Cobalt Nanoparticles Supported on Nitrogen-Doped Carbon: An Effective Non-Noble Metal Catalyst for the Upgrade of Biofuels

Article abstract of DOI:10.1002/cssc.201702078

A new method has been developed for the deoxygenation of vanillin to produce 2-methoxy-4-methylphenol (MMP) as a promising liquid fuel over a heterogeneous non-noble metal catalyst. Cobalt nanoparticles supported on nitrogen-doped carbon (Co/N-C-600) exhibit high activity and stability for the deoxygenation of vanillin into MMP under mild conditions (150 °C, 10 bar H₂). Nearly quantitative MMP yield is obtained in isopropanol after 8 h at 150 °C and 10 bar H₂ pressure. According to the distribution of products with time, the deoxygenation of vanillin into MMP mainly proceeds through the hydrogenation of vanillin into vanillyl alcohol and the subsequent hydrogenolysis of vanillyl alcohol into MMP, of which the latter is the rate-determining step, owing to a much higher activation energy. Moreover, after being recycled several times, the loss of catalytic activity is negligible, which demonstrates that the Co/N-C-600 catalyst shows good resistance to deactivation.

Full text of DOI:10.1002/cssc.201702078

Accepted Article
Title: Nitrogen-doped carbon supported Co catalysts: An effective
none-noble metal catalyst for the upgrade of biofuels
Authors: Liang Jiang, Peng Zhou, Chanjuan Liao, Zehui Zhang, and
Shi wei Jin
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10.1002/cssc.201702078
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Nitrogen-doped carbon supported Co catalysts: An effective
none-noble metal catalyst for the upgrade of biofuels
Liang Jianga, Peng Zhoua, Chanjuan Liaob, Zehui Zhanga*, Shiwei Jina
Abstract: A new method has been developed for the deoxygenation
of vanillin to produce 2-methoxy-4-methylphenol (MMP) as
role in the catalytic efficiency of the heterogeneous catalysts.
For example, Wang and co-workers found that the N-doped
carbon-supported Pd catalyst exhibits much higher activity in the
hydrodeoxygenation of vanillin than other Pd catalysts supported
on metal oxides and active carbon.[16] Obviously, the high cost of
noble metal catalysts made them an unsatisfactory choice.
a
promising liquid fuel over a heterogeneous non-noble metal catalyst.
The nitrogen-doped carbon supported Co nanoparticles (Co/N-C-
600) exhibited high activity and stability for the deoxygenation of
vanillin into MMP under mild conditions (150 oC, 10 bar H2). Nearly
quantitative MMP yield was obtained in iso-propanol after 8 h at 150
oC and 10 bar H2 pressure. According to the distribution of products
with time, the deoxygenation of vanillin into MMP mainly underwent
the hydrogenation of vanillin into vanillyl alcohol and the subsequent
hydrogenolysis of vanillyl alcohol into MMP, and the latter was the
rate-determining step, which had a much higher active energy.
Moreover, after being recycled several times, the loss of catalytic
activity was negligible, which demonstrated that the Co/N-C-600
catalyst had the property of resistance to deactivation.
Currently, great efforts have been devoted to the
development of non-noble metal catalysts to replacement noble
metal catalysts for chemical transformations. A few none-noble
metal catalysts have been reported for the hydrodeoxygenation
of vanillin into MMP.[20-21] Due to the intrinsic low activity of the
none-noble metal catalysts, the reduction of vanillin over none-
noble metal catalysts are restricted by their low resistance to
deactivation and low energy efficiency caused by the severe
reaction conditions. In addition, the selectivity of MMP is also a
great challenge. For example, the CoMo/Al2O3 catalyst showed
poor catalytic performance towards the reduction of vanillin,[22]
which produced low vanillin conversion of 53% and carbon
balance of 75% and provided mixtures of over four compounds
with MMP selectivity of 43.4% after 4 h under harsh conditions
(300 oC and 5 MPa H2). Obviously, it is highly desirable to
develop new non-noble heterogeneous catalysts with high
activity and selectivity for the hydrodeoxygenation of vanillin into
MMP under mild conditions.
Currently, much emphasis has been stressed on chemists to
develop chemicals reaction in sustainable and environmental-
friendly ways, due the shrink of the non-renewable fossil fuel
resources and the global concern on the environmental
pollutions.[1-3] One of the major aspects in sustainability is the
use of renewable resources to supply chemicals and fuel to the
society. Biomass is one of the most abundant renewable
resources, and the only material renewable resources.[4]
Through biorefiney, biomass can be upgraded into chemicals
and fuels using physical, chemical and biological methods.[5-7]
Recently, nitrogen-containing carbon materials have
received considerable attention as fascinating materials for the
immobilization of metal nanoparticles.[23, 24] The incorporation of
nitrogen atoms in the carbon structure can enhance chemical,
electrical, and functional properties,[25] therefore, nitrogen-doped
carbon materials supported metal catalysts exhibited excellent
properties such as high catalytic activity and high stability. In our
previous work, we have prepared nitrogen-doped carbon
supported Co catalysts (Co/N-C-T, T represent the pyrolysis
temperature), and the Co/N-C-T showed high catalytic activity
towards the transfer hydrogenation of nitro compounds and
imines. It is anticipated that this catalyst can also be used for the
hydrodeoxygenation of vanillin into MMP with high activity and
high selectivity.
The hydrogenation of unsaturated functional groups such as
carbon–carbon double bonds and the hydrogenolysis of carbon–
oxygen bonds are both important reactions in synthetic
chemistry and biomass biorefiney,[8-10] particularly in the liquid
fuels sector, in which catalysts are very essential in the
reduction of oxygen content. 4-Hydroxy-3-methoxybenzaldehyde
(vanillin), a common component of pyrolysis oil derived from the
lignin fraction, can be hydrogenated to MMP, a potential future
biofuel. The transformation of vanillin has been early performed
using molecular Ru catalysts.[11] To overcome the drawbacks of
homogeneous catalysts, great efforts have been devoted to the
search of heterogeneous catalysts for this reaction. Noble metal
catalysts such as Pd,[12-17] Au,[18] and Ru[19] have been reported
for the hydrodeoxygenation of vanillin into NMP. Among them,
Pd nanoparticles deposited on various supports have been
mainly employed for up-grading of vanillin, and the nature and
surface structure of the support was found to play a significant
Results and Discussion
Catalytic hydrodeoxygenation of vanillin in various
solvents
In our previous work, we have prepared the nitrogen-doped
carbon supported Co nanoparticles (Co/N-C-T) by the prolysis of
ZIF-67 at different temperatures, which was formed by the
coordination of Co2+ with 2-methylimidazole ligand. It was found
that Co/N-C-600 showed the highest catalytic activity, as it has
the smallest size of Co nanoparticles and the highest nitrogen
content.[26] Therefore, the Co/N-C-600 catalyst was used as for
the deoxygenation of vanillin. Firstly, the effect of the reaction
solvents was studied to investigate the effect of solvents on the
[a]
[b]
Liang Jiang, Dr. P. Zhou, Prof. Z. H. Zhang and S. W. Jin
Key Laboratory of Catalysis and Materials Sciences of the Ministry
of Education, South-Central University for Nationalities
Wuhan, 430074, P. R. China.
E-mail: zehuizh@mail.ustc.edu.cn Tel.: +86-27-67842572. Fax:
+86-27-67842572.
Chanjuan Liao
College of Resources and Environment, Hunan Agricultural
University
Changsha, 410128, P. R. China.
E-mail: cjliao@hunau.edu.cn
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10.1002/cssc.201702078
ChemSusChem
Full Paper
vanillyl alcohol) could not effectively dissolve in water, therefore,
a difficult contact of the catalyst with the substrates was also
present. Under the mixed solvents (H2O : DMF=9 mL : 1 mL) to
ensure the substrates could effectively dissolve in the mixed
conversion of vanillin and the product selectivity, as different
solvents have different properties such as polarity, boiling points
and acid-base prosperities. As shown in Table 1, the catalytic
performance of Co/N-C-600 was observed to be greatly affected
by the reaction solvents. Generally, high conversions
(88.6~98.8%) were achieved in oxygen-containing organic
solvents (Table 1, Entries 1~4), while that was much lower in
weak polar solvents without oxygen atoms (Table 1, Entries 5~
6). The excellent conversions in oxygen-containing organic
solvents should be attributed to the following reasons. One the
one hand, the substrate, the intermediate of vanillyl alcohol, as
well as the hydrophilic Co/N-C-600 were well dispersed in the
oxygen-containing organic solvents, while that was not the case
in the weak polar solvents of toluene and hexane. The well
dispersion of the Co/N-C-600 catalyst in the reaction solvent
resulted in improving the exposure of the catalyst to the
substrate (vanillin) and the intermediate (vanillyl alcohol),
thereby increasing the catalytic performance significantly. In
addition, the oxygen-containing solvents might also have the
ability to stabilize the intermediate of vanillyl alcohol via
hydrogen bonding, which was believed to be favorable for the
reduction on the catalytic surface.[28]
solvents,
a higher conversion of vanillin of 94.6% was
acheived(Table 1, Entries 10 vs 7). Interestingly, the products
distribution was also significantly affected by the reaction
solvents. MMP was detected to be the main product with
alcohols as the reaction solvents, while vanillin alcohol was
mainly produced in other solvents such as tetrahydrofuran (THF),
ethyl acetate and water. However, the total selectivity of MMP
and vanillin alcohol was 68.8%. The byproduct was formed by
the etherification of vanillin alcohol with ethanol with a selectivity
of 28.8%. Iso-propanol was the best solvent for the
deoxygenation of vanillin over the Co-N/C-600 catalyst, which
afforded 98.2% conversion of vanillin with MMP and
vanillyl alcohol selectivity of 85.1% and 14.8%, respectively
(Table 1, Entry 3). Compared with the results in ethanol, no
ether product was observed in iso-propanol, possibly due to the
larger steric hindrance of iso-propanol than ethanol (Table 1,
Entries 3 vs 4). No vanillin conversion was observed in iso-
propanol at 10 bar H2 without adding the Co/N-C-600 catalyst
(Table 1, Entry 9), suggesting that the catalytic performance
derived from the Co/N-C-600 catalyst. Cobalt nanoparticles
serve as the active sites to activate the H2 and nitrogen atoms
should serve as a base site to promote the heterolytic cleavage
of H2.[29] Iso-propanol can also be used a hydrogen donor for
Table 1. The effect of the reaction solvents on the dydrodeoxygenation of
vanillin
transfer hydrogenation reactions,[30] therefore,
a
control
experiment was also carried out under 10 bar N2 pressure. No
vanillin conversion was observed in iso-propanol at 10 bar N2
(Table 1, Entry 8), suggesting that the Co/N-C-600 catalyst did
not promote the transfer hydrogenation of vanillin by iso-
propanol at 150 oC, but activated H2 to reduce vanillin. Therefore,
iso-propanol was used as the reaction solvent for the
deoxygenation of vanillin in the following experiments.
Conv
[27]
Entry
Solvent
ε/εo
Sel.1(%)
Sel.2 (%)
. (%)
88.6
98.5
98.2
98.8
24.6
10.7
77.9
0
1
2
THF
7.4
67.7
51.7
14.8
9.9
24.2
33.8
85.1
58.9
24.5
25.8
34.9
-
Ethyl acetate
Iso-propanol
Ethanol
6.0
17.9
24.5
2.4
3
The effect of the hydrogen pressure on the
deoxygenation of vanillin
4
5
Toluene
70.5
68.9
61.8
-
6
Hexane
1.9
Vanillyl alcohol selectivity
MMP selectivity
7
H2O
80.1
17.9
17.9
-
120
Vanillin conversion
105
8b
9c
10d
11
Iso-propanol
Iso-propanol
H2O+DMF
DMF
90
75
60
45
30
15
0
0
-
-
94.6
29.3
62.7
36.7
90.3
90.2
3.5
a Reaction conditions: vanillin (1 mmol), H2 (10 bar), Co/N-C-600 (20 mg),
Solvent (10 mL), 150 oC, 4 h. b The reaction was performed under 10 bar N2
atmosphere under otherwise the same conditions. c The reaction was
performed under the same conditions except no catalyst added. d The reaction
was performed under the mixed solvents (H2O : DMF=9 mL : 1 mL).
1
5
2.5
7.5
10
20
15
H2 pressure (bar)
Fig. 1 The results of the vanillin conversion and the selectivity of the products
at different H2 pressure. Reaction conditions: Vanillin (1 mmol), Co/N-C-600
(20 mg), iso-propanol (10 mL), 150 oC.
Although H2O has the strongest polarity, the catalytic
performance of Co/N-C-600 towards the deoxygenation of
vanillin in water was poorer as compared with the oxygen-
containing organic solvents (Table 1, Entries 7 vs 1~4). One of
the main reasons should be that the substrates (vanillin and
To detect the relationship between hydrogen pressure and
the catalytic efficiency of the Co/N-C-600 catalyst, the
deoxygenation of vanillin was performed with the initial H2
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10.1002/cssc.201702078
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o
pressure from 1 to 20 bar at 150 C for 4 h. As shown in Fig. 1,
vanillin conversion and products selectivity were greatly affected
by the H2 pressure, especially at low H2 pressure range. After 4
h, vanillin conversion greatly increased from 24.5% at 1 bar to
97.1% at 5 bar. The selectivity of vanillyl alcohol decreased from
71.0% at 1 bar and 1.4% at 20 bar, while the selectivity of MMP
gradually increased from 24.5% at 1 bar to 98.6 % at 20 bar.
The increase of vanillin conversion and the decrease of the
vanillyl alcohol selectivity should be due to the fact that the
higher concentration of H2 in the reaction solution at higher H2
pressure accelerated the rate of the hydrogenation of vanillin
into vanillin alcohol and the subsequent hydrolysis of
vanillyl alcohol into MMP. The effect of the hydrogen pressure
on the conversion and products distribution was observed to
much more obvious especially at low H2 pressure from 1 bar to 5
bar, which means the H2 concentration tends to be constant at
higher pressure beyond 5 bar. Complete conversion of vanillin
conversion and excellent MMP yield of 98.6% were produced
after 10 h at 150 oC and 20 bar H2 pressure. A particular note is
that the deoxygenation of vanillin over the Co/N-C-600 catalyst
can be performed atmospheric H2 pressure enables the use of
common glass reactors, which would show great promising in
the practical application without the resort of specialized
instrument. Thus, this method has a promising potential in
industrial application from economical, environmental and safe
viewpoints. To the best of our knowledge, there was not a report
that one catalyst can be performed under atmospheric pressure
for the transformation of vanillin into MMP.
When the temperature rose from 25 to 110 oC, the conversion of
vanillin improved remarkably from 4.1% to 97.4%, demonstrating
that the reduction of vanillin was sensitive to the reaction
temperature below 110 oC. The reaction temperature also
showed a great influence on the products distribution. Vanillyl
alcohol was the only product at low temperature from 25 to 70
oC, while MMP was started to be observed when the reaction
was performed at 90 oC. These results suggested the
hydrogenolysis of vanillyl alcohol into MMP was much more
difficult than the hydrogenation of vanillin into vanillyl alcohol,
requiring higher reaction temperature to overcome the energy
barrier. Further increasing the reaction temperature from 90 to
170 oC, the selectivity of vanillyl alcohol gradually decreased and
that of MMP gradually increased, especially at high reaction
o
temperature range of 130 C~170 oC. For all cases, the carbon
balance was almost up to 100%, suggesting that the side
reactions such as the decarbonylation and polycondensation
were not present in our catalytic system.[20-22] The highest MMP
yield was produced in 99.0% at full vanillin conversion after 4 h
at 170 oC and 10 bar pressure.
(a)
0.0
-0.2
T=60C k=0.00125 min-1
-5.6
-0.4
-5.8
T=70C k=0.00227 min-1
-6.0
-6.2
-0.6
-6.4
Ea=48.09 kJmol-1
T=80C k=0.00334 min-1
-6.6
-6.8
-0.8
0.00284 0.00288 0.00292 0.00296 0.00300
Catalytic deoxygenation of vanillin at different
temperatures
1/T
0
50
100
150
200
250
300
Time (min)
Fig. 2 depicts the results of the reduction of vanillin over the
Co/N-C-600 catalyst in iso-propanol at different reaction
temperatures under 10 bar H2 pressure. It is observed that the
reaction temperature significantly influenced the conversion of
vanillin and selectivity of products. It is noted that the reaction
can be performed even at room temperature, albeit the reaction
rate was slow. To the best of our knowledge, there have been
no reports on the reduction of vanillin over non-noble metal
catalysts with H2 at room temperature.
(b)
0.0
T=120C k=0.00297 min-1
-0.2
-0.4
-0.6
-0.8
-1.0
-4.8
T=130C k=0.00415 min-1
T=140C k=0.00705 min-1
-5.1
-5.3
-5.5
-5.7
Ea=56.92 kJmol-1
0.00244 0.00248 0.00252 0.00256
Vanillyl alcohol
MMP
1/T
100
80
60
40
20
0
100
80
60
40
20
0
0
20 40 60 80 100 120 140 160
Time (min)
Fig. 3 Kinetic studies of the hydrogenation of vanillin into vanillyl alcohol (a),
and the hydrogenolysis of vanillyl alcohol into MMP. The inset shows the
corresponding Arrhenius plot. Reaction conditions: substrate (1 mmol), Co/N-
C-600 catalyst (20 mg), iso-propanol (10 mL), H2 pressure (40 bar).
Furthermore, kinetic studies were investigated into order to
provide more insights into an overall outlook of the
transformation of vanillin into MMP. The hydrogenation of
vanillin into vanillyl alcohol, and the hydrogenolysis of vanillyl
alcohol were individually studied at two different temperature
ranges and under 40 bar H2 pressure. At 40 bar H2 pressure, the
H2 concentration in iso-propanol can be considered to be
50
25
70
130
170
90 110
150
Temperature (oC)
Fig. 2 The results of the deoxygenation of vanillin at different temperatures.
Reaction conditions: Vanillin (1 mmol), Co/N-C-600 (20 mg), iso-propanol (10
mL), and 10 bar H2.
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10.1002/cssc.201702078
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constant, thus the reaction can be presumed to be pseudo-first
order reaction. Fig. 3 (a) shows the plot of ln(Ct/C0) versus time
at 60, 70 and 80 C for the hydrogenation of vanillin into vanillyl
alcohol, and the reaction rate constant k was determined to be
0.00125, 0.00227 and 0.00334 min-1 for 60, 70 and 80 oC,
the transformation of vanillin into MMP must proceed via path B
and/or C. In order to investigate the reaction pathway, the time
course of deoxygenation of vanillin over the Co/N-C-600 catalyst
was recorded. Fig. 4 plots the conversion of vanillin and the
yields of vanillyl alcohol and MMP as a function of reaction time.
The reaction was accompanied by a rapid increase in vanillyl
alcohol, and the maximum vanillyl alcohol reached 69.3% at an
o
respectively. Similar, the reaction constant
k
for the
hydrogenolysis of vanillyl alcohol into MMP were calculated to
be 0.00297, 0.00415, and 0.00705 min-1 for 120, 130 and 140 oC, early reaction time of 30 min. The vanillin conversion was also
respectively. According to Arrhenius plot, the activation energy
of the hydrogenation of vanillin into vanillyl alcohol and the
hydrolysis of vanillyl alcohol into MMP were calculated to 48.09
and 56.92 kJ mol-1, respectively. The data once again revealed
that the hydrogenolysis of vanillyl alcohol into MMP was much
more difficult than the hydrogenation of vanillin into vanillyl
alcohol. There were few reports on the kinetic studies of the
transfer hydrogenation of vanillin into NMP. The activation
energy of the deoxygenation of vanillin over the Co/N-C-600
catalyst was a little higher than that over the noble metal Ru/C
catalyst (41.2 kJ/mol).[31]
greatly enhanced at an early reaction stage with 96.5% after 60
min. There results once again revealed that the hydrogenation
of vanillin into vanillyl alcohol was much easier than the
hydrogenolysis of vanillyl alcohol into MMP. MMP was observed
with a yield of 4.9% even in the first 10 min, which might be
produced from hydrogenolysis of hydrogenolysis alcohol (Path
B) or the direct hydrogenolysis of C=O as well (Path C). But it
can be concluded that the reaction should most probably
undergo the hydrogenation/hydrogenolysis process (Path B)
according to the Fig. 4, which was different from the direct
hydrogenolysis pathway over Pd nanoparticles loaded on of
MIL-101(Cr) [31]. It is also observed that MMP yield increased
much lower at the late reaction stage, due to the lower
concentration of vanillyl alcohol at that period. The highest MMP
yield was obtained in 96.6% with vanillin conversion of 99.6%
after 480 min at 150 oC and 10 bar H2.
Study of the possible reaction pathway
[H]
O
O
OH
-H2O
[H]
[H]
Path A:
R'
R'
R'
R'
R'
R'
R'
R'
R
R
R
R
R
R
[H]
[H]
OH
Path B:
Path C:
Reusability of the Co/N-C-600 catalyst
R
O
R'
R
R
100
80
60
40
20
0
Scheme 1. Reaction pathways for the deoxygenation of the carbonyl groups.
The transformation of carbonyl group into methyl group
processes via the following three pathways (Scheme 1):
hydrogenation/dehydration
hydrogenation/hydrogenolysis
(Path
B),
A),
direct
(Path
and
hydrogenolysis of the C=O bond (Path C). In the case of the
100
90
80
1
2
3
4
5
70
Recycling number
vanillin conversion
MMP yield
vanillyl alcohol yield
60
Fig. 5 Reusability of the Co/N-C-600 catalyst in the deoxygenation of vanillin.
Reaction conditions: vanillin (1 mmol), H2 (20 bar), catalyst (20 mg), iso-
propanol (10 mL), 150 oC.
50
40
30
20
10
0
As a heterogeneous catalyst, one of the most important merits is
that the catalyst should be recycled and reused without the loss
of its catalytic activity. Therefore, the recycling of the Co/N-C-
600 catalyst was investigated. As reported in our previous work,
the Co/N-C-600 catalyst can be facilely recovered from the
reaction mixture with the assist of an external magnet. The spent
Co/N-C-600 catalyst was washed with water, ethanol and iso-
propanol, respectively. Finally, the humid catalyst was used
directly for the next run under the identical conditions. As
demonstrated in Fig. 5, MMP yields were the same during the
recycling experiments. On the meanwhile, there was no
0
100
200
300
400
500
Time (min)
Fig. 4 Trend of reactants and product proportion as a function of reaction time.
Reaction conditions: vanillin (1 mmol), H2 (1 MPa), Co/N-C-600 (20 mg), iso-
propanol (10 mL), 150 oC.
transformation of vanillin into MMP, it should not undergo the
hydrogenation/dehydration pathway (Path A), as no hydrogen
atom is adjacent to the hydroxyl group in vanillyl alcohol. Thus,
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10.1002/cssc.201702078
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Full Paper
Conclusion
detectable cobalt in the reaction solution. These results
confirmed that the Co/N-C-600 catalyst was highly stable without
losing its activity and selectivity. The high stability of Co
nanoparticles in the Co/N-C-600 catalyst should be due to the
strong electronic interaction between Co nanoparticle and
nitrogen atoms.
In summary, we have developed an effective method for the
upgrade of vanillin into MMP, a common component in lignin-
derived bio-oil. The as-made Co/N-C-600 catalyst exhibits high
catalytic activity and selectivity in tandem hydrogenation–
deoxygenation of vanillin into MMP. The high catalytic activity of
the Co/N-C-600 catalyst should be unique characteristics of the
presence of nitrogen atoms and Co nanoparticles. The cobalt
nanoparticles should be act as the active center to absorb and
activate the H2, and N atoms act as Lewis base to help
heterocleavage of H2. Several reaction parameters have been
optimized, and it was found that the reaction temperature, the
hydrogen pressure and the reaction solvent showed a great
effect on the catalytic performance of Co/N-C-600 catalyst.
Comparison of the performance of CO/N-C-600 with
other catalytic systems
We have compared our developed method for the
deoxygenation of vanillin with other reported methods, which are
summarized in Table 2. It is worth noting that the catalytic
performance of the Co/N-C-600 catalyst was comparable or
even superior to some of the currently reported noble metal
catalysts (Table 2, Entries 1 vs 2 & 3) or higher than the reported
none-noble catalysts (Table 2, Entries 1 vs 4 & 5 ). For example,
Resasco and co-workers developed an emulsion system for the
reduction of vanillin over the Pd/SWNT/SiO2 catalyst, which
produced full vanillin conversion and ~95% selectivity of MMP
after 3 h at high temperature of 200 oC and 3.45 bar H2 pressure
(Table 2, Entry 2).[17] Our Co/N-C-600 catalyst can produced
nearly quantitative MMP yield after 4 h at a lower temperature of
o
Nearly quantitative MMP yield was obtained after 8 h at 150 C
and 10 bar H2 pressure. The deoxygenation of vanillin into MMP
mainly underwent the hydrogenation of vanillin into vanillyl
alcohol and the subsequent hydrogenolysis of vanillyl alcohol
into MMP, and the latter was the rate-determining step, which
had a much higher active energy. In addition, the Co/N-C-600
catalyst could be reused seven times without any loss in activity
and selectivity. The high catalytic efficiency of the Co/N-C-600
catalyst holds promising potential as a greener alternative to
noble-metal catalysts for the biofuel upgrade process.
o
170 C and 10 bar H2. Obviously, the use of non-noble metal
catalysts at lower temperature showed a great promising
potential in the practical application. Compared with the other
non-noble metal catalysts, the Co/N-C-600 catalyst was superior
to the currently reported non-noble catalysts. For example,
although active carbon supported molybdenum carbide
(Mo2C/AC) nanoparticles produced high vanillin conversion at 20
bar H2 and 100 oC, but the selectivity of MMP and the total
carbon balance were too low.[20] Many undetected side reactions
were present in the Mo2C/AC catalytic system. Although
hydrotalcite-supported Cu could produce MMP with 90%
selectivity at full conversion of vanillin, but it was performed
under harsh conditions at 180 oC and 40 bar H2 with a long
reaction time of 18 h.[21] It was believed the high catalytic activity
of the Co/N-C-600 catalyst was attributed to the synergetic effect
of nitrogen and Co nanoparticles in the activation of H2 to
promote the reduction process as we discussed in our previous
work. The nitrogen atoms would act as a Lewis base to
accelerate the heterolytic cleavage of H2 to generate the NH+
and Co-H- , which were the active species for the reduction of
polar unsaturated bonds.[26]
Experimental Section
Materials
All of the solvents were purchased from Sinopharm Chemical
Reagent Co., Ltd. (Shanghai, China). All of the chemicals were
purchased from Aladdin Chemicals Co. Ltd. (Beijing, China).
Water used in all experiments was purified by a Milli-Q system
(Millipore).
Catalytic activity test
In a typical run, 40 mL autoclave was charged with iso-propanol
(10 mL), vanillin (1 mmol), and Co/N-C-600 catalysts (20 mg).
Then the autoclave was purged five times with hydrogen to
completely remove the air. After being sealed, the autoclave was
charged with 10 bar H2 at room temperature, then it was heated
from room temperature to 150 oC and kept at 150 oC for 4 h with
a mechanic stirring at 1000 RPM (Revolutions per minute).
Afterwards, the autoclave was cooled to room temperature,
depressurized. The chemicals in the reaction solution were
analyzed by gas chromatography with toluene as the internal
standard.
Table 2. The results of our method and other methods for the deoxygenation
of vanillin
T.
H2
(bar)
Con.
(%)
Sel.
1(%)
Sel.
2(%)
Entry
Catalyst
Ref.
(oC)
This
work
Co/N-C-
600
1
2
3
4
5
170
200
150
100
180
10
3.45
10
100
100
98
0.5
~5.0
21
99
~95
79
Analytic methods
Pd/SWNT
/SiO2
17
16
20
21
It was performed on Agilent 7890A GC with autosampler and a
flame ionization detector. A HP-5 capillary column (30 m × 530
μm × 1.5 μm) was used for the separation of reaction mixtures.
The temperature of the column was initially kept at 80 °C for 3
min, and then was increased at a rate of 20 °C min-1 to 220 °C.
The products were identified by Agilent 6890N GC/5973MS as
Pd/C
Mo2C/AC
HT/Cu
20
99.6
100
2.4
00
57.4
90
40
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10.1002/cssc.201702078
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Full Paper
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10.1002/cssc.201702078
ChemSusChem
Full Paper
Liang Jiang, Peng Zhou, Chanjuan Liao, Zehui Zhang*,
Shiwei Jin
Page 1. – Page 6.
Nitrogen-doped carbon supported Co catalysts: An
effective none-noble metal catalyst for the upgrade of
biofuels
The nitrogen-doped carbon supported Co nanoparticles (Co/N-C-600)
exhibited high activity and stability for the deoxygenation of vanillin
into MMP under mild conditions. Nearly quantitative MMP yield was
obtained in iso-propanol after 8 h at 150 oC and 10 bar H2 pressure.
Moreover, the Co/N-C-600 catalyst demonstrated high stability.
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