Polyaniline-supported iron catalyst for selective synthesis of lower olefins from syngas

Accepted Manuscript

Polyaniline-supported iron catalyst for selective synthesis of lower olefins from syngas

http://www.journals.elsevier.com/ journal-of-energy-chemistry/

Bang Gu , Shun He , Wei Zhou , Jincan Kang , Kang Cheng , Qinghong Zhang , Ye Wang PII: DOI: Reference:

S2095-4956(17)30108-0 10.1016/j.jechem.2017.04.009 JECHEM 304

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

13 February 2017 3 April 2017 5 April 2017

Please cite this article as: Bang Gu , Shun He , Wei Zhou , Jincan Kang , Kang Cheng , Qinghong Zhang , Ye Wang , Polyaniline-supported iron catalyst for selective synthesis of lower olefins from syngas, Journal of Energy Chemistry (2017), doi: 10.1016/j.jechem.2017.04.009

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ACCEPTED MANUSCRIPT Polyaniline-supported iron catalyst for selective synthesis of lower olefins from syngas

Bang Gu, Shun He, Wei Zhou, Jincan Kang, Kang Cheng*, Qinghong Zhang, Ye Wang*

State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National Engineering Laboratory for Green

Engineering, Xiamen University, Xiamen 361005, Fujian, China

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Chemical Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical

*Corresponding author. Tel: +86-592-2187470; Fax: +86-592-2183047.

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E-mail: [email protected]; [email protected].

Abstract

Uniform iron nanoparticles dispersed on polyaniline have been used as catalysts for the direct conversion of synthesis gas into lower olefins. As compared to active carbon and

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N-doped active carbon, polyaniline as a support of Fe catalysts showed higher selectivity of lower olefins (C2-4=). The C2-4= selectivity reached ~50% at a CO conversion of 79% over a 10

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wt% Fe/polyaniline catalyst without any promoters. The XRD, H2-TPR, TEM and HRTEM studies revealed that the presence of nitrogen-containing groups in polyaniline structure could

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promote the dispersion and reduction of iron oxides, forming higher fraction of iron carbides with smaller mean sizes and narrower size distributions. The propylene-TPD result indicates

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that the use of polyaniline support facilitates the desorption of lower olefins, thus suppressing

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the consecutive hydrogenation to form undesirable lower paraffins.

Keywords: Polyaniline; Iron catalyst; Lower olefins; Fischer-Tropsch; Electron effect

1. Introduction Lower olefins (ethylene, propylene, and butenes) are key building blocks in the chemical industry. They are mainly produced from steam cracking of naphtha [1]. A lot of efforts have been made to develop alternative processes to synthesize lower olefins from non-petroleum 1

ACCEPTED MANUSCRIPT carbon resources, such as coal, natural gas, renewable biomass and even waste [2,3]. Fischer-Tropsch (FT) synthesis is a key process for the transformation of various carbon resources into hydrocarbons including lower olefins via syngas. Iron-based catalysts are usually used for the FT synthesis to lower olefins (FTO) because of their high selectivity towards lower olefins, low cost, and high water-gas-shift activity, which is favorable to adjust the H2/CO ratio for biomass- or coal-derived syngas [4].

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It is well known that the products of FT process follow the Anderson-Schulz-Flory (ASF) distribution because of the polymerization mechanism [5]. According to the ASF distribution, the maximum selectivity of C2–C4 hydrocarbons (including both olefins and paraffins) is approximately 57% with a chain growth probability factor (α) of 0.46. Many studies have been devoted to designing functional Fe catalysts for enhancing the selectivity of lower

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olefins. The supported Fe catalysts with specific promoters and moderate iron-support interactions have recently been demonstrated to be promising. Among various promoters, potassium [6,7], sodium [6,8], copper [9,10], manganese [11,12] and zinc [12] are often used. These promoters are considered to work as “electronic modifiers”, which tune CO and H2

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chemisorption behaviors on catalyst surfaces [13-15]. The selectivity of lower olefins could be improved significantly by using these promoters. For example, de Jong and co-workers

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reported the conversion of syngas to lower olefins over a modified Fe/α-Al2O3 catalyst containing appropriate contents of sulfur and sodium, and the lower olefins selectivity could

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reach 60% at a low CO conversion (~1%) [16,17]. The inert α-Al2O3 support showed week interaction with iron species [18]. It was also found that the proximity between iron and

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promoters (Na plus S) was a key parameter for methanation and C–C chain growth [16], which determined the hydrocarbon distribution.

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Besides inert metal oxide supports such as α-Al2O3, carbon materials also have relatively

weak interactions with the active metal and allow for the easy chemical modification. Various types of carbon materials, such as activated carbon [19,20], mesoporous carbon [21,22], graphene [7,23,24], carbon nanotubes [25], and carbon nanofibers [26], have been demonstrated to be excellent catalyst supports for FT synthesis. Similar to metal oxide-supported catalysts, the Fe catalysts supported on carbon materials are also sensitive to promoters, especially alkali metals. Thus, the catalysts are usually heavily modified with 2

ACCEPTED MANUSCRIPT different modifiers to achieve a considerable selectivity of lower olefins. Recently, a few groups have investigated the use of N-doped carbon materials as catalyst supports for FT synthesis without introducing any promoters [13,27,28]. It is expected that the N atoms in carbon matrix may not only act as an electron donor like an alkali metal but also serve as anchoring sites for the precursor of Fe catalysts [13,29,30]. Both the CO conversion activity and the selectivity of lower olefins could be improved by the doping of N into carbon

with well-defined N groups for FT synthesis are still scarce.

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materials. However, the studies devoted to the application of N-containing carbon materials

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Scheme 1. Chemical structure of polyaniline (PANI).

Polyaniline (PANI) is a kind of conducting polymer that composed of benzenoid and quinonoid units with unique π-conjugated structure (Scheme 1). During the past decade, PANI has been exploited for catalysis due to its good stability, high conductivity, nontoxicity and

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low cost [31,32]. However, so far there has been no report on the utilization of PANI as a support for FT synthesis. Here, we report a first study to harness PANI as a support of Fe

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catalyst for the conversion of syngas into lower olefins. We will demonstrate that the electron-rich nitrogen groups in PANI can enhance the dispersion and reduction of iron

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species, favoring the CO conversion activity. Moreover, PANI can facilitate the desorption of

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lower olefins from catalyst surfaces, beneficial to the selective formation of lower olefins.

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2. Experimental

2.1. Materials and catalyst preparation PANI was synthesized by a sonochemical method [33]. Typically, 2.8 g of aniline and 6.4

mg of ferrous chloride (FeCl2∙H2O) were first dissolved in 300 mL hydrochloric acid (1 M) and 75 mL distilled water, respectively. The two solutions were then mixed together with stirring, followed by ultrasonic treatment in an ultrasonic bath (100W/40 kHz). Then, a H2O2 aqueous solution (6 wt%, 68 mL) was added dropwise into the solution of aniline and FeCl2 to initiate the polymerization reaction at 25 oC. The molar ratio of FeCl2/aniline/H2O2 was 3

ACCEPTED MANUSCRIPT kept at l/600/2400. The addition of H2O2 aqueous solution lasted for about 30 min, after which the ultrasonic treatment further continued for 4 h to complete the polymerization. The green powders were recovered by filtration and thorough washing with deionized water until the filtrate became colorless. The recovered solid was purified with a queous ammonia solution (5 wt%) for 3 h, followed by washing thoroughly with distilled water. The obtained powdery solid was dried in vacuum at 50 oC for 12 h. For comparison, the N-doped active

at 900 oC for 12 h in the flow of NH3 (50 mL/min).

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carbon was synthesized using a post-synthesis method through treatment of an active carbon

Supported Fe catalysts were prepared by an impregnation method using Fe(NO3)3 aqueous solution as a precursor of iron. The Fe loading was fixed at 10 wt%. After the impregnation, the sample was dried in vacuum at 50 oC for 12 h followed by thermal

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treatment at 355 oC for 4 h under a flow of nitrogen (50 mL/min). 2.2. Catalyst characterization

X-ray powder diffraction (XRD) patterns were recorded on a RigakuUltima IV diffractometer (Rigaku, Japan). Cu Kα radiation (40 kV and 30 mA) was used as the X-ray

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source. The average crystallite size of iron oxides (Fe2O3or Fe3O4) was evaluated by the

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Scherrer equation using the diffraction peak of Fe2O3 at 2θ of 33.2o or Fe3O4 at 2θ of 35.4o. N2 physisorption measurements were carried out on a Micromeritics Tristar 3020 Surface Area and Porosimetry analyzer. Prior to N2 adsorption, the sample was degassed at 250 oC for

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2 h. Transmission electron microscopy (TEM) measurements were performed on a Phillips

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Analytical FEI Tecnai 30 electron microscope operated at an acceleration voltage of 300 kV. More than 100 particles were counted to estimate the average Fe particle size and the standard

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deviation from TEM images. ICP-OES analyses were performed on a Thermo Electron IRIS Intrepid II XSP instrument. Elemental analysis experiments were carried out in Vario ELIII instrument. FT-IR studies were performed with a Nicolet 6700 instrument equipped with an MCT detector.

H2 temperature-programmed reduction (H2-TPR) experiments were carried out on an apparatus equipped with a thermal conductivity detector. The TPR profile of the catalyst (100 mg) before reduction was obtained by heating the sample from 50 oC to 600 oC at a rate of 10 o

C/min in a flow of H2/Ar (5 vol% H2) mixture. Temperature-programmed desorption (TPD) 4

ACCEPTED MANUSCRIPT of CO2 and C3H6 were measured on a Micromeritics AutoChemII 2920 instrument. Typically, 100 mg of samples were used for each measurement. For CO2-TPD, the sample was pretreated at 150 oC in a He flow for 1 h to remove traces of water and impurities. Then, the adsorption of CO2 was performed at 25 oC for 1 h. The gaseous and weakly adsorbed CO2 were subsequently removed by purge with He for an additional 1 h. Subsequently, the temperature was raised from 25 to 500 oC at a rate of 10 oC/min and the CO2-TPD profile was

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recorded by a mass spectrometer with a signal of m/z = 44. The C3H6-TPD was carried out for the Fe catalyst after the reaction. In brief, the catalyst was first treated at 300 oC in a He flow for 6 h. Then, the adsorption of C3H6 was performed at room temperature for 1 h. After purge with He for an additional 1 h, the temperature was raised from 25 to 600 oC at a rate of 30 o

C/min. The C3H6-TPD profile was recorded by a mass spectrometer with a signal of m/z =41.

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2.3. Catalytic reaction

The syngas conversion was performed on a high-pressure fixed-bed flow reactor made by Xiamen HanDe Engineering Co., Ltd. Typically, the catalyst (0.20 g) with grain sizes of 250-600 μm (30–60 mesh) loaded in the stainless-steel reactor (inner diameter, 8 mm) was

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pretreated in a H2 gas flow (30 mL/min) at 350 oC for 4 h before reaction. After the reactor

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was cooled down to 50 oC, a syngas with a H2/CO ratio of 1/1 and a pressure of 0.5–2.0 MPa was introduced into the reactor. Argon with a concentration of 4% in the syngas was used as an internal standard for the calculation of CO conversion. The temperature was raised to the

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desired reaction temperature (310–350 oC) to start the reaction. The products were analyzed

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by a gas chromatograph (HuaaiGC-9560), which was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). A TDX-01 packed column was

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connected to the TCD, and a KB-Al2O3/Na2SO4 capillary column was connected to the FID. The selectivity presented in this work was calculated on a molar carbon basis for CO hydrogenation. The carbon balance for each reaction was better than 90%.

3. Results and discussion 3.1. Structures of supports and supported Fe catalysts Table 1 shows some physical properties of supports and fresh supported Fe catalysts. Active carbon (AC) and N-doped active carbon (N-AC) possessed high surface areas and total 5

ACCEPTED MANUSCRIPT pore volumes. The average diameters of mesopores evaluated by the BJH method were in the range of 3.3–3.5 nm for AC and N-AC. PANI showed a relatively lower surface area (90 m2/g) and total pore volume (0.23 cm3/g) than AC or N-AC. However, a larger mean mesopore diameter of 14 nm was observed for PANI, and this would facilitate the dispersion of iron nanoparticles. After the loading of iron, the surface areas decreased. This is probably caused by the loading of iron oxide particles inside the pores of supports. The decrease in surface

confirmed that the iron loading was around 10 wt%.

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areas was more significant for AC- or N-AC-supported catalysts. Our ICP measurements

DFee

Fecontentf

(nm)

(wt%)

-

-

-

3.5

-

-

-

0.23

14

-

-

-

138

0.24

7.8

14

18

10

Fe/N-AC

113

0.22

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Table 1. Physical properties of supports and supported Fe catalysts.

6.8

7.6

9.4

11

Fe/PANI

48

0.10

10

6.3

7.1

11

Vtotb

Dmesoc

DFed

(m2/g)

(cm3/g)

(nm)

(nm)

AC

983

0.47

3.3

N-AC

638

0.41

PANI

90

Fe/AC

BET surface area.

b

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a

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SBETa

Sample

c

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Single point desorption total pore volume of pores, P/P0=0.975.

The porediameter in the mesoporous region evaluated by the BJH method.

d e f

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The diameter of iron oxide from XRD analysis.

Average particle size of iron oxide by TEM.

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The Fe content from ICP-OES.

Fig. 1 shows the XRD patterns for supported Fe catalysts before reaction. The diffraction lines at 2θ of 33.2o and 35.6o are assignable to the hematite phase (Fe2O3), while the diffraction line at 2θ of 35.4o can be ascribed to magnetite phase (Fe3O4). The Fe/AC catalyst contained mainly Fe2O3 phase after thermal treatment, whereas the Fe/N-AC and Fe/PANI catalysts only showed XRD peaks belonging to Fe3O4 with mixed Fe2+ and Fe3+ in the 6

ACCEPTED MANUSCRIPT octahedral sites of spinel structure. This indicates the occurrence of an auto-reduction during thermal decomposition of Fe(NO3)3 in N2 atmosphere on N-AC and PANI. This agrees with a previous report that the presence of nitrogen groups in mesoporous carbon materials could facilitate the reduction of cobalt oxide under inert Ar atmosphere [34]. The crystallite size of iron oxide particles was evaluated to be 14 nm by using the Scherrer equation for the Fe/AC catalyst (Table 1). The crystallite sizes of iron oxide particles estimated from XRD decreased to7.6 and 6.3 nm for the Fe/N-AC and Fe/PANI, respectively. This can be interpreted by

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considering that the nitrogen groups may serve as the anchoring sites for iron precursors [34,35]. Considering the large difference in surface areas between AC and PANI (Table 1), we can conclude that the dispersion of Fe is determined more by the surface chemistry of carbon

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materials than by the surface area.

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Fig. 1. XRD patterns for Fe/AC, Fe/N-AC and Fe/PANI catalysts before reaction.

Fig. 2 shows TEM micrographs for the Fe catalysts loaded on different supports. The

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Fe/AC catalyst displayed big Fe particles with a wide particle size distribution. The mean size was estimated to be 18 nm by counting more than 100 Fe particles. As compared to Fe/AC, the Fe/N-AC catalyst showed smaller iron nanoparticles with a mean particle size of 9.4 nm and a more uniform particle size distribution (Fig. 2b). Our Fe/PANI catalyst possessed the most uniform distribution of Fe particles (Fig. 2c). The mean size of Fe particles in the Fe/PANI catalyst was 7.1 nm. These results suggest that the presence of nitrogen groups in carbon supports has a positive effect on the dispersion of Fe species. Probably, the intrinsic hydrophobicity of inert carbon supports makes it difficult to anchor the metal particles. It has 7

ACCEPTED MANUSCRIPT been reported that the nitrogen doping in carbon materials can reduce the surface hydrophobicity due to the strong electron-donating behavior of nitrogen, and thus the

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dispersion of metal or metal oxide particles can be enhanced [27,36].

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Fig. 2.TEM micrographs and iron particle distributions. (a) Fe/AC, (b) Fe/N-AC, (c) Fe/PANI.

High reducibility of iron species is one of the essential factors for obtaining high FT

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synthesis activity [37-39]. We investigated the reduction behavior of iron species loaded on different supports. Fig. 3 shows H2-TPR profiles of our catalysts. Because PANI may undergo

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decomposition at high temperatures [40], we conducted H2-TPR measurements be low 600 oC. Two reduction peaks were observed for all the three catalysts. The reduction of iron oxides

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may occur step by step as follows: Fe2O3 →Fe3O4→FeO→Fe [41]. However, each step may not have a distinctive peak at a definite temperature because of different metal-support interactions, measuring conditions and the existence of promoters [42]. Generally, the first TPR peak at 250–450 oC is assignable to the reduction of Fe2O3 to Fe3O4 or Fe3O4 to FeO, and the second peak (>450 oC) can be ascribed to the reduction of FeO to Fe0 [41]. Fig. 3 clearly shows that the presence of nitrogen in PANI and N-AC can facilitate the reduction of iron species. Such a reduction-promoting effect is more significant when PANI is used as the 8

ACCEPTED MANUSCRIPT support. This result agrees well with the improved reduction behavior of iron catalysts

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supported on N-doped carbon nanostructures [27].

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Fig. 3. H2-TPR profiles for iron oxides loaded on AC, N-AC and PANI. 3.2. Fischer-Tropsch synthesis

The catalytic performances of Fe catalysts loaded on several types of carbon materials as

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well as on SiO2 have been compared. As displayed in Table 2, CO conversions were in a range of 50%–80% over all the catalysts examined. Fe catalysts loaded on carbon materials

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have been reported to be more active than that loaded on SiO2, because of higher carburization degrees of Fe on carbon supports [22]. For the Fe/CNT and Fe/AC catalysts

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without nitrogen doping, the selectivity of lower olefins (C2-4=) was lower than 30%. The selectivity of lower olefins increased to 36% for the Fe/N-AC catalyst with nitrogen doping.

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The Fe/PANI catalyst showed the highest CO conversion (79%) and the highest C2-4= selectivity (47%). In addition, the olefin-to-paraffin ratio in C2–C4 (C2-4=/C2-40) was

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significantly higher for the Fe/N-AC and Fe/PANI catalysts. The Fe-based catalysts without promoters usually show broadly distributed products with low C2-4= selectivity (<35%) [1,23,27]. Therefore, the nitrogen group contained in the support should play crucial roles in suppressing the hydrogenation of C2-4= to C2-40.

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ACCEPTED MANUSCRIPT Table 2. Catalytic performances of supported Fe catalysts for FT synthesisa.

Catalyst

CO2

conv.

select.

Hydrocarbon selectivity(%)

C2-4=/

CH4

C2-4=

C2-4o

C5+

C2-4o

(%)

(%)

Fe/SiO2

50

45

29

25

25

21

1.0

Fe/CNT

75

44

25

29

31

15

0.94

Fe/AC

62

41

30

28

25

17

1.1

Fe/N-AC

73

42

27

36

19

18

1.9

Fe/PANI

79

44

24

47

14

15

3.4

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a

CO

Reaction conditions: W = 0.2 g, H2/CO = 1, P = 2 MPa, T = 350 oC, time on stream =30 h, F=30 mL/min.

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We investigated the effect of reaction parameters, i.e., reaction temperature and pressure, on catalytic behaviors of the Fe/PANI catalyst. High C2-4= selectivity of 45%–55% could be obtained under all reaction conditions employed in this work. When the temperature was increased from 310 to 350 oC, CO conversion increased sharply from 8% to 79% and the C2-4=

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selectivity declined slightly from 50% to 45% (Fig. 4a). At the same time, the selectivities of CH4 and C2-40 increased, while that of C5+ hydrocarbons decreased. On the other hand, when

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the pressure increased from 0.5 to 2.0 MPa, CO conversion increased linearly and theC2-4= selectivity reduced gradually but was still > 45% even at the highest pressure employed (Fig.

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4b). At the same time, the selectivities of CH4 and C2-40 increased. The selectivity of C5+

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changed in the same trend with that of C2-4=, decreasing gradually upon increasing the pressure. Therefore, although the reaction parameters influence catalytic performances, the

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Fe/PANI catalyst shows high C2-4= selectivity in a wider range of operation conditions.

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Fig. 4. Catalytic performances of Fe/PANI under different operation conditions. (a) Different temperatures, (b) different pressures. Reaction conditions: W = 0.2 g, H2/CO = 1, P = 0.5-2.0

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MPa, T = 310–350 oC, F=30 mL/min.

We investigated the stability of the Fe/PANI catalyst at a pressure of 1 MPa and a

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temperature of 350 oC. A CO conversion of 20% was observed at the initial reaction stage, and it gradually increased during the reaction (Fig. 5). The observed increase in CO

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conversion may stem from the phase transformation of iron species from oxides to iron carbides [43,44]. The C2-4= selectivity increased in the initial 10 h, but it then decreased

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slowly with prolonging the time on stream. A high level (> 45%) of C2-4= selectivity could be sustained even after 100 h of reaction. The conversion of CO was 36% after 100 h of reaction.

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Fig. 5. Stability of the Fe/PANI catalyst. Reaction conditions: W = 0.2 g, H2/CO = 1, P = 1 MPa, T = 350 oC, F=30 mL/min.

3.3. The structures of catalysts after reaction

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Fe-based catalysts are known to undergo evolution during the FT reaction [45]. Fig. 6 shows the XRD patterns for the used catalysts after 30 h of reaction. The Fe/AC catalysts contained mixed iron carbide phases together with a small amount of Fe3O4. The XRD peaks belonging to iron oxides could not be observed for the Fe/N-AC and Fe/PANI catalysts after

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reactions. The Fe/PANI catalyst displayed strong iron carbide peaks, which could be attributed to Fe5C2, Fe2C and Fe3C phases. It is generally accepted that iron carbides are the

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active phases of iron-based catalysts for FT synthesis [39,46]. However, it is still controversial that which type of iron carbides is more active for FT synthesis [47]. The current consensus is that an active iron catalyst under working conditions contains a high fraction of iron carbides

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[4,48]. We found that the Fe/PANI catalyst with higher CO conversion activity and higher C2-4= selectivity was composed of iron carbides with a higher fraction and with better

AC

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crystallinity.

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Fig. 6. XRD patterns for Fe/AC, Fe/N-AC and Fe/PANI catalysts after reaction.

Fig. 7 shows the FT-IR spectra of PANI, Fe/PANI and the used Fe/PANI (Fe/PANI-R) catalysts. The IR bands at 1574 cm-1 and 1488 cm-1 are attributable to the C=N and C=C stretching modes of vibration for the quinonoid and benzenoid units, respectively. The band at 1300 cm-1 can be assigned to the C–N stretching mode of benzene ring. The C–H bending

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modes of the benzenoid ring and the quinonoid ring are at 1245 and 1124 cm-1, respectively

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[49,50]. Although the loading of iron on PANI led to small shifts of some IR bands ascribed to PANI, the three samples all showed the characteristic bands belonging to PANI. These observations confirmed that PANI was successfully synthesized and the PANI structure could

AC

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be maintained after the heat treatment under N2 and under the FT reaction.

Fig. 7. FT-IR spectra for PANI, Fe/PANI and the Fe/PANI after 30 h of reaction. 13

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The loss of nitrogen is a serious problem for N-containing carbon materials during high-temperature and high-pressure reactions. Table 3 displays the elemental contents of supports, the fresh catalysts and the used catalysts. The N contents for N-AC and PANI supports were 9.1 and 12 wt%, respectively. The N content decreased slightly after the loading of iron onto either N-AC or PANI. However, the change of the N content was quite different for the Fe/N-AC and Fe/PANI after reaction. The N content of the Fe/N-AC catalyst

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decreased dramatically from 8.8 to 2.8 wt% after reaction. This indicates that the N in the Fe/N-AC catalyst is unstable during the catalytic reaction. On the other hand, the N content in the Fe/PANI catalyst only changed slightly after FT reaction. This observation in combination with the FT-IR result for the catalyst after reaction clarifies that the Fe/PANI catalyst is highly stable. This is a quite unique, since the decrease in the N content after FT reactions has been

stability of N in the Fe/PANI catalyst.

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generally observed [28]. We believe that the robust structure of PANI contributes to the high

Table 3. Elemental composition of the supports and Fe based catalysts before and after

Residue

(%)

(%)

9.1

1.5

8.5

0.10

12

4.7

17

0.15

74

8.8

1.1

17

0.10

53

9.1

3.1

35

0.15

Fe/N-AC-R

70

2.8

0.9

26

0.03

61

8.9

3.5

27

0.13

N

(%)

(%)

81

PANI

67

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N-AC

Fe/PANI

CE

Fe/N-AC

Fe/PANI-R

a

C

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Samples

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H

AC

reactiona.

N/Cb

Presented by weight percentage.

b

Presented by molar ratio.

To understand the significant role of N-containing groups in the Fe/PANI catalyst in improving the C2-4= selectivity, we investigate the surface properties of the PANI as well as AC and N-AC using CO2-TPD and C3H6-TPD techniques. Fig. 8(a) shows that AC without N-doping cannot chemisorb acidic CO2 molecules, whereas PANI and N-AC display 14

ACCEPTED MANUSCRIPT significant CO2 chemisorption. The desorption peaks of CO2 from PANI and N-AC occurred at 200 and 250 oC, respectively. It is generally accepted that the basic support is favorable for the synthesis of C2-4=, since the basicity is beneficial to the desorption of C2-4= [23,27]. We further measured the C3H6-TPD. The results demonstrates that the desorption of C3H6 from the Fe/PANI occurs at around 110 oC (Fig. 8b). This temperature is lower than those for the Fe/N-AC (~130 oC) and the Fe/AC (170 oC). Thus, the desorption of lower olefins is the

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easiest for the Fe/PANI. We believe that this can keep the largest fraction of C2-4= from subsequent hydrogenation on the Fe/PANI surface, contributing to its highest C2-4= selectivity

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and highest ratio of olefins/paraffins (Table 2).

Fig. 8. TPD profiles for the supports and catalysts. (a) CO2-TPD for AC, N-AC and PANI supports, (b) C3H6-TPD for the used Fe/AC, Fe/N-AC and Fe/PANI catalysts.

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ACCEPTED MANUSCRIPT The sintering of Fe particles and the coke formation are proposed to be the two main reasons for the deactivation of FT catalysts [1,51,52]. We have characterized our catalysts after 30 h of reaction by using TEM and HRTEM techniques. As displayed in Fig. 9, the mean diameter of iron particles was 27 nm for the Fe/AC after reaction. This mean size was bigger than that (18 nm) for the fresh Fe/AC catalyst (Fig. 2a). Thus, serious sintering of Fe particles occurred for this catalyst during the reaction and a carbon shell surrounding the iron

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particles was also observed. The sintering of iron particles and carbon-shell formation were not significant for the Fe/N-AC catalyst after reaction. The Fe/PANI catalyst still had the highest dispersion of iron particles after reaction among these three catalysts. The mean iron particle size only increased slightly to 9.9 nm from 7.1 nm for the fresh catalyst. However, the carbon shell surrounding the iron carbide particles could still be observed. This may be

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responsible for the slow decrease in the C2-4= selectivity with time on stream (Fig. 5).

Fig. 9. TEM micrographs, HRTEM micrographs and particle size distributions for supported Fe catalysts after 30 h of reaction. (a) Fe/AC, (b) Fe/N-AC, (c) Fe/PANI.

Our HRTEM results showed that the Fe/N-AC and Fe/PANI catalysts after reaction were 16

ACCEPTED MANUSCRIPT mainly composed of iron carbide phases, in particular Fe5C2, whereas the used Fe/AC catalyst contained both Fe5C2 and Fe3O4 particles. This agrees with the XRD results. Conductive polymers have attracted much attention in catalysis, but few studies have been devoted to the utilization of conductive polymers for FT synthesis. This work presents the first study using PANI, a well-known conductive polymer, as a support of Fe catalysts for FT synthesis. Through our studies, we have gained the following insights that may guide the

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design of efficient FT catalysts for lower olefins synthesis (Scheme 2). First, PANI can provide abundant anchoring sites for iron precursors and stabilize iron particles during severe FT reaction conditions. Second, the presence of nitrogen-containing functional groups could facilitate the reduction of iron oxides, which is a crucial step for obtaining high carburization

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degree for Fe-based catalysts under reaction conditions and high FT reaction activity. Our characterizations show that the most active working catalyst (Fe/PANI) contain mixed the highest fraction of iron carbides (Fe5C2, Fe3C and Fe2C), and no iron oxide phases were observed from both XRD and HRTEM for this catalyst. In addition, the presence of

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nitrogen-containing groups induce the basicity of the catalyst, facilitating the desorption of the formed lower olefins. Furthermore, the nitrogen groups can help inhibit the sintering of iron

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particles. We believe that all these favorable properties contribute to the higher CO conversion

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and enhanced lower olefin selectivity for the Fe/PANI catalyst.

Scheme 2. The functions of PANI as a catalytic support for lower olefins synthesis from syngas.

4. Conclusions PANI was successfully synthesized by a simple sonochemical method. The Fe/PANI catalyst displayed excellent catalytic performance in Fischer-Tropsch synthesis with lower 17

ACCEPTED MANUSCRIPT olefin selectivity as high as 50%. The CO conversion and lower olefins selectivity over the Fe/PANI catalyst were much higher than those over the Fe/AC and Fe/N-AC catalysts. The use of PANI as a support led to uniform and higher dispersion of Fe particles, and facilitated the reduction of iron oxide and the formation of iron carbides. These eventually resulted in higher catalytic activity. The intrinsic basicity of the PANI also facilitated the desorption of lower olefins from the catalyst, favoring the selectivity of lower olefins. We demonstrated that

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the nitrogen groups in PANI are very stable under severe Fischer-Tropsch reaction conditions. The robust structure and excellent performances of the Fe/PANI catalyst provide a new possibility to design advanced Fischer-Tropsch catalysts for the synthesis of lower olefins without additional promoters.

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Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21503174, 91545203, 21433008, 21403177 and 21673188) and China Postdoctoral Science

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Foundation (No. 2016T90596).

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ACCEPTED MANUSCRIPT Description portion :

Polyaniline support with abundant surface electron-rich nitrogen groups can enhance the dispersion and reduction of iron species, thus favoring the CO conversion and lower olefins selectivity in Fischer-Tropsch synthesis.

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Graphic Abstract

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