Cobalt-Iron-Manganese Catalysts for the Conversion of End-of-Life-Tire-Derived Syngas into Light Terminal Olefins

Article abstract of DOI:10.1002/chem.201704191

Co-Fe-Mn/γ-Al₂O₃ Fischer–Tropsch synthesis (FTS) catalysts were synthesized, characterized and tested for CO hydrogenation, mimicking end-of-life-tire (ELT)-derived syngas. It was found that an increase of C₂-C₄ olefin selectivities to 49 % could be reached for 5 wt % Co, 5 wt % Fe, 2.5 wt % Mn/γ-Al₂O₃ with Na at ambient pressure. Furthermore, by using a 5 wt % Co, 5 wt % Fe, 2.5 wt % Mn, 1.2 wt % Na, 0.03 wt % S/γ-Al₂O₃ catalyst the selectivity towards the fractions of C₅₊ and CH₄ could be reduced, whereas the selectivity towards the fraction of C₄ olefins could be improved to 12.6 % at 10 bar. Moreover, the Na/S ratio influences the ratio of terminal to internal olefins observed as products, that is, a high Na loading prevents the isomerization of primary olefins, which is unwanted if 1,3-butadiene is the target product. Thus, by fine-tuning the addition of promoter elements the volume of waste streams that need to be recycled, treated or upgraded during ELT syngas processing could be reduced. The most promising catalyst (5 wt % Co, 5 wt % Fe, 2.5 wt % Mn, 1.2 wt % Na, 0.03 wt % S/γ-Al₂O₃) has been investigated using operando transmission X-ray microscopy (TXM) and X-ray diffraction (XRD). It was found that a cobalt-iron alloy was formed, whereas manganese remained in its oxidic phase.

Full text of DOI:10.1002/chem.201704191

DOI: 10.1002/chem.201704191
Full Paper
&
Syngas Conversion
Cobalt-Iron-Manganese Catalysts for the Conversion of End-of-
Life-Tire-Derived Syngas into Light Terminal Olefins
Jan P. Falkenhagen,[a] Lise Maisonneuve,[b] Pasi P. Paalanen,[a] Nathalie Coste,[b]
Nicolas Malicki,[b] and Bert M. Weckhuysen*[a]
Abstract: Co-Fe-Mn/g-Al2O3 Fischer–Tropsch synthesis (FTS)
catalysts were synthesized, characterized and tested for CO
hydrogenation, mimicking end-of-life-tire (ELT)-derived
syngas. It was found that an increase of C2-C4 olefin selectivi-
ties to 49% could be reached for 5 wt% Co, 5 wt% Fe,
2.5 wt% Mn/g-Al2O3 with Na at ambient pressure. Further-
more, by using a 5 wt% Co, 5 wt% Fe, 2.5 wt% Mn, 1.2 wt%
Na, 0.03 wt% S/g-Al2O3 catalyst the selectivity towards the
fractions of C5+ and CH4 could be reduced, whereas the se-
lectivity towards the fraction of C4 olefins could be improved
to 12.6% at 10 bar. Moreover, the Na/S ratio influences the
ratio of terminal to internal olefins observed as products,
that is, a high Na loading prevents the isomerization of pri-
mary olefins, which is unwanted if 1,3-butadiene is the
target product. Thus, by fine-tuning the addition of promot-
er elements the volume of waste streams that need to be re-
cycled, treated or upgraded during ELT syngas processing
could be reduced. The most promising catalyst (5 wt% Co,
5 wt% Fe, 2.5 wt% Mn, 1.2 wt% Na, 0.03 wt% S/g-Al2O3) has
been investigated using operando transmission X-ray mi-
croscopy (TXM) and X-ray diffraction (XRD). It was found that
a cobalt-iron alloy was formed, whereas manganese re-
mained in its oxidic phase.
Introduction
mainly conventional fuel. However, 58% of the total ELT were
re-used, either directly, retreaded or sent for material recovery.
There, the tires are ground or shredded and the resulting gran-
ulates and powders are then commonly used for civil engineer-
ing applications, for example, in road construction or for ero-
sion and sound barriers. This however has raised health con-
cerns as toxic compounds (e.g., polycyclic aromatic hydrocar-
bons) have been detected in these final applications.[16] In
other words, it would be advantageous if alternative applica-
tion routes would be found for the use of these large amounts
of ELT materials.
The Fischer–Tropsch synthesis (FTS) is a versatile production
route for the conversion of syngas, that is, a mixture of CO and
H2, into high-quality fuels and waxes, which has been commer-
cialized in various countries, including South Africa and
Qatar.[1–14] Through pyrolysis or steam reforming, virtually all
carbon sources, for example, natural gas, coal, waste, biomass
as well as recycled rubber, can be converted into syngas to
subsequently produce (renewable) fuels and chemicals.
Interestingly, currently large amounts of possible hydrocar-
bon feedstock, such as end-of-life tires (ELT), are poorly valor-
ised and could be considered as a valuable source for syngas
generation and subsequent FTS processing. In Europe, that is
the EU27 as well as Norway, Switzerland and Turkey, the
amount of ELT in 2012 was estimated to be equal to 3.6 million
tons, of which 95% were recovered.[15] Of these 3.4 million
tons of ELT, 37% were sent for energy recovery, whereas the
vast majority was used by the cement industry to substitute
Interestingly, competing processes to valorize ELT, such as
carbon substitution as used in the steel industry, de-vulcaniza-
tion or pyrolysis, have yet to overcome a lot of challenges and
do not see widespread application to date. In the case of py-
rolysis, for example, the yielded products (i.e., solid char, pyro-
lytic liquids and gases) are often ill-defined and in turn their
quality often does not justify the process costs. Hence, this
process is used less and less because of oil prices-with a year-
on-year reduction of 25% in 2012/2013.[15] Consequently, to cir-
cumvent these problems, it makes sense to look for alternative
options to convert ELT into a high quality chemical feedstock.
Here, syngas is a promising option because it opens up new
synthesis routes towards fuels and important platform chemi-
cals, such as methanol, ammonia or light olefins. The latter are
especially interesting because the boom in the supply of non-
conventional oil sources in recent years has led to lighter hy-
drocarbon feedstocks and has reduced the global availability
of important base chemicals, such as light olefins, including
1,3-butadiene. This creates opportunities for on-purpose pro-
[a] Dr. J. P. Falkenhagen, P. P. Paalanen, Prof. Dr. B. M. Weckhuysen
Inorganic Chemistry and Catalysis
Debye Institute for Nanomaterials Science, Utrecht University
Universiteitsweg 99, 3584 CG Utrecht (The Netherlands)
E-mail: b.m.weckhuysen@uu.nl
[b] Dr. L. Maisonneuve, Dr. N. Coste, Dr. N. Malicki
Manufacture FranÅaise des Pneumatiques Michelin
Centre de Technologie de Ladoux
23 place des Carmes-Dꢀchaux, 63040 Clermont-Ferrand (France)
Supporting information and the ORCID number(s) for the author(s) of this
article can be found under https://doi.org/10.1002/chem.201704191.
Chem. Eur. J. 2018, 24, 1 – 11
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Figure 1. Block diagram for the conversion of synthesis gas derived from ELT into 1,3-butadiene for the production of synthetic rubbers for new tires, as ex-
plored in this research work. This includes the processing of potential recycle/valorisation streams, which should be minimized by increasing the selectivity to-
wards light terminal olefins, including 1-butene, which can be further dehydrogenated to 1,3-butadiene. This approach requires the development of a sulfur-
resistant Fischer–Tropsch-to-olefins (FTO) catalyst, which has a high selectivity towards C4 olefins.
cesses because, for example, by 2025, 10% of the global buta-
diene production is expected to stem from the on-purpose de-
hydrogenation of 1-butene.[16]
This work has recently regained interest[24,27] and been extend-
ed to supported FTS catalysts.[28–31]
To date, most of the published studies focus on monometal-
lic catalysts, which are catalysts featuring a single dominant
FTS-active metal, with additional promotors to enhance activi-
ties and selectivities.[7] However, despite promising results only
a limited number of studies on mixed bimetallic systems has
been published.[32] Among these, bimetallic Fe-Co catalysts are
of special interest because it has been shown that catalysts
based on the combination of these elements may exhibit
higher activities[33–37] or more favorable selectivities than their
monometallic counterparts.[36,38]
The application of the FTS reaction to selectively produce
light olefins from syngas has therefore gained a lot of interest
recently and been named the Fischer–Tropsch-to-olefins (FTO)
process.[17] With the help of FTO to produce 1-butene, followed
by an additional catalytic dehydrogenation step, ELT-derived
syngas could potentially thus be converted into 1,3-butadiene.
This 1,3-butadiene can in principle then be re-used to produce
new high-quality synthetic rubber for tires in a sustainable
low-waste process, providing a closed carbon production
circle. This approach, which we have explored in this research
work, is illustrated in Figure 1. Fischer–Tropsch synthesis (FTS)
is known to be catalyzed by Co- or Fe-based catalyst materials
and extensive research has been carried out on these systems.
Depending on the used catalyst and the related process condi-
tions different reaction products are obtained.
In this work, we have synthesized a series of Co-Fe/g-Al2O3
FTS catalysts with Na, Mn and S as additives for the production
of lower terminal olefins, including 1-butene. We chose this
combination of active elements and promotor elements to:
(a) tune the product selectivity towards C4 olefins at high tem-
perature; (b) enhance the water–gas shift (WGS) reactivity,
making it possible to use a H2-poor syngas feed;[39] and (c) test
the resistance of the catalysts towards poisoning through S-
species, enabling the use of end-of-life tires derived syngas.
Hence, in this article, it will be shown that the fraction of C4
olefins can be increased by adding Mn and Na as promoter el-
ements. Moreover, we demonstrate that a high Na loading pre-
vents the isomerization of primary olefins, which coincides
with S enrichment in the catalyst material. Consequently, by
adding Mn and Na as promoter elements, the expected yield
towards C4 olefins could be improved, and then the volume of
by-products reduced. To get more fundamental insight into
the nature of the active phase during FTS operation, operando
transmission X-ray microscopy (TXM) and X-ray diffraction
(XRD) has been used under realistic reaction conditions, that is,
at 10 bar and 3008C. It was found that a Co-Fe alloy was
formed in the active catalyst, whereas Mn stayed in its oxidic
state.
In a high temperature (HT) regime (i.e., the HT-FTS process,
T=320–3508C) Co-based catalysts would predominantly pro-
duce methane, whereas Fe-based catalysts mainly yield light
olefins and paraffins. In a low temperature (LT) regime (i.e., the
LT-FTS process, T=200–2508C), both for Co- and Fe-catalysts
high molecular mass products are obtained, extending into
the range of waxes. Moreover, Co is intrinsically more active
than Fe[5,8] and consequently, finding a catalyst material that
combines selectivities obtained in Fe-based processes with ac-
tivities obtained in Co-based processes would represent a
breakthrough.
However, making use of ELT-derived syngas poses serious
challenges to a FTS process. Due to the vulcanization process,
ELTs contain large amounts of S (typically 1.2–1.8 wt.%),[18]
which has long been known as a poison for both Fe and Co
FTS catalysts even when present in concentrations as low 1–
2 mgmꢀ3 [19–24]
Hence, the required clean-up of ELT-derived
.
syngas to yield an appropriately low S content would become
expensive. In contrast, early reports[25,26] claim that the addition
of S (in combination with alkali elements) to catalysts based
on Group 8 metals lengthened lifetime, increased olefin con-
tent and reduced the production of high boiling hydrocarbons,
enhancing both activity and selectivity of the treated catalysts.
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even adding small amounts of Na and S (i.e., 0.2 wt.%/
0.06 wt.%, ca. 1 S atom per every 100 Co or Fe atoms in the
catalyst) leads to significant changes in activity as well as the
C2, C3 and C4 olefin selectivity (Table 1). Furthermore, increasing
the S content up to 0.12% (ca. 1 per 50 Co and Fe atoms)
leads to a promotion effect rather than to catalyst poisoning.
The activity (as expressed by the MTY) increases to 3.75ꢁ
10ꢀ5 molCO·gmetalꢀ1·sꢀ1 for the best catalyst in this series (i.e.,
5Co5Fe 2.5Mn0.2Na0.06S/g-Al2O3), whereas a C4 olefin selectivi-
Results and Discussion
Effect of the Co:Fe ratio and Mn promotion on catalyst se-
lectivity and activity at atmospheric pressure
Table 1 shows the CO conversion, metal–time yields (MTY), and
selectivities (CO2-free) for a series of Co-Fe/g-Al2O3 Fischer–
Tropsch synthesis (FTS) catalysts under investigation in this
work. These catalysts have been prepared with different
Co:Fe:Mn ratios as well as in the presence and absence of Na
and S as additives. Table S1 (Supporting Information) summa-
rizes the elemental composition of the catalysts as well as their
reduction behavior, as measured with temperature pro-
grammed reduction with hydrogen (H2 TPR). The experiments
were run under differential conditions at low conversion to ex-
clude the interference of secondary reactions. The catalytic
performances were collected after 8 h time on stream at at-
mospheric pressure, 2708C and a H2:CO ratio of 1:1. Here, in
comparison to 2.5Co7.5Fe2.5Mn/g-Al2O3 and 7.5Co2.5Fe2.5Mn/
g-Al2O3, 5Co5Fe2.5Mn/g-Al2O3 shows a slightly higher metal–
time yield (MTY) and C4 olefin selectivity as well as lower meth-
ane selectivity. Interestingly, for these Mn-modified catalysts, a
much less pronounced dependence of the selectivities on the
Co:Fe ratio is observed than previously reported for Mn-free
Co-Fe FTS catalysts.[38]
ty
of
14.8%
is
observed,
compared
to
4.6ꢁ
10ꢀ5 molCO·gmetalꢀ1·sꢀ1 and a C4 olefin selectivity of 13.9% for
the sulfur-free catalysts (i.e., 5Co 5Fe2.5Mn/g-Al2O3), respective-
ly.
Similar behavior has been reported in the literature for Fe-
based catalysts, whereas for Co-based systems, poisoning was
observed.[24,27] However, because of the very different methods
used (i.e., the use of spiked gas streams, impregnation of sup-
ported catalysts with S-containing solutions and co-precipita-
tion or impregnation for bulk catalysts) the results remain diffi-
cult to compare. Nevertheless, Fe-based catalysts are relatively
more resistant towards S poisoning, which can be explained
through the more favorable formation of a cobalt sulfide
phase in the case of Co-based catalysts.[24,40] For example, Co/
Al2O3 poisoning has been reported for doping the material
with as little as 10 ppm of sulfur (i.e., 1 S atom per 8300 Co
atoms), when the catalyst was impregnated with an ammoni-
um sulfide solution[41] or even lower levels when the feed gas
was spiked with S-containing compounds.[42–45] Further increas-
ing the Na loading in this series of catalysts to weight loadings
beyond 0.2 wt.% leads to a shift in the chain length propaga-
tion factor, to an increase of the C5+ and to a decrease of the
CH4 selectivity together with a pronounced loss in activity.
Effect of sodium and sulfur doping on catalyst selectivity
and activity at atmospheric pressure
The effect of Na and S promotion on catalyst selectivity and
activity was studied as well. Prior to the melt infiltration using
the respective transition-metal nitrates, the g-Al2O3 support
was impregnated with a solution containing the appropriate
amounts of ammonium sulfate and sodium nitrate. By follow-
ing this procedure, it was ensured that the added S was ad-
sorbed and distributed equally across the catalyst. Under the
safe assumption that all S will be adsorbed,[24] the selected S
loadings correspond to catalyst poisoning occurring in a typi-
cal Fe-based FTS plant under operating conditions employing
syngas with a S content in the feed of typically <5 ppb over
several years.[27] Comparing the results for the Na/S-doped FTS
catalysts tested at atmospheric pressure, it is obvious that
Effect of Mn promotion on catalyst selectivity, activity and
water–gas shift activity at 10 bar
To reach complete syngas conversion and to avoid gradients in
the ratio of the partial pressures of the reactant gases over the
reactor, the H2/CO usage ratio of the catalytic reaction, calcu-
lated according to Equation (1),must match the H2/CO feed
ratio at the inlet of the reactor.
Table 1. Activity data for a series of Co-Fe-Mn/g-Al2O3 FTS catalysts after 8 h time on stream at atmospheric pressure, 2708C and a H2:CO ratio of 1:1.
Catalyst
Conversion
[%]
MTY
Selectivity
[%]
[10ꢀ5 mol CO·gmꢀ1·sꢀ1
]
CH4
C2-C3O
C2-C3P
C4O
C4P
C5+
2.5Co7.5Fe2.5Mn
5Co5Fe2.5Mn
7.5Co2.5Fe2.5Mn
5Co5Fe2.5Mn
0.2Na0.06S
0.2Na0.12S
0.4Na0.09S
0.6Na0.03S
0.6Na0.09S
0.8
2.1
2.1
0.9
2.3
2.3
24.7
23.7
24.4
34.7
32.6
31.6
0.9
0.7
0.9
12.8
13.9
13.5
0.7
0.5
0.6
26.2
28.6
29.0
3.4
3.0
2.1
0.6
1.5
0.8
3.7
3.3
2.3
0.7
1.7
0.9
23.1
23.4
19.8
14.0
19.1
18.6
34.4
33.8
34.8
28.8
33.4
29.5
2.0
3.4
1.5
0.7
1.0
0.7
14.8
15.4
14.9
15.6
15.0
14.6
0.7
1.0
0.6
0.6
0.5
0.6
24.9
23.0
28.2
40.2
30.6
35.8
1.2Na0.03S
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molH2inꢀmolH2out
molCOinꢀmolCOout
ð1Þ
Table 2. Activity data for promoted, unpromoted and partially poisoned Co-
Fe-Mn/g-Al2O3 catalysts after 24 h on stream at 10 bar, 3008C, a H2/CO ratio of
Usage ratio ¼
1:1 and a GHSV of 4000 mLn·gcatꢀ1·hꢀ1
.
For catalysts having no water–gas shift activity, according to
Equation (2), the observed usage ratio is around 2.1, as deter-
mined by the stoichiometry of the FTS reaction itself.[4] Making
use of a catalyst, that is active for the WGS reaction, allows the
use of lower H2/CO ratios in the syngas feed, for example, H2-
poor syngas as usual syngas derived from biomass or as one
may expect from rubber gasification.[39]
Catalyst
Conversion HC yield
[%]
Usage MTY
ratio
CO2
[%]
[gHC·kgcatꢀ1·hꢀ1] [molH2/ [10–
molCO
]
4 mol·gmetalꢀ1·sꢀ1
]
2.5Co7.5Fe2.5Mn 39.4
5Co5Fe2.5Mn 54.5
7.5Co2.5Fe2.5Mn 64.8
5Co5Fe2.5Mn
0.2Na0.06S
0.2Na0.12S
0.4Na0.09S
0.6Na0.03S
0.6Na0.09S
1.2Na0.03S
248
381
438
1.3
1.4
1.3
0.8
1.1
1.3
37.1
30.1
32.0
CO þ H2O $ CO2 þ H2
ð2Þ
41.7
32.9
35.3
44.0
29.7
44.1
285
223
235
289
172
247
1.5
1.6
1.5
1.4
1.5
1.0
0.8
0.6
0.7
0.9
0.6
0.9
31.2
31.4
33.3
33.8
41.8
43.6
Here, a supported Co-Fe catalyst could be expected to com-
bine beneficial properties of both Co and Fe catalysts exhibit-
ing both a high FTS and WGS activity. However, previous stud-
ies have shown, that under the conditions used for the LT-FTS
process (up to 2508C) WGS activities of Co:Fe catalyst materials
are negligible.[4,36,38,46] Thus, to improve the WGS activity, man-
ganese oxide has been added to the catalyst materials as a po-
tential chemical promoter[10] and the catalytic studies were
conducted at a higher temperature.
the olefinicity of the lighter hydrocarbon fractions is reduced,
hinting towards a higher influence of secondary hydrogenation
reactions at higher pressure. In agreement with a higher chain
length propagation probability, a higher amount of C5+ frac-
tions is observed. It is evident that compared to the
2.5Co7.5Fe2.5Mn/g-Al2O3 and 7.5Co2.5Fe2.5Mn/g-Al2O3 catalyst
materials, 5Co5Fe2.5Mn/g-Al2O3 exhibits reduced selectivities
towards the (Figure 1) undesired CH4, C2-C3 and C4P fractions
(Table 3) and increased C5+ selectivity. Moreover, the selectivity
towards the desired C4 olefin fraction, including 1-butene, is
similar for the three catalysts. Looking more closely at the
product distribution towards individual C4 components within
the C4 olefin fraction, the 5Co5Fe2.5Mn/g-Al2O3 catalyst is
more selective towards 1-butene as the desired terminal olefin
and less selective towards internal olefins (i.e., cis- and trans-2-
butene). Based on the hydrocarbon yields per time unit for the
tested catalyst materials (Table 4), 5Co5Fe2.5Mn/g-Al2O3 was
selected as the most promising candidate for further optimiza-
tion due to its comparably high 1-butene productivity as well
as its low productivity towards internal olefins and branched
C4 hydrocarbons and its intermediate CH4 productivity.
Table 2 summarizes the catalytic data of a series of Co-Fe-
Mn/g-Al2O3 FTS catalysts. The conversion, the hydrocarbon
yield, the usage ratio and the time yields for the different Co-
Fe-Mn/g-Al2O3 catalysts have been investigated under industri-
ally relevant conditions at 10 bar and 3008C. Under these con-
ditions, the catalyst materials under investigation reach inter-
mediate conversion levels between 39 and 65% and selectivi-
ties towards CO2 of 30–37%, proving that the Co-Fe-Mn/g-
Al2O3 catalysts are indeed active for promoting the WGS reac-
tion. Moreover, the observed usage ratios are in the range of
1.3–1.4 and therefore well below the expected stoichiometric
ratio and the ratio used in a typical industrial process.[36] The
first trend that becomes obvious from Table 2 is that for the
Mn-promoted Fe-Co catalysts at 10 bar, the conversion and the
MYTs increase with increasing cobalt content. This trend is
more pronounced at 10 bar than at atmospheric pressure.
Compared to the results for the tests run at atmospheric pres-
sure, the selectivities towards lower olefins observed at 10 bar
are significantly reduced for all catalysts tested. Furthermore,
Table 3. Selectivities for promoted, unpromoted and partially poisoned Co-Fe-Mn/g-Al2O3 catalysts after 24 h on stream at 10 bar, 3008C, a H2/CO ratio of
1:1 and a GHSV of 4 Ln·gcatꢀ1·hꢀ1
.
Catalyst
Selectivity [%C]
CH4 C2O C2P
C3O C3P C4O C4P C5+
iso-Butane n-Butane trans-2-Butene 1-Butene iso-Butene cis-2-Butene
2.5Co7.5Fe2.5Mn 25.8 0.8
5Co5Fe2.5Mn 19.4 0.8
7.5Co2.5Fe2.5Mn 23.9 0.7
5Co5Fe2.5Mn
6.7
3.6
4.1
9.3 3.3
7.9 1.7
9.3 2.0
8.2 2.8 43.0 0.1
8.0 1.8 56.9 0.0
9.2 2.1 48.7 0.0
2.7
1.7
2.1
1.6
1.1
1.5
4.3
5.2
5.5
0.6
0.5
0.5
1.6
1.2
1.6
0.2Na0.06S
0.2Na0.12S
0.4Na0.09S
0.6Na0.03S
0.6Na0.09S
1.2Na0.03S
25.1 0.8
26.5 0.9
23.9 0.9
21.4 1.2
26.2 1.3
18.1 2.0
8.2 10.8 7.0
9.0 10.0 8.8
8.3 11.4 6.7 12.1 4.3 32.3 0.1
6.0 11.8 3.3 11.8 3.0 41.6 0.0
10.2 14.0 7.7 14.5 4.9 21.3 0.0
6.6 13.5 3.0 12.6 2.8 41.4 0.1
11.9 4.7 31.5 0.1
11.6 5.3 27.8 0.3
4.6
5.1
4.1
3.0
4.9
2.7
4.2
4.6
4.4
2.5
4.9
2.1
3.4
2.7
3.4
6.0
4.2
7.2
0.9
1.0
1.0
0.8
1.3
0.9
3.4
3.3
3.3
2.5
4.1
2.3
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Table 4. Hydrocarbon yields for promoted, unpromoted and partially poisoned 5Co5Fe2.5Mn/g-Al2O3 catalyst materials catalysts after 24 h on stream at
10 bar, 3008C, a H2/CO ratio of 1:1 and a GHSV of 4000 mLn·gcatꢀ1·hꢀ1
.
Hydrocarbon yield [gHC·kgcatꢀ1·hꢀ1
CH4 C2O C2P C3O C3P C4O C4P C5+ iso-
Butane Butane Butene
]
n-
trans-2-
1-
iso-
cis-2-
C2-C4O/
[HC-(C2-
C4O)]
1-butene/C2-
C4O
Butene Butene Butene
2.5Co7.5Fe2.5Mn 73.3 2.0 17.9 23.0 8.6 20.3 7.3 95.2 0.2
5Co5Fe2.5Mn 84.6 3.1 14.6 30.1 6.6 30.7 6.9 204.7 0.2
7.5Co2.5Fe2.5Mn 119.7 3.2 19.4 40.7 9.4 40.3 9.5 195.5 0.2
5Co5Fe2.5Mn
7.1
6.8
9.4
4.0
4.3
6.7
10.7
19.9
24.2
1.5
1.7
2.3
4.0
4.6
7.0
0.22
0.20
0.24
0.24
0.31
0.29
0.2Na0.06S
0.2Na0.12S
0.4Na0.09S
0.6Na0.03S
0.6Na0.09S
1.2Na0.03S
82.1 2.2 25.2 31.0 21.0 33.9 13.9 76.0 0.3
67.8 2.0 21.6 22.4 20.6 26.0 12.4 50.5 0.6
64.4 2.2 21.0 26.8 16.5 28.5 10.4 65.0 0.3
70.8 3.6 18.6 34.1 10.0 34.1 8.9 109.1 0.0
51.6 2.2 18.8 24.1 13.9 25.0 8.7 27.7 0.0
51.1 4.9 17.5 33.5 7.9 31.1 7.1 93.9 0.2
13.6
11.8
10.1
8.9
8.7
6.9
12.0
10.3
10.3
7.1
8.5
5.2
9.7
6.1
8.1
17.2
7.3
17.8
2.6
2.2
2.3
2.2
2.2
2.3
9.6
7.4
7.9
7.2
7.0
5.6
0.31
0.29
0.32
0.33
0.47
0.39
0.14
0.12
0.14
0.24
0.14
0.26
Effect of sodium and sulfur doping on catalyst selectivity
and activity at 10 bar
drocarbons increase significantly, especially compared to the
Na/S-free samples. Most importantly, for the Na/S-doped cata-
lyst materials, the selectivity towards the desired C4 olefin frac-
tion is improved from 8.0% in the Na/S-free 5Co5Fe2.5Mn/g-
Al2O3 material to 12.6% in the 5Co5Fe2.5Mn1.2Na0.03S/g-Al2O3
sample, whereas the C5+ selectivity is reduced. This corre-
sponds to a marked reduction in the hydrocarbon yields
(Table 4) for the CH4 and the C5+ fractions, whereas the C4
olefin productivity is maintained at 0.03 kg·kgcatꢀ1·hꢀ1.
In a next step of our study, a set of 5Co5Fe2.5Mn/g-Al2O3 cata-
lyst materials based on a g-Al2O3 support doped with different
absolute levels and ratios of Na and S was investigated. De-
pending on the respective catalyst, conversion levels between
30 and 44% were achieved-compared to 55% for the Na/S-
free 5Co5Fe2.5Mn/g-Al2O3 catalyst (Table 2). Here, the poison-
ing effect on the catalyst activity due to S doping is partly
compensated by a positive influence due to an increasing Na
loading. For the usage ratios, with the notable exception of
5Co5Fe2.5Mn1.2Na0.03S/g-Al2O3, values between 1.4 and 1.6
are observed. Interestingly, lower usage ratios are achieved
with catalyst materials, which are rich in Na, whereas an op-
posing trend is observed with regard to S doping.
For the distribution of individual compounds within the C4
fraction, there is a clear trend that the amount of formed ter-
minal olefins increases with an increasing Na/S ratio, whereas a
lower ratio leads to higher amounts of internal olefins
(Figure 2). Chemically, this can be rationalized as a reduction of
acidity of the alumina support due to the promotion with Na
and therefore a reduced activity of the support material for
olefin isomerization.[47,48] Therefore, adding Na to the catalyst
materials helps to preserve the catalytic performance from poi-
soning by S, inhibiting the isomerization of terminal olefins on
the catalyst support.
Table 3 summarizes the selectivities for the different studied
catalysts. It was found that the CH4 selectivity, exhibited by the
Na/S doped catalysts, while initially being higher than for the
Na/S-free catalysts drops with increasing Na content. With an
increasing Na/S promoter ratio, both the olefinicity of this frac-
tion (Figure 2) and the absolute selectivity towards C2-C3 hy-
Figure 2. Relationship between the Na/S ratio and the ratio of terminal and internal olefins (left); the olefinicity of C2-C3 and C4 product fractions, respectively
for a series of 5Co5Fe2.5Mn/g-Al2O3 catalyst materials (right).
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Active phase determination with operando transmission X-
ray microscopy (TXM) and X-ray diffraction
In the fresh catalyst material, Fe is exclusively present in the
oxidation state 3+, both as maghemite and hematite in a
ratio of around 80/20. However, due to the large similarities of
the XANES spectra of hematite and CoxFexO4 spinels, a clear
distinction between these materials is not possible from our
data.[58] Co species are found as a mixture of both CoII and CoIII
species, namely in CoFe2O4 and Co3O4 phases. This is corrobo-
rated by XRD measurements, in which differences in the XRD
patterns for the Co3O4 and CoFe2O4 phases are observed (Fig-
ures S8–S10, Supporting Information).
As TXM beamtime is very precious, we have made a selection
process to determine which catalyst should be investigated
with this method. Two different metrics were used keeping the
overall aim as expressed in Figure 1 in mind. To reduce the
volume of waste streams of little value in an industrial process,
the CH4, C2–C4 paraffin and C5+ hydrocarbon fractions should
be suppressed, whereas a maximum yield of C2–C4 olefins (C2-
C4O) is desired. That is, the yield ratio of valuable C2–C4O
streams compared to waste streams [all hydrocarbons pro-
duced with the exception of C2–C4 olefins: C2-C4O/(HC-(C2-
C4O)] should be improved. Here, the two best performing cata-
lyst materials are 5Co5Fe2.5Mn0.6Na0.09S/g-Al2O3 and 5Co5-
Fe2.5Mn1.2Na 0.03S/g-Al2O3, respectively (Table 4). At the same
time, 1-butene is especially suitable for further conversion to
1,3-butadiene by catalytic dehydrogenation (e.g., by the Mitsu-
bishi butene to crude butadiene (BtcB) process), which makes
it the most valuable component of the C4 stream in our ap-
proach offering an alternative to established processes for the
production of 1,3-butadiene.[49–53] Therefore, a maximum 1-
butene yield within the C4 olefin fraction is desired and as a
secondary metric the ratio of 1-butene/C2-C4O can be used to
identify the most promising catalyst. From Table 4, it is evident
that in this domain, 5Co5Fe2.5Mn1.2Na0.03S/g-Al2O3 is per-
forming significantly better than 5Co5Fe2.5Mn0.6Na0.09S/g-
Al2O3, making it the best choice for further characterization.
Therefore, 5Co5Fe2.5Mn 1.2Na0.03S/g-Al2O3 was selected for
the operando TXM study.
As soon as the catalyst is heated in a stream of H2 and He,
reduced phases are observed. After the reduction at a constant
temperature of 4008C, the majority of both Co (59%) and Fe
atoms (70%) are then reduced to their metallic state, probably
forming an alloy, which could be confirmed by operando XRD
measurements (Figure S11), making use of a laboratory set-up
developed recently in our laboratory.[63] In both cases, little
change is seen between the two datasets recorded at different
times during the reduction indicating that the process was
complete after the first spectrum was recorded. However, par-
tially reduced metal(II) oxide species (i.e., FeO and CoO), which
could stem from very small particles exhibiting strong metal
support interactions (SMSI) were still detected. In the case of
Fe, even a FeSx species is observed, which could stem from the
S that was added to the catalyst during impregnation or im-
purities in the syngas feed. After the reduction phase the tem-
perature of the reactor was lowered and the system was pres-
surized with syngas (H2:CO 1:1) to 10 bar. At this point the
mass spectrometric analysis of the effluent gas from the reac-
tor demonstrates that methane, ethylene, ethane and propane
were formed in the FTS reaction (Figure S12, Supporting Infor-
mation), that is, a working catalyst was truly studied in this op-
erando TXM experiment.
To compromise between measurement time and data quali-
ty, energy points were selected so that the recording time of
each scan was approximately 30 min. Before the operando
TXM experiment started the fresh catalyst was characterized at
room temperature by measuring Mn, Fe and Co K-edge TXM
image stacks. For the identification and quantification of Co,
Fe and Mn species, present bulk X-ray absorption near-edge
structure (XANES) spectra (Figures S1–S4 in the Supporting In-
formation) were generated by averaging all pixels. The data
was least squares fitted to the spectra of Co, Fe and Mn refer-
ence materials. A full list of Co, Fe and Mn species, which were
considered to be present during the experiment together with
their XANES spectra, is included in the Supporting Information
(Figures S5–S7).
The least square fits for both the Fe and the Co K-edge
spectra indicate that the present phases in the catalyst shift
upon introduction of syngas into the reactor. For Fe, the major
species is still Fe metal with a contribution of about 59–64%,
whereas wꢂstite and maghemite contribute about 25–28%
and around 12–13%, respectively. For Co, most of the contri-
bution to the fitted data stems from Co metal (ꢁ70–77%),
whereas the remaining amount consists of CoO (ꢁ23–30%).
Thus, the spectral fits both for the Fe and Co K-edge XANES
spectra demonstrate that the degrees of reduction of the Fe
and Co phase remain similar upon exposure to syngas.
For the fresh catalyst sample the quantification of the pres-
ent Mn phases yields a mixture of predominantly MnIII and
MnIV and oxide phases with a Mn/O ratio of 1.7 (Figure 3),
which is in line with characterization results earlier published
for manganese nitrate-derived Mn/g-Al2O3 catalyst materials.[54]
Under reducing conditions, this composition changes during
temperature ramping to 4008C within 30 min to give about a
65/35 mixture of manganosite and Mn3O4, therefore a mixture
of MnII and MnIII species.[55] This ratio remains almost constant
during reduction and FTS, confirming the presence of oxidic
Mn species, which are known to catalyze the WGS reac-
tion.[56,57]
From 2D TXM, further conclusions can be drawn (Figure 3).
In the fresh catalyst material, Co, Fe and Mn are seemingly ho-
mogeneously distributed across the section of the measured
catalyst particle that is in the field of view (FOV) (ca. 25ꢁ
25 mm) of the performed TXM experiment. The fact that Mn
seems to be concentrated in the center part of the catalyst
particle is due to the spherical shape of the particle, that is,
the sample thickness increases from the top-left to the
bottom-right corner of the FOV. Because of the low 2.5 wt.%
loading of Mn on the material pixels in the shell region are fil-
tered out. Thus, to extract the most relevant chemical informa-
tion, regions of interest (indicated as white squares) were used
for pixel averaging to yield the XANES spectra. The fresh cata-
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Figure 3. Top: A series of 2D TXM images of a 5Co5Fe2.5Mn1.2Na0.03S/g-Al2O3 catalyst particle as a function of the different treatments applied: fresh materi-
al at room temperature; during reduction; during FTS at 3008C and 10 bar pressure in syngas with a H2/CO of 1:1. The white squares highlight the regions of
interested, which were used for generating the XANES spectra. The times-on-stream are stated as inside the respective images. Bottom: Co, Fe and Mn bulk
XANES spectra (solid lines) together with their least-squares fits (dashed lines) during the in situ experiment. Co, Mn and Fe-K edge phase quantification was
obtained by fitting the obtained bulk XANES spectra with spectra of reference compounds.
lyst particle consists of Co3O4 and CoFe2O4, Fe2O3 (both as
hematite and maghemite), MnO2, Mn3O4 and Mn2O3 (Figure 3).
In the initial phase during the temperature ramp, the reduction
of the Co species does not occur completely homogeneously
(Figure S2, Supporting Information). However, no clear differ-
ence between catalyst particle core and shell was observed.
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With proceeding reduction, the local differences become much
less pronounced and the investigated metal species become
almost homogeneously distributed across the particle.
Experimental Section
Catalyst synthesis: Supported catalysts with a nominal metal load-
ing of 10 wt% (Fe and Co combined) were prepared by incipient
wetness impregnation and melt infiltration. First, aqueous solutions
of appropriate concentrations of NaNO3 and NH4SO4 were used to
fill the pores of aliquots (1.00 g) of the g-Al2O3 (170 m2 gꢀ1, pore
volume 0.44 mLgꢀ1, Puralox SCCa 5/170, Sasol Germany) support
material. The materials were then dried at 1008C (2 h) in flowing
air and calcined at 4008C (28Cminꢀ1, 8 h). In a second step, these
materials were infiltrated with a melt of appropriate amounts of
the transition-metal nitrate precursors at 638C. The materials were
then dried and calcined. The catalyst samples have been character-
ized by temperature programmed reduction (TPR) with H2 and the
experimental details as well as the data are summarized in Table S1
and Figure S13 (Supporting Information).
Conclusion
A series of Co-Fe-Mn/g-Al2O3 materials was synthesized and
tested as catalysts for Fischer–Tropsch synthesis (FTS). The aim
was to use S-rich end-of-life-tire (ELT)-derived syngas with a
low H2/CO ratio of 1:1 to facilitate the production of lower ole-
fins, including 1-butene.[39] It was assumed that MnOx should
act here as water–gas shift (WGS) catalyst, as was proposed in
our past work on Co-Mn/TiO2 FTS catalysts.[55,56,64,65] However,
for these Co-Mn/TiO2 FTS catalysts, it was already found that
the role is complex as MnOx was considered to be both a
structural and electronic promotor, resulting in higher metal
dispersions and lower hydrogenation activity. These findings
are in line with a recent paper of Han and co-workers who
found that the addition of MnOx to a Fe/SiO2 FTS catalyst in-
creased the reaction rate (by for example, enhancing the disso-
ciation adsorption of CO) and the olefin selectivity.[66] Further-
more, these authors observed that MnOx improved the disper-
sion of supported FeOx, facilitating its reduction and enhanch-
ing the carburization of FeOx. Interestingly, Xiaohao and co-
workers have recently investigated the addition of MnOx to a
Co/SiO2 FTS catalyst.[67] These authors found that MnOx affects
the formation of Co2C, a phase which has been recently shown
to have a high selectivity towards lower olefins.[68]
Catalyst testing: Atmospheric pressure catalytic experiments were
carried out in a fixed bed reactor and kept to conversions <5% to
ensure differential operation of the catalyst bed. A H2/CO feed
ratio of 1 and a GHSV (Gas Hourly Space Velocity) of 18 Ln·gcatꢀ1·hꢀ1
were used. 20.0 mg of the calcined catalysts (75–150 mm sieve frac-
tions) were diluted with 80.0 mg of silicon carbide (75–150 mm
sieve fractions) and pre-reduced in pure H2 at 4008C (18Cminꢀ1
,
16 h, 60 Ln·gcatꢀ1·hꢀ1). The temperature was lowered and syngas
was introduced into the reactor. Results after 8 h on stream at
2708C are reported. The produced hydrocarbons were quantified
using a Varian CP-3800 GC equipped with a FID detector. For calcu-
lating the performance hydrocarbon fractions from C1 to C10 were
analysed. Selectivities are calculated free from CO2; % C represents
the percentage of carbon atoms in a product fraction with respect
to all converted CO molecules. Metal–time yield (MTY) is defined
as mol of CO converted to hydrocarbons per gram of cobalt/iron.
10 bar catalytic experiments were conducted employing a custom-
built fixed bed reactor. Here, a quartz reactor tube with an inner di-
ameter of 6 mm was used. Typically 150 mg of catalyst were dilut-
ed with SiC to give a bed length of 30 mm. Catalysts were pre-re-
duced (18Cminꢀ1, 4008C, 12 h) at ambient pressure using a 90/10
H2/He mixture with a GHSV of 22 Ln·gcatꢀ1·hꢀ1, the reactor was then
cooled to 2108C at which point it was pressurized with syngas (H2/
CO/He 40:40:20) to reach 10 bar. The GHSV was then reduced to
4 Ln·gcatꢀ1·hꢀ1 and the temperature raised to 3008C (28Cminꢀ1). For
the analysis of products a modified Thermo Scientific Trace 1300
GC with one FID (C1–C5, C6+) and two TCD channels (for perma-
nent gases) was used. He was used as an internal standard to cal-
culate the flow through the reactor. This flow was then used to de-
termine the conversion, productivities and selectivities. The selec-
tivities towards the hydrocarbon products are given free from CO2.
Some details on these catalytic experiments, including a chromato-
graphic analysis of the product composition, are shown in Figur-
es S14 and S15 in the Supporting Information.
In our current work, it was further found that by doping a
5Co5Fe2.5Mn/g-Al2O3 material with additional 1.2 wt.% of Na
and 0.03 wt.% of S, the selectivity towards C4 olefins could be
significantly improved, whereas, compared to the Na/S-free
catalysts, C5+ and CH4 fractions are reduced at a comparable
C4 olefin productivity of 0.03 kg·kgcatꢀ1·hꢀ1. Furthermore, the
Na/S ratio has been identified as a factor influencing the iso-
merization of 1-alkenes. Thus, by adding Na as a promoter, the
isomerization of 1-alkenes can be inhibited and the ratio of 1-
alkenes to internal and iso-alkenes is maintained even if S is
present. In other words, under S-rich conditions, the selectivity
can be directed towards the target product 1-butene as the
primary product in the C4 olefin fraction and therefore the
amount of waste streams that need to be recycled, treated or
upgraded is reduced.
Catalyst characterization: TXM has been conducted on the Beam-
line 6-2c at the Stanford Synchrotron Radiation Lightsource (SSRL)
at SLAC/Stanford University (Menlo Park, CA, USA).[59] In TXM, mon-
ochromatic X-radiation from the synchrotron source is focused
onto the sample by a capillary condenser. The transmission image
is formed by a zone plate onto a CCD camera. The image has an
energy dependent field of view of about 25 mmx25 mm and a spa-
tial resolution of about 30 nm. By changing the energy during
imaging, XANES spectra are collected for each pixel. These XANES
spectra give information about the local chemical environment
and oxidation states of the element(s) of interest, that is., Fe, Mn
and Co. For this experiment, FTS catalyst particles are placed in a
thin-walled borosilicate glass capillary with a diameter of 200 mm
TXM has been used to chemically image a single 5 wt% Co,
5 wt% Fe, 2.5 wt% Mn, 1.2 wt% Na, 0.03 wt% S/g-Al2O3 cata-
lyst particle with a spatial resolution of about 30 nm in 2D and
to identify, localize and quantify the Co, Fe and Mn species
present. These TXM measurements have been carried out for
the fresh catalyst material and both during catalyst activation
and under FT synthesis conditions. The initial reduction leads
to a fast formation of a homogeneously distributed cobalt-iron
alloy as the active catalytic phase, which could be confirmed
by a separate operando XRD experiment. Furthermore, it was
found that manganese stays oxidic during FTS.
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and a wall thickness of 10 mm, which is attached to a custom
made reactor holder. The particles are held in place within the iso-
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Manuscript received: September 6, 2017
Version of record online: && &&, 0000
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FULL PAPER
Na and S can be used as additives to
improve the selectivity towards C2–C4
olefins for the Fischer–Tropsch-to-olefins
reaction over Mn-promoted Co-Fe/g-
Al2O3 catalysts. It was found that Na
prevents the isomerization of primary
olefins, such as 1-butene. Operando
transmission X-ray microscopy and X-ray
diffraction indicates the formation of a
Co-Fe-alloy, whereas Mn stayed in its
oxidic phase.
&
Syngas Conversion
J. P. Falkenhagen, L. Maisonneuve,
P. P. Paalanen, N. Coste, N. Malicki,
B. M. Weckhuysen*
&& – &&
Cobalt-Iron-Manganese Catalysts for
the Conversion of End-of-Life-Tire-
Derived Syngas into Light Terminal
Olefins
Chem. Eur. J. 2018, 24, 1 – 11
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ