Synthesis of bulk and supported nickel phosphide using microwave radiation for hydrodeoxygenation of methyl palmitate

Article abstract of DOI:10.1039/c5ra09020b

In this paper, we proposed a novel method for preparing bulk and supported Ni₂P catalysts under mild conditions. Ni₂P and Ni₂P/SiO₂ were synthesized from nickel hypophosphite precursors at 230 °C for 5 min using a CEM Discover microwave reactor, and the initial reaction temperature is about 202 °C. The catalysts were characterized using XRD, TEM, SEM, XPS, BET, carbon monoxide chemisorption, and the catalytic performance was tested for hydrodeoxygenation (HDO) of methyl palmitate in a fixed-bed reactor. Interestingly, microwave irradiation does not result in sintering of Ni₂P particles. The principal products of the HDO reaction for both catalysts are pentadecane and hexadecane. Isomerization products were not detected, and other by-products content is very low (<1%). The HDO results demonstrate that the catalyst prepared using a microwave has better activity than that prepared using calcination.

Full text of DOI:10.1039/c5ra09020b

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PAPER
Synthesis of bulk and supported nickel phosphide
using microwave radiation for hydrodeoxygenation
of methyl palmitate
Cite this: RSC Adv., 2015, 5, 53623
ab
Yinan Xue,ab Qingxin Guan
and Wei Liab
*
In this paper, we proposed a novel method for preparing bulk and supported Ni2P catalysts under mild
conditions. Ni2P and Ni2P/SiO2 were synthesized from nickel hypophosphite precursors at 230 ꢀC for 5 min
using a CEM Discover microwave reactor, and the initial reaction temperature is about 202 ꢀC. The
catalysts were characterized using XRD, TEM, SEM, XPS, BET, carbon monoxide chemisorption, and the
catalytic performance was tested for hydrodeoxygenation (HDO) of methyl palmitate in a fixed-bed
reactor. Interestingly, microwave irradiation does not result in sintering of Ni2P particles. The principal
products of the HDO reaction for both catalysts are pentadecane and hexadecane. Isomerization products
were not detected, and other by-products content is very low (<1%). The HDO results demonstrate that the
catalyst prepared using a microwave has better activity than that prepared using calcination.
Received 14th May 2015
Accepted 10th June 2015
DOI: 10.1039/c5ra09020b
www.rsc.org/advances
showed 100% conversion and 85% selectivity to n-pentane, with
higher oxygen removal activity and less C–C bond cracking
1. Introduction
activity than commercial noble metal Ru/C and Pd/Al2O3 catalysts
based on the same amount of active sites.8
In recent years, synthesis of renewable fuel from biomass has
attracted considerable attention. The HDO reaction is an
important reaction for the preparation of renewable fuel.
However, conventional hydrogenation catalysts (such as CoMo/
Al2O3 and NiMo/Al2O3) do not possess sufficient stability under
HDO reaction conditions, because the sulfur content will be
gradually decreased during the reaction.1 Therefore, more and
more studies focus on seeking noble metal catalysts and other
novel catalytic materials to promote HDO efficiently.
Metal phosphides, a class of interstitial compounds, have
attracted considerable attention for their unexpected properties
in catalysis, magnetic applications, and applications in telecom-
munications, optoelectronic devices, lithium batteries, and solar
cells.2,3 In particular, Ni2P used as hydrodesulfurization catalyst
shows excellent activity and resistance to sulfur poisoning
performance.2 In recent studies, Ni2P as a highly effective HDO
catalyst shows good activity and stability in converting biomass to
renewable oil.4–10 J. Moon and coworkers studied the active sites
of Ni2P/SiO2 catalyst for HDO of guaiacol by using XAFS and DFT
study. They revealed that the active site of Ni2P is composed of a
threefold hollow Ni and P sites which lead to adsorption of H or
OH groups.7 A. Iino and coworkers studied the reaction mecha-
nism of Ni2P/SiO2 catalyst for HDO of 2-methyltetrahydrofuran at
a medium pressure of 0.5 MPa. They concluded that Ni2P/SiO2
As a class of new catalytic materials, the synthesis and appli-
cations of metal phosphides underwent tremendous development
in the recent decades. A variety of methods for synthesizing metal
phosphides have been reported.11–26 Generally, the synthesis of
metal phosphide has become highly convenient, which has
broken through the limitations of high temperature and heating
rate.19,20 In this paper, we proposed an alternative path to the
synthesis of nickel phosphide. Ni2P and Ni2P/SiO2 were synthe-
ꢀ
sized from nickel hypophosphite precursor at 230 C for 5 min
using a CEM DISCOVER microwave reactor. Interestingly, micro-
wave radiation does not result in sintering of Ni2P particles. The
HDO results show that the catalyst prepared using microwave
having better activity than that prepared using calcination.
Microwave is an electromagnetic wave with a frequency of
2.45 GHz and commonly used in materials synthesis.27–29 It is
mainly through the polarization of electromagnetic waves on
the polar molecules to achieve a uniform temperature inside the
sample. In our previous studies, bulk and supported Ni2P could
be synthesized from the decomposition of nickel hypo-
phosphite precursor in a static protecting gas atmosphere at
250 C.20 Herein, we use microwave radiation instead of tradi-
ꢀ
tional tube furnace to synthesize bulk and supported Ni2P. The
reaction may occur at a high temperature is expressed in ionic
formula as follows:
aCollege of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry
(Ministry of Education), China
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Nankai University, Tianjin, 300071, China. E-mail: qingxinguan@nankai.edu.cn;
Fax: +86-22-23504436; Tel: +86-22-23504436
6Ni2+ + 8H2PO2 ¼ 3Ni2P + P + 4H3PO4 + 4H+
(1)
ꢁ
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250–290 W). The product was cooled to ambient temperature
under Ar and was washed several times with deionized water to
remove ion impurities, aer which the wet material was dried at
2. Experimental methods
SiO2 was purchased from Tianjin Chemist Scientic Ltd.,
China. It has a specic surface area of 279 m2 gꢁ1, a pore volume
of 0.8 cm3 gꢁ1. Other reagents were analytical pure grade and
purchased from Alfa Aesar.
ꢀ
120 C for 3 h.
2.2. Characterization
The characterization was performed for passivated sample,
which was exposed to the air and without any reduction treat-
ment. Powder X-ray diffraction (XRD) was performed on a
Bruker D8 focus diffractometer, with Cu Ka radiation at 40 kV
and 40 mA. The transmission electron microscope (TEM)
images were purchased using a Philips Tecnai G2 F-20 eld
emission gun TEM. Scanning electron microscope (SEM)
images were obtained using a Tescan Vega3 SBH. Brunauer–
Emmett–Teller (BET) specic surface areas were obtained using
nitrogen adsorption/desorption measurements at 77 K with a
BELSORP-Mini instrument. Samples were degassed at 300 ꢀC
for 3 h before measurement. The surface area was calculated
using a multi-point BET model. X-ray photoelectron s pectro-
scopy (XPS) was carried out using a Kratos Axis Ultra DLD
spectrometer employing a monochromated Al-Ka X-ray source
(hn ¼ 1486.6 eV), hybrid (magnetic/electrostatic) optics and a
multi-channel plate and delay line detector (DLD). All XPS
spectra were recorded using an aperture slot of 300 ꢂ 700
microns, survey spectra were recorded with a pass energy of
80 eV, and high-resolution spectra with a pass energy of 40 eV.
In order to subtract the surface charge effect, the C1s peak was
xed at a binding energy of 284.6 eV. The CO chemisorption was
performed with Micromeritics Chemisorb 2750 gas-adsorption
equipment. The sample was loaded into a quartz reactor and
2.1. Synthesis of bulk and supported nickel phosphides
2.1.1. Calcination method. The preparation of bulk Ni2P is
illustrated as follows. Nickel chloride (NiCl2$H2O) and sodium
hypophosphite (NaH2PO2$H2O) were dissolved in deionized
water under stirring with a mole ratio of P/Ni ¼ 1.5/1. Aer
stirring for 1 h, the solution was evaporated slowly to dehydrate,
ꢀ
and was dried at 80 C for 8 h to obtain the precursor. At the
beginning of the thermal decomposition, the air in the reactor
was removed by owing Ar, aer which the precursor was
treated at 250 ꢀC for 30 min in a static Ar atmosphere (heating
rate: 5–20 ꢀC minꢁ1). The product was cooled to ambient
temperature under Ar and was washed several times with
deionized water to remove ion impurities, aer which the wet
ꢀ
material was dried at 120 C for 3 h.
The preparation of Ni2P/SiO2 is illustrated as follows.
NiCl2$H2O and NaH2PO2$H2O were dissolved in deionized water
under stirring with a mole ratio of P/Ni ¼ 1.5/1. Aer stirring for
1 h, the solution was impregnated onto the SiO2 using incipient
wetness impregnation. Then, the wet solid were evaporated
slowly to dehydrate, and were dried at 80 ꢀC for 8 h to obtain the
precursor. At the beginning of the thermal decomposition, the air
in the reactor was removed by owing Ar, aer which the
ꢀ
precursor was treated at 250 C for 30 min in a static Ar atmo-
sphere (heating rate: 5–20 ꢀC minꢁ1). The product was cooled to
ambient temperature under Ar and was washed several times
with deionized water to remove ion impurities, aer which the
wet material was dried at 120 ꢀC for 3 h.
ꢀ
pretreated in 10% H2/Ar at 450 C for 3 h. Aer cooling in He,
pulses of 10%ꢀCO/He in a He carrier (25 mL (NTP) minꢁ1) were
injected at 35 C through a loop tube.
2.1.2. Microwave method. The preparation of bulk Ni2P is
illustrated as follows NiCl2$H2O and NaH2PO2$H2O were dis-
solved in deionized water under stirring with a mole ratio of P/Ni
¼ 1.5/1. Aer stirring for 1 h, the solution was evaporated slowly
to dehydrate, and was dried at 80 ꢀC for 8 h to obtain the
precursor. At the beginning of microwave radiation, the air in the
reactor was removed by owing Ar, aer which the precursor was
2.3. Catalytic activity test
HDO catalytic activities were evaluated using 30 wt% methyl
palmitate in decalin. HDO reaction was carried out in a xed-bed
microreactor. The catalyst was pelleted, crushed, and sieved with
20–40 mesh; 1.0 g of the catalyst was diluted with SiO2 to a
volume of 5.0 mL in the reactor. Prior to the reaction, catalysts
were pretreated in situ with owing H2 (100 mL minꢁ1) for 3 h.
The testing conditions for the HDO reaction were 1 MPa, weight
hourly space velocity (WHSV) ¼ 6 hꢁ1, and H2/oil ¼ 1000. Liquid
products were collected every hour aer a stabilization period of
6 h. Both feed and products were analyzed with an Agilent 7890A/
5975C GC-MS equipped with a ame ionization detector and a
HP-5 column. The TOF was calculated using eqn (2):
ꢀ
radiated at 230 C for 5 min in a static Ar atmosphere (heating
power of CEM Discover reactor is 250–290 W). The product was
cooled to ambient temperature under Ar and was washed several
times with deionized water to remove ion impurities, aer which
the wet material was dried at 120 ꢀC for 3 h.
The preparation of Ni2P/SiO2 is illustrated as follows.
NiCl2$H2O and NaH2PO2$H2O were dissolved in deionized
water under stirring with a mole ratio of P/Ni ¼ 1.5/1. Aer
stirring for 1 h, the solution was impregnated onto the SiO2
using incipient wetness impregnation. Then, the wet solid were
evaporated slowly to dehydrate, and were dried at 80 ꢀC for 8 h
FA0 XA
TOF ¼
(2)
W COuptake
to obtain the precursor. At the beginning of microwave radia- where FA0 is the molar rate of reactant fed into the reactor
tion, the air in the reactor was removed by owing Ar, aer (mmol sꢁ1), W is the catalyst weight (g), COuptake is the uptake
which the precursor was radiated at 230 ꢀC for 5 min in a static of chemisorbed CO (mmol gꢁ1), and XA is the reactant
Ar atmosphere (heating power of CEM Discover reactor is conversion (%).
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can be synthesized at a mole ratio of P/Ni ¼ 1.5. Similarly, the
weak diffraction peaks of Ni12P5 were detected when the mole
ratio of P/Ni is 1.34 (eqn (1)). Different with our previous results,
the diffraction peaks of Ni5P4 were detected when the mole ratio
of P/Ni is 1.75, which indicates that P is a slight excess for the
synthesis of bulk Ni2P in this mole ratio. This result also shows
that the microwave method has better utilization of phosphorus
than calcination method.
3. Results and discussion
3.1. Synthesis of bulk and supported nickel phosphides
Similarly, NaH2PO2$H2O and NiCl2$6H2O were used as a raw
material. Fig. 1a shows the XRD patterns of the samples
prepared at different temperatures for 10 min from precur-
sors with a P/Ni mole ratio of 1.5. The result indicates that
Ni2P can be synthesized at 230 and 250 ꢀC. The strong
diffraction peaks of NaCl were detected when the temperature
is 200 ꢀC, which indicates the precursor was not decomposed
and 200 ꢀC is lower than the lowest reaction temperature
To study the effect of irradiation time on the bulk Ni2P
product, the precursors with different irradiation times at
ꢀ
230 C were analysed by XRD. The diffraction peaks of Ni12P5
were detected in the precursor aer being radiated at 230 ꢀC for
2 min, and these diffraction peaks disappeared aer 5 min
(Fig. 3). Crystal structure of Ni2P was not damaged when irra-
diation time extended to 10 minutes. The result indicates that
there is a rapid crystal phase transition process from Ni12P5 to
Ni2P, and Ni2P can be synthesized in 5 min.
ꢀ
(Fig. 1b). From the temperature curves of 230 and 250 C, we
can clearly see that this is an exothermic reaction, and the
initial reaction temperature is between 202 and 206 ꢀC. When
a decomposition reaction occured, the measured temperature
suddenly increased, and then rapidly came down to the
setting values.
In our previously published paper,20 Ni2P was synthesized in
the mole ratio of P/Ni range of 1.5 to 1.75 using a calcination
method. To study the inuence of the microwave irradiation
deeply, a series of samples were prepared with different mole
ratios of P/Ni. As shown in Fig. 2, we can clearly see that Ni2P
Fig. 4 shows the SEM images of bulk Ni2P prepared using
different methods. Surprisingly, microwave radiation at 230 ꢀC
did not cause serious sintering of metal particles, which might
because microwave radiation is very uniform. Although irradi-
ation will cause sudden increase in temperature, but the
apparent temperature does not exceed 300 ꢀC. On the contrary,
the average particle size of Ni2P synthesized using calcination
method is signicantly larger than that of synthesized using the
microwave method.
Fig. 5 shows the XRD patterns and TEM images of 30 wt%
Ni2P/SiO2 catalysts prepared using different methods. Two
supported catalysts having similar weak diffraction peaks of
Ni2P (PDF# 65-1989). The XRD results show that Ni2P/SiO2
catalysts can be synthesized at a mole ratio of P/Ni ¼ 1.5
using different methods. From the TEM images, we can
clearly see that Ni2P particles prepared using microwave
method had signicantly better dispersion. Subsequent CO
uptake and HDO test also show Ni2P particles are well
dispersed on SiO2.
XPS analysis was carried out to observe the valence of Ni and
P elements on 30 wt% Ni2P/SiO2 surfaces (Fig. 6). The Ni2p
Fig. 1 (a) The XRD patterns of the samples prepared at different
temperatures for 10 min from the precursors with a mole ratio of P/Ni
¼ 1.5, (b) microwave temperature patterns of the samples in different Fig. 2 The XRD patterns of bulk Ni2P prepared with different mole
setting values.
ratios of P/Ni (microwave radiation at 230 ꢀC for 10 min).
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Fig. 3 The XRD patterns of bulk Ni2P prepared with different micro-
wave radiation time (microwave radiation at 230 ꢀC, P/Ni ¼ 1.5).
spectrum of Ni2P mainly involves two peaks. The rst one is
centered at 852.9 eV and assigned to Nid+ of Ni2P, and the
second one is at 856.3 eV, corresponding to Ni2+ ions interacting
possibly with phosphate ions as a consequence of a supercial
passivation. For P2p, binding energy at 129.7 eV is attributed to
Pdꢁ of Ni2P and 133.0 eV is attributed to surface nickel phos-
phate species.19,26 Some difference is that the binding energy of
calcination sample shi slightly to a higher value than that of
microwave sample. Temperature and time of synthesis of
Ni2P/SiO2 using calcination method are higher than that using
the microwave method. The shi of binding energy may be
explained by a stronger interaction between Ni2P with SiO2 at
high temperature.
Fig. 5 The XRD patterns and TEM images of 30 wt% Ni2P/SiO2 cata-
lysts prepared using different methods, (a) XRD patterns, (b) prepared
using calcination method (P/Ni ¼ 1.5, 250 ꢀC, 30 min), and (c) prepared
using microwave method (P/Ni ¼ 1.5, 230 ꢀC, 5 min).
Fig. 4 The SEM images of bulk Ni2P prepared using different methods,
(a) and (b) prepared using calcination method (P/Ni ¼ 1.5, 250 ꢀC, Fig. 6 The XPS spectra of 30 wt% Ni2P/SiO2 catalysts prepared using
30 min), (c) and (d) prepared using microwave method (P/Ni ¼ 1.5, different methods: (a) Ni2p core level spectra of Ni2P/SiO2, (b) P2p
230 ꢀC, 5 min).
core level spectra of Ni2P/SiO2.
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Table 1 The physical properties and HDO activities of different cata-
lysts (testing conditions: 1 MPa, 290 ꢀC, WHSV ¼ 6 hꢁ1, H2/oil ¼ 1000)
as,BET
CO
Conversion
(%)
TOF
Samples
(m2 gꢁ1
)
(mmol gꢁ1
)
(sꢁ1
)
SiO2
279
118
117
—
6.0
8.0
—
27.0
78.3
—
0.08
0.18
30 wt% Ni/SiO2
30 wt% Ni2P/SiO2
(calcination)
30 wt% Ni2P/SiO2
(microwave)
120
9.8
99.4
0.19
Scheme 1 The HDO route of methyl palmitate over Ni2P/SiO2.
As shown in Fig. 7, Ni2P/SiO2 prepared using microwave
method has a signicantly higher HDO activity at low reaction
temperatures. The principal products of the HDO reaction for
two catalysts are pentadecane and hexadecane. Isomerization
products were not detected, and the content of other by-
products is very low (<1%). BET specic surface area of two
catalysts was 117, and 120 m2 gꢁ1, respectively (Table 1).
However, Ni2P/SiO2 prepared using microwave method has
higher CO uptake, which also indicates that catalyst prepared
by microwave had a better dispersion of Ni2P particles. Two
3.2. Catalytic activity
To study the effect of two preparation methods on catalyst
performance, 30 wt% Ni2P/SiO2 catalysts were synthesized and
tested at different temperatures. HDO catalytic activities were
evaluated using 30 wt% methyl palmitate in decalin (testing
conditions: 1 MPa, WHSV ¼ 6 hꢁ1, H2/oil ¼ 1000). According to
published papers,9,30–33 we give a possible HDO route of methyl
palmitate over Ni2P/SiO2 (Scheme 1).
ꢀ
catalysts have similar TOF value at 290 C, which is 0.18 and
0.19 sꢁ1. For comparison, 30 wt% Ni/SiO2 were prepared
using the incipient wetness impregnation. As shown in Table
1, Ni/SiO2 shows very low conversion (27.0%) and TOF value
(0.08 sꢁ1).
To clearly understand the advantage of the microwave
method, comparison chart of the two methods was given. As
shown in Scheme 2, two methods use same precursor and
protective gas. The difference is that the microwave method
requires lower reaction temperature and shorter time.
Including heating and cooling steps, the entire microwave
process is only about een minutes, which is far below the
calcination method.
Fig. 7 The catalytic activities of 30 wt% Ni2P/SiO2 catalysts at different
temperatures (a) prepared using the calcination method (P/Ni ¼ 1.5,
250 ꢀC, 30 min), and (b) prepared using the microwave method (P/Ni ¼
1.5, 230 ꢀC, 5 min).
Scheme 2 Pathway of two methods.
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´
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13 J. A. Cecilia, A. Infantes-Molina, E. Rodrıguez-Castellon and
In this paper, Ni2P and Ni2P/SiO2 were synthesized from nickel
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Discover microwave reactor, and the initial reaction tempera-
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Acknowledgements
This work was nancially supported by the NSFC (21376123,
U1403293), MOE (IRT-13R30 and 113016A), NSFT
(15JCQNJC05500), and the Research Fund for 111 Project
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