master equation formulation at 250–2,500 K. The results clearly point out that the 1,5 H atom transfer reaction of 1-hexyl radical with exothermicity proceeds through the lowest reaction barrier, whereas the decomposition processes are thermodynamically unfavorable with large endothermicity. The temperature effect is important on the relative importance of different reactions in the 1-hexyl system. In the low-temperature range of 250–900 K, isomerization reactions, especially 1,5 H atom transfer reaction of 1-hexyl radical, are dominating and responsible for over 82.17% of all the reactions, due to their smaller reaction barriers than those of the decomposition reactions. Furthermore, an equilibrium process involving the isomeric forms of the hexyl radicals appearing at relative low temperature was validated theoretically. However, isomerization and decomposition processes are kinetically competitive and simultaneously important under normal pyrolysis conditions.

Kinetic Monte Carlo simulations were carried out for Fischer–Tropsch synthesis in a wide range of industrially relevant reaction conditions. The macroscopic performance of different reaction conditions obtained from simulations quite agrees with experimental trends. In all examined conditions, H and CO are dominant species on catalyst surface. The rest intermediates only exist in small amount, and are controlled by H and CO coverages. Activity and selectivity for Fischer–Tropsch synthesis and water gas shift reaction are determined by surface H/CO ratio. Reaction conditions can directly change surface coverage of H, CO, and vacant sites, and consequently exert an influence on macroscopic performance of iron catalyst.

This paper designs four cases to investigate the performances of the polygeneration processes, which depend on the commercially ready technology to convert coal to liquid fuels (CTL) and electricity with CO2 sequestration. With Excel-Aspen Plus based models, mass and energy conversion are calculated in detail. The simulation shows that the thermal efficiency is down with the synfuels yield decrease though the electricity generation is increased. It also suggests that the largest low heat value (LHV) loss of coal occurs in the gasification unit. From the comparison of the four cases, prominent differences of coal energy transition appear in water–gas shift (WGS) units, Fischer–Tropsch (FT) synthesis and combined cycle processes. CO2 capture and vent are discussed and the results show that the vent amount of CO2 increases with the increase of percentage of the syngas going to produce electricity. The results also show that the ratio of carbon captured to total carbon increases from 58% to 93% which is an important contribution to cutting down the greenhouse gas vent.

A new group contribution (GC) method has been developed to estimate parameters of the simplified perturbed-chain statistical associating fluid theory (sPC-SAFT) for hydrocarbon. A key advantage of this method is that the binary interaction parameter between groups has been adopted in estimating the molecular parameter of pure component. Besides, parameters of all single groups are set to be the same for simplifying the model. Using genetic algorithm, twelve group interaction parameters are estimated on the basis of 71 sets of pure components of sPC-SAFT parameters. Tests and comparisons with other methods were performed in calculating the PVT and phase equilibrium data of heavy and branched hydrocarbon systems. The results show that the present GC method is better than the other predictive approaches in calculating parameters of sPC-SAFT.

The effect of co-feeding CO2 on the catalytic properties of an Fe–Mn catalyst during Fischer–Tropsch synthesis (FTS) was investigated in a spinning basket reactor by varying added CO2 partial pressure in the feed gas. It was found that co-feeding CO2 to syngas did not decrease the activity of the catalyst, on the contrary, a dramatic increase of the activity and an increase of methane selectivity were observed over the catalyst after removal of CO2 from the feed gas. The addition of CO2 led to an increase in olefin/paraffin ratios of low carbon hydrocarbons and a slight decrease in C19+ selectivity. It also slightly decreased CO2 formation rate on the catalyst by increasing the rate of reverse step of the water–gas shift (WGS) reaction and pushing the reaction towards equilibrium, and did not remarkably influence the hydrocarbon formation rate. However, the co-feeding CO2 can significantly increase the water formation rate and the overall oxygenate formation rate under these reaction conditions.

A study has been carried out to investigate the effects of binder SiO2 content on catalytic behavior of spray-dried precipitated Fe/Cu/K/SiO2 catalysts for Fischer–Tropsch synthesis (FTS). The catalysts were characterized by means of N2 physisorption, H2 temperature-programmed reduction (TPR), scanning electron microscopy (SEM), and Mössbauer effect spectroscopy (MES). The Fischer–Tropsch synthesis performances (activity, selectivity and stability) of the catalysts were studied in a slurry-phase continuously stirred tank reactor (CSTR). The results indicated that the increase of SiO2 content stabilizes Fe3O4 phase and suppresses the further reduction and carburization of the catalysts in syngas. Long time on stream FTS performances showed that the catalyst with SiO2 improves its reaction stability. The selectivities to light hydrocarbons (methane, C2–C4, C5–C11) are enhanced whereas those to heavy hydrocarbons (C12+) are suppressed with increasing SiO2 content. The results were explained to the interactions of Fe–SiO2 and K–SiO2. From the present study, it is found that a catalyst with composition of 100Fe/5Cu/4.2K/25SiO2 on mass basis displays both better FTS performances and a good attrition resistance, which is suitable for the use in CSTRs or SBCRs (Slurry Bubble Column Reactors) for FTS reaction.

A systematic study was undertaken to investigate the effects of Al2O3/SiO2 ratio on reduction, carburization and catalytic behavior of iron-based Fischer–Tropsch synthesis (FTS) catalysts promoted with potassium and copper. The catalysts were characterized by N2 physical adsorption, CO2 temperature-programmed desorption (TPD), H2 temperature-programmed reduction (TPR) and Mössbauer effect spectroscopy (MES). CO2-TPD indicated that Al2O3 binder has stronger acidity than SiO2 binder and weakens the surface basicity of the catalysts. H2-TPR profiles suggested that the lower Al2O3/SiO2 ratio promotes the reduction of Fe2O3→Fe3O4. With further increasing Al2O3/SiO2 ratio, the transformation of Fe2O3→Fe3O4 shifts to higher temperatures. The MES results showed that the increase of Al2O3/SiO2 ratio leads to the relatively large crystallite size of α-Fe2O3 and inhibits carburization of the catalyst. During reaction tests in a fixed bed reactor it was found that a maximum in catalyst activity is noted at the Al2O3/SiO2 ratio of 5/20 (weight basis). The selectivity to olefins shows a rapid decrease and the formations of methane and light hydrocarbons are promoted with increasing Al2O3/SiO2 ratio. The oxygenate selectivity in total products increases with increasing Al2O3/SiO2 ratio.

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 CH2 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.

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, H2/CO feed ratio of 0.66–2.0 and space velocity of 0.66–2.65×10−3 Nm3 kgcat−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.

It was found that air-exposure suppressed the sequent reduction behavior before a Fe–Mn–Cu–K/SiO2 Fischer-Tropsch synthesis (FTS) catalyst was loaded in reactor. Thermogravimetry and mass spectrometry analysis (TG–MS), in situ diffuse reflectance infrared Fourier transform (DRIFT) analysis, CO2 temperature-programmed desorption (TPD), in situ syngas reduction and Mössbauer spectroscopy were used to reveal the intrinsic relationship between air-exposure and reduction behavior of catalysts. The result of in situ reduction indicates that the air-exposure restrains the reduction of Cu-promoted catalyst and suppresses the formation of active sites. Results of TG–MS indicate that water and CO2 are adsorbed on surface of catalysts when catalysts were exposed to air. Copper promotion enhances the selective adsorption of CO2 in air. The species formed upon CO2 adsorption are irreversible surface carbonates, which cannot be removed under the typical or higher temperature used in FTS reaction. The surface carbonates formed in air-exposure restrain the role of Cu in reduction and lead to the low extent of reduction or carburization.Air-exposure suppresses the reduction behavior of Cu-promoted catalyst in Fischer-Tropsch synthesis, whereas, it does not affect that of Cu-free catalyst. The intrinsic reason of this phenomenon was investigated.

A novel process involving the coupling of the hydrogenation of furfural and the dehydrogenation of 1,4-butanediol has been studied in the vapor phase for the synthesis of 2-methylfuran (2-MF) and γ-butyrolactone (γ-BL) over the same Cu-based catalyst. It was found that hydrogen and heat energy are utilized with high efficiency in this process.

The kinetic experiments of Fischer–Tropsch synthesis (FTS) over an industrial Fe–Cu–K catalyst are carried out in a micro-fixed-bed reactor under the conditions as follows: temperature of 493–542 K, pressure of 10.9–30.9 bar, H2/CO feed ratio of 0.98–2.99, and space velocity of 4000–10 000 h−1. The effects of secondary reactions of olefins are investigated by co-feeding C2H4 and C3H6. A detailed kinetics model taking into account the increasingly proven evidence of the olefin re-adsorption mechanism is then proposed. In this model, different sites are assumed for FTS reactions and water gas shift (WGS) reaction, respectively. Rate expressions for FTS reactions are based on the carbide polymerisation mechanism, in which olefin re-adsorption is considered to be a reverse step of olefin desorption reaction. Rate expression for WGS reaction is based on the formate mechanism. An integral reactor model considering both FTS and WGS kinetics is used to describe the reaction system, and the simultaneous estimation of kinetic parameters is conducted with non-linear regression procedure. The optimal model shows that the rate determining steps in FTS reactions proceed via the desorption of hydrocarbon products and the adsorption of CO and the slowest step in WGS reaction is the desorption of gaseous carbon dioxide via formate intermediate species. The activation energies of FTS reactions and WGS reaction are in good agreement with literature values.

Fe/SiO2 catalysts with different Fe/Si molar ratios were used to investigate the effects of silica on chemical/structural properties and Fischer–Tropsch synthesis (FTS) performance of iron-based catalysts. In the chemical aspect, silica interacts with Fe species by the formation of FeOSi structure, which further transforms into an Fe2SiO4 phase during FTS reaction. The interaction largely disturbs the electronic structure of Fe atoms in iron oxide phases and in turn resists the reduction and activation of catalysts. In the structural aspect, silica increases the dispersion of Fe species and inhibits the aggregation of active iron particles. Addition of silica largely changes the adsorption sites of catalysts, i.e., decreases the number of weak H adsorption sites but improves the adsorption strengths of H, C, and O on reduced or carburized catalysts. With increasing amounts of silica, the chemical and structural effects cause the firstly decrease and then the increase of the initial FTS activity and the selectivities of heavy hydrocarbons and olefins during the Fischer–Tropsch synthesis. In addition, an important finding is that a proper amount of silica apparently suppresses the methane selectivity and stabilizes the iron carbide in the FTS reaction.Graphical abstractSilica affects iron Fischer–Tropsch catalysts in several ways. As a chemical promoter, it inhibits reduction and carburization of the iron, while in structural sense, it improves the iron dispersion and stabilizes the active phase. Finally, it suppresses the formation of methane. Download high-res image (157KB)Download full-size imageHighlights► Chemical and structural effects of silica on iron-based catalysts were investigated. ► Silica interacts with Fe to form FeOSi bond and disturbs the electron structures of Fe atoms. ► Silica largely decreases the iron particles and strengthens H, C, and O adsorptions on iron sites. ► High-silica-promotion largely suppresses the formation of methane.

•Two 4000 bbl/d demonstration scale projects using the MTFT technology are being successfully operated in China.•DFT studies on adsorption and reaction mechanisms on iron phases can provide guidance to development of F–T catalysts.•Kinetics based on theoretical and experimental results can be very important tools for F–T reactor and process development.Recent advance using Synfuels China's Fischer–Trospch (F–T) synthesis technology has been made in the 4000 bbl/d coal-to-liquids (CTL) plants in China. Fundamental studies with more than twenty years solid data and experience accumulation in catalysis and kinetic studies have paved the way to the successful demonstration of Synfuels China's medium temperature FT (MTFT) synthesis process. Density Functional Theory (DFT) together with the sophisticated catalyst property characterization tools has been routinely applied during catalyst development. Fundamental R&D efforts integrating all aspects of chemical engineering have greatly been enhanced by combining the fundamental tools covering the F–T synthesis mechanism, reaction engineering, and process optimization.

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.Enwrapping the catalyst particle with a polar solvent, polyethylene glycol (PEG), can significantly increase the selectivity to α-olefins, especially to those with long molecule chains.Download full-size imageResearch Highlights► The α-olefin selectivity increases markedly with the addition of PEG. ► The α-olefin content becomes almost independent of carbon number. ► The PEG-enwrapped catalyst can realize product separating during reaction. ► This study provides a potential route of producing higher linear α-olefins. ► It reveals the importance of controlling the catalytic environment of FTS reaction.

Unpromoted and Mo-promoted iron catalyst precursors (100Fe, 100Fe5Mo, and 100Fe10Mo) were prepared by a coprecipitation method, and were subsequently characterized by laser Raman spectroscopy, Mössbauer spectroscopy, at 298 K and 20 K, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The effect of pretreatment with H2, CO, and H2/CO = 0.67 at 280 and 350 °C was investigated. Specifically, the reduction/carburization behaviors and the evolution of iron phases and Mo species in the catalysts during the pretreatment and during the FTS reaction were extensively studied. During pretreatment, the catalyst structure experienced an extensive restructuring process that was strongly dependent on the pretreatment protocols and Mo promoter loading levels. The microscopic information confirmed this effect on the iron active-site dispersion and the catalytic performance.Graphical abstractPretreatment induced migrations of Mo affect the size distribution and local environments of iron active sites.Download high-res image (233KB)Download full-size imageHighlights► Systematic pretreatment study of Fe and Fe–Mo FT catalysts. ► Evolution of Fe and Mo phases in the catalysts during activation and FTS reaction. ► Activation-induced shift in promotional effects of Mo affected the size of active Fe sites. ► The dispersion effect of Mo favored a high catalyst activity. ► The epitaxial crystal growth effect of Mo showed positive effect on catalyst selectivity.

The mechanisms of H- and OH-assisted CO activation and the consecutive C–C coupling on the flat Co(0001) surface have been computed at the level of periodic RPBE density functional theory. The potential energy surfaces show that the OH-assisted route with the formation of COH, CHOH and CHO is kinetically more preferred than the H-assisted route with CHO formation; however, both routes coincide at the same point with the CHO intermediate which leads to CH3OH formation. Considering the rather low surface OH coverage under H-rich conditions, the OH-assisted route might not be operative. Along with the minimum energy path of CH3OH formation [CO + 4H → CHO + 3H → CH2O + 2H → CH3O + H → CH3OH], the dissociation and the consecutive C–C coupling [CHO → CH + O and CH + CO; CH2O → CH2 + O and CH2 + CO; CH3O → CH3 + O and CH3 + CO] are not competitive kinetically and thermodynamically. This indicates that the flat Co(0001) surface does not represent the active site for Co-based Fischer–Tropsch synthesis; this is supported by the experimentally observed surface reconstruction and carbide formation under reaction conditions. In addition, water-gas shift reaction [CO + OH → COOH → CO2 + H; CO + O → CO2] is also not operative on the Co(0001) surface.

The adsorption and activation of CO, H2O, CO2 and H2 on the clean as well as O, OH and H precovered Fe(100) surface at 0.25 ML coverage and Fe(111) surface at 0.33 ML coverage were computed on the basis of density functional theory (GGA-PBE) to investigate the catalytic activity of metallic iron. On the clean Fe(100) and Fe(111) surfaces, the most favorable reactions are H2O and CO dissociation, and the surface intermediates are the coadsorbed C + O + 2H; CO hydrogenation is kinetically favored and thermodynamically disfavored, whereas CO direct oxidation and COOH formation are kinetically much unfavorable. The O- and OH-precovered Fe(100) surfaces at 0.25 ML coverage and Fe(111) surfaces at 0.33 ML coverage do not promote CO2 formation from the CO direct oxidation and COOH formation from CO and OH coupling, whereas the 0.33 ML O-precovered Fe(111) surface suppresses CO dissociation. In contrast to the 0.25 ML H precovered Fe(110) surface, which does not show a significant effect in the surface reactions, the 0.33 ML H precovered Fe(111) surface can lower the barriers for CO oxidation and hydrogenation, whereas the barrier of COOH formation is increased. It was concluded that the metallic iron surfaces, in particular, the O and OH precovered iron surfaces are not appropriate catalysts for the water-gas shift reaction; however, CO2 dissociation is kinetically and thermodynamically much favorable on all these surfaces. For a given reaction on different surfaces, there is no linear Brønsted–Evans–Polanyi (BEP) correlation between the barriers and reaction energies.

Comprehensive spin-polarized density functional theory (DFT) combined with ab initio molecular dynamic (AIMD) simulations have been performed to explore the structures, energies, and diffusion behavior of platinum on Fe5C2 surfaces with importance in Fischer–Tropsch (F–T) catalysis. The results indicate that the morphology of the Pt promoter on Hägg carbide highly depends on the surface structure of the Fe5C2. The Fe5C2(111) surface shows a stronger Pt affinity than the Fe5C2(100) surface, while the Fe5C2(100) surface has stronger Ptn aggregation ability. Under the realistic iron-based F–T reaction conditions, the adsorbed Pt atoms can diffuse easily, and tend to aggregate following the line-band-layer mode on Fe5C2(100). While on the Fe5C2(111) surface, the adsorbed Pt atoms prefer sitting on the most stable sites, and form dispersed structures rather than aggregated structures until the coverage of Pt is above 2/7 (the surface atom ratio of Pt to Fe). In addition, the three-dimensional structures are not favored on both surfaces because of the stronger Fe–Pt interaction over the Pt–Pt bonding. The adsorbed Pt atoms possess negative charges and the stronger electron transfer is observed on the Fe5C2(100) surface, especially at the low coverage.