Hierarchical structured α-Al2O3 supported S-promoted Fe catalysts for direct conversion of syngas to lower olefins

Article abstract of DOI:10.1039/c5cc00786k

Hierarchical structured α-Al₂O₃ is shown to be able to effectively disperse and immobilize iron species, in comparison with commercial α-Al₂O₃. After promotion using an appropriate amount of sulfur, iron catalysts exhibit not only enhanced Fischer-Tropsch synthesis activity and selectivity toward lower olefins, but also increased resistance against carbon deposits.

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Hierarchical structured α-Al2O3 supported S-promoted Fe catalysts for
direct conversion of syngas to lower olefins
Xiangping Zhou,a Jian Ji,a Di Wang,a Xuezhi Duan,a,* Gang Qian,a De Chenb and Xinggui Zhoua
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Hierarchical structured α-Al2O3 is shown to be able to
effectively disperse and immobilize iron species, in
comparison with commercial α-Al2O3. After promotion with
an appropriate amount of sulfur, the iron catalyst exhibits not
10 only enhanced Fischer-Tropsch synthesis activity and
selectivity toward lower olefins, but also increased resistance
against carbon deposits.
The Fischer-Tropsch-to-Olefins (FTO) process has recently
received increasing attention, because it is an alternative, non-oil-
15 based route for the direct production of lower olefins from coal-,
natural gas- or biomass-derived syngas.1 The process is generally
performed at relatively high temperature to shift selectivity
toward short-chain hydrocarbons.1,2 Relative to other FTO
catalysts, low-cost Fe catalysts are more attractive not only for
20 the lower methanation activity at high temperature, but also for
the higher selectivity toward lower olefins, stronger resistance
toward contaminants in syngas and higher water-gas shift activity,
which allows for the use of CO-rich syngas without H2/CO ratio
adjustment.1-3
50 Fig. 1 Typical FESEM images of α-Al2O3-H (a and b) and α-Al2O3-C (c),
and their XRD patterns (d).
25
Although unsupported (also referred to as bulk) Fe catalysts
modified by promoters have exhibited promising selectivity
toward lower olefins,4 their poor mechanical stability is
prohibitive to practical applications.5 Various supported Fe
catalysts with good mechanical stability have been tested.1-3,6
residence time and high temperature, which would prevent the
oxidation of iron by H2O and/or CO2, minimize the secondary
reactions and facilitate the desorption of olefins.2b,8 This would
55 allow us to probe the sulfur promoter effects of Fe/α-Al2O3
catalysts on FTO performance and formation of carbon deposits,
aiming to gain knowledge for rational design of highly efficient
and carbon-deposits-resistant Fe-based FTO catalysts.
α-Al2O3-H support was obtained from the calcination of
60 hierarchical structured boehmite (Fig. S1, ESI†) prepared by a
two-step solvothermal method described earlier,9 and α-Al2O3-C
was used as a reference support. Both supports did not contain
detectable impurities (Fig. S2, ESI†). Their morphologies were
characterized by field emission scanning electron microscopy
65 (FESEM), and the results are shown in Figs. 1a-1c. α-Al2O3-H
exhibits a hierarchical structure in the form of nano- or
submicron-sized rods integrated as micron-sized needle leaves,
whereas α-Al2O3-C appears as micron-sized particles with
irregular shapes. They show different ratio of XRD peak
70 intensities (Fig. 1d) and thus different crystal plane orientation. In
addition, α-Al2O3-H (11 m2·g-1) has higher N2-BET surface area
than α-Al2O3-C (2 m2·g-1).
30 Promoted iron nanoparticles dispersed on supports such as α-
Al2O3 and carbon materials that have weak interaction with the
metal would produce high yields of lower olefins.1,2a-2c,6e-6h In
particular, promotion with sulfur would result in superior
selectivity toward lower olefins.2d,6e However, these catalysts
35 usually underwent severe deactivation by carbon deposits.7
α-Al2O3 is a good support worthy of exploration for FTO
reaction owing to its proven merits in wide industrial applications.
It is also a good support in research of the deactivation
mechanism of FTO reaction, because of the easy identification of
40 the carbon deposits. For increased dispersion and immobilization
of iron particles, it is better to have a high surface area. Moreover,
it should be very pure to exclude the possible influences of other
elements (e.g., Mn, Si and Cr).2a
Herein, we report the development of hierarchical structured α-
45 Al2O3 (i.e., α-Al2O3-H) with a high surface area as a support to
effectively disperse and immobilize (S-promoted) iron catalyst in
comparison to commercial α-Al2O3 (α-Al2O3-C). These catalysts
are tested under FTO conditions of low CO conversion, short
Subsequently, Fe catalysts were prepared with these two
supports by using ammonium iron citrate as iron precursor
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Fig. 3 FTY (a) and product selectivity (b) as a function of TOS on Fe/α-
50 Al2O3-H (▲) and Fe/α-Al2O3-C (○) catalysts. Reaction conditions: 350 oC,
Fig. 2 (a) H2-TPR profiles of calcined Fe/α-Al2O3-H (red) and Fe/α-
Al2O3-C (blue) as well as bulk α-Fe2O3 (black). Elemental mapping
images of calcined Fe/α-Al2O3-H (b) and Fe/α-Al2O3-C (c) samples. Fe,
5 Al and O elements are colored in blue, green and red, respectively.
-1
1 bar and H2/CO=1 with a GHSV=9000 mL·h-1·gcat
.
demonstrated that with the increase of iron particle size, the CH4
=
=
selectivity decreases, and the C2 -C4 selectivity almost does not
change.6e Therefore, it can be concluded that the encapsulating
55 carbon rather than the agglomeration of iron particles is
dominating reason resulting in the deactivation.
(Tables S1 and S2, ESI†). Legible diffraction peaks of α-Fe2O3
(Fig. S3, ESI†) are seen in the XRD patterns of calcined Fe/α-
Al2O3-H and Fe/α-Al2O3-C samples. Fig. 2a shows H2-TPR
profiles of two calcined samples. Both of them exhibit three main
10 reduction peaks. For the first reduction peak, the shift to lower
temperature is seen over calcined Fe/α-Al2O3-H sample compared
to bulk α-Fe2O3, which could be related to iron oxides dispersed
by support;10 the similar reduction temperatures between calcined
Fe/α-Al2O3-C and bulk α-Fe2O3 indicate the presence of
15 unsupported iron particles in calcined Fe/α-Al2O3-C. Generally,
reduction peak of high temperature is associated with metal
oxides strongly interacting with support.11 Therefore, larger peak
area for the third reduction peak of calcined Fe/α-Al2O3-H
compared to calcined Fe/α-Al2O3-C suggests stronger interaction
20 of α-Al2O3-H with iron oxides. Furthermore, high-resolution
transmission electron microscopy (HRTEM) and elemental
mapping were used to study the dispersion and immobilization of
iron oxides on support, and the results are shown in Fig. S4 (ESI†)
and Figs. 2b-2c. Obviously, almost all the iron oxide particles are
25 highly dispersed and immobilized on α-Al2O3-H support. On the
contrary, on α-Al2O3-C support, both supported-type and
unsupported-type (bulk) iron oxides are observed. These results
demonstrate that α-Al2O3-H is a promising support to disperse
and immobilize iron catalysts.
Fe/α-Al2O3-H catalysts with different content of S promoter by
using ferrous sulfate as S source (Table S1-S2, ESI†) were
further tested for FTO process. Table 1 gives a comparison of
60 FTO performances between Fe/α-Al2O3-H catalysts with S
promoter and Fe/α-Al2O3-H catalyst without S promoter under
TOS=19 h. It is found that all the Fe/α-Al2O3-H catalysts with S
promoter compared to Fe/α-Al2O3-H catalyst exhibit not only
significantly enhanced FTY and selectivity toward lower olefins,
65 but also reduced selectivity toward CO2 and CH4. This indicates
remarkable promotional effects of sulfur on the FTO performance.
Inspired by recent findings that S promoter can enhance FTO
performance of Fe-based catalysts, but meanwhile produce more
carbon deposits,2b we employed thermogravimetric analysis
70 (TGA) to determine content of carbon deposits on three used S-
promoted Fe/α-Al2O3-H catalysts (Table 1 and Fig. S7, ESI†).
Compared to the used Fe/α-Al2O3-H catalyst (~6.3 wt % carbon
deposits), the used Fe/α-Al2O3-H-3S and Fe/α-Al2O3-H-5S
catalysts exhibit significantly enhanced contents of carbon
75 deposits, which are 11.1 and 12.4 wt%, respectively. More
30 The two calcined samples were in-situ reduced in the reactor at
o
350 C for 2 h under ambient pressure, and then syngas was
introduced for FTO process. Fig. 3 shows iron time yield (FTY,
the number of CO molecules converted to hydrocarbons per gram
of iron per second) and product selectivity as a function of time
35 on stream (TOS) of the two catalysts. Although Fe/α-Al2O3-H
=
catalyst exhibits higher initial FTY and C2 -C4= selectivity than
Fe/α-Al2O3-C catalyst, they undergo severe deactivation in terms
of significantly decreasing FTY with the increase of TOS.
Subsequently, the change in the geometric properties between
40 fresh and used Fe/α-Al2O3-H catalysts was studied by HRTEM to
identify dominating reason for the deactivation. As shown in Figs.
S5-S6 (ESI†) and Fig. 4a, the used catalyst exhibits larger mean
particle size than the fresh catalyst, and the surfaces of most iron
particles in the used catalyst are encapsulated by graphitic carbon
45 layers. Additionally, with the increase of TOS, Fe/α-Al2O3-H
=
=
catalyst shows remarkable decreasing C2 -C4 selectivity and
increasing CH4 selectivity. Recently, de Jong and coworkers have
Fig. 4 Typical HRTEM images of used Fe/α-Al2O3-H (a), Fe/α-Al2O3-H-
3S (b), Fe/α-Al2O3-H-5S (c) and Fe/α-Al2O3-H-8S (d) catalysts.
2
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Table 1 FTO performances of Fe-based catalysts and properties of carbon deposits on the used catalysts. a
Selectivity (%C)
Carbon deposits
CO conversion
(%)
FTY
CO converted to
CO2 (%C)
Catalyst
(10-6 molCO∙gFe-1∙s-1)
=
0
C2=-C4 C20-C4 C5+
Content (wt%)b
Type
CH4
Fe/α-Al2O3-H
0.4
1.8
2.3
0.9
1.0
11.5
15.1
5.9
86
47
43
43
78
38
38
29
18
55
55
68
2
6
6
2
2
1
6.3
11.1
12.4
1.1
Encapsulating carbon
Fibrous carbon
Fe/α-Al2O3-H-3S
Fe/α-Al2O3-H-5S
Fe/α-Al2O3-H-8S
1
Fibrous carbon
~1
Isolated carbon
-1
a Reaction conditions: 350 oC, 1 bar, TOS=19 h and H2/CO=1 with a GHSV=9000 mL·h-1·g cat
;
b Obtained from TGA results of the used catalysts (Fig. S7, ESI†).
interestingly, when the content of sulfur promoter is increased up
to 0.08 wt%, the used Fe/α-Al2O3-H-8S catalyst shows very small
amount of carbon deposits (i.e., 1.1 wt%). Notably, Fe/α-Al2O3-
45 stable. In addition, this catalyst has much higher FTY and
selectivity toward lower olefins than Fe/α-Al2O3-H catalyst. All
of these results indicate that an appropriate amount of sulfur
promoter would yield highly active, selective and stable Fe-based
FTO catalysts. Moreover, it is noted that the above different Fe-
50 based catalysts contain comparable Na contents (Table S2, ESI†),
which might have a similar synergistic effect with S promoter for
Fe FTO catalyst.2b However, the effects of S concentration are
still apparent in the present work.
In summary, we have demonstrated that hierarchical structured
55 α-Al2O3 compared to commercial α-Al2O3 effectively disperses
and immobilizes iron species. Sulfur concentration has a crucial
impact on FTO performance and carbon deposits type of S-
promoted Fe/Al2O3-H catalysts. Fe/Al2O3-H-8S catalyst is found
to exhibit not only enhanced Fischer-Tropsch synthesis activity
5
=
H-8S catalyst shows an unprecedented high C2 -C4= selectivity up
to 68 %C. This might shed new light on the development of
highly efficient and carbon-deposits-resistant Fe FTO catalysts
by adjusting sulfur content.
Furthermore, HRTEM measurements of fresh and used S-
promoted catalysts were performed to determine the level of
sintering of iron particles and the type of carbon deposits (Table
1 and Figs. 4b-4d). Although iron particles on the three fresh
catalysts have similar mean size (Fig. S8, ESI†), those on the
10
15 three used catalysts have different level of sintering. The used
Fe/α-Al2O3-H-3S and Fe/α-Al2O3-H-5S catalysts contain two
kinds of iron particles: one is supported on α-Al2O3-H, the other
detaches the support surfaces and is located at the tips of fibrous 60 and selectivity toward lower olefins, but also increased resistance
carbon (also referred as to carbon nanofibers). Previous studies
20 reported that the growth of fibrous carbon is based on a delicate
balance between carbon formation rate and carbon diffusion
rate.12 Under the same FTO conditions, higher CO conversions of
Fe/α-Al2O3-H-3S and Fe/α-Al2O3-H-5S than Fe/α-Al2O3-H
catalyst may explain why the addition of sulfur promoter changes
25 the type of carbon deposits from encapsulating carbon to fibrous
carbon. In addition, on the used Fe/α-Al2O3-H-8S catalyst, almost
no carbon nanofibers are observed. The detected carbon deposits
by TGA are most likely to originate from the precipitation of
dissolved carbon in metal particles during the cooling process or
30 carbon islands,13 which is donated as “isolated carbon”. The
above systematic studies reveal the remarkable sulfur promoter
effects on the type of carbon deposits.
against carbon deposits. These findings might open new avenues
for the rational design of Fe FTO catalysts by adjusting promoter
content.
This work is financially supported by the National Basic
65 Research Program of China (2012CB720500), the Natural
Science Foundation of China (21306046), the Fundamental
Research Funds for the Central Universities (WA1514013 and
WB1514056) and the 111 Project of Ministry of Education of
China (B08021).
70 Notes and references
a State Key Laboratory of Chemical Engineering, East China University
of Science and Technology, 130 Meilong Road, Shanghai 200237, China.
E-mail: xzduan@ecust.edu.cn
b Department of Chemical Engineering, Norwegian University of Science
75 and Technology, N-7491 Trondheim, Norway
Fig. S9 (ESI†) shows FTY as function of TOS over the three
S-promoted Fe/α-Al2O3-H catalysts. On Fe/α-Al2O3-H-3S and
35 Fe/α-Al2O3-H-5S catalysts, FTY significantly decreases followed
by increases with the reaction time. On the basis of the
encapsulating carbon as the dominating reason for the
deactivation of Fe/α-Al2O3-H, the decreasing FTY could be
†Electronic Supplementary Information (ESI) available: Experimental
details, Table S1-S2 and Figs. S1-S9. See DOI: 10.1039/b000000x
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