Enhancing the light olefin selectivity of an iron-based Fischer-Tropsch synthesis catalyst by modification with CTAB

Article abstract of DOI:10.1039/c8ra04622k

The effects of the surfactant hexadecyltrimethylammonium bromide (CTAB) on the catalytic performance of a manganese-promoted iron (FeMn) catalyst for the Fischer-Tropsch to olefin (FTO) reaction were investigated. The use of the CTAB-assisted FeMn catalyst resulted in the production of light olefin (C_2-4) selectivity of up to 55.45% with a ratio of olefin to paraffin among the C₂-C₄ hydrocarbons as high as 7.75 under industrially relevant conditions (320 °C, 1.0 MPa, H₂/CO ratio of 1.5 (v/v), GHSV = 4200 h^-1). The characterization results indicate that CTAB has a great influence on the structure, composition, chemical state, and catalytic performance of the iron-based catalyst. Most interestingly, a greater amount of Mn promoter was found to be dispersed on the surface of α-Fe₂O₃, rather than being dissolved into the α-Fe₂O₃ lattice when CTAB was employed, which contributed towards enhancing the promotional effects of the Mn promoter, leading to the formation of certain surface-specific activity sites.

Full text of DOI:10.1039/c8ra04622k

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Enhancing the light olefin selectivity of an iron-
based Fischer–Tropsch synthesis catalyst by
modification with CTAB†
Cite this: RSC Adv., 2018, 8, 32073
a
b
a
Yingxin Liu, Chao Huo and Huazhang Liua
*
Chuanxue Zhu,
The effects of the surfactant hexadecyltrimethylammonium bromide (CTAB) on the catalytic performance of
a manganese-promoted iron (FeMn) catalyst for the Fischer–Tropsch to olefin (FTO) reaction were
]
investigated. The use of the CTAB-assisted FeMn catalyst resulted in the production of light olefin (C2–4
)
selectivity of up to 55.45% with a ratio of olefin to paraffin among the C2–C4 hydrocarbons as high as 7.75
under industrially relevant conditions (320 ꢀC, 1.0 MPa, H2/CO ratio of 1.5 (v/v), GHSV ¼ 4200 hꢁ1). The
characterization results indicate that CTAB has a great influence on the structure, composition, chemical
state, and catalytic performance of the iron-based catalyst. Most interestingly, a greater amount of Mn
promoter was found to be dispersed on the surface of a-Fe2O3, rather than being dissolved into the a-
Fe2O3 lattice when CTAB was employed, which contributed towards enhancing the promotional effects of
the Mn promoter, leading to the formation of certain surface-specific activity sites.
Received 30th May 2018
Accepted 3rd September 2018
DOI: 10.1039/c8ra04622k
rsc.li/rsc-advances
FTO process.4,5,10 Other negative factors, such as carbon depo-
sition and catalyst stability, remain severe problems for the FTO
reaction. Generally, iron-based and cobalt-based catalysts are
1. Introduction
Light olens (C2–4]), which include ethylene, propylene and
butylenes, are widely used in the chemical industry as key
building blocks.1,2 Traditionally, light olens are produced by
naphtha cracking or uid catalytic cracking, raising many
concerns about the availability of petroleum-based feedstocks
and their serious impact on the environment.3,4 Therefore,
alternative routes for their production have been explored. For
example, light olens can be produced via direct routes,
namely, the oxide–zeolite (OX–ZEO) process and Fischer–
Tropsch to olens (FTO) reaction.2,3,5–7 In both of these
processes, synthesis gas, a mixture of CO and H2, which can be
derived from coal, natural gas, and renewable biomass, is
employed as a feedstock.8 The FTO reaction has received
renewed interest in recent decades because of its simplied
operation conditions and low energy consumption.4,9 However,
the FTO reaction is based on the surface polymerization of CHx
species to form hydrocarbons, and the selectivity of C2–C4
hydrocarbon products is limited by the so-called Anderson–
Schulz–Flory (ASF) distribution, which is no more than 58%.
Furthermore, high methane selectivity is a big challenge for the
expected to be used for highly efficient FTO reactions.11,12 Iron-
based catalysts are commonly applied in the FTO reaction, due
to them being favorable for light olen production, low cost and
having a high activity for the water–gas shi reaction (WGS).11,13
Fischer–Tropsch chemistry is very complex and subtle, and
can be greatly inuenced by reaction conditions and catalyst
properties.14–16 Recently, various iron-based catalysts have been
designed for improving light olen selectivity by proper catalyst
modication, such as optimizing the catalyst composition and
structure.17–21 The modication of iron-based FTO catalysts with
promoters has been shown to be an effective way of tuning the
selectivity of products.20,22,23 As an electron promoter, manga-
nese can enhance the selectivity of olens by suppressing
methane selectivity and the a-olen secondary reaction.19
Manganese has specic effects on iron-based catalyst proper-
ties, including changing the surface basicity and the electronic
state of the active phase, and effecting the dispersion of iron,
which consequently affect the catalyst reduction and carbon-
ization activities.19,23–25 Recently, the modulation of surfactant
has been reported to exert a great inuence on nucleus forma-
tion and crystal growth during co-precipitation by adsorbing on
the formed particle surfaces, which can efficiently protect the
particles from agglomeration and even tailor the surface prop-
erties of the catalyst.5,26–28
aInstitute of Industrial Catalysis, Zhejiang University of Technology, Hangzhou
310032, Zhejiang, PR China. E-mail: zhucx@zjut.edu.cn; chaohc@zjut.edu.cn;
hzliu@zjut.edu.cn; Fax: +86-571-88320815; Tel: +86-571-88320815
bResearch and Development Base of Catalytic Hydrogenation, College of
Pharmaceutical Science, Zhejiang Technology, Hang-zhou 310032, Zhejiang, PR
China. E-mail: yxliu@zjut.edu.cn
Iron carbides have been reported to be the catalytic active
phase of iron-based catalysts during the Fischer–Tropsch
synthesis (FTS) reaction in many studies.29–31 Suitable iron
carbide particle size or shell thickness was found to be essential
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra04622k
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for high catalytic activity.14,32 However, the exact role that equation, and the total pore volume and average pore size were
different carbides play in syngas conversion, via the FTS or FTO determined using the Barrett–Joyner–Halenda (BJH) method.
reactions, is controversial. This is why the eld of FTS has
The structures of the catalysts were determined using an X-
¨
received wide attention. As reported, the use of c-Fe5C2 (Hagg ray diffractometer (XRD) (Rigaku, Thermo ARL SCINTAGX
˚
carbide) favours the production of C5+ hydrocarbons and TRA) equipped with CuKa radiation (l ¼ 1.54050 A, 40 kV, 40
oxygenates,29,33 while q-FeC3 (iron carbide) results in the mA) in the 2q angle range from 10–80ꢀ at a scanning speed of
formation of light olens.19 Even so, the understanding of iron 4ꢀ minꢁ1. The in situ XRD patterns of the catalyst were recorded
carbide species in the FTO reaction has met with limited under a ow of H2 (30 mL minꢁ1) during the stepwise heating of
ꢁ1
ꢀ
ꢀ
success.
In our work, the catalysts were prepared using a surfactant
the sample from 25 to 500 C at a rate of 5 C min
.
The actual compositions of the samples were analyzed using
hexadecyltrimethyl ammonium bromide (CTAB)-associated X-ray uorescence (XRF, Rigaku ZSX Primus II) and energy-
metal salt solution precursor via a co-precipitation route. The dispersive spectrometers (EDS, Thermo NORAN, USA)
addition of CTAB led to effective control of the size and shape of attached to a scanning electron microscope. The near surface
the formed metal oxide nanoparticles,34–39 which had a great chemical information of the materials was analyzed by X-ray
inuence on the nature of the catalyst surface and its perfor- photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD,
mance. A narrow particle-size distribution with smaller parti- Japan) using an Al Ka X-ray source, where the base pressure of
cles was obtained when sufficient surfactant (CTAB) was the chamber was less than 2 ꢄ 10ꢁ8 Pa. The binding energies
employed, which did well in inhibiting carbon deposition (BEs) were calibrated relative to adventitious carbon using the C
during the FTO reaction. Moreover, a larger amount of Mn 1s peak at 284.8 eV.
promoter was found at the surface of a-Fe2O3, rather than being
The morphology of the catalysts was characterized by
dissolved into the a-Fe2O3 lattice, resulting in a low interaction transmission electron microscopy (TEM) on a Hitachi H-600
between Fe–Mn, which contributed to the promotional effects electron microscope operated at an accelerating voltage of 100
of Mn on the iron-based FTO catalyst.19,40 Here, the effects of kV. The TEM specimens were prepared by ultrasonically sus-
CTAB on the catalyst properties, involving the structure, surface pending a powder sample in ethanol, and then drops of the
chemical state and catalytic performance were investigated in suspension were deposited on a carbon-coated copper grid and
detail using several characterization methods.
dried at room temperature before analysis.
The reduction behavior was measured by hydrogen
temperature-programmed reduction (H2-TPR) experiments
carried out in an ASAP 2020 (Micromeritics, America) using
5 vol% H2/95 vol% Ar (ow rate 50 mL minꢁ1) as the reducing
agent. About 30 mg of sample was pretreated under an Ar ow
(20 mL minꢁ1) at 200 ꢀC for 2 h, and for the reduction, the
sample was heated from room temperature to 700 ꢀC at a rate of
10 ꢀC minꢁ1. The TPR proles were recorded using the response
of the thermal conductivity detector (TCD) of the effluent gas.
The basicity of the samples was determined by stepwise
temperature-programmed desorption of carbon dioxide using
the same equipment as that employed for the H2-TPR
measurements. About 50 mg of catalyst was loaded into the
reactor and was heated under a ow of He (50 mL minꢁ1) from
room temperature to 700 ꢀC. The samples were subsequently
cooled to 50 ꢀC under the He ow, and then exposed to CO2 for
1 h, followed by purging with He for 1 h to remove the weakly
adsorbed species. Aer this step, the temperature was increased
2. Experimental section
2.1 Synthesis of the catalysts
The FeMn catalysts were prepared via
a modied co-
precipitation method. Typically, an appropriate amount of
iron(III) nitrate nonahydrate (Fe(NO3)3$9H2O) and man-
ganese(II) nitrate tetrahydrate (Mn(NO3)2$4H2O) were dissolved
into deionized water to form a 0.136 M metal nitrate solution
(the nominal molar ratio of Mn/Fe was 0.08). This salt solution
was introduced into a solution of CTAB (35ꢀ0 mL) to obtain
a mixed solution, which was preheated to 60 C. Then, a solu-
tion of NH3$H2O (1.0 M, 300 mL) was slowly dropped into the
above mixed solution in a beaker (pH ꢂ 9.5) and was mechan-
ically stirred (400 rpm) for 1.5 h. A constant temperature of 60 ꢃ
ꢀ
0.5 C was maintained during the precipitation process. Aer
aging for 12 h at room temperature, the obtained suspension
was ltered, wasꢀhed thoroughly with deionized water, dried
ꢁ1
ꢀ
to 700 ꢀC at a rate of 10 C min
.
ꢀ
overnight at 120 C and then calcined at 400 C for 5 h under
static air. The obtained catalysts were denoted as FeMn–CTAB-
g, in which g is the molar ratio of CTAB to Fe atoms. For
comparison, the catalyst was prepared without CTAB following
a similar procedure, and was named as FeMn–H2O.
2.3 Catalyst evaluation
The FTS reaction was performed in a xed-bed reactor with an
inner tube diameter of 10 mm. Generally, the catalyst (60–100
mesh, 1.0 mL) was mixed with quartz sand (60–100 mesh, 5.0
mL) and then loaded into a stainless-steel reactor (the catalyst
2.2 Catalyst characterization
The texture properties of the samples were measured by was diluted with the same size quartz sand to remove any
nitrogen physisorption at ꢁ196 ꢀC in ASAP 2010 (Micromeritics, temperature gradient within the catalyst bed). The gas ow was
USA). Prior to the adsorption measurements, the samples were controlled by mass ow controllers (MFC). The reaction prod-
evacuated under vacuum at 200 ꢀC for 6 h. The specic surface ucts passed over a 160 ꢀC hot trap and a ꢁ1 ꢀC cooling trap
area was calculated using the Brunauer–Emmett–Teller (BET) under working pressure, resulting in the collection of aqueous,
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liquid oil, and solid wax products. The residual outlet gas was of the FeMn mixed oxide using a CTAB-associated iron and
analyzed online by gas chromatography (GC). The catalyst was manganese oxide co-precipitation method, by suppressing
reduced prior to the reaction in H2 at 400 ꢀC under 1.0 MPa for particle aggregation in the preparation process, is an effective
12 h with a gas hourly space velocity (GHSV) of 1500 hꢁ1 method.34,35 Furthermore, compared with that of the FeMn–H2O
(1500 mL hꢁ1 mLcatꢁ1). Aer that, the reactor was cooled down catalyst, there was a greater increase in the intensity of the peak
to 200 ꢀC. Subsequently, the feed ow was switched to synthesis of the (110) plane than that of the (104) plane upon the addition
gas (volume ratio H2/CO ¼ 1.5), with the pressure maintained at of CTAB, which is anomalous to the standard stick pattern
1.0 MPa, and the reactor was heated to 320 ꢀC at 4 ꢀC minꢁ1 with (JCPDS card no. 033-0664). This result implies that the addition
a GHSV of 4200 hꢁ1. The gaseous reaction products were of CTAB in the preparation of iron-based catalysts dramatically
analyzed online using a gas chromatograph (SHIMADZU GC inuences the growth orientation of the a-Fe2O3 nanocrystals,
2014ATF) equipped with two columns and two detectors. The resulting in the preferential exposure of the (110) plane.
analysis of the H2, CO, CO2, and CH4 content of the outlet gases
The N2 adsorption–desorption isotherms (Fig. 1b) show that all
was performed using a carbon molecular sieve column (TDX-1) of the samples exhibit type IV isotherms with clear H2(b) hysteresis
with a thermal conductivity detector (TCD), using He as the loops due to capillary condensation in the mesoporous materials.32
carrier gas. The hydrocarbon (C1–C5) products were analyzed Two peaks can be observed from the pore size distribution curve of
using a KCl modied alumina capillary column (19095P-K25) the FeMn–H2O catalyst (Fig. S1†), suggesting the co-existence of
with an argon carrier and a hydrogen ame ionization two types of pores. The small pores centered at around 5–12 nm
detector (FID). The selectivity was calculated as the percentage are generated from the intra-aggregation of Fe2O3 and the large
of equivalent carbons present in the hydrocarbon product (C%). pores observed at around 80 nm are caused by inter-aggregation of
The CO conversion was calculated on a carbon-atom basis, as the catalyst.32,43 It is noteworthy that the size of the intra-aggregated
follows:
pores slightly shis towards a smaller pore size, while the inter-
aggregated pores become broader and then disappear upon the
addition of CTAB, indicating that the aggregated particle size
signicantly decreased and the aggregation of the catalysts was
effectively suppressed.43
COinlet ꢁ COoutlet
CO conversion ¼
ꢄ 100%
(1)
COinlet
where COinlet and COoutlet represent the moles of CO at the inlet
and outlet, respectively.
The BET surface areas and total pore volumes of the FeMn
catalysts increased signicantly from 73.22 m2 gꢁ1 and 0.23 m3
gꢁ1 for the FeMn–H2O catalyst to 135.65 m2 gꢁ1 and 0.30 m3 gꢁ1
for the FeMn–CTAB-0.8 catalyst (Table S1†). The molar ratios of
Mn/Fe on the surfaces of the catalysts were measured by XPS
and were found to be much higher than that of the bulk
determined by XRF, which suggests that Mn was mainly
dispersed on the surface of the a-Fe2O3 particles (Table S1†). No
residual elemental bromine (Br) was detected, which indicates
that the Br species was completely removed during decompo-
sition of CTAB by calcination (400 ꢀC, 5 h). The TGA-MS result in
Fig. S2† proves that the surfactant was entirely evaporated and
that no residual carbon source from the CTAB was present in
the catalyst, as observed by a lack of signal for CO/CO2.
The redox properties of the catalysts are shown in Fig. 1c.
The reduction peaks in the range of 280–390 ꢀC, 390–550 ꢀC and
The CO2 selectivity was calculated according to the equation:
CO2
outlet
CO2 selectivity ¼
ꢄ 100%
(2)
COinlet ꢁ COoutlet
where COoutlet represents the moles of CO2 at the outlet.
The selectivity of the individual product CnHm was obtained
from the equation:
nCnHm
CnHm selectivity ¼
ꢄ 100%
(3)
COinlet ꢁ COoutlet ꢁ CO2
outlet
where CnHm outlet represents the moles of C of the product at the
outlet.
3. Results and discussion
The XRD patterns of the calcined FeMn catalysts exhibit similar 550–630 ꢀC in the H2-TPR prole of the FeMn–H2O catalyst were
peaks at 2q values of 24.2, 33.2, 35.6, 40.9, 49.5, 54.1, 62.4, and ascribed to the processes of a-Fe2O3 to Fe3O4, Fe3O4 to FeO and
64.0ꢀ (Fig. 1a), which can be assigned to the (012), (104), (110), FeO to metallic Fe,23,32,41,42 respectively. The rst reduction peak
(113), (024), (116), (214), (300) planes of a-Fe2O3 (hematite gradually divided into two distinct peaks upon the addition of
ꢀ
phase, PDF#87-1166), respectively. In addition, no MnOx can be CTAB. The peak in the range of 280 to 345 C corresponded to
observed in all of the XRD patterns. However, the elemental the reduction of a-Mn2O3 to Mn3O4,23,41,42 of which the peak area
analysis results showed that the Mn/Fe ratios on these sample gradually enhanced, as presented in Table S2,† suggesting that
surfaces are in the range of 0.12–0.17 (Table 1). This suggests there was a larger amount of a-Mn2O3 exposed on the surface.
that MnOx was highly dispersed on the surface of the catalysts,41 The other peak in the range of 345 to 390 ꢀC was assigned to the
or that the size of the crystalline MnOx is out of the XRD reduction of a-Fe2O3 to Fe3O4,23,32 however, the peak area
detection limit.42 The broad peaks of the FeMn–CTAB catalysts showed the opposite trend to that of the previous peak (Table
indicate a very small crystallite size.32 Calculated via the S2†), which might be due to the higher coverage of Mn promoter
Scherrer equation based on the (104) plane, the average a-Fe2O3 on the surface iron atoms, limiting the contact between the
nanoparticle size was found to signicantly decrease from active Fe phase and H2.19,40
18.1 nm for FeMn–H2O to 6.1 nm for FeMn–CTAB-0.8 (Table
The intensity of the second peak of FeMn–H2O decreased
S1†). These results suggest that reducing the nanoparticle size when a low amount of CTAB was employed, and disappeared
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Fig. 1 (a) XRD patterns, (b) N2 adsorption–desorption isotherms, (c) H2-TPR profiles, (d) CO2-TPD profiles, (e) Fe 2p XPS spectrum, (f) Mn 2p XPS
spectrum of the calcined FeMn catalysts with different amounts of CTAB.
upon the further addition of CTAB, which indicates that the Fe2O3 surface.23,24,42 From the H2-TPR proles, it was revealed
Fe3O4 was directly reduced to metallic Fe, and that FeO was not that the third reduction peak intensity remarkably increased
formed as an intermediate because it was unstable.23,41 It is with peak temperature dropping to the lowest 596 ꢀC for FeMn–
possible that the increase in the amount of CTAB added CTAB-0.8 catalyst, caused by the higher dispersion of the metal
inhibited the incorporation of Mn2+ ions into the FeO lattice, oxides, smaller nanoparticles size, and larger BET areas were
leading to a signicant decrease in the interaction between Fe– obtained upon addition of CTAB.24,41 The reduction process of
Mn and a large amount of MnOx species dispersing on the a- the calcined FeMn–CTAB-0.8 catalyst in H2 was also studied by
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Table 1 Catalytic performance of the FeMn catalysts with different amounts of added CTABa
Hydrocarbon selectivity (C mol%, CO2-free)
Catalysts
CO conv. (%)
CO2 sel. (%)
CH4
C2–4 olens
C2–4 paraffins
C5+ oxygenatesc
O/Pb
FeMn–H2O
91.31
88.30
73.57
55.41
34.76
63.70
41.04
36.99
29.65
23.62
14.0
15.29
13.59
11.38
14.69
18.04
12.31
30.16
29.81
35.25
44.13
55.45
37.72
13.75
10.78
9.08
9.18
7.16
6.67
40.80
45.82
44.29
32.0
19.36
43.30
2.19
2.77
3.88
4.81
7.75
5.66
FeMn–CTAB-0.15
FeMn–CTAB-0.3
FeMn–CTAB-0.6
FeMn–CTAB-0.8
FeMn–CTAB-1.0
21.13
a
Catalysts were in situ reduced at 400 ꢀC for 12 h and tested at 320 ꢀC, 1.0 MPa, GHSV ¼ 4200 hꢁ1, H2/CO ¼ 1.5 (v/v), at TOS ¼ 20 h. b O/P represents
the molar ratio of olen to paraffin in the C2–C4 range of hydrocarbons. c C5+ are the hydrocarbons that were analyzed online by gas chromatography
(GC); oxygenates are from the 160 ꢀC hot trap and ꢁ1 ꢀC cooling trap.
in situ XRD, as exhibited in Fig. S3,† which further revealed the atoms were exposed on the surface, owing to the lower and
reduction of a-Fe2O3 via the processes of a-Fe2O3 to Fe3O4, higher number of particles of both Fe and Mn oxides.
Fe3O4 to FeO and FeO to metallic Fe.
Furthermore, regardless of whether the Mn promoter was
The catalyst surface basicity was investigated by CO2-TPD, as located on the a-Fe2O3 surface, or dissolved into the a-Fe2O3
shown in Fig. 1d. The rst peak at a low temperature of around lattice, it did not result in a signicant change in the Mn/Fe
60 ꢀC in the CO2-TPD prole can be attributed to the desorption ratio on the catalyst surface.
of CO2 weakly adsorbed in the bulk a-Fe2O3 phase.24,42 The other
The morphologies of the calcined FeMn–H2O and FeMn–
two peaks in the range of 130 to 330 ꢀC and 330 to 660 ꢀC can be CTAB-0.8 catalysts were measured by TEM, as shown in Fig. 5. A
attributed to the desorption of CO2 that interacted moderately much smaller nanoparticle size was observed for the samples
with the surface medium-strong basic sites and the strongly with CTAB. The average particle size of the FeMn–H2O catalyst
chemisorbed CO2 that had some bearing on the surface strong was 18.7 nm, estimated by randomly selecting 200–300 particles
basic sites,24,42 respectively. The peak intensities in the high in the range of 10 to 35 nm (Fig. 2a), while for the FeMn–CTAB-
temperature area of the CO2-TPD prole increased signicantly 0.8 catalyst, the nanoparticles were found to be in the range of
upon the addition of CTAB (Table S3†), which indicates that 3–10 nm, except for a few particles with a larger size of around
CTAB remarkably enriched the basic sites on the catalyst 19.0 nm. The average size of the nanoparticles was 4.79 nm,
surface. This may be due to the surface imperfections on the which can be observed in the particle size distribution (PSD) in
small particles as well as a greater amount of MnOx species Fig. 2c. Combining the information from the TEM images and
exposed to the surface, enhancing the number of basic sites.43,44 the corresponding PSD (Fig. S4†), it can be considered that the
Moreover, the peak position shied to a higher temperature, addition of CTAB effectively decreases the nanoparticle size.34–38
which suggests an increase in basicity.42
Moreover, Fig. S4d† shows that a narrower particle-size distri-
The Fe 2p XPS spectra of the catalysts are shown in Fig. 1e. bution was obtained when sufficient CTAB was employed. The
The peaks positioned at around 710.28 and 724.08 eV (Table reason for this might be that CTAB effectively prevents the
S4†) correspond to the Fe 2p3/2 and Fe 2p1/2 levels of Fe3+
,
particles from agglomerating during the co-precipitation
respectively, together with Fe3+ satellite peaks detected at process.34–38,45 HR-TEM analyses demonstrated that such nano-
around 718.78 eV, verifying the formation of the a-Fe2O3 phase particles are composed of a-Fe2O3, with specically exposed
for all of the samples.32,40 Notably, there was a decrease in the Fe interplanar spacing facets of (006) (2.29 A), (110) (2.52 A), and
˚
˚
˚
2p3/2 peak intensity upon an increase in the amount of CTAB (104) (2.70 A) (Fig. 2d). No MnOx was detected in the samples.
employed, implying there was a decrease in the number of Fe These are results were found to be in good agreement with the
atoms on the surface.9,40 More noteworthy is that the Fe 2p3/2 XRD results. The selected area electron diffraction (SAED)
peak position shied to a lower binding energy upon an pattern of FeMn–H2O revealed it to be polycrystalline, while that
increase in the CTAB to Fe atom molar ratio in the preparation, of the FeMn–CTAB-0.8 catalyst showed its single crystalline
indicating a higher electron density of surface Fe atoms nature.
(Fig. 1e).9,19,40
EDS elemental mapping analyses were further employed to
Fig. 1f reveals that Mn promoter is located on the surface of gain an insight into the actual distributions of Fe and Mn in the
the catalysts. The Mn 2p XPS peak positions imply that various calcined FeMn–CTAB sample. As shown in Fig. 3a, it was found
oxidized manganese species form upon the addition of CTAB, that the Fe patch corresponds to big nanoparticles, while the Mn
such as Mn3+ and Mn4+ for the promoted catalysts, as shown in patch was detected in the surrounding small nanoparticles and the
Fig. S5,† which suggests that electron transfer occurs between O patch almost exhibits the same shape. These results strongly
the surface Mn and Fe atoms.9 It is worth pointing out that the indicate that the Mn species is located on the surface of Fe2O3.
molar ratio of Mn/Fe at the surface of the FeMn–CTAB-0.8
A schematic diagram of the preparation process is shown in
sample slightly decreased even if a greater number of Mn Fig. 4. It can be seen that the cationic CTAB surfactant facilely
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Fig. 2 TEM images of the calcined FeMn–H2O and FeMn–CTAB-0.8 as-prepared catalysts, where d refers to the average particle size.
adsorbed on the precipitated particle surfaces through the observed at a 2q value of 25ꢀ (carbon PDF#26-1077), which was
attraction of opposite charges, leading to the formation of due to the Boudouard reaction (2CO / C + CO2) in the catalytic
micelle shells, in which the hydrophobic tail group points reaction process.40 The HR-TEM image and Fe 2p XPS spectrum
towards the center nanoparticles and the hydrophilic head of the spent FeMn–CTAB-0.8 catalyst further proved that the
group points towards the aqueous solution.37,38 The formed sample mainly consisted of Fe3O4 and iron carbides, as well as
micelle shells protect the particles from agglomerating during carbon deposits (Fig. 5b and c). The diffraction peaks of the
the preparation process, and were completely evaporated by spent catalyst were sharper than those of the as-synthesized a-
calcination, with no residual CTAB found in the catalyst.34–39 Fe2O3 nanoparticles, suggesting that the average size of iron-
Regular micelle shells were obtained with a moderate amount containing particles increased aer the reaction. The TEM
of CTAB. However, since the precipitation process was unstable results further proved that the average particle size increased
and constantly changed, this led to a disordered distribution of from 4.79 to 37.8 nm, as shown in Fig. 2c and S6e.† It is note-
the surfactant. Therefore, the formed micelle shells became worthy that the iron carbides formed during the FTO reaction
irregular when an excess amount of CTAB was introduced, as were present in more than one phase and that this was
¨
shown in Fig. 4c, which although was not as effective as the dependent on the CTAB content. For example, c-Fe5C2 (Hagg
former process, it also prevented the particles from agglomer- carbide, PDF#89-2544) was found in the spent FeMn–H2O
ating. This is the reason why the particle size and BET surface catalyst, while Fe7C3 (iron carbide, PDF#17-0333) and q-Fe3C
area increased for the FeMn–CTAB-1.0 catalyst. This suggests (iron carbide, PDF#89-2005) were observed in the spent FeMn–
that the optimum molar ratio of CTAB to Fe in the catalyst CTAB-0.8 catalyst, which was veried by XRD and HR-TEM, as
preparation is 0.8.
The structural characterization of the spent FeMn catalysts surface sites formed when CTAB was employed.
was carried out aer 60 h on stream in the FTO reaction. Fig. 5a The TEM images in Fig. S6d† show that overlapped particles
shown in Fig. 5a and c. This indicates that certain specic
shows that the Fe3O4 and iron carbide species are the dominant slightly aggregated, with a size distribution mainly ranging
phases in the samples. In addition, carbon deposition was from 20 to 55 nm. The average particle size was 37.8 nm for the
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Fig. 3 TEM mapping of the calcined FeMn–CTAB-0.8 catalyst.
spent FeMn–CTAB-0.8, which was larger than that of the other
The Fe 2p XPS spectra of the spent FeMn–CTAB-0.8 catalyst
spent samples, as can be seen in Fig. S6,† while the spent shows two sets of peaks from the Fe3O4 and c-Fe5C2 phases in
FeMn–CTAB-1.0 had the smallest mean size of 24.4 nm among the range of the Fe 2p3/2 (710.10 eV) and Fe 2p1/2 (723.75 eV)
all of the spent samples. It can be considered that small parti- orbital signals (Fig. 5b), suggesting the reduction and carbon-
cles tended to aggregate, while the narrow particle-size distri- ization of a-Fe2O3,32 which is in good agreement with the XRD
bution, with an appropriate particle size of about 9.2 nm, for (Fig. 5a) results. Fig. S7b† shows that the O 1s peak appears at
FeMn–CTAB-1.0 resulted in a decrease in the extent of the a binding energy of 529.08 eV, and can be deconvoluted into
particle aggregation. The HR-TEM images of FeMn–CTAB-0.8 MnO and Fe3O4, indicating the coexistence of iron–manganese
exhibit the specic (220) and (311) planes of Fe3O4, the (31ꢁ1) oxide and carbide phases on the surface. Fig. S7d† shows the
plane of c-Fe5C2, the (101) plane of q-Fe3C, and the (202) plane formation of q-Fe3C aer the reaction.
of Fe7C3. Furthermore, Fig. 5c and d show that the presence of
Table 1 summarizes the catalytic activities and hydrocarbon
iron carbides located on the surface of Fe3O4 with a size of 4.3– distribution of the FeMn–H2O and FeMn–CTAB-g catalysts (g ¼
9.4 nm, which would be very likely to form Fe3O4@iron carbide 0.15/0.3/0.6/0.8/1.0) under the industrially relevant conditions
core–shell structure nanoparticles.
of 320 ꢀC, 1.0 MPa, GHSV ¼ 4200 hꢁ1, and a H2/CO (v/v) ratio of
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Fig. 4 Schematic illustration of the preparation process for the FeMn–CTAB catalysts.
1.5, at a TOS ¼ 20 h. As presented in Table 1, the use of the which the ratio of olen to paraffin (denoted as O/P) in the C2–
FeMn–H2O catalyst resulted in 91.31% CO conversion and C4 range hydrocarꢁb1ons was as low as 2.19, and the FTY was 1.29
15.29% CH4 selectivity, as well as 30.16% C2–4 selectivity, of ꢄ 10ꢁ5 molCO gFe sꢁ1. For the FeMn–CTAB catalysts, the CO
]
Fig. 5 (a) XRD patterns of the spent FeMn catalysts, (b) Fe 2p XPS spectrum of the spent FeMn–CTAB-0.8 sample, and (c and d) (HR)-TEM images
of the spent FeMn catalysts.
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conversion and FTY gradually decreased upon the addition of system.19 Thus, we prepared a series of FeMn–CTAB catalysts
CTAB, even though there was an increase in the BET surface with different loadings of Mn promoter and studied their FTO
area due to many of the Fe activity sites being covered by Mn catalytic performance. The catalyst activity, CO conversion and
promoter (Fig. 3a). The same results were obtained upon adding products distributions are summarized in Table S5.† It can be
]
the amounts of Mn promoter shown in Table S5† (the molar seen that the C2–4 selectivity of FeMn–CTAB-0.8 was at its
]
ratio of CTAB/Fe is 0.8). While the C2–4 selectivity and O/P highest (about 61.21%) when the loading of Mn was 0.1 (Mn/
value were found to gradually increase upon the addition of Fe, mol molꢁ1). In addition, the catalyst activity and CO
CTAB. When the addition of CTAB was 0.8 (CTAB/Fe, mol conversion gradually decreased upon an increase in the Mn
]
molꢁ1), the C2–4 selectivity was at its highest at 55.45%, with loading, which could be due to the partial coverage of Mn over
an O/P value of as high as 7.75. At the same time, the selectivity the Fe activity sites, which prevents the reactant from making
for C5+ hydrocarbons and all oxygenate products (from the contact with the activity centers (Fig. 3a).40
160 ꢀC hot trap and ꢁ1 ꢀC cooling trap) signicantly decreased.
It has been proved that a larger amount of MnOx covered the
Combining the above characterization results, it is consid- active Fe surface when CTAB was employed, leading to a loss in
ered that some specic active sites formed on the surface of the the number of active Fe sites that are used for CO dissociative
FeMn–CTAB catalysts during the preparation process,14–16,19 and adsorption.19,40,42 In addition, the active sites were occupied by
resulted in the formation of specic iron carbides, such as c- the absorbed CO because of strong surface basicity, which
Fe2C5, q-Fe3C, and Fe7C3 phases, when exposed to the syngas, decreased the rate of CO dissociation.14,46 All of these factors
which might be the main reason for the different catalytic account for the decrease in the CO conversion and catalytic
performances observed for the FeMn–CTAB catalysts. In activity over the FeMn–CTAB catalysts. However, the addition of
particular, for FeMn–CTAB-0.8, the formation of the q-Fe3C and an excess of CTAB brought about an increase in the CO
Fe7C3 phases was benecial to the CO dissociation absorption, conversion and catalyst activity (CTAB/Fe ¼ 1.0, mol molꢁ1), but
]
but suppressed the H2 absorption and a-olen readsorption a distinct decrease in the C2–4 selectivity and O/P value, as
]
over the FeMn–CTAB catalysts, contributing to high C2–4
observed in Table 1, which might be due to the higher exposure
selectivity.19,40 Many factors may be the cause for the formation of the Fe active phase, as well as the presence of specic iron
of these active phases, especially the specic effects of the Mn carbides that formed during the FTO reaction (such as c-
promoter.25 For example, Mn could affect the electronic state of Fe2C5).24,41
the surface carbonaceous species, leading to the formation of
Fig. 6a–d show the catalytic performance during the reac-
a special iron carbide phase (q-Fe3C) in the particular catalytic tion. It was found that the catalytic activity and the product
Fig. 6 Catalytic performance of the FeMn–H2O and FeMn–CTAB-g (g ¼ 0.15/0.3/0.6/0.8/1.0) catalysts as a function of time: (a) catalytic activity,
(b) CO2 selectivity, and product selectivities of (c) FeMn–H2O, and (d) FeMn–CTAB-0.8. Reaction conditions: 320 ꢀC, 1.0 MPa, H2/CO ¼ 1.5 (v/v),
GHSV ¼ 4200 hꢁ1 and TOS ¼ 60 h.
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selectivity changed alongside the on-stream time. Fig. 6a shows industrially relevant conditions (320 ꢀC, 1.0 MPa, H2/CO ratio of
that the catalytic activity of all of the calcined catalysts was low 1.5 (v/v), GHSV ¼ 4200 hꢁ1). In addition, it further conrms the
at the beginning of the reaction, and increased rapidly in the role of specic iron carbides in the formation of light olens
initial period, then tended to stabilize at a high level. Fig. 6c and and also helps to understand the relationship between the
d show the CO conversion and the hydrocarbon product selec- catalyst structure and its catalytic performance. Moreover, the
tivity of the FeMn–H2O and FeMn–CTAB-0.8 catalysts as CTAB surfactant-assisted Fe catalyst can be used for the high
a function of the on-stream time. In contrast, the favorable selectivity of light olens in the industrially appealing FTO
formation of C2–4] was observed when CTAB was employed. For process.
]
the FeMn–CTAB-0.8 catalyst, the C2–4 selectivity rose to
a maximum (55.45%) and slightly decreased when the reaction
proceeded over a longer time, and was then maintained at
Conflicts of interest
around 43.5%, which was higher than that of the FeMn–H2O
catalyst (around 30%). Meanwhile, the formation of C5+
There are no conicts to declare.
hydrocarbons and oxygenate products increased continuously
over the rst 40 h and seemed to stabilize at 34.8%, which was Acknowledgements
lower than the value observed for the FeMn–H2O catalyst
Financial support from the Zhejiang Provincial Natural Science
Foundation of China (Grant No. LY18B060015) is
acknowledged.
(around 43.5%). The CH4 selectivity, as well as the formation of
C
2–4 paraffins, was constant low for both of the catalysts.
Before the reaction, the fresh catalysts were reduced in H2 at
400 ꢀC under 1.0 MPa for 12 h, and a large amount of metallic
iron formed on the initial catalyst surface, as observed from the
H2-TPR and in situ XRD measurements (Fig. 1c and S3†). On the
other hand, the characterization revealed that various carbides
species formed on the catalyst surfaces when exposed to syngas,
which might be induced by the specic surface active sites of
the FeMn catalysts.14–16,19,40,42 It has been reported that metallic
iron and iron carbides display distinct CO dissociation barriers
and binding strengths for C and O atoms,14,19 leading to
different CO conversions and catalytic activities between the
initial reaction stages and aer the reaction has proceeded for
a longer time (Fig. 6a–d).14,32,33,40,42 The same applies to the
product selectivity, which is inuenced not only by the initial
state of the catalyst surface, but also longer reaction times.14–16
These results further demonstrated that the changed iron
catalyst surface, including the growth of iron carbide particles,
carbide structure, carbon accumulation (Fig. 5a–c), and binding
strength between some atoms, had a great inuence on the
catalytic performance.14–16,40
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