Iron foam catalysts for forming olefins via Fischer–Tropsch synthesis

Abstract

Despite major advances in the direct formation of light olefins (C₂–C₄) by Fischer–Tropsch synthesis (FTS), understanding the actions of promoters and iron carbides remains a difficult endeavor, since the observed results are impacted by an extensive range of complexity and unpredictabilities. In this study, macroporous iron foam (Fe foam) was used as a precursor for the FTS catalyst. The purpose was to reduce the effects of diffusion rate and heat transfer as well as to minimize the influence of promoters on the coverage of active sites. The iron foam was further promoted with sodium (Na), cesium (Cs), rubidium (Rb), and potassium permanganate (KMnO₄) to annotate the role of iron carbides in the production of light olefins during FTS. Based on the results of various characterization, such as XRD, XPS, H₂-TPR, SEM, Mössbauer spectroscopy and HR-TEM, it was found that the KMn/Fe foam catalyst with the maximum content of the active carbide phase (θ-Fe₃C) resulted in the highest light olefin and all olefin selectivity (48.0% and 78.4%) and O/P ratio (14.4 for light olefins) among all catalysts in the current study.

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1. Introduction

Industrial energy relies on fossil fuels. Fossil fuels serve as the primary source for economic progress in several countries.¹ In 2014, over 90% of the demand for energy was met by fossil fuel sources. The demand for fossil-based natural fuels is projected to grow by around 50% or even more by 2030.² The present pace of demand cannot be met by natural oil resources and has triggered the necessity of finding alternative renewable energy resources to alleviate energy scarcity.³ Furthermore, renewable energy sources are better for the environment than fossil fuels.⁴ In addition, over 71% of greenhouse gas emissions, which contribute to increasing global temperatures and impede attempts to combat climate change, are caused by the combustion of natural fuels.^2,5

Light olefins, which are alkenes with carbon atoms ranging from C₂ to C₄, are essential components in the production of several useful compounds. These include fuels, fibers, polymers, solvents, cosmetics and plastics. The extensive chemical derivatives of ethylene (C₂H₄) and propylene (C₃H₆) make them the most commercialized products, and their demand is always on the rise. Several pathways may be employed for the synthesis of light olefins, with natural crude oil resources serving as the main carbon sources in processes such as thermal cracking and fluid catalytic cracking.⁶ However, the high energy demand and adverse environmental effects become the major concerns.

The global need to minimize petroleum use and safeguard the environment has spurred interest in exploring alternative methods such as Fischer–Tropsch synthesis (FTS).⁷ This methodology offers a promising solution for the direct conversion of syngas from biomass and natural gas, with the specific objective of producing chemicals such as light olefins (C₂–C₄).⁸ Fe-based FTS catalysts possess several notable characteristics, including a relatively lower cost, limited hydrogenation reactivity, enhanced water–gas shift reaction, and reduced likelihood of carbon chain elongation.⁹

For Fe-based catalysts, multiple researchers have suggested that the physical characteristics of the material are essential factors in determining the efficiency of FTS and the selectivity towards light olefins.^10,11 For decades, significant emphasis has been devoted to elucidating the relationship between structure and performance, which subsequently influences the synthesis of highly stable and active catalysts.^12,13 Torres Galvis et al.¹⁴ revealed that under similar reaction parameters, the increase in the iron carbide particle size for promoted catalyst samples significantly enhances olefin selectivity. However, at the smallest particle size of 2 nm, increase in methane formation was observed for both promoted and unpromoted catalyst samples.

Generally, a smaller pore size promotes the even distribution of the metal that is being supported.^15,16 Nevertheless, it also leads to insufficient diffusion of both products and reactants, which can have a negative impact on the catalytic efficiency of catalysts. In contrast, an increased pore size facilitates diffusion rate and inhibits 1-olefin re-adsorption,^15,17,18 resulting in a considerable rise in olefin-to-paraffin ratio (O/P) of C₂–C₄ hydrocarbons while transitioning from smaller to larger pores. In order to minimize diffusion limits and ensure optimal FTS performance, the crucial parameter to tune is the pore size.¹⁹ To address this issue, many techniques have been devised to mitigate diffusion constraints in FT S, such as employing monolithic catalysts and utilizing eggshell catalysts and large diameter pores.^20,21 Hence, it is anticipated that catalysts with macropores will alleviate internal diffusion constraints in fixed-bed FTS reactors.¹⁹ According to Becker et al.,²² the incorporation of transport pores with larger pore diameters has the potential to boost reaction rates, thereby lowering methane selectivity. Lu et al.²³ investigated macroporous Cu–Fe catalysts for high alcohol synthesis and achieved impressively high selectivity for 1-alcohols as good diffusion of macroporous tunnels. Zhang's team¹⁷ studied the effect of pore size at the same iron particle size. The results exhibited a marked enhancement in O/P and light olefin selectivity as the pore size of the catalysts was increased, suggesting that the O/P ratio was more pronounced in the case of macroporous catalyst.

It was predicted that the generation rate of light olefins would increase in proportion to the diameter of the support pores. On the other hand, FTS is known for its significant exothermic nature, which is due to a typical reaction enthalpy of −165 kJ mol_CO⁻¹.²⁴ Insufficient regulation of temperature can lead to a transition towards the generation of lighter hydrocarbons due to hot spot formation.^25,26 However, the presence of better heat transfer properties in catalyst structures mitigates the intensity of hot areas. For enhancing intra-bed heat transfer within the FTS, possible methods involve the use of supercritical medium, the implementation of metal monolith catalyst structures, the utilization of metallic foams and the use of folded packing with either open or closed cross-flow patterns.^27–32 The recent developments in macroporous structures that incorporate metallic materials are garnering attention in the research community as a result of their enhanced heat conductivity and improved load-bearing capacity.³³ The recent developments in macroporous structures that incorporate metallic materials are garnering attention in the research community as a result of their enhanced heat conductivity and improved load-bearing capacity.³³ According to prevailing views in the literature the defining feature of the structures is their porosity, which typically ranges from 80% to 95%,^34,35 leading to their classification as porous metals. Fratalocchi et al.²⁶ conducted the comparative analysis of aluminum foam catalysts in a fixed-bed reactor. The outcomes demonstrated that metal foam supports have the capability to be utilized as improved reactor internals to enhance the rate of intensely exothermic processes. Almeida et al.²⁵ reported that foam is characterized by high durability and superior mass and heat transfer properties. The high porosity and non-uniform configuration of metallic foams facilitate effective gas–surface interactions, ensuring intensified reactions.³⁶ Therefore, it is believed that the Fe foam with macro-pores as a precursor of FTS catalysts could mitigate the impact of heat transfer and diffusion limitation on product distribution of the FTS reaction.

In a high-temperature FTS, the effectiveness of Fe catalysts predominantly arises from iron carbides.³⁷ A crucial function of the support lies in preserving finely dispersed iron in its carbidized state throughout the FTS reaction. Torres Galvis et al.³⁸ examined the influence of support characteristics affecting the catalytic performance of Fe-based catalysts. The results indicated the extensive Fe dispersion across these support materials. The catalyst samples exhibited CO conversion rates between 77% and 88%; however, the Fe/γ-Al₂O₃ catalyst had a significantly lower CO conversion of 10%. The increased CO conversion, elevated selectivity for light olefins, and reduced methane selectivity were achieved using catalysts supported by CNF and α-Al₂O₃. Furthermore, the addition of promoters to bulk Fe and weak metal–support interaction favors enhanced selectivity towards light olefins.^39,40

Usually, χ-Fe₅C₂ is regarded as the primary active phase in the FTS process.⁴¹ However, recent investigations have found that θ-Fe₃C contributes to a higher selectivity for light olefins (C₂–C₄).^37,42

Moreover, the addition of promoters can be employed to further improve the selectivity and stability of the active carbide phase.⁴³ The utilization of alkali metals as promoters has been found to be highly successful in reducing the secondary reaction of produced olefins, hence resulting in an increased selectivity towards olefins.^44,45 However, the complex interplay among alkali metals and iron varies as the atomic number increases, which leads to consistent effects on reductions, adsorptions, and carburization when implemented as catalyst promoters in Fe-based FTS catalysts.^8,46 On the other hand, our previous studies^37,42,47 revealed a favorable influence of Mn on the formation of light olefins. The results revealed that the introduction of Mn had an impact on the electronic state and stability of carbonaceous species on the surface, leading to the formation of a distinct phase of iron carbide (θ-Fe₃C).

Besides, different types of metal foams such as aluminum foam, nickel foam etc. have been reported in the literature for a wide range of applications.^31,48 Metal foams are advantageous because their high porosity minimizes the pressure drop, and their high thermal conductivity facilitates rapid heat transfer and minimizes the risk of localized “hot spots” in case of exothermic reactions.⁴⁹ The unique qualities of Fe foam, such as its macroporosity characterized by a three-dimensional structure and abundant active metal sites, make it an optimum choice for creating a cost-effective and highly efficient catalyst,⁵⁰ and to the best of the authors' knowledge it has not been reported as a catalyst for FTS to form light olefins. Herein, the aim of the current study is to utilize Fe foam as a macroporous catalyst for FTS to selective formation of light olefins. The sole purpose of utilizing Fe foam is to serve as a catalyst to generate a stable active iron carbide phase required to achieve high selectivity for light olefins with high O/P ratios of C₂–C₄ hydrocarbons. Different alkali metal promoters (sodium (Na), rubidium (Rb), cesium (Cs)) and KMnO₄ have been employed to study the effect of each promoter and the synergic effect on the catalytic performance and active phase generation towards the selectivity for light olefins.

2. Materials and methods

2.1 Catalyst preparation

The procedure for preparing the Fe foam (40–60 mesh) involved several steps. First, 3 g Fe foam (Duxin Friction Powder Limited, China, purity >95%) was subjected to a treatment with 25 ml of HCL (1 mol L⁻¹) and allowed to stand for 30 minutes to eliminate impurities and rust. Subsequently, the treated Fe foam was thoroughly washed with deionized water and ethanol to remove any residual hydrochloric acid from its surface. Following the washing steps, the Fe foam was placed in an ultrasonic cleaner for 15 minutes, after which it was rinsed three times with deionized water. Furthermore, the foam was subjected to vacuum filtration for 30 minutes and washed with 1000 ml of deionized water during filtration. Finally, the filtered foam was dried in an oven at 120 °C for 12 hours to obtain the dry Fe foam.

Fe foam was promoted by Na, Rb and Cs by impregnation methods and the catalysts were denoted as Na/FF, Rb/FF and Cs/FF, respectively. To synthesize the Na/FF sample, 0.4 g of NaOH was added to 100 ml of deionized water to make 0.1 mol L⁻¹ NaOH solution. Subsequently, the process involved placing the Fe foam in a beaker, measuring 10 mL of the NaOH solution using a volumetric flask. The NaOH solution was added dropwise onto the 3 g Fe foam. The mixture was then allowed to stand for 5 hours. Following this, the solution underwent filtration, and the resulting material was dried at 120 °C for 12 hours.

Furthermore, Fe foam was impregnated with CsNO₃ (purity 99%) and RbNO₃ (purity 99%) solutions by a similar method to that of the Na/Fe foam catalyst. 0.022 g of CsNO₃ were dissolved in 5 ml of deionized water. The solution was then added dropwise to 3 g of Fe foam to make Fe foam soaked with the solution, under mild stirring with a glass rod to ensure the even distribution of Cs on the Fe foam. The mixture was then allowed to stand for 5 hours. Following this, the solution underwent filtration, and the resulting material was dried at 120 °C for 12 hours. Rb/FF was prepared by taking a calculated amount of 0.04 g of RbNO₃ following the same procedure as that for Cs/FF.

The Fe foam promoted with K and Mn, denoted as KMn/FF, was prepared through a defined process. To prepare KMn-promoted Fe foam, 1.58 g of potassium permanganate powder were introduced into a beaker and dissolved in 100 ml of deionized water to formulate a 0.1 mol L⁻¹ KMnO₄ solution. Following the solution preparation, a small quantity of diluted nitric acid (1.0 mol L⁻¹) was incorporated to adjust the solution's pH to 1, followed by thorough mixing. 3 g Fe foam was then immersed in the acidic potassium permanganate solution and allowed to stand for 22 hours. After filtration, the resulting material underwent vacuum drying at 60 °C for 12 hours.

2.2 Catalyst characterization

A range of characterization techniques were employed to analyze the physical and chemical properties of certain catalysts.

The crystalline structure of the samples underwent characterization using an Ultima IV powder X-ray diffractometer (XRD) employing Cu Kα radiation. The testing parameters included scanning the Cu Kα light source in the range of 5–90° at a rate of 5° min⁻¹, with a voltage of 40 kV and a current of 150 mA. The initial XRD data were processed using MDI Jade 6 software and subsequently compared with the standard card (PDF) to ascertain the crystal phase of the sample.

X-ray photoelectron spectroscopy (XPS) was employed for discerning the electronic states of elements situated on a sample's surface. The instrument utilized is the Thermo ESCALAB 250XI model. The determination conditions include the use of a mono Al Kα source with monochromatic energy set at 1486.6 eV. To ensure accuracy, the binding energy of all elements was adjusted using the C 1s standard peak (284.8 eV).

H₂ temperature-programmed reduction (H₂-TPR) was conducted using the AutoChem II 2920 apparatus. Fresh particles of the sample (20–40 mesh) were loaded into a quartz tube, with a connected thermocouple directly measuring the sample temperature. Initially, argon was employed for purging at 120 °C for 0.5 hours (heating rate 10 °C min⁻¹). Subsequently, when the sample temperature descended below 50 °C, purging with an Ar mixture containing 5% H₂ was initiated for 0.5 hours. The temperature was then held at 800 °C for 1 hour (heating rate 5 °C min⁻¹), with the consumption of H₂ monitored throughout this period by a thermal conductivity detector (TCD).

The morphology and physical structure of the samples underwent examination through scanning electron microscopy (SEM) using a Zeiss SUPRA 55 instrument. Test conditions included extra high tension (EHT) within the range of 10.00–20.00 kV, working distance (WD) set between 8 and 13 mm, and signal configured as InLens. The electron microscope was outfitted with an energy spectrum, and the Oxford INCA software facilitated the surface scanning of the samples to acquire the element distribution spectrum.

The evaluation of macroporous samples was conducted through mercury injection porosimetry (MIP) using an Autopore V9620 mercury injection instrument.

Mössbauer spectroscopy analyses were conducted (room temperature) using an MR-351 constant-acceleration Mössbauer spectrometer from FAST, Germany. As a radioactive source, 57Co (Rh) was employed. The identification of components relied on their quadruple splitting, isomer shifts, and magnetic hyperfine fields. The assessment of phase compositions was achieved by analyzing the regions corresponding to the absorption peaks. High-resolution transmission electron microscopy (HR-TEM) was employed to discern the lattice of the active phase in the catalyst. The instrument utilized for this purpose was the JM-3010 model from Japan JEOL, operating at an acceleration voltage of 300 kV.

2.3 FTS catalytic reaction

FTS reaction tests using various catalysts were conducted in a fixed-bed stainless-steel reactor of 10 mm in inner diameter and 400 mm in length. In each experiment, 1.0 g of 40–60 mesh samples was loaded along with quartz wool before the initiation of the reaction.

Syngas (with an H₂/CO ratio of 1) including 5% argon served as an internal standard. The catalyst, once prepared, was introduced into the fixed bed-reactor. Subsequently, the temperature of the reactor was elevated to 350 °C at a gradual rate of 2 °C min⁻¹ under a pressure of 0.1 MPa and a GHSV of 5100 mL g_cat.⁻¹ h⁻¹. In situ reduction was then conducted for 10 hours using syngas. Following this, the reactor was allowed to cool to room temperature.

Preceding the FTS reactions, the reactor pressure was adjusted to 1.0 MPa using syngas, and the reactor temperature was gradually increased to 350 °C at a rate of 10 °C min⁻¹. The subsequent catalyst tests were conducted under the following operational conditions: 350 °C temperature, 1.0 MPa pressure, H₂/CO ratio of 1, and a GHSV of 2200 mL g_cat.⁻¹ h⁻¹.

The products of the Fischer–Tropsch synthesis (FTS) reaction underwent analysis through two GC-2014C gas chromatographs from Shimadzu, each equipped with different detectors. The effluent gases (CO, CO₂, CH₄) were online analyzed using a thermal conductivity detector (TCD), while a flame ionization detector (FID) was employed for the online analysis of hydrocarbons (C₁–C₅) using a Porapak-Q column. For the analysis of liquid products, a silicone SE-30 column was used offline on GC-FID.

3. Results and discussion

3.1 SEM and pore distribution

Fig. 1 shows the morphology and elemental analysis of unpromoted and promoted Fe foam by scanning electron microscopy (SEM) and EDS elemental mapping. Fig. 1(a1) indicates that Fe foam has a special foam-based structure with large pore size, which is beneficial to enhance the diffusion rate and advantageous in preventing agglomeration and localized hot spots during high-temperature FTS reactions,^49,51–53 contributing to high selectivity for olefins.

Fig. 1 SEM images and EDS analyses of various fresh catalysts. (a1–c1) Fe foam; (a2–c2) Na/Fe foam; (a3–c3) Rb/Fe foam; (a4–c4) Cs/Fe foam; (a5–d5) KMn/Fe foam.

Fig. 1(c1) displays the elements in fresh Fe foam in which metallic iron is the major component of the Fe foam. Additionally, Fig. 1(a2–d5) represent SEM images and element distribution of Fe foam modified by various promoters. The results indicated a uniform distribution of alkali metal promoters and manganese on the Fe foam. This uniformity facilitates our investigation into the roles of alkali metals, manganese, and the macroporous foam structure in the formation of the active phase during Fischer–Tropsch synthesis reactions. The pore distribution of the KMn/FF catalyst was determined by MIP and shown in Fig. 2, and it was proved that the macroporous structure was maintained during the reduction process and FTS reaction.

Fig. 2 Mercury intrusion in KMn/FF samples. (a) Log differential intrusion and pore size diameter. (b) Pore size distribution histogram.

3.2 X-ray diffraction (XRD) analysis and HR-TEM

So far, researchers have identified and deliberated upon various types of iron carbides that have the potential to impact the performance of FTS. χ-Fe₅C₂ is commonly regarded as the primary active phase,^54,55 and the carbon-deficient iron carbide phase θ-Fe₃C has proven to be advantageous in the generation of light olefins in the FTS reaction.^37,42,56 In order to evaluate the catalyst phase transition that occurs during the reaction, the catalysts were characterized using XRD in their initial, reduced, and post-reaction states.

As shown in Fig. 3, the fresh Fe foam and the promoted Fe foam catalysts only showed an obvious diffraction peak of Fe (PDF 06-0696), and 44.7°, 65.0° and 82.3° are attributed to (110), (200) and (211) of metallic Fe, respectively.⁵⁷ However, no diffraction peaks of iron oxide and promoter species were observed in the fresh Fe foam samples, suggesting that the foam iron carrier is mainly composed of metallic iron and the results are in line with SEM analysis. The Fe foam samples obtained after reduction and reaction also maintain good elemental Fe phase as shown in Fig. 3(a–d). It is worth noting that after reduction and reaction a wide diffraction zone appeared between 2θ = 40° and 50°, which was not found in the fresh Fe foam sample, and within this range primary peaks of various iron carbide species, including Fe₅C₂ and Fe₃C, were observed.^47,58

Fig. 3 XRD pattern of the reduced and spent catalysts. (a and b) XRD pattern of the reduced FF, Na/FF, Rb/FF, Cs/FF and KMn/FF; (c and d) XRD pattern of the spent FF, Na/FF, Rb/FF, Cs/FF and KMn/FF.

As shown in Fig. 3(c and d), it can be clearly seen that for the spent catalysts, Na-, Rb-, Cs- and KMn-promoted catalysts showed more intense peaks and broader regions between 2θ = 40° and 50°, indicating the formation of more carbide species as compared to the unpromoted Fe foam. Yang et al.⁸ reported that the catalytic ability and the formation of active carbide phases enhanced with the increase in the atomic number of alkali metals because of their stronger electron-donating ability to iron. However, for the synergic effect of KMnO₄, K facilitates the homogeneous generation of iron carbides by enhancing catalytic activity,⁵⁹ and Mn enhances catalyst stability.³⁷

Importantly, no prominent peak of other elements was observed (Fig. 3(a–d)). These results might be attributed to the homogeneous dispersion or low content of the promoter species as compared to the metallic iron. Xiong et al.⁶⁰ utilized Mn-, K- and Cu-promoted iron-based catalysts for FTS. The results indicated that no diffraction peak for the promoter elements was observed. Ma et al.⁶¹ studied Na-promoted Fe–Zr catalyst for light olefin synthesis and found no prominent peak of promoter species. The current study results are in line with previous literature.

Further insights into the function of the spent catalysts in the generation of iron carbide species were examined using high-resolution transmission electron microscopy (HR-TEM) as shown in Fig. 4.

Fig. 4 HR-TEM images of the spent catalysts: (a) Fe foam, (b) Na/FF, (c) Rb/FF, (d) Cs/FF, (e) KMn/FF.

The presence of θ-Fe₃C species was prominently noted alongside χ-Fe₅C₂ in all the promoted catalysts (Na/Fe foam, Cs/Fe foam, Rb/Fe foam, KMn/Fe foam). This observation suggested that the alkali metal promoters play a role in shaping the iron carbides, thereby contributing to the development of θ-Fe₃C species. It is plausible to assert that the promotion of Fe-foam with alkali metals and KMnO₄ not only elucidates the impact of promoters on the generation of iron carbide, specifically θ-Fe₃C, but also clarifies the precise role of θ-Fe₃C species in the formation of light olefins during the FTS reaction.

3.3 X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) was performed to detect the surface chemical formulation of Fe foam catalysts including the reduced and spent samples. For the FTS reaction, the presence of iron carbide phases on the surface of the catalyst is directly related to the catalyst performance.⁴¹

The spectra of Fe 2p_3/2 and Fe 2p_1/2 have been observed for the reduced and spent catalysts as shown in Fig. 5. For the reduced Fe foam samples, the results indicated that the peak at around 707 eV is attributed to Fe₅C₂,^37,62 and the peak at 707.9 eV belongs to Fe₃C.^41,63 Besides, the Fe⁰ species appeared at 706.01 eV (ref. 64) and the peak at around 717 eV corresponds to the satellite peak.^41,47 However, the spent catalysts did not show Fe⁰ species on the surface of Fe foam, which was likely to be converted to carbides or oxide during the FTS reaction. Both χ-Fe₅C₂ and θ-Fe₃C phases were detected on the spent samples as shown in Fig. 5 and the results are in line with the XRD and HR-TEM analysis. In addition, for Cs-promoted Fe foam, the position of Cs 3d_1/2 and Fe 2p_1/2 peaks was close to each other, resulting in the overlapping of Cs and Fe peaks and a broader strong peak has been observed (Fig. 5c), which made it difficult to fit the peaks of Fe 2p1/2 for Cs/FF, so only Fe 2p3/2 fitting had been considered for all samples (Fig. 5a–c). However, it has been clearly observed from the XPS results of spent catalysts that the carbide phases of the promoted Fe foam samples showed a slight gradual shift to higher binding energies than the Fe foam as shown in Table 1 and Fig. 5b. This might be attributed to the fact that the promoter effect has enhanced the electron transfer between Fe and metal promoters during the FTS reaction⁴¹ and more electron donation occurs with the increase in the atomic number of the alkalis, resulting in a shift to higher binding energies in the order KMn/FF > Cs/FF > Rb/FF > Na/FF, contributing to forming more olefins during the FTS reaction.

Fig. 5 XPS spectra of Fe 2p_3/2 for reduced (a) and spent (b) catalysts.

Table 1 The XPS spectra of Fe 2p_3/2 for reduced and spent Fe foam catalysts

Catalysts	Binding energy Fe 2p3/2					Fe5C2/Fe3C ratio
	Fe^2+	Fe^3+	Fe^0	Fe5C2	Fe3C
Reduced FF	709.6	711.8	706	706.9	707.9	3.97
Spent FF	710.3	711.9	—	707	707.9	2.24
Reduced Na/FF	709.6	711.5	706.1	707	707.9	1.24
Spent Na/FF	710.3	711.9	—	707.1	708.05	0.82
Reduced Rb/FF	709.8	711.4	706	707	708	1.1
Spent Rb/FF	710.3	712.0	—	707.1	708.1	0.78
Reduced Cs/FF	709.7	712	706.1	707	708	0.76
Spent Cs/FF	710.6	712.0	—	707.2	708.2	0.73
Reduced KMn/FF	709	711.5	706.1	707	707.9	0.77
Spent KMn/FF	710	711.8	—	707.2	708.3	0.52

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It is noteworthy that the Fe₅C₂/Fe₃C ratio of the samples after reaction is lower than that before reaction (Table 1). Liu et al.^37,42 reported that the presence of θ-Fe₃C enhances production of light olefins. In another study, Ma et al.⁶⁵ reported that θ-Fe₃C demonstrates enhanced capability for C–C chain growth and reduced selectivity towards CH₄. Therefore, the increased θ-Fe₃C species on the promoted catalysts would contribute to higher selectivity for light olefins and reducing the formation of methane during the FTS reaction.

3.4 H₂ temperature-programmed reduction (H₂-TPR)

The reduction behavior of the prepared fresh catalyst was studied using H₂ temperature-programmed reduction (H₂-TPR), and the results are shown in Fig. 6. It is reported that the peak at around 300–400 °C, represents the first step reduction peak of Fe₂O₃ to Fe₃O₄, and the peak at 500–700 °C represents the second step reduction peaks of Fe₃O₄ to FeO and Fe⁰.⁶⁶ As shown in Fig. 6, the initial peaks at approximately 300–400 °C for Rb/FF and Cs/FF indicated the initial reduction phase of Fe₂O₃ to Fe₃O₄ with minimal H₂ consumption at slightly higher temperature as compared to unpromoted Fe foam. These results demonstrated that the inclusion of alkalis marginally suppresses the reduction of α-Fe₂O₃. Additionally, a slight increase in temperature was also observed for Cs/FF compared to Rb/FF, indicating that alkali metals with higher atomic numbers enhance the inhibitory impact⁸ and facilitate the formation of more carbon-poor carbide phase (θ-Fe₃C) during the FTS reaction as shown in XPS analysis (Fig. 5b). It is noteworthy that for KMn/FF, the Mn(III)→Mn(II) reduction peak was observed in the range of 200–300 °C with lower H₂ consumption playing an important role in lowering the reduction degree of the catalyst, facilitating the generation of C-poor carbide phase (θ-Fe₃C).⁴⁷

Fig. 6 H₂-TPR profiles of various Fe foam catalysts.

As illustrated in Fig. 6, during the second step of high-temperature reduction (FeO→Fe⁰), the promoted catalysts exhibited greater H₂ consumption and higher peak strength compared to the unpromoted Fe foam. This observation is consistent with the XPS results, where the Fe⁰ peak of the promoted catalysts after reduction was higher than that of the unpromoted Fe foam. Nevertheless, in comparison to the unpromoted Fe foam, the promoted catalysts (Rb/FF, Cs/FF, KMn/FF) exhibited a little deviation towards elevated temperature. However, no such shift towards high temperature has been observed for Na/FF. This phenomenon can be explained by the presence of intense interactions between iron species and heavier alkalis due to high surface concentration of heavier alkalis, which may slightly hinder the reduction of Fe–O on the surface or outer layer of the catalyst. Nevertheless, the attenuation of the Fe–O bond in catalysts was the result of the ineffectual interactions between alkali promoters and the overall bulk iron species, resulting in a high consumption of H₂ (enhanced reduction).⁴⁶ These metallic Fe species eventually transform into carbides during the reaction process, thereby promoting the generation of olefins.

Therefore, it can be postulated that although the alkali metal and KMn promoters enhanced electron transfer, their surface concentration contributed to developing Fe–alkali and Fe–Mn interactions. This resulted in a slight decrease in the first step reduction, facilitating the formation of the θ-Fe₃C phase, which played an important role in enhancing light olefin selectivity. These results are consistent with the XPS analysis.

3.5 Mössbauer spectroscopy

Mössbauer spectroscopy enabled the precise determination of values and composition for iron-based catalysts, particularly iron carbides.⁶⁷ Mössbauer spectroscopy was employed to thoroughly examine the reduced and spent catalysts. The α-Fe phase has been found in both post-reduction and post-reaction analysis, suggesting that the iron foam is mainly composed of metallic iron with good stability during the reaction, in line with XRD analysis. The Mössbauer spectra of all reduced and spent catalysts reveal the simultaneous presence of both χ-Fe₅C₂ and θ-Fe₃C (as shown in Fig. 7 and Table 2). The results align well with the findings from XPS analysis. In the post-reduction analysis, the results indicated a very low θ-Fe₃C content (8.26%) compared to χ-Fe₅C₂ (15.87%) for unpromoted Fe foam. The θ-Fe₃C content increased after promotion and further increased with the atomic number of the alkalis, following the order Cs/FF > Rb/FF > Na/FF, reaching a maximum of 15.17% for KMn/FF (Table 2).

Fig. 7 Mössbauer spectra of various reduced (a) and spent (b) catalysts.

Table 2 Mössbauer parameters of the reduced catalysts

Catalyst	Phase	IS (mm s^−1)	QS (mm s^−1)	Hhf (kOe)	Area (%)
Reduced FF	χ-Fe5C2	0.09	−0.42	226	6.47
		0.01	−0.15	190	8.74
		0.4	−0.8	136	0.88
	Fe3C	0.2	0.01	208	4.22
		0.3	0.23	207	1.53
	α-Fe	0.01	0.00	329	73.24
	Fe^2+(spm)	0.70	2.0		1.05
	Fe^3+(spm)	0.00	1.7		3.87
Reduced Na/FF	χ-Fe5C2	0.05	−0.74	226	1.85
		0.11	−0.2	193	12.1
		0.001	−0.35	136	2.3
	Fe3C	0.2	0.09	208	8.8
		0.19	0.07	207	0.1
	α-Fe	0.004	0.00	328	70
	Fe^2+(spm)	0.70	0		1.49
	Fe^3+(spm)	0.00	1.7		3.3
Reduced Rb/FF	χ-Fe5C2	0.31	−0.8	241	2.76
		0.1	−0.2	187	8.83
		0.21	−0.47	136	1.97
	Fe3C	0.19	0.03	208	10.14
		0.34	0.34	204	1.88
	α-Fe	0.002	0.00	33	69.61
	Fe^2+(spm)	0.70	2		0.46
	Fe^3+(spm)	0.00	1.64		4.35
Reduced Cs/FF	χ-Fe5C2	0.29	−0.64	257	1.9
		0.11	−0.26	188	9.84
	Fe3C	0.22	0.06	208	8.09
		0.33	0.36	204	2.94
	α-Fe	0.003	0.00	330	72.39
	Fe^2+(spm)	1.10	1.9		0
	Fe^3+(spm)	0.00	1.64		4.84
Reduced KMn/FF	χ-Fe5C2	0.17	−0.8	257	1.99
		0.1	−0.21	188	8.27
		0.18	−0.43	134	1.29
	Fe3C	0.18	−0.004	208	10.21
		0.29	0.19	204	4.95
	α-Fe	0.004	0.00	331	67.43
	Fe^2+(spm)	0.74	1.44		0.57
	Fe^3+(spm)	0.00	1.63		5.29

---

In the post-reaction analysis, not much rise in θ-Fe₃C content was observed for unpromoted Fe foam. However, for promoted Fe foam samples, a reduction in χ-Fe₅C₂ and an increase in θ-Fe₃C content were observed, which correlated with the increasing atomic number of the alkalis. In the spent catalyst samples, the synergistic effect of K and Mn resulted in the highest θ-Fe₃C content for KMn/FF (21.37%) among all samples (Table 3), which would impact the generation of light olefins. The increase in θ-Fe₃C content and reduction in χ-Fe₅C₂ content in the spent catalyst samples clearly indicated the dynamic transformation of iron species during the FTS reaction, resulting in the synthesis of more C-poor θ-Fe₃C.

Table 3 Mössbauer parameters of the spent catalysts

Catalyst	Phase	IS (mm s^−1)	QS (mm s^−1)	Hhf (kOe)	Area (%)
Spent FF	χ-Fe5C2	0.18	−0.02	210	5.56
		0.08	−0.18	189	9.36
		0.19	−0.4	132	0.95
	Fe3C	0.2	0.02	208	5.03
		0.29	0.19	206	3.23
	α-Fe	0.003	0.00	331	71.1
	Fe^2+(spm)	0.83	0.001		0.3
	Fe^3+(spm)	0.001	1.66		4.45
Spent Na/FF	χ-Fe5C2	0.16	−0.8	258	1.81
		0.08	−0.1	189	8.44
		0.17	−0.5	136	0.83
	Fe3C	0.18	−0.003	208	10.55
		0.31	0.26	204	3.03
	α-Fe	0.002	0.00	330	69.3
	Fe^2+(spm)	0.7	1.89		0.32
	Fe^3+(spm)	0.006	1.64		5.72
Spent Rb/FF	χ-Fe5C2	0.23	−0.8	243	1.26
		0.11	−0.21	187	8.79
		0.17	−0.49	135	0.89
	Fe3C	0.19	0.0003	208	12.38
		0.35	0.35	204	3.6
	α-Fe	0.002	0.00	330	67.87
	Fe^2+(spm)	0.71	1.78		0.61
	Fe^3+(spm)	0.001	1.64		4.6
Spent Cs/FF	χ-Fe5C2	0.21	−0.8	253	1.21
		0.11	−0.2	188	9.26
		0.2	−0.2	127	1.05
	Fe3C	0.19	−0.01	208	12.25
		0.33	0.29	204	6.31
	α-Fe	0.003	0.00	331	65.89
	Fe^2+(spm)	0.8	1.05		0
	Fe^3+(spm)	0.001	1.64		4.03
Spent KMn/FF	χ-Fe5C2	0.22	−0.8	254	1.49
		0.1	−0.2	180	5.22
	Fe3C	0.19	0.016	208	15.05
		0.21	0.06	205	6.32
	α-Fe	0.0002	0.00	330	67.23
	Fe^2+(spm)	1.15	1.89		0
	Fe^3+(spm)	0.82	1.65		4.69

---

3.6 FTS reaction performance

The catalytic performance of various Fe foam catalysts is compared in Table 4. The results unequivocally demonstrated a substantial enhancement in both light olefin and all olefin selectivity upon the addition of promoters onto Fe foam. The unpromoted Fe foam determined 26.8% light olefin selectivity and 39.6% all olefin selectivity, respectively. However, the KMn/FF catalyst realized the best olefin selectivity as 48.0% light olefin selectivity and 78.4% all olefin selectivity, with the highest CO conversion and good stability as illustrated in Fig. 7. This outcome underscored the influential role of promoters in facilitating electron transfer and influencing the abundance of stable carbide phases, thus contributing to the enhanced catalyst performance, as proved by XPS and Mössbauer results.

Table 4 FTS reaction performance of various Fe foam catalysts

Catalysts	CO conv. (%)	CO2 sel. (%)	Selectivity (c-mol%).					C2–C4 (O/P)	α
			CH4	C2–C4 paraffin	C2–C4 olefin	C5+	All olefins
Reaction conditions: 350 °C, 1 g catalyst, 1 MPa, H2/CO = 1, 2200 ml gcat.^−1 h^−1, 10 h.
FF	9.1	40.7	40.3	18.7	26.8	8.2	39.6	1.43	0.66
Na/FF	18.1	36.5	12.5	3.20	34.1	45.8	76.6	10.6	0.78
KMn/FF	50.9	44.8	11.3	3.33	48.0	39.6	78.4	14.4	0.81
Rb/ F	30.4	38.2	11.8	3.05	37.1	50.7	71.5	12.1	0.84
Cs/FF	25.9	42.0	16.1	3.25	41.2	39.2	70.6	12.6	0.76

---

On the other hand, with the introduction of promoters, a notable reduction in methane selectivity was observed, decreasing from 40.3% of the unpromoted Fe foam to 11.3% of KMn/FF (Table 3). Zhang et al.⁶⁸ also reported that the catalyst featuring a high proportion of the χ-Fe₅C₂ phase has demonstrated a progressive rise in undesirable CH₄ selectivity. In contrast, the existence of θ-Fe₃C assumes a crucial role in impeding the methanation process, leading to the attainment of the lowest CH₄ selectivity. As the KMn/FF catalyst showed the lowest χ-Fe₅C₂ content as determined by XPS and Mössbauer spectroscopy, it significantly decreased methane selectivity during the FTS reaction.

Alkalis have been documented to promote the adsorption of CO on the surface of catalysts, either molecularly or dissociatively.^46,69 The CO adsorption effects of alkalis are likely attributable to their basic characteristics or electron donation capabilities. Prior studies have documented that the application of alkali can decrease the surface functioning of the transition metals and enhance the dissociation of CO on these types of metals.⁴⁶ The significant basicity of alkalis, which have relatively low ionization potentials, is predominantly responsible for this phenomenon. Alkalis have the ability to readily donate outer-shell electrons to transition metals. The presence of a high number of electrons in iron species will make it easier for CO to adsorb on the surface of the catalyst, through either molecular or dissociative processes.

Moreover, the surface concentration of alkali elements grows in direct correlation with their atomic number. By forming close bonds with iron species, alkalis that are highly concentrated are able to facilitate the adsorption and dissociation of CO more efficiently.⁴⁶ Alkalis (Na, Rb, Cs) are acknowledged for their contribution to improving the catalytic efficacy of Fe-based FTS catalysts. Alkalis function by altering the electronic characteristics of Fe surfaces, hence enhancing the adsorption of reacting agents like CO and hydrogen. These altered interactions enhance the production of light olefins by the optimization of intermediate species throughout the reaction process. Furthermore, the alkali's effect on the catalyst's surface basicity contributes to the stabilization of active carbides, enhancing hydrocarbon chain elongation while inhibiting methane generation, hence promoting the synthesis of light olefins. The presence of intense interactions between iron species and heavier alkalis (Rb, Cs) due to high surface concentration of heavier alkalis effectively enhance the adsorption and dissociation of CO, resulting in increased CO conversion. However, these intense interactions resulted in a slight decrease in the first step reduction, facilitating the formation of the θ-Fe₃C phase as explained by H₂-TPR analysis. Consequently, Cs- and Rb-promoted Fe foam resulted in more θ-Fe₃C content, attributed to their strong surface interactions as compared to Na-promoted Fe foam.

As shown in the XPS results (Table 1 and Fig. 5), the promotion effects enhanced electron transfer between metal atoms and alkalis. Consequently, the efficacy of alkalis in enhancing CO adsorption would augment in proportion to their atomic number, as their surface concentration would correspondingly increase, leading to the formation of a greater number of active carbide phases on the catalyst. Evidently, sodium inhibited the FTS activity, whereas cesium, potassium, and rubidium enhanced it.

It is noteworthy that the maximum CO conversion per hour has been observed for KMnO₄-promoted FF (Fig. 8). The Mössbauer results revealed that the maximum content of the active carbide phase (θ-Fe₃C) has been found in KMn/Fe foam (spent catalyst), resulting in the highest light olefin C₂–C₄ selectivity (48%) and highest O/P ratio of 14.4 in the current study (Table 3). Due to the synergistic influence of KMnO₄ as a promoter, K enhances the catalytic activity by facilitating CO adsorption, while Mn hinders chain length elongation by developing Fe–Mn interactions and minimizes secondary hydrogenation.⁴⁷ The greater redox characteristics of the oxygen-dense manganese oxide species, along with the existence of alkali (K) promoter, boost the electron transfer between active sites, facilitating the development of efficient active carbide phase (Fe₃C). This synergy promotes catalytic stability and suppresses CH₄, leading to increasing both light olefin and all olefin selectivity.

Fig. 8 CO conversion vs. time on stream for various catalysts. Reaction conditions: 350 °C, 1 g catalyst, 1 MPa, H₂/CO = 1, 2200 ml g_cat.⁻¹ h⁻¹, 10 h.

Drawing conclusions from the outcomes, it can be postulated that the macroporous nature of iron foam, being a metal foam, facilitates accessible gas species in gas–solid interactions and improves diffusivity. This is attributed to the fact that the large pores facilitate the bulk molecular diffusion and minimize the restrictive impact of Knudsen diffusion, which might occur due to the small pore size.⁷⁰ Knudsen diffusion transpires at pore diameters much lower than the mean free route of the gas molecules and hinders efficient gaseous diffusion across the pores, whereas molecular diffusion takes place in bigger pore diameters, with a pore width of 100 nm often serving as the demarcation between the two.^71,72 Furthermore, the macroporous nature of the catalyst encourages turbulence and boosts interphase heat transfer.^73,74 These factors contributed to the formation of more θ-Fe₃C in alkali metal-promoted Fe foam due to the interconversion ability between χ-Fe₅C₂ and θ-Fe₃C. Notably, 350 °C is the favored temperature for the generation of θ-Fe₃C.⁶⁸ Combined with SEM and MIP analysis it can be speculated that the addition and uniform distribution of promoters did not affect the macroporous structure of the catalyst throughout the reduction and reaction process which plays an important role in enhancing diffusion and an impactful role in achieving high olefin-to-paraffin ratios. H₂-TPR analysis indicated that the addition of promoters hinders the first step reduction. This hindrance is not favorable for carburization and facilitates the formation of more carbon-deficient θ-Fe₃C phase. The H₂-TPR findings were corroborated by XPS and Mössbauer analyses, which demonstrated an increase in electron transfer with the introduction of promoters, resulting in a shift of the carbide phases to elevated binding energies. This suggests that the promoters alter the electronic properties of the Fe surface. Moreover, the Fe₅C₂/Fe₃C ratios in the promoted catalyst samples were decreased relative to that of the unpromoted Fe-foam, resulting in improved selectivity for light olefins. The FTS catalytic performance (Table 4) is in good agreement with the characterization results.

In addition to the influence of the structural features of iron foam, the favorable effect of KMnO₄ is ascribed to its capacity to act as a stabilizing anchor for the iron phase. KMnO₄ exhibits the ability to simultaneously fulfill dual functions concurrently. Manganese assists in stabilizing iron particles, whereas potassium facilitates the uniform formation of iron carbides on the catalyst's surface, leading to the highest θ-Fe₃C formation, enhancing the stability of the catalyst and the active sites.⁷⁵ Thus, Fe foam facilitates an optimal set of reaction conditions for examining the inherent function of θ-Fe₃C in the production of light olefins.

Fig. 9(a and b) demonstrate the relationship between the iron carbide content of spent catalysts and the obtained O/P ratios after 10 h of reaction time based on XPS data (Fig. 9a) and Mössbauer parameters (Fig. 9b). As shown in Fig. 9a, the results clearly indicated that during the reaction, the Fe₅C₂/Fe₃C ratio was reduced and a prominent increase in O/P ratio has been observed, justifying the distinct role of θ-Fe₃C in enhancing the light olefin production. It can be clearly seen that the minimum O/P ratio was found to be 1.43 for unpromoted Fe foam at an Fe₅C₂/Fe₃C ratio of 2.24. However, as the Fe₅C₂/Fe₃C ratio reduced to 0.82 for Na-promoted Fe foam, a prominent increase in O/P ratio up to 10.6 was observed. Consequently, the reduction in the Fe₅C₂/Fe₃C ratio to 0.78 and 0.73 for Rb/FF and Cs/FF resulted in the rise of the O/P ratio to 12.1 and 12.6, respectively. Importantly, the minimum Fe₅C₂/Fe₃C ratio was found to be 0.52 for KMn/FF, resulting in the maximum O/P ratio of 14.4. Similarly, in Fig. 9b, it can be clearly observed that the increase in O/P ratio is directly related to the increase in the θ-Fe₃C content, suggesting that θ-Fe₃C is advantageous to forming olefins in the FTS reaction.

Fig. 9 Influence of iron carbide content on the O/P value. (a) Based on XPS data; (b) based on Mössbauer parameters.

In comparison with the previously reported studies of mesoporous catalysts, KMnO₄-promoted Fe foam as a macroporous catalyst with high θ-Fe₃C active phase content showed much improved results in terms of the olefin-to-paraffin (O/P) ratio, which was found to be 14.4 in the present study. Dossary et al.⁷⁶ utilized Fe₂O₃/MgO mesoporous catalysts for light olefin synthesis via FTS at H₂/CO = 1, reaction temperature of 250 °C and reaction pressure of 2.02 MPa. The results indicated an increase in the active iron carbide phase after the addition of Cu promoter. However, the maximum olefin-to-paraffin ratio was found to be 1.5. In another study, Yang et al.⁸ synthesized FeMnZr mesoporous catalysts promoted with alkali metals. At optimum reaction conditions of 320 °C, H₂/CO = 2 and 1.0 MPa, the promoted catalyst samples facilitated the formation of the active carbide phase (ε′-Fe_2.2C). However, the maximum olefin-to-paraffin ratio of 6 was observed for K-promoted catalyst samples and light olefin selectivity reached up to 29.2%. Furthermore, our previous study¹⁵ reported a comparison between silica mesoporous (Fe/S5) and macroporous catalyst (Fe/S80-E); the results indicated a threefold increment in the O/P ratio along with a notable enhancement in C₂–C₄ olefin selectivity and decreased CH₄ selectivity relative to the mesoporous catalyst.

Compared to previously published mesoporous catalysts for FTS, Fe foam as a macroporous catalyst exhibited markedly improved performance, especially regarding the olefin-to-paraffin ratio. Although mesoporous catalysts have been extensively researched for their efficacy in several catalytic processes, they frequently have comparatively low O/P ratios, which might be attributed to 1-olefin re-adsorption due to limited diffusion. Conversely, macroporous catalysts, characterized by their larger pore design, enhance the diffusion of reactants and products relative to the more constrictive mesoporous materials. Moreover, the θ-Fe₃C phase is crucial in augmenting catalytic efficiency by enabling the selective activation of reactants and fostering preferred reaction pathways. This interplay between the macroporous structure and the θ-Fe₃C phase renders Fe foam an exceptionally efficient and selective catalyst for processes necessitating high olefin production. The catalytic performance surpasses that of traditional mesoporous catalysts regarding the olefin-to-paraffin ratio and offers a viable option for FTS applications necessitating strong olefin selectivity, representing a notable progression in catalytic technology.

4. Conclusion

In the present work, a simple approach has been used to prepare Na-, Cs-, Rb- and KMnO₄-promoted iron foam as FTS catalysts. The macroporous nature of iron foam played an important role in avoiding diffusion limitations and the coverage of active sites due to agglomeration. Alkalis have been documented to promote the adsorption of CO on the surface of catalysts, resulting in enhanced CO conversion. However, heavier alkali metals (Rb and Cs) performed better for FTS synthesis by increasing CO conversion as compared to Na. The Mössbauer results revealed the presence of the highest χ-Fe₅C₂ content in Cs/Fe foam among all promoted catalyst samples, resulting in increased CH₄ selectivity, and the analysis justified the presence of the θ-Fe₃C phase (high content in spent catalyst samples). Among Cs, Rb and Na, heavier alkalis (Cs and Rb) contributed more to the formation of the active carbide phase (θ-Fe₃C) as proved by XPS and Mössbauer analysis, resulting in higher olefin-to-paraffin ratio (O/P) and light olefin selectivity. However, the synergistic influence of KMnO₄ as a promoter, where K enhances catalytic activity while Mn hinders chain length elongation by developing Fe–Mn interactions, showed the maximum content of the θ-Fe₃C phase, resulting in the highest olefin selectivity and O/P ratio (in light olefins) among all catalysts in the current study. The favorable phase composition and structural properties of iron foam catalysts emphasize their potential as FTS catalysts, particularly in altering product distributions to promote the production of olefins.

Data availability

All data that support the findings of this study are included within the article.

Author contributions

Basit Ali: investigation, data curation, writing – original draft. Jielang Huang: investigation, data curation, writing – original draft. Jing Zhang: investigation, data curation. Shouli Sun: conceptualization, supervision. Yi Zhang: conceptualization, supervision, funding acquisition, writing – review and editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is supported by the National Natural Science Foundation of P. R. China (U20B2022, 22078006 and 22478020), the Bingtuan Science and Technology Program (2021DB006), and the National Key R&D Program of China (2022YFC2105301).

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Footnote

† B. A. and J. H. equally contributed to this article.

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