Enhanced Conversion of Syngas to Gasoline-Range Hydrocarbons

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Enhanced Conversion of Syngas to Gasoline-Range Hydrocarbons over Carbon Encapsulated Bimetallic FeMn Nanoparticles Guangyuan Ma, Xianzhou Wang, Yanfei Xu, Qiong Wang, Jie Wang, Jianghui Lin, Hongtao Wang, Chenglong Dong, Chenghua Zhang, and Mingyue Ding ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00932 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Energy Materials

Enhanced Conversion of Syngas to Gasoline-Range Hydrocarbons over Carbon Encapsulated Bimetallic FeMn Nanoparticles Guangyuan Ma[a]‡, Xianzhou Wang[b,c]‡, Yanfei Xu[a], Qiong Wang[a], Jie Wang[a], Jianghui Lin[a], Hongtao Wang[a], Chenglong Dong[a], Chenghua Zhang[b,d]*, Mingyue Ding[a,e]* a

School of Power and Mechanical Engineering, Hubei International Scientiﬁc and Technological

Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan 430072, China b

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

Sciences, Taiyuan 030001, China c

d

University of Chinese Academy of Sciences, Beijing 100049, China National Energy Center for Coal to Liquids, Synfuels China Co., Ltd., Huairou District,

Beijing, 101400, China e

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai

University, Tianjin 300071, China

ACS Paragon Plus Environment

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KEYWORDS: FeMn@C core-shell catalyst; Fischer-Tropsch synthesis; confinement effect; gasoline range products; carbon encapsulation; manganese promoter; synthesis gas

ABSTRACT: Adjusting the hydrocarbons product distribution is the focus of Fischer-Tropsch synthesis (FTS) reaction. A novel FeMn@C core-shell catalyst with bimetallic FeMn nanoparticles encapsulated in carbon shell was synthesized by one-step solvothermal method. In the FTS reaction, this catalyst exhibited higher catalytic activity and C5+ hydrocarbons (especially for 63.3% C5-12 gasoline range products) selectivity as well as excellent stability compared to the Fe@C or traditional FeMn/SiO2 catalyst. The superior activity could be attributed to the enrichment of Fe elements at the margin stemmed from the replacement of central Mn species, promoting the formation of more active iron carbides by combining with the surrounding carbonaceous matter. Particularly, “confinement effect” of the core-shell structure for FeMn@C facilitated the polymerization of light olefins produced by Mn-modified Fe nanoparticles, resulting in more heavy hydrocarbons and enhanced stability. This work showed new insights to develop a potential FTS catalyst candidate.

1. INTRODUCTION Growing concern on alternative processes for producing liquid fuels and chemicals has been spurred with rapid depletion of crude oil 1, in which Fischer-Tropsch synthesis (FTS) technology used for converting syngas (CO + H2) derived from coal, biomass or natural gas into fuel hydrocarbons and commodity chemicals, is receiving increasing interest for both academic and industrial applications

2,3

. Generally, FTS products are followed the Anderson-Schulz-Flory

(ASF) distribution, which are typically straight chain hydrocarbons. Therefore, designing novel catalysts and new strategies to break the ASF distribution become more and more important 4,5.


2

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Compared to other Co, Ru and Ni active metals for Fischer-Tropsch synthesis, the Fe-based catalyst is more attractive for FTS with a lower H2/CO ratio of syngas derived from biomass and/or coal because of its superior water-gas shift (WGS) activity, lower cost and flexible product distribution

6,7

. In order to improve the selectivity of target products, numerous

promoters including potassium 8, copper 9, sodium

10

, manganese

11

etc. are widely used to

optimize the spatial structure and electronic characters of active iron species. Especially, Mn promoter is often added into Fe-based catalysts for modifying the selectivity of C2-4 olefins 12 or C5+ heavy hydrocarbons

13

via its structural and electronic characteristics. In addition, a smaller

active Fe nanoparticles size may improve the FTS catalytic activity and target products selectivity 14,15, whereas these nanoparticles are easily gathered by high surface energy, resulting in the rapid deactivation for FTS

16

. Supported metal catalysts can effectively inhibit metal

nanoparticles aggregation by strengthening the metal-support interaction 17, in which metal oxide supports including SiO2, Al2O3, TiO2, and so on are adopted widely to stabilize and disperse Fe nanoparticles

18,19

. However, high dispersion of iron nanoparticles on the surface of oxide

supports results in a strong metal-support interaction, which easily suppresses the iron species reduction and formation of active iron sites, decreasing the FTS catalytic activity 20. Carbon-based material as an excellent support for utilization in the catalysis field has been attracted increasing attention due to its adjustable structure and weak interaction of metal-support. Numerous carbon-based materials including activated carbon (AC), carbon nanotubes (CNTs), carbon nanofibers (CNF), carbon spheres etc. have been used as supports for iron-based FTS catalysts

21,22

, which present a higher catalytic activity and a higher C5+

hydrocarbons selectivity via well dispersion and optimized reducibility of active iron species. However, high-temperature carburization of iron species dispersed on carbon supports results in


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a broad distribution of iron nanoparticles size. Moreover, in order to obtain functionalized groups for anchoring metal species, pretreatment in acid or base solutions for these carbon-based materials is necessary, which brings seriously environmental pollution 23. Hydrothermal carbonization of biomass-derived compounds such as glucose, sucrose and starch may prepare carbonaceous spheres, which provides an environmentally friendly method to produce carbon materials

24,25

. Several important advantages offered by carbonaceous spheres

compared to conventional carbon materials include core-shell structure, tunable nano-sizes and rich surface O-H and C=O groups, which play a critical role in weakening the metal-support interaction and suppressing the agglomeration of metal nanoparticles. To date, carbon encapsulated single iron nanoparticles (FexOy, Fe3O4 etc.) have been designed and applied in the FTS field, which display excellent C5+ hydrocarbons selectivity and good stability

7,26

. Mn

promoter as introduced above is currently utilized to optimize the FTS performance of traditional iron-based catalysts, whereas bimetallic FeMn nanoparticles encapsulated in carbonaceous spheres have never been reported so far. Herein, we reported for the first time a novel FeMn@C core-shell catalyst with bimetallic FeMn nanoparticles encapsulated in carbonaceous spheres, which showed high catalytic activity and superior selectivity of gasoline range hydrocarbons in the FTS reaction. More importantly, this catalyst presented excellent stability with time on stream. Therefore, this optimum catalyst for FTS shows good potential in practically industrial applications. 2. EXPERIMENTAL SECTION 2.1 Catalysts preparation


4

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2.1.1 Synthesis of Fe3O4 nanoparticles The Fe3O4 nanoparticles were synthesized by a hydrothermal process. FeCl3·6H2O (2.70 g) was dissolved in ethylene glycol (80 mL) to form a clear solution. Under vigorous stirring, urea (6 g) was added to the solution. The mixture was vigorously stirred for 30 min, and then transferred into a Teflon-lined stainless-steel autoclave. After being heated at 200 °C for 12 h, the sample was collected and washed three times with deionized water and ethanol, and then dried finally at 60 °C for 12 h. 2.1.2. Synthesis of FeMnx@C carbon encapsulated spheres The FeMnx@C catalyst was synthesized by a hydrothermal process. FeCl3·6H2O and MnCl2·4H2O were dissolved in ethylene glycol (80 mL) to form a clear solution. The quantities of dissolved Mn precursor in the solution were adjusted to achieve various molar ratios of Mn to Fe (x = 0, 0.1, 0.5, 1.0, 1.5). Under vigorous stirring, urea (6 g) and glucose (0.4 g) were added into the solution. The mixture was vigorously stirred for 30 min, and then transferred into a Teflon-lined stainless-steel autoclave and heated at 200 °C for 12 h. The black samples were collected and washed three times with deionized water and ethanol, and then dried finally at 60 °C for 12 h. 2.1.3 Synthesis of FeMn/SiO2 catalyst The traditional FeMn/SiO2 catalyst was prepared by a coprecipitation method. FeCl3·6H2O and MnCl2·4H2O (molar ratio = 10:1) were mixed by using an ammonium solution as a precipitation reagent to form a solution. After precipitation, the precipitate was filtered and washed with deionized water and ethanol. Subsequently, the sample was remixed with silica sol in vigorous


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stirring for 4 h, and then filtered and washed with deionized water and ethanol for three times. After being dried at 80°C for 12 h, the sample was calcined at 500 °C for 5 h. 2.2 Catalyst characterization The elemental concentrations in the calcined catalysts were determined by inductively coupled plasma optical emission spectroscopy (ICPOES) using an Atom-scan 16 spectrometer (TJA, USA). Textural properties of the samples were measured by N2 physisorption at -196 °C using a Micromeritics ASAP 2420 instrument. The samples were degassed under vacuum at 200 °C for 4 h prior to measurement. The specific surface area of the catalysts was calculated by Brunauer-Emmett-Teller (BET) method and pore size distribution was analyzed by Barrett-Joyner-Halenda (BJH) method. The scanning electron microscopy (SEM) images were obtained on a QUANTA 400 scanning electron microscope to characterize the surface morphology of the catalysts. Energy dispersive X-ray spectroscopy (EDX) was performed on a GENESIS EDX detector. High resolution transmission electron microscopy (HRTEM) experiments were performed in a Philips CM200 high resolution transmission electronic microscopy with 200 kV accelerating voltage. Powder X-ray diffraction (XRD) was performed with a PANalytical X’Pert Pro diffractometer with Cu Kα radiation operated at 40 kV and 100 mA. XPS spectra of the catalysts were checked by Thermo Scientific K-alpha XPS system, using an Al Ka X-ray source (1486.6 eV). The C 1s as a reference signal was adjusted to 284.6 eV. Raman spectra of the samples were measured using a single monochromator renishaw system 1000 equipped with a thermoelectrically cooled CCD detector and holographic super-notch filter. Infrared Fourier transform spectroscopy were collected using an infrared spectrometer (Thermo NICOLET 5700 FTIR Spectrometer), in the wavenumber range 4000-400 cm-1 with KBr pellets. H2-TPR measurements were performed in a quartz tube equipped with a thermal conductivity


6

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detector (TCD). The samples were pretreated with high purity N2 at 350 °C and a flow rate of 30 mL/min for 1.5 h to remove water and other contaminants. The samples were cooled down to room temperature, and then 5% H2/N2 was introduced into the system at a flow rate of 30 mL/min. The TCD signal and sample temperature were recorded while the temperature being ramped to 800 °C at a heating rate of 10 °C/min (held at 800 °C for 30 min). 2.3 Catalytic evaluation The catalytic tests were conducted in a fixed-bed reactor. Typically, 0.5 g of catalyst (40-60 mesh) diluted with an equivalent amount of quartz sand with the same size were loaded into the reactor. The catalyst was reduced in H2 atmosphere at 350 °C, 0.1 MPa and 2000 h-1 for 10 h. After on-line activation described above and cooling to 200 °C, syngas (H2 (47.5 %)/ CO (47.5 %)/ N2 (5%), vol) was introduced into the reactor, followed by a rise of reaction temperature with a rate of 1 °C/min. The reaction was carried out at the desired temperature, gas hourly space velocity (GHSV) and pressure. The operation conditions were as follows: temperature of 300 360 °C, GHSV of 5000 - 15000 h-1 and pressure of 1.0 - 4.0 MPa. Reaction products were separated by a hot trap (kept at 100 °C) and a cold trap (kept at 1 °C). Gaseous products were detected online by a gas chromatograph (FULI GC 97) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Porapak Q and 5A MolSieve packed column were connected to TCD, while RB-PLOT Al2O3 capillary column was connected to FID. Liquid products were detected on a gas chromatograph (FULI GC 97) with a FID using RB-5 capillary column. CO conversion and product selectivity were calculated on a carbon-atom basis.


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3. RESULTS AND DISCUSSION A novel carbon-encapsulated Fe-Mn nanocatalyst was prepared by simple one-step solvothermal method without calcination, as illustrated in Scheme 1. As ferric chloride and urea were dissolved initially into ethylene glycol (EG), ferric chloride was firstly transformed to FeOOH, and then continually to FeCO3 via the decomposition of urea under hydrothermal conditions of boiling EG 5. After adding glucose in above homogeneous solution, small carbonaceous colloids with surface functional groups (O-H and C=O etc.) were formed by the dehydration of glucose 7. Subsequently, the FeCO3 nanoparticles combined with carbonaceous colloids to form FeCO3-in-C microstructures through Coulombic interactions, which were self-assembled to iron oxide-carbon core-shell spheres by further dehydration of surface functional groups of carbonaceous species

27

. As manganese chloride and ferric chloride were added simultaneously

into the homogeneous solution, the Fe-Mn solid solution was firstly formed via the strong iron-manganese interaction

28

, and then combined with carbonaceous colloids to obtain

bimetallic FeMn nanoparticles encapsulated by carbon layers. The strong migration capacity of Mn species promoted the assembling of Mn elements at the center and derived Fe elements shifting towards the margin inside the core-shell structure. Moreover, the formation of carbonaceous spheres with tunable sizes in the nano or micrometer range via the hydrothermally treatment of glucose provided the confinement space for suppressing the aggregation of FeMn nanoparticles.


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Scheme 1. Illustration of the formation process of the Fe@C and FeMn@C As shown in Figure S1 (supporting information), SEM image presented uniform spherical morphology with an average size of about 480 nm for as-prepared Fe3O4 nanoparticles, which was prepared by a hydrothermal synthesis method. XRD result (Figure S2a) showed all the diffraction peaks of the Fe3O4 catalyst at 2θ of 30.20°, 35.55°, 43.17°, 62.64°, 57.05°, corresponding to the bulky Fe3O4 (JCPDS No. 99-0073). A characteristic Fe 2p3/2 peak at 710.2 eV with a shoulder Fe 2p1/2 peak at 723.5 eV shown in XPS pattern (Figure S2b) illustrated the formation of Fe3O4 phase on the surface layers. From Figure S3 it can be seen that SEM image of iron nanoparticles encapsulated by carbon shell displayed the spherical morphology with the average size of about 28 µm, which was much bigger than that of as-prepared Fe3O4 nanoparticles. The bulky structure shown in XRD pattern (Figure S4a) presented the diffraction peaks of FeCO3 at 2θ of 22.5°, 43.9°, 53.6°, 79.2°, whereas no diffraction peaks of Fe3O4 were observed. The full scan XPS spectra (Figure S4b) revealed the appearance of C, O, N and Fe elements on the surface layers. XPS result (Figure S4c) further demonstrated that the Fe 2p peak of Fe@C nanoparticles exhibited 0.3 eV shift of higher binding energy compared to the Fe3O4 nanoparticles, indicating no formation of Fe3O4 both in the bulk and surface regions for carbon


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encapsulated iron nanoparticles. This suggested that carbonaceous colloids formed via the dehydration of glucose combined with FeCO3 nanoparticles to form FeCO3-in-C microstructures with the strong interaction, suppressing further reduction of FeCO3 to Fe3O4. Moreover, EDS element mapping images result (Figure S5) illustrated the presence and distribution of Fe, C and O elements. Both the Fe and C elements were dispersed homogeneously in the Fe@C spheres without the appearance of layer-by-layer growth, revealing that large number of small FeCO3-in-C crystallites that nucleate with high surface energy had a significant tendency to aggregate rapidly into large Fe@C nanoparticles 29. SEM image (Figure 1a) showed that the morphology and average size of FeMn@C (the Mn/Fe molar ratio is 0.1) were almost the same as the Fe@C spheres. TEM image depicted in Figure 1b presented the homogeneous distribution and polymerization of small nanoparticles to large aggregates, implying that incorporation of Mn promoter into Fe@C spheres did not change the aggregation behavior of Fe@C nanoparticles. HRTEM image of FeMn@C spheres (Figure 1c) showed lattice fringes with inter plane spacing of 0.28 and 0.30 nm, corresponding to the (104) plane of FeCO3 and Fe2.73Mn0.27O4, respectively, revealing the formation of Fe-Mn solid solution inside the FeMn@C spheres

12

. The existence of FeCO3 phase was further confirmed by XRD

and XPS results (shown in Figure S6a and 6c). C, O, N, Fe and Mn elements were displayed on the surface of FeMn@C (Figure S6b). The corresponding EDX mapping (Figure 1d-g) exhibited also the distribution of Fe, C, Mn and O elements, respectively. A higher density of manganese elements was observed at the center than around the periphery, whereas a contrary phenomenon was displayed for iron elements, which demonstrated stronger migrated capacity of Mn elements in comparison with Fe elements inside spheres, resulting in the aggregation of Mn species in the center, whereas deriving Fe elements towards the periphery. Besides, EDS line scan profiles in


10

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Figure 1h and 1i presented that the signals of Mn elements were mainly detected in the center region, whereas that of Fe elements were observed mostly in the outer area, which further proved the strong migrated capacity of Mn elements.

Figure 1. (a) SEM image of FeMn@C; (b) TEM image of ground FeMn@C; (c) HRTEM image of the edge of FeMn@C; (d-g) SEM-EDS element mapping of FeMn@C; (h) line-scanning image of FeMn@C; (i) the corresponding compositional profile along the red line in (h) N2 adsorption-desorption isotherm plots were used to analyze the pore nature of the FeMn@C spheres. As shown in Figure 2a, both the FeMn@C and Fe@C spheres displayed typical type IV plots with an H2-type hysteresis loop, which were characteristic of “ink-bottle”-like mesoporous materials

30

. The “ink-bottle”-like pores stemmed from the aggregation of small carbon

encapsulated Fe or FeMn nanoparticles, which were consistent with the SEM and TEM results


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mentioned above (Figure 1). Textual properties calculated from the BET method (listed in Table S1) showed that BET surface area and pore size of FeMn@C spheres were 219.5 m2/g and 3.32 nm, respectively, which were similar to that of Fe@C, verifying the stability of spherical morphology for carbon encapsulated iron nanoparticles after adding Mn promoter. Surface functional groups of carbon encapsulated Fe and FeMn nanoparticles were characterized by FTIR (Figure 2b). The broad band at v = 3450 cm-1 was attributed to the hydroxy group (-OH) stretching vibration 7, and the bands at v = 1640 and 1390 cm-1 were ascribed to C=C and -COOgroups

7,31

, respectively. These functional groups stemmed from the dehydration of glucose in

obtaining carbonaceous shell encapsulated on the core of Fe or FeMn particles. Carbonaceous species on the surface layers were further analyzed by XPS spectra. As shown in Figure 2c, the C 1s spectrum could be resolved into three same peaks. The peak of binding energy at 289.0 and 285.4 eV was ascribed to C-O-C and -CO3 bands, respectively 32, with the other peak at 284.5 eV corresponding to C=C band 33. It was apparent that the intensity of C=C band for FeMn@C was higher than that of Fe@C, implying that incorporation of Mn promoter into carbon encapsulated Fe nanoparticles facilitated the formation of surface carbonaceous deposition. This phenomenon was further confirmed by Raman spectra. From Figure 2d it was found that two broad bands centered at 1342 and 1581 cm-1 were observed for FeMn@C and Fe@C, which were designated usually as D- and G- type carbons, respectively 34. The D type carbon is attributed to amorphous carbon with the G type carbon corresponding to graphitic carbon. A higher intensity of D and G type carbons was observed for FeMn@C compared to Fe@C, suggesting that the adding of Mn promoter in Fe@C increased the amounts of surface carbon layers. In addition, the intensity ratio of the D band to the G band (iD/iG) of FeMn@C was 0.699, which was comparative with the iD/iG value of Fe@C (0.693), demonstrating that adding Mn in Fe@C didn’t change the


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graphitization degree of surface carbonaceous species although the amount of carbon layers increased. 120 110

(a)

Fe@C FeMn@C

(b)

-OH

-COOFe-O C=C -OH C-H

-CH2 FeMn@C

Transmittance (a.u.)

90 80 70 60 dV/dlog(w) Pore Area (cm3/g)

Absorbed amount (cm-3g-1)

100

50 40 30 20 10

0.60

Fe@C FeMn@C

0.55 0.50 0.45 0.40 0.35

Fe@C

0.30 0.25 0.20 0.15 0.10 0.05 0.00 1

10

100

Pore size (nm)

0 0.0

0.2

0.4

0.6

0.8

1.0

4000

Relative Pressure (P/P0)

(c)

-CO3 288.8eV

3600

3200

2800

2400

2000

1600

1200

800

400

Wavenumber (cm-1)

C-O-C 285.4eV

C-C 284.5eV

(d)

G

D Fe@C FeMn@C

-1

1342cm 1581cm-1

Fe@C Intensity(a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FeMn@C

298

296

294

292

290

288

286

284

282

280

200

400

600

Binding energy (eV)

800

1000

1200

Raman Shift (cm-1)

1400

1600

1800

Figure 2. (a) N2 adsorption-desorption isotherms and BJH pore diameter distribution plots (inset); (b) FTIR spectra; (c) C1s XPS spectra; (d) Raman spectra H2-TPR results (Figure S7) showed that the single Fe3O4 nanoparticles presented two distinct peaks in the range of 400 and 700 °C, which were attributed to the reduction of Fe3O4 to FeO, and FeO to metallic Fe, respectively 7. By comparison, the reduction peaks of iron species for the Fe@C and FeMn@C catalysts shifted to lower temperature, illustrating the enhanced reduction of carbon encapsulated Fe or FeMn nanoparticles. This could be attributed to electron transport between metal core and carbon shell, changing the chemical environment of Fe or FeMn


13


nanoparticles, similar to the confined effect of iron oxide encapsulated in the CNTs

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35

.

Furthermore, the reduction peaks of iron species of Fe@C shifted slightly towards higher temperature after adding Mn promoter, which could be ascribed to the formation of Fe-Mn solid solution for the FeMn@C catalyst, as presented in Figure 1, strengthening the Fe-Mn interaction in the core and suppressing the reduction of Fe species 36. Fischer-Tropsch synthesis performances of the as-prepared samples were investigated under industrially relative reaction conditions (340 °C, 2.0 MPa, H2/CO = 1). As shown in Figure 3a, the FeMn@C catalyst presented higher catalytic activity (92.34% of CO conversion) than the traditional FeMn/SiO2 catalyst (27.74%). The FTY value (the number of CO moles converted to hydrocarbons per gram of iron per second) of FeMn@C was 56.61 µmolCOgFe-1s-1, which was 3.3 times more than that of FeMn/SiO2 (16.94 µmolCOgFe-1s-1), indicating that carbon-encapsulated FeMn nanoparticles resulted in the formation of more iron active sites compared to the traditional FeMn/SiO2 catalyst. It is generally accepted that iron carbides play an important role in providing active sites for Fischer-Tropsch synthesis despite some arguments existed on the nature of active sites

37

. XRD results (Figure S8) showed that the FeMn@C catalyst presented

main diffraction peaks of iron carbides (mainly ε-Fe2C, JCPDS No. 36-1249), while no obvious diffraction peaks of iron carbides were observed for the traditional FeMn/SiO2 catalyst, demonstrating more iron active sites formed with the carbon-encapsulated FeMn catalyst. This could be attributed the interaction between carbonaceous shell and iron metal species, promoting the formation of iron carbides for FTS 7. Especially, the catalytic activity of FeMn@C was higher than that of Fe@C (about 1.6 times based on the FTY value), which was not in agreement with previous results that adding Mn into traditional Fe catalysts strengthened the Fe-Mn interaction, suppressing the activation of iron oxide and leading to lower catalytic activity 38. It is


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possible that the difference of position and environment of Mn elements inside the carbon-encapsulated iron catalysts changed the FTS catalytic activity. SEM and TEM results mentioned above (Figure 1) indicated that Mn promoter possessed the strong migrated capacity in the carbon encapsulated FeMn nanoparticles, which resulted in the migration of Mn elements into the center and derived Fe elements towards the outside. As a result, the amount of Fe species on the surface layers increased, which led to the formation of more iron carbides by combining with carbonaceous species on the shell, promoting the FTS catalytic activity. As shown in Figure S9, XPS result demonstrated that the intensity of iron carbides peak (at 707.6 eV) for the used FeMn@C catalyst was higher than that of the used Fe@C, further verifying more active iron carbides formed by the addition of Mn promoter into Fe@C. 70

70

(a)

C5+ C2-C4

56.61

22.04 35.16

30

13.73 23.84

20

14.10

16.94 4.64

10

0

7.33

10.73

Fe@C

FeMn@C

CH4

C20-C40

C2=-C4=

C50-C120

56.61

C13+

CH4 47.63

50

45.30

22.04 40

33.61 32.19

28.11

11.95

13.12

3.49

4.07

1.0 0.5 Mn/Fe molar ratio

1.5

30

20

23.84 15.59 10.73

4.26 FeMn/SiO2

C5=-C12=

C2-C4

54.47

10

8.04

55

(c)50

60

C5+

CH4

50

40

(b) FTY (µmolCOgFe-1s-1)

FTY (µmolCOgFe-1s-1)

60

0

70 (d)65

5.27

0.1

CH4

0

0

C2 -C4

=

=

C2 -C4

0

C5 -C12

0

=

=

C5 -C12

+

C13

60 45

55

Product Selectivity (%)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 35 30 25 20 15

50 45 40 35 30 25 20 15

10

10 5 0

5

Fe@C

FeMn@C

FeMn/SiO2

0

FeMn0.1@C

FeMn0.5@C

FeMn1.0@C

FeMn1.5@C


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Figure 3. FTY value of (a) Fe@C, FeMn@C and FeMn/SiO2; (b) FeMnx@C (x = 0.1, 0.5, 1.0, 1.5). Product selectivity of (c) Fe@C, FeMn@C and FeMn/SiO2; (d) FeMnx@C (x = 0.1, 0.5, 1.0, 1.5). In a further step, the amount of Mn loading in the FeMn@C catalyst had obvious effect on the FTS activity. From Figure 3b and Table S2 it could be seen that the catalytic activity presented a decreasing trend with the increasing of Mn loading. The FTY value decreased gradually from 56.61 µmolCOgFe-1s-1 of FeMn0.1@C to 45.30 µmolCOgFe-1s-1 of FeMn1.5@C, accompanied with the decrease in CO2 selectivity from 48.93 to 20.97%, illustrating that the adding of more Mn contents in the FeMn@C catalyst resulted in the decrease in both the FTS catalytic activity and WGS reaction activity. An increasing intensity of Fe-Mn solid solution diffraction peaks with the increase of Mn loading was observed in XRD patterns (Figure S10), which strengthened the Fe-Mn interaction and suppressed the reduction of iron species. H2-TPR results (Figure S11) indicated that accompanied with the increase of Mn loading the reduction peaks of iron species in the FeMn@C catalysts shifted slowly towards higher temperature, confirming the reduction difficulty of iron species over the FeMn@C catalyst with higher Mn loading. Combined with the activity results on carbon encapsulated Fe or FeMn nanoparticles (Figure 3a and b), it is considered that a balance between the Fe-Mn interaction and surface iron amounts could be existed for the FeMn@C core-shell catalyst. As the amount of Mn loading was low (the FeMn0.1@C catalyst), the interaction of Fe-Mn was weakened, which had a negligible effect on the activation of iron species. The assembling of Fe elements in the surface region stemmed from the replacing of central Mn elements resulted in the formation of more active iron carbides by the combination of surrounding carbonaceous species, promoting the catalytic activity. In contrast, the adding of more Mn loadings in FeMn@C strengthened the Fe-Mn interaction, which played a


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primary role in suppressing the reduction of iron species, and decreasing active iron sites for FTS. Besides, adding Mn into Fe@C facilitated the formation of surface carbon layers, which had been verified by the XPS and LRS results (Figure 2c and 2d). The C/Fe molar ratio on the surface layers calculated by XPS (Figure S12) showed an increasing trend from 3.13 of FeMn0.1@C to 11.80 of FeMn1.0@C, revealing the formation of more surface carbon layers on the surface of FeMn nanoparticles over the FeMn@C catalyst with higher Mn loading. These surface carbon layers could block the catalyst channel, restrained the diffusion of probe molecules and further decreased the FTS catalytic activity and WGS reaction activity. The product distribution of as-prepared catalysts was further summarized in Figure 3c and 3d. As shown in Figure 3c, the traditional FeMn/SiO2 catalyst presented a higher selectivity of CH4 and C2-4 hydrocarbons, while a lower C5+ hydrocarbons selectivity. 25.15% of CH4 and 47.47% C2-4 hydrocarbons with 2.81 of the C2-4=/C2-4o ratio, as well as 27.38% of C5+ hydrocarbons were obtained after reaction for 30 h, indicating that the traditional FeMn/SiO2 catalyst promoted the formation of light olefins while restrained that of heavy hydrocarbons. This was in good accordance with the results that the Mn modified Fe-based catalyst facilitated the formation of a higher C2-4 olefins but a lower C5+ hydrocarbons by restraining the secondary hydrogenation and chain growth of olefins

39

. By comparison, the selectivity of CH4 and C2-4 hydrocarbons

decreased while that of C5+ hydrocarbons increased for the FeMn@C core-shell catalyst, indicating that carbon encapsulated FeMn nanoparticles promoted carbon chain growth of light hydrocarbons 26. This could be attributed to the enhanced interaction between iron nanoparticles and the surrounding carbonaceous layers, which played an important role in promoting carbon chain growth ability of hydrocarbons 37.


17


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Interestingly, the selectivity of C5+ hydrocarbons presented an increasing trend with the increase of Mn loading in the FeMn@C catalyst. As shown in Figure 3d, the C5+ hydrocarbons selectivity increased from 38.93% of FeMn0.1@C to 67.58% of FeMn1.0@C, accompanied with the gradual decrease of the CH4 and C2-4 hydrocarbons selectivity from 18.96 and 42.11% to 7.33 and 25.09%, respectively, indicating that the increasing Mn content in FeMn@C facilitated obviously the product distribution shifting towards heavy hydrocarbons. From Table S2 it was found that the ratio of C2-4=/ C2-4o increased gradually from 2.26 of FeMn0.1@C to 6.49 of FeMn1.0@C, although the selectivity of total C2-4 hydrocarbons presented a decreasing trend, implying that FeMn nanoparticles encapsulated in carbon layers possessed the capacity for CO dissociation and the weakening of olefin rehydrogenation, promoting the formation of light olefins

12

. Therefore, promoting product distribution shifting from light hydrocarbons towards

C5+ hydrocarbons for the FeMn@C catalyst with higher Mn loading could be ascribed to “confinement effect” of core-shell structure, which extended the residence time of light olefins formed inside the shell, promoting the olefins polymerization for producing C5+ hydrocarbons. As shown in Table S2, the C5-12=/ C5-12o ratio increased gradually from 2.71 of FeMn0.1@C to 6.80 of FeMn1.0@C, revealing the enhanced olefin polymerization with adding more Mn loading into the FeMn@C catalyst. On the other hand, the FeMn@C catalyst with higher Mn loading facilitated the formation of more carbonaceous species on the surface layers, which combined with iron species at the margin to further strengthen the carbon chain growth ability of hydrocarbons, promoting the formation of C5+ hydrocarbons. In addition, as the Mn/Fe ratio increased to 1.5, the carbon-chain growth of light hydrocarbons was suppressed, which could be attributed to excessive adding of Mn promoter in FeMn@C, decreasing the amounts of active iron carbides and impeding the production of long-chain hydrocarbons 26. More interestingly, the


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FeMn1.0@C catalyst presented the excellent selectivity of C5-12 hydrocarbons (gasoline range products) as high as 63.13% (Figure 3d), which was better than the values reported to date for carbon encapsulated iron nanoparticles, including FexOy@C, Fe3O4@C and Fe-in-CNT etc. 7,26,35,40

. The addition of Anderson-Schulz-Flory (ASF) plots of as-prepared catalysts were shown

in Figure S13. The naked Fe3O4 nanoparticles exhibited an α value of 0.56, lower than that of 0.62 for Fe@C catalyst, demonstrating that carbon encapsulated Fe nanoparticles facilitated the production of long-chain hydrocarbons. Moreover, the FeMn1.0@C catalyst displayed the highest α value of 0.70, confirming the promoted effect of Mn promoter on the product of C5+ hydrocarbons. According to the ASF model predicting the product distribution, the maximum selectivity of C5-12 hydrocarbons (including paraffins and olefins) is approximately 45% 7. The selectivity of C5-12 hydrocarbons for the FeMn1.0@C catalyst in the present study reached about 63%, which was deviated greatly from the typical ASF distribution, indicating that carbon encapsulated FeMn nanoparticles promoted obviously the secondary reaction of olefins oligomerization, shifting product distribution towards heavy hydrocarbons.

C50-C120

94.18

C5=-C12=

C13+

100

(b)80

CH4

0

C2 -C4

90.69

94.44

0

=

C2 -C4

89.08

=

0

0

C5 -C12

=

C5 -C12

=

C13

100

70

80

79.39

50

60

40 40

30 20

80 60

68.73

50

60

40 40

30 20

20

20 10

10 0

+

88.91

CO Conversion (mol%)

C2=-C4=

92.59

70 60

C20-C40


CH4


(a)80 Product Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

320 340 Temperature (°C)

360

0

0

5000

8000 10000 GHSV (h-1)

15000

0


19


(d) C2 -C4

0

= 2

C -C4

92.53

70

=

0

C5 -C12

0

95.69

=

C5 -C12

=

C13

+

82.97

90 80

60 50

60

40 40

30

100 95

100

95.76

20 20

85

Conversion (mol%)

CH4

0


(c)80 Product Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

CO H2

80 75 70 65 60

10

55 0

1.0

2.0 3.0 Reaction pressure (MPa)

4.0

0

50 0

20

40

60

80

100

Time on stream (h)

Figure 4. Catalytic performance of the FeMn@C catalysts in different reaction conditions (a) Temperature; (b) GHSV; (c) Reaction pressure. (d) CO conversion and product selectivity with time on stream. FTS performances of the FeMn@C catalyst were further modified by adjusting reaction conditions such as reaction temperature, GHSV and reaction pressure and so on. As shown in Figure 4a, CO conversion increased from 79.39 to 94.44 % with the increasing of reaction temperature from 300 to 360 °C. The selectivity of C5+ hydrocarbons presented a decreasing trend from 63.78% to 47.19% with the increase of temperature from 300 to 360 °C, whereas an increasing trend was observed for the selectivity of CH4 and C2-4 hydrocarbons from 8.10 and 28.12% to 18.20 and 34.61%, respectively, which could be ascribed to the cracking reaction of long-chain hydrocarbons in higher reaction temperature

41

. Increasing GHSV from 5000 to

15000 h-1 results in a slight decrease of CO conversion from 90.69 to 68.73% (Figure 4b), while the increasing of FTY value from 48.90 to 107.03 µmolCOgFe-1s-1 (Figure S14), which was consistent with general FTS results that increasing GHSV could improve the yield of produced hydrocarbons. Besides, increasing GHSV shortened the contact time of reactants, which decreased the selectivity of C5+ hydrocarbons, while increased that of CH4 and C2-4 hydrocarbons


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by reducing the contact frequency of light hydrocarbons, and restraining their carbon chain growth reaction. Increasing CO conversion from 82.97 to 95.76% with an increasing trend of C5+ hydrocarbons selectivity and a decreasing selectivity of CH4 and C2-4 hydrocarbons accompanied with increasing pressure from 1.0 to 4.0 MPa (Figure 4c) suggested that higher reaction pressure was in favor of the formation of heavy hydrocarbons. In addition to the evaluation of catalytic activity and hydrocarbons selectivity, the long-term stability of FeMn@C was assessed under industrially testing conditions (340 °C, 2.0 MPa and H2/CO = 1). An excellent stability was displayed for the FeMn@C catalyst with no significant deactivation being observed with 100 h on stream (Figure 4d), revealing the excellent stability of carbon encapsulated FeMn nanoparticles in the FTS reaction. SEM image result (Figure S15) showed a stable core-shell structure of FeMn@C with nanoparticles size around 30 µm after reaction, which was comparative to the fresh catalyst, indicating the excellent stability of FeMn@C spheres with the suppressing of FeMn nanoparticles sintering by confinement effect of carbon layers 34. 4. CONCLUSION In summary, we developed a novel FeMn@C core-shell catalyst with hydrothermal carbon-coated FeMn nanoparticles. This catalyst displayed a highly catalytic activity and C5+ hydrocarbons selectivity as well as excellent stability for FTS reaction, much better than the Fe@C or the traditional FeMn/SiO2 catalyst. The excellent catalytic activity might be ascribed to the strong migrated capacity of Mn promoter, promoting the assembling of Mn elements at the center, and deriving Fe elements towards the margin, which combined with carbonaceous species on the surface shell to form more active iron carbides. The superior selectivity of C5+ hydrocarbons could be attributed to “confinement effect” of the core-shell structure of FeMn@C, which extended the residence time of intermediate light olefins produced by Mn-modified Fe


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nanoparticles inside the shell, improving the secondary reaction of olefins oligomerization. Especially, a highly selectivity of 63.13% C5-12 hydrocarbons (gasoline range products) was obtained for the Fe-Mn1.0@C core-shell catalyst, breaking obviously the ASF distribution. Besides, the stability was further improved by carbon encapsulation, which suppressed the agglomeration of FeMn nanoparticles. Overall, this facile method for preparing the FeMn@C core-shell catalyst with enhanced catalytic activity and C5+ selectivity provided a promising way for the industrial application. ASSOCIATED CONTENT Supporting Information. Supporting Information includes Figures and Tables. SEM/XRD/XPS of Fe3O4 catalysts (Fig. S1 and S2). SEM image, XRD pattern, XPS spectra and SEM-EDS mapping images of Fe@C (Fig. S3, S4 and S5). XRD pattern and XPS spectra of FeMn@C (Fig. S6). H2-TPR profiles of Fe3O4, Fe@C and FeMn@C (Fig. S7). XRD patterns and XPS spectra of the used FeMn@C and FeMn/SiO2 (Fig. S8 and S9). XRD patterns, H2-TPR profiles and C/Fe molar ratio of surface layer of FeMnx@C (Fig. S10, S11 and S12). The ASF distribution of Fe3O4, Fe@C and FeMn1.0@C (Fig. S13). The effect of GHSV on the FeMn@C catalyst (Fig. S14). SEM image of the used FeMn@C (Fig. S15). Textural properties of Fe@C and FeMn@C (Table S1). Catalytic Performance of FeMnx@C catalysts (Table S2). AUTHOR INFORMATION Corresponding Author E-mail address: [email protected] (Mingyue Ding).


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E-mail address: [email protected] (Chenghua Zhang). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. NOTES The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the Science and Technology project of Guangdong Province (2016A050502037), and Fundamental Research Funds for the Central Universities (2042017kf0173, 2042017kf0200). REFERENCES (1) Torres Galvis, H. M.; Bitter, J. H.; Khare, C. B.; Ruitenbeek, M.; Dugulan, A. I.; de Jong, K. P. Supported Iron Nanoparticles as Catalysts for Sustainable Production of Lower Olefins. Science. 2012, 335 , 835-838. (2) Sartipi, S.; Parashar, K.; Makkee, M.; Gascon, J.; Kapteijn, F. Breaking the Fischer-Tropsch Synthesis Selectivity: Direct Conversion of Syngas to Gasoline over Hierarchical Co/H-ZSM-5 Catalysts. Catal. Sci. Technol. 2013, 3, 572–575. (3) Roberto, C. V; Raveendran, S. N.; Daniel, C.; Stéphane, C.; Andrei, K.; Amadeus, R.; Johannes, T.; Andreas, J.; Gadi, R. De Novo Design of Nanostructured Iron–Cobalt Fischer– Tropsch Catalysts. Angew. Chemie Int. Ed. 2013, 52, 4397–4401. (4) Torres Galvis, H. M.; Koeken, A. C. J.; Bitter, J. H.; Davidian, T.; Ruitenbeek, M.; Dugulan, A. I.; de Jong, K. P. Effects of Sodium and Sulfur on Catalytic Performance of Supported Iron Catalysts for the Fischer–Tropsch Synthesis of Lower Olefins. J. Catal. 2013, 303, 22–30. (5) Zhao, B.; Zhai, P.; Wang, P.; Li, J.; Li, T.; Peng, M.; Zhao, M.; Hu, G.; Yang, Y.; Li, Y.-W.; et al. Direct Transformation of Syngas to Aromatics over Na-Zn-Fe5C2 and Hierarchical HZSM-5 Tandem Catalysts. Chem 2017, 3, 323–333. (6) Ding, M.; Yang, Y.; Wu, B.; Li, Y.; Wang, T.; Ma, L. Study on Reduction and Carburization Behaviors of Iron Phases for Iron-Based Fischer–Tropsch Synthesis Catalyst. Appl. Energy 2015, 160, 982–989. (7) Jun‐ling, T.; Ming‐yue, D.; Qian, Z.; Yu‐lan, Z.; Chen‐guang, W.; Tie‐jun, W.; Long‐ long, M.; Xin‐jun, L. Design of Carbon‐Encapsulated Fe3O4 Nanocatalyst with Enhanced Performance for Fischer–Tropsch Synthesis. ChemCatChem 2015, 7, 2323–2327.


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(40) Guczi, L.; Stefler, G.; Geszti, O.; Koppány, Z.; Kónya, Z.; Molnár, É.; Urbán, M.; Kiricsi, I. CO Hydrogenation over Cobalt and Iron Catalysts Supported over Multiwall Carbon Nanotubes: Effect of Preparation. J. Catal. 2006, 244, 24–32. (41) Bortnovsky, O.; Sazama, P.; Wichterlova, B. Cracking of Pentenes to C2–C4 Light Olefins over Zeolites and Zeotypes: Role of Topology and Acid Site Strength and Concentration. Appl. Catal. A Gen. 2005, 287, 203–213.


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Table of contents (TOC) A novel FeMn@C core-shell catalyst with bimetallic nanoparticles encapsulated in carbonaceous spheres exhibits high catalytic activity and superior selectivity of gasoline range hydrocarbons.


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Enhanced Conversion of Syngas to Gasoline-Range Hydrocarbons

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