The effect of pore size or iron particle size on the formation of light olefins in Fischer-Tropsch synthesis

Article abstract of DOI:10.1039/c5ra02319j

The surface modification of a silica support was applied in solely comparing the effects of iron particle size or pore size on the formation of light olefins. Smaller iron or iron carbide particles are advantageous in forming light olefins and O/P of C₂-C₄ is more sensitive to the pore size of the catalysts. This journal is

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COMMUNICATION
The effect of pore size or iron particle size on formation of light olefins
in Fischer–Tropsch synthesis
Yi Liu,a Jian-Feng Chenb and Yi Zhang*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
size with different particle size, or a fixed particle size of active
50 metals with different pore sizes.
The surface modification of silica support was applied in
solely comparing the effects of iron particle size or pore size
on forming light olefins. Smaller iron or iron carbide particle
is advantageous to form light olefins and O/P of C2-C4 is more
In the present work, in order to investigate the sole effect of
pore size or iron particle size on formation of light olefins in
Fischer–Tropsch synthesis (FTS), the various iron supported
catalysts with pore size of 5 nm, 50 nm and 80 nm were prepared
55 by surface modification of silica support by ethylene glycol to
realize the similar iron particle size. The similar properties of iron
active phase on different catalysts guaranteed solely comparing
the effect of pore size on formation of light olefins. For sole
comparing effects of particle size, the conventional impregnation
60 method was applied to prepare the catalysts contained large iron
particle size and the same pore size.
10 sensitive to the pore size of catalysts.
Fischer–Tropsch synthesis has long been known as one of the key
technologies for producing ultra-clean fuels from non-crude-oil
feedstocks. Comparing to ruthenium or cobalt catalysts, iron-
based catalysts are advantageous due to their high selectivity of
15 olefins, low costs, and tolerance to high temperatures and
1
moisture environments. Previously, researchers showed that
syngas can be converted to olefins via F-T synthesis, by using
bulk or supported iron-based catalysts. 2-4
For supported iron-based catalysts, several researchers have
20 suggested that the texture properties of the support have a
(a)
significant influence on the FTS activity and light olefins
5-8
selectivity.
Generally, small pore size is advantageous to
formation of small supported metal particles and promotes the
9-11
dispersion of supported metal.
However, the small pore size
25 of supports results in poor diffusion efficiency of reactants and
products, which is a disadvantage to catalytic performance of
catalysts. In contrast, the support with a larger pore size improves
the reducibility, improves the diffusion of reactants and products,
suppresses the re-adsorption of 1-olefins, and leads to high heavy
5, 7, 12
30 hydrocarbon content in the products.
However, the
increased chain-growth probability occurring on the larger pore
catalyst might be attributed to the combined effects of larger
metal particle size and larger pore size. It was found that both the
dispersion of the supported metal and the pore size of the catalyst
35 showed their close relationship with the propagation of the
carbon chain.
(b)
Although a few studies were carried out on the influence of the
iron particle size and the pore size of support on the FTS reaction
activity and selectivity, controversy persists, because these
40 observations were the results of complex interplay among many
factors. These factors included not only the re-adsorption
probability of α-olefins in the confined space and the diffusion
situation, but also the effect of changes in the site density, such as
the changes in loading, reducibility and the particle size of active
45 metal. 1 Hence, it is difficult to evidence the effects of the support
porosity on activity and selectivity, since the metal dispersion or
the metal particle size also depends on the pore size distribution.
More insights are expected to prepare catalysts with a fixed pore
Fig. 1 (a) Pore size distribution of silica supported iron-based catalysts as
65 prepared, (b) X-ray diffractograms of silica supported iron-based catalysts
as prepared.
The textural properties of obtained catalysts were characterized
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by N2 physisorption and mercury intrusion porosimetry. As
significantly from original FeMn/S80 and FeMn/S50 catalysts
shown in Fig. 1a, the pore size distributions of all iron-based 15 and were very close to that of FeMn/S5, as shown in Table 1 and
catalysts show monomodal pore size distributions, indicating
there are not newly formed pores inside the original pores
contributing to solely compare effects of pore size on light olefin
formation. XRD was used to analyze the crystalline phases
Fig. 1b. It is proved that the pretreatment of silica supports by
ethylene glycol remarkably modified the properties of the silica
surface and increased the isolated SiOH ratio on the silica surface,
5
13
resulting in lower crystalline size of supported iron oxide. On
present in the catalysts after calcination. As show in Fig. 1b, no 20 the other hand, it is considered that during the calcination step,
peaks assigned to manganese species were observed on these
samples, due to very small loading of Mn. It is found that the
the heat uniformly released from thermal decomposing of oxyl
group derived from ethylene glycol was quickly absorbed by the
precursor of catalyst to supply energy for decomposition of
nitrates, which efficiently restrained the aggregation of the iron
10 particle size increase from 8.1 to 17.5 nm for samples with
constant metal loading on increasing the pore size from 5 to 80
nm. For the ethylene glycol pretreated catalysts, the particle size 25 oxides, resulting in smaller particle size. 14
of the FeMn/S80-E and FeMn/S50-E catalysts decreased
Table 1 Various properties of silica supported iron-based catalysts.
Catalysts
Pore Size (nm)
Pore Volume
(cm3/g)
Specific Surface
Area (m2/g)
d(Fe2O3)
(nm)a
d(Fe2O3)
(nm)b
d(Fe5C2)
(nm)c
Mn/Fe atomic
ratiod
Fe
reducibility
(%)e
FeMn/S80
FeMn/S50
FeMn/S5
FeMn/S50-E
FeMn/S80-E
84.6
51.9
4.9
54.0
83.1
0.93
0.95
0.80
0.91
0.93
70.4
111.4
372.8
78.2
17.5
13.7
8.1
8.2
8.2
17.1
13.8
7.9
8.5
8.0
18.2
15.2
8.2
8.3
8.2
0.37
0.29
0.09
0.10
0.09
41.9
39.4
32.0
29.0
73.5
29.9
a Fe2O3 crystallite size as determined by X-ray diffraction. b Fe2O3 crystallite size as determined by TEM. c Fe5C2 crystallite size as determined by TEM
after reduction. d Determined by XPS, the stoichiometric Mn/Fe atomic ratio of all catalysts is 0.05. e Calculated by H2-TPR.
distribution of various iron-based catalysts. The results are in
agreement with the XRD analysis, indicating that the average iron
(a)
(b)
(d)
particle size increases with increasing pore size. Meanwhile, for
the catalysts pretreated by EG, significant differences in Fe2O3
40 particle size have been observed (Fig. 2c and e). The FeMn/S80-
E and FeMn/S50-E catalysts exhibited homogeneous remarkable
smaller particle size than that of FeMn/S80 and FeMn/S50,
respectively. The TEM images of various reduced iron catalysts
were compared in Fig. S2. As shown in Table 1, the size of iron
45 carbide kept the same trend to the iron particle size of various
catalysts.
The iron phase composition and dispersion of the fresh
catalysts were also determined by Mössbauer spectroscopy. As
shown in Fig. 3, the MES spectra of the fresh FeMn/S50 catalyst
50 include a sextet and a doublet, whereas the MES spectra of the
FeMn/S50-E and FeMn/S5 catalysts include only a doublet.
According to the MES parameters listed in Table S1, the sextet is
assigned to the magnetic α-Fe2O3 with crystallites size larger than
13.5 nm. 15 The doublet is typical for the superparamagnetic (spm)
(c)
(e)
55 Fe3+ ions on the non-cubic sites with the crystallite diameters
30
16,17
smaller than 13.5 nm.
The FeMn/S50 consists of 62.8%
ferromagnetic α-Fe2O3 and 37.2% Fe3+ (spm) and the FeMn/S50-
E and FeMn/S5 are composed of 100% Fe3+ (spm). This result
suggests that the average Fe2O3 particle size of FeMn/S50 were
60 larger than those of FeMn/S50-E and FeMn/S5. In addition, for
the FeMn/S50-E and FeMn/S5 catalysts, the ratio of the atoms
located on the „„surface‟‟ of the crystallites to the atoms located
in the „„bulk‟‟ of the crystallites was similar. This result indicates
that the average iron oxide particle sizes were almost identical for
65 FeMn/S50-E and FeMn/S5, 18, 19 and it is well consistent with the
results of TEM and XRD characterization. Based on these
findings, the similar particle structure of catalysts with different
pore size contributes to solely investigate the pore size effects,
and the same pore size of catalysts with different iron particle
Fig. 2 TEM micrographs for silica supported iron-based catalysts on (a)
FeMn/S5, (b) FeMn/S50, (c) FeMn/S50-E, (d) FeMn/S80 and (e)
FeMn/S80-E as prepared.
35
Fig. 2 and Fig. S1 show the TEM images and the particle size
2
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size is advantageous to solely comparing the particle size effects 35 catalysts was measured by temperature programmed reduction
on selectivity of light olefins.
(TPR). The H2-TPR profiles of the catalysts are presented in Fig.
5. It is clearly shown that the reduction process of the catalysts all
occurs in two main stages. The first stage at low temperature
(250-400℃) is attributed to the transformations of Fe2O3→Fe3O4
40 and part of Fe3O4→FeO, whereas the second stage at higher
temperature (400-550 ℃ ) correspond to the reduction of the
remainder of Fe3O4 and FeO, and the peak above 600℃ should
be ascribed to the reduction of wustite (FeO) or Fe2SiO4, which
strongly interacted with SiO2. 24, 25 In addition, it is found that the
45 temperature of the reduction peak shifts to high temperature with
a decrease of Fe2O3 particle size, due to strong interaction with
silica support of small iron particle. On the other hand, the
catalysts with similar iron particle size, such as FeMn/S5,
FeMn/S50-E and FeMn/S80-E, exhibited very similar reduction
50 profile, contributing to the similar properties of active phase
during FTS reaction. As compared in Table 1, the reducibility of
FeMn/S50 and FeMn/S80 was higher than that of catalysts with
smaller iron particles. In addition, based on the XPS results
(Table 1), it was observed that the Mn/Fe atomic ratio on the
55 catalyst surface increased from 0.09 of FeMn/S5 to 0.37 of
FeMn/S80 when the Fe2O3 particle size increased from 8.1 to
17.5 nm. It is considered that the different reducibility and Mn/Fe
atomic ratio of the catalysts with large iron particle could
influence the reaction performance during FTS reaction.
Fig. 3 ssbauer spectra of the fresh iron-based catalysts: (a) FeMn/S50;
5
(b) FeMn/S50-E; (c) FeMn/S5.
In order to identify the chemical properties of iron particle, the
XPS analysis was carried out to determine the surface chemical
composition and electronic structures of iron particle with
different size. The Fe 2p spectrum (Fig. 4) displays the main
10 peaks of Fe 2p3/2 at the binding energies between 710.4 and 711.3
eV, together with a very weak satellite structure between the
spin–orbit doublet. This result indicates that Fe species are
20, 21
characteristic of trivalence in α-Fe2O3.
Concerning the
binding energy of Fe2p of various catalysts, it slightly decreased
15 with increased iron particle size. Meanwhile, the catalysts with
the almost same particle size showed the same binding energy. It
is considered that the large iron particle of FeMn/S80 and
FeMn/S50 catalyst increased electron density of surface iron
atom, leading to lower binding energy. On the other hand, it has
20 been reported that manganese oxide could act as surface basicity
as alkali metal does, 22, 23 namely, manganese oxide might act as a
strong electron donor. Therefore, the Fe/Mn ratio of the iron
particle surface was determined by XPS also. As compared in
Table 1, the Fe/Mn ratio was increased with increasing iron
25 particle size, and FeMn/S80-E, FeMn/S50-E and FeMn/S5
catalysts exhibited the almost same Fe/Mn ratio, indicating that
Mn enriched on the surface of larger iron particle and would
exhibited different promotional effects for larger iron particles,
comparing to smaller one. Meanwhile, for the FeMn/S80-E,
30 FeMn/S50-E and FeMn/S5 catalysts, the similar Fe/Mn ratio
indicated the promotional effects of added Mn on EG pretreated
catalysts were similar to the non-pretreated one, contributing to
solely comparing the effects of pore size on light olefin formation.
The reduction behavior of the silica supported iron-based
60
Fig. 4 XPS spectra of Fe 2p on silica supported iron-based FTS catalysts
as prepared.
Fig. 5 H2-TPR profiles of silica supported iron-based catalysts.
65
To investigate the sole effects of pore size and particle size on
the catalytic performance, the obtained catalysts were applied to
fixed-bed FTS reaction. As compared in Fig. S3, the FeMn/S80-E
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catalyst realized the highest CO conversion in all catalysts.
Meanwhile, both FeMn/S50-E and FeMn/S80-E catalysts
exhibited excellent stability during 30 h reaction. As reported in
prepared by conventional impregnation method.
As shown in Fig. 6 b and Table 2, when the iron particle size
was similar for different catalysts, the selectivity of light olefin
significantly increased with the increased pore size of catalysts,
7
our previous literature, the pore size of catalyst could influence
5
the catalytic activity because of the differences in the diffusion of 55 from 30.2% for FeMn/S5 to 54.6% for FeMn/S80-E. And the
both reactants and products. On the other hand, smaller iron
ratio of olefin to paraffin (O/P) in C2-C4 hydrocarbon also
increased with increasing pore size. Furthermore, the formation
of CH4 was suppressed with increasing pore size, which
decreased from 24.2% to 13.3% for FeMn/S5 and FeMn/S80-E,
carbide clusters of fine dispersed iron catalysts are less prone to
26
deactivation.
Therefore, for FeMn/S80-E catalyst, the largest
pore size and very small iron carbide particle size is advantageous
10 to realize the high activity and good stability. Because 60 respectively. However, the liquid hydrocarbons (C5+) selectivity
FeMn/S80-E and FeMn/S80 catalysts contained the same pore
size, it is considered that smaller iron particle size is
advantageous to obtain high activity and stability for iron based
FTS catalyst.
Although a few studies were carried out on the influence of the
iron particle size and the pore size of support on the FTS reaction
selectivity, controversy persists, because these observations are
the result of complex interplay among many factors. In this study,
the sole effect of pore size or iron particle size was investigated
was similar for different catalysts as around 19%, as shown in
Table 2.
(a)
60
50
40
30
20
10
0
25
20
15
10
5
15
20 for forming light olefins in Fischer–Tropsch synthesis (FTS).
Table 2 Catalytic activity and product selectivity of silica supported iron-
based catalysts. a
Catalysts CO CO2
conv. (%CO
Hydrocarbon
selectivity
C2-C4
Olefin/P
araffin
(%) conv.)
(c-mol %, CO2-free)
CH4
8.5
11.7
24.2
14.8
13.3
C=
C 2-4
10.7
15.5
27.2
14.2
11.7
C5+
2-4
0
FeMn/S80 40.0 33.9
FeMn/S50 44.0 34.1
FeMn/S5 21.9 24.3
FeMn/S50-E 48.3 32.9
FeMn/S80-E 50.5 33.4
48.2
43.5
30.2
51.2
54.6
32.6
29.3
18.4
19.8
20.4
4.50
2.81
1.11
3.61
4.67
0
10
20
30
40
50
60
70
80
90
Pore size (nm)
(b)
60
25
20
15
10
5
a Reaction condition: 573 K, 1.0 MPa, H2/CO=1, W/F(CO+H2+Ar) =5 g-
cat. h mol-1, Weight of catalyst =0.5 g.
50
40
30
20
10
0
25
The selectivities of light olefins and methane as function of
pore size were compared in Fig. 6. For the catalysts obtained
from conventional preparation method, as shown in Fig. 6a, the
selectivity of light olefins increased from 30.2% for FeMn/S5 to
48.2% for FeMn/S80, and methane selectivity significantly
30 decreased from 24.2% to 8.5% for FeMn/S5 and FeMn/S80,
respectively. The tendency of olefin to paraffin ratio (O/P) was
also increased from 1.11 to 4.50 for FeMn/S5 and FeMn/S80 as
compared in Table 2. However, the FeMn/S80 catalyst formed
much more liquid products (C5+) than FeMn/S5 catalysts, as the
35 selectivity of liquid products (C5+) was 32.6% and 18.4%,
respectively. It is well know that the support with a larger pore
size favors the diffusion of reactants and products, suppresses the
re-adsorption of 1-olefins, and leads to high heavy hydrocarbon
0
0
10
20
30
40
50
60
70
80
90
Pore size (nm)
65
Fig. 6 Product selectivity as a function of pore size: (a) nonpretreated
catalysts; (b) EG-pretreated catalysts; light olefin ( ); methane ( ); iron
carbide particle size ( ).
For FeMn/S50-E and FeMn/S80-E catalysts, however, the
5
content in the product. Therefore, the catalysts with larger pore
70 methane selectivity was remarkably enhanced than that of
FeMn/S50 and FeMn/S80 catalysts, when the iron carbide
particle size significantly decreased. Meanwhile, the methane
selectivity of the FeMn/S50-E and FeMn/S80-E catalysts was
very similar as 14.8% and 13.3%, respectively. On contrast, the
75 methane selectivity of FeMn/S50 catalyst was higher than that of
FeMn/S80 catalyst. The selectivity of liquid hydrocarbon also
exhibited similar tendency. As compared in Fig. 6 and Table 2,
the FeMn/S50-E and FeMn/S80-E catalysts contained the almost
same iron carbide particle size (Fig. 6 b), and the FeMn/S50
80 catalyst exhibited smaller iron carbide particle size than
40 size, such as FeMn/S50 and FeMn/S80, formed much more liquid
hydrocarbon and less methane. However, as the improved
diffusion efficiency in larger pore, the second reactions of formed
light olefins were suppressed, leading to the increased light olefin
selectivity with the increased pore size.
45
It should be point out that the FeMn/S80 catalyst contained
both large pore size and large iron particle size, and the reaction
performance of this catalyst should be the synergetic effects of
large pore size and particle size. It is considered that the lowest
methane selectivity contributes the highest light olefin and liquid
50 hydrocarbon selectivity for FeMn/S80 catalyst in catalysts
4
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FeMn/S80. On the other hand, the FeMn/S80-E and FeMn/S80
particles was advantageous to form more light olefins.
catalysts had much larger pore size than the FeMn/S50-E and
FeMn/S50 catalysts. Base on these results, it is considered that
the selectivity of methane and liquid hydrocarbons (C5+) strongly
depended on iron particle size or iron carbide particle size,
namely, smaller iron particle or iron carbide particle was much
beneficial to forming light hydrocarbons including methane.
Concerning the selectivity of light olefins, the FeMn/S5,
FeMn/S50-E and FeMn/S80-E catalysts exhibited the increased
10 light olefins selectivity with the increased pore size, as illustrated
in Fig. 6 b. Because these catalysts contained the same iron
particle size, and it is proved that those iron particles had similar
bulk and surface properties as proved by XRD, TEM, XPS, TPR
and Mössbauer spectroscopy. It is believed that the large pore
15 size could contribute to the formation of light olefins via
45 Conclusions
The sole effect of iron particle size or pore size was investigated
on formation of light olefins in Fischer–Tropsch synthesis (FTS).
The obtained catalysts with the same pore size and similar
properties of iron active phase guaranteed solely comparing the
50 effect of iron particle size on formation of light olefins. The
smaller iron particle is much beneficial to forming light
hydrocarbons including methane. Furthermore, smaller iron
particle is advantageous to form more light olefins. Meanwhile,
the olefin to paraffin ration (O/P) of C2-C4 hydrocarbons is more
55 sensitive to pore size of catalysts due to suppressing the second
reaction of formed olefins.
5
suppressing the second reaction of formed olefins including
27
Acknowledgment
hydrogenation, isomerization and chain-growth reaction.
On
the other hand, as illustrated by Fig. 7, the olefin to paraffin ratio
(O/P) of C2-C4 hydrocarbons significantly increased from small
20 pore catalyst to large pore catalyst regardless of iron carbide
particle size, indicating the O/P of C2-C4 hydrocarbons was more
sensitive to pore size of catalysts.
This work was supported by National Natural Science Foundation
of China (Nos. 91334206, 51174259), Ministry of Education
60 (NCET-13-0653), National “863” program of China (No.
2012AA051001 and 2013AA031702).
Notes and references
60
55
50
45
40
35
30
25
10
9
8
7
6
5
4
3
2
1
a State Key Laboratory of Organic–Inorganic Composites, Department of
Chemical Engineering, Beijing University of Chemical Technology,
65 Beijing 100029, China. Fax: 86-10-64423474; Tel: 86-10-64447274; E-
mail: yizhang@mail.buct.edu.cn
Pore of 80nm
Pore of 50nm
b Research Centre of the Ministry of Education for High Gravity
Engineering and Technology, Beijing University of Chemical Technology,
Beijing 100029, China. E-mail: chenjf@mail.buct.edu.cn
O/P of 80nm Pore
O/P of 50nm Pore
70 † Electronic Supplementary Information (ESI) available: Experimental
details, Mössbauer parameters of the fresh iron-based catalysts, TEM
micrographs for the catalysts after reduction and CO conversion as a
function of time on stream. See DOI: 10.1039/b000000x/
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40 FeMn/S80-E catalyst had larger pore size than FeMn/S50-E 100
catalyst. Therefore, it is considered that the formation of light
olefins on silica supported FeMn was more sensitive to iron or
iron carbide particle size, namely, smaller iron or iron carbide
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