Synthesis of higher alcohols from syngas over Fe/K/β-Mo2C catalyst

Article abstract of DOI:10.1007/s10562-010-0288-1

Fe/K/β-Mo₂C catalysts were prepared by the temperature-programmed-reaction (TPRe) method and tested for higher alcohols synthesis (HAS). The catalysts exhibited high catalytic activity and selectivity to higher alcohols (C₂ ⁺OH). The effect of Fe/Mo molar ratio on the catalytic performance of HAS was investigated and the best one was at Fe/Mo molar ratio of 1/14. It could be concluded that the Fe promoter exerted strong promotion for carbon chain growth, especially for the stage of C ₁OH to C₂OH. The "Fe₃Mo₃C" and "Fe₃C" phases formed over Fe promoted K/β-Mo ₂C catalysts, which might be responsible for the high activity of higher alcohols and hydrocarbons synthesis, respectively. Graphical Abstract: The typical change in CO conversion and selectivity of alcohols with time on stream over Fe/K/β-Mo₂C (Fe/Mo=1/14) catalyst is shown in Figure. After about 72 h, a pseudo steady state was obtained. During the experimental period (192 h), the catalyst showed a constant activity and selectivity.

Full text of DOI:10.1007/s10562-010-0288-1

Catal Lett (2010) 136:9–13
DOI 10.1007/s10562-010-0288-1
Synthesis of Higher Alcohols from Syngas over Fe/K/b-Mo C
2
Catalyst
Ning Wang
Dong Jiang
•
•
Kegong Fang
•
Minggui Lin
•
Debao Li Yuhan Sun
•
Received: 18 November 2009 / Accepted: 28 January 2010 / Published online: 2 March 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Fe/K/b-Mo C catalysts were prepared by the
as chemicals [1, 2]. However, the application of such an
2
temperature-programmed-reaction (TPRe) method and
tested for higher alcohols synthesis (HAS). The catalysts
exhibited high catalytic activity and selectivity to higher
attractive synthesis still suffers from the lack of high
performance catalysts. It’s the reason that develop-
ment of catalysts with high efficiency and selectivity for
higher alcohols has been one of the key objectives of
research.
?
alcohols (C2 OH). The effect of Fe/Mo molar ratio on the
catalytic performance of HAS was investigated and the
best one was at Fe/Mo molar ratio of 1/14. It could be
concluded that the Fe promoter exerted strong promotion
According to the literature, molybdenum carbide
exhibits some properties similar to those of noble metals
[3] and the application of molybdenum carbide in the
field of catalysis has attracted more and more attention.
Molybdenum carbide catalysts have been recognized as
being effective for the methanation and/or Fischer–
Tropsch synthesis of light hydrocarbons [4, 5]. While
Lecercq et al. [6] reported that the formation of alcohols
is related to the surface stoichiometry and to the extent
of carburization over the molybdenum carbide catalyst.
Promotion of molybdenum carbide with K CO has been
for carbon chain growth, especially for the stage of C OH
1
to C OH. The ‘‘Fe Mo C’’ and ‘‘Fe C’’ phases formed over
2
3
3
3
Fe promoted K/b-Mo C catalysts, which might be
2
responsible for the high activity of higher alcohols and
hydrocarbons synthesis, respectively.
Keywords Fe/K/b-Mo C catalyst Á
2
Higher alcohols synthesis Á Syngas
2
3
found to greatly enhance the selectivity to alcohols
composed of linear C –C [7]. Thus, molybdenum car-
1
Introduction
1
7
bide could be a potential catalytic material for alcohol
synthesis. Generally, the Fischer–Tropsch elements (Fe,
Co and Ni) were used as promoters because of their
strong ability to promote carbon chain growth in HAS
The higher alcohols synthesis (HAS) via syngas is con-
sidered as a potential alternative for clean fuels as well
[
8–10].
In our previous work, the K/Fe/b-Mo C catalyst was
2
YN .. WS u an n (g &Á K) . Fang (&) Á M. Lin Á D. Jiang Á D. Li Á
State Key Laboratory of Coal Conversion, Shanxi Institute
of Coal Chemistry, Chinese Academy of Sciences, 030001
Taiyuan, Shanxi, People’s Republic of China
e-mail: kgfang@sxicc.ac.cn
studied for mixed alcohols synthesis and exhibited
potential catalyst for HAS [11]. But the reaction condi-
tions and compositions are not optimized for the catalyst.
So the catalyst is expected to perform better under
appropriate conditions. Thus, Fe doped K/b-Mo C cata-
2
Y. Sun
e-mail: yhsun@sxicc.ac.cn
lysts with different Fe content were developed for HAS
in the present work. They display a better performance
for HAS and the characteristics of Fe/K/b-Mo C for
higher alcohols synthesis from syngas are also
2
N. Wang
Graduate University of the Chinese Academy of Sciences,
1
00039 Beijing, People’s Republic of China
investigated.
123

1
0
N. Wang et al.
2
Experimental
2
.1 Catalyst Preparation
The catalysts prepared through Temperature-Programmed-
Reaction (TPRe) method [12]. In detail, the b-Mo C was
2
prepared by direct carburization of the MoO . The Fe/K/b-
3
Mo C catalyst was prepared as follows: firstly, Fe(NO ) Á
2
3 3
9
H O and (NH ) Mo O Á4H O aqueous solutions were
2
4 6
7
24
2
prepared separately and then mixed with required Fe/Mo
molar ratio under constant stirring for 2 h, followed by aging,
filtrating, drying, and then calcined at 773 K for 4 h. Fol-
lowing grinding to a fine powder, the oxide precursor was
converted to FeMo carbide via TPRe. K CO modification
2
3
was accomplished by physically mixing K CO with the final
2
3
carbide, and then the mixture was calcined at 773 K for 2 h
under Ar. In all cases, the K/Mo molar ration was of 0.2 [13].
Fig. 1 XRD patterns of K/b-Mo
content. (a) Fe/Mo = 0 (b) Fe/Mo = 1/16 (c) Fe/Mo = 1/14 (d) Fe/
2
C-based catalysts with different Fe
Mo = 1/12 (e) Fe/Mo = 1/10
2
.2 Characterization Methods
BET surface areas of catalysts were determined by N2
had definitive phase of the molybdenum carbide with
adsorption at 77 K using ASAP-2000 Micromeritics
hexagonal close packed (HCP) structure [14] (2h =
instrument. X-ray powder diffraction (XRD) patterns of the
samples were obtained on a Rigaku D/Max 2500 powder
diffractometer using Cu Ka radiation as the X-ray source.
Elemental analysis was acquired on Atomscan 16 ICP-AES
instrument (TJA, USA). The X-ray photoelectron spectra
3
4.48°, 38.08°, 39.48°, 52.18°, 61.58°, 69.68°, 74.68°
and 75.68° for b-Mo C [1 0 0], [0 0 2], [1 0 1], [1 0 2],
2
[1 1 0], [1 0 3], [1 1 2] and [2 0 1], respectively). For
K/b-Mo C catalyst, the peaks at 2h value of 49.5 and
6
2
6.7 might be related to ‘‘K–Mo–C’’ entities [13]. For
(
XPS) was recorded on a XSAM800 spectrometer (Kratos,
Fe/K/b-Mo C catalysts, the peak intensity corresponding
2
UK) using an Al Ka X-Ray source, the base pressure of the
to the b-Mo C decreased with increasing Fe/Mo molar
2
-
7
chamber was less than 2 9 10 Pa. The binding energy
was corrected with the energy of C1s (284.8 eV) for carbon
contaminant.
ratio. Further investigation revealed that the weak signals
at 42.6° and 46.5° might correspond to Fe C [15], and
3
weak signals at 44.5° might correspond to Fe Mo C
3
3
[
16], which formed during the preparation (Fig. 1B).
2
.3 Catalyst Testing
The surface compositions of Fe/K/b-Mo C catalysts are
2
listed in Table 1. The Fe/Mo ratio and the K/Mo increased
The catalytic reactions were carried out using a stainless
steel fixed-bed reactor with 1.0 mL (40–60 mesh) of cat-
alyst. Both gaseous and liquid products were analyzed off-
line by gas chromatography. H , CO, CH and CO were
for the Fe/K/b-Mo C catalysts compared with the K/b-
Mo C catalyst. The surface Fe content was all higher than
2
2
its nominal preparation, it could be concluded that there is
2
4
2
b-Mo C segregation of the Fe/K phases to the surface, and
2
determined by thermal conductivity detector (TCD) with a
TDX-101 column. The water and methanol in liquids were
also detected by TCD with a GDX-401 column. The
alcohols were analyzed by flame ionization detector (FID)
with a Porapack-Q column. The carbon balance and mass
balance were 100 ± 5%.
Table 1 Surface composition of the K/b-Mo
calculated from XPS data)
2
C-based catalysts
(
Catalyst
n(Fe)/n(Mo)
Surface composition/mol%
Atom ratio
Mo
Fe
K
C
O
Fe/
Mo
K/ Fe/
Mo Mo
a
0
11.76 –
6.43 52.78 29.03 –
0.55
–
3
Results and Discussion
1
1
1
1
a
/16
/14
/12
/10
9.45 1.29 5.79 51.02 32.42 0.13 0.61 0.058
8.68 1.93 5.29 52.28 30.77 0.22 0.61 0.067
7.61 2.52 5.59 54.03 30.41 0.33 0.71 0.077
7.13 2.85 5.04 53.32 32.67 0.39 0.67 0.095
3
.1 Structural Properties of Fe/K/b-Mo C Catalysts
2
The XRD patterns of samples are shown in Fig. 1. In
Fig. 1A, it can be seen that K/b-Mo C-based catalysts all
Based on ICP results
2
1
23

Synthesis of Higher Alcohols from Syngas over Fe/K/b-Mo
2
C Catalyst
11
711.6 eV
Mo 3d
Fe 2p
724.6 eV
(
e)
d)
c)
(e)
706.9 eV
(
(
(d)
232.5 eV
(b)
a)
(c)
(b)
228.4 eV
235.5 eV
(
240
238
236
234
232
230
228
226
224 732 729 726 723 720 717 714 711 708 705 702
Binding energy/eV
Binding energy/eV
Fig. 2 XPS spetra of Mo 3d and Fe 2p with different Fe/Mo molar ration. (a) Fe/Mo = 0 (b) Fe/Mo = 1/16 (c) Fe/Mo = 1/14 (d) Fe/Mo =
/12 (e) Fe/Mo = 1/10
1
probably that behaving as a support for the dispersed iron-
containing phases. Namely, the iron promoter is enriched
on the surface. Also, the surface K content was all higher
than its theory preparation, which suggested that K pro-
moter was enriched on the surface of catalyst.
exhibited two large peaks, at 724.6 and 711.6 eV for the Fe
2p1/2 and Fe 2p3/2 signals [18]. Simultaneously, a narrow
Fe 2p3/2 peak appearing at 706.9 eV is characteristic of
iron in iron carbides [19]. The iron carbide could increase
the selectivity to oxygenates [20]. In addition, the peaks of
Fe 2p shifted to lower binding energy as the iron content
increased. It also indicated that there was a strong effect
between iron and molybdenum. In terms of catalytic per-
formances of CO hydrogenation reaction, Fe promoter, as a
electronic promoter, released the electron density and
transferred to molybdenum. Thus, the increased proportion
of molybdenum in an oxidative environment might be
favorable to the alcohols synthesis.
3
.2 XPS Characterization
The Mo 3d and Fe 2p XPS spectra are displayed in
Fig. 2. They showed the presence of Mo 3d in different
chemical state. The three main peaks with binding energy
(
B.E.) located at 228.4, 232.5, and 235.5 eV. According
to the literature, the peak at 228.4 eV was unambigu-
ously attributed to Mo atoms in the carbide form [17].
4
?
The peak at B.E. of 232.5 eV was attributed to Mo
species [16]. The peak at 235.5 eV was assigned to Mo
3.3 Effect of the Fe Promoter on the Catalytic
Performance
6
?
species, which was characteristic of oxidized phase,
resulting from the passivation step [14]. When adding Fe
The catalytic performance for HAS in steady state over the
to K/b-Mo C catalyst, the B.E. of Mo 3d shifted to lower
K/b-Mo
K/b-Mo
2
2
C-based catalysts are given in Tables 2 and 3. For
C catalyst, the CO conversion was 40.46% and the
2
value. However, the position of Mo 3d shifted to higher
B.E. value again with the Fe/Mo molar ratio increased.
Simultaneously, the intensity of the peaks became
weaker.
yield of alcohols achieved 0.10 g/mL h. In alcohols prod-
ucts (see Table 3), the methanol selectivity was high (about
40.40%). For Fe/K//b-Mo C catalyst, the optimum of
2
The XPS spectra of Fe 2p of Fe/K/b-Mo C sample is
also presented in Fig. 2. The XPS spectra of Fe 2p
Fe/Mo molar ratio was about 1/14. At the Fe/Mo molar
ratio of 1/14, the CO conversion increased to 50.25% and
2
2
Table 2 Performance of CO hydrogenation over Fe/K/b-Mo C catalysts
Catalyst n(Fe)/n(Mo)
CO conv. (%)
Yield (g/(mL h))
Alcohols
Selectivity (C%)
Hydrocarbons
Alcohols
Hydrocarbons
CO
2
0
40.46
38.93
50.25
46.72
36.91
0.10
0.11
0.14
0.12
0.08
0.11
0.17
0.22
0.21
0.15
15.24
19.20
22.69
18.03
16.37
34.34
25.60
27.60
28.15
27.91
50.24
55.19
49.70
53.81
55.71
1
1
1
1
/16
/14
/12
/10
-
1
2
Reaction conditions: T = 593 K, P = 7.0 MPa, GHSV = 4,000 h , n(H )/n(CO) = 2.0
123

1
2
N. Wang et al.
Table 3 Alcohols distribution over Fe/K/b-Mo
2
C catalysts
obtained. During the experimental period (192 h), the
catalyst showed a constant activity and selectivity. In
addition, other catalysts also exhibited similar trends.
Generally, molybdenum carbide-based catalyst has an
induction period in Fischer–Tropsch reaction [21]. For Fe/
Catalyst
n(Fe)/n(Mo)
Alcohols distribution (wt%)
?
MeOH
EtOH
PrOH
BuOH
5
C OH
0
40.40
31.98
28.71
27.94
25.14
48.03
49.75
52.72
48.40
46.33
9.56
13.46
13.63
15.36
17.63
1.90
4.17
4.17
6.73
9.04
0.11
0.64
0.62
1.57
1.86
K/b-Mo C catalyst, a part of the surface oxygen on the
1
1
1
1
/16
/14
/12
/10
2
catalyst probably reacted with carbidic carbon and was
removed from the uppermost surface layer during this
induction period [22]. It was also reported that during the
induction, some surface carbon atoms of carbide and some
deposited carbon species were eliminated [22].
-
1
,
Reaction conditions: T = 593 K, P = 7.0 MPa, GHSV = 4,000 h
n(H )/n(CO) = 2.0
2
3.4 Effect of Iron on Chain Propagation
the selectivity to alcohols also increased. As the Fe content
?
further increased, the selectivity to C2 OH, especially the
The Anderson-Schulz-Flory (A-S–F) distribution of
hydrocarbons and alcohols is given in Fig. 4. Over both
propyl alcohol and butyl alcohol, increased remarkably, but
the selectivity to hydrocarbons greatly increased. Thus, it
could be concluded that Fe promoter has dual function in
CO hydrogenation, one is to favor the higher alcohols, and
the other is to promote the hydrocarbons synthesis.
Namely, there is a synergistic interaction between the Fe
and Mo, and the ‘‘Fe Mo C’’ new phase might be
K/b-Mo
C and Fe/K/b-Mo C catalysts, the distribution of
2
2
hydrocarbons obeyed linear. While alcohols showed a
significant deviation of methanol, they did not obey the
traditional linear A-S–F distribution. Except for methanol,
it was worth noting that the C
liner distribution plot. For Fe/K/b-Mo
chain-growth probability (P in Fig. 4) of alcohols was of
0.19, which was higher than that of K/b-Mo C catalyst
–C
alcohols still followed a
2
4
C catalyst, the
3
3
2
responsible for the high activity of higher alcohol synthe-
sis. The remarkable effect of Fe promoter might be ascri-
bed to its own intrinsically activity for carbon monoxide
2
(0.12). It seemed that iron promoter exerted strong pro-
hydrogenation especially methanation, and the ‘‘Fe C’’
3
motion for carbon chain growth, especially for the stage of
new phase might responsible for the synthesis of
hydrocarbons.
C
1
OH to C
2
OH. It was noted that the chain-growth prob-
C catalyst was
ability of hydrocarbons of Fe/K/b-Mo
2
The typical change in CO conversion and selectivity of
increased from 0.29 to 0.33. Combined with the XRD
results, the chain-growth probability of hydrocarbons
alcohols with time on stream over Fe/K/b-Mo C (Fe/
2
Mo = 1/14) catalyst is shown in Fig. 3. Initially, the cat-
alyst showed high CO conversion and relatively low
selectivity to alcohols. Gradually, the CO conversion
decreased and alcohol selectivity increased with time on
stream. After about 72 h, a pseudo steady state was
might be attributed to the phase Fe C. And the high
3
?
selectivity to alcohols, especially for C
to new phase ‘‘Fe Mo C’’ in Fe/K/b-Mo
ther with the ‘‘K–Mo–C’’ entities. It could be speculated
that the ‘‘Fe Mo C’’ and ‘‘K–Mo–C’’ species might be the
active sites for HAS.
OH was assigned
C catalyst, toge-
2
2
3
3
3
3
8
7
7
6
6
5
5
4
4
0
5
0
5
0
5
0
5
0
30
25
CO Conv.
4
Conclusion
Total alcohols
C1OH
+
C2 OH
A novel Fe/K/b-Mo C catalyst was prepared and tested for
2
20
HAS. The catalyst revealed high performance for HAS via
syngas. The catalytic performance over the catalyst was
obviously influenced by the Fe/Mo molar ratio. The opti-
mized Fe/Mo molar ratio was of 1/14 for HAS. Further-
more, the iron promoter exerted strong promotion for C–C
growth. The phase, ‘‘Fe Mo C’’, might be responsible for
15
1
5
0
0
3
3
?
the high selectivity of C2 OH. And the ‘‘Fe C’’ phase
3
favors the hydrocarbons synthesis over the Fe/K/b-Mo C
catalyst.
2
2
4
48
72
96
120 144 168 192
Time on stream/h
Acknowledgments The authors gratefully acknowledge the finan-
cial support from National Natural Science Foundation of China
Fig. 3 Stability test of Fe/K/b-Mo C catalyst (Fe/Mo = 1/14)
2
1
23

Synthesis of Higher Alcohols from Syngas over Fe/K/b-Mo
2
C Catalyst
13
a
b
4
4
2
0
PHC=0.33
2
PHC=0.29
0
4
-2
4
2
0
2
4
P
ROH=0.12
2
0
P
ROH=0.19
-
-
-
2
1
2
3
4
5
1
2
3
4
5
Carbon number
Fig. 4 A-S–F plots of alcohols and hydrocarbons over the catalysts. a K/b-Mo
Carbon number
C b Fe/K/b-Mo C (Fe/Mo = 1/14)
2
2
under key Project No. 20590363 and Shell on Higher Alcohols
Synthesis Package.
10. Xiang ML, Li DB, Xiao HC, Zhang JL, Li WH, Zhong B, Sun
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1. Xiang ML, Li DB, Li WH, Zhong B, Sun YH (2007) Catal
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