Highly dispersed molybdenum carbide nanoparticles supported on activated carbon as an efficient catalyst for the hydrodeoxygenation of vanillin

Article abstract of DOI:10.1039/c5ra00866b

Characterized by XRD and TEM, highly dispersed molybdenum carbide (Mo₂C) nanoparticles with a diameter of 1-4 nm were effectively synthesized on activated carbon at 700 °C. The Mo₂C-based catalyst exhibited high activity and stability for the hydrodeoxygenation (HDO) of vanillin under mild conditions (100 °C, 1.0 MPa of H₂, 3 h) in aqueous solution. According to the distribution of products with time, a HDO mechanism involving vanillyl alcohol as an intermediate product was proposed. Moreover, after being recycled several times, the loss of catalytic activity was negligible, which demonstrated that the Mo₂C-based catalyst had the property of resistance to deactivation.

Full text of DOI:10.1039/c5ra00866b

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Highly dispersed molybdenum carbide
nanoparticles supported on activated carbon as an
efficient catalyst for the hydrodeoxygenation of
vanillin
Cite this: RSC Adv., 2015, 5, 43141
*
Lili He, Yu Qin, Hui Lou and Ping Chen
Characterized by XRD and TEM, highly dispersed molybdenum carbide (Mo2C) nanoparticles with a
diameter of 1–4 nm were effectively synthesized on activated carbon at 700 ꢀC. The Mo2C-based
catalyst exhibited high activity and stability for the hydrodeoxygenation (HDO) of vanillin under mild
conditions (100 ꢀC, 1.0 MPa of H2, 3 h) in aqueous solution. According to the distribution of products
with time, a HDO mechanism involving vanillyl alcohol as an intermediate product was proposed.
Moreover, after being recycled several times, the loss of catalytic activity was negligible, which
demonstrated that the Mo2C-based catalyst had the property of resistance to deactivation.
Received 15th January 2015
Accepted 15th April 2015
DOI: 10.1039/c5ra00866b
www.rsc.org/advances
deactivation and low energy efficiency caused by the severe
reaction conditions. Other catalysts include: zeolites,12,13 tran-
sition metal catalysts14 and so on. Among these catalysts, Mo2C-
based catalysts have been extensively studied due to their
inexpensive cost and platinum-like characteristics.15–17 Addi-
tionally, their attractive properties such as coke resistance, high
selectivity, water-stability and chemical stability18,19 also make
them excellent candidates for many reactions, such as hydro-
genation,15 hydrodesulfurization,20 hydrocarbon isomeriza-
tion,21 methane reforming22 and so on.
Our previous research concerning the high catalytic activity
of Mo2C supported on AC (Mo2C/AC) in the HDO of vegetable
oils demonstrated that the permeated carbon in the lattice of
the Mo metal caused elongation of the Mo–Mo distance and an
increase in the d-band electron density at the Fermi level of
molybdenum, which was proved to be effective for
hydrogenisation.23,24
In this paper, we used a carbothermal hydrogen reduction
(CHR) method based on TPR to prepare Mo2C/AC catalyst at 700
ꢀC, and we explored the effects of different reaction conditions
on the HDO of vanillin, which is one typical aldehyde
compound produced during oil pyrolysis derived from the
lignin fraction. The results of catalytic tests conducted at mild
conditions in aqueous solution stated that the Mo2C-based
catalyst showed high selectivity toward C]O bonds, and
possessed the properties of water-stability and coke resistance.
1. Introduction
Nowadays, the continuous requirement for energy, which is
mainly obtained from fossil fuel sources, has caused the
exhausting of petroleum reserves. With the arising world-wide
environmental concerns, the search for renewable feedstocks
as a replacement for traditional fossil fuels to meet the huge
future needs is required urgently. Due to their environmental-
friendliness, world-wide abundance and renewable nature
within a relatively short cycle,1,2 bio-oils, as a prospective
resource for replacing fossil fuels have attracted more and more
attention. However, due to their high oxygen content,3 bio-oils
must be up-graded to meet the requirements of the current
infrastructure; carboxylic acids, hydroxyketones, hydroxy-
aldehydes, phenolic compounds and dehydrosugars constitute
the main oxygenated compounds in bio-oils. Accordingly, to
achieve the desired composition an active catalyst must be
applied here. Previous researches have already made great
efforts in searching for an appropriate catalyst for up-grading
bio-oils. Noble metal catalysts, such as Pd,4,5 Ru,6,7 Rh6 and
Pt8,9 heterogeneous and homogeneous catalysts deposited on
various supports have been employed for up-grading bio-oils,
however, the limitations of high cost, coke deposition and low
selectivity toward carbon–oxygen bonds made them an unsat-
isfactory choice. As alternatives to the noble metal catalysts,
sulde NiMo and CoMo catalysts10,11 with various surface
chemistries have also been widely researched, however, their
application in industry is restricted by their low resistance to
2. Experimental section
2.1 Materials
Department of Chemistry, Xixi Campus, Zhejiang University, Hangzhou 310028,
Zhejiang, P. R. China. E-mail: c224@zju.edu.cn; Fax: +86-571-88273283; Tel: +86-
571-88273283
All reagents were commercially available and were used as
supplied
without
further
purication.
HNO3,
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ꢀ
(NH4)6Mo7O24$4H2O (AHM), vanillin, raw activated carbon, range of 80–180 C aer being pressurized with hydrogen to a
ethanol, anisole, glucose and deionized water were used relevant pressure (0.6 MPa, 1.0 MPa, 2.0 MPa), and then reacted
throughout the experiments.
for a certain time.
Regeneration of the catalyst. The catalyst ltered from the
mixture aer reaction was reuxed in ethanol at 85 ꢀC for 4
hours; the separated catalyst was treated at 290 ꢀC in the ow of
air for 2 hours to remove the aꢀdsorbate; then reduction of the
2.2 Methods
2.2.1 Preparation of catalysts
Catalyst preparation. Supported on AC, the different loading catalyst was carried out at 700 C for 2 hours in the ow of H2
of Mo2C catalysts were accomplished using the CHR method. using the CHR method, and the reduced catalyst was passivated
Prior to impregnation, the carbon was treated with HNO3 to in a ow of 1.0% O2/N2 for 12 hours at room temperature.
remove amorphous carbon and to increase the number of
2.2.3 Analytical methods. Aer each reaction, the gas
surface oxygen-containing functional groups which was useful products were collected and qualitatively analyzed on a GC (sp-
for the metal deposition and dispersion.25 The pretreatment of 6800A) equipped with a packed column (TDX-01), while the
the raw activated carbon was carried out by mixing raw activated qualitative analysis of the liquid products was performed using
carbon (4.00 g) and 6 M HNO3 (100.0 mL) at 80 ꢀC for 3 hours. GC-MS (HP 6890GC and 5973MSD) with a HP-5 column.
Aer the mixture being cooled to room temperature, the sample Thereaer, with anisole as the internal standard, the quantita-
was washed with plenty of water and dried at 120 ꢀC for one tive analysis was carried out using high performance liquid
night. Incorporation of Mo onto the carbon support to prepare chromatography (HPLC) equipped with Hypersil BDS (C18
the catalysts with different loading of Mo2C was completed by chromatographic column with the diameter of 5 mm, 150 mm ꢂ
mixing activated carbon (2.00 g), the corresponding amount of 4.6 mm) at an ultraviolet ray wavelength of 270 nm. The mass
AHM and 15 mL of deionized water in a 100 mL round- balance was dened as the molar ratio of products aer the
bottomed ask, followed by sonication at 120 W for 1.5 hours reaction to the total vanillin in the feed (mass balance ¼ (P + M)/
and removal of the water at 60 ꢀC with a rotatory evaporator. The M0, where P was the molar mass of the products, M was the
sample was then dried at 120 ꢀC for 12 hours. All of these amount of vanillin aer the reaction and M0 was the amount of
catalysts were carbonized at 700 ꢀC to obtain the desired vanillin in the feed, respectively).
molybdenum phase. The carbonization process was performed
2.2.4 Catalyst characterization. Raman spectra were
in a H2 ow by heating to 450 ꢀC from room temperature at the obtained using a LabRam HR800 at an excitation wavelength of
ramping rate of 5 ꢀC minꢁ1 and 1 ꢀC minꢁ1 from 450 ꢀC to 514.5 nm. X-ray diffraction (XRD) patterns were recorded on a
700 ꢀC, and holding for 2 hours at the nal temperature. Before RIGAKUD/MAX2550/PC diffractometer using Cu Ka radiation
exposure to air, the catalyst was passivated in a ow of 1.0% O2/ (l ¼ 0.15406 nm) at a scan rate of 5ꢀ minꢁ1 over the range of 10ꢀ
N2 for 12 hours at room temperature. Herein, the as-synthesized to 80ꢀ. Transmission electron microscopy (TEM) was performed
catalyst was denoted as Mo2C/AC, and the theoretical percent- with a FEI Tecnai F30 microscope at an accelerating voltage of
ages of Mo2C to carbon in the prepared catalysts were 10.0, 20.0, 300 kV. The acidity of the catalysts was evaluated using NH3-
30.0, 50.0 and 80.0%, respectively. The CHN analysis result temperature-programmed desorption (NH3-TPD) with a BEL
revealed that the content of C, H and N in 20.0% Mo2C/AC was CAT-B instrument. Before adsorption, the samples were acti-
68.8, 0.75 and 0.64%, respectively. The theoretical content of C vated in helium at 500 C (10 C minꢁ1 ramp) for 1 hour, then
in 20.0% Mo2C/AC was 84.0%, so the percentage carbon loss cooled to 100 ꢀC, and saturated with NH3 for 30 min. Subse-
during the preparation was 47.1%. According to the previous quently, the samples were purged with He at 50 mL minꢁ1 for 15
ꢀ
ꢀ
reports,26 in the process of preparing Mo2C through
temperature-programmed reaction, CH4, CO and H2O would be adsorbed state, and then the temperature was increased to
a
min to remove NH3 in the gas phase and in the physically
ꢁ1
ꢀ
ꢀ
released as gaseous products, which would account for the 550 C at increments of 10 C min
carbon loss. According to the CHN analysis, the actual content
.
of Mo2C in 20.0% Mo2C/AC was 31.7%, but in this article we still
denoted the catalyst as 20.0% Mo2C/AC.
Preparation of the bulk Mo2C catalyst. Glucose was employed
3. Results and discussion
3.1 Characterization of Mo2C/AC catalysts
as the carbon source in the preparation of the bulk Mo2C The XRD patterns of the catalysts with different loading are
catalyst. The CHR method was used with some modication to shown in Fig. 1. In agreement with the standard XRD patterns of
guarantee that the glucose was pyrolyzed to carbon. Specically, hexagonal close-packed b-Mo2C, the diffraction peaks located at
the sampꢁle1 was heated to 400 ꢀC at the rate of 5 ꢀC minꢁ1, then 34.4ꢀ, 38.0ꢀ, 39.4ꢀ, 61.5ꢀ and 69.6ꢀ, illustrating that b-Mo2C was
ꢀ
1 C min to 700 ꢀC and held for 2 hours.
successfully formed at 700 ꢀC. Additionally, using Scherrer’s
2.2.2 Experimental procedure
formula, the average size of the 20.0% Mo2C/AC crystallites at
HDO reactions. All reactions were carried out in a 100 mL (101) was 2.6 nm, which was in good agreement with the average
stainless steel autoclave equipped with a mechanical stirrer. For value predicted by TEM (2.6 nm), while the value was 15.0 nm
a typical run, the catalyst (0.050 g for the loaded catalyst, and for 50.0% Mo2C/AC, suggesting that the particle size increased
0.100 g for the bulk catalyst) was introduced into the reactor with loading.
together with reagent (0.100 mmol of vanillin) and water (20.0
The TEM images of 20.0% Mo2C/AC shown in Fig. 2, together
mL). Subsequently, the samples were heated to a temperature with XRD results, revealed that the Mo2C particles with an
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The TEM images of catalysts with different loading were shown
in Fig. 3, from which it was easily concluded that the particles
were well-distributed on the surface with low loading, while
with increased loading the stacking of the particles resulted in
agglomeration.
In the Raman spectra of 20.0% Mo2C/AC and untreated AC
shown in Fig. 4, aer loading, the increase in the intensity of the
D band at ꢃ1350 cmꢁ1, a defect-induced Raman feature
revealing the imperfect crystalline structure of the material, was
related to the oxygen-containing functional groups introduced
by the treatment with HNO3 and the passivation with 1.0% O2/
N2.
Compared with AC, the NH3-TPD results displayed in Fig. 5
revealed that the 20.0% Mo2C/AC catalyst had a NH3 desorption
peak located at a temperature around 179 ꢀC, indicating that the
loading of active components imported weak acid sites, which
could be explained by the presence of Mo–OH on the catalyst
surface. Furthermore, according to previous reports, the weak
acidity of the catalyst not only contributes to the adsorption of
reactant and the process of hydrogenation, but also accounts for
less mass balance.27
Fig. 1 XRD patterns of (a) 50.0% Mo2C/AC and (b) 20.0% Mo2C/AC.
As can be seen in Table 1, the Brunauer–Emmett–Teller
(BET) results of AC and the Mo2C/AC catalysts with different
loading showed that loading of Mo2C caused the reduction of
SBET, which was induced by the slight destruction of the AC
surface in the carbonization procedure of preparing Mo2C. Aer
loading, the pore volume increased within a certain range of
loading due to the insertion of Mo2C particles, while with high
loading the overlapping particles would block the pores and
result in a decreased pore volume.
Fig. 2 TEM images and the diameter distribution of the 20.0% Mo2C/
The N2 adsorption–desorption isotherm of the Mo2C/AC
catalyst is shown in Fig. 6. The isotherm was of type II accord-
ing to IUPAC classication. The adsorption and desorption
branches of the isotherm coincide, that is, there was no
adsorption–desorption hysteresis. Depending on the surface
properties of the catalyst, it can be considered as the stage of
monolayer formation.
AC catalyst.
average diameter of 2.6 nm were well dispersed on the support.
Fig. 2b showed the TEM micrographs of Mo2C with the d-
spacing values of 0.221 nm for the (101) crystallographic planes.
Fig. 3 TEM images (a–d) of the Mo2C/AC catalysts with a loading of
10.0, 30.0, 50.0 and 80.0%, respectively.
Fig. 4 Raman spectra of (a) 20.0% Mo2C/AC and (b) untreated AC.
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3.2.2 The distribution of products with time. As can be
seen in Fig. 7, in the rst 1.5 hours, the reaction was accom-
panied by a rapid increase in the conversion of vanillin and
yield of vanillyl alcohol, while a slow increase in the yield of p-
creosol, suggesting that vanillin was mainly hydrogenated to
vanillyl alcohol in the rst step. As the reaction proceeded
further, the main initial product, vanillyl alcohol, was gradually
converted to p-creosol via hydrogenolysis; meanwhile, the
conversion increased with time until it reached approximately
100%. Based on the experimental results, we speculated that the
reaction was a consecutive reaction consisting of two rst-order
reactions. In the rst step the vanillin adsorbed by acid sites was
hydrogenated to vanillyl alcohol. The second step was
completed by the hydrogenation and dehydration of vanillyl
alcohol to p-creosol. Correspondingly, the HDO mechanism was
proposed and shown in Fig. 8a, in which p-creosol was
produced by a side reaction. Additionally, based on the small
amount of CO detected in the gas products, the mechanism of
the reaction to guaiacol was shown in Fig. 8b.
Fig. 5 NH3-TPD profiles of the 20.0% Mo2C/AC catalyst and AC.
Table 1 Textural parameters for AC and the Mo2C/AC catalysts with
different loading
3.2.3 The effect of different reaction conditions. The
experimental results of reactions catalyzed by the Mo2C/AC
catalysts with different loading were shown in Fig. 9. With a
93.2% conversion of vanillin and a 50.1% yield of p-creosol
Pore volume
Sample
SBET (m2 gꢁ1
)
(cm3 gꢁ1
)
ꢀ
under mild conditions (1.0 MPa, 100 C, 3 h), the highest effi-
AC
967
945
794
683
494
418
0.16
0.26
0.22
0.14
0.10
0.07
ciency vanillin HDO reaction was achieved over the 20.0%
Mo2C/AC catalyst. Associated with the increase in conversion of
vanillin and yield of p-creosol, the increase in loading induced
an increase in catalytic activity within a certain range of loading
percentage, but no more increase was observed when the
loading was higher than 50.0%. Based on the XRD and TEM
results, it can be said that the stacking of the active particles
accounted for the agglomeration and low catalytic activity.
Compared with the result for catalysis by bulk Mo2C, the high
activity of Mo2C/AC implied that the AC played a part in
dispersing the active sites and changing the characteristics of
the catalysts.
10.0% Mo2C/AC
20.0% Mo2C/AC
30.0% Mo2C/AC
50.0% Mo2C/AC
80.0% Mo2C/AC
Results displayed in Fig. 10 demonstrated that the reaction
temperature had a signicant inuence on the conversion of
vanillin and yield of products. When the temperature rose from
80 ꢀC to 100 ꢀC, the conversion of vanillin improved remarkably
from 51.4% to 98.6%, demonstrating that, at this range of
temperature, the reaction was thermodynamically controlled.
When conducted at temperatures higher than 100 ꢀC, the
increase in conversion with temperature was negligible, sug-
gesting that the HDO reaction was under kinetic control at these
conditions. The reaction temperature also had inuence on the
distribution of products. Hydrogenation of vanillin to vanillyl
alcohol was the main reaction at a low temperature (80 ꢀC),
while, owing to the promoted rate of dehydration by the
elevated reaction temperature, p-creosol with a yield of 63.7%
became the dominating product at 100 ꢀC. Further increasing of
temperature offered an increase in the yield of p-creosol, as well
as in the yield of guaiacol. A high yield of p-creosol of 82.0% was
achieved at 180 ꢀC. According to the previous reports, as the
temperature elevated, the accelerated speed of dehydrogenation
would cause an increase in polycondensation,28 which was
Fig. 6 Nitrogen adsorption–desorption isotherm of the 20.0% Mo2C/
AC catalyst.
3.2 HDO of vanillin
3.2.1 The blank reaction. The blank experiment with no
catalyst or AC was ran at 100 ꢀC with 1.0 MPa of H2 for 3 hours to
verify the active part for this reaction. The results shown in
Table 2 clearly indicated that the HDO reaction could not
proceed without the catalyst and the effect of the AC was
negligible.
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Table 2 The results of the blank reactiona
Yield (%)
t
(h)
Conv.
(%)
Mass balance
(%)
Catalyst
Mass (g)
T (ꢀC)
p-Creosol
Vanillin alcohol
No
AC
Mo2C
—
0.05
0.10
100
100
150
3
3
6
0
3.0
33.8
0
0
12.9
0
3.0
17.1
100
100
99.9
a
Reaction conditions: vanillin (0.1 mmol), water (20 mL), p(H2) ¼ 1.0 MPa.
Fig.
9 The results for the reactions catalyzed by catalysts with
Fig. 7 The changes in the concentration of products and reactant
over time in the presence of 50.0% Mo2C/AC at 100 ꢀC and 0.6 MPa of
H2.
different loading. Reaction conditions: 50.0% Mo2C/AC, p(H2) ¼ 1.0
MPa, 100 ꢀC for 3 hours. A is for the conversion of vanillin. B and C are
for the yield of p-creosol and vanillyl alcohol, respectively.
Fig. 8 The mechanisms of the vanillin HDO (a) and decarbonylation
(b).
believed to be responsible for the loss in mass balance.
Considering energy efficiency and distribution of products, the
optimal temperature was at 100 C.
To inspect the relationship between hydrogen pressure and
the rate of hydrogenation, tests with the initial pressure from 0.6
MPa to 2.0 MPa were conducted at 100 ꢀC for 3 hours. As shown
in Fig. 11, the results indicated that increasing of hydrogen
Fig. 10 Results of the reactions carried out at different reaction
temperatures. Reaction conditions: 50.0% Mo2C/AC, p(H2) ¼ 1.0 MPa,
for 3 hours. A is for the conversion of vanillin. B and C are for the
selectivity of p-creosol and vanillyl alcohol, respectively.
ꢀ
pressure could accelerate the step of dehydration to p-creosol 1.0 MPa to 2.0 MPa was attributed to the capacity for adsorption
signicantly. However, the gradual increase in the conversion of of hydrogen. In view of the equipment and the capacity for
vanillin and yield of p-creosol with the pressure increase from adsorption, the optimal hydrogen pressure was at 1.0 MPa.
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and were highly dispersed on the support. Taking the experi-
mental results and energy efficiency into account, the optimal
conditions for the HDO reaction catalyzed by the Mo2C/AC
catalyst were considered to be a temperature of 100 ꢀC, 1.0
MPa of H2 for 3 hours, and 20.0% for catalyst loading. The
Mo2C/AC catalysts were shown to have the character of water-
stability, and high selectivity to the C]O bond under mild
conditions. Moreover, due to the fact that it can be recycled
several times with slight loss of activity, it can be said that the
Mo2C/AC catalyst is coke resistant and possesses promising
prospects for being used for up-grading bio-oil in the future.
Acknowledgements
This work was nancially supported by the National Basic
Research Program of China (2013CB228104).
Fig. 11 The effect of H2 pressure on the HDO reaction. Reaction
conditions: 50.0% Mo2C/AC, 100 ꢀC for 3 hours. A is for the conversion
of vanillin. B and C are for the selectivity of p-creosol and vanillyl
alcohol, respectively.
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