Effect of MgFe-LDH with Reduction Pretreatment on the Catalytic Performance in Syngas to Light Olefins

Abstract

MgFe-layered double hydroxides (LDH) were widely used as catalysts for Fischer–Tropsch synthesis to produce light olefins, in which the state of Fe-species may affect the resulting catalytic active sites. Herein, the typical MgFe-LDH was hydrothermally synthesized and the obtained MgFe-LDH was pretreated with H₂ at different temperatures to reveal the effects of the state of Fe-species on the catalytic performance in Fischer–Tropsch synthesis. MgFe-LDH materials were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), H₂ temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). It was found that a MgO-FeO solid solution would be formed with the increase of the reduction temperature, which made the electrons transfer from Mg atoms to Fe atoms and strengthened the adsorption of CO. The pre-reduced treatment toward Mg-Fe-LDH enabled the FeC_x active sites to be easily formed in situ during the reaction process, leading to the high conversion of CO. CO₂ temperature-programmed desorption (CO₂-TPD) and H₂ temperature-programmed desorption (H₂-TPD) analysis confirmed that the surface basicity of the catalysts was increased and the hydrogenation capacity was weakened, the secondary hydrogenation of the olefins was inhibited, and therefore as were the enhancement of O/P in the product and the high selectivity of light olefins (42.7%).

1. Introduction

As the main structural units and raw materials of many fine chemical products, light olefins (C₂–C₄) are traditionally mainly produced by the steam cracking and fluid catalytic cracking of naphtha [1,2]. With the increasing depletion of petroleum resources and the implementation of sustainable development strategies, it is more attractive to produce light olefins from syngas, which is extracted from non-petroleum carbon resources such as natural gas, coal and renewable biomass. The production of light olefins includes indirect routes such as methanol-to-olefins (MTO), dimethyl ether-to-olefins (DMTO), the dehydration of alcohols, and direct route FischerTropsch to olefins (FTO). Since the direct route is more economical than the indirect route after a long research period, the FTO route that directly converts syngas to lower olefins is expected to replace the traditional production route [3,4,5,6].

Since the FischerTropsch synthesis (FTS) process follows the Anderson–Schultz–Flory (ASF) distribution [7,8], resulting in wide production distribution, the selectivity of C₂–C₄ hydrocarbons is usually lower than 58% [6]. Therefore, the FTO reaction usually needs to be carried out at a high temperature (>200 °C) to obtain a higher selectivity to light olefins [6,8]. However, with the increase in reaction temperature, the side reaction Water Gas Shift (WGS), which is one of the primary generation pathways of CO₂ [9,10], will be kinetically accelerated [11], resulting in over 40% of CO being converted to CO₂ in the majority of FTO catalysts [2,8,12,13,14,15,16]. In previous work, Bao et al. and Wang et al. proposed the concept of metal oxide and zeolite (OX-ZEO) catalysts that coupled CO activation and C-C coupling processes at different active sites [17,18]. High olefin selectivity (80%) was obtained over OX-ZEO catalysts at a reaction temperature of 400 °C, but with a low CO conversion (<20%) and a fairly high CO₂ selectivity (≈50%).

Among all the studied metals, such as Fe, Co and Ru [6], Fe-based catalysts attract more attention due to their lower price and higher reactivity of FTO and WGS reactions [19,20,21]. In addition, Fe-based catalysts are easier to use to generate more initial olefins in a wide range of reaction temperatures and H₂/CO ratios [22,23,24,25]. Since de Jong and colleagues discovered that supported iron-based catalysts have excellent selectivity (61%) for light olefins in FTO [8], research on iron-based catalysts has increased rapidly [13,14,26,27,28]. In the research of various promoters and supports, the addition of alkaline-earth metal Mg has been found to promote the catalyst activity and olefin selectivity and also inhibit the formation of CO₂ by suppressing the WGS reaction [29,30,31,32]. Niemelä and Krause reported that Mg-modified Co/SiO₂ catalysts could reduce CO₂ production owing to the change in the electronic structure of the active sites [29]. The effect of alkaline-earth metal on the catalytic performance of Fe-based catalysts was studied by Luo and Davis, and they found that Mg could suppress the water–gas shift reaction, thereby inhibiting the rate of CO₂ formation [30]. Li and colleagues reported a similar effect of Mg on precipitated Fe/Cu/K/SiO₂ catalysts [31]. Zong et al. designed Mg and K dual-decorated iron catalysts supported on reduced graphene oxide (rGO), where the modification of Mg could suppress the formation of magnetite during FTO that inhibited the water–gas shift reaction, leading to the reduce of CO₂ selectivity [32].

Layered Double Hydroxide (LDH) is a unique anionic compound that belongs to a class of two-dimensional materials. The general chemical formula of LDH is [M²⁺_1−xM³⁺_x(OH)₂]^x+(Aⁿ⁻)_x/n·mH₂O, where M²⁺ represents a divalent cation such as Mg²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, and M³⁺ represents a trivalent cation such as Fe³⁺, Co³⁺, Al³⁺, etc. [33,34,35]. The divalent and trivalent metal cations in LDH are highly dispersed in the laminate and arranged in an orderly manner [36]. Therefore, the calcination of LDH under air atmosphere or its reduction under heating conditions will lead to in situ topological transformations to form metal oxides or metal–metal oxide composites [37,38]. The preparation of highly dispersed supported catalysts by calcining or reducing LDH precursors provides a new method to obtain catalysts with specific morphologies and surface structures [39,40,41]. The interaction between the active phase and support is greatly enhanced, and the particle size and stability of the catalysts can also be well controlled [42]. Ma et al. [43] synthesized the SMSI-type TiO_2−x overlayers decorated Ni nanoparticle catalysts via calcination and then reduced the NiTi-LDH precursor, and obtained the CO conversion of ~19.8% with ~64.6% selectivity to C₂₊ paraffins in the FTS process at 220 °C under atmospheric pressure. Recently, Han and coworkers prepared FeMgAl catalysts with a controllable Mg/Al ratio by precipitation method, which inhibited CO₂ selectivity to 28% when the CO conversion was 20% and the light olefin selectivity was 56%. It was proved that MgO and Al₂O₃ play a key role in regulating the surface/bulk properties of iron species, revealing the effect of Mg/Al ratio on CO and H₂ adsorption/reactivity activities and proposing a balanced strategy to control carbon chain growth capacity (α), the ratio of olefins/paraffins (O/P) and primary/secondary CO₂ formation pathways [44].

In this contribution, we successfully prepared a series of Fe-x (x represents the reduction temperature) catalysts by reducing MgFe-LDH with hydrogen at different temperatures. The state of the iron phase in Fe-x catalysts was investigated in detail by changing the reduction temperatures, and the structures and properties of various catalysts were characterized by XRD, SEM, TEM, H₂-TPR, XPS, CO₂-TPD, and H₂-TPD technologies. The catalytic performance of such catalysts was evaluated on the FTO reaction, and the effect of reaction conditions on the catalytic results was also investigated, such as temperature, the ratio of H₂/CO, and space velocity.

2. Results and Discussion

2.1. Catalyst Characterization

2.1.1. Crystalline Structure

Figure 1a confirmed the successful synthesis of MgFe-LDH nanosheets. The H₂ reduction process of the MgFe-LDH nanosheet at different temperatures (from 300 to 600 °C) was tracked by X-ray diffraction (XRD) in Figure 1b. Fe-300 (reduction temperature was 300 °C) lost the characteristic diffraction peaks of the LDH precursor and showed the diffraction peaks of MgO (JCPDS No. 45-0496) at 42.9° (200) and 62.3° (220), and γ-Fe₂O₃ (JCPDS No. 04-0755) at 35.6° (311); this was mainly due to the removal of some interlayer anions and water molecules. When the reduction temperature rose to 400 °C (Fe-400), the crystal facets shifted to lower angles, located at 42.5° and 61.7°, respectively, between diffraction peaks of MgO and FeO, indicating that FeO (2.14 Å), gradually dissolved into the crystal lattice of MgO (2.10 Å), led to MgO-FeO (JCPDS No. 77-2366) solid solution being formed [45,46,47]. As the reduction temperature further increased to 500 °C (Fe-500), the (110) facet of Fe⁰ (JCPDS No. 87-0722) appeared at 44.7°. With the increase in reduction temperature, the diffraction peak intensities of MgO-FeO solid solution and Fe⁰ gradually increased. This showed that the reduction degree of the iron phase becomes higher, which is described in detail in the H₂-TPR characterization in Section 2.1.5.

2.1.2. Textural Properties

N₂ adsorption–desorption isotherms of the samples and the physical properties extracted from them were shown in Figure 2 and

Table 1. The primary pore structure properties of the catalysts.

Samples	Surface Area (m2·g−1)	Pore Volume (m3·g−1)	Pore Size (nm)
Fe-300	50.6	0.28	13.1
Fe-400	70.9	0.34	11.6
Fe-500	28.6	0.14	9.0
Fe-600	9.7	0.05	5.8

, respectively. All the catalysts showed type IV isotherms according to the IUPAC classification [48], all of them with mesoporous structures. When the relative pressure was low, the adsorption and desorption branches almost coincided due to the monolayer adsorption of N₂ on the mesoporous catalysts. Hysteresis loops occurred because the adsorption branch rose faster than the desorption branch at moderate relative pressures [49]. In addition to this, all the samples exhibited H3-type hysteresis loops, which we believed were caused by the accumulation of flake particles. As shown in Table 1, the specific surface area and pore volume of Fe-400 increased, possibly due to the further removal of interlaminar anions and bound water. When the reduction temperature was further increased (>500 °C), the specific surface area and pore volume suddenly decreased, which may be related to the aggregation of MgO-FeO species [50].

2.1.3. SEM Analysis

SEM images of MgFe-LDH and Fe-x catalysts are shown in Figure 3. MgFe-LDH precursors were made of typical flower-like nanosheets, and the reduced catalysts below 500 °C maintained the nanosheet structures. Due to the stacking and inter-embedding of the nanosheets, all of the samples showed irregular slits holy structures [51]. When the reduction temperature was below 500 °C, it can be seen that the catalysts still maintained the sheet structure of LDH. The sheet structures were destroyed in the Fe-600 catalyst, indicating that high temperatures led to pore structure reorganization, which was consistent with the trend observed in the specific surface areas [50].

2.1.4. TEM Analysis

TEM images of the MgFe-LDH precursor and Fe-x catalysts are shown in Figure 4 and Figure S1. MgFe-LDH showed hexagonal lamellar structures of approximately 150 nm (Figure S1a). In the images of Fe-300, it can be seen that parts of the sheet structures fragmented into small particles, which were composed of MgO (200) and γ-Fe₂O₃ (311) (Figure 1b). Notably, a large number of pores appeared in the Fe-400 images (Figure S1b), which was mainly due to the layer collapse that occurred with the evolution of intercalated H₂O and small molecules during the conversion of LDH to metal oxides [52,53]. For Fe-500, only one lattice fringe with an average distance of 2.14 Å can be seen, which belonged to FeO-MgO (Figure 1d). Since the ionic radius and lattice constant of MgO and FeO are similar, they can form an ideal solid solution in all composition ranges [54,55]. When the reduction temperature was further increased to 600 °C, FeO diffused from the surface layer of the solid solution to the bulk phase (Figure S1c) [56].

2.1.5. H₂-TPR Measurement of the Catalysts

The reduction behavior of the Fe-x series catalysts was tested by H₂ temperature-programmed reduction (TPR), and the results were shown in Figure 5. It can be seen that the reduction of the catalysts was mainly divided into two stages. The peak α between 250 and 500 °C represented the transition from Fe₂O₃ to Fe₃O₄, and included some Fe₃O₄ to FeO transitions. Due to the poor stability of FeO, when the reduction temperature was higher than 500 °C Fe₃O₄ and FeO were reduced to Fe⁰ at the same time, which was recorded as peak β [20,57]. The peak α and peak β shifted to lower temperature regions from sample Fe-300 to Fe-500. This may be due to the electrons gradually transferring from Mg to Fe during the process of structural transition, resulting in an easier reduction of FeO_x. This phenomenon would be demonstrated in the XPS characterization in Section 2.1.6. However, for Fe-600 the reduction peaks shifted significantly to a high temperature. This may be due to the high temperature intensifying the diffusion of FeO from the surface layer of the sample to the bulk phase, increasing the depth of FeO entering the MgO particles, making it difficult to reduce [56]. In addition, the positions of the two reduction peaks, peak α and peak β of the catalysts, as well as the H₂ consumption, were listed in

Table 2. Reduction peak temperature and H₂ consumption of the catalysts.

Samples	Peak α		Peak β
	T/°C	n (H2)/(mmol/g)	T/°C	n (H2)/(mmol/g)
Fe-300	421	2.773	708	2.625
Fe-400	393	1.679	723	2.748
Fe-500	358	0.678	707	3.470
Fe-600	399	0.339	742	2.156

. As the reduction temperature increases, the H₂ consumption of peak α decreases significantly, indicating that a large amount of Fe₂O₃ was reduced to FeO, which coincided with the XRD characterization results.

2.1.6. XPS Analysis of the Catalysts

The surface electronic structure of the catalysts was measured by XPS. As shown in Figure 6a, two peaks of the samples, at 710.8 eV and 724.4 eV, are attributed to Fe2p_3/2 and Fe2p_1/2 of the Fe₂O₃ phase, respectively [58,59,60]. Starting with Fe-400, there are two peaks attributed to FeO at 709.6 eV and 723.2 eV [60]. The peak value of FeO in Fe-500 became stronger. In addition, Fe-500 and Fe-600 also had two peaks attributed to Fe⁰ at 706.7 eV and 719.8 eV, and the peak value of Fe⁰ was stronger. Combined with the characterization results of XRD and H₂-TPR, the iron phase in Fe-300 was mainly Fe₂O₃, which gradually transformed to FeO and Fe⁰ with the increase in the reduction temperature. In Table 2, the peak β H₂ consumption (2.156 mmol/g) of Fe-600 is significantly lower than that of Fe-500 (3.470 mmol/g), indicating that when the reduction temperature was increased to 600 °C, the Fe⁰ phase increased significantly, so the consumption of H₂ during H₂-TPR process decreased. As can be seen in Figure 6b, the peaks that appeared at 49.7 eV belonged to MgO. As the reduction temperature increased, the binding energy of Mg atoms shifted toward higher positions; this was because electrons could transfer from Mg atoms to Fe atoms. Obviously, Fe-500 has the lowest Mg atomic valence, which is speculated to be due to the highest proportion of the MgO-FeO solid solution phase in Fe-500, and the existence of this phase is more conducive to the electron transfer from Mg atoms to Fe atoms.

2.1.7. Chemisorption Properties of the Catalysts

The surface basicity of the catalysts was studied by CO₂ temperature-programmed desorption (CO₂-TPD), and the results were shown in Figure 7a. All of the samples had two desorption peaks at about 90 °C and 285 °C, the former corresponding to weakly physically adsorbed CO₂ and the latter to weakly chemisorbed CO₂ [60,61,62]. With the increase in the reduction temperature, the intensity of the desorption peak at 90 °C increased significantly, and reached the highest value at Fe-500, indicating that the CO₂ amount of weak physical adsorption increased, and the weak basicity of the catalyst surface increased. With the increase in the reduction temperature, the intensity of the desorption peak at 285 °C also increased slightly, indicating that the medium basicity of the catalyst surface was slightly enhanced and its degree was not as obvious as that of weak physical adsorption. When the reduction temperature is increased to 600 ° C, the weak basicity of Fe-600 decreases significantly. The enhancement of the alkalinity of the catalyst surface is conducive to the adsorption and dissociation of CO, which leads to the increase of CO conversion and the length of the carbon chain, in addition to the ratio of olefins/paraffins.

The adsorption capacity to hydrogen of the catalysts was studied by H₂ temperature-programmed desorption (H₂-TPD), and the results are shown in Figure 7b. According to previous studies, the desorption peak located at 100–300 °C represented the chemisorption of H₂ [60,63]. From Fe-300 to Fe-500, the desorption temperature gradually decreased, indicating that the adsorption capacity for H₂ became weaker with the increase of the reduction temperature. Fe-500 had the lowest H₂ desorption temperature, indicating that its adsorption and dissociation ability to H₂ was the weakest, which can inhibit the secondary hydrogenation reaction of olefins generated during FTO process well, thereby improving the selectivity of olefins. According to XPS, this may be due to the electrons transferred from Mg atoms to Fe atoms, resulting in higher electron density of the catalysts [60]. However, the desorption peak of Fe-600 shifted slightly to a high temperature, indicating that the adsorption capacity of H₂ was enhanced. This would promote the secondary hydrogenation of the olefins, resulting in the increased selectivity of paraffins during the FTO process.

2.2. Catalytic Performance

2.2.1. The Activity and Selectivity of the Catalysts

The catalytic performances of the catalysts were shown in

Table 3. Catalytic performance of the catalysts.

Samples	CO Conversion (%)	CO2 Selectivity (%)	Hydrocarbon Distribution (%)				O/P (C2–C4)
			CH4	C2–C4=	C2–C40	C5+
Fe-300	8.11	13.27	30.47	35.97	22.22	11.34	1.62
Fe-400	9.20	3.93	27.49	44.74	21.45	6.32	2.09
Fe-500	29.94	19.43	24.11	42.68	14.96	18.25	2.85
Fe-600	7.10	5.87	31.06	42.11	18.98	7.85	2.22

Reaction conditions: H₂/CO = 2.0, GHSV = 2000 h⁻¹, T = 300 °C, P = 2.0 MPa.

. The carbon balance of all the reactions was above 95%. From the sample Fe-300 to Fe-500, the CO conversion increased significantly from 8.11% to 29.94%. However, for Fe-600, the CO conversion suddenly dropped to 7.10%. Combined with the characterization results of XRD, H₂-TPR and XPS, due to the formation of MgO-FeO solid solution, the electrons transferred from Mg atoms to Fe atoms, resulting in the high electron density of Fe atoms, which was beneficial to the adsorption of CO, thereby generating FeC_x active sites in situ during the reaction process and ultimately improving CO conversion. At the same time, XPS, CO₂-TPD and H₂-TPD analysis proved that the strong basicity and strong electron density on the surface of the Fe-500 catalyst could inhibit the adsorption of H₂; that is, it could weaken the hydrogenation reaction, which would not only reduce the selectivity of CH₄ but also inhibit the secondary hydrogenation reaction of the olefins, thereby improving the ratio of olefins/paraffins (O/P) of the product [60,64]. For Fe-600, due to the stacking of lamellae when the temperature was above 600 °C, many active sites were covered, resulting in a significant decrease in CO conversion.

2.2.2. The Effects of the Reaction Conditions over the Fe-500 Catalyst

As we all know, reaction temperature is one of the important factors affecting the selectivity of the light olefins, so we evaluated the CO hydrogenation performance of the Fe-500 catalyst at different reaction temperatures, and the results are listed in Figure 8a. With the increase in reaction temperature, the CO conversion increased sharply and the selectivity of CO₂ also increased significantly. For the distribution of hydrocarbon products, the increase of reaction temperature led to the movement of products in the direction of low carbon, the selectivity of CH₄ increased, and the selectivity of C₅₊ decreased. In addition, a larger proportion of alkanes was observed, which was due to the increase in temperature, leading to the secondary hydrogenation of alkenes on the surface of the catalyst to form alkanes [65]. In contrast, we chose the reaction temperature of 330 °C for the next study.

As shown in Figure 8b, the effect of the H₂/CO ratio of feed gas on catalytic performance has also been studied. With the increase of the H₂/CO ratio, the CO conversion rate first increased and then decreased, and the CO₂ selectivity showed a similar trend. When the H₂/CO ratio was 2:1, CO conversion and CO₂ selectivity reached the highest point: 78% and 43%, respectively. In this process, CH₄ selectivity slightly increased and C₅₊ selectivity slightly decreased. The value of O/P changes obviously, which is the opposite trend to the change of CO conversion. With the increase of the H₂/CO ratio, the content of C₂–C₄ olefins decreases first and then increases. To sum up, we chose the H₂/CO ratio of 1.5:1 for the next study.

In addition, we also studied the effect of reaction space velocity on catalytic performance, as shown in Figure 8c. With the increase of reaction space velocity, CO conversion increased slightly and CO₂ selectivity decreased. For hydrocarbons in the product, CH₄ selectivity was almost unchanged (≈20%) and C₅₊ selectivity was significantly increased from 10% to 24%. The proportion of C₂–C₄ hydrocarbons decreased from 68% to 53%, with O/P slightly decreased. When GHSV was 1000 h⁻¹, C₂–C₄ olefins selectivity was 51%, with O/P 3.10. In summary, the optimal reaction conditions of the Fe-500 catalyst were 330 °C, H₂/CO = 1.5:1, GHSV = 1000 h⁻¹. We compared the data with previous works, which were listed in Table S1.

3. Materials and Methods

3.1. Catalyst Preparation

3.1.1. Synthesis of the MgFe-CO₃-LDH Precursor

MgFe-CO₃-LDH was prepared by the hydrothermal method. In brief, 2.7602 g Na₂CO₃ was dissolved into 60.7 mL of distilled water, named solution A. Then, 10.4 mL of Mg(NO₃)₂ aqueous solution (2 mol/L) was added into solution A under vigorous stirring. Similarly, 5.2 mL of Fe(NO₃)₃ (2 mol/L) aqueous solution was added to the above solution. Finally, 3.9 mL of NaOH solution (9.67 mol/L) was added to adjust the pH value, with constant stirring. The obtained mixed solution was transferred into a 100 mL Teflon-lined autoclave, and then moved into the laboratory oven for further heating at 100 °C for 3 days. The obtained red-brown solid was then filtered, washed with deionized water, and dried at 100 °C for 12 h. Thus, the precursor MgFe-CO₃-LDH was obtained.

3.1.2. Synthesis of Fe-X Catalysts

MgFe-CO₃-LDH was reduced in an H₂/Ar (10/90, v/v) flow at different reduction temperatures (300, 400, 500, 600 °C) for 180 min with a heating rate of 2 °C/min. The products were named Fe-x, where x refers to the reduction temperature.

3.2. Catalyst Characterization

Powder X-ray diffraction (XRD) patterns of all samples were determined using a Rigaku Ultima IV (Rigaku Corporation, Tokyo, Japan) powder diffractometer and tested with Cu Kα radiation at 40 kV and 40 mA.

The textural properties of the samples were obtained by recording the N₂ adsorption–desorption isotherms at −196 °C on the Micromeritics ASAP 2020 PLUS HD88 (Micromeritics, Norcross, GA, USA), and the samples were pretreated at 110 °C for 10 h before testing. The specific surface area and pore size distribution of the samples were calculated by Brunauer–Emmett–Teller and Barrett-Joyner-Halenda methods, respectively.

Scanning electron microscopy (SEM) images were obtained with a JSM-7001F (Japan Electronics Co., Ltd., Tokyo, Japan) instrument with a voltage of 3.0 kV and 30,000 times amplification.

High-resolution transmission electron microscopy (HRTEM) images of the samples were acquired on a JEOL JEM 2100F (Japan Electronics Co., Ltd., Tokyo, Japan) with an acceleration voltage of 200 kV.

The H₂ temperature-programmed reduction (H₂-TPR) curves of the samples were used to record the reducibility of the catalysts on Micromeritics AutoChem II 2920 (Micromeritics, Norcross, GA, USA). A total of 50 mg of catalyst was put into a quartz tube, pretreated with He at 110 °C for 1 h, then cooled to 50 °C for 30 min. Additionally, it was then switched to a 4.85% H₂/Ar mixture; after the baseline was stable, the temperature was increased to 900 °C by the rate of 10 °C/min and H₂ consumption was recorded.

The chemical state of the elements on the surface of the catalysts was detected by X-ray photoelectron spectroscopy and tested on ThermoFischer ESCALAB 250Xi (Thermo Fischer Scientific, Waltham, MA, USA) equipped with a monochromatized Al Kα radiation source. The operating voltage was 12.5 kV, and the current was 16 mA. Data calibration was performed using the peak C1s of 284.6 eV.

CO₂ temperature-programmed desorption (CO₂-TPD) curves were recorded on the TPR-tested equipment. A total of 50 mg of the catalyst was pretreated at 500 °C for 1 h, and then cooled to 50 °C. The samples were exposed to a flow of CO₂ for 40 min and then switched to a flow of He purged for 40 min to remove physically adsorbed CO₂ species. Finally, the temperature was raised to 500 °C at a heating rate of 10 °C/min, and the desorption signal of CO₂ was recorded by a thermal conductivity detector (TCD).

H₂ temperature-programmed desorption (H₂-TPD) curves were recorded on the TPD-tested equipment. A total of 50 mg of the catalyst was pretreated at 500 °C for 1 h and then cooled to 50 °C. The samples were exposed to a flow of H₂ for 40 min and then switched to a flow of He purged for 40 min to remove physically adsorbed H₂ species. Finally, the temperature was raised to 500 °C at a heating rate of 10 °C/min, and the desorption signal of H₂ was recorded by a thermal conductivity detector (TCD).

3.3. Catalytic Activity Tests

CO hydrogenation reactions were performed in a stainless fixed-bed reactor loading 1 mL of catalyst (40–60 mesh). The feed gas (H₂/CO = 2, molar ratio) was introduced into the reactor with a pressure of 2 MPa and GHSV of 2000 h⁻¹. Nitrogen with a concentration of 10% in the syngas was used as an internal standard for the calculation of CO conversion. The reaction temperature was then ramped slowly to 300 °C. The permanent gases (H₂, N₂, CO, CH₄), light hydrocarbon (CH₄, C₂–C₄), and those gases (CO, CH, CO₂) that can be converted into methane were monitored by an online HXSP GC-950 (Haixinsepu Instrument Co., Ltd, Shanghai, China) gas chromatograph (GC) equipped with the 5A molecular sieve, modified Al₂O₃, and TDX-01 columns. The oil and wax products were separated by a cold trap (0 °C) and a hot trap (150 °C). The oil products were analyzed using an Agilent 7890B (Agilent Technologies Co., Ltd, Santa Clara, CA, USA) GC with a DB-1 column. The product selectivity was calculated based on the carbon balance, and the carbon balance of each test in this work was above 95%.

CO conversion ($X_{CO}$) and product selectivity are calculated as follows:

$$X_{CO}=\left(1-\frac{\frac{S_{N_2,in}}{S_{N_2,out}}}{\frac{S_{CO,in}}{S_{CO,out}}}\right)\times 100\%$$

(1)

$S_{N_2,in}$: the peak area of N₂ in the inflow; $S_{N_2,out}$: the peak area of N₂ in the outflow; $S_{CO,in}$: the peak area of CO in the inflow; $S_{CO,out}$: the peak area of CO in the outflow.

$$Selectivit{y}_{C{O}_2}=\frac{n_{C{O}_2}}{n_{C{O}_2}+\sum n_{C_i}}\times 100\%$$

(2)

$n_{C{O}_2}$: carbon number of CO₂ in the products; $n_{C_i}$: carbon number of corresponding products.

$$Selectivit{y}_{C_i}=\frac{n_{C_i}}{\sum n_{C_i}}\times 100\%$$

(3)

$$\frac{O}{P}=\frac{n_{C_{2-4}}^o}{n_{C_{2-4}}^p}$$

(4)

$n_{C_{2-4}}^o$: carbon number of C_2–4 olefins in the products; $n_{C_{2-4}}^p$: carbon number of C_2–4 paraffins in the products.

4. Conclusions

In conclusion, a series of Fe-x catalysts were prepared by reducing the MgFe-LDH precursor at different temperatures (300–600 °C). The Fe-500 catalyst acquired a CO conversion rate of 30%, light olefins selectivity of 43%, and a low CO₂ selectivity (19%) under the reaction conditions of 300 °C, H₂/CO = 2:1, GHSV = 2000 h⁻¹, 2 MPa. After a series of characterizations, it was proved that, with the increase in reduction temperature, the catalyst gradually evolved into FeO-MgO solid solution and Fe⁰. The formation of the MgO-FeO solid solution is conductive to the electrons transfer from Mg atoms to Fe atoms, so it enhanced the adsorption of CO to generate FeC_x active sites in situ during the reaction process, thereby improving CO conversion. In addition, the increase of the basicity and electron density on the surface of the catalysts weakened the adsorption of H₂, inhibited the secondary hydrogenation reaction of the olefins while inhibiting the formation of methane, and improved the selectivity of light olefins and the O/P value of the product. We optimized the reaction conditions of the Fe-500 catalyst, and finally determined that the suitable reaction conditions were 330 °C, H₂/CO = 1.5:1, and GHSV = 1000 h⁻¹. Under these conditions, the CO conversion was 47%, the selectivity of light olefins was 51%, and the O/P value was 3.10. This study provides new routes for the preparation of novel FTO catalysts.