Hydrodeoxygenation of methyl palmitate over MCM-41 supported nickel phosphide catalysts

Article abstract of DOI:10.1016/j.cattod.2015.03.010

In this paper, bulk and MCM-41-supported Ni₂P catalysts were synthesized by the one-step direct reduction of metal phosphate precursors. The role of cations and anions in the synthesis of Ni₂P was studied. Catalytic properties for the hydrodeoxygenation (HDO) of methyl palmitate were first investigated in a continuous-flow fixed-bed microreactor. The resulting catalysts were characterized using XRD, TEM, BET, XPS, and carbon monoxide chemisorption. Results indicated that Ni₂P can be synthesized at 600 °C from its phosphate precursors with a mole ratio of Ni/P = 2/1. Based on the analytical result of GC-MS, we propose a possible HDO reaction mechanism for the HDO of methyl palmitate on MCM-41-supported nickel phosphide catalysts.

Full text of DOI:10.1016/j.cattod.2015.03.010

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Hydrodeoxygenation of methyl palmitate over MCM-41 supported
nickel phosphide catalysts
Qingxin Guan, Fanfan Wan, Fei Han, Zihe Liu, Wei Li∗
College of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin), Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, bulk and MCM-41-supported Ni2P catalysts were synthesized by the one-step direct reduc-
tion of metal phosphate precursors. The role of cations and anions in the synthesis of Ni2P was studied.
Catalytic properties for the hydrodeoxygenation (HDO) of methyl palmitate were first investigated in
a continuous-flow fixed-bed microreactor. The resulting catalysts were characterized using XRD, TEM,
BET, XPS, and carbon monoxide chemisorption. Results indicated that Ni2P can be synthesized at 600 ◦C
from its phosphate precursors with a mole ratio of Ni/P = 2/1. Based on the analytical result of GC-MS,
we propose a possible HDO reaction mechanism for the HDO of methyl palmitate on MCM-41-supported
nickel phosphide catalysts.
Received 11 November 2014
Received in revised form
16 December 2014
Accepted 5 March 2015
Available online xxx
Keyword:
Hydrodeoxygenation
Methyl palmitate
Nickel phosphide
Phosphate
© 2015 Elsevier B.V. All rights reserved.
Pentadecane
1. Introduction
furnace at 600 ◦C. This step will limit the application of some carrier,
such as carbon, which will react with oxygen at high temperatures
The global energy shortage has become an important factor
restricting the development of the global economy. In recent
times, biomass has attracted much attention as a sustainable
resource with advantages over fossil fuels such as renewability,
lower expense, a short CO2 cycle, and protection of the environ-
ment. Many routes for biomass conversion to liquid transportation
fuels have been considered [1]. In the 1980s, first-generation
biodiesel was reported, its main component is a fatty acid methyl
ester (FAME). Second-generation biodiesel can be obtained by
hydrodeoxygenation (HDO) and isomerization of FAME, which is
similar to a diesel fuel component and is composed of alkanes [2,3].
To date, most of the attention has been focused on the HDO reac-
tions, mainly involving the cleavage of C–O and C–C bonds [4–7].
In recent years, however, transition metal phosphide catalysts have
attracted much attention for HDO [8–12].
and waste energy. In this paper, bulk and MCM-41-supported Ni2P
catalysts were synthesized by the direct reduction of metal phos-
phate precursors in one step. The role of cations and anions in the
synthesis of metal phosphides also was studied. Compared with the
traditional temperature-programmed reduction method [18–20]
and direct reduction (DR) method [17], the metal phosphate pre-
cursors could be directly reduced in a H2 atmosphere and skip the
step of calcination in a muffle furnace at 600 ◦C. Various Ni2P/MCM-
41 catalysts were synthesized and tested for the HDO of methyl
palmitate, which is a derivative of palm oil and widely distributed
in nature. Compared with palmitic acid, methyl palmitate has a
lower melting point (30 ◦C), and is more suitable for use in labora-
tory studies. However, all available reports on the HDO of methyl
palmitate describe experiments carried out in a stainless steel batch
autoclave reactor [21,22]. We now report on an interesting study
of MCM-41-supported nickel phosphide (NixP/MCM-41) catalysts
for the HDO of methyl palmitate in a fixed-bed microreactor.
In our previous studies, metal phosphide catalysts were syn-
thesized by the thermal decomposition of metal hypophosphite
and oxide precursors [13–17]. The oxide precursors were, however,
obtained from the calcination of metal phosphate in air in a muffle
2. Experimental methods
∗
Corresponding author at: Key Laboratory of Advanced Energy Materials Chem-
MCM-41 was purchased from Tianjin Chem. Sci. Ltd., China.
It has a specific surface area of 840.6 m2 g−1, a pore volume of
0.8 cm3 g−1, and a BJH average pore size of 3.3 nm. The other
istry (Ministry of Education), College of Chemistry, Nankai University, No. 94 Weijin
Road, Tianjin, China. Tel.: +86 22 23508662; fax: +86 22 23508662.
E-mail address: weili@nankai.edu.cn (W. Li).
http://dx.doi.org/10.1016/j.cattod.2015.03.010
0920-5861/© 2015 Elsevier B.V. All rights reserved.
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reagents were of analytical pure grade and purchased from Tianjin
Guangfu Fine Chemical Research Institute, China.
were used to evaluate the HDO activity. The TOF was calculated
using Eq. (1):
FAo
XA
TOF =
,
(1)
W COuptake
2.1. Synthesis of bulk and supported nickel phosphides
where FAo is the molar rate of reactant fed into the reactor
(␮mol s−1), W the catalyst weight (g), COuptake the uptake of
chemisorbed CO (␮mol g−1), and XA is the reactant conversion (%).
The synthesis of nickel phosphide involves two main steps: (1)
the metal phosphate precursor is obtained by incipient wetness
impregnation, followed by drying, and (2) the phosphate precur-
sor is converted to nickel phosphide in flowing H2. In a typical
experiment, bulk nickel phosphide was obtained as follows. Ini-
tially, 5.82 g (0.02 mol) Ni(NO3)2·6H2O and 0.98 g (0.01 mol) H3PO4
were dissolved in 20 mL of deionized water and stirred for 1 h.
Then, the slurry was evaporated at 120 ◦C to obtain the precur-
sor. Subsequently, the precursor was reduced to nickel phosphide
under flowing H2 (60 mL min−1) at 600 ◦C for 1 h. Finally, the prod-
uct was cooled to ambient temperature under flowing H2, and
then passivated for 1 h under flowing 1% O2/N2. Similar to the syn-
thesis of bulk nickel phosphide, supported nickel phosphide was
prepared by reducing the supported phosphate precursor under
flowing H2. In order to evaluate the effects of cations and anions,
Ni(NO3)2·6H2O and H3PO4 can be replaced by other salts (such as
NiCl2 and Na3PO4).
3. Results and discussion
3.1. Synthesis of bulk and supported nickel phosphides
In our previous studies, metal phosphide catalysts were synthe-
sized by the direct reduction of oxide precursors [17], which uses a
low H2 flow speed and is not affected by the heating rate. The reduc-
tion temperature of the metal phosphate precursors was same as
that used in the DR method.
In this paper, the role of cations and anions in the synthesis of
nickel phosphides was first studied. Fig. 1 shows the XRD patterns
of washed and unwashed samples of nickel phosphides synthe-
sized at 600 ◦C from NiCl2 and Na3PO4 precursors with a mole ratio
of Ni/P = 10/5. Diffraction peaks of Na4P2O7 (PDF# 1-356) and NaCl
(PDF# 5-628) were detected in the unwashed sample. In addition
to generating NaCl, sodium ions also generate Na4P2O7, which con-
sumes some of the phosphate. This will lead to the generation of less
phosphorus in nickel phosphide. The XRD result of a washed sample
proved this; the XRD pattern showed that the sample was a mixture
of Ni12P5, Ni5P2, and Ni. In Fig. 2, Na2HPO4 and Na3PO4 were used
as the phosphorus sources. Fig. 2 shows the XRD patterns of bulk
nickel phosphides synthesized at 600 ◦C from different precursors
with different mole ratios of Ni/P. All the results indicate that the
phosphorus content of nickel phosphide increased with a decrease
in mole ratio of Ni/P. Compared with Na2HPO4, the effect of Na3PO4
on the synthesis of nickel phosphide was more significant. Fig. 3
shows the XRD patterns of bulk nickel phosphides synthesized at
600 ◦C from different precursors with a mole −ra1tio of Ni/P = 10/5.
2.2. Characterization
In our studies, powder X-ray diffraction (XRD) was performed
on
a Bruker D8 focus diffractometer, with Cu K␣ radiation
at 40 kV and 40 mA. Transmission electron microscopy (TEM)
images were acquired using a Philips Tecnai G2 F-20 field emis-
sion gun electron microscope. The Brunauer–Emmett–Teller (BET)
specific surface areas were obtained using nitrogen adsorp-
tion/desorption measurements at 77 K with
a BELSORP-mini
instrument. X-ray photoelectron spectroscopy (XPS) was carried
out using a Kratos Axis Ultra DLD spectrometer employing a
monochromated Al-K␣ X-ray source (hv = 1486.6 eV), hybrid (mag-
netic/electrostatic) optics and a multi-channel plate and delay line
detector (DLD). All XPS spectra were recorded using an aperture
slot of 300 ␮m × 700 ␮m, survey spectra were recorded with pass
energy of 80 eV, and high-resolution spectra with pass energy of
40 eV. In order to subtract the surface charge effect, the C1s peak
was fixed at a binding energy of 284.6 eV. The carbon monoxide
(CO) chemisorption was performed with Micromeritics ChemiSorb
2750 gas adsorption equipment. The sample was loaded into a
quartz reactor and pretreated in 10% H2/Ar at 450 ◦C for 3 h. After
The results indicate that Cl−1, OH−1, and NO3
had no effect on
the synthesis of nickel phosphide. Their similarities are that, at reac-
tion temperature, they can generate volatile substances, which can
be removed by the flowing H2. The above results show that the
non-volatile cations can affect the generation of nickel phosphide.
Fig. 4 shows the XRD patterns of 15 wt% NixP/MCM-41 catalysts
synthesized at 600 ◦C from Ni(NO3)2 and H3PO4 precursors with
different mole ratios of Ni/P. We can clearly see that the diffrac-
tion peaks of Ni12P5 and Ni2P were detected from the precursor
cooling in He, pulses of 10% CO/He in a He carrier (25 mL min−1
were injected at 30 ◦C through a loop tube.
)
2.3. Catalytic activity test
The HDO catalytic activities were evaluated using 50 wt% methyl
palmitate in decalin. The HDO reaction was carried out in a
continuous-flow fixed-bed microreactor. The height of the fixed-
bed microreactor is 700 mm, which has a constant temperature
range of 200 mm. The inner diameter of fixed-bed microreactor
is 15 mm. The catalyst was pelleted, crushed, and sieved with
20–40 mesh. Then 2.0 g of the catalyst was diluted with SiO2 to
a volume of 10.0 mL in the reactor. Prior to the reaction, catalysts
were pretreated in situ with flowing H2 (100 mL min−1) for 3 h. The
testing conditions for the HDO reaction were 3 MPa, weight hourly
space velocity (WHSV) = 3 h−1, and H2/oil = 1000. Liquid products
were collected every hour after a stabilization period of 6 h. Both
feed and products were analyzed with an Agilent 7890A/5975C GC-
MS equipped with a flame ionization detector and an HP-5 column.
The methyl palmitate conversion and turnover frequency (TOF)
Fig. 1. The XRD patterns of washed and unwashed samples synthesized at 600 ◦
from NiCl2 and Na3PO4 precursors with a mole ratio of Ni/P = 10/5.
C
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Fig. 4. The XRD patterns of 15 wt% NixP/MCM-41 catalysts synthesized at 600 ◦
from Ni(NO3)2 and H3PO4 precursors with different mole ratios of Ni/P.
C
5 to 20 wt%. The result indicates that the synthesis of supported
nickel phosphide was affected by loading and mole ratios of Ni/P.
Fig. 6 shows the TEM images of NixP/MCM-41 catalysts synthe-
sized at 600 ◦C from Ni(NO3)2 and H3PO4 precursors with a mole
ratio of Ni/P = 0/5, and with different loadings: (a) 5 wt%, (b) 10 wt%,
(c) 15 wt%, and (d) 20 wt%. The results clearly show that size of
the nanoparticles of nickel phosphides increased as the loading
increased from 5 to 20 wt%. Increasing the loading will make the
active component significantly sintering.
Fig. 7 shows the XPS spectra of 15 wt% NixP/MCM-41 synthe-
sized at 600 ◦C from Ni(NO3)2 and H3PO4 precursors with a mole
ratio of Ni/P = 10/5. The Ni2p core level spectrum of nickel phos-
phide involves two peaks. The first one is assigned to Ni␦+ of nickel
phosphide and is centered at 852.9 eV. The second one is centered
at 857.0 eV, corresponding to Ni2+ ions possibly interacting with
phosphate ions as a consequence of superficial passivation. Mean-
while, the P2p binding energy of nickel phosphide was observed at
129.4 eV for P␦− of nickel phosphide and 134.5 eV for surface nickel
phosphate species [23]. According to the result in Fig. 5, we can see
that Ni12P5 is the main product for 15 wt% NixP/MCM-41. However,
the binding energy of Ni2p at 852.9 eV and P2p at 129.4 eV is not
obvious, which may be due to surface of Ni12P5 is oxidized as a
consequence of superficial passivation.
Fig. 2. The XRD patterns of bulk nickel phosphides synthesized at 600 ◦C from dif-
ferent precursors with different mole ratios of Ni/P.
with mole ratios of Ni/P = 10/5, and 10/8, respectively. The results
indicate that, compared with bulk nickel phosphide, the synthesis
of supported nickel phosphide requires that the precursor has a
slight excess of phosphorus.
Fig. 5 shows the XRD patterns of NixP/MCM-41 catalysts synthe-
sized at 600 ◦C from Ni(NO3)2 and H3PO4 precursors with a mole
ratio of Ni/P = 10/5, and with different loadings. We can clearly see
that the diffraction peaks of Ni12P5 were detected from the pre-
cursor with 10 wt% loading. Under the same mole ratio, diffraction
peaks of Ni2P were detected at 20 wt% loading. The phosphorus con-
tent of nickel phosphides increased as the loading increased from
Table 1 shows the physical properties of different samples. Com-
pared with a MCM-41 support, all the catalysts showed a significant
Fig. 5. The XRD patterns of NixP/MCM-41 catalysts synthesized at 600 ◦C from
Ni(NO3)2 and H3PO4 precursors with a mole ratio of Ni/P = 10/5, and with different
loadings.
Fig. 3. The XRD patterns of bulk nickel phosphides synthesized at 600 ◦C from dif-
ferent precursors with a mole ratio of Ni/P = 10/5.
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Fig. 6. The TEM images of NixP/MCM-41 catalysts synthesized at 600 ◦C from Ni(NO3)2 and H3PO4 precursors with a mole ratio of Ni/P = 10/5, and with different loadings:
(a) 5 wt%, (b) 10 wt%, (c) 15 wt%, and (d) 20 wt%.
Table 1
decrease in specific surface area. The specific surface area of the
The physical properties and HDO catalytic activities of different samples (testing
sample decreased with loading amount increased. This is mainly
because the increasing loading will lead to obviously sintering of
nickel phosphide, which can be seen in the TEM images.
conditions: 3 MPa, 290 ◦C, WHSV = 3 h−1, H2/oil = 1000).
Catalyst
SBET
(m2 g−1
CO uptakes
Conversion
(%)
TOF (s−1
)
)
(␮mol g−1
)
MCM-41
840.6
581.1
542.4
492.2
450.6
–
–
–
5 wt% NixP/MCM-41
10 wt% NixP/MCM-41
15 wt% NixP/MCM-41
20 wt% NixP/MCM-41
2.8
3.0
3.1
2.9
79.7
84.6
84.5
79.7
0.44
0.44
0.43
0.43
3.2. Catalytic activity
The HDO catalytic activities were evaluated using 50 wt% methyl
palmitate in decalin. The methyl palmitate conversion, selectivity,
and TOF were used to evaluate the HDO activity. Unless otherwise
noted, the NixP/MCM-41 catalysts were synthesized at 600 ◦C from
Ni(NO3)2 and H3PO4 precursors with a mole ratio of Ni/P = 10/5.
Fig. 8 shows the catalytic activities of 15 wt% NixP/MCM-41 before
and after activation (testing conditions: 3 MPa, WHSV = 3 h−1
,
H2/oil = 1000). The result indicates that the catalyst activity was
similar after reduction at 400 ◦C for 3 h.
Fig. 9 shows the catalytic activities of catalysts at different
loadings. The results in Fig. 9a show that the HDO conversion of
all the catalysts gradually increased as the reaction temperature
increased. The results in Fig. 9b show that the selectivity of pen-
tadecane (C15) and hexadecane (C16) exhibited opposite trends as
Fig. 7. The XPS spectra of 15 wt% NixP/MCM-41 synthesized at 600 ◦C from Ni(NO3)2
and H3PO4 precursors with a mole ratio of Ni/P = 10/5.
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Fig. 8. The catalytic activities of 15 wt% NixP/MCM-41 before and after activation
(testing conditions: 3 MPa, WHSV = 3 h−1, H2/oil = 1000).
the loading increased. As shown in Fig. 5, at the same mole ratio
of Ni/P, the phosphorus content of nickel phosphides increased as
the loading increased. For NixP/MCM-41 catalysts, the increasing
loading will also lead to obviously sintering of nickel phosphide.
The decreased selectivity of C16 may be due to the increasing
phosphorus content of nickel phosphides and sintering of nickel
phosphide. Table 1 shows the TOFs of NixP/MCM-41 catalysts with
different loadings. The results indicate that they had 2.8, 3.0, 3.1,
and 2.9 ␮mol g−1 CO uptakes; the TOFs at 290 ◦C were 0.44, 0.44,
Fig. 10. The catalytic activities of catalysts prepared with different mole ratios of
Ni/P: (a) conversion at different reaction temperatures, (b) selectivity at 290 ◦
(testing conditions: 3 MPa, WHSV = 3 h−1, H2/oil = 1000).
C
0.43, and 0.43 s−1, respectively. The active sites of different catalysts
have similar TOF values.
Fig. 10 shows the catalytic activities of catalysts prepared with
different mole ratios of Ni/P. As shown in Fig. 10a and b, the
HDO conversion and selectivity of C16 gradually decreased with
decreasing mole ratios of Ni/P in the precursor. Meanwhile, the
amount of C15 gradually increased. Similar to the results in Fig. 9,
Fig. 9. The catalytic activities of catalysts at different loadings, (a) conversion at
different reaction temperatures, (b) selectivity at 290 ◦C (testing conditions: 3 MPa,
WHSV = 3 h−1, H2/oil = 1000).
Fig. 11. The catalytic activities of 15 wt% NixP/MCM-41 at different WHSVs (testing
conditions: 3 MPa, 290 ◦C, H2/oil = 1000).
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Fig. 12. The catalytic activities of 15 wt% NixP/MCM-41 at different reaction tem-
Fig. 13. The catalytic activities of 15 wt% NixP/MCM-41 and 15 wt% Ni/MCM-
peratures (testing conditions: 3 MPa, WHSV = 3 h−1, H2/oil = 1000).
41 at different reaction temperatures (testing conditions: 3 MPa, WHSV = 3 h−1
H2/oil = 1000).
,
our studies, 15 wt% NixP/MCM-41 was synthesized at 600 ◦C, and
15 wt% Ni/MCM-41 was synthesized at 350 ◦C. Hence, nanoparticle
of Ni may have a better dispersion than Ni2P.
the decreased selectivity of C16 may be due to the increasing phos-
phorus content of nickel phosphides. These results indicate that a
decreasing mole ratio of Ni/P in the precursor is unfavorable to the
HDO of methyl palmitate.
We have carried out experiments to get different intermediates
by decrease reaction temperature and increase WHSV. However,
only palmitic acid and hexadecanol were detected but had a low
content. We did not detect other intermediates, such as hexade-
canal, hexadecane, and pentadecene. Based on the above GC-MS
results and published similar reports, we propose a possible HDO
reaction mechanism of methyl palmitate on MCM-41-supported
nickel phosphide catalysts. As shown in Fig. 14, the reaction path-
way of the HDO of methyl palmitate involves nine steps. Methyl
palmitate could produce hexadecanal by removal of methanol (step
1). Simultaneously, methyl palmitate could produce palmitic acid
by hydrogenation followed by methane removal (step 2) [24]. Only
one path to produce hexadecanol, which is obtained by hydro-
genation of hexadecanal (step 3) [25]. Hexadecanol could produce
hexadecene by dehydration (step 4), and hexadecane could be gen-
erated from hexadecene by a hydrogenation (step 5) [26]. Palmitic
acid could directly generate pentadecane by a decarbonation (step
9) [21]; it could also generate hexadecanal by dehydration (step
6) [6,27]. Hexadecanal could directly generate pentadecene by
removel of the CO (step 7) [28], which could then be hydrogenated
to produce pentadecane (step 8). It is worth noting that palmitic
acid and hexadecanol were detected but had a very low content
(<1.5%), due to high catalytic activity.
Fig. 11 shows the catalytic activities of 15 wt% NixP/MCM-41 at
different WHSVs (testing conditions: 3 MPa, 290 ◦C, H2/oil = 1000).
It was clearly evident that HDO conversion and selectivity of
C16 gradually decreased as the WHSV increased. Meanwhile, the
amount of C15 and intermediate state products (such as palmitic
acid and hexadecanol) gradually increased.
Fig. 12 shows the catalytic activities of 15 wt% NixP/MCM-
41 at different reaction temperatures (testing conditions: 3 MPa,
WHSV = 3 h−1, H2/oil = 1000). HDO conversion and selectivity of
C15 gradually increased as the reaction temperature increased.
Meanwhile, the amounts of C16, palmitic acid, and hexadecanol
gradually decreased.
For comparison, we studied the catalytic activities of 15 wt%
NixP/MCM-41 and 15 wt% Ni/MCM-41. Fig. 13 shows the cat-
alytic activities of 15 wt% NixP/MCM-41 and 15 wt% Ni/MCM-41
at different reaction temperatures (testing conditions: 3 MPa,
WHSV = 3 h−1, H2/oil = 1000). The result indicates that Ni/MCM-
41 catalyst having better catalytic activity at low temperatures.
However, their catalytic activity becomes similar with increasing
reaction temperature. Difference in activity may be due to the
different catalyst preparation temperature. High preparation tem-
perature can cause sintering of the catalytic active components. In
Fig. 14. Reaction pathway of the HDO of methyl palmitate.
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4. Conclusions
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In summary, bulk and MCM-41-supported Ni2P catalysts were
synthesized by the direct reduction of metal phosphate precursors
in one step. The XRD results indicate that Ni2P can be synthesized at
600 ◦C from its phosphate precursors with a mole ratio of Ni/P = 2/1.
The synthesis of supported nickel phosphide, compared with bulk
nickel phosphide, requires the precursor to have a slight excess of
phosphorus. The role of cations and anions in the synthesis of Ni2P
was studied. The results indicate that Cl−1, OH−1, and NO3 had
−1
no effect on the synthesis of nickel phosphide. Their similarities are
that, at reaction temperature, they can generate volatile substances,
which can be removed by the flowing H2. The non-volatile cations
(Na+) can affect the generation of nickel phosphide. The catalytic
properties of the HDO of methyl palmitate were first investigated in
a continuous-flow fixed-bed microreactor. The results of the HDO
indicate that MCM-41-supported nickel phosphide catalysts exhib-
ited excellent activities. Finally, based on the analytical results of
GC-MS, we propose a possible HDO reaction mechanism for the
HDO of methyl palmitate on MCM-41-supported nickel phosphide
catalysts.
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Acknowledgments
This work was financially supported by the NSFC (21376123,
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Please cite this article in press as: Q. Guan, et al., Hydrodeoxygenation of methyl palmitate over MCM-41 supported nickel phosphide
catalysts, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.010