Higher alcohol synthesis over nickel-modified alkali-doped molybdenum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions

Article abstract of DOI:10.1016/j.cattod.2014.12.003

Ethanol and higher alcohols are one of the most interesting alternatives to replace fossil fuels in the transportation sector. Nickel-modified alkali-doped molybdenum sulfide is a potential catalyst for the conversion of syngas to mixed alcohols. In this work, K-Ni-MoS₂ catalysts were synthetized by coprecipitation in aqueous solution or in microemulsions, followed by alkali doping. The influence of the preparation route in CO hydrogenation was investigated at 91 bar, 340/370 °C and GHSV = 2000-14,000 NmL/h g_catalyst. The catalysts were also characterized by TGA, ICP, XPS, nitrogen adsorption, XRD, SEM-EDX and TEM. The novel microemulsion catalyst outperformed the conventional one, resulting in higher yields of ethanol and higher alcohols. The higher activity and selectivity was attributed to a higher concentration of promoters on the microemulsion catalyst surface, together with a lower degree of crystallinity.

Full text of DOI:10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
Catalysis Today xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Higher alcohol synthesis over nickel-modified alkali-doped
molybdenum sulfide catalysts prepared by conventional
coprecipitation and coprecipitation in microemulsions
Rodrigo Suárez Parísa,∗, Vicente Montesb, Magali Boutonneta, Sven Järåsa
a KTH—Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemical Engineering and Technology, Stockholm, Sweden
b University of Córdoba, Organic Chemistry Department, Córdoba, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Ethanol and higher alcohols are one of the most interesting alternatives to replace fossil fuels in
the transportation sector. Nickel-modified alkali-doped molybdenum sulfide is a potential catalyst
for the conversion of syngas to mixed alcohols. In this work, K-Ni-MoS2 catalysts were synthetized
by coprecipitation in aqueous solution or in microemulsions, followed by alkali doping. The influ-
ence of the preparation route in CO hydrogenation was investigated at 91 bar, 340/370 ◦C and
GHSV = 2000–14,000 NmL/h gcatalyst. The catalysts were also characterized by TGA, ICP, XPS, nitrogen
adsorption, XRD, SEM-EDX and TEM. The novel microemulsion catalyst outperformed the conventional
one, resulting in higher yields of ethanol and higher alcohols. The higher activity and selectivity was
attributed to a higher concentration of promoters on the microemulsion catalyst surface, together with
a lower degree of crystallinity.
Received 3 October 2014
Received in revised form
24 November 2014
Accepted 1 December 2014
Available online xxx
Keywords:
Syngas
Higher alcohols
K/MoS2
Nickel
Microemulsion
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
efficient and cheaper catalyst has become essential to make this
process commercially attractive.
Environmental concerns and the shortage of fossil fuels have
recently increased the search for new energy sources. Second-
generation biofuels, including ethanol, are becoming a promising
alternative to conventional fuels, while avoiding problems related
to food and feed production [1]. Second-generation ethanol can
be produced by means of either a biochemical or a thermochemi-
cal route. The thermochemical route has a higher flexibility, since
a wide variety of feedstocks (e.g. lignocelullosic biomass, waste
streams) can be used as raw materials to produce syngas [2]. Syngas
is then catalytically converted to ethanol and higher alcohols in the
so-called higher alcohol synthesis (HAS). Ethanol can be used not
only as a neat fuel or fuel additive, but also as a hydrogen carrier
in fuel cells [3]. Higher alcohols also have a great potential as fuel
additives [4,5].
Although noble metal catalysts (mainly Rh-based) exhibit high
ethanol selectivities, they are unattractive for commercial purposes
due to the reduced availability and high cost of the metals [6].
On the other hand, non-noble catalytic systems have been widely
studied and they are usually classified into three categories [3,6]:
modified methanol catalysts, modified Fischer-Tropsch catalysts
and modified molybdenum-based catalysts.
Molybdenum sulfide catalysts are one of the most promising
options for HAS due to their sulfur resistance, high water–gas shift
activity and slow deactivation by coke deposition [6,7], thus avoid-
ing the need for ultra-desulfurization or water separation units. In
addition, they are suitable for processing syngas with low H2/CO
ratios, typically obtained when gasifying biomass [8]. Modification
with promoters is however essential to enhance the yield of the
desired products, one of the main challenges in HAS [3,6,9].
Alkali metals (Li, Na, K, Cs) have proven to shift selectivity
from hydrocarbons to alcohols [6]. According to Santiesteban [10],
the reaction scheme over alkali-doped molybdenum sulfide cata-
lysts is based on a CO-insertion mechanism. The function of the
alkali is to decrease the rate of hydrogenation of alkyl species and
increase the rate of CO insertion [11]. Further addition of 3d transi-
tion metals (Co, Ni) increases the selectivity to ethanol and higher
alcohols [3].
No commercial plants applying the aforementioned technology
have been implemented yet, since the catalytic systems developed
so far suffer from low catalytic activity, poor product distribution
or severe reaction conditions [6]. Therefore, the search for a more
∗
Corresponding author at: Teknikringen 42, SE-10044, Stockholm, Sweden.
Tel.: +46 8790 8243.
E-mail address: rosp@kth.se (R. Suárez París).
http://dx.doi.org/10.1016/j.cattod.2014.12.003
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
Table 1
2
The microemulsion technique appears to be a suitable method
to manufacture the catalyst. A microemulsion (ME) is an optically
transparent and thermodynamically stable solution which consists
of spherical aqueous nanodroplets stabilized by a layer of surfactant
molecules [12]. Metal salts can be solubilized inside the aqueous
core of the nanodroplets and then precipitated to form particles
in nano-sized range and with narrow size distribution [12]. Cata-
lysts prepared using MEs have shown enhanced properties in many
applications when compared with conventional catalysts [13].
There are several studies regarding the conversion of syngas
over unsupported Ni-modified K-doped molybdenum sulfide cat-
alysts [14–18], but none of them applies the ME technique in the
catalyst synthesis. The aim of this work is to develop and study the
performance of a novel Ni-modified K-doped molybdenum sulfide
catalyst prepared through coprecipitation in MEs and compare it
with an analogous catalyst prepared by conventional coprecipita-
tion. Two additional conventional catalysts, promoted only with
nickel or potassium, have also been synthetized, characterized and
tested, in order to get a better understanding of the individual effect
of each promoter and be able to explain the differences between the
ME catalyst and the conventional one.
Composition of the ME systems.
Phase
Compound (s)
Composition (wt%)
Oil
Iso-octane
53
15
12
20
Surfactant
Co-surfactant
Water
CTAB
1-Butanol
ME1: water,
Ni(CH3COO)2·4H2O, acetic
acid; ME2: water, (NH4)2MoS4
the precipitate was first washed with a mixture of chloroform and
methanol (mass ratio 1:1) and, then, twice with methanol. The rest
of the preparation procedure is analogous to that used for the con-
ventional catalyst: drying, crushing and sieving, alkali doping and,
finally, thermal decomposition under H2 flow. The temperature
ramp during the H2 treatment was set to 0.5 ◦C/min in this case, so
as to ensure complete elimination of the catalyst precursor residues
not removed during the washing steps.
2.2. Catalyst characterization
A thermogravimetric analysis (TGA) of the non-calcined ME cat-
alyst was performed on a Netzsch STA 449 F3 Jupiter instrument.
The sample was subjected to the same temperature program as
in the thermal decomposition (0.5 ◦C/min to 450 ◦C, held 90 min),
under H2 flow. An equivalent test was also carried out on the bipro-
moted conventional catalyst.
The contents of nickel, potassium and molybdenum in the cat-
alysts were determined using inductively coupled plasma-mass
spectroscopy (ICP-MS), following EPA methods 200.7 and 200.8
[22,23].
X-ray photoelectron spectroscopy (XPS) data were recorded on
4 mm × 4 mm pellets, 0.5-mm thick, obtained by gently pressing
the powdered materials. Prior to analysis, the pellets were out-
gassed in the instrument pre-chamber at 150 ◦C to a pressure below
2 × 10−8 Torr, in order to remove chemisorbed volatile species. XPS
spectra were recorded on a Leibold-Heraeus LHS10 spectrome-
ter, equipped with an EA-200MCD hemispherical electron analyzer
with a dual X-ray source, using Mg K␣ (1253.6 eV) at 120 W and
30 mA, with C(1s) as energy reference (284.6 eV).
2. Methods
2.1. Catalyst preparation
The bipromoted catalysts, containing both nickel and potas-
sium, were synthetized by coprecipitation and subsequent alkali
doping. The conventional catalyst was obtained through coprecip-
itation in aqueous solution, while the ME catalyst was obtained by
mixing two water-in-oil MEs. Detailed information about the cat-
alyst preparation procedures is shown below. The monopromoted
catalysts were synthetized using the same procedure as for the
conventional catalyst, but excluding the doping step with nickel
(K-MoS2) or potassium (Ni-MoS2).
2.1.1. Conventional catalyst
The conventional catalyst (K-Ni-MoS2) was prepared by adding
dropwise, under continuous stirring, an aqueous solution of
Ni(CH3COO)2·4H2O to an aqueous solution of (NH4)2MoS4. The
resulting black precipitate was aged for 24 h and then recov-
ered by centrifugation. The recovered precipitate was first washed
with deionized water (miliQ) and, subsequently, with a mixture
of ethanol and deionized water (mass ratio 1:1). After each wash,
the powder was recovered by centrifugation. The washed powder
was dried at 50 ◦C and then crushed and sieved to a pellet size of
45–250 ␮m. Alkali doping was achieved by mechanically mixing
the dried catalyst precursor with K2CO3 (45–250 ␮m). The final
catalyst was obtained after a thermal treatment at 450 ◦C (ramp:
20 ◦C/min) for 90 min, under H2 flow. A second sieving was per-
formed to discard particles with a pellet size above 250 ␮m that
could be formed during the heat treatment. The final sample was
kept in a tightly closed container before being characterized and
tested.
Nitrogen adsorption measurements were performed in
a
Micromeritics ASAP2000 unit. The samples were outgassed by
evacuation at 250 ◦C for a minimum of 4 h before being analyzed.
2.1.2. Microemulsion catalyst
Two analogous water-in-oil ME systems were prepared: the first
ME (ME1) contained the nickel salt and acetic acid; the second ME
(ME2) contained the molybdenum salt. The detailed composition
of the systems is shown in Table 1. MEs with similar composition
were previously used for catalyst preparation [19–21].
The ME catalyst (ME K-Ni-MoS2) was prepared by adding drop-
wise, under continuous stirring, ME1 to ME2. The coprecipitation
step is illustrated in Fig. 1. The black precipitate was aged for 24 h
and then recovered by centrifugation. In order to remove surfactant
and hydrocarbon residues, the washing procedure was modified:
Fig. 1. Coprecipitation in MEs.
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
3
Table 2
Catalyst composition (fresh catalysts).
Catalyst
Nominal ratio (mol/mol)
ICP analysis (mol/mol)
EDX analysis (mol/mol)
XPS analysis (mol/mol)
Ni/Mo
K/Mo
Ni/Mo
K/Mo
Ni/Mo
K/Mo
Ni/Mo
K/Mo
K-Ni-MoS2
ME K-Ni MoS2
Ni-MoS2
0.50
0.50
0.50
0
1.50
1.50
0
0.45
0.40
0.50
0
1.19
1.51
0
0.61
0.55
0.70
0
1.42
1.21
0
0.05
0.18
0.14
0
1.50
3.49
0
K-MoS2
1.50
1.50
1.15
5.16
Data were collected at liquid nitrogen boiling temperature (77 K).
Surface area was calculated using the Brunauer–Emmett–Teller
(BET) method with data collected at relative pressures between
0.06 and 0.2.
The patterns of X-ray diffraction (XRD) were recorded on a
Siemens D5000 using Cu K␣ radiation as the X-ray source (40 kV,
30 mA). The measurements were performed at 2ꢀ values from 5◦ to
60◦, and at a rate of 0.12◦ min−1. The most probable phases in the
samples were identified using the EVA software (version 13.0.0.2,
2007).
Scanning electron microscopy (SEM) images were obtained
using a JEOL JSM-6300 unit equipped with an energy-dispersive
X-ray (EDX) detector. It was operated at an acceleration voltage of
20 keV with a resolution of 65 eV.
Transmission electron microscopy (TEM) images were obtained
using a JEOL JEM-1400 microscope. All samples were mounted on
3 mm holey carbon copper grids.
Fig. 2. TGA curves for the decomposition of the bipromoted catalysts.
Fig. 3. N2-adsorption isotherms.
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
4
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
together with an external aluminum jacket, allowed for a temper-
ature profile along the bed within 0.5 ◦C of the set point.
A premixed syngas with H2/CO ratio = 1 and 4% N2 as internal
standard was used in the experiments. The reaction conditions
were: P = 91 bar, T = 340/370 ◦C. Gas hourly space velocity (GHSV)
was varied between 2000 and 14,000 NmL/h gcatalyst in order to
obtain data for different conversion values. Activity and selectivity
were measured on stable catalysts, after at least 100 h on stream
(stabilization conditions: 91 bar, 340 ◦C, 6000 NmL/h gcatalyst). At
the operating conditions used for this study, no appreciable deacti-
vation was observed after the stabilization period, as will be shown
in Section 3.
Product analysis was carried out with an on-line GC, equipped
with a TCD and two FID detectors. The presented data represents
the average of at least three gas composition measurements at the
same reaction conditions and with carbon mass balance closures
higher than 98%. More details about the reaction and analysis setup
can be found in [24].
Fig. 4. XRD diffraction patterns.
3. Results and discussion
2.3. Catalytic testing
3.1. Characterization of fresh catalysts
CO hydrogenation reactions were performed in a high-pressure
fixed-bed tubular reactor operating in down-flow mode. Approxi-
mately, 0.7 g catalyst, diluted with 3.3 g SiC (pellet size: 53–80 ␮m),
were used in each test. The reactor was heated by an electric furnace
and the temperature was regulated by a cascade control, with one
sliding thermocouple in the catalyst bed and a second thermocou-
ple in the oven. The dilution with SiC and this control architecture,
3.1.1. Thermal decomposition
The TGA curves of the bipromoted catalysts are shown in Fig. 2.
Similar patterns are obtained for both samples, with the largest
weight loss occurring at temperatures between 200 and 275 ◦C,
that is attributed to the decomposition of the precursors under
H2 atmosphere. The greater decrease in the ME catalyst is due to
the removal of surfactant and oil residues. As reported before for
Fig. 5. Representative SEM micrographs of the fresh bipromoted catalysts.
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
5
Fig. 6. Representative TEM micrographs of the fresh bipromoted catalysts.
analogous ME systems, there is no significant weight loss beyond
300 ◦C, which indicates a complete elimination of the residues
[25,26].
greater differences among the catalysts. The higher concentration
of both promoters on the surface of the ME catalyst is especially
remarkable, as compared to the conventional one.
3.1.2. Bulk and surface composition
3.1.3. Textural characteristics
The measured Ni/Mo and K/Mo molar ratios in the fresh (H2-
treated) catalysts are given in Table 2, along with the nominal ratios.
ICP and EDX analyses, which evaluate the bulk catalyst compo-
sition, show a good agreement between the nominal and the actual
Ni/Mo and K/Mo ratios, with small deviations. On the other hand,
XPS analyses, which give insight into the surface composition, show
N2-adsorption isotherms for the four catalysts are shown in
Fig. 3. Type II isotherms with H3-type hysteresis loops (IUPAC clas-
sification) are observed on all the catalysts containing potassium.
This behavior is characteristic of solids containing mainly macrop-
ores and consisting of aggregates or agglomerates of particles with
slit-shaped pores with different sizes and shapes [27]. In contrast,
Fig. 7. CO conversion as a function of GHSV, at T = 340 ◦C (left) and T = 370 ◦C (right).
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
6
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
Fig. 8. Product distribution (GHSV = 6000 NmL/h gcatalyst) at T = 340 ◦C (left) and T = 370 ◦C (right).
Fig. 9. Methanol STY as a function of GHSV, at T = 340 ◦C (left) and T = 370 ◦C (right).
Ni-MoS2 exhibits a type IV isotherm, evidencing the mesoporous
nature of the material [27]. It seems that potassium doping changes
greatly the pore structure.
33.5◦, 39.5◦ and 58.2◦ [30–32]. The incorporation of nickel into
the catalyst generates peaks mainly at 21.7◦, 31◦, 46◦ and 53.7◦,
that are attributable to the formation of NixSy phases [31]. Potas-
sium doping results in two big peaks around 9.5◦ and 19◦, which
have previously been assigned to KxMoS2(H2O)y, formed due to the
contact of the samples with humid air during manipulation and
analysis [14]. Smaller peaks at about 30◦ and 31◦ could correspond
to K2SO4 [14,33] or K-Mo-S phases [18,30,34]. The superimposi-
tion and broadness of the diffraction peaks do not allow for a more
comprehensive analysis.
Table 3 shows the BET surface area of the catalysts, derived
from the N2-adsorption measurements. Potassium doping leads
to a decrease in surface area, as previously reported [14,28]. The
novel ME catalyst has lower surface area than the conventional one,
possibly due to the higher potassium content, detected by ICP. Even
though higher surface area values have been reported for K-Ni-
MoS2 catalysts [15,18], the conditions used for the decomposition
of ammonium tetrathiomolybdate can influence the final structure
of the material and thus its surface area [29].
3.1.5. Scanning and transmission electron microscopy
SEM micrographs do not show large morphological differences
among the bipromoted catalysts. They are composed of agglomer-
ates, showing irregular shapes with different sizes, as can be seen
in Fig. 5, left. Nevertheless, higher resolution SEM images suggest
3.1.4. Bulk phase identification
The XRD diffraction patterns of the catalysts are presented in
Fig. 4. Even though all the samples are amorphous to a large extent,
the degree of crystallinity is especially low in the catalyst prepared
with MEs, for which almost no peaks were detected.
Table 4
In the conventional K-Ni-MoS2 catalyst, poorly-crystallized
Summary of operating conditions during the catalytic test.
hexagonal MoS2 is identified with peaks at 2ꢀ values of 14.4◦,
Period (s)
Temperature (◦C)
GHSV (NmL/h gcatalyst)
Stabilization
1, 5, 11
2
3
4
6, 10
7
8
9
340
340
340
340
340
370
370
370
370
6000
6000
2000
10,000
14,000
6000
2000
10,000
14,000
Table 3
BET surface area of the catalysts as determined by N2-adsorption.
Catalyst
BET surface area (m2/g)
K-Ni-MoS2
ME K-Ni-MoS2
Ni-MoS2
3
1
43
1
K-MoS2
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
7
Fig. 10. Ethanol STY as a function of GHSV, at T = 340 ◦C (left) and T = 370 ◦C (right).
a lower surface area in the ME catalyst, in accordance with the
N2-adsorption measurements (Fig. 5, right).
TEM micrographs (Fig. 6) show the typical layered structure of
MoS2, with randomly oriented crystallites. The conventional cata-
lyst appears to have a higher crystallinity, as already detected by
XRD. Even though the resolution of the instrument did not allow for
a thorough study of the nano-scale morphology, the conventional
catalyst exhibits a higher stacking and length of the slabs. In con-
trast, the ME catalyst is composed of smaller crystallite domains.
Ferrari et al. [33] have recently suggested that small crystallites
may have a positive effect in the selectivity in HAS.
3.2. Catalytic testing: activity, selectivity and stability
CO conversion over the different catalysts at 340 and 370 ◦C is
presented in Fig. 7. The trends are the same for both temperatures.
The biggest differences appear at low space velocity values, being
Ni-MoS2 the most active catalyst in CO hydrogenation. However, as
will be shown below, it produces mainly hydrocarbons and, there-
fore, it is not useful for HAS. Comparing the bipromoted samples,
the novel ME catalyst shows significantly higher activity.
Fig. 12. CO conversion over the ME catalyst as a function of time on stream.
The product distribution (alcohols, other oxygenates, hydrocar-
bons) at GHSV = 6000 NmL/h gcatalyst is shown in Fig. 8, indicating
that potassium promotion is essential to shift the selectivity from
hydrocarbons to alcohols. Promotion only with nickel is not suf-
ficient. Nickel is a well-known methanation catalyst and Ni-MoS2
yields hydrocarbons, with methane, ethane and propane compris-
ing more than 80% of the products.
Figs. 9 and 10 represent methanol and ethanol space-time yields
(STY) over the potassium-doped catalysts, evidencing the impor-
tant effect of nickel promotion on shifting the selectivity from
methanol to higher alcohols. If the bipromoted catalysts are com-
pared, ethanol production is clearly enhanced over the ME catalyst.
Fig. 11. 1-Propanol and iso-butanol STY as a function of GHSV, at T = 340 ◦C (left) and T = 370 ◦C (right).
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
8
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
Fig. 13. Representative SEM micrographs of the spent bipromoted catalysts.
Table 5
Catalytic performance of ME K-Ni-MoS2 at different times on stream (T = 340 ◦C, GHSV = 6000 NmL/h gcatalyst).
Time on stream (h)
CO conversion (%)
Selectivity (%C, CO2-free)
Methanol
Ethanol
Higher alcohols
Other oxygenates
Methane
Higher hydrocarbons
113
141
189
17.5
18.6
19.2
34.0
35.0
35.5
25.1
22.9
22.3
15.2
15.5
14.3
6.9
6.0
5.9
14.3
15.7
16.6
4.5
4.9
5.4
Concerning the alcohol distribution over the bipromoted cat-
alysts, the most abundant high alcohols are, apart from ethanol,
1-propanol and iso-butanol. Minor amounts of other alcohols are
also obtained. The ME catalyst results again in higher productivities
of the aforementioned compounds, as represented in Fig. 11.
It has been reported that the selectivity on K-Ni-MoS2 cata-
lysts is very dependent on CO conversion levels [14]. Therefore,
the higher selectivities to ethanol and higher alcohols over the ME
catalyst could simply be attributed to the higher conversion val-
ues obtained for each space velocity, which would lead to a higher
long-to-short alcohol chain ratio. However, the analysis of results at
similar conversions still shows an improved performance of the ME
catalyst. For example, for a CO conversion of 7.6%, the conventional
catalyst results in a selectivity to ethanol and higher alcohols of
33.0% (%C, CO2-free); however, for a slightly lower conversion, 7.3%,
the selectivity increases up to 35.2% (%C, CO2-free) with the novel
ME catalyst. This suggests that the new preparation method does
not only lead to an increase in catalyst activity but also in selectivity
to the desired products. Nevertheless, the differences in selectivity
are not too big and no definitive conclusions can be drawn tak-
ing into account the small uncertainties that may be present in the
measurements.
The stability of the ME catalyst has also been examined
after an initial stabilization period of approximately 100 h on
stream. Temperature and GHSV were modified over the follow-
ing 100 h, as indicated in Table 4. No important deactivation
was observed after the stabilization period, with only minor
changes in activity, as shown in Fig. 12. Note that the transitions
in the graph correspond to a change of the operating condi-
tions. Examining the results at the reference conditions (T = 340 ◦C,
GHSV = 6000 NmL/h gcatalyst, i.e. periods 1, 5 and 11), there is a grad-
ual increase in CO conversion, but this tendency is fairly slow (see
Table 5).
In what concerns selectivity, the trends are also very slow. The
selectivity values at the reference conditions have been summa-
rized in Table 5. There is an increase in methanol and hydrocarbon
selectivity, while the selectivity to the desired products (ethanol
and higher alcohols) steadily decreases. Similar patterns were also
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
9
Table 6
The characterization results showed that the surface area of the
microemulsion catalyst was lower. XRD patterns and TEM analysis
evidenced a higher crystallinity in the conventional sample, while
the microemulsion catalyst had an amorphous structure or con-
sisted of very small crystallites. Despite a similar composition of
the bulk catalysts, XPS results indicated a higher concentration of
nickel and potassium on the microemulsion catalyst surface. The
testing of catalysts containing only one of the promoters proved
not only the importance of these elements to shift the selectivity
to the desired products, but also their synergistic effect on unsup-
ported molybdenum sulfide catalysts. Taking this into account, it
is highly likely that the enrichment of nickel and potassium on the
microemulsion catalyst surface is one of the main reasons for its
better performance.
Catalyst composition (spent catalysts)a.
Catalyst
EDX analysis (mol/mol)
Ni/Mo K/Mo
XPS analysis (mol/mol)
Ni/Mo K/Mo
Spent K-Ni-MoS2
Spent ME K-Ni-MoS2
0.02 (0.61) 1.23 (1.42)
0.18 (0.55) 1.42 (1.21)
0.06 (0.05) 2.90 (1.50)
0.26 (0.18) 5.27 (3.49)
a
The corresponding ratios in the fresh catalysts are given in brackets.
observed in the conventional sample (not shown) and have been
reported before for K-Ni-MoS2 catalysts [35].
3.3. Characterization of spent catalysts
In order to try to get a better understanding of the reasons
of the improved performance of the novel ME catalyst, the spent
bipromoted samples were characterized by means of EDX and XPS
(Table 6). The spent samples were subjected to the same operating
conditions (and similar reaction times) before being character-
ized: an initial stabilization period, followed by the analysis of two
reaction temperatures and four different space velocities for each
temperature, as summarized in Table 4.
Acknowledgements
KIC InnoEnergy and Junta de Andalucía (P09-FQM-4781 project)
are acknowledged for the financial support. The authors are grate-
ful to Robert Andersson and Luis Lopez (KTH) for their help in the
beginning of this work. Thank you to Lina Norberg Samuelsson
(KTH) for her help with the TGA analyses.
Considering the EDX results in the fresh and spent catalysts,
there is an important removal of nickel during the reaction. It is very
likely due to the formation of Ni(CO)4, a compound that is thermo-
dynamically stable under the temperature and pressure conditions
within this study [36]. As a result of the formation of nickel car-
bonyls, nickel could be transported down the catalyst bed and,
eventually, removed.
Also noteworthy is the higher concentration of nickel on the sur-
face of the spent ME catalyst, as compared to the conventional one,
a phenomenon that was also observed in the fresh catalysts. The
analysis of SEM images (Fig. 13) shows a greater change in morphol-
ogy, after syngas exposure, in the conventional K-Ni-MoS2, with
the formation of small flakes on the surface. The same morphol-
ogy is also identified in the catalyst without nickel, K-MoS2, which
could be an additional indication of a higher nickel deficiency in
the conventional catalyst. No appreciable morphological changes
are detected in the ME catalyst.
Regarding potassium, there is a migration from the bulk to the
surface of the catalysts after syngas exposure. This migration has
been reported before on K-MoS2 [33,37,38] and K-Co-MoS2 cata-
lysts [39,40]. DFT calculations also support the idea that potassium
atoms are more energetically favored on the catalyst surface [41].
As for the nickel, the level of potassium on the surface of the spent
ME catalyst is higher than on the conventional sample.
References
[1] R.E.H. Sims, W. Mabee, J.N. Saddler, M. Taylor, Bioresour. Technol. 101 (2010)
1570–1580.
[2] G. Evans, C. Smith, in: A. Sayigh (Ed.), Comprehensive Renewable Energy, Else-
vier, Oxford, 2012, pp. 155–204.
[3] J.J. Spivey, A. Egbebi, Chem. Soc. Rev. 36 (2007) 1514–1528.
[4] J. Campos-Fernández, J.M. Arnal, J. Gómez, M.P. Dorado, Appl. Energy 95 (2012)
267–275.
[5] S. Kumar, J.H. Cho, J. Park, I. Moon, Renew. Sustainable Energy Rev. 22 (2013)
46–72.
[6] V. Subramani, S.K. Gangwal, Energy Fuels 22 (2008) 814–839.
[7] V.R. Surisetty, A.K. Dalai, J. Kozinski, Appl. Catal. A: Gen. 404 (2011) 1–11.
[8] M.J.A. Tijmensen, A.P.C. Faaij, C.N. Hamelinck, M.R.M. van Hardeveld, Biomass
Bioenergy 23 (2002) 129–152.
[9] S. Zaman, K.J. Smith, Catal. Rev. Sci. Eng. 54 (2012) 41–132.
[10] J.G. Santiesteban, Alcohol Synthesis From Carbon Monoxide and Hydrogen Over
MoS2-Based Catalysts, Lehigh University, Bethlehem, PA, 1989.
[11] X. Xiaoding, E.B.M. Doesburg, J.J.F. Scholten, Catal. Today 2 (1987) 125–170.
[12] S. Eriksson, U. Nylén, S. Rojas, M. Boutonnet, Appl. Catal. A: Gen. 265 (2004)
207–219.
[13] M. Boutonnet, S. Lögdberg, E. Elm Svensson, Curr. Opin. Colloid Interface Sci.
13 (2008) 270–286.
[14] R. Andersson, M. Boutonnet, S. Järås, Appl. Catal. A: Gen. 417–418 (2012)
119–128.
[15] D. Li, C. Yang, N. Zhao, H. Qi, W. Li, Y. Sun, B. Zhong, Fuel Process. Technol. 88
(2007) 125–127.
[16] D. Li, N. Zhao, H. Qi, W. Li, Y.H. Sun, B. Zhong, Catal. Commun. 6 (2005) 674–678.
[17] D.B. Li, C. Yang, H.R. Zhang, W.H. Li, Y.H. Sun, B. Zhong, in: B. Xinhe, X. Yide (Eds.),
Studies in Surface Science and Catalysis, vol. 147, Elsevier, 2004, pp. 391–396.
[18] H. Qi, D. Li, C. Yang, Y. Ma, W. Li, Y. Sun, B. Zhong, Catal. Commun. 4 (2003)
339–342.
4. Conclusions
[19] B.J. Palla, D.O. Shah, P. Garcia-Casillas, J. Matutes-Aquino, J. Nanoparticle Res. 1
(1999) 215–221.
[20] E.E. Svensson, S. Nassos, M. Boutonnet, S.G. Järås, Catal. Today 117 (2006)
484–490.
[21] S. Nassos, E.E. Svensson, M. Nilsson, M. Boutonnet, S. Järås, Appl. Catal. B: Env-
iron. 64 (2006) 96–102.
[22] US EPA, Trace elements in water, solids, and biosolids by inductively coupled
plasma-atomic emission spectrometry, in: EPA Method 200.7 (revision 5.0),
2001.
[23] US EPA, Determination of trace elements in waters and wastes by inductively
coupled plasma-mass spectrometry, in: EPA Method 200.8 (revision 5.4), 1994.
[24] R. Andersson, M. Boutonnet, S. Järås, J. Chromatogr. A 1247 (2012) 134–145.
[25] X. Liu, J. Wang, L.-M. Gan, S.-C. Ng, J. Ding, J. Magnet. Magnet. Mater. 184 (1998)
344–354.
[26] N. Das, R. Majumdar, A. Sen, H.S. Maiti, Mater. Lett. 61 (2007) 2100–2104.
[27] G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catal. Today 41 (1998)
207–219.
[28] T.Y. Park, I.-S. Nam, Y.G. Kim, Ind. Eng. Chem. Res. 36 (1997) 5246–5257.
[29] Y. Iwata, Y. Araki, K. Honna, Y. Miki, K. Sato, H. Shimada, Catal. Today 65 (2001)
335–341.
[30] V.R. Surisetty, Y. Hu, A.K. Dalai, J. Kozinski, Appl. Catal. A: Gen. 392 (2011)
166–172.
In this work, a novel nickel-modified potassium-doped molyb-
denum sulfide catalyst was synthetized through coprecipitation in
microemulsion systems, characterized and tested for higher alco-
hol synthesis. This catalyst outperformed an analogous catalyst
prepared through coprecipitation in aqueous solution.
In order to compare the performance of both catalysts, CO
hydrogenation tests (H2/CO ratio = 1) were carried out at 91 bar,
340 and 370 ◦C, and varying the gas hourly space velocity to obtain
data for different conversion levels. One important remark is the
stability of the tested catalysts, since no significant deactivation
was observed after an initial period of about 100 hours on stream.
Activity and selectivity changes after this point were unimportant if
all the operating conditions were kept constant. The microemulsion
catalyst showed an improvement in both conversion and selectiv-
ity to ethanol and higher alcohols, resulting in higher yields of the
desired products for the studied operating conditions.
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003

G Model
CATTOD-9376; No. of Pages10
ARTICLE IN PRESS
10
R. Suárez París et al. / Catalysis Today xxx (2014) xxx–xxx
[31] D. Li, C. Yang, H. Qi, H. Zhang, W. Li, Y. Sun, B. Zhong, Catal. Commun. 5 (2004)
605–609.
[32] J. Iranmahboob, D.O. Hill, H. Toghiani, Appl. Surf. Sci. 185 (2001) 72–78.
[33] D. Ferrari, G. Budroni, L. Bisson, N.J. Rane, B.D. Dickie, J.H. Kang, S.J. Rozeveld,
Appl. Catal. A: Gen. 462–463 (2013) 302–309.
[36] W.M. Shen, J.A. Dumesic, C.G. Hill Jr., J. Catal. 68 (1981) 152–165.
[37] H.C. Woo, I.S. Nam, J.S. Lee, J.S. Chung, Y.G. Kim, J. Catal. 142 (1993) 672–690.
[38] H. Xiao, D. Li, W. Li, Y. Sun, Fuel Process. Technol. 91 (2010) 383–387.
[39] J. Iranmahboob, D.O. Hill, H. Toghiani, Appl. Catal. A: Gen. 231 (2002) 99–108.
[40] J. Iranmahboob, H. Toghiani, D.O. Hill, Appl. Catal. A: Gen. 247 (2003) 207–218.
[41] A. Andersen, S.M. Kathmann, M.A. Lilga, K.O. Albrecht, R.T. Hallen, D. Mei, J.
Phys. Chem. C 115 (2011) 9025–9040.
[34] V.R. Surisetty, A.K. Dalai, J. Kozinski, Appl. Catal. A: Gen. 385 (2010) 153–162.
[35] R. Andersson, M. Boutonnet, S. Järås, Fuel 107 (2013) 715–723.
Please cite this article in press as: R. Suárez París, et al., Higher alcohol synthesis over nickel-modified alkali-doped molyb-
denum sulfide catalysts prepared by conventional coprecipitation and coprecipitation in microemulsions, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.12.003