Potassium promoted core-shell-structured FeK@SiO2-GC catalysts used for Fischer-Tropsch synthesis to olefins without further reduction

Article abstract of DOI:10.1039/c9nj03947c

Fe-based Fischer-Tropsch synthesis (FTS) catalysts, promoted by graphitic carbon (GC) and potassium, were directly prepared by a novel modified sol-gel method without further reduction. The effects of the GC promoter and K contents on the catalyst structures and FTS performance were systematically studied. The significant improvement of FTS performance was attributed to GC, which acted as a reductant for synthesizing metallic Fe⁰ from Fe³⁺ in the chelating complexes, and effectively enhanced the strength of SiO₂ channels. Meanwhile, an appropriate amount of K ions promoted CO chemisorption and inhibited H₂ chemisorption by affecting the electronic properties of iron, resulting in a lower hydrogenation capacity and a higher olefin selectivity. Transmission electron microscopy and CO-TPD characterization proved that FeK@SiO₂-GC catalysts had well-defined core-shell structures and higher CO chemisorption. The FTS results indicated that the introduction of a small amount of GC and K (K/Fe/GC, 1.5/100/100) improved the reduction and dispersion of iron during the calcination process, and significantly enhanced the FTS activity. The CO conversion and C₂-C₄ olefin selectivity of the catalyst increased rapidly from 21.1% and 23.7% to 53.5% and 41.3% after GC and K promotion. The GC and K promoted Fe-based catalysts prepared by a modified sol-gel method, which omits the complex and high energy consumption reduction process, can be used directly for highly efficient FTS and thus will be more promising in the future.

Full text of DOI:10.1039/c9nj03947c

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PAPER
Potassium promoted core–shell-structured
FeK@SiO -GC catalysts used for Fischer–Tropsch
2
Cite this: New J. Chem., 2020,
synthesis to olefins without further reduction
44, 87
a
a
b
b
b
Zhijiang Ni, * Xuefei Zhang, Jirong Bai, Zhilei Wang, Xi Li
Yanhu Zhang*
and
c
Fe-based Fischer–Tropsch synthesis (FTS) catalysts, promoted by graphitic carbon (GC) and potassium,
were directly prepared by a novel modified sol–gel method without further reduction. The effects of the
GC promoter and K contents on the catalyst structures and FTS performance were systematically studied.
The significant improvement of FTS performance was attributed to GC, which acted as a reductant for
0
3+
synthesizing metallic Fe from Fe in the chelating complexes, and effectively enhanced the strength of
SiO channels. Meanwhile, an appropriate amount of K ions promoted CO chemisorption and inhibited H
2
2
chemisorption by affecting the electronic properties of iron, resulting in a lower hydrogenation capacity
and a higher olefin selectivity. Transmission electron microscopy and CO-TPD characterization proved that
2
FeK@SiO -GC catalysts had well-defined core–shell structures and higher CO chemisorption. The FTS
results indicated that the introduction of a small amount of GC and K (K/Fe/GC, 1.5/100/100) improved
the reduction and dispersion of iron during the calcination process, and significantly enhanced the FTS
Received 30th July 2019,
Accepted 14th November 2019
2 4
activity. The CO conversion and C –C olefin selectivity of the catalyst increased rapidly from 21.1% and
23.7% to 53.5% and 41.3% after GC and K promotion. The GC and K promoted Fe-based catalysts prepared
DOI: 10.1039/c9nj03947c
by a modified sol–gel method, which omits the complex and high energy consumption reduction process,
can be used directly for highly efficient FTS and thus will be more promising in the future.
rsc.li/njc
The corresponding catalytic activity is considered to depend
on the appropriate iron size in iron carbide and high disper-
1
. Introduction
Lower olefins (C –C alkenes) are very important raw materials sion. In order to achieve high surface active sites, some refined
2
4
in chemical industries for the production of polymers, solvents, approaches have been explored to prepare metal nanoparticles
1
–3
cosmetics, and so on. Lower olefins are manufactured in the with a high surface/interior atom ratio on the high-surface-area
4
–6
14
15
16
17
industry primarily by the methanol to olefins (MTO) process.
oxide supports, such as SiO
2
,
Al
2
O
3
,
TiO
2
and ZnO being
However, MTO technology involves a number of technological the most frequently used. The strong interaction between oxide
steps which reduce the overall conversion efficiency. In addition, support and highly dispersed metal nanoparticles easily result
the catalyst undergoes obvious deactivation. Thus, in compar- in the formation of non-reducible compounds, which deviate
ison with traditional technologies, Fischer–Tropsch synthesis from the original properties of metal deposits, and inhibit the
1
9
(
FTS) provides a new sustainable route for manufacturing lower formation of metal active phase. Bukur et al. found that the
7
–9
olefins. Iron, cobalt and ruthenium are active metals for the FTS activity was decreased by the addition of silica to precipi-
FTS, but only iron and cobalt are used industrially. Iron catalysts tate Fe (on the basis of 1 parts of Fe) as follows: unpromoted
are considered to be more favorable than cobalt catalysts for the unsupported 4 0.08 SiO
2 2 2
4 0.24 SiO 4 1 SiO . The order can
production of C –C hydrocarbons from coal or biomass because be explained by a lower extent of reduction of Fe and lower
2
4
3
,10–13
of their low cost and low methane selectivity.
effectiveness of potassium due to its interaction with silica.
20
Zhang et al. studied the reduction behavior of silica-supported iron
catalysts by using a H -TPR method, and found that the reduction of
2
a
School of Mechanical Engineering & Urban Rail Transit, Changzhou University,
Changzhou, 213001, China. E-mail: nizhijiang@126.com
iron oxides was suppressed which leads to the formation of iron(II)
silicate during the reduction. Even so, the traditional reduction
b
c
Department of Environmental Science and Engineering, Fudan University,
Shanghai 200433, China
21
process for 10 hours is essential and important before reaction,
but it increases the cost and complexity of FTS. Therefore, the
addition of the corresponding promoters to improve metallic
Advanced Manufacturing & Equipment Institute School of Mechanical Engineering,
Jiangsu University, Zhenjiang 212013, China. E-mail: zhyh@ujs.edu.cn
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iron reducibility and dispersibility, and no need for a reduction
process will have a greater market value in the Fischer–Tropsch
industry.
Many studies have been conducted to improve the metal
reducibility and dispersibility during the auto-combustion,
1
3
which include doping with alkali metals and expensive noble
2
2,23
24
metals,
and the pH value of the precursors. But these
methods cannot avoid the reduction process before reaction. In
2
5,26
our previous work,
we found that GC was supposed to be a
significant promoter for iron-based FTS catalysts, owing to Fig. 1 Scheme of FeK @SiO -GC catalyst formation during synthesis.
x
2
reducibility and electron conductivity properties similar to those
22,23
of expensive noble metals (Pt and Ru).
Meanwhile, we system-
process, 4.95 g of Fe(NO ) Á9H O, Â wt% KNO (Â wt% denoted:
atically studied the effects of GC on the FTS performances of iron-
based catalysts with a mole ratio of promoter/iron = 0.15, 0.3 and
3 3
2
3
K to Fe ratios of 0, 0.5%, 1%, 1.5% and 2%) and polyvinylpyrro-
lidone (PVP, 2.5 g) were mixed in ethanol solution, and trans-
ferred into a Teflon-lined stainless autoclave, which was loaded
in an oven at 180 1C for 4 h. After this, distilled water (100 mL),
ethanol (125 mL), hexadecyltrimethylammonium bromide
0.45. We found that an appropriate amount of GC significantly
weakened the bonding strength between the metal and oxide
leading to the ease of reduction, finally reduced the selectivity of
4 2 4
CH (19.7%) and facilitated the selectivity of C –C olefins (40.7%)
(CTAB, 1.25 g) and ammonia (5 mL, 25 wt% water solution)
obviously. Moreover, iron-based catalysts often contain small
27,28
29 were subsequently added into the suspension and stirred for
1 hour. Then, 5 mL of TEOS was added slowly under mild
continuous stirring at room temperature for another 24 h. The
silica-coated nanocomposites were washed with ethanol several
times. The final product was dried at 60 1C and then heated to
amounts of potassium
and some other metals such as Mn,
20
30
31
Cu, Zn and Mg as promoters to improve its activity and
selectivity, since potassium has a stronger basicity and influences
the adsorption of reactants on the active sites. This leads to some
effects on the FTS activity, the enhancement in the selectivity to
olefins, the suppression of the formation of methane, and the
selectivity shift to higher molecular weight products. Nearly all
iron-based FTS catalysts contain more or less potassium as one of
the promoters, which can donate electrons to iron and facilitate
CO chemisorption, since CO tends to accept electrons from iron.
Meanwhile, hydrogen with a higher surface coverage was inclined
to donate electrons to iron, and the electrons donated to iron from
potassium weakened the strength of the Fe–H bond. Hence,
potassium strengthened the Fe–C bond and weakened the Fe–H
7
00 1C for 5 h in flowing argon to carbonize the as-made samples
completely. The obtained catalysts with different K values are
denoted as FeK @SiO -GC, FeK0.5@SiO -GC, FeK @SiO -GC,
FeK1.5@SiO -GC and FeK @SiO -GC.
0
2
2
1
2
2
2
2
For comparison, the same previous synthesis method of the
FeK @SiO -GC catalyst was performed, and the final step was
0
2
calcination at 550 1C for 3 h in flowing air and denoted as the
Fe @SiO catalyst (Fig. 1).
2
O
3
2
2
.2 Fischer–Tropsch synthesis
The FTS reaction was performed in a tubular fix-bed reactor
i.d. = 10 mm) at 340 1C, with a total pressure of 2 MPa and a
32–34
bond.
The overall effects of potassium on the behavior of iron-
based FTS catalysts have been extensively investigated and are well
established over different catalyst systems.
The present work focuses on a systematic understanding of
the effects of potassium promotion on core–shell-structured
(
H /CO ratio of 1. About 0.5 g of the catalyst was loaded into the
2
isothermal region of the reactor for all the reaction tests. The
autoreduced samples were treated in an argon flow at 150 1C
for 6 h and the hydrogen-reduced samples were treated in a
hydrogen flow at 450 1C for 6 h, and then cooled down to
ambient temperature before switching to premixed syngas.
All the FTS reaction tests of the as-prepared catalysts were
performed twice in order to reduce the relative errors of
catalytic activity and product selectivity within 10%. After the
FTS reaction, the gas effluents were analyzed online by using a
TDX-01 packed column with a TCD, and the amount of each
product based on the carbon balance was calculated. Mean-
while, the gas hydrocarbons were analyzed online using a PONA
capillary column with an FID.
2
FeK@SiO -GC catalysts under industrial relevant operation
conditions. The catalyst characterization results indicated that
the introduction of graphitic carbon significantly improved the
reduction of iron during autocombustion and can help
to provide hydrophobic mesoporous channels with higher
hydrothermal stability. The as-prepared catalyst without further
reduction exhibited a high FTS activity comparable to that
of the conventional H
graphitic carbon promoter and different potassium contents
on the Fe@SiO -GC catalysts and FTS performance of the
catalysts have been studied.
2
reduced catalyst. The effects of the
2
2
. Experimental
3. Results and discussion
3.1 Catalyst analysis
2.1 Preparation of FeK
x
@SiO
2
-GC catalysts
@SiO
The synthesis procedure for the FeK
x
2
-GC catalyst was The crystallite structures of the iron-based catalysts were examined
performed by a hydrothermal deposition method. In a typical by XRD and the results are shown in Fig. 2. Fig. 2a displays the
8
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Fig. 2 XRD pattern of (a) Fe@SiO
2
-GC catalysts with different GC contents, (b) Fe@SiO
2
-GC catalysts with different calcination temperatures and (c)
2
Fe@SiO -GC catalysts with different K doping levels.
XRD patterns of the unprompted and different GC-content
promoted catalysts. First, there were obvious Fe O peaks
2
3
in the XRD pattern of the unprompted Fe
The diffraction peaks at 2y values of 33.2, 35.6, 49.5, 54.0,
2.4 and 63.91 (labeled as ‘‘m’’, Fig. 2a) marked with ‘‘m’’ are
readily assignable to hematite (a-Fe , JCPDS 33-0664) on the
Fe @SiO catalyst. From the other XRD patterns in Fig. 2a,
we found that Fe gradually reduced to metallic Fe with the
2 3 2
O @SiO sample.
6
2 3
O
2
O
3
2
2 3
O
addition of GC contents. Meanwhile, the diffraction peaks of
metallic Fe became more and more intense with the increase of
GC contents, which also indicates that the precursor of PVP has
good reducibility in the process of high temperature calcination
0
35
(
C + Fe
2
O
3
- CO
2
+ Fe ). Then, we studied the reducibility of
-GC samples by changing the calcination
iron in the Fe@SiO
2
2 1 2
Fig. 3 Raman spectra of the Fe@SiO -GC and FeK @SiO -GC catalysts.
temperature under the same amount of GC modification. As
shown in Fig. 2b, the intensity of the metallic Fe peaks (three
peaks at around 44.6, 65.0 and 82.31) becomes more obvious the vibration of carbon species (Fig. 3). Meanwhile, the GC
with the increase of the calcination temperature, and this content and the pyrolysis process conditions in FeK @SiO -GC
x
2
2 1
indicates that the higher the reduction of metals the higher catalysts are the same. Therefore, the Fe@SiO -GC and FeK @
the degree of graphitization of carbon. Fig. 2c shows the XRD SiO
patterns of the FeK @SiO
levels and the other XRD patterns of the FeK
lysts with different K contents. The three peaks at around 44.6, SiO
5.0 and 82.31 (labeled as ‘ ‘E ’’) are indexed to the (110), (200) vibration modes of CQC in GC. Another peak at 1364 cm is
2
-GC samples have typical Raman characterization and have
-GC catalysts with different K doping a certain general illustrative. The band at 1597 cm (G-band)
À1
x
2
x
@SiO
2
-GC cata- clearly shows the formation of graphitic structures of FeK
0
@
2
-GC and FeK @SiO -GC catalysts belonging to the stretching
1
2
À1
6
and (211) reflection of iron (JCPDS no. 65-4899), respectively. related to the vibration of carbon species in the plane termina-
Moreover, the crystallite size of metallic iron on FeK @SiO -GC, tion of the disordered graphite. Generally, the relative intensity
0
2
FeK0.5@SiO
SiO -GC derived from the full width at half-maximum (FWHM) tion degree of the carbon material: a smaller I
of the most intensive reflection at a 2y value of 44.61 and the higher degree of atom ordering. The I /I ratios of FeK
Scherrer equation is ca. 13.9, 15.7, 16.9, 18.5 and 20.1 nm, GC and FeK @SiO -GC were 1.01 and 1.24, indicating that the
respectively. There was no obvious change in the crystallite size carbon in Fe@SiO -GC catalysts has a good degree of graphitiza-
2
-GC, FeK
1
@SiO
2
-GC, FeK1.5@SiO
2
-GC and FeK
2
@
ratio of these two bands (I
D
/I
G
) was used to assess the graphitiza-
/I means a
@SiO
2
D G
D
G
0
2
-
1
2
2
of metallic Fe after the modification of K. As reported, the iron tion, which may significantly contribute to the electron transfer
phase exists in the XRD pattern as the Fe O form. However, between the iron core and graphitic carbon.
2
3
metal Fe phases were observed from the above XRD patterns,
which showed the reduced phases of iron oxides (Fe and were investigated using the TEM images. As shown in Fig. 4a,
the TEM images of the Fe @SiO catalyst show a typical core–
shell morphology. This revealed that PVP added as a stabilizer
-GC catalysts, we performed Raman spectroscopy to define was an effective reagent for the synthesis of monodisperse
The morphology and microstructure of the synthesized samples
2 3
O
3
6
FeO) through reduction (GC).
To further discuss the graphitization degrees of the FeK
SiO
2
O
3
2
x
@
2
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2 3 2 0 2 2 2 2
Fig. 4 TEM images and particle size distribution of (a) Fe O @SiO , (b) FeK @SiO -GC, (c) FeK0.5@SiO -GC, (d) FeK1.0@SiO -GC, (e) FeK1.5@SiO -GC and
2
(f) FeK2.0@SiO -GC catalysts.
3
7
38,39
Fe O nanoparticles. After introducing GC into the pores of channel from the decomposition of the PVP stabilizer.
Com-
-GC catalyst without K doping, the
consistent with our previous research. Fortunately, the particle surface area and pore volume of FeK @SiO -GC increased greatly,
2
3
0 2
silica, the core–shell structures remain relatively intact, which are pared with the FeK @SiO
26
x
2
2
À1
size of metallic Fe decreases from 21 nm to 16 nm with the
S
BET increased from 84.17 to 138.88 m g (FeK1.0@SiO
2
-GC) and
-GC), which agree
the growth of iron particles (Fig. 4a and b). The TEM images in well with the results of Dry’s research. The above results show
Fig. 4c–f show that FeK0.5@SiO -GC, FeK1.0@SiO -GC, FeK1.5 that the increase of the K content is beneficial for the migration of
SiO -GC and FeK @SiO -GC nanoparticles have a core–shell Fe species from the pore to the surface layer, and enhances the
3
À1
addition of GC, which indicates that GC can effectively inhibit PV from 0.1107 to 0.7117 cm g (FeK1.5@SiO
2
18
2
2
@
2
2.0
2
structure with a 4 nm-thick silicon-shell, respectively. As the K interaction between K and Fe. This may promote the dispersion of
content increases, the size of the iron core increases from 11 nm the Fe species on the surface of the catalysts, resulting in a slight
2 2
(FeK0.5@SiO -GC) to 20 nm (FeK2.0@SiO -GC), which is in agree- increase in the surface area. In particular, the pore volume reached
ment with the results of XRD analysis. The three-dimensional a maximum value with 1.5% K doping, which indicated that the
porous core–shell structure can play an antioxidant and anti- best Fe–K interaction was obtained at 1.5% K doping. Fig. 5b shows
agglomeration role, and the mesoporous pores are further the thermogravimetric analysis (TGA) of FeK @SiO -GC catalysts
x 2
enhanced by GC modification. Meanwhile, potassium is a typical under an air atmosphere at different metallic K doping levels.
metal promoter, which can effectively promote the dispersion of Among them, the significant weight loss of FeK @SiO -GC,
0
2
metallic Fe particles and reduce the particle size.
FeK0.5@SiO -GC, FeK @SiO -GC, FeK @SiO -GC and FeK2.0@
2 1.0 2 1.5 2
2
The textural properties of the silica support, and calcined and SiO -GC catalysts in the range of 400–650 1C could be attributed
carburized Fe-based catalysts are shown in Table 2 and Fig. 5a. to GC burning in air. Specific statistics are shown in Table 1, the
As shown in Fig. 5a, the Fe @SiO and FeK @SiO -GC nano- weight loss values of FeK @SiO -GC (x = 0, 0.5, 1.0, 1.5, 2.0) are
particles exhibit a type-IV isotherm with a long hysteresis loop at 12%, 17%, 16%, 15.5% and 14%, respectively, which indicate the
a relative pressure (P/P ) of 0.4–1.0, which is characteristic of GC content of the catalyst.
mesoporous materials according to the classification of Interna- The reduction behaviors of catalysts were investigated using
tional Union of Pure and Applied Chemistry (IUPAC). Moreover, H -TPR (Fig. 6a). As shown in Fig. 6a, the H -TPR profile of
O
2 3
2
x
2
x
2
0
2
2
the mesoporous silica shell has a narrow pore size distribution Fe O @SiO shows two hydrogen consumption peaks at 394
2
3
2
centered at 3.8 nm calculated from the Barrett–Joyner–Halenda and 516 1C, which are assigned to the consecutive reduction
0
(
2 3
BJH) method (Fig. 5a, inset). Meanwhile, the surface area and pore of Fe O - FeO and FeO - Fe , respectively. Between the
volume slightly decrease after graphitized carbon doping, owing to two peaks of hydrogen consumption, the second broad peak
the residual GC attached onto the surface of the mesoporous illustrates the strong interaction of highly-dispersed FeO with
9
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Fig. 5 (a) N
2
adsorption–desorption and pore size distribution curves, and (b) TG profiles of the FeK@SiO
2
-GC catalysts with different K doping levels.
Table 1 Pore characteristic of FeK
K doping levels
x
@SiO
2
-GC catalysts with different
2
H reduction peak gradually move toward the high temperature
region (Fig. 6a). This is because that a large amount of
K particle covers the surface of iron with the increase of the
K doping amount, which affects the dissociative adsorption of
a
b
c
GC content K/Fe
S
BET
PV
APS
2
À1
3
À1
Sample
Fe @SiO
(%)
(wt%) (m g
)
(cm g
)
(nm)
H
2
on the surface of iron, and decreases H
2
reducibility.
2
O
3
2
—
12
—
0
367.22
0.2953
0.1107
0.5715
0.4854
0.7117
0.3445
3.00
5.80
17.07
13.98
24.58
FeK @SiO
0
2
-GC
84.17
133.86
138.88
115.81
106.16
The CO-TPD curves of FeK @SiO -GC catalysts are shown in
x
2
FeK0.5@SiO
FeK1.0@SiO
FeK1.5@SiO
FeK2.0@SiO
2
2
2
2
-GC 17
0.46
0.98
1.52
2.03
Fig. 6b. As shown in Fig. 6b, the CO desorption peaks are all
located in the range of 200–700 1C. For the FeK0.5@SiO -GC
-GC 15
-GC 11
-GC 20
2
12.98 catalyst, the desorption curve of CO shows a strong desorption
peak at 287 1C, and the intensity of the CO desorption peak
increased significantly and moved toward a high temperature
a
b
c
XRF. Pore volume. Average pore size.
4
0
with the increase of the K content. Zhang et al. reported that
the Si–OH groups of the SiO support (e.g. iron phyllosilicate). the CO adsorption on the Fe surface produces four desorption
2
For the FeK @SiO -GC catalysts, the small reduction profiles are peaks, which are defined as three molecular states at 23, 67
x
2
x 2
assigned to FeO - Fe reduction. However, the FeK @SiO -GC and 157 1C, and dissociation states at high temperatures. The
samples did not find the FeO phase in the XRD pattern, probably desorption temperature of CO on the FeK @SiO -GC samples is
x
2
because the metallic iron was slightly oxidized in air, and the much higher than the desorption temperature of the CO mole-
hydrogen consumption peaks of FeK @SiO -GC catalysts were cule on the Fe surface and is close to that of the CO desorption
all attributed to FeO - Fe . It can be observed from Fig. 6a that temperature. Benziger et al. estimated the activation energy
x
2
0
41
42
the reduction temperature of Fe@SiO
is concentrated at about 384 1C, whereas the H2 reduction 20 kJ mol (about 2.28 eV). The principle calculation of CO
2
-GC (without K doping) of dissociation CO desorption on the Fe surface to be 220 Æ
À1
temperature of FeK @SiO -GC is higher than that of the adsorption on the Fe C surface shows that the CO adsorption
x
2
5 2
Fe@SiO
00 1C, which is mainly due to the interaction between K and the CO desorption temperature on the surface of iron carbide is
Fe, which can inhibit the iron reduction by H . In addition to about 500 1C, it is very close to the CO desorption temperature in
the Fe@SiO -GC catalyst, the H reduction temperature this study. Therefore, the desorption peak on the FeK2.0@SiO -GC
increase with the doping amount of the K promoter, and the catalyst may be due to the strong binding of CO on the surface
2
-GC catalyst, and the lowest temperature can reach energy of the Fe atom is generally lower than À2.10 eV [191]. If
4
2
2
2
2
2 x 2
Fig. 6 (a) H -TPR profiles and (b) CO-TPD curves of the FeK @SiO -GC catalysts with different K doping levels.
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of iron carbide. Therefore, the desorption peak of FeK
NJC
x
@SiO
2
-GC be one reason for the higher CO conversion. Second, according
x 2
catalysts may be attributed to the strongly bonded CO on the to the results of TEM (Fig. 4), FeK @SiO -GC also promoted the
surface of iron carbide. It can be seen that metallic K increases the dispersion of Fe particles because the GC-modified interior
adsorption of CO on FeK @SiO -GC catalysts to a great extent, pore-walls protected the core–shell structures, which prevented
x
2
which is consistent with the result of XRD characterization.
the collapse of mesoporous structures and effectively increased
the probability of contact between the CO molecules and Fe
3
.2 Catalytic tests
The Fe @SiO and FeK
doping levels were studied in FTO under industrially relevant CO conversion.
particles. Furthermore, FeK
-GC catalysts with different K particles smaller than Fe
x
@SiO -GC show metallic Fe nano-
2
@SiO , which can help to increase
2
2
O
3
2
x
@SiO
2
2
O
3
conditions (H
2
/CO = 1, 2 MPa, 350 1C). Before the FTS reaction,
x 2
As can be seen in Table 2, the CO conversion of FeK @SiO -
À1
all the catalysts were calcined in Ar (50 mL min ) for 4 h at GC (x = 0, 0.5, 1.0, 1.5, 2.0) increases with the increase of K
120 1C. All the tests were conducted twice, and the relative doping from 0% to 1.5%, beyond this potassium concentration,
errors under the same conditions were less than 10%. Table 2 a monotonic decrease in catalyst activity is observed with the
and Fig. 7 summarize the hydrocarbon product distribution, increase of potassium. Thus, it can be indicated that an
the lower olefin selectivity, and other performances of all the appropriate amount of K promoted Fe@SiO
catalysts. CO conversion is a rough measure of the FTS reaction pared by a self-assembly method (FeK1.5@SiO
of the catalysts. As can be seen in Table 2, FeK @SiO -GC high FTS activity with a CO conversion of 53.5%, as demon-
x = 0, 0.5, 1.0, 1.5, 2.0) catalysts exhibit a higher CO conversion strated by Brien et al. K o¨ lbel postulated that the catalysts
than Fe O @SiO (35.8, 30.9, 35.7, 53.5, 44.8 and 21.1%, containing potassium had a higher concentration of active sites
2
-GC catalyst pre-
2
-GC) exhibited a
x
2
4
3
44
(
2
3
2
respectively). First, according to the results of XRD (Fig. 2), than that on un-promoted catalysts, and therefore accelerated
FeK @SiO -GC (x = 0, 0.5, 1.0, 1.5, 2.0) exhibit a large amount of the FTS activity. With further increase in the potassium con-
metallic Fe rather than Fe , which was attributed to the tent, the active sites may be blocked by potassium, resulting in
higher reducibility of Fe
x
2
x y
O
x
O
y
after GC modification. This may a decline in the catalyst activity. Furthermore, there was no
obvious activity loss in the 100 hours of reaction time on
stream, indicating a high stability of this catalyst (Fig. 7a). It
Table 2 Comparison of the Fischer–Tropsch synthesis catalytic perfor- is believed that the promotion of K on dispersion of Fe (Fig. 4)
mances between the FeK
x
@SiO
2
-GC and Fe
2
O
3
@SiO
2
catalysts
played important roles in the high activity of FeK1.5@SiO -GC.
2
c
Compared with CO conversion of the un-promoted catalyst
Selectivity (%)
–C
(
FeK @SiO -GC, 38.1%), the CO conversion of FeK @SiO -GC
0 2 0.5 2
CO conv.
(%)
C
2
4
C
2
–C
4
d
2
and FeK1.0@SiO -GC was lower, which was 30.9% and 35.7%,
Catalyst
CH
4
olefins paraffins
C
5+
O/P
respectively. It could be because that the addition of potassium is
in favor of carbon deposition on the surface, which leads to the
a
b
Fe
Fe
FeK
FeK
2
O
O
3
3
@SiO
@SiO
2
21.1
40.6
35.8
38.1
30.9
35.7
53.5
44.8
32.5 23.7
22.9 36.0
28.9 32.2
21.1 39.1
16.3 34.7
17.1 39.3
16.3 41.3
13.3 39.6
23.4
17.4
12.9
11.7
11.9
9.6
20.4 1.01
23.7 2.07
2
2
a
b
@SiO
@SiO
26.0 2.49 formation of inactive carbon covering the active sites on the surface
0
2
-GC
-GC
34,45
0
2
28.4 3.35
37.1 2.91
33.9 4.08
and thus leads to a further decline in the FTS activity.
Mean-
a
a
a
a
FeK0.5@SiO
FeK1.0@SiO
FeK1.5@SiO
FeK2.0@SiO
2
2
2
2
-GC
-GC
-GC
-GC
while, it is found from CO-TPD (Fig. 6b) that the chemisorption
6.8
5.4
35.6 6.06 ability of FeK2.0@SiO -GC is the strongest. Combined with the
2
41.8 7.40
investigation of Miller and Moskovits, there is a competition
a
À1 catÀ1
2
between dissociative CO chemisorption and H adsorption on the
Reaction condition: 340 1C, 2 MPa, H
2
/CO = 1, GHSV = 4.5 L h
g ,
À1
TOS = 100 h, and calcination in Ar (50 mL min ) for 4 h at 120 1C active sites of catalysts, which results in a maximum in conversion
b
À1
before the FTS reaction. Reduced in 5% H
2
/Ar (50 mL min ) for
c
as a function with the change in the potassium content.
Lower olefins are the primary products of the FTS reaction
over iron-based catalysts. Table 2 and Fig. 7b show that the
1
6 h at 400 1C before the FTS reaction. Hydrocarbon selectivity was
d
normalized with the exception of CO
paraffin in all of the C2–4 hydrocarbons.
2
.
O/P is the ratio of olefin to
x 2
Fig. 7 (a) Conversion of CO and (b) hydrocarbon selectivities with time on stream on the FeK @SiO -GC catalysts with different K doping levels.
9
2 | New J. Chem., 2020, 44, 87--94
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2
Fig. 8 (a) TEM-images of the used FeK@SiO -GC-1 catalyst and (b) particle size distribution.
ratio of olefin to paraffin in all of the C2–4 hydrocarbons effectively prevents the aggregation and sintering of the metal
O/P ratio) increases monotonically (2.49, 2.91, 4.08, 6.06 and Fe particles during the Fischer–Tropsch synthesis.
.40, respectively) with increasing K content. Several factors
may affect the O/P ratio. (1) As shown in the CO-TPD test
(
7
4
. Conclusions
(
Fig. 6b), K promotion increases CO chemisorption and decreases
H chemisorption (strengthening of the Fe–C bond and weakening
2
Fe-based FTS catalysts containing GC and potassium promoters
were synthesized using a sol–gel autocombustion method,
burnt in an argon atmosphere, and directly applied to the
FTS reaction without further reduction. GC and K as promoters
had a remarkable influence on the physical properties, and
morphology of the FeK @SiO -GC catalysts as well as catalytic
32–34
of the C–O and Fe–H bonds).
and H chemisorption decreases at the same time with the increase
of the K content in the FeK @SiO -GC catalysts. Thus, the C/H ratio
on the catalyst surface increases with the increase of the K content
of the iron catalyst, and the higher C/H ratio will lead to a lower
saturation degree of the product. Consequently, a secondary
reaction of olefins was inhibited, and the olefin selectivity in
C –C hydrocarbon increased. (2) The amount of strong basic
sites is increased with the incorporated K promoter, and leads
to the increase of lower olefin selectivity.
above, the K promoter also improves the dispersion of metallic
Fe, reduces its crystallite size during preparation and FTS
performance. The interdispersion of Fe and K atoms (ions)
weakens the connection of the carbon chain on the adjacent
CO chemisorption will increase
2
x
2
x
2
stability and activity of FTS performances, which varied
depending upon the utilized K contents. Due to a combination
2
4
of higher iron dispersion and reducibility, FeK1.5@SiO
a K/Fe ratio of 1.5 wt% exhibited the highest CO conversion
53.5%) and relative C –C olefin selectivity (41.3%) compared
with the conventional Fe O @SiO catalyst, which calcites in Ar
2
-GC with
2
8,46
(3) As-mentioned
(
2
4
2
3
2
for 4 h at 120 1C before the FTS reaction. The Fe–SiO catalysts
2
can be directly applied in a highly effective FTS reaction without
the complicated and energy consumption reduction process,
and therefore this method will be widely applied in designing
other nanoparticle catalysts.
3
3
iron atoms and leads to the increase of light olefin selectivity.
The synergetic effects of these factors result in an appropriate
amount of K promoter for the lower olefin selectivity.
2
The FTS activity of FeK1.5@SiO -GC (53.5%) prepared by a
one-step reduction method is higher than that of the conven-
Conflicts of interest
Acknowledgements
tional Fe
Meanwhile, Table 2 also shows that Fe@SiO
further reduction exhibited a similar activity and selectivity to
that by H reduction before the FTS reaction. Thus, it can be
2 3 2 2
O @SiO (40.6%) by H reduction before the reaction.
2
-GC without There are no conflicts to declare.
2
concluded that the GC promoted Fe–SiO2 (FeK1.5@SiO -GC)
2
catalyst prepared by the auto-combustion method may not
undergo the complex and high energy consumption reduction
process, and exhibited higher activity and selectivity to the
This work was carried out with financial support from the
National Natural Science Foundation of China (Grant No.
21905031) and the Science Foundation of Changzhou University
2 3 2
Fe O @SiO catalyst prepared by the conventional method, as
(ZMF18020299). This research was also supported by Yancheng
it effectively reduced the cost of FTS industrial processes.
To study the stability and sustainability of the FeK@SiO
Huanbo Energy Technology Co., Ltd.
2
-GC
catalyst, we collected and analyzed the FeK@SiO
2
-GC-1 after
1
00 h reaction. The TEM image of FeK@SiO -GC-1 (Fig. 8a)
2
References
found that the metal particles had not aggregated after 100 h
reaction. The size of metal Fe particles is about 12–16 nm
1 X. Zhang, L. Zhong, G. Zeng, Y. Gu, C. Peng, F. Yu, Z. Tang
and Y. J. C. E. J. Sun, Chem. Eng. J., 2018, 351, 12–21.
2 F. Yu, T. Lin, X. Wang, S. Li, Y. Lu, H. Wang, L. Zhong and
Y. Sun, Appl. Catal., A, 2018, 563, 146–153.
(Fig. 8b), and does not increase significantly. This may be
because the outer shell of the mesoporous silica layer is tightly
coated with the metal Fe particles, and the unique structure
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020
New J. Chem., 2020, 44, 87--94 | 93

View Article Online
Paper
NJC
3
4
S. M. G. Lama, J. L. Weber, T. Heil, J. P. Hofmann, R. Yan, 25 Z. Ni, S. Kang, J. Bai, Y. Li, Y. Huang, Z. Wang, H. Qin and
K. P. de Jong and M. Oschatz, Appl. Catal., A, 2018, 568,
13–220.
Z. Liu, S. Ren, X. Yu, X. Chen, G. Wang, X. Wu, G. Yu,
X. Li, J. Colloid Interface Sci., 2017, 505, 325–331.
26 Z. Ni, H. Qin, S. Kang, J. Bai, Z. Wang, Y. Li, Z. Zheng and
X. Li, J. Colloid Interface Sci., 2018, 516, 16–22.
2
M. Qiu, C. Yang and Y. J. C. S. Sun, Catal. Sci. Technol., 2018, 27 M. Luo, R. J. O’Brien, S. Bao and B. H. J. A. C. A. G. Davis,
, 423–427. Appl. Catal., A, 2003, 239, 111–120.
J. W. Park, J. Y. Lee, K. S. Kim, S. B. Hong and G. Seo, Appl. 28 Z.-x. Wang, T. Dong, T. Kan and Q.-x. Li, Chin. J. Chem.
Catal., A, 2008, 339, 36–44. Phys., 2008, 21, 141–150.
P. Losch, M. Boltz, C. Bernardon, B. Louis, A. Pal ˇc i ´c and 29 B. T. Teng, X. H. Guo, Y. Li, J. Chang and L. Tian, J. Mol.
V. Valtchev, Appl. Catal., A, 2016, 509, 30–37. Catal. A: Chem., 2007, 272, 182–190.
F. Jiao, J. Li, X. Pan, J. Xiao, H. Li, H. Ma, M. Wei, Y. Pan, 30 B. C. Enger, Å.-L. Fossan, Ø. Borg, E. Rytter and A. J. J. o. C.
8
5
6
7
Z. Zhou and M. Li, Science, 2016, 351, 1065–1068.
C. L ´o pez and A. Corma, Science, 2012, 335, 835–838.
H. M. T. Galvis and K. P. D. Jong, ACS Catal., 2013, 3,
Holmen, J. Catal., 2011, 284, 9–22.
8
9
31 A. N. Pour, S. M. K. Shahri, H. R. Bozorgzadeh, Y. Zamani,
A. Tavasoli and M. A. J. A. C. A. G. Marvast, Appl. Catal., A,
2008, 348, 201–208.
2130–2149.
1
1
0 H. Schulz and M. Claeys, Appl. Catal., A, 1999, 186, 71–90.
1 L. A. Cano, M. V. Cagnoli, J. F. Bengoa, A. M. Alvarez and
S. G. Marchetti, J. Catal., 2011, 278, 310–320.
2 D. Pe n˜ a, A. Cognigni, T. Neumayer, W. van Beek, D. S. Jones,
M. Quijada and M. Rønning, Appl. Catal., A, 2018, 554, 10–23.
32 M. E. Dry, T. Shingles, L. J. Boshoff and G. J. J. J. o. C.
Oosthuizen, J. Catal., 1969, 15, 190–199.
33 D. B. Bukur, X. Lang, D. Mukesh, W. H. Zimmerman,
M. P. Rosynek and C. J. I. Li, Ind. Eng. Chem. Res., 1990,
29, 1588–1599.
1
1
1
1
3 J. Li, X. Cheng, C. Zhang, Q. Chang, J. Wang, X. Wang, Z. Lv, 34 H. Arakawa and A. T. J. I. Bell, Ind. Eng. Chem. Res., 1983, 22,
W. Dong, Y. Yang and Y. Li, Appl. Catal., A, 2016, 528, 131–141. 97–103.
4 S. Li, N. Yao, F. Zhao and X. Li, Catal. Sci. Technol., 2016, 6, 35 J. Matos, J. L. Brito and J. J. A. C. A. G. Laine, Appl. Catal., A,
188–2194. 1997, 152, 27–42.
5 G. Jacobs, M. C. Ribeiro, W. Ma, Y. Ji, S. Khalid, 36 J. Matos, J. L. Brito and J. Laine, Appl. Catal., A, 1997, 152,
2
P. T. A. Sumodjo and B. H. J. A. C. A. G. Davis, Appl. Catal.,
A, 2009, 361, 137–151.
6 J. Li, G. Jacobs, T. Das and B. H. J. A. C. A. G. Davis, Appl.
Catal., A, 2002, 236, 67–76.
7 Z. Pan, M. Parvari and D. B. J. A. C. A. G. Bukur, Appl. Catal.,
A, 2014, 480, 79–85.
8 J. F. Cotter, Hudson Rev., 1982, 35, 471–472.
9 D. B. Bukur, D. Mukesh and S. A. Patel, Ind. Eng. Chem. Res.,
27–42.
37 R. Xie, C. Wang, X. Lin, W. Hui, T. Zhao and Y. Sun, Catal.
Lett., 2014, 144, 516–523.
38 R. Xie, H. Wang, P. Gao, X. Lin, Z. Zhang, T. Zhao and
Y. Sun, Appl. Catal., A, 2015, 492, 93–99.
39 V. R. Galakhov, A. S. Shkvarin, A. S. Semenova, M. A. Uimin,
A. A. Mysik, N. N. Shchegoleva, A. Y. Yermakov and
E. Z. Kurmaev, J. Phys. Chem. C, 2010, 114, 22413–22416.
40 Z. Zhang, W. Dai, X.-C. Xu, J. Zhang, B. Shi, J. Xu, W. Tu and
Y.-F. Han, AIChE J., 2017, 63, 4451–4464.
1
1
1
1
1990, 29, 194–204.
2
0 C. H. Zhang, Y. Yang, B. T. Teng, T. Z. Li, H. Y. Zheng,
H. W. Xiang and Y. W. J. J. o. C. Li, J. Catal., 2006, 237, 41 A. A. Khassin, T. M. Yurieva, V. V. Kaichev, V. I. Bukhtiyarov,
4
05–415.
A. A. Budneva, E. A. Paukshtis and V. N. Parmon, J. Mol.
Catal. A: Chem., 2001, 175, 189–204.
2
2
1 Y. Cheng, J. Lin, K. Xu, H. Wang, X. Yao, Y. Pei, S. Yan,
M. Qiao and B. J. A. C. Zong, ACS Catal., 2016, 6(1), 389–399. 42 J. Benziger and R. J. Madix, Surf. Sci., 1980, 94(1), 119–153.
2 R. Phienluphon, L. Shi, J. Sun, W. Niu, P. Lu, P. Zhu, 43 R. J. O’Brien and B. H. J. C. L. Davis, Catal. Lett., 2004, 94,
T. Vitidsant, Y. Yoneyama, Q. Chen and N. J. C. S. Tsubaki,
Catal. Sci. Technol., 2014, 4, 3099–3107.
3 N. Tsubaki, S. Sun and K. J. J. o. C. Fujimoto, J. Catal., 2001,
1–6.
44 H. K o¨ lbel and F. J. C. I. T.-C. Engelhardt, Chem. Eng.
Technol., 2010, 22, 97–104.
2
1
99, 236–246.
45 F. S. Karn, J. F. Schultz, R. E. Kelly and R. B. J. Anderson, Ind.
Eng. Chem. Res., 1963, 2, 43–47.
2
4 L. Shi, W. Shen, G. Yang, X. Fan, Y. Jin, C. Zeng, K. Matsuda
and N. J. J. o. C. Tsubaki, J. Catal., 2013, 302, 83–90.
46 Y. Yang, Appl. Catal., A, 2004, 266, 181–194.
9
4 | New J. Chem., 2020, 44, 87--94
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2020