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. 2021 Feb 8;6(7):4968–4976. doi: 10.1021/acsomega.0c06008

Direct Conversion of Syngas to Light Olefins through Fischer–Tropsch Synthesis over Fe–Zr Catalysts Modified with Sodium

Zhunzhun Ma †, Hongfang Ma †, Haitao Zhang †, Xian Wu †, Weixin Qian †,*, Qiwen Sun ‡, Weiyong Ying †
PMCID: PMC7905929  PMID: 33644604

Abstract

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Fe–Zr–Na catalysts synthesized by coprecipitation and impregnation methods were implemented to investigate the promoting effects of Na and Zr on the iron-based catalyst for high-temperature Fischer–Tropsch synthesis (HTFT). The catalysts were characterized by Ar adsorption–desorption, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, CO temperature-programmed desorption, H2 temperature-programmed desorption, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy (MES). The results indicated that Na changed the active sites on the catalyst surface for the CO and hydrogen adsorption, owing to the electron migration from Na to Fe atoms, which resulted in an enhanced CO dissociative adsorption and a decrease in hydrogen adsorption on the metallic Fe surface. The decreased H/C ratio on the catalyst surface accounted for the increased chain propagation and weakened hydrogenation of light olefins. Besides, Na could also facilitate the carbonization of catalysts and protect the iron carbide against oxidation, which provides more active sites for HTFT reaction and is beneficial to the C–C coupling. Zr could decrease the hematite crystallite size and stabilize the active phase to improve the HTFT activity. At an optimal Na loading of 1.0 wt %, the Fe–Zr–1.0Na catalyst exhibited the highest light olefin selectivity of 35.8% in the hydrocarbon distribution at a CO conversion of 95.2%.

1. Introduction

Light olefins (C2= ∼ C4=) are the important organic chemical intermediates that have been widely used to synthesize plastics, solvents, synthetic rubbers, and so on.1 The light olefins are traditionally obtained from the thermal cracking of naphtha. Taking the rapidly growing demand for light olefins and the depleted petroleum resources into account, the high-temperature Fischer–Tropsch synthesis (HTFT) is considered as a promising process; this process directly converts the syngas to light olefins and has the advantages of energy conservation, sufficient feedstocks, and simple operation.2 In comparison with the cobalt-based catalyst, the iron-based catalyst has the characteristics of low price, wide operation conditions, and the high selectivity of light olefins.3

Numerous metals have been investigated as promoters to improve the selectivity of light olefins, such as alkali metals,4−6 alkaline earth metals,7,8 rare earth metals,9,10 manganese,11,12 and zinc.13 Li et al.2 reported that Na-doped Co–Mn catalysts could stimulate the formation of the Co2C; the Co2C nanoprisms dramatically enhanced the selectivity of light olefins and were regarded as the active site for Fischer–Tropsch to olefin (FTO). Li et al.14 discussed the diffusion of alkali metals in the bulk of catalysts. Alkali metals accumulated on the surface of the catalyst after reduction, especially for Na and Li. An et al.15 proposed that the residual sodium had a negative effect on the Fe/Cu/K/SiO2 catalyst, resulting from the severe aggregation of Fe2O3 and inhibitive carburization. Zhai et al.16 also researched the effect of residual sodium on the Fe–Zn catalyst. They found that the Na promoter distributed differently on iron carbide and zinc oxide. Na-modified Fe5C2 could weaken the adsorption of olefins to inhibit the hydrogenation of olefins, increasing the olefin/paraffin ratio. Alkali metals have a significant influence on FT reaction performance, but the role of Na in the formation of the active phase is controversial.

The structure of the precipitated iron catalyst would be fragmented due to the change in the iron phase during the Fischer–Tropsch synthesis.17 Owing to the strong interactions between zirconium and iron, the Zr-promoted iron-based catalyst could exhibit the stable activity of FTS.18 Moreover, Zhang et al.19 found that the zirconium promoter could suppress the hydrogenation of olefins and inhibit the chain growth ability. Ma et al.20 discussed that Zr could decrease the crystal size, promote dispersion of α-Fe2O3, and provide abundant surface defects of catalysts. Cho et al.21 prepared the mesoporous Fe–Zr mixed oxide catalysts using KIT-6 as a hard template. The results showed that the strong interaction between iron nanoparticles and ZrO2 structural additives led to the formation of stable χ-Fe5C2. Besides, the stable mesoporous structures improved the diffusion rate of the heavy hydrocarbons, which decreased the coke deposition and maintained the catalytic activity.

Herein, a series of Fe–Zr catalysts with different Na loadings were synthesized by coprecipitation and impregnation methods. The role of Na on the Fe–Zr catalyst in Fischer–Tropsch synthesis was discussed. The catalysts were characterized by Ar adsorption–desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-programmed desorption (H2-TPD, CO-TPD), X-ray photoelectron spectra (XPS), and Mössbauer spectroscopy (MES).

2. Results and Discussion

2.1. Structure and Morphology of the Catalysts

The Ar-physisorption isotherms of the Fe–Zr–xNa catalysts are presented in Figure S1. All the catalysts exhibit the type IV isotherms that indicate the mesoporous materials.22 The BET surface area (SBET), average pore size (Dp), total pore volume (Vp), composition, and crystallite size (dFe2O3) of the fresh catalysts are listed in Table 1. The BET surface area decreases from 59.5 to 21.9 m2·g–1, while the average pore size increases from 12.3 to 37.4 nm with increasing the Na loading from 0 to 1.5 wt %. The total pore volume of the fresh catalyst is seldom influenced by the Na loadings, suggesting that the catalyst pores might not be blocked.23 The BET surface area of the Fe–Zr–1.0Na sample is approximately twice that of the Fe–1.0Na sample. The hematite crystallite size of the Fe–Zr–1.0Na sample is 27.9 nm and is smaller than that of the Fe–1.0Na sample, which proves that zirconium is a suitable structural-type promoter that could segregate the hematite crystallite and prohibit the growth of the hematite crystallite.

Table 1. Textural Property of the Fresh Catalysts.

catalyst Na loadinga (wt %) SBET (m2·g–1) Vp (cm3·g–1) Dp (nm) dFe2O3b (nm)
Fe–Zr–0Na   59.5 0.25 12.3 22.9
Fe–Zr–0.25Na 0.23 35.6 0.26 20.3 23.8
Fe–Zr–0.5Na 0.50 32.2 0.26 21.9 25.0
Fe–Zr–1.0Na 0.92 24.8 0.23 27.5 27.9
Fe–Zr–1.5Na 1.50 21.9 0.24 37.4 37.0
Fe–1.0Na 0.95 12.3 0.15 41.9 47.0
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a

Determined by ICP–AES.

b

Average crystallite size estimated by the Scherrer equation.

The XRD patterns of the fresh samples after calcination are displayed in Figure 1A. It clearly illustrates that the main phase in all the fresh catalysts is the hematite (2θ values of 24.1, 33.0, 35.6, 40.9, 49.5, 54.0, 57.6, 62.4, and 64.1°).24 With the modification of sodium, the diffraction peaks at 2θ values of 30.3, 50.8, and 60.2° appear on the Fe–Zr–1.5Na catalyst, which could be ascribed to the tetragonal ZrO2 phase (PDF#50-1089).25 The increase in Na loading destroys the structure of the Fe–Zr solid solution and weakens the role of Zr in dispersing the hematite crystallite, resulting in the growth of the hematite crystallite. There are no diffraction peaks for the sodium phase in all the XRD patterns, which could result from the low concentration of the Na atom on the catalyst.26

Figure 1.

Figure 1

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XRD patterns of the catalysts: (A) fresh catalyst and (B) after the reaction.

The XRD patterns of the spent catalysts are depicted in Figure 1B. The diffraction peaks at 30.0, 35.5, 43.0, 53.5, 56.9, and 62.4° could be confirmed as Fe3O4 (PDF#19-0629). The diffraction peaks at 39.4, 41.0, and 44.0° are on behalf of χ-Fe5C2 (PDF#51-0997),27 which is generally considered as the active phase in the iron-based FTS.28 With increasing Na loading, the diffraction peaks of iron carbide at 39.4 and 41.0° emerge. The ascent of the diffraction peak intensity at 44.0° and descent of the diffraction peak intensity located at 35.5° indicate that the content of χ-Fe5C2 increases while the amount of magnetite decreases on the Na-promoted catalysts after the FT synthesis with the increase in Na loading. The results uncover that the sodium additive could facilitate the carbonization of the iron phase. However, considering the low peak intensity of carbide species and the limited precision of the XRD quantitative analysis, the variation of carbide species content on Fe–Zr–xNa catalysts could not be accurately obtained from XRD results. Therefore, iron phase compositions after reaction were accurately quantified by Mössbauer spectroscopy.

The morphologies of the Fe–Zr–xNa catalysts are described in Figure 2. The catalysts are composed of approximately spherical nanoparticles. Combined with the results of Ar physisorption, the Na-free and Na-promoted catalysts possess the mesopores and intergranular pores, which is beneficial to the improvement of catalytic activity with the merits of low mass transfer resistance and inhibition of the coke deposition.21 With the modification of sodium, the Fe–Zr–1.0Na catalyst has a wider particle size distribution, and the average particle size measured by SEM increases. It has been reported that the Na promoter could easily migrate on the catalyst, resulting from the lower melting point and atomic radius than iron oxide,14 which might cause the severe aggregation of hematite on the Na-promoted samples. Besides, the particle size of the Fe–1.0Na catalyst sharply increases, compared with the Fe–Zr–1.0Na catalyst. It further implies that zirconium could restrict the aggregation of hematite grain and disperse the hematite crystallite, leading to a significant increase in the BET surface area on the Fe–Zr–1.0Na catalyst.

Figure 2.

Figure 2

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SEM images of (A) Fe–Zr–0Na, (B) Fe–Zr–1.0Na, and (C) Fe–1.0Na.

2.2. Chemisorption Properties of Catalysts

CO-TPD profiles of the reduced catalysts are shown in Figure 3. There are three distinct desorption peaks of CO on the Na-free sample, which represent the low-temperature sites (about 100 °C), medium-temperature sites (about 180 °C), and high-temperature sites (above 300 °C). The low and medium desorption peaks could be considered as the molecular CO desorption, while the high-temperature desorption peak could be considered as the desorption of dissociative CO species.14,29 With increasing Na content, the medium-temperature desorption peak dramatically decreases; the dissociative adsorption peak moves toward the higher temperature and the peak area significantly increases, revealing the promoting effect of Na on the dissociative adsorption of CO. Na additive could alter the CO adsorption sites and enhance the CO bond cleavage on the iron phase. Therefore, Na-promoted catalysts could provide more active sites for the dissociation of CO, leading to the increase in carbon species concentration on the catalyst surface; it could be ascribed to the reinforcement of the Fe–C bond influenced by sodium as an electron contributor.10,14 The hydrocarbons can be formed by the polymerization of −CH2– intermediates that are the recombination of dissociative H and C atoms according to the surface carbide mechanism,30 the increase in CO dissociative adsorption would improve the catalytic activity.

Figure 3.

Figure 3

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CO-TPD profiles of the reduced catalysts.

Figure 4 describes the H2-TPD profiles of the reduced catalysts. The Na-free sample has three desorption peaks, implying three different kinds of adsorption sites on the reduced catalyst surface. The desorption peak at about 100 °C (Hα) could account for the H species weakly adsorbed on the shallow hole or on the top of the catalysts to form the weak Fe–H bond. The desorption peak at about 200 °C (Hβ) might be considered as the desorption of H species from the deep hollow or the defect on the surface of the metal iron. Moreover, the desorption peak at above 300 °C (Hγ) was probably assigned to the split of OH species on the surface of unreduced oxides.31,32

Figure 4.

Figure 4

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H2-TPD profiles of the reduced catalysts.

The adsorbed H species of Hα would be more reactive than that of Hβ, resulting from the lower desorption temperature of Hα.33 The location of the Hα desorption peak is less influenced by Na loading, while the Hβ desorption peak moves to low temperature with the increase in Na loading. The desorption peak areas of both Hα and Hβ significantly decrease with increasing sodium content. The results show that the increase in Na loading leads to the decrease in active sites for H species adsorption and weakens the Fe–H bond energy on the metallic iron surface. The decrease in H species concentration on the catalyst might hinder the hydrogenation of the olefins, resulting in the higher selectivity of olefins.

The desorption peak of Hγ shifts toward low temperature, and the corresponding hydrogen adsorption amount decreases dramatically with increasing Na loading. Hγ could be on behalf of the cleavage of OH on the catalyst surface, such as on the unreduced ZrO2.34 The inhibiting effect of the Na additive on the H species adsorption results from the increase in electron density of metallic iron and the decrease in electron vacancies on the catalyst surface, as the alkali metals are the electron donators.35

2.3. Electronic Effect

Figure 5 depicts the XPS spectra of the reduced Fe–Zr–xNa catalysts. As shown in Figure 5A, the peaks located at binding energies of 711.1 and 724.8 eV represent the typical peaks of Fe 2p3/2 and Fe 2p1/2, respectively. Besides, the peaks observed at 707.2 and 719.8 eV might be liable for Fe0 2p3/2 and Fe0 2p1/2, accounting for the iron phase transformation during the reduction process.36,37

Figure 5.

Figure 5

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XPS spectra of the reduced Fe–Zr–xNa catalysts: (A) Fe 2p and (B) Zr 3d.

The change in the binding energy results from the variation of the electron cloud density of the central atom.38 The Na-promoted samples have the lower binding energy of Fe 2p, which may be ascribed to the higher electronegativity of Fe (1.83) in comparison with Na (0.93). Iron atoms can deprive Na atoms of electrons, increasing the electron cloud density of iron atoms. Therefore, sodium could be regarded as the electron contributor, and the electron-enriched iron species could enhance the dissociation of CO and inhibit H2 adsorption, as verified by CO-TPD coupled with H2-TPD.5,35

The Zr 3d XPS spectra are shown in Figure 5B. For the Fe–Zr–0Na sample, the location of Zr 3d5/2 at 182.1 eV suggests that Zr4+ in the ZrO2 is the primary form on the catalyst surface,39 indicating that ZrO2 is quite stable and is difficult to reduce.20 With the addition of sodium, the binding energy of Zr 3d5/2 decreases, owing to the charge transfer from Na to Zr, which might change the Fe–Zr interaction and destroy the Fe–Zr solid solution, as determined by XRD.

2.4. Iron Species of Catalysts after the Reaction

The iron species of the spent catalysts are analyzed by the Mössbauer spectra. As shown in Figure S2, the MES spectra could be divided into a central doublet and five sextets. Table 2 summarizes the corresponding Mössbauer spectra parameters. The sextets with Hhf values of approximately 491 and 459 kOe could represent the tetrahedral site (A site) and octahedral site (B site) in Fe3O4, respectively. The Hhf values located on about 217, 183, and 105 kOe could be ascribed to χ-Fe5C2.5 The iron phase compositions of Fe–Zr–xNa catalysts after reaction are mainly Fe3O4 and χ-Fe5C2, which agrees with the results obtained from XRD.

Table 2. Mössbauer Spectra Parameters of the Spent Catalysta.

    Mössbauer parameters
catalyst phase Hhf (kOe) IS (mm/s) QS (mm/s) area (%)
Fe–Zr–0Na Fe3+ (spm)   0.34 1.04 4.8
  Fe3O4 (A) 491.47 0.30 –0.03 33.5
  Fe3O4 (B) 459.79 0.66 0.01 42.9
  χ-Fe5C2 217.29 0.23 –0.16 6.9
    183.62 0.18 –0.03 8.9
    105.70 0.14 –0.02 3.0
Fe–Zr–0.25Na Fe3+ (spm)   0.28 1.14 3.3
  Fe3O4 (A) 490.92 0.30 –0.03 30.5
  Fe3O4 (B) 459.49 0.66 0.02 41.6
  χ-Fe5C2 216.73 0.25 –0.13 10.2
    183.34 0.23 0.04 10.5
    106.72 0.14 0.09 3.9
Fe–Zr–0.5Na Fe3+ (spm)   0.35 1.09 3.9
  Fe3O4 (A) 490.68 0.30 –0.03 22.4
  Fe3O4 (B) 459.28 0.66 0.02 36.5
  χ-Fe5C2 216.61 0.24 –0.14 13.0
    183.80 0.21 0.04 17.6
    104.97 0.09 0.01 6.6
Fe–Zr–1.0Na Fe3+ (spm)   0.34 1.08 3.8
  Fe3O4 (A) 490.22 0.30 –0.02 19.3
  Fe3O4 (B) 458.57 0.66 0.02 27.3
  χ-Fe5C2 216.62 0.26 –0.11 17.7
    182.71 0.18 –0.05 21.2
    107.52 0.23 –0.07 10.7
Fe–Zr–1.5Na Fe3+ (spm)   0.18 0.94 4.2
  Fe3O4 (A) 487.60 0.30 –0.04 4.4
  Fe3O4 (B) 455.96 0.71 0.09 8.5
  χ-Fe5C2 215.61 0.27 –0.10 21.6
    175.13 0.26 –0.03 45.8
    101.41 0.12 –0.04 15.5
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a

Reaction condition: 320 °C, 1.5 MPa, H2/CO = 2, 10,000 mL/(h·g Cat), and 48 h.

With the increase in Na loading from 0 to 1.5 wt %, the content of magnetite in the iron species decreases from 76.4 to 12.9%, while the amount of χ-Fe5C2 increases from 18.8 to 82.9%; it further confirms that the sodium promotes the formation of iron carbide. As determined by XPS and CO-TPD results, the sodium promoter could donate the electrons to iron atoms, resulting in the enhancement of the Fe–C bond and the increase in CO dissociative adsorption, which facilitates the carbonization of iron species. The reoxidation of reduced iron phases may take place under the hydrothermal condition, and the content of iron carbides and iron oxides could reach a balanced state during the Fischer–Tropsch synthesis.40,41 However, alkali promoters protect iron species against oxidation by the water.42 XPS results manifest that the sodium promoter could strongly interact with iron species to form electron-enriched iron species, which might prohibit the oxidation of iron carbide.43 Therefore, Na could favor the facile formation of χ-Fe5C2 and avoid the oxidation of the active phase, implying that sodium would promote the FT reaction activity.

2.5. Catalytic Performance

Fe–Zr–xNa samples were used to test the HTFT synthesis performance. The CO conversion and hydrocarbon distributions of the samples are displayed in Table 3. The CO conversion of the Na-free sample is only 80.6%, while the Na-promoted catalysts exhibit a high CO conversion ranging from 94.7 to 98.1%. The increase in CO conversion on the Na-promoted catalysts could be ascribed to the enhancement of CO chemisorption and facile formation of iron carbide, as verified by CO-TPD and MES. Na could increase the CO dissociative adsorption sites and facilitate the formation of the χ-Fe5C2, which is the main active phase for the FT reaction.44

Table 3. Activity and Selectivity of the Catalystsa.

        hydrocarbon distribution (% C)
   mass balance (%)
catalyst XCO (%) XH2 (%) SCO2 (%) CH4 C2=–C4= C20–C40 C5+ O/Pb C
Fe–Zr–0Na 80.6 50.7 37.8 24.7 22.1 22.5 30.7 0.98 96.3
Fe–Zr–0.25Na 96.1 54.6 37.7 26.6 20.8 22.9 29.7 0.91 96.7
Fe–Zr–0.5Na 98.1 58.2 34.4 22.7 26.8 15.2 35.3 1.76 97.2
Fe–Zr–1.0Na 95.2 50.2 36.5 13.5 35.8 6.3 44.4 5.68 96.4
                   
Fe–Zr–1.5Na 94.7 51.6 37.4 12.4 31.5 5.0 51.1 6.30 97.0
Fe–1.0Na 40.9 30.1 34.9 15.7 32.3 6.2 45.8 5.21 95.5
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a

Reaction condition: 320 °C, 1.5 MPa, H2/CO = 2, 10,000 mL/(h·g Cat), and 48 h.

b

The molar ratio of the alkenes to alkanes in the C2–C4 hydrocarbons.

The FT synthesis could be reasonably considered as a surface-catalyzed process, including chain initiation, propagation, and termination; the chain termination is completed by the hydrogenation of intermediates to form alkanes or the dehydrogenation of intermediates to form alkenes. With increasing the Na loading from 0.25 to 1.5%, the selectivity of CH4 rapidly decreases from 26.6 to 12.4% and the selectivity of heavy hydrocarbons (C5+) expresses an opposite tendency to methane. Besides, the increase in the O/P ratio from 0.91 to 6.30 implies that Na could inhibit the secondary hydrogenation reaction. As depicted in Figure S3, the increase in the chain growth probability factor (α) from 0.65 to 0.76 further suggests that Na could enhance the chain propagation. The competitive chemisorption between hydrogen and CO on the catalyst surface might contribute to this phenomenon.30 Na significantly alters the active sites for the dissociative adsorption of CO and hydrogen, leading to the enhanced CO dissociative adsorption and weakened hydrogen adsorption on the catalyst surface with the Na loading increasing from 0.25 to 1.5%, as determined by H2-TPD and CO-TPD; this might prohibit the hydrogenation of intermediates and promote the chain propagation.23 Na could also facilitate the carbonization of iron species, which might be responsible for the enhancement of C–C coupling. Moreover, XPS results show that Na stimulates the formation of electron-enriched iron species. The electron-enriched iron species could inhibit the hydrogenation of olefins and sharply increase the O/P ratio in the product, resulting from the weak adsorption of olefins on the catalyst surface.16

As discussed above, the formation of light olefins is deeply influenced by the Na promoter, which could inhibit the secondary hydrogenation reaction and enhance the chain propagation. The inhibitive effect of Na on hydrogenation is beneficial for the product of light olefins, while the chain propagation has a negative effect on the formation of light olefins. Therefore, it is feasible to obtain the optimal selectivity of light olefins by controlling the amount of Na loading, which could adjust the Fe–Na interaction. The weak Fe–Na interaction would be in favor of the formation of methane and paraffins, while the strong Fe–Na interaction would increase the selectivity of heavy hydrocarbons. At an optimal Na loading of 1.0 wt %, the Fe–Zr–1.0Na catalyst exhibits the highest light olefin selectivity.

Compared with the Fe–Zr–1.0Na sample, the Fe–1.0Na sample shows the lower FTS activity. The XPS and Ar physisorption results reveal that zirconium is the stable structure promoter that could hinder the growth of hematite crystallites and significantly increase the specific surface area, which might stabilize the active phases and provide more active sites for FT reaction. Therefore, the synergetic effect between zirconium and sodium leads to higher FT activity and the optimal selectivity of light olefins in the hydrocarbon distribution on the Fe–Zr–1.0Na catalyst.

The stability of the Fe–Zr–1.0Na catalyst is depicted in Figure S4. The CO conversion of the Fe–Zr–1.0Na catalyst decreases with the longer time on stream. The TEM images of the spent Fe–Zr–1.0Na catalyst are shown in Figure S5. After a 48 h reaction, the particle sizes of the spent Fe–Zr–1.0Na catalyst are similar to that of the fresh catalyst. The Fe–Zr–1.0Na catalyst shows superior mechanical stability. The deactivation of the Na-promoted catalyst might be attributed to the carbon deposition.45

3. Conclusions

In this paper, the effects of promoters on the structure, chemisorption characteristics, and catalytic performance of HTFT reaction were studied. The Na-promoted samples have a larger average pore size, improving the diffusion of reactants. Na is the electron promoter that possesses strong interaction with Fe, showing the electron transfer from Na to Fe. The electron-rich iron species alter the active sites for the dissociative adsorption of CO and hydrogen, resulting in the increase in CO dissociative adsorption and decrease in hydrogen adsorption on the catalyst surface. Therefore, the H/C ratio on the catalyst surface declines with increasing Na loading, resulting in the enhanced chain propagation and a decrease in the hydrogenation of light olefins. Moreover, Na could facilitate the carbonization of catalysts and protect the iron carbide against oxidation, which provides more active sites for HTFT reaction and is in favor of the C–C coupling. Zr is the stable structure promoter that could prohibit the growth of hematite crystallites and significantly increase the specific surface area, which might stabilize the active phases and provide more active sites for HTFT reaction. At an optimal Na loading of 1.0 wt %, the Fe–Zr–1.0Na catalyst exhibited the highest light olefin selectivity of 35.8% in the hydrocarbon distribution at a CO conversion of 95.2%.

4. Experimental Section

4.1. Preparation of the Catalyst

Na-doped Fe–Zr catalysts were prepared by coprecipitation and impregnation methods. Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 98.5%, Macklin), zirconium(IV) nitrate pentahydrate (Zr(NO3)4·5H2O, 98%, Adamas-beta), and sodium carbonate anhydrous (Na2CO3, 99.9%, Macklin) were used as metal sources. Ammonia solution (25–28%, AR, Adamas-beta) was used as the precipitant. The mixed metal salt solution (molar ratio Fe/Zr = 100/15) and alkali solution were added drop by drop into a beaker under continuous stirring. The pH of the slurry was controlled at 9.0 ± 0.1 and the temperature was maintained at 70 °C during the precipitation process. After being aged for 4 h at 70 °C, the precursor was purified with deionized water, subsequently dried at 110 °C for 12 h, and calcined at 500 °C for 4 h.

Na-doped Fe–Zr catalysts were synthesized by the incipient wetness impregnation method. The Fe–Zr precursor was impregnated with Na2CO3 aqueous solutions of desired concentrations. Then, the samples were aged at room temperature for 24 h, dried at 110 °C for 12 h, and calcined at 500 °C for 4 h. The samples were signed as Fe–Zr–xNa (x = 0, 0.25, 0.5, 1.0, and 1.5), where x is the mass fraction (%) of Na. The Fe–xNa catalyst was obtained in the same way without the addition of Zr. Finally, the catalysts were sieved to desired mesh particles for catalytic tests.

4.2. Catalyst Characterization

Ar physisorption was performed at 87 K on a Micromeritics ASAP 2020. The sample was degassed in the vacuum before analysis. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The cumulative pore volume and average pore diameter were obtained by the Barrett–Joyner–Halenda (BJH) procedure.

Inductively coupled plasma atomic emission spectroscopy (ICP–AES, Agilent 725) was used to determine the compositions of samples.

The XRD patterns were obtained through a rotating anode X-ray powder diffractometer (D/MAX2550 VB/PC). The samples were scanned in the 2θ ranging from 10 to 80°.

The SEM images were obtained by field emission SEM (ZEISS Sigma 300). The TEM images were obtained by JEM-2100F microscopy.

The CO-TPD and H2-TPD experiments were performed on the chemisorption analyzer (Micromeritics AutoChem II2920). The catalyst (0.2 g) was reduced with H2 at 350 °C for 5 h before the analysis. Then, the catalyst was in touch with 5% CO/95% He or 10% H2/90% Ar flow for 30 min at 60 °C. Next, the catalyst was blown with He or Ar flow to obtain a stable baseline. Then, the tail gas was analyzed using a thermal conductivity detector (TCD) with simultaneously increasing the temperature from 60 to 800 °C (β = 10 °C/min).

X-ray photoelectron spectroscopy (XPS) was performed using the spectrometer (Thermo ESCSLAB 250Xi, USA) equipped with Al Kα radiation (hv = 1486.6 eV, 150 W).

Mössbauer spectroscopy of the spent catalysts was performed on the Mössbauer spectrometer (Wissel, Germany) at room temperature using the 57Co (Pd) source. The spectra were obtained over 512 channels in the mirror image format. The spectral data were fitted by the nonlinear least squares method. The parameters of isomer shift (IS), quadruple splitting (QS), and magnetic hyperfine field (Hhf) were used to distinguish iron species.

4.3. Catalyst Activity Evaluation

The HTFT reactions were evaluated in a fixed-bed reactor with an 8 mm inner diameter. About 0.3 g of the catalyst (40–60 mesh) mixed with 1.0 g of SiO2 (40–60 mesh, Macklin) was placed in the isothermal area of the fixed-bed reactor. Before the reaction, the catalyst was reduced with H2 at ambient pressure (350 °C, 5000 mL/(h·gCat), 5 h). The reaction condition is at 320 °C, 1.5 MPa, CO/H2/N2 = 30/60/10, and 10,000 mL/(h·gCat). The tail gas flowed through a hot trap (180 °C, 1.5 MPa) and a cold trap (0 °C, 1.5 MPa) to separate water, oil, and wax. Then, the uncondensed gaseous products were online analyzed by gas chromatography (GC Agilent 7890A). After the reaction was stable for 48 h, the wax, water, and oil phases were collected and analyzed by gas chromatography. The CO conversion (XCO), CO2 selectivity (SCO2), and hydrocarbon distribution (SCiHj) of the hydrocarbons were calculated as follows.

4.3. 1
4.3. 2
4.3. 3

where Nin and Nout are the molar flow rates of the specific species in the inlet and outlet of the reactor, respectively, and the CiHj represented the hydrocarbons with the carbon number of i.

Acknowledgments

The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (grant no. 21706068), the National High Technology Research and Development Plan of China (863 plan, 2011AA05A204), and the Fundamental Research Funds for the Central Universities (no. 50321012017013).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c06008.

  • Ar-physisorption isotherms of the Fe–Zr–xNa catalysts; Mössbauer spectra of Fe–Zr–xNa catalysts after the reaction; product distributions for Fe–Zr–xNa catalysts; stability of the Fe–Zr–1.0Na catalyst with time on stream; and TEM images of the Fe–Zr–1.0Na catalyst (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c06008_si_001.pdf (840.8KB, pdf)

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