Optimization of unsupported CoMo catalysts for decarboxylation of oleic acid

Article abstract of DOI:10.1016/j.catcom.2015.03.034

Hydrodeoxygenation (HDO) processes have been developed to remove the oxygenated compounds in lipids. However, the HDO process consumes excess hydrogen. As opposed to the HDO process, decarboxylation does not require hydrogen. In this study, decarboxylation of oleic acid without hydrogen was carried out over unsupported CoMo catalysts. Unsupported CoMo catalysts were prepared by a co-precipitation method. The Co/Mo ratio was systematically varied to optimize unsupported CoMo catalyst. The catalyst properties were studied using various characterization techniques and related to the activity results in decarboxylation.

Full text of DOI:10.1016/j.catcom.2015.03.034

Catalysis Communications 67 (2015) 16–20
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Optimization of unsupported CoMo catalysts for decarboxylation of
oleic acid
a
a
a
a
a
a
Jae-Oh Shim , Dae-Woon Jeong , Won-Jun Jang , Kyung-Won Jeon , Seong-Heon Kim , Byong-Hun Jeon ,
a,
b
b
c
d,
Hyun-Seog Roh ⁎, Jeong-Geol Na , You-Kwan Oh , Sang Sup Han , Chang Hyun Ko ⁎
Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon 220-710, South Korea
Biomass and Waste Energy Laboratory, Korea Institute of Energy Research (KIER), Daejeon 305-343, South Korea
Petroleum and Gas Research Center, Korea Institute of Energy Research (KIER), Daejeon 305-343, South Korea
a
b
c
d
School of Applied Chemical Engineering, Chonnam National University, Gwangju 500-757, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrodeoxygenation (HDO) processes have been developed to remove the oxygenated compounds in lipids.
However, the HDO process consumes excess hydrogen. As opposed to the HDO process, decarboxylation does
not require hydrogen. In this study, decarboxylation of oleic acid without hydrogen was carried out over unsup-
ported CoMo catalysts. Unsupported CoMo catalysts were prepared by a co-precipitation method. The Co/Mo
ratio was systematically varied to optimize unsupported CoMo catalyst. The catalyst properties were studied
using various characterization techniques and related to the activity results in decarboxylation.
Received 22 January 2015
Received in revised form 19 March 2015
Accepted 21 March 2015
Available online 27 March 2015
Keywords:
Decarboxylation
Oleic acid
©
2015 Published by Elsevier B.V.
Unsupported CoMo catalyst
Co/Mo ratio
1
. Introduction
upgrading process. The decarboxylation reaction removes a carboxyl
group by releasing CO and producing a hydrocarbon, as illustrated by
2
Recently, the growing threat of oil depletion and climate change has
Reaction (1) [2]. Therefore, interest in non-sulfide decarboxylation cat-
alyst has grown significantly.
focused attention on the world's energy consumption. In particular,
consumption of liquid transportation fuels has surged during the 20th
century, due to an increase in the number of automobiles [1]. Therefore,
alternative fuels such as biodiesel have received much attention in
recent years. Biodiesels are based on lipids derived from various biomass
resources [2]. However, biodiesel contains a large amount of oxygen
compounds [3–6]. High oxygen contents in biodiesel result in thermal
instability, corrosiveness, and a low heating value [4–6]. Therefore, the
quality of biodiesel can be improved by oxygen elimination technology.
The hydrodeoxygenation (HDO) process is commonly applied for
the elimination of oxygen in lipids derived from biodiesel. CoMoS/γ-
Decarboxylationreaction
ð1Þ
:
R−COOH→R−H þ CO ðgÞ; ΔH ¼ þ9:2 kJ=mol
2
Recently, researchers have tried to overcome the disadvantages of
CoMo and NiMo sulfide catalysts. Bui et al. revealed that cobalt promot-
ed sulfide Mo catalyst is highly sensitive to support effects [8]. They
pointed out that the ZrO
sion and deoxygenation pathways [8]. However, for Al
2
support directly influences guaiacol conver-
and TiO sup-
O
2 3
2
ported CoMo sulfide catalyst systems, unexpected reactions have been
observed. Several studies have tried to avoid any unnecessary effects
of the support. Therefore, unsupported CoMo and NiMo sulfides are
used in the decarboxylation reaction [1,9]. Furthermore, Eijsbouts
et al. reported that the catalytic performance of unsupported CoMo cat-
alysts is greatly enhanced compared to supported CoMo catalysts [10].
Thus, the first aspect of this work is to develop unsupported CoMo
catalysts for the decarboxylation reaction of oleic acid.
2 3 2 3
Al O and NiMoS/γ-Al O catalysts are commonly used for the HDO
process because hydrotreating catalysts are known to be active in the
sulfide form [3,6]. However, CoMo and NiMo sulfide catalysts have crit-
ical problems such as rapid deactivation by coke deposition and a need
for the addition of sulfur donor compounds (H
sulfur content in the initial bio-feedstock [7]. Moreover, CoMo and
NiMo sulfide catalysts require a large amount of H [7].
2
S) because of the low
2
To overcome these disadvantages, catalytic decarboxylation with a
minimum consumption of hydrogen can be considered for biodiesel
The second aspect of this work is to study the effect of Co/Mo ratio on
the catalytic performance in decarboxylation. Several studies have
found that the promoter (such as Co and Ni) over Mo ratio has an im-
pact on the catalytic performance for decarboxylation [1,9,11]. Ruinart
de Brimont et al. reported that the Ni/Mo ratio has a significant effect
⁎
Corresponding authors.
E-mail addresses: hsroh@yonsei.ac.kr (H.-S. Roh), chko@jnu.ac.kr (C.H. Ko).
http://dx.doi.org/10.1016/j.catcom.2015.03.034
566-7367/© 2015 Published by Elsevier B.V.
1

J.-O. Shim et al. / Catalysis Communications 67 (2015) 16–20
17
on the decarboxylation selectivity [1]. Also, Wang et al. found that the
Co/Mo ratio is an important factor for catalytic activity because surface
composition changed due to variations in Co/Mo ratio [9]. In this
study, the Co/Mo ratio was systematically varied to optimize catalyst.
The effect of Co/Mo ratio on the catalytic performance has been studied
using various characterization techniques and is related to the activity
results in decarboxylation. Therefore, the final aim of this study is to
optimize unsupported CoMo catalysts with a non-sulfide nature for
decarboxylation reaction of oleic acid without hydrogen.
room temperature. Liquid products were collected after filtering the
solid phase catalysts. The liquid products were analyzed using a gas
chromatograph (HP 6890N) equipped with a flame ionization detector
and a capillary column (HP-5, 30 m).
3. Results and discussion
3.1. Catalyst characterization
Fig. 1 shows the XRD patterns of CoMo catalysts with various Co/Mo
2
. Experimental
ratios. According to XRD patterns, both MoO
3 4
and CoMoO phases can
be detected for all the prepared catalysts [24,25]. According to the
literature, carboxylate complexes are decarboxylated at the surface of
2
.1. Catalyst preparation
CoMoO
sponding to CoMoO
CoMoO peak at 26.3° decreased with increasing amounts of cobalt.
The crystallite size of the CoMo catalysts was calculated from CoMoO
peaks using the Debye–Scherrer equation, and the results are listed in
Table 1. The crystallite size of CoMoO increased with increasing
amounts of cobalt. As a result, Co0.1Mo0.9 catalyst exhibits the smallest
crystallite size of CoMoO , while Co0.8Mo0.2 shows the largest.
4
[26]. The diffractograms of CoMo catalysts show peaks corre-
CoMo catalysts were prepared by a co-precipitation method.
4
. The full width at half maximum (FWHM) of
Stoichiometric quantities of (NH
4
)
6
Mo
7
O
24·4H
2
O (99%, Fluka) and
4
Co(NO ·4H O (98%, Aldrich) were combined in distilled water. To
3
)
2
2
4
this solution, 28.8% ammonium hydroxide was added at 80 °C. After
aging for 4 h, they were thoroughly washed with 2 L of distilled water
to remove any impurities and air-dried for 12 h followed by drying at
4
1
10 °C. The prepared catalysts were calcined at 900 °C for 5 h.
4
Table 1 shows the characteristics of CoMo catalysts with various
Co/Mo ratios. Among the prepared catalysts, Co0.5Mo0.5 shows the
highest BET surface area. On the contrary, Co0.1Mo0.9 shows the lowest
BET surface area. The BET surface area increased with increasing
amounts of cobalt up to 50%. However, the BET surface area was de-
creased with increasing amounts of cobalt greater than 50%. Generally,
unsupported catalysts have much less surface area than supported
catalysts [27].
2
.2. Characterization
The BET surface area was measured by nitrogen adsorption at
196 °C using an ASAP 2010 (Micromeritics). The XRD patterns were
−
recorded using a Rigaku D/MAX-IIIC diffractometer (Ni filtered Cu–K
radiation, 40 kV, 50 mA), and the crystallite size was estimated by
using the Scherrer equation [12–17]. Temperature programmed reduc-
tion (TPR) was carried out in an Autochem 2920 (Micromeritics) using
3
Catalyst acidity, measured by the NH -TPD method, is listed in
1
0 vol.% H
programmed desorption of ammonia (NH
evaluate the total acidity of the catalysts using an Autochem 2920
Micromeritics). Oxygen content was measured by elementary analysis
2
/Ar with a heating rate of 10 °C/min [18–23]. Temperature-
Table 1. The acidity increased with increasing cobalt oxide content. In
other words, Co0.8Mo0.2 shows the highest acidity and Co0.1Mo0.9
shows the lowest acidity. Infantes-Molina et al. reported that the exis-
tence of more Co2 ions results in the formation of amino complexes,
which yields a high acidity [28]. The addition of cobalt oxide in
the cobalt–molybdenum oxide results in the generation of acid sites.
According to the literature, cobalt promoted molybdenum sulfide
3
-TPD) was carried out to
+
(
using a Thermo Finnigan FLASH EA-1112 Elemental Analyzer (EA). The
Co/Mo ratio of the prepared catalysts was analyzed by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) using a Ther-
mo Scientific iCAP 6500. The analysis results are shown in Table 1.
(MoS
2
) catalyst showed a relatively higher acidity as compared to
[28]. Catalyst acidity is known to be important
non-promoted MoS
2
2
.3. Catalytic reaction
for deoxygenation reaction. Also, several recent studies suggested that
the acidity is a significant factor in the deoxygenation reaction [29,30].
Decarboxylation reactions were carried out in an autoclave reactor
2
Fig. 2 describes H -TPR patterns of CoMo catalysts with various
(
100 mL) operating in batch mode. The autoclave reactor was designed
Co/Mo ratios. CoMo catalysts have three reduction peaks. The first
peak appears in the temperature range between 250 °C and 425 °C.
This peak can be assigned to the reduction of Co species [31,32].
The second peak, appearing around 540 °C, can be attributed to the
for operation up to 100 bar and 450 °C. A multi-blade impeller mixed
the liquid reactant and solid catalyst. The temperature was measured
using a K-type thermocouple. In a typical batch experiment, 27.5 g of
oleic acid and 0.6785 g of catalyst (reactant/catalyst = 40/1 wt/wt)
were placed in the reactor. After oleic acid and catalyst were loaded in
the reactor, the reactor was flushed with nitrogen to remove the
remaining oxygen. Then, the reactor was heated from room tempera-
ture to 300 °C at a heating rate of 4.5 °C/min, and the reaction tempera-
ture was maintained for 3 h. The stirring speed was fixed at 300 rpm
during the reaction. The reactor was subsequently cooled down to
CoMoO4
*
o
MoO
3
*
o
*
*
* *
Co0.1Mo0.9
*
* *
*
Co0.2Mo0.8
Table 1
Co O
3 4
Co O4
3
Characteristics of CoMo catalysts with various Co/Mo ratios.
Co0.5Mo0.5
a
Acidityc
(μmolNH3/gcat)
3
Co O4
Catalyst
BET S.A.
CoMoO
4
crystallite
Analyzed
2
sizeb (nm)
Co/Mo ratiod
(
m /g)
Co O4
3
Co0.1Mo0.9
Co0.2Mo0.8
Co0.5Mo0.5
Co0.8Mo0.2
1.30
1.41
4.24
2.21
41.8
46.2
50.6
53.2
741
1013
5870
0.099:0.901
0.198:0.802
0.497:0.503
0.798:0.202
Co0.8Mo0.2
70 80
12991
20
30
40
50
60
a
Estimated from N
2
adsorption at −196 °C.
b
c
2θ (degree)
Estimated from XRD.
Estimated from NH -TPD.
Estimated from ICP-AES.
3
d
Fig. 1. XRD patterns of CoMo catalysts with various Co/Mo ratios.

18
J.-O. Shim et al. / Catalysis Communications 67 (2015) 16–20
o
C
588
100
88.1
82.7
Co0.1Mo0.9
80
60
40
o
C
574
67.9
61.7
Co0.2Mo0.8
o
5
17 C
Co0.5Mo0.5
o
5
43 C
20
Co0.8Mo0.2
5.2
0
200
400
600
800
Blank
Co0.8Mo0.2
Co0.5Mo0.5
Co0.2Mo0.8
Co0.1Mo0.9
o
Temperature ( C)
Fig. 3. Oleic acid conversion over CoMo catalysts with various Co/Mo ratios (reaction con-
dition: 300 °C, reactant/catalyst = 40/1, 1 atm, N condition).
2
Fig. 2. H -TPR patterns of CoMo catalysts with various Co/Mo ratios.
2
reduction of CoMoO
corresponds to the reduction of MoO
follows these steps: MoO → MoO → Mo [31,32]. Interestingly, the
reduction temperature of CoMoO species in the Co0.5Mo0.5 catalyst
was the lowest among those of the other catalysts. The addition of
Mo oxide (more than 50%) in the Co oxide matrix inhibits the reduc-
tion of the Co–Mo oxide species. Mo may polarize the Co–O bonds,
making them more ionic, and consequently more difficult to reduce
4
[31,32]. The last peak, appearing at 800 °C,
3
species, wherein the reduction
conversion. This result indicates that oleic acid conversion was strongly
dependent upon the Co/Mo ratio. Oleic acid conversion of CoMo
3
2
4
catalysts follows the order: Co0.5Mo0.5 N Co0.2Mo0.8 N Co0.1Mo0.9
Co0.8Mo0.2 N blank.
N
Fig. 4 depicts the selectivity to C17 compounds (decarboxylation
reaction products of oleic acid without hydrogen) over CoMo catalysts
with various Co/Mo ratios. Interestingly, the change in selectivity to
[
31]. In our previous result, reduction property is an important factor
C17 compounds follows the same trend as the conversion of reactant.
of deoxygenation [6]. Thus, it is expected that the Co0.5Mo0.5 cata-
lysts should have higher activity than others.
In our previous results, the catalytic performance is related to oxygen
removal efficiency [6]. To check oxygen removal efficiency of CoMo
catalysts with various Co/Mo ratios, elemental analysis was carried
out. Table 2 exhibits oxygen removal efficiency of CoMo catalysts with
various Co/Mo ratios. Co0.5Mo0.5 catalysts exhibited the highest oxygen
removal efficiency (67.8%) among the tested catalysts. Interestingly,
even though the oleic acid conversion and C17 selectivity of Co0.8Mo0.2
are much lower than that of Co0.2Mo0.8, Co0.8Mo0.2 exhibits much higher
oxygen removal efficiency than Co0.2Mo0.8 due to the strong acidity of
Co0.8Mo0.2. This result suggests that the strong acidity helps further
improve the oxygen elimination. However, Wang et al. suggests that
the excess acidity of the catalyst was responsible for the cracking,
which is in good agreement with our C17 selectivity result [34]. There-
fore, excess acidity was not entirely beneficial because of a decrease in
3
.2. Activity tests
Scheme 1 displays the possible deoxygenation reaction pathways of
oleic acid. According to the literature, the oleic acid conversion path-
ways under inert conditions mainly consist of two steps [33]. In the
first reaction step, oleic acid is decarboxylated to 8-heptadecene. In
the second step, 8-heptadecene is hydrogenated to heptadecane.
Also, oleic acid can be hydrogenated, which forms stearic acid [4].
The generation of hydrogen in inert conditions is given by the forma-
tion of di-unsaturated fatty acid for hydrogenation [33]. Additionally,
possible side reactions are cracking reactions, ketonization, polymeriza-
tion, and aromatization.
C17 selectivity.
Fig. 3 shows oleic acid conversion over CoMo catalysts with various
Co/Mo ratios. All catalysts exhibited higher oleic acid conversion than
the blank test. Co0.5Mo0.5 catalyst exhibited about 88% oleic acid conver-
sion. On the contrary, other catalysts showed relatively low oleic acid
Table 3 exhibits liquid yield of CoMo catalysts with various Co/Mo
ratios. Co0.5Mo0.5 catalyst exhibited the lowest liquid yield (34.2%)
among the prepared catalysts. It should be noted that the highest C17
selectivity of Co0.5Mo0.5 catalyst is correlated with the lowest liquid
O
OH
Oleic acid
Hydrogenation
Decarboxylation
O
OH
8-heptadecene
Stearic acid
Hydrogenation
Decarboxylation
Heptadecane
Heptadecane
Scheme 1. Reaction scheme for the decarboxylation of oleic acid to hydrocarbon.

J.-O. Shim et al. / Catalysis Communications 67 (2015) 16–20
19
30
Table 3
Liquid yield of CoMo catalysts with various Co/Mo ratios.
a
25
Catalyst
Liquid product (g)
Gaseous product (g)
Liquid yield (%)
Co0.1Mo0.9
Co0.2Mo0.8
Co0.5Mo0.5
Co0.8Mo0.2
19.4
16.0
9.4
8.1
11.5
18.1
14.7
70.4
58.0
34.2
46.6
20.7
2
0
19.5
12.8
16.1
14.8
a
15
Gaseous product expressed as gram (g) is calculated by measuring initial weight of
reactant (27.5 g) minus weight of liquid product.
10
because it is difficult to define all the specific products. The other
9
possible other compounds are more short hydrocarbons (~C ) and ox-
5
0
ygenate compounds (due to polymerization, ketonization, and aromati-
zation products).
3
.7
The abovementioned reaction data can be explained as follows.
Firstly, the higher activity of Co0.5Mo0.5 is mainly correlated with the
highest BET surface area. A high surface area is usually helpful to
enhance catalytic activity due to the exposure of surface active centers
to reactants. The BET surface area value of the Co0.5Mo0.5 catalyst was
about 2 to 3 times higher than the others. Thus, the increase of active
sites in Co0.5Mo0.5 catalyst results in the highest oleic acid conversion
and C17 selectivity. Secondly, reducibility of the CoMo catalyst can be
related to the decarboxylation reaction. According to the TPR results,
Blank
Co0.8Mo0.2
Co0.5Mo0.5
Co0.2Mo0.8
Co0.1Mo0.9
Fig. 4. C17 selectivity over CoMo catalysts with various Co/Mo ratios (reaction condition:
00 °C, reactant/catalyst = 40/1, 1 atm, N condition).
3
2
yield. Na et al. reported that the liquid yield of the catalyst decreased
with increasing degree of decarboxylation because oxygen in oleic
acid was removed in the form of CO
2
[35]. The second highest C17 selec-
4
the CoMoO species of Co0.5Mo0.5 can be reduced at the lowest temper-
tivity was observed for the Co0.2Mo0.8 catalyst, which had a 58.0% liquid
yield. The C17 selectivity of the Co0.8Mo0.2 catalyst, which has a 46.6% liq-
uid yield, is lower than that of the former catalyst. This is mainly due to
the fact that the cracking reaction is influenced by the highest acidity of
the Co0.8Mo0.2 catalyst.
Earlier reports reveal that cobalt metal serves as a promoter in the
formation of the catalyst active phase, and its character is important
4
for catalytic activity [9]. In this study, the CoMoO species is formed by
cobalt promoter addition, and this is the active species in the decarbox-
ylation reaction. However, the increase of cobalt oxide content (less
than 50%) in molybdenum resulted in a decrease in oleic acid conver-
sion, C17 selectivity, and oxygen removal efficiency. Thus, the Co/Mo
ratio influences the catalytic performance of CoMo catalysts.
ature of 512 °C. It is most likely that the easier reducibility of Co0.5Mo0.5
results in the highest oleic acid conversion among the prepared CoMo
catalysts. This result is supported by the fact that heptadecane and
stearic acid species were found in the reaction products (Table 4). In
other words, CoMoO
the reaction. Therefore, easier reducibility of CoMoO
4
can be reduced through a hydrogenation step in
species probably
4
has a beneficial effect on catalytic performance. Thirdly, the decarboxyl-
ation reaction affects the catalyst acidity. Strong acidity has a beneficial
effect in elimination of oxygen. However, the presence of excess acidity
can deteriorate catalytic performance, resulting in a decrease in the oleic
acid conversion and C17 selectivity.
4. Conclusions
To check the carbon balance of reaction products over Co0.5Mo0.5
catalyst, all products were collected for GC analysis. However, the
exact carbon balance could not be determined due to technical limita-
tions. A loss of gas phase products is a main reason for these technical
limitations. Thus, a carbon balance was calculated based on a quantita-
tive analysis of the liquid phase product. The composition of reaction
products over Co0.5Mo0.5 catalyst is listed in Table 4. The amount of 8-
heptadecene (12.1%) is the largest among the single compositions.
This result indicates that decarboxylation is the main reaction pathway.
Moreover, the existence of heptadecane (6.1%) and stearic acid (5.2%)
even in inert conditions proved that hydrogenation occurred as in the
reaction mechanism proposed. This result is in good agreement with
the previous literature results [34]. In addition, it can be said that the
prepared catalyst in this study can be reduced even under inert
conditions. Thus, catalytic performance may be affected by the reduc-
ibility of the catalyst. Oleic acid (11.9%) is the residue which did not
The catalytic performance of non-sulfide unsupported CoMo cata-
lysts depends on the Co/Mo ratio. Co0.5Mo0.5 catalyst exhibits the
highest oleic acid conversion, C17 selectivity, and oxygen removal effi-
ciency. The higher conversion, C17 selectivity, and oxygen removal effi-
ciency of this catalyst in decarboxylation reaction without hydrogen are
correlated with the highest BET surface area and easier reducibility of
4
CoMoO species. Moreover, catalyst acidity influences the decarboxyl-
ation reaction and oxygen removal efficiency. Additionally, decarboxyl-
ation is the main reaction pathway in inert condition. As a consequence,
Co0.5Mo0.5 catalyst can be a promising decarboxylation catalyst for the
biodiesel upgrading process.
Acknowledgments
This research was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded by the
Ministry of Science, ICT and Future Planning (2013R1A1A1A05007370).
9
react with the catalyst. Furthermore, shorter hydrocarbons (C –C16, die-
sel fraction, 8.6%) were formed via cracking reaction. However, we can-
not determine the composition of the remaining 56.1% of compounds
Table 4
Table 2
Composition of collected liquid products over Co0.5Mo0.5 catalyst.
Oxygen contents of products and oxygen removal efficiency.
Reactant
Percentage
100.0%
Products
Percentage
Oxygen contents (%)
Oxygen removal efficiency (%)
Oleic acid
Oleic acid
Stearic acid
Heptadecane (C17
8-Heptadecene (C17
11.9%
5.2%
6.1%
12.1%
8.6%
56.1%
100.0%
Oleic acid (reactant)
Blank test
Co0.1Mo0.9
Co0.2Mo0.8
Co0.5Mo0.5
11.5
10.4
6.7
5.0
3.7
–
9.6
)
41.7
56.5
67.8
60.0
)
9
C –C16 hydrocarbons
Other compounds
Total
Co0.8Mo0.2
4.6
Total
100.0%

20
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