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Article

Probing the Roles of Residual Sodium in Physicochemical Properties and Performance of FeAlNa Catalyst for Fischer–Tropsch Synthesis

by
Wenlong Song
1,†,
Qiqi Zhu
1,†,
Kangzhou Wang
2,*,
Rui Zhu
1,
Qingxiang Ma
1,
Tiansheng Zhao
1,
Qingjie Guo
1,
Xinhua Gao
1 and
Jianli Zhang
1,*
1
State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry & Chemical Engineering, Ningxia University, Yinchuan 750021, China
2
School of Materials and New Energy, Ningxia University, Yinchuan 750021, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2023, 13(7), 1081; https://doi.org/10.3390/catal13071081
Submission received: 9 June 2023 / Revised: 5 July 2023 / Accepted: 6 July 2023 / Published: 9 July 2023
(This article belongs to the Special Issue Catalytic Transformation of Low-Carbon Resources)
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Abstract

:
Although Fe-based catalysts have made significant progress in Fischer–Tropsch synthesis, the effect of residual sodium on the structural properties and catalytic performance of Fe-based catalysts has been controversial. Herein, we report the positive role of residual sodium in the structural properties and performance of FeAlNa catalysts for olefins synthesis from syngas. Meanwhile, the as-prepared catalysts were characterized by the multiple characterization technique to reveal the positive role of residual sodium on the structural properties. The characterization results revealed that the residual sodium improved the reduction behavior of Fe species and adsorption ability of CO, and inhibited the secondary hydrogenation due to its weak adsorption ability of H2. Moreover, the residual sodium inhibited the interaction between Fe and Al. Importantly, a high olefins/paraffins ratio of 6.19 and low CH4 selectivity of 12.8% were achieved on the residual sodium modified FeAlNa catalyst. An in-depth understanding of the structural properties and catalytic performance of residual sodium on FeAl-based catalysts can provide a theoretical basis for the development of novel efficient catalysts and large-scale applications for olefins synthesis from syngas.

1. Introduction

Olefins are key building blocks for the industrial production of polymers, lubricants, detergents, cosmetics, etc. [1,2,3]. Currently, it is mainly produced by cracking naphtha or gasoline [4,5]. With the depletion of crude oil resources and increasing environmental issues, the production of olefins via the Fischer–Tropsch synthesis (FTS) route can reduce the dependence on petroleum resources [6,7,8,9]. Therefore, the synthesis of olefins via the FTS route has attracted extensive attention as a renewable feedstock to replace petroleum considering both environment concerns and potential economic opportunity, the key of which is to develop catalysts with high olefins selectivity and catalyst stability.
Several transition metals, such as Ru, Ni, Co, and Fe, exhibited excellent catalytic performance for FTS reaction [10,11,12,13,14]. Ru-based catalysts have high catalytic activity, but limited large-scale applications due to their low availability and high price [10]. Ni-based catalysts have similar catalytic activity to Ru-based catalysts, but their strong hydrogenation ability results in the production of a large amount of CH4 [11]. Both Co- and Fe-based catalysts are widely used for FTS reaction, but the former catalysts with high methanation activity can produce large amounts of undesirable CH4 [14]. Fe-based catalysts are not only the active center of the FTS reaction, but can also catalyze the water–gas conversion (WGS) reaction, which means it has unique advantages in catalyzing the hydrogenation of hydrogen-deficient (H2/CO < 2) syngas [11,13]. Moreover, it has higher olefins selectivity and lower CH4 selectivity in FTS reaction [13].
Alkali metal (K, Na, Li, etc.) promoters were introduced into the synthesized catalysts to improve the catalytic activity and olefins selectivity for FTS reaction [15,16,17,18,19]. Alkali metals can easily contribute their electrons on the s-orbital to the d-orbital of Fe, thereby accelerating the dissociation and adsorption of CO, but affecting the dissociation and adsorption of H2 on the active site [17,19]. Furthermore, the catalyst preparation method has an important impact on the catalytic performance. Fe-based catalysts were usually prepared by co-precipitation due to this method producing catalysts with high specific surface area and facilitating the large-scale production of catalysts. However, the alkali metal sodium was difficult to completely remove, and it has a great influence on the catalytic performance of the FTS reaction [20,21,22,23]. Jun et al. [20] investigated the effect of residual sodium on the catalytic activity of Cu/ZnO/Al2O3 in CO2 hydrogenation to methanol. The results indicated that residual sodium inhibited the interaction between CuO phase and ZnO support, thus reducing Cu dispersion. The residual sodium in the Fe/Cu/K/SiO2 catalyst can promote the aggregation of Fe2O3 particle and inhibit the formation of Fe5C2 as the active center, which has a negative impact on FTS reaction [21]. Zhai et al. [22] revealed the effect of residual sodium on the catalytic performance of Fe–Zn catalysts. The results demonstrated that residual sodium could weaken the adsorption of primary olefins on the Fe5C2, and it inhibited the hydrogenation ability of the catalyst to improve the selectivity of olefins. Therefore, the role of residual sodium on the structural properties and catalytic performance of Fe-based catalysts has been controversial, and a clear understanding of its role is of crucial significance for the development of Fe-based catalysts in FTS reactions.
In this work, a series of residual sodium-modified FeAlO catalysts was fabricated by a simple co-precipitation method, and their catalytic performance of olefins synthesis from syngas were evaluated. The positive roles of residual sodium on the physicochemical properties, structural properties, the interaction between Fe and Al, catalytic activity, and olefins selectivity of the residual sodium-modified FeAlO catalyst were elucidated in detail via multiple characterization techniques.

2. Results and Discussion

2.1. Structure and Textural Properties

We performed in situ XRD characterization to reveal the changes of the catalyst phase during the reduction process, and the results are shown in Figure 1. The reaction process was continuously fed with syngas, and the diffraction peaks of FeAl-20Na catalyst were not significantly changed when the reaction temperature increased from 25 °C to 300 °C. The diffraction peaks of Fe2O3 (JCPDS 33-0664) disappeared, while the diffraction peaks of Fe3O4 (JCPDS 75-1610) were significantly enhanced when the reduction temperature was continuously increased to 400 °C (Figure 1a). Compared to the FeAl-20Na catalyst, the diffraction peaks of the FeAlNa catalyst were significantly weaker than those of FeAl-20Na catalyst. However, when the reduction temperature was increased to 300 °C, the diffraction peak of Fe2O3 significantly decreased, and it was all converted to Fe3O4 at 400 °C (Figure 1b). These results suggest that residual sodium can promote the reduction of Fe2O3.
The XRD patterns of as-prepared catalysts are shown in Figure 2a. The diffraction peaks at 24.1°, 33.4°, 35.8°, 41.0°, 49.6°, 54.3°, 57.9°, 62.6°, and 64.3° were attributed to the hexagon oxides of α-Fe2O3 phase (JCPDS 33-0664) [24]. No characteristic diffraction peaks of Na species were observed in sodium-containing catalysts, indicating that Na species were uniformly dispersed or/and low concentration. Compared with FeAl, the diffraction peak intensity of FeAl-xNa did not change significantly, indicating that rising impregnated sodium almost had no effect on the structure of the catalyst. However, the diffraction peak intensity of FeAlNa declined relative to those of FeAl. The average crystallite sizes of α-Fe2O3 were calculated from the diffraction peak at 33.3° using the Scherer formula, and the results compared in Table 1. Compared with FeAl, FeAlNa displayed a low average crystallite size (14.5 vs. 23.1 nm), while FeAl-xNa exhibited an opposite trend. Meanwhile, the crystallite size of FeAl-xNa first increased to a value of 26.5 nm and then decreased with further increasing the content of impregnated sodium. These results indicated that the residual sodium inhibited the nucleation process of α-Fe2O3 [25].
N2 adsorption–desorption isotherms of the as-prepared catalysts are shown in Figure 1b, and the textural properties are summarized in Table 1. In Figure 2b, FeAlNa and FeAl-xNa exhibited typical IV isotherm with H2-type and H3-type hysteresis loop, respectively, which was assigned to the mesoporous structure formed by the particle accumulation [26]. Compared with FeAlNa, FeAl-xNa displayed a narrow pore size distribution probably due to impregnated sodium having blocked part of the mesoporous structure (Figure S1) [27]. Compared with FeAl, the average pore size of FeAl-xNa increased, while the pore volume and specific surface area decreased due to the increment in crystallite dimension. Meanwhile, with increasing impregnated sodium, the specific surface area increased, and the pore volume and average pore size first increased and then decreased. These results were similar to the previously reported results [27]. Among these catalysts, FeAlNa exhibited a higher specific surface area of 76.6 m2/g and lower pore size and pore volume of 0.17 nm and 9.0 cm3/g, respectively, due to the high dispersion of residual sodium in the bulk phase and smaller crystallite size.
SEM images of synthesized catalysts are shown in Figure 3. FeAlNa catalysts exhibited large particles with different particle sizes attached to their surfaces, which have a wide particle size distribution (Figure 3a). The FeAl catalyst exhibits significant agglomeration of large particles (Figure 3b). FeAl-xNa catalysts are composed of irregular particles, in which FeAl-10Na displayed a narrow particle size distribution (Figure 3c), while FeAl-20Na and FeAl-30Na suffered from particle agglomeration (Figure 3d,e), suggesting that the introduction of excessive sodium made the iron phase aggregate on the catalyst surface. The impregnated sodium highly dispersed on the synthesized FeAl catalyst (Figure S2). Furthermore, the mapping images indicated that the residual sodium highly dispersed on the FeAl catalyst, which could suppress the interaction between Fe and Al species.
We employed the HR-TEM characterization technique to reveal the microstructure of the catalysts, and the results are shown in Figure 4. The particle size distribution of FeAl-20Na and FeAlNa catalysts was relatively homogeneous, but a small number of particles showed agglomeration. Compared with FeAl-20Na and FeAlNa catalysts, FeAlNa catalyst exhibits smaller particle sizes (16.9 nm vs. 35.4 nm). This result indicated that the residual sodium could be effective for smaller particle sizes, which is beneficial to the carbonation of Fe species. The crystal plane of (012), (104), and (110) can be clearly observed in Figure 4, indicating that the synthesized catalyst is mainly in the form of α-Fe2O3, which was consistent with in situ XRD results.

2.2. Chemical State Analysis

To further analyze the effect of Na on the interaction between Fe and Al species in the catalyst, XPS analysis was performed on the reference FeAl and as-prepared catalysts. The binding energy of the synthesized catalysts was corrected using C 1s peak at 284.8 eV. The Fe 2p spectra of the as-prepared catalysts are shown in Figure 5a. The FeAl sample exhibited two peaks at 724.5 eV and 710.7 eV, which were assigned to Fe 2p3/2 and Fe 2p1/2 orbitals, respectively [28]. Compared with the reference FeAl sample, the Fe 2p peak of FeAl-xNa samples shifted to lower binding energy due to the higher electronegativity of Fe compared to Na [29]. However, the Fe 2p peak of FeAlNa samples showed no obvious change compared to the FeAl sample. Similarly, the Al 2p peak of samples containing Na shifted to lower binding energy due to the higher electronegativity of Al compared to Na (Figure S3). The O 1s peak of FeAl exhibited a peak at 529.7 eV and a shoulder peak at 531.1 eV (Figure 5b). Compared with FeAl, the shoulder peak of FeAlNa catalyst clearly weakened, while FeAl-xNa catalysts displayed a stronger shoulder peak. Meanwhile, with increasing the fraction of impregnated Na, the O 1s peaks shifted to lower binding energy and the shoulder peak intensity increased. These results indicated that residual sodium inhibited the interaction between Fe and Al, while the impregnated Na had the opposite effect [30].

2.3. Reduction Behavior

H2-TPR was performed to analyze the reduction behavior of fresh as-prepared catalysts, and the results are shown in Figure 6. The FeAl catalyst exhibited a stronger reduction peak at 420 °C and a weak high temperature reduction peak, which were attributed to the reduction of Fe2O3 to Fe3O4 and Fe3O4 to FeO or metallic Fe, respectively [31]. The FeAl-xNa catalyst had higher reduction temperature than those of the FeAl catalyst, which might be assigned to the reduction of iron oxide that interacts with Al species. Meanwhile, with increasing the fraction of impregnated Na, the reduction temperature first decreased and then increased, indicating lower reduction extent of Fe2O3. This could be attributed to the introduction of sodium blocking part of Fe2O3 on the surface and further strengthening the interaction between iron and aluminum [32]. The increased loading content of sodium leads to a decrease in the active site of H species adsorption and weakens the Fe-H bond energy on the surface of metallic iron. The decrease of H species concentration on catalyst surface may inhibit the secondary hydrogenation of primary olefin, resulting in higher selectivity of olefin [33]. However, the reduction temperature of FeAlNa catalyst shifted to lower reduction temperature compared to FeAl catalyst, indicating that residual Na promoted the reduction ability of the as-prepared catalyst.

2.4. Surface Adsorption Behavior

The effect of residual Na and impregnated Na on the CO adsorption behaviors of as-prepared catalysts were investigated by CO-TPD. As shown in Figure 7a, two CO desorption peaks were observed on the FeAl catalyst, in which there was a lower temperature desorption peak corresponding to the weak CO adsorption and a higher temperature desorption peak corresponding to the strong CO adsorption. The FeAlNa catalyst possessed a strong CO desorption peak in the temperature range of 400–500 °C, suggesting that residual Na had strong adsorption ability of CO adsorption [34]. The strong adsorption of CO on FeAlNa catalyst could be attributed to the weak interaction between Fe and Al species caused by residual Na. Therefore, the residual Na can change the adsorption site of CO and promote the rupture of C-O bond in iron phase. However, the desorption peak of FeAl-20Na catalysts shifted to high temperature, while the desorption peak intensity decreased. In addition, the desorption peak intensity declined with increasing the fraction of impregnated Na, suggesting that the impregnated Na also improved the CO adsorption, but a higher fraction of impregnated Na suppressed the CO adsorption. Among them, the residual Na promoted the adsorption of CO many times higher than the impregnated Na. As an electron promoter, Na enhanced the Fe-C bond on the catalyst surface and provided more active sites for CO dissociation, thus increasing the concentration of carbon species on the catalyst surface [35]. According to the mechanism of surface carbides, CO and H2 adsorbed on the surface of the catalysts dissociate and adsorb and then transfer to carbon species and hydrogen radicals, which combine on the surface of the catalyst to form -CH2- monomer and form hydrocarbons through polymerization [36]. Therefore, the strong CO adsorption promoted the catalytic activity of the catalysts.
The surface basicity of catalysts was characterized by CO2-TPD. As shown in Figure 7b, FeAl catalyst exhibited two CO2 desorption peak at 94 °C and 314 °C, which were attributed to the CO2 desorption from weak base site and strong base site, respectively [37]. The former was mainly the desorption of CO2 from the hydroxyl group on the catalyst surface [38]. Compared with the FeAl, the CO2 adsorption temperature of the FeAl-20Na catalyst shifted to a higher temperature, suggesting that impregnated Na improved the base properties. Moreover, with an increase in the fraction of impregnated Na, the CO2 desorption peak intensity declined while the adsorption peak temperature showed no obvious change. However, the FeAlNa catalyst possessed stronger CO2 desorption peaks at high temperature and low temperature than those of the FeAl catalyst. These results indicated that residual Na and impregnated Na improved the surface basicity of synthesized catalyst, which is beneficial to the olefins synthesis from syngas [39].

2.5. Catalytic Performance

The CO hydrogenation performance of the as-prepared catalysts was evaluated under given reaction conditions to investigate the effect of residual sodium on catalytic performance, and the results are summarized in Table 2. The FeAl catalyst exhibited the lowest CO conversion of 23.8%, olefins selectivity of 14.0%, and O/P ratio of 0.68, and paraffin was the main product in total hydrocarbon distribution. Meanwhile, it displayed a very low olefins yield, only 3.3%. In contrast, the FeAl-xNa catalyst had a higher catalytic activity and olefins selectivity, especially lower CH4 selectivity in FTS reaction. Over the FeAl-xNa catalysts, the CO conversion ranged from 48.0 to 96.5%, which was positively correlated with the increasing trend of impregnated sodium content. However, the selectivity of olefins decreased significantly with the increase of impregnated sodium content, indicating that the higher content of impregnated sodium was not beneficial to the production of olefins. Meanwhile, with the increase of impregnated sodium content, the C5+ selectivity decreased from 43.3% to 30.0%. The olefins yield first increased and then decreased, and O/P ratio decreased as the impregnated sodium content increased. Unfortunately, the olefins yield and O/P ratio were lower than 38% and 4.2, respectively. Among these catalysts, FeAlNa catalyst displayed the best catalytic performance with the CO conversion of 98.7% and the C2=–C4= and olefins selectivity of 37.1% and 57.0%, respectively. Importantly, this catalyst also possessed the highest olefins yield of 56.3% and O/P ratio value of 6.2, as well as the lowest CH4 selectivity of 12.8%.
Compared with the FeAl-xNa catalyst, the FeAlNa catalyst displayed a higher O/P ratio, suggesting that sodium suppressed the secondary hydrogenation of generated primary olefins. In addition, the FeAlNa catalyst had the highest O/P ratio, indicating that the negative effect of residual sodium on secondary hydrogenation is significantly stronger than that of impregnated sodium. Compared with the FeAl-20Na catalyst, the C2=–C4= selectivity of FeAlNa catalyst did not obviously change, but the C5+ selectivity and olefins selectivity increased. This result indicated that the effect of residual sodium in promoting chain growth ability was significantly stronger than that of impregnated sodium. For the FeAlNa catalyst, the olefins selectivity significantly increased to 57.0%. However, the olefin selectivity of the FeAl catalyst was significantly lower than that of alkanes selectivity. This result can be explained by the fact that residual sodium improved the basic properties of the catalyst surface, and the sodium-free catalyst does not have sufficient surface basic sites (Figure 5b) [40].
These excellent catalytic properties presented above were closely related to the physicochemical properties of the catalysts modified by residual sodium in FTS reactions. Residual sodium was revealed to be an effective promoter to effectively inhibit methane production and promote C-C coupling reactions. At the same time, it could inhibit the occurrence of secondary reactions of generated primary olefins to improve the production of olefins and increase the O/P ratio. This can be explained by the fact that residual sodium significantly enhanced the dissociation adsorption of CO and improved the reduction ability and carbonization ability of the catalyst, and gave it a high CO conversion. At the same time, it could inhibit the secondary reaction of generated primary olefins and promote the chain growth reaction, resulting in higher selectivity towards olefins products with high O/P ratio as well as lower CH4 selectivity.

3. Conclusions

In this work, the positive role of residual sodium in the structural properties and catalytic performance of the FeAlNa catalysts for FTS reaction was revealed. The residual sodium significantly improved the dissociative adsorption of CO and reduction behavior and carbonization ability of Fe species and inhibited the interaction between Fe and Al species and the secondary hydrogenation of generated primary olefins. Importantly, the residual sodium-modified FeAl catalyst exhibited the maximal O/P ratio of 6.2, and outstanding C2=–C4= selectivity of 37.1% and olefins yield of 56.3% with a higher CO conversion of 98.7% and lower CH4 selectivity of 12.8%. This work may shed new light on the design multifunction FeAl catalyst for FTS reaction and provides an understanding of the positive roles of residual sodium in the physicochemical properties and catalytic performance of FeAlNa catalyst.

4. Experimental

4.1. Catalyst Preparation

4.1.1. Residual Sodium-Modified FeAl Catalyst

Residual sodium-modified FeAl catalyst was fabricated by the co-precipitation method with a molar ratio of Fe:Al:NaOH + Na2CO3 = 60:20:3. In brief, an appropriate amount of iron nitrate nonahydrate (Fe(NO3)3·9H2O, AR, Sinopharm Chemical Reagent Co., Ltd. Xi’an, China) and aluminum nitrate nonahydrate (Al(NO3)3∙9H2O, AR, Sinopharm Chemical Reagent Co., Ltd. Xi’an, China) was dissolved into deionized water of 100 mL to form uniform solution A. Meanwhile, sodium hydroxide (NaOH, AR, Sinopharm Group Chemical Reagent Co., Ltd. Xi’an, China) and sodium carbonate (Na2CO3, AR, Sinopharm Group Chemical Reagent Co., Ltd. Xi’an, China) were added into 130 mL deionized water under stirring to form solution B. The B solution and A solution were added dropwise into 100 mL deionized water under stirring at room temperature and pH = 10 (precise adjustment by peristaltic pump). The precipitate was collected by centrifugation and washed with deionized water 3 times, and then dried at 80 °C overnight and calcined at 550 °C for 4 h under air atmosphere with a heating rate of 2 °C/min. The ICP test showed that residual sodium accounted for 1.5% of the catalyst (Table S1).

4.1.2. Sodium-Modified FeAl Catalysts

The FeAl catalyst was prepared by the hydrothermal method with a molar ratio of Fe:Al:urea = 3:1:30. Typically, calculated amounts of Fe(NO3)3·9H2O, Al(NO3)3∙9H2O, and urea (CO(NH2)2, AR, Sinopharm Chemical Reagent Co., Ltd.) were added to deionized water of 100 mL and stirred at room temperature for 5 min, and then the above-mentioned mixture solution was transferred to a Teflon-lined autoclave reactor for crystallization at 120 °C for 10 h. Then, the obtained precipitate was washed with deionized water until neutral and dried in air at 100 °C for 6 h. Finally, the sample was calcined at 550 °C for 4 h in static air to obtain FeAl powder.
Sodium-modified FeAl catalysts were synthesized by the impregnation method. In brief, a certain amount of Na2CO3 was dissolved into the deionized water to form a uniform solution. Subsequently, the Na2CO3 solution was dropped into the above-prepared FeAl sample under ultrasound, and the sample was dried at 100 °C overnight. The obtained samples were denoted as FeAl-xNa (the catalyst changes little before and after calcination, H2-TPR profiles of FeAl-20Na and calcined FeAl-20Na-1 sample, Figure S4), where x (10, 20, 30) stands for the preset weight percentage of sodium in the whole catalyst (FeAl-20Na catalyst and FeAlNa catalyst have the same Na content).

4.2. Characterizations

The phase changes during the reduction process were performed on an in situ X-ray diffractometer (XRD) using Rigaku SmartLab (Rigaku-corporation, Tokyo, Japan) instrument with Cu Kα radiation. Typically, syngas was introduced into the reaction chamber for 30 min and heated to the reduction temperature at a ramp rate of 20 °C·min−1 at atmospheric conditions. The scan was carried out from 3° to 85° at a speed of 5°·min−1. X-ray diffraction (XRD) patterns of as-prepared catalysts were determined using a Rigaku D/MAX2200PC (Rigaku-corporation, Tokyo, Japan) instrument with Cu Kα radiation (λ = 0.154 nm) at 40 kV and 40 mA. The scanning speed was 10°·min−1 over the range between 3° and 85°.
The morphology of the catalyst was characterized by ZEISS GeminiSEM 300 scanning electron microscope (Zeiss, Jena, Germany). Prior to the test, the Oxford Quorum SC7620 (Quorum, Wallington, UK) sputtering coater was used to spray gold for 45 s with the working current of 10 mA. In energy spectrum mapping, the acceleration voltage is 15 kV and the detector is SE2 secondary electronic detector. The microstructure was performed by high-resolution transmission electron microscopy (HR-TEM; FEI Talos F200X, Thermo Fisher, Waltham, MA, USA) at an accelerating voltage of 200 kV.
The textural properties of the samples were analyzed by N2 physisorption on a JW-BK132F (JWGBW, Beijing, China) instrument. Prior to the measurement, the sample was degassed at 350 °C for 6 h, and the N2 adsorption–desorption isotherms of the samples were obtained at −196 °C. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) equation. The pore size distribution was obtained from the desorption isotherm by Barrett–Joyner–Halenda (BJH) method.
H2-temperature programmed reduction (TPR) was performed on a Micromeritics AutoChem II 2920 (Micromeritics, Washington, DC, USA) instrument to analyze the reducibility of the calcined catalysts. Before the reduction, a sample of 0.05 g was packed into the quartz tube and pretreated with pure He flow of 30 mL·min−1 at 350 °C for 1 h, followed by cooling to 50 °C. Then, the gas was switched to 10 vol.% H2/He mixture (30 mL·min−1) and the temperature of sample increased to 800 °C at a rate of 10 °C·min−1.
CO temperature-programmed desorption (CO-TPD) tests were performed on AutoChem II Micomeritics (Micromeritics, Washington, DC, USA) equipped with thermal conductivity detector (TCD). Before the test, samples of 0.1 g were loaded into a U-shaped quartz tube. The sample was reduced in 10 vol.% H2/Ar atmosphere at 300 °C for 1 h, and then cooled to 50 °C in pure He flow of 30 mL·min−1 for 1 h. Subsequently, CO pulse adsorption was carried out at 50 °C until saturation, and then pure He flow continued to purge for 1 h. The sample was heated from 50 °C to 800 °C at a rate of 10 °C·min−1, and the CO desorption curve was recorded synchronously every 1 s.
CO2 temperature-programmed desorption (CO2-TPD) was also conducted on AutoChem II Micomeritics measurement. Prior to the measurement, the as-prepared sample of 0.1 g was reduced using 10 vol.% H2/He flow of 30 mL·min−1 at 300 °C for 1 h, followed by the sample being cooled to 50 °C under pure He atmosphere. Subsequently, the 10 vol.% CO2/He was introduced into the reactor at 50 °C for 1 h, and then the physically adsorbed CO2 molecular was removed under pure He (30 mL·min−1) for 0.5 h. Finally, the sample was heated from 50 °C to 600 °C with a rate of 10 °C·min−1, and the TCD signals were recorded.
X-ray photoelectron spectroscopy (XPS) was performed to measure the state of surface elements on a ThermoFisher ESCALAB250 (Thermo Fisher, Waltham, MA, USA) spectrometer with a monochromatized Al Kα anode at operating voltage of 12 kV and current 12 mA. C 1s peak at 284.8 eV was used as internal standard to correct binding energy. The XPS spectra of the corresponding atoms were obtained by fitting the curves and subtracting the background with Casa XPS microwave.

4.3. Catalytic Performance Tests

CO hydrogenation experiments of as-prepared catalysts were performed on a fixed-bed reactor with an inner diameter of 4 mm under continuous mode. In a typical experiment, as-prepared catalysts of 0.5 mL (0.3940 g) were loaded into the reactor and reduced at 300 °C in a H2/CO (molar ratio of 2) flow of mL·min−1 for 3 h at atmosphere pressure. Subsequently, the temperature of the catalyst was cooled to the reaction temperature of 280 °C and the pressure increased to 1.5 MPa, followed by the syngas (H2/CO molar ratio of 2) being introduced into reactor with the weight hourly space velocity (WHSV) of 1000 mL·g−1·h−1. The products were analyzed using on-line gas chromatograph (GC-2014C) equipped with TCD attached to TDX-01 packed column (2 m × 3 mm) to analyze H2, CO, CH4, and CO2, and flame ionization detector (FID) attached to Rtx-1 capillary column (60 m × 0.32 mm × 5 μm) to analyze the hydrocarbons distribution, respectively. Results were calculated as follows:
CO   conversion = C O i n l e t − C O o u t l e t C O i n l e t   ×   100 %
CO 2   selectivity = C O 2   o u t l e t C O i n l e t − C O o u t l e t   ×   100 %
S i = M o l e   o f   C i   p r o d u c t × i ∑ 1 i M o l e   o f   C i   p r o d u c t × i   ×   100 %
where C O i n l e t t—the mole of CO2 at the inlet, C O o u t l e t —the mole of CO at the outlet, C O 2   o u t l e t t—the mole of CO2 at the outlet, Si—the selectivity of hydrocarbon with carbon number i at the outlet, and C i —the hydrocarbon with carbon number i at the outlet.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13071081/s1, Figure S1. The pore distribution of as-prepared catalyst; Figure S2. Mapping images of FeAl-20Na catalyst; Figure S3. Al 2p spectra of the reference FeAl and as-prepared catalysts; Figure S4. H2-TPR profiles of reference FeAl-20Na and calcined FeAl-20Na-1 sample.

Author Contributions

Conceptualization, Q.G., X.G. and J.Z.; methodology, W.S. and Q.Z.; formal analysis, Q.M., T.Z., Q.G. and X.G.; investigation, Q.Z., R.Z. and W.S.; resources, W.S. and Q.Z.; data curation, W.S. and Q.Z.; writing—original draft preparation, Q.Z. and W.S.; writing—review and editing, K.W. and J.Z.; supervision, Q.M., X.G., T.Z. and J.Z.; project administration; funding acquisition, J.Z., X.G. and Q.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (21968025, 21965029), the Natural Science Foundation of Ningxia (2022AAC03040), the Graduate Innovation Program of Ningxia University (CXXM202327), the Innovation Leadership Program in Sciences and Technologies of Ningxia (2020GKLRLX09), Fourth Batch of Ningxia Youth Talents Supporting Program (TJGC2019022), and West Light Foundation of the Chinese Academy of Sciences (XAB2019AW02).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 01081 g001 550
Figure 1. In situ XRD patterns of (a) FeAl-20Na and (b) FeAlNa.
Figure 1. In situ XRD patterns of (a) FeAl-20Na and (b) FeAlNa.
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Catalysts 13 01081 g002 550
Figure 2. (a) XRD patterns of as-prepared catalysts and (b) N2 adsorption–desorption isotherms of synthesized catalysts.
Figure 2. (a) XRD patterns of as-prepared catalysts and (b) N2 adsorption–desorption isotherms of synthesized catalysts.
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Catalysts 13 01081 g003 550
Figure 3. SEM images of the catalyst (a) FeAlNa, (b)FeAl, (c) FeAl-10Na, (d) FeAl-20Na, (e) FeAl-30Na, and (f) mapping images of FeAlNa and Fe, Al and Na correspond to red, green and blue respectively and come from the pinkl rectangle.
Figure 3. SEM images of the catalyst (a) FeAlNa, (b)FeAl, (c) FeAl-10Na, (d) FeAl-20Na, (e) FeAl-30Na, and (f) mapping images of FeAlNa and Fe, Al and Na correspond to red, green and blue respectively and come from the pinkl rectangle.
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Catalysts 13 01081 g004 550
Figure 4. TEM images of the synthesized catalysts (a) FeAl-20Na and (b) FeAlNa. The insets represent the corresponding particle size distribution. HR-TEM images of the synthesized catalysts crystal face spacing (vertical distance of parallel lines between arrows) (c) FeAl-20Na and (d) FeAlNa.
Figure 4. TEM images of the synthesized catalysts (a) FeAl-20Na and (b) FeAlNa. The insets represent the corresponding particle size distribution. HR-TEM images of the synthesized catalysts crystal face spacing (vertical distance of parallel lines between arrows) (c) FeAl-20Na and (d) FeAlNa.
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Catalysts 13 01081 g005 550
Figure 5. Fe 2p spectra (a) and O 1s spectra (b) of the reference FeAl and as-prepared catalysts.
Figure 5. Fe 2p spectra (a) and O 1s spectra (b) of the reference FeAl and as-prepared catalysts.
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Catalysts 13 01081 g006 550
Figure 6. H2-TPR profiles of reference FeAl and as-prepared catalysts.
Figure 6. H2-TPR profiles of reference FeAl and as-prepared catalysts.
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Catalysts 13 01081 g007 550
Figure 7. (a) CO-TPD profiles and (b) CO2-TPD profiles of the reference FeAl and synthesized catalysts.
Figure 7. (a) CO-TPD profiles and (b) CO2-TPD profiles of the reference FeAl and synthesized catalysts.
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Table
Table 1. Textural properties of reference FeAl and as-prepared catalysts.
Table 1. Textural properties of reference FeAl and as-prepared catalysts.
SamplesBET Surface Area (m2/g) aPore Size (nm) bPore Volume (cm3/g) aCrystallite Size (nm) c
FeAl60.70.2315.2923.1
FeAl-30Na53.10.259.5524.6
FeAl-20Na50.10.3112.4626.5
FeAl-10Na44.70.2511.1925.1
FeAlNa76.60.179.0014.5
a Obtained by BET method. b Calculated by BJH method. c Calculated from the strongest peak at 33.3° using Scherrer equation.
Table
Table 2. Catalytic performance of the as-prepared catalysts for FTS reaction a.
Table 2. Catalytic performance of the as-prepared catalysts for FTS reaction a.
CatalystsCO Conv. (%)CO2 Sel. (%)O/P bCH4 c (%)C2–C4= (%)C2–C40 (%)C5+ (%)Olefins (%)Olefins Yield (%)
FeAl23.821.90.726.517.826.129.614.03.3
FeAlNa98.733.16.212.837.16.044.157.056.3
FeAl-10Na48.051.34.113.235.08.543.356.227.0
FeAl-20Na91.338.32.418.732.513.335.540.837.3
FeAl-30Na96.541.52.121.133.115.830.037.135.8
a Reaction conditions: H2/CO = 2.0, 280 °C, 1.5 MPa, 1000 mL·g−1·h−1, TOS = 28 h. b O/P, the ratio of olefins to paraffin in C2–C4 hydrocarbons. c Hydrocarbon products distribution on CO2 free basis.
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Song, W.; Zhu, Q.; Wang, K.; Zhu, R.; Ma, Q.; Zhao, T.; Guo, Q.; Gao, X.; Zhang, J. Probing the Roles of Residual Sodium in Physicochemical Properties and Performance of FeAlNa Catalyst for Fischer–Tropsch Synthesis. Catalysts 2023, 13, 1081. https://doi.org/10.3390/catal13071081

AMA Style

Song W, Zhu Q, Wang K, Zhu R, Ma Q, Zhao T, Guo Q, Gao X, Zhang J. Probing the Roles of Residual Sodium in Physicochemical Properties and Performance of FeAlNa Catalyst for Fischer–Tropsch Synthesis. Catalysts. 2023; 13(7):1081. https://doi.org/10.3390/catal13071081

Chicago/Turabian Style

Song, Wenlong, Qiqi Zhu, Kangzhou Wang, Rui Zhu, Qingxiang Ma, Tiansheng Zhao, Qingjie Guo, Xinhua Gao, and Jianli Zhang. 2023. "Probing the Roles of Residual Sodium in Physicochemical Properties and Performance of FeAlNa Catalyst for Fischer–Tropsch Synthesis" Catalysts 13, no. 7: 1081. https://doi.org/10.3390/catal13071081

APA Style

Song, W., Zhu, Q., Wang, K., Zhu, R., Ma, Q., Zhao, T., Guo, Q., Gao, X., & Zhang, J. (2023). Probing the Roles of Residual Sodium in Physicochemical Properties and Performance of FeAlNa Catalyst for Fischer–Tropsch Synthesis. Catalysts, 13(7), 1081. https://doi.org/10.3390/catal13071081

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