FTS Reaction Research Articles

Direct CO2 hydrogenation over Na and CeO2-promoted Iron and Cobalt-based Catalysts Supported on MCM-41

This contribution has studied iron or cobalt-based Fischer-Tropsch catalysts, promoted with CeO2 and Na and supported on MCM-41 mesoporous silica in the direct CO2 hydrogenation to hydrocarbons. The cobalt-based catalyst is more active in the presence of ceria as a promoter, presenting long-chain hydrocarbons under reaction conditions of 350 °C, 40 bar, H2/CO2 = 3, and GHSV = 6000 mL g-1 h-1. The characterization of the catalysts suggests that CoCeNa/M exhibits a strong Co-O-Si bond formation, preventing the cobalt oxide reduction to Co2+ species, which can be associated with methane production. On the other hand, iron-based catalysts present higher CO and CH4 concentrations, as shown via in situ DRIFTS, suggesting a possible iron carbide phase, which can be formed by the severe FTS reaction conditions, producing a high concentration of water and competing with Sabatier reaction.

Cobalt‐Potassium Synergistic Modification Effects on Fe‐Based Catalysts for CO2 Hydrogenation to Low‐Carbon Olefins

ABSTRACTTo enhance the CO2 hydrogenation activity and low‐carbon olefin selectivity of Fe‐based catalysts, a strategy involving the use of metal promoters to modulate the structure of active metal centers in catalyst preparation was proposed. The incipient wetness impregnation method was used to introduce modifying agents K and/or Co into the Fe/Al2O3 catalyst. Characterization techniques such as XRD, BET, SEM, HRTEM, H2‐TPR, XPS, CO2‐TPD, NH3‐TPD, and TG were employed to investigate the effects of the modifying agents on the dispersion, reducibility, electronic properties, and acid–base properties of active metal species. Furthermore, the influence of K and Co modification on the CO2 hydrogenation activity and low‐carbon olefin selectivity of Fe/Al2O3 catalysts was explored. The results revealed that the introduction of K generated more basic sites and electron‐rich active metal centers in the catalyst, facilitating the adsorption and activation of CO2, while suppressing the hydrogenation of olefins and the formation of methane, thus improving the selectivity towards low‐carbon olefins. Co promoter facilitated the dispersion of Fe species, exposing more active sites and enhancing the FTS reaction activity, leading to more CO being converted into C2+ hydrocarbon products. Under the synergistic effect of K and Co, the CO2 conversion activity of the Fe‐based catalyst significantly increased, achieving a CO2 conversion rate of 37%, while the selectivity towards C2‐C4= increased to 31.9%.

Core–Shell catalyst particles for tandem catalysis: An experimental/numerical approach towards optimal design

Tandem catalysis is a promising approach to intensify chemical processes and increase their efficiency. On the other hand, the design of efficient, optimal and targeted tailored tandem catalysts is yet so challenging as the optimal catalyst loading is difficult to assess a priori. In this article, we present a concise route towards the design of optimal core–shell tandem catalyst particles on the example of coupled RWGS and FTS reactions for any specific spherical morphology. The route features five consecutive steps including: tandem system identification, catalyst synthesis, i.e. mono- and tandem-functional, catalyst characterization and initial performance test, kinetic modeling with parameter estimation if necessary, and optimal design of catalysts. The initial step features thermodynamic equilibrium calculations for RWGS and FTS showing a common operational window. Then, Pt and Co are selected as active metals and the formulation of the tandem catalyst is designed. For the second step, the synthesis route for the tandem catalyst Pt,CeO2@SiO2-Co, and the mono-functional catalysts, Pt@SiO2 and CeO2@SiO2-Co are presented. For the third step, all catalysts were tested for CO2 hydrogenation as an exemplary tandem process. A reduced transport model from literature was adjusted for RWGS and FTS reactions with kinetic expression from literature to enable numerical optimization. The kinetic parameters are estimated based on the performance tests of the mono-functional reference materials, i.e. Pt@SiO2 for RWGS and CeO2@SiO2-Co for FTS. The model is validated by cross comparison to the data from the tandem reaction setup. In the fifth step, the model was used for the numerical optimization of the catalyst loading on core and shell leading to the identification of the optimal design, resulting in a significant increase of C2+ Yield.

Selectivity control between reverse water-gas shift and fischer-tropsch synthesis in carbon-supported iron-based catalysts for CO2 hydrogenation

CO2 hydrogenation to chemicals and fuels has the potential to alleviate CO2 emissions and displace fossil resources simultaneously via consecutive RWGS and FTS reactions, also known as CO2-FTS. As Fe-based catalysts are active and selective for both reactions, their bifunctionality requires a delicate balance between the RWGS and FTS. In this work, we investigated the thermodynamic constraints of RWGS and CO2-FTS, the influence of CO2 conversion on selectivity and the influence of Fe nanoparticle size within the range of 4.7 to 10.3 nm. An inert carbon support was selected to rule out metal-support interaction and promoting effects of the support. Catalytic performance was evaluated at 300 °C, 11 bar, H2/CO2/Ar = 3/1/1, 600 to 72000 mL·gcat-1·h-1. At a CO2 conversion level below RWGS equilibrium conversion of 23 %, RWGS was found to be the primary and dominant reaction. No primary Sabatier reaction was observed. At higher CO2 conversion till the CO2-FTS threshold of 42 %, the secondary FTS reaction became dominant. Notably, a non-linear relation between CO2 conversion and CO selectivity was discovered. Comparing two catalysts with identical 5 wt% Fe loading but different average Fe nanoparticle size (6.6 and 8.4 nm), the 8.4 nm Fe catalyst was at least two times more active than the 6.6 nm Fe catalyst. In situ Mössbauer spectroscopy suggested a positive correlation between particle size, carburization and selectivity towards long-chain hydrocarbons. For these potassium-promoted carbon-supported Fe-based catalysts, nanoparticles of at least 8 nm are required for the formation of Fe carbides and improved reactivity.

Fischer-tropsch synthesis: effect of temperature and iron-cobalt ratio in Fe-Co/meso-HZSM-5 catalyst on liquid product distribution

The Fischer-Tropsch synthesis converted hydrogen and carbon monoxide into linear hydrocarbons as liquid fuel. Iron and cobalt were used as polymerization catalyst, that impregnated on HZSM-5. The Fe-Co/HZSM-5 could be applied as bifunction catalyst which combined polymerizing synthesis gas and long chain hydrocarbon cracking for making biofuel. The objective of this study is observing the effect of temperature and composition of iron and cobalt combination, supported by HSZM-5 (Fe-Co/HZSM-5) catalyst on fuel product composition. The results obtained from this study would be used to find optimum condition for various iron and cobalt ratio in the catalyst. The mesoHZSM-5 was prepared from ammonium ZSM-5 over calcination, desilication, and dispersion. The mixed solution consisted of Co(NO3)2.6H2O and Fe(NO3)3.9H2O were used as precursor for incipient wetness impregnation on HZSM-5. The catalyst performance was observed in a continuous fixed bed reactor using Fe-Co/meso-HZSM-5 catalyst with synthesis gas at various composition iron and cobalt ratio (10–40 % wt. Fe in Co), various temperature (225–275 °C) at 20 bars. All catalysts were reduced in situ in the reactor. The 10Fe-90Co/mesoHZSM-5 catalyst was more suitable for FTS at 250 °C with alkane (20.49 %) as the main product and alcohol as the by-product (79.51 %). The others catalysts composition of 20–40 % Fe (by weight) in Fe-Co were more suitable for FTS at 225–250°C because under these conditions, alkanes as the main product were obtained in relatively higher compositions compared to other compounds. The mechanism of paraffins, olefins, aldehydes and alcohols formation in this FTS reaction followed the hydrogen assisted CO dissociation with CO-insertion mechanism

Inhibiting water-gas-shift reaction in the Fischer–Tropsch synthesis: Unraveling the roles of phase and facet over iron carbides

An effective method, regulating the phase and facet of FexCy catalysts, is proposed in this study to adjust the catalytic performance of WGSR/FTS and thus affect CO2 selectivity in the FTS reaction. The role of phase and facet over FexCy catalysts in regulating the reaction mechanisms and catalytic activity of WGSR/FTS in the FTS reaction are clearly unraveled. The results conclude that the local electronic environment of Fe atoms and the position of d-band center arisen from the phase and facet difference of FexCy play the determined role in the regulation of catalytic activity of WGSR/FTS over different FexCy catalysts. Among them, χ-Fe5C2(510) facet is screened out as the optimal catalyst to effectively inhibit CO2 formation and to present excellent FTS activity under the FTS conditions. This study gives out a theoretical guide for the design of FexCy catalysts with the specific phase and facet to regulate the activity of FTS and WGSR and therefore greatly decrease CO2 selectivity in the FTS reaction.

Effect of Fe3O4 content on the CO2 selectivity of iron-based catalyst for Fischer-Tropsch synthesis

In this study, a series of catalysts with different Fe3O4 to iron carbide ratios were obtained by carburizing the α-Fe2O3 precursor prepared by co-precipitation method, under various carburization conditions. XRD, Mössbauer spectroscopy, XPS, and Raman spectroscopy were used to characterize the bulk and surface phase compositions of the Fe-based catalysts. The results show that increasing the carburization temperature and prolonging the carburization time lead to higher iron carbide concentration. To explore the active phase of CO2 formation, the catalysts were tested under different reaction conditions by tuning either CO conversion or H2O partial pressure. It turns out that the catalytic performance of the Fe-based catalyst in the FTS and water-gas shift (WGS) reactions is influenced by both the content of iron carbide and the degree of carbon deposition. Under typical Fischer-Tropsch reaction condition, the CO2 selectivity is determined by the CO conversion rather than the Fe3O4 content in the catalyst, meaning that the WGS reaction is here limited by the kinetic factors. On the contrary, adding H2O to the reaction gas results in the trend that higher CO2 selectivity is promoted by higher content of Fe3O4 in the Fe-based catalyst. It seems that Fe3O4 is the main active phase for the WGS reaction in the iron-based catalyst for FTS. These results provide a new insight into the active phase of CO2 generation on the Fe-based catalysts, which could be the theoretical basis for the design of new industrial FTS catalysts with low CO2 selectivity.

Enhanced stability of a fused iron catalyst under realistic Fischer–Tropsch synthesis conditions: insights into the role of iron phases (χ-Fe5C2, θ-Fe3C and α-Fe)

The performance and stability of fused-Fe catalyst in FTS reaction are enhanced through the control synthesis of iron phases (χ-Fe5C2, θ-Fe3C and α-Fe).

Highly robust and efficient MnZnFe2O4 decorated fibrous KCC-SiO2 catalyst for the synthesis of light olefins from syngas

In this study, high light olefin (C2–C4) selectivity is obtained through FTS reaction and using a novel and robust catalyst of KCC-SiO2 fibrous nanospheres decorated with MnZnFe2O4 NPs.

N-doped ordered mesoporous carbon (N-OMC) confined Fe3O4-FeCx heterojunction for efficient conversion of CO2 to light olefins

Converting CO2 into light olefins is full of challenges and opportunities, which was limited by poor activity catalysts. Herein, Fe3O4-FeCx heterojunction active site confined in N-doped graphene shell was synthesized on the surface of N-OMC. Alkali metal and N atom was used for modifying the surface electron density, which could suppress the excessive hydrogenation of CHx and improve the adsorption of CO2, thus increasing the yield of light olefins. The 0.8Fe-0.1K@N-OMC showed CO2 conversion of 54.5%, C2=–C4= selectivity of 65.63%, and a stability of 100 h (3.0 MPa, 320 °C, and GHSV = 4800 mL/gcat/h). The reaction mechanism was investigated using various characterization techniques, which demonstrates that the excellent catalytic active was originated from Fe3O4-FeCx heterojunction. The CO was first produced by the RWGS reaction catalyzed by Fe3O4, and then converted to light olefins on the neighboring FeCx active site by FTs reaction. Confinement effect of N-doped graphene shell inhibits the reoxidation and agglomeration of heterojunction active components, which boosts the catalyst' stability. This work proposes a simple and feasible synthesis strategy to construct Fe3O4-FeCx heterojunction catalysts, which provides insights into catalysts design and the reaction mechanism of CO2 conversion to light olefins.

Dual catalysis by homogeneous/heterogeneous ruthenium species

In this Chem issue, Han et al. devised a system relying on tandem heterogeneous and homogeneous catalysis by ruthenium species for the CO2 hydrogenation to hydrocarbons with remarkable efficiency. In particular, 5-atom (or longer) chain hydrocarbons were obtained with remarkably high selectivity (71.1%).

Effect of Ba on the catalytic performance of Co-Ru/Al2O3 catalyst for Fischer-Tropsch synthesis

Alkali earth metals and Ru are famous selectivity promoters in cobalt-based Fischer-Tropsch synthesis. Ba has displayed as an effective selectivity promoter, however, the presence of Ba leads to a dropped CO conversion yield. To combine the advantages of Ba and Ru in promoting selectivity, Ba-modified alumina was fabricated and used as the supports for Co and Co-Ru catalysts. Promotion of Co/Al2O3 by a combination of Ru and Ba achieved an increase of CO conversion and a low CH4 selectivity due to the enhanced CO dissociation than unmodified anda single additive modified catalyst. The influences of other alkali earth metal additives were also investigated, which had a similar impact that of Ba on the Co-Ru catalysts for promoting FTS reaction.

Carburized cobalt catalyst for the Fischer–Tropsch synthesis

Catalysts with Co, Co3C, and Co2C phases are synthesized by controlling the carburization time. The catalyst with Co3C phase exhibits the highest activity in FTS reaction. DFT calculations reveal the high intrinsic activity of Co3C.

The Effect of Potassium on TiO2 Supported Bimetallic Cobalt–Iron Catalysts

The effect of potassium addition on Fischer–Tropsch catalysts containing 10 wt% cobalt and 2 wt% iron supported on pure TiO2 was studied using a continuous flow reactor at atmospheric pressure, a syngas feed with H2/CO = 1.7 and GHSVsyngas = 1944 mLsyngas gcat−1 h−1. The FTS reaction was performed in a range of temperature 275–350 °C. Differences in textural, structural, chemical and redox properties of the materials were evaluated by N2 adsorption/desorption isotherms, XRD, XPS, and TPR. As compared to the catalyst without potassium, forming large quantity of methane at each of the three temperatures, the potassium promoted catalysts formed less methane and consistent amount of alcohols. Moreover, the potassium containing samples produced more of the heavier hydrocarbons and more CO2 at the higher temperature as compared to the potassium free sample. According to the structural-activity relationship potassium acted as both, structural and electronic modifier.

Fischer-Tropsch synthesis to lower α-olefins over cobalt-based catalysts: Dependence of the promotional effect of promoter on supports

Promoters, such as Na and Mn, play an important role in governing the active phase and catalytic performance of cobalt-based FTS catalyst. In this work, the dependence of the promotional effect of promoters on supports has been investigated by using the carbon and silica supported cobalt-based catalysts in the presence of Na, Mn, and both Na and Mn promoters, respectively. The characterization results of H2-TPR, CO-TPD and XRD profiles together with the FTS performances demonstrate the strong dependence of the promotional effect of promoter on the type of support used. The carbon support, which shows weak cobalt-support interaction, facilitates the manifestation of the effect of promoter. In detail, Mn can promote the formation of Co2C sites at the conditions of 260 °C, 0.5 MPa and lower H2/CO ratio, resulting from the promoted CO adsorption and subsequent dissociation. Thus, the suppressed adsorption of H2 and the promoted surface dissociative C* concentration enhances the selectivities to lower olefins (C2=-C4=) and C5+ with a decrease in methane selectivity. Interestingly, the further addition of Na promoter shows synergistic effect of Na and Mn promoter on remarkably increasing selectivity to lower olefins due to the obviously suppressed chain growth. This synergistic effect could be observed only at the weak cobalt-carbon interaction with a more obvious effect at lower H2/CO ratio in feed gas that favors the increase of Co2C sites with mainly exposed facets of (111) and (020). The presence of (020) facet might be more crucial to enhance the formation of lower olefins. However, over silica support with strong cobalt-silica interaction, the promotional effect of Mn is obviously suppressed, which leads to only CoO and metallic Co sites in the FTS reaction and much weaker impact on product selectivity. The further addition of Na into Mn-promoted cobalt catalysts shows similar effect as Na promoter alone in the product selectivity where the chain growth is highly promoted to remarkably suppress methane selectivity and enhance selectivity to C5+ rather than desired lower olefins. In contrast to carbon support, the strong cobalt-silica interaction leads to less promotional formation of CO2 via the Boudouard reaction (2CO*→C* + CO2) due to the increased non-dissociative adsorption of CO.

Development of fixed bed reactor for applications in GTL-FPSO: The effect of dilution material for control of reaction heat

The catalyst bed of Fischer-Tropsch Synthesis reactor was prepared by the mixing Ru/Co/Al2O3 catalyst with dilution materials (SiC, alumina or glass bead) with different thermal conductivity. The FTS reaction was carried out to investigate the effect of dilution materials for controlling the reaction heat in the shell and tube type fixed bed reactor with an oil circulation system. When the temperature of circulating oil is relatively low, it was found that the reactivity of the FTS reactor is stabilized as the heat conductivity of the diluted material is lower. It was found that when the temperature of circulating oil is slightly lower than or similar to the initiate reaction temperature of FTS reaction which well known as literatures (230 °C), the effect of difference in thermal conductivity of diluent materials in FTS reaction become weaker. Also, based on the CFD simulation results, it can help understanding the tendency of increasing the oil temperature to reduce the catalyst performance difference according to the thermal conductivity of the catalyst bed. It was concluded that if the oil temperature is high enough, the diluent material only act as the dispersion material of the exothermic heat value per unit volume of the catalyst bed in the shell and tube fixed bed FTS reactor system.

Comprehensive understanding of SiO2-promoted Fe Fischer-Tropsch synthesis catalysts: Fe-SiO2 interaction and beyond

The physico-chemical properties of SiO2-promoted Fe-based Fischer-Tropsch synthesis catalysts were traditionally considered to be governed by Fe-SiO2 interaction. Here we found that the configuration between SiO2 and iron species also played a pivotal role in determining the structure and thus FTS performance of the catalysts. Fe@Si catalyst with embedded structure, impregnated Fe/Si catalyst as well as precipitated Fe-Si catalyst was fabricated by different methods respectively, and they were thoroughly characterized by multiple techniques. The results indicated that, despite the weaker Fe-SiO2 interaction, the construction of SiO2 shell outside the iron species core in Fe@Si catalyst strongly inhibited the reduction of iron oxides due to the confining effect of SiO2 shell, which increased the diffusion resistance of H2O generated in the reduction process. However, the sintering of iron species was effectively hindered even during FTS reaction by the physical separation of SiO2 shell. In contrast, iron species in impregnated Fe/Si and precipitated Fe-Si catalyst experienced aggregation with different degrees. The reduction behavior of these two catalysts were well correlated with the strength of Fe-SiO2 interaction. The FTS performance showed that both Fe@Si and Fe/Si catalyst exhibited higher initial activity but deactivated gradually, while the activity of Fe-Si catalyst displayed an opposite trend. These phenomena were discussed in terms of the variation of iron carbides content at different stages, which was determined not only by Fe-SiO2 interaction, but also by the manner that SiO2 and iron species constructed. The present study contributed a new understanding of the structure-performance relationship for SiO2-promoted Fe-based FTS catalysts.

Structural evolution of large Fe3O4microspheres on graphene oxide for efficient conversion of syngas into α-olefins

Large porous Fe3O4microspheres on graphene oxide under FTS reaction conditions can evolve into much smaller iron carbide nanocapsules, which show excellent and stable activity and high α-olefin productivity.

Equilibrium morphology evolution of FCC cobalt nanoparticle under CO and hydrogen environments

Metal nanoparticles could change their shape and exposed certain facet under specific reaction conditions, resulting in greatly influence the performance of catalysts in heterogeneous catalysis. Combining spin-polarized density functional theory and ab initio atomistic thermodynamics, the equilibrium morphology evolution of FCC cobalt nanoparticle had been investigated under CO and hydrogen environments. The phase diagram clearly revealed the stable coverage could be affected by temperature and CO and hydrogen partial pressure. According to the surface energies of four surfaces, Wulff construction was introduced to characterize the morphologies of the Co catalyst at diverse temperatures and pressures. At the typical temperature (675 K) of hydrogen reduction, the estimated surface proportion ratio of the Co(3 1 1) exposed the B5 site could exist a marked increase, which was also consistent with the available Co catalyst reduced under experimental conditions. Our results would give direct insights into the understanding and quantitative description of structure evolution as well as active facets of the Co nanoparticles under the realistic FTS reaction condition.

Development of fixed bed reactor for application in GTL-FPSO: The effect of nitrogen and carbon dioxide contents in feed gas on Fischer-Tropsch synthesis reaction over Ru/Co/Al2O3 catalyst

Abstract The FTS reactions over Ru/Co/Al2O3 catalyst were performed in the shell and tube type fixed-bed reactor with an oil circulation system in order to investigate the effect of N2 and CO2 content. It was found that the N2 gas does not affect the product selectivity of the FTS reaction over Ru/Co/Al2O3 catalyst in the fixed-bed reactor at pilot scale but it has decreased the CO conversion. It was also found that the CO2 has affected the product selectivity of the FTS reaction through the chain growth reaction. It eventually has led the product distribution to shift to the short chain hydrocarbons in FTS reaction. As a result it is concluded that removing the impurities from the syngas in commercial GTL and GTL-FPSO plants is essential to enhance the economic efficiency of process.