Effect of methanol addition on higher alcohol synthesis over modified molybdenum sulfide catalysts

Article abstract of DOI:10.1016/j.catcom.2015.04.018

One of the main problems in higher alcohol synthesis is the poor product distribution. Cofeeding of methanol, together with the synthesis gas, has been suggested in order to increase the yield of ethanol and higher alcohols. In this work, the effect of methanol addition on K-MoS₂ and K-Ni-MoS₂ catalysts was studied at 71 bar, 340 °C and GHSV = 6000 N mL/h · g_catalyst. Under these conditions methanol recycle is not a viable option for boosting higher alcohol production. The main result was an increase in methane yield, while the effect in higher alcohols was negative or not significant.

Full text of DOI:10.1016/j.catcom.2015.04.018

Catalysis Communications 67 (2015) 103–107
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Effect of methanol addition on higher alcohol synthesis over modified
molybdenum sulfide catalysts
Rodrigo Suárez París ⁎, Magali Boutonnet, Sven Järås
KTH — Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemical Engineering and Technology, Stockholm, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
One of the main problems in higher alcohol synthesis is the poor product distribution. Cofeeding of methanol,
together with the synthesis gas, has been suggested in order to increase the yield of ethanol and higher alcohols.
In this work, the effect of methanol addition on K–MoS and K–Ni–MoS catalysts was studied at 71 bar, 340 °C
2 2
and GHSV = 6000 N mL/h · gcatalyst. Under these conditions methanol recycle is not a viable option for boosting
higher alcohol production. The main result was an increase in methane yield, while the effect in higher alcohols
was negative or not significant.
Received 5 February 2015
Received in revised form 10 April 2015
Accepted 14 April 2015
Available online 16 April 2015
Keywords:
Syngas
©
2015 Elsevier B.V. All rights reserved.
Higher alcohols
K/MoS
Nickel
2
Methanol cofeeding
1
. Introduction
has been reported in a patent from The Dow Chemical Company [11].
However, in the latter case, no information regarding hydrocarbon pro-
duction is shown.
Catalytic conversion of carbon monoxide and hydrogen (syngas) to
ethanol and/or higher alcohols is an attractive alternative for producing
fuels and fuel additives that can partially replace fossil fuels in the trans-
portation sector. Syngas can be produced from coal, natural gas or bio-
mass, which results in a great flexibility [1,2]. Modified molybdenum
sulfide catalysts are among the most interesting catalytic systems for
higher alcohol synthesis (HAS). In addition to their sulfur resistance,
they present high activity for the water–gas shift reaction and the rate
of coking is slow [1,3].
The aim of this work is to study the effect of cofeeding methanol, in
various concentrations, over K–MoS and K–Ni–MoS catalysts. To the
2 2
authors' knowledge, there is no published information about this effect
on nickel-modified alkali-doped molybdenum sulfide catalysts. The op-
erating conditions in the study have been chosen in order to simulate
the conditions of an industrial plant [12].
The carbon–carbon bond formation to transform C
1
to C
2
species
2. Experimental
is the rate-determining step in HAS. Therefore, homologation or
cofeeding of lower alcohols has been proposed to increase the produc-
tion of higher alcohols [3,4].
An increased yield of ethanol and higher alcohols upon methanol
addition has been reported mainly on Cu/ZnO catalysts [4–6]. Isotope la-
beling experiments indicate that both methanol and CO are incorporated
into the ethanol product [7,8]. However, there is still no full agreement
about the effect of methanol addition. Calverley and Smith have shown
a decrease in higher alcohol yield when methanol is cofed together with
2.1. Catalyst preparation and characterization
Two unsupported molybdenum-based catalysts were synthetized, con-
taining either one or two promoters. The bipromoted catalyst (K–Ni–MoS
was obtained by coprecipitation of Ni(CH COO) ·4H O and (NH MoS
and further doping with K CO . The preparation procedure is described in
2
)
3
2
2
4
)
2
4
2
3
detail in a previous paper by our group [13]. Briefly, the coprecipitation
step produced a black precipitate that was aged, washed and recovered
by centrifugation. This precursor was then dried, crushed, sieved to a pellet
2 3 2 3
syngas over K CO /Cu/ZnO/Cr O [9].
Regarding modified molybdenum sulfide catalysts, the information is
even scarcer. Santiesteban et al. found improved ethanol yields upon
injection of methanol, together with an increased production of methyl
formate, methyl acetate and, specially, methane [10]. A similar behavior
size of 45–250 μm and doped with K
mixture. The final catalyst was obtained after thermal treatment at 450 °C
(ramp: 20 °C/min) for 90 min, under H flow (GHSV = 1200 N mL/
h · gcatalyst). Thereafter, a second sieving was performed to discard particles
with a pellet size above 250 μm that could be formed during the heat treat-
ment. The final sample was kept in a tightly closed container before being
tested.
2 3
CO (45–250 μm) through mechanical
2
⁎
Corresponding author at: Teknikringen 42, SE-100 44, Stockholm, Sweden.
E-mail address: rosp@kth.se (R. Suárez París).
http://dx.doi.org/10.1016/j.catcom.2015.04.018
566-7367/© 2015 Elsevier B.V. All rights reserved.
1

104
R. Suárez París et al. / Catalysis Communications 67 (2015) 103–107
The monopromoted catalyst (K–MoS
2
) was synthetized using the
Table 2
Summary of experimental conditions used in the catalytic tests.
same procedure, but excluding the coprecipitation step and thus the
doping with nickel.
Period
Psyngas
(bar)
PHe
PMeOH
(bar)
He added
(mol%)
MeOH added
(mol%)
Detailed catalyst characterization is provided elsewhere, including
(bar)
2
TGA, ICP, XPS, N -adsorption, XRD, SEM–EDX and TEM analyses [13].
Stabilization
68.2
54.5
54.5
54.5
54.5
54.5
54.5
54.5
54.5
54.5
0
0
0
3.6
0
7.1
0
1.8
0
1.1
0
0
20
15
20
10
20
17.5
20
18.5
20
0
0
5
0
10
0
2.5
0
1.5
0
In this previous study by our group, it was shown that both materials
have a low surface area and contain mainly macropores. A number of
key features are presented in Table 1. As shown, the promoters have
been successfully incorporated during catalyst preparation.
1
2
3
4
5
6
7
8
9
14.2
10.6
14.2
7.1
14.2
12.4
14.2
13.1
14.2
2
.2. Catalytic testing
CO hydrogenation reactions were performed in a high-pressure
fixed-bed tubular reactor operating in down-flow mode. Approximately
.8 g catalyst, diluted with 7.5 g SiC (pellet size: 53–80 μm), was used in
1
the tests. The reactor was heated by an electric furnace and the tempera-
ture was regulated by a cascade control, with a sliding thermocouple in
the catalyst bed and a second thermocouple in the oven. This control ar-
chitecture, together with the dilution with SiC and an external aluminum
jacket, allowed for a temperature profile along the bed within ± 0.5 °C of
the set point.
required time to get the desired flow of methanol through the reactor
when the pump is started or stopped. All these factors may contribute
to a greater or lesser extent to the fluctuations observed after a change
in conditions. However, each period was kept for at least 8 h in order to
be able to reach a pseudo-steady state.
The changes in alcohol and hydrocarbon space-time yields (STY) are
presented in Fig. 2. Unexpectedly, the introduction of methanol in the
reactor system does not result in improved yields of alcohols, but de-
creases the production of both ethanol and 1-propanol. On the other
hand, methanol has the opposite effect on hydrocarbon STY. Methanol
cofeeding increases their production, this effect being more pronounced
with increasing methanol concentration.
2 2
A premixed syngas with H /CO ratio = 1 and 4 mol% N as internal
standard was used in the experiments. The reaction conditions were:
P = 71 bar, T = 340 °C and GHSV = 6000 N mL/h · gcatalyst. The catalysts
were first stabilized, under the aforementioned conditions, to ensure
steady-state was reached. The stabilization period varied from about
2 2
20 h on stream (K–MoS ) to 40 h on stream (K–Ni–MoS ).
After the stabilization period, a He pocket (20 mol%) was introduced
A complete product distribution is shown in Table 3, together with
CO and net methanol conversions. 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%. The net conversion of methanol is appreciably higher than
CO conversion. The selectivity to the different products follows the
same trend that was described above for ethanol, 1-propanol, methane
and ethane yields. Methanol cofeeding clearly reduces the production of
the different alcohols, while increasing hydrocarbon selectivity. Methane
in order to keep the syngas partial pressure constant when methanol
was cofed. Thereafter, different concentrations of methanol were
added by means of a HPLC dosing pump (Gilson 307). The liquid was
evaporated before being mixed with the gaseous stream, immediately
prior to the reactor. The different conditions used in the study are
summarized in Table 2. Note that in between each period of metha-
nol addition a period without methanol cofeeding was run to check
for deactivation.
Product analysis was carried out using an on-line GC, equipped with
one TCD and two FID detectors. The carbon mass balance closures were
typically higher than 98%. More details about the reaction and analysis
setup can be found elsewhere [14].
comprises up to 65% of the products (C%, CO
anol addition.
2
-free basis) upon 10% meth-
3.2. Methanol addition on K–Ni–MoS
2
3
. Results
The incorporation of a second promoter, nickel, on K–MoS changes
2
the previously shown trends, regarding both activity and selectivity. In
this case, catalyst activity is also stable throughout the experiment,
with values of about 2.5% in the periods without methanol cofeeding
(Fig. 3). It has been previously reported that the incorporation of nickel
3
.1. Methanol addition on K–MoS
2
The effect of methanol addition on CO conversion over the mono-
promoted catalyst is shown in Fig. 1. It should be noted that catalyst ac-
tivity is fairly constant during the 6 days of operation: after introducing
the He pocket, the conversion remains stable around 13% in the periods
without methanol cofeeding. Addition of methanol slightly influences
the conversion of CO, with values of around 10% for the different con-
centrations of added methanol.
2
on alkali-doped MoS catalysts, despite increasing the selectivity to
The transient periods observed in Fig. 1 when changing the condi-
tions (and also in the subsequent Figs. 2–4) might be explained by a
combination of different factors: slow change in catalyst performance
with time on stream; residence time in the reactor and the subsequent
piping to the GC; time between each GC injection (approx. 90 min); and
Table 1
The main physicochemical properties of the tested catalysts.
Catalyst
ICP analysis
mol/mol)
EDX analysis
(mol/mol)
XPS analysis
(mol/mol)
BET surface area
(m /g)
2
(
Ni/Mo K/Mo Ni/Mo K/Mo Ni/Mo K/Mo
K–MoS
K–Ni–MoS
2
0
0.45
1.50
1.19
0
0.61
1.15
1.42
0
0.05
5.16
1.50
1
3
2
2
Fig. 1. Effect of methanol addition on CO conversion (K–MoS ).

R. Suárez París et al. / Catalysis Communications 67 (2015) 103–107
105
2
Fig. 2. Effect of methanol addition (K–MoS ) on: ethanol and 1-propanol STY (left); methane and ethane STY (right).
taken into consideration (Fig. 4-right). Namely, there is a slight increase
in alcohol production at the cost of a huge increase in methane STY.
The performance of the bipromoted catalyst when different
amounts of methanol are added to the system is summarized in
Table 4. A marked decrease in ethanol and higher alcohol selectivity
is also obtained in this case. However, a direct comparison of selec-
tivity values with the monopromoted catalyst cannot be made, since
the results obtained with nickel-modified alkali-doped MoS cata-
2
lysts depend on the CO conversion level [15] (and CO conversion
values are rather different in both catalysts).
4. Discussion
Cofeeding of methanol is generally assumed to be a means of increas-
ing the yield of higher alcohols, but there is no complete agreement about
this effect on the different catalysts used in HAS. As previously mentioned,
only few studies have addressed this topic despite the great variety of
catalyst options.
2
Fig. 3. Effect of methanol addition on CO conversion (K–Ni–MoS ).
ethanol and higher alcohols, results in a lower activity [13]. Addition of
Although isotope labeling experiments reveal that methanol can be
methanol increases CO conversion on K–Ni–MoS2, unlike what occurs
2
converted to ethanol over MoS -based catalysts [10], our results mostly
with K–MoS
2
. The increase is especially remarkable at high methanol
show a negative effect when methanol is introduced in the reaction
system under realistic conditions of pressure and temperature (71 bar
and 340 °C). The yield of ethanol and higher alcohols is particularly re-
concentrations: CO conversion increases from 2.5% to more than 10%
when 10% methanol is added to the syngas feed.
Regarding the effect of methanol on alcohol STY, the effect is still
negative for low methanol concentrations. However, when the concen-
tration increases to 5% and specially 10% the yields of both ethanol and
2
duced when methanol is cofed over K–MoS .
A simplified scheme of the reaction pathways for the formation of
alcohols and hydrocarbons from syngas is shown in Fig. 5. In view of
the results, it can be suggested that the methanol added to the system
competes for adsorption sites with other reaction intermediates. It
1
-propanol are somewhat enhanced, as can be seen in Fig. 4-left. How-
ever, the great increase in hydrocarbon STY, mostly methane, must be
2
Fig. 4. Effect of methanol addition (K–Ni–MoS ) on: ethanol and 1-propanol STY (left); methane and ethane STY (right).

Table 3
2
Catalytic performance of K–MoS for different methanol concentrations in the feed.
2
Selectivity (C%, CO -free)
MeOH added
mol%)
CO conversion
(%)
Net MeOH conversion a
(%)
CO
(%)
2
selectivity
Methanol
Ethanol
1-Propanol
1-Butanol
i-Butanol
Other
alcohols
Other
oxygenates
Methane
Ethane
Propane
Higher
alkanes
Alkenes
(
0
1
2
5
1
12.6
11.3
10.7
11.3
11.7
n/a
40.5
54.8
55.4
50.2
48.2
27.6
22.6
19.9
12.3
10.6
15.2
8.7
6.8
4.5
2.6
14.7
8.0
6.2
3.9
2.3
2.4
1.2
0.9
0.5
0.4
2.0
1.1
0.8
0.4
0.3
1.3
0.7
0.5
0.3
0.2
2.1
1.3
1.1
0.7
0.4
30.6
50.0
57.1
60.0
64.7
1.0
4.7
5.5
14.4
14.3
0.3
0.2
0.2
2.0
3.4
0.0
0.0
0.0
0.3
0.4
2.7
1.5
1.1
0.6
0.4
.5
.5
40.9
65.7
86.7
92.6
0
a
Net methanol conversion is calculated as the difference between moles of methanol at the inlet and outlet of the reactor per mole of fed methanol.
Table 4
Catalytic performance of K–Ni–MoS
2
for different methanol concentrations in the feed.
Selectivity (C%, CO
2
-free)
MeOH added
mol%)
CO conversion
(%)
Net MeOH conversion a
(%)
CO
(%)
2
selectivity
Methanol
Ethanol
1-Propanol
1-Butanol
i-Butanol
Other
alcohols
Other
oxygenates
Methane
Ethane
Propane
Higher
alkanes
Alkenes
(
0
1
2
5
1
3.1
3.6
3.9
5.8
10.5
n/a
45.7
60.7
60.1
55.8
55.7
40.5
28.9
24.2
13.3
12.9
31.1
15.5
11.4
6.1
6.9
3.8
3.0
1.6
1.5
1.2
0.6
0.4
0.2
0.2
0.5
0.4
0.4
0.2
0.3
0.6
0.3
0.2
0.1
0.1
5.2
3.3
2.6
1.6
1.7
12.1
37.9
46.5
56.6
54.2
0.6
4.1
5.2
7.5
8.5
0.2
2.5
2.8
4.3
5.0
0.1
1.3
1.4
2.4
3.1
1.0
1.4
2.0
6.0
8.3
.5
.5
58.7
74.2
89.4
91.2
0
4.1
a
Net methanol conversion is calculated as the difference between moles of methanol at the inlet and outlet of the reactor per mole of fed methanol.

R. Suárez París et al. / Catalysis Communications 67 (2015) 103–107
107
2
not significant (K–Ni–MoS ), while the amounts of methane and higher
hydrocarbons produced in the reaction are clearly increased. Therefore,
adding methanol to molybdenum sulfide-based catalysts under the
studied operating conditions is not a suitable option for increasing the
yield of ethanol and higher alcohols.
Acknowledgments
KIC InnoEnergy is acknowledged for financial support. Thanks to
Christina Hörnell for improving the English in the manuscript. The
authors are also grateful to Javier Barrientos and Jorge Velasco (KTH)
for inspiring scientific discussions.
Fig. 5. Reaction pathways for the conversion of syngas to alcohols and hydrocarbons.
2
was previously reported that the chemisorption of CO and H and their
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